Conformations and Vibrational Assignments of the (Fluorosulfonyl

Article pubs.acs.org/JPCC

Conformations and Vibrational Assignments of the (Fluorosulfonyl)(trifluoromethanesulfonyl)imide Anion in Ionic Liquids Guinevere A. Giffin, Nina Laszczynski, Sangsik Jeong, Sebastian Jeremias, and Stefano Passerini* Institute of Physical Chemistry and MEET Battery Research Center, University of Muenster, Corrensstrasse 28, 48149 Muenster, Germany S Supporting Information *

ABSTRACT: Investigations of the (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI) anion, incorporated in various ionic liquids, by means of density functional theory (DFT) methods and differential scanning calorimetry (DSC), Xray 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 kJ·mol−1. The energy barrier to conversion between the anti and syn/gauche conformers is between 10 and 14 kJ·mol−1 and lower than 10 kJ·mol−1 for rotations around the SNSF and SNSC dihedral angles, respectively. The FTFSI anion has a characteristic vibration at 730 cm−1 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 solid−solid phase transition at 96.9 °C. The FTFSI expansion−contraction mode at 745 cm−1 in KFTFSI form I shifts to 741 cm−1 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.

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INTRODUCTION Room-temperature ionic liquids, i.e., salts that exist in a liquid state at room temperature, are a unique class of materials that are under focus as electrolyte components for lithium batteries. Ionic liquid (IL)-based electrolytes have many desirable properties such as good thermal and chemical stability, low melting point, and negligible volatility.1,2 These properties make them a good alternative to organic liquid electrolytes despite slightly lower conductivities and higher viscosities that can limit the rate capabilities of IL-based batteries.1,2 Two anions that have received significant attention are bis(t rifluoromethan esulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI). TFSI-based electrolytes have a higher thermal and electrochemical stability than those containing FSI, but FSI-based electrolytes have a lower viscosity and higher conductivity.3 In addition, FSI is a better solid− electrolyte interface former than TFSI.4 However, both anions have a tendency to form crystalline complexes in some IL/salt mixtures, which limits the useful temperature range of the electrolyte mixtures.4−6 To reduce the overall anion symmetry, which decreases the likelihood of the formation of crystalline phases, and to take advantage of the benefits derived from both anions, a relatively new anion has been proposed.7,8 This anion, (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI), has one half that is like FSI and the other half like TFSI. The structures of the three anions are shown in Figure 1. Quantum chemical calculations have suggested that FTFSI has a lower © 2013 American Chemical Society

Figure 1. Structures of FTFSI, TFSI, and FSI.

oxidative stability than either TFSI or FSI and that a LiFTFSI ion pair dissociation energy is between that of LiFSI and LiTFSI.9 LiFTFSI has a lower melting point than either LiTFSI or LiFSI and in fact has one of the lowest melting points of lithium salts with anions having a molecular weight below 400.10 As a molten salt, LiFTFSI has a transport number close to one (0.94) and has recently been tested as an intermediatetemperature electrolyte for a solvent-free lithium battery.11 In Received: August 27, 2013 Revised: October 21, 2013 Published: October 22, 2013 24206

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Figure 2. Optimized geometries of the FTFSI anion.

ILs containing FTFSI, the properties are generally an intermediate between those of TFSI and FSI ILs.8 However unlike ILs containing the latter symmetric anions, FTFSI ILs remain amorphous below −100 °C.8 It is well-known that both the TFSI and FSI anions exist in two different conformations: one with C2 symmetry where the CF3/F moieties are on opposite sides of the anion (trans), and another with C1 symmetry where the CF3/F are on the same side of the anion (cis).12−15 In both cases, the C2 rotamer is the low-energy structure but the two geometries are separated by less than 5 kJ·mol−1.14,15 Spectroscopic studies and crystal structure analysis have determined that both conformers are present in the liquid and solid states.16−18 Spectroscopy has also been extensively used to examine the ionic interactions involving the anion in the IL electrolyte mixtures.18−20 However, this type of study requires a good knowledge of the vibrational assignments of the interacting species. This work uses density functional theory (DFT) studies to determine the geometries that exist for the FTFSI anion and to calculate the theoretical vibrational spectrum. These results are used in the assignment of the experimental vibrational bands of the FTFSI anion in several pyrrolidinium-based ILs. In addition, the existence of polymorphic crystalline phases in potassium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (KFTFSI) are investigated with differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), and Raman spectroscopy.

The KFTFSI salt (Provisco CS Ltd., Brno, Czech Republic) was dried for 3 days at 10−8 Pa (turbomolecular pump) at room temperature. The water content was 20 kJ·mol−1) are encountered likely due to the steric effects occurring when the CF3 and SO2F moieties are in close proximity. FTFSI Vibrational Spectrum. The theoretical vibrational spectra, which were calculated for the three low-energy geometries, and the experimental spectra of three pyrrolidinium ILs containing the FTFSI anion are shown in Figure 4. The calculated vibrational frequencies and mode assignments can be found in Tables S1−S3 in the Supporting Information. A few striking differences are seen in the band frequencies of the three rotamers. The most notable is in the intense band between 691 and 700 cm−1 (unscaled). This mode, which is attributed to the expansion and contraction of the entire FTFSI anion, is comparable to the characteristic TFSI mode located at 713 and 716 cm−1 (calculated)16 for the C2 and C1 rotamers, respectively. The FTFSI mode is shifted to lower frequencies, and the separation between the conformers is larger. In both cases, the anti (C2) geometry has the higher frequency. The trend in the frequency separation is generally consistent with the calculated frequencies published for the FSI anion, but the positions of the conformers are reversed; i.e., C1 has a higher frequency than C2.17 However, care should be taken in making a direct comparison of the published FSI data17 and the TFSI16 or FTFSI data because different DFT functionals and basis sets have been used. There is a broad band at 730 cm−1 in the experimental spectrum of the FTFSI ILs, broader than that in the spectrum of TFSI ILs. The breadth of the band, which is representative of the large frequency difference between the FTFSI rotamers, suggests that the FTFSI anion may be found in all three geometries in the ILs. In TFSI, the expansion−contraction mode is particularly sensitive to the anion coordination in solvate structures.30 It is expected that the same will be true for FTFSI. The frequency of 730 cm−1 will correspond to the “free anion” (FTFSI in a solvent-separated ion pair), and any solvate species where the FTFSI is coordinated to metal cations (contact ion pairs or aggregates) should occur at higher frequencies. It should be noted that in Pyr12O1FTFSI this peak is broader and shifted to slightly lower frequencies due to a Pyr12O1+ mode that underlies the more intense FTFSI band. At frequencies lower below 700 cm−1, there are several small differences between the three calculated spectra. The most striking cases are the pattern of the four bands around 280−350 cm−1 and the three bands centered at 380−410 cm−1. These peaks are also found in the spectra of TFSI and are sensitive to anion conformation. Studies on the conformation evolution of the anion with temperature perturbation focus on these

chosen so that the geometries and vibrational frequencies would be directly comparable to those published for the TFSI anion by Herstedt et al.16 All calculations were made using Gaussian 09.27 The potential energy surface was constructed by varying one of either the SNSF or SNSC dihedral angle in 10° steps while holding the other constant. The calculated Raman spectra were obtained by applying Gaussian functions with a full width at half-maximum (fwhm) of 5 cm−1 to the calculated frequencies and intensities. The vibrational mode assignments were made by animating each mode in GaussView.28

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RESULTS AND DISCUSSION FTFSI Geometry. The geometry optimization of the starting structures resulted in three FTFSI geometries: “syn”, “gauche”, and “anti” rotamers. The optimized geometries are shown in Figure 2, and the Cartesian coordinates can be found in the Supporting Information. Of the 37 initial structures, the syn, gauche, and anti conformers were found as the optimized geometry 21, 2, and 14 times, respectively. The anti conformation can be likened to the C2 conformers of both the TFSI and FSI anions.12,13,15,16 The syn conformer of FTFSI is more like the C1 form of FSI than that of TFSI, which has a gauche conformation. The difference between the C1 conformers of TFSI (gauche) and FSI (syn) can explain the presence of the additional low-energy geometry for FTFSI. It seems that rotation of the CF3 group about the NS bond results in both the syn and gauche geometries in addition to the anti rotamer. It is not correct to express the FTFSI geometry by point group notation because in fact all three rotamers have C1 symmetry due to the inherent asymmetry of the anion. The syn form of FTFSI has an energy that is 2.9 kJ·mol−1 higher than the anti conformer, while the gauche form is 1.1 kJ·mol−1 higher. This small energy difference implies that the anion will likely exist as a mix of conformers in the IL, where interconversion should easily occur. The conversion pathways between the conformers can be examined by the determination of the energy barriers to rotation and the investigation of the potential energy surface (PES). The PESs of both FSI and TFSI have been previously published by at least two different groups.12,15,29 The general overview of the FTFSI PES, shown in Figure 3, is more similar

Figure 3. Potential energy surface as a function of the SNSF and SNSC dihedral angles for the FTFSI anion. The cross-sections corresponding to the angles in the optimized syn, gauche, and anti rotamers can be found in Figure S1 of the Supporting Information. 24208

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Figure 4. (a) Calculated Raman spectra for the optimized geometries of the FTFSI anion. The spectra were obtained by applying Gaussian functions with a fwhm of 5 cm−1 to the calculated frequencies and intensities. (b) Raman spectra of three FTFSI-containing ILs.

Table 1. Experimental Vibrational Frequencies of FTFSI− in Pyrrolidinium-Based ILs

bands.13,31,32 Similar studies have been done using bands in the same region for FSI.14 One important point to consider in such studies is that peak fitting of this region is particularly challenging due to the overlapping of bands from each rotamer and in some instances the superposition of bands from the IL cations, e.g., the Pyr14 cation. In the experimental spectra, at least five distinct contributions can be seen in the group of bands around 280−350 cm−1. At the high-frequency end of this range, the shoulder at 338 cm−1 is likely an indicator of the syn conformer while the peak at 348 cm−1 is probably attributable to the gauche and/or anti geometries. The bands at ca. 380− 410 cm−1 may also be useful to track conformer changes, but this is not immediately clear from the spectra of the ILs as they are broader and more overlapped than those at 280−350 cm−1. Finally, there is an additional band between 615 and 630 cm−1 in the calculated gauche and anti spectra. However, in the experimental spectra the intensity of these peaks is so low that in a practical sense these bands are likely to be of little use to distinguish between rotamers. A proposed assignment of the experimental spectra is given in Table 1. These assignments were made using the previously published assignments for TFSI33 and FSI17,34 combined with the assignments of the calculated spectra given in Tables S1−S3 of the Supporting Information. The bands associated with the cations have not been included. The peaks in the spectra attributed to the anion are very comparable in all three ILs, and the only real differences are derived from the different cations. The similarity of the FTFSI contributions suggests that there is little difference in the FTFSI environment in the different ILs. Polymorphs of KFTFSI. The existence of more than one anion geometry introduces the possibility that there can be polymorphic crystalline phases in the solid state. A study on alkali metal FSI salts has demonstrated this point.17 KFTFSI has been studied here to determine if the same effect occurs with the FTFSI anion. The DSC, powder XRD, and Raman spectra of KFTFSI are shown in Figures 5, 6, and 7, respectively. The first heating cycle of the pristine material has a single endothermic peak at 101.6 °C (peak maximum). The crystalline phase that melts in this event will be referred to as form I. In the second heating cycle, two peaks are clearly evident, one at 96.9 °C and one at 101.5 °C. The first peak may be attributed to a solid−solid phase transition, which suggests the presence of a second crystalline phase (form II).

freqa (rel intensb)

assignmentc

161 (6) 177 (11) 210 (1) 283 (55) 307 (40) 324 (41) 337 (17) 348 (32) 404 (14) 467 (10) 530 (10) 538 (11) 554 (8) 572 (13) 606 (2) 641 (2) 729 (64) 761 (30) 815 (13) 1152 (12) 1184 (32) 1238 (68) 1342 (12) 1373 (9)

δ(SNS) δ(SNS) τ(SO2) + ω(CF3) ω/τ(SO2) + ω(CF3) ω/τ(SO2) ω(CF3) + ω/τ(SO2) τ(SO2): “syn” conformer τ(SO2): “trans/gauche” conformer τ/δ(SO2) + ω(CF3) δ(SO2F) δas(CF3) + δ(SO2)/δ(SO2F) δas(CF3) + δ(SO2) δas(CF3) δas(CF3) + δ(SO2) δ(SO2) + δas(CF3) δ(SNS) expansion−contraction of FTFSI− δs(CF3) + ν(SF) ν(SF) + νs(SNS) νs(SO2) + νas(SNS) νs(SO2) + νs(CF3) νas(CF3) νas(SO2) νas(SO2)

Frequencies in cm−1. bRelative intensities as compared to the most intense band at 2973 cm−1. cThe abbreviations denote the following: ν, stretch; δ, bend; ω, wag; τ, twist; s, symmetric; as, asymmetric. a

The formation of the two crystalline phases was examined by varying the cooling rate. One sample was quenched from the melt and another was cooled from the melt at a rate of 10 °C· min−1. In the quenched sample, there is only a single endothermic event with a peak maximum of 101.1 °C. The small temperature decrease is likely due to the presence of a disordered fraction or small amount of form II, which act as impurities and slightly reduce the melting point of form I. In the c10 sample, there is a smaller endothermic peak at 94.9 °C, which gives evidence of the solid−solid phase transition and the existence of crystalline form II below that temperature. A similar trend was seen in NaFSI, which undergoes a solid−solid 24209

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Single-crystal X-ray diffraction studies of NaFSI indicated that while both crystalline phases belong to the same space group, the anion geometries and the overall crystal packing are different.17 The cis FSI conformer is found in form II, while the trans FSI conformer is found at 113K and cis−trans disorder at 298 K in form I.17 In this study, powder XRD rather than single-crystal XRD is used to investigate if KFTFSI has two crystalline forms. A comparison of the diffraction patterns of the pristine and c10 samples, which show significant differences, supports the idea that there are two distinct crystalline phases. Most notably, the highest intensity reflection is located at 9.1 2θ in the pristine material (form I) and at 8.7 2θ in c10 (form II). There are also several other differences in the diffraction pattern. The pattern of the quenched sample appears to have some contributions from both crystalline phases. This supports the idea derived from the DSC measurements that a small amount of form II acts as an impurity and reduces the melting point of form I. Raman spectroscopy is sensitive not only to the anion conformation but also to the crystalline form. In the solid state, the symmetry influencing the vibrations must be the symmetry of the crystal.35 Therefore, without knowing the space group, site group symmetries and number of molecules per unit cell, it is not possible to conclusively assign all of the peaks in the Raman spectrum.35 As a result, caution should be taken when drawing conclusions about the geometry of the FTFSI anion in the crystal simply from the Raman spectra. There are a couple of interesting points that can be deduced from the Raman spectra of KFTFSI shown in Figure 7. Many bands in the pristine sample are shifted in frequency or not present in the c10 sample, while some new bands have appeared at other frequencies. Most noticeable are the disappearance of bands at 1326 and 406 cm−1 (pristine), the appearance of bands at 1319 and 393 cm−1 (c10), and the separation of the band at 179 cm−1(pristine) into two bands at 169 and 185 cm−1 (c10). These spectral changes imply that the two samples have different crystalline phases (pristine = form I; c10 = form II) as a result of the thermal treatment, which is in agreement with the DSC and XRD data. It is not immediately apparent if these crystalline phases contain different anion geometries or simply have different unit cells. The spectrum of the quenched sample is similar to that of form I (pristine) but also contains contributions characteristic of form II (c10). This result is also consistent with the idea that the quenched sample is primarily crystal form I, but contains small amounts of form II. The spectrum of the c10 sample was collected again 14 days after the initial measurement. The new spectrum (c10 + 14d) more closely resembles that of the quenched or pristine sample than that of the original c10, which suggests that crystalline form II is a metastable phase and slowly converts to form I over time. The possibility that the conversion from form II to form I was hastened by heating from the laser during the Raman measurement cannot be ruled out. The peak found at 730 cm−1 in the ILs (Figure 4b), which corresponds to free FTFSI anions, is shifted to 741−745 cm−1 in KFTFSI. The shift to higher frequencies is the result of ionic interactions between K+ and FTFSI as contact ion pairs or aggregate solvates. The peak position is lower for crystalline form II (c10) than for form I (pristine). This shift may be due to one of the two following effects or may be a combination of both: (1) The FTFSI anion is in a different geometry in form II (syn or gauche) than in form I (anti); (2) there is a weaker ionic interaction between K+ and FTFSI− in crystalline form II

Figure 5. DSC profiles of KFTFSI.

Figure 6. Powder X-ray diffraction patterns of KFTFSI.

Figure 7. Raman spectra of KFTFSI (c10, cooled at 10 °C·min−1; c10 + 14d, spectrum collected 14 days after c10 measurement).

phase transition at 375K (form II to form I) and then melts at 391 K (form I to liquid).17 24210

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(3) Appetecchi, G. B.; Montanino, M.; Passerini, S. Ionic liquid-based electrolytes for high energy, safer lithium batteries. ACS Symp. Ser. 2012, 1117, 67−128. (4) Paillard, E.; Zhou, Q.; Henderson, W. A.; Appetecchi, G. B.; Montanino, M.; Passerini, S. Electrochemical and Physicochemical Properties of Py14FSI-Based Electrolytes with LiFSI. J. Electrochem. Soc. 2009, 156, A891−A895. (5) Henderson, W. A.; Passerini, S. Phase Behavior of Ionic Liquid− LiX Mixtures: Pyrrolidinium Cations and TFSI− Anions. Chem. Mater. 2004, 16, 2881−2885. (6) Zhou, Q.; Fitzgerald, K.; Boyle, P. D.; Henderson, W. A. Phase Behavior and Crystalline Phases of Ionic Liquid-Lithium Salt Mixtures with 1-Alkyl-3-methylimidazolium Salts. Chem. Mater. 2010, 22, 1203−1208. (7) Matsumoto, H.; Terasawa, N.; Umecky, T.; Tsuzuki, S.; Sakaebe, H.; Asaka, K.; Tatsumi, K. Low Melting and Electrochemically Stable Ionic Liquids Based on Asymmetric Fluorosulfonyl(trifluoromethylsulfonyl)amide. Chem. Lett. 2008, 37, 1020−1021. (8) Reiter, J.; Jeremias, S.; Paillard, E.; Winter, M.; Passerini, S. Fluorosulfonyl-(trifluoromethanesulfonyl)imide Ionic Liquids with Enhanced Asymmetry. Phys. Chem. Chem. Phys. 2013, 15, 2565−2571. (9) Scheers, J.; Jónsson, E.; Jacobsson, P.; Johansson, P. Novel Lithium Imides; Effects of -F, -CF3, and -CN Substituents on Lithium Battery Salt Stability and Dissociation. Electrochemistry 2012, 80, 18−25. (10) Kubota, K.; Nohira, T.; Hagiwara, R.; Matsumoto, H. Thermal Properties of Alkali (Fluorosulfonyl)(trifluormethylsulfonyl) amides. Chem. Lett. 2010, 39, 1303−1304. (11) Kubota, K.; Matsumoto, H. Investigation of an Intermediate Temperature Molten Lithium Salt Based on Fluorosulfonyl(trifluoromethylsulfonyl)amide as a Solvent-Free Lithium Battery Electrolyte. J. Phys. Chem. C 2013, 117, 18829−18836. (12) Canongia Lopes, J. N.; Shimizu, K.; Pádua, A. A. H.; Umebayashi, Y.; Fukuda, S.; Fujii, K.; Ishiguro, S.-i. Potential Energy Landscape of Bis(fluorosulfonyl)amide. J. Phys. Chem. B 2008, 112, 9449−9455. (13) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y .; Is hi guro, S. Con for mat ion al Equilibr ium of Bis (trifluoromethanesulfonyl) Imide Anion of a Room-Temperature Ionic Liquid: Raman Spectroscopic Study and DFT Calculations. J. Phys. Chem. B 2006, 110, 8179−8183. (14) Fujii, K.; Seki, S.; Fukuda, S.; Kanzaki, R.; Takamuku, T.; Umebayashi, Y.; Ishiguro, S. Anion Conformation of Low-Viscosity Room-Temperature Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(fluorosulfonyl) Imide. J. Phys. Chem. B 2007, 111, 12829−12833. (15) Johansson, P.; Gejji, S. P.; Tegenfeldt, J.; Lindgren, J. The Imide Ion: Potential Energy Surface and Geometries. Electrochim. Acta 1998, 43, 1375−1379. (16) Herstedt, M.; Smirnov, M.; Johansson, P.; Chami, M.; Grondin, J.; Servant, L.; Lassègues, J. C. Spectroscopic Characterization of the Conformational States of the Bis(trifluoromethanesulfonyl)imide Anion (TFSI−). J. Raman Spectrosc. 2005, 36, 762−770. (17) Matsumoto, K.; Oka, T.; Nohira, T.; Hagiwara, R. Polymorphism of Alkali Bis(fluorosulfonyl)amides (M[N(SO2F)2], M = Na, K, and Cs). Inorg. Chem. 2012, 52, 568−576. (18) Zhou, Q.; Boyle, P. D.; Malpezzi, L.; Mele, A.; Shin, J.-H.; Passerini, S.; Henderson, W. A. Phase Behavior of Ionic Liquid-LiX Mixtures: Pyrrolidinium Cations and TFSI − AnionsLinking Structure to Transport Properties. Chem. Mater. 2011, 23, 4331−4337. (19) Han, S.-D.; Allen, J. L.; Jonsson, E.; Johansson, P.; McOwen, D. W.; Boyle, P. D.; Henderson, W. A. Solvate Structures and Computational/Spectroscopic Characterization of Lithium Difluoro(oxalato)borate (LiDFOB) Electrolytes. J. Phys. Chem. C 2013, 117, 5521−5531. (20) Lassegues, J.-C.; Grondin, J.; Aupetit, C.; Johansson, P. Spectroscopic Identification of the Lithium Ion Transporting Species in LiTFSI-Doped Ionic Liquids. J. Phys. Chem. A 2009, 113, 305−314.

than in form I. Although it may be desirable to assign the geometry of the FTFSI anion in the crystalline phases on the basis of the frequency of the 741−745 cm−1 peak, the crystal structures for form I and form II must be determined to definitively associate a particular FTFSI anion geometry with a crystalline phase.

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CONCLUSION Results of DFT calculations (B3LYP/6-31G**) indicate that there are three likely geometries of the FTFSI anion that are separated by less than 3 kJ·mol−1. The energy barrier to conversion between the anti and syn/gauche conformers is 10− 14 kJ·mol−1 and lower than 10 kJ·mol−1 for rotations around the SNSF and SNSC dihedral angles, respectively. The vibrational spectrum was calculated for each rotamer and was used in combination with the previously published assignments for TFSI and FSI to assign the FTFSI modes in the experimental Raman spectra of three ILs. The band at 730 cm−1 is assigned to the expansion and contraction of the entire FTFSI anion. This band, like its TFSI analogue, is sensitive to ionic coordination. A DSC, powder XRD, and Raman study of KFTFSI indicates that the salt exists in two crystalline phases. Form II converts to form I via a solid−solid phase transition at 96.9 °C. The FTFSI expansion−contraction mode at 745 cm−1 in form I shifts to 741 cm−1 in form II. It can be hypothesized that this shift could be due to different anion geometries or ionic interaction strengths in the two crystalline forms.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing Cartesian coordinates of the optimized syn, gauche, and anti conformers from the DFT calculations (B3LYP/6-31G**) and calculated vibrational frequencies (unscaled), intensities, and mode assignments for the three optimized geometries, figure showing potential energy surface cross-sections corresponding to the dihedral angles in the syn, gauche, and anti conformers, and text giving the full author list for ref 27. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by BMBF (Bundesministerium fuer Bildung und Forschung) within the project “MEET Hi-END Materialien und Komponenten für Batterien mit hoher Energiedichte” (Foerderkennzeichen: 03X4634A).

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REFERENCES

(1) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621−629. (2) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A. Energy Applications of Ionic Liquids. Energy Environ. Sci. [Online Early Access]. 2013, DOI: 10.1039/c3ee42099j. Published Online: http://dx.doi.org/10.1039/C3EE42099J. 24211

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp408565b | J. Phys. Chem. C 2013, 117, 24206−24212