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 (Ns) 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*≈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 NEC and NDMC 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 LiCoO 2 cathode surface.