Dynamic heterogeneity and mechanism of ionic transport in NaTFSI-based WiSE and superconcentrated NaTFSI–EmimTFSI hybrid aqueous electrolytes

We investigate the impact of simultaneous addition of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIMTFSI) ionic liquid and excess NaTFSI salt on the microscopic dynamics and transport properties of NaTFSI-based water-in-salt electrolyte (WiSE). From the analysis of the translational dynamics of the ions, we observe that the Na+ ions are the fastest diffusing ionic species stemming from their longer correlated motion with water molecules in the 8 m WiSE. As observed in experiments, the simulated ionic conductivity of the electrolyte decreases monotonically with increasing overall concentration of the electrolyte from 8 to 80 m by adding EMIMTFSI and NaTFSI together. The analysis of distinct diffusion coefficients reveals an overall anti-correlated collective motion of both coions and counterions in the WiSE (8 m) and hybrid electrolytes (50 and 80 m). Among these collective motions, the anion-anion (TFSI--TFSI-) exhibits most pronounced anti-correlated motion. In the hybrid electrolytes, a longer residence time for the Na+ ions in the vicinity of water molecules is observed, signifying the dominance of the vehicular mechanism of Na+ ion conduction. On the other hand, Na+ and EMIM+ cations both adapt structural exchange mechanisms for their conduction when present in close proximity to the TFSI- anions in both WiSE and hybrid electrolytes. The presence of longer average hydrogen bond lifetimes between water molecules reveals stronger hydrogen bonds due to their small cluster-like existence in the hybrid electrolytes. The slower relaxation of the rotational time autocorrelation function for water and the EMIM+ cation with increasing salt concentration implies restriction in their motions due to the surrounding species. Pronounced dynamic heterogeneity in the motion of Na+ ions is observed in the hybrid electrolytes as compared to that in WiSE because of very different solvation environments in their vicinity. A longer correlation timescale for Na+ ions and water molecules is revealed through the analysis of distinct van Hove correlation functions in the hybrid electrolytes.