Electric Double Layer Theory of Interfacial Ionic Liquids for Capturing Ion Hierarchical Aggregation and Anisotropic Dynamics

Abstract

Compared to aqueous ions, ionic liquids (ILs) consist entirely of cations and anions, and they function in energy conversion and storage owing to unique properties. Classical electric double layer (EDL) theory born a century ago gives essential insights into aqueous interfaces, whereas it fails in ILs due to overlooking ionic geometries and interactions, leaving a theoretical gap in describing IL interfaces. This study captures IL fingerprints by combining nuclear magnetic resonance experiments, quantum chemistry investigations, and first-principles molecular simulations. Based on these findings, an EDL theory is proposed to reflect hierarchical aggregation structure and anisotropic dynamics for interfacial ILs. It involves four elements including ion geometries, ion–ion interactions, ion–wall interactions, and temperature and interfacial electric field effects. Specifically, polyatomic ions with nonuniform shapes are cornerstones for successive elements. Cation–anion non-oxygen hydrogen bonds and cation–cation π⁺–π⁺ stacking generate bound clusters. Ion–wall adsorption causes hierarchical aggregation patterns, and cation–wall parallel stacking creates anisotropic dynamics at interfaces. Moreover, the thermal-weakened ionic interactions and electric field-enhanced interfacial electron transfer alter the aggregation states and dynamic properties of interfacial ILs. This theory is adaptable to various IL–wall combinations, advancing electrolyte design for next-generation electrochemical energy devices.