2

A new electrolyte solvent has been studied and developed to refrain from inherent flammability and safety for lithium ion batteries and lithium ion capacitors. Some flame-retardant solvents such as fluorinated carbonate 1 phosphate 2 have already been reported. However, those solvents did not show enough flammability performance, yet leading to gas evolutions and eventually decreasing their ionic conductivity. At the previous meeting of ECS 3 we proposed a branched hydrofluoroether (HFE), 2-trifluoromethyl-3-methoxyperfluoropentane (TMMP, Fig.1 (A)) as an electrolyte solvent to provide enough non-flammable characteristic as well as to improve rate capability (Fig. 3 (A)), cycleability and low-temperature performance. Using LiPF6, however, the TMMP dosage in EC+DEC electrolyte is limited to less than 30 vol. %, due to low miscibility of the TMMP (Fig. 2). In this study, we investigated an alternative HFE, 2-(trifluoro-2-fluoro-3-difluoropropoxy)-3-difluoro-4-fluoro-5-trifluoropentane (TPTP, Fig. 1(B)), to use LiPF6 in the effective nonflammable electrolyte composition (> 50 vol %). Charge-discharge characteristics were tested on sample coin cells composed of a graphite anode and a LiCoO2 cathode in various electrolyte compositions with and without HFE (Fig. 3). In Fig.3 (A) LiBETI-system, 79 % of the capacity (obtained at a lower rate of 0.2 C) was maintained in the TPTP-based electrolyte (EC/DEC/TPTP (5/45/50)) at a rate of 6C. In Fig.3 (B) LiPF6-system, 94 % of the capacity @ 0.2 C was maintained in the presence of the TPTP at a rate of 9 C. Activation energy for desolvation of Li was measured with Ni microelectrode in the various concentration of the LiBETI. In concentration of 1 M, the value for the TPTP-based electrolyte (38 kJ mol-1) was lower than that for EC/DE