An in situ electrochemical impedance spectroscopy was applied to the investigation of LiCoO2 electrode in ethylene and ethyl methyl carbonates electrolyte. The electrode impedance was plotted on the three-dimensional diagram, which has real, imaginary and time axes, with the measurements of the charge (deintercalation) and discharge (intercalation) curves. The formation of the solid electrolyte interface (SEI) on LiCoO2 electrode was discussed by the variation of impedance spectra. It was found that the SEI structure was constructed on LiCoO2 electrode and its resistance changed by the reversible process. Moreover, the effects of vinylene carbonate and ethylene sulfite as additives into the electrolyte were investigated. (c) 2005 Elsevier B.V. All rights reserved.