Fluorosilane compounds with oligo(ethylene oxide) substituent as safe electrolyte solvents for high-voltage lithium-ion batteries

Two fluorosilanes with oligo(ethylene oxide) unites were synthesized through hydrosilylation of chlorosilane with allyl substituted oligo(ethylene oxide) ether followed by fluorination with potassium fluoride. The synthesized fluorosilane compounds exhibited lower viscosity, higher dielectric constant and higher oxidation potential, compared with their non-fluorination counterparts. Difluoro(3-(2-(2-methoxyethoxy)ethoxy)propyl)methylsilane (DFSM₂), one of the two compounds, was evaluated as high-voltage and thermal stable electrolyte co-solvent with the conventional carbonate-based electrolytes. Using an optimized electrolyte of 1M LiPF₆ in EC/DFSM₂/EMC (2/3/5 in vol.) with addition of 5 wt% fluoroethylene carbonate (FEC), high-voltage LiCoO₂(LCO)/graphite full cell displayed outstanding cycling stability of 92.5% capacity retention after 135 cycles at 4.4 V upper cutoff voltage. Characterized by differential scanning calorimetry (DSC) analysis, the DFSM₂-based electrolyte demonstrated higher thermal stability with lithiated graphite anode and delithiated LCO cathode, thus better safety feature compared with the conventional electrolyte.