Influence of LiTFSI Addition on Conductivity, Diffusion Coefficient, Spin

Article pubs.acs.org/jced

Influence of LiTFSI Addition on Conductivity, Diffusion Coefficient, Spin−Lattice Relaxation Times, and Chemical Shift of One-Dimensional NMR Spectroscopy in LiTFSI-Doped Dual-Functionalized Imidazolium-Based Ionic Liquids Tzi-Yi Wu,† Yi-Hsuan Wang,‡ Shyh-Gang Su,‡ Yuan-Chung Lin,§ Chung-Wen Kuo,∥ Jeng-Kuei Chang,⊥ and I-Wen Sun*,‡ †

Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan, ROC ‡ Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan, ROC § Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC ∥ Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan, ROC ⊥ Institute of Materials Science and Engineering, National Central University, Taoyuan 32001, Taiwan, ROC S Supporting Information *

ABSTRACT: An ionic liquid (IL) 1-allyl-3-(2-methoxyethyl)imidazolium bis(trifluoromethylsulfonyl)imide ([AMO][TFSI]) is prepared, and the effect of the addition of LiTFSI into [AMO][TFSI] on the transport and physicochemical properties is studied herein. The diffusion coefficients of 1H, 7Li, and 19F are determined using pulsed-gradient spin−echo NMR to study the dynamics of all ion species. The neat [AMO][TFSI] and LiTFSI-doped [AMO][TFSI] give approximately straight lines for the relationship of D vs Tη−1, demonstrating that the Stokes−Einstein equation holds for the ionic diffusivity in the binary system. NMR T1 relation time measurements show the 1H-T1 and 19F-T1 of LiTFSI-doped [AMO][TFSI] decrease with the increase of Li salt concentration, which is due to the viscosity increases and the formation of stable coordination adducts of Li and TFSI when the salt concentration increases. From the study of chemical shift in one-dimensional NMR spectroscopy, an upfield shift in 1 H and 19F spectra is observed in ILs with increasing lithium salt concentration; the formation of ion clusters is the dominant effect after the addition of LiTFSI in [AMO][TFSI].

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triflate, and bistrifluorosulfonylimide anions.4 Among them, most investigations on ILs are related to imidazolium derivatives due to their low viscosity and high conductivity. Moreover, taskspecific ionic liquids (TSIL) are interesting materials for particular applications.5 Kimizuka and Nakashima used etherbased ILs to dissolve glucose;6 the ether linkage acts as a hydrogen bond acceptor for the hydroxyl group of carbohydrates, which

INTRODUCTION Ionic liquids (ILs) are constructed wholly of ions, and many of them at temperatures below 100 °C are liquids. Latterly, ILs as a possible substitute for volatile solvents have caused a breakthrough in the field of solvents.1 Their intrinsic properties, such as negligible volatility and nonflammability, make them conspicuous candidates as electrolytes in lithium ion batteries,2 an electrochromic device,3 and solar cells. So far the most commonly investigated ILs ordinarily contain imidazolium, ammonium, pyridinium, pyrrolidinium, and phosphonium cations and tetrafluoroborate, hexafluorophosphate, © XXXX American Chemical Society

Received: May 14, 2014 Accepted: January 16, 2015

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DOI: 10.1021/je500431h J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

phase contains unreacted material, which was decanted. The bottom phase was added with 50 mL of diethyl ether and stirred thoroughly. The diethyl ether was decanted followed by addition of fresh diethyl ether, and this step was repeated three times. The solvent was evaporated and dried at 80 °C under vacuum for 2 days. Yield: 81 %. 1H NMR (300 MHz, D2O, ppm): δ 3.40 (s, 3H, -CH3), 3.86 (t, 2H, -CH2-), 4.43 (t, 2H, −N−CH2-), 4.85 (d, 2H, -N−CH2-), 5.37−5.47 (m, 2H, -CHCH2), 6.03−6.12 (m, 1H, -CHCH2), 7.53 (d, 1H, imidazolium proton), 7.57 (d, 1H, imidazolium proton), 8.85 (s, 1H, imidazolium proton). Elem. Anal. Calcd for C9H15ClN2O: C, 53.33 %; H, 7.46 %; N, 13.82 %. Found: C, 53.18 %; H, 7.38 %; N, 13.84 %. Synthesis of 1-Allyl-3-(2-methoxyethyl)imidazolium Bis(trifluoromethylsulfonyl)amide ([AMO][TFSI]). LiTFSI (100 mmol, 28.7 g), [AMO][Cl] (100 mmol, 20.3 g), and 50 mL of deionized water were added to a flask. The mixture was stirred at 60 °C for 3 h. After cooling, an oily product was formed, which was extracted with chloroform. The solvent was evaporated and the oily liquid was dried in a vacuum at 313.15 K to eliminate the water. Yield: 91 %. 1H NMR (400 MHz, DMSO-d6, ppm): δ 3.33 (s, 3H, -CH3), 3.75 (t, 2H, -CH2-), 4.38 (t, 2H, N−CH2-), 4.84 (d, 2H, N−CH2-), 5.40−5.45 (m, 2H, -CHCH2), 6.00−6.10 (m, 1H, -CHCH2), 7.44 (d, 1H, imidazolium proton), 7.53 (d, 1H, imidazolium proton), 8.63 (s, 1H, imidazolium proton). 19 F NMR (400 MHz, DMSO-d6): 83.96 ppm. Elem. Anal. Calcd for C11H15F6N3O5S2: C, 29.53 %; H, 3.38 %; N, 9.39 %. Found: C, 29.45 %; H, 3.36 %; N, 9.34 %. The structure of [AMO][TFSI] is shown in Figure 1.

increases the solubility of carbohydrates. Allyl-based ILs possess lower viscosity and stronger dissolution abilities for cellulose than those of other ILs.7 To better understand the character of ILs and broaden their applications in electrolytes, it is necessary to realize their thermophysical properties. The design of novel product developments based on ILs can be carried out when their thermophysical properties, such as conductivity, surface tension, and thermal stability, are amply characterized. Moreover, the translational diffusion coefficient (D) and spin−lattice relaxation time (T 1 ) of liquids are two of the most important thermophysical properties for investigating the interactions in liquids.8 PGSE-NMR is an effective method for the analysis of D; this can be attributed to it being able to measure the diffusion coefficients at various temperatures. Furthermore, NMR supplies temperature-dependent T1 of 1H and 19F nuclei for the cations and anions. In the present study, a dual-functionalized imidazolium-based IL with ether- and allyl-containing imidazolinium was synthesized and the effect of the addition of LITFSI on the thermophysical properties was studied. Such an IL contains asymmetric functional groups at the side chain of imidazolinium, which is commonly accompanied by low melting point, low viscosity, and high conductivity. The diffusion coefficients of 1-allyl-3-(2-methoxyethyl)imidazolium cation (DAMO+), TFSI anion (DTFSI−), and lithium ion (DLi+) at different LiTFSI concentrations were measured using 1H, 7Li, or 19F NMR, respectively. DAMO+ and DTFSI− are plotted versus 1/η by the Stokes−Einstein (SE) equation. The rotational motions of ILs were investigated using 1H and 19F spin−lattice relaxation (T1) measurements at various temperatures.

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EXPERIMENTAL SECTION Method. The density of a neat IL (or a binary mixture) was measured using a dilatometer, which was calibrated by measuring the density of neat glycerin at (30, 40, 50, 60, 70, and 80) °C. An IL was placed into the dilatometer up to the mark, the top of the capillary tube (located on the top of the dilatometer) was sealed, and the dilatometer (with capillary tube) was placed into a thermostatic water bath. The main interval between the two marks in the capillary tube was 0.01 cm3, and the minor interval between the two marks was 0.001 cm3. From the correction coefficient of glycerin in the capillary tube at different temperatures, the density of neat IL or LiTFSI-doped IL by the expanded liquid volume in the capillary tube at various temperatures was calculated. Each sample was measured at least three times to determine the average value. The viscosities of the ILs were estimated using a calibrated Ostwald viscometer (Cannon-Fenske glass capillary viscometers, CFRU, 9721-A50), the viscometer was also placed in a thermostatic water bath. The time of flow was measured with a stopwatch (0.01 s precision). For each IL, the experimental viscosity was estimated by averaging three to five measurements of flow time. The conductivity of the ILs was measured using a conductivity meter LF 340 and a standard conductivity cell TetraCon 325 (Wissenschaftlich-Technische Werkstätten GmbH, Weilheim, Germany). The cell constant was estimated by calibration using an aqueous 0.01 M KCl solution. Synthesis of 1-Allyl-3-(2-methoxyethyl)imidazolium Chloride ([AMO][Cl]). To a two-necked flask fitted with a reflux condenser, 2-chloroethyl methyl ether (89.8 g, 0.95 mol) and 1-allylimidazole (64.9 g, 0.6 mol) were added; the mixture was stirred at 80 °C for 24 h until two phases formed. The top

Figure 1. Structure of 1-allyl-3-(2-methoxyethyl)-imidazolium bis(trifluoromethylsulfonyl)amide ([AMO][TFSI]).

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RESULTS AND DISCUSSION Viscosity and Conductivity of ILs. As shown in Table 1 and Figure 2, the temperature dependence of density data presents a linear relationship at (303.15 to 353.15) K. The equation ρ = a + bT (1) is used to express the temperature dependence of density for ILs in Figure 2, where a and b are the density at 0 K and the volume expansion coefficient, respectively. The fitting parameters of eq 1 are summarized in Table 2. The densities increase with the increase of LiTFSI molar fraction for the binary systems; for instance, LiTFSI-doped [AMO][TFSI] (xLiTFSI = 0.1007, ρ = 1.4650 g·cm−1 at 30 °C) shows higher density than neat IL (ρ = 1.4422 g·cm−1 at 30 °C), and xLiTFSI = 0.4001 (ρ = 1.5570 g·cm−1 at 30 °C) shows higher density than xLiTFSI = 0.3254 (ρ = 1.5298 g·cm−1 at 30 °C). In LiTFSI-doped ILs, small lithium ions fit into the interstices among all ionic species upon the addition of LiTFSI into [AMO][TFSI], attractive interaction occurred between neighboring ionic species, and therefore LiTFSI-doped ILs show higher density than that of neat [AMO][TFSI]. The relative viscosity of Li-doped [AMO][TFSI]/neat [AMO][TFSI] is displayed in Figure 3 and Table 1. It is worth noting that the η of Li-salt-doped [AMO][TFSI] is higher than that of neat [AMO][TFSI]; a reasonable B



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Table 1. Experimental Values of ρ, η, σ, and Λ in ILs at Different Temperatures and Pressure p = 0.1 MPaa xLiTFSI = 0.1007

neat [AMO][TFSI] ρ

T K

−3

(g·cm )

309 1.4370 314 1.4323 317 1.4296 323.5 1.4235 328 1.4194 334 1.4139 338.5 1.4099 344 1.4049 348 1.4013 354.2 1.3958 neat [AMO][TFSI]

K

xLiTFSI = 0.1760

ρ

T

−3

(g·cm )

309 1.4597 315 1.4539 319.5 1.4496 325.2 1.4442 327.2 1.4423 334.2 1.4357 338.1 1.4320 343.2 1.4273 349 1.4219 354.5 1.4169 xLiTFSI = 0.1007

K

xLiTFSI = 0.2508

ρ

T

T −3

(g·cm )

K

308 1.4801 313 1.4754 318 1.4707 323 1.4660 327.2 1.4620 334 1.4557 338 1.4521 344 1.4466 349 1.4420 355 1.4366 xLiTFSI = 0.1760

xLiTFSI = 0.3254

ρ

T −3

(g·cm )

308 1.5053 313 1.5004 317 1.4964 324 1.4895 328 1.4856 334 1.4798 338 1.4759 345 1.4692 349 1.4654 355 1.4598 xLiTFSI = 0.2508

K

xLiTFSI = 0.4001

ρ −3

(g·cm )

308 1.5253 313 1.5203 317 1.5163 323 1.5104 327 1.5065 334 1.4997 337 1.4969 343 1.4911 347 1.4873 356 1.4788 xLiTFSI = 0.3254

T

ρ

K

(g·cm−3)

309.24 1.5511 314 1.5463 318 1.5423 324 1.5362 328 1.5322 334 1.5263 339 1.5214 344 1.5165 349 1.5116 355 1.5058 xLiTFSI = 0.4001

T

η

T

η

T

η

T

η

T

η

T

η

K

(mPa·s)

K

(mPa·s)

K

(mPa·s)

K

(mPa·s)

K

(mPa·s)

K

(mPa·s)

309 23.7 314 20.3 317 17.8 323.5 14.6 328 13.1 334 11.2 338.5 10.0 344 8.8 348 8.2 354.2 7.0 neat [AMO][TFSI] T K

σ

309 38 315 27.9 319.5 22.8 325.2 19.6 327.2 17.7 334.2 15.3 338.1 12.9 343.2 11.4 349 10.3 354.5 9.5 xLiTFSI = 0.1007 T

−1

(mS·cm )

308.7 5.46 313.3 6.27 318.1 7.2 323.3 8.28 327.9 9.35 332.9 10.56 337.6 11.8 343 13.32 348.1 14.75 352 16.01 neat [AMO][TFSI]

K

σ

308 56.2 313 44.4 318 35.8 323 29.6 327.2 25.4 334 20.4 338 18.2 344 15.4 349 13.7 355 11.9 xLiTFSI = 0.1760 T

−1

(mS·cm )

308.9 4.25 314.4 5.14 319.2 5.96 324.4 6.94 328.9 7.78 334.0 8.85 339.0 9.86 344.5 11.06 348.3 11.93 354.9 13.5 xLiTFSI = 0.1007

K

308 72.2 313 57.1 317 47.8 324 37.4 328 32.3 334 25.9 338 22.9 345 19.2 349 17.1 355 14.6 xLiTFSI = 0.2508

σ

T −1

(mS·cm )

K

308.2 3 313.9 3.72 319.6 4.53 322.4 4.93 328.7 5.88 334.5 6.91 338.9 7.6 343.8 8.58 348.6 9.54 354.9 10.9 xLiTFSI = 0.1760

σ

308 116.4 313 89.4 317 75.1 323 56.8 327 49.0 334 37.1 337 32.0 343 27.3 347 24.0 356 18.9 xLiTFSI = 0.3254 T

−1

(mS·cm )

308.7 2.28 314.4 2.84 317.9 3.27 324.6 4.1 328.5 4.67 333.4 5.35 337.4 6 343.5 6.98 348.2 7.84 352.5 8.58 xLiTFSI = 0.2508

K

σ −1

(mS·cm )

308.8 1.598 313.5 2.0 319 2.48 323.9 3.0 328.8 3.53 333.1 4.05 338.5 4.74 343.3 5.39 348.5 6.2 353.3 6.94 xLiTFSI = 0.3254

309.2 221.6 314 168.3 318 138.5 324 100.9 328 82.1 334 63.2 339 54.2 344 44.8 349 38.0 355 31.7 xLiTFSI = 0.4001 T

σ

K

(mS·cm−1)

308.2 0.856 313.4 1.126 318.1 1.411 324.1 1.832 327.5 2.14 334.3 2.75 339.3 3.23 344.4 3.85 348.7 4.36 353.5 4.98 xLiTFSI = 0.4001

T

Λ

T

Λ

T

Λ

T

Λ

T

Λ

T

Λ

K

(S·cm2·mol−1)

K

(S·cm2·mol−1)

K

(S·cm2·mol−1)

K

(S·cm2·mol−1)

K

(S·cm2·mol−1)

K

(S·cm2·mol−1)

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0

1.407 1.659 1.938 2.245 2.582 2.947 3.343 3.768 4.224 4.711

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0

0.996 1.213 1.451 1.709 1.985 2.278 2.587 2.910 3.247 3.596

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0

0.664 0.820 0.994 1.186 1.394 1.618 1.857 2.110 2.376 2.655

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0

0.476 0.598 0.736 0.891 1.063 1.252 1.457 1.678 1.914 2.165

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0

0.300 0.389 0.492 0.609 0.739 0.882 1.039 1.207 1.387 1.578

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0

0.152 0.204 0.267 0.341 0.428 0.528 0.640 0.766 0.906 1.059

a Standard uncertainties (u) are u(xLiTFSI) = 0.0002, u(T) = 0.02 K, and u(p) = 0.02 MPa, and the combined expanded uncertainties (Uc) are Uc(ρ) = 0.0002 g·cm−3, Uc(η) = 0.5 %, and Uc(σ) = 1% with a 0.95 level of confidence.

a Vogel−Tamman−Fulcher (VTF) and modified VTF equations are recommended.9 The modified VTF equation is presented as

explanation relies on ion clusters formation and a tighter structure in LiTFSI-doped [AMO][TFSI]. Since the Arrhenius plots of ln η vs T−1 are slightly nonlinear, the η(T) data fitted by C



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Figure 3. Temperature dependence of viscosity for ILs.

Figure 2. Temperature dependence of ρ for ILs.

candidates for application in capacitors and lithium ion batteries. The viscosity sequence for ILs increases in the following order: neat [AMO][TFSI] < (xLiTFSI = 0.1007) < (xLiTFSI = 0.1760) < (xLiTFSI = 0.2508) < (xLiTFSI = 0.3254) < (xLiTFSI = 0.4001). The addition of a Li salt to [AMO][TFSI] increased viscosity due to the ionic clusters formation between Li+ and TFSI− and particular interactions between Li+ and ether-containing imidazolinium. For collision theory, the Arrhenius equation can be expressed as

Table 2. Adjustable Parameters of ρ for ILs at Various LiTFSI Concentrations ρ

a

xLiTFSI

a

104b

R2 a

neat [AMO][TFSI] xLiTFSI = 0.1007 xLiTFSI = 0.1760 xLiTFSI = 0.2508 xLiTFSI = 0.3254 xLiTFSI = 0.4001

1.719 1.750 1.765 1.804 1.824 1.857

−9.121 −9.411 −9.263 −9.694 −9.692 −9.909

0.9999 0.9998 0.9998 0.9998 0.9998 0.9999

Correlation coefficient.

η− 1 =

ηo

⎡ −E ⎤ η = ηo exp⎢ a ⎥ ⎣ RT ⎦

(4)

⎛ E ⎞⎛ 1 ⎞ ln η = −⎜ a ⎟⎜ ⎟ + ln ηo ⎝ R ⎠⎝ T ⎠

(5)

or

⎡ −B ⎤ exp⎢ ⎥ T ⎣ (T − To) ⎦

−1

(2)

For transition state theory, the Eyring equation can be expressed as12

and the VTF equation is expressed as ⎡ −B ⎤ η−1 = ηo−1 exp⎢ ⎥ ⎣ (T − To) ⎦

(3)

⎛ k T ⎞ VH VS ln η = ln⎜ B ⎟ − + ⎝ h ⎠ RT R

(6)

⎛ ηh ⎞ ⎛ VH ⎞⎛ 1 ⎞ VS ⎟ + ⎟⎜ ln⎜ ⎟ = −⎜ ⎝ R ⎠⎝ T ⎠ k T R ⎝ B ⎠

(7)

or

where η o , B, and T o are adjustable parameters. The corresponding adjustable parameters are summarized in Table 3; the values of the correlation coefficient, R2, are more than 0.999, which reveals that the VTF equation performs well in fitting the viscosity data. [AMO][TFSI] shows lower viscosity (η = 30.1 mPa·s at 30 °C) than reported ILs ([C4mim][HCOO], η = 138.5 mPa·s at 30 °C;10 [BMIm][NO3], η = 123.5 mPa·s at 30 °C);11 such low viscosity ILs are good

where kB is the Boltzmann constant. The Ea values of ILs were evaluated from the relationship of η vs T using the Arrhenius equation; the ΔS and ΔH values of them were evaluated using the Eyring equation, and they are

Table 3. VTF Equation Parameters of η and σ η

a

σ

ηo

To

B

σo

To

B′

xLiTFSI

(mPa·s)

K

K

R2 a

(mS·cm−1)

K

K

R2 a


0.317 1.684 0.594 0.329 0.157 0.229

192.1 252.2 216.9 195.5 184.1 195.3

505.1 176.8 414.4 606.5 818.8 784.0

0.999 0.999 0.999 0.999 0.999 0.999

487.3 178.9 193.8 264.0 163.7 334.7

169.8 205.6 202.7 196.3 212.2 200.4

624.7 386.4 439.7 534.3 447.2 643.7

0.999 0.999 0.999 0.999 0.999 0.999

Correlation coefficient. D



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Table 4. Ea, ΔS, and ΔH Evaluated Using η vs T and σ vs T η

σ

ΔS

Ea −1

−1

ΔH −1

ΔS

Ea −1

−1

ΔH

−1

−1

xLiTFSI

(kJ·mol )

(J·mol ·K )

(kJ·mol )

(kJ·mol )

(J·mol ·K )

(kJ·mol−1)


24.23 27.33 29.79 30.68 36.07 38.88

−306.42 −313.21 −317.85 −318.54 −328.31 −335.46

26.98 30.08 32.53 33.43 37.65 41.63

22.41 22.74 24.83 27.33 29.63 35.05

−167.22 −168.10 −163.99 −158.34 −153.73 −141.12

19.67 19.99 22.09 24.59 26.89 32.31

Table 6. Comparison of the Activation Energies for Ea,η and Ea,Λa Ea,η

Ea,Λ

xLiTFSI

(kJ·mol−1)

(kJ·mol−1)

α

αEA


24.23 27.33 29.79 30.68 36.07 38.88

23.30 24.34 26.32 28.83 31.41 36.89

0.94 0.86 0.85 0.92 0.87 0.92

0.96 0.89 0.88 0.94 0.87 0.95

α is obtained from the general Walden plots, and αEA is estimated from Ea,Λ/Ea,η.

a

listed in Table 4. The Ea, |ΔS|, and ΔH for the LiTFSI-doped [AMO][TFSI] (xLiTFSI = 0.1007, Ea = 27.33 kJ·mol−1, |ΔS| = 313.21 J·mol−1·K−1, and ΔH = 30.08 kJ·mol−1) are greater than those of neat [AMO][TFSI] (Ea = 24.23 kJ·mol−1, |ΔS| = 306.42 J·mol−1·K−1, and ΔH = 26.98 kJ·mol−1). LiTFSI-doped [AMO][TFSI] containing a greater molar fraction of LiTFSI shows larger Ea and ΔH values than those of LiTFSI-doped [AMO][TFSI] containing small xLiTFSI. Figure 4 shows the temperature dependence of the conductivity of ILs with various concentrations of the LiTFSI; the conductivity decreases gradually with increasing LiTFSI concentration, indicating a significant increase of viscosity in the binary mixture after the addition of lithium salt. The VTF equation (eq 8) describes the temperature dependence on the conductivity of ILs.

Figure 4. Temperature dependence of σ for ILs.

⎡ − B′ ⎤ σ = σo exp⎢ ⎥ ⎣ (T − To) ⎦

(8)

In this relationship, σo is the parameter regarding the number of charge carriers, B′ sometimes denotes the pseudoactivation energy, which is similar to the Arrhenius parameters, and T0 represents the ideal Tg. The VTF fitting parameters of

Figure 5. Temperature dependence of Λ for ILs.

Table 5. Ea, ΔS, and ΔH Evaluated Using Λ vs T and ΛNMR vs T Λ

ΛNMR

Ea

ΔS

ΔH

Ea

ΔS

ΔH

xLiTFSI

(kJ·mol−1)

(J·mol−1·K−1)

(kJ·mol−1)

(kJ·mol−1)

(J·mol−1·K−1)

(kJ·mol−1)


23.30 24.34 26.32 28.83 31.41 36.89

−174.09 −173.23 −170.04 −164.54 −159.66 −147.23

20.58 21.62 23.60 26.11 28.69 34.18

25.12 27.32 28.52 31.63 35.19 38.56

−163.54 −159.34 −157.88 −151.33 −143.86 −136.82

22.41 24.60 25.80 28.91 32.47 35.84

E



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estimated Ea, ΔS, and ΔH values are shown in Table 4; all of them increase with the addition of a lithium salt. From the experimental values of density and conductivity in the IL mixtures, the molar conductivity (Λ) can be calculated from the following equation:

temperature dependence on the conductivity for the ILs are listed in Table 3. [AMO][TFSI] shows higher conductivity (σ = 4.54 mS·cm−1 at 30 °C) than reported ILs ([C4mim][PF6], σ = 2.40 mS·cm−1 at 30 °C;13 [MPI][TFSI], σ = 3.31 mS·cm−1 at 30 °C);14 such ILs with high ionic conductivity are potential electrolytes for the use in electrochemical devices. The conductivities decrease in the order of neat [AMO][TFSI] > (xLiTFSI = 0.1007) > (xLiTFSI = 0.1760) > (xLiTFSI = 0.2508) > (xLiTFSI = 0.3254) > (xLiTFSI = 0.4001). This trend is consistent with the increase in viscosity (i.e., the trend of conductivity is precisely opposite to that of the viscosity). The

Λ=σ

M ρ

(9)

As shown in Figure 5, the observed temperature dependences of Λ are well-fitted using the following VTF equation: ⎡ −B′ ⎤ Λ = Λo exp⎢ ⎥ ⎣ (T − To) ⎦

(10)

where Λo, B′, and To are the adjustable parameters, and they are summarized in Table S1 in the Supporting Information. The estimated Ea, ΔS, and ΔH values are summarized in Table 5, and the Ea, ΔS, and ΔH values of Λ show similar inclination with σ. Moreover, the ΔS of temperature-dependent conductivity, molar conductivity, become less negative with increasing LiTFSI concentration, whereas the ΔS of temperature-dependent viscosity become more negative with increasing LiTFSI concentration; this can be attributed to the η decreases with increasing temperature. On the contrary, σ and Λ increase with increasing temperature. The ΔS of Λ vs T decreases slowly when xLiTFSI = 0−0.1760, whereas ΔS of Λ vs T decreases significantly when xLiTFSI = 0.1760−0.4001; this can be attributed to more coordination adducts of [Li(TFSI)n+1]n− that are formed when xLiTFSI = 0.1760−0.4001. The Walden rule shows the relationship of equivalent conductivity (Λ) and the fluidity (or reciprocal viscosity (1/η)),

Figure 6. Walden plots of ILs. The diagonal line is the ideal line obtained from 0.01 M KCl aqueous solution.

Figure 7. Temperature dependence of (a) DAMO+, (b) DTFSI−, (c) DLi+, and (d) ΣDixi in ILs. F



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Table 7. Experimental Values of DAMO+, DTFSI−, and DLi+ for ILs at Various Temperatures and Pressure p = 0.1 MPaa T/K

xLiTFSI = 0.1007

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0 353.0

3.76 × 10−7 4.68 × 10−7 5.73 × 10−7 6.88 × 10−7 7.77 × 10−7 9.28 × 10−7 10.59 × 10−7 12.44 × 10−7 13.72 × 10−7 15.97 × 10−7 17.18 × 10−7

2.85 × 10−7 3.59 × 10−7 4.51 × 10−7 5.39 × 10−7 6.44 × 10−7 7.75 × 10−7 9.11 × 10−7 10.52 × 10−7 11.69 × 10−7 13.76 × 10−7 14.66 × 10−7

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0 353.0

2.63 × 10−7 3.31 × 10−7 3.85 × 10−7 4.79 × 10−7 5.52 × 10−7 6.44 × 10−7 8.02 × 10−7 9.11 × 10−7 10.46 × 10−7 11.61 × 10−7 13.62 × 10−7

1.8 × 10−7 2.27 × 10−7 2.82 × 10−7 3.43 × 10−7 4.11 × 10−7 4.88 × 10−7 5.77 × 10−7 6.84 × 10−7 7.73 × 10−7 8.66 × 10−7 10.7 × 10−7

303.0 308.0 313.0 318.0 323.0 328.0 333.0 338.0 343.0 348.0 353.0 a

neat [AMO][TFSI]

2.08 × 10−8 2.67 × 10−8 3.31 × 10−8 4.18 × 10−8 5.13 × 10−8 6.16 × 10−8 7.32 × 10−8 8.65 × 10−8 1.01 × 10−7 1.17 × 10−7 1.32 × 10−7

xLiTFSI = 0.1760 DAMO+/(cm2·s−1) 2.27 × 10−7 2.87 × 10−7 3.63 × 10−7 4.38 × 10−7 5.21 × 10−7 6.34 × 10−7 7.57 × 10−7 8.78 × 10−7 10.03 × 10−7 11.69 × 10−7 12.66 × 10−7 DTFSI−/(cm2·s−1) 1.39 × 10−7 1.70 × 10−7 2.15 × 10−7 2.58 × 10−7 3.14 × 10−7 3.79 × 10−7 4.47 × 10−7 5.34 × 10−7 6.10 × 10−7 6.93 × 10−7 8.32 × 10−7 DLi+/(cm2·s−1) 1.64 × 10−8 2.24 × 10−8 2.75 × 10−8 3.55 × 10−8 4.33 × 10−8 5.11 × 10−8 6.07 × 10−8 7.14 × 10−8 8.32 × 10−8 9.61 × 10−8 1.09 × 10−7

xLiTFSI = 0.2508

xLiTFSI = 0.3254

xLiTFSI = 0.4001

1.62 × 10−7 2.13 × 10−7 2.71 × 10−7 3.37 × 10−7 4.16 × 10−7 4.95 × 10−7 5.92 × 10−7 7.06 × 10−7 8.32 × 10−7 9.54 × 10−7 10.54 × 10−7

1.07 × 10−7 1.45 × 10−7 1.89 × 10−7 2.43 × 10−7 2.98 × 10−7 3.65 × 10−7 4.56 × 10−7 5.26 × 10−7 6.18 × 10−7 7.47 × 10−7 8.32 × 10−7

6.96 × 10−8 9.65 × 10−8 1.28 × 10−7 1.70 × 10−7 2.18 × 10−7 2.75 × 10−7 3.33 × 10−7 4.14 × 10−7 4.85 × 10−7 5.90 × 10−7 6.74 × 10−7

8.01 × 10−8 1.06 × 10−7 1.40 × 10−7 1.70 × 10−7 2.20 × 10−7 2.53 × 10−7 3.04 × 10−7 3.84 × 10−7 4.49 × 10−7 5.27 × 10−7 6.07 × 10−7

4.63 × 10−8 6.45 × 10−8 8.69 × 10−8 1.07 × 10−7 1.43 × 10−7 1.72 × 10−7 2.12 × 10−7 2.69 × 10−7 3.22 × 10−7 3.80 × 10−7 4.53 × 10−7

2.96 × 10−8 4.17 × 10−8 5.69 × 10−8 7.60 × 10−8 9.67 × 10−8 1.26 × 10−7 1.54 × 10−7 1.92 × 10−7 2.34 × 10−7 2.81 × 10−7 3.26 × 10−7

1.07 × 10−8 1.41 × 10−8 1.84 × 10−8 2.33 × 10−8 2.91 × 10−8 3.59 × 10−8 4.40 × 10−8 5.23 × 10−8 6.25 × 10−8 7.30 × 10−8 8.40 × 10−8

6.64 × 10−9 9.15 × 10−9 1.22 × 10−8 1.59 × 10−8 2.04 × 10−8 2.53 × 10−8 3.07 × 10−8 3.76 × 10−8 4.45 × 10−8 5.28 × 10−8 6.28 × 10−8

4.62 × 10−9 6.38 × 10−9 8.62 × 10−9 1.14 × 10−8 1.48 × 10−8 1.87 × 10−8 2.34 × 10−8 2.87 × 10−8 3.49 × 10−8 4.19 × 10−8 4.95 × 10−8

The standard uncertainty for the determination of the diffusion coefficient was 1 %.

which is related to ionic mobilities. The Λ and η can be expressed as follows,15 Λη α = C

Self-Diffusion Coefficient of ILs. Temperature dependence of DAMO+, DLi+, and DTFSI− can be evaluated using PGSENMR technique due to the presence of protons and the absence of fluorine and lithium in the chemical structure of AMO+ and the presence of fluorine and the absence of protons and lithium in the chemical structure of TFSI−. The DAMO+ and DTFSI− can be determined by studying the 1H and 19F nuclei, respectively. Moreover, the self-diffusion coefficient of Li + can be determined by observing the 7Li nuclei owing to the presence of lithium and the absence of fluorine and protons. Figure 7 shows the temperature dependence of DAMO+, DLi+, DTFSI−, and the product (ΣDixi) of individual diffusion coefficients multiplies their molar fraction in the [AMO][TFSI] and LiTFSI binary system, where xi is the molar ratio of individual ions and Di is the self-diffusion coefficient of each ion. As shown in Table 7, the DAMO+,DLi+, and DTFSI− of ILs follow the sequence (xLiTFSI = 0) > (xLiTFSI = 0.1007) > (xLiTFSI = 0.1760) > (xLiTFSI = 0.2508) > (xLiTFSI = 0.3254) > (xLiTFSI = 0.4001), implying that the viscosity increases and the formation of stable coordination adducts of Li and TFSI, such as [Li(TFSI)n+1]n−, upon addition of LiTFSI. The VTF equation fits well the DAMO+, DLi+, DTFSI−, and ΣDixi at 30−80 °C as follows,

(11)

where C indicates the Walden product. α is the slope of the temperature-dependent data in the Walden plot; the α values of ILs are shown in Table 6. Another way that yields α is Ea,Λ/Ea,η; Ea,Λ and Ea,η are listed in Table 4 and Table 5. Table 6 compares α values estimated from the slopes of the Walden plots in Figure 6 with those evaluated from the ratio (Ea,Λ/Ea,η) of the temperature-dependent Ea. The values of α evaluated from two methods are in good agreement. As shown in Figure 6, the positions of LiTFSI-doped [AMO][TFSI] are less than the Λη of a 0.01 M KCl aqueous solution, which is completely dissociated and its ions have the same mobility. Compare the discrepancy between the position of LiTFSI-doped [AMO][TFSI] and the ideal line of Walden plots; the deviation does not show significant change with the addition of LiTFSI to [AMO][TFSI] in the IL binary system, and this may be attributed to the association of neighboring ions that cannot be observed from the Walden plots for the binary system. G



Article

Table 8. Ea, ΔS, and ΔH Evaluated Using DAMO+ vs T and DTFSI− vs T DAMO+

DTFSI-

ΔS

Ea −1

ΔH

−1

−1

ΔS

Ea −1

−1

−1

ΔH −1

xLiTFSI

(kJ·mol )

(J·mol ·K )

(kJ·mol )

(kJ·mol )

(J·mol ·K )

(kJ·mol−1)


26.97 29.40 30.90 33.37 36.34 40.28

−287.50 −281.70 −278.74 −273.21 −266.78 −257.30

24.25 26.69 28.18 30.66 33.62 37.56

29.04 30.84 31.64 35.65 40.02 42.54

−283.88 −280.95 −280.66 −271.62 −261.63 −256.94

26.32 28.13 28.93 32.93 37.31 39.82

Table 9. Ea, ΔS, and ΔH Evaluated Using DLi+ vs T and ΣDixi vs T ΣDixi

DLi+ Ea

ΔS

ΔH

Ea

ΔS

ΔH

xLiTFSI

(kJ·mol−1)

(J·mol−1·K−1)

(kJ·mol−1)

(kJ·mol−1)

(J·mol−1·K−1)

(kJ·mol−1)


32.94 33.04 36.63 39.40 42.05

−291.90 −293.17 −285.16 −279.76 −274.14

30.22 30.32 33.91 36.68 39.33

27.84 30.04 31.24 34.35 37.90 41.27

−280.31 −276.12 −274.66 −268.10 −260.64 −253.60

25.12 27.32 28.52 31.63 35.19 38.56

Table 10. Ion Transference Number (t) at 303.15 K for ILs neat [AMO][TFSI] xLiTFSI = 0.1007

xLiTFSI = 0.1760

xLiTFSI = 0.2508

xLiTFSI = 0.3254

xLiTFSI = 0.4001

⎡ − B′ ⎤ D = Do exp⎢ ⎥ ⎣ (T − To) ⎦

ion

t

AMO+ TFSI− AMO+ Li+ TFSI− AMO+ Li+ TFSI− AMO+ Li+ TFSI− AMO+ Li+ TFSI− AMO+ Li+ TFSI−

0.589 0.411 0.585 0.005 0.410 0.569 0.009 0.422 0.595 0.013 0.392 0.597 0.018 0.385 0.570 0.025 0.405

Figure 8. tLi of ILs at various LiTFSI concentrations.

anions display different self-diffusion coefficients for ILs.18 Moreover, DLi+ is smaller than those of DAMO+ and DTFSI−; this can be attributed to the coordination of lithium ion with several bis(trifluoromethylsulfonyl)imide anions, resulting in slower migration of Li+ than those of AMO+ and TFSI−. Figure 8 shows the tLi of ILs at different LiTFSI concentrations; the Li+ transference number increases with increasing molar fraction of LiTFSI in the [AMO][TFSI] and LiTFSI binary system. LiTFSI-doped [AMO][TFSI] shows higher Li+ transference number (tLi = 0.025 at 30 °C and xLiTFSI = 0.4001) than reported for LiTFSI-doped IL (tLi of LiTFSI-doped [MPI][TFSI] = 0.024 at 30 °C and xLiTFSI = 0.4015).16 The ILs with high Li+ transference number would be potential candidates for application in electrolytes of lithium ion batteries. Stokes−Einstein Relationship in ILs. The following Stokes−Einstein equation can predict the diffusion coefficients (D) of ILs,

(12)

where the constants Do, B′, and To are adjustable parameters. The fitting parameters of the DAMO+, DLi+, DTFSI−, and ΣDixi are summarized in Table S2 and Table S3 in the Supporting Information. On the basis of the D vs T plot, the evaluated Ea, ΔS, and ΔH are shown in Table 8 and Table 9. The transference numbers (ti) of all ion species can be evaluated from ti =

x iDi Σx iDi

(13)

where xi is the molar ratio of individual ions; the calculated ti values of AMO+, Li+, and TFSI− at 303.15 K are shown in Table 10. AMO+ shows larger transference numbers than those of TFSI− in ILs, implying the structure and shape of cations and

D= H

kT cπηrs

(14) DOI: 10.1021/je500431h J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Figure 11. Dependencies of Ea for η−1, σ, Λ, DAMO+, DTFSI−, and DLi+ in ILs.

Figure 9. Relationship between D and T/η for the (a) AMO+ cation and (b) TFSI− anion in ILs.

Figure 10. Temperature dependence of ΛNMR for ILs.

Figure 12. (a) Temperature dependence of longitudinal relaxation times (T1) for ILs using 1H nucleus; (b) LiTFSI concentration dependence of the longitudinal relaxation times (T1) using 1H nucleus.

where k is the Boltzmann constant, c is a constant, and rs denotes the Stokes radius of the diffusing species. The change in D with Tη−1 of ILs presents approximately straight lines in Figure 9, demonstrating that eq 14 holds for the ionic diffusivity in the ILs. However, the slopes of the lines are not consistent for the ILs;

this can be attributed to the constant c ranges between 4 and 6 for the stick and slip boundary conditions, respectively.17 Molar Conductivity Calculated from PGSE-NMR. The ΛNMR can be estimated using the following Nernst−Einstein equation, I


Journal of Chemical & Engineering Data ΛNMR =

Ne 2(x AMO+D AMO+ + DTFSI− + x Li+D Li+) kT

Article

dependence of the ΛNMR calculated from the experimental D and eq 15 is shown in Figure 10; the fitting parameters of the VTF equation are summarized in Table S1 in the Supporting Information. The ΛNMR values were obtained from all of the diffusing species, including correlated ones (i.e., neutral ion pairs), detected during the diffusion NMR measurement. On the other hand, the Λ values were estimated according to the migration of the charged species under an electric field. Therefore, the ΛNMR values were larger than those evaluated from experimental conductivity (Λ). Temperature Dependence of the Dynamic Properties. Figure 11 shows the Ea of 1/η, σ, Λ, DAMO+, DLi+, and DTFSI− at various LiTFSI molar fractions; all of them showed an increased tendency with increasing LiTFSI molar fractions, indicating the viscosity tends to increase and stable coordination adducts of Li and TFSI, such as [Li(TFSI)n+1]n−, form with increasing LiTFSI concentration, and these effects result in a decrease of the translational motions. The activation energies of 1/η, DAMO+, DLi+, and DTFSI− are larger than those of the σ and Λ at different lithium salt concentrations. For instances, Ea,Λ of neat [AMO][TFSI] at (30 to 80) °C was 23.30 kJ·mol−1 (in Table 5), which was smaller than those of the Ea,D at (30 to 80) °C (about 26.97 kJ·mol−1 for DAMO+ and 29.04 kJ·mol−1 for DTFSI−). Relaxation Time of Ionic Liquid Electrolytes. Figure 12a and Figure 13a show the temperature dependencies of the relaxation time (T1) of AMO+ and TFSI− for ILs, respectively, the C-2 hydrogen atom of imidazolium cation was used to determine 1H-T1. As the temperature of the system was increased, the viscosity decreases and the rotational velocities of AMO+ and TFSI− increase. The motion moves further away from resonance, resulting in a less efficient relaxation; hence, high 1 H-T1 and 19F-T1 values are observed at high temperature. The 1H-T1 of AMO+ and 19F-T1 of TFSI− at various Li salt concentrations are shown in Figure 12b and Figure 13b, respectively. Generally, the 1H-T1 and 19F-T1 values of ILs decrease with increasing LiTFSI concentration (Table 11), which was due to the viscosity increasing when the salt concentration increases (i.e., the motion of the molecules slows). Moreover, the formation of stable coordination adducts of Li and TFSI, such as [Li(TFSI)n+1]n−, is another effect when LiTFSI is added in [AMO][TFSI]. These effects result in the motion moving to a frequency closer to the resonant frequency of the spins and hence producing a more efficient relaxation.

(15)

where N is the Avogadro number and e denotes the electric charge on each ionic carrier (1.602 × 10−19 C). The temperature

Figure 13. (a) Temperature dependence of longitudinal relaxation times (T1) for ILs using 19F nucleus; (b) LiTFSI concentration dependence of the longitudinal relaxation times (T1) using 19F nucleus.

Table 11. Experimental Values of T1 Using 1H and 19F Nuclei for the Neat [AMO][TFSI] and LiTFSI-Doped [AMO][TFSI] at Various Temperatures T/K

neat [AMO][TFSI]

xLiTFSI = 0.1007

xLiTFSI = 0.1760

xLiTFSI = 0.2508

xLiTFSI = 0.3254

xLiTFSI = 0.4001

1.215 1.37 1.508 1.755 1.976 2.248

1.194 1.352 1.489 1.662 1.847 2.079

1.25 1.378 1.539 1.719 1.922 2.165

0.6213 0.7223 0.7513 0.9005 1.043 1.117

0.5802 0.6426 0.7027 0.8264 0.8818 0.9302

0.5871 0.6592 0.7159 0.7879 0.8602 0.985

1

303.0 313.0 323.0 333.0 343.0 353.0

1.308 1.473 1.674 1.901 2.23 2.574

1.262 1.427 1.619 1.879 2.161 2.478

303.0 313.0 323.0 333.0 343.0 353.0

0.8366 1.067 1.146 1.461 1.657 2.181

0.7378 0.8078 0.9926 1.211 1.313 1.691

H-T1/s 1.242 1.398 1.528 1.762 2.007 2.281 19 F-T1/s 0.7165 0.7367 0.9134 1.06 1.203 1.443 J



Article

Table 12. Ea, ΔS, and ΔH Evaluated by T1 vs T Using 1H and 19F Nuclei T1 of 1H

T1 of 19F

ΔS

Ea −1

−1

ΔH −1

ΔS

Ea −1

−1

−1

ΔH −1

xLiTFSI

(kJ·mol )

(J·mol ·K )

(kJ·mol )

(kJ·mol )

(J·mol ·K )

(kJ·mol−1)


12.05 12.09 10.81 10.97 9.69 9.77

−212.23 −212.35 −216.70 −216.33 −220.57 −220.03

9.33 9.37 8.09 8.25 6.98 7.06

16.09 14.70 12.95 10.70 8.85 8.83

−202.45 −208.41 −214.56 −222.72 −229.34 −229.35

13.38 11.98 10.24 7.98 6.13 6.11

Table 13. Coefficients of the Polynomiala for the Correlation of T1 in a Function of LiTFSI Concentration in ILs Using 1H and 19F Nuclei 1

H-T1

19

a

F-T1

T/K

c0/s

c1/s

c2/s

c3/s

c4/s

303 313 323 333 343 353 303 313 323 333 343 353

1.3081 1.473 1.6747 1.9029 2.2322 2.5769 0.8358 1.0679 1.1447 1.4604 1.6568 2.1799

−0.998 −0.6543 0.0657 0.0986 −0.6588 −1.0372 −1.7373 −5.0769 −1.6407 −2.4576 −6.3003 −6.7546

8.6106 3.178 −9.8696 −5.4249 −1.1037 −0.0719 13.166 33.533 4.6073 −0.2026 42.733 30.68

−38.321 −14.715 34.965 5.9857 −13.745 −20.127 −56.931 −105.14 −28.906 4.7283 −157.54 −127.89

55.301 23.431 −32.112 10.074 39.268 50.661 77.532 116.55 52.441 1.6261 194.12 186.91

T1 = c0 + c1x + c2x2 + c3x3 + c4x4.

Table 14. Experimental Values of 1H and 19F Chemical Shift for ILs at Various Temperatures T/K

neat [AMO][TFSI]

xLiTFSI = 0.1007

xLiTFSI = 0.1760

xLiTFSI = 0.2508

xLiTFSI = 0.3254

xLiTFSI = 0.4001

1

303.0 313.0 323.0 333.0 343.0 353.0

8.3953 8.5193 8.6336 8.7391 8.8389 8.9387

8.3638 8.4892 8.6107 8.7325 8.8335 8.9343

303.0 313.0 323.0 333.0 343.0 353.0

−80.8436 −80.5474 −80.2582 −80.0292 −79.7987 −79.5934

−80.8685 −80.5601 −80.2773 −80.0579 −79.8579 −79.6543

H-δ/ppm 8.3366 8.4725 8.5925 8.7078 8.8142 8.9147 19 F-δ/ppm −80.8773 −80.5729 −80.2859 −80.0719 −79.8653 −79.6638

Accordingly, low 1H-T1 and 19F-T1 values are observed when the salt concentration increases. The observed temperature dependences of 1H-T1 and 19F-T1 are fitted using the following equation: ⎡ − B′ ⎤ T1 = T1o exp⎢ ⎥ ⎣ (T − To) ⎦

8.3092 8.4451 8.5668 8.6784 8.7833 8.8841

8.2814 8.4157 8.5374 8.6516 8.7567 8.8603

8.2504 8.3826 8.5080 8.6236 8.7298 8.8329

−80.9643 −80.6543 −80.3646 −80.1479 −79.9343 −79.7284

−81.0376 −80.7221 −80.4294 −80.2016 −79.9929 −79.7931

−81.0744 −80.7473 −80.4461 −80.2211 −80.0101 −79.8046

energies for diffusion; this can be attributed to that although there is a significant translational component of the hydrogen (or fluorine) relaxation, there is also a smaller but still significant rotational component. The fits of 1H-T1 and 19F-T1 at various LiTFSI concentrations were calculated using the following fourth-order polynomial equation,

(16)

T1 = c0 + c1x + c 2x 2 + c3x 3 + c4x 4

where T1o, B′, and To are the adjustable parameters; the corresponding fitting parameters of 1H-T1 and 19F-T1 for the ILs are summarized in Table S4 in the Supporting Information. According to the relationship of 1H-T1 vs T and 19F-T1 vs T, the evaluated Ea, ΔS, and ΔH values are shown in Table 12. The activation energies of the T1 are lower than the activation

(17)

where x is the mole fractions of LiTFSI and c0, c1, c2, c3, and c4 are the fit coefficients; the fitting parameters are summarized in Table 13. Chemical Shift of One-Dimensional NMR Spectroscopy. Parts a and b of Figure 14 show the temperature and K



Article

Figure 15. (a) Temperature dependence of 19F chemical shift for ILs; (b) LiTFSI concentration dependence of 19F chemical shift.

Figure 14. (a) Temperature dependence of 1H chemical shift for ILs; (b) LiTFSI concentration dependence of 1H chemical shift.

Similar to the 1H spectrum, a downfield shift in the 19F spectrum was observed in the binary system with increasing temperature, indicating a decrease in electron density around the 19F nuclei of bis(trifluoromethylsulfonyl)amide. On the other hand, an upfield shift in the 19F spectrum was observed in neat [AMO][TFSI] and LiTFSI-doped [AMO][TFSI] with increasing lithium salt concentration; the formation of ion clusters is the dominant effect after the addition of LiTFSI in [AMO][TFSI]. From the primary data of the 19F chemical shift for the ILs, the 19F chemical shift decreases slowly when xLiTFSI = 0−0.1760, whereas the 19F chemical shift decreases significantly when xLiTFSI = 0.1760− 0.3254, implying the chemical associations of ions take place when xLiTFSI = 0.2508−0.3254.

LiTFSI concentration dependence of 1H NMR chemical shift for ILs, respectively. The C-2 hydrogen atom of imidazolium cation was used to determine the 1H NMR chemical shift. A downfield shift in the 1H spectrum has been observed in the binary system with increasing temperature (Table 14); a downfield shift is indicative of a decrease in electron density around the 1H nuclei of imidazolium which is due to the distance between imidazolium, bis(trifluoromethanesulfonyl)imide, and lithium ion increases with increasing temperature. On the other hand, an upfield shift in the 1H spectrum was observed in neat [AMO][TFSI] and LiTFSI-doped [AMO][TFSI] with increasing lithium salt concentration; this can be attributed to an increase in electron density around the 1H nuclei of imidazolium, which is due to ion clusters formation causing a tighter structure in the binary system. The AMO+ can interact with TFSI− to form a cluster, and the clusters [Li(TFSI)n+1]n− can be formed by Li+ and TFSI−. However, the molar ratio of [AMO][TFSI] decreases with increasing lithium salt concentration; the electron density between the 1H nuclei of imidazolium becomes weak, which causes the 1H chemical shift to move downfield. After the competition between these two kinds of interactions, the former is the dominant effect and the chemical shift of the 1H NMR signal moves upfield finally. Parts a and b of Figure 15 show the temperature and LiTFSI concentration dependence of the 19F NMR chemical shift, respectively, for neat [AMO][TFSI] and LiTFSI-doped [AMO][TFSI].

■

CONCLUSION The transport properties (conductivity, viscosity, and selfdiffusion coefficients) and physicochemical properties (T1 and chemical shift of one-dimensional NMR spectroscopy) were measured in the [AMO][TFSI]/LiTFSI binary system at various lithium salt concentrations and various temperatures. The conductivity and self-diffusion coefficients decrease associated with viscosity increases after the addition of LiTFSI in the ILs. The longitudinal relaxation times for 1H and 19F nuclei exhibit the same type of temperature and salt concentration dependence. The values of T1 were seen to increase with temperature and decrease with salt concentration. 1H NMR and 19F NMR L



Article

(13) Geng, Y.; Chen, S.; Wang, T.; Yu, D.; Peng, C.; Liu, H.; Hu, Y. Density, viscosity and electrical conductivity of 1-butyl-3-methylimidazolium hexafluorophosphate + monoethanolamine and + N, Ndimethylethanolamine. J. Mol. Liq. 2008, 143, 100−108. (14) Wu, T. Y.; Hao, L.; Chen, P. R.; Liao, J. W. Ionic conductivity and transporting properties in LiTFSI-doped bis(trifluoromethanesulfonyl)imide-based ionic liquid electrolyte. Int. J. Electrochem. Sci. 2013, 8, 2606−2624. (15) Xu, W.; Angell, C. A. Solvent-free electrolytes with aqueous solution-like conductivities. Science 2003, 302, 422−425. (16) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical properties and structures of room temperature ionic liquids. 1. Variation of anionic species. J. Phys. Chem. B 2004, 108, 16593−16600. (17) Cussler, E. L. Diffusion: Mass Transfer in Fluid System; Cambridge University Press: New York, 1984.

chemical shift also show the same type of temperature and salt concentration dependence; the addition of LiTFSI in the ILs induces an upfield shift in 1H and 19F spectra.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing molar conductivity data, self-diffusion coefficient data, and parameters of T1, text describing materials, sample preparation, formation of LiTFSI adducts, and accompanying references, and a figure showing the optimized geometric trans structure of [Li(TFSI)2]−. 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]. Tel.: +886-6-2757575 ext. 65355. Funding

We thank the Ministry of Science and Technology of Taiwan for financially supporting this project. This research also received funding from the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan, ROC. Notes

The authors declare no competing financial interest.

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REFERENCES

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