On the Nature of the Electrochemical Cycling Stability of Non-Stoichiometric LaNi sub 5 -Based Hydride-Forming Compounds. I. Crystallography and Electrochemistry

Single-phase, non-stoichiometric La(Ni/Cu) sub x compounds (5.0 < =x < =6.0) have been prepared by annealing the solids at the appropriate temperatures within the homogeneity regions of the materials phase diagrams. The effects of both the non-stoichiometric composition and the chemical composition (nickel:copper ratio) on the crystallographic and electrochemical properties have been investigated. A special substitutional mechanism is presented which can account for the crystallographic data. This mechanism involves the partial replacement of lanthanum atoms by dumb-bell pairs of Ni atoms, whereas Cu atoms are argued to occupy preferentially the crystallographic positions surrounding these dumb-bells. Electrochemical parameters, such as the storage capacity, cycling stability and discharge efficiency, have been determined and are found to be strongly dependent on both the non-stoichiometric and the chemical composition of the compounds. Microscopic investigations of the electrochemically cycled electrodes revealed an unequivocal correlation between particle size reduction and cycling stability. A model is proposed which can account for the cycle life behavior of these non-stoichiometric compounds. This model, in which the electrode surface area and the materials oxidation rate constant play an essential role, has been tested using electrodes with different surface areas.