Physicochemical and electrochemical properties of the (fluorosulfonyl)(trifluoromethylsulfonyl)amide ionic liquid for Na secondary batteries

In this study, thermal, physical, and electrochemical properties of the ionic liquid electrolyte system, Na[FTA]-[C₃C₁pyrr][FTA], (FTA^‒ = (fluorosulfonyl) (trifluoromethylsulfonyl)amide and C₃C₁pyrr⁺ = N-methyl-N-propylpyrrolidinium) have been investigated for Na secondary batteries. The asymmetric FTA^‒ structure provides a wide liquid-phase temperature range, especially at low x(Na[FTA]) (x(Na[FTA]) = molar fraction of Na[FTA]) and low temperature range. Glass transition at 170–209 K is the only observed thermal behavior in the x(Na[FTA]) of 0.0–0.4. Temperature dependence of viscosity and ionic conductivity obeys the Vogel‒Tammann‒Fulcher equation, and the correlation between molar conductivity and viscosity follows the fractional Walden rule. The anodic potential limits are above 5 V vs. Na⁺/Na at 298 and 363 K. The noticeable effects of x(Na[FTA]) are observed in the electrochemical performance of Na metal and hard carbon electrodes. In both cases, a moderate concentration, x(Na[FTA]) of 0.2–0.3, enables favorable charge-discharge behavior. At 363 K, the discharge capacities of the hard carbon electrode at x(Na[FTA]) of 0.3 are 260 and 236 mAh g⁻¹ at the current densities of 20 and 200 mA g⁻¹, respectively. The optimum cycling performance occurs at x(Na[FTA]) = 0.3, providing satisfactory capacity retention and high average Coulombic efficiency.