Effects of Lithium Salts on Shear Relaxation Spectra of Pyrrolidinium

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Effects of Lithium Salts on Shear Relaxation Spectra of Pyrrolidinium-Based Ionic Liquids Tsuyoshi Yamaguchi, Ken-ichi Mikawa, Shinobu Koda, Nobuyuki Serizawa, Shiro Seki, Kenta Fujii, and Yasuhiro Umebayashi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp3032308 • Publication Date (Web): 29 May 2012 Downloaded from http://pubs.acs.org on June 3, 2012

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The Journal of Physical Chemistry

Effects of lithium salts on shear relaxation spectra of pyrrolidinium-based ionic liquids Tsuyoshi Yamaguchi,1,* Ken-ichi Mikawa,1 Shinobu Koda,1 Nobuyuki Serizawa,2 Shiro Seki,2 Kenta Fujii3 and Yasuhiro Umebayashi4,† 1

Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, Aichi 464-8603, Japan

2

Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan

3

Neutron Scattering Laboratory, Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8565, Japan

4

Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

E-mail: [email protected] (T. Yamaguchi) TITLE RUNNING HEAD: Shear relaxation of ionic liquid with lithium salt CORRESPONDING AUTHOR: E-mail: [email protected], Tel: +81-52-789-3592, Fax: +8152-789-3273.

†

Present address: Graduate Scholl of Science and Technology Niigata University, 8050, Igarashi, 2-no-

cho, Nishi-ku, Niigata City, 950-2181, Japan

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The shear relaxation spectra of the solutions of lithium salts in ionic liquids composed of N-methyl-Npropylpyrrolidinium

cation

paired

with

bis(trifluoromethanesulfonyl)amide

(TFSA-)

or

bis(fluorosulfonyl)amide (FSA-) anions are determined from 5 to 205 MHz at various concentrations of lithium salts. The addition of lithium salt retards the shear relaxation, together with the increase in the shear viscosity. The normalized spectra reduces to a single curve when plotted against the product of the frequency and zero-frequency shear viscosity, which indicates that the increase in the shear viscosity by lithium salts is ascribed to the increase in the relaxation time. The difference in the shear viscosity of TFSA- and FSA-based ionic liquids is also elucidated in terms of the shear relaxation time. The relationship with previous studies on ionic mobility and liquid structure is also discussed.

KEYWORDS: Ionic liquid, shear viscosity, lithium, pyrrolidinium

1.

Introduction

Ionic liquid is one of the current topics that attract attention of many researchers. Owing to its peculiar properties such as low volatility by molecular design of cations and anions, applications of ionic liquids to various fields have been tested so far.1,2,3,4 In addition, understandings of physicochemical properties of ionic liquids are in progress at present.5,6,7,8,9 One of the promising applications of ionic liquids is the solvent for the electrolyte of lithium battery. High ionic conductivity, high electrochemical stability and moderate solubility of lithium salt into ionic liquid are suitable for the electrolyte, and negligible volatility and negligible flammability contribute to enhance the safety of the battery. One of the drawbacks of ionic liquids for the electrolyte of lithium battery is its high viscosity. The mobility of ions is usually low in solvents of high viscosity. In addition to the high viscosity of neat ionic liquid, the dissolution of lithium salt leads to further increase in viscosity, which accompanies the reduction of the mobility of lithium ion.10,11,12,13,14,15,16,17 ACS Paragon Plus Environment

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The use of bis(fluorosulfonyl)amide (FSA)-based ionic liquid is a way to reduce the problem described above. 18,19,20,21,22,23,24 The shear viscosity of neat FSA-based ionic liquid is usually smaller than that of neat bis(trifluoromethanesulfonyl)amide (TFSA)-based one. In addition, the increase in the shear viscosity of FSA-based ionic liquid with the addition of LiFSA is rather small relative to the corresponding TFSA system. In order to realize the advantage of FSA-based ionic liquid on molecular level, various studies have been performed including NMR, 25 , 26 , 27 Raman, 28 , 29 Infrared, 30 X-ray diffraction29 and molecular dynamics simulation.31 Shear viscosity is one of dynamic properties of liquid that reflects the structure relaxation. It is given by the time-integral of the time correlation function of shear stress according to the Kubo-Green formula, and it is roughly given as the product of the high-frequency shear modulus, G∞, and the relaxation time,

τ. The effects of lithium and FSA- ions on the shear viscosity should also be explained in terms of the changes in G∞ and τ.32 Shear viscosity can be extended to the function of frequency, and the ordinary shear viscosity is its zero-frequency limiting value. We can obtain the information on G∞ and τ from the shear relaxation spectrum, and the measurement of the shear relaxation is one of the popular tools to investigate the viscoelastic properties of soft matters. Shear relaxation measurement has also been applied to ionic liquids by some groups. 33,34,35,36,37 Yamaguchi and coworkers found that the replacement of PF6- anion with TFSA- one reduces shear viscosity through the relaxation time.35 The effect of the introduction of oxyethylene chain was also explained by the relaxation time.37

On the other hand, the lengthening of the alkyl chain of an

imidazolium cation leads to both the increase in the relaxation time and the decrease in the shear modulus, and the shear viscosity is increased because the effect of the relaxation time is overwhelming.35 In addition to these rheological studies, the high-frequency viscometry based on quartz crystal microbalance (QCM) technology was applied to ionic liquids to probe the local viscosity near a reacting electrode.38


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In this work, we apply the shear impedance spectroscopy to the solutions of lithium salts in FSA- and TFSA-based ionic liquids in order to understand how the dissolution of lithium ion affects the shear viscosity of ionic liquids from the rheological point of view. The counter anions of the lithium salt are the same as those of ionic liquids, so that the systems consist of three ionic species. We also compare the FSA- and the TFSA-based ionic liquids to examine the mechanisms of the smaller shear viscosity of the neat FSA-based ionic liquid and the weaker effect of lithium ion on it.

2.

Experimental method

2.1. Sample preparation Ionic liquid samples, N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P13TFSA) and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (P13FSA), were purchased from Dai-ichi Kogyo Seiyaku Co., Ltd (Japan). LiTFSA and LiFSA were purchased from Kishida Chemical and Piotrek (Japan), respectively. The Li-salt-doped binary solutions were prepared by adding the LiTFSA and the LiFSA to the P13TFSA and the P13FSA, respectively. The concentrations of the lithium salts were 0, 0.25, 0.50 and 0.75 mol kg-1 for LiTFSA + P13TFSA, and 0, 0.25, 0.50, 0.75 and 1.00 mol kg-1 for LiFSA + P13FSA. The ionic liquids were dried in a vacuum chamber at 323 K for more than 48 h and stored in a dry argon-filled glovebox before measurements.

2.2. Density and Viscosity measurements Density (ρ) and viscosity (η) were measured using a thermoregulated Stabinger-type density/viscosity meter (SVM3000G2, Anton Paar). The temperature was controlled in the range of during heating from 10 °C to 80 °C at 5 °C intervals. The stability of the temperature during the measurement was within ±0.005°C from the target temperatures.

2.3. Shear impedance spectroscopy ACS Paragon Plus Environment

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The frequency-dependent shear viscosity, η(ν), was measured at frequencies from 5 to 205 MHz with a shear impedance spectrometer, which was based on QCM technology and described in detail elsewhere.37,39 Briefly, the electric response of an AT-cut quartz crystal before and after the contact with sample liquid was measured with a vector network analyzer (ZVL3/03, Rohde & Schwartz), and the shear viscosity was determined from the changes of the resonance frequency and bandwidth induced by the sample liquid. The value directly obtained in the experiment was ρη(ν), which was divided by the mass density, ρ, to calculate the shear relaxation spectrum, η(ν). The temperature was controlled at 25.0 ± 0.1 °C by flowing thermostatted water through the cell, and the temperature of the sample was monitored with a thermistor immersed into the sample. The experimental error was estimated to be |∆η(ν)/η(ν)| < 0.1 at 10 - 100 MHz, and |∆η(ν)/η(ν)| < 0.2 at other frequencies. The measurement from 5 to 205 MHz completes within an hour, and the drift of η(ν) within the experimental time was negligible compared with the experimental error. All the measurements were performed twice, and the averaged values were employed.

3.

Results and discussion

1.55 1.5 -3

ρ [g cm ]

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1.45 1.4

1.35 1.3 0

0.2

0.4 0.6 0.8 m [mol kg-1]

1

1.2

Li

Figure 1. The density of the solutions of LiTFSA in P13TFSA (red circles) and LiFSA in P13FSA (blue squares) at 25°C are plotted against the molality of lithium salts, mLi. ACS Paragon Plus Environment

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The Journal of Physical Chemistry 350 250 200 150

0

η [mPa s]

300

100 50 0 0

0.2

0.4 0.6 0.8 m [mol kg-1]

1

1.2

Li

Figure 2. The shear viscosity, η0, of the solutions of LiTFSA in P13TFSA (red circles) and LiFSA in P13FSA (blue squares) at 25°C are plotted against the molality of lithium salts, mLi. The density and shear viscosity are shown in Figs. 1 and 2, respectively, as the functions of the molality of lithium salts. Only the values at 25°C are shown for the use in the analysis of shear relaxation spectra, and the values at other temperatures are available as Supporting Information. The addition of lithium salts results in the increase of both density and viscosity. The density varies almost linearly to the molality, whereas the viscosity increases rather exponentially. Comparing the FSA- and TFSA-based systems, the shear viscosity of neat P13FSA is smaller than that of neat P13TFSA, and the increase in the viscosity with the addition of lithium salt is smaller in P13FSA than in P13TFSA. On the other hand, the variations of the mass density are similar in both systems. 350 300 η(ν) [mPa s]

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250 200 150 100 50 0 10

100 ν [MHz]

Figure 3.

The shear relaxation spectra, η(ν), of the solutions of LiTFSA in P13TFSA.

The

concentrations are 0 mol kg-1 (red circles), 0.25 mol kg-1 (blue squares), 0.50 mol kg-1 (green diamonds) and 0.75 mol kg-1 (black triangles), respectively. The real and imaginary parts are plotted with filled and ACS Paragon Plus Environment

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open symbols, respectively, and the zero-frequency limiting values are shown by the horizontal dotted lines.

100 η(ν) [mPa s]

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80 60 40 20 0 10

100 ν [MHz]

Figure 4. The shear relaxation spectra, η(ν), of the solutions of LiFSA in P13FSA. The concentrations are 0 mol kg-1 (red circles), 0.25 mol kg-1 (blue squares), 0.50 mol kg-1 (green diamonds), 0.75 mol kg-1 (black up-pointing triangles) and 1.00 mol kg-1 (purple down-pointing triangles), respectively. The real and imaginary parts are plotted with filled and open symbols, respectively, and the zero-frequency limiting values are shown by the horizontal dotted lines. Figures 3 and 4 show the shear relaxation spectra of the TFSA- and the FSA-based solutions, respectively. The frequency-dependent shear viscosity, η(ν), is a complex variable that possesses real and imaginary parts as η(ν) = η'(ν) - iη"(ν). The shear relaxation spectra exhibit a typical relaxation behavior, that is, the real part is a monotonically decreasing function of frequency while the imaginary part shows a maximum at an intermediate frequency. The frequency at which the imaginary part shows a maximum corresponds to the shear relaxation frequency approximately. For example, the relaxation frequency of 0.50 mol kg-1 solution of LiTFSA in P13TFSA is about 100 MHz, which indicates that the relaxation time is about 1.6 ns. The peaks of the imaginary parts are not visible when the value of η0 is low, because relaxation frequency is higher than the upper limit of our experiment, 205 MHz. The real parts, η'(ν), of all the samples appear to converge to η0 at 5 MHz within the experimental error, which guarantees the consistency between the measurements of steady-state and high-frequency shear viscosity. The amplitude of the relaxation is comparable with the zero-frequency shear viscosity, η0, as is clearly 7 ACS Paragon Plus Environment


observed in Fig. 3. It indicates that the liquid dynamics that causes the shear relaxation observed here is the process that determines η0. The addition of lithium salts increases the shear viscosity together with the increase in the relaxation time. The FSA salt possesses lower viscosity and smaller relaxation time compared with the TFSA salt. Since the shear viscosity, η0, is approximately given by the product of G∞ and τ, it is quite natural that the more viscous a liquid is, the longer the relaxation time becomes. The plots of the normalized shear relaxation spectra, η(ν)/η0, against frequency reduced in various ways tell us the roles of G∞ and τ in the variation of η0. In particular, the plot of η(ν)/η0 against η0ν reduces to a single master curve when the relaxation time is solely responsible to the change in the shear viscosity. 1.2

0

1 η(ν) / η

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0.8 0.6 0.4 0.2 0 0.1

1 η0ν [MPa]

10

Figure 5. The normalized shear relaxation spectra, η(ν)/η0, of neat P13TFSA (red circles) and P13FSA (blue squares) are plotted against η0ν. The real and imaginary parts are plotted with filled and open symbols, respectively. Figure 5 compares the normalized spectra of neat P13TFSA and P13FSA on the η0ν-axis. The two spectra reduce to a single curve on this axis, indicating that the difference in the values of η0 of the two liquids is ascribed to that in the relaxation time, τ. In other words, the replacement of TFSA- anion with FSA- one lowers the shear viscosity through the promotion of the shear relaxation. Yamaguchi and coworkers showed that higher viscosity of PF6- salt than that of TFSA- one is related to the longer relaxation time of the former,35 which is in harmony with our present result. ACS Paragon Plus Environment

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Both TFSA- and FSA- anions are flexible molecules that possess intramolecular degrees of freedom. Fujii, Umebayashi and coworkers studied the conformational equilibrium of these anions in ionic liquids intensively with X-ray diffraction, Raman spectroscopy and MD simulation.31,28,31,40,41 Both anions have two conformations, respectively, and the isomers are in dynamic equilibrium in ionic liquids under ambient condition. Comparing TFSA- and FSA-, the difference in the dihedral angles of two isomers are smaller for TFSA- than that for FSA-, and the activation barrier between the isomers of TFSA- is higher than that of FSA-. We guess that the larger deformation of FSA- provides larger number of relaxation paths, which decreases the structural relaxation time. However, there are other factors, such as ionic size, mass, and detailed interionic interaction, that may affect the relaxation time, and further studies will be required to relate the molecular structure of ions with the relaxation time. 1.2

η(ν) / η

0

1 0.8 0.6 0.4 0.2 0 1

10 η ν [MPa]

100

0

Figure 6. The normalized shear relaxation spectra, η(ν)/η0, of LiTFSA + P13TFSA mixtures at various concentrations are plotted against η0ν. The meanings of the symbols are the same as those in Fig. 3.

1.2 1 0

0.8

η(ν) / η

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0.6 0.4 0.2 0 0.1

1 η ν [MPa]

10

0


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Figure 7. The normalized shear relaxation spectra, η(ν)/η0, of LiFSA + P13FSA mixtures at various concentrations are plotted against η0ν. The meanings of the symbols are the same as those in Fig. 3. Figures 6 and 7 shows the normalized spectra of the TFSA- and the FSA-based solutions, respectively, plotted against η0ν. The spectra of both solutions reduce to their respective master curves. The change in the shear viscosity as the function of the concentration of lithium salts is thus elucidated in terms of relaxation time. The lithium ion works to prevent the structural relaxation of ionic liquids, thereby the shear viscosity is increased. Its effect on the shear viscosity of FSA-based ionic liquid is smaller than that of TFSA-based one, because the slowing down of the structural relaxation of the former is weaker than that of the latter. Dissolution of lithium salts into non-ionic solvents such as propylene carbonate (PC) also increases the viscosity of the solution. The shear relaxation spectra of the solution of lithium perchlorate (LiClO4) in PC was measured by Yamaguchi and coworkers, and they reported that the increase in the relaxation time accompanies that of the viscosity.39 Detailed analysis shows that the dependence of the viscosity on the concentration of LiClO4 is roughly explained from the view of the relaxation time.42 Therefore, lithium salts increases the viscosity of ionic and non-ionic liquids in a similar manner. Pitawala and coworkers reported that the temperature dependence of the ionic conductivity of the mixtures of lithium salts and ionic liquids at various concentrations of the lithium salts fall onto a master curve as the function of reduced temperature, that is, the temperature divided by the glass transition temperature.13 The existence of the master curve indicates that the lithium salt hardly affects the conduction mechanism, and the role of the lithium salt is to increase the glass transition temperature by increasing the relaxation time. Our present results are therefore in harmony with theirs in that the lithium salt decreases the fluidity of the solution through the relaxation time. Yamaguchi and coworkers measured shear relaxation spectra of various ionic liquids35,37 and their mixtures with water. 43 They showed that the change in the relaxation time is the reason for that in shear viscosity in many cases. The result in the present work is thus in harmony with previous ones, and the


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shear viscosity of ionic liquid systems appears to reflect mainly its relaxation time. Given that the dissolution of small lithium ion brings strong electrostatic interaction into the solution, however, the small variation of G∞ is rather surprising, because G∞ reflects the strength of instantaneous intermolecular interaction. We consider that small increase in G∞ is overwhelmed by the large increase in τ. Large variation of the relaxation time under the small changes in liquid structure and molecular interaction is one of the characteristics of supercooled liquids near glass transition temperature, and we guess that the dynamics of ionic liquids has something to do with that of supercooled liquids. Next we turn to the difference in the concentration dependence of shear viscosity of TFSA- and FSAbased solutions demonstrated in Fig. 2. The liquid structure of the mixture of lithium salts with ionic liquids have been studied with vibrational spectroscopy, NMR spectroscopy, X-ray and neutron diffraction experiments, and molecular dynamics (MD) simulation. 44 , 45 , 46 , 47 In particular, Fujii, Umebayashi and coworkers demonstrated that lithium ion is solvated strongly by two TFSA- anions in TFSA-based ionic liquids, and the solvated lithium ion may be regarded as a Li(TFSA)2- complex. On the other hand, the lithium ion in FSA-based ionic liquids is coordinated by three FSA- anions to form Li(FSA)32- structure. The coordination of two TFSA- ions is bidentate, whereas both monodentate and bidentate coordinations were observed in FSA-based ionic liquids. The distance between Li+ and the oxygen atoms of anions is longer for TFSA- than for FSA-, and the temperature dependence of the coordination number of the latter is larger than that of the former. These results indicate that the rigidity of the Li(TFSA)2- structure is higher than that of the Li(FSA)32- one. The presence of the rigid aggregate can be an obstacle against the shear flow, thereby the structural relaxation time increases with the addition of lithium salt. According to the idea described above, the small increase in the shear viscosity with the addition to the FSA-based ionic liquid compared to the TFSA-based one is ascribed to the lower rigidity of Li(FSA)32- solvation structure. It is of course a speculation, and we need to clarify at first what kind of structure is involved in the shear relaxation in order to discuss the microscopic origin of the change in the relaxation time. We consider that the comparison between shear relaxation and quasi-elastic scattering experiments is a suitable method to find the microscopic structure related to the ACS Paragon Plus Environment

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shear relaxation, 48 , 49 because the wavelength of the structure relevant to shear viscosity should be characterized by the relaxation time equal to that of shear viscosity. A study along this direction is now in progress, and we expect it will allow us to understand the shear relaxation on a solid microscopic basis. One of the most important quantity of the lithium salt-ionic liquid mixture as the electrolyte for lithium battery is the mobility of lithium ion. The mobility of ions has been studied with pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), which revealed that the self-diffusion coefficients of ions follow the variation of shear viscosity of the solution.11,30 Since the increase in shear viscosity accompanies the decrease in ionic mobility, the microscopic mechanism of shear viscosity likely affects the ionic conduction mechanism. The time scale of the microscopic process involved in the ionic conduction can be inferred from the frequency-dependence of the ionic mobility. Although the PFG-NMR cannot determine the frequency-dependent self-diffusion coefficient in the 100 MHz region, the dielectric relaxation spectrum reflects the frequency-dependent electric conductivity in the corresponding frequency region.50,51 The ionic conductivity is expected to increase with frequency at the frequency where shear relaxation occurs, and the relaxation frequency of ionic conductivity will decrease with the addition of lithium salt as is the case of shear relaxation. The relationship between the shear viscosity and ionic conductivity has been discussed in terms of the ratio of their relaxation times,10,33,52 which can be determined experimentally from the comparison between shear relaxation and conductivity dispersion. Since the dielectric relaxation of ionic liquids in the 100 MHz region is also affected by the reorientational relaxation of ions, however, the resolution of the assignment of the dielectric relaxation is inevitable before comparing the frequency dependence of ionic conductivity and shear viscosity quantitatively.5051,5152

4.

Conclusion


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The frequency-dependent shear viscosity of two solutions of lithium salts in ionic liquids, LiTFSA + P13TFSA and LiFSA + P13FSA, are determined from 5 to 205 MHz at various concentrations of lithium salts. All the solutions show a relaxation in the 100 MHz region, and the relaxation shifts to lower frequency with increasing the viscosity of solution. All the spectra reduce to a single curve when plotted against the product of frequency and zero-frequency shear viscosity, which indicates that the variation of the shear viscosity is caused by that of the structural relaxation time. P13FSA is less viscous than P13TFSA because the structural relaxation of the former is faster than that of the latter. The lithium salt increases the viscosity of solution through the increase in the relaxation time. The increase in the shear viscosity of LiFSA + P13FSA with the addition of lithium salt is weaker than that of LiTFSA + P13TFSA because the effect of lithium ion on the relaxation time is weaker in the former solution. In particular, the difference in the solvation structure of lithium ion in TFSA- and FSA-based ionic liquids revealed in previous studies affects the concentration dependence of shear viscosity through the structural relaxation time. The comparison with neutron or X-ray quasielastic scattering will help us relating the liquid structure and shear relaxation, and the study in this direction is currently in progress. In addition to providing us the information on the mechanism of shear viscosity, we consider that it is also useful to study the ionic mobility through the comparison between shear relaxation and frequency-dependent conductivity.

Supporting Information Available: The numerical values of density, viscosity and shear relaxation spectra are available free of charge via the Internet at http://pubs.acs.org.


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TOC Graphic:

1.2

P13 TFSA + LiTFSA

0

1 η(ν) / η

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Page 18 of 18

0.8 0.6 0.4

+

Higher Li Concentration

0.2 0 1

10 η ν [MPa]

100

0


18

Effects of Lithium Salts on Shear Relaxation Spectra of Pyrrolidinium

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