Composite positive electrode material, preparation method thereof and positive electrode sheet

Abstract

The application discloses a kind of composite positive electrode material and its preparation method and positive plate, the composite positive electrode material includes positive electrode material matrix, and carbon coating layer is coated in the positive electrode material matrix, and intermediate growth layer is also arranged between the positive electrode material matrix and the carbon coating layer;The intermediate growth layer contains phosphate particles.Compared with prior art, by setting intermediate growth layer between positive electrode material matrix and carbon coating layer, the adhesion area of carbon coating material is increased, so that carbon coating layer is more firm, and carbon coating effect is better.Specifically, in the sintering process of carbon coating, the intermediate growth layer with the same crystal structure as the positive electrode material matrix locally fuses and grows on the surface of the positive electrode material matrix, so that the carbon coating achieves the effect similar to in-situ coating, ensuring the close combination between the carbon coating layer and the positive electrode material matrix, greatly enhancing the surface structure of the composite positive electrode material, and effectively prolonging the service life of the battery.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a composite cathode material, its preparation method, and cathode sheet. Background Technology

[0002] In the field of energy storage and supply, lithium-ion batteries have gradually established their dominant position due to their superior energy density, long cycle life, lack of memory effect, and extremely low self-discharge rate. The intricate structure of lithium-ion batteries encompasses the positive electrode, negative electrode, separator, electrolyte, and a series of precision components. Among these, the positive electrode material, as the core component, plays a decisive role in the overall performance of the battery. Currently, the selection of positive electrode materials for commercial lithium-ion batteries is quite extensive, mainly including lithium cobalt oxide, ternary materials, lithium manganese oxide, lithium iron phosphate, and emerging materials such as lithium manganese iron phosphate, which is gradually moving towards the forefront of application.

[0003] Lithium iron phosphate (LFP) materials stand out among numerous cathode materials due to their suitable discharge voltage, theoretically high specific capacity, excellent thermal and electrochemical stability, broad electrolyte compatibility, abundant raw material sources, and environmentally friendly properties. Of particular note is the innovative application of nanotechnology and carbon coating technology, which significantly improves the conductivity of LFP materials, making them one of the most favored cathode materials. Building on this, lithium manganese iron phosphate (LFP), as a derivative variant of LFP, has also become a key focus of current research and development.

[0004] In the synthesis of phosphate-based cathode materials, the high-temperature carbothermal reduction method dominates, cleverly achieving in-situ carbon coating and ensuring excellent coating effects. However, compared to these traditional methods, other synthesis strategies such as solvothermal synthesis exhibit unique advantages in terms of precise control of material particle size, optimized morphology design, and fine adjustment of composition. However, solvothermal synthesis typically requires the prior synthesis of phosphate materials followed by subsequent carbon coating treatment, i.e., non-in-situ carbon coating. However, the adhesion of the carbon coating layer in non-in-situ carbon coating is not good enough, leading to problems such as decreased thermal stability, shortened cycle life, and accelerated capacity decay of the cathode material.

[0005] Given the aforementioned defects in current phosphate-based cathode materials, it is indeed necessary to provide a technical solution to address these problems. Summary of the Invention

[0006] The purpose of this invention is to provide a composite cathode material with a stable surface structure and good conductivity.

[0007] To achieve this objective, the present invention provides the following solution:

[0008] A composite cathode material includes a cathode material matrix and a carbon coating layer covering the cathode material matrix, wherein an intermediate growth layer is disposed between the cathode material matrix and the carbon coating layer; the intermediate growth layer contains phosphate particles.

[0009] Preferably, the particle size of the positive electrode material matrix is ​​a, the average particle size of the intermediate growth layer is b, and a satisfies the relationship: 0 < a ≤ 600 nm; b satisfies the relationship: 0 < b ≤ 200 nm.

[0010] Preferably, the chemical formula of the cathode material matrix is ​​LiM 1-x N x PO4, wherein M is at least one of Fe, Mn, Ni, and Co, and N is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co, and Mn, and 0 ≤ x ≤ 0.1;

[0011] Preferably, the chemical formula of the phosphate particles is LiA. 1-y B y PO4, wherein A is at least one of Fe, Ni and Co, and B is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co and Mn, and 0 ≤ y ≤ 0.1.

[0012] Preferably, the mass of the intermediate growth layer is 0.5% to 30% of the total mass of the composite cathode material.

[0013] Preferably, the mass of the carbon coating layer is 0.5% to 3.0% of the total mass of the composite cathode material.

[0014] Preferably, the D90, D50, and D10 of the phosphate particles satisfy the relationship: (D90-D10) / D50≥2.

[0015] Preferably, the D90, D50, and D10 of the positive electrode material matrix satisfy the relationship: (D90-D10) / D50≤2.

[0016] This invention also provides a method for preparing a composite cathode material, comprising the following steps:

[0017] Step 1: Weigh the lithium source, M source, N source and phosphorus source respectively, use water or organic liquid as solvent, transfer them to a high-pressure reactor and mix thoroughly. Seal and heat under an inert atmosphere to carry out a solvothermal reaction. After cooling, filtering, washing, drying and pulverizing, the cathode material matrix is ​​obtained.

[0018] Step 2: Add lithium source, source A, source B, phosphorus source and carbon source to the first premixing tank, and add pure water at the same time, and stir and premix to obtain the first premixed slurry;

[0019] Step 3: Add the cathode material matrix and pure water to the second premixing tank and stir to obtain the second premixed slurry;

[0020] Step 4: First, grind the first premixed slurry to obtain the third slurry; then add the second premixed slurry and mix it with the third slurry, continue grinding to obtain the fourth slurry, and dry it to obtain the composite material cathode precursor;

[0021] Step 5: Sinter the composite cathode material precursor at a temperature of 500-880℃ for 4-10 hours to obtain the composite cathode material.

[0022] Preferably, in step four, the grinding particle size of the third slurry is D50≤300nm.

[0023] The present invention also provides a positive electrode sheet, comprising a positive electrode material, a conductive agent and a binder, wherein the positive electrode material is the aforementioned composite positive electrode material.

[0024] The present invention also provides a secondary battery, comprising a separator, a negative electrode, an electrolyte, a battery casing, and the aforementioned positive electrode.

[0025] Compared to existing technologies, the advantages of this invention are as follows: By setting an intermediate growth layer between the cathode material substrate and the carbon coating layer, this invention increases the adhesion area of ​​the carbon coating material, making the carbon coating layer more robust and achieving a better carbon coating effect. Specifically, during the carbon coating sintering process, the intermediate growth layer, which has the same crystal structure as the cathode material substrate, locally fuses and grows on the surface of the cathode material substrate, achieving an effect similar to in-situ coating. This ensures a tight bond between the carbon coating layer and the cathode material substrate, greatly enhancing the surface structure of the composite cathode material and effectively extending the battery's lifespan. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of a composite cathode material according to an embodiment of the present invention.

[0027] Among them, 1. positive electrode material substrate; 2. intermediate growth layer; 3. carbon coating layer. Detailed Implementation

[0028] To make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] According to a first aspect of the present invention, a composite cathode material is provided, comprising a cathode material substrate 1 and a carbon coating layer 3 covering the cathode material substrate 1, wherein an intermediate growth layer 2 is further disposed between the cathode material substrate 1 and the carbon coating layer 3; the intermediate growth layer 2 contains phosphate particles.

[0030] The cathode material substrate 1 and the intermediate growth layer 2 have the same crystal structure.

[0031] The carbon coating layer 3 is distributed throughout all the gaps between the intermediate growth layer 2 and the cathode material substrate 1. The intermediate growth layer 2 increases the adhesion area of ​​the carbon coating material, making the carbon coating layer 3 more robust and improving the carbon coating effect. During the sintering process of carbon coating, the intermediate growth layer 2, which has the same crystal structure as the cathode material substrate 1, partially fuses and grows on the surface of the cathode material substrate 1, achieving an effect similar to in-situ coating. This ensures a tight bond between the carbon coating layer 3 and the cathode material substrate 1, greatly reducing the risk of coating layer detachment or damage, and effectively extending the battery's lifespan.

[0032] The composite cathode material obtained by this invention has a stable surface structure and good conductivity. Lithium-ion batteries made using it as the cathode active material have excellent electrochemical performance.

[0033] In one embodiment of the present invention, the particle size of the positive electrode material substrate 1 is 'a', and the average particle size of the intermediate growth layer 2 is 'b'. 'a' satisfies the relationship: 0 < a ≤ 600 nm. For example, 'a' can be 1 nm, 10 nm, 50 nm, 60 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, or 600 nm. When better rate performance is required, the positive electrode material substrate 1 needs a smaller particle size. When higher compaction density is required, the particle size of the positive electrode material substrate 1 needs to be larger. However, because phosphate materials have poor conductivity, excessively large particles will significantly reduce the specific capacity of the material. Considering all factors, the given range is below 600 nm. 'b' satisfies the relationship: 0 < b ≤ 200 nm. For example, 'b' can be 1 nm, 10 nm, 50 nm, 60 nm, 100 nm, 150 nm, or 200 nm. As the intermediate growth layer 2, in addition to its primary function of connecting the substrate material and the carbon coating layer, it also serves to fill the gaps between the cathode material substrate 1 and increase the compaction density of the composite cathode material. Its particles should be small so that they have better activity in sintering and fusing with the cathode material substrate 1. At the same time, it is best to have a wide particle size distribution range so that it can effectively fill the gaps between the cathode material substrate 1.

[0034] In one embodiment of the present invention, the chemical formula of the cathode material substrate 1 is LiM 1-x Nx PO4, wherein M is at least one of Fe, Mn, Ni, and Co, and N is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co, and Mn, and 0 ≤ x ≤ 0.1.

[0035] In one embodiment of the present invention, the chemical formula of the phosphate particles is LiA. 1-y B y PO4, wherein A is at least one of Fe, Ni and Co, and B is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co and Mn, and 0 ≤ y ≤ 0.1.

[0036] In this invention, the design of the cathode material substrate 1 and the intermediate growth layer 2 is further optimized, both of which can contain elemental doping at specific sites. Specifically, the Li sites can be doped with one or a combination of Na and K, while the P or O sites can be doped with one or more of N, F, S, etc. This doping strategy not only enriches the composition of the material but may also further improve the electrochemical performance of the material by adjusting the electronic structure and ionic conductivity.

[0037] The compositions of the cathode material substrate 1 and the intermediate growth layer 2 can be the same or different according to actual needs to optimize overall performance. In particular, the intermediate growth layer 2 is preferably a manganese-free, structurally stable phosphate material. When this intermediate growth layer 2 covers the surface of the manganese-containing phosphate cathode material substrate 1, it can effectively inhibit the dissolution of manganese and reduce side reactions with the electrolyte, thereby improving the cycle stability and safety of the battery.

[0038] Furthermore, since the cathode material substrate 1 has already formed a good crystalline structure during the preparation process, the sintering temperature and sintering time can be reduced and shortened in the subsequent carbon coating sintering step. This improvement not only simplifies the production process but also significantly reduces production energy consumption and improves production efficiency.

[0039] In one embodiment of the present invention, the mass of the intermediate growth layer 2 is 0.5% to 30% of the total mass of the composite cathode material. A mass percentage of the intermediate growth layer 2 within the range of 0.5% to 30% of the total mass of the composite cathode material is suitable. If its percentage is too low, effective connection between the cathode material substrate 1 and the carbon coating layer 3 cannot be fully achieved, thus affecting the overall performance of the material. Conversely, if its percentage is too high, it will not only suppress the excellent properties inherent in the cathode material substrate 1 itself, such as high activity and high capacity, but may also lead to a decrease in the material's compaction density, which is detrimental to the battery's energy density and cycle stability.

[0040] In one embodiment of the present invention, the mass of the carbon coating layer 3 is 0.5% to 3.0% of the total mass of the composite cathode material. The mass percentage of the carbon coating layer 3 in the composite cathode material should be controlled between 0.5% and 3.0%. If the percentage is lower than the lower limit of this range, it will be difficult to form a continuous and complete carbon coating layer 3, which will not effectively protect the cathode material matrix 1 and improve its conductivity. Conversely, if the percentage exceeds the upper limit of this range, it will lead to a significant decrease in the proportion of electrochemically active components, which will adversely affect key performance indicators such as specific capacity and compaction density, reducing the energy storage capacity and volumetric energy density of the battery. Therefore, precise control of the proportion of the carbon coating layer 3 is crucial for balancing the electrochemical performance and physical properties of the composite cathode material.

[0041] In one embodiment of the present invention, the D90, D50, and D10 of the phosphate particles satisfy the relationship: (D90-D10) / D50≥2. The intermediate growth layer 2 has a small particle size and a wide distribution range. As the intermediate growth layer 2, in addition to its primary function of connecting the matrix material and the carbon coating layer, it also fills the voids between the cathode material matrix 1 and increases the compaction density of the composite cathode material. Its particles should be small to ensure good sintering and fusion with the cathode material matrix 1. Ideally, the particle size distribution range should be wide to effectively fill the voids between the cathode material matrix 1.

[0042] In one embodiment of the present invention, the D90, D50, and D10 of the cathode material matrix 1 satisfy the relationship: (D90-D10) / D50≤2. The cathode material matrix 1 is synthesized by hydrothermal or solvothermal methods and has uniform particle morphology and size. This uniformity is crucial for improving the electrochemical performance and cycle stability of the material.

[0043] According to a second aspect of the present invention, a method for preparing a composite cathode material is also provided, comprising the following steps:

[0044] Step 1: Weigh the lithium source, M source, N source and phosphorus source respectively, use water or organic liquid as solvent, transfer them to a high-pressure reactor and mix thoroughly. Seal and heat under an inert atmosphere to carry out a solvothermal reaction. After cooling, filtering, washing, drying and pulverizing, the cathode material matrix 1 is obtained.

[0045] Step 2: Add lithium source, source A, source B, phosphorus source and carbon source to the first premixing tank, and add pure water at the same time, and stir and premix to obtain the first premixed slurry;

[0046] Step 3: Add the positive electrode material matrix 1 and pure water to the second premixing tank and stir to obtain the second premixed slurry;

[0047] Step 4: First, grind the first premixed slurry to obtain the third slurry; then add the second premixed slurry and mix it with the third slurry, continue grinding to obtain the fourth slurry, and dry it to obtain the composite material cathode precursor;

[0048] Step 5: Sinter the composite cathode material precursor at a temperature of 500-880℃ for 4-10 hours to obtain the composite cathode material.

[0049] The preparation method of this invention is simple and efficient in process design, and uses widely available and easily accessible materials. Specifically, this preparation method increases the adhesion area of ​​the carbon coating layer 3 by adding an intermediate growth layer 2, ensuring a more robust and reliable formation of the carbon coating layer 3. This achieves uniform and tight coating of the carbon layer on the surface of the composite cathode material, which greatly enhances the carbon coating effect. Compared with traditional methods, the composite cathode material prepared by this invention not only has stronger carbon layer bonding force but also effectively prevents the detachment of active materials, significantly enhancing the structural stability of the material.

[0050] Furthermore, since the cathode material substrate 1 has already formed a good crystalline structure during the preparation process, the sintering temperature and sintering time can be reduced and shortened in the subsequent carbon coating sintering step. This improvement not only simplifies the production process but also significantly reduces production energy consumption and improves production efficiency.

[0051] In one embodiment of the present invention, in step one, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium monohydrogen phosphate, lithium dihydrogen phosphate, and lithium phosphate; the M source is at least one of an oxide, phosphide, or phosphate of element M; and the phosphorus source is at least one of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid. It should be noted that the lithium source, M source, and phosphorus source can simultaneously provide two or more of the materials of Li, M, and P. During the preparation process, the lithium source, M source, and phosphorus source should be added according to the specific element ratio of the materials.

[0052] In one embodiment of the present invention, in step two, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium monohydrogen phosphate, lithium dihydrogen phosphate, and lithium phosphate; the A source is at least one of an oxide, phosphide, or phosphate of element A; and the phosphorus source is at least one of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid. It should be noted that the lithium source, A source, and phosphorus source can simultaneously provide two or more of the materials of Li, A, and P. During the preparation process, the lithium source, A source, and phosphorus source should be added according to the specific element ratio of the materials.

[0053] In one embodiment of the present invention, in step two, the carbon source is a mixture of organic carbon source and inorganic carbon source. The organic carbon source is selected from at least one of glucose, sucrose, PEG, PVP and citric acid. The inorganic carbon source includes at least one of pyrolyzed carbon, CNT, graphene and SP. The proportion of inorganic carbon source in the total carbon content does not exceed 30%.

[0054] According to one embodiment of the present invention, in step one, the surface of the positive electrode material substrate 1 is activated; the surface activation treatment is at least one of acid treatment, alkali treatment, and high-energy ball milling treatment. The main purpose of activating the surface of the positive electrode material substrate 1 is to promote good bonding and adhesion between the intermediate growth layer 2 and the surface of the host material, thereby ensuring that the intermediate growth layer 2 can be deposited and grown more effectively and uniformly on the surface of the host material.

[0055] According to one embodiment of the present invention, in step four, the grinding particle size of the third slurry is D50≤300nm.

[0056] In a third aspect of the present invention, a positive electrode sheet is also provided, comprising a positive electrode material, a conductive agent and a binder, wherein the positive electrode material is the aforementioned composite positive electrode material.

[0057] In a fourth aspect of the invention, a secondary battery is also provided, comprising a separator, a negative electrode, an electrolyte, a battery casing, and the aforementioned positive electrode.

[0058] The negative electrode includes a negative current collector and a negative active material layer coated on at least one surface of the negative current collector. The negative active material layer may be one or more of the following, including but not limited to graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium.

[0059] The graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is typically a structure or component that collects current. The negative electrode current collector can be any material suitable for use as a negative electrode current collector in a secondary battery, for example, it can be, but is not limited to, metal foil, and more specifically, it can be, but is not limited to, copper foil.

[0060] The secondary battery also includes an electrolyte, which comprises an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-resistant electrolytes; or it can be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; or it can be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include, but are not limited to, at least one of film-forming additives, conductive additives, flame-retardant additives, overcharge-resistant additives, additives for controlling the H2O and HF content in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0061] The present invention will be further described below through specific embodiments.

[0062] Example 1

[0063] This embodiment provides a composite cathode material, wherein the cathode material substrate 1 is LiFePO4, and the surface of the cathode material substrate 1 has an intermediate growth layer 2 of LiFePO4 particles and a carbon coating layer 3 grown on its surface. The mass of the intermediate growth layer 2 accounts for 10% of the mass of the composite cathode material, and the weight of the carbon coating material accounts for 1.5% of the mass of the composite cathode material. The average particle size of the cathode material substrate 1 is 500 nm, the average particle size of the intermediate growth layer 2 particles is 120 nm, and the D90, D50, and D10 of the phosphate particles satisfy the relationship: (D90-D10) / D50=3.

[0064] The preparation method of the positive electrode composite material is as follows:

[0065] Step 1: Weigh lithium hydroxide, ferrous sulfate, and phosphoric acid in a molar ratio of Li:Fe:P = 3:1:1. Mix them evenly in a high-pressure reactor using pure water as a solvent. Replace the air in the reactor with nitrogen, seal the reactor, heat it to 180°C and keep it at that temperature for 6 hours. After cooling, open the reactor and collect the material. Filter to separate the solid product, wash it with pure water, dry and pulverize it to obtain the cathode material matrix 1 (LiFePO4).

[0066] Step 2: Weigh lithium carbonate, iron phosphate, glucose, and PEG4000 according to the weight ratio and add them to the first premixing tank. At the same time, add pure water and stir to premix to obtain the first premixed slurry. The solid content of the first premixed slurry is 40%. The lithium carbonate and iron phosphate are mixed in a molar ratio of 1.01:1, and the glucose and PEG4000 are mixed in a mass ratio of 8:2.

[0067] Step 3: Add the positive electrode material matrix 1 and pure water to the second premixing tank and stir to obtain the second premixed slurry. The solid content of the second premixed slurry is 40%.

[0068] Step 4: First, grind the first premixed slurry for 4 hours to obtain the third slurry; then add the second premixed slurry and mix it with the third slurry, continue grinding for 30 minutes to obtain the fourth slurry, and spray dry it to obtain the composite material cathode precursor;

[0069] Step 5: Sinter the composite cathode material precursor in high-purity nitrogen at 700℃ for 6 hours to obtain the composite cathode material.

[0070] Preparation of secondary batteries:

[0071] The composite cathode material, conductive carbon black and PVDF are mixed in a mass ratio of 95:3:2 and dispersed in N-methylpyrrolidone to form a slurry. After stirring, coating, drying, rolling and slitting, the cathode sheet is obtained.

[0072] Artificial graphite is used as the negative electrode active material. Conductive carbon black and PVDF are mixed in a mass ratio of 94:3:3 and dispersed in NMP to form a slurry. After stirring, coating, drying, rolling and slitting, the negative electrode sheet is obtained.

[0073] The negative electrode, positive electrode, and separator are interleaved and wound together, then subjected to terminal welding, aluminum foil packaging, electrolyte injection, encapsulation formation, and degassing molding steps to finally produce a soft-pack lithium-ion battery with a designed capacity of 2500mAh. The electrolyte consists of 1mol / L LiPF6 as the solute and EC, DMC, and DEC as the solvent in a volume ratio of 1:1:1.

[0074] Example 2

[0075] Unlike Example 1, the intermediate growth layer 2 is LiNiPO4, and its mass accounts for 3% of the mass of the composite cathode material.

[0076] The rest is exactly the same as in Example 1, and will not be repeated here.

[0077] Example 3

[0078] Unlike Example 1, the cathode material substrate 1 is LiMn. 0.7 Fe 0.3The PO4 material, a composite cathode material, has a carbon content of 1.6%, and 0.5% CNT is added as an inorganic carbon source during synthesis. The sintering temperature is 650℃.

[0079] The rest is exactly the same as in Example 1, and will not be repeated here.

[0080] Example 4

[0081] Unlike Example 1, the cathode material substrate 1 is LiMn. 0.6 Fe 0.39 Mg 0.01 The PO4 material has an intermediate growth layer 2 of LiNiPO4, which accounts for 2% of the mass of the composite cathode material. 0.5% graphene is added as an inorganic carbon source during the material synthesis, and the sintering temperature is 650℃.

[0082] The rest is exactly the same as in Example 1, and will not be repeated here.

[0083] Example 5

[0084] Unlike Example 4, the intermediate growth layer 2 is LiCoPO4.

[0085] The rest is exactly the same as in Example 4, and will not be repeated here.

[0086] Example 6

[0087] Unlike Example 3, the cathode material substrate 1 is LiMnPO4.

[0088] The rest is exactly the same as in Example 3, and will not be repeated here.

[0089] Example 7

[0090] Unlike Example 1, in step one, the surface of the positive electrode material substrate 1 is also activated. The surface activation method is to ball mill the main material LiFePO4 at high speed for 3 hours at a ball milling speed of 800 rpm.

[0091] The rest is exactly the same as in Example 1, and will not be repeated here.

[0092] Example 8

[0093] Unlike Example 1, the average particle size of the cathode material substrate 1 is 280 nm, and the average particle size of the intermediate growth layer 2 particles is 50 nm.

[0094] The rest is exactly the same as in Example 1, and will not be repeated here.

[0095] Example 9

[0096] Unlike Example 1, the D90, D50, and D10 of the phosphate particles satisfy the following relationship:

[0097] (D90-D10) / D50=5.

[0098] The rest is exactly the same as in Example 1, and will not be repeated here.

[0099] Comparative Example 1

[0100] Unlike Example 1, Comparative Example 1 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0101] The rest are the same as in Example 1, and will not be repeated here.

[0102] Comparative Example 2

[0103] Unlike Example 2, Comparative Example 2 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0104] The rest are the same as in Example 2, and will not be repeated here.

[0105] Comparative Example 3

[0106] Unlike Example 3, Comparative Example 3 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0107] The rest are the same as in Example 3, and will not be repeated here.

[0108] Comparative Example 4

[0109] Unlike Example 4, Comparative Example 4 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0110] The rest are the same as in Example 4, and will not be repeated here.

[0111] Comparative Example 5

[0112] Unlike Example 5, Comparative Example 5 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0113] The rest are the same as in Example 5, and will not be repeated here.

[0114] Comparative Example 6

[0115] Unlike Example 6, Comparative Example 6 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0116] The rest are the same as in Example 6, and will not be repeated here.

[0117] Comparative Example 7

[0118] Unlike Example 7, Comparative Example 7 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0119] The rest are the same as in Example 7, and will not be repeated here.

[0120] Comparative Example 8

[0121] Unlike Example 8, Comparative Example 8 does not have an intermediate growth layer 2, and the cathode material substrate 1 is directly carbon-coated and sintered.

[0122] The rest are the same as in Example 8, and will not be repeated here.

[0123] Comparative Example 9

[0124] Unlike Example 9, the D90, D50, and D10 of the phosphate particles satisfy the following relationship:

[0125] (D90-D10) / D50=1.

[0126] The rest are the same as in Example 9, and will not be repeated here.

[0127] Performance testing:

[0128] The powder compaction density of the cathode materials in the above embodiments and comparative examples was tested, and the test results are shown in Table 1.

[0129] The electrochemical performance of the secondary batteries in the above examples and comparative examples was tested, and the test results are shown in Table 1.

[0130] The specific performance testing steps and data processing methods are as follows.

[0131] Powder compaction density test: Take a certain mass of sample and put it into a special tablet mold. Place the mold on the powder compaction density tester, and apply a pressure of 30KN under program control. The test software will give the compaction density value.

[0132] Capacity and cycle performance testing: At 25℃, the electrode was charged at a constant current of 0.5C (1250mA) to the charging cutoff voltage, then charged at a constant voltage to 0.05C (125mA), and then discharged at 0.5C (1250mA) to 2.0V. This charge-discharge cycle was repeated 1000 times, and the discharge capacity at the first cycle and the discharge capacity at the 1000th cycle were measured. The cutoff voltage varies depending on the positive electrode active material; for example, when the positive electrode active material is lithium iron phosphate, the charging cutoff voltage is set to 3.7V; when the positive electrode active material is lithium manganese iron phosphate, the charging cutoff voltage is set to 4.5V; and when the positive electrode active material is a cobalt or nickel-containing phosphate material, the charging cutoff voltage is set to 5.0V.

[0133] First-cycle discharge specific capacity (mAh / g) = first-cycle discharge capacity (mAh) / mass of positive electrode active material (g);

[0134] 1000-cycle capacity retention = (1000th cycle discharge capacity / 1st cycle discharge capacity) × 100%.

[0135] Rate testing: At 25℃, charge at a constant current of 0.5C (1250mA) to the cutoff voltage, maintain the voltage at 0.05C (125mA), and then discharge at 0.5C (1250mA) to 2.0V. Repeat this cycle 10 times and calculate the average discharge energy, recorded as the 0.5C cycle discharge energy. Similarly, charge at a constant current of 0.5C (1250mA) to the cutoff voltage, maintain the voltage at 0.05C (125mA), and then discharge at 10C (25000mA) to 2.0V. Repeat this cycle 10 times and calculate the average discharge energy, recorded as the 10C cycle discharge energy. The cutoff voltage varies depending on the positive electrode active material; for example, when the positive electrode active material is lithium iron phosphate, the charging cutoff voltage is set to 3.7V; when the positive electrode active material is lithium manganese iron phosphate, the charging cutoff voltage is set to 4.5V; and when the positive electrode active material is a cobalt or nickel-containing phosphate material, the charging cutoff voltage is set to 5.0V.

[0136] Rate discharge energy retention rate = (10C cycle discharge energy / 0.5C cycle discharge energy) × 100%.

[0137] Table 1

[0138]

[0139]

[0140] As can be clearly seen from the test data shown in Table 1, the data from Examples 1 to 8 are significantly better than those from Comparative Examples 1 to 8. This result indicates that introducing an intermediate growth layer 2 between the cathode material substrate 1 and the carbon coating layer 3 effectively increases the adhesion area of ​​the carbon coating material. This design allows the carbon coating layer 3 to adhere more firmly to the cathode material substrate 1, thereby improving the overall effect of carbon coating. Furthermore, this structural improvement also promotes the increase in the compaction density of the composite cathode material powder.

[0141] When this optimized composite cathode material is applied to secondary batteries, Examples 1 to 8 show significant advantages over Comparative Examples 1 to 8 in terms of first-cycle discharge specific capacity, rate discharge energy retention, and 1000-cycle capacity retention. This further confirms that the introduction of the intermediate growth layer 2 not only enhances the stability of the carbon coating layer 3 but also significantly improves the electrochemical performance of the secondary battery.

[0142] The test data from Examples 4-5 and Example 6 clearly show that the test data from Examples 4-5 are superior to those from Example 6. This result indicates that the intermediate growth layer 2 is preferably a manganese-free, structurally stable phosphate material. When this intermediate growth layer 2 covers the surface of the manganese-containing phosphate cathode material substrate 1, it can effectively suppress the dissolution of manganese and reduce side reactions with the electrolyte, thereby improving the cycle stability and safety of the battery.

[0143] A comparison of the test data of Example 9 and Comparative Example 9 shows that the data of Example 9 are better than those of Comparative Example 9. This indicates that a wider particle size distribution range in the intermediate growth layer 2 will have a better effect. This is mainly because, as the intermediate growth layer 2, in addition to its main function of connecting the matrix material and the carbon coating layer, it also plays a role in filling the gaps between the cathode material matrix 1 and increasing the compaction density of the composite cathode material. The wide particle size distribution range can effectively fill the gaps between the cathode material matrix 1.

[0144] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A composite cathode material, characterized in that, The cathode material includes a cathode material substrate and a carbon coating layer covering the cathode material substrate. An intermediate growth layer is further disposed between the cathode material substrate and the carbon coating layer. The intermediate growth layer contains phosphate particles and is partially fused and grown on the surface of the cathode material substrate. The cathode material substrate and the intermediate growth layer have the same crystal structure. The chemical formula of the cathode material substrate is LiM. 1-x N x PO4, wherein M is at least one of Fe, Mn, Ni, and Co, and N is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co, and Mn, and 0 ≤ x ≤ 0.1; the chemical formula of the phosphate particles is LiA. 1-y B y PO4, wherein A is at least one of Fe, Ni and Co, and B is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co and Mn, and 0 ≤ y ≤ 0.1; The preparation method of the composite cathode material includes the following steps: Step 1: Weigh the lithium source, M source, N source and phosphorus source respectively, use water or organic liquid as solvent, transfer them to a high-pressure reactor and mix thoroughly. Seal and heat under an inert atmosphere to carry out a solvothermal reaction. After cooling, filtering, washing, drying and pulverizing, the cathode material matrix is ​​obtained. Step 2: Add lithium source, source A, source B, phosphorus source and carbon source to the first premixing tank, and add pure water at the same time, and stir and premix to obtain the first premixed slurry; Step 3: Add the cathode material matrix and pure water to the second premixing tank and stir to obtain the second premixed slurry; Step 4: First, grind the first premixed slurry to obtain the third slurry; then add the second premixed slurry and mix it with the third slurry, continue grinding to obtain the fourth slurry, dry it, and obtain the composite cathode material precursor; Step 5: Sinter the composite cathode material precursor at a temperature of 650-880℃ for 4-10 hours to obtain the composite cathode material.

2. The composite cathode material according to claim 1, characterized in that, The particle size of the positive electrode material matrix is ​​a, and the average particle size of the intermediate growth layer is b. The particle size of a satisfies the following relationship: 0 < a ≤ 600 nm; the particle size of b satisfies the following relationship: 0 < b ≤ 200 nm.

3. The composite cathode material according to claim 1, characterized in that, The mass of the intermediate growth layer is 0.5% to 30% of the total mass of the composite cathode material.

4. The composite cathode material according to claim 1, characterized in that, The mass of the carbon coating layer is 0.5% to 3.0% of the total mass of the composite cathode material.

5. The composite cathode material according to claim 1, characterized in that, The D90, D50, and D10 of the phosphate particles satisfy the relationship: (D90-D10) / D50≥2.

6. The composite cathode material according to claim 1, characterized in that, The D90, D50, and D10 of the cathode material matrix satisfy the following relationship: (D90-D10) / D50≤2.

7. The composite cathode material according to claim 1, characterized in that, In step four, the grinding particle size of the third slurry is D50≤300nm.

8. A positive electrode sheet, comprising a positive electrode material, a conductive agent, and a binder, characterized in that, The cathode material is the composite cathode material according to any one of claims 1-7.

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