Ion conduction in zwitterionic-type molten salts and their polymers

TLDR

The authors synthesized zwitterionic imidazolium salts bearing covalently bound sulfonate or sulfonamide groups, including vinyl‑functionalized variants that were polymerized. Although the salts form molten‑state materials, their ions do not migrate under bulk potential gradients, but adding equimolar LiTFSI restores conductivity, boosting it from 10⁻⁹ to 10⁻⁵ S cm⁻¹ at 50 °C (and up to 10⁻⁴ S cm⁻¹ for sulfonamide variants) and yielding similarly high conductivities in polymerized forms.

Abstract

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 10−9 S cm−1 at 25 °C, but jumped to 10−5 S cm−1 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 10−4 S cm−1 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 10−5 S cm−1 at 50 °C following the addition of an equimolar amount of LiTFSI to the imidazolium cation unit.