Improving the oxygen redox stability of NaCl-type cation disordered Li2MnO3 in a composite structure of Li2MnO3 and spinel-type LiMn2O4

Abstract

Li₂MnO₃, a layered lithium-rich manganese oxide, is one of the potential high-capacity positive electrode materials among the Li containing transition metal oxide materials but its low electrical conductivity restricts its electrochemical activity. In our previous study, Li–Mn cation disorder in Li₂MnO₃ resulting in the formation of a NaCl-type structure was found to improve its electrochemical activity, i.e., a high initial discharge capacity of 320 mA h g⁻¹. However, it still suffered from a poor cycling performance probably due to another problem, e.g., its structural instability, i.e., oxygen emission during charging. In this study, we found that the structural stability of NaCl-type Li₂MnO₃ was significantly improved by forming a composite with spinel-type LiMn₂O₄ by mechanical milling. The initial discharge capacity of the composite was close to 400 mA h g⁻¹ and its 15th discharge capacity still showed more than 80% of the initial one, while less than 50% for NaCl-type Li₂MnO₃. The oxide ions in the composite were found to be more stable than those of NaCl-type Li₂MnO₃ during the electrochemical redox reaction, which was confirmed by O K-edge XAS measurements, resulting in the higher reversible capacity and better cycling performance.