Formulation of Ionic Liquid Electrolyte to Expand the Voltage Window of Supercapacitors

While their high-power performance exceeds that of batteries, supercapacitors lack the energy density capability for some energy storage applications. Since the energy density of the device is proportional to the square of operating potential window (OPW), an effective way to increase the energy density of the device is by increasing the potential window. Ionic liquid electrolytes are beneficial for this reason since they can theoretically operate at up to 6 V, though experimentally, this value is between 3-4 V depending on the properties of electrode materials. By employing a novel electrochemical technique to analyze symmetric supercapacitors of carbon electrodes and ionic liquid electrolytes, we have found one of the possible reasons for smaller OPWs of ionic liquids in practical applications. 1 We observed that for a symmetric device with electrodes equal in size and weight, the different properties of cations and anions in the electrolyte results in an asymmetric performance of the two similar electrodes. By performing a systematic study with ionic liquid electrolytes and model electrode materials, we show that a symmetric behavior of the electrodes can be achieved by formulating an ionic liquid mixture containing different cations and anions. In our model electrochemical device, onion like carbon (OLC) was used to fabricate the device electrodes and two ionic liquids, EMI-TFSI and EMI-BF4, and their mixtures were used as electrolytes. This electrolyte mixing results in an extended OPW of the device and a higher energy density. 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).