Thermal and Transport Properties of Na[N(SO2F)2]-[N-Methyl-N-propylpyrrolidinium][N(SO2F)2] Ionic Liquids for Na Secondary Batteries

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-N-propylpyrrolidinium) 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.