Comparative study of M[N(SO₂F)(SO₂CF₃)]–[N-butyl-N-methylpyrroridinium][N(SO₂F)(SO₂CF₃)] (M = Li, Na, K, Rb, Cs) ionic liquid electrolytes

Abstract

We systematically evaluated the physicochemical properties of a series of M[FTA]–[C₄C₁pyrr][FTA] ionic liquids (ILs) (M = alkali metal, FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide, C₄C₁pyrr = N-butyl-N-methylpyrrolidinium) as electrolytes for alkali metal-ion batteries. First, the viscosity (η), ionic conductivity (σ), and density (ρ) of the M[FTA]–[C₄C₁pyrr][FTA] ILs at x(M[FTA]) = 0.20 (x(M[FTA]) = molar fraction of M[FTA]) were measured. The σ values ranged from 1–3 mS cm⁻¹ at 298 K and increased as follows: Na < Li < K < Rb < Cs, which indicated that the Li-based IL did not obey the trend predicted by the charge densities of alkali metal cations. Second, the Li-based IL exhibited slightly lower vertical intercept values than the other FTA-based ILs in the Walden plots obtained using the results of η, σ, and ρ measurements. Third, the electrochemical stability of the ILs was investigated by cyclic voltammetry, and the redox potentials of the alkali metals (E(M⁺/M)) were determined. The E(M⁺/M) values of the FTA-based ILs increased as follows: Cs < Rb < K < Li < Na. Subsequently, we compared the obtained E(M⁺/M) values with those of other general electrolytes, such as propylene carbonate (PC)-based electrolytes and aqueous solutions. The trend in E(M⁺/M) values of the FTA-based ILs was similar to that of PC-based electrolytes and was significantly different from that of aqueous solutions. In particular, the FTA- and FSA-based ILs (FSA = bis(fluorosulfonyl)amide) presented the most negative E(Na⁺/Na) and E(K⁺/K) values among various electrolytes, which indicated that utilization of these IL electrolytes for the development of Na- and K-ion batteries would present significant advantages.