Understanding ion transport in electrolytes is crucial for fabricating high-performance batteries. Although several ionic liquids have been explored for use as electrolytes in Na secondary batteries, little is known about the transport properties of Na + ions. In this study, the thermal and transport properties of Na[FSA]-[C 3 C 1 pyrr][FSA] (FSA − : bis(fluorosulfonyl)amide and C 3 C 1 pyrr + : N-methyl-Npropylpyrrolidinium) ionic liquids were investigated in order to determine their suitability for use as electrolytes in Na secondary batteries. In the x(Na[FSA]) range of 0.0-0.5 (x(Na[FSA]) = molar fraction of Na[FSA]), a wide liquid-phase temperature range was observed at close to room temperature. The viscosity and ionic conductivity of this system, which obey the Vogel-Tamman-Fulcher equation, increases and decreases, respectively, with an increase in x(Na[FSA]). Further, its viscosity and molar ionic conductivity satisfy the fractional Walden rule. The apparent transport number of Na + in the investigated ionic liquids, as determined by the potential step method at 353 K, increases monotonously with an increase in x(Na[FSA]), going from 0.08 for x(Na[FSA]) = 0.1 to 0.59 for x(Na[FSA]) = 0.7. The Na + ion conductivity, determined by multiplying the ionic conductivity with the apparent transport number, is an indicator of Na + ion transport in Na secondary batteries and is high when x(Na[FSA]) is in the 0.2-0.4 range.

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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Therapeutic study, Level III.

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