With increasing application demands of electronics and electric vehicles, the energy density of lithiumion batteries (LIBs) is expected to be higher and higher. The silicon-based anode materials have triggered global research interest due to low operating voltage and high specific capacity. However, for the Si-based anode, the large volume change during cycling causes cracking and pulverization of Si particles, leading to the sluggish kinetics and poor cycle life. In this work, fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LiFSI) are used as synergistic functional additives to enhance the performance of silicon–carbon (Si–C) composite anode in pouch cell. The properties of solid electrolyte interphase (SEI) formed on the surface of Si–C composite anode have been systematically investigated. The images of different electrolytes infiltration and gas production after formation are analyzed with ultrasonic transmission scanning technique. DFT calculations are used to illustrate the mechanism. All date collection is at pouch cell level, which is more persuasive.