Structural modifications and electrochemical behaviour of the β(II)-Ni(OH)2/β(III)-NiOOH redox couple upon galvanostatic charging/discharging cycling

Among the various crystallographic phases of nickel hydroxide, the β-form is the most widely used as the active mass of the positive electrode of nickel-based secondary cells. However, the exact electrochemical and structural behaviour of the β(II)/β(III) redox system upon cycling and ageing remains to be clarified. This work reports the electrochemical behaviour under galvanostatic conditions of two non-doped Ni(OH)2 samples having different initial crystallography and morphology. The evolution of these features is investigated as a function of the cycling conditions and more particularly as a function of the reduction state achieved on discharge. Electrodes cycled at the 1st discharge plateau show that charged/discharged active materials are very similar. However, when discharge is continued down to the so-called ‘second discharge plateau’, drastic changes are observed as an increase of the crystallisation degree and of the compactness of the Ni(OH)2 powder. Some insights into the nickel electrode redox behaviour are given. It appears that the nickel electrode operates, upon charge, as a single-phase or as a mixed-phase system depending on the depth of discharge.