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

Abstract

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 0.12–0.16. 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_<i>n</i>(TFSI)_<i>m</i>]^{(<i>m</i>−2<i>n</i>)–} 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