Mechanistic model of charge retention in AB₂-based nickel metal hydride systems

Self-discharge and charge retention in nickel metal hydride (Ni-MH) batteries, particularly those based on AB₂-hydride storage alloys, are critically related to these battery's reliability in practical applications. Recently we have successfully developed simulation capability for Ni-MH batteries and shown that the hydrogen evolution-recombination cycle is crucial to battery performance. We, however, could not predict its cycle life behavior yet. This work is the first attempt to collect experimental data to help us identify possible mechanisms associated with the self-discharge and charge retention. Eventually, we hope that cycle life prediction can be developed. By carefully analyzing the electrode potential changes with cell pressure and cell voltage changes with time, we found that there are three different stages of capacity loss and proposed the corresponding reactions. A predictive model derived from the phenomenological description and mechanistic understanding from this work was constructed and the prediction was compared with the experimental result in good agreement.