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 γ-NiOOH → β-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 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 γ-NiOOH ⇄ β-Ni(OH)2 phase transformation, so that a phase, called βexγ and retaining some of the layers buckling of the γ-phase, was isolated for the first time. Upon oxidation, the βexγ-phase was found to directly reconvert to the γ(III)-phase