Lithium Ion Transport and Solvation in N-Butyl-N-methylpyrrolidinium

Article pubs.acs.org/JPCC

Lithium Ion Transport and Solvation in N‑Butyl‑N‑methylpyrrolidinium Bis(trifluoromethanesulfonyl)imide− Propylene Carbonate Mixtures Ruben-Simon Kühnel and Andrea Balducci* MEET Battery Research Center & Institute of Physical Chemistry, University of Muenster, Corrensstr. 28/30, 48149 Muenster, Germany ABSTRACT: Ionic liquid−solvent mixtures are electrolytes for electrochemical energy storage devices that can combine low flammability with high conductivity. Here, we report about (lithium) transport properties of a nd Li + solvation in N -butyl-N -methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI)−propylene carbonate mixtures containing LiTFSI. Transport properties are investigated by viscosity, conductivity, and diffusion measurements. Interestingly, Li+ ion diffusion is enhanced the most upon PC addition to PYR14TFSI−LiTFSI. Raman measurements show a decreasing contribution of TFSI− to the coordination of Li+ upon PC addition to the electrolyte while the total degree of ion dissociation goes through a minimum.

■

properties and Li+ ion solvation of IL−solvent mixtures containing a lithium salt is available in the literature. Therefore, we decided to investigate these properties over a broader range of compositions for a model electrolyte containing a wellknown, thermally and electrochemically very stable IL (PYR14TFSI), a common polar electrolyte solvent liquid at room temperature (PC), and LiTFSI as lithium salt. The electrolytes were investigated in terms of viscosity, conductivity, diffusion coefficients, and by Raman spectroscopy.

INTRODUCTION Ionic liquids (ILs) doped with a lithium salt are alternative electrolytes for lithium-ion batteries (LIBs) that offer some advantages compared to conventional liquid electrolytes.1 Because of their ionic nature, ILs display a very low vapor pressure which leads to nonflammability (only the thermal decomposition products are flammable)2 and the possibility of using them in “open” systems like lithium-air batteries.3 Furthermore, some ILs, e.g., the ones containing pyrrolidinium cations and anions of the bis[(perfluoroalkyl)sulfonyl]imide type, are electrochemically more stable than conventional electrolytes and are hence potentially better compatible with high-voltage cathode materials.4 The main drawback of ILbased electrolytes is the rather low Li+ ion conductivity due to the typically relatively high viscosity of ILs as a result of the Coulomb interactions between the ions.5 A lot of research has been devoted to developing ILs with low viscosity. However, so far these ILs could be obtained only at the expense of lower thermal and/or electrochemical stability, and the obtained viscosities are still higher than those of conventional liquid electrolytes.6,7 In the last years, mixtures of conventional solvent-based electrolytes and ILs have been proposed in several works as a solution to overcome the lithium-ion transport limitations of IL-based electrolytes while retaining a lower flammability than that of conventional electrolytes.6,8−13 Knowledge about the (lithium) transport properties of such electrolytes is of great importance for optimizing the composition of the electrolytes. So far, the reported studies on lithium transport properties of IL−solvent mixtures either focused on small amounts of solvents added to an IL-based electrolyte12−15 or did not include the IL-free electrolyte.16 In general, only little experimental data concerning transport © 2014 American Chemical Society

■

EXPERIMENTAL METHODS Electrolyte Preparation. PYR14TFSI was prepared as reported previously.17 LiTFSI (3M, battery grade) was dried under vacuum at 100 °C prior to use; PC (UBE, battery grade) was used as received. Five (1 − x) PYR14TFSI−(x) PC−0.3 M LiTFSI electrolytes were prepared by first mixing PYR14TFSI and PC in the intended ratio and then adding 0.3 M of LiTFSI. Electrolytes with a PC weight fraction x of 0, 0.2, 0.5, 0.8, and 1 were considered for this study. The corresponding molar ratios of Li+ to TFSI− to PC can be found in Table 1. The electrolytes were prepared in an argon-filled glovebox (MBRAUN; H2O and O2 levels, D(Li+). This order was also found for small amounts of the solvents toluene, EC, vinylene carbonate, or tetrahydrofuran added to a similar IL−LiTFSI electrolyte.12 The fact that Li+ was the species with the smallest diffusivity independent of electrolyte composition must be related to the Li+ solvation shell, leading to the biggest effective radius for the Li+-containing species because of the strong interaction between Li+ and TFSI− and/or PC that can be expected. However, Li+ diffusivity also profited the most from PC addition to the electrolyte. For example, the Li+ ion diffusion coefficient increased almost five times by the substitution of 20% of the IL by PC, whereas that of PYR14+ increased only 3.5 times. The relative increase in diffusion coefficient was especially pronounced at low temperatures, in line with the trends found for the effect of solvent addition on the conductivity of the electrolyte. To obtain a deeper understanding of the effect of PYR14TFSI substitution by PC, relative diffusion coefficients with respect to those obtained in PYR14TFSI−0.3 M LiTFSI were calculated for the different ions. Furthermore, apparent transference numbers and Li+ conductivities as well as the degree of ionicity were calculated for the different electrolytes. Figure 3 shows relative diffusion coefficients obtained by dividing the diffusion coefficients for the PC-containing electrolytes by those for PYR14TFSI−0.3 M LiTFSI. Obviously, the Li+ diffusivity increased the most, while the increase was the smallest for PYR14+. TFSI− diffusion increased more than PYR14+ diffusion but less than Li+ diffusion. This is an interesting finding that was previously reported for small

ciDi ∑i ciDi

∑i

ciDi ∑i ciDi

(2)

Here, ti, ci, and Di are the apparent transference number, concentration and diffusion coefficient of ion i, respectively. The ion concentrations were calculated by considering the densities of the electrolytes, which are also reported in Table 1. Furthermore, the diffusion coefficients were interpreted in terms of the Nernst−Einstein model (eq 3). σNMR =

e2 kT

∑ NV,iDi i

(3)

The Nernst−Einstein equation connects the measured diffusion coefficients with the conductivity of the electrolyte assuming that all ions fully contribute to the conductivity of the electrolyte. In this equation, e is the elementary charge, k the Boltzmann constant, T the absolute temperature, and NV,i is the number density of ion i calculated by multiplying the ion concentration with the Avogadro constant. Figure 2 shows the temperature dependence of the selfdiffusion coefficients for the PYR14+, TFSI−, PC, and Li+ species. The solvent PC was always the fastest diffusing species 5744

dx.doi.org/10.1021/jp5005264 | J. Phys. Chem. C 2014, 118, 5742−5748

The Journal of Physical Chemistry C

Article

study. As expected, the Li+ and TFSI− transference numbers steadily increased with the PC weight fraction while the values for PYR14+ decreased. The Li+ transference number increased from 0.023 to 0.037, 0.057, 0.12, and 0.39 by substituting 20%, 50%, 80%, and 100% of PYR14TFSI by PC, respectively. As expected, the latter value is comparable to values reported for other LiTFSI-based organic electrolytes.21 From the application in LIBs point of view, the contribution of Li+ to the conductivity of the electrolyte is an important property. Hence, apparent Li+ conductivities were calculated by multiplying the apparent Li+ transference number with the measured ionic conductivity (Figure 5). In this case, the Figure 3. Ratio of the diffusion coefficients for (1 − x) PYR14TFSI− (x) PC−0.3 M LiTFSI electrolytes by the diffusion coefficients for PYR14TFSI−0.3 M LiTFSI.

amounts of solvents added to an IL-based electrolyte.12 Substitution of 20% of PYR14TFSI by PC for PYR14TFSI−0.3 M LiTFSI led to a larger increase in diffusivity than that derived from the change in viscosity for Li+, while it was smaller for PYR14+. These differences indicate changes in ion aggregation and/or solvation. It will be shown below in this work that the Li+ solvation shell changed with the PC fraction of the electrolyte. Concerning the cation of the IL, the relatively small increase in relative diffusion coefficients indicates an attractive PC−PYR14+ interaction, which was suggested previously.12 Further substitution of PYR14TFSI by PC lead to less pronounced differences in the changes in diffusivity for the different species. Furthermore, the changes in diffusivity became more similar to the changes in viscosity with increasing PC content of the electrolyte. This can be understood by considering that a PC content of only 20 wt % corresponds already to an approximate molar ratio of PYR14TFSI to PC of 1:1. Hence, increasing the PC content from 20 to 50 wt % and beyond corresponds to a relatively strong dilution of the IL in PC. Therefore, it is not surprising that the relative changes in diffusion coefficients are more similar to each other and to the change in viscosity when increasing the PC content from 20 to 50 wt % and beyond. Figure 4 shows apparent transference numbers calculated from the diffusion coefficients considering the concentrations of each ion. The lithium transference number was the lowest independent of the electrolyte composition, which is in part due to the low LiTFSI concentration of 0.3 M chosen for this

Figure 5. Temperature dependence of apparent lithium conductivity for (1 − x) PYR14TFSI−(x) PC−0.3 M LiTFSI electrolytes.

apparent Li+ transference numbers were calculated from diffusion coefficients at 10, 20, 30, and 50 °C that were obtained from VTF fits of the measured diffusion coefficients to account for slightly differing temperatures at which the D values were measured for the different species and electrolytes (especially important for the measurements at around 50 °C). Although these values cannot be taken as true Li+ conductivities because of the fact that the diffusive motion is different from the ion motion in the presence of an applied electric field, this property can serve as a relative measure for the ability of the electrolyte to conduct Li+. Similar to the trend found for Li+ diffusivity, the apparent Li+ conductivity initially strongly increased by the substitution of 20% of PYR14TFSI by PC from 0.032 to 0.13 mS cm−1 (values at 20 °C) while further substitution increased it to a lesser extent. Values of 0.40, 0.82, and 1.2 mS cm−1 were obtained for the electrolytes containing 50%, 80%, and 100% of PC, respectively. These values illustrate once more that substitution of relatively small amounts (e.g., 20 wt %) of an IL by a solvent can significantly improve the performance of the electrolyte. We showed in our previous work that the capacity of LiFePO4 electrodes is significantly improved when switching from PYR14TFSI−LiTFSI to a mixture containing 20% of PC.10 Ion pairing and other forms of ion aggregation are a common phenomenon for ILs. It was shown by several groups that the measured ionic conductivity (σAC) is lower than the one based on the diffusion coefficients calculated with the Nernst− Einstein equation (σNMR). Furthermore, it was suggested to consider the quotient of σAC and σNMR, the so-called ionicity, as a measure for the level of ion aggregation.22−24 High ionicity should correspond to a low level of aggregation. Addition of a lithium salt to an IL usually decreases the ionicity because of

Figure 4. Apparent transference numbers for (1 − x) PYR14TFSI−(x) PC−0.3 M LiTFSI electrolytes at 20 °C. 5745

dx.doi.org/10.1021/jp5005264 | J. Phys. Chem. C 2014, 118, 5742−5748

The Journal of Physical Chemistry C

Article

troscopy. Li+ coordination by TFSI− anions can be investigated by analyzing the strong band at ca. 740 cm−1. This band corresponds to the expansion and contraction of the whole TFSI− anion and is known to shift to higher wavenumbers upon coordination of TFSI− to Li+.25,27 Because of the relatively small energy difference of this band between “free” and coordinated TFSI−, the band representing coordinated TFSI− is often visible only as a shoulder. Figure 7 shows the obtained Raman spectra in the region of this TFSI− band. The contributions of coordinated and “free” TFSI− were obtained by peak deconvolution and are also shown in the graphs. Obviously, the contribution of TFSI− to the coordination of Li+ decreased upon substitution of PYR14TFSI by PC. To quantify the average number of TFSI− anions (n) in the supposed [Li(TFSI)n(PC)m](n−1)− complexes, we multiplied the ratio of total TFSI− (cTFSI−) to Li+ (cLi+) concentration with the fraction of the integrated intensity of the component corresponding to coordinated TFSI− (Icoord) assuming that the Raman scattering coefficient does not change from “free” to coordinated TFSI− (eq 4).28 c − Icoord n = TFSI c Li+ Icoord + Ifree (4)

the high charge density of Li+, which favors ion pairing.23 Figure 6 shows σAC, σNMR, and the degree of ionicity.

Figure 6. Measured ionic conductivity (△), conductivity calculated from self-diffusion coefficients with the Nernst−Einstein equation (○), and ratio of the two (ionicity) (■) for (1 − x) PYR14TFSI−(x) PC−0.3 M LiTFSI electrolytes at 20 °C.

Figure 8 shows the average number of TFSI− anions coordinating one Li+ ion assuming that each TFSI− anion is

Surprisingly, the ionicity initially decreases upon substitution of PYR14TFSI by PC, indicating a higher degree of aggregation (see above). This finding was counterintuitive; it was expected that a polar solvent like PC would effectively reduce the Coulombic interaction between the ions, leading to increased ionicity. When substituting further IL by PC, the ionicity went through a minimum and then increased again. The increasing ionicity when diluting the IL further could be expected, as fewer ions have to be separated by relatively more PC molecules. Raman Spectroscopy. Because of its high charge density, Li+ is strongly interacting with TFSI− anions in PYR14TFSI− LiTFSI.25 In PC−LiTFSI electrolytes, Li+ is preferentially solvated by PC molecules. Only in concentrated solutions does interaction between Li+ and TFSI− become relevant.26 There is some information about Li+ coordination in IL−solvent mixtures available in the literature suggesting a mixed coordination by TFSI− and (polar) solvent molecules.12,15,16 However, to the best of our knowledge, a study on Li+ solvation covering a broader range of mixture compositions including the two binary electrolytes is missing. Therefore, we carried out a preliminary investigation of Li+ solvation in the five considered PYR14TFSI−PC−0.3 M LiTFSI electrolytes via Raman spec-

Figure 8. Average number of TFSI− anions (n) coordinating each Li+ ion for (1 − x) PYR14TFSI−(x) PC−0.3 M LiTFSI electrolytes, derived from the Raman intensities of the characteristic TFSI− band at ca. 740 cm−1.

Figure 7. Raman spectra in the 730−760 cm−1 spectral range of (1 − x) PYR14TFSI−(x) PC−0.3 M LiTFSI electrolytes. Each figure contains the experimental spectrum (circles), the sum of the fitting curves (thin black lines), and the fitting curves representing “free” (thick black lines) and coordinated (thick gray lines) TFSI−. 5746

dx.doi.org/10.1021/jp5005264 | J. Phys. Chem. C 2014, 118, 5742−5748

The Journal of Physical Chemistry C

Article

TFSI− anions present in the first coordination shell of Li+ for the supposed [Li(TFSI)n(PC)m](n−1)− complexes decreased with the PC fraction of the electrolyte from n = 2 for PYR14TFSI−LiTFSI to 1.2 for the 1:1 mixture with PC and to zero for PC−0.3 M LiTFSI. Hence, the effective charge of solvated Li+ initially decreased with the PC content of the electrolyte and increased again when Li+ was coordinated by less than one TFSI− anion. This finding was also in line with the degree of ionicity, which indicated that ion aggregation initially increased upon PYR14TFSI substitution by PC, went through a maximum, and decreased again.

coordinating only one Li+ ion. However, it has to be kept in mind that there is an ongoing debate on the geometry of the [Li(TFSI)n](n−1)− complexes present in ionic liquids. Some authors reported bidentate Li+ coordination by four oxygen atoms of two TFSI− anions; others reported monodentate (four oxygen atoms of four TFSI− anions) or mixed mono- and bidentate coordination depending on the LiTFSI concentration of the electrolyte.28,29 The situation is further complicated by the cis−trans isomerism of TFSI−.28 However, this work does not aim to contribute to this discussion. Therefore, the number of TFSI− anions (n) per Li+ cation derived from the Raman measurements using eq 4 should be considered only as an average number and not in terms of the only existing “real” number of TFSI− present in the first coordination shell of Li+ for the considered electrolytes. The complexity of Li+ solvation in liquid electrolytes is high, and most likely different complexes, longer and shorter lived ones, exist. For the solvent-free electrolyte, n = 2 was found within the error of the fit, in line with the values reported in the literature.30 In the absence of the IL, no contribution of TFSI− to the coordination of Li+ was found, in line with Wang et al.26 The mixtures showed decreasing n with increasing PC weight fraction of the electrolyte. When the PC weight fraction was 0.2, 0.5, and 0.8, the value of n was 1.2, 0.5, and 0.1, respectively. Together with the decrease in n, the peak representing TFSI− coordination to Li+ shifted to smaller wavenumbers. This indicates not only fewer TFSI− anions present in the Li+ solvation shell of the mixtures but also a weakened interaction of the remaining TFSI− anions and Li+. Relating n to the molar fraction of TFSI− with respect to the sum of the TFSI− and half the PC concentration (assuming that two PC molecules substitute one TFSI− anion and that each PC molecule coordinates one Li+ ion), which is, e.g., about 0.7 for the electrolyte containing PC in a weight fraction of 0.2 (see Table 1), indicates a small preference for Li+ coordination by PC for the mixtures. The reduced number of TFSI− anions present in the Li+ solvation shell should correspond to an increased coordination by solvent molecules. PC was reported to coordinate Li+ through the skeletal oxygen atoms, whereas other groups reported coordination through the carbonyl oxygen atom.26,31 A corresponding Raman band that is sensitive to Li+ coordination also exists: the skeletal bending mode of PC at ca. 712 cm−1.26 Although this peak had a small shoulder corresponding to coordinated PC for the PC−0.3 M LiTFSI electrolyte, peak deconvolution could not be carried out with a reasonable error because of partial overlap of this band with the stronger TFSI− band. Nevertheless, mixed Li+ coordination by TFSI− and PC for the PYR14TFSI−PC−0.3 M LiTFSI mixtures can be reasonably assumed from the decreasing number of TFSI− coordinated to Li+ with increasing PC weight fraction of the electrolyte. In the literature, it was estimated for PC−LiTFSI electrolytes that each Li+ ion is coordinated by 4−5 PC atoms.26 Hence, it can be expected that this number is lower for the mixtures containing PYR14TFSI.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Guinevere Giffin for helpful discussions concerning the Raman measurements. The authors thank the University of Münster and the Ministry of Innovation, Science and Research of North Rhine-Westphalia (MIWF) within the project “Superkondensatoren und LithiumIonen-Hybrid-Superkondensatoren auf der Basis ionischer Flüssigkeiten” and the Bundesministerium für Bildung and Forschung (BMBF) within the project IES (Contract 03EK3010) for the financial support.

■

REFERENCES

(1) 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. 2014, 7, 232−250. (2) Diallo, A.-O.; Morgan, A. B.; Len, C.; Marlair, G. An innovative experimental approach aiming to understand and quantify the actual fire hazards of ionic liquids. Energy Environ. Sci. 2013, 6, 699−710. (3) Monaco, S.; Soavi, F.; Mastragostino, M. Role of Oxygen Mass Transport in Rechargeable Li/O2 Batteries Operating with Ionic Liquids. J. Phys. Chem. Lett. 2013, 4, 1379−1382. (4) Gao, X.-W.; Feng, C.-Q.; Chou, S.-L.; Wang, J.-Z.; Sun, J.-Z.; Forsyth, M.; MacFarlane, D. R.; Liu, H.-K. LiNi0.5Mn1.5O4 spinel cathode using room temperature ionic liquid as electrolyte. Electrochim. Acta 2013, 101, 151−157. (5) Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567−5580. (6) Arbizzani, C.; Gabrielli, G.; Mastragostino, M. Thermal stability and flammability of electrolytes for lithium-ion batteries. J. Power Sources 2011, 196, 4801−4805. (7) Zhou, Q.; Henderson, W. A.; Appetecchi, G. B.; Montanino, M.; Passerini, S. Physical and Electrochemical Properties of N-Alkyl-Nmethylpyrrolidinium Bis(fluorosulfonyl)imide Ionic Liquids: PY13FSI and PY14FSI. J. Phys. Chem. B 2008, 112, 13577−13580. (8) McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. Electrochemical Properties of Imidazolium Salt Electrolytes for Electrochemical Capacitor Applications. J. Electrochem. Soc. 1999, 146, 1687−1695. (9) Guerfi, A.; Dontigny, M.; Charest, P.; Petitclerc, M.; Lagacé, M.; Vijh, A.; Zaghib, K. Improved electrolytes for Li-ion batteries: Mixtures of ionic liquid and organic electrolyte with enhanced safety and electrochemical performance. J. Power Sources 2010, 195, 845−852. (10) Kühnel, R.-S.; Böckenfeld, N.; Passerini, S.; Winter, M.; Balducci, A. Mixtures of ionic liquid and organic carbonate as electrolyte with improved safety and performance for rechargeable lithium batteries. Electrochim. Acta 2011, 56, 4092−4099.

■

CONCLUSIONS Substitution of PYR14TFSI by PC in PYR14TFSI−0.3 M LiTFSI had a pronounced effect on the mobility of the ions. Interestingly, Li+ diffusion profited the most from this dilution while the increase in PYR14+ diffusivity was limited, probably because of an attractive interaction involving PC. The change in Li+ diffusivity could be related to a change in the Li+ solvation shell. Raman measurements showed that the average number of 5747

dx.doi.org/10.1021/jp5005264 | J. Phys. Chem. C 2014, 118, 5742−5748

The Journal of Physical Chemistry C

Article

(11) Kim, K.; Cho, Y.-H.; Shin, H.-C. 1-Ethyl-1-methyl piperidinium bis(trifluoromethanesulfonyl)imide as a co-solvent in Li-ion batteries. J. Power Sources 2013, 225, 113−118. (12) Bayley, P. M.; Lane, G. H.; Rocher, N. M.; Clare, B. R.; Best, A. S.; MacFarlane, D. R.; Forsyth, M. Transport properties of ionic liquid electrolytes with organic diluents. Phys. Chem. Chem. Phys. 2009, 11, 7202−7208. (13) Deshpande, A.; Kariyawasam, L.; Dutta, P.; Banerjee, S. Enhancement of Lithium Ion Mobility in Ionic Liquid Electrolytes in Presence of Additives. J. Phys. Chem. C 2013, 117, 25343−25351. (14) Le, M. L. P.; Cointeaux, L.; Strobel, P.; Leprêtre, J.-C.; Judeinstein, P.; Alloin, F. Influence of Solvent Addition on the Properties of Ionic Liquids. J. Phys. Chem. C 2012, 116, 7712−7718. (15) Bayley, P. M.; Best, A. S.; MacFarlane, D. R.; Forsyth, M. The effect of coordinating and non-coordinating additives on the transport properties in ionic liquid electrolytes for lithium batteries. Phys. Chem. Chem. Phys. 2011, 13, 4632−4640. (16) Hardwick, L. J.; Holzapfel, M.; Wokaun, A.; Novák, P. Raman study of lithium coordination in EMI-TFSI additive systems as lithium-ion battery ionic liquid electrolytes. J. Raman Spectrosc. 2007, 38, 110−112. (17) Kühnel, R.-S.; Balducci, A. Comparison of the anodic behavior of aluminum current collectors in imide-based ionic liquids and consequences on the stability of high voltage supercapacitors. J. Power Sources 2014, 249, 163−171. (18) Stilbs, P. Fourier transform pulsed-gradient spin-echo studies of molecular diffusion. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1−45. (19) Fox, E. T.; Paillard, E.; Borodin, O.; Henderson, W. A. Physicochemical Properties of Binary Ionic Liquid−Aprotic Solvent Electrolyte Mixtures. J. Phys. Chem. C 2013, 117, 78−84. (20) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (21) Hayamizu, K.; Aihara, Y.; Arai, S.; Martinez, C. G. PulseGradient Spin-Echo 1H, 7Li, and 19F NMR Diffusion and Ionic Conductivity Measurements of 14 Organic Electrolytes Containing LiN(SO2CF3)2. J. Phys. Chem. B 1999, 103, 519−524. (22) Noda, A.; Hayamizu, K.; Watanabe, M. Pulsed-Gradient Spin− Echo 1H and 19F NMR Ionic Diffusion Coefficient, Viscosity, and Ionic Conductivity of Non-Chloroaluminate Room-Temperature Ionic Liquids. J. Phys. Chem. B 2001, 105, 4603−4610. (23) Nicotera, I.; Oliviero, C.; Henderson, W. A.; Appetecchi, G. B.; Passerini, S. NMR Investigation of Ionic Liquid−LiX Mixtures: Pyrrolidinium Cations and TFSI− Anions. J. Phys. Chem. B 2005, 109, 22814−22819. (24) MacFarlane, D. R.; Forsyth, M.; Izgorodina, E. I.; Abbott, A. P.; Annat, G.; Fraser, K. On the concept of ionicity in ionic liquids. Phys. Chem. Chem. Phys. 2009, 11, 4962−4967. (25) Castriota, M.; Caruso, T.; Agostino, R. G.; Cazzanelli, E.; Henderson, W. A.; Passerini, S. Raman Investigation of the Ionic Liquid N-Methyl-N-propylpyrrolidinium Bis(trifluoromethanesulfonyl)imide and Its Mixture with LiN(SO2CF3)2. J. Phys. Chem. A 2005, 109, 92−96. (26) Wang, Z.; Gao, W.; Huang, X.; Mo, Y.; Chen, L. Spectroscopic studies on interactions and microstructures in propylene carbonate LiTFSI electrolytes. J. Raman Spectrosc. 2001, 32, 900−905. (27) 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. (28) Lassègues, 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. (29) Borodin, O.; Smith, G. D.; Henderson, W. Li+ Cation Environment, Transport, and Mechanical Properties of the LiTFSI Doped N-Methyl-N-alkylpyrrolidinium+TFSI− Ionic Liquids. J. Phys. Chem. B 2006, 110, 16879−16886. (30) Umebayashi, Y.; Mitsugi, T.; Fukuda, S.; Fujimori, T.; Fujii, K.; Kanzaki, R.; Takeuchi, M.; Ishiguro, S.-I. Lithium Ion Solvation in Room-Temperature Ionic Liquids Involving Bis-

(trifluoromethanesulfonyl) Imide Anion Studied by Raman Spectroscopy and DFT Calculations. J. Phys. Chem. B 2007, 111, 13028− 13032. (31) Johansson, P.; Edvardsson, M.; Adebahr, J.; Jacobsson, P. Mixed Solvent and Polymer Coordination in PAN and PMMA Gel Polymer Electrolytes Studied by Ab Initio Calculations and Raman Spectroscopy. J. Phys. Chem. B 2003, 107, 12622−12627.

5748

dx.doi.org/10.1021/jp5005264 | J. Phys. Chem. C 2014, 118, 5742−5748