Evidence for direct γ-NiOOH⇆β-Ni(OH)2 transitions during electrochemical cycling of the nickel hydroxide electrode

We report on a detailed study of the gamma-NiOOH --> beta-Ni(OH)(2) phase transformation previously proposed as the origin of the second low voltage plateau, occasionally observed during the discharge of Ni-based alkaline batteries. This was done by means of intermittent galvanostatic as well as low rate potentiodynamic techniques coupled with X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. X-ray data confirmed that the second plateau originates from the direct reduction of gamma(III) into beta(II). Although this phase transformation always occurred, independently of the discharge rate, near 0.8 V with an overvoltage of 400 mV, electrochemical measurements showed that its equilibrium voltage is 1.25 V. TEM measurements gave direct evidence of a textural memory effect during the gamma-NiOOH reversible arrow beta-Ni(OH)(2) phase transformation, so that a phase, called beta(ex gamma) and retaining some of the layers buckling of the gamma-phase, was isolated for the first time. Upon oxidation, the beta(ex gamma)-phase was found to directly reconvert to the gamma(III)-phase.