The electrochemical cycling properties of a Si electrode were drastically improved by the introduction of an ethylene carbonate (EC)-rich electrolyte solution. Cross-sectional morphologies of fully charged Si electrodes were observed with scanning electron microscopy (SEM). The increment of EC in the electrolyte solution showed a tendency to prohibit the deterioration of the Si electrode during Li+ ion insertion. We analyzed the surface films formed on a Si electrode through Fourier transform infrared (FT-IR) spectroscopy. The solvation shell structures of Li+ ions in electrolyte solutions were studied with FT-IR spectroscopy, and the apparent solvation numbers for Li+ ions by EC molecules were calculated. (C) 2008 Elsevier B.V. All rights reserved.