Ionic Transport Studies on (PEO) 6 NaPO3 Polymer

8/2/2019 Ionic Transport Studies on (PEO) 6 NaPO3 Polymer

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Ionic transport studies on (PEO)6:NaPO3 polymerelectrolyte plasticized with PEG400

Amrtha Bhide, K. Hariharan *

Solid State Ionics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India

Received 3 June 2007; received in revised form 12 July 2007; accepted 31 July 2007Available online 8 August 2007

Abstract

Conduction characteristics of the poly(ethylene oxide) based new polymer electrolyte (PEO)6:NaPO3, plasticized withpoly(ethylene glycol) are investigated. Free standing flexible electrolyte films of composition (PEO)6:NaPO3 + x wt.% -PEG400 (30 6 x 6 70) are prepared by solution casting method. A combination of X-ray diffraction (XRD), optical micros-copy and differential scanning calorimetry (DSC) studies have indicated enhancement in the amorphous phase of polymerdue to the addition of plasticizer. Further, a reduction in the glass transition temperature observed from the DSC result hasinferred increase in the flexibility of the polymer chains. The cationic transport number tNa of 0.42 determined throughcombined acdc technique has confirmed ionic nature of conducting species. Ionic conductivity studies are carried out as afunction of composition and temperature using complex impedance spectroscopy. The electrolyte with maximum PEG400

content has exhibited an enhancement in the conductivity of about two orders of magnitude compared to the host polymerelectrolyte. The complex impedance data is analyzed in conductivity, permittivity and electric modulus formalism in orderto throw light on transport mechanism. A solid state electrochemical cell based on the above polymer electrolyte with aconfiguration Na|SPE|(I2 + acetylene black + PEO) has exhibited an open circuit voltage of 2.94 V. The discharge charac-teristics are found to be satisfactory as a laboratory cell. 2007 Elsevier Ltd. All rights reserved.

Keywords: Polymer electrolyte; Ionic conductivity; Poly(ethylene oxide); Poly(ethylene glycol)

1. Introduction

Polymer metal salt complexes have gained tech-nological importance as electrolyte materials forthe solid state electrochemical devices such as bat-teries, fuel cells, electro chromic windows and supercapacitors. Solid polymer electrolytes (SPE) are

found to be advantageous compared to the conven-

tional solid electrolytes in view of the flexibility, easeof preparation into required geometries and betterelectrode electrolyte contacts, [1]. The high mole-cular weight poly(ethylene oxide) (PEO) resumesmuch attention as the host for the polymer electro-lytes, because of its ability to dissolve a variety ofalkali salts MX (M alkali metal, X anion), elec-trochemical stability and beneficial structure forsupporting ion migration [2,3]. Apart from Li+ ionconducting polymer electrolytes, certain Na+ ion

0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.eurpolymj.2007.07.038

* Corresponding author. Tel.: +91 44 22574856; fax: +91 4422574852.

E-mail address: [email protected] (K. Hariharan).

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conducting electrolyte systems have been exploredin order to understand the structure, conductionmechanism and in application such as solid statesodium batteries [36].

Recently we have reported a new polymer metal

salt complex based on PEO complexed with the saltNaPO3 (sodium metaphosphate) [7]. Among the dif-ferent (PEO)

n:NaPO3 (n = [ethylene oxide]/Na

3,4,6,8 and 10) compositions, (PEO)6:NaPO3 hasexhibited the highest ionic conductivity of2.8 108 S cm1 at 351 K with a cationic transportnumber of 0.23. This electrolyte is found to be non-hygroscopic in nature, exhibiting significant ionicconductivity when compared with the polycrystal-line NaPO3 and its glassy counterpart. Howeverthe above transport characteristics such as, ionicconductivity and transport number need to be

improved for a device application. Several effectiveapproaches have been adopted to enhance the con-duction characteristics of the PEO based polymerelectrolytes. Among these, (i) composite polymerelectrolytes (CPE) with the dispersion of nano orsub micron sized non-conducting inert fillers [8,9]and (ii) plasticized polymer electrolytes with non-aqueous, high dielectric constant organic solventsor low molecular weight polymers have been widelyreported [10,11]. With the addition of the plasticiz-ing additives such as, propylene carbonate (PC),

ethylene carbonate (EC), dimethyl carbonate(DMC) and low molecular weight polyethyleneglycols [1114], the polymer electrolytes continueto retain the solid nature. Such electrolytes arecalled as hybrid polymer electrolyte systems. Mostof the investigations on polymer electrolytes withplasticizing additives have revealed the role ofdielectric constant, viscosity of the plasticizer andthe concentration on the enhancement of electro-chemical properties of the solid polymer electrolytes(SPE) [15,16]. However, the transport mechanismwas found to be complex.

In the present investigation, we have chosen forthe plasticizer, a low molecular weight poly(ethyleneglycol), PEG400 which is a non-volatile, moderatelyviscous liquid with a dielectric constant of 12.5. Themain objective is to investigate the effect of the lowmolecular weight polymer PEG400 on the surfacemorphology, crystallinity and conduction character-istics of the polymer electrolyte (PEO)6:NaPO3. Theplasticized polymer electrolytes have been charac-terized through X-ray diffraction (XRD), opticalmicroscopy, differential scanning calorimetry

(DSC) and Fourier transform infrared spectroscopy

(FTIR) techniques. Transport number measurementand the temperature dependent conductivity studieshave been carried out through complex impedancespectroscopy technique. A solid state primary bat-tery based on the best conducting polymer electro-

lyte with a configuration Na|SPE|(I2 + acetyleneblack + PEO) has been fabricated.

2. Experimental

2.1. Preparation of polymer electrolytes

Poly(ethylene oxide) (Mw = 4 106, Aldrich) was

dried in vacuum oven at 50 C for 12 h and sodiummetaphosphate at 150 C for about 24 h prior touse. Appropriate quantities of PEO and NaPO3

required for ether oxygen to alkali ion ratio(O/Na) 6:1 were dissolved in acetonitrile and mag-netically stirred in order to obtain a homogeneoussolution. Then the low molecular weight poly(ethyl-ene glycol) (Mw = 400, Aldrich) has been added andfurther stirred. Finally completely homogenizedsolution was poured into PTFE petri dish and vac-uum dried at 50 C for 48 h to remove all the tracesof solvent. The plasticized electrolytes thus obtainedhad a thickness of about 100150 lm and were pre-served in a vacuum desiccator for further investiga-

tions. The amount of PEG added into the polymerelectrolyte has been expressed as weight percent(wt.%) of PEO present and the different composi-tions of electrolyte systems have been representedas (PEO)6:NaPO3 + x wt.% PEG400. Free standing,homogeneous polymer electrolytes were obtainedfor the plasticizer content in the range, 30 6 x 670 wt.% PEG400.

2.2. Fabrication of solid state sodium battery

The positive electrodes were fabricated by castingthe slurry containing 60 wt.% of I2, 30 wt.% of acet-ylene black granules (DENKA Black, Singapore)and 10 wt.% of PEO on stainless steel foils from sus-pensions of the ingredient materials in acetonitrile.Anode electrode of thickness about 100 lm hasbeen prepared by pressing sodium metal onto astainless steel foil. The cell was fabricated by sand-wiching the best conducting polymer electrolytefilm, between the electrodes in a swagelock assem-bly, under inert gas filled glove box. Discharge char-acteristics were monitored using Keithley 614 high

impedance electrometer.

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2.3. Characterization techniques

The optical micrographs of the polymer electro-lytes have been recorded using a Axioskop2 Mat(Zeiss) microscope attached with Polaroids. X-ray

diffraction patterns of the uncomplexed PEO anddifferent compositions of plasticized polymer elec-trolytes have been recorded by PANalytical X-raydiffractometer in the 2h range 1050 using CuK

a

radiation. Thermal behavior of the complexes hasbeen studied using NETZSCH DSC (200 Phox),over a range of100 to 100 C. Electrolyte samplesof about 56 mg were sealed in aluminum pans andexperiments were carried out under nitrogen gasatmosphere. The samples were first heated fromroom temperature to 100 C (I run), cooled to

100 C (run II), then again heated to 100 C (run

III) at a heating rate of 10 C min1. FTIR absorp-tion spectra were recorded by a Perkin-Elmer spec-trometer in the frequency range 3000500 cm1.

The cation transference number tNa has beenevaluated through the combined complex imped-ance and DC polarization measurement technique[17]. For this, the samples were sandwiched betweena pair of non-blocking sodium metal electrodesunder the inert atmosphere. Initially the impedancespectra of the cell have been recorded using a HP4192A impedance analyzer in the frequency range

5 Hz to 13 MHz. Then the cell has been polarizedwith a small dc potential (V) and the time evolutionof the resulting current was recorded using a highinput impedance electrometer. Further the cell wasallowed to polarize for about 10 h, until a steady

state current (Is) has been reached. The cationictransference number has been obtained using the

relation

tNa Rb

V=Is Rct ; 1

where Rb and Rct represent the bulk and electrodeelectrolyte charge transfer resistance of the polymerelectrolyte respectively. For the electrical conductiv-ity studies, fresh films were cut into circular shape ofabout 10 mm diameter and sandwiched between twofinely polished blocking silver electrodes to form a

symmetric cell Ag|polymer electrolyte|Ag. Theimpedance measurements have been carried out inthe temperature range of 302343 K under nitrogengas atmosphere.

3. Results and discussion

3.1. Surface morphology

Optical micrographs of the uncomplexedPEO, host polymer electrolyte (PEO)6:NaPO3 and

Fig. 1. Optical micrographs of spherulitic structures of (a) PEO; (b) (PEO)6:NaPO3 and (c)(e) (PEO)6:NaPO3 + x wt.% PEG400 with

x = 30, 50 and 70.

A. Bhide, K. Hariharan / European Polymer Journal 43 (2007) 42534270 4255


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plasticized electrolytes are shown in Fig. 1. Micro-graph of the PEO (Fig. 1a) shows well definedspherulitic structures separated by dark regions,indicating semi-crystalline nature of the polymer.With the addition of NaPO3 the spherulitic struc-

tures of PEO are found to be less prominent dueto the encroachment of adjacent spherulites(Fig. 1b). A significant change in the morphologyof polymer electrolytes is observed with the additionof plasticizer (Fig. 1ce). All the electrolyte systemshave consistently shown increase in number ofspherulites with a reduction in their size, indicatinggradual enhancement in the amorphous phase of thepolymer.

3.2. X-ray diffraction

Fig. 2(ae) shows the XRD pattern of uncom-plexed PEO and polymer electrolytes with differentconcentrations of the plasticizer. The XRD pattern

of uncomplexed PEO (Fig. 2a) shows two broadBragg peaks around 20 and 23 indicating semi-crystalline nature of the polymer. These diffractionpeaks become broader and less intense with theaddition NaPO3 salt (Fig. 2b). This can be attrib-

uted to increase in amorphous phase of the polymerarising due to destruction of ordered arrangementsof polymer chains and interaction of ether oxygenof PEO and Na+ ion. With the addition ofPEG400, the XRD patterns have exhibited furtherbroadening and reduction in the intensity of theBragg peaks with a diffuse background indicatingsignificant reduction in the crystalline phase.

3.3. Thermal analysis

Fig. 3 shows typical DSC traces of (PEO)6:

NaPO3 and(PEO)6:NaPO3 + 50 wt.% PEG400 poly-mer electrolytes. Table 1 summarizes the variousthermal properties of the plasticized polymer elec-

10 20 30 40 50

(e)

(d)

(c)

(b)

(a)

Intensity(arb.units)

2 (deg)

Fig. 2. XRD pattern of (a) PEO; (b) NaPO3; (c) (PEO)6:NaPO3 and (d)(e) (PEO)6:NaPO3 + x wt.% PEG400 with x = 30 and 70.



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trolytes such as, glass transition temperature (Tg),melting temperature (T

m) and melting enthalpy

(DHm) of the crystalline phase of the polymer, eval-uated during the heating process from 100 C to100 C (run III). The plots clearly indicate absenceof chemical reaction of host polymer electrolytewith plasticizer. Further, the retention of the plasti-cizer in the polymer electrolyte upon thermal cyclingis confirmed. The polymer electrolytes have exhib-ited an endothermic hump around 65 C and apeak around 55 C corresponding to the glasstransition temperature and the melting temperatureof crystalline phase of the polymer respectively.

The additional endothermic peak around 6 C

-100 -80 -60 -40 -20 20 40 60 80 100

(a)

Tg

Tg

(b)

Temperature (C)

(arb.units)

III

II

I

Tm

Tm

DSC

signalEndo

0

Fig. 3. DSC traces of (a) PEO6:NaPO3 and (b) (PEO)6:NaPO3 + 50 wt.% PEG400.

Table 1Thermal properties of the PEO, PEO6:NaPO3 and the plasticizedpolymer electrolytes (PEO)6:NaPO3 + x wt.% PEG400

Composition(PEO)6:NaPO3 +x, wt.% PEG400

Tg, C Tm, C Melting enthalpy(DHm), J/g

Pure PEO 61.1 63.7 133.9PEO6:NaPO3 53.1 62.3 79.930 64.6 58.4 65.240 65.6 56.1 63.350 66.2 55.2 56.860 67.0 54.8 52.670 70.2 50.6 46.6



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observed in the DSC trace of plasticized polymerelectrolyte is attributed to the melting of PEG400.The Tg of these samples decreased from 53.1 Cfor PEO6:NaPO3 to 70.2 C for the electrolytewith the maximum content of PEG400. The gradualdecrease in Tg value with the increase in PEG con-tent has suggested an enhancement in the flexibilityof polymer chains. The enthalpy of melting of crys-talline phase of the polymer is found to decreasewith increase in plasticizer content. This indicatesreduction in crystalline phase of the polymer elec-trolyte. The enthalpy of melting of the plasticized

electrolyte reduces to a minimum of 46.6 J/g for

the electrolyte with maximum content of PEG400,compared with 79.9 J/g for (PEO)6:NaPO3 hostmatrix and 133.9 J/g for the uncomplexed PEO.These results substantiate the Bragg peak broaden-ing and decrease in the spherulitic structuresobserved in the XRD pattern and optical micro-graphs respectively.

3.4. Fourier transform infrared spectroscopy

Comparison of FTIR spectra of plasticized poly-mer electrolytes, plasticizer free polymer metal salt

complex, uncomplexed PEO and PEG is presented

3000 2500 2000 1500 1000 500

T

ransmittance(%)

Wavenumber (cm-1)

(d)

(c)

(b)

(a)

(e)

Fig. 4. Comparison of FTIR spectra of (a) PEO; (b) (PEO)6:NaPO3; (c)(d) plasticized polymer electrolyte (PEO)6:NaPO3 +x wt.% PEG400 with x = 50 and 70, respectively and (e) PEG400.



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in Fig. 4. Both the plasticized polymer electrolyteand host polymer electrolyte have exhibited com-mon spectral features, except a decrease in the inten-sity of the bands. A shift in the position of C@Ostretching mode of PEO observed at about

1802 cm

1

[18] for PEO6:NaPO3, is due to the inter-action of ether oxygen and the Na+ ion. However,with the addition of the plasticizer, no furtherchange in the position of the above mode is seen.This infers that the PEG does not interact with etheroxygen of PEO chains. A detailed analysis of theFTIR spectra has not been carried out due to poorresolution of the spectra and common IR active

bands of PEO and PEG400. However, from theXRD, DSC and FTIR results it can be concludedthat PEG merely resides in the host polymer matrix,providing a free volume for easier chain motion ofthe polymer chains.

3.5. Transport number measurement

Fig. 5 shows typical plots of the complex imped-ance data measured for the symmetric cells Na|SPEplasticized with 70 wt.% PEG|Na and Na|PEO6:Na-PO3|Na at 337 K. The impedance plots show twosemicircles over the frequency range of measurement.

0 500 1000 1500 2000

0

500

1000

Z' (k)

-Z"(

k)

-Z"(k)

Z' (k)

0 10 12 14 16

0

2

4

6

8

10

2 4 6 8

b

a

Rb

CdlCPE

Rct

w

Fig. 5. Impedance plot for (a) Na|PEO6:NaPO3|Na and (b) Na|(PEO)6:NaPO3 + 70 wt.% PEG400|Na cell at 337 K. Inset (b) Equivalent

circuit of the plasticized electrolyte with non-blocking Na electrodes.



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The electrical circuit elements attributed to the evo-lution of the complex impedance spectrum have beendetermined using EQUIVCRT non-linear fittingprogramme [19]. The first semicircle appearing athigh frequency region has been associated with thebulk resistance (Rb) and the constant phase element(CPE). The skewed semicircle appearing at the lowerfrequency region has been attributed to the interfa-cial resistance (Ri) in parallel with the double layercapacitance (Cdl). Interfacial resistance includescharge transfer resistance (Rct) and a surface filmresistance due to the passive layer on the sodium

electrode. The above observed features have been

modeled to an equivalent circuit as shown in Fig.5b (inset). The cell was maintained at the same tem-perature for about 4 h and impedance measurementswere carried out periodically in order to check theconsistency of the data. Since the value of total inter-facial resistance was found to be constant over a per-iod of time, the contribution of the surface filmresistance has been neglected. Thus the bulk resis-tance (Rb) of value of 10.2 kX and stable chargetransfer resistance (Rct) of 5.6 kX have been deter-mined. Then the symmetric cell was polarized forabout 12 h by applying a small dc potential of

100 mV and the steady state current (Is) of 3.3 lA

0 10 20 30 40 50 60 70

0

10

20

30

40

50

60

70

80

90

100

110

-Z"(k)

Z' (k)-Z"(k)

Z' (k)

333 K

329 K

325 K

0 2 4 6

0

1

2

3

4

5

6

7

341 K

337 K

1 3 5 7

Fig. 6. Impedance plots of the plasticized polymer electrolyte (PEO)6:NaPO3 + 50 wt.% PEG400 as a function of temperature.



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was noted. Using the above values in Eq. (1), the cat-ionic transference number is found to be 0.42, whichis much higher than that for (PEO)6:NaPO3tNa 0:23. Similarly the transference number val-ues of other compositions are found to be compara-

ble, lying in the range of 0.390.42.

3.6. Ionic conductivity studies

Fig. 6 shows the complex impedance plots ofthe electrolyte sample (PEO)6:NaPO3 + 70 wt.%PEG400 at different temperatures. Each spectrum

comprises of a semi circle at high frequency region,

2.9 3.0 3.1 3.2 3.3

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

log(

)(Scm-1)

x = 70

x = 60

x = 50

x = 40

x = 30

340 330 320 310 300

1000/T(K-1)

T(K)

1000/T (K-1)

2.9 3.0 3.1

-8.50

-8.25

-8.00

-7.75 (PEO)6:NaPO3

log

(Scm-1)

Fig. 7. Temperature dependent conductivity of plasticized electrolytes (PEO)6:NaPO3 + x wt.% PEG400 (30 < x < 70). Inset: temperature

dependent conductivity of (PEO)6:NaPO3. The solid lines are only to guide the eye.



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with their centers below the real axis. These semicir-cles are attributed to the bulk property of the elec-trolyte i.e. bulk resistance (Rb) with a non-idealcapacitor usually known as constant phase element(CPE) which accounts for the observed depression

of semicircles. The spike appearing at low frequencyregion is modeled to a capacitor Cdl correspondingto the double layer capacitance at the electrolyteelectrode interfaces. The diameter of the semicircle

is found to decrease with increase in temperature,indicating an activated conduction mechanism. Byknowing the value of bulk resistance and the dimen-sions of the electrolyte films, conductivity valueshave been evaluated.

Inset ofFig. 7 shows temperature dependent con-ductivity of (PEO)6NaPO3 host matrix. The plot hasexhibited a clear distinction in the slopes around335 K, above which the polymer electrolyte starts

7.6 7.8 8.0 8.2 8.4

-3.9

-3.8

-3.7

-3.6

103/(T-T0) (K-1)

log(T1/2)(S/cmK

-1/2)

103/T (K-1)

log()(Scm

-1)

2.90 2.95 3.00 3.05

-4.92

-4.88

-4.84

-4.80

-4.76

Fig. 8. Temperature dependent conductivity data (PEO)6:NaPO3 + 70 wt.% PEG400 and linear fit obtained for the VTF equation.



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to soften. The activation energy for ion migration isfound to be 0.72 eV in the temperature region belowthe softening point, compared to 0.40 eV in the elas-tomeric rich phase. Fig. 7 shows temperature depen-dent conductivity of different plasticized electrolyte

samples. These plots have shown sluggish transitionaround the softening point (330 K), about which atendency for a curvature has been observed. Suchbehavior has been often observed for elastomericphase rich solid electrolytes obeying VTF (VogelTammanFulcher) relation [20]

r T AT12 exp BkT T0

; 2

where T and A are the absolute temperature andthe fitting constant respectively. B is considered asthe apparent activation energy. T0 is the Vogeltemperature or ideal glass transition temperature,at which free volume of the polymer tends to zero

value.Fig. 8 shows the temperature dependent conduc-tivity plots of the composition (PEO)6:NaPO3 +70 wt.% PEG400 and linear plot of log (rT

1/2) versus1000/(T T0). The best linear fit has been obtainedat T0 % Tg = 204 K, exhibiting an apparent activa-tion energy of 0.05 eV. The electrolyte compositionswith PEG content ofx = 0.6 and 0.5 have exhibited

1 3 5 7

-7.2

-7.0

-6.8

-6.6

-6.4

-5.9

-5.8

-5.7

-5.6

-5.5

-5.4

-5.3

-5.2

log()(Scm-1)

log (Hz)

341 K337 K

333 K

329 K

325 K

2 4 6

Fig. 9. Frequency dependent conductivity of the composition (PEO)6:NaPO3 + 50 wt.% PEG400 at different temperature.



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apparent activation energy of about 0.07 eV. How-ever, linearization of the conductivity data of othercompositions was not satisfactory. At low tempera-ture region the conductivity data of all the composi-tions have obeyed Arrhenius behavior, suggesting a

thermally activated conduction process governed bythe equation

r r0 exp EakBT

; 3

where r0 is the pre exponential factor, Ea is the acti-vation energy, kB and Tare the Boltzmann constantand absolute temperature respectively. Fitting the

data to the above equation the activation energiesare found to be around 0.42 eV. These activationenergy values agree well with the value of plasticizerfree electrolyte above the softening point. This indi-cates that addition of plasticizer promotes mobility

of the ions by providing easier path ways.The above conduction characteristics show anenhancement in the conductivity and mobility ofthe ion, with a constant cationic transport numberindependent of plasticizer content. These featuresindicate that PEG plays role in enhancement inthe amorphous phase, without facilitating dissocia-tion of the salt.

1 3 5 7

1

2

3

4

5

6

log(')

log () (Hz)

343 K

335 K

327 K

2 4 6

Fig. 10. Plot of log e 0 with the frequency of the composition (PEO)6:NaPO3 + 50 wt.% PEG400.



13/18

3.7. Frequency dependent conductivity

Analysis of frequency dependent electricalresponse of solid electrolytes with disordered struc-ture is a versatile approach for understanding the

nature of ionic transport. For this the collectedcomplex impedance data has been analyzed in dif-ferent formalisms such as complex conductivity(r(x)), permittivity (e) and electric modulus (M).These complex electrical quantities are interrelatedaccording to the following equation:

M 1

e jxC0Z 4

where x is the angular frequency and j ffiffiffiffiffiffiffi1

p. The

vacuum capacitance is expressed as C0 e0Ad , wheree0 is the permittivity of the free space, A and d arethe area of cross section and thickness of the samplerespectively. The analysis of complex permittivity

data provides information about electrode polariza-tion. On the other hand, the analysis of modulusdata assumes importance, as the conductivity relax-ation becomes prominent due to the suppression ofthe electrode effects.

In the present study, temperature dependentimpedance data of a typical composition (PEO)6:NaPO3 + 50 wt.% PEG400 has been analyzed inthe above mentioned formalisms. Fig. 9 shows the

1 3 5 7

-1

0

1

2

3

4

5

6

-1

0

1

2

3

4

5

6

log("-(0)/0)

log(")

log () (Hz)

343 K

327 K

2 4 6

Fig. 11. Plot of log e 0 0 and log [e 0 0 r(0)/xe0] with the frequency of the composition (PEO)6:NaPO3 + 50 wt.% PEG400.



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variation in the real part of the frequency dependentconductivity at different temperatures. At lower fre-quencies below 10 kHz, the conductivity is found toincrease with increase in temperature. A frequencyindependent conductivity plateau is observed in

the frequency range 1050 kHz. At higher frequen-cies, dispersion in the conductivity value isobserved. The conductivity dispersion is found tobe less predominant at lower temperatures and asthe temperature increases, the frequency at whichthe dispersion becomes prominent shifts to higherfrequency region. In other words, the bulk relaxa-tion shifts to higher frequencies with increase intemperature.

Fig. 10 shows the plot of log e 0 and log x at dif-ferent temperatures. The high dielectric permittivityvalues observed at low frequency region can be

attributed to the build up of space charge nearthe electrodeelectrolyte interface which blocksthe charge transport. At higher frequencies thepermittivity values of the material is found todecrease rapidly and saturate, as the dipoles in the

macromolecules hardly be able to orient in thedirection of the applied field. Similar trend in thevariation of imaginary part of the permittivity (e 00)with frequency is seen from Fig. 11. In the case ofpolymer electrolytes, the above entity (e 00) associ-ated with dielectric loss comprises of relaxation lossdue to ionion interaction and the bulk conductionloss. For the polymer electrolytes with reasonableconductivity, the dielectric relaxation peaks due topermanent dipoles are suppressed by polarizationof mobile species present in the material [21]. There-fore in order to obtain true dielectric relaxation loss

1 2 3 4 5 6 7

0.0

0.4

0.8

1.2

1.6

343 K

337 K331 K

325 K

M'

M"

(10-3)

log () (Hz)

1 32 4 5 6 7

0.00

0.01

0.02

0.03

0.04

325 K

331 K

337 K

343 K

Fig. 12. Frequency dependent electric modulus of the composition (PEO)6:NaPO3 + 50 wt.% PEG400 at different temperature.



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as a function of frequency, the dc conductivity con-tribution of the loss r0

-e0is subtracted from the total

dielectric loss e 0 0. Thus the corrected dielectric loss isobtained from the relation

ecorr

e00

r

0

-e0 : 5It is clearly indicated from the ecorr versus frequencyplots (Fig. 11) that the dipole relaxation loss ismuch smaller compared to e 0 0. The maximum ofthe corrected dielectric relaxation peaks are foundto shift to higher frequency with increase in temper-

ature, origin of which can be attributed to the en-ergy absorbed by the permanent dipoles or ionpairs present in the electrolyte system.

The frequency dependence of M0 and M0 0 at dif-ferent temperatures is shown in Fig. 12. It is seen

from the figure that at lower frequenciesM

0 ap-proaches to zero indicating negligible contributionof electrode polarization. At higher frequencies,

the M0 is found to increase gradually with a ten-dency for saturation. The observed dispersion is

mainly due to conductivity relaxation spread over

range of frequencies. The low values of M0 0, in the

2.9 3.0 3.1 3.2

-6.5

-6.0

-5.5

-5.0

Conductivity data

log()(Scm-1)

1000/T (K-1)

log(max

)(Hz)

2.9 3.0 3.1 3.2

5.2

5.4

5.6

5.8

6.0

6.2

6.4

max of modulus peak

Fig. 13. Conductivity and frequency of peak position of the modulus data as a function of temperature. The solid lines are only to guide

the eye.



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low frequency region indicate negligible contribu-

tion of the electrode polarization to the electric

modulus. At higher frequencies M0 0

spectra show

broad and asymmetric peak with a maximum at afrequency xmax. This Non-Debye behavior can be

interpreted as being consequence of distributions

of relaxation time and due to non-exponential ap-

proach of electrical functions, defined by the

stretched Kohlrausch function [22].

/ t exp t=s b; 6

where s is the characteristics relaxation time and0 < b < 1 represents the departure of plot from lin-ear exponential (b = 1). The b parameter deter-mined using the full width at half maxima value ofM0 0 peak [23] is found to be around 0.61 and inde-pendent of temperature. These features can beattributed only to the conductivity relaxation sincethe polymer chains exhibit dielectric relaxations atvery high frequencies of the order of GHz [24,25].It is seen from the frequency dependent M0 0 plotthat the frequency corresponding to M00max shifts tohigher frequencies with increase in temperature.

The plot of log (xmax) versus 1000/T presented in

Fig. 13 behaves similar to the temperature depen-

dent conductivity, exhibiting two different regions

above and below the softening point of the polymer.

Further, the activation energy corresponding to the

two regions are found to be comparable with the

values obtained from the conductivity data. Theseresults substantiate that the M0 0 relaxation peakscorresponds to the conductivity relaxation.

3.8. Solid state sodium battery

Based on the best conducting polymer electrolyte(PEO)6:NaPO3 + 70 wt.% PEG400, a primary solidstate electrochemical cell has been fabricated witha configuration, Na|solid polymer electrolyte|(I2:acetylene black:PEO, 60:30:10). For the above con-

figuration, the half cell reactions can be written as:At the anode:

Na ! Na e

and at the cathode:

I2 2e ! 2I: 7The over all reaction can be written as

Na 12

I2 ! NaI: 8

From the value of Gibbs free energy of formation of

NaI DGf 286:1 kJ=mol [26], the theoretical

0 10 20 30 40 50 60 70 80 90

1.8

2.0

2.2

2.4

2.6

2.8

3.0

OCV

500 k load

Voltage(V)

Time (h)

Fig. 14. Discharge characteristics of solid state electrochemical cell Na|(PEO)6:NaPO3 + 70 wt.% PEG400|(I2:acetylene black:PEO).



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open circuit voltage (OCV) has been calculatedusing the relation

E0 DGf

nF; 9

where n and F are the number electrons involved inthe reaction and Faraday constant (96,500 C/mol),respectively.

In the present investigation, the cell has exhibitedan OCV of 2.94 V, which is in good agreement withthe theoretically calculated value of 2.96 V. Fig. 14shows the open circuit voltage (OCV) and dischargecharacteristics of the primary cell with a constantload of 500 kX and the other cell parameters havebeen presented in Table 2. The initial drop observedin the voltage can be attributed to the polarization

of the cell, which arises from the concentration gra-dient of the reactants and products at the electrodesurface. The above laboratory cell demonstrates thepractical application of the electrolyte in solid statebattery with a plateau region of about 24 h at roomtemperature.

4. Conclusions

Plasticization of the polymer electrolyte PEO6:NaPO3 has been successfully carried out with theaddition of the low molecular weight polymerPEG400. A significant decrease in the degree ofcrystallinity of the polymer electrolyte has been evi-denced through XRD, DSC and optical microscopystudies. An enhancement in the cationic transporttNa number has been observed from a value of

0.23 for the host polymer electrolyte to the valuesin the range 0.390.42 for the plasticized electrolyteswith different concentration of PEG400. Tempera-ture dependent conductivity studies infer segmentalmotion assisted ionic transport in the elastomericrich phase governing VTF type behavior exhibiting

an ionic conductivity of 8.9 10

7 S cm

1 at 310 K.

The characteristics of the primary cell Na|SPE|(I2 +acetylene black + PEO) have demonstrated theapplication of the plasticized electrolyte in the solidstate sodium battery.

References

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Table 2Cell parameters of Na|(PEO)6:NaPO3 + 70 wt.% PEG400|(I2 + acetylene black + PEO)

Cell parameter Numerical value

Open circuit voltage 2.94 VShort circuit current 380 lA

Area of the cell 1.13 cm2

Current density 336 lA cm2

Weight of the cell 0.87 gPower density 1.28 W Kg1

Time of plateau region (load 500 kX) 25 hDischarge capacity 9.5 mA h1



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sodium tetrafluoroborate and sodium tetrahydroborate. JAm Chem Soc 1982;104:624751.

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Ionic Transport Studies on (PEO) 6 NaPO3 Polymer