K2CO3 is evaluated as a film-forming additive in 1 mol dm−3 LiPF6–EC:DMC electrolyte in lithium-ion batteries. It is found that the cyclic performance of graphite electrode is improved in the electrolyte with K2CO3, due to the reduction of EC during the first lithiation process is largely suppressed. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) are used in order to better understand the reaction mechanism. The results show that, K2CO3 successfully modifies the SEI film to a worm mesh structure with better viscoelasticity to accommodate the volume changes of graphite electrode during electrochemical cycles, and makes it easier for lithium ions to intercalate into and deintercalate from the graphite electrode, thus contributes to better performance of graphite electrode.