Magnesium - based hydrogen storege materials modified by mechanical alloying

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Acta mater. Vol. 47, No. 14, pp. 3737±3743, 1999 # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain S1359-6454(99)00249-9 1359-6454/99 $20.00 + 0.00

MAGNESIUM-BASED HYDROGEN STORAGE MATERIALS MODIFIED BY MECHANICAL ALLOYING N. CUI, P. HE and J. L. LUO{ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 (Received 13 April 1999; accepted 4 August 1999) AbstractÐThe e€ects of mechanical alloying on microstructure and electrochemical performance of a Mg± Ni±Y±Al hydrogen storage alloy in 6 M KOH solution were studied. The ball-milled powders were examined by X-ray di€raction (XRD), transmission electron microscopy (TEM), selected-area electron di€raction (SED) and energy dispersion spectrometry (EDS). TEM and EDS results clearly reveal that the smaller nickel clusters or particles were well dispersed on the surface of larger magnesium alloy particles by mechanical grinding for 72 h. With an increase in milling time to 240 h, the nickel clusters or particles disappeared and a new monophase alloy with amorphous structure was formed. The electrochemical capacity of the modi®ed material signi®cantly increased with increasing milling time within 72 h and then dropped to nearly nil when the milling time reached 240 h. The capacity decay, however, was always improved with increasing grinding time. Further analysis and discussion were made based on d.c. polarization and a.c. impedance spectroscopy measurement results. # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Mechanical alloying; Amorphous materials; Hydrides; Ni±MH battery; Kinetics

1. INTRODUCTION

Batteries can store energy in a clean, convenient and ecient manner. There is a growing need for high-speci®c power, high-speci®c energy and lowcost battery systems suitable for use in spacecraft, military and defense, communication, electric vehicles, power tools and consumer appliances. Currently, nickel/cadmium batteries have the largest share of the rechargeable battery market. Due to the relatively low capacity and environmental concerns, more ecient and safe substitutes for cadmium are urgently needed. Metal (alloy) hydride materials have shown great promise for providing a solution to the aforementioned problems [1±3]. During the last decade, Ni±MH (metal hydride) batteries using a hydrogen storage alloy as the negative electrode material have been developed and are therefore expected to be attractive commercial rechargeable batteries of the next generation and promising candidates for propulsion of electric vehicles [4±6]. Only rare-earth system (AB5-type) and zirconium±titanium±vanadium system (AB2 Laves phase-type) hydrogen storage alloys have been used as negative electrode materials for the commercial production of Ni±MH batteries [3, 7, 8]. However, these materials have a low hydrogen storage capacity resulting in a low electrode energy density. {To whom all correspondence should be addressed.

In addition, for rare-earth system alloys, the quality and stability of production is hard to control since the compositions of misch metals (mixed rare-earth metals) used as the main raw materials always change with di€erent supplies. For the Zr±Ti±V system, its application and dissemination are also limited because the main element, V, is very costly and toxic. Thus, the development of light-weight and economical hydride negative electrode materials is of great importance for the large-scale application of Ni±MH batteries. Recently, magnesium-based hydrogen storage alloys have shown their great potential for use as the hydride electrode materials for Ni±MH batteries because of their higher theoretical electrochemical capacity (999 mAh/g) and lower cost [9±14]. Many e€orts have been made to improve their electrochemical hydriding/dehydriding performance in alkaline solution, such as alloy element substitution [9, 10, 15], surface microencapsulation of alloy powder [9, 16, 17], oxide additions [11, 15], alloy composites [9, 18] and mechanical alloying [13, 14]. Our previous study indicated that the additions of yttrium and aluminum to Mg2Ni alloy might remarkably improve its electrode capacity, rate capability and durability [10]. However, the electrode performance of this material was still far from practical application. Mechanical alloying (MA) has proved to be an e€ective method to synthesize and modify the hydrogen storage alloy electrode materials [13, 14, 19±21]. Zaluski et al. [22] studied the catalytic e€ect

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of Pd on hydrogen absorption of mechanically alloyed Mg2Ni, LaNi5 and TiFe in gas±solid systems. They found that the Pd catalyst acted e€ectively when present in the form of small clusters or particles dispersed on the surface of the absorbing materials in the nanocrystalline state. However, the additive Pd is too costly for mass production. Recently, Kohno et al. showed that the discharge capacity of Mg2Ni alloy was apparently improved by mechanical alloying in an amorphous state, but the electrode showed a quick capacity decay [13]. The purpose of this work is to systematically study the e€ect of crystalline structure on the electrochemical hydriding and dehydriding behavior. For this purpose, Mg1.95Y0.05Ni0.92Al0.08 was selected as the primary alloy in this study and modi®ed with MA to obtain the ®nal materials with various crystalline structures. 2. EXPERIMENTAL DETAILS

The Mg1.95Y0.05Ni0.92Al0.08 primary alloy was prepared by the conventional vacuum powder metallurgical technique [10, 15]. The alloy pellets obtained were crushed and ground mechanically into powder below 250 mesh. The resulting primary alloy powder mixed with 10 wt% ®ne nickel powder (