The formation of solid electrolyte interface (SEI) film on natural and ionic polymer-encapsulated graphite during an initial intercalation process was investigated by ac impedance and scanning electron microscopy. Results showed that the SEI film formed on encapsulated graphite could effectively block cointercalation of solvent molecules and easier accommodate the volume change of graphite particles than that on natural graphite. Thus, the physical properties of the SEI film formed on encapsulated graphite such as ionic conductivity, morphology, and thickness underwent less change than that on the natural graphite. (C) 2003 The Electrochemical Society.