Hydrofluoroethers (HFEs) of 2-trifluoromethyl-3-methoxyperfluoropentane (TMMP) and 2-(trifluoro-2-fluoro-3-difluoropropoxy)-3-difluoro-4-fluoro-5-trifluoro pentane (TPTP) were investigated as cosolvents of ethylene carbonate (EC) +diethyl carbonate (DEC). Both lithium bis(pentafluoroethylsulfonyl)imide and lithium hexafluorophosphate (LiPF6) are soluble in EC+DEC+TPTP (5:45:50 in volume), while LiPF6 is not soluble in EC+DEC+TMMP (5:45:50). The enhanced discharge properties of Li-ion batteries (typically graphite/LiCoO2) in those electrolyte systems were evaluated and compared with the conventional carbonate mixed ones. The TPTP-mixed electrolyte [1 M LiPF6/EC+DEC+TPTP (5:45:50)] gave a high capacity retention of 80% at an extremely high rate of 12C. An EC+DEC-based electrolyte normally exhibited poor retention of less than 40%. Such a rate-enhancing effect of HFEs (TPTP and TMMP) was manifested by observation over a wide temperature range from -20 to +25 degrees C. The striking enhancement was observed at -20 degrees C and at the first 10% dosage and level off at 50%. The activation energy, as substantiated by microvoltammetry, for the Li+ desolvation process decreases from 45-100 kJ mol(-1) (over the Li+ concentration range of 0.01-2 M, respectively) to 25 kJ mol(-1) by adding TMMP or TPTP. This implies that in the presence of HFEs, the kinetics for the rate-determining intercalation process is enhanced by an accelerated Li+ transport with a reduced solvation number.