Synthesis, phase stability, and electrochemically driven transformations in the LiCuO₂-Li₂CuO₂ system

Abstract

Trivalent LiCuO₂ was prepared by oxidative leaching of divalent Li₂CuO₂ in Br₂-acetonitrile solutions at ambient temperature. Both phases were found to be thermally air unstable and the onset temperatures (100-150 °C) relatively low for their transformation into CuO and lithium salts. Electrochemically driven lithium insertion/ extraction into/from both Li₂CuO₂ and LiCuO₂ were followed by in situ X-ray diffraction that revealed, for both starting materials, that the Li₂CuO₂↔ LiCuO₂ transitions always proceed through a succession of biphasic and monophasic processes involving three type phases: Li_1+yCuO₂, "Li_1.5CuO₂", and Li_2-zCuO₂. Such a strategy can therefore be used for the preparation of stable copper mixed valence oxides. This study also revealed a very large polarization linked to the Li_1.5CuO₂ ↔ Li₂CuO₂ transition. Although so far not explained, this fact enabled us to test the respective reversibility of the LiCuO₂↔ Li_1.5CuO₂ and Li_1.5CuO₂↔ Li₂CuO₂ transitions. It was found that performing cycling on both transitions comes with a continuous decrease in the cell capacity while good capacity retention can be obtained when one limits the electrochemical work to the LiCuO₂↔ Li_1.5CuO₂ process.