(Invited) Ionic Liquid Mixture Electrolytes to Increase the Performance of Electrochemical Capacitors

While known for a higher power density than batteries, electrochemical capacitors are limited by their energy density for some energy storage applications. Since the energy density of the device is proportional to the square of operating potential window, an effective way to increase energy density of the device is by increasing the window. Ionic liquid electrolytes are beneficial for this reason since they can theoretically operate at up to 6 V, though experimentally, the value is between 3-4 V, depending on the properties of electrode materials. By employing a novel electrochemical technique to study symmetric supercapacitors with carbon electrodes and ionic liquid electrolytes, we have found one of the possible reasons for the observed smaller operating potential window of ionic liquids in practical applications. We observed that even for a symmetric device, the different properties of the cation and anion results in an asymmetric performance of the two electrodes. However, a solution to this asymmetry is found by mixing two ionic liquids with the same cation, which ultimately results in a higher operating potential window of the device and therefore a higher energy density. 1 Onion-like carbon (OLC) was used as the electrode material due to its large (>500 m 2 /g) outer surface available for ion adsorption and double layer formation, and no internal porosity that could restrict the movement of ions. 2 However, electrode materials can also be made from high surface area carbons that contain a network of pores (e.g., activated or carbide-derived carbon) in which ions are adsorbed during charging. Though they boast a large operating potential window, ionic liquids are known to contain large and bulky ions. This can make it difficult to use an ionic liquid on a porous carbon with a range of pore sizes, some being smaller than the ion size, even though the specific surface area of the electrode material measured by gas adsorption is high and therefore the energy storage capability is more impressive. It has also been shown that the capacitance of porous electrodes is maximized when the ion size and pore size are equal. 3 In this case, the ion is small enough to fit inside the pores while still large enough to take advantage of the surface area within the pore walls. While materials with a large outer surface allow a more accessible surface for adsorbing large ions, their capacitance is limited by their specific surface area. By designing an electrolyte based on mixed ionic liquids, we can match the ions in the mixture to the multiple pore sizes of the electrode material. Recently, it was found that the capacitance of porous carbon could be increased with the presence of multiple ions in the electrolyte mixture. Imidazolium-based ionic liquids are chosen based on their variable alkyl chain length which alters the cation size and bis(trifluoromethylsulfonyl)- imide (TFSI) is chosen as the anion for all ionic liquids. Different porous carbons with varying pore size distributions are used to illustrate the effect of ionic liquid mixture electrolytes. Finally, it has been shown previously that ionic liquid mixture electrolytes can be used to increase the operating temperature window of a supercapacitor to -50 °C – 100 °C. 4 Ionic liquid mixtures have therefore been shown to improve the potential window, the capacitive performance, and the temperature window of supercapacitors, becoming an important area of study in the field of energy storage. 1. Van Aken, K. L., Beidaghi, M. & Gogotsi, Y. Formulation of Ionic-Liquid Electrolyte to Expand the Voltage Window of Supercapacitors. Angew. Chemie 127, 4888–4891 (2015). 2. McDonough, J. K. et al. Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes. Carbon N. Y. 50, 3298–3309 (2012). 3. Lin, R. et al. Solvent effect on the ion adsorption from ionic liquid electrolyte into sub-nanometer carbon pores. Electrochim. Acta 54, 7025–7032 (2009). 4. Lin, R. et al. Capacitive Energy Storage from -50 to 100 C Using an Ionic Liquid Electrolyte. J. Phys. Chem. Lett. 2, 2396–2401 (2011). Figure 1