Structural Effects of Water Addition in Triglyme-Based Solvate Ionic Liquid Electrolytes

Diluting solvate ionic liquids (SILs) with small amounts of water combines the advantages of this electrolyte class with those of water-in-salt (WIS) electrolytes. This yields a new class of hybrid electrolytes: water-in-solvate-ionic-liquid (WISIL) electrolytes, as introduced in our previous work (Philipp et al. ChemPhysChem 2026, 27, e70353). Our study uses a combination of Fourier-transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulations to examine hydrogen bonding in water-diluted mixtures of the SIL formed by the equimolar mixture of [Li][NTf₂] and triglyme (G3) at varying water content. An exchange of OH and OD in water allows for the separation of overlapping OH bands and their assignment to different water environments: quasi-free OH groups, water coordinated between G3 and [NTf₂]^-, and water-water hydrogen bonds (HB). The spectra reveal a gradual transition from predominantly dispersed water molecules in the WISIL regime at low water content to extended water-water HB networks at higher water content. MD simulations corroborate these findings by showing that water initially inserts into the cationic [Li(G3)]⁺ complex and replaces one of the donor functions of a G3 oxygen atom toward [Li]⁺, resulting in [Li(G3)(H₂O)]⁺ with a long-lived HB between the metal-coordinated water and G3. These complexes progressively disintegrate upon dilution, resulting in increased mobility and bulk-like water behavior. This study emphasizes the narrow compositional window in which adding water beneficially alters the physicochemical properties of SIL electrolytes.