Correlation between cointercalation of solvents and electrochemical intercalation of lithium into graphite in propylene carbonate solution

Correlation of solvent cointercalation and electrochemical intercalation of Li into graphite was studied. Cointercalation of dimethylsulfoxide (DMSO), 2-methyltetrahydrofuran (2-MeTHF), dimethoxymethane (DMM), diethoxymethane (DEM), 1,2-diethoxyethane (DEE) and 1,2-dibutoxyethane (DBE) into graphite with Li was investigated by the solution method to find out that DMSO, DMM, DEM, and DEE can cointercalate into graphite with Li and that 2-MeTHF and DBE cannot. By using the functional density theory, steric hindrance of solvated lithium ion was found to be predominant for cointercalation. Electrochemical Li intercalation into graphite was studied in propylene carbonate (PC) solution containing 1 mol dm(-3) LiClO4 in the presence of various amounts of the above solvents. By the addition of DMSO in the PC electrolyte, solvent decomposition at around 1.0 V (vs. Li/Li+) was thoroughly suppressed, and electrochemical intercalation of Li took place. Suppression of the solvent decomposition was dependent on the amount of DMSO. This is because competing cointercalation of DMSO suppressed the cointercalation of PC which causes the exfoliation of graphite, leading to the formation of the stable solid electrolyte interface. Suppression of cointercalation of PC was also observed by the addition of DMM, DEM, and DEE in a limited condition. Addition of 2-MeTHF and DBE into the PC electrolyte is not available for electrochemical intercalation of lithium. These results show that cointercalation plays an important role for electrochemical intercalation of lithium into graphite. (C) 2003 The Electrochemical Society.