Urea Disrupts Lithium Solvation Shells: Multinuclear NMR and MD Insights into Aqueous Electrolytes Based on Deep Eutectic Solvents

The solvation properties of electrolytes are one of the primary drivers of the performance of concentrated aqueous-based batteries. In this study, we provide an understanding of the lithium solvation and its dynamics in an electrolyte derived from a deep eutectic solvent (comprising LiClO4, urea and water) by effectively correlating multinuclear (1H, 17O, 35Cl, and 7Li) diffusion nuclear magnetic resonance (NMR) spectroscopy with molecular dynamics (MD) simulations. The freely accessible static field gradient from a commercial NMR magnet was used to determine diffusion coefficients for all population-averaged constituents─Li+, ClO4–, urea, and water─at various concentrations. The diffusion ratios DLi+/Durea ≈ 1 and DLi+/DClO4– ≈ 1 for the concentrated 1:3:2 electrolyte suggest correlated ionic motion and transport dominated by a vehicular character rather than hopping mechanisms. Ab initio MD simulations reveal mixed Li solvation shell configurations, with substantial urea coordination (∼66% of Li+ with one or two urea molecules in the 1:3:2 system) and an increasing occurrence of contact ion pairs with concentration. This results in relatively slow ionic mobility with a cation transference number around 0.5. Ion cluster analysis reveals a systematic increase in the occurrence of contact ion pairs with concentration, yet cluster sizes remain far below percolation thresholds, consistent with vehicular transport. Residence time analysis further quantifies this picture: the characteristic diffusional length exceeds the inner-sphere solvation shell radius (Lc/Ls > 1) across all concentrations, providing evidence for vehicular-dominated transport. 17O diffusion NMR measurements (D17O/D1H > 1.2 in a concentrated system) provide insights into water dynamics despite significant experimental challenges from quadrupolar relaxation. These findings provide molecular-level insights into aqueous electrolytes based on deep eutectic solvents to further guide the rational design of aqueous battery systems.