Cause of the Memory Effect Observed in Alkaline Secondary Batteries Using Nickel Electrode

By repeating shallow discharging and overcharging of alkaline secondary batteries using nickel electrode, a lowering of working voltage is observed on the discharge curve, which is called a memory effect. The X-ray diffraction pattern of the charged-state of the normal nickel electrode contained only diffraction peaks due to β-NiOOH. A charged-state nickel electrode showing lowered discharge voltage had diffraction peaks due to γ-NiOOH in addition to those due to β-NiOOH. The formation of γ-NiOOH can be attributed to be the main cause for the memory effect commonly observed in alkaline secondary batteries such as nickel cadmium and nickel hydrogen batteries. By XRD analysis and AC impedance study, γ-NiOOH was found to be initially formed at the current collector side; it grows to the solution side in the course of shallow discharge-charge cycling. Therefore, if the amount of γ-NiOOH formed is small, only β-NiOOH can be detected, even when the memory effect is observed. In this case, γ-NiOOH can be detected by shaving the surface of the electrode using a piece of emery paper to remove covering β-NiOOH. This γ-NiOOH disappears within a few cycles of the normal charge-discharge cycling, resulting in the elimination of the memory effect. It may be concluded that the cause of the memory effect is mainly due to the formation of γ-NiOOH.