ON THE NATURE OF THE ELECTROCHEMICAL CYCLING STABILITY OF NONSTOICHIOMETRIC LANI5-BASED HYDRIDE-FORMING COMPOUNDS .1. CRYSTALLOGRAPHY AND ELECTROCHEMISTRY

Single-phase, non-stoichiometric La(Ni/Cu)x compounds. (5.0 less-than-or-equal-to x less-than-or-equal-to 6.0) have been prepared by annealing 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 (Ni-to-Cu 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 La 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 behaviour 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.