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 gamma-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. (C) 2009 Elsevier B.V. All rights reserved.