Comparative Study on Transport Property of Asymmetric Fluorosulfonyl(trifluoromethylsulfonyl)Amide-Based Phosphonium Ionic Liquids

Abstract

Room-temperature ionic liquids (RTILs), i.e. organic molten salts with melting points below ambient temperature, have been regarded as potential electrolytes for various electrochemical systems, because RTILs have unique physicochemical properties such as favorable solubility of organic and inorganic compounds, relatively high ionic conductivity, no measurable vapor pressure, high thermal stability, low flammability, etc. The quaternary phosphonium cation based RTILs have attracted attention in recent years due to their high chemical and thermal stabilities. We have proposed and synthesized a class of low viscous and highly conductive RTILs based on triethylalkylphosphonium cations in combination with typical sulfonylamide-based anions, e.g. bis(trifluoromethylsulfonyl)amide (N(SO2CF3)2 -, TFSA) and bis(fluorosulfonyl)amide (N(SO2F)2 -, FSA) anions.1,2 Our recent approaches include a design and synthesis of low-melting phosphonium based RTILs together with asymmetric fluorosulfonyl(trifluoromethylsulfonyl)amide (N(SO2F)(SO2CF3)-, FTA) anion. In this work, we report the transport property and unique viscosity behavior of several FTA anion based phosphonium RTILs as shown Fig. 1. The FTA-based phosphonium RTILs was synthesized by aqueous ion exchange reaction of the precursor quaternary phosphonium halides with lithium FTA. The crude RTILs obtained were hydrophobic and purified by washing with pure water several times in order to remove residual metal and halide ions. The RTILs thus prepared were dried under high vacuum for at least 1 day. The physicochemical properties of RTILs, e.g. density, viscosity, conductivity (ac impedance method), were measured at elevated temperatures under argon atmosphere.Several low-melting phosphonium RTILs were successfully prepared by combination with FTA anion (P2225, P2228 and P4441-FTA). All FTA-based phosphonium RTILs showed typical temperature dependent behaviors for density, viscosity and electrical conductivity. The conductivities of FTA-based phosphonium RTILs were higher and lower than those of corresponding TFSA- and FSA-based phosphonium RTILs, respectively; however, it should be noted that the viscosities of FTA-based phosphonium RTILs were the lowest in the phosphonium RTILs as depicted in Fig. 2. To our knowledge, such a unique viscosity behavior has not been previously reported in the FTA-based ammonium based RTILs. The detailed transport property and electrolytic behaviors such as the Arrhenius plots of viscosity and conductivity with VTF fitting and the Walden plots will be also discussed in comparison with both corresponding TFSA- and FSA-based RTILs.References1) K. Tsunashima, et al, Electrochem. Commun., 9, 2353 (2007).2) K. Tsunashima, et al, Electrochem. Commun., 13, 178 (2011). Figure 1