Steep capacity loss of discharged state metal-hydride electrode and its mechanism

Considerable irreversible capacity loss of LaNi3.6Co0.7Mn0.4Al0.3 electrode was observed after standing at 100% or 95% depth of discharge (DOD) in open circuit for 2 or 5 days, respectively. The mechanism was systematically studied. Electrochemical impedance spectroscopy (EIS) showed considerably increased charge transfer resistance (Rct) for the electrodes after standing. X-ray diffraction (XRD), open-circuit potential (OCP), scanning electron microscope (SEM), energy disperse X-ray analyzer (EDS) and X-ray photoelectron spectroscopy (XPS) analyses were conducted to investigate the details during standing. The results demonstrated that there were two successive oxidations occurred, which acted as the conjugated reactions for hydrogen evolution reaction (HER), on the alloy surface during the standing process. The first oxidation was attributed to the electrochemical desorption of hydrogen, which was essentially the self-discharge process of electrode and irrelevant to the irreversible capacity decay. The second one corresponded to the oxidation of elemental Ni with flocculent deposits on the alloy surface, which primarily determined the rapid capacity and kinetics deterioration of the alloy electrode.

► Great irreversible capacity loss was observed for MH/Ni electrode after standing at partial charged state for only several days.
► Two oxidations occurred successively during standing, which acted as conjugated reactions for hydrogen evolution reaction.
► MH/Ni batteries should be stored in partial charged state to avoid the long-term failure, even after a short-time storage.