J. Phys. Chem. B 2008, 112, 2991-2995
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Cation-Anion Interactions in 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate-Based Ionic Liquid Electrolytes Christopher M. Burba,*,† Nathalie M. Rocher,‡ Roger Frech,§ and Douglas R. Powell§ Department of Natural Sciences, Northeastern State UniVersity, Tahlequah, Oklahoma 74464-2302, Department of Materials Engineering, Monash UniVersity, Clayton, Victoria 3800, Australia, and Department of Chemistry and Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019-3051 ReceiVed: August 15, 2007; In Final Form: December 7, 2007
An important step in developing ionic-liquid-based electrolytes for lithium rechargeable batteries is obtaining a molecular-level understanding of the ionic interactions that occur in these systems. In this study, 1-ethyl3-methylimidazolium trifluoromethansulfonate ([C2mim]CF3SO3) is complexed with LiCF3SO3, and the local structures of the CF3SO3- and [C2mim]+ ions are investigated with infrared and Raman spectroscopy. The isolation and subsequent refinement of a Li[C2mim](CF3SO3)2 crystal provides further insight into the structure of the [C2mim]CF3SO3-LiCF3SO3 solutions. Minor changes are observed in the infrared and Raman spectra of dilute [C2mim]CF3SO3-LiCF3SO3 solutions compared to pure [C2mim]CF3SO3. However, a suspension of very small Li[C2mim](CF3SO3)2 crystallites forms at a solution composition of [C2mim]CF3SO3:LiCF3SO3 ) 10:1 (mole ratio), placing an upper limit on the solubility of LiCF3SO3. Essentially no changes are observed in the vibrational modes of the [C2mim]+ cations over the entire range of LiCF3SO3 compositions studied, suggesting that the addition of these compounds does not significantly perturb the local structure of the [C2mim]+ cations. The salt used in this study has a common anion with the ionic liquid; thus, the ion cloud surrounding the [C2mim]+ ions, which must be primarily composed of CF3SO3- anions, is not significantly altered with the addition of LiCF3SO3.
Introduction Room-temperature ionic liquids are becoming an increasingly popular class of solvents for a number of diverse applications.1 Most room-temperature ionic liquids consist of a bulky cation (usually a pyridinium, imidazolium, or quaternary ammonium ion derivative) and an anion that possesses a highly delocalized charge (e.g., AlCl4-, PF6-, CF3SO3-, N(CF3SO2)2-, etc.). The resulting compounds have sufficiently low lattice energies to exist in a molten state at room temperature. In large part, the success of ionic liquids can be attributed to their low vapor pressures and flammabilities, making them ideal replacements for volatile organic solvents. The tremendous synthetic versatility available in designing constituent ions allows researchers to tailor other important solvent properties of the ionic liquid (e.g., degree of hydrophobicity or hydrophilicity, viscosity, conductivity, and electrochemical window). Thus, an ionic liquid can be designed to meet the needs of a particular application. In recent years, increased attention has been focused on using ionic liquids as solvents for electrolytes used in fuel cells, lithium rechargeable batteries, and electrochemical capacitors. Lithium rechargeable batteries traditionally employ either a nonaqueous liquid electrolyte or a solid-state polymer electrolyte. Most liquid electrolyte formulations consist of dissolved lithium salts in a mixture of volatile organic solvents (e.g., ethylene carbonate, dimethyl carbonate, etc.),2,3 whereas LiN(CF3SO2)2 dissolved in poly(ethylene oxide) is an example of * To whom correspondence should be addressed. E-mail: burba@ nusok.edu. † Northeastern State University. ‡ Monash University. § University of Oklahoma.
a polymer electrolyte. A principle goal in lithium battery research is to enhance battery safety by eliminating flammable components while simultaneously increasing the overall capacity of the cell. Polymeric electrolytes are especially interesting in this regard because the flammable organic solvents and the heavy metal casings used to contain liquid electrolytes are eliminated. Although this strategy does improve safety, commercialization of solid-state polymer electrolytes is largely hindered by their very low room-temperature conductivities.4 Ionic liquids have been suggested for both classes of lithium battery electrolytes. In one approach, appropriate lithium salts are dissolved in an ionic liquid and the resulting electrolyte might be useful as a drop-in replacement for traditional liquid electrolytes. The high ionic conductivities, low vapor pressures, and low flammabilities make ionic-liquid-based electrolytes ideal substitutes for the highly flammable organic solvents, thereby increasing safety. A different strategy is to incorporate an ionic liquid into a solid-state polymer electrolyte. Shin et al. used this approach to platicize poly(ethylene oxide) with an ionic liquid to boost the ionic conductivity of the electrolyte.5,6 The resulting film delivered good conductivities near room temperature while maintaining reasonable mechanical properties. The nature of the ionic species present in a polymer electrolyte is thought to have an important effect on the ionic conductivity.7-9 However, the contribution of the different ionic species to the ionic conductivity has been controversial, and research is still very active in this area. A key step in developing ionic-liquid-based electrolytes is to understand how lithium salts interact with the constituents of an ionic liquid on a molecular level. In this regard, vibrational spectroscopy is extremely useful for probing ionic interactions because the vibrational modes of an ion are very sensitive to its local structure (i.e., the immediate
10.1021/jp076577l CCC: $40.75 © 2008 American Chemical Society Published on Web 02/14/2008
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potential energy environment surrounding the ion).10-17 In this study, infrared and Raman spectra of LiCF3SO3 dissolved in [C2mim]CF3SO3 are measured to elucidate the fundamental ion-ion interactions within these solutions. Both [C2mim]CF3SO3 and LiCF3SO3 are important candidates for lithium rechargeable battery applications. The vibrational modes of the CF3SO3anion have been extensively investigated for a variety of systems, and those studies provide a good starting point for exploring the ionic liquid solutions.12,14,18-20 Experimental Methods Lithium trifluoromethanesulfonate (LiCF3SO3) and 1-ethyl3-methylimidazolium trifluoromethanesulfonate ([C2mim]CF3SO3) were obtained from Aldrich. Hereafter, the trifluoromethanesulfonate anion is abbreviated as “triflate.” The [C2mim]CF3SO3 precursor was used as received, while LiCF3SO3 was dried at 120 °C under vacuum for 48 h. All of the compounds were stored and manipulated under an argon atmosphere (VAC, e1 ppm H2O). Solutions were prepared by mixing weighed amounts of [C2mim]CF3SO3 with LiCF3SO3, and each solution was stirred for a minimum of 24 h prior to spectroscopic analysis. Compositions are reported as mole ratios of the [C2mim]+ and Li+ cations ([C2mim]CF3SO3:LiCF3SO3). Crystals suitable for X-ray crystallography were isolated from a 2:1 [C2mim]CF3SO3-LiCF3SO3 sample that was heated to approximately 70 °C under vacuum and then slowly cooled back to room temperature over ∼72 h. A colorless, needle-shaped crystal (0.38 × 0.09 × 0.03 mm) was selected for X-ray analysis.21 Intensity data were collected at 100(2) K on a Bruker diffractometer with an APEX CCD detector.22,23 The structure was solved by direct methods and refined by full-matrix least squares based on F2.24 A complete description of the crystal structure, including all calculated bond lengths and angles, is provided as Supporting Information. Infrared spectra were collected using a Bruker IFS 66v Fourier transform infrared (FTIR) spectrometer (KBr beam splitter and DTGS detector). The crystalline sample was prepared with standard KBr pellet techniques, and the spectrum was recorded under vacuum. Liquid samples were placed between two ZnSe windows and measured under dry air purge. All spectra were recorded between 500 and 4000 cm-1 with a 1 cm-1 resolution. FT-Raman spectroscopy was performed with an FRA 106/S mounted on a Bruker Equinox 55 spectrometer. Samples were sealed inside thin NMR tubes under an argon atmosphere. The 1064 nm line of a Nd:YAG laser was used as the excitation source, and data were recorded at 2 cm-1 resolution. Results and Discussion [C2mim]CF3SO3-LiCF3SO3 Crystal. The [C2mim]CF3SO3-LiCF3SO3 crystal forms a monoclinic unit cell in the P21/c space group with four asymmetric units in the cell (Z ) 4). The formula unit for the crystal is Li[C2mim](CF3SO3)2. An extended unit cell of the Li[C2mim](CF3SO3)2 crystal is provided in Figure 1. Two symmetrically inequivalent triflate anions are identified in the Li[C2mim](CF3SO3)2 crystal structure, with each anion coordinating two different lithium ions. The [C2mim]+ cations form parallel stacks along the crystallographic a axis and do not directly coordinate the triflate anions in the crystal structure. The ethyl group of the [C2mim]+ cation adopts a nonplanar conformation in the Li[C2mim](CF3SO3)2 crystal, similar to most ionic liquid crystals that contain the [C2mim]+ cation.25 The lack of significant imidazolium-triflate interactions is in contrast to the [C2mim]CF3SO3 crystal, where
Figure 1. Unit cell representation of the Li[C2mim](CF3SO3)2 crystal projected along the c axis. Individual atoms are denoted by different symbols and colors (carbon, black circle; nitrogen, blue circle with right-to-left slash (/); oxygen, red circle with left-to-right slash (\); fluorine, green solid circle; lithium, purple circle with “×”). Hydrogen atoms are not included for clarity.
Figure 2. Li[C2mim](CF3SO3)2 crystal structure emphasizing the coordination environment of the lithium ions. Hydrogen atoms are not included for clarity.
TABLE 1: Li-O Bond Lengths (Å) for the Li[C2mim](CF3SO3)2 Crystal Li(1)-O(1BA) Li(1)-O(3A)
1.906(11) 1.932(11)
Li(1)-O(2B) Li(1)-O(1AA)
1.934(12) 1.937(12)
significant C-H‚‚‚O interactions are observed in the crystalline phase of the ionic liquid.26,27 Lithium ions occupy tetrahedral sites in the Li[C2mim](CF3SO3)2 crystal structure; each lithium ion is coordinated by four oxygen atoms originating from four independent triflate anions. The local environment of the lithium ions is depicted in Figure 2, and individual Li-O bond lengths are summarized in Table 1. Three of the Li-O bond lengths have an average length of 1.934 ( 0.003 Å, while the fourth Li-O bond is shorter at 1.906(11) Å. The average of all four Li-O bonds in the Li[C2mim](CF3SO3)2 crystal is 1.927 ( 0.014 Å. The crystal structure of lithium triethylammonium bis(trifluoromethanesulfonate) is very similar to that of Li[C2mim](CF3SO3)2.28 Both compounds contain Li+ ions that are coordinated by four O atoms from four different CF3SO3- anions. However, Lerner
Ion-Ion Interactions in Ionic Liquid Electrolytes and Bolte28 report hydrogen bonding between the NH group of triethylammonium cation and an oxygen atom from a CF3SO3anion, whereas the crystal structure of Li[C2mim](CF3SO3)2 lacks significant hydrogen-bonding interactions between the imidazolium and triflate ions. The Li-O bond lengths for Li[C2mim](CF3SO3)2 are comparable to those found in other crystal structures containing the CF3SO3- anion.29-31 For example, the unit cell of the 2-methoxyethyl ether-LiCF3SO3 crystal (diglyme-LiCF3SO3)30 is composed of four dimers, two of which are identical and slightly different from the other two identical dimers. Lithium ions are 5-fold coordinate and interact with three ether oxygen atoms from a diglyme molecule and two oxygen atoms from two different triflate anions in the asymmetric unit of each dimer. The average Li-O bond distances for the two diglyme-LiCF3SO3 dimers (2.047 ( 0.083 and 2.038 ( 0.071 Å) are slightly longer than the Li-O bond lengths in the Li[C2mim](CF3SO3)2 crystal. The subtle differences in the bond distances are probably due to different coordination environments for the two systems. In the lithium triethylammonium bis(trifluoromethanesulfonate) system discussed above, the Li-O bond lengths are very similar to those of the Li[C2mim](CF3SO3)2 compound, where three of the four bonds have an average length of 1.918(5) Å and there is a slightly larger bond at 1.964(4) Å. However, in this case, each triflate anion is coordinated to only one lithium cation and does not lead to the formation of “dimers”. In a crystalline compound, site symmetry effects and the correlated motion of ions within the unit cell determine the number of vibrational modes, their symmetry species, and thus their spectral activity. Therefore, the factor group correlation method is used to analyze the vibrations of the triflate anions of Li[C2mim](CF3SO3)2.32 To conserve space, only results for the νs(SO3) and δs(CF3) modes are discussed. Triflate anions occupy C1 sites within the unit cell of Li[C2mim](CF3SO3)2 (space group C2h5). Thus, eight νs(SO3) and eight δs(CF3) modes are expected according to theory. These vibrations are enumerated among the irreducible representations of the C2h point group as 2Ag + 2Bg + 2Au + 2Bu. Modes having Ag or Bg symmetry are Raman active and infrared inactive, while Au and Bu modes are Raman inactive and infrared active. Therefore, four Ramanactive and four IR-active vibrational modes are predicted for νs(SO3). Similarly, there are four possible Raman-active and four possible IR-active modes for δs(CF3). Infrared and Raman spectra of crystalline Li[C2mim](CF3SO3)2 are presented in Figures 3 and 4. Spectra of the [C2mim]CF3SO3-LiCF3SO3 solutions are also included in the figures for comparison. According to the Li[C2mim](CF3SO3)2 crystal structure, triflate anions are coordinated by two different Li+ ions. Therefore, each triflate anion vibrates as a [Li2CF3SO3]+ aggregate species in the crystalline phase from a spectroscopic point of view. Bands in the Raman spectrum of the crystalline compound (Figure 3) at 1060, 1043, and 1033 cm-1 are assigned to νs(SO3). It should be noted that a [C2mim]+ mode probably occurs at 1026 cm-1.33 The infrared spectrum of Li[C2mim](CF3SO3)2, also shown in Figure 3, consists of several overlapping bands (1043, 1040, and 1031 cm-1) that are also assigned to νs(SO3). In Figure 4, two bands are observed in the Raman spectrum of the crystal (766 and 757 cm-1) and at least three bands can be resolved between 770 and 750 cm-1 in the infrared spectrum. These bands are all assigned to δs(CF3) motions of the triflate anion. It is not unusual that the number of experimentally detected bands is less than the number predicted by the factor group analysis since some modes may have small dipole moment derivatives (weak IR intensities) or small
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Figure 3. Infrared and Raman spectra of νs(SO3) for the [C2mim]CF3SO3-LiCF3SO3 solutions and the Li[C2mim](CF3SO3)2 crystal.
Figure 4. Infrared and Raman spectra of δs(CF3) for the [C2mim]CF3SO3-LiCF3SO3 solutions and the Li[C2mim](CF3SO3)2 crystal.
polarizability derivatives (weak Raman intensities) or coincidentally overlap one another. [C2mim]CF3SO3-LiCF3SO3 Solutions. Vibrational spectra of the [C2mim]CF3SO3-LiCF3SO3 solutions are compared to the spectra of Li[C2mim](CF3SO3)2 and neat [C2mim]CF3SO3 in Figures 3 and 4. Raman spectra of pure [C2mim]CF3SO3 contain single triflate bands in the νs(SO3) and δs(CF)3 regions (1032 and 757 cm-1, respectively). However, the infrared spectrum of neat [C2mim]CF3SO3 consists of a broad νs(SO3)
2994 J. Phys. Chem. B, Vol. 112, No. 10, 2008 band roughly centered at 1030 cm-1 (Figure 3), while a weaker band can be resolved near 1017 cm-1 that is assigned to a vibration of the [C2mim]+ cation.34 In Figure 4, infrared spectra of neat [C2mim]CF3SO3 consist of a narrow δs(CF)3 band (756 cm-1) superimposed on a relatively broad [C2mim]+ band.34 The frequencies of the νs(SO3) and δs(CF3) modes are relatively high in pure [C2mim]CF3SO3. In other LiCF3SO3based electrolyte systems, the appearance of νs(SO3) and δs(CF3) bands at similar frequencies signaled the presence of ion pairs or aggregate species (for an example, see ref 19). The frequencies of the νs(SO3) and δs(CF3) modes are related to the local chemical environment of the triflate anions. Differences in the coordination environment of CF3SO3- can result in significant differences in the electronic distribution within the triflate molecule, affecting the force constants and thus the frequencies of the νs(SO3) and δs(CF3) modes.19 In pure [C2mim]CF3SO3, high electrostatic forces must exist between the constituent ions as triflate anions can only interact with the [C2mim]+ cations. This constitutes a very different local environment for CF3SO3- anions compared to systems where triflate salts are dissolved in a neutral solvent.12,14,18,35,36 In those systems, the ions are primarily surrounded by the solvent molecules, and the strength of the ion-ion interactions is reduced, particularly for high dielectric solvents. Several workers have examined the role of lithium solvation with vibrational spectroscopic methods in a related ionic liquid system, [C2mim]TFSI complexed with LiTFSI, where TFSI) N(CF3SO2)2-.25,37-39 In that system, lithium ions strongly interact with the TFSI- anions, forming [Li(TFSI)2]- aggregate species.38 However, Raman spectroscopic measurements clearly demonstrate preferential lithium solvation by neutral solvent molecules when the LiTFSI-[C2mim]TFSI ionic liquid system is mixed with compounds such as ethylene carbonate, vinylene carbonate, or a glyme (CH3(OCH2CH2)nOCH3 with n ) 2-4). Moreover, the addition of the neutral solvent molecules increases the relative population of “free” TFSI- anions compared to the [Li(TFSI)2]- aggregate species. Essentially no changes are observed in the νs(SO3) and δs(CF3) bands at low LiCF3SO3 molar compositions (
