In situ neutron diffraction study of deuterium gas absorption by AB5 + y alloys used as negative electrode materials for Ni-MH batteries

LaNi(5)-type alloys store reversibly hydrogen and are used as negative electrode materials in Ni-MH batteries. Substitutions on La and Ni crystallographic sites have led to competitive materials with complex formulae Mm(Ni(4.3-x)Mn(0.4)Al(0.3)Co(x))(1+y) (Mm: mishmetal). Materials involving an unexpected metastable phase gamma show the best cycle lives. This phase occurrence depends on the mishmetal composition, the cobalt rate and the over-stoichiometry. It is observed as a transitory phase only for charge in electrochemical process. To confirm the appearance of this phase during gas loading, in beam D(2) gas absorption has been performed on two materials for which the gamma phase is expected. Phase amounts and cell volumes have been measured by in situ neutron powder diffraction analysis under controlled gas pressure as a function of the state of charge. (C) 2004 Elsevier B.V. All rights reserved.