ELECTROREDUCTION OF GRAPHITE IN LICLO4-ETHYLENE CARBONATE ELECTROLYTE- CHARACTERIZATION OF THE PASSIVATING LAYER BY TRANSMISSION ELECTRON-MICROSCOPY AND FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Electrochemical intercalation of unsolvated lithium into pitch carbon fibres P100 and natural graphite UF4 has been carried out in LiClO4-et hylene carbonate electrolyte. The reversible electrochemical capacity for a current equal to 7 mu A/mg is 260 mAh/g for P100 carbon fibres a nd about 350 mAh/g for UF4 graphite, respectively. During the first di scharge (reduction) an electrochemical capacity greater than the theor etical value (372 mAh/g) corresponding to LiC6 is obtained. This exces s of capacity can be related to the formation of a passivating layer o n the carbon surface. Analysis of this layer by means of transmission electron microscopy (electron diffraction, electron energy loss spectr oscopy, and imaging) and Fourier-transform infrared spectroscopy has s hown that this layer is composed of lithium carbonate Li2CO3 and alkyl carbonates of lithium ROCO(2)Li. Formation of Li2CO3 occurs at potenti als in the 1-0.8 V range versus Li+/Li, and formation of lithium alkyl carbonates then follows at potentials below 0.8 V. We then attributed the voltage plateau at 0.9 V versus Li+/Li observed in the electrochem ical waves to the reduction of ethylene carbonate into Li2CO3. Transmi ssion electron spectroscopy revealed the presence of lithium chloride in the electrolyte which appears as small rods.