Intermediate-temperature ionic liquid NaFSA-KFSA and its application to sodium secondary batteries

The physicochemical properties of an intermediate-temperature ionic liquid (ITIL), NaFSA-KFSA (xNaFSA = 0.56, xKFSA = 0.44, and FSA = bis(fluorosulfonyl)amide), were investigated to test the potential of the ITIL as an electrolyte for sodium secondary batteries operating at intermediate temperatures (333–393 K). The viscosity, ionic conductivity, and density of this ITIL, measured at 363 K, were 435 cP, 3.3 mS cm−1, and 2.14 g cm−3, respectively. Cyclic voltammetry revealed that the electrochemical window is as wide as 5.2 V at 363 K, and that reversible electrochemical deposition/dissolution of sodium metal occurs at the cathode limit potential. A Na/NaFSA-KFSA/NaCrO2 cell was constructed and its charge–discharge properties investigated at 353 K. The discharge capacity at the 1st cycle was 77.3 mA h (g-NaCrO2)−1 at 15 mA (g-NaCrO2)−1. Except for the initial few cycles, the coulombic efficiencies were higher than 99.9% for 100 cycles, and 89% of the initial discharge capacity was maintained after 100 cycles. Considering its non-volatility, non-flammability, and low cost, this inorganic ITIL is highly promising as a new class of electrolyte for sodium secondary batteries.

► NaFSA (56 mol%)-KFSA (44 mol%) is a non-flammable and inexpensive inorganic ionic liquid.
► Electrochemical window of NaFSA-KFSA is as wide as 5.2 V at 363 K.
► A Na/NaFSA-KFSA/NaCrO2 cell exhibited good cycle performance at353 K
► The discharge capacity was 77.3 mA h g−1 at 15 mA g−1.
► The coulombic efficiency was higher than 99.9% for 100 cycles.