Novel cross-linked copolymer gel electrolyte supported by hydrophilic polytetrafluoroethylene for...
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Author's Accepted Manuscript
Novel cross-linked copolymer gel electrolytesupported by hydrophilic polytetrafluor-oethylene for rechargeable lithium batteries
Qingwen Lu, Jianhua Fang, Jun Yang, RongrongMiao, Jiulin Wang, Yanna Nuli
PII: S0376-7388(13)00696-0DOI: http://dx.doi.org/10.1016/j.memsci.2013.08.029Reference: MEMSCI12357
To appear in: Journal of Membrane Science
Received date: 22 April 2013Revised date: 14 August 2013Accepted date: 15 August 2013
Cite this article as: Qingwen Lu, Jianhua Fang, Jun Yang, Rongrong Miao, JiulinWang, Yanna Nuli, Novel cross-linked copolymer gel electrolyte supported byhydrophilic polytetrafluoroethylene for rechargeable lithium batteries, Journalof Membrane Science, http://dx.doi.org/10.1016/j.memsci.2013.08.029
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Novel cross-linked copolymer gel electrolyte supported by
hydrophilic polytetrafluoroethylene for rechargeable lithium
batteries
Qingwen Lu, Jianhua Fang, Jun Yang�, Rongrong Miao, Jiulin Wang, Yanna Nuli
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan
Road, Shanghai 200240, China
Abstract
A novel hydrophilic polytetrafluoroethylene (PTFE)-supported gel polymer
electrolyte (GPE) membrane based on the cross-linked poly(ethylene glycol) and
poly(glycidyl methacrylate) block copolymer (PEG–b–PGMA) is successfully
prepared by cationic ring-opening polymerization and followed by in situ
cross-linking process. The poly(ethylene glycol) side chains of PEG–b–PGMA
interact with the liquid electrolyte and hold it inside the membrane, while the
hydrophilic and highly-porous PTFE membrane offers mechanical support for the
cross-linked GPE. The ionic conductivity of the optimized GPE-3 reaches 1.30 × 10�3
S cm�1 at 25 °C and is high enough to be applied in lithium secondary batteries. The
GPE is electrochemically stable up to 4.5 V versus Li/Li+. Moreover, the optimized
GPE membrane demonstrates non-flammability and good dimensional stability at
elevated temperature, which can improve the safety of the cell. The Li/LiFePO4 cell
using the GPE-3 exhibits stable cycling behavior and superior rate performance
comparable to the cell based on conventional liquid electrolyte. Therefore, the
� Corresponding author. Tel./fax: +86 21 5474 7667 E-mail address: [email protected] (J. Yang)
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reported GPE is very promising for the use in rechargeable lithium batteries.
Keywords:
Cationic ring-opening polymerization; Hydrophilic microporous
polytetrafluoroethylene membrane; Curable block copolymer; Gel polymer electrolyte;
Rechargeable lithium battery
1. Introduction
Rechargeable lithium (or Li-ion) polymer batteries have been regarded as
important next-generation power sources for electric vehicles and stationary energy
storage systems[1, 2]. Traditional liquid electrolytes with organic solvents possess
high ion conductivity. However, they are unsafe because of their several shortcomings,
such as high volatility, easy leakage and flammability. Conventional poly(ethylene
oxide) (PEO)-based all-solid-state polymer electrolytes have been extensively studied,
and exhibit the conductivities ranging from 10�7 S cm�1 to 10�5 S cm�1 at room
temperature, which is not sufficient for practical application [3]. Gel polymer
electrolytes (GPEs) are just good alternatives between all-solid-state polymer
electrolytes and conventional liquid electrolytes. Various polymers including PEO [4],
poly(propylene oxide) (PPO)[5], poly(methylmethacrylate) (PMMA)[6],
poly(acrylonitrile) (PAN)[7] and poly(vinylidene fluoride) (PVdF)[8] have been
investigated as GPE matrixs and high ionic conductivity with other desirable
properties have been achieved by incorporation of ceramic fillers and polar plasticizer
[9, 10]. For example, Oh and Amine [11] prepared a poly(ethylene
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oxide)-co-poly(propylene oxide) random copolymer (abbr. as P(EO–PO)) based GPE
having ionic conductivity higher than 10�3 S cm�1 and good battery performance.
However, these GPE membranes mostly present poor mechanical properties because
they have been softened by uptake of liquid electrolytes. This drawback might cause
problems of winding tension and internal short-circuits during the cell assembly and
operation, and it is one of the most important reasons for preventing them from being
used in practical rechargeable lithium batteries.
On the other hand, chemical cross-linking leads to the formation of an
irreversible gel. Thus, chemically cross-linked GPEs show good thermal and
dimensional stability. The cross-linked polyether system is regarded as one of the
most potential gel bases for GPEs because of the ideal interaction between lithium ion
and ethylene oxide (EO) unit. However, the mechanical properties of chemically
cross-linked GPEs are also unsatisfactory. In order to enhance mechanical strength of
cross-linked GPEs, the microporous polyolefin separators have been employed as a
dimensional support to reinforce the cross-linked GPEs. [12-14]. Such
membrane-supported cross-linked GPEs show sufficient mechanical strength for the
fabrication of lithium batteries. Nevertheless, the porosity of the commercial
microporous polyolefin separators is generally not high, which limits ionic
conductivity of the composite films.
As well known, microporous polytetrafluoroethylene (PTFE) membrane has
been widely used in the proton exchange membrane fuel cells [15], lithium-air
batteries[16], membrane distillation[17, 18] and water purification systems[19]
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because of its outstanding mechanical and thermal stability, good toughness and
chemical inertness. In addition, the high porosity up to 80% is obtainable for this kind
of membrane. These excellent performances indicate that it could be a stable support
to reinforce the GPEs. In this work, the hydrophilic microporous PTFE membrane
was first used as a dimensional support to enhance the mechanical strength of the
cross-linked PEG–b–PGMA gel electrolyte. The block copolymer PEG–b–PGMA
was prepared via the cationic ring-opening polymerization from glycidyl methacrylate
(GMA) and methoxypolyethylene glycols (PEG) oligomer. The PEG–b–PGMA
consists of two chemically dissimilar segments: the ethylene oxide (EO) chains act as
ionophilic units and the glycidyl methacrylate with double bonds serve as
cross-linking groups. Methoxypolyethylene glycol was chosen because the ethylene
glycol side chains have high affinity to the liquid electrolyte, thus keeping it inside the
membrane to avoid cell leakage. The mechanical, heat-resistant and electrochemical
properties of this gel polymer electrolyte system were systematically investigated.
Furthermore, a Li/LiFePO4 cell using the optimized GPE-3 was assembled and tested.
2. Experiment
2.1 Materials
Glycidyl methacrylate (GMA), ethylene glycol dimethacrylate (EGDA) and
methoxypolyethylene glycols (PEG) oligomer with the number-averaged molecular
weight of 1000 Da were purchased from Aladdin. Lithium
bis-tri�uoromethanesulphonimide (LiTFSI, purity: 99%, Shenzhen Capchem
Technology Co., Ltd.) was heated at 100 °C under vacuum prior to electrolyte
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preparation. Dichloromethane (CH2Cl2) was refluxed by CaH2 before use. Boron
trifluoride diethyl etherate (BF3(OEt)2, purity: 98%, Aladdin) was dried with activated
molecular sieve. Other materials, such as benzoyl peroxide (BPO), methanol and
diethyl ether were used as received. 1.0 M liquid electrolyte was made by dissolving a
certain quality of LiTFSI in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1
in volume, Shenzhen Capchem Technology Co., Ltd.). Hydrophilic microporous
PTFE membranes (thickness: 25 μm; porosity: 80%; pore size: 1 μm) were purchased
from Shanghai Minglie Chemical Industry Science and Technology Co., Ltd.
Commercial polyethylene (PE) separators (thickness: 20 μm; porosity: 37%) were
purchased from ENTEK International Ltd. Commercial carbon-coated LiFePO4 was
from Phostech Lithium Company (average particle size: 0.2 μm; carbon content: 2
wt.%).
2.2 Synthesis of PEG–b–PGMA curable block copolymer
PEG–b–PGMA was synthesized according to a one-pot method in previous study
[20]. The schematic PEG–b–PGMA synthesis procedure is illustrated in Fig.1a. The
cationic ring-opening polymerization of GMA with PEG and BF3(OEt)2 was carried
out in a dried three-neck flask equipped with a magnetic stirrer flask under argon gas.
In a typical reaction, PEG (15 g, 0.015 mol; Mn=1000 Da) and GMA (14.20 g, 0.1
mol) were dissolved in 80 ml of dried CH2Cl2. When the mixture was cooled to
–12 °C in an ice–salt bath, BF3(OEt)2 (1.6 ml, 0.012 mol) was quickly dropped into it
by syringe. After 50 min, a little methanol was added to the mixture to end the
cationic polymerization. The resulting block copolymer was concentrated with a
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rotator evaporator and isolated by pouring the polymerization mixture into a large
excess of ether. The resulting block copolymer was dissolved into methanol and
reprecipitated in ether at least three times. A transparent, jellylike, viscous liquid was
obtained. Then, this copolymer was kept in a refrigerator. The samples were
freeze-dried before the measurement of 1HNMR.
2.3 Preparation of gel polymer electrolyte
Fig.1b illustrates a flow chart of the preparation procedure for in situ
polymerization of the PTFE-supported cross-linked PEG–b–PGMA electrolyte
membrane. This composite GPE membrane was prepared by a radical initiated
reaction in the microporous PTFE membrane soaked with a homogeneous precursor
electrolyte solution consisting of a curable PEG–b–PGMA, EGDA cross-linking agent,
a liquid electrolyte (1M LiTFSI in ethylene carbonate/dimethyl carbonate, 1/1, v/v)
and benzoyl peroxide (BPO) as a thermal radical initiator, which was cured at 80 °C
for 12 h. The exact weight ratio of PEG–b–PGMA: EGDA: BPO was 100:2:0.5.
Hereinafter, GPE-1, GPE-2, GPE-3 and GPE-4 were respectively prepared by soaking
with the PTFE membrane in the precursor solutions consisting of different
PEG–b–PGMA concentration (5.0, 10.0, 15.0 and 20.0 wt.%). All the samples were
prepared in a glove box under an argon atmosphere.
2.4 Sample analysis
The surface morphology of the pristine hydrophilic microporous PTFE
membrane, the GPE-1, GPE-2, GPE-3 and GPE-4 film were observed by JEOL
JSM-7401F field emission scanning electron microscope (FE-SEM). Pore size, pore
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size distribution, and porosity of the GPE-3 composite film were tested using a model
CFP1100AI Capillary Flow Porometer (CFP) manufactured by PMI. 1HNMR spectra
were recorded on a Varian Mercury Plus 400 MHz instrument with CDCl3 as the
solvent. The mechanical properties of the PE separator and the GPE-3 membrane
were measured from stress–strain tests using Instron 4465 instrument with a tensile
speed of 5 mm min�1.
2.5 Electrochemical Measurements
To determine the uptake amount of liquid electrolyte, the resulting membrane
was washed with methanol for several times, and dried under vacuum for 12 h at 80°C.
Then, the PTFE-supported polymer membrane was immersed in electrolyte solution
for 1 h. Subsequently, the excess solution on the surface of the membrane was slightly
absorbed using filter paper. The uptake amount was calculated from the weight
difference of the samples before and after the immersion step [21].
The ionic conductivity was measured by ac impedance spectroscopy using a
CHI660C electrochemical analyzer in the frequency range from 100 KHz to 1 Hz at
temperatures between 25 and 80 °C. Electrochemical impedance spectroscopy (EIS)
was measured using a frequency response analyzer (CHI660C) with an
electrochemical interface in the frequency range from 100 KHz to 0.01 Hz. Cyclic
voltammetry measurements were done in Swagelok cell by sandwiching the
electrolyte between stainless steel as working electrode and lithium metal as reference
and counter electrode at 25°C. The voltage scan rate was 10 mV s�1 in the potential
range from –1.0 to 5.0 V.
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LiFePO4 based cathode was prepared by pasting a mixture of active material,
carbon black (Super-P) and PVDF as binder at a weight ratio of 80:10:10 on Al foil.
CR2016-types of coin cells were assembled in glove box containing less than 10 ppm
H2O or O2 for electrochemical evaluation. The cycling performances of LiFePO4/Li
cells with the GPE-3 and the liquid electrolyte were measured on a Land CT2001
battery test system at 25°C.
3. Results and discussion
3.1 Physical properties of the gel polymer electrolytes
Fig.2 shows the 1H-NMR spectrum of the PEG–b–PGMA. The signals at 3.38
ppm (g) and 3.6 ppm (f) correspond to the protons of methoxy (–OCH3) and PEO
units, respectively. Additionally, the chemical shifts at 6.13 ppm and 5.58 ppm (e) are
assigned to the proton resonance of the double bond of the methacrylate group. These
results indicate that the GMA was successfully grafted to PEG by the cationic
ring-opening polymerization.
Electrolyte uptake of a polymer membrane is important for a GPE to have high
ionic conductivity and tightly related to the cross-linked PEG–b–PGMA content in the
composite membrane. Table 1 shows the cross-linked PEG–b–PGMA content,
thickness, electrolyte uptake and ionic conductivity of the different composite
electrolyte membrane at 25°C. The GPE-3 membrane accommodates the largest
amount of liquid electrolyte and exhibits the highest ionic conductivity of 1.30 ×10�3
S cm-1 at 25°C, which is sufficient for rechargeable lithium battery. Decrease of ionic
conductivity with enhancing the content of cross-linked PEG–b–PGMA is mainly due
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to the fact that the ionic motion is restricted with increasing the length of cross-linked
polymer chains. Moreover, it is noted that the lowest conductivity is related to GPE-2,
not to GPE-1. The absorbed liquid electrolyte in PEO gel filled PTFE microporous
membrane should exist in two states. A part is in swollen PEO gel space and the other
is in free liquid state within the micropores of the composite membrane. It is observed
that there are more non-uniform micropores on the surface of GPE-1 membrane than
on the GPE-2 membrane (Fig.4c-d). Therefore, the higher conductivity for GPE-1,
instead of GPE-2, should be attributed to more free liquid state electrolyte in the
composite membrane. The highest ionic conductivity for GPE-3 is probably related to
the synergetic effect between liquid electrolyte uptake of cross-linked PEG–b–PGMA
and liquid electrolyte accommodation in micropores (Fig.4e). In contrast, the PE
membrane soaked in the same liquid electrolyte exhibited a reduced ionic
conductivity of 0.12×10�3 S cm�1 at 25°C. The low ionic conductivity is attributed to
the fact that the PE separator only adsorbs a little liquid electrolyte because it is
inherently hydrophobic and possesses relatively low porosity. In the following study,
the optimized GPE-3 will be further evaluated.
Fig.3 illustrates the temperature dependence of ionic conductivity of GPE-3. The
ionic conductivity increases with the temperature in accord with Arrhenius
relationship.
Fig.4 presents the SEM images of the original hydrophilic porous PTFE
membrane, PE separator, GPE-1, GPE-2, GPE-3 and GPE-4 membrane. The pristine
PTFE membrane in Fig. 4a presents highly porous and fibrous network structure,
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which possesses large free space to accommodate PEG–b–PGMA. The commercial
PE separator has a smooth surface and uniform porous structure with the pore size of
ca. 0.10 �m (Fig.4b). Its porosity is about 37%. As shown in Fig. 4c-f, the micropore
density decreases significantly and the surface smoothness is improved with the
enhancement of the PEG–b–PGMA content. Because both PTFE membrane and
PEG–b–PGMA oligomer are hydrophilic, they have good affinity and are easy to be
incorporated. Moreover, in situ cross-linking may improve the mechanical and
structural stability of the composite film.
Fig.5 shows the pore size distribution of the GPE-3 membrane. Its mean flow
pore size is located at about 0.159 �m with a narrow pore-size distribution. 60% of its
pore dimensions are at ca. 0.150 �m, and about 28% of them are at ca. 0.160 �m. The
membrane porosity is 35.8%.
Fig.6 shows the wetting behavior of the commercial PE separator and GPE-3
membrane. When a drop of liquid electrolyte is applied on the PE separator, it formed
a bead (Fig.6a). In contrast, it’s quick spreading out occurred on the GPE-3 membrane
(Fig.6b). This indicates that the composite membrane exhibits better affinity to the
polar electrolyte than the polyolefin separator. Polyolefin separator is an inherently
hydrophobic polymer, thus it usually does not possess good wettability to polar liquid
electrolytes. However, the surface of the PTFE-supported cross-linked membrane is
hydrophilic and the ethylene oxide (EO) chains are ionophilic with a strong
interaction between ethylene oxide (EO) and liquid electrolyte. Therefore, it is apt to
accommodate the liquid electrolyte to form an effective GPE.
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Fig.7a demonstrates the morphology changes of the GPE-3 membrane and PE
separator before and after being stored at 150°C for 0.5h. The PE separator suffers
from a high degree of shrinkage during its exposure to high temperature environment,
while the GPE-3 membrane shows a little change in the morphology. The GPE-3
membrane is thermally stable, because the PTFE backbone has a high melting
temperature of more than 320°C and the cross-linked PEG–b–PGMA possesses good
thermal and dimensional stability. In fact, the high temperature safety and
electrochemical performance of a battery are mostly related to the dimensional
stability of electrolyte separator, which is especially critical for the large-size and high
power lithium secondary batteries. The easy deformation of PE separators at elevated
temperature approaching to the melting point may cause internal short-circuiting or
even lead to thermal runaway. Furthermore, the flammability of the GPE-3 membrane
was tested. Fig.7b shows that alcohol lamp fire does not cause any combustion
behavior although this membrane absorbed the conventional flammable liquid
electrolyte. There are some reasons for its perfect flame retarding ability. First of all, it
is well known that the PTFE membrane cannot easily catch on fire due to its excellent
self-extinguishing properties. Also, a high concentration of the non-flammable LiTFSI
component is another good flame retardant. On the other hand, it was observed that
the GPE-3 membrane tended to shrink and produce a large amount of char, as soon as
it was put into the flame. Because the flammable solvent was mainly kept inside the
cross-linked copolymer by means of the strong interaction between the solvent
molecules and the ethylene oxide chains of cross-linked copolymer, the membrane
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shrinkage and surface’s scorch may form a protection layer to prevent inflammation
of the absorbed solvent. Therefore, the composite membrane cannot catch on fire. The
GPE-3 membrane with better thermostability and non-flammability is highly desirable
for the development of high power rechargeable lithium batteries.
Another important physical property of battery separator is its mechanical
performance. Fig.8 presents the stress–strain curves of the PE separator and the
GPE-3 membrane at room temperature, respectively. The tensile strength of the PE
separator is 79.0 MPa with an elongation-at-break value at 266.5%. However, the
tensile strength of the GPE-3 membrane decreases to 21.1 MPa due to the high
porosity of the PTFE membrane, which is usually still high enough to be used as
separator in rechargeable lithium batteries. Its elongation-at-break value reaches
380.2%, indicating its better toughness than the PE separator. The good toughness will
reduce the risk of the flexural deformation of the membrane, which is favorable for
application in rechargeable lithium batteries.
3.2 Electrochemical behaviors of the gel polymer electrolytes
Fig.9 shows the electrochemical stability windows of the prepared GPE-3 and
liquid electrolyte at 25 °C. In a potential range between �1.0 V and 5.0 V (vs. Li/Li+),
distinct reduction and oxidation peaks are observed, corresponding to the reversible
plating and stripping of metallic lithium. For the Li/GPE-3/SS cell, the small cathodic
current ranging from 1.5 V to 0 V vs. Li/Li+ might be assigned to electrochemical
reduction of the electrolyte [22] and the trace amount of water, which often lead to the
generation of a solid electrolyte interphase (SEI) protective layer on the anode. In the
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anodic direction, the current emerges at about 4.2 V vs. Li/Li+ for liquid electrolyte,
which is ascribed to the oxidative decomposition of the free solvent molecules. On the
other hand, oxidative degradation of the prepared GPE-3 takes place at around 4.5 V
vs. Li/Li+, because the PEO chains of cross-linked PEG-b-PGMA tend to decompose
at this voltage. Also, the decomposition of the liquid electrolyte cannot be excluded
here. A stability window up to 4.5 V would be sufficient for application in
rechargeable lithium batteries with certain cathodes, such as LiFePO4 and S-based
composites. The observed current density for the redox reaction is smaller in the
liquid electrolyte. This can be explained by the lower conductivity of liquid
electrolyte with the PE separator compared to the GPE-3.
Fig.10 exhibits the ac-impedance spectra of Li/PE-liquid electrolyte/Li and
Li/GPE-3/Li cells at different storage time. The real part of the impedance at the
highest frequency signi�es the bulk resistance of an electrolyte, and the amplitude of a
semicircle is representative of the interfacial resistance (Ri) between the electrodes
and electrolytes [23, 24]. The Ri in the liquid electrolyte increases quickly within the
first 13 days and then speed of the Ri change slows down (Fig. 10a). The Ri in the
GPE-3 is about 260 � cm2 at the 1st day, then increases with time like in the liquid
electrolyte, and becomes relatively stable after 13 days (Fig.10b). The lower Ri for the
cell using GPE-3 than that using PE-liquid electrolyte suggests better compatibility
between lithium electrode and GPE-3.
3.3 Evaluation of Li/LiFePO4 cell
Fig.11 shows the discharge capacities of the Li/LiFePO4 cells with GPE-3 and
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liquid electrolyte during cycling at room temperature, respectively. Both the cells
display good cycling performance with slight capacity fading throughout 80 cycles at
0.3C rate. Specific capacity of LiFePO4 cathode in GPE-3 is as high as 146.2 mAh g–1
after 80 cycles. The inset graph shows the initial and 50th cycle charge-discharge
profiles of the cells using PE-liquid electrolyte and GPE-3. For the Li/GPE-3/LiFePO4
cell, the quite flat voltage plateaus at 3.47 V for charging and 3.39 V for discharging
correspond to Fe3+/Fe2+redox couple reaction on the cathode. Moreover, the voltage
polarization of the GPE-3 cell is lower than that of the liquid electrolyte cell after 50
cycles. The low voltage polarization of Li/LiFePO4 cell with GPE-3 suggests its small
electrolyte resistance and interfacial resistances. Here corrosion of Al current collector,
which often takes place in lithium ion batteries with LiTFSI liquid electrolyte and
metallic oxide cathodes, can not be observed due to the upper potential limit to 4 V vs.
Li/Li+ [25] and the restricted liquid leakage of the GPE. Judging from the stable cycle
capacity and low voltage polarization, the GPE-3 as both electrolyte and separator
provides good electrochemical reversibility for the cell reactions.
Fig.12a shows the rate performances of the cells using PE-liquid electrolyte and
the GPE-3 at 25°C. After the cell was cycled at the initial rate of 0.2C for 5 cycles, the
current density was gradually increased in stages until 4C. With an enhanced
discharge rate, the capacity decreased regularly. For the cell with GPE-3, the
relatively stable capacities around 157, 147, 132, 120 and 109 mAh g–1 were obtained
at current rates of 0.2C, 0.5C, 1.5C, 3C and 4C, respectively. As the current rate
returned to 0.2C, most of the initial capacity can be retained. Fig.12b further exhibits
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the discharge profiles of Li/LiFePO4 cells with the GPE-3 membrane and PE separator
at different rates. It is clearly observed that the discharge voltage plateau declines with
increasing the current density for both the cells. However, the discharge voltage
plateau of Li/GPE-3/LiFePO4 cell is higher than that of Li/PE-liquid
electrolyte/LiFePO4 cell when the current rate is increased to 1.5C. The comparable
results for both the electrolyte systems indicate that the GPE-3 membrane might be
used for power lithium ion batteries.
4. Conclusions
We have reported the preparation, electrochemical characterization and cell tests
of a novel hydrophilic PTFE-supported cross-linked PEG–b–PGMA electrolyte
membrane for lithium secondary batteries. The highly porous PTFE membrane with
sufficient strength reinforces the dimensional stability of PEG–b–PGMA, which is
able to hold a large amount of liquid electrolyte via an interaction. The composite
membrane demonstrates superior thermal stability and toughness, non-flammability,
and good compatibility to lithium metal electrode. The optimized GPE-3 membrane
possesses the ionic conductivity of 1.30×10�3 S cm�1 at 25 °C and the electrochemical
stability window up to 4.5V. Moreover, the Li/LiFePO4 cells using the GPE-3 show
good cycling stability and rate performance, which are comparable to the cells based
on conventional liquid electrolytes. All these positive effects indicate that the
PTFE-supported cross-linked PEG–b–PGMA GPE could be a promising candidate for
rechargeable lithium batteries.
Acknowledgement
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This work was supported by National Natural Science Foundation of China (No.
21273146) and State Key 973 Program of the PRC (No.2014CB932303).
References
[1] J. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature
414 (2001) 359–367.
[2] M. Armand, J. M. Tarascon, Building better batteries, Nature 451 (2008) 652–657.
[3] W.-S. Young, J. N. Albert, A.B. Schantz, T. H. Epps III, Mixed-salt effects on the ionic
conductivity of lithium-doped PEO-containing block copolymers, Macromolecules 44 (2011)
8116–8123.
[4] F. B. Dias, L. Plomp, J. B. Veldhuis, Trends in polymer electrolytes for secondary lithium
batteries, J. Power Sources 88 (2000) 169–191.
[5] J. Song, Y. Wang, C. Wan, Review of gel-type polymer electrolytes for lithium-ion batteries, J.
Power Sources 77 (1999) 183–197.
[6] M. Rao, J. Liu, W. Li, Y. Liang, D. Zhou, Preparation and performance analysis of
PE-supported P(AN-co-MMA) gel polymer electrolyte for lithium ion battery application, J.
Membr. Sci. 322 (2008) 314–319.
[7] Y.-H. Liang, C.-C. Wang, C.-Y. Chen, Conductivity and characterization of plasticized polymer
electrolyte based on (polyacrylonitrile-b-polyethylene glycol) copolymer, J. Power Sources
![Page 18: Novel cross-linked copolymer gel electrolyte supported by hydrophilic polytetrafluoroethylene for rechargeable lithium batteries](https://reader035.fdocuments.us/reader035/viewer/2022080406/575094f91a28abbf6bbdc161/html5/thumbnails/18.jpg)
17
172 (2007) 886–892.
[8] Z. Cui, Y. Xu, L. Zhu, J. Wang, Z. Xi, B. Zhu, Preparation of PVDF/PEO-PPO-PEO blend
microporous membranes for lithium ion batteries via thermally induced phase separation
process, J. Membr. Sci. 325 (2008) 957–963.
[9] M. Rao, J. Liu, W. Li, Y. Liang, Y. Liao, L. Zhao, Performance improvement of poly
(acrylonitrile-vinyl acetate) by activation of poly(methyl methacrylate), J. Power Sources 189
(2009) 711–715.
[10] N. Oyama, Y. Fujimoto, O. Hatozaki, K. Nakano, K. Maruyama, S. Yamaguchi, K. Nishijima,
Y. Iwase, Y. Kutsuwa, New gel-type polyolefin electrolyte film for rechargeable lithium
batteries, J. Power Sources 189 (2009) 315–323.
[11] B. Oh, K. Amine, Evaluation of macromonomer-based gel polymer electrolyte for high-power
applications, Solid State Ionics, 175 (2004) 785–788.
[12] Y. H. Liao, X. P. Li, C. H. Fu, R. Xu, L. Zhou, C. L. Tan, S. J. Hu, W. S. Li,
Plypropylene-supported and nano-Al2O3 doped poly(ethylene oxide)–poly(vinylidene
fluoride-hexafluoropropylene)-based gel electrolyte for lithium ion batteries, J. Power
Sources 196 (2011) 2115–2121.
[13] J. Liu, W. Li, X. Zuo, S. Liu, Z. Li, Polyethylene-supported polyvinylidene fluoride–cellulose
acetate butyrate blended polymer electrolyte for lithium ion battery, J. Power Sources 226
(2013) 101–106.
[14] J.-A. Choi, Y. Kang, H. Shim, D.W. Kim, E. Cha, D.-W. Kim, Cycling performance of a
lithium-ion polymer cell assembled by in-situ chemical cross-linking with fluorinated
phosphorous-based cross-linking agent, J. Power Sources 195 (2010) 6177–6181.
![Page 19: Novel cross-linked copolymer gel electrolyte supported by hydrophilic polytetrafluoroethylene for rechargeable lithium batteries](https://reader035.fdocuments.us/reader035/viewer/2022080406/575094f91a28abbf6bbdc161/html5/thumbnails/19.jpg)
18
[15] Y. Zhao, H. Yu, D. Xing, W. Lu, Z. Shao, B. Yi, Preparation and characterization of PTFE
based composite anion exchange membranes for alkaline fuel cells, J. Membr. Sci.421-422
(2012) 311-317.
[16] J. Zhang, W. Xu, W. Liu, Oxygen-selective immobilized liquid membranes for operation of
lithium-air batteries in ambient air, J. Power Sources 195 (2010) 7438–7444.
[17] J. A. Franco, D. D. deMontigny, S. E. Kentish, J. M. Perera, G. W. Stevens,
Polytetrafluoroethylene (PTFE)-supported polypropylene membranes for carbon dioxide
separation in membrane gas absorption: hollow fiber configuration, Ind. Eng. Chem. Res. 51
(2012) 1376–1382.
[18] J. Zhang, J.-D. Li, S. Gray, Effect of applied pressure on performance of PTFE membrane in
DCMD, J. Membr. Sci. 369 (2011) 514–525.
[19] Q. Xu, Y. Yang, X. Wang, Z. Wang, W. Jin, J. Huang, Y. Wang, Atomic layer deposition of
alumina on porous polytetrafluoroethylene membranes for enhanced hydrophilicity and
separation performances, J. Membr. Sci. 415-416 (2012) 435–443.
[20] W. Huang, Y. Zhou, D. Yan, Direct synthesis of amphiphilic block copolymers from glycidyl
methacrylate and poly(ethylene glycol) by cationic ring-opening polymerization and
supramolecular self-assembly thereof, J. Polym. Sci., Part A: Polym. Chem. 43 (2005)
2038–2047.
[21] M.-H. Ryou, Y. M. Lee, K.Y. Cho, G.-B. Han, J.-N. Lee, D. J. Lee, J.W. Choi, J.-K. Park, A
gel polymer electrolyte based on initiator-free photopolymerization for lithium secondary
batteries, Electrochim. Acta 60 (2012) 23–30.
[22] X. Zhang, R. Kostecki, T. J. Richardson, J. K. Pugh, P. N. Ross, Electrochemical and infrared
![Page 20: Novel cross-linked copolymer gel electrolyte supported by hydrophilic polytetrafluoroethylene for rechargeable lithium batteries](https://reader035.fdocuments.us/reader035/viewer/2022080406/575094f91a28abbf6bbdc161/html5/thumbnails/20.jpg)
19
studies of the reduction of organic carbonates, J. Electrochem. Soc. 148 (2001)
A1341–A1345.
[23] C. Gerbaldi, J. R. Nair, S. Ahmad, G. Meligrana, R. Bongiovanni, S. Bodoardo, N. Penazzi,
UV-cured polymer electrolytes encompassing hydrophobic room temperature ionic liquid for
lithium batteries, J. Power Sources 195 (2010) 1706–1713.
[24] S. Liu, N. Imanishi, T. Zhang, A. Hirano, Y. Takeda, O. Yamamoto, J. Yang, Lithium dendrite
formation in Li/poly(ethylene oxide)–lithium bis(trifluoromethanesulfonyl)imide and
N-Methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide/Li Cells, J. Electrochem.
Soc. 157 (2010) A1092–A1098.
[25] M. Morita, T. Shibata, N. Yoshimoto, M. Ishikawa, Anodic behavior of aluminum in organic
solutions with different electrolytic salts for lithium ion batteries, Electrochim. Acta 47 (2002)
2787-2793.
Captions for figures and table:
Fig.1. (a) Schematic synthesis procedure of PEG–b–PGMA.
(b) Flow chart of preparation procedure of the PTFE-supported cross-linked
PEG–b–PGMA electrolyte via in situ polymerization.
Fig.2 1H-NMR spectrum of PEG–b–PGMA in CDCl3.
Fig.3 Temperature dependence of ionic conductivity of the GPE-3.
Fig.4 SEM images of the original hydrophilic porous PTFE membrane (a), PE
separator (b), GPE-1 membrane (c), GPE-2 membrane (d), GPE-3
membrane (e) and GPE-4 membrane (f).
Fig.5 Pore size distribution of the GPE-3 membrane.
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Fig.6 Wettability of the commercial PE separator (a) and the GPE-3 membrane (b) to
the liquid electrolyte.
Fig.7 The heat-resistance of polymer membranes: (a) High temperature test at 150°C;
(b) The flammability test of the GPE-3.
Fig.8 The stress–strain curves for PE separator and the GPE-3 membrane at room
temperature.
Fig.9 Cyclic voltammograms of the GPE-3 and liquid electrolyte at 25°C using Li as
reference and counter electrodes and stainless steel as working electrode.
Fig.10 Electrochemical impedance spectra of (a) Li/PE-liquid electrolyte/Li and (b)
Li/GPE-3/Li symmetric cells at 25°C.
Fig.11 Discharge capacities as a function of cycle number for the Li/LiFePO4 cells
with the GPE-3 and the liquid electrolyte at 25 °C, respectively. The inset
graph shows the initial and 50th cycle charge-discharge profiles of the cells
with the GPE-3 and the liquid electrolyte.
Fig.12 (a) Rate performances of the cells using the GPE-3 membrane and PE-liquid
electrolyte at 25 °C. (b) Discharge profiles of Li/LiFePO4 cells with the GPE-3
membrane and PE separator at different rates.
Table 1 The cross-linked PEG–b–PGMA content, thickness, electrolyte uptake and
ionic conductivity of the different membrane samples at 25°C.
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Table 1
Sample GPE-1 GPE-2 GPE-3 GPE-4
PEG–b–PGMA concentration in precursor solutions (wt.%)
5 10 15 20
Cross-linked PEG–b–PGMA content in the composite membrane (wt.%)
9.5 13.9 21.9 43
Thickness(�m) 30 34 39 55
Uptake (wt.%) 91.2 69.2 113.7 97.0
Conductivity (mS cm�1) 0.87 0.69 1.30 1.10
Research highlights � A novel PTFE-supported cross-linked GPE was prepared via in situ
polymerization. �
� This electrolyte shows non-flammability, superior thermal stability and
toughness. �
� The GPE membrane has good compatibility toward Li electrode.
� Li/LiFePO4 cell using GPE membrane displays excellent electrochemical
behavior. �
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Fig. 1
Figure 1
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Fig.2
Figure 2
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Fig. 3
Figure 3
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Fig. 4
Figure 4
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Fig. 5
Figure 5
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Fig. 6
Figure 6
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Fig. 7
Figure 7
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Fig.8
Figure 8
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Fig. 9
Figure 9
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Fig. 10
Figure 10
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Fig. 11
Figure 11
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Fig. 12
Figure 12
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A novel hydrophilic PTFE-supported gel polymer electrolyte (GPE) based on the
cross-linked poly(ethylene glycol) and poly(glycidyl methacrylate) copolymer was
successfully prepared by in situ polymerization. It shows non-flammability, good
dimensional stability and excellent electrochemical performance.
Graphical Abstract (for review)