Cathode electrolyte interface of lithium difluorobis(oxalato) phosphate at 4.4 V operation of LiCoO2 for high-energy lithium-ion batteries

Abstract For the electrolyte of lithium-ion batteries (LIBs), lithium difluorobis(oxalato) phosphate (Li-PF 2 C 4 O 8 ) salt supports high surface stability of the positive electrode of LiCoO 2 , even under a high potential of 4.4 V vs. Li/Li + for high-energy density. By adding a small amount of Li-PF 2 C 4 O 8 into the electrolyte, the cathode electrolyte interface (CEI) of LiCoO 2 is clearly modified in the early stage of the 1st cycle. The role of Li-PF 2 C 4 O 8 in electrolytes is systematically studied to investigate the effect of Li-PF 2 C 4 O 8 on the electrochemical performance of CEI. It is found that 5 wt.% of Li-PF 2 C 4 O 8 salts in the electrolyte leads to performance improvements, showing 153.5 mAh g −1 in the 400 th cycle, whereas LiCoO 2 without Li-PF 2 C 4 O 8 exhibited a poor capacity of 17.4 mAh g −1 at the same cycle. The derived CEI is generated from Li-PF 2 C 4 O 8 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.