N-Ethyl- N-propylpyrrolidinium Bis(fluorosulfonyl)amide Ionic Liquid Electrolytes for Sodium Secondary Batteries: Effects of Na Ion Concentration

A hybrid inorganic–organic ionic liquid based on sodium bis(fluorosulfonyl)amide (Na[FSA]) and [N-ethyl-N-methylpyrrolidinium] [FSA] ([C₂C₁pyrr][FSA]) is investigated as an electrolyte for sodium secondary battery operation over an extended temperature and Na⁺ ion fraction ranges. The phase diagram of the system reveals a wide liquid-phase temperature range at Na[FSA] mole fractions ranging from 0.3 to 0.7 near room temperature where the 0.7 mole fraction equates to a molar concentration of 5.42 mol L⁻¹. The viscosity and molar ionic conductivity are consistent with the fractional Walden rule, and the temperature dependence of these quantities obeys the Vogel–Tammann–Fulcher equation. The optimal Na[FSA] content of the ionic liquid occurs at mole fractions between 0.3 and 0.7 based on the sodium metal deposition/dissolution behavior and the rate and cycle properties of a NASICON-type cathode, Na₃V₂(PO₄)₃/C (NVPC). The greatest cycle efficiency εcycle of Na-metal deposition/dissolution is observed at x(Na[FSA]) = 0.6 (εcycle = 93.3%). Although Na/NVPC half-cell tests indicate a maximum rate and cycle performance at x(Na[FSA]) = 0.6 (83.5% retention at 100 C (11700 mA g⁻¹) and 80% retention after 4000 cycles at 2 C (234 mA g⁻¹)), NVPC/NVPC symmetric cell tests indicate that the greater Na[FSA] fraction provides better rate performance and that half-cell tests with a Na-metal electrode do not provide reliable data for the target electrode/electrolyte system