Studies of the first lithiation of graphite materials by electrochemical impedance spectroscopy

First lithiation of graphite electrode in 1 mol/L LiPF6-EC:DEC:DMC electrolyte was investigated by electrochemical impedance spectroscopy (EIS). The results illustrated that the first arc in the high-frequency range observed in the Nyquist diagram appears near 0.9 V in the initial lithiation of graphite electrode, and its diameter increases with the decrease of polarization potential. These EIS features were attributed to the formation and growth of SEI film. Appropriate equivalent circuit was proposed to fit the experimental EIS data. The fitting results revealed the process of the formation and growth of SEI film, and evaluated quantitatively the resistance of charge transfer, as well as the capacitance of double layer along with the increase of polarization potentials.