Dispersed Na0.6CoO2·yH2O and Li0.5CoO2 powders have been obtained at room temperature by rapid precipitation in aqueous solutions of LiOH or NaOH in the presence of a strong oxidizer. The precipitates are well crystallized and consist of nanoscale platelets with high specific area (above 100m2/g). The Li0.5CoO2 phase is stable in aqueous electrolytes whereas the Na0.6CoO2·yH2O rapidly converts to CoOOH in neutral electrolytes or pure water. It also transforms to anhydrous Na0.6CoO2 upon drying at moderate temperatures. Electrochemical studies show that at slow sweep rates the Na0.6CoO2·yH2O can store large amounts of charge in 10M NaOH from a combination of both faradic and capacitive reactions.