Effect of Asymmetric Performance of Ionic Liquid Electrolyte Ions on Maximum Operation Potential of Symmetric Supercapacitors

While known for their high-power performance compared to 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, increasing the potential window is an effective way to increase energy density of the device. 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 of carbon electrodes and ionic liquid electrolytes, we have found one of the possible reasons for smaller operation potential of ionic liquids in practical applications. We observed that even for a symmetric device with electrodes similar in size and weight, the different properties of cations and anions of the electrolyte results in an asymmetric performance of the two similar electrodes. By performing a systematical study on model electrode materials and ionic liquid electrolyte systems, we show that a symmetric behavior of the electrode can be achieved by mixing ionic liquids with different cation and anion properties. This ultimately results in a higher operation potential window of the device and higher energy density. In our model electrochemical device, onions like carbons (OLCs) were used to fabricate the device electrodes and two ionic liquids EMI-TFSI and EMI-BF4 and their mixtures were used as electrolytes.