Process for the preparation of orthorhombic iron phosphate dihydrate

Abstract

The application relates to the technical field of iron phosphate materials, in particular to a preparation method of orthorhombic iron phosphate dihydrate, which comprises the following steps: (1) mixing a ferrous source solution and a nitrogen-containing compound, stirring at 0 DEG C to 5 DEG C, and obtaining product A; (2) introducing an oxidizing gas into the product A, controlling the redox potential of the system to be 450 mV to 550 mV, adding a complexing agent after the oxidation reaction, and continuing to react, thereby obtaining product B; (3) adding a phosphate solution drop by drop into the product B, maintaining the pH value of the system to change in two stages, adjusting the pH value of the system to be 2.2 to 2.8 in the first stage, and adjusting the pH value of the system to be 1.2 to 2.2 in the second stage, thereby obtaining product C; and (4) vacuum drying the product C, and obtaining the orthorhombic iron phosphate dihydrate product. The orthorhombic iron phosphate dihydrate prepared by the method has a product purity of greater than or equal to 98.5%, a dihydrate crystallinity of greater than 92%, and an electrochemical performance which is 30% higher than that of a traditional monoclinic crystal.

Description

Technical Field

[0001] This invention relates to the field of iron phosphate materials technology, specifically to a method for preparing orthorhombic iron phosphate dihydrate. Background Technology

[0002] As a core energy storage component in the new energy field, the performance of the cathode material of lithium-ion batteries directly affects the energy density and cycle life of the batteries.

[0003] A lithium iron phosphate (LiFePO4) battery is a lithium-ion battery that uses lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material. The rated voltage of a single cell is 3.2V, and the charging cut-off voltage is 3.6V~3.65V. During charging, some lithium ions are released from the lithium iron phosphate and transferred to the negative electrode via the electrolyte, where they are embedded in the carbon material. Simultaneously, electrons are released from the positive electrode and travel from the external circuit to the negative electrode, maintaining the chemical reaction balance. During discharging, lithium ions are released from the negative electrode, travel through the electrolyte to the positive electrode, and simultaneously, electrons are released from the negative electrode and travel from the external circuit to the positive electrode, providing energy to the external environment.

[0004] Lithium iron phosphate (LiFePO4) has become the mainstream choice in the field of power batteries due to its low raw material cost, excellent thermal stability and environmentally friendly properties.

[0005] However, the electrochemical performance of lithium iron phosphate is highly dependent on the microstructure of its precursor, iron phosphate (FePO4·nH2O). Currently, industrial production of iron phosphate mainly relies on three processes: the ammonium process, the sodium process, and the iron process. Their common drawback is that they generally employ high-temperature solid-phase reaction or liquid-phase precipitation processes, resulting in iron phosphate that is mostly monoclinic with low crystal symmetry and tight layer stacking, leading to the following problems: (1) Limitations of crystal structure restrict the diffusion rate of lithium ions during charging and discharging (diffusion coefficient <10 at room temperature). -10 cm 2 / s), resulting in a severe degradation of the rate performance of the electrode material.

[0006] (2) The bottleneck of compaction density is that the non-close-packed characteristics of monoclinic crystal particles result in a tap density that is generally lower than 2.3 g / cm³. 3 This directly impacts the improvement of battery energy density. Studies have shown that orthorhombic crystal systems can optimize lithium-ion transport channels, but existing synthesis processes struggle to achieve controllable growth of this crystal form.

[0007] (3) Contradictions in industrial production, such as patent CN 102491302 A which discloses a battery-grade iron phosphate and its preparation method. This battery-grade iron phosphate is an anhydrous iron phosphate with orthorhombic crystal structure. Its preparation method uses an oxidation precipitation method with air as an oxidant. A pH adjusting agent solution is added to an aqueous solution of a mixture of divalent iron salt and phosphoric acid or phosphate, air is introduced, and the reaction is stirred to generate a crystalline complex containing ammonium ions, hydroxide ions and water of crystallization; then it is prepared by solid-liquid separation, washing, drying and calcination. Although the use of an air oxidation system in this patent reduces costs, the reaction kinetics are uncontrollable and the product morphology is not uniform (D 50 Fluctuations of ±20nm make it difficult to meet the consistency requirements of power batteries.

[0008] Orthorhombic iron phosphate dihydrate (orthogonal-FePO4·2H2O) has attracted much attention in the field of energy materials due to its unique crystal structure, especially in the field of lithium / sodium-ion battery cathode materials, where it shows significant advantages. As a lithium battery, its orthogonal structure anisotropy reduces volume expansion during charging and discharging. Compared with monoclinic lithium iron phosphate, it significantly reduces the generation of electrode material cracks and extends cycle life.

[0009] The traditional method for preparing orthorhombic ferric phosphate dihydrate is generally an oxidation precipitation method, which includes: oxidizing ferrous salts (such as FeCl2, Fe(NO3)2) to ferric salts, and then dissolving them in water with phosphates (such as NaH2PO4, (NH4)2HPO4) in a stoichiometric ratio. The pH of the solution is adjusted to 1.5-2.5, the temperature is maintained at 60-90℃ and stirred to obtain a precipitate. After washing and filtration, the precipitate is dried at 60℃-80℃ to obtain orthorhombic ferric phosphate dihydrate (FePO4·2H2O).

[0010] The technical defects of the above conventional methods are as follows: (1) The above methods use ferrous solution as the oxidation treatment between ferrous salts. Before oxidation, the ferrous salt solution is prone to iron ion self-oxidation side reaction in water, which affects the purity of the product of subsequent reaction. (2) The oxidation process is difficult to control, and it is easy to have insufficient oxidation or over-oxidation, resulting in by-products or impurities. (3) The oxidized iron ions are unstable. Iron ions may be locally supersaturated, leading to hydrolysis or agglomeration, which affects the uniformity of precipitation under acidic conditions and introduces amorphous impurities. Summary of the Invention

[0011] To address the problem of low purity in existing orthorhombic iron phosphate synthesis, this invention provides a method for preparing orthorhombic iron phosphate dihydrate, resulting in an orthorhombic iron phosphate dihydrate product with a purity ≥98.5%, a dihydrate crystallinity >92%, and electrochemical performance improved by more than 30% compared to the traditional monoclinic form.

[0012] The technical solution of this invention is as follows: This invention provides a method for preparing orthorhombic iron phosphate dihydrate, comprising the following steps: (1) Mix the ferrous source solution with a nitrogen-containing compound and stir at 0℃~5℃ to obtain product A; (2) Introduce oxidizing gas into product A and control the redox potential of the system to be 450 mV~550 mV. After the reaction, add a complexing agent to continue the reaction and obtain product B. (3) Add phosphate solution to product B to maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.2~2.8, and in the second stage, adjust the pH value of the system to 1.2~2.2 to obtain product C; (4) Product C is dried under vacuum to obtain an orthorhombic iron phosphate dihydrate product.

[0013] It should be further noted that in step (1), the ferrous source solution is at least one of ferrous chloride solution and ferrous nitrate solution. The ferrous chloride solution and ferrous nitrate solution provide a controllable ferrous salt precursor. Ferrous chloride (FeCl2) or ferrous nitrate (Fe(NO3)2) releases ferrous ions (Fe) upon dissolution. 2+ As the starting material for the oxidation reaction, it is gradually converted into iron ions through subsequent oxidation steps.

[0014] It should be further noted that in step (1), the nitrogen-containing compound is at least one of urea and triethanolamine; the mass ratio of ferrous source to urea in the ferrous source solution is 20~30:1. Urea or triethanolamine reacts with Fe... 2+ Formation of soluble complexes can inhibit Fe 2+ Premature oxidation or hydrolysis at low temperatures reduces the generation of byproducts and improves product purity.

[0015] It should be further noted that in step (2), the oxidizing gas is a mixture of oxygen and ozone, with a volume ratio of 3~5:1. The synergistic oxidation system formed by oxygen / ozone exhibits strong ozone oxidation capabilities, enabling "explosive oxidation" and rapidly reducing Fe. 2+ Concentration, reducing reaction resistance, and simultaneously at low Fe concentrations. 2+ At certain concentrations, oxygen prevents Fe from being oxidized through slow but persistent oxidation. 2+ Accumulation ensures complete oxidation reaction.

[0016] It should be further noted that in step (2), the complexing agent is a composite complexing agent of ammonium citrate and disodium ethylenediaminetetraacetate (EDTA-2Na), and the mass ratio of ammonium citrate to disodium EDTA-2Na is 2~3:1. The addition of citric acid and the EDTA-2Na composite complexing agent, along with Fe... 3+ Formation of stable complexes, inhibiting Fe 3+Hydrolyzes into Fe(OH)3 colloid, ensuring Fe 3+ It exists in a soluble form, providing a uniform nucleation environment for subsequent phosphate reactions.

[0017] It should be further noted that in step (2), the oxidation reaction takes 1 to 2 hours.

[0018] It should be further noted that in step (3), the phosphate solution is a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution, and the molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 1.5~2.5:1.

[0019] It should be further noted that in step (3), the pH value of the first stage system is maintained for 1 hour and the temperature is maintained at 50~60℃; the pH value of the second stage system is maintained for 1 hour and the temperature is maintained at 70~80℃.

[0020] It should be further noted that in step (4), the vacuum drying process conditions are: temperature 60℃~80℃, vacuum degree ≤10 Pa, and drying time 4~6 h.

[0021] It should be further explained that the specific steps include the following: (1) Dissolve 200g of ferric chloride in water, add 8g of urea and mix, stir at 0℃ to obtain product A; (2) An oxidizing gas is introduced into product A. The oxidizing gas includes oxygen and ozone in a volume ratio of 4:1. The oxidation-reduction potential of the system is controlled at 520 mV. After the reaction, a complex complexing agent of ammonium citrate and disodium ethylenediaminetetraacetate in a mass ratio of 2.5:1 is added to continue the reaction and obtain product B. (3) Add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B, and maintain the pH value of the system in two stages: in the first stage, adjust the pH value of the system to 2.5, maintain for 1 hour, and maintain the temperature at 55℃; in the second stage, adjust the pH value of the system to 2.5, maintain for 1 hour, and maintain the temperature at 75℃; to obtain product C; (4) Product C is dried under vacuum to obtain an orthorhombic iron phosphate dihydrate product.

[0022] The beneficial effects of this invention are as follows: (1) The method for preparing orthorhombic iron phosphate dihydrate provided by the present invention involves mixing a ferrous source solution with a nitrogen-containing compound, stirring at a low temperature of 0℃~5℃, and dissolving the ferrous source in Fe. 2+ It exists in the form of nitrogen-containing compounds acting as complexing agents, reacting with Fe at low temperatures (0-5℃). 2+ Formation of complexes, inhibiting Fe 2+ Spontaneous oxidation, avoiding Fe 2+ Premature oxidation or hydrolysis can produce byproducts.

[0023] (2) The preparation method of orthorhombic iron phosphate dihydrate provided by the present invention, for Fe 2+ Oxidation potential control (450-550mV) ensures Fe 3+ To ensure complete formation of Fe while avoiding over-oxidation or other side reactions, this invention incorporates a complexing agent during the oxidation process. The complexing agent reacts with Fe... 3+ Formation of stable complexes, inhibiting Fe 3+ Hydrolyzes into Fe(OH)3 colloid, ensuring Fe 3+ It exists in a soluble form, providing a uniform nucleation environment for subsequent phosphate reactions.

[0024] (3) The method for preparing orthorhombic iron phosphate dihydrate provided by the present invention maintains a two-stage pH gradient during the precipitation stage. In the first stage (pH 2.2-2.8, 50℃-60℃), a higher pH is used to promote the formation of FePO4 crystal nuclei, while a lower temperature limits the crystal growth rate, resulting in small and uniform seed crystals. In the second stage (pH 1.8-2.2, 70℃-80℃), a slightly lower pH is used to increase solubility and selectively dissolve unstable crystal faces. The increased temperature promotes the directional growth of crystals into a thermodynamically more stable orthorhombic crystal system. The present invention utilizes the synergistic effect of pH gradient and temperature to regulate the difference in crystal face energy, enabling crystals to grow along a specific direction, forming an orthorhombic structure, avoiding the generation of amorphous impurities, and improving crystallinity.

[0025] (4) The method for preparing orthorhombic iron phosphate dihydrate provided by the present invention slowly removes free water at low temperature (60℃-80℃) to avoid high temperature damage to the dihydrate structure, and maintains the stable existence of H2O molecules in the crystal lattice during drying to ensure that the product is a dihydrate. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a SEM image of the orthorhombic iron phosphate dihydrate obtained in Example 1 of this invention. Detailed Implementation

[0028] This invention provides a method for preparing orthorhombic iron phosphate dihydrate, comprising the following steps: (1) Mix the ferrous source solution with a nitrogen-containing compound and stir at 0℃~5℃ to obtain product A; (2) Introduce oxidizing gas into product A and control the redox potential of the system to be 450 mV~550 mV. After the reaction, add a complexing agent to continue the reaction and obtain product B. (3) Add phosphate solution to product B to maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.2~2.8, and in the second stage, adjust the pH value of the system to 1.2~2.2 to obtain product C; (4) Product C is dried under vacuum to obtain an orthorhombic iron phosphate dihydrate product.

[0029] It should be further noted that in step (1), the ferrous source solution is at least one of ferrous chloride solution and ferrous nitrate solution. The ferrous chloride solution and ferrous nitrate solution provide a controllable ferrous salt precursor. Ferrous chloride (FeCl2) or ferrous nitrate (Fe(NO3)2) releases ferrous ions (Fe) upon dissolution. 2+ As the starting material for the oxidation reaction, it is gradually converted into iron ions through subsequent oxidation steps.

[0030] In some specific embodiments, in step (1), the nitrogen-containing compound is at least one of urea and triethanolamine; the mass ratio of ferrous source to urea in the ferrous source solution is 20-30:1. Urea or triethanolamine reacts with Fe... 2+ Formation of soluble complexes can inhibit Fe 2+ Premature oxidation or hydrolysis at low temperatures reduces the generation of byproducts and improves product purity.

[0031] In some specific embodiments, in step (2), the oxidizing gas is a mixture of oxygen and ozone, with a volume ratio of oxygen to ozone of 3-5:1. The synergistic oxidation system formed by oxygen / ozone exhibits strong ozone oxidation capabilities, enabling "burst oxidation" and rapidly reducing Fe. 2+ Concentration, reducing reaction resistance, and simultaneously at low Fe concentrations. 2+ At certain concentrations, oxygen prevents Fe from being oxidized through slow but persistent oxidation. 2+ Accumulation ensures complete oxidation reaction.

[0032] In some specific embodiments, in step (2), the complexing agent is a composite complexing agent of ammonium citrate and disodium ethylenediaminetetraacetate (EDTA-2Na), with a mass ratio of ammonium citrate to disodium EDTA-2Na of 2~3:1. Citric acid and the EDTA-2Na composite complexing agent are added, along with Fe... 3+ Formation of stable complexes, inhibiting Fe 3+ Hydrolyzes into Fe(OH)3 colloid, ensuring Fe 3+ It exists in a soluble form, providing a uniform nucleation environment for subsequent phosphate reactions.

[0033] In some specific embodiments, the oxidation reaction time in step (2) is 1 to 2 hours.

[0034] In some specific embodiments, in step (3), the phosphate solution is a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution, with a molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate of 1.5~2.5:1. The phosphate solution provides phosphate ions to form a precipitate and also provides a buffer condition.

[0035] In some specific embodiments, in step (3), the first stage system is maintained at the pH value for 1 hour, and the maintenance temperature is 50~60℃. In some preferred embodiments, the maintenance temperature can be 50℃, 51℃, 52℃, 53℃, 54℃, 55℃, 56℃, 57℃, 58℃, 59℃, or 60℃. The second stage system is maintained at the pH value for 1 hour, and the maintenance temperature is 70~80℃. In some preferred embodiments, the maintenance temperature can be 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, 76℃, 77℃, 78℃, 79℃, or 80℃. The pH gradient and temperature work together to regulate the difference in crystal surface energy, causing the crystal to grow along a specific direction and form an orthogonal structure.

[0036] It should be further noted that in step (4), the vacuum drying process conditions are as follows: temperature is 60℃~80℃, and in some preferred embodiments, the temperature can be 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, 66℃, 67℃, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, 76℃, 77℃, 78℃, 79℃, or 80℃; vacuum degree ≤10 Pa; and drying time is 4~6 h. Controlling the drying conditions maintains the stable existence of H2O molecules in the crystal lattice, ensuring that the product is a dihydrate.

[0037] Example 1 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 200 g of ferrous chloride and dissolve it in 1 L of deionized water, add 8 g of urea, stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A; (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 4:1. The oxidation-reduction potential (ORP value) is controlled at 520 mV. After reacting for 90 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.5 and maintain it for 1 h at a temperature of 55°C. In the second stage, adjust the pH value of the system to 2.0 and maintain it for 1 h at a temperature of 75°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0038] Figure 1 The SEM image of the orthorhombic iron phosphate dihydrate prepared in Example 1 is shown.

[0039] Example 2 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 200 g of ferric nitrate and dissolve it in 1 L of deionized water. Add 8 g of urea and stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A. (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 4:1. The oxidation-reduction potential (ORP value) is controlled at 520 mV. After reacting for 90 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.5 and maintain it for 1 h at a temperature of 55°C. In the second stage, adjust the pH value of the system to 2.0 and maintain it for 1 h at a temperature of 75°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0040] Example 3 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 300 g of ferrous chloride and dissolve it in 1 L of deionized water. Add 15 g of urea and stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A. (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 3:1. The oxidation-reduction potential (ORP value) is controlled at 450 mV. After reacting for 60 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.3 and maintain it for 1 h at a temperature of 50°C. In the second stage, adjust the pH value of the system to 1.3 and maintain it for 1 h at a temperature of 80°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0041] Example 4 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 300 g of ferrous chloride and dissolve it in 1 L of deionized water. Add 10 g of urea and stir in an ice bath (temperature 0℃~5℃) for 60 minutes to obtain product A. (2) Introduce a mixture of oxygen (O2) and ozone (O3) into product A. The volume ratio of oxygen to ozone in the mixture is 5:1. Control the oxidation-reduction potential (ORP value) value to be 550 mV. After reacting for 120 minutes, add ammonium citrate-EDTA-2Na composite complexing agent. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 3:1. Continue the reaction to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 1.5:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.2 and maintain it for 1 h at a temperature of 50°C. In the second stage, adjust the pH value of the system to 1.2 and maintain it for 1 h at a temperature of 80°C to obtain product C. (4) After centrifugation, product C is dried at 80°C under vacuum for 4 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0042] Example 5 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 250 g of ferric nitrate and dissolve it in 1 L of deionized water. Add 10 g of urea and stir in an ice bath (temperature 0℃~5℃) for 30 minutes to obtain product A. (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 4.5:1. The oxidation-reduction potential (ORP value) is controlled at 450 mV. After reacting for 60 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.5 and maintain it for 1 h at a temperature of 55°C. In the second stage, adjust the pH value of the system to 2.0 and maintain it for 1 h at a temperature of 75°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0043] Example 6 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 250 g of ferrous chloride and dissolve it in 1 L of deionized water. Add 12 g of urea and stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A. (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 5:1. The oxidation-reduction potential (ORP value) is controlled at 520 mV. After reacting for 90 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.8 and maintain it for 1 h at a temperature of 50°C. In the second stage, adjust the pH value of the system to 2.2 and maintain it for 1 h at a temperature of 70°C to obtain product C. (4) After centrifugation, product C is dried at 60°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0044] Comparative Example 1 Comparative Example 1 is a ferric phosphate material prepared using the traditional ammonium process, and its crystal form is monoclinic.

[0045] Comparative Example 2 A method for preparing orthorhombic iron phosphate dihydrate includes the following steps: (1) Weigh 200 g of ferrous chloride and dissolve it in 1 L of deionized water, add 8 g of urea, stir at room temperature for 40 minutes to obtain product A; (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 4:1. The oxidation-reduction potential (ORP value) is controlled at 520 mV. After reacting for 90 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.5 and maintain it for 1 h at a temperature of 55°C. In the second stage, adjust the pH value of the system to 2.0 and maintain it for 1 h at a temperature of 75°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0046] Comparative Example 3 (1) Weigh 200 g of ferrous chloride and dissolve it in 1 L of deionized water. Stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A; (2) A mixture of oxygen (O2) and ozone (O3) is introduced into product A. The volume ratio of oxygen to ozone in the mixture is 4:1. The oxidation-reduction potential (ORP value) is controlled at 520 mV. After reacting for 90 minutes, an ammonium citrate-EDTA-2Na composite complexing agent is added. The mass ratio of ammonium citrate to EDTA-2Na in the composite complexing agent is 2.5:1. The reaction continues to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.5 and maintain it for 1 h at a temperature of 55°C. In the second stage, adjust the pH value of the system to 2.0 and maintain it for 1 h at a temperature of 75°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0047] Comparative Example 4 (1) Weigh 200 g of ferrous chloride and dissolve it in 1 L of deionized water, add 8 g of urea, stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A; (2) Introduce a mixture of oxygen (O2) and ozone (O3) into product A. The volume ratio of oxygen to ozone in the mixture is 4:1. Control the oxidation-reduction potential (ORP value) value to 520 mV and continue the reaction to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.5 and maintain it for 1 h at a temperature of 55°C. In the second stage, adjust the pH value of the system to 2.0 and maintain it for 1 h at a temperature of 75°C to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0048] Comparative Example 5 (1) Weigh 200 g of ferrous chloride and dissolve it in 1 L of deionized water, add 8 g of urea, stir in an ice bath (temperature 0℃~5℃) for 40 minutes to obtain product A; (2) Introduce a mixture of oxygen (O2) and ozone (O3) into product A. The volume ratio of oxygen to ozone in the mixture is 4:1. Control the oxidation-reduction potential (ORP value) value to 520 mV and continue the reaction to obtain product B. (3) Slowly add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B. The molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate is 2:1. Maintain the pH value of the system at 1.5~2.5 for 2 h and the temperature at 75℃ to obtain product C. (4) After centrifugation, product C is dried at 70°C under vacuum for 5 hours with a vacuum degree ≤10 Pa to obtain a light yellow orthorhombic iron phosphate dihydrate powder.

[0049] The purity and crystallinity of the iron phosphate materials prepared in each embodiment and comparative example were tested, and the specific data are shown in Table 1.

[0050] Table 1. Purity and crystallinity data of iron phosphate materials prepared in each example and comparative example.

[0051] Lithium iron phosphate was prepared from the iron phosphates obtained in Examples 1-6 and Comparative Examples 1-5 under the same conditions. The specific preparation method is as follows: Iron phosphate, deionized water, lithium carbonate, and glucose were mixed, with a molar ratio of 1:2:0.50:0.03. The mixture was milled to a particle size of 400 nm. The milled slurry was spray-dried, and then sintered at 700°C for 18 hours in a nitrogen atmosphere to obtain lithium iron phosphate. The lithium iron phosphate prepared from the iron phosphates in Examples 1-6 and Comparative Examples 1-5 was then used to prepare coin cells under the same conditions using conventional methods, and named sequentially as coin cells #1, #2, #3, #4, #5, #6, #7, #8, #9, #10, and #11. Their electrical performance was then tested under the same conditions using conventional methods in the art. The electrical performance test results of each button cell are shown in Table 2 below.

[0052] Table 2. Test results of electrical performance of each button cell

[0053] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing orthorhombic iron phosphate dihydrate, characterized in that, Includes the following steps: (1) Mix the ferrous source solution with a nitrogen-containing compound and stir at 0℃~5℃ to obtain product A. The nitrogen-containing compound is at least one of urea and triethanolamine. The mass ratio of ferrous source to urea in the ferrous source solution is 20~30:

1. (2) Introduce oxidizing gas into product A, control the oxidation-reduction potential of the system to be 450 mV~550 mV, add a complexing agent after the oxidation reaction to continue the reaction and obtain product B; (3) Add phosphate solution to product B and maintain the pH value of the system in two stages. In the first stage, adjust the pH value of the system to 2.2~2.8, maintain the pH value of the system at 1h for 1h, and maintain the temperature at 50~60℃. In the second stage, adjust the pH value of the system to 1.2~2.2, maintain the pH value of the system at 1h for 1h, and maintain the temperature at 70~80℃ to obtain product C. (4) Product C is vacuum dried to obtain orthorhombic iron phosphate dihydrate product.

2. The method for preparing orthorhombic iron phosphate dihydrate as described in claim 1, characterized in that, In step (1), the ferrous source solution is at least one of ferrous chloride solution and ferrous nitrate solution.

3. A method for preparing orthorhombic iron phosphate dihydrate as described in claim 1 or 2, characterized in that, In step (2), the oxidizing gas is a mixture of oxygen and ozone, with a volume ratio of oxygen to ozone of 3~5:

1.

4. A method for preparing orthorhombic iron phosphate dihydrate as described in claim 1 or 2, characterized in that, In step (2), the complexing agent is a composite complexing agent of ammonium citrate and disodium ethylenediaminetetraacetate, and the mass ratio of ammonium citrate to disodium ethylenediaminetetraacetate is 2~3:

1.

5. A method for preparing orthorhombic iron phosphate dihydrate as described in claim 1 or 2, characterized in that, In step (2), the oxidation reaction takes 1 to 2 hours.

6. A method for preparing orthorhombic iron phosphate dihydrate as described in claim 1 or 2, characterized in that, In step (3), the phosphate solution is a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution, with a molar ratio of ammonium dihydrogen phosphate to diammonium hydrogen phosphate of 1.5~2.5:

1.

7. A method for preparing orthorhombic iron phosphate dihydrate as described in claim 1 or 2, characterized in that, In step (4), the vacuum drying process conditions are: temperature 60℃~80℃, vacuum degree ≤10 Pa, and drying time 4~6 h.

8. The method for preparing orthorhombic iron phosphate dihydrate as described in claim 1, characterized in that, Specifically, the steps include the following: (1) Dissolve 200g of ferrous chloride in water, add 8g of urea and mix, stir at 0℃ to obtain product A; (2) An oxidizing gas is introduced into product A. The oxidizing gas includes oxygen and ozone in a volume ratio of 4:

1. The oxidation-reduction potential of the system is controlled at 520 mV. After the reaction, a complex complexing agent of ammonium citrate and disodium ethylenediaminetetraacetate in a mass ratio of 2.5:1 is added to continue the reaction and obtain product B. (3) Add a mixture of ammonium dihydrogen phosphate solution and diammonium hydrogen phosphate solution to product B, and maintain the pH value of the system in two stages: in the first stage, adjust the pH value of the system to 2.5, maintain for 1 hour, and maintain the temperature at 55°C; in the second stage, adjust the pH value of the system to 2.0, maintain for 1 hour, and maintain the temperature at 75°C; to obtain product C; (4) Product C is vacuum dried to obtain orthorhombic iron phosphate dihydrate product.

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