Frequency-dependent NMR relaxation : Insights into the Structure and Dynamics of ‘WiS’ Electrolytes

Angel Mary Chiramel Tony (University of Rostock, Germany)

LinkedIn: @Angel Mary Chiramel Tony; X: @AngelMaryCT1; Bluesky: @angeltony2000.bsky.social

Abstract: “The “Water-in-Salt Electrolytes” are promising candidates for safer high-voltage aqueous lithium-ion chemistry. Our system of concern for the NMR relaxation investigation is Li-bis(trifluoromethanesulfonyl)imide with H2O/D2O mixtures in 1:3, 1:4 and 1:5 ratios for shedding light on the structure and dynamics of this battery electrolyte as a model system. The frequency-dependant R1(spin-lattice) and R2(spin-spin) relaxation rates for several orders of magnitude is calculated with the computational framework employing MD simulations and applying a correction factor to account for the system size dependency and accessible time scales. Our approach is based on combining the analytical theory of Hwang and Freed (HF) for the long-range intermolecular contribution of the magnetic dipole-dipole correlation function with MD simulations . We show that the correlation functions due to the HF-theory do asymptotically converge with our MD simulation results at long times. We are successful in dissecting the intermolecular and intramolecular contribution of relaxation rates describing the translational and rotational dynamics with the NMR active nuclei 1H and 19F on water and anion molecules respectively. The results show that both longitudinal and transverse relaxation rates increase with increasing temperature and water content indicating enhanced dynamics. Morever the complimentery calculations shed light into the Li ion transport mechanism in the system. The key finding is that Li ion dynamics is compensated with anion repalcing water in highly concentrated systems making it an ideal candidate for battery electrolytes.”