Communication: Investigation of ion aggregation in ionic liquids and their solutions with lithium salt under high pressure

Abstract

X-ray scattering measurements were utilized to probe the effects of pressure on a series of ionic liquids, N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr-TFSI) (A = 3, 6, and 9), along with mixtures of ionic liquid and 30 mol. % lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. No evidence was found for crystallization of the pure ionic liquids or salt mixtures even at pressures up to 9.2 GPa. No phase separation or demixing was observed for the ionic liquid and salt mixtures. Shifts in the peak positions are indicative of compression of the ionic liquids and mixtures up to 2 GPa, after which samples reach a region of relative incompressibility, possibly indicative of a transition to a glassy state. With the application of pressure, the intensity of the prepeak was found to decrease significantly, indicating a reduction in cation alkyl chain aggregation. Additionally, incompressibility of the scattering peak associated with the distance between like-charges in the pure ionic liquids compared to that in mixtures with lithium salt suggests that the application of pressure could inhibit Li coordination with TFSI to form Li[TFSI] complexes. This inhibition occurs through the suppression of TFSI in the trans conformer, in favor of the smaller cis conformer, at high pressures.

Interplay of Structure and Dynamics in Lithium/Ionic Liquid Electrolytes: Experiment and Molecular Simulation

“…2,7,53,54 This peak is slightly shifted towards larger Q upon Li + salt addition (1.52 Å -1 for f = 0 and 1.57 Å -1 for f = 0.5) without modification of its shape and intensity. This contrasts with what is observed in other systems such as LiTFSI/Pyrrolidinium TFSI systems where a shift towards smaller Q was measured upon Li ion addition (this effect being related to the size of Li(TFSI) 2-anion 55 ). As already mentioned, the smaller peak at smaller Q is very sensitive to the amount of Li salt being added to the electrolyte.…”

Mesoscopic structural organization in fluorinated pyrrolidinium-based room temperature ionic liquids

“…More recently a series of Molecular Dynamics studies by Canongia Lopes and coworkers appeared [29][30][31][32], proposing, on the basis of computational work, a scenario where alkyl domains and fluorous ones tend to segregate from the charged three-dimensional network. Although not focusing on fluorinated anions that lead to the formation of fluorous domains, several studies have been dedicated to the exploration of structural properties in pyrrolidinium-based ILs, merging diffraction and computational techniques [33][34][35][36][37][38].…”

Communication: Anion-specific response of mesoscopic organization in ionic liquids upon pressurization

“…The interest toward this feature is witnessed by the wealth of studies addressing its nature and dependence upon chemical nature of the ions, [25][26][27][28][29][30] temperature, 13,15,31 mixture composition, [32][33][34][35][36][37][38][39][40][41] and, only recently, pressure. [42][43][44][45] High pressure studies are an important tool to explore the details of microscopic and mesoscopic correlations in complex materials. Pressure changes allow investigating intermolecular interactions, by inducing density changes without introducing the chaotic perturbation that temperature changes induce.…”

“…Pressure changes allow investigating intermolecular interactions, by inducing density changes without introducing the chaotic perturbation that temperature changes induce. As a matter of fact, while temperature dependence studies of morphology in RTILs have been reported in the past, 13,15,31 the pressure dependence of the mesoscopic organization in aprotic RTILs has been explored by means of experimental X-ray studies [42][43][44] and simulations [45][46][47][48][49][50] 42 Therein the progressive position shift and amplitude vanishing of the low Q peak centred at ca. 3 nm −1 had been observed.…”

“…S4 of the supplementary material, we show the pressure dependence of polar/apolar contributions to the computed X-ray and neutron scattering patterns for both [C 8 shows a similar behavior for both peaks at Q 2 and Q 3 that shift toward higher Q values, upon pressurization. 43 Figure 3 shows the pressure dependence of the intramolecular distance distribution function (ddf) between the ring nitrogen atom bearing the octyl chain and the terminal carbon of the octyl chain. A decrease of the amplitude of the large distance peaks can be observed, witnessing a progressive curling of the octyl chain; however, the change is not as large (Fig.…”