Preparation method of polyhedral single-crystal particle iron phosphate

Abstract

This invention discloses a method for preparing polyhedral single-crystal iron phosphate particles, belonging to the field of iron phosphate preparation technology. The method includes: diluting 85% concentrated phosphoric acid to 10%-30%, preheating to 40-80℃, adding iron powder to dissolve and prepare an iron-phosphorus solution with a P / Fe molar ratio of 1.5-3.0; preheating to 65-75℃, uniformly adding 120%-180% of the theoretical amount of hydrogen peroxide, and oxidizing at a constant temperature for 0.5-2 hours to form precursor crystal nuclei; heating to 75-85℃ at 1-3℃ / min, and aging at a constant temperature for 4-9 hours to transform crystals; filtering, washing, drying, calcining, and pulverizing to obtain the product. This invention achieves the controllable preparation of pure-phase, regularly morphologically regular polyhedral single-crystal iron phosphate particles through the synergistic control of low-concentration phosphoric acid, slow heating, and long-term aging, without adding any surfactants. Product D 50 The diameter is 4.5–6.5 μm, D 100 <80μm, sodium content <10ppm, potassium content <5ppm, green and environmentally friendly process, suitable for industrial production, can be used as a precursor for lithium-ion battery cathode material, and is suitable for the needs of new energy vehicles and energy storage fields.

Description

Technical Field

[0001] This invention relates to the field of iron phosphate preparation technology, specifically to a method for preparing polyhedral single-crystal iron phosphate particles. Background Technology

[0002] In recent years, with the widespread application of lithium-ion batteries in new energy vehicles and energy storage, the crystal structure and morphology of iron phosphate (FePO4), as a precursor to cathode materials, play a decisive role in the final electrochemical performance of lithium iron phosphate (LiFePO4). It is generally believed in the industry that single-crystal granular iron phosphate, compared to traditional plate-like or polycrystalline aggregates, can effectively reduce the number of grain boundaries, improve lithium-ion transport efficiency, and significantly enhance the material's compaction density and cycle stability.

[0003] Most methods for synthesizing iron phosphate produce a lamellar structure. To obtain a polyhedral single-crystal structure, Chinese patent CN115506006A proposes a method for preparing single-crystal iron phosphate by adding a specific surfactant, 1,2-propanediol. This method... 3+ The six-coordinate geometry (i.e., octahedral) configuration of the ion with 1,2-propanediol can be functionalized to form hydrated hydroxyferric phosphate single-crystal octahedrons, which can then be dehydrated at high temperature to form anhydrous single-crystal octahedral iron phosphate. However, this process requires the additional introduction of the surfactant 1,2-propanediol for morphology control; without the addition of 1,2-propanediol, the iron phosphate formed is always in the form of small flakes. Furthermore, the P / Fe ratio is high, increasing costs. In addition, this process uses scrap iron as the iron source to prepare the iron phosphate solution, with a dissolution time of 20-30 hours. After the filter cake is prepared, it needs to be calcined at high temperature for a long time, increasing production energy consumption and operating costs.

[0004] Currently, high-end lithium-ion batteries have increasingly stringent requirements for the purity, morphology, and particle size distribution of iron phosphate, which traditional preparation methods cannot achieve simultaneously. Therefore, developing a green and efficient single-crystal iron phosphate preparation process has significant industrial value. Summary of the Invention

[0005] Therefore, the present invention provides a method for preparing polyhedral single-crystal iron phosphate particles to solve the problems in the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: According to one aspect of the present invention, a method for preparing polyhedral single-crystal iron phosphate particles is provided, the method comprising: (1) Dilute 85% concentrated phosphoric acid to a mass concentration of 10%-30%, preheat to 40-80℃, add iron powder to dissolve, and prepare an iron-phosphorus solution with a P / Fe molar ratio of 1.5-3.0; (2) Preheat the iron-phosphorus solution to 65-75℃, and add hydrogen peroxide dropwise at a uniform rate while stirring. The amount of hydrogen peroxide added is equal to the amount of Fe oxidized. 2+ The required amount is 120%-180% of the theoretical amount. After the addition is complete, the oxidation reaction is carried out at a constant temperature for 0.5-2 hours to form precursor crystal nuclei. (3) Heat the sample to 75-85℃ at 1-3℃ / min and keep it at a constant temperature for 4-9 hours to obtain a single-crystal iron phosphate slurry; (4) The iron phosphate slurry is filtered, washed, dried and calcined to obtain anhydrous battery-grade polyhedral single crystal iron phosphate particles.

[0007] In this invention, 85% concentrated phosphoric acid is used, which is then diluted to a concentration of 10%-30% before the reaction to obtain the desired single-crystal particles. When the phosphoric acid concentration is higher than 30%, the ionic strength of the system increases significantly, the activity of iron ions and phosphate ions decreases, the local supersaturation during the oxidation precipitation process is uneven, the crystal nucleus growth becomes uncontrolled, and the grains grow abnormally. 50 Particle sizes exceeding 15 μm cannot yield well-formed polyhedral single crystals; when phosphoric acid concentration is below 10%, the system lacks sufficient acidity, leading to Fe... 2+ Insufficient dissolution easily leads to the formation of amorphous iron phosphate secondary phases, and the reaction rate is too slow to form complete single crystal particles. This invention clearly defines this concentration window, solving the technical problem of morphology loss of control caused by improper selection of phosphoric acid concentration in the prior art.

[0008] In existing technologies, to ensure a complete reaction, the minimum P / Fe molar ratio is 2.5, requiring a large amount of phosphoric acid and resulting in high costs. This invention prepares ferric phosphate by diluting concentrated phosphoric acid to 10%-30% to create a ferric phosphate solution, thus reducing the P / Fe molar ratio to 1.5. When the P / Fe molar ratio is below 1.5, the reaction is incomplete, iron powder remains, and the resulting product is amorphous. While a ratio above 3.0 is still possible, it increases costs; therefore, this invention controls the P / Fe molar ratio between 1.5 and 3.0.

[0009] Furthermore, in step (2), the hydrogen peroxide is added over a time of 30-45 minutes, and the stirring speed is 150-350 rpm. (Fe per unit time) 3+ The generation rate should not exceed the system diffusion rate to avoid amorphous iron phosphate inclusions.

[0010] As an example, the preferred heating rate is 1.5-2.5℃ / min; a heating rate below 1℃ / min is likely to cause the particle size distribution to broaden, while a heating rate above 3℃ / min is likely to induce secondary nucleation and form a bimodal distribution.

[0011] Furthermore, in step (2), the pH of the oxidation stage system is 0.7-1.2.

[0012] Furthermore, in step (3), the pH of the system naturally rises to 1.5-2.0 during the aging stage.

[0013] This invention utilizes the synergistic effect of "low-concentration iron-phosphorus solution (10%-30%) + peroxidation (120%-180%)". Premature hydrolysis is suppressed by controlling the pH during the oxidation stage (0.7-1.2), and slow polycondensation is driven by the natural pH increase during the aging stage (1.5-2.0). This mechanism of "homogeneous nucleation → ordered epitaxial growth" allows for the direct acquisition of well-shaped polyhedral single-crystal particles in an aqueous system without the introduction of any organic templates or dispersants, avoiding the risk of residual organic impurities and significantly reducing raw material costs.

[0014] Furthermore, in step (4), the washing water usage V_pure water:V_slurry = 3:1-5:1.

[0015] Furthermore, in step (4), the drying temperature is 90-130℃ and the drying time is 6-12h to obtain stable iron phosphate dihydrate single crystal particles.

[0016] Furthermore, in step (4), calcination is carried out in an air atmosphere with an air flow rate of 1-2 times the furnace volume per hour; the calcination temperature is 500-750℃, and the time is 2.5-3 hours; calcination time exceeding 3 hours results in significant particle agglomeration. This invention strictly controls the calcination time to 2.5-3 hours. This short-time calcination strategy is only used to remove water of crystallization, avoiding abnormal grain growth and hard agglomeration caused by prolonged high temperatures. The final product maintains excellent single-particle dispersion, D 50 The particle size was stably controlled between 4.5-6.5 μm, with no D. 100 Large particles (<80μm) perfectly balance the contradiction between phase transformation and morphology preservation.

[0017] According to another aspect of the present invention, a single-crystal ferric phosphate particle prepared by any of the methods described above is provided, wherein the ferric phosphate has a single-crystal pure-phase structure, contains no impurity phases, and has a polyhedral morphology. 50 The diameter is 4.5-6.5 μm, D 100 <80μm, sodium content <10ppm, potassium content <5ppm.

[0018] The present invention has the following advantages: This invention achieves the production of well-formed polyhedral single-crystal iron phosphate particles by precisely controlling the concentration of diluted phosphoric acid, the P / Fe molar ratio, and the staged synergistic effects of oxidation and aging temperatures, combined with a slow heating rate, without adding any organic surfactants such as 1,2-propanediol, crystallization agents, or dispersants. This reduces the amount of phosphoric acid used in the reaction process, lowers raw material costs and process complexity, and improves product purity.

[0019] This invention innovatively employs a low-temperature, long-time aging and crystal transformation technology, enabling the crystal to undergo a perfect transformation from an amorphous state to a single crystal state in the liquid phase through Ostwald aging, laying a good crystal structure foundation for subsequent heat treatment.

[0020] This invention achieves Fe within a unit time by precisely controlling the dropping time of hydrogen peroxide and the stirring speed. 3+ Matching the generation rate with the system diffusion rate avoids "explosive nucleation" caused by local supersaturation. Combined with a specific heating rate of 1-3℃ / min, it effectively avoids particle size distribution broadening (poor monodispersity) caused by excessively slow heating or secondary nucleation (bimodal distribution) caused by excessively rapid heating. This refined control of the flow field and reaction field ensures high stability of product performance between batches. Attached Figure Description

[0021] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0022] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0023] Figure 1 The present invention provides a method for preparing anhydrous battery-grade polyhedral single-crystal iron phosphate particles, as shown in the process flow diagram of Embodiment 1.

[0024] Figure 2 This is a scanning electron microscope (SEM) image of the iron phosphate dihydrate sample prepared in Example 2 of the present invention.

[0025] Figure 3 This is a scanning electron microscope (SEM) image of the iron phosphate dihydrate sample prepared in Example 3 of the present invention.

[0026] Figure 4 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample obtained in Example 3 of the present invention.

[0027] Figure 5 The X-ray diffraction (XRD) patterns of the dried and calcined iron phosphate samples obtained in Example 3 of this invention are shown.

[0028] Figure 6 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 1 of this invention.

[0029] Figure 7 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 2 of this invention.

[0030] Figure 8 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 3 of this invention.

[0031] Figure 9 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 4 of this invention.

[0032] Figure 10 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 5 of this invention.

[0033] Figure 11 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 6 of this invention.

[0034] Figure 12 This is a scanning electron microscope (SEM) image of the anhydrous calcined iron phosphate sample prepared in Comparative Example 7 of this invention. Detailed Implementation

[0035] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Unless otherwise specified in the embodiments of this invention, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products; different manufacturers and models of raw materials do not affect the implementation of the technical solution or the achievement of the technical effect of this invention.

[0037] Concentrated phosphoric acid: Analytical grade (AR), (Tianjin) XK13-011-00017 Phosphoric acid ≥85%, Tianjin Kemeo Chemical Reagent Co., Ltd. Iron powder: Analytical grade (AR), conforming to HGT3473-2003, iron ≥98%, Tianjin Aopusheng Chemical Co., Ltd.

[0038] Grinder: High-speed universal pulverizer, Beijing Yongguangming Medical Instrument Co., Ltd., model FW177.

[0039] Example 1 A method for preparing anhydrous battery-grade polyhedral single-crystal iron phosphate particles, the process flow diagram is as follows: Figure 1 As shown: (1) Dilute 85% concentrated phosphoric acid with deionized water to a mass concentration of 10% dilute phosphoric acid solution. Weigh 300g of the diluted phosphoric acid solution, preheat to 40℃, and slowly add iron powder at a stirring speed of 150rpm to dissolve it completely, so as to obtain an iron-phosphorus solution with a P / Fe molar ratio of 1.5.

[0040] (2) Preheat the above iron-phosphorus solution to 65°C, and add 30% hydrogen peroxide dropwise over 30 minutes while stirring at 200 rpm. The amount of hydrogen peroxide added is equal to the amount of Fe oxidized. 2+ 120% of the theoretical required amount. After the addition is complete, the mixture is oxidized at a constant temperature of 65℃ for 0.5 hours to form precursor crystal nuclei. The pH of the system during this stage is 0.7-1.0.

[0041] (3) After oxidation, the system was heated to 75°C at a heating rate of 1°C / min and aged at a constant speed of 200 rpm for 4 hours to obtain single-crystal iron phosphate slurry. During the aging stage, the pH of the system naturally rose to 1.5-1.8.

[0042] (4) Filter the filter cake and wash it with pure water at a volume of 3 times that of the ferric phosphate slurry. Dry the filter cake at 90°C for 6 hours to obtain ferric phosphate dihydrate single crystal particles. Place the dried product in an air atmosphere muffle furnace for calcination at an air flow rate of 1 times the furnace volume / h, a calcination temperature of 500°C, and a time of 2.5 hours.

[0043] (5) After calcination, the calcined product was pulverized using an FW177 high-speed pulverizer. The pulverizer chamber was filled with anhydrous iron phosphate to about 2 / 3 full. After closing the pulverizing chamber, the switch was turned on for 10 seconds and then off for 5 seconds. This process was repeated three times until the product was completely pulverized. (The parameters and preparation methods for pulverization that are not described in the following examples are consistent with those in this embodiment.) Anhydrous battery-grade polyhedral single-crystal iron phosphate product was obtained.

[0044] Example 2 A method for preparing anhydrous battery-grade polyhedral single-crystal iron phosphate particles: (1) Dilute 85% concentrated phosphoric acid to a mass concentration of 15%, preheat to 50°C, add iron powder to dissolve, and prepare an iron-phosphorus solution with a P / Fe molar ratio of 2.0.

[0045] (2) Preheat the iron-phosphorus solution to 70°C, and add hydrogen peroxide dropwise at a constant rate over 35 minutes while stirring at 250 rpm. The amount added is 140% of the theoretical amount. After the addition is complete, maintain the oxidation reaction at 70°C for 1 hour. The pH during the oxidation stage is 0.8-1.1.

[0046] (3) After oxidation, the temperature is increased to 80℃ at 1.5℃ / min and aged at 250 rpm for 6 hours for crystal transformation. During the aging stage, the pH naturally rises to 1.6-1.9.

[0047] (4) Filter the filter cake with 3 times the volume of pure water as washing agent for the ferric phosphate slurry. Dry the filter cake at 100℃ for 8 hours. Calcine the dried product in air atmosphere with an air flow rate of 1.5 times the furnace volume / h, at a calcination temperature of 575℃ for 3 hours.

[0048] (5) After calcination, the ferric phosphate was pulverized to obtain anhydrous battery-grade polyhedral single-crystal particles. The SEM image of the ferric phosphate dihydrate prepared in this embodiment is shown below. Figure 2 As shown.

[0049] Example 3 A method for preparing anhydrous battery-grade polyhedral single-crystal iron phosphate particles: (1) Dilute 85% concentrated phosphoric acid to a mass concentration of 20%, preheat to 60°C, add iron powder to dissolve, and prepare an iron-phosphorus solution with a P / Fe molar ratio of 2.5.

[0050] (2) Preheat the iron-phosphorus solution to 70°C, and add hydrogen peroxide dropwise at a constant rate of 200 rpm over 40 min, with the amount added being 150% of the theoretical amount. After the addition is complete, maintain the oxidation reaction at 70°C for 1 h. The pH during the oxidation stage is 0.9-1.2.

[0051] (3) After oxidation, the temperature is increased to 80℃ at 2.0℃ / min and aged at 200rpm for 7 hours for crystal transformation. During the aging stage, the pH naturally rises to 1.7-2.0.

[0052] (4) Filter and wash with pure water at 3 times the volume of ferric phosphate slurry. Dry the filter cake at 110℃ for 8 hours to obtain ferric phosphate dihydrate; calcine ferric phosphate dihydrate in an air atmosphere with an air flow rate of 2 times the furnace volume / h, a calcination temperature of 600℃ and a time of 3 hours to obtain calcined anhydrous ferric phosphate.

[0053] (5) After calcination, the iron phosphate was pulverized to obtain anhydrous battery-grade polyhedral single crystal particles.

[0054] The SEM image of the ferric phosphate dihydrate prepared in this embodiment is shown below. Figure 3 As shown, the SEM image of the anhydrous calcined ferric phosphate sample is as follows. Figure 4 As shown, the XRD patterns of the dried and calcined samples are as follows: Figure 5As shown in the figure, the product consists of regular polyhedral single-crystal particles, and the XRD pattern shows no impurity peaks, indicating it is pure-phase iron phosphate.

[0055] Example 4 A method for preparing anhydrous battery-grade polyhedral single-crystal iron phosphate particles: (1) Dilute 85% concentrated phosphoric acid to a mass concentration of 25%, preheat to 70°C, add iron powder to dissolve, and prepare an iron-phosphorus solution with a P / Fe molar ratio of 2.5.

[0056] (2) Preheat the iron-phosphorus solution to 70°C, and add hydrogen peroxide dropwise at a constant rate of 200 rpm over 45 min, with the amount added being 160% of the theoretical amount. After the addition is complete, maintain the oxidation reaction at 70°C for 1 h. The pH during the oxidation stage is 1.0-1.2.

[0057] (3) After oxidation, the temperature is increased to 80℃ at 2.5℃ / min and aged at 200rpm for 8 hours to transform crystals. During the aging process, the pH naturally rises to 1.8-2.0.

[0058] (4) Filter the filter cake with three times the volume of pure water as washing agent for the ferric phosphate slurry. Dry the filter cake at 120°C for 10 hours. Calcination is carried out in an air atmosphere with an air flow rate of twice the furnace volume per hour, a calcination temperature of 650°C, and a time of 2.5 hours.

[0059] (5) After gas calcination, the iron phosphate is pulverized to obtain anhydrous battery-grade polyhedral single crystal particles.

[0060] Example 5 A method for preparing anhydrous battery-grade polyhedral single-crystal iron phosphate particles: (1) Dilute 85% concentrated phosphoric acid to a mass concentration of 30%, preheat to 80°C, add iron powder to dissolve, and prepare an iron-phosphorus solution with a P / Fe molar ratio of 3.0.

[0061] (2) Preheat the iron-phosphorus solution to 75°C, and add hydrogen peroxide dropwise at a uniform rate over 45 minutes while stirring at 350 rpm. The amount added is 180% of the theoretical amount. After the addition is complete, maintain the oxidation reaction at 75°C for 2 hours. The pH during the oxidation stage is 1.0-1.2.

[0062] (3) After oxidation, the temperature is increased to 85℃ at 3.0℃ / min and aged at 350rpm for 9 hours to transform crystals. During the aging process, the pH naturally rises to 1.9-2.0.

[0063] (4) Filter the filter cake with pure water at a volume of 3 times that of the ferric phosphate slurry. Dry the filter cake at 130°C for 12 hours. Calcination is carried out in an air atmosphere with an air flow rate of 2 times the furnace volume per hour, a calcination temperature of 750°C, and a time of 3 hours.

[0064] (5) After calcination, the iron phosphate is pulverized to obtain anhydrous battery-grade polyhedral single crystal particles.

[0065] Comparative Example 1 The only difference between this comparative example and Example 3 is that in step (1), 85% concentrated phosphoric acid is diluted to a mass concentration of 40% (exceeding the range of 10%-30%), while the remaining steps and parameters are the same as in Example 3.

[0066] Result: Product D obtained 50 The particle size is 18.6 μm, as shown in SEM images. Figure 6 The crystal morphology was irregular, exhibiting a mixture of plate-like and spherical shapes, failing to yield regular polyhedral single crystals. This indicates that when the phosphoric acid concentration exceeds 30%, the system viscosity is too high, ion diffusion is hindered, and abnormal grain growth occurs, making it impossible to obtain the expected morphology.

[0067] Comparative Example 2 The only difference between this comparative example and Example 3 is that in step (1), 85% concentrated phosphoric acid is diluted to a mass concentration of 5% (below the lower limit of 10%), and the remaining steps and parameters are the same as in Example 3.

[0068] Results: The iron powder dissolution rate was significantly reduced, and some iron powder remained after 3 hours of reaction. The XRD pattern of the obtained product showed obvious amorphous ferric phosphate dispersion peaks, and the SEM image showed... Figure 7 The product exhibits amorphous flocculent agglomeration and fails to form single crystal particles. This indicates that when the phosphoric acid concentration is below 10%, the acid content of the system is insufficient, the iron powder is not fully dissolved, and amorphous secondary phases are easily formed.

[0069] Comparative Example 3 The only difference between this comparative example and Example 3 is that in step (3), the heating rate is 0.5℃ / min (less than 1℃ / min), and the other steps and parameters are the same as in Example 3.

[0070] Result: Product D obtained 50 The particle size is 6.8 μm, but the particle size distribution span (Span value) is significantly increased, with D10 at 1.2 μm and D90 at 15.3 μm, showing a marked broadening of the distribution. (SEM image follows) Figure 8 The data shows the coexistence of some abnormally large grains and fine particles. This indicates that the excessively slow heating rate caused the crystal nuclei to undergo Ostwald ripening prematurely in the heating transition region, resulting in a broadened grain size distribution.

[0071] Comparative Example 4 The only difference between this comparative example and Example 3 is that in step (3), the heating rate is 5℃ / min (higher than 3℃ / min), and the other steps and parameters are the same as in Example 3.

[0072] Results: Laser particle size analysis of the obtained product showed a distinct bimodal distribution, with the main peak located around 4.5 μm and the secondary peak around 0.8 μm. SEM results are shown below. Figure 9 The display shows a large number of fine secondary nucleation particles adhering to the surface of the main particles. This indicates that the excessively rapid heating rate triggered localized overheating and secondary nucleation, resulting in the presence of fine particles in the product and a deterioration in particle size distribution.

[0073] Comparative Example 5 The only difference between this comparative example and Example 3 is that in step (3), the constant temperature aging time is 2 hours (less than 4 hours), and the other steps and parameters are the same as in Example 3.

[0074] Results: The XRD pattern of the obtained product showed weak diffraction peak intensities and wide half-peak widths, indicating insufficient crystallinity. SEM images showed... Figure 10 The particles exhibit rough surfaces and indistinct crystal edges, with a morphology intermediate between amorphous and crystalline, failing to form a complete polyhedral single crystal morphology. This indicates that insufficient aging time prevented the crystals from achieving adequate crystal face development and defect repair.

[0075] Comparative Example 6 The only difference between this comparative example and Example 3 is that in step (4), the calcination time is 5 hours (more than 3 hours), and the other steps and parameters are the same as in Example 3.

[0076] Results: The specific surface area of ​​the obtained product decreased from 7.81 m² / g in Example 3 to 3.21 m² / g. SEM images are shown below. Figure 11 The study revealed a distinct sintering neck at the contact points of adjacent particles, with some particles having fused into hard agglomerates, thus compromising the independence of single-crystal particles. When this iron phosphate was used to prepare lithium iron phosphate using conventional methods, the compaction density decreased from 2.55 g / cm³ to 2.38 g / cm³. This indicates that excessively long calcination time led to particle fusion and agglomeration, severely impacting the subsequent material properties.

[0077] Comparative Example 7 The only difference between this comparative example and Example 3 is that in step (1), the P / Fe molar ratio of the prepared iron-phosphorus solution is 1.2, and the remaining steps and parameters are the same as in Example 3.

[0078] Results: The product contained 37.85% Fe, 19.92% P, Fe / P = 1.052, D50 = 5.83 μm, D100 = 78.42 μm, and BET = 7.26 μm. 2 / g, tap density = 1.03g / cm³, Na = 9.6ppm, K = 4.8ppm; iron powder was not completely dissolved, and a small amount of iron powder remained at the bottom of the slurry after the reaction. The filter residue after filtration was tested and found to be undissolved elemental iron; SEM results are as follows. Figure 12The results show that some particles have irregular morphologies and contain a small amount of amorphous impurities, failing to produce pure-phase polyhedral single crystal particles. This indicates that when the P / Fe molar ratio is below 1.5, the amount of phosphoric acid in the system is insufficient, failing to fully dissolve the iron powder. The residual iron powder impurities will disrupt normal crystal growth, leading to irregular morphology and decreased purity.

[0079] Test Example 1 The performance indicators of the iron phosphate products obtained in Examples 1-5 and Comparative Examples 1-7 are compared in Tables 1 and 2.

[0080] Table 1 Chemical composition and impurity content of ferric phosphate products in each embodiment

[0081] Table 2 Chemical composition and impurity content of each comparative example of iron phosphate products

[0082] As shown in Tables 1 and 2, in Examples 1-5, the P content remained stable at 20.86%-20.95%, the Fe content remained stable at 36.34%-36.46%, and the Fe / P molar ratio remained stable at 96.50%-96.91%, which highly matches the theoretical stoichiometric ratio of anhydrous iron phosphate. This indicates that the phosphoric acid concentration of 10%-30%, the P / Fe molar ratio of 1.5-3.0, and the segmented oxidation and aging process specified in this invention can precisely control the iron-phosphorus chemical ratio, and the product is pure-phase stoichiometric iron phosphate without iron-rich, iron-poor, or phosphorus-rich defects. Na: 2.29-8.98ppm < 10ppm; K: 0.84-5.18ppm < 5ppm; fully meeting the impurity limit requirements for iron phosphate precursors in high-end power batteries. The three-stage countercurrent washing, strict pH control, and slow crystallization in a low-acid system of this invention can efficiently remove alkali metal impurities introduced by the raw materials, preventing their accumulation within the crystal lattice. Cu, Ca, and Mn are all 0 ppm; Cr, Co, Mg, Ti, Ni, and Pb are all at extremely low levels; Al, although present in some quantity, is stable and controllable, with no abnormal exceedances; Zn impurities are stable in the range of 25–27 ppm with minimal fluctuations. Overall, it meets the battery-grade high-purity iron phosphate standard, and heavy metal impurities will not cause battery self-discharge, cycle degradation, lithium plating, or other failure issues. When all parameters of the embodiments of this invention fall within the scope of protection of this invention, the composition is accurate, the crystal form is complete, the impurities are extremely low, the specific surface area is optimal, and the morphology is a regular polyhedral single crystal, with excellent consistency in all indicators.

[0083] Comparative Example 2: P is only 19.83% and Fe is only 34.52%, both relatively low, indicating incomplete crystallization and a large presence of amorphous phases. Comparative Example 1: Fe / P = 97.10%, indicating iron richness and abnormally coarse grains. Comparative Examples 3 / 4 / 5: Although Fe / P ratios are close, all exhibit crystallization defects and are not complete single-crystal stoichiometric phases. Na in the comparative examples ranges from 4.08 to 12.56 ppm, while in Comparative Example 2, Na = 12.56 ppm, exceeding the standard by more than 10 ppm. K ranges from 2.03 to 7.23 ppm, while in Comparative Examples 1 and 2, K > 5 ppm, exceeding the standard. This indicates that the parameter deviations lead to abnormal crystal morphology, pore encapsulation, and impurities adsorbed on the crystal surface / inside the crystal lattice. Three-stage countercurrent washing cannot effectively remove these impurities, resulting in severe impurity enrichment, making the material unsuitable for power batteries. The comparative examples showed higher levels of Zn, Cr, Al, Pb, Ti, and Co than the examples; particularly, Comparative Example 2 (low phosphoric acid) showed a significant increase in Ti (42.18 ppm) and Al (95.67 ppm); Comparative Example 1 (high phosphoric acid) had a higher Ti (38.45 ppm) and Pb. This indicates that the process window outside this invention resulted in disordered crystal growth, making it highly susceptible to adsorption and enrichment of various metallic impurities, leading to a significant decrease in product purity. Comparative Example 6: P=20.91%, Fe=36.43%, Fe / P=96.60%. Although the chemical composition was similar to Example 3, the BET dropped sharply from 7.81 to 3.21 m² / g, indicating significant particle agglomeration and the destruction of single-crystal independence. Na=3.89 ppm, K=1.98 ppm. Due to the reduced surface area after particle fusion, impurities were trapped inside the agglomerates, making effective removal by washing impossible. Comparative Example 7: P=19.92%, Fe=37.85%, Fe / P=105.20%. The iron content was abnormally high, the phosphorus content was abnormally low, and the Fe / P ratio deviated significantly from the theoretical value. Na=9.60ppm and K=4.80ppm, significantly higher than the 3.54ppm and 0.86ppm of Example 3, respectively; Al=85.23ppm, also significantly higher than Example 3. This indicates that when the P / Fe molar ratio is below 1.5, the amount of phosphoric acid is insufficient, the iron powder is not fully dissolved, and impurities from the residual iron powder are introduced into the product. Furthermore, the irregular morphology leads to increased impurity adsorption.

[0084] Table 3 Physical properties and morphology of the iron phosphate products in each example and comparative example

[0085] As shown in Table 3, the products of Examples 1-5 exhibit excellent physical properties: particle size is controllable; D 50 Concentrated in the 4.63-6.60 μm range, which is within the target range of 4.5-6.5 μm; D 100 <80μm, no large particle agglomeration, suitable for battery electrode coating requirements. Good dispersibility: BET 7.58-8.17m 2 / g, tap density is 1.12-1.23g / cm³ 3This indicates that the particles are uniformly dispersed and have good packing performance, which can improve the compaction density of lithium iron phosphate in the subsequent process. The particles have regular morphology: all are polyhedral single-crystal particles, free of impurities and agglomerates, verifying the effectiveness of the synergistic process of "low-concentration phosphoric acid + slow heating + long-term aging" in this invention.

[0086] Comparative Example 1 (D) 50 =18.60μm), 2 (D 50 =32.45μm) Due to improper phosphoric acid concentration, the crystals grew abnormally or appeared as amorphous flocculent grains; Comparative Example 3 (D 100 =82.45μm), 4 (bimodal distribution) due to improper heating rate, the particle size distribution broadened or secondary nucleation occurred.

[0087] Comparative Example 2 (BET=25.36m) 2 / g, tap density = 0.68g / cm³ 3 It is amorphous or in small flakes and has extremely poor stacking properties.

[0088] Comparative Examples 1 (mixed morphology), 2 (amorphous flocculent), 5 (incomplete crystal form), 6 (particle agglomeration), and 7 (irregular morphology) all failed to produce regular polyhedral single crystals, confirming the criticality of the process parameters (phosphoric acid concentration, heating rate, aging time, etc.) of this invention.

[0089] Examples 1-5 of the present invention achieve precise control of the chemical composition, impurity content, physical properties, and morphology of the product by precisely controlling parameters such as phosphoric acid concentration (10%-30%), P / Fe molar ratio (1.5-3.0), heating rate (1-3℃ / min), and aging time (4-9h). In contrast, each comparative example shows varying degrees of performance defects due to deviation of a single parameter from the scope defined by the present invention, further demonstrating the scientific nature, rationality, and necessity of the process parameters of the present invention, highlighting its significant advantages over the prior art.

[0090] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing polyhedral single-crystal iron phosphate particles, characterized in that, The method includes: (1) Dilute 85% concentrated phosphoric acid to a mass concentration of 10%-30%, preheat to 40-80℃, add iron powder to dissolve, and prepare an iron-phosphorus solution with a P / Fe molar ratio of 1.5-3.0; (2) Preheat the iron-phosphorus solution to 65-75℃, add hydrogen peroxide dropwise at a uniform rate while stirring, and after the addition is complete, maintain the temperature for oxidation reaction for 0.5-2h to form precursor crystal nuclei; (3) Heat the sample to 75-85℃ at 1-3℃ / min and keep it at a constant temperature for 4-9 hours to obtain a single-crystal iron phosphate slurry; (4) The iron phosphate slurry is filtered, washed, dried and calcined to obtain anhydrous battery-grade polyhedral single crystal iron phosphate particles.

2. The preparation method according to claim 1, characterized in that, In step (2), the hydrogen peroxide is added over a period of 30-45 minutes and the stirring speed is 150-350 rpm.

3. The preparation method according to claim 1, characterized in that, In step (2), the amount of hydrogen peroxide added is Fe oxide. 2+ The required theoretical dosage is 120%-180%.

4. The preparation method according to claim 1, characterized in that, In step (2), the pH of the oxidation stage system is 0.7-1.

2.

5. The preparation method according to claim 1, characterized in that, In step (3), the pH of the system naturally rises to 1.5-2.0 during the aging stage.

6. The preparation method according to claim 1, characterized in that, In step (4), the washing water usage V_pure water:V_slurry = 3:1-5:

1.

7. The preparation method according to claim 1, characterized in that, In step (4), the drying temperature is 90-130℃ and the drying time is 6-12h to obtain stable iron phosphate dihydrate single crystal particles.

8. The preparation method according to claim 1, characterized in that, In step (4), the calcination temperature is 500-750℃ and the time is 2.5-3h.

9. The preparation method according to claim 8, characterized in that, In step (4), calcination is carried out in an air atmosphere with an air flow rate of 1-2 times the furnace volume per hour.

10. A single-crystal ferric phosphate particle prepared by any of the methods described in claims 1-9, characterized in that, The iron phosphate is a single-crystal pure-phase structure, free of impurities, and the particles have a polyhedral morphology. 50 The diameter is 4.5-6.5 μm, D 100 <80μm, sodium content <10ppm, potassium content <5ppm.

Images