The effect of an alkyl dicarbonate, diethyl 2,5-dioxahexane dicarboxylate (DEDOHC, C2H5OCO2C2H4OCO2C2H5), which is formed by an ester exchange reaction in a mix solvent system of ethylene carbonate (EC) and diethyl carbonate (DEC), on the performance of graphite/LiCoO2 lithium-ion cells was investigated. In the presence of 10 vol % DEDOHC in 1 M LiClO4/EC + DEC(1:1 v/v), discharge capacity decreased, in particular at a high charge/discharge rate. Capacity retention at a high charge/discharge rate also decreased during repeated charge and discharge cycles. These deteriorations in performance were attributed to an increase in cell impedance and deficiency of mobile lithium ion, which is caused by continuous consumption of lithium ion on graphite negative electrode. The results of half-cell tests revealed that the addition of DEDOHC causes serious degradation for the graphite half-cell. From ac impedance measurements it was found that surface film resistance on graphite electrode was fourfold higher in the solution containing DEDOHC, whereas charge-transfer resistance scarcely changed. Atomic force microscopy observation revealed the formation of dense and highly adhesive surface precipitate layer on graphite electrode in the presence of DEDOHC. Consequently, it was concluded that the increase in cell impedance, which leads to deterioration of charge and discharge performance, is mainly due to the increase in the resistance of the surface film formed from the reduction of DEDOHC on the graphite negative electrode. (c) 2005 The Electrochemical Society.