Stacking structures and electrode performances of rare earth-Mg-Ni-based alloys for advanced nickel-metal hydride battery

Rare earth-Mg-Ni-based alloys with stacking structures consisting of AB(5) unit (CaCu5-type structure) and A(2)B(4) unit (Laves structure) have received attention as negative electrode materials for advanced nickel-metal hydride (Ni-MH) battery. These alloy materials are very attractive because of high hydrogen storage capacity, low cobalt content and moderate plateau pressure, but have some difficulty to control the phase abundance and electrode performances. In this paper, relationship among composition, phase abundance, and electrochemical properties was investigated. Structural analysis was done using synchrotron X-ray diffraction patterns. In alloys such as La0.8Mg0.2Ni3.4-x-yCo0.3(MnAl)(x) (0 <= x <= 0.4), phase abundance was drastically changed with increasing amount of Mn and Al. In the range of 0.1 < x <= 0.2, hexagonal Pr5Co19-type (5:19H) or rhombohedral 1:4R phases were dominant. The Rietveld analysis suggested that Mg occupies La sites in A(2)B(4) unit, and Al has tendency to occupy Ni sites between A(2)B(4) unit and AB(5) unit or between AB(5) units in these types of phases. The developed alloys showed higher discharge capacity by 20% than the conventional one at a 0.2 C discharge rate. (C) 2007 Published by Elsevier B.V.