As electrolytes for sodium secondary batteries operating over a wide temperature range, Na[FSA]-[C(3)C(1)pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C(3)C(1)pyrr = 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]-[C(3)C(1)pyrr][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. (C) 2014 Elsevier B.V. All rights reserved.