Electrochemical intercalation of sodium in NaxCoO2 bronzes

Abstract

The chemical study of the Na_xCoO₂ 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<V<3.5V) have been measured as a function of the composition. The electrochemical intercalation of sodium in all these materials is reversible within their existence range. During the electrochemical intercalation a reversible charge is observed between phases having AABBCC and ABCABC oxygen packing. This can be explained by the relatively small displacement of the sheets. In contrast, for the material with AABB oxygen packing no transition occurs since a packing change requires breaking of cobalt-oxygen bonds, which is apparently impossible at room temperature.