ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 71 (2010) 329–335

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

� Corr

E-m

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

Molecular interaction and ionic conductivity of polyethyleneoxide–LiClO4 nanocomposites

Arup Dey, S. Karan, S.K. De �

Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

a r t i c l e i n f o

Article history:

Received 10 June 2009

Received in revised form

24 September 2009

Accepted 25 December 2009

Keywords:

A. Polymers

C. X-ray diffraction

D. Electrical conductivity

97/$ - see front matter & 2009 Elsevier Ltd. A

016/j.jpcs.2009.12.085

esponding author. Tel.: +91 33 24734971; fa

ail address: msskd@mahendra.iacs.res.in (S.K.

a b s t r a c t

Nanoparticle ceria (CeO2) has been used as inorganic filler in high molecular weight PEO2LiClO4 matrix

to improve the ionic conductivity of polymer–salt complex at ambient temperature. X-ray diffraction

and Fourier transform infrared spectra confirm the formation of PEO2LiClO4 salt complex and the

nanocomposite. Based on the FTIR data, the bands in the C–O–C stretching band and nðClO�4 Þ band have

been assigned and the effect of CeO2 on ion–ion and ion–polymer interactions in the polymer

electrolyte has been examined. Glass transition temperature and percentage of crystallinity are strongly

influenced by adding ceria nanoparticles. The highest conductivity of 2:43� 10�3 S cm�1 has been

achieved at room temperature for 10 wt% of CeO2. Temperature dependence of ionic conductivity

follows Arrhenius mechanism.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In the last few decades, there has been considerable interest inpolymer electrolytes due to their potential importance in thedevelopment of electric and load leveling vehicular applications[1,2]. Polyethylene oxide (PEO) based polymer electrolyte hasattracted great attention for high energy density and high powerlithium-ion batteries because of its ease formation of complexwith lithium salt, flexibility, stable mechanical properties,comparatively high mobility of charge carriers, etc. Pure PEOshows both crystalline and amorphous phase at room tempera-ture and the order of the conductivity is only about 10�7 S cm�1

[2–6]. Such a poor room temperature conductivity is due to thehigh concentration of crystalline phase present in pure PEO and isnot suitable for practical applications. It is well recognized thatincorporation of various low lattice energy lithium salts, LiX;X¼ ClO�4 , BF�4 , PF�6 etc. and liquid plasticizers like propylenecarbonate (PC), ethylene carbonate (EC) and polyethylene glycol(PEG) into the pure PEO improve the ionic conductivity about twoorders of magnitude [7,8]. While the mechanical properties andpotential stability of the thin film is far below the level of practicalapplications. On the other hand, irradiation or chemical cross-linking improves the mechanical properties [9–11] providingrigidity in the polymer chain but a consequent decay in ionicconductivity is observed.

An effective approach to achieve the balanced characteristicsof ionic conductivity, mechanical properties and electrolyte/

ll rights reserved.

x: +91 33 24732805.

De).

electrode interfacial stability is of current interest. Recently, it isreported in the literature that the addition of oxides such asAl2O3, TiO2, SiO2, etc. nano-sized particles reduce the crystallinityof the composite polymer electrolyte and improve its Liþ mobilityand mechanical stability [2,3,5,6,12–17]. Particle size and con-centration of the oxide fillers enhance ionic conductivity bymeans of reducing the polymeric crystallinity. However, it hasalso been reported that the glass transition temperature decreaseswith the addition of some fillers in polymer–salt complex andhence the crystallinity increases [18]. The effect of nano-sizedceramic filler in promoting ion transport has not yet wellunderstood. In this present work, nanoparticle CeO2 has beendispersed on PEO-base polymer electrolyte containing LiClO4 asthe salt. The ceramic filler CeO2 acts as solid plasticizer for PEO bykinetically inhibiting the chains crystallization. This helps tostabilize the amorphous phase of the polymer electrolyte at lowertemperature and leads to an increase of the range of ionicconduction. In addition, the Lewis acid–base interactions occurbetween CeO2 nanoparticle and both the lithium salt anion andthe PEO segments which result enhancement of the lithium iontransference number [19,20]. The effect of CeO2 nanoparticle onthe above polymer compositions is investigated using Fouriertransform infrared spectroscopy, differential scanning calorimeterand impedance spectroscopy.

2. Experimental

Nanoparticle CeO2 was prepared as described previously [21]using ammonium ceric nitrate, ðNH4Þ2½CeðNO3Þ6� (Qualigens,Mumbai, India) and citric acid monohydrate, both were in

ARTICLE IN PRESS

a

b

10 20 30 40 50 60

c

Inte

nsity

(a.u

)

2θ (degrees)

Fig. 1. X-ray diffraction patterns of (a) pure PEO; (b) PEO2LiClO4 and (c)

PEO2LiClO42CeO2 25 wt%.

e

dcth

erm

)

A. Dey et al. / Journal of Physics and Chemistry of Solids 71 (2010) 329–335330

analytical grade. Aqueous solution of ammonium ceric nitrate wasmixed with aqueous solution of citric acid with a constant citrateto nitrate ratio (C/N). The mixed homogeneous solution wasallowed to evaporate at about 200 3C with continuous stirringuntil the solution became viscous and turned into gel during theheating process. The gel slowly foamed, swelled and finally burntas a result of autoignition and within a few second whole samplewas fully burnt to produce yellow-colored CeO2 powder. Whenthe autoignition process completed the yielded powder was againcalcined at 200 3C for 2 h. The average particle size synthesized bythis process was about 10 nm.

Pure polyethylene oxide (PEO) (Alfa Aesar, molecular weight,Mw¼ 106) and LiClO4 (Aldrich) were weighed to the desiredoxygen:lithium ratio in PEO ½EO�=½Li� ¼ 8. The salt was dried at120 3C in a vacuum oven before use. In solution-casting technique,LiClO4 was first dissolved in methanol. After the salt wascompletely dissolved, a predetermined amount of PEO was addedand stirred using a magnetic stirrer continuously. Finally, CeO2

filler of different wt% (0, 5, 10, 15, 20 and 25) was dispersed in thesolution. The solution was thoroughly mixed and stirred at 60 3Cfor 24 h. The resulting homogeneous solution was poured intopolypropylene petri dish and vacuum dried at 50 3C for 48 h toremove all traces of the solvent. The thin films thus obtained werepreserved in a vacuum desiccator, prior to further characteriza-tion.

The macroscopic structure of different polymeric nanocompo-site films were examined by X-ray diffraction (XRD) analysis,which was performed at room temperature using a highresolution X’Pert PRO Panalytical X-ray diffractometer in therange 102603 using Cu Ka radiation.

Differential scanning calorimetry (DSC) was carried out onPerkin Elmer Diamond DSC in a temperature range between �80 3

and 100 3C with a scan rate of 20 3C=min under a constant flow(100 ml/min.) of nitrogen gas to avoid any contact of atmosphericmoisture. Pure indium was used for temperature and enthalpycalibration of the instrument.

The Fourier transform infrared (FTIR) spectra were obtainedusing a computer interfaced Shimadzu FTIR-8300 spectrometer inthe transmission mode from 400 to 1600 cm�1 wave numberrange. In this experiment the gelatinous polymer solution wascast on a KBr palette and dried in vacuum.

Ionic conductivity was measured using the ac impedancetechniques with Agilent 4192A frequency response impedanceanalyzer in the temperature range 28263 3C, where the givennanocomposite electrolyte films were sandwiched between twopolished stainless-steel blocking electrodes. The resistance of thenanocomposite polymer electrolyte films was obtained from thecross point at high frequency of frequency dependent impedancewith the real axis from where the conductivity was calculated.Temperature was monitored by Eurotherm temperature control-ler (Model no. 2404) using thermocouple sensor.

-80 -40 0 40 80

b

aHea

t Flo

w (E

ndo

Temperature (°C)

Fig. 2. DSC traces of (a) pure PEO; (b) PEO2LiClO4; (c) 5 wt% CeO2, (d) 10 wt% CeO2

and (e) 15 wt% CeO2 of PEO2LiClO42CeO2.

3. Results and discussion

The X-ray diffraction patterns of PEO2LiClO42CeO2 nanocom-posites are shown in Fig. 1. The characteristic peaks of crystallinePEO at 2y¼ 19:13 and 23:303 are assigned as (120) and (112)reflections (Fig. 1(a)). Fig. 1(b) shows the XRD pattern ofPEO2LiClO4 salt complex where the diffraction peaks becomebroader confirming the decrease in crystalline phase of PEO in thesalt-complex. Intensities of two peaks are almost equal. In case ofPEO2LiClO42CeO2 composites the peaks at 2y¼ 28:49, 33.02,47.45 and 56:13 correspond to the F-type structure of CeO2

(Fig. 1(c)). The average crystallite size (t) of the CeO2

nanocrystallites has been estimated by Scherrer formula [22]

given by

t¼0:9lb cosy

ð1Þ

where l is the wavelength of X-rays and b is the full width at halfmaxima of XRD peaks related to CeO2. The calculated crystallitesize of CeO2 is 10 nm. The peak associated with crystallinestructure of PEO at about 233 is split into different small peaksconsistent with other inorganic fillers [23,24]. The appearance ofsmall peaks correspond to the formation of different orderedphases in small domains. The peaks are not distinct due to

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a

Wavenumber (cm-1)

b

c

d

Abs

orba

nce

(a.u

) e

f

g

1600 1400 1200 1000 800 600

Fig. 3. FTIR spectra of (a) pure PEO; (b) PEO2LiClO4 and (c) 5 wt% CeO2 (d) 10 wt%

CeO2 (e) 15 wt% CeO2 (f) 20 wt% CeO2 (g) 25 wt% CeO2 of PEO2LiClO42CeO2.

aliz

ed In

tens

ity

A. Dey et al. / Journal of Physics and Chemistry of Solids 71 (2010) 329–335 331

sufficiently small crystalline domains. PEO–salt consists ofcrystalline PEO, crystalline PEO2LiClO4 and amorphous phase.The salt modifies the peak at about 233 as evident from Fig. 2(b).This clearly suggests that the crystalline region of PEO2LiClO4 isinfluenced due to the heterogeneous nucleation of inorganic filler.Moreover intensity of peak around 233 is lower than that of peakaround 193 in contrary to pure PEO. These indicate that aninteraction between CeO2 and PEO modifies the crystallinebehavior of pure PEO and PEO2LiClO4 phases.

Detailed structural changes of the polymer electrolyte withheat treatment has been studied by DSC thermogram. Fig. 2reveals the typical DSC traces of (a) pure PEO, (b) PEO2LiClO4, (c)PEO2LiClO42CeO2 5 wt%, (d) PEO2LiClO42CeO2 10 wt% and (e)PEO2LiClO42CeO2 15 wt%, respectively. For each curve the glasstransition temperature (Tg), melting temperature (Tm), percentageof crystallinity (wc) have been estimated.

Glass transition temperature, Tg of the polymer involves thefreezing of large scale molecular motion without the change instructure. At Tg a glassy phase of the sample becomes rubberyamorphous phase upon heating. It also gives an idea about themiscibility of strength of the molecular interaction within thecomplex. The value of Tg decreases gradually from �50 to �59 3Cwith the increase in nanofiller concentration as presented inTable 1. This implies that the amorphous phase becomes moreflexible to its ease conformation of ether chain for maximuminteraction with ceria nanoparticles. Moreover, single Tg

throughout the whole range of composites suggests thecomplete miscibility of the components.

In DSC curve, a minimum signifying an endothermic processcan be identified as melting of crystalline phase of the polymer.The melting endotherm of pure PEO (Tm � 713C) is broaden andhas pronounced decrement with the addition of salt which showsthat the crystallization behavior of PEO in presence of salt iscompletely different. The addition of salt influences the crystal-lization process of PEO chain. The interaction between PEO andsalt results in depression of melting point. The small endothermicpeak at lower temperature 35 3C is due to small fraction of un-complexed PEO. Both Tm1 and Tm2 vary in rather narrowtemperature ranges, i.e., between 30235 3C and 48252 3C withthe increase in filler concentration. The value of wc of thenanocomposite has been calculated from the relation,wc ¼DHm=DH�m, where DH�m ¼ 213:7 Jg�1, is the melting enthalpyof a completely crystalline PEO sample [25]. The lowest value ofcrystallinity has been found at 10 wt% of CeO2. Addition of saltand nanoparticle reduce the wc , or in other word to say, enhanceamorphous phase considerably which may be due to Lewis acid–base interaction between three moieties of PEO, LiClO4 and CeO2.The values of Tg , Tm1, Tm2, DHm and wc are summarized in Table 1.

FTIR spectrum has been utilized to probe the miscibility andinteraction in the PEO2LiClO42CeO2 nanocomposites as shown inFig. 3. A crystalline phase is confirmed by the presence of C–O–Cstretching triplet peak at 1147, 1110 and 1061 cm�1 with a

Table 1Composition and glass transition temperature (Tg), melting temperature (Tm),

change of enthalpy of melting (DHm) and percentage of crystalline (wc) of different

PEO2LiClO42CeO2 systems.

Composition Tg ( 3C) Tm1 ( 3C) Tm1 (3C) DHm (J/g) wc (%)

Pure PEO – – 71 155 72.53

PEO2LiClO4 �50 35 48 36.64 17.15

PEO2LiClO42CeO2 5 wt% �51 34 51 34.87 16.32

PEO2LiClO42CeO2 10 wt% �51 32 48 28.53 13.35

PEO2LiClO42CeO2 15 wt% �56 30 49 33.46 15.66

PEO2LiClO42CeO2 20 wt% �57 31 52 32.65 15.28

PEO2LiClO42CeO2 25 wt% �59 30 50 32.03 14.98

maximum at 1110 cm�1 in pure PEO [26]. The bands at 1242 and1281 cm�1 are ascribed to CH2 twisting mode for pure PEO. Thepresence of the CH2 wagging doublet at 1342 and 1360 cm�1 inpure PEO indicates the crystalline phase of PEO. The frequency ofthese CH2 vibrations depend on the molecular conformations ofpolymer backbone and crystallinity. In the crystalline state [27]the PEO chain contains seven chemical units (CH2CH2O) and twohelical turns with a crystallographic period of 19.6 A. Theconformations along this chain determine crystallinity of PEO.Experimentally it was observed that the intensity of doublet at1342 and 1360 cm�1 decreases with decreasing crystallinity.These two peaks collapse into a broad band around 1350 cm�1

in amorphous phase [26,28,29]. The doublet CH2 componentsmerge into single peak at 1352 cm�1 in the salt complex. Thissuggests that crystallinity of PEO decreases due to complexformation.

640 630 620 610

Rel

ativ

e N

orm

Wavenumber (cm-1)

Fig. 4. Peak fitting of FTIR spectra for nðClO�4 Þ in PEO2LiClO4 with 25 wt% of CeO2

polymer electrolyte in the region of 650–600 cm�1.

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1150 1100 1050 1000

Inte

nsity

(a.u

)

Wavenumber (cm-1)

Fig. 5. Deconvolution of FTIR spectra for C–O–C band in pure PEO.

b

Inte

nsity

(a.u

)

1200 1150 1100 1050 1000

a

Wavenumber (cm-1)

c

Fig. 6. Deconvolution of FTIR spectra for C–O–C band in (a) PEO2LiClO4; (b) 5 wt%

CeO2 and (c) 25 wt% CeO2 of PEO2LiClO42CeO2 in the region of 12002965 cm�1.

A. Dey et al. / Journal of Physics and Chemistry of Solids 71 (2010) 329–335332

The FTIR peak of nðClO�4 Þ band at the region of 6502600 cm�1

is frequently used to analyze ion–ion interactions in PEO2LiClO4

based polymer electrolytes. The effect of CeO2 nanoparticles onlithium salt dissolution in the polymer electrolytes was studied byfitting the nðClO�4 Þ mode in FTIR spectra to Gaussian–Lorentzianproduct function [30] with straight base line. The maximumintensity has been normalized to unity. Fig. 4 is a typical exampleof a fit of a Gaussian-Lorentzian peak to the experimental FTIRdata in the nðClO�4 Þ region for PEO2LiClO42CeO2 compositesystem with the highest concentration of filler. It is observedthat the nðClO�4 Þ mode is split into two maxima in the ranges of620–625 and 6302635 cm�1. It is suggested that the nðClO�4 Þbands centered between 620 and 625 cm�1 can be attributed tospectroscopically free ClO�4 anions whereas the band centeredbetween 630 and 635 cm�1 is associated with the presence ofLiþ2ClO�4 contact-ion pairs [31,32]. By taking the ratio of theintegral area of each peak to the total area, the fraction of freeanions can be calculated. The percentage of free anion forPEO2LiClO4 is 69. As 5 wt% CeO2 is added, the ratio of freeanions rapidly increases from 69 to 79%, suggesting that more freeLiþ is created. However, the fraction of free anions remainsalmost same as the wt% of CeO2 increases.

The most noticeable changes in FTIR spectrum are observed in100021100 cm�1 region which involves different vibrationalmodes of C–O–C. The fitted FTIR in spectral 100021200 cm�1

region of pure PEO is shown in Fig. 5. The fitted curve consists offive bands at 1032, 1059, 1094, 1112, 1147 cm�1 which areattributed to C–O–C symmetric and asymmetric stretching anddeformation modes. The integrated intensity ðIÞ of each band iscalculated as Ip

RbandAðnÞdn, where AðnÞ is the observed

absorbance at wave number n. The relative integratedintensities of major peaks are determined by the ratio I=Imax.The maximum value of I=Imax occurs at 1112 cm�1 which has fullwidth at half maximum (FWHM) of 26 cm�1. The deconvolutedFTIR spectra of PEO2LiClO4 complex, 5 and 25 wt% CeO2 samplesare shown in Fig. 6(a–c). The C–O–C stretching vibrations revealtwo broad bands at 1097 and 1012 cm�1 in contrary to singlebroad band in pure PEO. Moreover the values of FWHM are 51 and41 cm�1 which are larger than PEO. These indicate the complexformation of Liþ of LiClO4 with ether oxygen of PEO. The relativeintegrated intensities of four bands as a function of ceriaconcentration are shown in Fig. 7. The values of I=Imax for 1097and 1012 cm�1 bands are changed significantly with variation of

CeO2 content. These two bands attain maximum value of I=Imax

alternately in the entire concentration range of ceria. The valuesof I=Imax at 1057 cm�1 are very close to 1097 cm�1 of undoped and25 wt% CeO2 doped PEO complex. The relative integrated intensityof peak at 1132 cm�1 decreases monotonically with increase ofceria concentration. The positions of four peaks vary in thespectral regions of 101221015 cm�1, 105721058 cm�1,109721106 cm�1, 112021132 cm�1, respectively. Theinteractions among PEO, LiClO4 and CeO2 are associated withthe changes in intensity, shape and position of these stretchingmodes.

The conductivity of the polymeric nanocomposite is deter-mined at different CeO2 concentrations from the compleximpedance measurements. The conductivity (s) of the polymercomplex in the form of films has been calculated from the bulk

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0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

1120-1132 cm-1

1097-1106 cm-1

1057-1058 cm-1

1012-1015 cm-1

Rel

ativ

e In

tegr

ated

Inte

nsity

wt. % of CeO2

Fig. 7. Variation of relative integrated intensity with CeO2 of four peaks in the

spectral range 100021150 cm�1.

0 5 10 15 20 25-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

log(

σ) (S

-cm

-1)

CeO2 concentration (wt.%)

Fig. 8. Variation of conductivity with CeO2 concentration at room temperature

and solid line is the best fitted to Eq. (3).

2.9 3.0 3.1 3.2 3.3 3.4-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

PEO:LiClO4-CeO2 0 wt.%PEO:LiClO4-CeO2 5 wt.%PEO:LiClO4-CeO2 10 wt.%PEO:LiClO4-CeO2 15 wt.%PEO:LiClO4-CeO2 20 wt.%PEO:LiClO4-CeO2 25 wt.%

log(

σ) (S

-cm

-1)

1000/T(K-1)

Fig. 9. Temperature dependence conductivity plots of different polymer electro-

lyte composition based on PEO2LiClO42CeO2.

A. Dey et al. / Journal of Physics and Chemistry of Solids 71 (2010) 329–335 333

resistance (Rb) determined from the interception of the highfrequency semicircle with the real axis in the complex impedance(Z1 and Z2) plot given by

s¼ d

RbAð2Þ

where d and A are the thickness and area of the film, respectively.The calculated conductivity as a function of CeO2 concentrationsis displayed in Fig. 8 which shows that the room temperatureconductivity attains its maximum value of 2:43� 10�3 S cm�1 atabout 10 wt% of CeO2. The plot reveals that the ionic conductivityhas the tendency of enhancement at higher concentration of CeO2

with a minimum value at about 15 wt%. The increasing behaviorin two concentration regime as shown in Fig. 8, in the variation oflogðsÞ as a function of CeO2 plot may be the result of two differentcharge carriers, i.e, Liþ and ClO�4 . Similar observations have alsobeen found by us and many other researchers for the compositepolymer electrolytes [21,33–38]. The variation of conductivitywith ceria concentration can be described by the superposition of

linear and sigmoidal function

sðxÞ ¼mxþA2þA1�A2

1þeðx�x0Þ=dxð3Þ

where m represents gradient of linear part and A1, A2, x0 and dx

are constants of sigmoidal function. The best fitted data as shownin Fig. 8 clearly indicates that the conductivity as a function ofceria concentration follows Eq. (3).

The variation of ionic conductivity as a function of inversetemperature for PEO-based composite polymer electrolytes wasestimated and the nature of plots are depicted in Fig. 9, over thetemperature range from 28 to 63 3C (301 to 336 K). It can beobserved from Fig. 9 that the ionic conductivity for all thecompositions of PEO2LiClO42CeO2 system increases withincreasing temperature for the entire range i.e, the conductivitycurves shows positive temperature co-efficient. The nature of thecurves are consistent with Arrhenius type charge conduction inpolymer electrolyte nanocomposites and the conductivityrelationship follows the equation,

s¼ s0expð�Ea=kTÞ ð4Þ

where s0 is the pre-exponential factor, Ea is the activation energyand k is the Boltzmann’s constant. The slope of each straight linegives the activation energy which lies between 0.10 and 0.30 eV.Plot of activation energy with ceria concentration is depicted inFig. 10 and is similar to mirror image of conductivity plot.Activation energy is large for lower conductivity and is small forhigher conductivity. The curve in Fig. 10 follows Eq. (3) withdifferent fitting constants.

Presence of segmental motion and amorphous phase in PEOplay important roles in the enhancement of ionic conductivity.The glass transition (Tg) temperature decreases with increase ofceria content. The lower value of Tg indicates more flexibility ofether chain to attain the most probable molecular conformations.As a result of it, ionic conductivity is enhanced with the additionof nanosized ceria. The local motion of polymer segmentscontributes to ionic transport in PEO. The interaction of ceriawith PEO may suppress the segmental motion for ceria concen-tration 410 wt% which gives rise to decrease in conductivity.

Quantitative analysis of the FTIR spectra in 100021200 cm�1

region of nanocomposites as shown in Fig. 7 indicate that theintensities of some peaks are significantly different from PEO–saltcomplex. The C–O–C bands are sensitive to molecular chainconformation and crystallinity. The C–O–C stretching vibrations

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0 5 10 15 20 25 300.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Act

ivat

ion

Ene

rgy

(eV

)

CeO2 concentration (wt.%)

Fig. 10. Variation of activation energy with CeO2 concentration and solid line is

the best fitted to Eq. (3).

A. Dey et al. / Journal of Physics and Chemistry of Solids 71 (2010) 329–335334

are characteristics of trans conformation of PEO chain. Thebroadening (FWHM) of the bands indicates the solvation of Liþ

ions by ether linkages and chain conformation is disorder.Comparison between FTIR spectra of PEO (Fig. 5) andPEO2LiClO4 complex (Fig. 6) reveals that a new broad band at1012 cm�1 appears in PEO2LiClO4 phase. The second broad bandat 1097 cm�1 is related to pure PEO phase. Fig. 7 demonstratesthat the integrated intensities of peaks at 1097 and 1012 cm�1

change dramatically with ceria concentration. The intensity of1012 cm�1 is highest whereas that of 1097 cm�1 is lowest for10 wt% CeO2 sample. The most interesting fact is that themaximum ionic conductivity is obtained for 10 wt% CeO2

nanocomposite. This clearly manifests the strong interaction ofnanosized ceria with PEO2LiClO4 phase which gives rise tohighest ionic conductivity.

The insertion of nanoparticles into the polymer matrix createsan interface surrounding CeO2 which in other word, increases theamorphous region of PEO and hence enhances the conductivity.Ionic conduction in PEO2LiClO4 complex arises due to the motionof Liþ ion along the PEO chain through breaking and makingcoordination with ether oxygen. PEO polymers have Lewis baseoxygen which coordinates with the Lewis acid sites on the surfaceof nanosized CeO2. Ceria has a strong Lewis acid property. Theconcentration of ClO�4 ions remains almost constant. But ClO�4anions have large affinity to Ce4þ of CeO2 nanoparticles. Anionmigration may be facilitated with increase of ceria concentration.Recently, small angle neutron scattering experiments [39] suggestthat ionic conductivity increases due to the formation ofpercolating conducting pathways near to filler particles. Theore-tically it was also suggested that the ionic conductivity maximumwith insulating filler concentration in polymer nanocompositescan be described by percolation theory based on enhancedinterface conductivity [40,41]. The high conducting interfacebetween polymer matrix (PEO2LiClO4) and dispersed particles(CeO2) forms the percolating network. Both cations and anionscontribute to the ionic conductivity which gives rise to twoseparate percolation thresholds consistent with present observa-tion as a function of ceria concentration.

4. Conclusion

The PEO-based composite polymer electrolyte was preparedby the incorporation of CeO2 nanoparticle and the characteriza-

tions have been carried out by XRD, DSC, FTIR and impedancemeasurements. Quantitative FTIR studies show the existence ofstrong Lewis acid–base interaction between the components, i.e,PEO, LiClO4 and CeO2 in the PEO2LiClO42CeO2 system whichaffects the vibrational C–O–C bands substantially. The highestconductivity, 2:43� 10�3 S cm�1 at room temperature, has beenobtained for 10 wt% of ceria due to strong interaction withPEO2LiClO4 phase and is consistent with the lowest crystallinephase. The variation of room temperature ionic conductivity andthe activation energy as a function of CeO2 concentration havebeen analyzed by a combination of linear and sigmoidal function.High conducting interface region between nanosized CeO2 andpolymer gives rise to percolation of mobile ions and maximumconductivity. The incorporation of CeO2 nanoparticles in thepresent system increases the molecular interaction which resultsin enhancement in ionic conductivity.

Acknowledgment

This work is funded by the Council of Scientific and IndustrialResearch, Government of India, Scheme 03(1046)/05/EMR-II.

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