The electrochemical performance of a lithium ion battery depends basically on the characteristics of the solid electrolyte interphase (SEI) film that is formed on the negative electrode surface of the battery during its first charge-discharge process. The formation mechanism of the SEI film on surface of a MCF (mesophase-pitch-based carbon fiber) negative electrode was studied in the present paper. The studies have demonstrated the influence of some typical impurities on the formation of SEI film, and characterized the morphology and the composing of the SEI films using scanning electron microscopy and X-ray photoemission spectroscopy. The shape of SEI films overlaid on MCF has been observed distinctly. (c) 2005 The Electrochemical Society.