Tetrahydrofuran intercalated layered vanadium(III) phosphate electrode for aqueous zinc-ion battery

Layered VOPO4·2H2O is considered a good electrode material structure for the insertion and extraction of energy storage ions, but the lattice water may affect the electrochemical stability. To provide more active site and more stable layered structure, the lattice water was replaced by tetrahydrofuran (THF), and the THF intercalated VOPO4 was obtained. The interlayer spacing has changed from 7.4 to 6.3 Å after THF intercalation, and the layered structure of VOPO4-THF is well preserved. It offers enhanced structural stability during charge and discharge processes, minimizing electrode degradation. In addition, increasing the oxidation state of pentavalent vanadium in VOPO4-THF is also beneficial for enhancing the capacitance of electrode materials. Combining the stable layered structure with a high oxidation state, VOPO4-THF electrode material exhibits good capacity and long cycle life. Our work convert traditional VOPO4·2H2O electrode into a promising high-performance active electrode for zinc-ion batteries.


Introduction
Aqueous zinc-ion batteries are considered to be promising next-generation energy storage devices due to their inherent safety and competitive cost [1][2][3].Currently, research on zinc-ion batteries is mainly focused on developing highercapacity positive electrode materials and optimizing the stability of zinc negative electrode materials [4][5][6].The positive electrode material is the most critical factor affecting the performance of aqueous zinc-ion batteries, determining energy density, cycle life, and capacity.Nowadays, the most extensively studied positive electrode materials are Mnbased materials, V-based materials, and Prussian blue analogues [7][8][9].Among these, Mn-based materials and Prussian blue analogues face challenges in their application as positive electrode materials for zinc-ion batteries due to Mn dissolution caused by complex structural transformations and disproportionation reactions during cycling, as well as low capacity resulting from insufficient active centers and structural damage caused by Zn 2+ insertion/extraction processes [10,11].On the other hand, V-based materials, with their layered crystal lattice structure and variable valence states of vanadium, can serve as excellent positive electrode materials for aqueous zinc-ion batteries.For example, VO 2 , V 2 O 3 , V 2 O 5 , and V 5 O 13 materials have been reported to exhibit a high specific capacity of up to 300 mAh g −1 at 0.1 A g −1 [12][13][14][15].However, these V-based materials still face challenges such as limited reactive sites and volume expansion.Therefore, developing V-based positive electrode materials with fast reaction kinetics and structural stability is one of the key aspects in improving the performance of aqueous zinc-ion batteries.
Layered hydrated vanadium phosphate (VOPO 4 •2H 2 O), composed of PO 4 3− tetrahedra and [VO 6 ] octahedra connected by strong covalent bonds, is a potential candidate for good V-based positive electrode materials [16,17].Its unique layered structure can alleviate the problems of V-based positive electrode materials with slow reaction kinetics and poor structural stability to some extent.Nevertheless, the capacitance of VOPO   [20][21][22].These measures can enhance the specific capacitance of VOPO 4 •2H 2 O, but the lattice water may affect the electrochemical stability.In this work, we employ tetrahydrofuran (THF) organic reagent as intercalation material to replace the lattice water of VOPO 4 •2H 2 O, and the VOPO 4 -THF positive electrode was obtained.The interlayer spacing of VOPO 4 •2H 2 O has changed from 7.4 to 6.3 Å after THF intercalation, which is beneficial for its structural stability (Fig. 1).In addition, THF intercalation increases the content of pentavalent V, which is conducive to the redox reaction of VOPO 4 electrode and improves its capacitance.As a result, VOPO 4 -THF presents a good reversible capacity of 196.2 mAh g −1 at 0.1 A g −1 with an impressive energy density of 202.6 Wh kg −1 .Compared with VOPO 4 •2H 2 O, the capacitance of VOPO 4 -THF increased by four times.This work shows that the intercalation of THF optimizes the capacitance and cycling stability of VOPO 4 electrode materials, providing an alternative strategy to improve its electrochemical performance for zinc-ion batteries.

Preparation of VOPO 4 -THF
To prepare VOPO 4 -THF sample, VOPO 4 •2H 2 O was first prepared as follows: 4.8 g of V 2 O 5 , 26.6 mL of H 3 PO 4 , and 115.4 mL of deionized water were added into three-necked flasks separately.Then, 10 mL of HNO 3 (30%) was added and mixed evenly to stabilize the oxidation state of vanadium.The mixture was transferred to the reactor and maintained at 110 °C for 16 h.After cooling to room temperature, collect the yellow precipitate from the bottom, centrifuge, and wash it several times with deionized water and acetone separately.After drying, the bright yellow VOPO 4 •2H 2 O sample was obtained.

Characterization
The crystalline phase of VOPO 4 -THF and VOPO 4 •2H 2 O was investigated by powder X-ray diffraction (XRD) with Bruker D8 ADVANCE diffractometer (Cu Ka radiation, λ = 0.15418 Å) over the range 10-50° at a scan rate of 10°/min.Fourier transform infrared (FT-IR) spectra of VOPO 4 -THF and VOPO 4 •2H 2 O were collected on a Nicolet Avatar 370 spectrometer with KBr tablets.The chemical states of each element in the prepared samples were analyzed by X-ray photoelectron spectroscopy (XPS) on ESCALAB 250 (USA) using a monochromatic Al-K source.The morphology and microstructure of VOPO 4 -THF were observed by high-resolution transmission electron microscopy (HRTEM) (JEM-2100F) and field-emission scanning electron microscopy (FESEM) (JEOL JSM-6700F) equipped with element mapping.

Electrochemical measurements
The cathode material was prepared by mixing the active materials, acetylene black, and polyvinylidene fluoride (PVDF) in 1-methyl-2-pyrodanone (NMP) solutions in mass ratio of 7:2:1.The smooth ink was evenly coated on the stainless steel net and then dried in a vacuum oven at 60 °C for 12 h.The anode material was commercial zinc foil, and the electrolyte was 3 mol L −1 of Zn(CF 3 SO 3 ) 2 solution with the solvent of ultrapure water.The electrochemical performance of VOPO 4 -THF and VOPO 4 •2H 2 O electrode materials was evaluated by galvanostatic charge-discharge (GCD) tests, which were completed on the Neware electrochemical workstation, the voltage window of GCD is between 0.2 and 1.9 V, and the current density was set in the range of 0.05 ~ 2 A g −1 .

Results and discussion
The crystal structures of as-prepared VOPO 4 -THF and VOPO 4 •2H 2 O have been confirmed by XRD measurement as seen from Fig. 2a; the XRD patterns of them can be indexed to VOPO 4 •2H 2 O (PDF No. 36-1472) with the tetragonal P4/n space group [23,24].The (001), ( 200), (220) diffraction peaks can be identified in the XRD result of VOPO 4 -THF; these peaks all exhibit a certain degree of right shift compared to that of pure VOPO 4 •2H 2 O sample.According to the Bragg formula (2dsinθ = nλ) to calculate their layer spacing [25,26], it was found that the interlayer spacing of VOPO 4 -THF is 6.The surface elements and chemical states of the material before and after THF intercalation in VOPO 4 were explored by XPS technology.Figure 2c shows the survey XPS spectra of them, indicating that the characteristic binding energy of P 2p, P 2s, V 2p, V 2s, and O 1s can be detected [28].The C 1s peaks of VOPO 4 •2H 2 O may come from external carbon in the detected system.The intercalation of THF did not have a significant impact on the surface element distribution of VOPO 4 .However, it has a great influence on the valence state of vanadium.The high-resolution core-level XPS spectra of V 2p 3/2 in VOPO 4 -THF and VOPO 4 •2H 2 O have been presented in Fig. 2d, e, respectively.After comparison, the integral area ratio of V 5+ to V 4+ increases from 2.15 to 3.62 when intercalated by THF.It indicates that part The morphology and elemental characteristics of VOPO 4 -THF were detected by FESEM and element mappings.As displayed in Fig. 3a-d, the samples exhibit a micrometer-sized block-like morphology, with particle sizes around 10 μm and a relatively uniform distribution.From the magnified FESEM image, it can be observed that the blocklike VOPO 4 -THF is composed of stacked lamellar structures.Compared with that of VOPO 4 •2H 2 O sample (Figure S3, Supporting Information), the intercalated VOPO 4 -THF still retains a well-defined layered structure.
Besides, the element mappings in Fig. 3e-g reveal the presence of V, O, and P elements, with a homogeneous distribution and no detection of other impurity elements.Further characterization of the microstructure of VOPO 4 -THF material was performed using HRTEM.As shown in Fig. 4a-d, VOPO 4 -THF exhibits a layered stacking structure with a smooth surface at different magnifications, which is consistent with the results obtained from FESEM.In addition to the microscopic morphology, we compared the specific surface area of VOPO 4 •2H 2 O and VOPO 4 -THF (Table S1, Supporting Information).The specific surface area of VOPO 4 •2H 2 O is 5.3118 m 2 g −1 and VOPO 4 -THF is 7.3719 m 2 g −1 ; it is found that the specific surface area of VOPO 4 after THF intercalation is slightly larger, which can provide more active sites for zinc ions.
To evaluate the electrochemical performance of VOPO 4 -THF cathode material, we first measured the CV of VOPO 4 •2H 2 O at 1 mV s −1 and compared it with VOPO 4 -THF, and Figure S4 (Supporting Information) shows their CV results at 1 mV s −1 .It is obvious that VOPO 4 -THF has a larger integrated area, the storage of zinc ions is more abundant, and the assembled zinc-ion battery has a higher capacity.Moreover, the CV curves at different scanning speeds have been analyzed to quantify the Faradaic contribution, which were recorded at potential window of 0.2-1.9V with scan rates of 0.2-1 mV s −1 (Fig. 5a).Notably, three charge peaks and three discharge peaks of VOPO 4 -THF maintain well at higher sweep speeds.To deeply analyze the zinc-ion storage   5e shows the comparison of their GCD profiles.It can be observed that when the amount of THF added is 5.0 g, the discharge specific capacity of the electrode material reaches a maximum of 196.2 mAh g −1 .However, when the amount of THF is too low or too high, the discharge specific capacity is relatively low, such as the discharge specific capacitance of In addition, the cyclic stability of VOPO 4 •2H 2 O and VOPO 4 -THF electrode materials has been explored.As presented in Fig. 5f, the VOPO 4 -THF shows a significantly better capacitance retention rate of 76%, while the VOPO 4 •2H 2 O experiences a decrease in capacitance retention rate between the 70th and 100th cycle and maintains a rate below 50% after 200 cycles.This indicates that VOPO 4 -THF electrode materials show better stability than VOPO 4 •2H 2 O, which may be related to the more stable structure of VOPO 4 -THF after interlayer water being replaced by THF.
In addition, the stability of VOPO 4 -THF can also be reflected in Figure S5 (Supporting Information), which shows that the main characteristic peak of VOPO 4 -THF is clearly visible after 200 cycles of cycling and no other impurity peaks appear.Only with the insertion and removal of zinc ions, the layer spacing is changed, reflecting the stability of its structure.
The rate performance of VOPO 4 -THF at different current densities is shown in Fig. 5g.The reversible specific capacities of the VOPO 4 -THF electrode at 0.05, 0.1, 0.2, 0.5, 1, and 2 A g −1 are 236.3,186.9, 131.3, 101.4,75.8, and 65.4 mAh g −1 , respectively.When the current density is adjusted to 0.05 A g −1 , the initial reversible specific capacity reaches 196.6 mAh g −1 , indicating its good reversible redox kinetics with a coulombic efficiency remains almost 100%.The charging/discharging curves of VOPO 4 -THF for the first three cycles at 0.1 A g −1 are presented in Fig. 5h.According to the discharge curve calculation, the specific discharge capacities for the first, second, and third cycles are 196.2,197.7, and 192.9 mAh g −1 , respectively.The increasing trend indicates the battery activation process, while the subsequent decrease suggests stabilization.

Conclusion
In summary, THF intercalated VOPO 4 has been prepared to replace the lattice water in VOPO 4 •2H 2 O.After intercalation, the interlayer spacing of VOPO 4 •2H 2 O has changed from 7.4 to 6.3 Å, and part of V 4+ is oxidized to V 5+ in VOPO 4 -THF.More oxidation state of vanadium is conducive to the improvement of capacitance.In addition, the layered structure of VOPO 4 -THF is well preserved, facilitating the insertion and extraction of Zn 2+ .The optimal intercalation content of THF was explored in this work, the prepared optimal VOPO 4 -THF shows a good reversible capacity as high as 192.9 mAh g −1 at 0.1 A g −1 , and the highest energy density reaches 202.6 Wh kg −1 .This work provides a strategy for improving layered VOPO 4 electrode materials for zinc-ion batteries.

Fig. 1
Fig. 1 Structure diagram of VOPO 4 •2H 2 O and VOPO 4 -THF 3 Å, which is smaller than that of VOPO 4 •2H 2 O (7.4 Å).It indicates that the interlayer crystalline water in VOPO 4 •2H 2 O is replaced by THF that reduces the interlayer spacing.The comparison of FT-IR spectra of VOPO 4 -THF and VOPO 4 •2H 2 O further proves that the interlayer water is replaced in VOPO 4 -THF.As shown in Fig. 2b, the peaks at 968, 1613, and 3349 cm −1 is V-OH 2 interlayer H 2 O, H 2 O, and V-O bond, respectively [27].Compared with the FT-IR result of VOPO 4 •2H 2 O, there is almost no characteristic absorption peak of interlayer H 2 O in VOPO 4 -THF, suggesting the lack of interlayer water.THF organic molecules are composed of C, H, and O elements, and its molecular formula is C 4 H 8 O. Clearly, as shown in Figure S1 (Supporting Information), the scissor-like vibrational band CH 2 attributed to −CH at peak 1429 cm −1 .These characteristic peaks indicate the presence of THF in VOPO 4 -THF.In addition, TGA can also reflect the presence of THF on the side (Figure S2, Supporting Information).Since THF has a lower boiling point than H 2 O, VOPO 4 -THF will preferentially lose weight and stabilize as the temperature increases from 0 to 400 °C.As expected, the temperature at which VOPO 4 -THF reached stabilization was 180 °C, while VOPO 4 •2H 2 O was 190 °C.

Fig. 3 a
Fig. 3 a-d FESEM images of VOPO 4 -THF at different magnifications.e-g V, O, and P element mappings, respectively

Fig. 5 a
Fig. 5 a CV curves of VOPO 4 -THF at different scanning speeds, b relationship between scan rates and peak currents of VOPO 4 -THF, c CV fitting of pseudo-capacitance, d contribution ratio of VOPO 4 -THF at various scan rates, e GCD profiles of as-prepared VOPO 4 •2H 2 O Yifan Qiao and Jingjing Yuan wrote the main manuscript text.Yuchen Lu and Yihan Ren conducted a small portion of experiments and tests.Yifan Li and Zhihao Zhang prepared figures.Haiqun Chen checked this manuscript.All authors reviewed the manuscript.Funding The authors gratefully acknowledge the financial support from Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (No. BM2012110) and Changzhou University College Student Innovation and Entrepreneurship Training Program (No. 202210292015Y) and the start-up funding to J. J. Yuan by Changzhou University (ZMF22020057).