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 anneali ng at the appropriate temperatures within the homogeneity regions of t he materials phase diagrams. The effects of both the non-stoichiometri c composition and the chemical composition (Ni-to-Cu ratio) on the cry stallographic and electrochemical properties have been investigated. A special substitutional mechanism is presented which can account for t he crystallographic data. This mechanism involves the partial replacem ent of La atoms by dumb-bell pairs of Ni atoms, whereas Cu atoms are a rgued to occupy preferentially the crystallographic positions surround ing these dumb-bells. Electrochemical parameters, such as the storage capacity, cycling stability and discharge efficiency, have been determ ined and are found to be strongly dependent on both the non-stoichiome tric and the chemical composition of the compounds. Microscopic invest igations of the electrochemically cycled electrodes revealed an unequi vocal correlation between particle size reduction and cycling stabilit y. A model is proposed which can account for the cycle life behaviour of these non-stoichiometric compounds. This model, in which the electr ode surface area and the materials oxidation rate constant play an ess ential role, has been tested using electrodes with different surface a reas.