Introduction

Since the polymer electrolytes are capable of providing a higher output voltage, higher specific heat capacity, long cycle life, improved safety, and so on, their applications for Li secondary are very wide and outstanding. Hence, the studies on polymer electrolytes have reasonably increased in the recent decades [1–3]. The first polymer-based solid polymer electrolyte was suggested by Armand et al. [4]. Subsequently, the solid polymer electrolytes were considered for an ideal alternate to the liquid electrolytes. Starting from 1970, poly(ethylene oxide) (PEO) and alkali metal salt-based solid polymer electrolytes got great consideration [5–7]. However, their ionic conductivity at ambient temperature is very low and is yet scarcely used for practical applications at elevated current densities. Many attempts have been made to increase the room temperature ionic conductivity in the polymer matrix. These attempts include innovative polymer synthesis [8–10], cross-linking of two polymers [11–13], blending of two polymers [14, 15], formulation of plasticized polymer electrolytes [16–18], synthesis of composite polymer electrolytes by the addition of inorganic fillers [19–21], and the formation of porous electrolytes employing the phase inversion method [22–24].

Out of these above mentioned techniques, blending of another polymer with PEO has been identified to have a good compatibility. Blending can, however, have profound and sometimes unexpected effects on thermal stability, which cannot simply be predicted on the basis of the behavior of the components and their relative properties [25]. A common approach is to add low molecular weight plasticizers to the polymer electrolytes [7]. The plasticizers impart salt solvating power and high ion mobility to the polymer electrolytes. PEO is the most interesting base material because of its high chemical and thermal stability. It can solvate a wide variety of salts even at high concentrations [26].

On the other hand, the addition of vinylidene chloride unit with poly(acrylonitrile) (PAN) gives high mechanical stability. The ionic conductivity of PAN-based electrolytes is of the order of 10−5–10−6 S cm−1 with good thermal stability. PEO–oligomers with poly(vinylidene chloride-co-acrylonitrile)-based electrolytes were prepared by Kim et al. [27]. They reported the conductivity of the order of 10−4–10−5 S cm−1. In this work, we have chosen PVdC-co-AN as other polymer, because its mechanical and thermal stabilities are superior to other polymers.

Moreover, the ionic conductivity is an inherent property of high amorphousity and low viscosity. Since the viscosity of PEO is comparatively higher than that of PVdC-co-AN, the blending of PVdC-co-AN improves the ionic conductivity in addition to the enhancement of thermal and electrochemical stabilities. Barring that, to the best of the author’s knowledge, a very little work has been carried out with PVdC-co-AN in the field of polymer electrolytes. Hence, an attempt has been made to synthesize new polymer blend electrolyte which could find applications in rechargeable lithium batteries.

The present work explains the gel polymer electrolytes based on PEO/PVdC-co-AN/LiClO4 complex by the addition of different plasticizers using a well-known solvent casting technique. The cyclability and reversibility of the prepared electrolytes have been studied. The ionic conductivity at different temperatures of the gel electrolytes has also been reported.

Materials and methods

The solution casting technique was used to prepare the gel polymer electrolyte films. Poly(ethylene oxide) M w ∼ 8,000, poly(vinylidene chloride-co-acrylonitrile) M w ∼ 150,000, and LiClO4 were purchased from Sigma-Aldrich Chemicals Limited, USA, and they were used as received. PEO and PVdC-co-AN were dried at 50 °C under vacuum for 4 h to remove moisture. The salt was annealed at 60 °C under vacuum for 24 h. The plasticizers were also procured from Sigma-Aldrich, USA. The solvent tetrahydrofuran (THF) was acquired from E-Merck. Different gel polymer electrolytes were prepared with a constant ratio of PEO (64 wt%)/PVdC-co-AN (16 wt%)/LiClO4 (8 wt%)/X (12 wt%) (where X = EC, PC, gBL, DEC, DMC, and DBP). The polymers, salt, and plasticizer were dissolved separately and mixed together. The mixed solution was stirred for 24 h with the help of a magnetic stirrer to obtain a homogeneous mixture. The solvent THF in the solution was allowed to evaporate slowly. Finally, the homogeneous solution was cast on Petri dishes. The Petri dishes containing the solution were dried at 50 °C under vacuum for 12 h to remove the residual solvent. The resulting free standing films were smoothly removed from the Petri dishes and cut into required shapes. The thickness of the harvested films was in the order of about 0.3–0.4 mm.

The X’Pert PRO PANalytical X-ray diffractometer was used to do the structural analysis of the films at room temperature. The complex formation between the polymer complex and salt was ratified by SPECTRA RXI, PerkinElmer spectrophotometer in the range 400–4,000 cm−1. The ionic conductivity studies were carried out by placing the electrolyte film between the stainless steel blocking electrodes using a computer-controlled micro Autolab type III Potentiostat/Galvanostat in the frequency range 100 Hz–300 KHz. PerkinElmer PYRIS DIAMOND Thermogravimetry/Differential Thermal Analyzer (TG/DTA) was used to study the thermal stability of the blends from room temperature to 750 °C at a scan rate of 10 °C min−1. Mettler Toledo differential scanning calorimetry (DSC) (822e) instrument was employed to study the phase changes using differential scanning calorimetry. Using Cary Eclipse Fluorescence Spectrometer, the photoluminescence studies of the films were carried out. UV–visible analysis of the prepared films was done with (Lambda 35) PerkinElmer UV–visible spectrometer to calculate the optical band gap of the complexes. The surface morphology of the film having a maximum ionic conductivity was studied by JEOL, JSM-840A. AFM (A100SGS) was used to study the complex film having a maximum ionic conductivity. Computer-controlled micro Autolab type III potentiostat/galvanostat was employed for cyclic voltammetry and linear sweep voltammetry studies.

Results and discussion

X-ray diffraction analysis

Figure 1 delineates X-ray diffraction peaks of pure PEO, PVdC-co-AN, LiClO4, and prepared gel polymer electrolytes. The sharp, intense peaks at 2θ = 19.1 and 23.3° associated with the lattice planes (1 2 0) and (0 1 0) signify the semi-crystalline nature of PEO. The crystalline nature of PVdC-co-AN is affirmed by the diffraction angles at 2θ = 40.25°, 46.76°, and 67.94° compatible to (1 0 5), (2 0 2), and (2 1 2) lattice planes. The strong crystallinity of LiClO4 and BaTiO3 is discerned from the diffraction angles at 2θ = 18.36°, 23.2°, 27.5°, 32.99°, and 36.58° and 22°, 31°, 45°, 51°, 56°, 65°, 70°, and 75° pertaining to the lattice planes (1 0 1), (1 1 0), (2 0 0), (2 0 1), and (2 1 0) and (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0), and (3 0 1), respectively.

Fig. 1
figure 1

XRD patterns of the pure and prepared samples

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From Fig. 1, it is observed that with the addition of lithium salt and different plasticizers to the polymer blend, the intensity of these peaks decreases gradually. These observations apparently reveal that the polymer blend undergoes significant structural reorganization upon the addition of different plasticizers. The plasticizers might have induced significant disorder into the original polymers, and this is attributed to the interactions between polymer and the solvents, which resulted in polymer electrolytes with much lower crystallinity. Hence, the absence of peaks pertaining to lithium perchlorate salt in the complexes indicates the complete dissolution of the salt in the polymer matrix.

FTIR studies

Fourier transform infrared spectroscopy (FTIR) is a quite commonly used technique to examine the interactions in polymer blends [28, 29]. The immiscibility and the phase separation between the blending components will be identified by the spectra of pure individual components. On the other hand, the miscibility of the components is implied by the possibility of chemical interactions between the individual polymer chains, salt, and plasticizer. Generally, wave number shifts, increase or decrease in the intensity of the peaks, and broadening are taken as evidence of chemical interactions between the components in a blend and are indicative of miscibility.

Figure 2 illustrates the FTIR spectra of pure PEO, PVdC-co-AN, and salt and those of the prepared sample with different plasticizers, and Table 1 shows the various band assignments corresponding to the wave numbers. The broad vibrational peak around 3,445 cm−1 in the spectra pertaining to the –OH group [30] is attributed to the water or moisture absorption which might have happened during the sample loading for the spectral analysis. The peak 3,445 cm−1 is shifted to the wave numbers 3,454, 3,451, 3,454, 3,455, 3,448, and 3,452 cm−1 in all the prepared complexes. The well-built vibrations of C≡N and C=N bands in PVdC-co-AN [30] are represented by the peaks around 2,245 and 1,642 cm−1. These two peaks are shifted to the wave numbers 2,244, 2,251, 2,255, 2,244, and 2,240 cm−1 and 1,646, 1,648, 1,646, 1,653, and 1,646 cm−1, respectively, in the EC-, PC-, gBL-, DEC-, DMC-, and DBP-based gel polymer electrolytes.

Fig. 2
figure 2

FTIR spectra of the pure and the prepared samples

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Table 1 Assignments of FTIR absorption bands for the prepared membranes
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The symmetric and asymmetric C–H stretching forms of CH2 in PEO [31] is symbolized by the strong absorption band appearing around 2,800–2,950 cm−1 which shifts in the wave numbers 2,889, 2,891, 2,880, 2,883, 2,891, and 2,887 cm−1 in all the prepared electrolytes. The strong vibration of the ether oxygen group in PEO is indicated by the peak around 1,799 cm−1 [32] which is shifted to the wave numbers 1,797, 1,804, 1,797, 1,797, 1,806, and 1,802 cm−1. A strong vibrational peak at 1,468 cm−1 is attributed to CH2 stretching of pure PEO which is shifted to 1,460, 1,462, 1,462, 1,460, 1,463, and 1,462 cm−1, respectively [33]. It can be seen that the CH2 asymmetric wagging mode appearing at 1,346 cm−1 for pure PEO shifts to the wave numbers 1,356, 1,352, 1,359, 1,354,1,350, and 1,351 cm−1, respectively [34]. The –CH2 symmetric twisting mode is embodied by the peak near 1,242 cm−1 in pure PEO which has been shifted to the wave numbers 1,254, 1,249, 1,249, 1,247, 1,242, and 1,240 cm−1 in the samples SD1, SD2, SD3, SD4, SD5, and SD6, respectively [34]. The peak relating to the antisymmetric stretching vibration of the C–O–C bridge in PEO becomes visible close to 1,116 cm−1 [35], and it is shifted in the complexes to the wave numbers 1,111, 1,100, 1,098, 1,109, 1,107, and 1,107 cm−1, respectively. The peak corresponding to ClO4 − vibration is found around 625 cm−1 which is present in all the complexes.

New peaks pertaining to the absorption at 2,166, 1,965, and 1,460 cm−1 of EC-based complex, 2,744, 2,359, 2,165, 1,965, 1,462, and 1,245 cm−1 of PC-based complex, 2,511, 2,358, and 1,964 cm−1 of gBL-based complex, 2,513, 2,353, and 2,167 cm−1 of DEC-based complex, 2,742, 2,361, and 1,244 cm−1 of DMC-based complex, and 2,743, 2,165, and 1,720 cm−1 of DBP-based complex are also present in the FTIR spectra of the complexes.

The disappearance of the few characteristic peaks of the plasticizers and the appearance of the new peaks and shift in the peaks confirm the complex formation between polymers, salt, and different plasticizers.

Complex impedance analysis

The conduction mechanism is studied with the help of ac impedance spectroscopy by observing the contribution of polymeric chain, mobility, and carrier generation process [36]. Figure 3 depicts the room temperature ionic conductivity of the prepared gel polymer electrolytes. The theoretical analysis given by Watanabe and Ogata [37] stipulates for two semicircles to be present in the impedance spectrum of asymmetric cell. One semicircle at high-frequency region embodies the bulk electrolyte impedance, and the other at low-frequency region corresponds to the interfacial impedance. Reports say that the high frequency semicircles will not appear in the practical impedance plots of plasticized polymer electrolyte membranes [38, 39]. This feature reveals that the conductivity in these films is established mainly by the ions and which is calculated using the formula σ = l / (R b  · A), where A is the area of the film surface in contact with the electrodes, l is the thickness of the film, and the value of bulk resistance (R b) is taken on the X-intercept of the impedance plots. The film SD1 complexed with EC plasticizer is found to have maximum ionic conductivity value of 3 × 10−4 S cm−1 at 303 K. The rationale is clarified as follows.

Fig. 3
figure 3

Room temperature complex impedance plots of the prepared samples

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In general, the influence of plasticizer in the ionic conductivity of the polymer electrolyte films is determined by its own characteristics like dielectric constants, viscosity, polymer–plasticizer interaction, and ion–plasticizer interaction. In this view, the plasticizer EC possesses a high dielectric constant (89 at 40 °C) and comparatively low viscosity (1.9 cP). This may be the reason for the high ionic conductivity of the film SD1 complexed with EC. The plasticizers would dissolve enough charge carriers and provide more mobile medium for the ions so as to enhance the conducting behavior of the samples [40]. The conductivity is obtained in the decreasing order of the EC-, PC-, gBL-, DEC-, DMC-, and DBP-based gel polymer electrolytes which are in accordance with their dielectric constants 90, 64, 42, 28, 11, and 8.4, respectively. These results agree well with the earlier reports using different plasticizers [41, 42].

The ionic conductivity values of the sample observed over the temperature range 303–363 K have been plotted in Fig. 4. It could be visualized that the conductivity values increase with the increase of temperature regardless the nature of the sample as given in Table 2. This enhancement in the conductivity values with increase of temperature could be elucidated with the free volume theory [43]. Free volume is produced when the polymer expands with temperature and the polymer segments or molecules salvated with ions can move spasmodically into the free volume. The free volume around the polymer chain decides the overall mobility of the ions and the polymer segment, which represents the resultant ionic conductivity of the films. Hence, the ionic mobility and segmental mobility gear up the ion transport by overcoming the retarding effects of the ion clouds. The plot of the temperature-dependent ionic conductivity indicates that the ion conduction in the solid polymer electrolytes seems to obey the Vogel-Tammann-Fulcher (VTF) relation which describes the transport mechanism in a viscous matrix [44–47].

Fig. 4
figure 4

Temperature dependent ionic conductivity plots of the prepared samples

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Table 2 Ionic conductivity values of the prepared samples
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TG/DTA

All the physical phenomena associated with the weight changes are elaborately studied using TG/TDA. TG/DTA curves of EC-, PC-, gBL-, DEC-, DMC-, and DBP-based PEO/PVdC-co-AN/LiClO4 systems have been plotted as shown in Fig. 5(a-f).

Fig. 5
figure 5

a–f TG/DTA plots of the prepared blend electrolytes

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Table 3 illustrates weight changes with temperature. It is seen from Fig. 5 that the initial weight loss about 2–3 % is observed in the temperature range 76–88 °C. This may be due to the removal of residual solvent and moisture if any. The second weight losses attributing to the structural changes in the polymers occur between 272 and 289 °C which are 33, 35, 35, 38, 37, and 34 % for the samples SD1, SD2, SD3, SD4, SD5, and SD6, respectively. Then, there is no appreciable weight loss observed until the temperature reaches around 395–405 °C. Maximum thermal stability 222 °C is observed for the sample SD1.

Table 3 TG/DTA results of the prepared samples
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The DTA plots of the samples exhibit endothermic peaks between 67 and 71 °C. The second and third peaks are exothermic peaks in the range of 274–289 and 386–395 °C, respectively, which are well correlated with the weight loss of all the samples observed in TG curve.

DSC analysis

This technique is used to measure the difference in heat flow rate between the sample and inert reference as a function of time and temperature. The processes like glass transition, melting, evaporation, etc. are associated with the endothermic heat flow, whereas the physical events like crystallization, cure, oxidation, etc. are accompanied by an exothermic heat flow.

Figure 6 displays the DSC plots of the samples with various plasticizers. It is noticed that the samples SD1, SD2, SD3, SD4, SD5, and SD6 show a phase transition at 27, 28, 30, 31, 32, and 33 °C which are ascribed to the glass transition temperatures of the prepared complexes accordingly. The glass transition temperature of PEO is −53 to −45 °C and that of PVdC-co-AN is around 50 °C. Among the different complexes, the sample prepared with EC shows a minimum T g value which attributes to its maximum ionic conductivity. The decrease in the T g value also indicates the decrease of crystallinity upon the addition of plasticizer.

Fig. 6
figure 6

DSC curves of the prepared polymer electrolytes

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Photoluminescence studies

The polymer electrolyte films prepared with various plasticizers were subjected to photoluminescence analysis at an excitation wavelength of 280 nm. Figure 7 shows photoluminescence emission spectra of the prepared electrolytes. The carrier mobility of the electrolyte is inversely proportional to the local viscosity. But the local viscosity is directly associated with the emission intensity [48, 49].

Fig. 7
figure 7

Photoluminescence spectra of the prepared complexes

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In the present study, EC-based polymer electrolyte system has a minimum emission intensity as compared to other samples. This might be due to the maximum dissociation of the ionic charge carriers owing to its high dielectric constant. As the photoluminescence emission intensity decreases, the local viscosity of the surrounding polymeric media also decreases. This in turn increases the ionic mobility and hence conductivity. These results are exclusively concurring with the ionic conductivity values (Table 2).

UV–visible analysis

The optical absorption study of the polymer electrolyte films provides the band structure details of the solids. Generally, insulators and semiconductors are categorized into direct band gap materials and indirect band gap materials. Direct band gap materials are identified with the zero crystal momentum associated with the top of the valence band and the bottom of the conduction band. But, the bottom of the conduction band possesses non-zero crystal momentum in the case of indirect band gap materials.

In indirect band gap materials, a photon of the light magnitude of crystal momentum is associated with the transition from the valence band to the conduction band. The band gap of the materials is determined by the knowledge of the fundamental absorption pertaining to the transition from the valence band to the conduction band.

The energy of the incident photon (hv) and the absorption co-efficient α are related by the following expression

$$ \alpha =C\left[\raisebox{1ex}{${\left(\mathrm{h}\mathrm{v}-{E}_{\mathrm{g}}\right)}^n$}\!\left/ \!\raisebox{-1ex}{$\mathrm{h}\mathrm{v}$}\right.\right] $$

where C is a constant and E g is the band gap of the materials. The type of transition is determined by the exponent which assigns the values 1/2 for direct and 2 for indirect and for forbidden direct and forbidden indirect transitions, respectively [50].

The calculated values of (αhv)2 and \( {\left(\upalpha \mathrm{h}\mathrm{v}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.} \) were plotted against hv separately for all the samples as shown in Figs. 8 and 9, respectively. The direct band gap value was evaluated from (αhv)2 vs (hv) by extrapolating the linear portions of the curves to zero absorption. For the sample SD1, the direct band gap lies at 2.8 eV, while for the samples SD2, SD3, SD4, SD5, and SD6, they lie at 3.4, 3.7, 3.9, 4.5, and 5.0 eV, respectively. On the other hand, the indirect band gap values were obtained from plots of \( {\left(\upalpha \mathrm{h}\mathrm{v}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.} \) vs (hv) at the intercepts of the energy axis on extrapolating the linear portion of the curve to zero absorption value (Fig. 9). The indirect band gap value of the sample SD1, SD2, SD3, SD4, SD5, and SD6 are 1.6, 1.8, 2.0, 2.6, 3.2, and 4.0 eV, respectively.

Fig. 8
figure 8

Plots for direct band gap of the prepared samples

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Fig. 9
figure 9

Plots for indirect band gap of the prepared samples

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The direct and indirect band gap values have been displayed in Table 4. EC plasticizer-based gel polymer electrolyte is found to have a minimum band gap as compared to other plasticizer-based samples. However, all the complexes prepared with different plasticizers show a significant decrease in the band gap values for direct and indirect band gap calculations. This may be due to the formation of defects in the electrolytes which produce localized states in the optical band gap, and these states are accountable for the decrease in energy band gaps [51]. These results are in agreement with those obtained from conductivity studies.

Table 4 Direct and indirect band gap values of the prepared complexes
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Scanning electron microscopic study

Scanning electron microscope was employed to study the surface morphology of the film SD1 with maximum ionic conductivity. Figure 10(a, b) displays the surface morphology of the film with two different magnifications ×2,000 and ×100. It is observed that the addition of plasticizer has considerably reduced the roughness of the film. The smooth and homogeneous surface morphology indicates a remarkable increase in the amorphousity which is responsible for the increase in ionic conductivity of this film. This result is in good agreement with the X-ray diffraction and conductivity studies.

Fig. 10
figure 10

SEM analysis of the sample (SD1) having a maximum ionic conductivity at different magnifications. a ×2,000. b ×100

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Atomic force microscopic analysis

The atomic force microscope was used to study the two- and three-dimensional topographical images of the sample having a maximum ionic conductivity. Figure 11(a, b) represents the topographic images. From the figures, we are able to understand that there are number of pores within the scanned area of 50 × 50 μm. These small pores of the electrolyte are responsible for the enhancement of ionic conductivity. The pores helped to ensnare the large volume of the liquid in the pores accounting for the increased conductivity. The rms roughness value is found to be 200.9 nm. The increase of pore size and smoothness might be the rationale for the increased ionic conductivity of this film which agrees with the ionic conductivity results.

Fig. 11
figure 11

The topographic image of the sample (SD1) having a maximum ionic conductivity. a Two-dimensional image. b Three-dimensional image

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Cyclic voltammetric test

Since high ionic conductivity is not a sufficient property to make an electrolyte useful in practical terms, the electrochemical stability of the gel polymer electrolyte film (SD1) (PEO (64 wt%)/PVdC-co-AN (16 wt%)/LiClO4 (8 wt%)/EC (12 wt%)) exhibiting maximum ionic conductivity was explored using cyclic voltammetry. The cyclic voltagrams of the SS/gel polymer electrolyte/SS cell couple was taken at different scan rates (25–150 mV/s) as shown in Fig. 12. There is one poorly defined anodic peak around 0.28 V and one less prominent cathodic peak near −0.09 V. This may be due to the strong capacitive current [52]. These peaks are associated with the oxidation and reduction of LiClO4/polymer at the anode and cathode, respectively.

Fig. 12
figure 12

Cyclic voltammograms of the sample having a maximum ionic conductivity (SD1)

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It is also noticed that the area of the rectangles increases with the increase of scan rates. This reveals that the voltammetric current is directly proportional to the scan rate. The poor rectangular shape with the higher scan rate could be explained that the ion species from the polymer film may confront the difficulty to enter all the vacant sites in the active electrode. This pertains to the partial rate of movement in the electrolyte [53]. The overall performance of the polymer electrolyte film as separator is highly capacitive and electrochemically reversible.

Liner sweep voltammetric test

The electrochemical stability window of the gel polymer electrolyte having a maximum ionic conductivity was determined by linear sweep voltammetry with a constant scan rate (50 mV/s) [54] as shown in Fig. 13. The decomposition process of the polymer electrolyte results in the onset of current in the anodic higher voltage range and cathodic low voltage range [55]. The stability window for this film is observed around −2.8 to +2.6 V and also indicates there is no decomposition of any component in this potential range.

Fig. 13
figure 13

Linear sweep voltammetric plot of the sample having a maximum ionic conductivity (SD1)

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Conclusion

The gel polymer electrolytes consisting of PEO/PVdC-co-AN/LiClO4 with different plasticizers were prepared by solution casting technique. The reduction in the XRD peaks and the increase of amorphousity due to the addition of plasticizers were confirmed by X-ray diffraction technique. The complex formation and the interaction between polymer-plasticizer and ion-plasticizer were ascertained by FTIR analysis. The ionic conductivities were calculated using ac impedance analysis. The maximum room temperature ionic conductivity value is found to be 3 × 10−4 S cm−1 for the film prepared with EC plasticizer owing to its high dielectric constant. The sample (SD1) has the highest ionic conductivity which is supported by the lowest optical band gap in UV–visible spectroscopy. The two- and three-dimensional topographic images of the sample having a maximum ionic conductivity show the presence of number of pores which are responsible for high ionic conduction. The maximum thermal stability is found to be 222 °C for the sample SD1. The emission peaks observed from the photoluminescence spectra were in line with the conductivity results. The CV results show a strong capacitive behavior, cyclability, and reversibility of the above gel polymer electrolyte. The electrochemical window has been observed to be −2.8 to +2.6 V. Hence, these data suggest that the present electrolyte system is a worthy candidate for battery applications.