Effect of deep discharge on the electrochemical behavior of cobalt oxides and oxyhydroxides used as conductive additives in Ni-MH cells

When used as conductive additive at the positive electrode of Ni-MH batteries, the Na0.6CoO2 phase is converted, during the first charge, by oxidation, in a γ-hydrated cobalt oxyhydroxide, which exhibits promising performances. The behavior of these phases was studied in specific deep discharge or low potential storage conditions, through electrochemical short-circuit experiments. The evolution of the electrodes during the cycling was followed by X-ray diffraction and SEM analysis. These novel additives appear to be more efficient in these extreme conditions than the CoO or Co(OH)2 additives, commonly used in industrial devices.