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Materials Chemistry and Physics 146 (2014) 545e550

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Effect of SiO2 coating on polyethylene separator with differentstretching ratios for application in lithium ion batteries

K. Prasanna a, Chang-Soo Kimb, Chang Woo Lee a,*

aDepartment of Chemical Engineering, College of Engineering, Kyung Hee University, 1732 Deogyeong-daero, Gihung, Yongin,Gyeonggi 446-701, South KoreabDepartment of Materials Science and Engineering, University of WisconsineMilwaukee, Milwaukee, WI 53211, USA

h i g h l i g h t s

� Thickness is maintained around 25 mm after coating.� The stretched, coated separator shows reduced interfacial resistance.� The SiO2 coated separator possesses higher thermal properties.� The stretched, SiO2 coated separator shows improved Li-ion cell performance.

a r t i c l e i n f o

Article history:Received 15 September 2013Received in revised form8 February 2014Accepted 5 April 2014

Keywords:CeramicsCoatingsElectrochemical propertiesMechanical properties

* Corresponding author. Tel.: þ82 31 201 3825; faxE-mail address: cwlee@khu.ac.kr (C.W. Lee).

http://dx.doi.org/10.1016/j.matchemphys.2014.04.0140254-0584/� 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

To enhance the properties of polyethylene separators in lithium ion batteries, we tested separators withuni-axial stretching ratios of 180% and 300%. We also tested stretched separators coated with SiO2

ceramic substance to increase ionic conductivity and thermal stability without sacrificing mechanicalproperties. To test the thermal and tensile properties, thermomechanical analyzer (TMA) is employed. CR2032-type coin cells are prepared by sandwiching pristine and coated stretched separators, respectively,between the Li anode and Li[Ni1/3Co1/3Mn1/3]O2 cathode to evaluate the AC impedance and cyclingperformance. The coated separators are observed with superior ionic conductivity, thermal and tensileproperties. The cells prepared with coated separator have slightly higher discharge capacity and a bettercapacity retention ratio than the cells with pristine separators. These results suggest that the coatedseparator is a better option for lithium ion batteries.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Although high power-density and high energy-density batteriesare desired, there are safety issues with these batteries to beaddressed. Internal short-circuits can occur due to direct contactbetween the anode and cathode, which can cause explosions [1,2].The thermal and mechanical properties of the separators play a keyrole in avoiding internal short circuits. The most common separa-tors in lithium ion batteries are polyolefin separators, namelypolyethylene (PE) and polypropylene (PP) [3]. In general separatorsshould have high ionic conductivity, good mechanical and dimen-sional stability, high electrolyte wettability, sufficient physicalstrength to allow easy handling during battery assembly, goodthermal properties, and uniform thickness [4].

: þ82 31 204 8114.

Polyolefin separators used in lithium ion batteries have severalpositive and negative characteristics. Recent research has tried toreduce the negative characteristics of polyolefin separators. Themostimportant drawbacks of polyolefin separators are the thermal insta-bility and mechanical strength, which can result in deformationleading to fire or explosion [3,4]. Research has also focused onimproving ionic conductivity. To enhance separator properties, re-searchers have developed self-standing inorganic separators [3],stretched separators [5], nanocomposite-coated PE separators [6],non-woven-ceramic composite separators [7], and nanofiber-basedseparators prepared by electrospining [8]. Among these, coatingseparators with ceramic composite has drawn considerable attentionbecause it improves thermal and mechanical properties at low cost.The disadvantage of ceramic coating is its increased thickness.Increasing separator thickness reduces the active material weight [4]during battery assembly.

We compared the ionic conductivity and tensile strength of PEseparators uni-axially stretched to ratios of 180% and 300%. We also

K. Prasanna et al. / Materials Chemistry and Physics 146 (2014) 545e550546

had a dip coating on stretched separators with commerciallyavailable SiO2 ceramic composites to improve the ionic conduc-tivity and thermal properties. To compare pristine and coatedstretched PE separators, we use the following analyses: morpho-logical features using field emission scanning electron microscope(FE-SEM), crystallinity using X-ray diffractometer (XRD), wetta-bility using contact angle, tensile and thermal property using aTMA. To test their applicability to lithium ion batteries, we analyzedelectrochemical properties using AC impedance analyzer andchargeedischarge cycler.

2. Experimental

2.1. Preparation of ceramic coated separator

Commercially available SiO2 (Quartz, particle size of around400 nm) and polyvinylidene difluoride (PVDF, Kynar 761) weremixed in a 90:10 wt.% ratio. PVDF (0.2 g) was dissolved in 100 mlacetone, and then SiO2 powder (1.8 g) was added and vigorouslymixed with a magnetic stirrer for 2 h. The solution was then son-icated for 5 min. PE separators with stretching ratios of 180% and300% have mean pore size of 0.04 mm and 0.08 mm, and porosityratio of 40% and 45%, respectively, with the thickness of 21 �1 mm.Both these separators were dipped into the sonicated solution for1 min. SiO2 particles were attached to both sides of the PE sepa-rator. The coated separator was left at room temperature for 1 h toevaporate the solvent and was further vacuum dried at 50 �C for4 h. The final ceramic-coated separator was 25 � 1 mm thick, whenmeasured by FE-SEM, which is thicker than the pristine separator.

2.2. Characterizations

X-ray diffraction analysis was conducted to reveal the crystallinestructure of the pristine and coated PE separators using Cu Ka X-rayat 40 kV and 100 mA. The scan covered a 2q range of 15e40�.

The cross sectional image and the surface view of the coatedseparators was observed using low voltage FE-SEM (Leo Supra 55,Genesis 2000, Carl Zeiss). The samples were attached to the sampleholder with carbon tape, and silver coating was applied to enhanceelectronic conductivity.

A contact angle was measured using a tensiometer (Sigma 701,KSV Instruments Ltd) with the sessile drop method. Measurementswere taken by analyzing sitting drops on the pristine and coated PEseparators. Water droplets were limited to about 0.5 ml to preventdistortion of the spherical profile. Surface energies were calculatedby measuring the contact angle of the testing liquid on the samplesurfaces at room temperature.

Electrolyte uptake by the separators was determined by soakinga circular piece of membrane in the electrolyte solution 1M LiPF6 inEC:DEC (1:1, vol.%). Thewetted separator wasmeasured at differentsoaking intervals. Excess electrolytes on the separator surface wereremoved by wiping gently with tissue paper. All experiments wereperformed in an organic atmosphere. The electrolyte uptake % wascalculated using the following Eq. (1),

Electrolyte uptake % ¼ w2 �w1

w1� 100 (1)

where w1 is the initial separator weight, i.e. before soaking, and w2is the final separator weight, i.e. is after soaking.

For the ionic conductivity (s, mS cm�1) measurement, CR 2032-type coin cell was assembled with the separator immersed in the1 M LiPF6 in EC:DEC (1:1, vol.%) electrolyte and placed between twostainless steel (SS) electrodes (16 mm diameter). The cell has beenkept with no electrochemical run for the separator to get saturated

with the electrolyte for 24 h. Measurements were carried out usingan electrochemical analyzer (IVIUM Technologies Instruments)over the frequency range from 0.1 Hz to 100 kHz at 10 mV scan rate.The ionic conductivity is calculated using Eq. (2),

s ¼ lRbr2p

(2)

where l is the thickness and r is the radius of the sample separatorsused respectively. Rb is the bulk resistance obtained from ACimpedance measurements [9].

Dimensional changes and ultimate tensile strength of the pris-tine and coated separators were investigated by TMA (Q400 EM, TAInstruments). The separator samples were about 5 mm long and5 mm wide, and were held in the mini-instron type grips. A con-stant 2 g load was applied while the temperature was ramped at5 �C min�1 until the film ruptured. During this analysis shrinkageonset temperature, deformation temperature, and rupture tem-perature were recorded.

CR 2032-type coin cells were assembled by sandwiching theseparator alongwith the liquid electrolyte,1M LiPF6 in EC:DEC (1:1,vol.%), between the Li anode and Li[Ni1/3Co1/3Mn1/3]O2 cathode.The coin cells were assembled in a glove box filled with argon. ACimpedance spectroscopy was adopted to observe electrochemicalimpedance behavior using the electrochemical analyzer over thefrequency range from 0.1 Hz to 100 kHz at 10 mV scan rate. Halfcells were typically cycled between 2.7 V and 4.2 V at constant C-rate of 0.1 C at room temperature using an electrochemical cycler(BT2000, Arbin Instruments) for 100 chargeedischarge cycles tomeasure electrochemical capacity and cycling stability. Also thecells were charged at 0.1 C from 2.7 V to 4.2 V and discharged atdifferent C-rate namely 0.1, 0.5, 1, and 5 C from 4.2 V to 2.7 V.

3. Results and discussion

The crystalline structure, the surface morphology and thewettability of the prepared separators are explained using XRD, FE-SEM and contact angle analysis. Fig. 1 shows XRD patterns of (a)180% and 300% stretched-pristine PE separators and (b) 180% and300% stretched-coated PE separators. Fig. 1(a) presents sharp andintense peaks at 2q values of approximately 21.5� and 23.9� [10]and the intensity of PE peak decreased with increased stretchingratio. Fig. 1(b) shows the XRD pattern of coated PE separator (180%and 300% stretched), with a peak at a 2q value of approximately 27�,which represents the SiO2 [11] ceramic substance. The PE peakintensities decreased with the uptake of ceramic substance, asobserved in Fig. 1(b) where the 300% stretched, coated separatorshows significant decrease in PE peak intensities with the sharpincrease in SiO2 peak intensity. Based on the XRD patterns observedfor coated separators, it is concluded that the SiO2 particles maskthe PE separator and thereby reduce the PE peak intensity [11e13].On comparisonwith the coated and uncoated separators the overallcrystallinity is observed to be reduced for the coated separators.With regard to the literature, tensile strength is expected to in-creasewith the higher stretching ratio [14], and the ceramic coatingis likely to enhance the thermal properties along with improvedionic conductivity [14,15].

Fig. 2 shows FE-SEM images from (a) to (d) and contact anglesfrom (e) to (h) of the pristine and ceramic-coated stretched sepa-rators. Fig. 2(a) and (b) shows images of the 180% and 300%stretched separators, respectively, without ceramic coating. The300% stretched separator is observed with large pores and higherindentation than the 180% stretched separator. The surface of theceramic-coated separators is shown in Fig. 2(c) and (d), where theSiO2 particles are observed to be well inter-connected by the PVDF

15 20 25 30 35 40

300%-pristine

180%-pristine

Inte

nsity

(a.u

.)

2θ (degree)

PE

PE

(a)

15 20 25 30 35 40

180%-coated300%-coated

SiO

2

PE

PE

2θ (degree)

Inte

nsity

(a.u

.)

(b)

Fig. 1. XRD patterns of (a) 180% and 300% stretched-pristine PE separators; (b) 180%and 300% stretched-coated PE separators.

K. Prasanna et al. / Materials Chemistry and Physics 146 (2014) 545e550 547

binder. Even though the surface image of the 180% stretched,ceramic-coated and 300% stretched, ceramic-coated separatorsappears uniform, the XRD analysis proves that 300% stretched,ceramic-coated separator possess higher amount of ceramic

Fig. 2. FE-SEM images of (a) 180%-pristine; (b) 300%-pristine; (c) 180%-coated; (d) 300%-coa(h) 300%-coated separators.

substance which would have been influenced by the large poresand higher indentation of the 300% stretched separator. The pres-ence of ceramic substance is expected is to play a key role infacilitating the wettability nature through which the ionic con-ductivity and it is also expected to enhance the thermal stability.These hypotheses are supported by further studies.

Wettability of the polyolefin separator was confirmed usingcontact angle analysis. The surface energy of pristine and coatedseparators at both stretching ratios was calculated by measuringthe change in angle of the testing liquid, water. The angle decreasedwith increased stretching ratio and ceramic coating. Lower theangle higher the hydrophilic nature. The angles for stretched-pristine separators were 100.86� and 111.32� for 180% and 300%stretching, respectively, as shown in Fig. 2(e) and (f). 180% stretchedseparator had lower contact angle due to lower porosity value asdescribed by Wenzel and Cassie-Baxter model. The contact anglesof coated separators were 36.40� and 30.45� for 180% and 300%stretching, significantly lower than those of the pristine, respec-tively, as shown in Fig. 2(g) and (h), mainly due to the ceramicsurface coating. Lower angle is observed for the 300% stretched,coated separator due to more uptake of ceramic substance asproved by XRD patterns.

To determine the electrolyte characteristics of separator, elec-trolyte uptake percentage was calculated [17]. Fig. 3 shows theelectrolyte uptake percentage of separatorswith respect to the timesoaked in electrolyte. Electrolyte uptake percentage increased withan increased stretching ratio as well as with the ceramic coating.The uptake speed was also higher for ceramic-coated separatorsthan pristine separators. Ceramic coated separators were morewettable than uncoated separators due to the hygroscopic nature ofthe ceramic material and the unusual porous structure formed dueto the interstitial voids between the inter-connected SiO2 particles.The 300% stretched, coated separator posse’s higher electrolyteuptake percentage due to the higher porosity value of the 300%stretched separator along with presence of ceramic substance inlarge amount comparatively. The more wettability and electrolyteuptake would highly influence the ionic conductivity [15,18].

The 180% stretched and 300% stretched PE separators areobserved with ionic conductivity of 0.59 mS cm�1 and0.98 mS cm�1 which is lower than the 180% and 300% stretched,coated separators which is 1.32 mS cm�1 and 1.85 mS cm�1,respectively. The increase in ionic conductivity is attributed to the

ted separators. Contact angles of (e) 180%-pristine; (f) 300%-pristine; (g) 180%-coated;

Table 1TMA data for stretched-pristine and stretched-coated separators.

Property 180%-Pristine

300%-Pristine

180%-Coated

300%-Coated

Shrinkage onset temperature, �C 60 62 94 123Deformation temperature, �C 125 128 140 153Rupture temperature, �C 139 141 151 169

All temperatures are accurate to within �1 �C.

0

30

60

90

120

150

180

0 100 200 300 400 500

180%-pristine180%-coated300%-pristine300%-coated

Z' (Ω)

-Z"

(Ω)

Fig. 5. AC Impedance spectra of Li/separator/Li[Ni1/3Co1/3Mn1/3]O2 cells at open circuitpotential.

0

100

200

300

400

500

600

0 10 20 30 40 50 60

180%-pristine 300%-pristine180%-coated 300%-coated

Elec

trol

yte

Upt

ake

(%)

Time (minutes)

Fig. 3. Electrolyte uptake percentage of separators with respect to the time soaked inelectrolyte.

200180%-pristine

-1)

K. Prasanna et al. / Materials Chemistry and Physics 146 (2014) 545e550548

increase in the electrolyte uptake percentage and decrease incrystallinity [13,18,19].

Fig. 4 shows TMA data of stretched-pristine and stretched-coated separators. Thermal analysis was carried out using TMA,which illustrates the tensile response of the pristine and coatedseparators at constant load and varying temperatures with a ramprate of 5 �C min�1 in the transverse direction. As observed in Fig. 4,the coated separators are thermally more stable, as evidenced bythe thermal properties like having the shrinkage onset tempera-ture, deformation temperature, and rupture temperature. The dif-ferences in thermal properties are shown in Table 1. The thermalproperties do not change significantly with the stretching ratio, butthe tensile strength is expected to increase with the increase instretching ratio. The thermal properties increase significantly withthe ceramic coating [20,21] whichmeans the increase in safety. Thehigher stretching ratio and large amount of ceramic particle on the300% stretched, coated separator enhanced its tensile and thermalproperty [14e16] giving extra safety to lithium ion batteries.

For applicability of the pristine and coated separators for lithiumion batteries, their electrochemistry was analyzed bymeasuring ACimpedance spectroscopy and discharge capacity corresponding tocycle numbers. Fig. 5 shows AC Impedance spectra of Li/separator/Li[Ni1/3Co1/3Mn1/3]O2 cells at open circuit potential. The reducedinterfacial resistance using AC impedance spectroscopy indicatesthat coated separators take up more electrolytes than pristineseparators, as observed in the electrolyte uptake analysis. Reducedinterfacial resistance corresponds to higher ionic conductivity andenhances the charge/discharge reactions. Fig. 6 shows cycling

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

0 20 40 60 80 100 120 140 160

180%-pristine300%-pristine180%-coated300%-coatedD

imen

sion

al C

hang

e (μ

m)

Temperature (°C)

Fig. 4. TMA data of stretched-pristine and stretched-coated separators.

performances of Li/Li[Ni1/3Co1/3Mn1/3]O2 cells using the stretched-pristine and stretched-coated PE separators. The prepared half-cellwas cycled between 2.7 V and 4.2 V at a constant C-rate of 0.1 C. Atthis rate, the effect of a coated separator in a lithium battery is clearas shown in Fig. 6. The cell using coated separators had a slightlyhigher discharge capacity and more efficient cycling stability thanthe pristine separators. The coated separators had initial dischargecapacities of 170.91 mAh g�1 and 168.78 mAh g�1 and capacityretention ratios of 96.5% and 95.7% at 300% and 180% stretching,respectively. The 300% stretched, coated separator had a slightlyhigher initial discharge capacity and retention ratio, due to thelower interfacial resistance attained due to the higher electrolyteuptake percentage and improved ionic conductivity [22]. Thepristine separators with 300% and 180% stretching ratio hadcomparatively lower discharge capacities of 167.70 mAh g�1 and

100

120

140

160

180

0 10 20 30 40 50 60 70 80 90 100

300%-pristine180%-coated300%-coated

Spec

ific

Disc

harg

e C

apac

ity(m

Ah

g

Cycle Number

Fig. 6. Cycling performances of Li/Li[Ni1/3Co1/3Mn1/3]O2 cells using the stretched-pristine and stretched-coated PE separators.

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

4.3

0 50 100 150 200

0.1C0.5C1C5C

(a)

Vol

tage

(V)

Specific Discharge Capacity (mAh g-1)

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

4.3

0 50 100 150 200

0.1C0.5C1C5C

Vol

tage

(V)

Specific Discharge Capacity (mAh g-1)

(b)

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

4.3

0 50 100 150 200

0.1C0.5C1C5C

Specific Discharge Capacity (mAh g-1)

Vol

tage

(V)

(c)

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

4.3

0 50 100 150 200

0.1C0.5C1C5C

(d)

Vol

tage

(V)

Specific Discharge Capacity (mAh g-1)

Fig. 7. Discharge profiles of (a) 180%-pristine; (b) 300%-pristine; (c) 180%-coated; (d) 300%-coated separators. The cell was charged at 0.1C-rate from 2.7 V to 4.2 V and discharged at0.1, 0.5, 1 and 5C-rate.

K. Prasanna et al. / Materials Chemistry and Physics 146 (2014) 545e550 549

163.74 mAh g�1 and capacity retention ratios of 84.9% and 79.2%,respectively. The coated separators had better capacity and reten-tion ratio due to the ability of coated separators to encapsulate theelectrolyte within the matrix which contributes to a good ionconduction channel and high ionic conductivity of separator.Whereas the pristine separator may lack or leak electrolyte duringrepeated cycling. The cells with different separators were chargedat constant C-rate of 0.1 C and discharged at different C-rate of 0.1,0.5, 1, and 5 C as shown in Fig. 7. The obtained discharge profile atdifferent C-rate is almost similar for both the stretched, pristine andstretched, coated separators. The discharge capacity at various C-rates shows no significant changewith regard to the 180% and 300%stretched, pristine separators as shown in Fig. 7(a) and (b), whereascompared to the 180% and 300% stretched, coated separator(Fig. 7(c) and (d)), the stretched, pristine separator has slightlyhigher polarization effect and significant reduction in dischargecapacity at high C-rate. At the high C-rate of 5 C the 180% stretched,pristine separator has the lowest discharge capacity of104.85 mAh g�1, which is 63.39% lower than that of the dischargecapacity obtained at 0.1 C. Similarly at 5 C rate, the 300% stretched,pristine separator keep hold of 65.40%, the 180% stretched, coatedseparator keep hold of 84.45%, and 300% stretched, coated sepa-rator holds highest discharge capacity of 86.64% with that of thedischarge capacity obtained at 0.1 C. The 300% stretched, coatedseparators had significantly higher discharge capacity at high C-rateand better rate capability due to its efficient transport lithium ionsbetween the electrodes, using its high porous structure and its af-finity towards the used electrolyte.

4. Conclusions

We compared separatorswith uni-axial stretching ratios of 180%and 300%. Also we compared stretched separators that were in dipcoating with a ceramic substance, SiO2. The 300% stretched, SiO2-

coated separator showed improved wettability and higher ionicconductivity value of 1.85mS cm�1 due to the high porosity value ofthe 300% stretched separator and also due to the presence of SiO2particles in large amount compared to 180% stretched, SiO2-coatedseparator as proven by XRD pattern. The incredible percentage ofelectrolyte uptake by the 300% separator confirms its affinity to-wards the used electrolyte and its large surface area. The enhancedwettability, electrolyte uptake, and ionic conductivity of the 300%separator positively reflected on the electrochemical behavior oflithium ion batteries. The 300% separator showed slightly improveddischarge capacity of 170.91 mAh g�1 with excellent capacityretention ratio of 96.5% at 0.1 C and it also expressed better ratecapability behavior due to the reduced interfacial resistance. Withregard to the thermal properties, the 300% separator possessessignificantly higher shrinkage onset, deformation, and rupturetemperature of 123, 153, and 169 �C, respectively, and it wouldhighly enhance the safety of lithium ion batteries. As a consequencethe 300% stretched PE separator with SiO2 ceramic coating is rec-ommended as a potential alternative to the conventional PEseparator.

Acknowledgments

This research was supported by a grant (C12120710) fromGyeonggi Technology Development Program funded by GyeonggiProvince.

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