Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL/electrode interfaces. Here, we discover that surface-active ionic liquids (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional nonamphiphilic ILs (NAILs), for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the nonpolar surfactant tails, leading to unusual interfacial ion distributions. We reveal that at an intermediate degree of electrode polarization SAILs display optimal performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of nonpolar domains along the 2 surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behavior and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energydensity electrolytes based on spontaneous self-assembly behavior. Research interest in ionic liquids (ILs) as electrolytes for energy devices stems from several unique properties such as low volatility and flammability, as well as high electrochemical stability 1-5. An understanding of the molecular-level interactions between ILs and electrified interfaces is crucial for optimization of device performance 6. For instance, interfacial IL layers at charged surfaces govern the electric double layer (EDL) structure, a key factor determining the device energy density 2,4,6,7. The EDL structure with ILs is drastically different from that in aqueous and organic electrolytes 8-10 ; the complex ion ordering in ILs exhibits many subtleties, and remains an active area of debate 11-14. Here we present the first detailed investigation into electrocapacitive characteristics and fundamental EDL structures of an emerging IL class based on surface-active agents, or surface-active ILs (SAILs) 13,15-19. Our study reveals a novel material design principle for enhancing charge storage owing to the self-assembled nanostructures in amphiphilic liquids, and introduces a class of liquids with long-range ordering, having broad implications for diverse fields, ranging from interfacial science 20,21 to energy technologies 22,23. SAILs are inherently amphiphilic, and can self-assemble into nanostructures composed of distinct polar and nonpolar domains 13,15-19. Most previous studies on the IL EDL structure and IL-based energy devices focus on non-amphiphilic ILs (NAILs) where neither ion is based on a classical surfactant structure 6,24. Whereas nanostructuring was observed under confinement for some NAILs where one of the ions, usually the cation, bears moderate to long chai...
