Three-electrode LiCoO2/graphite pouch cells have been used to investigate the film formation mechanism of Fluoroethylene carbonate (FEC) for high-voltage performance. It is found that the cells exhibit improved cycling performance in 5% FEC containing electrolyte in the voltage range of 3.0–4.4 V. In situ electrochemical impedance spectroscopy (EIS) results reveal that the gradual film formation process in FEC electrolyte is not a negative lithium consumption process, different from that of the base electrolyte. The characterizations from scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) demonstrate that FEC facilitates the formation of stable solid electrolyte interfaces (SEI) simultaneously on anode and cathode of the LiCoO2/graphite pouch cells with different decomposition mechanisms, which can effectively protect electrodes and prevent electrolyte decomposition on both electrodes.