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 (chi(NaFSA) = 0.56, chi(KFSA) = 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 5.2V 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. (C) 2012 Elsevier B.V. All rights reserved.