Influence of solvent species on the charge-discharge characteristics of a natural graphite electrode

The charge-discharge characteristics of a natural graphite electrode are 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 charge-discharge characteristics in an electrolyte containing EC with a high volume fraction. In an electrolyte containing PC, however, the electrode cannot be charged and the solvent is decomposed. X-ray photoelectron spectroscopy is used to obtain information about the surface of natural 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 compound are formed in the bulk and on the surface of natural graphite when the volume fraction of PC is high. These results suggest that the thin 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.