INFLUENCE OF SOLVENT SPECIES ON THE CHARGE-DISCHARGE CHARACTERISTICS OF A NATURAL GRAPHITE ELECTRODE

The charge-discharge characteristics of a natural graphite electrode a re examined in a mixed solvent composed of ethylene carbonate (EC) and propylene carbonate (PC). The characteristics are influenced largely by the solvent species. Natural graphite electrode displays good charg e-discharge characteristics in an electrolyte containing EC with a hig h volume fraction. In an electrolyte containing PC, however, the elect rode cannot be charged and the solvent is decomposed. X-ray photoelect ron spectroscopy is used to obtain information about the surface of na tural graphite. A thin LiF layer, the decomposition product of lithium hexafluorophosphate (LiPF6), is formed on the surface of the natural graphite charged to 0.5 V (vs. Li/Li+) in an electrolyte containing a high volume fraction of EC. On the other hand, LiF and a carbonate com pound are formed in the bulk and on the surface of natural graphite wh en the volume fraction of PC is high. These results suggest that the t hin LiF layer, which is produced at a potential higher than 0.5 V (vs. Li/Li+) on the surface of natural graphite, enables the lithium ions to intercalate into the natural graphite without further decomposition of the electrolyte. (C) 1998 Elsevier Science S.A.