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 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 γ(III) into β(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 phase transformation, so that a phase, called β exγ and retaining some of the layers buckling of the γ‐phase, was isolated for the first time. Upon oxidation, the β exγ ‐phase was found to directly reconvert to the γ(III)‐phase.