Physicochemical and Electrochemical Properties of K[N(SO₂F)₂]-[N-Methyl-N-propylpyrrolidinium][N(SO₂F)₂] Ionic Liquids for Potassium-Ion Batteries

Abstract

The physicochemical and electrochemical properties of the binary ionic liquid, K[FSA]–[C₃C₁pyrr][FSA] (FSA = bis(fluorosulfonyl)amide; C₃C₁pyrr = N-methyl-N-propylpyrrolidinium), were investigated at 253–393 K, with the aim of developing a new electrolyte for potassium-ion batteries (K-ion batteries; KIBs). A phase diagram was constructed from the results of differential scanning calorimetry measurements and revealed that the melting point of this ionic liquid is below room temperature for compositions of x(K[FSA]) = 0–0.25 (x(K[FSA]) = molar fraction of K[FSA]). The viscosity, ionic conductivity, and density were measured for x(K[FSA]) = 0–0.25. The ionic conductivity when x(K[FSA]) = 0.20 was 4.8 mS cm⁻¹ at 298 K, which is higher than that for the equivalent sodium and lithium ionic liquids. Cyclic voltammetry measurements of M[FSA]–[C₃C₁pyrr][FSA] ionic liquids (x(M[FSA]) = 0.20; M = K, Na, or Li) indicated that potassium metal deposition/dissolution occurs at a more negative potential than that for lithium and sodium deposition/dissolution, suggesting that KIBs with a high operating voltage can be constructed using K[FSA]–[C₃C₁pyrr][FSA] as an ionic liquid electrolyte.