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

Abstract

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 (ΔG^∗_dep) for desolvation of lithium ion (Li⁺) 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 ΔG^∗_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, ΔG^∗_dep was found lower (ΔG^∗_dep≈35 kJ mol⁻¹) for TPTP-mixed electrolytes (LiBETI/EC+DEC+TPTP (5:45:50)) and much lower (ΔG^∗_dep≈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.