A new high performance ionic liquid mixture electrolyte for large temperature range supercapacitor applications (−70 °C to 80 °C) operating at 3.5V cell voltage

In this work, we propose a new eutectic ionic liquid mixture based on 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide to be employed as electrolyte in large temperature range supercapacitor applications. The electrolyte exhibited excellent thermal properties in terms of high temperature decomposition (440 °C), a very low melting temperature (no phase transition up to −70 °C) as well as a wide electrochemical window (4.4 V vs Pt). Accordingly, a symmetric coin cell supercapacitor device was performed using porous Ni-foam electrodes in presence of the corresponding mixture at an extended working temperature range from −70 °C to 80 °C. The electrochemical performance of the device was characterised by a wide electrochemical window of 3.5 V maintaining an excellent electrochemical double layer capacitive behavior in all the range. In addition, areal capacitance values between 5 μF cm−2 (−70 °C) and 100 μF cm−2 (80 °C) were obtained at a high scan rate of 20 V s−1, evidencing the rapid and extraordinary electrochemical response even at such extreme temperatures with an excellent cycling stability.