Liquid Structure of NaTFSI-EmimTFSI Hybrid Aqueous Electrolytes: Beyond the Solubility Limit of NaTFSI-Based WiSE

Abstract

Limitations caused by lower solubility of the sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) salt in water are one of the major concerns for NaTFSI-based water-in-salt electrolytes (WiSEs). Recent endeavors in this direction have discovered that the addition of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EmimTFSI) ionic liquid (IL) boosts the solubility of NaTFSI salt in water to 30 m. In the present study, with the help of all-atom molecular dynamics simulations, we have investigated the structural changes in these highly concentrated NaTFSI-EmimTFSI-based hybrid aqueous electrolytes. Our results reveal that the Na⁺ ions prefer to be surrounded by the oxygen atoms of the anions in the hybrid electrolyte, unlike in the WiSE, where they predominantly interact with the oxygen atoms of water molecules. With the addition of the IL, the number of anions around anions is found to increase. Consequently, the anionic network begins to dominate over the water network present in the WiSE. The solvation environment of the Emim⁺ cations consists of the TFSI^– anions, leading to the presence of charge ordering, which remains present for all the concentrations of the hybrid electrolytes. Interestingly, the Emim⁺ cations and water molecules tend to stay away from each other. The deviation in the behavior of water from its bulk-like characteristics is maximum in the 80 m hybrid electrolyte. The analysis of the simulated X-ray scattering structure function reveals the presence of an intermediate-range ordering in the 7 m WiSE. However, the addition of the EmimTFSI IL with excess NaTFSI leads to the loss of intermediate-range ordering and the appearance of charge ordering in the hybrid electrolytes.