Enhanced ion conduction in imidazolium-type molten salts

alpha, omega-Diimidazoliumalkylene salts (C-n(ImX)(2); n = 2-8) were prepared as a new type of room temperature molten salts. By changing their counter anion from bromide to bis(trifluoromethylsulfoneimide), the salt property was drastically changed and was revealed to be due to the formation of room temperature molten salts except for the system with n = 2. DSC measurement suggested that C-n(ImTFSI)(2) with n = 3 and 4 formed super cooled liquids and those with n = 6 and 8 were amorphous (showing no melting point). These amorphous compounds showed ionic conductivity of about 10(-3) S cm(-1) at 30 degrees C. The salts with n = 6 and 8 showed much higher ionic conductivity than others. This high ionic conductivity was kept even after the addition of lithium salt (LiTFSI). Comparing with the similar room temperature molten salts, alpha, omega-diimidazolium-poly(ethylene oxide) salts, C-n(ImTFSI)(2) showed little temperature dependence of ionic conductivity even when the matrix contains lithium salt. It was based on the low activation energy of matrix derived from the absence of strong ion-dipole interaction between lithium ion and ether oxygens. (C) 2000 Elsevier Science Ltd. All rights reserved.