Enhancing Capacitance by Increasing Electric Double Layer Disorder in Ionic Liquid Mixtures

Supercapacitors, possessing rapid charge-discharge time scales and cycle lifetimes, provide opportunities to mitigate fluctuations from intermittent energy sources. However, while supercapacitors exhibit lower energy densities to lithium-ion batteries, advancing supercapacitor energy density is necessary to improve their utility in applications requiring rapid operating time scales. Supercapacitor energy densities critically hinge on interfacial capacitances and electrolyte voltage windows. Ionic liquids have been heavily investigated as supercapacitor electrolytes due to promising properties like high electrochemical stability, nonflammability, and nonvolatility. Yet, energy densities of ionic liquid-derived supercapacitors are often limited by low interfacial capacitances due to pronounced ion-ion correlations. Here, we show capacitance can be significantly enhanced in ionic liquids over wide voltages by mixing ionic liquids to improve electric double layer entropy. Importantly, capacitance increases exhibited by mixtures persist over the entire electrochemical stability window, showing like-charged co-ions often play similar roles to oppositely charged counterions when determining double layer capacitance. By leveraging electrochemical impedance spectroscopy and surface forces measurements, we conclude that capacitive enhancements originate from changes in short-range nanostructures in ionic liquids, where atomic force microscopy measurements reveal that mixing ionic liquids results in more compact bound ion layers with increased disorder and compressibility. Notably, we find that long-range decay lengths are statistically similar between neat ionic liquids and an equimolar mixture at potentials where we report up to 150% capacitance enhancement, indicating decoupling between the bulk electrostatic decay length and interfacial capacitance. Overall, our results reveal that modulating compositional entropy of ionic liquid double layers provides avenues to increase supercapacitor energy densities and advance next-generation energy storage.