Iron phosphate material, preparation method therefor, positive electrode material, positive electrode sheet, and secondary battery

Abstract

The present application relates to the field of lithium-ion batteries, and provides an iron phosphate material, a preparation method therefor, a positive electrode material, a positive electrode sheet, and a secondary battery. The iron phosphate material satisfies an evaluation index q in the range of 0.02 μm-1 to 0.15 μm-1. The evaluation index q is calculated using the following formula: q = t / Dv90, where t is the iron-to-phosphorus ratio of the iron phosphate material, and Dv90 is the particle size value corresponding to a 90% cumulative volume distribution percentage when the particle sizes of iron phosphate particles of the iron phosphate material are sorted from smallest to largest. By controlling the value of the evaluation index q, the present application improves the efficiency of sand milling or ball milling in the preparation process of lithium iron phosphate, facilitates the preparation of positive electrode materials such as lithium iron phosphate materials and lithium manganese iron phosphate materials that meet particle size requirements and iron-to-phosphorus ratio requirements, reduces material blockage during material conveyance, and improves the electrochemical performance of positive electrode materials such as lithium iron phosphate materials and lithium manganese iron phosphate materials prepared therefrom.

Description

Iron phosphate materials, their preparation methods, cathode materials, cathode plates and secondary batteries

[0001] This application claims priority to Chinese patent application 202411190704.X, filed on August 27, 2024. The entire contents of the aforementioned Chinese patent application are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery technology, specifically to an iron phosphate material, its preparation method, a cathode material, a cathode electrode sheet, and a secondary battery. Background Technology

[0003] Lithium iron phosphate (LiFePO4) is an important cathode material widely used in lithium-ion batteries. LiFePO4 exhibits a very stable charge-discharge platform, structural stability during charge and discharge, and is non-toxic, non-polluting, and usable at high temperatures. Its raw materials are widely available, and it demonstrates excellent electrochemical performance, making it the most widely used cathode material in lithium-ion batteries.

[0004] As the most important precursor to lithium iron phosphate, iron phosphate's particle size distribution not only affects the compaction density and electrical properties of lithium iron phosphate, but also the efficiency of processing procedures such as batching, ball milling, sand milling, and spraying. A wide particle size distribution with large particles can lead to material blockage and difficulty in grinding during material transport; its iron-to-phosphorus ratio also affects the specific capacity of lithium iron phosphate.

[0005] To avoid these problems, existing technologies include methods to remove free water from the iron phosphate filter cake and surface modification using a disc mill drying device to improve the processing performance of iron phosphate. However, iron phosphate prepared by this method suffers from severe material agglomeration, large particles, and small specific surface area. Furthermore, the grinding time for lithium iron phosphate precursors is long, affecting lithium iron phosphate production efficiency. Alternatively, polymerized ferric salts can be used as an iron source to prepare iron phosphate. Although the product performance is relatively stable, the production process of polymerized ferric salts is more complicated than using titanium dioxide byproducts directly, affecting production efficiency. Additionally, the phosphorus in the concentrated phosphoric acid solution prepared during the aging process does not enter the iron phosphate, resulting in excessive consumption of phosphoric acid.

[0006] On the other hand, ferric phosphate primary particles are nanoparticles. Due to their small particle size, the filter cake is relatively viscous and prone to agglomeration during the flash drying and calcination process in a rotary kiln, resulting in a wide particle size distribution and a large number of large particles in the finished ferric phosphate product. Current processes mostly involve flash drying to remove free water, obtaining dihydrate ferric phosphate, which is then fed into a rotary kiln to remove water of crystallization and undergo crystal transformation. Because of the large particle size, this requires an additional pulverization step. While the pulverization step can deagglomerate the material, it also damages the particle morphology to some extent, affecting material transport and electrochemical performance during batching. Furthermore, the pulverization step is energy-intensive, increasing costs.

[0007] Based on the above, further research is still needed to provide iron phosphate materials that are easy to process and have good electrochemical performance.

[0008] Summary of the Invention

[0009] In view of the technical problems existing in the background art, this application provides an iron phosphate material, its preparation method, a cathode material, a cathode electrode sheet and a secondary battery, aiming to solve the technical problem of how to improve the processing performance and electrochemical performance of iron phosphate material.

[0010] In a first aspect, embodiments of this application provide an iron phosphate material, which satisfies the evaluation index q in the range of 0.02 μm. -1 ~0.15μm -1 The evaluation index q is calculated using the following formula: q = t / Dv 90

[0011] In the formula, t is the iron-to-phosphorus ratio of the iron phosphate material, and Dv 90 This refers to the particle size value corresponding to a volume distribution percentage of 90% when the iron phosphate particles in the iron phosphate material are sorted from smallest to largest.

[0012] In the technical solution of this application embodiment, the electrochemical performance and particle size of the iron phosphate material are comprehensively considered by the above-mentioned evaluation index q, that is, the ratio of iron-phosphorus ratio and particle size of the iron phosphate material. In the process of extensive research, the researchers of this application found that iron phosphate materials whose evaluation index q meets the above range are beneficial to improving the efficiency of sand milling or ball milling in the preparation process of lithium iron phosphate and lithium manganese iron phosphate, which facilitates the preparation of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate that meet the particle size and iron-phosphorus ratio requirements. It can reduce material blockage in the material conveying process and is beneficial to improving the electrochemical performance of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared therefrom.

[0013] Further, the particle size aggregation index of the iron phosphate material is less than or equal to 10; or, the iron-to-phosphorus ratio of the iron phosphate material is 0.97–0.99; or, the mass fraction of iron in the iron phosphate material is 36.2%–36.7%; or, the mass fraction of phosphorus in the iron phosphate material is 20.55%–20.75%; or, the specific surface area of ​​the iron phosphate material is 6 m². 2 / g~10m 2 / g.

[0014] In this embodiment, when the particle size aggregation index of the iron phosphate material is less than or equal to 10, the particle size of the iron phosphate material is relatively uniform. Iron phosphate materials with an iron-to-phosphorus ratio of 0.97 to 0.99, as well as iron phosphate materials with iron or phosphorus content within the above range, can significantly improve the performance of cathode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared with them, and the battery performance. Furthermore, iron phosphate materials with the above specific surface area can ensure the performance of cathode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared subsequently.

[0015] Furthermore, the iron phosphate material is monoclinic anhydrous iron phosphate; the Dv of the iron phosphate material 10 The diameter ranges from 0.7 μm to 1.8 μm; the Dv of iron phosphate materials 50 The diameter of the iron phosphate material is 2μm to 5μm. 90 The size ranges from 8μm to 30μm.

[0016] Iron phosphate materials with the above-mentioned morphological and particle size characteristics are beneficial to improving the electrochemical performance of cathode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared from them.

[0017] Secondly, embodiments of this application provide a method for preparing ferric phosphate material, the method comprising: mixing a first oxidant, a phosphate solution and a first ferrous solution to obtain a first ferric phosphate slurry; removing the solvent from the first ferric phosphate slurry to obtain an amorphous ferric phosphate solid; mixing the amorphous ferric phosphate solid with the first solvent to obtain a second ferric phosphate slurry; after the second ferric phosphate slurry undergoes a first stage of aging, adding a second ferrous solution and a second oxidant to obtain a third ferric phosphate slurry; and dehydrating the third ferric phosphate slurry to obtain the ferric phosphate material.

[0018] In the technical solution of this application embodiment, during the aging step of the ferric phosphate slurry, smaller particles have higher solubility than larger particles in the slurry. The smaller particles dissolve, while the larger particles grow. At this point, a second ferrous solution and a second oxidant are added, reacting with phosphorus in the slurry. This skips the nucleation process, allowing the larger particles to grow directly on the smaller particles, ultimately resulting in ferric phosphate with a uniform particle size distribution. Simultaneously, by adding the second ferrous solution during the aging step to alter the iron-to-phosphorus ratio in the ferric phosphate slurry, the iron-to-phosphorus ratio of the finished anhydrous ferric phosphate is increased, reaching a ratio of 0.975 or higher. The ferric phosphate material prepared using the above method does not require pulverization, and the resulting material has a uniform and narrow particle size distribution, which is beneficial for energy conservation. This preparation method can prepare any of the above-mentioned iron phosphate materials, and its evaluation index q meets the above range. It is beneficial to improve the efficiency of sand milling or ball milling in the preparation process of lithium iron phosphate and lithium manganese iron phosphate, facilitates the preparation of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate that meet the particle size requirements, can reduce material blockage in the material conveying process, and is beneficial to improve the electrochemical performance of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared by it.

[0019] Furthermore, the molar ratio of iron in the second ferrous solution to that in the first ferrous solution is (0.05-0.12):1; the molar ratio of iron in the second ferrous solution to that in the second oxidizing agent is 2:1-1:1.

[0020] In this embodiment, selecting appropriate amounts of the second ferrous solution and the second oxidant is beneficial for the full utilization of iron and further improving the iron-to-phosphorus ratio of the iron phosphate material, as well as increasing the efficiency of the conversion of ferrous iron to ferric iron.

[0021] Further, the second oxidant and the second ferrous solution are added dropwise to the second ferric phosphate slurry.

[0022] In this embodiment, by selectively adding the second oxidant and the second ferrous solution dropwise, the particle size uniformity of the iron phosphate material can be further improved.

[0023] Furthermore, the solid-liquid ratio of the second ferric phosphate slurry is 1:4-1:9. Preferably, before the first stage of aging, the second ferric phosphate slurry is mixed with a phosphoric acid solution. Preferably, the ratio of the amount of phosphorus in the phosphoric acid solution to the amount of iron in the second ferric phosphate slurry is 1:5-1:10. Preferably, the temperature of the first stage of aging is 80℃-95℃.

[0024] In this embodiment, the first stage of aging under the above conditions is beneficial to accelerate the aging speed of the ferric phosphate slurry, speed up the production cycle, obtain better preliminary ferric phosphate crystal particles, and also provide better reaction conditions for subsequent processing, further improving the preparation efficiency and product quality of ferric phosphate.

[0025] Further, the ratio of the amount of the first oxidant to the amount of iron in the first ferrous solution is 2:1-3:1; the ratio of the amount of iron in the first ferrous solution to the amount of phosphorus in the phosphate solution is 1:(1-1.3); preferably, the pH value of the phosphate solution is 5-7.5; preferably, the pH value of the first ferrous solution is 3-4.5; preferably, the phosphate solution and the first oxidant are added dropwise to the first ferrous solution.

[0026] In this embodiment, by controlling the appropriate amounts of the first oxidant, the iron element in the first ferrous solution, and the phosphorus element in the phosphate solution, a high iron-to-phosphorus ratio of iron phosphate material can be obtained, and under suitable pH conditions, it is beneficial to the conversion of iron phosphate. Adding the phosphate solution and the first oxidant in a dropwise manner can make the iron phosphate particles in the formed first iron phosphate slurry more uniform.

[0027] Further, the ferrous salts in the first and second ferrous solutions are each independently selected from any one or more of ferrous sulfate, ferrous chloride, and ferrous oxalate; preferably, the concentration of iron in the first ferrous solution is 0.5 mol / L to 1 mol / L; the first oxidant and / or the second oxidant is a hydrogen peroxide solution, preferably, the mass fraction of hydrogen peroxide in the hydrogen peroxide solution is 25% to 35%; the phosphate is selected from any one or more of monoammonium phosphate and calcium dihydrogen phosphate, preferably, the concentration of phosphorus in the phosphate solution is 1 mol / L to 1.5 mol / L.

[0028] In this embodiment, using the aforementioned ferrous solution or oxidant is beneficial for improving reaction efficiency and reducing production costs.

[0029] Thirdly, embodiments of this application provide a positive electrode material, the raw materials for preparing the positive electrode material comprising any of the above-mentioned iron phosphate materials, or the raw materials for preparing the positive electrode material comprising iron phosphate materials prepared by any of the above-mentioned iron phosphate material preparation methods.

[0030] In this embodiment, since the iron phosphate material meets the specific requirements of the above-mentioned evaluation index q value, it can better balance the relationship between the iron-phosphorus ratio and particle size of iron phosphate, which is conducive to the preparation of lithium iron phosphate, lithium manganese iron phosphate and other materials, simplifies the preparation process of lithium iron phosphate, lithium manganese iron phosphate and other materials, and improves the electrochemical performance of cathode materials such as lithium iron phosphate and lithium manganese iron phosphate.

[0031] Furthermore, the cathode material is selected from lithium iron phosphate and / or lithium manganese iron phosphate.

[0032] In this embodiment, the cathode material selected from lithium iron phosphate and / or lithium manganese iron phosphate is prepared from lithium iron phosphate material that meets the evaluation index q. This not only simplifies the preparation process but also provides better electrochemical performance.

[0033] Fourthly, this application provides a positive electrode sheet containing the aforementioned positive electrode material.

[0034] In this embodiment, the positive electrode sheet contains a positive electrode material prepared from the aforementioned iron phosphate material, thus exhibiting superior electrochemical performance and lower cost.

[0035] Fifthly, embodiments of this application provide a secondary battery containing the aforementioned positive electrode plate.

[0036] In this embodiment, because the iron phosphate of this application has superior performance, the cathode material prepared using it also has good electrochemical performance. Therefore, when it is used as a cathode electrode component in a secondary battery, the resulting secondary battery also has comprehensively improved electrochemical performance, thus enabling it to be well applied in multiple application scenarios.

[0037] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0038] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0039] Figure 1 is a schematic diagram of the preparation process of the iron phosphate material provided in this application;

[0040] Figure 2 shows the scanning electron microscope (SEM) results of the anhydrous iron phosphate prepared in Example 2 of this application;

[0041] Figure 3 is a SEM image of the anhydrous iron phosphate prepared in Example 3 of this application;

[0042] Figure 4 is a SEM image of the anhydrous iron phosphate prepared in Example 4 of this application;

[0043] Figure 5 shows the SEM results of anhydrous iron phosphate prepared in Comparative Example 1 of this application;

[0044] Figure 6 shows the SEM results of anhydrous iron phosphate prepared in Comparative Example 2 of this application;

[0045] Figure 7 shows the X-ray diffraction (XRD) results of the anhydrous iron phosphate materials prepared in Examples 2 to 4 of this application;

[0046] Figure 8 shows the XRD results of the iron phosphate dihydrate materials prepared in Examples 2 to 4 of this application;

[0047] Figure 9 shows the XRD results of the anhydrous iron phosphate materials prepared in Comparative Examples 1 and 2 of this application. Detailed Implementation

[0048] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0050] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0051] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0052] As analyzed in the background section of this application, existing iron phosphate materials suffer from poor processing performance and insufficient electrochemical performance. To address this problem, the first aspect of this application provides an iron phosphate material that satisfies an evaluation index q within a range of 0.02 μm. -1 ~0.15μm -1 The evaluation index q is calculated using the following formula: q = t / Dv 90

[0053] In the formula, t is the iron-to-phosphorus ratio of the iron phosphate material, and Dv 90 This refers to the particle size value corresponding to a volume distribution percentage of 90% when the iron phosphate particles in the iron phosphate material are sorted from smallest to largest.

[0054] This application comprehensively considers the electrochemical performance and particle size of iron phosphate materials by using the aforementioned evaluation index q, which is the ratio of iron-phosphorus ratio to particle size. During the research process, the researchers of this application found that iron phosphate materials whose evaluation index q meets the above range are beneficial to improving the efficiency of sand milling or ball milling in the preparation process of lithium iron phosphate, lithium manganese iron phosphate, etc., making it easier to prepare positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate that meet the requirements of particle size and iron-phosphorus ratio. Moreover, it can reduce material blockage during the material transportation process and is beneficial to improving the electrochemical performance of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate containing it.

[0055] Specifically, the evaluation index q can be 0.02μm. -1 0.03μm -1 0.04μm -1 0.05μm -1 0.06μm -1 0.07μm -1 0.08μm -1 0.09μm -1 0.10μm -1 0.11μm -1 0.12μm -1 0.13μm -1 0.14μm -1 0.15μm -1 Preferably, the evaluation index q is 0.03 μm. -1 ~0.13μm -1 .

[0056] In some typical embodiments of this application, the particle size aggregation index (PDI) of the above-mentioned iron phosphate material is less than or equal to 10. The particle size aggregation index is a parameter used to quantify the uniformity of particle size distribution, and its calculation formula is PDI = (Dv... 90 -Dv 10 ) / Dv 50 Among them, Dv 10 Dv represents the particle size value corresponding to a volume distribution percentage of 10% when the iron phosphate particles in the iron phosphate material are sorted from smallest to largest. 50This refers to the particle size value corresponding to a volume distribution percentage of 50% when the iron phosphate particles are sorted from smallest to largest. A lower particle size aggregation index indicates a more uniform particle size distribution, while a higher index indicates greater particle size variation. In this embodiment, when the particle size aggregation index of the iron phosphate material is less than or equal to 10, the particles are relatively uniform in size and have a narrow particle size distribution, facilitating subsequent processing and preparation of lithium iron phosphate and lithium manganese iron phosphate, and improving their performance. For example, the particle size aggregation index of the aforementioned iron phosphate material is 2, 4, 6, 8, or 9; preferably, it is 1 to 7; and more preferably, it is 1 to 5.

[0057] In some embodiments of this application, the iron-to-phosphorus ratio of the iron phosphate material is 0.97 to 0.99, where the iron-to-phosphorus ratio is the molar ratio of iron to phosphorus in the iron phosphate material. An iron-to-phosphorus ratio within this range not only enables the prepared lithium iron phosphate or lithium iron manganese phosphate to have higher specific capacity and conductivity, resulting in batteries containing it with higher cycle life and output voltage, but also better complements the Dv of iron phosphate. 90 The values ​​are matched to ensure that lithium phosphate has a suitable evaluation index q value, thereby guaranteeing the high performance of iron phosphate.

[0058] In some embodiments of this application, in order to further improve the performance of iron phosphate, the mass fraction of iron in the iron phosphate material is 36.2%-36.7%; the mass fraction of phosphorus in the iron phosphate material is 20.55%-20.75%.

[0059] Preferably, the specific surface area of ​​the iron phosphate material is 6m². 2 / g~10m 2 The iron phosphate material with the above-mentioned specific surface area can more effectively guarantee the performance of subsequent prepared cathode materials such as lithium iron phosphate and lithium manganese iron phosphate. More preferably, the specific surface area of ​​the iron phosphate material is 6.8 m² / g. 2 / g~10m 2 / g.

[0060] In some embodiments of this application, the iron phosphate material is monoclinic anhydrous iron phosphate. Anhydrous iron phosphate with this crystal form facilitates the subsequent preparation of lithium iron phosphate or lithium iron manganese phosphate with suitable morphology, and in particular, it can significantly improve battery performance compared to amorphous iron phosphate. Preferably, the Dv of the iron phosphate material... 10 The micrometer diameter (Dv) is 0.7 μm to 1.8 μm, more preferably 0.8 μm to 1.7 μm, and even more preferably 1.0 μm to 1.7 μm; the Dv of the iron phosphate material is... 50 The diameter is 2μm to 5μm, more preferably 2.5μm to 5μm, and even more preferably 3μm to 5μm; the Dv of the iron phosphate material90 The particle size is 8μm to 30μm, more preferably 8μm to 25μm, and even more preferably 8μm to 20μm. When the particle size of iron phosphate is too large, it increases the difficulty of subsequent preparation of lithium iron phosphate or lithium iron manganese phosphate. If the particle size of iron phosphate is too small, it is prone to agglomeration during the preparation of lithium iron phosphate or lithium iron manganese phosphate, resulting in a low discharge specific capacity of the battery prepared with it as the positive electrode material. When the particle size of iron phosphate material meets the above range, it can further improve the overall performance of iron phosphate material.

[0061] According to another typical embodiment of this application, a method for preparing ferric phosphate material is provided. The method includes: mixing a first oxidant, a phosphate solution, and a first ferrous solution to obtain a first ferric phosphate slurry; removing the solvent from the first ferric phosphate slurry to obtain amorphous ferric phosphate solid; mixing the amorphous ferric phosphate solid with the first solvent to obtain a second ferric phosphate slurry; aging the second ferric phosphate slurry in a first stage, then adding a second ferrous solution and a second oxidant to obtain a third ferric phosphate slurry; and dehydrating the third ferric phosphate slurry to obtain the ferric phosphate material.

[0062] In the above preparation method, during the aging step of the ferric phosphate slurry, smaller particles have higher solubility than larger particles. As the smaller particles dissolve, the larger particles grow. At this point, a second ferrous solution and a second oxidant are added, reacting with phosphorus in the slurry. This skips the nucleation process, allowing the larger particles to grow directly on the smaller particles, ultimately resulting in ferric phosphate with a uniform particle size distribution. Simultaneously, the addition of the second ferrous solution during the aging step alters the iron-to-phosphorus ratio in the ferric phosphate slurry, thereby increasing the iron-to-phosphorus ratio of the finished anhydrous ferric phosphate, achieving a ratio above 0.975. This method eliminates the need for pulverization, resulting in a material with a uniform and narrow particle size distribution, which is beneficial for energy conservation. This preparation method can prepare any of the above-mentioned iron phosphate materials, and its evaluation index q meets the above range. It is beneficial to improve the efficiency of sand milling or ball milling in the preparation process of lithium iron phosphate or lithium iron manganese phosphate, and facilitates the preparation of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate that meet the particle size requirements. Moreover, it can reduce material blockage in the material conveying process and is beneficial to improve the electrochemical performance of positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared by it.

[0063] The solvents for the first and second ferrous solutions can be selected from existing technologies. For example, the solvents for the first and second ferrous solutions can be water, which can be pure water, deionized water, or distilled water. This is not only relatively inexpensive and readily available, but also relatively environmentally friendly.

[0064] In some embodiments of this application, the ferrous salts in the first and second ferrous solutions are each independently selected from any one or more of ferrous sulfate, ferrous chloride, and ferrous oxalate, exhibiting good solubility and being relatively inexpensive and readily available. Preferably, the concentration of the first ferrous solution is 0.5 mol / L to 1 mol / L, and the concentration of the second ferrous solution is also 0.5 mol / L to 1 mol / L, which is beneficial for forming ferric phosphate particles with relatively uniform particle size. Preferably, the ferrous salts in the first and second ferrous solutions are selected from the same ferrous salt, and the concentrations of the first and second ferrous solutions are the same, so that the first and second ferrous solutions can be prepared together, simplifying the process and reducing costs.

[0065] In some preferred embodiments of this application, the first oxidant and / or the second oxidant is a hydrogen peroxide solution, which can not only oxidize ferrous iron to ferric iron at a suitable rate, but also does not introduce new impurities, facilitating the preparation of high-purity ferric phosphate. When the first oxidant and / or the second oxidant is a hydrogen peroxide solution, the oxidizing agent in the first oxidant is hydrogen peroxide, and / or, the oxidizing agent in the second oxidant is hydrogen peroxide. Preferably, the mass fraction of hydrogen peroxide in the hydrogen peroxide solution is 25 wt% to 35 wt%.

[0066] In some embodiments of this application, the phosphate is selected from one or more of monoammonium phosphate and calcium dihydrogen phosphate. Preferably, the concentration of phosphorus in the phosphate solution is 1 mol / L to 1.5 mol / L. It should also be noted that, in order to achieve a suitable pH value, the phosphate solution may contain phosphoric acid.

[0067] In some embodiments of this application, the ratio of the amount of oxidizing agent in the first oxidant to the amount of iron in the first ferrous solution is 2:1 to 3:1, which is sufficient to fully oxidize ferrous iron to ferric iron. For example, the ratio of the amount of oxidizing agent in the first oxidant to the amount of iron in the first ferrous solution is 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, or 3:1, but is not limited to these ratios.

[0068] In some preferred embodiments of this application, the ratio of the amount of iron in the first ferrous solution to the amount of phosphorus in the phosphate solution is 1:(1-1.3), which is beneficial for forming a phosphate material with a high iron-to-phosphorus ratio. For example, the ratio of the amount of iron in the first ferrous solution to the amount of phosphorus in the phosphate solution is 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2, 1:1.25, or 1:1.3, but is not limited to these ratios. Preferably, the ratio of the amount of iron in the first ferrous solution to the amount of phosphorus in the phosphate solution is 1:(1-1.2), more preferably 1:(1-1.1), which can reduce material consumption.

[0069] Preferably, the pH value of the phosphate solution is 5-7.5, which is beneficial to improving the preparation efficiency of ferric phosphate. For example, the pH value of the phosphate solution is 5, 5.5, 6, 6.5, 7 or 7.5.

[0070] Preferably, the pH value of the first ferrous solution is 3-4.5, such as 3, 3.2, 3.5, 3.8, 4, 4.2 or 4.5, which can precipitate the metal cation impurities in the first ferrous solution and prevent them from entering the ferric phosphate, thus increasing the impurity element content in the ferric phosphate.

[0071] The pH value of the aforementioned phosphate or ferrous solution can be adjusted by adding a pH adjuster to meet the aforementioned pH range. The pH adjuster can be selected from existing technologies. Preferably, a pH adjuster that does not introduce new impurities is used, such as ammonia, phosphoric acid, or sodium hydroxide.

[0072] Preferably, the phosphate solution and the first oxidant are added dropwise to the first ferrous solution. The phosphate solution and the first oxidant can be added dropwise to the first ferrous solution separately, or they can be mixed thoroughly before being added to the first ferrous solution to form amorphous ferric phosphate. Preferably, the dropping time is 15-40 minutes to prepare the first ferric phosphate slurry. Exemplarily, the dropping time is 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, or 40 minutes, but is not limited to these.

[0073] The solvent in the first ferric phosphate slurry can be removed, for example, by pressing and rinsing the first ferric phosphate slurry to obtain a filter cake, which is an amorphous ferric phosphate solid.

[0074] The amorphous iron phosphate solid is mixed with a first solvent to obtain a second iron phosphate slurry, which is used for subsequent aging and other treatments. The first solvent can be selected from existing technologies, such as pure water, deionized water, or distilled water. In some embodiments of this application, the amorphous iron phosphate solid is mixed with pure water and pulped to obtain the second iron phosphate slurry. Preferably, the solid-liquid ratio of the second iron phosphate slurry is 1:4-1:9, that is, the mass ratio of iron phosphate to the first solvent in the second iron phosphate slurry is 1:4-1:9. As examples, the solid-liquid ratio of the second iron phosphate slurry is 1:4, 1:4.5, 1:5, 1:5.5, 1:6, 1:6.5, 1:7, 1:7.5, 1:8, 1:8.5, or 1:9, but is not limited to these.

[0075] In some typical embodiments of this application, before the first stage of aging, the second ferric phosphate slurry is mixed with a phosphoric acid solution to adjust the pH value of the slurry. This accelerates the aging rate of the second ferric phosphate slurry, speeds up the production cycle, and the phosphoric acid can further react with the iron in the subsequently added second ferrous solution to convert it into ferric phosphate. Preferably, the mass fraction of phosphoric acid in the phosphoric acid solution is 70%-90%. For example, the mass fraction of phosphoric acid in the above-mentioned phosphoric acid solution is 75%, 80%, or 85%, but it is not limited to these.

[0076] In some preferred embodiments of this application, the preferred ratio of phosphorus in the phosphoric acid solution to iron in the second ferric phosphate slurry is 1:5 to 1:10. This significantly promotes aging and allows the aging process to continue effectively after the subsequent addition of the second ferrous solution and the second oxidant, resulting in high utilization. For example, the ratio of phosphorus in the phosphoric acid solution to iron in the second ferric phosphate slurry can be 1:5.5, 1:6, 1:6.5, 1:7, 1:7.5, 1:8, 1:8.5, 1:9, or 1:9.5, but is not limited to these ratios.

[0077] In some embodiments of this application, the phosphoric acid solution is mixed with the second ferric phosphate slurry and pulped for 20-30 minutes, followed by heating for the first stage of aging. For example, the mixing and pulping time of the phosphoric acid solution with the second ferric phosphate slurry is 22 minutes, 25 minutes, 26 minutes, etc., but is not limited to these.

[0078] In some embodiments of this application, the temperature of the first stage of aging is 80℃-95℃, which has high aging efficiency and is beneficial to crystal grain growth. For example, the temperature of the first stage of aging is 83℃, 85℃, 88℃, 90℃, 93℃, etc., but is not limited to these.

[0079] During the first aging process described above, the second ferric phosphate slurry will change color as aging progresses, for example, from pale yellow to pinkish-white. This color change indicates a change in the crystal form of ferric phosphate, transforming it from amorphous ferric phosphate to monoclinic ferric phosphate. In some embodiments of this application, the aforementioned color change occurs after heating to the temperature stated in the first section and holding at that temperature for 30-70 minutes (for example, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 65 minutes, etc., but not limited to these).

[0080] In some typical embodiments of this application, the second oxidant and the second ferrous solution are added 3 to 60 minutes after the second ferric phosphate slurry changes color (e.g., from pale yellow to pinkish-white), resulting in ferric phosphate particles with relatively uniform particle size. Preferably, the second oxidant and the second ferrous solution are added 5 to 20 minutes after the second ferric phosphate slurry changes color (e.g., from pale yellow to pinkish-white) (for example, it can be 6 minutes, 8 minutes, 10 minutes, 12 minutes, 15 minutes, 18 minutes, etc., but is not limited to these).

[0081] During the research process, the researchers of this application discovered that when the ferric phosphate slurry is first aged and changes color, only a portion of the ferric phosphate transforms from amorphous to monoclinic crystals. At this time, the addition of ferrous sulfate and hydrogen peroxide reacts with the phosphorus in the ferric phosphate slurry, directly generating monoclinic ferric phosphate on small particles. This has a minor impact on the growth of large ferric phosphate particles in the slurry. However, after a period of aging and color change, more than half of the ferric phosphate transforms from amorphous to monoclinic crystals. Since small particles in the ferric phosphate slurry have higher solubility than larger particles, the small particles dissolve, and the large particles grow. At this time, the addition of ferrous sulfate and hydrogen peroxide reacts with the phosphorus in the ferric phosphate slurry, directly generating monoclinic ferric phosphate on small particles. This monoclinic ferric phosphate competes with the large particles for small particles, preventing the large particles from adsorbing small particles and growing even larger. After a longer period of aging and color change, the large particles have basically completed their growth. At this time, the addition of ferrous sulfate and hydrogen peroxide reacts with the phosphorus in the ferric phosphate slurry, generating monoclinic ferric phosphate on small particles. The resulting ferric phosphate material has a particle size Dv. 90 Dv 100 The larger the particle size, the weaker the effect on improving the size uniformity of iron phosphate particles.

[0082] In some preferred embodiments of this application, adding the second oxidant and the second ferrous solution dropwise to the second ferric phosphate slurry is beneficial for further improving the uniformity of the ferric phosphate particles. The second ferrous solution and the second oxidant can be added simultaneously, or the second ferrous solution can be added before the second oxidant; the addition of the second oxidant can be stopped simultaneously with the addition of the second ferrous solution, or the addition can be stopped after the addition of the second ferrous solution has finished.

[0083] In some embodiments of this application, the second oxidant is mixed with the second ferrous solution and then added to the second ferric phosphate slurry after the first stage of aging; preferably, in order to prevent the formation of ferric hydroxide colloid after the second ferrous solution and the second oxidant are mixed, which would have an adverse effect on the subsequent growth of ferric phosphate crystals, acid is added to the second ferrous solution to lower its pH value.

[0084] In some preferred embodiments of this application, the molar ratio of iron in the second ferrous solution to iron in the first ferrous solution is (0.05-0.12):1, which is beneficial for the full utilization of iron and further improves the iron-to-phosphorus ratio of the ferric phosphate material. For example, the molar ratio of iron in the second ferrous solution to iron in the first ferrous solution is 0.06:1, 0.08:1, 0.1:1, etc., but is not limited to these.

[0085] The amount of the second oxidant can be referenced to the amount of the first oxidant used in preparing the first ferric phosphate slurry. For example, the molar ratio of iron in the second ferrous solution to the oxidizing agent in the second oxidant is 2:1 to 1:1. For instance, the molar ratios of iron in the second ferrous solution to the oxidizing agent in the second oxidant are 1.1:1, 1.2:1, 2:1.5, 1.5:1, 1.6:1, 1.8:1, etc., but are not limited to these.

[0086] In some embodiments of this application, after adding the second oxidant and the second ferrous solution, a second aging process is performed. Preferably, the second aging time is 40 min to 100 min, and the second aging temperature is 80℃ to 95℃. For example, the second aging time is 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, etc., but is not limited to these. For example, the second aging temperature is 82℃, 85℃, 88℃, 90℃, 92℃, etc., but is not limited to these.

[0087] The third ferric phosphate slurry obtained after the above treatment is dehydrated to obtain the ferric phosphate material. The specific dehydration process can be selected from existing technologies, such as sequential solvent removal and calcination.

[0088] In some typical embodiments of this application, the above-mentioned dehydration treatment includes pressing, drying, and calcination to obtain the above-mentioned iron phosphate material. Preferably, the drying is performed by flash evaporation.

[0089] The agglomeration of materials occurs primarily due to two factors. The first agglomeration is mainly caused by capillary action during the drainage process in the drying stage. The second agglomeration occurs during the calcination process, where the removal of water of crystallization and the crystal transformation process involve strong forces such as capillary action, hydrogen bonds, and chemical bonds, causing particles to connect and form hard agglomerates. This second agglomeration builds upon the agglomeration during the first drying stage. If the particle size distribution is narrow and uniform during drying, particle aggregation can be avoided, reducing the particle size gradient. Particles that are too large or too small will agglomerate during calcination, forming even larger particles. Based on this agglomeration mechanism, this application, by adding a second ferrous solution and a second oxidant, can also obtain ferric phosphate dihydrate with a narrow particle size distribution, thus avoiding particle aggregation and reducing the particle size gradient. Furthermore, pressing can yield filter cakes with low moisture content, effectively controlling the moisture content of the pressed filter cakes. Flash evaporation not only achieves high water removal efficiency but also helps control the particle size of the flash-evaporated particles within a narrow range. This produces uniformly sized dried powder, which is then fed into a rotary kiln for calcination at a suitable temperature, yielding anhydrous ferric phosphate with a narrow particle size distribution. This controlled method produces anhydrous ferric phosphate without the need for pulverization, resulting in a uniform and narrow particle size distribution.

[0090] In some embodiments of this application, the moisture content of the filter cake obtained after pressing is 40%-60% (for example, the moisture content of the filter cake obtained after pressing is 42%, 45%, 48%, 50%, 52%, 55%, 56%, 58%, etc., but not limited to these), which can further reduce particle agglomeration in subsequent processing, making the dried particles more uniform, thereby resulting in a narrower particle size distribution of the calcined ferric phosphate material. In some embodiments of this application, the above-mentioned third ferric phosphate slurry is washed and then pressed.

[0091] Preferably, the inlet air temperature of the flash evaporation is 300℃-600℃ (for example, the inlet air temperature can be 350℃, 400℃, 450℃, 500℃, 550℃, etc., but is not limited to this), and the outlet air temperature is 140℃-200℃ (for example, the outlet air temperature can be 150℃, 155℃, 160℃, 165℃, 170℃, 175℃, 180℃, 185℃, 190℃, 195℃, etc., but is not limited to this); by adjusting the inlet air temperature and outlet air temperature of the flash drying process, not only can all free water be removed, but also a small amount of crystal water can be removed. Preferably, the crystal water content (by mass fraction) of the dried material is 13%-17% (for example, the crystal water content of the dried material is 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, etc., but not limited to this), which can further reduce particle agglomeration in the subsequent calcination process and make the particle size of the iron phosphate material more uniform.

[0092] In some embodiments of this application, the solvent-removed dried material is uniformly fed into a rotary kiln. The temperature of the first temperature zone of the rotary kiln is 300℃-400℃, the temperature of the second temperature zone is 500℃-600℃, the temperature of the third / fourth / fifth temperature zones is 600℃-700℃, and the temperature of the sixth temperature zone is 300℃-400℃, resulting in better sintering effect. Preferably, the residence time in each temperature zone is 20 minutes to 25 minutes (for example, the residence time in each temperature zone can be independently selected from 21 minutes, 22 minutes, 23 minutes, 24 minutes, etc., but is not limited thereto).

[0093] According to another typical embodiment of this application, a positive electrode material is provided, wherein the raw materials for preparing the positive electrode material include any of the above-mentioned iron phosphate materials, or the raw materials for preparing the positive electrode material include iron phosphate materials prepared by any of the above-mentioned iron phosphate material preparation methods.

[0094] Because the iron phosphate material of this application meets the specific requirements of the above-mentioned evaluation index q value, it can better balance the relationship between the iron-phosphorus ratio and particle size of iron phosphate, which is conducive to the preparation of lithium iron phosphate, lithium manganese iron phosphate and other materials, simplifies the preparation process of lithium iron phosphate, lithium manganese iron phosphate and other materials, and improves the electrochemical performance of cathode materials such as lithium iron phosphate and lithium manganese iron phosphate.

[0095] In some typical embodiments of this application, the cathode material is selected from lithium iron phosphate and / or lithium manganese iron phosphate. Cathode materials selected from lithium iron phosphate and / or lithium manganese iron phosphate, being prepared from iron phosphate that meets the evaluation index q, not only have a simplified preparation process but also exhibit better electrochemical performance.

[0096] According to another typical embodiment of this application, a positive electrode sheet is provided, which contains the above-described positive electrode material.

[0097] The positive electrode of this application contains a positive electrode material prepared from the aforementioned iron phosphate material, thus exhibiting superior electrochemical performance and lower cost.

[0098] According to another typical embodiment of this application, a secondary battery is provided, which includes the above-described positive electrode.

[0099] Because the iron phosphate in this application has excellent performance, the positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate prepared using it also have good electrochemical performance. Therefore, when it is used as a positive electrode component in secondary batteries, the resulting secondary batteries also have comprehensively improved electrochemical performance, thus enabling them to be well applied in multiple application scenarios.

[0100] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0101] 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 this application.

[0102] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0103] I. Preparation Method

[0104] Example 1

[0105] Anhydrous iron phosphate material was prepared using the process flow shown in Figure 1, as detailed below:

[0106] (1) Dissolve ferrous sulfate heptahydrate in water, add alkaline solution (sodium hydroxide solution with a mass fraction of 32%) to adjust the pH of the solution to 3.7, filter to obtain a clear ferrous sulfate solution, add water to prepare a first ferrous solution with a molar concentration of 1 mol / L for later use.

[0107] (2) Dissolve monoammonium phosphate in water, add 20wt% ammonia water to adjust the pH of the solution to 7, and add water to prepare a phosphate solution with a molar concentration of 1.5mol / L for later use.

[0108] (3) Add the first oxidant (27% hydrogen peroxide) to the prepared phosphate solution, mix well to obtain an oxidant-phosphate mixed solution, wherein the ratio of the amount of hydrogen peroxide to the amount of Fe in the first ferrous solution is 2.4:1.

[0109] (4) Using the first ferrous solution as the base liquid, the oxidant phosphoric acid mixed solution is added dropwise to the first ferrous solution using a peristaltic pump for 20 min. The molar ratio of Fe in the first ferrous solution to P in the oxidant phosphoric acid mixed solution is 1:1.02. The synthesis reaction is carried out by stirring for 50 min to obtain the first ferric phosphate slurry. The first ferric phosphate slurry is then filtered and washed (i.e., washed once) to obtain a washed filter cake, which is an amorphous ferric phosphate solid.

[0110] (5) Add the first solvent (pure water) to the amorphous iron phosphate solid and slurry to obtain the second iron phosphate slurry. The solid-liquid ratio of the second iron phosphate slurry is 1:5. Add a small amount of phosphoric acid solution (concentration of 85%) to the second iron phosphate slurry. The molar ratio of phosphoric acid in the phosphoric acid solution to iron in the first ferrous solution is 0.12:1. Continue slurrying for 20 minutes, then raise the temperature to 93℃ for the first stage of aging. Keep the temperature until the slurry color changes from light yellow to pinkish white. After another 5 minutes, add the prepared second ferrous solution (1 mol / L ferrous sulfate solution) and the second oxidant (27% hydrogen peroxide by mass) dropwise to the second iron phosphate slurry. At this time, the molar ratio of Fe in the added ferrous sulfate to Fe in the first ferrous solution is 0.05:1, and the molar ratio of hydrogen peroxide to the added ferrous sulfate is 1.5:2. Continue to keep the temperature for 60 minutes for the second stage of aging to obtain the third iron phosphate slurry.

[0111] (6) The third ferric phosphate slurry was filtered by a filter press to separate the solid and liquid to obtain filter cake and second washing mother liquor. Then the filter cake was rinsed with water, i.e., second washing. After the second washing was completed, it was pressed to obtain a second washing filter cake with a moisture content of 45%. The second washing mother liquor was subjected to elemental analysis. The test results are shown in Table 1.

[0112] (7) The second washing filter cake is conveyed to the forced feeder at the flash evaporation inlet by a belt. The inlet air temperature is 400℃ and the outlet air temperature is 150℃. The second washing filter cake is dried to obtain a dried material with a crystal water content of 15%.

[0113] (8) The dried material is evenly fed into the rotary kiln for sintering. The temperature is set as follows: the temperature of the first temperature zone is 350-400℃, the temperature of the second temperature zone is 550-600℃, the temperature of the third / fourth / fifth temperature zones is 650-700℃, and the temperature of the sixth temperature zone is 350-400℃; the residence time of each temperature zone is 25 minutes.

[0114] (9) After sieving and demagnetizing, the iron phosphate material, i.e. anhydrous iron phosphate, is obtained.

[0115] Example 2

[0116] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that the molar ratio of Fe in the second ferrous solution added in step (5) to Fe in the first ferrous solution was 0.1:1, and the other steps were the same as in Example 1.

[0117] The dried material obtained in step (7), i.e., ferric phosphate dihydrate, was subjected to XRD test. The test results are shown in Figure 8. The obtained ferric phosphate material (anhydrous ferric phosphate) was subjected to SEM test and XRD test. Its SEM image is shown in Figure 2, and its XRD image is shown in Figure 7.

[0118] Example 3

[0119] Anhydrous ferric phosphate was prepared according to the method in Example 1, with the following difference: In step (5), the molar ratio of Fe in the second ferrous solution to Fe in the first ferrous solution was 0.09:1, and the second oxidant (hydrogen peroxide with a mass fraction of 27%) was first added to the second ferrous solution (1 mol / L ferrous sulfate solution) for pre-oxidation, and then added dropwise to the second ferric phosphate slurry. The timing of the addition was the same as in Example 1. To prevent the formation of ferric hydroxide colloid during oxidation, a small amount of dilute sulfuric acid was added before adding hydrogen peroxide to lower the pH of the reaction system. The molar ratio of sulfuric acid in the dilute sulfuric acid to ferrous sulfate in the ferrous sulfate solution was 1:2. The other steps were the same as in Example 1.

[0120] In this embodiment, the dried material obtained in step (7), i.e., ferric phosphate dihydrate, was subjected to XRD test, and the test results are shown in Figure 8. The obtained ferric phosphate material (anhydrous ferric phosphate) was subjected to SEM test and XRD test, and its SEM image is shown in Figure 3 and its XRD image is shown in Figure 7.

[0121] Example 4

[0122] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: in step (5), the molar ratio of Fe in the second ferrous solution to Fe in the first ferrous solution was 0.1:1, and the second oxidant (hydrogen peroxide with a mass fraction of 27%) was added to the second ferrous solution (1 mol / L ferrous sulfate solution) to oxidize in advance. In order to prevent the formation of ferric hydroxide colloid during the oxidation process, a small amount of dilute sulfuric acid was added before adding hydrogen peroxide to lower the pH value of the reaction system. The molar ratio of sulfuric acid in the dilute sulfuric acid to ferrous sulfate in the ferrous sulfate solution was 1:2. Other steps were the same as in Example 1.

[0123] In this embodiment, the dried material obtained in step (7), i.e., ferric phosphate dihydrate, was subjected to XRD test, and the test results are shown in Figure 8. The obtained ferric phosphate material (anhydrous ferric phosphate) was subjected to SEM test and XRD test, and its SEM image is shown in Figure 4 and its XRD image is shown in Figure 7.

[0124] Example 5

[0125] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that the molar ratio of Fe in the first ferrous solution to Fe in the second ferrous solution added in step (5) was 0.12:1, and the other steps were the same as in Example 1.

[0126] Example 6

[0127] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: the molar ratio of Fe in the first ferrous solution to Fe in the second ferrous solution added in step (5) was 0.1:1, and the addition time was 15 minutes after the first stage of aging slurry changed color. Other steps were the same as in Example 1.

[0128] Example 7

[0129] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: in step (5), the molar ratio of Fe in the first ferrous solution to Fe in the second ferrous solution was 0.1:1, and the addition time was 30 minutes after the first stage of aging slurry changed color. Other steps were the same as in Example 1.

[0130] Example 8

[0131] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: the molar ratio of Fe in the first solution to Fe in the second ferrous solution added in step (5) was 0.1:1, and the addition time was 40 min after the first stage of aging slurry changed color. Other steps were the same as in Example 1.

[0132] Example 9

[0133] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: the molar ratio of Fe in the first ferrous solution to Fe in the second ferrous solution added in step (5) was 0.1:1, and the addition time was 3 minutes after the first stage of aging slurry changed color. Other steps were the same as in Example 1.

[0134] Comparative Example 1

[0135] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: in step (5), the molar ratio of Fe in the first ferrous solution to Fe in the second ferrous solution was 0.1:1, and the second oxidant (hydrogen peroxide with a mass fraction of 27%) was added to the second ferrous solution (1 mol / L ferrous sulfate solution) for pre-oxidation. Ferric hydroxide colloid was generated during the oxidation process and then directly added to the second ferric phosphate slurry. Other steps were the same as in Example 1.

[0136] The SEM image of the iron phosphate material prepared in this comparative example is shown in Figure 5, and the XRD pattern is shown in Figure 9.

[0137] Comparative Example 2

[0138] Anhydrous ferric phosphate was prepared according to the method in Example 1, except that: no second ferrous solution and second oxidant were added during the aging process in step (5), and the other steps were the same as in Example 1;

[0139] The SEM image of the iron phosphate material prepared in this comparative example is shown in Figure 6, and the XRD pattern is shown in Figure 9.

[0140] Test methods

[0141] 1. Elemental analysis of the mother liquor from the second washing stage and elemental analysis of the iron phosphate material.

[0142] The contents of Fe, P, Al, Ca, and other elements in the secondary washing mother liquor or the various elements in the iron phosphate material were tested using an inductively coupled plasma optical emission spectrometer (ICP-OES). The test results of each element in the secondary washing mother liquor are shown in Table 1 below, and the test results of each element in the iron phosphate material are shown in Table 2 below.

[0143] 2. Testing of performance parameters of iron phosphate materials

[0144] BET surface area test: The gas adsorption BET method was used.

[0145] Particle size: The iron phosphate materials prepared in the above examples and comparative examples were measured using a laser particle size analyzer.

[0146] Iron content determination: Under acidic conditions, stannous chloride is used to remove most of the Fe in the sample solution. 3+ Reduced to Fe 2+ If necessary, add hydrogen peroxide to eliminate excess Sn. 2+ Using sodium tungstate solution as an indicator, titanium trichloride was used to further react Fe... 3+ Reduced to Fe 2+ Then, excess titanium trichloride is reacted with potassium dichromate solution. Finally, ferrous iron is titrated with a standard potassium dichromate solution to determine the iron content.

[0147] Phosphorus content determination: Quinomolybdate-limonene gravimetric method. Under acidic conditions, orthophosphate reacts with quinomolybdate-limonene precipitant to form a yellow quinoline phosphomolybdate precipitate. After filtration, washing, drying, and weighing, the phosphorus content can be calculated.

[0148] The iron-to-phosphorus ratio (Fe / P) is obtained by calculating the ratio of the number of moles of iron to the number of moles of phosphorus in the iron phosphate material.

[0149] pH value determination: Refer to GB / T 9724, General Rules for pH Value Determination of Chemical Reagents.

[0150] 3. Performance testing of iron phosphate materials

[0151] First, the iron phosphate and lithium source (lithium carbonate) prepared in the above embodiments and comparative examples were sintered to obtain the corresponding lithium iron phosphate cathode materials. Then, the prepared lithium iron phosphate cathode materials were mixed with conductive carbon powder and PVDF binder at a mass ratio of 90:5:5, homogenized, and coated onto aluminum foil. After drying at 100°C, they were rolled using a roller press, and then die-cut into 14mm diameter electrode sheets using a punching machine. The mass of the active material was obtained by weighing and deducting the mass of the aluminum foil. After drying the cathode sheet, CR2032 coin cells were assembled in a UNlab-type inert gas chamber from Braun GmbH, Germany. The assembly sequence was: negative electrode shell, lithium sheet, electrolyte, separator, electrolyte, positive electrode sheet, gasket, spring, and positive electrode shell. The charge-discharge electrochemical performance of the CR2032 coin cells was tested using a Wuhan Landian CT2001A battery testing system, with a voltage range of 2.0-4.6V.

[0152] II. Analysis of Test Results for Each Embodiment and Comparative Example

[0153] Table 1

[0154] As shown in Table 1, Comparative Example 2, due to the absence of ferrous sulfate and hydrogen peroxide in the aging step, and the addition of a small amount of phosphoric acid during the aging step, resulted in a higher phosphorus content (3048.58 ppm) in the ferric phosphate slurry compared to iron, leading to a high phosphorus content in the second washing mother liquor. This resulted in the waste and loss of this phosphorus element. Example 1, by adding a small amount of ferrous sulfate and hydrogen peroxide in the aging step, significantly reduced the phosphorus content in the second washing mother liquor. However, compared to other examples, such as Examples 2 to 5, the phosphorus content was still relatively high. Examples 2, 3, 4, and 5, which added more ferrous sulfate and hydrogen peroxide in the aging step, significantly reduced the phosphorus content in the second washing mother liquor to below 100 ppm. The phosphorus content in the second washing mother liquor of Examples 2, 4, and 5 was even around 20 ppm.

[0155] Table 2

[0156] As can be seen from Table 2, the addition of ferrous sulfate and hydrogen peroxide during the aging process does not affect the elemental content in anhydrous ferric phosphate, and both are within the acceptable range.

[0157] Table 3

[0158] Table 4

[0159] As can be seen from Tables 3 and 4, in Comparative Example 1, because ferrous sulfate was oxidized first and then added to the aged ferric phosphate, ferric hydroxide colloid was generated during the addition process. This resulted in a wider overall particle size distribution and uneven particle size distribution of the anhydrous ferric phosphate, leading to a lower evaluation index q value. Consequently, the resulting lithium iron phosphate material, used as a cathode material in a secondary battery, exhibited significantly lower charge and discharge specific capacities compared to the Example. In the Example, after adding ferrous sulfate and an oxidant during the aging step, the particle size D90 of the anhydrous ferric phosphate decreased significantly, the overall particle size distribution narrowed, and the particle size became more uniform, resulting in an evaluation index q value of 0.02 μm. -1 ~0.15μm -1 Within the specified range, the obtained lithium iron phosphate material, when used as the positive electrode material in secondary batteries, resulted in significantly higher charge and discharge specific capacities in the tested secondary batteries. Furthermore, as shown in Table 3, the preparation method for lithium iron phosphate material provided in this application can obtain materials with uniform particle size and narrow particle size distribution without the need for crushing or other operations. This is beneficial for saving energy consumption and improving the efficiency of sand milling and ball milling during lithium iron phosphate preparation, while reducing material blockage during material transport.

[0160] Specifically, in the preparation of lithium iron phosphate (LFP), the particle size of the precursor iron phosphate material has a significant impact on the electrical performance of LFP. Larger particle sizes result in lower discharge capacity and poorer cycle performance in LFP batteries; conversely, smaller particle sizes lead to particle agglomeration during battery fabrication, also resulting in lower discharge capacity. Therefore, by rationally controlling the particle size distribution (PDI) and particle size of the iron phosphate material, achieving a uniform particle size distribution and a lower PDI, the likelihood of agglomeration in subsequent processes is reduced, which is beneficial for improving the electrical performance of LFP batteries.

[0161] Furthermore, an excess of phosphorus relative to iron promotes the growth of primary particles. By controlling the iron-phosphorus ratio of iron phosphate materials within a suitable range, it is beneficial to control the primary particle size of the prepared cathode materials such as lithium iron phosphate and lithium manganese iron phosphate, thereby resulting in superior electrical performance of the corresponding cathode materials.

[0162] In summary, by controlling the evaluation index q = iron-phosphorus ratio / PDI, at 0.02 μm -1 ~0.15μm -1 Within the range, lithium iron phosphate materials obtained through iron phosphate materials exhibit superior performance in terms of capacity and cycle performance.

[0163] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

A type of iron phosphate material, characterized in that, The iron phosphate material satisfies the evaluation index q within the range of 0.02 μm. -1 ~0.15μm -1 , The evaluation index q is calculated using the following formula: q=t / Dv 90 In the formula, t is the iron-to-phosphorus ratio of the iron phosphate material, and Dv 90 The particle size value is the particle size value corresponding to a volume distribution percentage of 90% when the iron phosphate particles of the iron phosphate material are sorted from smallest to largest. The iron phosphate material according to claim 1 is characterized in that, The particle size aggregation index of the iron phosphate material is less than or equal to 10; The iron-to-phosphorus ratio of the iron phosphate material is 0.97 to 0.99; The iron content of the iron phosphate material is 36.2%-36.7% by mass. The phosphorus content of the iron phosphate material is 20.55%-20.75% by mass; The specific surface area of ​​the iron phosphate material is 6m². 2 / g~10m 2 / g. The iron phosphate material according to claim 1 or 2 is characterized in that, The iron phosphate material is monoclinic anhydrous iron phosphate; The Dv of the iron phosphate material 10 Its thickness ranges from 0.7 μm to 1.8 μm; The Dv of the iron phosphate material 50 The size ranges from 2μm to 5μm. The Dv of the iron phosphate material 90 The size ranges from 8μm to 30μm. A method for preparing an iron phosphate material, characterized in that, include: The first oxidant, phosphate solution and first ferrous solution are mixed to prepare the first ferric phosphate slurry; The solvent in the first ferric phosphate slurry is removed to obtain amorphous ferric phosphate solid; The amorphous iron phosphate solid is mixed with a first solvent to obtain a second iron phosphate slurry; After the second ferric phosphate slurry is aged in the first stage, a second ferrous solution and a second oxidant are added to obtain the third ferric phosphate slurry. The third ferric phosphate slurry is dehydrated to obtain the ferric phosphate material. The method for preparing iron phosphate material according to claim 4 is characterized in that, The molar ratio of iron in the second ferrous solution to iron in the first ferrous solution is (0.05-0.12):1; The molar ratio of iron in the second ferrous solution to the oxidizing agent in the second oxidant is 2:1 to 1:

1. The method for preparing iron phosphate material according to claim 4 or 5 is characterized in that, The solid-liquid ratio of the second ferric phosphate slurry is 1:4-1:9; Preferably, the second oxidant and the second ferrous solution are added dropwise to the second ferric phosphate slurry; Preferably, before the first stage of aging, the second ferric phosphate slurry is mixed with the phosphoric acid solution. The ratio of the amount of phosphorus in the phosphoric acid solution to the amount of iron in the second ferric phosphate slurry is 1:5 to 1:

10. Preferably, the temperature of the first aging stage is 80℃-95℃. The method for preparing iron phosphate material according to claim 4 is characterized in that, The ratio of the amount of oxidizing substance in the first oxidant to the amount of iron in the first ferrous solution is 2:1 to 3:

1. The ratio of the amount of iron in the first ferrous solution to the amount of phosphorus in the phosphate solution is 1:(1-1.3). Preferably, the pH value of the phosphate solution is 5-7.5; Preferably, the pH value of the first ferrous solution is 3-4.5; Preferably, the phosphate solution and the first oxidant are added dropwise to the first ferrous solution. The method for preparing iron phosphate material according to claim 4 is characterized in that, The ferrous salts in the first ferrous solution and the second ferrous solution are each independently selected from any one or more of ferrous sulfate, ferrous chloride, and ferrous oxalate; preferably, the concentration of iron in the first ferrous solution is 0.5 mol / L-1 mol / L, and the concentration of iron in the second ferrous solution is 0.5 mol / L-1 mol / L. The first oxidant and / or the second oxidant is a hydrogen peroxide solution, preferably, the mass fraction of hydrogen peroxide in the hydrogen peroxide solution is 25% to 35%; The phosphate is selected from any one or more of monoammonium phosphate and calcium dihydrogen phosphate. Preferably, the concentration of phosphorus in the phosphate solution is 1 mol / L to 1.5 mol / L. A positive electrode material, characterized in that, The raw materials for preparing the positive electrode material include the iron phosphate material according to any one of claims 1 to 3, or the raw materials for preparing the positive electrode material include the iron phosphate material prepared by the method for preparing the iron phosphate material according to any one of claims 4 to 8. The cathode material according to claim 9 is characterized in that, The cathode material is selected from lithium iron phosphate and / or lithium manganese iron phosphate. A positive electrode sheet, characterized in that, It contains the positive electrode material as described in claim 9 or 10. A secondary battery, characterized in that, It contains the positive electrode sheet as described in claim 11.

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