High-pressure synthesis and electrochemical properties of tetragonal LiMnO2

Tetragonal structured LiMnO2 (t-LiMnO2) samples were synthesized under pressures above 8 GPa and investigated as a positive electrode material for lithium-ion batteries. Rietveld analyses based on X-ray diffraction measurements indicated that t-LiMnO2 belongs to a γ-LiFeO2-type crystal structure with the I41/amd space group. The charge capacity during the initial cycle was 37 mA h g−1 at 25 °C, but improved to 185 mA h g−1 at 40 °C with an average voltage of 4.56 V vs. Li+/Li. This demonstrated the superiority of t-LiMnO2 over other lithium manganese oxides in terms of energy density. The X-ray diffraction measurements and Raman spectroscopy of cycled t-LiMnO2 indicated an irreversible transformation from the γ-LiFeO2-type structure into a LixMn2O4 spinel structure by the displacement of 25% of the Mn ions to vacant octahedral sites through adjacent octahedral sites.