A metal-air battery includes an anode and a passivation layer formed on the anode. The passivation layer functions as electrolyte and cathode, so that no additional electrolyte or cathode is included. During discharge, metal cations derived from oxidation of the anode migrate across the passivation layer, and react with nucleophilic gas and electrons received from a gas diffusion layer. The metal-air battery, by virtue of having no added electrolyte or cathode, is compact and contains no volatile materials.