Electrochemical impedance spectroscopy (EIS) was performed during the first charge of a graphite/lithium metal test cell to determine the effect of an additive to a polysiloxane-based (PS-B) electrolyte on passive film formation on graphite surface. The graphite electrode was separated from the lithium metal electrode by a porous polyethylene membrane immersed in the PS-B electrolyte containing dissolved vinyl ethylene carbonate (VEC) and lithium bis(oxalato) borate (LiBOB). The presence of the VEC additive suppressed the formation of a gel-like passive film on the graphite electrode, and increased the discharge capacity [H. Nakahara, A. Masias, S.Y. Yoon, T. Koike, K. Takeya, Proceedings of the 41st Power Sources Conference, Philadelphia, 14–17 June, 2004, p. 165]. EIS analysis of the charged graphite electrode revealed that the addition of VEC decreased the passive film resistance, the film capacitance, and the charge transfer resistance. The dependence of film resistance on cell voltage corresponded only to island-like film formation, as opposed to gel-like film formation. This finding was supported by SEM observations that revealed no gel-like film formation [H. Nakahara, A. Masias, S.Y. Yoon, T. Koike, K. Takeya, Proceedings of the 41st Power Sources Conference, Philadelphia, 14–17 June, 2004, p. 165]. The VEC addition decreased the film resistance and the charge transfer resistance, which increased the discharge capacity of the graphite electrode.