Comprehensive studies of the electrical and thermal properties of NaxCoO2−y cathode material in the characteristic points of the discharge curve i.e. on the pseudo-plateaus and on the potential jumps, show non-monotonous variations of its transport properties (metallic/semiconducting/metallic/semiconducting/metallic) during sodium intercalation, which indicates that the density of states near the Fermi level is either finite or vanishing, depending on alloy composition. This remarkable effect is coherently supported by results of KKR-CPA calculations of NaxCoO2−y, accounting for oxygen defects as well as sodium ordering.