As electrolytes for sodium secondary batteries operating over a wide temperature range, Na[FSA]-[C3C1pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C3C1pyrr = N-methyl-N-propylpyrrolidinium) ionic liquids have been investigated. The effects of Na ion concentration (0–60 mol% Na[FSA]) and operation temperature (253–363 K) on the viscosity and ionic conductivity and charge–discharge performance of Na/Na[FSA]-[C3C1pyrr][FSA]/NaCrO2 cells are studied. Results show that Na ion concentration strongly affects the rate capability of the cells, and that the best rate capability at 363 K is obtained at 40 mol% Na[FSA]. The operation temperature also significantly influences the charge–discharge performance, especially at low temperatures. At operation temperatures below 273 K, 25 mol% Na[FSA] is found to be the optimum Na ion concentration. There exist different optimum ranges of Na ion concentration depending on the operation temperatures.