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

Abstract

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 (Qrecha) 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[LixMn2–x]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 Qrecha 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.