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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 Mg 2+ ions: one in a bidentate coordination to a single Mg 2+ and one in a bridging geometry between two Mg 2+ ions. On average, each Mg 2+ 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 n (TFSI) m ( m −2 n )– 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.
Jeremias et al. (2014) studied this question.