AB2 AND AB5 METAL HYDRIDE ELECTRODES - A PHENOMENOLOGICAL MODEL FOR THE CYCLE LIFE

The variation of the capacities of different metal hydride electrodes as a function of the number of charge-discharge cycles, i.e. the cycle life curve, were quantified using a phenomenological model. This model allows a numerical fit of the measured discharge capacities as a function of the number of cycles. The model determines five independent parameters which describe the electrode cycle life. These parameters are the total and the active start-up capacity, and the activation, oxidation and reduction constants. These are the most interesting characteristics of the electrode for battery applications. This model was applied to the cycle life curve of different AB2 and AB5-type electrodes. We found that these parameters depend on the applied discharge current. We must be able to explain this effect. Using X-ray photoelectron spectroscopy depth profiles, we observed strong changes in the surface composition which, together with the increased surface area as a result of pulverization, is responsible for the activation.