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

Abstract

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.

Electrolytes toward High‐Voltage Na3V2(PO4)2F3 Positive Electrode Durable against Temperature Variation

“…This observation is consistent with temperature dependence observed in some electrodes using ionic liquid electrolytes. [ 9,11,12,60–64 ]…”

Solid Electrolyte Interphases on Sodium Metal Anodes

“…Furthermore, the Na + concentration or transport numbers rather than the ionic conductivity plays a more important role in the formation of SEI layers utilizing ILs. [124,125] To be specific, the increase in the Na[bis(fluorosulfonyl)amide (FSA)] mole fraction provides a stable supply of Na + at the electrode, leading to a robust SEI film and dendrite-free Na deposition morphology. Moreover, the operation temperature is an important factor affecting battery performance because it alters the coordination interaction between Na + and the solvent anions.…”

“…b) Cycle efficiency of Na plating/stripping in Na[FSA]‐[C 2 C 1 pyrr][FSA] with different Na[FSA] amounts at 298 and 363 K. Reproduced with permission. [ 124 ] Copyright 2019, American Chemical Society. c) Role of temperature condition in affecting appearance of the plated Na metal in Na[FSA]‐[C 2 C 1 im][FSA].…”

Preparation and electrochemical properties of ionic-liquid-modified Na3SbS4 membrane composite electrolytes

“…Ionic liquids (ILs) possess the advantages of nonflammability, low vapor pressure, and good stability. In addition, ILs with good fluidity and wettability can suitably alleviate the issue of high interfacial impedance in SEs, ILs thus have good application prospect in energy storage field [26][27][28]. For example, the wettability characteristic of bis(trifluoromethanesulfon)imide lithium (BMP-TFSI) can reduce the interfacial impedance of cold-pressed Li 7 La 3 Zr 2 O 12type electrolyte [29]; the incorporation of LiTFSI-Pyr 13 TFSI improves the interfacial stability of sulfide toward Li metal [30].…”