For application of electrolyte materials in energy storage devices their transport properties are essential. Commonly determined transport coefficients are conductivity, viscosity and diffusion coefficients, where the latter are ion-specific quantities, available by multinuclear Pulsed-Field-Gradient (PFG)-NMR diffusion. This lecture presents ion transport studies in advanced electrolytes for Li ion batteries, i.e. (i) ionic liquid (IL)- based liquid electrolytes and (ii) ternary polymer gel electrolytes consisting of ionic liquid, polymer and Li salt, which are suitable materials to yield a compromise between sufficient conductivity and mechanical stability. Here, multinuclear PFG-NMR of 7Li, 19F (anions) and 1H (cations) serves to analyze the transport processes as influenced by complex mutual interactions of the constituents. For example, in systems based on the poly(ionic liquid) PDADMATFSI a detailed comparison of relative diffusivities of anions and Li showed that charged chains are superior polymeric constituents in ternary gels in terms of their ability to enhance Li+ conduction. Transport numbers calculated from PFG-NMR diffusion coefficients thus serve to optimize materials for transport of specific ion species. Furthermore, to identify the conductivity contribution of a specific ion species remains a challenge, since from diffusion coefficients transference numbers can only be obtained under the assumption of completely uncorrelated ion motion. In order to determine true transference numbers, the electrophoretic mobility µ has to be known. Here, electrophoretic NMR (eNMR) allows to directly measure the electrophoretic mobility µ of NMR-active ions. Again, multinuclear studies (e.g. 1H, 7Li, 19F) yield a full characterization of all ionic species of typical liquid electrolytes. Various examples such as pure ionic liquid, IL/Li salt mixtures, and glyme-based solvate ionic liquids are demonstrated, providing unique information on the mobility of any single ion species and thus allowing conclusions on correlated motion of different ion species.

Studying ion transport in polymer electrolytes by NMR methods

255th National Meeting (2018)

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