L Journal of Alloys and Compounds 330–332 (2002) 787–791 www.elsevier.com / locate / jallcom Influence of stoichiometry and composition on the structural and electrochemical properties of AB -based alloys used as negative 5 1y electrode materials in Ni–MH batteries a, b a c d * ´ M. Latroche , Y. Chabre , A. Percheron-Guegan , O. Isnard , B. Knosp a UPR 209, LCMTR, ISCSA, IFR 1780, CNRS,2 –8, rue Henri Dunant, 94320, Thiais, France b ´ ` LSP , Universite J. Fourier Grenoble I et CNRS, BP 87, 38402, St Martin d’ Heres, France c CRG- A, D1B, Institut Laue-Langevin, BP 156X, 38042, Grenoble, France d SAFT, Direction de la Recherche, 111 Bd. Alfred Daney, 33074, Bordeaux, France Abstract Negative electrodes of Ni–MH batteries are made of AB -type based pseudo-binary phases such as MmNi Mn Al Co 5 3.55 0.4 0.3 0.75 (Mm5mischmetal). Recently, the appearance of an intermediate g 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 Co 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 g 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.  2002 Elsevier Science B.V. All rights reserved. Keywords: Intermetallic alloys; Hydrides; Electrode material; Neutron diffraction 1. Introduction life without significant capacity loss for hundreds of cycles [7,8]. In a previous paper [9], we have presented results Ni–MH batteries have now widely replaced Ni–Cd obtained on different Co-containing electrodes. The ap- batteries in the huge market of portable goods. Most of the pearance of an intermediate g phase during the charge negative electrodes of such batteries are made of LaNi - process for the Co-rich alloys was observed. This phase 5 type alloys [1,2] and improved materials have been presents the same structure as the parent intermetallic obtained from complex pseudo-binary phases such as compound (hexagonal CaCu type structure with P6 / mmm 5 MmNi Mn Al Co [3,4]. Use of mischmetal was space group) but with an intermediate cell volume between 3.55 0.4 0.3 0.75 formerly only for economic purpose since this rare earth the a and b phases which allows one to estimate its mixture is cheaper than pure lanthanum, but it appears that hydrogen content at |3 H/f.u. It was proposed that the the mischmetal composition is also of great importance for occurrence of this intermediate phase, which involves a cycle lifetime [5]. Even when using mischmetal, alloy cost two step process in the a to b transformation, leads to a is still high, especially because of cobalt addition which significant reduction in the decrepitation induced by the represents 10% of the weight but 45% of the total alloy large volume expansion during charge–discharge cycles price. However, interest in cobalt addition has been and the subsequent sensitivity to corrosion. Recently, demonstrated in the past [6] and recent studies have shown Cocciantelli et al. [10,11] reported on the influence of the that it plays an essential role in the improvement of cycle stoichiometric ratio B / A on the performance of negative electrodes. They came to the conclusion that the use of over-stoichiometric alloys is a promising way to reduce corrosion and that it should compensate for a Co content *Corresponding author. Fax: 133-1-4978-1203. E-mail address: michel.latroche@glvt-cnrs.fr (M. Latroche). reduction. Consequently, the question arises if the en- 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01490-6