Structural investigation of layered Li1-δMnxCr1-xO2 by XANES and in situ XRD measurements

A wide solid solution of lithium manganese chromium oxides, LiMnxCr1-xO2 (0 less than or equal to x less than or equal to 0.6), having the layered alpha-NaFeO2 structure, was synthesized by employing the emulsion drying method, and the Mn and Cr oxidation states of the prepared powders were identified by combination of the Rietveld analysis of the X-ray diffraction (XRD) data and X-ray absorption near edge spectroscopic (XANES) analysis. A reversible structural change of LiMn0.6Cr0.4O2 was observed using the in situ XRD technique after the first charge. With increasing Mn contents, the reversible capacity increased due to the redox couples of Mn3+/4+ and Cr3+/6+, as observed by the XANES spectra measured as a function of the Li contents during the first cycle. The rechargeable capacity for LiMn0.6Cr0.4O2 was about 140 mAh g(-1) between 4.3 and 2.7 V vs. Li for 30 cycles. The measured chemical diffusion coefficient for LiMn0.6Cr0.4O2 was much higher than those for LiAlxMn2-xO4 and LiAlxCo1-xO2. This emulsion drying synthesis is an excellent powder preparation alternative method of high capacity cathode materials to be used in a Li-ion secondary battery. (C) 2003 The Electrochemical Society.