Unraveling local structures of Salt-in-Water and Water-in-Salt electrolytes via ab initio molecular dynamics

Water-in-salt electrolytes (WiSE) are attractive for electrochemical energy storage applications owing to their wide electrochemical stability windows and inherent safety. The high concentration of salts is widely known to suppress the decomposition of water, which otherwise limits the stability of conventional aqueous salt-in -water electrolytes (SiWE). Nevertheless, the microstructural features of WiSE that lead to this enhanced stability have not yet been fully elucidated. In this work, ab initio molecular dynamics simulations were performed to study the energetic, structural, and spectroscopic properties of LiTFSI SiWE and WiSE solutions. A detailed mapping of water-water and water-anion hydrogen bonding and cation-anion electrostatic interactions is reported for both electrolytes. Structural features are presented in terms of both radial and spatial distribution functions. Analysis of IR and vibrational power spectra reveal key differences in the intermolecular in-teractions in SiWE and WiSE that arise from modified solvation shell structures. The results obtained herein reveal the most important structural and spectroscopic differences between the electrolytes in normal and superconcentrated concentrations.