Investigation of Structure and Dynamics of Choline Chloride/Urea/LiCl Deep Eutectic System by Nuclear Magnetic Resonance Spectroscopy

Lithium-ion battery electrolytes have been widely used in the electrochemical market. However, conventional electrolytes still face challenges such as flammability, poor thermal stability, and environmental concerns. Deep eutectic solvents (DESs) demonstrate promising potential in electrochemistry due to their excellent electrochemical performance and thermal stability. Despite progress in research on DESs/lithium salt systems, investigations into their molecular structure and dynamic behavior are still required. This study selected typical DES-Reline as the model system and investigated the molecular/ionic interactions, molecular structure and dynamic behavior of the species involved in choline chloride-urea deep eutectic solvent (Reline)/LiCl mixtures. First, viscosity, conductivity, diffusion coefficients, and relaxation times revealed that the species in the Reline/LiCl system exhibited dynamic characteristics typical of a high-viscosity liquid. Increasing the LiCl concentration would reduce the mobility of the molecules and ions. Based on the chemical shifts and lineshape observed in 1D 15N and 13C nuclear magnetic resonance (NMR) spectra, the urea signals were found to broaden exponentially, and their chemical shifts gradually moved downfield as the LiCl concentration increased. Furthermore, urea demonstrated higher sensitivity to LiCl concentration compared to choline ions. Two-dimensional 1H-1H correlation spectra (2D 1H-1H COSY) indicated that the addition of LiCl significantly weakened the proton spin coupling between urea and choline, while Fourier transform infrared (FT-IR) spectroscopy results showed no significant change in the overall strength of hydrogen-bonding network in Reline/LiCl system. Additionally, two distinct lithium species with markedly different mobilities were probed through 7Li NMR, which were assigned to Li+ and (LiCl3)2–. The two species coexisted in Reline under the chemical equilibrium of Li++3Cl–⇌(LiCl3)2‒. The Van’t Hoff plot yielded ΔS of ‒101.93 J/mol and ΔH of –31.34 kJ/mol for this equilibrium. Accordingly, it revealed that approximately 59% of lithium existed as (LiCl3)2– in Reline/LiCl mixture, interacting indirectly with urea molecules and choline ions via Cl–. This indirect interaction was confirmed by 2D 7Li-1H heteronuclear Overhauser effect spectra (2D 7Li-1H HOESY). Finally, based on the cross-relaxation rate constant (σIS), it was revealed that with the increase of LiCl fraction, the interaction between Li+ and choline-CH3 group weakened, while the interaction with urea-NH2 group strengthened progressively. This study aimed to elucidate the mechanism of the interactions at molecular level, providing a theoretical basis for designing high-performance DESs-based electrolytes and offering a reference methodology for experimental characterization of similar systems.