3.1. Structure and morphology
The crystal structure of the prepared material is first determined by x-ray diffraction (XRD). The prepared samples were found to be in good agreement with the WO3 card (PDF#72–0677) and the CoWO4 (PDF#72–0479) card (Fig. 1.a). The lower peak of CoWO4 is due to the low content of ZIF-67 compared to WO3.
The Raman test exhibited two peaks (Fig. 1.b), D-band and G-band, associated with disordered carbon atoms related to defects in carbon atoms (D-band) and SP2 vibrations of carbon atoms (G-band), respectively. The ratio of ID to IG (ID/IG=1.14) was able to demonstrate the increase in defects, and in order to effectively demonstrate the pore size distribution and specific surface area of the material, nitrogen adsorption experiments were performed. Figure 1.c shows the typical VI-type adsorption curve. The specific surface area of CoWG was 168.6 m2 g− 1 with an average adsorption pore size of 5.8 nm, and the pore size was mainly distributed between 3 and 5 nm, which was able to be obtained from the results of BET (Fig. 1.d). The BET proves that the composite material has a large specific surface area, which is conducive to the full infiltration of electrolyte. The high porosity ensures the diffusion of ions, and the rGO can promote the transfer of electrons, which ensures excellent ion penetration and effective charge transfer for the electrochemical reaction.
The SEM images were taken. Figure 2.a and b are CoWG and Fig. 2.c and d are WO3. By comparison, it can be found that the addition of rGO and ZIF-8 can significantly reduce the aggregation effect. In Fig. 2.b, it can be found that ZIF-67 with hollow structure (indicated by the dashed yellow circles) is on rGO. Compared to complete graphene wrapping, which may spatially prevent ion diffusion, rGO as a substrate and using ZIF-67 as an isolation layer maintains abundant internal porosity. At once, it can promote electron transfer without inhibiting ion diffusion. This lays the foundation for the ionic diffusion performance of CoWG.[20, 29]
The lattice stripes and diffraction haloes of CoWG were analyzed by high-resolution transmission electron microscopy (HRTEM). The lattice stripes in Fig. 3.a correspond to the (0 2 0) crystal plane of WO3, and the illustration on the top right shows the lattice stripes after the fast Fourier transform (FFT). The diffracted haloes in Fig. 3.b belong to the (2 0 0) (-2 2 2) and (0 2 2) crystal plane, respectively. TEM (c and d) of CoWG in Fig. 3 show that WO3 particles and ZIF-67 frame are evenly distributed on the rGO, which obviously solves the aggregation effect. This also proves that WO3 is the main component of CoWG. In order to analyze the way CoWO4 is present in it, XPS tests were performed next.
Elemental Composition and Valence of CoWG by X-ray Photoelectron Spectroscopy (XPS). Figure 4.a show the high-resolution W 4f spectra. When the binding energies are 38.25 and 36.1 eV, it can be interpreted as two peaks of W 4f5/2 and W 4f7/2.[30, 31] In the high-resolution spectra of C1s, the peaks at 284.8, 285.6 and 286.5 eV are C-C, C-O and C = O bonds (Fig. 4.b). The formation of C-O and C = O bonds were due to the residual oxygen functional groups in rGO. These results clearly indicate that most of the oxygen-containing functional groups in the GO were removed during hydrothermal reaction to form rGO. Figure 4.c shows the high-resolution spectrum of O 1s. The bond energies 530.9, 531.4 and 533.4 eV correspond to W-O, Co-O and O-H bonds, respectively. High-definition spectra of Co 2p (Fig. 5.d). The main peak of high-resolution Co 2p consists of two peaks at 781.4 and 797.9 eV, which are related to the spin orbits of Co 2p3/2 and Co 2p1/2, respectively. [19, 32, 33]
3.2 Electrochemical properties
The electrochemical study of the electrode with high porosity was carried out and the lithium storage mode and reaction mechanism were analyzed. Figure 5.a shows the CV curve of CoWG. It can be found that there is a large peak current at 0.5V, but it disappears after the cycle, which proves the formation of SEI film and stabilizes after the cycle. The electrochemical properties of CoWG and WO3 were compared.[34, 35] Fig. 5.b and c show the charge and discharge curves of WO3 and CoWG. There are discharge platforms in the first discharge process, which also proves the irreversible process during the reaction. The large amount of irreversible consumption of Li+ in the first cycle also proves the formation of SEI film in the first cycle. Figure 5.d shows the magnification performance of WO3 and CoWG, and it can be found that CoWG has a higher reversible specific capacity than WO3 at different magnifications. Figure 5.e is the electrochemical performance of 100 cycles at a current density of 100 mA g− 1. The figure clearly shows that CoWG has higher initial coulombic efficiency and cycle retention. After 100 cycles, it still has a specific capacity of 607.8 mAh g− 1, while WO3 decays to 248.3 mAh g− 1. This can be attributed to the addition of ZIF-67 as an isolation layer and rGO effectively alleviates the volume expansion of WO3 during cycling and improves the conductivity. At a current density of 800 mA g− 1, it still has a specific capacity of 248.3 mAh g− 1 after 300 cycles, showing excellent cycle stability. The improvement of specific capacity during cycling can be attributed to the activation of electrode materials and the reversible formation/decomposition of SEI film, which can provide additional lithium interface storage at low potential through the so-called pseudocapacitive mechanism.
The lithium-ion diffusion coefficients of CoWG and WO3 were compared by galvanostatic intermittent titration technique (GITT). Figure 6.1 shows the GITT plot and Fig. 6.2 shows the lithium-ion diffusion coefficient. Prior to the GITT test, 5 cycles of activation were performed, every 6 minutes of discharge, a 1h resting period was performed. The lithium-ion diffusion coefficient is calculated by Eq. (1):
$${D}_{{Li}^{+}}=\frac{4}{\pi \tau }{\left(\frac{m{V}_{m}}{MA}\right)}^{2}{\left(\frac{\varDelta {E}_{s}}{{\varDelta E}_{\tau }}\right)}^{2}$$
1
In this equation, \(\tau\) denotes the standing time, \(m\), \(M\), \({V}_{m}\) and \(A\) denote the active substance mass, molar mass, molar volume and electrode geometric area, respectively. \({\varDelta E}_{s}\) is the voltage change during relaxation and \({\varDelta E}_{\tau }\) is the change during the current pulse.[36, 37]
The diffusion coefficients in Fig. 6.2 clearly show that the diffusion coefficient of CoWG for charge/discharge cycle is superior compared to that of WO3. This also proves that the addition of high porosity ZIF-67 not only alleviates the agglomeration but also facilitates the diffusion of lithium-ion.
During the first discharge, three groups of voltages, 0.75 V, 0.45V and 0.01V, were selected to test the influence of SEI film formation on impedance (Fig. 7.a). It can be found that the first semicircle of SEI film in the figure increases first and then decreases with the decrease of voltage, and reaches stability at 0.01V, which proves that SEI is formed by the reaction between electrode material and electrolyte at the solid-liquid interface.[38] Fig. 7.b is the SEM image after a cycle. Compared with fresh electrodes (Fig. 2.a, b), the electrode surface after the first discharge was covered with a layer of SEI film (Fig. 7.b).
The contribution values of diffusion behavior and capacitance behavior were calculated by cyclic testing CV curves at different scanning rates(Fig. 8). It can be found that with the increase of the scanning rate, the peak current is offset, which is caused by the polarization reaction (Fig. 8.a). The assessment of Li+ storage can be conducted based on equations (2) and (3).
$$log \left(i\right)= blog \left(v\right)+ log \left(a\right)$$
3
Where \(i\) and \(v\) denote the current and scan rate, respectively. Formula (4) allows for a quantitative determination of the ratio of capacitance contribution to Li+ diffusion contribution in the electrode.
$$i\left(V\right) = {k}_{1}v + {k}_{2}{v}^{1/2}$$
4
In the equation \(v\) is the scan rate, \({k}_{1}\) and \({k}_{2}\) are the coefficients with the surface capacitive effects and the diffusion control contribution, respectively.
With the increase of the scanning rate, the capacitance behavior ratio shows a gradient increase. When the scanning rate increases from 0.5 to 5 mv s− 1, the capacitance behavior ratio increases from 22–86%, which is also the reason why CoWG has good magnification performance.
By the EIS test, it was found that CoWG had a smaller initial impedance due to the improved conductivity of the electrode material with the addition of rGO. In high frequency region, CoWG has better ion diffusion performance than WO3. It was also proved that the addition of rGO and ZIF-67 enhanced the reaction kinetics during charge and discharge.