A key strategy to form a LiF-based SEI layer for a lithium-ion battery anode with enhanced cycling stability by introducing a semi-ionic C F bond

A solid electrolyte interphase (SEI) on an anode is a critical issue in lithium-ion batteries because it is related to cycling stability. In this study, we introduce a semi-ionic CF bond on the surface of graphite (SICF) via plasma fluorination to introduce a LiF-based SEI layer on the anode during the first cycle. In the charge-discharge profiles and cyclic voltammetry curves, a peak related to the LiF-based SEI formation was clearly observed for SICF. In particular, SICF had an excellent long-term cycling stability of 98.8% for 100 cycles (1.0 C-rate). From the anodes of the disassembled coin cells, it was found that semi-ionic CF bonds improved the formation of a stable LiF-based SEI layer and decreased the number of side reactions with HF, which was produced from PF5. Moreover, SICF exhibited a lower volume expansion compared to that of the pristine anode and the anode with covalent CF bonds. Therefore, introducing a semi-ionic CF bond via plasma fluorination is a key strategy for forming a LiF-based SEI layer on the graphite anode surface that enhances the cycling stability of lithium-ion batteries.