Positive electrode material and its manufacturing method, secondary battery

Abstract

The present application provides a positive electrode material, a method for producing the same, and a secondary battery. [Solution] The positive electrode material includes a lithium iron phosphate active material and a coating layer, the lithium iron phosphate active material includes a plurality of secondary particles, a plurality of first primary particles are distributed among at least some of the secondary particles, the coating layer includes a first coating layer coated on the surface of the secondary particles, the first coating layer is coated on the surface of the secondary particles in a surface-coated state and has a network structure. The surface of the secondary particles has a coating layer that is surface-coated and has a network structure, which can improve the capacity, rate performance and cycle performance of the positive electrode material, and the present application provides a method for matching the first primary particles with the secondary particles, thereby improving the press density of the positive electrode material, reducing the aggregation of the positive electrode material particles, and improving the capacity and rate performance of the positive electrode material.

Description

[Technical field]

[0001] The present application relates to the technical field of secondary batteries, and in particular to a positive electrode material and a method for producing the same, and a secondary battery. [Background technology]

[0002] In recent years, with the development of technology and the promotion of the concept of "carbon neutralization", the demand for new energy is growing rapidly. Among them, lithium ion batteries have the advantages of high energy density, no memory effect, long cycle life, etc., and are widely used in consumer products, power batteries, energy storage and other industries. Common lithium ion battery positive electrode materials mainly include lithium iron phosphate, ternary materials and lithium cobalt oxide.

[0003] Lithium iron phosphate has attracted wide attention since its proposal in 1997, and occupies a place in the positive electrode material of lithium ion batteries due to its excellent stability, high rate charging, non-toxicity, and long cycle life properties. However, lithium iron phosphate also has significant defects such as small tap density and press density, low electrical conductivity, and low ion diffusivity, which restrict the further widespread application of lithium iron phosphate in the positive electrode material.

[0004] Therefore, how to prepare lithium iron phosphate materials with high dynamic performance, high capacity and excellent cycle performance is the technical focus in this field. Summary of the Invention [Problem to be solved by the invention]

[0005] In order to overcome the above drawbacks, the present application provides a positive electrode material capable of improving the capacity, rate performance, cycle performance, and low-temperature performance of the positive electrode material, a manufacturing method thereof, and a secondary battery. [Means for solving the problem]

[0006] In a first aspect, an embodiment of the present application provides a positive electrode material comprising a lithium iron phosphate active material and a coating layer, the lithium iron phosphate active material comprising a plurality of secondary particles, with a plurality of first primary particles distributed among at least some of the secondary particles, the coating layer comprising a first coating layer coated on surfaces of the secondary particles, the first coating layer coating the surfaces of the secondary particles in a surface-coating state, and having a network structure.

[0007] In a second aspect, the present embodiment comprises: Mixing an iron source and a microgel to obtain a first precursor, the first precursor including a microgel and an iron source distributed inside and outside the microgel, the microgel including a carbon chain polymer; and mixing the first precursor, a phosphorus source, a lithium source, and a carbon source, and then performing a heat treatment to obtain a positive electrode material.

[0008] In a third aspect, embodiments of the present application provide a secondary battery comprising either the cathode material described in the first aspect or the cathode material produced by the method described in the second aspect. Effect of the Invention

[0009] The present technical solution has at least the following technical effects compared with the prior art: the positive electrode material of the present application has a first coating layer on the surface of the secondary particles in a surface-coated state, and the first coating layer has a network structure, the surface-coated first coating layer can reduce the electrolyte from entering the inside of the positive electrode material, inhibit the electrolyte from eroding the positive electrode material, reduce the occurrence of side reactions between the electrolyte and the active material of the positive electrode material, and improve the initial efficiency and capacity of the positive electrode material, the network structure first coating layer can avoid the problem of ion transport inhibition caused by the coating layer formed in the conventional coating method being too thick, the network coating layer forms a conductive network between the secondary particles, improves the conductivity of the positive electrode material, and is favorable for reducing the contact resistance between particles and the electrode polarization, and the network coating layer is distributed on the surface of the secondary particles to provide sufficient ion transport channels for the charge / discharge process of the positive electrode material, and can improve the lithium desorption / insertion ability of the positive electrode material, thereby improving the rate performance and cycle performance of the positive electrode material. The plurality of first primary particles distributed among the secondary particles can improve the specific surface area of ​​the positive electrode material, shorten the lithium ion transport distance, and improve the dynamic performance of the positive electrode material. Figure 1 shows a schematic structural diagram of the positive electrode material of the present application, and as can be seen from Figure 1, the positive electrode material of the present application includes a plurality of secondary particles and a plurality of first primary particles distributed among the secondary particles, and the matching between the first primary particles and the secondary particles can improve the press density of the positive electrode material, and improve the capacity, rate performance, and cycle performance of the positive electrode material. [Brief description of the drawings]

[0010] The present application will now be further described with reference to the figures and examples. [Figure 1] FIG. 1 is a schematic diagram showing the structure of a positive electrode material according to the present application. [Diagram 2] 1 is a flowchart for producing a positive electrode material according to the present application. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In order to better understand the technical solution of the present application, the embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0012] It should be clear that the described embodiments are only some of the embodiments of the present application, but not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by a person skilled in the art without creative labor shall fall within the scope of protection of the present application.

[0013] The terms used in the examples of this application are merely for the purpose of describing particular examples and are not intended to limit the present application. As used in the examples of this application and the appended claims, the singular forms "a," "one," "the," and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0014] It should be understood that the term "and / or" used in this specification is merely a relational relationship describing related objects, and indicates that three relations may exist, for example, A and / or B may indicate three situations: A exists alone, A and B exist simultaneously, and B exists alone. Also, the character " / " in the text generally indicates that the related objects before and after it are in an "or" relationship.

[0015] As a positive electrode material, lithium iron phosphate has the advantages of high rate charging, long cycle life and high stability. Compared with other positive electrode materials, lithium iron phosphate has poor dynamic performance and weak electrical conductivity, which limits the output characteristics of lithium ion batteries supported by lithium iron phosphate. To address this issue, the particle size of lithium iron phosphate can be reduced by nano-sizing to shorten the transportation distance of lithium ions, thereby improving the problem. However, nano-sizing lithium iron phosphate greatly increases the difficulty of manufacturing the positive electrode material, and nano-sizing lithium iron phosphate has poor dispersion performance and is difficult to improve the press density. As the degree of nano-sizing lithium iron phosphate increases, the specific surface area of ​​lithium iron phosphate increases, and the side reactions on the surface increase, which reduces the electrochemical performance and service life of the lithium iron phosphate positive electrode material applied to the battery.

[0016] In view of this, an embodiment of the present application provides a positive electrode material comprising a lithium iron phosphate active material and a coating layer, wherein the lithium iron phosphate active material comprises a plurality of secondary particles, with a plurality of first primary particles distributed among at least some of the secondary particles, and the coating layer comprises a first coating layer provided so as to coat the surfaces of the secondary particles, the first coating layer coats the surfaces of the secondary particles in a surface-coating state, and the first coating layer has a mesh-like structure.

[0017] In the above technical solution, the positive electrode material of the present application has a first coating layer on the surface of the secondary particles in a surface-coated state, and the first coating layer has a network structure; the first coating layer coated on the surface can reduce the electrolyte from entering the inside of the positive electrode material, inhibit the electrolyte from corroding the positive electrode material, and reduce the occurrence of side reactions between the electrolyte and the active material of the positive electrode material, thereby improving the initial efficiency and capacity of the positive electrode material; the first coating layer with a network structure can avoid the problem of ion transport inhibition caused by the coating layer formed in the conventional coating method being too thick; the network coating layer forms a conductive network between the secondary particles, improves the conductivity of the positive electrode material, and is favorable for reducing the contact resistance between particles and electrode polarization; the network coating layer is distributed on the surface of the secondary particles, provides sufficient ion transport channels for the charge and discharge process of the positive electrode material, and can improve the lithium desorption / insertion ability of the positive electrode material, thereby improving the rate performance and cycle performance of the positive electrode material. The plurality of first primary particles distributed among the secondary particles can improve the specific surface area of ​​the positive electrode material, shorten the transport distance of lithium ions, and improve the dynamic performance of the positive electrode material. Figure 1 shows a schematic structural diagram of the positive electrode material of the present application, and as can be seen from Figure 1, the positive electrode material of the present application includes a plurality of large-sized secondary particles and a plurality of small-sized first primary particles distributed among the secondary particles, and the matching between the first primary particles and the secondary particles can improve the press density of the positive electrode material, and improve the capacity, rate performance, and cycle performance of the positive electrode material.

[0018] In the present application, the first coating layer is coated on the surface of the secondary particles in a surface-coating state, and the first coating layer has a mesh-like structure; as will be understood, the first coating layer can completely coat the surface of the secondary particles, and the outer surface of the first coating layer has a vein structure like a leaf, and the outer surface of the first coating layer has a surface portion like the epidermis of a leaf and a vein portion (protrusion portion) like the veins of a leaf, with the surface portion being thin and the vein portion being thick, and the vein portions being connected to each other to present a mesh-like structure.

[0019] In some embodiments, the secondary particles include a plurality of second primary particles, and the secondary particles are particles formed by agglomeration of the second primary particles, and preferably, the secondary particles are aggregates formed by agglomeration of the second primary particles. As can be understood, a plurality of second primary particles are agglomerated to form secondary particles, and the surface of the secondary particles has a first coating layer with a network structure, and the first coating layer can provide sufficient ion transport paths, and the median diameter of the second primary particles is smaller than the median diameter of the first primary particles, and the lithium ion diffusion distance of the second primary particles having a smaller particle size is shorter than that of the first primary particles, which can improve the lithium ion transport efficiency of the second primary particles, improve the electrical conductivity, and further improve the rate performance of the positive electrode material; and the second primary particles having a smaller particle size can increase the solid-liquid transport interface between the positive electrode material and the electrolyte, thereby improving the dynamic performance and low-temperature performance of the positive electrode material.

[0020] As will be understood, the median diameter (D50) indicates the particle diameter corresponding to when the cumulative particle size distribution percentage of one sample reaches 50%. In some embodiments, the median diameter of secondary particles can be measured by a method such as dynamic light scattering, and the median diameter of primary particles can be measured by performing microtopography by a method such as scanning electron microscope (SEM) or transmission electron microscope (TEM), and the particle size distribution can be statistically calculated from the captured image.

[0021] As can be understood, the first primary particles and the second primary particles are both crystals grown from a crystal nucleus, i.e., single crystal particles, and their internal structures have perfect lattices. The only difference between the first primary particles and the second primary particles is their location and particle size.

[0022] In some embodiments, the median diameter of the first primary particles is 100 nm to 800 nm, and specifically may be 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, or 800 nm, etc., and within the above range, the median diameter of the first primary particles is small, which is advantageous for improving the press density and dispersion performance of the positive electrode material, and can improve the capacity of the positive electrode material.

[0023] In some embodiments, the median diameter of the second primary particles is 50 nm to 700 nm, specifically, it may be 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, or 700 nm, etc., and within the above range, the particle size of the second primary particles is small, which contributes to the insertion and desorption of lithium ions during the charge and discharge process of the positive electrode material, and can improve the rate performance of the positive electrode material. If the median diameter of the second primary particles is smaller than 50 nm, the aggregation of the second primary particles is serious, which is disadvantageous for improving the cycle performance of the positive electrode material, and also reduces the dispersion performance when the positive electrode material is prepared into a slurry, which is disadvantageous for improving the processing performance of the application of the positive electrode material to a secondary battery. If the median diameter of the second primary particles is larger than 700 nm, the diffusion length of the lithium ions is long, which is disadvantageous for improving the dynamic performance of the positive electrode material.

[0024] In some embodiments, the median diameter of the secondary particles is 400 nm to 2000 nm, and specifically may be 400 nm, 600 nm, 800 nm, 1000 nm, 1300 nm, 1500 nm, 1800 nm, 2000 nm, etc., and within the above range, the particle size of the secondary particles is appropriate, which can reduce side reactions of the positive electrode material, improve the lithium ion transport performance, and improve the overall electrochemical performance of the positive electrode material.

[0025] In some embodiments, the coating layer further comprises a second coating layer at least partially distributed on the surface of the first primary particle. That is, in this embodiment, the surface of the secondary particle and the first primary particle are both provided with a coating layer, the existence of the second coating layer can effectively block the contact between the positive electrode material active material and air and electrolyte, and reduce side reactions, and the median diameter of the first primary particle is small, so that the coating layer coated on the surface of the first primary particle has a large specific surface area, which is favorable for improving the cycle life of the positive electrode material.

[0026] In some embodiments, the first coating layer includes a carbon layer, and the carbon material in the carbon layer may be soft carbon, hard carbon, amorphous carbon, or the like.

[0027] In some embodiments, the second coating layer includes a carbon layer, and the carbon material in the carbon layer may be soft carbon, hard carbon, amorphous carbon, or the like.

[0028] In some embodiments, the average thickness of the first coating layer is greater than that of the second coating layer, the secondary particles have a large particle size, and the first coating layer covering the secondary particles has a large average thickness, which on the one hand ensures effective isolation between the active material of the positive electrode material and the electrolyte and reduces the occurrence of side reactions, and on the other hand, the thickness of the vein portion of the first coating layer having a large average thickness is favorable for the network coating layers on the surfaces of the multiple secondary particles to come into contact with each other to form a conductive network, thereby improving the rate performance of the positive electrode material. The second coating layer covering the first primary particles has a small average thickness, which is favorable for shortening the diffusion distance between the lithium ions and the first primary particles, thereby reducing the difficulty of desorbing / inserting lithium from / to the positive electrode material, thereby improving the cycle performance and rate performance of the positive electrode material.

[0029] In some embodiments, the average thickness of the first coating layer is 1 nm to 15 nm, and may be, for example, 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 12 nm, or 15 nm. Within the above range, the average thickness of the first coating layer is appropriate, and the thickness of the vein portion of the first coating layer is thick, thereby realizing effective coating of the secondary particles, and the thickness of the surface portion is thin, thereby shortening the diffusion length of lithium ions, allowing the positive electrode material to quickly desorb / insert lithium during the charge / discharge process, and improving the kinetic performance of the positive electrode material. As will be understood, the average thickness of the first coating layer is the average value of measuring at least 10 sets of maximum thicknesses of the first coating layer and at least 10 sets of minimum thicknesses of the first coating layer, and as will be understood, the maximum thickness and minimum thickness of the first coating layer can be measured by the following method, that is, after cutting the positive electrode material, in the image shown in a scanning electron microscope (SEM) of the obtained cross section, the maximum thickness of the first coating layer is the thickness of the vein portion (protruding portion) of the network structure of the first coating layer, and the minimum thickness of the first coating layer is the thickness of the vein portion (recessed portion) of the network structure of the first coating layer.

[0030] Preferably, the maximum thickness of the first coating layer is greater than the average thickness of the second coating layer, i.e., the thickness of the protruding portion of the network structure of the first coating layer is thick, which can ensure the stability of the structure of the positive electrode material, suppress the occurrence of side reactions on the surface of the positive electrode material, and improve the conductivity of the positive electrode material. The minimum thickness of the first coating layer is smaller than the thickness of the second coating layer, i.e., the thickness of the recessed portion of the network structure of the second coating layer is thin, which is favorable to reducing the difficulty of lithium desorption / insertion of secondary particles, and can improve the dynamic performance of the positive electrode material.

[0031] In some embodiments, the average thickness of the second coating layer is 1 nm to 10 nm, specifically, it may be 1 nm, 3 nm, 5 nm, 8 nm, or 10 nm, etc., and within the above range, the second coating layer can completely cover the surface of the first primary particles, effectively cover the first primary particles, and shorten the diffusion length of lithium ions, allowing the positive electrode material to quickly detach / insert lithium during charging / discharging, and improving the dynamic performance of the positive electrode material. As can be understood, the second coating layer is uniformly coated on the surface of the first primary particles, the positive electrode material is cut, and the cross section obtained by cutting is measured in the diagram shown by a scanning electron microscope (SEM), and the average thickness of the second coating layer refers to the arithmetic average value of the thickness of the second coating layer at at least 10 sets of different positions.

[0032] In some embodiments, the mass ratio of the first coating layer in the positive electrode material is 0.3wt%-2.5wt%, specifically, it may be 0.3wt%, 0.6wt%, 0.9wt%, 1.5wt%, 1.8wt%, 2.2wt% or 2.5wt%, etc., and of course, it may be other values ​​within the above range, and the present application is not limited thereto. Within the above specified range, a coating layer of appropriate thickness can be produced, and the cycle performance, capacity and rate performance of the positive electrode material can be improved.

[0033] In some embodiments, the mass ratio of the second coating layer in the positive electrode material is 0.1wt%-2.0wt%, specifically, it may be 0.1wt%, 0.5wt%, 1wt%, 1.2wt%, 1.6wt%, 1.8wt%, or 2.0wt%, etc., and of course, it may be other values ​​within the above range, and the present application is not limited thereto. Within the above specified range, a coating layer of appropriate thickness can be produced, and the cycle performance, capacity and rate performance of the positive electrode material can be improved.

[0034] In some embodiments, the specific surface area of ​​the positive electrode material is 7 m 2 / g~15m 2 / g, specifically, 7m 2 / g, 8m 2 / g, 9m2 / g, 10m 2 / g, 11m 2 / g, 12m 2 / g, 13m 2 / g, 14m 2 / g or 15m 2 / g, and controlling the specific surface area of ​​the positive electrode material within the above range is advantageous in improving the cycle performance and safety performance of a secondary battery produced from the positive electrode material.

[0035] In some embodiments, the press density of the cathode material is 2.3 g / cm 3 ~2.9g / cm 3 Specifically, 2.3 g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 or 2.9 g / cm 3 etc., and within the above range, the positive electrode material of the present application has a high press density, which is advantageous for improving the capacity of the positive electrode material.

[0036] In some embodiments, the median diameter of the positive electrode material is 0.3 μm to 2.5 μm, and specifically may be 0.3 μm, 0.8 μm, 1.2 μm, 1.8 μm, 2.2 μm, or 2.5 μm, or may be other values ​​within the above range, and is not limited thereto.

[0037] In some embodiments, the mass ratio of the secondary particles to the first primary particles is 1:50 to 20:1, and may be, specifically, 1:50, 5:40, 10:30, 20:25, 40:8, or 50:1, etc., and being within the above range indicates that the positive electrode material of the present application has an appropriate number of secondary particles and first primary particles, and particle size matching deposition is performed between the secondary particles and the first primary particles, which is advantageous for improving the press density of the positive electrode material.

[0038] The present application further provides a method for producing the above-mentioned positive electrode material, and FIG. 2 shows a flowchart for producing the positive electrode material of the present application, Mixing an iron source and a microgel to obtain a first precursor, the first precursor including a microgel and an iron source distributed inside and outside the microgel, the microgel including a carbon chain polymer; and mixing the first precursor, the phosphorus source, the lithium source and the carbon source, and then performing a heat treatment to obtain a positive electrode material.

[0039] In the above technical solution, the present application premixes an iron source with a microgel, the microgel being a micro- or nano-level gel particle and a polymer microparticle having an intramolecular cross-linked structure, the microgel has ion adsorption properties, and by mixing with the iron source, a part of the iron source can be adsorbed inside the microgel, that is, a part of the iron source in the first precursor is distributed inside the microgel, and a part of the iron source is distributed outside the microgel, and the first precursor is mixed with a phosphorus source, a lithium source, and a carbon source, and then heat-treated, the microgel can provide two different reaction environments for the in-situ synthesis of lithium iron phosphate, and the iron source and the lithium source located outside the microgel react in-situ to generate lithium iron phosphate single crystal particles, that is, the first primary particles, and at the same time, the carbon source is coated on the surface of the first primary particles during the heat-treatment process to form a second coating layer. The iron source and the lithium source located inside the microgel can also react in-situ to generate lithium iron phosphate, but due to the limited growth environment and limited diffusion inside the microgel, the growth of lithium iron phosphate is inhibited, and the lithium iron phosphate inside the microgel tends to generate small second primary particles, and the second primary particles are easily aggregated to form secondary particles. At the same time, the microgel containing the carbon source and the carbon chain polymer is carbonized during the heat treatment process to form a first coating layer on the surface of the lithium iron phosphate, and the microgel with an intramolecular cross-linked structure is located on the surface of the first coating layer after carbonization, so that the surface of the first coating layer has a net structure. The net structure of the first coating layer forms a conductive network between the secondary particles, which is advantageous for improving the conductivity of the positive electrode material and reducing the contact resistance between particles and electrode polarization. The net-like coating layer is distributed on the surface of the secondary particles, which provides sufficient ion transport channels for the positive electrode material during the charge and discharge process, improves the lithium desorption / insertion ability of the positive electrode material, and improves the rate performance and cycle performance of the positive electrode material.The present application provides a method for producing lithium iron phosphate by pre-mixing an iron source with a microgel, which is a simple process and easy to obtain raw materials. In the process of forming lithium iron phosphate in-situ, the iron source forms lithium iron phosphate with two different growth environments, i.e., secondary particles and first primary particles, thereby improving the press density and gram capacity of the positive electrode material, and improving the cycle performance, capacity and rate performance of the positive electrode material.

[0040] The manufacturing method of the present invention will now be described in detail.

[0041] In step S100, an iron source and a microgel are mixed to obtain a first precursor, the first precursor including a microgel and an iron source distributed inside and outside the microgel, and the microgel including a carbon chain polymer.

[0042] In some embodiments, the mass ratio of the cross-linking monomer in the microgel is 0.1%-10%, specifically 0.1%, 0.5%, 1%, 3%, 5%, 8% or 10%, etc., and it can be understood that the microgel is obtained by copolymerizing the cross-linking monomer with the cross-linking agent, and the mass ratio of the cross-linking monomer affects the adsorption performance of the microgel. As long as it is within the above specified range, the microgel has a suitable cross-linking degree and can adsorb a suitable amount of the iron source into the inside of the microgel, and the remaining iron source is located outside the microgel, which further provides two different synthetic environments for lithium iron phosphate materials. In the inside of the microgel, the limited environment tends to generate second primary particles, and at the same time, the second primary particles are prone to aggregate and form secondary particles, and outside the microgel, the first primary particles tend to generate, and the cooperation between the secondary particles and the first primary particles can improve the press density, electrical conductivity and lithium insertion degree of the positive electrode material, which is favorable to improve the cycle performance, rate performance and capacity of the positive electrode material. If the mass ratio of the cross-linking monomer in the microgel is too small, the microgel is difficult to form a stable restricted space, which is disadvantageous to obtaining two different lithium iron phosphate materials.If the mass ratio of the cross-linking monomer in the microgel is too large, the iron source is difficult to diffuse into the microgel, which results in too little raw material inside the microgel, which is disadvantageous to producing lithium iron phosphate secondary particles.

[0043] In some embodiments, the particle size of the microgel is 300 nm to 2500 nm, specifically, it may be 300 nm, 500 nm, 800 nm, 1200 nm, 1600 nm, 2000 nm, 2200 nm, or 2500 nm, etc., and within the above specified range, it can provide a certain space for embedding the iron source inside the microgel, which is advantageous for obtaining two different lithium iron phosphate materials. If the particle size of the microgel is too large, it becomes difficult for the lithium iron phosphate raw material to diffuse inside the microgel, which is disadvantageous for the formation of dense secondary particles, and if the particle size of the microgel is too small, the spatial region by the microgel is too small, which makes it difficult to form secondary particles.

[0044] In some embodiments, the microgel is a polymer microparticle having an intramolecular crosslinked structure, and the polymer in the microgel comprises a carbon-containing polymer, and for example, the microgel comprises at least one of polyacrylic acid-based microgel, polystyrene-based microgel and polyacrylamide-based microgel. The microgel can not only provide two different reaction environments in the reaction process of lithium iron phosphate, but also form a net-like coating layer after carbonization after heat treatment, which can avoid the problem of ion transport inhibition caused by the coating layer formed in the conventional coating method being too thick; the net-like coating layer can provide sufficient ion transport passages and improve the lithium desorption / insertion ability of the positive electrode material, thereby improving the rate performance and cycle performance of the positive electrode material; and the net-like coating layer can form a conductive network between secondary particles, which is advantageous for improving the conductivity of the positive electrode material and reducing the contact resistance between particles and electrode polarization.

[0045] In some embodiments, the mass ratio of the iron source to the microgel is 100:0.01 to 100:1.36, and may be, for example, 100:0.01, 100:0.05, 100:0.1, 100:0.3, 100:0.8, 100:1.0, 100:1.25, or 100:1.36. In a swollen state, the density of the microgel is much smaller than that of an inorganic material such as an iron source, and therefore the added mass of the microgel is much smaller than the added mass of the iron source, thereby allowing the microgel to provide a suitable volume of production environment for the iron source.

[0046] In some embodiments, the iron source comprises a soluble iron source, the soluble iron source comprising at least one of iron nitrate, iron chloride, iron sulfate, and iron oxalate.

[0047] In some embodiments, the iron source is present in the form of a solution, which is advantageous in that the solution has good fluidity and a portion of the iron source is adsorbed inside the microgel, and the microgel is in a swollen state and provides a production environment of an appropriate volume for the iron source.

[0048] In some embodiments, the solution concentration of the iron source is 1 mol / L or more, and may be, for example, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, or 7 mol / L.

[0049] In some embodiments, mixing of the iron source with the microgel is carried out under stirring conditions, which favors uniform dispersion of the iron source.

[0050] In step S200, performing a heat treatment after mixing the first precursor, the phosphorus source, the lithium source and the carbon source includes the following steps.

[0051] In step S201, a phosphorus source is added to the first precursor, and a reaction is carried out under conditions of 60° C. to 90° C. to obtain a precipitate, which is then subjected to solid-liquid separation, washing and drying to obtain a second precursor.

[0052] In the above step, a phosphorus source is added to the first precursor, and a part of the phosphorus source enters the inside of the microgel, and the phosphorus source reacts with the iron source located inside the microgel and the iron source located outside the microgel at 60°C to 90°C to generate iron phosphate precipitate.

[0053] In some embodiments, the phosphorus source comprises at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and sodium phosphate.

[0054] In some embodiments, the phosphorus source is added to the first precursor in the form of a solution, thereby favoring a thorough mixed reaction between the phosphorus source and the iron source.

[0055] In some embodiments, the method of solid-liquid separation includes at least one of filtration and suction filtration.

[0056] In some embodiments, the cleaning solvent comprises at least one of deionized water and an alcoholic solvent, the alcoholic solvent including ethanol, propanol, and the like.

[0057] In some embodiments, the drying temperature is 50°C to 150°C, and specifically may be 50°C, 60°C, 80°C, 90°C, 110°C, 130°C, or 150°C, or the like.

[0058] In some embodiments, the drying time is 1 h to 6 h, and specifically may be 1 h, 2 h, 3 h, 4 h, 5 h, or 6 h, etc.

[0059] In step S202, a lithium source and a carbon source are added to the second precursor, and the second precursor is heated at 400° C. to 700° C. for 5 h to 8 h.

[0060] In this step, a lithium source and a carbon source are added to the second precursor and heated at 400°C to 700°C. The lithium source can react with the iron phosphate generated in step S201 to generate lithium iron phosphate. Due to the presence of the microgel, the microgel can provide two different reaction environments, some lithium iron phosphate is generated inside the microgel, and some lithium iron phosphate is generated outside the microgel. The growth of lithium iron phosphate inside the microgel is restricted, and lithium iron phosphate particles with small particle size tend to be generated. The lithium iron phosphate particles aggregate to generate secondary particles, and the growth of lithium iron phosphate located outside the microgel is not restricted, so lithium iron phosphate primary particles tend to be generated. The addition of the carbon source serves as a reducing agent for the second precursor and the lithium source, on the one hand, and the carbon source can be carbonized under heating conditions to coat the surfaces of the primary particles and secondary particles, and at the same time, the microgel can also be carbonized under heating conditions to generate a coating layer with a network structure, forming a first coating layer on the surface of the secondary particles. As can be understood, the first coating layer is obtained by coating the surface of the secondary particles with both the carbon source and the microgel, and the microgel is located on the surface of the first coating layer to form a network structure, and the presence of the network structure can reduce the electrolyte from entering the inside of the positive electrode material to cause side reactions, suppress the electrolyte from eroding the positive electrode material, and improve the initial efficiency and capacity of the positive electrode material. It can also avoid the problem of ion transport inhibition caused by the coating layer formed in the conventional coating method being too thick, and the network-like coating layer can provide sufficient ion transport passages and improve the lithium desorption / insertion ability of the positive electrode material, and the network-like coating layer can form a conductive network between the secondary particles, improve the conductivity of the positive electrode material, and is favorable for reducing the contact resistance between particles and the electrode polarization. The second coating layer is obtained by coating the surface of the primary particles with the carbon source, and the second coating layer can be coated on the surface of the primary particles to avoid direct contact between the primary particles in the second coating layer and the electrolyte, improve the conductivity of the primary particles, and improve the rate performance of the positive electrode material.

[0061] In some embodiments, the carbon source comprises at least one of sucrose, glucose, and polyethylene glycol. As will be understood, the carbon source of the present application is a reducing carbon source, and specifically may be any one of sucrose, glucose, and polyethylene glycol, or may be a mixture of polyethylene glycol and sucrose, a mixture of polyethylene glycol and glucose, a mixture of polyethylene glycol and sucrose, or a combination of polyethylene glycol and glucose.

[0062] In some embodiments, the lithium source is at least one of lithium carbonate, lithium phosphate, lithium hydroxide, and lithium chloride.

[0063] In some embodiments, the mass ratio of the iron source, the phosphorus source, and the lithium source is 1:(1-4):(1-1.5), and may specifically be 1:1:1, 1:2:1.2, 1:3:1, or 1:4:1.5, etc.

[0064] In some embodiments, the ratio of the iron source to the carbon source is 1 mol:(10-100)g, specifically, 1 mol:10g, 1 mol:30g, 1 mol:50g, 1 mol:70g, 1 mol:90g or 1 mol:100g. Within the above range, a coating layer of appropriate thickness can be formed on the surface of the finally produced lithium iron phosphate, a dense carbon coating can be provided, and it is favorable for the insertion and desorption of lithium ions during the charge and discharge process of the lithium iron phosphate material, and the overall electrochemical performance of the positive electrode material can be improved. If the amount of carbon source added is too large, the coating layer will be too thick, which will increase the internal resistance of the material and reduce the total amount of active material. If the amount of carbon source added is too small, the carbon coating will be non-uniform, which will affect the conductivity of the positive electrode material.

[0065] In some embodiments, after adding the lithium source and the carbon source to the second precursor, the method further comprises polishing the second precursor to which the lithium source and the carbon source have been added before heating, which favors the production of lithium iron phosphate with a small particle size and improves the dispersibility and electrical conductivity of the lithium iron phosphate.

[0066] In some embodiments, the mode of grinding includes ball milling or sand milling.

[0067] In some embodiments, the heating temperature is 400° C. to 700° C., and may be specifically 400° C., 450° C., 500° C., 560° C., 610° C., 650° C., 680° C., or 700° C., and is not specifically limited to the numerical values ​​listed, and other numerical values ​​not listed within the numerical ranges apply as well. If the heating temperature is less than 400° C., the crystallinity of the resulting lithium iron phosphate and the pressing performance of the powder are inferior.

[0068] In some embodiments, step S202 further includes adding a dispersion solvent during the process of adding the lithium source and the carbon source to the second precursor, which is advantageous for uniformly dispersing the lithium source, the carbon source and the first precursor to obtain high-quality lithium iron phosphate and a coating layer that is uniformly distributed on the surface of the lithium iron phosphate.

[0069] In some embodiments, the dispersant comprises at least one of deionized water and an alcohol solvent.

[0070] In some embodiments, the heating is performed in a protective gas atmosphere comprising at least one of nitrogen gas, argon gas, helium gas, and neon gas.

[0071] In some embodiments, the heating rate is 1° C. / min to 10° C. / min, and may be specifically 1° C. / min, 2° C. / min, 3° C. / min, 4° C. / min, 5° C. / min, 6° C. / min, 7° C. / min, 8° C. / min, 9° C. / min, or 10° C. / min, etc., and is not specifically limited to the recited numerical values, and other unrecited numerical values ​​within the numerical range also apply.

[0072] An embodiment of the present application further provides a secondary battery, such as a lithium ion battery or a sodium ion battery, where the battery includes a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet.

[0073] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder, and the positive electrode active material is the lithium iron phosphate positive electrode material produced by the above-mentioned production method.

[0074] The positive electrode current collector may be an Al foil, or other positive electrode current collectors commonly used in this field.

[0075] The conductive agent of the positive electrode sheet may include at least one of conductive carbon black, layered graphite, graphene, or carbon nanotubes.

[0076] The binder in the positive electrode sheet may contain at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethylcellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0077] The negative electrode sheet includes a current collector and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, a conductive agent, and a binder.

[0078] The current collector of the negative electrode sheet may include at least one of copper foil, aluminum foil, nickel foil, or a carbon-fluorine current collector.

[0079] The negative electrode active material may include at least one of soft carbon, hard carbon, artificial graphite, natural graphite, amorphous carbon, a silicon-oxygen compound, and lithium titanate.

[0080] The binder may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polyamideimide, polysiloxane, polystyrene butadiene rubber, epoxy resin, polyester resin, polyurethane resin, and polyfluorene.

[0081] The conductive agent may include at least one of conductive carbon black, ketjen black, acetylene black, carbon nanotubes, VGCF (Vapor Grown Carbon Fiber), or graphene.

[0082] In some embodiments, the separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene comprises at least one selected from high density polyethylene, low density polyethylene, or ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene have excellent short circuit prevention effects and can improve the stability of the battery by the shutdown effect. In some embodiments, the thickness of the separator is within a range of about 5 μm to 500 μm.

[0083] In some embodiments, the lithium ion battery may further include an electrolyte. In some examples, the electrolyte may include, but is not limited to, at least two of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), and propyl propionate (PP). The electrolyte may also include at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), and a dinitrile compound as an electrolyte additive. In some examples, the electrolyte may further include a lithium salt.

[0084] In some embodiments, a positive electrode sheet, a separator, and a negative electrode sheet are wound or laminated in this order to form an electrode member, which is then packaged, for example, in an aluminum plastic film, and an electrolyte is injected, chemically formed, and packaged to produce a battery. The produced battery is then subjected to a performance test and a cycle test. EXAMPLES

[0085] The following are exemplary, but non-limiting, examples of the present application.

[0086] Example 1

[0087] The method for producing the lithium iron phosphate cathode material includes the following steps.

[0088] (1) Add 404 g of Fe(NO) to 1 L of deionized water. 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 h to obtain a first precursor, the microgel being polyacrylic acid microgel, the particle size of the microgel being 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel being 2%.

[0089] (2) 68.3 mL of H 3 PO 4The solution (85 wt%) was added dropwise to the first precursor solution in step (1) and stirred for 20 min. The resulting mixed solution was then placed in a water bath at 90 °C for 3 h and heated. The resulting white precipitate was filtered, washed first with deionized water and ethanol, and then dried at 100 °C for 4 h to obtain the second precursor, FePO 4 2H 2 O crystals were obtained.

[0090] (3)FePO 4 2H 2 O crystal and LiOH H 2 O and polyethylene glycol (PEG), where FePO 4 2H 2 O crystals were used as the iron and phosphorus sources, and LiOH H 2 O was used as the lithium source, polyethylene glycol (PEG) was used as the carbon source and reducing agent, and LiOH H 2 O and FePO 4 2H 2 The molar ratio of FePO to O is 1:1, and the amount of polyethylene glycol used is FePO 4 2H 2 There are 40 g of polyethylene glycol per mole of O. 4 2H 2 O, LiOH H 2 A small amount of deionized water was dropped onto the mixture of O and PEG, which was then polished and heated at 650 °C for 6 h to obtain the positive electrode material.

[0091] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0092] Example 2

[0093] The differences from Example 1 are: (1) 404 g of Fe(NO 3 )3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 0.5%.

[0094] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0095] Example 3

[0096] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 5%.

[0097] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0098] Example 4

[0099] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 8%.

[0100] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0101] Example 5

[0102] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 300 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 2%.

[0103] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0104] Example 6

[0105] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1000 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 2%.

[0106] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0107] Example 7

[0108] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 1.2120 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 2500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 2%.

[0109] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0110] Example 8

[0111] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 0.6060 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 2%.

[0112] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0113] Example 9

[0114] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 2.7270 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 2%.

[0115] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0116] Example 10

[0117] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 Add O and stir to completely dissolve, then add 5.4944 g of microgel and disperse for 2 hours to obtain a first precursor, in which the microgel is polyacrylic acid microgel, the particle size of the microgel is 1500 nm, and the mass ratio of the acrylic acid crosslinking monomer in the microgel is 2%.

[0118] In this embodiment, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes a plurality of secondary particles and primary particles located between the secondary particles, the surfaces of the secondary particles have a first coating layer, the first coating layer is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coating state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, the second coating layer is a carbon layer.

[0119] Comparative Example 1

[0120] The differences from Example 1 are: (1) 404 g of Fe(NO 3 ) 3 9H 2 O was added and completely dissolved by stirring to obtain the first precursor.

[0121] In this comparative example, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes primary particles, and the surfaces of the primary particles have a carbon coating layer.

[0122] Comparative Example 2

[0123] The method for producing the lithium iron phosphate cathode material includes the following steps.

[0124] (1) Add 202 g of Fe(NO) to 1 L of deionized water. 3 ) 39H 2 Add 34.1 mL of HO and stir to completely dissolve the iron nitrate solution. 3 PO 4 The solution (85 wt%) was added dropwise to the iron nitrate solution and stirred for 20 min. The resulting mixed solution was then placed in a water bath at 90 °C and heated for 3 h. The resulting white precipitate was filtered, washed first with deionized water and ethanol, and then dried at 100 °C for 4 h to obtain FePO 4 2H 2 O crystals were obtained. 4 2H 2 O crystal, LiOH·H 2 O and polyethylene glycol (PEG), where FePO 4 2H 2 O crystals were used as the iron and phosphorus sources, and LiOH H 2 O was used as the lithium source, polyethylene glycol (PEG) was used as the carbon source and reducing agent, and LiOH H 2 O and FePO 4 2H 2 The molar ratio of O to FePO was 1:1, and the amount of polyethylene glycol used was 1:1. 4 2H 2 There are 40 g of polyethylene glycol per mole of O. 4 2H 2 O, LiOH H 2 A small amount of deionized water was dropped onto the mixture of O and PEG, polished, and heated at 650°C for 6 h to obtain the first precursor.

[0125] (2) 202 g of Fe(NO) in 1 L of deionized water 3 ) 3 9H 2 Add 34.1 mL of HO and stir to completely dissolve the iron nitrate solution. 3 PO 4 The solution (85 wt%) was added dropwise to the iron nitrate solution and stirred for 20 min. The resulting mixed solution was then placed in a water bath at 90 °C and heated for 6 h. The resulting white precipitate was filtered, washed first with deionized water and ethanol, and then dried at 100 °C for 4 h to obtain FePO 4 2H 2 O crystals were obtained. 42H 2 O crystal, LiOH·H 2 O and polyethylene glycol (PEG), where FePO 4 2H 2 O crystals were used as the iron and phosphorus sources, and LiOH H 2 O was used as the lithium source, polyethylene glycol (PEG) was used as the carbon source and reducing agent, and LiOH H 2 O and FePO 4 2H 2 The molar ratio of O to FePO was 1:1, and the amount of polyethylene glycol used was 1:1. 4 2H 2 There are 40 g of polyethylene glycol per mole of O. 4 2H 2 O, LiOH H 2 A small amount of deionized water was dropped onto the mixture of O and PEG, polished, and heated at 650 °C for 12 h to obtain the second precursor.

[0126] (3) The first precursor and the second precursor were directly mixed to obtain a positive electrode material.

[0127] In this comparative example, the chemical composition of the positive electrode material is LiFePO 4 / C, the positive electrode material includes primary particles having two different particle size distributions, i.e., primary particles a and primary particles b, and the surfaces of the primary particles having the two different particle sizes both have a coating layer, which is a carbon layer.

[0128] Performance test:

[0129] The lithium iron phosphate materials obtained in each of the Examples and Comparative Examples were subjected to the following performance tests.

[0130] (1) Press density test: A powder press density test (based on GB / T 24533-2009) was carried out on the lithium iron phosphate materials obtained in each of the examples and comparative examples.

[0131] (2) The surface morphology of the sample, the size of the positive electrode material particles, the size of the first primary particles, the size of the second primary particles, the thickness of the first coating layer, and the thickness of the second coating layer were observed using a Hitachi S4800 scanning electron microscope, and the average thickness of the first coating layer and the average thickness of the second coating layer were calculated based on the measurement results.

[0132] (3) The specific surface area of ​​the materials was measured using a Micromeritics Tristar 3020.

[0133] (4) Electrochemical performance test: The lithium iron phosphate material obtained in each example and comparative example was used as the positive electrode active material, and the positive electrode active material: conductive carbon black: polyvinylidene fluoride (PVDF) was mixed in a mass ratio of 90:5:5, and N-methylpyrrolidone (NMP) was used as a solvent to form a slurry, which was then applied to an aluminum foil and vacuum dried at 90 ° C to obtain a positive electrode sheet, which was then assembled into a coin-type half cell. (The lithium salt of the electrolyte was 1 mol / L LiPF6, and the solvent was a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1. The obtained battery was subjected to a charge-discharge test in an environment of 25 ± 2 ° C., the charge-discharge voltage range was 3.7 to 3.5 V, the current was 0.1 C, and the test data was recorded. The test results are shown in Tables 1 and 2.

[0134] Table 1 shows the parameters of the positive electrode materials produced in each of the examples and comparative examples.

[0135] [Table 1]

[0136] Table 2 shows the performance parameters of the positive electrode materials produced in each of the examples and comparative examples.

[0137] [Table 2]

[0138] The positive electrode materials produced in Examples 1 to 10 can provide two different reaction environments for the production of lithium iron phosphate by adding microgel to the iron source in advance, thereby obtaining a plurality of secondary particles and primary particles distributed among the secondary particles, and the surfaces of the secondary particles are covered with a coating layer having a network structure, thereby improving the capacity, rate performance and cycle performance of the positive electrode material. The present application can improve the press density of the positive electrode material, reduce the aggregation of the positive electrode material particles, and improve the capacity and rate performance of the positive electrode material by matching the first primary particles with the secondary particles.

[0139] In Examples 1 to 4, the mass ratio of the cross-linking monomer in the microgel is different, and the cross-linking degree of the microgel is different. With the increase in the cross-linking degree of the microgel, the swelling degree of the microgel in the solution decreases, and the internal space decreases. At the same time, the transport speed of the reaction raw materials in the microgel is suppressed, the particle size of the precursor grown into the microgel is further reduced, and the particle size of the second primary particles is reduced, which is favorable to shortening the transport distance of lithium ions in the solid phase, and can improve the dynamic properties of the positive electrode material.

[0140] In Examples 1 and 5 to 7, as the particle size of the microgel increases, the volume of the secondary particles that can be accommodated inside the microgel increases, the particle size of the formed secondary particles increases, and due to the influence of the transportation of the reaction raw materials, the particle size of the second primary particles becomes slightly smaller, and the above particle size change contributes to some extent to the improvement of the press density of the positive electrode material.

[0141] In Examples 1 and 8 to 10, the amount of microgel added was different, and as the amount of microgel added increased, the proportion of secondary particles increased, and the press density tended to increase and then decrease.

[0142] The positive electrode material prepared in Comparative Example 1 contains only primary particles and a carbon layer coated on the surface thereof. Comparing Comparative Example 1 with Example 1, the positive electrode material of Comparative Example 1 has a low press density and a large specific surface area, which results in many side reactions on the surface. This shortens the service life when applied to a lithium iron phosphate secondary battery, and makes it difficult to meet the requirement for higher energy density for a battery of the same volume.

[0143] The positive electrode material prepared in Comparative Example 2 includes large-sized lithium iron phosphate particles and small-sized lithium iron phosphate particles. Compared with Example 1, Comparative Example 2 needs to first prepare large-sized lithium iron phosphate particles and small-sized lithium iron phosphate particles, and then mix them together. This makes the operation procedure complicated, and the solid-phase mixing effect is poor, and the mixing time is long, making it difficult to improve the production efficiency. In addition, the large-sized particles used in Comparative Example 2 have a longer lithium ion solid-phase transport path and poor dynamic performance compared to the secondary particles of the present application, which is unfavorable for high-rate scene applications.

[0144] The present application describes the detailed process equipment and process flow of the present application through the above embodiment, but the present application is not limited to the above detailed process equipment and process flow, that is, it does not mean that the present application must be implemented depending on the above detailed process equipment and process flow. Those skilled in the art should understand that any improvement to the present application, equivalent replacement of each raw material of the product of the present application and addition of auxiliary components, selection of specific methods, etc. are all within the protection scope and disclosure scope of the present application.

Claims

1. A positive electrode material comprising a lithium iron phosphate active material and a coating layer, the lithium iron phosphate active material includes a plurality of secondary particles, and a plurality of first primary particles are distributed among at least some of the secondary particles; a first coating layer coated on a surface of the secondary particle, the first coating layer being a carbon layer, the first coating layer being coated on the surface of the secondary particle in a surface-coating state, and the first coating layer having a mesh-like structure.

2. 2. The positive electrode material according to claim 1, wherein the secondary particles include a plurality of second primary particles, and a median diameter of the second primary particles is smaller than a median diameter of the first primary particles.

3. The positive electrode material according to claim 2, characterized in that the first primary particles have a median diameter of 100 nm to 800 nm, and / or the second primary particles have a median diameter of 50 nm to 700 nm, and / or the secondary particles have a median diameter of 400 nm to 2000 nm.

4. 2. The positive electrode material according to claim 1, wherein the coating layer further includes a second coating layer at least partially distributed on a surface of the first primary particles, and an average thickness of the first coating layer is greater than an average thickness of the second coating layer.

5. The cathode material according to claim 4, characterized in that the second coating layer is a carbon layer, and / or the average thickness of the first coating layer is from 1 nm to 15 nm, and / or the average thickness of the second coating layer is from 1 nm to 10 nm.

6. The positive electrode material according to claim 4, characterized in that a mass ratio of the first coating layer in the positive electrode material is 0.3 wt % to 2.5 wt %, and / or a mass ratio of the second coating layer in the positive electrode material is 0.1 wt % to 2.0 wt %.

7. The specific surface area of ​​the positive electrode material is 7 m 2 / g to 15m 2 / g, and / or the press density of the positive electrode material is 2.3 g / cm 3 ~2.9g / cm 3 and / or the median diameter of the positive electrode material is 0.3 μm to 2.5 μm, and / or the mass ratio of the secondary particles to the first primary particles is 1:50 to 20:

1.

8. A method for producing a positive electrode material according to any one of claims 1 to 7, comprising the steps of: Mixing an iron source and a microgel to obtain a first precursor, the first precursor including a microgel and an iron source distributed inside and outside the microgel, the microgel including a carbon chain polymer; a step of mixing the first precursor, a phosphorus source, a lithium source, and a carbon source and then performing a heat treatment to obtain a positive electrode material, and in a process of mixing the first precursor, the phosphorus source, the lithium source, and the carbon source and then performing a heat treatment, a part of the phosphorus source, the lithium source, and the carbon source are adsorbed inside a microgel of the first precursor and react in situ with an iron source inside the microgel to generate the secondary particles, and at the same time, the microgel and the carbon source inside the microgel are carbonized in the heat treatment process to form a first coating layer having a network structure on the surface, the first coating layer coating the surfaces of the secondary particles, and a part of the phosphorus source, the lithium source, and the carbon source react in situ with an iron source distributed outside the microgel to generate the first primary particles.

9. the mass ratio of the crosslinking monomer in the microgel is 0.1% to 10%; and / or the particle size of the microgel is between 300 nm and 2500 nm; and / or the microgel comprises at least one of a polyacrylic acid-based microgel, a polystyrene-based microgel, and a polyacrylamide-based microgel; and / or the mass ratio of the iron source to the microgel is from 100:0.01 to 100:1.36; and / or the iron source comprises a soluble iron source, the soluble iron source comprising at least one of iron nitrate, iron chloride, iron sulfate, and iron oxalate; and / or the iron source is in the form of a solution, and the solution concentration of the iron source is 1 mol / L or more; The method for producing a positive electrode material according to claim 8, characterized in that the mixing of the iron source and the microgel is carried out under stirring conditions.

10. The step of performing a heat treatment after mixing the first precursor, the phosphorus source, the lithium source, and the carbon source includes the steps of: adding a phosphorus source to the first precursor, reacting the first precursor under conditions of 60° C. to 90° C. to obtain a precipitate, and subjecting the precipitate to solid-liquid separation, washing and drying to obtain a second precursor; adding a lithium source and a carbon source to the second precursor and heating at 400° C. to 700° C. for 5 h to 8 h; the phosphorus source comprises at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and sodium phosphate; The carbon source comprises at least one of sucrose, glucose, and polyethylene glycol; the lithium source is at least one of lithium carbonate, lithium phosphate, lithium hydroxide, and lithium chloride; the mass ratio of the iron source, the phosphorus source and the lithium source is 1:(1-4):(1-1.5); The ratio of the iron source to the carbon source is 1 mol:(10 to 100) g; 9. The method for producing a positive electrode material according to claim 8, wherein a dispersion solvent is further added in the process of adding a lithium source and a carbon source to the second precursor, and the dispersion solvent includes at least one of deionized water and an alcohol solvent.

11. A secondary battery, A secondary battery comprising the positive electrode material according to any one of claims 1 to 7.

Images