To analyze the phase composition of the synthesized materials, XRD characterization was performed on the target and comparison materials. Figure 2(a) shows the XRD spectra of the comparison material ZnO@NF, the diffraction peaks at 2θ = 32.1°, 34.5°, 36.5°, and 57.2° correspond to the (100), (002), (101), and (110) crystal planes of ZnO (JDPDS NO. 75-1526), respectively, demonstrating that ZnO on the surface of NF was successfully synthesized. The XRD spectra of the NiCo2O4@NF composites are shown in Fig. 2(b). The diffraction peaks at 2θ = 44.7°, 52.0°, and 76.5° of the composites correspond to the (111), (200), and (220) crystal planes of Ni (JDPDS NO.87–0712), respectively. The diffraction peaks at diffraction angles 2θ = 18.9°, 31.2°, 38.4°, 59.1°, and 65.0° are attributed to the (111), (220), (222), (511), and (440) crystal planes of NiCo2O4 (JDPDS NO. 73-1702). Figure 2(c) shows the XRD spectral characterization of the target material ZnO/NiCo2O4@NF, which was successfully synthesized as a ZnO/NiCo2O4@NF composite grown in situ on NF by comparing with the PDF card. The intensity of the diffraction peaks of the synthesized material is relatively low due to the three strong diffraction peaks of the monolithic nickel, but still, obvious diffraction peaks can be seen in this figure, and the presence of the elements Ni, Co, and O can also be proved by the Mapping characterization of NiCo2O4@NF composite material shown in Fig. 2(d), and the distribution of the three elements is very homogeneous on the NF, which once again confirms the synthesis of NiCo2O4@NF composites and shows that the NiCo2O4@NF material grows uniformly on the NF.
To observe the morphological characteristics of the composites, the target materials and two comparison materials were subjected to scanning electron microscopy (SEM) characterization of the target material and the two comparison materials. Figure 3 (a, b) show the morphological features of the NiCo2O4@NF composite material at different magnifications. A unique loose and porous nano-flake structure is formed in situ on the foam nickel substrate, its pores diameter is 40–70 µm. The nano-flakes exhibit a thickness of only a few tens of nanometers and are irregularly distributed on the surface of the NF. Figure 3 (c, d) present the morphological features of the reference material ZnO at different magnifications. ZnO exhibits a disordered distribution of short rod structures on the microscale, randomly dispersed on the NF substrate. The appearance of a few conical clusters in the image may be attributed to the corrosion of the foam nickel substrate by the ZnO solution during the preparation process, resulting in the formation of a small amount of NiO morphology.
Figure 3 (e, f) display the surface morphology of the ZnO/NiCo2O4@NF composite material. It is evident from the images that a layer of short rod-like ZnO structures has grown on the surface of the NiCo2O4@NF composite material. The diameter of these rods ranges from 40 to 60 nanometers, with lengths varying between 100 and 400 nanometers. Notably, a comparison between Fig. 3 (b) and Fig. 3 (e) reveals a significant increase in the synthesized ZnO when ZnO grows on the surface of the NiCo2O4@NF composite material. This increase can be attributed to the fact that the sheet-like porous structure of the NiCo2O4@NF composite material, with its diameter-matched pores of 400–700 nanometers, provides a favorable surface for the attachment and anchoring of ZnO nanorods. Consequently, the quantity of hydrothermally synthesized nano-rods is enhanced. The uniform coating of ZnO on the surface of the NiCo2O4@NF composite material is beneficial for improving the fragility associated with its loose and porous structure, thereby enhancing structural stability.
The target material was utilized as a lithium-ion negative electrode material and assembled into a button cell to evaluate its electrochemical performance. Figure 4(a) illustrates the cyclic voltammetry (CV) curves of the NiCo2O4@NF electrode, ZnO, and ZnO/NiCo2O4@NF electrode within the voltage range of 0.01-3.00 V at a scan rate of 0.1 mV s− 1. The reduction peak of NiCo2O4@NF appears at 1.06 V during the negative scan, corresponding to the reduction process of Ni2+ and Co2+ to metallic Ni and Co, as indicated by the discharge plateau at 1.3 V in Fig. 4(b) of the NiCo2O4@NF discharge curve. In Fig. 4(a), the oxidation peaks observed at 0.3, 0.4, 0.55, 0.7, and 1.4 V during the positive scan for ZnO and ZnO/NiCo2O4@NF are associated with the oxidation of metallic Zn to Zn2[21]. The oxidation peak observed at 2.25 V for ZnO/NiCo2O4@NF corresponds to the oxidation of metallic Ni and Co to Ni2+ and Co2+. The reduction peak at 1.1 V during the negative scan for ZnO/NiCo2O4@NF corresponds to the discharge plateau at 1.35 V in the galvanostatic charge-discharge curve. As shown in Fig. 4(a), The peak potential gaps for NiCo2O4@NF, ZnO/NiCo2O4@NF, and ZnO are 1.217 V, 1.15 V, and 1.143 V, respectively. It is evident that after the anchoring of ZnO, NiCo2O4@NF composite material has a stronger anodic/cathodic peak intensity and a smaller potential gap between the reduction and oxidation peaks which indicates faster Li+ diffusion kinetics and lower polarization[22].
Cycling and rate capability tests were conducted to investigate the electrochemical performance of the ZnO/NiCo2O4@NF electrodes in button battery. Figure 5(a) presents the capacity and stability tests of the electrodes after 100 cycles at a current density of 100 mA∙g− 1. The NiCo2O4@NF electrode, ZnO electrode, and ZnO/NiCo2O4@NF electrode exhibited initial discharge specific capacities of 1516.6/1116.9 mAh∙g− 1, 1133.1/722.6 mAh∙g− 1, and 1460.1/944.6 mAh∙g− 1, respectively. The corresponding first-cycle coulombic efficiencies were 73.6%, 63.8%, and 64.7%. After 100 charge-discharge cycles, the discharge-specific capacities of the NiCo2O4@NF, ZnO@NF, and ZnO/NiCo2O4@NF electrodes reached 313.8, 245.4, and 475.2 mAh∙g− 1, respectively. The capacity retention rates after 100 cycles, relative to the second cycle, were 28.2%, 34.6%, and 51.5% for the three electrodes, respectively. The ZnO/NiCo2O4@NF electrode exhibited the highest capacity and capacity retention rate after 100 cycles. The addition of ZnO enhanced the stability of the material, resulting in slower capacity decay during cycling.
Figure 5(b) shows the rate capability test results of the NiCo2O4@NF, ZnO@NF, and ZnO/NiCo2O4@NF electrodes. At current densities of 100, 200, 400, 800, and 1600 mA∙g− 1, the discharge capacities of the NiCo2O4@NF electrode were 1409.8, 1261.8, 1021.1, 836.9, and 658.4 mAh∙g− 1, respectively. The corresponding specific capacities of the ZnO@NF electrode were 541.2, 377.5, 296.8, 244.6, and 204.5 mAh∙g− 1, while the ZnO/NiCo2O4@NF composite electrode exhibited specific capacities of 828.6, 657.2, 499.8, 432.4, and 357.3 mAh∙g− 1. When the current density was restored from 1600 mA∙g− 1 to 100 mA∙g− 1, the capacities of the NiCo2O4@NF electrodes were 917.0, 342.5, and 540.1 mAh∙g− 1, respectively. The capacity loss rates, relative to the initial cycle at 100 mA g− 1, were 35.0%, 36.7%, and 33.9% for the NiCo2O4@NF, ZnO@NF, and ZnO/NiCo2O4@NF composite electrodes, indicating relatively good reversibility for the ZnO/NiCo2O4@NF composite electrode. The improved reversibility is likely attributed to the anchoring of the ZnO nanorod with high mechanical strength on the surface of the ZnO/NiCo2O4@NF composite electrode, which enhances its structural stability under high current conditions.
To further analyze the factors influencing the electrochemical kinetics of the materials, cyclic voltammetry (CV) tests were conducted on the three electrodes at different scan rates. Figure 6 (a, c, e) displays the CV curves of the NiCo2O4@NF, ZnO@NF, and ZnO/NiCo2O4@NF electrodes at scan rates of 0.2, 0.4, 0.6, 0.8, and 1.0 mV∙s− 1. With different scan rates, the CV curves of the three electrodes exhibit similar shapes, with an increase in the magnitude of oxidation/reduction peaks and slight shifting, indicating minor polarization of the electrode materials. Figure 6 (b, d, f) presents the plots of logv versus logi for the cathodic and anodic scans. The relationship between the current density i and the scan rate ν follows the equation logi = b*logν + loga, where the value of adjustable parameter b reflects the controlling process during electrode charge and discharge. The b values for the NiCo2O4@NF electrode are 0.44 and 0.6, while for the ZnO@NF electrode, the b values are 0.68 and 0.7. As for the ZnO/NiCo2O4@NF electrode, the b values are 0.5 and 0.64. Overall, the NiCo2O4@NF electrode is primarily controlled by the diffusion process, while the ZnO@NF electrode is controlled by a combination of capacitance and diffusion processes. The ZnO/NiCo2O4@NF electrode exhibits b values between those of the other two electrodes, indicating that the diffusion process remains the primary control mechanism.
Finally, the electrochemical impedance spectroscopy of the three electrodes was further discussed and analyzed. Figure 7(a) shows the Nyquist plots of the NiCo2O4@NF, ZnO@NF, and ZnO/NiCo2O4@NF electrodes before the charge-discharge cycling test. The plots for all three electrodes exhibit similar shapes, consisting of a semicircle and a sloping line. The semicircle in the high-frequency region represents the charge transfer impedance, while the sloping line in the low-frequency region indicates the Warburg diffusion impedance. From the graph, it can be observed that the charge transfer impedances of those electrodes are 908.5, 1162.9, and 245.8 Ω, respectively. The ZnO/NiCo2O4@NF electrode exhibits the smallest charge transfer impedance, which may be attributed to the formation of a dense layer of ZnO nanorods, increasing the contact area with the electrolyte.
Figure 7(b) shows the diffusion impedances of Li+ within the materials for the three electrodes. It can be seen that the ZnO/NiCo2O4@NF electrode has the lowest diffusion impedance, indicating a relatively lower impedance within the material. Figure 7(c) presents the impedance test curves of the NiCo2O4@NF, ZnO@NF, and ZnO/NiCo2O4@NF electrodes after 30 charge-discharge cycles. It is worth noting that the NiCo2O4@NF electrode exhibits a semicircle and a sloping line, while the ZnO@NF and ZnO/NiCo2O4@NF electrodes consist of two semicircles and a sloping line. The presence of two semicircles in the latter two electrodes indicates the existence of two interface electron transfer processes. The appearance of two semicircles in the ZnO@NF electrode is attributed to the interface between the rod-like ZnO and the accompanying clustered NiO with the electrolyte, while the two semicircles in the ZnO/NiCo2O4@NF electrode arise from the interfaces formed between the rod-like ZnO and the porous sheet-like NiCo2O4@NF material with the electrolyte. The smallest semicircle in the Nyquist plot of the ZnO/NiCo2O4@NF electrode indicates the lowest charge transfer impedance, while Fig. 7(d) also shows the smallest diffusion impedance after 30 cycles among the three electrode materials, reflecting optimal interface charge transfer rate and bulk lithium-ion transport rate.