Anion-Dependent Perturbations in Ionic Liquid Structure Probed By High-Energy X-Ray Scattering

Abstract

Room-temperature ionic liquids (ILs) are molten salts at room temperature, and their wide electrochemical windows and high ionic conductivities make them promising next-generation electrolytes for energy storage devices. The vast potential cation-anion pairings result in tunable structure and organization of the bulk ILs, which ultimately dictates their properties. However, the amorphous structure of bulk ILs remains poorly understood due to the typically large and flexible nature of the molecules and complex interactions between them. We employed high-energy synchrotron X-ray scattering to study the structure of a set of archetypal imidazolium-based ILs. We sought to understand perturbations in the structure across 3 popular anions possessing distinctly different geometric features and dipole moments (i.e., PF6 -, OTf-, TFSI-), while also systematically varying the length of the alkyl chain on the cation. In particular, we collected scattering at both high and low angles to compare local and longer-range structural order, respectively, and we leveraged pair distribution function (PDF) analysis to transform our X-ray data to real-space density fluctuations for intuitive inspection and quantitative comparison with simulations. Both classical and ab-initio molecular dynamics (MD) cross-validated these detailed intra- and intermolecular density distributions rendered by our X-ray experiments. This enabled us to use MD to decouple and characterize cation-anion, cation-cation, and anion-anion interactions as well as H-bonding type interactions for each IL studied. Developing such analytical frameworks for detailed analysis of bulk IL structure as a function of different ions is important toward developing guidelines for engineering cations and anions for targeted applications. Furthermore, understanding the influence of interchanging ions is a prerequisite for applications in which ILs interface with other species, such as other types of ions, nanoparticles, or electrode interfaces.