Abstract: The formation of solid electrolyte interface（SEI） film on the surface of LiCoO₂，the cathode material of lithium ion battery was investigated by cyclic voltammetry curves and electrochemical impedance spectroscopy（EIS）.Cycle voltammetry curves showed the SEI film of LiCoO₂ formed in the first cycle.EIS results showed the SEI film formed at low rate and  normal temperature were denser and had lower resistance value.The storage time also had influence on the resistance of lithium ion battery，and the resistance value with 7 storage days was lower than that of 1 day.

Key words: solid electrolyte interface film, LiCoO₂, cyclic voltammetry curves, electrochemical impedance spectroscopy（EIS）