Preparation of a ROMP-type imidazolium-functionalized norbornene ionic liquid block copolymer and the electrochemical property for lithium-ion batteries polyelectrolyte membranes

Article abstract of DOI:10.1039/c5ra04860e

Imidazolium-functionalized norbornene ionic liquid block copolymers are synthesized via ring-opening metathesis polymerization (ROMP). The solid polyelectrolytes were prepared by blending the copolymers with various contents of lithium bis(trifluoromethanesulphonyl)imide. The effects of the polymerized ionic liquid (PILs) compositions on the properties, morphology, and ionic conductivity of the polyelectrolytes are investigated. The polymer with 50.7 mol% PIL displays a long-range ordered lamellar structure, and its corresponding polyelectrolyte achieves the best ionic conductivity with 7.44 × 10^-6 S cm^-1 at 30 °C and 4.68 × 10^-4 S cm^-1 at 100 °C, respectively. The ionic conductivity is enhanced by 1 order of magnitude compared to the polyelectrolytes with lower polymerized ionic liquid block compositions. In addition, the polyelectrolytes are electrochemically stability up to 4.2 V versus Li/Li⁺, indicating their suitable electrochemical property for lithium-ion battery application.

Full text of DOI:10.1039/c5ra04860e

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PAPER
Preparation of a ROMP-type imidazolium-
functionalized norbornene ionic liquid block
copolymer and the electrochemical property for
lithium-ion batteries polyelectrolyte membranes†
Cite this: RSC Adv., 2015, 5, 43581
a
ab
Hongyu Zhua and Defu Chenc
*
Juan Wang, Xiaohui He,
Imidazolium-functionalized norbornene ionic liquid block copolymers are synthesized via ring-opening
metathesis polymerization (ROMP). The solid polyelectrolytes were prepared by blending the copolymers
with various contents of lithium bis(trifluoromethanesulphonyl)imide. The effects of the polymerized
ionic liquid (PILs) compositions on the properties, morphology, and ionic conductivity of the
polyelectrolytes are investigated. The polymer with 50.7 mol% PIL displays a long-range ordered lamellar
structure, and its corresponding polyelectrolyte achieves the best ionic conductivity with 7.44 ꢀ 10ꢁ6
S cmꢁ1 at 30 ꢂC and 4.68 ꢀ 10ꢁ4 S cmꢁ1 at 100 ꢂC, respectively. The ionic conductivity is enhanced by 1
order of magnitude compared to the polyelectrolytes with lower polymerized ionic liquid block
compositions. In addition, the polyelectrolytes are electrochemically stability up to 4.2 V versus Li/Li+,
indicating their suitable electrochemical property for lithium-ion battery application.
Received 19th March 2015
Accepted 7th May 2015
DOI: 10.1039/c5ra04860e
www.rsc.org/advances
have also been characterized. For example, Elabd et al.11,12
synthesized a series of PIL diblock copolymers, and the ionic
conductivity is 0.88 mS cmꢁ1 at 25 ꢂC. With the similar PIL
1. Introduction
Solid polymer electrolytes (SPEs) have been extensively studied
for several decades as an alternative for the traditional liquid
electrolyte in lithium-ion batteries due to their superior safety
and mechanical properties.1–4 However, the SPEs exhibit fairly
low ionic conductivity at room temperature, well below the 10ꢁ3
S cmꢁ1 minimum required for practical battery electrolytes,5
and thus limits their feasibility for commercial application. So
the key challenge is to improve the ionic conductivity of the
solid polymer electrolytes.
Polymerized ionic liquids (PILs) display the outstanding
performance of ionic liquids such as the unique nonvolatile,
non-ammability, large electrochemical window, high ionic
conductivity, and the good lm-forming of polymers,6–8 and
thus they have been considered as a new type of materials of
solid polymer electrolytes. Recently, various PIL electrolytes
have been synthesized, such as tetraalkylammonium-type
poly(IL)s,9,10 imidazolium-based poly(IL)s,11–16 and poly-
(pyrrolidinium),6,17 and the corresponding physical and chem-
ical properties, ionic conductivity, and microphase separation
compositions, the ionic conductivity of the block copolymers
increased 2 orders of magnitude compared to the random
copolymers, and the block copolymers lm can form strong
microphase-separated to further increase ionic conductivity.
Moreover, Yang18 synthesized a novel polymeric ionic liquid
owning the well thermal and electrochemical stability and high
lithium ion conductivity simultaneously. Thus, the PIL elec-
trolytes have been already applied to lithium ion battery.9,10,13,19
In this work, a series of imidazolium-functionalized nor-
bornene ionic liquid block copolymers were synthesized via
ring-opening metathesis polymerization (ROMP) (Scheme 1).
We expect that the block copolymers can achieve excellent lm-
forming properties from the polynorbornene backbone and
high ionic conductivity from a long-range ordered imidazolium
phase. The thermal properties, morphology and ionic conduc-
tivity of the block copolymer electrolytes are investigated as a
function of ionic liquid block copolymers and lithium salt
contents.
aSchool of Materials Science and Engineering, Nanchang University, 999 Xuefu Avenue,
Nanchang 330031, China. E-mail: hexiaohui@ncu.edu.cn
bJiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 2.1. Materials
2. Experimental section
999 Xuefu Avenue, Nanchang 330031, China
cSchool of Civil Engineering and Architecture, Nanchang University, 999 Xuefu Avenue,
6-Bromo-1-hexanol, 1-methylimidazole, ethyl vinyl ether, 5-
norbornene-2-carboxylic acid (NBCOOH, 98%, mixture of
endo and exo isomers), 4-dimethylaminopyridine (DMAP),
lithium bis(triuoromethanesulphonyl)imide (LiTFSI), Grubbs'
Nanchang 330031, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra04860e
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Scheme 1 Synthesis routes for monomers and copolymers.
catalyst rst generation (Cl2(PCy3)2Ru]CH-Ph), 1-(3-dimethyl-
aminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI) are
used as received from Energy Chemical, methyl-5-norbornene-
2-carboxylate (NBCOOCH3) is puried by distillation over
CaH2 at a reduced pressure under a dry nitrogen purge just
before used.
2.3. Synthesis of 5-norbornene-2-carboxylate-1-hexyl-3-
methyl-imidazolium bis((triuoromethyl)sulfonyl)amide
(NIm-TFSI) (2)
Compound 1 (6.5 g, 0.022 mol) and 1-methylimidazole (2.6 mL,
0.033 mol) are reuxing in acetonitrile for 18 h. Aer cooling to
room temperature, the reaction mixture is concentrated under
reduced pressure to afford yellow viscous oil and then washed
with Et2O (3 ꢀ 100 mL). The resulting oil is dissolved in H2O
(100 mL), LiTFSI (6.53 g, 0.023 mol) is added, and the mixture is
stirred at room temperature for 12 h. The yellow oil is extracted
with dichloromethane and the organic layer is washed with
water and dried over anhydrous MgSO4. Monomer 2 is obtained
as yellow oil (10.08 g, 80%). 1H NMR (400 MHz, CDCl3): d ¼ 8.77
(s, 1H), 7.30 (d, 2H), 6.17–5.88 (m, 2H), 4.17 (t, 2H), 4.05 (t, 2H),
3.98 (m, 1H), 3.94 (s, 3H), 3.16 (s, 1H), 2.99 (s, 1H), 2.85 (m, 1H),
2.13–2.22 (m, 2H), 1.92–1.79 (m, 2H), 1.68–1.21 (m, 7H).
2.2. Synthesis of 5-norbornene-2-carboxylate-6-
bromohexane (NBr) (1)
The synthesis of monomer is shown in Scheme 1(A). In a 100
mL round-bottomed ask equipped with a magnetic stir bar, the
EDCI (8.05 g, 0.042 mol) and DMAP (catalytic amount) are added
to the 30 mL CH2Cl2 solution of the dissolved 5-norbornene-2-
carboxylic acid (4.7 g, 0.034 mol) and 6-bromo-1-hexanol (5 g,
0.028 mol). Stirring the mixture at room temperature for 5 h,
then the reaction is quenched by adding water (30 mL). The
crude product is extracted three times with CH2Cl2 and the
organic layer is dried with anhydrous MgSO4. Filtered, and
concentrated by rotary evaporator to afford oil product. The
crude mixture is further puried by column chromatography on
silica gel by using hexanes/EtOAc (20 : 1) to yield a yellow oil (7.2
2.4. Synthesis of homopolymer of methyl-5-norbornene-2-
carboxylate (NB-COOCH3) (PMN) (4)
The synthesis of homopolymer is shown in Scheme 1(B). All
g, 85%). 1H NMR (400 MHz, CDCl3): d ¼ 6.15–5.90 (m, 2H), 4.05 polymerizations are performed under a nitrogen atmosphere,
(t, 2H), 3.50 (m, 1H), 3.36 (t, 2H), 3.16 (s, 1H), 2.99 (s, 1H), 2.85 tetrahydrofuran (THF) is distilled from sodium prior to using.
(m, 1H), 2.13–2.22 (m, 2H), 1.92–1.79 (m, 2H), 1.68–1.21 (m, 7H). Grubbs' catalyst rst generation [Cl2(PCy3)2Ru]CH-Ph]
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(67.0 mg, 0.08 mmol) is dissolved in 10 mL THF rst,
NBCOOCH3 (0.609 g, 4 mmol) is added to the solution by
syringe under stirring. The reaction mixture is stirred at 30 ꢂC,
aer 5 h, the reaction is terminated by ethyl vinyl ether
(equivalent) and continue to stir for 1 h. Then the polymer
solution is precipitated in hexanes, puried, and dried under
vacuum to get the brown solid.
2.5. Synthesis of ionic liquid block copolymers of methyl-5-
norbornene-2-carboxylate-block-5-norbornene-2-carboxylate-
1-hexyl-3-methyl-imidazolium bis((triuoromethyl)sulfonyl)-
amide (5–9)
The synthesis of ionic liquid block copolymers is shown in
Scheme 1(C) and the typical polymerization reaction is as
follows. Grubbs' catalyst (167.40 mg, 0.2 mmol) is dissolved in
10 mL THF, and then NBCOOCH3 (1.272 g, 8 mmol) is added to
the solution by syringe under stirring. The reaction mixture is
stirred at 30 ꢂC, and the NBCOOCH3 reacted completely aer 5
h, which detected by 1H NMR; the monomer 2 (1.167 g, 2 mmol)
is added and continue stirring for 24 h, the reaction is then
terminated by ethyl vinyl ether (equivalent) and continue stir-
ring 1 h. The polymer solution is precipitated in hexanes,20–22
puried, and dried under vacuum to get the brown solid (1.995
g, 84%). Similar chemical shis are observed for polymers 5–9.
Fig. 1 The photograph of electrolyte membrane.
determined by using a Waters 2410 gel permeation chromato-
graph (GPC) with a refractive index detector in tetrahydrofuran
(THF) and using a calibration curve of polystyrene standards.
Thermo gravimetric analysis (TGA) is performed on a TA-600 for
thermal analysis at a heating rate of 20 ꢂC minꢁ1 under nitrogen
with a sample weight of 4–10 mg. The differential scanning
calorimeter (DSC, Perkin-Elmer DSC 7) is measured to deter-
mine the glass transition temperatures (Tg) of copolymers at a
heating/cooling rate of 10 ꢂC minꢁ1. The X-ray diffraction (XRD)
study of the samples is carried out on a Bruker D8 Focus X-ray
diffractometer operating at 30 kV and 20 mA with a copper
2.6. Preparation of polyelectrolyte membranes
The polyelectrolyte membranes are prepared by separately dis-
solving the block copolymers and lithium salt (LiTFSI) in THF
for 12 h, and the mixed compositions are in four different
proportions (Table 1). Then the solutions are casted on PTFE
molds to prepare the polyelectrolyte lms. The lms are dried in
the nitrogen atmosphere at room temperature for 24 h, and
subsequently dried under vacuum for 48 h at 40 ꢂC. The
photograph of polyelectrolyte membrane is illustrated in Fig. 1.
The lms were about 150 mm thickness (Table 1).
target (l ¼ 1.54 A) and at a scanning rate of 1 minꢁ1. The
microphase separation studying is carried on JEOL JEM-2100F
transmission electron microscope (TEM). The samples are
coated on Cu grids and stained by exposing to the vapor of a 5
wt% aqueous solution of RuO4 for 2 h to enhance contrast
degree between phases before testing.23 The ionic conductivities
of polymer electrolyte membranes are performed on electro-
chemical impedance spectroscopy (EIS) over a frequency range
of 100–1 MHz with the amplitude of 10 mV at the temperature
range from 30 to 100 ꢂC. The ionic conductivity (s) is calculated
from the following equation (eqn (1)):
ꢂ
˚
2.7. Measurements
1H NMR spectra are recorded in deuterated solvents on a Bruker
ADVANCE 400 NMR Spectrometer. 1H NMR chemical shis are
reported in ppm downeld from tetramethylsilane (TMS)
reference using the residual protonated solvent as an internal
standard. Molecular weights of the PIL block copolymers are
s ¼ L/RS
(1)
Table 1 Compositions and thickness of polyelectrolyte membranes
Sample (PIL mole
percentage)
where the R represents the bulk electrolyte resistance, L means
the sample thickness, and S is the area of the polymer electro-
lyte lm.
LiTFSIa (%)/thicknessb (mm)
c
c
c
c
c
21.3 mol%
24.2 mol%
31.0 mol%
38.7 mol%
50.7 mol%
5/150
5/148
5/150
5/148
5/150
—
—
—
—
15/152
15/150
15/150
—
—
c
10/146
10/152
10/154
20/150
20/148
20/148
3. Results and discussion
3.1. Characterization of the monomer and polymers
a
b
The weight ratio of LiTFSI/copolymer (wt%). The thickness of
c
The chemical structure of 5-norbornene-2-carboxylate-1-hexyl-3-
electrolyte membranes. The electrolyte membranes are not prepared
due to the ionic conductivity is very low with the 5% LiTFSI content.
methyl-imidazolium bis((triuoromethyl)sulfonyl)amide (NIm-
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monomer polymerization (Fig. 2(b)). Proton absorption peaks of
imidazolium ring in the monomer and ionic liquid block
copolymers appear around 8.70 (CH), 7.26 to 7.33(CH]CH) and
3.92 (CH3) are similarly. The NBCOOCH3 homopolymer has no
protons absorption peak between 7.26 to 8.70 ppm. And as the
ionic liquid block compositions increase, the proton absorption
peak areas increase gradually. The successful synthesis of the
monomer and copolymers were well conrmed by the 1H NMR
spectra. The Mn of the copolymers obtained by GPC and Poly-
merized Ionic Liquids (PILs) block molar percentages are
calculated24 by 1H NMR analysis. The related images are pre-
sented in the ESI (Fig. S1–S6†) and the corresponding results are
exhibited in Table 2.
3.2. Thermal properties
The thermal properties of copolymers are investigated by ther-
mogravimetry analysis (TGA) and differential scanning calo-
rimetry (DSC). All block copolymers exhibit good thermal
stability with decompositions temperatures (5% weight loss)
over 390 ꢂC (Fig. 3), which is more stable than that of the
homopolymer (Table 2). The differential scanning calorimeter
(DSC) curves are displayed in Fig. 4(a). All the samples are
heated to 250 ꢂC to remove thermal history rst. Homopolymer
has only one distinct glass transition and the PIL bock copoly-
mers have two glass transitions that belong to the different
blocks, these results indicate that the block copolymers have
strong microphase-separated.11 With the content of PIL block
increases, the intensity of Tg1 decreases, while the intensity of
the Tg2 shows the opposite tendency. Moreover, the Tgs
decreases aer blending with lithium salt because the large
anions TFSIꢁ has weaker association with cation,25 and incre-
mental content of lithium salt will further reduce the Tgs
(Fig. 4(b)).
Fig. 2 1H NMR of NIm-TFSI (a) and copolymers (b).
3.3. Morphology of the copolymers
The microstructure of the copolymers is studied by X-ray
diffraction (XRD) (Fig. 5). All the pure copolymer lms
casting from their THF solutions displayed two weak diffrac-
tion peaks in the angles of ꢃ5.6ꢂ and ꢃ21.8ꢂ. The rst reec-
tion peak at 2q ¼ 21.8ꢂ is the fundamental building of
copolymer backbone, and the broad peaks indicate the
copolymers exhibiting the amorphous structure.26 It remains
TFSI), poly (methyl-5-norbornene-2-carboxylate) (PMN), and PIL
diblock copolymers are conrmed by 1H NMR spectra shown in
Fig. 2. For the imidazolium-functionalized norbornene mono-
mer, the characteristic protons absorption peaks of double
bond in norbornene ring appear at 6.12 and 5.88 ppm (Fig. 2(a)).
But the absorption peaks transfer to 5.50–5.13 ppm aer the
Table 2 Molecular characteristics of copolymers
Feed ratio of PIL
mol%
Actual ratio
of PIL mol%
PIL diblock copolymersa
Mn (g molꢁ1
)
PDI
Td (ꢂC)
Tg1 (ꢂC)
Tg2 (ꢂC)
PMN
0
20
25
33
40
50
0
27 874
33 229
33 252
33 502
34 637
37 674
1.04
1.06
1.93
1.04
1.17
2.10
281
385
391
394
327
400
—
41.39
48.98
48.98
51.24
51.24
51.24
21.3 mol%
24.2 mol%
31.0 mol%
38.7 mol%
50.7 mol%
21.3
24.2
31.0
38.7
50.7
ꢁ1.33
ꢁ3.30
ꢁ4.30
ꢁ4.98
ꢁ4.98
a
Numbers correspond to PIL mol% that is calculated by 1H NMR spectroscopy.
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Fig. 3 TGA thermogram of PIL diblock copolymers.
Fig. 5 XRD patterns of copolymers.
unchanged prole when the ratio of PIL composition is varied.
However, the intensity of peak at 2q ¼ 5.6ꢂ increases along with
the enhanced content of the PIL part, suggesting a more
regular packing of side chain.27,28 Transmission electron
microscopy (TEM) was performed to further study the
copolymers micro-morphology. The TEM images of copolymers
are present in Fig. 6. The copolymers with 21.3 mol% PIL and
31.0 mol% PIL show some degree of microphase-separated
morphology, but they have no long-range periodic order.
While the 50.7 mol% PIL copolymer exhibits good lamella
morphology with long-range periodic order clearly, this may be
due to the difference electron densities between poly-
norbornene main chain and PIL side chains.29 These
microphase-separated morphologies are expected to provide a
good passage for Li+ transporting in the membranes.
3.4. Ionic conductivity
As the homopolymer is brittle and can not form lm indepen-
dent, all ionic conductivities are obtained from PIL block
copolymers electrolyte membranes. Fig. 7 exhibits the temper-
ature dependence of ionic conductivity of polyelectrolyte with
different PIL ratio ranging from ca. 21.3 to 50.7 mol% with 5%
LiTFSI. Obviously, the ionic conductivity increases with the rise
of temperature and PIL molar ratios. This trend may be due to
the stronger segmental motion caused by high temperature,
which is benecial for ionic transport.30,31 Besides, the PIL as
the ion transporters plays an important role in improving ionic
conductivity. The results show that the ionic conductivity of the
polyelectrolyte with 38.7 mol% and 50.7 mol% PIL increase ca. 1
order of magnitude compared to the polyelectrolyte with lower
PIL compositions. Fig. 8 reveals the ionic conductivity of 50.7
mol% PIL with different LiTFSI contents. The increasement of
LiTFSI content can raise the concentration of Li ions, and result
in enhancing the ionic conductivity. On the other hand, the
block copolymer with 50.7 mol% PIL compositions exhibits
good lamella morphology and it will provide a xed ion trans-
port channels32 and help Li ions to transport easier and faster.
The polyelectrolyte containing 50.7 mol% PIL and 20% LiTFSI
offers the best ionic conductivity with 7.44 ꢀ 10ꢁ6 S cmꢁ1 at 30
ꢂC and 4.68 ꢀ 10ꢁ4 S cmꢁ1 at 100 ꢂC, respectively. However, the
room conductivity achieved in our study is lower compared with
the literature reported homo-PIL, and it is agree with the results
that the conductivity value of solid polymer electrolytes is lower
Fig. 4 DSC thermograms of copolymers (a) and polyelectrolyte of
50.7 mol% PIL with different lithium salt contents (b).
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Fig. 6 TEM of copolymers: (a) 21.3 mol% PIL; (b) 31.0 mol% PIL; (c) 50.7 mol% PIL.
than that of polymer gel electrolytes.9,10,13,33,34 On the other side, the gas constant and T is the absolute temperature. Ea is
like the other similarity PIL block copolymers,11,12,35 these calculated from the slope of the tting lines with the temper-
copolymers do not apply to lithium ion battery.
ature range of 373–303 K. The calculated Ea values are 27.56 kJ
The ionic conductivities of other copolymer electrolytes with molꢁ1 (50.7 mol% PIL/5% LiTFSI polyelectrolyte membrane),
different lithium salt contents are also measured (Table 3). As 26.77 kJ molꢁ1 (50.7 mol% PIL/10% LiTFSI polyelectrolyte
expected, the ionic conductivities increase with the enhance- membrane), 24.78 kJ molꢁ1 (50.7 mol% PIL/15% LiTFSI poly-
ment of PIL compositions and LiTFSI contents.
electrolyte membrane) and 24.39 kJ molꢁ1 (50.7 mol% PIL/20%
Because the cation is xed on the polymer chain and only the LiTFSI polyelectrolyte membrane), respectively. The Ea values
anion is mobile, PIL block copolymer electrolyte is single-ion of polyelectrolyte membranes have a small change, and it
conductor. Therefore, the connectivity of conducting (i.e., ions agrees with that the lower activation energy getting the higher
ion-to-ion hopping distance) has signicant impact on ion ionic conductivity.36
transport.11 In the PIL block units, the high concentration of ion
conductors is helpful for shorting the anion-to-anion hopping
distance, and leading to low hopping energy and the efficiency
3.5. Electrochemical stability
Electrochemical stability window of the polyelectrolyte is
characterized by linear sweep voltammetry using the cell
SS|polymer electrolyte|Li. In the measurement, we choose two
typicality high ionic conductivity polyelectrolytes, and the
results are shown in Fig. 9. Both of polyelectrolytes exhibit
suitable electrochemical stability. Although the poly-
electrolyte with PIL ratio of 31.0 mol% is relatively more
(2) stable due to the less content of ionic liquids. The poly-
electrolyte of 50.7 mol% PIL compositions decomposed at
of ion transport. Consequently, the increased PIL compositions
made the local concentration of conducting ions up-shi and
thus enhanced the ionic conductivity.
From Fig. 7 and 8, It can be found that the ionic conductivity
approximately linear increases with the temperature increasing,
which obeys the Arrhenius equation (eqn (2)):
ꢀ
ꢁ
ꢁEa
sðTÞ ¼ A exp
RT
where s is the ionic conductivity, A is the pre-exponential about 4.2 V versus Li/Li+ at 30 ꢂC, demonstrating it can be
factor, Ea is the activation energy of ion conduction, and R is applied to lithium ion batteries.
Fig.
8 Temperature dependence of ionic conductivity of poly-
Fig.
7 Temperature dependence of ionic conductivity of poly-
electrolyte of 50.7 mol% PIL with various LiTFSI components.
electrolyte with 5% LiTFSI.
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Table 3 Ionic Conductivity of polyelectrolyte with different lithium
salt contents
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4. Conclusion
In this study, we synthesized a series of ionic liquid block
copolymers via ring-opening metathesis polymerization, and the
copolymers show good thermal stability and the obvious two
glass transition temperatures that indicating the strong micro-
phase separation. Especially, the 50.7 mol% PIL copolymer
exhibits good lamella morphology with long-range periodic
order clearly in TEM image. In this system of polyelectrolyte, the
higher PIL block compositions as well as lithium salt contents
are benecial for ionic conductivity. The polyelectrolyte with
50.7 mol% PIL compositions and 20% LiTFSI offers the best
ionic conductivities with 7.44 ꢀ 10ꢁ6 S cmꢁ1 at 30 ꢂC and 4.68 ꢀ
10ꢁ4 S cmꢁ1 at 100 ꢂC, and the values are largely improved
compared to other low PIL compositions block polymers.
Moreover, the polyelectrolyte also exhibits good electrochemical
stability and decomposes at about 4.2 V versus Li/Li+. Consid-
ering that the good corresponding properties of the block
copolymers, such as stability and ionic conductivity, they will be
one of the potential candidate materials for lithium ion battery.
¨
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