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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-Npropylpyrrolidinium) 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 ioncic conductivity and charge-discharge performance of Na/Na[FSA]-[C 3 C 1 pyrr][FSA]/NaCrO 2 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 2 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.

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