Ternary Mixtures of Sulfolanes and Ionic Liquids for Use in High Temperature Supercapacitors

Ionic liquids are a natural choice for supercapacitor electrolytes. However, their cost is currently high. In the present work, we report the use of ternary mixtures of sulfolane, 3-methyl sulfolane, and quaternary ammonium salts (quats) as low-cost alternatives. Sulfolane was chosen because it has a high Hildebrand solubility parameter (delta(H) = 27.2 MPa1/2) and an exceptionally high dipole moment (mu = 4.7 D), which means that it mixes readily with ionic liquids. It also has a high flash point (165 degrees C), a high boiling point (285 degrees C), and a wide two-electrode (full-cell) voltage stability window (>7 V). The only problem is its high freezing point (27 degrees C). However, by using a eutectic mixture of sulfolane with 3-methyl sulfolane, we could depress the freezing point to -17 degrees C. A second goal of the present work was to increase the electrical conductivity of the electrolyte beyond its present-day value of 2.1 mS cm(-1) at 25 degrees C, currently provided by butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (BTM-TFSI). We explored two methods of doing this: (1) mixing the ionic liquid with the sulfolane eutectic and (2) replacing the low-mobility TFSI anion with the high-mobility MTC anion (methanetricarbonitrile). At the optimum composition, the conductivity reached 12.2 mS cm(-1) at 25 degrees C.