Solvation Structure and Dynamics of Li+ in Ternary Ionic Liquid–Lithium Salt Electrolytes (original) (raw)
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The Journal of Physical Chemistry B, 2007
The solvation structure of the lithium ion in room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMI + TFSI-) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP + TFSI-) has been studied by Raman spectroscopy and DFT calculations. Raman spectra of EMI + TFSIand BMP + TFSIcontaining Li + TFSIover the range 0.144-0.589 and 0.076-0.633 mol dm-3 , respectively, were measured at 298 K. A strong 744 cm-1 band of the free TFSIion in the bulk weakens with increasing concentration of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSIions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSIions. It has been established in our previous work that the TFSIion involves two conformers of C 1 (cis) and C 2 (trans) symmetries in equilibrium, and the dipole moment of the C 1 conformer is significantly larger than that of the C 2 conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C 1-TFSI-) 2 ]-, [Li(C 1-TFSI-)(C 2-TFSI-)]-, and [Li(C 2-TFSI-) 2 ]were examined using the theoretical DFT calculations. It is concluded that the C 1 conformer is more preferred to the C 2 conformer in the vicinity of the lithium ion.
Batteries
Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li–PF6, Li–BF4, and Li–TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation–anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation–anion combination and of different fractions of lithium in the solutions to assess their relative ...
Physical chemistry chemical physics : PCCP, 2016
Herein, we discuss the study of solvation dynamics of lithium-succinonitrile (SN) plastic crystalline electrolytes by ultrafast vibrational spectroscopy. The infrared absorption spectra indicated that the CN stretch of the Li(+) bound and unbound succinonitrile molecules in a same solution have distinct vibrational frequencies (2276 cm(-1)vs. 2253 cm(-1)). The frequency difference allowed us to measure the rotation decay times of solvent molecules bound and unbound to Li(+) ion. The Li(+) coordination number of the Li(+)-SN complex was found to be 2 in the plastic crystal phase (22 °C) and 2.5-3 in the liquid phase (80 °C), which is independent of the concentration (from 0.05 mol kg(-1) to 2 mol kg(-1)). The solvation structures along with DFT calculations of the Li(+)-SN complex have been discussed. In addition, the dissociation percentage of lithium salt was also determined. In 0.5 mol kg(-1) LiBF4-SN solutions at 80 °C, 60% ± 10% of the salt dissociates into Li(+), which is bound...
Lithium solvation in dimethyl sulfoxide-acetonitrile mixtures
We present molecular dynamics simulation results pertaining to the solvation of Li+ in dimethyl sulfoxide-acetonitrile binary mixtures. The results are potentially relevant in the design of Li-air batteries that rely on aprotic mixtures as solvent media. To analyze effects derived from differences in ionic size and charge sign, the solvation of Li+ is compared to the ones observed for infinitely diluted K+ and Cl− species, in similar solutions. At all compositions, the cations are preferentially solvated by dimethyl sulfoxide. Contrasting, the first solvation shell of Cl− shows a gradual modification in its composition, which varies linearly with the global concentrations of the two solvents in the mixtures. Moreover, the energetics of the solvation, described in terms of the corresponding solute-solvent coupling, presents a clear non-ideal concentration dependence. Similar nonlinear trends were found for the stabilization of different ionic species in solution, compared to the ones exhibited by their electrically neutral counterparts. These tendencies account for the characteristics of the free energy associated to the stabilization of Li+Cl−, contact-ion-pairs in these solutions. Ionic transport is also analyzed. Dynamical results show concentration trends similar to those recently obtained from direct experimental measurements
We employ molecular dynamics (MD) simulation and experiment to 12 investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrroli-13 diniumbis(trifluoromethylsufonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrroli-14 dinium bis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium 15 boron tetrafluoride ([EMIM][BF]), as a function of Li-salt mole fraction (0.05 # xLi+16 # 0.33) and temperature (298 K # T # 393 K). Structurally, Li+ is shown to be solvated 17 by three anion neighbors in [pyr14][TFSI] and four anion neighbors in both 18 [pyr13][FSI] and [EMIM][BF4], and at all levels of xLi+ we find the presence of lithium 19 aggregates. Pulsed field gradient spin-echo NMR measurements of diffusion and 20 electrochemical impedance spectroscopy measurements of ionic conductivity are made 21 for the neat ionic liquids as well as 0.5 molal solutions of Li-salt in the ionic liquids. Bulk 22 ionic liquid properties (density, diffusion, viscosity, and ionic conductivity) are obtained 23 with MD and show excellent agreement with experiment. While the diffusion exhibits a 24 systematic decrease with increasing xLi+, the contribution of Li+ to ionic conductivity increases until reaching a saturation doping 25 level of xLi+ = 0.10. Comparatively, the Li+ conductivity of [pyr14][TFSI] is an order of magnitude lower than that of the other 26 liquids, which range between 0.1 and 0.3 mS/cm. Our transport results also demonstrate the necessity of long MD simulation 27 runs ($200 ns) required to converge transport properties at room temperature. The differences in Li+ transport are reflected in 28 the residence times of Li+ with the anions (!Li/−), which are revealed to be much larger for [pyr14][TFSI] (up to 100 ns at the 29 highest doping levels) than in either [EMIM][BF4] or [pyr13][FSI]. Finally, to comment on the relative kinetics of Li+ transport 30 in each liquid, we find that while the net motion of Li+ with its solvation shell (vehicular) significantly contributes to net diffusion 31 in all liquids, the importance of transport through anion exchange (hopping) increases at high xLi+ and in liquids with large32 anions
The journal of physical chemistry. B, 2008
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 show...
Coordination and interactions in a Li-salt doped ionic liquid
2014
We report on the coordination and interactions in the LiTFSI doped ionic liquid PyR 14 TFSI over a large concentration range, 0.01 ≤ x ≤ 0.4, using Raman spectroscopy. We find that the concentration dependence of the average number of TFSI anions coordinating to one Li-ion (N TFSI/Li) can be divided into three regimes. For low concentrations, x ≤ 0.05, we find that a large number TFSI anions coordinate each Li-ion, N N 2. The number decreases with increasing salt concentration and the interaction between the Li-ion and the TFSI anions is rather weak in this concentration range. At intermediate concentrations, 0.1 ≤ x ≤ 0.2, the number of TFSI anions coordinating each Li-ion (N TFSI/Li) is~2 pointing towards the formation of [Li(TFSI) 2 ] − ionic clusters. At higher concentrations, x N 0.2, N TFSI/Li decreases further indicating the transition to more complex structures with Li-ions bridging TFSI anions. We also show that the evolution of the microscopic structure as a function of Li-salt concentration is mirrored in the behaviour of macroscopic properties such as the ionic conductivity and the glass transition temperature, which also show a crossover in the same concentration range.