Method for preparing a sheet iron phosphate from an iron hydroxyphosphate intermediate and use thereof

Abstract

The application discloses a method for preparing flaky iron phosphate from iron hydroxyphosphate intermediate and flaky iron phosphate prepared by the method, and belongs to the technical field of lithium ion battery cathode materials. The method comprises the following steps: S1, preparing an iron salt solution; S2, preparing a phosphorus salt solution; S3, reacting the iron salt solution as a bottom solution with the phosphorus salt solution to generate an iron hydroxyphosphate precursor; S4, performing crystal transformation treatment on the precursor under acidic oxidation conditions to obtain a crystalline iron hydroxyphosphate intermediate; S5, reacting the intermediate with an oxidizing agent in an acidic environment to obtain iron phosphate dihydrate; and S6, calcining to obtain flaky anhydrous iron phosphate. The crystalline iron hydroxyphosphate intermediate is used as a crystal structure template to induce iron phosphate dihydrate to form a flaky morphology in a directional manner, and the obtained flaky iron phosphate has a relatively short lithium ion diffusion path, and when used for preparing lithium iron phosphate cathode materials, the rate performance and electrochemical performance of the lithium iron phosphate cathode materials can be significantly improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a method for preparing flake-shaped iron phosphate from a hydroxy ferric phosphate intermediate and the prepared flake-shaped iron phosphate. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as a high-safety, long-life, and low-cost cathode material for lithium-ion batteries, owes its performance largely to the physicochemical properties of its precursor, iron phosphate (FePO4). Studies have shown that the microstructure of iron phosphate (such as particle size, shape, and pore structure) directly affects the final cathode material's packing density, lithium-ion diffusion pathways, and electrochemical performance. Therefore, developing preparation techniques that can precisely control the morphology of iron phosphate is crucial for improving the overall performance of lithium iron phosphate batteries.

[0003] Currently, the industry has conducted numerous explorations in controlling the morphology of iron phosphate, mainly focusing on optimizing particle size distribution and constructing special structures. However, these existing technologies are usually accompanied by significant process complexity or inherent defects. Particle size distribution technology: For example, patent CN119176533A attempts to construct a gradation system composed of large and small particles of specific sizes by precisely controlling reaction conditions to optimize tap density. The limitations of this method are: a narrow process window, extremely stringent requirements for the dynamic balance of reaction parameters, and a high likelihood of fluctuations in the morphology and proportion of products between different batches, resulting in poor reproducibility and stability, which is detrimental to large-scale stable production.

[0004] Biomass template method: For example, patent CN112436132A utilizes natural plant components as pore-forming agents and carbon sources to prepare porous iron phosphate. The limitations of this method are: the biomass raw materials are complex and non-uniform, potentially introducing uncontrollable impurities, leading to uneven pore structures and large batch-to-batch variations; furthermore, the subsequent processing steps are cumbersome, posing challenges to the feasibility and economic viability of industrial-scale production.

[0005] In summary, while existing morphology control technologies have made some progress, they generally share a common challenge: achieving specific morphologies often requires the introduction of complex process controls or non-standard raw materials. This leads to poor process stability, low reproducibility, high costs, or the introduction of uncertainties, making it difficult to meet the stringent requirements of consistency, stability, and economy for large-scale industrial production. Therefore, developing a robust, easily controllable, and precisely induced target morphology method for iron phosphate preparation has become an urgent need in this field. Summary of the Invention

[0006] In view of the common bottlenecks of difficult control and poor stability in the morphology control process in the above-mentioned background technology, the present invention aims to provide a brand-new solution.

[0007] The core technical problem to be solved by this invention is: how to avoid the drawbacks of existing complex control processes and achieve precise and reliable induction and control of iron phosphate products (especially those with a flake-like morphology) through a structurally well-defined, stable and controllable intermediate.

[0008] Specifically, this invention provides a method for preparing flake ferric phosphate from a hydroxyferric phosphate intermediate and the prepared flake ferric phosphate, comprising the following steps: S1. Prepare an iron salt solution; S2. Prepare phosphate salt solution; S3. Add the iron salt solution to the reactor as the base liquid and heat it. Add the phosphate salt solution to the reactor under continuous stirring and react for a period of time to obtain slurry A. After filtration and washing, obtain the hydroxyferric phosphate precursor. S4. Add pure water to the above-mentioned ferric hydroxyphosphate precursor and stir to form a uniform slurry. Then add pH adjuster and oxidant, heat and keep warm to obtain slurry B. After filtration and washing, obtain crystalline ferric hydroxyphosphate intermediate (Fe3(PO4)2(OH)2). The crystalline ferric hydroxyphosphate intermediate is a crystal structure template that induces ferric phosphate dihydrate to form a plate-like morphology. S5. Add pure water to the above crystalline ferric hydroxyphosphate intermediate and stir to form a uniform slurry. Then add phosphoric acid, oxidant and pH adjuster, heat and keep warm to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. S6 and ferric phosphate dihydrate are dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate.

[0009] Preferably, the iron concentration of the iron salt solution in S1 is 0.5-2 mol / L, and the pH of the iron salt solution is adjusted to 2-3 by adding an inorganic acid. The iron source is one or more of ferrous sulfate, ferrous nitrate, and ferrous chloride, and the inorganic acid is one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0010] Preferably, the phosphorus concentration of the phosphate salt solution in S2 is 0.5-2 mol / L, and the pH of the phosphate salt solution is adjusted to 6.0-12.0 by adding a pH adjuster. The phosphorus source is one or more of phosphoric acid, ammonium phosphate, monoammonium phosphate, and diammonium phosphate, and the pH adjuster is one or more of ammonia water, ammonia gas, and sodium hydroxide.

[0011] Preferably, the Fe:P ratio of the iron source and phosphorus source in S1 and S2 is 1:0.5-1:2.

[0012] Preferably, the bottom liquid temperature in S3 is 30-50℃, the stirring rate of the reactor is 100-600rpm, the feeding time of the phosphate salt solution is 20-40min, the reaction time is 20-40min, and the washing is completed when the conductivity of the washing water is ≤3000μs / cm.

[0013] Preferably, the molar ratio of the iron source to the oxidant in S4 is 1:0.2-1, the oxidant is one or more of sodium persulfate, hydrogen peroxide, and perchloric acid, the pH adjuster is one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid, the pH is adjusted to 1-6, the heating temperature is 80-100℃, the holding time is 0.5-6h, and the washing ends when the conductivity of the wash water is ≤1500μs / cm.

[0014] Preferably, in step S5, the molar ratio of the iron source to phosphoric acid is 1:0.2-1.5, the molar ratio of the iron source to the oxidant is 1:0.2-1, the oxidant is one or more of sodium persulfate, hydrogen peroxide, and perchloric acid, the pH adjuster is one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid, the pH is adjusted to 1.0-1.8, the heating temperature is 80-100℃, the holding time is 1-6h, and the washing ends when the conductivity of the washing water is ≤1500μs / cm.

[0015] Preferably, the drying temperature in step S6 is 100-200℃, the drying time is 2-6h, the calcination temperature is 450-750℃, and the calcination time is 2-10h.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The method provided by this invention uses ferric hydroxyphosphate as an intermediate. Its crystal structure can be precisely controlled during the conversion to ferric phosphate. The conversion crystallization induces the formation of lamellar ferric phosphate, which helps to shorten the distance for electron and lithium ion conduction and diffusion within the material, thereby improving Li-energy efficiency. + The migration efficiency is improved, thereby effectively enhancing the electrochemical performance of the cathode material. In addition, the hydroxyferric phosphate structure introduced in this invention is stable and controllable, with excellent process repeatability and low equipment requirements. It can be directly applied to existing iron phosphate process production lines without complicated technical modifications, making it suitable for industrial production.

[0017] 2. This invention uses hydroxyferric phosphate Fe3(PO4)2(OH)2 with a defined crystal structure as a key intermediate. Its specific crystal structure serves as a template or directing agent during the conversion process, stably and repeatedly guiding the generation of iron phosphate with a plate-like morphology, thereby avoiding the randomness and instability of morphology control in traditional processes.

[0018] 3. The entire process route of this invention is based on conventional liquid-phase reaction and heat treatment steps, without the need for complex dynamic control or special template agents. The raw materials are inexpensive and readily available, and it has high compatibility with existing iron phosphate production lines. It also has excellent process stability, reproducibility and large-scale production potential. Attached Figure Description

[0019] Figure 1 The image shows the SEM image of the iron phosphate prepared in Example 1 of this invention. Figure 2 The image shows the SEM image of the iron phosphate prepared in Example 2 of this invention. Figure 3 This is the SEM image of the iron phosphate prepared in Example 3 of this invention; Figure 4 The image shows the SEM image of the iron phosphate prepared in Comparative Example 1 of this invention. Figure 5 This is the SEM image of the iron phosphate prepared in Comparative Example 2 of this invention; Figure 6 This is the SEM image of the iron phosphate prepared in Comparative Example 3 of this invention. Detailed Implementation

[0020] Example 1 This embodiment provides a method for preparing flaky ferric phosphate from a hydroxyferric phosphate intermediate, the method comprising the following steps: S1. Dissolve ferrous sulfate heptahydrate in deionized water, and add concentrated sulfuric acid to adjust the pH to obtain an iron salt solution; wherein the concentration of iron in the solution is 0.5 mol / L, and the pH of the solution is 2; S2. Dissolve ammonium dihydrogen phosphate in deionized water, add ammonia to adjust the pH to obtain a phosphate salt solution; wherein, the concentration of phosphorus in the solution is 0.5 mol / L, and the pH of the solution is 6; S3. The iron salt solution of S1 is added to the reactor as the bottom liquid and heated. The phosphate salt solution of S2 is added to the reactor under continuous stirring and reacted for a period of time to obtain slurry A. After filtration and washing, the hydroxyferric phosphate precursor is obtained. The bottom liquid temperature is 30℃, the stirring rate is 600rpm, the phosphate salt addition time is 20min, the reaction time is 20min, and the washing condition is that the conductivity of the washing water is ≤3000μs / cm. S4. Add pure water to the above-mentioned ferric hydroxyphosphate precursor and stir to form a uniform slurry. Then add pH adjuster and oxidant, heat and keep warm to obtain slurry B. After filtration and washing, obtain crystalline ferric hydroxyphosphate intermediate (Fe3(PO4)2(OH)2). The pH of the slurry is adjusted to 4.0, the molar ratio of iron source to hydrogen peroxide is 1:0.2, the heating temperature is 80℃, the holding time is 4h, and the washing condition is that the conductivity of the washing water is ≤1500μs / cm. S5. Add pure water to the above-mentioned hydroxyferric phosphate and stir to form a uniform slurry. Then add phosphoric acid, hydrogen peroxide and sulfuric acid, heat and keep warm to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. The molar ratio of iron source to hydrogen peroxide is 1:0.33, the molar ratio of iron source to phosphoric acid is 1:0.4, the pH of the slurry is adjusted to 1.8, the heating temperature is 95℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S6 and ferric phosphate dihydrate were dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate. The drying temperature was 150℃ and the drying time was 4 hours, while the calcination temperature was 620℃ and the calcination time was 10 hours.

[0021] Example 2 This embodiment provides a method for preparing flaky ferric phosphate from a hydroxyferric phosphate intermediate, the method comprising the following steps: S1. Dissolve ferrous sulfate heptahydrate in deionized water, and add concentrated sulfuric acid to adjust the pH to obtain an iron salt solution; wherein the concentration of iron in the solution is 0.5 mol / L, and the pH of the solution is 2; S2. Dissolve ammonium dihydrogen phosphate in deionized water, add ammonia to adjust the pH to obtain a phosphate salt solution; wherein, the concentration of phosphorus in the solution is 0.5 mol / L, and the pH of the solution is 6; S3. The iron salt solution of S1 is added to the reactor as the bottom liquid and heated. The phosphate salt solution of S2 is added to the reactor under continuous stirring and reacted for a period of time to obtain slurry A. After filtration and washing, the hydroxyferric phosphate precursor is obtained. The bottom liquid temperature is 50℃, the stirring speed is 600rpm, the phosphate salt addition time is 30min, the reaction time is 30min, and the washing condition is that the conductivity of the washing water is ≤3000μs / cm. S4. Add pure water to the above-mentioned ferric hydroxyphosphate precursor and stir to form a uniform slurry. Then add pH adjuster and oxidant, heat and keep warm to obtain slurry B. After filtration and washing, obtain crystalline ferric hydroxyphosphate intermediate (Fe3(PO4)2(OH)2). The pH of the slurry is adjusted to 4.0, the molar ratio of iron source to hydrogen peroxide is 1:0.2, the heating temperature is 90℃, the holding time is 4h, and the washing condition is that the conductivity of the washing water is ≤1500μs / cm. S5. Add pure water to the above-mentioned hydroxyferric phosphate and stir to form a uniform slurry. Then add phosphoric acid, hydrogen peroxide and sulfuric acid, heat and keep warm to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. The molar ratio of iron source to hydrogen peroxide is 1:0.33, the molar ratio of iron source to phosphoric acid is 1:0.6, the pH of the slurry is adjusted to 1.4, the heating temperature is 95℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S6 and ferric phosphate dihydrate were dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate. The drying temperature was 150℃ and the drying time was 4 hours, while the calcination temperature was 620℃ and the calcination time was 10 hours.

[0022] Example 3 This embodiment provides a method for preparing flaky ferric phosphate from a hydroxyferric phosphate intermediate, the method comprising the following steps: S1. Dissolve ferrous sulfate heptahydrate in deionized water, and add concentrated sulfuric acid to adjust the pH to obtain an iron salt solution; wherein the concentration of iron in the solution is 0.5 mol / L, and the pH of the solution is 2; S2. Dissolve ammonium dihydrogen phosphate in deionized water, add ammonia to adjust the pH to obtain a phosphate salt solution; wherein, the concentration of phosphorus in the solution is 0.5 mol / L, and the pH of the solution is 6; S3. The iron salt solution of S1 is added to the reactor as the bottom liquid and heated. The phosphate salt solution of S2 is added to the reactor under continuous stirring and reacted for a period of time to obtain slurry A. After filtration and washing, the hydroxyferric phosphate precursor is obtained. The bottom liquid temperature is 40℃, the stirring rate is 600rpm, the phosphate salt addition time is 40min, the reaction time is 40min, and the washing condition is that the conductivity of the washing water is ≤3000μs / cm. S4. Add pure water to the above-mentioned ferric hydroxyphosphate precursor and stir to form a uniform slurry. Then add pH adjuster and oxidant, heat and keep warm to obtain slurry B. After filtration and washing, obtain crystalline ferric hydroxyphosphate intermediate (Fe3(PO4)2(OH)2). The pH of the slurry is adjusted to 4.0, the molar ratio of iron source to hydrogen peroxide is 1:0.2, the heating temperature is 100℃, the holding time is 4h, and the washing condition is that the conductivity of the washing water is ≤1500μs / cm. S5. Add pure water to the above-mentioned hydroxyferric phosphate and stir to form a uniform slurry. Then add phosphoric acid, hydrogen peroxide and sulfuric acid, heat and keep warm to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. The molar ratio of iron source to hydrogen peroxide is 1:0.33, the molar ratio of iron source to phosphoric acid is 1:0.8, the pH of the slurry is adjusted to 1.0, the heating temperature is 95℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S6 and ferric phosphate dihydrate were dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate. The drying temperature was 150℃ and the drying time was 4 hours, while the calcination temperature was 620℃ and the calcination time was 10 hours.

[0023] Comparative Example 1 The difference between this comparative example and Example 3 is that the crystal transformation temperature is 60°C, as detailed below: S1. Dissolve ferrous sulfate heptahydrate in deionized water, and add concentrated sulfuric acid to adjust the pH to obtain an iron salt solution; wherein the concentration of iron in the solution is 0.5 mol / L, and the pH of the solution is 2; S2. Dissolve ammonium dihydrogen phosphate in deionized water, add ammonia to adjust the pH to obtain a phosphate salt solution; wherein, the concentration of phosphorus in the solution is 0.5 mol / L, and the pH of the solution is 6; S3. The iron salt solution of S1 is added to the reactor as the bottom liquid and heated. The phosphate salt solution of S2 is added to the reactor under continuous stirring and reacted for a period of time to obtain slurry A. After filtration and washing, the hydroxyferric phosphate precursor is obtained. The bottom liquid temperature is 30℃, the stirring rate is 600rpm, the phosphate salt addition time is 30min, the reaction time is 30min, and the washing condition is that the conductivity of the washing water is ≤3000μs / cm. S4. Add pure water to the above-mentioned ferric hydroxyphosphate precursor and stir to form a uniform slurry. Then add pH adjuster and oxidant, heat and keep warm to obtain slurry B. After filtration and washing, obtain an amorphous intermediate. The pH of the slurry is adjusted to 4.0, the molar ratio of iron source to hydrogen peroxide is 1:0.2, the heating temperature is 60℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S5. Add pure water to the above amorphous intermediate and stir to form a uniform slurry. Then add phosphoric acid, hydrogen peroxide and sulfuric acid, heat and keep warm to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. The molar ratio of iron source to hydrogen peroxide is 1:0.33, the molar ratio of iron source to phosphoric acid is 1:0.6, the pH of the slurry is adjusted to 1.4, the heating temperature is 95℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S6 and ferric phosphate dihydrate were dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate. The drying temperature was 150℃ and the drying time was 4 hours, while the calcination temperature was 620℃ and the calcination time was 10 hours.

[0024] Comparative Example 2 The difference between this comparative example and Example 3 is that the crystal transformation temperature is 70°C, as detailed below: S1. Dissolve ferrous sulfate heptahydrate in deionized water, and add concentrated sulfuric acid to adjust the pH to obtain an iron salt solution; wherein the concentration of iron in the solution is 0.5 mol / L, and the pH of the solution is 2; S2. Dissolve ammonium dihydrogen phosphate in deionized water, add ammonia to adjust the pH to obtain a phosphate salt solution; wherein, the concentration of phosphorus in the solution is 0.5 mol / L, and the pH of the solution is 6; S3. The iron salt solution of S1 is added to the reactor as the bottom liquid and heated. The phosphate salt solution of S2 is added to the reactor under continuous stirring and reacted for a period of time to obtain slurry A. After filtration and washing, the hydroxyferric phosphate precursor is obtained. The bottom liquid temperature is 30℃, the stirring rate is 600rpm, the phosphate salt addition time is 30min, the reaction time is 30min, and the washing condition is that the conductivity of the washing water is ≤3000μs / cm. S4. Add pure water to the above-mentioned hydroxyferric phosphate precursor and stir to form a uniform slurry. Then add pH adjuster and oxidant, heat and keep warm to obtain slurry B. After filtration and washing, obtain low-crystallinity hydroxyferric phosphate intermediate (Fe3(PO4)2(OH)2). The pH of the slurry is adjusted to 4.0, the molar ratio of iron source to hydrogen peroxide is 1:0.2, the heating temperature is 70℃, the holding time is 4h, and the washing condition is that the conductivity of the washing water is ≤1500μs / cm. S5. Add pure water to the above-mentioned hydroxyferric phosphate and stir to form a uniform slurry. Then add phosphoric acid, hydrogen peroxide and sulfuric acid, heat and keep warm to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. The molar ratio of iron source to hydrogen peroxide is 1:0.33, the molar ratio of iron source to phosphoric acid is 1:0.6, the pH of the slurry is adjusted to 1.4, the heating temperature is 95℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S6 and ferric phosphate dihydrate were dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate. The drying temperature was 150℃ and the drying time was 4 hours, while the calcination temperature was 620℃ and the calcination time was 10 hours.

[0025] Comparative Example 3 The difference between this comparative example and Example 3 is that there is no crystal transformation step, as detailed below: S1. Dissolve ferrous sulfate heptahydrate in deionized water, and add concentrated sulfuric acid to adjust the pH to obtain an iron salt solution; wherein the concentration of iron in the solution is 0.5 mol / L, and the pH of the solution is 2; S2. Dissolve ammonium dihydrogen phosphate in deionized water, add ammonia to adjust the pH to obtain a phosphate salt solution; wherein, the concentration of phosphorus in the solution is 0.5 mol / L, and the pH of the solution is 6; S3. The iron salt solution of S1 is added to the reactor as the bottom liquid and heated. The phosphate salt solution of S2 is added to the reactor under continuous stirring and reacted for a period of time to obtain slurry A. After filtration and washing, the hydroxyferric phosphate precursor is obtained. The bottom liquid temperature is 30℃, the stirring rate is 600rpm, the phosphate salt addition time is 30min, the reaction time is 30min, and the washing condition is that the conductivity of the washing water is ≤3000μs / cm. S4. Add pure water to the above-mentioned hydroxyferric phosphate precursor and stir to form a uniform slurry. Then add phosphoric acid, hydrogen peroxide and sulfuric acid, heat and hold at the temperature to obtain slurry C. After filtration and washing, ferric phosphate dihydrate is obtained. The molar ratio of iron source to hydrogen peroxide is 1:0.33, the molar ratio of iron source to phosphoric acid is 1:0.6, the pH of the slurry is adjusted to 1.4, the heating temperature is 95℃, the holding time is 4h, and the washing is completed when the conductivity of the washing water is ≤1500μs / cm. S5 and ferric phosphate dihydrate were dried and calcined to obtain high-purity battery-grade anhydrous ferric phosphate. The drying temperature was 150℃ and the drying time was 4 hours, while the calcination temperature was 620℃ and the calcination time was 10 hours.

[0026] Results Analysis The anhydrous ferric phosphates prepared in Examples 1-3 and Comparative Examples 1-3 were characterized by scanning electron microscopy (SEM), and the results are as follows: Figures 1 to 6 As shown. Among them, Figures 1 to 3 The SEM images of the iron phosphate prepared in Examples 1, 2, and 3 are shown respectively. It can be seen that the products have obvious flaky morphology and clear and uniformly distributed lamellar structure. Figure 4 and Figure 5 The SEM images of Comparative Example 1 and Comparative Example 2 are shown respectively. The morphology is irregular granular or incomplete lamellar, indicating that it is difficult to form regular crystalline ferric hydroxyphosphate intermediates at lower temperatures, thus affecting the induction effect of lamellar structure. Figure 6 The SEM image for Comparative Example 3 shows that the product has almost no flaky features, further confirming the key role of crystalline intermediates in the formation of flaky morphology.

[0027] The iron phosphate prepared in Examples 1-3 and Comparative Examples 1-3 was ground and sintered once to form lithium iron phosphate material, which was then made into a button cell. Its electrochemical performance was tested, and the test results are shown in Table 1 below.

[0028] Table 1. Test items and test results for Examples 1-3 and Comparative Examples 1-3

[0029] This invention prepares flake-shaped iron phosphate via a hydroxyferric phosphate intermediate method and investigates the effects of key process parameters on the electrochemical performance of the final lithium iron phosphate cathode material. The results show that the crystallization temperature in step S4 and the acidification pH value in step S5 are the two core factors determining the material's performance.

[0030] As can be seen from the data in Table 1, the electrochemical performance of Example 2 and Example 3 is basically equivalent. The initial discharge specific capacities of Example 2 at 0.1C, 0.5C, and 1C are 160.85, 156.22, and 153.67 mAh / g, respectively, while those of Example 3 are 160.82, 156.16, and 153.64 mAh / g, respectively. The capacity difference between the two at each rate is less than 0.06 mAh / g, indicating almost no difference. Considering both cost and performance, Example 2 is the optimal solution (S4 crystallization temperature 80℃, S5 acidification pH=1.4). Its lower acid dosage can reduce production costs and wastewater treatment pressure, while maintaining excellent electrochemical performance comparable to Example 3.

[0031] When the S4 crystallization temperature dropped to 70℃ (Comparative Example 2) and 60℃ (Comparative Example 1), the intermediate gradually changed from a semi-crystalline state to an amorphous state, and could not effectively induce a regular plate-like structure. The 1C capacity dropped to 143.44mAh / g and 142.34mAh / g, respectively, which was about 6.7% lower than that of Example 2. This confirmed that high temperature crystallization (≥80℃) is the key threshold for obtaining crystalline intermediates and thus improving electrochemical performance.

[0032] Comparative Example 3, which omits the S4 crystal conversion step, has a 1C capacity of only 130.57 mAh / g, which is about 15% lower than that of Example 2, further demonstrating the necessity of the intermediate crystal conversion step.

[0033] In summary, this invention utilizes crystalline hydroxyferric phosphate intermediates to induce the formation of lamellar iron phosphate, enabling the stable preparation of lithium iron phosphate cathode materials with excellent electrochemical performance. Among these, Example 2 (S4 crystallization temperature 80℃, S5 acidification pH=1.4) offers superior cost advantages while ensuring performance and is the recommended process condition.

Claims

1. A method for preparing flaky ferric phosphate from a hydroxyferric phosphate intermediate, characterized in that, Includes the following steps: S1. Prepare an iron salt solution; S2. Prepare phosphate salt solution; S3. Using an iron salt solution as the base solution, react with a phosphate salt solution to generate a hydroxyferric phosphate precursor. S4. The hydroxyferric phosphate precursor is subjected to a crystallization treatment under acidic oxidation conditions to obtain a crystalline hydroxyferric phosphate intermediate; the chemical formula of the crystalline hydroxyferric phosphate intermediate is Fe3(PO4)2(OH)2. S5. The crystalline ferric hydroxyphosphate intermediate is reacted with an oxidant in an acidic environment and washed to obtain ferric phosphate dihydrate; S6. Calcining the ferric phosphate dihydrate yields flaky anhydrous ferric phosphate. The crystalline ferric hydroxyphosphate intermediate serves as a crystal structure template, inducing ferric phosphate dihydrate to form in a plate-like morphology.

2. The method according to claim 1, characterized in that, In step S1, the iron concentration of the iron salt solution is 0.5-2 mol / L, and the pH of the iron salt solution is adjusted to 2-3 by adding an inorganic acid. The iron salt is one or more of ferrous sulfate, ferrous nitrate, and ferrous chloride, and the inorganic acid is one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. In step S2, the phosphorus concentration of the phosphate salt solution is 0.5-2 mol / L, and the pH of the phosphate salt solution is adjusted to 6.0-12.0 by adding a pH adjuster; the phosphate salt is one or more of phosphoric acid, ammonium phosphate, monoammonium phosphate, and diammonium phosphate, and the pH adjuster is one or more of ammonia water, ammonia gas, and sodium hydroxide.

3. The method according to claim 1, characterized in that, In step S3, the molar ratio of iron source in the iron salt solution to phosphorus source in the phosphate salt solution is Fe:P = 1:0.5-1:2; the reaction temperature is 30-50℃; the feeding time of the phosphate salt solution is 20-40 min; and the reaction time is 20-40 min.

4. The method according to claim 1, characterized in that, In step S4, the temperature of the crystal transformation treatment is 80-100℃, and the holding time is 0.5-6 hours; the pH is adjusted to 1-6 during the crystal transformation treatment.

5. The method according to claim 1, characterized in that, In step S5, the pH value of the acidic environment is 1.0-1.8, and the reaction temperature is 80-100℃.

6. The method according to claim 1, characterized in that, In steps S4 and S5, the oxidant is one or more of sodium persulfate, hydrogen peroxide, and perchloric acid.

7. The method according to claim 1, characterized in that, In step S4, the molar ratio of iron source to oxidant is 1:0.2-1; in step S5, the molar ratio of iron source to oxidant is 1:0.2-1.

8. The method according to claim 1, characterized in that, Step S5 also includes the addition of phosphoric acid, wherein the molar ratio of iron source to phosphoric acid is 1:0.2-1.

5.

9. The method according to claim 1, characterized in that, The calcination temperature in step S6 is 450-750℃, and the calcination time is 2-10 hours.

10. The flaky ferric phosphate prepared by the method according to any one of claims 1-9, characterized in that, The sheet-like iron phosphate is anhydrous iron phosphate or iron phosphate dihydrate, and its microstructure is sheet-like, used as a precursor for lithium-ion battery cathode materials.

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