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 Li Mn₂O₄ spinel structure by the displacement of 25% of the Mn ions to vacant octahedral sites through adjacent octahedral sites.