ORIGINAL PAPER

Structural, thermal and ion transport studieson nanocomposite polymerelectrolyte-{(PEO + SiO2):NH4SCN} system

Kamlesh Pandey & Mrigank Mauli Dwivedi &Mridula Tripathi & Markandey Singh & S. L. Agrawal

Received: 17 August 2007 /Revised: 21 November 2007 /Accepted: 18 January 2008 / Published online: 4 March 2008# Springer-Verlag 2008

Abstract Development and characterisation of polyethyl-ene oxide (PEO)-based nanocomposite polymer electrolytescomprising of (PEO-SiO2): NH4SCN is reported. Forsynthesis of the said electrolyte, polyethylene oxide hasbeen taken as polymer host and NH4SCN as an ionic chargesupplier. Sol–gel-derived silica powder of nano dimensionhas been used as ceramic filler for development of nano-composite electrolytes. The maximum conductivity ofelectrolyte ∼2.0×10−6 S/cm is observed for samplescontaining 30 wt.% silica. The temperature dependence ofconductivity seems to follow an Arrhenius-type, thermallyactivated process over a limited temperature range.

Keywords Polymer nanocomposite . PEO: SiO2.

Ionic transference number . Electrical conductivity

Introduction

With the rapid growth of portable electronic devices, thedemand of compact lightweight, high-capacity, solid-staterechargeable batteries have also increased tremendouslyover the years [1]. The polymer electrolytes have recentlybecome a hot contender due to their significant theoreticalinterest as well as practical importance for the developmentof solid-state electrochemical devices beside other distinc-tive properties [2–3]. A large number of solid polymericelectrolytes with appreciably high ionic conductivity havebeen investigated in the past three decades by complexingpolar polymers [e.g. polyethylene oxide (PEO) and poly-propylene oxide (PPO)], having strong solvating abilitywith a number of alkali, alkaline and transition metal salts(e.g. LiClO4, Mg(ClO4)4, LiI, NaI, AgNO3 etc) [4–5].Amongst these polymers, the polyethylene oxide is asemicrystalline polymer at room temperature and has anexceptional property to dissolve high concentration of awide variety of dopants [6].

Recently, particular attention has been devoted tointroduce some structural modification in polymer elec-trolyte in order to increase their electrical conductivityand improve their thermal, mechanical and electrochem-ical properties to provide commercial acceptability inelectrochemical devices. Various techniques (like plasti-cization, co-polymerization, etc.) have been adopted toachieve the desired objective in these polymer electro-lytes [7–8]. In this process, another class of polymerelectrolyte referred to as “composite polymer electrolyte(CPE)” has been developed. These polymer electrolytesare dispersed with ceramic or inorganic or high molecularweight organic fillers to enhance electrical conductivityand to improve thermal, mechanical and electrochemicalstability of the polymer film. Such a dispersion of fillers

Ionics (2008) 14:515–523DOI 10.1007/s11581-008-0210-7

K. Pandey (*) :M. M. DwivediNational Centre of Experimental Mineralogy and Petrology,University of Allahabad,Allahabad 211 002, Indiae-mail: kp542831@gmail.com

M. M. Dwivedie-mail: mmdwivedi@gmail.com

M. TripathiDepartment of Chemistry, CMP Degree College,Allahabad, Indiae-mail: mt68@rediffmail.com

M. Singh : S. L. AgrawalDepartment of Physics, Awadhesh Pratap Singh University,Rewa, Madhya Pradesh, India

S. L. Agrawale-mail: sla_ssi@rediffmail.com

was first suggested by Weston and Steele [9]. Since then, anumber of inorganic or ceramic and organic additives havebeen reported [10–13]. Recently, few works have beenreported on the synthesis and characterization of nano-composite polymer system [14–16] where due to smallparticle size, the electrochemical, magnetic and opticalbehaviour have been shown to improve drastically [17].These materials have become more attractive and usefulfor the medical, technical and industrial use [18]. Promp-ted by these considerations, attempts have been made hereto develop a proton or ammonium ion conducting nano-composite electrolyte based on host polymer PEO.

The present paper reports a nanocomposite polymerelectrolyte PEO: SiO2: NH4SCN system. The effect ofceramic additive (silica) and a salt ammonium thiocyanateon PEO, with respect to morphology and electricalconductivity have been investigated via Differential scan-ning calorimetry (DSC), X-ray diffraction (XRD), Opticalmicroscopy, scanning electron microscopy (SEM), infrared(IR) spectroscopy and ionic transport measurement. Thechange in the morphology and structure of CPE was studiedby XRD, optical microscopy and SEM. The electricalconductivity has been evaluated from complex–impedanceplot and interpreted as a function of composition andtemperature of material.

Experimental

The polymer PEO (M.W. ∼6×105, ACROS Organics) andthe salt Ammonium thiocyanate (NH4SCN, Rankem India)of AR grade were used in synthesis of electrolyte. For thesynthesis of ceramic filler (SiO2), tetraethyl orthosilicate(TEOS, Aldrich) was used as precursor material, ethanol assolvent and Ammonia solution as a catalyst. To obtain nanofillers, TEOS was hydrolyzed through the two-step hydro-lysis process [19]. The pH of the solution was kept at ∼10(i.e. basic medium) to obtain the filler particles. A part ofthis solution was jellified and dried in the form of powder.The refractive index (RI) of this powder was measured(RI=1.426) by index-matching technique [20] to confirmformation of silica powder. Rest part of the solution wassubsequently admixed stoichiometrically in PEO solution(PEO dissolved in de-ionised (DI) water at 40 °C) andstirred for 12 h continuously. This gelatinous polymersolution was finally cast in a polypropylene petri dish. Tosynthesize the polymer electrolyte films, vacuum-driedNH4SCN was also added stoichiometrically in solution ofPEO and SiO2 in DI water. The solution cast film wasfinally dried at room temperature for obtaining freestanding films of CPE.

Structural behavior of PEO–SiO2 and PEO–SiO2–NH4SCN system was studied with the aid of XRD, IR

spectra and SEM or optical microscopy. The XRD patternwas recorded between 2θ values 15–60° at room temper-ature using Phillips X-Pert diffractometer. The Infraredspectrums were recorded on Perkin Elmer IR Spectropho-tometer in a range 400–4,000 cm−1. The optical micro-graphs of the film were recorded using computer controlledLeica DMLP polarizing microscope and the ScanningElectron Micrograph of the films were taken by Jeol JXA8100 EPMA at 15 kV. DSC data were collected with aPerkin Elmer DSC unit in the temperature range RT to150 °C at a heating rate of 5 °C/min under N2 atmosphereto access thermal behaviour of composite polymer electro-lyte films. The transport number (tion) was determined bythe Wagner’s method of polarisation [21]. In this method, afixed d.c. potential (×0.5 V) was applied across the sampleand the current was monitored with time for a sufficientlylarge time to allow the sample to get fully polarised. Theelectrical conductivity was evaluated from complex–im-pedance plot obtained using computer controlled Hioki(Japan) LCZ HI Tester (model 3520-01). During allmeasurements, humidity level was maintained to eliminateits effect. The humidity level was maintained by a constanthumidity chamber, in which an oversaturated salt solution(i.e. Ca(NH)3. 4H2O for humidity 51%) was used formaintaining the humidity level.

Result and discussion

Structural and thermal studies

X-ray diffraction

Figure 1 depicts the XRD pattern of water-casted PEO filmdoped with different SiO2 contents with pristine PEO film.All the curves show the presence of background modula-tion—a feature prevalent in polymeric systems. Though thecrystal structure of PEO is well known, the XRD pattern ofPEO film was recorded to ascertain its structure andmorphology. The XRD pattern of pure PEO film showssharp and intense peak at 2θ=19° and 23°, whichcorrespond to values reported elsewhere [22]. The unit cellof polyethylene oxide is a repeat unit of (O–CH2–CH2)having bond length of the order of 19.2 Å [23]. Existenceof broad peaks, in addition to few sharp reflections, furtherconfirms its partial crystalline and partial amorphousnature. Owing to this type of morphology, pristine-solventfree polymer electrolytes fail to deliver high conductivityvalues. To overcome the problem of partial crystalline andpartial amorphous structure, ceramic filler SiO2 was dopedin pristine electrolyte. The intercalation of polymer chainwith ceramic filler (silica) usually increases the interlayerspacing of ceramic material. This effect leads to shift of

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diffraction peaks towards the lower 2θ value which arerelated through the Bragg’s relation

1 ¼ 2d sin θ ð1Þ

The XRD pattern of different compositional ratios of PEOand Silica with addition of salt is shown in Fig. 1a and b,respectively. From Fig. 1a, it is clear that addition of ceramic

filler (SiO2) in polymer host (PEO) reduces the intensity ofthe main peaks (in PEO, 2θ=19° and 23°) followed bybroadening of the peak area, which is an indication ofreduction in degree of crystallinity. When SiO2 concentrationexceeds 50 wt.% in PEO: SiO2, peaks get submerged inbroadening and few new peaks of SiO2 appear. From thisdiffractogram, it is also inferred that at lower weightpercentage of SiO2, the crystallite size of silica is large but

Fig. 1 a XRD Pattern of xPEO+(1−x) SiO2 film for different x values (wt.%). b XRD Pattern of 95[xPEO+(1−x) SiO2]+5 NH4SCN film fordifferent x values (wt.%)

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as SiO2 concentration increases, the dispersal becomeshomogeneous followed by reduction in size. At higher ratioof SiO2 due to the cluster formation, the peak of SiO2

reappears and crystallinity decreases. Moreover, addition ofsilica shifts diffraction maxima to lower 2θ value. Themagnitude of shift varies with doping ratio. When salt isdoped to form CPE, no new peak appears; instead, theexisting peaks of PEO:SiO2 reappear at the same position butwith reduced intensities (Fig. 1b). Samples with the higherweight content of NH4SCN give a completely amorphousfilm. As a result, polymer–salt interaction cannot be ruledout. The average crystallite size of crystals in differentcompositions were calculated (given in Table 1) by theScherrer formula [24], and it was found to be ∼30–50 nm.Thus, transformation of crystalline phase into amorphousphase is concluded from XRD measurements.

SEM and optical study

Optical micrographs of pure PEO and with the addition ofceramic and salt are shown in Fig. 2. In this figure, distinctspherulites are observed in the pure polymer, which isindicative of the lamellar microstructure of the purepolymer. The dark boundaries observed between thespherulites show presence of partial amorphous phase(Fig. 2a). Morphology of the film changes substantiallyupon addition of SiO2 in different weight percentage(Fig. 2b). Micrographs also show the heterogeneousdispersion for low percentage of ceramic additive whichtransforms into homogeneous distribution at higher SiO2

contents (Fig. 2c). After the addition of salt, no significantchange is observed in the optical micrograph (Fig. 2d).

The SEM image of the different compositions is shownin Fig. 3. Pure PEO film revealed partial crystallinestructure of PEO (Fig. 3a). The crystalline structure formeddue to longer evaporation time. Addition of ceramic fillermakes the original entity lose this character (Fig. 3b).

Higher SiO2 ratio distribution in PEO becomes homoge-neous followed by cluster formation. Scanning electronimages show that the addition of SiO2 disturbs thecrystalline nature of pure matrix (Fig. 3c). Similarly, dopingof salt shows the same type of structure with separate entity(Fig. 3d). The higher ratio of SiO2 content shows the morehomogeneous with a particle of the limit of nanometer size(∼100 nm) which is suggestive of nanosize format.

Infrared spectroscopic study

Infrared spectra of PEO:SiO2 film and x (PEO:SiO2):(1−x)NH4SCN salt (in the range of 4,000–400 cm−1) are shownin Fig. 4 and peak assignments are given in Table 2. Themain feature of the PEO: SiO2 spectrum is presence of thebroad peaks at 3,700 and 1,620 cm−1 due to –OH stretchingand –OH bending. The existence of peaks at 3,000–3,100,1,700, 1,410 and 1,390 cm−1 are related to C–H stretching,C–H in plane bending, C–O stretching and ν CH2–O,respectively. Other silica related peaks are at 941 and802 cm−1 [25]. Crystalline PEO has a triplet peak at 1,149,1,109, 1,061 cm−1 and another peak at 1,280 cm−1 (relatedto –C–H twisting) [26]. All these peaks completely vanishafter introduction of ceramic filler (SiO2) in present studies,which also indicates reduction in crystallinity of the system.It is evident that SiO2 acts as inert filler in PEO matrix,which only modifies the morphology of the system.

Addition of salt NH4SCN gives few new peaks at 2,000–2,100 and 860 cm−1, which indicates formation ofcrystalline complex of x (PEO:SiO2):(1−x) NH4SCN. Thenew band at 2,100 cm−1 is ascribable to the contact ion pairand solvent-separated diamers. The salt addition reducesthe intensities of different original peaks. Other importantfeature in IR spectrum is the relative intensity of the band at2,060 cm−1, which increased at the expense of 2,025 cm−1.This indicates the disintegration of crystalline PEO/ PEO:SiO2 phase and results in decrease of crystallinity. At alocal level, after the addition of NH4SCN, the intensity ofband 860 cm−1 slightly enhanced, which particularly weresensitive to the local conformation of O–C–C–O tortionalangle. This type of complexation suggests gauche coordi-nation. For CH2 wagging modes at 1,343, 1,360 cm−1, theintensity decreases drastically and is replaced by a sharpband around 1,350 cm−1 upon the addition of NH4SCN.The reduced intensities of various characteristic peaks(triplet band of crystalline PEO at 1,280, 1,360 and3,100 cm−1) confirms the fall in degree of crystallinityafter addition of ceramic filler and salt.

Differential scanning calorimetry

The DSC curve of Pure PEO, PEO: SiO2 with differentfiller concentration and x (PEO:SiO2):(1−x) NH4SCN are

Table 1 Calculated average particle size of undoped and doped PEO:SiO2 film for different compositions

Sample Averageparticlesize (nm)

Pure PEO film 45.290 wt.% PEO+10 wt.% SiO2 film 23.780 wt.% PEO+20 wt.% SiO2 film 22.870 wt.% PEO+30 wt.% SiO2 film 20.760 wt.% PEO+40 wt.% SiO2 film 21.220 wt.% PEO+80 wt.% SiO2 powder 31.8[90 wt.% PEO+10 wt.% SiO2]95: (NH4SCN)5 film 32.5[80 wt.% PEO+20 wt.% SiO2]95: (NH4SCN)5 film 25.7SiO2 powder 50–60

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Fig. 3 SEM image of pure PEOfilm (a), 90 PEO+10 SiO2

film (b), 70 PEO+30 SiO2 film(c) and 95[90 PEO+10 SiO2]+5NH4SCN film (d)

Fig. 2 Optical micrographs ofpure PEO film (a), 90 PEO+10SiO2 film (b), 70 PEO+30 SiO2

film (c) and 95[90 PEO+10SiO2]+5 NH4SCN film (d)

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shown in Fig. 5a and b, respectively. Diffractogram of purePEO film shows two peaks, one endothermic around 69 °Cand another exothermic around 105 °C. The former peak isrelated to the melting of pure PEO while the latter peak isrelated to evaporation of adsorbed water. After 125 °C, thesample starts dissociating. Addition of ceramic filler (SiO2)in PEO matrix continuously reduces the melting tempera-ture of the resulting system. The melting temperature ofpolymer changes from 69 °C to 64 °C after the addition of30 wt.% SiO2. Further crystallinity of the host polymer isseen to reduce as indicated by enthalpy change afteraddition of filler. This effect is possibly due to theformation of more amorphous domains with partial mis-cibility of filler with polymer host. Similar observation hasbeen recorded even after the addition of salt. A furtherreduction in melting temperature (Tm) was noticed in caseof composite polymer electrolytes. Such a reduction ispossibly due to the formation of amorphous PEO: NH4SCNcomplexes witnessed in IR and XRD studies. The ΔHm and

the calculated crystallinity of PEO:SiO2 and with NH4SCNare lower than that of PEO film casted in water and is inagreement of XRD results. This also suggests that silica iscompatible with PEO and changes the morphology ofsystem through reduction in crystallinity of the complex.The enhancement of amorphosity is favourable for the ionicmobility and, hence, its ion transport behaviour in the PEO-based solid polymer electrolyte.

Ion transport studies

Polarisation studies

In the present study, the likely mobile species are protonicand, thus, thick silver electrodes were used in WagnerPolarisation method to assess the nature of ion transport andevaluate total transference number from current time plot.The variation of current with time for two differentcomposition i.e. x[90 PEO:10 SiO2]:(1−x)NH4SCN andx[80 PEO:20 SiO2]:(1−x)NH4SCN are given in Fig. 6a andb, respectively. From these plots, the initial current iinitialand final current ifinal is evaluated, and total ionictransference number (tion) was calculated using the relation

tion ¼ iinitial � ifinaliinitial

ð2Þ

The calculated values of tion for different compositefilms are listed in Table 3. From the table, it is apparentthat polyethylene oxide with the filler SiO2 only isessentially a nonionic material, but after doping sampleswith ammonium thiocyanate, nature of material changesand become completely ionic. These values are at bestqualitative due to (a) nonavailability of ideal blockingelectrode for protonic or gaseous species and (b) uncertain-ty in the measurement of initial current due to quick onsetof polarisation.

Table 2 Assignment of different IR peaks of 90PEO:10SiO2 film and 95[90PEO: 10SiO2]: 5NH4SCN film

Peak position in 90 PEO:10 SiO2 film Peak position in 95(90 PEO:10 SiO2):5 NH4SCN film Assignments

Broad peak at 3,650–3,800 cm−1 Broad peak 3,650–3,800 cm−1 ν OH, adsorbed H–O–H,Broad peak at 3,100 cm−1 — =C–H stretching2,902 cm−1 — νas CH2, ν C–Hx organic groups—— 2,100 cm−1 SCN1,780 cm−1 — organics, C–O1,600 cm−1 1,620 cm−1 H–O–H, molecular1,410,1390 cm−1 — C–H in plane bending, ν CH2–O941 cm−1 — νas Si–OH—— 860 cm−1 SCN peak802 cm−1 — νs Si–O–Si, νas Si–C, Si–O–Si–CnHmBroad peak at 780 cm−1 Broad peak at 780 cm−1 with reduced intensity ν SiO–C2H5

Fig. 4 Infrared spectra of a 90 PEO+10 SiO2 and b 95 [90 PEO+10 SiO2]+5 NH4SCN film (pure PEO spectrum in inset [24])

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Electrical conductivity measurement

The variation of electrical conductivity (σRT) of thecomposite polymer electrolyte film as a function of fillercomposition (i.e. SiO2 content) is shown in Fig. 7. Thisindicates that the conductivity increases with the increase ofSiO2 content up to 30 wt.% (σmax=2.37×10

−6 S/cm) and,

thereafter, it starts decreasing with the increasing concen-tration of filler before saturating at large SiO2 content.Similar effect has also been observed with increasingconcentration of salt. This is possibly due to modificationor increase of transition temperature of the polymercomposite electrolyte with increase in filler or dopantconcentration. In addition, the mechanical stability of thecomposite materials has also been found to improve withincreasing concentration (up to 30% only) of filler and salt.Because morphology of films is directly linked to mobilityof charge carrier to migrate upon the application of electricfield, ionic conductivity of polymer electrolytes is bound tobe influenced by morphology of the system. In view of thisexplanation, conductivity of polymer electrolyte modifiedwith nanofillers enhances until 40 wt.% SiO2 and thendiminishes. Here, it is remarkable that as SiO2 contentincreases, the brittleness of film increases. Beyond 40 wt.%

Fig. 6 Current vs time curve for a x[90 PEO:10 SiO2]:(1−x)NH4SCN and b x[80 PEO:20 SiO2]:(1−x) NH4SCN

Fig. 5 a DSC curve for xPEO+(1−x) SiO2 with different x values (xin wt.%). b DSC curve for [xPEO+(1−x) SiO2] undoped and dopedwith 5 wt.% NH4SCN

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addition of SiO2, it is impossible to synthesise the film;rather, the system is like fine powder.

The temperature dependence of the conductivity i.e. σ vs1/T plot of x[90 PEO:10 SiO2]:(1−x)NH4SCN and x[80PEO:20 SiO2]: (1−x)NH4SCN are shown in Fig. 8.Siekierski [27] has studied ion transport properties ofPEO-NH4SCN system. He has shown a variation of ionicconductivity from 3×10−9 S/cm for (PEO)40NH4SCN to 7×10−7 S/cm for (PEO)20NH4SCN. After addition of nano-filler, a further increase of twofold is noticed in conductiv-ity. Further, these plots show insignificant variation inconductivity PEO:SiO2 (90:10 and 80:20 wt.% ratio) over awide range below melting temperature of PEO. After themelting temperature (∼66 °C) of PEO, the conductivitystarts increasing with some fluctuations. This is indicativeof unstable nature of the PEO:SiO2 film after the Tm ofPEO. The addition of salt NH4SCN shows a sudden jumpin the conductivity after Tm, in each of the four compositionshown in Fig. 8. This has been explained on the basis ofsemicrystalline to amorphous phase transition. The almostlinear variation in conductivity as a function of temperaturefollows apparently an Arrhenius type thermally activatedprocess below Tm (in all samples); above Tm (only with saltcomposition films), it exhibits Vogel–Tamman–Fulchertype character. The calculated values of activation energyof different composition are given in Table 4, whichsuggests continuous fall in activation energy on account

of morphological improvement in films upon addition ofnanosized fillers.

Conclusions

The experimental studies through XRD, IR, SEM and DSCshowed that ceramic filler SiO2 was able to decrease thecrystalline content and enhanced salt dissociation of x(PEO:SiO2):(1−x)NH4SCN. The intercalated silica in PEOpolymer host also produces a huge interfacial area withbetter mechanical and thermal property of the solidcomposite electrolyte. The XRD data and SEM show theaverage particle size in the film to be in nanosize format.The composite polymer electrolyte film of composition ofx[90 PEO:10 SiO2]:(1−x)NH4SCN and x[80 PEO:20 SiO2]:(1−x)NH4SCN show reasonably good ionic conductivitywith the complete ionic nature. The temperature depen-dence of conductivity of SPEs show the Arrhenius typethermally activated process with two different activationenergies, before and after the melting temperature.

Fig. 8 Variation of conductivity with temperature in x[90 PEO:10 SiO2]:(1−x) NH4SCN and x[80 PEO:20 SiO2]:(1−x) NH4SCNpolymer films

Fig. 7 Composition dependence of conductivity of PEO: SiO2

polymer film

Table 4 Activation energy of different nano-polymer compositeelectrolyte films

Sample Activation energy (eV)

[90 PEO+10 SiO2]95:(NH4SCN)5 33.5[90 PEO+10 SiO2]90:(NH4SCN)10 3.5[80 PEO+20 SiO2]95:(NH4SCN)5 7.5[80 PEO+20 SiO2]90:(NH4SCN)10 1.8

Table 3 Ionic transference number for the nanocomposite polymericfilm

Sample Ionic transference number

90 PEO:10 SiO2 0.0095[90 PEO:10 SiO2]:5 NH4SCN 0.9090[90 PEO:10 SiO2]:10 NH4SCN 0.9280 PEO:20 SiO2 0.0095[80 PEO:20 SiO2]:5 NH4SCN 0.9190[80 PEO:20 SiO2]:10 NH4SCN 0.94

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