Electrochemical intercalation of sodium in NaxCoO2 bronzes

The chemical study of the NaxCoO2 system (0.5⩽x⩽1) has shown previously the existence of bronze-type phases with layer structure. In each material the lattice is built up by sheets of edge-sharing CoO6 octahedra allowing Na+ ions to be intercalated with trigonal prismatic surrounding (AABBCC or AABB oxygen packing) for small values of x and octahedral environment (ABCABC oxygen packing) for larger ones. As these materials are mixed conductors, they have been used as cathodes in Na batteries at room temperature. Discharge potentials (2.0