VOPO4⋅2H2O: Large-Scale Synthesis and Zinc-Ion Storage Application

Introduction

With the aggravation of the serious energy crisis, it is particularly important to maximize energy utilization through energy storage and conversion technologies (Palanisamy et al., 2016; Guo Q. et al., 2020; Yang et al., 2020; Zhang S. et al., 2020). Rechargeable batteries recently have received tremendous solicitude as a result of their broad prospect and great potential in energy storage application such as portable electronic device, electric vehicles, and large-scale grid (Xia et al., 2018; Ao et al., 2019; Xiong et al., 2020). Due to the extremely finite resources and steeply rising prices of lithium and its increasingly concerned safety issues, lithium-ion batteries (LIBs) barely meet the growing energy demand and seriously hinder their further growing for large-scale electrochemical energy storage (EES) in the modern society (Chen L. et al., 2019; Lin et al., 2019; Li C. et al., 2020; Wang et al., 2020).

While, aqueous rechargeable alkaline metal ion batteries based on environmentally friendly water electrolytes give prominence to the merits of low price, good safety and high ion conductivity (two orders of magnitude higher than that of traditional non-aqueous rechargeable batteries), which in turn become the most promising alternatives to traditional LIBs in large-scale EES (Chen D. et al., 2019; Li G. et al., 2020). Among these rising aqueous rechargeable batteries, aqueous zinc ion batteries (ZIBs) are characterized as the potential representative owing to the low redox potential (−0.76 V vs. SHE.), high theoretical specific capacities (819 mAh g^–1 and 5845 mAh cm^–3), rich reserves (as the fourth “common” metal in the crust of earth), low cost and outstanding water compatibility of the zinc mental (Xia et al., 2017; Song et al., 2018; Li et al., 2019). However, the current available cathode materials of ZIBs deliver unsatisfactory performance [e.g., the low discharge capacity of Prussian blue analogs (Ke et al., 2017), the poor cycling stability of MnO₂ polymorphs (Sun et al., 2017), etc.], seriously restricting the development and commercialization of aqueous ZIBs. Recently, vanadium-based electrode materials show impressive electrochemical properties in alkaline metal ion batteries because of their multiple valence changes and high electrochemical activity in chemical energy storage (Deng et al., 2020; Xie et al., 2020; Zhang X. et al., 2020). Among various vanadium-based electrode materials (Yue et al., 2016; Guo X. et al., 2020; Liu et al., 2020), layered VOPO₄⋅2H₂O has been widely concerned. This electrode material provides adjustable layer spacing to accommodate the intercalated zinc ions, and with the help of water, the zinc ions are more easily intercalated into the VOPO₄⋅2H₂O material (Huang et al., 2014; Peng et al., 2017; Tang et al., 2020). Additionally, the VOPO₄⋅2H₂O material can also store electric charge during anionic redox reaction process (Yang et al., 2020). There are many recent reports about VOPO₄⋅2H₂O electrode material applied in aqueous ZIBs, but it is invariably found that the prepared VOPO₄⋅2H₂O material showed low output in production and unsatisfactory performance in zinc storage property, which is in a difficult position to meet the demand of large-scale EES application. For instance, Shi et al. studied the zinc storage performance of the VOPO₄⋅2H₂O cathode synthesized by a reflux method. And it is shown that the VOPO₄⋅2H₂O cathode only released 60 mAh g^–1 at a high electric current density of 5 A g^–1 and an unobtrusive cycle-life with only 500 cycles at 2 A g^–1 (Shi et al., 2019). Consequently, it is essential to produce a high performance VOPO₄⋅2H₂O cathode material with large-scale production.

Herein, a facile route has been designed for the large-scale synthesis of VOPO₄⋅2H₂O sample by a simple solid-state method, which is in great favor of extending the application of aqueous ZIBs in large-scale storage devices. The VOPO₄⋅2H₂O electrode delivers an excellent zinc storage ability with high reversible discharge specific capacity (165 mAh g^–1 at 0.05 A g^–1) and good rate characteristics (90 and 75 mAh g^–1 at 2 and 5 A g^–1, respectively) in the voltage window of 0.4–1.6 V (versus Zn²⁺/Zn). Moreover, the VOPO₄⋅2H₂O electrode exhibits a superior long cycle performance with stable Coulombic efficiency evolution during the repetitive electrochemical process (i.e., 76% capacity retention up to the 1000th cycle at a high electric current density of 5 A g^–1).

Materials and Methods

Synthesis of VOPO₄⋅2H₂O Sample

In a typical procedure, a suspension consisting of 1440 mL deionized water (DI H₂O), 60 g vanadium pentoxide (V₂O₅) powders, and 360 mL concentrated phosphoric acid (85% H₃PO₄) was transferred to an agate tank and ball-milled at an appropriate speed of 500 rmp min^–1 for 30 h. After the program completed, the yellow-greenish products were centrifuged and washed by water and acetone several times. Conclusively, the resulting product was dried in a vacuum (60°C for 8 h) to yield VOPO₄⋅2H₂O sample. Here, it is noteworthy that the total mass of the as-obtained VOPO₄⋅2H₂O power prepared by this simple method can reach to a mass production of ∼80 g, which is nearly 100 times of the yield via the traditional reflux method (Shi et al., 2019).

Characterization

The polycrystall X-ray diffraction (XRD, Rigaku SmartLab) with Cu Kα X-ray source (λ = 0.154056 nm) was employed to accurately determine the crystallographic structure of the prepared sample. The fourier transform infrared spectroscope (FTIR, Nicolet 6700) was detected within a wavenumber interval of 4000–500 cm^–1. The TG curve was obtained by a thermal gravimetric analysis (TGA/DSC3 +) instrument from 25 to 600°C in air atmosphere (heating rate: 10°C min^–1). The X-ray photoelectron spectroscopy (XPS, Escalab 250Xi) was used to analysis phase composition and chemical state of element for the final product. The field-emission scanning electron microscopy (FESEM, Hitach SU-8220) and transmission electron microscopy (TEM, JEM-2100F) instrument were used to measure the morphology for the synthesized sample.

Electrochemical Measurement

The working electrode (i.e., cathode) contained 60 wt% active material VOPO₄⋅2H₂O, 30 wt% conductive carbon nanotubes and 10 wt% binder polyvinylidene fluoride in N-methyl pyrrolidone solvent. After thorough stirring, the homogenous slurry was coated on titanium foils and dried in a vacuum oven at 60°C for a whole night. The CR2032-type coin cells were used to assemble button cells, including the above working cathode, the metallic zinc anode and 3 M Zn (CF₃SO₃)₂ electrolyte as well as glass-fiber separator in air atmosphere. Galvanostatic charge–discharge cycling and rate performance tests were investigated within the voltage test window of 0.4–1.6 V (vs Zn²⁺/Zn) by using NEWARE system. Electrochemical impedance spectroscopy (EIS) studies were executed with the button cell on an electrochemical workstation (Multi-Autolab M204) within the frequency domain of 10⁵–10^–2 Hz. Cyclic voltammetry (CV) were also tested by the Multi-Autolab M204 measurement at different potential scan rates scanning from 0.1 to 0.5 mV s^–1. It is mentioned here that, before electrochemical testing, the VOPO₄⋅2H₂O cathode was initially activated by discharging to 0.4 V.

Results and Discussion

The crystallographic structure and phase purity of the resultant product is measured by XRD technique. As shown in Figure 1A, all observed reflections completely matched with the standard pattern of the tetragonal VOPO₄⋅2H₂O (space group: P4/n, JCPDS card: 84-0111). And no detectable impurity peaks can be found, indicating the high purity of VOPO₄⋅2H₂O crystal (Zhou et al., 2016). The FT-IR analysis is carried out to confirm the existence of various groups in the VOPO₄⋅2H₂O sample, and its spectrum is exhibited in Figure 1B. The absorption peaks located at about 684 cm^–1 is attributed to the V-O extension vibration. The peaks at 950 and 1080 cm^–1 can be related to the P-O stretching vibration in PO₄ group (Bao et al., 2011). The absorption peaks of P–OH and O–H stretching vibration in H₂O are observed at 3600 and 3400 cm^–1, respectively. The peak at 1616 cm^–1 corresponds to the H–O–H bending vibration of interlayer H₂O (Zhou et al., 2014). These characteristic peaks are proved to be in agreement with the chemical composition of VOPO₄⋅2H2O (Patel et al., 2003). Additionally, the water content of the final product is confirmed by the TG analysis in the air test environment within the operating temperature window of 25–600°C. As shown in the illustration of Figure 1B, the weight loss percentage of the final product between 25 and 150°C is calculated to be 18.5%, which conforming with the evaporation loss of two crystal water molecules in per mole of as-obtained sample (Zhou et al., 2018; Hyoung et al., 2019). The surface element analysis and chemical states for as-prepared composite were further identified via XPS spectra. As demonstrated in Figure 1C, P, O, and V elements were presented in VOPO₄⋅2H₂O survey spectrum, and the high-resolution XPS spectra of P2p and O1s are displayed in Supplementary Figure 1 (Supplementary Material), which are in good accordance with the reported result in the recent literature (Verma et al., 2019). According to the high-resolution V2p XPS spectrum in Figure 1D, it is clearly observed two sharp peaks located at the binding energy of 518.5 and 516.6 eV, according with V2p_3/2 of V⁵⁺ and V⁴⁺, respectively. And the other peaks at 525.8 and 524.6 eV are associated with V2p_1/2 of V⁵⁺ and V⁴⁺, respectively (Chen et al., 2018).

FIGURE 1

Figure 1. The chemical and physical characterization of VOPO₄⋅2H₂O sample. (A) XRD pattern; (B) FT-IR spectrum and TG curve (inset); (C) the survey XPS spectrum; (D) XPS high-resolution spectrum of V 2p.

The morphology and detailed structure of VOPO₄⋅2H₂O sample are investigated by FESEM and TEM analysis. The VOPO₄⋅2H₂O sample displays irregular bulk morphology with the size of 0.5- 2 μm from the SEM images (Figures 2a,b). The TEM images in Figures 2c,d indicate that this bulk material contains many nanosheets. Moreover, a lattice fringe with the d-spacing of ≈0.36 nm can be also investigated in the HRTEM image (Figure 2e), corresponding to the (0 0 2) crystal plane of the tetragonal VOPO₄⋅2H₂O.

FIGURE 2

Figure 2. The microstructure and morphology of the VOPO₄⋅2H₂O sample. (a,b) SEM images under different scanning resolutions; (c,d) TEM images; (e) HRTEM image.

In order to systematically investigate the zinc storage performance of VOPO₄⋅2H₂O cathode in large-scale EES applications, a series of electrochemical tests based on coin cells employing 3 M Zn(CF₃SO₃)₂ aqueous electrolyte are carried out within the voltage window of 0.4–1.6 V (versus Zn²⁺/Zn). The typical galvanostatic charge-discharge profiles from the 1st to 50th cycles are revealed in Figure 3A. Two stable flat charge voltages at 0.7 and 1 V appeared under the driven forward by zinc ions over cycles (Shi et al., 2019). The associated differential capacity curves are also displayed in Supplementary Figure 2, Supporting information. And two apparent redox peaks of 0.58/0.76 and 0.92/1.05 V are associated with the two-step Zn²⁺ (de)intercalation processes. Moreover, the representative cyclic voltammetry test of the VOPO₄⋅2H₂O electrode at 0.1 mV s^–1 scan rate in the voltage range of 0.4–1.6 V (vs Zn²⁺/Zn) is presented in Supplementary Figure 3, Supporting information. The two apparent redox peaks are clearly observed in the CV curves, which are almost identical to the result obtained from the associated differential capacity curves. Figure 3B exhibits the capacity and coulombic efficiency of VOPO₄⋅2H₂O cathode at 0.05 A g^–1 in the voltage range of 0.4–1.6 V versus Zn²⁺/Zn. Obviously, the VOPO₄⋅2H₂O cathode exhibited a disappointing cycling stability at the low electric current density of 0.05 A g^–1, this is mainly due to the presence or co-intercalation of water in the active material and the side reaction of the electrolyte caused during the longer Zn²⁺ insertion process at the low current density, the zinc storage capacity of VOPO₄⋅2H₂O electrode is on the decline in the previous activation stages, leading to the discharge specific capacity of the VOPO₄⋅2H₂O cathode decayed from 165 to 108 mAh g^–1 over 50 cycles.

FIGURE 3

Figure 3. The electrochemical performance of the VOPO₄⋅2H₂O sample at low current density of 0.05 A g⁻¹. (A) Reversible charge-discharge profiles; (B) capacity and coulombic efficiency evolution.

Additionally, the rate performance and its associated voltage profiles of the VOPO₄⋅2H₂O electrode under various current densities are exhibited in Figures 4A,B. The VOPO₄⋅2H₂O electrode displays high reversible specific capacities of 165, 114, 108, 103, 97, and 90 mAh g^–1 (according to the discharge specific capacities at 1st, 6th, 11th, 16th, 21th, and 26th cycles) at 0.05, 0.1, 0.2, 0.5, 1, and 2 A g^–1, respectively. Even at a high electric current density of 5 A g^–1, the VOPO₄⋅2H₂O cathode can still achieve an attractive reversible capacity of 75 mAh g^–1. And a reversible specific capacity of 125 mAh g^–1 remains when the electric current density is changed back to 0.05 A g^–1, manifesting the reversible redox kinetic of VOPO₄⋅2H₂O electrode. Furthermore, the cycling behavior of the VOPO₄⋅2H₂O electrode has been examined at the high electric current density of 5 A g^–1 displayed in Figure 4C and Supplementary Figure 4, Supporting information. The VOPO₄⋅2H₂O electrode not only holds the general shape of the galvanostatic charge-discharge profiles during the long term cycle (1000 times) at 5 A g^–1, but also retains a stable reversible specific capacity of 57 mAh g^–1 with 76% of capacity retention after 1000 cycles, which is exceptional to that of recently covered VOPO₄⋅2H₂O based aqueous ZIBs (Shi et al., 2019).

FIGURE 4

Figure 4. The electrochemical characteristics of VOPO₄⋅2H₂O sample. (A) Rate capabilities at different current densities from 0.05 to 5 A g^–1; (B) typical galvanostatic charge-discharge curves; (C) the long-term cycling performance at high current density of 5 A g^–1 (the cell is initially activated for five cycles at 0.05 A g^–1).

Furthermore, EIS analysis is also measured to explore the electrochemical kinetics of VOPO₄⋅2H2O cathode for zinc storage behavior. As shown in Figure 5A, the Nyquist plot of the VOPO₄⋅2H₂O electrode is evaluated at the 3rd fully charged state. It includes the semicircle part of high-middle frequency range and the slant part of low frequency, which represent charge transfer impedance (R_ct) between the VOPO₄⋅2H₂O electrode and electrolyte and Warburg resistance (W) corresponded to the zinc ion diffusion in active material, respectively. Equivalent circuit model is established by EIS analysis and shown in the inset of Figure 5A. It has been calculated that the R_ct-value of VOPO₄⋅2H₂O electrode is simulated to be around 184 Ω, smaller than that of recent reported VOPO₄⋅2H₂O electrodes in aqueous ZIBs (Shi et al., 2019). It is remarkable that the smaller R_ct-value indicates the more relax desolvation at the interphase and faster zinc ions diffusion. Here, the apparent diffusion coefficient of Zn²⁺ (D_Zn^2+) for the VOPO₄⋅2H₂O electrode is explored and calculated by applying the Equation: $D_{{\mathit{Zn}}^{2+}}=0.5R^2{T}^2/S^2{n}^4{F}^4{C}^2{\mathrm{\sigma }}^2$ (Yang et al., 2019). In the formula, R, T, and F stand for the universal gas constant, absolute temperature and the Faraday constant, corresponding to the related values of 8.314 J mol^–1 K^–1, 298.15 K, and 96,485 C mol^–1, respectively. S, n, and C are delegated the active surface area of the cathode, the number of electrons migration and the concentration of zinc ions, respectively. The Warburg coefficient σ is related to the gradient of the fitting linear of Z′-ω^–1/2 based on the standard equation: Z′ = R + σω^−1/2 (Chen S. et al., 2019). Where ω astricts the low frequency area, R is a frequency independent kinetic parameter, whose value is approximately equal to that of R_ct in the Nyquist plot. Figure 5B displays the linear fitting of Z′-ω^–1/2 and shows a faster diffusion process (3.2 × 10^–14 cm² s^–1) of zinc ions during the repeated charge/discharge process. Overall, such small charge transfer impedance and large apparent diffusion coefficient are favorable to promote the (de)intercalation of zinc ions, exhibiting outstanding large current performance and excellent rate capability. The cyclic voltammetry (CV) technique was performed to further understand the zinc storage performance of the as-obtained VOPO₄⋅2H₂O cathode. The CV profiles at different potential scanning rates ranging from 0.1 to 0.5 mV s^–1 are revealed in Figure 5C, two well-defined sharp redox reaction peaks are observed, corresponding to the two-step Zn²⁺ (de)intercalation behavior, which also are well consisted with the voltage platforms appeared in galvanostatic charge-discharge curves. What’s more, the sharp defined redox peaks could still be kept at the high scanning rate of 0.5 mV s^–1, although the redox peaks increased in width and height with the increment of the scan rate, suggesting the good reaction kinetics and excellent rate performance of VOPO₄⋅2H₂O cathode. Furthermore, the zinc ion diffusion coefficient $D_{{\mathit{Zn}}^{2+}}$ can be also obtained base on the data from the CV profiles and applying of the Randles-Sevcik equation (Wei et al., 2017):

$i_P=(2.65\times {10}^5)n^{3/2}{\mathit{SD}}_{{\mathit{Zn}}^{2+}}^{1/2}C{\mathrm{\upsilon }}^{1/2}$

FIGURE 5

Figure 5. The kinetics behavior of the VOPO₄⋅2H₂O sample. (A) Nyquist plot; (B) fitting line of Z′−ω^–1/2; (C) CV curves at various scan rates; (D) the relationship of the specific current of the oxidation peak marked by a circle in panel (C) and the square root of scan rate.

wherein i_p is the specific peak current, n stands for the number of electrons migration, S is the actual contact area between the cathode active material and electrolyte, C is referred to the concentration of Zn²⁺, and υ is scanning rate. $D_{{\mathit{Zn}}^{2+}}$ is estimated from the slope of fitting linear of the specific current i_p along with the square root of the scan rate υ^1/2. Based on the slope value of i_p-υ^1/2 in Figure 5D, the zinc ion diffusion coefficient ($D_{{\mathit{Zn}}^{2+}}$) of VOPO₄⋅2H₂O cathode is calculated to be ∼2.0 × 10^–13 cm² s^–1, according well with the value of Zn²⁺ coefficient obtained from EIS measurement. And the fast charge transfer kinetics of Zn²⁺ (de)intercalation for the VOPO₄⋅2H₂O cathode is further highlighted via the applying of CV technique.

Conclusion

In this study, we have developed a facial solid state reaction to prepare the VOPO₄⋅2H₂O sample with industrial mass production grade. When the VOPO₄⋅2H₂O is applied as cathode material of rechargeable aqueous ZIBs, it displays high reversible specific capacity (165 mAh g^–1 at 0.05 A g^–1 within the voltage window of 0.4–1.6 V (vs. Zn²⁺/Zn)) and superior rate capability (90 and 75 mAh g^–1 at 2 and 5 A g^–1, respectively), as well as outstanding stable cyclicity (76% of capacity retention after 1000 cycles at 5 A g^–1). In addition to the excellent zinc storage performance, the VOPO₄⋅2H₂O cathode can easily achieve mass production based on simple effective regulation and control in synthesis, showing great development potential in the field of large-scale EES.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

XR directed the project. XZ, DY, and WL performed the experiment, analyzed data, and wrote the manuscript. All authors contributed to the discussion.

Funding

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 51972067 and 51802044) and Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2019B151502039).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenrg.2020.00211/full#supplementary-material

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