Analysis of the passive surface film on a graphite electrode charged in polysiloxane-based electrolyte

In this study, electrochemical impedance spectroscopy (EIS) was performed during the first charge of a graphite/lithium metal test cell. The graphite electrode was separated from the lithium metal electrode by a porous polyethylene membrane maintaining polysiloxane-based electrolyte with the dissolved lithium bis(oxalato)borate (LiBOB). Analysis of the EIS data indicated that films with distinct electrical properties were present on the graphite surface. The value of each component in the equivalent circuit used in the data analysis depended on the cell voltage. The magnitude of the resistance and capacitance of the surface films changed at 1.8 V and 1.4 V, and the magnitude of the charge transfer resistance and double-layer capacitance changed at ∼1.8 V. The results of the EIS analysis correlated with the observed morphological changes of the graphite surface, which included formation of an island-like film and a gel-like film, as reported previously [H. Nakahara, A. Masias, S.Y. Yoon, T. Koike, K. Takeya, Proceedings of the 41st Power Sources Conference in Philadelphia, 14–17 June, 2004, p. 165].