We synthesized a series of imidazolium cations containing covalently-bound anionic sites, such as sulfonate or sulfonamide groups. These zwitterionic imidazolium salts were found to form molten salts just like ordinary imidazolium salts. However, regardless of the high ion density, these ions cannot migrate along potential gradients induced in the bulk. This is a new and unique characteristic in molten salts. When other salts were added to this, the ions generated from the newly added salts were able to behave as carrier ions. The ionic conductivity of a pure molten salt was 10 29 S cm 21 at 25 ‡C, but jumped to 10 25 S cm 21 by adding an equimolar amount of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at 50 ‡C. The zwitterionic salt having a sulfonamide group instead of sulfonate had an ionic conductivity of 10 24 S cm 21 at 50 ‡C after adding an equimolar amount of LiTFSI. These zwitterionic imidazolium salts having vinyl groups were synthesized and polymerized. In spite of their rubber-like properties they showed excellent ionic conductivities of around 10 25 S cm 21 at 50 ‡C following the addition of an equimolar amount of LiTFSI to the imidazolium cation unit.Compound 2. A mixture of N-vinylimidazole (5.0 g, 53.0 mmol) and 1,3-propanesultone (6.5 g, 53.0 mmol) in acetone (200 mL) was stirred at room temperature for 5 days
Examples of a new class of onium salts bearing the tetrafluoroborate anion (BF 4 Ϫ ) were prepared by neutralization of the corresponding amines. This method greatly simplifies the preparation of many organic molten salts. Some of the resulting neutralized amines displayed an ionic conductivity greater than 10 Ϫ2 S cm Ϫ1 at room temperature. The largest ionic conductivity was observed in 2-methyl-1-pyrroline neutralized by HBF 4 . In addition, the ionic conductivity above the melting point is well described by the Vogel-Tamman-Fulcher formula. There is a correlation between the ionic conductivity of neutralized amines and of the corresponding alkylated onium salts having the same heteroaromatic rings. These neutralized amines are convenient models of onium salts having high ionic conductivity.
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