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 has become a widely used method in this field. However, to identify the conductivity contribution of a 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 µ of NMR-active ions. During a PFG-NMR experiment an electric voltage is applied and the ion mobility is obtained from its drift velocity in the electric field.
With our eNMR setup we recently presented a systematic mobility study of the cations and anions in seven different Ionic Liquids (IL).1 The electrophoretic mobilities strongly depend on cation and anion structure, i.e. they increase for the cation type with shorter alkyl chain length. Finally, not only the contribution of each ionic species to the conductivity, but also the degree of correlated ion motion depends on molecular structure.
We further report on multinuclear eNMR studies of Li salt-containing electrolytes, i.e. IL/Li salt mixtures as well as glyme-based solvate ionic liquids. In both types of systems interesting correlations between different ionic species are identified. For example, in binary mixtures of Li salt and IL a negative mobility of Li+ may occur, implying a drift direction opposite to the expectation for a cation. On the other hand, in glyme-based solvate ILs, a strong correlation of Li+ and glyme mobility is an effect of the Li+ coordination by oligoethers. Here, interesting dependencies of ionic correlations on Li salt concentration can be identified.
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.