The influence of Mn on the crystallography and electrochemistry of nonstoichiometric ABS-type hydride-forming compounds

To design Go-free, low-pressure, hydride-forming compounds for application in rechargeable nickel metal hydride batteries, nonstoichiometric AB(x) mat erials were investigated. The influence of both the Mn content and the degr ee of nonstoichiometry on the crystallography, electrochemical cycling stab ility, and electrode morphology were studied. The investigated composition was in the range of La(Ni1-zMnz)(x) with 5.0 less than or equal to x less t han or equal to 6.0 and 0 less than or equal to xz less than or equal to 2. 0. The annealing temperature was essential in preparing homogeneous compoun ds. In agreement with geometric considerations, both the a and c axis of th e hexagonal unit cell increase with increasing Mn content. In contrast, the a axis decreases with increasing degree of nonstoichiometry. As proved by neutron-diffraction experiments, the introduction of dumbbell pairs of Ni o r Mn atoms on the La positions in the crystal lattice is responsible for th is behavior. The electrochemical cycling stability is found to be strongly dependent on both the chemical and nonstoichiometric composition. Electroch emically stable materials are characterized by the absence of a significant particle-size reduction upon electrode cycling, reducing the overall oxida tion rate. Unstable materials suffer from severe mechanical cracking throug h which the oxidation rate is increased. The improved mechanical stability is attributed to the reduced discrete lattice expansion. The most stable co mpound has a partial hydrogen pressure of only 0.1 bar, which matches well with that desirable in practical NIMH batteries. Neutron-diffraction experi ments confirmed the hypothesis that La atoms are replaced by dumbbell pairs of Ni, in the case of the binary LaNi5.4, and by Mn atoms in the case of t he Mn-containing nonstoichiometric compounds. Electron-probe microanalyses and density measurements support the dumbbell hypothesis. (C) 1999 The Elec trochemical Society. S0013-4651(98)12-048-7. All rights reserved.