Surface film formation on graphite negative electrode in lithium-ion batteries

lit situ atomic force microscopic (AFM) observation of the basal plane of highly oriented pyrolytic graphite was performed during cyclic voltammetry at a slow scan rate of 0.5 mV s(-1) in 1 mol dm(-3) LiClO4 dissolved in a mixture of ethylene carbonate and diethyl carbonate. In the potential range 1.0-0.8 V, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the surface. These two features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively, and may have a role in suppressing further solvent cointercalation. At potentials more negative than 0.65 V, particle-like precipitates appeared on the basal plane surface. After the first cycle the thickness of the precipitate layer was 40 mn, and increased to 70 nn after the second cycle. The precipitates were considered to be mainly organic compounds that are formed by the decomposition of solvent molecules, and they have an important role in suppressing further solvent decomposition on the basal plane. (C) 2001 The Electrochemical Society.