Raman spectroscopy of carbon-coated LiCoPO₄ and LiFePO ₄ 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 Li ₃Fe₂(PO₄)₃ and Fe₂O ₃, 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.