Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts - The

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Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts Keigo Kubota, Zyun Siroma, Hikaru Sano, Susumu Kuwabata, and Hajime Matsumoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11281 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry

Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts Keigo Kubota,*,a Zyun Siroma,a Hikaru Sano,a Susumu Kuwabataa,b and Hajime Matsumoto*,a,b a) Department of Energy and Environment, Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b) Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

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ABSTRACT The self-diffusion coefficients of the lithium cation (D(Li+)) and counter anion (D(anion)) in the molten lithium amide, such as lithium bis(fluorosulfonyl)amide (Li[FSA]) and lithium fluorosulfonyl(trifluoromethylsulfonyl)amide (Li[FTA]), were measured by a pulsed-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) method at 150 ºC. Relationships between viscosity and the resulting self-diffusion coefficient (the D(Li+) is 1.4×10–11 m2·s–1 and the D(anion) is 5.5×10–12 m2·s–1 for the Li[FSA] and the D(Li+) is 1.7×10–12 m2·s–1 and the D(anion) is 6.6×10–13 m2·s–1 for the Li[FTA]), the Li[FTA] and Li[FSA] have a stronger intercalation between ions than the high temperature lithium molten salts. However, their ionic conductivities are higher than those estimated by D(Li+) and D(anion), specifically, that of the Li[FTA] is three times higher. Therefore, the Li[FTA] would have special conductive behavior, which leads to the superionic nature on the Walden Plot and high rate performance against much high viscosity in lithium secondary battery. Such unique conduction and battery behaviors using the Li[FTA] might be due to the robust and asymmetric structure of the FTA–.


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INTRODUCTION

The liquid electrolyte of a lithium secondary battery is usually composed of a lithium salt and an organic solvent.1 In general, the organic solvent must be able to dissolve the lithium salts, most of which have melting points above 200 °C, but the organic solvent has some problems, such as flammability and high vapor pressure. Recently, a solid electrolyte has been developed to improve the safety and durability of the battery.2 However, the liquid electrolytes leave a margin for improvement considering their productivity and compatibility with composite electrodes, which cannot be combined with the solid electrolytes. To improve the safety of the liquid electrolytes, room temperature ionic liquids (RTILs) have attracted much interest due to their non-flammability and low vapor pressure.3 Based on such a background, we have attempted to develop a low-melting lithium molten salt, which melts below the melting point of the lithium metal (180 ºC), as the electrolyte without any organic solvent or organic cation of the RTIL. It is ultimately a simple constructions like a liquid electrolyte, which is composed of only a lithium cation

(Li+)

and

counter

anion.

(fluorosulfonyl)(trifluoromethylsulfonyl)amide

We with

reported a

structural

that

a

lithium

asymmetry

(Li[FTA],

Li[CNO4F4S2]), formed a molten salt electrolyte above the melting point of 100 ºC, which might be the lowest value of all the reported robust amide anions.4,5 It showed a stable charge and discharge with the lithium metal or composite electrodes without any organic solvents, RTILs, and the other additive salts at 110 ºC and its rate performance was better than that of an organic electrolyte at 25 ºC.5 This result is contrary to the high viscosity and poor ionic conductivity of the Li[FTA] (17000 mPa·s and 0.2 mS·cm–2 at 110 ºC) based on the organic solvents at 25 ºC (usually four orders magnitude lower viscosity and three orders magnitude higher conductivity


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than the Li[FTA]). The Li[FTA] can be used at 45 ºC by the addition of a cesium salt with the same FTA anion.6 The cation mixture showed a better rate performance than the organic electrolyte at the same temperature (65 ºC).6 These molten salt electrolytes are expected to be the electrolytes of novel molten salt batteries operating from around room temperature to over 100 ºC. To elucidate the unique rate performance versus the poor transport property, the self-diffusion coefficients (D) of the Li+ (D(Li+)) and counter anion (D(Li+)) for the Li[FTA] are required because it is a significant parameter for the mobility of ions and intimately interrelated with the viscosity and ionic conductivity. The D for the various electrolytes of the lithium secondary battery, which are a mixture of the lithium salts and solvents, such as organic solvent or RTILs, were reported in many papers7-10 by a pulsed-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) method at room temperature. In contrast, the D of the conventional lithium molten salts at high temperature (above 300 ºC), such as halides or nitrates used in a thermal battery,11 were mostly measured by a capitally method12 or molecular-dynamics simulation.13,14 Recently, the PGSE-NMR method was performed at high temperature for a few alkali metal molten salts.15-17 In this paper, we report the D of the Li+ (D(Li+)) and counter anion (D(Li+)) for the Li[FTA] measured by PGSE-NMR at 150 ºC. Furthermore, we also measured another robust and small amide such as a lithium bis(fluorosulfonyl)amide (Li[FSA], Li[NO4F2S2]) with a relatively lowmelting lithium salt (m.p.: 140 ºC),18 to investigate the effect of the structural asymmetry of amide anions on the transport property of the lithium single molten salt system. We discuss these results and the other transport properties, such as the viscosity and ionic conductivity, using the Stokes-Einstein equation and Nernst-Einstein equation. These results of


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the Li[FSA] and Li[FTA] were then compared to those of the high temperature lithium molten salts, the other low-melting lithium molten salts (lithium ionic liquids10) synthesized from LiBH4, oligoethylene glycol monomethyl ether (n = 7.2), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (Li[C36.8H65.6O18.4BF12]) or pentafluorophenol (PFP) (Li[C42.8H63.6O18.4BF10]), and mixtures of the RTILs or organic solvents.


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EXPERIMENTAL SECTION

The Li[FTA] and Li[FSA] were synthesized by a neutralization reaction of their carbonates and anion acids, which were prepared using a strong acid ion-exchange resin.5 The diffusion coefficients of 7Li and 19F were measured at 150 ºC using the pulsed gradient spin-echo (PGSE) NMR (JEOL Ltd., ECA400 in UBE Scientific Analysis Laboratory). The PGSE experiments were performed using a pulse sequence modified with bipolar pulses and longitudinal eddy current delay (BPP-STE-LED).19 The samples were inserted into a standard 5 mm diameter NMR tube. The height of the sample was less than 10 mm in order to avoid convection effects due to the presence of temperature gradients. The NMR probe was a T40TH5AT/FG2D (JEOL Ltd.). The samples were heated from room temperature to 150 ºC at the rate of 1 ºC·min–1. The NMR measurement started after not less than 12 hours from reaching 150 ºC. The external references were a LiCl aqueous solution (1 mol·dm–3) for 7Li at 0 ppm and hexafluorobenzene (C6F6) for 19F at –164.9 ppm. The values of the parameters for the pulse sequence of the Li[FSA] are G = 0.1~0.8 T·m–1 for the strength of the gradient pulse, δ = 5~10 ms for the pulse width, and ∆ = 600~1500 ms for the diffusion time corresponding to the interval between the two gradient pulses. For the Li[FTA], the values of the parameters are G = 0.1~0.43 T·m–1, δ = 5~10 ms, and ∆ = 200~1000 ms. The diffusion coefficients were obtained by a regression analysis of the signal attenuation curve according to the Stejskal equation.20 The 7Li NMR and

19

F NMR spectra of the Li[FSA] and Li[FTA] are shown in Figure 1. For

the 7Li NMR, a single peak of the Li+ at 1.57 ppm was observed in each salt. For the 19F NMR, a single peak of the FSO2 side chain at 56.6 ppm was observed in the Li[FSA]. On the other hand, the same peak of the FSO2 side chain at 59.8 ppm and another peak at –73.7 ppm, attributed to


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the CF3SO2 side chain, were observed for the Li[FTA]. These results well agreed with those of ionic liquids containing the FTA anion.


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Figure 1. 7Li-NMR and

19

F-NMR spectra of (a) 7Li of Li[FSA], (b)

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19

F of Li[FSA], (c) 7Li of

Li[FTA] and (d) 19F of Li[FTA].


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RESULTS AND DISCUSSION

Diffusion Coefficient of Lithium Molten Salts

For the measurement of the D by the PGSE-NMR at high temperature, the first point to confirm is that no convection occurred in the sample during the measurement.16 The dependence of the self-diffusion coefficient of Li+ (D(Li+)) and counter anions (D(anion)) in Li[FSA] and Li[FTA] on ∆ at δ = 7 ms is shown in Figure 2. If the convection occurs, the D may decrease with the increase in ∆. Based on the result, the D(Li+) and D(anion) are independent of ∆. Therefore, there was no effect of convection that occurred in this measurement. The D(Li+) is 1.4×10–11 m2·s–1 and the D(anion) is 5.5×10–12 m2·s–1 for the Li[FSA] and the D(Li+) is 1.7×10–12 m2·s–1 and the D(anion) is 6.6×10–13 m2·s–1 for the Li[FTA]. They are average values of the NMR results for the various parameters in which the relative uncertainty was less than 10 %. For the 1-1 lithium molten salt, such as the Li[FSA], Li[FTA], high temperature lithium molten salts,12,17 Li[HFIP] and Li[PFP],10 the D(Li+) and D(anion) are summarized in Table 1. When compared to the other lithium molten salts, the D values of the Li[FSA] and Li[FTA] are one-hundredth and one-thousandth of those of the high temperature molten salts and 2~10 times higher than the Li[HFIP] and Li[PFP] at room temperature. However, there is a significant difference in the operating temperature (T in Table 1). The D of LiF, LiCl and LiNO3 having high melting points above 200 ºC, must be measured at a higher temperature than the melting points and the D of the Li[HFIP] and Li[PFP], which are room temperature molten salts, are measured at room temperature. Thus, the comparison of D itself can be explained by the difference in the operating temperature and it is difficult to discuss the effect of the anion species.


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On the other hand, the ratio of D(Li+)/D(anion) increases with an increase in the anion size among the high temperature molten salts, Li[FSA] and Li[FTA], which apparently have larger anions than halides and nitrates as shown in Figure 3. This is explained that the diffusion of the free Li+, which is clustered with no anion, and becomes relatively faster with an increase in the anion size. However, the D(Li+)/D(anion) of Li[HFIP] and Li[PFP] are close to unity, which are lower than the Li[FTA], though the HFIP– and PFP– are much higher than the FTA– as shown in Figure 3. Therefore, the diffusion of Li+ in the Li[HFIP] and Li[PFP] would be prevented from the complex structure of the HFIP– and PFP–. The Li[FSA] was reported as not only a low-melting lithium molten salt, but as mixtures with various additional materials. The D of the Li[FSA] mixture, such as solvated ionic liquids6 with glyme (triethylene glycol dimethyl ether (G3, C8H18O4) and tetraethylene glycol dimethyl ether (G4, C10H22O5), mixtures of a lithium salt and RTIL (N-methyl-N-propyl pyrrolidinium (P13, [C8H18N]+) FSA),18 and mixtures of a lithium salt and organic solvent (ethylene carbonate (EC, C3H4O3) and diethyl carbonate (DEC, C5H10O3)),7 and also summarized in the bottom part of Table 1. No apparent tendency of the D(Li+) is found between the single Li[FSA] and its mixtures. However, the D(anion) of the single Li[FSA] at 150 ºC is lower than those of all of its mixtures at room temperature versus the temperature dependence because the coulomb interaction of FSA– from Li+ decreased by solvation with Li+ of the organic solvents or insertion of the other large organic cation. On the other hand, the D(Li+)/D(anion) values of their mixtures are below unity, which is much lower than that of the single Li[FSA]. This is not explained by the decreased interaction because it will enhance both the D(Li+) and D(anion). For the organic solvent mixtures, the solvents, of which the D are twice as much as the Li+, form a cluster with the Li+.7 For the RTIL mixture, the Li+ forms an anion complex,21 such as Li[FSA]32–, because


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the mixture contains a number of FSA– ions for each Li+. Their sizes become much larger than the single Li+. Therefore, one of the features of the low-melting lithium molten salt is the high ratio D(Li+)/D(anion) being greater than unity because they contain no organic solvent and only one anion for one Li+. Comparing the Li[FSA] and Li[FTA], there is a 10-times difference in the D(Li+) and D(anion), respectively, at the same temperature and similar anion structure. The approximately same ratios of the D(Li+)/D(anion), which are much higher than unity, indicated that the ion size of Li+ in Li[FTA] is not changed by solvation of an organic solvent or coordination of anions the same as the Li[FSA]. Therefore, in the Li[FTA], the diffusion of Li+ and FTA– is slow due to the strong interaction with each other, which would be much higher than that in the Li[FSA].


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Figure 2. Dependence of self-diffusion coefficient on diffusion time: ●, Li+ of Li[FSA]; ○, FSA– of Li[FSA]; ■, Li+ of Li[FTA]; □, FTA– of Li[FTA].


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Table 1. Self-diffusion coefficients8-10,12,17,21 and viscosity5,8-10,21,23 of lithium molten salts.


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Figure 3. Structure of anions reported in this paper.


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Relationship with Viscosity Based on Stokes-Einstein Equation

It is difficult to discuss the interaction between ions by only the D in the lithium molten salts because the temperature dependence is much greater than the others. Therefore, we discuss the relationship between the D of each ion and experimental viscosity (η) using the Stokes-Einstein equation:

D = kT/πarη

(1)

where k is the Boltzmann constant, T is the temperature, r is the hydrodynamic radius of each ion, and a is a theoretical constant, which were empirically obtained between 4 to 6 for the slip and stick boundary conditions, respectively. For the Li+, the value of kT/πrη was plotted versus D(Li+) using the ionic radius22 of the Li+ (Figure 4) in order to determine the applicability of the Nernst-Einstein equation. The conventional high temperature lithium salts12,23 correspond to the slip condition (a = 4). The Li[FSA]-glyme solvated ionic liquids4 are located above the line of a = 6. This is because the radius of the Li+ is larger than the elemental one by solvation of the glyme. The mixture of Li[FSA] and RTIL is reported to have the same result because of the anion complex.21 On the other hand, the single Li[FSA], which contains no glyme and one FSA– for one Li+, is located between the lines of a = 4 and 6. Thus, the Li+ is an elemental ion in the Li[FSA], but its interaction with an anion is relatively stronger than those of the high temperature lithium molten salts.


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The Li[FTA] is located in a region above the a = 6 line though it is a single lithium salt the same as the Li[FSA]. Unlike the solvated ionic liquid, the size of the Li+ in the Li[FTA] was found not to be changed, the same as the Li[FSA] in Table 1. In addition, the low-melting Li[HFIP] and Li[PFP],10 which were reported that the Li+ appears to induce strong coulombic interactions with the borate anion, show a much greater deviation Thus, for the lithium molten salts having no cluster and ionic complex, this result suggests that the interaction increases with the increase in the size or complexity of the anion.


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Figure 4. (a) Applicability of the Stokes-Einstein equation in 1:1 lithium molten salts, and (b) a close-up view of the region occupied by low-melting salts: □, LiF;17,23 ◊, LiCl;12,23 △, LiNO3;12,23 ●, Li[FSA];5 ■, Li[FTA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]-G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]-EC-DEC.9


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Relationship with Ionic Conductivity Based on Nernst-Einstein Equation

The Li[FSA] and Li[FTA] has a stronger interaction than the high-temperature lithium molten salt in a free diffusion state. Thus, when applying a current or an electric field, the ionic conductivity can be calculated by the D using the Nernst-Einstein equation based on the assumption that each ion is completely dissociated:

σcalc = (e2F2c(D(cation) + D(anion))/RT

(2)

where e is the electric charge of the ions, F is Faraday’s constant, c is the concentration of the lithium salt, and R is the gas constant. The σcalc using the Nernst-Einstein equation is derived for noninteracting ions, as in an infinitely dilute solution.24 The σcalc and experimental ionic conductivity5,7-10,23 (σexp) are plotted in Figure 5. The high temperature lithium salts have very high σexp values, however, they are lower than the expected value from the D because of ionic association. The ratio of σexp/σcalc is about 0.8, which suggests a degree of dissociation. This value is higher among the 1:1 molten salts, such as alkali metal molten salts12,25 and RTILs containing one species of an organic cation and anion.7 In contrast, the Li[HFIP] and Li[PFP], which have a very strong interaction, the same as the Li[FTA], show greater deviations from the fully dissociated line and the ratio of σexp/σcalc is below 0.4. This indicated that the stronger interaction leads to the association of ions upon applying an electric field. The Li[FSA] and Li[FTA] at 150 ºC have intermediate ionic conductivities between the high temperature lithium molten salt and the low-melting Li[HFIP] and Li[PFP]. This is apparently


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explained by the temperature. However, the Li[FSA] is located above the fully-dissociated line. This means that the practical conductivity exceeds the best possible value in the fully-dissociated state. This result is contrary to the previous discussion that the interaction of the Li[FSA] is stronger than those of the high temperature molten salts. Thus, the Li[FSA] undergoes a specific migration mechanism upon applying an electrical field, which cannot be observed by the measurements in a free diffusion state, such as the viscosity measurement and NMR. The deviation of Li[FSA] from the Nernst-Einstein equation is slight. It is not improbable that there is an error in the NMR or conductivity measurement. For some high temperature alkali metal molten salts, the σexp/σcalc slightly exceeds unity, but this deviation was determined be a statistical error.25 However, the Li[FTA] shows a much greater deviation. As shown in Figure 5, it is apparently separated from the fully-dissociated line. The ratio of σexp/σcalc is 3.1. This result is completely opposite of the Li[HFIP] and Li[PFP], which are classified as the Li[FTA] at the D-η correlation in Figure 4. This result suggests that the ions, in particular the small Li+, specifically undergo a fast migration when applying the electrical field unlike the free diffusion with no electrical field. A more elaborate investigation is necessary about the movement of ions in these molten salts regarding the free diffusion or upon applying the electrical field. A more detailed investigation, such as PGSE-NMR using an electric field,26 will be required.


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Figure 5. Applicability of the Nernst-Einstein equation in 1:1 lithium molten salts: □, LiF;17,23 ◊, LiCl;12,23 △, LiNO3;12,23 ●, Li[FSA];5 ■, Li[FTA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]-G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]EC-DEC.9


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Superionic Behavior Based on Walden Plot

The relationship of the molar conductivity (Λexp) derived by σexp and the experimental density data and η is directly represented by the Walden plots27 (Figure 6). The fully-dissociated liquids, such as a diluted aqueous KCl solution of 1 mol dm–3, show a behavior denoted by the dashed line in Figure 6 (ideal line). Another comparative example with no report about the D, such as lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA] in Figure 3, Li[C6NO4F6S2]) molten salt and a high concentration (over 4 mol·dm–3) of Li[FSA] in acetonitrile (AN, C2H3N), denoted as a “superconcentrated electrolyte”,28 are also shown in Figure 6. The high temperature molten salts, Li[FSA] and Li[FTA], are located slightly above the ideal line. The Li[TFSA], Li[HFIP] and Li[PFP] belongs to the “poor ionic liquids” domain. Thus, a much larger and complex anion provides migration of Li+ by their interaction. The mixtures with the Li[FSA] are also located in the poor ionic liquids domain. This result suggests that additional materials will improve the viscosity and ionic conductivity of the Li[FSA], however, they also negate the superionic nature of the lithium molten salt. Among all the electrolytes, a greater deviation of the Li[FTA] from the ideal line suggests a specific conductive mechanism. As a result, the ions in the Li[FTA] with no electrical field slowly diffuse due to the strong interaction, but when applying on the electrical field, they migrate faster than the free diffusion. In particular, the small Li+ would quickly migrate through the large FTA–.


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Figure 6. Walden plot of log(molar conductivity, Λ) against log(reciprocal viscosity, η–1) for 1:1 lithium molten salts: □, LiF;23 ◊, LiCl;23 △, LiNO3;23 ●, Li[FSA];5 ■, Li[FTA] ];5 ×, Li[TFSA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]-G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]-EC-DEC;9 □, Li[FSA]-AN.28


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CONCLUSION

The diffusion coefficient measurements of the cation and anion for the Li[FSA] and Li[FTA] were successful using the PGSE-NMR method at 150 ºC. Based on the viscosity using the Stokes-Einstein equation, the low-melting lithium molten salts with the large anions, such as the FSA–, FTA–, HFIP– and PFP–, have a stronger interaction than the high-temperature lithium molten salts with the robust small anions. However, related to the ionic conductivities using the Nernst-Einstein equation, the conductivity of the Li[FSA] and Li[FTA] is higher than the predicted value in the fully-dissociated state. This result suggests a faster special movement of ions in these salts upon applying an electric field than that during the free diffusion. Compared with the single Li[FSA] molten salt and its mixtures with solvents, the special conductive behavior is removed by the addition of solvents. In particular, the special conductive behavior of the Li[FTA] appears to be three times faster than the estimated value in free diffusion. Considering that the interaction energy between the Li+ and amide anions estimated by ab-initio calculations was not very different,29 the unique transport property of the Li[FTA] might be caused by its structural asymmetry of the FTA– with a perpetual dipole moment in the presence of a certain electric field. It would be related to the reported good rate performance in the Li[FTA] molten salt battery considering much high viscosity of the Li[FTA]. As the electrolytes of a lithium secondary battery, the Li[FTA] are expected to show an interesting performance.


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ACKNOLEDGEMENT A part of this study was supported by the Advanced Low Carbon Technology Research and Development Program Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of the Japan Science and Technology Agency (JST) and also a part of this study was conducted in cross-appointment project between Osaka University and AIST.


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AUTHOR INFORMATION Corresponding Author Name: Keigo Kubota E-mail: [email protected] Phone: +81727519047

Name: Hajime Matsumoto E-mail: [email protected] Phone: +81727519426

Co-Author Name: Zyun Siroma E-mail: [email protected]

Name: Hikaru Sano E-mail: [email protected]

Name: Susumu Kuwabata E-mail: [email protected]

Notes The authors declare no competing ﬁnancial interest.


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REFERENCES (1) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (2) Fergus, J. W. Ceramic and Polymeric Solid Electrolytes for Lithium-ion batteries. J. Power Sources, 2010, 195, 4554–4569. (3) Sakaebe, H.; Matsumoto, H. In Electrochemical Aspects of Ionic Liquids,; Ohno, H., Ed. WileyInterscience: Hoboken, NJ, 2005, Chapter 16. (4) Matsumoto, H.; Terasawa, N.; Umecky, T.; Tsuzuki, S.; Sakaebe, H.; Asaka, K.; Tatsumi, K. Low Melting and Electrochemically Stable Ionic Liquids Based on Asymmetric Fluorosulfonyl(trifluoromethylsulfonyl)amide. Chem. Lett., 2008, 37, 1020-1021. (5) Kubota, K.; Matsumoto, H. Investigation of an Intermediate Temperature Molten Lithium Salt Based on Fluorosulfonyl(trifluoromethylsulfonyl)amide as a Solvent-Free Lithium Battery Electrolyte J. Phys. Chem. C, 2013, 117, 18829-18836. (6) Kubota,

K.;

Matsumoto,

H.

Cation

Mixtures

of

Alkali

Metal

(Fluorosulfonyl)(trifluoromethylsulfonyl)Amide as Electrolytes for Lithium Secondary Battery. J. Electrochem. Soc., 2014, 161, A902-A907. (7) Noda, A.; Hayamizu, K.; Watanabe, M. Pulsed-Gradient Spin-Echo 1H and 19F NMR Ionic Diffusion Coefficient, Viscosity, and Ionic Conductivity of Non-Chloroaluminate RoomTemperature Ionic Liquids. J. Phys. Chem. B, 2001, 105, 4603-4610. (8) Ueno, K.; Yoshida, K.; Tsuchiya, M.; Tachikawa, N.; Dokko, K.; Watanabe, M. Glyme−Lithium Salt Equimolar Molten Mixtures: Concentrated Solutions or Solvate Ionic Liquids? J. Phys. Chem. B, 2012, 116, 11323-11331.


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(9) Takekawa, T.; Kamiguchi, K.; Imai, H.; Hatanoa, M. Physicochemical and Electrochemical Properties of the Organic Solvent Electrolyte with Lithium Bis(fluorosulfonyl)Imide (LiFSI) As Lithium-Ion Conducting Salt for Lithium-Ion Batteries. ECS Trans., 2015, 64, 11-16. (10) Shobukawa, H.; Tokuda, H.; Tabata, S.; Watanabe, M. Preparation and Transport Properties of Novel Lithium Ionic Liquids. Electrochim. Acta, 2004, 50, 1-5. (11) Guidotti, R. A.; Masset, P. Thermally Activated (“Thermal”) Battery Technology Part I: An Overview. J. Power Sources, 2006, 161, 1443-1449. (12) Janz, G. J.; Bansal, N. P. Molten Salts Data: Diffusion Coefficients in Single and MultiComponent Salt Systems. J. Phys. Chem. Ref. Data, 1982, 11, 505-693. (13) Ciccotti, G.; Jacucci, G., MacDonald, I. R. Transport Properties of Molten Alkali Halides. Physical Review A, 1976, 13, 426-436. (14) Koishi, T.; Tamaki, S.; J. Chem. Phys., A Theory of Transport Properties in Molten Salts. 2005, 123, 194501-1-11. (15) Matenaar, U.; Richter, J.; Zeidler, M. D. High-Temperature–High-Pressure NMR Probe for Self-Diffusion Measurements in Molten Salts. J. Magnetic Resonance, 1996, A122, 72-75. (16) Rollet, A. L.; Sarou-Kanian, V.; Bessada C. Self-diffusion Coefficient Measurements at High Temperature by PFG NMR. C. R. Chimie, 2010, 13, 399-404. (17) Sarou-Kanian, V.; Rollet, A. L.; Salanne, M.; Simon, C.; Bessada, C.; Madden, P. A. Diffusion Coefficients and Local Structure in Basic Molten Fluorides: in Situ NMR Measurements and Molecular Dynamics Simulations. Phys. Chem. Chem. Phys., 2009, 11, 11501-11506. (18) Huang, J.; Hollenkamp, A. F. Thermal Behavior of Ionic Liquids Containing the FSI Anion and the Li+ Cation. J. Phys. Chem. C, 2010, 114, 21840-21847.


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(19) Alexej, J.; Norbert M. Suppression of Convection Artifacts in Stimulated-Echo Diffusion Experiments. Double-Stimulated-Echo Experiments. J. Magnetic Resonance, 1997, 125, 372–375. (20) Stejskal, E. O.; Tanner, J. E. Spin Diffusion Measurements: Spin Echoes in the Presence of a Time-Dependent Field Gradient. J. Chem. Phys., 1965, 42, 288-292. (21) Hayamizu, K.; Tsuzuki, S.; Seki, S.; Fujii, K.; Suenaga, M.; Umebayash, Y. Studies on the Translational and Rotational Motions of Ionic liquids Composed of N-methyl-N-propylpyrrolidinium

(P13)

Cation

and

Bis(trifluoromethanesulfonyl)amide

and

Bis(fluorosulfonyl)amide Anions and Their Binary Systems Including Lithium Salts. J. Chem. Phys., 2010, 133, 194505-1-12. (22) Shannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta Cryst., 1969, B25, 925-946. (23) Janz, G. J. Thermodynamic and Transport Properties for Molten Salts: Correlation Equations for Critically Evaluated Density, Surface Tension, Electrical Conductance, and Viscosity Data. J. Phys. Chem. Ref. Data, 1988, 17, 1-309. (24) Harris, K. R. Relations between the Fractional Stokes-Einstein and Nernst-Einstein Equations and Velocity Correlation Coefficients in Ionic Liquids and Molten Salts. J. Phys. Chem. B, 2010, 114, 9572–9577. (25) Ciccotti, G.; Jacucci, G. Transport Properties of Molten Alkali Halides. Physical Review, 1976, 13, 426-436. (26) Umecky, T.; Saito, Y.; Matsumoto, H. Direct Measurements of Ionic Mobility of Ionic Liquids Using the Electric Field Applying Pulsed Gradient Spin-Echo NMR. J. Phys Chem. B Lett., 2009, 113, 8466-8468.


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(27) Yoshizawa, M.; Xu, W.; Angell, C. A. Ionic Liquids by Proton Transfer: Vapor Pressure, Conductivity, and the Relevance of ∆pKa from Aqueous Solutions. J. Am. Chem. Soc., 2003, 125, 15411-15419. (28) Tsuzuki, S.; Kubota, K.; Matsumoto H. Cation and Anion Dependence of Stable Geometries and Stabilization Energies of Alkali Metal Cation Complexes with FSA−, FTA−, and TFSA− Anions: Relationship with Physicochemical Properties of Molten Salts. J. Phys. Chem. B, 2013, 117, 16212−16218.


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TOC Graphic


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128x60mm (96 x 96 DPI)



Figure 1. 7Li-NMR and

19

F-NMR spectra of (a) 7Li of Li[FSA], (b) of Li[FTA].

19

F of Li[FSA], (c) 7Li of Li[FTA] and (d)

323x239mm (96 x 96 DPI)


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19

F

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Figure 2. Dependence of self-diffusion coefficient on diffusion time: ●, Li+ of Li[FSA]; ○, FSA– of Li[FSA]; ■, Li+ of Li[FTA]; □, FTA– of Li[FTA]. 163x139mm (96 x 96 DPI)



Figure 3. Structure of anions reported in this paper. 237x130mm (96 x 96 DPI)


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Figure 4. (a) Applicability of the Stokes-Einstein equation in 1:1 lithium molten salts, and (b) a close-up view of the region occupied by low-melting salts: □, LiF;17,23 ◊, LiCl;12,23 △, LiNO3;12,23 ●, Li[FSA];5 ■, Li[FTA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]-G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]-EC-DEC.9 165x136mm (96 x 96 DPI)



Figure 4. (a) Applicability of the Stokes-Einstein equation in 1:1 lithium molten salts, and (b) a close-up view of the region occupied by low-melting salts: □, LiF;17,23 ◊, LiCl;12,23 △, LiNO3;12,23 ●, Li[FSA];5 ■, Li[FTA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]-G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]-EC-DEC.9 162x136mm (96 x 96 DPI)


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Figure 5. Applicability of the Nernst-Einstein equation in 1:1 lithium molten salts: □, LiF;17,23 ◊, LiCl;12,23 △, LiNO3;12,23 ●, Li[FSA];5 ■, Li[FTA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]-EC-DEC.9 156x136mm (96 x 96 DPI)



Figure 6. Walden plot of log(molar conductivity, Λ) against log(reciprocal viscosity, η–1) for 1:1 lithium molten salts: □, LiF;23 ◊, LiCl;23 △, LiNO3;23 ●, Li[FSA];5 ■, Li[FTA] ];5 ×, Li[TFSA];5 *, Li[HFIP] (n = 7.2);10 +, Li[PFP] (n = 7.2);10 and mixtures: ○, Li[FSA]-G3;8 □, Li[FSA]-G4;8 ○, Li[FSA]-[P13][FSA];21 ○, Li[FSA]EC-DEC;9 □, Li[FSA]-AN.28 158x142mm (96 x 96 DPI)


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Table 1. Self-diffusion coefficients8-10,12,17,21 and viscosity5,8-10,21,23 of lithium molten salts. 287x170mm (96 x 96 DPI)


Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts - The

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