Ternary Potassium Single Cation Ionic Liquid Electrolyte for Potassium Secondary Batteries

Potassium single cation ionic liquids (K-SCILs), which solely contain K⁺ as the cationic species, realize exceptionally high K⁺ concentrations and exhibit unique physicochemical and electrochemical properties. However, K-SCILs tend to have high melting points due to the smaller size of K⁺ than those of bulky organic cations, resulting in high operating temperatures for battery applications. In this study, a K-SCIL with a melting point below that of K metal (64 °C) was developed by evolving a binary system to a ternary one. The resulting K-SCIL, K[FSA]_0.33[FTA]_0.33[TfO]_0.33 (FSA^-: bis(fluorosulfonyl)amide, FTA^-: (fluorosulfonyl)(trifluoromethanesulfonyl)amide, and TfO^-: trifluoromethanesulfonate), has a low melting point of 50 °C with a high K⁺ concentration of 9.3 mol dm^-3 at 55 °C. It allows the safe handling of K metal and exhibits improved solid K metal deposition/dissolution compared to a conventional organic electrolyte. The K-SCIL does not involve the formation of a K⁺ concentration gradient near the electrode surface, which is demonstrated by the applicability of large currents exceeding a limiting current density assumed by calculation. Furthermore, stable K⁺ intercalation/deintercalation into/from graphite was successfully demonstrated at 55 °C, highlighting the potential of this K-SCIL for advanced potassium battery applications.