Ionic Liquids for Supercapacitive Energy Storage: A Mini-Review

Ionic liquids (ILs), composed of bulky organic cations and versatile anions, have sustainably found widespread utilizations in promising energy-storage systems. Supercapacitors, as competitive high-power devices, have drawn tremendous attention due to high-rate energy harvesting and long-term durability. The electric energy of supercapacitors is stored through the ion dynamics and physicochemical interactions at the electrolyte/electrode interface. To satisfy the high-energy request for building better supercapacitors, ILs stand out by virtue of the characteristic negligible vapor pressure and molecular designability, coupled with several fascinating features including a highly ionized environment, good thermal/chemical stability, and universal solubility/affinity. This mini-review offers an overview of recent IL utilizations in both electrolyte exploitation and electrode synthesis for supercapacitors. On the role of IL-based electrolyte components, three representative types (i.e., neat IL electrolytes, IL mixtures, and IL (quasi-)solid-state electrolytes) are applied to put aside the water-splitting roadblock, thus affording high charge storage under wide electrochemical stability potentials. On the other hand, the involvement of ILs in material science is described as microstructure-directing agents, heteroatom dopants, and carbon precursors, respectively, for the purpose of boosting the interfacial physicochemical interactions toward superior electrode capacitances. Finally, current challenges and future outlooks associated with IL media/materials are summarized for next-generation supercapacitor applications.