Thermal and transport properties of Na[N(SO<inf>2</inf>F)<inf>2</inf>]-[ N -Methyl- N -propylpyrrolidinium][N(SO<inf>2</inf>F)<inf>2</inf>] ionic liquids for Na secondary batteries

Abstract

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]-[C3C1pyrr][FSA] (FSA–: bis(fluorosulfonyl)amide and C3C1pyrr+: 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.