Evolution of Local Structure around Manganese in Layered LiMnO2 upon Chemical and Electrochemical Delithiation/Relithiation

Mn K-edge X-ray absorption spectroscopic (XAS) analyses have been performed to probe the evolution of electronic and crystal structures of layered LiMnO2 upon chemical and electrochemical delithiation/relithiation. According to the X-ray absorption near-edge structure studies, it becomes clear that the trivalent manganese ion in LiMnO2 is significantly oxidized by acid treatment and is not fully recovered by subsequent lithiation reaction with n-BuLi. The extended X-ray absorption fine structure results presented here demonstrate that the local structure around manganese in LiMnO2 is changed from a layered α-NaFeO2-type structure to a spinel-like one upon chemical delithiation reaction. It is also found from the XAS analyses for the cycled LiMnO2 that the electrochemical charge−discharge process gives rise not only to the partial oxidation of manganese ion but also to the migration of Mn into the interlayer lithium site, resulting in the coexistence of the layered structure and the spinel one. Such results highlight the lattice instability of layered manganese oxide for the chemical and electrochemical extraction of lithium, which is responsible for the remarkable capacity fading and the formation of two plateaus at around the 3 and 4 V regions after the first electrochemical cycle. On the basis of the present experimental findings, we are now able to suggest that the electrochemical performance of layered LiMnO2 can be improved by blocking the Mn migration path through cationic substitution.