On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based hydride-forming compounds : part I crystallography and electrochemistry

Single-phase, non-stoichiometric La(Ni/Cu)x compounds (5.0x6.0) 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 (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 dumbbell 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.