Structural and Electrochemical Analyses on the Transformation of CaFe 2 O 4 -Type LiMn 2 O 4 from Spinel-Type LiMn 2 O 4

Abstract

Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFe 2 O 4 -type LiMn 2 O 4 (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 LiMn 2 O 4 spinel (SP-LMO) was decomposed into three phases of Li 2 MnO 3 , MnO 2 , and Mn 2 O 3 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 Li 2 MnO 3 phase in the sample (=9.1 wt %). In addition, the Li-rich spinels, Li[Li x Mn 2-x ]O 4 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 Li 2 MnO 3 , MnO 2 , and Mn 2 O 3 . Details of the structural transformation from SP-LMO to CF-LMO and the effect of Mn ions on the 1D conduction pathway are discussed.