Improvement by heating of the electronic conductivity of cobalt spinel phases, electrochemically synthesized in various electrolytes

The nature of the alkaline electrolyte (based on KOH, NaOH, LiOH), in which Co3O4 spinel type phases are synthesized by electrooxidation of CoO, is shown to play a key role on the composition, the structure and the electronic conductivity of the materials. In the materials, prepared in pure LiOH electrolyte or in mixed ternary electrolyte (KOH, NaOH, LiOH), Co4+ ions are present in the octahedral framework, which entails electronic delocalization in the cobalt T2g band and a high conductivity. The structure of the sample, synthesized in KOH, is on the opposite closer to that of ideal Co3O4, with only Co3+ in the octahedral sublattice, which leads to a semi-conducting behavior. Whatever the initial material, a thermal treatment induces an increase of the Co4+/Co3+ ratio in the octahedral network, resulting in a significant increase of the electronic conductivity.