Thestability of the Na_0.6CoO₂ phase, as intended to be usedas a conductive additive at the positive electrode of Ni–MHbatteries, was tested in electrochemical cycling conditions, in KOH alkalineelectrolyte. Simple aging of Na_0.6CoO₂ in the electrolyte entails itsspontaneous transformation into a mixture of y and B(III) cobalt oxyhydroxides. This reaction, occurring within the solid state, is basedon an original mechanism, which involves cationic exchange processes andpasses through an intermediate interstratified phase. Electrochemical oxidation of Na_0.6CoO₂in the alkaline electrolyte, as well as chemical oxidation, leadsto a y cobalt oxyhydroxide, which can be totally reducedto B(III), depending on the reduction conditions. It is importantto understand such an evolution mechanism of the Na_0.6CoO₂ additivebecause it will have a significant and positive influence onthe electrochemical cycling of the positive electrode in specific deepdischarge or low potential storage conditions.Read less <