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

Abstract

LiCoO2 powders synthesized by a sol/gel process followed by an annealing heat treatment in the range 400–900°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 post-annealing temperature. The gradual LT–HT phase transformation can therefore be observed: a pure Fd3m phase is obtained at 400°C, and then the two LiCoO2 forms coexist from 500°C to below 700°C, the R-3m relative amount increasing from 20% at 500°C to 90% at 600°C, to be 100% at 700°C. Cyclic voltammetric measurements confirmed this evolution, showing the typical behaviour of the high performance R-3m layered phase from 600°C. These results put forward an original and appropriate use of Raman spectroscopy in the field of electrode materials for lithium batteries.