Anomalous Increase in Double Layer Capacitance in Ionic Liquid-Metal Salt Blends

Abstract

Supercapacitors provide exiting opportunities for the future of energy storage due to a combination of fast charge-discharge kinetics, high safety, and long device lifetimes. Yet, current supercapacitors have energy densities that are orders of magnitude lower than those of lithium-ion batteries, relegating them to niche uses such as regenerative braking systems. While ionic liquids have shown promise as next generation electrolytes in supercapacitors due to their nonflammable nature, large electrochemical stability window, and predicted large capacitances, neat ionic liquids have capacitances ranging from 5 to 20 µFcm-2 which does not compete with current supercapacitors made with cheaper electrolytes. In this talk, I will discuss how we have substantially increased the capacitance of ionic liquid-electrode interfaces though the addition of metal salt additives containing Li, Na, K, and Mg ions. We maintain the large voltage window while increasing the capacitance to up to 80 µFcm-2 at both positive and negative polarizations of gold electrodes. By conducting complementary surface structuring studies, we elucidate molecular assembly at charged surfaces to understand ion behavior when only surrounded by other ions in solution. We hypothesize that the introduction of metal salts interrupts ion aggregates that exist in concentrated electrolytes, reducing the screening lengths in ionic liquids and thus increasing capacitance. Overall, our study demonstrates that ionic liquids may have transformative promise for next-generation supercapacitors, and highlights new paradigms for tuning interfacial properties via modulation of ion coordination in concentrated electrolytes.