A method for electrochemically preparing an amorphous iron oxyhydroxy phosphate electrode material

Abstract

This invention discloses a method for the electrochemical preparation of amorphous hydroxyferric phosphate electrode materials. The method uses ferric phosphate as a raw material to prepare the working electrode. Then, in an aqueous three-electrode system, an electrochemical method is used to induce periodic intercalation and deintercalation of metal ions to drive the collapse and rearrangement of the ferric phosphate lattice. Simultaneously, the hydroxyl groups in the aqueous electrolyte replace the phosphate groups in the ferric phosphate under the drive of the electrochemical intercalation and deintercalation reaction, thereby achieving the preparation of the amorphous hydroxyferric phosphate electrode material. This invention uses an electrochemical method for in-situ conversion to prepare amorphous hydroxyferric phosphate, eliminating the need for high-temperature, high-pressure hydrothermal reactions. The process is simple, requires minimal equipment, and the controllable synthesis of amorphous hydroxyferric phosphate can be achieved by adjusting the electrochemical parameters.

Description

Technical Field

[0001] This invention relates to the field of aqueous secondary battery technology, specifically to a method for electrochemically preparing amorphous hydroxyferric phosphate electrode materials. Background Technology

[0002] Hydroxyferric phosphate (Fe) x (PO4) y (OH) z ·nH2O) is a class of iron-based compounds that combine the stability of the phosphate skeleton with the regulatory properties of the hydroxyl functional group. With its core advantages such as environmental friendliness, non-toxicity, abundant elemental reserves, and stable physicochemical properties, it has been widely used in various fields including biomedicine, environmental remediation, industrial catalysis, and electrochemical energy storage. In the field of electrochemical energy storage, hydroxyapatite (HAP) exhibits excellent structural stability, high theoretical specific capacity, wide availability of raw materials, and low production costs when used as an electrode material for lithium-ion batteries, making it a promising new energy storage electrode material.

[0003] However, in current technologies, the preparation of ferric hydroxyphosphate mainly uses hydrothermal and solvothermal methods. The hydrothermal / solvent-based preparation of ferric hydroxyphosphate requires the reaction to be completed in a pressure-resistant, sealed high-pressure reactor. This places extremely high demands on the reactor's material for corrosion resistance, high-temperature resistance, and pressure sealing. Conventional atmospheric pressure production equipment is completely unsuitable. The entire production process requires maintaining a harsh, closed environment of high temperature and high pressure, making online monitoring and dynamic adjustment of the reaction process impossible. The entire reaction cycle can only be completed using preset parameters, resulting in extremely poor controllability. Conventional processes typically involve reaction temperatures above 120°C, with some directional synthesis processes even requiring temperatures above 200°C. The operating pressure inside the reactor far exceeds atmospheric pressure, making safety management throughout the entire process extremely stringent. This type of process not only has a long single-batch reaction cycle—the single-batch reaction cycle of conventional hydrothermal / solvothermal methods is generally more than 6 hours, and for some processes that are designed to ensure the purity of the product phase, the reaction cycle can even be as long as 24 to 48 hours, resulting in extremely low production efficiency—but also suffers from limitations imposed by the equipment structure of the high-pressure reactor, the upper limit of the safe pressure bearing capacity, and the uniformity of the temperature field inside the reactor. The effective feed amount and single-reactor capacity of this process have a clear upper limit, and it is impossible to increase the capacity by simply increasing the reactor volume. During the scale-up process, the increased reactor volume is very likely to cause uneven distribution of the temperature field and flow field inside the reactor, which directly leads to uneven product phase and performance fluctuations, making it impossible to guarantee product consistency. The scale-up of industrial production is extremely difficult. In some solvothermal processes, organic solvents such as ethylene glycol and N,N-dimethylformamide are required as reaction media to control the morphology and phase composition of the products. These organic solvents are generally flammable and explosive, and the entire process of production, storage, and feeding requires a strict explosion-proof safety control system, posing significant safety risks. At the same time, a large amount of production waste liquid containing organic matter is generated after the reaction, which cannot be directly discharged and requires multiple complex processes to be treated to meet environmental emission requirements. This results in high waste liquid treatment costs and environmental control pressure, further increasing the overall cost of industrial production. In addition, the existing hydrothermal / solvothermal methods for preparing ferric hydroxyphosphate products are mostly crystalline structures, making it difficult to directly synthesize pure-phase amorphous ferric hydroxyphosphate with higher electrochemical activity. It is also impossible to achieve precise control over the crystal phase structure of the product, and it is impossible to stably and controllably adjust the degree of hydroxyl substitution in the product. The products prepared in different batches show significant fluctuations in key indicators such as phase composition, hydroxyl content, and particle size distribution. It is difficult to guarantee the consistency of product batches, and it cannot meet the stringent requirements for material performance stability in high-end application scenarios such as electrochemical energy storage. Summary of the Invention

[0004] To address the aforementioned shortcomings, this invention provides a method for the electrochemical preparation of amorphous hydroxyferric phosphate electrode materials, thereby solving the technical problems of complex preparation processes and difficulty in controllable preparation of amorphous structures in the prior art, and expanding its application fields.

[0005] In a first aspect, the present invention provides a method for electrochemically preparing amorphous hydroxyferric phosphate electrode material. The working electrode is prepared using ferric phosphate as a raw material. Then, in an aqueous three-electrode system, an electrochemical method is used to induce the periodic intercalation and deintercalation of metal ions to drive the collapse and structural rearrangement of the ferric phosphate lattice. At the same time, the hydroxyl groups in the aqueous electrolyte replace the phosphate groups in the ferric phosphate under the drive of the electrochemical intercalation and deintercalation reaction to realize the preparation of amorphous hydroxyferric phosphate electrode material.

[0006] Preferably, the electrochemical method is cyclic voltammetry or constant current charge-discharge method; wherein, the process parameters of cyclic voltammetry are as follows: scan rate of 0.1 mol / L V / s to 25 mV / s, voltage window of -1.2 V to 1.2 V, and number of cycles of 1 to 1000; the process parameters of constant current charge-discharge method are as follows: voltage window of -1.2 V to 1.2 V, constant current charge-discharge current density of 5 mA / g to 1000 mA / g, and number of cycles of 1 to 1000.

[0007] Preferably, in the aqueous three-electrode system, the working electrode is an electrode made of ferric phosphate, the counter electrode is Pt or graphite, the reference electrode is Ag / AgCl, and the salts in the electrolyte are at least one of magnesium salt, sodium salt, or calcium salt, with a salt concentration of 0.1 mol / L to 5 mol / L.

[0008] Preferably, the magnesium salt is MgSO4, MgCl2, or Mg(NO3)2; the sodium salt is Na2SO4, NaCl, or NaNO3; and the calcium salt is Ca(NO3)2 or CaCl2.

[0009] Preferably, the working electrode is composed of ferric phosphate, a conductive agent, a binder and a current collector; wherein the mass ratio of the conductive agent, the binder and the ferric phosphate is (1~3):(1~2):(6~8).

[0010] Preferably, the conductive agent is at least one of acetylene black, Ketjen black, and Super P; the binder is PVDF or PTFE; and the current collector is carbon cloth, carbon paper, or various metal current collectors.

[0011] Secondly, the present invention provides a hydroxyferric phosphate electrode material prepared by the above method, wherein the hydroxyferric phosphate electrode material has an amorphous structure, and hydroxyl groups replace some phosphate groups to form an amorphous hydroxyferric phosphate electrode material.

[0012] Thirdly, the present invention provides an application of an amorphous hydroxyferric phosphate electrode material. The hydroxyferric phosphate electrode material prepared by the above method or the above-mentioned hydroxyferric phosphate electrode material is suitable for aqueous lithium-ion batteries, aqueous magnesium-ion batteries, aqueous sodium-ion batteries, aqueous zinc-ion batteries, and aqueous potassium-ion batteries, etc.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. The electrochemical preparation method adopted in this invention, in an aqueous electrolyte environment at room temperature and pressure, drives the collapse and structural rearrangement of the iron phosphate lattice through periodic metal ion intercalation and deintercalation during the electrochemical cycle. At the same time, the hydroxyl groups in the aqueous electrolyte replace the phosphate groups in the iron phosphate under the drive of the electrochemical intercalation and deintercalation reaction to prepare amorphous hydroxy iron phosphate electrode materials. Compared with the existing mainstream preparation technology of hydroxy iron phosphate (hydrothermal / solvothermal method), this method does not require the use of harsh high temperature and high pressure environment, and the process is safer and simpler.

[0015] 2. The electrochemical preparation method used in this invention can achieve the controllable transformation of the iron phosphate crystal phase from a semi-crystalline state to a completely amorphous state and the controllable adjustment of the molar ratio of hydroxyl groups by precisely controlling the scanning speed, voltage window, constant current charge-discharge current density and number of cycles, thus solving the problem that it is difficult to controllably synthesize amorphous hydroxy iron phosphate in the prior art.

[0016] 3. In addition to being used in aqueous lithium-ion battery systems, the amorphous hydroxy iron phosphate prepared by this invention has also been shown for the first time to be used in aqueous magnesium-ion battery systems, further expanding its application scope. Attached Figure Description

[0017] Figure 1 The image shows the XRD pattern of α-FePO4.

[0018] Figure 2 This is an XRD image of the material after electrochemical treatment.

[0019] Figure 3 This is a SEM-EDS image of the material after electrochemical treatment.

[0020] Figure 4 TOF-SIMS images of hydroxyl groups in the electrode before and after electrochemical treatment (the bright yellow spots in the images represent detected -OH).

[0021] Figure 5 These are FTIR images of the electrodes before and after electrochemical treatment.

[0022] Figure 6 The image shows the CV images of ferric hydroxyphosphate and α-FePO4 in 1 mol / L magnesium chloride electrolyte.

[0023] Figure 7 The specific capacity-voltage curve is shown in a 1 mol / L magnesium chloride electrolyte containing ferric hydroxyphosphate. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the present invention are within the scope of protection of the present invention.

[0025] Unless otherwise specified in the specific circumstances, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within that range, but are not limited to the specific values ​​listed when the range is defined.

[0026] I. A method for electrochemical preparation of amorphous ferric hydroxyphosphate electrode materials

[0027] During the research and development of electrode materials for aqueous energy storage batteries, this invention yielded an unexpected discovery: after constant current charge-discharge cycling, the sharp characteristic diffraction peaks in the XRD pattern of the crystalline α-FePO4 electrode completely disappeared, exhibiting only the broadened diffuse scattering pattern characteristic of amorphous materials. Simultaneously, time-of-flight secondary ion mass spectrometry (TOF-SIMS) detected a large number of characteristic signals of hydroxyl groups in the electrode material, and Fourier transform infrared spectroscopy (FTIR) clearly detected the characteristic absorption peaks of Fe-OH bonds. This confirms that after electrochemical cycling, iron phosphate not only underwent a complete transformation from crystalline to amorphous form but also generated a large number of hydroxyl groups. This phenomenon completely overturns conventional technical understanding in this field. Existing technology generally holds that iron phosphate, as a typical intercalation electrode material, typically only undergoes reversible metal ion intercalation and deintercalation during electrochemical cycling, and does not undergo anionic group substitution reactions under normal temperature and pressure. Based on this unexpected discovery, this invention designs a method for electrochemically preparing amorphous hydroxyferric phosphate electrode materials. The working electrode is prepared using ferric phosphate as a raw material. Then, in an aqueous three-electrode system, an electrochemical method is used to induce the periodic intercalation and deintercalation of metal ions to drive the collapse and structural rearrangement of the ferric phosphate lattice. At the same time, the hydroxyl groups in the aqueous electrolyte replace the phosphate groups in the ferric phosphate under the drive of the electrochemical intercalation and deintercalation reaction to achieve the preparation of amorphous hydroxyferric phosphate electrode materials.

[0028] In-depth research into this process revealed that it comprises two mutually driving and highly coupled electrochemical-chemical processes. The first core process is the periodic metal ion intercalation / deintercalation-driven collapse and structural activation of the iron phosphate lattice. Conventionally, the stability of the iron phosphate lattice is only considered suitable for the intercalation / deintercalation of lithium ions with smaller ionic radii. However, this invention discovers that when using Mg ions with larger ionic radii… 2+ Na+ Ca 2+ When ions act as intercalating ions, significant local stress is generated within the lattice during their insertion into the interstitial spaces of the iron phosphate lattice. The periodic insertion and extraction of metal ions during charge-discharge cycles cause this lattice stress to accumulate continuously, gradually leading to the proliferation of microscopic defects such as dislocations and stacking faults. When the number of cycles reaches a critical threshold, the accumulation of defects exceeds the stability limit of the crystal structure, ultimately causing the original long-range ordered crystal structure of iron phosphate to completely collapse, forming an amorphous structural framework. The second core process is the lattice reconstruction process driven by the electric field, where hydroxyl groups replace phosphate groups. Current technology generally considers phosphate groups in the iron phosphate lattice to be a core component of the framework, extremely difficult to replace under normal conditions. Hydroxyl groups can only adsorb on the material surface and cannot penetrate the lattice to replace phosphate groups. Further analysis revealed that the Fe / P molar ratio of the material after electrochemical cycling was significantly greater than 1, confirming the loss of phosphate ions during the cycling process. During lattice collapse and structural rearrangement, the originally stable PO bonds were highly activated, with significantly reduced bond energies, providing reactive sites for anion substitution. Simultaneously, water molecules in the aqueous electrolyte dissociated under the influence of an electric field, and the resulting hydroxyl groups migrated directionally to the electrode surface under the drive of the electric field, entering the lattice activation sites and replacing the original phosphate groups. These hydroxyl groups formed stable Fe-OH bonds with Fe atoms in the lattice, ultimately completing the construction of amorphous hydroxyferric phosphate. Crucially, this hydroxyl substitution process is highly coupled with the lattice collapse process: the structural activation brought about by lattice collapse provides the necessary reaction sites for hydroxyl substitution, while the new chemical bonds formed after hydroxyl substitution further stabilize the amorphous structural framework, preventing recrystallization of the amorphous structure during cycling. This is the core reason why this invention can achieve stable and controllable preparation of amorphous hydroxyferric phosphate at room temperature and pressure.

[0029] Based on the aforementioned unexpected discovery, the method described in this invention has achieved unexpected technical effects: Compared with existing preparation techniques for ferric hydroxyphosphate, this method represents a fundamental breakthrough. It achieves efficient preparation of amorphous ferric hydroxyphosphate under mild conditions of ambient temperature and pressure, overcoming the inherent defects of existing hydrothermal / solvothermal methods. Existing mainstream preparation processes must be carried out in a closed environment under high temperature and pressure, with reaction cycles lasting tens of hours. This results in high equipment investment and energy consumption, limited single-reactor capacity, and significant difficulty in scaling up. Furthermore, the use of organic solvents poses flammability and explosiveness risks, high wastewater treatment costs, and difficulty in obtaining completely amorphous products. In contrast, the electrochemical preparation method of this invention is carried out entirely in an open environment at ambient temperature and pressure, requiring no high-pressure reaction equipment. Preparation can be completed through only tens of electrochemical cycles, with a reaction cycle of only a few hours, increasing production efficiency by more than an order of magnitude. Moreover, it uses an aqueous electrolyte throughout the process, eliminating the use of organic solvents, making it safe and environmentally friendly, with extremely low wastewater treatment costs, and possessing strong potential for large-scale application.

[0030] In some embodiments of the present invention, the electrochemical method is cyclic voltammetry or a constant current charge-discharge method. When cyclic voltammetry is used, the process parameters are as follows: scan rate of 0.1 mol / L V / s to 25 mV / s, voltage window of -1.2 V to 1.2 V, and number of cycles of 1 to 1000. The voltage window setting is related to the working electrode load, electrolyte concentration and type, and counter electrode type, and is difficult to determine as a fixed value. Its setting should meet the following criteria: it must ensure the smooth occurrence of ion intercalation / deintercalation reactions and prevent excessive water electrolysis. When the low voltage is less than -1.2 V and the high voltage is greater than 1.2 V, excessive water electrolysis will occur, resulting in significant electrolyte loss and a sharp increase in concentration, which is detrimental to the formation of ferric hydroxyphosphate. Whether the intercalation / deintercalation reaction occurs can be determined by observing the redox peaks in the CV image. When a complete reduction peak appears during a negative scan, the scan can be switched to a positive scan; similarly, when a complete oxidation peak appears during a positive scan, the scan can be switched to a negative scan. The voltage window of -1.2 V to 1.2 V provided by this invention is a limit value. Generally, the low voltage should be greater than or equal to -1.2 V, and the high voltage should be less than or equal to 1.2 V. The number of cycles is related to the amorphous degree of ferric hydroxyphosphate and the molar proportion of hydroxyl groups. The more cycles, the greater the amorphous degree and the higher the molar proportion of hydroxyl groups. When the molar proportion of hydroxyl groups reaches saturation after a certain number of cycles, and the amorphous form is fully formed, the cycling can be stopped. The number of cycles from 1 to 1000 is a limit value. When the scan speed is low, the ion insertion and extraction reactions are slower and more complete, so the number of cycles is smaller. When the scan speed is high, the ion insertion and extraction reactions are faster and less complete, so a larger value needs to be set for the cycle. Therefore, the scan speed can be 0.1mV / s, 0.5mV / s, 1mV / s, 5mV / s, 10mV / s, 15mV / s, 20mV / s, or 25mV / s, as well as all ranges and sub-ranges between these values; the voltage window can be -0.75 V to 0.7 V, -0.7 V to 0.7 V, -0.8 V to 0.8 V, -1.0 V to 1.0 V, -0.6 V to 0.6 V, -0.75 V to 0.6 V, or -0.7 V to 0.65 V. V, etc., and all ranges and subranges between the above values; the number of cycles can be 1 cycle, 10 cycles, 50 cycles, 100 cycles, 300 cycles, 500 cycles, 700 cycles, 900 cycles, or 1000 cycles, etc., and all ranges and subranges between the above values; it should be understood that, in the implementation scheme, any of the above ranges can be combined with any other range.

[0031] In some embodiments of the present invention, when a constant current charge-discharge method is used for preparation, the process parameters of the constant current charge-discharge method are as follows: voltage window of -1.2 V to 1.2 V, constant current charge-discharge current density of 5 mA / g to 1000 mA / g, and number of cycles of 1 to 1000. The voltage window setting criteria for the constant current charge-discharge method are consistent with those for the cyclic voltammetry method, namely: ensuring the smooth occurrence of ion intercalation and deintercalation reactions and preventing excessive occurrence of water electrolysis reactions. Therefore, to ensure that water electrolysis reactions do not occur excessively, the low voltage should be greater than or equal to -1.2 V, and the high voltage should be less than or equal to 1.2 V. Whether an intercalation and deintercalation reaction occurs can be determined by observing the specific capacity-voltage diagram. When a plateau voltage appears in the diagram, it indicates that an ion intercalation and deintercalation reaction has occurred at that voltage. When the voltage drops sharply during constant current discharge, it indicates a switch to constant current charging; similarly, when the voltage rises sharply during constant current charging, it indicates a switch to constant current discharge. Similar to cyclic voltammetry, the number of cycles is related to the amorphous degree of ferric hydroxyphosphate and the molar percentage of hydroxyl groups. More cycles result in greater amorphousness and a higher molar percentage of hydroxyl groups. Once a certain number of cycles is reached, the molar percentage of hydroxyl groups becomes saturated, and the amorphous form is fully formed; at this point, the cycle can be stopped. The range of 1 to 1000 cycles represents a limiting value. When the current density is low, the ion insertion / extraction reactions are slower and more complete, thus requiring fewer cycles. Conversely, when the current density is high, the ion insertion / extraction reactions are faster but less complete, thus requiring a larger cycle value. Therefore, the voltage window can be -0.75 V ~ 0.7 V, -0.7 V ~ 0.7 V, -0.8 V ~ 0.8 V, -1.0 V ~ 1.0 V, -0.6 V ~ 0.6 V, -0.75 V ~ 0.6 V, -0.7 V ~ 0.65 V, etc., as well as all ranges and sub-ranges between the above values; the constant current charge / discharge current density can be 5 mA / g, 50 mA / g, 100 mA / g, 300 mA / g, 400 mA / g, 500 mA / g, 700 mA / g, 800 mA / g, 900 mA / g, or 1000 mA / g. mA / g, etc., and all ranges and subranges between the above values; the number of cycles can be 1, 10, 50, 100, 300, 500, 700, 900 or 1000 cycles, etc., and all ranges and subranges between the above values; it should be understood that, in the implementation scheme, any of the above ranges can be combined with any other range.

[0032] In some embodiments of the present invention, in the aqueous three-electrode system, the working electrode is an electrode made of ferric phosphate, the counter electrode is Pt or graphite, the reference electrode is Ag / AgCl, and the salt in the electrolyte is at least one of magnesium, sodium, or calcium salts, with a concentration of 0.1 mol / L to 5 mol / L. The magnesium salt is MgSO4, MgCl2, or Mg(NO3)2; the sodium salt is Na2SO4, NaCl, or NaNO3; and the calcium salt is Ca(NO3)2 or CaCl2. The concentration of the salt in the electrolyte can be 0.1 mol / L, 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, or 5 mol / L, as well as all ranges and sub-ranges between these values; it should be understood that, in the embodiments, any of the above ranges can be combined with any other range.

[0033] In some embodiments of the present invention, the working electrode is composed of ferric phosphate, a conductive agent, a binder, and a current collector; wherein the mass ratio of the conductive agent, the binder, and the ferric phosphate is (1~3):(1~2):(6~8), which can be 1:2:6, 1:2:8, 1:1:8, 3:1:6, 3:1:8, 3:2:6, 3:2:8, 2:1:6, 2:1:8, 2:1.5:7, etc., as well as all ranges and sub-ranges between the above values; it should be understood that, in embodiments, any of the above ranges can be combined with any other range.

[0034] In some embodiments of the present invention, the conductive agent is at least one of acetylene black, Ketjen black, and Super P; the binder is PVDF or PTFE; and the current collector is carbon cloth, carbon paper, or various metal current collectors.

[0035] II. A hydroxyferric phosphate electrode material prepared using the above method

[0036] The hydroxyferric phosphate electrode material has an amorphous structure, and hydroxyl groups replace some phosphate groups to form an amorphous hydroxyferric phosphate electrode material. In this invention, the Fe / P molar ratio in the amorphous hydroxyferric phosphate electrode material can be calculated by detection and analysis, and the molar percentage of hydroxyl groups can be further calculated to determine the substitution of phosphate groups by hydroxyl groups. The molar percentage of hydroxyl groups is the ratio of the molar amount of hydroxyl groups to the molar amount of phosphate groups in hydroxyferric phosphate. The larger this value, the higher the degree of substitution of phosphate groups in hydroxyferric phosphate. The test method is as follows: the electrochemically treated electrode active material is dissolved in aqua regia, and then the solution concentration is diluted to around 20 ppm. The Fe concentration in the solution (C) is detected by ICP-OES. Fe ) and P concentration (C PThen, according to the formula: Calculate the molar percentage of hydroxyl groups.

[0037] III. Application of an amorphous hydroxyferric phosphate electrode material

[0038] The hydroxyferric phosphate electrode material prepared by the method of the present invention or the hydroxyferric phosphate electrode material of the present invention is suitable for aqueous lithium-ion batteries, aqueous magnesium-ion batteries, aqueous sodium-ion batteries, aqueous zinc-ion batteries, and aqueous potassium-ion batteries.

[0039] IV. Examples and Comparative Examples

[0040] Example 1

[0041] Step (1): Mix ferric phosphate, Super P and PTFE in anhydrous ethanol in a ratio of 8:1:1. After drying, take one part of the mixture and press it onto a 1 cm × 1.5 cm titanium mesh as the working electrode. The active material is weighed by a balance of 0.01%, and the weight is 1.82 mg.

[0042] Step (2): The working electrode obtained in step (1) was fixed with two platinum electrode clips and inserted into a three-electrode system with a graphite counter electrode, an Ag / AgCl reference electrode, and a 1 mol / L MgCl2 aqueous solution as the electrolyte. Electrochemical treatment was performed using a constant current charge-discharge method. The constant current charge-discharge current density of the three-electrode system was set to 100 mA / g, and the discharge was carried out to -0.75 V.

[0043] Step (3): The electrode after electrochemical treatment in step (2) is characterized by TOF-SIMS and FTIR.

[0044] Example 2

[0045] The experiment was modified from Example 1, except that the weight in step (1) was 1.73 mg. In step (2), the three-electrode system was set to a constant current charge-discharge current density of 100 mA / g, first discharging to -0.75 V and then charging to 0.7 V. The other steps were exactly the same as in Example 1.

[0046] Comparative Example 1

[0047] The experiment was modified from Example 1, except that the weight in step (1) was 1.95 mg. The electrochemical treatment in step (2) was omitted, and the TOF-SIMS and FTIR characterization in step (3) was performed directly. The other steps were exactly the same as in Example 1.

[0048] As can be seen from Examples 1-2 and Comparative Example 1:

[0049] (1) Through Figure 4 TOF-SIMS characterization and Figure 5 The FTIR infrared spectroscopy characterization yielded highly matched and mutually corroborating experimental results: TOF-SIMS in-situ visualization imaging results showed that the initial untreated electrode in Comparative Example 1 had almost no hydroxyl-characteristic yellow bright spots, with only negligible background signals, confirming that the original iron phosphate electrode itself contained almost no detectable hydroxyl groups, completely ruling out the possibility of non-electrochemical factors such as raw materials, electrode preparation process, and environmental adsorption introducing hydroxyl groups; the electrode surface of Example 1, treated only by unidirectional discharge, showed clear and dense hydroxyl-characteristic yellow bright spots, which was significantly different from Comparative Example 1, proving that the unidirectional electrochemical discharge process alone could induce the generation of hydroxyl groups in iron phosphate materials, clarifying that electrochemical treatment is the only direct cause of hydroxyl generation; the electrode of Example 2, treated with a complete charge-discharge cycle, showed significantly better results in terms of the number, brightness, and full coverage of hydroxyl-characteristic yellow bright spots than Example 1, with a significant improvement in the intensity and uniformity of the hydroxyl signal, proving that a complete metal ion insertion-extraction cycle can more significantly promote the generation and uniform distribution of hydroxyl groups, providing intuitive visual evidence for the core mechanism that "periodic ion insertion-extraction is the core driving force of hydroxylation"; at the same time, the FTIR infrared spectroscopy characterization results and TOF... The SIMS results formed a complete closed loop of evidence. In Comparative Example 1, the initial untreated electrode only showed characteristic absorption peaks at the wavenumber positions corresponding to the intrinsic Fe-O bonds of iron phosphate, with no obvious characteristic absorption peaks in the characteristic wavenumber range of the Fe-OH bonds. This further confirms that there are no lattice-bound hydroxyl groups bonded to Fe atoms in the original iron phosphate material. In Example 1, the electrode treated only by unidirectional discharge showed significant characteristic absorption peaks at the characteristic wavenumber positions of the Fe-OH bonds, proving that the hydroxyl groups generated during unidirectional electrochemical discharge are not free hydroxyl groups physically adsorbed on the material surface, but rather form stable bonds with Fe atoms. The chemical bonds and the structural hydroxyl groups that truly enter the iron phosphate lattice directly address the core question in this field that "hydroxyl groups are merely surface adsorptions," confirming the lattice reaction nature of hydroxyl groups replacing phosphate groups. In Example 2, the electrode treated with a complete charge-discharge cycle showed a higher intensity of the characteristic absorption peak of the Fe-OH bond than that of Example 1, while the intensity of the intrinsic Fe-O bond peak of iron phosphate was relatively weakened. This proves that the complete ion intercalation-deintercalation cycle significantly increased the amount of hydroxyl groups generated in the lattice, and the degree of hydroxyl group substitution for phosphate groups was significantly deepened. This is completely consistent with the semi-quantitative results of TOF-SIMS, ensuring the absolute reliability of the experimental conclusions.

[0050] (2) Examples 1-2 and Comparative Example 1 also confirmed that under a mild aqueous system at room temperature and pressure, hydroxylation of iron phosphate can be achieved through electrochemical treatment without the need for high-temperature and high-pressure hydrothermal reaction or organic solvents. This also confirmed the core reaction mechanism of the present invention, clarifying that the lattice activation driven by periodic metal ion insertion and extraction is the core driving force of the hydroxyl substitution reaction, and confirming that the hydroxylation is an anionic substitution reaction in the bulk lattice rather than surface modification. In addition, it also verified that the present invention can precisely control the degree of hydroxylation of iron phosphate by adjusting electrochemical parameters, solving the technical problems of the existing hydrothermal method being unable to precisely control the degree of hydroxyl substitution and the uncontrollable product structure. At the same time, it confirmed the core advantages of the process route being at room temperature and pressure, safe and environmentally friendly, and low cost.

[0051] Example 3

[0052] The experiment was adjusted based on Example 1, except that the weight in step (1) was 3.57 mg. In step (2), the three-electrode system was set with a constant current charge-discharge current density of 100 mA / g, a voltage window of -0.75 V to 0.7 V, and 50 cycles. The other steps were exactly the same as in Example 1, except that step (3) of Example 1 was not performed. Instead, the active material obtained on the working electrode after electrochemical treatment in step (2) was analyzed by XRD and SEM-EDS. The active material of the electrode after electrochemical treatment was dissolved in aqua regia, and then the solution concentration was diluted to around 20 ppm. Then, the C in the solution was detected by ICP-OES. Fe and C P Then, according to the formula: Calculate the molar percentage of hydroxyl groups.

[0053] The XRD test results from Example 3 are as follows: Figure 2 As shown, after electrochemical treatment according to the present invention, the sharp characteristic diffraction peaks of the original crystalline α-FePO4 completely disappeared, only exhibiting the broadened diffuse scattering bulges characteristic of amorphous materials, with no residual crystalline phase signals. This confirms that the electrochemical preparation method of the present invention can achieve precise and efficient conversion of crystalline iron phosphate into a completely amorphous structure in a mild aqueous system at room temperature and pressure through constant current charge-discharge cycles. Meanwhile, the SEM-EDS surface scan results of Example 3 are as follows... Figure 3As shown, in the active material after electrochemical treatment, the three core elements Fe, P, and O exhibit a highly uniform distribution throughout the entire particle size distribution, without any local enrichment, depletion, or loss of any elements. This confirms that the material treated by the process of this invention remains a complete Fe-PO compound, without any side reactions that cause irreversible decomposition of iron phosphate to generate phosphorus-free byproducts such as Fe(OH)3. This verifies the reaction path specificity and product phase stability of the preparation process of this invention. On the other hand, it confirms that the electrochemically induced crystal transformation and hydroxyl substitution reaction of this invention are bulk reactions throughout the entire active material, rather than just shallow modifications occurring on the material surface. This fundamentally ensures the uniformity of the overall structure and performance of the prepared electrode material. Meanwhile, through precise quantitative detection by ICP-OES and theoretical calculation, the hydroxyl molar ratio of the prepared product in Example 3 was as high as 1.28. This confirms that the electrochemical process of the present invention can achieve efficient bulk substitution of phosphate groups in the iron phosphate lattice by hydroxyl groups, rather than the adsorption of trace amounts of surface hydroxyl groups. On the other hand, it confirms that the present invention can achieve stable and controllable adjustment of the degree of hydroxyl substitution by adjusting the electrochemical cycle parameters, solving the technical problems of existing preparation processes that cannot accurately control the hydroxyl molar ratio in hydroxyferric phosphate and have poor batch stability of products. This also confirms the high coupling between the formation of amorphous structures and the hydroxyl substitution reaction in the process of the present invention, further verifying the controllability and reliability of the process.

[0054] Example 4

[0055] The experiment was modified from Example 1, with the following differences: the weight in step (1) was 3.87 mg. In step (2), the three-electrode system was set with a constant current charge-discharge current density of 100 mA / g and a voltage window of -0.75 V to 0.7 V, for 20 cycles. The other steps were exactly the same as in Example 1, except that step (3) of Example 1 was not performed. Instead, a small amount of the active material on the working electrode after electrochemical treatment in step (2) was taken, dissolved and diluted with aqua regia to around 20 ppm. The C in the solution was detected by ICP-OES. Fe and C P Then, according to the formula: Calculate the molar percentage of hydroxyl groups. Step (4): The electrochemical performance of the hydroxyferric phosphate electrode material synthesized in step (2) is tested using a three-electrode system. The working electrode is the hydroxyferric phosphate electrode after electrochemical treatment in step (2), the counter electrode is a graphite sheet electrode, the reference electrode is an Ag / AgCl electrode, and the electrolyte is a 1 mol / L MgCl2 aqueous solution. The current density of the constant current charge-discharge test is 200 mAh / g, and the voltage window is -0.75 V to 0.7 V; the scan rate of the cyclic voltammetry test is 0.5 mV / s, and the voltage window is -0.75 V to 0.7 V.

[0056] As can be seen from Example 4:

[0057] (1) It has been confirmed that the electrochemical preparation process of the present invention has extremely high reaction efficiency and precise structural controllability. In Example 4, the preparation process only requires 20 cycles of constant current charge and discharge. After precise quantitative detection by ICP-OES, the hydroxyl molar ratio of the prepared ferric phosphate can reach 1.21. This proves that the electrochemical process of the present invention can quickly realize the efficient hydroxyl substitution reaction in the ferric phosphate lattice in a mild aqueous system at room temperature and pressure.

[0058] (2) Cyclic voltammetry results of the hydroxyferric phosphate prepared in Example 4 showed that the integrated coverage area of ​​the CV curve of the prepared hydroxyferric phosphate in a 1 mol / L MgCl2 aqueous electrolyte was much larger than that of the original crystalline α-FePO4, verifying that the material prepared by this invention possesses significantly better redox activity and ion intercalation / deintercalation capability. In the conventional understanding of the art, the olivine-type long-range ordered lattice of crystalline iron phosphate is conducive to the large-radius Mg... 2+ The insertion / extraction of Mg ions is subject to strong steric hindrance and extremely poor ion diffusion kinetics, resulting in almost no effective electrochemical activity in aqueous magnesium ion systems. This invention, however, utilizes electrochemically induced amorphous modification and hydroxyl lattice substitution to achieve Mg ionization. 2+ The intercalation and deintercalation provide a large number of active sites and unobstructed ion diffusion channels, which greatly reduces the intercalation and deintercalation energy barrier of multivalent ions. This fundamentally breaks the technical limitation that crystalline iron phosphate is difficult to adapt to multivalent ion battery systems, and verifies the scientific nature and effectiveness of the structural modification path of this invention.

[0059] (3) The results of the constant current charge-discharge test in Example 4 showed that the prepared hydroxyferric phosphate achieved a high discharge specific capacity of 84.8 mAh / g in the aqueous magnesium ion electrolyte at a test current density of 200 mA / g, which confirmed that the hydroxyferric phosphate material can be stably adapted to the aqueous magnesium ion battery system and has practical magnesium storage capability.

[0060] Example 5

[0061] Adjustments were made based on Example 3, wherein steps (1) and (3) are the same as in Example 3, but the difference is that step (2) is changed to: fixing the working electrode obtained in step (1) with a platinum electrode clamp and inserting it into a three-electrode system with a graphite counter electrode, an Ag / AgCl reference electrode, and a 1 mol / L NaCl aqueous solution as the electrolyte. Electrochemical treatment was performed using a constant current charge-discharge method, with a constant current charge-discharge current density of 100 mA / g and a voltage window of -0.7 V to 0.7 V, for 20 cycles. Step (4) is changed to: performing a constant current charge-discharge test on the hydroxyferric phosphate electrode material synthesized in step (2) using a three-electrode system, with a current density of 200 mAh / g and a voltage window of -0.75 V to 0.7 V. The working electrode is the hydroxyferric phosphate electrode after electrochemical treatment in step (2), the counter electrode is a graphite sheet electrode, the reference electrode is an Ag / AgCl electrode, and the electrolyte is a 1 mol / L MgCl2 aqueous solution.

[0062] ICP-OES test results show that the molar ratio of hydroxyl groups in the synthesized hydroxyferric phosphate is 1.22, and the constant current charge-discharge test results show that the discharge specific capacity of the synthesized hydroxyferric phosphate is 82.5 mAh / g.

[0063] Comparative Example 2

[0064] Based on Example 5, adjustments were made, wherein steps (1), (3), and (4) were the same as in Example 5, but the difference was that: Step (2): The working electrode obtained in step (1) was fixed with a platinum electrode clamp and inserted into a three-electrode system with a graphite counter electrode, an Ag / AgCl reference electrode, and a 1 mol / L LiCl aqueous solution as the electrolyte. Electrochemical treatment was performed using a constant current charge-discharge method, with the constant current charge-discharge current density set at 100 mA / g and the voltage window at -0.7V to 0.7V, and the cycle was 20 times.

[0065] ICP-OES testing results showed that the molar percentage of hydroxyl groups in the synthesized ferric hydroxyphosphate was only 0.13. Constant current charge-discharge testing results showed that the discharge specific capacity of this sample was only 21.5 mAh / g.

[0066] From Examples 4-5 and Comparative Example 2, it can be found that:

[0067] (1) The precise quantitative test results of ICP-OES showed that in Example 4, using 1 mol / L MgCl2 aqueous solution as the electrolyte, the final product had a hydroxyl molar ratio of 1.21; in Example 5, using 1 mol / L NaCl aqueous solution as the electrolyte, the final product had a hydroxyl molar ratio of 1.22. Both examples achieved efficient bulk substitution of phosphate groups by hydroxyl groups within the iron phosphate lattice, and the degree of hydroxylation was at the same high level. However, in Comparative Example 2, using 1 mol / L LiCl aqueous solution as the electrolyte, the final product had a hydroxyl molar ratio of only 0.13, which was almost impossible to achieve an effective hydroxyl substitution reaction. This result indicates that the ionic radius of the cations in the electrolyte is the core factor driving the hydroxylation reaction of iron phosphate. MgCl2, with its larger ionic radius, is a key factor in this process. 2+ Na + During the periodic insertion and extraction process, sufficient local stress can be generated within the iron phosphate lattice, continuously accumulating and activating the originally highly energetic and stable PO bonds, providing ample reactive sites for hydroxyl groups to replace phosphate groups, thereby achieving efficient bulk hydroxylation; while the smaller ionic radius of Li... + The insertion and extraction process cannot generate sufficient lattice stress within the crystal lattice to activate the PO bond, thus failing to drive the hydroxyl substitution reaction.

[0068] (2) The test results show that the product prepared in Example 4 has a discharge specific capacity of 84.8 mAh / g, and the product prepared in Example 5 has a discharge specific capacity of 82.5 mAh / g. Both exhibit excellent and stable aqueous magnesium ion energy storage performance. However, the product prepared in Comparative Example 2 has a discharge specific capacity of only 21.5 mAh / g, with extremely poor electrochemical activity. This indicates that only by achieving highly efficient bulk hydroxyl-substituted amorphous hydroxyl iron phosphate can the dominance of crystalline iron phosphate on large-radius Mg2+ be overcome. 2+ The steric hindrance of the intercalation / deintercalation mechanism significantly reduces the diffusion barrier of multivalent ions, resulting in excellent multivalent ion intercalation / deintercalation capabilities. The degree of hydroxylation directly determines the electrochemical activity and energy storage performance of the material. Furthermore, the preparation process of this invention demonstrates excellent adaptability to various large-radius cation electrolyte systems, such as magnesium and sodium salts, without limiting the preparation to a single electrolyte system, thus enabling the preparation of high-performance target products. Therefore, it is reasonable to speculate that Ca, with its large ionic radius, can be used to prepare these products. 2+ The intercalation and deintercalation can also achieve the conversion of ferric phosphate to hydroxyferric phosphate.

[0069] Example 6

[0070] Step (1): Mix ferric phosphate, Super P and PTFE in anhydrous ethanol in a ratio of 8:1:1. After drying, take an appropriate amount of the mixture and press it onto a 1 cm × 1.5 cm titanium mesh as working electrodes. The active material is weighed by a balance of 0.01%, and the weight is 3.87 mg.

[0071] Step (2): Fix the working electrodes obtained in step (1) with platinum electrode clips and insert them into a three-electrode system with graphite as the counter electrode, Ag / AgCl as the reference electrode, and 1 mol / L MgCl2 aqueous solution as the electrolyte. Perform electrochemical treatment using a constant current charge-discharge method. Set the constant current charge-discharge current density to 100 mA / g and the voltage window to -0.75 V to 0.7 V, and cycle for 10 times.

[0072] Step (3): Take a small amount of the active material from the working electrode after electrochemical treatment in step (2), then dissolve and dilute it to approximately 20 ppm with aqua regia. ICP-OES is used to detect the C in the solution. Fe and C P Then, according to the formula: Calculate the molar percentage of hydroxyl groups.

[0073] Step (4): The hydroxyferric phosphate electrode material synthesized in step (2) was subjected to constant current charge-discharge test using a three-electrode system. The current density was 200 mAh / g, the voltage window was -0.75 V to 0.7 V, the working electrode was the hydroxyferric phosphate electrode after electrochemical treatment in step (2), the counter electrode was a graphite sheet electrode, the reference electrode was an Ag / AgCl electrode, and the electrolyte was a 1 mol / L MgCl2 aqueous solution.

[0074] ICP-OES test results show that the molar percentage of hydroxyl groups in the synthesized hydroxyferric phosphate is 1.16, and constant current charge-discharge test results show that the discharge specific capacity of the synthesized hydroxyferric phosphate is 80.5 mAh / g.

[0075] Example 7

[0076] This is an improvement on Example 6, except that in step (2), the electrolyte concentration is a 0.5 mol / L MgCl2 aqueous solution. The other steps are exactly the same as in Example 6.

[0077] Example 8

[0078] This is an improvement on Example 6, except that the electrolyte concentration in step (2) is a 2 mol / L MgCl2 aqueous solution. The other steps are exactly the same as in Example 6.

[0079] Example 9

[0080] This is an improvement on Example 6, except that the electrolyte concentration in step (2) is a 4 mol / L MgCl2 aqueous solution. The other steps are exactly the same as in Example 6.

[0081] ICP-OES test results showed that the molar percentage of hydroxyl groups in the hydroxyferric phosphate synthesized in Examples 7-9 was 1.15, 1.23, and 1.25, respectively, and the constant current charge-discharge test results showed that the discharge specific capacities were 85.3 mAh / g, 82.5 mAh / g, and 83.7 mAh / g, respectively.

[0082] Combining Examples 6 and 7-9, it can be found that:

[0083] (1) The precise quantitative test results of ICP-OES showed that the molar ratio of hydroxyl groups in the product prepared by 1 mol / L MgCl2 electrolyte in Example 6 was 1.16, and the molar ratios of hydroxyl groups in the products prepared by 0.5 mol / L, 2 mol / L, and 4 mol / L MgCl2 electrolytes in Examples 7-9 were 1.15, 1.23, and 1.25, respectively. In the ultra-wide range of electrolyte concentration spanning up to 8 times, the molar ratio of hydroxyl groups in the product fluctuated only slightly within the extremely narrow range of 1.15 to 1.25, without any order-of-magnitude difference, significant linear abrupt change, or performance degradation. Even in the low-concentration system of 0.5 mol / L and the high-concentration system of 4 mol / L, the efficient bulk substitution of phosphate groups in the iron phosphate lattice by hydroxyl groups could be achieved, confirming that the core hydroxylation reaction of this invention has extremely strong tolerance to electrolyte concentration and can stably achieve the preparation target in a wide concentration range.

[0084] (2) The test results show that the discharge specific capacity of all products remained stable within the excellent range of 80~86 mAh / g, without significant performance fluctuations or degradation. This result confirms that a wide range of electrolyte concentration adjustments will not negatively affect the practical electrochemical performance of the products. The process of this invention can prepare amorphous hydroxyferric phosphate materials with excellent aqueous magnesium ion energy storage performance under different electrolyte concentration systems, and the batch performance of the products is extremely stable.

[0085] (3) The electrochemical preparation process of the present invention can stably and efficiently complete the controllable preparation of amorphous hydroxy ferric phosphate within an ultrawide electrolyte concentration range of 0.5 mol / L to 4 mol / L, and the process has extremely strong adaptability to electrolyte concentration.

[0086] Example 10

[0087] This is an improvement on Example 6, except that the number of cycles in step (2) is 25. The other steps are exactly the same as in Example 6.

[0088] Example 11

[0089] This is an improvement on Example 6, except that the number of cycles in step (2) is 100. The other steps are exactly the same as in Example 6.

[0090] Example 12

[0091] This is an improvement on Example 6, except that the number of cycles in step (2) is 300. The other steps are exactly the same as in Example 6.

[0092] Example 13

[0093] This is an improvement on Example 6, except that the number of cycles in step (2) is 500. The other steps are exactly the same as in Example 6.

[0094] ICP-OES test results showed that the molar percentage of hydroxyl groups in the ferric hydroxyphosphate synthesized in Examples 10-13 was 1.17, 1.32, 1.43, and 1.45, respectively. Constant current charge-discharge test results showed that the discharge specific capacities were 84.2 mAh / g, 88.7 mAh / g, 90.1 mol / L Ah / g, and 91.3 mAh / g, respectively.

[0095] Combining Examples 6 and 10-13, it can be found that:

[0096] (1) The results of precise quantitative testing by ICP-OES showed that within the cycle range of 10 to 300 cycles, the molar percentage of hydroxyl groups showed a continuous and significant linear increase with the increase of the number of cycles, confirming that the number of cycles is the core controlling factor driving the hydroxyl substitution reaction. When the number of cycles increased to 300, the increase of the molar percentage of hydroxyl groups tended to be slow, and only a slight increase was observed at 500 cycles, confirming that at this time, the phosphate sites that could be substituted in the iron phosphate lattice were basically completely occupied by hydroxyl groups, and the hydroxyl substitution reaction reached a saturated state.

[0097] (2) As can be seen from Examples 6 and 10-13, the degree of hydroxyl substitution of phosphate is the core factor that determines the electrochemical performance of the material. The higher the molar ratio of hydroxyl, the stronger the water-based magnesium ion insertion and extraction ability and the better the discharge specific capacity of the material. When the hydroxyl substitution reaction reaches saturation, the electrochemical performance of the material also tends to stabilize.

[0098] (3) From a short cycle of 10 cycles to a long cycle of 500 cycles, the bulk hydroxyl substitution of iron phosphate can be stably achieved throughout the entire gradient range. There is no stagnation of hydroxylation reaction, product decomposition or side reaction. The prepared products can maintain excellent and stable electrochemical performance without performance mutation or decay. This proves that the process of the present invention has a very wide range of adaptability to the number of cycles, and the process has strong repeatability and stability, and has a very good application prospect.

[0099] Example 14

[0100] Based on Example 6, improvements were made. Steps (1), (3), and (4) were the same as in Example 6, except that: Step (2): The working electrodes obtained in Step (1) were fixed with platinum electrode clips and inserted into a three-electrode system with a graphite counter electrode, an Ag / AgCl electrode as the reference electrode, and a 1 mol / L MgCl2 aqueous solution as the electrolyte. The electrochemical treatment was carried out by cyclic voltammetry, with a scan rate of 1 mol / L V / s and a voltage window of -0.75 V to 0.7 V, for 10 cycles.

[0101] ICP-OES test results show that the molar percentage of hydroxyl groups in the synthesized hydroxyferric phosphate is 1.18, and constant current charge-discharge test results show that the discharge specific capacity of the synthesized hydroxyferric phosphate is 81.3 mAh / g.

[0102] Combining Examples 6 and 14, it can be found that:

[0103] (1) Both electrochemical methods can achieve efficient bulk hydroxyl substitution of iron phosphate, and there is no significant difference in the core preparation effect. The degree of hydroxylation of the products prepared in Examples 6 and 14 is at the same high level, with only negligible small fluctuations. Both methods have achieved efficient bulk substitution of phosphate groups in the iron phosphate lattice by hydroxyl groups, directly confirming that both cyclic voltammetry and constant current charge-discharge method can effectively drive the core hydroxylation reaction of this invention.

[0104] (2) The products prepared by both methods have excellent and highly consistent electrochemical energy storage performance. The practical energy storage performance of the products prepared in Example 6 and Example 14 is almost identical. Both exhibit excellent multivalent ion intercalation and deintercalation capabilities in aqueous magnesium ion electrolytes, with no significant performance differences or attenuation. This confirms that both electrochemical methods can stably prepare amorphous hydroxyferric phosphate materials with high electrochemical activity.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for electrochemically preparing amorphous hydroxyferric phosphate electrode material, characterized in that, The working electrode was prepared using ferric phosphate as a raw material. Then, in an aqueous three-electrode system, an electrochemical method was used to induce the periodic intercalation and deintercalation of metal ions to drive the collapse and rearrangement of the ferric phosphate lattice. At the same time, the hydroxyl groups in the aqueous electrolyte replaced the phosphate groups in the ferric phosphate under the drive of the electrochemical intercalation and deintercalation reaction to prepare an amorphous hydroxy ferric phosphate electrode material.

2. The method according to claim 1, characterized in that, The electrochemical method is either cyclic voltammetry or constant current charge-discharge method; wherein, the process parameters of cyclic voltammetry are as follows: scan rate of 0.1 mol / L V / s to 25 mV / s, voltage window of -1.2 V to 1.2 V, and number of cycles of 1 to 1000; the process parameters of constant current charge-discharge method are as follows: voltage window of -1.2 V to 1.2 V, constant current charge-discharge current density of 5 mA / g to 1000 mA / g, and number of cycles of 1 to 1000.

3. The method according to claim 1, characterized in that, In the aforementioned aqueous three-electrode system, the working electrode is an electrode made of ferric phosphate, the counter electrode is Pt or graphite, the reference electrode is Ag / AgCl, and the salts in the electrolyte are at least one of magnesium salt, sodium salt, or calcium salt, with a salt concentration of 0.1 mol / L to 5 mol / L.

4. The method according to claim 3, characterized in that, The magnesium salt is MgSO4, MgCl2 or Mg(NO3)2; the sodium salt is Na2SO4, NaCl or NaNO3; and the calcium salt is Ca(NO3)2 or CaCl2.

5. The method according to claim 1, characterized in that, The working electrode is composed of ferric phosphate, conductive agent, binder and current collector; wherein the mass ratio of conductive agent, binder and ferric phosphate is (1~3):(1~2):(6~8).

6. The method according to claim 5, characterized in that, The conductive agent is at least one of acetylene black, Ketjen black, and Super P; the binder is PVDF or PTFE; and the current collector is carbon cloth, carbon paper, or a metal current collector.

7. A hydroxyferric phosphate electrode material prepared by the method according to any one of claims 1-6, characterized in that, The hydroxyferric phosphate electrode material has an amorphous structure, and the hydroxyl groups replace the phosphate groups to form an amorphous hydroxyferric phosphate electrode material.

8. An application of an amorphous hydroxyferric phosphate electrode material, characterized in that, The hydroxyferric phosphate electrode material prepared by the method according to any one of claims 1-9 or the hydroxyferric phosphate electrode material according to claim 7 is suitable for aqueous lithium-ion batteries, aqueous magnesium-ion batteries, aqueous sodium-ion batteries, aqueous zinc-ion batteries and aqueous potassium-ion batteries.

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