Iron phosphate and method for producing the same, lithium iron phosphate, battery

Abstract

The application belongs to the technical field of battery materials, and discloses iron phosphate and a preparation method thereof. By controlling the proportion of divalent iron ions in the iron source solution, iron phosphate with different primary particle sizes can be controllably synthesized. The synthesis method is simple, reliable and easy to operate, and the existing production equipment and production line can be directly used. The iron phosphate yellow materials with different primary particles are mixed, and then subjected to crystallization, aging, washing, drying and sintering to obtain iron phosphate with a size grading of primary particles. The application proposes a novel grading method for grading iron phosphate from the microcosmic level of the size grading of primary particles. The application also discloses lithium iron phosphate prepared from the iron phosphate with the size grading of primary particles, and a battery.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a method for preparing iron phosphate. Background Technology

[0002] Iron phosphate, with the molecular formula FePO4, is a crucial raw material for the production of lithium iron phosphate (LFP). Battery material manufacturers commonly employ a high-temperature solid-state process using iron phosphate and lithium salts to produce LFP. With the widespread adoption of new energy vehicles and the development of energy storage batteries, the demand for LFP and the need for product iteration have further increased. Consequently, the demand for high-quality iron phosphate has also experienced explosive growth. The performance of LFP largely depends on the crystal properties of the iron phosphate product. High-performance iron phosphate products are beneficial for the preparation of high-energy-density LFP batteries. Therefore, obtaining high-performance and batch-stable iron phosphate products is a major research challenge in current technology. Summary of the Invention

[0003] In view of the problems existing in the prior art, the first objective of the present invention is to provide a method for preparing iron phosphate with controllable particle size in a primary process; the second objective of the present invention is to provide iron phosphate with primary particle size gradation and its preparation method; the third objective of the present invention is to provide lithium iron phosphate and a battery.

[0004] To achieve the above objectives, the present invention provides the following specific technical solutions.

[0005] First, this invention provides a method for preparing ferric phosphate with controllable particle size, comprising: Step S1: Prepare an iron source solution containing ferrous ions and ferric ions; add an oxidant to the phosphorus source solution to obtain a mixed solution; Step S2: Add the mixed solution to the iron source solution to synthesize iron phosphate yellow. Step S3: After the yellow ferric phosphate is re-pulped, phosphoric acid is added, and the mixture is crystallized and aged to obtain white ferric phosphate. Step S4: The white ferric phosphate material is washed, dried, and sintered to obtain ferric phosphate.

[0006] In a further preferred embodiment, an iron source solution comprising ferrous ions and ferric ions is prepared by: first, preparing an iron salt solution containing ferrous ions, and then adding an appropriate amount of oxidant to the iron salt solution according to the ratio of ferrous ions to ferric ions in the iron source solution.

[0007] In a further preferred embodiment, the iron ions are derived from at least one of ferrous chloride, ferrous sulfate, ferrous nitrate, and ferrous acetate.

[0008] In a further preferred embodiment, the phosphorus source is at least one selected from phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

[0009] In a further preferred embodiment, the oxidant is at least one of potassium permanganate, potassium dichromate, sodium hypochlorite, and hydrogen peroxide, with hydrogen peroxide being more preferred.

[0010] In a further preferred embodiment, the iron content in the iron source solution is 4.0 wt% to 6.0 wt%.

[0011] In a further preferred embodiment, the phosphorus content in the phosphorus source solution is 3.0 wt% to 4.0 wt%.

[0012] In a further preferred embodiment, an oxidant is added to the phosphorus source solution to ensure that the ferrous ions in the iron source solution can be completely oxidized.

[0013] In a further preferred embodiment, during the synthesis of iron phosphate yellow, the temperature of the reaction system is adjusted to 40~70℃, and the pH value of the reaction system is 1.5~4.0.

[0014] In a further preferred embodiment, the mixed solution is completely added to the iron source solution within 30 to 120 minutes.

[0015] In a further preferred embodiment, the solid content of the pulp obtained by re-pulping the iron phosphate yellow is 8~20wt%.

[0016] In a further preferred embodiment, the crystallization and aging temperature is 80~99℃; during crystallization and aging, the pH value of the reaction system is 0.8~2.5.

[0017] In a further preferred embodiment, in step S3, the molar ratio of P in the added phosphoric acid to Fe in the iron phosphate yellow is n(P):n(Fe) = (0.2~0.6):1.

[0018] In a further preferred embodiment, the sintering temperature is 540~580℃.

[0019] In a further preferred embodiment, when the target primary particle size of ferric phosphate is 40-100 nm, the mass of ferrous ions in the iron source solution accounts for 85-100% of the total iron mass; when the target primary particle size of ferric phosphate is 200-500 nm, the mass of ferrous ions in the iron source solution accounts for 30-70% of the total iron mass; and when the target primary particle size of ferric phosphate is 500-1200 nm, the mass of ferrous ions in the iron source solution accounts for 5-30% of the total iron mass.

[0020] Secondly, this invention provides a method for preparing ferric phosphate with a primary particle size distribution, comprising: According to steps S1 and S2 of the aforementioned method for preparing ferric phosphate with controllable particle size, iron source solutions with different mass ratios of divalent iron ions are prepared to obtain different ferric phosphate yellow materials. Mix at least two of the different iron phosphate yellow pigments to obtain a mixed yellow pigment; The mixed yellow material is processed according to steps S3 and S4 of the aforementioned method for preparing ferric phosphate with controllable primary particle size to obtain ferric phosphate with primary particle size gradation.

[0021] In a further preferred embodiment, an iron source solution with ferrous ions accounting for 80-95% of the total iron mass is prepared to obtain ferric phosphate yellow material A; an iron source solution with ferrous ions accounting for 5-30% of the total iron mass is prepared to obtain ferric phosphate yellow material B; yellow material A and yellow material B are mixed in a certain proportion to obtain mixed yellow material, wherein yellow material A accounts for 70-90% of the total mass of the mixed yellow material and yellow material B accounts for 10-30% of the total mass of the mixed yellow material.

[0022] Based on the same inventive concept, the present invention provides ferric phosphate with a primary particle size distribution obtained by the above preparation method.

[0023] In a further preferred embodiment, the proportion of ferric phosphate with a primary particle size of 40-450 nm is 70-95 wt%, and the proportion of ferric phosphate with a primary particle size of 500-1200 nm is 5-30 wt%.

[0024] In addition, the present invention provides a lithium iron phosphate prepared using the above-mentioned primary particle size distribution of iron phosphate.

[0025] The present invention also provides a battery comprising the above-mentioned lithium iron phosphate.

[0026] Compared with the prior art, one or more technical solutions of the present invention can achieve at least one of the following beneficial effects: This invention enables the controlled synthesis of iron phosphate with different particle sizes by controlling the proportion of divalent iron ions in the iron source solution. The synthesis method is simple, reliable, easy to operate, and can be directly utilized with existing production equipment and production lines.

[0027] This invention proposes a novel gradation method for iron phosphate by grading it at the microscopic level of particle size distribution.

[0028] Lithium iron phosphate (LiFePO4) prepared by primary particle size gradation of iron phosphate inherits the characteristics of iron phosphate. Large-particle LiFePO4 is obtained by sintering large-particle-size iron phosphate, while small-particle-size LiFePO4 is obtained by sintering small-particle-size iron phosphate. The small-sized LiFePO4 particles fill the gaps between the large-sized particles, effectively increasing the compaction density of the material. Attached Figure Description

[0029] Figure 1 The image shows the XRD pattern of the iron phosphate prepared in Example 1.

[0030] Figure 2 The image shows the XRD pattern of the iron phosphate prepared in Example 2.

[0031] Figure 3 The image shows the XRD pattern of the iron phosphate prepared in Example 3.

[0032] Figure 4 The image shows the XRD pattern of the iron phosphate prepared in Example 4.

[0033] Figure 5 The image shows the XRD pattern of the iron phosphate prepared in Example 5.

[0034] Figure 6 The image shows the XRD pattern of the iron phosphate prepared in Example 6.

[0035] Figure 7 The image shows a SEM image of the iron phosphate prepared in Example 1.

[0036] Figure 8 The image shows a cross-sectional SEM image of the iron phosphate prepared in Example 1.

[0037] Figure 9 This is a SEM cross-sectional image of the iron phosphate prepared in Example 2.

[0038] Figure 10 This is a SEM cross-sectional image of the iron phosphate prepared in Example 3.

[0039] Figure 11 This is a SEM cross-sectional image of the iron phosphate prepared in Example 4.

[0040] Figure 12 This is a SEM cross-sectional image of the iron phosphate prepared in Example 5.

[0041] Figure 13 This is a SEM cross-sectional image of the iron phosphate prepared in Example 6.

[0042] Figure 14 This is a SEM cross-sectional image of the iron phosphate prepared in Example 7.

[0043] Figure 15The image shows a cross-sectional SEM image of the iron phosphate prepared in Example 8.

[0044] Figure 16 The image shows a cross-sectional SEM image of the iron phosphate prepared in Example 9.

[0045] Figure 17 This is a SEM cross-sectional image of the iron phosphate prepared in Example 10. Detailed Implementation

[0046] The primary particles of iron phosphate have a crucial impact on its properties. The size, density, and packing of the primary particles affect the tap density and hardness of the secondary aggregates, thus influencing their subsequent use in the synthesis and production of nanoscale lithium iron phosphate. Lithium iron phosphate prepared from iron phosphate with small primary particles has higher capacity but relatively lower tap density; while lithium iron phosphate prepared from iron phosphate with large primary particles has higher tap density but relatively lower capacity.

[0047] The applicant discovered in their research that iron and phosphorus sources react to form ferric phosphate. The particle size of the ferric phosphate particles formed in this precipitation reaction mainly depends on the saturation of the solution and the reaction temperature during precipitation formation; the higher the reaction temperature, the smaller the primary particles formed. The reaction between ferric ions and phosphorus sources is a direct precipitation reaction, with only the precipitation reaction being exothermic, resulting in relatively larger primary particles. The reaction between ferrous ions and phosphorus sources involves both precipitation and redox reactions, both of which are exothermic, thus the reaction temperature is higher than that of ferric ions and phosphorus sources, resulting in relatively smaller primary particles.

[0048] Therefore, this invention achieves the regulation of primary ferric phosphate particle size by controlling the molar ratio of divalent and trivalent ferric ions in the iron source solution during the synthesis process.

[0049] Specifically, some embodiments of the present invention provide a method for preparing ferric phosphate with controllable particle size in a single step, comprising: Step S1: Prepare an iron source solution containing ferrous and ferric ions according to the primary particle size target value of ferric phosphate; add an oxidant to the phosphorus source solution to obtain a mixed solution; Step S2: Add the mixed solution to the iron source solution to synthesize iron phosphate yellow. Step S3: After the yellow ferric phosphate is re-pulped, phosphoric acid is added, and the mixture is crystallized and aged to obtain white ferric phosphate. Step S4: The white ferric phosphate material is washed, dried, and sintered to obtain ferric phosphate.

[0050] In some specific embodiments, an iron source solution comprising ferrous and ferric ions is prepared by: first, preparing an iron salt solution containing ferrous ions; and then, adding an appropriate amount of oxidant to the iron salt solution according to the molar ratio of ferrous and ferric ions in the iron source solution. Other methods for preparing an iron source solution comprising ferrous and ferric ions are also within the scope of the present invention. For example, those skilled in the art can directly dissolve a certain amount of ferrous salt or ferric salt in water to obtain an iron source solution comprising ferrous and ferric ions.

[0051] In some specific embodiments, the iron salt is at least one selected from ferrous chloride, ferrous sulfate, and ferrous nitrate. Ferrous sulfate is more preferably preferred.

[0052] In some specific embodiments, the phosphorus source is at least one selected from phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, and sodium phosphate. Ammonium dihydrogen phosphate is more preferably preferred.

[0053] In some specific embodiments, the oxidant is at least one of potassium permanganate, potassium dichromate, sodium hypochlorite, and hydrogen peroxide, more preferably hydrogen peroxide. Hydrogen peroxide generates a large number of microbubbles during the oxidation process, which helps to reduce the size of the primary particles of ferric phosphate.

[0054] Under the technical concept of this invention, there are no limitations on the concentrations of the iron source solution and the phosphorus source solution used in the synthesis of iron phosphate; conventional concentrations are sufficient. In some specific embodiments of this invention, the iron content in the iron source solution is 4.0 wt% to 6.0 wt%, for example, 4.0 wt%, 5.0 wt%, and 6.0 wt%; and the phosphorus content in the phosphorus source solution is 3.0 wt% to 4.0 wt%, for example, 3.0 wt%, 3.5 wt%, and 4.0 wt%.

[0055] In a further preferred embodiment, an oxidant is added to the phosphorus source solution to ensure that the ferrous ions in the iron source solution can be completely oxidized. The amount of oxidant used is 0.7-1.0 molar ratio of ferrous ions in the iron source solution during the synthesis reaction.

[0056] In some specific embodiments, when synthesizing iron phosphate yellow, the temperature of the reaction system is adjusted to 40~70℃, such as 40℃, 50℃, 60℃, 70℃, etc.; the pH value of the reaction system is 1.5~4.0, such as 1.5, 2.0, 3.0, 4.0, etc.

[0057] In some specific embodiments, the mixed solution is completely added to the iron source solution within 30-120 minutes. The feeding rate should not be too fast, otherwise it will affect the dispersion effect of the material; if the feeding rate is too slow, it will result in low production efficiency and capacity.

[0058] In some specific embodiments, the solid content of the slurry obtained by re-pulping the iron phosphate yellow is 8-20%, such as 8%, 10%, 12%, 15%, 20%, etc.

[0059] In some specific embodiments, the crystallization and aging temperatures are 80~99℃, such as 80℃, 85℃, 88℃, 90℃, 95℃, 99℃, etc.; during crystallization and aging, the pH value of the reaction system is 0.8~2.5, such as 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, etc.

[0060] In some specific embodiments, in step S3, the molar ratio of P in the added phosphoric acid to Fe in the iron phosphate yellow is n(P):n(Fe) = (0.2~0.6):1.

[0061] In some specific embodiments, the sintering temperature is 540~580℃.

[0062] Under the technical concept of this invention, those skilled in the art can obtain ferric phosphate products with different primary particle sizes by adjusting the different proportions of ferrous and ferric ions in the iron source solution. In other words, to obtain ferric phosphate products with different primary particle sizes, those skilled in the art can continuously experiment to obtain a suitable proportion of ferrous and ferric ions in the iron source solution. In some specific embodiments, when the target primary particle size of ferric phosphate is 40-100 nm, the mass of ferrous ions in the iron source solution accounts for 85-100% of the total iron mass; when the target primary particle size of ferric phosphate is 200-500 nm, the mass of ferrous ions in the iron source solution accounts for 30-70% of the total iron mass; when the target primary particle size of ferric phosphate is 500-1200 nm, the mass of ferrous ions in the iron source solution accounts for 5-30% of the total iron mass.

[0063] Furthermore, to balance certain electrical properties of lithium iron phosphate, existing technologies typically employ a mixture of lithium iron phosphate particles with different sizes to obtain graded lithium iron phosphate. For example, CN115650200A discloses a method for preparing high-energy-density lithium iron phosphate materials. This method involves coarsely grinding a lithium source, iron / phosphorus source, and carbon source, followed by fine grinding to obtain large-particle slurry with a particle size D50 of 0.6~1.0 μm and small-particle slurry with a particle size D50 of 0.15~0.5 μm. The large and small particle slurries are then mixed at a mass ratio of 1:1 to 1:9 to obtain a mixed slurry. This mixed slurry is then sieved, spray-dried to obtain a precursor, and finally sintered to obtain the high-energy-density lithium iron phosphate material. For example, CN114314550A discloses a high-energy-density lithium iron phosphate, comprising at least one large-particle lithium iron phosphate and at least one small-particle lithium iron phosphate, with a mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate of 1~10:10~19, and a particle size ratio of the large-particle lithium iron phosphate to small-particle lithium iron phosphate of 2~6:1. Furthermore, there are reports of preparing lithium iron phosphate by first grading the precursor—iron phosphate. For instance, CN117486185A discloses a method for preparing a lithium iron phosphate cathode material, comprising: taking at least two parts of iron phosphate, mixing and dissolving the two parts of iron phosphate with a lithium source, a carbon source, a dopant, and deionized water in a certain proportion to obtain a mixed solution; wherein the two parts of iron phosphate have at least one of different particle sizes, different specific surface areas, and different iron-phosphorus molar ratios; and subjecting the mixed solution to wet grinding, spray drying, sintering, and crushing to obtain the lithium iron phosphate cathode material.

[0064] However, the above methods only address the macroscopic gradation of lithium iron phosphate (LFP) particles of different sizes. Grading LFP particles of different sizes may lead to poor product stability, especially when controlling particle size through sintering temperature. During the sintering of large LFP particles, even slight temperature fluctuations can easily cause electrochemical performance degradation, and for large-scale production equipment, even minor temperature adjustments often require lengthy debugging periods. Furthermore, when preparing LFP by grading iron phosphate particles of different sizes, the grinding process during preparation breaks down secondary agglomerates, making it difficult to retain the particle size distribution characteristics. Subsequent drying and sintering processes often fail to adequately preserve the particle size distribution of the iron phosphate in the LFP.

[0065] Therefore, based on the different battery capacity and compaction density of lithium iron phosphate prepared from iron phosphate with different primary particle sizes, and the above-mentioned method for preparing iron phosphate with controllable primary particle size, some embodiments of the present invention provide iron phosphate with primary particle size gradation, characterized in that, in the iron phosphate, the proportion of iron phosphate with a primary particle size of 40~450nm is 70-95wt%, and the proportion of iron phosphate with a primary particle size of 500~1200nm is 5-30wt%.

[0066] The method for preparing the primary particle size distribution of ferric phosphate includes: According to steps S1 and S2 of the aforementioned method for preparing ferric phosphate with controllable particle size, iron source solutions with different molar ratios of ferrous ions and ferric ions are prepared to obtain different ferric phosphate yellow materials. Mix at least two of the different iron phosphate yellow pigments to obtain a mixed yellow pigment; The mixed yellow material is processed according to steps S3 and S4 of the aforementioned method for preparing ferric phosphate with controllable primary particle size to obtain ferric phosphate with primary particle size gradation.

[0067] In some specific embodiments, an iron source solution with ferrous ions accounting for 80-95% of the total iron mass is prepared to obtain ferric phosphate yellow material A; an iron source solution with ferrous ions accounting for 5-30% of the total iron mass is prepared to obtain ferric phosphate yellow material B; yellow material A and yellow material B are mixed in a certain proportion to obtain mixed yellow material, wherein yellow material A accounts for 70-90% of the total mass of the mixed yellow material and yellow material B accounts for 10-30% of the total mass of the mixed yellow material.

[0068] Based on the same inventive concept, some embodiments of the present invention provide ferric phosphate with primary particle size distribution obtained by the above preparation method. In some specific embodiments, the proportion of ferric phosphate with primary particle size of 40-450 nm is 70-95 wt%, and the proportion of ferric phosphate with primary particle size of 500-1200 nm is 5-30 wt%.

[0069] Lithium iron phosphate was prepared using the above-mentioned primary particle size distribution of iron phosphate. The prepared lithium iron phosphate was then assembled into a battery.

[0070] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0071] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0072] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0073] Example 1 Ferrous sulfate was dissolved in water to prepare an iron salt solution with an iron content of 4 wt%.

[0074] Hydrogen peroxide is slowly added to the iron salt solution until the mass of ferrous ions in the iron salt solution is 80% of the total mass of iron, thus obtaining the iron source solution.

[0075] Ammonium dihydrogen phosphate was dissolved in water to prepare a phosphorus source solution with a phosphorus content of 3.0 wt%. Hydrogen peroxide was added to the phosphorus source solution to obtain a mixed solution, wherein the molar ratio of H2O2 in the hydrogen peroxide to ferrous ions in the iron source solution was 0.7:1.

[0076] The mixed solution was slowly added to the iron source solution to carry out the reaction, ensuring that the temperature of the reaction system was 50℃, the pH value was 2.0, and that the mixed solution was added completely within 60 minutes. After the reaction was completed, a yellow material was obtained.

[0077] Wash the yellow material multiple times until the conductivity of the wash water is less than 10000 μs / cm.

[0078] The washed yellow material was re-watered and pulped until the solid content of the pulp was 10wt%.

[0079] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of added phosphoric acid to iron in the yellow pulp was 0.3:1. The reaction system was maintained at a temperature of 80°C and a pH of 1.0. After the reaction was completed, white pulp was obtained.

[0080] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 550℃ for 2 h. After sintering, the ferric phosphate product is obtained.

[0081] Example 2 Dissolve ferrous nitrate in water to prepare an iron salt solution with an iron content of 5 wt%.

[0082] Hydrogen peroxide is slowly added to the iron salt solution until the mass of ferrous ions in the iron salt solution accounts for 50% of the total mass of iron, thus obtaining the iron source solution.

[0083] Ammonium phosphate was dissolved in water to prepare a phosphorus source solution with a phosphorus content of 3.5%. Hydrogen peroxide was added to the phosphorus source solution to obtain a mixed solution, wherein the molar ratio of H2O2 in the hydrogen peroxide to ferrous ions in the iron source solution was 0.75:1.

[0084] The mixed solution was slowly added to the iron source solution to carry out the reaction, ensuring that the temperature of the reaction system was 40℃, the pH value was 3.0, and that the mixed solution was added completely within 90 minutes. After the reaction was completed, a yellow material was obtained.

[0085] Wash the yellow material multiple times until the conductivity of the wash water is less than 10000 μs / cm.

[0086] The washed yellow material was re-watered and pulped until the solid content of the pulp was 8wt%.

[0087] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.4:1. The reaction system was maintained at a temperature of 85°C and a pH of 1.2. After the reaction was completed, white pulp was obtained.

[0088] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 540℃ for 3 h. After sintering, the ferric phosphate product is obtained.

[0089] Example 3 Ferrous chloride was dissolved in water to prepare an iron salt solution with an iron content of 6 wt%.

[0090] Hydrogen peroxide is slowly added to the iron salt solution until the mass of ferrous ions in the iron salt solution accounts for 20% of the total mass of iron, thus obtaining the iron source solution.

[0091] Ammonium dihydrogen phosphate was dissolved in water to prepare a phosphorus source solution with a phosphorus content of 4.0 wt%. Hydrogen peroxide was added to the phosphorus source solution to obtain a mixed solution, wherein the molar ratio of H2O2 in the hydrogen peroxide to ferrous ions in the iron source solution was 0.8:1.

[0092] The mixed solution was slowly added to the iron source solution to allow the reaction to proceed, ensuring the reaction system temperature was 60℃, the pH value was 1.5, and the mixed solution was added completely within 30 minutes. After the reaction was complete, a yellow material was obtained.

[0093] Wash the yellow material multiple times until the conductivity of the wash water is less than 10000 μs / cm.

[0094] The washed yellow material was re-watered and pulped until the solid content of the pulp was 12wt%.

[0095] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.5:1. The reaction system was maintained at a temperature of 88°C and a pH of 0.8. After the reaction was completed, white pulp was obtained.

[0096] Wash the white filter cake until its conductivity is less than 200 μS / cm. Dry the filter cake at 100°C for 10 hours, then send it to sinter at 560°C for 2 hours. After sintering, the ferric phosphate product is obtained.

[0097] Example 4 Dissolve ferrous sulfate in water to prepare an iron salt solution with an iron content of 5 wt%.

[0098] Hydrogen peroxide is slowly added to the iron salt solution until the mass of ferrous ions in the iron salt solution accounts for 80% of the total mass of iron, thus obtaining the iron source solution.

[0099] Phosphoric acid was dissolved in water to prepare a phosphorus source solution with a phosphorus content of 3.0 wt%. Hydrogen peroxide was added to the phosphorus source solution to obtain a mixed solution, wherein the molar ratio of H2O2 in the hydrogen peroxide to ferrous ions in the iron source solution was 0.85:1.

[0100] The mixed solution was slowly added to the iron source solution to carry out the reaction, ensuring that the temperature of the reaction system was 70℃, the pH value was 4.0, and that the mixed solution was added completely within 120 minutes. After the reaction was completed, a yellow material was obtained.

[0101] Wash the yellow material multiple times until the conductivity of the wash water is less than 10000 μs / cm.

[0102] The washed yellow material was re-watered and pulped until the solid content of the pulp was 15wt%.

[0103] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.25:1. The reaction system was maintained at a temperature of 90°C and a pH of 1.5. After the reaction was completed, white pulp was obtained.

[0104] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 570℃ for 2 h. After sintering, the ferric phosphate product is obtained.

[0105] Example 5 Ferrous sulfate was dissolved in water to prepare an iron salt solution with an iron content of 4 wt%.

[0106] Hydrogen peroxide is slowly added to the iron salt solution until the mass of ferrous ions in the iron salt solution accounts for 50% of the total mass of iron, thus obtaining the iron source solution.

[0107] Ammonium dihydrogen phosphate was dissolved in water to prepare a phosphorus source solution with a phosphorus content of 3.5 wt%. Hydrogen peroxide was added to the phosphorus source solution to obtain a mixed solution, wherein the molar ratio of H2O2 in the hydrogen peroxide to ferrous ions in the iron source solution was 0.9:1.

[0108] The mixed solution was slowly added to the iron source solution to carry out the reaction, ensuring that the temperature of the reaction system was 50℃, the pH value was 3.0, and that the mixed solution was added completely within 60 minutes. After the reaction was completed, a yellow material was obtained.

[0109] Wash the yellow material multiple times until the conductivity of the wash water is less than 10000 μs / cm.

[0110] The washed yellow material was re-watered and pulped until the solid content of the pulp was 20wt%.

[0111] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.20:1. The reaction system was maintained at a temperature of 95°C and a pH of 2.0. After the reaction was completed, white pulp was obtained.

[0112] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 100°C for 12 hours, then send it to sinter at 580°C for 2 hours. After sintering, the ferric phosphate product is obtained.

[0113] Example 6 Ferrous sulfate was dissolved in water to prepare an iron salt solution with an iron content of 6 wt%.

[0114] Hydrogen peroxide is slowly added to the iron salt solution until the mass of ferrous ions in the iron salt solution accounts for 20% of the total mass of iron, thus obtaining the iron source solution.

[0115] Ammonium dihydrogen phosphate was dissolved in water to prepare a phosphorus source solution with a phosphorus content of 3.0 wt%. Hydrogen peroxide was added to the phosphorus source solution to obtain a mixed solution, wherein the molar ratio of H2O2 in the hydrogen peroxide to ferrous ions in the iron source solution was 0.95:1.

[0116] The mixed solution was slowly added to the iron source solution to carry out the reaction, ensuring that the temperature of the reaction system was 50℃, the pH value was 2.0, and that the mixed solution was added completely within 90 minutes. After the reaction was completed, a yellow material was obtained.

[0117] Wash the yellow material multiple times until the conductivity of the wash water is less than 10000 μs / cm.

[0118] The washed yellow material was re-watered and pulped until the solid content of the pulp was 10wt%.

[0119] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.2:1. The reaction system was maintained at a temperature of 99°C and a pH of 2.5. After the reaction was completed, white pulp was obtained.

[0120] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 120℃ for 8 hours, then send it to sintering plant and sinter at 550℃ for 2 hours. After sintering, the ferric phosphate product is obtained.

[0121] The ferric phosphates prepared in Examples 1-6 were subjected to XRD tests. The XRD scanning range was 5-80°, and the scanning speed was ≤2° / min. The XRD spectra are shown below. Figures 1-6 As shown. The obtained XRD pattern was refined in Jade software. The full width at half maximum (FWHM) of the strongest diffraction peak (denoted as β) was calculated from the refined XRD pattern. This data was read from the device software and substituted into the Scherrer formula D=Kλ / (βcosθ), where K is the shape factor, K=1, and λ is the incident wavelength of the X-rays. Using Cu target Kα rays, λ=0.15418nm, the corresponding grain size D was obtained.

[0122] Using the ferric phosphate prepared in Example 1 as a standard, SEM tests were performed on it (e.g. Figure 7 As shown in the figure, the test was repeated three times, with each test using a different test area to ensure sample representativeness and reduce error. The dimensions of the iron phosphate material in the SEM image were measured and statistically analyzed using the scale bar, yielding an average primary particle size L = 101.7 nm. The correlation coefficient between the average primary particle size L and the grain size D of iron phosphate was a, a = L / D = 5.646. The average primary particle size of the iron phosphate prepared in Examples 2-6 was calculated using L = a * D. The results are shown in Table 1.

[0123] Table 1 Figures 8-13 The images shown are cross-sectional SEM images of the iron phosphates prepared in Examples 1 to 6, respectively.

[0124] Example 7 The yellow pigment obtained in Example 1 and the yellow pigment obtained in Example 3 were mixed to form a mixed yellow pigment, wherein the yellow pigment obtained in Example 1 accounted for 15 wt% and the yellow pigment obtained in Example 3 accounted for 85 wt%.

[0125] The mixed yellow material was washed multiple times until the conductivity of the wash water was less than 10000 μs / cm.

[0126] The washed mixed yellow material was re-watered and pulped until the solid content of the pulp was 10wt%.

[0127] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron in the yellow pulp was 0.6:1. The reaction system was maintained at a temperature of 80°C and a pH of 1.0. After the reaction was completed, white pulp was obtained.

[0128] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 550℃ for 2 h. After sintering, the ferric phosphate product is obtained.

[0129] Example 8 The yellow pigments obtained in Example 2 and Example 3 were mixed to form a mixed yellow pigment, wherein the yellow pigment obtained in Example 2 accounted for 25 wt% and the yellow pigment obtained in Example 3 accounted for 75 wt%.

[0130] The mixed yellow material was washed multiple times until the conductivity of the wash water was less than 10000 μs / cm.

[0131] The washed mixed yellow material was re-watered and pulped until the solid content of the pulp was 10wt%.

[0132] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.5:1. The reaction system was maintained at a temperature of 90°C and a pH of 2.0. After the reaction was completed, white pulp was obtained.

[0133] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 550℃ for 2 h. After sintering, the ferric phosphate product is obtained.

[0134] Example 9 The yellow pigment obtained in Example 4 and the yellow pigment obtained in Example 6 were mixed to form a mixed yellow pigment, wherein the yellow pigment obtained in Example 4 accounted for 20 wt% and the yellow pigment obtained in Example 6 accounted for 80 wt%.

[0135] The mixed yellow material was washed multiple times until the conductivity of the wash water was less than 10000 μs / cm.

[0136] The washed mixed yellow material was re-watered and pulped until the solid content of the pulp was 10%.

[0137] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.3:1. The reaction system was maintained at a temperature of 95°C and a pH of 0.8. After the reaction was completed, white pulp was obtained.

[0138] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 550℃ for 2 h. After sintering, the ferric phosphate product is obtained.

[0139] Example 10 The yellow pigments obtained in Example 5 and Example 6 were mixed to form a mixed yellow pigment, wherein the yellow pigment obtained in Example 5 accounted for 30 wt% and the yellow pigment obtained in Example 6 accounted for 70 wt%.

[0140] The mixed yellow material was washed multiple times until the conductivity of the wash water was less than 10000 μs / cm.

[0141] The washed mixed yellow material was re-watered and pulped until the solid content of the pulp was 10wt%.

[0142] Phosphoric acid was added to the pulp after beating to initiate a reaction. The molar ratio of phosphoric acid (P) to iron (Fe) in the yellow pulp was 0.4:1. The reaction system was maintained at a temperature of 85°C and a pH of 1.5. After the reaction was completed, white pulp was obtained.

[0143] Wash the white material until the conductivity of the filter cake is less than 200 μS / cm. Dry the filter cake at 110℃ for 10 h, then send it to sintering and sinter at 550℃ for 2 h. After sintering, the ferric phosphate product is obtained.

[0144] Figures 14-17 The images shown are SEM cross-sectional images of the iron phosphate products obtained in Examples 7 to 10, respectively. It can be seen that the size of the primary particles of the products is different.

[0145] Lithium iron phosphate was prepared from the iron phosphate products prepared in Examples 1 to 10 by the following method: the iron phosphate product and lithium carbonate were mixed at a molar ratio of Fe:Li = 1:1.03, and then glucose, a carbon source, was added. The amount of glucose added was 10 wt% of the total material. The mixture was then heat-treated under a nitrogen atmosphere, with the temperature increased to 450°C at a rate of 2°C / min and held for 2 hours. The temperature was then increased to 780°C at a rate of 2°C / min and held for 8 hours. Finally, lithium iron phosphate was obtained by air jet milling.

[0146] The compaction density and electrochemical performance of lithium iron phosphate products were tested using the following methods: Compacted density test: 1. Weigh approximately 1.00g of the sample using weighing paper on a balance; 2. Load the weighed sample into the mold, and place the mold with the material in the designated position in the middle of the compaction density meter.

[0147] 3. The material is compressed using a 3T pressure, and then the compaction density is calculated by dividing the weight by the volume after compression.

[0148] Electrochemical testing: Lithium iron phosphate, prepared from the iron phosphate products obtained in the above embodiments, was used as the positive electrode active material. It was mixed with conductive agent acetylene black (AB) and binder polyvinylidene fluoride (PVDF) in a mass ratio of 90:5:5, and mixed with N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode slurry. This slurry was then coated onto aluminum foil to obtain a positive electrode sheet. A coin cell was assembled in a glove box using the lithium sheet as the negative electrode. The electrolyte solution was 1 M LiPF6 in EC:DEC (volume ratio 4:6), and the separator was a commercial electrolyte separator with a diameter of 16 mm. Current charge-discharge tests were conducted at room temperature (25°C) using the Sinovel Battery testing system.

[0149] The test results are shown in Table 2.

[0150] Table 2 As shown in Table 2, the iron phosphate obtained in Examples 1-3 and Examples 4-6 had a relatively uniform primary particle size distribution. With the increase of the primary particle size of iron phosphate, the compaction density of lithium iron phosphate increased significantly. However, the increase in primary particle size also led to a longer lithium-ion diffusion path, resulting in poorer electrochemical performance of lithium iron phosphate.

[0151] Example 7: The yellow materials obtained in Examples 1 and 3 were batched to obtain lithium iron phosphate. The compaction density, 0.1C discharge specific capacity, and 1C discharge specific capacity of the lithium iron phosphate prepared from the lithium iron phosphate obtained in Example 7 were at the middle values ​​of the above parameters of the lithium iron phosphate prepared from the lithium iron phosphate obtained in Examples 1 and 3. The same applies to Examples 8 to 10. By using a primary particle size gradation method, smaller primary particles can fill the gaps between larger primary particles, effectively compensating for the insufficient compaction density of lithium iron phosphate with small primary particles. At the same time, smaller primary lithium iron phosphate particles can also improve the poor electrical performance of larger primary lithium iron phosphate particles, resulting in a higher level of compaction density and discharge specific capacity.

[0152] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing ferric phosphate with controllable particle size in a single step, characterized in that, include: Step S1: Prepare an iron source solution containing a determined ratio of ferrous ions and ferric ions according to the primary particle size target value of ferric phosphate. An oxidizing agent is added to the phosphorus source solution to obtain a mixed solution; Step S2: Add the mixed solution to the iron source solution to synthesize iron phosphate yellow. Step S3: After the yellow ferric phosphate is re-pulped, phosphoric acid is added, and the mixture is crystallized and aged to obtain white ferric phosphate. Step S4: The white ferric phosphate material is washed, dried, and sintered to obtain ferric phosphate.

2. The method for preparing ferric phosphate with controllable particle size as described in claim 1, characterized in that, The iron ions are derived from at least one of ferrous chloride, ferrous sulfate, ferrous nitrate, and ferrous acetate; and / or, The phosphorus source is at least one selected from phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; and / or, The oxidant is at least one of potassium permanganate, potassium dichromate, sodium hypochlorite, and hydrogen peroxide.

3. The method for preparing ferric phosphate with controllable particle size as described in claim 1 or 2, characterized in that, The iron source solution contains 4.0 wt% to 6.0 wt% iron; and / or, The phosphorus source solution contains 3.0 wt% to 4.0 wt% phosphorus; and / or, The amount of oxidant used is 0.7-1.0 molar ratio of divalent iron in the iron source solution during the synthesis reaction.

4. The method for preparing ferric phosphate with controllable particle size as described in claim 1, characterized in that, In step S2, when synthesizing iron phosphate yellow, the temperature of the reaction system is adjusted to 40~70℃ and the pH value of the reaction system is 1.5~4.

0.

5. The method for preparing ferric phosphate with controllable particle size as described in claim 1, characterized in that, In step S3, the solid content of the slurry obtained by re-pulping the iron phosphate yellow is 8~20wt%; and / or, the crystallization and aging temperature is 80~99℃; during the crystallization and aging, the pH value of the reaction system is 0.8~2.

5.

6. The method for preparing ferric phosphate with controllable particle size as described in claim 1 or 5, characterized in that, In step S3, the molar ratio of P in the added phosphoric acid to Fe in the iron phosphate yellow is n(P):n(Fe) = (0.2~0.6):1; and / or, In step S4, the sintering temperature is 540~580℃.

7. The method for preparing ferric phosphate with controllable particle size as described in claim 1, characterized in that, Step S1 determines the ratio of ferrous ions (Fe2+) to ferric ions in the iron source solution based on the primary particle size target value of ferric phosphate. Specifically: When the primary particle size of ferric phosphate is 40-100 nm, the mass of ferrous ions in the iron source solution accounts for 85-100% of the total iron mass; when the primary particle size of ferric phosphate is 200-500 nm, the mass of ferrous ions in the iron source solution accounts for 30-70% of the total iron mass; when the primary particle size of ferric phosphate is 500-1200 nm, the mass of ferrous ions in the iron source solution accounts for 5-30% of the total iron mass.

8. Ferric phosphate with a single particle size distribution, characterized in that, In the ferric phosphate, the proportion of ferric phosphate with a primary particle size of 40~450nm is 70-95wt%, and the proportion of ferric phosphate with a primary particle size of 500~1200nm is 5-30wt%.

9. A method for preparing ferric phosphate with primary particle size distribution, including: Several iron source solutions containing ferrous ions and ferric ions are prepared, wherein the mass ratio of ferrous ions to the total iron mass is different in different iron source solutions. Steps S1 and S2 of the method for preparing ferric phosphate with controllable particle size according to any one of claims 1 to 6 yield different ferric phosphate yellow materials. Mix at least two of the different iron phosphate yellow pigments to obtain a mixed yellow pigment; The mixed yellow material is processed according to steps S3 and S4 of the method for preparing ferric phosphate with controllable primary particle size according to any one of claims 1 to 6 to obtain ferric phosphate with primary particle size gradation.

10. The method for preparing ferric phosphate with primary particle size distribution as described in claim 9, characterized in that, Prepare an iron source solution with ferrous ions accounting for 80-95% of the total iron mass to obtain ferric phosphate yellow material A; prepare an iron source solution with ferrous ions accounting for 5-30% of the total iron mass to obtain ferric phosphate yellow material B; mix yellow material A and yellow material B in a certain proportion to obtain mixed yellow material, wherein yellow material A accounts for 70-90% of the total mass of the mixed yellow material and yellow material B accounts for 10-30% of the total mass of the mixed yellow material.

11. A lithium iron phosphate, characterized in that, It is prepared by using the primary particle size distribution of ferric phosphate as described in claim 8 or the primary particle size distribution of ferric phosphate obtained by any of the preparation methods described in claims 9-10.

12. A battery, characterized in that, Including the lithium iron phosphate as described in claim 11.

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