Valence change by in situ XAS in surface modified LiMn2O4 for Li-ion battery

The surface-modified cathode material in Li-ion battery was synthesized to decrease the side reactions at the interface between the cathode electrode and electrolyte. It is aimed to reduce the fading rate and to enhance the electrochemical performance, particularly at high C rate. In this study, microstructure, valence change and variation of bonding state in the surface-modified LiMn2O4 were examined and probed. Both the LiCuxMn2−xO4-coated LiMn2O4 and Li2O–2B2O3 (LBO)-coated LiMn2O4 were synthesized by chemical solution method in this study. Field-emission SEM plan-view observation revealed the particles with the size of 7–8 μm for both LiCuxMn2−xO4-coated LiMn2O4 and LBO-coated LiMn2O4, exhibiting a well-developed octahedral structure with sharp edges. Nevertheless, from the cross-section view of both LiCuxMn2−xO4-coated LiMn2O4 and LBO-coated LiMn2O4, it was observed that the lager particles consisted of many smaller ones in the sub-micrometer range. It was demonstrated that LiCuxMn2−xO4-coated LiMn2O4 and LBO-coated LiMn2O4 exhibited two distinct types of surface modification on the basis of the detailed analysis of FESEM and HRTEM. The XANES of Cu and Mn K-edge spectrum for LiCuxMn2−xO4-coated LiMn2O4 showed that the valence of Cu and Mn was close to Cu2+ and Mn4+. Furthermore, the oxidation state of Mn was reversibly increased and decreased during charge. It was further revealed in this study that the trend of the variation for the bonding length of Mn–O and Mn–M (M = Mn or Cu) was in agreement with the oxidation state of Mn, which was decreased with Li deintercalation while increased with Li intercalation during cycling. On the basis of the in situ XAS data, it was evidenced that Mn transferred toward Mn4+ to minimize the Jahn–Teller distortion, and thus the electrochemical property was improved.