Raman and FTIR Spectroscopy investigations of carbon-coated LixFePO4 materials
Résumé
Raman and Fourier transform infrared (FTIR) spectroscopy investigations were performed on carbon-coated LiFePO4 materials differing by the temperature of their thermal treatments (575 and 800°C) and by their electrochemical performance, with that obtained at a higher temperature showing larger reversible capacity and better capacity retention at high rates. Raman spectra gave information on the carbon located at the surface of the LiFePO4 particles, which was shown for the two samples to be highly disordered with small in-plane correlation lengths (<3 nm). A UV Raman study has shown that these carbon coatings contain almost no sp3-type carbon hybridization. This study has also highlighted again that the sp3-type C/sp2-type C ratio cannot be determined straightforwardly from Raman spectra recorded with visible excitation (such as 632.8 nm), and thus that no direct correlation can be done between the Raman band intensity ratio ID/IG and the sp3-type C/sp2-type C ratio; a UV Raman study is necessary to get the true information on the sp3-type C contribution. The baseline and absolute intensity of the FTIR spectra were shown to be sensitive to changes in the electronic conductivity of the C-LiFePO4 samples. Furthermore, good crystallinity was maintained for LixFePO4 materials upon cycling, showing good reversibility of the lithium deintercalation/intercalation reaction.