Ionic liquids and γ-butyrolactone mixtures as electrolytes for supercapacitors operating over extended temperature ranges

Abstract

Physicochemical and electrochemical properties of three different ionic liquids (1-propyl-1 methylpyrrolidinium bis(fluorosulfonyl)imide – Pyr₁₃FSI, 1-butyl-1-methylpyrrolidinium bis(trifluoro methanesulfonyl)imide – Pyr₁₄TFSI and 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide – EMITFSI) were investigated and compared with binary mixtures of those ionic liquids with γ-butyrolactone (GBL). It was found that the highest conductivity for each mixture was obtained for a concentration close to 50 wt%. Then thermal and transport properties for the three neat ionic liquids and the three mixtures with GBL at 50 wt% were evaluated from −50 °C to 100 °C. The addition of GBL enhances the conductivity and fluidity of the mixtures, especially at low temperature. For instance, at −50 °C the ionic conductivity for EMITFSI/GBL is still as high as 1.9 mS cm⁻¹ and its viscosity is 70 mPa s. Another advantage of the solvent addition is that it suppresses the melting transition and allows applications down to −50 °C. A drawback is the slight reduction of the electrochemical stability window of the electrolyte.