Chelating Additives Reversing the Lithium Migration Direction in Ionic Liquid Electrolytes

Binary electrolytes of Li salt in ionic liquid (IL) are interesting systems for battery application. Typical for these systems is a strong Li-anion coordination causing vehicular Li+ transport in negatively charged Li-anion clusters and negative Li+ transference numbers. We investigate the influence of the additives tetrahydrofuran, monoglyme and triglyme on the Li+ migration behavior in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (EMImTFSA) based mixtures via electrophoretic NMR. When adding coordinating additives, we partly observe a reversal of the lithium migration direction and an electrophoretic drift of the neutral molecule. Using the same O:Li ratio, this effect is strongly depending on the chelating ability of the respective additive. Strongly coordinating additives form a chelate with the Li ion and render its drift velocity and its transference number positive, while the nonchelating additive tetrahydrofuran has a weaker effect on the Li drift. By choice of a suitable additive, it is therefore possible to decompose negatively charged lithium-anion clusters while maintaining ionic liquid-like properties. These findings provide a mechanistic explanation for the beneficial effect of additives on Li+ transport in IL-based battery cells.