Synthesis and physical properties of new fluoroether sulfones

Article abstract of DOI:10.1016/j.jfluchem.2018.10.014

Eight new strategically designed fluoroether sulfone solvents have been synthesized through different synthetic pathways for potential applications in lithium-sulfur battery electrolytes. The structures of these compounds have been confirmed by ¹H-NMR, ¹³C-NMR and elemental analysis. Several 1,2-dimethoxyethane (DME)-based electrolyte formulations have been prepared with these solvents to derive an electrolyte with good ionic conductivity (>5 mS cm^?1 at 25 ℃). The viscosity of these new additives has also been determined as it is directly related to the ionic conductivity. Three of these solvents that contain a trifluoromethyl group displayed acceptable ionic conductivity.

Full text of DOI:10.1016/j.jfluchem.2018.10.014

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Authors: Zheng Yue, Xinyi Mei, Hamza Dunya, Qiang Ma,
Christopher McGarry, Braja K. Mandal
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Synthesis and physical properties of new fluoroether sulfones
Zheng Yue, Xinyi Mei, Hamza Dunya, Qiang Ma, Christopher McGarry and Braja K. Mandal*
Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, United States
Graphical abstract
Highlights
 Eight new fluoroether sulfone compounds have been synthesized and characterized.
 Some of the derivatives displayed low viscosity compared to similar non-fluoro analogs.
 Several binary electrolytes of these compounds with 1,2-dimethoxyethane have been
prepared.
 Some of these electrolytes displayed good ionic conductivity suitable for Li-S batteries.

ABSTRACT
Eight new strategically designed fluoroether sulfone solvents have been synthesized through
different synthetic pathways for potential applications in lithium-sulfur battery electrolytes. The
structures of these compounds have been confirmed by 1H-NMR, 13C-NMR and elemental analysis.
Several 1,2-dimethoxyethane (DME)-based electrolyte formulations have been prepared with
these solvents to derive an electrolyte with good ionic conductivity (>5 mS cm-1 at 25℃). The
viscosity of these new additives has also been determined as it is directly related to the ionic
conductivity. Three of these solvents that contain a trifluoromethyl group displayed acceptable
ionic conductivity.
Keywords:
Fluoroether sulfones
Electrolytes
Viscosity
Ionic conductivity
1． Introduction
Carbonate-based solvents, such as ethylene carbonate (EC) and dimethyl carbonate (DMC),
are exclusively used to prepare electrolytes for lithium-ion batteries (LIBs) [1–3]. These
electrolytes are not suitable for use in lithium-sulfur batteries (LSBs) because of direct chemical
reactions between the polysulfide intermediates (e.g., Li2S6, Li2S4, etc.) form during the discharge
process and organic carbonates. As of now, only ether-based organic solvents, such as 1,2-
dimethoxyethane (DME), 1,3-dioxolane (DOL) and tetraethylene glycol dimethyl ether

(TEGDME) have been found resilient to polysulfide intermediates [4–7]. However, one problem
with these ether solvents is that they dissolve too much of polysulfide intermediates, which cross-
over through the separator to the anode and undergo irreversible parasitic reactions—a
phenomenon better known as the polysulfide shuttle (PSS) effect [6–10].
To suppress or eliminate the PSS effect, extensive efforts have been made by designing novel
ether-based solvent systems for the LSB electrolytes. Partially fluorinated ethers have been
dominated as the co-solvent in most efforts. This category of solvents includes 1,1,2,2-
tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether [11,12], 1,1,2,2-tetrafluoroethyl 2,2,2-
trifluoroethyl ether [13,14], bis(2,2,2-trifluoroethyl) ether [15] and ethyl-1,1,2,2-tetrafluoroethyl
ether [16]—all of which displayed good PSS-suppressing effects by decreasing the solubility of
polysulfide intermediates in the electrolytes, and helping the formation of solid electrolyte
interface (SEI) layers on the surfaces of the lithium anode and sulfur-carbon composite cathode
[12,17].
In a recent publication [18], we have synthesized and investigated the viability of a new series
of partially fluorinated ethers as a co-solvent for LSB electrolytes. These fluoroethers possess
similar structures but feature gradually increasing molecular weights and numbers of fluorine
atoms. The density and viscosity of these compounds were measured. The best result was obtained
with the electrolyte formulation containing 1M LiTFSI in DME:TFEG (2,2,2-trifluoroethyl methyl
ether ethylene glycol) = 1:2 wt.%. Charge-discharge 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). The results of the
electrochemical stability testing from the cyclic voltammetry studies was very impressive (in
excess of 4 V) compared to the operating voltage of LSBs (2.8 to 1.6 V).

Inspired by the recent development of fluoroether solvents for LSB electrolytes, we
envisioned introducing a sulfone group in our recently reported fluoroether molecules with the
hope that the resulting fluoroether sulfones would complement the properties of both sulfone (high
dielectric constant) and fluoroether groups, leading to superior LSB electrolytes. Following this
assumption, we have synthesized eight new fluoroether sulfone molecules containing 1,1,1-
trifluoroethoxy or 1,1,2,2-tetrafluoropropoxy groups. In this paper, we report the synthesis,
structural characterization and key basic physical properties, such as viscosity and ionic
conductivity, of these new compounds.
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. Ethylene carbonate (Sigma Aldrich), p-toluenesulfonyl
chloride (Sigma Aldrich), ethanethiol (Sigma Aldrich), 2-propanethiol (Sigma Aldrich),
trifluoromethanesulfonyl chloride (Sigma Aldrich), 2-(methylthio) ethanol (Alfa Aesar), lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI) (Alfa Aesar), 2, 2, 2-trifluoroethan-1-ol (SynQuest),
2, 2, 3, 3-tetrafluoropropan-1-ol (SynQuest). Hydrogen peroxide solution 50% (w/w) in H2O
(Sigma Aldrich), acetic acid (Fisher Scientific), sulfuric acid, 98% (Fisher Scientific) and sodium
hydroxide (Fisher Scientific) were used as received.
2.2. Synthetic Procedures
2.2.1. Synthesis of 5a and 5b: Tosylates 2a and 2b were synthesized by the reaction of tosyl
chloride with the corresponding fluorinated alcohols (Fig. 1). In a typical procedure, 5.5 mmol 2-

methylthio ethanol in 15 mL anhydrous ethyl ether was added dropwise into a suspension of 9.0
mmol NaH in 30 mL of ethyl ether with ice-water bath and stirring. This mixture was stirred 30
min and allowed to warm to room temperature. After no more H2 was released, a solution of 5.0
mmol compound 2a (or 2b) in 15 mL anhydrous ethyl ether was added dropwise. The mixture was
stirred 24h at room temperature. Then 25 mL water was added to dissolve all solid products. The
water layer was separated and extracted with 2 × 15 mL dichloromethane. All organic layers were
combined and washed with 2 × 10 mL brine solution. The organic layer was dried with anhydrous
MgSO4, and solvent was removed in a rotovap. The remaining was a light yellow, oil-like liquid,
which was distillated under vacuum (5 mmHg) to give colorless products 5a (or 5b).
2.2.2. Synthesis of 5c, 5d, 5e and 5f: As shown in Fig.2, compound 3a & 3b were prepared
from CF3CH2OH or HCF2CF2CH2OH according to the method reported by Kent et al [19],. Then
3a & 3b were tosylated following the same procedure for 2a & 2b, formed 4a & 4b, respectively.
Then 5c & 5e was synthesized by the reaction of 4a with NaH and CH3CH2SH & (CH3)2CH2SH,
following the same procedure as of 5a & 5b. 5d & 5f were also synthesized from 4b. All products
were purified by vacuum distillation prior to proceed the next step.
2.2.3. Synthesis of 5g and 5h: R1-OTf was prepared by the reaction of
trifluoromethanesulfonyl chloride (TfCl) and CF3CH2OH. For the reaction of R1-OTf with
thiourea, we followed the method reported by Dai et al [20] (Fig. 3). R2-OTs was used to prepare
4a. The raw products 5g & 5h were not purified by vacuum distillation and used directly for the
next step, because of their high boiling points.
2.2.4. General procedure for the oxidation of sulfides to sulfones: 6 mL of glacial acetic acid
was added slowly into 10 mL 50% H2O2 solution with stirring in an ice-water bath. 10 drops of
concentrated sulfuric acid were added subsequently. After that the ice-water bath was removed

and the mixture was stirred at room temperature for 2h. This mixture was then cooled again with
an ice-water bath. 10 mmol of the sulfides (5a, 5b, 5c, 5d, 5e, 5f, 5g & 5h) was added slowly.
After all reactants were added, the ice-water bath was removed and stirring continued for 1h at
room temperature. Then the mixture was diluted with 30 mL of DI water, neutralized with
NaHCO3 and extracted by DCM. After drying with anhydrous MgSO4, the solvent was removed
and the product (6a, 6b, 6c, 6d, 6e, 6f, 6g & 6h) was purified by Al2O3 column chromatography
with CH2Cl2 as an eluent.
2.3. Characterizations and Measurements
1
13
2.3.1. NMR & elemental analysis: H NMR and C NMR spectra (300 MHz Bruker NMR
Spectrometer) and elemental analysis (Perkin Elmer 2400 CHNS Elemental Analyzer) were used
to confirm the structures and purity of the products.
Compound 5a: 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.85 (q, JHF=8.7 Hz, 2H, CH2), 3.79
(t, JHH=6.6 Hz, 2H, CH2), 2.70 (t, JHH=6.6 Hz, 2H, CH2), 2.15 (s, 3H, CH3); 13C NMR (75.47 MHz,
CDCl3): δ(ppm) 124.0 (q, JCF=279 Hz, CF3), 72.2 (CH2), 68.4 (q, JCF=32.7 Hz, CH2), 33.3 (CH2),
16.1(CH3). Anal. calcd. for C5H9F3OS (%): C, 34.48; H, 5.21; S, 18.41. Found: C, 34.39; H, 5.17;
S, 18.35.
1
Compound 6a (TFEMS): H NMR (300.13 MHz, CDCl3): δ(ppm) 4.04 (t, JHH=5.1 Hz, 2H,
CH2), 3.88 (q, JHF=8.5 Hz, 2H, CH2), 3.25 (t, JHH=5.1 Hz, 2H, CH2), 2.96 (s, 3H, CH3); 13C NMR
(75.47 MHz, CDCl3): δ(ppm) 123.6 (q, JCF=279 Hz, CF3), 68.6 (q, JCF=32.7 Hz, CH2), 66.4(CH2),
55.0 (CH2), 43.1(CH3). Anal. calcd. for C5H9F3O3S (%): C, 29.13; H, 4.40; S, 15.55. Found: C,
29.02; H, 4.34; S, 15.50.

1
Compound 5b: H NMR (300.13 MHz, CDCl3): δ(ppm) 5.94 (tt, JHF=53.3 Hz, JHF=4.8 Hz,
1H, HCF2), 3.85 (t, JHF=12.6 Hz, 2H, CH2), 3.74 (t, JHH=6.6 Hz, 2H, CH2), 2.68 (t, JHH=6.6 Hz,
13
2H, CH2), 2.68 (q, JHH=7.4 Hz, 2H, CH2), 2.14 (s, 3H, CH3); C NMR (75.47 MHz, CDCl3):
δ(ppm) 112.4 (tt, JCF=30.2 Hz, JCF=204 Hz, CF2H), 109.1 (tt, JCF=34.0 Hz, JCF=249 Hz, CF2), 71.9
(CH2), 68.0 (t, JCF=30.2 Hz, CH2), 33.3 (CH2), 16.0(CH3). Anal. calcd. for C6H10F4OS (%): C,
34.95; H, 4.89; S, 15.55. Found: C, 34.87; H, 4.92; S, 15.48.
1
Compound 6b (TFPMS): H NMR (300.13 MHz, CDCl3): δ(ppm) 5.88 (tt, JHF=53.0 Hz,
JHF=4.4 Hz, 1H, HCF2), 3.97 (t, JHH=5.3 Hz, 2H, CH2), 3.85 (t, JHF=12.9 Hz, 2H, CH2), 3.22 (t,
JHH=5.3 Hz, 2H, CH2), 2.91 (s, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 112.6 (tt,
JCF=26.4 Hz, JCF=166 Hz, CF2H), 109.1 (tt, JCF=37.7 Hz, JCF=249 Hz, CF2), 67.8 (t, JCF=22.6 Hz,
CH2), 66.2(CH2), 54.8 (CH2), 42.8(CH3). Anal. calcd. for C6H10F4O3S (%): C, 30.25; H, 4.23; S,
13.46. Found: C, 30.22; H, 4.14; S, 13.42.
Compound 5c: 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.87 (q, JHF=8.7 Hz, 2H, CH2), 3.77
(t, JHH=6.9 Hz, 2H, CH2), 2.73 (t, JHH=6.9 Hz, 2H, CH2), 2.61 (q, JHH=7.5 Hz, 2H, CH2), 1.25 (t,
JHH=7.5 Hz, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 124.0 (q, JCF=279 Hz, CF3), 72.6
(CH2), 68.4 (q, JCF=32.7 Hz, CH2), 30.8 (CH2), 26.5 (CH2), 14.8(CH3). Anal. calcd. for C6H11F3OS
(%): C, 38.29; H, 5.89; S, 17.04. Found: C, 38.22; H, 5.84; S, 17.01.
1
Compound 6c (TFEES): H NMR (300.13 MHz, CDCl3): δ(ppm) 4.04 (t, JHH=5.3 Hz, 2H,
CH2), 3.87 (q, JHF=8.5 Hz, 2H, CH2), 3.22 (t, JHH=5.2 Hz, 2H, CH2), 3.08 (q, JHH=7.5 Hz, 2H,
CH2), 1.40 (t, JHH=8.1 Hz, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.6 (q, JCF=279
Hz, CF3), 68.6 (q, JCF=32.7 Hz, CH2), 66.3(CH2), 52.2 (CH2), 49.4 (CH2), 6.4 (CH3). Anal. calcd.
for C6H11F3O3S (%): C, 32.73; H, 5.03; S, 14.56. Found: C, 32.66; H, 5.00; S, 14.47.

1
Compound 5d: H NMR (300.13 MHz, CDCl3): δ(ppm) 5.94 (tt, JHF=53.4 Hz, JHF=4.8 Hz,
1H, HCF2), 3.85 (t, JHF=12.6 Hz, 2H, CH2), 3.73 (t, JHH=6.6 Hz, 2H, CH2), 2.72 (t, JHH=6.0 Hz,
2H, CH2), 2.58 (q, JHH=7.4 Hz, 2H, CH2), 1.26 (t, JHH=7.4 Hz, 3H, CH3); 13C NMR (75.47 MHz,
CDCl3): δ(ppm) 112.4 (tt, JCF=30.2 Hz, JCF=204 Hz, CF2H), 109.1 (tt, JCF=34.0 Hz, JCF=249 Hz,
CF2), 72.3 (CH2), 68.0 (t, JCF=30.2 Hz, CH2), 30.8 (CH2), 26.5 (CH2), 16.1(CH3). Anal. calcd. for
C7H12F4OS (%): C, 38.18; H, 5.49; S, 14.56. Found: C, 38.12; H, 5.43; S, 14.45.
1
Compound 6d (TFPES): H NMR (300.13 MHz, CDCl3): δ(ppm) 5.87 (tt, JHF=53.0 Hz,
JHF=4.4 Hz, 1H, HCF2), 3.94 (t, JHH=5.4 Hz, 2H, CH2), 3.83 (t, JHF=12.9 Hz, 2H, CH2), 3.16 (t,
JHH=5.3 Hz, 2H, CH2), 3.00 (q, JHH=7.5 Hz, 2H, CH2), 1.30 (t, JHH=7.5 Hz, 3H, CH3); 13C NMR
(75.47 MHz, CDCl3): δ(ppm) 112.6 (tt, JCF=34.0 Hz, JCF=279 Hz, CF2H), 109.2 (tt, JCF=34.0 Hz,
JCF=249 Hz, CF2), 67.8 (t, JCF=22.6 Hz, CH2), 66.0 (CH2), 51.9 (CH2), 49.1 (CH2), 6.3(CH3). Anal.
calcd. for C7H12F4O3S (%): C, 33.33; H, 4.80; S, 12.71. Found: C, 33.27; H, 4.73; S, 12.66.
Compound 5e: 1H NMR (300.13 MHz, CDCl3): δ(ppm) 3.85 (q, JHF=8.7 Hz, 2H, CH2), 3.75
(t, JHH=6.9 Hz, 2H, CH2), 2.96 (m, JHH=6.6 Hz, 1H, CH), 2.73 (t, JHH=6.9 Hz, 2H, CH2), 1.25 (d,
JHH=6.6 Hz, 6H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 124.0 (q, JCF=279 Hz, CF3), 72.7
(CH2), 68.4 (q, JCF=32.7 Hz, CH2), 35.3 (CH), 29.7 (CH2), 23.4(CH3). Anal. calcd. for C7H13F3OS
(%): C, 41.57; H, 6.48; S, 15.86. Found: C, 41.51; H, 6.44; S, 15.76.
1
Compound 6e (TFEiPS): H NMR (300.13 MHz, CDCl3): δ(ppm) 4.05 (t, JHH=5.4 Hz, 2H,
CH2), 3.87 (q, JHF=8.7 Hz, 2H, CH2), 3.26 (t, JHH=5.3 Hz, 2H, CH2), 3.22 (m, 1H, CH), 1.39 (t,
JHH=6.9 Hz, 6H, CH3); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.6 (q, JCF=279 Hz, CF3), 68.4
(q, JCF=32.7 Hz, CH2), 66.1 (CH2), 54.4 (CH), 50.0 (CH2), 15.0 (CH3). Anal. calcd. for C7H13F3O3S
(%): C, 35.89; H, 5.59; S, 13.69. Found: C, 35.81; H, 5.55; S, 13.63.

Compound 5f: 1H NMR (300.13 MHz, CDCl3): δ(ppm) 5.95 (tt, JHF=53.4 Hz, JHF=4.8 Hz, 1H,
HCF2), 3.85 (t, JHF=12.6 Hz, 2H, CH2), 3.73 (t, JHH=6.6 Hz, 2H, CH2), 2.96 (m, JHH=6.6 Hz, 1H,
CH), 2.73 (t, JHH=6.6 Hz, 2H, CH2), 1.27 (d, JHH=6.6 Hz, 6H, CH3); 13C NMR (75.47 MHz, CDCl3):
δ(ppm) 112.4 (tt, JCF=30.2 Hz, JCF=204 Hz, CF2H), 109.1 (tt, JCF=34.0 Hz, JCF=249 Hz, CF2), 72.5
(CH2), 68.0 (t, JCF=30.2 Hz, CH2), 35.3 (CH), 29.7 (CH2), 23.4(CH3). Anal. calcd. for C8H14F4OS
(%): C, 41.02; H, 6.02; S, 13.69. Found: C, 40.96; H, 6.02; S, 13.63.
1
Compound 6f (TFPiPS): H NMR (300.13 MHz, CDCl3): δ(ppm) 5.88 (tt, JHF=53.0 Hz,
JHF=4.4 Hz, 1H, HCF2), 3.99 (t, JHH=5.6 Hz, 2H, CH2), 3.85 (t, JHF=12.8 Hz, 2H, CH2), 3.19 (t,
JHH=5.7 Hz, 2H, CH2), 3.14 (m, 1H, CH), 1.33 (d, JHH=6.9 Hz, 6H, CH3); 13C NMR (75.47 MHz,
CDCl3): δ(ppm) 112.6 (tt, JCF=34.0 Hz, JCF=279 Hz, CF2H), 109.2 (tt, JCF=34.0 Hz, JCF=249 Hz,
CF2), 67.9 (t, JCF=30.2 Hz, CH2), 65.9 (CH2), 54.3 (CH), 49.7 (CH2), 14.9 (CH3). Anal. calcd. for
C8H14F4O3S (%): C, 36.09; H, 5.30; S, 12.04. Found: C, 36.10; H, 5.24; S, 12.01.
Compound 6g (TFETFS): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 4.09 (t, JHH=5.1 Hz, 2H,
CH2), 3.95 (q, JHF=9.3 Hz, 2H, CH2), 3.90 (q, JHF=8.4 Hz, 2H, CH2), 3.42 (t, JHH=5.1 Hz, 2H, CH2);
13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.5 (q, JCF=279 Hz, CF3), 121.4 (q, JCF=279 Hz, CF3),
68.6 (q, JCF=35.2 Hz, CH2), 66.1 (CH2), 57.2 (q, JCF=32.7 Hz, CH2), 54.9 (CH2). Anal. calcd. for
C6H8F6O3S (%): C, 26.28; H, 2.94; S, 11.69. Found: C, 26.17; H, 2.90; S, 11.64.
Compound 6h (TFEMOES): 1H NMR (300.13 MHz, CDCl3): δ(ppm) 4.03 (t, JHH=5.6 Hz, 2H,
CH2), 3.86 (q, JHF=8.7 Hz, 2H, CH2), 3.79 (t, JHH=5.6 Hz, 2H, CH2), 3.36 (t, JHH=5.4 Hz, 2H, CH2),
3.34 (s, 3H, CH3), 3.26 (t, JHH=5.4 Hz, 2H, CH2); 13C NMR (75.47 MHz, CDCl3): δ(ppm) 123.7
(q, JCF=279 Hz, CF3), 68.5 (q, JCF=32.7 Hz, CH2), 66.2 (CH2), 66.0 (CH2), 58.9 (CH3), 54.9 (CH2),
54.7 (CH2). Anal. calcd. for C7H13F3O4S (%): C, 33.60; H, 5.24; S, 12.81. Found: C, 33.53; H,
5.17; S, 12.75.

2.3.2. Viscosity: The viscosities of all synthesized molecules were measured with the
Brookfield DV3T viscometer containing CPA-40Z spindle, and a Fisher Scientific Isotemp 1013s
recycling heater was used to keep the temperature stable at 25℃.
2.3.3. Ionic Conductivity: The ionic conductivities of the electrolyte solutions were measured
by Mettler Toledo S230-Kit conductivity meter in an environmental chamber from 25℃ to 70℃.
The conductivity meter was calibrated against a standard 0.01 mol L−1 KCl aqueous solution with
conductivity 1.314 mS cm−1 at 25.0°C.
3. Results and Discussion
3.1. Molecular Design and Synthetic Strategy
The alkyl sulfones have been extensively investigated as a single or co-solvent for high-
voltage LIBs [21,22], because of their high degree of LiTFSI dissolution power (>1M), high
dielectric constant, low toxicity, low volatility, and superior electrochemical stability (>4V). The
most studied sulfones are tetramethylene sulfone (TMS) [23,24] and ethyl methyl sulfone (EMS)
[23]. 1M LiPF6 in EMS solution has been applied in many early-stage researches on LSBs [23,25–
27]. Unfortunately, these alkyl sulfone-based electrolytes displayed poor ionic conductivity (<2.7
mS cm-1) [28], because of their high viscosity. Subsequently, several researchers have tried to
improve their viscosity by modifying the molecules with oligoether [29], cycloalkyl [30] or
branched alkyl groups [21]; however, these efforts did not exceed acceptable ionic conductivity
(>5 mS cm-1) for LSBs. As a result, single sulfone-based solvents have been proved unsuitable for
use in LSBs.
Another method to improve the strength of sulfone-based solvents is to mix them with a low-
viscosity, low melting point co-solvent. Several researchers have conducted comparative studies

on mixtures of sulfone-based and ether-based solvents as electrolytes in LSBs. Yoon et al studied
a binary electrolyte system (TMS/DME) for LSBs [31]. The ionic conductivity was markedly
improved, but the PSS effect is still significant as indicated from the cell cycling data. Very
recently, Su et al reported the synthesis of two fluoroalkyl sulfone molecules, trifluoromethyl ethyl
sulfone (FMES) and trifluoromethyl propyl sulfone (FMIS) [32]. These proved to have reduced
viscosity and enhanced wettability to the electrodes and the separator, compared to their non-
fluorinated counterparts. This study has motivated us to synthesize the proposed fluoroether
sulfones.
All new fluoroether sulfones were synthesized according to the schemes outlined in Figs. 1-
3. In the first step, an appropriate alcohol was either tosylated or tryflated for Williamson ether
synthesis to produce corresponding thioether derivatives. In the second step, the thioether
derivatives were transformed to fluoroether sulfones by a simple oxidation process involving
hydrogen peroxide in the acetic acid medium (Fig. 1). Most of the starting alcohols were
commercially available. As depicted in Fig. 2, the alcohols, 3a and 3b, were prepared by treating
the readily available alcohol with ethylene carbonate and sodium hydroxide [19]. The access to a
few thioethers (5g and 5h) also necessitated the adoption of a different synthetic protocol involving
the treatment of a triflate derivative with thiourea [20] (Fig. 3).

Fig. 1. Synthetic strategy to TFEMS (6a) and TFPMS (6b).
Fig. 2. Synthetic strategy to TFEES (6c), TFPES (6d), TFEiPS (6e) and TFPiPS (6f).

Fig. 3. Synthetic strategy to TFETFS (6g) and TFEMOES (6h).
3.2. Melting Point
All fluoroether sulfones possess higher molecular weights than that of TMS. Surprisingly, the
melting point of TMS is higher (27.5℃), while TFEES (6c), TFEiPS (6e), TFPiPS (6f), TFETFS
(6g) and TFEMOES (6h) showed melting points lower than room temperature (liquid at 25℃).
Compounds TFEMS (6a), TFPMS (6b) and TFPES (6d) turned out to be waxy solids. The
trifluoromethyl group may also have negative effect on the melting point. When we compare the
two pairs of molecules with similar structures (Table 1), in the case of small molecules MEMS
and TFEMS, the -CF3 group increases the melting point. However, in the case of larger molecules
DMES and TFEMOES, the -CF3 group causes the opposite effect. For small molecules, we believe
the molecular weight plays a more important role to affect the melting point. Whereas in larger
molecules, the steric hindrance and electron repulsion brought on by the -CF3 group outweigh the
impact of molecular weight.
When we compare the fluoroether sulfone molecules with similar structures, TFEES and
TFEiPS are liquid at room temperature, while lower molecular weight TFEMS is solid at room

temperature. This is because an isopropyl group has one more branch than an ethyl group, which
also has one more branch than a methyl group; and increasing asymmetricity in the molecule
prevents the process of crystallization. As a result, the melting point is decreased. Similar melting
temperature sequence is also displayed by TFPMS, TFPES and TFPiPS. In general, the
introduction of longer ether chains lowers the melting point, because the attached ether group
increases the flexibility of the molecule, but it is also associated with negative effects. In a recent
study, Sun & Angell [29] reported the melting points of a series of sulfone molecules with
oligoether chains: CH3OCH2CH2SO2CH3 (MEMS, 15.0 ℃ ), CH3OCH2CH2SO2C2H5 (EMES,
2.0℃), CH3(OCH2CH2)2SO2C2H5 (EMEES, <0℃), CH3(OCH2CH2)3SO2C2H5 (EMEEES, <0℃)
and CH3OCH2CH2SO2CH2CH2OCH3 (DMES, 47.0 ℃). Although the melting point gradually
decreases with the increase of the oligoether chain, the ionic conductivity of 1M LiTFSI solution
with these sulfone molecules tends to decrease gradually.

Table 1
Comparison of melting points of new fluoroether sulfones with similar compounds
m.p.
Molecular weight
Ethersulfones
MEMS [29]
Structure
(g mol-1)
(℃)
138.19
15.0
Waxy
solid at
25oC
TFEMS
206.18
DMES [29]
TFEMOES
182.06
250.05
47.0
Liquid
at 25oC
3.3. Viscosity
As summarized in Table 2, the viscosities of our fluoroether sulfones were measured at 25℃
except for the molecules, which are solid or semisolid at the room temperature (TFEMS, TFPMS
and TFPES). Typically, the viscosity tends to be higher with the increase of molecular weight, as
the product with the lowest molecular weight, TFEES, showed the lowest viscosity (16.14 cP).
However, it is still too viscous to be applied as a single solvent for LSB electrolytes. Therefore,
these fluoroether sulfones must be formulated with a low viscosity thinner solvent, such as DME.
High viscosity slows down the transport of ions in the electrolyte, leading to bad electrochemical
reaction kinetics and poor cell performance.
Several years ago, Sasaki et al [33] synthesized three new fluoroethers to develop an effective
LSB electrolyte: ethylene glycol 2-fluoroethyl methyl ether (FEG, MW = 122.1 g/mol), ethylene

glycol 2, 2-difluoroethyl methyl ether (DFEG, MW = 140.1 g/mol) and ethylene glycol 2, 2, 2-
trifluoroethyl methyl ether (TFEG, MW = 158.1 g/mol). Although the molecular weight of TFEG
is the highest among them, it showed much less viscosity than the other two fluoroethers (0.78 cP
vs. 1.01 cP for FEG and 1.06 cP for DFEG). To explain this phenomenon, they assumed the high
electronegativity of the trifluoromethyl group could provide larger steric hindrance and electron
repulsion, which decreases the interactions between molecules. We believe this conclusion is also
applicable to our fluorinated sulfone molecules. By contrast, -CF2CF2H group does not display the
same effect as for -CF3, which is ascribed by the high melting point and viscosity of TFPMS,
TFPES and TFPiPS. Our study also shows that the -CF3 group dramatically decreases the viscosity
compared to the -CF2CF2H group present in the molecule.

Table 2
Viscosities of new fluoroether sulfone molecules
MW
(g mol-1)
ηa
Code
Structure
Comment
(cP)
b
TFEMS (6a)
206.18
220.21
_
New
compound
TFEES (6c)
TFEiPS (6e)
16.14
19.45
New
compound
234.24
New
compound
b
TFPMS (6b)
TFPES (6d)
238.20
252.23
_
New
compound
b
_
New
compound
TFPiPS (6f)
266.25
57.29
New
compound
TFETFS (6g)
274.01
250.05
22.41
32.36
New
compound
TFEMOES (6h)
New
compound
a Measured at 303.15 K.
b Crystalline solid or semi-solid at room temperature.

3.4. Ionic Conductivity
The ionic conductivity of an electrolyte is inversely related to the viscosity. When a larger
portion of thinner co-solvent, such as DME, is applied, the conductivity of the solution increases
due to the decrease of solution viscosity, which improves the kinetics of the electrochemical
process. However, the solubility of polysulfide intermediates also increases with the increase in
DME proportion, which means that the formulated electrolyte will be less effective to suppress the
PSS effect. As mentioned earlier, the higher the fluoroether content the better the electrolyte for
suppressing PSS. Since the fluoroether sulfones are highly viscous, a balance between ionic
conductivity and the proportion of fluoroether sulfone is paramount. In this study, three fluoroether
sulfones (TFEES, TFEiPS and TFETFS) showed promising potential as LSB electrolytes. When
they are formulated with 1:1 weight proportion with DME (low viscosity solvent, 1.1 cP) the ionic
conductivities of the corresponding electrolytes exceeded the basic minimum ionic conductivity
value, >5 mS cm-1 at 25℃. At higher temperatures, the ionic conductivity values are very good.

Table 3
Viscosity and ionic conductivity of the binary electrolytes containing 1M LiTFSI
Sulfones
DME/Sulfone ratio
(wt./wt.)b
ηc
χ (mS cm-1)
(cP)
25℃
5.76
3.31
5.18
3.05
5.12
2.52
30℃
6.35
3.73
5.71
3.47
5.54
2.91
35℃
7.02
4.15
6.28
3.88
6.13
3.34
40℃ 45℃ 50℃
TFEES
TFEES
TFEiPS
TFEiPS
TFETFS
1/1
1/2
1/1
1/2
1/1
1/2
6.98
11.90
7.50
7.59
4.59
6.88
4.41
6.54
3.79
8.20
5.11
7.45
4.93
7.04
4.24
8.74
5.61
7.98
5.46
7.38
4.70
13.43
6.80
TFETFS
11.51
a Literature data.
b Contains 1M LiTFSI.
c Measured at 303.15 K.
4. Conclusion
In conclusion, we have described the synthesis of eight new fluoroether sulfones through different
synthetic pathways for potential applications in lithium-sulfur battery electrolytes. TFEMS,
TFPMS and TFPES are solid/semisolid at room temperature while the other five, TFEES, TFEiPS,
TFPiPS, TFETFS and TFEMOES are liquid. Their room temperature viscosities were measured.
Several 1M LiTFSI-based binary electrolytes composed of DME and our fluoroether sulfones as
the co-solvent were prepared, and their ionic conductivities were measured from 25℃ to 50℃.
Three binary electrolytes have displayed ionic conductivity in excess of 5 mS cm-1 at 25℃.

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