A method for preparing battery-grade iron phosphate

Abstract

The application discloses a preparation method of battery-grade iron phosphate, which comprises the following steps: adding a mixed aqueous solution of monomers and an initiator into a ferrous salt solution, uniformly mixing to obtain a mixed bottom solution; respectively introducing a phosphate solution and hydrogen peroxide into the mixed bottom solution to generate an oxidation precipitation reaction; after the oxidation precipitation reaction is completed, the system starts to be heated to generate an aging reaction; after the aging is completed, the product is filtered, washed and dried to obtain iron phosphate dihydrate; and high-performance battery-grade iron phosphate can be obtained after calcination. The application constructs a "synthesis-polymerization-aging" synergistic reaction system, the precursor iron phosphate obtained after the synthesis reaction is uniformly distributed with polymerized monomers on the surface, the high molecular chains or networks generated in real time during the in-situ polymerization reaction are used as dynamic control agents, the morphology and particle size of the iron phosphate in the subsequent aging process are accurately controlled, and thus the high-performance battery-grade iron phosphate material with clear structure, moderate specific surface area and high tap density is obtained.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material preparation technology, specifically relating to a method for preparing high-performance battery-grade iron phosphate. Background Technology

[0002] Iron phosphate (FePO4) is a key precursor for preparing high-performance lithium iron phosphate (LFP) cathode materials for lithium-ion batteries. Its purity, crystal morphology, particle size distribution, tap density, and batch stability directly determine the consistency of the electrochemical performance of LFP materials. The "ammonia method" is one of the mainstream industrial methods for preparing iron phosphate due to its advantages such as low cost, resource utilization of the byproduct ammonium sulfate, and easy control of the iron-phosphorus ratio in the product. However, the traditional ammonium process still faces the following insurmountable technical bottlenecks: 1. Uncontrollable particle morphology and size: During the reaction, primary iron phosphate particles are prone to disordered agglomeration, forming irregularly shaped, poorly compacted micron-sized agglomerates, resulting in low tap density; 2. Wide particle size distribution and poor batch stability: Local supersaturation and temperature fluctuations in the reaction system lead to uneven nucleation and growth rates, resulting in poor batch-to-batch reproducibility; 3. Dependence on external dispersants: To improve dispersibility, surfactants are often added, such as the preparation method of iron phosphate and lithium iron phosphate disclosed in CN103887499A, as well as iron phosphate and lithium iron phosphate. The introduction of surfactants in this patent may introduce impurities that are difficult to remove (such as sodium and potassium ions), and carbon residues or abnormal grain growth may occur during high-temperature calcination, affecting the purity and electrical properties of the final product. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to provide a method for preparing battery-grade iron phosphate based on in-situ polymerization-assisted regulation of the surface morphology of iron phosphate.

[0004] Technical Solution: The preparation method of battery-grade iron phosphate of the present invention includes the following steps: adding a mixed aqueous solution of monomer and initiator to a ferrous salt solution and mixing evenly to obtain a mixed base liquid; introducing a phosphate solution and hydrogen peroxide into the mixed base liquid respectively to induce an oxidation precipitation reaction; after the oxidation precipitation reaction is completed, heating the system to induce an aging reaction; after the aging reaction is completed, filtering, washing, and drying the product to obtain an iron phosphate precursor; calcining the precursor to obtain battery-grade iron phosphate, wherein the battery-grade iron phosphate has a tap density greater than 1 g / cm³. 3 Specific surface area ranges from 5 to 6.5 m² 2 High-performance FePO4 per g.

[0005] Preferably, the ferrous salt includes at least one of ferrous sulfate (FeSO4), ferrous phosphate (Fe3(PO4)2), and ferrous chloride (FeCl2), and the concentration of the ferrous salt solution is 1.2-1.6 mol / L. By uniformly mixing the ferrous salt solution with the aqueous solution of monomer and initiator, the monomer and initiator are uniformly distributed on the surface of the precipitate during the subsequent oxidation reaction, avoiding the agglomeration of iron phosphate particles in the later process. This lays the foundation for obtaining a spherical battery-grade iron phosphate product with a suitable specific surface area and uniform particle size dispersion.

[0006] The monomer is a water-soluble vinyl carboxylic acid monomer, added at 2%-5% of the molar amount of ferrous ions. The carboxyl groups in the structure of the water-soluble vinyl carboxylic acid monomer have a strong complexation effect with ferric ions, which can uniformly inhibit the anisotropic growth of crystals during subsequent precipitation, and is conducive to the formation of isotropic spherical primary iron phosphate particles. Moreover, the vinyl carboxylic acid monomer can be completely thermally decomposed into gas during subsequent calcination, avoiding the introduction of impurities that affect the performance of iron phosphate products. When the monomer addition is too small, it will lead to insufficient polymerization, and it will be unable to form effective steric hindrance on the surface of iron phosphate, thus failing to inhibit particle agglomeration and having limited effect on morphology control. When the monomer addition is too large, it will lead to excessive system viscosity, making subsequent stirring and filtration difficult. In addition, excessive monomer decomposition during calcination will generate a large amount of gas, causing the iron phosphate particles to crack and become hollow, making it difficult to control the specific surface area of ​​the finished product.

[0007] The initiator is a water-soluble thermal decomposition initiator, added at 0.2%-1% of the total monomer mass. This amount is used to ensure complete polymerization of the monomer. The water-soluble thermal decomposition initiator can thermally decompose into gas and escape during subsequent calcination. If a small amount of initiator is not completely decomposed, it can be completely washed away with water during subsequent washing to avoid residual metal ions or other impurities, ensuring high product purity and accurate iron-phosphorus stoichiometry.

[0008] Preferably, the water-soluble vinyl carboxylic acid monomer is one of acrylic acid, methacrylic acid, and itaconic acid. The aforementioned water-soluble vinyl carboxylic acid monomers are commonly used and readily available.

[0009] Preferably, the water-soluble thermal decomposition initiator is one of ammonium persulfate, potassium persulfate, and azobisisobutyramidine hydrochloride. The aforementioned water-soluble thermal decomposition initiators are commonly used, have good polymerization initiation effects, and are easy to obtain.

[0010] Preferably, after the mixing is completed, the mixed base liquid is heated to 45-55°C to preheat the subsequent oxidation precipitation reaction and ensure the stability of the reaction process.

[0011] Preferably, the hydrogen peroxide has a mass concentration of 8%-12%. The hydrogen peroxide is used to oxidize divalent ferrous ions to trivalent ferric ions. Its concentration directly affects the oxidation rate and the controllability of the reaction, ultimately determining the purity, particle size, and performance of the ferric phosphate product. Under the co-current feed conditions described in this invention, this concentration ensures stable reaction. Typically, 0.5 mol of hydrogen peroxide is required to oxidize 1 mol of ferrous ions. However, to ensure complete oxidation of ferrous ions, the amount of hydrogen peroxide should be greater than or equal to 0.6 times the total molar amount of iron, i.e., an excess of 20%.

[0012] Preferably, the phosphate includes at least one of ammonium dihydrogen phosphate (NH4H2PO4), triammonium phosphate ((NH4)3PO4), and diammonium hydrogen phosphate ((NH4)2HPO4). The concentration of phosphate ions in the phosphate solution is 1.8-2.4 mol / L. The feed volume is controlled to ensure that the total molar amount of phosphate ions in the reaction system is 1.05-1.1 times the total molar amount of ferrous ions, i.e., the feed molar ratio is Fe:P = 1:1.05-1.1 in the system. The phosphate is used to precipitate the ferric ions obtained from oxidation in the solution, control the phosphate concentration to be excessive, ensure the iron-to-phosphorus ratio of the finished iron phosphate product is qualified, and ensure the complete reaction of the system.

[0013] The phosphate solution and hydrogen peroxide are introduced simultaneously and in parallel into the mixed base liquid. This parallel introduction is used to control the pH stability of the system. A stable pH environment can ensure uniform nucleation and growth of ferric phosphate and provide a stable reaction medium for in-situ polymerization, enabling precise control of particle morphology, particle size, and product purity. If the phosphate solution and hydrogen peroxide are mixed and then introduced, it will cause large fluctuations in the pH of the system, which will have a significant impact on the morphology and physicochemical properties of the final product, resulting in smaller primary particles and a lower tap density of the finished ferric phosphate product.

[0014] Preferably, the pH is 1.1-1.3. When the pH is too low, the ferric phosphate precipitate cannot be completely precipitated, resulting in a decrease in yield. When the pH is too high, the ferric phosphate crystallization is easily out of control, resulting in a decrease in crystallinity and a reduction in product purity and crystallinity.

[0015] The oxidation precipitation reaction is carried out at a temperature of 55-60℃ and a reaction time of 60-80 min. At this time, ferrous ions in the solution are oxidized to ferric ions and combine with phosphate ions to form ferric phosphate precipitate, which is then precipitated from the solution. The monomer and initiator are uniformly distributed on the surface of the precipitate.

[0016] The aging reaction is carried out at a temperature of 85-88℃, and the aging reaction continues for 1.5-2 hours after the material turns white. When the temperature rises to 80℃, the monomers on the aforementioned precipitate surface undergo in-situ polymerization under the action of the initiator, generating polymer chains. This "dynamically grown" polymer can be uniformly adsorbed on the surface of the primary ferric phosphate crystal nuclei, effectively preventing irreversible agglomeration between particles through strong steric hindrance, thus eliminating the generation of hard agglomeration of ferric phosphate particles at the source, and forming a "soft contact" secondary structure, resulting in a suitable specific surface area, uniform particle size dispersion, and good consistency in the finished ferric phosphate. During the aging process, the yellow ferric phosphate precursor dissolves and recrystallizes, i.e., the material turns white, indicating that the amorphous ferric phosphate crystal form transformation is complete at this point. After the material turns white, aging continues to allow the ferric phosphate crystals to grow to the ideal morphology. Stirring continues during the aging process to promote particle growth and ensure the final particle size and spherical morphology of the product.

[0017] Furthermore, the heating time for the aging reaction is ≤60 min, and the heating time to 80℃ is 15-30 min. This heating time is used to regulate the polymerization effect. When the temperature rises too quickly, the polymerization rate is too fast, leading to a sudden increase in system viscosity and local overpolymerization, causing the iron phosphate particles to form irregular hard agglomerates, resulting in uncontrolled morphology and particle size. When the temperature rises too slowly, the monomer cannot polymerize during the primary particle formation stage of iron phosphate, failing to provide steric hindrance, thus making it difficult to inhibit particle agglomeration and weakening the in-situ regulation effect. By controlling the monomer polymerization efficiency and rate, the kinetic matching relationship between the polymerization reaction and the precipitation reaction can be precisely controlled, thereby controlling the final structure of the particles. Furthermore, the morphology of the primary iron phosphate particles can be programmably customized through monomer molecular polymerization assistance.

[0018] The calcination temperature is 550-585℃, and the calcination time is 2.5-3.5h.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) It combines the iron phosphate preparation process with in-situ polymerization technology to construct a “synthesis-polymerization-aging” synergistic reaction system. After the oxidation synthesis reaction, the monomer and initiator are evenly distributed on the precipitate surface. The polymer chains or networks generated in real time during the reaction process are used as dynamic regulators to achieve precise control of iron phosphate nucleation, growth and assembly, thereby obtaining high-performance iron phosphate materials with clear structure, uniform size, moderate specific surface area and high tap density; (2) The amount of monomer and initiator used is only used for iron phosphate morphology regulation, and they can be completely thermally decomposed into gas during the subsequent calcination process to avoid metal and other impurities from remaining, ensuring the high purity of iron phosphate products and accurate iron-phosphorus stoichiometry; (3) It avoids the use of surfactants and avoids the introduction of impurities that are difficult to remove, which may lead to residual carbon or abnormal grain growth during high-temperature calcination, affecting the purity and electrical properties of the final product. Attached Figure Description

[0020] Figure 1 A scanning electron microscope (SEM) image of the ferric phosphate product prepared in an embodiment of the present invention;

[0021] Figure 2 This is a scanning electron microscope (SEM) image of the iron phosphate product prepared in the comparative example of this invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the embodiments and comparative examples. Without additional explanation, all reagents used are commercially available reagents.

[0023] Example 1

[0024] (1) Solution preparation: Use analytical grade ferrous sulfate to prepare a ferrous salt solution with a concentration of 1.6 mol / L; use NH4H2PO4 and phosphoric acid to prepare a phosphate solution with a concentration of 2.4 mol / L (control the feed volume to ensure that the total molar ratio of phosphate ions to ferrous ions in the total system is 1.1:1); take 8% hydrogen peroxide for later use (at this time, the molar amount of hydrogen peroxide should be 0.75 times the total molar amount of ferrous ions in the system); use acrylic acid with a molar amount of 4% ferrous ions as monomer and ammonium persulfate with a mass of 0.2% of the total mass of monomer as initiator to prepare a mixed aqueous solution.

[0025] (2) After introducing the ferrous salt solution into the reactor, add the mixed aqueous solution to the ferrous salt solution and stir at 200 r / min for 30 min to obtain a mixed bottom liquid. Then, heat the reactor to 50°C. Simultaneously and concurrently introduce the phosphate solution and hydrogen peroxide into the reactor, controlling the addition time for each to be 60 min. At this time, an oxidation precipitation reaction occurs in the system, and the oxidation precipitation reaction temperature is 55°C.

[0026] (3) After the oxidation synthesis reaction is completed, the reactor is heated to 88°C, and the heating time is controlled within 45 min, of which the time to heat to 80°C is 15 min. During this step, the system undergoes an aging reaction. After the material turns white, it continues to age for 2 h. The obtained product is filtered, and the filter cake is washed until the conductivity of the washing liquid is below 100 μs / cm. The filter cake is placed in a drying oven and dried at 150°C for 6 h to obtain ferric phosphate dihydrate. The ferric phosphate dihydrate is calcined at 550°C for 3 h, pulverized and demagnetized to obtain battery-grade anhydrous ferric phosphate.

[0027] Example 2

[0028] (1) Solution preparation: Prepare a ferrous salt solution with a ferrous salt concentration of 1.2 mol / L using analytical grade ferrous chloride; prepare a phosphate solution with a phosphate concentration of 1.8 mol / L using (NH4)3PO4 and phosphoric acid (control the feed volume to ensure that the total molar ratio of phosphate ions to ferrous ions in the total system is 1.05:1); prepare 12% hydrogen peroxide by mass (at this time, the molar amount of hydrogen peroxide should be 0.75 times the molar amount of ferrous salt); prepare a mixed aqueous solution using 3% ferrous ion molar amount of methacrylic acid as monomer and 0.5% potassium persulfate by mass of total monomer as initiator.

[0029] (2) After introducing the ferrous salt solution into the reactor, add the mixed aqueous solution to the ferrous salt solution and stir at 200 r / min for 30 min to obtain a mixed bottom liquid. Then, heat the reactor to 45°C. Simultaneously and concurrently introduce the phosphate solution and hydrogen peroxide into the reactor, controlling the addition time for each to be 70 min. At this time, an oxidation precipitation reaction occurs in the system, and the oxidation precipitation reaction temperature is 55°C.

[0030] (3) After the oxidation synthesis reaction is completed, the reactor is heated to 85°C for a time not exceeding 60 min, including 25 min for reaching 80°C. During this step, the system undergoes an aging reaction. After the material turns white, it is aged for another 1.5 h. The resulting product is filtered, and the filter cake is washed until the conductivity of the washing liquid is below 100 μS / cm. The filter cake is placed in a drying oven and dried at 150°C for 6 h to obtain ferric phosphate dihydrate. The ferric phosphate dihydrate is calcined at 565°C for 2.5 h, pulverized, and demagnetized to obtain battery-grade anhydrous ferric phosphate.

[0031] Example 3

[0032] (1) Solution preparation: Use analytical grade ferrous phosphate to prepare a ferrous salt solution with a concentration of 1.4 mol / L; use (NH4)2HPO4 and phosphoric acid to prepare a phosphate solution with a concentration of 2.0 mol / L (control the feed volume to ensure that the total molar ratio of phosphate ions to ferrous ions in the total system is 1.1:1); take 10% hydrogen peroxide for later use (at this time, the molar amount of hydrogen peroxide should be 0.75 times the molar amount of ferrous salt); use itaconic acid with a molar amount of 5% ferrous ions as monomer and azobisisobutyramidine hydrochloride with a mass of 1% of the total mass of monomer as initiator to prepare a mixed aqueous solution.

[0033] (2) After introducing the ferrous salt solution into the reactor, add the mixed aqueous solution to the ferrous salt solution and stir at 200 r / min for 30 min to obtain a mixed bottom liquid. Then, heat the reactor to 55°C. Simultaneously and concurrently introduce the phosphate solution and hydrogen peroxide into the reactor, controlling the addition time for each to be 80 min. At this time, an oxidation precipitation reaction occurs in the system, and the oxidation precipitation reaction temperature is 60°C.

[0034] (3) After the oxidation synthesis reaction is completed, the reactor is heated to 87°C, and the heating time is controlled within 45 min, of which the time to heat to 80°C is 30 min. During this step, the system undergoes an aging reaction. After the material turns white, it continues to age for 2 h. The obtained product is filtered, and the filter cake is washed until the conductivity of the washing liquid is below 100 μs / cm. The filter cake is placed in a drying oven and dried at 150°C for 6 h to obtain ferric phosphate dihydrate. The ferric phosphate dihydrate is calcined at 585°C for 3.5 h, pulverized and demagnetized to obtain battery-grade anhydrous ferric phosphate.

[0035] Comparative Example 1: The difference from Example 1 is that in step (2), an equal amount of 2-hydroxyethyl methacrylate is used instead of acrylic acid, while the other steps remain unchanged.

[0036] Comparative Example 2: The difference from Example 1 is that in step (1), the amount of acrylic acid added is 6% of the molar amount of ferrous ions, while the other steps remain unchanged.

[0037] Comparative Example 3: The difference from Example 1 is that in step (1), the ferrous salt solution also contains polyvinylpyrrolidone at a theoretical 2% mass of ferric phosphate, while the other steps remain unchanged.

[0038] Comparative Example 4: The difference from Example 1 is that in step (2), the phosphate solution and hydrogen peroxide are added after being mixed in advance.

[0039] Comparative Example 5: The difference from Example 1 is that in step (3), the time for heating the reactor to 80°C is 5 minutes.

[0040] Comparative Example 6: The difference from Example 1 is that in step (3), the time for heating the reactor to 80°C is 45 minutes.

[0041] Comparative Example 7: The difference from Example 1 is that it does not undergo in-situ polymerization, that is, in step (2), a mixed aqueous solution of monomer and initiator is not added to the ferrous salt solution, and the other steps remain unchanged.

[0042] Comparative Example 8: The difference from Example 1 is that in step (1), the amount of acrylic acid added is 1% of the molar amount of ferrous ions, while the other steps remain unchanged.

[0043] The ferric phosphate products prepared in the examples and comparative examples were subjected to the following performance tests, and the results are shown in Table 1. Figure 1 and Figure 2 As shown:

[0044] (1) The specific surface area was tested according to GB / T 19587-2017 using a fully automatic specific surface area and porosity analyzer;

[0045] (2) The tap density was tested using a tap density meter in accordance with GB / T 5162-2022;

[0046] (3) The content of metal ion impurities in iron phosphate was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) in accordance with GB / T 30903-2014 standard;

[0047] (4) The morphology, particle size distribution and microstructure of anhydrous iron phosphate powder were tested using a high-resolution desktop scanning electron microscope in accordance with GB / T 27766-2011 standard.

[0048] Table 1. Performance test results of the ferric phosphate products prepared in the examples and comparative examples. Serial Number <![CDATA[Specific surface area m 2 / g]]> <![CDATA[Tap density g / cm 3 > Cuppm Nippm Alppm Crppm Znppm Mgppm Mnppm Nappm Kppm Example 1 6.471 1.05 0 0 27.4 2.5 8.5 50.5 112.7 17.2 21.5 Example 2 6.122 1.02 0 0 21.6 1.7 5.2 39.9 109.4 11.8 27.3 Example 3 5.843 1.04 0 0 25.9 3.1 7.9 43.6 115.6 13.9 19.1 Comparative Example 1 4.779 0.79 0 0 18.6 2.4 5.5 55.7 105.6 15.3 21.1 Comparative Example 2 3.995 0.99 0 0 25.7 1.5 6.5 49.5 102.1 13.2 17.5 Comparative Example 3 8.772 0.59 0 0 21.4 2.5 8.5 50.5 112.7 17.2 21.5 Comparative Example 4 7.012 0.71 0 0 22.9 3.2 7.1 33.6 111.1 16.9 18.1 Comparative Example 5 3.339 1.01 0 0 19.6 2.7 6.5 53.7 115.6 15.9 16.6 Comparative Example 6 8.227 0.61 0 0 22.1 4.4 4.7 61.7 107.1 14.2 17.9 Comparative Example 7 6.359 0.76 0 0 19.1 5.1 4.1 51.4 109.3 16.2 15.3 Comparative Example 8 5.335 0.71 0 0 18.5 4.1 3.7 47.3 111.6 12.5 17.1

[0049] From Table 1, Figure 1 and Figure 2 It can be seen that when the in-situ polymerization synergistic process for preparing ferric phosphate was used in Examples 1-3, the obtained ferric phosphate products all exhibited high performance characteristics such as high tap and moderate specific surface area. Furthermore, the ICP results showed that the monomers and initiators used in the in-situ polymerization reaction could be removed during washing and sintering, resulting in low content of related metal ions and high purity of the finished ferric phosphate product. Figure 1 The scanning electron microscopy results show that the water-soluble vinyl carboxylic acid monomers used in the preparation process can effectively control the morphology and particle size of ferric phosphate. For example, the primary ferric phosphate particles prepared in Example 1 have a spherical structure and a large particle size, which is consistent with the characteristics of high tapping. At the same time, the secondary particles do not show much agglomeration. In Examples 2 and 3, the monomer type, dosage and related process parameters were adjusted within the standard range, and the test results of the ferric phosphate products prepared by them all met expectations.

[0050] Comparing Examples 1-3 with Comparative Example 1, it can be seen that when the ester monomer is replaced with one that is not a water-soluble vinyl carboxylic acid monomer, the prepared iron phosphate product fails to exhibit high tap characteristics, and its particle morphology is more plate-like. This indicates that the carboxyl group in the structure of the water-soluble vinyl carboxylic acid monomer has a strong complexation effect with the iron ion, which can uniformly inhibit the anisotropic growth of crystals during the subsequent precipitation process, and is conducive to the formation of isotropic spherical primary iron phosphate particles.

[0051] Comparing Examples 1-3 with Comparative Examples 2, 7, and 8, it can be seen that in Comparative Example 2, when the water-soluble vinyl carboxylic acid monomer was in excess, the specific surface area of ​​anhydrous ferric phosphate was low, and there was more secondary particle agglomeration. This was mainly because the excessive amount of monomer added led to an increase in the viscosity of the reaction system, making stirring and filtration difficult. At the same time, the polymer decomposition during calcination produced too much gas, which easily caused particle cracking and its gas pore-forming ability to fail, making it difficult to control the specific surface area. In Comparative Example 8, when the amount of water-soluble vinyl carboxylic acid monomer was low (1%), the in-situ polymerization coating effect was poor, and it was impossible to form effective steric hindrance on the surface of ferric phosphate, resulting in limited morphology control. Therefore, the excess amount of water-soluble vinyl carboxylic acid monomer should not exceed 5% of the molar amount of ferrous ions. In Comparative Example 7, the in-situ polymerization reaction was completely abandoned, the overall particle distribution was more dispersed, the primary particle size was small, the tapping was low, and the primary particle morphology could not present a spherical shape.

[0052] Comparing Examples 1-3 and Comparative Example 3, it can be seen that Comparative Example 3, by adding the surfactant polyvinylpyrrolidone to the ferrous solution, significantly improved the specific gravity of the finished product ferric phosphate, but the tap density was lower (below 0.6 g / cm³). 3 The small particle size of primary iron phosphate particles indicates that the addition of surfactants leads to abnormal grain growth of iron phosphate during calcination, affecting the purity and electrical properties of the final product.

[0053] Comparing Examples 1-3 and Comparative Example 4, it can be seen that when the feeding method of hydrogen peroxide and phosphate solution in the oxidation precipitation reaction is changed, the feeding process after mixing the phosphate solution and hydrogen peroxide leads to large pH fluctuations in the system, which has a significant impact on the morphology and physicochemical properties of the final product. Figure 2 It can be seen that the primary particles in the electron microscope scan are relatively small, and the tap density of the finished iron phosphate product is relatively low.

[0054] Comparing Examples 1-3 and Comparative Examples 5-6, it can be seen that the heating time of the aging reaction has a significant impact on the performance of the ferric phosphate product. When the temperature rises too quickly, the prepared product exhibits the disadvantage of an imbalance between the surface area and the tapping volume. This is because the overall reaction time is unbalanced from synthesis to in-situ polymerization and then to aging. The in-situ polymerization reaction fails to control the morphology of ferric phosphate, resulting in an irregular morphology. When the temperature rises too slowly, the holding time between synthesis and aging will be too long, and the slurry will remain in the low-temperature zone for a long time, resulting in disordered grain growth and ultimately leading to a larger D50 particle size in the finished ferric phosphate product.

[0055] In summary, the ferric phosphate prepared in Comparative Examples 2-8 only showed good performance in one of the two indicators: specific surface area or tap density, while the other indicator was significantly reduced. The product prepared by this method, however, achieves a good balance between the two indicators. In ferric phosphate preparation, specific surface area and tap density are usually negatively correlated. Especially for spherical ferric phosphate, due to the larger primary particles, a higher specific surface area is required, often at the expense of tap density. This method can fundamentally inhibit the agglomeration of ferric phosphate particles and achieve controllable preparation of ferric phosphate morphology and particle size. It maintains a high specific surface area while preserving a high tap density, without introducing harmful impurities, which is of great significance for the future development of ferric phosphate products.

Claims

1. A method for preparing battery-grade iron phosphate, characterized in that, Includes the following steps: A mixed aqueous solution of monomer and initiator is added to a ferrous salt solution and mixed thoroughly to obtain a mixed base solution; Phosphate solution and hydrogen peroxide are introduced into the mixed base liquid to induce an oxidation precipitation reaction. After the oxidation precipitation reaction is completed, the system is heated to induce an aging reaction. After the aging reaction is completed, the product is filtered, washed, and dried to obtain an iron phosphate precursor. The precursor is then calcined to obtain battery-grade iron phosphate.

2. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that, The monomer is a water-soluble vinyl carboxylic acid monomer, and its addition amount is 2%-5% of the molar amount of ferrous ions.

3. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that, The initiator is a water-soluble thermal decomposition initiator, and its addition amount is 0.2%-1% of the total mass of the monomer.

4. The method for preparing battery-grade iron phosphate according to claim 2, characterized in that, The water-soluble vinyl carboxylic acid monomer is one of acrylic acid, methacrylic acid, and itaconic acid.

5. The method for preparing battery-grade iron phosphate according to claim 3, characterized in that, The water-soluble thermal decomposition initiator is one of ammonium persulfate, potassium persulfate, or azobisisobutyramidine hydrochloride.

6. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that, The phosphate solution and hydrogen peroxide are introduced simultaneously and in parallel into the mixed base liquid.

7. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that, The oxidation precipitation reaction is carried out at a temperature of 55-60℃ for 60-80 minutes.

8. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that, The aging reaction is carried out at a temperature of 85-88℃, and the material needs to continue aging for 1.5-2 hours after it turns white.

9. The method for preparing battery-grade iron phosphate according to claim 8, characterized in that, The heating time for the aging reaction is ≤60 min, and the heating time to 80℃ is 15-30 min.

10. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that, The calcination temperature is 550-585℃, and the calcination time is 2.0-3.5h.

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