Raman spectroscopy of carbon-coated LiCoPO4 and LiFePO4 olivines

The effect of laser power on the Raman spectra of two carbon-coated nano-powders of LiCoPO4 and LiFePO4 olivine cathode materials were investigated. In the ambient atmosphere at a moderate laser power, the phenomenon of the removal of the carbon coating layer from both samples was detected. The olivine structure of LiCoPO4-C powder therefore remains unchanged during the prolonged exposure to a 4.3 mW laser beam. The mild removal of the carbon layer makes it possible to analyze the details of the LiCoPO4 structure in air without interference from carbon. LiFePO4-C powder, together with carbon layer gasification, undergoes oxidative decomposition by the oxygen with the formation of Li3Fe2(PO4)(3) and Fe2O3, even at a laser power of 1 mW. Thus, care should be taken when measuring and interpreting the Raman spectra of this material both in air and in an inert atmosphere, as obvious decomposition of the LiFePO4 olivine structure takes place even at a moderate power of the excitation laser. A comparative study of the stability of these two carbon-coated nano powders under laser beam irradiation and heating was carried out with the use of TGA-mass spectrometry. (C) 2011 Elsevier B.V. All rights reserved.