Physical and electrochemical properties of the Na[FSA]–[C2C1im][FSA] (C2C1im+:1-ethyl-3-methylimidazolium and FSA−:bis(fluorosulfonyl)amide) ionic liquids have been investigated in view of their application as electrolytes for sodium secondary batteries operating in a wide temperature range. The Na[FSA]–[C2C1im][FSA] ionic liquids in the range of 0.0 ≤ x(Na[FSA]) ≤ 0.5 are in the liquid state at room temperature, where x(Na[FSA]) is the mole fraction of Na[FSA]. In the case of x(Na[FSA]) = 0.3, the ionic conductivity, viscosity, and electrochemical window at 298 K are 5.4 mS cm−1, 78 mPa s, and 5.1 V, respectively. Sodium metal deposition/dissolution test in the ionic liquid at x(Na[FSA]) = 0.3 resulted in average cycle efficiencies of 69% and 96% at 298 K and 363 K, respectively, at a current density of 1.0 mA cm−2.