Abstract
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. Among all cathode materials, LiMn2O4 exhibits lower cost, acceptable environmental characteristics and better safety property than other cathode materials. The research focus is aimed to reduce the capacity fading and to enhance the electrochemical performance of spinel LiMn2O4, particularly at high C rate. In this study, the microstructure and electrochemical property in the surface-modified LiMn2O4 were examined and probed. The Li2O-2B2O3 (LBO)-coated LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 were synthesized by either solid-state method or chemical solution method. From the cross section view of LiCuxMn2-xO4-coated LiMn2O4 and LBO-coated LiMn2O4 observed with FE-SEM, it was demonstrated that the lager particles consisted of many smaller ones in the sub-micrometer range. It was argued that LiCuxMn2-xO4-coated LiMn2O4 and LBO-coated LiMn2O4 exhibited two distinct types of surface modification on the basis of the detailed analysis of HRTEM. In addition, the location of Cu in spinel LiCuxMn2-xO4-coated LiMn2O4 was at 16d site revealed by HRTEM. In addition, the electrochemical behavior was examined by using two-electrode coin cells. First of all, the capacity fading can be reduced by the technique of surface modification. The 0.4 wt％ LBO-coated km110 powder retained 93％ of its original discharge capacity after 10 cycles. Furthermore, the capacity fading of 0.3 wt％ LBO-coated Li1+xMn2O4 cathode material was 7％ after 20 cycles, showing much better cycleability than the un-coated one of 15％. The resistance of the LBO-coated Li1+xMn2O4 was also smaller than the un-coated one, indicating that the side reaction at the interface between the cathode and electrode could be diminished. Besides, for the LiCuxMn2-xO4-coated LiMn2O4, the fading rate of LiMn2O4 at 0.2 C was reduced 2.25％ after 10 cycles by surface modification. At higher C rate of 0.5 C, the decrease of fading rate was more obvious at 5.16％ after 25 cycles. The phase transformation of both base LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 during charging at 0.1 C, 0.5 C and 1C rate from 3 V to 4.5 V was confirmed by the in situ synchrotron X-ray diffractometer (in situ XRD). The plateau potential difference between the base LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 composite was 50 mV. The decrease of the plateau can be related to the fact that the kinetics of the LiCuxMn2-xO4-coated LiMn2O4 composite cathode material was faster than that of the uncoated material. 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+, respectively. Furthermore, the oxidation state of Mn was reversibly increased and decreased during charge. The EXAFS was further revealed 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 by the technique of surface modification, and thus the better electrochemical property was achieved.