This paper presents a novel approach to an explanation of apparently different characters of the discharge/charge curve in Li x CoO 2 (monotonous curve in large range of lithium content) and Na x CoO 2−y systems (step-like curve). Comprehensive experimental and theoretical studies on crystallographic, electronic and electrical transport properties of the Li x CoO 2 battery cathode material are reported. In the case of Na x CoO 2−y it was evidenced that the origin of the observed step-like character of the discharge/charge curve of Na x CoO 2−y is due to the specific features of the electronic structure, arisen from the presence of the oxygen vacancies and sodium ordering.