Influence of stoichiometry and composition on the structural and electrochemical properties of AB5+y-based alloys used as negative electrode materials in Ni-MH batteries

Negative electrodes of Ni-MH batteries are made of AB(5)-type based pseudo-binary phases such as MmNi(3.55)Mn(0.4)Al(0.3)Co(0.75) (Mm=mischmetal). Recently, the appearance of an intermediate gamma phase during the charge process for Co-rich alloys was reported. The occurrence of this phase leads to a significant reduction of the decrepitation induced by large volume expansion during charge-discharge cycles and subsequent sensitivity to corrosion. It was also reported that stoichiometries larger than five improve the cycle life of materials with low Cc content. In the present work, the structural and electrochemical behaviour of over-stoichiometric low Co content electrode material has been investigated by in situ neutron diffraction. This technique allows bulk analysis of the working electrode and gives information on the involved phases, their relative amount and their structural properties. From this analysis, it has been possible to relate the appearance of the intermediate gamma phase to the composition and the stoichiometry of the intermetallic compound. Subsequently, a tentative phase diagram has been established as a function of cobalt content, amount of cerium and stoichiometry ratio B/A. (C) 2002 Elsevier Science BY All rights reserved.