For application of electrolyte materials in energy storage devices their transport properties are essential. Multinuclear (e.g. 1H, 7Li, 19F) Pulsed-Field-Gradient (PFG)-NMR diffusion has become a widely used method in this field. However, to identify the conductivity contribution of specific ion species remains a challenge, since the electrophoretic mobility µ has to be known.
Electrophoretic NMR (eNMR) allows to directly measure the electrophoretic mobility of ions with NMR-active nuclei. During a PFG-NMR experiment an electric voltage is applied and the ion mobility is obtained from its drift velocity in the electric field. Provided that challenges arising from high conductivities and subsequent resistive heating can be overcome, even concentrated electrolyte systems can be investigated.
The lecture reviews our multinuclear eNMR studies on electrolytes for Li battery applications, e.g. Li salt-in-Ionic Liquid systems. Surprisingly, a negative mobility of Li+ may occur, implying a drift direction opposite to the expectation for a cation. This was attributed to a vehicular transport mechanism of Li in net negatively charged anion clusters and has strong implications for battery operation. In some systems a transition from a vehicular to a structural transport mechanism can be achieved by compositional variation.
We further report on various liquid electrolyte systems with organic additives and glyme-based solvate ionic liquids. Here, in addition to ion drift velocities, even a drift of uncharged molecular components in the electric field can be identified by 1H eNMR. Thus, conclusions on their coordination to Li+ ions are possible, sheding light on the mechanisms governing the Li transport.
In summary, electrophoretic NMR elucidates transport mechanisms on a molecular level, and provides unique information; in particular, where correlated motion of different ion species is involved.