Novel Electrochemical Energy Storage Devices: Materials, Architectures, and Future Trends [1 ed.] 3527345795, 9783527345793

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Novel Electrochemical Energy Storage Devices

Novel Electrochemical Energy Storage Devices Materials, Architectures, and Future Trends

Feng Li Lei Wen Hui-ming Cheng

Authors Feng Li

Institute of Metal Research Chinese Academy of Sciences 72 Wenhua Road Shenyang 110016 China

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Lei Wen

Institute of Metal Research Chinese Academy of Sciences 72 Wenhua Road Shenyang 110016 China Hui-ming Cheng

Institute of Metal Research Chinese Academy of Sciences 72 Wenhua Road Shenyang 110016 China Cover

Cover Image: © TarikVision/Getty Images

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A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2021 WILEY-VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34579-3 ePDF ISBN: 978-3-527-82104-4 ePub ISBN: 978-3-527-82106-8 oBook ISBN: 978-3-527-82105-1 Typesetting SPi Global, Chennai, India Printing and Binding

Printed on acid-free paper 10 9 8 7 6 5 4 3 2 1

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Contents Preface xiii Abbreviations 1 1.1 1.2 1.3 1.4 1.5

2 2.1 2.1.1 2.1.2 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.3 2.4 2.4.1 2.4.2 2.4.3 2.4.4

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Introduction 1 Energy Conversion and Storage: A Global Challenge 1 Development History of Electrochemical Energy Storage 3 Classification of Electrochemical Energy Storage 4 LIBs and ECs: An Appropriate Electrochemical Energy Storage 6 Summary and Outlook 10 References 10 Materials and Fabrication 15 Mechanisms and Advantages of LIBs 15 Principles 15 Advantages and Disadvantages 16 Mechanisms and Advantages of ECs 18 Categories 18 EDLCs 18 Pseudocapacitor 20 Hybrid Capacitors 21 Roadmap of Conventional Materials for LIBs 22 Typical Positive Materials for LIBs 23 LiCoO2 Materials 23 LiNiO2 and Its Derivatives 25 LiMn2 O4 Material 26 LiFePO4 Material 27

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2.4.5 2.4.6 2.5 2.5.1 2.5.2 2.6 2.6.1 2.6.2 2.6.3 2.7 2.7.1 2.7.2 2.7.3 2.8 2.8.1 2.8.2 2.9 2.9.1 2.9.2 2.9.3 2.9.4 2.9.5 2.9.6 2.10 2.10.1 2.10.2 2.10.3 2.11

3 3.1 3.1.1 3.1.2 3.1.3

Lithium–Manganese-rich Materials 28 Commercial Status of Main Positive Materials 28 Typical Negative Materials for LIBs 29 Graphite 29 Soft and Hard Carbon 31 New Materials for LIBs 33 Nanocarbon Materials 33 Alloy-Based Materials 35 Metal Lithium Negative 39 Materials for Conventional ECs 39 Porous Carbon Materials 40 Transition Metal Oxides 41 Conducting Polymers 42 Electrolytes and Separators 42 Electrolytes 42 Separators 45 Evaluation Methods 46 Evaluation Criteria for LIBs 46 Theoretical Gravimetric and Volumetric Energy Density 46 Practical Energy and Power Density of LIBs 47 Cycle Life 48 Safety 48 Evaluation Methods for ECs 49 Production Processes for the Fabrication 50 Design 50 Mixing, Coating, Calendering, and Winding 51 Electrolyte Injecting and Formation 51 Perspectives 51 References 53 Flexible Cells: Theory and Characterizations 67 Limitations of the Conventional Cells 67 Mechanical Properties of Conventional Materials Limitations of Conventional Architectures 68 Limitations of Electrolytes 69

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3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.4 3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.5.5 3.5.6 3.6

4 4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3 4.4 4.4.1 4.4.2

Mechanical Process for Bendable Cells 69 Effect of Thickness 70 Effect of Flexible Substrates and Neutral Plane 71 Mechanics of Stretchable Cells 72 Wavy Architectures by Small Deformation Buckling Process 72 Wavy Architectures by Large Deformation Buckling Process 74 Island Bridge Architectures 75 Static Electrochemical Performance of Flexible Cells 76 Dynamic Performance of Flexible Cells 77 Bending Characterization 78 Stretching Characterization 78 Conformability Test 79 Stress Simulation by Finite Element Analysis 79 Dynamic Electrochemical Performance During Bending 83 Dynamic Electrochemical Performance During Stretching 85 Summary and Perspectives 90 References 90 Flexible Cells: Materials and Fabrication Technologies 95 Construction Principles of Flexible Cells 95 Substrate Materials for Flexible Cells 95 Polymer Substrates 96 Paper Substrate 97 Textile Substrate 98 Active Materials for Flexible Cells 98 CNTs 98 Graphene 99 Low-Dimensional Materials 99 Electrolytes for Flexible LIBs 101 Inorganic Solid-state Electrolytes for Flexible LIBs 102 Solid-state Polymer Electrolytes for Flexible LIBs 104

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4.5 4.6 4.6.1 4.6.2 4.6.3 4.7 4.7.1 4.7.2 4.7.3 4.8 4.8.1 4.8.2 4.9 4.9.1 4.9.2 4.9.3 4.9.4 4.9.5

5 5.1 5.2 5.3 5.4 5.4.1 5.4.2 5.5 5.5.1

Electrolytes for Flexible ECs 104 Nonconductive Substrates-Based Flexible Cells 107 Paper-Based Flexible Cells 108 Textiles-Based Flexible Cells 112 Polymer Substrates-Based Flexible Cells 117 CNT and Graphene-Based Flexible Cells 121 Free-standing Graphene and CNTs Films for SCs 121 Free-standing Graphene and CNT Films for LIBs 122 Flexible CNTs/Graphene Composite Films for the Cells 125 Construction of Stretchable Cells by Novel Architectures 127 Stretchable Cells Based on Wavy Architecture 127 Stretchable Cells Based on Island-Bridge Architecture 129 Conclusion and Perspectives 130 Mechanical Performance Improvement 131 Innovative Architecture for Stretchable Cells 132 Electrolytes Development 132 Packaging and Tabs 132 Integrated Flexible Devices 133 References 133 Architectures Design for Cells with High Energy Density 147 Strategies for High Energy Density Cells 147 Gravimetric and Volumetric Energy Density of Electrodes 149 Classification of Thick Electrodes: Bulk and Foam Electrodes 151 Design and Fabrication of Bulk Electrodes 153 Advantages of Bulk Electrodes 153 Low Tortuosity: The Key for Bulk Electrodes 155 Characterization and Numerical Simulation of Tortuosity 157 Characterization of Tortuosity by X-ray Tomography 157

Contents

5.5.2 5.6 5.7 5.7.1 5.7.2 5.7.3 5.7.4 5.8 5.8.1 5.8.2 5.8.3 5.8.4 5.8.5 5.8.6 5.9 5.9.1 5.9.2 5.9.3 5.10 5.10.1 5.10.2 5.10.3 5.10.4 5.11 5.12 6 6.1 6.1.1 6.1.2 6.1.3 6.2

Numerical Simulation of Tortuosity on Rates by Commercial Software 158 Fabrication Methods for Bulk Electrodes 159 Thick Electrodes with Random Pore Structure 160 Pressure-less High-temperature Sintering Process 160 Cold Sintering Process 161 Spark Plasma Sintering Technology 162 Brief Summary for Sintering Technologies 165 Thick Electrodes with Directional Pore Distribution 165 Iterative Extrusion Method 165 Magnetic-Induced Alignment Method 168 Carbonized Wood Template Method 168 Ice Templates Method 172 3D-Printing for Thick Electrodes 173 Brief Summary for Bulk Electrodes 175 Carbon-Based Foam Electrodes with High Gravimetric Energy Density 178 Graphene Foam 179 CNTs Foam 181 CNT/Graphene Foam 181 Carbon-Based Thick Electrodes 182 Low Electronic Conductive Material/Carbon Foam 182 Large Volume Variation Materials/Carbon Foam 186 Compact Graphene Electrodes 188 Summary for Carbon Foam Electrodes 189 Thick Electrodes Based on the Conductive Polymer Gels 191 Summary and Perspectives 193 References 195 Miniaturized Cells 205 Introduction 205 Definition of the Miniaturized Cells and Their Applications 205 Classification of Miniaturized Cells 206 Development Trends of the Miniaturized Cells 207 Evaluation Methods for the Miniaturized Cells 209

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6.2.1 6.2.2 6.3 6.4 6.4.1 6.4.2 6.5 6.5.1 6.6 6.7 6.7.1 6.7.2 6.7.3 6.7.4 6.8 6.9 6.10 6.11 6.11.1 6.11.2 6.12 6.12.1 6.12.2 6.13 6.14

7 7.1 7.1.1

Evaluation Methods for Electric Double-layer m-ECs 210 Evaluation methods for m-LIBs and m-ECs 211 Architectures of Various Miniaturized Cells 212 Materials for the Miniaturized Cells 213 Electrode Materials 213 Electrolytes for the Miniaturized Cells 214 Fabrication Technologies for Miniaturized Cells 215 Fabrication of Miniaturized Cells with 2D Parallel Plate Configuration 216 Fabrication Technologies for 2D Interdigitated Cells 220 Printing Technologies for 2D Interdigitated Cells 222 Advantages of Printing Technologies 222 Classification of Printing Techniques 222 Screen Printing for Miniaturized Cells 224 Inkjet Printing 228 Electrochemical Deposition Method for 2D Interdigitated Cells 228 Laser Scribing for 2D Interdigitated Cells 231 In Situ Electrode Conversion for 2D Interdigitated Cells 234 Fabrication Technologies for 3D In-plane Miniaturized Cells 236 3D Printing for 3D Interdigitated Configuration Cells 236 3D Interdigitated Configuration by Electrodeposition 239 Fabrication of Miniaturized Cells with 3D Stacked Configuration 240 3D Stacked Configuration by Template Deposition 241 3D Stacked Configuration by Microchannel-Plated Deposition Methods 245 Integrated Systems 247 Summary and Perspectives 249 References 250 Smart Cells 263 Definition of Smart Materials and Cells 263 Definition of Smart Cells 263

Contents

7.1.2 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.3.6 7.4 7.4.1 7.4.2 7.5 7.5.1 7.5.2 7.6 7.6.1 7.6.2 7.6.3 7.7

Definition of Smart Materials 263 Type of Smart Materials 264 Self-healing Materials 264 Shape-memory Alloys 265 Thermal-responding PTC Thermistors 266 Electrochromic Materials 267 Construction of Smart Cells 268 Self-healing Silicon Anodes 268 Aqueous Self-healing Electrodes 271 Liquid-alloy Self-healing Electrode Materials 273 Thermal-responding Layer 274 Thermal-responding Electrodes Based on the PTC Effect 276 Ionic Blocking Effect-Based Thermal-responding Electrodes 278 Application of Shape-memory Materials in LIBs and ECs 280 Self-adapting Cells 280 Shape-memory Alloy-Based Thermal Regulator 281 Self-heating and Self-monitoring Designs 282 Self-heating 283 Self-monitoring 285 Integrated Electrochromic Architectures for Energy Storage 286 Integration Possibilities 286 Integrated Electrochromic ECs 287 Integrated Electrochromic LIBs 289 Summary and Perspectives 291 References 292 Index 301

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Preface Electrochemical energy storage in the cells (here these are lithium ion batteries (LIBs) and electrochemical capacitors (ECs)) has been recognized as the most promising technology for portable electronics as well as stationary and vehicle applications. Existing technologies still face performance and cost challenges, including barriers in specific energy, energy density, service life, and energy efficiency at high rates. Over the past decades, portable electronics have been used in every aspect of our daily life. One of the key components of future portable devices is the compatible cell with an ultrahigh energy density and specific features (e.g. miniaturization, integration, flexibility, and smart functions). Developing advanced cells always requires the discovery of new materials, new electrochemistry, and an increased understanding of the processes on which the devices depend. The overall performance of the cells is limited by the fundamental behavior of the used materials, including electrode active materials, electrolytes, separators, and other components. Unfortunately, the conventional fabrication technology and architectures of electrodes based on these materials have almost reached their limits, which cannot satisfy future requirements. Therefore, for the coming era of portable electronics, we urgently need to reconsider how we rationally design and intelligently fabricate advanced and intelligent cells. We need to not only construct novel configurations of materials, electrolytes, separators, and, as results, the cells to meet the desired criteria but also develop smart technologies to fabricate these electrochemical energy storage devices in an economically viable and time-efficient manner. In this book, we will present a comprehensive introduction of the developments of innovative materials, architectures and design considerations in the electrode, and cell configurations, together with the recent technologies used to achieve these novel designs. As we wanted to write a book for researchers, engineers, and students, we try our best to understand the current application of the cells in portable electronic products. The writing of this book was completed by Prof. Feng Li and Dr. Lei Wen, and Prof. Hui-ming Cheng revised it. The origin of this book is from the meeting of Prof. Li and Dr. Zai Yu in ChinaNano2017 at Beijing. Dr. Yu wished that we can write a book about our research. It is a hard work for us and new chance to think about our research insight. In 2018, we wrote an outline of the book and passed it to the

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Preface

reviewers for approval. After that, we began writing the book. Although Dr. Zai Yu has left Wiley, Ms. Aruna Pragasam is very happy to continue to contact and help us. On one occasion, Prof. Li went to Tsinghua University and talked with Prof. Qiang Zhang and Prof. Jiaqi Huang. We know that Dr. Shaoyu Qian is responsible for author of China region. She answered our questions with patience during the writing. Finally, we would like to thank all scientists who have been helpful in the preparation of this book and all colleagues who kindly devoted their time and efforts to contribute chapters and discussions. We thank Dr. Hongze Luo, from Council for Scientific and Industrial Research (CSIR), South Africa, for preparing the draft of Chapter 6; Dr. Zhigang Zhao, from Suzhou Institute of Nano-tech and Nano-Bionics (SINANO), China, for his help in the preparation of Electrochromic Cells section in Chapter 7; Drs. Ji Liang and Hou Feng, Mrs Hao Li, Nan Li, and Miss Pengyi Lu from Tianjin University, China, for their helpful discussion and initial drafting of Chapters 3 and 4; and Dr. Liqun Wang, Tianjin Normal University, for the helpful discussion in drafting Chapter 6. We would like to thank Mr. Haorui Shen and Huicong Yang, PhD candidates in our lab. Mr. Shen helped in the plotting of figures. Mr. Huicong Yang is the first reader of this book and gave valuable advices toward the entire book. We also thank the financial support from National Natural Science Foundation of China (Nos. 51525206, 51927803, 52020105010, 51972313, 52072378 and 51902316), MOST (2016YFA0200102 and 2016YFB0100100), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22010602), Liaoning Revitalization Talents Program (No. XLYC1908015), Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. Y201942) and China Petrochemical Cooperation (No. 218025). The Bureau of Industry and Information Technology of Shenzhen for the “2017 Graphene Manufacturing Innovation Center Project” (No. 201901171523).

xv

Abbreviations AAO AC AGV ALD CAS CCCD CMC CNF CNT CV CVD DEC DMC DME DOE DOL EBID EC EDLC EG EIS ELD EMC EPD ETPTA EV EVA FTO GCE GCP GIC GNS GO

anodized aluminum oxide activated carbon, alternative current automated guided vehicles atomic layer deposition Chinese Academy of Sciences constant current charge and discharge sodium carboxymethyl cellulose cellulose nanofiber carbon nanotubes cyclic voltammetry chemical vapor deposition diethyl carbonate dimethyl carbonate dimethoxyethane Department of Energy 1,3-dioxolane electron beam-induced deposition electrochemical capacitors, ethylene carbonate electric double-layer capacitors exfoliated graphene electrochemical impedance spectroscopy electrolytic deposition ethyl methyl carbonate electrophoretic deposition ethoxylated trimethylolpropane triacrylate electric vehicle ethylene vinyl acetate fluorine-doped tin oxide gel composite electrolyte graphene-coated paper graphite intercalation compound graphene nanosheet graphene oxide

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Abbreviations

HEV HFP IDC IHP IoT IREA ITO LBL LCO LDH LFP LIB LIBOB LIPON LLZO LMO LTO m-LIBs m-ECs MCMB mtoe NASICON NCA NEDO NiCd NiMH NMC NMP NW OHP P3DT P3OT P3OPy PAA PAAm PANI PC PDA PDMS PE PEDOT PEO PEG PEN PES

hybrid electric vehicle hexafluoropropylene International Data Corporation inner Helmholtz plane Internet of Things International Renewable Energy Agency indium tin oxide layer-by-layer LiCoO2 layer double hydroxides LiFePO4 lithium ion battery lithium bis(oxalato)borate lithium phosphorus oxynitrides Li7 La3 Zr2 O12 LiMn2 O4 Li4 Ti5 O12 micro lithium ion batteries micro electrochemical capacitors mesocarbon microbeads million ton oil equivalent sodium super ionic conductor LiNix Coy Al1−x−y O2 New Energy and Industrial Technology Development Organization nickel cadmium nickel metal hydride LiNix Mny Co1−x−y O2 N-methyl-2-pyrrolidone nano wire outer Helmholtz plane poly(3-decylthiophene) poly(3-octylthiophene-2,5-diyl) PSS poly(3-octylpyrrole) poly(styrenesulfonate) polyacrylic acid polyacrylamide polyaniline propylene carbonate polydopamine polydimethylsiloxane polyethylene poly(3,4-ethylenedioxythiophene) polyoxyethylene polyethylene glycol polyethylene naphthalate polyethersulfone

Abbreviations

PET PI PMMA PP PPO PPY PSS PTC PTFE PU PVA PVDF PVP rGO RF RTIL SE SEBS SEI SEM SHE SHP SPE SPRP TEABF4 TEM TMO MWCNTs UPy UV

polyethylene terephthalate polyimide polymethyl methacrylate polypropylene propylene oxide polypyrrole poly(styrenesulfonate) positive temperature coefficient polytetrafluoroethylene polyurethane polyvinyl alcohol polyvinylidene difluoride polyvinylpyrrolidone reduced graphene oxide radio frequency room-temperature ionic liquid solid electrolyte styrene–ethylene–butylene–styrene solid electrolyte interphase scanning electron microscope standard hydrogen electrode self-healing polymer solid polymer electrolyte Strategic Priority Research Program tetraethylammonium tetrafluoroborate transmission electron microscope transition metal oxides multi-walled carbon nanotubes ureidopyrimidinone ultraviolet

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1

1 Introduction The world is witnessing increasing requirements for energy to meet the needs of modern society. Due to the drastic climate warming around the world, developing innovative sustainable clean energy (wind, tidal, solar energy, etc.) with high energy efficiency is extremely important. However, various renewable energy to electricity are quite fluctuating over time, and developing reliable energy storage systems is an important way to solve these challenges. Therefore, to satisfy the increasing social and industrial demands, better electrochemical energy storage devices should be used. On this point, searching for novel electrochemical energy storage system with exceptional electrochemical properties for energy storage is essential. In this chapter, we will first give a brief introduction toward various electrochemical energy storage devices, including electrochemical capacitors (ECs) and lithium ion batteries (LIBs).

1.1 Energy Conversion and Storage: A Global Challenge Nobel chemistry prize winner Richard Smalley had said: “Energy is the single most important problem facing humanity today and energy is the largest enterprise on Earth” [1]. Nowadays, the energy generation still mainly relies on fossil fuels (oil, coal, and gas), which occupy 80% of total energy needs in the world. On the other hand, the fossil fuels are still to be the dominant primary energy resources for many years in the future. Therefore, limited supplies of the fossil fuels make it imperative that combustion-based energy sources should be replaced by clean and renewable energy [2]. These sustainable energies mainly include hydropower, solar, wind, geothermal, and tidal energy. Figure 1.1 shows the global energy consumption. In 2018, the growth rate of global energy consumption is 2.9%, which was the highest rate since 2010. Although fossil energy still occupies the most energy consumption around the world, the renewable energy also made a significant increase in recent years [4]. Gas and renewables have the most obvious increase among various energy sources since 2010 [3]. More importantly, the renewable energy sector also has significant social and economic impact. In 2018, at least 11 million people were employed in the renewable energy sector around the world. Compared with the data in 2016, the growth rate Novel Electrochemical Energy Storage Devices: Materials, Architectures, and Future Trends, First Edition. Feng Li, Lei Wen, and Hui-ming Cheng. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

1 Introduction

160 00 World energy consumption (MTOE)

2

Coal Renewables Hydroelectricity Nuclear energy Natural gas Oil

12 000

8000

4000

0

Figure 1.1

1996

2000

2004 2008 Years

2012

2016

Amounts of global energy consumption.

Solar photovoltaic

3605

Liquid biofuels

2063

Hydropower

Off-grid solar for energy access

2054

Wind energy

1160

Solar heating/ cooling

801

Solid biomass

787

Biogas Geothermal energy Municipal and industrial waste

94

CSP

34

334

41

Tide, wave and ocean energy

1 0

500

1000

1500

2000

2500

3000

3500 4000 Jobs (thousands)

Figure 1.2 Global renewable energy employment indexed by technology. Source: International Renewable Energy Agency [4]. © 2018, IRENA - International Renewable Energy Agency.

of employment was 5.3% [4]. As shown in Figure 1.2, the top five employment in the renewable energy sector on the list includes solar photovoltaic, liquid biofuels, hydropower, wind energy, and solar heating. Electricity has been considered as the most effective way to explore and utilize various renewable energies effectively. Compared with other forms of energy, electricity has many obvious advantages as follows [5]: (i) Convenience: electricity can be easily transformed into the desired forms of energy, such as heat, light, and mechanical energy. (ii) Easy control: electricity can simply be operated and tuned.

1.2 Development History of Electrochemical Energy Storage

Mobile devices

Robots, AGV

Electric vehicles

Aerospace

Renewable energy

Industry

Figure 1.3 vehicles).

Application of electrochemical energy storage, AGV (automated guided

(iii) Flexibility: electricity can be easily transferred by transmission line. (iv) Cheap: compared with other forms of energy, electricity is an economical form, which has been widely used for domestic and industrial applications. (v) Low transmission loss: electricity can be easily transmitted with high efficiency from the power plant to the user. Although electricity has many advantages, the renewable energy-based electricity is quite fluctuating over time. For example, the clouds constantly alter the output of solar energy systems and wind cannot blow at a fixed speed. Unfortunately, grid has a fixed frequency of 50 Hz, which was determined by turbines in power plants. These must be matched to avoid the fluctuation of grid. Therefore, the clean energy-based electricity requires to be stored and delivered for commercial usage. As a result, renewable energy calls for the development of electricity storage devices. Among these various electrochemical energy storage systems, ECs and various batteries have showed great potential not only in the powering portable electronics but also in the transportation sector. As shown in Figure 1.3, various electrochemical energy storage has been widely used in every aspect in our daily life, such as aerospace (satellites, rockets, and aircrafts), transportation (cars, trains, and ships), portable electronic gadgets (mobile phones, laptops, and digital cameras), and industry fields [6]. The ever-growing advancement of electrochemical energy storage technology has greatly promoted the development of human society. It can be anticipated that electrochemical energy storage materials and technology play more important role in human life.

1.2 Development History of Electrochemical Energy Storage As shown in Figure 1.4, the first electrochemical energy storage chemistry in history is Baghdad battery, which consisted of a ceramic pot, a tube of copper, a rod of iron, and vinegar electrolyte. This ancient battery has ∼2.0 volts of electricity [8].

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1 Introduction

Baghdad battery Iron head Asphalt seal Clay case 13.5 cm Electrolyte chamber Internal copper cylinder

www.aquiziam.com (C)

Figure 1.4

Replica of Baghdad battery found in Iraq. Source: Aquiziam [7].

The first modern battery was invented by the Italian scientist Volta in 1799, which was called as “Volta pile.” This battery was a stack of Ag and Zn disks, and the metal disks were separated by salt water-soaked cloth [9]. Volta found that the single pile could only produce 1.0–2.0 volts of electricity. To increase the voltage output, several “Volta piles” could be constructed side by side. The processes that occur in the device were later demonstrated by Humphry Davy and Michael Faraday, which described that it is the occurrence of chemical reactions that is responsible for the production of electricity [9]. This finding marked the emergence of the electrochemistry. Consequently, the research and development of various electrochemical energy storage systems became active in the nineteenth and twentieth centuries.[10] The simple history of electrochemical energy storage is shown in Figure 1.5.

1.3 Classification of Electrochemical Energy Storage The electrochemical energy storage has almost penetrated every aspect in our daily life, which can efficiently store and convert energy reversibly between chemical energy and electrical energy in an environmentally friendly way. There are many requirements that electrochemical energy storage needs to fulfill for various application fields, such as portable electronics, electric vehicles (EVs), and power tools. The main requirements for different electrochemical energy storage include high gravimetric/volumetric energy density, long cycle life, low cost, safety, and easy fabrication [11]. These attributes are mainly determined by the intrinsic properties of the materials and chemistry constituting the electrochemical

1.3 Classification of Electrochemical Energy Storage

Figure 1.5

Development of various electrochemical energy storage.

Figure 1.6 Classification of electrochemical energy storage. EDLC, electric double-layer capacitors.

Electrochemical energy systems

Electrochemical capacitors

Batteries

EDLC

Primary batteries

Pseudocapacitors

Secondary batteries

energy storage [12]. Based on the charge storage mechanism, the electrochemical energy storage technology has two main categories: ECs and batteries, as shown in Figure 1.6. As shown in Figure 1.6, ECs have two mechanisms to store electricity: double-layer capacitance and pseudocapacitance. Double-layer capacitance is based on ionic adsorption, whereas pseudocapacitance is an electrochemical process. Electrochemical batteries have two broad categories, primary and secondary batteries. A primary battery is one that cannot easily be recharged after one use. An example of a primary battery is the dry cell, which was commonly used to power remotes and clocks. In such cells, a Zn container acts as the negative and a carbon rod acts as the positive. A secondary battery can be recharged to their original pre-discharge status, such as LIBs, NiCd, and NiMH batteries.

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1 Introduction

1.4 LIBs and ECs: An Appropriate Electrochemical Energy Storage Batteries can store energy through reversible redox reactions in the bulk phase electrodes, whereas ECs can store amounts of energy at the electrolyte–electrode interface or through redox reactions on the surface of electrode [13, 14]. As shown in Figure 1.7, a battery usually deliver higher energy density and lower power density than that of ECs, whereas ECs are advantageous and potential candidates in a wide range of applications due to their high power densities (>10 kW kg−1 ), exceptional reversibility (90–95%), and ultra-long life span (>100 000 cycles) [15]. Among various secondary batteries, such as Pb-acid, Ni-Cd, and NiMH batteries, LIBs possess higher energy density and cycle life and have reasonably attracted the biggest commercial and research interest. LIBs have been widely used to power the portable electronics and have also enabled the rapid development of EVs and renewable energy sources. Compared with other secondary batteries, LIBs show many outstanding properties, such as high voltage and energy density, better cycle life, light weight, and low self-discharge rate [16]. Therefore, the development of LIBs has been a hotspot both in industry and in academy. In 2019, the Noble Prize in Chemistry rewards the study of the LIBs to Drs. M. Stanley Whittingham, John B. Goodenough, and Akira Yoshino. The LIBs concept emerged in the 1970s and was finally commercialized by Sony in the 1990s [17, 18]. The foundation of LIB began during the oil crisis by Stanley Whittingham, who found that TiS2 can accommodate lithium ions at a molecular level [19–21]. 107 Capacitors 106 Power density (W kg–1)

6

105 104 103

Electrochemical capacitors

102 Batteries 10 1 10–2

10–1

1 10 Energy density (Wh kg–1)

102

Fuel cells

103

Figure 1.7 Power density as a function of energy density for various electrochemical energy storage. Source: Libich et al. [13].

1.4 LIBs and ECs: An Appropriate Electrochemical Energy Storage

Lithium is the lightest metal (relative atomic weight is 6.94), and it has the lowest potential (−3.04 vs. standard hydrogen electrode, SHE); this property enables LIBs with higher energy density than that of other secondary battery [22]. Since 1970s, metallic lithium has been used as the negative material for lithium primary batteries. Unfortunately, metallic lithium is not suitable for secondary lithium batteries due to the uncontrolled lithium dendrite during cycling, which can pierce the separators to cause inner short-circuit, eventually causing smoking, firing, and explosion [23, 24]. The basic concept of “Rocking chair battery” was proposed in the late 1970s, which used a layer compound to replace the common lithium metal as negative electrode [16, 25]. In this configuration, lithium exists in an ionic state rather than a metal state; the lithium deposition can be greatly suppressed during the electrochemical process. Therefore, the safety of battery can be remarkably improved. According to this concept, Megahed and Scrosati [26] used LiWO2 as the negative, V2 O5 as the positive, and 1 mol l−1 LiClO4 /PC (propylene carbonate) as the electrolyte to confirm this rocking chair battery concept. However, the obtained LIB still showed low specific capacity and poor cycling, which failed to be commercialized. When a metal oxide rather than a metal sulfide acted as the positive, Dr. John Goodenough predicted that the positive may have greater potential. After a systematic investigation, he showed that LiCoO2 can produce as high as four volts potential in 1980 [27]. The finding of LiCoO2 was a revolutionary breakthrough and would result in more powerful batteries. In 1986, Auborn and Barberio [28] also assembled an LIB with LiCoO2 as the positive electrode, MoO2 as the negative electrode, and 1 mol l−1 LiPF6 /PC as the organic electrolyte. However, this battery has the intrinsic problems, such as low operating voltage and sluggish diffusion of lithium ions in the negative electrode. In the following years, the research and development of LIBs did not make significant progress. Until 1985, with the positive of Goodenough as a basis, Akira Yoshino constructed the first commercial LIB. Rather than the reactive metal lithium as the negative, petroleum coke was used as the negative electrode and LiCoO2 acted as the positive electrode to construct a new, high-voltage LIB [16]. This combination of positive and negative materials greatly extended the cycle life of LIBs and significantly improved its safety and voltage. This innovative design opened the door for the large-scale commercialization of LIBs, which has been regarded as a milestone in the history. Since its commercialization in 1991, LIBs have entered every aspect in our daily life and are the foundation of a wireless, fossil fuel-free society. The better LIBs have been an ongoing goal to satisfy future demands ranging from small-scale consumer electronics to large-scale EVs and grid storage. As shown in Figure 1.8, the energy density of 18 650 cylindrical cells slowly increased from ∼70 to ∼300 Wh kg−1 [29]. Nowadays, LIBs of 240–300 Wh kg−1 level have been commercialized and widely used in EVs. To develop better LIBs, major countries in the world have been invested lots of resources to conduct extensive and in-depth research in this field. For example, the “Battery 500” project was supported by the Department of Energy of United States to set a goal to achieve 500 Wh kg−1 in 2021. The “New Generation Battery” plan was also funded by New Energy and Industrial Technology

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1 Introduction

Figure 1.8 The history, current status, and development of LIBs. (NEDO, The New Energy and Industrial Technology Development Organization; GP, graphite; LFP, LiFePO4 ; NCM333, LiNi1/3 Co1/3 Mn1/3 O2 ; CATL, Contemporary Amperex Technology, Co. Ltd., Ningde, China; BYD, BYD Company Limited, Shenzhen, China; SKI, SK Innovation Co., LTD., Korea.)

Development Organization (NEDO) of Japan and planned to achieve 500 Wh kg−1 in 2030. In China, “Strategic Priority Research Program for Electric Vehicles with long range” was released by the Chinese Academy of Sciences (CAS). “Made in China 2025” project launched by the Chinese government set a goal to achieve 300 and 400 Wh kg−1 of LIBs in 2020 and 2025, respectively (as shown in Figure 1.8) [29]. In summary, a better volumetric/gravimetric energy density, lower cost, better safety, and cycle life are still important requirements to power mobile devices and improve the driving range of EVs. On the other hand, various renewable energies also require to be stored and delivered for commercial applications. In summary, these two problems call for developing appropriate LIBs to meet these requirements. Figure 1.9 shows the general trends for the present automobile battery research and development objectives with respect to the employed materials of negative, electrolyte, and positive. As shown in Figure 1.9, the energy density of first-generation LIBs for EVs is ∼130 Wh kg−1 , which is based on LiCoO2 , LiFePO4 , LiMn2 O4 , and graphite materials. By far, the second-generation LIBs, which are based on the high-capacity LiNix Mny Coz O2 (NMC) or LiNi0.8 Co0.15 Al0.05 O2 (NCA)-positive [31] and graphite materials, normally show practical energy densities up to c. 240 Wh kg−1 and was subsequently commercialized in TESLA Model S, shown in Figure 1.9. According to both Department of Energy (DOE) and SPRP (Strategic Priority Research Program) projects, a 400 Wh kg−1 is the roof-top in the third-generation LIBs due to the limitation in the capacity of conventional LIBs configuration. To achieve higher energy density, post LIBs chemistry has also been investigated, such as Li-S, Li-Air, and other systems. ECs are another important electrochemical energy storage devices, which first appeared in the middle of the twentieth century. Generally, ECs can be regarded as a complementary role for LIBs. In 1957, the first type of ECs was invented by Becker of General Electric. It was found that its capacitance is considerably higher than

1.4 LIBs and ECs: An Appropriate Electrochemical Energy Storage

DOE: US department of energy SPRP: Strategic priority research program, Chinese academy of sciences

SPRP goals DOE goals

Next generation batteries: Post Li-ion battery (Li-O2 or Li-S): > 3000 wh kg–1, > 3000 wh l–1 (Li-O2, DOE Expected) : 560 wh kg–1 (Li-S); 526 wh kg–1 (Li-O2) SPRP 2016

Post LIBs 2020-203X

ca. 400 wh kg–1 Si/C anode with High-Voltage cathode energy density: 300–400 wh kg–1, 800–1200 wh l–1

Graphite/NCA or NMC Energy density: 220 wh kg–1, 600 wh l–1

Si/C anode with High-voltage cathode energy density: 300 wh kg–1 (cell) / 2000 cycles

Graphite/NCA Energy density: 180 wh kg–1, (cell)/ 150 wh kg–1 (Package)

Graphite/Spinel or Phosphate Energy density: 130 wh kg–1

3rd Generation–LIBs Under research 2018–2020

2nd Generation LIBs Achieved by 2014–2015

Tesla model S

1st Generation LIBs Achieved by 2012–2013 Nissan LEAF

BYD e6

Figure 1.9 Progress of battery technologies based on DOE and SPRP projects. Source: Hong et al. [30]. Reproduced with permission of Wiley.

conventional capacitor due to the high specific surface area of porous carbon [13]. In 1969, a non-aqueous electrolyte was used in porous carbon-based ECs by Sohio, which enhanced the upper potential to 3 V [32]. Then in 1971, the capacitance behavior of RuO2 films was investigated by Trasatti and Buzzanca et al. [33]. In 1980s, Conway et al. [32, 34, 35] also conducted lots of investigations on the RuO2 type of EC, which shows a surface-redox pseudocapacitance. This field has been very active since about 1990s. Among various materials, carbon materials with high specific surface area, including activated carbons (ACs), carbon aerogels, carbon nanotubes (CNTs), and graphene, are unique class of materials for electric double-layer capacitors (EDLC)-type ECs and have been applied and investigated [36–38]. The energy storage mechanism in ECs takes place through either ion adsorption at the electrode/electrolyte interface or reversible faradaic reactions [13, 32, 39]. Based on their mechanisms of charge storage, ECs are classified into four broad categories. The first type includes electric double-layer capacitors or so-called EDLC ECs. The second type is pseudocapacitors or Faradaic ECs, which is based on reversible Faradic reactions. The third is called hybrid ECs. Hybrid ECs combines both previous EDLC and pseudocapacitors. The fourth category represents hybrid EC-LIBs devices, which is based on the combination of ECs, reversible faradic reactions, and LIB-type materials [40]. In Chapter 2, we will also give a brief introduction toward the mechanism of ECs. Table 1.1 shows summarization and comparisons of the important performances between LIBs and ECs. The important performances include energy and power density, self-discharge rate, cycle life, and the working temperatures. ECs can produce ultra-high power density and cycle performances, whereas LIBs usually possess higher gravimetric/volumetric energy density.

9

10

1 Introduction

Table 1.1

Comparison between ECs and LIBs.

Performances

ECs EDLC EC

Charge time (s) Cycle performance Cell voltage (V) Energy density (Wh kg−1 )

Pseudo EC

1–10

1–10

100

600

1 000 000

100 000

500 000

500–2000

0–2.7

2.3–2.8

2.3–2.8

3.6

3–5

10

180

250

−40–65

−20–60

Aprotic

Aprotic

Price (USD/kWh)

–10 000

–10 000

Operating temperature (∘ C)

−40–65

−40–65

Self-discharge per month (%) Electrolytes

LIBs Hybrid EC

60

60

Aprotic/protic

Protic

–140 4

Source: Libich et al. [13].

Based on Table 1.1, ECs and LIBs can provide unique solution to the electrochemical energy storage and could be considered as complementary technologies. Therefore, LIBs and ECs are the dominant electrochemical energy storage systems in modern society. Performances of LIBs and ECs strongly linked with the electrode materials used. With the booming development of materials, the performance of LIBs and ECs has also been progressing rapidly. In Chapter 2, the main part will be focused on materials of LIBs and ECs as the current advanced electrochemical energy storage.

1.5 Summary and Outlook Renewable energy sources, such as wind, solar tide, and geothermal, become extremely important in our modern society. Renewable energy sources must be first converted to secondary electricity before utilization. Therefore, developing electricity storage systems can be available to meet demand whenever needed would be the breakthrough in electricity distribution. Among electrochemical energy storage systems, ECs and LIBs have displayed great potential not only in portable electronics but also in the renewable energy transportation sector. ECs and LIBs have their own advantages; ECs can produce ultra-high power density and cycle life, whereas LIBs usually possess higher gravimetric/volumetric energy density.

References 1 Mike Lillich (2004). Nobel laureate Smalley speaks on global and nano energy challenges. https://www.purdue.edu/uns/html3month/2004/040902.Smalley .energy.html (accessed 18 June 2020). 2 Lund, H. (2007). Renewable energy strategies for sustainable development. Energy 32 (6): 912–919.

References

3 BP Company (2019). BP Statistical Review of World Energy, 68e. UK: BP Company. 4 International Renewable Energy Agency (2018). Renewable energy and jobs: Annual Review. UK: International Renewable Energy Agency. 5 Waseem Khan (2018). Importance of electricity. https://www.electronicslovers .com/2018/04/importance-of-electricity-in-our-daily-lives-andits-impact-onmodern-society.html (accessed 12 June 2020). 6 Peng, J.Y., Zu, C.X., and Li, H. (2013). Fundamental scientific aspects of lithium batteries (I)- thermodynamic calculations of theoretical energy densities of chemical energy storage systems. Energy Storage Science and Technology 2 (1): 55–62. 7 https://www.aquiziam.com/the-baghdad-battery/ (accessed 17 July 2020). 8 Pulpit Rock (2017). Baghdad battery. https://en.wikipedia.org/wiki/Baghdad_ Battery. (accessed 12 August 2020). 9 Wikipedia (2020). Voltaic pile. https://en.wikipedia.org/wiki/Voltaic_pile (accessed 12 July 2020). 10 Wikipedia (2020). Battery. https://en.wikipedia.org/wiki/Battery (accessed 20 July 2020). 11 Yang, Z.G., Zhang, J.L., Kintner-Meyer, M.C.W. et al. (2011). Electrochemical energy storage for green grid. Chemical Reviews 111 (5): 3577–3613. 12 Manthiram, A., Murugan, A.V., Sarkar, A. et al. (2008). Nanostructured electrode materials for electrochemical energy storage and conversion. Energy & Environmental Science 1 (6): 621–638. 13 Libich, J., Máca, J., Vondrák, J. et al. (2018). Supercapacitors: properties and applications. Journal of Energy Storage 17: 224–227. 14 Muzaffar, A., Ahamed, M.B., Deshmukh, K. et al. (2019). A review on recent advances in hybrid supercapacitors: design, fabrication and applications. Renewable & Sustainable Energy Reviews 101: 123–145. 15 Kotz, R. and Carlen, M. (2000). Principles and applications of electrochemical capacitors. Electrochimica Acta 45 (15-16): 2483–2498. 16 Li, M., Lu, J., Chen, Z.W. et al. (2018). 30 Years of lithium-ion batteries. Advanced Materials 30 (33): 24. 17 Goodenough, J.B. and Kim, Y. (2010). Challenges for rechargeable Li batteries. Chemistry of Materials 22 (3): 587–603. 18 Goodenough, J.B. and Park, K.-S. (2013). The Li-ion rechargeable battery: a perspective. Journal of the American Chemical Society 135 (4): 1167–1176. 19 Whittingham, M.S. (1976). Electrical energy-storage and intercalation chemistry. Science 192 (4244): 1126–1127. 20 Whittingham, M.S. (1978). Chemistry of intercalation compounds – metal guests in chalcogenide hosts. Progress in Solid State Chemistry 12 (1): 41–99. 21 Whittingham, M.S. (2004). Lithium batteries and cathode materials. Chemical Reviews 104 (10): 4271–4301.

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22 Cheng, X.B., Zhang, R., Zhao, C.Z. et al. (2017). Toward safe lithium metal anode in rechargeable batteries: a review. Chemical Reviews 117 (15): 10403–10473. 23 Cheng, X.B., Zhang, R., Zhao, C.Z. et al. (2016). A review of solid electrolyte interphases on lithium metal anode. Advanced Science 3 (3): 1500213. 24 Zhang, R., Li, N.W., Cheng, X.B. et al. (2017). Advanced micro/nanostructures for lithium metal anodes. Advanced Science 4 (3): 1600445. 25 Murphy, D.W. and Carides, J.N. (1979). Low voltage behavior of lithium/metal dichalcogenide topochemical cells. Journal of The Electrochemical Society 126 (3): 349–351. 26 Megahed, S. and Scrosati, B. (1994). Lithium-ion rechargeable batteries. Journal of Power Sources 51 (1–2): 79–104. 27 Mizushima, K., Jones, P.C., Wiseman, P.J. et al. (1980). Lix CoO2 – a new cathode material for batteries of high-energy density. Materials Research Bulletin 15 (6): 783–789. 28 Auborn, J.J. and Barberio, Y.L. (1986). Lithium intercalation cells without metallic lithium - MoO2 /LiCoO2 and WO2 /LiCoO2 . Journal of the Electrochemical Society (8): 133, C291–C291. 29 Lu, Y.X., Rong, X.H., Hu, Y.S. et al. (2019). Research and development of advanced battery materials in China. Energy Storage Materials 23: 144–153. 30 Hong, X.D., Mei, J., Wen, L. et al. (2019). Nonlithium metal–sulfur batteries: steps toward a leap. Advanced Materials 31 (5): 1802822. 31 Choi, J.W. and Aurbach, D. (2016). Promise and reality of post-lithium-ion batteries with high energy densities. Nature Reviews Materials 1 (4): 16013. 32 Conway, B.E., Birss, V., and Wojtowicz, J. (1997). The role and utilization of pseudocapacitance for energy storage by supercapacitors. Journal of Power Sources 66 (1-2): 1–14. 33 Trasatti, S. and Buzzanca, G. (1971). Ruthenium dioxide: a new interesting electrode material. Solid state structure and electrochemical behaviour. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 29 (2): A1–A5. 34 Angersteinkozlowska, H., Conway, B.E., Barnett, B. et al. (1979). Role of ion adsorption in surface oxide formation and reduction at noble-metals – general features of the surface process. Journal of Electroanalytical Chemistry 100 (1–2): 417–446. 35 Hadzijordanov, S., Angersteinkozlowska, H., Vukovic, M. et al. (1978). Reversibility and growth-behavior of surface oxide-film at ruthenium electrodes. Journal of the Electrochemical Society 125 (9): 1471–1480. 36 Pandolfo, A.G. and Hollenkamp, A.F. (2006). Carbon properties and their role in supercapacitors. Journal of Power Sources 157 (1): 11–27. 37 Shao, Y.L., El-Kady, M.F., Sun, J.Y. et al. (2018). Design and mechanisms of asymmetric supercapacitors. Chemical Reviews 118 (18): 9233–9280. 38 Zhu, Y.W., Murali, S., Stoller, M.D. et al. (2011). Carbon-based supercapacitors produced by activation of graphene. Science 332 (6037): 1537–1541.

References

39 Conway, B.E. (1991). Transition from supercapacitor to battery behavior in electrochemical energy storage. Journal of the Electrochemical Society 138 (6): 1539–1548. 40 Yu, Z.N., Tetard, L., Zhai, L. et al. (2015). Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy & Environmental Science 8 (3): 702–730.

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2 Materials and Fabrication As stated in Chapter 1, lithium ion batteries (LIBs) and electrochemical capacitors (ECs) have been widely investigated and utilized as power sources for various applications. The innovative materials and fabrications are crucial and required for developing and manufacturing next-generation high-performance LIBs and ECs. In this chapter, we present a comprehensive introduction for the developments of innovative materials, architectures, and design considerations in the electrode and cell configurations of LIBs and ECs, together with the recent technologies used to achieve these novel designs.

2.1 Mechanisms and Advantages of LIBs 2.1.1

Principles

A LIB consists of two electrodes, the negative and the positive, which are separated by a polymer separator soaked in liquid electrolytes with lithium salt, as shown in Figure 2.1. The principle of LIBs can be summarized as follows: (i) During charging process, Li+ is deintercalated from the positive material, then passed through the liquid electrolyte, and inserted into the negative; (ii) during the discharge process, Li+ is removed from the negative and then reinserted into the positive. That is to say, the rocking-chair-type LIB is a Li+ concentration gradient battery. During charging/discharging process, the Li+ continuously shuttles between the high concentration and the low concentration part. The positive and negative electrodes undergo redox reaction, respectively, which is shown in Figure 2.1. Take a typical LiCoO2 ||C system as an example, the overall electrochemical reaction is as follows: Positive reaction ∶ LiCoO2 = CoO2 + x Li+ + xe− Negative reaction ∶ 6C + x Li+ + x e− = Lix C6 All reaction ∶ 6C + LiCoO2 = Li1−x CoO2 + Lix C6 In a typical LIB with LiCoO2 positive, the total electrochemical reaction is usually reversible for a portion of lithium, which limits the depth of discharge. The LIB chemistry was first developed by Sony in 1990s. It can be seen from the Novel Electrochemical Energy Storage Devices: Materials, Architectures, and Future Trends, First Edition. Feng Li, Lei Wen, and Hui-ming Cheng. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

2 Materials and Fabrication

e– –

+

Cu

AI Li+

Separator

16

Anode (graphite)

Electrolyte

Cathode (LiCoO2)

Figure 2.1 Mechanisms of a LIB (LiCoO2 | Li+ electrolyte | graphite). Source: Goodenough and Park [1].

aforementioned reaction that the energy density of a LIB is determined by the Gibbs free energy change during the entire reaction. Therefore, the choice of suitable materials is the basis for the development of high-performance LIBs. As shown in Figure 2.2, currently, the commercial cells mostly include cylindrical cells, prismatic cells, and pouch cells. The cylindrical cells with the standard size 18650 are the typical products in the market. 18650 stands for the cell with 18 mm in diameter by 65 mm in length, and 0 means the shape of the cells is cylindrical. The prismatic and pouch-type LIBs are also widely used in various applications due to their small inactive volume and high design freedom at the module level. Compared with the cylindrical cells, the size of prismatic and pouch cells is easy to customize for the final product. As depicted in Figure 2.2, inside the cell, negative–separator–positive are stacked or wound together to yield the core, in which the negative material and positive material are coated on both sides of current collectors, respectively [2].

2.1.2

Advantages and Disadvantages

Similar to other batteries, the electrode materials determine the performance of LIBs, such as capacity, power density, and safety. For example, LIBs based on LiCoO2 positive provide higher capacity but poor safety due to its poor thermal stability. LiFePO4 is another important positive material for LIBs, which has 4–5 times longer cycle lifetimes, 8–10 times power density, and wider operating temperature range compared with LiCoO2 [3]. Although the properties of LIB vary from material to material, LIB has some common characteristics. Their main advantages compared with other rechargeable batteries are: (i) high energy density, which makes LIBs more practical in portable electronic devices. (ii) Low self-discharge rate (∼1.5% per month), which means that

2.1 Mechanisms and Advantages of LIBs

Separators

Cathode Separators Anode

Can

Cathode

Anode

(a) Cylindrical

Can (b) Prismatic

Exterior m m 0m m 10 90

300 mm 265 mm

Thickness: 10 mm Pouch

Separator Cathode Separator Anode Pouch

n Stacks of anode–separator–cathode

(c) Pouch

Figure 2.2 Three typical cell architectures. (a) Cylindrical cell, (b) prismatic cell, and (c) pouch cell. Source: Choi et al. [2].

the cell has a longer shelf life. (iii) Low memory effect: LIB has almost zero memory effect, which is a well-known phenomenon observed in Ni–Cd and NiMH batteries. Ni–Cd or NiMH batteries may gradually lose usable capacity when repeatedly recharged after being only partially discharged. (iv) Quick charging speed: LIBs has obvious higher charger rates compared with other secondary battery systems. (v) High working voltage: The open-circuit voltage of LIB is higher than other aqueous batteries, such as lead acid, NiCd, and NiMH batteries. (vi) Long life span: LIB can be charged/discharged over hundreds of cycles, and the LIBs can be 80% of their original capacity even after several thousand cycles. Disadvantages of LIBs are as follows: (i) Expensive: Generally, LIB is more expensive than the other batteries due to its complexity of manufacture and additional on-board circuitry. (ii) Sensitive to temperature: For LIB, heat usually causes the cells to degrade faster than they normally would be. Although LIBs have a wide working temperature range (−20 to 65 ∘ C), the recommended working and storing

17

18

2 Materials and Fabrication

temperature for the most LIBs is around 15–25 ∘ C. For example, LiFePO4 batteries must not be charged at temperatures below zero due to sluggish ionic kinetics, and LiMn2 O4 batteries must not be worked at higher than 55 ∘ C due to severe capacity fading caused by manganese dissolution. (iii) Aging effect: LIB is able to operate several thousand charge/discharge cycles. However, an unused LIB is not completely durable because it starts the degradation after manufacturing. (iv) Safety issues: LIB can be fired or exploded under overheating, overcharging, short circuit, or other abuse condition.

2.2 Mechanisms and Advantages of ECs 2.2.1

Categories

Generally, ECs also consist of two electrodes, which are separate by a membrane separator within the electrolyte. The performance of EC is mainly determined by the electrode materials [4]. The energy storage mechanism in ECs takes place through either ion adsorption at the electrode–electrolyte interface or reversible Faradaic processes on the surface [5]. Based on the mechanisms of charge storage, as stated in Chapter 1, ECs are classified into four broad categories. The first category of ECs includes electric double-layer capacitors or the so-called electric double layer capacitors (EDLCs). The second category includes pseudocapacitors or Faradaic ECs, which is based on reversible Faradic reactions. The third includes hybrid ECs. The fourth category represents hybrid EC-LIBs devices, which is based on the combination of ECs, reversible Faradic reactions, and intercalation-type materials [4].

2.2.2

EDLCs

EDLCs store charge through adsorption of ions by making use of an electrical double layer of charge developed at the electrode/electrolyte interface [6]. This kind of charge storage involves only nanometer distances and interaction allowing EDLCs to charge or discharge fully in seconds [7]. It is generally believed that the electrode materials for EDLCs require the highly accessible specific surface area with high electrical conductivity. The most investigated and favorable materials for EDLCs are mainly porous carbon materials, such as carbon nanotubes (CNTs), activated carbon, and graphenes [8]. Figure 2.3 illustrates three models in order to explain EDLs. As shown in Figure 2.3a, Helmholtz model is the simplest model to explain the spatial charge distribution on the interface between two layers. The charge of the solid electronic conductor is neutralized by oppositely charged ions at a d distance from the surface to the center of the ions. Gouy and Chapman developed theories of the diffuse layer, in which the ion concentration in the solution near the surface follows the Boltzmann distribution [9]. Combining both previous models, Stern improved the model showing that the ions have a finite size, giving an internal Stern layer (Helmholtz layer) and an outer diffuse layer (Gouy–Chapman layer). In Figure 2.3b,

2.2 Mechanisms and Advantages of ECs

POSITIVELY CHARGED SURFACE

POSITIVELY CHARGED SURFACE

Ψ0 + + + + + + + + + + + + + + +

+

– –

–

–

– –

– Ψ

Ψ0 + + + + + + + + + + + + + + +

–

+ –

–

–

– – –

– +

– –

–

–

– +

–

–

– –

+ –

+

– +

Ψ

–

–

–

– – OHP Stern layer Diffuse layer IHP

Diffuse layer

d (a)

–

POSITIVELY CHARGED SURFACE

– Ψ0 + – + – + +Ψ – – + – + – + – + – + – + – + – + – + – + – + –

(b) +

(c) Solvated cation

– Anion

Figure 2.3 EDL models. (a) Helmholtz, (b) Gouy–Chapman, and (c) Stern models . Source: Zhang and Zhao [8].

c, ψ is the potential, ψ 0 is the electrode potential, IHP is the inner Helmholtz plane, and OHP is the outer Helmholtz plane explained in the Stern model [8, 10]. Nowadays, Stern model has been considered as a typical description for EDL inside the solid/liquid electrode interface. Recently, this model was further modulated to be available for the solid/solid interfacial one by our group [11]. A solid/solid electrode interface often exists in the organic electrolyte-based systems due to the decomposition products of electrolyte or additive depositing on electrode surface. The formation of the solid/solid interface can effectively widen the stable voltage window of electrode. The modulated EDL is shown in the Figure 2.4, which has the same structure as Stern model that is composed of an internal Helmholtz layer and an outer diffuse layer. However, because of the ionic conduction and electronic insulation of solid electrolyte interface layer, this EDL is established by desolvated ions. This model can successfully explain the phenomenon that: First, the capacitance of electrodes increases after the solid/solid interface was formed, which is attributed to a shorter separated distance (d) of Helmholtz layer inside this interface; second, self-discharge rate of electrodes also decreases after a solid/solid interface was formed due to the stronger interaction force between the electrode surface and electrolyte ions inside this interface [11]. EDLC uses a liquid ion electrolyte instead of a solid insulating dielectric as the dielectric layer to store electric charges by forming an EDL on the surface of the electrode/electrolyte. In the simplest configuration, an EDLC consists of two electrodes immersed into an electrolyte and separated by separators.

19

20

2 Materials and Fabrication

Electrode

Figure 2.4 EDL model inside a solid/solid electrode interface. Source: Wang et al. [11].

Helmholtz layer (H) PF6–

Diffusion layer (D) Li+

Solvents

Charge storage mechanism in the EDLC is non-Faradaic. EDLC exhibits high energy density compared with conventional capacitors due to their very small charge separation distances and maximum effective surface space.

2.2.3

Pseudocapacitor

Owing to the electrostatic type of charge storage at the interfaces, EDLCs show excellent cyclic stability, which is useful in various applications ranging from hybrid vehicles to portable electronics [12]. However, EDLCs can still not satisfy the requirements for high energy density. The most common strategy to increase their energy storage capability is the integration of electrode materials that utilize fast Faradaic redox reactions, such as transition metal compounds, redox-active polymers, and hydroxides [13]. It was found that reversible redox reactions at the surface of appropriate materials show EDLC-like electrochemical features [14, 15]. Pseudocapacitor (or Faradaic capacitor) has totally different energy storage mechanism with EDLC. It involves reversible redox reactions on the electrode surface without bulk phase transformation. Generally, pseudocapacitors develop a different type of capacitance at the electrodes where the Faradaic charge depends linearly on the applied voltage exhibiting a capacitor-like behavior known as “pseudocapacitance,” which resembles a battery behavior but is different from the capacitive behavior of EDLCs [16]. Pseudocapacitive materials show the potential to obtain battery-level energy density combined with the power density and cycle life of EDLCs [15]. The characteristic behavior of different energy storage materials was summarized by Lukatskaya et al. [17]. As shown in Figure 2.5a, carbon-based EDLC shows nearly rectangular cyclic voltammetry (CV) curves and linear galvanostatic charge/discharge profiles. Unlike EDLCs, the phase transformation of the active material usually occurs during charge/discharge in the batteries, which can be characterized by distinct peaks in the CV and voltage plateaus in the charge/discharge curves as shown in Figure 2.5d. In contrast, without the phase transition, the pseudocapacitive materials present a continuous, highly reversible change in the oxidation state during charge/discharge, characterized by CVs with either significantly broadened peaks (intercalation pseudocapacitance, for example: T-Nb2 O5, Figure 2.5c) and little separation in peak position during charge/discharge or almost perfectly rectangular CVs (surface redox due to adsorption and/or fast intercalation of ions (for example:

2.2 Mechanisms and Advantages of ECs Surface redox

Intercalation

1

Q/Qmax

0

Current (i)

Current (i) 50 nm

Bulk 0.5 1

Q/Qmax

0

Potential (V) Potential (V)

Potential (V)

Potential (V)

Bulk 0.5

0

Potential (V) Potential (V)

Potential (V)

Potential (V)

50 nm

Battery

Pseudocapacitor Current (i)

Current (i)

Electrical double layer capacitor

50 nm

Bulk 0.5

1

Q/Qmax

0

5 nm Bulk 0.5

Q/Qmax

1

Mechanism No phase change

No phase change

No phase change

Phase change

Reversible ion adsorption

Continuous change in oxidation state

Intercalation + change in oxidation state

Intercalation + change in oxidation state

Intrinsic kinetics i~v

i~v

i~v0.5

Transition metal chemistry, large channeled structures. Examples: T-Nb2O5

High theoretical capacity. Example: LiCoO2, Si, LiFePO4

i~v Typical systems

High specific surface area materials. Example: Porous carbons (CDC, activated cabon), graphene, carbon onions

Transition metal chemistry; specific structures. Example: RuO2 (hydrated), birnessite MnO2, 2D Ti3C2

Porous carbon

Birnessite MnO2

T-Nb2O5

LiFePO4

c a

Challenges: Rapid ion access to each active site

(a)

(b)

(c)

(d)

Figure 2.5 Characteristic behaviors, such as CV, galvanostatic curves, mechanisms of typical materials. Source: Lukatskaya et al. [17]. Licensed under CC-BY-4.0.

hydrated RuO2 , birnessite MnO2 and MXene Ti3 C2 , Figure 2.5b) [18, 19]. It must be noted that pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions. However, it was recently demonstrated that a pseudocapacitive mechanism often occurs when conventional battery materials are downsized to nanosized particles, even for the typical LiCoO2 positive [20].

2.2.4

Hybrid Capacitors

Figure 2.6 shows the charge storage mechanisms of different hybrid capacitors. Figure 2.6a shows the plot of the electrode potentials (V+ and V− ) and cell potential (V) vs. specific capacity for symmetric (blue lines) and hybrid (red lines) configurations. Figure 2.6b is the typical symmetric configuration with activated carbon as both positive and negative electrodes. Figure 2.6c is the example of hybrid device consisting of an insertion metal oxide (MeO) negative combined with a high surface area of activated carbon positive (AC in the figure) [21]. The hybrid capacitors, which are the combination of EDLC and pseudocapacitor, have more enhanced performance. Contrary to the EDLC and the

21

2 Materials and Fabrication

5 Oxidative decomposition of electrolyte –

+

4 Potential (V vs. Li/Li+)

22

+

+

AC

+

+

– –

+

AC

2

AC

Li

AC

+

Li+

Reductive decomposition of electrolyte

10

20

30

– –

Li+ Li+

Li4Ti5O12 0

–

Li+ Li+

–

–

Li+

Li+

Li4Ti5O12 (1.55 V)

1

–

–

Li+ Li+

0

–

–

+

3

–

–

– – –

AC

40

Specific capacity (mAh g–1)

Figure 2.6 [21].

Hybrid capacitors and symmetric ECs, AC, activated carbon; Source: Naoi et al.

pseudocapacitor using symmetric design, the hybrid capacitor with asymmetric electrode configuration delivers higher specific capacitance associated with different storage mechanisms [21]. The two electrodes work with different mechanisms, which can effectively improve the working voltage range (Figure 2.6a), obviously improve the electrochemical performance, especially the energy density. In a typical hybrid capacitor, one of the electrodes is typically a pseudocapacitor electrode or battery-type electrode providing high energy density (Figure 2.6c). The hybrid capacitor is referred to as a lithium ion capacitor when the LIB-type electrolyte is used. Lithium ion capacitor combines the advantages of both LIB and EC. However, it is still a great challenge to balance the reaction kinetics between capacitive materials and intercalation-type electrodes [22, 23]. Due to the excellent electrochemical performances of graphene, the dual-graphene-based lithium ion capacitor has attracted wide attention. Recently, a dual-graphene-based lithium ion capacitor was proposed by charge injection strategy to maximize the energy density [24]. The hybrid capacitor showed the great potential for practical applications due to its energy density as battery with high power output and long cycle ability. In comparison to the present LIBs, the ECs, including EDLC, pseudocapacitor, and hybrid EC can deliver much greater power density and cycle life.

2.3 Roadmap of Conventional Materials for LIBs Like any other batteries, there are four core components inside a LIB, including positive and negative to supply and store Li+ , respectively, an electrolyte to transfer Li+ , and a polymer separator to keep the electrodes electronically separate. Figure 2.7 indicates the current material status of LIBs. The upper section above the dash line corresponds to positive materials, while the lower part contains various

2.4 Typical Positive Materials for LIBs

5

LiMn2O4

Voltage (V vs. Li+Li)

4

NMC/NCA

LiCoO2 LiFePO4

3

2 Li4Ti5O12

Metal oxides

1

Si

Graphite

Metal Li

0 0

200

400

3400

3600

3800

Specific capacity (mAhg–1)

Figure 2.7

Specific capacities and working voltages of different materials.

negative materials. For example, the potential of LiFePO4 and Li4 Ti5 O12 is ∼3.5V and ∼1.5V, respectively. Therefore, the voltage of a LiFePO4 //Li4 Ti5 O12 battery is 2.0V. The positive materials (LiCoO2 , LiMn2 O4 , LiFePO4 , LiNix Coy Mn1−x−y O2 , etc.) currently applied in commercial batteries allow for a nominal voltage of ∼4 V [25]. The upper limit of the electrochemical window of the electrolyte (alkyl carbonates/LiPF6 ) is about 5.0 V (vs. Li/Li+ ). While the negative materials in Figure 2.7 indicate four most promising groups: (i) graphite, (ii) tin and Si-based composites, (iii) metal oxides, and (iv) Li4 Ti5 O12 electrodes. Due to its flat voltage potential, graphite has been widely used for commercial LIBs. In the following section, we give a brief introduction toward various materials for LIBs.

2.4 Typical Positive Materials for LIBs As a key component of LIBs, the ideal positive material should have the following properties: (i) higher redox potential to ensure higher operating voltage; (ii) high lithium storage capacity; (iii) stable crystal structure to maintain long cycle life; (iv) high electronic and ionic conductivity to reduce the polarization; (v) good chemical stability and electrolyte compatibility; (vi) simple fabrication process, rich source of raw materials, and pollution-free. Currently, the dominant commercial positive materials mainly include LiCoO2 , LiNiO2 , LiMn2 O4 , LiFePO4 , LiNix Coy Mn1−x−y O2 , and their derivatives [26].

2.4.1

LiCoO2 Materials

The layered rock-salt structured LiCoO2 was recognized as the first viable positive material by Goodenough et al. [27, 28] in 1980, then successfully commercialized by Sony in 1991. Until now, LiCoO2 is still the most preferred positive active material

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Figure 2.8 Crystal lattice structure of layered LiCoO2 and LiNiO2 with R3m group.

c a

b

due to its superior cycle stability, excellent reversibility, high capacity, high volume energy density, and easy preparation. In the layered structured LiCoO2 , the Co ions are surrounded by six oxygen ions. The Li and Co ions are located in alternating planes, separated by oxygen layers as shown in Figure 2.8. The layered LiCoO2 possesses R3m space group and is indexed to a hexagonal crystal-type structure [28]. LiCoO2 has excellent electrochemical cycling performance between 3.5 and 4.2 V and possesses high theoretical capacity up to 274 mAh g−1 [28]. However, the practical capacity can only reach half the theoretical capacity because the reversible delithiation is limited to 0.5Li per LiCoO2 for safety reasons [29]. As shown in Figure 2.9, LiCoO2 has gradual phase transitions from H1 to H2, M1, H3, M2, and O1 phases. These phase transitions cause large anisotropic expansion 14.5

H3

H2

M1

H1 4.5

14.0 4.2 H2

M2 M1/M2

13.0

O1

3.9

H1 LiCoO2

LixCoO2

E

3.6

Co3+(4s0: a*1g) (4p0: t*1u)–O2–(2p6) Evac

Evac

eϕLCO= 5.0 ± 0.1 eV

12.5

1.3 ± 0.1 eV 1.1 ± 0.1 eV

Co3+(d 6: e*g)–O2–(2p6) μ=EF

Co3+(d 6: t2g)

3.3

ΔE=0.2 eV Δμ

μ=EF(4.2 V)

0.8 ± 0.1 eV

Co3+/4+(t2g)

O2–(2p6)

12.0 O1

H3

a (Å)

13.5 c (Å)

24

O2–(2p6)

N (ε)

3.0

N (ε)

11.5

2.7 0.0

0.2

0.4 0.6 x in LixCoO2

0.8

1.0

Figure 2.9 The variations in the a and c lattice parameters and phase transitions during Li+ extraction from LiCoO2 . Source: Wang et al. [27].

2.4 Typical Positive Materials for LIBs

and contraction [30]. As a result, the anisotropic dimensional change causes uneven stress and mechanical fracture within the particles. Therefore, the excessive deintercalation of Li+ over 0.5 per LiCoO2 leads to the crystal instability of LiCoO2 structure, which causes the cobalt atoms to migrate from the plane to the neighboring plane where the lithium atoms are located. In order to improve the crystal stability and increase the upper voltage limit, many strategies have been used [27]. For example, a La–Al co-doping was used to solve the issue of instability and increase the capacity of LiCoO2 at 4.5 V [31]. The dopants are found to reside in the crystal lattice of LiCoO2 , where La works as a pillar to increase the c-axis distance. Al acts as a positively charged center, which facilitates Li+ diffusion, stabilizes the structure, and suppresses the phase transition during cycling at 4.5 V. In another work, the trace Ti–Mg–Al codoping improved the cycling performance of LiCoO2 at 4.6 V (vs. Li/Li+ ) [32]. Mg and Al doping inhibits the undesired phase transition at voltages above 4.5 V. Even in trace doping, Ti can segregate obviously at the grain boundaries and modify the microstructure of the particles. Generally, the commercial LiCoO2 is prepared via a solid-state reaction between Co3 O4 and Li2 CO3 at high temperatures. To improve the electrochemical performance, many modification methods have been used, which mainly include elemental doping, surface, and coating [27, 33, 34]. Some of the major obstacles for its wide application in EV and hybrid electric vehicle (HEV) field are its high cost, poor safety, limited cobalt resources on earth, and toxicity [35].

2.4.2

LiNiO2 and Its Derivatives

Due to their high energy densities, the nickel-rich positive materials have been dominated in EV market [36]. LiNiO2 has layered structure similar to LiCoO2 with R3m space group, as shown in Figure 2.8 [37]. Compared with LiCoO2 , LiNiO2 has a higher theoretical capacity with lower cost, which was actively investigated as a high-capacity positive material to replace LiCoO2 for LIBs since 1990s [38]. The actual capacity of LiNiO2 can reach 190–210 mAh g−1 between 2.5 and 4.1 V. However, LiNiO2 also has severe issues, such as low thermal stability, difficult to synthesize, and less ordered structure, compared to LiCoO2 . LiNiO2 also has poorer cycle performance when it was charged to the higher voltage of above 4.3 V [39]. Cationic mixing also usually takes place in the LiNiO2 crystal structure, where Li layers are occupied by Ni2+ . This Li+ /Ni2+ mixing usually results in sluggish Li+ transportation kinetics during the electrochemical reaction [38]. To reduce the capacity fading during cycling, the nickel in LiNiO2 was substituted by manganese and cobalt to yield LiNix Mny Co1−x−y O2 [40]. One of the most successful candidates of this group is LiNi0.8 Mn0.1 Co0.1 O2 positive, which was called NCM811. In LiNix Coy Mnz O2 , the valency of the nickel, cobalt, and manganese cation is usually divalent, trivalent, and tetravalent, respectively. The electrochemically inert tetravalent manganese provides acceptable safety during charge/discharge. Trivalent cobalt can ensure the high electric conductivity and alleviate cation disorder. The redox couple of Ni2+ /4+ and Co3+ /4+ can achieve the capacity over 200 mAh g−1 at 4.3 V [36]. This material not only delivers higher

25

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2 Materials and Fabrication

capacity compared to LiNiO2 and LiCoO2 , but also has excellent cycle performance and low cost. Another important derivative of LiNiO2 is layered Li(Nix Co1−x) O2 material [41, 42]. In order to further improve the characteristics of Li(Nix Co1−x )O2 material, Al3+ doping was also used to suppress cationic mixing by preserving Li+ in lithium layers [43]. Until now, LiNi0.8 Co0.15 Al0.05 O2 (denoted as NCA) is now commercially available by Panasonic and Tesla. The NCA material has a practical specific capacity up to 190 mAh g−1 , corresponding to 0.7 Li extracted from LiNiO2 structure [44]. Currently, LiNiO2 and its derivatives have become one of the widely used positive materials due to the relatively abundant nickel resource, less cost, and high energy density. The most obvious competition is NCA adopted by Panasonic and TESLA, while LiNix Coy Mnz O2 is adopted by LG, SAMSUNG, and CATL. LiNi0.8 Co0.15 Al0.05 O2 is very similar to LiNi0.8 Mn0.1 Co0.1 O2 , whether it is related to Ni-rich chemistry, superior capacity, more complicated manufacturing, or stability issues. One advantage of the LiNi0.8 Mn0.1 Co0.1 O2 is its low cost due to the lower content of Co in the compound. While LiNi0.8 Co0.15 Al0.05 O2 exhibits better capacity retention and power characteristics compared with LiNi0.8 Mn0.1 Co0.1 O2 [45].

LiMn2 O4 Material

2.4.3

Spinel LiMn2 O4 is another alternative positive material to LiCoO2 due to its low cost, nontoxicity, and easy manufacture [46]. LiMn2 O4 was first proposed as a positive for LIBs in the early 1980s [46]. It has a typical spinel cubic structure with Fd-3m space group [47]. Li and Mn cations occupy both octahedral and tetrahedral sites, and the LiO4 tetrahedral sites share its face with MnO6 octahedral sites; this created voids subsequently allowing the Li+ transport in the 3D tunnel (Figure 2.10). LiMn2 O4 has a theoretical capacity of 148 mAh g−1 and two voltage plateaus at about 3 and 4 V during discharge. However, the actual capacity of LiMn2 O4 is only 110–120 mAh g−1 in order to maintain its structural stability. Although the practical capacity of LiMn2 O4 is slightly lower than that of LiCoO2 , it is very promising as a positive material for large-scale LIBs for Figure 2.10 Crystal structure of spinel LiMn2 O4 .

b c

a

2.4 Typical Positive Materials for LIBs

sustainable energy storage due to its simple fabrication process, low cost, abundance in natural resources, nontoxicity, and resistance to overcharge and safety [48]. However, the spinel material undergoes severe capacity fading during cycling, mainly due to the dissolution of Mn2+ into the electrolyte and the Jahn–Teller distortion of the unit cell [49]. Furthermore, the dissolved Mn2+ can move to the negative, where it can be reduced and converted to metallic Mn on the surface of graphite negative [50]. The metallic plating can block the Li+ intercalation and catalyze electrolyte decomposition at high temperatures (55 o C). Various strategies have been put forward to suppress the Mn dissolution and inhibit Jahn–Teller distortion during cycling. The research has shown that the doping of elements, such as Ni, Mg, Co, Cr, Al, and Fe, can effectively stabilize its crystal structure and suppress the Jahn–Teller effect [49]. LiMn2 O4 can also be modified by surface coating, such as metal oxides and fluorides, in order to reduce the dissolution loss of manganese [51]. Recently, it has been reported that LiBOB-based electrolyte can be used to replace LiPF6 to reduce the formation of trace HF in the electrolyte, which is also helpful for LiMn2 O4 cycling performance [52].

2.4.4

LiFePO4 Material

LiFePO4 is a natural mineral called triphylite, which was first explored by Björling and Westgren [53], and rediscovered as positive material for LIB by Goodenough and coworkers [54]. LiFePO4 has orthorhombic crystal structure with Pnma space group, in which FeO6 octahedral sites are separated by PO4 tetrahedral sites. Li+ would only be able to move in one direction through the channels of the unit cells as shown in Figure 2.11. Currently, LiFePO4 has been widely used in EVs and renewable energy storage due to its low cost, high thermal stability, high safety, excellent Figure 2.11 Crystal structure of LiFePO4 . Source: Chung et al. [55].

c

Li

a

b

Fe

0

P

27

28

2 Materials and Fabrication

cycling life, and environment friendliness. Olivine LiFePO4 has a theoretical capacity of 170 mAh g−1 with a flat discharge plateau at 3.4 V (vs. Li/Li+ ), and the volume variation is only 6.8% during the charge/discharge process [56]. Unfortunately, LiFePO4 has very low electronic and ionic conductivity [57]. Many efforts have been made to improve the power performance of LiFePO4 , such as decreasing the particle size, Li+ /Fe2+ sites doping, and coating LiFePO4 particles with conductive carbons. Carbon coating can significantly improve the electronic conductivity of LiFePO4 , providing pathways for electron transfer, subsequently improving the electrochemical properties [58, 59]. It has been demonstrated that doping of divalent (Mg2+ , Mn2+ , Ni2+ , and Cu2+ ) or supervalent ions (Al3+ , Cr3+ , Zr4+ , and Nb5+ ) either in Li site or in Fe site can significantly improve its electronic conductivity [60].

2.4.5

Lithium–Manganese-rich Materials

Lithium–manganese-rich layered transition metal oxide (TMO) has been considered as a another promising candidate with high specific capacities (higher than 200 mAh g−1 ) and excellent cycling performance within the high range of 2.0–4.8 V [61]. The chemical formula is written as xLi2 MnO3 ⋅(1−x) LiMO2 where M = Mn, Ni, Co, and x between 0 and 1. The integration of two or three oxide materials offers considerable electrochemical advantages over the commercialized single structured materials such as LiMnO2 , LiCoO2 , or LiNiO2 that include high discharge capacities (∼200 mAh g−1 ) with high average discharge voltage profiles (∼4.6 V). This is mainly due to the combined characteristics of LiCoO2 , LiNiO2 , and Li2 MnO3 [62–66]. On the other hand, since these lithium-rich materials possess a similar LiCoO2 layered structure with the addition of LiNiO2 and excess of Mn4+ through Li2 MnO3, these materials have enhanced thermal stability [62–66]. The capacity produced below 4.5 V (vs. Li/Li+ ) can be attributed to the LiMO2 component, whereas Li2 MnO3 serves as a capacity reservoir above 4.5 V. The Li2 MnO3 also offers structure stability with the presence of Mn4+ that occurs at higher operating voltages and achieves higher capacities due to the ability to extract two Li+ when charged at voltages >4.5 V [61]. Major obstacles that hinder the commercial application include: (i) voltage decay during cycling, (ii) Mn dissolution, and (iii) capacity fading during cycling. Among these issues, voltage plateau decay has been considered as a major obstacle, which is caused by phase transformation [67].

2.4.6

Commercial Status of Main Positive Materials

Figure 2.12 shows mass percentage of Chinese LIBs market shares of the leading materials in 2019 [68]. The overall market of LIB materials is 390 000 tons. The market share is ranked from LFP, NMC, LCO, NCA to LMO. Due to its higher energy density, materials with 80% is still under development. Overall, the commercialization of Ni-based layered oxide positive materials has been quite successful, representing a total of 48.8% of the Chinese battery materials market in 2019 [68], as shown in Figure 2.12.

2.5 Typical Negative Materials for LIBs

Figure 2.12 Mass percent of Chinese LIBs market shares of the positive materials in 2019.

2.5 Typical Negative Materials for LIBs An alternative negative material to replace metal lithium is the prerequisite for LIBs to commercialization because metal lithium forms uncontrollable dendrites, which can cause inner-short circuit and eventually cause the cell to catch fire and explosion. An ideal negative material for LIBs should meet the following characteristics: (i) large reversible lithium storage capacity, (ii) low and flat lithium intercalation/deintercalation potential, (iii) stable crystal structure and long cyclability, (iv) stable solid electrolyte interphase (SEI) on the surface, (v) excellent electronic and ionic conductivity, and (vi) cheap and pollution-free [69]. Currently, negative materials for LIBs can be divided into two main categories: (i) various carbonaceous materials, including graphite (natural graphite and artificial graphite), hard carbon and soft carbon; (ii) noncarbonaceous materials, mainly include TMOs, silicon-based materials, titanates tin-based materials, etc. Nowadays, various carbonaceous materials dominate the market of LIBs.

2.5.1

Graphite

Hérold et al. [70] reported that lithium can form intercalation compounds with graphite (graphite intercalation compound, GIC) in 1955. Success of carbon negative enabled the LIBs to become commercially viable since 1990s, and it is still the dominant choice of negative material. First-generation LIBs developed by Sony used coke as negative. Since 1997, most LIBs use natural or artificial graphite to attain a flatter discharge curve. Li+ is able to be inserted between graphene layers. Per six carbon atoms can accommodate one lithium, that is, LiC6 is formed. Generally, the Li+ inserts into a graphite on the edge plane of the graphite (as shown in Figure 2.13), while the insertion in the basal plane usually needs to overcome higher energy barrier. Studies have shown that with the lithium storage in graphite, a "stage phenomenon" usually occurs [71]. As shown in Figure 2.14, the so-called “stage phenomenon” means that the inserted Li+ is orderly distributed between the graphene layers. The number of graphene layers between the intercalation layers remains equal. The number of these layers is defined as “stage index.”

29

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2 Materials and Fabrication

Li+ Basal plane Graphene layers c-Axis

A Li+ Edge plane

B

+

Li

0.3354 nm A

Outline of unit cell

Figure 2.13

nm 0.246

2 nm

0.14

Scheme of Li+ insertion into graphite crystals.

Stage -1

Stage -2

Stage -3

Stage -4

Li layer Graphene layer

Figure 2.14

A schematic diagram showing the stage phenomena.

For example, when there is a Li+ intercalation layer per three graphene layers, the graphite intercalation compound is called a stage-3 compound. For LiC6 , it is a stage-1 compound. That is, Li+ is inserted between each graphene layers [71]. Due to the effects of stage phenomenon, graphite has obvious lithium insertion potentials at different stages. Each voltage plateau indicates that there are two-phase graphite intercalation compounds (with different stage indexes) coexisting in GIC. Figure 2.15 shows a typical discharge curve of graphite. It can be seen that with the insertion of Li+ , the gradual increase of x in Lix C6 leads to four distinct voltage plateaus at ∼0.21, ∼0.16, ∼0.14, and ∼0.09 V, respectively. Current negative materials used in commercial LIBs are graphitized carbonaceous materials, such as high-temperature graphitized mesocarbon microbeads (MCMB), natural and artificial graphites. MCMB has the advantages of initial efficiency, excellent cyclability, and moderate capacity (∼300 mAh g−1 ) [72, 73]. However, MCMB usually requires tedious fabrication procedures. Natural graphite, as a natural material that does not require special heat treatment to obtain higher

2.5 Typical Negative Materials for LIBs

Graphene layer

Li layer

0.3

E/V (vs. Li/Li+)

Stage III

Stage II

Stage I

0.2

0.1

s>IV+IV IV+III

III+IIL

IIL+II

II+I

0.0 0.22 III

Figure 2.15

0.34 IIL

0.5 II

1 X in Lix C6 I Stage

A diagram showing the discharged voltage profile of graphite.

regular graphitized structure, is suitable for reversible storage of Li+ . Therefore, due to its rich abundance in the earth crust, natural graphite has received extensive attention in recent years [74]. Main shortcomings of natural graphite include its low initial efficiency and poor cycle performance. Thus, various methods have been tried to modify natural graphite [75, 76]: (i) Constructing a core–shell structure. Surface of natural graphite was usually coated by a soft-carbon material (such as pyro carbon, pitch carbon, and resin carbon) or metal oxides (such as Al2 O3 and MgO) [77]; (ii) modifying the surface physicochemical state: mild and slight oxidation treatments were often used to introduce surface functional groups; (iii) doping: nonmetallic elements (such as B, F, P, N, S) are doped to improve the specific capacity and initial efficiency. It is generally believed that surface soft-carbon coating can effectively improve the comprehensive performance of natural graphite.

2.5.2

Soft and Hard Carbon

Both soft carbon and hard carbon belong to ungraphitized carbon materials. The difference between them is that the soft carbon can be easily converted into graphitized carbon at high temperatures. While the hard carbon is difficult to achieve graphitic phase transformation at high temperatures. Figure 2.16 shows the structural difference between soft and hard carbons. Soft carbon has crystallites orientation, while hard carbon has a kind of turbostratic structure. Figure 2.17 shows typical lithium storage capacity and charge/discharge curves of various carbon materials, which have three typical regions. Electrochemical performance of various carbon materials strongly relies on the heat treatment temperature, as shown in Figure 2.17a. The reversible capacity of the soft carbon

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2 Materials and Fabrication

(a)

(b)

Figure 2.16

A diagram showing the structure of (a) soft carbon and (b) hard carbon.

gradually decreases with the increase of heat treatment temperature, until it reaches its lowest value at around 1600 ∘ C, and then begins to rise slowly as the temperature rises further. As shown in region 1, once the heat treatment temperature is above 2400 ∘ C, the soft carbon (more precisely, it can be converted to graphitized carbon) exhibits a reversible capacity of 300–370 mAh g−1 , while the capacity of hard carbon 1000

1.5

Region 1

Some hard carbons Most soft carbons

800

Voltage (V)

Capacity (mAh g−1)

2

3

600 400

1

1.0

0.5

200 0 500

0.0 1000 1500 2000 2500 Heat treatment temperature (°C)

3000

(a)

0

200

600 800 400 Capacity (mAh g−1)

1000

(b) 1.5

2.0

Region 3

Region 2

Voltage (V)

1.5 Voltage (V)

32

1.0

1.0

0.5

0.5

0.0 (c)

0

200

400 600 800 Capacity (mAh g−1)

0.0

1000

0

200

400 600 800 Capacity (mAh g−1)

1000

(d)

Figure 2.17 Lithium storage capacity and charge/discharge curves of various carbon materials. (a) Relationship between reversible capacity and heat treatment temperature for carbon materials. Plots of voltage vs. reversible capacity for the second charge/discharge cycle of representative carbon materials: (b) graphite in (a) region 1; (c) low temperature soft carbon in (a) region 2; and (d) hard carbon in (a) region 3. Source: Dahn et al. [78].

2.6 New Materials for LIBs

Figure 2.18 in 2019.

Market share of negative materials

is only 200–300 mAh g−1 . At present, most of the commercial negative materials are soft carbon materials (i.e. artificial graphite) in region 1. Its charge/discharge curves are shown in Figure 2.17b. Region 2 represents low-temperature-derived soft carbon. From Figure 2.17c, this kind of soft carbon does not show obvious voltage plateau, and it also has significant voltage hysteresis. Region 3 is a kind of hard carbon. Hard carbon is another promising negative material, which has high specific capacity and low voltage plateau (Figure 2.17d) [78]. However, its large initial irreversible capacity still hinders its large-scale applications for high-performance LIBs, which is the region 2 in Figure 2.17a. Therefore, graphite and hard carbon had their advantages and disadvantages. Figure 2.18 shows the Chinese market share of various negative materials in 2019. Most current LIBs have already shifted toward graphite negatives, including artificial graphite and natural graphite, which occupied ∼95% market share [68].

2.6 New Materials for LIBs 2.6.1

Nanocarbon Materials

Although graphite of commercial LIBs has good cycle performance, it has low specific capacity and is difficult to meet the requirement toward high-performance LIBs. Due to their interesting properties, CNTs and graphene have attracted great attention in the LIBs field in recent years. CNTs, as unique one-dimensional nanotubular materials, have been extensively investigated for electrochemical energy storage [79]. When CNTs acted as the negative electrode in LIBs directly [80–82], they usually exhibit a very high lithium insertion capacity (500–2000 mAh g−1 ), but show high irreversible capacity and poor initial efficiency (200%) and poor cycle life. Various improvements have been attempted to improve its electrochemical performance: (i) preparation of tin/tin oxide nanomaterials [114]; (ii) construction of nanocomposite structures [115–117]; (iii) binary or multialloy systems [118, 119]. TMOs are another promising materials for LIBs due to the high theoretical capacity and safety characteristics. Based on the lithium storage mechanism, TMOs can be roughly classified into two subcategories [120, 121]: Conversion mechanism: Mx Oy + 2yLi+ + 2ye− ↔ yLi2 O + xM, such as Fe2 O3 , Co3 O4 , and NiO. Intercalation/deintercalation mechanism: Mx Oy + nLi+ + ne− ↔ Lin Mx Oy , and the oxides mainly include TiO2 , WO2 , and Li4 Ti5 O12 . This type of materials possess relatively low theoretical specific capacities; however, they can accommodate Li+ to form Lin Mx Oy and preserve excellent structural integrity during charge/discharge without structural collapse and ensure long cycle life. Among various intercalation/deintercalation reaction materials, Li4 Ti5 O12 has a cubic spinel structure, which provides a three-dimensional ionic pathway as shown in Figure 2.22 [123]. Li4 Ti5 O12 has received intense attention as a safe negative material for LIBs due to their many advantages, such as outstanding safety and excellent cycle life. Li4 Ti5 O12 has a theoretical capacity of 175 mAh g−1 and a stable lithium intercalation voltage of 1.55 V (vs. Li/Li+ ), which is higher than the reduction potential of most common electrolyte solvents and the formation of lithium dendrites [124–127]. It undergoes negligible structural variations during charge/discharge process, which imply high power characteristic and remarkable cyclic performance [128–130]. Li4 Ti5 O12 -based LIBs possess the advantages of higher safety and longer cycle life. These properties are of vital importance for HEVs and large-scale energy storage applications [131–133]. However, Li4 Ti5 O12 usually has poor rate performance due to its intrinsic poor electronic conductivity. Various strategies have been developed to enhance the electron conductivity of

37

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Figure 2.22 Crystal structure of Li4 Ti5 O12 . Source: Yi et al. [122].

b c

a

Li4 Ti5 O12 for improving the rate performance of Li4 Ti5 O12 . These strategies mainly include: fabrication of Li4 Ti5 O12 nanomaterials [134–136], coating of Li4 Ti5 O12 by conductive carbon [137–140], and element doping of Li4 Ti5 O12 at Li and Ti sites [141, 142]. Although these methods can improve the rate performance of Li4 Ti5 O12 to a certain extent, there are still many obstacles for commercial deployment. For example, nanosizing or carbon coating can effectively improve the rate performance of Li4 Ti5 O12 , but these methods usually result in low tap density and low thermos dynamic stability [143–145]. TiO2 is another typical noncarbon negative material with a theoretical specific capacity of 335 mAh g−1 , which has attracted extensive research interest because of its many advantages, such as abundant raw materials, low price, good safety, and no environmental friendliness. There are four crystal forms of TiO2 for LIBs, rutile, anatase, brookite, and TiO2 -B type [146]. TiO2 also has a small volume variation (2000 m2 g−1 ), controlled pore structure, excellent electrochemical stability and electrical conductivity, ease of process ability, and low cost. Porous carbon–based EDLC can deliver 1 000 000 cycles without obvious capacity fading [155]. According to the EDLC mechanism, carbon materials store energy by physical charge separation on the surface. The most important characteristic of various carbon materials is the interface between electrode and electrolyte. To increase specific capacitance, it is necessary to increase the surface area and tune porosity [155]. Main carbon materials for EDLC include: (i) activated carbon, (ii) CNTs, (iii) graphene, and (iv) carbon nanofibers. Activated carbon consists of discrete fragments of curved graphene sheets, in which pentagons and heptagons are randomly distributed throughout the hexagonal network [156]. The capacitance of the commercial EDLCs using activated carbons can reach levels of 100 F g−1 in organic electrolyte and 200 F g−1 in aqueous electrolyte. Activated carbon can be manufactured via two major steps, carbonization and activation. In the first step, the carbon-rich organic precursor undergoes heat treatment

2.7 Materials for Conventional ECs

to remove the noncarbon elements. Thereafter, the carbon material is physically or chemically activated by oxidizing gases or agents, such as H2 O, KOH, H2 SO4 , H3 PO4 , and ZnCl2 , respectively [157]. The chemically activated product has a high surface area and microporosity [157]. Activation is a mature process that can control the final properties of the material. Although the activated carbons have been commercialized as EC electrode materials, their applications are still hindered due to the limited energy storage and nonuniform pore distribution. CNTs can be classified into SWCNTs and MWCNTs, where SWCNTs are single graphene sheets rolled into cylinders and MWCNTs are concentric SWCNTs with increasing diameter. CNTs have high surface area, narrow pore size distribution, low resistivity, and high stability. However, the specific surface area of CNTs is significantly lower than that of activated carbon, resulting in lower energy density. Other limitations for commercial application of CNTs include their tendency to aggregate into bundles and higher cost [155]. Graphene is an allotrope of carbon in the form of planar sheets of sp2 -bonded carbon atoms densely packed in a hexagonal lattice [158]. Methods for graphene synthesis include chemical vapor deposition (CVD), mechanical and liquid phase exfoliation, chemical, thermal, or flame-induced reduction, epitaxial growth on metal surfaces, electrochemical synthesis, unzipping CNTs, and arc discharge [159]. Today, graphene is usually obtained by reducing graphene oxide (GO), which is commonly produced by the modified Hummers method from graphite. Graphene is an excellent electrode material for ECs because of its high surface area, high electrical conductivity, great flexibility, excellent mechanical and chemical properties, etc. The specific capacitances of graphene can reach levels of 135, 99, and 75 F g−1 in aqueous, organic, and ionic liquid electrolytes, respectively [160–162]. However, its tendency to aggregate in stacks of sheets limits the accessibility of electrolyte ions and degrade the unique properties of individual sheets [163]. A proposed solution that has been frequently studied is combining graphene with CNTs, because CNTs play as spacers between the graphene sheets to reduce restacking and agglomeration [164]. For example, Cheng et al. reported a graphene and SWCNT film by a blending process as electrodes in high energy density ECs. Specific capacitances reached 290.6 and 201.0 F g−1 for a single electrode in aqueous and organic electrolytes, respectively [164].

2.7.2

Transition Metal Oxides

Various TMOs are typical materials for pseudocapacitors. The TMO materials have been extensively investigated as electrochemically active materials for the next-generation ECs because of their high specific capacitance, low resistance, and high energy density [165]. Among the TMOs, RuO2 is the first reported and most studied because of its exceptionally high theoretical capacitance of 1200–2200 F g−1 , very high electrical conductivity, and excellent chemical stability [165]. However, its high cost has limited the practical application and prompted to look for other TMOs.

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In the recent years, single-component metal oxides and complex oxides, such as MnO2 , NiO, Co3 O4 , Fe2 O3 , and NiMn2 O4 , have been extensively investigated as potential electrode materials. However, the performance of these single-component TMOs is restricted by the poor electrical conductivity and low specific capacitances. Compared with single-component metal oxides, the mixed TMOs usually display higher specific capacitances, better rate capabilities, and long-term cycling stability owing to their multiple oxidation states and multimetal component system, which leads to rich electroactive sites and high electrical conductivity [166].

2.7.3

Conducting Polymers

Conductive polymers can be metallic conductive or semiconductive. Conducting polymers can be p-doped and n-doped by partial oxidation and partial reduction, respectively. Available conducting polymers can be p-doped easily and are most often conductive in their oxidized state. Therefore, research has been focused on incorporating conductive polymers into ECs as materials [167]. Conducting polymers used for electrodes have many advantages, and many conducting polymers exhibit high specific capacities and capacitances, while being able to deliver energy at a relatively fast rate. The most common conducting polymers, such as polypyrrole (PPY), polyaniline (PANI), and poly(3, 4-ethylenedioxythiophene) (PEDOT), have stable cycle performance and high theoretical capacitance [168, 169]. The conductive polymers for ECs can be found in the reviews and books [167].

2.8 Electrolytes and Separators 2.8.1

Electrolytes

Generally, the electrolyte used in LIBs is a lithium salt dissolved in a mixture of organic solvents. The electrolyte of a LIB not only delivers fast Li+ transport between electrodes but also stabilizes the electrode/electrolyte interfaces [170]. An ideal electrolyte should meet the following requirements: (1) (2) (3) (4)

Excellent ionic conductor and electronic insulator; Wide electrochemical window; Thermally stable within the operation temperatures; Low toxicity to limit environmental hazard [171].

The electrolyte for LIBs can be divided into liquid (organic liquid and room temperature ionic liquids), all solid polymer, gel polymer, and solid electrolytes. Nowadays, the organic liquid electrolyte still dominates the market in the LIBs field. In this section, we will briefly introduce the liquid electrolyte systems. Currently, the common liquid electrolytes used in commercial LIBs are nonaqueous solutions, in which 1M LiPF6 salt is dissolved in a mixture of organic carbonates solvents, in particular, mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC), propylene carbonate (PC), diethyl carbonate (DEC), and/or ethyl methyl

2.8 Electrolytes and Separators

Table 2.1

Typical cyclic and linear carbonates in LIBs.

Name

Structure O

O

Ethylene carbonate (EC)

T m.p (∘ C)

T b.p (∘ C)

𝜺 (25 ∘ C)

36.4

248

89.78

−48.8

242

64.92

4.6

91

3.107

−74.3

126

2.805

−53

110

2.958

O O

H 3C

O O

Propylene carbonate (PC)

O

Dimethyl carbonate (DMC)

H3C

O

O

CH3

O

Diethyl carbonate (DEC)

H3C

O

O

CH3

O

Ethyl methyl carbonate (EMC)

H3C

O

O

CH3

Source: Xu [170].

carbonate (EMC). The ideal electrolyte solvent for LIBs should meet the following requirements [170]: (1) high dielectric constant: it can dissolve salts in sufficient concentration; (2) low viscosity to facilitate rapid ionic transport; (3) chemical inertness toward electrodes and other components (such as current collector); (4) low melting point and high boiling point for maintaining a liquid state within a wide temperature range; (5) high safety, nontoxic, and inexpensive. Table 2.1 shows the key physical properties of the cyclic and linear carbonates. The alkyl carbonates were chosen because they have acceptable anodic stability for the 4 V positives electrode used in LIBs and have other characteristics such as high polarity, reasonable temperature range between melting point and boiling points, low toxicity, and acceptable safety features. The ideal electrolyte salt should meet the following requirements [69]: (1) (2) (3) (4) (5)

complete dissolution and dissociation of Li+ and anions in the solvent; dissociated Li+ with high mobility; inert toward solvent and other components; good thermal stability; nontoxic and inexpensive.

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Although many electrolytes have been investigated, only LiPF6 is widely used in commercial LIBs, because other lithium salts have fatal drawbacks in large-scale applications: for example, LiBF4 has low conductivity in solvents and can affect the stability of the SEI on the surface of the negative. LiAsF6 is more expensive and toxic. While LiClO4 is highly oxidizing and may cause a safety hazard. LiCF3 SO3 is corrosive toward aluminum current collectors. Recently, a new electrolyte salt–LiBOB (LiB(C2 O4 )2 ) has attracted intense attention because LiBOB does not produce acidic decomposition products and possesses high thermal stability (up to 300 o C) and low cost compared with LiPF6 [52]. However, this electrolyte salt has a low solubility and is not suitable for LIBs using in a low temperature. Although conventional liquid LIBs offer the advantages of high conductivity, they often suffer from poor safety. Replacement of liquid electrolytes with a solid electrolyte will not only solve the safety concerns of liquid electrolytes, but also offer possibilities for developing new battery chemistries. Therefore, searching novel solid Table 2.2

Summary of lithium-ion solid electrolyte materials.

Type

Materials

Conductivity Advantages

Oxide

Perovskite

10−5 –10−3

● ●

Li3.3 La0.56 TiO3

●

Disadvantages

High stability ● Nonflexible High mechanical strength ● Expensive High oxidation voltage

LiTi2 (PO4 )3 Li14 Zn (GeO4 )4 Garnet Li7 La3 Zr2 O12 Sulfide

Li2 S-P2 S5

10−7 –10−3

● ●

Li2 S-P2 S5 -MSx

●

Hydride

Halide

Borate or phosphate

Thin film

LiBH4 , LiBH4 -LiX (X=Cl, Br or I), LiBH4 -LiNH2 , LiNH2 , Li3 AlH6 and Li2 NH

10−7 –10−4

● ● ●

LiI, spinel Li2 ZnI4 10−8 –10−5 and anti-perovskite Li3 OCl

●

Li2 B4 O7 , Li3 PO4 and Li2 O-B2 O3 -P2 O5

10−7 –10−6

●

LiPON

10−6

●

● ● ● ●

Polymer

PEO

10−4 65–78 ∘ C)

● ● ● ●

High conductivity Good mechanical performance Low resistance

●

Low grain-boundary resistance Stable with lithium metal Good mechanical performance

●

● ●

● ●

Low oxidation stability Sensitive to moisture Poor positive compatibility Sensitive to moisture Poor compatibility with positive materials

Stable with lithium metal Sensitive to moisture ● Low oxidation voltage Good mechanical performance ● Low conductivity Facile manufacturing process Good reproducibility Good durability Stable with lithium metal Stable with positive materials Stable with lithium metal Flexible Easy to produce Low shear modulus

LiPON, lithium phosphorus oxynitrides; PEO, polyoxyethylene. Source: Manthiram et al. [175].

●

Relatively low conductivity

●

Expensive

●

Limited thermal stability Low oxidation voltage

●

2.8 Electrolytes and Separators

Table 2.3 Density, ionic resistivity, and voltage window for various electrolytes.

Density (g cm−3 )

Resistivity (𝛀cm)

Cell voltage (V)

KOH

1.29

1.9

1.0

H2 SO4

1.20

1.35

1.0

Propylene carbonate

1.20

52

2.5–3.0

Acetonitrile

0.780

18

2.5–3.0

Ionic liquids

1.3–1.5 (25 ∘ C)

28 (100 ∘ C)

∼4.5

Electrolyte

Source: Burke [7].

electrolytes for LIBs has been a hot spot in recent years, and much progress has been made in this field [172–174]. Manthiram et al. [175] summarized the performance metrics of various solid-electrolyte LIBs systems, as listed in Table 2.2. Nowadays, the most widely investigated solid-state electrolytes systems are oxide, sulfide, and polymer electrolytes. Detailed discussion can be found in the reviews [175, 176]. Different from LIBs, ECs have complex electrolyte composition. Three types of electrolytes can be used in ECs: aqueous, organic, and ionic liquids. The main characteristics of various electrolytes for ECs are summarized in Table 2.3. H2 SO4 and KOH solutions are the most common aqueous electrolytes. Unfortunately, aqueous electrolytes usually show narrow voltage window (∼1 V), and increasing the voltage window will result in water decomposition. The commercial ECs also use organic electrolytes, such as PC and acetonitrile, because of their wide cell voltage (∼3 V). Room-temperature ionic liquids are another important electrolyte system. Ionic liquids can provide the widest voltage window (∼ 5 V) and without thermal or chemical instability. However, the main disadvantage of ionic liquids is the insufficient ionic conductivity compared to the other two types of electrolytes. Detailed discussion of electrolytes for ECs can be found in the excellent reviews [177, 178].

2.8.2

Separators

The separator is sandwiched between the electrodes, which is essential to prevent direct contact while transferring ions between the two electrodes. Although the separator itself does not participate in any reactions, it plays an important role in determining electrochemical performance and ensures safety. The most commonly used separators are microporous separators, which consist of a polymeric separator or a nonwoven fabric mat. The considerations on choosing suitable separators for LIBs and ECs are as follows [179]: (1) Chemical stability: The separator must have sufficient chemical stability to the electrolyte and electrode materials. (2) Mechanical properties: The mechanical strength of the separator must be sufficient to withstand the tension of the winding operation during assembly.

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(3) Thickness: Separators should have suitable thickness. Although thinner separators are suitable for higher energy and power density, it usually results in lower mechanical strength and lower safety. While thick separator usually results in higher mechanical strength; however, the permeability is relatively poor. (4) Porosity and pore size: Proper porosity is very important to meet the permeability requirements, because the permeability requirements can maintain enough liquid electrolyte to maintain the ionic conductivity between the electrodes. (5) Thermal stability and automatic shutdown protection: Thermal shrinkage should be reduced to a minimum, when the temperature rises to the softening temperature. Once thermal runaway occurs, the separator must also be able to shut down automatically. (6) Wettability: The separator should be wet easily in the electrolyte and keep the electrolyte permanently. (7) Size stability: A separator should preserve its original sizes when it is soaked with liquid electrolyte [180]. According to the structure and composition, the separators can be classified into three major types [180]: (i) microporous polymer, (ii) nonwoven fabric mats, (iii) composite. Among these separators, microporous polymer has been widely used in commercial LIBs and ECs. Generally, the microporous separators are made from a polymer, such as polypropylene (PP), polyethylene (PE), and their blends or multilayer forms. Currently, commercially available microporous polyolefin separators are compatible with the LIBs and ECs. These separators can be cycled hundreds of times without significant degradation. The multilayer structures of PP and PE microporous separators have been designed with self-shutting mechanism. Self-shutting process usually occurs near the melting temperature of the polymer and turns the porous polymer film into a nonporous insulating layer. The research on the structure–property relationship of the microporous polymer separators can be found in the work by Venugopal et al. [181, 182].

2.9 Evaluation Methods 2.9.1

Evaluation Criteria for LIBs

The pursuit of high-performance, affordable, and durable LIBs has been a long-lasting goal. The following four metrics for the evaluation of LIBs are as follows: (i) energy density, (ii) power density, (ii) cycle life, and (iv) safety [183].

2.9.2

Theoretical Gravimetric and Volumetric Energy Density

The chemical reactions involving different reactants where charge transfer occurs can be used for electrochemical energy storage [184]. The chemical reaction is as follows: αA + βB → γC + δD

(2.1)

2.9 Evaluation Methods

The coupling of these chemical reactions results in a number of combination reactions and thus the possible redox systems. The theoretical potential (E) of these analogue battery systems can be calculated as follows: E = −Δf GΘ ∕nF

(2.2) GΘ is

In this equation, Δf the Gibbs free energy of the reaction, which involves charge transfer under standard condition; in other words, maximum electrical work of the reaction; n is the electron transfer number in the reaction; and F is the Faraday constant. Generally, the energy density can be expressed by the gravimetric energy density Wh kg−1 or the volumetric energy density (Wh L−1 ). The gravimetric energy density can be calculated as follows: 𝜀M = Δf GΘ ∕ΣM

(2.3)

ΣM is the sum of formula mole weights of the reactant. The volumetric energy density (𝜀v ) can be calculated as follows: 𝜀V = Δf GΘ ∕ΣVM

(2.4)

ΣV M is the sum of the molecular formula of the reactant. The specific lithium storage capacity of the materials in a LIB can be calculated from the following equation: For LIB, the specific lithium storage capacity of the material can be calculated by the following formula: Capacity = nF∕3.6 M

(2.5) (g mol−1 ).

where M is the mole weight of the reactant The gravimetric energy density, volumetric energy density, and theoretical voltage can be obtained from thermodynamic data according to Eqs. (2.2)–(2.5). Taking typical Li–S system as the example, relevant parameters can be calculated as follows: 2Li + S ←→ Li2 S The overall Gibbs energy change is –439.0 kJ mol−1 , and molecular weights of lithium and sulfur are 6.94 and 32.07, respectively. According to Eqs. (2.3) and (2.4), theoretical gravimetric energy density of Li–S battery is 𝜀M = (439 × 1000)∕(2 × 6.941 + 32) = 9568 J g−1 𝜀M = 9568 J g−1 = 2654 Wh kg−1 as 1 Wh = 3600 J For volumetric energy density, because molar volumes of lithium and sulfur are 0.0130 and 0.0167 L mol−1 , 𝜀V = (439 × 1000)∕(2 × 0.013 + 0.0167) = 10 281 600 J g−1 = 10 281 600 J L−1 = 2856 WL−1

2.9.3

Practical Energy and Power Density of LIBs

The power density of LIB means how it works at a high current. In the case of EVs, the energy density determines its travel range, while the power density determines

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its acceleration speed. The high power density corresponds to fast-to-rapid energy storage, but usually leads to a compromise in energy density [185, 186]. Usually, the energy and power densities of LIBs were determined by galvanostatic charge/discharge curves. The galvanostatic charge/discharge technique applies a constant current density (e.g., A g−1 ) and characterizes the response potential with respect to time. Typically, the cell is charged to a preset potential and the discharge process is monitored to evaluate the capacitance. At a constant discharge current i, the total energy stored in a LIB is tcutoff

ΔE =

∫0

V(t)idt

where tcutoff is the time to reach the cutoff potential. The specific gravimetric energy density (Wh kg−1 ) or volumetric energy density (Wh L−1 ) can be obtained as follows: Gravimetric energy density = ΔE∕Weight Gravimetric energy density = ΔE∕Volume where weight and volume are the values of practical devices.

2.9.4

Cycle Life

The cycle (or service) life of rechargeable batteries is a very important metric to evaluate long-term stability for large-scale applications. Usually, the cycle life of LIBs was determined by galvanostatic charge/discharge techniques. The cycle life of LIBs is usually related to the electrode stability during repeated charge/discharge cycling [185]. The mechanical instability caused by volumetric expansion leads to rapid capacity decay due to particle fracture and electrical insulation of the active materials. For instance, silicon materials suffer from large volumetric expansions up to 300% during lithiation, which results in poor cycle life. Uncontrolled side reactions at the electrode/electrolyte interface have also been identified as a primary reason for the capacity fading [187]. The decomposition of the organic electrolyte leads to continuous irreversible consumption of Li+ , and the capacity gradually decreases [188]. Therefore, there is a need to develop and optimize electroactive materials and electrode structures to maintain the stability of the electrodes and thus improve durability.

2.9.5

Safety

LIBs have been widely used for electronic devices, such as power laptops and mobile phones due to their small size, lightweight, and high energy density, since Sony commercialized LIBs in the 1990s. Nowadays, LIBs can be manufactured in larger packs for EVs and large-scale applications. Nissan Leaf EV is equipped with a 30 kWh battery pack, BYD e6 EV is equipped with a 60 kWh battery pack, and TESLA model S EV is equipped with a 90 kWh battery pack. Although these high-energy LIB battery packs provide a longer travel distance, it is quite dangerous to store such a large

2.9 Evaluation Methods

amount of energy in a small space, and several incidents of EVs related to LIBs have been reported in recent years [189]. Three typical abuse conditions include mechanical damage, thermal abuse, and electrical abuse (overcharge/discharge and short-circuits), which may cause LIB failure [190–192]. These types of abuse usually occur at the same time and relevant mandatory standards can be found in several books [193–195].

2.9.6

Evaluation Methods for ECs

Similar to LIBs, there are also some metrics for the evaluation of ECs. Pan et al. [196] summarized performance evaluation methods for ECs, as shown in Figure 2.25. There are three essential parameters, working voltage V O , cell capacitance CT , and equivalent series resistance RES . The three parameters are used to assess their energy and power performance. The main influencing factors and the corresponding test methods are also shown in Figure 2.25. These parameters are measured by CV, constant current charge/discharge, and electrochemical impedance spectroscopy. Moreover to the aforementioned parameters, retention capability, cycling stability, efficiency, self-discharge, and time constant that provide insights into the performance are also summarized in Figure 2.25. In order to standardize the evaluation method of ECs, many attempts

Experimental setup

Electrode thickness

Electrode density

Active material

Packaging

Current collector

Mass loading

Additives

Cell configuration

CCCD plot

Electrolyte material

Specific capacitance

RES IR drop

Operating voltage

AC @ 1 kHz

CV curve

Nyquist plot

Time constant

Dwelling time

Max energy density

Max power density

Usable energy density

Usable power density

Cell performance

AC by interpolation

Cycling stability

Figure 2.25 Key performance metrics, test methods, major affecting factors for the evaluation of ECs . CCCD, constant current charge and discharge; CV, cyclic voltammetry; RES , resistance of equivalent series; IR drop, dynamic voltage drop; AC, alternative current. Source: Zhang and Pan [196].

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have been made, which can be found in many national and international standards and reviews [196].

2.10 Production Processes for the Fabrication This section aims to provide a brief overview for manufacturing of LIBs and ECs [197]. LIB or EC has similar fabrication processes, usually manufactured by the following major processes: (i) Mixing, coating, calendering, and slitting processes to fabricate electrodes; (ii) winding/stacking processes of the electrodes and separator; (iii) insertion of the cell core and injection of the electrolyte into the cell case; (iv) sealing the cell; (v) formation and aging.

2.10.1 Design Figure 2.26 shows the performance requirements and cell design parameters. Important cell parameters include electrode composition, coating/calendering density, the balance of electrodes, electrolyte, and metal foil used. These parameters determine the performance of the final cells. Main design criteria and procedures for a conventional cell are as follows and architecture design procedures include: (1) Based on requirements of the customer to design dimensions and capacity parameters; (2) Design sizes and connecting ways of metal tabs; (3) Design safety devices based on safety requirements, such as positive temperature coefficient (PTC) resistors.

Figure 2.26 Performance requirements and design of conventional cells

2.11 Perspectives

Electrode composite and size design process: (1) According to the performance requirements for the cell to choose the material. For example, high energy density LIBs generally choose layered high nickel content positive, while high safety requirement LIBs generally choose LiFePO4 positive; (2) Reasonable composition and size design of the electrode. For example, high rate cells have higher content of conductive carbon additives (3–10%), while high energy density LIBs usually have lower content of carbon additives (1–3%); (3) Usually, high rate cells need lower coating and calendering density, and vice versa.

2.10.2 Mixing, Coating, Calendering, and Winding In the electrode manufacturing process, the electrode material slurry is coated onto metal foils, then calendered. Typically, the electrode consists of: (i) active materials, such as LiCoO2 , LiMn2 O4 , LiFePO4 , and graphite for LIBs and activated carbon for ECs; (ii) conductive agents: such as acetylene black, graphite powder and CNTs; (iii) binder, such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVdF). The electrode slurries are uniformly coated onto both sides of the current collector. The slot die, reverse roll coating, or blade coating equipment can be used for coating operation. After coating, the dried electrode is calendered to control thickness and increase the density of the electrode. The calendered electrode is cut to the desired width for cell fabrication. Thereafter, the winding machine combines two electrodes and separator strip and winds the combination into a compact cell core. Finally, the cell core is fastened tightly before inserting it into the case.

2.10.3 Electrolyte Injecting and Formation After inserting the cell core into the cell case, liquid electrolyte is added to the cell and then vacuum filled to ensure that the electrolyte permeates. Then, the cell is subject to a formation process to select out cells. Specially for LIBs, once the cell assembly is complete, the cell must be undergo at least one to three precisely controlled charge/discharge cycles to form stable SEI films on the surface of graphite negative materials. These charging/discharging processes were usually conducted at lower current, which is called the formation process. Constructing stable and efficient SEI is one of the most effective strategies to achieve a superior cycling performance for LIBs. For ECs, specific formation at low currents is also required to achieve better electrolyte permeation, less side reactions, and stable interfaces.

2.11 Perspectives In the past few decades, great efforts have been made to develop high-performance LIBs/ECs and their materials. LIBs have been successfully used in many fields

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Figure 2.27 Development of future cells. Integration: cells combined with other energy harvesting devices; Miniaturization, cells with smaller dimensions; Intelligence, cells with smart functions; Transparence, transparent cells. Source: Wen et al. [198].

such as portable electronics and EVs. However, further improvement of the electrochemical performance of LIBs remains a challenge to meet the continuously increasing energy density and power requirements in energy storage applications. Compared with LIB, EC has higher power density, shorter charge time, good discharge/charge cycle ability, and wide temperature range applicability, but lower energy density compared with LIBs. Overall, the unique properties of ECs can usually make up for the deficiencies of other power sources. However, challenges still need to be solved to further improve the performance of carbon-based electrodes. Moreover, portable electronics have been used in every aspect of our daily life over the past decades. One of the key components of future portable devices is the compatible LIBs and ECs with an ultrahigh energy density and specific features (e.g. miniaturization, integration, flexibility, and smart functions). Figure 2.27 shows the technological trends and possible applications of future cells. Unfortunately, the future requirements cannot be satisfied by the conventional fabrication technology, and architectures of electrodes based on these materials have almost reached their limits. Therefore, for the coming era of portable electronics, there is an urgent need to reconsider how to rationally design and intelligently

References

fabricate advanced LIBs, ECs, and intelligent devices. Despite the need to develop novel materials, electrolytes, separators, therefore the construction of devices that meet desired standards, there is also a need to develop smart technologies to manufacture these electrochemical energy storage devices in an economically feasible and time-efficient manner. This book provides a comprehensive introduction to the innovative materials in the electrode and cell configurations of LIB and EC and the latest technologies used to implement these novel designs. This book will introduce these five kinds of newly developed LIBs/ECs subsequently. This includes (i) flexible cell; (ii) cells with bulk electrodes in the height of millimeter scale with ultrahigh energy density; (iii) miniaturized 3D electrodes, and (iv) smart cells. In this book, apart from the comprehensive discussion on the typical examples of research and development on these electrode designs, it also includes the important materials for constructing of these electrode. Because the new electrode/material configurations always involve new fabrication techniques, such as freeze-casting, printing, sputtering, and holographic patterning. Up to now, numerous fabrication technologies have been evaluated and great progress has been made to develop new cells. In this book, these contents will be discussed on a case-by-case basis. A brief discussion and an outlook on the current challenges, future research trends, and possible opportunities in this fascinating area will be provided.

References 1 Goodenough, J.B. and Park, K.S. (2013). The Li-ion rechargeable battery: a perspective. Journal of the American Chemical Society 135 (4): 1167–1176. 2 Choi, J.W. and Aurbach, D. (2016). Promise and reality of post-lithium-ion batteries with high energy densities. Nature Reviews Materials 1: 16013. 3 Yuan, L.X., Wang, Z.H., Zhang, W.X. et al. (2011). Development and challenges of LiFePO4 cathode material for lithium-ion batteries. Energy & Environmental Science 4 (2): 269–284. 4 Yu, Z.N., Tetard, L., Zhai, L. et al. (2015). Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy & Environmental Science 8 (3): 702–730. 5 Libich, J., Máca, J., Vondrák, J. et al. (2018). Supercapacitors: properties and applications. Journal of Energy Storage 17: 224–227. 6 Conway, B.E. and Conway, B.E. (1999). Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications. Germany: Springer. 7 Burke, A. (2007). R&D considerations for the performance and application of electrochemical capacitors. Electrochimica Acta 53 (3): 1083–1091. 8 Zhang, L.L. and Zhao, X.S. (2009). Carbon-based materials as supercapacitor electrodes. Chemical Society Reviews 38 (9): 2520–2531. 9 Sposito, G. and Sposito, G. (2016). Encyclopedia of Geochemistry: A Comprehensive Reference Source on the Chemistry of the Earth (ed. W.M. White), 1–6. Germany, Cham: Springer, Springer International Publishing.

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10 González, A., Goikolea, E., Barrena, J.A. et al. (2016). Review on supercapacitors: technologies and materials. Renewable and Sustainable Energy Reviews 58: 1189–1206. 11 Wang, Y.Z., Shan, X.Y., Ma, L.P. et al. (2019). A desolvated solid–solid interface for a high-capacitance electric double layer. Advanced Energy Materials 9 (12): 1803715. 12 Evanko, B., Boettcher, S.W., Yoo, S.J. et al. (2017). Redox-enhanced electrochemical capacitors: status, opportunity, and best practices for performance evaluation. ACS Energy Letters 2 (11): 2581–2590. 13 Yan, J., Wang, Q., Wei, T. et al. (2014). Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities. Advanced Energy Materials 4 (4): 201300816. 14 Conway, B.E., Birss, V., and Wojtowicz, J. (1997). The role and utilization of pseudocapacitance for energy storage by supercapacitors. Journal of Power Sources 66 (1-2): 1–14. 15 Simon, P., Gogotsi, Y., and Dunn, B. (2014). Where do batteries end and supercapacitors begin? Science 343 (6176): 1210–1211. 16 Costentin, C., Porter, T.R., and Savéant, J.-M. (2017). How do pseudocapacitors store energy? Theoretical analysis and experimental illustration. ACS Applied Materials & Interfaces 9 (10): 8649–8658. 17 Lukatskaya, M.R., Dunn, B., and Gogotsi, Y. (2016). Multidimensional materials and device architectures for future hybrid energy storage. Nature Communications 7: 12647. 18 Simon, P. and Gogotsi, Y. (2008). Materials for electrochemical capacitors. Nature Materials 7 (11): 845–854. 19 Brousse, T., Belanger, D., and Long, J.W. (2015). To be or not to be pseudocapacitive? Journal of the Electrochemical Society 162 (5): A5185–A5189. 20 Okubo, M., Hosono, E., Kim, J. et al. (2007). Nanosize effect on high-rate li-ion intercalation in LiCoO2 electrode. Journal of the American Chemical Society 129 (23): 7444–7452. 21 Naoi, K., Naoi, W., Aoyagi, S. et al. (2013). New generation “nanohybrid supercapacitor”. Accounts of Chemical Research 46 (5): 1075–1083. 22 Kim, H., Cho, M.-Y., Kim, M.-H. et al. (2013). A novel high-energy hybrid supercapacitor with an anatase TiO2 –reduced graphene oxide anode and an activated carbon cathode. Advanced Energy Materials 3 (11): 1500–1506. 23 Armand, M. and Tarascon, J.M. (2008). Building better batteries. Nature 451: 652–657. 24 Weng, Z., Li, F., Wang, D.W. et al. (2013). Controlled electrochemical charge injection to maximize the energy density of supercapacitors. Angewandte Chemie-International Edition 52 (13): 3722–3725. 25 Marom, R., Amalraj, S.F., Leifer, N. et al. (2011). A review of advanced and practical lithium battery materials. Journal of Materials Chemistry 21 (27): 9938–9954. 26 Goodenough, J.B. and Kim, Y. (2010). Challenges for rechargeable Li batteries. Chemistry of Materials 22 (3): 587–603.

References

27 Wang, L.L., Chen, B.B., Ma, J. et al. (2018). Reviving lithium cobalt oxide-based lithium secondary batteries-toward a higher energy density. Chemical Society Reviews 47 (17): 6505–6602. 28 Mizushima, K., Jones, P.C., Wiseman, P.J. et al. (1980). Lix CoO2 (0500 μm. LiCoO2 ||Li4 Ti5 O12 sintered electrode cells were assembled with total combined thickness of negative, separator, and positive of up to 2.90 mm. These LIBs have improved the stability and high areal capacities, as high as 45 mAh cm−2 capacity at 1.28 mAh cm−2 . To further improve the ionic transportation, sacrificed template can also be used in bulk electrodes during spark plasma sintering [41]. Figure 5.14 shows the fabrication procedure for porous sintered electrodes. LiFePO4 (or Li4 Ti5 O12 )/NaCl/carbon black was first mixed and pelleted, as shown in Figure 5.14a, b. The 1 mm-thick pellet was spark plasma sintered, as shown in Figure 5.14c, d. Finally, the obtained pellets were then subjected to dissolution of NaCl in the water to obtain the porous electrodes, as shown in Figure 5.14e–i. Compared with conventional dense sintered

Figure 5.14 Scheme showing the fabrication procedures of thick porous electrodes using NaCl as templates. SEM images of (a) NaCl crystals, (b) LiFePO4 particles, (c) 3D view, and (d) cross section of the LiFePO4 -NaCl-C pellet, the NaCl templates embedded in the electrodes, (e) and (f) fractured surface of LiFePO4 -C electrode showing porosity after dissolution of NaCl, (g) optical photo of LiFePO4 -C electrode, (h) and (i) show uniform pores throughout the electrode. Source: Elango et al. [41]. Reproduced with permission of Wiley.

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LiFePO4 bulk electrodes, NaCl templates created large porosity (40%) with relatively uniform pores. The bulk electrodes show a remarkable specific areal capacity (20 mAh cm−2 ), which is four times higher than that of conventional coated LIBs (5 mAh cm−2 ) [41]. Besides bulk electrodes, all-solid-state LIBs by inorganic electrolytes were also fabricated by the spark plasma sintering technology [38].. The as-prepared all-solid-state LIB has a surface capacity of 2.2 mAh cm−2 , showing an attractive electrochemical performance at 80 ∘ C. In this all-solid-state LIB configuration, Li1.5 Al0.5 Ge1.5 (PO4 )3 acted as the solid-state electrolyte. Li3 V2 (PO4 )3 was used as both the negative and positive electrodes to build symmetric Li3 V2 (PO4 )3 ||Li1.5 Al0.5 Ge1.5 (PO4 )3 ||Li3 V2 (PO4 )3 batteries. Experimental results showed that this symmetric LIB can be fabricated in a few minutes by the spark plasma sintering to get the electrodes and the electrolyte simultaneously. Prototype of the Li3 V2 (PO4 )3 ||Li1.5 Al0.5 Ge1.5 (PO4 )3 ||Li3 V2 (PO4 )3 symmetric LIB obtained by the spark plasma sintering technique is shown in Figure 5.15a, b. Figure 5.15c shows the interface between electrolytes and electrode materials. The galvanostatic (a)

(c)

(b)

(d) 2.5 2 Cell potential (V)

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C/50 till 1,75 V D/50 C/50 till 2,35 V D/25 C/25 till 2,45 V

0.5 0 0

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2.5

Figure 5.15 (a) and (b) Prototype of Li3 V2 (PO4 )3 ||Li1.5 Al0.5 Ge1.5 (PO4 )3 ||Li3 V2 (PO4 )3 symmetric LIB fabricated by the spark plasma sintering technique, (c) interface of electrolyte/electrode, (d) charge/discharge curves tested at 80 ∘ C for a symmetrical Li3 V2 (PO4 )3 ||Li1.5 Al0.5 Ge1.5 (PO4 )3 ||Li3 V2 (PO4 )3 prototype (electrode composition: 25 Li3 V2 (PO4 )3 /60 Li1.5 Al0.5 Ge1.5 (PO4 )3 /15 conductive). Source: Delaizir et al. [38]. Reproduced with permission of Wiley.

5.8 Thick Electrodes with Directional Pore Distribution

curves of the prototype at C/50 or C/25 rates are shown in Figure 5.15d. The sintered LIBs had a areal capacities of 1.0, 2.0, and 2.2 mAh cm2 , depending on the different voltage, for 1.75, 2.35, or 2.45 V, respectively [38].

5.7.4

Brief Summary for Sintering Technologies

Sintering has lots of advantages, which are as follows: (i) Sintering technology is simple in preparation and good in stability, and the quality control method is simple and easy. (ii) During the sintering process, grain boundary and lattice diffusion occur among particles. Sintered bulk electrodes have better mechanical strength; (iii) Layer structured all-solid-state LIBs using inorganic electrolytes can be easily fabricated by sintering technologies; (iv) Sintered bulk electrodes usually show obviously calendered density than other kinds of bulk electrodes. Although aforesaid sintered bulk electrodes showed significantly high gravimetric and volumetric energy density, this kind of thick electrode did not show satisfied rate performance. This is mainly caused by the absence of enough macropores to store adequate electrolyte and highly tortuous architecture. As stated before, these significantly restricted the performance exertion. Therefore, the thick electrodes with low tortuosity value is extremely important for bulk electrodes. In the following section, we will mainly introduce thick electrodes with directional pore distribution.

5.8 Thick Electrodes with Directional Pore Distribution Although significantly high energy density has been achieved by aforesaid sintered bulk electrodes showed, rate performance is still not good enough. This unsatisfied rate performance is mainly caused by the high tortuosity of sintered electrodes. When the electrodes show highly tortuous architecture, it significantly increases the path distance for the ions inside the electrodes [42]. Therefore, the thick electrodes with low tortuosity are important for bulk electrodes as stated before. Nowadays, iterative extrusion, magnetic induced alignment, wood-template, ice-template, and 3D-printing methods have been used to construct directional pores in thick electrodes.

5.8.1

Iterative Extrusion Method

The key for low tortuosity electrode is to form conductive pathways that are totally composed of straight channels, and these straight channels are also parallel to the transport direction. Until now, two different template-free strategies were developed, both are effective in creating directional porous architectures. One method is based on iterative co-extrusion processes, which is also a common ceramic fabrication method. Using this method, the green ceramic samples with complex hollow shapes can be fabricated [43, 44]. Figure 5.16 shows the typical iterative coextrusion process. The initial solid rectangular prism consists of inner “M-type” Al2 O3 -containing plastic extrusion body and outer carbon-black extrusion

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Carbon black Alumina

First feed rod

Figure 5.16 Working mechanism of iterative coextrusion process. Source: Van Hoy et al. [44].

First reduction First extrudate

Second feed rod

Second reduction Second extrudate

body, and the initial prism is 10 mm width. By using a 5 : 1 die, the 250 mm long little “M” was fabricated and its width is 2 mm. The obtained 250 mm long extrusion body is cut into 25 pieces and reassembled. This reassembled feed rod has 25 objects. The second-stage extrusion with 5 : 1 die again formed a 2 mm-width prism, 250 mm long, but with cross section consisted of 25 “M” shapes, each 400 μm width. After several coextrusions, lots of microscopic objects can be fabricated, which is appropriate for building multilayer or porous architectures [44]. Using an iterative coextrusion and sintering method, the thick electrode was developed by minimizing electrode tortuosity, [45]. which maximizes power density at the given pore fraction and thickness. Compared with conventional calendered electrodes, the thick electrodes show approximately three times higher capacity per unit area (mAh cm−2 ) at 1C and 2C rates. Figure 5.17a shows the fabrication process of thick electrodes with controlled directional pores. Each iterative co-extrusion process reduces cross section of the electrode by constant factor, while keep the overall electrode density. After three extrusion processes and followed by sintering, the obtained electrodes have paralleled channels of ∼6 μm diameter with center spacing of 𝜆 ∼ 17 μm. The GEN 3 architecture is shown in Figure 5.17b. Figure 5.17c, d shows the electrochemical performance of 220 μm-sintered electrodes with directional porous architectures. At C/15 low discharge rate, the polarization and discharge capacity of all generation samples are almost identical. At high rates, it is clear that Gen 3 sintered electrode with low parallel porous channel spacing showed better capacity retention over the Gen 0–GEN 2 samples.

5.8 Thick Electrodes with Directional Pore Distribution

Figure 5.17 (a) Scheme of the electrode fabricated by iterative extrusion methods. Lower rows are images of the electrodes with different iterative extrusion times. Gen 0, Gen 1, Gen 2, and Gen 3 correspond to the initial feed rod and the assembled architectures after different iterative extrusion times, respectively, (b) SEM images of the Gen 3 electrode fabricated by third iterative coextrusion process, (c) relationship between specific capacity and discharge rates, and (d) discharge curves for electrodes with overall density of 62%. Source: Bae et al. [45]. Reproduced with permission of Wiley.

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5.8.2

Magnetic-Induced Alignment Method

Magnetic-induced alignment is the emerging and interesting method to introduce low tortuosity in electrodes. By external magnetic field during fabrication, the electrodes within align active materials were obtained. The alignment gives Li+ the highway to transit inside the electrodes and improves rate performance under high thickness. For example, Billaud et al. [42] developed an external magnetic method to align graphite flakes in the electrodes. Under high active materials loading (∼10 mg cm−2 ) and thickness (∼200 μm), the electrodes still showed satisfied rate performance. Figure 5.18 illustrates the fabrication mechanism of thick electrodes with aligned graphite flakes. As shown in Figure 5.18a, magnetic Fe3 O4 materials were uniformly coated on the outer of graphite flakes. Under strong external magnetic fields, these graphite materials can form flakes alignment during slurry casting. Most aligned graphite flakes are also vertical to the Cu foil. Unlike randomly calendered graphite flakes, these aligned graphite flakes formed the open Li+ diffusion path, which is vertical to the Cu current collectors (Figure 5.18b). Therefore, these electrodes showed significantly improved electrochemical performance. At 0.1 rate, both electrodes show almost identical voltage plateau and specific capacity and are ∼350 mAh g−1 . When being discharged at 1C, the electrodes with oriented graphite flakes had up to three times higher specific capacity and much lower over-potential, as shown in Figure 5.18c. Such good rate performance is attributed to the alignment of graphite flakes perpendicular to the Cu foil obviously decreasing the tortuosity of the Li+ transportation. The directional transportation pathways lead to higher charge transport kinetics and enable thick electrodes with more than 3 times higher area capacity (12, and 4 mAh cm−2 in conventional electrodes). Magnetic aligned nylon rods or emulsion droplets were also acted as sacrificial phases to form preferentially oriented pores in thick LiCoO2 electrodes [20]. As in Figure 5.19a, b, the external magnetic field causes parallel alignment of the magnetic nylon rods and magnetic emulsion droplets. Followed the subsequent sintering, the sacrificial nylon rods or emulsion droplets left directional pores in LiCoO2 electrodes (Figure 5.19c, d). Compared with calendered electrodes, the directional pores resulted in higher rates and enable electrodes with higher area capacity. As shown in Figure 5.19e, f, all these samples showed almost identical areal capacity at low rates. While at rates higher than 1C, two methods that prepared thick electrodes showed obviously better areal capacity. At 1C, the areal capacity can reach 8 mAh cm−2 , which is three times higher than that of conventional electrodes [20].

5.8.3

Carbonized Wood Template Method

Template method provides the novel way for fabricating various controllable and directional porous bulk materials, and recently, it has become an interesting topic in the design of advanced electrodes. Among various templates, carbonized wood can be considered as an effective template, which shows a kind of unique multi channeled anisotropic structures [46]. When using carbonized wood as templates, the

5.8 Thick Electrodes with Directional Pore Distribution

(a)

(b)

(c) Potential (V vs. Li+/Li)

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Reference aligned

Rate: C

Rate: C/10

1.0

0.5

0.0 0

100 200 300 400 Specific charge (mAh g–1)

0

50 100 150 Specific charge (mAh g–1)

Figure 5.18 Fabrication of bulk graphite electrodes with oriented flakes. (a) Graphite flakes coated with Fe3 O4 particles, (b) coating of graphite electrodes: without and with external magnetic field, (c) scheme shows the Li+ transportation pathways in both electrode architectures, (d) galvanostatic charge/discharge curves at 0.1C and 1C of aligned and conventional electrodes. Source: Billaud et al. [42]. Reproduced with permission of Springer Nature.

slurry consisted of active materials for cells was infiltrated into the hollow channels of the wood and carbonized to obtain the thick electrodes. Carbonized wood has many unique advantages as follows: (i) carbonized wood are hydrophilic, which is favorable for infiltrating aqueous slurry into hollow channels of templates, (ii) has lightweight, (iii) better intrinsic electronic conductivity, (iv) highly directional architectures and has full openly straight channels, which can provide high ionic transportation [47]. Therefore, carbonized wood can be used directly as self-standing and thick electrodes templates. By carbonized wood as sacrificial templates, Chen et al. [46] developed the high thick, lightweight and low-tortuosity 3D metal-free current collector. Figure 5.20a shows the fabrication procedure. First, aqueous LiFePO4 slurry was infiltrated into

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Suspension

(a)

Sintering

Consolidation

LiCoO2 suspension

Magnetized nylon rods

(b)

Magnetic field on Sintered electrode with aligned porosity

LiCoO2 suspension

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1 C-rate

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Figure 5.19 Thick electrodes construction using magnetic sacrificial aligned phases. (a) Magnetized nylon rods and (b) magnetic emulsion droplet. The magnetic field causes directional alignment of the nylon rods and emulsion droplets, (c) cross section of LiCoO2 electrodes by magnetic nylon microrods as sacrificial phases, (d) cross section of LiCoO2 electrodes using magnetic chained emulsions, (e) and (f) relationship between areal capacity and rates of a 310 μm-thick electrodes prepared by magnetic nylon rods (e) and magnetic emulsion droplets (f) as templates. Source: Sander et al. [20]. Reproduced with permission of Springer Nature.

the hollow channel of the nature wood and then carbonized to obtain the thick electrodes. As shown in Figure 5.20b, vertical and hollow channels in the architecture enable enough high materials loading and rate performance simultaneously. This ultrathick LiFePO4 electrode has large thickness of 800 μm, which active material loading is 60 mg cm−2 . The final specific areal capacity is 7.6 mAh cm−2 . Due to

5.8 Thick Electrodes with Directional Pore Distribution

Li+

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Infiltration Carbonization

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~1 mm ~15 μm ~15 μm

Ultrathick 3D electrode design

Conventional design

More electroactive materials Less dead materials Higher energy density

Less electroactive materials More dead materials Lower energy density

Fast ion/electron transport LFP nanoparticle Carbon matrix

Figure 5.20 (a) Design principle of ultrathick electrode using the carbonized wood as current collectors and (b) comparison between thick and conventional electrodes. Source: Chen et al. [46].

the confinement function of channels of carbonized wood, these thick electrodes also have longer cycling performance and excellent mechanical strength compared with conventional electrodes. In another work, LiCoO2 was also introduced to carbonized wood templates to replace LiFePO4 , as shown in Figure 5.21a–c [23]. When replaced coating electrodes of commercial LIB, vertical channels in the LiCoO2 thick electrodes also enable their higher energy density, which almost reaches ∼360 Wh kg−1 (Figure 5.21d, e). Carbonized wood also acted as the template to contain pseudocapacitance materials for high energy density ECs. Figure 5.22 shows the fabrication scheme of a wood-based high energy density ECs. One slab of wood was washed with HCl solution and carbonized at 1000 ∘ C to obtain the template with directional pores. Co(OH)2 nanoflakes were in situ electrodeposited inside the channels of carbonized carbon. Finally, two electrodes and polyvinyl alcohol (PVA)/KOH gel electrolytes were stacked to fabricate the devices. The ECs show high volumetric capacitance of 14.19 F cm−3 and energy density of 4.45 Wh l−1 , respectively, while maintaining a long cycle life of 10 000 cycles at 1.0 A g−1 . These results are far better than that of conventional coated electrodes [47].

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(a)

(b)

Long Li+ Poor electrolyte diffusion Li+ Li+ transport path

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Short Li+ Good electrolyte diffusion Li+ Li+ transport path

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Figure 5.21 Carbonized wood templates for ultrathick electrodes. (a) Wood and the wood anisotropic structure for the transportation of water, (b) conventional electrode with poor ionic diffusion and long Li+ transportation path, (c) thick electrode with vertical channels, (d) comparison between commercial electrode and thick electrode, and (e) relationship between thickness ratio and energy density of LiCoO2 -graphite cell. Source: Lu et al. [23]. Reproduced with permission of Wiley.

Electrodeposition

Carbonization

Natural wood

Carbonized wood

Cellulose paper (separator)

Co(OH)2@CW

Co(OH)2@CW (positive electrode)

Gel electrolyte

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Carbonized wood (negative electrode)

Figure 5.22 Fabrication of a wood-derived all-solid-state ECs. Source: Wang et al. [47]. Reproduced with permission of Wiley.

5.8.4

Ice Templates Method

Besides wood templates, the novel ice directional freezing process without post-sintering was developed to manufacture 900 μm thick positives with aligned pore arrays along the Li+ transport direction [48]. Directional freezing method is an interesting technology, which allows liquid water to freeze into ice from only one direction (usually one side of a container), so that it can be used as template in constructing porous electrode. Figure 5.23 shows the optical image of a directional ice, whose frozen direction can be clearly seen. As shown in Figure 5.24a, the homogeneously mixed aqueous suspension consisted of carbon black, LiCoO2 , and sodium carboxymethyl cellulose was directionally and quickly frozen by Cu cold rod; one end of this cold Cu cold rod was

5.8 Thick Electrodes with Directional Pore Distribution

Figure 5.23

A directional frozen ice.

immersed in liquid nitrogen. As shown in Figure 5.24b, c, directional ice crystals predominantly formed due to the steep temperature gradient. Finally, the highly directional ice dendrites may act as the sacrificial templates to form align pores for promoting Li+ transportation. Compared with the electrodes obtained by isotropic ice template or slurry coating, the reversible specific capacity of 900 μm thick electrode by directional frozen method was 142 mAh g−1 , which is almost identical to the conventional coated thin-film electrodes (Figure 5.24d). More importantly, directional frozen obtained thick electrodes showed a much higher areal capacity of about 14 mAh cm−2 than that of thin-film electrodes (Figure 5.24e). Similar to other electrodes with low tortuosity, the avoidance of sintering keeps submicron pores inside electrodes that promoted a relatively high electrode/electrolyte interfacial area and ensured efficient Li+ transportation [48].

5.8.5

3D-Printing for Thick Electrodes

3D printing is an innovative advanced additive manufacturing process, and it has recently attracted much interest in the energy storage field due to its capability to build complex 3D architectures. By 3D-printing, thick electrodes can be easily obtained by printing multiple layers and changing the nozzle and speed [49–51]. Compared with aforesaid thick electrodes using sintering technologies, the thick

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(a)

(c)

(b)

(d)

(e)

Figure 5.24 Illustration of the (a) directional frozen apparatus, (b) directional frozen process, (c) the thick LiCoO2 electrode made by directional frozen process, (d-e) electrochemical performance of various LiCoO2 electrodes at 0.1C, (d) specific capacities, and (e) areal capacities. Source: Huang et al. [48]. Reproduced with permission of Royal Society of Chemistry. (a)

(e)

(i)

(b)

(f)

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0.5 mm

0.5 mm

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Figure 5.25 3D-printed electrodes: (a) and (b) circle-grid pattern; (e) and (f) circle-ring pattern; (i) and (j) circle-line pattern. Source: Wang et al. [52]. Reproduced with permission of American Chemical Society.

5.8 Thick Electrodes with Directional Pore Distribution

electrodes obtained by 3D-printing have three advantages: (i) 3D-printing can build a lot of macropores, and these macropores could increase the surface area of the electrode and facilitate electrolyte penetration of active materials, (ii) electrode is composed of the filament, and it has shorter Li+ diffusion length, and (iii) electrode has better mechanical strength [52]. For example, the extrusion-based 3D-printing process was used to obtain a 3D patterned thick electrodes [52]. As a result, an ultrathick (1500 μm, eight layers) patterned electrode was fabricated, which has high areal capacity (∼7.5 mAh cm−2 ). Besides high areal capacity, these patterned electrodes also have complex shapes, as showed in Figure 5.25. Electrochemical characterization showed that the line-patterned electrode (Figure 5.25i, j) has much lower over potential at various cycling rates due to its smaller gaps between printing layers, which facilitates the ion and electron transport, particularly at high current densities. Besides planar printed bulk electrodes, bulk electrodes with interdigitated configuration can also be fabricated by 3D printing. This kind of bulk electrodes usually has higher power performance, which was caused by ionic parallel diffusion in the in-plane architecture design. For example, LiFePO4 ||Li4 Ti5 O12 bulk electrodes were fabricated by 3D-printing LiFePO4 and Li4 Ti5 O12 ink onto gold current collectors (Figure 5.26a) with a thickness of about 500 μm (Figure 5.26b). The eight-layer LiFePO4 ||Li4 Ti5 O12 LIB demonstrated an areal capacity of 1.5 mAh cm−2 at 1C (Figure 5.26c) [53]. In the summary, it is worth noting that the 3D-printing manufacture technique is more suitable for the preparation of prototype devices or laboratory-scale research due to less scalable nature compared to current industrial manufacture. It is expected that the future growth of additive manufacturing techniques will offer many opportunities for fast and large-scale manufacturing of thick electrodes at cost-effective price [7].

5.8.6

Brief Summary for Bulk Electrodes

In this section, we reviewed the fabrication of bulk electrodes with or without directional pores. These methods are primarily used for high energy density LIBs. Sintering technologies were mainly used for the fabrication of bulk electrodes without directional pores. The final sintered electrodes show high thickness and gravimetric/volumetric energy density. However, due to high tortuosity, sintered bulk electrodes usually do not show satisfied power performances. Compared with sintering technology, directional pores enable thick electrode with less tortuosity and increase the rate performance of thick electrodes without compromising the gravimetric/volumetric energy density. Until now, many methods were used to fabricate thick electrodes with directional porous architecture, which includes: (i) iterative extrusion method, (ii) magnetic-induced alignment method, (iii) carbonized wood templates method, (iv) ice templates method, and (v) 3D-printing process. Figure 5.27 compares different technologies for the fabrication

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(a)

Nozzle (30 μm)

Current collector (Au)

LTO

Glass Packaging

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Figure 5.26 (a) 3D-printing of Li4 Ti5 O12 and LiFePO4 electrodes, (b) SEM image of the 3D-printed electrodes. (c) Relationship between areal capacity and voltage of the eight-layer printed LiFePO4 ||Li4 Ti5 O12 cell at different rates. Source: Sun et al. [53]. Reproduced with permission of Wiley.

of bulk electrodes. Performance of various bulk electrodes can be evaluated based on five factors, which are as follows: (1) Thickness of electrodes: Experimental results show that these six methods can all produce electrodes at millimeter level. These thick electrodes also have areal capacity up to ∼10 mAh cm2 , which is about several times higher than that of conventional electrode of LIBs. Among these six methods, iterative extrusion, carbonized wood templates, and sintering technologies can produce the thickest bulk electrodes than other methods produced bulk electrodes. Magnetic induced alignment and ice template methods are usually based on aqueous materials slurry, which hinder the thickness improvement. Moreover, 3D-printing techniques also offer possibility to produce thick electrodes. However, it

5.8 Thick Electrodes with Directional Pore Distribution

(2)

(3)

(4)

(5)

must be noted that 3D-printing cannot provide high enough gravimetric/ volumetric energy density due to the macro pores inside the bulk electrodes. Ease of fabrication: Among these six methods, sintering technology is the simplest method for the preparation of bulk electrodes. More efforts should be conducted to reduce the cost and complexity of other methods. Electronic conductivity: Due to their high conductivity, carbonized wood templates-derived bulk electrodes usually show highest electronic conductivity, which is favorable for electrochemical performance improvement of bulk electrodes. While sintered bulk electrodes usually show poor conductivity due to the absence of conductive carbon in sintering. Calendered density: Iterative extrusion, carbonized wood template, and conventional sintering techniques usually undergo a high-temperature process. So, grain boundary and lattice diffusion occur among particles during the fabrication. Therefore, these methods that produce bulk electrodes may show higher calendered density. Directional pores: Due to the existence of highly directional pores inside electrodes, iterative extrusion, 3D-printing, and carbonized wood template method (a)

Thickness of electrodes

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Figure 5.27 Comparison between different technologies for the fabrication of bulk electrodes. (a) Iterative extrusion, (b) magnetic-induced alignment, (c) carbonized wood template, (d) ice template, (e) 3D-printing, and (f) sintering technology.

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can produce bulk electrodes with low tortuosity, while conventional sintering usually has tortuous holes inside electrodes.

5.9 Carbon-Based Foam Electrodes with High Gravimetric Energy Density In the aforesaid section, we mainly focused on the design and fabrication of thick electrodes, which is achieved by sintering and template methods. These obtained electrodes usually possess high gravimetric and volumetric energy density simultaneously. Even though delicate porous structure has been obtained by different electrode fabrication processes to improve the kinetics in thick bulk electrodes, the result has not matched the researchers’ satisfaction. Therefore, in this section, we will review another important architecture of thick electrode, 3D foam electrodes, which has a much faster kinetics than that of thick bulk electrode. This kind of architecture usually show high specific capacity, thickness, and mass fraction in electrode, while usually has lower calendered density. Therefore, these 3D porous electrodes show high gravimetric energy density and power density, while usually possess low volumetric energy density due to low calendering density. In this section, we will summarize the design and synthesis of 3D foam electrodes for the cells. As stated before, for thick electrode with high active materials loading, only a portion of active materials is involved in electrochemical reactions due to the poor electronic/ionic transportation, as shown in Figure 5.28a [55–57]. The 3D electrode architecture contains 3D conductive network, which can provide highly efficient ionic/electronic transportation pathway (Figure 5.26b) and is useful for the utilization of active materials [54]. In the next section, we will introduce main 3D continuous conductive networks and their applications in the cells. These 3D conductive networks mainly include: (i) conductive carbon network, such as graphene and CNT framework and (ii) conductive polymer networks. Among these frameworks, carbon-based foam electrodes Li+

Li+

Li+

Li+

Li+

Li+

Li+

Li+

Li+

Electrolyte

Electrode material e–

e–

e–

e–

e–

e–

Current collector

e–

e–

e–

e–

e–

Figure 5.28 (a) Conventional 2D planar electrodes show poor electronic and ionic diffusion pathways; (b) charge transportation pathways in the 3D thick electrode show continuous electronic/ionic transportation across the entire electrode thickness. Source: Sun et al. [54].

5.9 Carbon-Based Foam Electrodes with High Gravimetric Energy Density

possess excellent conductivity and higher active materials loading, which is more suitable for bulk electrodes with high gravimetric energy density.

5.9.1

Graphene Foam

Graphene, the rising star in material field, has many outstanding properties and has attracted enormous amount of research. Graphene is a monolayer of carbon atoms, tightly bounded in a hexagonal honeycomb lattice. Graphene is one of the allotropes of carbon in the form of a plane of sp2 -bonded atoms with a molecular bond length of 0.142 nm. Its outstanding properties include high surface area (∼2630 m2 g−1 ), high porosity (up to 99.8%), excellent electrical conductivity, mechanical flexibility, and extraordinary chemical stability. And because of the properties mentioned above, the applications of graphene in the cells have attracted more and more attentions. As a new form of graphene, graphene foam combines the unique mechanical and electrical properties of two-dimensional graphene, yielding the light, highly conductive bulk material with excellent strength and flexibility. Generally, graphene foam can be synthesized via (i) chemical vapor deposition (CVD) method and (ii) self-assembly processes. Graphene foam can be synthesized by the chemical vapor deposition method using the metallic template, such as Ni foam. As an example, Chen et al. [58] reported the direct synthesis of graphene foam architecture by chemical vapor deposition using commercial nickel foam as templates. After removing the Ni template, self-standing graphene foam with tuned properties and morphologies can be obtained. The graphene foam is composed of few layers of graphene. Its specific weight is ∼0.6 mg cm−2 , and its porosity are about 99.7%. More importantly, the electrical conductivity of the graphene foam is ∼6 orders of magnitude higher than chemically derived graphene-based materials. Although Ni foam is the most common commercial templates, it still has large pore diameter (50–100 μm) and cannot be easily tuned. Therefore, besides Ni foam templates, Cu foam [59], Cu–Ni alloy foam [60], and anodized aluminum oxide (AAO) [61] with novel morphology and tunable size/pore diameter have also been explored as templates for graphene foams. Another important way to obtain graphene foam is the self-assembly of GO. In the typical procedure, GO is first dispersed in the solution and treated by a series of procedures, involving (i) gelation, (ii) reduction processes, and (iii) special drying techniques; then, 3D graphene foam is finally obtained [62]. Until now, many different gelation and reduction methods have been used on this strategy, including chemical electrochemical and hydrothermal reduction [63], flow-directed assembly [64], evaporation-induced self-assembly [65], Langmuir–Blodgett technique [66], and layer-by-layer assembly [67]. Generally, after gelation and reduction of GO dispersion, special drying process should be used to remove water and organic molecules. Among different drying methods, freeze-drying and supercritical drying are the most effective methods to remove water without destroying the GO framework [62]. The as-prepared graphene foam usually showed excellent super-elasticity, high compressibility, and

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5 Architectures Design for Cells with High Energy Density

(a)

(b)

(c)

(d)

Figure 5.29 Design and compressive elasticity of graphene foam. (a) Fabrication of carbon–graphene foam, (b) SEM image of carbon–graphene foam. Scale bar, 100 mm, (c) TEM image shows that the lamella consisted of graphene and amorphous carbon, Scale bar, 2 nm. (d) Stress–strain curves of carbon–graphene foam. Source: Gao et al. [68]. Reproduced with permission of Springer Nature.

superior fatigue resistance simultaneously. Moreover, microscale structures were also introduced into graphene networks to further enhance mechanical properties. For example, Gao et al. [68] synthesized the graphene foam with an unique hierarchical porous, as shown in Figure 5.29. It was obtained by bidirectional freezing process to obtain a chitosan–GO scaffold composed of parallel flat lamellas with long-range alignment. By subsequent annealing, the flat lamellas crumbled into the waved multi-arch morphology. Derived from the designed unique lamellar multi-arch microstructure, the obtained monolithic carbon material exhibits spring-like super-elasticity, high compressibility, and superior fatigue resistance simultaneously. Fabrication procedures, TEM morphologies, and its mechanical performances are shown in Figure 5.29b–d. Detailed discussion and introduction about the fabrication of graphene foam can be found in several excellent reviews [69–73]. Graphene has many outstanding properties. For example, CVD-derived graphene consists of 3D interconnected network of high-quality graphene layers and the electrical conductivity of the CVD graphene foam is as high as ∼1000 S m−1 . Moreover, various kinds of graphene foam are extremely light (∼0.1 mg cm−2 ). It possesses high porosity of ∼99.7% and high specific surface area. Therefore, compared with conventional bulk materials, graphene foam is the ideal platform to build foam electrodes, which can be used as the fast transport channel for charge carriers to improve the kinetics of electrodes [72].

5.9 Carbon-Based Foam Electrodes with High Gravimetric Energy Density

5.9.2

CNTs Foam

Several recent excellent papers reviewed the fabrication and application of CNT foams [74–76]. The main procedures for fabricating CNTs foam are similar to the graphene foam: the first step is the preparation of aqueous or non-aqueous CNT gels. Then, by freezing or supercritical drying, CNT foams can be obtained [74]. To make CNT gels, uniformly dispersed CNT is the prerequisite. However, CNT is difficult to disperse in most solvent. Therefore, lots of dispersants have been often used to effectively disperse CNT [75, 77]. These polymers aid the dispersion of CNT and act as structural binders to hold the foams formed after drying. Thermal annealing can significantly improve their electrical and mechanical properties and increase their surface area and porosity [78]. CVD method is another effective method for the fabrication of CNT foam. For example, Hata et al [79] used an efficient CVD method to fabricate CNT forest, which can be seen as CNT foam. Gui et al. obtained a sponge-like CNT consisting of self-assembled, interconnected CNT skeletons, with high porosity (>99%) and structural flexibility (Figure 5.30b) [80]. In another work, water-stimulated enhanced catalytic activity also leads to massive growth of vertically aligned CNT with heights up to 2.5 mm [81].

5.9.3

CNT/Graphene Foam

Although graphene and CNT foams usually have similar fabrication methods, there are obvious differences between them as follows [74]: (i) GO usually shows excellent gelation capability than CNT due to their rich surface functional groups, which enables graphene foams ease of preparation and excellent mechanical performance; (ii) CNT foam was usually more conductive than graphene foam. CNT and graphene can be synergistically hybridized in the simultaneous procedure

Figure 5.30 CNT foam bent at a large angle. Source: Gui et al. [80]. Reproduced with permission of Wiley.

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5 Architectures Design for Cells with High Energy Density

to form CNT/graphene materials. This hybrid architecture has the advantage of both CNT and graphene [74]. Similar to their individual components, the CNT/graphene foam can also be fabricated by CVD templates and special drying method. For instance, ∼1.0 mm thick CNT/graphene foam was synthesized by two-step CVD method [82]. First, graphene foam was synthesized by Ni foam as templates. CNTs were grown on graphene foam by the second CVD process. Finally, the Ni templates were etched by HCl solution to obtain the CNT/graphene foams. CNT/graphene foam hydrogel precursors were also dried by supercritical CO2 to obtain the CNT/graphene hybrid aerogel [83]. The resultant CNT/graphene foam has an excellent electrical conductivity of 7.5 S m−1 . An ultralight and highly compressible CNT/graphene foam was also fabricated by a freeze-drying method [84]. First, CNT and GO were dispersed in aqueous solution, followed by freeze-drying and hydrazine reduction. The as-prepared CNT/graphene foam exhibits an interconnected, porous framework with crinkly sheets. Compared with their individual components, CNT/graphene foam provide better compressibility and conductivity. CNT/graphene foam usually has a complete recovery ability after 50–82% compression [84]. In this hybrid architecture, bridges between the CNT and graphene prevent the sliding of graphene. Moreover, small amount of CNT in this foam is enough to obtain excellent reversible compressibility, which can totally recover after compression at 𝜀 ≤ 63% for 1000 times [84]. In summary, CNT/graphene foam can provide unique advantages over the individual CNT and graphene. These excellent properties enable their implementation as substrates for foam electrodes of the cells.

5.10 Carbon-Based Thick Electrodes As stated before, CNTs, graphene, and CNT/graphene foam shows outstanding electronic conductivity and highly porous structure. This unique architecture provides fast ions and electrons transportation speed, which is ideal for the thick electrodes. In this section, we give a brief introduction toward applications of graphene and CNT foams in thick electrodes. Table 5.3 summarized applications of the conductive network, mainly include graphene foam and CNT foam for the cells with high active materials loading. As shown in Table 5.4, current CNT/graphene foam electrodes are prepared by several methods: the drop-casting method, in situ deposition, and hydrothermal methods. Compared with the former, in situ deposition and hydrothermal methods provide higher materials loading and stronger interfacial strength, which is an ideal method to form thick electrodes.

5.10.1 Low Electronic Conductive Material/Carbon Foam One of the primary advantages of CNT/graphene foam is their super electronic and ionic conductivity because the graphene or CNT foams have both highly conductive pathway and interconnected networks for electrons/ions. Meanwhile,

Table 5.3

Application of graphene foam and CNT foam for the cells with high active materials loading.

Active materials

Synthesis and content of graphene or CNT foams

Fabrication method of the electrode

LiFePO4

CVD, 12%, GF

In situ hydrothermal growth

CVD, 20%, GF

In situ deposition

Li4 Ti5 O12

Rate performance

Areal performance (mAh cm−2 )

Volumetric performance (mAh cm−3 )

References

75 mAh g−1 , at 200C

/

/

[85]

/

/

[86]

/

[87]

135 mAh g−1 , at 200C Fe3 O4

190 mAh g−1 @60C 350 mAh g−1 @10C

MoS2

CVD, 15%, GF

P123-assisted solution-phase

1235.3 mAh g−1 @200 mA g−1

/

LiFePO4

CVD, 1.5%, GF

Drop-casting onto graphene foam

36 mAh g−1 @2540 mA g−1

12 mg cm−2 , 2 mAh cm−2

Silicon

Self-assembly, 15.1%, GF Drop-casting onto graphene foam

983 mAh g−1

1.5 mg cm−2

1016

[89]

V2 O5 /PEDOT

CVD, GF

265 mAh g−1 at 5C, 168 mAh g−1 at 60C

1.5∼2 mg cm−2

/

[90]

Co3 O4

Self-assembly, 24.7%, GF In situ growth

1000 mAh g−1 @50 mA g−1

5–8 mg cm−2

[91]

Fe3 O4

Self-assembly, ∼50 %, GF Hydrothermal growth 400 mAh g−1 @150mA g−1

15 mg cm−2

[92]

In situ growth

[88]

900 mAh g−1 @4800 mA g−1 177 mAh g−1

3.9 mAh cm−2

272

[93]

Hydrothermal growth 830 mAh g−1 @100 mA g−1

/

/

[95]

790 mAh g−1 @37.2mA g−1

/

/

[96]

/

/

[98]

0.60 mg cm−2

/

[99]

Nb2 O5

Self-assembly, ∼30%, GF Self-assembly

Fe2 O3

Self-assembly, 24.7%, GF Hydrothermal growth 995 mAh g−1

SnO2

Self-assembly, 70%, GF

[94]

/

CNT foam

/

MoS2

CNT foam, 34%

Hydrothermal growth 935 mAh g−1 @100 mA g−1

Co3 O4

CNT foam, 80.6%

In situ thermolysis

MnO2

CVD CNT foam, 50%

Hydrothermal growth 1250 mAh g−1 @400 mA g−1

Fe2 O3

CNT-GF foam, 70%

500 mAh g−1 @500mA g−1 −1

−1

Solvothermal growth 1000 mAh g @200 mA g

[97]

0.65 mg cm−2

[100]

5 Architectures Design for Cells with High Energy Density

CVD-derived graphene foam or CNT foam usually shows excellent electrochemical stability compared with widely used metal foils. Therefore, CNT/graphene foam can significantly improve the rate performance of low electronic conductive material, especially for LiFePO4 and Li4 Ti5 O12 . As shown in Table 5.4, the maximum rate of Li4 Ti5 O12 /graphene foam electrode reached to 200 C, which corresponds to full discharge within 18 seconds [85]. In another work, Ji et al. [88] loaded LiFePO4 onto graphite foam. Due to high conductivity (∼1.3 × 105 S m−1 ) and lightweight (∼9.5 mg cm−3 ) of graphene foam, the specific capacity of the LiFePO4 /graphene foam was 23% higher than that of the LiFePO4 /Al electrode. In another strategy, the bottom-up strategy to mesoporous nanostructure Fe3 O4 /graphene foams was used [86]. When it acted as the negative for LIBs, it shows high reversible capacity and rates. At 1C rate, the reversible capacity is about 785 mAh g−1 and this cell did not show obvious capacity fading after 500 cycles. The maximum rates of this materials reached 60C, which has potential for fast discharge applications. As stated above, besides high rate performance, CNT or graphene foam can be used to build bulk electrodes. For example, Choi et al. [101]. fabricated the thick electrodes based on CNT and cellulose nanofibril. Figure 5.31a shows the fabrication procedure. First, the aqueous cellulose nanofibril suspension was poured onto

(a)

(b)

10 8

Conventional LFP cathode

PE

Cathode SEA CNF

PE

4 2

LFP/ PVdF/ C

LFP/ SWNT

LTO/ PVdF/ C

0

2.5

(d)

2.0 AI foil

AI foil

6

Conventional LTO anode

Anode SEA CNF LTO/ SWNT

Voltage (V)

(c) Areal mass (mg cm–2)

184

1.5 1.0 0.5 0.0

h-Nanomat Cell Conventional Cell

0

10

20 30 40 50 60 Capacity (mAh gcell–1)

70

Figure 5.31 Thick electrodes based on the CNT conductive network. (a) Fabrication procedure for the CNT-based foam electrode, (b) ion and electron transportation pathways, (c) areal mass capacity comparison between CNT-based current collector and conventional designs. (d) Specific capacity comparison between CNT-based current collector and Al foil laminated design. Source: Choi et al. [101].

5.10 Carbon-Based Thick Electrodes

a filter paper and vacuum filtrated, and then cellulose nanofibril film was obtained. On the top of the formed cellulose nanofibril paper, the slurry of SWCNTs and active materials was further introduced by the vacuum infiltration method. Figure 5.31b shows its architecture, which enable a drastic weight reduction of the device. The final LIB showed a ∼2 times higher gravimetric capacity than coated electrodes (Figure 5.31c). Figure 5.31d shows the specific capacity comparison; gravimetric specific discharge capacity of this cell (60 mAh gcell −1 ) is significantly higher than that of conventional LiFePO4 ||Li4 Ti5 O12 batteries (∼27 mAh gcell −1 ). The conductive nanofiber framework was fabricated by the electrostatic self-assembly of carbon black on cellulose nanofibers [102]. The fabrication procedure is shown in Figure 5.32a. First, the bleached pulp was 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized, and the carbon black was added to the suspension to obtain the conductive cellulose nanofiber suspension. LiFePO4 was mixed with the conductive cellulose nanofiber suspension and followed by freeze-drying. During this process, the conductive cellulose nanofibers can tightly hold the LiFePO4 materials together during the freeze-drying process, shown in Figure 5.32b. After further mechanically pressing, the final thick LiFePO4 electrode has a high volumetric energy density (538 Wh l−1 ) and area capacity (8.8 mAh cm−2 ). (a)

(b)

Figure 5.32 Fabrication of conductive cellulose nanofibers and the architecture of the thick electrodes.CNF, cellulose nanofibers; (a) Self-assembly between cellulose nanofibers and carbon black. (b) The hierarchical architecture of the final thick electrode. Source: Kuang et al. [102].

185

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5 Architectures Design for Cells with High Energy Density

–

–

+

HAP NWs

KB NPs

CFs

LFP NPs UCFR-LFP electrode slurry

HAP/KB hybrid networks Electrostatic self-assembly

–

Stirring

Electrostatic self-assembly

16.4 mAh cm–2

Filtration

Ultrahigh-capacity 750 °C Thermally stable

Free-standing UCFR-LFP electrode Fire-resistant

Fast electron and ion transporting behaviors

Figure 5.33 Fabrication procedures of the thick LiFePO4 electrode. (1) Self-assembly of the hydroxyapatite nanowire/carbon black frameworks, (2) fabrication of the electrode slurry, and (3) vacuum filtration process. Source: Li et al. [103].

Hydroxyapatite nanowires, carbon black, carbon fiber, and LiFePO4 were also self-assembled to form the thick LiFePO4 electrodes [103]. As shown in Figure 5.33, electrostatic self-assembly happened between hydroxyapatite nanowires and carbon blacks, which formed conductive and porous framework. Second, the negative LiFePO4 and carbon fiber are added into the positive hydroxyapatite nanowire suspension to prepare slurry. Followed by vacuum filtration, the thick electrode is obtained. The resultant thick (1.35 mm) electrodes have significantly high active materials loading (108 mg cm−2 ) and specific areal capacity (16.4 mAh cm−2 ).

5.10.2 Large Volume Variation Materials/Carbon Foam It is well known that high capacity negative material, such as silicon and transition metal oxide, usually shows poor electronic conductivity and often undergoes large volume expansion/contraction during repeated cycles. This kind of volumetric variation resulted in loss of electrical contact and rapid capacity decay. To solve this problem, integrating negative materials with graphene or CNT is a well-known strategy. As an example, Co3 O4 nanoparticles were incorporated into graphene foam, thereby creating the heterogeneous Co3 O4 /graphene foam. At high current rate of 1000 mA g−1 , the capability retention rate is 71%. The P123-assisted method was also adopted to in situ grow MoS2 architectures onto graphene foam. The MoS2 @graphene foam electrode shows a high discharge capacity of 1235.3 mAh g−1 at a current density of 200 mA g−1 , maintaining 85.8% of the initial capacity after 60 cycles [87]. In another work, the nitrogen-doped graphene/Fe3 O4 architectures were fabricated and Figure 5.34 shows the fabrication process [92]. Urea, GO, and FeCl3 ⋅6H2 O

5.10 Carbon-Based Thick Electrodes

Magnetic Fe3O4 nanoparticles Graphene oxide sheet

FeCI3-6H2O, urea 180 °C, 16 h hydrothermal

3D N-G/Fe3O4 hydrogel

GO aqueous suspension

Figure 5.34 Fabrication procedure of nitrogen-doped graphene/Fe3 O4 hydrogels. (a) Aqueous mixture of urea, GO, and FeCl3 ⋅6H2 O, (b) self-assembly and reduction of the nitrogen-doped graphene/Fe3 O4 foam. Source: Chang et al. [92]. Reproduced with permission of Royal Society of Chemistry.

CVD growth

MoS2 growth

CNT coating

Ni etching

Ni foam

Figure 5.35

GF

GF@CNT

GF@CNT@MoS2

Fabrication procedure of MoS2 @CNT@graphene foam. Source: Ren et al. [97].

were first mixed in water, then followed by self-assembly under hydrothermal reaction. After freeze-drying and heating at 500 ∘ C, the thick electrode is obtained. As stated before, CNT/graphene foam can provide unique advantages over the individual CNT and graphene, which has been proved in CNT/graphene ternary composite materials. As shown in Figure 5.35, CNT was first coated on graphene foam, then coated with MoS2 nanoparticles to obtain the CNT/graphene foam [97]. Similar to individual components, the foam not only enlarges the electrode/electrolyte contact and shortens Li+ diffusion distance but also has enough void space to buffer the volumetric variation of MoS2 particles. More importantly, the CNT-coated layer can further strengthen graphene foam. Due to these advantages, this MoS2 @ CNT@GF foam shows the better rate performances and cycle stability than individual CNT or graphene component. It must be noted that in such structure, the main role of CNT and graphene networks is volumetric buffer matrix and the interconnected graphene sheets prevent the aggregation of transition metal oxides or sulfides during cycling. To further increase the materials loading, rationally adjusted porosity of the foam electrodes is crucial. For example, the holey-graphene/Nb2 O5 with good high-rate performance at high mass loading (>10 mg cm−2 ) was synthesized [93]. First, the

187

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5 Architectures Design for Cells with High Energy Density

Nb2 O5 is uniformly decorated on GO. In order to introduce hierarchically porous structure, GO was etched and mixed with the aforesaid Nb2 O5 /GO to produce the composite material. By porosity optimization, the holey-graphene/Nb2 O5 can accommodate significant increase in mass loading. At 2A g−1 discharging currents, the Nb2 O5 /holey-graphene shows little difference in discharge capacity for materials loadings ranging from 1 to 11 mg cm−2 .

5.10.3 Compact Graphene Electrodes Although aforesaid CNT or graphene-based foam electrodes with high active materials loading usually show better gravimetric energy density, these materials normally suffer from relatively low volumetric performance and their high gravimetric energy densities cannot be translated to the practical devices. The CNT or graphene-based foam electrode with high volumetric performance is highly important to facilitate the materials and devices development for energy storage applications. To overcome this, the graphene compact electrodes by using the evaporationinduced drying of graphene hydrogel were fabricated and the calendered density of this graphene bulk electrodes reaches 1.58 g cm−3 , which is about 70% the theoretical density of graphite [104]. The graphene combines two advantages: (i) porous microstructure and (ii) high density. Therefore, this compact graphene electrode shows the high volumetric capacitance up to 376 F cm−3 . Figure 5.36 shows the fabrication process of porous graphene foam and high-density graphene bulk from graphene hydrogel. During freeze-drying, phase separation in the graphene hydrogel leads to the repulsion of graphene nanosheets from ice crystals. Finally, the porous graphene network is well retained and shows very low density. While for the evaporation-induced drying method, the strong interaction between water and graphene results in the obvious shrinkage of the graphene hydrogel to form final graphene bulk electrodes. More importantly, this graphene bulk not only consisted of interlinked and flexible nanosheets but also retains its porous properties [104].

Figure 5.36 Fabrication of porous graphene foam and high-density graphene bulk from graphene hydrogel. Source: Tao et al. [104].

5.10 Carbon-Based Thick Electrodes

Based on the novel compact graphene bulk, the graphene bulk electrodes with high volumetric energy density was also developed [105–109]. Nowadays, these compact graphene composites are mainly for ECs. As an example, the dense graphene electrode for ECs was fabricated by integrating PANI and compact graphene bulk. The final electrode has the density over 1.5 g cm−3 and exhibits high volumetric capacitance up to 800 F cm−3 . Figure 5.37 illustrates the main process for high-density bulk PANI/graphene electrodes. First, the homogeneous GO solution was hydrothermally treated to obtain the interconnected porous GO hydrogel, as shown in Figure 5.37a. This GO hydrogel that contained ANI monomers was in situ polymerized in HCl/(NH4 )2 S2 O8 aqueous solution (Figure 5.37b, c). Followed by HI reduction and vacuum drying, the hydrogel was transferred to the denser packing of the composite (Figure 5.37d, e). The final density of PANI/graphene bulk electrode is about 1.5 g cm−3 , which almost reaches the most compact architecture of PANI/graphene (Figure 5.38f). One of the major disadvantages of this evaporation-induced compact GO is their small pore size, which is difficult to accommodate organic or ionic liquid electrolytes. Therefore, this compact GO electrode is not usually suitable in organic electrolyte system. In order to precisely tune the pore size of compact graphene electrode over a wide range, Li et al. [3] used the novel template strategy to obtain the ultra-thick and dense electrode for symmetric ECs. In this method, zinc chloride acted as the sacrificial pore former to tune the specific surface area of bulk graphene from 370 to ∼1000 m2 g−1 , while the bulk electrode retains its high density from 1.6 to 0.6 g cm−3 . Figure 5.38a shows its fabrication process. The graphene hydrogel was first soaked in ZnCl2 aqueous solution, followed by vacuum drying and heat treatment. This architecture balanced porosity and density of graphene electrodes; the sliced bulk graphene electrode has thickness up to 400 μm (Figure 5.38b) and shows volumetric capacitance of 150 F cm−3 in ionic liquid electrolyte. This volumetric capacitance corresponds to a volumetric energy density of ∼65 Wh l−1 for EC, as shown in Figure 5.38c.

5.10.4 Summary for Carbon Foam Electrodes In brief summary, we reviewed the recent progress of carbon-based foam electrodes. CNT, graphene, and CNT/graphene foam show outstanding electronic conductivity and highly porous structure. This unique architecture provides high ions and electrons transportation, which is the ideal for thick electrodes. One of the major disadvantages of the carbon foam electrodes is the relatively low volumetric performance, and the high gravimetric energy densities cannot be translated to the practical devices. Therefore, the CNTs or graphene-based bulk electrode with high volumetric performance is urgently needed. Nowadays, graphene compact electrodes obtained by evaporation-induced drying method are a very interesting strategy, which combines the advantages of high rate and high volumetric energy density.

189

(b)

Concentration retention (%)

(a) Adsorption of ANI

Go hydrogel

110

0.013 M 0.027 M 0.08 M 0.16 M 0.24 M

100 90 80

(c) Polymerization of ANI

70

Reduction of GO

60 50 40 0

2

4

6

Time (h)

ANI

(d)

(e)

PANI/graphene (f)

1.7

Density (g cm–3)

1.6

Shrinking

1.5 1.4 1.3 1.2 1.1 1.0

PANI/graphene

PANI/graphene monolith

0

15 30 45 PANI loading (%)

60

Figure 5.37 Synthesis of high-density PANI/graphene bulk electrodes. (a) GO hydrogel was fabricated by the hydrothermal method. (b) Immersing GO hydrogel in ANI solution. (c) In situ polymerization of ANI/GO hydrogel and followed by HI reduction. (d) Evaporation-induced drying condensed the PANI/graphene hydrogel, (e) bulk electrodes and (f) relationship between density and PANI loading. Source: Xu et al. [110]. Reproduced with permission of Wiley.

5.11 Thick Electrodes Based on the Conductive Polymer Gels

2.5 cm

ZnCl2 vacuum heat solution drying treatment

1.4 cm

0.6 cm 0.4 cm

Evol (W∙h∙L–1)

200μm 400μm 600μm 800μm

80

Electrodes thickness This work 400 Evol Evol based on device 300 Evol based on two electrodes

60

200

120

pellet electrodes

100

40 100

20 0 (b)

(c)

on G M oS 2 Ti 3C 2 GOTi 3C 2r HGF LSG carbiber -CC PaG y E e f M cla a-M pap E

0

Electrode thickness/μm

(a)

IT M

Figure 5.38 (a) Preparation process of the porosity adjustable graphene bulk electrodes; (b) the graphene bulk electrodes with different thickness; and (c) the electrochemical performance comparison between a compact graphene electrode and reported materials. Source: Li et al. [3]. Reproduced with permission of Royal Society of Chemistry.

5.11 Thick Electrodes Based on the Conductive Polymer Gels Besides graphene foams and CNT foams, the continuous conductive polymer network provides an optional way to construct foam electrodes. Unlike CNT and graphene foams, the conformal in situ polymerization layer can be formed on the surface of active materials and active materials are embedded in the polymer matrix to form the integrated electrode. Polymer matrix in these electrodes not only provides continuous electron pathways and hierarchical pores for ionic transportation [111], but also acts as a binder [54]. Currently, conductive polymers used in the cells for building foam electrodes mainly include polypyrrole [111, 112], PANi [113], and nanocarbons/polypyrrole gel [114]. In conventional cells, polymer binders bridge active materials and carbon black to ensure the conductivity. Conductive carbons within the active material are only a kind of point-to-point contact, which impede the improvement of electronic conductivity. In order to remedy this, Shi et al. [111] designed a conductive PPy/Fe3 O4 gel framework, which acted as the high capacity negative material for LIBs. Figure 5.39 shows the comparison between electrode and conductive polymer integrated electrodes. It was shown that the continuous polymer framework provides the excellent electrical connection among particles. These conductive gel-based Fe3 O4 foam electrodes show the obviously improved rate and cycle performance. Compared with conventional binder, this foam electrode has higher mass ratio of active materials and therefore obtained higher specific capacities.

191

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5 Architectures Design for Cells with High Energy Density

(a)

(b)

Fe3O4 nanoparticle

Carbon particle

Conductive polymer coated Fe3O4 NP

Figure 5.39 Conventional electrode and conductive polymer-based foam electrodes. (a) Point-to-point contacts impede effective ionic and electronic transportation, (b) conductive polymer gel framework in the foam electrode facilitates the transport of ions and electrons. Source: Shi et al. [111].

Electrolyte

(a)

N H N H Polymer monomers

N H

Mix with

In situ polymerization

C-LFP

Crosslinker

Initiator

Current collector

Figure 5.40 Fabrication and structural properties of C-LiFePO4 /C-PPy hybrid foam. Source: Shi et al. [112].

In another work, the LiFePO4 -gel foam electrode was developed by in situ polymerization method [112]. Figure 5.40 illustrates the fabrication and structural properties of C-LiFePO4 /C-PPy foam electrodes. Conductive PPy polymer was in situ polymerized on the surface of active materials and bridged by molecules with various functional groups, and this polymer matrix connected the active materials. Due to their natural conductive properties, PPy polymers provide continuous electronic highways within inorganic–organic framework. Hierarchical pores formed within the polymer network facilitate electrolyte infiltration, as well as the ionic transportation within the electrode. Moreover, in situ coated polymer layer effectively prevents aggregation of both inorganic and organic components and achieves full access to individual particles [112].

5.12 Summary and Perspectives

Huang et al. [113] used conductive and electrochemically active polymer to replace conductive additive and binder and aim to enhance rate performance of LIB materials. PANI and PPy were used as the conductive polymers. Electrochemical deposition and chemical polymerization methods were used to fabricate the LiFePO4 /polymer positives. It was shown that obviously improved capacity and rate performance can be obtained in the positives. Among these polymer composite materials, the electrodeposited carbon-coated LiFePO4 /PPy material shows the best performance. As stated in Chapter 2, silicon is a high capacity negative material for highperformance LIBs and its theoretical capacity reaches 4200 mAh g−1 . Unfortunately, silicon undergoes severe volume expansion (∼300%) during cycling, which results in poor cycling stability. In order to overcome this, Liu et al. [114] developed Si particle/conducting polymer/CNT hybrid material for LIBs. This hierarchical conductive hydrogel/CNT framework offers a continuous electron transport matrix and high porosity to buffer the volume expansion of Si particles. By the architectures, the excellent cycling performance is obtained with 86% capacity retention over 1000 cycles. This strategy opens the way for fabricating robust, high-performance LIB materials with obvious volume expansion [114]. In brief summary, this conductive polymer-based network has attracted intense interest over the past few decades due to its high conductivity, flexible structure, and tunable hierarchical structures; these conductive polymer gels show great potential for the construction of foam electrodes [115]

5.12 Summary and Perspectives The ideal cells should have many comprehensive properties, such as rate capability, energy density, cost, cycle life, and temperature response. All these requirements have radically driven the technological development over the past 150 years. For current cells, higher energy densities are an eternal quest to power mobile devices or extend the cruising range of electric vehicles [116]. The energy density of the cells is determined by the nature attributes of the specific energy storage systems. The active materials with high specific capacities and high/low working voltage (high/low voltage for positive/negative material) have been the focus in the cells. Besides materials development, a promising way to improve the performance of the cells is the usage of innovative architectures to meet the higher energy density and power density requirements. The design of conventional electrodes is based on metal foil coating technologies. It has several disadvantages: (i) higher inactive materials content and (ii) poor electronic and ionic conductivity. These two disadvantages hinder the further development of higher energy density and power density. To solve this, we reviewed two innovative architecture: (i) thick electrodes and (ii) foam electrodes. The former architecture possesses higher gravimetric and volumetric energy density by reducing content of inactive components and increasing the thickness of electrodes. While the latter architecture usually shows outstanding gravimetric energy density and power density by the usage of graphene/CNT foam current collectors.

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In the thick electrodes section, we summarized the conventional strategies toward higher energy density of LIBs and reviewed key points for the design of thick electrodes. It was found that low tortuosity is crucial for the development of thick electrodes. For thick electrodes, the urgent problems are its ionic conductivity: thick electrodes usually result in poor ionic conductivity, which usually cause low rate performances. By optimized architectures and lower tortuosity of the cells, rate performances of thick electrodes can be significantly improved. We then focus on the design and fabrication of 3D CNT/graphene and polymer foam electrodes. Due to rich porous structure and ultra-high electronic conductivity, these kinds of electrodes can accommodate high active materials loading and provide high electronic/ionic conductivity. Nowadays, great progress has been achieved in CNT/graphene or polymer foam electrodes. In this part, we give a brief introduction about the fabrication of CNT and graphene foam and summarize the fabrication of composites. Although significant progresses have been made in recent years, many critical problems of foam electrodes are still remained to be solved. Main problems are as follows: (i) Volumetric energy density: current carbon-based porous electrodes usually show lower volumetric energy density, future research should be conducted to enhance the density of foam electrodes without compromising active materials loading. Currently, due to capillary interaction, graphene compact offers promising solution to build foam electrodes. (ii) Interaction between carbon foam or polymer foam and active materials [117]: structural control, surface functionalization of CNT/graphene foam, and the bonding between active materials and the carbon surface are still to be optimized. Understanding the synergistic effect between carbon foam and active materials has been the target and needs more comprehensive studies in both academy and industry field. Although carbon foam electrodes possess many urgent problems to be solved, it still becomes increasingly competitive in energy storage applications [72]. The gravimetric/volumetric energy density of the cells depend on the following three parameters based on the cell level (Figure 5.2) [118]: (1) Intrinsic capacity of active materials: As stated above, the gravimetric capacitance has been one of the most important parameters to determine the theoretical energy density of the cells. (2) Porous architecture: For a given chemistry system, the volumetric energy density of the electrode strongly depends on the porous architecture of electrode itself [3, 119, 120], while porous architecture strongly links with the calendering process during cells fabrication. (3) Calendering can reduce the void of the electrodes and therefore can increase the volumetric capacitance of electrodes. However, high calendering density could also destruct the porous structure and hinder ion transport [121]. On the other hand, although low calendering density can enhance the ionic transport inside the electrode, volumetric energy density of cells decreases. Therefore, it is important to balance the porosity and calendering density; the former is related to ionic transport, while the latter is related to the volumetric capacitance [122–124].

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(4) Electrode compositions: During the fabrication processes of cells, carbon black can increase the electrical conductivity and electrolyte adsorption and binder provides adhesion between the active materials, conductive additives, and current collectors. These non-active additives could also affect the electrode porosity that influences electronic and ion transport [125–128]. The small amount of carbon black causes a low electrical conductivity. And large amounts will reduce the percentage of active materials in the electrode, resulting in the low capacitance of the electrode. Therefore, it is also important to use appropriate amount of additives for the cells with higher volumetric/gravimetric energy density.

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6 Miniaturized Cells 6.1 Introduction 6.1.1

Definition of the Miniaturized Cells and Their Applications

Recently, the development of microelectronic devices and continuous miniaturization of the devices have evolved rapidly, such as microelectromechanical systems, micro medical implantations, smart medicine [1–3], wireless sensors, and nanoscale actuators [4, 5], which have reshaped every facets of our lifestyle. Nowadays, the Internet of Things (IoTs) describe billions of interrelated physical objects for exchanging data over the internet around the world. IoTs can be used as sensors or could be controlled remotely via the network and internet [6]. The technology analyst company International Data Corporation predicted that totally 41.6 billion IoT units will be available by 2025. The miniaturized energy storage technologies’ compatible IoT units are urgently to be investigated for such desired applications. Unfortunately, the lack of suitable miniaturized power sources remains one of the major obstacles to the daily use of microelectronic devices. The most of the microelectronic devices have been still powered by relatively bulky power sources [7–10], which limit their ability of being independent working unit, and always resulted in smallest packaged size greatly restricted by the size of power sources [7, 8]. Recently, the development of the miniaturized cells for microelectronic devices has attracted a lot of research attention. Advances made in devices processing together with developments in nanomaterials have enabled the development of novel miniaturized cells. For the miniaturized cells, the volumetric energy density is more important than gravimetric energy density because the size is of the most important. The microelectronic devices need rechargeable miniaturized cells within a scale in square centimeter or in square millimeter; the typical volume is in the range of 1–10 mm3 . These miniaturized cells should also possess high-power (W m−3 ) and high-energy-density (Wh cm−3 ) characteristics [11, 12]. The different types of energy systems have been furtherly investigated for miniaturized cells. Owing to their specific advantages, miniaturized or micro lithium-ion batteries (m-LIBs) Novel Electrochemical Energy Storage Devices: Materials, Architectures, and Future Trends, First Edition. Feng Li, Lei Wen, and Hui-ming Cheng. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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6 Miniaturized Cells

Lithium batteries

Bulk size

Smart phones

Laptops

Microbattery

Medical devices Microelectronics Integrated circuits

Electric vehicles

Reduced size

Figure 6.1 Examples of miniaturized cells and the form of cells used. Source: Wang et al. [13]. Reproduced with permission of Elsevier.

and micro electrochemical capacitors (m-ECs) are the ideal power sources for microelectronic devices, which can be on the millimeter scale or less, as shown in Figure 6.1. Various miniaturized cells have been developed by few companies, such as Panasonic, PowerPaper, Combat, Infinite Power Solutions, and ST Microelectronic devices. Generally, the market of miniaturized cells is small, but it should grow within the following years. For example, the researchers developed miniaturized lithium primary batteries for Juvenile salmon acoustic telemetry system at Pacific Northwest National Laboratory [14]. Panasonic commercialized the pin-type batteries for microelectronic applications as shown in Figure 6.1 [13]. The development in the miniaturized cells is intimately linked to the energy storage chemistry, microelectronics, and fabrication technologies [15].

6.1.2

Classification of Miniaturized Cells

The miniaturized cells can store and deliver energy in the form of electrochemical energy in the micrometer or millimeter size for microelectronic devices. Generally, miniaturized cells mainly include the m-LIBs and m-ECs. As stated in Chapter 2, LIBs and ECs possess different energy storage mechanisms. LIBs store energy by slow reversible redox reaction, which usually deliver higher energy density but lower power density, whereas ECs store energy by fast Faradaic reaction or electrolyte ions absorb/desorb mechanism, which endow ECs with ultrahigh power density than that of LIBs. In principle, the m-LIBs are better than other batteries chemistries due to their high operating voltage and high energy density compared with other conventional secondary batteries [16]. Nowadays, the energy

6.1 Introduction

Figure 6.2

Performance of the miniaturized cells and other typical electronic components.

density requirements of the existing and emerging microelectronic devices crucially impose the need for m-LIBs with unprecedented volumetric energy and power densities. The m-ECs have also been regarded as another alternative candidate for the miniaturized cells. The m-ECs can be charged and discharged much more rapidly and last longer, although store less energy than m-LIBs. Many m-ECs have been developed as power sources for microelectronic devices. Recently, the m-ECs are also developed to pair with m-LIBs and energy harvesting microsystems for meeting the peak power requirements, such as the wireless sensors or communication transmitters [17]. To satisfy these applications, the miniaturized cells must be able to store and deliver the sufficient energy and high power within the limited area, as shown in Figure 6.2.

6.1.3

Development Trends of the Miniaturized Cells

The conventional LIBs and ECs are fabricated by sandwiching a polymer separator between positive and negative, which consists of slurry cast active materials, conductive additives, and binders on the metallic current collector (generally Cu and Al). The conventional LIBs and ECs are normally very big and rigid [18], but their size has to be miniaturized effectively for microelectronic devices. Moreover, the fabrication processes become more and more incompatible when the size of the energy storage devices reduces. As a result, the miniaturized cells differ from the bulk-sized counterparts greatly in their architectures, fabrication processes, and materials. One of the major difficulties is to find the way to package components of LIBs and ECs in the sub mm3 range without sacrificing the energy and power density [19].

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6 Miniaturized Cells

Figure 6.3 The variation of volumetric energy densities of different packaged LIBs.

Figure 6.3 shows that the energy and power densities of different types of LIBs in volume. It was shown that energy densities decreased with the reduced packaging sizes. The main reason is that the space for active materials gets significantly reduced when the packaging size of energy storage devices reduced. Moreover, the inactive materials including the separator, additives, current collectors, and outside packaging materials begin to occupy more ratio of space in small package volume. It is still challenging to package the energy storage devices volumetric efficient in microscale. Another issue, the small footprint size of those devices results in lower energy density of the conventional energy storage devices with two-dimensional (2D) planar architectures. Due to the limited space of the devices, the third dimension must be effectively utilized to take more energy storing materials on the small footprint. Usually, the thickness of electrodes of LIBs is from several nanometers to ∼100 μm and is called thin-film 2D m-LIBs or m-ECs. While 3D m-LIBs or m-ECs mean the thickness of electrodes which is in the range of several hundreds of microns typically from 0.1 to 1.0 mm. Unfortunately, the power and rate capability of the 3D miniaturized cells with thick electrodes is significantly limited by slow kinetics of diffusion and electronic conduction. In order to build the practical 3D miniaturized cells, the thickness of the electrodes has to be increased keeping the transport distances for the ions and electrons [12]. The architectures design based on 3D interdigitated and interpenetrating electrodes can meet this requirement [20]. Since developing miniaturized cells is to power microelectronic devices, the technique of fabricating these devices is directly with the help of state of micro fabrication technique [21, 22]. The following section will give the various technologies for the fabrication of 2D and 3D miniaturized cells. As schematically shown in Figure 6.4, the development of (i) materials selection, (ii) optimized cell design, and (iii) fabrication methods requires special attention in the search for high-performance miniaturized cells and significant research efforts have been dedicated to these four aspects for these micro-devices [23, 24]. As stated in Chapter 1, the current researches for conventional LIBs and ECs are mainly on

6.2 Evaluation Methods for the Miniaturized Cells

Fabrication methods

Miniaturized cells Novel materials

New architecture

Figure 6.4 Schematic illustration of the factors that require development for the high performance miniaturized cells.

materials and chemistry development, i.e. (i) materials optimization, (ii) developing stable electrolytes, and (iii) developing new energy storage chemistry (such as Li-S, Li-Air, etc.). Unlike conventional LIBs and ECs, the research and development for miniaturized cells are mainly focused on the fabrication processes and architectures design of electrodes and devices. The progress and the prospects of miniaturized cells based on these four aspects, especially for m-LIBs and m-ECs, will be discussed in Chapter 6. The ideal miniaturized cells should possess high electrochemical performance with good durability and have innovative attributes with innovative properties for specific purposes. Integrating the miniaturized cells with additional smart functions, such as various sensors and energy harvesting devices, is also of great significance to the portable, wearable, and smart electronic devices. In this regard, the advances have been made in the fields, such as self-charging, self-responding, and self-monitoring energy devices. Therefore, integrating smart functions can make miniaturized cells more suitable under various circumstances and enable many unprecedented applications [14]. The integration of miniaturized cells will also be introduced at the end of Chapter 6.

6.2 Evaluation Methods for the Miniaturized Cells Similar to “bulk” electrochemical energy storage systems, the electrochemical performance of the miniaturized cells can also be evaluated in terms of

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their (i) gravimetric, (ii) volumetric, and (iii) areal energy/power densities [25]. Unfortunately, there are no standards available, which makes the reported performance less compared [26]. Conventional methods to assess the performances of an electrochemical energy storage system are to normalize its performances by the weight and volume. The thickness and density of the electrodes, as well as mass of the other inactive components, determine the gravimetric and volumetric performance of the miniaturized cells [27]. The volume characteristics can also be used to characterize the miniaturized cells because they allow for the comparison of different electrode materials, regardless of thickness. However, when it comes to large differences in thickness, volume characteristics can also provide misleading information. The volume characteristics do not change from the thin electrode film to thick electrode film. The process cannot be scaled up to result in the larger thickness of the electrode, especially when the expression of nanoscale electrodes produced by specific techniques [26]. For m-LIBs and m-ECs confined to electronic circuits, the available surface area of these devices is limited. In this circumstance, it is essential to consider standardizing the area and volume of these miniaturized cells to its footprint area [25, 26]. It is more appropriate to use footprint area normalized volumetric and areal energy/power densities of the miniaturized cells for electrochemical performance evaluation. In the following section, the introduction toward the evaluation methods for electrochemical performance of m-LIBs and m-ECs will be given [26].

6.2.1

Evaluation Methods for Electric Double-layer m-ECs

For standard electric double-layer capacitors with CV curves, the specific normalized areal capacitances CA , in mF cm−2 , and volumetric specific capacitances Cv , in mF cm−3 , based on the CV curves can be calculated by Eqs. (6.1) and (6.2). V

CA =

f 1 I(V)dV ∫ 2 × S × v × (Vf − Vi ) Vi

CV =

f 1 I(V)dV 2 × V × v × (Vf − Vi ) ∫Vi

(6.1)

V

(6.2)

In Eq. (6.1), S is the entire surface area of electrodes in cm2 , V is the total volume of electrodes in cm3 , v is the voltage scan rate in V s−1 , V f and V i are the potential V

range of CV curves, and I(V) is the voltammetric current in amperes. ∫V f I(V)dV is i the integrated area from CV curves. The specific areal CA in mF cm−2 and volumetric capacitance CV in F cm−3 can be calculated from the galvanostatic charge/discharge measurements by Eqs. (6.3) and (6.4). I S × (dV∕dt) C CV = A d

CA =

(6.3) (6.4)

6.2 Evaluation Methods for the Miniaturized Cells

where I is the discharge current in amperes and dV/dt is the slope of galvanostatic discharge curves. d is the thickness of active materials. S is the total area of the active electrodes. The specific areal EA in μWh cm−2 and volumetric energy densities EV in Wh cm−3 can be calculated from Eqs. (6.5) and (6.6): EA =

(ΔV)2 1 × CA × 2 3600

(6.5)

EV =

(ΔV)2 1 × CV × 2 3600

(6.6)

where ΔV = V max − V drop is the range of discharge potential, V max is the maximum voltage, and V drop is the voltage drop indicated by the difference between the first two data points in the discharge curves. The specific areal PA in mW cm2 and volumetric PV in W cm3 power densities can be obtained from (6.7) and (6.8). EA × 3600 Δt E PV = V × 3600 Δt where Δt is discharge time (in s).

(6.7)

PA =

6.2.2

(6.8)

Evaluation methods for m-LIBs and m-ECs

Usually, the galvanostatic charge/discharge curves are used to evaluate electrochemical performance parameters for m-LIBs and pseudocapacitors m-EC. For m-LIBs and pesudocapacitor m-ECs, the capacity at a fixed current I can be expressed as Eq. (6.9) [28, 29]. Q

Δt

Q=

Idt =

∫0

∫0

(6.9)

dq

The energy in a fully charged device depends on the discharge current I dis . It can be obtained by measuring the time Δt(I dis ) for its complete discharge at a constant dq Idis = dt . Q

Δt

Energy =

∫0

IV(t)dt =

∫0

V(q)dq

(6.10)

Therefore, the specific areal capacitances CA in mAh cm−2 , volumetric capacitances Cv in mAh cm−3 , specific areal EA in μWh cm−2 , volumetric energy densities EV in Wh cm−3 , specific areal PA in mW cm2 , and volumetric PV in mW cm3 power densities can be expressed as follows: Δt

CA =

∫0

IV(t)dt∕S

(6.11)

IV(t)dt∕V

(6.12)

Δt

CV =

∫0

211

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6 Miniaturized Cells Δt

E A = CA

∫0

Vdt∕Δt

(6.13)

Vdt∕Δt

(6.14)

Δt

E V = CV

∫0

EA × 3600 Δt E PV = V × 3600 Δt

(6.15)

PA =

(6.16)

6.3 Architectures of Various Miniaturized Cells The architecture of electrodes and devices significantly affects the output performance of the miniaturized cells. According to the architectures, the miniaturized cells can be classified into four types based on the patterns and orientations of microelectrodes on a single substrate [30, 31]: (i) 2D parallel plate configuration. As shown in Figure 6.5a, the 2D parallel plate configuration usually consists of multiple stacked layers. These multilayer stacked layers start from the bottom substrate, the current collector, then anode, electrolyte, cathode, and top current collector in a vertical direction via sputtering and layer-by-layer deposition. (ii) 2D (a)

High power Low areal utilization

Low power High area utilization

(b)

2D interdigitation

2D plate 3D interdigitation

3D stacking Miniaturized cells

High areal energy High power

(c) Substrate

Current collector

High areal energy Low power Anode

Electrolyte

(d) Cathode

Figure 6.5 Classification of four types of miniaturized cells. (a) 2D parallel plate configuration, (b) 2D interdigitated configuration, (c) 3D interdigitated configuration, and (d) 3D stacked configuration.

6.4 Materials for the Miniaturized Cells

interdigitated configuration. Figure 6.5b shows a 2D interdigital configuration. This configuration has an interdigitated geometry, which consists of two adjacent electrodes separated by micron gaps in parallel arrangement on one side of the substrate. (iii) 3D interdigitated configuration: 3D interdigitated configuration devices consist of vertically aligned isolated pillar microelectrodes of interdigital electrodes on current collectors/substrates as shown in Figure 6.5c. (iv) 3D stacked configuration: As shown in Figure 6.5d, 3D stacked configuration is also a kind of multiple stacked layers, but 3D stacked configuration consists of all components on a 3D vertically aligned substrate. As shown in Figure 6.5, various miniaturized cells show different advantages and disadvantages as described below, respectively. The 2D parallel plate configuration and the 3D stacked configuration can fully utilize the footprint of the substrate, while the area of the microelectrodes in the 2D and 3D interdigital configurations in the plane is reduced by the interval between two adjacent microelectrodes. However, 2D parallel plate configuration and 3D stacked configuration usually show lower power performance because the ions must move vertically though the entire thickness of the cell. Contrary to the parallel state architecture, 2D interdigitated and 3D interdigitated configuration usually has higher power performance, which caused by ionic parallel diffusion in the in-plane architecture design. Moreover, 3D stacked and interdigitated architectures usually have high active materials mass loading [30]. In order to avoid short circuits, the miniaturized cells with stacked configuration require gel or solid electrolytes as separators, while 2D or 3D interdigitated miniaturized cells do not have separators, and both architectures work well with liquid and solid electrolytes [30].

6.4 Materials for the Miniaturized Cells 6.4.1

Electrode Materials

As stated in the following parts, there are numerous methods for the fabrication of the miniaturized cells with different architectures to increase the energy and power density as well as the cycling lifetime and frequency response. Similar to conventional ECs, various pseudocapacitive and carbon materials have also been used to construct m-ECs. Various m-ECs usually based on carbon materials achieved high rate capability and high frequency response [32, 33]. The carbon materials, including graphene and carbon nanotubes (CNTs) [34, 35], onion-like carbon [25], activated carbon [36], and carbide-derived carbon [37, 38], have been intensively used to fabricate m-ECs. Carbon materials have many advantages, especially higher specific surface area, high electronic conductivity, high chemical stability, easy processing, and nontoxicity. As a one-atom thick 2D carbon sheet, graphene shows great potential in m-ECs due to its excellent electrical conductivity and large surface area (2630 m2 g−1 ) [39–41]. Graphene has high specific capacitance, which is among the best carbonaceous materials for m-ECs. The macroscopic structure of graphene can be manipulated into new architectures to improve the electrochemical properties, as shown in Figure 6.6.

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6 Miniaturized Cells

Graphene sheet

Honeycomb

Wrinkled sheet

Hydrogel

Nanomesh

3D porous

Porous sheet

Vertically oriented

Figure 6.6 Schematic illustration of typical graphene macroscopic structures. Source: El-Kady et al. [42].

In order to improve the areal and volumetric performance, various pseudocapacitive materials (such as metal oxides, RuO2 [37, 43], Co3 O4 , MnO2 [44], VS2 [45], etc.) and conductive polymers (such as polypyrrole [46], polyaniline [47], etc.) have been reported for m-ECs. As the result, the reported pseudocapacitive m-ECs exhibited promising volumetric capacitance, but with low charge and discharge efficiency, power capability, and frequency response [48, 49]. Various conducting polymers have been used for m-EC materials [50–52]. These polymers can be easily prepared by chemical or electrochemical methods, which show good electrical conductivity. These conducting polymers [51, 53, 54] mainly include polypyrrole, polyaniline, polythiophene, the composites with CNTs, and inorganic battery-type materials [55]. Those composite materials can improve the conductivity, cycle ability, specific capacitance, mechanical stability, and process ability [51, 56]. More importantly, conducting polymers compositing graphene and CNTs are flexible and strain resistant, which can retain their electrochemical properties in the flexible, stretchable, and even transparent m-ECs for wearable microelectronic devices. The choices of materials are also strongly dependent on fabrication technologies. For example, the chemical vapor deposition might be the first consideration to grow carbon materials in graphene-based m-ECs, such as CNTs, graphene, because they have more freedom to design synthesis process for depositing the electrode materials. Similar to m-ECs, materials for m-LIBs are also strongly dependent on fabrication technologies and architectures. For example, the conventional cathode and anode materials were mainly used for the fabrication of 2D m-LIBs. The positive electrodes include LiCoO2 , LiMn2 O4 , LiFePO4 , LiNi0.8 Co0.2 O2 , etc., the negative electrodes mainly involves metal lithium [57, 58]. The discussion of various materials in detail for conventional ECs and LIBs was presented in Chapter 2.

6.4.2

Electrolytes for the Miniaturized Cells

The electrolytes of the miniaturized cells are mainly the liquid electrolytes and solid-state electrolytes [59, 60]. The choice of electrolytes for the miniaturized

6.5 Fabrication Technologies for Miniaturized Cells

cells also depends on their architectures. Due to safety and packaging concerns, conventional liquid electrolytes are not suitable for stacked 2D miniaturized cells. Therefore, stacked 2D miniaturized cells must be operated in inorganic solid-state electrolytes, which usually involve ion-conductive LiNbO3 -SiO2 [61], Li2 S-P2 S5 [62], lithium lanthanum titanium oxides (Li3x La(2/3)−x TiO3 ) [63], lithium phosphorus oxynitride (LiPON) [64], and Li14 Zn(GeO4 )4 [65]. For liquid electrolytes, safety concerns call for effective packaging to prevent electrolyte leakage [66]. Unfortunately, liquid electrolyte often results in rugged and bulky packaging layer, which limits their applicability in wearable miniature systems. Therefore, liquid electrolytes should not be regarded as promising electrolytes for the miniaturized cells. In order to solve this, various gel-type electrolytes and solid-state electrolytes have been developed to meet the safety, architecture and packaging requirements [67]. Moreover, the polymer electrolytes act as both electrolyte and separator [30], which can significantly improve the mechanical stability of the miniaturized cells. Furthermore, the solid polymer electrolyte matrix can prevent electrolyte leakage and ensure robust electrolyte functionality without encapsulation [68, 69]. For example, the all solid-state m-LIBs were fabricated by the interdigital patterns of LiFePO4 microspheres/graphene as cathode and Li4 Ti5 O12 nanospheres/graphene as anode, without polymer binders and separators [69]. The resultant all solid-state m-LIBs exhibited excellent volume energy density, excellent high-temperature performance, and excellent flexibility. In another work, the polymer electrolyte consisting of ionic liquid 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide and triblock copolymer (poly(styrene-b-methylmethacrylate-b-styrene) was used for all solid-state m-ECs, which achieved excellent performances [70]. The ionic liquid has several advantageous characteristics as electrolyte solutions, including non-volatility, nonflammability, wide electrochemical windows, and high ionic conductivity. In addition, the polymer matrix results in the solid-state electrolyte mechanically stable for flexible operation without leakage. A flexible bipolar all solid-state m-LIB was fabricated through multistage printing assisted by ultraviolet (UV) curing.[71] The flexible/noncombustible gel electrolyte of sebaconitrile-based electrolyte and semi-interpenetrating polymer network skeleton instead of inorganic electrolytes. The printed bipolar m-ECs have excellent flexibility, charge/discharge performance, and abuse tolerance resistance (nonflammability). Figure 6.7 shows the scheme of the procedure used for fabricating the stencil-printed gel composite electrolytes, the gel composite electrolytes, and chemical structure of its main components.

6.5 Fabrication Technologies for Miniaturized Cells In the past decades, the manufacture of energy storage devices was mainly based on the conventional methods due to their mature procedures [72]. As stated in Chapter 2, the conventional LIBs and ECs using liquid electrolyte are not suitable for the miniaturized cells due to the restrictions for architecture design, size, and inherent

215

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6 Miniaturized Cells Stencil printing & UV curing

GCE paste

GCE

1 M liBF4 in sebaconitrile Semi-IPN polymer network ( Al2O3

ETPTA

PVdF-HFP) SBN

ETPTA

n

PVdF-HFP

Figure 6.7 Scheme of fabricating stencil-printed gel composite electrolytes (GCE), the composite gel electrolytes, and chemical structure of the main components. Source: Kim et al. [71].

risk of leakage, which give rise to the development of m-LIBs and m-ECs fabrication technology [73, 74]. In order to build the miniaturized cells, the innovative fabrication methods should be developed such as laser direct writing, photolithography, and various printing methods [75–77]. Herein, a brief review on the main technologies for the fabrication of the miniaturized cells will be discussed among the different architectures.

6.5.1 Fabrication of Miniaturized Cells with 2D Parallel Plate Configuration The first type of lithium solid thin-film batteries was invented by Hitachi and Maxell in 1980s [78]. Since then, many thin-film deposition was adopted for developing m-LIBs [79–81]. Several vapor deposition techniques have been used to prepare thin-film materials for m-LIBs, which mainly include pulsed laser deposition, direct current (DC), and radio frequency (RF) sputtering [82–86]. For example, the functional thin-film solid-state m-LIBs were constructed by sputtering TiS2 cathode, oxide-sulfide solid electrolyte, and Li anode layer via high vacuum vapor deposition by Energizer Company in 1992 [79]. The completed m-ECs have the thickness of about 10 μm and operating voltage about 2.5 V. The m-ECs showed ∼100% cathode utilization at current density range of 10–135 μA cm−2 and ∼100% cycling efficiency over 1000 cycles. All solid-state 2D stacked m-LIBs and m-ECs are one of the choices for future microelectronic devices, which has broad temperature stability due to the absence of an organic electrolyte solution [87]. For 2D stacked m-LIBs, sufficient energy must also be stored in a limited space. Increasing the thickness of active materials is an effective way to increase the capacity per unit area, although there is theoretical limitation due to the diffusion length of operation time. To further improve the practical energy density and rate performance, many planar m-LIBs were developed by semiconductor manufacturers since 2007, such as Cymbet, Oak Ridge Micro Energy, Excellatron, Infinite Power Solutions, Front Edge Technology, etc. The magnetron sputtering technology has been widely used in industrial processes because high-quality thin films can be obtained at low temperature.

6.5 Fabrication Technologies for Miniaturized Cells

Figure 6.8 Schematic diagram of RF magnetron sputtering.

Substrate and film growth

Sputtering gas

Sputtering target

Sputtering technique bombards targets through energetic ions and sprays atoms into the plasma, then impinging a thin layer on the substrate. Magnetron sputtering techniques include RF magnetron sputtering and DC magnetron sputtering. RF magnetron sputtering does not require an electrically conductive electrode as the target. Therefore, in principal, any material can be deposited using RF magnetron sputtering. RF magnetron puttering is a suitable technique for layer deposition of different materials with controlled thickness. Figure 6.8 shows the basic mechanism of magnetron sputtering process, in which a deposition target is bombarded with energetic ions of inert gases, such as argon or helium. The powerful collision of the energetic ions with the target ejects target metal atoms into space. Thereafter, the atoms are deposited on the substrate and formed a film. The target is cooled by water and therefore produces little radiant heat [88]. As an effective film depositing technology, RF sputtering is a suitable technique to build 2D stacked miniaturized cells in the thickness below 1 μm, especially for m-LIBs [89–92]. It was found that RF magnetron sputtering improved the density and homogeneity of thin films of LiCoO2 compared with the spray pyrolysis technique [89]. The all solid-state 2D stacked m-LIBs were fabricated by RF magnetron sputtering deposition, in which the positive, negative, and electrolyte were LiCoO2 film, lithium metal, and glassy LiPON, respectively [93]. The prepared polycrystalline LiCoO2 film exhibited the strong preferred orientation after high-temperature sintering. The fabricated m-LIBs can provide 30% of the maximum capacity between 4.2 and 3 V at a discharge current of 10 mA cm−2 , and almost no capacity fading over thousands of cycles at more moderate discharge–charge. A primary thin-film lithium battery was also fabricated on a chip [93]. The thickness of the thin-film m-LIBs was in the order of tens of μm, and areas are in the cm2 range. The prepared battery is in the form of Volta pile, with alternating layers of LiMn2 O4 /or LiCoO2 , LiPON, and lithium metal. The maximum potential is 4.2 V with continuous/maximum current output is approximately 1–5 mA cm−2 . The final m-ECs are shown in Figure 6.9. The prepared LiMn2 O4 polycrystalline thin films showed excellent electrochemical performance than that of bulk LiMn2 O4 materials [94]. It is mainly due to the small grain size and porosity of these 3D films, which reduces the diffusion path length for Li+ . The LiMn2 O4 thin films prepared though RF magnetron sputtering and thermal annealing in oxygen atmosphere demonstrated good intercalation

217

6 Miniaturized Cells

Protective coating

15 µm

218

Anode

Cathode current collector

Electrolyte Cathode Anode current collector

Substrate

(a)

Figure 6.9

(b)

(c)

(d)

On-chip thin-film m-LIBs. Source: Bates et al. [93].

kinetics and cycling performance. The prepared LiMn2 O4 film had a capacity of approximately 50 μAh cm−2 μm at 200 μA cm−2 and retained over 98% capacity after 1000 cycles [95]. RF magnetron sputtering technology was also used to fabricate LiFePO4 films. The LiFePO4 thin films formed on Pt or Pt/Ti/quartz glass at 400 ∘ C showed good electrochemical performance. It exhibited an initial discharge capacity of 56 μAh/(cm2 μm) and good cycling performance at a current density of 10 μA cm−2 between 3.0 and 4.3 V [96]. The all solid-state 2D stacked m-LIBs were fabricated using crystalline sputtered V2 O5 thin films as cathode with LiPON as the solid electrolyte, and the prepared thick film (2.4 μm) has stable specific capacity of 30 μAh cm−2 [97].

6.5 Fabrication Technologies for Miniaturized Cells

Carbon thin film layer

Silicon thin film layer

Current collector

(a)

Silicon thin film layer Carbon thin film layer Silicon thin film layer Current collector

(b)

Figure 6.10 Schematic diagram of layer-by-layer thin-film m-LIB electrodes. Source: Reyes Jiménez et al. [98].

The Si-based thin-film electrodes for high energy density m-LIBs were prepared by magnetron sputtering. Figure 6.10 shows the layer-by-layer electrode structures, where amorphous carbon layers are used to improve mechanical strength [98]. The silicon film electrodes with amorphous carbon layers exhibited significantly improved electrochemical properties in terms of capacity retention and efficiency. The amorphous carbon middle layer also reduced volume expansion and enhanced the mechanical stability of Si during lithium insertion/extraction for better integrity. One of the first generation m-ECs using DC magnetron sputtering deposit thin-film RuO2 electrodes was reported in 2001 [43, 99]. The LiPON electrolyte was deposited by reactive sputtering on top of bottom RuO2 electrode at room temperature. Figure 6.11 shows the fabrication of the m-EC using RuO2 films by sputtering technology. The resulting stacked films showed typical double layer capacitor, higher current resistance drop, and faster capacity decrease, which was explained by large ion size and its slow mobility [99]. To further improve the areal and volumetric capacitance of RuO2 electrodes, all solid-state thin-film RuO2 capacitors have been fabricated by tungsten co-sputtered ruthenium oxide electrodes (W-RuO2 ) film by DC magnetron sputtering. Compared with the amorphous RuO2 electrode-based thin-film RuO2 capacitors, the W-RuO2 electrodes exhibited higher specific discharge capacitance and more stable cyclability. For example, the capacitance per volume of the W-RuO2 -based m-EC

219

220

6 Miniaturized Cells

Figure 6.11

Fabrication of the m-EC using RuO2 films by sputtering technology.

was 54.2 mF cm−2 μm, while that of the RuO2 electrodes was 30.4 mF cm−2 μm [100].

6.6 Fabrication Technologies for 2D Interdigitated Cells The fabrication methods for 2D planar miniaturized cells can be divided into two categories [26]. As shown in Figure 6.12a, the first category involves technologies based on transferring the electrode material onto the patterned current collectors. These technologies include screen printing, inkjet printing, and electrophoretic deposition (EPD). In the second category (Figure 6.12b), the electrode materials are synthesized directly from the solution or by the chemical transformation of the pre-coated material, such as laser scribing, electrode conversion, and electrolytic deposition during the manufacture of the miniaturized cells. Figure 6.13 compares different technologies for the fabrication of 2D interdigitated cells. It was shown that performance of 2D interdigitated cells can be evaluated based on three factors, which are as follows: (1) Thickness of interdigitated electrodes: as an important parameter, thickness of the interdigitated electrodes determines the specific areal energy density of miniaturized cells. As shown in Figure 6.13, the screen printing technology can offer the highest layer thickness in the range of 200–1000 μm. While the inkjet printing and laser scribing can offer the layer thickness in the range of 10–100 μm. For the last three ways, the electrode thickness is between screen printing and inkjet printing. (2) Resolution of interdigitated electrodes: For the miniaturized cells, the power density of the miniaturized cells with interdigitated architecture can be significantly enhanced by reducing the gap between two electrodes. Among these six

6.6 Fabrication Technologies for 2D Interdigitated Cells

Reservoir

Laser scribing

Inkjet printing Drops

Laser

Screen printing Electrode conversion Squeegee Image Screen

Paste

Chlorination

Working electrode

Carbide

Carbide-derived carbon

Printed image Counter electrode Electrophoretic deposition

Electrolytic deposition

Charged particles

(a)

Reference electrode

(b)

Figure 6.12 Typical fabrication processes of 2D planar miniaturized cells. (a) Processes based on the electrode material integration from an existing powder within the micro devices: (b) processes based on in situ synthesis of the active material. Source: Kyeremateng et al. [26]. Reproduced with permission of Springer Nature.

Figure 6.13 Comparison of different technologies for 2D interdigitated cells. (a) Screen printing, (b) inkjet printing, (c) laser scribing, (d) electrophoretic deposition, electrolytic deposition, and electrode conversion.

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methods, inkjet printing and laser scribing provide the best resolution (∼0.1 μm) than other four methods. (3) Ease of fabrication: among these four methods, screen printing, inkjet printing, and laser scribing are the simplest method for the preparation of 2D interdigitated electrodes. More importantly, inkjet printing and laser scribing can produce 2D electrodes with complex pattern.

6.7 Printing Technologies for 2D Interdigitated Cells 6.7.1

Advantages of Printing Technologies

The printing technology has great potential for the fabrication of m-LIBs and m-ECs due to the variety of printing inks [72, 101]. Comparing with other fabrication techniques, the printing techniques have two characteristic advantages [102]: (i) The material is printed layer by layer on the substrate to make the required device or structure, which is a kind of additive manufacturing process. This advantage makes the printing process pollution-free because it requires low energy consumption and generates little material waste and pollution. (ii) The functionality of the device is related not only to the printed material but also to the substrate material, which allows a variety of flexible or stretchable materials as substrates instead of conventional rigid substrates. Combining these two advantages together, the miniaturized cells can be manufactured with various printing technologies easily.

6.7.2

Classification of Printing Techniques

Nowadays, main printing technologies for m-LIBs and m-ECs include ink-jet printing, screen printing, and extrusion 3D-printing techniques. These various printing techniques show different printed layer thickness and printing resolution, which determined by rheological behavior of inks. In the next following section, we will give a brief review about classification of printing techniques. The rheological behavior of the fluid is the parameter of its anti-deformation ability at a given rate. As shown in Figure 6.14, there are two kinds of inks: (i) inks with Newtonian behavior: Newtonian behavior is the liquids whose viscosity remains constant at different shear rates, usually only in low molecular weight liquids, such as water, solvents, and highly diluted polymer solutions (Figure 6.14a) [103]. (ii) inks with non-Newtonian behavior: All the other fluids show non-Newtonian behaviors, which can be further subdivided into time-related and time-independent viscosity behaviors. More detailed discussions about rheological behavior of fluids can be found in the references [104–106]. In general, the inks with thixotropic or pseudoplastic behavior have obvious advantages for printing (Figure 6.14a). The viscosity of the inks with thixotropic or pseudoplastic behavior will be greatly reduced during the printing processes, which allows the ink to flow through the printer easily. Moreover, the viscosity of the resulting printing ink will be gradually or immediately recovered after removing the printing press, enabling the formation of desired pattern quickly [102].

6.7 Printing Technologies for 2D Interdigitated Cells

Newtonian

Time-dependent Non-Newtonian

(a)

Thixotropic Viscosity

Viscosity

Viscosity

Rheopectic

Shear rate

Shear rate

Shear rate

Time-independent Non-Newtonian

Pseudoplastic

Viscosity

Pseudoplastic

Viscosity

Viscosity

Dilatant

Shear rate

Shear rate

Shear rate

(b) Layer thickness (μm)

103 102 Screen printing 101

10–1

Printing resolution (μm)

100

101

102

103 104 105 Ink viscosity (cP)

106

107

108

103 102

Inkjet printing

3D printing Screen printing

101 100 10–1 10–2 10–1

(d)

Inkjet printing

100

10–2 10–1

(c)

3D printing

100

101

102

104 105 103 Ink viscosity (cP)

106

107

108

Figure 6.14 (a) Different shear profiles for various fluids. (b, c) Comparison of the features. Source: Li et al. [102].

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Rheological behavior and printing technologies determine the resolution and layer thickness of the printed cells. As shown in Figure 6.14b, the extrusion-based 3D printing can offer the highest layer thickness in the range of 10–1000 μm, compared with other printing technologies, such as screen printing and inkjet printing. Therefore, extrusion-based 3D printing technique is a viable way to build 3D interdigitated configuration with high areal energy density, as shown in Figure 6.5c. For the miniaturized cells, the power density of the miniaturized cells with interdigitated architecture can be significantly enhanced by reducing the gap between two electrodes. The optimized architecture with high-resolution patterns can enhance the performances of the miniaturized cells due to reduced ohmic losses and better impregnation by the electrolyte, higher accessibilities, and compactness [25, 107, 108]. As shown in Figure 6.13c, the most reported inkjet printing and screen printing technologies have resolution in the range of 1–100 μm, which is very suitable for the construction of 2D interdigitated cells, as shown in Figure 6.5b. In summary, the layer thickness and resolution of the printed devices depend on the four factors: (i) printing technologies, (ii) rheological behavior of inks, (iii) substrates, and (iv) posttreatment [102].

6.7.3

Screen Printing for Miniaturized Cells

Screen printing is a printing technique, in which the ink is transferred onto the substrate through the porous mesh, except in areas where the ink cannot penetrate through closed templates. The ink is applied to the substrate by placing the mesh over the material. The blade or scraper moves across the screen to fill the opening hole with ink and then reverse travel the screen to instantly touch the substrate along the contact line. The ink will only pass through the open area of the mesh, forming an image on the substrate. Screen printing is still one of the most versatile printing technologies, although the design flexibility of screen printing is relatively low. Screen printing can be used for a wide range of substrates, including paper, textiles, plastics, glass, metal, and many other materials [109–111]. Screen printing allows electrodes to be deposited and composed simultaneously, reducing processing time and complexity. Screen printing can be used to print patterns at a large scale in a short period of time at uniform thicknesses. The screen printing is suitable for large-scale and mass production due to its much faster printing speed (∼70 m min−1 ) compared with the other printing techniques [112]. This is a reliable technique for patterning on a variety of substrates such as paper, cloth, plastic, glass etc. Recently, screen printing technology has also opened new way to manufacture m-LIBs and m-ECs. As mentioned before, various processes, such as RF magnetron sputtering, have been used to fabricate LiCoO2 films for m-LIB [113, 114]. By changing the deposition conditions, the properties of the LiCoO2 film can be easily changed. Another limitation of RF magnetron sputtering technology is the low discharge capacity due to the limited thickness of the cathode film [91, 113, 115, 116]. Therefore, unlike RF magnetron sputtering techniques, screen-printing techniques can fabricate thicker

6.7 Printing Technologies for 2D Interdigitated Cells

patterned electrodes for miniaturized cells. For example, the thick film LiCoO2 cathode about 6 μm was fabricated by the screen printing technique directly, which does not need a post annealing [117]. Screen printing can be used to fabricate 2D interdigitated cells from the variety of materials, such as carbon materials, metals, metal oxides, polymers, and biomaterials [84]. Screen printing is an easy-to-manufacture, scalable, and cost-effective method for manufacturing m-ECs [118]. By simply connecting devices in series, parallel, or both, the screen printing method can enhance the capacitance and voltage [91, 119]. As an example, the planar m-ECs using graphene materials were fabricated by screen printing technique, which showed ultrahigh voltage output, remarkable mechanical flexibility, and superior modularization [120]. The fascinating features of graphene combined with advanced screen printing technology greatly simplify the manufacturing process and facilitate the integration of planar m-ECs into printed microelectronic devices. The m-ECs with serially connected 130 cells delivered a voltage of more than 100 V, which indicates their superior modularization. In another work, this screen printing technology was used to build an all solid-state m-EC on flexible substrate [121]. The typical screen printed m-EC consisted of the printed Ag electrode, MnO2 /onion-like carbon as active material and polyvinyl alcohol: H3 PO4 as the solid-state electrolyte. The printed solid-state flexible m-ECs exhibited a high capacity of 7.04 mF cm−2 at a current density of 20 μA cm−2 , mainly originating from the contribution of the active MnO2 /onion-like carbon. The screen-printed m-EC also retained 80% of the specific capacity after 1000 cycles. Moreover, the fabricated m-ECs showed excellent high mechanical flexibility when the devices were bent to a radius of 3.5 mm. Screen printing process was also used to construct asymmetric m-ECs. For example, the planar asymmetric m-ECs with Ti3 C2 Tx as the negative, Co–Al layered double hydroxide nanosheets as the positive, and polyvinyl alcohol/KOH as the gel electrolyte were fabricated by two-step screen printing process [122]. The fabricated m-ECs exhibited high voltage of 0.40–1.45 V, enhanced areal energy densities of 8.8 μWh cm−2 compared to other reported normally 5000 S cm−1 , and the final screen-printed m-EC can provide a high volumetric capacitance of 338 F cm−3 , volumetric energy density of 18.8 mWh cm−3 , and power density of 40.9 W cm−3 [123]. In addition, the printed m-EC maintained high capacitance of 91.6% after 8000 cycles and had significant mechanical flexibility, showing an initial capacitance of 88.6% after 2000 bending cycles. This method does not require additional metal-based contacts and interconnects and shows great application potential in wearable system-on-a-chip. Since 2013, Wu et al. have been conducted on m-LIBs and m-ECs based on screen printing technique [69, 120, 124–139]. The screen-printed 2D interdigitated m-LIBs or m-ECs exhibited excellent volume energy density, flexibility, high-temperature performance, and integration of bipolar batteries. For example, the planar m-LIBs were fabricated based on the interdigital patterns of LiFePO4 /graphene as cathode

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LTO LTO

LFP

LFP

EG

LFP LTO

EG

EG

EG Vacuum

Figure 6.15 Schematic fabrication of flexible m-LIBs using Li4 Ti5 O12 anode and LiFePO4 cathode. Source: Zheng et al. [69].

and Li4 Ti5 O12 /graphene as anode working in ionogel electrolyte without polymer binder and separator. Figure 6.15 shows the mask-assisted fabrication of m-LIBs using LiFePO4 cathode and Li4 Ti5 O12 on flexible substrate. The fabricated m-LIBs delivered a high volumetric energy density of 125.5 mWh cm−3 and excellent rate capability due to the multidirectional Li+ diffusion mechanism. Moreover, m-LIB has excellent miniaturization and simplified control of fabrication. Besides m-LIBs, Wu et al. fabricated series graphene-based m-ECs based on screen-printing techniques, which can be found in some papers and reviews [120, 124–139]. Graphene, fluorine-modified graphene, graphene/polyaniline, B-doped graphene, nitrogen-doped graphene, S-doped graphene, graphene/thiophene, and transition metal oxides/graphene were used to construct m-ECs with high power density and areal energy density. For example, a fluorine-modified graphene was fabricated by an electrochemical exfoliation of graphite, as shown in Figure 6.16a–d. This fluorine-modified graphene was used to construct m-ECs by a screen-printing technology, as shown in Figure 6.16e–h. The final m-ECs show exceptional cyclability with ∼93% after 5000 cycles, robust mechanical flexibility with 100% of capacitance retention bended at 180∘ . Using highly conducting graphene ink, the screen printing technologies were also used to construct graphene-based ultrahigh-voltage integrated m-ECs on flexible substrates, as shown in Figure 6.17. As a result, the output voltage and capacitance of integrated m-ECs are readily adjustable through connection in well-defined arrangements of integrated m-ECs. As a proof of concept, an energy storage pack of integrated m-ECs with 130 m-EC units can output a high voltage exceeding 100 V, which demonstrate superior modularization and performance uniformity.

6.7 Printing Technologies for 2D Interdigitated Cells

(a)

(b) Electrochemical intercalation

(c) Exfoliation

Fluorination

(f)

(e) Mask removal

Transfer to PET

(g)

(d) Filtration

(h)

Figure 6.16 Construction of fluorine-modified graphene-based m-ECs. (a−c) Electrochemical exfoliation of graphite to fluorine-modified graphene via intercalation, fluorination, and exfoliation (red balls: BF4 − anion; blue balls: H2 O molecule). (d) Fluorinemodified graphene ink. (e) Fabrication of fluorine-modified graphene patterns. (f) As-fabricated m-EC on the PTFE membrane. (g) Transfer of m-EC onto the PET substrate. (h) Picture of nine m-ECs obtained on a flexible PET substrate. Source: Zhou et al. [134]. Reproduced with permission of American Chemical Society.

Screen print

In

te gr at

io

n

~100 V

Figure 6.17 Fabrication of an ultrahigh-voltage integrated m-ECs on flexible substrates by a screen printing technology. Source: Shi et al. [120].

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6.7.4

Inkjet Printing

Inkjet printing is a well-known printing technique to deposit tiny droplets of ink onto paper, plastic, or other substrates. Since the late 1970s, the inkjet printer contorted by computer has been widely used to reproduce digital images. Nowadays, three manufacturers, Canon, Epson, and HP, dominate the majority of inkjet printer market. Typically, HP and Canon use heating plate system that creates vapor bubbles to form and eject ink droplets through the nozzle. In contrast, Epson uses an array of piezoelectric plates that drive droplets through the nozzle. In both printer types, nozzle sizes are in the range of 20–30 μm and droplets are in a range of 10–20 pL. The main attractive feature of inkjet printing is that it is a noncontact, additive patterning and maskless method approach, which allows the deposition of versatile thin films, and the designs of films can be changed easily. Other advantages of the technology include low costs, less material waste, and scalability for large area manufacturing [140]. Therefore, it has been recently used as a major fabrication method for assembling miniaturized cells [141–145]. For example, the porous composite LiFePO4 electrode for m-LIB was prepared by inkjet printing [146]. The aqueous ink copolymer consists of the low molecular weight poly-acrylic-co-maleic acid copolymer having more suitable rheological properties for inkjet printing. The thin and porous electrode films prepared by ink-jet printing show high rate charge/discharge performance and good cycle performance. In addition to these outstanding electrochemical properties, this work also highlights the importance of rheological properties of the inks. Figure 6.18 shows the all solid-state asymmetric in-plane m-ECs based on lamellar potassium cobalt phosphate hydrate (K2 Co3 (P2 O7 )2 ⋅2H2 O) nanocrystal whiskers and graphene by inkjet printing [147]. Using inkjet printing, complex in-plane m-ECs were easily obtained in the simple and cost-effective manner. The fabricated m-ECs exhibited a high specific capacitance of 6.0 F cm−3 , long cycling stability up to 5000 cycles, and maximum energy density of 0.96 mWh cm−3 . Figure 6.19 shows the all solid-state planar m-ECs on the paper substrates by ink-jet printing [148]. First, the digitally designed interdigital electrode patterns were printed on paper substrate with rGO ink to form a conductive matrix. The negative electrode was then printed with activated carbon-Bi2 O3 ink, and the positive electrode was printed with rGO-MnO2 ink. Each ink used the different printer nozzles on the half of the pre-printed conductive pattern to form an asymmetric design. Polyvinyl alcohol-KOH electrolyte ink was printed on the electrode pattern and cured to complete the device. The prepared planar m-EC showed excellent electrochemical performance, high energy density of 13.28 mWh cm−3 at a power density of 4.5 W cm−3 .

6.8 Electrochemical Deposition Method for 2D Interdigitated Cells The EPD or electrolytic deposition (ELD) is another liquid-based deposition technique. The redox reaction can be triggered by applying a potential between

6.8 Electrochemical Deposition Method for 2D Interdigitated Cells

KOH/PVA Active material Silver PET Graphene nanosheets

Current density (mA cm–2)

2 PET

K2Co3(P2O7)2.2H2O nanocrystal whiskers

(a)

1

0

–1 Scan rate--30 mV s–1 –2 –0.2

0.2

0.6 0.4 Current--60 mA cm–3

0.2 0

50

100 150 Time (s)

Specific capacitance (F cm–3)

0.8

0.0

0.4 0.6 Potentail (V)

0.8

1.0

1.2

10 1.07 V 0.70 V 0.60 V 0.50 V

1.0 Potential (V)

0.0

(b) 1.2

(c)

1.07 V 0.80 V 0.70 V 0.60 V 0.50 V

8 6 4 2 0

200

(d)

0.5

0.6

0.7 0.8 0.9 Potential (V)

1.0

1.1

Figure 6.18 (a) Schematic view of the printed flexible m-EC, (b) CV curves, (c) galvanostatic charge/discharge curves, and (d) specific capacitances with the increase of the potential window. Source: Pang et al. [147].

Step 2

Step 1

Conducting ink

Negative electrode ink

Positive electrode ink

Step 3

Electrolyte ink

Inkjet-printed negative Inkjet-printed electrolyte Band paper substrate Conducting patterns on Inkjet-printed negative and positive electrodes bond paper substrate electrode

Figure 6.19 Schematic illustration of the stepwise fabrication of the printed m-ECs using different electrode materials and electrolyte. Source: Sundriyal and Bhattacharya [148].

the counter electrode and the conductive surface of the material expected to be deposited on it. These two methods are commonly used to apply metal protective films on industrial scale, but they can also be used to produce various battery layers on a laboratory scale. EPD is based on the suspension of particles in the solvent, whereas ELD is based on solution of salts, i.e., ionic species [149].

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V Electrolyte

V Charged particle

Suspension

Metal cations Anions Electrode Electroplating

Figure 6.20

Electrophoretic deposition

Schematic illustration of (a) cathodic EPD and (b) anodic EPD.

EPD is a colloidal process in which the suspended particles are pushed from the suspension medium onto the substrate through electric field. EPD was discovered in 1808, then practically used to deposit ThO2 particles on the Pt cathode emitter for electron tube [150]. Thereafter, EPD has attracted wide interest as the material processing technology for wide range of technical applications. EPD enables the production of unique microstructures and nanostructures as well as novel and complex material combinations. In recent years, EPD has also been used to produce composite materials for the miniaturized cells. Figure 6.20 shows a schematic illustration of the two types of EPD processes, cathodic EPD and anodic EPD. For cathodic EPD, the particles are positively charged; deposition occurs on the cathode [151]. On the contrary, the deposition of negatively charged particles on the anode for anodic EPD [151]. By modification of the surface charge on the particles, the EPD process is very versatile for miniaturized cells applications. ELD process uses the current to reduce dissolved metal cations forming thin film on the electrode. ELD process itself involves creating electrolytic cells, which consists of two electrodes immerging in the electrolyte solution. When the current is turned on, the positive ions in the electrolyte move to the negatively charged electrode (called the cathode). When the positive ions reach the cathode, they combine with electrons and lose their positive charge. At the same time, the negative ions reach the positive anode, they transfer electrons to the positive anode and lose their negative charge [149]. For example, Figure 6.21 shows the high-performance on-chip m-EC by highly dispersed rGO/polypyrrole on interdigital-like electrodes by EPD [152]. The electrode film was easily and uniformly structured on the substrate. The fabricated m-EC demonstrated high volumetric capacitance ∼147.9 F cm−3 , high energy density ∼13.15 mWh cm−3 at a power density of 1300 mW cm−3 and retained ∼ 71.7% of the initial capacitance after 5000 cycles. EPD has been considered as an inexpensive and relatively simple method for the preparation of thin-film m-LIBs [153–155]. By EPD of CuS film on the nickel or gold-coated silicon substrate, the fabricated m-LIBs delivered the excellent peak-power capability of 50 mW cm−2 and stable electrochemical behavior [153]. In another work, the m-LIBs were fabricated using LiFePO4 thin-film cathodes by EPD method in 2012 [155]. The role of polymers and surface-active additives in electrolytic baths, voltages, and deposition schemes has been studied. The role

6.9 Laser Scribing for 2D Interdigitated Cells

RGO/PPy RGO/PPy RGO/PPy RGO/PPy RGO/PPy RGO/PPy RGO/PPy RGO/PPy

As-deposited RGO/PPy

Titanium sheet

PET substr ate

Electrophoretic deposition

te

Titanium sheet

Figure 6.21

As-deposited RGO/PPy Gel electrolyte

PET substr a

PET substr ate

Solidified

Solid-state electrolyte

Titanium sheet

Schematics of the fabrication process of m-EC by EPD. Source: Liu et al. [152].

of polymers and surfactant additives in electrolytic bath, voltage, and deposition protocol has been studied to obtain highly adhesive, compact pristine LiFePO4 and polymer-LiFePO4 films for the planar and 3D m-LIBs. The fabricated m-LIBs showed a peak-pulse-power capability of 175 mW cm−2 and stable electrochemical behavior for more than 200 cycles at 100% depth of discharge. In summary, EPD and ELD processes are promising ways to build 2D interdigitated m-LIBs and m-ECs because these processes can easily control the thickness and morphology of the film deposition by simply adjusting the deposition time and applied potential. One of the major disadvantages of EPD and ELD techniques is their low resolution. To solve this, a simple approach was demonstrated to fabricate the on-chip m-ECs with enhanced performances by minimizing the gap between positive and negative [107]. Figure 6.22 shows that the current collector was first patterned onto a silicon oxide wafer and protected by insulating mask layer using photolithography technique. Thereafter, the material deposition was facilitated at the wafer level, and the device was implemented by lift-off process. This method avoided the necessity of depositing electrode material onto a micro-sized patterned current collector, which usually results in deposition on unwanted areas with minimum gap of about 1 μm. The fabricated devices showed a high capacitance 3 mF cm−2 using a hydrous RuO2 electrode and a high scan rate ability up to 10 000 V s−1 for multi-walled CNT-based m-EC, which was five orders of magnitude higher than those of conventional ECs.

6.9 Laser Scribing for 2D Interdigitated Cells Laser scribing has been used to add, remove, and modify different types of materials without physical contact between the laser and the material. Laser scribing can be used to generate complex patterns with high resolution and controlled composition. Laser scribing technology has also been used in 2D interdigitated cells. Laser engineering structured electrodes significantly improve cycle retention and increase the power density and energy density of miniaturized cells [156–160].

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SiO2 layer

Figure 6.22 Fabrication process high-resolution pattern by lift-off process. Source: Dinh et al. [107].

Si wafer cross-linked photoresist

Ti/Au deposition

hRuO2 electrodeposition

Lift-off

Graphene was reduced from GO by direct laser using standard LightScribe DVD optical drive [161]. A GO film on a flexible substrate is placed on top of LightScribeenabled DVD disc. GO film changes from golden brown to black as it reduced to laser-scribed graphene. The GO immediately changed into well-exfoliated few-layered laser-scribed graphene film by low-power infrared laser. Then, a symmetric m-EC consisted of two identical laser-scribed graphene electrodes, ion-porous separator, and electrolyte. The produced graphene films with excellent mechanical property showed high electrical conductivity (1738 S m−1 ) and specific surface area (1520 m2 g−1 ). The m-ECs fabricated with these electrodes exhibited ultrahigh energy density in different electrolytes while maintaining high power density and excellent cycle stability. Moreover, the fabricated m-ECs maintained excellent electrochemical property under high mechanical stress and are therefore expected to be used for high-power, flexible microelectronic devices.

6.9 Laser Scribing for 2D Interdigitated Cells

Electrolyte LIG electrodes Kapton tape Silver paint Polymide

(a)

(b)

(c)

Figure 6.23 (a) The digital photograph of laser-induced graphene-based m-ECs with twelve interdigital electrodes; scale bar in 1 mm. (b) SEM image of laser-induced graphene electrodes; scale bar in 200 μm, and (c) schematic diagram of laser-induced graphene-based m-EC architecture. Source: Lin et al. [162]. Reproduced with permission of Springer Nature.

Figure 6.24 Fabrication of planar m-ECs using transferred chemical vapor deposition graphene films and laser scribing. Source: Ye et al. [156].

In another work, 3D porous graphene films were prepared from polyimide by laser scribing in 2014 [162]. The prepared porous graphene was directly used to fabricate 2D interdigitated m-ECs. Figure 6.23 shows the digital photograph of laser-induced graphene-based m-EC with 12 interdigital electrodes. The experimental results showed that the graphene can be readily patterned by laser direct writing process to interdigitated electrodes for in-plane m-ECs with specific capacitances of >4 mF cm−2 and power densities of approximately 9 mW cm−2 . Figure 6.24 shows the planar m-ECs based on CVD graphene films fabricated by laser scribing [156]. The m-ECs exhibited simultaneously ultrahigh energy density of 23 mWh cm−3 and power density of 1860 W cm−3 in an ionogel electrolyte. The work demonstrated that the laser scribing is a highly efficient approach to fabricate the large areal m-ECs with diverse planar geometry, exceptional flexibility, and capability of customer-designed integration. Besides GO reduction method, lignin can be directly transformed into graphene by laser-scribing method, as shown in Figure 6.25 [163]. Specifically, lignin films were directly converted into graphene electrodes by one-step CO2 laser-scribing. The obtained graphene electrodes have hierarchical porosity, high electrically conductivity (6.2 S cm−1 ), and high specific surface area (338.3 m2 g−1 ). These electrodes prepared by laser-scribing can be used directly as m-EC electrode without the need for a

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Figure 6.25 Schematic diagram of laser scribed graphene electrodes for m-ECs. Source: Zhang et al. [163].

binder and current collector. The fabricated m-ECs exhibited high areal capacitance (25.1 mF cm−2 ), high volumetric energy density (1 mWh cm−3 ), and high volumetric power density (2 W cm−3 ).

6.10 In Situ Electrode Conversion for 2D Interdigitated Cells The miniaturized cells can also be manufactured by patterning and etching standard materials and then converting them into nanoporous carbon as an active material [164]. For example, pyrolytic photoresist-derived carbon has become an attractive electrode material for electrochemical applications due to its good electrochemical properties and excellent properties, including easy manufacturing processes, high rate performances, and high specific capacitance. As shown in Figure 6.26, the patterned electrodes were prepared though three primary steps: photoresist deposition, patterning, and pyrolysis. First, the photoresist was spun-coated to a thickness of 10 μm on the SiO2 /Si wafer. Thereafter, the interdigitated electrodes were patterned by UV lithography and heated at high temperatures to obtain the pyrolytic photoresist-derived carbon, resulting device has an excellent long-term cycling stability. The maximum energy density of 3 mWh cm−3 was higher than that of commercial thin-film LIBs, with the maximum power density of 26 W cm−3 . The combination of the pyrolytic photoresist with an

6.10 In Situ Electrode Conversion for 2D Interdigitated Cells

Lithographic patterning Photoresist Substrate

Substrate

Pyrolysis

Application of ionogel Ionogel Substrate

Figure 6.26 [165].

Substrate

Schematic illustration of the m-ECs fabrication process. Source: Wang et al.

ionogel electrolyte is promising for applications in integrated energy storage for microelectronic devices [165]. Another typical carbon material, carbide-derived carbon was produced by selectively etching metals from metal carbides by chlorine at high temperatures [166]. The carbide-derived carbon has been shown to perform well as an active material in conventional ECs [167]. The carbide-derived carbon has two attractive advantages for the fabrication of m-EC: (i) The conductive precursor carbide can be deposited in uniform and thick films by chemical vapor deposition and physical vapor deposition techniques [168]. (ii) The chlorination process can be carried out at low temperatures, and the final coating adheres well to the perfect interface on the atom, thereby minimizing device impedance. In another work, an m-EC based on carbide-derived carbon film was constructed by chlorine-containing plasma etching. The volumetric capacitance (∼60 F cm−3 ) for carbide-derived carbon film of ∼50 μm in tetraethylammonium tetrafluoroborate (TEABF4 ) was similar to a ∼300 μm traditionally fabricated electrode from carbon powders (∼50 F cm−3 ). Similarly, the volumetric capacitance for films thicker than ∼100 μm in 1M H2 SO4 electrolyte was closed to what was measured on a ∼300 μm thick traditionally fabricated electrode (∼80 F cm−3 ) [166]. A graphene-based 2D interdigitated m-ECs were also fabricated based on micropatterning of in situ reduced GO films on both rigid and flexible substrates. Figure 6.27 shows the fabrication procedure of the m-EC on a silicon wafer. First, a GO film was spin-coated on an oxygen plasma treated silicon wafer. Then, the GO film was reduced by CH4 plasma. Finally, lithography techniques were applied to fabricate graphene-based interdigital microelectrode patterns through

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(b)

(a)

Oxygen plasma

(c) Go film

CH4 plasma reduction

Spin coating Si wafer

MPG film

700 °C Si wafer

Si wafer Masking and Au sputtering

(f)

El ec tro lyt e

236

Si wafer

Au collector

(e)

Au collector

(d)

Au collector MPG film

Gel electrolyte Solidified

Oxygen plasma etching Si wafer

Si wafer

Figure 6.27 (a–f) Fabrication of m-ECs integrated onto a silicon wafer. (a) Oxygen plasma surface treatment of silicon and then GO solution were spin-coated on surface-modified silicon, (b) CH4 plasma reduction, (c) deposition of patterned gold current collector, (d) oxygen plasma etching, (e) drop casting of gel electrolyte, and (f) gel electrolyte solidification. Source: Wu et al. [169].

the deposition of gold current collectors. This 2D interdigitated m-EC allows for working at ultrahigh rates up to 1000 V s−1 , three orders of higher than that of traditional ECs.

6.11 Fabrication Technologies for 3D In-plane Miniaturized Cells Although 2D miniaturized cells can be miniaturized and integrated, those 2D geometries are limited in energy density and power density for various microelectronic devices. Therefore, 3D m-LIBs are considered as an effective way to solve this challenge in order to achieve high energy and power density within the footprint area [170, 171]. 3D geometry can significantly maximize the volume of active material in unit area while keeping its thickness small for rapid Li diffusion. As shown in Figure 6.5c,d, there are two kinds of architectures for 3D miniaturized cells: 3D interdigitated configuration and 3D stacked configuration. Compared with the 2D configuration, the 3D architecture can maximize the energy and power through a unique architecture design and increase the load of active materials within a given footprint area, while achieving short diffusion paths and large capacity. The number of different technologies for 3D miniaturized cells has been proposed over the past decade. The fabrication of 3D miniaturized cells in complete 3D configuration is a particularly challenging in the initial stage of this technology field. Nevertheless, significant results have been recently achieved in this new field due to careful architecture and the new fabrication methods. In the following section, we will give an overview of various fabrication technologies toward 3D miniaturized cells.

6.11.1 3D Printing for 3D Interdigitated Configuration Cells The 3D printing is a novel printing technology, which provides the unique way for rapid prototyping of many applications due to its unique advantages compared to

6.11 Fabrication Technologies for 3D In-plane Miniaturized Cells

conventional manufacturing methods, such as in geometrical shape design as well as rapid prototyping, especially complex 3D structure constructions. 3D printing can be defined as a digitally controlled process to fabricate 3D successive layers of material until final structure is created [172]. The 3D printing process is type of additive manufacturing process, which builds 3D object by successively depositing material layer by layer [173]. The development of 3D printing technology has been driven by the key factors such as rapid prototyping, the ability to print large structures, reducing printing defects, and enhancing mechanical properties. Today, 3D printing techniques have been developed to meet the needs of printing complex structures with high resolution. As shown in Figure 6.28, there are typical five kinds of 3D printing: (i) fused deposition modeling, (ii) power bed and inkjet head 3D printing, (iii) stereolithography, (iv) selective laser sintering, and (v) 3D plotting/direct write [174]. Figure 6.28a shows the most common 3D printing methods primarily using polymer filaments known as fused deposition modeling by controlled extrusion Filament B

Filament A

Inkjet print head(binder delivery) Roller

Part

Drive wheel Extrusion nozzle Part Print bed

Material spool

Fabrication paltform

Powder bed Powder supply platform V Fabrication platform (b)

V

(a)

Laser source Laser source

Scanner system

Scanner system Roller Part

Fabrication platform V

(c)

Liquid photopolymer

Powder bed

Powder supply platform

(d)

V Fabrication platform

Pneumatic pressure/stepper motor

Material (liquid, paste, hydrogel) V Plotting medium Part

(e)

Figure 6.28 Schematic diagram of typical 3D printing process (a) fused deposition modelling, (b) power bed and inkjet head 3D printing, (c) stereolithography, (d) selective laser sintering, and (e) 3D plotting/direct write. Source: Wang et al. [174].

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of thermoplastic filaments. Figure 6.28b shows the powder-liquid 3D printing based on powder processing. Figure 6.28c shows the stereolithography used the photopolymer that can be cured by UV laser. The UV laser is controlled in desired path to inject into the resin container, and the photo curable resin is polymerized into 2D patterned layer. After each layer is cured, the platform is lowered and then another layer of uncured resin is ready to be patterned. Figure 6.28d shows the selective laser sintering technique based on powder processing. A laser beam with controlled path replaces the use of a liquid adhesive, which scans the powder to sinter by heating. The 3D plotting is based on extruding viscous material from a pressurized syringe to create 3D shape. A comprehensive review of these methods was discussed in the review paper by Ngo et al. [175]. The photolithography/etching and 2D inkjet printing are confined to two dimensions. 3D printing offers almost lots of possibilities for rapid 3D prototyping. Therefore, it has been considered for application in the wide range of fields, from mechanical engineering, medicine, and materials science to chemistry. 3D printing is also considered as a revolutionary and attractive process for manufacturing electrochemical energy storage devices, and recently 3D-printed cells have been reported [176]. The 3D printing technology can accurately control device spatial geometries and architectures and enhance both energy and power density of the miniaturized cells. In addition, 3D printing can provide excellent controllability on the electrode thickness through a greatly simplified and low-cost process. The 3D interdigitated m-LIB architectures composed of Li4 Ti5 O12 anode and LiFePO4 cathode were fabricated by 3D printing concentrated inks as shown in Figure 6.29 [176]. Convenient 3D printing technology allows functional inks to be accurately patterned in a silky form ranging from 100 μm2 to 1 m2 , with a minimum feature size as small as 1 μm. The 3D-printed m-LIB exhibited high areal energy and power densities for applications in autonomously powered microelectronic devices. A convenient fabrication process was successfully demonstrated for 3D printing of m-ECs.[177] The widths and heights of the 3D-printed patterns can be easily adjusted by 3D printing parameters, such as the diameter of the nozzle, applied printing pressure, CNT contents, and working distance. The ink should exhibit shear thinning behavior and viscoelasticity, allowing each layer to maintain its shape, while still providing sufficient fluidity for the substrate and interlayer adhesion [178]. Typically, the printable ink system consists of active materials, inactive organic binder/additives, and solvents. The significant progress has been made in ink writing for the miniaturized cells based on CNT, graphene, MoS2 , and several other materials; however, the most printable inks have been made through the use of additives as co-solvents or surfactants to adjust their rheological properties, which can affect the electrical and electrochemical properties of the prepared electrodes and may need removal after printing [179–182]. To solve this, all solid-state m-ECs were fabricated via 3D printing of additive-free and MXene water-based ink [183]. The all solid-state m-ECs benefited from the high electrical conductivity and excellent electrochemical properties of 2D Ti3 C2 Tx MXene and 3D electrode architecture, which provided high area and volume energy density. It turned out that high-concentration MXene ink exhibits ideal

6.11 Fabrication Technologies for 3D In-plane Miniaturized Cells

Nozzle (30 μm)

LTO LFP

Au

200 μm

2 mm

Figure 6.29 Fabrication, SEM morphology, and optical picture of m-LIB with 3D interdigitated architectures. Source: Sun et al. [176]. Reproduced with permission of Wiley.

viscoelasticity at room temperature and can be used for extrusion printing, it can be used to manufacture m-ECs with various structures and electrode thicknesses on various substrates. This 3D-printed m-ECs showed excellent electrochemical performance with extremely high areal capacitance of ∼1035 mF cm−2 and the maximum energy density of m-ECs reached 51.7 μWh cm–2 .

6.11.2 3D Interdigitated Configuration by Electrodeposition 3D interdigitated configuration cells can also be fabricated by electrodeposition method. Figure 6.30 shows the fabricated high-performance 3D m-LIBs based on interdigitated 3D bi-continuous nanoporous electrodes, which were obtained from nickel scaffold by electrodepositing nickel through polystyrene opal self-assembled on a glass substrate with interdigitated gold templates [184]. The m-LIBs with a volume of 0.03 mm3 had power density of 7.4 mW cm−2 μm−1 at 870C rate. The 3D microarchitecture optimizes both ion and electron transportation for high power delivery, resulting in high power and scalable expansion into larger areas. Photolithography, also known as optical lithography or UV lithography, is the process used in micro-processing to pattern the thin film or the bulk of substrate (or wafer). Photolithography uses light to transfer the geometric pattern from photomask (or optical mask) to photosensitive chemical photoresist on the substrate. A series of chemical treatments etch the exposure pattern into the material or deposit new material in the desired pattern on the material under the photoresist [185]. In

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Figure 6.30 (a) Schematic fabrication process, (b) the electrodes consisted of electrolytically active layer in red and yellow coated on a bi-continuous nickel scaffold in blue, (c) SEM cross section of the interdigitated electrodes spanning two periods, and (d) top-down SEM of the interdigitated electrodes. Source: Pikul et al. [184].

complex integrated circuits, the CMOS wafer may experience up to 50 photolithography cycles. Figure 6.31 shows the procedure to fabricate the complete carbon/polymer 3D m-LIBs by photolithography and electrodeposition technology [186]. This process involved three steps: patterning the photoresist using photolithography, pyrolyzing the patterned photoresist to form carbon electrode arrays and carbon current collectors, and electrochemical polymerization of dodecyl benzene sulfonate-doped polypyrrole on one set of 3D carbon arrays. The electrodeposited electrode array showed better gravimetric reversible lithium intercalation than that of 2D electrodeposited films.

6.12 Fabrication of Miniaturized Cells with 3D Stacked Configuration Demonstrating 3D all solid-state miniaturized cells has been a long-standing goal of developing miniaturized cells with high power and high areal energy density. Ideally, the 3D stacked m-LIBs can maximize the volume of active material per unit area, while keep its thickness less to achieve rapid lithium diffusion. There are

6.12 Fabrication of Miniaturized Cells with 3D Stacked Configuration

Figure 6.31 Schematic fabrication of carbon/polypyrrole 3D interdigitated configuration m-LIB. Source: Min et al. [186].

Photoresist (PR)

SiO2

Si Lithography

PR arrays

Pyrolysis C

Current collectors Electrochemical deposition C

PPY

two main procedures for the fabrication of 3D stacked configuration, one is based on sequentially deposition onto 3D current collectors, and the other is based on microchannel plated deposition methods.

6.12.1 3D Stacked Configuration by Template Deposition To increase the effective surface area of miniaturized cells, 3D current collector can be formed on conventional planar substrate. Such structure can be prepared by template combined with classical solid-state deposition techniques. After removing the template, the 3D morphology is obtained, which can act as 3D current collectors, and then the active material layers can be subsequently deposited on it, as shown in Figure 6.32. To obtain full 3D stacked m-LIBs, several factors should be considered: (i) enough space should exist in the structure to accommodate all the layers that will be deposited to make the 3D m-LIBs, (ii) The aspect ratio of the 3D current collectors must be high enough to obtain a significant area increase, (iii) conformal coating of all battery layers, and (iv) electronically insulating between all active material layers. Therefore, architectures of 3D current collectors and deposition techniques play key roles for the fabrication of 3D stacked m-LIBs. The electrodeposition of aluminum nanopillars onto planar aluminum substrate and the surface area enhancement (A) is calculated as [188]: 𝜋pdh A=1+ 2 (6.17) ip sin 𝜃

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Template

Figure 6.32 3D m-LIBs based on template deposition of nanorods. Source: Oudenhoven et al. [187].

Pores filled

Template selectively removed

Battery layers deposited

Figure 6.33 Architecture of 3D m-LIB. Source: Talin et al. [189].

L3D

Cu LiPON LiCoO2 Pt

Si

s is the spacing between the rods, measured between the centers of the rods, 𝜃 is the angle of the pattern in which the rods are positioned, d and h are the diameter and height of the deposited columns, respectively. A surface area enhancement factor of 30 can be obtained by typical values of h = 10 μm, d = 200 nm, s = 500 nm, and 𝜃 = 60∘ . The 3D solid-state m-LIB consisted of Si micro columns was sequentially deposited by physical vapor deposition as shown in Figure 6.33 [189]. As shown in Figure 6.34, this 3D m-LIB consisted of cylindrical or conial Si micropillars onto with layers corresponding to the current collector, cathode, electrolyte, and anode was deposited sequentially. The power performance of these 3D m-LIBs greatly lags behind that of similarly prepared planar batteries, and the cause of this poor power performance was the combination of 3D m-LIB structural inhomogeneity and low electrolyte ionic conductivity, which together resulted in a highly nonuniform internal current density distribution and poor utilization of cathode.

6.12 Fabrication of Miniaturized Cells with 3D Stacked Configuration

Figure 6.34 FIB cross section of 3D m-LIB by physical vapor deposition. Source: Talin et al. [189]. Reproduced with permission of American Chemical Society.

Figure 6.35 SEM image of aluminum nanorods. Source: Cheah et al. [190]. Reproduced with permission of American Chemical Society.

In another work, the nanostructured 3D m-LIB has been fabricated, which consisted of current collector of aluminum nanorods (Figure 6.35), the uniform TiO2 layer (17 nm) covering the nanorods made using atom layer deposition, electrolyte, and metallic lithium. The capacity of 3D architecture m-LIB is ten times as compared to 2D counterpart for the same footprint area, as shown in Figure 6.36. Figure 6.37 shows the 3D m-LIB based on imprinted microelectrodes and integrated through layer-by-layer stacking [191]. LiMn2 O4 microelectrodes were imprinted on indium tin oxide-coated glass surface by solvent-assisted imprinting (i) Indium tin oxide served as a cathode current collector. The cathode array was coated layer by layer with drop-casted poly (dopamine acrylamide)-co-poly (ethylene glycol) methyl ether methacrylate as separator/gel polymer electrolyte after thermal annealing in N2 flow, (ii) Li4 Ti5 O12 /mesoporous carbon composite as a

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Figure 6.36 The capacity of geometrical surface area in mAh cm−2 of 3D m-LIB and 2D m-LIB. The galvanostatic cycling current is 0.001 mA. Source: Cheah et al. [190].

0.02

Capacity (mAh cm–2)

244

0.015 3D micro-battery 0.01

0.005 2D micro-battery 0

0

10

20 30 Cycle number

40

50

Figure 6.37 Schematic illustration of interdigitated microelectrode array (a) m-LIB fabrication (b): (i) fabrication of micro-cathode array via solvent-assisted imprinting. (ii) Coating of polymer separator. (iii) Coating of Li4 Ti5 O12 /mesoporous carbon electrode. (vi) Evaporation of aluminum current collector. Source: Li et al. [191].

counter electrode, (iii) finally, the thin layer of aluminum was thermally evaporated on top as an anode current collector, and (vi) the active footprint of m-LIB was determined by the overlapping area of the two current collectors and is 20 mm2 . Although the template deposition method can result in higher energy density, there are two disadvantages for 3D current collector deposition method. The final structure of the rod will be dense because most membranes have open structure, which will greatly limit the possibility of subsequent active material layer deposition.

6.12 Fabrication of Miniaturized Cells with 3D Stacked Configuration

Moreover, these rods are very fragile and highly sensitive to mechanical damage during charge and discharge [187].

6.12.2 3D Stacked Configuration by Microchannel-Plated Deposition Methods To fabricate 3D stacked configuration, another strategy is microchannel-plated deposition method to form more robust 3D structure. Silicon, metal, glasses, or polymers can be used to build microchannel-plated architecture. Figure 6.38 shows the fabricated m-LIB based on microchannel-plated deposition method. This architecture can obtain a good surface area enhancement without compromise mechanical strength. The surface area increase (A) can be obtained as follows [21]: ) ( d 𝜋d +2 (6.18) t − A= 2 (d + s)2 d, s, and t represent the microchannel diameter, the inter channel spacing, and the substrate thickness, respectively. The surface area enhancement is close to 23 with typical values of d = 50 μm, s = 10 μm, and t = 500 μm, when compared to a single-sided planar configuration [21]. As an example, 3D m-LIB composite cathode with microchannels was also fabricated. The composite thin-film cathodes were electrodeposited from the bath modified by polyethylene glycol dimethyl ethers and polyethylene oxide additives of different molecular weights and concentrations. Its reversible capacity reached 3.5 mAh cm−2 , which is 20–30 times higher than that of the planar 2D thin-film cells with the same footprint [192]. The important technological obstacle to 3D stacked configuration is the conformal deposition of thin films on 3D structures. It was proposed that atomic layer deposition (ALD) was a powerful technique that can enables conformably coat thin film Figure 6.38 Schematic illustration of 3D thin-film m-LIB based on microchannel plates. Source: Oudenhoven et al. [187].

Contact

Substrate

Contact

Substrate

Electrode 2 Electrolyt Electrode 1

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(a)

(f)

(g)

(b)

(c)

(e)

(h)

(d)

(i)

Figure 6.39 Fabrication and characterization of 3D m-LIBs. Source: Pearse et al. [193]. Reproduced with permission of American Chemical Society.

on complex substrate [87]. The anatase TiO2 negative electrode was coated on 3D tubes with Li3 PO4 as an electrolyte. The surface capacity was significantly increased 105 times, from 3.5 μAh cm−2 for a planar configuration up to 0.37 mAh cm−2 for 3D configuration. In another work, m-LIB was fabricated by ALD process. This 3D cylindrical pores were first etched on the Si substrates using deep reactive ion etching method (Figure 6.39a,b), and five m-LIB layers were subsequently deposited by ALD without pattering (Figure 6.39c–f). This m-LIB consisted of electrochemically pre-lithiated LiV2 O5 cathode, thin Li2 PO2 N solid electrolyte, and SnNx anode, as shown in Figure 6.39h,i. Figure 6.39g shows the optical picture of the m-LIBs. The multilayered battery structure enabled m-LIBs to deliver 37 μAh cm−2 ⋅μm with only 0.02% per-cycle capacity loss. Liquid electrolytes are still used in most 3D m-LIBs because of their high ionic conductivities, which results in excellent performances at room temperatures. However, the safety and leakage issues greatly limit the practical applications. In order to solve this problem, the solid polymer electrolytes based on poly(trimethylene carbonate) have also been implemented in 3D m-LIBs as shown in Figure 6.40 [194]. The LiFePO4 -coated carbon foams and Cu2 O-coated Cu nanopillars were coated with solid polymer electrolytes and used in m-LIBs. It was found that the functionalized poly(trimethylene carbonate) with hydroxyl end groups can form a uniform and well-covering layer on LiFePO4 -coated carbon foams. This m-LIB composed of Cu2 O-coated Cu nanopillars and solid polymer electrolyte prepared from a higher ionic conductive copolymer was constructed and showed good

6.13 Integrated Systems

Li-metal foil

(a)

LiFePO4-Coated carbon foam

Solid polymer electrolyte

3D-microbatteries

Li-metal foil

Cu pillars (b)

Cu2O coating

Figure 6.40 Fabrication of all solid-state Li-polymer 3D m-LIBs based on (a) LiFePO4 coated carbon foam electrode and (b) Cu2 O-coated Cu nanopillar electrode. Source: Sun et al. [194].

performance at ambient temperature and 60 ∘ C, and the footprint areal capacity of the cells was approximately 0.02 mAh cm−2 . In summary, the advantages of this microchannel plated deposition method possesses higher mechanical strength and feasibility than that of template deposition method [192]. The main challenge of this method is to find simple techniques and conditions to obtain conformal 3D deposition [187].

6.13 Integrated Systems In addition to direct use of the power sources for microelectronic devices, the future m-LIBs and m-ECs need to be combined with other energy harvest devices and sensors, such as solar cells, nanogenerators, and various sensors to develop integrated devices. The final devices could be made more rapidly and seamlessly when the microelectronic devices and miniaturized cells can be integrated together. Future microelectronic devices also require miniaturized cells to have multifunctionality and self-power ability. One of the effective strategies to meet the needs of specific wearable and portable applications is to implement an all-in-one flexible system on lightweight and bulky single-plane substrate. From this point of view, integrating or merging multiple functions into one device to enable it to adapt to a changing environment is essential for the development of miniaturized cells. As shown in Figure 6.41, Si-based solar cell was employed to connect paper-based integrated device of tandem miniaturized devices and polyaniline-based gas sensor to demonstrate that paper-based m-ECs can bridge solar cells and outlets. The integrated device consisted of the tandem m-EC bridging solar cell and gas sensor, which stores solar energy, therefore sustainably self-power the sensor [195].

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Tandem MSC A

Solar cell

PANI nanorods

Gas sensor

Figure 6.41 Integrated device consisted of the tandem m-EC bridging solar cell and gas sensor. Source: Guo et al. [195].

Strain sensor Solar cell

Figure 6.42 The stretchable m-EC array with integrated strain sensors and m-ECs. Source: Yun et al. [196].

Micro-supercapacitor Light Solar cells

Supercapacitors 5 mm Strain sensor

Figure 6.42 shows a fabricated stretchable array of high-performance solid-state m-ECs, which can be charged with integrated Si-based solar cells [196]. The m-EC achieved an aerial capacitance of 5.17 mF cm−2 and retained 80% of the initial capacitance after 5000 charge/discharge cycles. The composite film of fragmentized graphene foam and polydimethylsiloxane was used to fabricate the strain sensor. Twelve parallel connected m-ECs, a strain sensor, and solar cells were integrated on a single deformable polymer substrate. Attaching the entire integrated device to the wrist, the integrated strain sensor could detect both externally applied strain and the arterial pulse using the energy stored in the m-ECs from the solar cells. Figure 6.43 shows self-powered sensor system for gas detection in the wearable wristband fashion fabricated by inkjet printing [197]. The wearable wristband integrated device mainly consisted of printed silver interconnects, amorphous silicon

6.14 Summary and Perspectives

Printable supercapacitors

CH3COOLi/PVA electrolyte

Charging MnO2 + Li+ + e–

MnOOLi

Solar cells

Discharging

Inkjet printing nozzles

Switches Printed MnO2/rGO/PEDOT:PSS Active layer

Molecules

Ni SMD LED Printable SnO2 gas sensor

Voltage regulator Printed Ag interconnects

Figure 6.43 Schematic illustration of fabrication procedures and system operational mechanism of the flexible and wearable integrated self-powered smart sensor system. Source: Lin et al. [197].

solar cells array as energy harvesting and conversion module, planar MnO2 -based m-ECs as energy storage devices, and SnO2 gas sensor with a LED indicator as active and power consumption units. The stored solar energy can directly power the sensor and light up the LED indicator or can be stored in m-ECs. The use of flexible self-powered integrated micro devices enables all flexible components of the energy harvester, energy storage, and energy consumption to be integrated on small area flexible substrate, which is essential for specific and emergency applications in unpredictable situations of no external power or external electricity-cut environments, such as smart clothing, IoTs, and healthcare sensors [30].

6.14 Summary and Perspectives The m-LIBs and m-ECs are the most promising miniaturized cells to power the microelectronic devices, which can also be integrated on the semiconductor chips to meet the needs of the future smart microelectronic devices. The development is still in its early stage and many challenges to be overcome in the future. The main developments and challenges for miniaturized cells are as follows: (1) Fabrication technologies development: development of miniaturized cells do not only depend on fundamental research but also require advanced microfabrication technologies. Many microfabrication technologies have been evaluated, and significant progress has been made in improving surface energy, especially with 3D electrode designs. (2) Energy density development: one of the main challenges of miniaturized cells today is its low areal energy density. Another promising approach is to use 3D architectures for these tiny devices. Utilizing 2D materials, such as graphene,

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MXene, MoS2 , also has great potential for miniaturized cells with high energy density due to their outstanding properties. (3) The future miniaturized cells can also be combined with other various sensors to develop integrated devices. As stated before, combining m-ECs with pressure/gas sensors can achieve energy storage and multifunctions simultaneously [156]. Integrating or merging multiple functions into one device so that it can adapt to the changing environment is essential for the development of miniaturized cells providing consumers with more choices. Overall, despite the many challenges to be overcome, the miniaturized cell will come into use and provide promising applications in microelectronic field.

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174 Wang, X., Jiang, M., Zhou, Z. et al. (2017). 3D printing of polymer matrix composites: a review and prospective. Composites Part B: Engineering 110: 442–458. 175 Ngo, T.D., Kashani, A., Imbalzano, G. et al. (2018). Additive manufacturing (3D printing): a review of materials, methods, applications and challenges. Composites Part B: Engineering 143: 172–196. 176 Sun, K., Wei, T.-S., Ahn, B.Y. et al. (2013). 3D printing of interdigitated Li-ion microbattery architectures. Advanced Materials 25 (33): 4539–4543. 177 Yu, W., Zhou, H., Li, B.Q. et al. (2017). 3D Printing of carbon nanotubes-based microsupercapacitors. ACS Applied Materials & Interfaces 9 (5): 4597–4604. 178 Fu, K., Wang, Y., Yan, C. et al. (2016). Graphene oxide-based electrode inks for 3D-printed lithium-ion batteries. Advanced Materials 28 (13): 2587–2594. 179 Compton, B.G. and Lewis, J.A. (2014). 3D-Printing of lightweight cellular composites. Advanced Materials 26 (34): 5930–5935. 180 Guo, Y., Patanwala, H.S., Bognet, B. et al. (2017). Inkjet and inkjet-based 3D printing: connecting fluid properties and printing performance. Rapid Prototyping Journal 23 (3): 562–576. 181 Ambrosi, A. and Pumera, M. (2016). 3D-printing technologies for electrochemical applications. Chemical Society Reviews 45 (10): 2740–2755. 182 Moyano, J.J., Gomez-Gomez, A., Perez-Coll, D. et al. (2019). Filament printing of graphene-based inks into self-supported 3D architectures. Carbon 151: 94–102. 183 Orangi, J., Hamade, F., Davis, V.A. et al. (2019). 3D printing of additive-free 2D Ti3 C2 Tx (MXene) ink for fabrication of micro-supercapacitors with ultra-high energy densities. ACS Nano 14 (1): 640–650. 184 Pikul, J.H., Gang Zhang, H., Cho, J. et al. (2013). High-power lithium ion microbatteries from interdigitated three-dimensional bicontinuous nanoporous electrodes. Nature Communications 4: 1732. 185 Wikipedia (2019). Photolithography. https://en.wikipedia.org/wiki/ Photolithography (accessed 17 July 2019). 186 Min, H.S., Park, B.Y., Taherabadi, L. et al. (2008). Fabrication and properties of a carbon/polypyrrole three-dimensional microbattery. Journal of Power Sources 178 (2): 795–800. 187 Oudenhoven, J.F.M., Baggetto, L., and Notten, P.H.L. (2011). All-solid-state lithium-ion microbatteries: a review of various three-dimensional concepts. Advanced Energy Materials 1 (1): 10–33. 188 Perre, E., Nyholm, L., Gustafsson, T. et al. (2008). Direct electrodeposition of aluminium nano-rods. Electrochemistry Communications 10 (10): 1467–1470. 189 Talin, A.A., Ruzmetov, D., Kolmakov, A. et al. (2016). Fabrication, testing, and simulation of all-solid-state three-dimensional Li-ion batteries. ACS Applied Materials & Interfaces 8 (47): 32385–32391. 190 Cheah, S.K., Perre, E., Rooth, M. et al. (2009). Self-supported three-dimensional nanoelectrodes for microbattery applications. Nano Letters 9 (9): 3230–3233. 191 Li, W., Christiansen, T.L., Li, C. et al. (2018). High-power lithium-ion microbatteries from imprinted 3D electrodes of sub-10 nm LiMn2 O4 /Li4 Ti5 O12 nanocrystals and a copolymer gel electrolyte. Nano Energy 52: 431–440.

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192 Golodnitsky, D., Nathan, M., Yufit, V. et al. (2006). Progress in three-dimensional (3D) Li-ion microbatteries. Solid State Ionics 177 (26-32): 2811–2819. 193 Pearse, A., Schmitt, T., Sahadeo, E. et al. (2018). Three-dimensional solid-state lithium-ion batteries fabricated by conformal vapor-phase chemistry. ACS Nano 12 (5): 4286–4294. 194 Sun, B., Asfaw, H.D., Rehnlund, D. et al. (2018). Toward solid-state 3D-microbatteries using functionalized polycarbonate-based polymer electrolytes. ACS Applied Materials & Interfaces 10 (3): 2407–2413. 195 Guo, R., Chen, J., Yang, B. et al. (2017). In-plane micro-supercapacitors for an integrated device on one piece of paper. Advanced Functional Materials 27 (43): 1702394. 196 Yun, J., Song, C., Lee, H. et al. (2018). Stretchable array of high-performance micro-supercapacitors charged with solar cells for wireless powering of an integrated strain sensor. Nano Energy 49: 644–654. 197 Lin, Y., Chen, J., Tavakoli, M.M. et al. (2019). Printable fabrication of a fully integrated and self-powered sensor system on plastic substrates. Advanced Materials 31 (5): 1804285.

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Definition of Smart Materials and Cells

7.1.1

Definition of Smart Cells

Smart devices are electronic devices, usually connected to other devices or networks through wireless protocols such as Bluetooth, Wi-Fi, and 5G. Smart devices often use built-in sensors to quickly detect and respond to external changes, and a variety of smart devices can interact and autonomously operate, such as smart phones, autonomous vehicles, and drones [1, 2]. Although various smart devices have been widely used in all aspects of our daily life, their core power supply components have hindered their development. Conventional cells are still based on chemistry systems, such as inorganic cathode/anode materials, polymer separators, organic electrolytes, and metal current collectors. On the other hand, the conventional cell design and materials cannot respond to the external/internal stimuli, such as mechanical, light, and environmental variation [3, 4]. Therefore, investigation on smart cells has become a hot research spot in recent years. In 1995, Intel and Duracell integrated conventional cell packs with battery management system for laptops, which was considered as the first example of the smart cell. By now, the battery management system becomes the essential part of battery pack [5, 6]. Although it could measure the charge level and health condition of the cell via detecting the internal voltage/current output, it still lacks the capability of detecting the interior physical and chemical status of each component. In summary, it is highly expected to develop smart cells and smart materials to achieve self-response functions, which is critical to enhance the functionality and safety of cells.

7.1.2

Definition of Smart Materials

The properties of emerging smart materials can change significantly in a controllable way when these undergo relevant stimulation such as thermal, electrical, environmental, mechanical, and magnetic changes [7–9]. These altered manifestations can quickly return to their original states once the external stimulation disappears. Smart Novel Electrochemical Energy Storage Devices: Materials, Architectures, and Future Trends, First Edition. Feng Li, Lei Wen, and Hui-ming Cheng. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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materials have been widely used in various applications, such as sensors, actuators, switches, and controllable drug taking. The smart materials with conventional cells, the resultant smart cells, can spontaneously detect and/or respond to abnormalities and stimulation at an early stage; therefore, the safety characteristics and functionality can also be improved significantly. In this chapter, the main smart materials/design and their applications will be comprehensively introduced from four aspects: (i) self-healing materials or cells, (ii) electrochromic cells, (iii) self-monitoring and self-heating cells, and (iv) self-adapting cells.

7.2

Type of Smart Materials

7.2.1

Self-healing Materials

As an innovative material, smart materials can imitate the functions of living creatures to self-protect, respond, and heal when responding to external damage [10]. Figure 7.1 shows the three self-healing strategies from the molecular to the macro scale. Until now, most self-healing materials are based on intrinsic self-healing mechanism, which can detect and repair itself through reversible bonds without any manual intervention when damaged. The self-healing mechanisms in the polymer chain mainly include dynamic non-covalent bonds and covalent bonds, as shown in Figure 7.1a [10, 11]. The self-healing process of dynamic non-covalent bonds is typical supramolecular assembly process, which includes electrostatic cross-linking, metal coordination bonding, hydrogen-bonding, etc. [11]. Based on the mobility of polymer chains and (a)

(b)

Catalyst Microcapsule

Liquid healing agent

Polymerized healing agent

Crack

(c)

Figure 7.1 Multiscale methods for the self-healing functions in polymers. Source: Patrick et al. [10].

7.2 Type of Smart Materials

kinetics of the supramolecular assembly, the self-healing procedure usually takes place in the order of minutes to hours. To accelerate the self-healing kinetics, the external stimulation can be used to enhance the chain dynamic and self-assembly activities [11]. The dynamic covalent self-healing mechanism mainly includes disulfide,[12] acylhydrazone [13], ester [14, 15], olefin [16], imine [17, 18], and Diels–Alder reaction [19]. Because the kinetics of covalent formation are slow, external stimulations, such as heat, chemical environment change, catalyst, and light, are always used to accelerate the self-assembly process [11]. Therefore, the self-healing materials can also be divided into autonomic healing and non-autonomic healing as follows: (i) autonomic healing: completely self-healing without external intervention. (ii) Nonautonomic healing: require external stimulation, such as temperature and radiation. Different from the intrinsic self-healing polymers, microencapsulation is another self-healing mechanism. A microencapsulation-based self-healing system to autonomically heal cracks was reported at 2001 by White group [20]. This self-healing material contains the microencapsulated healing agent, which can be released when the crack breaks. The polymerization of the self-healing component is obtained by the embedded catalyst, which binds to the crack surface. The self-healing material yielded up to 75% recovery in toughness, and this microencapsulation self-healing mechanism is suitable for other brittle inorganic materials, such as glasses and ceramics [20]. As shown in Figure 7.1b, the self-healing can be achieved by embedding microencapsulated healing agents in the epoxy resin base. The upcoming fracture ruptures the microcapsules and releases the healing agent to the fracture plane through capillary action. Finally, the polymerization of the self-healing component is achieved by contact with the embedded catalyst to bond the cracked surface. The microcapsule self-healing method can also be applied to various polymer matrices and various types of functional liquid healing components, such as liquid-metal healing agent. Although the microencapsulation self-healing strategy has fast response speed, it is difficult to accomplish multiple healing cycles (usually only one cycle) due to the limited healing agents used in the healed part. As an alternative way shown in Figure 7.1c, the microvascular self-healing systems composed of refillable channels with healing components. The vasculature can deliver healing agents in a complementary way and therefore has the capability of multiple healing cycles.

7.2.2

Shape-memory Alloys

As well-known smart materials, shape-memory materials have the ability to “remember” the shape after being plastically deformed. The most prominent and widely used shape-memory materials are shape-memory alloys. Shape-memory alloys can return to its pre-deformed shape within a recoverable range by heating or other external stimulation [21, 22]. Their shape-memory effect originates from the existence in such materials of two stable crystal structures: a high temperature-favored austenitic phase and a low temperature-favored martensitic

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phase. Deformations of the low temperature phase are recovered completely during the solid–solid transformation to the high temperature phase [23]. The first commercial shape-memory alloy is the equiatomic Ni–Ti alloy, which was found in 1963. Due to the combination of a desirable transition temperature close to body temperature, superelasticity, biocompatablility, and two-way shape-memory capability, Ni–Ti shape-memory alloy has been widely used. Nowadays, other types of shape-memory alloys include Cu–Al–Ni, Ti–Ni, and Ni–Al alloys [24]. Compared with shape-memory alloys, shape-memory polymeric materials have the advantages of high elastic deformation (up to more than 200% for most of the materials), low cost, low density, and excellent biocompatibility and biodegradability. They also have a broad range of application temperatures that can be tailored, tunable stiffness, and are easily fabricated. These two materials (metal alloys and polymers) also possess distinct applications due to their intrinsic differences in mechanical, viscoelastic, and optical properties [23]. For shape-memory alloys or polymers, recent applications are mainly focused on medical areas, such as biodegradable structures, actuators, and smart stents [23]. Mechanisms and applications of various shape-memory materials can be found in some reviews [25–27]. Nowadays, various shape-memory alloys began to find its promising applications in lithium ion batteries (LIBs) or electrochemical capacitors (ECs) field. (i) Shape-adapting cells: the first application field is shape-adapting cells. This kind of shape-adapting cells can self-recover its shape once the temperature rises to the critical point. (ii) Thermal regulator: another application field of shape-memory materials is thermal regulator, which is a kind of passive thermal regulator to address the critical need for adaptive thermal management in LIB applications [28, 29].

7.2.3

Thermal-responding PTC Thermistors

As a kind of smart material, thermal-responding positive temperature coefficient (PTC) materials show obvious increasing resistance with the temperature. Electrode materials with PTC effect can cut off the current autonomously, which can significantly improve the safety characteristics of cells. To make the electrode material have thermal self-response function on PTC effect, the simplest strategy is to directly mix the PTC material with the active electrode material. Generally, the PTC materials are usually used as surface coating agents for the electrode materials. When the internal temperature of the cells rises to critical temperature, it will suddenly change from electrical conductor to insulator through phase transition process. The sudden nonlinear increase in resistance destroys the conductive network between electrode materials and sharply reduces the peak current inside the cells, thereby cutting off the complex chemical chain reaction by thermal runaway [30, 31]. Therefore, the appropriate PTC material is the core of the thermal-responding electrodes. In this case, advanced PTC materials first should have high response speed at the critical temperature. Besides, the high electronic conductivity at room temperatures, high electrochemical and chemical stability in electrochemical environments, and critical temperature are also necessary for excellent PTC [32].

7.2 Type of Smart Materials

The resistance of silicon-based PTC materials usually has linear relation with temperature, indicating much lower response speed; therefore, it is not suitable to prevent thermal runaway. On the other hand, the ceramic PTC materials, such as various rare-earth-doped BaTiO3 , usually show high critical temperature higher than 120 ∘ C, it is not suitable for the cells [32]. By contrast, the polymer PTC materials usually contain carbon black embedded in the polymer [33]. When it is in normal conditions, the carbon black particles provide effective conductive pathways as the better contact [34]. Once it is heated above the Curie temperature, the carbon black particles will be separated from each other as the polymer expands. The poor contract and sharp increasing resistance cuts off the current in the cells and prevents the subsequent thermal runaway.

7.2.4

Electrochromic Materials

Electrochromic materials are characterized by the adjustable colors as a result of a charge transfer (redox) process induced by applied electrical potentials. Materials meeting the requirement of electrochromism should have at least two redox states associated with distinct optical spectra. For example, a combination of two types of electrochromic metal oxides, characterized by anodic coloring and cathodic coloring, can be adopted in electrochromic devices. The main classes of such active materials include metal oxides (especially tungsten oxide), viologens (4,4-bipyridylium salts), conjugated polymers, and metal coordination complexes. Among these electrochromic materials, metal oxides and polymers have also been widely used in electrochromic LIBs and electrochemical capacitors. Transition metal oxides are the most common active components for electrochromic applications, including the oxides of tungsten, molybdenum, nickel, vanadium, titanium, cobalt, copper, and niobium. Based on the multivalence nature of such compounds, the oxides of tungsten, nickel, molybdenum, and iridium are the most heavily studied and display the most intense changes in color, ranging from blue or gray to transparent. WO3 , a n-type semiconductor with a wide bandgap, is the most intensively investigated inorganic electrochromic materials [35]. The earliest recorded electrochemical color change of tungsten oxide was in 1930, followed by the first demonstration of an electrochromic device presented by Deb in 1969, which is generally regarded as the founding moment of smart window technology [35]. In typical WO3 electrode, cations (e.g., H+ and Li+ ) can be reversibly inserted and extracted under bias potentials, which is the origin of the electrochromic processes. Electrochromic oxides can be divided into two categories, characterized by anodic and cathodic coloring, respectively. Similar to WO3 , V2 O5 , MoO3 , and TiO2 also show cathodic coloring, whereas Ni(OH)2 is an anodically colored material, which shows a color change from pale green to brown under positive voltages in basic electrolytes. Besides transition metal oxides, the conducting polymers with great range of colors are also available [36]. Polymeric thiophenes, pyrroles, and polyaniline, as common electrochromic materials, have attracted the most attention for

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electrochromic electrochemical capacitors [37, 38]. Detailed introduction about their electrochromic performance can be found in the reviews [39–41].

7.3

Construction of Smart Cells

Integrating the smart materials with the conventional cells, the resultant smart cells can spontaneously detect and respond to abnormalities and stimulation at the early stage; therefore, their cycle performance, cycle life and functionality can also be improved significantly. Main smart cells fall into five categories: (i) self-healing electrodes: the self-healing strategies are used to endow materials with the ability to prevent defaults growth at an early stage and inhibit catastrophic failure; (ii) thermal-responding electrodes: this kind of electrode could sense the temperature and switch to cut off the electron flow when thermal runaway happens; (iii) shape-recovery cells: this kind of cells can self-recover the original shape or size in the memory process when subjected to temperature field; (iv) self-heating and self-monitoring cells: the self-heating design can be demonstrated to be a much more appropriate strategy to heat cells, which enable the cells can work in low temperatures. While the self-monitoring design can detect the inner short-circuit at an early stage; (5) electrochromic cells: this kind of cells has the ability of dynamic color changing, which can act as indicators for energy status displays.

7.3.1

Self-healing Silicon Anodes

The electrode material of LIB usually repeats volume expansion/contraction during charging and discharging [42, 43]. This leads to electrode material degradation by mechanical fatigue and damage [44]. The micro-cracks generated in the active materials can destroy the conductive network, and the propagation of the cracks could eventually lead to the ultimate failure of the materials [45]. The ability to repair the cracks can prevent further expansion and extend the life of the materials. Self-healing polymers can respond to micro-damages and heal the degradation by initiating repair mechanisms [10, 20]. Although silicon negative has very high theoretical lithium storage capacity, it has volume expansion of about 300% when fully lithiated [46, 47]. The inherent shortcoming of Si hinders the commercial application for LIB. Nano silicon materials or Si/C composite negatives are promising solutions. Compared with bulk silicon negative, lots of silicon nanostructures, including nanowires or nanoparticles, have higher mechanical stability during cycling [48, 49]. Silicon nanostructures can be embedded in the conductive carbon materials, which can effectively alleviate the volume expansion/contraction [50–52]. Despite improved cycle stability, the huge volume change of nanowires can still cause the fading issue. The smart self-healing materials are an alternative strategy to remedy the fading problem of silicon negatives [11, 53, 54]. Table 7.1 lists the self-healing polymers in silicon negatives for LIBs. Figure 7.2 is the design principle for the self-healing electrode by embedding Si microparticles in randomly branched amorphous polymer with hydrogen bonds.

7.3 Construction of Smart Cells

Table 7.1

Typical self-healing polymers in silicon negatives for LIBs.

Silicon sizes

Chemical architecture

Conventional binders

Self-healing polymers

3–8 μm

Branched hydrogenbonding polymer

47%, 27%, and 14% after 20 cycles with alginate, CMC, and PVDF binders

80% after 90 cycles

[55]

120–200 nm

β-Cyclodextrin

27.1% retention rate with alginate binders

68.7% after 200 cycles

[56]

0.8 μm

PEG-Functionalized SHP

40% after 150 cycles

80% after 150 cycles

[53]

0.9 μm

Hydrogen-bonding directed SHP

45% after 100 cycles

83% after 100 cycles

[57]

1–5 μm

Hydrogen-bonding directed SHP

73.1% after 100 cycles

91.8% after 100 cycles

[58]

50–70 nm

UPy–PEG–UPy

0.3% decay per cycle

0.04 % decay per cycle

[59]

∼50 nm

β-CDp /6AD

23–29% after 150 cycles

90 % after 150 cycles

[60]

References

SHP, self-healing polymer; CDp, cyclodextrin; CMC, sodium carboxymethylcellulose; PEG, polyethylene glycol; AD, adamantane; LIBs, lithium ion batteries. Source: Wen et al. [61].

Scheme 1 Lithiation

Scheme 2 Lithiation

Self-healing

Silicon materials

Lithiated silicon materials

Traditional polymer binders

Self-healing coating

Figure 7.2

Design and architecture of the self-healing electrode. Source: Wang et al. [55].

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As shown in the scheme 1 of Figure 7.2, the conventional bulk Si negative has significant pulverization by traditional polymer binders, caused by huge volumetric expansion/contraction during cycling. However, the self-healing polymer chain can stretch and rebuild the binder itself, buffer expansion without breaking at the crack interface, as shown in the scheme 2 of Figure 7.2. The self-healing electrodes showed excellent cycling performance. After 90 cycles, the capacity retention rate reached 80%. Conventional binders, such as polyvinylidene difluoride (PVDF), carboxymethyl cellulose, and alginate, showed obviously poor stability; its retention rate was only 14%, 27%, and 47% after 40 cycles, respectively. Besides the hydrogen bond-based self-healing mechanism, dynamic ion–dipole and host–guest polymers have also been used to construct electrodes with self-healing ability. For example, the supramolecular binder prepared by cross-linking between β-cyclodextrin and dendritic gallic acid was reported to protect Si negative [60]. Interaction between β-cyclodextrin and adamantane was in this kind of self-repair function. Figures 7.3a,b shows the chemical structure and self-healing mechanism. The dynamic cross-linking provides tight bond

(a)

α-CD

β-CD

5.7 Å

7.8 Å

γ-CD

Glycerol side chain

7.9 Å

II

9.5 Å

Hyperbranched α,β, γ-CDp

(b) Binder layer

Si

Lithiation

Delithiation

+Li+, +e–

–Li+, –e–

Volume expansion

Volume contraction

Good integrity

SuperP

Dynamic crosslinker Hydrogen bonding

II 6AD

Control guest molecule Supramolecular crosslinking

1AD

Figure 7.3 (a) Molecular architectures of hyperbranched α-, β-, and γ-cyclodextrin. (b) Cross-linking interactions between 6 adamantane and β-cyclodextrin, molecular structures of molecules incorporating into the adamantane structure. Source: Kwon et al. [60].

7.3 Construction of Smart Cells

O

O

O

H N

O n

O

O

H

N H

N H

O

N

N

N

N

O

H N

N

H

H

O

O N H

O

Figure 7.4 Proposed principle for the self-healing process of UPy-PEG polymers. Source: Yang et al. [59].

between silicon negative and binder, which can improve integrity and stability, solid electrolyte interphase (SEI) formation, and cycle than PVDF binder. Figure 7.4 shows the proposed mechanism for the self-healing process of ureidopyrimidinone-grafted polyethylene glycol (UPy-PEG-UPy) binders. When the electrode is scratched and placed alone, the reconstruction of the tetrahydro bond of the UPy dimer is driven by dimerization. Moreover, when used as the binder, the Si negative shows good charge/discharge characteristics with first coulomb efficiency of 81% and the reversible capacity of 1454 mAh g−1 after 400 cycles. This self-healing procedure of UPy–PEG adhesive is attributed to the re-establishment of quadruple hydrogen bond when the electrode suffers from the cracks. The performance is contributed by the self-healing procedure of the binder, which automatically heals micro-cracks once they propagate. The freestanding silicon negative with the homogenous dispersion of silicon and multi-walled carbon nanotube (MWCNT) in a hydrogen bond-based self-healing polymer matrix was produced, which had drastic improvement in the capacity and cycling stability due to the better self-healing characteristics of the polymer support [58]. The self-healable supramolecular polymers were synthesized by copolymerization of tert-butyl acrylate and ureido-pyrimidinone monomer followed by hydrolysis, which significantly reduced the volume expansion of silicon particles [62]. Figure 7.5 illustrates the schematic design of the self-healable silicon negative by polyacrylic acid/uriedo-pyrimidinone (PAA-Upy) as the binder. Huge expansion of Si during charging caused the break of the non-covalent crosslinking of UPy dimers, and these crosslinking networks can be reconstructed during delithiation. The initial discharge capacity of the self-healing silicon electrode reached 4194 mAh g−1 . After 110 cycles, the specific capacity still reached 2638 mAh g−1 , which shows significant enhancement of the electrochemical performance in comparison with that of silicon negative with normal binders [62].

7.3.2

Aqueous Self-healing Electrodes

The self-healing aqueous LIB was also reported, in which the aligned CNT films attached with LiTi2 (PO4 )3 and LiMn2 O4 on the self-healing polymer support served as negative and positive, respectively [63]. Li2 SO4 /sodium carboxymethy cellulose served as electrolyte and separator. After several cutting and self-healing cycles

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(a)

(b) Breaking Reform

PAA-UPy (c)

Lithiation Si

LixSi Delithiation

Figure 7.5 (a) Structure of PAA–UPy, (b) UPy–UPy dimers self-healing based on quadruple hydrogen bonding, (c) huge expansion of Si negative during charging results in the break of the non-covalent crosslinking of UPy dimers, and these crosslinking networks can be reconstructed during delithiation. Source: Zhang et al. [62].

(a)

(b)

As fabricated

(c) After cutting

5 mm

After self-healing

5 mm

5 mm

Figure 7.6 From (a–c), pictures of the self-healing aqueous electrode after breaking, cutting, and self-healing cycle. Source: Zhao et al. [63]. Reproduced with permission of Springer Nature.

(Figure 7.6a–c), the electrochemical and mechanical performance can be well preserved. Compared with random CNTs, the aligned CNTs had better self-healing efficiency due to its good alignment. Due to the role of borate ester bonding, the SA-g-DA/Li2 SO4 hydrogel electrolyte also had self-healing properties for LiV3 O8 and LiMn2 O4 electrodes after mechanical fracture. The LIB could promptly maintain its mechanical and electrochemical performance when subjected to repeated cleavage. After 20 minutes of self-healing, the LIB could still recover 98.7% of its initial mechanical strength and showed high self-healing efficiency. In addition, the LIB had high elastic strain and could also be customized to any geometry while keeping its electrochemical properties [64].

7.3 Construction of Smart Cells

7.3.3

Liquid-alloy Self-healing Electrode Materials

The liquid metal is a promising candidate in the field of self-healing owing to its excellent fluidity and high surface tension and conductivity. The fluidity is useful to enhance the cycling performance of materials with large volume variation. As a kind of cryogenic liquid metal with low toxicity, Ga alloy-based liquid metal is one of the most common liquid alloys [65]. Gallium (Ga) with extremely low melting point (29.8 ∘ C) has attracted sattention as anode material. One Ga can accommodate two Li atoms upon full lithiation, which has theoretical capacity of 760 mAh g−1 by forming the Li2 Ga alloy. Figure 7.7 shows the theoretical discharge curve of Ga in the range between 0.0 and 2.0 V. Three intermetallic phases, Li2 Ga7 , LiGa, and Li2 Ga, are formed during the electrochemical reaction between Ga and Li [66]. The theoretical capacity from Ga to Li2 Ga corresponds to 769 mAh per gram of Ga. The liquid metal Ga undergoes the crystallization process during lithiation process and transforms to solid electrode. During delithiation process, the solid alloy phase transform to the liquid state. During delithiation, the cracks are mainly formed in the electrode, and the cracks can be self-healed by the solid-to-liquid phase transformation. The liquid metal strategy by Ga metal can be generalized to other materials since many alloys with low melting temperature can potentially accommodate large amounts of Li. For example, the room-temperature liquid Sn–Ga alloy with self-healing characteristics can avoid cracking and show better cycling performance [66]. At the current density of 200, 500, and 1000 mA g−1 , the specific capacity of the liquid Sn–Ga alloy was 775, 690, and 613 mA h g−1 , respectively. Even after 4000 cycles, this liquid alloy did not have obvious decay and show high specific capacity of 400 mAh g−1 . As shown in Figure 7.8a,b, in the liquid–alloy composition, the incorporation of Sn can reduce the melting temperature of Ga, and the graphene/CNT matrix can improve the electronic conductivity of the liquid alloy and hinder the Sn–Ga alloy from gathering or detaching from the metal current collector during cycling (Figure 7.8c,d).

B

1.5 Potential vs. Li/Li+ (V)

Figure 7.7 Theoretical curve and discharge curves of Ga metal.

1.0

0.5

Li2Ga7 Li2Ga

LiGa 0.0

0.0

0.2

0.4 0.6 x in Li2xGa

0.8

1.0

273

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7 Smart Cells (a) 1 mM 1ATC9 Sonication

300 °C Stir

(b)

(c)

(d)

Ga12.6SN1.0

Sn

Ga

900 °C

Ga12.6SN1.0

Sn

Ga

GO, CNT

O 1ATC9: HS

N H

CH2(CH2)7CH3

Figure 7.8 (a) Fabrication procedure for the rGO/CNT-supported liquid Sn–Ga alloy negative, (b) Sn and Ga in the solid state and the Ga–Sn alloy in the liquid state at room temperature, (c) the liquid metal alloy after sonication, (d) liquid alloy/carbon matrix negative. Source: Wu et al. [65]. Reproduced with permission of Royal Society of Chemistry.

7.3.4

Thermal-responding Layer

Employing thermal-responding PTC layer between the positive material layer and the Al current collector is an effective as well as a direct way to improve safety of batteries [67, 68]. Based on the rapid increase of the electric resistance at high temperatures, PTC composite electrodes could sense the temperature and act as thermal switch to cut off the electron flow when thermal runaway happens. For example, the carbon black mixed with PMMA as PTC layer was embedded between Al foil and LiCoO2 cathode to form the sandwiched Al/PTC/LiCoO2 electrode [67]. In the temperature range of 80–120 ∘ C, the resistance of the PTC electrode increased rapidly, which can promptly inhibit the current through the electrode [68]. The mixed Ni particle with PVDF layer as PTC layer applied between Al current collector and the LiFePO4 layer, which increased the resistance greatly and absolutely, cut off the electrochemical process at relatively low temperature at 90 ∘ C. On the other hand, the conductive Ni particles can improve the rate and cycle performance of the electrode, which means that the PVDF/Ni can afford lower electrode polarization and good interface [69]. The sandwiched Al/P3OT/LiCoO2 was assembled by coating an ultra-thin p-type conducting polymer and poly(3-octylthiophene-2, 5-diyl) (P3OT) on the Al foil, as shown in Figure 7.9a [70]. This sandwiched layer endowed the reversible electrochemical activity and appropriate transition temperature of the P3OT polymer with obvious increasing resistance and thermal self-cutting effect between 90 and 100 ∘ C, as shown in Figure 7.9b,c. At normal operation temperature, the P3OT layer is electrochemically oxidized to conducting state by p-doping of anions from the organic electrolyte at the positively charged cathode. Once the temperature reaches specific point, the P3OT layer transforms promptly to the insulating state via de-doping process, therefore preventing thermal runaway at the beginning. Moreover, the ultra-thin structure can avoid the excessive occupation of the internal space, keeping the inherent energy density of the material as much as possible.

7.3 Construction of Smart Cells

(a) Voltage (V vs. Li+/Li)

4.4

(b)

4.2 4.0 3.8 3.6

25 °C 55 °C 90 °C

3.4 3.2 3.0 0

30 60 90 120 150 180 Capacity (mAh g–1) (c)

Resistivity (Ω cm)

200 000 150 000 100 000 50 000 0 20

40

60

80 100 120 Temperature (°C)

140

160

Figure 7.9 (a) Fabrication of the layered Al/P3OT/LiCoO2 cathode, (b) charge/discharge profiles of the LiCoO2 -PTC electrode at different temperatures, and (c) resistivity of p-doped P3OT at different temperatures; the inset demonstrates self-cutting behaviors of the P3OT film. Source: Ji et al. [70]. Reproduced with permission of Royal Society of Chemistry.

In fact, when thermal runaway takes place, it is usually a sudden process, which requires the PTC layer with rapid response to prevent the following procedure. However, the aforementioned epoxy carbon-based PTC usually has low phase transition process and low response speed. Furthermore, the large aggregated solid particles (>10 μm) of this type of PTC materials cause insufficient contact between the PTC and active material. Besides, the thick PTC layer can lead to the larger space of the cell and further decrease the energy density [70]. To solve this issue, the fast and reversible thermo-responsive polymer switching self-regulating material has been sandwiched between LiCoO2 and Al current collector (Figure 7.10a) [71]. As shown in Figure 7.10b, this thermo-responsive polymer switching was produced from nanostructured spiky Ni particles with graphene as the filler and polyethylene (PE) with high thermal expansion coefficient as the matrix. The spiky nanostructure of the metal particles provides high electronic conductivity at normal operating temperatures, whereas at overheating state, the polymer matrix can expand and separate the conductive particles, resulting in breaking of the conductive pathways. Therefore, this thermo-responsive polymer switching film can shut down the LIB above the critical temperature. Because the conductivity

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(c) 150

(+)

GrNi

Cathode

(–)

Normal state

Capacity (mAh g–1)

(b)

(a)

Anode

276

100 Battery at 25 °C

50

Battery at 70 °C

0

Polymer TRPS

0 Overheating

10 20 30 Cycle number

40

Figure 7.10 (a) Architecture of safe LIB based on the thermo-responsive polymer switching film-coated Al current collector, LiCoO2 positive and metal lithium negative, (b) self-shutting mechanism of the TRPS material, and (c) capacity of the safe LIB at 25 and 70 ∘ C. Source: Chen et al. [71].

changes of the thermo-responsive polymer switching film is reversible, the LIB can be repeatedly cut off and revived, as shown in Figure 7.10c.

7.3.5

Thermal-responding Electrodes Based on the PTC Effect

To endow electrode with the function of thermal self-response based on PTC effect, the simplest strategy is to mix the PTC materials with active materials directly. Generally, the PTC materials are usually used as surface coating agents for active materials in this strategy. When the internal temperature of LIB rises to critical point (i.e., Curie temperature), it suddenly changes from the electrical conductor to an insulator through phase transition process. This characteristic of sudden non-linear electrical resistance increase determines the conductive network of the active materials and sharply reduces the maximum current inside the LIB, thereby cutting off the complex chemical reaction caused by thermal runaway [30, 31]. Therefore, the appropriate PTC material is the core of the temperature-response electrodes. First, advanced PTC materials should have high response speed at the Curie temperature. The appropriate Curie temperature and high electrochemical and chemical stability in electrochemical environments are also necessary for excellent PTC, besides high electronic conductivity at room temperatures [71]. To date, a variety of materials have been applied as PTC materials in temperature-response electrodes, and typical PTC materials can be classified into thermistors, polycrystalline ceramic materials [32], and polymer PTC materials [72]. The resistance of thermistors usually has linear relation with temperature, indicating much lower response speed for this type of materials, therefore unfavorable to prevent thermal runaway. On the other hand, the ceramic PTC materials, such as BaTiO3 doped with rare-earth elements and other compounds, usually show high critical temperature (above 120 ∘ C), which is unsuitable for LIBs as well [32]. In contrast, unlike these PTC materials, polymer PTC materials usually contain carbon black embedded in the polymer [33]. When it is in normal conditions, the carbon black particles provide effective conductive pathways as the better contact [34]. Once it is heated above

7.3 Construction of Smart Cells

the Curie temperature, the carbon black particles will be separated from each other as the expansion of the polymer. This causes poor contact and a sharp increase in the resistance, thus further switching off the current in LIBs and preventing the thermal runaway. In short, the suitable critical temperature and compatibility with conventional LIBs have promoted research on various possible advanced polymer-PTC-based positive electrode materials, including high-density PE [31, 73], polymethyl methyl acrylate (PMMA), [67] PVDF [69], ethylene vinyl acetate (EVA) [74], etc. Unfortunately, mechanically mixed process does not produce intact contact between active ingredient and PTC, which will deteriorate the self-response speed of the final electrodes. To solve this problem, carbon black is used as the second conductive material in the high-density PE resin to form LiCoO2 composite cathode [31]. The size of the carbon black particles is smaller than that of the PTC material, which increases the contact between the PTC material and the active material. The PTC-based positives had excellent rate performance and cycle stability. When the temperature is higher than 130 ∘ C, the positives also showed obvious resistance increase during internal short circuit [30]. Furthermore, the current can be cutoff almost completely during short-circuit test at ∼145 ∘ C, which demonstrated its high safety of such PTC-based LiCoO2 cells. The relationship between the composition of the PTC-based positive electrode and the safety and discharge characteristics was also studied [73]. Obviously, the overcharge tolerance is enhanced with the increase of PTC material loadings in the materials. However, the working temperature of PTC-based materials, that is 100–130 ∘ C, is still much higher than that of solvent evaporation and SEI film decomposition [75]. On the other hand, the temperature around 130 ∘ C is very close to softening temperature of the polymer separators, which makes it even more challenging to be used in practical LIBs. Therefore, more effective strategies for lowering the Curie point of the PTC materials or searching for more suitable PTC materials with lower Curie point are necessary. The EVA-based LiFePO4 composite was developed by embedding EVA additives into the LiFePO4 positive via either directly mixing the components or sandwiching the PTC coating between the LiFePO4 positive and the Al foil [74]. The obtained electrodes have a critical temperature of 90 ∘ C, which is lower than the critical runaway point (∼140 ∘ C). Therefore, the LiFePO4 /EVA materials had a significant current-limiting function above 90 ∘ C. However, with the introduction of nonconductive plastic matrix may also deteriorate rate capability and energy density. In this regard, the conductive framework is much more promising, for instance, the highly conductive poly(3-Octylpyrrole) : poly(styrenesulfonate) (P3OPy : PSS). In this concept, the PTC effect is achieved by the P3OPy from conductive p-doped state to de-doped insulating state at high temperature. The P3OPy/carbon black exhibit a high electronic conductivity of 30 S cm−1 and active thermal shutdown function at ≥120 o C [76]. Based on this type of electrode with directly mixing the components or sandwich-structured layers, the chain reactions by thermal runaway are usually

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(a)

(b) 4.0 Voltage vs. Li (V)

278

3.6 3.2 30 °C 80 °C 110 °C

2.8 2.4

0

50

100 150 200 Capacity (mAh g–1)

250

300

350

Figure 7.11 (a) TEM image of in situ P3DT coated LiCoO2 positive and (b) charge/discharge curves of LiCoO2 @p-doped P3DT at different temperatures. Source: Xia et al. [78]. Reproduced with permission of Royal Society of Chemistry.

exothermal and occur at the electrolyte/highly oxidized positive interface. In this case, only mixing the positive and the PTC material or sandwiching the PTC layer does not fully protect the LIB from the exothermic chain reaction, and it will also result in the formation of thick PTC with several tens of micrometers, making it difficult to build a conductive matrix between PTC and electrode materials, further decreasing rate performances of the final electrodes and cells [77]. The uniform layer of poly(3-decylthiophene) (P3DT) was coated on the surface of the LiCoO2 electrode by spray drying method to solve this problem [78]. Different from typical insulating polymer PTC, p-doped P3DT shows practical PTC effect. Electronic conductivity of p-doped P3DT could be significantly increased from ∼10−3 S cm−1 at room temperature to ∼10−6 S cm−1 at 80–100 ∘ C. On the other hand, the compact in situ coated P3DT polymer with a thickness of about 20 nm can also protect LiCoO2 particles (Figure 7.11a). Thus, the in situ coated P3DT layer provided thermal self-shutdown function directly in the range of 80–110 ∘ C (Figure 7.11b), and this temperature is below the critical temperature of the exothermic reaction of LIB.

7.3.6

Ionic Blocking Effect-Based Thermal-responding Electrodes

Besides internal self-activating thermal shut down of PTC composite cathode materials, ionic blocking effect-based thermal-responding electrodes was also studied for the capability of shutting down the LIB at critical temperatures by automatically blocking the ionic pathways. The hyper-branched architecture self-terminated oligomer was coated on the Li(Ni0.4 Co0.2 Mn0.4 )O2 positive material, which was developed to suppress the risk of thermal runaway [79, 80]. The functional polymer with dendritic (hyperbranched) and nanoscale structure can passivate the activity of charged positive, therefore slowing the migration of lithium ions, and further inhibiting thermal runaway at the initial stage of temperature increase.

7.3 Construction of Smart Cells

The thermally sensitive polymer-based polymer was prepared by mixing poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and PVDF [81]. As the formation of amorphous regions in the mixed binder, the softening and swelling are much more prominent than the regular PVDF binder when the temperature is above 110 ∘ C. In this design, the relatively more crystalline PVDF phase can stabilize the structure of the material, while the amorphous HFP phase determines the dissolution and swelling degree, which influences the formation of amorphous regions when the temperature rise. Therefore, such amorphous regions will generate when the temperature of the electrode is over 110 ∘ C and further cause the sharp volume increase, followed by the subsequent electrolyte swelling and poor ionic transport. The use of this blended polymer binder can improve the ability to suppress thermal runaway without sacrificing the cycling performance of the LIB. In the related study, the heat generated in the cell could be significantly dropped by ∼50% after nail piercing. In addition to the materials with inherent self-responsiveness, the functionalized microspheres incorporated in polymers were also used to improve the safety, such as paraffin wax microspheres [82], PE microspheres, and polydopamine (PDA)-coated PE microspheres [83]. In this design, the polymer microspheres were covered on graphite negatives or polymer separators. As shown in Figure 7.12, such polymer microspheres will undergo thermal transition into liquid state and wet the interface of electrode materials when the temperature of the cell is over the polymer melting temperature (65 ∘ C for paraffin wax microspheres and 110 ∘ C for PE microspheres), forming the ion-insulating barrier, which will shut off the ions migration and further prevent the subsequent reactions of thermal runaway over 98% loss of the capacity [82]. In this concept, a variety of polymer microspheres can be used in this process to optimize the trigger temperature, shutdown rate, and thermomechanical stability in some designs for specific aims. PDA has been proven capable of improving the PE microspheres dispersion in N-methyl-2-pyrrolidone (NMP) solution, which endows the LIBs with the capability of rapid autonomous shutdown function [83]. These

Separator

Capsule Coated anode Anode

Hot anode

Figure 7.12 Embedded thermo-responsive polyethylene microspheres in LIB (left). With the increase of temperature, the incorporated polyethylene microspheres gradually melt and cover the separator and electrode, blocking the ionic transportation and preventing the thermal runaway of the cell (middle and right). Source: Baginska et al. [82]. Reproduced with permission of Wiley.

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polymer microspheres work similarly to the conventional shutdown separators, but they could also suddenly in situ generate the conformal ionic barrier on the electrode. Although these ion-blocking microspheres can effectively improve the safety of LIB, the practical application of this strategy is severely limited by the irreversible process because it cannot restore its original state totally after activating the protective effect.

7.4 Application of Shape-memory Materials in LIBs and ECs 7.4.1

Self-adapting Cells

As mentioned earlier, the repeated charge/discharge cycling overtime is usually accompanied by volume contraction/expansion, which is likely to bring about molecular damages and the subsequent electrochemical performance deterioration. Besides the internal molecular damage, external mechanical fluctuation also causes damage to the LIBs with macroscale shape transition. As a consequence, mechanical fatigue, short circuits, and eventual thermal runaway could take place. Therefore, it is also of significance to eliminate such possible risks for safer batteries at the early stage, such as the development of smart shape-recovery materials. Recently, shape-memory alloys, also known as smart metal or smart alloys, have attracted widespread attention as a kind of shape-recovery materials. It represents a series of alloys that may restore their original geometry shape in memory process between two conversion stages when subjected to relevant stimulation, such as temperature field. The transition process is known as the shape-memory effect, which is generally categorized into three shape-memory characteristics, the one-way shape-memory effect, two-way shape-memory effect, and pseudoelasticity [21, 22]. These types of shape-memory alloys mainly include Fe-based, Cu-based, and TiNi alloys, such as Fe–Mn–Si, Cu–Zn–Al, Cu–Al–Ni, and Ti–Ni alloys [24]. Fe-based and Cu-based shape-memory alloys usually show instability, brittleness, and poor thermo-mechanic performance. In contrast, TiNi alloys are more comfortable and safer to handle and much more preferable for most applications, especially for the aqueous ECs [84, 85]. The graphene-coated Ti-Ni metal foil was applied as the anode for the shape-memory ECs with the MnO2 /Ni foil as cathode and the PVDF-HFP-[EMIm]BF4 or PVA-KOH as gel electrolyte. It provides a low trigger temperature to activate the shape self-recovery function as the transition temperature of Ti–Ni substrate is about 15 ∘ C [86]. Just as shown in Figure 7.13a, the EC could slowly return to its initial round shape once touched the human wrist. Similarly, Huang et al. [87] constructed a shape-memory fiber EC by Ni–Ti wires with shape-memory capacity as the skeleton and substrate for the active materials of MnO2 nano-flakes and polypyrrole (PPy). Such functional fiber ECs could recover the initial state in a few seconds without obviously compromising capacitive performance, indicating unlimited potential for various applications other than energy storage. In addition to the shape-memory alloy, polyurethane (PU) could

7.4 Application of Shape-memory Materials in LIBs and ECs

In refrigerator

Touch with a human wrist

Return to initial shape

Figure 7.13 A shape-recovery process of EC triggered by human wrist. Source: Liu et al. [86]. Reproduced with permission of Wiley.

also endow devices with shape-recovery capability [88]. It served as the substrate for aligned CNT sheets in the fiber-shaped EC, which could be deformed to specific shapes as required. Once the temperature is higher than the thermal phase transition temperature, it can automatically return to its original shape and size with stable electrochemical performance. By now, the research on shape-recovery LIBs is still at the early stage and accompanied by various challenges based on different functional materials especially integrated with batteries or other energy storage devices. However, the shape-recovery material is very likely to be one of the trends for the future application of electronic devices, especially for the LIBs and ECs with autonomous responding capability toward external stimulation, such as temperature and pressure.

7.4.2

Shape-memory Alloy-Based Thermal Regulator

In extreme temperatures, the poor performance of LIBs hindered their wider applications. A critical challenge in LIB thermal management systems is that cold and hot ambient environments call for opposite requirements: thermal transmission at high temperature for LIB cooling and thermal isolation at low temperature to retain the LIBs’ internally generated heat. These requirements cause an inevitable compromise of performances at low or high temperatures. Therefore, it has been difficult to manage LIBs’ temperature for extreme temperature conditions using conventional linear thermal components [28]. In a recent work, a shape-memory alloy-based passive thermal regulator was designed. This shape-memory alloy-based thermal regulator can stabilize LIB temperature in both hot and cold temperatures. Figure 7.14a shows the design principle of the shape-memory alloy-actuated thermal regulator, where the yellow lines show the shape-memory alloy wire. As shown in Figure 7.14b, the red and blue lines are stress–strain curves of shape-memory alloy at temperatures above and below the transition temperature, and the grey line indicates the stress in the shape-memory alloy wire. At low temperatures, the thermal regulator separates the two plates, i.e. OFF mode (Figure 7.14a,b). Therefore, at OFF mode, thermal isolation at low temperature can retain the LIBs’ internally generated heat. While at high temperatures, the shape-memory alloy wire began to contract (from the blue curve to the red curve in Figure 7.14b) and pull the two surfaces together, the intact contact results in drastically better interfacial thermal transport. Figure 7.14c shows

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(a)

(c)

(b)

Figure 7.14 Mechanism of the shape-memory alloy-based thermal regulator. (a) Design of the shape-memory alloy-actuated thermal regulator, (b) mechanisms of ON and OFF modes, and (c) the changes to wire length, gap size, contact pressure, and thermal conductance during this switching process. Source: Hao et al. [28]. Reproduced with permission of Springer Nature.

the variations to wire length, gap size, contact pressure, and thermal conductance during this switching process, which can adjust its properties as a function of the temperature. Therefore, this smart thermal regulator can be regarded as an ideal LIB passive thermal management systems. Demonstration with a battery pack consisting of commercial 18 650 cylindrical LIBs shows that this thermal regulator increases cold-weather capacity by more than threefold simply by retaining the battery’s self-generated heat, while also keeping the module from overheating in hot environments even at 2C discharge rates.

7.5

Self-heating and Self-monitoring Designs

The aforementioned sections mainly focused on material modification, while further development of advanced LIBs not only requires seeking or design new materials but also need to develop new chemistry to better understand and interpret the electrochemical processes. Therefore, in addition to the investigation of active materials, separators, electrolytes, and eventual devices, novel configurations, including self-heating and self-monitoring, can also satisfy the increasing demand for future cells.

7.5 Self-heating and Self-monitoring Designs

7.5.1

Self-heating

Generally, the capacity and rate characteristics of LIBs would decrease dramatically at low temperatures because of the sluggish electrochemical kinetics for ion diffusion transport and the increasing SEI film resistance, especially when the temperature is below −20 ∘ C [89]. For example, the capacity of graphite at −20 ∘ C is just 12% of that at room temperature, but once it is charged at room temperature, 94% of capacity could be gained. This phenomenon is caused by the slow kinetics of Li+ diffusion in graphite at low temperature, and the delithiation process could be much more easily influenced by temperature compared with lithiation process [90]. For another example, the LiFePO4 could only maintain 40–70% of its initial capacity when the ambient temperature is below −20 ∘ C regardless of its excellent rate performance at room temperature [91]. Unfortunately, in very large extent, lithium dendrites would generate when the kinetics become sluggish at low temperatures, further leading to safety concerns [92]. In this regard, achieving practical application in cold climate for LIBs is a pretty challenging issue as the result of the poor rate performance at low temperatures [93]. As LIBs are conventionally composed of multiple components including positives, electrolytes, separators, and negatives, efforts to improve the low-temperature performance should involve all the component materials in LIBs. Therefore, various strategies have been proposed to deal with this issue in the field of designing novel nanomaterials, optimizing architectures, and developing low-temperature electrolytes [93]. In addition, using the conventional electric current to generate heat is also an effective alternative method to keep relatively stable and normal operating temperature for the LIBs at a low temperature. However, such conventional heating method could hardly lead to uniform temperature distribution inside the cell, easily sluggish responding speed [94, 95]. To meet the requirement for high-energy LIBs in hybrid EVs, hybrid electric vehicles (HEVs), or EVs, the batteries have become much larger and thicker, which show poor electrochemical performance at low temperature, especially for these large-scale applications. The direct strategy for avoiding cell damage at low temperatures is to raise the working temperature by heating. However, the conventional heating methods easily cause local over-heating and/or low heating rate especially for large LIB packs [96]. Recently, a fast self-heating strategy was developed, and the self-heating methods and the conventional heating methods were compared, particularly for the heating rates and temperature distribution in terms of performance [97]. In this comparison, the conventional heating methods include internal heating by resistance and external heating via resistive heaters. It was found that both external and internal conventional methods are slow in heating speed. For the conventional external heating method, the heating process even requires 20 minutes to ensure a temperature increase of 30 ∘ C. In contrast, for the self-heating LIBs, the heating speed can even reach 1–2 ∘ C s−1 . This heating speed is about 40–50 times faster than that the conventional external heating method [98]. In this regard, the self-heating is demonstrated to be much more appropriate strategy to heat battery, especially for large-size thick

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(a)

(b)

Figure 7.15 Configuration and performance of self-heating LIB under 1 C rate at −20 ∘ C. Source: Wang et al. [99].

cells. Since the heating components are inserted inside the LIB, the heating efficiency of self-heating method is almost 100%, and the temperature distribution is uniform. Based on the advantages of the automatic heating method, the concept of full-climate battery by internal self-heating materials was proposed based on the advantages of self-heating method [99]. The 50-μm-thick nickel foil sheet with two tabs was inserted into the LIB to provide the efficient and rapid self-heating function as shown in Figure 7.15a. It is worth noting that one tab is attached to the negative electrode and another tab extends beyond the cell used as the activation terminal. When it is at low temperature, the switch will be opened, followed by flowing of electrons to the nickel foil to produce large amounts of ohmic heat. The generated heat could quickly heat the core from −20 to 0 ∘ C within 20 seconds with 3.8% of the capacity of the LIB (Figure 7.15b), and 30 seconds from −30 to 0 ∘ C with 5.5% cell capacity. Once the normal operating temperature is reached, the switch closes, and the battery transforms to a normal state. In addition, the battery exhibits excellent stability after 500 self-heating test cycles at −30 ∘ C. On the basis of the above study, the self-heating LIB by two-sheet Ni foil inside of the battery showed significantly reduction of the heating time to 12.5 seconds from −20 to 0 ∘ C with only 2.9% cell capacity consumption [100]. In this strategy, the two-sheet Ni foil design gave rise to the much more uniform temperature distribution as well as more rapid heating speed. In addition, the inserted nickel foil can also be used as a temperature sensor owing to the linear relation between the temperature increase and the electrical resistance. To unravel the self-heating process, theoretical analysis is also applied on the basis of the electrochemical-thermal coupled model [100]. It shows that the ohmic heat generated on the nickel heating component represents most of the generated heat, which is considered to be the key parameter in determining the heating speed [101]. Furthermore, the temperature distribution is usually uniform in the active area,

7.5 Self-heating and Self-monitoring Designs

while obvious hot spots appear on the activation terminal inside the self-heating LIB [102]. All the theoretical and experimental analyses demonstrate the effective uniform heat distribution across the reaction area for these internal nickel foils inside the LIBs. At −30 ∘ C, the self-heating LIBs can recover 80% of capacity in less than 14 minutes without lithium plating [101, 102]. Moreover, such LIBs can be applied for most of the smart applications at low temperature environment. Particularly, smart self-heating architectures are of vital importance for the batteries at low temperatures, especially for the large-scale application, which effectively prevents the degradation of batteries. In terms of the investigations about the internal and external architecture design both theoretically and experimentally, the distinct differences could be obtained between the self-heating and external heating of LIBs. First, the heating components are obviously located in different parts of the battery, embedded inside the cell for self-heating, while for the conventional heating method, it is located outside the cell. In addition, the location of the heating component largely determines the temperature field and response speed. The position of the heating components largely determines the temperature distribution and response speed. In summary, the self-heating method can provide more uniform temperature field and higher responding speed. However, it is still a great challenge to ensure high uniform temperature distribution and safety for the application of large-scale battery pack due to the complexity of the internal architecture [103, 104].

7.5.2

Self-monitoring

The self-internal heating strategy with the heating components inside the cell core requires neither additional circuit components nor heat transfer system, leading to low cost and high reliability. However, the safety issue is still a concern especially for the large pack applications, such as HEVs and EVs. Generally, the safety and stability of cells mainly rely on the electrochemical states of the electrodes. Lots of reported catastrophic accidents are usually linked with electrolyte decomposition and lithium dendrite formation on the negative material surface at low potentials. Unfortunately, it is tough to monitor the isolated working potentials of the positive and the negative for practical LIBs in real time. In this regard, the self-detection is an important role in autonomously monitoring the individual states of the electrodes, which will support the LIBs to endure the abuse conditions, thereby avoiding further safety outcome [105]. To solve the abovementioned problem, the smart lithium-ion EC with built-in safety monitor was fabricated by additional lithium metal electrode as voltage modulator and two voltage sensors to form the feedback cycle (Figure 7.16a,b) [106]. In this regard, three functions can be provided by such proof-of-concept smart lithium-ion EC. First, it could effectively boost energy of the carbon supercapacitors by enhancing the energy density. And the pre-lithiation process makes it possible to modulate the potential of the electrodes, leading to the potential extending from 2.8 to 4.3 V (Figure 7.16c,d). Second, it could also probe the potential variations on both the positive and the negative electrodes versus the lithium voltage modulator by monitors V1 and V2 as shown in Figure 7.16b,e. V1 < 0 V demonstrates lithium deposition on the

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(b)

(c)

(d)

(e)

(f)

Figure 7.16 Design, functions, and performances of smart lithium ion EC. Source: Shan et al. [106]. Reproduced with permission of Elsevier.

negative materials during cycling, and V2 > 4.5 V indicates the possible electrolyte decomposition. Based on the feedbacks, the smart lithium-ion EC can be shut down before the possible accident happens. Moreover, the extra lithium reference electrode can also be used as an additional lithium source to restore the capacity of the degraded device (Figure 7.16f). According to the research, with about 40% capacity loss after 280 cycles, the capacity of the smart lithium-ion capacitor could reach to 105.3 mAh g−1 with an ∼100% Coulombic efficiency under the regeneration function achieved by this extra lithium electrode. Furthermore, this smart function could be universally applied, which is also applicable for the activated carbon lithium-ion ECs besides the graphene-based lithium ion ECs. Therefore, Li voltage modulator provides an alternative remedy for safer as well as more durable LIBs. This strategy is particularly suitable for those unstable anode materials, such as tin silicon-based materials. Specifically, it is well known that the high-capacity negative materials without flat charge/discharge curve can exhibit better durability in a narrower voltage range rather than full range [43]. In other words, the excessive volume change can be reduced during charge and discharge by the lower or upper cutoff voltage, therefore extending the cycle life [107]. In this case, such voltage self-monitoring function can be applied to monitor and further tune the potential of alloy electrodes, thereby obviously enhancing the cycling stability of LIBs with high capacity. Despite the achievement of many smart designs, further advancements are still required especially for the practical applications [108].

7.6 Integrated Electrochromic Architectures for Energy Storage 7.6.1

Integration Possibilities

Electrochromism is the reversible change in the optical features of certain materials under small electric stimulation. Based on the dynamic color changing, the

7.6 Integrated Electrochromic Architectures for Energy Storage

Electron flow

Ion flow

TCO

Electrochromic material Solid or gel electrolyte Counter-electrode Transparent conductor (TCO)

Figure 7.17 Schematic illustrating the typical structures of electrochromic devices. Source: Runnerstrom et al. [109]. Licensed under CC-BY-3.0.

electrochromic materials are particularly attractive as indicators for energy status displays with energy storage [109]. As many common characteristics are shared between electrochromic devices and LIBs or ECs, LIBs and cells are suitable to be integrated with electrochromic materials. It is well known that the charge storage in the cells generally originate from Faradic reactions both on the surface and inside the electrode. As shown in Figure 7.17, a typical electrochromic device is a multilayer unit constructed from several fundamental components, including two transparent conductive layers on supporting substrates, an electrochromic component layer, and a counter electrode layer separated by an electrolyte layer. When Faradic reaction occurs, the accompanying color change in certain electrode materials is the phenomenon known as electrochromism. The electrochromic process occurs also via the injection and ejection of charge. Due to similar architecture of electrochromic devices and LIBs or ECs, electrochromic devices are suitable to be fused with the cells. Over the past few years, many electrochromic cells based on transition metal oxides, conducting polymers, organic/inorganic composites, and other materials have been reported [40, 109–114].

7.6.2

Integrated Electrochromic ECs

The pseudocapacitor, an important type of EC, is based on the surficial redox reactions of the electrode. Electrochromism denotes the reversible changes of color via reversible redox reactions driven by an externally applied voltage and shares the common characteristics with pseudocapacitance. First, electrochromic devices and pseudocapacitors have the same sandwich structure composed of a working electrode layer, electrolyte layer, and counter electrode layer. Second, their working principles are both based on redox reactions of the electrode material. Thus, if an

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electrode material showed different colors between oxidized and reduced states, it could work as an EC and electrochromic devices simultaneously. Given such a material, electrochromic function and supercapacitance could be integrated into one device, which would function not only for energy storage but also as an electrochromic window. Significantly, the present level of capacity stored in the ECs could be visually monitored by a color change. In the past few years, many electrochromic ECs have been reported based on transition metal oxides, conducting polymers, organic/inorganic hybrids, and other materials. Tungsten oxide is one of the most promising transition metal oxides for electrochromic applications because of the high optical contrast, low working potential, fast switching, and good cyclic durability. Tungsten oxide has also been extensively investigated for pseudocapacitors and is therefore an ideal candidate for electrochromic supercapacitors. Electrochromic supercapacitors was constructed based on WO3 nanoparticles on FTO [115], WO3 films on Ti foil substrates [116], and WO3 films on silver NW substrates [113]. The obtained EC supercapacitors exhibited impressive electrochemical performances and high specific capacitance up to 639.8 F g−1 . Figure 7.18a depicts the capacitance variation of 100-nm-thick WO3 films on Ti foil substrates upon different applied potentials, associated with the color changing from light blue to deep blue [116]. Single-crystal tungsten oxide quantum dots were also used as active materials for electrochromic supercapacitors, which have relatively high specific capacitance (45 mAh g−1 ) and excellent electrochromic performance [110]. Besides tungsten oxide, other transition metal oxides, including NiO, TiO2 [111, 118], MnO2 , Co3 O4 , V2 O5 , and others, have also been applied in electrochromic supercapacitors and have likewise exhibited good capacity together with acceptable optical modulation. Although transition metal oxides are commonly studied materials for capacitive and electrochromic applications, they have poor mechanical properties upon repeated bending, which restricts their application in flexible devices. In contrast, conductive polymers show the advantages of good conductivity, flexibility, low cost, and even provide larger gravitational capacitance than inorganic materials, especially for some polymers, such as polyaniline (PANI), polythiophenes, poly(3, 4-ethylenedioxythiophene) (PEDOT) : PSS, and PPy. An energy storage smart windows was also fabricated using ordered PANI NWs, which integrated (a) 16 Capacitance (mF)

288

Double layer capacitance

(b)

Pseudicapacitance

12 8 4 0

0

–0.1 –0.2 –0.3 –0.4 –0.5 Potential (V vs. Ag/AgCI)

Figure 7.18 (a) Capacitance constituent and photographs of 100 nm WO3 under different applied potentials from 0 to 0.5 V with an interval of 0.1 V. Source: Yang et al. [116]. (b) The photographic images of the flexible PANI film at different oxidation states, presenting noticeable multicolor property of transparent, pale yellow, green, and blue. Source: Zhou et al. [117]. Reproduced with permission of Elsevier.

7.6 Integrated Electrochromic Architectures for Energy Storage

electrochromism and supercapacitance functions in one flexible device with high areal capacitance of 400 F cm−3 and inaugurated a new era for energy storage smart windows [114]. Flexible PANI films on ITO/PET substrates were prepared, which possessed large optical modulation ability with multicolor displays showing reversible color changes including transparent, yellow, green, blue, and purple at different oxidation states (Figure 7.18b). Further, the device had a specific capacitance of 473.3 F g−1 , with the level of energy storage directly indicated by its colors [117]. Both organic and inorganic electrochromic materials have their own advantages in electrochromic supercapacitors. The organic/inorganic hybrid approach allows the favorable properties of different components to be synergistically combined, providing an advance over single organic or inorganic materials. Organic/inorganic hybrids suitable for electrochromic supercapacitors are usually transition metal oxide/conducting polymer systems. The hybridization offers to improve the performances of energy storage devices by overcoming the drawbacks of each component while integrating their advantages. For example, PANI/WO3 composites, which showed higher coloration efficiency and specific capacitance in comparison with the pure components, were fabricated through electro polymerization of aniline monomers onto a spin-coated WO3 film. Additionally, the nanocomposite film had good stability due to the chemical bonding between WO3 particles and the PANI matrix [119]. A smart supercapacitor electrode was fabricated, which could display a color change as a response to different levels of remaining capacity. Based on the pattern of the metal oxide W18 O49 on a PANI background, both of which shared similar redox processes in sulfuric acid aqueous electrolyte under varied potentials, excellent electrochemical and electrochromic behaviors were achieved. A solid-state WO3 -PANI asymmetric wearable supercapacitors was fabricated by stacking WO3 and PANI as the negative and positive electrodes. These two complementary materials contributed to the excellent EC and electrochemical performances of the supercapacitors, and the energy storage level could be directly read out by a color change visible [120]. In another work, WO3 /PEDOT:PSS was assembled as a cathode for an asymmetric supercapacitor, which functioned as an electrochromic supercapacitor by allowing the permission of light and heat to be adjusted, and as an energy storage device capable of lighting up an LED indicator [121].

7.6.3

Integrated Electrochromic LIBs

Compared with the ECs, LIBs have the characteristics of higher energy density but lower energy delivery speed. Based on the similarity of the sandwich structure and the reversible electrochemical reaction between the LIBs and the electrochromic devices, the electrochromic LIB can also be realized. The electrode materials and color changes of typical electrochromic LIBs are summarized in Table 7.2. The range of electrode materials that can be used for electrochromic LIBs is narrower than the range of electrode materials that can be used for electrochromic ECs because of the need for additional matching between the two electrodes to provide high-voltage output. The most widely used active materials for electrochromic LIBs

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Table 7.2

Typical electrochromic LIBs.

Cathode

Anode

Chromatic transition

Electrolyte Capacitor

References

TiO2

Li+

222 mAh g−1 at 168 mA g−1

Mo-doped WO3

H+

55.89 mAh g−1 Colorless to blue at 1 A g−1

[123]

MoO3 –W0.71 Mo0.29 O3

Li+

41.9 mAh g−1 at 0.1 A g−1

41.9% contrast

[124]

Transparent to dark-blue

[122]

Li3 Fe2 (PO4 )3

Li4 Ti5 O12 Li+

Transparent to blue-gray

[125]

LiMn2 O4

Li4 Ti5 O12 Li+

Colorless to dark-blue

[112]

Green to orange NiO

WO3

75 mAh g−1 at 1 A g−1

Li+

Colorless to dark-blue

[126]

are TiO2 , WO3 , Li4 Ti5 O12 , and LiMn2 O4 . For example, electrochromic materials have been deployed as either cathode or anode active components in electrochromic LIBs in the past few years. Li4 Ti5 O12 , showing a chromatic transition from colorless to dark blue during Li-ion intercalation, can be used as the anode for lithium-ion electrochromic LIBs materials, accompanied by Li3 Fe2 (PO4 )3 and LiMn2 O4 acting as cathodes [112]. In another work, the anisotropic TiO2 (B) nanocrystals produced through a surfactant-assisted nonaqueous sol–gel route were constructed as a continuous network with rod-shaped profile. As-prepared TiO2 (B) nanocrystals can be used as electrodes and efficiently store Li with excellent capacitive properties. As presented in Figure 7.19a, the prepared TiO2 (B) electrodes showed an high Li+ storage capacity up to 222 mAh g−1 at a current density of 168 mA g−1 and notable coloration efficiency of 131 cm2 C−1 at 1.5 V with the color changing from colorless (a)

(b) 2.5 V –2.5 V

60

0.0 –0.5 –1.0 –1.5

(c)

70

B NRs

0.5

Transmittance (%)

Voltage (V) vs Ag/AgCl

290

0

50

100 150 200 Capacity mAhg–1

250

50 40 30 20

Bleached state

Colored state

10 0

400

500 600 700 Wavelength (nm)

800

Figure 7.19 (a) Li+ storage performance and photographs of ultrathin TiO2 (B) nanorodbased electrochromic cells. Source: Giannuzzi et al. [122]. Reproduced with permission of American Chemical Society. (b) Transmittance curves of the electrochromic cell and (c) photograph of the electrochromic cell, which can light the LED. Source: Li et al. [124]. Reproduced with permission of Elsevier.

7.7 Summary and Perspectives

to dark blue [122]. A solid-state electrochromic LIB based on an assembly of WO3 nanowire arrays as the anode, with reduced-graphene-connected bilayer NiO nanoflake arrays as the cathode, and LiClO4 -polyvinyl alcohol as polyelectrolyte. The electrochromic LIB as a power source exhibited a capacity of 75 mAh g−1 at 1 A g−1 , with the remaining capacity of the electrochromic LIB visually indicated by a color change between transparent and blue [126]. It is expected that the doping of metal ions with lower oxidizing capacity can improve the coloration efficiency and energy storage kinetics of metal oxide nanomaterials [127]. For example, Mo ions-doped WO3 nanowires array acted as the active material for a bi-functional electrochromic battery in H+ electrolyte, which exhibited improved electrochromic performance and excellent cycling stability compared with pristine WO3 [126]. The MoO3 –W0.71 Mo0.29 O3 nanowires was reported as a potential material for electrochromic LIB electrodes [124]. The assembled electrochromic LIBs by MoO3 –W0.71 Mo0.29 O3 material as the working electrode and NiO material as the counter electrode showed good energy storage performance and high optical contrast modulation (Figure 7.19b,c) [124]. In summary, electrochromic energy storage devices functioning in interactive modes are at the core of efforts to realize intelligent LIBs and ECs, which could be usefully adopted in buildings and automobiles by visualizing dynamic color displays in response to varying levels of energy stored. Restricted by the high transmittance requirement, the capacity of electrochromic energy storage devices is still not sufficient for compatibility with traditional LIBs or ECs of the same volume. Moreover, a high coloration efficiency and fast switching require a large optical response with low energy density, which is contradictory to the high charge density requirement of energy storage devices. Presently, a compromise between the needs of these two kinds of devices is accepted in technologies combining electrochromism and energy storage. On the other hand, despite the significant improvements with regard to electrochromism in the past years, electrochromic materials still have limitations for use in displays due to the poor color-tuning versatility. In fact, the electrochromic switching states are limited to a rather narrow range of colors, especially for inorganic materials. The challenge is to find new electrochromic materials or devices enabling multicolor modulation, which would greatly extend the opportunities for electrochromic materials in different applications such as displays and smart windows.

7.7

Summary and Perspectives

Smart features stimulate the development of smart electrochemical energy storage devices to satisfy the demand for more elaborate apparatus in the future. Smart cells can detect and respond to outer stimuli, such as temperature increase, abnormal capacity drop, interior short circuits, and other types of external stimuli. As discussed above, apart from the conventional materials and strategies, a series of promising smart materials and novel architectures have been conceived for safer and functional LIBs and ECs both theoretically and experimentally, drawing much

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attention in this field both from the academic community and industry. Nevertheless, significant challenges still exist. As mentioned above, the future development of smart LIBs and ECs will mainly focus on several aspects as follows. (1) As for performance improvement of LIBs and ECs, one of the main challenges is developing smart materials without sacrificing other performance. When operated at high rates, PTC composite materials and coated current collectors have low response speed, which may severely degrade the self-shutdown capability.. Therefore, enhancing the response speed for such smart cathode materials is urgently required. Self-healing chemistries give cells novel properties such as long life, flexibility, and self-healing characteristics. Future cells must possess high self-healing efficiency under external stimulation and should not undergo any side-reactions under current cells chemistry. (2) Integration with other smart sensors will definitely become a major trend in future development for the practical demand for multifunctional energy storage devices. Therefore, smart LIBs will not only play the roles of power supplies but also provide more other functions in the future, via integrating with other smart materials, such as temperature, voltage, SOC sensors, and sensors with ionic response functions. Therefore, it is of extreme importance for future smart LIBs to monitor and offer more information in terms of working status. Besides, it is also required to be compatible with other novel materials, devices, and even systems to achieve a multifunctional capacity. (3) Based on their dynamic color changing, electrochromic materials are particularly attractive as indicators for energy status displays when combined with the cells. Future innovative electrochromic materials with high coloration efficiency and fast switching should be used for practical applications.

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301

Index a

ice templates method 172–173 iterative extrusion method 165–167 magnetic induced alignment method 168 disadvantages 152–153 fabrication methods 159–160 LiFePO4 -based 161 low tortuosity 155–157 performance evaluation 176–178 with random pore structure 160–165 cold sintering process 161–162 presure-less high-temperature sintering process 160–161 spark plasma sintering 162–165

active materials 68, 98–101, 148, 208, 276 all solid-state asymmetric in-plane m-ECs 228, 229 aqueous polymeric gels 104–105 aqueous self-healing electrodes 271–272 automobile battery research and development 8 autonomic healing 265

b battery management system 263 bendability 95 bendable cells flexible substrates and neutral plane 71–72 mechanical process 69–70 thickness effect 70–71 brittle materials 68 Bruggeman relation 156 bulk electrodes advantages 152, 153 architecture 152 based on CNT conductive network 184 based on conductive polymer gels 191–193 with directional pore distribution 165–178 carbonized wood template method 168–172 3D printing of 173–175

c calendering density 194 of electrode 149, 151–153 carbide-derived carbon 235 carbon based foam electrodes CNT foams 181 CNT/graphene foam 181–182 graphene foam 179–180 carbon-based foam electrodes 189 disadvantages 189 carbonized wood template method 168–172 carbon nano tubes (CNTs) 33 carbon/polymer 3D m-LIBs 240 carbon/polypyrrole 3D interdigitated configuration m-LIB 241 cellulose nanofibril paper 108

Novel Electrochemical Energy Storage Devices: Materials, Architectures, and Future Trends, First Edition. Feng Li, Lei Wen, and Hui-ming Cheng. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

302

Index

chemical vapor deposition (CVD) for CNT foam fabrication 181 graphene foam fabrication 179–180 C-LiFePO4 /C-PPy foam electrodes 192 CNT and graphene based flexible cells 121–122 flexible CNTs/graphene composite films 125–127 free-standing films for ECs and LIBs 121–122 free-standing graphene and CNT films for LIBs 122–124 CNT foam application 183 CNT foams 181 CNT/graphene foam 181–182 advantages 182, 187 CNT/graphene foam electrodes 182 CNTs 98 cold sintering process 161–162 commercial paper 108 compact graphene electrodes 188–189 conducting polymers, for electrochemical capacitors 42 conductive cellulose nanofiber fabrication 185 conductive nanofiber framework 185 conductive polymer based foam electrodes 191–193 conductive PPy/Fe3 O4 gel framework 191 construction of thermal-responding layer 274–276 construction principles of flexible cells 95 conventional cells 51, 263 architecture design procedures 50 disadvantages of architecture 68–69 electrolytes 69 mechanical properties 67–68 electrode composite and size design process 51 electrode manufacturing process 51 electrolyte injecting and formation 51

mixing, coating, calendering and winding 51 performance requirements and design 50 cycle performance 77 cyclic voltammetry (CV) 77 cylindrical LIBs 16

d directional freezing method 172–174 double layer capacitance 5 dual-graphene-based lithium ion capacitor 22 dynamic covalent self-healing mechanism 264–265

e electric double layer capacitor (EDLC) activated carbon for 40 carbon nanotubes for 41 graphene electrode material for 41 electric double-layer capacitors 9 electric double-layer capacitors (EDLCs) charge storage mechanism 20 electrode materials for 18–20 Gouy-Chapman model 18–19 Helmholtz model 18 liquid ion electrolyte 19 solid/solid electrode interface 19–20 Stern model 19 electric double-layer miniaturized electrochemical capacitors, valuation methods for 210–211 electricity, advantages 2 electrochemical batteries 5 electrochemical capacitor (ECs) 6, 149 electric double-layer capacitors 18 electrolytes for 45 energy storage mechanism 9, 18 evaluation methods for 49 history of 8 vs. lithium-ion batteries 9, 10, 52 performance 18 separators for 45

Index

electrochemical deposition method, for 2D interdigitated cells 228–231 electrochemical energy storage application 3 classification of 4–5 development history 3–4 power vs. energy density 6–10 electrochemical energy storage system 1 electrochemical impedance spectroscopy (EIS) 77 electrochromic materials 267–268 electrochromic oxides 267 electrochromism architecture of 287 characteristics 287 definition 286–287 LIBs 289–291 organic and inorganic 289 pseudocapacitor 287–289 transition metal oxides 288 tungsten oxide 288 electrode manufacturing process 51 electrode materials, for miniaturized cells 213–214 electrodeposited carbon-coated LiFePO4 /PPy material 193 electrodeposition method 3D interdigitated configuration cell fabrication, 239–240 electrolytes for flexible ECs aqueous polymeric gels 104–106 inorganic solid materials 106 non-aqueous polymer gel 106, 108 for flexible LIBs inorganic solid-state electrolytes 102–103 solid-state polymer electrolytes 104 for miniaturized cells 214–215 electrolytic deposition (ELD) 228–231 electronic gadgets 67 electrophoretic deposition (EPD) 228–231 electrospinning 98 energy generation 1

ethoxylated trimethylolpropane triacrylate (ETPTA) 104 evaporation-induced compact GO disadvantages of 189 extrusion-based 3D printing technique 175, 224

f Faradaic capacitor 20 Faradaic ECs 9 Faradic reaction 287 flexible cells active materials CNTs 98–99 graphene 99 low-dimensional materials 99–101 bendable cells flexible substrates and neutral plane 71–72 mechanical process 69–70 thickness effect 70–71 construction of stretchable cells based on island-bridge architecture 129–131 stretchable cells based on wavy architecture 127–129 construction principles of 95 conventional cells 67–68 dynamic electrochemical performance of bending 83–85 bending characterization 78 conformability test 79 stress simulation by finite element analysis 79–82 stretching 86–88 stretching characterization 78–79 electrolytes 101 electrolytes development 132 innovative architecture designs 132 integrated flexible devices 133 mechanical performance improvement 131 nonconductive substrates 107–121

303

304

Index

flexible cells (contd.) packaging and tabs 132–133 static electrochemical performance of 76–77 stretchable cells 72–76 substrate materials paper 97 polymer 96–97 requirements 96 textile 98 flexible CNTs/graphene composite films 125–127 flexible electronic technology 67 fluorine-modified graphene based m-ECs 226, 227 foam electrodes 151–153 advantages 153 architecture 152 carbon based with high gravimetric energy density 178–179 CNT foam 181 CNT/graphene foam 181–182 graphene foam 179–180 disadvantages 152 fossil energy 1 free-standing graphene and CNT films for LIBs 122–124 Free-standing graphene and CNTs films for SCs 121–122 freeze-drying method 179

g galvanostatic charge/discharge 77 gel polymer electrolyte 104 global energy consumption 1–2 graphene 99 based 2D interdigitated m-ECs 235 based m-ECs 226 compact electrode 188 composites 34 macroscopic structures 214 properties of 179 graphene foam application 183 bidirectional freezing process 180

chemical vapor deposition method 179 design and compressive elasticity of 180 drying process 179 properties of 180 self-assembly of GO 179 graphite, for LIBs 29–31 gravimetric capacity exertion (cp ), of LIB material 150 gravimetric energy density, of electrodes 149–151

h hard carbon 31–33 heterogeneous Co3 O4 /graphene foam 186 high-density bulk PANI/graphene bulk electrode synthesis 189, 190 high energy density cells electrodes gravimetric and volumetric energy density of 149–151 thick electrode, classification of 151–153 factors related to 154 strategies for 147–149 architecture design 148 materials and chemistry development 147–148 holey-graphene/Nb2 O5 foam 187 hybrid capacitors 21–22 charge storage mechanisms 21 dual-graphene-based lithium ion capacitor 22 and symmetric ECs 22 hybrid ECs 9 “hydrothermal lithiation” method 101 hyper-branched architecture self-terminated oligomer 278

i ice templates method 172–173 inkjet printing technique 228 feature of 228

Index

planar m-ECs on paper substrates 228 thin and porous electrode films 228 inks with Newtonian behavior 222 inks with non-Newtonian behavior 222 inks with thixotropic/pseudoplastic behavior 222–223 inorganic amorphous sulphides 102 inorganic ceramic electrolytes 103 inorganic solid materials 106 inorganic solid-state electrolytes 102–103 in-situ electrode conversion, for 2D interdigitated cells 234–236 integrated electrochromic architectures for energy storage, electrochromic devices 287 integrated flexible devices 133 integrated self-powered smart sensor system 248249 integrated Si-based solar cells 248 integrated systems 247–249 interdigitated microelectrode array 244 Internet of Things (IoTs) 205 ionic blocking effect based thermal response electrodes 278–280 island-bridge architectures 75–76 iterative extrusion method, for electrode fabrication 165–167

l large volume variation materials/carbon foam 186–188 laser induced graphene based m-ECs 233 laser scribing, for 2D interdigitated cells 231–234 layered Li(Nix Co1-x )O2 material 26 LiCoO2 material 23–25 LiFePO4 -gel foam electrode 192 LiFePO4 material 27–28 LiMn2 O4 polycrystalline thin films 217 Li-polymer 3D m-LIBs 247 liquid-alloy self-healing electrode materials 273–274 liquid electrolytes

for LIBs 42 for miniaturized cells 215 lithiation-delithiation processes 35–37 lithium ion batteries (LIBs) conventional materials for 22–23 LiCoO2 positive active material 23–25 LiFePO4 material 27–28 LiNiO2 and derivative 25–26 lithium-manganese-rich materials 28 material status of 22 negative materials 23 positive material properties 23 spinel LiMn2 O4 26–27 vs. electrochemical capacitors 52 electrochromism 289–291 energy density 16 mechanisms of 16 principle of 15 lithium-ion batteries (LIBs) 6–10, 147 advantages 16–17 alloy-based materials 35–39 Chinese market shares 28–29 commercialization 7 cycle life 48 cyclic and linear carbonates in 43 disadvantages 17–18 for electric vehicles 8 vs. electrochemical capacitors (ECs) 9–10 electrodes for 153 electrolyte used in 42 energy and power densities 208 energy and power density 47–48 evaluation of 46 graphene for 34 graphene hybrid materials 35 gravimetric and volumetric energy density 46–47 history, current status and development 8 intercalation/deintercalation reaction materials 37 Li4 Ti5 O12 based 37

305

306

Index

lithium-ion batteries (LIBs) (contd.) metal lithium negative for 39 nanocarbon materials 33–35 nanosized silicon materials for 36 negative materials for characteristics 29 Chinese market share 33 graphite 29–31 soft and hard carbons 31–33 porous carbon materials for 40–41 porous materials 36–37 properties 6 safety 48 separators for 45–46 silicon/carbon composite materials for 36 solid state electrolytes 45 thickness of electrodes 208 TiO2 for 38 transition metal oxides in 37–39 lithium ion capacitor 22 lithium-manganese-rich materials 28 Li4 Ti5 O12 /graphene foam electrode 184 low-dimensional materials (LIBs) architecture of 99 electrolytes inorganic solid-state electrolytes 102 solid-state polymer electrolytes 104 MXene phases 100 low electronic conductive material/ carbon foam 182–186

m magnetic induced alignment method 168 magnetron sputtering process 216–217 mesocarbon microbeads (MCMB) 30 metal lithium negative, for LIBs 39 microchannel plated deposition method 245–247 micro encapsulation based self-healing system 265 microvascular self-healing systems 265

miniaturized cells architectures of 212 classification of 206 3D 208 development trends of 207–209 2D interdigital configuration 212–213 3D interdigitated configuration 212–213 2D parallel plate configuration 212–213 3D stacked configuration 212–213 with 3D stacked configuration 240–247 microchannel plated deposition methods 245–247 template deposition 241–245 electrode materials 213–214 electrolytes for 214–215 evaluation methods 209–210 examples of 206 fabrication technologies for 215–220 high performance 209 integration of 209 market of 206 performance of 207 polymer electrolytes 215 research and development 209 volume characteristics 210 volumetric energy density 205 miniaturized electrochemical capacitors (m-ECs) 206, 207 all solid state asymmetric in-plane 228, 229 based on carbide-derived carbon film 235 carbide-driven carbon 235 carbon materials 213 conducting polymers 214 evaluation methods for 211 graphene material for 213 laser scribed graphene electrodes for 232, 234 pseudocapacitive materials 214 screen printed 225 on a silicon wafer 235–236

Index

solid-state 215 using RuO2 films 219–220 miniaturized lithium ion batteries (m-LIBs) 205, 206 evaluation methods for 211–212 mask-assisted fabrication 226 materials for 214 screen printing 225 solid-state 215 using Li4 Ti5 O12 anode and LiFePO4 cathode 226 MoO3-x -CNTs-cellulose nano fibers electrodes 111 MoS2 @ CNT@GF foam 187 multiwall carbon nanotubes (MWCNTs) 34 MXene phases 100 MXene water-based ink 238

n nanocarbon materials 68 nano fibers of cellulose (NFC) 110 nanosized silicon materials 36 nanostructured 3D m-LIB fabrication 243 neutral plane 69 nitrogen-doped graphene/Fe3 O4 hydrogels fabrication 186–187 N-methyl-2-pyrrolidone (NMP) solution 279 non-aqueous polymer gel 106, 107 non-autonomic healing 265 nonconductive substrates based flexible cells disadvantages of 108 paper-based flexible cells 108–112 physical methods 107 polymer 117–120 textiles-based flexible cells 112–117

o on-chip thin film m-LIBs 217, 218 organic and inorganic electrochromic materials 289 oxide solid-state electrolytes 102

p PANI/Au/paper electrode 111 paper-based flexible cells 108–112 paper substrate 97 paraffin wax microspheres 279 pesudocapacitor miniaturized electrochemical capacitors 211 photolithography 239–240 planar m-ECs using graphene materials fabrication 225 plasma-activated sintering 162 poly(3-decylthiophene) (P3DT) 278 poly(ethylene oxide) (PEO) 104 polyacrylic acid/uriedo-pyrimidinone (PAA-Upy) 271 polydimethylsiloxane (PDMS) 96 polydopamine (PDA) 279 polyethylene naphthalate (PEN) 96 polyethylene terephthalate (PET) 96 polyimide (PI) 96 polymer materials 68 polymer substrates 96–97 polymer substrates based flexible cells 117–120 polyurethane (PU) 96 porosity adjustable graphene bulk electrodes 189, 191 porous carbon materials, for LIBs 40 porous electrodes, tortuosity of 157 porous graphene foam fabrication 188 porous materials 36 portable electronics 52, 147 positive temperature coefficient (PTC) materials 266 pouch-type LIBs 16 presure-less high-temperature sintering process 160–161 primary battery 5 printing technology 107 prismatic LIBs 16 pseudocapacitance 5, 20, 21 pseudocapacitor 20, 287 pseudo-capacitors 9 transition metal oxides for 41 pulse electric current sintering 162

307

308

Index

pyrolytic photoresist-derived carbon electrode 234

r renewable energy 1 renewable energy based electricity 3 renewable energy sector, employment in 2 renewable energy sources 10 RF magnetron sputtering 216–218 limitation 224

s screen printing, for miniaturized cells advantages 225 asymmetric m-ECs construction 225 description 224 design flexibility 224 graphene based 226 vs. RF magnetron sputtering 224 substrate patterning 224 using highly-conducting graphene ink 226 secondary batteries 5, 6 self-healing materials 264 self-healing silicon anodes 268 self-powered smart sensor system 248–249 shape-adapting cells 266 shape-memory alloys 265 shape-memory materials in LIBs and Ecs alloy based thermal regulator 281–282 self-adapting cells 280–281 self-heating 283–285 self-monitoring 285–286 Si-based thin film electrodes 219 silicon/carbon composite materials 36 sintering, advantages of 165 Si particle/conducting polymer/CNT hybrid material 193 smart cells definition of 263 integrated electrochromic architectures for energy storage 286 smart devices 263

smart materials construction of aqueous self-healing electrodes 271–272 ionic blocking effect based thermal response electrodes 278–279 liquid-alloy self-healing electrode materials 273–274 self-healing silicon anodes 268–271 thermal-responding layer 274–276 thermal response electrodes based PTC effect 276–277 definition of 263–264 electrochromic materials 267 self-healing materials 264 shape-memory alloys 265 thermal-responding PTC thermistors 266 soft carbon 31, 32 solid-state polymer electrolytes 104 spark plasma sintering 162 spinel LiMn2 O4 material 26 stage index 29 stage phenomenon 29, 30 static electrochemical performance 76 stencil-printed gel composite electrolytes 215–216 stretchable cells innovative architecture 132 island-bridge architectures 75–76 wavy architectures large-deformation buckling process 74–75 small deformation buckling process 72–74 stretchable cells based on island-bridge architecture 129–131 stretchable cells based on wavy architecture 127–129 Stretchable devices 95 stretchable m-EC array with integrated strain sensors 248 substrate materials flexible cells paper 97

Index

polymer 96–97 requirements 96 textile 98 supercritical drying method 179

t tandem m-EC bridging solar cell and gas sensor 248 tap density 150 tap density, for electrodes 149 template deposition method, for miniaturized cells with 3D stacked configuration 241 textiles-based flexible cells 112 textile substrate 98 thermally sensitive polymer-based polymer 279 thermal regulator 266 thermal-responding layer 274 thermal-responding positive temperature coefficient (PTC) materials 266 thick bulk electrodes 152 thick electrodes 147 thick LiFePO4 electrode fabrication 186 thin film 2D m-ECs 208 thin film 2D m-LIBs 208 thin film solid state m-LIBs 216 3D conductive network 178 3D foam electrodes 153 3D in-plane miniaturized cell fabrication 236 3D printing 236 3D interdigitated configuration cell fabrication 239 3D porous graphene film preparation 233 3D printing process classification of 237 defined 236–237 3D interdigitated m-LIBs 238–239 fused deposition modelling 237 of m-ECs 238 powder-liquid 238 schematic illustration 237 selective laser sintering 238

stereolithography 238 for thick electrodes 173 3D solid-state m-LIB architecture 242 based on imprinted microelectrodes 243 power performance 242 tortuosity, in porous electrodes 156 tortuosity, of porous electrodes numerical simulation 158–159 X-ray tomography 157–158 transition metal oxides 267–288 transition metal oxides (TMOs), for pseudocapacitors 41 true density 149 tungsten oxide 288 2D interdigitated cells electrochemical deposition method for 228 fabrication technologies for 220–221 in-situ electrode conversion 234–236 laser scribing for 231 performance evaluation 220–222 printing technologies advantages 222 classification of 222 inkjet printing technique 228 and rheological behavior 222–224 screen printing 224 2D parallel plate miniaturized cell fabrication 216 2D planar miniaturized cells, fabrication of 220, 221

u ultrahigh-voltage integrated m-ECs, graphene based 226, 227 ureidopyrimidinone grafted polyethylene glycol (UPy-PEG-UPy) binders 271

v vacuum filtration method 111 volumetric energy density, of electrodes 149

309

310

Index

w wavy architecture 127 wavy architectures

wood-derived all-solid-state electrochemical capacitor 172 W-RuO2 -based m-EC 219

large-deformation buckling process 74–75

x

small deformation buckling process 72–74 WO3 267

y

Xerox paper 108

Young’s modulus

70

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