Fluorinated solvent molecule tuning to improve electrochemical performances of low-concentration electrolyte

The development of low-concentration electrolyte has a favorable application prospect due to its advantages of saving lithium resources. However, the large-scale application is limited by the formation of high-impedance organic electrode/electrolyte interphase (EEI) and the higher desolvation energy of Li+. Herein, a mixed flurorinated solvents of ethyl difluoroacetate (2F) / ethyl trifluoroacetate (3F) (3:7, in volume ratio) is designed to adjust the interaction between Li+ and the solvents, so as to achieve performance improvements of the low-concentration electrolyte system containing 0.5 M LiPF6. Results shows that 2F solvent greatly weakens the interaction between Li+ and the solvents, and 3F solvent further promotes anions to enter the solvation sheath to form Li(2F)3.0(PF6−)2.3(3F)0.5 structure. It promotes the Li+ desolvation, and brings about the formation of a LiF-rich cathode-electrolyte interface film on the surface of LiFePO4 (LFP) cathode due to the HOMO of the 2F solvent in the solvation sheath of Li+ will be increase as the involvement of PF6− in the solvation sheath of Li+. As a result, the cycling stability and rate performance (130 mAh g−1 at 4 C) of the LFP/Li half-cell are significantly improved. This work provides a new strategy to improve the electrochemical performances of low-concentration electrolytes by fluorinated solvent molecule tuning.