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ways to solve this problem is to use a mechanical support such as

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carbonate (EC)/diethyl carbonate (DEC) mixture was donated

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Contents lists available at ScienceDirect

evi

Radiation Physics

Radiation Physics and Chemistry 78 (2009) 505508PVDF-HFP to PEGDMA was 10/0, 9/1, and 8/2). Micro-porous PEE-mail address: ycnho@kaeri.re.kr (Y.-C. Nho).from TechnoSemichem Co., Ltd. Other solvents were reagent gradeand used as received.

To prepare the coating solution, PVDF-HFP and PEGDMA weredissolved in acetone at room temperature (the weight ratio of

0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.radphyschem.2009.03.035

Corresponding author. Tel.: +82635703060; fax: +82635703069.support the winding tension of anode/gel polymer electrolyte/cathode in an assembly process without a separator. One of the

were purchased from Aldrich and used as received. An electrolytesolution consisting of 1.0M of LiClO4 in 1:1 (v/v) ethylenebeen studied most extensively (Tarascon and Armand, 2001; Yuanet al., 2005; Pu et al., 2006; Kim et al., 2005; Kalyana Sundaramand Subramania, 2007; Chiu et al., 2007; Wang et al., 2006).However, gel polymer electrolyte has not been commercialized

2. Experimental

Poly(vinylidene uoride-co-hexauoropropylene) (PVDFand poly(ethylene glycol) dimethacrylate (PEGDMA, Mn Therefore, it is important to increase the safety of the batteries byimproving the thermal stability of the separator (Kanamura,2002).

Gel polymer electrolyte (GPE), which is attractive for lithium-ion batteries due to its high ionic conductivity and safety, has

The preparation and thermal/electrochemical properties of themodied membranes were investigated and discussed in thispaper.1. Introduction

In recent years, lithium secondarthe power source for a wide vaequipments such as cellular phondigital assistants (PDA), and laptop2005). Key requirements of the batteand safety. Up to now, porous polywidely as separators in most comteries. A weakness of the polyethylarge thermal shrinkage occurs unand the shrinkage may cause a shories have been used asof portable electronicital cameras, personaluters (Takemura et al.,e their storage capacitylms have been usedlized lithium-ion bat-E) separator is that ausual heat generationit between electrodes.

PE to prepare supported GPE (Kim et al., 2000). However, thesePE-supported GPE are difcult to fabricate due to the complicatedprocess. Therefore, to overcome its weakness, we consideredpolymer-coated polyethylene separators with a high performanceand an improved heat resistance by using a dip-coating process tobe handled easily, and by controlling the humidity condition andusing electron beam.

In the present study, polymer-coated polyethylene separatorswere prepared by a simple dip coating of PE separators withpoly(vinylidene uoride-co-hexauoropropylene)/poly(ethyleneglycol) dimethacrylate (PVDF-HFP/PEGDMA) mixture at differenthumidity levels (050%), followed by electron beam irradiation.Preparation and characterization of a Pseparator for lithium-ion polymer batte

Joon-Yong Sohn, Jong Su Im, Sung-Jin Gwon, Jae-H

Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeon

a r t i c l e i n f o

Keywords:

Polymer-coated membrane

Polymer electrolyte

Electron beam irradiation

Poly(vinylidene uoride-co-

hexauoropropylene)

Poly(ethylene glycol) dimethacrylate

Lithium battery

a b s t r a c t

In this study, polyethylene

poly(vinylidene uoride-

PEGDMA) mixtures at d

Micro-porous structures o

humidity levels (i.e. phase

humidity level. The ther

increasing EB absorption

PEGDMA-coated PE separa

condition showed a highe

separators.

journal homepage: www.els-si, Jeollabuk-do 580-185, Republic of Korea

arators were modied by dip coating of polyethylene (PE) separators in

exauoropropylene)/poly(ethylene glycol) dimethacrylate (PVDF-HFP/

ent humidity levels (050%), followed by electron beam irradiation.

coating layer were generated by performing dip-coating process at high

rsion process) and were found to be affected by the PEGDMA content and

l shrinkage of the prepared separators signicantly decreased with

se due to the formation of crosslinked networks of the PVDF-HFP/

s. It was also observed that the separators prepared under high humidity

quid electrolyte uptake and the ionic conductivity than the original PE

& 2009 Elsevier Ltd. All rights reserved.F-HFP/PEGDMA-coated PEby electron beam irradiation

er.com/locate/radphyschem

and Chemistry

P ama=ra mp=rp

(1)

to a phase inversion process which produces pores in the coatedpolymer layer of the separators. Furthermore, it can be concludedthat the hydrophilic nature of PEGDMA can assist this phaseinversion process during the drying process of the polymer-coatedPE separator. The porous structure of the prepared separators wasconrmed by SEM. We can see that more pores in the coatedpolymer are generated as the humidity level and PEGDMAincrease (data not shown).

Fig. 2 shows the liquid electrolyte (EL) uptake of the preparedmembranes as a function of the absorption dose. As shown in thisgure, the EL uptake of the prepared membranes increased withan increase in the humidity condition. The EL uptake of thecommercial PE separator was 175wt% and the EL uptake of thepolymer-coated PE separators prepared under R.H. 0% coatingcondition, was found to be lower than that of the bare PEseparators because the pores of the commercial PE separator werelled by the coating polymers without the formation of additional

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PEGDMA content (wt %)

Fig. 1. Porosity of PVDF-HFP/PEGDMA-coated PE separators as a function ofPEGDMA contents and humidity condition.

0

120

140

160

180

200

220

240

260

280E

L up

take

(%)

Absorption dose (kGy)

a b c d e f

PE separator

50 100 150 200

Fig. 2. Electrolyte uptake of PVDF-HFP/PEGDMA-coated PE separators as afunction of the absorption dose. (a) PVDF-HFP/PEGDMA(10/0)-coated PE separator

under R.H. 0%, (b) 8/2 under R.H. 0%, (c) 10/0 under R.H. 25%, (d) 8/2 under R.H.

25%, (e) 10/0 under R.H. 50%, and (f) 8/2 under R.H. 50%.

J.-Y. Sohn et al. / Radiation Physics and Chemistry 78 (2009) 505508506The liquid electrolyte uptake was determined from thedifference in the weights of the dried samples and their swollencounterparts after equilibrating the prepared separators in a liquidelectrolyte solution for 24h at room temperature. To remove theexcess electrolyte adhering to their surfaces, the equilibratedsamples were blotted using laboratory wipes and then quicklyweighed. These uptake measurements were carried out in an Ar-lled glove box (less than 5ppm O2 and H2O).

The ionic conductivity of the separators soaked with theelectrolyte solution at room temperature was determined by ACimpedance technique over the frequency range from 0.01 to100kHz using a Solatron SI 1260 frequency response analyzercombined with an SI 1287 electrochemical interface at a constantpotential of 10mV. The samples with area of A and thickness of Lwere sandwiched between two stainless steel blocking electrodesto measure their electrolyte resistance (Rb, O). The conductivity(s, S/cm) was then calculated from Eq. (2).

s L=RbA (2)The samples of the polymer-coated PE separators were cut into

4 cm4 cm pieces and then subjected for a thermal shrinkage.Thermal shrinkages were determined by measuring the dimen-sional change after heating the samples in a forced convectionoven at 150 1C for 1h.

3. Results and discussion

The porosity of the separator of lithium-ion battery is a veryimportant property since the pores of the separators provide thespace for electrolytes and lithium-ion movement between anodeand cathode for a current generation. In order to observe theporosity of the polymer-coated PE separators, the preparedseparators was impregnated with 1-butanol and then subjectedto a porosimetry analysis. Fig. 1 shows the porosity of the PVDF-HFP/PEGDMA-coated PE separators as a function of the PEGDMAcontents and humidity conditions. It was found that the porosityof the PVDF-HFP/PEGDMA-coated PE separators increasedgradually with increasing the PEGDMA content and increasedsignicantly with increasing humidity. The porosity of thepolymer-coated PE separator dried at R.H. 0% was lower thanthat of the commercial PE separator while the porosity ofthe separator dried at above R.H. 25% was higher than that ofthe commercial PE separator. This can be attributed to the factseparators (F12BMS, 12mm, Tonnen) were then dip coated in thecoating solution and dried under various humidity conditions(relative humidity (R.H.) 0%, 25%, and 50%). A relative humidity of0% was achieved in a Ar-lled glove box (less than 5ppm O2 andH2O) while relative humidities of 25% and 50% were achieved byusing a humidity control system.

The coated separators were sealed in PE packages and purgedwith dry nitrogen prior to electron beam irradiation using aconventional electron beam accelerator (EB-tech Co. Ltd., Daejeon,Korea) at a radiation dose of 50, 100, 150, and 200kGy, at a doserate of 10 kGy/pass at room temperature (acceleration voltage of1MeV and current density of 7.46mA).

The porosity (P) was determined using the following Eq. (1)where ma and mp represent the weights of the polymer-coated PEseparator after and before a impregnation with 1-butanol,respectively, ra and rp represent the densities of 1-butanol andthe dried polymer-coated PE separators, respectively

ma=rthat humidied atmosphere (in this case, at above R.H. 25%) leads0

30

35

40

45

50

Por

osity

(%)

Ref R.H. 0 % R.H. 25 % R.H. 50 %

5 10 15 20pores by a phase inversion process under R.H. 0% coating

condition. It was observed that the membranes prepared at R.H.25% and 50% coating conditions showed higher uptakes ofelectrolytes and this can be due to the formation of additionalpores by a phase inversion process at the higher humidityconditions. Since the presence of hydrophilic PEGDMA increasesthe porosity as shown in Fig. 1, membranes containing PEGDMAalso showed higher EL uptakes. Fig. 2 also showed that the ELuptakes of the membranes prepared at above R.H. 25% coatingcondition decreases with increasing the absorption dose. Thisbehavior is more severe when the membranes prepared in thepresence of PEGDMA (d, f in Fig. 2) were applied since morecrosslinked networks were formed by EB irradiation of thesamples containing PEGDMA. Although the EL uptakes decreasewith increasing irradiation dose, the irradiated membranesprepared at R.H. 50% (e, f in Fig. 2) showed higher EL uptakesthan the commercial PE separators.

The results of the ionic conductivity of the prepared mem-branes are presented in Fig. 3. Since the ionic conductivity ismainly dependent on the liquid electrolyte encapsulated in theseparators, it showed a similar behavior to the EL uptake resultsshown in Fig. 2. The ionic conductivity increased with an increasein the humidity but decreased with the increase in the absorption

In this study, we report that PVDF-HFP/PEGDMA-coated PEseparators were prepared successfully by a dip coating at different

irradiation dose were found to be important in determining liquidelectrolyte uptakes, ion conductivity, and thermal shrinkage of theprepared separators. The highly porous separators showed anenhanced electrolyte uptake and ionic conductivity. The EBtreatment of the separators containing PEGDMA was found togreatly improve the thermal shrinkage of the prepared separatorsby the formation of crosslinked networks. We believe that thissimple radiation processing for the modication of a commercialPE separator could be a very useful method to improve theelectrical and thermal properties of the separator.

Acknowledgement

This work was supported by the Nuclear R & D Program of theMinistry of Science & Technology, Korea.

Reference

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Kalyana Sundaram, N.T., Subramania, A., 2007. Microstructure of PVdF-co-HFPbased electrolyte prepared by preferential polymer dissolution process.J. Membr. Sci. 289, 16.

Kanamura, K., 2002. Lithium Secondary Battery Technology for the 21st Century.CMC Publishing, Tokyo, pp. 116124.

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2025

J.-Y. Sohn et al. / Radiation Physics and Chemistry 78 (2009) 505508 507dose. A steep decrease of the ion conductivity due to thecrosslinking of PEGDMA by EB irradiation was also observed. Itis inferred that the increased crosslinked networks of the coatedpolymer layer hindered the swelling of the separator thusretarded the mobility of the conducting ions. The separatorsprepared at above R.H. 25% showed higher ion conductivities thanthe original PE separators up to 200kGy.

Since a thermal shrinkage of a separator at a high temperaturecauses the internal short circuit of lithium-ion battery, measuringthe thermal shrinkage is very important to evaluate the thermalstability of the prepared separator. Fig. 4 shows the thermalshrinkage of the polymer-coated PE separator after heat treatmentof 150 1C for 1 h, as a function of the PEGDMA content. Thecommercial separator showed 76% thermal shrinkage while all thepolymer-coated PE separators showed a lower thermal shrinkagepercent. It was also found that the thermal shrinkage of themembrane irradiated at 200 kGy decrease with adding PEGDMAdue to the formation of crosslinked networks with the PVDF-HFP/PEGDMA and the PE separator, whereas the thermal shrinkage ofthe non-irradiated membrane increase with adding PEGDMA. This

0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

4.0x10-4

4.5x10-4

Ioni

c co

duct

ivity

(S/c

m)

Absorption dose (kGy)

PE separator

a b c d e f

50 100 150 200

Fig. 3. Ionic conductivity of PVDF-HFP/PEGDMA-coated PE separators as afunction of the absorption dose. (a) PVDF-HFP/PEGDMA(10/0)-coated PE separator

under R.H. 0%, (b) 8/2 under R.H. 0%, (c) 10/0 under R.H. 25%, (d) 8/2 under R.H.25%, (e) 10/0 under R.H. 50%, and (f) 8/2 under R.H. 50%.humidity levels (050%), followed by electron beam irradiation.Micro-porous structures generated during dip-coating process athigh humidity levels (above R.H. 25%), PEGDMA content, and EBresult indicates that the addition of a crosslinker, PEGDMA, duringa dip coating of PE separator and subsequent EB irradiation leadsto an enhanced thermal resistance.

4. Conclusions

20100PEGDMA content (wt %)

Fig. 4. Thermal shrinkage of PVDF-HFP/PEGDMA-coated PE separators as afunction of PEGDMA contents. (a) PVDF-HFP/PEGDMA-coated PE separator under

R.H. 0% at 0 kGy, (b) under R.H. 50% at 0 kGy, (c) under R.H. 0% at 200kGy, and (d)

under R.H. 50% at 200kGy.3035

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mal

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inka

ge (%

)

a b c d

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Takemura, D., Aihara, S., Hamano, K., et al., 2005. A powder particle size effect onceramic power based separator for lithium rechargeable battery. J. PowerSource 146, 779783.

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Wang, X.L., Mei, A., Li, M., et al., 2006. Effect of silane-functionalized mesoporoussilica SBA-15 on performance of PEO-based composite polymer electrolytes.Solid State Ionics 177, 12871291.

Yuan, F., Chen, H.Z., Yang, H.Y., et al., 2005. PAN-PEO solid polymerelectrolytes with high ionic conductivity. Mater. Chem. Phys. 89,390394.

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J.-Y. Sohn et al. / Radiation Physics and Chemistry 78 (2009) 505508508

Preparation and characterization of a PVDF-HFP/PEGDMA-coated PE separator for lithium-ion polymer battery by electron beam irradiationIntroductionExperimentalResults and discussionConclusionsAcknowledgementReference