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Novel ionic liquids comprised of a quaternary ammonium salt type cation have been applied to an electrolyte for high performance electric double-layer capacitors (EDLCs). The novel ionic liquids (IL-B; N, N-diethyl-N-methyl(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF 4 ) and IL-T; N, N-diethyl-N-methyl(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI)) are promising candidates for EDLC electrolytes in terms of a high decomposition voltage (wide voltage window) non-flammability, easy handling, non-volatility and low production costs. Notably, the wide voltage windowindicates that IL-B and IL-T are more advantageous in energy density than a typical propylene carbonate-based electrolytes (i.e., TEA-BF 4 /PC) and a conventional imidazolium type ionic liquid (i.e., 1-ethyl-3-methylimidazolium tetrafluoroborate, EMI-BF 4 ). The effectiveness of IL-B and IL-T on the application to EDLC electrolytes has been confirmed by using KOH-activated mesophase pitch-based carbon fibers (MPCFs) as an electrode material. The combination of IL-T (IL-B) and KOH-activated MPCFs has provided 56 F/g (51 F/g) of high specific capacitance at a maximum (1 mA/cm 2 discharge current density, 3.5V charging voltage), which is equivalent to 224 F/g (204 F/g) in a conventional three-compartment measuring system.In addition, the specific capacitance of both ionic liquids has increased proportional to the increase in the applied voltage from 2.5 to 3.5 V, in contrast to the decline observed for TEA-BF 4 /PC at 3.5 V. Furthermore, the mixture of the IL-B exhibiting high viscosity with propylene carbonate (1 M of IL-B in PC) has been found to provide an excellent capacitance behavior comparable to that observed for the pure IL-B. This indicates that the mixture has a great potential for application to EDLC electrolytes, similar to pure IL-B and IL-T.2

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