Synthesis and electrochemical property of sulfone-functionalized imidazolium ionic liquid electrolytes

Article abstract of DOI:10.1016/j.electacta.2013.01.040

Sulfone-functionalized imidazolium ionic liquids were synthesized from direct nucleophilic substitution for the first time. Detailed NMR analysis of the products revealed the competition pathways of classic S_N2 substitution and E2 elimination i

Full text of DOI:10.1016/j.electacta.2013.01.040

Electrochimica Acta 92 (2013) 392–396
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical property of sulfone-functionalized imidazolium
ionic liquid electrolytes
Wei Wenga, Zhengcheng Zhanga,∗, John A. Schlueterb, Khalil Aminea
a Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, IL 60439, United States
b Material Sciences Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, IL 60439, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Sulfone-functionalized imidazolium ionic liquids were synthesized from direct nucleophilic substitu-
tion for the first time. Detailed NMR analysis of the products revealed the competition pathways of
classic SN2 substitution and E2 elimination in the synthesis reaction. Impurities from E2 elimination
can easily be overlooked during the conventional method of ionic liquid preparation via SN2 substitu-
tion. Initial electrochemical examination of the synthesized ionic liquids shows good compatibility with
Received 18 December 2012
Received in revised form 9 January 2013
Accepted 9 January 2013
Available online 18 January 2013
Li1.1(Ni1/3Co1/3Mn1/3)0.9O2 cathode material.
Keywords:
Published by Elsevier Ltd.
Sulfone-functionalization
Ionic liquid
Electrolyte
Lithium ion battery
1. Introduction
IL electrolyte is the key factor that determines the electrochem-
ical performance of the batteries. One obvious way to improve
the interfacial properties is by adding solid-electrolyte-interphase
(SEI) forming additives (i.e. vinylidene carbonate). Besides that,
the introduction of various functional groups such as CN, OR,
CO2R has also been proved to be an effective method to
improve the electrochemical properties of lithium-ion batteries
(LIBs). In our previous studies [7], a disiloxane-functionalized
phosphonium based ionic liquid displayed exceptional stabil-
ity with lithium transition metal oxide cathode and graphite
anode.
In continuation of our previous work, this communication seeks
to study sulfone functionalized ILs as potential electrolyte for LIBs.
Ionic liquids with tethered sulfone groups are known. However,
only one study investigated the synthesis of sulfone tethered ionic
liquid via rather complicated sequences of reaction comprising
the oxidation of the corresponding thioether tethered cation using
strong oxidizing agent – m-chloroperoxybenzoic acid (mCPBA) [8].
Interestingly, the rather “remote” sulfone group was shown to have
important effects on IL solvent properties (i.e. polarity, H-bonding,
etc.). However, sulfone tethered ILs has not been evaluated as
lithium-ion electrolytes even though it possesses enhanced safety
characteristics. Herein, we reported the synthesis of new sulfone-
functionalized ionic liquids via a simplified synthetic route which
employs conventional nucleophilic displacement reaction [3]. The
competing pathways in the formation of sulfone tethered ILs has
been investigated and identified. Electrochemical property of the
synthesized ILs was examined as new lithium-ion battery elec-
trolytes.
Sulfur containing solvents such as dimethyl sulfoxide (DMSO)
and sulfolane have been widely used in various applications due
to their good solvation ability for a wide range of inorganic and
organic substrates [1]. The resonance structures of the S O dou-
ble bond afford dipolar character with negative charge centered
on the oxygen atom (Fig. 1). The high polarity and high dielectric
constant (DMSO: 46.7; sulfolane: 44) of the solvents make them
promising candidates for the dissociation of electrolyte salts. For
example, ethyl methyl sulfone (EMS) based electrolytes display a
better anodic stability (>5.5 V vs. Li+/Li) than the conventional car-
bonate solvents [2]. Good cycling performance can be achieved for
5 V LiNi0.5Mn1.5O4 cathode materials using a sulfone/linear carbon-
ate binary electrolyte [3]. More recently, DMSO has been studied as
an electrolyte solvent for Li-O2 batteries due to its superior stability
against superoxides [4,5].
Ionic liquids (ILs) represent an important family of solvents as
safe replacements for the carbonate solvents [6]. ILs have negligible
vapor pressure and good thermal stability. They are non-flammable
and can be easily recycled. Numerous studies have been conducted
on room temperature ILs based on quaternary ammonium or phos-
−
phonium cations and weakly-coordinating anions such as PF6
,
BF4−, and TFSI− [TFSI− = bis(trifluoromethanesulfonyl)imide]. It
has also been shown that the interface between electrode and
∗
Corresponding author. Tel.: +1 630 252 7868; fax: +1 630 972 4440.
E-mail address: zzhang@anl.gov (Z. Zhang).
0013-4686/$ – see front matter. Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.electacta.2013.01.040

W. Weng et al. / Electrochimica Acta 92 (2013) 392–396
393
Table 1
Crystal data and structure refinement of complex 1a.
Formula
MW
Cryst. syst.
T (K)
C7H13BrN2O2S
269.16
Triclinic
a (Å)
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
Z
6.7476(2)
7.5071(2)
11.0657(3)
101.982(1)
92.5430(1)
2
1.722
0.0262, 0.0751
0.0267, 0.0774
150(2)
0.71073 A
˚
Wavelength
Space group
Fig. 1. Resonance structures of DMSO (left) and representative solvents containing
O groups (right).
P − 1
4.133
272
S
ꢀ (mm−1
F(0 0 0)
GoF
)
ꢁ (g cm−3
)
R1, wR2 [I > 2␴(I)]
R1, wR2 (all data)
1.193
R1 = ꢂ ||Fo| − |Fc||/ꢂ|Fo|; wR2 = [ꢂw(Fo − Fc2)2/ꢂw(Fo2)2]1/2
;
2
2. Experimental
GoF = [ꢂw(Fo − Fc2)2/(Nobs − Nvar)]1/2
.
2
2.1. Chemical and materials
2.2.3. Synthesis of [ImC2HSO2CH3][TFSI] (2a)
All manipulations were carried out under a nitrogen atmo-
sphere, using standard Schlenk techniques and dried solvents. All
reagents were purchased from Aldrich and used as received. 1H and
13C NMR experiments were performed on a Bruker model DMX 500
NMR spectrometer (11.7 T).
To a solution of [ImC2HSO2CH3]Br (12.36 g, 0.046 mol) in
distilled water (30 mL) was added lithium bis(trifluorometha-
nesulfonyl)imide (13.2 g, 0.046 mol). The reaction mixture was
stirred for 24 h at ambient temperature. Distilled water (100 mL)
was added to the mixture and the bottom layer was extracted
with dichloromethane. The organic layer separated was washed
with fresh distilled water, dried over MgSO4. The crude solution
was stirred with decolorizing charcoal and filtered through a plug
of celite and then a plug of activated alumina. The solution was
concentrated by removing the solvent under reduced pressure and
dried at 100 ◦C under vacuum. Yield: 12 g (62%). 1H NMR (500 MHz,
CD3CN): ı 8.49 (s, 1H), 7.45 (br, s, 1H), 7.34 (br, s, 1H), 4.59 (t, J = 7 Hz,
2H, N CH2), 3.82 (s, 3H, N CH3), 3.57 (t, J = 7 Hz, 2H, CH2SO2), 2.95
(s, 3H, SO2CH3). 13C NMR (125 MHz, CD3CN): ı 136.7 (s), 123.8 (s),
2.2. Ionic liquid synthesis
2.2.1. Synthesis of 2-Bromoethyl methyl sulfone [9]
PBr3(20 g, 0.074 mol) was added slowly to 2-methylsulfonyl
ethanol (25 g, 0.2 mol) with vigorous stirring under Ar atmosphere.
The reaction mixture was stirred at room temperature for 4 days
to afford a dark-blue solution. Millipore water (30 mL) was care-
fully added to quench the reaction. The aqueous solution was
stored frozen for 1 h and crystalline compounds were formed from
slow thawing at ambient temperature. Yield: 16 g (43%). 1H NMR
(500 MHz, CDCl3): ı 3.70 (t, J = 7 Hz, 2H, BrCH2), 3.54 (t, J = 7 Hz, 2H,
CH2SO2), 3.00 (s, 3H, SO2CH3). 13C NMR (125 MHz, CD3Cl): ı 56.9
(s, BrCH2), 41.9 (s, CH2SO2), 21.1 (s, SO2CH3).
1
122.7 (s), 119.7 (q, JC–F = 319 Hz, TFSI), 52.6 (s), 42.8 (s), 41.1 (s),
36.0 (s).
2.2.4. Attempted synthesis of [ImC2MeSO2CH3]Br (1b)
A mixture of 1,2-dimethylimidazole (7.2 g, 0.075 mol) and 2-
bromoethyl methyl sulfone (14 g, 0.075 mol) was charged into a
round-bottom flask equipped with a condenser. The mixture was
stirred at 60 ◦C for 24 h. The resulting yellow and viscous oil was
analyzed by 1H NMR, which showed that 2a constitute ca. <20% of
the resulting mixture. 1H NMR (500 MHz, CDCl3): ı 7.95 (d, J = 2 Hz,
1H), 7.46 (d, J = 2 Hz, 1H), 4.77 (t, J = 6 Hz, 2H, N CH2), 3.92 (t, J = 6 Hz,
2.2.2. Synthesis of 1-methyl-3-(2-(methylsulfonyl)ethyl)-
1H-imidazol-3-ium bromide, [ImC2HSO2CH3]Br (1a)
A
mixture of 1-methylimidazole (2 g, 0.024 mol) and 2-
2H, CH2SO2). Identification and assignment of the full set of the 1
NMR resonance of 1b was not possible because of the presence of
other compounds.
H
bromoethyl methyl sulfone (4.62 g, 0.024 mol) was charged into a
round-bottom flask equipped with a condenser. The mixture was
stirred at 60 ◦C for 24 h. The resulting yellow and viscous oil was
washed with CH3CN and then dried under vacuum. Yield: 4.54 g
(83%). 1H NMR (500 MHz, d6-DMSO): ı 9.22 (s, 1H), 7.84 (t, J = 2 Hz,
1H), 7.72 (t, J = 2 Hz, 1H), 4.66 (t, J = 7 Hz, 2H, N CH2), 3.85 (s, 3H,
2.3. Instrumentation and Procedures
N
CH3), 3.82 (t, J = 7 Hz, 2H, CH2SO2), 3.01 (s, 3H, SO2CH3). 13C NMR
2.3.1. Electrochemical testing
The charge–discharge cycling performance was tested on a
Maccor Electrochemical Analyzer using 2032 coin cells with
(125 MHz, d6-DMSO): ı 137.3 (s), 123.6 (s), 122.7 (s), 52.5 (s), 42.6
(s), 41.0 (s), 35.9 (s).
Fig. 2. Reaction scheme for the synthesis of sulfone-functionalized ionic liquid.

394
W. Weng et al. / Electrochimica Acta 92 (2013) 392–396
Table 2
Selected bond lengths (Å) and bond angles (◦) for 1a.
S1 O1
S1 O2
S1 C6
S1 C7
1.4506(12)
1.4464(11)
1.7820(15)
1.7504(15)
1.462(2)
N1 C4
N2 C3
N2 C4
N2 C5
1.3294(18)
1.3821(19)
1.3339(19)
1.4690(18)
1.358(2)
N1 C1
C2 C3
N1 C2
1.383(2)
C5 C6
1.527(2)
O1 S1 O2
O1 S1 C6
O1 S1 C7
O2 S1 C6
O2 S1 C7
C6 S1 C7
C1 N1 C2
C1 N1 C4
C2 N1 C4
116.22(7)
C3 N2 C4
C3 N2 C5
C4 N2 C5
N1 C2 C3
N2 C3 C2
N1 C4 N2
N2 C5 C6
S1 C6 C5
108.94(12)
125.48(13)
125.52(12)
106.94(13)
106.79(13)
108.40(13)
113.49(12)
112.16(10)
108.48(7)
109.56(7)
109.09(7)
109.64(8)
103.03(7)
125.35(12)
125.52(13)
108.92(12)
1a
Fig. 4. 1H NMR spectrum of a typical reaction mixture of 1a, 3a and 4 in the region
of ı 9.0–3.0.
˚
Mok␣ radiation (ꢃ = 0.71073 A) with a frame exposure time of 30 s
and a detector distance of 5.00 cm. Five hemispheres were col-
lected with 0.30◦ ␻ scans. The raw intensity data were corrected
for absorption (SADABS) [10]. The structure was solved and refined
by using SHELXTL [11]. A direct-method calculation provided most
of atomic positions from the electron density map. Full-matrix
least squares/difference Fourier cycles were performed to locate
the remaining atoms. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were
placed in ideal positions and refined as “riding atoms” with rela-
tive isotropic displacement parameters. Structural and refinement
parameters are provided in Tables 1 and 2.
1b
4000
3000
2000
1000
Wavenumber (cm-1)
3. Results and discussion
Fig. 3. FT-IR spectra of 1a and 1b.
Li1.1[Ni1/3Co1/3Mn1/3
]
0.9O2(NCM) as the cathode, lithium metal as
Sulfone-functionalized imidazolium ionic liquids are synthe-
sized by quarternization of the imidazole ring with appropriate
halides followed by anion metathesis. Detailed analysis of the 1H
NMR spectrum of the reaction of 2-bromoethyl methyl sulfone [9]
with 1-methylimidazole as illustrated in Fig. 2 indeed revealed the
formation of the desired product 1a. Workup of the reaction mix-
ture and subsequent wash of the precipitate with diethyl ether
afford the pure 1a in a ∼80% yield. The CH2 CH2 linker between the
imidazole ring and sulfone group gave rise to a pair of triplet peak at
the anode, and a Celgard 3501 separator. The effective electrode
area was 1.6 cm2. The positive electrode was made of 84 wt% NCM,
8 wt% TB5500 carbon black, and 8 wt% polyvinylidene fluoride
(PVDF, Kureha 7208). Performance data were acquired and ana-
lyzed by the software associated with the instrument. FTIR spectra
were recorded on a Perkin-Elmer spectrum 100 instrument with
an attenuated total reflection (ATR) sampling accessory.
2.3.2. Crystal structure determination of 1a by single-crystal
X-ray diffraction
ı 4.66 ppm (N CH2) and 3.82 ppm (S CH2) respectively in the 1
H
NMR spectrum of 1a, indicating a significantly downfield shifting
from 2-bromoethyl methyl sulfone (ı 3.70 ppm and ı 3.54 ppm).
The chemical structure of 1a and 1b are characterized and con-
firmed by IR spectroscopy. FTIR spectra of 1a and 1b are shown in
Fig. 3. The IR peaks at 1316 and 1124 cm−1 in the IR spectrum of 1a
belong to the ␯asymSO2 and ␯symSO2 stretching bond respectively.
A colorless crystal of 1a was mounted onto the tip of a with
glass fiber with epoxy and placed on a Bruker APEX II 3-circle
diffractometer equipped with an APEX II detector. The crystal tem-
perature was maintained at 150(2) K through use of an Oxford
Cryostream 700 Plus LT device. The analysis was carried out using
Fig. 5. Competing SN2 reaction and E2 elimination in the synthetic process of precursor 1a and 1b.

W. Weng et al. / Electrochimica Acta 92 (2013) 392–396
395
Fig. 6. (a) Thermal ellipsoid plot and atomic numbering scheme for the crystal structure of 1a. Ellipsoids are drawn at 50% probability, with hydrogen atoms depicted as
spheres of arbitrary radius; (b) the ImC2HSO2CH3+ cations pack as centrosymmetric dimmers with CH–␲ interactions in the solid state. Carbon (black), nitrogen (blue), sulfur
(yellow), oxygen (red), and hydrogen (salmon) atoms are depicted as spheres of arbitrary radius. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of the article.)
4.5
4.0
3.5
3.0
500
400
300
200
100
0
140
120
100
80
(a)
(b)
60
40
Cycle2
Cycle 15
Cycle 25
20
0
0
5
10
15
20
25
0
20 40 60 80 100 120 140 160 180
Capacity (mAh/g)
Cycle Number
Fig. 7. (a) Charge/discharge voltage profiles of Li/NMC cell with 0.5 M LiTFSI/1b electrolyte; (b) capacity retention and the coloumbic efficiency of Li/NMC cells 0.5 M LiTFSI/1b
(C/20, testing temperature: 55 ◦C).
The frequency values agrees well with that observed for other
aliphatic sulfones (i.e. propyl sulfone: 1314 and 1132 cm−1). Imid-
azolium cations can be identified by the spectra features at
1174 cm−1 (imidazole C C, C N stretching), 1048 cm−1 (in-plane
antisymmetric C N stretching of N CH3). The FTIR spectrum of 1b
is more complicated due to the presence of sulfone group in TFSI
anions. The resonance peaks at 1360–1300 cm−1 can be ascribed to
1a crystallizes in triclinic space group P-1 with the asym-
metric unit containing one ImC2HSO2CH3 cation and bromide
+
anion. Crystal data and structure refinement of complex 1a is
summarized in Table 1. The molecular structure of 1a is illus-
trated in Fig. 6(a), and selected bond distance and angles are
provided in Table 2. While this is the first crystal structure
+
reported of the ImC2HSO2CH3 cation, the geometry of the
␯
asymSO2in the immidazolium cation while the resonance peak at
1,3-dimethylimidazolium moiety is similar to that previously
reported [12–14]. As illustrated in Fig. 6(b), in the solid state, the
ImC2HSO2CH3+ cations pack as centrosymmetric dimers stabilized
1283 cm−1 is associated with ␯asymSO2 from the TFSI anions.
The quarternization process of 2-bromoethyl methyl sulfone
with 1-methylimidazole generated byproducts and their struc-
tures were identified by the NMR spectroscopy. As shown in Fig. 4,
1H NMR resonance peaks in the vinyl proton region appear as a
doublet of doublet at ı 6.95 (J = 17 Hz and 10 Hz) and two dou-
blets at ı 6.3–6.1. This ABC pattern in the 1H NMR spectra is
characteristic of the monosubstituted terminal alkenes 4. 1H NMR
resonance arising from the imidaole ring region was identified
as three singlets with upfield shift from the corresponding pro-
tons of 1a. Based on the detailed NMR analysis, we proposed the
competitive pathways between classic SN2 substitution and E2
elimination during the synthesis of 1a and the reaction mechanism
is shown in Fig. 5. Apparently, the parent imidazole can act both
as a nucleophile and a base. Both SN2 mechanism and E2 mecha-
nism result in the release of Br−, which is a good leaving group. The
competition is kinetic in nature and can be affected by the sub-
strate. For 1-methylimidazole, the ratio of 1a/3a is 3/1. However
1,2-dimethylimidazole favors the elimination reaction. The ratio of
the resulting 1b/3b is about 1/8, which makes the isolation of pure
1b not feasible. This may be attributed by the stronger basicity and
increased steric bulkiness of 1,2-dimethylimidazole. The SN2/E2
competition pathways generally have been ignored in the synthe-
sis of ILs. There is a chance that the elimination product could be
carried over to the subsequent steps existing as trace of impurities
in the final product of ILs if no special cautions have been taken to
remove it.
˚
by a weak C H. . .␲ hydrogen bond (2.69 A between the ring cen-
troid and the hydrogen atom) between a methyl hydrogen atom
and the imidazolium ring ␲-system along the (0 1 0) direction. The
aromatic ring centroid separation is 4.6202(9) Å with the perpen-
˚
dicular separation between these rings being 3.3461(6) A and a
˚
slippage of 3.186 A.
The anion metathesis of isolated 1a with LiTFSI afforded the
final product of sulfone-substituted imidazolium ILs 2a. After
LiTFSI salt dissolved in 2a, an IL electrolyte containing 0.5 M
LiTFSI was examined as a potential electrolyte material for Li-
ion battery. A typical slope-like voltage profiles were observed
(Fig. 7(a)) in Li/Li1.1(Ni1/3Co1/3Mn1/3)0.9O2 (NCM) cells employing
0.5 M LiTFSI/2a as electrolyte. Fig. 7(b) is the representative capac-
ity retention profile of Li/NCM cell using the same electrolyte. The
initial specific charge and discharge capacities are about 175 and
165 mAh/g, respectively, which are close to the literature value.
The initial coulombic efficiency is low (∼94%) but maintains at
>98% in the subsequent cycles with a specific discharge capacity
of 145 mAh/g.
4. Conclusion
In conclusion, we investigated the first synthesis of sulfone-
functionalized ionic liquid from the direct quaternization reaction
(SN2) of imidazole and alkyl bromide. The competition pathways

396
W. Weng et al. / Electrochimica Acta 92 (2013) 392–396
between SN2 nucleophilic substitution and E2 elimination during
the synthesis of ionic liquid showed a substrate-dependent selec-
tivity. 1,2-Dimethylimidazole shows reversed preference of SN2
reaction as observed for 1-methylimidazole. Such side reactions
probably are less pronounced if simple alkyl halides are used as the
substrates. Our study thus magnifies the “side effect” of the most
common synthetic route for ILs and emphasizes the importance
of extra purification process to remove possible impurities. The
initial electrochemical examination in Li battery demonstrated
that sulfone-functionalized ionic liquid has good compatibility
with NCM cathode material.
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This work was supported by the Center for Electrical Energy
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funded by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences. Work in the Materials Science Division
was supported by UChicago Argonne, LLC, Operator of Argonne
National Laboratory (“Argonne”). Argonne, a U.S. Department of
Energy Office of Science laboratory, is operated under Contract No.
DE-AC02-06CH11357.
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