Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis(trifluoromethanesulfonyl)imide-Based Ionic Liquid Electrolytes

Inert polar aprotic electrolytes based on pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids were investigated for Mg battery applications. On a molecular scale, there are two TFSI populations coordinating Mg2+ ions: one in a bidentate coordination to a single Mg2+ and one in a bridging geometry between two Mg2+ ions. On average, each Mg2+ cation is surrounded by three to four TFSI anions. The electrolytes, in general, remain amorphous far below ambient conditions, which results in a wide useable temperature range in practical devices. There is a change in the ratio of bidentate:bridging TFSI and in the conductivity, viscosity, and diffusion behavior at a salt mole fraction of 0.120.16. At concentrations above this threshold, there is a more dramatic decrease of the diffusion coefficients and the conductivity with increasing salt concentration due to slower exchange of the more strongly coordinated bidentate TFSI. The mechanism of ion transport likely proceeds via structural diffusion through exchange of the bridging and free TFSI anions within adjacent [Mg-n(TFSI)(m)]((m-2n)) clusters and exchange of bidentate anions via a bidentate to bridging mechanism. The vehicular mechanism likely makes only a small contribution. At concentrations above approximately 0.16 mole fraction, the structural diffusion is more closely related to the tightly bound bidentate anions.