Physicochemical properties and application of concentrated KN(SO2F)2/sulfolane solution in high-voltage high-power K-ion batteries†

To achieve improved performance and practical applications of K-ion batteries (KIBs), which are considered next-generation batteries without resource constraints, electrolytes that can maximize the electrochemical performance of K-insertable electrode materials are crucial. In this study, a potassium bis(fluorosulfonyl)amide (KFSA)/sulfolane (SL) system was investigated as a potential electrolyte candidate for KIBs. The KFSA/SL solutions were found to be in the liquid state at room temperature, even at a highly concentrated KFSA/SL molar ratio of 1 : 1 (corresponding to 5.1 mol dm⁻³). A systematic spectroscopic analysis revealed that the KFSA/SL solution has a K⁺–SL–K⁺ bridge-type structure unique to SL-based electrolytes. In addition, the solutions remained in the liquid state at high concentrations by suppressing the crystallization of solvates, because the interaction between K⁺ and SL was weaker than that between Li⁺ or Na⁺ and SL systems. The concentrated KFSA/SL solution (1 : 1) exhibited superior electrochemical stability, which enabled stable cycling of the graphite negative electrodes and a high-voltage operation of the K₂Mn[Fe(CN)₆] and KVPO₄F positive electrodes. Furthermore, the superior cation-transport properties of the electrolyte owing to the bridge-type structure improved the rate capability of the electrode-active materials. This study advances the possibility of using novel electrolytes for realizing high-power, high-voltage KIBs.