Synthesis and electrochemical characteristics of Li[CoxLi(1/3-x/3)Mn(2/3-2x/3)]O-2 compounds

Li[CoxLi(1/3-x/3)Mn(2/3-2x/3)]O-2 (x = 0.1, 0.17, 0.20, 0.25, 0.33, and 0.5) compounds were prepared using a simple combustion method and studied by the X-ray diffraction method, electrochemical techniques, and X-ray absorption near-edge structure (XANES) analysis. The Rietveld analysis showed that these compounds could be classified as the alpha-NaFeO2 structure type. Both a and c parameters decreased linearly, while c/a increased with the increase of Co content. The oxidation states of Mn and Co ion in the Li[CoxLi(1/3-x/3)Mn(2/3-2x/3)]O-2 compounds were refined as 4+ and 3+, respectively, by XANES analysis. Irreversible voltage plateaus were observed in the initial charge profile of these compounds, as in the case of Li2MnO3-based oxides. The plateaus shifted to the higher voltage range, with the Co content decreased. The delivered discharge capacity gradually decreased from about 265 to 150 mAh/g as the Co content decreased from x 5 0.5 to 0.1. On cycling, the smooth discharge profile was converted to two subplateaus. The high Co content in the compounds suppressed the phase conversion during cycling, which led to a relatively stable cyclic performance. (C) 2004 The Electrochemical Society.