1 Introduction

In recent decades, the depletion of fossil fuels and environmental problems resulting from increasing greenhouse gas emissions have emerged as some of the most pressing global challenges. Lithium-ion batteries (LIBs) have become a promising alternative to carbon-based fuels, garnering significant attention for their high energy storage capacity and application as power components in various electronic devices, such as electric vehicles, laptops, and mobile phones [1,2,3,4]. However, to meet the ever-growing demand for portable energy storage devices, improvements in the electrochemical capacity, cyclic durability, and rate ability of LIBs are necessary. The anode electrode is a crucial component of LIBs that plays a significant role in their electrochemical performance. Among all types of anode materials, transition metal oxides (TMOs) are particularly promising alternatives [5,6,7,8]. CuO, in particular, is frequently used as an anode material due to its high electrochemical capacity, abundance, safety, low costs, and environmental friendliness. However, TMOs face several challenges, such as low electrical conductivity and significant volume change during consecutive charge/discharge processes as active anode materials. These problems often cause electrode pulverization, electrical disconnection, and consequent rapid capacity decay, thereby impeding the use of TMOs, including CuO, as anode materials in LIBs [9,10,11,12].

Numerous attempts have been made to optimize the size and morphology of CuO particles to overcome the aforementioned issues. To date, nanoparticles with various structures and morphologies, such as wires, tubes, plates, and dots, have been synthesized. Among the synthesized structures, CuO micro-flowers have been found to exhibit high capacity and stability, owing to their large surface area and short lithium-ion diffusion length [13,14,15,16,17,18,19]. Another approach to address the challenges of TMOs, such as pulverization and low conductivity, is to enhance their electrochemical properties by introducing organic conducting polymers, such as polyaniline and polypyrrole, as matrices or coating layers. These conducting polymers can accommodate volume changes during consecutive charge and discharge processes, thereby increasing the cyclability of the anode. Furthermore, conducting polymers can improve the conductivity of the active material and enhance electrode charge transfer [20,21,22,23].

In prior studies, researchers had developed Core–shell CuO@PPy composites using various techniques and investigated their electrochemical performance as Lithium-ion anode material. For instance, Yin et al. synthesized core–shell CuO nanobelts@PPy and examined their electrochemical properties as anode materials, exploring the influence of PPy shell thickness on the anode’s cyclability [24, 25]. In another study, Feng et al. synthesized CuO nanocrystals coated with a PPy shell, utilizing KCl as a dopant to address metal oxide capacity degradation in Lithium-ion anode materials [26]. While previous investigations have employed PPy as a coating layer on CuO particles, we have fabricated novel CuO micro-flower/PPy nanowire nanocomposites (NCs) by synthesizing PPy nanowires. Our study focuses on exploring the influence of PPy nanowires on the electrochemical performance of CuO particles.

2 Experimental section

2.1 Materials

Copper(II) sulfate pentahydrate (CuSO4.5H2O), ammonia solution 25% (NH4OH), pyrrole (C4H5N, Py), cetyltrimethylammonium bromide (C19H42BrN, CTAB), hydrochloric acid (HCl) solution, and anhydrous ethanol were purchased from Merck, and ammonium persulfate ((NH4)2S2O8) were purchased from Tetra-Chem.

2.2 Preparation of samples

2.2.1 Synthesis of CuO micro-flowers

For synthesizing flower-shaped CuO micro-particles, a 1.6 × 10–5 mM solution of Copper(II) sulfate pentahydrate in DI water was prepared by a magnetic stirrer. Then, a proper amount of ammonia 25% was added dropwise to the solution and stirred until reaching a pH of 11.5. Then, the solution was put in a microwave oven and irritated for 30 min at 900W. CuO micro-flower particles were synthesized and precipitated as a black powder (Fig. 1).

2.2.2 Synthesis of PPy nanowire

Polypyrrole nanowire particles were synthesized by the in situ chemical oxidative polymerization method in the presence of cetyltrimethylammonium bromide (CTAB) as template and ammonium persulfate (APS) as oxidant agent. In a typical process, 1.4 gr CTAB (0.1M) was first dissolved in 40-ml HCl (1M) and stirred for 1 h. Afterward, the solution was cooled to 0 °C, then 0.3-ml pre-distilled pyrrole monomer (was distilled and kept against exposure to light to eliminate residual polymerization) was added dropwise and stirred vigorously (1200 rpm). In this step, the viscosity of the solution increased dramatically. Second, the oxidant solution was prepared by dissolution of 0.27 gr APS in 40-ml HCl (1M) and cooled to 0 °C, and then added to the previous solution. The mixture was stirred for 24 h constantly in an ice bath. The achieved black precipitate was filtered and washed with a copious amount of DI water and ethanol till reaching pH 7 in the resulting suspension (Fig. 1).

To prepare CuO/PPy composites with different PPy weight ratios, a calculated amount of polypyrrole suspension and synthesized CuO micro-flowers were stirred for 2 h. The product was filtered and dried in the oven at 80 °C for 12 h. CuO@PPy nanocomposite with 3.5-, 5-, 6.2-, and 8.8-wt% PPy was prepared and used as anodic active material.

Fig. 1
figure 1

A schematic of CuO micro-flowers and PPy nanowires synthesis procedure

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3 Results and discussion

3.1 TGA

Thermogravimetric analysis (TGA) was performed on both bare CuO micro-flowers and CuO/PPy NCs with varying PPy weight ratios in the N2 atmosphere to quantify the amount of PPy in the nanocomposites. The samples were heated from 25 to 600 °C at a rate of 10 °C min−1. The TGA results are presented in Fig. 2a. The slight weight loss observed in both bare CuO and CuO/PPy NCs samples below 200 °C is likely attributed to the removal of surface hydroxyls or absorbed solvents. The weight loss between 200 and 600 °C in the NCs samples is predominantly due to the decomposition of PPy in the N2 atmosphere [27, 28]. In contrast, the bare CuO sample shows no significant weight loss in the tested temperature range. Based on the weight loss in the TGA curves, the mass fraction of PPy in the NCs samples was determined to be 3.5%, 5%, 6.2%, and 8.8%.

Fig. 2
figure 2

TGA (a), XRD (b), and FT-IR (c) curves of bare CuO micro-flowers and CuO/PPy NCs with different PPy weight ratios. CuO/PPy NCs synthesized with different polymerization times

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3.2 XRD

X-ray Diffraction (XRD) analysis was performed to investigate the crystalline structure of the synthesized samples. Figure 2b illustrates the XRD patterns of bare CuO, pure PPy nanowires, and CuO/PPy NCs with varying PPy mass ratios. All the characteristic diffraction peaks of bare CuO and NCs align well with the monoclinic phase of CuO (JCPDS No.05-0661), which exhibits diffraction peaks at 32.5°, 35.6°, 38.8°, 48.8°, 53.4°, 58.2°, 61.6°, 66.3°, and 68.1°, attributed to the (110), (1̅11), (111), (2̅02), (020), (202), (1̅13), (3̅11), and (220) planes of the monoclinic structure of CuO [29, 30]. The absence of any impurity diffraction peaks, such as Cu(OH)2 and Cu2O, confirms the high purity of the synthesized CuO micro-flowers and CuO NCs. Additionally, the pure PPy XRD curve displays a broad peak at 2θ ~ 23°, indicating the amorphous structure of PPy. However, this peak is scarcely noticeable in CuO/PPy samples due to its weak intensity in comparison to the CuO diffraction peak [24].

3.3 FT-IR

The chemical structure and composition of the synthesized samples were determined using Fourier transform infrared (FT-IR) spectra. As shown in Fig. 2c, two absorption bands at 497 cm−1 and 611 cm−1 were observed in all samples, which can be attributed to the Cu–O stretching vibration, indicating the successful formation of monoclinic CuO in the samples. Absorption peaks at 1620 cm−1 were identified as existing water molecules in the products. The peak at approximately 1462 cm−1 is assigned to the stretching vibration of the C–N bond in the PPy ring, while peaks near 1065 cm−1 and 789 cm−1 are ascribed to the C–H in-plane and out-of-plane vibrations in the PPy ring, respectively [31, 32].

3.4 BET

The Brunauer–Emmett–Teller (BET) gas-sorptometry technique was employed to determine pore volume, pore diameter, and specific surface area of the as-prepared CuO micro-flowers, as shown in Fig. 3. The N2 adsorption/desorption isotherms of the synthesized CuO micro-flowers exhibited the typical type IV behavior, which is indicative of mesoporous materials and is characterized by the presence of a hysteresis loop [33]. The BET specific surface area of the synthesized CuO micro-flowers was determined to be 14.504 m2 g−1, while the single point adsorption total pore volume was found to be 0.047757 cm3 g−1. Additionally, analysis of the corresponding Barrett–Joyner–Halenda (BJH) pore diameter distribution curve revealed an average pore size of 13.171 nm.

Fig. 3
figure 3

N2 adsorption–desorption isotherms and the pore size distribution of the CuO micro-flowers

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3.5 Morphology

The present study employed field-emission scanning electron microscopy (FESEM), EDS, and EDX to analyze the morphology, size, and chemical composition of bare CuO particles, PPy nanowires, and CuO/PPy NCs. As demonstrated in Fig. 4, CuO particles exhibit uniform micro-flowers consisting of intermingled ultrathin nanosheets, forming hierarchical nanostructures with a relatively large surface area. This unique structure has been shown to reduce the diffusion length of lithium ions and improve lithium storage. To synthesize CuO micro-flowers, Copper(II) sulfate was employed as a Cu precursor in alkaline aqueous media with the addition of ammonia to increase the solution’s pH. The reaction between Cu2+ and NH4OH led to the formation of Cu(OH)2 precipitates as initial nuclei. Upon microwave irradiation of the solution for 30 min at 900 W, CuO nanoflakes were produced, as indicated by reaction 2.

$${\text{Cu}}^{{{2} + }} + {\text{ 2NH}}_{{4}} {\text{OH }} \to {\text{ Cu}}\left( {{\text{OH}}} \right)_{{2}} \downarrow + {\text{ 2NH}}^{{{4} + }}$$
(1)
$${\text{Cu}}\left( {{\text{OH}}} \right)_{{2}} \left( {\text{s}} \right)\mathop{\longrightarrow}\limits^{\Delta }{\text{CuO }}\left( {\text{s}} \right) + {\text{H}}_{{2}} {\text{O }}\left( {\text{l}} \right)$$
(2)
Fig. 4
figure 4

Low-magnification and high-magnification FESEM, EDS elemental mapping images, and EDX of the synthesized CuO micro-flowers

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It is noteworthy that the presence of excess NH3 and NH4 in the solution may dissolve Cu(OH)2 precipitates, resulting in the formation of CuO particles via the complex reaction shown below:

$${\text{Cu}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ 4NH}}_{{3}} + {\text{ 2NH}}^{{{4} + }} \to \, \left[ {{\text{Cu}}\left( {{\text{NH}}_{{3}} } \right)_{{4}} } \right]^{{{2} + }} + {\text{ 2NH}}_{{3}} .{\text{H}}_{{2}} {\text{O}}$$
(3)
$$\left[ {{\text{Cu}}\left( {{\text{NH}}_{{3}} } \right)_{{4}} } \right]^{{{2} + }} + {\text{ 2OH }} \to {\text{ CuO}}\left( {\text{s}} \right) \, + {\text{ 4NH}}_{{3}} + {\text{ H}}_{{2}} {\text{O}}\left( {\text{l}} \right)$$
(4)

Therefore, it is essential to optimize the ammonia concentration to synthesize CuO nanoflakes successfully. A high ammonia concentration may lead to the dissolution of Cu(OH)2 precipitates, preventing the formation of CuO particles [15].

The microwave-assisted hydrothermal method, which is used, has emerged as a promising approach for synthesizing metal oxide nanostructures, offering a range of benefits. One of the key advantages is the ability to produce nanostructures with fine crystals and controllable morphology. By combining the hydrothermal method with microwave heating, this technique allows for the precise control over the size and shape of metal oxide nanostructures. The result is a high degree of purity and a narrow distribution of grain sizes, which is crucial for many applications. In addition to the refined crystal properties, the microwave hydrothermal method offers rapid synthesis and improved energy efficiency compared to traditional hydrothermal methods. The application of microwave heating reduces the reaction time significantly, enabling the production of metal oxide nanostructures in a shorter duration. Moreover, it reduces energy consumption, making the process more environmentally friendly [34,35,36].

The morphology and chemical composition of synthesized PPy particles were also studied by FESEM and EDX as depicted in Fig. 5. PPy particles exhibit nonwoven structures composed of nanowires with an average diameter of 30 nm, which were formed using cetyltrimethylammonium bromide (CTAB) as a soft template. CTAB is a cationic surfactant with both hydrophilic and hydrophobic moieties, which can form micelles of various sizes and shapes in aqueous media. The morphology of these micelles and, consequently, the morphology of the synthesized PPy can be controlled by varying the solution’s CTAB concentration, temperature, and pH. At specific CTAB concentrations, cylindrical micelles can be formed. By adding pyrrole monomers and ammonium persulfate (APS) to the media, bulk polymerization occurs in the interior of these threadlike micellar aggregates [37, 38]. Figure 6 illustrates the morphology, structure, and chemical composition of CuO/PPy NCs with varying PPy contents (3.5-, 5-, 6.2-, and 8.8-wt% PPy). These high-magnification images indicate that CuO micro-flowers and PPy nanowires are attached in many areas, forming well-distributed composites.

Fig. 5
figure 5

Low-magnification and high-magnification FESEM and EDX of the synthesized PPy nanowires

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Fig. 6
figure 6

FESEM images and EDX of the synthesized CuO/3.5-wt% PPy (a), CuO/5-wt% PPy (b), CuO/6.2-wt% PPy (c), and CuO/8.8-wt% PPy (d)

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3.6 Electrochemical properties of samples

The composite structure of CuO/PPy NCs renders them an auspicious candidate for energy storage applications. Consequently, the electrochemical performance of these composites was scrutinized as an active anode material for Li-ion batteries. For electrochemical analysis, the synthesized samples were deployed as the working electrode in a 2025 coin-type cell. A lithium sheet was used as the counter and reference electrode, while celgard 2300 was utilized as a separator. The cell electrolyte is composed of LiPF6 (1.0 mol L−1) in a 1:1 (w:w) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The entire cell assembly was conducted in an argon-filled glove box, followed by carrying out cyclic discharge/charge measurements on the prepared coin cells.

3.6.1 Discharge/charge profile

Figure 7 presents the galvanostatic discharge/charge profiles of the anodes composed of bare CuO micro-flowers and CuO/PPy NCs with various concentrations of PPy nanowires (3.5, 5, 6.2, and 8.8 wt%). It can be observed that the discharge–charge profiles of CuO/PPy NCs are analogous to that of bare CuO, indicating that the incorporation of organic polypyrrole does not alter the lithium storage behavior of CuO anodes. Specifically, the initial discharge and charge capacities of bare CuO anode are 1010 and 723 mAh g−1, respectively, at a rate of 0.2C. However, due to the formation of irreversible phases during the first charge process, the second discharge capacity decreased to 769 mAh g−1 and gradually dropped to 710 and 630 mAh g−1 after the fifth and tenth cycles, respectively.

Fig. 7
figure 7

Discharge/charge curves at different cycles of bare CuO micro-flowers, CuO/3.5-wt% PPy, CuO/5-wt% PPy, CuO/6.2-wt% PPy, and CuO/8.8-wt% PPy

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In the case of CuO/3.5-wt% PPy, as depicted in Fig. 7, the initial discharge and charge capacities are 931 and 670 mAh g−1, respectively, at 0.2 C and decrease to 673, 610, and 526 mAh g−1 for the second, fifth, and tenth cycles, respectively. For CuO/5-wt% PPy, the first and second discharge and charge capacities are 944, 676, 667, and 626 mAh g−1, respectively, demonstrating a marginal improvement compared to the CuO/3.5-wt% PPy sample. For CuO/6.2-wt% PPy, the first and second discharge and charge capacities are 943, 652, 627, and 588 mAh g−1, respectively. Lastly, for CuO/8.8-wt% PPy, the initial and second discharge and charge capacities are 915, 718, 688, and 571 mAh g−1, respectively.

3.6.2 Differential capacity versus voltage

To elucidate the redox behavior of the samples, Fig. 8 displays the differential capacity versus voltage (dQ/dV) plots of the first, second, fifth, and tenth cycles for both bare CuO micro-flowers and CuO/PPy NCs with varying PPy weight ratios. The peaks observed in the dQ/dV plots represent the plateaus present in the discharge–charge curves, which correspond to the lithiation and delithiation reactions of the samples. As depicted in Fig. 8, three reduction peaks can be observed in the first discharge curves of all samples. These peaks, located at around 2.1, 1.2, and 0.97 V, respectively, are attributed to the insertion of lithium-ion and the formation of [CuII1−xCuIx]O1−x/2 (0 ≤ x ≤ 0.4) solid solution, the formation of Cu2O, and the decomposition of Cu2O into Li2O and Cu. Furthermore, two oxidation peaks are observed in the first charge curves of all samples, appearing at approximately 2.3 and 2.7 V, respectively, which correspond to the delithiation process 2Cu + Li2O → Cu2O + 2Li and the partial oxidation of Cu2O into CuO [18, 39, 40].

Fig. 8
figure 8

Differential capacity versus voltage plots of bare CuO micro-flowers, CuO/3.5-wt% PPy, CuO/5-wt% PPy, CuO/6.2-wt% PPy, and CuO/8.8-wt% PPy

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3.6.3 Cyclic stability of samples

To investigate the impact of PPy nanowires on the cyclic stability and performance of the samples, cyclic discharge and charge were performed for 100 cycles. Figure 9 displays the cyclic performance of CuO/PPy NCs with varying PPy contents, in comparison to pure CuO micro-flowers. It is evident that the cyclic stability of CuO/PPy NCs has been significantly enhanced by increasing the PPy content from 3.5 to 6.2 wt%. Hybrid CuO/PPy NCs exhibit superior cyclic stability as compared to bare CuO micro-flowers. The discharge capacity of CuO micro-flowers was 776 mAh g−1 in the second cycle, which decreased to 89 mAh g−1 after 100 cycles. Thus, the capacity retention of bare CuO was only 11% (compared to the second cycle). In contrast, CuO/3.5-wt% PPy delivered a second discharge capacity of 678 mAh g−1, which was reduced to 128 mAh g−1 after the 100th cycle, with a capacity retention of 18%. Similarly, CuO/5-wt% PPy exhibited second and 100th discharge capacities of 668 mAh g−1 and 231 mAh g−1, respectively, with a retention capacity of 34%, demonstrating slight improvement over bare CuO and CuO/3.5-wt% PPy. CuO/6.2-wt% PPy displayed second and 100th discharge capacities of 627 mAh g−1 and 371 mAh g−1, respectively, with an optimum retention capacity of 60%, which is five times higher than that of the bare CuO sample, thus demonstrating the best cyclic performance among all synthesized NCs in this work. Furthermore, for CuO/8.8-wt% PPy, the retention capacity reached 29% with the second and 100th discharge capacities of 688 mAh g−1 and 200 mAh g−1, respectively.

Fig. 9
figure 9

Comparative cyclic performance of bare CuO micro-flowers and CuO/PPy with different PPy weight ratios

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The improved performance observed in CuO/PPy NCs anodes, in comparison with bare CuO anode, is primarily attributed to the presence of PPy nanowires, which enable accommodation of volume changes during consecutive discharge–charge processes and prevent the active anode material from pulverization and aggregation, thereby increasing its mechanical integrity[24, 25, 41]. As seen in Fig. 9, the capacity of CuO/8.8-wt% PPy increased gradually after the 50th cycle. This phenomenon is commonly observed for metal oxide active anode materials, and it is likely related to the formation of a Solid Electrolyte Interface (SEI) and activation of the anode material. During consecutive discharge–charge processes, the electrolyte decomposes on the surface of the anode, resulting in the formation of a polymeric gel-like layer on the surface of the anode, known as SEI. This layer contains electroactive compounds, such as lithium carbonate and methoxide, which deliver excess capacity at low potential through pseudo-capacitance-type behavior. At this stage, the decomposition quantity of SEI is greater than its formation during the discharge–charge process, resulting in the thickening of the SEI membrane and generating extra capacity. Moreover, successive discharge and charge cycles cause volume variation in the anode active material and increase available surface and Li+ diffusion [42, 43].

4 Conclusion

In this study, we have successfully synthesized CuO micro-flowers, PPy nanowires, and CuO/PPy NCs with varying PPy weight ratios. Our results demonstrate that the PPy content plays a critical role in both the structural characteristics and lithium storage properties of the hybrid CuO/PPy NCs. The presence of PPy nanowires serves to accommodate the significant volume changes experienced by CuO structures during the charge and discharge processes. It prevents anode pulverization due to the elastic nature of the polypyrrole. As a result of the formation of organic/inorganic nanocomposites of CuO/PPy NCs, we observe higher lithium storage capacity, better cyclic stability, and higher capacity retention. Specifically, we find that an increase in the weight percentage of polypyrrole leads to a significant improvement in discharge/charge capacities and cycling performance compared to bare CuO micro-flowers. Notably, we identify the optimized CuO/PPy NCs with 6.2-wt% PPy as having a high initial capacity of 943 mAh g−1 and retaining a high reversible value of 371 mAh g−1 after 100 cycles. These improved performance characteristics can be attributed to the beneficial structural features provided by PPy nanowires.