Synthesis of new fluorine-containing room temperature ionic liquids and their physical and electrochemical properties

Article abstract of DOI:10.1016/j.jfluchem.2018.05.008

An improved synthesis of fluorine-containing room temperature ionic liquids (FRTILs) is described. Twelve FRTILs are synthesized, in which the fluorinated cations based on N-methyl pyrrolidinium (MPy), 1-methly imidazolium (MIm) and diethylsulfide (DES) a

Full text of DOI:10.1016/j.jfluchem.2018.05.008

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Title: Synthesis of new fluorine-containing room temperature
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Authors: Xinyi Mei, Zheng Yue, Jim Tufts, Hamza Dunya,
Braja K. Mandal
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S0022-1139(18)30100-3
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1
Synthesis of new fluorine-containing room temperature ionic liquids and
their physical and electrochemical properties
Xinyi Mei, Zheng Yue, Jim Tufts, Hamza Dunya, Braja K. Mandal*
Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, United States
Professor Braja Mandal
Illinois Institute of Technology
Department of Chemistry
3101 S. Dearborn St. Rm. 328 LS
Chicago, IL 60616
Ph: 312-567 3446
Graphical abstract
Highlights
 Twelve fluorine-containing ionic liquids (FRTILs) were synthesized and characterized.
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 The density, miscibility, viscosity, and neat ionic conductivity are reported for each.
 Additionally, twelve electrolyte systems containing FRTILs and LiTFSI were tested for
flammability and ionic conductivity.
 All prepared FRTILs exhibited thermal stability up to 200 °C and non-flammability.
 The reported FRTIL-based electrolyte systems were determined to be electrochemically
suitable for use in lithium-sulfur batteries.
 The FRTIL-based electrolytes displayed a RT ionic conductivity between 7.44 and 10.51
mS/cm.
Abstract
An improved synthesis of fluorine-containing room temperature ionic liquids (FRTILs) is
described. Twelve FRTILs are synthesized, in which the fluorinated cations based on N-methyl
pyrrolidinium (MPy), 1-methly imidazolium (MIm) and diethylsulfide (DES) are associated with
anions, such as dicyanamide (DCA) or bis(trifluoromethane sulfonyl)imide (TFSI). The molecular
structures of these FRTILs were confirmed by FT-IR ，1H-NMR, 13C-NMR and elemental
analysis. The FRTILs have been investigated for their potential as electrolyte solvents, especially
for lithium-sulfur (Li-S) batteries. Thermal behaviors were also studied by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) showing excellent thermal stability,
exceeding 150 °C, for all FRTILs. Some of their physicochemical properties were determined,
including density, flammability, viscosity, and miscibility with polar and nonpolar solvents. Due
to the high viscosity of FRTILs, two low viscosity ILs (co-solvent thinners), [MImEt] [DCA] and
[MImEt] [TFSI], have been synthesized and used to dilute the FRTILs. The resulting ternary
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electrolyte systems, LiTFSI in 1:1 wt. % FRTILs/thinner, were characterized by their ionic
conductivity and electrochemical stability.
Keywords: Room temperature ionic liquids; Rechargeable lithium batteries; Electrolytes;
Fluorinated ionic liquids; Ionic conductivity; Electrochemical stability.
1.
Introduction
Rapid consumption of fossil fuels has motivated a substantial investigative effort in the
exploration for ecological renewable energy sources and energy storage systems [1,2]. Introduced
in 1991, Li-ion batteries (LIBs) have become the standard for electronic devices big and small,
ranging from hand held electronics to electric vehicles (EVs). Nevertheless, in terms of energy
density, safety, and cost, the current LIBs fail to meet the requirements for the growth demand of
EVs and hybrid electric vehicles (HEVs). Accordingly, the search of high-energy batteries is still
critical for the development and application of Li-based rechargeable batteries.
Li-S battery, characterized by the theoretical energy density of 2600 Wh kg-1, low cost,
natural abundance, and environmentally friendly [3] of the active element sulfur, appeared as one
of the most promising systems for the next generation energy storage devices [4]. However, the
commercial application of Li-S cells is limited by three major limitations: (i) complex cathode
design due to the inherent insulating nature and inactive electrochemical reactions of the sulfur
and (ii) the dissolution of the reaction intermediates [5], lithium polysulfides (Li2Sn), into the
electrolyte followed by capacity decay and low coulombic efficiency [6] and (iii) unstable anodic
structure. Intense scientific and technical efforts have been concentrated on solving these issues.
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The poor electrical conductivity and low utilization of sulfur have recently been addressed by
fabricating composite sulfur cathodes with conducting polymers [7], nanocarbons [8], and nano-
structured carbon materials [9]. By contrast, the dissolution of Li2Sn in the electrolyte still needs
to be prevented in order to achieve a long life cycle for Li-S batteries. During the discharge–charge
cycles, dissolved high-ordered polysulfide Li2Sn (4 < n ≤ 8) [10] could diffuse through the
separator and chemically react with Li foil, reduce to low-order insoluble polysulfide Li2Sn (1 < n
≤ 4) (as shown in Scheme 1) [11], leading to loss of active sulfur material [12], corrosion of Li
metal, poor cycle life, and poor charge-discharge efficiency [13].
Scheme 1. Electrochemical reactions on the cathode of Li-S cells [10–13]
The electrolyte plays a critical role in improving the performance of Li-S batteries. Non-
aqueous liquid electrolytes common in Li-ion batteries, such as ethylene carbonate (EC) and
diethyl carbonate (DEC), cannot be used in Li-S batteries because of their high reactivity with the
polysulfide anions [14]. The typical electrolyte mixture is composed of 1,3-dioxolane (DOL) and
1,2-dimethoxyethane (DME) as solvents and bis(trifluoromethane)sulfonimide lithium salt
(LiTFSI) in 1M concentration [2]. However, the low concentration of this electrolyte allows the
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Li2Sn species (especially, n ≥ 4) to be readily solubilized.
In this paper, we aim to develop a suitable electrolyte system capable of preventing the
electrode dissolution effectively. Unlike the traditional strategies that use polymer electrolytes and
inorganic solid electrolytes to form physical and kinetic barriers to Li2Sn dissolution, the design of
IL based electrolytes with lower solubility power of Li2Sn is a more straightforward approach. A
typical IL consists of bulky cations and charge-delocalized, bulky anions. The solvation power of
ionic species to dissolve Li2Sn is expected to be lower than that of organic electrolytes. In
conclusion, a tailored IL solvent should dissolve only as much of the active species (S8 and Li2Sn)
as required for operation, but could support sufficiently high Li salt concentration required for the
liquid electrolyte [6]. To this end, the solvation power toward the Li+ cation in Li2Sn, must be
minimized while ensuring adequate Li+ conduction through the originally dissolved Li salts in the
electrolyte [14].
Herein, we describe the design, synthesis, and characterization of twelve new asymmetrical
fluorinated ILs with one ethylene oxide (EO) unit, in which the cationic species are based on N-
methyl pyrrolidine (MPy), 1-methly imidazole (MIm) or diethylsulfide (DES), while anions are
dicyanamide (DCA) anion or bis(trifluoromethanesulfonyl)imide (TFSI) anion. This series of ILs
is named as fluorinated room temperature ionic liquids (FRTILs), because they exist in liquid state
at room temperatures [15].
ILs with fluorine-containing cations have been previously synthesized and covered in two
recently published reviews [16,17]. The most notable applications include anti-fungal drugs [18],
enhanced oxygen solubility [19], photovoltaics [20], surfactants [21] and super hydrophobic
coatings [22]. The present work represents the first report of FRTILs for lithium-sulfur battery
electrolytes.
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The design of these FRTILs takes many things into consideration. First, RTILs have
received extensive interest as potential electrolytes for lithium batteries, due to their favorable
physical and chemical properties, such as controlled miscibility, non-volatility, non-flammability,
and environmental friendliness [23]. The ability to modify physical and chemical properties of an
IL by tuning its structure through substitution and changes to the functionality of the anion or of
the cation [1], forms the basis of the so-called tailor-made properties.
Second, solvents containing specific fluorinated substituents are known to form stable solid
electrolyte interphase (SEI) on the electrode surface [24]. Other special attributes of fluorinated
solvents includes, superior flame retardant properties [25]; reduction of boiling temperature and
generally lower viscosity [26]. Moreover, the influence of inter-ion hydrogen bonding can be
diminished by introducing fluorine-containing groups into the cation and especially when
employing anions such as bis(trifluorosulfonyl) imide [27]. Therefore, through the combination of
a fluorinated functional group (-CF3 or -CF2CF2H) with an EO chain in the cation may result in
ILs that display non-volatility, non-flammability, relatively wide electrochemical windows [28]
and low viscosities and high conductivities [29]. The ethylene oxide (EO) unit is known as one of
the best solvating medium for lithium salts [23]. The -CH2CH2O- unit provides a suitable space
for Li-ion to coordinate with oxygen, and Li-ion migration is associated with the segmental
mobility of the EO chain [30]. Furthermore, a reduction of the symmetry, introduced by the
presence of the EO functionality, is known to cause a melting point drop. That is due to a reduction
in anion-cation interaction resulting from asymmetry and a delocalized charge. Moreover, as
mentioned above, the asymmetric structure could increase the lithium ion transport and ionic
conductivity.
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The dicyanamide (DCA)-based ILs presented here have many useful properties that make
them attractive as a potential solvent. DCA anion offers exceptionally low viscosity, which assists
heat and mass transport and enables higher conductivity. DCA-based ILs can be prepared
relatively inexpensively, because DCA is already a commodity anion, containing only carbon and
nitrogen, and is produced in large amounts for the pharmaceutical industry [27]. However, for
LIBs, the DCA anodic stability is below that of TFSI. The TFSI anion is an exceedingly weak base
due to strong delocalization of charge throughout the molecule. This feature contributes to its
stability vs. oxidation. The delocalized charge, weak basicity, and poor hydrogen-bonding ability
[31] produce weaker ion-ion interactions. The weak ion-ion interaction coupled with the flexibility
of the molecule tend to produce comparatively low viscosity and good thermal stability ILs [31].
Both the TFSI and DCA anions facilitate a stable charge/discharge of the Li–S cells.
Finally, the FRTILs are unlikely to serve as a single solvent system for the electrolyte,
because of their high viscosity (>50 cP at RT). To compensate, lower viscosity ILs those based on
1-ethyl-3-methylimidazole, such as [MImEt] [DCA] or [MImEt] [TFSI], can be added as a co-
solvent, described in this work as a ‘thinner’. To inform our selection, we thoroughly investigated
the electrolytic properties of a series of ternary electrolyte systems, LiTFSI in 1:1 wt%
FRTILs/thinner.
2
Experimental
2.1
Materials
2,2,2-Trifluoroethanol, 2,2,3,3-Tetrafluoro-1-propanol, were purchased from SynQuest
Labs, Inc. Sodium hydroxide, ethylene carbonate, 4-toluenesulfonyl chloride, lithium bis
(trifluoromethyl sulfonyl) imide, 1-methylpyrrolidine, 1-butyl imidazole and diethyl sulfide were
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purchased from Sigma-Aldrich Chemical Co., Ltd (USA). All the solvents were supplied by Fisher
Scientific Co., Ltd., (USA).
2.2
General procedures for the preparation of FRTILs
We have prepared twelve new FRTILs. The general molecular structures and abbreviations
of these compounds are listed in Table 1.
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Table 1. Appendix abbreviations of FRTILs
Anions
Cations
[MImEt][TFSI]
[MImEt][DCA]
[MPyEtTFE][TFSI]
[MPyEtTFP][TFSI]
[MImEtTFE][TFSI]
[MImEtTFP][TFSI]
[DESEtTFE][TFSI]
[DESEtTFP][TFSI]
[MPyEtTFE][DCA]
[MPyEtTFP][DCA]
[MImEtTFE][DCA]
[MImEtTFP][DCA]
[DESEtTFE][DCA]
[DESEtTFP][DCA]
The above abbreviation for FRTILs will be used in this paper: 1-ethyl-3-
methylimidazolium bis ((trifluoromethyl) sulfonyl) imide can be described as [MImEt][TFSI],
where MIm indicates the methylimidazolium group, Et represents the ethyl fragment, and TFSI
signifies the bis(trifluoromethyl) sulfonyl imide anion [32]. [MPyEtTFE][TFSI] as another
example, where MPy depicts the methylpyrrolidinium group, Et once again represents an ethyl
fragment, and TFE represents trifluoroethoxy group. In the case of [MPyEtTFP][TFSI], TFP
represents the tetrafluoropropoxy group. We believe this specific style is the most adaptable
approach to abbreviate lengthy names of ILs.
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2.2.1 General procedures for the purification of FRTILs
In general, the synthesis of FRTILs consists of a quaternization step followed by a
metathesis step to exchange anions. All the procedures used were standard literature methods with
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slight modifications. H NMR and C NMR spectra were recorded for all the new FRTILs
prepared, and they were shown to be analytically pure, but with small amounts of non-organic
colored impurities. Though there is no evidence that the impurities affect either the chemistry or
most of the physical properties of FRTILs, but may impact measurements involving electronic
absorption, luminescence, or Raman spectroscopy [33]. Consequently, we developed a
methodology for FRTILs decolorization after the final synthetic step rather than the halide
precursors.
2.2.2 Decolorization of FRTILs. Methanol was added to the final compound [MImEtTFE]
[TFSI] to make a homogenous solution. Activated carbon was added to the solution and the
resulting slurry was refluxed for 24 h at 70°C and then was cooled to room temperature. Tests
performed at lower temperature and shorter stirring time still gave slightly colored FRTILs. A
special column was designed for the purification and decolorization that was packed with the
following compounds: sand on the bottom to trap charcoal particles, flash chromatographic silica
gel in the middle for decolorizing/removal of polar and inorganic impurities, and charcoal on the
top for decolorizing [34,35]. The column was pretreated with methanol prior to its use. For the
majority of compounds, passage through the column rendered most eluates completely colorless.
However, in some cases color remained and the decolorizing charcoal step was repeated. Once
eluted, the eluate solvent was evaporated and the resulting liquid was dried under vacuum at 60 °C.
The process produced clear, colorless, inodorous, high-purity FRTILs. In such a case where a
second decolorizing step was insufficient, it was necessary to use a more polar mobile phase to
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promote better separation between the IL and the offending impurity; however, this typically
results in lower product yields [35]. Purified FRTILs were stored in sealed glass tubes in a
controlled environment.
2.3
General procedures for the preparation of MPy-based FRTILs
Scheme 2. Synthesis of tosylate intermediates.
2.3.1 Synthesis of tosylates. As shown in scheme 2, 2, 2, 2-Trifluoroethanol (TFE, 16.2 g, 161.9
mmol), NaOH (0.44g, 11 mmol) and Ethylene carbonate (9.75 g, 110.8 mmol) were dissolved in
40 ml tetraethylene glycol dimethyl ether (G4) and refluxed at 150 °C for 4h. Then the product
was distilled under vacuum. The distillate in the range of 112~150 °C was collected. A colorless
liquid product 2-(2, 2, 2-trifluoroethoxy) ethanol (TFEE， yield: 14.53 g, 90%) was obtained.
Then TFEE (14.53 g, 100.84 mmol), and 4-Toluenesulfonyl chloride (TsCl) (21.10 g, 111.1 mmol)
were dissolved in 100 ml dichloromethane (DCM). NaOH (8.07 g, 201.8 mmol) was added slowly
into the solution with vigorous stirring placed in an ice bath. After stirring overnight at room
temperature, another 50 ml DCM was added to dilute the mixture, and then washed with 50 ml
water one time and 20 ml brine two times. The organic layer was separated and dried over
anhydrous MgSO4. Then the solvent was removed to get the desired product: 2-(2, 2, 2-
trifluoroethoxy) ethyl 4-methylbenzenesulfonate (TFEEOTs, yield: 27.54 g, 91.6%), which was a
colorless viscous liquid that crystallized at room temperature.
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An analogous procedure was used to prepare 2-(2, 2, 3, 3-tetrafluoropropoxy) ethanol
(TFPE) starting with 2, 2, 3, 3-Tetrafluoro-1-propanol (TFP, 21.7 g, 164.3 mmol), NaOH (0.43 g,
10.8 mmol) and EC (9.65 g, 109.6 mmol), to give the desired compound as a colorless liquid: 2-
(2, 2, 3, 3-tetrafluoropropoxy) ethanol (TFPE, yield: 16.57 g, 85.8%). The corresponding tosylate
was prepared analogously from TFPE (16.57 g, 94.09 mmol) and TsCl (19.7 g, 103.7 mmol),
NaOH (7.52 g, 188 mmol) to give the desired compound 2-(2, 2, 3, 3-tetrafluoropropoxy) ethyl 4-
methylbenzenesulfonate (TFPEOTs, yield: 85.98 g, 91.4%), which was a colorless viscous liquid
that crystallized at room temperature.
Scheme 3. Synthetic Routes of MPy-based FRTILs
2.3.2 Synthesis of FRTILs [MPyEtTFE] [TFSI]. As shown in scheme 3, a flask charged with
TFEEOTs (8.0 g, 26.8 mmol) and 1-methylpyrrolidine (4.58 g, 53.8 mmol) was stirred and
refluxed overnight at 80oC. The mixture was then cooled to room temperature. The ILs are
insoluble in ethyl ether, and precipitate to form an insoluble sticky mass inside the flask. This
allows the use of an ether wash and decant procedure as a facile, high yield means of product
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purification. After repeating this washing procedure twice [36], the subsequent drying after the
final wash, yields the pure compound [MPyEtTFE] OTs (yield: 7.52 g, 73.1%) as a brown viscous
liquid.
[MPyEtTFE] OTs (2.87 g, 7.5 mmol) and LiTFSI (2.38 g, 8.3 mmol) were dissolved in
15 ml deionized water. The addition of LiTFSI to [MPyEtTFE] OTs caused the reaction to form
hydrophobic [MPyEtTFE] [TFSI] and hydrophilic LiOTs. The rapid formation of two liquid
phases clearly indicated that the reaction proceeded quickly. However, to assist the anion exchange
reaction, the two phases were dynamically stirred at room temperature. When the stirring was
interrupted the phase, separation took place immediately. The upper phase was composed of water,
LiOTs, and excess LiTFSI. The lower phase was composed mostly of [MPyEtTFE] [TFSI] ionic
liquid with a small amount of water and lithium salts, i.e., LiOTs, and LiTFSI. Typically, the upper
aqueous phase was clear and colorless while the lower had a slightly brownish coloration. The
metathesis reaction, in which the OTs anions are replaced by the TFSI anions is driven by the
intrinsic hydrophobicity of both the TFSI anion and the [MPyEtTFE] cation. These ions, in which
the charge is well-shielded by hydrophobic groups [MPyEtTFE] or delocalized by TFSI-, do not
easily form hydrogen bonds with water and tend to separate from the aqueous phase, forming
another denser, liquid, phase. The disappearance of the product from the aqueous solution drove
the exchange reaction to completion more easily. The solution was stirred overnight at room
temperature, extracted the solution with 30 ml dichloromethane (DCM). After removal of the
upper aqueous phase, the [MPyEtTFE] [TFSI] was washed three times with deionized water to
remove water-soluble salts and impurities such as LiOTs and excess LiTFSI [37]. The obtained
organic layer was washed three times with 10 ml water, dried over anhydrous magnesium sulfate
and active carbon. The residue was purified by column chromatography on alumina and
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concentrated under reduced pressure to give the final product [MPyEtTFE] [TFSI] (yield: 3.18
g, 86.4%) as a brown liquid.
1H NMR (300 MHz, D-acetone): δ (ppm) 4.24 (t, J=5.1 Hz, 2H), 4.13 (q, J=7.8 Hz, 2H),
3.85 (t, J=4.2 Hz, 2H), 3.75 (t, J=3.51 Hz, 4H), 3.29 (s, 3H), 2.39-2.05 (m, 4H). 13C NMR (300
MHz, CDCl3): δ (ppm) 122.8, 119.8 (q, J=159 Hz), 68.33, 66.2, 65.3, 62, 47.93, 21.2. Anal. calcd.
for C11H17F9N2O5S2: (%) C, 40.27; H, 5.91; N, 5.87; S, 5.38. Found by elemental analysis: C,
40.11; H, 5.95; N, 5.78; S, 5.34.
2.3.3 Synthesis of FRTILs [MPyEtTFP] [TFSI]. An analogous procedure was used to prepare
[MPyEtTFP] OTs starting with TFPEOTs (7.74 g, 23.4 mmol), 1-methylpyrrolidine (3.98 g, 46.7
mmol), to give the desired compound as a dark brown liquid [MPyEtTFP] OTs (yield: 7.62 g,
18.34 mmol). The corresponding ionic liquid was prepared analogously from [MPyEtTFP] OTs
(2.25 g, 5.4 mmol) and LiTFSI (1.85 g, 6.4 mmol) to give the desired compound as a dark brown
liquid [MPyEtTFP] [TFSI] (yield: 2.34 g, 82.4%), which was purified as previously discussed
for [MPyEtTFE] [TFSI].
1H NMR (300 MHz, D-acetone): δ (ppm) 6.34 (t, J=5.4 Hz, 1H), 4.22 (t, J=3.51 Hz, 2H),
4.09 (t, J=5.1 Hz, 2H), 3.87 (t, J=4.5 Hz, 2H), 3.59 (t, J=3.51 Hz, 4H), 3.29 (s, 3H), 2.35-2.04 (m,
4H). 13C NMR (300 MHz, D-acetone): δ (ppm) 128.0, 119.8 (q, J=159 Hz), 109.5, 70, 67.6, 57.8,
49.4, 35.7, 20.7. Anal. calcd. for C12H18F10N2O5S2: (%) C, 34.38; H, 4.72; N, 5.47; S, 8.34. Found
by elemental analysis: C, 34.23; H, 4.76; N, 5.37; S, 8.37.
2.3.4 Synthesis of FRTILs [MPyEtTFE] [DCA]. [MPyEtTFE] OTs (3.76 g, 9.8 mmol) was
dissolved in 15 ml methanol and passed through a column with 25 ml dried Dowex® 22 resin at
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the dropping rate of 1 drop/second. 60mL methanol was added to wash the column. The fractions
were combined, and the solvent was removed. The desired compound [MPyEtTFE] Cl (yield:
2.07g, 85.1%) was obtained as a brown viscous liquid.
First, NaN(CN)2 (0.34 g, 3.8 mmol) was totally dissolved in 30ml methanol. [MPyEtTFE]
Cl (2.07 g, 8.4 mmol) in 5 ml methanol was added to the NaN(CN)2 solution. The mixture was
stirred overnight at room temperature, followed by vacuum distillation to remove methanol until
a precipitate formed. Then the precipitate was removed by filtration and 15 ml acetone was added
to the filtrate. The solvent was then again removed by vacuum distillation until a precipitate
formed. This procedure was repeated several times until no precipitation formed. The filtrate was
concentrated under reduced pressure to give the final product [MPyEtTFE] [DCA] as a dark
brown liquid (yield： 1.63 g, 70.1%).
1H NMR (300 MHz, D2O): δ (ppm) 4.1 (t, J=5.1 Hz, 2H), 3.73 (q, J=7.8 Hz, 2H), 3.38 (t,
13
J=4.2 Hz, 2H), 3.18 (t, J=3.51 Hz, 4H), 2.87 (s, 3H), 2.37-1.91 (m, 4H). C NMR (300 MHz,
D2O): δ (ppm) 119.9, 65.9, 55.8, 51.7, 48.4, 40.6, 21.2. Anal. calcd. for C11H17F3N4O: (%) C,
48.56; H, 6.97; N, 10.96. Found by elemental analysis: C, 48.42; H, 6.89; N, 10.85.
2.3.5 Synthesis of FRTILs [MPyEtTFP] [DCA]. An analogous procedure was used starting with
[MPyEtTFP] OTs (1.91 g, 4.6 mmol), and yielding the desired compound as a yellow liquid
[MPyEtTFP] Cl (yield: 1.08 g, 83.6%). The corresponding ionic liquid was prepared analogously
from [MPyEtTFP] Cl (1.08, 3.9 mmol) and NaN(CN)2 (0.72 g, 8.1 mmol) to give the desired
compound as a dark brown liquid [MPyEtTFP] [DCA] (yield: 0.86 g, 71.8%).
1H NMR (300 MHz, D2O): δ (ppm) 6.15 (t, J=5.4 Hz, 1H), 4.05 (t, J=3.51 Hz, 2H), 3.59
(t, J=5.1 Hz, 2H), 3.51 (t, J=4.5 Hz, 2H), 3.07 (t, J=3.51 Hz, 4H), 3.03 (s, 3H), 2.16-1.96 (m, 4H).
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13C NMR (300 MHz, D2O): δ (ppm) 128.0, 119.9, 109.5, 67.2, 63.0, 58.1, 55.8, 48.4, 21.1. Anal.
calcd. for C12H18F4N4O: (%) C, 46.45; H, 5.85; N, 18.06. Found by elemental analysis: C, 46.48;
H, 5.78; N, 18.01.
2. 3 General procedures for the preparation of MIm-based FRTILs
Scheme 4. Synthetic Routes of MIm-based FRTILs
2.3.1 Synthesis of FRTILs [MImEtTFE] [TFSI]. As shown in scheme 4, TFEEOTs (7.41 g, 24.9
mmol) and 1-methylimidazole (4.16 g, 48.9 mmol) were mixed and refluxed overnight at 80oC
under stirring. The mixture was then cooled to room temperature. The solvent was removed by
vacuum evaporation, and the resulting residue was washed three times with 15 ml ethyl ether. The
remaining volatiles were removed under high vacuum to give the desired compound [MImEtTFE]
OTs (yield: 7.38 g, 78.1%) as a yellow viscous liquid.
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[MImEtTFE] OTs (2.46 g, 6.5 mmol) with LiTFSI (2.04 g, 7.1 mmol) were dissolved in
15 ml deionized water. The solution was stirred overnight at room temperature, and then the
product was extracted into 30 ml dichloromethane. The obtained organic layer was washed three
times with 10 mL of water, and then dried over anhydrous magnesium sulfate and active carbon.
The residue was purified by column chromatography on alumina and concentrated under reduced
pressure to give the final product [MImEtTFE] [TFSI] (yield: 2.16 g, 68.3%) as a yellow liquid.
1H NMR (300 MHz, D-acetone): δ (ppm) 9.05 (s, 1H), 7.85 (d, J=1.2 Hz, 1H), 7.72 (d,
J=1.2 Hz, 1H), 4.60 (t, J=4.5 Hz, 2H), 4.12 (m, 2H), 4.05 (t, J=6.9 Hz, 2H), 3.71 (s, 3H). 13C NMR
(300 MHz, D-acetone): δ (ppm) 138, 131, 122.8, 119.8 (q, J=159 Hz), 70.1, 67, 49.4, 37. Anal.
calcd. for C10H12F9N3O5S2: (%) C, 24.54; H, 2.47; N, 8.59; S, 13.11. Found by elemental analysis:
C, 24.43; H, 2.41; N, 8.48; S, 13.01.
2.3.2 Synthesis of FRTILs [MImEtTFP] [TFSI]. An analogous procedure was used to start with
TFPEOTs (7.12 g, 21.6 mmol), 1-methylimidazole (3.57 g, 41.9 mmol), and to give the desired
compound as a pale-yellow liquid [MImEtTFP] OTs (yield: 7.72 g, 86.8%). The corresponding
ionic liquid was prepared analogously from [MImEtTFP] OTs (2.46 g, 6.5 mmol) and LiTFSI
(2.04 g, 7.1 mmol) to give the desired compound as a pale-yellow liquid [MImEtTFP] [TFSI]
(yield: 2.16 g, 68.3%).
1H NMR (300 MHz, D-acetone): δ (ppm) 9.05 (s, 1H), 7.85 (d, J=1.2 Hz, 1H), 7.72 (d,
J=1.2 Hz, 1H), 6.27 (t, J=4.8 Hz, 1H), 4.57 (t, J=4.5 Hz, 2H), 4.11 (t, J=7.2 Hz, 2H), 3.98 (t, J=6.9
Hz, 2H), 3.71 (s, 3H). 13C NMR (300 MHz, D-acetone): δ (ppm) 138, 131, 119.8 (q, J=159 Hz),
70.1, 67, 49.4, 37. Anal. calcd. for C11H13F10N3O5S2: (%) C, 25.34; H, 2.51; N, 8.06; S, 12.30.
Found by elemental analysis: C, 25.28; H, 2.46; N, 8.16; S, 12.34.
17

18
2.3.3 Synthesis of FRTILs [MImEtTFE] [DCA]. [MImEtTFE] OTs (3.49 g, 9.2 mmol) was
dissolved in 15 ml methanol and passed through the column with 25 ml dried Dowex® 22 resin at
the dropping rate of 1 drop/second. 60mL methanol was added to wash the column. The fractions
were combined, and the solvent was removed. The desired compound [MImEtTFE] Cl (yield:
2.09 g, 93.1%) was obtained as a brown viscous liquid.
NaN(CN)2 (0.84 g, 9.4 mmol) was dissolved in 30ml methanol followed by addition of
[MImEtTFE] Cl (2.09 g, 8.6 mmol) dissolved in 5 ml methanol. The mixture was then purified
through forced precipitation of impurities using the method described in 2.2.4. The filtrate was
concentrated under reduced pressure to give the final product [MImEtTFE] [DCA] (yield： 1.66
g, 70.7%) as a brown liquid.
1H NMR (300 MHz, D2O): δ (ppm) 7.54 (s, 1H), 6.95 (d, J=1.2 Hz, 1H), 6.87 (d, J=1.2
Hz, 1H), 5.03 (t, J=4.5 Hz, 2H), 3.93 (m, 2H), 3.80 (t, J=6.9 Hz, 2H), 3.61 (s, 3H). 13C NMR (300
MHz, D2O): δ (ppm) 138, 131, 122.8, 121.5, 114.44, 70.0,49.1, 36.1, 33.2. Anal. calcd. for
C10H12F3N5O: (%) C, 43.64; H, 4.39; N, 25.45. Found by elemental analysis: C, 43.68; H, 4.42;
N, 25.33.
2.3.4 Synthesis of FRTILs [MImEtTFP] [DCA]. An analogous procedure was used starting with
[MImEtTFP] OTs (3.61 g, 8.8 mmol), and yielding the desired compound as a brown liquid
[MImEtTFP] Cl (yield: 2.23 g, 92%). The corresponding ionic liquid was prepared analogously
from [MImEtTFP] Cl (2.23, 8.1 mmol) and NaN(CN)2 (0.79 g, 8.9 mmol) to give the desired
compound as a brown liquid [MImEtTFP] [DCA] (yield: 1.73 g, 70.1%).
1H NMR (300 MHz, D2O): δ (ppm) 7.54 (s, 1H), 7.05 (d, J=1.2 Hz, 1H), 6.95 (d, J=1.2
Hz, 1H), 6.12 (t, J=4.8 Hz, 1H), 4.68 (t, J=4.5 Hz, 2H), 4.4 (t, J=7.2 Hz, 2H), 3.97 (t, J=6.9 Hz,
18

19
13
2H), 3.86 (s, 3H). C NMR (300 MHz, D2O): δ (ppm) 137, 124.7, 123.6, 122.7, 119.7, 109.5,
70.0, 67.3, 48.8, 35.8. Anal. calcd. for C11H13F4N5O: (%) C, 43.00; H, 4.26; N, 22.79. Found by
elemental analysis: C, 43.09; H, 4.16; N, 22.70.
2.4
General procedures for the preparation of DES-based FRTILs
Scheme 5. Synthetic Routes of DES-based FRTILs
2.4.1 Synthesis of FRTILs [DESEtTFE] [TFSI]. As shown in scheme 5, TFEEOTs (11.8 g, 39.6
mmol) and diethyl sulfide (7.14 g, 83.9 mmol) were mixed and refluxed at 70 °C under stirring for
3 days. After cooling to room temperature, the residue was washed three times with 15 ml ethyl
ether. The remaining volatiles were removed under high vacuum to give the desired compound
[DESEtTFE] OTs (yield: 4.8 g, 32.6%) as a pale-yellow liquid.
[DESEtTFE] OTs (0.83g, 2.2 mmol) with LiTFSI (0.85 g, 3 mmol) were dissolved in 15
ml deionized water. The solution was stirred overnight at room temperature, extracted the solution
with 30 ml dichloromethane. The obtained organic layer was washed three times with 10 ml of
19

20
water and then dried over anhydrous magnesium sulfate and active carbon. The residue was
purified by column chromatography on alumina and concentrated under reduced pressure to give
the final product [DESEtTFE] [TFSI] (yield: 0.57 g, 51.3%) as a pale-yellow liquid.
1H NMR (300 MHz, D-acetone): δ (ppm) 4.65 (m, 2H), 4.25 (t, J=5.1 Hz, 2H), 3.85 (t,
J=5.3 Hz, 2H), 3.7-3.4 (m, 4H), 1.55 (t, J=6.6 Hz, 6H). 13C NMR (300 MHz, D-acetone): δ (ppm)
129.5, 119.8 (q, J=159 Hz), 69.7, 66.4, 33.8, 30.3, 8.0. Anal. calcd. for C10H16F9NO5S3: (%) C,
24.15; H, 3.24; N, 2.82; S, 19.34. Found by elemental analysis: C, 24.08; H, 3.20; N, 2.73; S,
19.25.
2.4.2 Synthesis of FRTILs [DESEtTFP] [TFSI]. An analogous procedure was used to start with
TFPEOTs (12.59 g, 38.1 mmol), diethyl sulfide (7.2 g, 84.6 mmol), and to give the desired
compound as a yellow liquid [DESEtTFP] OTs (yield: 4.48 g, 28%). The corresponding ionic
liquid was prepared analogously from [DESEtTFP] OTs (2.61 g, 6.2 mmol) and LiTFSI (2.14 g,
7.4 mmol) to give the desired compound as a yellow liquid [DESEtTFP] [TFSI] (yield: 2.15 g,
65.5%).
1H NMR (300 MHz, D-acetone): δ (ppm) 6.3 (t, J=1.2 Hz, 1H), 4.25 (t, J=4.8 Hz, 2H), 3.8
(t, J=5.1 Hz, 2H), 3.65 (t, J=5.3 Hz, 2H), 3.5-3.3 (m, 4H), 1.55 (t, J=6.6 Hz, 6H). 13C NMR (300
MHz, D-acetone): δ (ppm) 127.8, 125.4, 119.8 (q, J=159 Hz), 69.7, 66.3, 34.0, 32.5, 8.2. Anal.
calcd. for C11H17F10NO5S3: (%) C, 24.95; H, 3.24; N, 2.65; S, 18.17. Found by elemental analysis:
C, 24.98; H, 3.28; N, 2.55; S, 18.06.
2.4.3 Synthesis of FRTILs [DESEtTFE] [DCA]. [DESEtTFE] OTs (2.31 g, 6.2 mmol) was
dissolved in 15 ml methanol and passed through a column with 25 ml dried Dowex® 22 resin at a
dropping rate of 1 drop/second. 60mL methanol was added to wash the column. The fractions
20

21
were combined and the solvent was removed. The desired compound [DESEtTFE] Cl (yield:
1.15g, 73.4%) was obtained as a dark yellow liquid.
NaN(CN)2 (0.44 g, 5.0 mmol) was totally dissolved in 15ml methanol, [DESEtTFE] Cl
(1.15 g, 4.5 mmol) in another 5 ml methanol was added together. The mixture was then purified
through forced precipitation of impurities using the method described in 2.2.4. The filtrate was
then concentrated under reduced pressure to give the final product [DESEtTFE] [DCA] (yield：
0.77 g, 60.3%) as a dark yellow liquid.
1H NMR (300 MHz, D2O): δ (ppm) 3.89 (m, 2H), 3.6 (t, J=5.1 Hz, 2H), 3.35 (t, J=5.3 Hz,
2H), 3.24-3.18 (m, 4H), 1.35 (t, J=6.6 Hz, 6H). 13C NMR (300 MHz, D2O): δ (ppm) 129.5, 119.9,
69.7, 66.4, 33.8, 30.3, 8.07. Anal. calcd. for C10H16F3N3OS: (%) C, 42.39; H, 5.69; N, 14.83; S,
11.32. Found by elemental analysis: C, 42.27; H, 5.62; N, 14.87; S, 11.26.
2.4.4 Synthesis of DILs [DESEtTFP] [DCA]. An analogous procedure was used starting with
[DESEtTFP] OTs (2.01 g, 4.8 mmol), and yielding the desired compound as a yellow liquid
[DESEtTFP] Cl (yield: 0.96 g, 70.4%). The corresponding ionic liquid was prepared analogously
from [DESEtTFP] Cl (0.96 g, 3.4 mmol) and NaN(CN)2 (0.33 g, 3.7 mmol) to give the desired
compound as a yellow liquid [DESEtTFP] [DCA] (yield: 0.66 g, 62.1%).
1H NMR (300 MHz, D2O): δ (ppm) 6.1 (t, J=1.2 Hz, 1H), 4.03 (t, J=4.8 Hz, 2H), 3.6 (t,
13
J=5.1 Hz, 2H), 3.35 (t, J=5.3 Hz, 2H), 3.3-3.1 (m, 4H), 1.35 (t, J=6.6 Hz, 6H). C NMR (300
MHz, D2O): δ (ppm) 129.4, 125.4, 119.9, 69.7, 66.3, 38.3, 33.6, 7.9. Anal. calcd. for
C11H17F4N3OS: (%) C, 41.90; H, 5.43; N, 13.33; S, 10.17. Found by elemental analysis: C, 41.81;
H, 5.36; N, 13.28; S, 10.22.
21

22
3
Results and Discussion
Characterization
3.1
To best optimize the electrolyte composition for use in a practical Li rechargeable battery,
we should take into account the flammability, conductivity, and viscosity of the electrolyte
components[38][38][37][37][36][36][35][35][34][34][33][33][33][31][31][31].
physicochemical properties of the FRTILs are listed in Table 1.
All
the
Table 1. Physicochemical properties of the FRTILs
Ionic conductivity
Neat (mS/cm)
Molecular
weight
(g/mol)
Appearance
(Before
decolorization)
Density Viscosity
Compounds
(g/ml)
(cp)
25 °C
1.23
1.50
2.28
1.43
3.55
3.65
5.30
2.00
5.19
1.05
2.20
1.92
70 °C
9.24
9.95
10.04
7.90
21.4
20.9
16.29
9.91
21.6
6.4
[MPyEtTFE][DCA] 278.27
[MPyEtTFP][DCA] 310.29
[MPyEtTFE][TFSI] 492.38
[MPyEtTFP][TFSI] 524.4
[MImEtTFE][DCA] 274.34
[MImEtTFP][DCA] 306.26
[MImEtTFE][TFSI] 488.35
[MImEtTFP][TFSI] 520.36
[DESEtTFE][DCA] 283.31
[DESEtTFP][DCA] 315.33
[DESEtTFE][TFSI] 497.42
[DESEtTFP][TFSI] 529.44
1.33
1.35
1.55
1.59
1.30
1.35
1.59
1.63
1.21
1.32
1.30
1.41
60.6
brown liquid
114.9
78.94
98.71
17.32
105
dark brown liquid
brown liquid
dark brown liquid
brown liquid
brown liquid
49.82
83.59
27.59
67.7
yellow liquid
pale yellow liquid
dark yellow liquid
yellow liquid
pale yellow liquid
yellow liquid
7.31
6.18
38.95
44.67
The density is the most regularly measured and reported physical property of ILs because
nearly every application requires knowledge of the density [13]. Table 1 contains density values
of FRTILs, as in general, these FRTILs are denser than either organic solvents and water with
values ranging between (1 and 1.6) g cm−3. The density properties follow quite consistent trends.
For a given cation, the density increases as the molecular weight of the anion increases, just as
FRTILs containing TFSI anions have a higher density than those containing DCA anions. For
22

23
instance, [MPyEtTFE][TFSI] (1.55 g cm−1 at 25°C) has higher density than [MPyEtTFE][DCA]
(1.33 g cm−1 at 25 °C), [MImEtTFE][TFSI] (1.59 g cm−1 at 25°C) has higher density than
[MImEtTFE][DCA] (1.30 g cm−1 at 25°C), [DESEtTFE][TFSI] (1.30 g cm−1 at 25°C) has higher
density than [DESEtTFE][DCA] (1.21 g cm−1 at 25°C). Since none of the anions have a
particularly long or awkward chain, it is possible that they all can occupy favorable, relatively
close-approach positions around the cation, resulting in the high densities [39]. On the other hand,
MIm-based FRTILs have a higher density than those of MPy-based FRTILs, and DES-based
FRTILs have the lowest density [40].
3.2
Flammability test
A battery might be placed in more hazardous conditions, for instance in the presence of
sparks or fire. In order to investigate the flammable behavior of the mixed electrolytes containing
FRTILs and [MImEt][DCA], we carried out a flammability test called self-extinguishing time
(SET) [41]. Each electrolyte was tested three times: the burner was switched on above the sample
for 5 s and then switched off. The ignition time after flame setting and the self-extinguish time
after removing the burner flame were measured as an indicator of the non-flammability index of
the sample [42] and the time it took for the flame to extinguish was normalized against liquid mass
to give the SET in second/gram [34]. The electrolyte was judged to be non-flammable, if the
electrolyte never ignited during the testing, or if the ignition of electrolyte ceased when the flame
was removed [38].
23

24
Figure 1. Development of the flames for different FRTILs based electrolytes during SET test
directly at the burner was turned on (at start) and was turned off (after 1s), respectively (see
Table 3 for the sample/formulation number).
It is clearly observed in Figure 1 that all the three series of electrolytes with various FRTILs
may be certified as non-flammable, since they cannot be ignited, meaning their SET is measured
24

25
to be 0 s. We also confirmed that neat FRTILs were completely non-flammable in nature, and
contribute to the high safety of the electrolyte system.
3.3
Solubility test
The synthesized FRTILs were tested for their solubility behavior both in polar and non-
polar solvents (shown in Table 5.2). FRTILs derived from [DCA] anions exhibited good solubility
in polar solvents, such as dimethyl sulfoxide (DMSO), acetonitrile (MeCN), methanol (MeOH),
water (H2O), acetone and tetrahydrofuran(THF). Simultaneously, all of them were partially
miscible in DCM and ethyl acetate (EtOAc), and insoluble in nonpolar solvents, such as hexane,
and diethyl ether. On the other hand, FRTILs derived from [TFSI] anion displayed different
solubility properties. They are very soluble in DCM and EtOAc, but not soluble in H2O [43]. So,
FRTILs cation paired with [DCA] increases the solubility of the ionic liquid in water, while
replacement with the [TFSI] anion dramatically decreases their water solubility. Moreover, the
hydrophobic [TFSI] anions based FRTILs possess some special properties: 1) inertness to organic
solvents and oxidizing agents; 2) resistance to corrosive acids and bases; and 3) resistance to
extremes of temperature and pressure [27].
25

26
Table 2. Comparison of solubility of FRTILs in various organic solvents
Miscibility
Compounds
DMSO MeCN MeOH H2O Et2O DCM EtOAc Acetone THF Hexane
[MImEt][DCA]
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
p
p
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
i
i
i
i
i
i
i
i
i
i
i
i
i
i
[MImEt][TFSI]
m
p
m
p
[MPyEtTFE][DCA]
[MPyEtTFP][DCA]
[MPyEtTFE][TFSI]
[MPyEtTFP][TFSI]
m
m
i
p
p
m
m
p
m
m
p
i
[MImEtTFE][DCA] m
m
m
i
[MImEtTFP][DCA]
[MImEtTFE][TFSI]
[MImEtTFP][TFSI]
[DESEtTFE][DCA]
[DESEtTFP][DCA]
[DESEtTFE][TFSI]
[DESEtTFP][TFSI]
m
m
m
m
m
m
m
p
p
m
m
p
m
m
p
i
m
m
i
p
p
m
m
m
m
i
i, immiscible; m, miscible; p, partly miscible
3.4
Ionic Conductivities.
All measurements were conducted in an environment controlled chamber from 25 °C to
70 °C using a two-electrode cell and a Gamry potentiostat. The conductivity values were then
calibrated to a standard 0.1 M KCl aqueous solution with conductivity of 13.14 mS cm−1 at 25.0 °C
[44].
As we know, the conductivity of neat ILs would decrease significantly upon mixing with
a lithium salt [23]. So, the lithium batteries fabricated using neat FRTILs as the electrolyte may
exhibit a more disappointing ionic conductivity upon addition of a lithium salt, which can be
ascribed to the following two reasons: (1) the increased viscosity at low temperature; (2) the largely
decreased lithium-ion diffusion rate across electrolyte system at low temperature. To overcome
26

27
this limitation of FRTILs, they must be blended with another co-solvent with low viscosity and
high electrochemical stability. Consequently, the ternary electrolyte systems of 0.5 M LiTFSI in a
mixture of FRTILs/[MImEt] [DCA] (the FRTILs: [MImEt] [DCA] weight ratio used was 1:1),
were developed. [MImEt][DCA] with a pretty low viscosity (14.82 cp at 25 °C) and high ionic
conductivity (27 mS cm-1 at 25 °C) acts as a thinner solvent for these FRTILs. The addition of
[MImEt][DCA] was able to enhance the mobility of the dissociated ions by decreasing the
viscosity of the mixture, and hence, increase the conductivity of the whole system, as compared
with the single solvent RTIL electrolyte [45].
The ternary mixtures were prepared by mixing the proper quantity of LiTFSI with
FRTILs/[MImEt][DCA], which were continuously stirred until we obtained homogeneous
samples (e.g., full dissolution of LiTFSI in ILs) [46]. The ternary mixtures of 0.5 M LiTFSI in
FRTILs/[MImEt] [DCA] were prepared as electrolytes, and their formulations were listed in Table
3. The ionic conductivity data of neat FRTILs are compared in Figures 2,4 and 6, and the ionic
conductivity data of LiTFSI containing FRTILs are presented in Figures 3,5 and 7.
27

28
Table 3. Formulations of the electrolytes with 0.5M LiTFSI in FRTILs/[MImEt][DCA]
Ionic conductivity
Components weight (g)
FRTILs [MImEt][DCA] LiTFSI
Neat FRTIL
abbreviation
(mS/cm)
25 °C
9.95
8.32
9.74
8.05
9.1
Compound
70 °C
29.6
23.9
24.6
25.3
30.8
28
Ea1
Ea2
Ea3
Ea4
Eb1
Eb2
Eb3
Eb4
Ec1
Ec2
Ec3
Ec4
[MPyEtTFE][DCA] 0.1002
[MPyEtTFP][DCA] 0.1016
[MPyEtTFE][TFSI] 0.0999
[MPyEtTFP][TFSI] 0.1005
[MImEtTFE][DCA] 0.1014
[MImEtTFP][DCA] 0.1005
[MImEtTFE][TFSI] 0.1007
[MImEtTFP][TFSI] 0.1033
[DESEtTFE][DCA] 0.1012
[DESEtTFP][DCA] 0.1015
[DESEtTFE][TFSI] 0.1008
[DESEtTFP][TFSI] 0.1015
0.1007
0.1003
0.1001
0.1
0.0233
0.0232
0.0216
0.0214
0.0236
0.0231
0.0215
0.0215
0.0245
0.0234
0.0227
0.0227
0.1007
0.1006
0.1005
0.1003
0.101
8.29
10.51
8.7
29.3
23.2
25
9.13
8.31
7.92
7.44
0.1007
0.1006
0.1002
30.4
22.8
21.7
Figure 2. Ionic Conductivity versus Temperature of neat MPy-based FRTILs.
28

29
Figure 3. Ionic Conductivity versus Temperature of MPy-based electrolytes.
Figure 4. Ionic Conductivity versus Temperature of neat MIm-based FRTILs.
29