Lithium-Sulfur Batteries: Materials, Challenges and Applications 0323919340, 9780323919340

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Lithium-Sulfur Batteries

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Lithium-Sulfur Batteries Materials, Challenges and Applications

Edited by

Ram K. Gupta Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States

Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

Huaihe Song State key Laboratory of Chemical Resource Engineering, College of Materials and Engineering, Beijing University of Chemical Technology, Beijing, China

Ghulam Yasin Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-91934-0 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisitions Editor: Kayla Dos Santos Editorial Project Manager: Zsereena Rose Mampusti Production Project Manager: Prem Kumar Kaliamoorthi Cover Designer: Miles Hitchen

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Contents

List of contributors

xiii

Part One Basic principles

1

1

3

2

3

Introduction to electrochemical energy storage technologies Ghulam Yasin, Sehrish Ibrahim, Shumaila Ibraheem, Ali Saad, Anuj Kumar and Tuan Anh Nguyen 1 Introduction 2 History and recent advances 3 Energy storage mechanisms 4 Conclusion and outlook References

3 4 5 6 7

Recent developments in lithiumesulfur batteries Harshit Mahandra, Guillermo Alvial-Hein, Hadi Sharifidarabad, Fariborz Faraji and Ovender Singh 1 Introduction 2 Structure and components of lithiumesulfur battery 3 Mechanism and electrochemical properties of lithiumesulfur battery 4 Polysulfide shuttle effect 5 Recent developments 6 Comparison with other lithium-ion batteries 7 Applications of lithiumesulfur batteries 8 Conclusion and future perspectives References

11

Chemistry and operation of lithiumesulfur batteries Vikram K. Bharti, Sony K. Cherian, Mayur M. Gaikwad, Anil D. Pathak and Chandra S. Sharma 1 Introduction 2 Cell chemistry of lithiumesulfur battery 3 Operation of lithiumesulfur batteries 4 Electrochemical characteristics and challenges of lithiumesulfur batteries 5 Polysulfide formation and conversion

37

11 12 14 16 17 27 28 30 30

37 39 42 44 50

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Contents

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Summary and outlook References

High-performance lithiumesulfur batteries: role of nanotechnology and nanoengineering Shiva Bhardwaj, Felipe Martins de Souza and Ram K. Gupta 1 Introduction 2 Working principle of lithiumesulfur batteries 3 Challenges in lithiumesulfur batteries 4 Role of nanotechnology and nanoengineering in lithiumesulfur batteries 5 Conclusion References

52 52

57 57 58 61 65 71 71

Mathematical modeling of lithiumesulfur batteries Shunli Wang, Lili Xia, Chunmei Yu, Josep M. Guerrero and Yanxin Xie 1 Introduction 2 Electrochemical modeling 3 Equivalent circuit modeling 4 Parameter identification of equivalent-circuit model 5 Model application 6 Chapter summary References

75

Nanocomposites for binder-free Li-S electrodes Qiongqiong Lu and Xinyu Wang 1 Introduction 2 Carbon nanotube-based nanocomposites for binder-free electrodes 3 Graphene-based nanocomposites for binder-free electrodes 4 Carbon nanofiber-based nanocomposites for binder-free electrodes 5 Mxene-based nanocomposites for binder-free electrodes 6 Hybrid nanocomposites for binder-free electrodes 7 Summary and outlook References

99

Separators for lithiumesulfur batteries Praveen Balaji T and Soumyadip Choudhury 1 Introduction 2 Working principles of lithiumesulfur batteries 3 Role of battery components in controlling ultimate performance 4 Separator requirements 5 Design strategies for separator engineering 6 Conclusions and future outlook References

75 78 81 86 91 94 95

99 99 103 107 109 112 115 116 121 121 122 123 123 129 147 147

Contents

vii

8

Progress on separators for high-performance lithiumesulfur batteries 157 Ruiping Liu and Jin-Lin Yang 1 Introduction 157 2 Critical benchmarks for lithiumesulfur battery interlayers 158 3 Recent studies of various interlayers 158 4 Perspectives and outlooks 173 Acknowledgments 173 References 174

9

Electrolytes for lithiumesulfur batteries Dheeraj Kumar Maurya and Subramania Angaiah 1 Introduction 2 Organic liquid electrolytes 3 Ionic liquid electrolytes 4 Polymer electrolytes 5 Inorganic ceramic electrolytes 6 Future perspective Acknowledgment References

179 179 181 185 186 189 193 194 194

Part Two Nanomaterials and nanostructures for sulfur cathodes

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Porous carbonesulfur composite cathodes Zhen Li and Bin He 1 Microporous carbon-based cathodes for lithiumesulfur batteries 2 Mesoporous carbon-based cathode for lithiumesulfur batteries 3 Hierarchical carbon-based cathode for lithiumesulfur batteries 4 Surface functionalized porous carbon for lithiumesulfur battery cathodes 5 Summary and perspective Acknowledgments References Recent advancements in carbon/sulfur electrode nanocomposites for lithiumesulfur batteries P. Rajkumar, K. Diwakar, R. Subadevi and M. Sivakumar 1 Introduction 2 Preparation of carbon/sulfur nanocomposites 3 Physical and electrochemical performance of carbon/sulfur nanocomposite cathodes 4 Conclusion Acknowledgments References

207 209 210 215 217 218 218

225 225 226 226 234 235 235

viii

12

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Contents

Advances in nanomaterials for sulfurized carbon cathodes Rodrigo V. Salvatierra, Dustin K. James and James M. Tour 1 Introduction 2 Sulfurized carbon basics 3 Elucidated structure and electrochemical profile 4 Recent progress of sulfurized carbon and future trends 5 Concluding remarks References

241

Grapheneesulfur composite cathodes Runwei Mo 1 Introduction 2 Challenges limiting the development of lithiumesulfur batteries 3 Graphene-based composites in the lithiumesulfur batteries 4 Conclusions 5 Outlook References

271

Grapheneesulfur nanocomposites as cathode materials and separators for lithiumesulfur batteries Wei Ni and Ling-Ying Shi 1 Introduction 2 Cathode material modifications 3 Modified separators and functional interlayers 4 Techniques and methods 5 Structural design 6 Challenges and perspective List of abbreviations Acknowledgments References Grapheneesulfur nanohybrids for cathodes in lithiumesulfur batteries P. Rajkumar, G. Radhika, K. Diwakar, R. Subadevi and M. Sivakumar 1 Introduction 2 Grapheneesulfur composites for lithiumesulfur batteries 3 Conclusion Acknowledgments References

241 243 247 253 261 262

271 272 272 280 281 283

289 289 290 295 295 296 302 305 306 306

315 315 316 328 329 329

Metaleorganic framework based cathode materials in lithiumesulfur batteries 333 M.K. Shobana and B. Jeevanantham 1 Introduction 333 2 Metaleorganic frameworks 336

Contents

3 4

17

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20

ix

Metaleorganic frameworks as sulfur hosts Conclusion References

337 352 352

MXene-based sulfur composite cathodes Jianli Wang and Wei-Qiang Han 1 MXene/sulfur cathodes in lithiumesulfur batteries 2 MXene-based composite/sulfur cathodes in lithiumesulfur batteries 3 MXene-derived oxide/sulfur cathodes in lithiumesulfur batteries 4 Heteroatom-doped MXene/sulfur cathodes in lithiumesulfur batteries 5 Novel structured MXene/sulfur cathodes in lithiumesulfur batteries References

361

Polymeric nanocomposites for lithiumesulfur batteries Annelise Jean-Fulcrand, Eun Ju Jeon, Schahrous Karimpour and Georg Garnweitner 1 Introduction 2 Fundamentals of polymers 3 Polymer nanocomposites for sulfur cathodes 4 Polymer electrolytes 5 Conclusion and outlook References

389

Design of nanostructured sulfur cathodes for high-performance lithiumesulfur batteries Masud Rana, Xia Huang and Bin Luo 1 Introduction 2 Redox processes and polysulfide characteristics of lithiumesulfur batteries 3 Design criteria for lithiumesulfur battery cathodes 4 Sulfur host materials for lithiumesulfur batteries 5 Outlook and conclusion References Nanostructured additives and binders for sulfur cathodes Duc-Luong Vu, Rakesh Verma and Chan-Jin Park 1 Introduction 2 Background of nanostructured additives and binders for sulfur cathodes 3 Nanostructured additives for sulfur cathodes 4 Lithiumesulfur battery binders 5 Conclusion and outlook References

361 364 372 375 379 384

389 390 396 409 414 415

425 425 426 428 433 445 446 453 453 453 455 471 476 477

x

Contents

Part Three Lithium metal anodes: materials and technology

487

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Lithium metal anode: an introduction Ghulam Yasin, Noor Muhammad, Shumaila Ibraheem, Anuj Kumar, Tuan Anh Nguyen and Sehrish Ibrahim 1 Introduction 2 Metallic lithium anode 3 Past and recent developments 4 Suppression strategies for lithium dendrites 5 Conclusion References Advanced carbon-based nanostructure frameworks for lithium anodes Yanbo Fang, Vamsi Krishna Reddy Kondapalli, Kavitha Joseph, Mahnoosh Khosravifar, Yu-Yun Hsieh, Paa Kwasi Adusei, Sathya Narayan Kanakaraj, Guangqi Zhang and Vesselin Shanov 1 Introduction 2 Carbon-based interlayers 3 Carbon-based lithium hosts 4 Summary and outlook Acknowledgments References Carbon-based anode materials for lithium-ion batteries Mahesh P. Bondarde, Rini Jain, Ji Soo Sohn, Kshama D. Lokhande, Madhuri A. Bhakare, Pratik S. Dhumal and Surajit Some 1 Introduction 2 Carbon allotropes as anodic material for lithium-ion batteries 3 Carbon as anode material for lithium-ion batteries 4 Carbon nanotube and carbon nanotube-based nanomaterial as anode 5 Graphene and graphene-based nanomaterial as anode material 6 Conclusions and future directions References

Part Four 24

489 490 491 492 492 493

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499 500 506 514 514 514 521

521 522 525 530 534 538 539

Applications and future perspectives

547

Lithiumesulfur batteries for marine applications Daljit Kaur, Manmeet Singh and Sharanjit Singh 1 Introduction 2 Types of batteries used in marine systems 3 Lithiumesulfur batteries

549 549 555 565

Contents

4 5 6

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Battery management system Performance, weight, and cost analyses of various batteries in hybrid and electric marine vessels Conclusion Nomenclature References

Two-dimensional layered materials for flexible electronics and batteries Anupama B. Kaul and Mohin Sharma 1 Introduction 2 Overview of two-dimensional layered materials 3 Additive manufacturing of two-dimensional layered materials for flexible electronics 4 Incorporation of solution-processed two-dimensional layered MXenes 5 Summary and conclusions Acknowledgments References Sustainability of lithiumesulfur batteries Zhiqiang Zheng, Guang Xia, Jiajia Ye, Zhanghua Fu, Xuting Li, Mark J. Biggs and Cheng Hu 1 Introduction 2 Fundamental aspects of lithiumesulfur battery sustainability 3 Improving lithiumesulfur battery sustainability 4 Conclusions References Recyclability and recycling technologies for lithiumesulfur batteries Fariborz Faraji, Misagh Khanlarian, Melina Roshanfar, Guillermo Alvial-Hein and Harshit Mahandra 1 Introduction 2 Lithiumesulfur battery 3 Recycling technologies 4 Conclusion and future perspective References Recyclability, circular economy, and environmental aspects of lithiumesulfur batteries Grazyna Simha Martynkov a, Gabriela Kratosova, Silvie Brozova and Sajjan Kumar Sathish 1 Introduction 2 Composition and construction of lithiumesulfur batteries

569 572 573 574 574

579 579 584 585 594 597 598 598 603

603 604 608 620 620

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627 629 633 644 645

653

653 654

xii

Contents

3 4 5 6 7

Index

Battery design and fabrication Recycling lithiumesulfur batteries Greener strategies for processing lithium-bearing e-waste Environmental impact and circular economy in the battery industry Future designs and outlook for natural solutions Acknowledgment References Further reading

659 660 665 666 666 669 669 672 673

List of contributors

Paa Kwasi Adusei Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Guillermo Alvial-Hein The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada Subramania Angaiah Electro-Materials Research Laboratory, Centre for Nanoscience and Technology, Pondicherry University, Puducherry, India Praveen Balaji T Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Madhuri A. Bhakare Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, Maharashtra, India Shiva Bhardwaj Department of Physics, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States Vikram K. Bharti Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India Mark J. Biggs

Heriot-Watt University, Edinburgh, United Kingdom

Mahesh P. Bondarde Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, Maharashtra, India Silvie Bro
zov a Department of Non-ferrous Metals, Refining and Recycling, Faculty of Materials Technology, VSB - Technical University of Ostrava, Ostrava, Czech Republic Sony K. Cherian Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India Soumyadip Choudhury Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Pratik S. Dhumal Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, Maharashtra, India

xiv

List of contributors

K. Diwakar Energy Materials Lab, Department of Physics, Alagappa University, Karaikudi, Tamil Nadu, India Yanbo Fang Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Fariborz Faraji The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada Zhanghua Fu Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China Mayur M. Gaikwad Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India Georg Garnweitner Institute for Particle Technology, Technische Universit€at Braunschweig, Braunschweig, Germany; Laboratory for Emerging Nanometrology, Technische Universit€at Braunschweig, Braunschweig, Germany Josep M. Guerrero Department of Energy Technology, Villum Investigator Center for Research on Microgrids (CROM), Aalborg, North Jutland Province, Denmark Ram K. Gupta Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States Wei-Qiang Han School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, China Bin He State Key Laboratory of Material Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China; State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, China Yu-Yun Hsieh Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Cheng Hu Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China Xia Huang Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, St. Lucia, QLD, Australia Shumaila Ibraheem Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China; College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China

List of contributors

xv

Sehrish Ibrahim College of Life Science and Technology, Beijing University of Chemical Technology, Beijing, China Rini Jain

SSES Amaravatis Science College, Pauni, Maharashtra, India

Annelise Jean-Fulcrand Institute for Particle Technology, Technische Universit€at Braunschweig, Braunschweig, Germany; Laboratory for Emerging Nanometrology, Technische Universit€at Braunschweig, Braunschweig, Germany B. Jeevanantham Department of Physics, School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India Eun Ju Jeon Institute for Particle Technology, Technische Universit€at Braunschweig, Braunschweig, Germany; Laboratory for Emerging Nanometrology, Technische Universit€at Braunschweig, Braunschweig, Germany Kavitha Joseph Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Dustin K. James States

Chemistry Department, Rice University, Houston, TX, United

Sathya Narayan Kanakaraj Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Schahrous Karimpour Institute for Particle Technology, Technische Universit€at Braunschweig, Braunschweig, Germany Anupama B. Kaul Department of Electrical Engineering, PACCAR Technology Institute, University of North Texas, Denton, TX, United States; Department of Materials Science and Engineering, University of North Texas, Denton, TX, United States Daljit Kaur

Department of Physics, DAV University, Jalandhar, Punjab, India

Misagh Khanlarian Department of Civil and Environmental Engineering, Faculty of Engineering and Design, Carleton University, Ottawa, ON, Canada Mahnoosh Khosravifar Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Vamsi Krishna Reddy Kondapalli Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Gabriela Krato
sov a Nanotechnology Centre, CEET, VSB - Technical University of Ostrava, Ostrava, Czech Republic Anuj Kumar Nano-Technology Research Laboratory, Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India Zhen Li State Key Laboratory of Material Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China

xvi

List of contributors

Xuting Li Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China Ruiping Liu China

China University of Mining & Technology (Beijing), Beijing, P.R.

Kshama D. Lokhande Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, Maharashtra, India Qiongqiong Lu Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V., Dresden, Germany Bin Luo Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, St. Lucia, QLD, Australia Harshit Mahandra The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada Felipe Martins de Souza Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States Gra
zyna Simha Martynkov a Nanotechnology Centre, CEET, VSB - Technical University of Ostrava, Ostrava, Czech Republic Dheeraj Kumar Maurya Electro-Materials Research Laboratory, Centre for Nanoscience and Technology, Pondicherry University, Puducherry, India Runwei Mo School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, Shanghai, China Noor Muhammad Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning, China Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam Wei Ni State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, ANSTEEL Research Institute of Vanadium & Titanium (Iron & Steel), Chengdu, Sichuan, China; Material Corrosion and Protection Key Laboratory of Sichuan Province, Sichuan University of Science and Engineering, Zigong, Sichuan, China; Vanadium and Titanium Resource Comprehensive Utilization Key Laboratory of Sichuan Province, Panzhihua University, Panzhihua, Sichuan, China Chan-Jin Park Department of Materials Science and Engineering, Chonnam National University, Gwangju, Gyeonggi, South Korea Anil D. Pathak Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India

List of contributors

xvii

G. Radhika Energy Materials Lab, Department of Physics, Alagappa University, Karaikudi, Tamil Nadu, India; Department of Physics, Rajalakshmi Institute of Technology, Chennai, Tamil Nadu, India P. Rajkumar Energy Materials Lab, Department of Physics, Alagappa University, Karaikudi, Tamil Nadu, India Masud Rana Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, St. Lucia, QLD, Australia Melina Roshanfar Department of Civil Engineering, Faculty of Engineering, University of Ottawa, Ottawa, ON, Canada Ali Saad Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China; College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Sajjan Kumar Sathish Nanotechnology Centre, CEET, VSB - Technical University of Ostrava, Ostrava, Czech Republic Vesselin Shanov Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States; Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, United States Hadi Sharifidarabad School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran Mohin Sharma Department of Electrical Engineering, PACCAR Technology Institute, University of North Texas, Denton, TX, United States Chandra S. Sharma Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India Ling-Ying Shi College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan, China M.K. Shobana Department of Physics, School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India Manmeet Singh Pradesh, India

Department of Mechanical Engineering, IIT Kanpur, Kanpur, Uttar

Sharanjit Singh Department of Mechanical Engineering, DAV University, Jalandhar, Punjab, India Ovender Singh Department of Chemistry, Chonnam National University, Buk-gu, Gwangju, Republic of Korea M. Sivakumar Energy Materials Lab, Department of Physics, Alagappa University, Karaikudi, Tamil Nadu, India

xviii

List of contributors

Ji Soo Sohn South Korea

Department of Mechanical Engineering, Yonsei University, Seoul,

Surajit Some Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, Maharashtra, India R. Subadevi Energy Materials Lab, Department of Physics, Alagappa University, Karaikudi, Tamil Nadu, India James M. Tour States

Chemistry Department, Rice University, Houston, TX, United

Rodrigo V. Salvatierra United States

Chemistry Department, Rice University, Houston, TX,

Rakesh Verma Department of Materials Science and Engineering, Chonnam National University, Gwangju, Gyeonggi, South Korea Duc-Luong Vu Department of Materials Science and Engineering, Chonnam National University, Gwangju, Gyeonggi, South Korea Jianli Wang School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, China Shunli Wang Southwest University of Science and Technology, Mianyang, Sichuan, China; Aalborg University, Aalborg, North Jutland Province, Denmark Xinyu Wang

Dalian Maritime University, Dalian, China

Guang Xia Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China Lili Xia China

Southwest University of Science and Technology, Mianyang, Sichuan,

Yanxin Xie China Jin-Lin Yang

Southwest University of Science and Technology, Mianyang, Sichuan, Tsinghua University, Beijing, P.R. China

Ghulam Yasin Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China; College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Jiajia Ye Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China Chunmei Yu Southwest University of Science and Technology, Mianyang, Sichuan, China

List of contributors

xix

Guangqi Zhang Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States Zhiqiang Zheng Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China

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Part One Basic principles

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Introduction to electrochemical energy storage technologies

1

Ghulam Yasin 1,2 , Sehrish Ibrahim 3 , Shumaila Ibraheem 1,2 , Ali Saad 1,2 , Anuj Kumar 4 and Tuan Anh Nguyen 5 1 Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China; 2College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China; 3College of Life Science and Technology, Beijing University of Chemical Technology, Beijing, China; 4Nano-Technology Research Laboratory, Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India; 5Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

1. Introduction Over the last few decades, considerable effort has been devoted to achieving clean, safe, and renewable energy sources for green energy production and sustainable storage to meet perpetually rising energy demand around the globe [1e9]. Energy sources are classified as either renewable or nonrenewable according to their intrinsic natures and inherent characteristics. Specifically, nonrenewable energy sourcesdfor example, fossil fuels and coals that exist worldwide to a limited extentdcannot be recreated within a short period. Conversely, renewable energy sources, comprising solar energy, geothermal energy, hydropower, wind power, tidal energy, nuclear energy, biomass, etc., are inexhaustible and can be generated as needed. Successful implementation of sustainable energy, particularly from renewable sources, requires next-generation systematized energy conversion and storage technologies. These technologies can be enabled using nanostructures and nanomaterials [10e19]. Electrochemical energy-storage technologies (EESTs), particularly rechargeable batteries and electrochemical capacitors, are promising candidates and are already used to efficiently power electronic gadgets, medical devices, and electric vehicles owing to their greatly desirable characteristics, such as excellent energy density and power density, high round-trip efficiency, extended cycle life, low relative costs, and ecologically benign natures [20e27]. Nevertheless, various challenges and limitations are still rooted mainly in the materials and technologies employed to connect renewable energy sources with electrochemical energy-storage systems (EESSs). Thus, the development of promising high-performance EESTs relies critically on new achievements and progress in multidisciplinary sciences, such as chemical, nanotechnology, physical, and materials sciences [28,29]. Among the various energy-storage technologies, the typical EESTs, especially lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and lithiumesulfur (LieS) batteries, have been widely explored worldwide and are considered the most

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00002-8 Copyright © 2022 Elsevier Inc. All rights reserved.

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Lithium-Sulfur Batteries

favorable, safe, green, and sustainable electrochemical energy-storage (EES) devices as future of renewable energy sources for humankind owing to their excellent efficiency, flexibility, and versatility [30e34]. However, formidable limitations and problems, such as relatively poor ion transport kinetics and inferior electronic conduction of anode/cathode materials, control the power and energy densities of rechargeable batteries. These limitations impede today’s battery manufacturers, who require extraordinary performance to meet the needs of grids storage and vehicles with ultralong driving ranges [35e41]. Therefore, next-generation batteries with excellent electrochemical performance, superior energy density, and high-power density are critically needed, and the research community has devoted a remarkable amount of attention to modern electrochemistry, nanotechnology, and materials science globally in the quest to develop such batteries.

2.

History and recent advances

Although some people claim the first battery was invented hundreds of years ago in the 1st century BCE, with the earlier discovery of the illustrious “Baghdad Battery,” a vessel-like battery found during an archeological excavation in the region near Baghdad that was therefore credited to the ancient Persian civilization. The earlier concepts and works of two Italian scientists, Luigi Galvani from the University of Bologna and Alessandro Volta from the University of Pavia, are almost universally acknowledged as the origins of the electrochemical cell (battery) at the end of the 18th century, which is the most commonly known type of battery worldwide [42]. There was a dispute between the two scientists about this important discovery. Galvani believed that electricity could be generated by animals. He conducted his classic experiment and noticed the manifest jerk of a frog’s leg when the typical series of two different metals were touched to it. On the other hand, Volta made the opposing argument that electricity could be produced by a “voltaic pile,” which was typically made by an alternative series of two different metals disks of silver and zinc separated by a cloth soaked in a solution of sodium chloride [42,43]. During the subsequent two centuries, the sustainable development and commercial utilization of EESSs progressively made them one of the greatest prevailing storage technologies, and they have been inducted efficiently into all parts of contemporary civilization. Moreover, since the first invention of a commercial LIB by Sony Corporation in 1991 [4,44,45], EESSs have again realized remarkable achievements for promising uses in various emerging areas, such as medical appliances, electronic gadgets, and electricity-powered vehicles, revealing the potential for use in future large-scale applications. Moreover, with the development and successful penetration of LIBs into the commercial market, rechargeable batteries have rapidly conquered the market by powering consumer electronics at a lower cost. Nowadays, these batteries are dynamically expanding and widening their potential applications in various emerging markets ranging from automobiles to large-scale energy storage for national grids (Fig. 1.1) [46].

Introduction to electrochemical energy storage technologies

5

Figure 1.1 A combined model illustration for microgrid and grid storage presents the relationship between the generation of renewable energy that supplies the grid, grid energystorage solutions, and energy supply for hybrid and pure-electric automobile transport. Adapted from D. Kundu, E. Talaie, V. Duffort, L.F. Nazar, The emerging chemistry of sodium ion batteries for electrochemical energy storage, Angew. Chem. Int. Ed. 54(11) (2015) 3431e3448, https://doi.org/10.1002/anie.201410376 with copyright permission from John Wiley and Sons.

3. Energy storage mechanisms The operating principles and typical energy-storage mechanisms of all EESSs follow the laws of thermodynamics. For example, in a heat engine, the fossil fuels have stored chemical energy, and a chemical redox reaction is initiated to convert or transform it into thermal energy, which is then used to perform mechanical work for propulsion procedures in an internal combustion engine. Similarly, the energy conversion process uses a somewhat analogous mechanism in EES devices, where typical electrochemical redox reactions take place on electrodes that enable the conversion of a device’s stored chemical energy into electrical energy to power the external gadget during the discharge cycle. For EES device storage, the mechanism is sometimes used to restore the chemical energy by converting electrical energy during the charging cycle (a reverse process). A reduction can be defined as the phenomenon of electrons’ addition to a species based on chemistry fundamentals. At the same time, oxidation is the process of electron removal from a species. The corresponding net effect is known as a chemical redox reaction, wherein the transfer of electrons occurs from a reductant species or electron donor species toward an oxidant species or electron acceptor species [47].

6

Lithium-Sulfur Batteries

In general, a net difference of the two typical reduction half-reactions could be indicated for any chemical redox reaction, and a redox couple is represented through the oxidized and reduced species in a distinctive half-reaction. Moreover, the transfer of electrons might be enabled by other proceedingsdfor instance, the transfer of an atom or ion. One can usually describe a couple as oxidation (Ox) over reduction (Red) (Ox/Red) with its resultant half-reaction, as given below [47]: Ox þ ne /Red Oxidation and reduction reactions are electrochemical processes that occur simultaneously in the cell; oxidation occurs at the anode, while reduction occurs at the cathode [47,48]. As the reaction continues, oxidation reaction releases the electrons as follows: At anode: Red1/Ox1 þ ne They travel to the cathode via an external circuit, which activates the reduction reaction as follows: At cathode: Ox2 þ ne /Red2 The reduction cycle induces a positive charge on the cathode by withdrawing electrons (equivalent to high potential). In the meantime, the oxidation process supplies the electrons toward the anode, making it a comparatively negative charge (equivalent to low potential). In the meantime, the oxidation process supplies the electrons toward the anode, making it a comparatively negative charge (equivalent to low potential). Consequently, the cathode is considered the higher potential electrode compared to the anode. Accordingly, this potential difference is a main driving force in practical batteries, which enable the electrons to perform electrical work via moving in an external circuit until the cell reaction, has not achieved the typical chemical equilibrium state. One should note that a certain number of electrons could transport between or within the two electrodes when the related potential difference is too large; as a result, it can perform a significant amount of electrical work. More importantly, a cell system that achieves the equilibrium state of its overall reaction does not perform any electrical work, resulting in the potential difference approaching zero.

4.

Conclusion and outlook

Renewable, green, low-cost, and environmentally benign energy conversion and storage technologies have paid much devotion nowadays to meet the perpetually growing energy demands of the increasing population of today’s modern society. Although, today’s LIBs could deliver more than double the energy density and electrochemical performance of the first commercial one presented in the market by the Sony Corporation in 1991. However, existing LIBs still face numerous challenges and limitations that should be resolved before using them in large-scale electricity storage. These issues include high costs, poor safety, short cycle life, and inadequate energy density and power density. Likewise, SIBs and LieS batteries face challenges that delay their

Introduction to electrochemical energy storage technologies

7

practical applications to replace LIBs for grid-scale energy storage. Most of these issues are associated with the anode side, which could be regulated to improve energy density and long cycle life. The development of advanced nanostructured anode materials is an effective strategy to expand energy resources over specific volumes and power levels at a significantly lower cost and is greatly needed for large-scale electricity storage applications. In addition, nanotechnology could contribute significantly to the increased storage performance and cycle life of cells by decreasing the diffusion paths of ions/electrons.

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[11] M. Nadeem, G. Yasin, M. Arif, H. Tabassum, M.H. Bhatti, M. Mehmood, U. Yunus, R. Iqbal, T.A. Nguyen, Y. Slimani, H. Song, W. Zhao, Highly active sites of Pt/Er dispersed N-doped hierarchical porous carbon for trifunctional electrocatalyst, Chem. Eng. J. 409 (2021) 128205, https://doi.org/10.1016/j.cej.2020.128205. [12] M. Nadeem, G. Yasin, M.H. Bhatti, M. Mehmood, M. Arif, L. Dai, Pt-M bimetallic nanoparticles (M ¼ Ni, Cu, Er) supported on metal organic framework-derived N-doped nanostructured carbon for hydrogen evolution and oxygen evolution reaction, J. Power Sources 402 (2018) 34e42, https://doi.org/10.1016/j.jpowsour.2018.09.006. [13] M. Nadeem, G. Yasin, M. Arif, M.H. Bhatti, K. Sayin, M. Mehmood, U. Yunus, S. Mehboob, I. Ahmed, U. Flörke, Pt-Ni@PC900 hybrid derived from layered-structure Cd-MOF for fuel cell ORR activity, ACS Omega 5 (5) (2020) 2123e2132, https:// doi.org/10.1021/acsomega.9b02741. [14] A. Kumar, D.K. Das, V.K. Vashistha, S. Ibraheem, G. Yasin, S. Gautam, V. Sharma, A novel CoN4-driven self-assembled molecular engineering for oxygen reduction reaction, Int. J. Hydrogen Energy 46 (52) (2021) 26499e26506, https://doi.org/10.1016/ j.ijhydene.2021.05.114. [15] A. Kumar, S. Ibraheem, T. Anh Nguyen, R.K. Gupta, T. Maiyalagan, G. Yasin, MolecularMN4 vs atomically dispersed MN4C electrocatalysts for oxygen reduction reaction, Coord. Chem. Rev. 446 (2021) 214122, https://doi.org/10.1016/j.ccr.2021.214122. [16] A. Kumar, V.K. Vashistha, D.K. Das, S. Ibraheem, G. Yasin, R. Iqbal, T.A. Nguyen, R.K. Gupta, M. Rasidul Islam, M-N-C-based single-atom catalysts for H2, O2 & CO2 electrocatalysis: activity descriptors, active sites identification, challenges and prospects, Fuel 304 (2021) 121420, https://doi.org/10.1016/j.fuel.2021.121420. [17] A. Kumar, G. Yasin, R.M. Korai, Y. Slimani, M.F. Ali, M. Tabish, M. Tariq Nazir, T.A. Nguyen, Boosting oxygen reduction reaction activity by incorporating the iron phthalocyanine nanoparticles on carbon nanotubes network, Inorg. Chem. Commun. 120 (2020) 108160, https://doi.org/10.1016/j.inoche.2020.108160. [18] A. Kumar, G. Yasin, V.K. Vashistha, D.K. Das, M.U. Rehman, R. Iqbal, Z. Mo, T.A. Nguyen, Y. Slimani, M.T. Nazir, W. Zhao, Enhancing oxygen reduction reaction performance via CNTs/graphene supported iron protoporphyrin IX: a hybrid nanoarchitecture electrocatalyst, Diam. Relat. Mater. 113 (2021) 108272, https://doi.org/ 10.1016/j.diamond.2021.108272. [19] G. Yasin, S. Ibrahim, S. Ibraheem, S. Ali, R. Iqbal, A. Kumar, M. Tabish, Y. Slimani, T.A. Nguyen, H. Xu, W. Zhao, Defective/graphitic synergy in a heteroatom-interlinkedtriggered metal-free electrocatalyst for high-performance rechargeable zinceair batteries, J. Mater. Chem. 9 (34) (2021) 18222e18230, https://doi.org/10.1039/D1TA05812F. [20] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (4) (2013) 1167e1176, https://doi.org/10.1021/ja3091438. [21] A. Yoshino, 1 - development of the lithium-ion battery and recent technological trends, in: G. Pistoia (Ed.), Lithium-Ion Batteries, Elsevier, Amsterdam, 2014, pp. 1e20. [22] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (16) (2008) 2930e2946, https://doi.org/10.1002/ anie.200702505. [23] S. Ullah, G. Yasin, A. Ahmad, L. Qin, Q. Yuan, A.U. Khan, U.A. Khan, A.U. Rahman, Y. Slimani, Construction of well-designed 1D seleniumetellurium nanorods anchored on graphene sheets as a high storage capacity anode material for lithium-ion batteries, Inorg. Chem. Front. 7 (8) (2020) 1750e1761, https://doi.org/10.1039/C9QI01701A.

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[24] S. Ullah, B.D.L. Campéon, S. Ibraheem, G. Yasin, R. Pathak, Y. Nishina, T. Anh Nguyen, Y. Slimani, Q. Yuan, Enabling the fast lithium storage of large-scalable g-Fe2O3/Carbon nanoarchitecture anode material with an ultralong cycle life, J. Ind. Eng. Chem. 101 (2021) 379e386, https://doi.org/10.1016/j.jiec.2021.05.045. [25] G. Yasin, M.A. Khan, W.Q. Khan, T. Mehtab, R.M. Korai, X. Lu, M.T. Nazir, M.N. Zahid, Facile and large-scalable synthesis of low cost hard carbon anode for sodium-ion batteries, Results Phys. 14 (2019) 102404, https://doi.org/10.1016/j.rinp.2019.102404. [26] G. Yasin, M. Arif, T. Mehtab, M. Shakeel, M.A. Mushtaq, A. Kumar, T.A. Nguyen, Y. Slimani, M.T. Nazir, H. Song, A novel strategy for the synthesis of hard carbon spheres encapsulated with graphene networks as a low-cost and large-scalable anode material for fast sodium storage with an ultralong cycle life, Inorg. Chem. Front. 7 (2) (2020) 402e410, https://doi.org/10.1039/C9QI01105F. [27] D. Yu, A. Kumar, T.A. Nguyen, M.T. Nazir, G. Yasin, High-voltage and ultrastable aqueous zinceiodine battery enabled by N-doped carbon materials: revealing the contributions of nitrogen configurations, ACS Sustain. Chem. Eng. 8 (36) (2020) 13769e13776, https://doi.org/10.1021/acssuschemeng.0c04571. [28] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nat. Nanotechnol. 12 (3) (2017) 194e206, https://doi.org/10.1038/nnano.2017.16. [29] J. Liu, Z. Bao, Y. Cui, E.J. Dufek, J.B. Goodenough, P. Khalifah, Q. Li, B.Y. Liaw, P. Liu, A. Manthiram, Y.S. Meng, V.R. Subramanian, M.F. Toney, V.V. Viswanathan, M.S. Whittingham, J. Xiao, W. Xu, J. Yang, X.-Q. Yang, J.-G. Zhang, Pathways for practical high-energy long-cycling lithium metal batteries, Nature Energy 4 (3) (2019) 180e186, https://doi.org/10.1038/s41560-019-0338-x. [30] F. Cheng, J. Liang, Z. Tao, J. Chen, Functional materials for rechargeable batteries, Adv. Mater. 23 (15) (2011) 1695e1715, https://doi.org/10.1002/adma.201003587. [31] C. Xu, B. Xu, Y. Gu, Z. Xiong, J. Sun, X.S. Zhao, Graphene-based electrodes for electrochemical energy storage, Energy Environ. Sci. 6 (5) (2013) 1388e1414, https://doi.org/ 10.1039/C3EE23870A. [32] J. Mei, T. Liao, G.A. Ayoko, J. Bell, Z. Sun, Cobalt oxide-based nanoarchitectures for electrochemical energy applications, Prog. Mater. Sci. 103 (2019) 596e677, https:// doi.org/10.1016/j.pmatsci.2019.03.001. [33] J.B. Goodenough, Electrochemical energy storage in a sustainable modern society, Energy Environ. Sci. 7 (1) (2014) 14e18, https://doi.org/10.1039/C3EE42613K. [34] H. Zhang, L.K. Ono, G. Tong, Y. Liu, Y. Qi, Long-life lithium-sulfur batteries with high areal capacity based on coaxial CNTs@TiN-TiO2 sponge, Nat. Commun. 12 (1) (2021) 4738, https://doi.org/10.1038/s41467-021-24976-y. [35] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions, Adv. Mater. 22 (35) (2010) E170eE192, https://doi.org/10.1002/ adma.201000717. [36] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (3) (2010) 587e603, https://doi.org/10.1021/cm901452z. [37] G. Yasin, N. Muhammad, T.A. Nguyen, P. Nguyen-Tri, Chapter one - nanobattery: an introduction, in: H. Song, R. Venkatachalam, T.A. Nguyen, H.B. Wu, P. Nguyen-Tri (Eds.), Nanobatteries and Nanogenerators, Elsevier, 2021, pp. 3e9. [38] G. Yasin, N. Muhammad, T.A. Nguyen, P. Nguyen-Tri, Chapter five - batterynanogenerator hybrid systems, in: H. Song, R. Venkatachalam, T.A. Nguyen, H.B. Wu, P. Nguyen-Tri (Eds.), Nanobatteries and Nanogenerators, Elsevier, 2021, pp. 61e68.

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[39] G. Yasin, N. Muhammad, A. Kumar, M. Tabish, M.U. Malik, M.T. Nazir, D. Liu, T.A. Nguyen, Chapter Eleven - nanostructured cathode materials in rechargeable batteries, in: H. Song, R. Venkatachalam, T.A. Nguyen, H.B. Wu, P. Nguyen-Tri (Eds.), Nanobatteries and Nanogenerators, Elsevier, 2021, pp. 293e319. [40] G. Yasin, M. Arif, M.A. Mushtaq, M. Shakeel, N. Muhammad, M. Tabish, A. Kumar, T.A. Nguyen, Chapter nine - nanostructured anode materials in rechargeable batteries, in: H. Song, R. Venkatachalam, T.A. Nguyen, H.B. Wu, P. Nguyen-Tri (Eds.), Nanobatteries and Nanogenerators, Elsevier, 2021, pp. 187e219. [41] R. Iqbal, G. Yasin, M. Hamza, S. Ibraheem, B. Ullah, A. Saleem, S. Ali, S. Hussain, T. Anh Nguyen, Y. Slimani, R. Pathak, State of the art two-dimensional covalent organic frameworks: prospects from rational design and reactions to applications for advanced energy storage technologies, Coord. Chem. Rev. 447 (2021) 214152, https://doi.org/ 10.1016/j.ccr.2021.214152. [42] B. Scrosati, History of lithium batteries, J. Solid State Electrochem. 15 (7) (2011) 1623e1630, https://doi.org/10.1007/s10008-011-1386-8. [43] A. Volta, On the electricity exited by the mere contact of conducting substances of different kind, letter to the Right Hon. Sir Joseph Banks, KBPRS, Phil, Trans. 2 (1800) 430. [44] M. Mohri, N. Yanagisawa, Y. Tajima, H. Tanaka, T. Mitate, S. Nakajima, M. Yoshida, Y. Yoshimoto, T. Suzuki, H. Wada, Rechargeable lithium battery based on pyrolytic carbon as a negative electrode, J. Power Sources 26 (3) (1989) 545e551, https://doi.org/ 10.1016/0378-7753(89)80176-4. [45] F.-F. Cao, H. Ye, Y.-G. Guo, Nanostrucutres and Nanomaterials for Lithium-Ion Batteries, Nanostructures and Nanomaterials for Batteries: Principles and Applications, Springer, Singapore, 2019, pp. 89e158. [46] D. Kundu, E. Talaie, V. Duffort, L.F. Nazar, The emerging chemistry of sodium ion batteries for electrochemical energy storage, Angew. Chem. Int. Ed. 54 (11) (2015) 3431e3448, https://doi.org/10.1002/anie.201410376. [47] S. Xin, H. Gao, Y. Li, Y.-G. Guo, Introduction to Electrochemical Energy Storage, Nanostructures and Nanomaterials for Batteries: Principles and Applications, Springer, Singapore, 2019, pp. 1e28. [48] P. Atkins, J.d. Paula, Physical Chemistry Thermodynamics, Structure, and Change, WH Freeman and Company, New York, 2014.

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Harshit Mahandra 1 , Guillermo Alvial-Hein 1 , Hadi Sharifidarabad 2 , Fariborz Faraji 1 and Ovender Singh 3 1 The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada; 2 School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran; 3Department of Chemistry, Chonnam National University, Buk-gu, Gwangju, Republic of Korea

1. Introduction Worldwide demand for energy is constantly increasing because of the accelerated economic and social growth experienced over the last decade. Current energy demand is predominantly satisfied by fossil fuels, which are becoming increasingly scarce and are not an environmentally friendly alternative [1]. Lithium-based technologies have been intended to help satisfy current energy demand because their components are abundant, cheap, nontoxic, and generate a large energy density. Nevertheless, the energy density offered by conventional Li-ion batteries is still not sufficient, which could lead to faltering demand in a future scenario. Recently, lithium-sulfur (LieS) batteries have emerged as a promising alternative to other energy sources, as these types of batteries provide a high theoretical specific capacity (1675 mAh g
1) and energy density (2600 Wh kg
1) while maintaining a fairly reasonable production cost and safe use for the environment [2]. Because of the tremendous advantages of LieS batteries, the scientific community has focused on improving this technology, reporting an exponential increase in the number of research articles and citations from 2011 to date [3]. In addition, the production of LieS battery technology is currently supported in North America, the European Union, and China through the US Department of Energy’s “Horizon 2020” initiative and the Chinese People’s Liberation Army’s Chemical Defense Institute, respectively. A LieS battery was produced by the Chinese Institute with a 330 Wh kg
1 specific energy and capable of 60% capacity after 100 cycles at 0.2 C [1]. Nonetheless, some significant drawbacks must be addressed related to sulfur theoretical capacity due to lithium sulfide (Li2S) production during operation and its sulfur-insulating nature and cathode volume expansion of more than 80%, resulting in insufficient capacity [1,3,4]. Additionally, the dissolution of polysulfides (PS) in the electrolyte has generated operational issues associated with active material loss and lower coulombic efficiency [4]. These current challenges for LieS batteries and their potential solutions have been summarized by Deng et al. [5] (Fig. 2.1). Future development

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00024-7 Copyright © 2022 Elsevier Inc. All rights reserved.

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Lithium-Sulfur Batteries

Figure 2.1 Current challenges and future development directions [5].

of LieS batteries (Fig. 2.1) is expected to grow as they integrate into commercial industries such as electric transportation and aerospace engineering on a massive scale. This chapter provides an overview of the current development of LieS batteries. The chapter starts with a complete description of battery components and later provides recent research developments in the cathode, anode, electrolyte, and other parts incorporated into the battery structure. Finally, the applications and future perspectives of LieS batteries have been identified.

2.

Structure and components of lithiumesulfur battery

A standard LieS battery generally comprises a sulfur cathode, a lithium (Li) metal anode, and an organic electrolyte [6]. The structure of coin and pouch cells and the variation in some operational parameters are shown in Fig. 2.2. Sulfur-based cathodes combined with lithium-metal anodes have usually been implemented in LieS batteries because they provide highly superior capacity (1675 mAh g
1) and energy density (2600 Wh kg
1) in LieS batteries compared with other similar batteries [2]. In addition, the high number of natural reserves available across the globe, the most

Recent developments in lithiumesulfur batteries

13

Figure 2.2 (A) LieS coin-shaped cell structure, (B) LieS pouch cell structure (C) dependency of the gravimetric energy density of LieS pouch batteries with areal sulfur loading and electrolyte/sulfur ratio (D) targeted specific energy of LieS pouch battery and its boundary condition [6].

affordable price, and the less-toxic environmental impact of sulfur have made more attractive LieS batteries. However, due to the poor conductive properties of sulfur as a cathodic material, other conductive materials, e.g., carbon additives, have been incorporated into the cathode structure to improve electrical conductivity and induce adequate electron flow [7]. Furthermore, binders have also been commonly added in the cathode to enhance their performance as binders establish network structures between carbon additives and sulfur particles, maintaining cathode integrity [8,9]. The severe shuttle effect of PS and considerable volume variation in the cathode during battery performance has tremendously affected LieS battery capacity and cathode stability, encouraging the implementation of improved cathodic materials such as nanocomposite-based technologies to enhance coulombic efficiency and cycling stability in the last few years [10]. In the case of the anode, Li metal has been chosen as the favored anodic material due to its considerable redox potential of
3.04 V vs. standard hydrogen electrode, favorable gravimetric density (0.59 g cm
3), and sufficient theoretical specific capacity (3860 mAh g
1) [11]. Nevertheless, issues related to the reactivity between Li

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Lithium-Sulfur Batteries

anode electrode with the electrolyte and growth of Li dendrites in the anodic surface have potentially reduced the efficiency, functionality, and safety of the LieS battery, causing a considerable loss of active material. Lately, other advances have arisen as potential solutions to the problems associated with Li anodes, including adding other compounds in the Li-based anode such as silicon or carbon, anode coating, and Li-free anode [11,12]. An electrolyte is another fundamental part of LieS batteries as they serve as a transport media for ions between the two electrodes. The most common electrolytes utilized in LieS batteries have been Li salts and ether solvents, including lithium bis(trifluoromethane)sulfonimide (LiTFSI) together with solvents, e.g., 1,3-dioxolane (DOL), 2-dimethoxyethane (DME), and tetra (ethylene glycol)-dimethyl ether (TEGDME) [13,14]. However, the shuttle effect has been reported with Li salts and ether solvents, resulting in an inefficient system. In the last few years, ionic liquids (ILs), solid-state electrolytes, and polymer electrolytes [13,14] have been evaluated to replace conventional electrolytes and consequently improve the performance of LieS batteries. In addition to the cathode, anode, and electrolyte, the separator is a fundamental part of a cell. It helps prevent short circuits between the two electrodes, protecting the electrode and preserving its functionality. Polypropylene (PP) and polyethylene (PE) membranes have been broadly employed as separations in LieS batteries [12].

3.

Mechanism and electrochemical properties of lithiumesulfur battery

The LieS battery comprises a sulfur cathode, a Li anode, an organic electrolyte, and a separator. The LieS redox process describes the complete electrochemical process: 16Li þ S8 H8Li2 S

(2.1)

A reductioneoxidation reaction occurs with the discharging of a LieS battery: 2Li þ S/Li2 S

(2.2)

Li is oxidized at the cathode, and sulfur is reduced at the anode during discharging due to the migration of Li ions from cathode to anode. Because, S8 is the most stable form of elemental sulfur, the discharging is a multistep reduction in S8 to generate various soluble (Li2Sn, 4  n  8) and insoluble (Li2Sn, n < 4) Li-PS. The specific reactions that occur in LieS batteries during the discharge process are represented below [8]. Soluble lithium polysulfides: S8 þ 2e
HS2
8

(2.3)

Recent developments in lithiumesulfur batteries

15

3S8 þ 8e
#4S2
6

(2.4)

2S6 þ 6e
#3S2
4

(2.5)

Insoluble lithium polysulfides:
4Liþ þ S2
4 þ 2e #2Li2 S2

(2.6)

2Liþ þ Li2 S2 þ 2e
#2Li2 S

(2.7)

However, LieS battery charging converts Li2S back into PS with less probability of S8 formation. The discharging reactions result in decreased electrochemical potential of the cathode, and charging leads to increased cathode electrochemical potential, gradually to the value of open-circuit voltage. The overall redox has an average voltage of 2.2 V vs. Li [8]. The energy stored in a battery in proportion to its weight is defined as energy density. It is expressed in watt-hours per kilogram (Wh kg
1) and can be calculated by multiplying nominal battery voltage to capacity per unit mass of the battery. Cathode material plays an important role in deciding the energy density of a battery. The theoretical specific energy density of LieS battery is three to five times higher than that of other Li ion batteries; therefore, it can be a better option for next generation in large operations. The shortcomings of self discharge, high internal resistance, and rapid capacity fading on cycling need to be overcome by constructing novel sulfur electrodes with nanostructures [15]. In the past few years, carbon-sulfur electrodes have been explored to overcome these shortcomings. Pei et al. [16] designed a highperformance separator is designed by coating with two-dimensional porous nitrogen-doped carbon nanosheets. This showed high cycling stability with 793 mAh g
1 specific capacity after 400 cycles at 0.5 C (sulfur loading ¼ 6.0 mg cm
2) and a high areal energy density of 12.1 mAh cm
2 after 100 cycles at 0.2 C at 12.0 mg cm
2 high sulfur loading. This supports the fact that commercial carbon-based sulfur cathodes show significantly enhanced performance. Li et al. [17] performed some engineering of a stable electrode-separator interface via in-situ vapor-phase polymerization of polypyrrole on commercial Celgard separator and claimed design success by high energy density and long cycle life through simple and scalable separator-electrode interface without significant increase in volume and mass. It was proved that use of the polypyrrole modified separator resulted in 250 stable cycles at 0.5 C with a low-capacity decay rate of 0.083% per cycle. Recently, Yin et al. [18] designed a three-dimensional free-standing sulfur cathode and fabricated by coaxially coating polar Ti3C2Tx flakes on sulfur-impregnated carbon cloth (Ti3C2Tx@S/CC) to prepare LieS batteries with high loading and high energy density (Fig. 2.3). The coating of Ti3C2Tx works as a polar and conductive protective layer by physical blocking and chemical anchoring of lithiumePSs. Outstanding cycling stability (746.1 mAh g
1 after 200 cycles at 1 C), superb rate performance (866.8 mAh g
1

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Lithium-Sulfur Batteries

Figure 2.3 (A) Fabrication process of the Ti3C2Tx@S/CC composite. High-magnification SEM images of (B) CC, (C) S/CC and (D) Ti3C2Tx@S/CC [18].

to 2 C), and high specific energy density (564.2 Wh kg
1 after 100 cycles at 0.5 C) were achieved. The enhanced capacities and cyclic stabilities will encourage practical applications of LieS batteries soon.

4.

Polysulfide shuttle effect

Shuttle effect is a phenomenon that involves the formation of soluble PS that migrate from the anode to the cathode and the subsequent formation of insoluble Li2S, which causes passivate the negative electrode and produces irreparable active material in the cell. The discharging/charging process is not simple due to the formation of several intermediate species that react to each other in the electrolyte. Some Li2Sx PSs (4 < x  8) are highly soluble in the electrolyte, which allows their diffusion from the cathode to the anode and results in undesired spontaneous reactions [1,19]. The PS species in the anode are further reduced to low-valence PS species. Afterward, the shorter PS moves back to the cathode where these species are again oxidized, generating a cycle or shuttle effect, as shown in Fig. 2.4. This effect is primarily responsible for the low coulombic efficiency of LieS batteries, which leads to selfdischarge and material loss. The self-discharge process might continue when the cell is in a rest state, as high-valence PS gradually diffuses and migrates to the anode due to a concentration gradient, further affecting the battery’s efficiency [1]. In addition, the Li2S generated during the discharge process is strongly insoluble in aprotic electrolytes (Fig. 2.4) and electrically insulating, augmenting internal resistance and consequently reducing the energy efficiency in the battery. Furthermore, Li2S species can form nonconductive slabs producing impoverished contact between electrodes and the electrolyte generating a volume expansion as high as 80% of the anodic electrode

Recent developments in lithiumesulfur batteries

17

Figure 2.4 Formation of PS shuttle in LieS battery [1].

that causes an attenuated battery capacity and safety risk [1,19]. Last but not least, the formation of Li2S in the internal part of the cathodic structure might also seriously affect conductive substrate influencing the battery performance [19]. The PS shuttle effect represents a critical bottleneck in LieS chemistry; therefore, researchers have lately devoted much attention to attenuating the shuttle effect during the discharge and charge process [6] to generate highly effective LieS batteries in terms of energy efficiency. Section 5 presents current developments proposed in the last few years to attenuate the PS shuttle effect and other drawbacks found in LieS cells.

5. Recent developments 5.1

Cathodes

A conventional cathode used in LieS cells considers sulfur the main constituent. Owing to the insulating nature of sulfur, LieS cathodes typically require a conductive host such as carbonaceous materials to be incorporated to improve the electronic conductivity and preserve an adequate electron flow in the electrode [7,10]. During battery discharge and charge cycles, various electrochemical reactions occur, forming a severe shuttle effect of PS, as explained in Section 4. Additionally, Li2S, a fully lithiated state of sulfur, has also been proposed to replace sulfur in the cathode as volume change can be better controlled [20]. Further, the higher melting point of Li2S above 930 C has given Li2S an advantage over sulfur, being suitable for carrying out encapsulation techniques at high temperatures forming more stable cathodic nanocomposites structures as described in Section 5.1.1. However, the replacement of sulfur has not

18

Lithium-Sulfur Batteries

solved problems related to shutting and conductivity; thus, more research is still required to address these significant drawbacks found in LieS batteries [21]. Recent developments made to minimize drawbacks associated with cathodes and consequently improve the performance of Li2S batteries have been based on the incorporation of sulfur/carbon (S/C)-based nanocomposites, polymer/carbon-based nanocomposites, nanocomposites including metaleorganic framework (MOF), metal oxides, and organosulfur [10].

5.1.1

Sulfur/carbon-based nanocomposites

Nanocomposites containing carbonaceous materials have been extensively investigated to enhance the performance of the sulfur-based cathodes due to the favorable conductive nature that contrasts the insulating behavior of sulfur and the improvement in the Li-ion transportation, delivering a superior electrochemical operation [4,10]. Depending on their structure, the carbonaceous materials have been grouped into one-dimensional (1D) carbon, two-dimensional (2D) carbon, and porous carbon providing different properties in the nanocomposite. Due to the melting point of sulfur below 120 C, effective methods carried out at higher temperatures to manufacture carbon-based nanostructures such as chemical vapor deposition cannot be performed as they affect sulfur integrity. Infiltration technique, precipitation-based method, and ball milling method (HEMM) have been alternatively reported. 1D carbon nanocomposites, including carbon nanotubes (CNTs) and carbon nanofibers (CNFs), have been implemented by researchers lately [4] as these materials confer excellent conductive and mechanical properties to the cathode by improving the kinetic reaction during operation. CNTs can be described as a class of CNFs having a stack of rolled graphene layers in their structure, forming cylinders of specific diameters. Thus, the properties of these types of carbonaceous materials depend on the radius, and the number of layers of graphene used in their structure. Ummethala et al. [22] synthesized CNT through an infiltration method to be utilized as a host for sulfur in the cathodic structure and reported improved PS retention due to the porous framework and cycling stability, generating a specific capacity of 137 mAh g
1 during the cell performance. Similarly, Fang et al. [23] demonstrated similar improvements when implementing single-wall carbon nanotubes (SWCNTs), as displayed in Fig. 2.5. Furthermore, the vapor permeation method has also been investigated to synthesize S/C composite taking advantage of the properties of carbon nanotubes (CNT) foams to develop cathodes with relatively high loading capacity [24]. The melt infiltration method has also been reported to synthesize S/C composites achieving better battery performance in terms of energy and power due to a lower loading of sulfur in the cathodes [25]. Two-dimensional carbon hosts derived from graphene, a carbon allotrope, have also been explored for their excellent stability and exceptional electrical conductivity. Not only graphene but also graphene oxide (GO), a derivative of graphite after oxidizing treatment, and reduced graphene oxide (RGO), the reduced form of GO, have been proposed to overcome drawbacks in LieS cells lately [4]. GO and RGO have different properties than graphene due to functional groups in the host structure

Recent developments in lithiumesulfur batteries

19

Figure 2.5 (A) Calculation of the theoretical sulfur content in carbon nanotubes (B) SWCNT network and the almost pure sulfur (APS) electrode representation (C) TEM image of SWCNT (D) TEM image of sulfur coated SWCNT in APS electrode [23].

and their impure nature. Ren et al. [26] tested a three-dimensional (3D) carbon host (55-PGC@SFPC/S) composed of RGO, CNTs, and porous carbon obtained from biomass was tested, and it optimized the appropriate level of RGO and CNTs, as these carbonaceous materials generated a more significant impact on cathodic electrochemical properties. Five percent of each RGO and CNT was found optimal to obtain a superior specific capacity of 1354 mAh g
1 at 0.1 C and help minimize the undesirable drawbacks commonly found in LieS cells. Porous carbon hosts generally encompass nanostructures not exceeding 100 nm of pore diameter, leading to better control of volume changes and PS encapsulation [4]. It is common to improve the polarity of the carbonaceous host, e.g., nitrogen doping nanostructure, to strengthen the bond with PS, which are strongly polar ions [4,27]. Wang et al. [27] proposed a nitrogen-containing porous carbonaceous sheet with Fe3C nanoparticles (Fe3C@NPCS) to improve battery limitations, reporting further enhancement due to the cooperative action of Fe3C nanoparticle carbon structure in forming a stable bond with PS and speed up electrochemical reactions.

20

5.1.2

Lithium-Sulfur Batteries

Sulfur/polymer-based nanocomposites

As previously stated, S/C-based nanocomposites implementation in LieS cathodes has been widely investigated because of the carbon characteristics that facilitate the battery performance. Nevertheless, the operational temperature required to synthesize carbon-containing composites exceeds four times the melting point of sulfur, causing detrimental issues in the sulfur structure [4]. On the other hand, conductive polymers have also been considered to improve cathode conductivity and stability because the synthesis of sulfur/polymer nanocomposites is below 100 C, being adequate to preserve the sulfur contained in the cathodes. In addition, due to the flexible nature and stability, the volume changes associated with the cathode during the discharge and charge operation of LieS batteries are better controlled as the polymer structure adequately encapsulates the sulfur attenuating the contact with the electrolyte [10]. Furthermore, the N and O-based functional groups also ease the immobilization of PS formed during battery operation, helping to minimize the shuttle effect. In the last few years, polythiophene, polyaniline (PANI), poly (3,4ethylenedioxythiophene) (PEDOT), and polypyrrole (PPY) have been typically explored to synthesize sulfur/polymer nanocomposites. Li et al. [28] contrasted the most used conductive polymers and found that the improved cathodic performance followed the increasing order of PANI < PPY < PEDOT due to the formation of chelating complexes with Li2S causing a tremendous advantage over the other conductive polymers. To attain higher electrical conductivity, sulfur particles have been embedded with polyacrylonitrile through a thermal method forming SPAN composites, which in turn has resulted in batteries with suitable stability with no shuttling after multiple cycles [29]. AC-S@PANI nanocomposite was proposed by Wang et al. [30] to minimize PS diffusion to the electrolyte effectively. The implementation of the polymeric structure resulted in 1453 mAh g
1 at 0.1 C specific capacity with excellent cycling stability followed by 200 discharge and charge cycles while further inhibiting PS migration from the cathode. More recently, Liu et al. [31] studied the addition of titanium dioxide (TiO2) in PEDOT-containing nanocomposites and found that the cathodic performance in terms of discharge capacity depended on the TiO2 particle size. The novel structure using 5 nm TiO2 effectively obstruct the migration of PS to the electrolyte, consequently improving the battery performance.

5.1.3

Other nanocomposites

Apart from the new developments regarding S/C and S/polymer-based nanocomposites, other types of composites encompassing MOF, metal oxides, and organosulfur have emerged as a new alternative to strengthen the electrochemical performance of LieS batteries. MOF, a hybrid composite compound by organic and inorganic matter, is characterized by having a porous structure with a considerable surface area together with metal clusters (metal nodes) and organic ligands (linkers) in the framework that make the composite functionally adjustable, improving the binding interaction toward PS and thus decreasing shuttle effect. MOF technologies have been applied by several research groups [32e35]. Hong et al. [32] explored a Cu-MOF that contained Cu(II)

Recent developments in lithiumesulfur batteries

21

having metal sides as nodes and H6TDPAT (2,4,6-tris (3,5-dicarboxylphenyl-amino)1,3,5-triazine) with N-based functional group as linkers to avoid diffusion of PS. Efficacious retention of PS and 745 mAh g
1 at 1 C capacity were attributed to the combined effect between Lewis acidic sides (linkers) and basic sides (nodes) in the structure that attached PS and Li-ions, respectively. Wang et al. [34] presented the addition of a MOF membrane containing Fc (1,1-ferrocenedicarboxylic acid), Zr (zirconium) together with carbon nanotubes (Fig. 2.6) as a promising alternative to inhibit the PS diffusion and improve battery capacity through a catalyst effect shown during the redox kinetics of PS. Furthermore, metal oxides have also been added to the structure of nanocomposites to strengthen the cathode material. The retention of PS is facilitated by the surface provided by the oxygen anion found in the metal oxide and the chemical and physical properties of each particular metal oxide used in the nanocomposite structure. Researchers have tested a wide variety of metal oxides, including but not limited to Al2O3, CeO2, La2O3, MgO, MoO2, NiFe2O4, SnO2, TiO2, SnO2, V2O3, and ZnO [10]. Zhu et al. [36] evaluated the effectiveness of V2O3 on a sulfur-based composite (Fig. 2.7), which resulted in an attenuation of PS diffusion to the electrolyte, acceleration in PS conversion, and a capacity of 973 mAh g
1 at 0.1 C. Wang et al. [35] investigated the addition of MoO2 particles inserted in nitrogen-containing carbon in sulfur-based cathodes. The study revealed that MoO2 had a favorable PS absorption due to stable chemical union and a cooperative effect between MoO2 and carbon, maximizing the use of the cathode by reducing the PS diffusion, stimulating PS conversion, and increasing electrical conductivity. Despite the benefits reported above, adding metal oxide particles in the nanocomposite structure augments the battery cost and reduces the specific energy because of no contribution of metal oxide. Organosulfur materials have also been utilized to enhance cathode performance due to their inexpensive manufacture and material versatility. A Summary of current

Figure 2.6 ZreFc metaleorganic framework/carbon nanotube membrane and the LieS cell setup utilizing this type of membrane to avoid PS diffusion and improve PS redox kinetics [34].

22

Lithium-Sulfur Batteries

Figure 2.7 Illustration of SeV2O3 composites demonstrating the improvement from adding V2O3 [36].

development using different organosulfur material has been discussed by Cairns and Hwa [4]. Nevertheless, deficient kinetic and stability issues associated with the organosulfur need to be further investigated.

5.2

Anodes

Li-based anodes have been the preferred choice for the LieS batteries manufacturer because this class of anodes has outstanding characteristics regarding redox potential, gravimetric density, and specific capacity, resulting in an excellent capacity for storing energy [11,37]. Nonetheless, significant issues have arisen, slowing down its massive use worldwide. On the one hand, due to its considerable chemical activity, Li has a greater probability of reacting with the electrolyte components, leading to ineffective efficiency per cycle. On the other hand, Li-based anodes are characterized by the generation of Li dendrites on the anode surface, affecting its battery stability and safety [11,38]. In the last few years, the scientific community has focused on addressing these problems that somehow make their commercialization more challenging. The addition of different compounds, protection technologies, and batteries with Li-free anode have been explored recently [11,37e39], which are summarized in Fig. 2.8.

5.2.1

Lithium-free anodes

Despite the excellent characteristics of Li anodes as described previously, challenges especially related to their safety need to be addressed. Recently, other options to replace anodic materials have been proposed, such as carbonaceous materials, tin (Sn), and silicon (Si), as they have substantial conductivity and coulombic efficiency, lower alteration in their volume, and a lower heaviness [11]. However, an alloy containing anodes is preferentially chosen over pure metals, e.g., Sn, due to the change in volume that limits their cycle life [39]. Jaumann et al. [40] investigated Si/C-based anodes and their interaction with ether electrolytes and observed a negative impact on the battery capacity due to a high degree of degradation of ether electrolytes. Hari Mohan et al. [41] explored a Sn/graphene (G)/RGO-based anode as Sn/G structure reduced the increase in volume and optimized the transfer of electrons, while RGO coating prevented the contact between

Recent developments in lithiumesulfur batteries

23

Figure 2.8 Summary of current developments to improve anode performance [11].

the electrolyte and anode, attenuating the solid electrolyte interphase (SEI) formation during continuous cycles. This in turn showed a good battery performance with a 413 mAh g
1 discharge capacity after 40 cycles and 336 wh kg
1 energy when implemented together with S/C cathode and LiTFSI electrolyte. Furthermore, Xu et al. [42] evaluated the LieS batteries with a Si-based anode utilizing lithium difluorophosphate (LiPO2F2)/N,N-dimethyltrifluoroacetamide and lithium hexafluorophosphate (LiPF6)/carbonate-based electrolyte (Fig. 2.9). The battery performance with both electrolytes depended on the electrolyte, resulting in higher conductivity and superior affinity with the anode and no formation of dendrites when considering LiFP6/carbonate electrolyte.

Figure 2.9 Illustration of batteries with Si-based anodes [42].

24

5.2.2

Lithium-Sulfur Batteries

Protection technology

In order to avoid the formation of SEI coating in the anode and unstable battery performance associated with dendrites formation in the anode surface and volume variation, anodic protection technologies (Fig. 2.8), have been suggested to prevent limited coulombic efficiency and battery lifetime [11,12,37,39,43] The protection techniques have included the addition of additives to the electrolyte [11,44], the formation of artificial film in the anode [45,46] and the incorporation of a current collector in the battery structure [47] to suppress dendrites formation and attenuate PS shuttle effect during the battery operation. Wu et al. [46] proposed utilizing sericin protein-based protection in the Li anode to enhance battery operation. Effective interaction between the electrolyte and the functional group, e.g., eCOOH, eOH, eNH, eNH2 found in the sericin structure, was reported resulting in a homogeneous accumulation of Li in the anode with no dendrite formation. The decrease in the thickness of the deposited Li layer led to better performance with 667.8 mAh g
1 at 1 C following 520 discharge/charge cycles. Lately, Akhtar et al. [45] improved the performance and lifespan of LieS batteries through the implementation of gelatin (2,2,2-trifluoroethanol (TFEA))-containing protection in the anode, which prevented dendrites formation as Li accumulation was homogeneously distributed due to the presence of oxygen and nitrogen polar functions groups (Fig. 2.10). Further, the structural protection also caused a PS immobilization avoiding the PS shuttle effect during the operation.

5.2.3

Compound technology

The compound technology comprises the use of Li-based anodes made up with other materials that help to minimize the challenges found in LieS batteries. In the same way, as in Li-free anode and protection technology, this technique seeks to weaken the formation of Li dendrites on the anode surface and maximize electron transfer to obtain a more efficient operation [11,37,39]. Several composite-based anodes have been explored in the last few years, encompassing Si-Li [48], SieCeLi [49], CeLi [50,51]. Zeng et al. [51] reported a notably enhancement regarding safety and corrosion issue by replacing the Li-based anode with graphene. Similarly, Zhang et al. [49] evaluated Li/graphene anode coating with Si film and pointed out excellent control of anode volume change together with attenuation of dendrites formation.

Figure 2.10 Addition of protection technology in the battery structure [45].

Recent developments in lithiumesulfur batteries

5.3

25

Electrolytes

Compared with cathodes, less research has been devoted to electrolytes toward improving LieS batteries to avoid principally shuttle effect and safety concerns. Electrolytes have a significant role in LieS batteries because they serve as a transport medium for ions between the two electrodes. An effective ionic conductivity, stable chemical reactivity, and remarkable affinity with both electrodes are the characteristics of an ideal electrolyte together with a low solubility for PS species helping evade the undesirable shuttle effect in the cells [8]. The most common form of electrolytes, i.e., Li salts and ether solvents have been indicated as usual compounds of LieS battery electrolytes [13,14]. Researchers have been extensively investigated LiTFSI mixed with solvents [51e53], e.g., DME, DOL, and TEGDME, commonly associated with unwanted shuttle effect and difference in viscosity caused by PS dissolution resulting in inefficient systems. Several attempts have been made to replace the conventional nonaqueous liquid electrolytic systems and different alternatives have been tried to improve the LieS battery performance.

5.3.1

Ionic liquids

ILs are an alternative to liquid electrolytes owing to their low flammability, depressible vapor pressure and substantial ionic conductivity [13]. The use of ILs in the batteries has prevented the diffusion of PS, and therefore, the shuttle effect. Nonetheless, challenges related to its elevated viscosity have made its implementation difficult in LieS batteries. Different systems containing ILs and Li salts have been explored for LieS batteries considering bis(trifluoromethanesulfonyl)imide (TFSI) as anions and 1-ethyl-3-methylimidazolium (EMIM), 1-butyl-3-methylimidazolium (BMIM), or 1-butyl-1-methylprrolidinium (PYR14), as cations in the ILs [54]. Wang et al. [55] accomplished a reduction in PS solubility and a substantial decrease in Li deposition on the anode surface by utilizing N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide with LiNO3 as an additive. This mitigates self-discharge and promote an excellent efficiency after 200 cycles.

5.3.2

Polymer electrolytes

Polymer electrolytes have also been considered to improve the performance of LieS batteries due to excellent characteristics regarding ion transportation similar to that of nonaqueous liquid electrolytic systems. The low probability of leakage, depreciable flammability, and considerable energy density have generated a greater interest in polymer electrolytes than conventional setups. Polymer electrolytes are generally composed of Li-based salts mixed with polyether solvents. The main classification of polymer electrolytes encompasses gel polymer electrolytes and composite polymer electrolytes (CPEs) [13]. It has been reported that polymeric electrolytes have the ability to behave not only as electrolytes but also as blinders and prevent dendrites generation, PS solubility problems, and safety issues. Due to the chemical stability, poly (ethylene) oxide (PEO)-based electrolytes have been recently investigated [56,57]. However, insufficient mechanical strength and ion conductivity are the problems

26

Lithium-Sulfur Batteries

that need to be addressed in future research. Shi et al. [57] proposed coating of a PEO-based electrolyte with a layer of Al2O3 which reduced Li deposition and thus the stability of the battery and coulombic efficiency and successfully implemented the technology in a punch cell with CPE/Al2O3.

5.3.3

Solid-state electrolytes

More recently, this electrolyte type has become a more attractive alternative to liquidstate electrolytic systems due to reducing safety problems associated with the high flammability, elevated solubility of PS, and greater probabilities of leakage found in LieS batteries with organic-based liquid electrolytes, resulting in a battery with a poor life span. In this way, solid electrolytes act as if they were a separator providing a safer configuration by avoiding short-circuit and PS migration to the cathode and therefore more favorable alternative for the safe manufacture of LieS batteries. Nevertheless, challenges related to compatibility between the Li anode and electrolyte along with insufficient ionic conductivity at ambient temperature require further research to improve the battery performance [13,14]. Both inorganic nonoxides and oxides have been used as precursors for this type of electrolytes. Perovskite-sort oxides, glass ceramics, and Li garnets are some of the kinds of solid electrolytic systems. Recently, garnet oxide electrolytes have attracted more attention [43,58,59] due to their superior electrochemical stability and ionic conductivity.

5.4

Separator

A separator represents a barrier that physically separates the anode and cathode and works as an insulator and ionic conductor, preventing short circuits while improving battery stability due to a PS retention, and therefore, minimizing the shuttle effect. Owing to their characteristics such as electrochemical and mechanical stability, suitable porosity, electrolytic wettability, and reduced price, polyolefin-based membranes, e.g., PP and PE membranes, have been extensively utilized. The recent developments that include separators in LieS batteries have been summarized by researchers recently [12]. Bai et al. [60] worked on enhancing LieS performance using a metal-organic framework (MOF@GO) separator, which acted as an ionic filter for PS, thus reducing the migration of unwanted ions to the anode and therefore producing negligible fading rate with better battery stability owing to minimize shuttle effect. Zheng et al. [61] evaluated a separator made with lightweight carbon flakes (CFs@PP) and reported an attenuation of the shuttle effect together with an improvement in the movement of electrons due to the separator nature, which in turn resulted in a 0.071% decline rate per cycle. More recently, Wang et al. [62] proposed a modification on a PP separator by including Fe3C/Fe@NC/G nanocomposite surrounded by carbon and graphene, effectively achieving a barrier for PS and catalytic action for PS conversion, which led to a decay rate of 0.062% after 500 cycles with a higher specific capacity of 1489 mAh g
1.

Recent developments in lithiumesulfur batteries

5.5

27

Binder

Binders have a primary function in LieS batteries because they help form a connection between carbon and sulfur and preserve the integrity of the cathode. However, binders can create inactive regions resulting in poor battery performance; therefore, selecting a binder is critical to ensure proper performance. Poly(tetrafluoroethylene) and polyvinylidene fluoride (PVDF) have been utilized frequently as binders; nevertheless, they have not been an efficient alternative. Novel binders and free battery binders have been reported to improve battery performance in the past few years [8,9]. Chen et al. [63] improved LieS performance by replacing a standard PVDF binder with lithium polyacrylate (LiPAA)/polyvinyl alcohol binder. The replacement of the common option by a more environmentally friendly and cheaper alternative resulted in enhanced PS adsorption and efficient Liþ transportation due to the robust conduct network formed in the battery. Further, Gu et al. [64] explored methylated amino resin (MAR) as binder material for S@MAR cathodes in LieS batteries. Due to the presence of the N functional group in the binder structure, a slight shuttle effect was reported by the capture of PS and facilitation of Li-ion conductivity. S@MAR also avoided a volume explanation of the cathode due to better mechanical properties. Additionally, Yoo et al. [65] suggested the combination of a conventional binder, poly (vinyl pyrrolidone) (PVP), and an elastic rubber polymer-carboxylated nitrile butadiene rubber, with an enhancement of 300% compared with the traditional option due to better trapping of PS and improved volume control.

6. Comparison with other lithium-ion batteries Second-generation or rechargeable batteries such as lead-acid and Li-ion batteries have been used continuously in portable electronics for more than a century due to their long service life, ease of use, and low maintenance cost [66,67]. At first, rechargeable Li cells were using lithium metal or LieAl (lithiumaluminum) alloys as their anodes and metal sulfides such as titanium (Ti), molybdenum (Mo), etc. as positive electrodes. In the following, the introduction of LiCoO2 as a positive electrode in commercial Li-ion batteries by Sony Corporation has brought Li-ion batteries one step closer to a portable energy source with high energy density, good electrochemical performance, and lack of capacity effects as the most important properties of these batteries [68]. This electrode (LiCoO2) was combined with a negative electrode made of carbon material that hosted Li-ions at low potentials. Their dominance in the market was such that they sought further advancement in cells by improving the positive electrode (cathode), but more recently, the production of nanocomposites made of Ti/C/Co or SneC has led to progress to huge surges in negative electrode materials in recent decades. These materials require a high-capacity positive electrode to provide the best performance [69]. In Li-ion batteries, the cathode is usually made of Li-metal oxide, and the anode is made of a carbon or Li material [70]. The operation of these batteries relies on electrochemical reactions between electrodes isolated and surrounded by electrolytes. During

28

Lithium-Sulfur Batteries

the charging process, Li ions leave the cathode and enter the anode, causing a graphite volume change up to 10% [71], and the opposite happens during the discharging process. Graphite, with a theoretical capacity of 372 mAh g
1, is used commercially as anode, and Li components such as LiCoO2 or LiMn2O4 could be used as a cathode [72]. Unlike anodes, the capacity of the cathode material is the main controller of the final performance of Li-ion batteries [71]. The positive electrodes of rechargeable Li cells are divided into several basic poly-anionic, Li-metal-oxide, LieS, and Li-air, of which lithium-metal-oxide can be called the most successful group of positive electrodes [68]. It is safe to say that the performance of the cathode determines the power and capacity of the battery; thus, the development of cathodic materials is vital for the future of energy resources. The cathode used in Li-ion batteries must have characteristics such as good conductivity, presence of metal ions with suitable electronegativity, a stable and reversible reaction with the entrance, and intercalation of Li [72]. In the last 3 decades, the energy storage market has been dominated by Li-ion batteries. However, due to the rapid growth of electric vehicles (EVs) and hybrid electric vehicles, further increase in the energy density of Li-ion batteries has attracted the attention of many researchers, and thus, electrode materials require high charge/ discharge capacity or higher cell operating voltage. On the other hand, the mechanism of current cathodic materials such as LiCoO2, LiMn2O4 and LiFePO4, are mainly based on insertion, which bounds the capacity to lower than 200 mAh g
1, and limit the ability to improve the energy density of Li-ion batteries. Therefore, sulfur cathode materials used in LieS batteries that have high specific theoretical capacity (1675 mAh g
1) and high energy density are one of the most favorable energy storage devices. In addition, its low sulfur cost and environmental friendliness, make LieS batteries a better candidate than advanced Li-ion batteries. A comparision of properties of LieS and different Li-ion batteries has been demonstrated in Table 2.1.

7.

Applications of lithiumesulfur batteries

It is safe to say that the commercialization of LieS batteries started in 2015. Recently, six major problemsdhigh sulfur content (70 wt%), high areal sulfur loading (5 mg cm
2), electrolyte to sulfur (E/S) ratio (4 Lmg
1), electrode size (1 cm2cell
1), discharge cutoff voltage (1.7 V), and cycle performance at a low C-ratedhave been reported for the commercialization of LieS batteries [75]. OXIS Energy was the first company in the world to attempt to build a large-scale LieS battery. The battery cathode is an S/C composite with a polymer connector that improves its performance over simple LieS battery electrodes. In this battery, the cathode surface is protected by a ceramic coating (a passive layer) to prevent chemical decomposition and the entrance of PSs into the electrolyte [76]. Sion Power brought together a team of world-class scientists and engineers to advance LieS battery technology. The result was the signing of a 3-year agreement

Type of battery

Anode

Cathode

Voltages (V/cell)

Capacity (Wh kgL1)

LS

Li metal

Sulfur

2.1e2.5

550

LCO LMO NMC LFP

Graphite Graphite Graphite Graphite

LiCoO2 LiMn2O4 LiNiMnCoO2 LiFePO4

3.0e4.2 3.0e4.2 3.0e4.2 2.5e3.6

150e200 100e150 150e220 90e120

40e500 (recently near 200) 500e1000 300e700 1000e2000 >2000

NCA LTO

Graphite Li2TiO3

LiNiCoAlO2 LMO or NMC

3.0e4.2 1.8e2.85

200e260 50e80

500 3000e7000

Cycle life

Thermal runaway

Applications

Ref.

e

EVs

[73]

150 C 250 C 210 C 270 C

Phones, tablets, laptops Power tools, medical devices EVs, e-bikes, industrial Portable and stationary needing high load currents and endurance Medical devices, industrial UPS, solar-powered street lighting

[74] [74] [74] [74]

150 C One of the safest Li-ion batteries

Recent developments in lithiumesulfur batteries

Table 2.1 Comparison of Li-ion and LieS batteries.

[74] [74]

29

30

Lithium-Sulfur Batteries

between Airbus Defense and Space and Sion Power Corporation. In this agreement, Sion Power has committed to providing and supplying LieS batteries for use in high-altitude quasi-satellite Airbus Defense aircraft [77]. An ultra-high-capacity LieS battery has been developed in the Department of Mechanical and Aerospace Engineering at Monash University in Melbourne, Australia, which is claimed to provide the energy needed for a smartphone for 5 days [78]. Sony, which introduced the first commercial Li-ion battery, plans to increase battery energy density up to 40% by replacing LieS batteries, thus producing new sulfur-based batteries to replace Li-ion batteries [79]. The commercial applications of LieS batteries, such as in EVs, are still under stress due to the limitations of LieS batteries and are still an open area for further research.

8.

Conclusion and future perspectives

In summary, this work provides a systematic description of advances in the global development of LieS batteries for energy applications. LieS batteries will hold their value in the future because of their unique characteristics, i.e., low cost, nontoxicity, availability, and most of all, greater discharge capacity (1675 mAh g
1) and energy density (2600 Wh kg
1). The solutions for the existing challenges, i.e., low conductivity, low lithium diffusivity, and short life cycle, will enhance their viability at a commercial scale. However, different works to enhance conductivity and minimize shuttle effect by changing properties of battery components, i.e., cathode, anode, and electrolyte, have been summarized here but still have considerable room for future research. Modification in the cells to improve their performance is a need but at about the same the cost should be kept at a reasonable price, e.g., the addition of metal oxide in the cathode. The most important issue in promoting mass commercialization is to significantly enhance the cycle life of LieS cells. Cells must attain dependable performance throughout a minimum of 200 cycles with a capacity reduction of no more than 60% to offer a marketable product. Increasing the cycle life to a target of 500 cycles would greatly expand the deployment possibilities for LieS cells. The use of thick, high-S loading cathodes, as well as the previously mentioned reductions in electrolyte volume and Li excess, will result in a higher volumetric energy density. This has been identified as a top priority for facilitating the widespread deployment of LieS batteries. However, to maximize the potential of LieS cells, research must be undertaken throughout the entire cell, including the electrolyte and shielded anode, as well as control and technical factors that enable larger-scale battery operation. In addition, new basic insights on the reaction process must be taken into account.

References [1] H. Zhang, X. Li, H. Zhang, LieS and LieO2 batteries with high specific energy, in: Li-S and Li-O2 Batteries with High Specific Energy, Research and Development, Springer Nature, 2017.

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Chemistry and operation of lithiumesulfur batteries

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Vikram K. Bharti, Sony K. Cherian, Mayur M. Gaikwad, Anil D. Pathak and Chandra S. Sharma Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India

1. Introduction Rechargeable batteries have emerged as one of the most promising options for electrical energy storage. Lithium-based batteries are the best candidate among all battery chemistries because of their high energy density, relatively low cost, and long cycle life. Lithium-ion (Li-ion) batteries are predominant in the current market and are based on an intercalation mechanism comprising metal oxide cathodes and graphite anodes [1]. Current Li-ion batteries are inadequate to meet the needs of tomorrow’s energystorage systems. High-energy-density rechargeable batteries are required to improve the driving range (distance after a recharge) of electric vehicles (EVs) and the capacity of grid-storage applications. However, the energy density of current commercial Li-ion battery technology is limited due to the intercalation chemistry of its cathode and anode material. The lithiumesulfur battery (LSB) is one of the most promising next-generation battery systems, with an extremely high theoretical gravimetric energy density of 2500 Wh kg
1 (Fig. 3.1). The high energy density of LSBs stems from the cathode and anode chemistry used. Sulfur cathodes have a high theoretical charge-storage capacity of 1675 mAh g
1, and the lithium-metal anode possesses a high theoretical capacity of 3860 mAh g
1. They are expected to provide two to four times the specific energy of conventional Li-ion batteries, making them excellent candidates for future energy-storage systems. The LSB system involves reversible redox conversion reactions between active sulfur and its end discharge product, lithium sulfide (Li2S). Dissolved polysulfides (Li2Sx: x ¼ 4e8) are formed during the discharge process by cleaving the SeS covalent bonds of solid cyclo octasulfur (S8). Then, through multistage reactions, it is further reduced into solid Li2S [3].

1.1

Background of lithiumesulfur battery

The LSB is one of the best candidates for next-generation energy-storage systems. Herbert and Ulam [4] pioneered elemental sulfur as positive electrode material for storage batteries and dry electric cells in 1962. Later, in 1966, Rao et al. [5] specifically

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00027-2 Copyright © 2022 Elsevier Inc. All rights reserved.

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Figure 3.1 Comparison of volumetric and gravimetric energy density of lithium-ion and lithiumesulfur batteries. Reproduced with permission from J.-G. Wang, K. Xie, B. Wei, Advanced engineering of nanostructured carbons for lithiumesulfur batteries, Nano Energy 15 (2015) 413e444. https:// doi.org/10.1016/j.nanoen.2015.05.006.

patented high-energy-density metalesulfur batteries using organic electrolytes. Subsequent developments focused chiefly on primary lithiumesulfur (LieS) cells. This period saw significant growth in electrolyte solvent identification for LSBs, such as saturated aliphatic amines, propylene carbonate, a mixture of tetrahydrofurane toluene, and mixtures of dioxolane-based electrolytes. The intermediate charged/discharged products, lithium polysulfides, are soluble in liquid electrolytes, significantly affecting cell performance. The solid-state electrolyte was also proposed for the LSB for safety reasons (limiting dendrite growth) and to bypass the severe limitations of the progressive dissolution of polysulfides in liquid electrolytes. The protection of the Li anode is significant in the field of LSBs. By using a lithium nitrate (LiNO3) additive, the Li anode surface can be protected from polysulfide species. The ether solvent forms a uniform solideelectrolyte interface (SEI) that helps prevent parasitic reactions between lithium and polysulfides [6]. A rapid increase in emerging applications development (such as EVs, military power supplies, and stationary storage systems for renewable energy) provoked higher demand for high-performing batteries. Thus, the investigation of high-energy dense LSBs has gained considerable interest. Researchers are mainly focused on developing more conductive sulfur-carbon composites and solid electrolytes. In addition, efforts have been focused on fabricating efficient electrodes, designing novel cell configurations, and understanding the degradation mechanism and limiting factors for longcycle-life LSBs. Following the pioneering work published by Nazar et al. [7] in

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nanostructured carbonesulfur cathode, it attracted drastic research interest in the field. The subsequent development in LieS rechargeable battery technology is enormous and is under development by industry and research organizations. Sion Power Incorporation is a notable industry in developing commercial LieS cells [8]. Most works in LSBs were devoted to the development of cathode composite electrodes and designing host matrices for sulfur impregnation. In addition to the efforts to control polysulfide dissolution using various cathode architectures, various recent research focused on finding suitable composite cathodes for effective polysulfide trapping by adsorption/ absorption within the electrode. Research in the field of LSBs is growing at a tremendous pace. The paradigm has shifted from developing a suitable host for sulfur confinement to using chemically synthesized dissolved polysulfides in either static or redox flow configurations. Alternatively, those polysulfides are even employed as electrolyte additives for improved cycling performance. LSB modeling and simulation can significantly enhance and analyze the cell design to study different operating conditions and cell design parameters. It is true at all length scales, from material discovery to predictive modeling for lifecycle analysis and the development of battery management systems. Now, most researchers have started using ab initio molecular dynamics and density functional theory to guide material selection for modeling the cell. In particular, the expanding size of the EV market seems to be the strongest motivation for making LSBs, the system of choice for the future [8].

1.2

Organization of the chapter

The chapter will start with a brief background of LSB technology, operating parameters, underlying theory, cell chemistry, and operations. Then, the underlying mechanism of polysulfide formation during cell reaction concerning voltage is discussed in detail. Further, we will discuss the reaction mechanism based on sulfur allotropes (mainly for S8 and S2eS6) and the role of catalysts in stabilizing the lithiumpolysulfide shuttle process for improving the electrochemical performance. Later, the LSB’s different thermodynamic and kinetic issues, including the insulating nature of S and Li2S, self-discharge, Li-anode, SEI, shuttle behavior of lithium polysulfides, volume change, and polarization of cell will be discussed. Finally, this discussion will elucidate the electrochemical conversion of cyclic S8 into lithium polysulfide (Li2Sx; 1  x  8) and the transformation of higher-order polysulfide Li2Sy (4  y  8) into lower-order polysulfide Li2Sz (1  z  2).

2. Cell chemistry of lithiumesulfur battery LSBs are expected to be viable candidates that can fulfill the energy requirements in the modern world. It is essential to understand the unique electrochemical reactions of LSBs to implement directed steps toward improving the technology. Generally, LieS cell configuration comprises a lithium anode, a porous polymeric membrane, an organic liquid electrolyte containing a lithium salt, and a sulfur composite

40

Lithium-Sulfur Batteries

Figure 3.2 Illustration of lithiumesulfur battery. Reproduced with permission from K. Propp, M. Marinescu, D.J. Auger, L. O’Neill, A. Fotouhi, K. Somasundaram, G.J. Offer, G. Minton, S. Longo, M. Wild, V. Knap, Multi-temperature state-dependent equivalent circuit discharge model for lithium-sulfur batteries, J. Power Sources 328 (2016) 289e299. https://doi.org/10.1016/j.jpowsour.2016.07.090.

cathode, as shown in Fig. 3.2. An LSB produces electricity through an electrochemical reaction involving sulfur and lithium to form Li2S, as shown in Fig. 3.2. In LieS systems during discharge, Li2S is formed by reducing sulfur. Contrarily, it oxidizes back to sulfur during charging. The two half-reactions are Li 4 Liþ þ e


(3.1)

S þ 2Liþ þ 2e
4 Li2S

(3.2)

The theoretical energy density with a potential of 2.15 V versus Li/Liþ is determined by the overall reaction: S8 þ 16Li 4 8Li2S;

E ¼ 2.15 V

(3.3)

However, lithium and sulfur form intermediate long-chain polysulfides before the Li2S formation. At the anode/electrolyte interface during discharge, Liþ ion is formed by oxidizing lithium (anode), which travels via the electrolyte. The electron arrives at the S cathode after traveling through the outer circuit. Sulfur is reduced at the cathode side, where electrons are added. A series of chemical steps proceed at the cathode to release energy from the elemental sulfur and form a series of intermediate long-chain followed by short-chain lithium polysulfide species [10]. As opposed to the intercalation chemistry of metal oxides, LieS redox reactions undergo conversion chemistry. Conversion chemistry in LSBs converts individual S8 molecules to Li2S, operating on

Chemistry and operation of lithiumesulfur batteries

41

a per-molecule basis instead of inserting lithium into a larger crystal structure. Ions can flow between electrodes while being electrically isolated by the separator. The electrolyte provides Liþ ions to keep electrons flowing, and the reaction will continue at the electrode. The LieS electrochemistry involves multiple complex reaction steps. Overall, the electrochemical reaction can be described by the reactions listed below [11]. S8 þ 2Liþ þ 2e
/ Li2S8; E ¼ 2.39 V

(3.4)

3Li2S8 þ 2Liþ þ 2e
/ 4Li2S6; E ¼ 2.37 V

(3.5)

2Li2S6 þ 2Liþ þ 2e
/ 3Li2S4;

(3.6)

E ¼ 2.24 V

Li2S4 þ 2Liþ þ 2e
/ 2Li2S2; E ¼ 2.2 V

(3.7)

Li2S2 þ 2Liþ þ 2e
/ 2Li2S; E ¼ 2.15 V

(3.8)

Elemental sulfur, S8, accepts two electrons from the external circuit and two lithium ions from the electrolyte, producing a polysulfide with composition Li2S8dsee Eq. (3.4). The lithium polysulfide products can dissolve in a selection of electrolytes [12]. The solubility of electrode material in the electrolyte is uncommon in Li-ion batteries and introduces several problems specific to the LSBs. These issues, specifically sulfur loss and polysulfide shuttling, will be discussed in the next section of this chapter. Sulfur in long polysulfide chains undergoes further reduction, splitting into shorter chains such as Li2S6 and Li2S4dsee Eqs. (3.5) and (3.6). Reduction of elemental sulfur to higher-order polysulfides (Li2Sn, n  4) with a total discharge capacity of 419 mAh g
1 exhibits a plateau at 2.4 V. These polysulfides are soluble in commonly used ether solvents [11]. The high-order polysulfides are further reduced to low-order polysulfides (Li2Sn, 1 < n < 4) with a total discharge capacity of 1256 mAh g
1, resulting in a lower voltage plateau (0.5 nm. The smaller sulfur molecules can fit in a microporous carbon matrix with a pore size of 0.5 nm. In sulfurized carbon, the short sulfur chains are covalently bound to the surface of a carbon matrix. Unlike cyclic S8 molecules, which undergo solideliquidesolid conversion reactions, no intermediate polysulfide is formed in short-chain sulfurdit is discharged and charged entirely through a solidesolid phase transition from S to Li2S. As a result, there is no upper discharge plateau corresponding to the transition from S8 to S2
4 . The voltage curves show only one discharge plateau (Fig. 3.4B) at 1.8 V, corresponding to the S2
4 to S2 transition [14]. In this regard, short-chain sulfur is also compatible with carbonate-based electrolytes. The polysulfide shuttle effect can be avoided entirely, resulting in high coulombic efficiency and stable capacity retention. But the energy density reduces because of the lower average voltage (1.9 V vs. Liþ/Li) of short-chain sulfur cathodes compared with elemental sulfur

44

Lithium-Sulfur Batteries

cathodes (2.2 V vs. Liþ/Li). Furthermore, it has significantly lower practical specific capacity because this type of sulfur cathode typically has a low sulfur content (typically less than 50 wt.%) [14,15].

3.3

Lithium sulfide

For the large-scale implementation of LSB, the use of a lithium metal anode, as discussed above, poses a significant safety issue. In this regard, Li2S, the final discharge product of sulfur, has been investigated as active material in the cathode. The lithiated Li2S can work with safer lithium-free anodes (e.g., graphite, Si/Sn-based anodes). However, the electrochemical activity of Li2S is even lower than that of sulfur. It requires a higher cutoff voltage of 3.8 V to overcome the large potential barrier (1 V) when charging begins and when activating Li2S (Fig. 3.4C), which results in low coulombic efficiency for the first cycle. Furthermore, Li2S cathodes are moisturesensitive and require a dry and inert environment during fabrication and manipulation that could impact industrial production [13]. However, the coupling of Li2S and lithium-free anodes has received little attention in the literature.

3.4

Catholyte

Dissolved polysulfides (catholyte) are used as the initial active material and termed a semiliquid battery system. Rauh et al. reported the first lithium polysulfide batteries in 1979 [16] (Fig. 3.4D). In this configuration, a conductive electrode (typically made of carbon materials) is used as a current collector for electron transport [16]. Instead of using solid sulfur as a precursor, lithium polysulfide in ether solvent as a catholyte or metallic lithium as an anode is used. The catholyte is designed for cycling only in the range between sulfur and Li2S4. As a result, challenges such as dendrite formation and volume change in conventional LSB can be avoided. Lithium polysulfide battery improves sulfur utilization, reduces electrode structural variation, distributes active material uniformly, and improves LieS redox reaction kinetics. Several studies have recently shown that lithium polysulfide systems have better redox kinetics due to the uniform distribution of active material. The higher reaction activity of liquid polysulfides over solid sulfur results in better sulfur utilization [13,16]. It has excellent cycle life and compatibility with flow battery design. Compared with the first three types of LSBs, the electrolyte mass in lithium polysulfide batteries is significantly high. This high value is due to the reaction between polysulfide and electrolyte, contributing to inactive mass and lower energy density. In practice, lithium polysulfide systems can be used in stationary flow batteries. However, when increasing the effective sulfur concentration in the catholyte, efforts should be made to reduce the mass of the conductive current collector [13].

4.

Electrochemical characteristics and challenges of lithiumesulfur batteries

LSBs are potential candidates to fulfill the current energy demand for high-energydensity electrochemical devices. LSBs bear an average voltage of 2.1 V with a

Chemistry and operation of lithiumesulfur batteries

45

Figure 3.5 Critical issues associated with lithiumesulfur battery. Modified and reproduced with permission from J.-G. Wang, K. Xie, B. Wei, Advanced engineering of nanostructured carbons for lithiumesulfur batteries, Nano Energy 15 (2015) 413e444. https://doi.org/10.1016/j.nanoen.2015.05.006.

corresponding theoretical energy density of 2500 Wh kg
1. However, commercialization has been hampered by several key challenges (Fig. 3.5). This section discusses the key challenges, their effects on performance, and possible ways to tackle these challenges.

4.1

Insulating nature of sulfur and lithium sulfide

Sulfur and its reduction product (Li2S) are electrically non-conducting and thus impede electronic conductivity. It results in low utilization of active sulfur during the electrochemical reaction. In an efficient redox reaction, sulfur must be incorporated into the conducting matrix, such as conducting polymers and carbon. Commercial demand is met with an optimized sulfur content of w70 wt.% mixed with 30 wt.% of the conductive matrix (carbon or conducting polymer). Conducting carbon (20e50 wt.%) is key to preparing the efficient cathode material for LSBs because of the viability to change its morphology as per requirement. Furthermore, a binding agent (3e15 wt%) is required to enhance the cohesion and adhesion of the electrode material to the current collector. However, an excess amount of binding agent and carbon can lead to decreased device energy density because these are electrochemically inactive components. So the efficient design of cathode materials requires limited binding agent volumes (typically 3e15 wt.%) and carbon (w20e50 wt.%) to meet the commercial demand of LSB. The major report from the researchers includes the usage of carbon-based materials with various morphology, such as carbon aerogel [17], carbon nanofiber [18], carbon nanotube [19], carbon nanobox [20], and carbon sphere [21].

46

Lithium-Sulfur Batteries

Nowadays, carbon derived from biomass is receiving more attention because of its tunable structure, porosity, and environmentally friendly approach. H. Chen et al. [22] have reported using walnut shells, peanut shells, and pistachio hulls as sulfur hosts, which significantly relieve the issue related to the insulating nature of sulfur. After investigating the properties of these different precursor carbons, they concluded that the carbon derived from the pistachio hull delivered the best electrochemical performance. After activating pistachio shell carbon, it delivered the specific capacity of 943 mAh g
1 at a current rate of 0.2 C and retained 619 mAh g
1 after 100 cycles. Later, Zhao-Yan Zhu et al. [23] have utilized the low-cost, green, and large-scale ballmilling of pollutant fly-ash and sulfur composite. The composite material was directly used as a cathode for LSB. The presence of microstructure in the material enables it to sustain volume changes. The presence of sulfur components in fly ash acts as a chemical anchor for polysulfide. The composite exhibited excellent electrochemical performance with a 0.068% capacity decay rate at a current rate of 0.2 C. Researchers later found that functional carbon materials have the dual roles of (1) providing electrical conductivity and (2) absorbing the intermediate polysulfides responsible for shutting behavior [13]. According to Hang-Yu Zhou et al. [24], nitrogen-doped graphene nanoribbon aerogels have been used as sulfur hosts. Benefited from the properties such as high specific surface area, high porosity, conductive network, and N-doping, the material exhibited a reversible capacity of 943 mAh g
1 and retained 813 mAh g
1 after 100 cycles at a current rate of 0.5 C. Nowadays, metal-transition compounds such as oxides, nitrides, phosphides, and sulfides are getting more attention due to their excellent adsorption and efficient catalytic activity to transform the long-chain polysulfides to short-chain polysulfides. A series of metal-oxides, sulfides, phosphides, and nitrides have been explored. However, the use of cobalt-based oxides (CoO, Co3O4, etc.) has gained more attention because of their excellent catalytic effect [25]. Lei Zhou et al. [26] reported exploring a double-shelled Co3O4/C nanocage as a cathode host. They believe that the double-shelled structure can enhance the adsorption capability and the sulfur’s electrical conductivity. As a result, the material has shown a cycle life of 500 with a capacity decay rate of 0.083% per cycle at a current rate of 1 C.

4.2

Shuttle behavior of lithium polysulfides

During the discharge action of LSB, long-chain lithium-polysulfides (Li2Sn, 4  n  8) are formed first, followed by subsequent reduction to short-chain lithium-polysulfides (Li2S2, Li2S). The rate kinetics for the formation of Li2Sn is higher than in Li2S2/Li2S formation. The Li2Sn produced initially are soluble in the most frequently used organic solvents and freely migrate between cathode and anode, leading to a so-called shuttle behavior. This shuttle effect has the most deteriorating effects, such as (1) loss of active materials, (2) passivation of the lithium anode, and (3) inferior cycling. However, the shuttle behavior is required to some extent to utilize all of the sulfur incorporated in the cathode. Whenever the sulfur present at the surface is consumed, the sulfur present in the bulk electrode can be utilized further [13]. So when designing the LieS battery, we must balance the shuttle effect to avoid excess loss of active sulfur. This loss can be avoided by capturing Li2Sn within the cathode

Chemistry and operation of lithiumesulfur batteries

47

region using a polysulfide suppressor. The polysulfide suppressor includes hydrophobic materials (such as carbon) and polar materials (such as heteroatom-doping, transition metal compounds). The polysulfides suppressor can be added to the cathode matrix or as a separate layer called an “interlayer.” Researchers have adopted various approaches, such as functionalizing carbon materials with heteroatom (N, O, P, S, B) doping. However, researchers soon realized that barely capturing long-chain polysulfides is not enough to boost LSB performance. Over the last decade, more emphasis has been given to electrocatalysts. The electrocatalyst plays a dual role in LSBs: (1) it adsorbs long-chain polysulfides, and (2) it converts them catalytically to shortchain polysulfides by forming a surface-bound complex, thiosulfate (S2O3)2
. Transition metal compounds (oxides, nitrides, and sulfides) are known for their various oxidation states. They can bind easily with the long-chain polysulfides and convert them to short-chain polysulfides. A series of transition metals compounds have been investigated in this regard. Weibang Kong et al. [27] have reported the usage of MnO2/Graphene/Carbon nanotube as an interlayer to inhibit the infamous shuttle behavior of polysulfides. Benefiting from the excellent barrier capability of this interlayer, the cell was cycled for 2500 cycles at a current rate of 1 C with an extremely slow capacity decay rate of 0.029% per cycle. Miaoran Li et al. [28] followed the previous work and reported using MoS2 on carbon sphere and utilized as a sulfur host. Benefited from the catalytic action of MoS2, the as-fabricated cell could operate to 600 cycles at the current rate of 0.5 C and deliver a capacity of 795 mAh g
1 with a capacity retention of 63.1%. Later, Xiaochun Gao et al. [29] reported using titanium nitride embedded in mesoporous carbon. Resulting from the strong catalytic effect of titanium nitride, the material suppressed shuttle behavior and led to a high capacity of 1264 mAh g
1 at 0.2 C with a negligible decay rate of 0.06% per cycle.

4.3

Volume change

The density of elemental sulfur is 2.07 g cm
3, whereas the density of the electrochemical end product is 1.66 g cm
3. This density difference gives rise to a volume change of up to 80% during lithiation and delithiation. The volume change results in pulverization and degradation of cathode materials, leading to the end of LSB cycle life. This issue can be avoided using a porous substrate such as a high surface area precursor to accommodate volume changes. The work reported by Jian Wei et al. [30] has designed a hollow V2O5 sphere that can sustain the volume changes and trap the longchain polysulfides. Furthermore, graphene oxide was mixed with as-prepared V2O5 to enhance electronic conductivity. Benefiting from the hollow structure and improved electronic conductivity, the material delivered 896 mAh g
1 after 100 cycles with a capacity decay rate of 0.015% per cycle at a current rate of 0.1 C. On the other hand, Ying Chu et al. [31] have designed a robust polymeric binder with multiple hydrogen bonds and used it for LSB. The multiple H-bonds formed an excellent adhesive layer and buffered the volume changes. As a result, the material demonstrated 629 mAh g
1 with high sulfur loading of 9.6 mg cm
2 at a current rate of 0.2 C.

48

4.4

Lithium-Sulfur Batteries

Self-discharge

Long-chain polysulfides in the LSB continuously dissolve in the electrolyte even in the rest state [32e34]. Due to the continuous dissolution of polysulfide, concentration gradient develops in the electrochemical system and diffuses to lithium anode, thus causing lithium anode deterioration and passivation leading to dead lithium [35]. One way to avoid this issue is to mix sulfur with materials to form chemical bonds that avoid dissolution during the rest state. Self-discharge leads to a continuous decrease in cell potential even in the rest state, resulting in cell failure. To avoid the issue of self-discharge, Jia-Qi Huang et al. [36] have utilized permselective graphene oxide membrane. The oxygen presence created a polar plane, and the carboxyl group acted as an active site for positively charged Liþ. Due to electrostatic interaction with the negatively charged species (S2
n ), it rejected the transportation of polysulfides, reducing self-discharge. The cyclic stability improved after using this permselective layer and the capacity decay rate decreased to 0.23% from 0.49%. In the other approach, Wangyu Li et al. [37] have reported using gel polymer electrolytes for a low-discharge LSB. The gel polymer electrolyte can restrain the dissolution of soluble polysulfides in the electrolyte and inhibit the self-discharge action. They have investigated electrochemical performance with quasi-solid-state batteries. The cell exhibited impressive cycling with 72% capacity retention at a current rate of 0.5 C after 100 cycles. Furthermore, they have tested the electrochemical performance with the cell after resting for 24 h, and the cell exhibited almost no capacity decay demonstrating the potential of gel polymer electrolyte.

4.5

Lithium anode dendrite

Lithium metal is undisputedly the most energy-dense anode candidate because of its low electrochemical potential (
3.04 vs. the SHE) and high-capacity density (3860 mAh g
1, w10-times higher than graphite). Lithium anode coupled with low-cost sulfur cathodes (1672 mAh g
1, w12 times higher than LiCoO2) yields a theoretical energy density of 2500 Wh kg
1. However, the use of lithium metal invites several challenges, such as dendrite formation and low lithium cycling efficiency. The dendrite formation and low lithium cycling result from the unstable SEI on the lithium anode. The unstable SEI cannot sustain the shape and volume changes during cycling, resulting in nonuniform lithium dissolution and deposition, leading to dendritic growth. In addition, the breakdown of the SEI layer results in exposure of fresh lithium to the electrolyte to form a new SEI layer, which decreases the lithium cycling efficiency. As a result, conservation of stable SEI is more difficult. Excess lithium, therefore, is required to couple with the sulfur cathode, which ultimately decreases the practical energy density of LSB. Nowadays, more emphasis is given to anode protection. In the work reported by Wei-Jing Chen et al. [38], a mixed di-isopropyl etherbased (mixed-DIPE) electrolyte was used to protect the Li-anode, as the fabricated electrolyte improves compatibility with Li-metal, enhances cyclability, and suppresses polysulfides shuttling. In addition, cyclic stability was compared with that of the routine electrolyte (1.0 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in

Chemistry and operation of lithiumesulfur batteries

49

1,3-dioxolane (DOL)/dimethoxyethane (DME)). The cycle life was doubled using modified rather than routine electrolytes. In another work by Jun-Yu Wei et al. [39], an organosulfur SEI was used to stabilize the Li-anode. The 3,5-bis(trifluoromethyl) thiophenol salt was used as an electrolyte additive to create the SEI layer on the Li-anode. The SEI layer containing organosulfur protects the Li-anode from corrosion due to intermediate polysulfides. The cell with high-sulfur loading of 4.5 mg cm
2, a lean electrolyte-to-sulfur ratio of 5.0 mL mg
1, and ultrathin Li-anode of 50 mm thickness operated for 82 cycles compared with 42 cycles for the routine electrolyte. This result demonstrates the advantage of the SEI layer formed with the chosen electrolyte additive.

4.6

Solideelectrolyte interface

Lithium metal itself is not stable in organic electrolytes. Along with the electrochemical cycling, it naturally forms an SEI on the lithium (anode) surface. Moreover, the diffusing short-chain lithium-polysulfides causes an additional parasitic reaction on lithium anode. The most commonly used electrolyte solvent for LSBs is a binary solvent system. The first solvent is linear ether (DME) that possesses high polysulfide solubility and fast polysulfide reaction kinetics but is more reactive with lithium metal. The second is the cyclic ether (DOL) which helps form a stable SEI layer on the surface of lithium metal. However, LiNO3 is used extensively as a co-salt along with LiTFSI to form a more stable SEI layer on the lithium metal. In addition, because of the impact of soluble polysulfides migrated from the sulfur cathode, the properties of the SEI layer on lithium anode are more complicated than Li-ion batteries. The work reported by Yan-Qiu Shen et al. [40] has shown the possibility of forming a stable and flexible hybrid SEI layer on the Li-anode by utilizing gel polymer electrolyte composed of polyvinylidene fluoride and organopolysulfide polymer. As a result of a stable SEI layer, the cell delivered an initial capacity of 943 mAh g
1 and retained 825 mAh g
1 after cycling for 100 cycles at the current rate of 0.1 C. The other work reported by Wei Guo et al. [41] used a bifunctional electrolyte additive, 1,3,5-benzenetrithiol (BTT), to create an SEI layer by organothiol transformation. The Li-metal reacts with BTT to form lithium 1,3,5-benzenetrithiolate as an anode protection film enabling smooth deposition/stripping. The cell with BTT delivered a specific capacity of 1239 mAh g
1 and long cycle life of 300 cycles while cycling at a current rate of 1 C.

4.7

Cell polarization

In most practical cases, the cell potential is different from the theoretically predicted cell potential. The deviation of cell potential from the equilibrium value is termed overpotential. The cause of this deviation is called polarization. It can be considered in the following manner: when current starts flowing in the cell, several phenomena occur at both electrodes. First, the ions must cross the barrier of the electric double layer (“activation overpotential”). Second, the ions consumed/generated at the electrodee electrolyte interface must be replenished by ions from the bulk electrolyte or

50

Lithium-Sulfur Batteries

transported to bulk solution to maintain charge neutrality (“concentration overpotential”). And finally, as the current flows through the cell, the resistance of the electrolyte, electrodes, and other contact resistances causes an ohmic drop (“resistance overpotential”).

5.

Polysulfide formation and conversion

In LSBs, the formation and conversion of lithium polysulfides is a key parameter. The long-chain polysulfides formed at higher voltage are soluble in the most frequently used ether-based electrolyte. These solvated polysulfides cause a series of serious bottlenecks for LSBs. This section briefly covers the formation of lithium polysulfides, their associated problems, and finally, their capturing strategies.

5.1

Reduction of elemental sulfur to lithium polysulfide

Sulfur is incorporated in elemental form “octasulfur (S8).” The sulfur is present in a charged state, and the reaction starts with its discharge action. During discharge action, S8 is reduced to a series of lithium-polysulfides (Li2Sx, 1  x  8). As the voltage drops with the progress of the electrochemical reaction, the chain length of sulfur reduces from S8 to S (i.e., from Li2S8 to Li2S). The formation of long-chain lithiumpolysulfides occurs at the voltage window of 3.0e2.0 V, whereas the formation of short-chain lithium-polysulfides takes place at the voltage window of 2.0e1.7 V. The end product Li2S is formed at 1.7 V [42].

5.2

Polysulfide precipitation and dissolution

The long-chain lithium-polysulfides formed at higher voltage are soluble in commonly used electrolyte solvents. Due to continuous dissolution in the cathode region, the concentration of long-chain lithium-polysulfides increases at the cathodeeelectrolyte interface, developing a concentration gradient and migrating to the anode. However, the short-chain lithium-polysulfides are formed on the reduction of long-chain lithium-polysulfides at a lower voltage. There are different phases involved in the complete discharge action. The discharge reaction starts with solid-phase S8, which converts to liquid phase long-chain lithium-polysulfides at higher voltage followed by subsequent reduction to solid-phase short-chain lithium-polysulfides. The end product is in the solid phase and has very low electrical conductivity. Therefore, they deposit at the cathode reducing the active sites to carry out the electrochemical reaction [43].

5.3

Polysulfide formation based on sulfur allotrope

The formation of polysulfides can be controlled by changing the initial state of sulfur used for LSBs. Among 30 existing allotropes of sulfur, octasulfur (S8) is more stable at room temperature. In addition, the other sources of sulfur used are (1) catholyte (in the

Chemistry and operation of lithiumesulfur batteries

51

form of liquid phase Li2S6); (2) short-chain sulfur molecules (obtained after heating at higher temperature generally, 250 C); and (3) solid-state (in the form of solid-phase Li2S). The usage of octasulfur gives rise to the formation of long-chain lithiumpolysulfides followed by short-chain lithium-polysulfides formations. However, when the initial material is Li2S6, Li2S8 formation is avoided, which has a great tendency toward solubility in electrolyte solvents. Meanwhile, when short-chain sulfur molecules are used as starting material, long-chain lithium-polysulfides formations are completely avoided, and only short-chain lithium-polysulfides are formed. However, the sulfur content and sulfur loading are less in the case of short-chain sulfur as per the commercial need.

5.4

Role of catalysts

To anchor long-chain lithium-polysulfides, initially, polysulfide trappers such as carbon in various forms and functional materials (especially heteroatom doping to carbon materials) have been used. These techniques improved the electrochemical performance to a large extent. However, the continuous accumulation of captured polysulfides increases the impedance of the LSB while long-cycling. Therefore, the catalysts that can convert these captured long-chain lithium-polysulfides are required to address this issue. In this regard, the transition metal compounds (namely, oxides, nitrides, and sulfides) are explored because of their great chemical affinity toward the long-chain lithium-polysulfides. Transition metal (such as Mn, Co, Ti) compounds first bind them and form a surface-bound intermediate, “thiosulfate (S2O3)2
.” Thiosulfate is electrochemically active throughout the electrochemical reaction and binds the forthcoming polysulfides. Li2S2 is released as a by-product of this reaction, decreasing the accumulated polysulfides (Reactions 1 and 2). The reaction for long-chain lithium polysulfide conversion to short-chain polysulfides can be explained using the Wackenroder reaction [44].

In this mechanism, the generated long-chain polysulfide reacts with thiosulfate by insertion of SeS bond to generate polythionate complex (I) and leave short-chain polysulfides (i.e., Li2S2 or Li2S) by disproportionate reaction, as shown in Reaction 1

52

Lithium-Sulfur Batteries

[45]. On the other hand, the disproportionate reaction can result in the generation of thionate complex (II) following the reaction:

However, the bipolythionate complex (I) formation is preferred due to its stability in aqueous media. Compared with the catalytic activity of transition metal nitrides and sulfides, the catalytic activity of transition metal oxide is lower. However, when the adsorption and catalysis are combined and compared, the transition metal-oxides surpass the affinity of nitrides and sulfides. The conversion reaction of thiosulfate to polythionate with Li2Sy as a byproduct is shown in Reactions 1 and 2.

6.

Summary and outlook

The rapid advancement of LieS technology has propelled it to the forefront of “beyond Li-ion” as it possesses high energy density due to the cell chemistry based on conversion reactions rather than on intercalation. Furthermore, the cell chemistry of lithiumesulfur is mainly dependent on the initial state of sulfur in the cathode, i.e., elemental sulfur (S8), short-chain sulfur, Li2S, and catholytes. However, the commercialization of LSBs is mainly hindered by pure utilization of sulfur (shuttle effect), insulating nature of sulfur, and dendrite formation on lithium anode. Nevertheless, the advancement of solid-state electrolyte technology and the improvement of custom modeling and diagnostic tools can help accelerate the implementation of LSBs. Although the first applications for LSBs are likely to be specialized, the technology’s increasing maturity over the next decade will ensure that LSBs help electrifies infrastructure across multiple sectors.

References [1] A. Manthiram, S.-H. Chung, C. Zu, Lithium-sulfur batteries: progress and prospects, Adv. Mater. 27 (12) (2015) 1980e2006, https://doi.org/10.1002/adma.201405115. [2] J.-G. Wang, K. Xie, B. Wei, Advanced engineering of nanostructured carbons for lithiumesulfur batteries, Nano Energy 15 (2015) 413e444, https://doi.org/10.1016/ j.nanoen.2015.05.006.

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[3] D. Parra, M. Swierczynski, D.I. Stroe, S.A. Norman, A. Abdon, J. Worlitschek, T. O’Doherty, L. Rodrigues, M. Gillott, X. Zhang, C. Bauer, M.K. Patel, An interdisciplinary review of energy storage for communities: challenges and perspectives, Renew. Sustain. Energy Rev. 79 (2017) 730e749, https://doi.org/10.1016/j.rser.2017.05.003. [4] D. Herbert, J. Ulam, Electric Dry Cells and Storage Batteries, US Patent 3043896, 1962. [5] M.L.B. Rao, Battery Employing, 1970. [6] D. Aurbach, E. Pollak, R. Elazari, G. Salitra, C.S. Kelley, J. Affinito, On the surface chemical aspects of very high energy density, rechargeable Li-sulfur batteries, J. Electrochem. Soc. 156 (8) (2009) A694eA702, https://doi.org/10.1149/1.3148721. [7] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat. Mater. 8 (6) (2009) 500e506, https://doi.org/10.1038/ nmat2460. [8] J.B. Robinson, K. Xi, R.V. Kumar, A.C. Ferrari, H. Au, M.M. Titirici, A.P. Puerto, A. Kucernak, S.D.S. Fitch, N.G. Araez, Z.L. Brown, M. Pasta, L. Furness, A.J. Kibler, D.A. Walsh, L.R. Johnson, C. Holc, G.N. Newton, N.R. Champness, et al., 2021 roadmap on lithium sulfur batteries, J. Phys. Energy 3 (3) (2021), https://doi.org/10.1088/25157655/abdb9a. [9] K. Propp, M. Marinescu, D.J. Auger, L. O’Neill, A. Fotouhi, K. Somasundaram, G.J. Offer, G. Minton, S. Longo, M. Wild, V. Knap, Multi-temperature state-dependent equivalent circuit discharge model for lithium-sulfur batteries, J. Power Sources 328 (2016) 289e299, https://doi.org/10.1016/j.jpowsour.2016.07.090. [10] A. Manthiram, Y. Fu, Y.-S. Su, Challenges and prospects of lithium-sulfur batteries, Acc. Chem. Res. 46 (5) (2013) 1125e1134, https://doi.org/10.1021/ar300179v. [11] R. Xu, J. Lu, K. Amine, Progress in mechanistic understanding and characterization techniques of Li-S batteries, Adv. Energy Mater. 5 (16) (2015), https://doi.org/10.1002/ aenm.201500408. [12] S.S. Zhang, Liquid electrolyte lithium/sulfur battery: fundamental chemistry, problems, and solutions, J. Power Sources 231 (2013) 153e162, https://doi.org/10.1016/ j.jpowsour.2012.12.102. [13] R. Fang, S. Zhao, Z. Sun, D.-W. Wang, H.-M. Cheng, F. Li, More reliable lithium-sulfur batteries: status, solutions and prospects, Adv. Mater. 29 (48) (2017) 1e25, https://doi.org/ 10.1002/adma.201606823. [14] S. Xin, L. Gu, N.-H. Zhao, Y.-X. Yin, L.-J. Zhou, Y.-G. Guo, L.-J. Wan, Smaller sulfur molecules promise better lithium’sulfur batteries, J. Am. Chem. Soc. 134 (45) (2012) 18510e18513, https://doi.org/10.1021/ja308170k. [15] S.S. Zhang, Sulfurized carbon: a class of cathode materials for high performance lithium/ sulfur batteries, Front. Energy Res. 1 (DEC) (2013), https://doi.org/10.3389/ fenrg.2013.00010. [16] L. Qie, C. Zu, A. Manthiram, A high energy lithium-sulfur battery with ultrahigh-loading lithium polysulfide cathode and its failure mechanism, Adv. Energy Mater. 6 (7) (2016), https://doi.org/10.1002/aenm.201502459. [17] C. Hernandez-Rentero, R. Cordoba, N. Moreno, A. Caballero, J. Morales, M. OlivaresMarín, V. Gomez-Serrano, Low-cost disordered carbons for Li/S batteries: a high-performance carbon with dual porosity derived from cherry pits, Nano Res. 11 (1) (2018) 89e100, https://doi.org/10.1007/s12274-017-1608-1. [18] Y. Song, H. Zhou, X. Long, J. Xiao, J. Yang, N. Wu, Z. Chen, P. Li, C. Chen, J. Liao, M. Wu, Dual-heterostructures decorated interweaved carbon nanofibers sulfur host for high performance lithium-sulfur batteries, Chem. Eng. J. 418 (March) (2021) 129388, https://doi.org/10.1016/j.cej.2021.129388.

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[19] H. Pan, K.S. Han, M.H. Engelhard, R. Cao, J. Chen, J.G. Zhang, K.T. Mueller, Y. Shao, J. Liu, Addressing passivation in lithiumesulfur battery under lean electrolyte condition, Adv. Funct. Mater. 28 (38) (2018) 1e7, https://doi.org/10.1002/adfm.201707234. [20] Y. Huang, X. Gao, Z. Zhang, S. Batool, X. Li, T. Li, Porous hollow carbon aerogelassembled core@polypyrrole nanoparticle shell as an efficient sulfur host through a tunable molecular self-assembly method for rechargeable lithium/sulfur batteries, ACS Sustain. Chem. Eng. 8 (42) (2020) 15822e15833, https://doi.org/10.1021/ acssuschemeng.0c02456. [21] W. Ge, L. Wang, C. Li, C. Wang, D. Wang, Y. Qian, L. Xu, Conductive cobalt doped niobium nitride porous spheres as an efficient polysulfide convertor for advanced lithiumsulfur batteries, J. Mater. Chem. 8 (13) (2020) 6276e6282, https://doi.org/10.1039/ d0ta00800a. [22] H. Chen, P. Xia, W. Lei, Y. Pan, Y. Zou, Z. Ma, Preparation of activated carbon derived from biomass and its application in lithiumesulfur batteries, J. Porous Mater. 26 (5) (2019) 1325e1333, https://doi.org/10.1007/s10934-019-00720-2. [23] Z.Y. Zhu, N. Yang, X.S. Chen, S.C. Chen, X.L. Wang, G. Wu, Y.Z. Wang, Simultaneously porous structure and chemical anchor: a multifunctional composite by one-step mechanochemical strategy toward high-performance and safe lithium-sulfur battery, ACS Appl. Mater. Interfaces 10 (48) (2018) 41359e41369, https://doi.org/10.1021/ acsami.8b14947. [24] H.Y. Zhou, Z.Y. Sui, S. Liu, H.Y. Wang, B.H. Han, Nanostructured porous carbons derived from nitrogen-doped graphene nanoribbon aerogels for lithiumesulfur batteries, J. Colloid Interface Sci. 541 (2019) 204e212, https://doi.org/10.1016/j.jcis.2019.01.067. [25] Q. Li, Y. Zhao, H. Liu, P. Xu, L. Yang, K. Pei, Q. Zeng, Y. Feng, P. Wang, R. Che, Dandelion-like Mn/Ni Co-doped CoO/C hollow microspheres with oxygen vacancies for advanced lithium storage, ACS Nano 13 (10) (2019) 11921e11934, https://doi.org/ 10.1021/acsnano.9b06005. [26] L. Zhou, H. Li, X. Wu, Y. Zhang, D.L. Danilov, R.A. Eichel, P.H.L. Notten, Doubleshelled Co3O4/C nanocages enabling polysulfides adsorption for high-performance lithium-sulfur batteries, ACS Appl. Energy Mater. 2 (11) (2019) 8153e8162, https:// doi.org/10.1021/acsaem.9b01621. [27] W. Kong, L. Yan, Y. Luo, D. Wang, K. Jiang, Q. Li, S. Fan, J. Wang, Ultrathin MnO2/ graphene oxide/carbon nanotube interlayer as efficient polysulfide-trapping shield for highperformance LieS batteries, Adv. Funct. Mater. 27 (18) (2017), https://doi.org/10.1002/ adfm.201606663. [28] M. Li, H. Peng, Y. Pei, F. Wang, Y. Zhu, R. Shi, X. He, Z. Lei, Z. Liu, J. Sun, MoS2 nanosheets grown on hollow carbon spheres as strong polysulfide anchor for high performance lithium sulfur batteries, Nanoscale (2020), https://doi.org/10.1039/d0nr05727d. [29] X. Gao, D. Zhou, Y. Chen, W. Wu, D. Su, B. Li, G. Wang, Strong charge polarization effect enabled by surface oxidized titanium nitride for lithium-sulfur batteries, Commun. Chem. 2 (1) (2019), https://doi.org/10.1038/s42004-019-0166-8. [30] J. Wei, B. Chen, H. Su, X. Li, C. Jiang, S. Qiao, H. Zhang, Graphene oxide-coated V2O5 microspheres for lithium-sulfur batteries, Ceram. Int. 47 (2021) 10965e10971, https:// doi.org/10.1016/j.ceramint.2020.12.216. [31] Y. Chu, X. Cui, W. Kong, K. Du, L. Zhen, L. Wang, A robust polymeric binder based on complementary multiple hydrogen bonds in lithium-sulfur batteries, Chem. Eng. J. 427 (2022), https://doi.org/10.1016/j.cej.2021.130844. [32] M. Jiang, B. Gan, Y. Deng, Y. Xiong, R. Tan, Suppressing self-discharge with polymeric sulfur in Li-S batteries, Materials 12 (1) (2018) 1e8, https://doi.org/10.3390/ma12010064.

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[33] H. Li, X. Wang, C. Qi, C. Zhao, C. Fu, L. Wang, T. Liu, Self-assembly of MoO3-decorated carbon nanofiber interlayers for high-performance lithium-sulfur batteries, Phys. Chem. Chem. Phys. 22 (4) (2020) 2157e2163, https://doi.org/10.1039/c9cp06287d. [34] M. Liu, Q. Li, X. Qin, G. Liang, W. Han, D. Zhou, Y.B. He, B. Li, F. Kang, Suppressing self-discharge and shuttle effect of lithiumesulfur batteries with V2O5-decorated carbon nanofiber interlayer, Small 13 (12) (2017) 1e7, https://doi.org/10.1002/smll.201602539. [35] S. Suriyakumar, A.M. Stephan, Mitigation of polysulfide shuttling by interlayer/permselective separators in lithium-sulfur batteries, ACS Appl. Energy Mater. 3 (9) (2020) 8095e8129, https://doi.org/10.1021/acsaem.0c01354. [36] J.Q. Huang, T.Z. Zhuang, Q. Zhang, H.J. Peng, C.M. Chen, F. Wei, Permselective graphene oxide membrane for highly stable and anti-self-discharge lithium-sulfur batteries, ACS Nano 9 (3) (2015) 3002e3011, https://doi.org/10.1021/nn507178a. [37] W. Li, Y. Pang, T. Zhu, Y. Wang, Y. Xia, A gel polymer electrolyte based lithium-sulfur battery with low self-discharge, Solid State Ionics 318 (July 2017) (2018) 82e87, https:// doi.org/10.1016/j.ssi.2017.08.018. [38] W.J. Chen, C.X. Zhao, B.Q. Li, Q. Jin, X.Q. Zhang, T.Q. Yuan, X. Zhang, Z. Jin, S. Kaskel, Q. Zhang, A mixed ether electrolyte for lithium metal anode protection in working lithiumesulfur batteries, Energy Environ. Mater. 3 (2) (2020) 160e165, https:// doi.org/10.1002/eem2.12073. [39] J.Y. Wei, X.Q. Zhang, L.P. Hou, P. Shi, B.Q. Li, Y. Xiao, C. Yan, H. Yuan, J.Q. Huang, Shielding polysulfide intermediates by an organosulfur-containing solid electrolyte interphase on the lithium anode in lithiumesulfur batteries, Adv. Mater. 32 (37) (2020) 1e7, https://doi.org/10.1002/adma.202003012. [40] Y.Q. Shen, F.L. Zeng, X.Y. Zhou, A. bang Wang, W. kun Wang, N.Y. Yuan, J.N. Ding, A novel permselective organo-polysulfides/PVDF gel polymer electrolyte enables stable lithium anode for lithiumesulfur batteries, J. Energy Chem. 48 (2020) 267e276, https:// doi.org/10.1016/j.jechem.2020.01.016. [41] W. Guo, W. Zhang, Y. Si, D. Wang, Y. Fu, A. Manthiram, Artificial dual solideelectrolyte interfaces based on in situ organothiol transformation in lithium sulfur battery, Nat. Commun. 12 (1) (2021) 1e13, https://doi.org/10.1038/s41467-021-23155-3. [42] Z. Wei, Y. Ren, J. Sokolowski, X. Zhu, G. Wu, Mechanistic understanding of the role separators playing in advanced lithium-sulfur batteries, InfoMat 2 (3) (2020) 483e508, https://doi.org/10.1002/inf2.12097. [43] Q. Pang, X. Liang, C.Y. Kwok, L.F. Nazar, Reviewdthe importance of chemical interactions between sulfur host materials and lithium polysulfides for advanced lithiumsulfur batteries, J. Electrochem. Soc. 162 (14) (2015) A2567eA2576, https://doi.org/ 10.1149/2.0171514jes. [44] X. Liang, C.Y. Kwok, F. Lodi-Marzano, Q. Pang, M. Cuisinier, H. Huang, C.J. Hart, D. Houtarde, K. Kaup, H. Sommer, T. Brezesinski, J. Janek, L.F. Nazar, Tuning transition metal oxide-sulfur interactions for long life lithium sulfur batteries: the “goldilocks” principle, Adv. Energy Mater. 6 (6) (2016) 1e9, https://doi.org/10.1002/aenm.201501636. [45] X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss, L.F. Nazar, A highly efficient polysulfide mediator for lithium-sulfur batteries, Nat. Commun. 6 (2015) 1e8, https://doi.org/ 10.1038/ncomms6682.

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High-performance lithiumesulfur batteries: role of nanotechnology and nanoengineering

4

Shiva Bhardwaj 1 , Felipe Martins de Souza 2 and Ram K. Gupta 2 1 Department of Physics, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States; 2Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States

1. Introduction Society has been experiencing a novel phenomenon in which the flux of information, connectivity, and high-tech devices has become an intrinsic part of culture and routine. This scenario became possible mostly due to the introduction of more accessible portable devicesdsmartphones, smartwatches, tablets, notebooks, etc.dthat are practical, convenient, and hence largely in demand. Furthermore, most nations have realized the need for alternative energy sources that provide a sustainable approach for grid energy storage and a shift from combustion to electric engines in transportation. Therefore, batteries that supply power for electronic devices, electric vehicles, and energy grids must have greater energy storage capacity, durability, and stability to satisfy high energy demands. The current technology of lithium-ion batteries (LIBs) based on lithium metal oxide cathodes and graphitic anode has become well established in the 30 years since it was pioneered by Sony [1]. However, the current needs for even higher performance have been demonstrated to be more than what LIBs can achieve [2]. Lithiumesulfur (LieS) batteries appear to be the solution for these issues, as they can deliver an energy density of around 2600 Wh kg
1, which is four times higher than LIBs, along with the use of abundant, less toxic starting materials. The difference in performance is related to the mechanism of the intercalation process, which differs from conventional LIBs. Even though it improves overall energy storage capacity, some issues must be handled before being used on a large scale. For example, during the (de)lithiation process of LieS batteries, a volume change of around 80% occurs when sulfur converts into lithium sulfide (Li2S) because the densities of the two materials are 2.03 and 1.63 g cm
3, respectively [3]. One strategy consists of increasing the surface area by using a porous or hollow structure. Another issue is the high resistance of lithium polysulfide (Li2Sx) (x ¼ 1e8) for electron and ion flux, as sulfur’s conductivity is 5  10
30 S cm
1 at room temperature. Despite this drawback, successful results were achieved by synthesizing a highly ordered nanostructure of carbonesulfur for a cathode that reached 0.2 S cm
1 along with satisfactory electrochemical performance [4]. The coulombic

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00028-4 Copyright © 2022 Elsevier Inc. All rights reserved.

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Lithium-Sulfur Batteries

efficiency of LieS can degrade more quickly than for LIBs because of the formation of soluble polysulfides that can lead to a decrease in active materials over the electrode’s surface. There has been some effort to tackle this issue, such as introducing LiNO3 to create a passive layer over the Li anode, preventing active sulfur detachment via a physical barrier, promoting chemical adsorption, or using different electrolytes [5,6]. Lastly, LIBs and LieS batteries share a concern for the high reactivity of the metallic Li anode toward liquid electrolytes. This effect leads to the formation of dendritic Li structures that eventually cause the battery to short circuit. Hence, it is a major concern, as it can lead to LieS batteries becoming unsafe. In response, the synthesis of composites of S with C, polymers, metal oxides, and various electrolytes has been used as a strategy to diminish metallic Li anode reactivity [7e9]. Despite the current challenges, the scientific community has obtained satisfactory results to tackle the current challenges, suggesting that LieS batteries can be used effectively for energy storage. Furthermore, Li2S and Li2Sx are regarded as viable candidates to obtain highperformance LieS batteries. Li2S, for instance, is a low-cost material that displays a theoretical capacity of around 1166 mAh/g [10]; Li2S has been widely studied as a result. An understanding of the lithiation mechanism plays an important role in LieS battery performance. Based on that, at the first step, a high energy barrier raises the potential to 3.45 V, followed by a decrease to 2.4 V related to the phase nucleation of polysulfides and the oxidation of Li2S. The oxidation process is facilitated by the presence of conducting materials in contact with Li2S. A plateau is then observed around 2.4 V followed by a gradual increase in voltage to 3.4 V and then 4.0 V. Another aspect of this mechanism is that the potential barrier is lower at small current densities and higher at high current densities, meaning that the Li2S delithiation process is thermodynamically favorable and kinetically unfavorable. Fig. 4.1 shows a schematic for the mechanism divided into four steps: steps (a) and (b), represented by Eq. (4.1); step (c), represented by Eqs. (4.2) and (4.3); and step (d), represented by Eq. (4.4):

2.

Li2S(s) / Li2
xS(s) þ xLiþ þ xe


(4.1)

yLi2S(s) / Li2Sy(l) þ (2y
2)Liþ þ (2y
2)e


(4.2)

Li2Sy(l) / y/8Li2S8(l) þ (2
y/4)Liþ þ (2
y/4)e


(4.3)

Li2Sy(l) / y/8Li2S8(l) þ (2
y/4)Liþ þ (2
y/4)e


(4.4)

Working principle of lithiumesulfur batteries

Batteries have three main components: an anode, a cathode, and an electrolyte between. The anode is a terminal composed of a metal where the oxidation process, loss of electrons, occurs during discharging, and hence, it behaves as a negative terminal. The opposite process occurs during charging as electrons are added to it, and therefore it undergoes the reduction process, where it behaves as a positive terminal that can

High-performance lithiumesulfur batteries

59

Figure 4.1 Schematic for the Li2S charging mechanism. (A) Before reaching the highest potential, Li2S is a single phase. (B) Li2
xS has a lower concentration of Li in the outer layer than in the core. (C) After surpassing the thermodynamic barrier, polysulfides are formed. (D) At the last step of the charging process, there is a higher concentration of polysulfides. Adapted with permission from Y. Yang, G. Zheng, S. Misra, J. Nelson, M.F. Toney, Y. Cui, High-capacity micrometer-sized Li2S particles as cathode materials for advanced rechargeable lithium-ion batteries, J. Am. Chem. Soc. 134 (2012) 15387e15394, Copyright (2012), American Chemical Society.

be composed, for instance, of metallic Li, Ni, or Pb. The cathode is a positive terminal and is generally composed of active material where reduction takes place during discharging. On the other hand, oxidation occurs at the cathode when the battery is in the charging process; hence, it behaves as the negative terminal. The electrolyte is the medium that allows the flow of charged species between the cathode and anode whenever a chemical reaction occurs; ions make use of the electrolyte conducting medium. The charging and discharging process are how chemical energy is converted into electrical energy. For the case of a battery, the electrical potential provided has a nearly constant value along with a longer discharging time. That process is explained in terms of the discharging and charging process. Discharging is a process by which the battery provides an electrical current. Chemical reactions occur between the electrolyte and the electrode interface. The active material from the anode releases electrons toward the cathode through an external circuit and positive ions (cations) in the electrolyte through what is known as an oxidation reaction. Meanwhile, at the cathode, acceptance of electrons takes place (reduction) through the external conducting wire completing the path for electrons to flow. Additionally, the electrolyte assists in transporting the charged species from the anode and the cathode such that the chemical potential equipoises simultaneous reactions to create the potential difference, causing a continuous flow of ions to take place through the electrolyte and electrons in an external circuit and maintaining a steady current flow.

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Lithium-Sulfur Batteries

Charging is the process by which chemical energy is stored in the system by providing an electrical potential. During the charging process, a reverse chemical reaction occurs, denoting the flow of electrons from the cathode to the anode via an external electric circuit, known as the oxidation reaction, during the charging exercise. The anode gains electrons such that reduction takes place over itdthe active materials of the anode and cathode revert to their original states. The chargeedischarge process can be performed several times in a traditional battery, leading to a minor decrease in its capacitance. LieS batteries have attracted considerable attention for a theoretical energy density five times higher than that of commercial LIBs (387 Wh kg
1 for LiCoO2/C battery). In practical terms, the energy density of a packaged LieS battery may provide an energy density of about 600 Wh kg
1. Thus, an electric vehicle running on that battery could cover 500 km [12]. The LieS battery arrangement usually consists of a lithium anode, a sulfur-based composite cathode, an organic electrolyte, and a polymer-based separator that prevents contact between the two electrodes, as shown in Fig. 4.2. The metallic lithium anode (negative electrode at the time of discharging) goes through oxidation, producing lithium ions while losing electrons. The lithium ions then move to the positive electrode (cathode) through the electrolyte, while the electrons travel to the positive electrode through the external electric circuit. Meanwhile, the sulfur-based anode accepts the electrons that lead to the chemical reaction through which energy is released along with the interaction of lithium. During discharging, sulfur atoms show a strong tendency to catenation, forming long homoatomic chains or homocyclic rings of various sizes. Octasulfur (cyclo-S8), for instance, crystallizes at 25 C as orthorhombic a-S8, and it is the most stable allotrope at room temperature. During an ideal discharge process, cyclo-S8 is reduced, and its ring can be opened, resulting in high-order Li2Sx (6 < x  8). As the discharge continues, lower-order Li2Sx (2 < x  6) are formed with the help of additional lithium. During electrochemical analysis, two discharge plateaus can usually be observed around 2.3 and 2.1 V, representing the conversions of S8 to Li2S4 and Li2S4 to

Figure 4.2 Scheme for LieS battery, its components, and the chargeedischarge process. Adapted with permission from A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Rechargeable lithiumesulfur batteries, Chem. Rev. 114 (2014) 11751e11787, Copyright (2014), American Chemical Society.

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61

Li2S, respectively. This effect is observed when ether-based liquid electrolytes are used. At the end of discharge, Li2S is formed, thereby generating an electrical current. Sulfur is reduced to produce Li2S by accepting the Liþ and electrons from the positive electrode. The reactions during discharge are represented by Eqs. (4.5)e(4.7): At the anode:

2Li / 2Liþ þ 2e


(4.5)

At the cathode:

S þ 2Liþ þ 2e
/ Li2S

(4.6)

Overall cell reaction: 2Liþ þ S / Li2S

(4.7)

When the battery is completely discharged, it requires charging from an external energy source such that a reverse chemical reaction occurs. From that process, Li2S dissociates through the same process as it occurs during discharging to form Li and S8 via the formation of intermediate Li2Sx and gets stored into its original form. This process happens because sulfur tends to catenate. This reverse reaction is demonstrated in Eq. (4.8): During charging: Li2S / S þ 2Liþ þ e


(4.8)

3. Challenges in lithiumesulfur batteries Despite its great potential to provide high energy density using cost-effective and common materials such as S, researchers face many issues with LieS technology that currently hinder its application on a larger scale. The main concern with LieS batteries is the poor electrical conductivity of sulfur (w10
30 S cm
1), making it an insulator for the flow of electrons. In addition, the intermediate products (i.e., Li2Sx) formed during the discharging process cause structural and morphological changes leading to detachment of active material resulting in low contact with the sulfur electrodes, hence reducing electrochemical performance. In addition, the dissolved polysulfides shuttle between the anode and cathode during battery functioning, reacting with both the lithium metal anode and the sulfur cathode. Moreover, the electrochemical conversion of S to Li2S involves structural and morphological changes as well as repeated dissolution and deposition of reactive species that tend to passivate both electrodes, leading to a significant increase in impedance. Many intermediate species like high-order Li2Sx and low-order Li2Sx are formed during the discharging process. These polysulfide ions can easily flow back and forth between the two electrodes, known as the redox shuttle effect between the LieS cathode and Li anode. This process decreases coulombic efficiency and leads to a fast selfdischarge. The reaction that occurs on the Li anode with the polysulfides through an electrochemical and chemical reduction can be described by Eqs. (4.9) and (4.10), respectively [6]: Electrochemical reduction: (n
1)Li2Sn þ 2Liþ þ 2e
/ nLi2Sn
1 Chemical reduction:

(n
1)Li2Sn þ 2Li / nLi2Sn
1

(4.9) (4.10)

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Lithium-Sulfur Batteries

These parasitic reactions cause problems such as consuming active sulfur species and corroding and polarizing the lithium anode. As soon as the shuttle effect increases, it causes the battery’s internal resistance to increase rapidly, and therefore capacity fades, resulting in a consequential reduction in coulombic efficiency. Moreover, it can also lead to limited rechargeability and poor charge efficiency. It occurs because, during the discharge process, all sulfur species are dissolved into the liquid electrolyte, which leaves numerous voids in the cathode, whereas the dissolved polysulfide can deposit back onto the cathode in the form of Li2S2 and Li2S. An understanding of this process can be seen in Fig. 4.3 that describes the formation of these polysulfides in a plot of time (h) versus voltage (V). During discharge, sulfur undergoes a reduction reaction leading to Li2S due to the acceptance of Liþ and electrons at the cathode. This process can be described by Eq. (4.11). During the charging process, the opposite occurs: S8 þ 16Liþ þ 16e
4 8 Li2S

(4.11)

Region I described the dissolution of S8 into the liquid electrolyte that leads to S2
4 which is described in Eq. (4.12): S8(s) / S8(l) þ 4e
/ 2S2
4

(4.12)

Figure 4.3 Initial voltage profile for LieS anode and carbon nanotube cathode at chargee discharge with current rate of 0.2 C. Adapted with permission from J. Yan, X. Liu, B. Li, Capacity fade analysis of sulfur cathodes in lithiumesulfur batteries, Adv. Sci. 3 (2016) 1600101, Copyright (2016) The Authors, some rights reserved; exclusive licensee WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Distributed under a Creative Commons Attribution License 3.0 (CC BY). https://creative commons.org/licenses/by/3.0.

High-performance lithiumesulfur batteries

63

The increase in S2
4 concentration into the electrolyte decreases the potential from the battery. It occurs because S2
4 also absorbs electrons, causing increased electrolyte viscosity that makes Liþ movement more difficult. This effect is reflected in the reverse peak circled in point 1. In Region II, S2
4 is converted into insoluble Li2S2 and Li2S. The reactions are described in Eqs. (4.13) and (4.14). Both Li2S and Li2S2 have low conductivity. This causes the discharge curve to maintain around 2e2.1 V until the cathode’s surface is covered by Li2S/Li2S2, which causes a voltage drop: þ
S2
4 þ 8Li þ 6e / 4Li2S

(4.13)

þ
S2
4 þ 4Li þ 2e / 2Li2S2

(4.14)

Region III then corresponds to the reduction of Li2S2 to Li2S described in Eq. (4.15): Li2S2 þ 2Liþ þ 2e
/ 2Li2S

(4.15)

During the charging process, a plateau is formed related to the oxidation of Li2S2 and Li2S into soluble long-chain polysulfides. This process causes the formation of the small peak circled at point 2. This series of reactions cause the electrolyte to be consumed and capacity to fade, as shown in Eq. (4.16) [15,16]: Li2Sn þ 2e
þ 2Liþ / Li2S þ Li2Sn
1

(4.16)

One strategy can be adopted by using binders that can retain the polysulfide species in the highly porous structure of the cathode during the cycling process of the lithiumesulfur battery to prevent capacity decay [12]. The conventional binders such as poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) cannot meet this requirement because of their swelling or gelling in contact with solvents in the electrolyte. In addition, sulfur reduction produces anionic polysulfide radicals as intermediate species, which react with many organic polymers. For example, simply ball-milling a slurry consisting of sulfur and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) in acetonitrile solvent leads to a strong odor of thiols, which suggests a possible reaction between the elemental sulfur and PVDF-HFP [6]. Sulfur and polysulfides react with polymers containing unsaturated C]C bonds such as natural and synthetic rubbers through a process known as “vulcanization.” Therefore, a qualified binder for the cathode of the LieS battery should not swell in the liquid electrolyte, should maintain the viscosity of the conducting medium constant, and must be chemically stable against all types of sulfur species. The formula to calculate the shuttle factor, first derived by Mikhaylik and Akridge, is presented in Eq. (4.17) to evaluate the degree of shuttle behavior, which includes the impact from the charge current and polysulfide diffusivity [13]: fc ¼

ks  qup  ½Stotal  Ic

(4.17)

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where ( fc) is the charge shuttle factor, Ic is the charge current, ks is the shuttle constant (heterogeneous reaction constant), qup is the specific capacity of sulfur contributed by the upper plateau, and [Stotal] is the total sulfur concentration. As known, the conversion of sulfur to a series of polysulfides causes a volumetric expansion and change in the morphologies. The repetitive occurrence of this process results in the formation of lithium dendrites. Dendrite formation is one of the major reasons for battery failure. This phenomenon consists of metallic microstructured (thin strands) deposited over the anode’s surface that cannot be absorbed by it within the required time. These dendrites then react with the electrolyte, causing it to degrade and trigger the loss of active lithium inside the battery. Such activity could cause the battery can become unsafe when exposed to higher C rates, which can cause a short circuit that leads to a device catching fire. In addition, the continuous evolution of a porous lithium structure leads to the corrosion of lithium metal, permanently decreasing its capacitance. Selfdischarge is another issue related to LieS batteries because it decides the practicality of energy storage devices. The intemperance of polysulfides is inexorable in the LieS battery, with a level of dissolution so high that self-discharge occurs even in the rest state. This self-discharge can be assessed to the slow dissolution rate of active material, which results in a decrement in capacity and a tendency to short-circuit. Based on that, Eq. (4.18) describes that the self-discharge behavior still closely correlates with the shuttle constant, which means that both the shuttle and the selfdischarge effect originate from the dissolution of the active material in the LieS battery system: d½SH  IC ¼
kS ½SH  dt qup

(4.18)

Electrolytes should be stable under the working conditions of the battery and perform the transport of charged species with minimum resistance. However, these effects vary depending on the employed electrolyte. For example, the liquid electrolyte can become more viscous after being exposed to several chargeedischarge cycles due to the release of polysulfides into the system. This effect leads to higher resistance for the transport of ions and therefore decreased battery efficiency. Another option gaining attention is using solid electrolytes to strictly provide ionic conduction as the ions move through its structural framework. However, the main challenge of making them applicable is decreasing the interfacial resistance between the solid electrolytes and electrode surfaces. In addition, it is necessary to decrease the effect of lithium plating/stripping that causes the capacity to deteriorate. Another picture comes in place by looking at the loading of sulfur compared with electrolyte ratio (E/S) as several researchers are attempting to reach a value of less than 10 mL mg
1 [12]. Progress in research has demonstrated that the organic carbonate-based electrolytes widely employed in LIBs are not suitable for LieS batteries because of their reactivity toward polysulfide anions. On the other hand, ether-based electrolytes present good compatibility because they can dissolve polysulfides and have high ionic conduction. Yet they still suffer from the shuttling effect. Some strategies have been employed to counter this issue, such as using solid electrolytes, salts, and a mix of solvents [17,18].

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Another case in which LieS batteries differ from LIBs is the Al current collector, which compromises safety if exposed to high temperature because it can exothermically react with sulfur. Yet Al is a convenient material due to its low cost, abundance, and recyclability. In response, researchers have employed carbon-coated Al foil to avoid the direct contact of Al with the sulfur-based electrode, improving the safety of LieS batteries.

4. Role of nanotechnology and nanoengineering in lithiumesulfur batteries Richard Feynman first discussed the concept of nanotechnology with his famous statement, “there is plenty of room at the bottom,” where he described the possibility of the synthesis of particles reaching a size around ten times larger than a single atom. The era of nanotechnology evolved just after the invention of the scanning electron microscope, which unveiled the visualization of individual atoms and bonds. Along with that came the discovery of buckminsterfullerene (C60), also known as a buckyball, along with graphene, carbon nanotube (CNT), and others. According to the National Nanotechnology Initiative, “Nanotechnology is science, engineering, and technology conducted at the nanoscale, which is about 1e100 nm.” Researchers concede that is a broad definition, opening the door to anyone from medical scientists to materials scientists to understand the term called nanotechnology. The main key for driving the research in battery technology is finding a suitable material that helps change the properties of the anode, cathode, and electrolyte/separator. Using nanotechnology in battery manufacturing offers many benefits. One example is coating the surface of an electrode with nanoparticles to increase the electrode surface area, thereby allowing more electrons and positive ions to flow through the electrode and the conducting medium, which generates an extra current. This helps increase the efficiency of hybrid vehicles by significantly reducing battery weight. Nanomaterials are also used to separate liquids inside the battery from the solid electrodes. This considerably improves battery shelf life, because when there is no load applied to the battery, this separation helps prevent the low-level self-discharge that generally occurs in all types of batteries. Various materials have been explored for batteries that help store high energy density, such as carbon-coated silicon nanowires, CNTs, lithium alloy/graphene foil, and phosphorene-graphene hybrid material. This exploration has shown that nanotechnology plays a vital role in improving the properties of LieS batteries and tackling some of their inherent drawbacks. In that sense, nanoengineering approaches have solved some initial challenges, such as high sulfur loading (>4 mg cm
2), lean electrolyte-to-sulfur ratio (300 cycles). The specific areal capacity depends on the sulfur utilization and the loading amount of sulfur in the cathode, as shown in Eq. (4.19) [12]: Areal capacity

    mAh CT  Sulfur utilization  Sulfur loading ¼ cm2 1000 (4.19)

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Lithium-Sulfur Batteries

CT is the theoretical specific capacity of sulfur (1672 mAh g
1), and sulfur utilization and loading are measured in percentage and mg cm
2, respectively. A strategy to obtain an optimized electrode design consisting of a conducting nanostructured material doped with sulfur may be an approach that diminishes the loss of active material conductivity. An example of this scenario is the utilization of sulfur composites with nanostructures such as graphene and CNT. Because these nanomaterials have a high surface area, large amounts of sulfur can be loaded whereas retaining the stable electrochemical performance for long cycles. Qie et al. [19] designed a layer-by-layer (LBL) cathode with six layers of carbon nanofibers to maximize the sulfur loading amount to 11.4 mg cm
2 and constituted about 56.3 wt.% of sulfur content. The cell demonstrated a high areal capacity of 11.4 mAh cm
2 at 0.1 C rate, more than twice that of commercial LIBs. Those satisfactory properties were achieved because the LBL formed a structure of carbon intercalated with sulfur where the first layer of carbon acted as a current collector, the middle ones served as conducting connecting layers, and the upper layer further enhanced the function as current collectors. Hence, the electron transfer step was facilitated. Concurrently, the large surface area of carbon allowed more sulfur to be embedded into its surface, which optimized its action. Finally, the layered structure held polysulfides to leave the structure and therefore improved battery cyclability. This type of approach has been applied to LieS batteries as a new strategy for diminishing polysulfide dissolution, and thus, LBL is considered a powerful approach that exerts nanometer control over film thickness. In addition, an ion-selective membrane can provide a uniform cocoon for sulfur/carbon composites. Properties of membranes are contingent on several parameters such as polyelectrolyte, ionic strength of assembly solution, and the number of layers. Bucur et al. [20] worked on a nanostructured sulfur cathode with a truffle-like structure in which sulfur particles were inserted in nanocarbon with ion-selective flexible nanomembrane decorated with conductive carbon. This nanostructure was used as a cathode as it possessed a loading of 65% sulfur that operated at a high rate of 2 C for more than 500 cycles at nearly 100% coulombic efficiency. The novelty of this work relied on a sulfur core structure infused with a hollow carbon structure that allowed the capacity properties of sulfur to take place, with electron transfer from carbon. In addition, the hollow structures of carbon and in some domains of sulfur gave room for the expansion of sulfur during (de)lithiation and limited polysulfide migration, justifying the high number of cycles. Another important aspect of this truffle structure was the addition of a positively charged polymeric membrane, which promoted selective migration of ions based on size and charge. Hence, Liþ could diffuse through the membrane, whereas polysulfides were blocked because of their sizes and negative charges, which were the opposite of the polymer and created attractive interactions with it. This improved battery performance has created more interest in nanoengineered cathodes. Another example was researched by Zhou et al. [21], who fabricated flexible graphene foam cathodes to obtain high sulfur loading of 10.1 mg cm
2. In this case, the cathodes obtained initial specific and areal capacities of 1000 mAh g
1 and 13.4 mAh cm
2. It also offered a reversible capacity of w450 mAh g
1 with cyclic stability as high as 1000 cycles. The design of this flexible battery also functions

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oppositely to that of Bucur et al. [20] in the sense that a graphene foam was used as the core structure to provide mechanical properties such as robusticity, mechanical support, and flexibility, as well as electrochemical properties due to its conductibility for electron-transfer and its open cell structure for the expansion of sulfur and allocation of electrolyte. The sulfur was introduced through simple slurry infiltration in a graphene/Ni foam coated with poly(dimethylsiloxane) filled with a 70 wt.% sulfur slurry. The approach is illustrated in detail in Fig. 4.4. Heteroatoms and polarized groups in carbon-based nanostructures, such as hydroxyl, carboxylic acid, or epoxy functional groups, enable polysulfide adsorption. Hence, graphene oxide can be used as a viable nanomaterial to improve the binding of polysulfides over the electrode surface. Wang et al. [22] adopted a simple technique wherein the freestanding structure of VS4 was directly combined with CNTs. As a result, the high electrical conductivity of CNTs and the large surface area of VS4 functioned as a porous structure that trapped polysulfides and catalyzed the conversion step for the (de)lithiation process. Therefore, prevent the deterioration of the LieS battery. Satisfactory properties were achieved through this facile approach, such as high cycling stability that reached

Figure 4.4 (A) Scheme for the fabrication of a flexible battery based on graphene foam using poly(dimethylsiloxane) (PDMS) as a binder filled with a sulfur slurry (S-PDMS/graphene foam). (BeE) Show the dimensions of the composite along with its flexibility after bending. Adapted with permission from G. Zhou, L. Li, C. Ma, S. Wang, Y. Shi, N. Koratkar, W. Ren, F. Li, H.-M. Cheng, A graphene foam electrode with high sulfur loading for flexible and high energy Li-S batteries, Nano Energy 11 (2015) 356e365, Copyright (2015), Elsevier.

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Figure 4.5 Synthetical approach for VS@NT. Step 1: Al was sputtered over the carbon paper (CP) fibers through atomic layer deposition (ALD). Step 2: Chemical vapor deposition (CVD) was performed to grow carbon nanotubes (CNTs) doped with nitrogen (NT). Step 3: The process was followed by the uniform growth of VS4 through the hydrothermal method. Adapted with permission from S. Wang, H. Chen, J. Liao, Q. Sun, F. Zhao, J. Luo, X. Lin, X. Niu, M. Wu, R. Li, X. Sun, Efficient trapping and catalytic conversion of polysulfides by VS4 nanosites for LieS batteries, ACS Energy Lett. 4 (2019) 755e762, Copyright (2019), American Chemical Society.

1200 cycles at 2 C and an areal capacity of 13 mAh cm
2 with an energy density of 243.4 Wh kg
1. Fig. 4.5 shows the synthetical approach for the synthesis of freestanding VS@NT. Current collectors in LieS batteries are conventionally Al foil, which is a convenient material due to its low cost, corrosion resistance, and electrochemical stability. However, the sulfur-based cathode can lose contact with the foil, incrementing the battery’s internal resistance. To provide a sufficient path for electrons and ions along with high sulfur loadings, a 3-D framework has been proposed as the current collector. Fang et al. [23] developed a hollow carbon fiber foam used as the 3-D current collector to accommodate high amounts of sulfur. The battery exhibited a high areal capacity of 12 mAh cm
2 after 150 cycles with a capacity retention of 70% at a sulfur loading of 21.2 mg cm
2. Nanoengineering anodes are the other end for battery assembly. Ideally, Li anodes should present high E/S ratios that grant the electrochemical battery stability over many cycles. However, ideally, this ratio should be less than 10, which is impractical. Hence, a viable option that can be employed to protect the Li anode can be performed by depositing nanolayers of material to passivate its surface. One example of this strategy is atomic layer deposition, which can accurately deposit a material to modify the interface between the anode and electrolyte. Kozen et al. [24] performed a controlled deposition of Al2O3 reaching a thickness of w14 nm, which enables effective protection of the Li anode against corrosion, shuttling effect, and prevent the formation of Li

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Figure 4.6 (A) Oxidation process of metal Li in air. (B) Effect of Al2O3 nanolayer over Li anode showing effective improvement in discharge capacity. (C) Schematics for the LieAl2O3 composite anode. Adapted with permission from A.C. Kozen, C.-F. Lin, A.J. Pearse, M.A. Schroeder, X. Han, L. Hu, S.-B. Lee, G.W. Rubloff, M. Noked, Next-generation lithium metal anode engineering via atomic layer deposition, ACS Nano 9 (2015) 5884e5892, Copyright (2015), American Chemical Society.

dendrites. The barrier effect provided by Al2O3 hindered the undesirable effect whereas permitted the flux of Liþ, leading to enhanced discharge capacity compared with the unprotected Li anode. An illustration of this process is provided in Fig. 4.6. Choi and colleagues performed other effective examples of sputtering by synthesizing a 2-D molybdenum disulfide (MoS2) deposited over the surface of Li anode [25]. This approach yielded a sulfur loading of 6 mg cm
2 along with a capacity retention of 1000 cycles at an E/S ratio of 10 mL mg
1. MoS2 nanolayer minimized the active material loss and helped retain the long cycle life capacity required at a practical level, which occurred due to its phase transition from semiconducting 2H to metallic 1H, which diminished the interfacial resistance and yet it allowed the flux of Liþ. Another strategy that can be adopted to decrease the shuttling effect is adding materials in organic electrolytes that can dissolve the polysulfide chains, such as pyrrole and triphenylphosphine. This is a facile approach as there is no requirement for chemical modification at the electrode’s surface [26]. Similarly, ether electrolytes can be mixed with N-methoxyethyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide to improve the stability of the solid electrolyte interface over the Li anode’s surface, which can prevent the formation of Li dendrites. These systems show stable discharge capacities at a rate of 0.1 C for as many as 100 cycles with areal loading of 4 mg cm
2 at an E/S ratio around 7.5 mL mg
1 [27]. The drawback for this strategy is the high cost of the solvents, making the process challenging to be scaled-up. Yet a common issue of liquid electrolytes is their toxicity and flammability. This obstacle led to an investment in solid-state electrolytes (SSEs) to further improve electrochemical properties and safety. With those goals in mind, a material commonly used as an SSE consists of Li2S and phosphorus pentasulfide (Li2SeP2S5). The advantages of this type of electrolyte lie in its transport mechanism, which works only by the movement of ions through its 3-D framework, whereas

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electrons do not flow. At the same time, polysulfides are also hindered from moving from one electrode to the other due to their larger size compared with the framework typically observed by some SSEs. The nanoengineered separator is also a major part of a battery to focus on. These days research in separators has gained rapid momentum and just because of widely attributed polyolefin-based separators. When it comes to entrapping polysulfides, carbon-based materials are promising, as they have ample pores to trap the polysulfide. Peng et al. [28] proposed a mesoporous cellular graphene-modified “Janus” separator to intercept the polysulfide crossover. This type of separation functions as an asymmetric structure composed of polypropylene as the insulators that prevent the formation of a passive layer of Li2Sx. In addition, the graphene framework adhered to the cathode functions as a highly conductive intermediator that allows the reactivation of the shuttling of Li2Sx back to maintain the ion channels. The mechanism for this process is schematized in Fig. 4.7. Such a modification ensured that the cell exhibits a high areal capacity of 5.5 mAh cm
2 with an aerial loading of 5.3 mg cm
2.

Figure 4.7 Scheme for (A) polypropylene separator and (B) Janus separator with graphene framework layer. The absence of a graphene framework allows the lithium polysulfide (Li2Sx) to deposit at the cathode’s interface, leading to a passive solid insulation layer. The graphene framework allowed the reduction of Li2Sx, whereas they are deposited over a conductive scaffold (Janus), promoting the penetration of Li2Sx into the cathode. Adapted with permission from H.-J. Peng, D.-W. Wang, J.-Q. Huang, X.-B. Cheng, Z. Yuan, F. Wei, Q. Zhang, Janus separator of polypropylene-supported cellular graphene framework for sulfur cathodes with high utilization in lithiumesulfur batteries, Adv. Sci. 3 (2016) 1500268, Copyright (2015) The Authors, some rights reserved; exclusive licensee WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Distributed under a Creative Commons Attribution License 3.0 (CC BY), https://creativecommons.org/licenses/by/3.0.

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5. Conclusion Throughout the discussion in this book chapter, it was noticeable that the implementation of LieS batteries had plenty of challenges that would hinder its applicability as a more efficient alternative than LIBs. The main challenges related to this technology include the decrease in shuttling effect due to the migration of polysulfides that move back and forth during the charge/discharge process that culminates in permanent loss of capacity due to the formation of a layer of inactive insoluble and insulator material. In addition, the drastic volume expansion that occurs from the (de)lithiation process during the conversion step could lead to the leakage of electrolyte or active materials. Another issue is the formation of dendritic lithium due to its high reactivity toward the electrolyte, which could lead to short-circuiting of the battery. Yet the scientific community has worked its way to overcome these drawbacks yielding satisfactory results that move toward a feasible application of LieS batteries. For instance, the development of several types of carbon-based materials with sulfur yielded composites that could effectively suppress the shuttling effect by limiting their migration as their high surface area hinders their movement. Beyond that, the porous or hollow structures that these carbonaceous structures present provide enough room for the expansion of sulfur during (de)lithiation by improving their stability. Lastly, effective properties to prevent the formation of dendritic lithium have been successfully employed, such as the formation of controlled thin nanolayer of metal oxides, i.e., Al2O3 or MoS2 that can allow the flux of Liþ yet protect Li anode surfaces, which promotes more cyclic stability for the battery, thus making for a safer device. Thus, it is likely that LieS batteries will soon compose the battery market due to their low-cost, high power density, and versatile synthetical approaches that can be usually performed with abundant materials.

References [1] M.R. Kaiser, Z. Han, J. Liang, S.-X. Dou, J. Wang, Lithium sulfide-based cathode for lithium-ion/sulfur battery: recent progress and challenges, Energy Storage Mater. 19 (2019) 1e15. [2] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [3] D.-W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.-M. Cheng, I.R. Gentle, G.Q.M. Lu, Carbonesulfur composites for LieS batteries: status and prospects, J. Mater. Chem. A. 1 (2013) 9382e9394. [4] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbonesulphur cathode for lithiumesulphur batteries, Nat. Mater. 8 (2009) 500e506. [5] S.-H. Chung, A. Manthiram, A natural carbonized leaf as polysulfide diffusion inhibitor for high-performance lithiumesulfur battery cells, ChemSusChem 7 (2014) 1655e1661. [6] S.S. Zhang, Liquid electrolyte lithium/sulfur battery: fundamental chemistry, problems, and solutions, J. Power Sources 231 (2013) 153e162. [7] M.R. Kaiser, Z. Ma, X. Wang, F. Han, T. Gao, X. Fan, J.-Z. Wang, H.K. Liu, S. Dou, C. Wang, Reverse microemulsion synthesis of sulfur/graphene composite for lithium/ sulfur batteries, ACS Nano 11 (2017) 9048e9056.

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[8] F. Li, M.R. Kaiser, J. Ma, Z. Guo, H. Liu, J. Wang, Free-standing sulfur-polypyrrole cathode in conjunction with polypyrrole-coated separator for flexible Li-S batteries, Energy Storage Mater. 13 (2018) 312e322. [9] K. Jeddi, K. Sarikhani, N.T. Qazvini, P. Chen, Stabilizing lithium/sulfur batteries by a composite polymer electrolyte containing mesoporous silica particles, J. Power Sources 245 (2014) 656e662. [10] Y. Son, J.-S. Lee, Y. Son, J.-H. Jang, J. Cho, Recent advances in lithium sulfide cathode materials and their use in lithium sulfur batteries, Adv. Energy Mater. 5 (2015) 1500110. [11] Y. Yang, G. Zheng, S. Misra, J. Nelson, M.F. Toney, Y. Cui, High-capacity micrometersized Li2S particles as cathode materials for advanced rechargeable lithium-ion batteries, J. Am. Chem. Soc. 134 (2012) 15387e15394. [12] E. Cha, M. Patel, S. Bhoyate, V. Prasad, W. Choi, Nanoengineering to achieve high efficiency practical lithiumesulfur batteries, Nanoscale Horizons 5 (2020) 808e831. [13] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Rechargeable lithiumesulfur batteries, Chem. Rev. 114 (2014) 11751e11787. [14] J. Yan, X. Liu, B. Li, Capacity fade analysis of sulfur cathodes in lithiumesulfur batteries, Adv. Sci. 3 (2016) 1600101. [15] Y. Diao, K. Xie, S. Xiong, X. Hong, Analysis of polysulfide dissolved in electrolyte in discharge-charge process of Li-S battery, J. Electrochem. Soc. 159 (2012) A421eA425. [16] R. Xu, I. Belharouak, X. Zhang, R. Chamoun, C. Yu, Y. Ren, A. Nie, R. ShahbazianYassar, J. Lu, J.C.M. Li, K. Amine, Insight into sulfur reactions in LieS batteries, ACS Appl. Mater. Interfaces 6 (2014) 21938e21945. [17] P. Mu, T. Dong, H. Jiang, M. Jiang, Z. Chen, H. Xu, H. Zhang, G. Cui, Crucial challenges and recent optimization progress of metalesulfur battery electrolytes, Energy Fuel. 35 (2021) 1966e1988. [18] J. Scheers, S. Fantini, P. Johansson, A review of electrolytes for lithiumesulphur batteries, J. Power Sources 255 (2014) 204e218. [19] L. Qie, A. Manthiram, A facile layer-by-layer approach for high-areal-capacity sulfur cathodes, Adv. Mater. 27 (2015) 1694e1700. [20] C.B. Bucur, J. Muldoon, A. Lita, A layer-by-layer supramolecular structure for a sulfur cathode, Energy Environ. Sci. 9 (2016) 992e998. [21] G. Zhou, L. Li, C. Ma, S. Wang, Y. Shi, N. Koratkar, W. Ren, F. Li, H.-M. Cheng, A graphene foam electrode with high sulfur loading for flexible and high energy Li-S batteries, Nano Energy 11 (2015) 356e365. [22] S. Wang, H. Chen, J. Liao, Q. Sun, F. Zhao, J. Luo, X. Lin, X. Niu, M. Wu, R. Li, X. Sun, Efficient trapping and catalytic conversion of polysulfides by VS4 nanosites for LieS batteries, ACS Energy Lett. 4 (2019) 755e762. [23] R. Fang, S. Zhao, P. Hou, M. Cheng, S. Wang, H.-M. Cheng, C. Liu, F. Li, 3D interconnected electrode materials with ultrahigh areal sulfur loading for LieS batteries, Adv. Mater. 28 (2016) 3374e3382. [24] A.C. Kozen, C.-F. Lin, A.J. Pearse, M.A. Schroeder, X. Han, L. Hu, S.-B. Lee, G.W. Rubloff, M. Noked, Next-generation lithium metal anode engineering via atomic layer deposition, ACS Nano 9 (2015) 5884e5892. [25] E. Cha, M.D. Patel, T.Y. Choi, W. Choi, Lithium-sulfur batteries: the effect of high sulfur loading on the electrochemical performance, ECS Trans. 85 (2018) 295e302. [26] Q. Pang, A. Shyamsunder, B. Narayanan, C.Y. Kwok, L.A. Curtiss, L.F. Nazar, Tuning the electrolyte network structure to invoke quasi-solid state sulfur conversion and suppress lithium dendrite formation in LieS batteries, Nat. Energy 3 (2018) 783e791.

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[27] G. Liu, Q. Sun, Q. Li, J. Zhang, J. Ming, Electrolyte issues in lithiumesulfur batteries: development, prospect, and challenges, Energy Fuel. 35 (2021) 10405e10427. [28] H.-J. Peng, D.-W. Wang, J.-Q. Huang, X.-B. Cheng, Z. Yuan, F. Wei, Q. Zhang, Janus separator of polypropylene-supported cellular graphene framework for sulfur cathodes with high utilization in lithiumesulfur batteries, Adv. Sci. 3 (2016) 1500268.

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Mathematical modeling of lithiumesulfur batteries

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Shunli Wang 1, 2 , Lili Xia 1 , Chunmei Yu 1 , Josep M. Guerrero 3 and Yanxin Xie 1 1 Southwest University of Science and Technology, Mianyang, Sichuan, China; 2Aalborg University, Aalborg, North Jutland Province, Denmark; 3Department of Energy Technology, Villum Investigator Center for Research on Microgrids (CROM), Aalborg, North Jutland Province, Denmark

1.

Introduction

The lithiumesulfur (LieS) battery is a new type of battery in which sulfur is used as the battery’s positive electrode, and lithium is used as the negative electrode. Compared with lithium-ion batteries, LieS batteries have many advantages such as lower cost, better safety performance, and environmental friendliness. Despite significant progress in LieS battery research, the batteries have low active material utilization, short cycle life, and poor stability; these shortcomings limit their implementation. Therefore, it is important to research LieS batteries in terms of theories, modeling, and so forth. Modeling is the focus of battery research and can reflect a battery’s dynamic and static characteristics. This chapter analyzes electrochemical and equivalent circuit models and applies the LieS batteries model.

1.1

Background of lithiumesulfur batteries

With the development, the demand for using and requirements of lithium-ion batteries has increased. Traditional lithium-ion batteries cannot fully meet the current application in certain fields, especially in electric vehicles (EVs). EVs are currently the most used application scenarios for lithium-ion batteries. Lithium-ion batteries have become a core part of EV research, and higher requirements have been placed on the discharge rate, energy density, and other performance characteristics of lithium-ion batteries. In recent years, researchers have noticed the application of sulfur in batteries. LieS batteries have an energy density (2600 Wh/Kg) 10 times that of lithium-ion batteries (200e250 Wh/Kg). In addition, sulfur is one of the most abundant elements, so it is inexpensive with a low contribution to environmental pollution. These factors indicate the potential of LieS in next-generation batteries. Currently, researchers are focused on improving the performance of LieS batteries from an electrochemical point of view, including studies on positive electrodes, separators, electrolytes, and negative electrodes. Existing modeling and analysis of LieS batteries are lacking, however. Battery model characteristics can be categorized to infer current problems of LieS batteries and provide theoretical support for improvements and applications.

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00005-3 Copyright © 2022 Elsevier Inc. All rights reserved.

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Such analysis includes state of charge (SOC), state of health (SOH), and state of power (SOP) estimation. Therefore, research on LieS battery modeling is of great significance.

1.2

Principle of the lithiumesulfur battery

The LieS battery refers to a new generation of secondary batteries charged and discharged based on the electrochemical reaction between a sulfur elemental positive electrode and a metal lithium negative electrode. This reaction is different from the lithium-ion deintercalation reaction that occurs during the charging and discharging of lithium-ion batteries. The process primarily involves breaking the SeS bond and the multielectron, multistep oxidation-reduction reaction of rebonding. The reaction mechanism is complicated, and the reaction equation is shown in Eq. (5.1): S8 þ 16LiH8Li2 S

(5.1)

According to Eq. (5.1), one S8 molecule can react with 16 lithium molecules to form the Li2S end product. The electrochemical reaction of the positive electrode during the actual charging and discharging process of LieS batteries is more complicated than indicated by the abovementioned reactions. It includes a multistep oxidationreduction reaction and is accompanied by a solideliquidesolid phase of intermediate products. The reaction equation of the positive electrode of the battery can be divided according to Eq. (5.2): 8 > > > S8 þ 2Liþ þ 2e H8Li2 S > > > > > > 3Li2 S8 þ 2Liþ þ 2e H4Li2 S6 > > < 2Li2 S6 þ 2Liþ þ 2e H3Li2 S4 > > > > > Li2 S4 þ 2Liþ þ 2e H2Li2 S2 > > > > > > : Li2 S2 þ 2Liþ þ 2e H2Li2 S

(5.2)

During the discharge process of the LieS battery, the elemental sulfur molecules in the solid phase react with lithium ions to form a soluble polysulfide polymer, Li2S8, and then Li2S8 is gradually reduced to Li2S6, Li2S4, and other soluble midproducts. With further progress of the electrochemical reaction, insoluble solid phases Li2S2 and Li2S are finally formed. The application of LieS batteries faces problems due to their internal reaction characteristics: (1) In actual use, the reaction process of the LieS battery produces a variety of lithium polysulfide intermediate products that are easily soluble in water and therefore cause the loss of active materials and thus result in poor battery cycle life. (2) The elemental sulfur and lithium sulfide produced by discharge are insulators, and their conductivity is poor, resulting in a low discharge rate of the battery.

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(3) During the reaction process, the volume of the electrode will change greatly, which will cause its instability. (4) During the charging process, the dissolved polysulfide intermediate product will form a diffusion phenomenon between the positive and negative electrodes of the battery due to the difference in the concentration gradient, the “shuttle reaction” will occur, and self-discharge is formed, which seriously restricts the coulombic efficiency of the LieS battery. (5) Due to the active electrochemical characteristics of the elementary substance, side reactions or lithium branches are prone to occur during the reaction process. The crystal forms an internal short circuit, which seriously affects the safety performance of the LieS batteries. At this stage, most research is on materials to solve the above problems, so less modeling research is being conducted on LieS batteries.

1.3

Modeling method

Modeling can effectively explore Li- S batteries’ dynamic and static characteristics, providing support for theoretical research and practical applications of the batteries. The internal electrochemical reactions of lithium-ion batteries are very complicated, and they are affected by the charging and discharging rate, SOC, and other impacts of the battery. Therefore, it is difficult to predict the battery status efficiently and accurately. Building the battery model can solve the above problems well. According to the model, the internal state parameters of the battery can be estimated through the measured external parameters. At present, battery modeling is divided into mathematical modeling, electrochemical modeling, and equivalent circuit modeling. Mathematical models usually abstract reality from physical objects, adapt empirical formulas and mathematical equations to describe the complex relationships between physical quantities within the research objects, and then use mathematical theories to optimize battery characteristics. The advantage of the mathematical model established by programming calculation software is that it greatly simplifies the parameter conversion process in the system design process. Its great disadvantage is that it is too cumbersome and must be simplified, and the application of simplified models often has large calculation errors. This method is often not universal and can thus be used to describe only the specific characteristics of the battery model. The electrochemical model is a model derived from electrochemical theory. A large number of coupled nonlinear equations can describe a battery’s thermodynamics and kinetics. In electrochemical model research, several differential equations can describe the change in ion concentration in the electrolyte material, the reaction process of electrodes, etc. The equations can directly describe changes in ions and voltages inside the battery. For example, Fick’s law [1] of diffusion is used to obtain the solid phase concentration of each electrode and the liquid phase concentration of the electrolyte, and Ohm’s law is used to calculate the potential of the electrode and the electrolyte. Another method of modeling is equivalent circuit modeling, which is a semiempirical and semitheoretical model. The equivalent-circuit model applies electronic components such as voltage sources, resistors, and capacitors to characterize the working characteristics of the battery. This type of model has the advantages of simplicity, ease of implementation, and high accuracy. In practice, equivalent-circuit models are

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widely applied. Compared with the electrochemical model, this model has no complicated mathematical description, so it is conducive to engineering applications [2]. SOC, SOH, SOP, and fault diagnoses have been realized mostly based on equivalent-circuit models. Common equivalent circuit models include the internalresistance (Rint) model, resistanceecapacitance (RC) model, Thevenin model, Partnership for a New Generation of Vehicles (PNGV) model, and composite equivalent circuit model [3]. This type of model adopts electronic components to characterize the dynamic and static characteristics of the battery.

2.

Electrochemical modeling

The electrochemical model is a kind of modeling based on the dynamics and thermodynamics inside the battery [4]. Electrochemical modeling can characterize the internal working characteristics of the battery. The electrochemical battery model contains a series of partial differential equations, which can provide all the information of the battery’s internal dynamics and thermodynamics.

2.1

Overview

The electrochemical model is based on the internal chemical mechanism to do modeling research on batteries. A differential equation is used to describe the changes in the internal materials of the battery. Commonly used electrochemical models include the Grahame, GouyeChapman, Bockris, and ButlereVolmer models. A schematic diagram of the electrochemical model is shown in Fig. 5.1. At present, most electrochemical models are based on the porous electrode theory [5]. The most classic one is the full-order two-dimensional model proposed by Doyle. This model adapts Fick’s second law to constrain the concentration of active materials in the battery and Ohm’s law to constrain the potential of the battery. In addition, a Electrolyte Anion

Figure 5.1 Schematic diagram of electrochemical model.

Cation

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79

large number of simplified battery electrochemical models (reduced-order electrochemical models) have been proposed to meet the application requirements of simple structure and convenient calculation, such as polynomial fitting models and singleparticle (SP) models. In applying the electrochemical model, Gottapu [6] achieved high-precision battery characterization by simplifying the traditional electrochemical model. Gao [7] established a practical electrochemical-thermal model and applied it in the battery management system with good results. In addition, many researchers have combined the Nernst and ButlereVolmer equations to accurately describe the dynamic and static characteristics of the battery while also reducing model complexity.

2.2

Porous electrode theory

Currently, most lithium-ion electrochemical models are based on porous electrode theory [8]. Porous electrode theory is the main theory to describe the internal electrode reaction of the battery, and it is also the main theoretical basis for the modeling of the electrochemical model of the battery. The substances participating in the electrode reaction include the active particles coated on the electrode, the electrolyte that infiltrates the pole piece and the diaphragm, and the solid-liquid phase interface. The electrode reaction mainly contains four reaction processes. The first is the charge transfer that occurs at the interface between the electrode and the electrolyte. The second electrode reaction is the diffusion of active electrode particles. Due to the concentration difference, the active electrode particles will migrate. The third electrode reaction is due to the structure with a certain capacitance formed at the contact surface between the electrolyte and the active particles so that charging and discharging behaviors similar to those of the capacitance will occur during the reaction. The last electrode reaction is the migration of the positive charge in the electrolyte and the migration of the negative charge of the electrons in the active particles. The porous electrode theory is based on the chemical reaction of the electrode and the formation of the porous structure of the electrode. There are some below important theories to guide the battery in electrochemical modeling: (i) The diffusion process of lithium particles in the active material in the electrode is controlled by the Fick diffusion theorem. (ii) The diffusion and migration of lithium ions in the electrolyte are controlled by the theory of concentrated solutions. (iii) The conductivity of the active material in the electrode is controlled by Ohm’s law. (iv) The electrochemical reaction at the solid-liquid interfaces where the active material in the electrode contacts the electrolyte is controlled by Faraday’s law and the Bolter-Volmer equation. (v) Simplify the nonfaradic reaction at the solid-liquid interface where the active material in the electrode contacts the electrolyte into an equivalent circuit.

2.3

Electrochemical model

The electrochemical model describes the transmission process and electrochemical reaction behavior of lithium ions inside the battery through partial differential equations. The pseudo-two-dimensional (P2D) [9,10] model is a commonly used

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battery mechanism model. The P2D model can describe the battery terminal voltage, SOC, temperature, cycle life, capacity decay, and other macroscopic phenomena by simulating the internal electricity of the lithium battery, as well as ion concentration distribution, potential distribution, local heat generation, solid electrolyte interface changes, and electrodes microscopic phenomena such as polarization. However, in practical engineering applications, P2D models face two problems. On the one hand, the number of parameters for electrochemical process modeling is large, all of which are battery material-level parameters, and the model parameters are difficult or unmeasurable. On the other hand, the strict P2D model is not easily realized through online calculation. The large consumption of computing resources makes it difficult to apply to the battery management system. Based on the P2D model modeling process, multiple particles form a solid-liquid two-phase porous electrode model. The researchers reduced the order of the P2D model by simplifying the model structure and proposed an SP model [11e13]. The SP model simplified the positive and negative porous electrodes into a single spherical particle, ignored changes in the concentration of lithium ions and potential changes in the electrolytic liquid phase, and reduced the order of the model from the perspective of modeling [1]. SP model can quickly simulate the battery charging and discharging process under the 1C rate. For the charging and discharging behavior at a high rate above 1C, the voltage accuracy of the SP model cannot meet the requirements. Based on the SP model, the researchers proposed an average value model similar to the SP model. The model ignores the concentration distribution of solid electrode particles. The ion concentration inside the spherical particles of the active material is equal to the average ion concentration in the entire electrode material, and only the potential difference caused by the liquid phase resistance is considered. Some researchers have considered the influence of the concentration distribution in the liquid phase on the potential and further improved the accuracy of the average model. The P2D model is an electrochemical model constructed by the structure of the battery negative electrode-diaphragm-positive electrode. The positive and negative electrodes are respectively formed by mixing a solid phase and a liquid phase; the solid phase is the active electrode material, and the liquid phase is the electrolyte. It is composed of polymer solid phase and liquid electrolyte with microscopic channels. The liquid electrolyte has the function of transporting ions., relevant assumptions should be made [14] as follows to establish the battery’s P2D model: (i) Only solid and liquid phases are considered inside the battery, and gas-phase substances are not. (ii) The solid-phase microstructure of the electrode is simplified to multiple uniform spherical particles with the same radius. (iii) Values for the lithium-ion solid-phase diffusion coefficient and lithium-ions are assumed. (iv) The transmission of ions in the liquid phase conforms to the theory of concentrated solutionsdonly the diffusion and migration processes are considered, and the convection and double-layer capacitance effects are not. (v) The transmission of ions only considers the thickness direction of the battery and not the ion transmission along the length and width of the electrode. (vi) The conductivity of the current collector is much higher than the electrode material.

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The SP model does not consider the change in the electrolyte, and each electrode is equivalent to a single spherical particle, thus simplifying the calculation of the liquid phase potential difference. SP model still uses Fick’s second law to describe the diffusion process of lithium ions inside spherical particles, and usually, secondorder ordinary differential equations are used to simplify partial differential equations. The ButlereVolmer equation is used to characterize the reaction rate of lithium-ion intercalation and deintercalation. According to the electrode potential, the terminal voltage can be obtained.

3.

Equivalent circuit modeling

The equivalent circuit model is a bridge that connects the external and internal characteristics of the battery. The equivalent circuit model uses the charge and discharge current, voltage, and operating temperature of the battery to estimate the internal characteristics of the battery, such as the open-circuit voltage (OCV), internal ohmic resistance, and polarization effects. Because the equivalent-circuit model has the advantages of high accuracy, simple modeling, and easy engineering realization, the equivalent-circuit model is mainly used in battery SOC estimation, SOH assessment, and SOP prediction, and it is widely used in practical applications [15e19].

3.1

Rint modeling

The equivalent circuit model is a semimechanical and semiempirical model that adopts electronic devices to describe the operating characteristics. The commonly used battery equivalent circuit models include the Rint, RC, Thevenin, PNGV, and composite models. The Rint model was proposed by the Idaho National Laboratory in the United States; its structure is shown in Fig. 5.2. The model is one of the first proposed equivalent circuit models to consist of a resistor and a voltage source. In this model, the resistor R0 is used to describe the characteristics of the internal ohmic resistance during operation, and the voltage source UOC is used to represent the battery’s OCV. Therefore, the terminal voltage of the model can be shown in Eq. (5.3): UL ¼ UOC  R0 •I

(5.3)

R0

I(t) +

UOC

UL -

Figure 5.2 Internal-resistance equivalent-circuit model.

RL

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The Rint model has the characteristics of simple structure and can effectively describe the ohmic characteristics of the model. However, because there are other phenomena of polarization effect points in the battery during use, there is a problem of poor accuracy in describing the working characteristics of the battery with a linear model.

3.2

Resistanceecapacitance modeling

The RC model is the standard lithium battery model used in the ADVISOR software released by the American National Renewable Energy Laboratory. The RC model is an equivalent circuit model obtained by improving the Rint model. Based on the Rint model, this model uses two capacitors and three resistors to characterize the operating characteristics of the battery. Compared with the Rint model, this model belongs to the nonlinear model. The RC circuit in the model can be used to characterize the polarization effect of the battery. Because this model can characterize the internal nonlinearity of the battery, it has higher accuracy. The structure of the model is shown in Fig. 5.3. In this model, capacitance Cb represents the battery’s energy storage capacity, and Cs is the surface capacitance of the battery, which is used to characterize the diffusion effect. In addition, Rt is ohmic resistance, Re and Rs are polarization internal resistances in the RC model. If the voltage of the capacitor Cb is UCb and the voltage of Cs is UCs, and the terminal current is used as the input, and the terminal voltage as the output, the state space expression of this model can be obtained as shown in Eq. (5.4): 8 2 3 3 2 2 3 1 1 Rs > 2 3 > >  > 6 7 7 6 > > 6 U_ Cb 7 6 ðRe þ Rs ÞCb ðRe þ Rs ÞCb 76 UCb 7 6 ðRe þ Rs ÞCb 7 > > 6 6 7 7 IL 7 6 >4 5þ6 > 74 7 5¼6 > > Re _ Cs 4 5 UCs 5 4 1 1 U > <  ðRe þ Rs ÞCb ðRe þ Rs ÞCs ðRe þ Rs ÞCs > > > 2 3 > > >     > U Cb > Rs Re Rs þ Re > 6 7 > > U ¼ þ R þ 4 5 L t IL > > Re þ Rs Re þ Rs Re þ Rs : UCs (5.4) IL Re

Rs

Is Cs

Ib

Rt UL

Cb

Figure 5.3 Resistanceecapacitance battery model.

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3.3

83

Thevenin modeling

The Thevenin model is also called the first-order RC model. The Thevenin model is one of the most commonly used equivalent circuit models in battery engineering applications. It has the advantages of model simplicity, few model parameters, high accuracy, and convenient identification of model parameters. It is commonly used in battery SOC estimation, SOP prediction, fault diagnosis, and so on. This equivalent circuit model adds a resistor-capacitor parallel circuit based on the Rint model to characterize the polarization reaction inside the battery. Unlike the RC model, the Thevenin model directly connects the resistorecapacitor parallel circuit and internal resistance in series. The model is simpler but has higher accuracy, and it can effectively describe the battery’s dynamic and static operating characteristics. It is shown in Fig. 5.4. In the Thevenin model, R0 is used to describe the ohmic characteristics of the battery, and Rp and Cp are the polarization resistance and polarization capacitance of the battery, respectively, which are used to describe the polarization effect of the battery. In addition, UOC is an ideal voltage source used to characterize the OCV of the battery. Because there is a close relationship between the OCV and SOC, the relationship between the OCV and SOC is usually used to characterize the OCV or UOC. The mathematical description of the Thevenin model is shown in Eq. (5.5): 8 1 1 > < U_ p ¼  Up þ IL Rp Cp Cp > : UL ¼ UP þ R0 •IL þ UOC

(5.5)

In Eq. (5.5), Up is used to represent the polarization voltage of the RC parallel circuit in the model, and IL is the terminal current. In the figure, the relationship between UOC and SOC is often obtained through experiments to estimate the OCV based on the real-time SOC. Therefore, the model has only three parameters to identify, which is one reason for the model’s wide use.

Rp

I(t)

R0 + UOC

Ip(t) Cp

UL -

Figure 5.4 The Thevenin model.

RL

84

3.4

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Partnership for a new generation of vehicles modeling

The PNGV model was the standard battery model in the PNGV Battery Test Manual in 2001, and it was also used as the standard battery model in the FreedomCAR Battery Test Manual in 2003. It is an improvement on the Thevenin model. In this model, UOC is an ideal voltage source representing the OCV of the battery. R0 is the battery ohmic resistance, which is used to describe the ohmic effect during the charging and discharging stages of the battery. Rp and Cp are the polarization resistance and polarization capacitance of the battery, respectively, and are used to describe the polarization effect of the battery. Compared with the Thevenin model, a capacitor, Cb, is added in this model. It is leveraged to describe the change in OCV generated by the time accumulation of current. The structure of the model is shown in Fig. 5.5. This model is a typical linear lumped parameter circuit that can predict battery terminal voltage changes under the condition of hybrid pulse power characteristics. In the PNGV model, voltage changes in capacitor Cb can be used to reflect changes in the OCV by the SOC and characterize the battery’s direct-current response. This model makes up for the shortcomings of the Thevenin model. The terminal voltage description of the model is shown in Eq. (5.6): UL ¼ UP þ R0 •IL þ UOC þ Ub

(5.6)

where Ub is the voltage of capacitor Cb, IL is the current loading, and UP is the polarization voltage of the RC parallel circuit.

3.5

Improved electrical modeling

The above model can effectively characterize most battery characteristics when it is working, but there are still shortcomings. When the battery is in use, complex chemical reactions occur inside, and none of the above models can fully consider the battery’s dynamic characteristics. For example, the OCV during charge and discharge has hysteresis. The internal resistance is different during charge and discharge. The selfdischarge inside the battery cannot be effectively characterized in the above model. This section explains some improved equivalent-circuit models.

RP

IL(t)

R0

Cb UOC

CP

Ip(t)

+ UL -

Figure 5.5 The Partnership for a New Generation of Vehicles model.

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Mathematical modeling of lithiumesulfur batteries

85

RP2

RP1

IL(t)

R0 + UP1 -

+ UP2 +

UOC

CP1

CP2

UL

RL -

Figure 5.6 Second-order Thevenin model.

Because the above models have their shortcomings, the above equivalent-circuit model is usually improved according to the use situation. For example, because the polarization effects in the battery include concentration polarization and electrochemical polarization effects, a parallel RC loop of the Thevenin model cannot effectively characterize the two polarization effects. Therefore, researchers have put forward the second-order Thevenin model, which has higher accuracy than the traditional Thevenin model. This model is shown in Fig. 5.6. In this model, the parallel circuit formed by RP1 and CP1 is used to characterize the concentration polarization effect, and the parallel circuit formed by RP2 and CP2 is used to characterize the electrochemical polarization effect of the battery. UP1 and UP2 represent the respective voltages of the two RC parallel circuits. The resistance R0 is used to characterize the internal resistance effect of the battery, the ideal voltage source UOC characterizes the OCV of the battery, and IL is the load current. The mathematical description of the model is shown in Eq. (5.7): 8 U U dU U dU > < IL ¼ 0 ¼ P1 þ CP1 P1 ¼ P2 þ CP2 P2 R0 RP1 dt RP2 dt > : UL ¼ UP1 þ UP2 þ R0 •IL þ UOC

(5.7)

In addition, many scholars have adopted the PNGV model with the second-order RC parallel circuit, also to good results. Because internal resistance differs during the charging and discharging phases, researchers have proposed an improved method based on the Thevenin equivalent circuit that uses two resistors to characterize the internal resistance of the batterydone to characterize the internal resistance of the discharge and the other to characterize the internal resistance of the battery during charging. This structure is shown in Fig. 5.7. In this figure, the difference from the traditional model lies in the characterization of the internal resistance of the model. When charging or discharging, only one diode is turned on in the part that characterizes the internal resistance of the model, so different resistances characterize it during charging and discharging. When the battery is discharged, the internal resistance is characterized by a branch composed of an ideal diode D0 and a resistor R0. D1 and R1 characterize the internal resistance effect during the charging process.

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RP

D0

R0 IL(t)

+ UP +

UOC

CP

D1

R1

UL

RL -

Figure 5.7 Second-order Thevenin model.

In addition to the abovementioned improved equivalent circuit model, there are also improved models such as fractional-order models [20e22], dynamic high-order models, and hybrid models [19,23,24]. They are improved based on the original model to make up for some shortcomings of the traditional model.

4.

Parameter identification of equivalent-circuit model

For battery model research, modeling is an important point, and another important point is the acquisition of model parameters. The accuracy of the model’s parameters is related to the accuracy of the model’s characterization of battery characteristics. This section will introduce model parameter identification.

4.1

Overview

There are three main types of parameter identification for equivalent models. One is offline parameter identification [25], including the curve fitting and point selection methods. This type of method relies on battery characteristic test experiments. The second category is based on data-driven methods [26,27], mainly neural network algorithms. This type of method uses a large amount of experimental data to build neural networks, and the training results can only be used in conditions similar to training conditions. The third category is online parameter identification methods, which are based on the least squares (LS) algorithm, including the recursive least squares (RLS), forgetting factor recursive least squares, and recursive extended least squares algorithms [28].

4.2

Exponential curve fitting

The exponential curve fitting method is a method to estimate the equivalent circuit model parameters based on the battery characteristic test experiment. A battery’s working characteristics must be understood when estimating model parameters. Commonly used experiments include the hybrid pulse power characterization (HPPC) test, rate test, etc.

Mathematical modeling of lithiumesulfur batteries

(a)

HPPC test voltage

87

(b)

HPPC test current

Figure 5.8 Voltage and current of hybrid pulse power characterization (HPPC) test. (A) HPPC test voltage (B) HPPC test current.

The exponential curve fitting method is usually based on the HPPC experiment for parameter identification. The HPPC experiment can be used not only for parameter identification but also for obtaining the OCVeSOC curve of the battery. The commonly used HPPC experiment is shown in Fig. 5.8. The above figure is the voltage and current of the HPPC test experiment. The current causes the change in voltage. The current corresponding to U0eU7 in the figure is I0eI7. Assuming that the voltage changes from I0 to I1, it is in the discharging stage. According to experiments, when the current changes from I0 to I1, the battery voltage will drop from U0 to U1, and when the current rises from I2 to I3, the voltage will increase from U2 to U3. These changes are caused by the ohmic effect of the battery. Therefore, the Ohm internal resistance R0 of the battery can be obtained according to the changes in voltage and current at these two periods. The equation is shown in Eq. (5.8): R0 ¼

ðU0  U1 Þ þ ðU3  U2 Þ 2I1

(5.8)

In the HPPC test, the voltage U1eU2 changes slowly when the current I1eI2 occurs. The voltage change in this section is primarily caused by the polarization effect inside the battery. Taking the Thevenin model as an example, and the zero-state response expression is shown in Eq. (5.9):   t UL ¼ UOC  IR0  IRP 1  es

(5.9)

where s is the time constant of the RC loop in the model, t is the sampling time, UOC is the OCV of the model, and I is the load current. Therefore, Eq. (5.9) can be transformed into the exponential equation of Eq. (5.10):   t y ¼ a  bI  cI 1  ed

(5.10)

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In Eq. (5.10), a ¼ UOC, b ¼ R0, c ¼ RP, d ¼ s. According to the voltage curve U1eU2 and the corresponding current, the parameters in Eq. (5.10) can be obtained by curve fitting to realize the identification of model parameters. In the same way, this method can also be used for parameter identification of another equivalent circuit model.

4.3

Least squares method

The LS algorithm is one of the most common parameter identification methods, which is based on the principle of minimum mean square error. The discrete system can be expressed in the form shown in Eq. (5.11): y ¼ a 1 x 1 þ a2 x 2 þ / þ a N x N þ b þ e

(5.11)

where a1, a2, ., aN are the parameters to be identified in the model, x1, x2, ., xN are system variables, and b is the constant. According to the principle of minimum mean square error, Eq. (5.12) can be obtained: J¼

l X

e2 ðiÞ ¼ ½yð1Þ  a1 x1 ð1Þ  .aN xN ð1Þ  b2

i¼1

þ.½yðlÞ  a1 x1 ðlÞ  .aN xN ðlÞ  b2

(5.12)

Because the error e is a constant, the partial derivative is calculated, and its partial derivative is 0. If the system is expressed in the form of a matrix, Eq. (5.13) can be obtained: 8 vJ > > < ¼ 2X T ðY  XqÞ ¼ 0 vq > 1 T  > : X Y q ¼ XT X

(5.13)

where X is the matrix formed by the variable x, Y is the matrix formed by the output, and q is the parameter to be identified.

4.4

Recursive least squares

The LS algorithm is an offline parameter identification method, which cannot meet the needs of online parameter identification. On this basis, the researchers proposed the RLS algorithm, which is improved based on the LS. It is the most extensive method in the online parameter identification method of battery equivalent circuit model. This section will take the Thevenin model as an example to introduce the method. In the Thevenin model, the OCV is usually obtained through experiments. Therefore,

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the difference between the terminal voltage and the OCV is used as the output, and the load current is used as the input to obtain a single-input single-output system. According to Laplace transformation, it is shown in Eq. (5.14): 8 > < Ua ðsÞ ¼ UOC ðsÞ  UL ðsÞ > : Ua ðsÞ ¼ R0 IðsÞ þ RP IðsÞ þ v 1 þ ss

(5.14)

In Eq. (5.14), s is the time constant, and v is the measurement noise of the model. According to the bilinear transformation, the discrete equation of the model to be identified can be obtained: 8 < yðkÞ ¼ ayðk  1Þ þ bIðkÞ þ cIðk  1Þ :

a¼s

b ¼ ðR0 þ RP Þ

c ¼ sR0

(5.15)

where y(k) is the discrete variable of Ua(s), I(k) is the current, and a, b, and c are the coefficients related to the parameters R0, RP, and CP. The LS form of Eq. (5.15) is shown as Eq. (5.16): 8 yðkÞ ¼ xðkÞT •qðkÞ > > > < > > > :

xðkÞ ¼ ½ yðk  1Þ

qðkÞ ¼ ½ a

b

IðkÞ

Iðk  1Þ T

(5.16)

c T

Then, the equations to calculate the Thevenin equivalent circuit model parameters can be represented by Eq. (5.15). After the results of parameter identification are obtained, the ohmic resistance R0, polarization capacitances CP, and polarization resistances RP can be derived: 8  > qðkÞ ¼ qðk  1Þ þ g•Pðk  1ÞxðkÞ yðkÞ  xT ðkÞqðk  1Þ > > > <  1 g ¼ xT ðkÞPðk  1ÞxðkÞ þ 1 > > > > : PðkÞ ¼ I  g•Pðk  1ÞxðkÞxT ðkÞ Pðk  1Þ

(5.17)

In addition, the RLS algorithm is identified by the measured voltage and current equivalent circuit model parameters, and there is measurement noise when measuring voltage and current. Therefore, many researchers increase the noise module to characterize the measurement noise of the battery model, to improve the accuracy of the model and parameter identification results.

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Neural network algorithm

The neural network algorithm is a data-driven method. The algorithm requires a large amount of experimental data as training samples to obtain a high-precision neural network model. Common neural network algorithms include the backpropagation (BP) neural network, the wavelet neural network (WNN), and so on. The neural network algorithm adjusts the weights of nodes and networks through training samples. The common neural network is a three-layer network: the input layer, the hidden layer, and the output layer. The structure is shown in Fig. 5.9. In the neural network model, Xi is the network’s input, and the number of inputs decides the number of input nodes in the model. The variable hj is the basic function of the neural network, and the user determines the number of hidden layer nodes. Yk is the output, and the output variables decide its nodes. The variables wij and wjk represent the weight from the input layer to the hidden layer and the hidden layer to the output layer, respectively. The BP neural network is a kind of error BP neural network with strong nonlinear approximation ability. The three-layer network can approximate and fit any continuous nonlinear function. In addition, BP neural network has a strong learning ability, can quickly adjust the weight of the network, and achieve rapid training of the model. Finally, the algorithm has good generalization ability and fault tolerance; it can effectively reduce the influence of external factors. However, the BP neural network algorithm also has some shortcomings. The BP neural network easily falls into the local minimum problem, which is sensitive to the initial weight of the network. Secondly, the BP neural network algorithm converges slowly. Finally, because the approximation ability and promotion ability of the network model are related to the typicality of the samples, it is very difficult to select typical samples to form the training set, so the BP neural network is dependent on data seriously. Unlike other neural network algorithms, the WNN replaces the sigmoid function of the hidden layer node in the neural network with a wavelet function. The weights from the input layer to the hidden layer and the threshold of the hidden layer of the neural network are replaced by the scaling factor and the time-shift factor of the wavelet

Input X1

wij

Hidden

X2

Xi Figure 5.9 Structure of the neural network.

h1

h2

hj

wjk

Output Y1

Y2

Yk

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91

function. Therefore, the WNN converges fast, avoids falling into the local optimum, and has the characteristics of local analysis in the domain. This algorithm can make up for the shortcomings of the BP neural network. When using neural networks to identify model parameters, the initial weights of neural network algorithms are very important. Therefore, neural network algorithms are usually combined with genetic and particle swarm optimization algorithms. The optimization algorithm can supply the optimal initial weight for the neural network, improve the algorithm’s convergence speed and approximation ability, and obtain better neural network training results.

5.

Model application

The purpose of battery modeling is to manage and control the battery and effectively exert the battery’s performance. The battery model is usually used to estimate the SOC, SOH, SOP, and so on.

5.1

State-of-charge estimation

The SOC is used to characterize the remaining power of the battery. The accurate SOC estimation can effectively prevent the battery from being overcharged and overdischarged to avoid battery damage. The SOC estimation methods [16,29] include ampere-hour (Ah) integration, OCV, and model-based. The Ah integration method is shown in Eq. (5.18): Rk SOCðkÞ ¼ SOCð0Þ þ

0

h•IðtÞdt Q

(5.18)

where SOC(k) is the estimated SOC value, SOC(0) is the initial value of SOC, Q is the battery capacity, and h is the Coulomb efficiency. The Ah integration method calculates the remaining SOC of the battery based on the discharging and charging current. This method is simple, but there is measurement noise during the current measurement, so this method will produce cumulative errors. As the use time increases, the cumulative errors will become larger and larger, and the deviation of SOC estimation will also increase. The model-based method is the most commonly used SOC estimation method. The model-based method is usually combined with the Ah integration method to correct the SOC error caused by noise through the model to improve the accuracy of SOC estimation. Commonly used methods are the extended Kalman filtering (EKF) algorithm [30e33], the unscented Kalman filtering (UKF) algorithm [34], and the particle filtering algorithm [35]. The EKF algorithm and the UKF algorithm are improved algorithms based on the Kalman filtering algorithm. The EKF algorithm uses the Taylor expansion formula to linearize the nonlinear model, and the UKF algorithm uses the unscented transform to process the nonlinear model. Due to the unscented

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transform, the calculation required is quite large, so EKF is more widely used than UKF in practical applications. The application principle of EKF in battery SOC estimation is shown in Eq. (5.19): 8 > > xkþ1=k ¼ Axk þ Bukþ1 > > > > > > > > Pkþ1=k ¼ APk AT þ Qk > > > > > >  > T T > > < Kkþ1 ¼ Pkþ1 C CPkþ1 C



þ Rk

1

> > > eykþ1 ¼ ykþ1  Cxkþ1=k þ Dukþ1 > > > > > > > > > xkþ1 ¼ xkþ1=k þ Kkþ1e ykþ1 > > > > > > > : Pkþ1 ¼ ðE  Kkþ1 CÞPkþ1=k



(5.19)

According to the model, Eq. (5.19) predicts the system state and error covariance at the kþ1 moment. The Kalman compensation gain is then calculated based on the error covariance. Then the initial prediction value is corrected according to the difference between the predicted observation value and the actual observation value with the Kalman compensation gain to realize the estimation of the SOC. In this equation, xk is the state vector; yk is the output vector; uk is the input vector; A is the system matrix; B is the input matrix; C is the output matrix; D is the feedforward matrix; Pk is the error covariance matrix; Kk is the Kalman gain. Qk and Rk are the system’s noise error covariance matrix and the model’s measurement noise error covariance matrix. Since the white noise condition cannot be met, there will be a certain estimation error when using the EKF algorithm to estimate the SOC. Therefore, an adaptive EKF algorithm is proposed that reduces the SOC estimation error by estimating the noise error covariance matrix. The calculation equation of the noise error covariance matrix Qk and Rk is shown in Eq. (5.20): 8 >   > > > > Qkþ1 ¼ ð1  dk ÞQk þ dkþ1 Kkeykþ1eyTkþ1 KkT > > > > > < eykþ1 ¼ ykþ1  Cxkþ1=k  Dukþ1 > >   > > > Rkþ1 ¼ ð1  dk ÞRk þ dkþ1 εkþ1 •εTkþ1 þ CPk CT > > > > > : εkþ1 ¼ ykþ1  Cxkþ1  Dukþ1

(5.20)

where dk is the forgetting factor, which can reduce the error caused by the excessive error of the data at the current moment.

5.2

State-of-health estimation

The SOH [26,36e38] is used to evaluate the aging degree of the battery. Aging will cause the changing of model parameters, so SOH is coupled with other state

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parameters related to model parameters, such as SOC estimation and SOP prediction. Aging will cause the internal resistance and capacity of the model to change; thus, estimating battery SOH related to the estimation accuracy of other state parameters is critical. The SOH calculation is divided into the equation based on the battery’s internal resistance and remaining capacity. The formula based on internal resistance is shown in Eq. (5.21): SOH ¼

R1  R0 R2  R0

(5.21)

where R0 is the internal resistance when the battery is dead, R1 is the battery’s internal resistance for aging estimation, and R2 is the resistance of the new battery. This definition of SOH by internal resistance can characterize the current aging state of the battery, but because the internal resistance is also affected by the charginge discharging rate of current and temperature, the use of internal resistance changes to estimate the SOH has a large error. The second definition of SOH by capacity is shown in Eq. (5.22): SOH ¼

C1 C0

(5.22)

where C1 represents the battery capacity and C0 is the initial capacity of a new battery. Compared with the battery’s internal resistance, changes in battery capacity are only affected by aging and temperature. Therefore, this method can better estimate battery SOH. However, according to the battery model, the battery’s internal resistance is easily obtained online, and it is quite difficult to estimate the capacity, which is usually obtained through experiments. At present, the most popular SOH estimation method is to estimate the internal resistance to calculate the SOH and combine the filtering algorithm to correct the SOH [37,39].

5.3

State-of-power prediction

The SOP is an important state parameter of the battery, representing its peak output power and input power at the current moment. The SOP [40,41] estimation of the battery is very important to use under special application conditions. For example, accurate SOP estimation can predict whether the robot can cross a certain obstacle or the load limit of the crewless aerial vehicle to avoid overcharging and overdischarging of the battery. At the same time, the calculation of peak input power also provides theoretical support for the fast-charging technology of the battery. SOP estimation methods are divided into experimental, data-driven, and modelbased methods [42e45]. The most commonly used experimental method is proposed in the United States Advanced Battery Consortium test manual, which provides battery SOP test steps. This method can test battery SOP, but it is only an offline identification method. In practice, it cannot be a real-time battery management system. The datadriven SOP methods include neural network algorithms, fuzzy control algorithms, and so on. The accuracy of these methods depends on the training samples, and it is

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difficult to consider all the application conditions of the battery. Therefore, the training results can only be used on specific occasions; they are not universal. The last SOP estimation method is an online estimation method based on the equivalent circuit model. This method is to estimate the SOP based on the parameters of the equivalent circuit model. Therefore, the model’s accuracy is determined by the accuracy of the parameter identification of the equivalent circuit model and is not affected by the operating conditions. The model-based method is currently the most widely used SOP estimation method. Next, the Thevenin model explains the SOP estimation method based on the equivalent-circuit model. According to the equivalent circuit model, the terminal voltage of the battery can be obtained, as shown in Eq. (5.23):

8 U ¼ UOC ðSOCÞ þ UP þ I•R0 > < L > : UP ¼ UP •e

t s

  t þ I•RP 1  es

(5.23)

When the battery’s terminal voltage reaches the chargeedischarge cutoff voltage, it cannot be charged and discharged. Therefore, according to the terminal voltage, the peak current of the battery during charging and discharging can be estimated. The SOP prediction equation is shown in Eq. (5.24): t 8 Umin  UOC  UP •es > > 0 1 P ¼ U • dis min > > > > t > > > R 0 þ R P @1  e  s A > > > < t > > > Umax  UOC  UP •es > > 0 1 ¼ U • P > max ch > > > t > > > R0 þ RP @1  es A :

(5.24)

In Eq. (5.24), Umin is the cutoff voltage during discharging, Umax is the cutoff voltage during charging. In addition, model-based SOP estimation methods are often used in conjunction with SOC-based SOP estimation methods. A model-based recursive estimation method is used to predict the long-term SOP and can predict the battery’s continuous charge and discharge SOP at a given time.

6.

Chapter summary

Modeling is the basis of LieS battery research and application, and the model can accurately describe the working state of the battery. The electrochemical model is a

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type of model obtained through the internal mechanism of the LieS battery. This type of model includes the P2D model, SP model, and so on. The electrochemical models have often been used in theoretical research on batteries. Another type of battery model is the equivalent circuit model, which uses the external characteristics of the battery to build an equivalent model using electronic components. Compared with the equivalent circuit model, the parameters of the electrochemical model can clearly describe the corresponding electrochemical reaction, and the meaning of each part of the equivalent circuit model is fuzzy. However, the equivalent circuit model has a simpler mathematical description, fewer model parameters, and is easy for engineering applications. Since the equivalent circuit model has fewer parameters, they are easier to identify than in the electrochemical model. HPPC experiments are commonly used in engineering applications to identify parameters or online parameter identification based on RLS. The equivalent circuit model is often used for SOC estimation, SOH prediction, SOP calculation, and battery management.

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Nanocomposites for binder-free Li-S electrodes

6

Qiongqiong Lu 1 and Xinyu Wang 2 1 Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V., Dresden, Germany; 2Dalian Maritime University, Dalian, China

1. Introduction To avoid low electron conductivity and electrochemically inactive binders and achieve higher energy density, binder-free nanocomposite electrodes with interconnected porous structures have attracted tremendous attention. In lithiumesulfur (LieS) batteries, binder-free electrodes based on various nanocompositesdsuch as carbon nanotube, graphene, carbon nanofiber (CNF), MXene, and hybridsdhave served as sulfur (S) species hosts, and lithium metal hosts have been explored and have demonstrated promising performance (Fig. 6.1) [1].

2. Carbon nanotube-based nanocomposites for binder-free electrodes Carbon nanotubes (CNTs) with a typical one-dimensional configuration are deemed as one or several rolled graphene sheets [2]. CNTs possess extraordinary electronic conductivity due to the covalently bonded sp2-carbon characteristic. In addition, CNTs exhibit a large aspect ratio, excellent mechanical properties, and good chemical stability [3]. CNTs and CNT-based nanocomposites can be assembled into various binderfree macroscopic film/foam electrodes by vacuum filtration, self-assembly, and freezedrying methods. The prepared binder-free CNT film/foam electrodes with a 3-Dconductive network and excellent mechanical properties improve S species utilization, suppress Li dendrite formation, and have the potential to be used directly as flexible electrodes in Li-S batteries.

2.1

Carbon nanotube-based nanocomposites for binder-free sulfur cathodes

Binder-free CNT film electrodes can be achieved from CNT suspensions with the help of surfactant agents or surface treatments to enhance CNT dispersion in the solution. For example, Manthiram’s group developed CNT@S composite film binder-free cathodes by a solution approach [4]. Isopropyl alcohol and Triton surfactant were used to improve the dispersion of CNTs in Na2S2O3 solution. After adding HCl solution into Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00003-X Copyright © 2022 Elsevier Inc. All rights reserved.

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Figure 6.1 Schematic summary of various nanocomposites for binder-free electrodes in lithiumesulfur batteries. Reprinted with permission from H. Sun; Z. Yan; F. Liu; W. Xu; F. Cheng; J. Chen. Selfsupported transition-metal-based electrocatalysts for hydrogen and oxygen evolution. Adv. Mater., 32 (3) (2020) 1806326. Copyright 2019 WILEY-VCH.

the mixture, the S particle was generated and attached to CNT to form CNT@S composites. Afterward, CNT@S composite binder-free cathodes were obtained via a filtering method. The binder-free cathodes with 40 wt% S delivered a high initial capacity of 1352 mAh g 1 at 1.0 C, and the capacity was still 68% after 100 cycles. In addition, Sun et al. prepared the well-dispersed porous CNT solution by controllable oxidation of CNT in air to improve CNT hydrophilicity and dispersion in solution (Fig. 6.2A) [5]. After facilely adding S powers in solution and drying as-obtained mixture suspension, the binder-free CNT@S nanocomposite film was fabricated. LiS batteries based on CNT@S film with 70 wt% S maintain a 760 mAh g 1 capacity after 100 cycles at 0.1 C. Though preparing the binder-free CNTs nanocomposite films through the solution method is facile and straightforward, there is a negative impact on the electron transfer between the S and CNTs when adopting surfactant agents or oxidation treatment. Therefore, it is critical to maintain the electrical conductivity of CNTs when preparing binder-free electrodes in Li-S batteries. Li et al. prepared SWCNT films with a

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Figure 6.2 (A) Schematic synthesis of porous carbon nanotube (CNT)@sulfur (S) solution. (B) Illustrations of single-walled carbon nanotube (SWCNT) films and SWCNT@S nanocomposite electrodes. (C) Optical image of the pristine SWCNT film with high flexibility. Reprinted with permission from (A) L. Sun; D. Wang; Y. Luo; K. Wang; W. Kong; Y. Wu; L. Zhang; K. Jiang; Q. Li; Y. Zhang; J. Wang; S. Fan. Sulfur embedded in a mesoporous carbon nanotube network as a binder-free electrode for high-performance lithium-sulfur batteries. ACS Nano, 10 (1) (2016) 1300e1308; (B,C) R. Fang; G. Li; S. Zhao; L. Yin; K. Du; P. Hou; S. Wang; H. M. Cheng; C. Liu; F. Li. Single-wall carbon nanotube network enabled ultrahigh sulfur-content electrodes for high-performance lithium-sulfur batteries. Nano Energy, 42 (2017) 205e214. Copyright 2015 Elsevier.

continuous-conductive network by the chemical vapor deposition (CVD) strategy [6]. The binder-free SWCNT@S nanocomposite was obtained after impregnating S into the SWCNT films by molten infusion method (Fig. 6.2B). Thanks to the uniform distribution of S in the high electron-conductive SWCNT films, the SWCNT@S electrode delivered a high square resistance (6 U cm 2) to the pristine CNT film (4 U cm 2). As a result, the interwoven, highly flexible structure of SWCNT films (Fig. 6.2C) provided the rapid paths for electron and ion transportation and promoted polysulfide trapping during the discharging/charging process. Subsequently, the Li-S batteries with a high S loading of 7.2 mg cm 2 provided a high initial discharge capacity of 1212 mAh g 1 at 0.17 A g 1 and capacity retention of 70% after 140 cycles. Heteroatom-doped CNTs can further enable chemisorption toward polysulfides and accelerate electrochemical reaction kinetics, which is beneficial for improving the electrochemical performance of Li-S batteries. For example, a composite film based on nitrogen (N)-doped CNTs (N-CNTs) and nano-S was obtained by the vacuum filtration strategy [7]. The binder-free N-CNTs@S cathodes exhibited a high capacity of 807 mAh g 1 at 0.2 C after 100 cycles, which was ascribed to the strong adsorption of polysulfides by the N atoms and the high electronic conductivity of N-CNTs. In addition to single heteroatom-doped CNTs, dual-heteroatom-doped CNTs were constructed to offer multiactive sites for chemisorption toward polysulfides. For example,

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Jin et al. prepared B- and O-doped CNTs (BO-CNTs) as the substrate for S cathodes [8]. The electrical conductivity of BO-CNTs was enhanced. More importantly, B and O atoms provided numerous active sites and showed strong adsorptive toward polysulfides, inducing an improved stable cycle life and rate capability.

2.2

Carbon nanotube-based nanocomposites for binder-free lithium anodes

Carbon nanotubes (CNTs) can accommodate the volume change, reduce local current density, and suppress the dendrite formation due to their highly conductive network, large surface area, and mechanically robust. Sun et al. prepared CNT paper via the CVD method with high conductivity of 1.1  105 S m 1 (Fig. 6.3A), serving as a binder-free framework to restore Li [9]. They found that CNT paper is robust and expandable, which is beneficial for withstanding the huge volume change during Li stripping/plating cycling. As a result, the CNT@Li electrode exhibits a high average coulombic efficiency of 97.5% at 1 mA cm 2 with 5 mAh cm 2 for 100 cycles and a stable voltage hysteresis of w120 mV at 2 mA cm 2 with 8.5 mAh cm 2 over 3000 h. In addition, Yang et al. used a commercial carbon nanotube sponge as a 3D host of Li anode to further study the nucleation behavior of Li on CNTs [10]. The high surface area of CNTs with porous structure (Fig. 6.3B) increases the density site for Li nucleation, and the “prelithiation” behavior of CNTs improved lithiophility, inducing uniform Li deposition.

Figure 6.3 (A) Optical image of carbon nanotube film prepared via the chemical vapor deposition method. (B) Scanning electron microscopy image (inset photograph) of 3-D porous carbon nanotube sponge. Reprinted with permission from (A) Z. Sun; S. Jin; H. Jin; Z. Du; Y. Zhu; A. Cao; H. Ji; L.J. Wan. Robust expandable carbon nanotube scaffold for ultrahigh-capacity lithium-metal anodes. Adv. Mater., 30 (32) (2018) 1800884. Copyright 2018 WILEY-VCH; (B) G. Yang; Y. Li; Y. Tong; J. Qiu; S. Liu; S. Zhang; Z. Guan; B. Xu; Z. Wang; L. Chen. Lithium plating and stripping on carbon nanotube sponge. Nano Lett., 19 (1) (2019) 494e499. Copyright 2019 American Chemical Society.

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3. Graphene-based nanocomposites for binder-free electrodes Owing to the unique structure of a single layer of sp2 bonded carbon atom, graphene possesses a large aspect ratio, extraordinary electronic conductivity, high specific surface area, as well as outstanding mechanical robustness and flexibility [11]. The excellent electronic conductivity of graphene enables fast electron transfer of the redox reaction. The plane configuration of graphene with high specific surface area makes it facile for combination with S and Li deposition. Furthermore, the outstanding mechanical robustness and flexibility of graphene are beneficial for accommodating the volume change. Therefore, graphene is considered a promising material to assemble binder-free electrodes for Li-S batteries.

3.1

Graphene-based nanocomposite for binder-free sulfur cathodes

Binder-free graphene@S nanocomposite electrodes were usually fabricated by vacuum filtration, hydrothermal, freeze-drying, or metal reduction methods. The primary binder-free graphene@S nanocomposites were reported by Jin et al. [12] Graphene sheets are firstly dispersed in water with ultrasonic treatment, and then the S nanoparticles are anchored onto the surface of graphene sheets using the Na2S2O3 chemical deposition method. Finally, the graphene@S nanocomposites films were obtained by the vacuum filtration strategy. Such films can be directly used as binder-free cathodes for Li-S batteries, and they delivered a capacity of w600 mAh g 1 at 0.1 C with capacity retention of 83% after 100 cycles. Graphene@S films were also fabricated by immersed graphene films prepared by vacuum filtration into a sulfur (S)/CS2 solution [13]. Furthermore, macroporous graphene@S nanocomposite paper was explored by a facile method. In this procedure, graphene oxide (GO) and S nanoparticles were mixed in water, and then the mixed solution was converted to GO@S precursor after the treatment of freeze-drying [14]. After that, reduced graphene oxide (rGO)@S film was obtained after low-temperature heat treatment and compression (Fig. 6.4A). The rGO@S film displayed a flexible ability (Fig. 6.4B). Owing to the uniform distribution of S nanoparticles on graphene sheets, the Li-S batteries based on the rGO@S film cathodes provided a high capacity of w1320 mAh g 1 at 0.1 A g 1. Furthermore, Yu’s group fabricated the binder-free graphene-based porous carbon (GPC)-S films as a sulfur cathode [15]. The as-GPC-S film with a 3-D hierarchical network and a conductivity of 3.25 S cm 1 induced rapid electron/ion transfer and immobilization of S. As a result, the GPC@S film displayed a stable cycle performance with capacity stabilized at 647 mAh g 1 at 1.0 C after 300 cycles. This remarkable cycle performance was ascribed to the 3-D porous carbon networks with hierarchical configuration. Threedimensional porous carbon networks promote the rapid transfer of electron/ions and restrict the dissolution of polysulfide into the electrolyte with micropore confinement. Furthermore, the graphene-derived porous carbon@S films were assembled in flexible Li-S batteries and displayed similar charge/discharge curves at both flat and bent levels.

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Figure 6.4 (A) Schematic illustration of preparation process of reduced graphene oxide (rGO) @sulfur (S) film. (B) Optical images of the prepared forms in various conditions. (C) Schematic process of fabricating binder-free rGO@S composite films. Reprinted with permission from (A, B) C. Wang; X. Wang; Y. Wang; J. Chen; H. Zhou; Y. Huang. Macroporous free-standing nano-sulfur/reduced graphene oxide paper as stable cathode for lithium-sulfur battery. Nano Energy, 11 (2015) 678e686. Copyright 2015 Elsevier. (C) J. Cao; C. Chen; Q. Zhao; N. Zhang; Q. Lu; X. Wang; Z. Niu; J. Chen. A flexible nanostructured paper of a reduced graphene oxideesulfur composite for high-performance lithiumesulfur batteries with unconventional configurations. Adv. Mater., 28 (2016) 9628e9636.Copyright 2016 WILEY-VCH.

However, the facile large-scale synthesis of binder-free graphene@S film electrodes is still challenging because the practical application of vacuum filtration and freeze-drying is limited by low efficiency and high cost. Binder-free graphene@S film was fabricated by a metal reduction method, which is promising for large-scale synthesis [16]. Firstly, S nanoparticles were deposited on the GO sheets using the reaction between HCl and Na2S2O3, obtaining the GO@S composites. After that, Zn foil was immersed into the GO@S solution. Because the potential of GO is higher than Zn, the GO sheet would be reduced by Zn metal. As a result, the rGO@S composite films were continuously reduced and assembled on the Zn surface (Fig. 6.4C). The size of rGO@S film depends on the size of Zn metal; thus, the rGO@S composite films are facile for large-scale production by using large area Zn metal. Due to superior Young’s modulus and extraordinary tensile strength of obtained nanostructured rGO@S composite film, the flexible sandwiched and cable-type LieS batteries based on the

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rGO@S composites offered a discharge capacity of 1187 mAh g 1 and 1360 mAh g 1 at 0.1 C, respectively. More importantly, the prepared sandwiched and cable-typeflexible batteries deliver a stable cycle performance even under a bending state. In addition to the active material of S combined with graphene film/foam, lithium sulfide (Li2S) with satisfied capacity is an alternative cathode material for Li-S batteries. More importantly, the Li-S batteries based on Li2S cathodes without lithium metal anode offer more flexibility and avoid safety issues. To fabricate rGO@Li2S paper, binder-free rGO paper was obtained by freeze-drying GO solution and following thermal treatment [17]. Afterward, Li2S was composited with rGO paper via dropping Li2S solution and drying under an inert atmosphere. Owing to rGO with numerous adhesive sites for Li2S, the Li2S agglomeration was prevented. As a result, binder-free electrodes based on nano-Li2S (25e50 nm) wrapped in rGO paper exhibited high Li2S utilization (96% at 0.1 C), superior electrochemical performance, and stable cycling. This excellent performance was ascribed to the high electrical conductivity (307 S cm 1) and structural flexibility of rGO@Li2S paper. Furthermore, Li2Scoated 3-D-doped rGO foams as a binder-free cathode were fabricated by a liquid infiltration/evaporation strategy [18]. In this work, B-doped and N-doped rGO hydrogels were prepared by adding boric acid and dicyandiamide, respectively, as dopant agents using a hydrothermal method. Li2S dissolved in anhydrous ethanol was dropped into rGO foams. The 3-D-conductive network and porous structure of graphene facilitated the electron and ion transport, and the doped heteroatoms contributed to a strong binding to S species. As a result, the batteries based on N-rGO@Li2S and B-rGO@Li2S electrodes delivered stable cycling life with high initial specific capacities of 800 mAh g 1 and 720 mAh g 1 at 0.3 C, respectively.

3.2

Graphene-based nanocomposites for binder-free lithium anodes

Graphene is a promising scaffold for Li anode since it possesses the advantage of a highly conductive network and relatively high surface area. Mukherjee et al. reported binder-free porous graphene networks fabricated by thermal reduced GO paper to entrap Li [19]. Benefiting from defect-induced plating of Li within the interior of the porous graphene structure, a high capacity above 850 mAh g 1 and extended testing for over 1000 charge/discharge cycles with coulombic efficiencies above 99% were obtained. In addition, Xie et al. designed a 3-D graphene@nickel (Ni) foam as a binder-free host by CVD graphene growth on Ni foam [20]. Due to the synergistic effects of high surface area and surface-coated graphene layer, 3-D graphene@ Ni foam showed lower effective current density, suppressed the growth of Li dendrites, and stabilized the solid electrolyte interphase film, which ensured a high coulombic efficiency of 92% at 1.0 mA cm 2 with 1.0 mAh cm 2 after 100 cycles. Moreover, layered LierGO film was fabricated via Li-assisted reduction of vacuum-filtrated GO film by Lin et al. [21] When putting GO film into molten Li, a “spark” reaction happened, where GO was reduced to rGO by molten Li, and molten Li was infused into formed nanogaps (Fig. 6.5A). The layered rGO with excellent lithiophilicity, large

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Figure 6.5 (A) Fabrication of layered lithium (Li)ereduced graphene oxide (rGO) composite film via Li-assisted reduction. (B) Illustration synthesis of nanoporous nitrogen (N)-doped grapheneeLi anode. Reprinted with permission from (A) D. Lin; Y. Liu; Z. Liang; H.-W. Lee; J. Sun; H. Wang; K. Yan; J. Xie; Y. Cui. Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes. Nat. Nanotechnol., 11 (2016) 626. Copyright 2016 Nature Publishing Group. (B) R. Zhang; X.R. Chen; X. Chen; X.B. Cheng; X.Q. Zhang; C. Yan; Q. Zhang. Lithiophilic sites in doped graphene guide uniform lithium nucleation for dendrite-free lithium metal anodes. Angew. Chem. Int. Ed., 56 (27) (2017) 7764e7768. Copyright 2018 WILEY-VCH.

surface area, and nanogaps guaranteed the uniform Li deposition and mitigated the volume change. As a result, the Li-rGO composite demonstrated a stable and low overpotential of w80 mV at 3 mA cm 2 with 1 mAh cm 2 over 100 cycles. In addition, Zhang et al. proved that N-functional groups are lithiophilic by density functional theory (DFT) calculation and experiment [22]. According to their DFT calculation results, pyrrolic N and pyridinic N (the two main N-containing functional groups of N-doped graphene) have relatively larger binding energies ( 4.46 and 4.26 eV) than graphene ( 3.64 eV) and Cu ( 2.57 eV), which means N atoms have strong interaction with lithium and thus can guild lithium nucleation leading to uniform lithium distribution. Consequently, the N-doped graphene matrix showed a high coulombic efficiency of 98% at a current density of 1 mA cm 2 with 1 mAh cm 2 over 200 cycles. Furthermore, a lithiophilic 3-D nanoporous N-doped graphene as a scaffold material for lithium anodes was reported by Huang et al. (Fig. 6.5B) [23]. Benefiting from a large lithiophilic surface, large porosity, and high conductivity of N-doped nanoporous graphene film, a small voltage hysteresis of 85 mV at 10 mA cm 2 with 5 mAh cm 2 over 127 cycles was obtained.

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4. Carbon nanofiber-based nanocomposites for binderfree electrodes CNFs can be used as hosts for binder-free S cathodes and Li metal anodes because of the interconnected network and excellent electrical conductivity, accommodating active materials and enhancing active material utilization.

4.1

Carbon nanofiber-based nanocomposites for binder-free sulfur cathodes

CNFs are usually prepared by electrospinning polyacrylonitrile (PAN) solution, followed by carbonization. For example, Fu et al. reported a freestanding CNF-S composite cathode via sulfur vapor condensation within electrospun CNF web [24]. Using a novel sulfur vapor deposition strategy, the CNF-S composite had a gradient sulfur distribution with different functions. The high-sulfur region served as a sulfur resource, while the carbon network decreased the internal charge transfer resistance. The lowsulfur region acted as a reservoir to localize the dissolved polysulfides, and the nonsulfur region was an inhibitor to prevent polysulfide shuttle between electrodes. Such cathode with sulfur loading of 2.6 mg cm 2 and high sulfur content of 65% exhibited a stable capacity of >700 mAh g 1 after 70 cycles at 100 mA g 1. In addition, Goodenough and colleagues obtained freestanding CNF/R/xS composite fabricated by applying cross-linked sulfur derived from vulcanized rubber as the active material and loaded it on an electrospun CNF network [25]. The cross-linked sulfur not only acts as a sulfur source but also can capture polysulfides during discharge. Such a cathode showed a high rate capability, i.e., a capacity of 880 mAh g 1 after 25 cycles at 5 C. Qiu’s group developed nano-Li2S particles embedded in N-doped CNF electrodes by electrospun and carbonization [26]. Firstly, the polymer composite films were obtained by electrospun of the mixed solution of Li2SO4 with PVP. Subsequently, the Li2SO4@PVP precursor films were with the treatment of stabilization by preoxidation and carbonization. After carbonization, nano-Li2S particles embedded in N-doped carbon fiber carbonaceous were obtained (Fig. 6.6A). The N-doped carbon fiber@Li2S films with a conductive and porous framework facilitated rapid electron/ ion transfer. The nano-Li2S embedded in N-doped carbon fibers can be directly served as the binder-free cathodes for Li-S batteries, and it exhibited an initial capacity of 720 mAh g 1 at 0.2C with Li2S mass loading of 3 mg cm 2. When the Li2S mass loading increases to 9 mg cm 2, an initial capacity of 520 mAh g 1 with a stable capacity retention of over 65% for 200 cycles at 1 C can be obtained. Furthermore, HyunWook Lee and coworkers also demonstrated an electrospun CNF matrix for a high sulfur loading cathode [27]. Intertwined CNF matrix can physically adsorb polysulfides via cohesive forces, CNF-S electrode with a high sulfur loading of 10.5 mg cm 2 showed a high initial areal capacity of 7.90 mAh cm 2 and retained an areal capacity of 7.14 mAh cm 2 after 100 cycles. The porous structure of CNF can be adjusted by adding a pore generator (such as PMMA, PS), template (such as SiO2) in electrospinning that can accommodate the

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Figure 6.6 (A) Schematic diagram of nitrogen (N)-doped carbon fiber@Li2S (Li2S@NCNF) cathodes. (B) Schematic diagram of the synthesis process of the lotus rootlike multichannel carbon (LRC)/sulfur (S)@ ethylenediamine-functionalized reduced graphene oxide (EFG) electrode. Reprinted with permission from (A) M. Yu; Z. Wang; Y. Wang; Y. Dong; J. Qiu. Freestanding flexible Li2S paper electrode with high mass and capacity loading for high-energy Li-S batteries. Adv. Energy Mat., 7 (17) (2017) 1700018. Copyright 2017 WILEY-VCH. (B) Z. Li; J. Zhang; Y. Chen; J. Li; X. Lou. Pie-like electrode design for high-energy density lithiumesulfur batteries. Nat. Commun. 6 (2015) 8850. Copyright 2015 Nature Publishing Group.

volume change in S species. Zhang’s group investigated the porous CNF as a sulfur host fabricated by electrospinning the PAN/PMMA solution and subsequent carbonization. In PAN/PMMA mixed nanofibers, PAN served as the carbon precursor, and PMMA acted as the pore generator. Due to the high electrical conductivity and high surface area of porous CNF, which helped immobilize sulfur and alleviate the polysulfide shuttle, such cathode with 42 wt% sulfur delivered an initial discharge capacity of 1400 mAh g 1 at 0.05 C and capacity retention of 85% after 30 cycles [28]. Furthermore, Lou’s group designed a three-dimensional interconnected lotus-rootlike multichannel carbon (LRC) by electrospinning PAN/PS solution and then carbonizing the spun paper (Fig. 6.6B) [29]. After the carbonization process, parallel channels are generated within the shells of carbonized PAN, resulting from PS decomposition. By loading sulfur and subsequently wrapping ethylenediamine-functionalized reduced graphene oxide (EFG) layer outside the LRC/S electrode, LRC/S@EFG composite was obtained. Because of the sulfur accommodation of LRC nanofiber and polysulfide suppression of the EFG layer, such a pie-like cathode exhibited a high capacity of 1314 mAh g 1 (4.7 mAh cm 2) at 0.1 C with excellent cycling stability. Moreover, areal capacity could be further enhanced to more than 8 mAh cm 2 with S mass loading of 10.8 mg cm 2 through three-layer electrode stacking.

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CNF binder-free film also can be fabricated by carbonizing the bacterial celluloses hydrogel. Bacterial celluloses have abundant hydrogen molecular bonds, and cellulose chains combine to form ultralong nanofibers with strong mechanical strength [30]. After the carbonization of cellulose hydrogel, a porous CNF aerogel with an interwoven network satisfying mechanical stability can be achieved and act as a substrate for binder-free S cathode in Li-S batteries [31]. After immersing the CNF aerogel in S/CS2 solution and dried, a uniform sulfur coating on the carbon nanofiber with a high sulfur loading of 81 wt% was obtained. The as-prepared carbon nanofiber@S composites offered a conductive framework and provided sufficient space to suppress the volume expansion during the electrochemical reaction process. Therefore, the nanostructured binder-free carbon nanofiber@S composite films showed an initial capacity of 976 mAh g 1 and capacity retention of 63.5% over 300 cycles at 800 mA g 1. Heteroatom can also be introduced into the carbon fiber aerogel to provide the active sites for the chemisorption of polysulfides. For example, Li et al. achieved N-doped carbon nanofiber aerogel by carbonizing bacterial cellulose under ammonia air [32]. The resultant film acted as a binder-free conductive substrate and was composited with the polysulfide, which served as active material. Such cathode with a high sulfur content of 90 wt% and mass loading of 6.4 mg cm 2 delivered superb capacity retention of 96% after 100 cycles at 0.2 C.

4.2

Carbon nanofiber-based nanocomposites for binder-free lithium anodes

CNF film is also a promising skeleton as a binder-free host for Li metal. For example, CNF film fabricated by vacuum filtration was employed as a Li metal host by Zhang et al. [33] Benefiting from a large surface area and high conductivity, CNF film demonstrated a high coulombic efficiency of 99.9% at 2.0 mA cm 2 with 0.5 mAh cm 2 for more than 300 cycles. In addition, a CNF-stabilized graphene aerogel film (G-CNT film) was developed as a binder-free host material for lithium deposition [34]. Benefiting from the features of G-CNF film, such as conductive scaffold, large surface area, porous and robust structure, thus G-CNF@Li film exhibits a high coulombic efficiency of 99% at an ultrahigh current density of 2 mA cm 2 with a high areal capacity of 10 mAh cm 2 for more than 700 h.

5. Mxene-based nanocomposites for binder-free electrodes MXene, one kind of 2-D material of layered transition metal carbides and nitrides, attracted huge research attention owing to its superior electric conductivity, stratified structure, and abundant surface-active sites [35]. In the S cathode, the excellent electronic conductivity of Mxenes can boost the redox kinetics of S species, enhancing the utilization of S species. In addition, a variety of functional groups (-OH, -O, -Cl, and -F) on MXenes can attract polysulfides and prevent the shuttle effect of

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polysulfides [36]. In Li metal anode, the functional groups of Mxene contribute to guiding uniform nucleation of Li anode. Furthermore, MXene could serve as the skeleton for binder-free S electrodes and Li metal electrodes due to its high mechanical tensile.

5.1

Mxene-based nanocomposites for binder-free sulfur cathodes

MXenes are extensively applied as S hosts due to their high electronic conductivity and rich functional group. For example, Xu’s group used an MXene solution and S composite mixture to form a binder-free MXene@S with porous channels by a vacuum filtration strategy [37]. The MXene sheets in the prepared composite films provided consecutive 3-D-conductive frameworks and numerous sites for trapping the polysulfides. As a result, the Li-S batteries based on binder-free MXene@S films delivered a capacity of 1029.7 mAh g 1 at 0.1 C and capacity retention of 92% after 200 cycles, demonstrating high capacity and long cycle life. In addition, S nanoparticle-decorated Ti3C2TX (Ti3C2Tx@S) ink was prepared by reaction between Na2Sx and HCOOH in Ti3C2TX solution [38]. The robust binder-free S@Ti3C2Tx films were obtained via the filtrating of viscous ink. Benefiting from the high electronic conductivity and chemical adsorption of polysulfides of polar Ti3C2Tx, binder-free MXene@S film cathodes showed small polarization, high capacity of 1244 mAh g 1, and outstanding cycle life of 800 cycles with 0.048% capacity loss per cycle at 0.1 C. Furthermore, Zhang et al. prepared a robust and conductive Ti3C2Tx paper via filtration method as S host [39]. Binder-free Ti3C2Tx/S paper with high electronic and mechanical conductivity was obtained by a physical evaporation approach (Fig. 6.7A). The Li-S battery based on obtained Ti3C2Tx/S paper delivered a high capacity of 1383 mAh g 1 at 0.1 C, an excellent rate property (1075 mAh g 1 at 2 C), and a stable cycle performance (0.014% decay per cycle after 1500 cycles). This superb electrochemical performance was ascribed to the unique properties of the Ti3C2Tx frameworks, which provided rapid electron transfer pathways and possessed good mechanical strength to suppress the volume expansion of S. Additionally, the interaction between the polar Ti3C2Tx and polysulfides prevented the shuttle effect of polysulfides, enhancing the utilization of S.

5.2

Mxene-based nanocomposites for binder-free lithium anodes

The Mxene can accommodate the volume change of Li, reduce local current density, prevent the side reaction between Li and polysulfides, and guide uniform Li deposition. Therefore, the Mxene has been considered as a promising material for Li metal anode. For example, Yang’s group developed an effective method to achieve the binder-free lamellar Ti3C2@Li films by a mechanical roll-to-roll strategy (Fig. 6.7B) [40]. Owing to the ductility of Li metals, the Ti3C2 atomic nanosheets were easily rolled into Li foils to obtain Ti3C2@Li films. The Ti3C2 served as the solid electrolyte

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Figure 6.7 (A) Fabrication of robust and conductive Ti3C2Tx/sulfur (S) paper. (B) Schematic process of binder-free Ti3C2@lithium (Li) electrodes by a roll-to-roll strategy. Reprinted with permission from (A) H. Tang; W. Li; L. Pan; K. Tu; F. Du; T. Qiu; J. Yang; C. P. Cullen; N. McEvoy; C. Zhang. A robust, freestanding MXene-sulfur conductive paper for longlifetime Li-S batteries. Adv. Funct. Mater., 29 (30) (2019) 1901907. Copyright 2019 WILEYVCH. (B) B. Li; D. Zhang; Y. Liu; Y. Yu; S. Li; S. Yang. Flexible Ti3C2 MXene-lithium film with lamellar structure for ultrastable metallic lithium anodes. Nano Energy, 39 (2017) 654e661. Copyright 2017 Elsevier.

interface to prevent the further reaction between Li and electrolyte. Moreover, the conductive Ti3C2 films ensured Li dendrite growth in nanotraps, inhibiting vertical growth leading to a short circuit. As a result, the binder-free Ti3C2@Li films delivered superb electrochemical properties in terms of low overpotential and stable voltage profiles with a long cycling life. Further to confirm the potentials of binder-free Ti3C2@Li anode for Li-S battery, the full cells were assembled with Ti3C2@Li anode and Scarbon cathode. The batteries exhibited a reversible capacity of 948 mAh g 1 and retained a capacity of 841 mAh g 1 after 100 cycles at 200 mA g 1. Even at an ultrahigh current density of 5.0 mA cm 2, the capacity of Ti3C2@Li//S full cell reached a capacity of 360 mAh g 1, which is much higher than that of Li//S full cell (90 mAh g 1).

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Hybrid nanocomposites for binder-free electrodes

Single component material suffers from the inherent issues for Li-S batteries, leading to inferior electrochemical performance. For instance, graphene and CNT materials tend to overlay together, obtaining the compacted graphene and CNT bundles with decreased surface area. CNF films with large surface area and microporous structures normally suffer poor electronic conductivity owing to abundant amorphous sp3 carbon [41]. To tackle the issue of individual issues, one solution is hybridization to integrate the individual features. Therefore, several materials can be used as the building blocks to construe the hybrid nanocomposite for binder-free electrodes in Li-S batteries.

6.1

Hybrid nanocomposites for binder-free sulfur cathodes

In comparison with the physical mix, integrating CNTs on graphene by an in situ growth method can enable high electronic conductivity of the composite. For example, Deng et al. reported hierarchical carbon films composited with CNTs and graphene by in situ growth of CNTs on graphene [42]. Firstly, GO suspension mixed with Ni acetate was filtered and freeze-dried, achieving porous GO foam with Ni acetate. Then, GO foam was reduced to rGO in Ar/H2 air; meanwhile, Ni particles attached to the rGO sheets catalytic CNTs growth under the C2H4 atmosphere, achieving the porous rGO/CNT films. Finally, the rGO/CNT films were immersed in the S/CS2 solution to obtain the rGO/CNT@S nanocomposite binder-free electrodes for Li-S batteries. It is worth noting that CNTs not only resist the overlay of rGO sheets but also promote charge/ion transfer kinetics. Therefore, the rGO/CNT@S binder-free cathode delivered an initial discharge capacity of 854 mAh g 1 at 0.5 C and capacity retention of 92.8% after 100 cycles, which is higher than that of the cathode without CNTs (capacity retention of 64.3% after 57 cycles). In addition, Yu and colleagues reported binderfree carbonized electrospun CNF films with CNTs for Li-S batteries [43]. The polyacrylonitrile and CNT solution were used for the electrospun precursor suspension. The carbon fiber/CNT nanocomposite films were obtained by electrospinning following stabilization and carbonization. After further activating with the KOH agent, porous nanocomposite films with superb conductivity and numerous micropores were achieved. More importantly, the typical electrospinning 1-D morphologies were retained, exhibiting satisfied mechanical flexibility and interwove network with shortened electron/ion ways. After thermal impregnating of S, the binder-free carbon fiber/ CNT@S cathodes were obtained, it showed a capacity of 637 mAh g 1 at w0.03 C after 100 cycles, which is much higher than that of cathodes without CNT and activation. Porous carbon materials with a conductive and porous structure were also hybridized into the graphene sheets. For instance, the N-doped double-shelled hollow carbon spheres (NDHCS) were firstly prepared by the template approach, and NDHCS@S was obtained by molten infusion of sulfur [44]. Graphene was introduced to form the graphene/NDHCS@S films by the filtration method to fabricate the binder-free cathodes. Graphene not only acted as mechanical and conductive substrates but also

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provided additional restrictions to shuttle polysulfides. Due to the synergistic effect of conductive graphene sheets and 3-D NDHCS, graphene/NDHCS@S films delivered an initial discharge capacity of 1360 mAh g 1 at 0.2 C, superior rate performance (430 mAh g 1 at 3 C), and stable cycle capability (62% capacity retention) after 200 cycles. This electrochemical performance is better than that of the original graphene@S films. Inorganic metal ions connected to negative ions by polar covalent bonds possess polar surfaces with powerful binding effects to the polysulfide composites. Therefore, inorganic materials are promising polar materials for hybridization in binder-free cathodes. For example, Wang et al. used the electrospun process to prepare the PAN composite films with titanium isopropoxide as a metal source [45]. After carbonizing, the N-doped porous carbon fibers (NPCFs) modified with TiO2 nanoparticles (5 nm) were obtained. After immersing in the S solution, the binder-free NPCFs/TiO2@S nanocomposite films were prepared and served as the cathodes for Li-S batteries (Fig. 6.8). NPCF/TiO2@S with 55% wt S content showed a high initial specific capacity of 1501 mAh g 1 at 0.1 C mAh and a superior rate capability of 668 mAh g 1 at 5 C coulombic efficiency of 98.7%. In addition, Mao et al. achieved the metaleorganic framework (MOF)/CNT hybrid films for S hosts [46]. MOF particles were immersed in a CNT interwoven framework to form an ideal configuration that enabled electric

Figure 6.8 Schematic illustration of nitrogen-doped porous carbon fiber (NPCF)/TiO2@sulfur (S) composite paper fabrication. Reprinted with permission from X. Song; T. Gao; S. Wang; Y. Bao; G. Chen; L.X. Ding; H. Wang. Free-standing sulfur host based on titanium-dioxide-modified porous-carbon nanofibers for lithium-sulfur batteries. J. Power Sources, 356 (2017) 172e180. Copyright 2017 Elsevier.

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conductivity and electrode integrity. Moreover, MOF particles offered a strong binding effect toward polysulfides by the porous structure and chemical confinement of metal ions. Various MOF particles can be applied by altering the precursors. The MOF/CNT hybrid films acted as a substrate for encapsulating the S, and a high mass loading of 11.3 mg cm 2 can be obtained. The MOF/CNT@S cathodes achieved a high capacity of 7.45 mAh cm 2 and stable cycle life even at various foldable levels.

6.2

Hybrid nanocomposites for binder-free lithium anodes

Some metal (Such as Au, Ag, Zn, Mg, etc.) showing low overpotential of Li nucleation is a promising coating to integrate with binder-free Li anode to regulate Li nucleation and deposition [47]. For example, Xue et al. reported silver nanowire and graphenebased hierarchical network as binder-free host to enable ultrahigh rate and superior long cycling of Li metal anode (Fig. 6.9A) [48]. Such a binder-free host with a hierarchical network (Fig. 6.9BeD) and conductive framework can facilitate the electron

Figure 6.9 (A) Illustration of procedure of hierarchical composite network based on silver nanowire and graphene. (BeD) Scanning electron microscopy images at various magnifications of the hierarchical network structure. Reprinted with permission from P. Xue; S. Liu; X. Shi; C. Sun; C. Lai; Y. Zhou; D. Sui; Y. Chen; J. Liang. A hierarchical silver-nanowire-graphene host enabling ultrahigh rates and superior long-term cycling of lithium-metal composite anodes. Adv. Mater., 30 (44) (2018) 1804165. Copyright 2018 WILEY-VCH.

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transportation, provide a lithiophilic site for Li deposition, accommodate the huge volume change, and relieve stress during repeated Li stripping/plating. As a result, stable cycling over 1000 cycles with small voltage hysteresis of 120 mV at a high current density of 40 mA cm 2 with 1 mAh cm 2 was obtained in symmetric cells. Oxides (Such as ZnO, SnO2, Al2O3, Si [49], etc.), which can chemically react with Li, are promising lithophilic coating on the binder-free substrate to improve wettability for molten Li and guide Li plating. The spontaneous reaction between oxides (such as ZnO) and molten Li endows more lithiophilic species (LixZn/Li2O), and the contact angle of molten Li on the binder-free substrate surface is reduced. For example, Song et al. fabricated CNFs comprising N/ZnO-doped carbon framework and carbon nanotube via coaxial electrospinning method as binder-free Li host [50]. Benefiting from the lithiophilic N/ZnO on the surface, uniform Li deposition was obtained. The porous and robust scaffold accommodated the Li and suppressed the Li dendrite formation. As a result, an ultralong life span of 2000 h with lower overpotentials at 1.0 mA cm 2 was obtained in symmetric cells. In addition, Zheng et al. explored lithiophilic Si nanoparticle coated electrospun CNF as a binder-free Li host [51]. Thanks to the lithiophilic Si coating, the wettability toward molten Li was improved, and the Li metal anode was prepared by the molten infusion method. Such composite exhibited small voltage hysteresis of 90 mV at 3 mA cm 2 with 1 mAh cm 2 over 80 cycles. Furthermore, an ultrafine (15 nm) TiN-decorated CNF network (CNF-TiN) was proposed as a binder-free Li host [52]. Benefiting from the synergetic effect of lithiophilic TiN and a highly conductive network, the uniform Li deposition is endowed. Consequently, a long-running lifespan of 600 h with a small overpotential of 30 mV at 1 mA cm 2 with 1 mAh cm 2 was obtained in symmetric cells.

7. Summary and outlook Through nanocomposite binder-free electrodes showed promising results in Li-S batteries, there are remaining challenges that need to be addressed for the practical application of Li-S batteries. 1. Approach for fabrication of nanocomposite binder-free electrodes frequently involves sophisticated structure design and synthesis, which is complex and time-consuming, limiting their potential for large-scale application. Therefore, a scalable and effective method of fabrication of nanocomposite binder-free electrodes for Li-S batteries should be developed. 2. Operando measurement and theoretical calculation are needed to further study the interaction between the functional materials and S species and the behavior of Li plating/stripping. 3. Since the active materials content, area loading, and electrolyte/sulfur ratio determine the energy density of whole cells, these parameters should be controlled to make Li-S batteries comparable with commercial lithium-ion batteries. On the cathode side, high S content (70 wt%), high sulfur area loading (5 mg cm 2), and low electrolyte/sulfur ratio (4 ul mg 1) should be required. On the anode side, limited Li (300 C), and are used in stationary applications [12]. S is physically isolated in these cells using a solideelectrolyte layer contained in the cathode compartment. In Li cells, if S is not contained in the presence of liquid electrolytes, the products of S lithiation, also known as lithium polysulfides (LiPS), become soluble and can leak from the cathode. This solideliquidesolid transition (S / soluble LiPS/ Li2S) is the S cathode conundrum. The production of LiPS is required to deliver high capacity, but it also leads to a major failure mechanism [13]. Most of the progress made on this technology is to mitigate the shuttle of these soluble LiPS from the cathode. Despite substantial progress in basic research, cell making, and attempted commercialization [14,15], batteries with S cathodes and Li metal as the anode still present serious cycling stability issues and difficulty approaching its theoretical gravimetric capacity. Combining S with carbon and other functional materials has been one of the major attempted methods to solve the insulating nature of S and slow the diffusion of LiPS. Trapping S, controlling electrolytes, or adding barriers to slow the diffusion of LiPS, have led to some success, but not as consistently as when S is directly bonded to a carbon framework. When S is covalently bound to carbon, it is generically called a sulfurized carbon (SC) cathode. Early attempts to produce SC date from 1981 with sulfurization of carbon using SO2 [16]; however, Wang reported the first known high stability SC cathode using elemental S and a polymer in 2002 [17]. Since these early works, this variant of S-based cathodes has gained more attention, and performance has substantially improved [18e20]. SC is a nanomaterial that consists of S species covalently bonded to a carbon framework which does not generate soluble LiPS, the root cause of capacity fading in cathodes based on elemental S. For those reasons, SC cathodes present high stability (>1000 cycles at 70%) with highcapacity retention and low self-discharge. The electrochemical signature of SC when lithiated is distinct from elemental S cathodes; this distinction has produced a wealth of studies to discern the actual molecular structure of SC and the mechanisms for its lithiation. These studies are ongoing. SC cathode technology is advancing on all fronts, from synthesis to nanocompositing and electrode improvement, leading to high S content and higher efficiencies in S utilization. Addressing progress on SC requires an understanding of the SC structure, which has recently been refined through combined theoretical and experimental studies. This chapter is organized to report nanomaterials preparation based on the SC synthesis, emphasizing the structural models and experimental aspects. The chapter is divided into three parts: (1) a synthesis and characterization section that highlights the preparation and properties of SC, (2) the most recent elucidation of SC structure and its associated electrochemical behavior, and (3) progress on SC performance where special emphasis is given to experimental methods that challenge formerly established SC descriptions. We use the term SC to include molecular structures where S is covalently bonded to the carbon-containing framework. Therefore, SC is not restricted here to a single production strategy or a particular type of precursor.

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2. Sulfurized carbon basics The synthesis of SC typically involves the pyrolysis of a nitrogen-containing polymer, such as polyacrylonitrile (PAN), in the presence of an excess of elemental S. The pyrolysis is conducted under N2 or Ar, up to a temperature above the melting point of S, ranging from 200 C to 500 C, for a period of 3e10 h [21e23]. Electrodes are cast over regular aluminum foils or 3-D scaffolds, using either organic solvents such as N-methyl-2-pyrrolidone or water-based slurries [24] with the addition of a conductive additive. SC synthesis produces materials with an S content of 30%e60%, which delivers a range of gravimetric capacities, from 150 to 800 mAh per gram of SC (Fig. 12.1) [25]. Since the carbon and other elements are part of the molecular structure of SC as much as S, reporting gravimetric capacity based on S alone can be misleading. Fig. 12.1 shows the energy density (Wh kg
1) and gravimetric capacity of SC (Ah g
1) versus the reported S content from 50 selected works in the literature of SC, covering almost 20 years. Most of the works use PAN as the carbon source. Gravimetric capacity was extracted from the literature at a 0.1Ce0.2C discharge rate. The cathode energy density was computed using a discharge voltage of 1.85 V, representing the average value found in the SC literature, which ranges from 1.8 to 1.9 V versus Li/Liþ after the first discharge. To compare SCs with Li-ion cathode technology, two generic LMO cathodes were considered, an NMC-type, 200 mAh g
1 at 3.9 V, and a

Figure 12.1 Cathode energy density (Wh kg
1) and gravimetric capacity (mAh g
1) versus sulfur (S) content in sulfurized carbon (SC). Reported values from 50 works in the SC literature. To compute the cathode energy density, the measured gravimetric capacity of SC was multiplied by 1.85 V, an average discharge voltage for SC, and lithium mass was included. For comparison purposes, red and blue dotted lines show the calculated energy densities of generic lithiated metal oxide (LMO) cathode versus lithium (Li) metal.

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LiFePO4 cathode, 180 mAh g
1 at 3.2 V. For a fair comparison with these LMO cathodes, Li mass was included in the energy density calculation of SC, assuming a Li2S stoichiometry for fully lithiated SC. Naturally, a trend of higher capacities is observed with higher S content. Nonetheless, a sensible difference in capacity is observed for the same S content, indicating variable S utilization, mostly in the 40%e50% S content range. Even if SC is prepared using similar precursors and temperatures, it can deliver different capacities, which indicates a high level of sensitivity to the preparation conditions. In general, all SC cathodes display high stability (>90% capacity retention over 100 cycles) independent of the preparation method, although the stability may be linked to the choice of electrolyte and Li metal anode stability. The numbers in Fig. 12.1 indicate that despite the normally assumed low S content in SC, improvements in S content and S utilization are evident, to the extent that SC delivers competitive energy densities compared to state-of-the-art LMO cathodes. The room for improvements is clear. A theoretical S cathode containing 80% S content with 80% S utilization relative to S theoretical gravimetric capacity (marked with a star in Fig. 12.1) delivers nearly double the energy density of an LMO high-performance cathode. A few studies have shown that synthesis at lower temperature ranges, 250 Ce300 C, than the boiling point of S, 444 C for a-S8, produce SC with higher S content but reduced cycling stability [26,27]. At temperatures higher than 400 C, S content in the range of 35%e40% produces stable electrochemical cathodes in carbonate-based electrolytes. The content measured by elemental analysis may also include elemental S condensed on SC, which can be removed by rinsing with toluene or by Soxhlet extraction [28]. The temperature of pyrolysis is linked to different SC structures as observed by time-of-flight secondary ion mass spectrometry [26,28] but also can influence the average size of the S chains bonded to the carbon framework [29]. Higher temperatures are linked to a higher degree of graphitization in the carbon framework, thus enhancing the electrical conductivity of SC chains [30]. SC conductivity, either before or after lithiation [31,32] (10
4 S cm
1), is higher than S (10
30 S cm
1) and PAN (10
12 S cm
1) [18]. Synthesis time affects S content, but few studies addressed the effects on the electrochemical properties. Longer synthesis times are linked to a higher degree of dehydrogenation and S loss. A synthesis time of 2e4 h was indicated as optimal, resulting in similar SC [33]. There are no magnetic studies on SC and extremely limited studies on the mechanical properties. During lithiation, an expansion of 22% was observed in SC electrodes during the first cycle at capacities of w4.8 mAh cm
2, which is attenuated to a reversible w12% in the subsequent cycles [34]. When SC powder is converted to electrode films, the electrode density can reach w0.6e0.8 g cm
3. Higher synthesis temperatures decrease SC powder density and increase surface area [31]. With proper compression, electrodes can reach densities of w1.5 g cm
3, enabling cathodes with volumetric capacities above 1000 mAh cm
3 [35], thus surpassing LMOs with capacities in the range of 500e800 mAh cm
3 [36]. Although the densities in SC electrodes are comparable to elemental S cathodes, the volume expansion operates over different materials [37]; the lithiation on SC is distributed through small S chains, while lithiation in S/C induces a local expansion of the small S8 crystals.

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Methods to mix S and the precursor polymer reportedly influence the quality of the resulting SC. For instance, the mixture between S and PAN is not sufficiently well characterized to understand the fate of the precursor polymer or S after mixing. Differential scanning calorimetry shows that cyclization of PAN occurs at lower temperatures when S is present [38]. S and PAN mixtures can be hand mixed [39], milled [40], milled and then compressed into pellets [31], or PAN can be produced as fibers by electrospinning and later sprinkled with S to produce SC fibers, which eventually lead to the production of freestanding SC [41,42]. Acrylonitrile can be polymerized in the presence of S [43]. S can also be infused by using S solutions in carbon disulfide [44]. Since the PAN molecular weight is known to affect the final quality of the material, it is suggested that the S and PAN mixing method may influence the precursor by affecting the PAN characteristics [45]. Several studies have paid particular attention to the polymer precursor, typically a nitrogen-containing polymer such as PAN. In addition to PAN, polyaniline (PANI) [46,47], polypyrrole (PPy) [48], and covalent triazine frameworks [49,50] have shown cycling stability and S content comparable to SC from PAN. When PANI and PPy were used, these polymers were not pyrolyzed in the presence of S; instead, S replaced chlorine in chlorinated analogs of PANI and PPy, resulting in higher S content but inferior cycling stability. Products resulting from direct pyrolysis of PANI and S have demonstrated high stability and distinct electrochemical profiles during discharge [51e53], which could be associated with the ability of PANI to be reorganized to polyaromatic structures when compared to PAN [30]. Interestingly, SC prepared using polymers or molecules with no nitrogen, such as polyethylene glycol [54] and primary alcohols (1eCnH2nþ1OH; n ¼ 5e10) [55], do not produce as stable cathodes even at similar or higher S content. S-containing copolymers with much higher S content can be produced using styrene monomers in the presence of polysulfide radicals by an inverse vulcanization process; however, they display more pronounced capacity fading [56,57]. In the case of S and PAN mixtures, the polymers are decomposed in the presence of S. In these examples, the higher nitrogen content may enable not only SeC bonds, but also SeN bonds, or nitrogen can participate in the lithiation reaction, as recently proposed by experimental and theoretical studies [58]. Alternatively, nitrogen can induce different interactions with oxygenated groups to enhance S incorporation [59]. The nitrogen is important even if the precursor is not a polymer. Carbonized PAN can be used as a precursor to prepare SC with similar S content as if PAN was used [28,60]. However, the reaction is dependent on the pyrolysis temperature. There is limited research on the S purity needed to produce SC; generally, highly purified sublimated S is used for the reaction in mass ratios of 3e60 to the precursor polymer [26]. In electrode preparation, it was demonstrated that polyacrylic acid (PAA) binders are more compatible with SC cathodes than PVDF, despite most cathodes being prepared using PVDF. The carbonyl groups of PAA might interact more efficiently with the SC surface and current collectors, producing fewer cracks after cycling and little to no delamination from the aluminum foil [61]. In terms of electrolytes for rechargeable

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batteries, SCs are versatile in accepting both carbonate and ether-based electrolytes, although less stability is generally reported using ethers, such as 1,2dimethoxyethane (DME) and 1,3-dioxolane (DOL) with LiTFSI or LiFSI salts [22,29]. Solvent-in-salt electrolytes based on DME/DOL (4 M LiFSI in DME) do not result in the same electrochemical instability compared to electrolytes at low concentration (1 mol L
1) [62]. Elemental S cathodes are not compatible with carbonates such as ethylene carbonate (EC) and diethyl carbonate (DEC) because the generated LiPS react with carbonate solvents [63]. In addition, polysulfides can also substitute fluorine if LiPF6 salt is used [64]. In general, SC cathodes are highly compatible with carbonate electrolytes; the normal compositions of LiPF6 in 1:1 EC:DEC produce cathodes with good electrochemical stability attributed to the solidesolid conversion of S chains to Li2S and little to no dissolution of LiPS. Since the stability of Li metal anodes is highly dependent on the electrolyte, the observed capacity fading in lithiumesulfurized carbon (LieSC) cells can also be attributed to the Li failure, not necessarily SC. In terms of spectroscopic and physical characterization, SC has a distinctive signature that does not vary substantially from each preparation method. Raman spectroscopy of SC consistently shows the presence of sp2 domains, as seen by broad D and G bands (1360 and 1580 cm
1, at 1.54 eV) and a pronounced ID/IG ratio [65]. Less reported is that SC also displays a G’ band at w2740 cm
1, characteristic of graphite stacking [26]. Modes related to S are found in 300 (in-plane CeS), 370 (CeS), 473 (SeS), and 936 cm
1 (S ring deformation) [22,66]. The spectra are distinct from elemental S electrodes after melt-diffusion. FTIR spectra of SC enable more direct comparison with the precursor PAN, highlighting the C-N bonds and the evolution of CeS bonds based on the degree of polymer decomposition [22,67,68]. Specific band ratios in FTIR spectra can be used as the temperature signatures used in the synthesis [27]. UV-Vis spectra of SC are rarely reported but indicate a broad absorption related to the SC conjugated structure [68]. Thermogravimetric analysis (TGA) characterization shows a mass loss above 600 C, indicating CeS cleavage (740 kJ mol
1). Mass losses at lower temperatures ( 2) does not seem to affect SC electrochemical stability despite the expansion and even allows its use as an anode with extra capacity [83,84]. The three-dimensional aspect of SC is overlooked in SC models because of its complexity. These models do not consider the secondary organization resulting from the twisting, coiling, or stacking of SC chains that can lead to the production of intrinsic pores in the SC particle [85,86]. These three-dimensional structures are

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Figure 12.4 Chemical modification pathway of sulfurized carbon (SC) chains during lithiation. Modifications are followed by the corresponding galvanostatic charge/discharge curve. Reproduced with permission of Elsevier from C.-J. Huang, J.-H. Cheng, W.-N. Su, P. PartoviAzar, L.-Y. Kuo, M.-C. Tsai, M.-H. Lin, S. Panahian Jand, T.-S. Chan, N.-L. Wu, P. Kaghazchi, H. Dai, P.M. Bieker, B.-J. Hwang, Origin of shuttle-free sulfurized polyacrylonitrile in lithiumsulfur batteries, J. Power Sources 492 (2021) 229508.

the basis for alternative explanations on the first cycle irreversible capacity, where a cathodeeelectrolyte interphase (CEI) is supposedly formed over the pores of SC from the electrolyte decomposition. BET characterization of SC indicates a surface area of 8e20 m2 g
1 and micro/mesoporous structure (0.1e3 nm pore size) [67,87]. During CEI formation, CeS bonds of SC are cleaved, but the CEI seals the pores with an insoluble ion-conductive layer, which traps both Li2S and later S species. Several reports support this model by merely trapping elemental S in small pores (w0.5 nm) [88,89]. The resulting electrochemical response is similar (one-plateau

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Figure 12.5 (A) Molecular model of lithiated sulfurized carbon (SC) with Li/S ratio ¼ 2.9. (B) Volume change and density versus Li/S molar ratio of SC. At Li/2e2, which corresponds to discharge to 1.0 V versus Li/Liþ, the expected expansion for an SC with 42% S content is nearly 56%. Reproduced with permission of ACS from S. Perez Beltran, P.B. Balbuena, Sulfurized polyacrylonitrile (SPAN): changes in mechanical properties during electrochemical lithiation, J. Phys. Chem. C 125 (2021) 13185e13194.

discharge profile), and the production of soluble LiPS is avoided as much as in SC produced by S and PAN mixture pyrolysis. The rationale behind this design is that S becomes trapped in pores that are sufficiently small to avoid generating larger LiPS (Li2S4, Li2S6) or to be solvated by larger anions [89]. Ab initio experiments demonstrated that the main difference between the one- and two-plateau models might be the solvated S species [90]. Experimentally, this approach was achieved by several methods, such as melting infusion into carbon nanotube (CNT) inner cavities, coating electrodes with evaporated films, and S precipitation, among others [78]. The argument for the existence of a CEI supports the well-known behavior of SC in terms of electrolytes. LiPS can be formed within the pores and react, producing an insoluble layer sealing the pore; this insoluble layer could be produced more easily using carbonates rather than ether-based electrolytes [20,23,91]. However, experimental and theoretical studies argue that typical carbonate electrolytes used in SC cathodes present high stability in the electrochemical window typically used in SC testing (1e3 V, vs. Li/Liþ) and should not produce a CEI [92]. Some carbonate electrolytes can produce a CEI, but they must be discharged to nearly 0 V (vs. Li/Liþ) to decompose solvents into a thin CEI over S particles, thus enhancing stability. For instance, Lee et al. reported that deep discharging (to 0.1 V, vs. Li/Liþ) regular elemental S cathodes in FEC effectively produced a CEI layer, therefore protecting and trapping S species. The electrochemical signature of the resulting cathode presents the typical one-plateau discharge, and the cathode was stable for more than 1000 cycles without LiPS generation [93]. Alternatively, recent DFT studies simulated that carbon of the SC chain could react with solvents from the electrolyte, forming CeO bonds during dehydrogenation [81]. The establishment of a comprehensive model for SC is still under development.

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4. Recent progress of sulfurized carbon and future trends Research in SC has progressed on many fronts, not only in increments of S content and utilization but also in modifications in the synthetic procedure. These modifications can produce SC nanomaterial with properties that can potentially solve SC limitations, such as low energy efficiency and discharge voltage. These advancements are reviewed and critically analyzed in this last part, considering its potential to turn SC into functional electrodes.

4.1

Sulfurized carbon morphology

Commercial LMO and graphite have defined dimensions and shapes designed to maximize compactness and optimize other features, such as power density and electrolyte ratio [94e97]. This last one is a crucial factor for high energy density. The synthesis of SC type has established a mixture between a precursor (typically PAN) and S; however, a direct relationship between the SC particle size distribution and the parent precursor was never evaluated since both particles and agglomerates are reported. A direct association is found when PAN is produced as fiber with a well-defined diameter and length. Frey et al. reported using 30 mm diameter PAN/PMMA textile fibers as a source of SC [69]. The resulting SC maintained the same shape after pyrolysis. Although the PMMA was eliminated during the pyrolysis process, the blending with PAN reduced its crystallinity, which facilitates S incorporation. Similarly, a blend with polystyrene (PAN/PS) produced pores in the SC fibers, which enable more effective incorporation of active material, in that case, SeS2 [67]. Carbon fibers and CNTs can be mixed and blended with PAN, producing unique morphologies such as embedded CNTs in SC agglomerates or carbon-fiber-infused SC [98,99]. One of the main restraints for LieS achieving high energy density is using a minimal amount of electrolyte. A large intake of electrolytes is required if cathode porosity is high, which substantially reduces the energy density of LieS batteries [100]. Kim et al. reported that SC could be sintered (Fig. 12.6A). The typical mixture of S and PAN was compressed into pellets after mixing, which were later pyrolyzed into binder-free SC electrodes. Since SC was sintered, the empty volume fraction in these electrodes was reduced, enabling the cathode to operate at a reduced electrolyte ratio [31]. In another example, SC cathodes using cross-linked guar gum binder reduced the porosity of electrodes from 52% to 13% during compression, reaching a density of w1.5 g cm
3, minimizing the electrolyte intake (Fig. 12.6B) [35]. SC shapes can also be controlled by infusing the polymer precursor in templates. Liu reported that acrylonitrile could be polymerized in mesoporous silica (SBA-15) followed by pyrolysis with excess S [101]. The resulting mesoporous SC has welldefined shapes and enables a high S content compared with regular S and PANbased synthesis. The interesting aspect of this work is that SC particles of 20e50 nm were produced that deliver impressively high rates at high capacities.

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Figure 12.6 (A) Discharge voltage of conventional sulfurized carbon (SC) electrode (SP conventional), compact SC electrodes (SP450 compact), and SC electrodes obtained from S and PAN mixture as pellets (SP450 and SP600). (B) Scheme of electrode morphology. (C) SEM images of SC electrodes before and after applying pressure. (D) Increment of volumetric capacity (mAh cm
3) based on electrode compression measured by density. (A-B) Reproduced with permission of RSC from H. Kim, C. Kim, M.K. Sadan, H. Yeo, K.-K. Cho, K.-W. Kim, J.-H. Ahn, H.-J. Ahn, Binder-free and high-loading sulfurized polyacrylonitrile cathode for lithium/sulfur batteries, RSC Adv. 11 (2021) 16122e16130. (C-D) Reproduced with permission of Elsevier from J. Chen, H. Zhang, H. Yang, J. Lei, A. Naveed, J. Yang, Y. Nuli, J. Wang, Towards practical LieS battery with dense and flexible electrode containing lean electrolyte, Energy Storage Mater. 27 (2020) 307e315.

4.2

Nanocomposites with sulfurized carbon

In this strategy, nanostructures such as carbon nanomaterials are added to the S and PAN mixture and later become part of the SC particles. Many works have demonstrated similar results, an increment in conductivity, and higher S utilization than regular SC obtained by powder processing. Recent reviews cover this type of nanocomposite [18,20]. For instance, PAN fibers containing CNTs were produced by electrospinning and converted to SC fibers with CNT embedded in its structure [76,102]. Interestingly, Li2S nanoflakes, identified by ex situ XRD and SEM, were observed at the surface of the SC fiber when fully lithiated. The addition of moleculare organic framework with CoS2 on SC shows improved Li uptake performance, producing stable capacities exceeding 8 mAh cm
2 [103]. It is speculated that since both

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materials have lithiation processes in the same voltage range, CoS2 acts as a redox mediator (RM). SC has also benefited from the use of 3-D substrates [24,104]. Liu demonstrated that SC coated on hollow 3-D graphene structures are sandwiched to produce high areal capacity electrodes. Cathodes with more than 19 mAh cm
2 were demonstrated, one of the highest values with SC. Although the absence of metals is one of the advantages of S cathodes, the addition of small quantities of Fe has demonstrated improved functionality. Haridas et al. added Fe2O3 to the S and PAN mixtures, resulting in FeS/SC nanocomposites, stable in ether- and carbonate-based electrolytes [105].

4.3

Polymer precursor modification

It is known that branched PAN results in structural defects, such as pores, when converted to carbon fibers [71], which can compromise mechanical properties. This same effect can be beneficial to incorporate more S and increase utilization of PAN active sites. Using a similar rationale, Lei et al. reported that using a cross-linked PAN with S resulted in SC with higher S content and better S utilization [106]. The PAN was cross-linked with hydrazine and had a higher surface area, which the authors attribute as the reason for much higher S content (54%) compared to non-cross-linked SC (42%) prepared under the same synthetic conditions (Fig. 12.7A). The cathode

Figure 12.7 (A) Production of sulfurized carbon (SC) using cross-linked polyacrylonitrile (PAN). (B) Scheme of producing O and N-rich environment for SC with more active sites for S bonding. O-bonds are provided by the co-pyrolysis of PAN and perylene-3,4,9,10tetracarboxylic dianhydride. (A) Reproduced with permission of ACS from J. Lei, J. Chen, A. Naveed, H. Zhang, J. Yang, Y. Nuli, J. Wang, Sulfurized polyacrylonitrile cathode derived from intermolecular cross-linked polyacrylonitrile for a rechargeable lithium battery, ACS Appl. Energy Mater. 4 (2021) 5706e5712. (B) Reproduced with permission of NAS from C. Luo, E. Hu, K.J. Gaskell, X. Fan, T. Gao, C. Cui, S. Ghose, X.-Q. Yang, C. Wang, A chemically stabilized sulfur cathode for lean electrolyte lithium sulfur batteries, Proc. Natl. Acad. Sci. U.S.A. 117 (2020) 14712e14720.

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delivered >800 mAh g
1 based on the SC mass, which is one of the highest reported for SC. Adding a molecular additive to the S and PAN mixture is an effective method to modify the number of active sites to bind with S. Luo et al. reported that adding perylene-3,4,9,10-tetracarboxylic dianhydride to the regular S and PAN mixture could produce both CeS and SeO bonds that increased the S content to 60% [63]. SC cathodes delivered capacities at w600 mAh g
1 (1e3 V, vs. Li/Liþ) based on SC mass at >5 mAh cm
2 with high stability for 200 cycles (Fig. 12.7B).

4.4

Sulfurized carbon doping

Adding heteroatoms in the small S chains can be a potential method to reduce the high hysteresis of SC and increase energy efficiency. One natural modification is the use of other chalcogens, Se and Te. Despite its similarity with S, Se and Te have higher density and conductivity and can produce eutectic mixtures with S that facilitate SC production using the same methods [65,67]. SeSx and PAN mixtures resulted in SC with enhanced performance in rate and active material content. Similar results were obtained in Te-doped SC (Te0.04S0.96@pPAN) in Li and Na batteries [107,108]. UVVis measurements indicated that Se- and Te-doped SCs were more compatible with ether-based electrolytes than regular SC (SPAN). Galvanostatic intermittent titration measurements indicated that the dopants could substantially increase the lithium diffusion in SC and thus accelerate the redox reaction, minimizing the production of soluble LiPS. The incorporation of small quantities of Te in SC (Te0.045S0.955PAN) resulted in higher discharge voltage (w1.9 V) compared to regular SC (1.85e1.88 V) [109]. SC with approximately 1:1 S:Se proportions produced cathodes with higher rate capabilities, lower voltage hysteresis, and higher active material content (w60%) [110]. Completely selenized carbon produced lower Se content dependent on the synthesis temperature and did not deliver a capacity as high as SC [111,112]. Coheating an S and PAN mixture with iodine demonstrated that ‘I’ could participate in the CEI as LiI, improving the ionic conductivity of SC [113]. The drawback of using Se or Te is that their toxicity lessens most commercial-scale interest, especially when compared to the very low toxicity of S.

4.5

Redox mediators

RMs are a class of electroactive molecules that display reversible redox behavior and can improve the S utilization of SC. An RM is typically used when a material is composed of larger particles or agglomerates, and there is a large resistance to electrons reaching the core of the particle, where the active material is present and underutilized. RM can be reduced/oxidized on conductive surfaces, diffused in the electrolyte, used to transport charges to these areas of difficult access, and be recycled in sequence. In general, SC has this kind of morphology with particles and agglomerates, in which active material is found at its core but not connected electrically. There is limited information regarding Li diffusion in single SC particles. Special morphologies

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such as hollow SC fibers or SC fibers with CNTs have demonstrated superior performance based on the assumption that electrolyte and conductive materials can reach a larger area of the SC material [65,76,114]. There are very few reports of RM and SC. Chao et al. demonstrated that a modified anthraquinone (1,5-bis(2-(2-[2methoxyethoxy]ethoxy)ethoxy)anthra-9,10-quinon (BEAQ)) added in either carbonate or ether-based electrolyte reduces the polarization of SC cathodes (Fig. 12.8A and B). The authors demonstrated the positive RM effect even when no carbon additive was present in the SC electrode [115]. BEAQ acts by assisting the delithiation process of lithiated SC. In another work, Li et al. added soluble LiPS (Li2S8) to the electrolyte with SC cathodes [116]. LiPS acted as an RM during the charge reaction, thus producing two competing pathways for converting Li2S by reforming either CeS bonds in SC or S8 in the LiPS. The addition of small amounts of LiPS led to a 10% increase in capacity, verified for over 400 cycles.

Figure 12.8 (A) Proposed redox mediator (RM) mechanism in sulfurized carbon (SC) cathodes. (B) Cyclic voltammograms of Li|SC and Li|BEAQ. Reproduced with permission of Wiley-VCH from C.-X. Zhao, W.-J. Chen, M. Zhao, Y.-W. Song, J.-N. Liu, B.-Q. Li, T. Yuan, C.-M. Chen, Q. Zhang, J.-Q. Huang, Redox mediator assists electron transfer in lithiumesulfur batteries with sulfurized polyacrylonitrile cathodes, EcoMat. 3 (2021) e12066.

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Liquid electrolytes and sulfurized carbon

The well-known compatibility of SC with either carbonate or ether-based electrolytes in certain conditions must also be evaluated in terms of the anode stability, which is normally Li metal. The choice of electrolyte needs to produce a stable solid electrolyte interphase (SEI) on Li metal, enabling a LieSC cell to operate without dendritic growth, which tends to require a large excess of Li metal and a large excess of electrolyte to compensate for the low anode electrochemical reversibility. In terms of SC, the same choice of electrolyte needs to guarantee high cycling stability in the SC, which has been abundantly demonstrated, but it also increases the S utilization while minimizing the electrolyte required to wet all inner surfaces of SC. Therefore, the rational choice of liquid electrolyte (salts, solvent, and additives) needs to balance the effects on both SC and Li, which is necessary to achieve high gravimetric and volumetric energy density. In addition to several reviews on electrolytes for Li metal anodes [117,118], Chen et al. recently prepared a comprehensive background review of electrolytes specifically designed for LieSC systems [119]. A good example was recently provided with fluorinated additives and LieSC. Electrolytes with high salt concentration are known to mitigate the production of dendrites in Li metal but, with few exceptions, these electrolytes resut in high viscosity liquids. Based on the work of Mayer et al., the capacity of SC decreases as electrolyte viscosity increases, which is supported by temperature studies [120]. The addition of fluorinated cosolvent (1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether) in 6 mol L
1 LiFSI/DME produced a low viscosity electrolyte that could wet both a SC cathode and enable Li metal with good reversibility (99.4% coulombic efficiency). These same conditions allowed the cell to operate with low electrolyte ratio (2 mL mg
1 of S) [121]. With the same rationale, Cai et al. studied the effect of methyl propionate additive in LieSC cells to increase compatibility with both anodes and cathodes, achieving LieSC cells that operate at
40 C with w80% retention of the capacity obtained at room temperature [122]. Regarding Li metal stability, SC films can also be used as artificial SEI, enabling the deposition of dense columnar Li films with no production of dendrites [123].

4.7

Solid electrolyte and sulfurized carbon

Solid electrolytes have become one of the potential alternatives to enable functional lithium metal anodes in cells. Because of the inherent safety advantages, solid electrolytes have shown promising ability to control the formation of dendrites given that some conditions are satisfied, such as compactness, stable interface with Li metal, absence of voids, and good mechanical stability [124]. Using S cathodes, solid electrolytes can also provide a compartmentalized cell structure, keeping the LiPS only on the cathode side of the cell, thus avoiding the shuttle of those species. In the most typical assembly of a solid-state electrode, S, conductive additive, and the solid electrolyte powder are mixed and turned into a pellet [125e127]. This method does not afford a good contact, and S utilization is low. Adding a combination of solid and liquid electrolytes enables better contact of ions with active material. In addition, the volume expansion of S when converted to Li2S can lead to a local pulverization

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of the solid electrolyte matrix, a process also known as chemo-mechanical failure [128], making the material nonviable for further lithiation. With these considerations, SC carbon material presents an excellent option for solid-state batteries since the C/N framework of SC can buffer the volume expansion, and LiPS issues are not a problem. Attempts to incorporate solid-state electrolytes with SC cathodes are rare. In general, an assisting gel-polymer electrolyte mediates the contact between the SC electrode and Li metal (Fig. 12.9A and B) [129e131]. In these works, a combination of polymer and Li salt (PEO and LiTFSI) are mixed with SC and cast over current collectors. Solidstate electrolyte (Li3.25Ge0.25P0.75S4 - LGPS) or polymer electrolyte (PEO-LiBH4)

Figure 12.9 Examples of solid electrolyte and sulfurized carbon (SC) cathodes. (A) Scheme of cell construction using cathode with liquid electrolyte (CLE) and cathode with gel polymer electrolyte (CGPE); (B) Corresponding cycling stability in cells. Specific capacity reported in SC mass. (C) Scheme of SC cathode construction with solid electrolyte using typical construction and added polymer electrolyte with conductive polymer framework; (D) Corresponding cycling stability. Specific capacity reported in S mass (SCs has 42 wt% S). (B) Reproduced with permission of Elsevier from Y. Liu, D. Yang, W. Yan, Q. Huang, Y. Zhu, L. Fu, Y. Wu, Synergy of sulfur/polyacrylonitrile composite and gel polymer electrolyte promises heat-resistant lithium-sulfur batteries, iScience 19 (2019) 316e325. (D) Reproduced with permission of Wiley-VCH from M. Li, J.E. Frerichs, M. Kolek, W. Sun, D. Zhou, C.J. Huang, B.J. Hwang, M.R. Hansen, M. Winter, P. Bieker, Solid-state lithiumesulfur battery enabled by thio-LiSICON/polymer composite electrolyte and sulfurized polyacrylonitrile cathode, Adv. Func. Mater. 30 (2020) 1910123.

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can be placed as an electrolyte layer on top of the cathode (D) [132]. The LGPS is embedded in PEO and a plasticizer (Pyr1,4TFSI), as shown in D, but contact with SC is limited. Depending on the liquid electrolyte, polymeric membranes (PVDF/ PMMA, PEG) have higher stability compared to the pure liquid electrolyte [87,129]. In these works, higher temperatures were tested (60 C), and the polymer electrolyte helped produce more stable Li metal anodes, which can support the enhanced electrochemical stability. The introduction of selenium has also induced better compatibility of SC with solid electrolytes [133].

4.8

Other organosulfur cathodes

An early paper on SC suggested that the maximum content of S in SC produced by PAN is 56% [33], based on gravimetric studies of the synthesis. S content has significantly improved in recent years, enabling SC cathodes to outperform LMO and elemental S cathodes, at least in gravimetric energy density. Higher S content is required to outperform LMO in terms of volumetric energy density [134]. Looking at S content only, S copolymers have reported much higher S content, still including the CeS/SeS bonding framework. These S copolymers are typically obtained by an inverse vulcanization process, originally described by Pyun [135]. Vulcanization is the cross-linking of polymer chains using a small quantity of S, while inverse vulcanization involves the same cross-linking but with a high mass proportion of S and a polymer or other compound in a much smaller quantity [57]. Achieving high S content in this process is simple; however, cathodes become unstable because of the LiPS shuttle when the CeS bonding framework is cleaved at the lithiation process. There are several examples of S copolymers. Carbyne polysulfide structures were produced by heating S and dehydrochlorinated polyvinylidene chloride at 350 C, resulting in SC with 54% of S [136]. TGA, Raman, and Fourier-transform infrared (FTIR) spectroscopy indicate a molecular framework composed of C-Sx-C (2 < x < 4) in which the same one-plateaued discharge profile is observed in both carbonate and ether electrolytes. Similar behavior was observed when S was mixed with phenol-formaldehyde resin [137]. According to the authors, the random nature of bonds in the resin was important to produce a three-dimensional copolymer with S2eS3 chains. Based on the temperature control of pyrolysis, the S chain size could be regulated. Yan reported that higher S contents of 71% and 90% were achieved by treating S with poly(tetrafluorohydroquinone) and tetrafluoro-p-benzoquinone, which can be processed at lower temperatures in the range of the melting point of S (w200 C). A high capacity of w900 mAh g
1 was measured in both coin and pouch cells [138]. S copolymers with graphene (90% S content) could also demonstrate exceptional stability attributed to the presence of CeS mediated by graphene sheets [139]. An interesting approach was to covulcanize polymer (PAN) and other compounds to achieve higher S content. With this strategy, S can be incorporated as small chains on the SC, and the added monomers can create a parallel network with S, thus increasing the overall S content. Chen et al. reported using vulcanization accelerators (2-mercaptobenzothiazole) in the S and PAN mixture pyrolysis, which increased the S

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Figure 12.10 (A) Schematic diagram of covulcanized sulfurized carbon (SC) containing sulfur (S) bonded to SC and polysulfides from the vulcanization agent. (B) Charge and discharge curves (at 0.1C) of covulcanized SC with diphenyl guanidine (DG), high-speed zinc N-ethylN-phenyldithiocarbamate (ZDB), tetramethylthiuram disulfide (TD), 2,2dithiobis(benzothiazole) (DBB), ethylenethiourea (ET) and 2-mercaptobenzothiazole (MBTs). Reproduced with permission of Elsevier from Y. Wang, Y. Shuai, K. Chen, Diphenyl guanidine as vulcanization accelerators in sulfurized polyacrylonitrile for high performance lithium-sulfur battery. Chem. Eng, J. 388 (2020) 124378.

content in SC by 8% [38]. Wang et al. demonstrated that diphenyl guanidine could increase the S content, producing an SC with 52% of S, 15% higher than SC prepared under the same conditions with only PAN (Fig. 12.10A and B) [140]. A high capacity of w850 mAh g
1 was delivered during 200 cycles. In another related work, the authors replaced S with a S copolymer in the reaction of PAN to produce SC [40]. The copolymerization of S was made in the presence of p-benzoquinone dioxime (PBD), and PAN was confined inside a microporous carbon. Using the S copolymer instead of pure S in the synthesis resulted in SC carrying small S chains already containing the PBD as a cross-linking agent, which resulted in more robust CEI and higher electrochemical stability.

5. Concluding remarks SC cathodes have achieved sophistication and performance that rival regular elemental S cathodes beyond their well-known cycling stability. The S content, utilization, and functionality of electrodes have been expanded. We highlight experiments and theoretical studies on the participation of the C/N framework in the lithiation mechanism, which has been well-received and should be strengthened to support new methods of SC preparation. A more adequate understanding of the three-dimensional structure of SC is also necessary to design high-power electrodes and enable the minimal use of liquid electrolytes, as has recently been addressed. The 3-D structure can also be an opportunity for increased S content based on pore volume, as incipient works have started to suggest.

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Grapheneesulfur composite cathodes

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Runwei Mo School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, Shanghai, China

1. Introduction Given sulfur’s low cost, safety, and abundance, lithiumesulfur (LieS) batteries are considered promising candidates for future advanced energy-storage devices [1e5]. Compared with commercial lithium-ion batteries, LieS batteries exhibit a higher energy density (as high as 2600 W h kg 1) mainly due to the conversion reaction between lithium and sulfur [6e8]. Therefore, replacing commercial lithium-ion batteries with LieS batteries as energy-storage batteries for electric vehicles can greatly increase driving distance. LieS batteries must be fully developed to solve the problem of traditional energy depletion, which could also greatly promote the rapid development of electric vehicles. After years of development, LieS batteries have made great progress [9e15]. However, the current electrochemical performance of LieS batteries has failed to meet commercialization requirements. To improve the electrochemical performance of LieS batteries, researchers in recent years have carried out a variety of optimization studies. One of the mainstream research directions is to combine sulfur and lithium metal with composite substrate materials with excellent mechanical strength and high conductivity. Among them, graphene has a high specific surface area, high conductivity, and outstanding mechanical strength, so it has attracted much attention as a composite substrate material. In terms of crystal structure, graphene is a two-dimensional material consisting of a layer of sp2 bonded carbon atoms [16e18]. The electron mobility at room temperature is 15,000 cm2 V 1 S 1, and the theoretical surface area is 2630 m2 g 1 [19e22]. Graphene also has outstanding flexibility that can alleviate the volume change in the sulfur electrode during the electrochemical reaction and provide the necessary conditions for the production of flexible electrochemical energy-storage devices [23e26]. From the synergistic effect of the composite, graphene as an ideal matrix material could construct an excellent conductive network structure for the sulfur electrode, which can greatly increase the utilization rate of the sulfur electrode during the process of electrochemical reaction. In addition, graphene can achieve multiple assembly methods through the regulation of interface properties. On the one hand, the structure obtained by assembly exhibits a high specific surface area, and on the other hand, a continuous multilevel micro-nano pore structure is constructed, which can inhibit the diffusion of polysulfides during the charging and discharging process [27e30].

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00012-0 Copyright © 2022 Elsevier Inc. All rights reserved.

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A composite grapheneesulfur electrode design is an effective strategy to improve the electrochemical performance of LieS batteries. The composite design of two or more materials could solve the problem of sulfur, thereby realizing an electrochemical energy-storage battery with high power, high capacity, and long cycle life. Therefore, to achieve the above research goals, it is necessary to make key breakthroughs are needed in the large-scale manufacturing process and innovative structural design. This chapter systematically introduces the challenges limiting the development of LieS batteries and the latest research progress of graphene-based composites in the LieS batteries. It is worth noting that the innovative structure and effective strategy of grapheneesulfur composite electrode construction are introduced with emphasis. This lays the foundation for the application of commercial LieS batteries in the future.

2.

Challenges limiting the development of lithiumesulfur batteries

This section focuses on several challenges that limit the development of LieS batteries, including (1) the insulating properties of sulfur substances, which lead to poor rate performance and irreversible capacity loss during long chargeedischarge cycles [31]; (2) lithium polysulfide’s solubility in water in the electrolyte, which produces a shuttle effect leading to poor coulombic efficiency and a short cycle life during the electrochemical reaction [32]; (3) the tremendous volume change in sulfur in the lithiation process, which leads to rapid capacity decay and electrode degradation [33]; and (4) lithium dendrite formation during the chargeedischarge process, which can easily short-circuit the battery and thereby pose a safety hazard [34]. The combined effect of the above factors has led LieS batteries to have poor coulombic efficiency, low reversible capacity, rapid capacity decay, and poor safety performance [35,36]. Effective strategies have been studied in recent years to overcome the above problems, including (1) combining sulfur substances with various conductive materials to reduce the large internal resistance [37]; (2) mixing sulfur with fixed soluble polysulfide additives to inhibit its “shuttle effect” [38]; (3) Structural design to limit the volume expansion of sulfur, thereby stabilizing the electrode architecture [39]; and (4) The structure design of the solid-liquid interface realizes the uniform deposition of lithium metal and forms a stable solid electrolyte interface, thereby improving the safety of the battery [40]. Nevertheless, many scientific and engineering problems still must be solved to realize the commercialization of LieS batteries.

3.

Graphene-based composites in the lithiumesulfur batteries

3.1

Graphene oxideesulfur composite cathodes

Graphene oxide (GO) has been proven to be an excellent matrix material for anchoring electrochemically active substances in electrode composites [41]. GO has a high

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dispersibility in various solvents, making it possible to realize hybrid materials of various shapes and sizes during the compounding process [41]. Therefore, GO is considered an ideal matrix material for compounding with sulfur [42e46]. In 2019, Mo’s group realized a variety of composite structures of sulfur nanoparticles and graphene through experimental design, in which sulfur nanoparticles exhibit different dispersity and diameters [42]. The results prove that S NPs are firmly and uniformly fixed on the surface of graphene nanosheets, and there is no obvious aggregation phenomenon. It can be seen through electron microscopy that the size of sulfur nanoparticles uniformly compounded on graphene is in the range of 5  3 nm. The composite was used as the cathode of LieS batteries for the electrochemical performance test. After being cycled for 500 cycles at 0.5 A g 1, its reversible specific capacity remains as high as 1058 mAh g 1. The excellent electrochemical performance of the composite electrode is mainly attributed to the unique structure of nanoparticles and high dispersion. It is conducive to rapid charging and discharging kinetics, which can improve the utilization rate of the electrode. It also can alleviate the volume expansion of the sulfur electrode during electrochemical testing. Furthermore, Cao and collaborators reported a method for manufacturing highly flexible self-supporting GOeS composite electrodes based on the simultaneous reduction and assembly of graphene oxide sheets with sulfur nanoparticles on the metal surface [43]. The nanostructured GOeS composite electrode was used as the cathode of the lithiumesulfur battery for electrochemical performance tests. It showed a high initial discharge capacity of 1302 mAh g 1, and the discharge capacity is still as high as 978 mAh g 1 over 200 cycles of charging and discharging under 0.1 C. This excellent electrochemical performance is mainly attributable to the composite structure having a cross-linked porous network structure, thereby exhibiting outstanding mechanical properties, rapid electron and ion transmission rate, and an enhanced ability to inhibit the diffusion of polysulfides. It is worth noting that the GOeS composite electrode has excellent mechanical and electrochemical performance, so the shape of its flexible energy-storage device can be designed according to application needs. The composite structure was assembled with a cable-type and soft-package LieS battery, and its electrochemical performance was tested, which showed high initial mass-specific capacities of 1360 and 1187 mAh g 1 under a chargeedischarge rate of 0.1 C. Subsequently, researchers developed a variety of strategies to synthesize high-performance GOeS composite electrodes and use them as LieS battery cathodes for electrochemical performance studies [44e46].

3.2

Grapheneesulfur composite cathodes

As a typical two-dimensional material, graphene has a large specific surface area, outstanding structural stability, and excellent electrical conductivity, which has caused extensive research by researchers in recent years [47]. The immobilization of nanostructured sulfur on graphene sheets in LieS batteries can significantly improve its electrochemical performance. As we all know, compared with GO, G or reduced graphene has higher electrical conductivity, so that the utilization rate of active materials can be improved during the process of charging and discharging [47]. Thus,

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compounding sulfur with reduced graphene oxide (rGO) or graphene (G) through various compounding strategies has become an important topic in LieS battery research. It is worth noting that researchers have developed many effective methods to directly reduce GO to rGO or G. In 2013, Zhang’s research group reported a breakthrough in the preparation of rGO materials [47]. The specific experimental procedure successfully prepared the rGO hydrogel by reducing the GO solution under hydrothermal conditions for 10 h at 150 C, and then potassium hydroxide was added to the rGO hydrogel at 800 C under the protection of an inert atmosphere. The activation treatment is performed to finally synthesize a porous graphene sheet. Using this porous graphene sheet as a matrix material can load up to a 67% sulfur content, approximately three times the previously reported result. It is worth noting that this sulfur/porous graphene composite electrode exhibits excellent cycle stability, in which the reversible capacity is still as high as 685 mAh g 1 after 100 cycles under a chargeedischarge rate of 1 C. This composite strategy improves the electrochemical performance of lithiume sulfur batteries, which provides a feasible way to prepare high-performance SeG composite electrodes. Based on the above research results, the assembly of two-dimensional graphene sheets into graphene hydrogels with a three-dimensional porous network structure has gradually become a research hotspot. The main reason is that the threedimensional porous continuous graphene structure can achieve excellent mechanical strength, large surface area, and fast mass and electron transmission [48,49]. For example, Xu and his colleagues added CoCl2 to the graphene aqueous solution and then successfully synthesized 3-D rGO material under hydrothermal conditions. The prepared 3-D rGO material achieved a sulfur loading of up to 73 wt% [48]. In 2014, the research group also successfully synthesized sulfur-anchored graphene (SeG) composite using a one-pot hydrothermal method, in which w5 nm sulfur nanoparticles are firmly anchored on the graphene sheet [49]. In this research work, the electrochemical performance of SeG composite material was compared with another graphene-S composite material, in which the latter used traditional thermal annealing under 450 C to reduce GO. The initial capacity of this SeG composite electrode is as high as 1400 mAh g 1 under a current density of 335 mAh g 1, while the initial capacity of other graphene-S composite electrodes is only 1050 mAh g 1 under the same test conditions. In addition to reducing GO to rGO and then combining sulfur and rGO to prepare electrode materials for lithiumesulfur batteries, graphene can be prepared by electrochemical exfoliation, chemical vapor deposition (CVD), and other methods [50e52]. Once obtained, it can be used as a sulfur-supported matrix material in application research for LieS batteries. Among the preparation methods, Wei’s group chose mesoporous metal oxide as the catalyst template and prepared an intrinsically unstacked double-layer templated graphene using the CVD method. The graphene material and sulfur were composited, exhibiting excellent rate capability for the LieS batteries [52]. Specifically, when the chargeedischarge rate was 5 and 10 C, the specific capacity after 200 cycles was still as high as 832 mAh g 1 and 628 mAh g 1, respectively. It can be seen from the results of this study that different graphene preparation methods will have a significant impact on the electrochemical performance of LieS batteries.

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In 2020, Chen et al. first prepared a composite material of PB and rGO through a practical design and then used it as the main material of the sulfur hybrid (S@PB@ rGO), in which sulfur was composited to an rGO and Na2Co[Fe(CN)6] framework [53]. The composite structure was used as a lithiumesulfur battery cathode for electrochemical performance testing. The initial capacities of the S@PB@rGO composite electrode were 1163, 918, 721, 439, and 816 mAh g 1 under the charging and discharging rates of 0.1, 0.5, 1, 2, and 0.5 C, respectively. The composite material exhibits excellent electrochemical performance, and the main reasons are analyzed as follows: (1) PB has excellent adsorption properties, so it can be used as a sulfur host to adsorb polysulfides and sulfur on the surface. (2) During the process of charging and discharging, the porous structure of PB not only alleviates the volume expansion of the sulfur electrode but also provides a fast transmission channel for the insertion and extraction of lithium ions, thus exhibiting excellent cycling stability in electrochemical tests. (3) After being compounded with rGO, the shuttle effect of polysulfides can be inhibited. The main reason for analysis is that rGO can adsorb long-chain polysulfides. In summary, the composite structure design of sulfur and conductive rGO or G matrix alleviates the large volume expansion during the electrochemical charge and discharge process and inhibits the shuttle effect of lithium polysulfide, thereby greatly improving the cycling stability and rate performance of LieS batteries. Compared with SeGO composite material, SerGO or SeG composite material as the cathode of the LieS batteries generally shows higher rate performance and better cycle stability. The reason for the analysis is that rGO or G material has better conductivity.

3.3

Grapheneesulfurecarbon composite cathodes

Graphene has a large specific surface area, so it can be used as a matrix material to support sufficient sulfur materials. However, in the actual preparation process, the graphene nanosheets are prone to serious stacking and obvious agglomeration, reducing the sulfur material’s supporting area. The composite material was used in the electrochemical performance test of lithiumesulfur battery, and its rate capability and specific capacity showed a serious attenuation phenomenon [54]. To solve the above problems, the Kaskel group reported for the first time a self-supporting carbide-derived carbon/ carbon nanotube (CNT)/sulfur composite cathode, which showed excellent electrochemical performance. Because this dual-carbon material mixture has more excellent electrical conductivity and cycle stability, researchers have extensively studied it [54]. Researchers have become increasingly interested in the structural design of dualcarbon materials in LieS batteries in recent years. Among them, CNTs have been extensively studied in many fields due to their large specific surface area and high electrical conductivity. However, CNTs do not exhibit excellent performance when used as a conductive matrix because of their one-dimensional structure. The basic structural units of CNTs and graphene are both hexagonal honeycomb lattices of carbon, which are representative carbon allotropes. It is worth noting that the unique structure of CNTs with a large aspect ratio can provide a fast channel for electrons and lithium ions. As a typical two-dimensional structure, graphene nanosheets will prevent

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the rapid transmission of lithium ions to a certain extent due to their planar structure under high current density conditions [55]. The surface properties of the graphene material were analyzed, and the characterization results showed that it has a variety of pore structures and surface functional groups and defects. A variety of atoms can be doped on the graphene surface, limiting the diffusion of polysulfides. In addition, graphene with a rich pore structure can also penetrate the electrolyte. Through the above analysis, it can be shown that CNTs and graphene have complementary characteristics in many research fields. Through the reasonable design of experiments and the use of their synergistic effects, their respective excellent performances can be combined cleverly. Therefore, designing a CNT/graphene composite architecture is considered an effective method to improve the electrochemical performance of LieS batteries. Among them, the amount and form of sulfur loaded in the composite material will also directly affect the electrochemical performance of LieS batteries. Hwa’s research group successfully prepared GO/CNT/S composites by freeze-drying. Specifically, sulfur and its sulfides were first added during the entire electrode preparation process, and finally, impurities were removed [24]. Based on the advantages of this preparation method, the experimental results show that CNTs are cleverly embedded. Especially when the S content is as high as 87%, there is no obvious S agglomeration. In addition, G
omez-Urbano first prepared a GOeCNT composite matrix and then embedded S materials on the GOeCNT composite matrix by the melt-diffusion strategy [56]. The composite material exhibits excellent electrochemical performancedthat is, its specific capacity is still as high as 500 mAh g 1 after 100 cycles under the condition of a chargeedischarge rate of 0.1 C. Based on the abundant functional groups on the surface of graphene, the surface properties of the G/CNT composite matrix can be rationally designed. Specifically, modifying the surface properties of graphene by heteroatom doping, including single-atom doping or diatomic codoping, can significantly improve the adsorption performance of the G/CNT composite matrix. Su’s group successfully converted Prussian blue into an N-doped G/CNT composite matrix by ingenious design [57]. Experimental results show that the composite material doped with nitrogen has more active sites, which is beneficial to promote the capture of polysulfides, thereby significantly improving the long-cycle stability of LieS batteries. To make the preparation method more economical, green, and environmentally friendly, Wu’s group chose egg white as the inherent N/P codoping to successfully prepare composite materials [58]. The experimental results show that the uniform codoping of N/P elements is realized on the carbon skeleton, thereby improving the chemical adsorption capacity of the composite material. Sun’s group developed a method of codoping carbon nanowalls with N and S to decorate graphene. This composite material achieves the purpose of efficiently capturing polysulfides [59]. The electrochemical performance of the composite material as a LieS battery cathode showed excellent cycle stability; that is, its capacity decay rate was only 0.078% after 100 cycles under a chargeedischarge rate of 0.2 C. The above results indicate that the heteroatom modification of carbon materials can significantly improve the electrochemical performance of the battery. In addition to the research on one-dimensional carbon nanomaterials, the spherical structure of carbon materials has caused extensive research. The nano-spherical carbon

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material as a matrix can accommodate a large amount of S material and improve the contact area with the electrolyte, which is conducive to the rapid transport of lithium ions. Zhou’s research group successfully prepared a graphene-coated N-doped doubleshell hollow carbon sphere composite architecture (G-NDHCS-S), which was then used to capture sulfur materials [60]. The use of graphene in the composite material provides a highly electronic conductive network structure and mechanical properties, so the composite material does not need additional binders and conductive additives. The research work has greatly improved the electrochemical properties of the composite electrode through the design of a porous double-shelled hollow architecture. Through the electrochemical performance test of the composite material, the results showed that the initial discharge capacity is as high as 1360 mAh g 1 under a chargee discharge rate of 0.2 C. Jia’s group also successfully synthesized carbon nanosphere/ graphene composites and studied its electrochemical performance [61]. The specific surface area of the composite material is as high as 3200 m2 g 1, which provides conditions for high sulfur loading. The result exhibits that the mass content of sulfur in the composite structure is as high as 74.5%. The composite electrode exhibits excellent cycle stabilitydthat is, the specific capacity still reaches 916 mAh g 1 over 100 chargeedischarge cycles. The above results show that the spherical structure of carbon materials as a carrier can improve the electrochemical performance of LieS batteries, which has very important research value.

3.4

Grapheneesulfurepolymer composite cathodes

In addition to the composite structure design of various carbon materials and graphene, the composite of polymer and graphene as a matrix to support sulfur particles is also an important research direction. The doped polymer has a significant effect on improving the electrochemical performance of LieS batteries. On the one hand, the polymer can limit the dissolution and diffusion of polysulfides through chemical bond interactions, and on the other hand, it can also relieve the volume expansion of sulfur electrodes during the electrochemical reaction [62]. In 2011, Cui’s research group designed a G/S composite material by wrapping polymer-coated sulfur particles with lightly oxidized GO sheets [62]. The prepared composite electrode exhibits excellent cycle stabilitydthat is, the specific capacity can reach 600 mAh g 1 after 100 chargee discharge cycles. Recently, researchers have studied the influence of different polymer/sulfur composites on electrochemical performance, such as polyacrylonitrile (PAN) [63], polyaniline [64], and polypyrrole [65]. The main reason for the analysis is that different polymers have different particle habits and electrochemical stabilities. In 2012, Yin’s research group successfully prepared a PAN/graphene (PAN/G) composite material through in-situ polymerization for the first time and then composited it with sulfur materials and tested its electrochemical performance as a lithiumesulfur battery cathode [66]. The electrochemical results show that the PAN-S/G electrode exhibits excellent electrochemical performancedthat is, the first reversible specific capacity is as high as 700 mAh g 1. Subsequently, Yin’s team successfully prepared a dual-mode sulfur-based composite by combining sulfur with reduced GO nanosheets and pyrolyzed PAN nanoparticles. Through material characterization analysis, the

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sulfur content in the composite material was as high as m 81.7 wt% [67]. The composite material was made into a positive electrode, and the electrochemical performance test was carried out in a LieS battery. The composite electrode exhibited good electrochemical properties, in which the initial discharge capacity is as high as 1000 mAh g 1, and the utilization rate of sulfur is as high as 70%. In addition, Chen’s group prepared sulfur/polypyrrole/graphene nanosheets composites through structural design. The initial capacity of the composite material is as high as 1415.7 mAh g 1, and its reversible capacity can still reach 641.5 mAh g 1 after 40 cycles under a chargeedischarge rate of 0.1 C [68]. Subsequently, Qiu’s and his colleagues successfully synthesized a sulfur/graphene/mesoporous carbon composite with a sandwich architecture [65]. The prepared composite as a LieS battery cathode exhibits excellent high-rate cycle stability, especially under the high chargeedischarge rate of 1e3 C, the capacity decay rate per cycle is only 0.05%. It is worth noting that recently researchers have also developed a large number of other sulfur/polymer/graphene composites used as LieS battery cathodes for electrochemical performance, such as polyimide/S/G composites [69], polydopamine coated S/G composites [70], and poly(dimethylsiloxane)/S graphene foam composites [71]. The above research results show that, compared with sulfur/graphene composite electrodes, polymer-coated sulfur/graphene composite electrodes exhibit better electrochemical properties in LieS batteries.

3.5

Grapheneemetal sulfide composite cathodes

In recent years, researchers have aroused widespread interest in the study of metal sulfides in LieS batteries, mainly because they have obvious advantages compared with metal oxides, such as the following: (1) compared with metal oxides, metal sulfides have higher conductivity, which can improve the utilization of materials; (2) compared with metal oxides, metal sulfides could avoid overlapping with the operating voltage window, which is mainly due to the reduced lithiation voltage; and (3) compared with metal oxides, metal sulfides and Li2Sx have a stronger interaction, which can lower the barrier and catalyze the reaction process. Metal sulfides have electrochemical activity to store lithium to contribute additional capacity and provide polar sites to limit the migration of polysulfides. It is worth noting that adding metal sulfides can improve the catalysis and capture capabilities in LieS batteries [72]. Based on the characteristics of metal sulfides that catalyze redox reactions, researchers have recently carried out work on several metal sulfides as catalysts to improve the electrochemical performance of LieS batteries. Yuan’s research group reported a composite material of cobalt disulfide and graphene and studied its lithiume sulfur battery [73]. CoS2 shows a catalytic redox reaction that is mainly attributed to its conductive thiophilic carrier. Specifically, the researchers used mechanical grinding to mix graphene and CoS2 to prepare CoS2/graphene composites. The experimental results showed that the introduction of CoS2 into the composite material improved the electrochemical performance of the LieS battery to a certain extent, but it did not achieve the expected effect. The analysis reason believes that the structural design

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of the composites improved the electronic conductivity, but this preparation method cannot achieve a uniform distribution of CoS2 particles on the graphene framework. Therefore, achieving a uniform distribution of metal sulfide and graphene is of great significance for improving the electrochemical performance of the composite electrode. As a typical two-dimensional material, MoS2 has attracted wide interest from researchers due to its low cost, high surface area, and abundant active sites [74]. Lin and his colleagues successfully prepared a composite of rGO and MoS2 nanosheets [74]. The characterization and analysis of MoS2 nanosheets showed that the number of layers is about six to eight layers. Since MoS2 and rGO have similar two-dimensional structures, the composites exhibited excellent contact compatibility during structural design conducive to exerting a synergistic effect between MoS2 and rGO. The composites exhibited high rate capability and outstanding cycle performance. Its specific capacity can still reach 826.5 mAh g 1 under a chargeedischarge rate of 8 C. It is worth noting that the capacity decay rate of each cycle is only 0.083% for 600 cycles under a chargeedischarge rate of 0.5 C. Furthermore, Wei’s research group successfully prepared S/MoS2 nanoparticles/rGO composites by hydrothermal reaction. The composite as a lithiumesulfur battery cathode exhibited excellent cycle stabilitydthat is, under the condition of a chargeedischarge rate of 2 C, after 300 cycles of testing, the single-cycle capacity decay rate is only 0.011% [75]. The above research work confirmed that MoS2 as a metal sulfide could improve the electrochemical performance of LieS batteries. Recently, nickel sulfide (Ni3S2) has received widespread attention in energy storage due to its high electrical conductivity. Guo’s research group innovatively designed and prepared Ni3S2/(N, S)-rGO composites through Ni3S2 and N/S codoped modified rGO [76]. In the composite, Ni3S2 and (N, S)-rGO exert a good synergy that enhances composite material conductivity and improves the polysulfide capturing effect. Among them, Ni3S2 plays a role in redox catalysis, and the three-dimensional network structure of rGO can inhibit the shuttle effect of lithium polysulfide. The composition of the composite material was analyzed and characterized, and the content of Ni3S2 was 28.2 wt%. The composites exhibited excellent electrochemical properties when used in the cathode of LieS battery. The electrode can stably cycle up to 1000 cycles under a chargeedischarge rate of 3 C, and the capacity decay rate per cycle is only 0.023%. To better meet the needs of practical applications, the researchers also made electrodes with high sulfur loading. When the sulfur loading of the electrode is 5.8 mg cm 2, the capacity retention rate of the electrode is still as high as 72.5% under a chargee discharge rate of 1 C after 200 cycles. In short, introducing graphene, CNT, polymers, or metal sulfides into sulfur-based composite materials can build a continuous conductive network, thereby enhancing its high rate capability; it can also inhibit the shuttle effect of polysulfides, thereby greatly improving long-cycle performance. Through the composite structure design of multiple materials, the electrochemical performance of the LieS battery can be improved through the synergistic effect, which lays the foundation for future practical applications.

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Conclusions

In summary, this chapter focuses on the application research of graphene-based sulfur composites in LieS batteries, including GOeS composite cathodes, graphene-S composite cathodes, graphene-S-carbon composite cathodes, graphene-S-polymer composite cathodes, and grapheneemetal sulfide composite cathodes. Material structure design aims to achieve LieS batteries with high energy density and excellent cycle performance, thereby promoting the practical application of LieS batteries. Graphene is an ideal sulfur material carrier because of its high specific surface area, excellent conductivity, and strong trapping ability for polysulfides. After years of research, the composite structure of sulfur cathode and graphene can take many forms, such as core-shell structure, sandwich structure, and simple physical hybrid structure, and so on. The introduction of graphene into the composite structure can significantly improve the electrochemical performance of LieS batteries. The analysis of the reason is mainly due to the excellent controllable assembly capabilities and flexibility of graphene. The introduction of other carbon materials into the graphene/sulfur composite material can realize a multistage continuous porous conductive network architecture, which can further improve the conductivity of the composite and increase the sulfur loading. In the LieS batteries, graphene in the composite material can alleviate the bulk expansion of sulfur, promote reaction kinetics, and improve the dissolution of polysulfides in the electrolyte, thereby significantly improving and rate performance and cycle stability. In the S/graphene composite structure, graphene captures soluble polysulfides based on electrostatic interactions and significantly relieves sulfur material volume changes during charging/discharging. It is worth noting that the composite material can also inhibit the shuttle effect of polysulfides, which is mainly due to the strong interaction between graphene and polysulfides. In addition, considering that graphene has tunable chemical reactivity and electronic properties, graphene can be functionalized or doped with heteroatoms to improve the electrochemical properties of LieS batteries. In addition to the above binary composite system, other carbon materials and polymers can be further introduced to construct a ternary composite system to further improve the conductivity of the composite materials and inhibit the shuttle effect of polysulfides. When the graphene-based ternary composite system is used as the cathode of the LieS batteries, the electrochemical test results show high specific capacity, excellent cycle stability, and outstanding rate performance. Compared with other carbon materials and polymers, the introduction of metal particles into the binary composite system has a unique redox catalytic mechanism for optimizing the electrochemical performance of LieS batteries. Metal sulfides play chemical bonding and catalytic roles in the LieS battery system that improve reaction kinetics and inhibit the shuttle effect of polysulfides. The research results prove that the combination of physical and chemical effects can significantly improve LieS battery cycle stability and rate performance.

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5. Outlook Various structural designs have indeed improved the electrochemical performance of LieS batteries to a certain extent, but there are still many challenges to the practical application of LieS batteries. For example, much research has reported that pyridine-N has a strong bonding force in LieS batteries, but it still cannot precisely control the content and type of N doping. Boron doping in graphene can significantly improve the electrochemical performance of lithiumesulfur batteries, but there are still problems such as complex doping process, high cost, and difficult characterization accuracy. It is worth noting here that the interaction mechanism between graphene and transition metals bonds has not been fully studied. The introduction of metal sulfides into S/graphene composite materials shows effective catalytic conversion and strong chemical bonding in LieS batteries to inhibit the shuttle effect of polysulfides, but the redox catalytic mechanism has not been clearly explained. The size and dispersion of the catalyst play a very important role in improving the electrochemical performance of lithiumesulfur batteries. However, there are still problems to be overcome for its practical application, especially the coordinated development of the preparation process, batch stability, environmental friendliness, and economic benefits. It is expected that the practical application of high-performance LieS batteries will be realized. To further promote the development of LieS batteries, some important challenges discussed below in the use of graphene in LieS batteries must be resolved. In addition, the basic prospects for realizing the practical application of LieS batteries are analyzed.

5.1

Enabling uniform sulfur distribution

The main strategy for manufacturing G/S cathode materials is to melt and cast sulfur into the graphene matrix. However, this strategy usually shows the uneven distribution of sulfur in the graphene matrix. Due to the long charge transfer distance and the low conductivity of sulfur, the sulfur far away from the G/S interface exhibits poor reaction kinetics. Therefore, it is necessary to design and optimize the sulfur-based composite structure, which can enhance the electrochemical properties of the sulfur cathode. After optimizing the composite structure, as much as possible to ensure that each or S8 molecule can contact the graphene matrix, thereby significantly improving the utilization of active materials. It is worth noting that developing technology for growing a sulfur layer on graphene is an important research direction. If the distance between the graphene layers is controlled within a distance that can only deposit a few layers of sulfur molecules, it is expected that sulfur can be utilized to the maximum.

5.2

Optimize the preparation method and production cost of graphene

At present, a variety of graphene preparation methods have been developed, and the pore structure and interface properties of graphene can also be controlled. However,

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the preparation method requires a harsh reaction environment or complicated reaction steps, which causes high production costs and limits its mass production and application. To reduce the cost of graphene production, it is necessary to optimize existing processes and develop environmentally friendly synthetic methods. First, the interaction between the interface is used to develop a method for preparing a highly conductive soft template and graphene composite material, which can be retained in the graphene structure without the need for cumbersome template removal steps. Second, based on the adsorption caused by different electronegativity and the crosslinking caused by reactions among functional groups, it is possible to develop a spontaneous chemical reaction between a template and graphene under room temperature and avoid using high temperatures. Third, exploring efficient and green methods to produce graphene nanosheets on a large scale will be an important research direction.

5.3

Improving the volume energy density of composite material

At present, graphene/sulfur electrodes prepared by common methods have larger voids, which reduces the volumetric energy density of the composite electrode. To further improve the utilization of space, there are mainly the following two improvement methods. On the one hand, a third material can be introduced into the G/S binary composite system to fill the gaps in the composite material. On the other hand, increasing the density of functional groups, increasing the number of soft templates, and reducing the size of the hard template can result in a denser graphene/sulfur composite material. By improving the density of the G/S composite material, the volumetric energy density of the electrode can be increased.

5.4

In-depth study of the mechanism of structural instability

By compounding graphene as the matrix with sulfur, the specific capacity and cycle performance of the composite material are improved to a certain extent, but there is still a capacity decay in the reported research work. There are two main reasons for capacity degradation. One is the dissolution of sulfur in the LieS battery system, and the other is the collapse of the graphene-based porous architecture caused by the large-volume expansion of sulfur during the charge and discharge process. To further verify whether the graphene-based porous architecture of sulfur collapses during charging and discharging, in-situ characterization techniques can be used to observe the structural changes of the composite electrode. In situ characterization methods can more intuitively explain the reasons for capacity fading so that the composite structure can be further optimized to solve the problem of structural instability.

5.5

Scientific design of the composite structure

Introducing graphene into the sulfur-based composite material can improve the electric field’s conductivity and uniformity, but there is still room for further improvement. Through the scientific design of the composite structure, this can explore new strategies to improve the electrochemical properties of LieS batteries. With smart

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manufacturing technology (like 3-D printing), one side of the graphene surface is modified and designed so that two sides of the graphene exhibit different sulfophilic and lithiophilic characteristics. The design purpose of this composite structure is to simultaneously limit the shuttle of polysulfides and the growth of lithium dendrites, thereby significantly improving the electrochemical performance of LieS batteries.

5.6

Improving electrode mechanical strength

High-performance LieS batteries have potential applications in wearable electronic devices. To meet the requirements of wearable devices for electrode flexibility, composite materials must exhibit excellent mechanical strength. However, most reported G/S composite materials did not exhibit satisfactory mechanical strength, which has been attributed mostly to the weakness of the cross-linking points between the graphene sheets. The introduction of conductive polymer into the G/S composite system can improve the overall mechanical properties of the composite structure. In addition, it is worth noting that the introduction of self-healing materials into composite materials can enhance the mechanical properties of the composite material and improve the structural stability of the composite material under repeated bending and stretching. In addition to the improved methods above, combining sulfur-based composite materials with solid electrolytes realizes high-performance flexible energy-storage devices. In short, through in-depth research from the perspective of material design, mechanism analysis, and device assembly, it is expected to significantly improve the electrochemical performance of LieS batteries, thereby greatly promoting their practical application.

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[74] H. Lin, L. Yang, X. Jiang, G. Li, T. Zhang, Q. Yao, G.W. Zheng, J.Y. Lee, Electrocatalysis of polysulfide conversion by sulfur-deficient MoS2 nanoflakes for lithium-sulfur batteries, Energy Environ. Sci. 10 (2017) 1476e1486. [75] H. Wei, Y. Ding, H. Li, Q. Zhang, N. Hu, L. Wei, Z. Yang, MoS2 quantum dots decorated reduced graphene oxide as a sulfur host for advanced lithium-sulfur batteries, Electrochim. Acta 327 (2019) 134994. [76] D.Y. Guo, Z.H. Zhang, B. Xi, Z.S. Yu, Z. Zhou, X.A. Chen, Ni3S2 anchored to N/S codoped reduced graphene oxide with highly pleated structure as a sulfur host for lithiumsulfur batteries, J. Mater. Chem. A 8 (2020) 3834e3844.

Grapheneesulfur nanocomposites as cathode materials and separators for lithiumesulfur batteries

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Wei Ni 1,3,4 and Ling-Ying Shi 2 1 State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, ANSTEEL Research Institute of Vanadium & Titanium (Iron & Steel), Chengdu, Sichuan, China; 2College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan, China; 3Material Corrosion and Protection Key Laboratory of Sichuan Province, Sichuan University of Science and Engineering, Zigong, Sichuan, China; 4Vanadium and Titanium Resource Comprehensive Utilization Key Laboratory of Sichuan Province, Panzhihua University, Panzhihua, Sichuan, China

1. Introduction Lithiumesulfur batteries represent one of the most promising next-generation battery candidates owing to their superior theoretical capacity and low cost compared with those of conventional Li-ion batteries [1e4]. However, poor conductivity, relatively high volumetric expansion during the charge/discharge processdespecially the gradual dissolution of lithium polysulfide intermediates (Li2Sn, 3
n
8)dand subsequent cross-deposition on the Li anode have hampered their further application [5e9]. Graphene, an amazing 2-D single-atom-layer material, has emerged as one of the most promising conductive matrices for high-capacity LieS batteries due to its unique 2-D structure, high surface area, superior electric conductivity, mechanical strength, structural stability, and flexibility [10e12]. Although it does not mean the emergence of graphene resurged the LieS batteries, it greatly advanced and is still promoting the development of high-performance LieS batteries. LieS batteries resurged following the blockbuster work of the Nazar group in 2009 on polymer-modified highly ordered mesoporous carbon/sulfur-based composite as cathodes for LieS batteries [13]. The incorporation of nonpolar graphene with sulfur does not necessarily address the critical concerns about LieS batteries, including polysulfides shuttling (e.g., by simple ballmilling hybridization) [6,14], but the synergistic effects such as polymer modification, chemical modification or doping [11,15], separator modification (or interlayer design) [16], and electrolyte optimization [2,17] to physically/chemically confine polysulfides (or inhibit polysulfide diffusion; illustrated in Fig. 14.1) [2,18,19]. In addition, anode

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00020-X Copyright © 2022 Elsevier Inc. All rights reserved.

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Figure 14.1 Mechanisms of various methods for polysulfide diffusion inhibition. Reprinted with permission from C. Deng, Z. Wang, S. Wang, J. Yu, Inhibition of polysulfide diffusion in lithiumesulfur batteries: mechanism and improvement strategies, J. Mater. Chem. 7 (20) (2019) 12381e12413, Copyright 2019 The Royal Society of Chemistry.

and battery design have together enhanced LieS battery performance and are bringing them into practical application [3,20e25]. Graphene is an ideal carbon material to host the electroactive sulfur in the cathode and plays a delicate role in the bifunctional polysulfide-adsorption layer (between the cathode and separator) and the anode [7,16,25]. The following sections are followed by the categories of these modifications and the dimensions of recently reported cathodes, i.e., from 0-D to 3-D, as well as the functional layers to alleviate the shuttle effect and enhance the coulombic efficiency and cyclic performance.

2.

Cathode material modifications

The weak interaction/affinity between sulfur (or polar polysulfides) and nonpolar carbons is not enough to alleviate/avoid polysulfide dissolution in a long-term cycle life (e.g., >100 cycles) [5,26]. Thus, modification of the carbons, including graphene, is in urgent need. And functionalized graphene with an enhanced ability to uniformly/ chemically absorb polysulfides has gained increasing attention [7]. These typical strategies are detailed in the sections that follow.

2.1

Polymer modifications

To further confine sulfur particles or polysulfide intermediates among the graphene layers, polymer modifications are usually utilized. This polymer modification includes amphiphilic poly(ethylene glycol) (PEG) [27], amylopectin [5], and

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cetyltrimethylammonium bromide (CTAB) [28], which are usually used to improve the cyclability of LieS batteries. These polymers are dissolved to wrap the related graphene/sulfur composites. Dai and Cui et al. first reported the mildly oxidized graphene oxide (GO) sheets/carbon black (CB)-wrapped PEG-functionalized submicrometer sulfur particles. By providing an electric conducting path, accommodating the volume expansion of sulfur particles, and trapping soluble polysulfides, the resulting grapheneesulfur composites (w70 wt.% sulfur, w15 wt.% GO, and w8 wt.% CB) demonstrate improved capacities to as high as 600 mAh g1 over 100 cycles at 0.2C in a specific ether-based electrolyte of 1M LiTFSI in DOL/DME (with a potential cutoff window of 1.7e2.5 V, 13% and 9% decay from 10th to 100th cycle at a rate of 0.2 and 0.5C, respectively, 1C ¼ 1675 mAh g1; unless otherwise noted, the following capacity values are all based on the mass of elemental sulfur) (Fig. 14.2) [27]. The addition of PEG helps trap the polysulfides and provides a flexible cushion to accommodate some of the stress and volume change.

Figure 14.2 (A) Illustration of the synthesis of graphene-wrapped sulfur particles composite. (B and C) The corresponding SEM images of grapheneesulfur composite at low and high magnifications. (D and E) Typical charge/discharge voltage profiles and cycling performance of the grapheneesulfur cathode material with PEG coating at various rates. Reprinted with permission from H.L. Wang, Y. Yang, Y.Y. Liang, J.T. Robinson, Y.G. Li, A. Jackson, et al., Graphene-wrapped sulfur particles as a rechargeable lithium-sulfur battery cathode material with high capacity and cycling stability, Nano Lett. 11 (7) (2011) 2644e2647, Copyright 2011 American Chemical Society.

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Chemical modifications

GO, usually a precursor for graphene synthesized by (modified) Hummers’ method, has original functional groups such as epoxide, hydroxyl, carbonyl, and carboxyl groups that may play a role in immobilizing some free polysulfides to some degree [5,29e34]. Sulfur/polysulfides can be chemically grafted to the GO matrix through a facile chemical reactionedeposition method. For example, the chemically immobilized sulfur on 2-D GO shows high and stable reversible capacity of 950 mAh g1 and superior coulombic efficiency of around 96.7% over 50 deep cycles at 0.1C in an electrolyte of 1M LiTFSI in PYR14TFSI/PEGDME (1.0e3.0 V, 66 wt.% S in the GO/S nanocomposite) [35]. The microemulsion approach is scalable for large production and the ultrauniform coating of S to the GO surface via binding of S to the S─S bonds of the rippled rings of the 2-D honeycomb lattice. However, owing to the lower electronic conductivity of GO (originally 1.29  103 S cm1 and 0.316 S cm1 after being heat-treated at 155 C for 12 h), an additional conductive agent such as CB is introduced and lowers the sulfur content in the cathode (decreasing to 46.2 wt.%). To relieve the capacity fading over long-term cycles, other more efficient chemical modifications are required, such as amino-functionalization (Fig. 14.3) [36], sulfonate group modification [30,37], and alkali-activated highly porous graphene [38,39]. Heteroatom-doping is another highly effective way to enhance sulfur utilization and polysulfide adsorption due to graphene’s defects and tuned electronic structures (chemical

Figure 14.3 (A and B) SEM and TEM images of EFGeS nanocomposite (EFG: ethylenediamine-functionalized rGO). (C) Rate capabilities of EFGeS and rGOeS nanocomposites (1.5e3.0 V) and (D) Cycling performance and coulombic efficiency of EFGeS nanocomposite (60 wt.% S) at 1, 2, and 4 C. (E) Long-term cycling performance of EFGeS nanocomposite and rGOeS composite. Reprinted with permission from Z.Y. Wang, Y.F. Dong, H.J. Li, Z.B. Zhao, H.B. Wu, C. Hao, et al., Enhancing lithium-sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide, Nat. Commun. 5 (2014) 5002, Copyright Springer Nature.

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polarity). These typical examples include nitrogen [11,40,41], boron [42,43], sulfur, and codoping [39,44e47]. Alternatively, a related chemical modification could be made to the pristine graphene nanosheets from other resources/techniques [48].

2.3

Sulfur-rich polymers

Besides the elemental sulfur and Na2S2O3 derived sulfur, the sulfur-rich polymers are also of note. Although the conventional sulfur-containing compounds/polymers such as organic disulfides, disulfide-containing polymers, and some organic polysulfides (vulcanized polymers) have been put aside due to the lower sulfur content and capacity thereof [49e51], another new kind of polysulfides (called sulfur-embedded polymers, sulfur copolymers, organosulfide polymers or inverse vulcanization copolymers) based on chemically polymerized sulfur have once attracted great attention followed the publication of Pyun et al. work on Nature Chemistry in 2013 [49e56]. However, the latter polysulfides do not seem to be the focus of practical application nowadays due to the relatively complicated synthesis techniques, intermediate process, and inadequate long-term cycle performance and energy density, although much research has been done to advance this new concept and strategy.

2.4

Other composites

The carbon hybrids/composites by incorporation of graphene with other carbon allotropes such as amorphous porous carbon (with pore acting as polysulfides reservoir) [14,57,58], heteroatom-doped carbon layer [59,60], carbon nanotubes [61], carbon (nano)fibers [62,63], hierarchical carbons [64], as well as other polar transition-metal compounds (mainly oxides, chalcogenides, nitrides, MXenes) [65,66], MOFs [67], and conducting polymers [68e70], is also a critical strategy to enhance the (chemi)sorption and redox kinetics, improve the sulfur utilization rate and reduce the shuttle effect [66,71e74]. Worth noting is that these composites may show greatly enhanced cycling performance (e.g., ultralow capacity fading over 1000 cycles) by the confinement or affinity of the short-chain (covalent) sulfur/polysulfides into the microchannels or onto the surface [60,75,76]. And the introduction of transition metal compounds or single-atom catalysts (SACs) via the adsorptionecatalysis mechanism has also greatly enhanced the utilization of sulfur (or redox kinetics) as well as suppressing the shuttle effect (Fig. 14.4A and B) [74,77,78,80e83]. However, the unnecessary weight increase induced by these functional additives should be minimized or balanced for enhanced performance and low cost. These contents will be referred to in related chapters. Here, a typical example is provided to illustrate SACs anchored on graphene with maximal exposed active centers and optimized electronic environments (i.e., optimal electrocatalysis) for superior energy efficiencies. Cobalt SAC-embedded N-doped graphene (CoeN/G) with a bifunctional CoeNeC coordination center shows superior electrocatalysis for Li2S formation and decomposition, and the as-prepared S@ CoeN/G can deliver a high specific capacity of 1210 mAh g1 with an ultrahigh S loading of 90 wt.% and exhibits long-term cycle stability over 500 cycles (coulombic efficiency of w99.6%), as well as high areal capacity (5.1 mAh cm2) and low capacity

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Figure 14.4 Illustrations of (A) The catalysis effect for accelerated redox kinetics and (b) Single-atom catalyst (SAC) boosted electrochemical conversion in LieS batteries. (C and D) TEM and HAADF-STEM images of CoeN/G (CoeN/G: Co-containing SAC embedded in Ndoped graphene framework). (E) Cycling performance of S@CoeN/G cathodes, compared with S@N/G, S@Co/G, and S@rGO electrodes. (F) Cycling performance of S@CoeN/G cathodes with varied S areal loadings of 2.0, 3.4, and 6.0 mg cm2 at 0.2C. (A and B) Reprinted with permission from Y. Chen, X.C. Gao, D.W. Su, C.Y. Wang, G.X. Wang, Accelerating redox kinetics of lithium-sulfur batteries, Trends Chem. 2 (11) (2020) 1020e1033, Copyright 2020 Elsevier Inc. and J. Wang, L.J. Jia, J. Zhong, Q.B. Xiao, C. Wang, K.T. Zang, et al., Single-atom catalyst boosts electrochemical conversion reactions in batteries, Energy Storage Mater. 18 (2019) 246e252, Open Access, Published by Elsevier B.V. 2018. (CeF) Reprinted with permission from Z.Z. Du, X.J. Chen, W. Hu, C.H. Chuang, S. Xie, A.J. Hu, et al., Cobalt in nitrogen-doped graphene as single-atom catalyst for high-sulfur content lithium-sulfur batteries, J. Am. Chem. Soc. 141 (9) (2019) 3977e3985, Copyright 2019 American Chemical Society.

decay rate of 0.029% per cycle at 0.2C with 6.0 mg cm2 sulfur loading (Fig. 14.4CeF) [79]. A similar NieN/G modified separator was also fabricated, showing high performance comparable to cathode materials [84]. At one time, Li2S and its composites were reported as cathode materials (Si or Sn as the anode) to replace the sulfur composites for avoiding the lithium dendrite effect [85,86]; however, it reduces the energy density of LieS batteries. Now more effective strategies have been proposed to address these lithium dendrite-related security concerns.

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3. Modified separators and functional interlayers The porous PP/PE film separators usually used in commercial Li-ion batteries were once mainly adopted for LieS batteries; however, these simple inert polyolefin-based separators do not contribute to the reduction/elimination of dissolved polysulfides to deposit on the opposite Li anode [6,87]. With the emergence of the bifunctional interlayer concept (usually a conductive interlayer sandwiched between the cathode and separator to suppress polysulfide diffusion) initiated by the Manthiram group [1], more work has been focused on this subject. Overall performance, especially cycle stability, is significantly enhanced via inhibition of the shuttling effect, e.g., polysulfide-interception or the (intact) adsorption mechanism [45,88]. These modified separators/additional layers include graphene [89,90], GO [91,92], functionalized graphene [45], graphene composites (e.g., with sodium lignosulfonate [87], dendrimer [93], GO/Nafion [94], doped carbon layer [95], boron nitride nanosheets [96], or other inorganic 2-D materials [65,66,72,73]), porous carbon paper (pyrolyzed filter paper [6]) and so on [16,74,97,98]. The incorporation of GO onto the separator may increase the open-circuit voltage (i.e., anti-self-discharge) besides the suppressed shuttle effect and improved coulombic efficiency and cycling stability (Fig. 14.5) [92]. The adoption of some negatively charged groups (e.g., sulfonic) in graphene composites may further enhance the suppression of transport by polysulfide anions across the separator without compromising Liþ ion kinetics, thus resulting in a smoother and robust cycling performance up to 1000 cycles [87]. The introduction of functional (thin and selective) interlayer structure in these LieS batteries significantly enhanced their cycle performance due to the relieved shuttle effect and decreased internal charge transfer resistance, and some superior examples even demonstrate unexpected stability up to 1000 cycles with ultralow decay per cycle (e.g., 0.0037% per cycle) [96]. Generally, these modified separators/additional layers play a role partially similar to selective membranes/channels for polysulfide anions (nicknamed ionic shields or ionic sieves) [99e101].

4. Techniques and methods For the fabrication of sulfur/graphene composites, besides some conventional techniques/methods such as milling [6], solution-diffusion [102e104], vapor- or meltdiffusion [43,63,105], in situ pyrolysis/hydrolysis [106], or solution synthesis [20,107,108], more new approaches are emerging, e.g., reverse microemulsion synthesis [109], these new methods usually endow the LieS batteries with higher sulfur utilization, long cycle life and higher energy density due to the greatly reduced sulfur particle size as well as the functional protective/adsorptive coating for suppressed shuttle effect. Overall, the sulfur in the composites usually exist in the amorphous form via the additional melt- or vapor-diffusion process, it may also exist in micro-/nanocrystalline morphology, and no further thermal treatment is needed, although the amorphous form with smaller sulfur (particle) size generally endows the cathode with higher redox kinetics and improved performance [27,32]. The polysulfide/graphene composite may be considered a kind of intermediate of the S/graphene composite.

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Figure 14.5 (A) Illustration of a GO-membrane-incorporated LieS battery, where GO membrane is sandwiched between cathode and anode to efficiently prohibit the shuttle effect. (B) Permselectivity of the GO membrane for high order polysulfides. (C) Galvanostatic charge/ discharge profiles with and without GO membrane at a rate of 0.1C. (D) Open circuit voltage profiles with and without GO membrane showing a distinct self-discharge behavior. J.Q. Huang, T.Z. Zhuang, Q. Zhang, H.J. Peng, C.M. Chen, F. Wei, Permselective graphene oxide membrane for highly stable and anti-self-discharge lithium-sulfur batteries, ACS Nano 9 (3) (2015) 3002e3011, Open Access, Published by American Chemical Society 2015 [92].

5.

Structural design

5.1

Material structures

The graphene may also be fabricated into different dimensions or structures via varied methods or precursors, such as 0-D graphene quantum dots (GQDs) [110], 1-D graphene nanoscrolls [105,111], graphene nanoribbon [69,112,113], 2-D graphene nanosheets (for unstacked double-layer graphene, see Fig. 14.6) [114], (porous) graphene paper (for graphene nanoribbon paper, see Fig. 14.7) [63,113], and 3-D hierarchical structures (via 3D printing [53], self-caging nanochemistry [30], hydrothermal selfassembly [57,103,115], electrochemical assembly [116], microwave plasma CVD [117], layer-by-layer assembly [64], ice-templating/freeze-drying [41,111], sprayfrozen assembly [118], laser-scribing [119], microwave heating [120], templateassisted CVD or pyrolysis [121e126]), and some other complex graphene hollow

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Figure 14.6 (A) Illustration for the synthesis of unstacked double-layer templated graphene (DTG), with layered double hydroxide (MgAl LDH) nanoflake-derived mesoporous metal oxide as a template. (B and C) TEM and HRTEM images of the cast DTG flakes. (D and E) Chargeedischarge profiles and cycle performance of the DTG/S cathode at different rates. Reprinted with permission from M.Q. Zhao, Q. Zhang, J.Q. Huang, G.L. Tian, J.Q. Nie, H.J. Peng, et al., Unstacked double-layer templated graphene for high-rate lithium-sulphur batteries, Nat. Commun. 5 (2014) 3410, Copyright Springer Nature.

micro/nanostructures (hollow graphene nanoshell ensemble [127,128], core-shell or yolk-shell structure [27,129]). For spray-frozen assembled hierarchical open porous spherical hybrid of rGO/nanorod-like sulfur, see Fig. 14.8 [118]. Overall, the physical confinements and chemical interaction could be further enhanced by specific graphene/ sulfur configurations and doping by functional groups/heteroatoms [21].

5.2

Electrode structures

In general, based on the structure/morphology, sulfur/graphene composite electrodes can be divided into three categories, i.e., 1-D, 2-D, and 3-D electrodes. 1-D fibrous and 2-D flexible film-based electrodes are attracting much interest due to the rapid development of flexible electronics that require corresponding flexible

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Figure 14.7 (A and B) Schematic and digital images showing the preparation procedure of flexible and freestanding S/rGO nanoribbon paper (S/RGONRP), and (CeF) The corresponding SEM and TEM images. Reprinted with permission from Y. Liu, X. Wang, Y. Dong, Y. Tang, L. Wang, D. Jia, et al., Self-assembled sulfur/reduced graphene oxide nanoribbon paper as a free-standing electrode for high performance lithium-sulfur batteries, Chem. Commun. 52 (87) (2016) 12825e12828, Copyright 2016 The Royal Society of Chemistry.

energy-storage devices [4,130]. These typical examples or prototypes include graphene/sulfur composite film electrodes (for 2-D graphene composite cathodes see Fig. 14.9) [104,131e134], and nanostructured paper for LieS batteries with unconventional configurations (cable-type or soft-packaged) [135]. Despite the preformed 3-D structures of 3-D aerogel-based cathodes, they may be destroyed or deteriorated in conventional or inappropriate configurations of the cells/batteries (for the fibrous hybrid foam of graphene and sulfur nanocrystals, see Fig. 14.10) [43,57,102,103,136]. The 3D printing approach is emerging as an interesting way to accurately fabricate diverse, complex electrode architectures. For example, a cathode of well-designed periodic microlattices of sulfur copolymer/graphene composite was obtained by 3D printing the precursor ink. The interconnected graphene provides an excellent electronic pathway, and the integrated cathode demonstrates a high reversible capacity and good cycle performance (Fig. 14.11) [53]. And the performance could be enhanced by the incorporation of other synergistic modifications. Compared to the conventional electrodes of Li-ion batteries with metal current collectors, the freestanding, flexible and electroactive current collectors are now attracting great interest, e.g., those based on graphene, carbon nanotube, or carbon (nano)fiber [4,111]. For example, the all-graphene-based cathode of LieS batteries could be constructed by using highly conductive graphene paper as current collector, highly porous

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Figure 14.8 (A) Schematic of the synthesis of radially assembled open-porous spherical rGO/ nanosulfur particles (R-rGO/nS). (BeE) SEM, TEM and dark-field TEM images of the asprepared R-rGO/nS. (FeI) EDS mapping images of R-rGO/nS showing the uniform distribution of C, S, and O elements. Reprinted with permission from J.S. Yeon, S. Yun, J.M. Park, H.S. Park, Surface-modified sulfur nanorods immobilized on radially assembled open-porous graphene microspheres for lithium-sulfur batteries, ACS Nano 13 (5) (2019) 5163e5171, Copyright 2019 American Chemical Society.

graphene as host for sulfur, and partially oxygenated graphene as adsorption interlayer (filtered/coated onto commercial polymer separator) [7], this kind of integrated electrode not only enhances the energy density of the LieS battery but also extends its application to flexible/wearable devices. And similar graphene-based composite fibers for flexible/wearable ultralight LieS batteries have also been recently investigated [137].

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Figure 14.9 (A and B) Digital photos and (C and D) SEM images of a freestanding honeycomblike S@rGO ultrathin nanocomposite membrane with multiscale sulfur particles. (E and F) Surface and cross-sectional SEM images of an rGOeS composite film with nanosized sulfur particles. (G) Rate performance of the rGOeS composite film cathodes compared with rGOeS mixture, and (H) The corresponding cycle performance at 0.1C. (AeD) Reprinted with permission from W. Ni, J. Cheng, X. Li, Q. Guan, G. Qu, Z. Wang, et al., Multiscale sulfur particles confined in honeycomb-like graphene with the assistance of bio-based adhesive for ultrathin and robust free-standing electrode of LieS batteries with improved performance, RSC Adv. 6 (11) (2016) 9320e9327, Copyright 2016 The Royal Society of Chemistry. (EeH) Reprinted with permission from S. Luo, M. Yao, S. Lei, P. Yan, X. Wei, X. Wang, et al., Freestanding reduced graphene oxideesulfur composite films for highly stable lithiumesulfur batteries, Nanoscale 9 (14) (2017) 4646e4651, Copyright 2017 The Royal Society of Chemistry.

5.3

Battery structures

The LieS batteries may be of many different configurations or morphologies, which could be, for example, hard-pack or soft-pack batteries [3], flexible energy-storage devices (unconventional 1-D/2-D configurations) [4,135], liquid electrolyte-based batteries, gel polymer electrolyte-based batteries [138], or all-solid-state batteries. And some electrolytes/additives both alleviate the shuttle effect and suppress lithium dendrite growth [28,138]. For example, flexible and freestanding nanostructured rGO/S composite with an internal cross-linked porous network with soft-packaged and cable-type configurations showed comparable performance to conventionally configured coin cells, which

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Figure 14.10 (A) Schematic illustration for preparing fibrous grapheneesulfur (GeS) hybrid self-supporting electrode. (BeD) STEM image and corresponding sulfur map of the squared region of the typical GeS63 hybrid (S loading of 63 wt.%), as well as the HRTEM image. Reprinted with permission from G.M. Zhou, L.C. Yin, D.W. Wang, L. Li, S.F. Pei, I.R. Gentle, et al., Fibrous hybrid of graphene and sulfur nanocrystals for high-performance lithium-sulfur batteries, ACS Nano 7 (6) (2013) 5367e5375, Copyright 2013 American Chemical Society.

shows a promising performance application to be integrated with flexible and wearable electronics (Fig. 14.12) [135]. And the conventional metal current collectors could be replaced by flexible and lightweight graphene foams/films to enhance the energy density and flexibility of the LieS batteries. For the profiles of all-graphene LieS batteries with graphene current collector and graphene-coated separator (GCC/S þ Gseparator), see Fig. 14.13 [139,140]. Theoretically, all-solid-state LieS batteries can completely inhibit polysulfide dissolution and shuttling, as well as Li dendrite formation, compared to liquid electrolytes. Via the delicate design to reduce the stress/strain and interfacial resistance but not significantly lower the reaction kinetics, Yao et al. fabricated a kind of highperformance all-state-solid-state LieS batteries, of which the cathode consists mostly of conformally coated S on reduced graphene oxide (rGO) [141]. The as-configured LieS battery shows a high rate capacity comparable to specific liquid electrolytes and could maintain an excellent cycle performance, i.e., 830 mAh g1 at 1C after 750 cycles. However, it should be mentioned that the all-solid-state batteries are temperature-dependent and will play best at a relatively high temperature (60 C) and show somewhat decreased voltage plateaus (Fig. 14.14).

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Figure 14.11 (A) Schematic illustration of the procedure of 3D printing sulfur copolymergraphene (3-DP-pSG) architectures. (BeG) SEM images of hierarchical 3-DP-pSG architectures and the corresponding elemental mapping images of S and C. Reprinted with permission from K. Shen, H.L. Mei, B. Li, J.W. Ding, S.B. Yang, 3D printing sulfur copolymer-graphene architectures for Li-S batteries, Adv. Energy Mater. 8 (4) (2018) 1701527, Copyright 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

6.

Challenges and perspective

Graphene-based materials have been widely exploited for the application of highperformance LieS batteries. Compared with other conductive counterparts, they have demonstrated superior advantages with promising applications. However, most research is based on early prototypes, large-scale energy-storage devices, and versatile adaptation are urgently needed for eventual wide application. The primary challenges for graphene-based materials in LieS batteries are as follows: (i) Compared with mesoporous carbon, graphene can afford much higher mass loading for the cathode composites via overcoming the limitation of sulfur vapor diffusion and high cost

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Figure 14.12 (A) Schematic illustration of the fabrication of freestanding rGOeS composite film. (B and C) Photos of the as-prepared flat and folded rGOeS composite films. SEM images of (D and E) The cross-section and (F and G) The rGOeS composite film surface. (H and I) Rate performance and cycling performance of rGOeS composite film electrode compared with rGOeS mixture electrode. Reprinted with permission from J. Cao, C.Q. Zhao, N. Zhang, Q. Lu, X. Wang, et al., A flexible nanostructured paper of a reduced graphene oxideesulfur composite for high-performance lithiumesulfur batteries with unconventional configurations, Adv. Mater. 28 (43) (2016) 9629e9636, Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. for mesoporous materials. However, the surface of graphene should be tuned to relieve the polysulfides to escape from the relatively large layer space while not sacrificing the conductivity of the graphene skeleton. The interlayer or functional components are important for enhancing cycling performance, although the thickness should be controlled to avoid the significant deadweight (unnecessary weight increase), resulting in decreased gravimetric/ volumetric specific capacities of LieS batteries. The design of composite materials and cathodes with highly dispersed nanosized sulfur or smaller sulfur molecules anchored on graphene layers represents an alternative direction for high-performance LieS batteries. (ii) For binders, the conventionally used polyvinylidene fluoride (PVDF), which needs Nmethyl-2-pyrrolidone (NMP) solvent, may be replaced by other advantageous binders because the toxic NMP could dissolve the sulfur and thus destroy the preconstructed nanostructures; it also possesses no bonding between binder and polysulfides and therefore

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Figure 14.13 (A and B) Schematic illustrations of the LieS batteries with two different electrode configurations (GCC/S: graphene current collector coated with S; G-separator: graphene membrane-coated separator). The paler yellow color represents the alleviated shuttle effect. (C) Rate performance and (D) Cycling performance of the LieS batteries with different configurations. Reprinted with permission from G.M. Zhou, S.F. Pei, L. Li, D.W. Wang, S.G. Wang, K. Huang, et al., A graphene-pure-sulfur sandwich structure for ultrafast, long-life lithium-sulfur batteries, Adv. Mater. 26 (4) (2014) 625e631, Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. could not alleviate polysulfide dissolution [5,71]. Some bioresource-based macromolecules (biopolymers), polar organics, and other advanced binders with superior affinity/conductivity may be promising alternatives [5,71,85,142]. A 3-D porous/hollow binder-free cathode could enhance the electronic conductivity of the electrode, although the tap density and volumetric energy density may be severely reduced. (iii) Appropriate cutoff voltage windows are critical for the long-term cycling stability, e.g., 1.5/ 1.7 to 2.8/3.0 V would be the optimal for the LieS batteries due to the avoiding of the formation of solid/insoluble “dead” Li2S/Li2S2 in the cathode, which partially decreases the sulfur utilization and the capacity thereof. (iv) The areal sulfur loading rather than the gravimetric specific capacity is critical for practical application. However, for many (initially) reported works, the low sulfur content (e.g., 95%

[82]

100%

[17] Lithium-Sulfur Batteries

Abbreviations: DVB, divinyl benzene; P3HT, polythiophene; PETEA, pentaerythritol tetraacrylate; PMAT, poly(m-aminothiophenol); S-DIB, sulfur-1,3-di-isopropenylbenzene.

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Figure 18.13 Inverse vulcanization process to prepare poly(S-pentaerythritol tetraacrylate (PETEA)) copolymer.

capacity of 823 mAh g1 after 100 cycles. The polymeric sulfur suppresses the accumulation of insoluble polysulfide and confines the migration of insoluble species. By varying the DIB content, the performance of batteries could be enhanced; a content of 10% was reported as the optimal composition exhibiting an initial capacity of 1225 mAh g1 and a remaining capacity of 1005 mAh g1 after 100 cycles [69]. Furthermore, the production of the sulfur-copolymer is easily scalable [70]. Using the same DIB-based sulfur-copolymer, Oleshko et al. confirmed the presence of sulfur and Li2S inside the copolymer matrix via transmission electron microscopy and field-emission scanning-electron microscopy [71]. However, the visual investigation could not reveal whether the sulfur was covalently bonded to the copolymer and participated actively electrochemically. The poly(S-r-DIB) and the interaction between polysulfides and copolymers were characterized in-depth by Solid-State Nuclear Magnetic Resonance, X-ray Absorption Near-Edge Structure, and X-ray diffraction [72]. Thereby, it could be proven that the use of polymeric sulfur results in reversibly recovered sulfur and the trapping of the active species into the polymeric network. However, one main disadvantage of these organic electrode materials is the low conductivity of polysulfur molecules, limiting the battery capacity. Therefore, it is of interest to prepare chemically stable sulfur-based organic materials with high electrical conductivity by copolymerizing the sulfur with electrically conductive polymers. The most investigated conductive polymeresulfur copolymer is obtained by the vulcanization of PANI and is termed SPAN. Wang and Fanoush observed that SPAN with polycarbonate electrolytes delayed the dissolution of polysulfides [73,74]. Oschmann et al. copolymerized allyl-terminated polythiophene (P3HT) with sulfur (Fig. 18.14A) [75]. The allyl end group of P3HT plays a critical role in the homogeneous distribution of sulfur along the P3HT chain. By covalently bonding sulfur to P3HT, strong interactions can be invoked between the polysulfides and the polymer, and the electrical conductance is increased, hence improving overall battery performance. Similarly, Zeng et al. synthesized a sulfurepolythiophene copolymer that was then encapsulated in a PEDOT:PSS thin layer [76]. The coated copolymer exhibits a superior conductivity, which translates to an enhanced capacitance and rate performance. In another work, sulfur was copolymerized with poly(m-aminothiophenol) as a thiol and

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Figure 18.14 (A) Schematics of the synthesis and proposed microstructure for S-coupled polythiophene (P3HT) copolymer and its enhanced cycling performance by copolymerization [75], and (B) Schematic of the processing Na/GPE/poly(S-PETEA)@C and cycling performance of Na/LE/S@C, Na/LE/poly(S-PETEA)@C and Na/GPE/poly(S-PETEA)@C at 0.1 C [17]. Abbreviation: S-PETEA, sulfurated-pentaerythritol tetraacrylate. (A) Adapted from B. Oschmann, J. Park, C. Kim, K. Char, Y.-E. Sung, R. Zentel, Copolymerization of polythiophene and sulfur to improve the electrochemical performance in lithiumsulfur batteries, Chem. Mater. 27 (2015) 7011e7017. (B) Adapted from D. Zhou, Y. Chen, B. Li, H. Fan, F. Cheng, D. Shanmukaraj, T. Rojo, M. Armand, G. Wang, A stable quasisolid-state sodiumesulfur battery, Angew. Chem. Int. Ed. 57 (2018) 10168e10172.

amine-rich polymer. An enhancement of the performance compared with noncovalently bound sulfur was observed [77]. Ji’s group synthesized two different sulfurrich copolymers with grafted PANI fragments [78,79]. In one case, conductive PANI segments were grafted onto a linear sulfur copolymer backbone, whereas in the other method, PANI was grafted to a cross-linked sulfur copolymer chain. Both copolymers were compared with a similar sulfur copolymer without grafted conductive segments. The results showed that the addition of the conductive polymer side chains increased the capacity, the C-rate capability, and the cycling stability. The direct interaction between the conductive polymer segment and the sulfur improves lithiumion diffusion and reduces the interfacial charge-transfer resistance. Moreover, the cross-linked S-PANI structure seems to successfully contain the polysulfides, which is confirmed by good capacity retention. A different approach to be considered to improve the electrical conductivity of the sulfur copolymer without integration of conductive polymers is the combination with nanostructured conductive carbon, forming both a physical and chemical confinement.

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J. Park et al. postulated that linking a sulfur-copolymer to GO would improve the electrical conductivity, and inhibit the volume expansion and the polysulfide dissolution [80]. They functionalized GO with oleylamine, and poly(S-r-DIB) was conjugated with oleylamine-rGO (O-rGO) to synthesize the conductive copolymer block. At a Crate of 0.5 C, the poly S-r-DIB-O-rGO cathode achieved an initial discharge capacity of 1265 mAh g1, and the capacity retention was also improved to 81.7% after 500 cycles. Another attempt to prepare a structured carbon/sulfur-copolymer to obtain a synergistic effect was presented by G. Hu et al., who used anodic aluminum oxide templates to process a hybrid cathode in which the S-r-DIB copolymer is filled in CNTs [81]. These strategies combine both physical and chemical ways to inhibit the polysulfide and improve electrical conductivity. I. Gomez et al. reported fast and easy inverse vulcanization of sulfur with divinyl benzene (DVB). The copolymerization of DVB with a high sulfur content was performed within 5 min [82]. The poly(S-DVB) copolymer network showed enhanced mechanical stability and acted as a sulfur reservoir, hence extending cell life. The capacity faded by just 0.04% over 1600 cycles at 0.5 C. An unconventional approach is to use a cross-linked polymer to synthesize a sulfurcopolymer. Zhou et al. synthesized a poly(S-PETEA)-based cathode for Na-S batteries (Fig. 18.14B) [17]. The sulfur-copolymer cathode retained the sulfur species by chemical binding and successfully inhibited the shuttle effect. A synthesized PETEA-tris[2-(acryloyloxy)ethyl] isocyanurate-based gel electrolyte was assembled with the polymeric sulfur cathode. The developed quasi-solid-state Na-S cell achieved an initial discharge capacity of 877 mAh g1 at 0.1 C. This approach shows a feasible pathway for developing Na-S batteries and reveals that these PETEA-based polymeric sulfur cathodes can potentially be applied in LSBs. The chemical confinement through inverse vulcanization copolymerization has proven to be a highly promising solution for the design of LSBs. Moreover, the combination of physical and chemical confinement was shown to enable the complete suppression of the polysulfide shuttle.

4. Polymer electrolytes 4.1

Solid polymer electrolytes

Substituting a liquid electrolyte by an SPE is a promising solution to overcome the limitations in LSBs, but it also introduces new design challenges because the SPE must be integrated into the cathode to provide conductive lithium-ion pathways for the electrochemical reaction. Combined with a lithium metal anode, the overall energy density of a cell can be improved because of its lower density and good processability as thin layers [19,83]. The internal resistance decreases with the reduction in electrolyte thickness, resulting in faster reaction kinetics within the cell. SPEs are electrochemically stable and nonflammable, which is important concerning the safety of batteries. Compared with solid inorganic electrolytes, they are low-cost, easily processable, and mechanically flexible, allowing the electrolyte to bend and fold [7]. Morever, depending on the shear modulus of the SPE, lithium dendrite growth can be suppressed [84,85].

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SPEs are another alternative to physically and chemically suppress the polysulfide shuttle. For this purpose, they must have a dense molecular structure without significantly changing viscosity when the operating temperature increases. Additionally, the functional groups of the polymer chain can interact with the polysulfides [86e88]. When designing polymer-based all-solid-state lithiumesulfur batteries, the SPE is used as the binder within the cathode to ensure the cohesion between the different components of the cathode composite and good adhesion to the current collector. SPEs are required to withstand significant mechanical stresses caused by the volume expansion. At the same time, the SPE enables essential lithium-ion pathways in the cathode that are crucial to access the entirety of the active material for the electrochemical reaction [89e91]. To achieve excellent performance in LSBs, an SPE requires a good ionic conductivity (>104 S cm1), appropriate mechanical strength and flexibility, and eventually the ability to trap polysulfides [7]. Among all SPEs, PEO has been the dominant material since 1970 because of its ability to dissolve various lithium salts [92]. Moreover, PEO and its derivatives have superior properties for high lithium-ion conductivity, such as low glass transition temperatures (Tg ¼ 66 C) and good chain flexibility [93]. When tailoring SPEs to improve the ionic conductivity, it is required to maintain adequate mechanical and thermal stability. In the case of PEO, the desired ionic conductivity is achieved at temperatures above its Tm (64 C). Unfortunately, an increase in temperatures changes the viscosity of the polymer and is detrimental to its mechanical properties. Recent approaches to increase ionic conductivity while simultaneously conserving thermal and mechanical stability include the addition of fillers and the synthesis of copolymers supporting a fully amorphous structure [19,31,75]. Moreover, fillers in HPEs enhance the interface stability of the cathode composite. In an early study, two different SPEs composed of PEO and poly(ethylenemethylene) oxide (PEMO) were integrated into the cathode composite, and their electrochemical performances were investigated. PEMO is a derivative of PEO and is fully amorphous at room temperature. Both SPEs were prepared by the solvent casting method, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was used as the lithium salt [94]. PEO-LiTFSI achieved the highest ionic conductivity (4.9
104 S cm1 at 90 C) and exhibited the highest initial discharge capacity (722 mAh g1cathode). In comparison, PEMO-LiTFSI led to a low initial discharge capacity of 243 mAh g1cathode due to the low ionic conductivity of PEMO (1.2
104 S cm1 at 60 C). The voltage profiles of PEMO-LiTFSI did not show the two characteristic plateaus of standard LSBs because of the poor active material utilization, which was restricted by the low ionic conductivity. In contrast, PEOLiTFSI allowed access to almost all available active material (1600 mAh g1sulfur). However, the capacity fade was significantly higher, indicating that polysulfides dissolve in the PEO-based SPE. The increase in operating temperature is beneficial for the ionic conductivity and active material utilization, but the loss of mechanical strength and enhancement of polysulfide dissolution kinetics lead to the rapid fade in capacity. Nevertheless, the potential of SPEs in LSBs, and the importance of high ionic conductivity (>5
104 S cm1) was demonstrated [94].

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Eshetu et al. investigated the influence of the type of lithium salt on the electrochemical performance of a PEO-based sulfur composite by comparing LiTFSI with lithium (fluorosulfonyl) (trifluoromethanesulfonyl)imide (LiFTFSI). It was observed that PEO-LiFTFSI was more stable and allowed to reach an initial discharge capacity of 800 mAh g1sulfur at a current density of 0.1 C and a coulombic efficiency of 99% over 60 cycles at 70 C [95]. This can partly be explained by the formation of a stable SEI layer at the cathode-electrolyte interface between FTFSI cations and polysulfides, protecting the cell from overcharging. Nevertheless, polysulfide dissolution persisted. A PVDF-based SPE was integrated into the sulfur cathode for its ability to completely suppress the polysulfide shuttle. Unfortunately, PVDF shows low ionic conductivity and must be improved to sufficiently access the active material. Therefore, Shan et al. modified the PVDF-based SPE by blending it with cellulose acetate, 1-butyl-1-methyl-pyrrolidine bis-trifluoromethyl sulfonimide (Py14TFSI) interface stabilizer, and LiTFSI. The material showed an ionic conductivity of 1.45
104 S cm1 at 25 C and high tensile strength. The significant feature of this SPE is the complete suppression of the polysulfide shuttle, which was confirmed by density functional theory (DFT) modeling and postmortem X-ray photoelectron spectroscopy measurements. The cell exhibited a high initial discharge capacity of 1245.9 mAh g1sulfur at 25 C, which faded to 600 mAh g1sulfur after 50 cycles [96]. DFT calculations confirmed the strong interaction of Py14TFSI cations with polysulfides, preventing their dissolution. When combined with a low donor number polymer, such as PVDF, the migration of polysulfides is effectively hindered [91]. Considering that SPEs show good performance in LSBs at higher temperatures, and only cells with PEO-based SPEs achieve high discharge capacities, this work revealed the potential of other polymers such as PVDF to be applied even at room temperature [97].

4.2

Hybrid solid polymer electrolytes

Another approach to enhance the properties of SPEs is by combining at least two components forming an HPE system. HPEs can increase active material utilization, lower the interface resistance between the cathode and the electrolyte, and potentially suppress polysulfide shuttle [95,98e101]. As previously stated, the ionic conductivity of PEO-based SPEs depends mainly on the amorphous phase of the polymer. Therefore, one approach is to lower the crystallinity of PEO with the addition of various fillers, such as passive inorganic fillers (e.g., BaTiO3, TiO2, SiO2, Al2O3, ZrO2, and MgO) [102e105] or active inorganic fillers e.g., Li7La3Zr2O12 (LLZO) and Li7P3S11 (LPS) [99,109]. Passive fillers only influence the level of the crystalline phase, whereas active fillers additionally contribute to lithium-ion transport. In many studies, the effect of the size, concentration, and shape of the filler particles on the properties of the SPE was investigated [104,106]. Common polymers for HPEs are PAN, PVDF, and PEO, with PEO-based HPEs being the most studied due to PEO’s distinctive properties [107,108]. It is important to note that the exact mechanism of lithium-ion transport is still up for debate regarding active fillers. So far, three different lithium-ion transport mechanisms are assumed to take place in

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Figure 18.15 (A) Fast lithium-ion conduction pathway alongside connected space charge regions of Li7La3Zr2O12 (LLZO) particles. (B) Distribution of the lithium vacancy concentration at the surface of the inorganic filler attracting lithium-ions to the surface of the space charge region.

HPEs simultaneously: first, the lithium-ions migrate through the polymer matrix; second, the ion transport alternates between the polymer and the particles; and finally, the lithium-ion migrates through the space charge region, which is located at the interface between the particles and the polymer (Fig. 18.15A). The latter was observed to be the predominant transport mechanism in several studies [109e113]. Firstly, the addition of passive fillers into a matrix polymer for the SPE-based cathode composites is discussed. Passive fillers enhance the electrochemical performance in terms of higher discharge capacity, lower capacity fade, and to some extent because of their ability to adsorb polysulfides [90,100,101]. The adsorption of polysulfides by some metal oxides was demonstrated in liquid electrolyte-based sulfur cathodes [100,114], and a similar effect was observed when metal oxides were integrated into a polymer to form an HPE [90,115]. Mamorstein et al. developed an HPE composed of PEGDME, LiTFSI, and fumed silica. The PEGDME HPE achieved a notable high ionic conductivity of 1.5
103 S cm1 at room temperature, which is attributed to the use of a short polymer chain length and the addition of fumed silica, which both reduce crystallinity. At room temperature, cells with this HPE achieved an initial discharge capacity of 376 mAh g1cathode, and a capacity fade of 50% after the first cycle. However, the capacity retention was improved when compared to standard PEO-based cathode. Although the ionic conductivity was higher than with PEO, the active material utilization was 50% lower [94]. This study highlights that ionic conductivity is not the only factor contributing to the access of the active material. In another study, the electrochemical performance and interfacial stability of HPEs comprising PEO10-LiCF3SO3 and TiO2 (0 wt.%e15 wt.%) were investigated with a sulfur-based cathode composite. The addition of TiO2 fillers had a positive effect on the battery’s overall performance, which was demonstrated by an improved ionic conductivity, better sulfur utilization, excellent initial discharge capacity of 1600 mAh g1sulfur at 90 C, and extended cycling stability. It was assumed that the

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TiO2 HPE formed a stable SEI layer suppressing the polysulfide shuttle [116]. Unfortunately, no additional measurements were conducted to validate this assumption. Another work incorporated the nanoclay montmorillonite (MMT) into PEO-LiTFSI to enhance the ionic conductivity and suppress the polysulfide shuttle. The incorporation of MMT increased the amorphous phase in the HPE, resulting in better ionic conductivity. The cathode was composed of a presynthesized active material of PANesulfureMg0.6Ni0.4O, acetylene black, and PEO-LiTFSI/MMT HPE, which delivered an initial discharge capacity of 998 mAh g1sulfur at 60 C and exhibited good capacity retention after the second cycle. The effect of the addition of PEO/ MMT on the suppression of the diffusion of polysulfides can be explained with the voltage profiles. The typical stepwise conversion reaction shows two characteristic plateaus. The first plateau indicates the formation of soluble polysulfide species, and the second plateau corresponds to the conversion of insoluble species. When MMT is added to the SPE, the voltage profile displays only one plateau, indicating no conversion of the solid polysulfide species into liquid soluble ones [117]. A much more promising approach is the incorporation of active fillers into the polymer matrix by using both oxidic and sulfidic materials. Commonly studied active oxide fillers include Li1.3Al0.3Ti1.7(PO4)3 (LATP), LLTO, and LLZO. The latter is the most widely investigated due to its superior ionic conductivity. A comparison between active and passive fillers in a PEO-LiClO4 SPE has been made using LATP and TiO2, respectively. The ionic conductivity was improved by two orders of magnitude when using LATP. Other works investigated the effect of the addition of active fillers on the electrochemical performance, and in all cases, the sulfur utilization increased significantly, while the capacity fade decreased [99,118,119]. Nonetheless, operando spectroscopic measurements revealed that polysulfides still migrate to the anode when using, for example, titanium-doped LLZO (LLZTO) as an active filler [118]. The precise interaction between HPEs and cathode composites remains unclear, and contradicting interpretations were reported based on secondary measurements. Some works incorporate fillers only in the electrolyte and not in the cathode, whereas other works conduct only electrochemical measurements without the support of spectroscopic data, e.g., the binding energy or possible interaction between active fillers and polysulfides. Overall, there is a lack of informative data regarding the interface interaction between HPEs and active material (sulfur and polysulfides), which explains the absence of a consensus. Tao et al. introduced a novel design strategy for HPEs comprising Al3þ/Nb5þcodoped cubic LLZO nanoparticles in the PEO matrix, combined with a porous carbon foam (LLZO@C) modified with doped LLZO nanoparticles as ion conductive agent in the cathode. As a result, the HPE with 15 wt% LLZO possessed the highest ionic conductivity of 1.1
104 S cm1 at a moderate temperature of 40 C. Moreover, the cycling performance of the cathode with LLZO@C revealed exceptional discharge capacities of >900 mAh g1sulfur at moderate 37 C and stable cycling behavior over 200 cycles at 0.05 C. The capacity fade after 80 cycles was above 10%, which is exceptionally low compared with other recent studies. Coulombic efficiency was stable at 100% over 200 cycles. The authors state that these outcomes are related to the superior ionic conductivity in the electrolyte and the reduced interface resistance between the HPE

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and the LLZO@C. The cathode without LLZO achieved an initial capacity 40% lower than that of the cathode modified with LLZO. Unfortunately, no further chemical analysis was performed to confirm these assumptions [99]. Recently, Wang et al. addressed the problem of an inefficient ionic and electronic network within polymer-based cathode composites. An electron/ion dual-conductive framework was introduced by an HPE infiltrated as a liquid precursor into a porous rGO/S-CNT composite, successfully constructing continuous penetrating ionic and electron pathways. The PEO-LLZTO HPE achieved an ionic conductivity of 0.84
104 S cm1 at 30 C. Compared with a standard sulfur cathode setup, the composite exhibited a 50% higher initial discharge capacity of 506 mAh g1sulfur even at 1 C and 925 mAh g1sulfur after 100 cycles at 0.1 C. The enhancement in sulfur utilization is related to the highly stable ionic conductive network within the cathode. The influence of this network was also observed in voltage profiles for different C-rates. The characteristic plateaus were visible for 0.05e1 C in cells with the dual ionic/electron conductive cathode, whereas the plateaus disappeared for the standard sulfur cathode composite. The fluctuation in the charge curves indicates a decrease in the electrochemical kinetics related to the low ionic conductivity of the standard sulfur cathode. The high capacity retention of the dual ionic/electron conductive cathodes is attributed to the possible confinement of polysulfides by LLZTO [120]. In recent years, increasing attention has been paid to HPEs made with active sulfidic fillers. LPSs, LGPSs, and LSiPSs are the most studied fillers among sulfidic materials [7] and have been incorporated into PEO-based SPEs [119,121,122]. Even though the ionic conductivity of sulfidic fillers is better compared with oxidic fillers, they are generally not favored because of their instability and toxicity when reacting with atmospheric humidity. The cycling stability and performance improvement using LPS-based HPEs were demonstrated by Li et al., who incorporated 1 wt.% LSPS into a PEO-LiTFSI SPE. The resulting HPE achieved an ionic conductivity of 1.69
104 S cm1 at 50 C. The aluminum current collector was replaced by nickel foam to prevent corrosion. At 60 C, the initial capacity increased slightly from 934 mAh g1sulfur (PEOLiTFSI) to 1015 mAh g1sulfur (LSPS-PEO-LiTFIS). A 100% coulombic efficiency over 40 cycles was obtained with the HPE, whereas the SPE malfunctioned after 16 cycles because of the high overpotentials [119]. Currently, no studies have investigated whether or not sulfide particles in SPE-based cathodes directly interact with polysulfides. It has been found that polysulfides can be chemically anchored only when a pure sulfidic solid electrolyte is used [123].

5.

Conclusion and outlook

This chapter has discussed the development and application of polymer nanocomposites for lithiumesulfur batteries, particularly for sulfur-based composite cathodes. We have detailed how their electrical, chemical, mechanical, and ionic properties can improve battery performance. The various challenges faced by LSBs require the development of polymer composites specially designed and optimized for several of the above properties.

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For example, integrating electrically conductive polymers into a sulfur-based cathode can enhance sulfur utilization and rate capacity, functionalized polymers can trap polysulfides to delay capacity fade, flexible polymers can withstand volume expansion, and ionically conductive polymers can inhibit polysulfide diffusion while maintaining Li-ion transport. In the future, the demand for sustainable energy-storage devices will continuously grow, further penetrating the portable electronics and mobility sectors. Moreover, new application areas such as full-electric short-range aviation will arise. Therefore, the next generation of batteries must become lighter, smaller, and more powerful. These attributes can be developed by optimizing existing battery materials or with alternative electrochemical systems. Research progress in polymeric nanocomposites alone cannot overcome these limitations. However, this work can significantly enhance the synergy between polymers and cathode composites to elevate overall performance, including developing low-temperature SPEs and self-healing materials that retain structural integrity during cycling.

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[109] Z. Li, H.M. Huang, J.K. Zhu, J.F. Wu, H. Yang, L. Wei, X. Guo, Ionic conduction in composite polymer electrolytes: case of PEO:Ga-LLZO composites, ACS Appl. Mater. Interfaces 11 (1) (2019) 784e791, https://doi.org/10.1021/acsami.8b17279. [110] H. Zhai, P. Xu, M. Ning, Q. Cheng, J. Mandal, Y. Yang, A flexible solid composite electrolyte with vertically aligned and connected ion-conducting nanoparticles for lithium batteries, Nano Lett. 17 (5) (2017) 3182e3187, https://doi.org/10.1021/ acs.nanolett.7b00715. [111] W. Liu, S.W. Lee, D. Lin, F. Shi, S. Wang, A.D. Sendek, Y. Cui, Enhancing ionic conductivity in composite polymer electrolytes with well-aligned ceramic nanowires, Nat. Energy 2 (5) (2017) 1e7, https://doi.org/10.1038/nenergy.2017.35. [112] Z. Wan, D. Lei, W. Yang, C. Liu, K. Shi, X. Hao, L. Shen, W. Lv, B. Li, Q.H. Yang, F. Kang, Y.B. He, Low resistanceeintegrated all-solid-state battery achieved by Li7La3Zr2O12 nanowire upgrading polyethylene oxide (PEO) composite electrolyte and PEO cathode binder, Adv. Funct. Mater. 29 (1) (2019) 1e10, https://doi.org/10.1002/ adfm.201805301. [113] W. Liu, N. Liu, J. Sun, P.C. Hsu, Y. Li, H.W. Lee, Y. Cui, Ionic conductivity enhancement of polymer electrolytes with ceramic nanowire fillers, Nano Lett 15 (4) (2015) 2740e2745, https://doi.org/10.1021/acs.nanolett.5b00600. [114] B. Ding, L. Shen, G. Xu, P. Nie, X. Zhang, Encapsulating sulfur into mesoporous TiO2 host as a high performance cathode for lithiumesulfur battery, Electrochim. Acta 107 (2013) 78e84, https://doi.org/10.1016/j.electacta.2013.06.009. [115] H. Marceau, C.S. Kim, A. Paolella, S. Ladouceur, M. Lagacé, M. Chaker, A. Vijh, A. Guerfi, C.M. Julien, A. Mauger, M. Armand, P. Hovington, K. Zaghib, In operando scanning electron microscopy and ultravioletevisible spectroscopy studies of lithium/ sulfur cells using all solid-state polymer electrolyte, J. Power Sources 319 (2016) 247e254, https://doi.org/10.1016/j.jpowsour.2016.03.093. [116] J.H. Shin, K.W. Kim, H.J. Ahn, J.H. Ahn, Electrochemical properties and interfacial stability of (PEO)10LiCF3SO3-TinO2n - 1 composite polymer electrolytes for lithium/ sulfur battery, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 95 (2) (2002) 148e156, https://doi.org/10.1016/S0921-5107(02)00226-X. [117] Y. Zhang, Y. Zhao, D. Gosselink, P. Chen, Synthesis of poly(ethylene-oxide)/nanoclay solid polymer electrolyte for all solid-state lithium/sulfur battery, Ionics (Kiel). 21 (2) (2015) 381e385, https://doi.org/10.1007/s11581-014-1176-2. [118] Y.X. Song, Y. Shi, J. Wan, S.Y. Lang, X.C. Hu, H.J. Yan, B. Liu, Y.G. Guo, R. Wen, L.J. Wan, Direct tracking of the polysulfide shuttling and interfacial evolution in all-solidstate lithiumesulfur batteries: a degradation mechanism study, Energy Environ. Sci. 12 (8) (2019) 2496e2506, https://doi.org/10.1039/c9ee00578a. [119] X. Li, D. Wang, H. Wang, H. Yan, Z. Gong, Y. Yang, Poly(Ethylene oxide)-Li10SnP2S12 composite polymer electrolyte enables high-performance all-solid-state lithium sulfur battery, ACS Appl. Mater. Interfaces 11 (25) (2019) 22745e22753, https://doi.org/ 10.1021/acsami.9b05212. [120] J. Wang, X. Yan, Z. Zhang, R. Guo, H. Ying, G. Han, W.Q. Han, Rational design of an electron/ion dual-conductive cathode framework for high-performance all-solid-state lithium batteries, ACS Appl. Mater. Interfaces 12 (37) (2020) 41323e41332, https:// doi.org/10.1021/acsami.0c10463.

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Design of nanostructured sulfur cathodes for high-performance lithiumesulfur batteries

19

Masud Rana, Xia Huang and Bin Luo Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, St. Lucia, QLD, Australia

1. Introduction Lithium-ion battery technology has played an essential role in powering portable electronics and electric vehicles [1]. However, state-of-art lithium-ion batteries still cannot fully meet the stringent requirements for electric vehicles and large-scale energy-storage applications, especially energy density improvement and cost reduction [2]. Among the contenders for next-generation electrochemical energy-storage systems, lithiumesulfur batteries (LSBs) are particularly promising owing to their high theoretical energy density (2600 Wh kg
1, 2800 Wh L
1) and the abundant sulfur feedstock [3]. However, the commercial application of LSBs has been hindered by several barriers, among which the shuttling effect is considered the most critical [4]. The shuttling problem is the dissolution, accumulation, and migration of polysulfides (PSs), leading to rapid capacity loss, poor cycling stability, and self-discharge of LSBs [5]. Rational sulfur cathode design based on PS characteristics is one of the most efficient approaches to mitigate the shuttle effect [6e8]. This chapter first introduces the intrinsic properties of PSs, followed by a demonstration of the sulfur cathode design criteria to address the shuttle effect, including the following: U Suitable pore structures and specific surface area of the nanostructured cathode materials that can physically block the PS from cathode to anode diffusion U High electronic and ionic conductivity with relatively low electrolyte/sulfur ratios U Chemically interactive materials (metals, functional groups, dopants, etc.)- to chemically suppress the PS from diffusion from cathode to anode U Electrocatalytic effect of nanostructured cathode materials that can accelerate the electrochemical reactions

Then, some representative studies on rational sulfur cathode design for highperformance LSBs are summarized, including carbonaceous materials, metal-based compounds, and polymer composites. Finally, a brief conclusion and the outlook for the development of LSBs toward practical application will be provided.

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00009-0 Copyright © 2022 Elsevier Inc. All rights reserved.

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Redox processes and polysulfide characteristics of lithiumesulfur batteries

A traditional LSB cell consists of the sulfur cathode, separator, lithium anode, and ether-based electrolyte (Fig. 19.1) [9]. The sulfur cathode experiences a complicated multistep electrochemical redox process upon discharge. The solid S8 is progressively reduced to highly soluble long-chain PSs (Li2S8, Li2S6), to smaller Li2Sx species (x ¼ 3, 4), and then to insoluble lithium sulfides (Li2S2 and Li2S) [10,11]. The long-chain PSs diffuse from the cathode to anode and are chemically reduced by the lithium anode to either short-chain PSs that diffuse back to the cathode side driven by concentration gradient (namely, the shuttle effect) or solid lithium sulfides that will passivate on the lithium surface [12]. The dissolution and migration of PSs are considered the origin of fast capacity decay and low coulombic efficiency due to active material loss, anode corrosion, and electrolyte degradation [13]. On the other hand, the insulating solid sulfur and lithium sulfides can accumulate to form thick passivation layers on both electrode surfaces, interrupting the electronic and ionic transport pathways for further electrochemical redox reactions [14,15]. Therefore, it is crucial to design an efficient sulfur host to control the diffusion of higher-order PSs and reactivate solid sulfur species during the repeated charge/discharge of LSBs.

Figure 19.1 Conceptual diagram with various fundamental components and mechanisms of lithiumesulfur batteries in ideal condition (A) and (B) practical operation [9]. (A, B) Reproduced with permission from M. Rana, et al., The role of functional materials to produce high areal capacity lithium sulfur battery, J. Energy Chem. 42 (2020) 195e209, Copyright 2020, Elsevier.

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The Li2Sx (x ¼ 3, 4, 6, 8) molecules exhibit a ringlike structure with two Liþ ions on opposite sides of the ring. The distance of the LieS bond has been shown to shorten measurably while reducing into low-order PSs, implying stronger intermolecular interactions between Liþ and S
x (higher binding energies) [16]. The PS transport is highly related to the average charge transfer between Liþ and S
x within the PS structure. More specifically, the charge transfer is substantially decreased from 0.316 e per Liþ of Li2S3 to 0.236 e per Liþ of Li2S8, leading to much weaker interactions between Liþ and S2
8 . Therefore, the dissolution of these PSs is likely a competitive process wherein the Li2S8 with the lowest binding energy between Liþ and S2
8 exhibits the highest solubility in the electrolyte among all the soluble PSs (Fig. 19.2) [17]. The diameter and electronegativity of Liþ is 0.45 nm and 0.97, respectively [18], while Liþ tends to transport and interact with S8 to produce Li2S8 with a maximum length of 2 nm, and the length of Li2S4 becomes 1.6 nm as the potential is applied to LSBs [19]. Despite these research efforts, an agreement on the molecular nature of PSs is far from being reached, with various uncertainties and anticipations remaining due to the complicated interactions of the sensitive PSs with the electrolyte [20]. For example, it has been reported that PS anions become solubilized in the electrolyte and diffuse across the battery to cause the shuttle effect [21e23], while other researchers have claimed that the PS species in the solution remained as lithiated neutral species [24e27]. In addition, the electric field present in LSBs adds additional possibilities to transport these high-order anionic PS species to the anode [28,29]. Further effort is required to better understand how the PS behaves in the bulk electrolyte, which is important for shedding light on data for use in future LSB optimization.

Figure 19.2 Sulfuresulfur and lithiumesulfur bond lengths in angstrom (Å) as well as their structural views [17]. Reproduced with permission from L.-C. Yin, et al., Understanding the interactions between lithium polysulfides and N-doped graphene using density functional theory calculations, Nano Energy 25 (2016) 203e210, Copyright 2016, Elsevier.

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3.

Design criteria for lithiumesulfur battery cathodes

3.1

Pore structure

Pore structure, including volume and diameter, is a crucial parameter to control the diffusion of PS through a physical separation and permit the passage of Liþ during LSB cycling. The pores are distributed in three sizes: micropores 2 nm, mesopores of 2 to 50 nm, and macropores 100 nm (Fig. 19.3). The pore diameter is proportional to the pore volume and inversely proportional to the total surface area of the materials. Typically, porous materials can be characterized based on the pore size, shape, and surface area. The porosity of materials can be determined based on the physical pores called interparticle pores. On the other hand, internal pores can be distinguished as microspores (2 nm), mesoporous materials (2e50 nm), and macrospores (50 nm). The physical pore allows the passage of PSs to be macroporous compared with the dimensions of Li2S8 (w2 nm) and Li2S4 (w1.6 nm). Considering the dimensions of PSs, micropores seem to be effective in controlling the PS diffusion. It is established that nonpolar host materials interact with PSs in van der Walls interactions, and at room temperature, the PSs are intended to diffuse again. The materials with internal pore diameters less than the PS dimension (w2 nm) would help mitigate PS diffusion at both van der Walls interactions and physical separations. Another point is that the materials with smaller pore

Figure 19.3 Conceptual schematics showing the differences among micropores, mesopores, and macropores and their characteristics to allow Liþ and mitigate the passage of PSs [5].

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diameters and surface areas do not have enough space to impregnate the sulfur, leading to low sulfur loading. Therefore, the pore diameters also need careful attention in the design of PS host materials.

3.2

Surface area

High sulfur loading has been pursued as one of the key factors to promote LSBs into commercial application. Especially, sulfur content (wt%) and areal loading (mg cm
2) are important parameters for LSBs that need to be considered in LSB research. Usually, a high active material content of z90 wt% is used in commercial LIB cathodes. However, in LSBs, the sulfur content in sulfur-based nanocomposites is limited to 50 wt%e80 wt%. The host materials must have a high surface area to accommodate more sulfur content (wt%) and higher areal loading (mg cm
2). On top of these benefits, the high surface area also promotes Liþ conduction during cycles and allows a low electrolyte-to-sulfur ratio.

3.3

Ionic conductivity

Ionic conductivity in the electrolyte of LSBs is crucial to promote electrochemical reactions. The ionic conductivity of the electrolyte is an essential parameter to be considered for charging LSBs at a high C-rate. This electrical conductivity is due to the motion of ionic charge (Liþ) in the electrolyte. The most commonly used salt in the LSB electrolyte is lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI) and lithium nitrate (LiNO3) as a co-salt. The purpose of using LiTFSI is to promote the Liþ during the charge and discharge to have accelerated ionic exchange during charge/discharge. The LiNO3 is mostly used to suppress the shuttle issue in LSBs. The LiNO3 reacts with Li to form a robust surface layer on the Li anode that protects the Li anode from reacting with the dissolved PS. The ionic conductivity of the electrolyte depends on the temperature, ionic concentration, and viscosity of the electrolyte [30,31]. Hence, the high ionic conductivity could be explained in the electrolyte with less viscous at high temperatures. The addition of co-salt LiNO3 additive and formation of PSs in LSBs also affect the viscosity of the electrolyte in LSBs. Hence, there is a trade-off between the diffusion of PSs and the ionic conductivity controlled by the concentration of LiNO3. The highest ionic conductivity was obtained for LSBs electrolyte with the presence of 1 M (LiTFSI) in TEGDME/DOL ¼ 33: 67(volume ratio) based electrolyte at ambient temperature [32].

3.4

Electrical conductivity

The sulfur is the active component in LSBs to store/release energy in LSBs. The electrochemical reaction of S8 and Liþ produces PSs in the LSBs that cause the shuttle effect. The final discharge product of this electrochemical reaction is Li2S after successive electrochemical reactions. The sulfur has very poor electrical conductivity 5  10
30 S cm
1 at 25 C. On the other hand, all the PSs and their ultimate solid discharge products of Li2S are also electronically insulating (electronic resistivity

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Figure 19.4 The importance of high electrical conductivity to reactivate PSs in lithiumesulfur batteries.

ca. 10
30 S cm
1 [33]), which also impedes the ionic exchange of Liþ in the electrolyte (Liþ diffusivity in Li2S is 10
15 cm2 s
1) [34]. The insulating nature of both necessitates incorporating electrically highly conductive materials to maintain smooth electron access to the sulfur and Li2S to promote the electrochemical kinetics (Fig. 19.4).

3.5

Chemical interactions

The shuttle effect is one of the leading causes of substantial vulnerability of active materials losses, high overpotential losses, and rapid capacity decay [14]. Nonpolar carbon-based materials have shown tremendous performance over the years to mitigate the shuttle issue in LSBs. However, the nonpolar carbon materials accommodate PSs into their structure only through the van der Walls interaction, and PSs tend to be unbound at room temperature. Hosting materials with chemically interactive elements or functional groups are another alternative to chemically interact with the PSs and mitigate the shuttle issue in LSBs. There are different types of chemical interactions, as discussed below.

3.5.1

Polarepolar interactions

Due to the inherently polar nature of PSs, they do not show strong chemical interactions with typical nonpolar carbon materials [35]. The nonpolar surface of carbon serves as a physical adhesion of PSs that does not afford strong binding with PSs. Different types of chemically interactive materials have been explored to establish polarepolar interactions with PSs, including modified carbon, functionalized

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Figure 19.5 Conceptual mechanism of (A) Polarepolar (B) Lewis acidebase and (C) Catenation interactions [9]. Reproduced with permission from M. Rana, et al., The role of functional materials to produce high areal capacity lithium sulfur battery, J Energy Chem. 42 (2020) 195e209, Copyright 2020, Elsevier.

polymeric, and inorganic materials [36] (Fig. 19.5). Sufficient electrical conductivity must be maintained for these polar host materials to ensure proper sulfur use and reactivation, especially for LSBs with thick sulfur electrodes. The reduced graphene oxide (rGO) is a pretty good example with different polar host materials such as carboxyl (eCOOH), hydroxyl (eOH), and ester groups (CeOeC) to bind PS [37,38]. Other different functional polar groups such as sulfonic (SO
3 ), nitrile (eC^N), imine (eN^), and amine (eNH2) groups also exhibit strong polarepolar binding sites for PS. Various metal oxides such as Mg0.6Ni0.4O and Al2O3 are also examples of polare polar interactions with PS [35]. Electronegative heteroatom elements (N, O, S) have mostly been reported as effective anchoring sites, including nitriles, amines, pyrrolidones, esters, and thiophenes [39]. Depending on the interactive nature of these electron-rich elements (LieN and Sn2
with a polar host), the interactions between polar host and heteroatoms are identified as polarepolar and Lewis acidebase. Hou et al. [40] carried out a systematic study on the higher electronegativity of F (3.98), O (3.44), and N (3.07) atoms against Li atoms (0.98) and reported that the lone extra electron pair are electron-rich donors with filled p orbitals that naturally act as Lewisbase sites to interact with PSs through LieN bonding. Conversely, electron-rich B, F, S, P, and Cl monodopants in the carbon matrix saw insignificant interactions with PS.

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Lewis acidebase polysulfide interactions in lithiumesulfur batteries

Beyond the polarepolar bonding, the Lewis acidebase nature of the PS host is another effective way to mitigate the shuttle effect. The PS anions (S2
n ) are soft Lewis bases with lone sulfur electron pairs. Therefore, PS host materials with Lewis acidebase characteristics strongly interact with PSs [41]. Generally, the electron-rich P orbitals of Lewis acidebase materials interact with PSs through dipoleedipole electrostatic interaction and thereby effectively prevent PS shuttle [42]. Metaleorganic frameworks (MOFs), MXene phases, and substoichiometric metal chalcogenides come from the class of Lewis acidsebases. The MXene phase introduces abundant Lewis acid Ti sites and hydroxyl groups and interestingly introduces metal-sulfur (SeTieC) binding at the interface socalled Lewis acidebase interactions [39]. The electronegative metals (Ni, Co, Fe, Cr, Mo, Mn) and substoichiometric oxides (Ti4O7 and Co9S8) and other sulfides (CoS2 and FeS2) are considered in this category with Lewis acid characteristic [39,43,44]. Different metal oxides also interact with the PS through LieO/S bonding [45].

3.5.3

Catenation interactions

PSs produce polythiosulfate triggered by metal oxides such as MnO2, VO2, and CuO to mediate the PS migration [46]. For example, in MnO2, Mn(iv) ions undergo a redox 2
reaction with PS S2
n anions to produce functional thiosulfate groups (S2O3 ) on the 2
surface. Later, the long-chain Sn (x > 4) is catenated into the SeS bond of S2O2
3 species to form polythionate (O3SeSn
2eSO3)2
, eventually forming short-chain Li2S and Li2S2. These catenated SeS chains in the polythionate are electrochemically active during subsequent dischargeecharge processes. The polythiosulfate preserves the ionic conductivity of PS (10
5 to 10
6 S cm
1), leading to Liþ-ion diffusion.

3.5.4

Electrocatalysis

The slow conversion reactions from the soluble long-chain PSs to insoluble shortchain PSs have been identified as the shuttle effect’s origin [47]. Using electrocatalysts to accelerate sulfur redox kinetics is a promising approach to migrate PS accumulation for increased cycling stability and to promote the rate capability of LSBs [48]. Inspired by the solar cell and redox flow cells where the reactions are promoted by using electrocatalytic electrodes, using catalytic current collector in LSBs was initially proposed by Leela et al. [49] LSBs using Ni coated on Al foil as the current collector exhibited the smallest polarization than those of using Pt and Au coated current collectors, suggesting Ni a promising electrocatalyst for sulfur conversions. The performance of LSBs was further enhanced by using graphene with well-dispersed catalyst nanoparticles as the electrode [50]. Various metal-based compounds and doped-carbon materials have been developed as electrocatalysts for LSBs since then, which significantly promoted the performance of LSBs [51]. Furthermore, the knowledge on performance enhancement mechanisms through using different electrocatalysts has been continuously deepened by using a wide range of advanced material characterization techniques and theoretical calculations [52].

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4. Sulfur host materials for lithiumesulfur batteries In responding to the abovementioned criteria for high-performance LSBs, many materials have been developed/introduced as sulfur hosts over the past several decades. Advances in understanding sulfur redox mechanisms and knowledge about the structureeproperty relationships in LSBs also drive material design and fabrication [53]. In the early stage, improving the conductivity of insulating sulfur species and physical confining the soluble PSs are regarded as the keys to improving the sulfur utilization and cycling life of LSBs, and thus, various carbon materials and conductive polymers have been developed as sulfur hosts based on their high conductivity, high specific surface areas, and tunable pore structures [3]. It was not until the recognition that the weak physical confinement of PS lasts less than 200 cycles that the research focus shifted to various metal-based materials that provide stronger chemical interactions with PS to endow long cycling stability to 1000 cycles [35]. In the past few years, electrocatalysts have been used to facilitate the redox kinetics of sulfur conversion reactions predominantly to suppress the shuttle effect [48]. Instead of systematically summarizing the advances achieved in the past decades, representative research will be discussed in the following context regarding the development of carbon materials, metal-based compounds, and polymers for sulfur cathodes.

4.1

Carbon-based materials

Carbon materials feathered with high conductivity, tunable porosity, and high specific surface area have received extensive attention to increasing sulfur utilization and cycling stability at the early stage. Various carbon materials have been developed while continuously optimizing the fabrication process to produce desirable structures and morphologies to enhance battery performance [54,55]. Among carbon materials, graphene is a two-dimensional, one-atom-thick, highly conductive material with various additional benefits high surface area, chemical stability, and mechanical strength and flexibility, making it useful for electrochemical energy-storage applications [56]. Liu et al. [57] developed a highly conductive graphene sheet to load more active sulfur. The crumpled graphene sheets with high flexibility ensured excellent porous structures to afford sufficient space for the lithiation and expansion of sulfur and greatly improved the electrical conductivity for significant sulfur utilization. The graphene sheet with a high specific surface area of 987 m2 g
1 exhibited decent electrochemical performance with an initial specific capacity of 1354 mAh g
1 and retention capacity of 735 mAh g
1 after 500 cycles at 1 C. Recently, graphene quantum dots have been considered favorable heterogeneous sites to PS nucleation and dendrite suppression for LSBs [58]. In this research, Raman spectroscopy revealed that the regulated electric field between electrolyte and electrode interface is beneficial to suppress the dendrite formation in LSBs. Recently, hydrogen-substituted graphdiyne (HsGDY)/graphene with an ultraspecific surface area of 2184 m2 g
1 has been used to achieve superior electrochemical performance from LSBs [59]. The acetylenic active sites in HsGDY can trap the Liþ of PSs owing to the strong adsorption, as shown in Fig. 19.6A.

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Figure 19.6 (A) PS molecules interactions on the hydrogen substituted graphdiyne (HsGDY)/ graphene [59]. (B) Proposed self-caging mechanism for the growth of yolk-shell graphene@ sulfur particles [60]. (C) sulfur adsorbed on (A) nitrogen-doped carbon with pyridinic NeCOOH functional group and (B) nitrogen-free carbon with eCOOH group [61]. (A) Reproduced from S. Kong, et al., Hydrogen-substituted graphdiyne/graphene as an sp/sp 2 hybridized carbon interlayer for lithiumesulfur batteries, Nanoscale 13 (6) (2021) 3817e3826 with permission from The Royal Society of Chemistry. (B) Reproduced with permission from P. Yu, et al., Template-free self-caging nanochemistry for large-scale synthesis of sulfonatedgraphene@ sulfur nanocage for long-life lithium-sulfur batteries. Adv. Funct. Mater. (2021) 2008652, Copyright 2021, Wiley-VCH). (C) Reproduced with permission from J. Song, et al., Nitrogen-doped mesoporous carbon promoted chemical adsorption of sulfur and fabrication of high-areal-capacity sulfur cathode with exceptional cycling stability for lithium-sulfur batteries, Adv. Funct. Mater. 24 (9) (2014) 1243e1250, Copyright 2014, Wiley-VCH.

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The strong physical adsorption and chemical interactions promoted the conversion reaction of PSs to further mitigate the shuttling problem. Considering the large volume changes, the shuttle effect, and poor conductivity of the electrode, Yu et al. [60] have reported interesting research on the sulfonated graphene (SG) as a sulfur nanocase (Fig. 19.6B). The SG@S nanocage greatly improved the conductivity of the composite and effectively trapped the volume changes of the active sulfur particles during operation. Thus, the SG@S composite electrodes exhibited superior material functions and structural stability with a minimal capacity fading rate of 0.019% per cycle at 0.5 C for over 2000 cycles. A mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite has been reported as a promising cathode for advanced LSBs. The highly electronegative nitrogen doping in the MPNC material can effectively accelerate the chemical adsorption of PSs [61]. The calculated interaction energies of sulfur on nitrogen-doped carbon and nitrogen-free carbon are
56.88 kCal mol
1 and
41.92 kCal mol
1, respectively, indicating that nitrogen doping can enhance the stabilization of sulfur in the eCOOH group in the carbon (Fig. 19.6C). On the other hand, the microporous structure ensured physical encapsulation of PSs into their surface area to mitigate their shuttling from cathode to anode. By this mutual contribution of the MPNC host materials, 95% retention capacity was achieved. Li et al. [62] developed a facile and sustainable route to produce porous conductive carbon through the carbonization of banana peel. The carbon-sulfur cathode achieved outstanding electrochemical performance at high areal loading. The major contribution of this research is subject to the highly conductive carbon to promote the electron transfer and the hierarchically porous structure to encapsulate sulfur with good accessibility to electrolyte penetration. The research community has long focused on making good uses of the high conductivity and high specific surface area of carbonaceous materials to increase sulfur utilization and cycling stability (through both physical and chemical interaction with PS) of LSBs, while the electrocatalytic capability of carbon-based materials has not been clearly revealed until recently by Zhen et al. [63] The electrocatalytic sulfur reduction reaction has been systematically investigated using different heteroatomdoped holey graphene framework (HGF) as a model, including pure HGF, nitrogendoped, sulfur-doped, nitrogen and sulfur dual-doped HGF. In combination with DFT calculation and various well-developed experimental approaches in oxygen reduction reaction area (e.g., rotating disk electrode), it is revealed that the initial sulfur to soluble PS conversion is facile with low activation energy, while the long-chain PS to insoluble PS transformation must overcome high activation energy, which leads to PS accumulation and shuttle effect. The nitrogen and sulfur dual-doped HGF significantly reduced the charge transfer resistance and activation energy of sulfur reduction reactions, endowing LSB with high-rate capability and long-term cycling stability. This work fundamentally demonstrated how materials fascinate the redox kinetics of sulfur conversion reaction to promote the rate capability and improve the life span of LSBs, suggesting the rational design of electrocatalysis, a promising strategy for high-performance LSBs.

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Metal compound-based sulfur cathodes

To further enhance chemical interaction between PSs, many metal-based compounds have been proposed as sulfur hosts, e.g., metal oxides, sulfides and nitrides, and MOFs [64].

4.2.1

Metal oxides

Because of the oxygen anion (O2
), metal oxides typically have a strong polar surface, providing abundant active polar sites for PS adsorption [65]. TiO2 with versatile morphologies (e.g., nanoparticles, nanofibers, and mesoporous hollow spheres) is among the most widely explored metal oxides in LSBs [66e68]. Despite the strong chemical bonding with PS, the insulating nature of TiO2 has impeded the redox reactions of PS anchored on the TiO2 surface [69]. In this regard, the conductive Magnéli phase TinO2n
1 (3 < n < 10) was proposed as a sulfur host, which endowed the LSBs with superior rate performance and capacity retention compared with that of using carbon and TiO2 as the sulfur hosts [70,71]. The enhanced performance was attributed to the high bulk metallic conductivity of Ti4O7 and the strong bonding between the lowcoordinated Ti sites of Ti4O7 and sulfur species. Different from the widely applied chemisorption, a new chemical approach was proposed to inhibit the PS dissolution by Nazar et al. d-MnO2 nanosheets were demonstrated as a highly efficient PS reservoir through reacting with PS to generate thiosulfates (S2O2
3 ), which reversibly acted as a redox shuttle that converts highly soluble long-chain PS to insoluble short-chain sulfur species via disproportionation, endowing the LSB with a low capacity decay rate of 0.036% per cycle for over 2000 cycles [72]. It was later revealed that the formation of active thiosulfates was directly correlated to the redox potentials of the host materials. Materials with a redox potential in the range of 2.4e3.05 V versus Li/Liþ react with PS to generate thiosulfates/polythionates, materials with potentials below w1.5 V versus Li/Liþ do not react with PS (e.g., Co3O4 and Ti4O7), while materials with too high redox potentials (>3.05 V vs. Liþ/Li) oxidize PS to form inactive sulfate and thiosulfate (Fig. 19.7A) [46]. For example, LSBs using VO2-graphene (the redox potential of VO2, 2.79 V) as the sulfur host exhibited a low capacity decay rate of 0.058% per cycle for 1000 cycles, while V2O5-graphene (the redox potential of V2O5, 3.4 V) delivered poor cycling stability. The active shuttle mediator formation mechanisms proposed in this work open a new avenue for designing highperformance sulfur hosts.

4.2.2

Metal sulfides

Although the chemical interaction between PS metal sulfides is not as strong as that with metal oxides (Fig. 19.7B) [64], metal sulfides with inherent metallic conductivity or electrochemically catalysis on sulfur conversion reactions are of great attraction for performance enhancement of LSBs [75]. Coupling theoretical calculations with experimental research, Cui and co-workers demonstrated that TiS2, CoS2, and VS2 significantly facilitated the oxidation of Li2S (with reduced overpotential at the initial stage of the charge, Fig. 19.7C), which greatly outperformed that of SnS2, Ni3S2, and

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FeS [73]. LBSs with TiS2, CoS2, or VS2 delivered greatly enhanced rate capability and capacity retention, which was attributed to these metal sulfides’ high conductivity and strong chemical interaction with PSs (Fig. 19.7D). Considering both the adsorption and active catalytic sites are largely restricted to the edges of two-dimensional nanosheets [76], Wang et al. initially proposed a nanoconfined conversion process to synthesis single-to several-layered TiS2 nanosheets sandwiched by porous carbon (TiS2@NSC) [74]. Compared with the bulk TiS2 and porous carbon (NSC) counterparts, TiS2@NSC-based LSBs exhibited a greatly reduced energy barrier at the initial charge process (Fig. 19.7E), leading to a superior capacity of 920 mAh g
1 over 120 cycles at 0.2 C under lean electrolyte condition. In addition to the strong PS bonding and efficient catalysis, electrochemical active metal sulfides can contribute additional capacity when their redox potential window lies in the same range as LSBs. An intercalationeconversion hybrid cathode was constructed by combing the Chevrelphase Mo6S8 (128 mAh g
1 in 1.7e2.8 V vs. Li/Liþ) with S8 (at w1:1 by weight), which delivered jointly high gravimetric and volumetric energy density of 366 Wh kg
1 and 581 Wh L
1, respectively, under practical operating conditions with a low carbon content of 10 wt% and low electrolyte-active material ratio of 1.2 mL mg
1 [77]. Mo6S8 played an imperative role for the impressive performance including, reducing the utilization of carbon due to its high conductivity, increasing the volumetric energy density owing to its high density (5.04 vs. 2.07 g cm
3 of S8), suppressing PS dissolution via forming strong bonding between the lithiation Mo6S8 with PS. Metal sulfides have riveted extensive attention in LSBs owing to their desirable properties as sulfur hosts [65,78]. The continuous development of novel metal sulfides with controllable morphology to maximize their functionality as sulfur hosts will benefit LSBs with further performance enhancement and a deeper understanding of the structure-property relationships. In addition, the Mo6S8eS8 hybrid cathode has established an ideal model to make the most of metal sulfides in LSBs toward practical applications.

4.2.3

Metal nitrides

Metal nitrides with high electrical conductivity can be highly beneficial to sulfur utilization. A conductive porous vanadium nitride/reduced graphene composite (VN/G, Fig. 19.8A) was demonstrated as a promising conductive framework to ensure strong anchoring for PS and fast PS conversion [79]. Despite a much low specific surface area of VN/G (37 m2 g
1) than that of rGO (296 m2 g
1), VN/G-based LSB exhibited lower polarization and faster redox reaction kinetics than rGO-based LSBs (Fig. 19.8B and C) thanks to the superb electrical conductivity of VN/G (1150 S m
1 vs. w240 S m
1 of rGO). Titanium nitride is another attractive sulfur host owing to its high conductivity and strong PS affinity, which endows the corresponding LSBs with high-rate capability [82,83]. It was demonstrated by experimental and computational methods that the ultrastrong bonding of TiN surface and sulfur leads to the spontaneous fragmentation of sulfur into short-chain sulfur species, dramatically improving the redox kinetics of the sulfur conversion reactions

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Figure 19.7 (A) Chemical reactivity of various metal oxides with lithium polysulfides as a function of their redox potential versus Li/Liþ. (B) Binding energies of S8, Li2S6, Li2S4, Li2S2 clusters with various layered structure materials. (C) The initial charge voltage profiles of Li2S cathodes using various metal sulfides as the catalysts. (D) Digital image of the polysulfide (0.005 M Li2S6) adsorption test using carbon and various metal sulfides. (E) The initial charge potentials of a representative cycle in TiS2@N, S dual-doped carbon (NSC), NSC/TiS2eC, and NSC-based lithiumesulfur batteries.

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(Fig. 19.8D) [80]. Besides the intrinsic properties (high conductivity, strong PS interaction, and efficient catalysis toward sulfur conversion), the performance of LSBs can be largely affected by the morphology of host materials. For example, nanoparticles have the advantages of highly exposed active surfaces, while charge transport in these physically assembled nanoparticles is less efficient than in the continuous 2D surfaces [84]. However, there are limited layered metal nitrides [85]. Luo et al., for the first time, demonstrated a topochemical conversion process to synthesis ultrathin TiN nanosheets with abundant pores across the surface (namely nanomesh) (Fig. 19.8E and F) [81]. Compared with the traditional TiN nanoparticle counterparts, the TiN nanomesh exhibits superior catalysis for PS conversion, leading to impressive rate capability for the corresponding LSBs. Metal nitrides possess many desirable properties as sulfur hosts, e.g., high conductivity, strong PS interaction, and efficient electrocatalysis. A growing number of metal nitrides are incorporated into LSBs [86e88]. However, compared with the highly developed synthesis processes for metal oxides and sulfides, it remains challenging to fabricate nanostructured metal nitrides to maximize sulfur accommodation capability, adsorption, and active catalytic sites. To make good use of the intrinsic properties of metal nitrides for further performance enhancement of LSBs, more efforts are required in the rational design of the synthesis approach for metal nitrides.

4.2.4

Metaleorganic frameworks

MOFs represent a class of porous hybrid materials composed of organic linkers and inorganic joints [89]. The tunable pore size, versatile metal nodes, and organic ligands of MOFs make them a promising candidate to accompany sulfur to suppress PS dissolution via physical confinement and chemical bonding [90,91]. Aiming to enhance sulfur confinement for better cycling stability, Tarascon et al. in 2011 proposed using an MOF as a confined matrix for sulfur impregnation [92]. Chromium trimesate Materiaux InstitutLavoisier-100(Cr), MIL-100(Cr) was chosen as a suitable candidate owing to its large pore volume (w1 cm3 g
1) and the hierarchical porous structure with two types of mesoporous cages (w25 to 29Å) connected by microporous hexagonal (9Å) and pentagonal windows (5Å) (Fig. 19.9A). The large pore volume hosts sulfur as well as the liquid electrolyte, which allows high ionic conductivity, while the small windows confine the PS from diffusion. A complete

= (A) Reprinted with permission from X. Liang, et al., Tuning transition metal oxideesulfur interactions for long life lithium sulfur batteries: the “Goldilocks” principle, Adv. Energy Mater. 6 (6) (2016) 1501636, Copyright 2016, Wiley-VCH. (B) Reprinted with permission from Q. Zhang, et al., Understanding the anchoring effect of two-dimensional layered materials for lithiumesulfur batteries, Nano Lett. 15 (6) (2015) 3780e3786, Copyright 2015 American Chemical Society. (D) Reprinted with permission from G. Zhou, et al., Catalytic oxidation of Li2S on the surface of metal sulfides for Li
S batteries. Proc. Natl. Acad. Sci. 114 (5) (2017) 840e845, Copyright 2017, PNAS. (E) Reprinted with permission from X. Huang, et al., Sandwich-like ultrathin TiS2 nanosheets confined within N, S codoped porous carbon as an effective polysulfide promoter in lithium-sulfur batteries, Adv. Energy Mater. 9 (32) (2019) 1901872, Copyright 2019, Wiley-VCH.

Figure 19.8 (A) Schematic of the synthesis of porous VN nanoribbon/graphene (VN/G) composite (B) Cyclic voltammetry profiles of the VN/G composite and reduced graphene oxide (RGO) cathodes. (C) Galvanostatic discharge/charge curves of the VN/G and rGO cathodes at 0.2 C. (D) Comparison of the rate-performance between the bare cathode and C/TiN cathode with an acetylene black mesh interlayer (TiN-ABM), (inset, the binding geometry of Li2S4 on the surface of sulfur-TiN composite). (E) High-resolution transmission electron microscopy of TiN nanomesh, (F) Schematic of the nanoconfined topochemical conversion from Ti3C2Tx@polydopamine nanosheet to the ultrathin TiN nanomesh@carbon. (C) Reprinted with permission from Z. Sun, et al., Conductive porous vanadium nitride/ graphene composite as chemical anchor of polysulfides for lithium-sulfur batteries, Nat. Commun. 8 (1) (2017) 1e8, Copyright 2017, Nature. (D) Reprinted with permission from T.-G. Jeong, et al., Heterogeneous catalysis for lithiumesulfur batteries: enhanced rate performance by promoting polysulfide fragmentations, ACS Energy Lett. 2 (2) (2017) 327e333, Copyright 2017, American Chemical Society. (F) Reprinted with permission from X. Huang, et al., Nanoconfined topochemical conversion from MXene to ultrathin non-layered TiN nanomesh toward superior electrocatalysts for lithium-sulfur batteries, Small (2021) 2101360, Copyright 2021, Wiley-VCH.

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Figure 19.9 (A) Cycling performance of MIL-100(Cr)/sulfur, SBA-15/sulfur and mesoporouscarbon/sulfur composites at 0.1 C. (B) Crystal structure of Ni6(BTB)4(BP)3 and (C) the proposed Lewis acidebase interactions between Ni6(BTB)4(BP)3 and PSs. (D) The activation of sulfur loading into binuclear copper paddlewheel units with benzene-1,3,5-tricarboxylate linkers (CuBTCs) with the preferable binding sites of sulfur, and (E) the cycling performance of lithiumesulfur batteries using CuBTCs of various particle sizes and carbon as sulfur hosts. (F) Scheme of the fabrication process of Ni3(HITP)2 and its application in LBS. (A) Reprinted with permission from R. Demir-Cakan, et al., Cathode composites for LieS batteries via the use of oxygenated porous architectures, J. Am. Chem. Soc. 133 (40) (2011) 16154e16160, Copyright 2011, American Chemical Society. (B and C) Reprinted with permission from J. Zheng, et al., Lewis acidebase interactions between polysulfides and metal organic framework in lithium sulfur batteries. Nano Lett. 14 (5) (2014) 2345e2352, Copyright 2014, American Chemical Society. (D and E) Reprinted with permission from A.E. Baumann, et al., Promoting sulfur adsorption using surface Cu sites in metaleorganic frameworks for lithium sulfur batteries, J. Mater. Chem. 6 (11) (2018) 4811e4821, Copyright 2018, The Royal Society of Chemistry. (F) Reprinted with permission from D. Cai, et al., A highly conductive MOF of graphene analogue Ni3 (HITP) 2 as a sulfur host for high-performance lithiumesulfur batteries, Small 15 (44) (2019) 1902605, Copyright 2019, Wiley-VCH.

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infiltration of sulfur within MIL-100(Cr) pores was achieved (no crystalline sulfur detected by X-ray powder diffraction) through a melt-diffusion heat treatment at 155 C with a precisely controlled sulfur to MIL-100 (Cr) ratio (w48% sulfur) considering the pore volume of MIL-100(Cr), sulfur density and its volume expansion upon lithiation. MIL-100(Cr)/S@155 exhibited enhanced capacity retention compared with sulfur cathodes using mesoporous carbon and SBA-15 as the hosts (Fig. 19.10A), which was ascribed to the unique pore structure of MIL-100(Cr) for PS confinement, and the oxygenated framework of MIL-100(Cr) which forms a weak bonding with PS, respectively. To improve sulfur amount impregnated in MOF, Ni6(BTB)4(BP)3 (BTB is benzene-1,3,5-tribenzoate, and BP is 4,40 -bipyridine) with a high pore volume of 2.15 cm3 g
1 (to 82 wt% sulfur) and superior specific surface area of 5243 m2 g
1, was proposed as a sulfur host [41]. Ni6(BTB)4(BP)3/S composite with w60 wt% sulfur was used as the cathode to balance conductivity and exhibited high capacity retention of 89% for 100 cycles at 0.1 C (from 689 to 611 mAh g
1). The superb cycling stability came from both the confinement from the pores (Fig. 19.9B), and more importantly, the Lewis acidebase interactions between the Ni(II) center of the Ni-MOF (Lewis acid) and PS (Lewis base) (Fig. 19.9C). When Co6(BTB)4(BP)3 was used as the sulfur host, LSBs suffered inferior cycling stability. This work revealed the importance of the metal center of MOF in suppressing the shuttle effect of LSBs, and the Lewis acidebase interactions proposed here provide new insights into the design of efficient sulfur hosts. Along with the growing recognition of the imperative role of the chemical interactions between sulfur species and the host structures in achieving long-term cycling stability of LSBs [100], more research has been put into a deeper understanding of the MOF-sulfur interactions [101]. Using binuclear copper paddlewheel units with benzene-1,3,5-tricarboxylate linkers (CuBTCs) as a platform, Thoi et al. identified that sulfur preferably bonded to a Cu paddlewheel unit (Fig. 19.9D), and thus, CuBTCs with a higher density of Cu sites (CuBTCs of smaller particle size) leads to better capacity retention compared with those with larger particle sizes (e.g., 1.6 and 5.9 mm) (Fig. 19.9E) [93]. Despite the advantages, such as the unique pore structures and rich central metal ions and organic ligands, the insulting nature of most MOF has largely limited the sulfur utilization, rate-performance, and energy density of MOF-based sulfur cathodes. Intrinsically conductive MOF is an emerging research field, which can be achieved when the organic linkers are fully conjugated and form strong interactions with the metal center [102,103]. Among these conductive MOFs, the two-dimensional Ni3(HITP)2 (HITP is 2,3,6,7,10,11hexaiminotriphenylene) exhibits a record-high conductivity of 2 and 40 S$cm
1 at bulk (pellet) and surface (film), respectively [104]. When used as a sulfur host, Ni3(HITP)2/S composite (Fig. 19.8F, with a sulfur content of 65.5 wt%) delivered a high initial capacity of 1303 mAh g
1 at 0.2 C with capacity retention of 65% after 100 cycles [94]. While the past decade has witnessed exciting progress in using MOF as sulfur hosts to improve the performance of LSBs, a more comprehensive and deeper understanding of sulfureMOF interactions is required to achieve precisely and rationally designed MOF-S composites for high-performance LSBs. Furthermore, cost-effective and commercial-scale synthesis technologies for MOF and MOFeS composite fabrication are required for their practical applications.

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Figure 19.10 (A) Fabrication of yolkeshell sulfurepolyaniline composite. (B) Scheme of coating poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) onto CMK-3/sulfur composite to suppress polysulfide diffusion. (C) Scheme of the lithium storing mechanisms of the sulfurized polyacrylonitrile (SPAN). (D) Scheme of the synthesis of poly(sulfur-random1,3-diisopropenylbenzene) copolymers through inverse vulcanization process. (E) Scheme of the synthesis of hyper-cross-linked polymer/sulfur composites. (A) Reprinted with permission from W. Zhou, et al., Yolkeshell structure of polyaniline-coated sulfur for lithiumesulfur batteries, J. Am. Chem. Soc. 135 (44) (2013) 16736e16743, Copyright 2013, American Chemical Society. (B) Reprinted with permission from Y. Yang, et al., Improving the performance of lithiumesulfur batteries by conductive polymer coating, ACS Nano 5 (11) (2011) 9187e9193, Copyright 2011, American Chemical Society. (C) Reprinted with permission from W. Wang, et al., Recognizing the mechanism of sulfurized polyacrylonitrile cathode materials for LieS batteries and beyond in AleS batteries, ACS Energy Lett. 3 (12) (2018) 2899e2907, Copyright 2018, American Chemical Society. (D) Reprinted with permission from A.G. Simmonds, et al., Inverse vulcanization of elemental sulfur to prepare polymeric electrode materials for LieS batteries, ACS Macro Lett. 3 (3) (2014) 229e232, Copyright 2014, American Chemical Society. (E) Reprinted with permission from J.H. Zeng, et al., Sulfur in hyper-cross-linked porous polymer as cathode in lithiumesulfur batteries with enhanced electrochemical properties, ACS Appl. Mater. Interfaces 9 (40) (2017) 34783e34792, Copyright 2017, American Chemical Society.

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Polymer-based sulfur cathodes

The introduction of polymers into sulfur-electrodes started from using conductive polymers to increase the conductivity of sulfur electrodes and inhibit the dissolution of PSs. Chen et al. initially prepared the sulfur-polythiophene composite through in situ chemical oxidative polymerization of thiophene on the surface of sulfur powder, which (w60% sulfur) delivered a high initial capacity of 1168 mAh g
1 with a capacity restation of 70% after 50 cycles [105]. The polymerization process was further optimized, increasing sulfur content to 72% and extending the cycling life to 80 cycles (70% capacity retention) [106]. Although the diffusion of PSs was effectively suppressed by coating a polymer on the sulfur surface, it is difficult to keep the intact coreeshell structure against long-term cycling owing to the large volumetric expansion of sulfur upon lithiation. A yolk-shell polyaniline-coated sulfur composite (58 wt% sulfur) was prepared to address this issue through vulcanizing (heating) the coreeshell polyaniline-coated sulfur structure (82 wt% sulfur) (Fig. 19.10A), leading to superior cycling stability with a high capacity of 765 mAh g
1 after 200 cycles at 0.2 C, corresponding to a capacity retention of 70% [95]. Constructing sulfurcarbon composites has long been the most widely used strategy to improve electrode conductivity and suppress PS diffusion by physical confinement. To further prolong the life span of LSBs, coating a conducting polymer onto these sulfur-carbon composites was proposed to provide additional chemical bonding to PSs. Using polyethylene glycol to functionalize the surface of CMC-3/S was applied in the milestone work of Nazar and co-works in 2009 [107]. It was later reported by Cui et al. that the capacity retention of CMC-3/S composite was increased from 70%/100 cycles to 80%/100 cycles after coating the PEDOT:PSS conducting polymer (Fig. 19.10B) [96]. Incorporating sulfur into the matrix of conductive polymers at a molecular level represents another widely used strategy to improve the cycling stability and rate capability of LSBs. Sulfurized polyacrylonitrile (SPAN) was initially designed and prepared by Wang et al. by heating PAN and sulfur at 280 Ce300 C under argon gas protection [108]. The SPAN composite with 53% sulfur exhibited a singleplateau discharge curve with an initial capacity of 850 mAh g
1 at 0.2 mA cm
2, which remained 600 mAh g
1 after 50 cycles. This pioneering work has triggered extensive interest in using sulfurized polymers as an active component for lithium storage [109e111]. For example, by introducing graphene nanosheets (GNS) as a three-dimensional current collector, a pyrolyzed PAN-S/GNS (S, 47 wt% and GNS, 4 wt%) delivered a superior capacity of 1500 mAh g
1 at 0.1 C, and 800 mAh g
1 at 6C [112]. Different from the solid-liquid-solid conversions in the conventional LSBs, these SPAN composites can work in carbonate-based electrolytes and exhibit a direct solidesolid redox process with a single discharge plateau. The reason for the unique redox process as well as the superior performance was later revealed that the cleavage of SeS bond generates thiyl radical in the first cycle, and then the electrons of the thiyl radical delocalized on the pyridine backbone followed by the formation of conjugative structure, which react with lithium ions through a reversible lithium coupled electron transfer process, forming an ion-coordination bond (Fig. 19.10C) [97].

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Despite the exciting progress achieved in using sulfurized polyacrylonitrile as the active components, the sulfur content was usually below 50%, which limited the achievable energy density of LSBs. Pyun et al. have initially proposed the application of inverse vulcanization process (copolymerizing elemental sulfur with vinylic monomers) to prepare highly stable and processable sulfur-rich polymers (Fig. 19.10D) [113]. The poly(S-r-DIB) (DIB represents 1,3-diisopropenylbenzene) with a sulfur content of up to 90 wt% exhibited a conventional solideliquidesolid conversion process and achieved a high initial capacity of 1100 mAh g
1 at 0.1 C with a supercapacity restation of 75% after 200 cycles. The performance was further improved by optimizing the synthesis process and manipulating the ratio of polymer and sulfur, with 1005 mAh g
1 remaining after 100 cycles and 635 mAh g
1 after 500 cycles [98]. The superior cycling stability was attributed to organosulfur units, which suppressed the irreversible deposition of insoluble lithium sulfides. This synthetic process demonstrated in this work has opened a door for the fabrication and application of polymeric-sulfur for lithium storage [114e116]. In addition to incorporating sulfur chains into the polymer matrix at a molecular level through polymerization [117], porous polymers with high specific areas and large volume are promising hosts for sulfur and are similar to MOFs as sulfur hosts. A hyper-cross-linked porous polymer with a specific surface area of 1980 m2 g
1 and a pore volume of 2.61 cm3 g
1 was demonstrated to accommodate 80 wt% sulfur through the melt-diffusion process (Fig. 19.10E), and the as-obtained HCP/S composite exhibited a high initial capacity of 1333 mAh g
1 at 0.2 C with 658 mAh g
1 remained after 120 cycles at 0.5 C [99]. Compared with the pristine sulfur cathode, the superior performance of HCP/S was attributed to the polymer’s cross-linked porous framework, which shortened the ionic and electronic conductive pathways and migrated the PS shuttle effect. Polymers with the advantages of low cost and versatile chemistries have long been widely applied in constructing robust sulfur cathodes for high-performance LSBs, including conductive coating polymers on the surface of sulfur (composites), embedding sulfur-chains (both short and long) into polymers matrix through chemical reactions and infiltrating sulfur into the abundant pores of polymers using the meltdiffusion process. These as-developed polymeresulfur composites exhibited ether the solidesolid conversion or the traditional solid-liquid-solid redox reactions. It is found that a trade-off must be managed between sulfur content and battery performance, such as sulfur utilization and cycling stability. Novel polymers with large pore volumes to confine sulfur species, high conductivity to improve sulfur utilization, and strong chemical interactions with PS to suppress capacity decay are highly desirable, but developing them remains challenging.

5. Outlook and conclusion In this chapter, the desired parameters for sulfur cathode design toward highperformance LSBs have been demonstrated, followed by a brief introduction of the representative research works in the design and fabrication of various host materials

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to answer these criteria. An ideal sulfur host should have high conductivity, strong chemical bonding with PSs, and an efficient catalytic effect on sulfur conversion reactions. To maximize the functionality of the intrinsic property of sulfur hosts in improving sulfur utilization, cycling stability, and rate-capability, rational structural and morphological design is required, such as incorporating metal-based compounds into carbon matrix to achieve well-dispersed adsorption and active catalytic sites and constructing ultrathin 2-D materials to enable abundant exposed active sites. In the past decade, the cathode design has gained the most research interest among all the components of LSBs, and the research community has witnessed significant advances in performance enhancement of LSBs through continuously optimizing sulfur cathodes and gaining knowledge on the fundamental sulfur redox processes and the structure-property relationships. The ultimate target is to push LSB technology toward practical applications. Along with the growing recognition of this goal, the LSB community has put more emphasis on, for example, sulfur loading (>5 mg cm
2), sulfur-electrolyte ratio (6.4), such as NMP, DMA, and DMF, were investigated. Solvents with lower Pi values, such as cyclohexanone and terpineol (C/T) and isopropyl alcohol (IPA), were also explored. In Section 3.1.1, we discuss the importance of characterizing the viscosity and surface energy of the fluids, and in Section 3.1.2, our analysis of graphene dispersions is discussed in the context of two different graphite starting templatesda graphite rod and a commercially available graphite powder. Then in Section 3.2, we delve into the device applications of graphene for flexible electronics, and in Sections 3.3 and 3.4, we cover the use of these materials in grapheneeMoS2 heterostructure flexible optoelectronic devices and solution-dispersed WS2 and h-BN, respectively.

3.1.1

Fluid viscosity and surface energy

A key property of inks making them viable for printing is their ability to generate droplets, which intimately depends on the ink h, g, and r and nozzle diameter a. The printing procedure is likely to be sensitive to both the lateral flake size and the dispersion concentration. Once ejected, drop behavior is also influenced by choice of the substrate, where the droplet contact angle qc [55] is directly related to the substrateeink surface energies; a large qc will cause the ink to dewet the substrate, whereas a small qc will lead to poorly controlled features of the printed lines. In addition, the distance between the substrate and the inkjet head must be optimized to guarantee both homogeneous printing and the highest possible resolution. A substrate very close to the nozzle will lead to secondary drops that scatter off during the impact of the primary drop due to the initial drop-jetting pressure, which will affect the homogeneity of the final printed features. A parameter often used to predict fluid jettability is the sopffiffiffiffiffiffiffiffi called Ohnesorge number Oh ¼ h= gra [56]. In most situations, r and a are constant for given fluids and printer settings, suggesting that Oh is intimately dependent on h and g. Five solvents with different surface tensions ranging from 23 to 41 mJm2 and polarities from 3.9 to 6.7 have been studied, as shown in Fig. 25.5A, comprising various functional groups and molecular weights. The h and solvent polarity play an important role in determining optimum ink droplet formation and printability. Previous investigations demonstrate that using EC in the nanoparticle dispersion improves ink stability and pattern uniformity and does not leave significant residue after annealing. The h and qc can be engineered by adding EC to all five solvents to yield h in a range of 8e12 cP, which is within the regime appropriate for inkjet printing (5e15 cP), as

Figure 25.5 (A) Structural formulas of the five solvents studied in this work; (B) Graph showing the change in viscosity of the five solvents with the addition of ethyl cellulose (EC) and area of recommended viscosity values for inkjet printing. Inset shows successful drop formation and ejection from the printer nozzle with minimal satellite droplets; (C) IeV curves of graphite rod (GR) dispersed in five solvents after being drop-cast on SiO2 wafer and annealed for 1 h at 350 C; (D) SEM micrograph film resulting from dispersion of GR in dimethylformamide (DMF); (E) SEM micrograph film resulting from dispersion of GR in methyl-2-pyrrolidone (NMP); and (F) IeV curves of NMP unfiltered and filtered ink after being drop-cast on SiO2 wafer and annealed for 1 h at 350 C (inset shows SEM image and dispersion vial).

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shown by the shaded region in Fig. 25.5B. The top inset in Fig. 25.5B shows the obtained ink droplet for graphite nanoparticles in NMP with 4 wt% EC, demonstrating the drop dynamics of the prepared ink with minimal satellite droplets [54].

3.1.2

Graphene dispersion

Two graphite sources were used as the starting templates for generating the graphite/ graphene dispersions, namely commercially available graphite rod (GR), which is an untreated or “natural” graphite, and graphite powder (graphite in an artificially compressed powder form). The GR dispersions were drop-cast onto SiO2 substrates and annealed at 350 C for 60 min, and their electronic transport properties were measured. Fig. 25.5C illustrates an IeV characteristic of GR dispersion in all five solvents, while the inset depicts GR film resistance. Using two-terminal measurements, the GR dispersion in DMF exhibits the lowest resistance (1.6 kU), while C/T yields the highest resistance (18 kU) for the same electrode spacing and approximate thickness. The image in Fig. 25.5D indicates a uniform film morphology for the DMF sample, unlike the NMP sample shown in Fig. 25.5E, which exhibits a rougher and more nonuniform film morphology (see yellow arrows). While the GR dispersed in DMF exhibits the lowest resistance in drop-cast films compared with other solvents, it also requires the addition of w6 wt.% of EC compared with w4 wt.% required for NMP to yield optimum viscosity and printability (10 cP). Since a particle size below 200 nm is necessary to avoid nozzle clogging in our inkjet printer, the dispersion (GR þ NMP solvent þ EC) was filtered using a 0.2 mm syringe filter. Electronic transport measurements were conducted for the unfiltered and filtered inks, as shown by the IeV characteristic in Fig. 25.5F. While the filtered ink exhibited a higher resistance, this is not surprising given the removal of more graphite precipitates during the filtration process. The top inset in Fig. 25.5F shows the relatively dark black color of the filtered ink in the vial, which suggests a highconcentration nanoparticle dispersion, while the bottom inset shows the relatively smooth surface morphology of the drop-cast ink onto SiO2 that explains its reasonable electronic transport characteristics. Additional details related to the Reynolds and Weber numbers can be used to further engineer the ink fluidic properties for optimal printing, as discussed in greater detail elsewhere [54].

3.2

Flexible electronic devices based on two-dimensional graphene

Our graphene ink formulations have been successfully used in the design and fabrication of high-power resistive structures printed on both rigid and flexible substrates with the potential to deliver close to 10 W of power [57]. One of the important parameters in the inkjet printing of 2-D materials is the role of the annealing temperatureetime profile on morphological and electronic transport characteristics.

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Figure 25.6 SEM images showing the microstructure of the inkjet-printed lines after annealing for 1 h (AeC) and 2 h (EeG) at 250, 350, and 450 C, respectively, while (D) depicts the resistance variation with annealing temperature for the two different annealing periods used.

The annealing temperatures considered here were w250, 300, 350, 400, and 450 C, and two annealing times were used, 1 h and 2 h. The top-surface SEM images were obtained from the channel area and are shown in Fig. 25.6AeC and EeG, while Fig. 25.6D shows the R variation as a function of the annealing temperature for the two times considered. The microstructure for the sample annealed for 1 h at 250 C (Fig. 25.6A) shows random particles embedded in a matrix structure presumed to be remnant EC. While EC possesses excellent membrane-forming ability and durability and is commonly used as a flexible coating for paper, cloth, and leather, the electrical properties of EC indicate that it is electrically insulating, which corroborates 2 MU as the highest R value, as noted in Fig. 25.6D. Thus, the SEM in Fig. 25.6A shows the excessive presence of EC when a low annealing temperature of 250 C was used. The microstructure of the samples annealed at higher temperatures (Fig. 25.6B and C) shows that this potentially insulating matrix is largely removed (less charging is seen in the SEM images of Fig. 25.6B and C compared with (A), for example), and a more uniform and conducting film has formed, which is corroborated by the lower R values seen in Fig. 25.6D for samples annealed at temperatures >300 C. Fig. 25.6C shows increased porosity in the microstructure for an annealing temperature of 450 C, consistent with the increased R values seen at 450 C in Fig. 25.6D. Annealing for 2 h at the same temperatures (250e450 C) resulted in significantly reduced R values, as shown in Fig. 25.6D. The R was reduced from 2 MU to 43 kU for samples annealed for 1 h and 2 h, respectively, at an annealing temperature of 250 C. The corresponding microstructures depicted in Fig. 25.6EeG show more film uniformity at all temperatures, which correlates to the enhanced electrical conductance values (Fig. 25.6D), in contrast to samples annealed for only 1 h. The longer annealing

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time likely accounts for the additional diffusion and coalescence of the nanomembranes to yield uniform films. The temperature-time profile of annealing on our samples demonstrated that while low-annealing temperatures can be traded for highannealing temperatures, the annealing time must increase to drive off the excessive remnant solvent. This decrease in annealing temperature is advantageous to widen the portfolio of materials that would otherwise be precluded in flexible electronics, given thermal stability issues at higher temperatures in some of these materials systems. These findings are equally relevant for using solution-processed graphene to form flexible batteries, including their use with surfactant-free and environmentally friendly inks that we have explored previously [58], to yield flexible devices for extreme thermal environments.

3.3

Flexible heterostructure graphene and molybdenum disulfide biocompatible photosensing devices

Now that we have highlighted the previous work on using graphene in flexible electronics platforms, we leverage it further toward an inkjet-printed, biocompatible, heterojunction photodetector using inks of photoactive molybdenum disulfide (MoS2) and electrically conducting graphene. The importance of such devices stems from their potential utility in age-related-macular degeneration (AMD), which is a condition where the photosensitive retinal tissue degrades with aging, eventually compromising vision. The biocompatible inkjet-printed 2-D heterojunction devices were photoresponsive to incoming broadband radiation in the visible regime, and the photocurrent Iph was scaled proportionally with the incident light intensity, exhibiting a photoresponsivity of R w 0.30 A/W. Strain-dependent measurements of Iph were also conducted with bending, which showed Iph w 1.16 mA with strain levels for curvatures of up to w0.262 cm1, indicating the feasibility of such devices for large-format arrays printed on flexible substrates, unlike conventional Si-implantable detectors that are rigid and nonconformable. Both mouse embryonic fibroblast (STO) and human esophageal fibroblast (HEF) were used for the biocompatibility analysis for inks drop-cast on two types of flexible substrates, polyethylene terephthalate and polyimide. The inks, formed using 2-D graphene and MoS2, were highly biocompatible on polyimide substrates for both STO and HEFda cell survival rate of up to 98% was measured for STO, while the cell confluence rate was between 70% and 98% [19].

3.3.1

Heterostructure flexible photodetector with graphene and molybdenum disulfide

The MoS2 ink was printed on the polyimide film, followed by the graphene ink on top of it. Fig. 25.7A shows the schematic diagram of large-format, inkjet-printed arrays of heterostructure devices on flexible substrates. Fig. 25.7B displays an array of actual printed devices over a large-format (w60  50 mm) area, where the inset on the right shows a magnified view of a single heterostructure device. Fig. 25.7C and D exhibit

Figure 25.7 (A) Design file showing the potential for scalability for printing such twodimensional (2-D) heterostructure devices for AMD over a large format using low-cost inkjet printing on flexible substrates; (B) Actual array of inkjet-printed heterostructure devices on flexible polyimide film over an area of 60  50 mm. Inset image on the right depicts a single inkjet-printed heterostructure device, where graphene electrodes were printed on top of the MoS2 layer; (C) Dependence of Iph and R on light intensity at 20 V; (D) D as a function of light intensity at 20 V. Inset shows the IeV characteristics at various light intensities, where Iph is seen to increase linearly with light intensity; (E) Relative Iph change as a function of light intensity at curvatures (1) 0.087 cm1, (2) 0.157 cm1, (3) 0.262 cm1. The bottom two insets depict the three-dimensional printed fixtures used, on which the devices under test were mounted for the strain-dependent measurements; (F) Iph and R as a function of curvature to gauge the effect of strain at 22 mW cm2; (G) Current versus probe distance data for a printed (C/T-MoS2) device illustrated in the top inset, where the 80 mm line widths that span 220 mm illustrate the potential for scalability using 2-D inks that can compete with conventional Si-implantable devices for AMD; (H) Optical microscopy image of the printed 80 mm line arrays, where the edges show good line resolution from the magnified image on the right.

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the corresponding photocurrent Iph, photoresponsivity R, and detectivity D as a function of light intensity (mW cm2), where a broadband white light source was used. The highest D for graphene-MoS2 heterojunction devices was measured as w3.6  1010 Jones. Both R (Fig. 25.7C) and D (Fig. 25.7D) are decreasing with increasing light intensity but exhibit saturation at higher power density levels, whereas Iph is increasing. The variation in Iph as a function of bias voltage at various light intensities is shown in the inset of Fig. 25.7D. The light intensity was increased to 18.5 mW cm2 on the photodetector device. At 18.5 mW cm2, Iph ¼ 2.1 mA (applied bias voltage w 20 V), showing a near-linear increase as light intensity increased.

3.3.2

Strain-dependent photoresponse

To validate the mechanical durability of the photodetectors on the flexible polyimide substrates, mechanical bending tests were carried out using three-dimensional (3-D) printed structures with five different radii of curvature: 0.072, 0.087, 0.112, 0.157, and 0.262 cm1. The experiment was performed under a bias voltage of 20 V, and the photocurrent response was measured as a function of strain with varying light intensity. The flexible heterostructure device was attached to the 3-D fixtures, as shown in the bottom two insets of Fig. 25.7Edfor example, curvatures of w0.087 and w0.157 cm1. The data in Fig. 25.7E show the device to be photoresponsive as a function of strain for curvatures of w0.087, w0.157, and w0.262 cm1, though Iph decreased with increasing strain as expected. The applied strain possibly increases the membrane-to-membrane separation, which explains the decrease in Iph with increasing strain. When the strain is applied, few-layer MoS2 causes mechanical deformation and produces piezoelectric charges at the heterostructure interface. The electric field at the interface and the increased metal-semiconductor junction provide a smaller driving force to separate photogenerated electron-hole pairs, which reduces Iph. Fig. 25.7F illustrates the dependence of Iph and R captured at a light intensity of w22 mW cm2 as a function of curvature. While the devices described in this work thus far are based on feature sizes >200 mm, Fig. 25.7G illustrates inkjet-printed circuitry on flexible polyimide substrates in the inset, where the achieved line widths are far smaller at w80 mm with absolute printed lengths approaching w20 cm. It can be seen from Fig. 25.7G that the current scales inversely with increasing probe separation in such structures, as expected. Even though this circuitry is printed with C/T-MoS2 ink, it translates readily to graphene and potentially interconnect applications. Fig. 25.7H shows the optical microscopy image of an array of printed 80 mm lines, where the edges show good line resolution from the magnified image on the right. This scaling toward lower line widths using printed 2-D materials can be pushed even further by tuning ink-substrate interactions, so the device features are comparable to those used in conventional Si implants for retinal prostheses.

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Tungsten disulfide and hexagonal boron nitride in additive manufacturing platforms for device applications

Other intriguing 2-D materials are WS2 and h-BN, which we now discuss for solution-processed platforms and flexible devices. First, within its bulk state, WS2 has an indirect bandgap of w1.4 eV but morphs to a direct bandgap of w2.1 eV when thinned to its monolayer form, which makes it attractive for highperformance optoelectronics and photodetectors operational in the visible regime. Conventional semiconductors such as Si and GaAs used in photodetectors are usually opaque, and their brittle nature limits their use to rigid substrates only. The bandgap in the visible part of the electromagnetic spectrum and the presence of Van Hove singularities in the electronic density of states enables WS2 nanosheets to exhibit transparency while displaying strong lightematter interactions, including on flexible substrates with bending. Here, we have conducted an extensive analysis of several exfoliation routes for forming eco-friendly dispersions of WS2 [20], which includes magnetic stirring (MS), shear mixing, and horn-tip (HT) sonication. After determining the optimal exfoliation route, grapheneeWS2egraphene heterostructure photodetectors were designed and fabricated, where WS2 is used as the semiconducting channel material, and graphene is used as a highly conducting electrode material. The dispersions were developed for formulating inks of WS2 using a mixture of environmentally friendly C/T as the solvent to subsequently print prototype nanodevices. Capacitancee voltage (CeV) and C-frequency ( f ) measurements were also conducted, which confirmed the excellent potential of solution-cast, trilayer grapheneeWS2egraphene heterostructures as a promising photodetector platform using additively manufactured inkjet printing. Secondly, for the realization of h-BN devices, a new exfoliation route was used to overcome the drawback of HT exfoliation by combining it with slow MS for h-BN bulk crystals. Inkjet printing of h-BN was then conducted [59], where various patterns were designed and printed, and a change from translucent to opaque was observed with increasing printing passes. All inkjet-printed graphene/h-BN/graphene capacitors were fabricated, and the leakage current density, JLeakage, was measured to be w72 nA mm2, while the capacitance density was w2.4 mF cm2.

4.

Incorporation of solution-processed two-dimensional layered MXenes

The solution-processed approaches discussed in Section 3 related to 2-D materials have enabled their wide use in flexible electronics applications. Another class of solution-processed 2-D materials, in addition to graphene and 2-D TMDCs and TMOs, is based on ternary compounds, specifically MXenes, that are potential LIB

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electrodes [60]. MXene materials or 2-D transition metal carbides/nitrides have the promise to increase the electrochemical performance of Li-based battery applications; compounds such as M2C (M ¼ Sc, Ti, V, and Cr, for example) are among this large family of materials yielding a high discharge capacity of >400 mA h g1 [61]. This class of 2-D materials can provide a range of working potentials, making them ideally suited for anodes or cathodes [62]. However, there is still some variation in reaching the maximum capacity with these compositions depending on the synthesis and processing approaches used. Since the discovery of Ti3C2Tx in 2011, MXenes have received a great deal of attention for their outstanding performance in energy and catalysis research, as well as for optoelectronics, biomedical, sensing, and electronic applications [63]. MXenes with the formula Mnþ1XnTx (n ¼ 1e3) are typically derived from the MAX phases by etching the A layer selectively, as shown in the process schematic [64] of Fig. 25.8, where M is a transition metal, A represents Group 13 or 14 elements in the periodic table, X denotes carbon or nitrogen, and Tx represents surface terminations [65]. Table 25.3 highlights the various etching methods used to produce MXenes. Overall, hydrofluoric acid (HF)-containing procedures are the most efficient and common, whereas other approaches, particularly HF-free formulations, are somewhat sparse but rapidly evolving. Because etchants play a critical role in MXene surface terminations, various etching procedures can change the etching result and MXene characteristics. HF-free techniques make it possible to make MXenes without F terminations, and new characteristics of MXenes with different terminations are being studied further. MXenes possess some unique properties, such as large/tunable interlayer spaces and high aspect ratios. Fig. 25.9 illustrates the elemental distribution for the MAX phases and typical processes involved in the synthesis of MXenes. Also shown are three M1.33X, eleven M2X, nine M3X2, and ten M4X3 MXenes and one M5X4 MXene [66]. Materials in the MXene family demonstrate outstanding hydrophilicity and exceptional conductivity (e.g., w 9880 S cm1 for Ti3C2Tx and w3250 S cm1 for V2CTx) [67e69] with a variety of terminations (eO, OH, and eF) and surface chemistries, making them desirable for energy-

Figure 25.8 (A) Schematic illustration of etching of MAX phase using hydrofluoric acid (HF). (B) Interlayer spacing at different temperatures when etching the MAX phase with HF. Reproduced with permission from M.K. Aslam, Y. Niu, M. Xu, MXenes for non-lithium-ion (Na, K, Ca, Mg, and Al) batteries and supercapacitors, Adv. Energy Mater. 11 (2020) 2000681.

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Table 25.3 Variation in etching approaches used to synthesize MXenes. Sr. No. 1

Etching method Wet chemical etching

2

Molten salt etching

3

Electrochemical etching

Etchant

MXene types

HFcontaining

HF, HF þ HCl

Fluoride salt/acid

Salt: LiF, NaF, KF, NH4F, CsF, CaF2, etc. Acid: HCl, H2SO4

Fluoride salt Alkali treatment UV-induced Fluoride salt Lewis acid salt

NH4HF2, NH4F NaOH

Ti3C2Tx, V2CTx, Ti2CTx, Nb2CTx, Mo2CTx, Ti2NTx, (Ti0.5,Nb0.5)2CTx, Mo1.33CTx, Nb1.33CTx, W1.33CTx, V(2-x) CTx, (V0.5Cr0.5)3C2Tx, Ti3CNTx, Mo2TiC2Tx, Mo2Ti2C3Tx, Ta4C3Tx, V4C3Tx, Nb4C3Tx, Zr3C2Tx, etc. Ti3C2Tx, V2CTx, Nb2CTx, Mo2CTx, Ti2CTx, Ti3CNTx, Cr2TiC2Tx, W1.33CTx, etc. Ti3C2Tx Ti3C2Tx

UV light þ H3PO4 KF þ LiF þ NaF ZnCl2, CuCl2, NiCl2, FeCl2, AgCl, CoCl2, CdCl2, CdCl2, CdBr2, etc HCl, NH4Cl þ TMAOH

Mo2CTx Ti4N3Tx Ti3C2Tx, Ti3CNTx, Nb2CTx, Ta2CTx, Ti2CTx, etc. Ti2CTx, Ti3C2Tx

Reproduced with permission from F. Ming, H. Liang, G. Huang, Z. Bayhan, H.N. Alshareef, MXenes for rechargeable batteries beyond the lithium-ion, Adv. Mater. 33 (2021) 2004039.

storage devices and supercapacitors. Another advantage of MXenes is that they can be utilized as a flexible sulfur cathode with a conductive binder and backbone [70]. The 1T-2H MoS2eC MXene used as an electrode also has outstanding mechanical characteristics, allowing it to survive repeated bending without affecting electrochemical performance [21].

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Figure 25.9 (A) Fragment of periodic table showing the “M,” “A,” and “X” elements of the known MAX phases. (B) Typical preparation process of M3X2 MXene from the corresponding M3AX2 phase. (C) Reported MXenes to date. Reproduced with permission from F. Ming, H. Liang, G. Huang, Z. Bayhan, H.N. Alshareef, MXenes for rechargeable batteries beyond the lithium-ion, Adv. Mater. 33 (2021) 2004039.

5. Summary and conclusions In summary, 2-D layered materials can improve the electrical conductivity and electrochemical characteristics of LIBs, and with their layered structure, they have the potential to act as cathode and anode electrodes to improve cyclability as well as volumetric and gravimetric capacities. The solution-processing techniques for 2-D layered materials were discussed in the context of inkjet printing adapted for battery platforms,

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including flexible, conformable applications. Other enhancements from incorporating 2-D layered materials include increased mechanical stability to avoid pulverization, and dendrite production can be minimized through uniform Liþ ion electrodeposition. A key design concern is thermal safety, which can be improved with the appropriate selection of the 2-D material employed in the batteries. Furthermore, the contributions of 2-D materials as current collectors are critical for increasing LIB energy density and life span. Other electrode materials can be introduced by controlling the expansion/ shrinkage of high-capacity electrode materials during cycling with 2-D protective layers. In conclusion, 2-D materials are well positioned to influence several directions of LIB research to enhance electrochemical performance and life span, which should provide fundamental technological breakthroughs in the future.

Acknowledgments We thank the Office of Naval Research (grant number ONR N00014-20-1-2597) for their support, and A.B.K. is also grateful to the PACCAR Technology Institute at UNT and the Endowed Professorship that further supported the work.

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Zhiqiang Zheng 1 , Guang Xia 1 , Jiajia Ye 1 , Zhanghua Fu 1 , Xuting Li 1 , Mark J. Biggs 2 and Cheng Hu 1 1 Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong, P.R. China; 2Heriot-Watt University, Edinburgh, United Kingdom

1. Introduction Lithiumesulfur batteries (LSBs) have garnered significant attention for their high theoretical energy density (w2600 Wh kg
1) and cost-effectiveness, as well as the environmental friendliness of sulfur [1]. These merits make LSBs one of the most promising candidates to succeed lithium-ion batteries (LIBs). However, the commercial application of LSBs is hindered by several obstacles [2e4]. (1) The dissolution of lithium polysulfides (LiPSs) in the electrolyte induces the loss of active materials and a severe shuttle effect leading to low sulfur utilization, rapid capacity degradation, and poor coulombic efficiency (CE). (2) The insoluble sulfur and final discharge products are electrical and ionic insulators that impede electron and ion transfers and limit the kinetics of electrochemical reactions. (3) Lithium (Li) dendrites induced by inhomogeneous deposition on anodes bring about serious safety problems, cause rapid anode failure, and lead to electrolyte depletion. Great efforts have been made to solve the abovementioned issues. These include dedicated cathode designs, separator modifications, electrolyte engineering, and anode protections. Laboratory-developed LSBs have achieved great progress with high discharge capacities, excellent rate capabilities, and stable cycling performance. It is recognized that further research efforts to advance LSB commercialization should be devoted to structures ranging from coin cells to pouch cells because of the huge gap between them [5]. Cell parameter standards have been established for realizing high energy densities (>500 Wh kg
1), including high sulfur content (>80%), high areal sulfur loading (>80%), a low electrolyte-to-sulfur (E/S) ratio (350 Wh kg
1) have been proven available [12,13], but the average production costs of these cells are yet to be established. Predicting cell costs at the material design stage is a significant input in evaluating the cost-effectiveness of practical LSBs. Taking the work of Yang et al. for LSB cost estimation [14], Table 26.1 shows the baseline costs of the main components in LSB pouch cells with corresponding weight and cost ratios. The values were calculated based on a high sulfur loading (5 mg cm
2), limited excess lithium (50 wt%), and a low E/S ratio (3 mL mg
1). Under the condition of limited use of lithium and electrolytes, these two components are still the costliest, accounting for 55.4% and 30.5% of the total cell cost, respectively. Based on the above parameters, the LSB pouch cell cost is only Table 26.1 Baseline costs of lithiumesulfur battery raw materials with corresponding weight and cost ratios. Component

Baseline cost ($ kgL1)

Weight ratio (%)

Cost ratio (%)

Electrolyte Sulfur Lithium metal Current collector Carbon Membrane Binder Others (taps, Al laminate film, etc.)

12 0.22 100 18.5 15 2 10 10

50.4 16.8 11.0 7.3 4.2 3 2.3 5

30.5 0.2 55.4 6.8 3.2 0.3 1.2 2.5

Reproduced with permission from X. Yang, X. Li, K. Adair, H. Zhang, X. Sun, Structural design of lithiumesulfur batteries: from fundamental research to practical application, Electrochem. Energy Rev. 1 (3) (2018) 239e293. Copyright 2019, the author(s).

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$32.9 kWh
1 when only the raw materials are considered. The cost increases to $58.8e$78.3 kWh
1 when 70%e80% available capacity and 20%e30% nonmaterial expenses are assumed [14]. However, this is an ideal scenario, as the cost of lithium anodes and sulfur cathodes will typically increase under practical conditions. Moreover, it is quite challenging to achieve very low E/S ratios (10 mL mg
1 are widely employed to achieve high electrochemical performance in LSBs [8], which pushes the cell cost up. Furthermore, lithium metal has a much higher price than conventional graphite anodes ($12 kg
1) [15]. The real cost of lithium metal anodes is also determined by the thickness of lithium foils. Although the price of lithium ingots is w$80 kg
1 [16], lithium foils are much more expensive ($100e300 kg
1 for 200 mm thickness and $250-1000 kg
1 for 20 mm thickness) [10] because of additional manufacturing costs. In addition to cell production costs, cycle life is an important factor influencing cost-effectiveness. Commercial LIBs can achieve stable cycling for thousands of cycles, while high-energy-density LSBs (>300 Wh kg
1) are currently only stable for a few hundred cycles [7,17]. Controlling the costs of electrolytes and lithium anodes is expected to reduce total cell costs, while prolonging LSB cycle life will also contribute to improved cost-effectiveness.

2.2

Safety

Safety acts as the prerequisite for the large-scale application of energy-storage devices, especially for electric vehicles. Because of the use of highly flammable electrolytes and reactive lithium metal anodes, the safety hazards of LSBs must be carefully evaluated. Ether-based electrolytes are the most widely used liquid electrolyte system for LSBs, with a common composition of 1M lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) dissolved in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with LiNO3 as the electrolyte additive. The flash points of DOL and DME are
6 and 1 C, respectively. The high flammability of the electrolytes makes it easy for LSBs to catch fire when they suffer heat shocks. The electrolytes are also highly volatile due to the low boiling points (75 C for DOL and 84 C for DME), which increases the risk of explosions, and the addition of LiNO3 makes the situation even worse [18]. The use of lithium metal anodes is another factor that can cause serious safety problems. The notorious lithium dendrites are likely to penetrate the separators and

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lead to short circuits, possibly causing fires and explosions [19]. The “dead lithium” and solid electrolyte interphase (SEI) debris detached from the current collector exhibit high surface areas and are highly reactive when exposed to moisture or water [11]. Lithium anodes also suffer from large volume changes that lead to anode pulverization and cell swelling [20]. This increases cell instability and creates challenges for the design of battery packs. The high flammability of lithium and electrolytes makes LSBs sensitive to various abuse conditions, such as crushing, penetration, overcharge, and external short circuits [11]. These conditions break the stable state of cells, leading to internal short circuits and thermal runaway while triggering safety hazards such as smoke, gas ejection, fires, and even explosions [21]. It is vitally important to evaluate the safety of pouch cells under abuse conditions to propose strategies to design safer and more stable LSBs. Huang et al. comprehensively evaluated LieS pouch cell safety under abuse conditions, including nail penetration, impact, external short circuit, and cell overcharge [22]. They identified an important self-protection mechanism, “insulated seal,” enabled by LiPSs in LSB pouch cells. In fact, this mechanism enables safer LSB performance than LIB performance under internal short circuits. Furthermore, LieS pouch cells performed better in safety during the overcharge test owing to the disproportionation reaction of long-chain LiPSs, which suppresses the oxygen evolution reactions on the cathode side. The study also proved that a separator with high thermal conductivity contributes to fast heat dissipation and effectively decreases the risk of thermal runaway. In summary, safety is a high priority for LSB commercialization and sustainable development. The design of dendrite-free lithium anodes and less-flammable electrolytes acts as an effective strategy to achieve this goal. Further studies on the failure mechanisms of LieS pouch cells under abuse conditions may provide additional insights for developing safer LSBs.

2.3

Environmental impacts

Environment protection has received increasing attention in recent years, especially in developing countries. Although LSB applications have positive environmental impacts, the manufacturing processes of materials and cells may impose environmental burdens, such as toxic raw materials and high energy costs. Electrode materials with complex synthesis routes could have negative impacts on the environment. Sulfur is naturally abundant and nontoxic, but sulfur immobilizers and catalysts often contain heavy metals such as Co [23], Ni [24], Sb [25], and Cd [26]. In addition, sulfur host synthesis involves toxic solvents such as methanol, methylbenzene, trichloromethane, and hydrochloric acid. These liquid wastes must be handled carefully to avoid possible water and soil pollution. The use of volatile organic compounds may cause air pollution. Organic solvents such as CS2 are used to dissolve sulfur during cathode preparation. Furthermore, polyvinylidene fluoride (PVDF) is still the most commonly used binder, and N-methyl-2-pyrrolidone (NMP) is the typical organic solvent during the preparation of cathode slurry.

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Although NMP exhibits a relatively high boiling point, leakage to the surrounding atmosphere may occur during the electrode drying process. The chemical composition of electrolytes is another issue that increases environmental burdens. Most electrolyte solvents have strong corrosivity and are harmful to soil and water ecosystems. Some lithium salts, including LiTFSI and bis(trifluoromethane)sulfonimide lithium salt (LiN(SO2CF3)2), are harmful to aquatic life with longlasting effects [27]. In addition, the production process of lithium salts may bring about serious environmental impacts. LiTFSI production involves large quantities of toxic methyl chloride [28], which causes ozone depletion. Scientific protocols must be developed to assess the environmental impacts of LSBs. Life cycle assessment (LCA) is widely used to evaluate the environmental impacts of batteries [29], providing quantitative assessments of resource consumption and environmental releases along the entire life cycle of battery devices. In an LCA study, Lopez et al. compared the environmental impacts of five LSBs with different sulfur cathode materials [30]. As shown in Fig. 26.1, 11 indicators were employed

Figure 26.1 Environmental impacts of selected lithiumesulfur batteries evaluated by life cycle assessment. Reproduced with permission from S. Lopez, O. Akizu-Gardoki, E. Lizundia, Comparative life cycle assessment of high performance lithium-sulfur battery cathodes, J. Clean. Prod. 282 (2021) 311. Copyright 2020, Elsevier.

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to comprehensively evaluation of the environmental impacts. LSBs using the grapheneecarbon nanotube (CNT)eCoesulfur cathode had the greatest environmental impacts, while the lowest appeared to be the sodium carboxymethylcellulose (NaCMC)esulfur cathode composed of 70% sulfur, 20% activated carbon, and 10% NaCMC. The greater environmental burden caused by the grapheneeCNTeCoe sulfur cell was attributed primarily to multiple cathode components synthesized through complex and high energy-consuming processes. It was also found that for all five batteries, the largest contribution to global warming potential comes from the electrolyte, and the second comes from energy consumption for material processing and cell manufacturing. The environmental impacts of electrolytes were found to originate from the use of organic solvents and the production of LiTFSI. Therefore, reducing the E/S ratio is expected to be highly effective in reducing the environmental burdens of LSBs. The preparation process of lithium foils as LSB anodes involves mining, extraction, extrusion, and roll milling. These processes are associated with the consumption of energy, fossil fuels, and water resources, which results in substantial environmental impacts. The impact assessment of lithium metal anodes can be performed based on footprint family indicators. Wang et al. revealed that the production of lithium foils contributes to 58.95%, 55.21%, and 49.05% of the total ecological, carbon, and water footprints of LSBs, respectively [31]. Therefore, reducing excess lithium is beneficial for both energy density and practical LSB sustainability. To sum up, reducing environmental impacts is essential to sustainable LSB development. For mass production, toxic raw materials should be avoided to the greatest extent possible in cathodes and electrolytes. Facile and green electrode preparation processes are preferred in practical production. Reducing the use of electrolytes and lithium will significantly alle viate associated environmental burdens. In addition, LSB recycling methods remain to be developed and should act as the final and key step toward sustainable LSBs.

3.

Improving lithiumesulfur battery sustainability

3.1

Cathode materials

A series of materials have been developed as effective sulfur hosts for the cathode, including carbon materials [32], polymers [33], metal, and metal compounds [34]. However, sulfur hosts generally contain high-value or toxic components, hindering their sustainable large-scale application. Although PVDF is currently the most widely used binder for LSBs, the toxic and volatile organic solvents (e.g., NMP) used to dissolve PVDF impose potential hazards to human health and the environment. In this section, recently developed sustainable sulfur hosts and green biomassderived polymer binders are discussed.

Sustainability of lithiumesulfur batteries

3.1.1

609

Sulfur hosts

Biomass is a class of naturally derived raw materials with the advantages of low cost and eco-friendliness. Biomass-derived carbons have attracted substantial interest as sulfur hosts because of their variable structures, abundant functional groups, and heteroatoms [35]. Research on biomass-derived carbons for LSB cathodes has focused on structural regulation, surface chemistry modification, and composite materials design. Biomass-derived carbons with large surface areas and pore volumes are beneficial for accommodating high sulfur content while ensuring sufficient transport pathways for electrons and ions. Chuang et al. reported a cathode material based on carbonized cotton prepared through the one-step carbonization of natural cotton [36]. As shown in Fig. 26.2A, the carbonized cotton presents a cross-linked architecture with macro/micropores that effectively accommodate sulfur to ensure a high sulfur loading of 61.4 mg cm
2. Modifying the carbon surface with high polarity endows it with improved chemical adsorption for LiPSs. As biomass materials are generally rich in carbohydrates or proteins with abundant functional groups [40], heteroatom-doped carbons from biomass are more easily obtained through controlled carbonization. Wu et al. prepared a trimodal hierarchical porous material with N, O-codoped carbon frameworks [37]. As shown in Fig. 26.2B, the material was derived from the carbonization of bagasse with p-SiO2 loaded. It was demonstrated that the hierarchical porous structure constructed by the hard template method promotes charge transfer and alle viates the volume expansion of sulfur during charge/discharge cycling. N and O heteroatoms with strong chemical adsorption toward LiPSs effectively suppress the shuttle effect and lead to homogeneous deposition of Li2S. Combining biomass-derived carbons with other materials of strong polarity and catalytic activity is a promising route for synthesizing multifunctional composites as sulfur hosts. Liu et al. prepared a ginkgo-nut derived honeycomblike structure with N, S-codoped carbons decorated by CoS2 or NiS2 (GC-MS2) [38]. As shown in Fig. 26.2C, the synthesis was enabled by templated carbonization and further hydrothermal treatment for metal growth. The honeycomblike hierarchical architecture with N and S doping facilitates rapid charge transport and restricts LiPSs through physical and chemical adsorption. MS2 nanoparticles anchoring on the carbon skeleton enhance the interaction with LiPSs and accelerate the redox reactions of active sulfur species. Although biomass materials are renewable and relatively cheaper, the carbonization and functionalization processes are worthy of more attention in further reducing the associated energy consumption. Using natural or industrial wastes to synthesize LSB cathodes also conforms to the purposes of sustainable development. Zhu et al. employed fly ash as the sulfur host and prepared a fly ash/sulfur composite through a low-cost ball-milling process (Fig. 26.2D) [39]. Due to the hierarchical porous structure of fly ash and its strong affinity to LiPSs, the cathode exhibited stable cycling with a low capacity decay

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Figure 26.2 Fabrication process of (A) carbon-cotton cathode. (B) Trimodal hierarchical porous material with N, O-codoped carbon frameworks (TD-HDC) and TD-HDC/sulfur composite. (C) Honeycomblike porous GC-MS2. (D) Fly ash/sulfur composite. (A) Reproduced with permission from S.H. Chung, C.H. Chang, A. Manthiram, A carbon-cotton cathode with ultrahigh-loading capability for statically and dynamically stable lithium-sulfur batteries, ACS Nano 10 (11) (2016) 10462e10470. Copyright 2016, American Chemical Society. (B) Reproduced with permission from D. Wu, J. Liu, J. Chen, H. Li, R. Cao, W. Zhang, Z. Gao, K. Jiang, Promoting sulphur conversion chemistry with tri-modal porous N, O-codoped carbon for stable LieS batteries, J. Mater. Chem. 9 (9) (2021) 5497e5506. Copyright 2021, The Royal Society of Chemistry. (C) Reproduced with permission from J. Liu, S.H. Xiao, Z. Zhang, Y. Chen, Y. Xiang, X. Liu, J.S. Chen, P. Chen, Naturally derived honeycomb-like N,S-codoped hierarchical porous carbon with MS2 (M¼Co, Ni) decoration for high-performance Li-S battery, Nanoscale 12 (8) (2020) 5114e5124. Copyright 2020, The Royal Society of Chemistry. (D) Reproduced with permission from Z.Y. Zhu, N. Yang, X.S. Chen, S.C. Chen, X.L. Wang, G. Wu, Y.Z. Wang, Simultaneously porous structure and chemical anchor: a multifunctional composite by one-step mechanochemical strategy toward high-performance and safe lithium-sulfur battery, ACS Appl. Mater. Interfaces 10 (48) (2018) 41359e41369. Copyright 2018, American Chemical Society.

rate of 0.042% per cycle at 1C. Li et al. developed a composite of Fe2P@N,P-doped carbon (Fe2P@NPC) by biologically recycling metal using bacteria from electroplating sludge [41]. The Fe2P@NPC/S cathode presented a high discharge capacity (1555.7 mAh g
1 at 0.1C), excellent rate capability (679.7 mAh g
1 at 10C), and stable cycling performance (761.9 mAh g
1 at 1C after 500 cycles). Encouraged by the pioneering works, there are still plenty of opportunities to reuse wastes in preparing LSB cathodes for sustainable development.

3.1.2

Binders

Aqueous polymer binders using water as the solvent appears to be a promising route to replace PVDF. Recent studies have focused on developing aqueous binders with

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designed functionality [42], such as high bonding strength, mitigated LiPS shuttling, and promoted transfers of electrons and ions. Biomass-derived polymer binders are potential candidates. Liu et al. developed a mechanically robust binder through the intermolecular binding effect of guar gum and xanthan gum for high-loading LSBs [43]. This low-cost biopolymer binder with abundant oxygen-containing groups effectively restrained the shuttle effect and delivered a high areal specific capacity of 26.4 mAh cm
2 at a high sulfur loading of 19.8 mg cm
2. Pang et al. reported a crack-free and high-loading cathode (14.9 mg cm
2) by crossing-linking only 5 wt% of carboxymethyl cellulose (CMC) binder with sulfur composites using traditional slurry coating processing [44]. The stable and compact cathode enabled by cross-linked CMC reduces electrolyte use while bringing the E/S ratio down to 3.5 mL mg
1. Considering that the binder content in commercial LIB electrodes is limited to 3 wt% [45], it is practically significant to reduce the weight ratio of binders in future research on high-energy-density LSBs.

3.2

Separators and electrolytes

The application of practical LSBs is closely related to the sustainable development of advanced separators and electrolytes. Considering the safety and cost of LSBs, the design of low-cost, eco-friendly, and safe separators and electrolytes is crucial. In addition, a lower E/S ratio leads in the direction of safer, cheaper, and greener LSBs.

3.2.1

Separators

Applying modified separators or functional interlayers is a popular strategy to mitigate the shuttle effect of LiPSs and improve the sluggish kinetics of sulfur conversions. Biomass materials are competitive candidates here as well in terms of sustainability. Chen et al. designed a protein-based interlayer and found that the sidechain length of denatured proteins critically affects LiPS trapping [46]. As shown in Fig. 26.3A, denatured zein proteins possess both long and short sidechains, while gelatin proteins consist mainly of short sidechains. It was revealed that short-branched gelatin exposes more LiPS trapping sites than the long zein sidechains do (Fig. 26.3B). Benefiting from the gelatin-based interlayer, a high-loading sulfur cathode (9.4 mg cm
2) delivered a high areal capacity of 8.2 mAh cm
2 over 100 cycles at 0.1 A g
1. Another separator function is preventing thermal runaway and enhancing battery safety performance. Researchers have attempted to design safe separators by adding flame-retardant materials, such as ammonium polyphosphate [47] and polyphosphazene [48]. The use of these refractory materials improves the thermal stability of LSBs and enhances cell performance through the effective chemical adsorption of LiPSs.

3.2.2

Electrolytes

Typical ether-based electrolytes with low flash and boiling points are flammable and volatile during cell operation. The heterogeneous lithium deposition in liquid

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Figure 26.3 (A) Typical polypeptide chains of denatured zein and gelatin proteins. (B) Illustration of the effects of protein sidechain groups on polysulfide trapping sites. Reproduced with permission from M. Chen, C. Li, X. Fu, W. Wei, X. Fan, A. Hattori, Z. Chen, J. Liu, W.H. Zhong, Let it catch: a short-branched protein for efficiently capturing polysulfides in lithiumesulfur batteries, Adv. Energy Mater. 10 (9) (2020) 1903642. Copyright 2020, Wiley-VCH.

electrolytes may cause dendrite growth and even cell explosion upon short-circuiting. The demand for reliable LSBs encourages the exploration of safer and more stable electrolytes. Cell safety can be significantly improved with nonflammable electrolyte solvents such as diethylene glycol dimethyl ether [49] and triethyl phosphate [50]. Employing flame-retardant additives is a simple method to achieve safer electrolyte systems. For example, a thermally stable electrolyte was obtained by adding Py1,4 TFSI in the common DOL/DME-based electrolyte [51]. In addition, electrolyte additives play a key role in forming the SEI on the surface of lithium. Li et al. revealed the synergistic effect of LiNO3 and Li2S8 in forming a stable and uniform SEI layer that suppresses dendrite growth and reduces electrolyte decomposition [52]. Other additives such as KNO3 [53] and LaNO3 [54] were also proved to stabilize lithium anodes and prevent dendrite growth.

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Solid-state LSBs have gained wide attention for their improved safety features. Several excellent reviews [55e59] are available summarizing recent developments in solid-state electrolytes (SSEs) for LSBs, including polymer electrolytes, inorganic solid electrolytes, and composite electrolytes. Low Liþ conductivity is the main drawback of solid polymer electrolytes [56]. The introduction of liquid electrolytes into polymer electrolytes (i.e., gel polymer electrolytes) improves Liþ conductivity. Inorganic solid electrolytes possess higher Liþ conductivity. However, additional development is required to address the large interfacial resistance and rigid properties for oxide-based SSEs as well as the chemical and electrochemical instabilities for sulfide-based SSEs [59]. In addition, more effort should be made to explore low-cost SSEs with facile fabrication processes.

3.2.3

Lower electrolyte-to-sulfur ratios

Electrolytes contribute a large part of the total cell cost in LSBs. They also act as a major source of LSB safety hazards and environmental burdens. Reducing electrolyte use is a realizable approach to achieve high-energy-density and sustainable LSBs. The amount of electrolyte added to the cell is closely related to its properties. Low-viscosity electrolytes have better electrode wettability. Shin et al. employed 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether as a cosolvent and diluent to an electrolyte of 6M LiTFSI in DME [60]. The diluted electrolyte had lower viscosity, so less electrolyte was needed to wet the electrodes. At an E/S ratio of 2 mL mg
1, the LieS battery stably cycled for 100 cycles. The electrolyte solvation chemistry is another key factor for lean-electrolyte LSBs [61]. In a practical cell under a lean-electrolyte condition, saturation of the electrolyte by LiPSs may occur and result in poor sulfur utilization and sluggish conversion kinetics [62]. High-donor electrolytes with high LiPS solubility can effectively increase sulfur utilization [63]. For example, Baek et al. demonstrated 1,3-dimethyl-2-imidazolidinone (DMI) as a high-donor electrolyte for LSBs [64]. DMI with high LiPS solubility enables a new reaction route involving a sulfur radical (S3
). A 3-D deposition morphology of Li2S was obtained compared with the 2-D filmlike morphology in DOL and DME. Therefore, a high specific capacity of 1595 mAh g
1 (0.03C) was delivered at a low E/S ratio of 5 mL mg
1. The use of high-donor nitrates (NO
3 ) [65] was also proved to promote LiPS dissolution and enable high sulfur utilization of above 1200 mAh g
1 at a low E/S ratio of 5 mL mg
1. Interestingly, using electrolyte solvents with low LiPS solubility is also a novel strategy to reduce the E/S ratio. Pang et al. found that a sparingly solvating electrolyte based on concentrated glycol ether transforms the sulfur conversion mechanism from dissolutioneprecipitation to a quasi-solid-state pathway [66]. This electrolyte inhibits LiPS dissolution and shuttling as well as its parasitic reaction with the lithium anode, thus reducing electrolyte consumption. For nonsolvating electrolytes [67], the redox reaction of sulfur from a solidesolid reaction does not rely on the volume of electrolytes, but sluggish kinetics limit cell performance. Exploring effective redox mediators or catalysts for the potential application of sparingly solvating or nonsolvating electrolytes will enable further improvements.

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Protection of lithium metal anodes is also necessary for lean-electrolyte LSBs because the parasitic reactions between lithium and concentrated LiPSs cause continuous electrolyte depletion and rapid failure of lithium anodes. Lithium anode protection is also a requirement for safer LSBs and is discussed in the next section.

3.3

Lithium metal anodes

Lithium metal anodes have been regarded as the “Holy Grail” for rechargeable batteries because of their ultrahigh theoretical specific capacity and low density, and because they possess the lowest negative potential [68]. However, serious safety hazards caused by the lithium dendrites have hindered their practical application for many years. The continuous reactions between lithium and electrolytes also lead to their gradual consumption. In addition, the use of thick lithium foils leads to lower energy densities and higher safety risks. Therefore, improving lithium anode safety and reducing excess lithium are essential to enable practical and sustainable LSBs.

3.3.1

Lithium metal protection

A stable and uniform SEI on the lithium anode is beneficial to prevent the formation of lithium dendrites and protect the lithium metal from attack by the electrolyte. An ideal passivation layer on the lithium metal should be thin but robust and possess high Liþ conductivity [69]. Besides in situ SEI regulation through electrolyte engineering, artificial SEIs have emerged as effective methods to stabilize lithium anodes [70]. Jing et al. developed a porous Al2O3 protective layer on the surface of lithium using a simple spin-coating method [71]. This artificial SEI prevents the side reactions between lithium and the electrolyte. Cha et al. explored a scalable approach to fabricate 2D MoS2 coating on the surface of lithium via magnetron sputtering [72]. The uniformly deposited MoS2 layer shows strong adhesion to the lithium metal and inhibits the growth of lithium dendrites. Further efforts should be made to explore green and low-cost artificial SEIs to meet the need for scale-up preparations. Constructing rational hosts for lithium is another important method to suppress dendrite growth. A porous host also mitigates the volume change of the anode during Li deposition and stripping. Carbon-based materials such as CNTs [73], graphene [74], and carbon foams [75] are promising lithium hosts. Biomass-derived carbon materials have also been demonstrated. Jin et al. reported rice husk-derived carbons (RCs) as the lithium host [76]. As shown in Fig. 26.4A, the abundant F-containing groups in RCs are favorable for the formation of lithium fluoride in the SEI, which could effectively suppress lithium dendrites. Liu et al. employed porous carbons derived from watermelon flesh as the lithium host with homogenized lithium deposition and suppressed lithium dendrites [79]. Pairing with the cathode of NeCo9S8/S, an LSB pouch cell delivered high energy density (325 Wh kg
1) and power density (1412 W kg
1). In addition to carbon hosts, current collectors based on 3-D porous metals have been used to suppress lithium dendrites. Huang et al. fabricated hybrid current collectors by the in situ growth of Cu2S nanowires (NWs) inside commercial Cu foams [77]. As shown in Fig. 26.4B, the lithiophilic Cu2S/NWs increase the contact area between

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Figure 26.4 (A) Controllable deposition process of lithium enabled by rice husk-derived carbons. (B) Lithium plating behaviors with the bare Cu foam and 3-D Cu2S nanowires/Cu current collectors. (C) Fabrication process of metal-coated carbon fabric electrodes. (A) Reproduced with permission from C. Jin, O. Sheng, W. Zhang, J. Luo, H. Yuan, T. Yang, H. Huang, Y. Gan, Y. Xia, C. Liang, J. Zhang, X. Tao, Sustainable, inexpensive, naturally multifunctionalized biomass carbon for both Li metal anode and sulfur cathode, Energy Storage Mater. 15 (2018) 218e225. Copyright 2018, Elsevier. (B) Reproduced with permission from Z. Huang, C. Zhang, W. Lv, G. Zhou, Y. Zhang, Y. Deng, H. Wu, F. Kang, Q.H. Yang, Realizing stable lithium deposition by in situ grown Cu2S nanowires inside commercial Cu foam for lithium metal anodes, J. Mater. Chem. 7 (2) (2019) 727e732. Copyright 2019, The Royal Society of Chemistry. (C) Reproduced with permission from J. Chang, J. Shang, Y. Sun, L.K. Ono, D. Wang, Z. Ma, Q. Huang, D. Chen, G. Liu, Y. Cui, Y. Qi, Z. Zheng, Flexible and stable high-energy lithium-sulfur full batteries with only 100% oversized lithium, Nat. Commun. 9 (1) (2018) 4480. Copyright 2018, The Author(s).

the current collector and electrolyte with homogeneous Liþ flux, thus enabling dendrite-free deposition. Moreover, the current collector favors the formation of a stable SEI containing Li2S/Li2S, which reduces the side reactions between lithium and the electrolyte.

3.3.2

Limited excess lithium

Although excess lithium is necessary to compensate for its high reactivity [11], the amount used in most currently produced cells is far more than could be included in a high-energy-density cell. This probably results from the fact that lithium foils are mostly available with thicknesses of a few hundred micrometers [16]. It is suggested that researchers turn to the use of thinner lithium foils with limited excess (30 mm or 6 mAh cm
2) to better identify the technical challenges of practical LSBs [78].

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Restricting excess lithium metal is also an inevitable requirement for safer and more sustainable LSBs with high gravimetric and volumetric energy densities. For stable cycling with limited lithium use, efforts should be made to decrease the loss of lithium and increase the plating and stripping CE. First, a rationally designed cathode should be able to confine LiPSs within the cathode region and promote the conversion of sulfur species, especially in high sulfur loadings and contents. Second, lithium dendrites should be effectively suppressed to avoid the accumulation of “dead lithium.” Third, effective protection of the lithium metal anodes is necessary to reduce the side reactions between lithium and the electrolyte. All of these steps should be implemented simultaneously to minimize the loss of lithium and enhance its utilization. The number of studies related to achieving stable LSBs using limited lithium has increased in recent years [78,80,81]. For example, Chang et al. reported a flexible and high-energy-density LSB (288 Wh kg
1 and 360 Wh L
1) using 100% excess lithium [78]. As shown in Fig. 26.4C, metal-coated carbon fabrics (CFs) were designed as lithium and sulfur hosts. The CF structure enables high flexibility and decreases the local current density of the electrodes. As the lithium host, Cu-coated CF induces uniform deposition of lithium without dendrite formation, leading to an excellent CE of >99.89% over 400 cycles. As the sulfur host, Ni-coated CF has strong capability to anchor Li2S and promote LiPS reduction kinetics, enabling a CE of >99.82% for the cathode over 400 cycles.

3.4

Scalable electrode preparations

Expensive raw materials and complex synthetic processes are often involved in successfully developing LSBs with superior electrochemical performance, not to mention the potential safety hazards and environmental burdens. Many lab-scale experiments of LSB coin cells cannot be directly translated to practical pouch cells due to economic and environmental concerns [5]. It is therefore necessary to conduct material and cell designs with future scale-up in mind. Recent developments in the potential technologies of scalable preparations for sulfur cathodes and lithium anodes are discussed here.

3.4.1

Cathode preparation

The preparation of sulfur cathodes generally includes the synthesis of sulfur-based composites and the following cathode fabrication. To disperse insulating sulfur into conductive hosts such as porous carbons, various sulfur-loading techniques have been developed, including melt-diffusion [82], ball milling [83], dissolution crystallization [84], and chemical deposition [85]. The merits and demerits of various synthetic methods of sulfur-based composites are summarized by Huang et al. (Fig. 26.5) [86]. The most favorable method for sulfur incorporation depends on the structural characteristics of the conductive host. Considering the mass production of sulfur-based composites, expensive and harmful materials, and high-energy-consumption processes should be avoided. As seen in Fig. 26.5, supercritical fluid, vacuum mixing, and

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Figure 26.5 Merits and demerits of different synthetic methods for sulfur-based composites. Reproduced with permission from L. Huang, J. Li, B. Liu, Y. Li, S. Shen, S. Deng, C. Lu, W. Zhang, Y. Xia, G. Pan, X. Wang, Q. Xiong, X. Xia, J. Tu, Electrode design for lithiumesulfur batteries: problems and solutions, Adv. Funct. Mater. 30 (22) (2020) 1910375. Copyright 2020, Wiley-VCH.

electrodeposition methods are currently not suitable for practical applications due to higher costs and the dependence on specific facilities. Dissolution crystallization usually involves toxic solvents such as CS2, so efforts should be paid to ensure safety and environmental protection requirements. Chemical deposition is a method for in situ growth of sulfur nanoparticles on host materials based on simple chemical reactions [86]. Extra treatment should be employed to prevent toxic gas emissions, including H2S and SO2 [18]. Ball milling and melt-diffusion are promising methods for scalable production owing to their lower costs and eco-friendliness, but further efforts should be made to increase the homogeneity of sulfur incorporated into the hosts. For cathode fabrication, the two main approaches are conventional slurry coating on 2-D current collectors and the construction of freestanding 3-D current collectors. The method of slurry coating is widely used and compatible with the production lines of LIBs. Typically, sulfur-based composites, conductive agents, and binders are mixed in a solvent to form a slurry and then coated on a 2D metal foil. However, thick cathodes with high sulfur loadings often suffer from high internal resistance, low sulfur utilization, and cracking and flaking defects. Only a very small portion (around 5%) of the high-loading cathodes are built with Al-foil current collectors, especially when the sulfur loading is higher than 8 mg cm
2 [87]. More effective sulfur hosts, binders, and conductive agents are required to tackle these problems. An effective strategy for fabricating thick electrodes is integrating organized nanoparticles into microsized secondary structures [18]. Ye et al. designed a promising sulfur host for thick electrodes through a modular assembly of scattered Ketjen black nanoparticles into an oval-like microstructure [88]. The secondary structure enabled a specific discharge capacity of 8.417 mAh cm
2 with a high sulfur loading of

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8.9 mg cm
2. The binder plays an important integrity role, especially for a thick sulfur cathode. Exploring effective aqueous binders is crucial to fabricating a thick and robust electrode. In addition, interconnected conductive networks will be beneficial to increase sulfur utilization under high-loading conditions [2]. Adding a small quantity of CNTs or graphene as conductive agents has also been found to be a facile approach to decrease the internal resistance of thick cathodes [2]. Three-dimensional current collectors such as carbon foams and carbon cloths have emerged as the support matrixes for sulfur cathodes due to the ability to accommodate high sulfur content. However, many of these 3-D current collectors are prepared via time-consuming or costly processes, such as freeze-drying [89] and chemical vapor deposition [90], limiting their mass production potential Moreover, large electrolyte volumes are required to wet these 3-D porous architectures, and their low bulk density limits the volumetric capacity of the cathodes. Further works are required to develop novel and facile techniques to achieve the large-scale synthesis of 3-D current collectors with rational structures.

3.4.2

Anode preparation

Lithium foil is the most widely used anode material for LSBs. As lithium is highly active in ambient air, the manufacturing of lithium foils must be in an inert or lowmoisture atmosphere, which significantly increases the cost of processing. For cost savings, passivation treatment of the lithium surface is a promising strategy [91]. Zhang et al. developed an air-stable and waterproof lithium metal anode with waxe poly(ethylene oxide) (PEO) packaging via a simple and scalable dip-coating approach [92]. As shown in Fig. 26.6, the sealing effect of wax protects lithium from air and moisture corrosion, while the PEO in the coating ensures uniform Liþ flux and prevents the growth of lithium dendrites. Thin lithium foils with only several tens of micrometers are required for practical LieS pouch cells with high energy density and cell safety. The high cost to produce these thin lithium foils may make LSBs lose the competitive advantage of cost [93]. This encourages the exploration of novel techniques for manufacturing thin lithium foils using melt processing and vapor deposition [69,93]. The use of 3-D hosts for lithium avoids the processing of expensive lithium foils. However, the preparation of lithium anodes with 3D hosts involves electrodeposition or melt infusion of lithium, which are still technically challenging for scale-up. Further efforts should focus on scalable pre-lithiation technologies. Using Li2S as the cathode to couple with Li-free anode is a possible approach to overcome the issues of cost and safety associated with lithium foils [94], but the air-sensitivity of Li2S must be handled by designing proper cathode structures and processing methods.

3.5

Recycling of lithiumesulfur batteries

Recycling is an important step in the battery life cycle to form a closed loop [95]. Recycling spent batteries aims to prevent them from causing severe and persistent pollution while recovering valuable elements such as Li and Co.

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Figure 26.6 Working mechanism of the waxepoly(ethylene oxide) coating on the lithium metal anode. Reproduced with permission from Y. Zhang, W. Lv, Z. Huang, G. Zhou, Y. Deng, J. Zhang, C. Zhang, B. Hao, Q. Qi, Y.-B. He, F. Kang, Q.-H. Yang, An air-stable and waterproof lithium metal anode enabled by wax composite packaging, Sci. Bull. 64 (13) (2019) 910e917. Copyright 2019, Elsevier.

Although LSBs have not yet achieved wide application, recycling methods should be considered in advance. LIB recycling technologies could provide useful guidance for LSBs. Various recycling technologies have been developed for LIBs, including pyrometallurgical and hydrometallurgical methods [96e98]. Generally, pretreatments such as discharge and separation are necessary to reduce safety risks during battery disassembly and enhance the efficiency of further recycling of electrode materials. Pyrometallurgical recycling is a high-temperature smelting process that turns spent batteries into alloys composed of Co, Ni, Cu, and Fe through thermal reductions. A slag containing Li, Al, and Mn is also produced and requires further treatment and extraction [98]. The hydrometallurgical process effectively recycles valuable metals (Li, Co, Ni, etc.) from spent LIBs. It involves metal leaching and subsequent extractions [97]. Compared with pyrometallurgical recycling, this recycling method enables high metal purities with lower energy consumptions. The most valuable material for the recycling of LSBs is the lithium metal in the anodes. The hydrometallurgical process may be suitable for extracting lithium. Schwich et al. evaluated the recycling potential of LSBs using thermal and hydrometallurgical methods [99] and found that up to 93% of lithium could be recycled in the form of Li2CO3. Pretreatments are especially important because of the high safety hazards associated with battery disassembly induced by the highly flammable lithium and electrolytes. An ideal LSB recycling process should maximize lithium recovery

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but also consider other metals, such as Cu in the current collectors. Sustainable recycling technologies with controllable environmental impacts will be the development direction.

4.

Conclusions

Sustainability should be considered simultaneously with the development of highenergy-density LSBs. This chapter evaluates LSB sustainability from the perspectives of cost-effectiveness, safety, and environmental impacts. Cell cost is a primary factor determining market competitiveness. The safety performance of LSBs requires more assessments under abuse conditions, and understanding the failure mechanism is beneficial for exploring safer LSB systems. Reducing the environmental burdens of LSBs is in line with the development of clean energy. LCA is an effective method to evaluate the environmental impacts of LSBs. For more sustainable LSBs, the development of sulfur cathodes, separators, electrolytes, and lithium anodes should meet the requirements of lower costs, higher safety standards, and reduced environmental impact. Renewable biomass materials and their derivatives are promising materials for cathodes, separators, and lithium anodes. Reducing the electrolyte-to-sulfur ratio and limiting excess lithium are effective methods for achieving cheaper, safer, and greener LSBs. The practical application of LSBs will face challenges from the mass production of electrode materials and the fabrication of electrodes. Additional efforts are required within these steps to develop sustainable synthesis and manufacturing techniques with lower costs and minimal environmental burdens.

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[77] Z. Huang, C. Zhang, W. Lv, G. Zhou, Y. Zhang, Y. Deng, H. Wu, F. Kang, Q.H. Yang, Realizing stable lithium deposition by in situ grown Cu2S nanowires inside commercial Cu foam for lithium metal anodes, J. Mater. Chem. 7 (2) (2019) 727e732. [78] J. Chang, J. Shang, Y. Sun, L.K. Ono, D. Wang, Z. Ma, Q. Huang, D. Chen, G. Liu, Y. Cui, Y. Qi, Z. Zheng, Flexible and stable high-energy lithium-sulfur full batteries with only 100% oversized lithium, Nat. Commun. 9 (1) (2018) 4480. [79] Y. Liu, Y. Barnscheidt, M. Peng, F. Bettels, T. Li, T. He, F. Ding, L. Zhang, A biomassbased integral approach enables Li-S full pouch cells with exceptional power density and energy density, Adv. Sci. (2021) 2101182. [80] W. Xue, Z. Shi, L. Suo, C. Wang, Z. Wang, H. Wang, K.P. So, A. Maurano, D. Yu, Y. Chen, L. Qie, Z. Zhu, G. Xu, J. Kong, J. Li, Intercalation-conversion hybrid cathodes enabling LieS full-cell architectures with jointly superior gravimetric and volumetric energy densities, Nat. Energy 4 (5) (2019) 374e382. [81] C. Zhao, G.L. Xu, Z. Yu, L. Zhang, I. Hwang, Y.X. Mo, Y. Ren, L. Cheng, C.J. Sun, Y. Ren, X. Zuo, J.T. Li, S.G. Sun, K. Amine, T. Zhao, A high-energy and long-cycling lithium-sulfur pouch cell via a macroporous catalytic cathode with double-end binding sites, Nat. Nanotechnol. 16 (2) (2021) 166e173. [82] H. Xu, Q. Jiang, B. Zhang, C. Chen, Z. Lin, Integrating conductivity, immobility, and catalytic ability into high-N carbon/graphene sheets as an effective sulfur host, Adv. Mater. 32 (7) (2020) e1906357. [83] J. Xu, J. Shui, J. Wang, M. Wang, H.K. Liu, S.X. Dou, I.Y. Jeon, J.M. Seo, J.B. Baek, L. Dai, Sulfur-graphene nanostructured cathodes via ball-milling for high-performance LithiumSulfur batteries, ACS Nano 8 (10) (2014) 10920e10930. [84] P. Yu, Z.C. Xiao, Q.Y. Wang, J.K. Pei, Y.H. Niu, R.Y. Bao, Y. Wang, M.B. Yang, W. Yang, Advanced Graphene@Sulfur composites via an in-situ reduction and wrapping strategy for high energy density lithiumesulfur batteries, Carbon 150 (2019) 224e232. [85] Y. Li, Q. Guan, J. Cheng, B. Wang, Ultrafine nanosulfur particles sandwiched in little oxygen-functionalized graphene layers as cathodes for high rate and long-life lithiumsulfur batteries, Nanotechnology 31 (24) (2020) 245404. [86] L. Huang, J. Li, B. Liu, Y. Li, S. Shen, S. Deng, C. Lu, W. Zhang, Y. Xia, G. Pan, X. Wang, Q. Xiong, X. Xia, J. Tu, Electrode design for lithiumesulfur batteries: problems and solutions, Adv. Funct. Mater. 30 (22) (2020) 1910375. [87] S.H. Chung, C.H. Chang, A. Manthiram, Progress on the critical parameters for lithiumesulfur batteries to be practically viable, Adv. Funct. Mater. 28 (28) (2018) 1801188. [88] Y. Ye, F. Wu, Y. Liu, T. Zhao, J. Qian, Y. Xing, W. Li, J. Huang, L. Li, Q. Huang, X. Bai, R. Chen, Toward practical high-energy batteries: a modular-assembled oval-like carbon microstructure for thick sulfur electrodes, Adv. Mater. 29 (48) (2017) 1700598. [89] G. Hu, C. Xu, Z. Sun, S. Wang, H.M. Cheng, F. Li, W. Ren, 3D graphene-foam-reducedgraphene-oxide hybrid nested hierarchical networks for high-performance Li-S batteries, Adv. Mater. 28 (8) (2016) 1603e1609. [90] G. Zhou, L. Li, C. Ma, S. Wang, Y. Shi, N. Koratkar, W. Ren, F. Li, H.M. Cheng, A graphene foam electrode with high sulfur loading for flexible and high energy Li-S batteries, Nanomater. Energy 11 (2015) 356e365. [91] J. Wu, L. Yuan, Z. Li, X. Xie, Y. Huang, Air-stable means more: designing air-defendable lithium metals for safe and stable batteries, Mater. Horiz. 7 (10) (2020) 2619e2634. [92] Y. Zhang, W. Lv, Z. Huang, G. Zhou, Y. Deng, J. Zhang, C. Zhang, B. Hao, Q. Qi, Y.B. He, F. Kang, Q.-H. Yang, An air-stable and waterproof lithium metal anode enabled by wax composite packaging, Sci. Bull. 64 (13) (2019) 910e917.

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Recyclability and recycling technologies for lithiumesulfur batteries

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Fariborz Faraji 1 , Misagh Khanlarian 2 , Melina Roshanfar 3 , Guillermo Alvial-Hein 1 and Harshit Mahandra 1 1 The Robert M. Buchan Department of Mining, Queen’s University, Kingston, ON, Canada; 2 Department of Civil and Environmental Engineering, Faculty of Engineering and Design, Carleton University, Ottawa, ON, Canada; 3Department of Civil Engineering, Faculty of Engineering, University of Ottawa, Ottawa, ON, Canada

1. Introduction Over the years, demand for energy and energy-storage units has been growing at a breathtaking rate. Increased energy consumption, portable electronic devices, and concerns about environmental issues related to reliance on fossil fuels are some reasons for this rapid rise [1,2]. The development of rechargeable batteries with high-density storage capacities, long lifetimes, and low manufacturing costs has suggested the emergence of lithium-sulfur (LieS) batteries as one of the most promising options [3]. This revolutionary type of battery has many advantages over previous rechargeable generations, i.e., a high energy capacity of approximately 1670 mAh g
1 and impressive energy density of about 2500 kW kg
1 (both theoretical values), an eco-friendlier nature, and low costs from the application of sulfur instead of expensive metals [4]. Based on the promising features of LieS batteries, an increase in applications of LieS batteries is expected. As a result, a large volume of spent LieS batteries will be produced and will require a decent recycling plan [5]. LieS battery functionality is based on the mobility of lithium between the anode and the cathode using polysulfide (PS) compounds (S2
n , n between 2 and 8) (Fig. 27.1). The overall reaction in charge and discharge of a simple LieS battery can be expressed in the form of Eq. (27.1) [6]: S8 þ 16Liþ þ 16e


ƒƒƒƒ! ƒƒƒƒ 8Li2 S

Charge=discharge

(27.1)

Researchers have listed various reasons for a LieS battery unit to lose its functionality. First, with time, electrode surfaces become covered by sulfur and lithium-PSs that are not electrically conductive. Therefore, the rate of charge and discharge becomes increasingly slower [4]. Second, after several charge/discharge cycles, the volume of chemicals in the battery expands, and the whole unit is destroyed, e.g., the

Lithium-Sulfur Batteries. https://doi.org/10.1016/B978-0-323-91934-0.00014-4 Copyright © 2022 Elsevier Inc. All rights reserved.

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Figure 27.1 Simplified view of charge and discharge processes in lithiumesulfur batteries.

movement of LieS compounds from the electrolyte (Li2S, density ¼ 1.66 g cm
3) to the cathode (a-S8, density ¼ 2.07 g cm
3) [7]. Third, lithium PS, as the carrier of lithium ions, gradually dissolves in the electrolyte and reacts with LieS battery components, which damages the batterydthis is known as the shuttle effect [8]. Fourth, in every charging phase, there is uncontrolled growth of lithium on the anode that occasionally leads to wild dendrite formation and cell malfunction [9]. Fifth, the battery may explode in response to undesirable reactions between lithium and the electrolyte that form a large volume of gas inside the battery [10]. There is extensive research to tackle each of the problems and extend the working life of LieS batteries by working on the chemistry and the state of the art in battery design. All the development can postpone the accumulation of battery waste but not terminate it. Therefore, it is necessary to think ahead and plan for viable recycling approaches to address future needs. Various materials within LieS batteries are worth recycling. Lithium, organic compounds, various forms of carbon, various metals other than lithium, and sulfur/PSs are examples of common materials present in spent LieS batteries. Since LieS batteries are new forms of energy-storage devices, few processes have been designed specifically for their recycling; however, plenty of successfully implemented processes for the recycling other forms of energy-storage units can be adapted for spent LieS battery recycling. Technologies such as pyrometallurgy, hydrometallurgy (including biohydrometallurgy), and solvometallurgy combined with physical, mechanical, chemical, and thermal pretreatment techniques are the available options [11e13]. This chapter introduces the structure and materials applied in building LieS batteries. Next, the available recycling technologies applicable for spent LieS batteries are discussed, and illustrations of spent LieS battery recycling are provided. Afterward, advancements and new technologies in battery recycling and their applicability for spent LieS batteries recycling are discussed. In the end, a perspective about the current and future trends in the recycling of LieS batteries is proposed.

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2. Lithiumesulfur battery The LieS battery is a new technology that is still under development. Various versions of this battery continue to come out every year with new designs and different materials [3]. LieS batteries are the most recent energy-storage devices introduced in the 1960s and implemented after 2009 [14,15]. The development of LieS batteries has been fast, and it is expected that several generations of development will occur during widespread application [16]. For a reliable recycling strategy, the components of LieS batteries must be known. The structure of LieS batteries is not universal, but they have mutual components such as sulfur-based cathodes, lithium anodes, electrolytes, separators, and casings (Fig. 27.2). In addition, they may contain binders, interlayers, and collectors. Depending on the battery technology, additional features (e.g., such as catalysts) can be found within the structures of batteries [18].

2.1

Recyclable materials in spent lithiumesulfur batteries

There are various recyclable materials in the main components of the cathodes, anodes, electrolytes, binders, and separators as well as the cases and collectors of LieS batteries. Carbon (graphite, graphene, etc.), sulfur, polymers, organic materials from binders and separators, metals, and metal oxides from electrodesdmainly lithium from the electrolyte and aluminum, copper, and nickel from the casing are the most important recoverable materials.

2.1.1

Cathode materials

For a viable recycling plan, it is necessary to identify the valuable materials and the way they are enclosed in the structure of the battery. Conventional cathode materials are made simply of carbon and sulfur. The concentration of sulfur in such cathodes is in the range of 50 wt.%e70 wt.%, and the remainder comprises carbon and binders [19]. Carbon can be composited by sulfur or coated on the surface of sulfur [7]. The source of carbon can be any of mesoporous/hollow carbon spheres, carbon nanotubes (CNTs), graphene, graphene oxide, graphene oxide/CNT interlayers, carbonaceous materials, metal oxides and sulfides, conductive polymers, and materials in which sulfur was trapped in the host [7,20,21]. The host should have high porosity and make a

Figure 27.2 Schematic view of a general structure of a lithiumesulfur battery [17].

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good composite with sulfur [22]. Polar carbon host cathodes such as heteroatom-doped carbon or hybrids with transition metal compounds can be considered a sufficient cathode material because of their good interactions with lithium PSs [18]. In semiliquid batteries, liquid PS is a cathode that provides high energy density [21]. Various types of carbon have attracted considerable attention because carbon materials can be used as a bifunctional PS-adsorption layer between the sulfur electrode and separator [22]. Polymers, along with carbon as a coating or in the form of a composite, are additional cathode materials that can be used in LieS batteries. The materials applied in the structure of this type of cathode include forms of sulfur, carbon, polymers, and metals. In this cathode, a conductive polymer is usually prepared with a carbon/sulfur composite. Some examples of polymers used in this application are poly(ethylene glycol), poly(thiophene), poly(pyrrole), poly(3,4-ethylenedioxythiophene), poly(acrylonitrile), and poly(styrene sulfonate) [7,23]. Besides carbon, recycling of the organic materials from the waste of these cathodes is favorable. Carbon materials derived from biomass (CDBs) can be an appropriate host because of their inherited specific structure. There are several CDBs, such as cotton, kapok, and 3-D cross-linked bacterial cellulose. These are fibrous CDBs that bring about a decent porosity for trapping sulfur. Another example of a CDB with a high specific area is micro/mesoporous coconut shell carbon [24]. A puffed rice carbon is another host for sulfur in cathodes and can be produced from the popcorn process. By using nickel nanoparticles and forming PRC/Ni composites, a cathode with a highly reversible capacity can be created [24]. Conductive carbon is an attractive cathode material because of its high electrical conductivity, prevention of dissolved PSs, and high porosity [22]. A nitrogen-doped CNT/graphene host provides better specific capacity and rate performance for LieS battery cathodes [25]. In addition to nitrogen doping, phosphorus, boron, and sulfur doping can improve cathode performance [26]. Metal-carbon cathodes are a newer generation of cathode materials in which metals are also employed in the cathode structure. The metals can be in the form of oxides, elements, or sulfides. In some cathodes, carbon hosts are modified with metal oxides such as MxOy/C, Mg0.6Ni0.4O, SiO2, MgO, Al2O3, NiO2, NiOeNiCo2O4, CeO2, as well as oxides of La and Ca [24,25]. Due to the poor electron conductivity of metal oxides, carbon/semiconductive metal oxides and other metals (such as Pt, Fe, Co, and Ni) have been developed as a host for sulfur in LieS battery cathodes [7,26]. Carbon/metal sulfide hosts are another technology and can contain ZnSeFeS, Co3S4e MnS, VS2, CoS2, TiS2, FeS, SnS2, and Ni2S3 metal sulfides [7,21,26]. Combinations of different hosts have also been practiced; cathode materials Co3S4eMnS nanotube, WS2eWO3, TiO2eNi3S2, VO2eVN, TiNeVN, and multilayers of Ti3C2/Li2S are some examples [21,25]. Therefore, the waste generated from spent LieS batteries also can recover metal values from cathode materials.

2.1.2

Anode

The anode for LieS batteries is simply lithium in elemental form; however, as this metal is not hard enough to hold its shape in the battery structure, it is usually supported by a supplementary copper foil. In this structure, about one-fourth of the anode

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is lithium, and the rest is copper (as copper has a higher density) [27]. Direct exposure of lithium to the electrolyte shortens the lifetime of the LieS battery; therefore, other materials are also employed in the anode structure [7]. The addition of some customized separators, the use of different protecting interlayers, the addition of additives to the electrolyte, and the use of a current collector are some examples of ways to control the pattern of lithium growth on the anode and extend the functionality and lifetime of the battery [28]. Table 27.1 shows some materials used in the anode of lithiumesulfur batteries that can be considered for recycling, where lithium is the main element of interest [19,23,24,26,29]. The rest of the materials are usually employed as protective layers for anodes to prevent unwanted reactions with the PS in the electrolyte [7,30]. Most protective materials are made of carbon in graphene or graphite forms [29]. Metals in the anode and its protective interlayer can be in different forms, i.e., elemental, oxide, sulfide, or nitridede.g., TiO2, Nb2O5, MoS2, Sb2S3, Co9S8, Mo2N, Sn, and Li alloys (e.g., fibrous Li7B6) [18,21,26,29,31]. Another approach is providing a solid electrolyte interphase (SEI) layer at the interfacial of the anode and electrolyte [7,21]. Lithium nitrate (LiNO3) is one of the protective layers in this application [32]. Polymer materials, such as monomer poly(ethylene glycol) dimethacrylate, can also be applied as SEIs [7]. Some studies have reported that sulfur powder can protect lithium anodes and be applied on the surface of anodes [7].

2.1.3

Electrolyte

The electrolyte in the structure of LieS batteries plays a pivotal role, as it is responsible for the transfer of lithium ions between the anode and cathode. The main material in the electrolyte is polysulfide (S2
n , n between 2 and 8) and can be prepared in different interlayer types of (1) liquid electrolyte, (2) polymer electrolyte, and (3) all-solid-state electrolyte [6,33]. Each electrolyte type has its advantages; liquid electrolytes usually contain any dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), or dimethylformamide (DMF) and can provide better control over battery electrochemistry [34]. Polymerized or polymer gel electrolytes are made of materials such as poly(ethyleneoxide)-modified poly(vinylidene fluoride-co-hexafluoropropylene), pentaerythritol, polyethylene oxide, and polypropylene oxide comprising carboxylate and Table 27.1 Recyclable anode materials of lithiumesulfur batteries [4,18,21,23,24,26,31,38,39]. Material

Examples

Metallic Carbon

Li, Au, Si, Ag, Sn, MnO2, Co, Cu3N, ZnO, Cu, CueNi hybrid, AlF3, and Li7B6 Graphite, carbon nanotubes (CNTs), graphene, grapheneeCNT hybrids, carbon nanofibers, porous carbon, graphite particles, graphite microtubes, and TiCecarbon hybrid Conductive polyaniline and polypyrrole, and monomer poly(ethylene glycol) dimethacrylate

Polymers

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Lithium-Sulfur Batteries

sulfonate groups [35]. A benefit of using this type of electrolyte is the prevention of unwanted reactions between the electrolyte and the electrodes (shuttle effect) [6]. Solid-state electrolytes are made of inorganic materials and are advantageous as they minimize the risk of combustion and increase battery life (oxidation of anode is inhibited). Despite their advantages, solid-state batteries have a slow charge and discharge kinetic, which can be partially overcome by adding catalysts (titanium, lanthanum, tellurium, etc.) [20,36]. It is worth mentioning that a combination of the interfaces has been tried in some LieS batteries and provided all the advantages simultaneously [37]. Another conducting salt that can be used as an electrolyte is lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2, commonly known as LiTFSI) [19]. LiTFSI can be used with LiNO3 in the 1,2-dimethoxyethane/1,3-dioxolane (DME/DOL) electrolyte [26]. Regarding solid-state electrolytes, starch, which consists of repeated glucose monomers with glycosidic linkages, can be used [24]. To develop the stretchable properties of the electrolyte, g-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560) can be added to make a cross-link in starch [24]. Silicon-containing polymer, garnettype lithium-ion conducting oxides, LISICON-type conductors, lithium sulfide glasses (e.g., Li2SeP2S5, Li2SeSiS2, or Li2S-GeS2-P2S5), Li7La3Zr2O12, Li0.5La0.5TiO3, Li10GeP2S12, Li1.3Al0.3Ti1.7(PO4)3, and Li3OCl are the other solid-state electrolytes [23,38]. Polymers such as poly(ethylene oxide) polymer, poly(ethylene glycol) (dimethyl ether) polymer, and poly(vinylidene fluoride-co-hexafluoropropylene), , can also be used as electrolytes [7]. Additives that enhance LieS battery electrolyte properties include LiNO3, P2S5, phenyl diselenide, and nanosized zirconia [21,23,29]. Here, polymers and lithium can be considered valuable materials for recycling.

2.1.4

Other lithiumesulfur battery configurations

In addition to the main components of LieS batteries, some other battery configurations can be worth recycling, i.e., casing, separators and interlayer, and binders are some of features. Cases of LieS batteries are made primarily of steel for cylinder cells and aluminum for pouch cells [29]. Aluminum and carbon nanofibers are used mostly as collectors for the cathode, and copper and sometimes nickel are used as collectors for the anode [29,39]. To enhance the charge transfer rate, some collectors are made of porous and conductive substrates (metals and carbon) [4]. Separators and interlayers are the other important parts of LieS batteries and are designed to limit the shuttle effect, battery self-discharge, and lithium dendrite growth on the anode [40]. Various materials can be used as separators, for instance, polyethylene, poly(tetrafluoroethylene), polypropylene (PP), polyolefin, graphene, poly(dopamine), glassy fiber paper, black phosphorus, polysulfides, sulfonated acetylene black, carboxyl functional groups, graphene layers with Li4Ti5O12 nanoparticles, microporous PP matrix layer, GO barrier, Nafion, a metaleorganic framework/GO composite film, and single-walled CNTs [24,26,29]. Materials such as nonconductive carbon, polypropylene, hydroxide nanosheet/graphene oxide, anodized aluminum oxide, graphdiyne nanosheets imbued on a polypropylene membrane, and carbon nitride

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could be used as separators and interlayers in LieS batteries [6]. Carbons such as nanotubes, microporous, nanofibers, and cellulose fibers, as well as poly(pyrrole), can be used as interlayers [24,25]. Binders are responsible for the effective connection between different battery components; thus, they should be adhesive, chemically stable, and capable of enduring multiple charge and discharge cycles [4]. Materials such as poly(acrylonitrileemethyl methacrylate), natural gelatin polymer, polyacrylic acid, polyacrylamide, waterborne polyurethane polymer with phytic acid, and dissolvable ionic cross-linked polymer can be used as binders in LieS batteries [4,5,32]. Gelatin, N-methyl-2-pyrrolidone (NMP), polyvinyl pyrrolidone (PVP), Na-alginate, gum arabic, PVP blends with Nafion, polyamidoamine dendrimers, polycationic-cyclodextrins, poly (acrylic acid), poly- (ethylene oxide), and carboxymethyl-cellulose:styrene-butadiene rubber (CMC:SBR), soy protein, and sodium-alginate are the other binders that have been investigated so far [23,24,29]. Generally, binders in LieS batteries are at low levels, and as a result, they are not usually considered for recycling [29].

3. Recycling technologies Due to the wide range of materials used in battery manufacturing and different types of batteries, several methods are available for recycling or reusing batteries. In this section, the general technologies for battery recycling are discussed and supported by referring to some examples of practical recycling of spent LieS batteries.

3.1

Available recycling technologies

The prevalent technologies for battery recycling can be categorized into the following main groups: physiomechanical, pyrometallurgical, and hydrometallurgical technologies (Fig. 27.3) [41]. Because of the multimaterial structure of batteries (metals, carbon, sulfur, and organics), each part demands a different recycling process; therefore, recycling usually starts with a physical treatment to separate and sort the incorporated parts [42]. This step can instantly recover some parts of the battery, including some plastics, iron (from casing), and some carbon materials; however, most battery materials need further processing [43]. Depending on the chemical and physical properties of the separated parts and based on the goal of recycling (which of the metals, organics, carbons, etc. are demanded for recycling), the next step can be either pyrometallurgy or hydrometallurgy. In pyrometallurgy, high temperature is applied in an appropriate atmosphere (air, inert gas, or vacuum), and the main product is an alloy, while in hydrometallurgy, materials are reacted with reagents in the aqueous phase, and products are recovered in pure forms [44]. The methods can also be combined (hybrid technologies) to make an efficient recycling process [45]. Most of the recycling techniques were originally developed for lithium-ion batteries but are still applicable for spent LieS batteries.

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Figure 27.3 A general flow sheet of battery recycling processes.

3.1.1

Physiomechanical battery treatment

Fig. 27.4 shows the physiomechanical steps in the early stages of battery recycling. It starts with battery discharging, followed by size reduction, and is completed by sorting and classification [46]. Before starting any of the processes, batteries are usually discharged to eliminate the risks of ignition and explosion [47]. Short-circuiting the batteries by soaking them in a saline solution or applying cryogenic materials (e.g., liquid nitrogen) are common discharging treatments before battery recycling [48,49]. After safe discharge of batteries, their incorporated materials need to be taken apart. There are different materials with various properties in the structure of LieS batteries, which must be separated, sorted, and concentrated to be effectively recycled; therefore, dismantling and size reduction steps are necessary to liberate the battery materials [50]. Different types of wet and dry crushers, shredders, grinders, ball mills, etc. are available for the size reduction step [51]. Size reduction can also be selective, as

Figure 27.4 Physiomechanical steps in battery recycling [46].

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most electrode materials tend to accumulate in the fine fraction, while the rest of the materials, including most of the interlayers, current collectors, and plastic parts, remain coarse [52]. To increase the efficiency of the size reduction step, the methods can be applied after a chemical or thermal pretreatment step. Removing the binders using organic solvents such as NMP (or N-pyrolTM), dimethyl carbonate (DMC)OC(OCH3)2, DMF-(CH3)2NC(O)H), DMAc-CH3C(O)N(CH3)2, and DMSO(CH3)2SO) [53] or heating the materials (up to 850  C) are some of the chemical and thermal pretreatments [54]. Sorting and classification techniques including size separation, gravity separation, cyclones, magnetic separation, electrostatic separation, airflow separation, froth floatation, and their combinations are some widely developed methods for battery recycling and are shown in Fig. 27.5 [55,56]. They are usually performed after dismantling and size reduction steps to provide a concentrated feed for the next recycling steps. Size separation such as sieving simply splits finer particles from the coarser ones [51]. Given that more valuable materials (most metals) accumulate in the finer fraction, this step can provide an initial satisfactory separation [57]. Gravity separation, cyclone, and airflow separation divide materials based on their density, shape, and size [58,59]. Shaking tables, airflow separators, cones, fluidized bed separators, air tables, cyclones, and wind shakers are all examples of materials separation based on their density [60e63]. Magnetic and electrostatic separation benefits form electrical and magnetic differences to separate magnetic/nonmagnetic and electrostatic/nonelectrostatic components [64,65]. Hydrophile and hydrophobic materials can take apart by flotation [66]. Graphite and sulfidic compounds are natural hydrophobic materials in the battery structure and can be recovered through flotation [67,68]. Efficiency and selectivity of the flotation process can be defined by wettability of the particles, stability and size of the bubbles, and hydrophobicity of the particles, which can be modified using different collectors (additives that adhere to the particles and alter their surface properties), and frothers (chemicals that can influence stability and size of the bubbles) [69e71]. Some of the common collectors in battery recycling are kerosene, n-dodecane, and synthetic hydrocarbon oils, and some common frothers are methyl isobutyl carbinol (MIBC) and MIBC- C6H14O [71e75].

Figure 27.5 Sorting and separation techniques for battery recycling.

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3.1.2

Lithium-Sulfur Batteries

Pyrometallurgical methods

In pyrometallurgy, materials are recovered through high-temperature procedures. Advantages of this method are the ability to treat the large size of feed, fast processing rate, and low sensitivity of the process to the composition and size of the feed [76,77]. The most common pyrometallurgical techniques in battery recycling are incineration, pyrolysis, molten salt, and reduction smelting which are compared in Fig. 27.6 [76]. Incineration is an intermediate step before final recovery during which battery materials are exposed to fire, and the organic and carbon parts are burnt off. As a result, metallic and metal oxide fractions are liberated, and carbon/sulfur content is decreased [78]. In this process, high temperature (about 1000  C) is provided by burning the organic and carbon materials available in the battery materials and an extensive volume of toxic gases, including halogenated fumes, dioxins, furans, SOx, CO, and CO2, is released [56,76,78]. Although incineration is easy to operate, it cannot recover the organic materials from the battery, not an environmentally friendly process, and in the case of the LieS battery, where carbon, sulfur, and organics are the main parts, it cannot be a viable recycling process. Pyrolysis is another thermal process in battery recycling where materials are heated up without oxygen (vacuum or an inert gas like argon, N2, etc.) [79]. The outputs of the process are (1) a solid fraction containing metallic compounds and carbon materials, (2) an oil that is the organic materials in reusable forms (3-hydroxypentanoic acid, hydroxyl halides, and benzene), and (3) a gas phase containing mostly useful gases of CO, H2, and CxHy that can be considered fuel [80e82]. Considering nothing is destroyed over pyrolysis, an important advantage of the process is its ability to recover nearly every part of the battery with a relatively low level of environmental damage. In the molten salt recycling process, the battery is treated by submerging it in a molten salt mixture, usually at eutectic concentration. Some applicable salts for battery treatment are AlCl3eNaCl, NaNO3eKNO3, and NaOHeKOH [83]. In this technique,

Figure 27.6 General pyrometallurgical processes and their operational conditions applicable in battery recycling.

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the organic materials of the battery (especially binders and separators) are reacted with the salts, and a metal-rich fraction is recovered; the formation of the dangerous halogenated fumes is limited, and most of the carbon parts of the battery remain intact. As a result, for recycling of spent LieS batteries, the molten salt method cannot recover the organic parts but can safely (low level of hazardous fumes) liberate the metallic fraction and recover the carbon parts. Reduction smelting is one of the main high-temperature processes for metal production. It reduces metal oxides by reacting them to carbon (or CO) at high temperatures above 1400  C [76]. This is a well-developed process for recovering mainly cobalt and nickel from spent lithium-ion batteries; however, it cannot recover lithium, carbon, or any organic materials [84]. Therefore, it may not be a decent choice to recycle spent LieS batteries unless they contain a high concentration of valuable metals other than lithium.

3.1.3

Hydrometallurgical methods

Hydrometallurgy is a method of metal recovery/recycling from ores and waste materials at relatively low temperatures in aqueous phases [85]. Leaching, purification, and recovery are the main stages of a typical hydrometallurgical process, as shown in Fig. 27.7. Leaching is the dissolution of the target metal using different leaching reagents, including inorganic acids, organic acids, alkaline solutions, reductants, oxidants, and complexation agents [87]. In the recycling of battery materials through hydrometallurgy, metals are the main products, and the rest of the materials, mostly

Figure 27.7 A typical hydrometallurgical route for battery recycling [85,86].

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organic compounds and carbon, remain as residues to be recovered separately [88]. Wang and Friedrich [89] managed to leach nearly all of the lithium, nickel, and cobalt from lithium-ion battery materials at optimum leaching conditions of 80  C, 2 mol L
1 sulfuric acid or 4 mol L
1 hydrochloric acid, 50 g L
1 hydrogen peroxide, and 2 h reaction time, and recovered 99.8% pure graphite as residue. There are different reagents applicable for metals leaching from battery materials. Sulfuric acid, nitric acid, and hydrochloric acid are the most common mineral acids, and citric acid, acetic acid, and oxalic acid are some common organic acids used in the leaching step of battery recycling [87]. Metals leaching could be based on different mechanisms of acidolysis (reaction with Hþ or OH
), complexolysis (metal is mobilized after reacting to a complexing agent), and electrolysis (leaching based on reduction or oxidation reactions) [90]. Because leaching materials and the reagents can have different interactions, the leaching step could be selective [91]. Zhang et al. [91] managed to selectively leach over 99% lithium and 96% manganese from a nickel cobalt manganese (NMC) type lithium-ion battery using 2.75 mol L
1 H3PO4 at 40  C for 10 min while only 4.5% cobalt and 1.2% nickel was leached. Bio-hydrometallurgy is another way to recover lithium from spent LieS batteries. Bio-hydrometallurgy is an environmentally friendly, inexpensive, and feasible method to dissolve the metal contents of spent batteries by producing various organic acids. In this method, metal complexes are formed at ambient temperature and pressure by employing microorganismsdbacteria and fungi [92]. It is reported that lithium can be leached and recovered almost completely from spent batteries using a mixture of organic acids produced through fungal bioleaching using Aspergillus niger [93]. Bioleaching-mediated metal recovery using the autotrophic bacterium Acidithiobacillus ferrooxidans achieved 89% Li recovery [94]. Xin et al. [95] reported 80% Li recovery by bioleaching of Li from spent lithium-ion batteries using sulfur-oxidizing bacteria. Biohydrometallurgy has proved to be an efficient leaching method for both low and high concentrations of metals and is also an environmentally friendly approach; however, the extraction process in this method is usually long, from days to months. In terms of purification, solvent extraction (using organic solvents to selectively separate metals), selective adsorbents, and precipitation are the main processes used in hydrometallurgical recycling of the spent batteries [96]. Di-(2-ethylhexyl)phosphoric acid (D2EHPA) and Cyanex® 272 are two common extractants in recycling that can separate Mn, Co, and Al [97]. Acorga® M5640 and LIX® 84-I are examples of chelating solvents mostly used to extract Fe, Cu, and Al. In many cases, a combination of these solvents is used to achieve a synergistic effect [85,86,98]. For recycling an NMC battery, Keller et al. [97] employed D2EHPA to separate manganese from nickel and cobalt in a sulfate medium. At 100 g L
1 extractant, pH 2.2, and 0.01 mol L
1 of metal sulfates (each), 84% of manganese with high selectivity of 40 relatives to nickel and 22 relatives to cobalt was recovered. Recovery is the last step in hydrometallurgy that can deliver pure final products. Metals could be recovered from the solution in the forms of less water-soluble precipitates by changing the pH of the solution, adjusting the temperature, saturating the medium by evaporating the water content, making insoluble salts by adding carbonate,

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sulfide, phosphate, oxalate, etc., cementation (adding less noble metals to the medium to let the more noble species precipitate out), or electrowinning (providing electron to the system so the cations can precipitate) [99]. In practice, a hybrid method is applied to obtain the highest purity and recovery. For example, in a battery recycling study [89], copper was selectively recovered from a leach solution through cementation by adding iron powder then adjusting the temperature at 60  C; the impurities of iron and aluminum were removed by adjusting the pH at about 3.5e4; next, cobalt, nickel, and manganese were selectively precipitated as cobalt hydroxide, nickel carbonate, and manganese sulfide at 40  C by adding NaOH, Na2CO3, and Na2S, respectively. In the end, pure Li2CO3 was recovered from the solution by increasing the temperature up to 95  C and supplying Na2CO3.

3.2

Recycling of spent lithiumesulfur batteries

In practice, a combination of the available technologies is employed for efficient recycling of spent LieS battery materials. The major parts of LieS batteries are the cathode, which is 50 wt.%e80 wt.% sulfur, 20 wt.%e50 wt.% carbon, and small quantities of binder, the anode (usually pure lithium), and minor parts of the electrolyte, separator, and casing [19,29]. Recycling each material may require dissimilar strategies. Considering that LieS batteries are new technologies, there is not much research about their recycling. Here, some of the few reports are summarized. Accurec Recycling GmbH in Germany is one of the few companies that has a process for the recycling of spent LieS batteries. Unfortunately, there is not much detail available, and some parts of the battery, such as electrolytes, binders, and separators, are barely recovered [29]. As shown in Fig. 27.8, this is a combination of mechanical, pyrometallurgical, and hydrometallurgical processes that starts with pretreatment, including sorting, disassembling, and discharging (in NaCl solution) the spent batteries. In the next step, sulfur, electrolytes, solvents, and volatile hydrocarbons are removed through a pyrolysis process in a vacuum furnace at 250  C and captured through a distillation process. After the thermal treatment, the batteries are milled and grounded. Afterward, steel from the casing and aluminum and copper from current collectors are separated from the ground material using a series of mechanical separation processes, including a vibrating screen, magnetic separator, and zig-zag classifier. The remaining fraction undergoes a reduction smelting process where graphite is consumed, and lithium goes to the slag phase. Because some lithium-ion batteries are also added as a feed of this process, the outcome of the reduction smelting is an alloy of cobalt. In later steps, the slag is crushed mechanically, Li is leached using H2SO4 and precipitated as Li2CO3 [29,59,96]. Sulfur is one of the main components of LieS batteries, which can be recovered and purified based on its low melting point (118  C) [100]. Therefore, by heating the spent LieS battery material up to 145e200  C, sulfur is selectively melted and separated. To prevent the loss of sulfur as SO2, a neutral atmosphere (e.g., argon or vacuum) is advised [29]. Carbon materials are the main component in the cathode of LieS batteries and are worthy of retrieval. A technique that helps recover carbon from battery materials is leaching, followed by applying high temperature on the remaining solid residue

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Figure 27.8 Accurec Recycling GmbH route for recycling spent lithiumesulfur battery.

[101]. Ribeiro et al. [102] prepared graphite oxide by recycling graphite from spent battery materials. In this study, they leached the pretreated cathode materials in 2 mol L
1 citric acid and 3% v/v H2O2 at 80  C for 3 h. Next, the solid residue was filtered and dried at 450  C for 2 h under an argon atmosphere, which resulted in the complete synthesis of graphite oxide with proper electrochemical properties. Similarly, Gao et al. [101] recovered pure graphite (99.6%) from the spent graphite of battery materials by washing it in a 20% sulfuric acid solution for 20 min, and subsequently, they rinsed the acid and dried the solid materials, and finally heated them at 1500  C for 2 h under an argon atmosphere. It is also possible to valorize the cathode materials by synthesizing a new product from the waste battery. Ma et al. [103] proposed a method to recover graphene from the electrodes of spent LieS batteries and use it to produce a sulfur-doped graphene electrocatalyst for oxygen-reduction applications. To do so, they first charged and discharged the LieS battery for 100 cycles to reach the desired sulfur-doped graphene composition. Subsequently, they disassembled the battery and washed its cathode with a mixture of 1,2-dimethoxyethane and 1,3-dioxolane to remove the PSs and dried it at 60  C. Afterward, the aluminum collector and binder were removed by soaking the

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materials in an NMP solution at 80  C. The remainder from this process became the feed for sulfur-doped graphene electrocatalyst. Lithium can be melted from the spent anodes of LieS batteries and used in new lithium anode manufacturing. The melting point of metallic lithium is 180  C, which means it can easily be separated from carbon materials through the melting process [104]. Lin et al. [105] successfully synthesized a lithium anode by embedding graphene oxide into molten lithium. In another study, Liu et al. [104] synthesized dendrite-free lithium anode by coating molten lithium on a polyamide matrix. Because of the high reactivity of lithium, spent batteries can be industrially processed using a low-temperature process (cryomilling) currently in use by Toxco Inc. (USA) and BDC Inc. (Canada) [106]. In the cryomilling process, after cooling the lithium-bearing waste batteries in liquid nitrogen, they are mechanically shredded [107]. In the next stage, the shredded materials are washed with water, and lithium is dissolved as lithium hydroxide [106]. Hydrometallurgical methods can be used to recycle lithium from spent anodes of LieS batteries. Different leaching reagentsdmainly include inorganic acids, organic acids, and alkaline solutionsdhave been studied to recover lithium so far [30,55]. Zhang et al. [108] developed a hydrometallurgical process for the separation and recovery of lithium from spent batteries using hydrochloric acid. After the leaching stage, lithium can be precipitated as lithium carbonate using sodium carbonate at about 100  C. Sulfuric acid and nitric acid also can be used to leach lithium from the spent batteries [109]. Contestabile et al. [110] reported that lithium could be recovered by inserting anode into an isobutyl alcohol/water. Lithium is dissolved as LiOH which can be precipitated as lithium carbonate by bubbling CO2 gas. By heating the solution, equilibrium shifted to the formation of carbonate ions which precipitate lithium as Li2CO3. Lithium also can be leached using alkaline solutions such as ammonia [111]. Organic acids have been used for leaching valuable metals from spent batteries to reduce the footprints of the recycling processes [87]. Strong organic acids, such as citric acid, malic acid, aspartic acid [112], formic acid [113], ascorbic acid [114], oxalic acid [115], and glycine [116] have been reported to be able to leach lithium from spent batteries. Also, different studies have been reported that in the presence of weak organic acids such as acetic acid, lactic acid, and gluconic acid, where lithium can be leached with almost high efficiencies [117,118]. A combination of thermal treatment, mechanical treatment, and hydrometallurgical process was applied by Schwich et al. [19] to recover valuable materials from spent LieS batteries. As shown in Fig. 27.9, the cells were first pyrolyzed at 500  C under an argon atmosphere (1 h) to avoid reactions and the loss of carbon and sulfur as CO2 and SO2 gases. During pyrolysis, volatile components of the electrolyte, separator, and binder evaporate, and some H2S is captured, too. Next, materials were separated and then ground in a glovebox to prevent the oxidation of metallic lithium. Subsequently, the powder was sieved, and a size fraction smaller than 1 mm was used for hydrometallurgical treatment. The hydrometallurgical process consisted of consecutive steps of leaching the active mass powder and filtration of the carbon material, precipitation of aluminum as Al(OH)3, and carbonation precipitation of Li as Li2CO3. The leaching

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Figure 27.9 Flowchart of a process for lithiumesulfur battery recycling.

step employed HNO3 and H2O2 and resulted in the release of some SO2 and H2S gases, which were employed to precipitate copper as Cu2S in a separate process. NaOH was added to precipitate aluminum as well as some iron. At the last step, pure lithium carbonate was produced after concentrating the solution by adding Na2CO3 at 100  C.

3.3

Novel technologies

Some newer technologies have been employed for more efficient battery recycling. Applying organic solvents in a water-free environment is a new branch in metals recovery from different sources. This process is known as solvometallurgy and has some advantages, such as limited water use, negligible or no leachate generation, high selectivity in extraction that removes the need for a purification step, little or

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no use of strong reagents, and close to zero waste from the process [119]. Deep eutectic solvents (DESs) are green, affordable, and selective chemicals with high functionality in metals recovery and are suitable reagents for treating complicated battery materials in both leaching and purification steps [120]. Tran et al. [121] used choline chloride and ethylene glycol to synthesize a DES and employed it for metals recovery from spent lithium-ion batteries. In this solvent, graphite, polyvinylidene fluoride (PVDF) binder, and aluminum current collector were not soluble; therefore, some pretreatment steps were skipped, and over 90% of lithium and cobalt was recovered in a single leaching step. In addition, the graphite, binder, and aluminum fraction stayed in a recoverable form for further treatment. Application of a mixture of choline chloride and citric acid at the eutectic concentration for metal leaching from the cathode of spent lithium-ion battery is a successful example of using DES for metal recovery and was studied by Peeters et al. [120]. In this investigation, copper was used as a reducing agent for cobalt leaching and resulted in complete metals leaching in the forms of chloride complexes (98% cobalt, 94% copper, and 93% lithium). In the next step, copper was completely recovered from the solution using LIX® 984; subsequently, cobalt was separated (85% efficiency) using Aliquat 336 (methyl(trioctyl) ammonium chloride), and lithium remained in the solution. Ionic liquids (ILs) are novel chemicals that can effectively recover metals from battery materials [122]. They are salts that are liquid at temperatures below 100  C and have remarkable qualities such as high thermal and chemical stability, high solvent capacity, low vapor pressure, nonflammability, excellent extractants, and ecofriendliness [123]. In the case of battery recycling, Zante et al. [122] employed both IL and DES to purify a leach solution containing lithium, cobalt, manganese, and nickel in a sulfate media. In the first step, they employed IL 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide and managed to selectively extract 99% of the manganese from the solution. Next, they used IL trihexyltetradecylphosphonium chloride and extracted ca. 90% of cobalt from the solution with no impurity. In the last stage, a DES synthesized from decanoic acid and lidocaine resulted in full separation of nickel from the solution while lithium remained. In an attempt to prepare graphene from the graphite part of a spent battery, Lei et al. [124] employed AlCl3 and 1-ethyl-3-methylimidazolium chloride (an IL) mixture. In this practice, the graphite was electrochemically treated (over 20 charge and discharge cycles) in the abovementioned solution at 2.0 V and a current density of 10 mA g
1, resulting in a five-layer graphene film with uniform thickness and the desired flexibility. Sulfur can also be recovered using ILs. Based on the good solubility of sulfur in some aromatic and aliphatic hydrocarbons, Ren et al. [125] investigated the mixture of toluene and different ILs to selectively recover this element. The investigated ILs include 1-butyl-3-methylimidazolium hexafluorophosphate, 1-hexyl3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1-hexyl-3-methylimidazolium tetrafluoroborate. In this comparison, the best IL for sulfur recovery was 1-hexyl-3-methylimidazolium hexafluorophosphate.

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Conclusion and future perspective

Recycling is an act of remediating the waste by valorizing them in the form of some raw materials. Therefore, it has both environmental and economic incentives for its initiation. LieS batteries are from the most recent generations of energy-storage facilities expected to enjoy widespread use. Therefore, planning some viable recycling strategies for implementing a sustainable world is highly reasonable. The materials in spent LieS batteries vary depending on battery generation, and the processes to recover the most valuable materials from spent batteries perform well in this work. LieS batteries are still in development, and not many recycling routes are specifically designed for their recycling. However, recycling the older generation of batteries is well established and can be adjusted to recycle spent LieS batteries or accept them as auxiliary feed. Mainstream techniques of physiomechanical treatment, pyrometallurgy, and hydrometallurgy are the available routes for battery recycling. In this list, the first route is mainly a pretreatment step that prepares the feed for pyrometallurgical and hydrometallurgical techniques, although it can yield some carbon, sulfur, iron (from casing), lithium (from anode), and plastics after processes such as electromagnetic separation, air separation, flotation, etc. Pyrometallurgy is the process of using high temperatures to produce metals from large size feed in the shortest time. This process is not sensitive to the type of feed and burns the organic and carbon materials as a fuel and reducing agent in normal operations. Therefore, the normal setup of pyrometallurgy cannot be a viable option for treating spent LieS battery, as it cannot recover most of its components. Nevertheless, pyrolysis treatment is used, which operates without oxygen and retrieves almost all carbon, organic, and metallic materials. Hydrometallurgical routes are the only option that can deliver pure final products. During the process, metals are mobilized in an aqueous phase after reacting to some reagents, carbon and sulfur parts are left and separated by filtration, and metals are selectively precipitated as pure final products. In general, the available recycling techniques focus on the metal and carbon parts and are not efficient in recovering electrolytes, binders, and separators from spent batteries. Advancement in recycling technologies is moving toward applying DESs and ILs as effective reagents for green, affordable, and selective chemicals with high functionality in metals and organic recovery. Recent studies revealed that reagents could also recover organic materials and sulfur from spent batteries. Another important trend in the future of recycling is converting waste materials to functional final products rather than simply producing raw materials. Producing graphene from the graphite of end-life battery materials is an example. Recycling spent LieS batteries will help to reduce the environmental burden of hazardous waste and improve the economy. The involvement of biohydrometallurgy in spent LieS batteries recycling will result in fruitful results with several advantages such as minimization of waste production, no toxic gas emission, and clean extraction resulting in potential element recovery. The summarized information will be of keen interest for researchers working in the field of battery waste recycling and will provide future directions for further research.

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Recyclability, circular economy, and environmental aspects of lithiumesulfur batteries

28

Gra
zyna Simha Martynkov a 1 , Gabriela Krato
sova 1 , Silvie Bro
zova 2 and 1 Sajjan Kumar Sathish 1 Nanotechnology Centre, CEET, VSB - Technical University of Ostrava, Ostrava, Czech Republic; 2Department of Non-ferrous Metals, Refining and Recycling, Faculty of Materials Technology, VSB - Technical University of Ostrava, Ostrava, Czech Republic

1. Introduction Secondary batteries can be recharged; that is, they can have their chemical reactions reversed by supplying electrical energy to (charging) the cell. Secondary batteries age during each cycle, so they are not indefinitely rechargeable. This is the primary reason great care should be paid from the start of research on batteries until their ends of life from a materials and construction point of view, ensuring reliable resource use and a safe ending for each battery component. Lithium is one of the earth’s limited resources, and thus, the recycling of lithium sourced from used lithiumesulfur (LieS) batteries is important if LieS is further commercialized. The disposal and recycling of spent LieS batteries are hazardous. After cycling, the pulverized lithium metal is highly reactive, and the organic solvent is quite flammable. Hence, lithium can easily catch fire when spent LieS batteries are opened and exposed to air. Detailed studies should be undertaken, and special attention should be paid to this concern. An LieS battery using elemental sulfur as the active cathode material has the highest theoretical specific gravity in gravimetry (2600 Wh kg
1) and capacitive density (2800 Wh L
1) based on just the weight of the active materials. Prototype LieS housing cells can already deliver specific energy of up to 600 Wh kg
1, which is much more than existing lithium-ion batteries (LIBs) (250 Wh kg
1) and can provide electric cars with ranges that exceed 300 miles. LieS batteries also have the advantages of high power output, rich sulfur reserves, and low costs and have become one of the most important factors for competition in rechargeable battery technology. [7,46]. Future research on commercial LieS batteries should focus on the following points: (i) Modifying lithium metal with multifunctional and cost-effective current collectors or stabilized coating layers to buffer the infinite volume changes of lithium and protect it from corrosion; lithium dendrites should be given special attention and suppressed as much as possible to avoid safety problems

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(ii) Developing sulfur cathode preparation craft using common carbon with cathode materials that exhibit good electrochemical performance, are low-cost, and add to the competitiveness of commercially available LieS batteries (iii) Exploring novel separators to minimize lithium polysulfide transport, suppress the shuttle effect, and enhance the efficient use of sulfur (iv) Selecting suitable electrolytes and additives for pouch cells, both of which should be tested and verified under a certain amount of sulfur loading and a limited amount of lithium; solid electrolytes will be the ultimate choice for commercially available LieS batteries (v) Optimizing sulfur mass loading to maximize pouch cell capacity, as it is unrealistic to blindly pursue a sulfur content higher than 10 mg cm
2 in pouch cellsdthe sulfur mass loading of commercially available LieS batteries should be cautiously designed for competitive energy density, cycling life, and low cost (vi) Requiring reliable test methods to assess LieS battery safety; more advanced technologies in materials processing, cell manufacturing, cell packaging, and cell control and management must be developed

2.

Composition and construction of lithiumesulfur batteries

The following discussion concerns the anode, cathode, electrolyte, interlayer, additive, mass loading, and cell safety of LieS batteries in both coin cells and pouch cells [57]. The commercialization process of LieS batteries has developed very slowly, and hence, no product has yet come to market. The general composition of an LieS battery is shown in Fig. 28.1.

2.1

Lithium anode

Lithium metal is an ideal anode material because of its high specific capacity of 3,860 mAh g
1 and low potential of
3.04 V (vs. a standard hydrogen electrode).

Figure 28.1 Basic components of a lithiumesulfur battery.

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However, the lithium anode suffers from dendritic growth, significant volume changes during repeated cycles, and low coulombic efficiency. Significant volume changes can lead to pulverization of the anode and expansion of the pouch case, which may cause sudden failure and serious safety problems with the cells [16,47]. Lithium powder can lose electrical contact with the lithium foil. When lithium is limited and very active, the loss of contact can lead to a rapid deterioration in electrochemical performance that generates substantial heat and causes ignition with exposure to air. This problem is rarely observed in button cells because of the excessive amount of lithium, low energy capacity, and stainless-steel case. However, in housing cells, the aluminum-plastic housing is not as strong as the stainless-steel housing, and the energy capacity is 1000 times higher than in coin cells; therefore, we should pay special attention to the design of LieS bag cell housings. This situation further aggravates lithiumeelectrolyte side reactions, resulting in electrolyte exhaustion, a sudden drop in capacity, and ultimately, cell failure. With a bulk density of 0.534 g cm
3, lithium foil can deliver a theoretical areal capacity of w1 mAh cm
2 with a thickness of 5 mm. Typically, in coin cell research, the energy capacity of the cathode is