Super-Enhanced Lithium-ion Transport by an Effective Shift of Solvation Shell Structure in Branched Hydrofluoroether Electrolyte

New hydrofluoroethers (HFEs), 2-trifluoromethyl-3-methoxy-perfluoro-pentane (TMMP) and 2-(trifluoro-2-fluoro-3-difluoropropoxy)-3-difluoro-4-fluoro-5-trifluoropentane (TPTP) were investigated as nonflammable electrolyte solvents for lithium ion batteries. Activation energies (Delta G*(dep)) for desolvation of lithium ion (Lit) and lithium-solvation number (N-s) were evaluated for 1 M LiBETI/EC + DEC electrolytes with and without TMMP or TPTP in order to understand the mechanism of rate-capability improvement. The Delta G*(dep) for the carbonate-mixed electrolyte (0.01-2 M of LiBETI/EC + DEC (50:50 in volume)) increased from 45.7 to 98.3 kJ mol(-1) with a decrease in the LiBETI concentration. In comparison, Delta G*(dep) was found lower (Delta G*(dep) approximate to 35 kJ mol(-1)) for TPTP-mixed electrolytes (LiBETI/EC + DEC + TPTP (5:45:50)) and much lower (Delta G*(dep) approximate to 25 kJ mol(-1)) for TMMP-mixed electrolytes (LiBETI/EC + DEC + TMMP (5:45:50)). Adding 60 vol% TPTP in EC + DMC (50:50) reduced both N-EC and N-DMC by 40%, exemplified by Raman spectroscopy with different concentrations of LiBETI (0.05-1.5 M) and DFT calculation. Thus, the rate-capability improvement by adding TMMP or TPTP is exerted by both enhancements of the intercalation kinetics and Li+ diffusion at the LiCoO2 interface, where HFEs preferentially exist by their affinity with the hydrophilic LiCoO2 cathode surface.