Comparative Study of M[N(SO2F)(SO2CF3)]-[N-Butyl-N-methylpyrroridinium][N(SO2F)(SO2CF3)] (M = Li, Na, K, Rb, Cs) Ionic Liquid Electrolytes

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 (eta), ionic conductivity (sigma), and density (rho) of the M[FTA]-[C4C1pyrr][FTA] ILs at x(M[FTA]) = 0.20 (x(M[FTA]) = molar fraction of M[FTA]) were measured. The s 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 eta, sigma, and rho 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.