Collective Microscopic Dynamics in Aqueous LiTFSI from Dilute to Water-in-Salt Electrolytes: Insights from Quadrupolar NMR Relaxation and Molecular Simulations

Abstract The dynamics in electrolytes evolve dramatically with concentration, from dilute solutions to water-in-salt electrolytes (WISE). NMR relaxation provides insights into the local collective dynamics around specific nuclei, complementary to macroscopic properties such as conductivity and viscosity, but modeling is necessary to interpret experiments in terms of microscopic motion. Here, we combine first-principles calculations, classical molecular dynamics simulations, and experimental measurements to investigate the microscopic origin of the quadrupolar relaxation of 7Li+ in aqueous LiTFSI from dilute to WISE. We show that the strong increase in relaxation rate with concentration is dominated by a progressive slowing down of electric field gradient fluctuations at the Li+ site. While the ultrafast inertial contribution to the relaxation remains relatively unchanged due to the persistent preferential coordination of Li+ by water over TFSI, the slow collective relaxation mode becomes increasingly dominant in the concentrated regime. Further analysis reveals the emergence of long-lived dynamics characteristic of strongly correlated ionic environments. We show the central role of the bulky, strongly coordinating TFSI anion, which promotes persistent Li–TFSI associations, reduced ion mobility, and enhanced collective structural relaxation. Finally, we assess the relevance of the Stokes–Einstein–Debye model and demonstrate the breakdown of the simple hydrodynamic picture.