Composite phosphate-based positive electrode material, preparation method and application thereof

Abstract

The application belongs to the technical field of battery materials, and particularly relates to a composite phosphate-based positive electrode material and a preparation method and application thereof. The application comprises the following steps: mixing and grinding raw material components including a phosphorus source, an iron source, a manganese source, a lithium source and mixed carbon source I with a solvent I to obtain a solid-liquid mixed raw material; after one-time drying treatment, one-time sintering treatment is performed under an inert atmosphere; secondary grinding treatment is performed on the one-time crystallization product and mixed carbon source II and a solvent II; after secondary drying treatment, secondary sintering treatment is performed under an inert atmosphere to obtain the composite phosphate-based positive electrode material; the mixed carbon source comprises a high-molecular carbon source and a low-molecular carbon source; the temperature of the one-time sintering treatment is higher than that of the secondary sintering treatment. Through the mixed carbon source, secondary carbon coating and secondary sintering, the carbon content on the surface of the one-time crystallization material is reduced, the particle growth and uniform carbon coating are realized, and the performance of the composite material, such as the compaction density, the electrical conductivity, the capacity and the initial efficiency, is improved.

Description

Technical Field

[0001] This application belongs to the field of battery materials technology, and in particular relates to a composite phosphate-based cathode material, its preparation method and application. Background Technology

[0002] Lithium manganese iron phosphate (LFP) is a novel phosphate-based lithium-ion battery cathode material formed by doping lithium iron phosphate (LFP) with a certain proportion of manganese. The doping of manganese effectively combines the advantages of both iron and manganese. Furthermore, manganese and iron are both located in the fourth period of the periodic table and are adjacent to each other, possessing similar ionic radii and some chemical properties; therefore, doping does not significantly affect the original structure. LFP is a product of the combination of lithium manganese phosphate and lithium iron phosphate, fully integrating the advantages of both. LFP's high voltage platform results in higher energy density, and its cycle performance and safety performance are comparable to LFP, while its low-temperature performance is superior. Theoretically, LFP's energy density is 15%–20% higher than that of LFP, and battery products made using LFP have a 5%–10% lower cost. In terms of safety, LFP, being a phosphate system, exhibits thermal runaway performance comparable to LFP. Moreover, it has better low-temperature performance than LFP, retaining nearly 80% of its capacity at -20℃.

[0003] However, the addition of manganese results in a smaller particle size for the finished lithium manganese iron phosphate product. Furthermore, to improve the poor conductivity of lithium manganese iron phosphate, more carbon source is often needed, leading to a compaction density typically around 2.3 g / cm³. 3 Around 2.5 g / cm³, compared to the compaction density of lithium iron phosphate (2.5 g / cm³). 3 ~2.7g / cm 3 However, in practical applications, its advantage of higher energy density has not been fully utilized, which has limited its large-scale industrial application.

[0004] Therefore, there is still an urgent need to improve the compaction density of phosphate-based cathode materials such as lithium manganese iron phosphate and increase their specific capacity. Summary of the Invention

[0005] The purpose of this application is to provide a composite phosphate-based cathode material, its preparation method, and its application, aiming to solve, to some extent, the problem that the compaction density of existing phosphate-based cathode materials such as lithium manganese iron phosphate needs to be improved.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, this application provides a method for preparing a composite phosphate-based cathode material, comprising the following steps:

[0008] A solid-liquid mixed raw material is prepared by mixing and grinding raw material components including phosphorus source, iron source, manganese source, lithium source and mixed carbon source I with solvent I in one step; wherein, the mixed carbon source I includes high molecular weight carbon source I and low molecular weight carbon source I.

[0009] After the solid-liquid mixture is dried once, it is sintered once under an inert atmosphere to obtain a primary crystallized product.

[0010] The primary crystallization product is subjected to secondary grinding with mixed carbon source II and solvent II to obtain a mixed slurry; wherein, the mixed carbon source II includes high molecular weight carbon source II and low molecular weight carbon source II;

[0011] After the mixed slurry undergoes a secondary drying process, it is then subjected to a secondary sintering process under an inert atmosphere to obtain a composite phosphate-based cathode material; wherein the temperature of the primary sintering process is higher than the temperature of the secondary sintering process.

[0012] In some possible implementations, the amount of mixed carbon source I added is 6% to 7% of the theoretical yield of the composite phosphate-based cathode material;

[0013] In some possible implementations, the amount of the mixed carbon source II added is 7% to 8% of the theoretical yield of the composite phosphate-based cathode material.

[0014] In some possible implementations, the mass ratio of the high molecular weight carbon source I to the low molecular weight carbon source I in the mixed carbon source I is (0.5 to 3):1.

[0015] In some possible implementations, the mass ratio of the high molecular weight carbon source II to the low molecular weight carbon source II in the mixed carbon source II is (0.5 to 3):1.

[0016] In some possible implementations, the polymeric carbon source I and the polymeric carbon source II are each independently selected from at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and carbon black.

[0017] In some possible implementations, the low molecular weight carbon source I and the low molecular weight carbon source II are each independently selected from at least one of glucose, sucrose, fructose, starch, corn sugar, phenolic resin, and oleic acid.

[0018] In some possible implementations, the temperature of the first sintering is 700℃~850℃, and the sintering time is 10~30 hours.

[0019] In some possible implementations, the secondary sintering temperature is 600℃~780℃ and the sintering time is 10~30 hours.

[0020] In some possible implementations, the solid-liquid mixture raw material has a particle size of 0.3 μm to 0.5 μm and a solid content of 50% to 60%.

[0021] In some possible implementations, the mixed slurry has a particle size of 0.5 μm to 1.5 μm and a solid content of 40% to 50%.

[0022] In some possible implementations, the conditions for the primary drying process include: performing a single spray drying at a temperature of 180°C to 250°C and a drying frequency of 300Hz to 360Hz.

[0023] In some possible implementations, the primary drying process is a single spray drying process, wherein the particle size D50 of the single spray drying is 10 μm to 50 μm.

[0024] In some possible implementations, the conditions for the secondary drying process include: performing secondary spray drying at a temperature of 180℃~250℃ and a drying frequency of 300Hz~360Hz.

[0025] In some possible implementations, the secondary drying process employs secondary spray drying, wherein the particle size D50 of the secondary spray drying is 10 μm to 50 μm.

[0026] In some possible implementations, the raw material component may also include a doped metal source.

[0027] In some possible implementations, the doped metal source includes at least one of magnesium, aluminum, cobalt, copper, zinc, molybdenum, niobium, tungsten, titanium, nickel, alum, fluorine, phosphorus, and boron sources.

[0028] In some possible implementations, the particle size D50 of the composite phosphate-based cathode material is 0.5 μm to 1.5 μm.

[0029] In some possible implementations, the carbon material in the composite phosphate-based cathode material has a mass percentage content of 1.5% to 1.8%.

[0030] In some possible implementations, the content of the doped metal element in the composite phosphate-based cathode material is 100ppm to 10000ppm.

[0031] Secondly, this application provides a composite phosphate-based cathode material, which is prepared by the above method and includes an active core and a carbon material coating layer, wherein the active core includes lithium manganese iron phosphate.

[0032] Thirdly, this application relates to the application of a composite phosphate-based cathode material, in which the composite phosphate-based cathode material prepared by the above method or the above-mentioned composite phosphate-based cathode material is applied to cathode sheets and / or secondary batteries.

[0033] The method for preparing composite phosphate-based cathode materials provided in the first aspect of this application includes, on the one hand, a secondary grinding process. This grinding process improves the mixing uniformity of the raw material components and refines their particle size, which is beneficial for subsequent reactions. The secondary grinding process also enables secondary carbon coating, improving the coating effect of the mixed carbon source on the crystalline product and enhancing the compaction density and conductivity of the composite phosphate-based cathode material. On the other hand, a mixed carbon source is used, in which a high-molecular-weight carbon source forms a complex with a low-molecular-weight carbon source. This synergistic effect improves the coating effect on the phosphate-based active material and better controls the particle growth rate of the composite phosphate-based cathode material, thereby increasing its compaction density and conductivity. Furthermore, a secondary sintering process is performed, with the temperature of the primary sintering process being higher than that of the secondary sintering process. The mixed carbon source added during the primary sintering process can participate in the reduction reaction of metals such as manganese and iron, and simultaneously deposit a carbon coating layer on the surface of the primary crystallized phosphate-based active material. The carbon source is added in a secondary manner for coating, thus reducing the carbon source content in the primary sintering. This reduces the risk of excessive reduction of metals and the formation of byproducts. It also reduces the restriction on the growth of primary crystallized phosphate-based active material particles by carbon coating, allowing the phosphate-based active material particles to grow larger and increasing the compaction density and capacity of the composite phosphate-based cathode material. During the secondary sintering process, the degree of carbonization of the carbon material is increased, improving the crystallinity of the phosphate-based active material and forming a uniform and complete carbon coating layer, thereby improving the electrochemical properties of the composite phosphate-based cathode material, such as conductivity and capacity.

[0034] The composite phosphate-based cathode material of this application is prepared using the above-mentioned method. It mainly utilizes the synergistic effect of multiple carbon additions and sintering processes to increase the compaction density of the composite phosphate-based cathode material, especially lithium manganese iron phosphate, by promoting particle growth. Specifically, this is achieved through mixing carbon sources and secondary carbon coating, reducing the surface carbon content of the primary crystallized material, thus enabling the growth of phosphate-based active material particles and achieving a uniform carbon coating effect, thereby improving the electronic conductivity of the composite phosphate-based cathode material. By controlling the secondary sintering process conditions, the growth of primary crystallized particles is further promoted, thereby improving the compaction density, conductivity, capacity, and first-efficiency electrochemical performance of the composite phosphate-based cathode material.

[0035] The composite phosphate-based cathode material provided in this application has high compaction density and high specific capacity, and can be directly applied to cathode sheets or further applied to secondary batteries to improve the energy density, cycle life and other electrochemical performance of cathode sheets and secondary batteries. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a schematic flowchart of the preparation method of the composite phosphate-based cathode material provided in the embodiments of this application;

[0038] Figure 2 This is a SEM image of the composite lithium iron phosphate cathode material provided in Example 1 of this application;

[0039] Figure 3 This is a SEM image of the composite lithium manganese iron phosphate cathode material provided in Comparative Example 3 of this application. Detailed Implementation

[0040] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0041] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0042] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0043] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0044] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0045] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass mentioned in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0046] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0047] The first aspect of this application provides a method for preparing a composite phosphate-based cathode material, as shown in the attached figure. Figure 1 As shown, it includes the following steps:

[0048] S10. The raw material components, including phosphorus source, iron source, manganese source, lithium source and mixed carbon source I, are mixed and ground with solvent I in one step to obtain a solid-liquid mixed raw material; wherein, the mixed carbon source I includes high molecular weight carbon source I and low molecular weight carbon source I;

[0049] S20. After drying the solid-liquid mixture, sinter it under an inert atmosphere to obtain a primary crystallized product.

[0050] S30. The primary crystallization product is subjected to secondary grinding with mixed carbon source II and solvent II to obtain a mixed slurry; wherein, mixed carbon source II includes high molecular weight carbon source II and low molecular weight carbon source II;

[0051] S40. After the mixed slurry is subjected to secondary drying treatment, it is subjected to secondary sintering treatment under an inert atmosphere to obtain a composite phosphate-based cathode material; wherein the temperature of the primary sintering treatment is higher than the temperature of the secondary sintering treatment.

[0052] It should be noted that mixed carbon source I includes high molecular weight carbon source I and low molecular weight carbon source I, and mixed carbon source II includes high molecular weight carbon source II and low molecular weight carbon source II. High molecular weight carbon sources refer to carbon-containing compounds with larger molecular weights, belonging to macromolecular carbon sources, and typically have complex molecular structures and high molecular weights. Low molecular weight carbon sources refer to carbon-containing compounds with smaller molecular weights, typically having simpler molecular structures and lower molecular weights. In the embodiments of this application, the molecular weight of the high molecular weight carbon source is at least higher than that of the low molecular weight carbon source.

[0053] The method for preparing the composite phosphate-based cathode material provided in the first aspect of this application has at least the following beneficial effects:

[0054] On the one hand, a two-stage grinding process is adopted. Firstly, the raw material components, mixed carbon source, and solvent are thoroughly ground to achieve primary carbon coating. This grinding process improves the mixing uniformity of the raw material components and refines their particle size, which is beneficial for subsequent reactions. Secondly, the primary crystallized product is ground a second time with the mixed carbon source and solvent to achieve secondary carbon coating. This grinding process refines the particle size of the primary crystallized product and improves the coating effect of the mixed carbon source on the crystallized product, thereby improving the compaction density and conductivity of the composite phosphate-based cathode material and other electrochemical properties.

[0055] On the other hand, a mixed carbon source is used, in which the high molecular weight carbon source is a macromolecular carbon source, which forms a complex with the low molecular weight carbon source. The synergy between the two is beneficial to improving the coating effect of phosphate-based active materials and can better control the particle growth rate of composite phosphate-based cathode materials, thereby improving the compaction density and conductivity of composite phosphate-based cathode materials.

[0056] On the other hand, a secondary sintering process is performed, with the primary sintering temperature higher than the secondary sintering temperature. The mixed carbon source added during the primary sintering process can participate in the reduction reactions of metals such as manganese and iron, and simultaneously deposit a carbon coating layer on the surface of the primary crystallized phosphate-based active material. In this application, the mixed carbon source is added secondary for coating, thus reducing the carbon source content in the primary sintering. This reduces the risk of excessive metal reduction and the generation of byproducts; it also reduces the restriction on the growth of the primary crystallized phosphate-based active material particles by the carbon coating, allowing the phosphate-based active material particles to grow larger, resulting in a primary crystallized product with a more suitable particle size, and improving the compaction density and capacity of the composite phosphate-based cathode material. During the secondary sintering process, the carbon source not only carbonizes under high temperature conditions, increasing the degree of carbonization and improving the crystallinity of the phosphate-based active material, but also forms a uniform and complete carbon coating layer on the surface of the phosphate-based active material, improving the electrochemical performance of the composite phosphate-based cathode material, such as conductivity and capacity.

[0057] In step S10 above:

[0058] In some possible implementations, the phosphorus source includes at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0059] In some possible implementations, the lithium source includes any two or more lithium salts selected from lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium acetate, and lithium sources.

[0060] In some possible implementations, the iron source includes one or more of ferric nitrate, ferrous sulfate, ferric phosphate, ferrous oxalate, ferric oxide, and iron(II,III) oxide.

[0061] In some possible implementations, the manganese source includes one or more of manganese carbonate, manganese oxalate, manganese sulfate, manganese nitrate, manganese dioxide, and manganese tetroxide.

[0062] In some possible implementations, the raw material components used in the above embodiments of this application all have a purity of ≥99%, and all of the above raw material components have good solubility and dispersion properties, which is beneficial for the preparation of phosphate-based active materials.

[0063] In some possible implementations, the raw material components also include a doped metal source, which can also improve the crystal structure of phosphate-based active materials and enhance the electrochemical properties of composite phosphate-based cathode materials, such as conductivity.

[0064] In some possible implementations, the doping metal source includes at least one of magnesium, aluminum, cobalt, copper, zinc, molybdenum, niobium, tungsten, titanium, nickel, vanadium, fluorine, phosphorus, and boron sources. These doping metal sources can be in the form of nitrates, sulfates, phosphates, carbonates, oxides, hydroxides, etc. Doping with these metal elements can improve the crystal structure of phosphate-based active materials, further enhancing the electrochemical properties of composite phosphate-based cathode materials, such as conductivity.

[0065] In some possible implementations, the amount of mixed carbon source I added is 6% to 7% of the theoretical yield of the composite phosphate-based cathode material. In this embodiment, because the carbon source is added through a secondary sintering coating, the amount of carbon source added during the initial mixing and grinding with the raw material components can be reduced. This reduces the carbon source content in the initial sintering, thereby lowering the risk of excessive metal reduction and the generation of byproducts. Furthermore, it reduces the restriction on the growth of the primary crystallized phosphate-based active material particles by the carbon coating, allowing the phosphate-based active material crystals to grow larger, resulting in a primary crystallized product with a more suitable particle size, and improving the compaction density and capacity of the composite phosphate-based cathode material.

[0066] For example, the amount of mixed carbon source I added can be any point value or a range between any two points, such as 6%, 6.3%, 6.5%, 6.8%, 7%, etc., which are typical but not limiting, for the theoretical yield of composite phosphate-based cathode materials.

[0067] In some possible implementations, the mass ratio of polymeric carbon source I to low-molecular-weight carbon source I in the mixed carbon source I is (0.5–3):1. In this case, the low-molecular-weight carbon source is easily soluble, while the polymeric carbon source has a high decomposition temperature and high carbon content. Using this mixing ratio results in better carbon coating amount and coating effect, as well as better graphitization.

[0068] For example, in the mixed carbon source I, the mass ratio of high molecular weight carbon source I to low molecular weight carbon source I can be any typical but non-limiting point value or an interval between any two point values, such as 0.5:1, 1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 2:1, 2.5:1, 3:1, etc.

[0069] In some possible implementations, the polymeric carbon source I is selected from at least one of: polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and carbon black. The polymeric carbon sources used in the embodiments of this application all have high molecular weights and long carbon chain structures, and after carbonization, they can form two-dimensional linear or three-dimensional sheet-like carbon materials, which have a better coating effect on phosphate-based active crystalline products, thereby improving the electrochemical performance such as conductivity of the composite phosphate-based cathode material. In addition, these polymeric carbon sources can also function as grinding and dispersing agents. PEG carbon sources have large molecular weights, high decomposition degrees, high graphitization degrees, and high viscosity in the molten state. PVP can also act as a dispersant; adding a small amount of PVP can simultaneously serve as a dispersant and a carbon source. Carbon black is a finished carbon product. PVA combines the coating effect of PEG with the dispersing effect of PVP.

[0070] In some possible implementations, the low-molecular-weight carbon source I is selected from at least one of glucose, sucrose, fructose, starch, corn sugar, phenolic resin, and oleic acid. These low-molecular-weight carbon sources used in the embodiments of this application can act as reducing agents in the subsequent primary sintering process, participating in the reduction reactions of metals such as manganese and iron, and also as dispersants, improving the dispersion performance of each raw material component in the solid mixed raw material and assisting in subsequent drying processes such as spray granulation.

[0071] In some possible implementations, solvent I includes organic solvents such as water, ethanol, methanol, ethylene glycol, and isopropanol, or a mixture of water and organic solvents. These solvents exhibit good solubility and dispersion properties for components including phosphorus sources, iron sources, manganese sources, lithium sources, and mixed carbon source I. In some embodiments, the mass percentage of organic solvent in the mixed solvent is 10% to 60%, specifically 10%, 20%, 30%, 40%, 50%, 60%, etc.

[0072] In some possible implementations, the particle size of the solid-liquid mixture is 0.3μm to 0.5μm, and the solid content is 50% to 60%. After a single mixing and grinding process, the particles in the solid mixture are refined to a D50 of 0.3μm to 0.5μm. At the same time, the solid content of the solid-liquid mixture is 50% to 60%, which is beneficial for subsequent drying processes such as spray drying to obtain a one-time spray material with uniform composition and suitable particle size.

[0073] For example, in a solid-liquid mixture, the particle size can be any typical but non-limiting point value or a range between any two points, such as 0.3μm, 0.4μm, or 0.5μm; the solid content can be any typical but non-limiting point value or a range between any two points, such as 50%, 55%, or 60%.

[0074] In step S20 above:

[0075] In some possible implementations, the conditions for a single drying process include: performing a single spray drying at a temperature of 180°C to 250°C and a drying frequency of 300Hz to 360Hz. Under these spray conditions, the solid-liquid mixture is fully and uniformly atomized. For example, the temperature for a single spray drying can be any typical but non-limiting value, such as 180°C, 200°C, 220°C, or 250°C, or a range between any two such values; the frequency can be any typical but non-limiting value, such as 300Hz, 310Hz, 320Hz, 330Hz, 340Hz, or 350Hz, or a range between any two such values.

[0076] In some possible implementations, the primary drying process employs a single spray drying, with a particle size D50 of 10 μm to 50 μm. In the embodiments of this application, after the solid-liquid mixed raw material is dried by spray drying or other methods, the dried product is spherical particles with secondary agglomeration of primary particles. The measured particle size D50 at this point is the particle size of these secondary agglomerated spherical particles. The test results show that obtaining materials with small particle sizes and relatively high uniformity after primary spray drying is beneficial for improving the efficiency of subsequent primary sintering. For example, the particle size D50 of primary spray drying includes typical but not limiting values ​​such as 10 μm, 20 μm, 30 μm, 40 μm, and 50 μm, or an interval between any two values.

[0077] In some possible implementations, the primary sintering temperature is 700℃~850℃, and the sintering time is 10~30 hours. Under these sintering conditions, phosphate-based active material crystals are formed and grown. The primary sintering temperature is slightly higher than the secondary sintering temperature, and the relatively higher sintering temperature is more conducive to the growth of primary crystalline particles. The mixed carbon source added during the primary sintering process can participate in the reduction reaction of metals such as manganese and iron, and can also deposit a carbon coating layer on the surface of the primary crystalline phosphate-based active material. Furthermore, in this embodiment, the mixed carbon source is added in two stages for coating, thus reducing the carbon source content in the primary sintering. This reduces the risk of excessive reduction of metals and the generation of by-products; at the same time, it reduces the restriction of carbon coating on the growth of primary crystalline phosphate-based active material particles, allowing the phosphate-based active material crystals to grow larger, obtaining primary crystalline products with more suitable particle size, and improving the compaction density and capacity of the composite phosphate-based cathode material.

[0078] For example, the sintering temperature can be any typical but non-limiting point value or a range between any two points, such as 700℃, 720℃, 750℃, 800℃, 820℃, 850℃, etc., and the sintering time can be any typical but non-limiting point value or a range between any two points, such as 10 hours, 15 hours, 20 hours, 25 hours, 30 hours, etc.

[0079] In some possible implementations, the phosphate-based active material can be lithium manganese iron phosphate, lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate-based materials doped with other metal elements, lithium iron phosphate-based materials doped with other metal elements, lithium manganese phosphate-based materials doped with other metal elements, etc. Doping with metal elements can further improve the electrochemical performance of the phosphate-based cathode material. The phosphate-based cathode materials in the embodiments of this application have broad selectivity and strong practicality, and can meet different application requirements.

[0080] In step S30 above:

[0081] In some possible implementations, the amount of mixed carbon source II added is 7% to 8% of the theoretical yield of the composite phosphate-based cathode material. The amount of carbon source added during the secondary grinding process in the embodiments of this application, which is mixed with the primary crystallization product, can fully perform secondary carbon coating on the primary crystallization product, improve the carbon coating effect, and thereby improve the electrochemical properties such as conductivity and compaction density of the composite cathode material.

[0082] For example, the amount of mixed carbon source II added can be any point value or a range between any two points, such as 7%, 7.5%, 8%, etc., which are typical but not limiting for the theoretical yield of composite phosphate-based cathode materials.

[0083] In some possible implementations, the mass ratio of polymeric carbon source II to low-molecular-weight carbon source II in the mixed carbon source II is (0.5–3):1. In this case, the low-molecular-weight carbon source is easily soluble, while the polymeric carbon source has a high decomposition temperature and high carbon content. Using this mixing ratio results in better carbon coating amount and coating effect, as well as better graphitization.

[0084] For example, in the mixed carbon source II, the mass ratio of high molecular weight carbon source II to low molecular weight carbon source II can be any typical but non-limiting point value or an interval between any two point values, such as 0.5:1, 1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.8:1, 2:1, 2.5:1, 3:1, etc.

[0085] In some possible implementations, the polymeric carbon source II is selected from at least one of polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and carbon black. The polymeric carbon sources used in the embodiments of this application all have high molecular weights and long carbon chain structures, and after carbonization, they can form two-dimensional linear or three-dimensional sheet-like carbon materials, which have a better coating effect on phosphate-based active crystalline products, thereby improving the electrochemical performance such as conductivity of the composite phosphate-based cathode material.

[0086] In some possible implementations, the low-molecular-weight carbon source II is selected from at least one of glucose, sucrose, fructose, starch, corn sugar, phenolic resin, and oleic acid. These low-molecular-weight carbon sources used in the embodiments of this application can act as reducing agents in the subsequent primary sintering process, participating in the reduction reactions of metals such as manganese and iron, and also as dispersants, improving the dispersion performance of each raw material component in the solid mixed raw material and assisting in subsequent drying processes such as spray granulation.

[0087] In some possible implementations, solvent II includes organic solvents such as water, ethanol, methanol, ethylene glycol, and isopropanol, or a mixture of water and organic solvents. These solvents have high dissolution and dispersion effects on both the primary crystallization product and the mixed carbon source II. In some embodiments, the mass percentage of organic solvent in the mixed solvent is 10% to 60%, specifically 10%, 20%, 30%, 40%, 50%, 60%, etc.

[0088] In some possible implementations, the particle size of the mixed slurry is 0.5μm to 1.5μm, and the solid content is 40% to 50%. After secondary mixing and grinding, the particle size of the mixed slurry is refined to a D50 of 0.5μm to 1.5μm, while the solid content of the mixed slurry is 50% to 60%, which is beneficial for subsequent drying methods such as spray drying to obtain secondary spray material with uniform composition and appropriate particle size.

[0089] For example, in the mixed slurry, the particle size of the material includes any typical but non-limiting point value or an interval between any two point values ​​such as 0.5μm, 0.8μm, 1μm, 1.2μm, and 1.5μm, and the solid content can be any typical but non-limiting point value or an interval between any two point values ​​such as 50%, 55%, and 60%.

[0090] In step S40 above:

[0091] In some possible implementations, the conditions for secondary drying include: secondary spray drying at a temperature of 180°C to 250°C and a drying frequency of 300Hz to 360Hz. Under these spray conditions, the mixed slurry is fully and uniformly atomized. For example, the temperature for secondary spray drying can be any typical but non-limiting value, such as 180°C, 200°C, 220°C, or 250°C, or a range between any two values; the frequency can be any typical but non-limiting value, such as 300Hz, 310Hz, 320Hz, 330Hz, 340Hz, or 350Hz, or a range between any two values.

[0092] In some possible implementations, the secondary drying process employs secondary spray drying, with the particle size D50 of the secondary spray-dried material ranging from 10 μm to 50 μm. In this embodiment, the mixed slurry is processed using a secondary spray drying process. After spray drying and granulation, the dried product consists of spherical particles formed by secondary agglomeration of the particles. The measured particle size D50 at this point represents the particle size of these spherical particles formed by secondary agglomeration. The test results show that obtaining spray-dried material with small particle size and relatively high uniformity after secondary spray drying is beneficial for improving the efficiency of subsequent secondary sintering. For example, the particle size D50 of the secondary drying process includes any typical but non-limiting point value or an interval between any two point values, such as 10 μm, 20 μm, 30 μm, 40 μm, and 50 μm.

[0093] In some possible implementations, the secondary sintering temperature is 600℃~780℃, and the sintering time is 10~30 hours. In this case, the secondary sintering temperature in the embodiments of this application is slightly lower than the primary sintering temperature. During the secondary sintering process, the carbon source not only carbonizes under high temperature conditions, increasing the degree of carbonization of the carbon material and improving the crystallinity of the phosphate-based active material, but also forms a uniform and complete carbon coating layer on the surface of the phosphate-based active material, improving the electrochemical performance of the composite phosphate-based cathode material, such as conductivity and capacity.

[0094] For example, the temperature of the secondary sintering can be any typical but non-limiting point value or a range between any two points, such as 600℃, 620℃, 650℃, 700℃, 750℃, 780℃, etc., and the sintering time can be any typical but non-limiting point value or a range between any two points, such as 10 hours, 15 hours, 20 hours, 25 hours, 30 hours, etc.

[0095] In some possible implementations, the particle size D50 of the composite phosphate-based cathode material is 0.5 μm to 1.5 μm. Under this particle size condition, composite phosphate-based cathode material particles of different sizes can form a gradation, thereby better improving the compaction density and specific capacity of the composite phosphate-based cathode material. For example, the particle size D50 of the composite phosphate-based cathode material can be any typical but non-limiting point value such as 0.5 μm, 1 μm, 1.5 μm, or an interval between any two point values.

[0096] In some possible implementations, the mass percentage of carbon material in the composite phosphate-based cathode material is 1.5% to 1.8%. This content ensures both improved conductivity and stability of the composite phosphate-based cathode material, while also maintaining the content of phosphate-based active materials, thereby ensuring the capacity of the composite phosphate-based cathode material. For example, the mass percentage of carbon material in the composite phosphate-based cathode material can be any typical but non-limiting value, such as 1.5%, 1.6%, 1.7%, or 1.8%, or a range between any two values.

[0097] In some possible implementations, the content of the doped metal element in the composite phosphate-based cathode material is between 100 ppm and 10,000 ppm. At this doping level, the crystal structure of the phosphate-based active material can be effectively improved, further enhancing the electrochemical performance of the composite phosphate-based cathode material, such as conductivity; while also ensuring the structural stability and cycle stability of the composite phosphate-based cathode material. For example, the content of the doped metal element in the composite phosphate-based cathode material can be any typical but non-limiting value, such as 100 ppm, 200 ppm, 300 ppm, 500 ppm, 600 ppm, 800 ppm, or 1000 ppm, or a range between any two values.

[0098] Secondly, embodiments of this application provide a composite phosphate-based cathode material, which is prepared by the above method and includes an active core and a carbon material coating layer, wherein the active core includes lithium manganese iron phosphate.

[0099] The composite phosphate-based cathode material in this application is prepared using the above-described method. It primarily utilizes the synergistic effect of multiple carbon additions and sintering processes to increase the compaction density of the composite phosphate-based cathode material, especially lithium manganese iron phosphate, by promoting particle growth. Specifically, this is achieved through mixing carbon sources and secondary carbon coating, reducing the surface carbon content of the primary crystallized material, thus enabling the growth of active material particles such as lithium manganese iron phosphate, while simultaneously achieving a uniform carbon coating effect and improving the electronic conductivity of the composite phosphate-based cathode material. Furthermore, by doping with metal elements and controlling the secondary sintering process conditions, the growth of the primary crystallized particles is further promoted, thereby improving the compaction density, conductivity, capacity, and initial efficiency of the composite phosphate-based cathode material.

[0100] Thirdly, the present application describes the application of a composite phosphate-based cathode material, in which the composite phosphate-based cathode material prepared by the above method or the above-described composite phosphate-based cathode material is applied to cathode sheets and / or secondary batteries.

[0101] The composite phosphate-based cathode material provided in this application has high compaction density and high specific capacity, and can be directly applied to cathode sheets or further applied to secondary batteries to improve the energy density, cycle life and other electrochemical performance of cathode sheets and secondary batteries.

[0102] In some possible implementations, a positive electrode is provided, comprising a current collector and a positive electrode active layer formed on the surface of the current collector, wherein the positive electrode active layer contains the aforementioned composite phosphate-based positive electrode material. The positive electrode of this application, due to the use of the aforementioned composite phosphate-based positive electrode material in the positive electrode active layer, improves the energy density, cycle life, and other electrochemical performance of the positive electrode.

[0103] In some possible implementations, the preparation of the positive electrode active layer includes the following steps: mixing the above-mentioned composite phosphate-based positive electrode material, conductive agent and binder to form an electrode slurry, coating the electrode slurry onto the current collector, and then preparing the positive electrode sheet through steps such as drying, rolling and die cutting.

[0104] In some possible implementations, the mass percentage of the composite phosphate-based cathode material in the cathode active layer of the cathode sheet is 90% to 95%. Specifically, the mass percentage of the composite phosphate-based cathode material in the cathode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0105] In some possible implementations, the current collector of the positive electrode includes, but is not limited to, any one of copper foil or aluminum foil.

[0106] In some possible implementations, the binder content in the positive electrode active material layer is 2wt% to 5wt%. In specific embodiments, the binder content can be typical but not limited to 2wt%, 3wt%, 4wt%, 5wt%, etc.

[0107] In some possible implementations, the binder includes one or more of the following: polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0108] In some possible implementations, the conductive agent content in the positive electrode active material layer is 1 wt% to 5 wt%. In specific embodiments, the conductive agent content can be a typical but not limited content such as 3 wt%, 4 wt%, or 5 wt%.

[0109] In some possible implementations, the conductive agent includes graphite, carbon black, acetylene black, graphene, carbon fiber, and C. 60 And one or more of carbon nanotubes.

[0110] In some possible implementations, a secondary battery is provided, which includes the aforementioned positive electrode. The secondary battery provided in this application, due to including the aforementioned positive electrode with excellent electrochemical performance such as high energy density and long cycle life, improves the energy density, cycle stability, and other electrochemical performance of the secondary battery.

[0111] This application does not specifically limit the negative electrode, electrolyte, separator, etc. in the secondary battery of the embodiments, and can be applied to any battery system.

[0112] In some possible implementations, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (e.g., coke), and hard carbon, or nitrides, tin-based oxides, tin alloys, and nano-anode materials. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0113] In some possible implementations, the steps for making the negative electrode sheet include: mixing the negative electrode active material with conductive agents such as conductive carbon black, binders such as carboxymethyl cellulose and styrene-butadiene rubber, and solvents such as water in a mass ratio of (80-99):(1-5):(2-10):100 to form a negative electrode slurry, then degassing under vacuum, discharging the material, coating it on a coating machine, and obtaining the negative electrode sheet after rolling, slitting, and die-cutting.

[0114] In some possible implementations, the membrane is capable of blocking electrons while allowing ions to pass through. Exemplary membranes include, but are not limited to, at least one material selected from polypropylene fibers, polyacrylonitrile fibers, polyvinyl formal fibers, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fibers, and poly(p-phenylene terephthalamide).

[0115] In some possible implementations, the electrolyte comprises at least one soluble metal salt. In some specific embodiments, the metal salt includes LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 At least one of SO2(N)[m, n], where m and n are natural numbers. These electrolytic salts can ensure high ionic conductivity of the electrolyte and do not undergo harmful side reactions with electrode materials, electrolyte, diaphragm, etc., and have good chemical stability.

[0116] In some possible implementations, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0117] In some possible implementations, the battery cell types include lithium-ion batteries, as well as novel batteries such as lithium-air batteries and lithium metal batteries.

[0118] In some possible implementations, the battery cells of this application can be assembled into a battery module. The battery module can contain multiple battery cells, the specific number of which can be adjusted according to the application and capacity of the battery module. Furthermore, the battery module may also include a housing with a receiving space in which multiple battery cells are received.

[0119] In one possible implementation, battery cells and / or battery modules can also be assembled into a battery pack, and the number of battery cells or battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0120] To enable those skilled in the art to clearly understand the above-described implementation details and operations, and to demonstrate the significant advancements in the performance of the composite phosphate-based cathode material, its preparation method, and its application in the embodiments of this application, the following examples illustrate the above technical solutions.

[0121] Example 1

[0122] A composite lithium iron phosphate cathode material, the preparation of which includes the following steps:

[0123] 1. According to the molar ratio of 4:6:2.15:6, iron phosphate, manganese nitrate, lithium carbonate, and lithium dihydrogen phosphate are mixed and set aside. Then, according to the theoretical yield of composite manganese iron lithium phosphate, glucose, PVP, TiO2, and NiCO3 are weighed in a mass ratio of 3.87%, 3%, and 4000 ppm respectively. PVP is added to the first liquid (50% ethylene glycol and 50% deionized water), and the mixture is stirred thoroughly for 30 minutes. Then, glucose, phosphorus source, iron source, manganese source, lithium source, and dopant elements are added. According to the solid content of 40% to 50%, the first liquid (50% ethylene glycol and 50% deionized water) is continued to be added to prepare a sand milling slurry. The slurry is transferred to a sand mill for sand milling, and the particle size of the sand mill output is controlled between 0.4 and 0.5 μm to obtain the first solid-liquid mixture.

[0124] 2. The solid content of the first solid-liquid slurry was adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying was tested. The particle size D50 of the secondary particles was controlled between 10 and 20μm. The sintering was carried out in a tube furnace at 800℃ for 20 hours, while nitrogen was introduced at a gas flow rate of 400ml / min. The slurry was then crushed by an air jet mill at 50Hz to obtain the primary crystallization product.

[0125] 3. After pulverizing the primary crystallization product, add it to the first solution (50% ethylene glycol and 50% deionized water), along with 3% glucose and 4.26% polyethylene glycol (PEG). After thorough mixing, add it to the first liquid (50% ethylene glycol and 50% deionized water) with a solid content of 40%–50% to prepare a sand milling slurry. Transfer the slurry to a sand mill and mill it, controlling the output particle size to be between 0.78 and 0.8 μm to obtain the second solid-liquid mixture.

[0126] 4. The solid content of the obtained second solid-liquid slurry is adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying is tested. The particle size D50 of the secondary particles is controlled between 10 and 20μm. The sintering is carried out in a tube furnace at 720℃ for 10 hours, while nitrogen is introduced at a gas flow rate of 400ml / min. Then, the sintering is carried out by an air jet mill at 55Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0127] Example 2

[0128] A composite lithium iron phosphate cathode material, the preparation of which includes the following steps:

[0129] 1. According to the molar ratio of 4:6:2.15:6, ferrous oxalate, manganese oxalate, lithium carbonate, and lithium dihydrogen phosphate were mixed and set aside. Then, according to the theoretical yield of composite lithium manganese iron phosphate, glucose, PVP, TiO2, and NiCO3 were weighed in a mass ratio of 3.87%, 3%, and 4000 ppm respectively. PVP was added to the first liquid (50% ethylene glycol and 50% deionized water), and the mixture was stirred thoroughly for 30 minutes. Then, glucose, phosphorus source, iron source, manganese source, lithium source, and dopant elements were added. According to the solid content of 40% to 50%, the first liquid (50% ethylene glycol and 50% deionized water) was continued to be added to prepare a sand milling slurry. The slurry was transferred to a sand mill and sand milled, and the particle size of the sand mill output was controlled between 0.4 and 0.5 μm to obtain the first solid-liquid mixture.

[0130] 2. The solid content of the first solid-liquid slurry was adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying was tested. The particle size D50 of the secondary particles was controlled between 10 and 20μm. The sintering was carried out in a tube furnace at 800℃ for 20 hours, while nitrogen was introduced at a gas flow rate of 400ml / min. The slurry was then crushed by an air jet mill at 50Hz to obtain the primary crystallization product.

[0131] 3. After pulverizing the primary crystallization product, add it to the first solution (50% ethylene glycol and 50% deionized water), along with 3% glucose and 4.26% PEG. After thorough mixing, add it to the first liquid (50% ethylene glycol and 50% deionized water) with a solid content of 40%–50% to prepare a sand milling slurry. Transfer the slurry to a sand mill and mill it, controlling the output particle size to be between 0.78 and 0.8 μm to obtain the second solid-liquid mixture.

[0132] 4. The solid content of the obtained second solid-liquid slurry is adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying is tested. The particle size D50 of the secondary particles is controlled between 10 and 20μm. The sintering is carried out in a tube furnace at 720℃ for 10 hours, while nitrogen is introduced at a gas flow rate of 400ml / min. Then, the sintering is carried out by an air jet mill at 55Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0133] Example 3

[0134] 1. Using the same preparation method as in Example 1, a first solid-liquid mixture was obtained. The first crystallized product was then obtained using the same sintering method as in Example 1.

[0135] 2. The pulverized product from the first crystallization is added to the first solution (50% ethylene glycol and 50% deionized water), along with 1% glucose and 6.83% PEG. After thorough mixing, the product is added to the first liquid (50% ethylene glycol and 50% deionized water) at a solid content of 40%–50% to prepare a sand milling slurry. This slurry is then transferred to a sand mill for sand milling, and the particle size of the milled material is controlled to be between 0.78 and 0.8 μm to obtain the second solid-liquid mixture.

[0136] 3. The solid content of the obtained second solid-liquid slurry is adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying is tested. The particle size D50 of the secondary particles is controlled between 10 and 20μm. The particles are sintered in a tube furnace at 720℃ for 10 hours, while nitrogen is introduced at a gas flow rate of 400ml / min. The particles are then crushed by an air jet mill at 55Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0137] Example 4

[0138] 1. Using the same preparation method as in Example 1, a first solid-liquid mixture was obtained. The first crystallized product was then obtained using the same sintering method as in Example 1.

[0139] 2. The pulverized product from the first crystallization is added to the first solution (50% ethylene glycol and 50% deionized water), along with 4.74% glucose and 2% PEG. After thorough mixing, the product is added to the first liquid (50% ethylene glycol and 50% deionized water) with a solid content of 40%–50% to prepare a sand milling slurry. This slurry is then transferred to a sand mill for sand milling, and the particle size of the milled material is controlled to be between 0.78 and 0.8 μm to obtain the second solid-liquid mixture.

[0140] 3. The solid content of the obtained second solid-liquid slurry is adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying is tested. The particle size D50 of the secondary particles is controlled between 10 and 20μm. The particles are sintered in a tube furnace at 720℃ for 10 hours, while nitrogen is introduced at a gas flow rate of 400ml / min. The particles are then crushed by an air jet mill at 55Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0141] Comparative Example 1

[0142] A composite lithium iron phosphate cathode material, the preparation of which includes the following steps:

[0143] 1. According to the molar ratio of 4:6:2.15:6, iron phosphate, manganese nitrate, lithium carbonate, and lithium dihydrogen phosphate are mixed and set aside. Then, according to the theoretical yield of composite manganese iron lithium phosphate, glucose, PVP, and PEG are weighed in a mass ratio of 6.87%, 3%, and 4.26%. PVP and PEG are added to the first liquid (50% ethylene glycol and 50% deionized water), and the mixture is stirred thoroughly for 30 minutes. Then, glucose, phosphorus source, iron source, manganese source, and lithium source are added. According to the solid content of 40% to 50%, the first liquid (50% ethylene glycol and 50% deionized water) is continued to be added to prepare a sand mill slurry. The slurry is transferred to a sand mill and sand milled, and the particle size of the sand mill output is controlled between 0.4 and 0.5 μm to obtain the first solid-liquid mixture.

[0144] 2. The solid content of the first solid-liquid slurry was adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying was tested. The particle size D50 of the secondary particles was controlled between 10 and 20μm. The sintering was carried out in a tube furnace at 800℃ for 20 hours, while nitrogen was introduced at a gas flow rate of 400ml / min. The sintering was then carried out by an air jet mill at 50Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0145] Comparative Example 2

[0146] A composite lithium iron phosphate cathode material, the preparation of which includes the following steps:

[0147] 1. According to the molar ratio of 4:6:2.15:6, iron phosphate, manganese nitrate, lithium carbonate, and lithium dihydrogen phosphate are mixed and set aside. Then, according to the theoretical yield of composite manganese iron lithium phosphate, glucose, PVP, TiO2, and NiCO3 are weighed in a mass ratio of 3.87%, 3%, and 4000 ppm respectively. PVP is added to the first liquid (50% ethylene glycol and 50% deionized water), and the mixture is stirred thoroughly for 30 minutes. Then, glucose, phosphorus source, iron source, manganese source, lithium source, and dopant elements are added. According to the solid content of 40% to 50%, the first liquid (50% ethylene glycol and 50% deionized water) is continued to be added to prepare a sand milling slurry. The slurry is transferred to a sand mill for sand milling, and the particle size of the sand mill output is controlled between 0.4 and 0.5 μm to obtain the first solid-liquid mixture.

[0148] 2. The solid content of the first solid-liquid slurry was adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying was tested. The particle size D50 of the secondary particles was controlled between 10 and 20μm. The sintering was carried out in a tube furnace at 800℃ for 20 hours, while nitrogen was introduced at a gas flow rate of 400ml / min. The slurry was then crushed by an air jet mill at 50Hz to obtain the primary crystallization product.

[0149] 3. After crushing the primary crystallization product, add it to the first solution (50% ethylene glycol and 50% deionized water), and simultaneously add 9% glucose. After thorough mixing, add it to the first liquid (50% ethylene glycol and 50% deionized water) according to a solid content of 40% to 50% to prepare a sand milling slurry. Transfer the slurry to a sand mill and mill it, controlling the output particle size of the sand milling to be between 0.78 and 0.8 μm to obtain the second solid-liquid mixture.

[0150] 4. The solid content of the obtained second solid-liquid slurry is adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying is tested. The particle size D50 of the secondary particles is controlled between 10 and 20μm. The sintering is carried out in a tube furnace at 720℃ for 10 hours, while nitrogen is introduced at a gas flow rate of 400ml / min. Then, the sintering is carried out by an air jet mill at 55Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0151] Comparative Example 3

[0152] A composite lithium iron phosphate cathode material, the preparation of which includes the following steps:

[0153] 1. The first sand milling was carried out under the same process conditions as in Example 1; the first sintering temperature was 700℃ for 20 hours, and other process conditions were the same.

[0154] 2. The pulverized product from the first crystallization is added to the first solution (50% ethylene glycol and 50% deionized water), along with 3% glucose and 4.26% PEG. After thorough mixing, the product is added to the first liquid (50% ethylene glycol and 50% deionized water) at a solid content of 40%–50% to prepare a sand milling slurry. This slurry is then transferred to a sand mill for sand milling, and the particle size of the milled material is controlled to be between 0.78 and 0.8 μm to obtain the second solid-liquid mixture.

[0155] 3. The solid content of the obtained second solid-liquid slurry is adjusted to 50%. After spray drying at a feed temperature of 200℃ and a drying frequency of 340Hz, the particle size of the secondary particles after drying is tested. The particle size D50 of the secondary particles is controlled between 10 and 20μm. The particles are sintered in a tube furnace at 720℃ for 10 hours, while nitrogen is introduced at a gas flow rate of 400ml / min. The particles are then crushed by an air jet mill at 55Hz to obtain the finished composite lithium manganese iron phosphate cathode material.

[0156] Furthermore, to verify the progressiveness of the embodiments of this application, the following performance tests were performed on the above embodiments and comparative examples:

[0157] 1. The morphology of the composite lithium manganese iron phosphate cathode materials prepared in Example 1 and Comparative Example 3 was observed using scanning electron microscopy. The test results for Example 1 are attached. Figure 2 The SEM image shows the test results for Comparative Example 3. Figure 3 The SEM images are shown. It can be seen that the finished product of this application contains a large number of small particles with good sphericity, as well as large particles with a blocky structure, forming a dense packing. This structure is beneficial for the finished product to have high compaction and good electrochemical performance. In contrast, the film of Comparative Example 3 is dominated by large particles, and the particle gradation effect is poor.

[0158] 2. The compaction density, particle size D50, carbon content, and doped metal element content of the composite lithium manganese iron phosphate cathode materials prepared in the above embodiments and comparative examples were tested respectively.

[0159] ① The compaction density was tested using a UTM7305 electronic testing press;

[0160] ② The particle size D50 was tested using a Malvern 3000 laser particle size analyzer;

[0161] ③ The carbon content was tested using a high-frequency carbon-sulfur analyzer.

[0162] The test results are shown in Table 1 below:

[0163] Table 1

[0164]

[0165] As can be seen from the above test results, the composite lithium manganese iron phosphate cathode materials prepared in the embodiments of this application all have high compaction density. With the overall carbon content remaining unchanged, carbon coating through two grinding and two sintering processes can better improve the conductivity of the composite lithium manganese iron phosphate. Furthermore, the above test results show that the following aspects can be considered to improve the compaction of lithium manganese iron phosphate: Firstly, the addition of polymeric carbon sources such as PEG polymers reduces the polarity of water to a certain extent and forms a uniform and smooth carbon precursor coating at the microscale during the sand milling slurry stage. This facilitates the formation of a uniform carbon coating on the lithium iron phosphate surface. When the material is subjected to pressure, the carbon coating layer acts as a lubricant, making it easier for small particles to fill the pores and improving the overall compaction of the material. Secondly, high-temperature single sintering and low carbon coating conditions promote the growth of crystal particles (due to the addition of manganese in lithium manganese iron phosphate, ...).

[0166] They are difficult to grow, so special, unconventional methods are needed.

[0167] 3. The composite phosphate-based cathode materials prepared in the above embodiments and comparative examples are applied to lithium-ion batteries, and their electrochemical performance is tested. The preparation method of the lithium-ion battery includes the following steps:

[0168] ① The preparation of the positive electrode is as follows: The composite phosphate-based positive electrode material prepared according to the above embodiment or comparative example by mass ratio: SP:PVDF:NMP=93.5:2.5:4:100 is mixed evenly in a ball mill for 2 hours to obtain a positive electrode slurry. The prepared positive electrode slurry is added to aluminum foil and evenly scraped with a scraper. After the positive electrode is dried at 130℃, it is rolled under a pressure of 10Mpa to obtain a rolled electrode. A Φ15mm round piece is cut from the middle area and weighed.

[0169] ② The battery assembly process is as follows: The prepared positive electrode is attached to the positive electrode metal shell with conductive adhesive, a lithium metal sheet is used as the electrode, a Celgard 2400 microporous membrane is used as the separator, and a 1.0 mol / L LiPF6 solution is used as the electrolyte. The solvent of the electrolyte is a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) in a volume ratio of 1:1:1. The battery is assembled into a button cell in a glove box.

[0170] The electrochemical performance of the above coin cells was tested using a LAND electrochemical tester. The charging termination voltage was 3.75V and the discharge cutoff voltage was 2.0V.

[0171] The lithium-ion batteries using the composite phosphate-based cathode materials prepared in the above embodiments and comparative examples were subjected to charge-discharge tests at 0.1C and 1C rates at room temperature (25°C).

[0172] The test results are shown in Table 2 below:

[0173] Table 2

[0174]

[0175] As can be seen from the above test results, the embodiments of this application reduce the carbon content on the surface of the primary crystallized material by controlling the processes of mixed carbon source, secondary carbon coating, and secondary sintering, thereby achieving particle growth and uniform carbon coating. This not only improves the compaction density of the prepared composite lithium manganese iron phosphate cathode material, but also does not affect the electrochemical performance of the cathode material. When the prepared composite lithium manganese iron phosphate cathode material is applied to lithium-ion batteries, the cathode material can simultaneously exhibit good charge and discharge specific capacity, resulting in better application performance.

[0176] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a composite phosphate-based cathode material, characterized in that, Includes the following steps: A solid-liquid mixed raw material is prepared by mixing and grinding raw material components including phosphorus source, iron source, manganese source, lithium source and mixed carbon source I with solvent I in one step; wherein, the mixed carbon source I includes high molecular weight carbon source I and low molecular weight carbon source I; the mass ratio of high molecular weight carbon source I to low molecular weight carbon source I in the mixed carbon source I is (0.5~3):

1. After the solid-liquid mixture is dried once, it is sintered once under an inert atmosphere to obtain a primary crystallized product. The primary crystallization product is subjected to secondary grinding with mixed carbon source II and solvent II to obtain a mixed slurry; wherein, the mixed carbon source II includes high molecular weight carbon source II and low molecular weight carbon source II; the mass ratio of the high molecular weight carbon source II to the low molecular weight carbon source II in the mixed carbon source II is (0.5~3):1; After the mixed slurry undergoes a secondary drying process, it is then subjected to a secondary sintering process under an inert atmosphere to obtain a composite phosphate-based cathode material. The temperature of the primary sintering process is higher than that of the secondary sintering process. The primary sintering temperature is 800℃~850℃, and the sintering time is 10~30 hours. The secondary sintering temperature is 600℃~780℃, and the sintering time is 10~30 hours.

2. The method for preparing the composite phosphate-based cathode material as described in claim 1, characterized in that, The raw material components also include a doped metal source; And / or, the amount of mixed carbon source I added is 6% to 7% of the theoretical yield of the composite phosphate-based cathode material; And / or, the amount of mixed carbon source II added is 7% to 8% of the theoretical yield of the composite phosphate-based cathode material.

3. The method for preparing the composite phosphate-based cathode material as described in claim 2, characterized in that, The doped metal source includes at least one of magnesium source, aluminum source, cobalt source, copper source, zinc source, molybdenum source, niobium source, tungsten source, titanium source, nickel source, alum source, fluorine source, phosphorus source, and boron source; And / or, in the composite phosphate-based cathode material, the content of doped metal elements is 100ppm to 10000ppm.

4. The method for preparing the composite phosphate-based cathode material according to any one of claims 1 to 3, characterized in that, The polymeric carbon source I and the polymeric carbon source II are each independently selected from at least one of the following: polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and carbon black; And / or, the low molecular weight carbon source I and the low molecular weight carbon source II are each independently selected from at least one of glucose, sucrose, fructose, starch, corn sugar, phenolic resin, and oleic acid.

5. The method for preparing the composite phosphate-based cathode material as described in claim 4, characterized in that, The solid-liquid mixture raw material has a particle size of 0.3μm to 0.5μm and a solid content of 50% to 60%. And / or, in the mixed slurry, the particle size of the material is 0.5μm to 1.5μm, and the solid content is 40% to 50%.

6. The method for preparing the composite phosphate-based cathode material as described in claim 5, characterized in that, The conditions for the primary drying process include: spray drying at a temperature of 180℃~250℃ and a drying frequency of 300Hz~360Hz. And / or, the primary drying process is a single spray drying, wherein the particle size D50 of the single spray drying is 10 μm to 50 μm; And / or, the conditions for the secondary drying process include: secondary spray drying at a temperature of 180℃~250℃ and a drying frequency of 300Hz~360Hz; And / or, the secondary drying process employs secondary spray drying, wherein the particle size D50 of the secondary spray drying is 10μm to 50μm.

7. The method for preparing the composite phosphate-based cathode material according to any one of claims 1 to 3, 5 or 6, characterized in that, The particle size D50 of the composite phosphate-based cathode material is 0.5 μm to 1.5 μm; And / or, in the composite phosphate-based cathode material, the mass percentage of carbon material is 1.5% to 1.8%.

8. A composite phosphate-based cathode material, characterized in that, The composite phosphate-based cathode material is prepared by the method described in any one of claims 1 to 7, comprising an active core and a carbon material coating layer, wherein the active core comprises lithium manganese iron phosphate.

9. An application of a composite phosphate-based cathode material, characterized in that, The composite phosphate-based cathode material prepared by any one of claims 1 to 7 or the composite phosphate-based cathode material as described in claim 8 is applied to cathode sheets and / or secondary batteries.

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