Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=gpom20

Download by: [27.251.197.194] Date: 18 June 2016, At: 07:08

International Journal of Polymeric Materials andPolymeric Biomaterials

ISSN: 0091-4037 (Print) 1563-535X (Online) Journal homepage: http://www.tandfonline.com/loi/gpom20

Electrical performance of lithium ion basedpolymer electrolyte with polyethylene glycol andpolyvinyl alcohol network

Mayank Pandey, Girish M. Joshi & Narendra Nath Ghosh

To cite this article: Mayank Pandey, Girish M. Joshi & Narendra Nath Ghosh (2016) Electricalperformance of lithium ion based polymer electrolyte with polyethylene glycol and polyvinylalcohol network, International Journal of Polymeric Materials and Polymeric Biomaterials,65:15, 759-768, DOI: 10.1080/00914037.2016.1163569

To link to this article: http://dx.doi.org/10.1080/00914037.2016.1163569

Published online: 15 Jun 2016.

Submit your article to this journal

Article views: 2

View related articles

View Crossmark data

INTERNATIONAL JOURNAL OF POLYMERIC MATERIALS AND POLYMERIC BIOMATERIALS 2016, VOL. 65, NO. 15, 759–768 http://dx.doi.org/10.1080/00914037.2016.1163569

Electrical performance of lithium ion based polymer electrolyte with polyethylene glycol and polyvinyl alcohol network Mayank Pandeya, Girish M. Joshia and Narendra Nath Ghoshb

aPolymer Nanocomposite Laboratory, Department of Physics, School of Advanced Sciences, VIT University, Vellore, India; bDepartment of Chemistry, BITS, Pilani, K K Birla Goa Campus, Goa, India

ABSTRACT A solid polymer electrolyte based on lithium hydroxide (LiOH) added with polyethylene glycol and polyvinyl alcohol polymers was synthesized by solution casting. The structural variation with respect to loading wt% of LiOH reveals the semicrystalline property of polymer electrolyte. The differential scanning calorimetry data shows the onset of crystalline to amorphous transition, which occurs nearly to the melting peak, for higher salt content. The structural properties and cross-linking between polymer and salt were demonstrated by polarized optical microscopy. The polymer electrolytes were subjected to AC impedance analysis spectra for obtaining the ionic conductivity at different temperature. The charge carriers relax much faster for higher lithium salt concentration based polymer electrolyte and produces higher conductivity. The highest room temperature conductivity 2.63 � 10� 5 S/cm is obtained for 8 wt% loading of lithium salt based polymer electrolyte, confirming their use in preparation of ion conducting devices.

GRAPHICAL ABSTRACT

ARTICLE HISTORY Received 28 December 2015 Accepted 5 March 2016

KEYWORDS Impedance spectroscopy; ionic conductivity; lithium ion; polymer electrolyte

1. Introduction

The lithium ion–based solid polymer electrolyte continues to draw research attention as a promising battery and other elec-trochemical device applications [1,2]. This is due to its good thermal stability and high electrical conductivity [3]. In recent years these solid polymer electrolyte have also replaces the conventional carbon based electrodes [4–6]. Solid polymer electrolytes have been successfully implemented in numerous electronic applications due to their electrical and chemically tunable properties [7,8]. Solid polymer electrolytes have unique properties such as excellent transport of charges along the polymer and strong UV-vis. absorption [9,10]. Many approaches such as addition of plasticizers, use of copolymers, doping of salt with large anions and blending of two polymers have been adopted for the development of solid polymer elec-trolyte and to enhance its conductivity.

Polyethylene glycol (PEG) is the most commonly used polar polymer for the development of polymer electrolyte. The PEG polymer exhibits strong ion-dipole interactions. Among other host polymer, PEG have been intensively studied for lithium ion batteries because it forms more stable com-plexes with inorganic salts and possesses higher solvating power for salt inspite of other polymers [11]. However, the application of PEG in polymer electrolyte is complicated due to its semicrystalline nature, which impedes the movement of polymer chain. Thus to obtain a composite film of PEG it has been cross-linked with polyvinyl alcohol (PVA) for present study due to its thermal and mechanical stability, nontoxic, water-soluble, and easy film-forming nature [12,13]. The chemical structure of PVA contains carbon chain backbone attached with hydroxyl group. It has been reported that the conductivity study of PVA demonstrates high values with

CONTACT Girish M. Joshi varadgm@gmail.com Polymer Nanocomposite Laboratory, Department of Physics, School of Advanced Sciences, VIT University, Vellore-632014, TN, India. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpom. © 2016 Taylor & Francis

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

other polymer system [14]. For developing a polymer electro-lyte, the addition of salt of different concentration with poly-mers is a major approach adopted by researchers. Lithium- based polymer electrolytes have been widely examined due to their extraordinary ion transfer properties, which have not been seen in other alkaline metal. The resultant ionic con-ductivity is highly dependent on the concentration of the added lithium salt, but the conduction mechanism does not involve all the ions of the salt [15].

In this work, we prepared lithium hydroxide (LiOH) based polyethylene glycol (PEG)/PVA polymer electrolyte. The strong interaction between PEG and PVA with lithium salt constitutes a strong bonding which enhances the electrical results of polymer electrolyte. In PEG/PVA/LiOH based poly-mer electrolytes, polymer chains coil around the Liþ and the O– atom in PEG chain provides a coordination site for Liþ

ions through Lewis acid-base interaction [16]. The hopping of Liþ ions from one coordination site to another within the semicrystalline phase has been obtained in the present study. Hence the main objective of the present study is to prepare polymer electrolyte for different concentration of LiOH. The structural variation and chemical complexation in polymer electrolyte with different concentration of salt have been ana-lyzed by XRD and FTIR, respectively. UV result demonstrates the optical absorbance characteristic of prepared polymer elec-trolyte. The thermal stability of present material is demon-strated by using differential scanning calorimetry (DSC). The ion conductivity is subjected by AC impedance spectroscopic analysis. The temperature dependence conductivity of polymer electrolyte follows the Arrhenius relationship, which also represents the hopping of ions in polymer matrix. The fre-quency dependent AC conductivity is evaluated at different temperature. Some interesting results of present polymer elec-trolyte indicate great promise for the further improvement in the field of electrochemical device application.

2. Experimental

2.1. Materials

LiOH is a white hygroscopic crystalline inorganic compound (MW-41.96, SD Fine Chem Limited, Mumbai, India), and we added to it PEG white powder (MW-8000, LR grade, SD Fine Chem Limited) and PVA white granule (MW-1,25,000, LR grade, SD Fine Chem Limited) by solution casting tech-nique. Distilled water is used as a solvent for solubility and accuracy in results.

2.2. Preparation of PEG-PVA-LiOH polymer electrolyte

Different concentration of PEG (40–50 wt% loading in steps of 2 wt%) was dissolved in 20 ml distilled water and stir (350 rpm) at room temperature (30°C) for 8 h. Simultaneously, PVA (50 wt%) was dissolved in 20 mL distilled water and stir (350 rpm) at 70°C for 12 h. The different concentration of LiOH salt (2–10 wt% loading in steps of 2 wt%) was also dis-solved in 20 ml distilled water and stir (350 rpm) at room tem-perature (30°C) for 8 h. All the three aqueous solution of different loading wt% mix together and then stirred

(350 rpm) at room temperature (30°C) for 8 h until the homo-geneous solution is obtained. The homogeneous solution of different loading wt% of PEG-PVA-LiOH was poured into Teflon Petri dish and dried in an oven at 60°C for 12 h. The films has been further kept for drying in vacuum desiccators (for one week) so maximum amount of water content should be evaporated from the film and used for further characteriza-tions. The resultant smooth, uniform, transparent films are obtained. The thickness of the developed samples has been found to be in the range of 125–135 µm. The schematic illus-tration for strong interaction of PEG-PVA with the addition of LiOH is provided in Figure 1. The lithium ion is partially interconnected and shares an –OH bond with PVA and PEG polymer. Therefore, the salt and polymer with mono func-tional (-OH) end groups are interacted properly as shown in Figure 1. The strong interacted chains of polymer composite were complexed with lithium salts (LiOH) that possess excel-lent ion conduction mechanism.

2.3. Characterization methods

The X-ray diffraction was used to determine the crystalline structure of composite polymer electrolytes which is carried out by using Cu Ka radiation of wavelength k ¼ 1.54 Å pro-duced by Bruker AXS D8 focus advance X-ray diffraction meter (Rigaku, Japan, Tokyo) with Ni- filtered. The scans were taken in the 2h range from 10–80° with a scanning speed and step size of 1°/min and 0.01°, respectively. The chemical com-plexation between PEG-PVA-LiOH composite polymer elec-trolyte was characterized by FTIR spectroscopy (make Shimadzu-IR Affinity-1 spectrometer) in the wave number range of 500–4000 cm� 1 operated in a transmittance mode. The UV- Visible spectrum of polymer electrolyte was obtained by using Shimazdu UV-2401PC, UV–vis spectrophotometer in the range of 200–600 nm. The optical absorbance of prepared PEG-PVA-LiOH composite polymer electrolyte was recorded and energy band gap has evaluated from UV spectrum. The micro-structural properties and optical images of polymer electrolytes for 50 µm were obtained by Olympus BX61 optical microscopy. Differential scanning calorimetry (DSC)

Figure 1. Schematic illustration of cross-linking of PEG-PVA and interpenetration of LiOH.

760 M. PANDEY ET AL.

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

experiment was performed by using DSC-60 (Shimadzu, Japan) for thermal characterization of polymer electrolyte samples. The sample for DSC analysis was cut into small pieces of 2.5 mg weight from the original films. All the experiments were carried out from 50–200°C with heating rate of 10°C min� 1 in air atmosphere.

The ionic conductivity of composite polymer electrolytes was determined by impedance spectroscopy PSM-1735 impe-dance analyzer under varying range of temperature (30–150° C) and broadband frequency (50 Hz to 1 MHz). In the deter-mination of ionic conductivity, the sample of small size (silver pasted both sides) was placed between two fixture electrodes and kept inside dry temperature calibrator. For result accuracy all the samples has been tested three times and the average was drawn and analyzed.

3. Results and discussion

3.1. X-ray diffraction analysis

The change in structure and phase of PEG-PVA blend after it has been added with LiOH were analyzed by XRD. Figures 2 and 3 represents the XRD pattern of pure and LiOH interpe-netrated polymer electrolytes. The XRD spectra of pure and PEG-PVA blend agrees with the previously reported XRD patterns in the literature [17,18]. The sharp and well-defined diffraction peak of the LiOH as shown in Figure 2 indicates good crystallinity. The two dominant peaks obtained at 2h ¼

19.01° and 23.15° as shown in Figure 2 are related to crystal-line phase of PEG [19,20]. However, a relatively sharp and broad peak centered at 2h ¼ 19.25° (d ¼ 5.11 Å, D ¼ 0.25 Å) as shown in Figure 2, indicating a semicrystalline nature of PVA, contain crystalline and amorphous region. For low concentration of LiOH, the peaks are superimposed with the crystalline peaks of an PEG-PVA blend. The XRD pattern of polymer electrolyte for different loading wt% of LiOH is shown in Figures 3a–e. There are peaks at 2h ¼ 25 to 30° retained for LiOH. If examined closely the change in intensity has been obtained for different loading wt% of LiOH. The intensity of peaks decreases with respect to loading wt% of salt

suggesting a corresponding decrease in the crystalline structure of polymer electrolyte. As the loading wt% of salt increases the slight shift of the diffraction peaks and decrease in crystallinity are obtained. The structural parameters such as interlayer spacing (d) and crystalline size (D) were evaluated from XRD results. The average crystalline size D (Å) was eval-uated by using Scherrer’s formula:

D ¼0:9k

bCoshð1Þ

where k is X-ray wavelength (1.54 Å), b is a full width at half maximum (FWHM) intensity of the diffraction peaks, and h is the Bragg’s angle. The FWHM of main diffraction peak is used to evaluate the crystalline size of sample using the Scherrer equation. The evaluated values of interlayer spacing and crys-talline size of the prepared sample is tabulated in Table 1. The variation in the value of interlayer spacing (d) and crystalline

Figure 2. XRD diffraction pattern of pure PVA, PEG, and LiOH.

Figure 3. XRD diffraction pattern of PEG-PVA-LiOH for (a) 48:50:2, (b) 46:50:4, (c) 44:50:6, (d) 42:50:8, and (e) 40:50:10 loading wt%.

Table 1. Structural parameters of PEG/PVA/LiOH polymer electrolyte from XRD analysis.

Sample code Sample details (wt%) 2θ (°) d (Å) D (nm)

Pure PVA PVA::100 19.25 5.11 1.23 Pure PEG PEG::100 19.01 5.50 15.72

23.10 7.69 8.57 Pure LiOH LiOH::100 21.24 8.35 47.64

31.88 5.60 68.96 43.37 4.16 47.50

a PEG/PVA::50:50 19.37 5.08 67.27 23.15 4.27 11.92

b PVA/LiOH::50:50 19.92 4.94 3.65 23.35 4.23 23.10

c PEG/PVA/LiOH::48:50:2 19.36 5.08 17.14 23.77 4.14 13.11

d PEG/PVA/LiOH::46:50:4 19.06 5.16 14.95 23.20 4.25 8.42

e PEG/PVA/LiOH::44:50:6 19.68 5.03 9.63 23.57 4.18 7.69

f PEG/PVA/LiOH::42:50:8 19.45 5.06 9.97 23.63 4.18 7.73

g PEG/PVA/LiOH::40:50:10 19.48 5.06 14.66 23.64 4.18 8.74

INTERNATIONAL JOURNAL OF POLYMERIC MATERIALS AND POLYMERIC BIOMATERIALS 761

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

size (D) is observed as the wt% of salt increases in polymer electrolyte. Therefore the higher amorphous region of polymer electrolyte could provide more flexible polymer backbone, which promotes segmental mobility and increases the ionic conductivity.

3.2. FTIR analysis

FTIR spectroscopy is used to characterize the chain structure of polymer. It leads the way to observe the vibration energy of a covalent bond in the polymer host and the interaction occurs in the polymer salt complexes [21,22]. Figure 4 repre-sents the FTIR spectrum of polymer electrolyte for different loading wt% of LiOH with PEG-PVA polymer matrix. Figure 4i exhibits the –OH vibration and stretching peaks located at 3251 cm� 1 and 2910 cm� 1, respectively for PVA- PEG blend. The cross-linking between PEG-PVA polymers also exhibits major transmittance peaks at 1423 cm� 1 for C-H deformation, 1253 cm� 1 for C-O, and 1085 cm� 1 is located for C-O stretching. However the interpenetration of Li ions with PVA polymer represents the short hump broad peaks located at 3236 cm� 1 and 2937 cm� 1, which represents the –OH vibration and –OH stretching, respectively, shown in Figure 4i. The deep sharp peaks located at peak at

858 cm� 1 exhibits the acetyl C–O stretching which also support the existence of chemical interaction between PVA and LiOH [23]. Figure 4ii represents the FTIR spectroscopy of polymer electrolyte for different loading wt% of PEG-PVA-LiOH. The –OH vibration band of alcohols exhibits at 3258 cm� 1 for 2 wt% loading of LiOH is shifted toward the lower wave number and appeared as broad hydroxyl band for higher loading wt% of LiOH as shown in Figure 4ii (a–e). It gives a strong indication of specific interaction between the polymer matrix and dopant salt. The significant sharp peak due to stretching of the C-H bond in CH2 group located at 2897 cm� 1 is also shifted toward the lower wave number with increased intensity as a function of loading wt% of salt. The shift in C-O stretching from 1091 cm� 1 to 1098 cm� 1 with a sharp increase in peak supports the interpenetration of LiOH with PEG-PVA polymer system. The observed changes in the intensities of peaks, shifting of vibration bands and disappear-ance of some peaks indicates the complex formation between polymer and salt.

3.3. UV-visible spectra analysis

The optical property of polymer electrolyte has been studied by using UV-vis. spectroscopy technique. The optical absorp-tion spectra of polymer electrolyte were recorded in the region 190–250 nm as shown in Figure 5. In the UV region, the band between 200–230 nm has been observed for different loading wt% of PEG-PVA-LiOH with different absorption intensity and wavelength. The absorption band obtained in the region of 200–230 nm can be explained in terms of interaction or delocalization of the π-π* orbits. Figures 5a–e represents the shift in absorption band toward the higher wavelength with high intensity with respect to loading wt% of LiOH. These shifts in the absorption band are obtained due to inter-/ intramolecular hydrogen bonding between lithium ions with the adjacent OH group [24].

The absorption peaks are also related to transition of elec-tron. Therefore the shifting of absorption peaks toward the lower or higher wavelength is also depends on the transition

Figure 4. FTIR spectra of (i) PEG-PVA and PVA-LiOH and (ii) represents PEG-PVA- LiOH composites for (a) 48:50:2, (b) 46:50:4, (c) 44:50:6, (d) 42:50:8, and (e) 40:50:10 loading wt%.

Figure 5. UV-visible spectra of PEG-PVA-LiOH for (a) 48:50:2, (b) 46:50:4, (c) 44:50:6, (d) 42:50:8, and (e) 40:50:10 loading wt%.

762 M. PANDEY ET AL.

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

of electron. The optical absorption band represents the band structure of polymer electrolytes. Therefore to understand the band structure of polymer electrolyte, the band gap energy (Eg) has been evaluated. The fundamental absorption corre-sponding to the transition of phonons from valence band to conduction band is used to evaluate the band gap of polymer electrolyte [25]. The band gap energy (Eg) of the polymer elec-trolytes was evaluated by using Plank’s Einstein relation:

Eg ¼hCk

ð2Þ

where h is planks constant, c is speed of light, and k is cutoff wavelength. The estimated values of band gap energy (Eg) are varied from 4.74 to 5.28 eV as the loading wt% of LiOH increases. The variation in band gap energy (Eg) and shift in absorption peaks with increasing concentration of LiOH pro-viding an evidence for the incorporation of LiOH with PEG- PVA polymer matrix. The increase in band gap energy (Eg) can also be due to charge transfer complexes, which result with the enhancement of conductivity.

3.4. Differential scanning calorimetry analysis

The effect of LiOH on the crystallinity of PEG/PVA was inves-tigated through thermal properties of the polymer electrolytes using DSC. Figures 6 and 7 show the DSC endothermic curves for pure and Li ions based polymer electrolytes. The sharp endothermic peak at Tm (69°C), which is attributed to the melting point of PEG represents the semicrystalline phase. However, the sharp endothermic peaks are observed at 190° C which correspond to crystalline melting temperature (Tm) of pure PVA as shown in Figure 6. The Tm value for the PEG-PVA blend is decreases and shifts towards the lower melting temperature. Furthermore the glass temperature

evaluated from DSC thermograms for PEG, PVA, and PEG/ PVA blend is � 65°C, 85°C, and 70°C respectively. Figures 7a–e represent the DSC thermograms of polymer electrolytes for different loading wt% of LiOH salt. The addition of LiOH causes a change in the shape of the endothermic peak and the peak shifted towards the lower temperature. The addition of LiOH with different concentration also brings change in Tg values. The decrease in Tg value is obtained from 56°C to 40°C for 2 and 4 wt% loading of LiOH. However, for further increase in loading wt% (6 wt%) of LiOH, a shifts in Tg toward 53°C is observed and remains constant for 8 wt% loading of LiOH.

The previously mentioned changes also indicate the com-plexation process between LiOH salt and PEG/PVA polymers. The addition of salt causes peak broadening. The slight shift of Tm toward lower temperature on the addition of salt to the host polymer reveals the disruption of crystallinity of host

Figure 6. DSC Thermograms of pure PVA, PEG, (a) PVA/PEG::50:50, and (b) PVA/LiOH::50:50 loading wt%.

Figure 7. DSC Thermograms of PEG-PVA-LiOH for (a) 48:50:2, (b) 46:50:4, (c) 44:50:6, (d) 42:50:8, and (e) 40:50:10 loading wt%.

INTERNATIONAL JOURNAL OF POLYMERIC MATERIALS AND POLYMERIC BIOMATERIALS 763

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

polymer. The relative crystallinity (vc) was calculated using relationship:

vc ¼DHsample

m � 100DH0

mð3Þ

where the melting enthalpy (ΔHm) is the heat of fusion for the polymer electrolyte which can be calculated from the integral area of baseline of each melting curve [26]. The data of ΔHm and Tm, evaluated during the heating process from 50–200°C, are all summarized in Table 2.

The melting enthalpy for 100% crystalline PEG is equal to 206.8 J/g. The calculated values of vc are summarized in Table 2. The change in vc with respect to ΔH implies that the PEG crystallinity deteriorated due to the cross-linked pro-cess by the interpenetration of PVA and LiOH. When polymer reaches the melting point, the atoms begin to move (solid to liquid) and Li ion react freely with PEG. Therefore this PVA/PEG/LiOH (salt-polymer complex) reduces the Tm to lower temperature. The results of crystallinity obtained from DSC can also correlate with XRD results. The increase in load-ing wt% of LiOH in PVA/PEG polymer matrix decreases the degree of crystallinity, which reveals the presence of amorph-ous phase. This amorphous nature of polymer electrolyte is also observed in XRD results as well.

3.5. Optical microscopy analysis

The surface studies of polymer electrolyte films with and with-out Li are further characterized by polarized optical microscopy with magnification of 50�. Figure 8 represents the polarized optical microscopy images of PEG-PVA-LiOH samples. The optical image of cross linking polymer chain of PEG-PVA represents the Maltese cross-extinction pattern and very fine smectic scheliren texture as shown in Figure 8a.

The schlieren texture obtained for polymer blend revealing the significant buildup of amorphous phase. However the addition of LiOH salt with PVA polymer system represented spherulites with amorphous region (black) surrounding the polymeric surface as shown in Figure 8b. The spherulites size becomes small and the single-phase optical image has been obtained for 2 wt% loading of LiOH in polymer electrolyte as shown in Figure 8c. At higher concentration of LiOH the polarized optical images of polymer electrolyte exhibiting a non-uniform distribution of salt molecules as shown in Figure 8d–f. This can be due to the excess of Li salt from the forma-tion of PEG-PVA spherulites, which extracted the polymer electrolyte to the region of amorphous domain. The morpho-logical variations also influence the conductivity of polymer

electrolyte, which can be explained by effect of confinement on polymer mobility [27]. It is reported that the conduction ions always prefer amorphous region and thus the conduc-tivity gets enhanced [28]. The two-phase morphology of PEG-PVA-LiOH polymer electrolyte represents the increase in amorphicity, which improves the charge transfer mech-anism and hence also influence the conductivity results.

3.6. Impedance spectroscopy analysis

The interfacial compatibility between polymer and LiOH was further characterized through the interfacial resistance mea-sured by AC impedance spectroscopy. The complex impe-dance spectroscopy is widely used to calculate bulk electrical conductivity of polymer electrolyte films [29]. Figure 9 repre-sents the complex impedance Nyquist plot of PEG-PVA-LiOH polymer electrolyte. The impedance plot for different samples was obtained as a function of temperature. As the PEG-PVA blend has been concern, the well-defined semicircular pattern was obtained as shown in Figure 9a. The gradual decrease in the radius of semicircle with increase in temperature may be due to the dominance of the grain effect other than the grain boundary and electrode effect. However the complex impe-dance plot represents the two well-defined regions for different loading wt% of LiOH as shown in Figure 9b at lower temperature.

The semicircular region at higher frequency range related to the conduction process and the liner region attributed to the bulk effect of blocking electrode [30]. These results

Table 2. Thermal Properties of pure PEG, (a) PVA/PEG:: 50:50 and (b) 48:50:2, (c) 46:50:4, (d) 44:50:6, (e) 42:50:8, and (f) 40:50:10 loading wt% of PEG/PVA/LiOH.

Sample code Tm (°C) Tg (°C) DHm (J/g) χc (%)

Pure PEG 69 –65 206.8 100 a 66 60 73 35.6 b 62 56 202 98.0 c 58 40 181 87.8 d 67 53 139 67.4 e 59 53 128 62.1 f 63 64 125 60.6

Figure 8. Optical micrographs of (a) PVA/PEG:: 50:50, (b) PVA/LiOH::50:50 and (c) 48:50:2, (d) 46:50:4, (e) 42:50:8, and (f) 40:50:10 for different loading wt% of PEG-PVA-LiOH.

764 M. PANDEY ET AL.

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

suggest that due to presence of LiOH in polymer electrolyte the migration of ions may occur through the free volume of matrix polymer, which can also be represented by the resistor. The ionic migration and bulk polarization is in par-allel and therefore the semicircle pattern has been observed at a higher frequency. At higher loading wt% of LiOH the percolation temperature (323 K) has been obtained. After a percolation temperature the LiOH entangled polymer chain gets disturbed and reduces the radius of semicircle. From Figures 9a and 9b, it has been noted that the semicircular decreases with the increase in the salt concentration. This result suggests that only the resistive component of polymer electrolyte prevails when the concentration is increased. The capacitive nature disappears due to random orientation of the dipoles in the side chain. Therefore in present study the conduction process is not due to the orientation of electric dipole of polymer but to the hopping process of positive or negative charges of LiOH salt interpenetrated in PEG-PVA.

3.7. Ionic conductivity analysis

To investigate the mechanism of ionic conduction, the ionic conductivity was studied as a function of temperature from

30 to 150°C. The temperature dependent ionic conductivity of polymer electrolyte obeys the Arrhenius rule. Figure 10 represents the Arrhenius plot of logarithmic conductivity versus inverse temperature for PEG-PVA-LiOH polymer electrolyte. The ionic conductivity was calculated from the following equation:

r ¼t

ARbð4Þ

where t is the thickness of the polymer electrolyte, Rb is the bulk resistance, and A is the area of polymer electrolyte. The intercept point at Z′-axis at higher frequency in the complex impedance plot gives the resistance of bulk electrolyte (Rb). The ionic conductivity was evaluated from the measured resistance for the known area and thickness of polymer electrolyte films. Figure 10 illustrates that the ionic conduc-tivity of polymer electrolyte increases with increasing temperature.

The ionic conductivity of the polymer electrolytes depends on the concentration of the salts and their mobility in the medium [31]. This mechanism is interpreted as a hopping mechanism between coordinating sites, local structural relax-ation and segmental motion of polymer electrolyte [32]. As the temperature increases, the segmental motion of poly-meric chain occurs. This favors the hopping inter- and intra-chain movements of ions and due to this the conductivity of polymer electrolyte becomes high. The activation energy (Ea) a combination of the energy of defect formation and the energy of migration of Li ion is calculated from the slope of the linear fitted Arrhenius plot as tabulated in Table 3. The maximum room temperature conductivity 2.6 � 10� 5 S/cm was obtained for 8 wt% loading of LiOH and the activation

Figure 9. Impedance spectroscopy of (a) 48:50:2 and (b) 46:50:4 loading wt% of PEG-PVA-LiOH.

Figure 10. Temperature dependence ionic conductivity for (a) 48:50:2, (b) 46:50:4, (c) 42:50:8, and (d) 40:50:10 loading wt% of PEG-PVA-LiOH polymer electrolyte.

Table 3. Activation energy from Arrhenius plots of PEG/PVA/LiOH for (a) 48:50:2, (b) 46:50:4, (c) 44:50:6 and (d) 42:50:8 loading wt%.

Sample code Activation energy (Ea) (eV)

a 0.02 b 0.08 c 0.45 d 0.30

INTERNATIONAL JOURNAL OF POLYMERIC MATERIALS AND POLYMERIC BIOMATERIALS 765

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

energy has been found to be 0.30 eV. The low activation energy is due to the amorphous nature of polymer electrolyte that facilitates the fast Li ion motion in the polymer network. Hence, for lower temperature the free volume was decreased and because of less ionic mobility the ionic conductivity decreased.

The variation in temperature-dependent conductivity as a function of different wt% of LiOH is shown in Figure 11. The conductivity of polymer electrolyte increases with increase in LiOH concentration up to 8 wt%. The high con-ductivity is due to the high concentration of free Li ions in the electrolyte. The increase in mobile charge carrier also increases the amorphous nature of polymer electrolyte, which reduces the energy barrier by facilitating ion transport [33,34]. It is also observed that when salt concentration is increased from 8 to 10 wt%, the conductivity slightly decreases. The pre-vious report on Li ion and PVA polymer electrolytes contain-ing EC, PC plasticizers claims the maximum conductivity in order of 10� 4 S/cm [35,36]. However, in the present study the maximum room temperature ionic conductivity 2.63 � 10� 5 S/cm was obtained for 8 wt% loading of LiOH and the activation energy has been found to be 0.30 eV. This is due to the decrease in mobility and increasing disorder of polymer matrix for higher concentration of LiOH. The maximum conductivity obtained for 8 wt% loading of LiOH is due to increasing the number of charge carriers and decreasing ionic mobility. The effect of chain mobility on ionic conductivity is directly correlated with the free volume of polymer. As the free volume in polymer increases, the transportation of ions is more rapid in polymer electrolyte. The present electrolyte contains both cations and anions. However the increases in ionic conductivity as a function of temperature depend on the ionic motion. Thus, the addition of LiOH in polymer electrolyte increases the free volume space, which enhances the ionic conductivity. Thermal degra-dation of polymers is molecular deterioration as a result of overheating. At high temperatures the components of the long-chain backbone of the polymer can begin to separate (molecular scission) and react with one another to change

the properties of the polymer. Hence in present study the change in thermal degradation occurs due to the enrichment of LiOH in polymer electrolyte. The LiOH is an unstable in nature due to presence of OH group. Therefore the change in thermal degradation can easily correlated with the DSC results of PEG/PVA/LiOH polymer electrolyte. The thermal degradation occurs in present study may be due to the change in chemical reactions which lead to physical and optical property changes.

3.8. AC conductance spectra

The real part of AC conductivity (σ0) for 2, 4, and 8 wt% loading of LiOH in PEG/PVA polymer matrix at different temperature (30–100°C) is shown in Figures 12a–c. The real part of AC conductivity was evaluated by using dielectric loss according to the following relation.

r0 ¼ e0xe00 ð5Þ

The AC conductivity value increases nonlinearly with increase of frequency on logarithm scale. The AC conductivity spectra define two regions, a low frequency dispersive region and a plateau region. The region at frequencies lower than that of dispersive AC conductivity region defines the frequency inde-pendent plateau corresponding to direct current (DC) ionic conductivity. At higher frequency range (1–10 MHz) a plateau region is obtained for 4 wt% loading of LiOH in PEG/PVA polymer system, which is frequency independent value and also called equal to DC conduction. This is due to long-range ion dynamics resulting in the long-range ion transport mech-anism. Further at lower frequencies, the effect of electrode polarization is evidence by a large deviation from the DC pla-teau region [37]. However for higher loading wt% of LiOH, the plateau region is not observed significantly. Further it is found that the change in σ’ values with decrease of frequency in elec-trode polarization affected frequency region. Therefore the higher frequency region of σ’ is fitted by using Jonscher’s power law:

r0ðxÞ ¼ rdc þ Axn ð6Þ

where A is a pre-exponential factor and n is the fractional exponent ranging from 0 and 1. The power law fits the higher frequency region shown in inset graph of Figures 12a and 12c. The values of power factor are directly obtained from fitted graph. At high temperature and lower frequencies the ionic conductivity is high enough to produce significant build-up charges at the electrodes. This also reduces the effective field across the sample and hence the conductivity. The charge accumulation increases between electrode and electrolyte interface with decreases in frequency resultant the number of mobile ion decreases and eventually a drop in conductivity at low frequency. However, the conductivity is increases with frequency this is because the mobility of charge carrier is high at high frequency [38]. The evaluated results also describes that the DC conductivity increases with respect to temperature which is due to the free volume around the polymer chain which causes the mobility of ions and polymer segments [39].

Figure 11. Salt concentration dependence ionic conductivity for PEG-PVA-LiOH polymer electrolyte at different temperatures.

766 M. PANDEY ET AL.

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

4. Conclusions

The addition of LiOH with PEG/PVA results in excellent solid polymer electrolyte. The higher amorphous region

of polymer electrolyte obtained from XRD for higher wt% of LiOH provides more flexible polymer backbone, which promotes segmental mobility and increases the ionic con-ductivity. The scheliren texture obtained from optical microscope for polymer electrolyte reveals the significant buildup of amorphous phase, which also supports the XRD results. The high ionic conductivity of polymer electrolyte can be contributed by hopping inter and intra chain movements of ions and decrease in mobility and increasing disorder of polymer matrix for higher concen-tration of LiOH. The maximum room temperature ionic conductivity 2.63 � 10� 5 S/cm was obtained for 8 wt% load-ing of LiOH and the activation energy has been found to be 0.30 eV.

The results obtained from different characterization techni-ques in present study refer towards the further development of electric double-layer capacitor supercapacitor applications because Double-layer capacitance and pseudocapacitance both contribute inseparable to the total capacitance value of super-capacitors. However, the ratio of the two can vary greatly, depending on the design of the electrodes and the composition of the PEG/PVA/LiOH polymer electrolyte.

Acknowledgments

The authors are highly grateful for providing the electrical characterization facility under the Naval Research Board, Defense Research and Develop-ment Organization, New Delhi, under Project N0.259/Mat./11-12.

References

[1] Gadjourova, Z.; Andreev, Y. G.; Tunstall, D. P.; Bruce, P. G. Nature 2001, 412, 520.

[2] Thokchom, J. S.; Chen, C.; Abraham, K. M.; Kumar, B. Solid. State Ionics 2005, 176, 1887.

[3] Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. [4] Choi, W.; Harada, D.; Oyaizu, K.; Nishide, H. J. Am. Chem. Soc.

2011, 133, 19839. [5] Zhu, L. M.; Niu, Y. J.; Cao, Y. L.; Lei, A. W.; Ai, X. P.; Yang, H. X.

Electrochim. Acta 2012, 78, 27. [6] Xun, S. D.; Song, X. Y.; Battaglia, V.; Liu, G. J. Electrochem. Soc.

2013, 160, A849. [7] Nyholm, L.; Nystrom, G.; Mihranyan, A.; Stromme, M. Adv. Mater.

2011, 23, 3751. [8] Ibrahim, S.; Yasin, S. M. M.; Ahmad, R.; Johan, M. R. Solid State Sci.

2012, 14, 1111. [9] Kirsty, L.; Melvin, T. Z.; Hong, M.; Mehmet, S.; Fei, H.; Alex, K. Y.

Appl. Mater. Interface 2010, 2, 3153. [10] Bhargav, P. B.; Mohan, V. M.; Sharma, A. K.; Rao, V. V. R. N. Int. J.

Polym. Mater. 2007, 56, 579. [11] Ghosh, A.; Kofinas, P. ECS. Trans. 2008, 11, 131. [12] Prajapati, G. K.; Gupta, P. N. Nuclear. Inst. Methods rPhys. Res. Sec.

B. 2009, 267, 3328. [13] Rajeswari, N.; Selvesekarapandian, S.; Karthikeyan, S.; Nithya, H.;

Sanjeeviraja, C. Int. J. Polym. Mater. 2012, 61, 1164. [14] Rajendran, S.; Sivakumar, M.; Subadevi, R.; Nirmala, M. Phys. B

2004, 348, 73. [15] Karmakar, A.; Ghosh, A. Curr. Appl. Phys. 2012, 12, 539. [16] Lee, Y. S.; Shin, W. K.; Kim, J. S.; Kim, D. W. RSC Adv. 2015, 5, 18359. [17] Ravindran, D.; Vickraman, P. Int. J. Sci. Eng. Appl. 2012, 1, 72. [18] Xu, Y.; Jiao, X.; Chen, D. J. Phys. Chem. C 2008, 112, 16769. [19] Hashmi, S. A.; Kumar, A.; Maurya, K. K.; Chandra, S. J. Phys. D:

Appl. Phys. 1990, 23, 1307. [20] Jiang, Y. X.; Xu, J. M.; Zhuang, Q. C.; Jin, L. Y.; Sun, S. G. J. Solid

State Electrochem. 2008, 12, 353.

Figure 12. Frequency dependent ac conductivity for (a) 48:50:2, (b) 46:50:4, and (c) 42:50:8 loading wt% of PEG-PVA-LiOH polymer electrolyte at different tem-perature. Inset of (a) and (c) shows the fitted graph by using power law at higher frequency region.

INTERNATIONAL JOURNAL OF POLYMERIC MATERIALS AND POLYMERIC BIOMATERIALS 767

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6

[21] Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 3rd edn.; Brooks/Cole, Washington, DC, 2001.

[22] Kim, C. S.; Oh, S. M. Electrochim. Acta 2000, 45, 2101. [23] Bhavani, S.; Ravi, M.; Narasimha Rao, V. V. R. Int. J. Eng. Sci. Innov.

Technol. 2014, 3, 426. [24] Ibrahim, S.; Ahmad, R.; Rafie, J. M. J. Luminesc. 2012, 132, 147. [25] Abdullah, O. G.; Aziz, B. K.; Salh, D. M. Indian J. Appl. Res. 2013,

3, 477. [26] Ping-Lin, K.; Ching-An, W.; Chung-Yu, L.; Chin-Hao, T.; Chun-

Han, H.; Sheng-Shu, H. Appl. Mater. Interface 2014, 6, 3156. [27] Elmahdy, M. M.; Chrissopoulou, K.; Afratis, A.; Floudas, G.;

Anastasiadis, S. H. Macromolecules 2006, 39, 5170. [28] Halder, B.; Singru, R. M.; Maurya, K. K.; Chandra, S. Phys. Rev. B

1996, 54, 7143. [29] Singh, P. K.; Bhattacharya, B.; Nagarale, R. K. J. Appl. Poly. Sci.

2010, 118, 2976.

[30] Cech, O.; Thomas, J. E.; Sedlarikova, M.; Fedorkova, A.; Vondrak, J.; Moreno, M. S.; Vistin, A. Solid State Sci. 2013, 20, 110.

[31] Ramesh, S.; Liew, C. W.; Morris, E.; Durairaj, R. Thermochim. Acta 2010, 511, 140.

[32] Reddy, M. J.; Sreekant, T.; Subba Rao, U. V. Solid State Ionics 1999, 126, 55.

[33] Rajeswari, N.; Selvasekarapandian, S.; Prabu, M.; Karthikeyan, S.; Sanjeeviraja, C. Bull. Mater. Sci. 2013, 36, 333.

[34] Freitas, F. S.; Freitas, J. N.; Ito, B. I.; De Paoli, M. A.; Nogueira, A. F. Appl. Mater. Interface 2009, 1, 2870.

[35] Ulaganathan, M.; Rajendran, S. J. Appl. Electrochem. 2011, 41, 83. [36] Ulaganathan, M.; Rajendran, S. Ionics 2010, 16, 667. [37] Sengwa, R. J.; Dhatarwal, P.; Choudhary, S. Curr. Appl. Phys. 2015,

15, 135. [38] Ramesh, S.; Arof, A. K. Mater. Sci. Eng. B 2001, 85, 11. [39] Miyamoto, T.; Shibayama, K. J. Appl. Phys. 1973, 44, 5372.

768 M. PANDEY ET AL.

Dow

nloa

ded

by [

27.2

51.1

97.1

94]

at 0

7:08

18

June

201

6