Electrochimica Acta 171 (2015) 59–65

Effect of plasticizers on ionic conductivity and dielectric relaxation ofPEO-LiClO4 polymer electrolyte

S. Das, A. Ghosh *Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

A R T I C L E I N F O

Article history:Received 20 March 2015Received in revised form 24 April 2015Accepted 30 April 2015Available online 5 May 2015

Keywords:polymer electrolytesionic conductivitydielectric relaxation

A B S T R A C T

In this paper, we report the effect of different plasticizers, such as poly ethylene glycol, propylenecarbonate, ethylene carbonate and dimethyl carbonate on the ionic conductivity and dielectric relaxationof PEO-LiClO4 solid polymer electrolytes. The highest ionic conductivity is obtained for polymerelectrolyte plasticized with poly ethylene glycol. The temperature dependence of ionic conductivityfollows the well known Vogel–Tamman–Fulcher relation which indicates a strong coupling of ionic andsegmental motions for all compositions. The experimental data for the complex dielectric permittivityand electric modulus have been studied using Havriliak–Negami function for the understanding of iondynamics. Maximum dielectric strength is obtained for polymer electrolyte plasticized with polyethylene glycol. The modulus data have been analyzed using non-exponential Kohlrausch–Williams–Watts (KWW) function. It is observed that the non-exponential parameter b is quite lower than unity,suggesting highly a non-exponential relaxation exists in these materials. The temperature dependence ofrelaxation times obtained from dielectric and modulus formalisms also follow Vogel–Tamman–Fulcherrelation for all plasticizers in the compositions.

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1. Introduction

Solid polymer electrolytes are technologically importantmaterials for electrochemical devices [1–3]. Basically, a polymerelectrolyte consists of a host polymer (such as polyethylene oxide)doped with an alkali salt (such as LiClO4). The mechanical andelectrical properties of the polymer electrolytes are controlled byboth the host polymers and dopant salts. At room temperature, thehost polymers are generally biphasic consisting of both amorphousand crystalline phases [4]. It is well known that ionic conductionoccurs mainly in the amorphous phase. There are many techniques,such as inclusion of nanoparticles, doping of ionic liquid, etc. forenhancement of the amorphous phase and hence for theimprovement of the ionic conductivity of polymer electrolytes[5–8]. Plasticization is also an important way to improve ionicconductivity. Plasticization occurs when organic compounds withlow molecular weight and high dielectric constant are added topolymer matrix [9–13]. Recently, there is a great attention for thestudy of ion transport in these polymer electrolytes. Ion transportin these polymer electrolytes is not only controlled by ionicmotion, but also coupled with segmental motion of host polymers

* Corresponding author.E-mail address: sspag@iacs.res.in (A. Ghosh).

http://dx.doi.org/10.1016/j.electacta.2015.04.1780013-4686/ã 2015 Elsevier Ltd. All rights reserved.

[14]. A study of ion transport mechanism in polymer electrolytes isa great challenge from scientific viewpoint due to the intrinsiccomplexity of coupled motion. The impedance spectroscopy hasbeen widely used to get insight into ion transport in ion-conducting materials. Impedance spectroscopy data can beexpressed as different representations such as ac conductivity,dielectric permittivity, electric modulus, etc.

In this work, we have studied ionic conductivity and dielectricproperties of PEO-LiClO4 polymer electrolytes containing differentplasticizers such as polyethylene glycol (PEG), propylene carbonate(PC), ethylene carbonate (EC) and dimethyl carbonate (DMC).

2. Experimental technique

2.1. Materials

PEO (MW = 400000), LiClO4, PEG, EC, PC and DMC werepurchased from Sigma-Aldrich. PEO and LiClO4 were dried invacuum prior to use. Acetonitrile, also purchased from Sigma-Aldrich, was used as a solvent.

2.2. Preparation of solid polymer electrolytes

For preparation of solid polymer electrolytes, the molar ratio ofethylene oxide segments to lithium ions was kept at EO/Li = 18.

60 S. Das, A. Ghosh / Electrochimica Acta 171 (2015) 59–65

Appropriate amounts of PEO and LiClO4 were dissolved inacetonitrile and stirred in a magnetic stirrer. The solution becamethick after 24 hours due to evaporation of the solvent. The solutionwas then cast in a PTFE container and kept for 24 hours for normalevaporation. At last it was dried for 36 hours at 50 �C in vacuum toform free standing homogeneous film. For the preparation ofplasticized polymer electrolytes, a fixed amount (30 wt.%) of PEG,PC, EC and DMC was added separately to the solution of PEO andLiClO4 in acetonitrile under stirring condition. The same techniqueas stated above was followed to get films of thickness in the rangefrom 0.2 mm to 0.4 mm.

2.3. Impedance spectroscopy

The measurements of capacitance and conductance of the driedfilms were carried out in a RLC meter (Quad Tech, model 7600) inthe frequency range 10Hz–2 MHz in vacuum and in a widetemperature range with a temperature stability of �0.10 K. For theelectrical impedance measurement the samples were sandwichedbetween two stainless steel blocking electrodes of a conductivitycell and kept under vacuum (0.01 mbar) condition to avoidmoisture. The ionic conductivity of the films was obtained fromthe complex impedance plots. The conductivity (s) of the films wascalculated using the relation s = L/(RB*A), where L is the thicknessof films, A is the effective contact area of the electrodes and RB isthe bulk resistance obtained from the complex impedance plots.

Fig. 1. FE-SEM images of (a) PEO-LiClO4 (b) PEO-LiClO4 -30 wt.% EC (c) PEO-

2.4. Field emission scanning electron microscopy (FE-SEM)

The surface morphology of the platinum-coated polymer filmswas studied using a field emission scanning electron microscope(JEOL, model JSM-6700F).

3. Results and discussion

3.1. Surface morphology

Fig. 1 shows FE-SEM images for different plasticized polymerelectrolytes at different magnifications. The SEM images show theexistence of crystalline and amorphous regions. The images withmicro-pores are commonly observed due to fast evaporation ofacetonitrile during solution cast method. Reduction of crystallinephase in PEO is associated with the smooth surface morphology ofthe polymer electrolyte through the interaction between etheroxygen of PEO and Li+ ions. It is observed that surface morphologybecomes smooth for polymer electrolyte plasticized with PEGcompared to other plasticized electrolytes.

3.2. Ionic conductivity

The variation of ionic conductivity (s) for PEO-LiClO4-30 wt.%(PC, EC, DMC, PEG) polymer electrolytes with the reciprocaltemperature is shown in Fig. 2(a). It is noted that the plots in

LiClO4 -30 wt.% DMC (d) PEO-LiClO4 -30 wt.% PEG polymer electrolytes.

Fig. 2. (a) Ionic conductivity shown as a function of reciprocal temperature fordifferent plasticized PEO-LiClO4 electrolytes. Solid lines are fits of experimental datato Eq. (1).

Fig. 3. Ac conductivity spectra at different temperatures for PEO-LiClO4 - 30 wt.%DMC electrolytes. Solid lines are the fits of the experimental data to Eq. (2).

S. Das, A. Ghosh / Electrochimica Acta 171 (2015) 59–65 61

Fig. 2(a) for all plasticizers follow Vogel-Tamman-Fulcher (VTF)empirical formula given by [15–17]

s = s0T�1/2exp [-Ea/kB (T-T0)] (1)

where s0 is a pre-exponential factor, kB is the Boltzmann constant,Ea is the pseudo activation barrier related to the critical freevolume for ion transport, T0 is the Vogel scaling temperature,where the configuration entropy or the critical volume becomeszero and T is the absolute temperature. The excellent fits of theconductivity data presented in Fig. 2(a) to VTF formula areobtained in the entire temperature range, indicating clearly thatthe motion of Li+ ions is coupled with the polymer segmentalmotion in all polymer electrolytes. The values of the ionicconductivity at 298K and the parameters obtained from VTF fitsare shown in Table 1. It is noted in Table 1 that the activation energyEa exhibits a minimum and the ionic conductivity a maximum forpolymer electrolytes plasticized with PEG. The addition of DMC,which has lower dielectric constant (e = 3.1) than PEO (e � 5), hasno effect on the enhancement of the conductivity for PEO-LiClO4

electrolyte. But the plasticizers EC, PC and PEG have dielectricconstants of 89.78, 66.14 and 12.4 respectively, which are higherthan that of PEO and their ionic conductivity is higher than that ofPEO-LiClO4 electrolyte. Thus, it can be concluded that value ofdielectric constant of a plasticizer should be higher than that of thepolymer used, otherwise it shows anti-plasticization effect. Thehigher value of dielectric constant dissociates ion aggregates andthus helps in increasing the ionic conductivity.

Table 1Parameters obtained from VTF and power law fits for PEO-LiClO4 electrolytes with 30 w

Plasticizer(30 wt.%)

Ea (eV)(�0.002)

T0 (K)(�2)

Log1(�0T = 2

PEO-LiClO4 0.094 174 -5.8EC 0.140 151 -5.5PC 0.074 171 -4.8PEG 0.063 174 -4.7DMC 0.163 156 -6.3

3.3. Ac conductivity spectra

The frequency dependence of the ac conductivity at differenttemperatures for PEO-LiClO4-30 wt.% PEG electrolyte is shown inFig. 3. It is observed that in the low frequency region theconductivity is independent of frequency and corresponds to thedc ionic conductivity for all compositions and temperatures.However, the dispersion of the conductivity starts at higherfrequencies, indicating that a crossover from the dc ionic to thedispersive conductivity occurs at a particular frequency calledcrossover frequency. It is further noted that the crossoverfrequency shifts towards higher frequencies as the temperatureis increased. Other plasticized polymer electrolytes also showedsimilar frequency dependence.

The ac conductivity data for different electrolytes has beenanalyzed in the framework of the power law formalism [18,19]. Inthis formalism, the frequency dependent conductivity is given by

s0 (v) = s[1+(v/ vC) n] (2)

where s is the ionic conductivity, vC is the crossover frequency,and n is the power law exponent. The value of n is usually in therange, 0 < n < 1. We have fitted the frequency dependentconductivity data to Eq. (2) using s, vC and n as variableparameters. The values of the parameters s, vC and n obtained at298 K from the best fits are listed in Table 1. It is observed thathigher dc conductivity is associated to higher crossover frequencywhich is maximum for polymer electrolyte plasticized with PEG.The charge transport mechanism is related to the hopping of Li+

ions from one site to another favourable site. It is also noted thatthe value of n is less than unity.

t.% plasticizers.

0(s) (V�1 cm�1).04)98 K

Log10(vC) (rad s�1)(�0.04)T = 298 K

n(�0.01)T = 298 K

2 6.56 0.584 7.06 0.644 7.25 0.541 8.63 0.652 6.53 0.64

gHN=2

Fig. 4. Frequency dependence of (a) dielectric constant e0(v) and (b) dielectric losse00(v) at several temperatures for PEO-LiClO4-30 wt.% EC respectively. Frequencydependence of e0(v) for different plasticized polymer electrolytes at 298 K is shownin the inset of Fig. 4(a). The contributions of the conductivity and dielectricrelaxation at 258 K are shown by solid and dotted lines respectively in the inset of 4(b).

62 S. Das, A. Ghosh / Electrochimica Acta 171 (2015) 59–65

3.4. Dielectric relaxation

The dielectric relaxation is often studied in terms of a frequencydependent complex dielectric permittivity given by

e � ðvÞ ¼ e0ðvÞ � je00ðvÞ (3)

where e0 and e00 are the real and imaginary parts of the dielectricpermittivity, known as dielectric constant and dielectric lossrespectively. Figs. 4(a) and 4(b) show frequency dependentdielectric constant and loss respectively at several temperaturesfor the polymer electrolyte plasticized with EC. It is observed inFig. 4(a) that at lower frequencies the high dielectric constant valuecorresponds to space charge polarisation near electrode-electro-lyte interface and at higher frequencies shows a labelling-offdenoted as e1, which arises from the rapid polarization of atomsand electrons when a time dependent electric field is applied.

Table 2Dielectric strength (es-e1), aHN and gHN, p, Ea and T0 obtained from VTF fits for relaxa

Plasticizer (30 wt.%) De = es-e1(�1)T = 258 K

aHN

(�0.02)T = 258 K

PEO-LiClO4 19 0.86

EC 27 0.81

PC 21 0.76

PEG 40 0.79

DMC 14 0.65

Frequency dependent e0(v) for different plasticized polymerelectrolytes at 298 K is shown in the inset of Fig. 4(a). Additionof plasticizers to the polymer matrix increases localization ofcharge carriers including mobile Li+ ions which is responsible forhigher e0(v) value of plasticized polymer electrolytes. The value ofe0(v) increases with the increase in temperature as shown inFig. 4(a). Similar trend is noted in Fig. 4(b) for e00(v).

The present experimental data for e0(v) and e00(v) have beenanalyzed using Havriliak–Negami (HN) formalism. In this formal-ism the dielectric function is given by [20,21]

e� vð Þ ¼ e1 þ 11 þ ivtHNð ÞaHN� �gHN

" #(4)

where es and e1 are the static and high frequency dielectricconstants respectively, tHN is the relaxation time and aHN and gHN

are the shape parameters with condition 0 � aHN< 1 and 0 � aHN

gHN< 1. Ideal Debye relaxation is obtained, when shape param-eters are unity and the non-zero value corresponds to adistribution of relaxation time.

The real and imaginary parts of e*(v) are respectively expressedas,

e0ðvÞ ¼ e1 þ ðes � e1Þ 1 þ 2 vtHNð ÞaHNcos aHNp=2ð Þ þ vtHNð Þ2aHNh i�

�cos arctansinðaHNp=2Þ

½ðvtHNÞ�aHN þ cosðbHNp=2Þ� �� �

(5)

and

e00ðvÞ ¼ ðes � e1Þ 1 þ 2 vtHNð ÞaHNcos aHNp=2ð Þ þ vtHNð Þ2aHNh i�gHN=2

�sin arctansinðaHNp=2Þ

½ðvtHNÞ�aHN þ cosðaHNp=2Þ� �� �

þ Svp

(6)

An extra term Svp is added to Eq. (6) to account for the

contribution of electrode polarization as the dielectric relaxationpeaks due to permanent dipoles are concealed by polarization ofmobile ions in polymer material. The value of p is between 0 and1 and the unity value of p corresponds to ideal Ohmic behaviour.The solid lines in Figs. 4(a) and 4(b) are the best fits for e0(v) ande00(v) given by Eqs. (5) and (6) respectively to the experimentaldata. The parameters such as De = es-e1, aHN, gHN and exponent pobtained from the best fits are given in Table 2 for differentcompositions. The value of p is less than unity, indicating adivergence from ideal Ohmic behaviour in the polymer electro-lytes. In the inset of Fig. 4(b) the contribution of the ionicconductivity is shown by solid lines, while that of relaxation by thedotted lines. Similar behaviour is also observed for all othercompositions. It is observed in Table 2 that the dielectric strengthobtained for PEG plasticized polymer electrolyte is higher thanthose composition containing other plasticizers. It is also noted

tion time for PEO-LiClO4 electrolytes with 30 wt.% plasticizers.

gHN

(�0.02)T = 258 K

p(�0.05)T = 258 K

Ea (eV)(�0.002)

T0 (K)(�2)

0.52 0.89 0.040 1920.55 0.94 0.043 2070.54 0.95 0.016 2110.61 0.90 0.072 1730.62 0.96 0.061 189

Fig. 5. Reciprocal temperature dependence of the relaxation time tHN obtainedfrom dielectric relaxation for different plasticized PEO-LiCLO4 electrolytes. Solidlines are fits of experimental data to VTF formalism given in Eq. (1).

Fig. 6. Frequency dependence of (a) real part (M0) and (b) imaginary part (M00) ofcomplex electric modulus M* for PEO-LiCLO4-30 wt.% EC electrolytes. The solid linesare fits of the experimental data to Eq. (8).

Table 3Relaxation time tM, ahn and ghn and b for PEO-LiClO4 electrolytes with 30 wt.% plastic

Plasticizer (30 wt.%) Log10 (tM) (s)T = 253 K(�0.05)

ahn

T = 253 K(�0.02)

PEO-LiClO4 -4.44 0.74

EC -4.58 0.80

PC -5.82 0.88

PEG -3.68 0.86

DMC -5.91 0.81

S. Das, A. Ghosh / Electrochimica Acta 171 (2015) 59–65 63

that variation of dielectric strength is in the order (De)PEG > (De)EC > (De)PC > (De)0 > (De)DMC [(De)0 is the dielectric strengthwithout plasticizers]. The inverse of dielectric relaxation time tHNis plotted with reciprocal temperature in Fig. 5. It is observed thatthe temperature dependent relaxation time tHN is well fitted toVTF function and parameters obtained from the best fits are listedin Table 2. The relaxation time decreases due to faster segmentalmotion coupled with mobile ions for polymer electrolyte plasti-cized with PEG compared to other plasticizers.

3.5. Electric modulus

Ion transport in the polymer electrolytes has been furtherstudied using electric modulus formalism. Interfacial polarizationwhich is masked by the conductivity arises from the additive inconducting polymers and the dielectric permittivity is also high atlow frequencies. To overcome this difficulty, the electric modulushas been used to investigate and understand conductivityrelaxation mechanism. In the modulus formalism [22], an electricmodulus M* is defined as the inverse of the complex dielectricpermittivity e*

M�ðvÞ ¼ 1e�ðvÞ ¼ M0 þ iM00 ¼ e0

e02 þ e002þ i

e00

e02 þ e002(7)

where M0 and M00 are the real and imaginary parts of the complexmodulus M*. The frequency dependence of M0 and M00 spectra atdifferent temperatures for PEO- LiClO4-30 wt.% EC electrolyte isdepicted in Figs. 6(a) and 6(b) respectively. It is observed that thereal modulus M0 shows dispersion as the frequency is increasedand tends to saturate at M1 for higher frequencies. The imaginarymodulus M00 exhibits a single relaxation peak which shifts towardshigher frequencies with increasing temperature. At higherfrequencies (104Hz), the peak shifts because increase in tempera-ture results in faster ionic motion, leading to decrease in relaxationtime. Thus, the charge carriers are thermally activated. Thefrequency of the peak of M0 spectra corresponds to the conductivityrelaxation frequency vM. The conductivity relaxation time tM isobtained from the relation vM tM= 1 [22]. We also observe inFig. 6(b) that M00 spectra are much broader than what is expectedfor the ideal Debye peak and show asymmetric and skewedtowards the high frequency sides of the maxima. Such a broadnature of the peak may be taken as a consequence of thedistribution of relaxation times and is an indication of the non-Debye type relaxation. The temperature and frequency depen-dence of M0 and M00 for other samples also shows similar nature.

To study the non-Debye relaxation, we have used the empiricalHavriliak - Negami equation [20,23,24] given by

M�ðvÞ ¼ M1 þ Ms � M1ð Þ 1½1 þ ðivtMÞahn ghn

� �(8)

where Ms and M1 are respectively the high frequency and lowfrequency limiting values of electric modulus, tM is the relaxationtime, ahn and ghn are the shape parameters which describe thesymmetric and asymmetric broadening respectively of the

izers.

ghn

T = 253 K(�0.02)

bT = 253 K(�0.02)

Ea (eV)(�0.002)

T0 (K)(�2)

0.43 0.39 0.029 2070.37 0.39 0.051 1880.35 0.41 0.040 1880.38 0.42 0.098 1790.19 0.27 0.027 194

Fig. 7. (a) Reciprocal temperature dependence of relaxation time tM obtained frommodulus formalism for all plasticized polymer electrolytes. The solid lines are fits ofthe experimental data to VTF function. (b) Variation of ahn and ghn withtemperature. The solid lines are guide to the eye.

Fig. 8. (a) Temperature and time dependence of the KWW decay function F (t) forPEO-LiCLO4-30 wt.% PC electrolytes. Solid lines are the best fits of experimental datato Eq. (10). (b) Variation of b with temperature for different plasticized PEO-LiCLO4

polymer electrolytes.

64 S. Das, A. Ghosh / Electrochimica Acta 171 (2015) 59–65

isothermal modulus peak, such that 0 < ahn� 1 and 0 < ahn

ghn� 1. In Figs. 6(a) and 6(b) we have shown fits of theexperimental data for M0 and M00 to the real and imaginary partsof Eq. (8). The parameters obtained from the best fits are shown inTable 3. The inverse of relaxation time tM is plotted with reciprocaltemperature in Fig. 7(a). It is noted that the VTF function similar toEq. (1) has been fitted well to the temperature dependence of therelaxation time tM. The variation of ahn and ghn with temperatureis shown in Fig. 7(b) for EC plasticized polymer electrolytes. It maybe noted that the values of ahn and ghn are almost independent oftemperature and composition except at higher temperatures.

The complex modulus M*(v) can be expressed in terms ofFourier transform of a decay function F (t) as given below [23]

M�ðvÞ ¼ M1½1 �Z10

e�ivtð�@FðtÞ@t

Þdt (9)

where F (t) is the relaxation function giving the time evolution ofthe electric field in time domain given by, EðtÞ ¼ Eð0ÞFðtÞ withinthe materials. For ideal Debye case, F (t) shows an exponentialnature. But in the case of non-Debye type behaviour, F (t) isapproximated by a stretched exponential Kohlrausch–Williams–Watts (KWW) function [25,26] given by

FðtÞ � exp � ttKWW

� �b" #

(10)

where b is the stretched exponent lying between 0 to 1 and itsvalue is unity for the ideal Debye relaxation. Smaller is the value ofb, greater is the deviation from the Debye type relaxation. Thefunction F (t) in the time domain can be calculated using the

inverse transform of Eq. (9) given by

FðtÞ ¼ 2p

Z10

M0

vM1cosðvtÞdv (11)

We have shown in Fig. 8 (a) the experimental decay curveobtained from Eq. (10) for the PEO- LiClO4-30 wt.% PC polymerelectrolyte. The decay curves are fitted with KWW function and thevalues of exponent b obtained from the fits are listed in Table 3 fordifferent compositions. In Fig. 8(b), it may be noted that the valuesof b do not vary significantly with temperature and composition.The small values of b are an indication for highly non-exponentialrelaxation. The lower values of b also suggest co-operative motionresulting from strong ion-ion interaction in these electrolytes [27].Ngai [28] has proposed a relation between the power law exponentn and the stretched exponent b as, n = 1 � b. Though exponent nand b are obtained from two different methods, they are well validfor smaller range of ac conductivity data [29,30]. We note that thisrelationship between n and b is almost satisfied for the presentplasticized polymer electrolytes.

4. Conclusions

The enhancement of the ionic conductivity depends on thevalue of dielectric constant of plasticizers. The plasticizers havingdielectric constants higher than PEO help to increase the ionicconductivity of PEO-LiClO4 electrolyte. Addition of DMC in polymer

S. Das, A. Ghosh / Electrochimica Acta 171 (2015) 59–65 65

matrix acts as an anti-plasticization effect due to low dielectricconstant than that of the base polymer (PEO). The temperaturedependent ionic conductivity for all plasticized polymer electro-lytes follows well known VTF relation, confirming coupled motionof Li+ ions and segment of polymer chain. The dielectric strength isobserved to be high for PEG based polymer electrolyte. The inversetemperature dependence of the relaxation time obtained fromdielectric and modulus spectra follows the VTF nature. Thesmall values of stretched exponent b indicate non-exponentialrelaxation.

Acknowledgement

S. Das thanks Indian Association for the Cultivation of Sciencefor research fellowship.

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