Temperature Dependence of Self-Diffusion Coefficients of Ions and

Jun 13, 2012 - Kikuko Hayamizu*. National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Center 2, Tsukuba 305-8568, Japan. ABS...
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Temperature Dependence of Self-Diffusion Coefficients of Ions and Solvents in Ethylene Carbonate, Propylene Carbonate, and Diethyl Carbonate Single Solutions and Ethylene Carbonate + Diethyl Carbonate Binary Solutions of LiPF6 Studied by NMR Kikuko Hayamizu* National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Center 2, Tsukuba 305-8568, Japan ABSTRACT: The self-diffusion coefficients, D, of lithium, anions, and solvents in four binary-solution electrolytes of ethylene carbonate (EC)−diethyl carbonate (DEC) and three single-solution electrolytes of EC, DEC, and propylene carbonate (PC) including 1 M LiPF6 were measured by 1H, 7Li, and 19F NMR spectroscopy from (353 to 243) K or above freezing for seven solution electrolytes. In the single DEC electrolyte, DLi and DPF6 had almost the same values at every temperature, whereas DLi was smaller than DPF6 in the EC and PC solutions. In the binary EC− DEC electrolytes, as the ratio of EC increased, DLi gradually became smaller, while DPF6 remained almost unchanged in the temperature range studied. At 303 K, the degree of ion dissociation α was evaluated from the ionic conductivity, DLi and DPF6. The α value increased from 0.17 to 0.71 as the EC ratio increased from (0 to 100) %.

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INTRODUCTION Important solution electrolytes used for lithium ion batteries (LIB) are mixed solvent systems composed of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and so on. Although LiPF6 is well-known to have disadvantages such as thermal instability, HF (hydrogen fluoride) formation, and other reasons, it is still one of the most important lithium salts for producing practical LIB. Many studies have been reported on their physicochemical properties such as the ionic conductivity, dielectric constant, viscosity, and thermal properties. In particular, systematic studies on the ionic conductivity, viscosity, dielectric constant, and other properties have been reported for the PC−DEC and PC−EC binary solvent systems in wide ranges of solvent composition, lithium salt concentration, and temperature for LiPF6 and LiBF4.1−5 However, self-diffusion coefficients (D) of individual components (Dsolvent, Danion, and DLi) in the binary solution electrolytes including LiPF6 or LiBF4 have not yet been reported. We have reported D values of the components for the binary solvent systems composed of the PC-1,2-dimethoxyethane (DME) and PC-DEC systems including LiN(SO2CF3)2 (LiTFSA),6 where the values of Dsolvent, DTFSA, and DLi were measured by the pulsed-gradient spin−echo (PGSE) NMR method7 of 1H, 7Li, and 19F resonances, respectively. Also we reported D values in single-solvent electrolytes including various lithium salts.8−12 The D values of organic solution electrolytes used for LIBs have also been reported.13−17 To analyze the self-diffusion coefficients of the lithium electrolytes, we assumed that the classical Stokes−Einstein equation holds as © XXXX American Chemical Society

D=

kT cπηrs

(1)

where k is the Boltzmann constant, rs is the Stokes radius of the diffusing species, η is the viscosity, and the constant c theoretically ranges between 4 to 6 for slip and stick boundary conditions, respectively.7,18 Here since the individual selfdiffusion coefficients for the ions and solvents were measured, we assume that Dsolv =

kT cπηrssolv

and

Dion =

kT cπηrsion

(2)

solv where rion are the Stokes radii for the ions and solvents, s and rs respectively, and c and η are assumed to be the same for the all species. We have proposed an empirical R-parameter as8−12

R=

Dsolv r ion ∝ ssolv Dion rs

(3)

The diffusion coefficient of a species is inversely proportional to its hydrodynamic size, and R becomes a semiquantitative measure of the degree of ion solvation and ion association if the ion and/or ion pair diffuse as a unit for a longer time interval than the diffusion measurement. The self-diffusion coefficients of cations (D+) and anions (D−) measured by NMR are the weighted average values of the charged ions and the neutral (paired) ions. On the other hand, ionic conductivity measures the ion migration of charge Received: March 11, 2012 Accepted: May 31, 2012

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where g is the strength, δ is the duration of a gradient pulse, and Δ is the interval of the gradient pulses. In the present experiments, the data were obtained by setting Δ as 50 ms and varying the δ values between (0.1 and 2) ms, except for in the higher temperature region. Since the convection effects become significant at high temperatures and cause the apparent D to be larger with longer Δ, we measured D with varying Δ and confirmed that the data do not include the convection effect. The maximum g value was 12 Tm−1. The gradient settings used were chosen to cause sufficient signal attenuation to enable an accurate estimation of the diffusion coefficients. The errors of D estimated from the plots following to eq 5 were less than 5 %.21 Ionic Conductivity Measurements. The ionic conductivity was determined by the ac impedance method on a Solarton 1286 electrochemical Interface and 1255 frequency response analyzer controlled by a personal computer. The electrolyte samples were poured into a sealed glass cell with the Pt electrodes. The measurements were carried out from 100 Hz to 1 MHz at 303 K.

carrying ions. The ionic conductivity calculated by the Nernst− Einstein (NE) equation from the ion diffusion coefficients is always higher than the values determined by the electrochemical alternating current (ac) method. As shown in our previous papers,10,11 it is necessary to modify the NE equation by introducing the degree of ion association, ξ, such that σD =

Ne 2 (D+ + D−)(1 − ξ) kT

(4)

where N is the number of ions included in a unit volume without distinguishing ion dissociation states. For convenience we define the degree of dissociation as α = (1 − ξ) because it is more directly related to the conductivity. We have confirmed that ξ is extrapolated to zero at infinite dilution for the PC and GBL electrolytes including LiTFSA and LiBF4 studied in infinitesimal concentrations.10 In the present paper, the temperature-dependent Ds are studied for the binary EC− DEC and single-solvent EC, DEC, and PC systems including 1 M LiPF6. The basic properties of the three pure solvents are given in Table 1.

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RESULTS AND DISCUSSION Self-Diffusion Coefficient for Single-Solvent Electrolytes. The temperature-dependent Dsolv, DPF6, and DLi values are given in Table 2 for 1 M LiPF6 solution electrolytes for PC, EC, and DEC in the temperature range between (243 and 353) K, which is a slightly larger temperature range for LIBs. The solutions of PC and EC were frozen at (243 and 283) K, respectively. Arrhenius plots were made for the three samples in Figure 1 for (a) 1 M LiPF6 in DEC solution and (b) 1 M LiPF6 in EC and PC solutions on the same chart. The Arrhenius plots were curved except for DDEC in Figure 1a. When the EC solution was frozen at 283 K, the behaviors of D were similar in the EC and PC solutions above 293 K, where slightly larger D values were obtained in the EC than in PC solutions. Since EC is solid at ambient temperature, the preparation of PC solutions is easier. Here we found parallel relations in the D's between EC and PC solutions. The Vogel−Fulcher−Tamman (VFT) fit like D = Do exp(−B/(T − To)) was made, where Do, B, and To are fitting parameters. The values obtained are summarized in Table 3. Since the D's of EC were obtained in a narrow temperature range, we did not attempt fitting for the D's of the EC solution. DDEC was found to have a linear fit, and the activation energy was 15.2 ± 0.3 kJ mol−1. Self-Diffusion Coefficient for Binary-Solvent Electrolytes. The temperature-dependent values of Dsolvent, DPF6, and DLi for the EC−DEC binary electrolytes for four compositions are summarized in Table 4, and the Arrhenius plots are shown in Figure 2. The VFT fitting parameters of the DDEC, DEC, DPF6, and DLi are summarized in Table 5. In the EC/DEC 2:8 binary electrolyte, DDEC > DEC > DPF6 > DLi throughout the whole temperature range. When the EC ratio increased, the differences between DDEC and DEC became smaller, and they became almost the same in the high-temperature region for the EC/DEC 4:6 electrolyte and in the low-temperature region for the EC/DEC 6:4 electrolyte. In the EC/DEC 8:2 electrolyte, DEC was slightly larger than DDEC. Since the viscosity and dielectric constant of DEC and EC are quite different, it is surprising that the DDEC and DEC have similar values in the binary systems. We observed similar phenomena in PC−DEC binary systems but not in PC−DME binary systems.6 In the PC−DEC binary systems DDEC was always slightly larger than

Table 1. Basic Properties of the Solvents19 chemical name EC PC DEC

ethylene carbonate propylene carbonate diethyl carbonate

MF

BP/K

ηa d

C3H4O3

521

1.930

C4H6O3

515

2.530b

400

b

C5H10O3

0.748

εa

DN

40d

16.4

64.92b 2.820

15.1 c

16.0

a η (viscosity, mPa·s) and ε (dielectric constant) data. bAt 298 K. cAt 293 K. dAt 313 K.

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EXPERIMENTAL SECTION Sample Preparation. Lithium battery-grade EC, PC, DEC, and LiPF6 were purchased from Tomiyama Pure Chemical Industries Ltd., Tokyo. The sample preparation was carried out in a dry atmosphere to prevent the inclusion of moisture. The molar ratio of the mixed solvents was adjusted, and LiPF6 was added in a 1 M concentration. For NMR measurements, each electrolyte was placed into a 5 mm NMR microtube with a special insert to a height of 3 mm to prevent convection effects at higher temperatures,20 which sample tube was a modified one from a 5 mm NMR microtube (BMS-005J, Shigemi, Tokyo). NMR Measurements. The PGSE-NMR measurements were made using a TecMag Apollo system with a 4.7 T wide bore magnet equipped with JEOL gradient probes and a current amplifier. The temperature was varied with a JEOL GSH-200 console. Dsolvent, DPF6, and DLi were measured using 1 H, 19F, and 7Li NMR at (200, 184, and 77.7) MHz, respectively from (233 to 353) K, except for EC and PC single-solvent systems and a EC rich binary solvent system which were frozen above 233 K. All of the spectra did not include any impurity peaks. The simple Hahn spin−echo based sequence incorporating a field-gradient pulse in each τ period was used to measure D and was calculated by plotting the echo attenuation (E) according to eq 5 for a single diffusing species in an isotropic medium,7 E = exp(−γ 2g 2δ 2D(Δ − δ /3))

(5) B

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Table 2. Temperature-Dependent Dsolv, DPF6, and DLi in 1 M LiPF6 for Single-Solvent Systems of PC, EC, and DEC (10−10 m2·s−1) T/K 353 343 333 323 313 303 293 283 273 263 253 353 343 333 323 313 303 293 353 343 333 323 313 303 293 283 273 263 253 243

PC

PF6

1 M LiPF6 in PC 5.93 5.17 4.91 4.27 4.10 3.59 3.34 2.91 2.59 2.28 2.04 1.84 1.50 1.31 1.06 0.952 0.742 0.636 0.472 0.408 0.274 0.235 1 M LiPF6 in EC 7.10 5.80 5.97 5.10 5.04 4.30 4.04 3.63 3.30 2.81 2.59 2.14 1.91 1.60 1 M LiPF6 in DEC 14.5 7.43 12.6 6.21 10.7 5.30 9.34 4.38 7.50 3.77 5.80 2.75 5.10 2.43 4.21 2.02 3.37 1.71 2.54 1.17 2.07 0.77 1.47 0.52

Table 3. Fitting Parameters for VFT Equation for the Diffusion Coefficients D of Single Electrolytes of PC and DEC for D = Do exp(−B/(T − To) sample

Li

LiPF6 in PC 2.94 2.53 2.00 1.60 1.23 0.96 0.69 0.48 0.32 0.185 0.10

LiPF6 in DEC

component PC PF6 Li PF6 Li

D0/(m2·s−1) −8

2.04·10 1.74·10−8 1.38·10−8 2.13·10−8 1.86·10−8

B/T−1

To(T)

R2

759 748 807 868 821

138 140 141 98 106

0.99987 0.99979 0.99929 0.99793 0.99864

DPC. The similar values of the solvent diffusion coefficients in the binary systems suggest solvent−solvent interactions between DEC and EC or DEC and PC, because of the common substructure O−CO−O in DEC, EC, and PC. Composition Dependence of D. The composition dependences of DEC, DDEC, DPF6, and DLi are shown in Figure 3 for (a) 353 K, (b) 303 K, and (c) 263 K. The EC solution was frozen at 283 K. In the single DEC solution, DPF6 and DLi have similar values throughout the whole temperature range, as shown in Figure 1. When EC was added, DLi became smaller as the EC ratio increased. On the other hand, DPF6 was almost unchanged at every temperature. This indicates that EC molecules interact directly with Li ions while PF6 is almost independent of EC, especially at higher temperatures. DDEC decreased with the increase of the EC ratio, while the change of DEC was smaller. As pointed out above, DDEC and DEC have similar values in the binary solvents of EC/DEC 4:6 and 6:4 systems. It is noted that DEC becomes slightly larger than DDEC solutions with a higher EC ratio, in spite of the large differences in D values observed in single-solvent electrolytes at EC equal to (0 and 100) mol %. In Figure 4 DEC, DPF6, and DLi are plotted on the same chart against DDEC for the binary and single-solvent systems measured at (a) 263 K, (b) 303 K, and (c) 353 K. It is clearly shown that DLi is proportional to DDEC and the experimental ratio of DLi/DDEC is approximately 0.5. The most important function of DEC in a practical LIB is the reduction of viscosity in the mixed electrolytes to promote ionic conductivity. In this study, a good relationship between DLi and DDEC is consistent with the known concept that the combination of DEC and EC is suitable for practical usage. It is noted that DEC and DDEC are almost parallel and DLi is also proportional to DEC. On the other hand, DPF6 is not sensitive to solvent composition. In Figure 5, R-parameters, RLi = DEC/DLi and DDEC/DLi and RPF6 = DEC/DPF6 and DDEC/DPF6, are plotted against mol % of EC at 303 K. Similar plots are obtained for the data measured at (353 and 263) K. Since the values of DEC and DDEC were different in the EC/DEC 2:8 solution, the differences in the RLi and RPF6 values referred to EC or DEC are slightly different. The RLi is almost 2.2 and insensitive to the EC ratio. On the other hand, RPF6 decreased as the EC ratio increased. The van der Waals radii of Li, PF6, PC, and EC were proposed to be (0.076, 0.254, 0.276, and 0.258) nm, respectively,22,23 and the lithium ion is clearly much smaller than other species. The smallest DLi suggests that a lithium ion diffuses with an anion or solvents or both. The calculated van der Waals ratio of PF6 versus that of EC, rEC/rPF6, is 0.98. The experimental RPF6 changed from 2.1 to 1.2 with the change of the EC concentration from (0 to 100) %. Furthermore, RPF6 and RLi

3.40 2.83 2.33 1.82 1.53 1.13 0.85 7.10 5.75 5.00 4.33 3.40 2.70 2.27 1.83 1.60 1.06 0.69 0.48

Figure 1. Arrhenius plots of Dsolvent, DPF6, and DLi for (a) DEC solution, □, DEC; △, PF6; ▽, Li, and (b) PC and EC single-solvent systems, ○, EC; △, PF6; ▽, Li in the EC solution and ●, PC; ▲, PF6; and ▼, Li in the PC solution. The VFT fitting curves are shown. DDEC had a linear fit.

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Table 4. Diffusion Coefficients of Solvents, PF6, and Li in Four 1 M LiPF6 EC−DEC Binary Electrolytes (10−10 m2·s−1) T/K

DEC

EC

PF6

Li

DEC

EC

EC/DEC 2:8 353 343 333 323 313 303 293 283 273 263 253 243 233

11.1 9.56 8.41 6.80 5.89 4.57 3.65 2.82 2.15 1.60 1.09 0.740 0.446

9.96 8.22 7.44 5.90 4.98 3.80 3.02 2.28 1.66 1.20 0.800 0.521 0.296

353 343 333 323 313 303 293 283 273 263 253 243 233

7.76 6.47 5.58 4.52 3.73 2.99 2.40 1.88 1.30 0.933 0.583 0.317 0.164

8.25 6.92 5.93 4.97 3.98 3.15 2.50 1.87 1.35 0.900 0.589 0.313 0.156

PF6

Li

7.11 5.61 4.70 3.80 3.28 2.61 2.15 1.63 1.11 0.831 0.53 0.297 0.179

4.36 3.82 3.13 2.55 2.03 1.70 1.33 1.08 0.734 0.494 0.300 0.169 0.104

6.10 5.40 4.45 3.75 2.90 2.41 1.76 1.34 0.960 0.660

3.67 3.30 2.55 2.07 1.65 1.27 0.952 0.721 0.500 0.337

EC/DEC 4:6 6.85 5.63 4.93 4.00 3.40 2.66 2.08 1.63 1.18 0.935 0.568 0.362 0.204

5.00 4.25 3.64 2.83 2.38 1.88 1.47 1.10 0.830 0.633 0.390 0.238 0.132

9.31 7.84 6.57 5.48 4.41 3.60 3.06 2.39 1.79 1.27 0.868 0.538 0.239

9.32 7.76 6.34 5.49 4.49 3.5 2.94 2.29 1.68 1.16 0.768 0.451 0.229

6.24 5.16 4.48 3.49 3.00 2.41 1.95 1.39 0.980 0.621 0.386 0.207 0.126

4.00 3.41 2.83 2.20 1.85 1.41 1.11 0.780 0.538 0.361 0.230 0.120 0.0644

6.91 5.87 4.88 4.14 3.22 2.57 1.98 1.50 1.14 0.785

7.87 6.68 5.46 4.62 3.62 2.91 2.23 1.68 1.23 0.865

EC/DEC 6:4

EC/DEC 8:2

Figure 2. Arrhenius plots of DEC, DDEC, DPF6, and DLi for the EC−DEC binary solutions with EC/DEC ratios of (a) 2:8, (b) 4:6, (c) 6:4, and (d) 8:2 and the VFT fitting curves. □, DEC; ○, EC; △, PF6; ▽, Li.

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Table 5. Fitting Parameters for VFT Equation for Diffusion Coefficients D, D = Do exp(−B/(T − To)), for the Four Binary EC−DEC Systems Do/(m2·s−1) DEC EC PF6 Li

3.85·10−8 4.00·10−8 1.64·10−8 1.46·10−8

DEC EC PF6 Li

9.47·10−9 1.31·10−8 1.61·10−8 1.03·10−8

DEC EC PF6 Li

1.11·10−8 1.35·10−8 1.53·10−8 1.40·10−8

DEC EC PF6 Li

5.80·10−8 6.13·10−8 2.22·10−8 3.36·10−8

B/T−1 EC/DEC 2:8 895 912 742 796 EC/DEC 4:6 486 568 720 699 EC/DEC 6:4 557 583 696 795 EC/DEC 8:2 1209 1165 833 1157

To (T)

R2

100 106 122 119

0.99968 0.99968 0.99911 0.99906

151 143 128 133

0.99731 0.99909 0.99866 0.99852

148 147 136 130

0.99944 0.99973 0.99848 0.99967

80 86 120 95

0.99963 0.99986 0.99939 0.99933

Figure 4. ■, □, DEC; ▲, △, DPF6; and ▼, ▽, DLi are plotted versus DDEC for (a) 353 K, (b) 303 K, and (c) 263 K.

Figure 5. R parameters are plotted against mol % EC at 303 K. ▲, DDEC/DLi; ▼, DEC/DLi; △, DDEC/DPF6, and ▽, DEC/DPF6. The dotted lines are a guide to the eyes.

suggest that a lithium ion diffuses with accompanying PF6 and/ or solvent molecules. Ionic Conductivity at 303 K. Ionic conductivities σ for the seven electrolytes were measured at 303 K, and the concentration dependence of σ is shown in Figure 6a. The σ value at DEC 100 % was small, increased as the EC concentration increased, and showed a moderate maximum near EC/DEC 6:4. Similar phenomena were observed in DMC/PC and DEC/PC binary systems including 1 M LiTFSA.6 By using eq 4, the degree of ion dissociation α was calculated and plotted versus the concentration of EC in Figure 6b. The value of α increased with the increase of the EC ratio and reached 0.71 in the single solvent EC. Previously, we reported the α value for 1 M LiTFSA in PC to be 0.60.6 Greater ion dissociation was obtained in 1 M LiPF6 EC electrolyte. For reference, a slightly smaller value of α was obtained in the 1 M LiPF6 in PC, but it is larger than the value for 1 M LiTFSA in PC. RPF6 in the single EC electrolyte was 1.2, which is larger

Figure 3. Composition dependences of □, DDEC; ○, DEC; △, DPF6 and ▽,

DLi at (a) 353 K, (b) 303 K, and (c) 263 K.

have similar values in the DEC single solution. Then a PF6− ion is paired with a Li+ ion that is surrounded by a DEC solvent molecule. When EC is added, polar EC molecules enter coordinate sites of Li+ rather than DEC molecules, and the ion association between Li+ and PF6− becomes weaker. As the ratio of EC increased, RPF6 became smaller, suggesting that the ion association decreased. In the EC solution, RPF6 is about 1.3 and larger than the van der Waals ratio (rEC/rPF6), indicating that ion dissociation is not complete. Since the lithium ion is much smaller than the other species, the RLi values (about 2.1 to 2.3)

than the ratio of van der Waals radii. The larger RPF6 value is reflected by ion pairing effects. E

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(3) Ding, M. S.; Jow, T. R. Conductivity and viscosity of PC-DEC and PC-EC solutions of LiPF6. J. Electrochem. Soc. 2003, 150, A620− A628. (4) Ding, M. S. Conductivity and viscosity of PC-DEC and PC-EC solutions of LiBF4. J. Electrochem. Soc. 2004, 151, A40−A47. (5) Ding, M. S. Electrolytic conductivity and glass transition temperatures as functions of salt content, solvent composition, or temperature for LiBF4 in propylene carbonate + diethyl carbonate. J. Chem. Eng. Data 2004, 49, 1102−1109. (6) Hayamizu, K.; Aihara, Y. Ion and solvent diffusion and ion conduction of PC-DEC and PC-DME binary solvent electrolytes of LiN(SO2CF3)2. Electrochim. Acta 2004, 49, 3397−3402. (7) Price, W. S. NMR Studies of Translational Motion. Principles and Application; Cambridge University Press: Cambridge, 2009. (8) Hayamizu, K.; Aihara, Y.; Arai, S.; Garcia-Martinez, C. Pulsegradient spin-echo 1H, 7Li, and 19F NMR diffusion and ionic conductivity measurements of 14 organic electrolytes containing LiN(SO2CF3)2. J. Phys. Chem. B 1999, 103, 519−524. (9) Hayamizu, K.; Akiba, E. An evaluation method of liquid electrolytes for lithium batteries by the multinuclear pulsed-gradient spin-echo NMR: The diffusing radii of lithium ion and anions in organic solvents. Electrochemistry 2003, 71, 1052−1054. (10) Aihara, Y; Sugimoto, K.; Price, W. S.; Hayamizu, K. Ionic conduction and self-diffusion near infinitesimal concentration in lithium salt-organic solvent electrolytes. J. Chem. Phys. 2000, 113, 1981−1991. (11) Aihara, Y.; Bando, T.; Nakagawa, H.; Yoshida, H.; Hayamizu, K.; Akiba, E.; Price, W. S. The characteristic properties of ions for six lithium salts including lithium bis-(oxorateborate) dissolved in γbutyrolactone. J. Electrochem. Soc. 2004, 151, A119−A122. (12) Hayamizu, K.; Matsuo, A.; Arai, J. A divalent lithium salt Li2B12F12 dissolved in propylene carbonate (PC) studied by NMR methods. J. Electrochem. Soc. 2009, 156, A744−A750. (13) Kondo, K.; Sano, M.; Hiwara, A.; Omi, T.; Fujita, M.; Kuwae, A.; Iida, M.; Mogi, K.; Yokoyama, H. Conductivity and solvation of Li+ ions of LiPF6 in propylene carbonate solutions. J. Phys. Chem. B 2000, 104, 5040−5044. (14) Tsunekawa, H.; Narumi, A.; Sano, M.; Hiwara, A.; Fujita, M.; Yokoyama, H. Solvation and ion association studies of LiBF4propylenecarbonate and LiBF4-propylenecarbonate-trimethyl phosphate solutions. J. Phys. Chem. B 2003, 107, 10962−10966. (15) Takeuchi, M.; Kameda, Y.; Umebayashi, Y.; Ogawa, S.; Sonoda, T.; Ishigoro, S.; Fujita, M.; Sano, M. Ion-ion interactions of LiPF6 and LiBF4 in propylene carbonate solution. J. Mol. Liq. 2009, 148, 99−108. (16) Yang, L.; Xiao, A.; Lucht, B. L. Investigation of salvation in lithium ion battery electrolytes by NMR spectroscopy. J. Mol. Liq. 2010, 154, 131−133. (17) Zugmann, S.; Fleischmann, M.; Amereller, M.; Gschwind, R. M.; Winter, M.; Gores, H. J. Salt diffusion coefficients, concentration dependence of cell potentials and transference numbers of lithium difluoromono(oxalato)borate-based solutions. J. Chem. Eng. Data 2011, 56, 4786−4789. (18) Tyrrell, H. J. V.; Harris, K. R. Diffusion in Liquids: A Theoretical and Experimental Study; Butterworths: London, 1984. (19) Riddick, J. A.; Bunger, W. B.; Sakano, T. K., Eds. Organic Solvents, 4th ed.; John Wiley & Sons: New York, 1986. (20) Hayamizu, K.; Price, W. S. A new type of sample tube for reducing convection effects in PGSE-NMR measurements of selfdiffusion coefficients of liquid samples. J. Magn. Reson. 2004, 167, 328−333. (21) Hayamizu, K. On Accurate Measurements of Diffusion Coefficients by PGSE NMR Methods; http://www.ribm.co.jp/service/nmr_ document/pgse-nmr_eng.pdf (accessed March 1, 2012). (22) Ue, M. Mobility and ion association of lithium and quaternary ammonium salts in propylene carbonate and γ-butyrolactone. J. Electrochem. Soc. 1994, 141, 3336−3342. (23) Ue, M.; Murakami, A.; Nakamura, S. A convenient method to estimate ion size for electrolyte materials design. J. Electrochem. Soc. 2002, 149, A1385−A1388.

Figure 6. (a) Ionic conductivity σ and (b) degree of ion dissociation α are plotted against mol % EC at 303 K.

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CONCLUSION The self-diffusion coefficients D of the solvents, Li, and PF6 in the DEC−EC binary solvent systems including 1 M LiPF6 were measured in a wide temperature range by varying the relative ratio of DEC and EC. Also, temperature-dependent D values for 1 M LiPF6 single-solvent systems of DEC, PC, and EC were measured. The ion dissociation in the single DEC electrolyte is weak and is enhanced with the addition of EC. The factors behind the higher ion conductivity in the binary DEC−EC electrolytes were analyzed using DEC, DDEC, DLi, and DPF6. Although the physical properties are different in EC and DEC, they have the common substructure of the O−CO−O moiety. After mixing, DEC and DDEC have similar values in wide ranges of temperature and relative composition. The binary solvents may be homogeneous in microstructures. This property contributes to better performance of LIBs with EC−DECbased electrolytes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to H. Nakagawa for sample preparation and ionic conductivity measurement. The author deeply thanks to Y. Aihara and S. Seki for valuable discussion.

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REFERENCES

(1) Ding, M. S. Electrolytic conductivity and glass transition temperatures as functions of salt content, solvent composition, or temperature for LiBF4 in propylene carbonate + diethyl carbonate. J. Chem. Eng. Data 2003, 48, 519−528. (2) Ding, M. S. Liquid phase boundaries, dielectric constant, and viscosity of PC-DEC and PC-EC binary carbonates. J. Electrochem. Soc. 2003, 150, A455−A462. F

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