For the electrolyte of lithium-ion batteries (LIBs), lithium difluorobis(oxalato) phosphate (Li-PF2C4O8) salt supports high surface stability of the positive electrode of LiCoO2, even under a high potential of 4.4 V vs. Li/Li+ for high-energy density. By adding a small amount of Li-PF2C4O8 into the electrolyte, the cathode electrolyte interface (CEI) of LiCoO2 is clearly modified in the early stage of the 1st cycle. The role of Li-PF2C4O8 in electrolytes is systematically studied to investigate the effect of Li-PF2C4O8 on the electrochemical performance of CEI. It is found that 5 wt.% of Li-PF2C4O8 salts in the electrolyte leads to performance improvements, showing 153.5 mAh g−1 in the 400th cycle, whereas LiCoO2 without Li-PF2C4O8 exhibited a poor capacity of 17.4 mAh g−1 at the same cycle. The derived CEI is generated from Li-PF2C4O8 because it has a large number of phosphate species of the salt. The CEI is investigated in depth using systematic analyses involving the electrochemical method and surface analyses.