Trialkylsulfonium Fluorohydrogenate Giving the Highest Conductivity in Room Temperature Ionic Liquids

Abstract

electrochemistry and organic chemistry. 2‐4 Applications of ILs as safe electrolytes in electrochemical devices such as lithium batteries, solar cells, fuel cells, double‐layer capacitor, and plating have been investigated to replace presently used aqueous and organic solvents. 4‐8 Imidazolium cation is an aromatic cation and often gives a salt with lower melting point and higher ionic conductivity than those of nonaromatic cation‐based salts (e.g. pyrrolidinium, tetraalkylammonium, piperidinium, and phosphonium), whereas their electrochemical stability is sometimesinsufficientespeciallyagainstreductionbecauseofthehigh acidityofhydrogenattheC2‐carbonofimidazoliumring. 9,10 According to previous reports, small aliphatic trialkylsulfonium‐based ILs have relatively low melting points and high ionic conductivity comparable to imidazolium‐based ILs, (cf. Tm = 218 K and σ = 15.7 mS cm −1 at 298 K for S122N(SO2F)2, 11 Tm = 250 K and σ = 5.8 mS cm −1 at 298 K for S122N(SO2CF3)2, 12 no melting point and σ = 26.8 mS cm −1 at 298 K for S122N(CN)2, 13 Tm = 260 K and σ = 16.5 mS cm −1 at 298 K for EMImN(SO2F)2, 14 Tm = 257 K and σ = 8.4 mS cm −1 at 298 K for EMImN(SO2CF3)2, 14 and Tm = 261 K and 27 mS cm −1 at 293 K for EMImN(CN)2, 15 where EMIm, Tm and σ denote 1-ethyl3-methylimidazolium, melting point, and ionic conductivity, respectively). Fluorohydrogenate ((FH)nF − ) ILs based on aromatic and nonaromatic cations have been prepared and characterized by spectroscopic, physicochemical, and electrochemical methods. 16‐19 These ILs are nonvolatile at room temperature when they are thoroughly evacuated at room temperature and exhibit high ionic conductivities and low viscosities compared to other ILs based on popular anions (e.g. EMIm(FH)2.3F possessing Tm = 208 K, σ = 100 mS cm −1 and η = 4.9 cP at 298 K), where η denotes viscosity. 16 Considering the properties of the previously reported ILs of fluorohydrogenate anion or alkylsulfonium cation, combination of these two species is interesting to obtain low melting point and high ionic conductivity. This study reports synthesis and properties of ILs composed of a fluorohydrogenate anion (FH)2F − ) and a trialkylsulfonium (S111 ,S 112 ,S 122 ,o r S222) cation. Effects of cationic structure and alkyl chain length on physicochemical properties of the ILs are discussed.