Solid State Communications 149 (2009) 1282–1287

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Solid State Communications

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Effect of nanofillers on thermal and transport properties of potassiumiodide–polyethylene oxide solid polymer electrolyteArup 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 2 April 2009Received in revised form7 May 2009Accepted 8 May 2009by E.V. SampathkumaranAvailable online 20 May 2009

PACS:61.82.Pv72.20.-i

Keywords:A. PolymersB. Chemical synthesis

a b s t r a c t

In order to enhance the ionic conductivity of polyethylene oxide (PEO)–KI(80:20) based alkalinepolymer electrolytes, nanosized inorganic filler ZnS has been incorporated into PEO–KI matrixand the corresponding nanocomposite polymer electrolytes are synthesized by the usual solutioncasting procedure. Atomic force microscope image of composite polymer electrolyte exhibits that theintroduction of ZnS nanoparticles changes the surface morphology and aggregates them to form anarborization pattern. The prepared nanocomposite polymer electrolyte reveals an ionic conductivity ofabout 10−4 S cm−1 for 5 wt% ZnS at room temperature.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Solid polymer electrolytes have been a subject of great interestin recent years to the researchers in the field of polymerelectrolytes due to their potential applications in rechargeablebatteries, fuel cell, sensors, electrochromic display devices, smartwindows and other applications [1–4]. Polyethylene oxide (PEO)based polymer electrolytes are widely used in comparison to theceramic electrolytes because of their easy formation of thin filmsof desirable shape and size, flexibility during charge–dischargecycles, ability to form complex with various alkaline salts ofhigh concentrations, etc. Pure PEO has a poor conductivity(∼10−7 S cm−1) at room temperature but the conductivity can beimproved substantially by incorporating salts into the polymer [5,6]. Addition of salt to the polymer provides mobile ions andthe polymer host chains play a critical role in ionic transportprocess of the polymer electrolytes. Ion transport occurs throughamorphous region assisted by the segmental motion of thepolymer chains. However, increasing salt concentrations in thepolymer electrolytes’ ionic conductivity can be improved but theirmechanical properties and potential stability are not suitable forpotential applications.Among various methods, addition of nanosized inorganic fillers

into the polymer electrolytes is the recent trend to improve the

∗ Corresponding author. Tel.: +91 3324733073; fax: +91 3324732805.E-mail address:msskd@iacs.res.in (S.K. De).

0038-1098/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2009.05.021

ionic conductivity as well as mechanical properties and interfacialstability [3,7,8]. Different types of nanoparticles are dispersedinto the polymer–salt matrix for the formation of compositepolymer electrolytes. It is well accepted that the formation ofnanocomposite polymer electrolytes takes place through Lewisacid–base reactions [9–12]. The interaction of Lewis acid sites onthe surface of nanoparticles with base centres of ether oxygen inPEO leads to the formation of complex. Inorganic fillers posseslarge surface area which act as cross-linking centers for the PEOsegments. The ionic conductivity enhances because the strongLewis acidity of the inorganic nanofillers reduces the degree ofcrystallinity, in other words, induces more amorphous phase anddecreases the effect from ionic association. It is also observedthat the filler concentration and the nature of the filler play animportant role to improve the physical properties. There is hardlyany literature available on the detailed and systematic study ofnanosized ZnS dispersed composite polymer electrolyte. In thepresent work we report structural modification, thermal stabilityand electrical properties using KI salt for better understanding ofthe solid polymer electrolyte.

2. Experimental details

The nanocrystalline ZnS was prepared by the solvothermalmethod [13,14]. An appropriate amount of analytical gradethiourea was dissolved in absolute ethanol, then zinc acetate[Zn(CH3COO)22H2O] was added to the solution under continuous

A. Dey et al. / Solid State Communications 149 (2009) 1282–1287 1283

stirring. Here the amount of thiourea in excess of 10% that ofthe zinc acetate was taken so as to enable a complete reaction ofzinc acetate. The solution was poured into a Teflon-liner autoclaveof 100 ml capacity until 80% of the total volume was filled up.The autoclave was then sealed into a stainless steel tank andmaintained at 120 ◦C for 18 h. After gradual cooling to roomtemperature, a white precipitate was obtained. The samples werefiltered off and washed with distilled water, then dried undervacuum.Nanocomposite polymer electrolyte films were prepared by

solution-casting technique blending pure polyethylene oxide(PEO) (molecular weight, Mw = 106), potassium iodide (KI) saltand zinc sulfide (ZnS) in appropriate amounts. PEOandKIwere firstdissolved in methanol under continuous magnetic stirring at roomtemperature. When the bubbles disappeared and the polymericsolution became viscous, desired amount of ZnS was added andmixed with vigorous stirring. The homogeneous solution waspoured into a polypropylene petri dish and vacuum dried at 50 ◦Cfor 48 h to remove traces of the solvent completely. The thin filmswere then preserved in a vacuum desiccator.The macroscopic structure of different polymeric nanocompos-

ite films was examined by X-ray diffraction (XRD) analysis, whichwas performed at room temperature using a high resolution X’PertPRO Panalytical X-ray diffractometer in the range 10◦–60◦ usingCu Kα radiation. Atomic forcemicroscopic (AFM) studieswere per-formed using an Autoprobe CP Base Unit,Model No.AP0100. Differ-ential scanning calorimetry (DSC) was carried out on Perkin ElmerDiamond DSC in a temperature range between −60 ◦C and 80 ◦Cwith a scan rate of 20 ◦C/min under a constant flow (100 ml/min)of nitrogen gas to avoid any contact of atmospheric moisture. Pureindium was used for temperature and enthalpy calibration of theinstrument. The Fourier Transform Infrared (FTIR) spectra wereobtained by a computer interfaced Shimadzu FTIR-8300 spectrom-eter in the frequency range of 1500–800 cm−1. In this experi-ment the gelatinous polymer solution was cast on a KBr paletteand dried in vacuum. Ionic conductivity was measured using theac impedance techniques with Agilent 4192A frequency responseimpedance analyser in the temperature range 301–371 K, wherethe given nanocomposite electrolyte films were sandwiched be-tween two polished stainless steel electron blocking electrodes.The resistance of the nanocomposite polymer electrolyte filmswere obtained from the cross point at high frequency of frequencydependent impedance with the real axis from where the conduc-tivity was calculated. Temperature was monitored by Eurothermtemperature controller (Model No. 2404) using thermocouple sen-sor.

3. Results and discussion

The X-ray diffraction patterns of PEO–KI–ZnS complexes areshown in Fig. 1. A comparative study of XRD pattern showsthe existence of few differences in diffraction patterns of thecomplexed PEO films and that of pure PEO. The XRDpattern of purePEO film (Fig. 1(a)) shows two intensity maxima at 2θ = 19.13◦and 23.30◦ and are assigned as (120) and (112) planes of crystallinePEO, respectively. The characteristic peaks of PEO in PEO–KI film(Fig. 1(b)) are found to be less prominent and are broadenedaffirming the decrease in degree of crystallinity and complexationof PEO with KI salt due to destruction of ordered arrangementsof polymer chains and interaction of ether oxygen of PEO and K+ion. The XRD patterns of composite polymer electrolyte films areshown in Fig. 1(c) and (d), respectively. It is observed that threeadditional peaks appear at 2θ = 28.68◦, 47.88◦ and 56.68◦ inthe ZnS dispersal composite electrolyte films which correspondto the (111), (220), and (311) planes of cubic ZnS, respectively. Asthe wt% of the filler increases the peaks corresponding to the ZnS

Fig. 1. X-ray diffraction patterns of (a) pure PEO; (b) PEO–KI and (c) 5 wt% ZnS;(d) 15 wt% ZnS of PEO–KI–ZnS composite polymer electrolytes.

Table 1Composition and glass transition temperature (Tg ), melting temperature (Tm),change of enthalpy of melting (1H) and percentage of crystalline (Xc ) of differentPEO–KI–ZnS composite polymer electrolytes.

Sample Tg (◦C) Tm (◦C) 1H (J/g) Xc (%)

Pure PEO – 71 155 72.53PEO–KI −50 58 41.34 19.35PEO–KI–ZnS 3 wt% −50.6 56 35.17 16.45PEO–KI–ZnS 5 wt% −51.9 52 28.63 13.40PEO–KI–ZnS 8 wt% −51.6 53 34.56 16.17PEO–KI–ZnS 10 wt% −51.2 53 33.52 15.69PEO–KI–ZnS 12 wt% −46.0 55 35.45 16.59PEO–KI–ZnS 15 wt% −45.1 59 36.54 17.10

become prominent as portrayed in Fig. 1(d). The average particlesize of ZnS in different compositions of composite electrolytes hasbeen calculated by the Scherrer formula [15], and it is found to be∼10–30 nm.Two-dimensional AFM topographic images of pure PEO, PEO–KI

and PEO–KI–ZnS are shown in Fig. 2(a), (b) and (c), respectively.Fig. 2(a) exhibits a network of regular spherulites producingspirals and branches of well distributed surface contours whichsuggests the semicrystalline nature of pure PEO. Fig. 2(b) showsthat the surface morphology of the polymer matrix changes whenKI salt is added. The surface morphology changes dramaticallywith the introduction of nanofillers. Fig. 2(c) shows that thespirals and branches are completely absent here and a newsurface morphology is developed in which the nanoparticles areaggregated to form an arborization pattern.Fig. 3 shows the typical differential scanning calorimetry (DSC)

curves of pure PEO, PEO–KI and different concentrations of ZnSadded composite polymer electrolytes over a wide range oftemperature (−60 ◦C to 80 ◦C) and are summarized in Table 1.In each curve, the melting temperature is identified at the peakminimum of the endothermic process which is attributed tothe melting of crystalline phase. Glass transition temperature(Tg ) and melting temperature Tm of the polymer electrolytes aredetermined from the DSC curves. The enthalpy of melting (1H)is represented as energy in the form of heat absorbed per unitweight of the polymeric sample, and crystallinity (Xc) is estimatedfrom the ratio of the experimentally determined 1H to the valueof 213.7 J/g reported in the literature for the enthalpy of meltingof 100% crystalline PEO [16].It is observed from Table 1 that the Tg , Tm, 1H decrease upto

5 wt% of ZnS. The lowest value of crystallinity due to disruptionof PEO has been found at 5 wt%. Above 10 wt%, the Tg , Tm,and 1H increase with an increase in the concentration of ZnSnanoparticles, which play a role of cross-linking points in thepolymer electrolytes. Hence it may be concluded that the mobility

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4.00µm2.000.004.00µm2.000.00

a b

c

2.00µm1.000.00

0.10

µm /

Div

0.40

µm /

Div

0.20

µm /

Div

Fig. 2. (Color online) AFM topographic images of (a) pure PEO; (b) PEO–KI and (c) PEO–KI–ZnS composite polymer electrolytes.

Fig. 3. DSC traces of (a) pure PEO; (b) PEO–KI and (c) 5 wt% ZnS, (d) 10 wt% ZnS,(e) 15 wt% ZnS of PEO–KI–ZnS composite polymer electrolytes.

of the polymer chains decreases when the concentration of thecross-linking points, i.e., the filler, in the polymer electrolytesis considerably high because Tg is directly correlated with thesegmental motion of polymer chains.Several workers have utilized FTIR to investigate the interac-

tions in the polymer–salt complex and also the effect of addingfillers on it. Changes in the position and intensity of the bands,characteristic of groups or atoms, involves Lewis acid–base inter-action involving a filler, a polyether matrix and alkali metal ions(with cations acting as Lewis acid centers and anions as Lewis basecenters) [17]. Fig. 4 shows the infrared spectra for PEO–KI basedelectrolytes with dispersed ZnS filler. The most important absorp-

tion features present in PEO are CH2 bending at 1466 cm−1, CCstretching at 1147 cm−1, CH2 rocking and COC asymmetric stretch-ing at 962 cm−1, CH2 asymmetric rocking at 843 cm−1. Moreover,the band at 1242 cm−1 is attributed to CH2 twistingmode. Both thetriplet maxima at 1147, 1112 and 1061 cm−1, related to symmet-ric and asymmetric stretching C–O–C vibrations and the doubletat 1342 and 1360 cm−1 related to swinging vibrations of the CH2group are characteristic of the crystalline PEO phase. The shift inthe peak position of CH2 stretching mode to 953 cm−1 in PEO–KIindicates complex formation. Moreover, the presence of a singlepeak at 1350 cm−1, which can be assigned to swinging vibrationof C–H in CH2 group in the amorphous PEO phase, indicates the in-crease in the amorphous phase of the electrolyte upon the additionof the salt.The close inspection of 1200–1000 cm−1 region consisting

mainly of unresolved symmetric polyethylene C–O–C stretchingband is very important for PEO–salt complexes which shows theeffect of the filler and dopant salt on these vibrations. The FTIRspectra in 1000–1200 cm−1 region have been analyzed by fittingto Gaussian–Lorentzian product function with straight base line.The maximum intensity has been normalized to unity. Fig. 5 is atypical example of a fit for pure PEO. The maximum area is foundfor the peak at 1112 cm−1. Full-width at half-maximum (FWHM)of the maximum is 26 cm−1. The relative area with respect to themaximum area for other two peaks at 1094 and 1147 cm−1 are50% and 28% respectively. Fig. 6 shows the results of curve fittingFTIR spectra of PEO–KI, 3 wt% and 15 wt% ZnS doped samples. Thevalue of FWHMof themajor absorption band of PEO–KI is 51 cm−1.The broadening clearly indicates that K+ ion coordinates withether oxygen of PEO. The calculated area ratio as a function of ZnSconcentration is depicted in Fig. 7. The position (1110–1112 cm−1)of the maximum does not differ with ZnS content up to 8 wt%.

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Fig. 4. FTIR spectra of (a) pure PEO; (b) PEO–KI and (c) 5 wt% ZnS; (d) 10 wt% ZnS;(e) 15 wt% ZnS of PEO–KI–ZnS composite polymer electrolytes.

Fig. 5. Deconvolution of FTIR spectra for C–O–C band in pure PEO in the region of1200–1040 cm−1 .

Beyond this concentration, COC stretchingmodemaximum for PEOdecreases nearly to 1025 cm−1 with the increase of ZnS content inthe polymer–salt complex. The area ratios of peaks in the spectralregime 1073–1087 cm−1 and 1137–1147 cm−1 reveal maxima at5 wt% and 8 wt% ZnS concentration respectively. A new peak at1002–1005 cm−1 appears at 10 wt% ZnS and above. An area withsignificant contribution is also observed at 1015–1016 cm−1 for3 wt% and 8 wt% ZnS. The interactions among PEO, KI, ZnS areassociated with the changes in intensity, shape and position ofthese stretching modes [18].The variation in ionic conductivity of polymer electrolyte as a

function of the ZnS nanofiller concentration at room temperatureis shown in Fig. 8. The ionic conductivity increases with the ZnScontent and reaches an optimum value of 3.1 × 10−4 S cm−1at 5 wt% of ZnS. Further addition of ZnS, however, decreasesthe conductivity. Such a behavior in conductivity after 10 wt%ZnS concentration is due to aggregation of high concentrations ofZn+–K+ rich domains but it is still almost one order higher for15 wt% than that of PEO–KI polymer electrolyte. It is interestingto note here that log(σ ) vs concentration of ZnS plot shows twopeaks at 5 wt% and 10 wt%, respectively. Existence of two peaksfor the composite polymer electrolytes has already been reportedby us and several researchers [19–22]. The phenomenon can beexplained by the percolation model proposed by Bunde et al. [23]and the theory for an ion conducting polymer electrolyte suggestedby Lakshmi and Chandra [24,25]. It is well known fact that thepolymer electrolyte complex dispersed with nanofiller enhancesthe cation transference numbers at a considerable amount and

c

b

a

Fig. 6. Deconvolution of FTIR spectra for C–O–C band in (a) PEO–KI; (b) 3 wt%. ZnSand (c) 15 wt%. ZnS of PEO–KI–ZnS in the region of 1200–975 cm−1 .

Fig. 7. Variation of area ratio with ZnS of three peaks in the spectral range1150–1000 cm−1 .

hence, both cations and anions are mobile in such a polymerelectrolyte complex. According to Lakshmi and Chandra, the

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Fig. 8. Variation of conductivity with ZnS concentrations at room temperature.

Table 2Composition and activation energy (Ea) of different PEO–KI–ZnS compositepolymer electrolytes.

Sample Ea (eV)T > Tm T < Tm

PEO–KI 0.31 0.12PEO–KI–ZnS 3 wt% 0.23 0.09PEO–KI–ZnS 5 wt% 0.15 0.05PEO–KI–ZnS 8 wt% 0.23 0.06PEO–KI–ZnS 10 wt% 0.10 0.06PEO–KI–ZnS 12 wt% 0.11 0.13PEO–KI–ZnS 15 wt% 0.08 0.14

existence of two maxima in the present ion conducting compositepolymer electrolyte can be attributed to two separate percolationthresholds involving two different kinds of mobile species, cations(K+) and anions (I−).In Fig. 9, temperature dependence of conductivity i.e. log(σ ) vs

1/T plot of PEO–KI and electrolyte nanocomposites for differentconcentrations of ZnS indicate two regions (Region I and RegionII), above and below the melting temperature (Tm) of polymerelectrolyte. The conductivity increases with temperature upto themelting temperature. As the temperature reaches ∼60 ◦C, thereis a sudden rise in conductivity of the PEO–KI film, which is dueto the semicrystalline to amorphous phase transition and has alsobeen observed in the DSC thermogram in Fig. 3. Above 60 ◦C theconductivity increases again with temperature. The same behaviorhas been observed in all the compositions of nanocompositesystems. The existence of two regions in the conductivity vstemperature plot has been observed in a number of PEO basedpolymer electrolytes [26–30]. The linear region of the plot can beexpressed by the following general Arrhenius equation:

σ = σ0 exp(−Ea/kT ) (1)

where σ0 is the pre-exponential factor, Ea is the activation energyand k is the Boltzmann constant.The calculated activation energies(Ea) in two regions for PEO–KI and nanocomposite polymerelectrolyte systems are given in Table 2. The estimated activationenergy below and above the melting temperature lies between0.30 and 0.05 eV.

4. Conclusion

Investigations of the thermal and electrical properties of nano-sized ZnS dispersed PEO–KI system synthesized by the solutioncast technique in film form are carried out. Extensive thermal stud-ies show that the organic–inorganicmoieties arewellmiscible. The

Fig. 9. Temperature dependence conductivity plots of (a) PEO–KI and (b) 3 wt%ZnS; (c) 5 wt% ZnS; (d) 8 wt% ZnS; (e) 10 wt% ZnS; (f) 12 wt% ZnS; (g) 15 wt% ZnS ofPEO–KI–ZnS composite polymer electrolytes.

highest room temperature conductivity ∼3.1 × 10−4 S cm−1 hasbeen observed which is an order higher than PEO–KI salt complex.The existence of two peaks, a feature of the log(σ ) vs ZnS concen-tration plot has been explained in the light of percolation thresh-old theory. The temperature dependence conductivity displays theArrhenius type thermally activated process with two different ac-tivation energies, below and above the melting temperature. Theconductivity of PEO–KI–ZnS 5 wt% maintains its optimum valueall through the measured temperature range. The increase in ionicconductivity might be due to the active dissociation of the KI saltin the presence of nanosized ZnS.

Acknowledgements

This work is funded by the Council of Scientific and IndustrialResearch, Government of India, Scheme No: 03(1046)/05/EMR-II. S. Karan is thankful to the Council of Scientific and IndustrialResearch, Government of India for providing the fellowship.

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