The 'memory effect' on nickel oxide electrodes: electrochemical and mechanical aspects

Full discharge of nickel-cadmium batteries after sustained overcharge or repeated shallow discharge shows a voltage step generally known as the 'memory effect'. Nickel oxide electrode (NOE) gives a relevant contribution to the memory phenomenon, which has been related to the presence of the gamma-phase. This phase, in fact, had been found by XRD analysis in a charged NOE showing such an effect. This work deals with the mechanical behaviour of NOEs cycled so as to produce the memory effect. During the cycling of asymmetric electrodes, a beading moment occurs during discharge: this is caused by the expansion of the active mass. The force, normal to the plane of the electrode, which equilibrates with the bending moment can be measured, thus giving an indication of the stresses suffered by the sintered supporting structure. Significant differences in both the electrochemical and mechanical behaviour after sustained overcharge and after shallow discharge have been observed. The different behaviours can be related to the unlike distribution of the gamma-phase in the body of the electrode. It is suggested that the shallow cycling produces an accumulation of gamma-phase at the contact with the current collector, whereas during sustained overcharge all the active mass is changed into gamma-phase.