Dilute Ionic Liquid Electrolytes for High Energy Density Supercapacitors

Abstract

Supercapacitors are currently touted for their high power densities and cycle lives, allowing them to charge and discharge much more quickly than batteries, and to do so for a larger number of cycles. However, supercapacitors fail to compete with batteries in terms of energy density. Room temperature ionic liquids (RTILs) provide one possible way of improving energy density by moving beyond the operational voltages of conventional aqueous and organic electrolytes. Unfortunately, RTILs suffer from very poor conductivities due to their inherently large viscosities. For the most viscous ionic liquids, dilution with an organic solvent is quite necessary for electrochemical applications, and has been shown to improve the rate performance of devices. However, the competing effects of both ionic and electronic transport, as well as porosity, make it difficult to thoroughly analyze the effect that this has in devices. This work utilizes cyclic voltammetry and impedance spectroscopy in a model system to gain more insight into how dilution with Acetonitrile influences the properties and interfacial behavior of one RTIL (Pip14TFSI). A maximum in both ionic conductivity and intrinsic capacitance is observed at a concentration of 1.0 M, suggesting that previously seen changes in bulk properties with dilution are also accompanied by a change in the capacitive behavior of the interface. It is speculated that this maximum in intrinsic capacitance is attributed to both the increased dissociation of ions, as well as the tendency of solvent molecules to screen the electrostatic interactions between ions. Further measurements in devices with functionalized graphene electrodes indicate that dilution drastically improves the rate performance of RTIL based devices, but were unable to exceed an operating potential of 3.5 volts, indicating that impurities are a major issue in this two component system.