Inorganic–Organic Hybrid Ionic Liquid Electrolytes for Na Secondary Batteries

Sodium secondary batteries are attractive energy devices owing to the natural abundance of Na resources. The use of ionic liquid electrolytes can solve the safety problems associated with Na secondary batteries, especially for large-scale applications. The present study investigates the physical and the electrochemical properties of three inorganic–organic hybrid ionic liquid electrolytes for Na secondary batteries (Na[FSA]-[TMHA][FSA], Na[FSA]-[DBDM][FSA], and Na[FSA]-[AS(4.5)][FSA] (TMHA + = trimethylhexylammonium cation, DBDM + = dibutyldimethylammonium cation, AS(4.5) + = 5-azoniaspiro(4.5)nonane cation and FSA − = bis(fluorosulfonyl)amide anion) have been investigated. In all the three systems, the liquid phase is observed at room temperature in a certain range of the Na[FSA] fraction. The ionic conductivities and the viscosities of the three systems obey the Vogel–Tammann–Fulcher equation, and the highest ionic conductivity among the three is observed for the Na[FSA]-[AS(4.5)][FSA] system. Their cathode limit on a Cu plate is Na metal deposition around 0 V vs. Na + /Na and anode limit on a carbon electrode is observed at ∼5 V. The Coulombic efficiency of Na metal electrodeposition/dissolution is improved by elevation of temperature.