Physical Properties of a New Deep Eutectic Solvent Based on a Sulfonium Ionic Liquid as a Suitable Electrolyte for Electric Double-Layer Capacitors

We present in this study the physical properties of two deep eutectic solvent (DES) mixtures based on solid sulfonium bis[(trifluoromethyl)sulfonyl]imide (S111TFSI) aprotic ionic liquid and two different H-bond donors, formamide (FMD) and trifluoroamide (TFA), according to temperature. First, we investigated their thermal properties by differential scanning calorimetry , and the results revealed the formation of a deep eutectic solvent giving a wide liquid range from −40 to 270 °C for these mixtures which froze at a much lower temperature than either of the individual components. The densities, ionic conductivities, and viscosities of these DESs were measured according to temperature and then discussed by applying Arrhenius or Vogel–Tamman–Fulcher (VTF) equations, as well as the Walden classification. Thanks to their favorable transport properties, both S111TFSI/TFA and S111TFSI/FMD mixtures contribute to the formulation of the electrolytes with 1 mol·L–1 LiTFSI. The performances of these electrolytes were then estimated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge for activated carbon electrochemical double layer capacitor applications at 80 °C. The results showed that the selected H-bond donors allowed ion dissociation without solvation, increasing micropore accessibility and giving high capacitance values up to 350 F·g–1 in the case of formamide-based DESs. These unusual performances of the activated carbon material are debated with regards to the activation energy barrier to access the microporosity by ions in sulfonium-amides DESs.