Abstract

The controlled dilution of highly coordinated electrolyte systems with molecular solvents offers a promising route to tune transport and stability properties in advanced energy materials.[1] Here, we investigate the structural and dynamical evolution of solvate ionic liquid (SIL) electrolytes upon gradual water addition, with particular focus on the transition from a coordination-driven system, the water-in-solvate-ionic-liquid (WISIL) regime, to its breakdown at higher water contents.

Using a combined experimental and molecular dynamics simulation approach, we resolve how water is incorporated into the SIL formed by the equimolar mixture of [Li][NTf2] and triglyme (G3). At low water content, water remains highly dispersed and preferentially inserts into the cationic chelate complex [Li(G3)]+ coordination shell, forming long-lived [Li(H2O)(G3)]+ motifs that preserve the coordination-dominated local structure of the SIL. In this regime, the addition of water substantially lowers viscosity and enhances ionic conductivity while largely maintaining the wide electrochemical stability window (ESW) characteristic of the neat SIL.[2]

Upon further dilution, a distinct structural transition occurs: the coordinated complexes progressively disintegrate and extended water–water hydrogen-bond networks emerge[3], accompanied by increased molecular mobility and increasingly bulk-like water behavior. This transition is further characterized by predominantly hydrated lithium cations and a substantially narrowed ESW, approaching that of conventional aqueous electrolytes.

Our results identify a narrow compositional window in which water addition is beneficial and reveal the molecular mechanisms governing the crossover from dispersed mixing to hydrogen-bond network formation in complex electrolyte solutions. The findings connect microscopic mixing behavior with macroscopic transport and electrochemical properties, providing insights into solvent-driven structural transitions in highly concentrated electrolyte mixtures.

References

[1] K. Ueno, J. Murai, H. Moon, K. Dokko and M. Watanabe, J. Electrochem. Soc., 164, A6088 (2016).

[2] J. K. Philipp, D. Paschek, L. Kruse, A.-E. Surkus, B. Austrup, M. Schönhoff, R. Ludwig, ChemPhysChem, e70353 (2026, in print).

[3] J. K. Philipp, A. Strate, F. M. Januszewski, L. Möhring, L. Kruse, M. Bühl, D. Paschek, R. Ludwig, J. Phys. Chem. B (2026, submitted).