Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid

Abstract

Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N(-), bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions.

Low-frequency Raman spectra and fragility of imidazolium ionic liquids

“…The effect of adding Li + is comparable with the difference in viscosity between ͓emim͔͓BF 4 ͔ and ͓bmmim͔͓BF 4 ͔, but high in LiTf 2 N / ͓bmmim͔͓Tf 2 N͔ does not arise from any modification of imidazolium cation, instead it is due to strong interactions between small Li + cations and Tf 2 N anions. 26 Despite high room temperature viscosity, it is shown below ͑see Fig. 5͒ that I QES of LiTf 2 N / ͓bmmim͔͓Tf 2 N͔ is higher than that of pure ͓bmmim͔͓Tf 2 N͔.…”

“…2 The systems investigated in Ref. 26, namely, solutions of Li + cations in ͓bmmim͔͓Tf 2 N͔, provide a counterexample that low I QES is not simply the result of high viscosity. The viscosity of a 2.0M solution of LiTf 2 N in ͓bmmim͔͓Tf 2 N͔ is ϳ650 cP at room temperature, whereas ϳ 55 cP for pure ͓bmmim͔͓Tf 2 N͔.…”

“…The mixtures ͓bmmim͔͓Tf 2 N͔ / LiTf 2 N and ͓emim͔͓BF 4 ͔ / LiBF 4 are more fragile than the corresponding pure ionic liquids. 26,34 Anions experience a very asymmetric neighborhood in the presence of large imidazolium cations and small Li + cations so strong Li + -anion interactions occur in the mixtures. Despite slowing down the long-time structural relaxation, and the resulting high viscosity at room temperature, the mixtures are fragile because local short-time dynamics might be more temperature dependent at T g .…”

Polarization effects in molecular dynamics simulations of glass-formers Ca(NO3)2⋅nH2O, n=4, 6, and 8

“…35 Therefore, / NE lower than unity means that not all of the diffusive ionic motion are contributing to charge transfer, so that / NE is inversely related to the strength of ionic pairs. 37,38 Figure 14 shows single-particle time correlation functions of the center-of-mass velocity, C v ͑t͒ = ͗v i ͑t͒ . v i ͑0͒͘, calculated with the RIM and FCM for Ca͑NO 3 ͒ 2 ·8H 2 O at 350 K. The oscillatory behavior of C v ͑t͒ is due to the rattling of a given particle within the cages formed by the neighbors.…”

Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study

“…There have been some interesting studies on the Li + solvation and its transport properties in liquids, liquid electrolytes, and different kind of ionic liquids [48][49][50][51][52][53][54][55][56][57][58] . There were also some interesting studies explicitly on the structure and the dynamics of molten lithium nitrate (LiNO 3 ).…”