2, Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut, United States
3, Faculty of Biology, Chemistry & Earth Sciences, University of Bayreuth, Bayreuth, , Germany
Room temperature ionic liquids (ILs) have recently emerged as highly promising electrolytes for a wide range of emerging energy technologies, including next-generation supercapacitors and ion-batteries, due to their high thermal stability, ionic conductivity and wide electrochemical windows. The chemical and structural diversity of ILs creates a vast design space that could be exploited to optimize the device performance and stability. However, many mechanistic details remain enigmatic, including the fundamental nature of the cation-anion interactions and their relevance in determining structural and electronic properties of the liquids. Having this detailed information for the bulk liquid is a prerequisite for eventually deciphering the complexity the arises at nanostructured electrode interfaces, which are ubiquitous among energy storage devices. In this presentation, we combine high-level first-principles simulations and synchrotron X-ray characterization experiments to unravel the key structural, chemical and electronic properties of several archetypal ILs comprised of imidazolium-based ILs. In particular, we utilize extensive ab initio molecular dynamics simulations to probe the local density distribution and medium-range order of the ILs, which can be directly compared and validated by X-ray scattering measurements. Soft and tender X-ray absorption spectroscopy at the K-edge of fluorine, phosphorus, and sulfur contained on the anion also complements the chemical and electronic picture from the simulations. Our integrated theoretical and experimental approach relates these structural and chemical signatures with the intrinsic cation-anion interactions, by considering ILs with anions having significant differences in the molecular geometry, chemical composition, and charge distribution.
This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.