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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1

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: yangj723@sjtu.edu.cn (J. Yang)

2

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

3

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]

4

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

5

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

6

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

7

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.

8

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

9

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,

10

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.

11

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

12

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

13

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

14

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

15

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).

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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. �

Fig. 1

Figure 1

Fig.2

Figure 2

Fig. 3

Figure 3

Fig. 4

Figure 4

Fig. 5

Figure 5

Fig. 6

Figure 6

Fig. 7

Figure 7

Fig.8

Figure 8

Fig. 9

Figure 9

Fig. 10

Figure 10

Fig. 11

Figure 11

Fig. 12

Figure 12

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)