4 research outputs found

    Complex Nature of Ionic Coordination in Magnesium Ionic Liquid-Based Electrolytes: Solvates with Mobile Mg<sup>2+</sup> Cations

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    The Raman shifts of the TFSI<sup>āˆ’</sup> expansion-contraction mode in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium bisĀ­(trifluoromethanesulfonyl)Ā­imide ionic liquid (IL) electrolytes were analyzed to compare the ionic coordination of magnesium with lithium and sodium. In the Mg<sup>2+</sup>-IL electrolytes, the TFSI<sup>ā€“</sup> anions are found in three different potential energy environments, while only two populations of TFSI<sup>ā€“</sup> are evident in the Na<sup>+</sup>- and Li<sup>+</sup>-IL electrolytes. For Mg<sup>2+</sup>, the high frequency peak component is associated with a TFSI<sup>ā€“</sup> that is in a bidentate coordination with a single metal cation and can therefore be considered a contact ion pair (CIP) solvate. The mid frequency component is attributed primarily to bridging aggregate (AGG) TFSI<sup>ā€“</sup> solvate or a weakly bound monodentate CIP TFSI<sup>ā€“</sup>. The low frequency peak is well-known to be associated with ā€œfreeā€ TFSI<sup>ā€“</sup> anions. The average number of TFSI<sup>ā€“</sup> per Mg<sup>2+</sup> cation (<i>n</i>) is 3 to 4. In comparison, the value of <i>n</i> is 4 at very low concentrations and decreases with increasing salt mole fraction to 2 for Li<sup>+</sup> and Na<sup>+</sup>, where <i>n</i> of Na<sup>+</sup> is larger than that of Li<sup>+</sup> at any given concentration. The results imply the existence of anionic magnesium solvates of varying sizes. The identity of the Mg<sup>2+</sup> charge-carrying species is complex due to the presence of bridging AGG solvates in solution. It is likely that there is a combination of single Mg<sup>2+</sup> solvate species and larger complexes containing two or more cations. In comparison, the primary Li<sup>+</sup> and Na<sup>+</sup> charge-carrying species are likely [LiĀ­(TFSI)<sub>2</sub>]<sup>āˆ’</sup> and [NaĀ­(TFSI)<sub>3</sub>]<sup>2ā€“</sup> in the concentration range successfully implemented in IL-based electrolyte batteries. These solvates result in Mg<sup>2+</sup> cations that are mobile in the IL-based electrolytes as demonstrated by the reversible magnesiation/demagnesiation in V<sub>2</sub>O<sub>5</sub> aerogel electrodes

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

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    Inert polar aprotic electrolytes based on pyrrolidinium bisĀ­(trifluoromethanesulfonyl)Ā­imide ionic liquids were investigated for Mg battery applications. On a molecular scale, there are two TFSI<sup>ā€“</sup> populations coordinating Mg<sup>2+</sup> ions: one in a bidentate coordination to a single Mg<sup>2+</sup> and one in a bridging geometry between two Mg<sup>2+</sup> ions. On average, each Mg<sup>2+</sup> cation is surrounded by three to four TFSI<sup>ā€“</sup> 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<sup>ā€“</sup> and in the conductivity, viscosity, and diffusion behavior at a salt mole fraction of 0.12ā€“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<sup>ā€“</sup>. The mechanism of ion transport likely proceeds via structural diffusion through exchange of the bridging and ā€œfreeā€ TFSI<sup>ā€“</sup> anions within adjacent [Mg<sub><i>n</i></sub>(TFSI)<sub><i>m</i></sub>]<sup>(<i>m</i>āˆ’2<i>n</i>)ā€“</sup> 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

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    Investigations of the (fluorosulfonyl)Ā­(trifluoromethanesulfonyl)Ā­imide (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 kJĀ·mol<sup>ā€“1</sup>. The energy barrier to conversion between the anti and syn/gauche conformers is between 10 and 14 kJĀ·mol<sup>ā€“1</sup> and lower than 10 kJĀ·mol<sup>ā€“1</sup> for rotations around the SNSF and SNSC dihedral angles, respectively. The FTFSI anion has a characteristic vibration at 730 cm<sup>ā€“1</sup> 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 solidā€“solid phase transition at 96.9 Ā°C. The FTFSI expansionā€“contraction mode at 745 cm<sup>ā€“1</sup> in KFTFSI form I shifts to 741 cm<sup>ā€“1</sup> 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

    Crystalline Complexes of Pyr<sub>12O1</sub>TFSI-Based Ionic Liquid Electrolytes

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    This study examines the formation of previously unreported crystalline phases of <i>N</i>-methoxyethyl-<i>N</i>-methylpyrrolidinium bisĀ­(trifluoromethanesulfonyl)Ā­imide (Pyr<sub>12O1</sub>TFSI). The melting point of pristine Pyr<sub>12O1</sub>TFSI, determined by conductivity measurements, is between āˆ’20 and āˆ’17.5 Ā°C. Formation of this crystalline phase is difficult and only occurs under specific conditions. Pyr<sub>12O1</sub>TFSI readily forms 1:1 phases with both NaTFSI and MgĀ­(TFSI)<sub>2.</sub> The results of single crystal structure determinations are presented. The Na<sup>+</sup> crystalline phase provides clear evidence that the Pyr<sub>12O1</sub><sup>+</sup> cation can coordinate some metal ions, but this coordinative interaction does not occur with all metal cations, e.g., Mg<sup>2+</sup>, and in all states of matter, e.g., Na<sup>+</sup>-IL solutions. The TFSI<sup>ā€“</sup> ions are found in two different aggregate solvates in the Pyr<sub>12O1</sub>TFSI:NaTFSI 1:1 phase and in contact ion pair and aggregate solvates in the Pyr<sub>12O1</sub>TFSI:MgĀ­(TFSI)<sub>2</sub> 1:1 phase. The Pyr<sub>12O1</sub>TFSI:MgĀ­(TFSI)<sub>2</sub> crystalline phase gives insight into the local structure of the liquid electrolyte, where it is likely that a maximum of approximately 30% of the total TFSI<sup>ā€“</sup> can likely be coordinated in a bridging geometry, and the rest are in a bidentate coordination geometry. This ratio is determined from both the crystal structure and the Raman spectroscopy results
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