Unveiling the Influence of Ionic Liquid on the Interfacial Structure and Capacitive Performance of Water-in-Salt Electrolytes at Graphite Electrodes

Recent studies have shown that adding nonvolatile ionic liquids can significantly expand the electrochemical stability window of water-in-salt electrolytes (WiSEs). Our research examines the impact of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EmimTFSI) ionic liquid on the interfacial structure and capacitive performance of LiTFSI-based WiSEs using constant potential molecular dynamics simulations. For pure LiTFSI-based WiSEs, the differential capacitance (C-d) is found to be significantly higher near the negative electrode than the positive electrode due to the size disparity between the Li+ cation and TFSI- anion. We observe that the addition of bigger Emim(+) ions narrows the difference between C-d values at both the electrodes. We find that the distribution of abundant and highly structured TFSI- ions does not change much in response to the applied potential near the positive electrode. When the potential difference across the electrodes is increased, Li+ ions along with solvating water molecules form an additional layer near the negative electrode. A significant decrease in the water content near the electrodes at higher concentrations compared to that of pure LiTFSI WiSEs is observed, which explains the reduced water splitting observed in experiments. This study unveils the capacitive response of an archetypal WiSE and its hybrid mixture with EmimTFSI ionic liquid, boosting to future development of high-concentration aqueous electrolytes.