Structural and Electrochemical Analyses on the Transformation of CaFe₂O₄-Type LiMn₂O₄ from Spinel-Type LiMn₂O₄

Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFe₂O₄-type LiMn₂O₄ (CF-LMO), was synthesized at high pressures above 6 GPa, and its crystal structure and electrochemical properties were examined. CF-LMO exhibits a one-dimensional (1D) conduction pathway for Li ions, which is predicted to be superior to the three-dimensional conduction pathway for these ions. The stoichiometric LiMn₂O₄ spinel (SP-LMO) was decomposed into three phases of Li₂MnO₃, MnO₂, and Mn₂O₃ at 600 °C and then started to transform into the CF-LMO structure above 800 °C. The rechargeable capacity (Q _recha) of the sample synthesized at 1000 °C was limited to ∼40 mA h·g^-1 in the voltage range between 1.5 and 5.3 V because of the presence of a small amount of Li₂MnO₃ phase in the sample (=9.1 wt %). In addition, the Li-rich spinels, Li[Li Mn_2- ]O₄ with x = 0.1, 0.2, and 0.333, were also employed for the synthesis of CF-LMO. The sample prepared from x = 0.2 exhibited a Q _recha value exceeding 120 mA h·g^-1 with a stable cycling performance, despite the presence of large amounts of the phases Li₂MnO₃, MnO₂, and Mn₂O₃. Details of the structural transformation from SP-LMO to CF-LMO and the effect of Mn ions on the 1D conduction pathway are discussed.