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 0.12–0.16. 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

Conformations and Vibrational Assignments of the (Fluorosulfonyl)(trifluoromethanesulfonyl)imide Anion in Ionic Liquids

Investigations of the (fluorosulfonyl)­(trifluoromethanesulfonyl)­imide (FTFSI) anion, incorporated in various ionic liquids, by means of density functional theory (DFT) methods and differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Raman techniques are reported in this work. Theoretical studies using DFT methods (B3LYP/6-31G**) show that there are three likely anion geometries (syn, gauche, and anti) separated by less than 3 kJ·mol^–1. The energy barrier to conversion between the anti and syn/gauche conformers is between 10 and 14 kJ·mol^–1 and lower than 10 kJ·mol^–1 for rotations around the SNSF and SNSC dihedral angles, respectively. The FTFSI anion has a characteristic vibration at 730 cm^–1 assigned to the expansion and contraction of the entire anion that is sensitive to ionic interactions with metal cations. DSC, XRD, and Raman studies indicate that an alkali metal salt containing the FTFSI anion, KFTFSI, exists in two crystalline forms. Form II converts to form I via a solid–solid phase transition at 96.9 °C. The FTFSI expansion–contraction mode at 745 cm^–1 in KFTFSI form I shifts to 741 cm^–1 in form II. It can be hypothesized that this shift is due to the presence of different anion geometries or varying ionic interactions in the two crystalline forms

A Combined Theoretical and Experimental Study of the Influence of Different Anion Ratios on Lithium Ion Dynamics in Ionic Liquids

In this paper, we investigate via experimental and simulation techniques the transport properties, in terms of total ionic conductivity and ion diffusion coefficients, of ionic liquids doped with lithium salts. They are composed of two anions, bis­(fluorosulfonyl)­imide (FSI) and bis­(trifluoromethanesulfonyl)­imide (TFSI), and two cations, <i>N</i>-ethyl-<i>N</i>-methylimidazolium (emim) and lithium ions. The comparison of the experimental results with the simulations shows very good agreement over a wide temperature range and a broad range of compositions. The addition of TFSI gives rise to the formation of lithium dimers (Li⁺–TFSI^––Li⁺). A closer analysis of such dimers shows that involved lithium ions move nearly as fast as single lithium ions, although they have a different coordination and much slower TFSI exchange rates