Introduction

In recent years, there has been a growing interest in flexible and lightweight energy storage devices, particularly in wearable electronics, electric vehicles, and renewable energy applications. Polymer batteries have emerged as one of the top energy storage options due to their high energy density, capacity retention, and mechanical flexibility. One of the most common polymer materials used in battery applications is polyvinylidene fluoride (PVDF) [1,2,3]. PVDF is a semi-crystalline thermoplastic material that has excellent chemical resistance, good mechanical properties, and high dielectric strength. It exhibits piezoelectric and pyroelectric properties, making it suitable for sensor, actuator, and transducer applications. However, one of its limitations is its poor electrical conductivity, which can hinder its electrochemical performance in polymer batteries. To address the issue of poor electrical conductivity in PVDF, researchers have investigated incorporating conductive fillers [4,5,6].

Various published articles review the advances and prospects of PVDF-based polymer electrolytes for lithium batteries. The authors discuss the modified strategies, ion transport mechanisms, and applications of PVDF-based polymer electrolytes. They also compare the properties and performance of different PVDF-based polymer electrolytes with various additives, such as ceramic fillers, ionic liquids, nanofibers, and graphene [7, 8].

Furthermore, they investigate battery separators based on polyvinylidene fluoride (PVDF) polymers and copolymers. The authors focus on the effect of PVDF polymers on the lithium transport number, which is related to the selective interactions between the anion and the polymer matrix [3, 9,10,11]. They also analyze the influence of porosity and pore size on the gelation process and the solution uptake of the PVDF-based polymer electrolytes [12].

Polymers are widely used in various aspects of battery cells, such as active materials, membranes, and binders. As reported, the authors review the recent research on polymers for different types of batteries, such as lithium-ion, lithium-sulfur, lithium-air, sodium-ion, and redox-flow batteries. They also discuss the challenges and opportunities for polymer-based battery materials [8, 13,14,15].

The effect of morphological changes in copper-oxide fillers on the performance of solid polymer electrolytes for lithium-metal polymer batteries has been studied. The authors find that dendritic copper-oxide fillers can enhance the ionic conductivity, thermal stability, and electrochemical performance of PEO-based solid polymer electrolytes compared with spherical copper-oxide fillers [16]. They also demonstrate the flexibility and safety of the prepared solid polymer electrolytes [17, 18].

Also, the manufacturing technology and properties of a lithium polymer battery with a PVDF-based electrolyte doped with copper oxide nanoparticles are described. The authors report that copper oxide nanoparticles can improve the ionic conductivity and mechanical strength of the PVDF-based gel polymer electrolyte. They also analyze the charge-discharge characteristics and cyclic stability of the prepared lithium polymer battery [8, 13,14,15, 19, 20].

figure a

Silver nanoparticles offer excellent conductivity due to their high electron mobility, while the copper oxide nanoparticles enhance the structural stability of the overall composite. Additionally, the reduced graphene oxide provides a large surface area, further increasing the conductive pathways for electron transfer [21].

In this article, we explore the use of Ag-CuO/rGO nanoparticles as a filler for the PVDF polymer to improve its polar β phase and electrical conductivity for polymer battery applications. The combination of silver (Ag) and copper oxide (CuO) nanoparticles with reduced graphene oxide (rGO) shows great promise in enhancing the polar β phase of the PVDF matrix. The polar β phase is crucial for improving both mechanical and electrochemical properties, due to its strong interaction with lithium ions during charge/discharge cycles.

Experimental techniques

Raw materials

High-purity copper nitrate (Cu (NO3)2·3H2O), silver nitrate (AgNO3), sodium hydroxide pellets (NaOH), absolute methanol, concentrated sulfuric acid, sodium nitrate (NaNO3), hydrogen peroxide (30%), potassium permanganate, and 4% HCl aqueous solution were purchased from Sigma-Aldrich and used as received without further purification. Graphite flakes, poly (vinylidene fluoride) (PVDF) powder, and N-methyl-2-pyrrolidone (NMP, 99.5% purity) were purchased from Merck Chemical, India.

Nanoparticles preparation

We synthesized Ag-CuO nanoparticles by the co-precipitation method. We prepared a solution 0.5 M of AgNO3 and 0.5 M Cu(NO3)2·3H2O to obtain a 1:1 molar ratio of Ag:Cu. We added 3 M NaOH solution to the nitrates solution and stirred it magnetically for 15 min, forming a black precipitate. We collected, washed, and dried the precipitate in an oven. We followed the Hummers Jr. and Offeman method [22, 23] to produce reduced graphene oxide (rGO) from graphite flakes. We added 10% Wt of rGO powder to the nitrates solution and stirred it for another 30 min. We washed the resulting solution several times with distilled water and ethanol, and collected and dried the precipitated powder (Ag-CuO/rGO) at 60 °C. Finally, we calcined the powder at 600 °C for 4 h with a heating/cooling rate of 4 °C/min.

Ag-CuO-rGO/PVDF nanocomposite prepartion

The synthesized Ag-CuO/rGO nanoparticles were introduced into the PVDF polymer with different concentrations (0, 1, 1.5, 2, 2.5) Wt%. 10 g of PVDF powder were dissolved in 10 mL of NMP. The solution was stirred for 6 h at room temperature with NMP until it was completely transparent. A suspension of the required weight of nanoparticles in NMP was sonicated. The suspension was added to the PVDF solution and sonicated for 1 h. The final solution was then poured onto a clean Petri dish on a hotplate kept at 60 °C. The nanocomposite films were washed with distilled water to remove any contamination and to fully solidify. The nanocomposite films had thicknesses of 0.95 mm, 0.36 mm, 0.14 mm, 0.82 mm, and 0.7 mm for the Ag-CuO/rGO/PVDF film of concentrations (0%, 1%, 1.5%, 2%, and 2.5%) Wt. respectively.

Nanocomposite characterization

(XRD) The structural analyses of the nanopowder and the composite of the Ag-CuO/rGO/PVDF film were carried out using (XRD) Pruker D8 advance X-ray diffractometer with CuKα radiation of λ = 1.5418 Å. The X-ray diffraction pattern was recorded at room temperature in a wide range of Bragg angles 2θ (20o ≤ 2θ ≤ 80o) with 0.02º step size. Multi-point (St 1 on NOVA touch 4LX [s / n: 17,016,062,702]) isotherm (BET) was used to obtain the surface area, particle size and porosity type of Ag-CuO/rGO nanoparticles. The electrical and dielectric properties of Ag-CuO/rGO/PVDF film composites of concentrations (0, 1, 1.5, 2, 2.5)%Wt were measured in a wide range of frequencies between 100 Hz and 100 kHz using network impedance analyzer (KEYSIGHT-E4991B), the electrical parameters were calculated in temperature range from room temperature to 373 K. Fourier Transform Infrared Spectroscopy (FT-IR) instrument (Perkin Elmer) in the range of 4000–400 cm− 1 has been used to identify the chemical composition and functional groups of Ag-CuO/rGO/PVDF films of concentrations (0, 1, 1.5, 2, 2.5) Wt%.

Results and discussion

FTIR

Figure 1(a) shows the FTIR spectra of pure PVDF and PVDF filled with Ag-Cuo/rGO nanoparticles with different ratios, in the range from 4000 cm− 1 to 400 cm− 1. While Fig. 1(b) describes the functional group region of the investigated samples. Eight absorption bands appeared in the graph of all samples. These peaks correspond to the functional groups of the PVDF polymer. The bands and their related vibration modes are listed in Table 1.

Table 1 Ag-CuO/rGO/PVDF nanocomposite functional group wavenumber bands and its equivalent vibration modes.
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The effective bands range from 1600 cm to 1 to 350 cm-1. The centers and areas of these bands are listed in Table 2. The obtained position and assignments of the vibrational bands confirm agreement with the published data of pure PVDF. Most bands shifted or disappeared because of the incorporation of nanoparticles. It is known that PVDF mostly contains five crystalline phases with various structures: β, α, δ, γ, and є phases [32,33,34,35]. A dipole moment perpendicular to the polymer chain is created in the PVDF polymer chains. If these dipoles are arranged such that they are parallel, the crystal’s net dipole moment will be in the polar form of β, γ, and δ phases. On the other hand, if they are arranged in an antiparallel manner, the net dipole moment will vanish, and the crystal will be in the non-polar form of α and є phases. The production of the strong polar β phase in PVDF is due to the presence of the strongly negatively charged fluorine atoms, as compared to the hydrogen and carbon atoms. This creates a rather strong electric moment in the polymer [32]. Among these five phases, the β phase has the largest spontaneous polarization per unit cell. These advantages make the β phase important in different applications for its distinguished ferroelectric and piezoelectric properties [34].

The effect of the prepared nanoparticles on the relative fraction of β-phase was examined to determine the extent of improvement in this relative fraction by adding these nanoparticles.

To determine the relative fraction percent of β-phase in each sample, we have deconvoluted the FTIR spectra (1600 − 350 cm-1 regions) [36] (Fig. 1(c)).

\(F \left(\beta \right)\), values were calculated using the following relation: [37, 38]

$$F \left(\beta \right) \%= \frac{{A}_{\beta }}{\frac{{K}_{\beta }}{{K}_{\alpha }} {A}_{\alpha }+ {A}_{\beta }} \times 100$$
(1)

Aα and Aβ in Eq. (1) are the areas corresponding to absorption bands at 760 cm− 1 and 840 cm− 1 for α and β phases, respectively. Kα = 6.1 × 104 and Kβ = 7.7 × 104 cm2/mole, are the absorption coefficients for α and β phases. The obtained results are listed in Table 2. Figure 3 and Table 4 represent that the relative fraction of the β-phase slightly increased with the addition of the investigated nanoparticles up to 1.5%, and sharply increased in samples with ratios of 2 and 2.5. Indicating that the investigated nanoparticles incorporated into the PVDF chain structure ease the transformation to the β-phase.

Fig. 1
figure 1

(a-c): FTIR spectra of Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) % and its deconvolution pattern

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Table 2 The center wavenumber and areas of FTIR bands of Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt%
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XRD

Figure 2(a) confirms the formation of a single-phase XRD pattern of the Ag-CuO/rGO powder according to the ICDD reference cards [00-041-1104], [01-072-0607], and [00-005-0661].

It also indicates the presence of graphite and reduced graphene oxide (rGO). Where peaks at 2θ = 25.7°, 2θ = 28.7°, and 2θ = 51.06° are attributed to graphite, while the diffraction peaks at 2θ = 28.7° and 2θ = 51.06° correspond to rGO particles.

The phase identification of Ag-CuO/rGO/PVDF films with different weight percentages of Ag-CuO/rGO (0, 1, 1.5, 2, 2.5) Wt% was performed by X-ray diffraction (XRD). Figure 2(a) shows the XRD patterns of the films, which exhibit a single-phase structure. The peaks correspond to the ICDD card [01-072-1174] of the PVDF, which indicates that the incorporation of Ag-CuO/rGO did not affect the crystallinity of the PVDF matrix. The diffraction peaks indicated the presence of α and β phases in all the samples, where peaks at 18.30°, 26.50°, and 38.9° correspond to the α phase, and peaks at 19.85° and 35.90° correspond to the β crystal phase. The sharp and intense peak at 19.85° is due to the formation of the β-phase. Detailed data of XRD for Ag-CuO/rGO/PVDF was discussed in our previous work [39].

Table 3 The areas under the peaks of XRD of Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt%
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The relative fraction (\(F \left(\beta \right) )\) of β-phase of the investigated samples were obtained using the following relation: [36]

$$F \left(\beta \right)\%= \frac{{A}_{\beta }}{{A}_{\alpha }+ {A}_{\beta }} \%$$
(2)

We determined the area under the corrsponding diffraction peaks (\({A}_{\alpha } \;and\; {A}_{\beta }\)) by deconcoluting the XRD patterns as shown in Fig. 2(b) and the estimated data are listed in Table 3. The determined relative fraction of β-phase (\(F \left(\beta \right) )\) is listed in Table 4 and illustrated in Fig. 3. The table and figure confirm that the addition of the papered nanoparticles has changed the relative fraction of the β-phase and improved its value.

Fig. 2
figure 2

(a, b): (a) XRD diffraction pattern spectra, and (b) the deconvoluted pattern of Ag-CuO/rGO powder and Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt%

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Table 4 The relative fraction (\(F \left(\beta \right) )\) of β-phase determined from FTIR and XRD for Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt%
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BET

The N2 adsorption-desorption cycles of Ag-CuO/rGO nanoparticles at 77 K are illustrated in Fig. 4. The fig. confirms that the prepared nanoparticles are classified as mesoporous materials according to the IV isotherm type, accompanied by an H3-type hysteresis loop related to the IUPAC classification. The BET surface area (m²/g), total pore volume (cc/g), average pore size (nm), and average particle radius (nm) of Ag-CuO/rGO nanoparticles are listed in Table 5. The results identify a high surface area-to-volume ratio and small particle size.

Table 5 BET surface area (m2/g), Total Pore Volume (cc/g), Average Pore Size (nm) and Average Particle radius (nm) of Ag-CuO/rGO nanoparticles prepared powder
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Fig. 3
figure 3

The variation of relative fraction (𝐹 (𝛽) ) of β-phase with Ag-CuO/rGO concentration in PVDF

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Electrical parameters

Electrical parameters such as the real and imaginary parts of relative permittivity, loss tangent (tan δ), resistivity, and AC conductivity (σa.c) of Ag-CuO/rGO/PVDF at (0, 1, 1.5, 2, 2.5) weight% nanocomposite samples were evaluated from impedance measurements. The electrical behavior of pure PVDF and Ag-CuO/rGO/PVDF nanocomposites with different concentrations has been obtained and analyzed to determine the effect of the prepared nanofillers on the electrical properties of PVDF. The investigated nanocomposite samples were inserted between two copper conducting electrodes of 10 mm in diameter, forming a capacitor. The capacitance (C) of the investigated composite samples was measured at room temperature in the frequency range of 100 Hz to 100 kHz. The real part of the complex dielectric constant has been calculated using the following formula: [40, 41]

$${\epsilon }^{{\prime }}= ^{Cd} /{{\epsilon }_{o}A}$$
(3)

Where εo is the dielectric constant in a vacuum (εo = 8.854 × 10− 12 F/m), C, d, and A are the measured capacitance, sample thickness, and sample area, respectively [1]. While the imaginary part of the complex dielectric constant has been obtained from the formula: [41]

$$\epsilon = {\epsilon }^{{\prime }}-j{\epsilon }^{{\prime }{\prime }}$$
(4)

(Tan δ) is the measure of dissipation of electromagnetic energy signal in the material under investigation [42, 43].

$$\text{tan}\delta =\frac{{\epsilon }^{{\prime }{\prime }}}{{\epsilon }^{{\prime }}}$$
(5)

Also, the (σa.c) of the investigated samples has been obtained using the following formula: [44, 45]

$${\sigma }_{a.c.}=\; \omega\; {\epsilon }_{o}{\epsilon }^{{\prime }}\;\text{tan}\delta$$
(6)

Where, \(\omega =\left(2\pi f\right)\), f is the angular frequency of the applied electric field in Hertz [46].

Figure 5(a-e) shows the variation of the real part of the dielectric constant (ε\) with frequency in the range of 100 Hz to 100 kHz for pure PVDF and Ag-CuO/rGO/PVDF nanocomposites with different concentration films at various temperatures ranging from 293 to 373 K.

It is clear from the figure that ε\ is dependent on frequency, where ε\ sharply decreases with increasing frequency. Moreover, all the samples exhibit high linear dispersion in the low frequency region (100 Hz-1 kHz), followed by nonlinear behavior in the range of 1-20 kHz, and finally constant dispersion in the high-frequency region (20-100 kHz).

Figure 6(a-e) shows the dependence of the dielectric loss ε\\ on frequency in the same mentioned range of frequency and temperature. From the figure, ε\\ follows the same behavior as ε\ for all the investigated samples.

The decrease of ε\ and ε\\ with frequency in the region (1) is sharp due to the interfacial polarization, where dipoles have time to rearrange themselves in the direction of the applied field. By increasing the frequency, the polarization relaxation phenomenon weakens, resulting in a small extinction of the interfacial polarization. With further increase in frequency, the dipoles become saturated and cannot follow the variation in electric field direction.

By adding nanofillers to the PVDF polymer, free charges accumulate at the filler/polymer interface, making the interfacial polarization phenomenon predominant in the first region. At higher frequencies, the conductive nanofillers Ag-CuO/rGO form micro-capacitors distributed in the PVDF polymer, preventing the dielectric constant from decreasing.

Fig. 4
figure 4

The N2 adsorption–desorption cycles of Ag-CuO/rGO nanoparticles

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Fig. 5
figure 5

(a-e) the variation of dielectric constant ε\ with frequency in the range (100 Hz -100 kHz) for pure PVDF and Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt% in temperatures in the range (293 to 373 K)

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In the same manner, the impedance of pure PVDF and Ag-CuO/rGO/PVDF of different concentrations shown in Fig. 7(a-e) decreases with increasing frequency due to the production of free space charges at higher frequencies, which lowers the impedance.

The addition of the Ag-CuO/rGO nanoparticles increases conductivity and lowers impedance by creating a conductive path in the polymer matrix.

However, as the frequency of the applied electrical field increases, the impedance of the composite starts to rise. This impedance increase in the range of 3 – 20 kHz can be attributed to the accumulation of free charges within the polymer matrix. These free charges generate micro-capacitors, which store electrical energy in the form of an electric field across the dielectric material.

The accumulation of charges in the polymer matrix is primarily influenced by the presence of nanoparticles. These nanoparticles act as charge traps, capturing and immobilizing the free charges in their vicinity.

In the range of 20 to 100 kHz, the impedance appears to be in a steady state. This could be due to the fact that the charge carriers are stable and cannot respond to the fluctuations of the applied electric field.

The Arrhenius plots of the total conductivity of pure PVDF and Ag-CuO/rGO/PVDF nanocomposites of different concentrations are shown in Fig. 8(a-e). The activation energy (Ea) of the total conductivity of the investigated samples was estimated using the following relation [47]

$$\sigma = {\sigma }_{o} {e}^{\left(\frac{-{E}_{g}}{{K}_{B}T}\right)}$$
(7)

The slope of the linear relation between ln(σ) and 1000/T has been obtained for pure and doped PVDF samples with different concentrations at different frequencies (100 Hz-100 kHz). The activation energy Ea for A.C. significantly decreased with the addition of conducting nanoparticles, as shown in Fig. 9. The activation energy Ea for the samples investigated is listed in Table 6. The prepared nanoparticles Ag-CuO/rGO lowered the activation energy of the PVDF polymer by 10 times its original value.

Fig. 6
figure 6

(a-e) the variation of dielectric loss ε\\ with frequency in the range (100 Hz -100 kHz) for pure PVDF and Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt% in temperatures in the range (293 to 373 K)

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Fig. 7
figure 7

(a-e) the variation of impedance with frequency in the range (100 Hz -100 kHz) for pure PVDF and Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt% in temperatures in the range (293 to 373 K)

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Table 6 The A.C activation energy Ea of Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt%
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Fig. 8
figure 8

(a-e) the Arrhenius plots for pure PVDF and Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt % in temperatures in the range (293 to 373 K)

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Fig. 9
figure 9

The A.C activation energy Ea of Ag-CuO/rGO/PVDF nanocomposite with concentration (0, 1, 1.5, 2 and 2.5) Wt%

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Conclusion

In this article, we have demonstrated a simple and effective method to synthesize Ag-CuO/rGO nanoparticles and incorporate them into a PVDF matrix to fabricate high-performance polymer nanocomposites for different applications such as polymer battery applications. The Ag-CuO/rGO nanoparticles induced the formation of the polar β phase of PVDF. We have analyzed the vibrational bands of pure PVDF and PVDF/nanoparticle composites using FTIR spectroscopy. We have found that the incorporation of nanoparticles affects the position and intensity of the bands, indicating changes in the molecular structure and interactions of PVDF. We have also discussed the different crystalline phases of PVDF and their relation to the dipole moment and polarization of the polymer chains. We have highlighted the importance of the polar β phase of PVDF for its ferroelectric and piezoelectric properties, which can be enhanced by the addition of nanoparticles. The PVDF/Ag-CuO/rGO nanocomposites exhibited enhanced dielectric and electrical properties compared to pure PVDF. The electrical conductivity of the nanocomposites increased by four orders of magnitude at 2.5 Wt% Ag-CuO/rGO loading, reaching a value of 1.23 × 10− 3 S/cm. This remarkable improvement was attributed to the synergistic effect of the Ag-CuO/rGO nanoparticles and the polar β phase of PVDF, which formed conductive networks in the nanocomposites. The PVDF/Ag-CuO/rGO nanocomposites showed promising potential for polymer battery applications, as they can provide high energy density, capacity retention, and mechanical flexibility. This work provides new insight into the design and fabrication of high-performance polymer nanocomposites for energy storage devices.