Introduction

Due to the higher specific capacity than graphite, transition metal oxides (TMO, 600–800 mAh g−1) have attracted much attention recently and have been considered as one of the most promising anode materials for the next generation lithium ion batteries (LIB) [1,2,3,4,5,6,7]. Cupric oxide (CuO) is an important TMO, and it has been widely studied in photocatalysis [8, 9] and LIB [10,11,12,13]. When used as anode material, CuO usually displays large resistance and poor rate performance due to its poor electron and lithium conductivity. Moreover, the remarkable volume variation during the lithium intercalation/extraction leads to rapid capacity decay [14, 15]. Many strategies have been proposed to address these challenges and improve lithium storage performance, including preparing coated nanostructures [16,17,18], fabricating 3D hierarchical structures [19], introducing foam collectors [20,21,22,23], and designing free-standing electrode systems [24, 25]. Although these efforts have achieved tremendous success, CuO anodes exhibit a high irreversible capacity loss in each cycle because of the continuous formation of solid electrolyte interface (SEI) films. Moreover, once a high-current density is applied on the anodes, the electrode film will collapse instantaneously, and its capacity also drops rapidly to a low level, especially seriously in slurry electrodes. It is well known that in slurry electrodes, additional agents, such as conductors and binders, must be required to enhance contact between active materials effectively and integrate coatings with current collectors firmly. Undoubtedly, the binders not only reduce the energy density of the battery but also block the transmission of electrons and lithium, thus lowering the performances of battery. In view of these drawbacks, it is encouraged and crucial to develop free-standing electrodes for high performances of the batteries.

Free-standing electrodes are fabricated without using any polymer binders, and many researches have employed free-standing electrodes to overcome the problems mentioned above [26,27,28,29,30]. Some works have focused on incorporating CuO into a flexible matrix, such as carbon nanotube webs [31] and graphene sheets [32]. Others have employed advanced technologies for in-situ growth of CuO on the metal collectors, such as magnetron sputtering [33,34,35] and electrochemical deposition [36]. In this work, CuO anodes are fabricated by an electrochemical deposition method to form arrayed architectures, in which ordered interspaces among arrays can accommodate volume expansion efficiently. However, it is found that the ordered structure alone is not enough to ensure a high rate performance and a high lithium storage capacity, and usually conductive coating [37, 38] or employing solid electrolyte is necessary. Polypyrrole (PPy) is a favorite conductive polymer and has been used as conductive coating layers to improve the electrochemical performance of electrodes [39,40,41,42]. In numerous reports, PPy coating layers are usually prepared by electrochemical [43] or chemical [44, 45] oxidation techniques from pyrrole monomers in a liquid phase system. The electrochemical polymerization is not suitable for scaling up and cannot be used with non-conducting surfaces, while the traditional chemical oxidation has challenge for forming uniform coating layers due to the easy agglomeration of PPy. In this regard, we develop an evaporation method to make pyrrole vapor in situ polymerization on CuO arrays for the uniform coatings (as shown in Fig. 1). It is found that, with the help of this uniform PPy coating, the prepared CuO film can well maintain the stability of mechanical structures. Also, the uniform coatings can accelerate the transmission of lithium ions and electrons during the discharge/charge processes, hence harvesting a high lithium storage and better c-rate performance.

Fig. 1
figure 1

Schematic diagram of preparation process and the discharge/charge cycles for CuO@PPy integrated anode

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Experimental

Materials synthesis

In this work, CuO arrays were prepared by electrochemical deposition, which was performed on an electrochemical system (chi650E, Shanghai, China) by a typical three-electrode system. The working electrode was 304 stainless steel exposed one side of 1 × 1 cm2 conductive surface, and the counter electrode was a platinum foil with a large area. A saturated calomel electrode (SCE) was used as a reference electrode and the deposition solution was 15 mmol L−1 Cu(NO3)·3H2O. We carried out deposition experiments with a potentiostatic method at room temperature and negative voltage of − 0.82 V (vs. SCE) had been applied for 13 min. After deposition, the as-prepared films were removed from the bath immediately and rinsed with deionized water, and then dried in a stream of N2 gas before being calcined in an oven at 450 °C under air atmosphere for 150 min.

PPy Coatings were prepared by an evaporation method to make pyrrole vapor in situ polymerization on CuO arrays for the uniform coatings. The coating process is as shown in Fig. 1. The as-prepared CuO film was immersed in 0.1 mol L−1 FeCl3 solution to load Fe3+ ions. After being dried in the oven at 60 °C completely, the film was transferred into a sealed culture dish containing 0.5 mol L−1 pyrrole in ethanol solution for 24 h at room temperature. During this period, pyrrole monomers were slowly vaporized together with alcohol to form vapors, which reacted with Fe3+ adhering to the surface of CuO to form uniform PPy coatings. Before being dried completely, the film was washed with ultrapure water to remove the surplus Fe3+.

Materials characterization

The morphologies of the samples were examined by a field-emission scanning electron microscopy (FE-SEM; ZEISS Ultra, Germany) and crystallographic information for the sample was investigated with X-ray powder diffraction (XRD; Bruker D8 Advance, Germany).

Electrochemical measurements

The electrochemical measurements were carried out by using CR 2025 coin-type cells. The working electrode was the as-prepared films. Lithium foil was used as both the counter electrode and the reference electrode. The cell was assembled in a high purity argon-filled glove box. A microporous polypropylene membrane (Cellgard 2300) was used as the separator, and the electrolyte was LiPF6 (1.0 mol L−1) in a 1:1 (w:w) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Charge/discharge tests were performed using a LANHE battery testing system (CT2001A, Wuhan, China) at different current rate with a voltage windows of 0.05–3 V (vs. Li+/Li).

Results and discussion

It is challenging to obtain pure CuO samples by electrochemical deposition due to the variable valence of copper. Figure 2a displays the potentiodynamic curve helping to set an appropriate potential for preparing CuO samples. It can be seen that the curve is divided into four voltage regions roughly, which corresponds to different reactions. Combining with the color changing of the samples, we can infer that Cu2+ ions are mainly reduced to Cu at lower deposition potentials (range II) while are combined with OH− (NO3− + H2O + 2e− → NO2− + 2OH−) to form Cu(OH)2 or Cu (NO3)2·3Cu(OH)2 at higher deposition potentials (range IV). In view of the quality of the films and the efficiency and facility of the preparation process, we select − 0.82 V within the range III as the deposition potential in this work. XRD diffraction patterns indicate that samples prepared by electrochemical deposition before calcination are composed of Cu and Cu2O (Fig. 2b-ii) and have been changed to CuO completely after calcination at 450 °C for 150 min (Fig. 2b-iii). To further confirm the presence of PPy in CuO@PPy sample, the Fourier transform infrared (FT-IR) was carried out, and the result was shown in Fig. 2c. Clearly, the characteristic peaks at 2920 and 2865 cm−1 are ascribed to the stretching vibration of C‑H, 1580 and 1455 cm−1 are due to the antisymmetric and symmetric ring-stretching modes of the PPy rings [46], and 1385, 1280, 1045, 970 cm−1 are indexed to C‑N stretching, C‑N in-plane deformation, C=C‑H the in-plane and out-of-plane deformation vibrations in the pyrrole ring, respectively [47, 48]. There are two strong absorption bands observed at about 478 and 415 cm−1 in the low wavenumber region assigning to the Cu‑O stretching vibration [42]. The above results indicate that the CuO@PPy composite films are successfully prepared.

Fig. 2
figure 2

a Potentiodynamic curve to determine the preparation potential of CuO, b XRD diffraction patterns of stainless steel substrate (i), initial sample before calcinations (ii), CuO sample after calcinations (iii) and CuO@PPy sample (iv) and c the FT-IR spectroscopy of the CuO@PPy sample

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It can be seen from Fig. 3a that the prepared CuO film is composed of uniform particles to form ordered architectures, and the size of each particle is about 100–130 nm. More carefully, observing from Fig. 3b, we found these particles are aggregated from smaller nanoparticles (less than 15 nm). It is well known that, in order to solve the volume changing problems, one effective way is to create nanostructured CuO, and we believe the prepared CuO here can also play an important role in solving the capacity fading of CuO. Also, plenty of gaps can be found between particles, and undoubtedly these gaps can not only accommodate the volume expansion and relieve the stress caused by the expansion of CuO during the discharge processes but also provide convenience for in situ polymerization of pyrrole. Figures 3 c and d show SEM images of the coated CuO sample with PPy by the evaporation method. Clearly, no agglomeration of PPy can be observed and the coating layer is found to be uniform on CuO arrays. Due to the uniform coatings, ordered CuO@PPy nanostructures can well maintain the stability of mechanical structures during the charge/discharge processes. As shown in Fig. 3 e and f, it can be seen that the structures of the pure CuO have almost collapsed and few active materials remain on the electrode after 100 cycles. While for CuO@PPy anodes, the electrode structures maintain intact and the surface becomes more smooth. Undoubtedly, the protection of uniform PPy coating prevents CuO arrays falling off the substrate, suggesting good stability of the battery. Figure 3g‑i shows cross-sectional images of the samples at different states. It is notable that the large volume expansion of CuO anode has occurred during the first discharge process and the following cycles’ no significant volume changing can be observed. As illustrated in Fig. 1, CuO was gradually transformed to Cu2O and Li2O during the first discharge process, and the solid electrolyte interface (SEI) films were formed simultaneously, in which there is about 140% volume expansion. However, SEI films became stable and phase change occurred between Cu2O and Cu, and the volume is no longer to change [49]. Based on this result, it can be inferred that PPy coatings can not only stabilize SEI films but also maintain electrode structure intact in the following cycles.

Fig. 3
figure 3

a, b SEM images of pure CuO and c, d CuO@PPy anode before cycle and e, f after 100 cycles, respectively; the cross-sectional images of samples from pure CuO to g, h the 100th charge and i CuO@PPy

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To well understand the functions of the PPy coatings, the electrochemical performances as anode materials for LIBs were evaluated. The cyclic performances and coulombic efficiencies (Fig. 4c, d) indicate that the specific capacity as well as the stability have been improved greatly for CuO@PPy anodes. The better coincidence of the impedance curves of CuO@PPy after cycling also indicates that the addition of PPy can promote the formation of SEI effectively. It can be seen from Table 1 that arrayed structures make the stability of pure CuO improved in certain degree, but the specific capacity is not enhanced significantly, and the PPy coatings make the discharge specific capacities enhanced more than 20%, and it has reached to 35.1% at 0.2 C for the 100th cycle. The stability of CuO@PPy is as good as expected when the battery is performed at low-current charge/discharge cycles, the capacity is not loss at 0.2 C from the 1st to the 100th cycles, while it is only 3.4% and 6.9% for 1 C and 2 C, respectively. As to pure CuO anodes, the reason can be explained by the different mechanisms of electrochemical process shown as Fig. 4e, f. On the basis of reported results, the electrochemical process controlled by semi-infinite linear diffusion when the slope is near to 0.5 and dominated by surface capacitive-controlled process when it near to 1 [50]. At lower current densities, the electrochemical process controlled by semi-infinite linear diffusion and the capacity is mainly contributed to the embedding reactions of lithium in the bulk of CuO, in which the effect of phase transition is more significant. It is known that the phase transition may cause a large volume changing, which not only lead to the shedding of active materials but also the repeated forming of SEI films [51]. On the contrary, at higher current densities, the process is dominated by surface capacitive-controlled process, and the bulk diffusion of lithium ions is no longer the main reactions. The redox reactions on/near the surface of active materials are the main process, which is the pseudocapacitive charge storage [52]. In CV curves of CuO and CuO@PPy samples, the reduction peaks appearing at 0.67 V, 1.22 V can be assigned to the Cu2O + CuxOy → Cu + Li2O electrochemical reactions. At the same time, the oxidation peak appearing at 2.52 V is the transition of Cu → Cu2O. It can be seen from Table 2 that the binder-free PPy@CuO anode shows better performance by comparison with other people’s work [26, 29, 30] and better performance than some other anodes prepared by coatings [12, 14, 20].

Fig. 4
figure 4

The cyclic performances and coulombic efficiencies, impedance curves at different cycles, and CV curves at different scan rates from 1 to 5 mV s−1, and the determination of the slope values calculated by peak current and scan rate at reduction states (insert plots); a, c, e pure CuO and b, d, f CuO@PPy

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Table 1 Discharge specific capacities of the 1st and the 100th for CuO and CuO@PPy anodes at various current densities
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Table 2 Performance comparison with previously published literature
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Further investigation indicates that the amount of PPy coatings has a great influence on the electrochemical performance of CuO. Here, the coating amount of PPy is determined by the concentration of Fe3+, and the higher the concentration of Fe3+, the larger the coating amount of PPy. As shown in Fig. 5a, three type of CuO@PPy samples harvest higher specific capacities than the pure CuO sample from the initial cycle to the final cycle. Carefully analyzing, we find more than half of the enhanced capacity (about 150 mAh g−1) is contributed to the lithium storage of PPy, which have about 80–90 mAh g−1 of theoretical specific capacity [39, 53, 54]. The result of the study does not mean the more the amount of PPy is, the more the capacity from PPy is increased. Clearly, when the concentration of Fe3+ is reached to 0.4 M, the stability and the capacities of the battery are also dropped greatly. The reason is that the high concentration of Fe3+ can accelerate polymerization of pyrrole, leading to ununiform coatings and surface accumulation of PPy on CuO (as shown in Fig. 5a insertion). As a result, 0.1 M Fe3+ is best for uniform coating PPy on the surface of CuO arrays, and the battery has higher capacity and better stability. Moreover, as shown in Fig. 5b (▲ marked), the capacity fading is only to 12%, 11%, 9.7%, and 8.1% when the current increases from 0.2 C to 0.5 C, 1 C, 1.5 C, and 2 C. CuO@PPy with proper coating (such as 0.1 M) exhibits an excellent rate performance and a capacity retention of 94.7% when the current returned from 2 to 0.2 C.

Fig. 5
figure 5

The effect of PPy on the a stability and b c-rate performances of batteries

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Conclusions

An effective method was reported in this paper for preparing uniform PPy coatings of CuO arrays. The amount of the PPy coatings can be controlled by the concentration of Fe3+ easily and the stability and the capacity of CuO anodes can be improved greatly with the help of the uniform PPy coatings. Moreover, it is found that the large volume expansion of CuO anode has occured during the first discharge process, and the following cycles no volume changing can be observed. It can be inferred that PPy coatings can not only stabilize SEI films but also maintain electrode structure intact in following cycles.