Direct measurement of ion mobility by electrophoretic NMR and implications for correlated migration in liquid electrolytes

In studies of ion transport in electrolytes, multinuclear (¹H, ⁷Li, ¹⁹F) electrophoretic NMR (eNMR) allows to directly measure electrophoretic mobilities, extracting them from the ion drift velocity in an electric field.
For example, in Li salt-in-Ionic Liquid systems 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 of Li in net negatively charged Li-anion clusters. Their stoichiometry can be deduced from the effective charge. Employing compositional variations, such as using asymmetric anions or high Li salt concentrations, a transition from a vehicular to a structural transport mechanism of Li⁺ can be achieved.
In further liquid electrolytes, containing organic additives or in glyme-based solvate ionic liquids, a drift of uncharged molecules in the electric field can be identified by ¹H eNMR. Thus, conclusions on their correlated transport, due to coordination to Li⁺ are possible, sheding light on the mechanisms governing 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.