Positive electrode material, method for manufacturing the same, and secondary battery

Abstract

To provide a positive electrode material, a method for manufacturing the same, and a secondary battery.SOLUTION: A positive electrode material includes a lithium iron phosphate active material and a coating layer. The lithium iron phosphate active material contains 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 with which the surfaces of the secondary particles are coated. The first coating layer is applied to the surfaces of the secondary particles in a surface-coating state and has a network structure. The surfaces of the secondary particles are coated with a surface-coated coating layer having a network structure, which can improve the capacity, rate performance, and cycle performance of the positive electrode material. The matching of the first primary particles and the secondary particles improves the press density of the positive electrode material, reduces particle aggregation of the positive electrode material, and improves the capacity and the rate performance of the positive electrode material.SELECTED DRAWING: Figure 1

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 has grown rapidly. Among them, lithium-ion batteries have advantages such as high energy density, no memory effect, and long cycle life, and are widely used in industries such as consumer products, power batteries, and energy storage. Common positive electrode materials for lithium-ion batteries mainly include lithium iron phosphate, ternary materials, and lithium cobalt oxide.

[0003] Lithium iron phosphate has attracted widespread attention since its proposal in 1997, and occupies a leading position in the cathode materials of lithium-ion batteries due to its excellent stability, high-rate charging, non-toxicity, and long cycle life. However, lithium iron phosphate also has significant defects, such as low tap density and pressed density, low electrical conductivity, and low ion diffusivity, which limit its further widespread application in cathode materials.

[0004] Therefore, how to prepare lithium iron phosphate materials with high dynamic performance, high capacity and excellent cycle performance is a 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 method for manufacturing the same, 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, 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 coated on the surfaces of the secondary particles, and the first coating layer coats the surfaces of the secondary particles in a surface-coating state, and the positive electrode material has a network structure.

[0007] In a second aspect, the present embodiment comprises: A step 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; mixing the first precursor, a phosphorus source, a lithium source, and a carbon source, and then performing a heat treatment to obtain the 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. [Effects of the Invention]

[0009] Compared with the prior art, the present technical solution has at least the following technical advantages: In the positive electrode material of the present application, a first coating layer on the surface of the secondary particles is coated 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 reduces the infiltration of the electrolyte into the positive electrode material, suppresses the corrosion of the positive electrode material by the electrolyte, and reduces 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 network-structured first coating layer avoids the problem of ion transport being hindered by the coating layer formed in the conventional coating method being too thick; the network-like coating layer forms a conductive network between the secondary particles, improving the conductivity of the positive electrode material and advantageously reducing inter-particle contact resistance and electrode polarization; and the network-like coating layer, distributed on the surface of the secondary particles, provides sufficient ion transport paths during the charge and discharge process of the positive electrode material, improving 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 increase the specific surface area of the positive electrode material, shorten the lithium ion transport distance, and improve the kinetic performance of the positive electrode material. Figure 1 shows a schematic structural diagram of the positive electrode material of the present application. 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. The matching of the first primary particles with the secondary particles can increase the press density of the positive electrode material, thereby improving the capacity, rate performance, and cycle performance of the positive electrode material. [Brief explanation of the drawings]

[0010] The present application will now be further described with reference to the figures and examples. [Figure 1] 1 is a structural schematic diagram of a positive electrode material according to the present application. [Figure 2] 1 is a flowchart showing the production of a positive electrode material according to the present application. DETAILED DESCRIPTION OF THE INVENTION

[0011] In order to better understand the technical solution of the present application, the embodiments of the present application will be 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, and are not all of the embodiments, and all other embodiments that a person skilled in the art can obtain based on the embodiments of the present application without any creative work 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 scope of this application. As used in the examples of this application and the appended claims, the singular forms "a," "one," "the," "said," and "the" are intended to include the plural forms as well, unless the context clearly dictates 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. Furthermore, 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. However, compared to other positive electrode materials, lithium iron phosphate has poor dynamic performance and weak conductivity, limiting the output characteristics of lithium-ion batteries supported by lithium iron phosphate. To address this issue, nano-sizing the lithium iron phosphate particle size can shorten the lithium ion transport distance and improve the performance. However, nano-sizing lithium iron phosphate significantly increases the manufacturing difficulty of the positive electrode material, and nano-sized lithium iron phosphate has poor dispersion performance and is difficult to improve press density. As the lithium iron phosphate is nano-sized, the specific surface area of the lithium iron phosphate increases, increasing the number of surface side reactions, which reduces the electrochemical performance and service life of lithium iron phosphate positive electrode materials used in batteries.

[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 coating the surfaces of the secondary particles in a surface-coating state, and the first coating layer has a network structure.

[0017] In the above technical solution, the cathode 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 network-coated first coating layer reduces the infiltration of the electrolyte into the cathode material, inhibits the corrosion of the cathode material by the electrolyte, and reduces the occurrence of side reactions between the electrolyte and the active material of the cathode material, thereby improving the initial efficiency and capacity of the cathode material. The network-structured first coating layer avoids the problem of ion transport being hindered by excessively thick coating layers formed in conventional coating methods. The network-like coating layer forms a conductive network between the secondary particles, improving the conductivity of the cathode material and reducing inter-particle contact resistance and electrode polarization. The network-like coating layer is distributed on the surface of the secondary particles, providing sufficient ion transport paths during the charge and discharge process of the cathode material, and improving the lithium desorption / insertion ability of the cathode material, thereby improving the rate performance and cycle performance of the cathode material. The plurality of first primary particles distributed among the secondary particles can increase the specific surface area of the positive electrode material, shorten the lithium ion transport distance, and improve the kinetic performance of the positive electrode material. Figure 1 shows a schematic structural diagram of the positive electrode material of the present application. 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. The matching of the first primary particles with the secondary particles can increase the press density of the positive electrode material, thereby improving 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 network structure; as can 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 leaf-like vein structure, 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 form a network 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, preferably agglomerates formed by agglomeration of the second primary particles. As can be seen, the secondary particles are formed by agglomeration of a plurality of second primary particles, and the surfaces of the secondary particles have a network-structured first coating layer, and the first coating layer can provide sufficient ion transport paths. The median diameter of the second primary particles is smaller than that of the first primary particles, and the lithium ion diffusion distance of the second primary particles, which has a smaller particle size than the first primary particles, is shorter than that of the first primary particles, thereby improving the lithium ion transport efficiency of the second primary particles, improving the conductivity, and further improving the rate performance of the positive electrode material. In addition, the second primary particles, which have a smaller particle size, increase the solid-liquid transport interface between the positive electrode material and the electrolyte, thereby improving the kinetic performance and low-temperature performance of the positive electrode material.

[0020] As will be understood, the median diameter (D50) indicates the particle size 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 using a method such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and statistically determining the particle size distribution from the captured images.

[0021] As can be understood, both the first primary particles and the second primary particles are crystals grown from crystal nuclei, 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. 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, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, or 700 nm. Within this range, the second primary particles have a small particle size, which contributes to the intercalation and deintercalation of lithium ions during the charge and discharge process of the positive electrode material and improves the rate performance of the positive electrode material. If the median diameter of the second primary particles is smaller than 50 nm, the second primary particles will aggregate severely, which is detrimental to improving the cycle performance of the positive electrode material and also reduces the dispersion performance when preparing the positive electrode material into a slurry, which is detrimental to improving the processability of the positive electrode material for application in secondary batteries. If the median diameter of the second primary particles is larger than 700 nm, the diffusion length of lithium ions will be long, which is detrimental to improving the kinetic 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, or 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 particles. That is, in this embodiment, the surfaces of the secondary particles and the first primary particles are both provided with a coating layer. The presence of the second coating layer can effectively block contact between the positive electrode active material and air and the electrolyte, reducing side reactions. The median diameter of the first primary particles is small, so that the coating layer coated on the surface of the first primary particles has a large specific surface area, which is advantageous 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 the average thickness of the second coating layer, the particle size of the secondary particles is large, and the average thickness of the first coating layer coating the secondary particles is large, 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.On the other hand, the large average thickness of the first coating layer, which has a large thickness, helps to form a conductive network by allowing the network coating layers on the surfaces of multiple secondary particles to come into contact with each other, thereby improving the rate performance of the positive electrode material.The small average thickness of the second coating layer coating the first primary particles is advantageous for shortening the diffusion distance between lithium ions and the first primary particles, reducing the difficulty of lithium desorption / insertion from / into the positive electrode material, and 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 this 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 achieving effective coverage of the secondary particles, while the thickness of the surface portion is thin, thereby shortening the diffusion length of lithium ions and enabling the positive electrode material to rapidly desorb / insert lithium during the charge / discharge process, thereby improving the kinetic performance of the positive electrode material. It should be understood that 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. It should be understood that the maximum thickness and minimum thickness of the first coating layer can be measured by the following method: 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 (protrusion 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 (depression 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 thicker the protruding portions of the network structure of the first coating layer, the more stable the structure of the positive electrode material will be, suppressing side reactions on the surface of the positive electrode material and improving the conductivity of the positive electrode material. The minimum thickness of the first coating layer is less than the thickness of the second coating layer, i.e., the thinner the recessed portions of the network structure of the second coating layer, the less difficult it will be for lithium to be deintercalated / intercalated from the secondary particles, improving 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, and may be, for example, 1 nm, 3 nm, 5 nm, 8 nm, or 10 nm. Within this range, the second coating layer can completely cover the surface of the first primary particles, effectively cover the first primary particles, shorten the diffusion length of lithium ions, enable the positive electrode material to rapidly detach / insert lithium during charge / discharge, and improve the dynamic performance of the positive electrode material. As will be understood, the second coating layer is uniformly coated on the surface of the first primary particles, and 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). 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 different positions.

[0032] In some embodiments, the mass ratio of the first coating layer to the positive electrode material is 0.3 wt% to 2.5 wt%, and may be, for example, 0.3 wt%, 0.6 wt%, 0.9 wt%, 1.5 wt%, 1.8 wt%, 2.2 wt%, or 2.5 wt%, or may be other values within the above range, and the present application is not limited thereto. Within the above specified range, a coating layer with an appropriate thickness can be formed, 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 0.1 wt% to 2.0 wt%, specifically, 0.1 wt%, 0.5 wt%, 1 wt%, 1.2 wt%, 1.6 wt%, 1.8 wt%, or 2.0 wt%, and may be other values within the above range, and the present application is not limited thereto. Within the above specified range, a coating layer with an appropriate thickness can be formed, 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 pressed 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 If it is within the above range, the positive electrode material of the present invention 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, for example, 1:50, 5:40, 10:30, 20:25, 40:8, or 50:1. A ratio within this 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 occurs 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. 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, followed by 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, a polymer microparticle with an intramolecular cross-linked structure, the microgel having ion adsorption properties, and mixing with the iron source allows some of the iron source to be adsorbed inside the microgel, i.e., some of the iron source in the first precursor is distributed inside the microgel and some of the iron source is distributed outside the microgel, 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 lithium source located outside the microgel react in-situ to produce lithium iron phosphate single crystal particles, i.e., the first primary particles, and simultaneously, 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 lithium source located inside the microgel can also react in situ to produce lithium iron phosphate. However, the limited growth environment and limited diffusion inside the microgel inhibit the growth of lithium iron phosphate. The lithium iron phosphate inside the microgel tends to produce small secondary primary particles, which are also prone to agglomerate 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. The carbonized microgel, which has an intramolecular cross-linked structure, is located on the surface of the first coating layer, giving the surface of the first coating layer a network structure. The network-structured first coating layer forms a conductive network between the secondary particles, improving the conductivity of the positive electrode material and reducing interparticle contact resistance and electrode polarization. The network-like coating layer distributed on the surface of the secondary particles provides sufficient ion transport paths for the positive electrode material during the charge / discharge process, improving the lithium intercalation / deintercalation ability of the positive electrode material, and improving the rate performance and cycle performance of the positive electrode material.The present invention provides a method for pre-mixing an iron source with a microgel, which is simple in process and easy to obtain raw materials. During the in-situ formation of lithium iron phosphate from the iron source, two different growth environments for lithium iron phosphate, i.e., secondary particles and first primary particles, are formed. This improves the press density and gram capacity of the positive electrode material, and also improves the cycle performance, capacity, and rate performance of the positive electrode material.

[0040] The manufacturing method of the present invention will be specifically described below.

[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% to 10%, and may be, for example, 0.1%, 0.5%, 1%, 3%, 5%, 8%, or 10%. It is understood that the microgel is obtained by copolymerizing a cross-linking monomer with a cross-linking agent. The mass ratio of the cross-linking monomer affects the adsorption performance of the microgel. Within the above-specified range, the microgel has an appropriate degree of cross-linking and can adsorb an appropriate amount of the iron source into the microgel, with the remaining iron source located outside the microgel, providing a synthesis environment for two different lithium iron phosphate materials. The restricted environment within the microgel tends to generate second primary particles. At the same time, the second primary particles tend to aggregate to form secondary particles, and the first primary particles tend to generate outside the microgel. The cooperation between the secondary particles and the first primary particles improves the press density, electrical conductivity, and lithium insertion degree of the positive electrode material, which is advantageous for further improving 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 will not be able to form a stable restricted space, which is unfavorable for 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 will not be able to diffuse into the microgel, which will result in too little raw material inside the microgel, which is unfavorable for producing lithium iron phosphate secondary particles.

[0043] In some embodiments, the particle size of the microgel is 300 nm to 2500 nm, and may be, for example, 300 nm, 500 nm, 800 nm, 1200 nm, 1600 nm, 2000 nm, 2200 nm, or 2500 nm. A particle size within the above-specified range provides a certain amount of space for embedding the iron source within the microgel, which is advantageous for obtaining two different types of 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 into the microgel, which is disadvantageous for the formation of dense secondary particles. If the particle size of the microgel is too small, the spatial region created by the microgel is too small, making it difficult to form secondary particles.

[0044] In some embodiments, the microgel is a polymer microparticle having an intramolecularly crosslinked structure, and the polymer in the microgel comprises a carbon-containing polymer. For example, the microgel comprises at least one of polyacrylic acid-based microgel, polystyrene-based microgel, and polyacrylamide-based microgel. The microgel can provide two different reaction environments during the lithium iron phosphate reaction. It can also form a mesh-like coating layer after heat treatment and carbonization, which avoids the problem of ion transport being hindered by the coating layer formed in the conventional coating method, which is too thick. The mesh-like coating layer provides sufficient ion transport channels and improves the lithium desorption / insertion ability of the positive electrode material, thereby improving the rate performance and cycle performance of the positive electrode material. The mesh-like coating layer also forms a conductive network between the secondary particles, which improves the conductivity of the positive electrode material and is advantageous for reducing interparticle contact resistance 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 substance 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 production environment of an appropriate volume 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 because 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 distribution of the iron source.

[0050] In step S200, performing 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 the 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 portion of the phosphorus source enters the interior of the microgel. 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, respectively, to produce an 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, where the alcoholic solvent comprises ethanol, propanol, or 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, for example.

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

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

[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 reacts with the iron phosphate produced in step S201 to produce lithium iron phosphate. The presence of the microgel provides two distinct reaction environments: some lithium iron phosphate is produced inside the microgel and some outside the microgel. This limits the growth of lithium iron phosphate inside the microgel, resulting in the formation of small lithium iron phosphate particles. The lithium iron phosphate particles aggregate to form secondary particles, while the growth of lithium iron phosphate outside the microgel is not limited, resulting in the formation of primary lithium iron phosphate particles. The carbon source acts as a reducing agent for the second precursor and the lithium source. The carbon source is carbonized under heating conditions to coat the surfaces of the primary and secondary particles. At the same time, the microgel is carbonized under heating conditions to form a network-structured coating layer, forming a first coating layer on the surfaces of the secondary particles. The first coating layer is obtained by coating the surfaces of secondary particles with both a carbon source and a microgel, and the microgel is located on the surface of the first coating layer to form a network structure. The network structure reduces the electrolyte's infiltration into the cathode material and the occurrence of side reactions, suppresses corrosion of the cathode material by the electrolyte, and improves the initial efficiency and capacity of the cathode material. It also avoids the problem of ion transport inhibition caused by the coating layer being too thick in conventional coating methods. The network-like coating layer provides sufficient ion transport paths, improving the lithium desorption / insertion ability of the cathode material. The network-like coating layer also forms a conductive network between the secondary particles, improving the conductivity of the cathode material and reducing interparticle contact resistance and electrode polarization. The second coating layer is obtained by coating the surfaces of primary particles with a carbon source. The second coating layer prevents direct contact between the primary particles and the electrolyte inside the second coating layer, improving the conductivity of the primary particles and improving the rate performance of the cathode 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 combination such as a mixture of polyethylene glycol and sucrose, a mixture of polyethylene glycol and glucose, or a mixture of polyethylene glycol and sucrose or 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.

[0064] In some embodiments, the ratio of the iron source to the carbon source is 1 mol:(10-100) g, specifically 1 mol:10 g, 1 mol:30 g, 1 mol:50 g, 1 mol:70 g, 1 mol:90 g, or 1 mol:100 g. This range allows for the formation of a coating layer of appropriate thickness on the surface of the final lithium iron phosphate, providing a dense carbon coating that is beneficial for lithium ion insertion and desorption during the charge and discharge process of the lithium iron phosphate material and improving the overall electrochemical performance of the cathode material. Adding too much carbon source results in a coating layer that is too thick, increasing the internal resistance of the material and reducing the total amount of active material. Adding too little carbon source results in an uneven carbon coating, affecting the conductivity of the cathode material.

[0065] In some embodiments, after adding the lithium source and the carbon source to the second precursor, the method further includes polishing the second precursor to which the lithium source and the carbon source have been added before heating, which is advantageous for producing lithium iron phosphate with a small particle size and improves the dispersibility and 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 recited values, and other unrecited values within the ranges also apply. If the heating temperature is less than 400°C, the crystallinity of the resulting lithium iron phosphate and the pressing performance of the powder will be poor.

[0068] In some embodiments, step S202 further includes adding a dispersion solvent during the addition of the lithium source and the carbon source to the second precursor. The addition of the dispersion solvent is advantageous for uniformly dispersing the lithium source, the carbon source, and the first precursor, and for obtaining high-quality lithium iron phosphate and a coating layer 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 specifically may be 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, but is not 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, which 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. The positive electrode active material is the lithium iron phosphate positive electrode material produced by the above-described production method.

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

[0075] The conductive agent of the positive electrode sheet may include at least one of conductive carbon black, layered graphite, graphene, and 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 carboxymethyl cellulose, 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 contain 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), and 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, and ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene have excellent short-circuit prevention properties and can improve battery stability through their shutdown effect. In some examples, the thickness of the separator is within a range of approximately 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 sequentially wound or laminated to form an electrode member, which is then packaged, for example, in an aluminum-plastic film, and then an electrolyte is injected, chemically formed, and packaged to form a battery. The manufactured battery is then subjected to a performance test and a cycle test. [Example]

[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(NO3)3·9H2O to 1 L of deionized water and stir to dissolve completely. Add 1.2120 g of microgel and disperse for 2 hours to obtain the first precursor, which is polyacrylic acid microgel. The microgel particle size is 1500 nm and the mass ratio of acrylic acid crosslinking monomer in the microgel is 2%.

[0089] (2) 68.3 mL of H3PO4 solution (85 wt%) was added dropwise to the first precursor solution from step (1) 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 with deionized water and ethanol, and then dried at 100 °C for 4 h to obtain the second precursor, FePO4·2H2O crystals.

[0090] (3) FePO4·2H2O crystals, LiOH·H2O, and polyethylene glycol (PEG) were mixed, with FePO4·2H2O crystals serving as the iron and phosphorus source, LiOH·H2O as the lithium source, and polyethylene glycol (PEG) as the carbon source and reducing agent. The molar ratio of LiOH·H2O to FePO4·2H2O was 1:1, and the amount of polyethylene glycol used was 40g per 1mol of FePO4·2H2O. A small amount of deionized water was added to the mixture of FePO4·2H2O, LiOH·H2O, and PEG, followed by polishing and heating at 650°C for 6h to obtain the cathode material.

[0091] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0092] Example 2

[0093] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 1.2120 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1500 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 0.5%.

[0094] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0095] Example 3

[0096] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 1.2120 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1500 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 5%.

[0097] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0098] Example 4

[0099] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 1.2120 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1500 nm and an acrylic acid cross-linking monomer mass ratio of 8% in the microgel.

[0100] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0101] Example 5

[0102] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 1.2120 g of microgel was added and dispersed for 2 hours to obtain a first precursor, in which the microgel was polyacrylic acid microgel, the particle size of the microgel was 300 nm, and the mass ratio of the acrylic acid cross-linking monomer in the microgel was 2%.

[0103] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0104] Example 6

[0105] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 1.2120 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1000 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 2%.

[0106] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0107] Example 7

[0108] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 1.2120 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 2500 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 2%.

[0109] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0110] Example 8

[0111] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 0.6060 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1500 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 2%.

[0112] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0113] Example 9

[0114] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 2.7270 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1500 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 2%.

[0115] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0116] Example 10

[0117] The differences from Example 1 are: (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, and then 5.4944 g of microgel was added and dispersed for 2 hours to obtain a first precursor, which was polyacrylic acid microgel, with a particle size of 1500 nm and a mass ratio of acrylic acid cross-linking monomer in the microgel of 2%.

[0118] In this embodiment, the chemical composition of the positive electrode material is LiFePO4 / 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, which is a carbon layer, the carbon layer is coated on the surfaces of the secondary particles in a surface-coated state, and the surface of the carbon layer has a network structure, and the surfaces of the primary particles have a second coating layer, which is a carbon layer.

[0119] Comparative Example 1

[0120] The difference from Example 1 is that (1) 404 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred to completely dissolve, thereby obtaining a first precursor.

[0121] In this comparative example, the chemical composition of the positive electrode material is LiFePO4 / C, and the positive electrode material includes primary particles with a carbon coating layer on the surface of the primary particles.

[0122] Comparative Example 2

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

[0124] (1) 202 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred until completely dissolved, yielding an iron nitrate solution. 34.1 mL of H3PO4 solution (85 wt%) was added dropwise to the iron nitrate solution and stirred for 20 min. The resulting mixture was then heated in a 90 °C water bath for 3 h. The resulting white precipitate was filtered, washed with deionized water and ethanol, and then dried at 100 °C for 4 h to obtain FePO4·2H2O crystals. Furthermore, FePO4·2H2O crystals, LiOH·H2O, and polyethylene glycol (PEG) were mixed. Here, FePO4·2H2O crystals served as the iron and phosphorus source, LiOH·H2O served as the lithium source, and polyethylene glycol (PEG) served as the carbon source and reducing agent. The molar ratio of LiOH·H2O to FePO4·2H2O was 1:1, and the amount of polyethylene glycol used was 40 g per 1 mol of FePO4·2H2O. A small amount of deionized water was added dropwise to a mixture of FePO4·2H2O, LiOH·H2O, and PEG, followed by polishing and heating at 650°C for 6 h to obtain the first precursor.

[0125] (2) 202 g of Fe(NO3)3·9H2O was added to 1 L of deionized water and stirred until completely dissolved, yielding an iron nitrate solution. 34.1 mL of H3PO4 solution (85 wt%) was added dropwise to the iron nitrate solution and stirred for 20 min. The resulting mixture was then heated in a 90 °C water bath for 6 h. The resulting white precipitate was filtered, washed with deionized water and ethanol, and then dried at 100 °C for 4 h to obtain FePO4·2H2O crystals. Furthermore, FePO4·2H2O crystals, LiOH·H2O, and polyethylene glycol (PEG) were mixed. Here, FePO4·2H2O crystals served as the iron and phosphorus source, LiOH·H2O served as the lithium source, and polyethylene glycol (PEG) served as the carbon source and reducing agent. The molar ratio of LiOH·H2O to FePO4·2H2O was 1:1, and the amount of polyethylene glycol used was 40 g per 1 mol of FePO4·2H2O. A small amount of deionized water was added dropwise to a mixture of FePO4·2H2O, LiOH·H2O, and PEG, followed by polishing and heating 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 LiFePO4 / C, and the positive electrode material includes primary particles with two different particle size distributions, i.e., primary particles a and primary particles b. The surfaces of the primary particles with the two 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: The lithium iron phosphate materials obtained in each of the examples and comparative examples were subjected to a powder press density test (GB / T 24533-2009 was adopted).

[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 material 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. 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. The slurry was then applied to aluminum foil and vacuum dried at 90°C to obtain a positive electrode sheet. The resulting sheet 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 1:1 volumetric mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The resulting 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, and the current was 0.1 C. The test data was recorded, and 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 cathode materials prepared in Examples 1 to 10 were prepared by adding a microgel to the iron source in advance, which provided two different reaction environments for the production of lithium iron phosphate, resulting in a plurality of secondary particles and primary particles distributed among the secondary particles, with the surfaces of the secondary particles coated with a network-like coating layer, improving the capacity, rate performance, and cycle performance of the cathode material. The present application also improves the press density of the cathode material by matching the primary particles with the secondary particles, reducing particle aggregation of the cathode material, and improving the capacity and rate performance of the cathode material.

[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. As the cross-linking degree of the microgel increases, 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 further decreases, and the particle size of the second primary particles becomes smaller, which is advantageous to shorten 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 change in particle size contributes to some extent to an improvement in the press density of the positive electrode material.

[0141] In Example 1 and Examples 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 a large number of side reactions on the surface. This reduces the service life when applied to a lithium iron phosphate secondary battery, and makes it difficult to meet the demand for a higher energy density for a battery of the same volume.

[0143] The positive electrode material prepared in Comparative Example 2 contains large-sized lithium iron phosphate particles and small-sized lithium iron phosphate particles. Compared with Example 1, Comparative Example 2 requires first preparing large-sized lithium iron phosphate particles and small-sized lithium iron phosphate particles, and then mixing them. This makes the operation procedure complicated, the solid-phase mixing effect poor, and the mixing time long, making it difficult to improve production efficiency. Furthermore, the large-sized particles used in Comparative Example 2 have a longer lithium ion solid-phase transport path and poorer kinetic performance than the secondary particles used in the present application, making them unsuitable for high-rate applications.

[0144] Although the present application describes the detailed process equipment and process flow of the present application through the above examples, 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 improvements to the present application, equivalent replacement of each raw material of the product of the present application, 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; The coating layer includes a first coating layer coated on the surface of the secondary particle, the first coating layer coating the surface of the secondary particle in a surface-coating state, and the first coating layer has a network 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. 3. The cathode material according to claim 2, wherein a median diameter of the first primary particles is 100 nm to 800 nm, and / or a median diameter of the second primary particles is 50 nm to 700 nm, and / or a median diameter of the secondary particles is 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 the surface of the first primary particles, and the average thickness of the first coating layer is greater than the average thickness of the second coating layer.

5. The cathode material according to claim 4, wherein the first coating layer includes a carbon layer, the second coating layer includes a carbon layer, and / or the first coating layer has an average thickness of 1 nm to 15 nm, and / or the second coating layer has an average thickness of 1 nm to 10 nm.

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

7. The specific surface area of the positive electrode material is 7 m 2 / g~15m 2 / g, and / or the press density of the positive electrode material is 2.3 g / cm 3 ~2.9 g / 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: 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 cathode material, and in the process of mixing the first precursor, the phosphorus source, the lithium source, and the carbon source and then performing the 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 300 nm to 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 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 present in the form of a solution, and the solution concentration of the iron source is 1 mol / L or more; and / or mixing the iron source and the microgel 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 them 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 the second precursor at 400°C to 700°C for 5 hours to 8 hours; 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 to 4):(1 to 1.5); the ratio of the iron source to the carbon source added 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 during 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 cathode material according to any one of claims 1 to 7 or the cathode material produced by the production method according to any one of claims 8 to 10.

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