A large variety of pure p-Ni(OH), and cobalt-substituted nickel hydroxides Ni,_,Co(OH), of well-controlled chemical composition, particle size, and morphology have been prepared and characterized by x-ray, transmission electron microscopy, electrochemical, and Raman spectrometry studies. The hydroxide particle size and its Co-substituted content were found to govern the Ni-electrode capacity. Once these parameters were controlled, p-nickel hydroxide specimens able to reversibly exchange more than one electron per metal atom (Ni + Co) were routinely prepared. p-Ni,_1Co(OH), Raman spectra consists of five lines. The intensities of the ones located at 3605 and 515 cm' were found to change as a function of the particle size and the Co content. Indeed, the line at 3605 cm', ascribed to adsorbed water, appears and grows when the particle size decreases (e.g., when the surface/volume ratio increases), while the line located at 515 cm1 is enhanced by the presence of coprecipitated cobalt. A direct eorrelation between the intensity of the 515 cm' Raman line and the electrochemical capacity of the nickel hydroxide sample is found. Raman spectroscopy can then be used as a powerful nondestructive tool to differentiate "high capacity" from low capacity nickel hydroxide samples.

The nickel oxyhydroxide electrode (NOE) that acts as the positive electrode in Ni-based rechargeable alkaline batteries was studied. A survey of the influence of crystal-chemistry factors (nature and ratio of the various nickel hydroxide phases), cycling parameters (charge/discharge rates, charge/discharge cutoff voltages, and percentage of overcharge), and technological parameters (nature of the current collector, active material morphology, and type of additives) on the appearance of the second voltage plateau was performed. Direct experimental evidence shows that the appearance of the second plateau is directly linked to the amount of the y-phase present in the nickel oxyhydroxide electrode prior to its discharge. The occasional appearance of this phase in the electrode results from a poor active material/current collector interface related to the electrode-forming technology, or to secondary reactions that can lead to a physical disconnection of the active material upon cycling. Based on these findings, the presence of this y-phase accounts for the ohmic drop that has led to previous speculation of a barrier layer as the origin of the second plateau. Finally, tentative recommendations to eliminate or mitigate the appearance of the second plateau phenomena are given.

We report on a detailed study of the y-NiOOH --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 y(III) into P(I1). 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 y-NiOOH [3-Ni(OH)2 phase transformation, so that a phase, called Pexy and retaining some of the layers buckling of the y-phase, was isolated for the first time. Upon oxidation, the ,1-phase was found to directly reconvert to the y(III)-phase.