Physicochemical and Thermal Behaviors of Room-Temperature Phosphonium Ionic Liquids Based on Fluorosulfonyl(trifluoromethylsulfonyl)Amide Anion As Potential Electrolytes
Authors: Kikuchi Seiya , Nishikawa Kosuke , Kawaji Takatoshi , Tsunashima Katsuhiko , Hotta Hidekatsu , Yokobiki Akina , Tsuchida Yusuke , Funasako Yusuke , Yamada Hirohisa 
Published: December 2024

Room-temperature ionic liquids (RTILs), i.e. organic molten salts with melting points below 100 ◦C, have been intensively developed for electrolytic media in various electrochemical systems due to the fact that 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. Another advantage of RTILs is a purposive selection of the ion species and their designability. Although typical anion species employed for RTILs include sulfonylamide-based anions, e.g. bis(trifluoromethylsulfonyl)amide (N(SO2CF3)2
-, TFSA) and bis(fluorosulfonyl)amide (N(SO2F)2
-, FSA) anions, asymmetric fluorosulfonyl(trifluoromethylsulfonyl)amide (N(SO2F)(SO2CF3)-, FTA) anion have attracted attention in recent years. In our preliminary approach, FTA anion based RTILs in combination with quaternary phosphonium cationswe previously reported,1,2 examining their transport property behavior. In this work, we report design and synthesis of several FTA-based phosphonium RTILs (Fig. 1) as potential electrolytes available for electrochemical systems, summarizing their physicochemical and thermal characteristics including the thermal decomposition as well as the transport property behaviors.

The preparation and purification of the phosphonium RTILs were carried out according to the procedures described in our previously published papers.1-3 The phosphonium RTILs were prepared by aqueous ion exchange reactions of the precursor phosphonium bromides with lithium FTA. The resulting crude RTILs was extracted by dichloromethane, and then purified by washing with pure water several times until no residual bromide anion was detected with the use of AgNO3. The RTILs was dried under high vacuum for at least 1 day at 80 ◦C prior to use. The physicochemical and electrochemical properties such as conductivity (ac impedance method, using a two-Pt electrode cell), viscosity (cone-plate type viscometer), density, melting point (DSC), thermal decomposition temperature (TGA) were measured under dry argon atmosphere.

Several phosphonium RTILs melting at ambient temperatures were successfully prepared by combination with the FTA anion. Table 1 lists the physicochemical properties and thermal decomposition temperatures (10% weight loss) of the FTA-based phosphonium RTILs. All FTA-based phosphonium RTILs showed typical temperature-dependent behaviors for density, viscosity and electrical conductivity. It should be noted that P222(1O1)-FTA exhibited the lowest viscosity and the highest conductivity in the FTA-based phosphonium RTILs, which is attributed to the introducing effect of ether oxygen atom into the cation on the flexibility of the alkyl chain. On the other hand, the thermal decomposition temperatures of the FTA-based phosphonium RTILs tended to be relatively low (less than 300 ◦C) compared to those of TFSA-based RTILs. The detailed thermal decomposition behavior observed in the thermogravimetric traces will be discussed in comparison with both corresponding TFSA- and FSA-based RTILs.

References

1) K. Tsunashima, et al, Electrochem. Commun., 9, 2353 (2007).

2) K. Tsunashima, et al, Electrochemistry, 75, 734 (2007).

3) K. Tsunashima, et al, Electrochem. Commun., 13, 178 (2011).

Figure 1