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. (C) 2019 Elsevier Ltd. All rights reserved.