Sulfolane-Based Highly Concentrated Electrolytes of Lithium Bis

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C: Energy Conversion and Storage; Energy and Charge Transport

Sulfolane-Based Highly Concentrated Electrolytes of Lithium Bis(Trifluoromethanesulfonyl)amide: Ionic Transport, Li Ion Coordination and Li-S Battery Performance Azusa Nakanishi, Kazuhide Ueno, Daiki Watanabe, Yosuke Ugata, Yoshiharu Matsumae, Jiali Liu, Morgan L. Thomas, Kaoru Dokko, and Masayoshi Watanabe J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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The Journal of Physical Chemistry

Sulfolane-Based Highly Concentrated Electrolytes of

Lithium

Bis(trifluoromethanesulfonyl)amide:

Ionic Transport, Li Ion Coordination and Li-S Battery Performance Azusa Nakanishi,† Kazuhide Ueno,† Daiki Watanabe, Yosuke Ugata, Yoshiharu Matsumae, Jiali Liu, Morgan L. Thomas, Kaoru Dokko, and Masayoshi Watanabe*

Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya–ku, Yokohama, 240-8501, Japan

CORRESPONDING AUTHOR FOOTNOTE: To whom correspondence should be addressed. Telephone/Fax: +81-45-339-3955. E-mail: [email protected] †These two authors are equally contributed to this work.

ACS Paragon Plus Environment

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ABSTRACT Following our recent study demonstrating predominant Li ion hopping conduction in sulfolane (SL)-based highly concentrated electrolytes with LiBF4, LiClO4 and lithium bis(fluorosulfonyl)amide (LiFSA), here-in a systematic study on transport properties and Li ion coordination of SL-based electrolytes with lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was performed. In the highly concentrated region, Li ions clearly diffuse faster than SL and TFSA anions. The two oxygen atoms of the SL sulfonyl group tend to coordinate to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions, verifying the previous observation of the unusual Li ion conduction and its relevance to the SL- and anion-bridged, chain-like Li ion coordination structure for the SL-based concentrated systems with other Li salts. Moreover, addition of hydrofluoroether (HFE) to the SLbased concentrated electrolytes greatly enhances diffusion coefficients, but fragments the chain-like Li ion coordination to smaller clusters, leading to a reduced contribution of Li ion hopping to the overall Li ion conduction. The SL-based concentrated electrolyte and its mixtures with HFE showed lower lithium polysulfide solubility and higher rate capability for lithium-sulfur (Li-S) cells, compared with previously reported tetraglyme-based electrolytes. The SL-based electrolytes were found to manifest a significant improvement in Li ion mass transfer as a sparingly solvating electrolyte, enabling the solid-state sulfur redox reactions in high-performance Li-S batteries.


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Introduction Considerable efforts are being invested in developing novel Li ion conducting electrolytes with fast Li ion conduction and high thermal and electrochemical stabilities for high energy- and power density batteries. Among a variety of electrolyte materials ranging from liquid to solid (inorganic and polymeric) systems, liquid electrolytes with near-saturation Li salt concentrations have recently attracted growing attention since the high concentration of Li salts and scarcity of non-coordinating solvents in this type of liquid electrolyte confers various intriguing properties, including improved thermal stability, reduced vapor pressure, and enhanced electrochemical stability. However, these attractive attributes come at the cost of low ionic conductivity on account of high viscosity.1-3 Unique solvation structures, wherein almost all solvents coordinate to Li ions, are the key structural feature giving rise to the afore-mentioned properties of highly concentrated electrolytes. Many research groups have already demonstrated the improved performance of Li-ion cells with various highly concentrated electrolytes, such as higher rate charge-discharge performance and more stable charge-discharge cycling of the metallic Li anode and high-voltage cathodes, compared with analogous cells with conventional organic liquid electrolytes.4-11 Although high viscosities and slow Li ion transport have been recognized as common shortcomings for highly concentrated electrolytes, addition of nondisrupting, low-polarity diluent solvents that do not disrupt the unique Li ion solvation structures, such as fluorinated solvents, was found to be an effective method to reduce the viscosity and enhance the transport properties.12-14 Recent molecular dynamics simulations15-16 and diffusivity measurements by pulsed-field gradient (PFG-) NMR17-19 suggested that Li ion conduction in highly concentrated electrolytes involves not only the vehicular mechanism that relies on simple physical diffusion of the solvated Li ions, but also a Li ion hopping mechanism through Li ion exchange reactions from one coordinating site to another in a labile Li ion coordination structure. Decoupling Li ion transport from slow structural relaxation with a dominant hopping/exchange mechanism may be another possible route to achieve fast Li ion



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conduction in highly viscous liquid electrolytes. In our previous studies, diffusivity measurements of highly concentrated electrolytes comprised of sulfolane (SL) with LiBF4, LiClO4 and lithium bis(fluorosulfonyl)amide (LiFSA), have demonstrated that the Li ion diffusion is the fastest among the electrolyte components (i.e., Li ions, anions, and SL solvent molecules); this is a clear indication of Li ion diffusion occurring via a dominant Li ion hopping/exchange mechanism.18 Despite low ionic conductivity and high viscosity, the SL-based concentrated electrolytes enabled higher rate capability in a Li/LiCoO2 cell compared to an electrolyte with moderate, conventional, Li salt concentration of 1 mol dm−3. The improved rate performance was attributed to the hopping/exchange-based Li ion conduction. The wide appeal of highly concentrated electrolytes has also been extended to lithium-sulfur (LiS) batteries, which is one of the “beyond-Li-ion” batteries offering an inexpensive and high energy density alternative to circumvent the problems facing mature Li-ion technologies.20-24 Owing to the scarcity of non-coordinating solvent molecules and the common ion effect, the highly concentrated electrolytes effectively suppress dissolution of reaction intermediates (lithium polysulfides, Li2Sm) into the electrolyte,25-27 which is one of the major issues causing undesired side reactions and a loss of the active materials from the cathode for practical Li-S batteries. The limited Li2Sm solubility has led to a high Coulombic efficiency and a stable charge-discharge cycling of Li-S cells, and therefore the highly concentrated electrolytes and their mixtures with non-disrupting solvents are now regarded as “sparingly solvating electrolytes” suitable for achieving long life and high energy density practical LiS batteries.28 In the first part of this paper, we report the transport properties (self-diffusion coefficients, ionic conductivity and viscosity) and local Li ion coordination structures of binary systems of SL and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) to corroborate the previously reported unusual Li ion diffusion behavior in the SL-based highly concentrated electrolytes.18 In the second part, the SL-based concentrated electrolytes were diluted with a non-disrupting hydrofluoroether (HFE),12, 29 and effects


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of HFE on the transport properties and the coordination structure of Li ions were studied. Finally, charge-discharge performance of Li-S cells with the SL-based concentrated electrolytes and their mixtures with HFE was compared to that of the cells with tetraglyme (G4)-based electrolytes that we previously reported as sparingly solvating electrolytes for Li-S batteries.

Experimental Thermal transition temperatures, namely the melting point (Tm) and glass transition temperature (Tg), were determined using a differential scanning calorimeter (DSC6220, Seiko). The samples were hermetically sealed in aluminum pans in an Ar-filled glove box. The sample pans were first heated at 70 °C for 10 min, followed by cooling to −150 °C, and then heated from −150 °C to 70 °C at a scan rate of 10 °C min−1 under a nitrogen atmosphere. Tm and Tg were determined from the onset temperatures of the corresponding features in the heating thermograms. Battery-grade SL obtained from Kishida Chemical and 1,1,2,2–tetrafluoroethyl 2,2,3,3– tetrafluoropropyl ether (HFE) purchased from Daikin Industries were used as received. Battery-grade LiTFSA were kindly supplied by Solvay Japan and used without further purification. The SL-based concentrated electrolytes and the mixtures diluted with HFE were prepared by mixing LiTFSA and the solvents in the appropriate molar ratios and stirring overnight at room temperature in an Ar-filled glove box (VAC, dew point < −80 °C, [O2] < 1 ppm). PFG-NMR measurements were performed using a JEOL ECX-400 spectrometer with a 9.4 T narrow bore superconducting magnet equipped with a JEOL pulse field gradient probe and current amplifier to study the self-diffusion coefficients of each component: SL (1H, 399.7 MHz), TFSA anions (19F, 376.1 MHz), and Li cations (7Li, 155.3 MHz). The experimental procedure for PFG-NMR has been described in detail elsewhere.30



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The ionic conductivity (𝜎) of samples were determined by the complex impedance method using an impedance analyzer (VMP3, Biologic) in the frequency range of 500 kHz-1 Hz with a sinusoidal alternating voltage amplitude of 10 mV using a two platinized platinum electrodes cell (CG-511B, TOA Electronics). The density and viscosity were determined using a viscometer (SVM 3000, Anton Paar). Raman spectra of the samples were measured using a Raman spectrometer with a 532 nm laser (RMP-330, JASCO) with the spectral resolution of 4.5 cm−1. The instrument was calibrated using a polypropylene standard. The samples were sealed in a capillary tube, and their temperature was controlled using a Peltier microscope stage (TS62, INSTEC) with a temperature controller (mk1000, INSTEC). Single crystal X-ray structure analysis was performed on a Rigaku Mercury70 diffractometer using monochromatic Mo Kα radiation (λ= 0.71073 Å). The diffraction was measured at low temperature using a steady flow of −50 ℃ nitrogen gas. An empirical absorption correction was applied to the obtained data using multiscan averaging of symmetry equivalent data using spherical harmonics, implemented in the SCALES3 ABSPACK scaling algorithm (CrysAlisPro 1.171.38.43, Rigaku Oxford Diffraction, 2015). The crystallographic structure was solved by the direct method using SHELXT and all non-hydrogen atoms were refined anisotropically by the full-matrix least-squares method using SHELXL-2014/7.31-32 The saturation solubility of Li2S8 was determined by the procedure described elsewhere.33 In summary, the saturated solutions of Li2S8 were prepared by mixing stoichiometric amounts of S8 and Li2S at a molar ratio of 7:8 resulting in the desired nominal chemical structure. The solutions were stirred and heated at 60 °C for 5 days, and then maintained for 2 days at room temperature. The supernatant solution was then diluted with 1 mol·dm−3 LiTFSA in SL solution. The diluted Li2S8 solutions were electrochemically oxidized to S8 in a H-type two-electrode cell separated by a lithium-


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conducting ceramic (LICGC, Ohara). The maximum absorption of S8 oxidized from the polysulfides was recorded using a UV-Vis spectrophotometer (UV-2500PC, Shimadzu). The saturated solubility was calculated from the absorbance and the dilution factor. The sulfur composite cathode was prepared from a mixture of elemental sulfur (S8, Wako Chemical), porous carbon (Ketjen Black, EC600JD, Lion Corporation), and carboxymethyl cellulose (CMC2200, Daicel FineChem) at a weight ratio of 60:30:10 (hereafter abbreviated as S-KB cathode). KB and S8 were mixed using an agitating mortar, then transferred to a vial and maintained at 155 °C for 6 h to allow the diffusion of sulfur into the pores of KB.34 A slurry composed of S-KB composite, CMC binder, and pure water was spread on Al foil. The composite cathode was dried in an oven at 80 °C for 12 h and cut into a circular disk of 16 mm diameter. The typical loading of sulfur was ca. 0.7 mg cm−2. The 2032 coin-type cell was assembled in an Ar-filled glove box using the S-KB cathode, a Li metal anode (Honjo Metal, 16 mm in diameter), the electrolyte (80 μL), and a porous glass separator (GA55, Advantec). The galvanostatic charge/discharge measurements (HJ1001SD8, Hokuto Denko) were performed with cut-off potentials of 1.0 and 3.3 V for the discharge and charge steps, respectively, at 30 °C. Because the Li-S cell was prepared in the fully charged state, the charge/discharge cycle was defined as follows: 1st discharge → 2nd charge → 2nd discharge → 3rd charge → 3rd discharge, and so on. The Coulombic efficiency was defined as: Nth discharge capacity/Nth charge capacity. The specific capacity was calculated based on the mass of sulfur. Results and Discussion SL-LiTFSA binary system. A systematic study on thermal properties with DSC suggests that the SL-LiTFSA binary system forms a stable solvate at a 1:1 molar ratio (LiTFSA mole fraction of XLiTFSA = 0.5), and has a eutectic point (Te = −11.2 °C) between the pure SL and the 1:1 solvate at the solvent/Li salt molar ratio (x) of



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8. The phase diagram of the binary system (shown in the Supporting Information, SI, Figure S1) indicates that the SL-based electrolytes remain liquid at 30 °C in the composition range where x ≥ 2, and the transport properties were further studied in the range of 2 ≤ x ≤ 9. In our previous study,18 PFG-NMR has shown that SL-based highly concentrated electrolytes of a series of Li salts, such as LiBF4, LiClO4, and LiFSA exhibit an unusual Li ion diffusion behavior: self-diffusion coefficients of Li ions (DLi) was found to be higher than those of SL and the counter anions at x ≤ 3. A simple physical diffusion model (the so-called vehicular mechanism) premising translational motion of ‘solvated’ Li ion (by either SL or the anions) cannot account for the fastest Li ion diffusion. Therefore, a hopping/exchange mechanism was considered to significantly contribute to the intriguing Li ion diffusion behavior. Figure 1 shows the ratios of self-diffusion coefficients (DLi/DSL and DLi/DTFSA) and apparent Li transference numbers (tLi) based on the expression, tLi = DLi/(DLi + DTFSA) in the SL-based electrolytes with different compositions at 30 °C. In Table S1, the numerical data of DLi, DSL and DTFSA are listed along with density (d), ionic conductivity (σ), and viscosity (η) of the electrolytes. Generally, the diffusion behavior agrees well with that for the previously reported SL-based electrolytes with LiBF4. The DLi values for the present SL-LiTFSA system are similar to those for the reported SL-LiBF4 system at the same x. At cLi = 1.0 mol dm−3 (x = 9), both DLi/DSL and DLi/DTFSA are lower than unity; the diffusion coefficients follow the order: DSL > DTFSA > DLi, and Li ions are the slowest diffusive component. The same trend was also reported for typical organic electrolyte solutions with 1 mol dm−3 of Li salt

35-36

as well as the SL-LiBF4 system.18 With decreasing x (i.e., increasing cLi), DLi/DTFSA

becomes greater than 1, indicating that DLi becomes higher than the diffusion coefficient of the anion, while DSL is the highest at x ≥ 4. However, in the highly concentrated region (x ≤ 3), DLi clearly emerges as the highest value among the diffusion components as both DLi/DSL and DLi/DTFSA exceed unity. Li cations diffuse 1.6 times faster than TFSA anions at x = 2, resulting in a high tLi value of 0.61, which is the same value as that reported for the SL-LiBF4 system at x = 2.


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Figure 1. Diffusivity ratios (DLi/DSL and DLi/DTFSA) and apparent Li transference number (tLi) in SLbased electrolytes with different compositions (x = cSL/cLi) at 30 °C.

As suggested by the diffusivity data of the present study and the previous work18, the predominant Li ion hopping diffusion is likely a common behavior for SL-based concentrated electrolytes with Li salts having weakly coordinating anions including TFSA, BF4, ClO4, and FSA. In the previous report, local Li ion coordination was investigated using Raman spectroscopy combined with X-ray crystallography: the two oxygen atoms of the SO2 group of the SL coordinate to two different neighboring Li ions, forming a SL-bridged chain-like Li ion coordination. The counter anions also formed aggregated ion pairs (AGGs) or ionic clusters with Li ions.18 The SL-bridged chain-like structure and the ionic clusters were considered crucial to the unusual Li ion diffusion behavior dominated by the ion hopping/exchange mechanism, wherein Li ions are transferred from one coordinating site (on either SL or the anions) to another vacant site through ligand exchange in the labile Li ion coordination chains. Figure 2a shows Raman spectra in the range of 540-610 cm−1 for the SL-LiTFSA binary systems with different compositions. The Raman peak at 568 cm−1 for pure SL is assigned to the scissoring vibration mode of the SO2 moiety of SL.37 This peak becomes broader and shifts to a higher



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wavenumber because of Li ion coordination with decreasing x. Our previous Raman study on crystalline solvates of SL-LiBF4 (1:1) and SL-LiClO4 (2:1) suggested that the monodentate and bridging type SL molecules show characteristic peaks at 571 and 580 cm−1, respectively for the SO2 scissoring mode.18 Here, the broad peaks in the range of 565-590 cm−1 at lower values of x (x ≤ 3) are due to the variety of different Li ion coordination structures of SL in the liquid state, and clearly suggest that SL with either the monodentate or the bridging coordination accounts for a large proportion of the SL structures in the highly concentrated region of the SL-LiTFSA binary system. Raman spectra in the range of 740-750 cm−1 correspond to CF3 bending coupled with the S-N stretching vibration of the TFSA anions, and are known to be sensitive to differences in Li ion coordination.38-40 The band at 739-742 cm−1 is attributed to a solvent separated ion pair (SSIP) or an non-coordinating TFSA anion, whereas the band between 745-755 cm−1 originates from TFSA bound directly to Li ions in the form of a contact ion pair (CIP) or aggregate coordination (AGG). For the SL-LiTFSA binary system, the peak continuously shifts from 740 cm−1 to 746 cm−1 with decreasing x (Figure 2b), suggesting that TFSA anions form CIPs and AGGs in the SL-based concentrated electrolytes (x ≤ 3). A fine crystalline complex could be obtained at x = 1 as mentioned earlier, and its crystal structure is shown in Figures 3 and S2. The crystallographic data are also listed in Table S2. SL serves as the linker between two different Li ions, forming extended ‧‧‧SL‧‧‧Li+‧‧‧SL‧‧‧ chains (Figure 3a), similar to the reported SL-LiBF4 (1:1) and SL-LiClO4 (2:1) crystalline solvates.18 TFSA anions adopt a C2 (transoid) conformation and form highly aggregated ‧‧‧TFSA−‧‧‧Li+‧‧‧TFSA−‧‧‧ chains (Figure 3b), wherein two oxygen atoms of the sulfonyl groups coordinate to two different Li ions. This corroborates the above notion that both SL- and TFSA-bridged (i.e., AGG), chain-like Li ion coordination structures can be present to some extent in the concentrated liquid electrolytes based on the SL-LiTFSA binary system.


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Figure 2. Raman spectra of SL-LiTFSA binary mixtures at different SL/LiTFSA molar ratios (x) in the range of (a) 540-610 cm−1 for SL, and (b) 700-800 cm−1 for TFSA at 30 °C.

Figure 3. Single crystal structure of SL/LiTFSA complexes. SL- and TFSA-based chain-like Li ion coordination structures are highlighted in (a) and (b), respectively. Hydrogen atoms are omitted for clarity. Purple, Li; red, O; gray, C; yellow, S; light green, F; light blue, N. The crystallographic information file (cif) was deposited in the Cambridge Structure Database (CSD) as CCDC 1866670.

Figure 4 shows the salt concentration dependencies of ionic conductivity and viscosity for the SL-LiTFSA binary mixtures at 30 °C. The ionic conductivity increases as cLi increases below 1 mol dm−3, reaching a maximum value of 2.7 mS cm−1 at 1 mol dm−3, and then decreases down to 0.3 mS cm−1 with a further increase in cLi to 3.2 mol dm−3. The change in the viscosity from the dilute (12.5 mPa s) to concentrated (952 mPa s) regions is larger by one order of magnitude than the change in the



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conductivity. The SL-based concentrated electrolytes are even more viscous than the reported glyme-,30 acetonitrile (AN)-,7 and dimethyl sulfoxide41-based concentrated electrolytes at almost the same cLi of ~ 3 mol dm−3. The significantly higher viscosity is probably attributable to the chain-like Li ion coordination structures found in the SL-based concentrated electrolytes (Figures 2 and 3). In the highly concentrated region, the hopping/exchange mechanism can be involved in the ionic transport, and that likely contributes to the ionic conductivity of the SL-based concentrated electrolytes despite their remarkably high viscosity.

Figure 4. Isothermal conductivity and viscosity as a function of cLi at 30 °C of the SL-LiTFSA binary system.

SL-based concentrated electrolytes diluted with HFE. The high viscosity of the SL-based concentrated electrolytes needs to be addressed because it may hamper electrolyte wetting and filling of porous composite electrodes of lithium rechargeable batteries.42 In a previous work, addition of non-polar, low-viscosity, and non-flammable HFE to the G4-based molten Li salt complex electrolytes ([Li(G4)][TFSA]) resulted in more than one order of magnitude of reduction in viscosity and conductivity enhancement up to ~ 5 mS cm−1,25 where-in HFE is a non-coordinating inert solvent towards the local Li ion coordination structure.12,

29

We thus

expected that dilution of the SL-based concentrated electrolytes with non-disrupting HFE might result in a similar impact on the transport properties and the Li ion coordination structure.


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The concentrated SL-LiTFSA binary mixture (x = 2) was diluted with HFE at different molar ratios (y) of HFE/LiTFSA, and the salt concentration dependencies of ionic conductivity and viscosity at 30 °C are shown in Figure 5. The numerical data are also listed in Table S3. As expected, the viscosity drastically decreases with addition of HFE (an increase in y), and is reduced by almost two orders of magnitude at y = 4. However, the magnitude of the conductivity enhancement is not as high as that which might be expected from the viscosity change: the ionic conductivity reaches a maximum value of 0.9 mS cm−1 at y = 2 and 3, which is only two times higher than that without HFE (i.e. at y = 0). An analogously minor increase in the conductivity upon addition of HFE was also observed for other systems, such as SL-LiFSA-HFE42 and AN-LiTFSA-HFE.13, 43 The addition of low-polarity HFE may promote formation of rigid ion pairs, leading to a decrease in the number of charge carriers contributing to the ionic conduction. Moreover, the extended chain-like Li ion coordination structure in the SL-based concentrated electrolytes can be fragmented to smaller clusters in the presence of HFE. Therefore, the Li ion hopping/exchange mechanism would contribute less to the ionic conduction. All these factors are possible causes of the less-pronounced enhancement of conductivity despite the remarkable reduction of viscosity upon addition of HFE.

Figure 5. Isothermal conductivity and viscosity as a function of cLi at 30 °C for the SL-based electrolyte (x = 2) diluted with HFE. The effect of HFE on the Li ion coordination in the SL-based concentrated electrolytes was studied with Raman spectroscopy. As with the case of the G4-based concentrated electrolytes, the



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Raman peaks originating from HFE do not show any discernible shift in the mixtures of the SL-based concentrated electrolyte and HFE (x = 2, y = 1, 2, and 4), although Raman bands of HFE overlap with the bands of the SL-based electrolytes (Figure S3). This validates that HFE does not participate in Li ion coordination in the first solvation shell. To clarify the change in Raman spectra for SL and TFSA in more detail, HFE-derived bands are subtracted in a similar manner to that applied for the glymebased concentrated electrolytes diluted with HFE (Figure 6).44 As shown in Figure 6a, the difference spectra in the range corresponding to SO2 scissoring vibrations of SL have a very similar shape to the corresponding Raman spectra without HFE (x =2, y = 0), and the characteristic bands for the monodentate (571 cm−1) and bridging (580 cm−1) type SL molecules are observed in the spectra even in the presence of HFE (y = 0 ~ 4). Therefore, addition of HFE exerts little influence on the local Li ion coordination. On the other hand, the Raman peak positions corresponding to CF3 bending coupled with the S-N stretching vibration of TFSA anions slightly shift to higher wavenumber with increasing y (Figure 6b), indicating that the Coulombic ion-ion interaction between Li ions and TFSA anions is enhanced and the fraction of CIPs and AGGs increases in the presence of low-polarity HFE that decreases the dielectric constant of the system.

Figure 6. Difference Raman spectra of SL-LiTFSA (x = 2) diluted with different HFE/LiTFSA molar ratios of y in the range of (a) 540-600 cm−1 for SL, and (b) 710-770 cm−1 for TFSA at 30 °C. The HFE-


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derived bands are subtracted. The corresponding Raman spectra for the non-diluted electrolyte (x = 2, y = 0) are also shown for comparison.

Self-diffusion coefficients (DLi, DSL, and DTFSA) in the SL-LiTFSA-HFE ternary systems (x = 2, y = 0, 2 and 4) are plotted as a function of the reciprocal of viscosity (1/η) in Figure 7. The diffusion coefficients of all the components increase by more than one order of magnitude in association with the decrease in viscosity at y = 2 and 4. It is also noteworthy that DLi at y = 4 is higher than that in the SL-LiTFSA binary system (x = 9 and y = 0) at comparable cLi of ~ 1 mol dm−3. DSL and DTFSA show a similar increasing trend with addition of HFE, whereas the increase in DLi is smaller. Hence, Li ions diffuse the slowest among the components, and the diffusion coefficients follow the order: DSL > DTFSA > DLi in the diluted systems with HFE at y = 2 and 4. The tLi value estimated from the diffusion coefficients decreased from 0.61 at y = 0 to 0.49 at y = 4. This Li ion diffusion behavior (Figure 7) and the Raman spectra (Figure 6) corroborate the assumption that the chain-like Li ion coordination structure is fragmented into smaller clusters, leading to the reduced contribution of the Li ion hopping/exchange mechanism to the ionic conduction. The molar conductivity ratio (Λimp/ΛNMR) is a useful metric for evaluating the extent to which the diffusion of ionic species contributes to the actual ionic conduction and is relevant to the apparent degree of dissociation or correlative motion of ions in ionic conductors.35,

45-46

Λimp is the molar

conductivity, and ΛNMR is determined from DLi and DTFSA using the Nernst-Einstein equation: ΛNMR = F2(DLi + DTFSA)/RT, where F is the Faraday constant, R is the gas constant, and T is the absolute temperature. Figure 8 shows that the dependence of Λimp/ΛNMR on cLi. Λimp/ΛNMR for the SL-LiTFSA binary systems slightly decreases with increasing cLi, but remains high (0.7) even at x = 2. However, the addition of HFE to the SL-LiTFSA concentrated electrolyte (x = 2) significantly reduces Λimp/ΛNMR to 0.27 at y = 2, and 0.14 at y = 4, verifying that the minor (rather than large) increase in conductivity in Figure 5 is also attributed to ionic association in the SL-LiTFSA-HFE ternary systems.



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Figure 7. Self-diffusion coefficients plotted as a function of reciprocal of viscosity at 30 °C for the SL-LiTFSA binary system.

Figure 8. Molar conductivity ratio Λimp/ΛNMR of the SL-LiTFSA binary and SL-LiTFSA-HFE ternary electrolytes as a function of cLi

Li-S battery performance. In previous work, highly concentrated electrolytes were found to effectively suppress elution of the lithium polysulfide intermediates (Li2Sm), and thus achieve reversible charge-discharge reactions of Li-S cells with high specific capacity (i.e., high utilization rate of sulfur in the cathode) and high Coulombic efficiency.25-28 Moreover, the solubility of Li2Sm could be effectively reduced by addition


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of hydrofluoroether solvents such as HFE to molten solvate electrolytes43, 47-49 as well as ether-based organic electrolytes.50-51 Figure 9 shows the saturation solubility (represented as total atomic S concentration) of nominal Li2S8 in the SL-based electrolytes, (“nominal Li2S8” assumes a complete reaction between S8 and Li2S in 7:8 molar ratio without occurrence of disproportion reactions, see Experimental). The reported Li2S8 solubility in the G4-based solvate ionic liquid electrolyte (x = 1), [Li(G4)][TFSA], and its mixture with HFE, are also shown in Figure 9 for comparison.25 Note that Li2Sm with different polysulfide chain lengths are formed in the actual Li2S8 solutions because of complicated disproportionation reactions of Sm2−.52 UV-vis spectra of the saturated solutions (Figure S4) indicates that the spectral shape for the SL-LiTFSA-HFE ternary electrolyte was very similar to that for [Li(G4)][TFSA] diluted with HFE: namely, similar compositions of Sm2− species, such as S82−, S62−, S42−, S32−, and S22−, are stabilized in these electrolytes, according to the assignments reported in the literature.53-54 The Li2S8 solubility in the SL-based binary and ternary electrolytes is lower than those in the G4-based electrolytes. This is presumably due to the lower donor number (DN) of SL (14.8 kcal mol−1)55 compared with that of G4 (17 kcal mol−1).56 Nevertheless, the Li2S8 solubility in these electrolytes was more than two orders of magnitude lower than the reported values for organic electrolytes with conventional concentration such as 1 mol dm−3 of LiTFSA in G4 (~ 6 × 103 mM_S).33 Therefore, the SL-based concentrated electrolytes and their mixtures with HFE can be considered to behave as sparingly solvating electrolytes for Li-S batteries.



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Figure 9. Saturation solubility of Li2S8 (expressed in mM of atomic S) in the G4- and SL-based electrolytes, where Li2S8 is the nominal formula, assuming a complete reaction between S8 and Li2S without occurrence of disproportion reactions. The charge-discharge performance of Li-S cells with the SL-based concentrated electrolyte (x = 2) was compared to that with the G4-based concentrated electrolyte ([Li(G4)][TFSA]). Here, the SKB cathode with relatively low sulfur loading (0.7 mg cm−2) were used under electrolyte-rich conditions (80 μL) for the charge-discharge tests in order to simply elucidate the effects of the electrolyte on the battery performance. Given the solubility of the nominal Li2S8, (which is the most soluble composition of Li2Sm, m = 1~8), the cathode sulfur loading, and the electrolyte volume, both electrolytes potentially dissolve at the most ca. 10% of the active species of the S-KB cathode as Li2Sm, and thereby charge-discharge of Li-S cells with these sparingly solvating electrolytes indeed relies primarily on solid-phase redox reactions. Figure 10a shows the charge-discharge performance of LiS cells at different discharge current densities. The cell with the SL-based concentrated electrolytes (x = 2) exhibits lower capacity than the cell with [Li(G4)][TFSA] at current densities less than 0.15 mA cm−2. The lower conductivity and poor electrode wettability on account of higher viscosity of the SLbased electrolyte are assumed to be responsible for the lower capacity: 627 mPa s and 0.42 mS cm−1 for the SL-LiTFSA (x =2), c.f. 106 mPa s and 1.6 mS cm−1 for [Li(G4)][TFSA]. However, at higher current densities greater than 0.25 mA cm−2, the cell with the SL-based concentrated electrolytes (x = 2) delivers higher capacities. Coulombic efficiency is also higher for the cell with the SL-based electrolyte. This may be due either to the lower Li2S8 solubility or the more limited polysulfide shuttle in the highly viscous SL-based concentrated electrolyte.


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Figure 10. (a) Discharge capacity and Coulombic efficiency at different discharge current densities, and charge and discharge curves of the cells using the concentrated electrolytes, (b) SL-LiTFSA (x = 2) and (c) G4-LiTFSA (x = 1).

In the discharge curves at lower current densities (Figure 10b and c), two distinct voltage plateaus appear at approximately 2.3 and 2.0 V, and these are believed to correspond to the reduction of S8 to longer-chain Li2Sm (m ≥ 4), and the further reduction of Li2Sm to shorter Li2S2 and Li2S, for the upper and lower voltage plateau, respectively.53, 57 Despite the superior transport properties (ionic conductivity and viscosity) of the electrolyte, the cell with [Li(G4)][TFSA] shows larger polarization, and more importantly, the capacity derived from the second plateau significantly decreases with increasing current density. On the other hand, the capacity decay at higher current density is much alleviated for the cell with the SL-based electrolyte, although the increase in the polarization is non-



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negligible. Recent modeling studies of charge-discharge behaviors in Li-S cells suggest that larger discharge overpotential at higher rate is due either to electrode surface passivation by Li2S and/or Li2S2 associated with its precipitation or to limited mass transfer of reactants (i.e., dissolved Sm2− and Li ions).58-59 However, the observed difference in the rate performance should be of little relevance to the dissolution/precipitation of Li2Sm in these sparingly solvating electrolytes. As the reductive reaction at the lower voltage contributes a significant part of the capacity, a larger number of Li ions must be supplied from the electrolyte to the electrode for the progress of the reaction at the second plateau. Consequently, the second reduction process can be more limited by mass transfer of Li ion than the first reduction process. In previous work, the hopping conduction of Li ions in the SL-based concentrated electrolytes was suggested to be responsible for the improved rate capability of Li/LiCoO2 cells.18 Likewise, the enhanced Li ion mass transfer via the hopping/exchange mechanism can also play a vital role in the improved reaction kinetics in the Li-S cell with the SL-based electrolyte, such as improved rate properties and lower overvoltage for the second discharge plateau. Indeed, the Li ion diffusion limiting current density of the SL-LiBF4 concentrated electrolyte (x = 2) was reported to be as high as 3.5 mA cm−2 with a Li/Li symmetric cell,18 which is more than three times higher than that of [Li(G4)][TFSA] (1.1 mA cm−2).60 The effect of HFE on the charge-discharge performance of Li-S cell was further elucidated using the SL- and G4-based concentrated electrolytes diluted with HFE. With these electrolytes, the porous S-KB composite electrode can be easily infiltrated due to the decreased viscosity. The potentially solubilized sulfur species in the form of Li2Sm can also be reduced to less than 2% in the present Li-S cells. Figure 11a shows the charge-discharge performance of Li-S cells with the SL-LiTFSA-HFE (x = 2, y = 2) and G4-LiTFSA-HFE (x = 1, y = 4) ternary electrolytes. Similar to the G4-based electrolytes reported in our previous study,25 the discharge capacity and rate capability were improved for the SLbased electrolyte in the presence of HFE: discharge capacity at a current density of 2 mA cm−2 was increased from ca. 500 mAh g−1 to ca. 700 mAh g−1 by addition of HFE. Coulombic efficiency was


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also improved to 99% with the SL-LiTFSA-HFE ternary electrolyte owing to the lower solubility of Li2Sm. When compared to the cell with the G4-LiTFSA-HFE electrolyte, the cell with the SL-LiTFSAHFE electrolyte exhibits higher discharge capacity and better rate capability. The overvoltage during discharge is lower and the capacity derived from the second plateau is larger for the SL-based electrolyte (Figures 11b and c); however, it should be noted that the ionic conductivity of the SLLiTFSA-HFE (0.9 mS cm−1 at x = 2, y = 2) electrolyte is much lower than that of the G4-LiTFSA-HFE (5.4 mS cm−1 at x = 1, y = 4) electrolyte. The local Li ion coordination structure is maintained even in the presence of HFE as shown in Raman spectra (Figure 6). Although the Li ion hopping/exchange conduction was not apparent in the diffusion measurements with HFE dilution as discussed in the former section (Figure 7), the enhanced mass transfer of Li ions via hopping/exchange mechanism likely remains, contributing to the improved rate capability as well as the increased DLi by dilution with HFE. This may also be due to difference between the mechanism of bulk (random) diffusion and the mechanism for Li ion mass transfer during applied current/potential in the SL- and G4-based electrolytes. It is obvious that the SL-based electrolytes are superior to G4-based electrolytes as the sparingly solvating electrolyte for high-performance Li-S cells. Achieving Li-S cells with a high sulfur loading under reduced electrolyte conditions is essential for practical applications of the Li-S batteries with high energy density exceeding 300 Wh kg−1.61 Such an attempt with these sparingly solvating electrolytes is currently in progress, and will be reported in separate papers.



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Figure 11. (a) Discharge capacity and Coulombic efficiency at different discharge current densities, and charge and discharge curves of the cells using the ternary electrolytes, (b) SL-LiTFSA-HFE (x = 2, y = 2) and (c) G4-LiTFSA-HFE (x = 1, y = 4).

4. CONCLUSIONS The transport properties and Li ion coordination structures of the binary SL-LiTFSA and the ternary SL-LiTFSA-HFE systems were investigated. In the highly concentrated region (x ≤ 3), Li ion hopping conduction and SL-bridged, chain-like Li ion coordination with ionic clusters were found, similar to the earlier reported SL-based concentrated systems. Dilution of the SL-based concentrated electrolytes


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with HFE significantly reduced viscosity by almost two orders of magnitude at y = 4; however, the conductivity is not increased as much as expected from the decrease in viscosity, probably due to morepronounced ionic association in the presence of low-polarity HFE. The presence of HFE fragmented the SL-bridged, chain-like Li ion coordination structure to smaller clusters, whereas the local Li ion coordination structure with SL is maintained and the Li ion hopping/exchange mechanism may still persist. The SL-based binary and ternary electrolytes allowed for lower Li2Sm solubility, and higher rate capability of Li-S cells, compared with the G4-based electrolytes. The SL-based electrolytes were found to manifest a significant improvement in Li ion mass transfer as the sparingly solvating electrolyte for enabling the solid-state sulfur redox reactions in high-performance Li-S cells.

Supporting Information. Phase diagram of SL-LiTFSA binary system; numerical data of cLi, d, σ, η, DLi, DSL, DTFSA for the SLLiTFSA and the SL-LiTFSA-HFE at 30 °C; Crystallographic data of SL-LiTFSA = 1:1 crystalline solvate; Raman spectra of SL-LiTFSA-HFE ternary electrolytes; UV-vis spectra of SL-LiTFSA-HFE (x = 2, y = 2) and G4-LiTFSA-HFE (x = 1, y = 4) with saturated Li2S8. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This study was supported in part by the JSPS KAKENHI (Grant Nos. 16H06053 to K.U., 18H03926 and 16H06368 to K.D. and 15H05758 to M.W.) from the Japan Society for the Promotion of Science (JSPS), and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).



Author information Corresponding Author *Telephone/Fax: +81-45-339-3955. E-mail: [email protected]

Notes The authors declare no conflict of interest.


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Sulfolane-Based Highly Concentrated Electrolytes of Lithium Bis

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