Effect of Al2O3 on structural and dielectric properties of PVP-CH3COONa based solid polymer electrolyte films for energy storage devices.

Nanocomposite polymer (NCP) films were prepared by doping sodium acetate (CH3COONa) in polymer of polyvinyl pyrrolidone (PVP) by complete dispersion of aluminum oxide (Al2O3) with different wt% proportions using solution cast method. The acquired NCP films were systematically characterized. The crystalline structure of the prepared NCP films was confirmed by XRD. The little agglomeration and grain sizes involved in the films were analyzed by SEM. The chemical bond formation and interchange reaction between the host, dopant salt and the nanofiller were confirmed by FTIR and Raman. The lowest energy bandgap values were observed to be 3.0 eV for the synthesized film with wt% ratio of PVP + CH3COONa:Al2O3 (80:20:1%). The highest ionic conductivity was found to be 1.05 × 10-3 S/cm for the prepared film with wt% ratio of PVP + CH3COONa:Al2O3 (80:20:1%). From the charge discharge characteristics it was concluded that the film with wt% ratio of PVP + CH3COONa:Al2O3 (80:20:1%) possesses long durability when compared to the other prepared films.

Introduction

Nowadays scientists have showed much interest in the development of nanocomposite polymer (NCP) films due to their magnificent trademark nature and owing to great potential at physical and electrical properties. By doping the nanofillers in the host polymer, they show much influence on electrochemical properties. Due to this reason NCP films have been utilized in numerous potential applications, for example, secondary batteries, electro- chemical cells and bio-sensors [1, 2]. In spite of the fact that the battery innovation has enhanced for as far back as couple of decades, the invention of new devices, leads to execution having great efficiency and long durability. In perspective of that the researchers have focused on sodium based rechargeable batteries [3, 4]. Initially inorganic salt has been introduced in to the several polymer matrixes for the preparation of electrolytes which act as the medium in the batteries, after that the ionic conductivity of the electrolytes have been measured and reported [5]. Later the development has been done by incorporating the nanofillers and plasticizers in the host polymer in order to enhance the ionic conductivity. In the present situation nano-doped polymer batteries can be viewed as a basic power source because of their amazing properties. The following can be considered examples, great mechanical conduct, light weight, little spillage, long solidness and balance at temperatures [6, 7]. The mechanical properties can be enhanced by including the nanostructure inorganic fillers like SiO2, TiO2, ZrO2, Al2O3 or some of the plasticizers can be distributed in the polymer cross section [8, 9]. The high ionic trade of particles more than electrons presents in the polymer electrolyte and it was especially illuminated by Wagner's polarization strategy [10, 11].

The subsequent conductivity is controlled by the centralization of cations, anions and their mobilities. The convergence of cations and anions relies upon the separation of salt which is additionally controlled by the dielectric constant of the plasticizer or nanofillers [12]. The portability of particles relies upon the association with host polymer, plasticizer and free particles accessible in the electrolyte which prompts an expansion in ionic and segmental motion that will help particle transport and basically repays the impeding impact of the particle nature [13]. The movement of Na+ in strong polymer electrolyte, in view of the state, might be portrayed as long range development and short range development. The movement of the Na+ along the PVP chain through hopping from one site to straightaway is respected long range development. Since the segmental chain movement is moderate in PVP at room temperature, the movement of Na+ is quicker in strong state; consequently it contributes the most to the ionic conductivity at room temperature [14].

In the present examination polyvinyl pyrrolidone (PVP) polymer is taken as the host, due to its outstanding conducting attributes. PVP for the most part utilized in electronic and pharmaceutical enterprises because of its numerous potential applications. It is utilized as optoelectronic display material in microelectronics [15]. Sodium acetate (CH3COONa) is a natural earth abundant element and it is broadly utilized in the in plant of many applications like solar cells, sensors and electrolyte layers as long life batteries. Due to the low dispersal grid constants, CH3COONa can easily be soluble in natural solvents and in distilled water. Aluminum oxide (Al2O3) is utilized as filler in the NCP electrolyte to enhance mechanical conduct and level of roughness of the films, since aluminum particles creates the motion of the particles randomly, which rises the ionic conductivity.

Experimental

Materials and methods

Inorganic chemicals like sodium acetate (CH3COONa) with 98% purity, polyvinyl pyrrolidone (PVP) with (M.W: 36,000 g/mol) and aluminum oxide (Al2O3) with 99% purity were bought from the Sigma Aldrich chemicals, India. NCP films were synthesized by adding of proper wt% ratio of inorganic salt (sodium acetate) and by adding nanofiller in the host PVP polymer. All the chemicals were taken in different wt% ratios of (95:5:1), (90:10:1), (85:15:1) and (80:20:1). The triple distilled water was used as a solvent. The above mentioned chemicals were placed in a conical flask and kept at continuous stirring for 24 h for complete dissolution. After that the solution was poured in polypropylene petridishes and kept in a hot air oven at 60 °C for 48 h to evaporate the solvent. A set of PVP-CH3COOK:Al2O3 NCP electrolyte films were prepared at different wt% ratios. Finally the prepared films were kept in a vacuum desiccator to remove the moisture traces on the prepared films until further characterization.

Characterization

XRD was performed by utilizing a Philips analytical X'pert diffractometer in the diffraction 2θ angular range 10–80o. The morphological images and readied tests were done with FE-SEM, Carl Zeiss, Ultra 55 model. Fourier Transform Infrared (FTIR) spectra were recorded with Perkin Elmer Alpha-E Spectrophotometer in the wavenumber extending from 400 to 4000 cm−1. The chemical interaction between the polymer and the salt was analyzed by a Nano finder 30 SOLAR TII laser Raman spectroscope. Optical absorption studies were performed at room temperature using JASCO V-670 Spectrophotometer for the prepared films in the wavelength ranging from 150 to 800 nm. The dielectric and impedance estimations of the prepared films were carried out in the frequency between 5000 and 50000 Hz on LCR-HIOKI 3532-50 Heister. Cyclic voltammetry, electrochemical and discharge characteristics were performed on 600E electrochemical analyzer.

Results and discussion

XRD analysis

Fig. 1 exhibits the XRD pattern of pure PVP and sodium acetate doped PVP along with nanofiller Al2O3 NCP films with different wt% ratios (95:5:1), (90:10:1), (85:15:1) and (80:20:1). In the XRD pattern for pure PVP sample a wide peak was seen at 24.5o which demonstrated that it contains semicrystalline nature. The films with different wt% extents demonstrated the tops at 2θ = 29.0°, 30.9°, 38.2o and 44.2°. This obviously shows the dispersion of nanoparticles in the electrolytes. The comparable conduct in the XRD pattern was observed. It shows the entire scattering of salt taken in all the prepared films. Figure demonstrates that there is a couple of turns in the peaks has been seen, this might be because of the groups of salt organizing with the PVP. The FWHM of the prepared sample is calculated by using Debye-Scherrer's formula,Where k is a constant (k = 0.9), λ is wavelength of X-ray radiation (1.5405 Å), β is full width at half maximum (FWHM) intensity of the diffraction line and θ is diffraction angle. Based on the value of FWHM, the average crystallite size is estimated to be 18 nm, which is in the order of nanosize. Among all the wt% compositional films, less power peak intensity is watched for (80:20:1) wt% film. This might be because of the stage change from semicrystalline to amorphous nature and furthermore with the substitution of the crystalline particles in the polymer chains [16, 17]. A couple of changes occur in the XRD peaks i.e. as expanding the salt focus the crystalline idea of the films is reductions and few aggravation in the peaks has been watched. This might be because of the doping of nanofiller in the PVP polymer.

Fig. 1

XRD pattern of (a) Pure PVP, (b) PVP + CH3COONa: Al2O3 (95:5:1), (c) PVP + CH3COONa: Al2O3 (90:10:1), (d) PVP + CH3COONa: Al2O3 (85:15:1), (e) PVP + CH3COONa: Al2O3 (80:20:1).

SEM studies

The morphological pictures of various wt% proportions of NCP [PVP + CH3COONa: Al2O3]: x% were shown in Fig. 2a–e. Morphological pictures of pure PVP is shown in Fig. 2a and whatever is left of the images with different wt% creations were shown in Fig. 2b–e. All the SEM pictures were investigated at 10 μm. At the point when contrasted and alternate proportions on the polymer surface of film with minimal crystalline like structures were found in Fig. 2b–d. These little kinds of lumps shaped on account of the dissipating of salt and the nanofiller which lie in the polymer framework [18]. Regardless, at higher wt% association, no stage division was seen as a result of the deterioration of salt and the nanofiller for the creation (80:20:1) which was shown in Fig. 2e. This picture clarifies that the salt lumps and the dopant nanofiller were installed consistently in the PVP polymer. Accordingly porosity nature had been extended which provoke the development in the ionic conductivity.

Fig. 2

SEM images of (a) Pure PVP, (b) PVP + CH3COONa: Al2O3 (95:5:1), (c) PVP + CH3COONa: Al2O3 (90:10:1), (d) PVP + CH3COONa: Al2O3 (85:15:1), (e) PVP + CH3COONa: Al2O3 (80:20:1).

FTIR studies

FTIR spectrum is an important tool which is utilized to recognize the chemical bond formation and practical gathering between the dopant and host polymer. FTIR range recorded in the wavenumber extending from 450 to 4000 cm−1 is shown in Fig. 3. The peaks saw at 944.5, 1105.6 and 1569.2 cm−1 are assigned to aliphatic C–O bending, C–O stretching and C=O vibrations of nanocomposite PVP polymer film. By increasing the salt concentration to the polymer, the peak intensity and the phase shift changes take place and are seen in the spectra. The widening peak showed up somewhere in the range 2700–3600 cm−1 corresponds to C=O band. This might be credited to the redistribution of Na+ particles and ionic sets related with the carbonyl gathering (C=O) of polymer grid and oxygen molecules in Al2O3 which is framed because of the coordination of cation with the ester oxygen in the PVP + CH3COONa polymer complex. The peaks saw at 585, 1522 and 2682 cm−1 are allotted to aliphatic C–H2 twisting, C–H extending and C–H2 bending of nanocomposite polymer films with various wt% arrangements. It is assumed that the width of vibrational groups of nanocomposite polymer films at 541.8, 1049.8 and 2660.6 cm−1 change because of expanding wt% organizations in the PVP. At the point when the wt% of salt increments in the host polymer the weak forces are decreased. This presumes there is an expansion in the interlayer separate between polymer chains which change with the increment of dopant fixation [19]. The trademark band which is framed in the wavelength at 1096.9 cm−1 relates to C–H2 bowing of PVP. Furthermore, a vibrational band at 894.8 cm−1 is seen because of aliphatic C–H extending and CH2 bending of PVP [20, 21]. From the spectra it is clearly shown that with an increasing the salt concentration the complexation groups and their positions are marginally moved from their original positions and also noted that their peak intensities also changes. This might be due to cooperation of C–H twisting in PVP [22, 23, 24]. This outcomes the decrement in the straight forwardness of the prepared films.

Fig. 3

FTIR spectra of (a) Pure PVP, (b) PVP + CH3COONa: Al2O3 (95:5:1), (c) PVP + CH3COONa: Al2O3 (90:10:1), (d) PVP + CH3COONa: Al2O3 (85:15:1), (e) PVP + CH3COONa: Al2O3 (80:20:1).

Raman studies

Raman spectra of nanocomposite films with various wt% arrangements were shown in Fig. 4. The groups at 548.31, 693.86, 1187.24, 1531.21 and 1675.32 cm−1 are ascribed to C–H extending, C–H2 bowing, CH2 stretching of PVP, individually [25, 26]. Three groups at 548.31, 693.86 and 1187.24 cm−1 are trademark methods of CH extending and CH2 stretching of PVP. By including distinctive wt% creations of inorganic in the host, the band at 1187.24 cm−1 is decreased and new groups are additionally seen at 1531.21 and 1675.32 cm−1. This may be because of C=O extending and CH2 rocking of PVP. It was additionally seen that the force of Raman band at 548.31 and 693.86 cm−1 relates to the C–C extending, due to the presence of nano- filler in the host sample. The vibrational groups and intensity of the peaks changes may be due to the presence of Al2O3 nanofiller in NCP films. While expanding the Al2O3 nanofiller in NCP the Raman modes get augments at 1531.21 and 1675.32 cm−1 on account of C–H bowing vibrations. This adjustment in the power of Raman groups is expected to the solid columbic powers among nanofiller and the polymer sodium salt composites [27, 28].

Fig. 4

Raman spectra of (a) PVP + CH3COONa: Al2O3 (95:5:1), (b) PVP + CH3COONa: Al2O3 (90:10:1), (c) PVP + CH3COONa: Al2O3 (85:15:1), (d) PVP + CH3COONa: Al2O3 (80:20:1).

Optical absorption studies

UV-Vis spectroscopy is an essential apparatus which assumes a key job to quantify the optical bandgap in a vitality level. The excitations of molecules in vitality levels have been estimated by the optical assimilation spectroscopy [29].

The optical assimilation coefficient as an element of photon vitality for PVP + CH3COONa:Al2O3 strong polymer electrolyte films are shown in Fig. 5. The optical retention coefficient can be isolated into three locales, specifically, abnormal state ingestion, exponential area and powerless assimilation. Abnormal state ingestion is fundamental optical change. While, the powerless assimilation area is lower than the exponential district which rely upon the sample arrangement [30]. It is fascinating to see that the retention coefficient increments with expanding the photon vitality just as CH3COONa–Al2O3 focus. Extrapolation of the direct segment of the bends has been utilized to compute the estimations of ingestion edge. This assimilation edge diminishes with the expanding of the increments of CH3COONa focus. It is obvious that the estimations of the optical band hole diminish upon the expansion of sodium acetic acid derivation salt [31, 32]. These outcomes show the way that various states were presented between the valence and conduction groups, for example an expansion in the thickness of deformity states. Polymer films dependably contain a high convergence of unsaturated securities or imperfections. These deformities are in charge of the nearness of restricted states in the formless bandgap. The charge exchange edifices increment the optical conductivity by giving extra charges, which outcomes in a reduction of the optical energy bandgap (Eg).

Fig. 5

Optical absorption spectra of (a) PVP + CH3COONa: Al2O3 (95:5:1), (b) PVP + CH3COONa: Al2O3 (90:10:1), (c) PVP + CH3COONa: Al2O3 (85:15:1), (d) PVP + CH3COONa: Al2O3 (80:20:1).

In a vitality level when a photon is consumed at certain wavelength it experiences the change from lower to higher vitality level called assimilation process [33, 34]. UV-Vis spectroscopy of the readied strong polymer films were examined in the wavelength extending from 150 to 800 nm is shown in Fig. 5. An excitation peak has been distinguished at 350.24 nm in the wavelength region. This decrement in the wavelength might be expected to the dopant of inorganic salt in the host polymer along with the nanofiller. The optical bandgap of assimilation coefficient (α) is computed by the accompanying equation,Where 'α' is the absorbance and 'x' is the thickness of the film.

From the acquired qualities, plainly the disintegration of salt and the nanofiller is totally accomplished in polymer chains in a host cross section. The immediate and round about bandgap esteems moved to bring down energies [35].

Indirect bandgap vitality esteems are figured by the accompanying equation,Where Eg is the energy bandgap, C is a constant value.

Indirect bandgap esteems are ascertained from the diagram by plotting (αhυ)1/2 versus hυ as insert in Fig. 5 and the energy bandgap esteems are observed to be at 3.30, 3.25, 3.10 and 3.0 eV.

From the acquired values, the energy bandgap values are found to decrease consistently with expanding doping wt% grouping of CH3COONa + Al2O3 in the host PVP polymer. At the point when contrasted and the wt% compositional proportions, the most minimal bandgap esteem has been found for (80:20:1) wt% compositional proportion. This compositional proportion could improve the high ionic conductivity.

AC ionic conductivity studies

Nyquist plots of PVP and (PVP + CH3COONa:Al2O3) doped NCP films with different wt% proportions (95:5:1), (90:10:1), (85:15:1) and (80:20:1) are shown in Fig. 6a. The conductivity of the films was calibrated by sandwiching the sample in the silver plate holder which was connected to probes of LCR Heister. While measuring the conductivity, Cole-Cole plots with the variation of real and impedance plot have been acquired and are screened. The impedance plot for proposed wt% proportions is shown in the figure. The pure PVP of AC comprises of semi half circle, in which high recurrence credited to mass conductivity and the low recurrence partition attributed to the interfacial impedance. With the expansion in AC content at (80:20:1) wt%, demonstrating an increment in the versatile charge carriers affected by the exceedingly leading and bigger surface zone of sodium particles [36]. The plots for the higher substance of 20 wt % AC, for example, other wt% proportions comprise of high-recurrence half circle curve allocated to particle conduction, trailed by a low-recurrence line emerging from the anode polarization (Rf) [37, 38]. A regular normal for the impedance plot for the lower substance of AC can be fitted by a comparable circuit which appeared in the Fig. 6b. The distance across of the half circle is compared to the interfacial charge exchange obstruction (Rct), which more often reveals to the opposition of the electrochemical responses at the terminal surface and the Warburg impendence (W) credited to the dissemination obstruction of the particle movement in the host polymer cathode [39].

Fig. 6

(a): Cole-Cole plots of NCP films for different wt% ratios. (b): Schematic diagram of the corresponding equivalent circuit to the impedance plot of activated carbon (AC).

The ionic conductivity is figured by the accompanying connection,Where "Rb" is the bulk resistance.

The ionic conductivity and its values were calculated, from the above formula and the values are introduced in Table 1. Among all the wt% compositional ratios, the conductivity is observed to be high, which is found to be at 1.05 × 10−3 S/cm for the organization of (80:20:1%) film. The conductivity of the films is increased due to the salvation of the ions in the polymer chains, this intend in the decrement in the crystalline nature [40]. One of the reasons for higher wt% ratio of the film the non-porosity nature is formed in the films. As a result the glass transition of the film becomes low, which tends in the increase of the ionic conductivity abruptly [41]. Further by increasing the salt concentration the ionic conductivity may be decreased. This may be due to the disintegration of ions and the charge carriers and its mobility has been decreased. This indicates the reality the exchange of sodium particles occur openly in the host which acquires higher ionic conductivity.

Table 1

AC conductivity values of NCP films.

NCP films	Conductivity (Scm−1)
	at room temperature	at 373 K
Pure PVP	1.02 × 10−9	1.13 × 10−8
PVP + CH3COONa:Al2O3 (95:5:1%)	4.15 × 10−8	5.25 × 10−5
PVP + CH3COONa:Al2O3 (90:10:1%)	3.10 × 10−6	3.35 × 10−5
PVP + CH3COONa:Al2O3 (85:15:1%)	2.14 × 10−5	2.01 × 10−4
PVP + CH3COONa:Al2O3 (80:20:1%)	6.25 × 10−5	1.05 × 10−3

Dielectric properties

Dielectric spectra regarding log ω vs log σac have been accounted for in the present work. The variation of log ω vs log σac for various wt% creations for all the readied nanocomposite polymer electrolytes is appeared in Fig. 7. The dielectric constant and DC conductivity explains the relationship between these parameters (ε and σdc). This is can be interpreted based on the fact that the DC conductivity depends on charge carrier concentration and dielectric constant (ε) at room temperature [42]. It is seen that both dielectric constant and dielectric loss decreases with increasing frequency, but increases with increasing temperature. At all temperatures, high values of εl and εll were observed at low frequencies (region I), but these are relatively constant at high frequencies (region II). Since εl and εll increases with temperature, it may be inferred that temperature helps in producing more charged species preferably from the remaining undissociated salt or neutral ion aggregation [43]. The space-charge-induced enhancement effect provides local electric fields, which accelerate the transport of conduction ions. This effect has been observed and reported in numerous composite systems. The blocking effect, on the other hand, impedes the flow of conduction ions and thereby reducing σdc. It is also clearly observed that both dielectric constant and dielectric loss decrease in region II, i.e. above 358 K. This is attributed to the reduction of a large amount of sodium ions to sodium nanoparticles at these elevated temperatures. Thus from this work, it is understood that the dielectric study is an informative technique to predict the conductivity behavior of polymer electrolytes.

Fig. 7

Variation of the log ω vs log σac of (a) Pure PVP, (b) PVP + CH3COONa: Al2O3 (95:5:1), (c) PVP + CH3COONa: Al2O3 (90:10:1), (d) PVP + CH3COONa: Al2O3 (85:15:1), (e) PVP + CH3COONa: Al2O3 (80:20:1).

As increasing the logarithmic frequency the conductivity of the samples is seems to be increases. This might be because of the decreased extent of indistinct material prompting decrease in the greatness of scattering [44]. This demonstrates the expansion of sodium particles in the host polymer. It is observed from the spectra that, while expanding the logarithmic recurrence the conductivity appears to be high for the composition of (80:20:1) % ratio. This might be because of the variety inclination of dielectric steady and restriction of charge transporters [45]. The increase of DC conductivity by four orders can be more understood from the dielectric constant study. However, at high filler concentrations (5–10 wt%) the conductivity decreases. The occurrence of reaction between the Lewis acid-base interaction centers of the added filler and the electrolytic species result in lowering the ionic coupling and thus promotes the salt dissociation via a sort of “ion-filler complex” formation. The increase and decrease of DC conductivity can be interpreted in terms of space charge and blocking effect [46]. It is reported that the addition of ceramic filler to the polymer matrix may create a space charge layer at the filler-polymer interface and thus assists in ion transport. At higher filler concentrations the blocking effect will occur. The high value of dielectric constant at low frequency can be ascribed to electrode polarization effect. These high dielectric constants at low frequencies are responsible for the suppression of high frequency dielectric constant for all the compositions to a single bundle. The high frequency dielectric constant is important because it represents the intrinsic property of the material and is directly related to DC conductivity. Thus the increase of DC conductivity is associated with an increase of dielectric constant [47]. At lower recurrence scale, the dielectric steady is expanded by the conveyance of charge bearers which results the high ionic conductivity at the electrode–electrolyte interface. As expanding the logarithmic recurrence the dielectric steady qualities are step by step diminished however the dielectric consistent esteem is observed to be high for the example 80PVP:20LiI: Al2O3 (1 wt %), because of the development of room charge district at anode surface. This could be evident that the sodium salt is totally dissolved along with the nanofiller. Hence the polymer lattice offering rise to preparation of particles in the lattice grid and it enhances the electric conductivity [48].

Cyclic voltammetric studies

For the prepared NCP samples CV-studies have been calculated with different wt% proportions in the potential range from - 0.10 to 0.10 at output rate 25 mV s−1 which is shown in Fig. 8. From the figure the oxidation and isolated redox peaks have been observed and the potential peaks (oxidation crests) are seen at 0.01/0.05V. This might be because of the transformation of sodium in to sodium ions. The CV bend demonstrated the decrease crest at 0.01/-0.05V. The reversible oxidation/decrease crests have been framed because of breaking down of iodine and carbon. With the end goal to enhance the conductivity, iodine and carbon are utilized as cathode materials which impact the electrochemical execution and furthermore consequences for Na+ addition into and de-intercalation from the grid, i.e. sodium particles are removed from and embedded into the spinel stage in a two-advance process [49, 50, 51]. The oxidation top is credited to Na+ particles and set at half of the tetrahedral locales, while the decreased top is attributed to the expulsion of Na+ another portion of the tetrahedral destinations in which cooperation does not exist. The region of oxidation declines ceaselessly with continued cycling. The cycling performance does not even change by doping of nanofiller but their potential peak and their intensity is varied, this could leads to the long durability performance [52, 53]. In order to calculate the discharge performance of the NCP electrolytes, a battery has been fabricated with the combination of sodium metal (anode)/NCP electrolyte/iodine, carbon and electrolyte pieces are used as cathode materials which affect the electrochemical execution.

Fig. 8

Cyclic voltammetry of (a) PVP + CH3COONa: Al2O3 (95:5:1), (b) PVP + CH3COONa: Al2O3 (90:10:1), (c) PVP + CH3COONa: Al2O3 (85:15:1), (d) PVP + CH3COONa: Al2O3 (80:20:1).

The main charge/release cycle assessed galvanostatically at 25 mV s−1 for iodine and carbon utilized cathode with Na as an anode is shown in Fig. 8. The oxidation and reduction tops have been observed and the potential peaks (oxidation peaks) are seen at 0.01/0.05V. The release and charge forms are constrained by time, in contrast to the voltage cutoff, relating to 0.4 mol of reversible Na extraction/inclusion with a reversible limit of 75 mAh/g. The mid release and charge potential for arranged films was seen to be 2.98–5.5 V, individually. The release and accuse profile of the presence of voltage level was predictable with the recently revealed information for non-aqueous electrolytes [54]. In any case, the striking distinction seen while utilizing non-aqueous electrolyte is the deliverable charge limit which was 169 mAh/g, emphasis the hypothetical esteem (=158 mAh/g). This suggests the abundance limit in non-aqueous electrolyte batteries may represent electrolyte deterioration at the higher charging voltage of 2.8–5.5 V. The cycling execution for the prepared battery is expanded obviously outlining that the battery in aqueous media was rechargeable and achieved a steady estimation of 75 mAh/g after the ninth cycle. The sodium extraction and inclusion process was observed to be 85% reversible and the carbon cathode in fluid batteries was observed to be a potential contender for low-voltage battery application [55]. The improved limit has been observed in our present examination was to some extent ascribed to the dependability of the sodium batteries.

As the polarization is initiated at cathode layer, the sodium particles enter through the carbon and iodine which enhance the transport of ions and electrons and by the presence of nanofiller the ionic conductivity will improve. The territory of oxidation and decreasing crests diminished constantly with continued cycling. Figure shows that for wt% arrangement proportion of (80:20:1%) strong polymer electrolyte, the decreased crest at 2.8 V is vanished followed by 8 h with nonstop continued cycling because of the inclusion/extraction of sodium particles in the polymer network, which indicates the great reversibility and dependability [56, 57].

Electrochemical properties

Potential concerning charge time has been measured for the composition NCP film (80:20:1) wt% ratio and it is shown in Fig. 9. Figure shows that the existence voltage is raised from 2.98 V and settled to 5.5 V. The execution is raised up to 0.9 h and wound up consistently after time interval of 13 h. Exactly when a proportion of voltage is associated over the cell, the Na+ particles diffuse and trade from anode to the cathode layer through the electrolyte medium and the aggregate cycling course which was done at the room temperature. From the plot it is affirmed that the charge execution is performed well for the high compositional proportion [58, 59]. Figure shows the CV curves for multiple cycles of prepared films recorded in the voltage range from - 0.10 to 0.10 (as a standard electrode) with a slow scan rate of 25 μV/s. The CV curve for NCP films exhibited one oxidative peak (A1) at 60 mV during an anodic sweep corresponding to extraction of sodium from CH3COONa, producing CH3COO phase with charge compensation. During the reverse cathodic sweep at an identical scan rate one distinct cathodic peak (C1) at 169 mV was observed. The observed anodic and cathodic peaks are consistent with the obtained charge and discharge plateau corresponding to the cobalt redox couple. As it can be seen in figure during the second and fifth cycle, the oxidation and reduction peak decreased but beyond that the peaks were stabilized and maintained constant with virtually full rechargeability until the 25th cycle [60]. Thus, it was demonstrated that the sodium extraction and insertion mechanism is versatile and reversible using aqueous NaOH solution.

Fig. 9

Variation of time vs potential of NCP film for (80:20:1) wt% ratio.

OCV versus time for the combination of PVP + CH3COONa:Al2O3 (80:20:1%) is shown in Fig. 10. From the figure it is clearly observed that as extending the voltage in regards to time, the OCV settles from 2.8 V. This may be due to the distribution of particles in the prepared films [61, 62]. The charge/discharge performance is measured for the above mentioned cell combination setup. It is found out that the execution of the cell showed the OCV at 2.8 V for around 60 h when the test was finished. The thickness of the cell is maintained constant such that it shows a great influence on the cyclic performance of the cell. At higher level due to the disintegration of the particles the conductivity of the particles may decrease that overhaul the electrochemical adequacy through the intercalation and de-intercalation methodology of the Na+ particles. Everything happens due to the broad surface locale of the particles [63, 64]. Thus the NCP film with the wt% structures of PVP + CH3COONa:Al2O3 (80:20:1%) exhibited high ionic conductivity which can be executed as a cell.

Fig. 10

OCV vs time of NCP film for (80:20:1) wt% ratio.

Transference number

The transport properties of PVP + CH3COONa electrolyte framework is figured by utilizing Wagner's D.C. polarization procedure. At the point when a DC voltage (1.5 V) is connected around the SPE of a setup of Na(A)/polymer electrolyte/C of a polymer battery, the acquired DC current is screened and noted as for time. Comparable strategy is connected and figured for different examples. The transport properties of ionic and electronic flows as for time are ascertained by following equations [65, 66].Where it is the initial current and iele is the final residual current.

The current versus time plot of PVP + CH3COONa:Al2O3 NCP films for different wt% proportions are shown in Fig. 11. If the particle transport happens through the conductivity unwinding, the expansion in conductivity would then be able to be credited to the vibrational elements nearby the polymer spine and side chains. The raise of vibrational sufficiency can draw the coordination destinations nearer together and enable the particles to bounce from their involved site into a neighboring void site, where less vitality is utilized. At the end of the day, the ionic movement and polymer segmental movement are totally decoupled, i.e. the polymer segmental movement basically makes the coordination destinations drawing nearer together and in this manner particles could be effectively bounced starting with one site then onto the next. At the point when particle development and segmental portability are all around coupled, at that point particle transport happens at the electrode and electrolyte interfaces.

Fig. 11

Transport properties of NCP films for different wt% ratios.

From the plot, it is seen that the current is all of a sudden diminished as the time increments and after consistent state, long time of polarization happens in the samples. This decrement might be due to the stream of current over the anode interfaces [67]. From the figured information, it affirms that among all the stoichiometric proportions of polymer films for (80:20:1) wt% have the high ionic charge and moment number of exchange of electrons happens. The transport properties of all proportions of PVP + CH3COONa:Al2O3 strong polymer electrolytes and their information are exhibited in Table 2.

Table 2

Transport properties of NCP films.

NCP films	Transport properties
	tion	tele
Pure PVP	-----------	---------
PVP + CH3COONa:Al2O3 (95:5:1%)	0.96	0.04
PVP + CH3COONa:Al2O3 (90:10:1%)	0.97	0.03
PVP + CH3COONa:Al2O3 (85:15:1%)	0.98	0.02
PVP + CH3COONa:Al2O3 (80:20:1%)	0.99	0.01

Charge/discharge performance

The creation of the cell was completed with the blend of anode (Na+)/electrolyte (NCP)/(I + C + electrolyte follows) cathode and the galvanostatic charge/discharge execution of the wt% proportion of (80:20:1%) was improved and it is shown in Fig. 12. At introductory stage the decrement in the associated voltage has been observed. This might be because of the isolation of particles and the immediate consequence of the depolarization which is placed between the electrolytes. From the figure, it is seen that by growing the current, OCV is lessened exponentially from 2.98 V. This may be a result of the dissipating of salt and the nanofiller in the PVP polymer that has increased the transference of ions and everything occurs at the surface area of the particles [68, 69]. The expansion of nanoparticles in a strong polymer electrolyte improves its adaptability, processibility and henceforth utility. Because of the powers, the versatility of particles is increment transcendently consequently improves ionic conductivity [70]. On differentiating the results and the other wt% extents of the films, the NCP film (80:20:1%) has high ionic conductivity with stable execution of the cell.

Fig. 12

Discharge characteristics of (a) PVP + CH3COONa: Al2O3 (95:5:1), (b) PVP + CH3COONa: Al2O3 (90:10:1), (c) PVP + CH3COONa: Al2O3 (85:15:1), (d) PVP + CH3COONa: Al2O3 (80:20:1).

Conclusions

Nanocomposite polymer films are prepared by doping aluminum oxide by different wt% compositional ratios using solution cast technique. In the XRD pattern a wide peak has been seen at 24.5o which demonstrates that it contains semicrystalline nature. The value of FWHM, the average crystallite size is estimated to be 18 nm, which is in the order of nanosize. The films with different wt% extents demonstrated the tops at 2θ = 29.0°, 30.9°, 38.2o and 44.2°. When compared to all the SEM images the higher wt% association, no stage division was seen as a result of the deterioration of salt and the nanofiller for the creation of sample (80:20:1) x%. This picture clarifies that the salt lumps and the dopant nanofiller were installed consistently in the PVP polymer. FTIR showed that the width of vibrational groups of nanocomposite polymer films at 541.8, 1049.8 and 2660.6 cm−1 partially changes. This presumes there is an expansion in the interlayer separate between polymer chains which change with the increment of dopant fixation. Optical energy bandgap showed that energy bandgap esteems are observed to be at 3.30, 3.25, 3.10 and 3.0 eV. From the acquired values, the energy bandgap values are found to be decreasing consistently by expanding doping wt% grouping of CH3COONa + Al2O3 in the host PVP polymer. From the wt% composition studies the higher value of ion conductivity for composition 80PVP-20 CH3COONa+1% Al2O3 has been found to be 1.05 × 10−3 Scm−1. This high capacity value was reported to be irreversible; the capacity reduces to 75 mAh/g subsequent to successive cycles. By expanding the current, OCV is diminished exponentially from 2.98 V. The charge/discharge performance was also studied for the fabricated cell combination setup. It was seen that the execution of the cell showed the OCV at 2.98 V for around 140 h when the test was finished and were reported.

Declarations

Author contribution statement

M.C. Rao, M. Seshu Kumar: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.