Positive electrode material and preparation method therefor, positive electrode sheet and secondary battery

Abstract

A positive electrode material and a preparation method therefor, a positive electrode sheet and a secondary battery. The positive electrode material comprises first lithium cobalt oxide particles, second lithium cobalt oxide particles and third lithium cobalt oxide particles, wherein the Dv50 particle size R1 of the first lithium cobalt oxide particles, the Dv50 particle size R2 of the second lithium cobalt oxide particles and the Dv50 particle size R3 of the third lithium cobalt oxide particles satisfy the following relationship: R3>R2>R1; and the mass m1, m2 and m3 of the three lithium cobalt oxide particles satisfy the following relationship: 0.1%≤m3 / (m1+m2)≤1.1%. Compared with the prior art, the positive electrode material has the advantages that the particle size of the third lithium cobalt oxide particles is larger than the particle sizes of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles serve as the first stress point during rolling, so as to protect the first lithium cobalt oxide particles and the second lithium cobalt oxide particles and guarantee the particle integrity of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles under the conditions of a high rolling pressure and a high compaction density of an electrode sheet, and therefore the performance of a subsequent battery cell can be improved.

Description

Positive electrode material and preparation method thereof, positive electrode sheet and secondary battery Technical Field

[0001] The present invention relates to the field of secondary batteries, and in particular to a positive electrode material and a preparation method thereof, a positive electrode sheet and a secondary battery. Background Art

[0002] In the development of lithium-ion battery technology, lithium cobalt oxide (LCO), a commonly used cathode material, has been widely researched and applied due to its high energy density, good cycle life, and specific capacity. However, in recent years, the roll density of LCO has been gradually increased to improve battery energy density, which has brought a series of new challenges.

[0003] In this context, traditional lithium cobalt oxide particle designs and even manufacturing processes are no longer able to meet current application requirements. During the roller pressing process, the increased processing pressure causes most of the lithium cobalt oxide particles to be crushed, cracked, or even pulverized. This cracking and pulverization not only reduces the overall stability and structural compactness of the particles, but also has a serious negative impact on the subsequent performance of the battery cell, potentially reducing the cell's energy density, shortening the battery life, and affecting battery safety.

[0004] Existing technical solutions do not provide an effective solution to this problem. The current preparation method still inevitably causes the lithium cobalt oxide particles to break and pulverize during the roller pressing process. This will undoubtedly have a negative impact on the performance of lithium-ion batteries, especially energy density, and limit their development in applications requiring higher energy density.

[0005] Therefore, it is particularly important to provide a new technical solution that can effectively prevent lithium cobalt oxide particles from breaking and pulverizing during the rolling process, thereby improving the energy density of lithium-ion batteries.

[0006] Summary of the Invention

[0007] One of the objectives of the present invention is to address the shortcomings of the existing technology and provide a positive electrode material to improve the problem that the current lithium cobalt oxide positive electrode material is easily crushed, cracked or even pulverized under high rolling pressure, resulting in a decrease in battery cell performance.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A positive electrode material comprising first lithium cobalt oxide particles, second lithium cobalt oxide particles and third lithium cobalt oxide particles;

[0010] Among them, the particle size R1 of the first lithium cobalt oxide particle Dv50, the particle size R2 of the second lithium cobalt oxide particle Dv50 and the particle size R3 of the third lithium cobalt oxide particle Dv50 satisfy the relationship: R3>R2>R1; the mass m1 of the first lithium cobalt oxide particle, the mass m2 of the second lithium cobalt oxide particle and the mass m3 of the third lithium cobalt oxide particle satisfy the relationship: 0.1%≤m3 / (m1+m2)≤1.1%.

[0011] Preferably, R1, R2 and R3 satisfy the following relationship: 2.5 <R3 / R2<4.8;8.9<R3 / R1<29.2;1.9<R2 / R1<10.2。

[0012] Preferably, the value range of R1 is 2.0-6.0 μm; the value range of R2 is 13-20 μm; and the value range of R3 is 55-58 μm.

[0013] Preferably, the peak value I1 of the first lithium cobaltate particle, the peak value I2 of the second lithium cobaltate particle, and the peak value I3 of the third lithium cobaltate particle of the particle size distribution curve of the positive electrode material satisfy the relationship: I2>I1>I3, and the three are distributed in a "mountain" shape.

[0014] Preferably, the mass of the first lithium cobaltate particles accounts for 10% to 30% of the mass of the positive electrode material, the mass of the second lithium cobaltate particles accounts for 69% to 89.9% of the mass of the positive electrode material, and the mass of the third lithium cobaltate particles accounts for 0.1% to 1% of the mass of the positive electrode material.

[0015] Preferably, the first lithium cobaltate particles and the second lithium cobaltate particles are both doped with element M, and M is one or more of Al, Mg, Ti, La, Y, Zr, Ho, Ce, V, and Mo.

[0016] Preferably, the molar ratio of Li to Co in the first and second lithium cobaltate particles is 1.02-1.08, and the molar ratio of M to Co is 0.005-0.05; the molar ratio of Li to Co in the third lithium cobaltate particles is 1.04-1.09.

[0017] A second object of the present invention is to provide a method for preparing a positive electrode material, comprising the following steps:

[0018] S1, sintering the precursors of the first lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a high temperature for 16 to 18 hours to obtain the first lithium cobalt oxide particles;

[0019] S2, sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a high temperature for 18 to 20 hours to obtain the second lithium cobalt oxide particles;

[0020] S3, sintering the precursors of the third lithium cobalt oxide particles, Co3O4 and Li2Co3, at a high temperature for 20 to 22 hours to obtain the third lithium cobalt oxide particles;

[0021] S4. Evenly mix the first lithium cobaltate particles, the second lithium cobaltate particles, and the third lithium cobaltate particles prepared above to obtain the positive electrode material.

[0022] Preferably, the sintering temperature of S1 is 800-900°C; the sintering temperature of S2 is 900-1000°C; and the sintering temperature of S3 is 1000-1100°C.

[0023] A third object of the present invention is to provide a positive electrode sheet comprising the positive electrode material described in any of the above paragraphs.

[0024] A fourth object of the present invention is to provide a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator spaced between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet described above.

[0025] Compared with the prior art, the beneficial effect of the present invention is that: in the positive electrode material provided by the present invention, the third lithium cobalt oxide particles are larger than the first lithium cobalt oxide particles and the second lithium cobalt oxide particles. During rolling, the third lithium cobalt oxide particles are the first force points, thereby protecting the first lithium cobalt oxide particles and the second lithium cobalt oxide particles. Under high rolling pressure and high electrode compaction density, the particle integrity of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles is ensured, which can improve the performance of subsequent battery cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 is a particle size distribution test curve diagram of the positive electrode material of Example 1 of the present invention. DETAILED DESCRIPTION

[0027] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be described in further detail below with reference to specific implementation methods and accompanying drawings, but the implementation methods of the present invention are not limited thereto.

[0028] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature specified as "first" or "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0029] 1. Cathode materials

[0030] The first aspect of the present invention aims to provide a cathode material, including first lithium cobaltate particles, second lithium cobaltate particles, and third lithium cobaltate particles; wherein, the particle size R1 of Dv50 of the first lithium cobaltate particles, the particle size R2 of Dv50 of the second lithium cobaltate particles, and the particle size R3 of Dv50 of the third lithium cobaltate particles satisfy the relationship: R3 > R2 > R1, and the mass m1 of the first lithium cobaltate particles, the mass m2 of the second lithium cobaltate particles, and the mass m3 of the third lithium cobaltate particles satisfy the relationship: 0.1% ≤ m3 / (m1 + m2) ≤ 1.1%. The inventors found that when the electrode sheet with lithium cobaltate as the cathode material is rolled, the third lithium cobaltate particles with larger particle size will first承受 greater pressure, and the third lithium cobaltate particles will start to break and crack first, so as to protect the original first and second lithium cobaltate particles of the cathode material from cracking and breaking during rolling. Improve the performance of the subsequent battery cells.

[0031] The large particles referred to in the present invention mean particles with a particle size distribution between 50 and 63 μm, and the following relationships are satisfied among the three kinds of particles: 2.5 < R3 / R2 < 4.8; 8.9 < R3 / RI < 29.2; 1.9 < R2 / R1 < 10.2. Among them, the particle size of the third lithium cobaltate particles can be 50 μm, 51 μm, 52 μm, 53 μm, 54 μm, ;55 μm, 56 μm, 57 μm, 58 μm, 59 μm, 60 μm, 61 μm, 62 μm, 63 μm; it can include but is not limited to the listed values. Preferably, the particle size of the third lithium cobaltate particles is between 55 and 58 μm, and more preferably, it is 55 μm.

[0032] In some embodiments, the value range of the particle size R1 of Dv50 of the first lithium cobaltate particles is 2.0 to 6.0 μm; specifically, it can be 2.0 μm, 2.1 μm, 2.9 μm, 3.0 μm, 3.1 μm, 3.9 μm, 4.0 μm, 4.1 μm, 4.9 μm, □5.0 μm, 5.1 μm, 5.9 μm, 6.0 μm; it can include but is not limited to the listed values. Preferably, the particle size R1 of Dv50 of the first lithium cobaltate particles is 5 μm.

[0033] In some embodiments, the value range of the particle size R2 of Dv50 of the second particles is 13 to 20 μm; specifically, it can be 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm; it can include but is not limited to the listed values. Preferably, the particle size R2 of Dv50 of the second lithium cobaltate particles is 17.5 μm.

[0034] In some embodiments, the mass m1 of the first lithium cobalt oxide particles accounts for 10% to 30% of the mass of the positive electrode material; specifically, it can be 10%, 15%, 20%, 25%, or 30%; it can include but is not limited to the listed values. Preferably, the mass m1 of the first lithium cobalt oxide particles is 20%.

[0035] In some embodiments, the mass m2 of the second lithium cobalt oxide particles accounts for 69% to 89.9% of the mass of the positive electrode material, specifically 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 89.9%; it may include but is not limited to the listed values. Preferably, the mass m2 of the second lithium cobalt oxide particles is 79.5%.

[0036] In some embodiments, the mass m3 of the third lithium cobalt oxide particles accounts for 0.1% to 1% of the mass of the positive electrode material, specifically 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. The mass m3 may include, but is not limited to, the values ​​listed above. Preferably, the mass m3 of the third lithium cobalt oxide particles is 0.5%. Compared to the mass proportions of the first and second lithium cobalt oxide particles, the mass of the third lithium cobalt oxide particles is less because the third lithium cobalt oxide particles primarily serve to protect the first and second lithium cobalt oxide particles during rolling, and only a small amount is required to achieve this effect.

[0037] In some embodiments, the first and second lithium cobalt oxide particles are both doped with an element M, wherein M is one or more of Al, Mg, Ti, La, Y, Zr, Ho, Ce, V, and Mo. The first and second lithium cobalt oxide particles both contain the doping element M, while the third lithium cobalt oxide particles do not contain any doping element M. This is because the undoped third lithium cobalt oxide particles can improve crystallinity and thus their pressure resistance.

[0038] In some embodiments, the molar ratios of Li and Co in the first and second lithium cobalt oxide particles are both 1.02 to 1.08, and the molar ratios of M and Co are both 0.005 to 0.05; the molar ratio of Li and Co in the third lithium cobalt oxide particles is 1.04 to 1.09. Within the above ranges, the molar ratios of the elements are defined to yield a cathode material consistent with the present invention upon sintering, thereby achieving the aforementioned excellent electrical properties.

[0039] A second aspect of the present invention is to provide a method for preparing a positive electrode material, comprising the following steps:

[0040] S1, sintering the precursors of the first lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a high temperature for 16 to 18 hours, and breaking the agglomerated particles to obtain the first lithium cobalt oxide particles;

[0041] S2, sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a high temperature for 18 to 20 hours, and breaking the agglomerated particles to obtain the second lithium cobalt oxide particles;

[0042] S3, sintering the precursors of the third lithium cobalt oxide particles, Co3O4 and Li2Co3, at a high temperature for 20 to 22 hours, and breaking the agglomerated particles to obtain the third lithium cobalt oxide particles;

[0043] S4. Evenly mix the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles prepared above to obtain a final positive electrode material.

[0044] The above-mentioned positive electrode material of the present invention is prepared by this preparation method. The positive electrode material prepared by this preparation method is crushed to different degrees by regulating the sintering temperature and time as well as the specific components of the particles, so that the particles are separated, and then the three particles with different crushed particle sizes are mixed together. Compared with the traditional two-particle method, large-particle-size third lithium cobalt oxide particles are used during high-pressure rolling to protect the first lithium cobalt oxide particles and the second lithium cobalt oxide particles that perform their performance in the lithium cobalt oxide positive electrode sheet, thereby ensuring the particle integrity of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles, and improving the performance of subsequent battery cells.

[0045] The sintering temperature of S1 is 800-900°C; the sintering temperature of S2 is 900-1000°C; and the sintering temperature of S3 is 1000-1100°C. By controlling the sintering temperature within the above range, the first, second, and third lithium cobalt oxide particles can be fully sintered. The three types of particles have different particle sizes, and the sintering temperature and duration increase with increasing particle size, resulting in positive electrode material particles with good performance.

[0046] In some embodiments, in S4, the mass of the first lithium cobalt oxide particles accounts for 10% to 30% of the mass of the positive electrode material, the mass of the second lithium cobalt oxide particles accounts for 69% to 89.9% of the mass of the positive electrode material, and the mass of the third lithium cobalt oxide particles accounts for 0.1% to 1% of the mass of the positive electrode material. The mass ratio of the three mixed can be specifically 20:79.9:0.1, 20:79:1, 19.8:79.7:0.5, and 20:79.5:0.5; it may include but is not limited to the listed values. Preferably, the mass ratio of the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles is 20:79.5:0.5. Within this range, the protective effect of the third lithium cobalt oxide particles and the high temperature cycle performance of the positive electrode sheet are the best.

[0047] 2. Positive electrode

[0048] The third aspect of the present invention provides a positive electrode sheet, comprising the positive electrode material described above, specifically comprising a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector, wherein the positive electrode active material of the positive electrode active material layer is the positive electrode material described in the present invention.

[0049] The positive electrode current collector can be any material suitable for use as a positive electrode current collector for lithium-ion batteries in the art. For example, the positive electrode current collector can include but is not limited to metal foil, and more specifically can include but is not limited to aluminum foil.

[0050] 3. Secondary batteries

[0051] A fourth aspect of the present invention is to provide a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator spaced between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet described above.

[0052] The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer can be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microbeads, silicon-based materials, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium. 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; and 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 lithium-ion batteries. For example, the negative electrode current collector can be, but not limited to, metal foil, and more specifically, can be, but not limited to, copper foil.

[0053] The separator can be made of various materials suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more materials including but not limited to polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber.

[0054] The secondary battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB, as used in high-temperature electrolytes; can also be at least one of LiBF4, LiBOB, and LiPF6, as used in low-temperature electrolytes; can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI, as used in overcharge-preventing electrolytes; or 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; a chain carbonate, including DFC, DMC, or EMC; or a carboxylic acid ester, including MF, MA, EA, and MP. Additives include, but are not limited to, at least one of a film-forming additive, a conductive additive, a flame retardant additive, an overcharge prevention additive, an additive for controlling the H2O and HF content in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0055] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be described in further detail below with reference to specific implementation methods and accompanying drawings, but the implementation methods of the present invention are not limited thereto.

[0056] Example 1

[0057] A positive electrode material includes first lithium cobalt oxide particles, second lithium cobalt oxide particles and third lithium cobalt oxide particles; wherein, the particle size R1 of the first lithium cobalt oxide particle Dv50, the particle size R2 of the second lithium cobalt oxide particle Dv50 and the particle size R3 of the third lithium cobalt oxide particle Dv50 satisfy the relationship: R3>R2>R1; the mass m1 of the first lithium cobalt oxide particle, the mass m2 of the second lithium cobalt oxide particle and the mass m3 of the third lithium cobalt oxide particle satisfy the relationship: 0.1%≤m3 / (m1+m2)≤1.1%.

[0058] The preparation method of the positive electrode material is as follows:

[0059] S1, sintering the precursors of the first lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at 850°C for 17 hours, controlling the molar ratio of Li to Co to be 1.05, and the molar ratio of M to Co to be 0.02, and breaking the sintered particles to obtain first lithium cobalt oxide particles, wherein the first lithium cobalt oxide particles have a Dv50 particle size R1 of 5 μm;

[0060] S2, sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at 950°C for 19 hours, controlling the molar ratio of Li to Co to be 1.05, and the molar ratio of M to Co to be 0.02, and breaking the sintered particles to obtain second lithium cobalt oxide particles, wherein the second lithium cobalt oxide particles have a Dv50 particle size R2 of 17.5 μm;

[0061] S3, sintering the precursors of the third lithium cobalt oxide particles, Co3O4 and Li2Co3, at 1050°C for 21 hours, controlling the molar ratio of Li to Co to be 1.06, and breaking the sintered particles to obtain third lithium cobalt oxide particles, wherein the Dv50 particle size R3 of the third lithium cobalt oxide particles is 55 μm;

[0062] S4. Evenly mix the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles in a mass ratio of 20:79.5:0.5 to obtain a final lithium cobalt oxide product.

[0063] Example 2

[0064] The difference from Example 1 is that in step S4, the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles are uniformly mixed together in a mass ratio of 20:79.9:0.1.

[0065] The rest is the same as in Example 1 and will not be described again here.

[0066] Example 3

[0067] The difference from Example 1 is that in step S4, the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles are uniformly mixed together in a mass ratio of 20:79:1.

[0068] The rest is the same as in Example 1 and will not be described again here.

[0069] Example 4

[0070] The difference from Example 1 is that in step S4, the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles are uniformly mixed together in a mass ratio of 19.8:79.7:0.5.

[0071] The rest is the same as in Example 1 and will not be described again here.

[0072] Example 5

[0073] The difference from Example 1 is that the particle size Dv50 of the third lithium cobalt oxide particles prepared in step S3 is 56 μm;

[0074] The operating steps of S3 are as follows: sintering the precursors of the third lithium cobalt oxide particles, Co3O4 and Li2Co3, at a temperature of 1050°C for 22 hours, controlling the molar ratio of Li and Co to 1.06, and breaking the sintered particles to obtain third lithium cobalt oxide particles, wherein the Dv50 particle size R3 of the third lithium cobalt oxide particles is 56 μm; the rest is the same as in Example 1 and will not be repeated here.

[0075] Example 6

[0076] The difference from Example 1 is that the third lithium cobalt oxide particles prepared in step S3 have a particle size Dv50 of 58 μm;

[0077] The operation steps of S3 are as follows: sintering the precursors of the third lithium cobalt oxide particles, Co3O4 and Li2Co3, at 1100°C for 22 hours, controlling the molar ratio of Li to Co to be 1.07, and breaking the sintered particles to obtain third lithium cobalt oxide particles, wherein the Dv50 particle size R3 of the third lithium cobalt oxide particles is 58 μm;

[0078] The rest is the same as in Example 1 and will not be described again here.

[0079] Example 7

[0080] The difference from Example 1 is that the second lithium cobalt oxide particles prepared in step S2 have a Dv50 particle size of 13 μm;

[0081] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 900°C for 18 hours, controlling the molar ratio of Li to Co to be 1.03 and the molar ratio of M to Co to be 0.01, and breaking the sintered particles to obtain the second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 13 μm;

[0082] The rest is the same as in Example 1 and will not be described again here.

[0083] Example 8

[0084] The difference from Example 1 is that the second lithium cobalt oxide particles prepared in step S2 have a Dv50 particle size of 15 μm;

[0085] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 900°C for 18 hours, controlling the molar ratio of Li to Co to be 1.04 and the molar ratio of M to Co to be 0.01, and breaking the sintered particles to obtain the second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 15 μm;

[0086] The rest is the same as in Example 1 and will not be described again here.

[0087] Example 9

[0088] The difference from Example 1 is that the second lithium cobalt oxide particles prepared in step S2 have a Dv50 particle size of 19 μm;

[0089] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 1000°C for 19 hours, controlling the molar ratio of Li to Co to be 1.05 and the molar ratio of M to Co to be 0.02, and breaking the sintered particles to obtain the second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 19 μm;

[0090] The rest is the same as in Example 1 and will not be described again here.

[0091] Example 10

[0092] The difference from Example 1 is that the second lithium cobalt oxide particles prepared in step S2 have a Dv50 particle size of 20 μm;

[0093] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 1000°C for 20 hours, controlling the molar ratio of Li to Co to be 1.06 and the molar ratio of M to Co to be 0.03, and breaking the sintered particles to obtain second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 20 μm;

[0094] The rest is the same as in Example 1 and will not be described again here.

[0095] Example 11

[0096] The difference from Example 1 is that the first lithium cobalt oxide particles prepared in step S1 have a particle size Dv50 of 6 μm; the second lithium cobalt oxide particles prepared in step S2 have a particle size Dv50 of 16 μm;

[0097] The operation steps of S1 are as follows: sintering the precursors of the first lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 900°C for 18 hours, controlling the molar ratio of Li to Co to be 1.05 and the molar ratio of Co to M to be 0.02, and breaking the sintered particles to obtain the first lithium cobalt oxide particles, wherein the Dv50 particle size R1 of the first lithium cobalt oxide particles is 6 μm;

[0098] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 950°C for 18 hours, controlling the molar ratio of Li to Co to be 1.04, and the molar ratio of M to Co to be 0.01, and breaking the sintered particles to obtain the second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 16 μm;

[0099] The rest is the same as in Example 1 and will not be described again here.

[0100] Example 12

[0101] The difference from Example 1 is that the first lithium cobalt oxide particles prepared in step S1 have a particle size Dv50 of 4 μm; the second lithium cobalt oxide particles prepared in step S2 have a particle size Dv50 of 18 μm;

[0102] The operation steps of S1 are as follows: sintering the precursors of the first lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at 800°C for 17 hours, controlling the molar ratio of Li to Co to be 1.05 and the molar ratio of M to Co to be 0.02, and breaking the sintered particles to obtain the first lithium cobalt oxide particles, wherein the Dv50 particle size R1 of the first lithium cobalt oxide particles is 4 μm;

[0103] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at a temperature of 1000°C for 19 hours, controlling the molar ratio of Li to Co to be 1.05 and the molar ratio of M to Co to be 0.02, and breaking the sintered particles to obtain the second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 18 μm;

[0104] The rest is the same as in Example 1 and will not be described again here.

[0105] Example 13

[0106] The difference from Example 1 is that the first lithium cobalt oxide particles prepared in step S1 have a Dv50 particle size of 2 μm;

[0107] The operating steps of S1 are as follows: sintering the precursors of the first lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at 800°C for 17 hours, controlling the molar ratio of Li and Co to 1.02, and the molar ratio of M and Co to 0.01, and breaking the sintered particles to obtain the first lithium cobalt oxide particles, wherein the Dv50 particle size R1 of the first lithium cobalt oxide particles is 2 μm; the rest is the same as in Example 1 and will not be repeated here.

[0108] Example 14

[0109] The difference from Example 1 is that the third lithium cobalt oxide particles prepared in step S3 have a Dv50 particle size of 38 μm;

[0110] The operation steps of S3 are as follows: sintering the precursors of the third lithium cobalt oxide particles, Co3O4 and Li2Co3, at a temperature of 1000°C for 20 hours, controlling the molar ratio of Li to Co to be 1.04, and cracking the sintered particles to obtain third lithium cobalt oxide particles, wherein the Dv50 particle size R3 of the third lithium cobalt oxide particles is 38 μm;

[0111] The rest is the same as in Example 1 and will not be described again here.

[0112] Example 15

[0113] The difference from Example 1 is that the second lithium cobalt oxide particles prepared in step S2 have a Dv50 particle size of 11 μm;

[0114] The operation steps of S2 are as follows: sintering the precursors of the second lithium cobalt oxide particles, Co3O4 and Li2Co3, and the oxide of the metal element M at 900°C for 18 hours, controlling the molar ratio of Li to Co to 1.02 and the molar ratio of M to Co to 0.01, and breaking the sintered particles to obtain the second lithium cobalt oxide particles, wherein the Dv50 particle size R2 of the second lithium cobalt oxide particles is 13 μm

[0115] The rest is the same as in Example 1 and will not be described again here.

[0116] Comparative Example 1

[0117] The difference from Example 1 is that the third lithium cobalt oxide particles are not included, and the other aspects are the same as Example 1.

[0118] Comparative Example 2

[0119] The difference from Example 1 is that in step S4, the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles are uniformly mixed together in a mass ratio of 20:78.7:1.3.

[0120] The rest is the same as in Example 1 and will not be described again here.

[0121] Comparative Example 3

[0122] The difference from Example 1 is that in step S4, the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles are uniformly mixed together in a mass ratio of 20:79.95:0.05.

[0123] The rest is the same as in Example 1 and will not be described again here.

[0124] The positive electrode materials obtained in Examples 1 to 15 and Comparative Examples 1 to 3 were applied to positive electrode sheets.

[0125] The preparation method of the positive electrode sheet is as follows: the positive electrode material, SuperP, and PVDF are mixed in a ratio of 97.5:1.4:1.2, the solvent NMP is added, and the mixture is stirred and mixed evenly under the action of a vacuum mixer to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and then the positive electrode sheet is obtained after baking, cold pressing, and die-cutting.

[0126] The resulting positive electrode sheet is used in lithium-ion batteries. A lithium-ion soft-pack battery cell is made using graphite as the negative electrode active material and a polyethylene diaphragm coated with ceramic on one side as the separator.

[0127] The following performance tests were performed on the positive electrode sheets and lithium-ion batteries prepared in the examples and comparative examples respectively:

[0128] (1) Statistics of particle breakage after roller pressing

[0129] The number of broken particles in the obtained positive electrode sheet after rolling was counted. The lithium cobalt oxide electrode sheet after rolling was taken, and the cross-section of the sample was polished by an argon ion beam. The SEM of the electrode section was tested, and the number of cracked and broken first lithium cobalt oxide particles and second lithium cobalt oxide particles in 100 particles was counted by random sampling.

[0130] (2) Cyclic performance test

[0131] Place the soft-pack battery cell in a 45°C environment, charge it at a constant current and constant voltage of 3C to 4.53V, cut off the current at 0.05C, and discharge it at a constant current of 0.7C to 3.0V. This step of charge and discharge is recorded as one cycle. Repeat this cycle until the capacity retention rate of the battery cell is less than 80%. Stop testing the battery cell and record the cycle number of this battery cell.

[0132] The test results are shown in Table 1 below.

[0133] Table 1

[0134] From the comparison of the results in Table 1 above, it can be seen that compared with the positive electrode material in Comparative Example 1, the positive electrode material provided by the present invention can effectively protect the first lithium cobalt oxide particles and the second lithium cobalt oxide particles of the positive electrode material by adding third lithium cobalt oxide particles with a larger particle size, thereby reducing the number of particles broken under high rolling pressure, increasing the number of battery cycles, and effectively improving the high temperature cycle performance of the battery.

[0135] Among them, from the comparison between Example 1 and Examples 2 to 4 and Comparative Examples 2 to 3, it can be seen that even if the first lithium cobalt oxide particles, the second lithium cobalt oxide particles, and the third lithium cobalt oxide particles are used at the same time, the mass proportions of the three are different, the degree of protection of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles and the cycle performance of the battery will be affected. The lower the mass proportion of the third lithium cobalt oxide particles, the more particles are broken after rolling, and the worse the cycle performance of the battery; if the mass proportion of the third lithium cobalt oxide particles exceeds 1.1%, the cycle of the battery will also be reduced; it can be seen that the mass content of the third lithium cobalt oxide particles is above 0.1% to 1.1%, which will have a better protection effect and improve the high temperature cycle performance of the battery. Preferably, the mass content of the third lithium cobalt oxide particles is controlled at 0.5%.

[0136] From the comparison between Example 1 and Examples 7-13, it can be seen that the particle size Dv50 of the second lithium cobalt oxide particles will affect the high-temperature cycle number of the battery. The smaller the particle size, the larger the specific surface area of ​​the corresponding material. When the specific surface area is large, the contact area between the lithium cobalt oxide and the electrolyte will be larger, and the side reactions generated by the electrolyte and the surface of the lithium cobalt oxide at high temperature will also be more, which will affect the high-temperature cycle performance. Therefore, the smaller the particle size Dv50 of the second lithium cobalt oxide particles, the lower the cycle number of the battery.

[0137] From the comparison between Example 1 and Examples 9, 10, 12, and 15, it can be seen that the Dv50 particle size of the second lithium cobalt oxide particles will affect the number of broken particles after rolling. The larger the Dv50 particle size of the second lithium cobalt oxide particles, the more broken particles there are. The mass proportion of the second lithium cobalt oxide particles is the highest among the three particles. During rolling, the third lithium cobalt oxide particles are broken first, followed by the second lithium cobalt oxide particles with a particle size smaller than the third lithium cobalt oxide particles. The second lithium cobalt oxide particles are one of the main materials in the positive electrode sheet. The larger the particle size, the higher the probability of breakage. Therefore, the second lithium cobalt oxide particles must be controlled at 17.5 μm to play a role in improving battery performance.

[0138] In summary, the present invention protects the original first and second lithium cobalt oxide particles of the positive electrode material by introducing an "oversized" third lithium cobalt oxide particle, so that they do not crack or break during rolling. By controlling the particle size and mass ratio of the three particles, the performance of subsequent battery cells can be effectively improved.

[0139] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above. 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. In addition, although certain specific terms are used in this description, these terms are for convenience only and do not constitute any limitation to the present invention.

Claims

1. A cathode material, characterized in that, It includes a first lithium cobaltate particle, a second lithium cobaltate particle, and a third lithium cobaltate particle; Among them, the particle size R1 of Dv50 of the first lithium cobaltate particle, the particle size R2 of Dv50 of the second lithium cobaltate particle, and the particle size R3 of Dv50 of the third lithium cobaltate particle satisfy the relationship: R3 > R2 > R1, and the mass m1 of the first lithium cobaltate particle, the mass m2 of the second lithium cobaltate particle, and the mass m3 of the third lithium cobaltate particle satisfy the relationship: 0.1% ≤ m3 / (m1 + m2) ≤ 1.1%.

2. The cathode material according to claim 1, characterized in that, Among them, The following relationships are satisfied among R1, R2, and R3: 2.5 < R3 / R2 < 4.8; 8.9 < R3 / R1 < 29.2; 1.9 < R2 / R1 < 10.

2.

3. The cathode material according to claim 1 or 2, characterized in that, The value range of R1 is 2.0 - 6.0 μm; the value range of R2 is 13 - 20 μm; the value range of R3 is 55 - 58 μm.

4. The cathode material according to claim 1, characterized in that, The peak I1 of the first lithium cobaltate particle, the peak I2 of the second lithium cobaltate particle, and the peak I3 of the third lithium cobaltate particle of the particle size distribution curve of the positive electrode material satisfy the relationship: I2 > I1 > I3.

5. The cathode material according to claim 1, characterized in that, The mass of the first lithium cobaltate particle accounts for 10% - 30% of the mass of the positive electrode material, the mass of the second lithium cobaltate particle accounts for 69% - 89.9% of the mass of the positive electrode material, and the mass of the third lithium cobaltate particle accounts for 0.1% - 1% of the mass of the positive electrode material.

6. The cathode material according to claim 1, characterized in that, Both the first lithium cobaltate particle and the second lithium cobaltate particle are doped with element M, and M is one or more of Al, Mg, Ti, La, Y, Zr, Ho, Ce, V, Mo.

7. The cathode material according to claim 6, characterized in that, The molar ratio of Li and Co in the first lithium cobaltate particle and the second lithium cobaltate particle is 1.02 - 1.08, and the molar ratio of M and Co is 0.005 - 0.05; the molar ratio of Li and Co in the third lithium cobaltate particle is 1.04 - 1.

09.

8. The preparation method of the cathode material according to any one of claims 1 to 7, characterized in that It includes the following steps: S1. Sinter the precursor Co3O4, Li2Co3, and metal oxide of the first lithium cobaltate particle at high temperature for 16 - 18 h to obtain the first lithium cobaltate particle; S2. Sinter the precursor Co3O4, Li2Co3, and metal oxide of the second lithium cobaltate particle at high temperature for 18 - 20 h to obtain the second lithium cobaltate particle; S3. Sinter the precursor Co3O4 and Li2Co3 of the third lithium cobaltate particle at high temperature for 20 - 22 h to obtain the third lithium cobaltate particle; S4. Uniformly mix the prepared first lithium cobaltate particle, second lithium cobaltate particle, and third lithium cobaltate particle together to obtain the positive electrode material described above.

9. A positive electrode sheet, characterized in that, It includes the positive electrode material according to any one of claims 1 - 7.

10. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet, characterized in that, The positive electrode sheet is the positive electrode sheet according to claim 9.

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