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Room temperature ionic liquids (ILs) are the appealing research target as electrolytes for lithium batteries. The cation and anion structure of ILs influences their physical and chemical properties. In this study, an electron-donating branched substituent was introduced into imidazolium cations to promote charge delocalization and to improve the reduction stability of ILs. The ILs with branched substituent group at the third position of imidazolium cation, 1-allyl-3-isobutylimidazolium bis(fluorosulfonyl)amide ([ABisoIm][FSA]) and 1-allyl-3-tert-butylimidazolium bis(fluorosulfonyl)amide ([ABtertIm][FSA]), exhibited lower cathodic potential (−2.54 and −2.79 V vs. Fc/Fc+, respectively). [ABisoIm][FSA] and [ABtertIm][FSA] showed sufficiently high ionic conductivity (4.85 and 3.66 mS/cm at 298 K, respectively), although the viscosity increased with the introduction of bulky branched substituent. Lithium secondary batteries composed of electrolytes using [ABisoIm][FSA] and [ABtertIm][FSA] showed stable charge-discharge behavior.