Introduction

Lithium-ion batteries (LIBs) as one of the most promising rechargeable energy storage devices are widely applied in electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1,2,3]. The anode material often plays an important role in the determination of the energy density, safety, and cycling life of LIBs. Graphite has been widely used as the commercialized anode due to its excellent cycling stability and relatively small volume change of only 12% during lithiation/delithiation [4, 5]. However, this material offers low theoretical capacity (LiC6, 372 mAh/g) and low delithiation potential (0.05 V vs. Li+/Li) [6, 7]. Obviously, it is an unsuitable anode material for the next-generation LIBs required for smart electrical grid systems and wearable electronic devices. It is required to construct novel anode materials of higher Li-storage capability and operational safety than graphite. The primary candidates are transition metal oxides (TMOs) due to special lithium oxidation reduction storage mechanism in a reversible manner, resulting in high theoretical capacities [8].

Among the various TMOs, bimetallic oxide, with synergetic enhanced activities by modifying two components with each other, usually gains better performance. NiCo2O4 as one of bimetallic oxides with high theoretical specific capacity (891 mAh/g) is considered as a promising electrode material [1]. Moreover, NiCo2O4 possesses higher electronic conductivity compared with NiO or Co3O4, which is beneficial for the electron transfer during cycling [9]. In addition, the Co and Ni elements are low-toxicity during melting process, low cost and abundant in the earth, which improves the attraction of NiCo2O4. Nevertheless, during the charging/discharging process, the sluggish reaction kinetics and drastic volume change lead to poor rate performance and short cycling life, restricting its commercial application in LIBs. Numerous efforts have been made to tackle this issue. One is to construct nanostructured NiCo2O4 to reduce the diffusion distance of Li ion during charging/discharging process and thus alleviate the volume expansion/shrinkage [10,11,12]. For example, Anchali Jain et al. synthesized porous NiCo2O4 nanodisks through facile and straightforward hydrothermal process. The electrode delivered a discharge capacity of 673.9 mAh/g after 350 cycles at the current density of 0.5 C [13]. Jin et al. fabricated hydrangea-like NiCo2O4 through a solvothermal method. At a current density of 100 mA/g, the discharge specific capacity was up to 928 mAh/g after 100 cycles [14]. Although many methods have been developed to fabricate nanostructural NiCo2O4 anode materials with good Li ion storage properties, a simpler and proper synthetic route to a mass production of NiCo2O4 with high performance is still need to be explored.

Dealloying has been recognized to be an effective strategy for the fabrication of functional TMOs via selective dissolution of one or more active elements out of suitable precursor alloys [15, 16]. Ding prepared hierarchically porous MnOx microspheres by dealloying method followed by mediate temperature annealing. The porous MnOx microspheres showed a promising electrochemical performance as an advanced anode material in LIBs with a specific capacity of 757 mAh/g at 500 mA/g after 100 cycles [17]. Liu et al. designed porous CoFe2O4 nanoplates through dealloying Co4Fe8Al88 precursor ribbons. The as-obtained CoFe2O4 nanoplates exhibited high capacity, long cycle life, and good rate capability as an anode material for LIBs [18]. In present work, we provide a modified dealloying process to the fabrication of dendritic micro-nanostructural NiCo2O4 anode material. The different concentrations of H2O2 solution are controlled during the dealloying process, and two typical nanostructured products are obtained by this melt spinning and dealloying method. The electrochemical performances of these nanocomposites are investigated in details. The dendritic micro-nano NiCo2O4 electrode with high surface area shows high specific capacity and superior cycling reversibility, showing the encouraging application potential as an advanced anode material for LIBs.

Experimental method

The synthesis procedure of NiCo2O4 is schematically illustrated in Fig. 1 (see the supporting information for experimental details). Al-based CoNiAl alloy is chosen as the precursor alloy due to the more reactive property and low cost of Al. NiCo2O4 prepared by dealloying in sodium hydroxide is recorded as NiCo2O4-NaOH, while in the mixture solution of sodium hydroxide and hydrogen peroxide is marked as NiCo2O4-NaOH+H2O2. The annealed bimetallic oxides are recorded as NiCo2O4-NaOH-500 and NiCo2O4-NaOH+H2O2-500, respectively.

Fig. 1
figure 1

Schematic illustration of the formation process of NiCo2O4

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Results and discussion

Structural and morphologies

Thermogravimetric analysis (TGA) is carried out from room temperature to 600 °C with temperature gradient of 10 °C/min in air. Figure 2 shows the typical TGA curves of NiCo2O4-NaOH+H2O2-500, which can be used to demarcate the actual content of each component and investigate the thermal stability of NiCo2O4. The physically combined water molecules are the first to leave below 100 °C. The gradual weight loss of about 24.5% can be attributed to oxidation of the Ni/Co hydroxide as the temperature is increased to 500 °C and the remaining component is NiCoOx [19]. After 500 °C, no obvious decline in quality is observed, indicating that NiCoOx is relatively stable in composites.

Fig. 2
figure 2

TGA curve of the NiCo2O4-NaOH+H2O2-500

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To confirm the phase structure of NiCoOx composites, X-ray diffraction (XRD) is conducted, as shown in Fig. 3a. It is obvious that the major diffraction peaks agree with the standard reference pattern of NiCo2O4 (PDF# 73-1702). This result indicates that the utilized dealloying condition is favorable for the formation of pure NiCo2O4. XPS spectra are acquired to analyze the electronic structures of NiCo2O4. Figure 3b shows the Ni 2p XPS spectrum. The spectrum can be deconvoluted into four peaks, including Ni 2p3/2 (853.8 eV), Ni 2p1/2 (873.1 eV) and two satellite peaks [20]. An XPS high-resolution scan of the Co 2p core level is shown in Fig. 3c. The Co 2p3/2 peak and Co 2p1/2 peak are centered at 779.9 eV and 794.8 eV, being consistent with the bivalent oxidation state of Co. The peaks at 783.9 eV and 804.1 eV are corresponding shakeup satellites of Co 2p3/2 peak and Co 2p1/2 peak, respectively, which further confirmed that Co mainly exists in the Co2+ and Co3+ state [21]. Furthermore, the O 1 s core level spectra (Fig. 3d) can be resolved into three peaks, centered at 530.2, 531.8, and 533.0 eV, respectively. The low-binding-energy peak observed at 530.2 eV is attributed to OM oxygen, corresponding to O2- ions in transition metal oxide. The latter two peaks are assigned to OH and OH2, respectively [22]. The XPS results indicate that the metal elements in the NiCo2O4 composites exist in the form of Ni2+, Ni3+, Co2+, and Co3+, which are in good agreement with results of NiCo2O4 in the literature.

Fig. 3
figure 3

(a) XRD patterns of the dealloyed NiCo2O4-NaOH+H2O2-500 with reference to the standard spectrum card. XPS spectra of the NiCo2O4: (b) Ni 2p, (c) Co 2p, (d) O 1 s

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We have investigated the effect of H2O2 and annealing on the controllable morphology. The morphology and microstructure of as-prepared samples are investigated by SEM and TEM. Figure 4a and b show that the NiCo2O4-NaOH precursors are mainly irregular nanosheet with thickness of ~ 30 nm. As shown in Fig. 4c and d, in the present of H2O2, thinner nanosheets will form, which will eventually evolve into more uniform aggregates suggesting the role of H2O2 to provide centers of heterogeneous nucleation during processing [23]. After thermal decomposition at 500 °C, these nanosheets transform into dendritic structure along with the thin nanosheets turning into nanorods (Fig. 4e, f), forming a unique hierarchitecture with two degrees of structural characteristics at both nanometer and micrometer scales. Apparently, such in-site growth of dendritic structure not only improves the structural integrity but also effectively reduces the contact resistance among nanorods. TEM image provides the detailed microstructure of the obtained NiCo2O4-NaOH+H2O2-500 sample. As displayed in Fig. 4g, the sample consists of a large number of closely packed nanoflakes with a length of ~ 30–40 nm and a thickness of ~ 2–3 nm, which is in accordance with that from SEM observation. HRTEM image (Fig. 4h) provides more detail, in which the clear lattice fringes can be easily observed, with the spacing calculated to be ~ 0.25 nm, which can be ascribed to the (311) planes of NiCo2O4. Through the comparison of Fig. 4a–h, it is found that a multistage dendritic rod-shaped micro-nanostructure assembled from nanosheets can be controllably prepared by a modified dealloying method followed by mediate temperature annealing. A novel idea of designing micro-nanostructure is presented.

Fig. 4
figure 4

SEM images of the NiCo2O4 electrodes: (a–b) NiCo2O4-NaOH, (c–d) NiCo2O4-NaOH+H2O2, (e–f) NiCo2O4-NaOH+H2O2-500. (g) TEM image and (h) HRTEM image of NiCo2O4-NaOH+H2O2-500

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Figure 5a–d show the element mapping analysis of NiCo2O4-NaOH+H2O2-500. The distribution of Co, Ni, O, and Al are relatively uniform in the dendritic micro-nanostructure. As depicted in Fig. 5e, the elements in the selected region are Co, Ni, O, and Al, and most abundant O element is observed, which reconfirms that the main component is in the presence of NiCo2O4 [24,25,26]. The atom ratio of Ni and Co is approximately 1:2, which is close to that in the precursor ribbons [27]. According to the dealloying mechanism, the residual Al (several atom percent) is detectable in the as-dealloyed sample [28]. The results of element mapping and energy spectrum analysis indicate that NiCo2O4 material exists in oxidation state and the distribution of Co, Ni, and O elements is relatively uniform.

Fig. 5
figure 5

The element mapping images for (a–d): O, Al, Co, and Ni, respectively. (e) Energy spectrum analysis of NiCo2O4-NaOH+H2O2-500

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Electrochemical performance

Electrochemical tests including cyclic voltammetry (CV) and galvanostatic discharge-charge (GDC) cycling were conducted to evaluate the performance of the NiCo2O4 electrode. Figure 6a shows the CV curves of the first three cycles in the voltage range of 0.01–3.0 V (vs. Li+/Li) at a scan rate of 0.1 mV/s. In the first discharge process, the main peaks at 0.3 V and 0.75 V could be attributed to the reduction of Ni2+, Ni3+, Co2+, and Co3+ to their metallic states, respectively [19]. The small broad peak located at 0.10 V refers to the formation of a solid/electrolyte interface (SEI) generated from the decomposition of electrolyte. The electrochemical reaction of discharge can be shown as:

Fig. 6
figure 6

(a) CV curves of the first three cycles for the NiCo2O4-NaOH+H2O2-500 nanocomposites at a scan rate of 0.1 mV/s in the voltage window of 0.01–3.0 V (vs. Li+/Li). (b) Discharge/charge profiles of the three cycles for the NiCo2O4-NaOH+H2O2-500 nanocomposites at a current density of 100 mA/g. (c) Cycling performance of NiCo2O4-NaOH, NiCo2O4-NaOH-500, NiCo2O4-NaOH+H2O2, and NiCo2O4-NaOH+H2O2-500 at a current density of 100 mA/g

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NiCo2O4 + 8Li+ + 8e- → Ni + 2Co + 4Li2O

In the following cycles, the peaks shift to 0.85 V and 1.5 V. The obvious difference between the peaks in the first and subsequent cycles is mainly due to the irreversible capacity loss of the anode materials observed during the first cycle, as well as the polarization of the electrode material [29, 30]. In the charge process, there are three oxidation peaks at 0.47 V, 1.4 V, and 2.3 V, which probably related to the multistep oxidation of metallic Ni and Co to their oxide states, together with the decomposition of SEI film and Li2O [31, 32]. After the first cycle, the redox peaks almost remain consistent, indicating relatively good reversibility of the NiCo2O4 nanocomposites.

The initial three discharge/charge cycles of NiCo2O4 nanocomposites at 100 mA/g are displayed in Fig. 6b. During the first discharge process, it exhibits a conspicuous voltage plateaus between 1.0 and 0.6 V and a total capacity of ~ 868.5 mAh/g, while it delivers an initial charge capacity of 522.7 mAh/g. The irreversible capacity of the first cycle is attributed to the formation of SEI [33]. There are obvious charging platforms at 0.3–0.47 V, 0.47–1.4 V, and 1.4–2.3 V, attributing to the oxidation of Ni and Co and the decomposition of SEI [34, 35]. In the subsequent cycle, the discharge voltage platform shifts to 1.5 V. Four obvious discharge platforms are observed (between 3 and 1.5 V, 1.5–0.85 V, 0.85–0.3 V, and 0.3–0.2 V), which agrees with the CV results. The discharge/charge profiles almost overlap after the first cycle, indicating an excellent stability after the initial.

To better understand the electrochemical performance of the new electrode, we compared the cycling performance of NiCo2O4-NaOH, NiCo2O4-NaOH-500, NiCo2O4-NaOH+H2O2, and NiCo2O4-NaOH+H2O2-500 electrodes at the same current density, as shown in Fig. 6c. It is noteworthy that the NiCo2O4-NaOH+H2O2 electrode shows higher capacities than NiCo2O4-NaOH and NiCo2O4-NaOH-500 before 30 cycles, followed by a quick fall in capacity in prolonged cycles. The capacities of the uncalcined electrode all fade quickly after several cycles. Remarkably, the NiCo2O4-NaOH+H2O2-500 electrode exhibits excellent cyclic stability. These capacities of 1537.4 mAh/g and 1117.4 mAh/g are obtained at first and second cycle. Reversible capacity as high as 1019.6 mAh/g after 100 cycles can still be obtained at a current density of 100 mA/g, which is higher than the theoretical capacity. The SEM image indicates that the samples without H2O2 are mainly irregular nanosheets, while the samples with H2O2 have good crystallinity and exist as dendritic rod-shaped structures. After thermal decomposition at 500 °C, these nanosheet materials transform into micro-nanostructure, forming a unique hierarchitecture with two degrees of structural characteristics at both nanometer and micrometer scales. The improved cycling stability should be attributed to the micro-nanostructure assembled by dendritic rod, which can not only buffer the huge volume change of active material during discharge/charge process but also enhance the lithium-ion diffusion and electron transport [36, 37]. Table S1 compares the key electrochemical performance parameters of NiCo2O4 and the relevant TMOs between literature results and present work. The optimized NiCo2O4-NaOH+H2O2-500 nanocomposites have excellent cycling stability.

To better understand the superior cycling performance of NiCo2O4-NaOH+H2O2-500 electrode, the morphology after 100 discharge/charge cycles at 100 mA/g is investigated as presented in Fig. S1. The surface becomes rough, and some areas are clogged upon cycling. Nevertheless, the overall dendritic morphology is basically retained, indicating a good structure stability of NiCo2O4-NaOH+H2O2-500.

Dealloying time is an important parameter, and thus we investigate the electrochemical behaviors of NiCo2O4-NaOH+H2O2-500 materials obtained with controlled dealloying times (24 h, 48 h, and 64 h). For convenience, the fresh products dealloyed by different time are labeled as D-24 h, D-48 h, and D-64 h, respectively. Figure 7a depicts the cycling performance at the current density of 100 mA/g, both of which show a similar capacity trend. In particular, the D-64 h exhibits excellent cycle stability, retaining reversible specific capacity as high as 1016.9 mAh/g after 100 cycles, whereas comparable discharge capacities of 792.8 and 1009 mAh/g are obtained for D-24 h and D-48 h, respectively.

Fig. 7
figure 7

(a) Cycling performance of NiCo2O4-NaOH+H2O2-500 nanocomposites prepared with different dealloying time at 100 mA/g. (b) Rate capabilities of NiCo2O4-NaOH+H2O2-500 nanocomposites prepared by dealloying 24 h at a current density of 100 mA/g. (c) 48 h. (d) 64 h

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Apart from the high specific capacity and good cyclability, rate capability is another very important property for high-performance LIBs. Figure 7b–d show the rate performance of D-24 h, D-48 h and D-64 h electrodes at various current densities between 3.0 and 0.01 V. At the current densities of 100, 200, 500, and 1000 mA/g, the D-64 h anode shows the good rate capacity, with an average discharge capacity of 1385.2, 1160.3, 940.4, and 691.4 mAh/g, respectively. Another impressive result is the restoration of the D-64 h after high rate cycling. When the current density restores to 100 mA/g, the cell can recover a high reversible capacity of 1281.5 mAh/g. The reversible capacity of D-48 h is 867, 727.2, 530, 409.8, and 738 mAh/g, respectively, and meanwhile, the D-64 h anode shows the average capacity of 832.2, 695.1, 464, and 259.5 mAh/g, respectively. The results demonstrate that the D-64 h nanocomposites have the good cyclic stability and exceptional rate capability. It can ensure that the material structure will not be damaged in the process of rapid charge and discharge. Therefore, the D-64 h anode can accommodate the large current density changes.

For the direct and further understanding of the electrochemical superiority of the D-64 h electrode, electrochemical impedance spectroscopy (EIS) measurements for the three electrodes are conducted at an open-circuit voltage state using fresh cells and the resulting Nyquist profiles are presented in Fig. 8. In the high-frequency region, each curve consists of a depressed semicircle, representing the charge-transfer impedance of the cell. Meanwhile, a sloping line in low-frequency region exists in each plot, which is related to the mass transfer of Li ion [38]. It is clearly observed that the big difference focuses on the diameter of the three semicircles, which is proportional to the value of the charge-transfer impedance (Rct). As showed, the D-64 h anode exhibits the lowest Rct value. And the linear Warburg impedance (Zw) is a slope of approximately 45° in the low-frequency region corresponding to the Li ion diffusion process in the electrodes. These results are in good agreement with the electrochemical performance of D-64 h electrode. The outstanding electrochemical performance should be attributed to its unique structure and multi-components. With this structure, the special dendritic micro-nano structure provides a short diffusion length for Li ion, and offer available space to accommodate the volume changes of the during the cycling performance. Meanwhile, the mixed conductive metal Co/Ni in the composite facilitates electron transfer. Therefore, the capacity retention and kinetics are both improved.

Fig. 8
figure 8

EIS spectra of the NiCo2O4-NaOH+H2O2-500 nanocomposites prepared with different dealloying time

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Conclusions

In conclusion, dendritic micro-nanostructural NiCo2O4 materials are successfully synthesized by a simple melt spinning-dealloying method followed by mediate temperature annealing. The D-64 h electrodes deliver superb specific capacity, good rate performance, and outstanding cycling stability. A capacity of 1016.9 mA/g at a current density of 100 mA/g is obtained after 100 cycles. The special structure favors the enhanced electrochemical performance. On the one hand, the large number of nanosheets in NiCo2O4 significantly increases the contact area between the electrode and the electrolyte, thus shortening the Li-ion transmission path. On the other hand, the uniform distribution of micro-nanostructure is beneficial to reduce the strain caused by volume changes during the long-term discharge/charge cycle, which can greatly improve the cycling capacity. Owing to the superiorities of high lithium storage performances and easy preparation, the dendritic micro-nanostructural NiCo2O4 shows encouraging application potential as an advanced anode material for LIBs.