Monodisperse low-impurity-phase iron phosphate with high iron-phosphorus ratio and preparation method of monodisperse low-impurity-phase iron phosphate

Abstract

The invention provides monodisperse low-impurity-phase iron phosphate with a high iron-phosphorus ratio and a preparation method thereof, and relates to the technical field of lithium ion, the preparation method comprises the following steps: S1, preparing a phosphoric acid solution, a hydrogen peroxide solution, a ferrite solution and a phosphorus salt solution; s2, taking pure water as a base solution, respectively adding a hydrogen peroxide solution, a phosphorus salt solution and a ferrite solution, and reacting to obtain yellow slurry A; s3, carrying out solid-liquid separation and washing treatment on the yellow slurry A to obtain a yellow filter cake B, mixing the yellow filter cake B with a pure water solution, carrying out slurry mixing, adding a phosphoric acid solution, heating to 86-100 DEG C, carrying out a conversion reaction, and carrying out heat preservation to obtain white slurry C; after the slurry turns white, carrying out solid-liquid separation treatment to obtain a white filter cake C; s4, mixing the white filter cake C with pure water, carrying out size mixing, heating, carrying out heat preservation, and carrying out crystal curing, so as to obtain white size D; the prepared iron phosphate material is high in iron-phosphorus ratio, few in impure phase, high in primary particle dispersion degree, smooth in surface, free of fusion, uniform in pore and capable of spontaneously forming large and small particle grading.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ions, and specifically to a high-iron-phosphorus ratio iron phosphate with monodispersion and low impurity phase and a preparation method thereof. Background Art

[0002] With the rapid development of new energy vehicles and energy storage industries, lithium iron phosphate batteries have become the mainstream choice in the market due to their advantages such as high safety and long cycle life. To cope with the increasingly fierce market competition, the industry has put forward higher requirements for battery energy density, fast charging performance and cost-effectiveness. Research shows that the iron-lithium particle grading technology can significantly improve the tap density (the fourth-generation products have reached 2.6-2.7 g / cm³) by optimizing the ratio of large and small particles in the cathode material, thereby improving the battery energy density and fast charging ability. However, the physical and chemical properties and electrochemical performance of lithium iron phosphate highly depend on the quality of its precursor, iron phosphate, and the iron-phosphorus molar ratio (Fe / P), as the core index of iron phosphate, directly affects the material crystallinity, purity, sinter resistance and the final battery performance.

[0003] At present, high-iron-phosphorus ratio iron phosphate (Fe / P≥0.98) has become the key raw material for preparing small-particle lithium iron phosphate due to its excellent crystallinity and sinter resistance characteristics, which can effectively support the requirements of particle grading technology for high-rate charge and discharge. However, the existing preparation processes generally have the following problems: 1. The contradiction between process complexity and cost: Traditional methods require multi-step reactions, multiple slurry mixings and precise parameter controls, resulting in high production costs and low efficiency; 2. Material performance loss: Although some processes increase the iron-phosphorus ratio, the actual crystallinity is poor, leading to problems such as particle agglomeration and poor dispersibility; Taking typical patented technologies as examples: CN 118387849 A proposes a "controlled oxidation rate + seed induction" process, which realizes the improvement of the iron-phosphorus ratio and morphology control through staged reactions, but it is necessary to prepare two iron phosphate slurries and dilute and mix them. The process is redundant and the oxidation rate control window is narrow. The stability problem of hydrogen peroxide at high temperatures further restricts the process reliability; CN 118405675 A adopts the seed circulation and hydrogen peroxide substitution technology to reduce production costs, but the operation of separating and converting the slurry is cumbersome, and the particles are severely agglomerated after calcination, affecting the electrochemical performance of the material; CN 118405675 A optimizes the dispersibility through staged pulping and washing and pH regulation, but the secondary pulping requires the introduction of a pH regulator, increasing the burden of wastewater treatment. At the same time, a high-pH environment is prone to generate iron hydroxide colloid, leading to the risk of exceeding the standard of iron-lithium magnetic substances.

[0004] In summary, the prior art has not effectively balanced the contradictions among process simplification, cost control, and performance stability in the preparation of iron-rich phosphorous ratio iron phosphate. To address the above problems, the present invention proposes a method for preparing iron-rich phosphorous ratio iron phosphate with monodispersity and low impurity phases. Through innovative process design, the goals of high particle dispersibility and strong crystallinity are achieved, while reducing the operation complexity and production cost, providing key material support for the large-scale application of high-performance lithium iron phosphate batteries. Summary of the Invention

[0005] A method for preparing a monodisperse and low-impurity-phase iron-rich phosphorous ratio iron phosphate material, characterized by comprising the following steps: S1. Prepare a phosphoric acid solution with a concentration of 40 - 85 wt%, a hydrogen peroxide solution with a concentration of 10 - 30 wt%, an iron salt solution with a concentration of 1.0 - 1.7 mol / L, and a phosphorous salt solution with a concentration of 1.0 - 1.7 mol / L; S2. Add pure water as the bottom liquid into the reaction kettle, control the temperature of the bottom liquid at 30 - 50 °C, and add the hydrogen peroxide solution, the phosphorous salt solution, and the iron salt solution into the reaction kettle within 30 - 60 minutes respectively. After the feeding is completed, react for 10 - 30 minutes to obtain yellow slurry A; S3. After the yellow slurry A undergoes solid-liquid separation and washing treatment, obtain yellow filter cake B. Mix the yellow filter cake B with an aqueous solution of pure water for slurry adjustment, then add the phosphoric acid solution, and then raise the temperature to 86 - 100 °C for a conversion reaction, keep warm for 10 - 30 minutes to obtain white slurry C; After the slurry turns white for 10 - 20 minutes, perform solid-liquid separation treatment to remove free phosphoric acid and obtain white filter cake C; S4. Mix the white filter cake C with pure water for slurry adjustment, raise the temperature to 86 - 100 °C and keep warm for 90 - 180 minutes for crystal ripening to obtain white slurry D; S5. Perform solid-liquid separation and washing on the white slurry D to obtain a white filter cake D, and obtain the iron phosphate material through drying and calcination treatments on the white filter cake D.

[0006] Further, in the step S1, the pH of the iron salt solution is 1.5 - 3.0, and the pH of the phosphorous salt solution is 6.0 - 8.0.

[0007] Further, in the step S1, the iron salt solution is prepared from any one or a combination of ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder, and iron sheet; the phosphorous salt solution is prepared from any one or a combination of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

[0008] Further, in the step S2, the hydrogen peroxide solution added to the reaction kettle and the iron salt solution satisfy n(Fe):n(H2O2)=1:0.5 - 1:0.7.

[0009] Further, in the step S2, the phosphate solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(P)=1:0.9 - 1:1.1.

[0010] Further, in the step S3, the solid content after mixing the yellow filter cake B with pure water and phosphoric acid solution is controlled to be 10% - 15%.

[0011] Further, in the step S3, the molar ratio of the iron element content in the yellow filter cake B to the phosphorus element content in the phosphoric acid solution is controlled to be n(Fe):n(P)=1:0.5 - 1:0.9.

[0012] Further, in the step S3, the conductivity of the washing water after filtration and washing of the yellow slurry A is ≤5000 μS / cm; the solid content during the slurry mixing of the white filter cake C and pure water is controlled to be 10% - 30%.

[0013] Further, in the step S5, the conductivity of the washing water after filtration and washing of the white slurry D is ≤1000 μS / cm; the calcination process after drying the white filter cake D adopts a two - stage calcination system, with low - temperature calcination at 200 - 400°C for 1 - 3 h and high - temperature calcination at 500 - 750°C for 1 - 3 h.

[0014] Further, the methods and equipment for slurry solid - liquid separation, washing, drying, and calcination are all common methods and equipment in the iron phosphate industry.

[0015] On the other hand, the present invention provides an iron phosphate material prepared by a preparation method of a monodisperse and low - impurity - phase iron - rich phosphorus - ratio iron phosphate material. The primary particles of the iron phosphate material have clear boundaries, there is no obvious sintering neck between primary particles, and the primary particles have strong fire resistance.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The yellow slurry generated in the iron phosphate synthesis stage (S2) of the present invention is mainly composed of amorphous iron phosphate and ammonium ferric phosphate hydroxide. Conventional iron phosphate processes need to add phosphoric acid in the conversion stage (S3) to promote the crystallization of amorphous iron phosphate and ammonium ferric phosphate hydroxide into dihydrate iron phosphate. Taking amorphous iron phosphate as an example, the specific reaction formula can be expressed as: ① (room temperature - 80°C): FePO4 (amorphous) + H3PO4 → H3Fe(PO4)2 (transition phase)·xH2O; ② (80°C - 98°C): H3Fe(PO4)2·xH2O → FePO4·2H2O + H3PO4; After the slurry turns from yellow to white in the S3 stage, reaction ② mainly occurs in the system. At this time, phosphoric acid in the slurry is not conducive to the crystal growth and development of iron phosphate dihydrate. In step S3 of the present invention, solid-liquid separation treatment is carried out 10-20 minutes after the slurry turns white to remove phosphoric acid and obtain white filter cake C (mainly composed of transition phase and iron phosphate dihydrate); in the S4 stage, pure water is added and the temperature is raised again for transformation (raising the temperature to 86-100 °C and keeping warm for 90-180 minutes). At this time, the concentration of phosphate radicals in the system is extremely low, which easily breaks the dissolution-precipitation equilibrium of FePO4·xH3PO4 FePO4·2H2O +xPO4 3- + xH + , forcing the reaction to continuously proceed in the direction of iron phosphate dihydrate formation, promoting crystal development, and realizing the improvement of the iron-phosphorus ratio and the optimization of crystallinity.

[0017] 2. The anhydrous iron phosphate prepared by the present invention has a relatively high iron-phosphorus molar ratio (Fe / P), and the specific surface area (BET) is significantly increased under the same calcination system, indicating that the crystallinity and dispersibility of the anhydrous iron phosphate material prepared by the present invention are significantly enhanced. The core mechanism lies in the directional removal of phosphorus elements: in the stage when the slurry turns white, the present invention removes the high-phosphorus mother liquor to eliminate the competitive inhibition of free phosphate radicals (PO4 3- ) on the dephosphorization reaction, prompting the reaction equilibrium to shift towards the FePO4 formation direction; in addition, the low free phosphorus environment reduces the probability of lattice distortion, making the coordination of Fe 3+ and PO4 3- more complete, thereby improving the Fe / P ratio and crystal integrity.

[0018] 3. The present invention provides a monodisperse, low-heterophase iron phosphate with a high iron-phosphorus ratio and a preparation method thereof. Its particles have the characteristics of monodispersity and high crystallinity, which can be achieved without adding crystal seeds and other additives. Compared with the existing technology with complex reaction steps and the need to introduce additives such as template agents / complexing agents, this method significantly simplifies the process. The monodisperse structure can shorten the grinding time for preparing lithium iron phosphate and reduce energy consumption, while inhibiting particle agglomeration, promoting the uniform coating of the particle surface after the carbon source melts, and effectively avoiding problems such as the formation of flocculent carbon due to uneven carbon layer distribution, insufficient coating, and abnormal growth of micron-sized single crystals; the characteristics of high crystallinity and high iron-phosphorus ratio can meet the structural support requirements of particle grading technology for high-rate charge and discharge.

[0019] 4. The primary particles of the iron phosphate material prepared by the present invention have a high degree of dispersion, smooth surface without melting and merging, uniform pores, and spontaneously form a particle size grading of large and small particles.

[0020] 5. In step S3 of the present invention, solid-liquid separation is immediately carried out 10-20 minutes after the slurry turns white. At this time, free phosphoric acid has not fully participated in the subsequent reaction. After removal, it can be achieved that: avoiding excessive phosphoric acid from further inserting into the crystal lattice / wrapping and adsorbing inside the particles during the high-temperature conversion stage, and reducing the generation of impurity phases (pyrophosphate iron, Fe4(P2O7)3) during the subsequent high-temperature calcination process. The specific reaction formula is as follows: High-temperature calcination: 4FePO4 + 2H3PO4 → Fe4(P2O7)3 + 3H2O. Description of the Drawings

[0021] Figure 1 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Example 1; Figure 2 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Example 2; Figure 3 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Example 3; Figure 4 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Example 4; Figure 5 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Comparative Example 1; Figure 6 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Comparative Example 2; Figure 7 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Comparative Example 3; Figure 8 It is the SEM morphology diagram of anhydrous iron phosphate prepared in Comparative Example 4. Detailed Description of the Invention

[0022] Example 1 A preparation method of monodisperse and low-impurity-phase iron phosphate with a high iron-phosphorus ratio, the preparation process includes the following steps: Step 1: Synthesis of the precursor (1) Prepare the iron source solution: Dissolve ferrous sulfate heptahydrate (FeSO4·7H2O) in deionized water to prepare a 40 L ferrous sulfate solution with an iron element molar concentration of 1.4 mol / L; (2) Prepare the phosphorus source solution: Dissolve ammonium dihydrogen phosphate (NH4H2PO4) in deionized water to prepare a 40 L phosphorus salt solution with a phosphorus element molar concentration of 1.4 mol / L, and adjust the pH to 7.2 with ammonia water; (3) In a 100 L reaction kettle, start the stirrer and set the rotation speed to 35 Hz, and simultaneously pump in the ferrous sulfate solution, the phosphorus salt solution and the hydrogen peroxide (27 wt%) solution, and control the molar ratio of the three to be n(Fe):n(P):n(H2O2)=1:1:0.6; (4)Maintain the reaction temperature at 55 ± 2 °C and continue the reaction for 30 minutes to obtain a homogeneous yellow slurry A.

[0023] Step 2: Slurry conversion / crystalline phase reconstruction (1)Transfer the yellow slurry A to a centrifuge and wash it with deionized water until the conductivity of the filtrate is ≤ 3000 μS / cm to obtain a yellow filter cake B; (2)Add the yellow filter cake B, 40 kg of deionized water, and 5.2 kg of phosphoric acid (85 wt%) to a reaction kettle, stir and heat it to 95 °C at a heating rate of 2 °C / min, and keep the temperature constant for 10 minutes; (3)Centrifuge and separate the slurry while it is hot to remove the mother liquor and obtain a white filter cake C.

[0024] Step 3: Crystal ripening (1)Add the white filter cake C and 40 kg of deionized water to a reaction kettle, heat it to 95 °C at a rate of 3 °C / min and keep the temperature constant for 2 hours; (2)Centrifuge and filter the ripened slurry, wash it until the conductivity of the filtrate is ≤ 500 μS / cm to obtain a high-purity white filter cake D.

[0025] Step 4: Drying and staged calcination (1)Place the white filter cake D in a hot air circulation oven and dry it at 150 ± 5 °C for 6 hours to obtain an iron phosphate dihydrate (FePO4·2H2O) precursor; (2)Crush the dried product and place it in a muffle furnace for staged calcination: heat it to 300 °C at a rate of 5 °C / min and keep the temperature for 60 minutes to complete dehydration; heat it to 580 °C at a rate of 5 °C / min and keep the temperature constant for 4 hours, and then cool it to room temperature with the furnace to obtain a battery-grade anhydrous iron phosphate finished product.

[0026] Example 2 All process parameters in this example are the same as those in Example 1. The difference from Example 1 is that in Step 1, the pH of the phosphate salt solution is increased from 7.2 to 7.5, specifically as follows: A preparation method of monodisperse and low-heterophase iron phosphate with a high iron-to-phosphorus ratio, and the preparation process includes the following steps: Step 1: Precursor synthesis (1)Prepare an iron source solution: Dissolve ferrous sulfate heptahydrate (FeSO4·7H2O) in deionized water to prepare a 40 L ferrous sulfate solution with an iron element molar concentration of 1.4 mol / L; (2)Prepare a phosphorus source solution: Dissolve ammonium dihydrogen phosphate (NH4H2PO4) in deionized water to prepare a 40 L phosphate salt solution with a phosphorus element molar concentration of 1.4 mol / L, and adjust the pH to 7.5 with ammonia water; (3) In a 100 L reactor, turn on the stirrer and set the rotation speed to 35 Hz. Synchronously pump in the ferrous sulfate solution, phosphate solution, and hydrogen peroxide (27 wt%) solution, and control the molar ratio of the three as n(Fe):n(P):n(H2O2)=1:1:0.6; (4) Maintain the reaction temperature at 55±2 °C and continue the reaction for 30 minutes to obtain a homogeneous yellow slurry A.

[0027] Step 2: Slurry conversion / crystalline phase reconstruction (1) Transfer the yellow slurry A to a centrifuge and wash it with deionized water until the conductivity of the filtrate ≤ 3000 μS / cm to obtain a yellow filter cake B; (2) Add the yellow filter cake B, 40 kg of deionized water, and 5.2 kg of phosphoric acid (85 wt%) to the reactor, stir and heat it to 95 °C at a heating rate of 2 °C / min, and keep the temperature constant for 10 minutes; (3) Centrifuge and separate the slurry while it is hot to remove the mother liquor and obtain a white filter cake C.

[0028] Step 3: Crystal ripening (1) Add the white filter cake C and 40 kg of deionized water to the reactor, heat it to 95 °C at a rate of 3 °C / min and keep the temperature constant for 2 hours; (2) Centrifuge and filter the ripened slurry, wash it until the conductivity of the filtrate ≤ 500 μS / cm to obtain a high-purity white filter cake D.

[0029] Step 4: Drying and staged calcination (1) Place the white filter cake D in a hot air circulation oven and dry it at 150±5 °C for 6 hours to obtain an iron phosphate dihydrate (FePO4·2H2O) precursor; (2) After crushing the dried product, place it in a muffle furnace for staged calcination: heat it to 300 °C at a rate of 5 °C / min and keep the temperature constant for 60 minutes to complete dehydration; heat it to 580 °C at a rate of 5 °C / min and keep the temperature constant for 4 hours, and then cool it to room temperature with the furnace to obtain a battery-grade anhydrous iron phosphate finished product.

[0030] Example 3 All process parameters of this example are the same as those of Example 1. The difference from Example 1 is that in Step 2 of this example, the amount of phosphoric acid used is reduced to 3.5 kg, specifically as follows: A preparation method of monodisperse and low-heterophase iron phosphate with a high iron-phosphorus ratio, and the preparation process includes the following steps: Step 1: Precursor synthesis (1) Prepare an iron source solution: Dissolve ferrous sulfate heptahydrate (FeSO4·7H2O) in deionized water to prepare a 40 L ferrous sulfate solution with an iron element molar concentration of 1.4 mol / L; (2)Prepare the phosphorus source solution: Dissolve ammonium dihydrogen phosphate (NH4H2PO4) in deionized water to prepare a 40 L phosphate solution with a molar concentration of phosphorus of 1.4 mol / L, and adjust the pH to 7.2 with ammonia water; (3)In a 100 L reactor, start the stirrer and set the rotation speed to 35 Hz. Synchronously pump in the ferrous sulfate solution, the phosphate solution, and the hydrogen peroxide (27 wt%) solution, and control the molar ratio of the three as n(Fe):n(P):n(H2O2)=1:1:0.6; (4)Maintain the reaction temperature at 55±2 °C and continuously react for 30 minutes to obtain a homogeneous yellow slurry A.

[0031] Step 2: Slurry conversion / crystalline phase reconstruction (1)Transfer the yellow slurry A to a centrifuge and wash it with deionized water until the conductivity of the filtrate ≤ 3000 μS / cm to obtain a yellow filter cake B; (2)Add the yellow filter cake B, 40 kg of deionized water, and 3.5 kg of phosphoric acid (85 wt%) to the reactor, and stir and heat at a heating rate of 2 °C / min to 95 °C, and keep the temperature constant for 10 minutes; (3)Centrifuge and separate the slurry while it is hot to remove the mother liquor to obtain a white filter cake C.

[0032] Step 3: Crystal ripening (1)Add the white filter cake C and 40 kg of deionized water to the reactor, and heat up to 95 °C at a rate of 3 °C / min and keep the temperature constant for 2 hours; (2)Centrifuge and filter the ripened slurry, and wash it until the conductivity of the filtrate ≤ 500 μS / cm to obtain a high-purity white filter cake D.

[0033] Step 4: Drying and staged calcination (1)Place the white filter cake D in a hot air circulation oven and dry it at 150±5 °C for 6 hours to obtain a precursor of iron phosphate dihydrate (FePO4·2H2O); (2)After pulverizing the dried product, place it in a muffle furnace for stepped calcination: heat up to 300 °C at a rate of 5 °C / min and keep the temperature for 60 minutes to complete dehydration; heat up to 580 °C at a rate of 5 °C / min, keep the temperature constant for calcination for 4 hours, and cool it to room temperature with the furnace to obtain a battery-grade anhydrous iron phosphate finished product.

[0034] Example 4 All process parameters in this example are the same as those in Example 1. The difference from Example 1 is that: in this example, the heating rate in Step 2 is reduced from 2 °C / min to 1 °C / min, specifically as follows: A preparation method of monodisperse and low-heterophase iron phosphate with a high iron-phosphorus ratio, and the preparation process includes the following steps: Step 1: Precursor synthesis (1) Preparation of iron source solution: Dissolve ferrous sulfate heptahydrate (FeSO4·7H2O) in deionized water to prepare a 40 L ferrous sulfate solution with a molar concentration of iron element of 1.4 mol / L. (2) Preparation of phosphorus source solution: Dissolve ammonium dihydrogen phosphate (NH4H2PO4) in deionized water to prepare a 40 L phosphate solution with a molar concentration of phosphorus element of 1.4 mol / L, and adjust the pH to 7.2 with ammonia water. (3) In a 100 L reactor, start the stirrer and set the rotation speed to 35 Hz. Synchronously pump in the ferrous sulfate solution, phosphate solution and hydrogen peroxide (27 wt%) solution, and control the molar ratio of the three as n(Fe):n(P):n(H2O2)=1:1:0.6. (4) Maintain the reaction temperature at 55±2 °C and continue the reaction for 30 minutes to obtain a homogeneous yellow slurry A.

[0035] Step 2: Slurry conversion / crystalline phase reconstruction (1) Transfer the yellow slurry A to a centrifuge and wash it with deionized water until the conductivity of the filtrate ≤ 3000 μS / cm to obtain a yellow filter cake B. (2) Add the yellow filter cake B, 40 kg of deionized water and 5.2 kg of phosphoric acid (85 wt%) to the reactor, stir and heat it to 95 °C at a heating rate of 1 °C / min, and keep the temperature constant for 10 minutes. (3) Centrifuge and separate the slurry while it is hot to remove the mother liquor to obtain a white filter cake C.

[0036] Step 3: Crystal ripening (1) Add the white filter cake C and 40 kg of deionized water to the reactor, heat it to 95 °C at a rate of 3 °C / min and keep the temperature constant for 2 hours. (2) Centrifuge and filter the ripened slurry, and wash it until the conductivity of the filtrate ≤ 500 μS / cm to obtain a high-purity white filter cake D.

[0037] Step 4: Drying and staged calcination (1) Place the white filter cake D in a hot air circulation oven and dry it at 150±5 °C for 6 hours to obtain a precursor of iron phosphate dihydrate (FePO4·2H2O). (2) After crushing the dried product, place it in a muffle furnace for staged calcination: heat it to 300 °C at a rate of 5 °C / min and keep the temperature for 60 minutes to complete dehydration; heat it to 580 °C at a rate of 5 °C / min and keep the temperature for 4 hours, and then cool it to room temperature with the furnace to obtain a battery-grade anhydrous iron phosphate finished product.

[0038] Comparative example 1 The traditional preparation method is adopted in this comparative example, which is specifically as follows: Step 1: Precursor synthesis (1)Prepare the iron source solution: Dissolve ferrous sulfate heptahydrate (FeSO4·7H2O) in deionized water to prepare a 40 L ferrous sulfate solution with a molar concentration of iron element of 1.4 mol / L. (2)Prepare the phosphorus source solution: Dissolve ammonium dihydrogen phosphate (NH4H2PO4) in deionized water to prepare a 40 L phosphate salt solution with a molar concentration of phosphorus element of 1.4 mol / L, and adjust the pH to 7.2 with ammonia water. (3)In a 100 L reactor, start the stirrer and set the rotation speed to 35 Hz. Synchronously pump in the ferrous sulfate solution, phosphate salt solution, and hydrogen peroxide (27 wt%) solution, and control the molar ratio of the three as n(Fe):n(P):n(H2O2)=1:1:0.6. (4)Maintain the reaction temperature at 55±2°C and continue the reaction for 30 minutes to obtain a homogeneous yellow slurry A.

[0039] Step 2: Slurry conversion / crystalline phase reconstruction (1)Transfer the yellow slurry A to a centrifuge and wash it with deionized water until the conductivity of the filtrate ≤3000 μS / cm to obtain a yellow filter cake B. (2)Add the yellow filter cake B, 40 kg of deionized water, and 5.2 kg of phosphoric acid (85 wt%) to the reactor, stir and heat at a heating rate of 2°C / min to 95°C, and keep the temperature constant for 2 hours. (3)Centrifuge and filter the ripened slurry, and wash it until the conductivity of the filtrate ≤500 μS / cm to obtain a high-purity white filter cake C.

[0040] Step 3: Drying and staged calcination (1)Place the white filter cake C in a hot air circulation oven and dry it at 150±5°C for 6 hours to obtain a precursor of iron phosphate dihydrate (FePO4·2H2O). (2)After crushing the dried product, place it in a muffle furnace for staged calcination: Heat it to 300°C at a rate of 5°C / min and keep the temperature constant for 60 minutes to complete dehydration; Heat it to 580°C at a rate of 5°C / min, keep the temperature constant for 4 hours, and cool it to room temperature with the furnace to obtain a battery-grade anhydrous iron phosphate product.

[0041] Comparative Example 2 All process parameters of this comparative example are the same as those of Comparative Example 1. The difference from Comparative Example 1 is that in Step 1, the pH of the phosphate salt solution is increased from 7.2 to 7.5 in this comparative example.

[0042] Comparative Example 3 All process parameters of this comparative example are the same as those of Comparative Example 1. The difference from Comparative Example 1 is that in Step 2, the amount of phosphoric acid used is reduced to 3.5 kg in this comparative example.

[0043] Comparative Example 4 All process parameters of this comparative example are the same as those of Comparative Example 1. The difference from Comparative Example 1 is that: in this comparative example, the heating rate in Step 2 is reduced from 2 °C / min to 1 °C / min.

[0044] Experimental Part Samples of the anhydrous iron phosphate prepared in Examples 1-4 and Comparative Examples 1-4 were taken for testing the iron-phosphorus ratio and specific surface area. The results are shown in Table 1.

[0045] Table 1 Comparative Analysis of Iron-Phosphorus Ratio and Specific Surface Area of Each Sample

[0046] As shown in the data in Table 1, the iron-phosphorus molar ratio (Fe / P) of Examples 1-4 is increased by 1.5% - 4.0% compared with Comparative Examples 1-4, and the specific surface area (BET) is significantly increased under the same calcination system (Examples: 11.67 - 15.3 m² / g vs Comparative Examples: 2.96 - 8.92 m² / g). This phenomenon indicates that the crystallinity and dispersibility of the materials in the examples are significantly enhanced. The core mechanism lies in the directional removal of phosphorus elements: in the examples, by removing the high-phosphorus mother liquor during the stage when the slurry turns white, the competitive inhibition of the dephosphorization reaction by free phosphate ions (PO4 3- )is eliminated, promoting the reaction equilibrium to shift towards the direction of FePO4 formation; in addition, the low free phosphorus environment reduces the probability of lattice distortion, making the coordination of Fe 3+ and PO4 3- more complete, thereby increasing the Fe / P ratio and crystal integrity.

[0047] Samples of the anhydrous iron phosphate prepared in Examples 1-4 and Comparative Examples 1-4 were taken for iron phase analysis. The results are shown in Table 2. The proportions of the impurity phase iron pyrophosphate in Examples 1-4 are 0.46, 0.2, 0.21, and 0.6 respectively, which are generally lower than those of the comparative examples. This is mainly because after removing the high-phosphorus mother liquor during the growth stage of iron phosphate crystals, a low-phosphorus growth environment is created, reducing the adsorption of free phosphate ions, and thus reducing the occurrence of side reactions (formation of iron pyrophosphate) during the calcination stage.

[0048] Table 2 Iron Phase Analysis of Each Sample

[0049] Figure 1-8 The SEM morphologies of the calcined anhydrous iron phosphate in Examples 1-4 and Comparative Examples 1-4 are shown. It can be seen from the figure that in Examples 1-4: the degree of dispersion of primary particles is high, there is no obvious sintering neck between particles, the surface is smooth without melting and merging, the pores are uniform, and a size particle grading is spontaneously formed; Comparative Examples 1-4: Severe sintering and agglomeration occurred between the particles. The decrease in specific surface area was related to the formation of secondary particles, and the root cause was that the residual phosphate ions triggered liquid-phase sintering at high temperatures.

Claims

1. A preparation method of a monodisperse and low-impurity-phase iron-rich phosphorous iron phosphate material, characterized in that, It includes the following steps: S1. Configure a phosphoric acid solution with a concentration of 40 - 85 wt%, a hydrogen peroxide solution with a concentration of 10 - 30 wt%, a ferrous salt solution with a concentration of 1.0 - 1.7 mol / L, and a phosphate salt solution with a concentration of 1.0 - 1.7 mol / L; S2. Add pure water into the reaction kettle as the bottom liquid, control the temperature of the bottom liquid at 30 - 50 °C, and add the hydrogen peroxide solution, the phosphate salt solution, and the ferrous salt solution into the reaction kettle respectively within 30 - 60 min. After the feeding is completed, react for 10 - 30 min to obtain yellow slurry A; S3. After the yellow slurry A is subjected to solid - liquid separation and washing treatment, obtain yellow filter cake B. Mix the yellow filter cake B with an aqueous solution of pure water for slurry adjustment, then add the phosphoric acid solution, and then raise the temperature to 86 - 100 °C for the conversion reaction, keep the temperature for 10 - 30 min to obtain white slurry C; After the slurry turns white for 10 - 20 min, perform solid - liquid separation treatment to remove free phosphoric acid and obtain white filter cake C; S4. Mix the white filter cake C with pure water for slurry adjustment, raise the temperature to 86 - 100 °C and keep the temperature for 90 - 180 min for crystal ripening to obtain white slurry D; S5. Perform solid - liquid separation and washing on the white slurry D to obtain white filter cake D, and the white filter cake D is obtained through drying and calcination to obtain the said iron phosphate material.

2. The preparation method of a monodisperse and low-impurity-phase iron-rich iron phosphate material as claimed in claim 1, wherein, In the step S1, the pH of the ferrous salt solution is 1.5 - 3.0, and the pH of the phosphate salt solution is 6.0 - 8.

0.

3. The preparation method of a monodisperse and low-impurity-phase iron-rich iron phosphate material as described in claim 1, characterized in that, In the step S1, the ferrous salt solution is prepared from any one or a combination of ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder, and iron sheet; the phosphate salt solution is prepared from any one or a combination of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

4. The preparation method of a monodisperse and low-impurity-phase iron-rich iron phosphate material as claimed in claim 1, characterized in that, In the step S2, the hydrogen peroxide solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(H₂O₂)=1:0.5 - 1:0.

7.

5. The preparation method of a monodisperse and low-impurity-phase iron-rich iron phosphate material as claimed in claim 1, wherein In the step S2, the phosphate salt solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(P)=1:0.9 - 1:1.

1.

6. The preparation method of a monodisperse and low-heterophase iron-rich iron phosphate material as claimed in claim 1, characterized in that, In the step S3, the solid content after mixing the yellow filter cake B with pure water and the phosphoric acid solution is controlled to be 10% - 15%.

7. The preparation method of a monodisperse and low-impurity-phase iron-rich phosphorous iron phosphate material as claimed in claim 1, characterized in that, In the step S3, the molar ratio of the iron element content in the yellow filter cake B to the phosphorus element content in the phosphoric acid solution is controlled to be n(Fe):n(P)=1:0.5 - 1:0.

9.

8. The preparation method of a monodisperse and low-impurity-phase iron-rich iron phosphate material as claimed in claim 1, characterized in that, In the step S3, the conductivity of the washing water after the yellow slurry A is filtered and washed is ≤5000 μS / cm; the solid content during the slurry adjustment by mixing the white filter cake C with pure water is controlled to be 10% - 30%.

9. The preparation method of a monodisperse and low-heterophase iron-rich iron phosphate material as described in claim 1, characterized in that, In the step S5, the conductivity of the washing water after the white slurry D is filtered and washed is ≤1000 μS / cm; the calcination process after the white filter cake D is dried adopts a two - stage calcination system, with low - temperature calcination at 200 - 400 °C for 1 - 3 h and high - temperature calcination at 500 - 750 °C for 1 - 3 h.

10. An iron phosphate material prepared by the preparation method of a monodisperse and low - impurity - phase iron phosphate material with a high iron - phosphorus ratio as claimed in claims 1 - 9.

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