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2009, 113, 8466–8468 Published on Web 06/04/2009
Direct Measurements of Ionic Mobility of Ionic Liquids Using the Electric Field Applying Pulsed Gradient Spin-Echo NMR Tatsuya Umecky, Yuria Saito,* and Hajime Matsumoto National institute of AdVanced Industrial Science and Technology, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577 Japan ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: May 31, 2009
Ionic mobilities of the ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide,1-ethyl-3-methylimidazolium fluorosulfonyl-(trifluoromethylsulfonyl)amide, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) -amide, were measured using the electric field applying pulsed gradient spin-echo NMR technique. Observed mobilities were more than 1 order of magnitude greater than the values estimated from the diffusion coefficients measured under the equilibrium state without the electric field. Electric field dependence of the ionic mobility showed that the high mobility appeared above the threshould of the field strength with keeping the constant values. This indicates that the ions are orientated by the application of the electric field may be due to the dielectric polarization. Ionic liquids have been used for several fields of chemistry such as reaction and extraction solvents for organic and polymer syntheses, catalysis, and extraction for purification.1 They are pure salts of the liquid phase at room temperature characterized by the low vapor pressure, nonflammability and thermal and chemical stability. These advantages have been taken notice in the field of electrochemical devices such as the lithium secondary batteries, fuel cells and capacitors from the aspect of high safety performance.2 Recently, new types of ionic liquids have been developed actively for application to lithium electrolytes for strategies of improving the high viscosity that causes the low lithium mobility and chemical instability in the oxidation and reduction processes at the interface with electrodes.3 The most significant and essential point that should be considered in designing the electrolytes with the ionic liquid solvent, which comprise only salts, is to make sure the real structure of the carrier species which is responsible for charge transport in the battery system. It is acceptable that the equilibrium state of the electrolyte is dominated by the Coulombic interaction among the ionic species originated from the solute and solvent salts. In the case of lithium ionic liquid electrolytes, we have confirmed that the Li+ is solvated by several anions as Li(TFSI)2- and Li(TFSI)43- at the equilibrium state for the lithium bis(trifluoromethylsulfonyl)amide dissolved in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (LiTFSI/RMI-TFSI) by the evaluation of the size of the mobile species.4 Existence of the anion-coordinated lithium is also supported by the studies of the IR and Raman spectra and simulations.5 It is noticeable that the net charge of the anion-solvated lithium species is negative. The anionic species of lithium would, if it works as it is in the lithium cell system, move according to the potential gradient opposite to the lithium cations in the conventional neutral solvent. This situation is not appropriate 10.1021/jp9043946 CCC: $40.75
for the charge transport system which is held in the normal lithium cell.6 It is reasonable to think that this ionic situation is changed by the electric field of the cell working system because the electric field affects the Coulombic interaction among the ionic species leading to the change in the dissociation condition of the salt, solvation, and configuration of the ionic species. As a result, ionic mobility under the field would be different from the expected values from the diffusion coefficient which reflects the random walk feature without any applied field. In order to design the nanoscale structure of the electrolytes systematically, it is significant to investigate the ionic configuration, real structure of the mobile species, and the mobility of the ionic liquid solvents and their electrolytes under the environment of the electric field for the charge transport reaction. In this research, we represent for the first time the direct measurements of ionic mobility of the ionic liquid materials using the electric field applying pulsed gradient spin-echo NMR technique. We found anomalously high ionic mobility under the electric field compared with the diffusion coefficient values which reflect the random walk migration free from the external field. This reveals that the ionic species are affected by the applied field to be changed in ion configuration in the liquid. It is possible that the observed high mobility attributes to the orientation of the ionic species due to the dielectric polarization which attributes to the permanent and induced dipoles. In this study, we selected the ionic liquid materials, 1-ethyl3-methylimidazolium tetrafluoroborate (EMI-BF4), 1-ethyl3-methylimidazolium bis(fluorosulfonyl)amide (EMI-FSI), 1-ethyl-3-methylimidazolium fluorosulfonyl-(trifluoromethylsulfonyl)amide (EMI-FTI), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMI-TFSI), and 1-butyl3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMI 2009 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 113, No. 25, 2009 8467
TABLE 1: Macro Viscosity, Diffusion Coefficient, and Ionic Mobility of the Ionic Liquidsa EMI-FSI EMI-FTI EMI-TFSI EMI-BF4 BMI-TFSI η/mPa s Dobs(cation)b Dobs(anion)c µobs(cation)c µobs(anion)c µest(cation)c µest(anion)c
19 7.1 6.2 2.8 3.4 0.028 0.024
23 6.1 4.4 1.5 1.4 0.024 0.017
32 4.8 2.9 0.84 0.76 0.019 0.011
38 4.5 3.6 0.96 0.91 0.018 0.014
50 2.6 2.0 0.25 0.24 0.010 0.0078
a The experimental accuracy of the diffusion coefficient and mobility values was 95%. b /10-11 m2 s-1. c /10-7 m2 s-1 V-1.
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TFSI), which are potential solvents for lithium electrolytes. Table 1 represents the macro-viscosity, observed diffusion coefficient, and mobility of the cation and anion species. Estimated mobility (µest) was obtained by applying Dobs to the Einstein’s relation in order to compare the dynamic values measured with and without the field on the same unit.8 It should be noted that Dobs and the consequent µest have contributions from the dissociated ion and associated ion pair in the electrolyte of the equilibrium state.8,9 On the other hand, µobs is contributed only from the charged ionic species.8,9 Therefore, there is no need for the µobs to agree with the µest due to the difference in the contributing species. However, as it can be assumed that Dion and Dpair are in the same order of magnitude due to the same order in size of the ion and ion pair species, µobs and µest are comparable when the ionic situation does not change before and after the field application. Therefore, it is useful to compare µobs and µest to evaluate the electric field effect on the structure and configuration of the ionic species in the ionic liquid system. It is found from Table 1 that µobs is orderly greater than µest for all of the samples. We can rule out the possibility of the thermal convection for the cause of the large µobs due to the fact as follows. We measured the mobilities of the ionic species of the mixed salts, LiTFSI/EMI-TFSI and LiTFSI/BMI-TFSI. The µobs values of the systems were more than 1 order of magnitude greater than µest although the macro-viscosity is comparable to that of the electrolyte with the organic solvent, LiTFSI/EC+DEC/PMMA in which µobs is in the same order of µest. These facts reveal that the anomalous µobs is attributable to the structural feature of the ionic liquids. We elucidated the mobility change with the strength of the applied electric field, E in order to see the effect of the field strength on the situation of the ionic configuration in Figure 1. It is noted that the steep rise of µobs appeared at a particular E value, Et, for each sample. Below the field strength of Et, it was difficult to detect the echo decay attributed from the ions drift in response to the electric field due to the small echo change almost within the error. In the range of E g Et, µobs steeply increased and showed anomalously large value, with keeping constant independent of the field strength unless the applied field is strong enough to destroy the chemical structure of the liquids. This changing feature also supports that the thermal convection due to the applied field, which would induce the µobs increase with E, did not occur under the condition used in this research. We suppose that the rapid increase in µobs at Et and the large value compared with µest at E g Et reflect the change in the ion configuration of the ionic liquid. Without the field, the dissociated ions and associated ion pairs coexist randomly at the equilibrium state. Under the applied electric field, it is
Figure 1. Cation (hollow symbols) and anion (solid symbols) mobilities of the ionic liquids measured by the electric field applying pulsed gradient spin-echo NMR as a function of the applied electric field.
expected that the salt dissociation is promoted and the cation and anion are aligned to form the orientation polarization. It is acceptable that the ion oriented situation is advantageous for the ionic species to move smoothly through the space among the aligned species as the transport pathway. This is supported by the diffusive results of the ionic species in the liquid crystal solvent.9 We confirmed that the ion and neutral species in the liquid crystal electrolytes showed anomalously higher mobility in the nematic phase, in which the liquid crystal molecules are orientated in parallel to the direction of the magnetic field, than that in the isotropic phase of the higher temperature region.9 On the analogy of this result, we could expect that even in the liquid phase ion orientation takes place by the electric field application and is effective for fast ion transport probably due to the reduction of the friction resistance of random collision among the molecular and ionic species. We can also find that the range of the µobs values is fairly wide (3.4 × 10-7 to 0.24 × 10-7 m2 s-1 V-1) relative to the smaller difference in Dobs (6.2 × 10-11 to 2.0 × 10-11 m2 s-1). The larger the µobs, the lower the Et appeared. The degree of ion orientation and the magnitude of µobs after ion orientation would depend on the strength of the ionic interaction. This is confirmed from Table 1 that the lower the macroscopic viscosity, the higher the µobs was observed at lower Et. Furthermore, µobs depends on the degree of ion orientation which is associated with the polarization degree of the ionic species comprising the salt. Acknowledgment. This work was supported by R&D project for Li batteries (Li-EAD) by METI and NEDO. Supporting Information Available: Details of the material syntheses and the measurement method of diffusion coefficient and ionic mobility. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071. Ionic Liquids in Synthesis 2nd Edition; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, 2007. (2) Ohno, H. Electrochemical Aspect of Ionic Liquid; Ohno, H., Ed.; John Wiley & Sons, Inc.: New York, 2005; p 1. (3) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992, 965. Hagiwara, R.; Matsumoto, K. Electrochemical Aspect of Ionic Liquid; Ohno, H., Ed.; John Wiley & Sons, Inc.: New York, 2005; p 227. Xu, W.; Angell, C. A. Electrochem. Solid-State Lett. 2001, 4, E1. Hayashi, K.; Nemoto, Y.; Akuto, K.; Sakurai, Y. J. Power Sources 2005, 146, 689. McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. J. Electrochem. Soc. 1999, 146, 1687. Matsumoto, H.; Terasawa, N.; Umecky, T.; Tsuzuki, S.; Sakaebe, H.; Asaka, K.; Tatsumi, K. Chem. Lett. 2008, 37, 1020.
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(4) Saito, Y.; Umecky, T.; Niwa, J.; Sakai, T.; Maeda, S. J. Phys. Chem. B 2007, 111, 11794. Umecky, T.; Saito, Y.; Okumura, Y.; Maeda, S.; Sakai, T. J. Phys. Chem. B 2008, 112, 3357. (5) Borodin, O.; Smith, G. D. J Phys. Chem. B 206, 110, 4971. Nowinski, J. L.; Lightfoot, P.; Bruce, P. G. J. Meter. Chem. 1994, 4, 1579. Kameda, Y.; Umebayashi, Y.; Takeuchi, M.; Wahab, M. A.; Fukuda, S.; Ishiguro, S.; Sasaki, M.; Amo, Y.; Usuki, T. J. Phys. Chem. B 2007, 111, 6104. Borodin, O.; Smith, G. D; Henderson, W. J. Phys. Chem. B 2006, 110, 16879. Hardwick, L. J.; Holzapfel, M; Wokaun, A; Nova´k, P. J. Raman Spectrosc. 2007, 38, 110.
Letters (6) Bockris, J. O’M.; Reddy, A. K. N. Modern electrochemistry: ionics, 2nd ed.; Plenum Press: New York, 1998; p 702. (7) Umecky, T.; Saito, Y.; Tsuzuki, S.; Matsumoto, H. ECS Transactions, in press. (8) Kataoka, H.; Saito, Y.; Sakai, T.; Deki, S.; Ikeda, T. J. Phys Chem. B 2001, 105, 2546. Kataoka, H.; Saito, Y. J. Phys. Chem. B 2002, 106, 13064. (9) Saito, Y.; Hirai, K.; Murata, S.; Kishii, Y.; Kii, K.; Yoshio, M.; Kato, T. J. Phys. Chem. B 2005, 109, 11563.
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