Composite positive electrode material, preparation method thereof and positive plate

Abstract

The invention discloses a composite positive electrode material, a preparation method thereof and a positive plate. The composite positive electrode material comprises a positive electrode material matrix and a carbon coating layer coating the positive electrode material matrix, and a middle growth layer is further arranged between the positive electrode material matrix and the carbon coating layer; the intermediate growth layer contains phosphate particles. Compared with the prior art, the middle growth layer is arranged between the positive electrode material substrate and the carbon coating layer, so that the adhesion area of the carbon coating material is increased, the carbon coating layer is firmer, and the carbon coating effect is better. Specifically, in the sintering process of carbon coating, the middle growth layer with the same crystal structure as the positive electrode material matrix is locally fused and grows on the surface of the positive electrode material matrix, so that the carbon coating achieves the effect similar to in-situ coating, and the tight combination between the carbon coating layer and the positive electrode material matrix is ensured; the surface structure of the composite positive electrode material is greatly enhanced, and the service life of the battery is effectively prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a composite cathode material, a preparation method thereof, and a cathode sheet. Background Art

[0002] In the field of energy storage and supply, lithium-ion batteries have gradually established their dominant position due to their excellent energy density, long cycle life, memory-free effect, and extremely low self-discharge rate. The structure of lithium-ion batteries is sophisticated, including a cathode, an anode, a separator, an electrolyte, and a series of precision components. Among them, the cathode material, as the core component, plays a decisive role in the overall performance of the battery. Currently, the selection of commercial lithium-ion battery cathode materials is quite rich, mainly including lithium cobaltate, ternary materials, lithium manganate, lithium iron phosphate, and emerging materials such as lithium manganese iron phosphate that are gradually moving to the forefront of the application stage.

[0003] Lithium iron phosphate material stands out among many cathode materials due to its suitable discharge voltage, theoretically high specific capacity, excellent thermal stability and electrochemical stability, wide electrolyte compatibility, rich raw material sources, and environmental friendliness. Notably, the innovative application of nanotechnology and carbon coating technology has significantly improved the conductivity of lithium iron phosphate materials, making them one of the favored cathode materials. On this basis, lithium manganese iron phosphate material, as a derivative variant of lithium iron phosphate, has also become the focus of current research and development work.

[0004] In the synthesis route of phosphate cathode materials, the high-temperature carbothermal reduction method occupies the mainstream position, which ingeniously realizes the in-situ coating of the carbon layer and ensures excellent coating effects. However, compared with such traditional methods, other synthesis strategies such as solvothermal synthesis show unique advantages in the precise control of material particle size, the optimized design of morphology, and the fine adjustment of components. However, solvothermal synthesis usually requires the pre-synthesis of phosphate materials and then subsequent carbon coating treatment, that is, the so-called non-in-situ carbon coating. However, the adhesion effect of the carbon coating layer in non-in-situ carbon coating is not good enough, resulting in problems such as a decrease in the thermal stability of the cathode material, a shortening of the cycle life, and an acceleration of capacity decay.

[0005] In view of the above defects existing in current phosphate cathode materials, it is indeed necessary to provide a technical solution to solve the above problems. Summary of the Invention

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

[0007] To achieve this purpose, the present invention provides the following solutions:

[0008] A composite cathode material includes a cathode material matrix and a carbon coating layer coated on the cathode material matrix, and an intermediate growth layer is further provided between the cathode material matrix and the carbon coating layer; the intermediate growth layer contains phosphate particles.

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

[0010] Preferably, the chemical formula of the cathode material matrix is LiM 1-x N x PO 4 , where M is at least one of Fe, Mn, Ni, and Co, 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 PO 4 , where A is at least one of Fe, Ni, and Co, 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% - 30% of the total mass of the composite cathode material.

[0013] Preferably, the mass of the carbon coating layer is 0.5% - 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 cathode material matrix satisfy the relationship: (D90 - D10) / D50 ≤ 2.

[0016] The present invention also provides a preparation method of a composite cathode material, including the following steps:

[0017] Step 1: After separately weighing a lithium source, an M source, an N source, and a phosphorus source, using water or an organic liquid as a solvent, and transferring them to an autoclave for thorough mixing, sealing and heating in an inert atmosphere, carrying out a solvothermal reaction, and performing treatments such as cooling, filtering, washing, drying, and pulverizing to obtain a cathode material matrix;

[0018] Step 2: Adding a lithium source, an A source, a B source, a phosphorus source, and a carbon source to a first premixing tank, and simultaneously adding pure water, and carrying out stirring and premixing to obtain a first premixed slurry;

[0019] Step 3: Add the positive electrode material matrix and pure water into a second premixing tank and stir to obtain a second premixed slurry.

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

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

[0022] Preferably, in the step 4, the grinding particle size of the third slurry is D50 ≤ 300 nm.

[0023] The present invention also provides a positive electrode sheet, which includes a positive electrode material, a conductive agent, and a binder, and the positive electrode material is the above-mentioned composite cathode material.

[0024] The present invention also provides a secondary battery, which includes a separator, a negative electrode sheet, an electrolyte, a battery case, and the above-mentioned positive electrode sheet.

[0025] Compared with the prior art, the beneficial effect of the present invention is that: by providing an intermediate growth layer between the positive electrode material matrix and the carbon coating layer, the attachment area of the carbon coating material is increased, making the carbon coating layer more firm and the carbon coating effect better. Specifically, during the sintering process of the carbon coating, the intermediate growth layer having 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 an effect similar to in-situ coating, ensuring the tight combination between the carbon coating layer and the positive electrode material matrix, greatly enhancing the surface structure of the composite cathode material, and effectively extending the service life of the battery. Description of the Drawings

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

[0027] Among them, 1 is the positive electrode material matrix; 2 is the intermediate growth layer; 3 is the carbon coating layer. Detailed Embodiments

[0028] To make the technical solutions and advantages of the present invention clearer, the following will combine specific embodiments to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0029] In a first aspect according to the present invention, a composite cathode material is provided, which includes a cathode material matrix 1 and a carbon coating layer 3 coated on the cathode material matrix 1. An intermediate growth layer 2 is further provided between the cathode material matrix 1 and the carbon coating layer 3; the intermediate growth layer 2 contains phosphate particles.

[0030] Among them, the cathode material matrix 1 and the intermediate growth layer 2 have the same crystal structure.

[0031] The carbon coating layer 3 is distributed throughout all the void positions between the intermediate growth layer 2 and the cathode material matrix 1. The intermediate growth layer 2 increases the attachment area of the carbon coating material, making the carbon coating layer 3 more firm and having a better carbon coating effect. During the sintering process of the carbon coating, the intermediate growth layer 2 having the same crystal structure as the cathode material matrix 1 locally fuses and grows on the surface of the cathode material matrix 1, enabling the carbon coating to achieve an effect similar to in-situ coating, ensuring a tight combination between the carbon coating layer 3 and the cathode material matrix 1, greatly reducing the risk of the coating layer falling off or being damaged, and effectively extending the service life of the battery.

[0032] The composite cathode material obtained by the present invention has a stable surface structure and good electrical conductivity. A lithium-ion battery made using it as the cathode active material has excellent electrochemical performance.

[0033] In an embodiment according to the present invention, the particle size of the cathode material matrix 1 is a, and the average particle size of the particles in 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, 600 nm; when better rate performance of the material is required, the cathode material matrix 1 needs to have a smaller particle size. When a higher tap density of the material is required, the particle size of the cathode material matrix 1 needs to be larger. However, because phosphate materials have very poor electrical conductivity, too large particles will cause a significant reduction in the specific capacity of the material. Considering comprehensively, the given range is below 600. 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, 200 nm. As the intermediate growth layer 2, in addition to the most important role of connecting the matrix material and the carbon coating layer, it also plays a role in filling the voids between the cathode material matrix 1 and improving the tap density of the composite cathode material. Its particles should be small, so that it has better activity for sintering and fusing with the cathode material matrix 1. At the same time, it is preferably that the particle size distribution range is wide, so that the voids between the cathode material matrix 1 can be effectively filled.

[0034] In an embodiment according to the present invention, the chemical formula of the cathode material matrix 1 is LiM 1-x Nx PO 4 , wherein M is at least one of Fe, Mn, Ni, and Co, 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 according to the present invention, the chemical formula of the phosphate particles is LiA 1-y B y PO 4 , wherein A is at least one of Fe, Ni, and Co, B is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co, and Mn, and 0 ≤ y ≤ 0.1.

[0036] In the present invention, the design of the cathode material matrix 1 and the material of the intermediate growth layer 2 is further optimized, and they can both include element doping at specific positions. Specifically, the Li site can be doped with one or a combination of Na and K elements, while the P site or O site can be doped with one or more of N, F, S and other elements. This doping strategy not only enriches the composition of the material, but also may further improve the electrochemical performance of the material by adjusting the electronic structure and ionic conductivity.

[0037] The compositions of the cathode material matrix 1 and the intermediate growth layer 2 can be the same or different according to actual needs to optimize the overall performance. In particular, the intermediate growth layer 2 is preferably a phosphate material without manganese and with a stable structure. When this intermediate growth layer 2 covers the surface of the manganese-containing phosphate cathode material matrix 1, it can effectively inhibit the dissolution of manganese elements and reduce the side reactions with the electrolyte, thereby improving the cycle stability and safety of the battery.

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

[0039] In one embodiment according to the present invention, the mass of the intermediate growth layer 2 is 0.5% - 30% of the total mass of the composite cathode material. The mass ratio of the intermediate growth layer 2 within the range of 0.5% to 30% of the total mass of the composite cathode material is appropriate. If the ratio is too low, the effective connection between the cathode material matrix 1 and the carbon coating layer 3 cannot be fully realized, thereby affecting the overall performance of the material. On the contrary, if the ratio is too high, it will not only inhibit the excellent properties of the cathode material matrix 1 itself, such as high activity and high capacity, but also may cause a decrease in the tap density of the material, which is not conducive to the energy density and cycle stability of the battery.

[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 positive electrode material. The mass proportion of the carbon coating layer 3 in the composite positive electrode material should be controlled between 0.5% and 3.0%. If the proportion is lower than the lower limit of this range, it will be difficult to form a continuous and complete carbon coating layer 3, and it will not be possible to effectively protect the positive electrode material matrix 1 and improve its conductivity. On the contrary, if the proportion exceeds the upper limit of this range, the proportion of electrochemically active components will be significantly reduced, which will in turn have an adverse effect on key performance indicators such as gram capacity and compaction density, and reduce the energy storage capacity and volume energy density of the battery. Therefore, precise control of the proportion of the carbon coating layer 3 is crucial to balancing the electrochemical properties and physical properties of the composite positive electrode material.

[0041] In one embodiment of the present invention, D90, D50 and D10 of the phosphate particles satisfy the relationship: (D90-D10) / D50≥2. The particle size of the intermediate growth layer 2 is relatively small and has a relatively wide distribution range. As the intermediate growth layer 2, in addition to the most important function of connecting the base material and the carbon coating layer, it also plays the role of filling the gaps between the positive electrode material matrix 1 and improving the compaction density of the composite positive electrode material. Its particles should be small so that it has better activity in sintering and fusing with the positive electrode material matrix 1. At the same time, it is best that the particle size distribution range is wide, so that the gaps between the positive electrode material matrix 1 can be effectively filled.

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

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

[0044] Step 1: After weighing the lithium source, M source, N source and phosphorus source respectively, using water or organic liquid as solvent, transfer them to a high-pressure reactor and mix them thoroughly, seal and heat them in an inert atmosphere to carry out a solvent thermal reaction, and then cool, filter, wash, dry and crush them to obtain a positive electrode material matrix 1;

[0045] Step 2: adding the lithium source, source A, source B, phosphorus source and carbon source into a first premixing tank, and adding pure water at the same time, stirring and premixing to obtain a first premixed slurry;

[0046] Step 3: Add the positive electrode material matrix 1 and pure water into a second premixing tank and stir to obtain a 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, and continue grinding to obtain the fourth slurry, and dry it to obtain the composite cathode precursor.

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

[0049] The preparation method of the present invention is simple and efficient in process design, and the materials used are widely sourced and easy to obtain. Specifically, by adding the intermediate growth layer 2 in this preparation method, the attachment area of the carbon coating layer 3 is increased, ensuring that the formation of the carbon coating layer 3 is more firm and reliable, and achieving uniform and tight coating of the carbon layer on the surface of the composite cathode material. This improvement greatly enhances the effect of carbon coating. Compared with the traditional method, the composite cathode material prepared by the present invention not only has a stronger carbon layer binding force, but also can effectively prevent the shedding of active substances, significantly enhancing the structural stability of the material.

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

[0051] In an embodiment according to the present invention, in Step 1, 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 M element; 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 materials containing two or several of Li, M, and P, and the lithium source, M source, and phosphorus source should be added according to the specific element ratio of the materials during the preparation process.

[0052] In an embodiment according to the present invention, in Step 2, 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 A element; 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 materials containing two or several of Li, A, and P, and the lithium source, A source, and phosphorus source should be added according to the specific element ratio of the materials during the preparation process.

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

[0054] In an embodiment according to the present invention, in step one, the surface of the cathode material matrix 1 is subjected to an activation treatment; the surface activation treatment is at least one of acid treatment, alkali treatment, and high-energy ball milling treatment. The main purpose of performing the activation treatment on the surface of the cathode material matrix 1 is to promote good bonding and adhesion between the intermediate growth layer 2 and the surface of the main material, so as to ensure that the intermediate growth layer 2 can be deposited and grown on the surface of the main material more effectively and uniformly.

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

[0056] In the third aspect of the present invention, a cathode sheet is further provided, which includes a cathode material, a conductive agent, and a binder, and the cathode material is the above-mentioned composite cathode material.

[0057] In the fourth aspect of the present invention, a secondary battery is further provided, which includes a separator, an anode sheet, an electrolyte, a battery case, and the above-mentioned cathode sheet.

[0058] The anode sheet includes an anode current collector and an anode active material layer coated on at least one surface of the anode current collector. The anode active material layer may include, but is not limited to, one or several of graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium, etc.

[0059] Among them, the graphite may be selected from one or several of artificial graphite, natural graphite, and modified graphite; the silicon-based material may be selected from one or several of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material may be selected from one or several of elemental tin, tin oxide compounds, and tin alloys. The anode current collector is usually a structure or part for collecting current, and the anode current collector may be various materials suitable for use as the anode current collector of a secondary battery in the art. For example, the anode current collector may include, but is not limited to, metal foils, etc., and more specifically may include, but is not limited to, copper foils, etc.

[0060] The secondary battery further includes an electrolyte, and the electrolyte includes an organic solvent, an electrolyte lithium salt, and an additive. Among them, the electrolyte lithium salt may be LiPF 6 and / or LiBOB; it may also be LiBF 4 、LiBOB、LiPF6 at least one of; it can also be LiBF used in the overcharge prevention type electrolyte 4 , LiBOB, LiPF 6 , at least one of LiTFSI; it can also be LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 at least one of. The organic solvent can be cyclic carbonates, including PC, EC; it can also be linear carbonates, including DFC, DMC, or EMC; it can also be carboxylic acid esters, 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 prevention additives, additives for controlling the content of H 2 O and HF in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

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

[0062] Example 1

[0063] This example provides a composite cathode material, the cathode material matrix 1 is LiFePO 4 , and on the surface of the cathode material matrix 1, there are particles of the intermediate growth layer 2 of LiFePO 4 material and a carbon coating layer 3. Among them, the mass ratio of the intermediate growth layer 2 to the mass of the composite cathode material is 10%, and the weight ratio of the carbon coating material to the mass of the composite cathode material is 1.5%; the average particle size of the cathode material matrix 1 is 500 nm, the average particle size of the particles of the intermediate growth layer 2 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 cathode composite material is as follows:

[0065] Step 1: Weigh lithium hydroxide, ferrous sulfate, and phosphoric acid according to the molar ratio of Li:Fe:P = 3:1:1, use pure water as the solvent, mix them evenly in a high-pressure reaction kettle, displace the air in the kettle with nitrogen, seal it and heat it to 180 °C for 6 h, cool it and then open the kettle to collect the material, filter and separate the solid product, and wash it with pure water and then dry and pulverize it to obtain the cathode material matrix 1 (LiFePO 4 );

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

[0067] Step 3: Add the cathode material matrix 1 and pure water into the second premixing tank and stir to obtain a second premixed slurry with a solid content of 40%.

[0068] Step 4: First, grind the first premixed slurry for 4 hours to obtain a third slurry. Then, add the second premixed slurry and mix it with the third slurry, and continue grinding for 30 minutes to obtain a fourth slurry. After spray drying, a composite cathode precursor is obtained.

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

[0070] Preparation of secondary battery:

[0071] Mix the composite cathode material, conductive carbon black, and PVDF according to a mass ratio of 95:3:2, and disperse them in N-methylpyrrolidone to form a slurry. After stirring, coating, drying, rolling, and slitting, a positive electrode sheet is obtained.

[0072] Use artificial graphite as the negative electrode active material, mix conductive carbon black and PVDF according to a mass ratio of 94:3:3, and disperse them in NMP to form a slurry. After stirring, coating, drying, rolling, and slitting, a negative electrode sheet is obtained.

[0073] Wind the negative electrode sheet, positive electrode sheet, and separator alternately, and then go through the steps of terminal welding, packaging with aluminum foil encapsulation, injection of electrolyte, encapsulation and formation, and evacuation and forming to finally obtain a soft-packaged lithium-ion battery with a designed battery capacity of 2500 mAh. Among them, the solute of the electrolyte is 1 mol / L LiPF 6 , and the solvent is EC, DMC, and DEC with a volume ratio of 1:1:1.

[0074] Example 2

[0075] The difference from Example 1 is that the intermediate growth layer 2 is LiNiPO 4 , and its mass accounts for 3% of the mass of the composite cathode material.

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

[0077] Example 3

[0078] The difference from Example 1 is that the cathode material matrix 1 is LiMn 0.7 Fe0.3 PO 4 The material, the carbon content of the composite cathode material is 1.6%, 0.5% of CNT is added as an inorganic carbon source during synthesis, and the sintering temperature is 650 °C.

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

[0080] Example 4

[0081] Different from Example 1, the cathode material matrix 1 is LiMn 0.6 Fe 0.39 Mg 0.01 PO 4 The material, the intermediate growth layer 2 is LiNiPO 4 , and the proportion of the intermediate growth layer 2 in the mass of the composite cathode material is 2%. 0.5% of graphene is added as an inorganic carbon source during material synthesis, and the sintering temperature is 650 °C.

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

[0083] Example 5

[0084] Different from Example 4, the intermediate growth layer 2 is LiCoPO 4 .

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

[0086] Example 6

[0087] Different from Example 3, the cathode material matrix 1 is LiMnPO 4 .

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

[0089] Example 7

[0090] Different from Example 1, in step one, the surface of the cathode material matrix 1 is also subjected to an activation treatment, and the surface activation method is to subject the main material LiFePO 4 to high-speed ball milling for 3 h, and the ball milling speed is 800 rpm.

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

[0092] Example 8

[0093] Different from Example 1, the average particle size of the cathode material matrix 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 that of Example 1 and will not be repeated here.

[0095] Example 9

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

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

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

[0099] Comparative Example 1

[0100] Different from Example 1, in Comparative Example 1, there is no intermediate growth layer 2, and the carbon coating sintering is directly carried out on the cathode material substrate 1.

[0101] The rest is the same as in Example 1 respectively and will not be elaborated here.

[0102] Comparative Example 2

[0103] Different from Example 2, in Comparative Example 2, there is no intermediate growth layer 2, and the carbon coating sintering is directly carried out on the cathode material substrate 1.

[0104] The rest is the same as in Example 2 respectively and will not be elaborated here.

[0105] Comparative Example 3

[0106] Different from Example 3, in Comparative Example 3, there is no intermediate growth layer 2, and the carbon coating sintering is directly carried out on the cathode material substrate 1.

[0107] The rest is the same as in Example 3 respectively and will not be elaborated here.

[0108] Comparative Example 4

[0109] Different from Example 4, in Comparative Example 4, there is no intermediate growth layer 2, and the carbon coating sintering is directly carried out on the cathode material substrate 1.

[0110] The rest is the same as in Example 4 respectively and will not be elaborated here.

[0111] Comparative Example 5

[0112] Different from Example 5, in Comparative Example 5, there is no intermediate growth layer 2, and the carbon coating sintering is directly carried out on the cathode material substrate 1.

[0113] The rest is the same as in Example 5 respectively and will not be elaborated here.

[0114] Comparative Example 6

[0115] Different from Example 6, in Comparative Example 6, there is no intermediate growth layer 2, and the carbon coating sintering is directly carried out on the cathode material substrate 1.

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

[0117] Comparative Example 7

[0118] Different from Example 7, in Comparative Example 7, there is no intermediate growth layer 2, and carbon coating sintering is directly carried out on the cathode material substrate 1.

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

[0120] Comparative Example 8

[0121] Different from Example 8, in Comparative Example 8, there is no intermediate growth layer 2, and carbon coating sintering is directly carried out on the cathode material substrate 1.

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

[0123] Comparative Example 9

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

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

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

[0127] Performance test:

[0128] The cathode materials of the above examples and comparative examples were subjected to powder compaction density tests, and the test results are shown in Table 1;

[0129] The secondary batteries of the above examples and comparative examples were subjected to electrochemical performance tests, and the test results are shown in Table 1.

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

[0131] Powder compaction density test: Take a certain mass of the sample and put it into a special tablet pressing mold. Place the mold on a powder compaction density tester, and the program controls the application of a pressure of 30 KN. The test software gives the compaction density value.

[0132] Capacity and cycle performance test: At 25°C, charge at a constant current of 0.5C (1250 mA) until the charge cut-off voltage, then charge at a constant voltage until 0.05C (125 mA), and then discharge at 0.5C (1250 mA) to 2.0V. Repeat this charge-discharge cycle 1000 times, and measure the discharge capacity at the first cycle and the discharge capacity at the 1000th cycle. Among them, the cut-off voltage varies according to the different cathode active materials; for example, when the cathode active material is lithium iron phosphate, the charge cut-off voltage is set to 3.7V; when the cathode active material is lithium iron manganese phosphate, the charge cut-off voltage is set to 4.5V; when the cathode active material is a phosphate material containing cobalt or nickel, the charge cut-off voltage is set to 5.0V.

[0133] Discharge specific capacity in the first week (mAh / g) = Discharge capacity in the first cycle (mAh) / Mass of cathode active material (g);

[0134] Capacity retention rate after 1000 cycles = (Discharge capacity in the 1000th cycle / Discharge capacity in the first cycle) × 100%.

[0135] Rate performance test: At 25°C, charge at a constant current of 0.5C (1250 mA) until the cut-off voltage, charge at a constant voltage until 0.05C (125 mA), and then discharge at 0.5C (1250 mA) to 2.0V. Repeat 10 times to calculate the average discharge energy, which is recorded as the discharge energy of the 0.5C cycle; charge at a constant current of 0.5C (1250 mA) until the cut-off voltage, charge at a constant voltage until 0.05C (125 mA), and then discharge at 10C (25000 mA) to 2.0V. Repeat 10 times to calculate the average discharge energy, which is recorded as the discharge energy of the 10C cycle. Among them, the cut-off voltage varies according to the different cathode active materials; for example, when the cathode active material is lithium iron phosphate, the charge cut-off voltage is set to 3.7V; when the cathode active material is lithium iron manganese phosphate, the charge cut-off voltage is set to 4.5V; when the cathode active material is a phosphate material containing cobalt or nickel, the charge cut-off voltage is set to 5.0V.

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

[0137] Table 1

[0138]

[0139]

[0140] It can be clearly seen from the test data shown in Table 1 that the data of Examples 1 to 8 are significantly better than those of Comparative Examples 1 to 8. This result indicates that the introduction of the intermediate growth layer 2 between the cathode material matrix 1 and the carbon coating layer 3 effectively increases the attachment area of the carbon-coated material. This design enables the carbon coating layer 3 to adhere more firmly to the cathode material matrix 1, thereby enhancing the overall effect of carbon coating. Further, 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, the first-week discharge specific capacity, rate discharge energy retention rate, and 1000-week capacity retention rate of Examples 1 to 8 all show obvious advantages over those of Comparative Examples 1 to 8. 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 secondary batteries.

[0142] It can be clearly seen from the test data of Examples 4 to 5 and Example 6 that the test data of Examples 4 to 5 are all better than those of Example 6. This result indicates that the intermediate growth layer 2 is preferably a phosphate material without manganese and with a stable structure. When this intermediate growth layer 2 covers the surface of the manganese-containing phosphate cathode material matrix 1, it can effectively inhibit the dissolution of manganese elements and reduce the side reactions with the electrolyte, thereby improving the cycle stability and safety of the battery.

[0143] Comparing the test data of Example 9 and Comparative Example 9, the data of Example 9 are better than those of Comparative Example 9, indicating that better effects will be achieved when the particle size of the intermediate growth layer 2 has a relatively wide distribution range. This is mainly because as the intermediate growth layer 2, in addition to the main function of connecting the matrix material and the carbon coating layer, it also plays a role in filling the voids between the cathode material matrices 1 and increasing the compaction density of the composite cathode material. A wide particle size distribution can effectively fill the voids between the cathode material matrices 1.

[0144] According to the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions, or variations made by those skilled in the art based on the present invention all fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A composite positive electrode material, characterized in that: It comprises a positive electrode material matrix and a carbon coating layer coated on the positive electrode material matrix. An intermediate growth layer is arranged between the positive electrode material matrix and the carbon coating layer. The intermediate growth layer contains phosphate particles.

2. The composite positive electrode material according to claim 1, characterized in that The chemical formula of the positive electrode 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; And / or, 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, B is at least one of Mg, Ti, Al, Zr, Ba, V, Ni, Co and Mn, and 0≤y≤0.

1.

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

4. The composite positive electrode 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 positive electrode material.

5. The composite positive electrode 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 positive electrode material.

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

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

8. A method for preparing a composite positive electrode material, characterized in that: The following steps are involved: Step 1: After weighing the lithium source, M source, N source and phosphorus source respectively, transfer them to a high-pressure reactor with water or organic liquid as solvent and mix them thoroughly, seal and heat them under an inert atmosphere to carry out solvent thermal reaction, and then cool, filter, wash, dry and crush them to obtain a positive electrode material matrix; Step 2: adding the lithium source, source A, source B, phosphorus source and carbon source into a first premixing tank, and adding pure water at the same time, stirring and premixing to obtain a first premixed slurry; Step 3: adding the cathode material matrix and pure water into a second premixing tank and stirring to obtain a second premixed slurry; Step 4: first grind the first premixed slurry to obtain a third slurry; then add the second premixed slurry and mix with the third slurry, continue grinding to obtain a fourth slurry, and dry to obtain a composite material positive electrode precursor; Step 5: Sintering the composite positive electrode material precursor at a temperature of 500 to 880° C. for 4 to 10 hours to obtain a composite positive electrode material.

9. The method for preparing a composite positive electrode material according to claim 8, characterized in that: In the step 4, the grinding particle size of the third slurry is D50≤300nm.

10. A positive electrode sheet, comprising a positive electrode material, a conductive agent and a binder, characterized in that: The positive electrode material is the composite positive electrode material according to any one of claims 1 to 7 or a composite positive electrode material prepared by the method for preparing the composite positive electrode material according to any one of claims 8 to 9.

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