Raman study of the spinel-to-layered phase transformation in sol-gel LiCoO2 cathode powders as a function of the post-annealing temperature

LiCoO2 powders synthesized by a sol/gel process followed by an annealing heat treatment in the range 400-900 degrees C are systematically characterized using SEM, X-ray diffraction, FTIR and Raman spectroscopy. The composition of the final powder is found to be controlled by the heat treatment temperature. A thorough multipeak fitting analysis of Raman spectra gives access for the first time to the quantitative estimation of the R-3m and Fd3m relative amounts in the LiCoO2 powders as a function of the postannealing temperature. The gradual LT-HT phase transformation can therefore be observed: a pure Fd3m phase is obtained at 400 degrees C, and then the two LiCoO2 forms coexist from 500 degrees C to below 700 degrees C, the R-3m relative amount increasing from 20% at 500 degrees C to 90% at 600 degrees C, to be 100% at 700 degrees C. Cyclic voltammetric measurements confirmed this evolution, showing the typical behaviour of the high performance R-3m layered phase from 600 degrees C. These results put forward an original and appropriate use of Raman spectroscopy in the field of electrode materials for lithium batteries. (C) 2012 Elsevier B.V. All rights reserved.