Pulsed Field Gradient Nuclear Magnetic Resonance and Diffusion Analysis in Battery Research

Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) is a widely used method for determining the diffusion coefficient of ions and molecules both in the bulk and when confined (e.g., within porous materials). Due to the nature of diffusion phenomena and the correlation of these processes with the structures of isolated molecules or clusters, studies of diffusion can be used to extract both dynamic and structural information from complex mixtures, including battery electrolytes composed of cations, anions, and solvent molecules. PFG-NMR presents a powerful opportunity for battery scientists to quantify electrolyte properties, such as time scales for dynamics, transference numbers, and solvation structures of active ions that vary due to ion–ion and ion–solvent interactions. These measurements and the derived information about molecular interactions can ultimately be correlated with real battery performance. The purpose of this review is to provide readers with an overview of the basic principles and experimental considerations when undertaking PFG-NMR for battery electrolyte research. In this review, we will first (1) introduce basic PFG-NMR experiments, parameters, and the proper setup for acquiring accurate diffusion coefficients and (2) discuss artifacts that can arise in diffusion measurements, including their diagnosis and suppression. Second, we show the ultimate power of careful analyses of diffusion coefficients for extracting dynamic and structural properties of a wide range of electrolyte types (i.e., dilute, concentrated, polymer, and solid-state) through a review of selected literature. In addition, other NMR methods are briefly introduced, including relaxation measurements and Overhauser dynamic nuclear polarization (ODNP) NMR.