Relationship between composition, volume expansion and cyclic stability of AB5-type metalhydride electrodes

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Abstract

Metal hydride alloys as electrode material for battery application contain up to 15 at.% cobalt. Alloys without cobalt show a much shorter cycle life compared to cobalt containing alloys. The mechanism of how cobalt influences the cycle life is still not well understood. The aim of this work is to investigate the influence of cobalt on the properties of the electrode. A series of alloys with different cobalt content and several other substituents for nickel (Fe, Cu,...) were prepared in two different ways. A set of samples was conventionally melted. A second set of samples was prepared by gas atomization. The volume expansion upon hydriding was analyzed by means of X-ray diffraction. Electrochemical measurements, e.g., discharge capacity as a function of cycle number, were performed. The volume expansion upon hydriding decreases with increasing cobalt content of the alloy. Cobalt substitution for nickel improves the cycle life of an electrode, especially at elevated temperatures (40°C). However, alloys where cobalt is partially substituted by iron show an even better cyclic stability and rate capability.

Introduction

Cobalt is the most expensive element used today in the metalhydride alloys for battery applications. It is well known that cobalt in the alloy has a positive effect on the cycle life of metalhydride electrodes. However, the mechanism of the improvement of the cycle life through cobalt is still not clearly understood. The effect of substituting elements in the LaNi5−xMx (M=Ni, Mn, Cu, Cr, Al and Co) on the cycle life and the mechanical properties of the alloys was investigated by Sakai et al. [1]. The cycle life was improved for alloys with a small volume expansion upon hydriding, a low reversible capacity and a low Vickers hardness. The volume expansion upon hydriding and the Vickers hardness of the alloy have a pronounced influence on the rate of pulverization and the final grain size. The influence of the discharged capacity on the cycle life was studied by Boonstra et al. [2]and it was concluded, that corrosion is related to the discharge capacity used per cycle. Notten et al. [3]concluded, that the lattice expansion between the α-phase hydride and the β-phase hydride rather than the total lattice expansion upon hydriding was responsible for the mechanical stability of the powders.

In order to substitute cobalt with other elements without sacrificing the cycling stability, the influence of cobalt on the material's properties was investigated.

Section snippets

Experimental

The alloy samples LmNi4.3−xAl0.4Mn0.3Cox (Lm: lanthanum rich mischmetal: 51% La, 33% Ce, 12% Nd, 4% Pr) and LmNi3.8Al0.4Mn0.3Co0.3Fe0.2 were conventionally melted and gas atomized as well. Gas atomized alloys were produced by spraying the molten alloy in an inert gas chamber usually using argon atmosphere. The resulting powder consists of spherical particles with an average grain size of approximately 100 μm. X-ray diffraction was performed with copper radiation (Cu Kα λ=1.5406 Å). Channel

Results and discussion

The conventionally melted alloy LmNi3.6Al0.4Mn0.3Co0.7 (Fig. 1) has a reversible electrochemical capacity of 337 mAh g−1 at 20°C (326 mAh g−1 at 40°C) and a very good cycle stability, 0.05% at 20°C (0.2% at 40°C) loss of capacity per cycle. Furthermore this alloy shows a good rate capability, the 1C discharge capacity, i.e., the maximum discharge capacity which can be retrieved in one hour, is 325 mAh g−1 at 20°C (326 mAh g−1 at 40°C). The cycle stability was calculated from a fit of our previously

Conclusion

In this work we have shown a possibility of partly replacing cobalt with iron in the LmNi3.6Al0.4Mn0.3Co0.7 alloy system without sacrificing some of their electrochemical properties, such as capacity and rate capability.

Acknowledgements

Financial support from the EU-Project Brite-Euram (Contract No. BRE2-CT92-0219) is gratefully acknowledged.

References (5)

  • T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, A. Kato, H. Ishikawa and C. Iwakura, J. Less-Comm. Metals, 161 (1990)...
  • A.H. Boonstra, G.J.M. Lippits and T.N.M. Bernards, J. Less-Comm. Metals, 155 (1989)...
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