1 Introduction

Nowadays, lithium-ion batteries (LIBs) are extensively applied in the industrial and commercial applications (such as laptops, mobile communication equipment and electric vehicles) [1, 2]. To meet the increasing demands of the industries, the energy density and the cycle life of LIBs need to be further improved. Hence, many anode materials with superior electrochemical properties were reported and replaced the traditional graphite anode [3,4,5]. Among these materials, binary cobalt vanadates (such as Co3V2O8) have attracted much attention as an alternative to oxide-based anodes owing to the presence of multivalent vanadium element, interfacial effects and synergistic effect of Co and V ions [6, 7]. However, the decrease in volume expansion and mechanical strength of bulk material can reduce the storage efficiency of lithium upon cycling and impede their use in the commercial applications.

Several approaches have been employed to overcome these issues such as rational fabrication and synthesis of nanostructured materials [8,9,10,11,12]. The nanostructured materials allow the electrolyte throughout the electrode, provide shorter diffusion path length, restrict the volume expansion and enhance the contact area [13,14,15,16,17,18,19,20]. Thus, a variety of micro-/nanostructured Co3V2O8 materials have been reported, such as nanotubes [21], hollow and solid hexagonal micro-pencils [22], mesoporous nanoparticles [23] and porous microspheres [24]. For example, the Co3V2O8·nH2O hollow pencils exhibited impressive lithium storage capability, owing to their interfacial effects, multivalent vanadium ions and the reduction of volume expansion caused by the synergistic effects [25]. The multilayered Co3V2O8 nanosheets exhibited an outstanding specific capacity of 470 mAh·g−1 at 5.0 A·g−1 over 500 cycles, in which the reversible reaction of Co2+/Co0 and LixV2O5 acts as an electrochemical reaction, confirmed by ex situ transmission electron microscope (TEM). These nanosheets not only enhanced the contact area, but also assured favorable kinetics and a stable structure [26]. The porous Co3V2O8 nanosheets exhibited excellent lithium storage capacity. These layer-to-layer nanosheets with mesoporous structure and synergistic effect prevented the storage capacity from decaying and contributed in regaining the capacity [27]. Among various nanostructures, the hollow micro-/nanostructured electrodes have attracted remarkable attention due to the shorter diffusion path length and lager contact area [28, 29]. Luo et al. [30] recently synthesized interconnected Co3V2O8 hollow microspheres by a hydrothermal method followed by annealing; these microspheres displayed high cycling stability and rate capability (424 mAh·g−1 at 10 A·g−1 over 300 cycles). Wu et al. [31] recently synthesized uniform Co3V2O8 microspheres by a hydrothermal method followed by calcination. The Co3V2O8 microspheres exhibited high cycling stability and rate capability as anode materials owing to the hollow structure, synergistic effects, mechanical stability and their complex chemical composition.

In this work, double-shelled Co3V2O8 hollow nanospheres were fabricated using a solvothermal method followed by a thermal treatment. Scanning electron microscope (SEM) images show that the morphologies of Co3V2O8 nanostructures depend on the annealing temperatures. The formation of the double-shelled hollow nanospheres is the result of interaction between contraction force (Fc) and adhesion force (Fa) during oxidation. The double-shelled Co3V2O8 hollow nanospheres electrode exhibits an excellent electrochemical performance owing to the benefits of both the double-shell and the hollow morphology, which increases its potential for anode material in the LIBs.

2 Experimental

In the experiments, all the analytical grade reagents were used without further purification. First, 20 ml glycerin and 40 mL ethylene glycol were added into a 200-ml glass beaker and stirred for 2 h to form a limpid solution. Then, 3 mmol CoCl2·H2O and 2 mmol vanadyl acetylacetonate (VO(acac)2) were dissolved in the mixed solution by continuously stirring for 1 h. This solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and heated at 180 °C for 15 h. The obtained product was washed with deionized water thrice and then with anhydrous ethanol and later dried at 60 °C for 12 h. The Co3V2O8 samples were annealed at 350, 450 and 550 °C for 2 h, respectively. The products with different morphologies were marked as double-shelled hollow nanospheres (CVO–DSS), single-shelled hollow nanospheres (CVO-HS) and nanoparticles (CVO-NP), respectively.

X-ray diffraction (XRD; Bruker AXS, D8 diffractometer, Cu Kα radiation) was used to determine the crystal structure of the Co3V2O8 samples. Scanning electron microscope (SEM; JEOLJSM-7400F, Japan), X-ray energy-dispersive spectrometry (EDS; Oxford Instruments, INCA) which was attached to SEM, high-resolution transmission electron microscopy (HRTEM; JEOL-2010) and X-ray photoelectron spectrometer (XPS; VGESCA-LABMK II spectrometer) were used to detect the morphologies, element compositions and element distributions of these samples. Thermogravimetric analysis (TGA) was performed in air at a heating rate of 10 °C·min–1 from room temperature to 600 °C with an SDT Q600 TA Instruments thermal analyzer.

The Co3V2O8 electrodes were obtained by smearing the mixed slurry (the weight ratio of carboxymethylcellulose sodium, super P and Co3V2O8 sample is 1:1:8) on a Cu foil and dried at 100 °C for 12 h in vacuum. The mass loading of each Co3V2O8 sample was ~ 2.6–2.8 mg·cm2. The assembly of button battery (CR2032-type cells) was arranged in an argon-filled glove box. The separator used was an Celgard 2400 porous polypropylene film, and an electrolyte of l mol·L−1 LiPF6 consisting of ethylene carbonate and diethyl carbonate (1: 1 in volume) was used. Galvanostatic charge–discharge tests were conducted with a battery test system (LAND CT2001A, China). The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests were conducted using an electrochemical workstation (CHI 660A).

3 Results and discussion

3.1 Structure and composition characterization

Figure 1a shows the XRD patterns of the three Co3V2O8 samples. The characteristic peaks at 2θ = 35.3°, 43.5°, 57.7° and 63.2° correspond to the diffraction planes of (122), (042), (025) and (442) for the Co3V2O8 orthorhombic structure (JCPDS No. 74–1487), and no other peaks are observed. Figure 1c shows the XRD pattern of the precursor. The special peak at 2θ = 10.8° can be attributed to the metal alkoxides of the precursor. The thermogravimetry (TG) curve (Fig. 1b) represents the total precursor weight loss of 31.59%, owing to the evaporation of free water, adsorbed water and the decomposition of organic compounds [32].

Fig. 1
figure 1

a XRD patterns of CVO-DSS, CVO-HS and CVO-NP; b TGA curves of Co-V-based precursor; c XRD pattern and d SEM image of Co-V-based precursor

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XPS measurements were used to analyze the surface chemical composition and valence states of the CVO-DSS sample. The full spectrum peaks of Co, V and O are shown in Fig. 2a. The XPS spectrum of Co 2p shows two obvious peaks at 780.1 and 796.8 eV, representing Co 2p3/2 and Co 2p1/2, respectively, which could be attributed to Co2+ of the Co3V2O8 sample. The two peaks (783.1 and 798.1 eV) of Co3+ were located after peak fitting, corresponding to the previous reports [21, 22]. Two other peaks located at 524.7 and 516.9 eV of the V 2p spectrum, allocated to V 2p1/2 and V 2p3/2, respectively, correspond to V5+ of the Co3V2O8 sample, are shown in Fig. 2c. After peak fitting, two peaks of V4+ were located at 516.7 and 523.8 eV [23]. The O 1s spectrum indicated weak adsorbed-oxygen and doughty lattice-oxygen peaks located at 532.1 and 529.9 eV, respectively, as shown in Fig. 2d. These results confirm successful synthesis of pure Co3V2O8 phase.

Fig. 2
figure 2

XPS spectra of CVO-DSS: a survey spectrum, b Co 3d, c V 2p and d O 1s

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3.2 Morphology characterization

Figure 3a, b displays the morphology of CVO-DSS sample. The CVO-DSS displays uniform nanosphere morphology with a diameter of ~ 600 nm. The CVO-DSS maintained the nanosphere morphology of the Co-V-based precursor as observed by the SEM (in Fig. 1d). TEM and HRTEM measurements were used to further examine the microstructures of the Co3V2O8 samples. According to the TEM images (Fig. 3c, d), these nanospheres are of the size 500–800 nm. Strong contrast between the pale center and the dark edges indicates the obvious double-shelled hollow cavity. The HRTEM images, shown in Fig. 3d, e, indicate that the CVO-DSS demonstrates a particular d-spacing (0.254 nm) of (311) planes. The EDS elemental mapping shown in Fig. 3f confirms the uniform distribution of O, V and Co throughout the CVO-DSS sample. The morphology of the double-shelled Co3V2O8 hollow nanospheres can effectively mitigate the volume change, improve the lithium insertion–extraction and reduce the diffusion distance of Li+ [33, 34].

Fig. 3
figure 3

a and b SEM images of CVO-DSS; c and d low magnification TEM images of CVO-DSS, e HRTEM image of CVO-DSS; f SEM image and corresponding EDS elemental mappings of Co, V and O of CVO-DSS

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Figure 4 displays the SEM images of the other Co3V2O8 samples (single-shelled hollow Co3V2O8 nanospheres (CVO-HS) and Co3V2O8 nanoparticles (CVO-NP)) obtained at 450 and 550 °C, respectively. The CVO-HS shown in Fig. 4a, c exhibits single-shelled hollow morphology. However, when the calcination temperature reaches 550 °C, the as-prepared Co3V2O8 sample (CVO-NP) exhibits the nanoparticles morphology with the size of ~ 100–300 nm, as shown in Fig. 4b, d. Based on these results and the previous reports, the formation of the double-shelled hollow nanospheres can be considered as the result of the interaction between Fc and Fa during oxidation. Fc can induce an inward shrinkage of the Co-V-based precursor core during the decomposition of the organic species. Fa prevents the inward contraction of the precursor core. Similar phenomenon has been reported in a variety of nanostructured electrode materials [35,36,37].

Fig. 4
figure 4

SEM images of a CVO-HS and b CVO-NP; TEM images of c CVO-HS and d CVO-NP; e schematic illustration of temperature-dependent morphologies of Co3V2O8 samples

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3.3 Electrochemical performances

CV curve of the CVO-DSS material was evaluated at a scan rate of 0.2 mV·s−1, as shown in Fig. 5a. For the CVO-DSS electrode, two cathodic peaks at 0.65 and 0.05 V appear in the first intercalation of lithium ions. The peak at 0.65 V can be related to the decomposition of Co3V2O8. The pristine Co3V2O8 transformed into CoO and the LixV2O5 resulted from the intercalation of lithium ions into the Co3V2O8 lattice (Co3V2O8 + xLi+ + xe– → 3CoO + LixV2O5). As the insertion of Li+ ions increase, the peak at 0.05 V reduces CoO/Co0 and the lithiation of LixV2O5 occurs (CoO + 2Li+ + 2e– → Co + Li2O, LixV2O5 + yLi+ + ye– → Lix+yV2O5) [38]. During the positive voltage sweep, two peaks at ~ 1.31 and 2.37 V may depend on the extraction of lithium ions and hence the oxidation peak does not change in subsequent cycles. However, these cathodic peaks move at ~ 1.77, 1.02 and 0.35 V, respectively. The disappearance of the peak at 0.05 V confirms the irreversible reaction which can be attributed to the formation and partial disintegration of the SEI layer [39]. The overlapped scanning curves (excluding the first cycle) indicate that the CVO-DSS electrode exhibits reversible discharge/charge process and good stability.

Fig. 5
figure 5

a CV curves of CVO-DSS; charge/discharge profiles of b CVO-DSS, c CVO-HS and d CVO-NP; e cycling performance of CVO-DSS, CVO-HS and CVO-NP at a current density of 200 mA·g−1; f rate performance of CVO-DSS, CVO-HS and CVO-NP

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The galvanostatic charge–discharge (GCD) and the cycling performance were evaluated to understand the storage capacity of all the Co3V2O8 electrodes at a current density of 200 mA·g–1, as shown in Fig. 5b, c, d. The first discharge of CVO-DSS reveals a high capacity of 1366 mAh·g–1, and a reversible capacity of 1192/1210 mAh·g–1 can be obtained with an ideal coulombic efficiency of 87.3%. After 100 successive cycles, the CVO-DSS electrode indicates a good discharge capacity of 1210 mAh·g–1 and a capacity retention of 88.6% compared to the initial discharge capacity, as shown in Fig. 5e. For the CVO-HS and CVO-NP electrodes, the first discharge capacities are 1311 and 1180 mAh·g–1, respectively. However, the capacity decay is observed for in the CVO-HS and CVO-NP electrodes. They exhibit the discharge capacities of ~ 899 and 654 mAh·g–1 after the 100th cycle, only when the capacity retentions are 68.6% and 55.4% from the first cycle. It is clear that the cycling stability of the CVO-HS and CVO-NP electrodes is inferior to that of the CVO-DSS electrode. It is interesting to observe that the cycling performance of all the Co3V2O8 electrodes increases slightly starting from the second cycle and this repeats until the 45th cycle. Similarly, previous studies on metal oxides reported this phenomenon [25]. The discharge capacities of the CVO-HS and CVO-NP electrodes keep declining with the increase in the number of cycles. The capacity loss of hollow nanosphere and the nanoparticles electrodes could be associated with the pulverization and vigorous volume changes of nanostructures during the charge and discharge process. However, the CVO-DSS remains stable after 100 cycles. This result indicates that the double-shelled hollow nanospheres and porous structure can effectively limit the volume change and pulverization of the electrode materials. Figure 5f shows the rate capabilities of the CVO-DSS, CVO-HS and CVO-NP electrodes. The discharge capacities of 1382, 1278, 1315, 1106, 772, 605 and 519 mAh·g–1 can be acquired for the CVO-DSS at 200, 500, 1000, 2000, 5000, 10,000 and 20,000 mA·g−1, respectively. More interestingly, when the current density returns to 200 mA·g−1, the reversible capacity of 1109 mAh·g–1 can be obtained after 80 cycles. However, the CVO-HS and CVO-NP electrodes display unsatisfactory rate capabilities, especially at high rate.

EIS was utilized to analyze the interfacial property of the three Co3V2O8 electrodes in Fig. 6a. All the Nyquist plots are made up of a semicircles and slanted lines. The diameter of the semicircle represents the ability of charge transfer resistance (Rct); the slanted line displays the Warburg resistance (Zw) [40]. From the equivalent circuit in the inset of Fig. 6a (Rs is the electrolyte resistance), the Rct value of the CVO-DSS electrode is 56.3 Ω, which is less than that of the CVO-HS (78.5 Ω) and CVO-NP (88.7 Ω), indicating that the CVO-DSS exhibits a rapid charge transfer and lithium-ion diffusion, compared to the CVO-HS and CVO-NP electrodes. In addition, the linear fitting of the Warburg impedance of all the Co3V2O8 electrodes is shown in Fig. 6b. Lithium-ion diffusion coefficient (\(D_{{{\text{Li}}^{ + } }}\)) is closely related to the slope Aw of the fitting curve. The slope Aw affects the ability of lithium-ion diffusion coefficient (\(D_{{{\text{Li}}^{ + } }}\)) [41]. Equation (1) shows the relationship between \(D_{{{\text{Li}}^{ + } }}\) and Aw, and they are inversely proportional to each other [42]. The parameters E and F represent the open-circuit voltage and Faraday constant. x, S and Vm represent the Li+ concentration, surface area and molar volume of the active materials in Eq. (1).

$$D_{{{\text{Li}}^{ + } }} = 0.5\left[ {\frac{{V_{{\text{m}}} }}{{FSA_{{\text{w}}} }}\left( { - \frac{{{\text{d}}E}}{{{\text{d}}x}}} \right)} \right]^{2}$$
(1)
Fig. 6
figure 6

a EIS and b linear fitting of Warburg impedance of CVO-DSS, CVO-HS and CVO-NP (Z′, real part of impedance; Z″, imaginary part of impedance)

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The slope Aw of the CVO-DSS electrode is 22.6 Ω·s−1/2, which is less than that of the CVO-HS (56.2 Ω·s−1/2) and CVO-NP (65.3 Ω·s−1/2), indicating that the CVO-DSS exhibits rapid lithium-ion diffusion rate, compared to the CVO-HS and CVO-NP electrodes.

To further examine the long-term cycle at high current density, the double-shelled Co3V2O8 hollow nanosphere was tested, as shown in Fig. 7a. Though there is obvious capacity fade above 100 cycles, a satisfying reversible discharge and charge capacity of 847 and 834 mAh·g–1 can be achieved. It can also be decreased to 613 and 628 mAh·g–1 after 800 consecutive cycles at ultrahigh current density of 5000 mA·g−1. Furthermore, the TEM image of the CVO-DSS electrode after 100 cycles at current densities of 200 mA·g−1, as shown in Fig. 7b, indicating that the CVO-DSS electrode still retains the sphere-like morphology. The CVO-DSS electrode material can retain the morphology even after many cycles, which is the main reasons for its good cyclic stability. Thus, compared to the other existing literature as summarized in Table 1 [21, 24, 26, 27, 30, 31, 34, 43, 44], high lithium storage properties of the CVO-DSS electrode can be attributed to the following: (i) Uniform double-shelled hollow nanospheres effectively reduce the internal resistance and increase the diffusion rate of Li+ [45]; (ii) this particular nanostructure efficaciously increases conductivity of the electrode material and buffer the influence of volume expansion [46]; (iii) synergistic effect between Co and V ions can improve the electrochemical performance of the Co3V2O8 electrode material [47].

Fig. 7
figure 7

a Cycling performance of CVO-DSS at current densities of 5000 mA·g−1; b TEM image collected after 100 cycles of CVO-DSS

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Table 1 Comparison of electrochemical performances of Co3V2O8 with previously reported results for LIBs
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4 Conclusion

In summary, the Co3V2O8 electrode materials with different morphologies (double-shelled hollow nanosphere, single-shelled hollow nanosphere and nanoparticle) were synthesized successfully by using a solvothermal method followed by thermal treatment. The calcination temperature plays a crucial part in the formation of Co3V2O8 particles. The double-shelled Co3V2O8 hollow nanospheres (CVO-DSS) exhibit excellent lithium storage properties with a reversible capacity of ~ 1210 mAh·g–1 at 200 mA·g–1 after 100 cycles and also provide ~ 628 mAh·g–1 at 5000 mA·g–1 after 800 cycles. The double-shelled hollow nanosphere provides several advantages like increasing the Li+ diffusion coefficient and electronic conductivity, decreasing the electrode polarization and simultaneously balancing the volume expansion of Co3V2O8 during the cycling.