Transport and Electrochemical Properties of Three Quaternary

Article pubs.acs.org/jced

Transport and Electrochemical Properties of Three Quaternary Ammonium Ionic Liquids and Lithium Salts Doping Effects Studied by NMR Spectroscopy Kikuko Hayamizu,*,†,§ Seiji Tsuzuki,† and Shiro Seki‡ †

National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Center 2, Tsukuba, 305-8568, Japan Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan

‡

S Supporting Information *

ABSTRACT: Ionic liquids (IL) composed of a quaternary ammonium cation having an ether chain, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME) are electrochemically stable and used in electric double layer capacitors and also one of important candidates to use lithium secondary batteries. In this study, three DEME-based ILs with anions BF4, CF3BF3, and N(SO2CF3)2 were studied by measuring temperature dependences of ionic conductivity σ, viscosity η and density ρ. Also ion diffusion coefficients Danion and Dcation were obtained by NMR spectroscopy in the wide temperature range. Using the classical Stokes−Einstein (SE) and Nernst−Einstein (NE) equations, the relationships between η and D, and σ and D were evaluated. The lithium salt doping effects were studied by 7Li NMR spectroscopy. The lithium ion diffusion was slower than other ion diffusion at every temperature. Arrhenius-type plots of 7Li T1 showed minima in the three doped samples. Then one-jump distance of lithium ion was estimated in the temperature range between 273 and 373 K.

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INTRODUCTION Ionic liquids (ILs), which consist only of ions, have attracted more and more attention on account of their unique physical properties like viscosity, vapor pressure, thermal transition temperatures, conductivity, ion diffusion coefficient, etc. Initially, the studies were aimed at possible practical applications for electrolytes or solvents due to their desirable properties and variety of chemical structures. Electrochemical aspects of ionic liquids1 and physical properties have been reviewed,2 and databases are now available through the Internet.3 Quaternary ammonium cations form ILs with various anions. Due to the variety of substitution groups connected to a central nitrogen atom, many quaternary ammonium ILs have been synthesized and thermal and physicochemical studies were published for wide aliphatic quaternary ammonium salts.4−6 Among them a cation with an ether structure in a side chain, N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME), has been noticed to use energy devices such as electrochemical capacitors, lithium batteries, and field-effect transistors. Then five DEME-based bis(perfluoroalkanesulfonyl)amide N(CnF2n+1SO2)2 were synthesized, and physicochemical properties have been reported.6 Pressure effects on the Raman spectra have been studied for DEME-based BF4 and bis(trifluoromethylsulfonyl)amide (TFSA) ILs.7,8 © 2014 American Chemical Society

We chose the anion as TFSA and demonstrated the favorable performance of lithium secondary batteries using lithium salt doped DEME-TFSA.9−13 DEME-TFSA and DEME-TFSA-Li systems have been studied from the physicochemical and electrochemical aspects.14 NMR diffusion coefficients and T1 measurements have been made.15 Also, ab initio molecular orbital calculations where stable configurations of DEMETFSA-Li binary electrolyte showed an importance of the interaction between a lithium ion with an oxygen atom in the side chain of DEME.16 It should be noticed that ILs composed of an anion BF4 and a cation DEME or methylpyrrolidinium having an ether side chain are practically used as salts in electric double layer capacitors.17,18 In this paper we add two anions, BF4 and CF3BF3 and totally three DEME ILs including DEME-TFSA are studied for ionic conductivity (σ), viscosity (η), and density (ρ) over a wide temperature range in addition to ion diffusion coefficients (Danion and Dcation) by NMR spectroscopy. Also 7Li NMR measurements were performed for the lithium salt doped binary ILs. The chemical structures of a cation and three anions are shown below. Received: January 22, 2014 Accepted: May 19, 2014 Published: June 3, 2014 1944

dx.doi.org/10.1021/je500065k | J. Chem. Eng. Data 2014, 59, 1944−1954

Journal of Chemical & Engineering Data

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measurements, the samples were placed into 5 mm NMR microtubes (BMS-005J, Shigemi, Tokyo) to a height of 5 mm and sealed with epoxide resin in a glovebox to prevent moisture. The preparation and inclusion of samples were carried out in a dry-argon-filled glovebox. The 1H NMR spectra did not include any extra peaks such as H2O. A typical impurity detection limit for 1H NMR is near 1 mol percent. Based on this finding, we estimate the purity of the ionic liquids to be better than 99 mol percent. Ionic Conductivity Measurements. The temperature dependences of the ionic conductivity of the samples was measured in SUS/electrolyte/SUS symmetric blocking cells and determined by the complex impedance method, using an ac impedance analyzer Princeton Applied Research, PARSTAT2263, in the frequency range 200 kHz to 50 mHz with an applied voltage of 10 mV over the temperature range (353 and 233) K with cooling. Viscosity and Density Measurements. The viscosity (η/ mPas) and density (ρ/g cm−3) measurements were carried out using a thermo-regulated SVM3000G2 Stabinger-type viscosity and density/specific gravity meter (Anton Paar), respectively. The measurements were performed during cooling from (353 to 283) K with 5 K intervals. NMR Measurements. All NMR spectra were measured on a Tecmag Apollo with a 6.35 T wide bore magnet using a JEOL PFG probe and controlled by a JEOL console. The T1 measurements were performed by the inversion recovery (180°−τ−90°−Acq.) pulse sequence. The 1H, 19F, 7Li, and 11 B NMR spectra were measured at (270.2, 254.2, 105.0, and 86.7) MHz, respectively. A modified Hahn spin−echo-based sequence incorporating a gradient pulse in each τ period (PGSE) was used to measure the diffusion coefficients of DEME, anions and Li ion by 1H, 19F, and 7Li NMR, respectively. The echo attenuation, E, is related to the experimental variables and the diffusion coefficient D by

Since DEME has a specific structure, in advance of the study for DEME, we concentrated our studies on well-studied ILs such as 1-ethyl-3-methylimidazolium (EMIm)-based TFSA and bis(fluorosulfonyl)amide (FSA) of neat and binary systems including lithium salt,19,20 neat and binary N-methyl-Npropylpyrrolidinium (P13)-based TFSA and FSA systems,21 and BF4-based EMIm and 1-butyl-3-methylimidazolium (BMIm) ILs.22 Based on our previous studies, we will report on the neat and binary DEME-based IL systems for three anions. In the present paper, viscosity (η) and density (ρ) were newly measured for the three DEME ILs including DEMETFSA in the temperature range between 283 and 353 K. By adding new temperature-dependent data of η and ρ, the classical Stokes−Einstein (SE) and Nernst−Einstein (NE) equations and Walden plots were examined. The mutual relationships of experimental ion diffusion constants (Danion and Dcation) and η and molar conductivities of electrochemical measurements and those derived from NMR Danion and Dcation will be discussed. In order to use ILs for lithium secondary batteries, the doping of lithium salts is necessary. The 7Li NMR studies for lithium salt doped systems showed the Li diffusion was always the slowest among the component ions of the binary IL systems.15,19−21,23 Previously, we showed the lithium one-jump distance increases with temperature and lithium salt concentration in the DEME-TFSA-Li systems.15 In the present paper, the lithium salt doped samples were prepared such as DEMEBF4 + LiBF4, DEME-CF3BF3 + LiTFSA, and also DEME-TFSA + LiTFSA. The 7Li resonances were measured for diffusion and T1, and the Arrhenius plots of 7Li T1 exhibited T1 minima. The 7 Li correlation time for a one-jump was evaluated at each temperature, and lithium one-jump distances were estimated. Also, ab initio molecular orbital calculations were carried out for studying intermolecular interactions between ions.

E = exp(−γ 2g 2δ 2D(Δ − δ /3))

(1)

where γ is the gyromagnetic ratio of the observing nucleus, g is the strength of the pulse field gradient (PFG) of duration time δ, and Δ is the interval between the leading edges of the two PFG’s. Δ defines the time scale of the diffusion measurement, and in a homogeneous system D is independent of Δ. A single exponential diffusion plot following to eq 1 indicates free diffusion. However, above the ambient temperature for a longer interval time Δ, the diffusion measurements are more prone to convection artifacts leading to Δ-dependence of the measured diffusion coefficients. At lower temperatures, as described previously,20−22,24 the apparent D value is dependent on Δ and becomes larger for shorter Δ, which is opposite to the convection effects. The measurements were made for two or three different Δ values by setting Δ between (20 and 70) ms in the temperature range from (253 to 353) K controlled by the gas flow method. Reasonable values of D were obtained for suitable Δ at each temperature. An example of the measuring conditions for DEME-CF3BF3−Li at 298 K is following: Δ = 50 ms for the three ions, DEME (1H), g = 5.3 Tm−1, δ changed from (0.1 to 1.5) ms (15 points), D = 8.8·10−12 m2·s−1; BF3 (19F), g = 5.3 Tm−1, δ changed from (0.1 to 2) ms (20 points), D = 9.0·10−12 m2·s−1; Li (7Li), g = 6.6 Tm−1, δ changed from (0.2 to 4.0) ms (20 points), D = 4.2·10−12 m2·s−1. The largest g applied was 14.1 Tm−1 at 253 K. Although the root-meansquare (rms) errors of D values estimated by the diffusion plots

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EXPERIMENTAL SECTION Sample Preparation. N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)amide (DEME-TFSA) was purchased from Kanto Kagaku. DEMEBF4 and DEME-CF3BF3 were offered by Nisshinbo Industries, Inc. The lithium salts (LiTFSA and LiBF4) were purchased from Kishida Chemical Co., Ltd. and used to prepare the doped electrolytes of the concentration 0.32 mol kg−1 by adding LiBF4 in DEME-BF4 to prepare DEME-BF4−Li and LiTFSA to DEME-TFSA and DEME-CF3BF3 to prepare DEME-TFSA-Li and DEME-CF3BF3-TFSA-Li, respectively. LiCF3BF3 was not available. The present salt concentration was verified to be suitable in order to use lithium secondary batteries.12,13 First, the ILs and salts were dried in a vacuum chamber at 323 K more than 48 h and stored in a dry argon-filled glovebox ([O2] < 0.4 ppm, [H2O] < 0.1 ppm, Miwa Mfg. Co., Ltd.). The lithium salt doped samples were prepared by dissolving given amounts of a lithium salt in an IL, and allowing each mixture to stand by stirring at room temperature over 48 h. For NMR 1945



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following eq 1 are generally small and less than 0.5 %, several factors affect the accuracy of the absolute values of D, such as calibration of g, linearity of δ, proper setting of Δ, measuring temperature, convection effects and so on. The estimated standard uncertainties of the present D values were less than 2 %. We observed the D data under ordinary pressure (p = 0.1 MPa) because we cannot control the pressure. Computational Methods. The Gaussian 03 program25 was used for the ab initio molecular orbital calculations. The basis sets implemented in the Gaussian program were used. Electron correlation was accounted for at the MP2 level.26,27 The geometries of complexes were fully optimized at the HF/6311G** level. The MP2/6-311G** level interaction energy (Eint) was calculated by the supermolecule method. The basis set superposition error (BSSE)28 was corrected for all the interaction energy calculations using the counterpoise method.29 The stabilization energy by the formation of the complex (Eform) was calculated as the sum of the Eint and the deformation energy (Edef), which is sum of the increase of the energies of cation and anion by the deformation of the ion geometries associated with the complex formation. The Edef was calculated at the MP2/6-311G** level.

Figure 1. Arrhenius plots of the ionic conductivity for DEME-based ILs of BF4 (circle), TFSA (down-triangle), and CF3BF3 (up-triangle).

Table 2. VFT Parameters for the Ionic Conductivity, σ = σ0 exp(−B/(T − T0))

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RESULTS Ionic Conductivity. The numerical values of temperature dependent ionic conductivity of the three DEME-based ILs are given in Table 1 measured under ordinary pressure (p = 0.1

DEME-TFSA DEME-CF3BF3 DEME-BF4

Table 1. Ionic Conductivity (mS·cm−1) for DEME-TFSA, DEME-CF3BF3, and DEME-BF4 (p = 0.1 MPa)a temp/K

DEME-TFSA

DEME-CF3BF3

DEME-BF4

353.15 343.15 333.15 323.15 313.15 303.15 293.15 283.15 273.15 263.15 253.15 243.15 234.15

13.3 10.9 8.69 6.70 4.88 3.36 2.16 1.27 0.662 0.301 0.113 0.033 0.0083

14.2 11.6 9.37 7.20 5.30 3.73 2.49 1.56 0.897 0.474 0.220

11.6 8.57 6.12 4.14 2.62 1.53 0.821 0.381 0.156 0.055

σ0 (S·cm−1)

B (K−1)

T0 (K)

R2

0.621 ± 0.1 0.708 ± 0.2 2.14 ± 0.4

679 ± 21 749 ± 25 920 ± 34

174 ± 1 161 ± 2 176 ± 2

0.999 0.999 0.999

Density. Numerical values of temperature dependent density under ordinary pressure (p = 0.1 MPa) for the DEME-based TFSA, CF3BF3, and BF4 are given in Table 3, Table 3. Density (g·cm−3) for DEME-TFSA, DEME-CF3BF3, and DEME-BF4 (p = 0.1 MPa)a

a

Standard uncertainties u are u(temperature) = 0.01 K and u(ionic conductivity) = 0.5 %.

MPa). The Arrhenius plots of the ionic conductivities shown in Figure 1 are curved and fitted with a Vogel−Fulcher− Tammann (VFT) type relationship (i.e., σ = σ0 exp(−B/(T − T0)) where σ0, B, and T0 are the fitting parameters) and the best fitting parameters are given in Table 2. At high temperature the ionic conductivities of the three ILs gave the similar values. As the temperature decreased the ionic conductivity of DEME-BF4 dropped significantly and DEMECF3BF3 showed a little higher ionic conductivity rather than DEME-TFSA. At 283 K the ionic conductivities were (1.27, 1.56, and 0.381·10−3) S·cm−1 for the DEME based TFSA, CF3BF3, and BF4, respectively. The corresponding literature values at 283 K were larger than our values and (2.6, 3.0, and 1.3) mS·cm−1.4

temp/K

DEME-TFSA

DEME-BF3CF3

DEME-BF4

353.15 348.15 343.15 338.15 333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15 288.15 283.15

1.3573 1.3617 1.3661 1.3705 1.3750 1.3795 1.3840 1.3886 1.3932 1.3977 1.4023 1.4069 1.4115 1.4161 1.4206

1.1929 1.1966 1.2003 1.2040 1.2077 1.2115 1.2152 1.2189 1.2227 1.2264 1.2302 1.2340 1.2377 1.2415 1.2453

1.1418 1.1450 1.1482 1.1514 1.1547 1.1579 1.1611 1.1644 1.1677 1.1710 1.1744 1.1778 1.1813 1.1848 1.1882

a Standard uncertainties u are u(temperature) = 0.01 K and u(density) = 0.0005.

whose molecular weights are 427.4, 284.1, and 234.1, respectively. Linear temperature dependences of the density are shown in Figure 2 and the analysis was made for the density ρ versus temperature T (K) as ρ = b − aT. The best fitting parameters are given in Table 4. At 298 K the densities were (1.407, 1.234, and 1.178) g·cm−3 for DEME-based TFSA, CF3BF3, and BF4, respectively and the corresponding literature values were (1.42, 1.25, and 1.20) g·mL−1.4 The densities 1946



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Figure 2. Temperature dependent density for DEME-based TFSA (down-triangle), CF3BF3 (up-triangle), and BF4 (circle).

Table 4. Fitting Parameters of the Density for DEME-BF4, DEME-CF3BF3, and DEME-TFSA, ρ = b − aT b (g·cm−3) DEME-TFSA DEME-CF3BF3 DEME-BF4

1.4295 ± 0.0001 1.2567 ± 0.0001 1.1944 ± 0.0001

a (g·cm−3·K−1) −4

R2

(9.07 ± 0.02)·10 (7.486 ± 0.007)·10−4 (6.62 ± 0.03)·10−4

0.999 94 0.999 98 0.999 75

obtained are well correlated with the molecular weights described above. Viscosity. The numerical values of temperature dependent viscosity for the three ILs measured under ordinary pressure (p = 0.1 MPa) are given in Table 5. The viscosities η of the three

Figure 3. (a) Temperature-dependent viscosity η and (b) Arrhenius plots of η−1 (fluidity) for DEME-based TFSA (down-triangle), CF3BF3 (up-triangle), and BF4 (circle).

exp(−B/(T − T0)) . The best fitting parameters are given in Table 6. At 298 K the viscosities were (71.8, 105, and 422) mPas for DEME-based TFSA, CF3BF3, and BF4, respectively, and the corresponding literature values were (69, 108, and 426) cP.4 Ion Diffusion Coefficient. The NMR spectral patterns of 19 F, 11B, and 1H resonances are given in Figure S1 in the Supporting Information. Arrhenius plots of the ion diffusion coefficients of DEME (DDEME) and anions (DBF4, DCF3BF3, and DTFSA) are shown in Figure 4. The numerical values of ion diffusion coefficient for DEME and anions are given in Table 7. At 288 K, the values of DDEME = 7.7 and DCF3BF3 = 8.0·10−12 m2·s−1 for DEME-CF3BF3 and the corresponding literature values are 1.1 and 1.0·10−11 m2·s−1 at 289 K, respectively. For DEME-BF4, DDEME = 1.7 and DBF4 = 2.1·10−12 m2·s−1 at 288 K and the corresponding literature values are 3.5 and 4.2·10−12 m2·s−1 at 289 K, respectively.5 Usually, smaller diffusion constants are reliable, because many measuring factors of PGSE experiments cause larger experimental values. The relative values of DDEME/DBF4 are comparable. The ionic conductivities of the present ILs were also smaller than those of the literature values as described above. The plots can be fitted by the VFT-type equation D = D0 exp(−B/(T − T0)) and the best fitting parameters for each ion diffusion coefficient are given in Table 8, where those for the lithium salt doped ILs are included. In the high temperature region the plots are almost linear and the activation energies of the neat ILs were summarized in Table 9. Those estimated for the binary systems are also included in Table 9. In the neat samples, the activation

Table 5. Viscosity (mPas) for DEME-TFSA, DEME-CF3BF3, and DEME-BF4 (p = 0.1 MPa)a temp/K

DEME-TFSA

DEME-BF3CF3

DEME-BF4

353.15 348.15 343.15 338.15 333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15 288.15 283.15

11.4 12.8 14.5 16.6 19.1 22.2 26.1 31.0 37.4 45.7 56.8 71.8 92.7 123 166

16.7 19.0 21.7 24.9 28.8 33.6 39.5 47.0 56.4 68.5 84.1 105 133 170 223

30.5 36.0 43.0 52.0 63.7 79.1 99.7 128 167 222 303 422 604 890 1350

a

Standard uncertainties u are u(temperature) = 0.01 K and u(viscosity) = 0.35 %.

ILs are plotted versus temperature in Figure 3a. DEME-TFSA showed the smallest viscosity and the values of DEME-CF3BF3 were a little larger and changed in a parallel manner with DEME-TFSA. Clearly, DEME-BF4 gave the largest viscosity and increased remarkably as the temperature decreased. The Arrhenius-type plots for η−1 (fluidity) are shown in Figure 3b and the plots were well fitted by VFT equation η−1 = η0−1 1947



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Table 6. VFT Parameters of the Viscosity for DEME-BF4, DEME-CF3BF3, and DEME-TFSA, η−1 = η−1 0 exp(−B/(T − T0)) DEME-TFSA DEME-CF3BF3 DEME-BF4

η0−1 (mPas)−1

η0 (mPas)

B (K−1)

T0 (K)

R2

4.81 ± 0.02 9.65 ± 0.1 12.9 ± 1

0.208 0.104 0.077

697.2 ± 0.6 1055 ± 2 1073 ± 14

178.7 ± 0.1 145.6 ± 0.2 173.3 ± 0.8

1 1 0.999 99

BF4. When a cation size is larger than the counteranion size like N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)amide (P13−FSA)21 and 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIm-BF4),22 the relationship of the relative ion diffusion coefficients of anion and cation becomes inverse. In the present DEME-based ILs, the relationships of the DEME and anion diffusion coefficients are DDEME < DBF4, DDEME ≤ DCF3BF3 and DDEME > DTFSA. Lithium Salt Doping Effects on Ion Diffusion Coefficients. Generally, when the lithium salt is added to an IL, the viscosity increases.15,19−21 The behaviors of ion diffusion coefficients in the DEME-TFSA-Li systems with increasing salt concentration have been reported in our previous papers.15 For the neat DEME-BF4 and lithium salt doped DEME-BF4−Li, Arrhenius plots of the individual ion diffusion coefficients of DEME, BF4, and Li are shown in Figure 5, where a little decreases in DDEME and DBF4 with lithium salt doping were observed and much slower lithium diffusion (DLi) was clearly shown. The VFT parameters for the doped samples DEMEBF4−Li are included in Table 8. The Arrhenius plots of the diffusion coefficients for DEME-CF3BF3−Li and DEME-TFSALi systems are shown in Figures S2 and S3, respectively and the VFT fitting parameters are summarized as shown in Table 8. In Figure 6 Arrhenius plots of the lithium diffusion coefficients (DLi) for the three binary ILs are shown. The DLi(DEME-BF4− Li) was the smallest at all temperatures. At the high temperatures, the DLi(DEME-TFSA-Li) was the largest while in the low temperature DLi(DEME-CF3BF3−Li) was larger than DLi(DEME-TFSA-Li). 7 Li T1 of the Lithium Salt Doped Samples. Arrhenius plots of the 7Li T1 for the three lithium salt doped binary

Figure 4. Arrhenius plots of the ion diffusion coefficients of DEME (up-triangle) and anions (down-triangle) for the DEME-based TFSA, CF3BF3, and BF4.

energies of the DEME and anion within the same ILs were almost the same. The activation energies increase as DEMEBF4 > DEME-CF3BF3 > DEME-TFSA. The cation diffusion coefficients DDEME were (3.4·10−12, 1.2·10−11 and 1.7·10−11) m2·s−1 for the DEME-based BF4, CF3BF3, and TFSA at 298 K, respectively. At the same temperature the anion diffusion coefficients were DBF4 = 4.2·10−12, DCF3BF3 = 1.3·10−11, and DTFSA = 1.5·10−11 m2·s−1. Clearly, DEME-TFSA showed the largest values, next DEME-CF3BF3 and DEME-BF4 had the much smaller D values. This trend resembles to the Arrhenius plots of η−1 in Figure 3b. Generally, ILs show that the cation diffusion is faster than anion diffusion (Dcation > Danion). It is true for DEME-TFSA but DBF4 is larger than DDEME in DEME-

Table 7. Self-Diffusion Coefficients (m2·s−1) of Ions for DEME-TFSA, DEME-CF3BF3, and DEME-BF4 (p = 0.1 MPa)a DEME-TFSA

a

DEME-CF3BF3

DEME-BF4

temp/K

TFSA

DEME

CF3BF3

DEME

BF4

DEME

373 363 353 343 333 323 313 303 298 293 288 283 278 273 268 263 258 253

2.17·10−10 1.53·10−10 1.17·10−10 8.39·10−11 6.37·10−11 4.42·10−11 3.11·10−11 1.98·10−11 1.53·10−11 1.27·10−11 9.62·10−12 7.75·10−12 5.66·10−12 4.49·10−12 3.08·10−12 2.08·10−12 1.42·10−12 8.13·10−13

2.29·10−10 1.63·10−10 1.23·10−10 9.19·10−11 6.95·10−11 5.01·10−11 3.50·10−11 2.25·10−11 1.72·10−11 1.45·10−11 1.07·10−11 8.99·10−12 6.68·10−12 5.09·10−12 3.56·10−12 2.43·10−12 1.62·10−12 1.05·10−12

1.58·10−10 1.22·10−10 9.20·10−11 6.82·10−11 5.09·10−11 3.65·10−11 2.54·10−11 1.66·10−11 1.31·10−11 1.07·10−11 8.01·10−12 6.43·10−12 4.78·10−12 3.56·10−12 2.70·10−12 1.88·10−12 1.31·10−12 8.35·10−13

1.39·10−10 1.13·10−10 8.56·10−11 6.37·10−11 4.60·10−11 3.36·10−11 2.38·10−11 1.60·10−11 1.23·10−11 9.72·10−12 7.65·10−12 6.06·10−12 4.55·10−12 3.41·10−12 2.51·10−12 1.80·10−12 1.24·10−12 8.33·10−13

1.13·10−10 8.41·10−11 5.91·10−11 4.20·10−11 2.79·10−11 1.78·10−11 1.06·10−11 5.93·10−12 4.15·10−12 3.00·10−12 2.12·10−12 1.41·10−12 9.35·10−13 5.7·10−13

8.84·10−11 6.72·10−11 4.82·10−11 3.33·10−11 2.23·10−11 1.42·10−11 8.5·10−12 4.74·10−12 3.36·10−12 2.35·10−12 1.66·10−12 1.16·10−12 7.50·10−13 4.0·10−13

Standard uncertainties u are u(temperature) = 0.2 K and u(self-diffusion coefficient) is less than 2 %. 1948



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Table 8. VFT Fitting Parameters of the Ion Diffusion Coefficients for the Neat and Lithium Salt doped Binary Samples. D = D0 exp(−B/(T − T0)) sample DEME-TFSA DEME-CF3BF3 DEME-BF4 DEME-TFSA-Li

DEME-CF3BF3−Li

DEME-BF4−Li

ion DEME TFSA DEME CF3BF3 DEME BF4 DEME TFSA Li DEME CF3BF3 TFSA Li DEME BF4 Li

D0 (m2·s−1) −8

6.17·10 6.44·10−8 5.33·10−8 6.28·10−8 2.99× 10−8 6.23·10−8 5.16× 10−8 1.24·10−8 8.17·10−8 2.21·10−8 4.76·10−8 3.53·10−8 7.21·10−8 2.36·10−8 2.46·10−8 2.33·10−8

B (T−1)

T0 (K)

R2

1423 ± 112 1447 ± 134 1544 ± 48 1561 ± 65 1217 ± 83 1385 ± 40 1311 ± 131 878 ± 140 1511 ± 220 1265 ± 678 1464 ± 90 1410 ± 116 1924 ± 218 1147 ± 61 1121 ± 69 1233 ± 80

113 ± 7 124 ± 8 113 ± 3 113 ± 4 164 ± 5 154 ± 2 142 ± 8 176 ± 10 139 ± 12 136 ± 4 127 ± 5 128 ± 7 101 ± 11 173 ± 4 175 ± 4 174 ± 4

0.999 27 0.998 93 0.9999 0.999 82 0.999 63 0.999 94 0.999 17 0.996 64 0.998 65 0.999 69 0.999 61 0.9993 0.9992 0.999 74 0.999 65 0.9997

Table 9. Activation Energy Ea (kJmol−1) for Individual Ion Diffusion Coefficients sample DEME-BF4

DEME-TFSA

DEME-CF3BF3

neat sample BF4 DEME Li TFSA DEME Li CF3BF3 DEME Li

38.2 37.9 32.5 31.8 32.1 31.4 -

± 0.7 (313−373 K) ± 0.8 (313−373 K) ± 0.3 (273−373 K) ± 0.3 (273−373 K) ± 0.4 (273−373 K) ± 0.4 (273−373 K)

39.2 38.7 42.9 35.2 33.1 37.1 32.7 31.0 35.4

± ± ± ± ± ± ± ± ±

0.8 0.8 0.7 0.5 0.3 0.8 0.7 0.7 0.3

(313−373 (313−373 (313−373 (303−373 (303−373 (303−373 (293−373 (293−373 (263−373

K) K) K) K) K) K) K) K) K)

Figure 6. Arrhenius plots of the DLi of the binary samples with the lithium salt concentration of 0.32 mol·kg−1 for DEME-based BF4−Li (circle), CF3BF3−Li (up-triangle), and TFSA-Li (down-triangle). The data were fitted by the VFT equation and fitting parameters are given in Table 8.

Figure 5. Arrhenius plots of individual ion diffusion coefficients for BF4 (down-triangle), DEME (up-triangle), and Li (asterisk) of DEMEBF4−Li and neat DEME-BF4 where open symbols indicate the neat sample.

where ωq = 2πνq is the quadrupolar coupling constant. In the temperature range from (258 to 373) K, an Arrhenius-type plot of 7Li T1 showed a minimum in each sample. The relaxation minimum should occur when ω0τc = 2πν0τc = 0.616 (in the present study, for 7Li ν0 = 105.0 MHz). The 7Li correlation time τc(Li) at the minimum temperature is 9.34·10−10 s (934 ps). By the numerical calculation, we can estimate the quadrupolar coupling constant and τc (Li) at every temperature. The estimated 7Li quadrupolar coupling constants in the five ILs are summarized in Table S1 in the Supporting Information. From the estimated ωq value and experimental T1

samples are shown in Figure 7a. The calculation of lithium correlation time τc(7Li) was made under the assumption of the quadrupolar relaxation mechanism as described in our previous papers.15,20−22,24 The T1 for quadrupolar nuclei like 7Li (I = 3/ 2) undergoing isotropic reorientational diffusion is given by30 ⎞ ωq 2 ⎛ 4τc τc 1 ⎟ ⎜ = + 2 2 2 2 T1 50 ⎝ 1 + ω0 τc 1 + 4ω0 τc ⎠

lithium salt doped sample

(2) 1949



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Figure 7. Arrhenius plots of (a) the 7Li T1 and (b) the τc(Li) for the lithium salt doped binary ILs of DEME-BF4−Li (circle), DEMECF3BF3−Li (up-triangle), and DEME-TFSA-Li (down-triangle).

at a certain temperature, we can estimate the τc value at that temperature using eq 2. The temperature dependent τc values were plotted in Figure 7b. Since the Li+ can be assumed to be cubic symmetry, the main relaxation mechanism is translational diffusion for the quadrupolar 7Li. The τc(7Li) corresponds to one-flip time of the translational displacement. As shown in Figure 7b, the τc(Li) was the longest for DEME-BF4−Li, next DEME-CF3BF3−Li, and shortest in DEME-TFSA-Li. The activation energies obtained from the linear plots in Figure 7(b) τc(7Li) were (17.9 ± 0.5, 15.7 ± 0.4 and 19.3 ± 0.2) kJ· mol−1 for DEME-TFSA-Li, DEME-CF3BF3−Li, and DEMEBF4−Li, respectively. The activation energies for the lithium diffusion coefficients in Table 9 are (37.1, 35.4, and 42.9) kJ· mol−1 for the corresponding binary systems and more than twice time larger compared with the activation energies obtained from 7Li T1 measurements. Clearly, the lithium oneflip based on 7Li T1 can be thermally accelerated easily compared with the lithium diffusion in the longer observation time.

Figure 8. Ion diffusion coefficients are plotted against kT/πη for neat samples of (a) DEME and (b) the anions. The fitting as linear lines was made by a software Origin 8.

Table 10. Calculated Parameters from the SE Equation c·a [10−10 m]

radius (a) [10−10 m]

c

DEME

BF4 CF3BF3 TFSA DEME

10.65 ± 0.05 11.04 ± 0.07 11.38 ± 0.08 8.62 ± 0.06 10.22 ± 0.05 12.04 ± 0.07

3.49

3.05 3.16 3.26 3.80 3.89 3.66

2.27 2.63 3.29

the temperature range between (283 and 353) K as shown in Table 10. The empirical c values of DEME are 3.0−3.3 and the anions have larger c values. They are all smaller than 4 and outside the theoretical predictions. We have evaluated the experimental c values for various ions in ILs.20−22,24 The empirical c(cation) values are following: c(EMIm); 3.1 in EMIm-BF4, 2.8 in EMIm-TFSA and 2.5 in EMIm-FSA, c(P13); 3.0 in P13-TFSA and 3.2 in P13−FSA. c(BMIm); 3.1 in BMImBF4. The empirical c values for the anions are following: c(BF4); 3.8 in DEME-BF4, 4.4 in BMIm-BF4 and 4.5 in EMIm-BF4 and c(TFSA) 3.5−3.8 in five ILs and c(FSA) 2.8−3.1 in two ILs. Ionic Conductivity and Viscosity (Walden Plot). The molar conductivity was calculated by using the density data and the Walden plots of the molecular conductivity versus η−1 proposed by Angell and co-workers36 are shown in Figure 9. The Walden rule relates the molecular mobility (η−1) to the charged ions in solution electrolytes and can characterize ILs. The fully dissociated ions like diluted aqueous KCl solution give a behavior shown in a line in Figure 9. The deviations from the ideal plot, ΔW have been proposed to relate with the ion pairing of ILs.37 The deviations are in the order of DEME-BF4 < DEME-CF3BF3 < DEME-TFSA and they were 0.04, 0.14, and 0.24 at 303 K, respectively. Molar Conductivity (Nernst−Einstein Equation). From the self-diffusion coefficients of the cation and anion, the molar

DISCUSSION Ion Diffusion Coefficient and Viscosity (Stokes− Einstein Relationship). The classical Stokes−Einstein (SE) equation relating with the diffusion coefficient D to the viscosity η is given by kT cπηa

counter ion

BF4 CF3BF3 TFSA

■

D=

ion

(3)

where a is the Stokes radius for the diffusing species, k is the Boltzmann constant, and the constant c ranges between 4 to 6 for the slip and stick boundary conditions, respectively.31,32 Here, attempts were made to plot the ion diffusion coefficients D versus kT/πη in Figure 8 for (a) DEME and (b) the anions. Clearly linear relationships were obtained and the gradient corresponds to 1/ca in eq 3. The inverse of the gradients, c·a are given in Table 10. Here we assume that the radius a can be evaluated by van der Waals radius33 and the estimated a values for DEME, BF4, CF3BF3, and TFSA are given in Table 10.34,35 Automatically, the experimental c values can be calculated in 1950



Article

Figure 9. Walden plots (molar conductivity versus η−1 in the unit of poise−1) over the temperature range between (283 and 353) K for DEME-BF4 (circle), DEME-CF3BF3 (up-triangle), and DEME-TFSA (down-triangle). The line indicates the ideal KCl.

Figure 10. Molar conductivity for DEME-TFSA (down-triangle), DEME-CF3BF3 (up-triangle), and DEME-BF4 (circle). Open and solid symbols were calculated from NMR diffusion constants and ionic conductivity, respectively. The dotted lines indicate the linear plots for ΛNMR.

ionic conductivity can be calculated using the Nernst−Einstein (NE) equation,38

Table 11. Activation Energy Ea (kJ mol−1) for the Molecular Conductivities Λimp and ΛNMR

ΛNE =

zF 2 (D+ + D−) RT

(4)

DEME-BF4 DEME-CF3BF3 DEME-TFSA

where z is the ionic valency, F is Faraday’s constant, R is the gas constant, and D+ and D− are self-diffusion coefficients of the cation and anion under the assumption of complete ion dissociation. Since the NMR self-diffusion measurements reflect an averaged of the self-diffusion coefficients of a species in both free and associated forms, the value of the ΛNMR following eq 4 is always overestimated. The molar ionic conductivity (equivalent conductivity) Λimp can be obtained from the electrochemical impedance method by measuring only the charged ions. The Λimp/ΛNMR calculated as the ratio of Λimp relative to the ΛNMR from NMR diffusion measurements has been named as ionicity of ILs proposed by Tokuda et al.,39 and relates to the ratio of the charge carrying ions. The temperature dependent Λimp and ΛNMR are plotted versus temperature in Figure 10. Since the plots in Figure 10 are almost linear, the activation energies Ea were calculated and given in Table 11. The Ea(Λimp) and Ea(ΛNMR) are almost the same and similar to the Ea values of the diffusion coefficients given in Table 9 in the high temperature range. The Λimp/ΛNMR values are plotted versus temperature in Figure 11 for the DEME-based ILs. The Λimp/ΛNMR values are largest for DEME-BF4 and the DMECF3BF3 gave slightly larger values than DEME-TFSA. This trend is consistent with the Walden plot in Figure 9 where the deviation ΔW is smallest in DEME-BF4, then DEME-CF3BF3 became larger and DEME-TFSA gave the largest ΔW values. Thus, the relationship between the viscosity and ionic conductivity and that between the diffusion coefficient and ionic conductivity are consistent with each other. The stabilization energies (Eform) calculated for the LiBF4, LiCF3BF3, and LiTFSA complexes were (−144.1, −139.3, and −137.2) kcal·mol−1, respectively.5,16 The Eform calculated for the three complexes show that the magnitude of interaction between the cation and anion depends strongly on the anion. The static interaction in the BF4 complex is substantially larger than those in the CF3BF3 and TFSA complexes. The interaction in the TFSA complex is slightly weaker than that in the CF3BF3 complex. The same anion dependence of the

Λimp

ΛNMR

40.8 ± 1.3 28.7 ± 0.7 28.2 ± 1.1

41.6 ± 1.0 28.7 ± 0.4 28.9 ± 0.5

Figure 11. Λimp/ΛNMR values were plotted versus temperature for DEME-BF4 (circle), DEME-CF3BF3 (up-triangle), and DEME-TFSA (down-triangle).

E form was found for the EMIm complexes. 40 The Eform calculated for the EMIm complex with BF4 (−85.2 kcal/mol) is substantially larger than that for the EMIm complex with TFSA (−78.8 kcal/mol) and the Λimp/ΛNMR value for the EMIm-BF4 is larger than that for the EMIm-TFSA.40 In the present work the Λimp/ΛNMR value for the DEME-BF4 is also larger than those for the DEME-CF3BF3 and DEME-TFSA. We have reported that the Λimp/ΛNMR depends on the magnitude of the interaction between the cation and anion and on the shapes of mutual ions. The Λimp/ΛNMR becomes larger when the interaction is weaker and the shape of ion is close to a sphere. Probably the nearly round shape of BF4 is mainly responsible for the large Λimp/ΛNMR of the DEME-BF4 compared with that of the DEME-CF3BF4 and DEME-TFSA. 1951



Article

Above room temperature the Λimp/ΛNMR values decreased in the all ILs. Since the ionic conductivity is defined as the number of charged ions n multiplied by the velocity, ν. The ν must be parallel to the D, although the question remains that isolated and paired ions diffuse with the same velocity. Similarly, the D+ and D− can be measured independently, but these values are weighted averages of the isolated and paired ions, for example, D+ must be affected by a counteranion. Here we simply assumed that the discrepancy between Λimp and ΛNMR is related with the relative numbers of the charged and paired ions, and/ or the lifetime of the paired ions. Experimentally, although the temperature dependences of the Λimp/ΛNMR were not large, the decreasing tendency was commonly observed for the three ILs. The decrease of the Λimp/ΛNMR at the higher temperature consists with the increase of the deviation from the ideal Walden line at the high temperature. It is surprising that the ion-pairing tendency is supposed to increase as the increase of temperature in the present DEME ILs. The ion association scheme must change depending on temperature and further discussion is necessary. Lithium Salt Doping Effects. The lithium salt doped binary samples gave the slower diffusion coefficients of DEME and anions as shown in Figures 5, S2 and S3 for DEME-BF4− Li, DEME-CF3BF3−Li, and DEME-TFSA-Li systems, respectively. We have reported the salt concentration effects on the diffusion coefficients previously for the DEME-TFSA-Li and EMIm-BF4−Li systems.15,23 In the present systems, the lithium salt doping effects for the DEME diffusion were calculated as the ratio of DDEME(binary) relative to DDEME(neat) at 303 K and they were 0.80, 0.71, and 0.68 for DEME-BF4−Li, DEMECF3BF3−Li, and DEME-TFSA-Li, respectively, where the larger values correspond the smaller decrease of the diffusion coefficients. Also the effects on the anion D’s were 0.71, 0.67, and 0.60 for DBF4, DCF3BF3, and DTFSA, respectively. The lithium doping effects on the ion diffusion coefficients for the anions were larger compared with the DDEME and indicate a lithium ion interact preferably with anions in the individual systems. The magnitude of the lithium salt doping effects to the ion diffusion constants were in the order of DEME-BF4−Li < DEMECF3BF3−Li < DEME-TFSA-Li both in the cation and anions. The activation energies in Table 9 indicate that the Ea(DEME) and Ea(anion) increased slightly in the binary samples, and the Ea(Li) had the largest values in the individual systems. Since the 7Li resonance was sharp which indicates the lithium ion has cubic symmetry, and a possible 7Li relaxation mechanism is translational motion. The physical interpretation of τc(Li) in Figure 7(b) is the lithium jump from one place to another. The average distances of a single lithium jump was calculated by assuming that the DLi of the jump is the same as the measured value in μm space, i.e., ⟨Rone‑flip⟩ = (6Dτc(Li))1/2. The one-jump distances calculated are shown in Figure 12. The lithium jump distance became longer with increasing temperature and the mobility of the lithium in the ILs was in the order as DEME-TFSA-Li > DEME-CF3BF3−Li > DEME-BF4−Li. In the present study, the lithium one-jump distance was very short, and the distances longer than the van der Waals radius of the anions (in Table 10) were above 323, 313, and 343 K for DEME-BF4−Li, DEME-CF3BF3−Li and DEME-TFSA-Li, respectively. It must be noted that a lithium ion is generally solvated by two or three anions in binary ILs.20−22

Figure 12. Averaged Li one-jump distances versus temperature in the DEME-TFSA-Li (down-triangle), DEME-CF3BF3−Li (up-triangle), and DEME-BF4−Li (circle).

■

CONCLUSION The three ionic liquids composed of a quaternary ammonium cation having an ether chain, N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium (DEME) cation and different anions of BF4, CF3BF3, and TFSA were studied by measuring viscosity, density, ionic conductivity, and ion diffusion coefficient by NMR. The viscosity increased in the order of DEME-TFSA < DEME-CF3BF3 < DEME-BF4 and consistently the diffusion coefficient and density decreased in the order DEME-TFSA > DEME-CF3BF3 > DEME-BF4, while the ionic conductivity decreases in the order DEME-CF3BF3 > DEME-TFSA > DEME-BF4. From the Walden plots and the Nernst−Einstein model, the degree of the ion-pairing was in the order DEMETFSA > DEME-CF3BF3 > DEME-BF4 in the temperature range from (283 to 353) K. The 7Li resonance was observed for the lithium salt doped samples and the lithium diffusion coefficients were in the order of DEME-TFSA-Li > DEMECF3BF3−Li > DEME-BF4−Li above room temperature. For the lithium salt doped samples, the temperature dependences of lithium one-jump distance were estimated.

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ASSOCIATED CONTENT

S Supporting Information *

Estimation of quadrapolar coupling constants for the 7Li and 11 B nuclei, 1H, 19F and 11B spectral patterns. Arrhenius plots of the D’s of DEME-CF3BF3−Li and DEME-TFSA-Li. 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]. Present Address §

Institute of Applied Physics, University of Tsukuba, Tsukuba 305−8573, Japan. Notes

The authors declare no competing financial interest. 1952



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ACKNOWLEDGMENTS The authors express their sincere thanks to G. Masuda of Nisshinbo for offering us the samples.

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Transport and Electrochemical Properties of Three Quaternary

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