Structure, Energies, and Vibrational Frequencies of Solvated Li+ in

Nov 19, 2015 - Biomedical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. J. Phys. Chem. A , 2016, 120 (19), pp 3079–3087...
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Structure, Energies, and Vibrational Frequencies of Solvated Li+ in Ionic Liquids: Role of Cation Type Faina Dubnikova† and Yehuda Zeiri*,‡,§ †

The Fritz Haber Research Center for Molecular Dynamics, The Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ‡ Division of Chemistry, NRCN, P.O. Box 9001, Beer-Sheva 84190, Israel § Biomedical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel S Supporting Information *

ABSTRACT: This study examines the structure of five ionic liquids all of them containing bis[(trifluoromethyl)sulfonyl]imide (TFSI) as the anion with five different cations: Dimethylammonium, N-propylammonium, N-methyl-1-propylpiperidinium, N-methyl-3-methylpyridinium, and N-methylpyrrolidinium. This study is based on quantum chemical calculations of structure, energetics, and vibrational spectroscopy associated with solutions of Li+ in the five ionic liquids examined. We have shown that the Li−TFSI ion-pair stabilization is 2.5−4 fold larger than those of the ion pairs of five cations with TFSI. A large number of different species containing LikTFSInCtm (Ct represent one of five cations studied, k, n, m = 0−2) were examined in detail. The results suggest that Li−(TFSI)2 is a highly stable species and may be responsible for the transport of Li ions in these ionic liquids. The vibrational analysis suggests that the high stability of the Li−TFSI ion pair is mainly due to Coulomb interaction between the Li ion and two oxygen atoms bound to the two sulfur atoms in the TFSI anion. This O−Li−O bond exhibits stretching and bending modes that may allow monitoring of these ion pairs.

1. INTRODUCTION Lithium batteries have the highest energy efficiency of all existing electrochemical storage systems.1−4 These batteries have reached an advanced state of technological development and seem to be the power source of choice today. However, their widespread implementation as energy storage devices is hindered due to safety concerns surrounding the use of largescale lithium cells. Exothermic reaction of the electrolyte with the electrode materials can develop due to unforeseen events such as short circuits or local overheating. These types of events can result in a rapid increase of the battery temperature that may lead to fire or explosion. The electrochemical stability, nonvolatility, and thermal stability of ionic liquids (IL), described in many studies,1−4 may help to resolve this types of safety issues. The replacement of the conventional, flammable and volatile, organic solutions with IL-based, lithium ion-conducting electrolytes may greatly reduce the risk of thermal runaway. The use of IL can provide the lithium battery with the level of safety that is required for their large-scale application in significant applications. Extensive work in this direction is in progress, and the testing of ionic liquids as a new electrolyte media for future, highly safe, lithium batteries is under way in many laboratories.5−13 However, the results of these studies show large variations due to the use of a wide variety of ionic liquids as electrolytes. There are questions concerning the performance and cost of IL-based electrolytes. However, systems in which the lithium ion is solvated in ILs are intriguing from a basic scientific point of view, and they are also highly important practically. In © XXXX American Chemical Society

electrolytes based on a lithium salt dissolved in an IL electrolyte, the nature of the most important lithium carrying species becomes markedly different from that in the traditional organic liquid electrolytes. In traditional electrolytes, the lithium ion is solvated by neutral solvent molecules that possess electron donating functional groups leading to Lisolvent complexes. In most cases, even at high Li-salt concentration, the preferred coordination number of Li+ ions is often 4. For IL-based electrolytes, there are no neutral solvent molecules present; hence, the situation is quite different. The preferred coordination number of Li+ is still 4, and both anions from the IL and those originating from the lithium salt have to complete the solvation shell. The coordination number of 4 can be reached in different ways, as has been demonstrated in molecular dynamics (MD) simulations.14,15 These calculations suggest various lithium-carrying species including large aggregates. Dominance of triplet or higher multiplicity states of the Li-ion-containing complexes is observed in IR and Raman measurements and are strongly supported by quantum chemical calculations.16−21 However, several studies, such as that of Angenendt and Johansson,9 find no support for the existence of higher order multiplicity complexes, except at very low lithium salt/IL ratios.9,22 Therefore, the triplet formation Special Issue: Ronnie Kosloff Festschrift Received: October 29, 2015 Revised: November 17, 2015

A

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systems considered in the present work share a common anion, bis[(trifluoromethyl)sulfonyl]imide (TFSI). The Li+ carrier in five ILs systems with different cations were considered. The systems investigated are summarized in Table 1.

and its stability in the electrolyte solutions call for further investigated. In addition to the nature of lithium ion carrying species, the large excess of surrounding ions also contribute to the total ion conductivity. Ion-pair formation is an exception when suitable lithium salts are chosen, and ion pairs are only found for high lithium salt/IL ratios. Importantly, triple-ion complexes (cation with two anions) dominate for the most interesting ratios; however, it is also important to note that the nature of these triple-ion complexes can be tailored. The two constituent anions in each triple-ion complex are determined by both the choice of lithium salt and the choice of IL. There are basically two major categories of systems, depending whether the lithium salt and the IL have a common anion or not. If they do have a common anion, only one basic type of triple-ion complex can be obtained, [Li(An)2], but if they do not, the properties and concentrations of the two anions will determine the type of the dominant triple-ion complex, [Li(Ana)2], [Li(Anb)2], or [Li(Ana)(Anb)]. The present study focuses on a series of systems in which the IL and the Li salt have a common anion: Bis[(trifluoromethyl)sulfonyl]imide (TFSI). We employ quantum chemical calculations in the density functional (DFT) level to examine the nature and structure of different possible Li-ion-carrying species when different cations are used for the ILs. It should be emphasized that some of the ionic systems and complexes studied here have been investigated theoretically before. However, the reported studies in the literature were carried out with different computational methods and various basis sets. The present utilizes an identical method of calculation to all systems, hence allowing a more meaningful comparison. Moreover, the data described below can be used to generate a consistent set of force field parameters, which in turn can be used to study dynamic and transport properties of the systems with the possibility of a meaningful comparison between them.

Table 1. List of the Five Cations Considered Together with Their Structure and Abbreviations

For all these cations we performed DFT calculations to reveal the lowest energy structure of various complexes. The species considered were are of the general form: LikAnnCtm where An and Ct represent anion and cation, respectively, and k, n, m are integers in the range 0−3. The presentation of the results below will be subdivided into three subsections: Structure of the lowest energy configurations, vibrational spectra of the various species and the influence of cation variation on some of the complex properties. Lowest Energy Configurations. The energy, enthalpy, and free energy at room temperature were evaluated for the lowest energy configurations of a wide range of species. The TFSI anion was found to have two configurations: cis and trans. Both configurations of TFSI were considered for some of the complexes studied. However, for all the LikAnnCtm species examined the trans-TFSI isomer exhibited a slightly lower energy (up to approximately 3 kcal/mol). Because trans-TFSI has a lower energy than cis-TFSI, in most cases only the results for the trans-TFSI isomer are shown and the “trans-” label will be omitted in most of the remaining of the paper. The results obtained for a large number of LikAnnCtm complexes are listed in Table 2. The corresponding structures of all complexes reported in Table 2 are summarized in Table S1 of the Supporting Information. Careful inspection of the data presented in Table 2 shows a number of general trends. First, the binding between a Li ion and a TFSI anion is very strong, much stronger than the binding of TFSI to the five cations examined. The stabilization energy and free energy for the Li+···(TFSI)n (n = 1−3) complexes are shown in Figure 1. The stabilization energies shown are relative to the energy of the infinitely separated ions. The Li+···TFSI bond is mainly due to the interaction of the Li ion with two oxygen atoms, each bonded to one of the sulfur

2. COMPUTATIONAL METHOD DFT calculations were carried out using the Gaussian-09 package.23 The Becke−Lee−Yang−Parr functional24,25 together with Grimme dispersion correction BLYP-D26 was employed in the simulation reported in this study. This theoretical method together with an empirical dispersion correction is recommended in the literature as a reliable choice for calculations of the type reported here.27−29 Structures of adducts, with and without Li ion, were optimized using several different initial configurations, using the Dunning correlation consistent polarized valence double ξ (cc-pVDZ) basis set. To obtain more accurate energy values, all minimum energy structures found were further optimized using an augmented basis including diffuse functions (aug-cc-pVDZ).30,31 Vibrational frequency calculations were carried out also using the augmented basis set. The reported relative energies of the various adducts considered include the zero point energy (ZPE) correction of the corresponding species. Atomic charges and dipole moments were assigned by using the CHELPG scheme.32 The charge distributions of the different complexes were calculated and plotted using the GaussView Version 5 code.33 3. RESULTS AND DISCUSSION The main focus of the study is on the nature and structure of Li-ion-carrying species in different ILs. The Li-salt and IL B

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The Journal of Physical Chemistry A Table 2. Energy, Free Energy, and Entropy of Various LikAnnCtm Speciesa complex considered cis-TFSI···Li+ trans-TFSI···Li+ TFSI···Li+···TFSI (TFSI)3···Li+

TFSI···(Li+)2 (TFSI)2···(Li+)2

trans-TFSI···DMA DMA···trans-TFSI···DMA DMA···trans-TFSI···LI+

DMA···(trans-TFSI)2···LI+

cis-TFSI···PA trans-TFSI···PA PA···trans-TFSI···PA PA···cis-TFSI···LI+

PA···trans-TFSI···LI+

PA···(trans-TFSI)2···LI+

trans-TFSI···PDM PDM···trans-TFSI···PDM PDM···trans-TFSI···Li+

PDM···(trans-TFSI)2···LI+

trans-TFSI···PP13 PP13···trans-TFSI···LI+

trans-TFSI···PYRO PYRO···trans-TFSI···PYRO PYRO···trans-TFSI···LI+

PYRO···(trans-TFSI)2···LI+

complex stability relative to this decomposition channel cis-TFSI + Li+ trans-TFSI + Li+ TFSI···Li+ + TFSI TFSI + TFSI + Li+ (TFSI)2···Li+ + TFSI (TFSI)···Li+ + TFSI + TFSI TFSI+ TFSI + TFSI + Li+ TFSI···Li+ + Li+ TFSI + Li+ + Li+ TFSI..LI+ + TFSI..LI TFSI···Li+···TFSI + Li+ DMA trans-TFSI + DMA DMA··· trans-TFSI + DMA trans-TFSI···LI+ + DMA trans-TFSI··· DMA + Li+ trans-TFSI + LI+ + DMA trans-TFSI···Li+ + TFSI··· DMA (trans-TFSI)2···LI+ + DMA PA cis-TFSI + PA trans-TFSI + PA PA + trans-TFSI···PA cis-TFSI···Li+ + PA cis-TFSI..PA + Li+ cis-TFSI + Li+ + PA trans-TFSI···Li+ + PA trans-TFSI..PA + Li+ trans-TFSI + Li+ + PA trans-TFSI···Li+ + TFSI···PA (trans-TFSI)2···LI+ + PA PDM trans-TFSI + PDM PDM···trans-TFSI + PDM trans-TFSI···LI+ + PDM trans-TFSI··· PDM + Li+ trans-TFSI + LI+ + PDM trans-TFSI···Li+ + TFSI···PDM (trans-TFSI)2···LI+ + PDM PP13 trans-TFSI + PP13 trans-TFSI···LI+ + PP13 trans-TFSI···PP13 + LI+ trans-TFSI + LI+ + PP13 PYRO trans-TFSI + PYRO PYRO + trans-TFSI···PYRO trans-TFSI···LI+ + PYRO trans-TFSI··· PYRO + Li+ trans-TFSI + LI+ + PYRO trans-TFSI···Li+ + TFSI···PYRO (trans-TFSI)2···LI+ + PYRO

ΔE [kcal/mol]

ΔG [kcal/mol]

ΔS [cal/(K mol)]

−146.1 −149.3 −49.4 −198.7 +16.8 −32.6 −181.8 −54.2 −200.4 −42.7 −142.6

−138.8 −141.5 −36.2 −177.7 +32.4 −3.8 −145.3 −48.1 −187.6 −30.1 −135.4

−27.7 −29.3 −42.5 −71.8 −51.2 −93.1 −122.5 −25.1 −54.3 −40.0 −26.5

−108.5 −38.7 −21.4 −62.6 −170.7 −42.4 −101.5

−97.33 −28.4 −12.0 −55.4 −152.7 −27.9 −89.0

−36.7 −28.4 −31.8 −24.7 −61.1 −47.6 −41.8

−108.0 −108.4 −40.2 −21.8 −62.6 −170.5 −24.5 −65.9 −173.5 −40.8 −99.8

−95.3 −95.4 −28.7 −11.4 −55.0 −150.2 −12.6 −58.7 −154.1 −27.5 −86.8

−43.1 −43.7 −28.7 −43.2 −27.8 −70.7 −38.41 −24.1 −67.7 −42.3 −43.5

−103.4 −38.9 −24.9 −85.5 −176.0 −42.6 −84.0

−77.1 −25.2 −14.1 −78.4 −155.5 −27.7 −67.0

−45.1 −57.9 −39.9 −26.6 −71.5 −49.1 −57.8

−89.9 −26.2 −85.6 −175.5

−77.1 −14.2 −78.6 −155.7

−44.8 −42.2 −25.9 −70.6

−104.4 −35.5 −27.1 −71.9 −174.3 −36.0 −91.0

−90.7 −42.0 −13.7 −64.5 −155.2 −22.0 −76.5

−45.5 −22.6 −44.1 −27.9 −73.4 −45.0 −48.1

The free energy cited corresponds to 25 °C. The optimized structures of the different complexes listed below are presented in Table S1 in the Supporting Information. a

atoms as in the case of a single TFSI and an additional O atom on the second TFSI. The addition of a third TFSI anion leads to destabilization of the complex and its formation is unlikely. The addition of a second Li ion to the ion-pair Li+···TFSI leads to its stabilization by over 50 kcal/mol. Similar bond strength is

atoms. The addition of a second TFSI ion leads to the formation of a stable species. However, the additional stabilization of the Li+···(TFSI)2 complex due to the second anion is only a third of that of the Li+···TFSI bond. The binding in this case is mainly due to the Li-ion interaction with two O C

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Figure 3. Ratio between Li+ and Ct contributions to the complex stability for Li+···TFSI···Ct.

Figure 1. Stabilization energy (filled symbols) and free energy (open symbols) contributions of each anion added to the Li+···(TFSI)n complex. Squares represent the binding of Li+ to the first TFSI, circles to the second TFSI, and triangles to the third TFSI.

in Li+···TFSI···Ct. The binding ratios shown corresponds to the energies associated with the two decomposition routes: transTFSI···Ct + Li+ and trans-TFSI···Li+ + Ct, respectively. The data in Figure 3 clearly show that in the case of Ct = PA and DMA, the two smallest cations considered, the stabilization energy ratio is the smallest and it is larger for the other three cations. This difference is related to the cation size: a smaller Ct exhibits a larger ion-pair stabilization energy, hence the smaller ratio in Figure 3, and the stabilization energy of the ion pair is lower for the larger cations, hence the increasing ratio in Figure 3. This size dependence is expected because the TFSI−Ct binding is mainly due to the Coulomb interaction and it increases for smaller charge separations. The results presented in Table 2 show that the energy required for the decomposition of Li+−(TFSI)2−Ct complexes into two ion pairs: Li+−TFSI and TFSI−Ct is less than half the amount needed to obtain Li+−(TFSI)2 + Ct. This is an additional manifestation of the larger contribution of Li-ion interaction with the anions. Influence of Cation Identity. Let us examine the relative contribution of all cations examined to the binding of the different complexes. The energies and free energies associated with the different decomposition routes of the various LikAnnCtm species are summarized in Figure 4. The results presented in Figure 4 clearly show that the strongest bond is between TFSI and Li ion (black squares). The weakest bonds correspond to Ct = PDM and PP13, the two largest cations considered. The binding to the second cation (red circles) in all cases is much weaker than the binding to the first cation, typically the ratio between the two bonds is approximately 1:4. The dominant contribution of the Li ion to the stability of complexes containing both Li+ and Ct is exhibited in all the cases examined (compare green pointing-up triangles with blue pointing-down triangles and pink stars with purple hexagons). The behavior observed for the free energy values (Figure 4B) is almost identical to that of the total energies suggesting that the entropy contributions at room temperature are quite small. The variations in both ΔE and ΔG observed for the cations correlate very well with their size, a smaller cation is associated with larger contribution to the stabilization of the different complexes. The strong Li+−TFSI binding discussed above, suggests that the species with the Li-ion migration in these ionic liquids are complexes involving Li+ located between two TFSI anions. The much weaker interaction of this type of complex with the

obtained for the formation of a dimer of ion pairs, (Li+··· TFSI)2. The structures of these species, first three rows of Table S1 in the Supporting Information, shows that the Li ion(s) tend to be located between the TFSI anions forming bonds to the nearest oxygen atoms in the anion(s); see structures of Li−(TFSI)2, Li−(TFSI)3, and Li2−(TFSI)2. The charge distribution in three of these Li−TFSI complexes are shown in Figure 2 below. In the Li−TFSI ion pair the Li ion

Figure 2. Charge distribution in Li−TFSI (left), [Li2−TFSI]+ (middle), and [Li−(TFSI)2]− (right) complexes.

clearly forms bonds to the nearest two O atoms (bonded to different S atoms) to form a neutral ion pair (Figure 2 left structure). When two Li ions are present (Figure 2 middle structure) one of the Li ions forms bonds to two O atoms each attached to one of the S atoms while the second Li ion interacts mainly with a single O atom. This difference explains the markedly lower complex stabilization observed upon the addition of second Li+ to the Li−TFSI ion pair. In the case of Li−(TFSI)2 (Figure 2 right structure), the small Li ion is located between the two TFSI anions and forms strong bonds to one of the anions (the right TFSI anion here) as is clear by the reduced negative charge on the right side of the complex charge distribution. Thus, the difference in complex stabilization energy between Li−TFSI and Li−(TFSI)2 seen in Table 2, is clearly manifested in the charge distributions of the two complexes (Figure 2 left and right structures). In all the LikAnnCtm species examined that contain both Li+ and one of the cations, the contribution of the Li ion to the complex stability is much larger than that of the Ct. This is evident from the data presented in Table 2 if different complex decomposition routes are considered. Figure 3 shows the ratio between Li+ and a cation contribution to the complex stability D

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the bond due to the increased separation between the cation and the anion. Simultaneously, for most cations examine here, the decrease in Coulomb contribution to the binding is largely compensated by an increase in the contribution of newly formed hydrogen bonds. As a result, the binding of all Ct− TFSI ion pairs are similar except for the largest cation, PP13, where the binding in the PP13−TFSI ion pair is about 10−15% lower than the corresponding bond obtained for the other Ct− TFSI pairs. The interplay between the Coulomb attraction and hydrogen bonding in the Ct−TFSI ion pair is manifested in the charge distributions of the five ion pairs presented in Figure S1 (left column) in the Supporting Information. The cation−TFSI separation clearly varies with the size of the cation and the orientation of the H atoms to form as many as possible hydrogen bonds can be observed for all ion pairs. The addition of a Li+ to the Ct−TFSI pairs leads to a structure where the TFSI anion is located between the cation and the Li+ while the extra positive charge is distributed almost evenly over the whole complex with slightly higher concentration around the Li. The charge distributions of the Ct−TFSI−Li+ are shown in the right column of Figure S1 in the Supporting Information. The formation of hydrogen bonds in the Ct−TFSI ion pair and the charge polarization induced by the Li+ are expected to influence the vibrational modes associated with H atoms. This effect is discussed in the next subsection. Vibrational Spectra. The calculations performed in the present study included evaluation of both Raman and IR spectra of the different species considered. Examination of the vibrational characteristics of the six ions examined (TFSI and the five cations) exhibits a rich IR and Raman spectra for any one of the ions and complexes considered. The inclusion of Li+ in the various calculations leads to the appearance of new vibrational modes as well as a marked shifts in position of peaks that originate by modes close to the Li-ion binding site. The IR spectra of TFSI, TFSI−Li+, DMA, and DMA−TFSI−LI+ are presented in Figure 5 (DMA was chosen as a representative of the cations considered in this study). The spectra are cut at 1500 cm−1 because TFSI, the species with most pronounced vibrational modes, does not have any peaks at higher frequencies. The main TFSI vibrational bands observed are O−S−O asymmetrical stretch modes near 1350 cm−1, CF3 outof-plane bending and C−F stretching modes around 1250 cm−1, S−N−S asymmetrical stretch and O−S−O symmetrical stretch modes near 1170 cm−1 and near 1080 cm−1, S−N−S bending and CF3 bending modes in the range 770−820 cm−1, O−S−O bending together with CF3 scissoring near 580 cm−1, O−S−O scissoring near 500 cm−1, and low intensity modes at lower frequencies near 200 and 380 cm−1 that correspond to OSON torsion motion. Comparison of our IR spectrum for TFSI (both cis and trans) shows very good agreement with calculated data by Vitucci et al.34 who used B3LYP/6-31G** level of calculation. Good agreement is obtained for both 200− 700 and 700−1050 cm−1 regions where most IR peaks are. Raman spectra for the Li−TFSI ion pair, both experimental and calculated (using B3LYP/6-311+G(d,p)35 level of calculation) in the regions 720−780 and 250−450 cm−1 also are in very good agreement with the results of our calculations. Isolated DMA has two high intensity peaks near 3470 and 3520 cm−1 that split into three main peaks in the DMA−TFSI ion pair near 3090, 3120, and 3270 cm−1. This red shift of hydrogen modes is related to the formation of hydrogen bonds between the H atoms on the nitrogen of the DMA and the O and N atoms in the TFSI. However, for the DMA−TFSI−Li+

Figure 4. Energies (A) and free energies (B) associated with the different decomposition routes of the various LikAnnCtm species. The initial complex and the decomposition route considered is shown in the legend inset.

different cations examined, as is clearly observed in Table 2, supports this suggestion. The results presented in Figure 4 show that formation of a Li−TFSI ion pair results in weakening of the TFSI−Ct interaction dramatically (pointing-up triangles vs squares). Similar but much lesser weakening of the TFSI−Ct bond is also observed for Li−(TFSI) 2−Ct complexes (hexagonal vs squares in Figure 4). These findings may suggest that the Li−(TFSI)2 anions could serve as a main species by which Li+ transport occurs in ionic liquids. Next we turn to examination of the lowest energy configurations obtained for the different LikAnnCtm complexes. The calculated structures are shown in Table S1 of the Supporting Information. As expected, for all cations studied, the positively charged N atom in the cation is oriented toward a pair of O atoms on the TFSI. However, for all cations examined here, the N atom is surrounded by hydrogen atoms or by larger hydrocarbon groups. The steric interaction of the segments attached to the N atoms in both TFSI and all the cations leads to a structure in which the hydrogen-bonded nitrogen side in the cation is pointing toward the oxygen atoms located on the two sides of the N atom in the TFSI anion. Thus, the cation− TFSI binding is due to a combination of Coulomb attraction and hydrogen bonding. An increase in the size of the cation results in reduced contribution of the Coulomb interaction to E

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the environment surrounding the ion pair (i.e., existence of additional TFSI anions, Li+, or other cations). Some of these peaks are located in a frequency range outside the range spanned by most of the TFSI ion peaks, approximately 700− 1400 cm−1. These relatively low frequency, IR and Raman modes are observed for all complexes of the type LikAnnCtm with k, n, m ≥ 1 for all cations considered here. The frequencies at which the two peaks are located vary by a few tens of cm−1 in the ranges described above. In the high frequency range, 2700−3600 cm−1, all the IR and Raman peaks correspond to stretching and bending of the hydrogen atoms in the cations studied, because TFSI does not contain any hydrogens. The inclusion of Li+ results in most cases in minor shifts in the positions of these peaks. For example, in the case of TFSI−PA and TFSI−PYRO new peaks appear at 2977 and 2832 cm−1 that are related to the stretching mode of the N−H against an O atom in the TFSI. These peaks, with high Raman intensity, are both outside the range of most hydrogen related peaks (in the range 3000−3500 cm−1) and hence may be used to monitor the formation and existence of the two ion pairs. In the presence of Li+, the Raman peak characteristic of TFSI−PA pair at 2977 cm−1 is shifted to a higher frequency at 3170 cm−1 while the Raman peak at 2832 cm−1 of the TFSI−PYRO ion pair disappears. These variations are induced by the changes in the charge distribution of TFSI induced by the existence of the Li+ electric field. In the case of DMA two Raman peaks characterize the TFSI−DMA ion pair at 3122 and 3267 cm−1, respectively. The addition of Li+ shifts both peaks to higher frequencies by about 200 cm−1 to 3364 and 3494 cm−1, respectively. All the red and blue shifts and changes in the number of observed peak of the H atoms vibrations are dominated by the nature of the hydrogen bonds formed in these complexes. Inspection of the data in Table 3 shows that the addition of Li+ to each one of the TFSI−Ct ion pairs results in new unique vibrational modes associated with the interaction of the Li ion with the negatively charged groups or atoms in the TFSI−Ct ion pair (i.e., O, N and S atoms). In all cases the O−Li−O bending mode is observed together with various stretching modes related to the Li-ion bond with O, N, or S. In addition to these new peaks, the inclusion of the Li ion also results in a few shifts of higher frequency modes to new frequencies. Most of these shifted modes are associated with N−H stretching and bending.

Figure 5. Calculated IR spectra for (A) TFSI (black) and TFSI−Li+ (green) and (B) TFSI (black) DMA (red) and DMA−TFSI−Li+ (green).

complex only two high intensity peaks are observed in the calculations near 3360 and 3490 cm−1. This blue shift in the H atoms related features when Li+ is included is associated with changes in the hydrogen bonding in the Ct−TFSI pair due to charge polarization by the Li+. These marked shifts in the C−H modes positions manifest the strong interactions between the ions in the two complexes. Inspection of the spectra presented in Figure 5 show that the inclusion of a solvated Li+ is expected to induce substantial shifts in the positions of the ionic liquid IR absorption peaks as well as the observation of new features in the spectra. Table 3 below summarizes the main variations observed in the IR and Raman spectra compared to those of the isolated species or the TFSI−Ct ion pairs due to the addition of a Li ion. The interaction between Li+ and TFSI leads to the formation of a very strong bond between the Li ion and two oxygen atoms each bonded to one of the sulfur atoms. This O−Li−O bond results in two new bands in the Raman spectra, one corresponds to the symmetric stretch (in the range 400−500 cm−1) and the other to a bending mode of the O−Li−O system (in the range 280−380 cm−1). In the IR spectra, in addition to these two modes, one also obtains single Li−O bond stretches and the O−Li−O asymmetrical stretch. These changes allow characterization of the Li−TFSI bond formation. The exact frequency at which these peaks are observed may vary due to

4. CONCLUSIONS This study examined the structure of ionic liquids with bis[(trifluoromethyl)sulfonyl]imide (TFSI) as anion with five different cations: dimethylammonium (DMA), N-propylammonium (PA), N-methyl-1-propylpiperidinium (PP13), N-methyl3-methylpyridinium (PDM), and N-methylpyrrolidinium (PYRO). The study is based on DFT level quantum chemical electronic structure calculations. The main goal was to examine the structure and characteristics of ionic liquid solutions of Li+. The calculations show that the Li ion forms a very strong bond with TFSI and the addition of a second TFSI anion results in a marked stabilization of the complex. The binding in Li−TFSI ion pair is much stronger than all the bonds formed in the TFSI−Ct ion pairs with the five cations studied. This behavior is related to the localized charge on the Li ion as compared to the delocalized charge on the five cations examined (Figure 6) as well as to the shorter bond length in the ion pair as the F

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The Journal of Physical Chemistry A Table 3. Main Changes in Peak Positions and Appearance of New Peaks upon Addition of Li+ to Different Species frequency [cm−1]

IR intensity

Raman activity

frequency [cm−1]

mode assignment

TFSI···Li+ 382 37 2 O−Li−O bending 510 57 6 O−Li−O symmetrical stretching 603 177 0 Li−O single bond stretching 616 203 0 Li−O another single bond stretching 645 137 1 O−Li−O asymmetrical stretching DMA···TFSI 914 23 1 H−N−H rocking (replacing the mode at 813 in isolated DMA) 1701 214 8 H−N−H bending (replacing the mode at 1639 in isolated DMA) 3122 548 40 N−H stretch toward the TFSI direction* 3267 548 68 second N−H stretch toward the TFSI direction* * Replacing the modes at 3465 and 3528 of H−N−H symmetrical and asymmetrical stretch modes respectively DMA···TFSI··· Li+ 330 10 3 O−Li−O bending 404 40 2 Li−S stretch 449 72 0 Li−S stretch 471 56 0 Li−N stretching and N−S−O bending 503 65 1 Li−O stretching and O−S−O bending 686 94 1 Li−S stretching and S−N−S bending 871 34 1 H−N−H rocking (replacing the mode at 813 in isolated DMA) 1669 109 6 H−N−H bending (replacing the mode at 1639 in isolated DMA) 3364 459 89 N−H stretch toward the TFSI direction 3494 101 27 second N−H stretch toward the TFSI direction PA···TFSI 2977 1107 85 N−H stretch in the PA toward the O atom in TFSI (replacing the mode at 3513 in isolated PA) PA···TFSI···Li+ 345 12 2 N−Li−O bending 480 138 2 Li−S stretch 515 63 0 Li−O stretch combined with O−S−O bending 576 36 1 Li−O stretch combined O−S−O bending 3171 765 68 N−H stretch in PA toward the O atom in TFSI (replacing the mode at 3513 in isolated PA)

IR intensity

610

134

392 495 528 540 605 614

55 81 28 25 149 102

3263

68

381 486 532 542 597

50 90 9 24 101

605

136

627 3269

216 11

1588

79

2832

1787

382 484 531 540 594

54 92 11 30 119

602

84

628 3243

264 12

3249

9

Raman activity

mode assignment

PDM···TFSI 0 S−N−S bending toward the PDM direction (replacing the mode at 577 in isolated PDM) PDM···TFSI··· Li+ 1 O−Li−O bending 3 N−Li stretch 0 Li−O stretch combined with CF3 bending 0 Li−O stretch combined with CF3 bending 1 O−Li−O stretch 0 Li−O stretch combined with O−S−O bending PP13···TFSI 132 ring C−H stretch (replacing the mode at 3201 in isolated PP13) PP13···TFSI··· Li+ 1 O−Li−O bending 6 Li−N stretch 1 Li−O stretch combined with CF3 bending 0 Li−O stretch combined with CF3 bending 1 Li−O stretch combined with O−S−O bending 1 Li−O stretch combined with O−S−O bending 0 O−Li−O asymmetrical stretch 115 ring C−H stretch (replacing the mode at 3201 in isolated PP13) PYRO···TFSI 2 H−N−C bending (H oriented toward the TFSI direction) 211 N−H stretch in PYRO toward the N atom in TFSI instead (replacing the mode at 3492 in isolated PYRO) PYRO···TFSI··· Li+ 1 O−Li−O bending 6 Li−N stretch 1 Li−O stretch combined with CF3 bending 0 Li−O stretch combined with CF3 bending 1 Li−O stretch combined with O−S−O bending 1 Li−O stretch combined with O−S−O bending 0 O−Li−O asymmetrical stretch 36 C−H stretch in the CH3 group toward the TFSI direction 62 ring C−H stretch toward the TFSI direction

in these ionic liquids. The charge distribution in the Li− (TFSI)2 is show in Figure 7 (right side). Moreover, the formation of a Li−TFSI complex, Figure 7 (left side), results in weakening of the interaction between the TFSI anion and the surrounding cations examined. This behavior can add to the enhancement of Li+ mobility in the different ionic liquids. Vibrational analysis of the various species considered shows that Li ion forms the Li−TFSI pair by its interaction with two O atoms, each bound to one sulfur. This results in the formation of a new bond, O−Li−O that exhibits stretching and bending modes that can be observed in both IR and Raman spectroscopies. The addition of Li ion to different TFSInCtm complexes leads also to shifts in the frequency of some vibrational modes. These spectral modifications due to the

cation size decreases. The charge distribution in the TFSI anion and the five cations examined are shown in Figure 6. The electron charge in the TFSI is distributed mainly around the four electronegative oxygen atoms, hence localized at the two sides of the anion. In the five cations examined the positive charge is localized mainly around the nitrogen atoms in the cations. Thus, in the case of smaller cations, PA and DMA here, the positive charge around the N atom is located at an edge of the cation and a shorter TFSI−cation can be achieved. As a result, a stronger Coulomb interaction is obtained and the stabilization energy of the TFSI−Ct ion pair is larger. For the small Li+ cation, the highly localized charge on the Li leads to a marked enhancement of the binding in the Li−TFSI pair. Thus, the calculations here suggest that a highly stabilized Li− (TFSI)2 anion plays a dominant role in the mobility of Li ions G

DOI: 10.1021/acs.jpca.5b10588 J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b10588. Lowest energy structures of the complex presented in Table S1, charge distribution presented in Figure S1 and dipole moments of the various complexes presented in Table S2 (PDF)

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

REFERENCES

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Figure 6. Charge distributions in the TFSI anion and the five cations considered in this study.

Figure 7. Charge distribution in the Li−TFSI ion pair and the Li− (TFSI)2 anion.

addition of Li ion can be used to monitor the various species examined in the present study. The shifts in position and splitting of peaks related to H atom vibrations in the different complexes studied are associated with variations in the hydrogen bonds in the systems. Finally, the results of the calculations reported here can and will be used in the development of appropriate and reliable force fields to describe different systems considered in the present study. The force fields have to reliably describe the charge distributions in the various systems. The dipole moments of the various complexes are used to properly define these charge distributions. Hence, we include Table S2 in the Supporting Information, which contains the dipole moments of the various complexes considered here. The force fields developed using the data presented in this study will in turn be used to study the structure, dynamics, and transport in Li+− ionic liquid solutions, in particular, the nature of the species involved in the Li-ion transport in these ionic liquids, and to predict the influence of a cation type on the mobility of the Li+. H

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DOI: 10.1021/acs.jpca.5b10588 J. Phys. Chem. A XXXX, XXX, XXX−XXX