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

Rechargeable aqueous zinc ion batteries (ZIBs) have attracted increasingly solicitude in the application of large-scale electrochemical energy storage system (EES) as a result of their low-price, high security and environment-friendly. The synthesis of mass-produced electrode materials and the exploration of their potential electrochemical properties are essential steps to achieve superior large-scale EES. In this work, the large-scale preparation of vanadium oxyphosphate hydrate (VOPO4⋅2H2O) cathode material with impressively zinc storage ability is successfully demonstrated. Specially, it exhibits a high specific capacity of 165 mAh g–1 at 0.05 A g–1, and prominent rate property (90 and 75 mAh g–1 at 2 and 5 A g–1, respectively), as well as stable cyclability of 76% after 1000 cycles under a high current density of 5 A g–1 within the voltage window of 0.4–1.6 V (versus Zn2+/Zn). Moreover, the VOPO4⋅2H2O not only spreads superiority in electrochemical performance, but also shows the advantages of scalable production based on simple controllable adjustment in synthesis, which is expected to exhibit great development potential in the field of large-scale EES 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 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 largescale EES . 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;. 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 MnO 2 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 Xie et al., 2020;. Among various vanadium-based electrode materials (Yue et al., 2016;Liu et al., 2020), layered VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O material (Huang et al., 2014;Peng et al., 2017;Tang et al., 2020). Additionally, the VOPO 4 ·2H 2 O material can also store electric charge during anionic redox reaction process . There are many recent reports about VOPO 4 ·2H 2 O electrode material applied in aqueous ZIBs, but it is invariably found that the prepared VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O cathode synthesized by a reflux method. And it is shown that the VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O cathode material with large-scale production.
Herein, a facile route has been designed for the largescale synthesis of VOPO 4 ·2H 2 O sample by a simple solidstate method, which is in great favor of extending the application of aqueous ZIBs in large-scale storage devices. The VOPO 4 ·2H 2 O 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 Zn 2+ /Zn). Moreover, the VOPO 4 ·2H 2 O 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 ).

Synthesis of VOPO 4 ·2H 2 O Sample
In a typical procedure, a suspension consisting of 1440 mL deionized water (DI H 2 O), 60 g vanadium pentoxide (V 2 O 5 ) powders, and 360 mL concentrated phosphoric acid (85% H 3 PO 4 ) 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 VOPO 4 ·2H 2 O sample. Here, it is noteworthy that the total mass of the as-obtained VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O, 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 (CF 3 SO 3 ) 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 Zn 2+ /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 10 5 -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 VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O (space group: P4/n, JCPDS card: 84-0111). And no detectable impurity peaks can be found, indicating the high purity of VOPO 4 ·2H 2 O crystal (Zhou  (Zhou et al., 2014). These characteristic peaks are proved to be in agreement with the chemical composition of VOPO 4 ·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 Hyoung et al., 2019). The surface element analysis and chemical states for asprepared composite were further identified via XPS spectra. As demonstrated in Figure 1C, P, O, and V elements were presented in VOPO 4 ·2H 2 O 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 V 5+ and V 4+ , respectively. And the other peaks at 525.8 and 524.6 eV are associated with V2p 1/2 of V 5+ and V 4+ , respectively (Chen et al., 2018).
The morphology and detailed structure of VOPO 4 ·2H 2 O sample are investigated by FESEM and TEM analysis. The VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O.
In order to systematically investigate the zinc storage performance of VOPO 4 ·2H 2 O cathode in large-scale EES applications, a series of electrochemical tests based on coin cells employing 3 M Zn(CF 3 SO 3 ) 2 aqueous electrolyte are carried out within the voltage window of 0.4-1.6 V (versus Zn 2+ /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 Zn 2+ (de)intercalation processes. Moreover, the representative cyclic voltammetry test of the VOPO 4 ·2H 2 O electrode at 0.1 mV s −1 scan rate in the voltage range of 0.4-1.6 V (vs Zn 2+ /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 VOPO 4 ·2H 2 O cathode at 0.05 A g −1 in the voltage range of 0.4-1.6 V versus Zn 2+ /Zn. Obviously, the VOPO 4 ·2H 2 O 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 Zn 2+ insertion process at the low current density, the zinc storage capacity of VOPO 4 ·2H 2 O electrode is on the decline in the previous activation stages, leading to the discharge specific capacity of the VOPO 4 ·2H 2 O cathode decayed from 165 to 108 mAh g −1 over 50 cycles.
Additionally, the rate performance and its associated voltage profiles of the VOPO 4 ·2H 2 O electrode under various current densities are exhibited in Figures 4A,B. The VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O electrode. Furthermore, the cycling behavior of the VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O based aqueous ZIBs (Shi et al., 2019).
Furthermore, EIS analysis is also measured to explore the electrochemical kinetics of VOPO 4 ·2H2O cathode for zinc storage behavior. As shown in Figure 5A, the Nyquist plot of the VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O electrode is simulated to be around 184 , smaller than that of recent reported VOPO 4 ·2H 2 O electrodes in aqueous ZIBs (Shi et al., 2019). It is remarkable that the  (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 ).
smaller R ct -value indicates the more relax desolvation at the interphase and faster zinc ions diffusion. Here, the apparent diffusion coefficient of Zn 2+ (D Zn 2+ ) for the VOPO 4 ·2H 2 O electrode is explored and calculated by applying the Equation: D Zn 2+ = 0.5R 2 T 2 /S 2 n 4 F 4 C 2 σ 2 . 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 . 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 cm 2 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 VOPO 4 ·2H 2 O 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 welldefined sharp redox reaction peaks are observed, corresponding to the two-step Zn 2+ (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 VOPO 4 ·2H 2 O cathode. Furthermore, the zinc ion diffusion coefficient D 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 × 10 5 )n 3/2 SD 1/2 Zn 2+ Cυ 1/2 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 Zn 2+ , and υ is scanning rate. D 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 Zn 2+ ) of VOPO 4 ·2H 2 O cathode is calculated to be ∼2.0 × 10 −13 cm 2 s −1 , according well with the value of Zn 2+ coefficient obtained from EIS measurement. And the fast charge transfer kinetics of Zn 2+ (de)intercalation for the VOPO 4 ·2H 2 O 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 VOPO 4 ·2H 2 O sample with industrial mass production grade. When the VOPO 4 ·2H 2 O 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. Zn 2+ /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 VOPO 4 ·2H 2 O 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.