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Synthesis and electrochemical properties of partially fluorinated ether solvents for lithiumsulfur battery electrolytes
Journal of Power Sources 401 (2018) 271–277
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Synthesis and electrochemical properties of partially fluorinated ether solvents for lithiumesulfur battery electrolytes
T
Zheng Yuea, Hamza Dunyaa, Shankar Aryalb, Carlo U. Segreb, Braja Mandala,∗ a b
Department of Chemistry, Illinois Institute of Technology, Chicago, IL, 60616, USA Department of Physics & CSRRI, Illinois Institute of Technology, Chicago, IL, 60616, USA
H I GH L IG H T S
partially fluorinated ether compounds were synthesized and characterized. • Five properties and polysulfide solubility are provided. • Physical electrolyte formulations were tested for LieS battery electrolytes. • Several displayed good ionic conductivity and electrochemical stability. • Electrolytes • Improved cycling performances were achieved at high current density (0.5C).
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-sulfur batteries Partially fluorinated ethers Polysulfide shuttle effect Ionic conductivity Solid electrolyte interface Capacity fading
Fluorinated ethers have been used as a co-solvent in traditional ether-based electrolytes to suppress the polysulfide shuttle effect in lithium-sulfur batteries. In this work, five partially fluorinated ether compounds have been synthesized. The key properties, such as viscosity, ionic conductivity and polysulfide solubility of the electrolytes containing these co-solvents have been systematically studied. The electrolyte formulation showed best physical properties was tested in lithium-sulfur coin cells. The new fluoroether-based electrolytes displayed superior electrochemical performance compared to that of the traditional ether-based electrolytes.
1. Introduction Lithium-sulfur batteries (LSBs) are batteries composed of an elemental sulfur cathode and a lithium metal anode. Because of high theoretical specific capacity (1675 mAh g−1) of the sulfur cathode, and energy density (2600 W h kg-1), which is about five times higher than that of lithium-ion batteries (LIBs) [1], LSBs are considered to be the most promising candidate for the next generation energy storage devices. However, despite having extensive efforts since the concept of LSBs was introduced more than 30 years ago [2], a commercially viable LSB has yet conceived. The main barrier to the commercialization of LSBs is the rapid capacity fading and low coulombic efficiency in charge-discharge cycling, which leads to short battery life. Pioneering research has proved that the fast capacity fading is mainly caused by the insulating nature of elemental sulfur (electrical conductivity below 1 × 10−15 S m-1 at room temperature) and the polysulfide shuttle (PSS) effect [2,3]. During the battery discharge process, elemental sulfur in the cathode is first reduced to higher lithium polysulfide (Li2Sx,
∗
8 ≧ x ≧ 2) intermediates, which are soluble in the electrolyte system. Dissolved Sx−2 anions can then migrate through the electrolyte to the lithium metal anode, where they reduce to insoluble Li2S. This effect not only causes severe self-discharging, but also blocks the active material on the lithium anode surface, causing failure of the cell. Currently, the most widely used solvents for the LSB electrolytes are ethers, such as dimethoxy ethane (DME), dioxolane (DOL) and tetraethylene glycol diethyl ether (G4) [2,4]. The advantages of ether solvents are good solubility of lithium salts, comparatively high chemical and electrochemical stability, moderate dielectric constant (ε) and low viscosity (which leads to high ionic conductivity). However, their high donor ability also provides high solubility to polysulfide intermediates [4], leading to unwanted PSS phenomena. Recently, a new strategy has achieved great success to address PSS by mixing a large proportion (≥50%) of partially fluorinated ether solvent with the aforementioned traditional ether solvents. Several commercially available fluorinated ethers, such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) [5–8], bis(2,2,2-
Corresponding author. E-mail address: [email protected] (B. Mandal).
https://doi.org/10.1016/j.jpowsour.2018.08.097 Received 22 May 2018; Received in revised form 2 August 2018; Accepted 31 August 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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and allowed to warm to room temperature. After no more H2 evolution, a solution of 5.0 mmol of compound 3 in 15 mL anhydrous ethyl ether was added dropwise. Then the mixture was stirred 24 h at room temperature. Then 25 mL water was added to dissolve all solid. The water layer was separated and extracted with 2 × 15 mL dichloromethane. All organic layers were combined and washed with 2 × 15 mL brine. The organic layer was dried with anhydrous MgSO4, and the solvent was removed by rotary evaporation. The remaining was a yellow, oillike liquid. It was distilled under vacuum (5 mm Hg) to give colorless products 4a-e with yields from 28 to 42%.
trifluoroethyl) ether (BTFE) [9,10], ethyl-1,1,2,2-tetrafluoroethylether (ETFE) [11], 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE) [12] and 1,3-(1,1,2,2-tetrafluoroethoxy)propane (FDE) [13] have been studied as a co-solvent for LSB electrolytes. In general, polysulfides have much lower solubility in the partially fluorinated solvents, thereby significantly inhibiting the PSS effect. Recently, both Nazim et al. [9] (DOL/TTE) and Watanabe et al. [6] (G4/TTE) have used UV–vis spectroscopy as a quantitative tool to measure the concentration of polysulfides in electrolytes. Their investigations showed that long chain polysulfides had a much lower solubility in the partially fluorinated ether (TTE-containing electrolytes). A similar study using another fluorinated ether, FDE, by Sui et al. [13], also showed much lower solubility of polysulfides. Furthermore, partially fluorinated solvents have multiple functions in battery electrolytes. Chenxi et al. reported that TTE helps anode protection by forming mechanically stable solid electrolyte interface (SEI) layer [14] over the anode without the use of any electrolytic additive, such as LiNO3. The analysis of the SEI layer on the anode surface showed hierarchical compositions of LiF, which is believed to increase the stability of the SEI layer. Nazim et al. also investigated the surface morphology of the cycled sulfur electrode by SEM [9] and concluded that fluorinated ether compounds also helps forming protection layer on the sulfur cathode by reductive decomposition. In this work, we have synthesized five partially fluorinated ether compounds, 2,2,2-trifluoroethyl methyl ether ethylene glycol (TFEG), ethylene glycol 2,2,3,3-tetrafluoropropyl methyl ether (TFPG), ethylene glycol di(2,2,2-trifluoroethyl) ether (DTFEG), ethylene glycol 2,2,2trifluoroethyl-2,2,3,3-tetrafluoropropyl ether (TFEPG) and ethylene glycol di(2,2,3,3-tetrafluoropropyl) ether (DTFPG), using a simple, lowcost procedure. All of them have the similar structure R1-OCH2CH2OR2, where both R1 and R2 are fluoroalkyl groups with 3, 4, 6, 7 and 8 fluorine atoms. Their density and viscosity were determined. We formulated our solvents with different ratios of DME/DOL to make 1 M LiTFSI solution. The conductivity and polysulfide solubility of these new electrolytes were tested. We tested our electrolytes with conventional sulfur cathode, carried out charging-discharging for 50 cycles, and evaluated the Coulombic efficiency and capacity fading. We anticipate this research could reveal key structure-property relationships involving fluorinated ether compounds, and will provide impetus to design new battery co-solvents with much less PSS effect, higher ionic conductivity and superior cycling performance.
2.3. Characterization and measurement 1 H NMR and 13C NMR spectra (300 MHz Bruker NMR spectrometer) were used to confirm the purity (> 95%) and structures of the fluorinated ether compounds. TFEG (4a): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.89 (q, 3JHF = 8.8 Hz, 2H, CH2), 3.77 (t, 3JHH = 4.2 Hz, 2H, CH2), 3.54 (t, 3JHH = 4.5 Hz, 2H, CH2), 3.38 (s, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 124.1 (q, 1JCF = 279 Hz, CF3), 71.9 (2C, CH2), 68.8 (q, 2JCF = 42.8 Hz, CH2), 59.1 (CH3). Anal. calcd. for C5H9F3O2 (%): C, 37.98; H, 5.74; F, 36.05. Found: C, 37.89; H, 5.76; F, 36.01. TFPG (4b): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.94 (tt, 3JHF = 5.1 Hz, 2JHF = 53.4 Hz, 1H, CF2H), 3.89 (t, 3JHF = 12.6 Hz, 2H, CH2), 3.73 (t, 3JHH = 4.5 Hz, 2H, CH2), 3.54 (t, 3JHH = 4.5 Hz, 2H, CH2), 3.37 (s, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 112.4 (tt, 2JCF = 30.2 Hz, 1JCF = 204 Hz, CF2H), 109.1 (tt, 2JCF = 34.0 Hz, 1JCF = 249 Hz, CF2), 71.8 (CH2), 71.7 (CH2), 68.3 (t, 2JCF = 30.2 Hz, CH2), 59.0 (CH3). Anal. calcd. for C6H10F4O2 (%): C, 37.90; H, 5.30; F, 39.97. Found: C, 37.81; H, 5.24; F, 39.91. DTFEG (4c): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.90 (q, 3JHF = 8.7 Hz, 4H, CH2), 3.81 (s, 4H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.8 (q, 1JCF = 279 Hz, CF3), 71.9 (CH2), 68.8 (q, 2JCF = 32.7 Hz, CH2). Anal. calcd. for C6H8F6O2 (%): C, 31.87; H, 3.57; F, 50.41. Found: C, 31.79; H, 3.52; F, 50.35. TFEPG (4d): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.92 (tt, 3JHF = 4.8 Hz, 2JHF = 53.1 Hz, 2H, CF2H), 3.90 (q, 3JHF = 8.7 Hz, 4H, CH2), 3.81 (s, 4H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.8 (q, 1JCF = 279 Hz, CF3), 112.5 (tt, 2JCF = 34.0 Hz, 1JCF = 189 Hz, CF2H), 109.2 (tt, 2JCF = 37.7 Hz, 1JCF = 257 Hz, CF2), 71.8 (CH2), 71.7 (CH2), 68.6 (m, 2C, CH2). Anal. calcd. for C7H9F7O2 (%): C, 32.57; H, 3.51; F, 51.52. Found: C, 32.51; H, 3.54; F, 51.47. DTFPG (4e): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.91 (tt, 3JHF = 5.1 Hz, 2JHF = 53.1 Hz, 1H, CF2H), 3.90 (q, 3JHF = 8.7 Hz, 4H, CH2), 3.88 (t, 3JHF = 12.6 Hz, 4H, CH2), 3.78 (m, 4H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 112.5 (tt, 2JCF = 30.2 Hz, 1JCF = 189 Hz, CF2H), 109.2 (tt, 2JCF = 34.0 Hz, 1JCF = 249 Hz, CF2), 71.6 (2C, CH2), 68.6 (m, 2C, CH2). Anal. calcd. for C8H10F8O2 (%): C, 33.12; H, 3.47; F, 52.38. Found: C, 33.02; H, 3.38; F, 52.29. The density of the fluoroether compounds was determined by measuring the weight of 0.5 mL prepared fluorinated ether solvents, or 1 M LiTFSI solutions at room temperature. The viscosity of all samples was measured with a Brookfield DV3T viscometer containing spindle CPA-40Z, and a Fisher Scientific Isotemp 1013s recycling heater to keep the samples at 25 °C. The ionic conductivity was measured by using Mettler Toledo S230Kit conductivity meter in an environment chamber from 25 °C to 70 °C. The conductivity meter was calibrated with a standard 0.01 mol L−1 KCl aqueous solution with conductivity 1.314 mS cm−1 at 25.0 °C.
2. Experimental 2.1. Materials Sodium hydride (60% dispersion in mineral oil) was purchased from ACROS Organics, and washed 5 times with hexane to remove mineral oil before used. All other reagents and solvents were used without further purification: 2-methoxyethanol (Sigma Aldrich), Ethylene carbonate (Sigma Aldrich), p-toluenesulfonyl chloride (Sigma Aldrich), sodium hydroxide (Fisher scientific), lithium bis(trifluoromethylsulfonyl)imide (Alfa Aesar), elemental sulfur (Alfa Aesar), lithium sulfide (Alfa Aesar), 2, 2, 2-trifluoroethan-1-ol (SynQuest), 2, 2, 3, 3-tetrafluoropropan-1-ol (SynQuest). 2.2. General procedure for the synthesis of five fluoroether compounds The synthetic strategy to the fluoroether compounds used in this study is described in Scheme 1. Compounds 2a and 2b were prepared according to the method reported in a patent literature [15]. Tosylates 3a and 3b were synthesized by the reaction of tosyl chloride with the corresponding fluorinated alcohol. 5.5 mmol compound 2 in 15 mL anhydrous ethyl ether was added dropwise into a suspension of 9.0 mmol NaH (mineral oil was removed before use) in 30 mL of ethyl ether with ice-water bath and stirring. This mixture was stirred 30 min
2.4. Polysulfide solubility Li2S6 was produced using the following procedure: Li2S and elemental sulfur were mixed in the weight ratio of 3:8 and 1 g of the 272
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Scheme 1. General synthetic scheme for the fluorinated ether compounds.
compound. The viscosity of TFEPG is only slightly larger than TFPG, but much smaller than DTFPG. This evidence supports our assumption that compounds with eCF3 group tend to have lower viscosity. Yukio Sasaki measured the viscosity of ethylene glycol 2-fluoroethyl methyl ether (FEG), ethylene glycol 2, 2-difluoroethyl methyl ether (DFEG) and TFEG [4], and found that TFEG is much less viscous than the other two compounds. To explain this phenomenon, they assumed that 3 fluorine atoms attached to one carbon provides a larger steric hindrance and electronic repulsion, thus decreasing the interaction between molecules. By contrast, eCF2CF2H group dramatically increases the viscosity. The molecular weight of TFPG, which contains only one eCF2CF2H, is lower than that of DTFEG; however, its viscosity is much higher. High viscosity leads to low ionic conductivity. Our study indicates that eCF3 group should be included, while eCF2CF2H group should be excluded in the electrolyte design consideration.
mixture was dissolved in 5 mL DME and stirred overnight. The color changed from light yellow to red. Then the solvent was air dried. All operations were done in a glove box. The polysulfide-saturated solutions were made as follows: ∼20 mg Li2S6 was mixed with 0.5 mL sample, shaken vigorously, and let stand for 5 days. 20 μL of the solution's upper layer was diluted to 2 mL with anhydrous methanol, and the UV–vis spectrum was measured in the range of wavelength from 200 nm to 800 nm. Pure methanol was used as the blank. 2.5. Preparation of coin cells S/C spherical powder, (2–12 μm) from Sigma (w/w = 6.5:3.5) composite was manually ground for 20 min and prepared by the melt diffusion method at 155 °C for 12 h. The cathode slurry was prepared by mixing 75 wt% S/C, 15 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) in NMP solvent. The slurry was cast onto an aluminum foil substrate and then dried at 60 °C overnight. CR2032-type coin cells were assembled in an Ar filled glove box with oxygen content less than 1 ppm. The cells contained Celgard 2535 as the separator and lithium foils as both the counter and reference electrodes. All cells were cycled with 0.2C and 0.5C currents on a MTI cycler within a voltage range of 1.7–2.8 V at room temperature.
3.2. Ionic conductivity Typically, the conductivity of the solvents decreases with the increase in viscosity. The conductivity of 1 M LiTFSI solution composed of our fluorinated ether solvents are all higher than those containing TTE as the counterpart. As summarized in Tables 2 and 3, the conductivity of DME:co-solvent = 1:1 solution is much higher than that of DME:DOL:co-solvent = 1:1:1 solution. Among the fluorinated ether solvents, TFEG showed the best performance. The conductivity of DME:TFEG = 1:1 solution was 8.70 mS cm−1 and that of DME:TFEG = 1:2 solution was 7.97 mS cm−1 (Fig. 1). By contrast, the conductivity of 1 M LiTFSI in DME:DOL = 1:1 solution, which was most commonly used for LSBs, was ∼11 mS cm−1 in our measurement.
3. Results and discussions 3.1. Viscosity Among the five fluoroether compounds, TFEPG and DTFPG are new and only TTE is commercially available. TFEG and TFPG are previously reported, but not commercially available (Table 1). Therefore, we synthesized these compounds to complete the structure-property relationship. It is noteworthy that we report here the viscosities of TFPG, TFEPG, DTFEG and DTFPG for the first time. Only one literature reported the viscosity of TFEG [4] and its application related to LIBs. Typically, fluoroethers have higher viscosity than DME, because of their higher molecular weight. As summarized in Table 1, TFEG showed the lowest viscosity among the five compounds. DTFEG, despite its molecular weight being larger than TFPG, is the second lowest viscous
3.3. Polysulfide solubility To measure the solubility of polysulfides in electrolytes containing DME/fluorinated ether co-solvents, UV–Vis spectroscopy was applied. Fig. 2 shows the absorption curves of various solvent composition. Only the mixture of DME with TFEG and TFPG were tested because they performed much higher conductivity compared with the larger molecules. According to the literature [14], in organic solution S62− would 273
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Table 1 Physical properties of the fluorinated DME analogs. Mwa (g mol−1)
db (g cm−3)
ηc (cP)
Reference
DME
90.12
0.87
0.46
Commercially available
TTE
232.07
1.49
1.28
Commercially available
TFEG
158.12
1.18
0.91(0.78)
Not commercially available [4]
TFPG
190.14
1.30
2.26
Not commercially available [16]
DTFEG
226.12
1.34
1.40
Not commercially available [17]
TFEPG
258.13
1.40
2.31
New compound
DTFPG
290.15
1.42
4.50
New compound
Code
a b c
Structure
Molecular weight. Density at room temperature. Viscosity at 25 °C.
undergo a disproportionation reaction, as a result the solutions contain several different Sx2− species. The bands of the UV–vis absorption of Sx2− in organic solvents are assigned as follows: 490–500 nm for S82−, 450–470 nm for S62−, ∼420 nm for S42−, 340 nm for S32−, and ∼280 nm for S22−. Comparing the curve of DME:TFEG = 1:1, DME:TFPG = 1:1 and DME:TTE = 1:1, we can conclude that the rank of polysulfide solubility suppressing effect is TTE > TFPG > TFEG. However, DME:TFEG = 1:2 solution performed even lower absorption than DME:TTE = 1:1 solution except when the wavelength < 220 nm, which demonstrates the lower polysulfide solubility. As mentioned in Table 2 that DME:TFEG = 1:2 solution performed a much higher conductivity (7.97 mS cm−1) than that of DME:TTE = 1:1 solution (6.52 mS cm−1). This indicates 1 M LiTFSI in DME:TFEG = 1:2 may have even better performance than 1 M LiTFSI in DME:TTE = 1:1 as electrolyte for LSBs.
Table 2 Viscosity and ionic conductivity of 1 M LiTFSI electrolytes containing DME, DOL and fluoroether. Fluoroether co-solvent
blank TTE TFEG TFEG TFPG TFPG DTFEG DTFEG TFEPG DTFPG
Volume ratio DME/ DOL/Cosolvent
ηc (cP)
1/1 1/1/2 1/1/2 1/1/4 1/1/2 1/1/4 1/1/2 1/1/4 1/1/2 1/1/2
1.42 3.00 1.78 1.87 2.72 3.39 2.30 2.51 2.98 4.00
χd (mS cm−1) 25 °C
30 °C
35 °C
40 °C
45 °C
50 °C
10.9 4.01 7.44 5.92 5.26 3.51 4.78 2.86 4.15 3.39
11.6 4.29 7.78 6.27 5.58 3.87 5.10 3.11 4.46 3.70
12.3 4.58 8.22 6.63 5.97 4.26 5.44 3.35 4.79 4.04
13.0 4.90 8.61 6.98 6.36 4.63 5.76 3.60 5.12 4.40
13.7 5.22 9.04 7.37 6.74 4.98 6.09 3.86 5.47 4.76
14.3 5.56 9.45 7.68 7.18 5.35 6.44 4.13 5.82 5.10
3.4. Electrochemical performance
Table 3 Viscosity and ionic conductivity of 1 M LiTFSI electrolytes containing DME and fluoroether. Fluoroether co-solvent
TTE TTE TFEG TFEG TFPG TFPG DTFEG DTFEG TFEPG DTFPG TFEG/DTFPG
Volume ratio DME: Co-solvent
ηc (cP)
1/1 1/2 1/1 1/2 1/1 1/2 1/1 1/2 1/1 1/1 1/1/1
3.79 3.74 1.90 1.92 2.75 3.19 2.56 2.12 3.39 5.20 2.92
In this study, we chose to focus on the TFEG/DME (2:1) electrolyte, because it displayed the best PSS performance (i.e., lowest polysulfide solubility, Fig. 2). When we compare ionic conductivity, TFEG/DME (2:1) shows slightly lower value than that of the TFEG/DME (1:1) electrolyte (7.97 mS cm−1 vs. 8.70 mS cm−1), but exhibits higher value than all the other combinations. However, the polysulfide solubility of the most conductive TFEG/DME (1:1) electrolyte is much higher than other combinations we studied. It is important to note that our TFEG/ DME (2:1) electrolyte is superior to the previously reported TTE/DME (1:1) electrolyte in terms of both polysulfide solubility and ionic conductivity (6.52 mS cm−1). The cyclic voltammograms (CVs) of LieS cells with baseline (DME/ DOL) and fluorinated (TFEG/DME) electrolytes are presented in Fig. 3. The nature of CVs for both electrolytes is similar, which indicates there are no undesired/unexpected side reactions with the fluorinated electrolyte and redox potentials for sulfur reduction/oxidation on both baseline and fluorinated electrolytes are similar. The two reduction
χd (mS cm−1) 25 °C
30 °C
35 °C
40 °C
45 °C
50 °C
6.52 4.14 8.70 7.97 7.08 5.12 7.04 5.81 5.97 4.32 5.06
7.06 4.55 9.17 8.46 7.53 5.55 7.48 6.19 6.37 4.78 5.39
7.68 4.99 9.65 8.93 7.99 5.96 7.91 6.57 6.79 5.27 5.80
8.31 5.41 10.2 9.44 8.49 6.41 8.39 6.93 7.26 5.76 6.17
8.92 5.87 10.7 9.98 8.99 6.85 8.83 7.3 7.74 6.25 6.52
9.51 6.32 11.2 10.5 9.53 7.33 9.30 7.66 8.17 6.73 6.87
274
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Fig. 1. Conductivity of 1 M LiTFSI in different composition of solvents.
Fig. 2. UV–vis spectra of Li2S6-saturated 1 M LiTFSI solutions with different solvent combinations: DME/DOL = 1/1, DME/TFEG = 1/1, DME/TFPG = 1/1, DME/TTE = 1/1 and DME/TFEG = 1/2.
Fig. 3. Cyclic voltammograms of the first three cycles for LieS cell with the baseline and the TFEG/DME electrolyte (scanning rate of 0.2 mV s−1).
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Fig. 4. Electrochemical performance of LieS cells with baseline and fluoroether-based electrolytes. (a) Voltage profiles for the 1st and 50th cycle at 0.2C. (b) Voltage profiles for the 1st and 50th cycle at 0.5C. (c) Discharge capacity and coulombic efficiency over 50 cycles at 0.2C. (d) Discharge capacity and coulombic efficiency over 50 cycles at 0.5C.
In order to research the effect of our new electrolyte, we made two series of coin cells, one with the baseline electrolyte (DME/DOL = 1:1) and another with the best fluorinated electrolyte (TFEG/DME = 2:1). Fig. 4(a) and (b) represent galvanostatic discharge-charge curves at 0.2C and 0.5C rate for the 1st and 50th cycle. (1C = 1675 mAh g−1). LieS cells with the baseline and fluorinated electrolytes consist of 1.0 M LiTFSI DME/DOL (1:1 vol ratio) with 2 wt% LiNO3 and 1.0 M LiTFSI DME/TFEG (1:2 vol ratio) with 2 wt% LiNO3, respectively. Two typical discharge plateaus are observed at 2.2–2.3 V and 2.0–2.1 V for both baseline and fluorinated electrolytes. The higher voltage plateau corresponds to the reduction of sulfur to long-chain polysulfides (Li2Sx, where x = 4–8). The further reduction of long-chain polysulfides produces short-chain polysulfides (Li2S2 and/or Li2S) at the lower voltage. During the charging process, the single plateau at ∼2.4 V is assigned to the oxidation of Li2S to elemental sulfur. Based on the discharge curves, the S/C cathode with fluorinated electrolyte delivers a capacity of 870 mAh/g which is significantly higher than the corresponding capacity 606 mAh/g (by 44%) of the S/C cathode with the baseline electrolyte at 0.2C. The plateaus keep their shape after 50 cycles and the fluorinated electrolyte cell's capacity drops to 389 mAh/g (44.7% retention) while that of the baseline electrolyte drops to 302 mAh/g (49.8% retention). Thus, on capacity fading aspect, the fluorinated electrolyte didn't show superior performance at 0.2C, which is revealed in Fig. 4(c). However, with the fluorinated electrolyte the capacity fading is much improved at higher current density. As shown in Fig. 4(d), at the 0.5C rate, both cells show a gradual increase to the highest values after several cycles due to the activation effect for electrolyte diffusion. The fluorinated cell showed a slightly higher capacity than the baseline cell,
peaks represent two stages of electrochemical reactions during the discharging process [18,19]. The peak at higher voltage corresponds to the reduction of elemental S to the higher order polysulfides and the peak at lower voltage corresponds to the reduction of high order polysulfides to the lower order polysulfides. The single oxidation peak represents the formation of both polysulfides and final active sulfur Li2S8 or S. These CVs are consistent with the previously reported studies on baseline and electrolytes containing fluorinated compounds [12,14,17]. N. Azimi et al. [7] reported the CV for LieS battery with DOL/TTE and DOL/DME electrolytes, where an additional reduction peak for DOL/TTE at 1.8 V corresponding to TTE reduction is reported. However, in our modified electrolyte containing TFEG does not show such a feature, which indicates TFEG is electrochemically stable in the battery voltage range. Moreover, in contrast to the single oxidation peak at 2.47 V for our TFEG/DME electrolyte, they reported an additional shoulder-like oxidation peak at 2.35 V for TTE, which is also absent in the CV of our fluorinated electrolyte [7]. As shown in Fig. 3, the potential gap between the oxidation and reduction CV peaks of LieS cells with fluorinated (TFEG/DME) electrolyte is 0.479 V, ∼0.05 V smaller than that with DME/DOL electrolyte (0.533 V). This is due to the enhanced ionic conductivity, which ultimately decreases the over-potential and improves the reversibility of the cell [20]. From the discharge curve of the fluorinated electrolyte (Fig. 3), we have also observed that the peak at higher potential is much smaller compared to the peak at the lower potential, which is an evidence of the decreasing dissolution of high-order polysulfides, as reported by other groups with fluorinated electrolytes. 276
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655 mAh/g to 566 mAh/g, respectively. After 50 cycles, the fluorinated cell has a capacity retention of 70% which is better than the baseline cell, 55%. This is due to low solubility of polysulfides in the fluorinated electrolyte. To our knowledge, all of the published papers applying fluorinated ether solvents to LSBs [5–10,12,13,20] only reported the cycling performance at low current density (≤0.3C). Therefore, due to the improved kinetics, our fluoroether-based electrolyte could run at a higher current density with good capacity retention. Better kinetics of our electrolyte at higher current rate is also proved by the overpotential. As shown in Fig. 4(a), the heights of the second plateau for both electrolytes at 50th cycle are almost the same. However, in Fig. 4(b), the DME/TFEG = 1/2 electrolyte is obviously higher, indicating smaller overpotential than the baseline. The Coulombic efficiency (CE) of LieS cells with two electrolytes at 0.2C and 0.5C are shown in Fig. 4(c) and (d). Both cells showed a similar CE at 0.2C, but with the increased current at 0.5C, the cell containing our fluorinated electrolyte delivered a higher and more stable CE. Improved cycling stability has been reported previously in the research about other fluoroether-containing electrolytes [20], which possibly due to the formation of LiF, leading to increased SEI layer stability on both anode [11,13] and cathode surfaces [7].
[3] A.F. Hofmann, D.N. Fronczek, W.G. Bessler, Mechanistic modeling of polysulfide shuttle and capacity loss in lithium-sulfur batteries, J. Power Sources 259 (2014) 300–310, https://doi.org/10.1016/j.jpowsour.2014.02.082. [4] Y. Sasaki, G. Shimazaki, N. Nanbu, M. Takehara, M. Ue, Physical and electrolytic properties of partially fluorinated organic solvents and its application to secondary lithium batteries: partially fluorinated dialkoxyethanes, ECS Trans. 155 (2009) 23–31, https://doi.org/10.1149/1.3123124. [5] C. Zu, N. Azimi, Z. Zhang, A. Manthiram, Insight into lithium–metal anodes in lithium–sulfur batteries with a fluorinated ether electrolyte, J. Mater. Chem. a. 3 (2015) 14864–14870, https://doi.org/10.1039/C5TA03195H. [6] K. Dokko, N. Tachikawa, K. Yamauchi, M. Tsuchiya, A. Yamazaki, E. Takashima, J.W. Park, K. Ueno, S. Seki, N. Serizawa, M. Watanabe, Solvate ionic liquid electrolyte for Li-S batteries, J. Electrochem. Soc. 160 (2013) A1304–A1310, https://doi.org/ 10.1149/2.111308jes. [7] N. Azimi, W. Weng, C. Takoudis, Z. Zhang, Improved performance of lithium-sulfur battery with fluorinated electrolyte, Electrochem. Commun. 37 (2013) 96–99, https://doi.org/10.1016/j.elecom.2013.10.020. [8] N. Azimi, Z. Xue, N.D. Rago, C. Takoudis, M.L. Gordin, J. Song, D. Wang, Z. Zhang, Fluorinated electrolytes for Li-S battery: suppressing the self-discharge with an electrolyte containing fluoroether solvent, J. Electrochem. Soc. 162 (2014) A64–A68, https://doi.org/10.1149/2.0431501jes. [9] N. Azimi, Z. Xue, I. Bloom, M.L. Gordin, D. Wang, T. Daniel, C. Takoudis, Z. Zhang, Understanding the effect of a fluorinated ether on the performance of lithium-sulfur batteries, ACS Appl. Mater. Interfaces 7 (2015) 9169–9177, https://doi.org/10. 1021/acsami.5b01412. [10] M.L. Gordin, F. Dai, S. Chen, T. Xu, J. Song, D. Tang, N. Azimi, Z. Zhang, D. Wang, Bis(2,2,2-trifluoroethyl) ether as an electrolyte co-solvent for mitigating self-discharge in lithium - sulfur batteries, ACS Appl. Mater. Interfaces 6 (2014) 8006–8010, https://doi.org/10.1021/am501665s. [11] H. Lu, K. Zhang, Y. Yuan, F. Qin, Z. Zhang, Y. Lai, Y. Liu, Lithium/sulfur batteries with mixed liquid electrolytes based on ethyl 1,1,2,2-tetrafluoroethyl ether, Electrochim. Acta 161 (2015) 55–62, https://doi.org/10.1016/j.electacta.2015.02. 031. [12] H. Lu, Y. Yuan, K. Zhang, F. Qin, Y. Lai, Y. Liu, Application of partially fluorinated ether for improving performance of lithium/sulfur batteries, J. Electrochem. Soc. 162 (2015) A1460–A1465, https://doi.org/10.1149/2.0281508jes. [13] S. Gu, R. Qian, J. Jin, Q. Wang, J. Guo, S. Zhang, S. Zhuo, Z. Wen, Suppressing the dissolution of polysulfides with cosolvent fluorinated diether towards high-performance lithium sulfur batteries, Phys. Chem. Chem. Phys. 18 (2016) 29293–29299, https://doi.org/10.1039/C6CP04775K. [14] C. Barchasz, F. Molton, C. Duboc, J.C. Leprêtre, S. Patoux, F. Alloin, Lithium/sulfur cell discharge mechanism: an original approach for intermediate species identification, Anal. Chem. 84 (2012) 3973–3980, https://doi.org/10.1021/ac2032244. [15] V. Tortelli, I. Wlassics, C. Monzani, B.L. Kent, A. Veneroni, US 20140350181 A1, 2014. https://patents.google.com/patent/US20140350181A1/en?oq=US +20140350181A1. [16] T. Yamaguchi, R. Shimizu, C.-S. Jung, US20040038133A1, 2004. https://patents. google.com/patent/US20040038133?oq=US2004038133A1. [17] T. Nanno, M. Feguchi, US 20080306308 A1, 2008. https://patents.google.com/ patent/US20080306308A1/en. [18] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Amorphous silicon thin films as a high capacity anodes for Li-ion batteries in ionic liquid electrolytes, Electrochem. Commun. 9 (2007) 796–800, https://doi.org/10.1016/j.elecom.2006. 11.014. [19] G. Li, Z. Li, B. Zhang, Z. Lin, Developments of electrolyte systems for Lithiumâ €“Sulfur batteries: a review, Front. Energy Res. 3 (2015) 1–12, https://doi.org/10. 3389/fenrg.2015.00005. [20] H. Lu, Y. Yuan, Z. Hou, Y. Lai, K. Zhang, Y. Liu, Solvate ionic liquid electrolyte with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether as a support solvent for advanced lithium–sulfur batteries, RSC Adv. 6 (2016) 18186–18190, https://doi.org/10. 1039/C5RA24182K.
4. Conclusions We have synthesized and investigated the viability of a new serious of partially fluorinated ethers as a co-solvent for LieS battery electrolytes. They possess similar structures, but gradually increasing molecular weights and number of fluorine atoms. The density and viscosity of these compounds were measured. Our study shows that eCF3 group dramatically decrease the viscosity compared to the eCF2CF2H group present in the molecule. 1 M LiTFSI was dissolved in the mixture of our fluorinated ethers with DME and DOL in different portions. The viscosity and conductivity between 25 and 50 °C of these solutions were measured. We selected the combinations that showed the best performance and studied the polysulfide solubility by UV–vis spectroscopy. The best result was obtained with the formulation containing 1 M LiTFSI in DME:TFEG = 1:2. Charging-discharging tests demonstrated that this new electrolyte possesses better initial capacity, capacity retention and coulombic efficiency than the traditional DME:DOL = 1:1 solution, especially with a higher current density (0.5C). References [1] X. Fang, H. Peng, A revolution in electrodes: recent progress in rechargeable lithium-sulfur batteries, Small 11 (2015) 1488–1511, https://doi.org/10.1002/smll. 201402354. [2] Y.V. Mikhaylik, J.R. Akridge, Polysulfide shuttle study in the Li/S battery system, J. Electrochem. Soc. 151 (2004), https://doi.org/10.1149/1.1806394 A1969.
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Synthesis and electrochemical properties of partially fluorinated ether solvents for lithiumesulfur battery electrolytes
T
Zheng Yuea, Hamza Dunyaa, Shankar Aryalb, Carlo U. Segreb, Braja Mandala,∗ a b
Department of Chemistry, Illinois Institute of Technology, Chicago, IL, 60616, USA Department of Physics & CSRRI, Illinois Institute of Technology, Chicago, IL, 60616, USA
H I GH L IG H T S
partially fluorinated ether compounds were synthesized and characterized. • Five properties and polysulfide solubility are provided. • Physical electrolyte formulations were tested for LieS battery electrolytes. • Several displayed good ionic conductivity and electrochemical stability. • Electrolytes • Improved cycling performances were achieved at high current density (0.5C).
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-sulfur batteries Partially fluorinated ethers Polysulfide shuttle effect Ionic conductivity Solid electrolyte interface Capacity fading
Fluorinated ethers have been used as a co-solvent in traditional ether-based electrolytes to suppress the polysulfide shuttle effect in lithium-sulfur batteries. In this work, five partially fluorinated ether compounds have been synthesized. The key properties, such as viscosity, ionic conductivity and polysulfide solubility of the electrolytes containing these co-solvents have been systematically studied. The electrolyte formulation showed best physical properties was tested in lithium-sulfur coin cells. The new fluoroether-based electrolytes displayed superior electrochemical performance compared to that of the traditional ether-based electrolytes.
1. Introduction Lithium-sulfur batteries (LSBs) are batteries composed of an elemental sulfur cathode and a lithium metal anode. Because of high theoretical specific capacity (1675 mAh g−1) of the sulfur cathode, and energy density (2600 W h kg-1), which is about five times higher than that of lithium-ion batteries (LIBs) [1], LSBs are considered to be the most promising candidate for the next generation energy storage devices. However, despite having extensive efforts since the concept of LSBs was introduced more than 30 years ago [2], a commercially viable LSB has yet conceived. The main barrier to the commercialization of LSBs is the rapid capacity fading and low coulombic efficiency in charge-discharge cycling, which leads to short battery life. Pioneering research has proved that the fast capacity fading is mainly caused by the insulating nature of elemental sulfur (electrical conductivity below 1 × 10−15 S m-1 at room temperature) and the polysulfide shuttle (PSS) effect [2,3]. During the battery discharge process, elemental sulfur in the cathode is first reduced to higher lithium polysulfide (Li2Sx,
∗
8 ≧ x ≧ 2) intermediates, which are soluble in the electrolyte system. Dissolved Sx−2 anions can then migrate through the electrolyte to the lithium metal anode, where they reduce to insoluble Li2S. This effect not only causes severe self-discharging, but also blocks the active material on the lithium anode surface, causing failure of the cell. Currently, the most widely used solvents for the LSB electrolytes are ethers, such as dimethoxy ethane (DME), dioxolane (DOL) and tetraethylene glycol diethyl ether (G4) [2,4]. The advantages of ether solvents are good solubility of lithium salts, comparatively high chemical and electrochemical stability, moderate dielectric constant (ε) and low viscosity (which leads to high ionic conductivity). However, their high donor ability also provides high solubility to polysulfide intermediates [4], leading to unwanted PSS phenomena. Recently, a new strategy has achieved great success to address PSS by mixing a large proportion (≥50%) of partially fluorinated ether solvent with the aforementioned traditional ether solvents. Several commercially available fluorinated ethers, such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) [5–8], bis(2,2,2-
Corresponding author. E-mail address: [email protected] (B. Mandal).
https://doi.org/10.1016/j.jpowsour.2018.08.097 Received 22 May 2018; Received in revised form 2 August 2018; Accepted 31 August 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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and allowed to warm to room temperature. After no more H2 evolution, a solution of 5.0 mmol of compound 3 in 15 mL anhydrous ethyl ether was added dropwise. Then the mixture was stirred 24 h at room temperature. Then 25 mL water was added to dissolve all solid. The water layer was separated and extracted with 2 × 15 mL dichloromethane. All organic layers were combined and washed with 2 × 15 mL brine. The organic layer was dried with anhydrous MgSO4, and the solvent was removed by rotary evaporation. The remaining was a yellow, oillike liquid. It was distilled under vacuum (5 mm Hg) to give colorless products 4a-e with yields from 28 to 42%.
trifluoroethyl) ether (BTFE) [9,10], ethyl-1,1,2,2-tetrafluoroethylether (ETFE) [11], 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE) [12] and 1,3-(1,1,2,2-tetrafluoroethoxy)propane (FDE) [13] have been studied as a co-solvent for LSB electrolytes. In general, polysulfides have much lower solubility in the partially fluorinated solvents, thereby significantly inhibiting the PSS effect. Recently, both Nazim et al. [9] (DOL/TTE) and Watanabe et al. [6] (G4/TTE) have used UV–vis spectroscopy as a quantitative tool to measure the concentration of polysulfides in electrolytes. Their investigations showed that long chain polysulfides had a much lower solubility in the partially fluorinated ether (TTE-containing electrolytes). A similar study using another fluorinated ether, FDE, by Sui et al. [13], also showed much lower solubility of polysulfides. Furthermore, partially fluorinated solvents have multiple functions in battery electrolytes. Chenxi et al. reported that TTE helps anode protection by forming mechanically stable solid electrolyte interface (SEI) layer [14] over the anode without the use of any electrolytic additive, such as LiNO3. The analysis of the SEI layer on the anode surface showed hierarchical compositions of LiF, which is believed to increase the stability of the SEI layer. Nazim et al. also investigated the surface morphology of the cycled sulfur electrode by SEM [9] and concluded that fluorinated ether compounds also helps forming protection layer on the sulfur cathode by reductive decomposition. In this work, we have synthesized five partially fluorinated ether compounds, 2,2,2-trifluoroethyl methyl ether ethylene glycol (TFEG), ethylene glycol 2,2,3,3-tetrafluoropropyl methyl ether (TFPG), ethylene glycol di(2,2,2-trifluoroethyl) ether (DTFEG), ethylene glycol 2,2,2trifluoroethyl-2,2,3,3-tetrafluoropropyl ether (TFEPG) and ethylene glycol di(2,2,3,3-tetrafluoropropyl) ether (DTFPG), using a simple, lowcost procedure. All of them have the similar structure R1-OCH2CH2OR2, where both R1 and R2 are fluoroalkyl groups with 3, 4, 6, 7 and 8 fluorine atoms. Their density and viscosity were determined. We formulated our solvents with different ratios of DME/DOL to make 1 M LiTFSI solution. The conductivity and polysulfide solubility of these new electrolytes were tested. We tested our electrolytes with conventional sulfur cathode, carried out charging-discharging for 50 cycles, and evaluated the Coulombic efficiency and capacity fading. We anticipate this research could reveal key structure-property relationships involving fluorinated ether compounds, and will provide impetus to design new battery co-solvents with much less PSS effect, higher ionic conductivity and superior cycling performance.
2.3. Characterization and measurement 1 H NMR and 13C NMR spectra (300 MHz Bruker NMR spectrometer) were used to confirm the purity (> 95%) and structures of the fluorinated ether compounds. TFEG (4a): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.89 (q, 3JHF = 8.8 Hz, 2H, CH2), 3.77 (t, 3JHH = 4.2 Hz, 2H, CH2), 3.54 (t, 3JHH = 4.5 Hz, 2H, CH2), 3.38 (s, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 124.1 (q, 1JCF = 279 Hz, CF3), 71.9 (2C, CH2), 68.8 (q, 2JCF = 42.8 Hz, CH2), 59.1 (CH3). Anal. calcd. for C5H9F3O2 (%): C, 37.98; H, 5.74; F, 36.05. Found: C, 37.89; H, 5.76; F, 36.01. TFPG (4b): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.94 (tt, 3JHF = 5.1 Hz, 2JHF = 53.4 Hz, 1H, CF2H), 3.89 (t, 3JHF = 12.6 Hz, 2H, CH2), 3.73 (t, 3JHH = 4.5 Hz, 2H, CH2), 3.54 (t, 3JHH = 4.5 Hz, 2H, CH2), 3.37 (s, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 112.4 (tt, 2JCF = 30.2 Hz, 1JCF = 204 Hz, CF2H), 109.1 (tt, 2JCF = 34.0 Hz, 1JCF = 249 Hz, CF2), 71.8 (CH2), 71.7 (CH2), 68.3 (t, 2JCF = 30.2 Hz, CH2), 59.0 (CH3). Anal. calcd. for C6H10F4O2 (%): C, 37.90; H, 5.30; F, 39.97. Found: C, 37.81; H, 5.24; F, 39.91. DTFEG (4c): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.90 (q, 3JHF = 8.7 Hz, 4H, CH2), 3.81 (s, 4H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.8 (q, 1JCF = 279 Hz, CF3), 71.9 (CH2), 68.8 (q, 2JCF = 32.7 Hz, CH2). Anal. calcd. for C6H8F6O2 (%): C, 31.87; H, 3.57; F, 50.41. Found: C, 31.79; H, 3.52; F, 50.35. TFEPG (4d): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.92 (tt, 3JHF = 4.8 Hz, 2JHF = 53.1 Hz, 2H, CF2H), 3.90 (q, 3JHF = 8.7 Hz, 4H, CH2), 3.81 (s, 4H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.8 (q, 1JCF = 279 Hz, CF3), 112.5 (tt, 2JCF = 34.0 Hz, 1JCF = 189 Hz, CF2H), 109.2 (tt, 2JCF = 37.7 Hz, 1JCF = 257 Hz, CF2), 71.8 (CH2), 71.7 (CH2), 68.6 (m, 2C, CH2). Anal. calcd. for C7H9F7O2 (%): C, 32.57; H, 3.51; F, 51.52. Found: C, 32.51; H, 3.54; F, 51.47. DTFPG (4e): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.91 (tt, 3JHF = 5.1 Hz, 2JHF = 53.1 Hz, 1H, CF2H), 3.90 (q, 3JHF = 8.7 Hz, 4H, CH2), 3.88 (t, 3JHF = 12.6 Hz, 4H, CH2), 3.78 (m, 4H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 112.5 (tt, 2JCF = 30.2 Hz, 1JCF = 189 Hz, CF2H), 109.2 (tt, 2JCF = 34.0 Hz, 1JCF = 249 Hz, CF2), 71.6 (2C, CH2), 68.6 (m, 2C, CH2). Anal. calcd. for C8H10F8O2 (%): C, 33.12; H, 3.47; F, 52.38. Found: C, 33.02; H, 3.38; F, 52.29. The density of the fluoroether compounds was determined by measuring the weight of 0.5 mL prepared fluorinated ether solvents, or 1 M LiTFSI solutions at room temperature. The viscosity of all samples was measured with a Brookfield DV3T viscometer containing spindle CPA-40Z, and a Fisher Scientific Isotemp 1013s recycling heater to keep the samples at 25 °C. The ionic conductivity was measured by using Mettler Toledo S230Kit conductivity meter in an environment chamber from 25 °C to 70 °C. The conductivity meter was calibrated with a standard 0.01 mol L−1 KCl aqueous solution with conductivity 1.314 mS cm−1 at 25.0 °C.
2. Experimental 2.1. Materials Sodium hydride (60% dispersion in mineral oil) was purchased from ACROS Organics, and washed 5 times with hexane to remove mineral oil before used. All other reagents and solvents were used without further purification: 2-methoxyethanol (Sigma Aldrich), Ethylene carbonate (Sigma Aldrich), p-toluenesulfonyl chloride (Sigma Aldrich), sodium hydroxide (Fisher scientific), lithium bis(trifluoromethylsulfonyl)imide (Alfa Aesar), elemental sulfur (Alfa Aesar), lithium sulfide (Alfa Aesar), 2, 2, 2-trifluoroethan-1-ol (SynQuest), 2, 2, 3, 3-tetrafluoropropan-1-ol (SynQuest). 2.2. General procedure for the synthesis of five fluoroether compounds The synthetic strategy to the fluoroether compounds used in this study is described in Scheme 1. Compounds 2a and 2b were prepared according to the method reported in a patent literature [15]. Tosylates 3a and 3b were synthesized by the reaction of tosyl chloride with the corresponding fluorinated alcohol. 5.5 mmol compound 2 in 15 mL anhydrous ethyl ether was added dropwise into a suspension of 9.0 mmol NaH (mineral oil was removed before use) in 30 mL of ethyl ether with ice-water bath and stirring. This mixture was stirred 30 min
2.4. Polysulfide solubility Li2S6 was produced using the following procedure: Li2S and elemental sulfur were mixed in the weight ratio of 3:8 and 1 g of the 272
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Scheme 1. General synthetic scheme for the fluorinated ether compounds.
compound. The viscosity of TFEPG is only slightly larger than TFPG, but much smaller than DTFPG. This evidence supports our assumption that compounds with eCF3 group tend to have lower viscosity. Yukio Sasaki measured the viscosity of ethylene glycol 2-fluoroethyl methyl ether (FEG), ethylene glycol 2, 2-difluoroethyl methyl ether (DFEG) and TFEG [4], and found that TFEG is much less viscous than the other two compounds. To explain this phenomenon, they assumed that 3 fluorine atoms attached to one carbon provides a larger steric hindrance and electronic repulsion, thus decreasing the interaction between molecules. By contrast, eCF2CF2H group dramatically increases the viscosity. The molecular weight of TFPG, which contains only one eCF2CF2H, is lower than that of DTFEG; however, its viscosity is much higher. High viscosity leads to low ionic conductivity. Our study indicates that eCF3 group should be included, while eCF2CF2H group should be excluded in the electrolyte design consideration.
mixture was dissolved in 5 mL DME and stirred overnight. The color changed from light yellow to red. Then the solvent was air dried. All operations were done in a glove box. The polysulfide-saturated solutions were made as follows: ∼20 mg Li2S6 was mixed with 0.5 mL sample, shaken vigorously, and let stand for 5 days. 20 μL of the solution's upper layer was diluted to 2 mL with anhydrous methanol, and the UV–vis spectrum was measured in the range of wavelength from 200 nm to 800 nm. Pure methanol was used as the blank. 2.5. Preparation of coin cells S/C spherical powder, (2–12 μm) from Sigma (w/w = 6.5:3.5) composite was manually ground for 20 min and prepared by the melt diffusion method at 155 °C for 12 h. The cathode slurry was prepared by mixing 75 wt% S/C, 15 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) in NMP solvent. The slurry was cast onto an aluminum foil substrate and then dried at 60 °C overnight. CR2032-type coin cells were assembled in an Ar filled glove box with oxygen content less than 1 ppm. The cells contained Celgard 2535 as the separator and lithium foils as both the counter and reference electrodes. All cells were cycled with 0.2C and 0.5C currents on a MTI cycler within a voltage range of 1.7–2.8 V at room temperature.
3.2. Ionic conductivity Typically, the conductivity of the solvents decreases with the increase in viscosity. The conductivity of 1 M LiTFSI solution composed of our fluorinated ether solvents are all higher than those containing TTE as the counterpart. As summarized in Tables 2 and 3, the conductivity of DME:co-solvent = 1:1 solution is much higher than that of DME:DOL:co-solvent = 1:1:1 solution. Among the fluorinated ether solvents, TFEG showed the best performance. The conductivity of DME:TFEG = 1:1 solution was 8.70 mS cm−1 and that of DME:TFEG = 1:2 solution was 7.97 mS cm−1 (Fig. 1). By contrast, the conductivity of 1 M LiTFSI in DME:DOL = 1:1 solution, which was most commonly used for LSBs, was ∼11 mS cm−1 in our measurement.
3. Results and discussions 3.1. Viscosity Among the five fluoroether compounds, TFEPG and DTFPG are new and only TTE is commercially available. TFEG and TFPG are previously reported, but not commercially available (Table 1). Therefore, we synthesized these compounds to complete the structure-property relationship. It is noteworthy that we report here the viscosities of TFPG, TFEPG, DTFEG and DTFPG for the first time. Only one literature reported the viscosity of TFEG [4] and its application related to LIBs. Typically, fluoroethers have higher viscosity than DME, because of their higher molecular weight. As summarized in Table 1, TFEG showed the lowest viscosity among the five compounds. DTFEG, despite its molecular weight being larger than TFPG, is the second lowest viscous
3.3. Polysulfide solubility To measure the solubility of polysulfides in electrolytes containing DME/fluorinated ether co-solvents, UV–Vis spectroscopy was applied. Fig. 2 shows the absorption curves of various solvent composition. Only the mixture of DME with TFEG and TFPG were tested because they performed much higher conductivity compared with the larger molecules. According to the literature [14], in organic solution S62− would 273
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Table 1 Physical properties of the fluorinated DME analogs. Mwa (g mol−1)
db (g cm−3)
ηc (cP)
Reference
DME
90.12
0.87
0.46
Commercially available
TTE
232.07
1.49
1.28
Commercially available
TFEG
158.12
1.18
0.91(0.78)
Not commercially available [4]
TFPG
190.14
1.30
2.26
Not commercially available [16]
DTFEG
226.12
1.34
1.40
Not commercially available [17]
TFEPG
258.13
1.40
2.31
New compound
DTFPG
290.15
1.42
4.50
New compound
Code
a b c
Structure
Molecular weight. Density at room temperature. Viscosity at 25 °C.
undergo a disproportionation reaction, as a result the solutions contain several different Sx2− species. The bands of the UV–vis absorption of Sx2− in organic solvents are assigned as follows: 490–500 nm for S82−, 450–470 nm for S62−, ∼420 nm for S42−, 340 nm for S32−, and ∼280 nm for S22−. Comparing the curve of DME:TFEG = 1:1, DME:TFPG = 1:1 and DME:TTE = 1:1, we can conclude that the rank of polysulfide solubility suppressing effect is TTE > TFPG > TFEG. However, DME:TFEG = 1:2 solution performed even lower absorption than DME:TTE = 1:1 solution except when the wavelength < 220 nm, which demonstrates the lower polysulfide solubility. As mentioned in Table 2 that DME:TFEG = 1:2 solution performed a much higher conductivity (7.97 mS cm−1) than that of DME:TTE = 1:1 solution (6.52 mS cm−1). This indicates 1 M LiTFSI in DME:TFEG = 1:2 may have even better performance than 1 M LiTFSI in DME:TTE = 1:1 as electrolyte for LSBs.
Table 2 Viscosity and ionic conductivity of 1 M LiTFSI electrolytes containing DME, DOL and fluoroether. Fluoroether co-solvent
blank TTE TFEG TFEG TFPG TFPG DTFEG DTFEG TFEPG DTFPG
Volume ratio DME/ DOL/Cosolvent
ηc (cP)
1/1 1/1/2 1/1/2 1/1/4 1/1/2 1/1/4 1/1/2 1/1/4 1/1/2 1/1/2
1.42 3.00 1.78 1.87 2.72 3.39 2.30 2.51 2.98 4.00
χd (mS cm−1) 25 °C
30 °C
35 °C
40 °C
45 °C
50 °C
10.9 4.01 7.44 5.92 5.26 3.51 4.78 2.86 4.15 3.39
11.6 4.29 7.78 6.27 5.58 3.87 5.10 3.11 4.46 3.70
12.3 4.58 8.22 6.63 5.97 4.26 5.44 3.35 4.79 4.04
13.0 4.90 8.61 6.98 6.36 4.63 5.76 3.60 5.12 4.40
13.7 5.22 9.04 7.37 6.74 4.98 6.09 3.86 5.47 4.76
14.3 5.56 9.45 7.68 7.18 5.35 6.44 4.13 5.82 5.10
3.4. Electrochemical performance
Table 3 Viscosity and ionic conductivity of 1 M LiTFSI electrolytes containing DME and fluoroether. Fluoroether co-solvent
TTE TTE TFEG TFEG TFPG TFPG DTFEG DTFEG TFEPG DTFPG TFEG/DTFPG
Volume ratio DME: Co-solvent
ηc (cP)
1/1 1/2 1/1 1/2 1/1 1/2 1/1 1/2 1/1 1/1 1/1/1
3.79 3.74 1.90 1.92 2.75 3.19 2.56 2.12 3.39 5.20 2.92
In this study, we chose to focus on the TFEG/DME (2:1) electrolyte, because it displayed the best PSS performance (i.e., lowest polysulfide solubility, Fig. 2). When we compare ionic conductivity, TFEG/DME (2:1) shows slightly lower value than that of the TFEG/DME (1:1) electrolyte (7.97 mS cm−1 vs. 8.70 mS cm−1), but exhibits higher value than all the other combinations. However, the polysulfide solubility of the most conductive TFEG/DME (1:1) electrolyte is much higher than other combinations we studied. It is important to note that our TFEG/ DME (2:1) electrolyte is superior to the previously reported TTE/DME (1:1) electrolyte in terms of both polysulfide solubility and ionic conductivity (6.52 mS cm−1). The cyclic voltammograms (CVs) of LieS cells with baseline (DME/ DOL) and fluorinated (TFEG/DME) electrolytes are presented in Fig. 3. The nature of CVs for both electrolytes is similar, which indicates there are no undesired/unexpected side reactions with the fluorinated electrolyte and redox potentials for sulfur reduction/oxidation on both baseline and fluorinated electrolytes are similar. The two reduction
χd (mS cm−1) 25 °C
30 °C
35 °C
40 °C
45 °C
50 °C
6.52 4.14 8.70 7.97 7.08 5.12 7.04 5.81 5.97 4.32 5.06
7.06 4.55 9.17 8.46 7.53 5.55 7.48 6.19 6.37 4.78 5.39
7.68 4.99 9.65 8.93 7.99 5.96 7.91 6.57 6.79 5.27 5.80
8.31 5.41 10.2 9.44 8.49 6.41 8.39 6.93 7.26 5.76 6.17
8.92 5.87 10.7 9.98 8.99 6.85 8.83 7.3 7.74 6.25 6.52
9.51 6.32 11.2 10.5 9.53 7.33 9.30 7.66 8.17 6.73 6.87
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Fig. 1. Conductivity of 1 M LiTFSI in different composition of solvents.
Fig. 2. UV–vis spectra of Li2S6-saturated 1 M LiTFSI solutions with different solvent combinations: DME/DOL = 1/1, DME/TFEG = 1/1, DME/TFPG = 1/1, DME/TTE = 1/1 and DME/TFEG = 1/2.
Fig. 3. Cyclic voltammograms of the first three cycles for LieS cell with the baseline and the TFEG/DME electrolyte (scanning rate of 0.2 mV s−1).
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Fig. 4. Electrochemical performance of LieS cells with baseline and fluoroether-based electrolytes. (a) Voltage profiles for the 1st and 50th cycle at 0.2C. (b) Voltage profiles for the 1st and 50th cycle at 0.5C. (c) Discharge capacity and coulombic efficiency over 50 cycles at 0.2C. (d) Discharge capacity and coulombic efficiency over 50 cycles at 0.5C.
In order to research the effect of our new electrolyte, we made two series of coin cells, one with the baseline electrolyte (DME/DOL = 1:1) and another with the best fluorinated electrolyte (TFEG/DME = 2:1). Fig. 4(a) and (b) represent galvanostatic discharge-charge curves at 0.2C and 0.5C rate for the 1st and 50th cycle. (1C = 1675 mAh g−1). LieS cells with the baseline and fluorinated electrolytes consist of 1.0 M LiTFSI DME/DOL (1:1 vol ratio) with 2 wt% LiNO3 and 1.0 M LiTFSI DME/TFEG (1:2 vol ratio) with 2 wt% LiNO3, respectively. Two typical discharge plateaus are observed at 2.2–2.3 V and 2.0–2.1 V for both baseline and fluorinated electrolytes. The higher voltage plateau corresponds to the reduction of sulfur to long-chain polysulfides (Li2Sx, where x = 4–8). The further reduction of long-chain polysulfides produces short-chain polysulfides (Li2S2 and/or Li2S) at the lower voltage. During the charging process, the single plateau at ∼2.4 V is assigned to the oxidation of Li2S to elemental sulfur. Based on the discharge curves, the S/C cathode with fluorinated electrolyte delivers a capacity of 870 mAh/g which is significantly higher than the corresponding capacity 606 mAh/g (by 44%) of the S/C cathode with the baseline electrolyte at 0.2C. The plateaus keep their shape after 50 cycles and the fluorinated electrolyte cell's capacity drops to 389 mAh/g (44.7% retention) while that of the baseline electrolyte drops to 302 mAh/g (49.8% retention). Thus, on capacity fading aspect, the fluorinated electrolyte didn't show superior performance at 0.2C, which is revealed in Fig. 4(c). However, with the fluorinated electrolyte the capacity fading is much improved at higher current density. As shown in Fig. 4(d), at the 0.5C rate, both cells show a gradual increase to the highest values after several cycles due to the activation effect for electrolyte diffusion. The fluorinated cell showed a slightly higher capacity than the baseline cell,
peaks represent two stages of electrochemical reactions during the discharging process [18,19]. The peak at higher voltage corresponds to the reduction of elemental S to the higher order polysulfides and the peak at lower voltage corresponds to the reduction of high order polysulfides to the lower order polysulfides. The single oxidation peak represents the formation of both polysulfides and final active sulfur Li2S8 or S. These CVs are consistent with the previously reported studies on baseline and electrolytes containing fluorinated compounds [12,14,17]. N. Azimi et al. [7] reported the CV for LieS battery with DOL/TTE and DOL/DME electrolytes, where an additional reduction peak for DOL/TTE at 1.8 V corresponding to TTE reduction is reported. However, in our modified electrolyte containing TFEG does not show such a feature, which indicates TFEG is electrochemically stable in the battery voltage range. Moreover, in contrast to the single oxidation peak at 2.47 V for our TFEG/DME electrolyte, they reported an additional shoulder-like oxidation peak at 2.35 V for TTE, which is also absent in the CV of our fluorinated electrolyte [7]. As shown in Fig. 3, the potential gap between the oxidation and reduction CV peaks of LieS cells with fluorinated (TFEG/DME) electrolyte is 0.479 V, ∼0.05 V smaller than that with DME/DOL electrolyte (0.533 V). This is due to the enhanced ionic conductivity, which ultimately decreases the over-potential and improves the reversibility of the cell [20]. From the discharge curve of the fluorinated electrolyte (Fig. 3), we have also observed that the peak at higher potential is much smaller compared to the peak at the lower potential, which is an evidence of the decreasing dissolution of high-order polysulfides, as reported by other groups with fluorinated electrolytes. 276
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655 mAh/g to 566 mAh/g, respectively. After 50 cycles, the fluorinated cell has a capacity retention of 70% which is better than the baseline cell, 55%. This is due to low solubility of polysulfides in the fluorinated electrolyte. To our knowledge, all of the published papers applying fluorinated ether solvents to LSBs [5–10,12,13,20] only reported the cycling performance at low current density (≤0.3C). Therefore, due to the improved kinetics, our fluoroether-based electrolyte could run at a higher current density with good capacity retention. Better kinetics of our electrolyte at higher current rate is also proved by the overpotential. As shown in Fig. 4(a), the heights of the second plateau for both electrolytes at 50th cycle are almost the same. However, in Fig. 4(b), the DME/TFEG = 1/2 electrolyte is obviously higher, indicating smaller overpotential than the baseline. The Coulombic efficiency (CE) of LieS cells with two electrolytes at 0.2C and 0.5C are shown in Fig. 4(c) and (d). Both cells showed a similar CE at 0.2C, but with the increased current at 0.5C, the cell containing our fluorinated electrolyte delivered a higher and more stable CE. Improved cycling stability has been reported previously in the research about other fluoroether-containing electrolytes [20], which possibly due to the formation of LiF, leading to increased SEI layer stability on both anode [11,13] and cathode surfaces [7].
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4. Conclusions We have synthesized and investigated the viability of a new serious of partially fluorinated ethers as a co-solvent for LieS battery electrolytes. They possess similar structures, but gradually increasing molecular weights and number of fluorine atoms. The density and viscosity of these compounds were measured. Our study shows that eCF3 group dramatically decrease the viscosity compared to the eCF2CF2H group present in the molecule. 1 M LiTFSI was dissolved in the mixture of our fluorinated ethers with DME and DOL in different portions. The viscosity and conductivity between 25 and 50 °C of these solutions were measured. We selected the combinations that showed the best performance and studied the polysulfide solubility by UV–vis spectroscopy. The best result was obtained with the formulation containing 1 M LiTFSI in DME:TFEG = 1:2. Charging-discharging tests demonstrated that this new electrolyte possesses better initial capacity, capacity retention and coulombic efficiency than the traditional DME:DOL = 1:1 solution, especially with a higher current density (0.5C). References [1] X. Fang, H. Peng, A revolution in electrodes: recent progress in rechargeable lithium-sulfur batteries, Small 11 (2015) 1488–1511, https://doi.org/10.1002/smll. 201402354. [2] Y.V. Mikhaylik, J.R. Akridge, Polysulfide shuttle study in the Li/S battery system, J. Electrochem. Soc. 151 (2004), https://doi.org/10.1149/1.1806394 A1969.
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