Preparation Method of Lithium Iron Phosphate Cathode Material, Cathode Material, Electrode Sheet and Battery

Abstract

The present invention discloses a preparation method of a lithium iron phosphate cathode material, a cathode material, an electrode sheet and a battery, including mixing a lithium source, an iron source, a phosphorus source, a carbon source and a solvent uniformly, and obtaining a first slurry after grinding; wherein the molar ratio of element Fe to P in the first slurry is y:z; mixing a lithium source, an iron source, a phosphorus source, a carbon source and a solvent uniformly, and obtaining a second slurry after grinding; wherein the molar ratio of element Fe to P in the second slurry is b:c; and y and b satisfy: y:z≥b:c; drying the first slurry once to obtain a first spray material; using the first spray material as a base material, using the second slurry as a coating liquid, coating the second slurry on the first spray material, and performing secondary drying to obtain a second spray material; sintering and pulverizing the second spray material to obtain the lithium iron phosphate cathode material. The present invention can precisely control the size of the product lithium iron phosphate particles, improve the tap density of the lithium iron phosphate cathode material, and further improve the capacity and cycle performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a preparation method of a lithium iron phosphate cathode material, a cathode material, a pole piece and a battery. Background Art

[0002] Lithium-ion batteries are widely used in power batteries and energy storage fields due to their efficient and clean energy storage characteristics. Lithium iron phosphate batteries have advantages such as high capacity, low cost, long cycle life, and high safety. With the increasing requirement for the driving range of electric vehicles, the requirement for the energy density of the cathode material is also increasing. However, compared with ternary materials, the lower discharge voltage and tap density of lithium iron phosphate itself result in its lower energy density than ternary materials.

[0003] Currently, the energy density of the material is increased by improving the tap density of lithium iron phosphate. A common method for preparing lithium iron phosphate with high tap density is to prepare lithium iron phosphate with high tap density through multiple sinterings. The active material is sintered multiple times to prepare primary crystal phases, doping, and carbon coating in sequence, and finally lithium iron phosphate with high tap density is prepared. However, the above method has high energy consumption, a long material preparation cycle, and high cost.

[0004] Therefore, how to improve the tap density and energy density of lithium iron phosphate materials, while solving the problems of uneven particle size distribution and extremeization, and simplifying the preparation process and reducing energy consumption, has become an important research topic. Summary of the Invention

[0005] In view of this, the present invention aims to solve at least one of the technical problems in the related art. The present invention provides a preparation method of a lithium iron phosphate cathode material, a cathode material, a pole piece and a battery, which can accurately control the size of the produced lithium iron phosphate particles, thereby realizing the particle grading effect of the lithium iron phosphate cathode material, improving its tap density, and further improving the capacity and cycle performance of the battery.

[0006] To solve the above technical problems, the present invention is implemented as follows:

[0007] According to the first aspect of the present invention, the present invention provides a preparation method of a lithium iron phosphate cathode material, including the following steps:

[0008] S1: Mix a lithium source, an iron source, a phosphorus source, a carbon source and a solvent uniformly, and obtain a first slurry after grinding; wherein, the molar ratio of elements Fe and P in the first slurry is y:z;

[0009] Mix a lithium source, an iron source, a phosphorus source, a carbon source and a solvent uniformly, and obtain a second slurry after grinding; wherein, the molar ratio of elements Fe and P in the second slurry is b:c; and the y and b satisfy: y:z≥b:c;

[0010] S2: Perform primary drying on the first slurry to obtain a first spray material;

[0011] S3: Use the first spray material as the base material, use the second slurry as the coating liquid, coat it on the first spray material, and perform secondary drying to obtain a second spray material;

[0012] S4: Sinter and pulverize the second spray material to obtain the lithium iron phosphate cathode material.

[0013] In an optional embodiment, the chemical general formula of the first slurry is Li x Fe y P z O4, where x:y:z = (1.0~1.05):(0.97~1):1.0.

[0014] In an optional embodiment, the chemical general formula of the second slurry is Li a Fe b P c O4, where a:b:c = (1.0~id="1.05):(0.955~0.97):1.0.

[0015] In an optional embodiment, the preparation method of the lithium iron phosphate cathode material satisfies at least one of the features (1)~(8):

[0016] (1) The solid content of the first slurry is 20%~50%; (2) The particle size of the first slurry is 0.2μm ≤ D 50 ≤ 4μm; (3) The water content of the first spray material is 0.5%~5%; (4) The particle size of the first spray material is 15μm ≤ D 50 ≤ 30μm; (id="36) The solid content of the second slurry is 20%~50%; (6) The particle size of the second slurry is 0.5μm ≤ D 50 ≤ 2μm; (7) The water content of the second spray material is 0.5%~2.5%; (8) The particle size of the second spray material is 20μm ≤ D 50 ≤ 100μm.

[0017] In an optional embodiment, the preparation method of the lithium iron phosphate cathode material satisfies at least one of the features (1)~(5):

[0018] (1) The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium chloride or lithium nitrate; (2) The iron source includes one or more of iron phosphate, iron(III) oxide, ferrous oxalate, iron nitrate or iron chloride; (3) The phosphorus source includes one or more of iron phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid or lithium dihydrogen phosphate; (4) The carbon source includes one or more of glucose, polyethylene glycol, sucrose, starch or citric acid; (5) The solvent includes one or more of ultrapure water, methanol or ethanol.

[0019] In an optional embodiment, the preparation method of the lithium iron phosphate cathode material satisfies at least one of the features (1) to (5):

[0020] (1) In the first slurry, the addition amount of the carbon source accounts for 1.0 wt% - 1.5 wt% of the mass of the lithium iron phosphate cathode material; (2) In the second slurry, the addition amount of the carbon source accounts for 1.0 wt% - 1.5 wt% of the mass of the lithium iron phosphate cathode material; (3) The mass ratio of the first spray material to the solid substance of the second slurry is 0.15 - 1.5:1.0; (4) During the sintering process, the sintering temperature is 700°C - 800°C, the heating rate is 1°C / min - 5°C / min, and the sintering time is 6 h - 20 h; (5) The drying process adopts spray drying, wherein the inlet air temperature for the first drying is 200°C - 280°C, and the outlet air temperature is 80°C - 120°C; the inlet air temperature for the second drying is 150°C - 200°C, and the outlet air temperature is 60°C - 120°C.

[0021] In an optional embodiment, the first slurry further includes a first doping element; the first doping element includes one or more of a metal element, a non-metal element or a metalloid element.

[0022] In an optional embodiment, the addition amount of the first doping element is 500 ppm - 5000 ppm.

[0023] In an optional embodiment, the second slurry further includes a second doping element; the second doping element includes one or more of a metal element, a non-metal element or a metalloid element.

[0024] In an optional embodiment, the addition amount of the second doping element is 500 ppm - 5000 ppm.

[0025] According to the second aspect of the present invention, the present invention provides a lithium iron phosphate cathode material, including: the lithium iron phosphate cathode material is obtained by using the preparation method according to any one of the optional embodiments in the first aspect of the present invention.

[0026] In an alternative embodiment, the lithium iron phosphate cathode material includes first lithium iron phosphate particles and second lithium iron phosphate particles; a first carbon coating layer is coated on the surface of the first lithium iron phosphate particles, and a second carbon coating layer is coated on the surface of the second lithium iron phosphate particles; wherein, the iron-to-phosphorus ratio in the first lithium iron phosphate particles is greater than or equal to the iron-to-phosphorus ratio in the second lithium iron phosphate particles.

[0027] In an alternative embodiment, the lithium iron phosphate cathode material satisfies at least one of the characteristics (1) to (8):

[0028] (1) The particle size D of the lithium iron phosphate cathode material 10 ≥0.30 μm, 0.7 μm ≤ D 50 ≤1.5 μm, D 99 ≤12 μm; (2) The tap density of the lithium iron phosphate cathode material is 2.55 g / cm 3 ~2.70 g / cm 3 ; (3) The particle size of the first lithium iron phosphate particles is 0.2 μm ≤ D 50 ≤0.8 μm, and the particle size of the second lithium iron phosphate particles is 1.0 μm ≤ D 50 ≤2.0 μm; (4) The mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 0.1 to 1.5:1; (5) The iron-to-phosphorus ratio in the first lithium iron phosphate particles is 0.97 to 1; (6) The iron-to-phosphorus ratio in the second lithium iron phosphate particles is 0.955 to 0.97; (7) The thickness of the first carbon coating layer is 2 to 20 nm; (8) The thickness of the second carbon coating layer is 2 to 20 nm.

[0029] According to the third aspect of the present invention, the present invention provides a pole piece, including: the lithium iron phosphate cathode material obtained by the preparation method according to any one of the embodiments of the first aspect of the present invention, and / or the lithium iron phosphate cathode material according to any one of the embodiments of the second aspect of the present invention.

[0030] According to the fourth aspect of the present invention, the present invention provides a battery, including: the lithium iron phosphate cathode material obtained by the preparation method according to any one of the embodiments of the first aspect of the present invention, and / or the lithium iron phosphate cathode material according to any one of the embodiments of the second aspect of the present invention, and / or the pole piece according to any one of the embodiments of the third aspect of the present invention.

[0031] The preparation method of the lithium iron phosphate cathode material provided by the present invention further has the following beneficial effects:

[0032] 1. In the present invention, by coating the second slurry on the surface of the first spray material to form a second spray material with a core-shell structure, and respectively controlling the iron-phosphorus ratio in the first slurry and the second slurry, the particle size of the product lithium iron phosphate can be precisely controlled, thereby achieving the particle grading effect of the lithium iron phosphate cathode material, improving its tap density, and further enhancing the capacity and cycle performance of the battery.

[0033] 2. In a preferred embodiment of the present invention, a second spray material with a core-shell structure is obtained by fluidized bed spray granulation, and high-tap-density lithium iron phosphate is prepared after sintering. Small particle cores are prepared by fluidized spray, and large particle outer layers are prepared by fluidized bed granulation. By utilizing the temperature difference between the inner and outer layers and the difference in the Li / Fe ratio or Li / P ratio, particle grading of large and small particles is carried out. After pulverization, the material has good uniformity and a high specific capacity. The lithium ion battery further prepared with the lithium iron phosphate material as the cathode material can also obtain excellent performance. At the same time, the process flow of this preparation method is simple, the equipment requirements are low, and it is suitable for application in large-scale industrial production.

[0034] The additional aspects and advantages of the present invention will be partially given in the following description, partially become obvious from the following description, or be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The drawings herein are incorporated into the specification and constitute a part of this specification, showing embodiments consistent with the present invention, and are used together with the specification to explain the principles of the present invention. [[ID=Z12]]

[0036] Figure 1 The SEM diagram of the lithium iron phosphate cathode material provided in Embodiment 1 of the present invention is shown.

[0037] Figure 2 The XRD diagram of the lithium iron phosphate cathode material provided in Embodiment 2 of the present invention is shown.

[0038] Figure 3 The 0.1C charge-discharge curve diagram of the lithium iron phosphate cathode material provided in Embodiment 3 of the present invention is shown.

[0039] Figure 4 The schematic diagram of the preparation method of the lithium iron phosphate cathode material provided in the embodiments of the present invention is shown.

[0040] Through the above-mentioned drawings, specific embodiments of the present invention have been shown, and there will be more detailed descriptions hereinafter. These drawings and textual descriptions are not intended to limit the scope of the inventive concept in any way, but to illustrate the concept of the present invention to those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments of the present invention are only used to illustrate the present invention and not to limit the scope of the present invention.

[0042] The endpoints and any values disclosed in this article for a range are not limited to the exact range or value. These ranges or values should be understood to include values close to these ranges or values. For a numerical range, between the endpoint values of each range, between the endpoint values of each range or individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this article.

[0043] If there is no special instruction, all embodiments and optional embodiments of the present invention can be combined with each other to form a new technical solution.

[0044] If there is no special instruction, all technical features and optional technical features of the present invention can be combined with each other to form a new technical solution.

[0045] If there is no special instruction, all steps of the present invention can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out sequentially, or can also include steps (b) and (a) carried out sequentially. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b) and (c), or can also include steps (a), (c) and (b), or can also include steps (c), (a) and (b), etc.

[0046] If there is no special instruction, the "including" and "comprising" mentioned in the present invention mean open-ended, and can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or can only include or comprise the listed components.

[0047] The lithium iron phosphate cathode material with high tap density has high energy density, excellent cycle performance, etc. However, to prepare a lithium iron phosphate material with high density, it is necessary to control the particle size distribution of the lithium iron phosphate particles to achieve a grading effect between large and small lithium iron phosphate particles. To achieve the above grading effect, the lithium iron phosphate usually needs to be sintered multiple times to sequentially prepare primary crystal phases, doping, carbon coating, etc. This method has problems such as a long preparation cycle, high energy consumption, and it is not easy to precisely control the particle size of the lithium iron phosphate particles and the thickness of the carbon coating layer, resulting in poor consistency of the obtained lithium iron phosphate cathode material, and the sizes of large and small lithium iron phosphate particles tending to two extremes, etc., which in turn affect the capacity and cycle performance of the lithium iron phosphate cathode material.

[0048] In view of the above problems, the present invention provides a preparation method of a lithium iron phosphate cathode material, a lithium iron phosphate cathode material, a pole piece and a battery, and the specific solutions are as follows:

[0049] [Preparation Method of Lithium Iron Phosphate Cathode Material]

[0050] In some embodiments of the present invention, a preparation method of a lithium iron phosphate cathode material is provided, including the following steps:

[0051] S1: Mix a lithium source, an iron source, a phosphorus source, a carbon source and a solvent evenly, and obtain a first slurry after grinding; wherein, the molar ratio of element Fe to P in the first slurry is y:z;

[0052] Mix a lithium source, an iron source, a phosphorus source, a carbon source and a solvent evenly, and obtain a second slurry after grinding; wherein, the molar ratio of element Fe to P in the second slurry is b:c; and y and b satisfy: y:z≥b:c.

[0053] The lithium source includes but is not limited to one or more of lithium carbonate, lithium sulfate, lithium chloride, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium acetate or lithium nitrate; the iron source includes but is not limited to one or more of ferric chloride, ferric sulfate, iron carbonate, iron nitrate, ferrous acetate, ferrous citrate or iron phosphate; the phosphorus source includes but is not limited to one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, lithium dihydrogen phosphate or lithium hydrogen phosphate; the carbon source includes but is not limited to one or more of starch, glucose, citric acid, sucrose, cellulose, sorbitol or fructose; the solvent includes but is not limited to one or more of water, ethanol, methanol or ethylene glycol.

[0054] When preparing the first slurry and the second slurry, adding a carbon source is mainly that during the synthesis process, the carbon source can act as a reducing agent and undergo an oxidation-reduction reaction with iron phosphate / iron source (if it is divalent iron, no reduction is required, such as ferrous oxalate, ferrous sulfate, etc.) to reduce trivalent iron to divalent iron; in addition, due to the poor conductivity of lithium iron phosphate itself, adding a carbon source can act as a surface coating agent, and during the subsequent sintering process, a carbon coating layer can be formed to coat the surface of lithium iron phosphate, further improving the conductivity of the lithium iron phosphate cathode material and reducing the resistance of the lithium iron phosphate cathode material.

[0055] The grinding method can adopt dry grinding or stone grinding. Preferably, the grinding method can adopt a horizontal sand mill for wet mixing ball milling. Mixing and ball milling the above lithium source, iron source, phosphorus source, carbon source and solvent with zirconia spheres of 0.2mm~1.0mm can ensure that the particle size of the solid particles in the first slurry is uniform, promote the reaction activity of the subsequent lithium iron phosphate cathode material, and can avoid the influence of large raw material particles on the performance of the final product.

[0056] For the molar ratio y:z of elements Fe and P in the first slurry and the molar ratio of elements Fe and P in the second slurry being b:c, there is a relationship y:z≥b:c, that is, the iron-phosphorus ratio in the first slurry is greater than or equal to the iron-phosphorus ratio in the second slurry; by controlling the iron-phosphorus ratios in the first slurry and the second slurry, the particle size of the lithium iron phosphate cathode material can be effectively controlled, enabling the obtained lithium iron phosphate cathode material to form a particle size gradation, thereby effectively improving the comprehensive performance of the battery. In a preferred embodiment, by adjusting the Li / Fe or Li / P ratio (Li ratio) in the first slurry and the second slurry, it also affects the particle size of the lithium iron phosphate cathode material. Generally speaking, increasing the Li content ratio will reduce the particle size of the lithium iron phosphate cathode material, and decreasing the Li content ratio will increase the particle size of the lithium iron phosphate cathode material; at the same time, by adjusting the Li content ratio, the capacity and cycle performance of the battery can also be improved.

[0057] S2: Perform primary drying on the first slurry to obtain the first spray material.

[0058] Primary drying can be carried out by methods such as freeze drying or heat drying; preferably, primary drying can be carried out by spray drying. Spray drying ejects the sand-mixed slurry through pressure, centrifugation, or air flow (similar to the principle of a sprayer); the first slurry is atomized into droplets, and the water in the droplets gradually evaporates under the action of the inlet air temperature to form individual spray particles, that is, the first spray material; in the first spray material particles, there are mainly small particles of iron phosphate, and substances such as lithium carbonate and carbon source exist around the small particles of iron phosphate.

[0059] S3: Use the first spray material as the base material, and use the second slurry as the coating liquid to coat the first spray material and perform secondary drying to obtain the second spray material.

[0060] Use the obtained first spray material as the base material and the second slurry as the coating liquid for granulation to obtain the second spray material with a core-shell structure, that is, the second slurry coats the surface of the first spray material. After secondary drying, a solid component is formed with the core being the first spray material and the shell being the second slurry. The preferred granulation method can be fluidized bed spray granulation. The water in the spray droplets will not immediately evaporate and dry, and the droplets will adhere to the base material, that is, the first spray material, to form a coating layer; or during the fluidization process, it promotes the adhesion between particles to form large spray-fluidized particles, and various raw materials inside the large particles are evenly dispersed without raw material stratification.

[0061] S4: Sinter and pulverize the second spray material to obtain the lithium iron phosphate cathode material.

[0062] When sintering the second spray material with a core-shell structure, the carbon source inside the spray balls of the second spray material will first decompose under high temperature to generate carbon dioxide, carbon monoxide, organic carbon atmosphere and carbon. The carbon acts as a reducing agent to reduce ferric iron to ferrous iron and participates in the process of converting iron phosphate and lithium carbonate into lithium iron phosphate. The residual carbon will coat the surface of lithium iron phosphate to form a carbon coating layer. At the same time, the particles of the second spray material will fuse at high temperature to form lithium iron phosphate particles of different sizes. For the second spray material with a core-shell structure, during sintering, the temperature of the outer shell is high and it tends to grow into large lithium iron phosphate particles; the temperature of the inner core is low and it tends to grow into small lithium iron phosphate particles. Further, based on the iron-phosphorus ratio, large and small particles are generated using the internal and external temperatures, thus achieving a grading effect.

[0063] During the preparation process, different iron-phosphorus ratio formulations will affect particle formation. Ferric phosphate with a high iron-phosphorus ratio has low reaction activity and tends to form small particles; ferric phosphate with a low iron-phosphorus ratio has high reaction activity and is conducive to growth and tends to form large particles. By controlling the Fe / P ratio, Li / Fe ratio or Li / P ratio in the first slurry and the second slurry respectively in the above steps, and controlling the Fe / P ratio, Li / Fe ratio or Li / P ratio in the core and shell of the second spray material, the particle size of the product lithium iron phosphate can be precisely controlled to form a grading effect, further improving the tap density of the lithium iron phosphate cathode material and enhancing the capacity and cycle performance of the battery.

[0064] Furthermore, referring to Figure 4 , in order to control the final product lithium iron phosphate cathode material to have an excellent grading effect and high tap density, the phosphorus source, iron source and lithium source in the second spray material are coated in the carbon source layer. Many small particles (such as lithium carbonate, iron phosphate, etc.) will be generated inside a spray ball of the second spray material. The carbon source bonds the large and small particles together to form a second spray material spray ball. After sintering, the lithium iron phosphate particles of various sizes inside the spray ball will separate in the bonded carbon layer. In this way, the lithium iron phosphate with different particle sizes formed in the inner and outer layers will be mixed evenly, thus achieving particle size grading.

[0065] In some embodiments, the chemical general formula of the first slurry is Li x Fe y P z O4, where x:y:z = (1.0~1.05):(0.97~1):1.0.

[0066] The chemical general formula of the first slurry can be LiFe 0.97 PO4, Li 1.01 Fe 0.98 PO4, Li 1.02 Fe 0.99 PO4, Li 1.03 Fe0.97 PO4 or Li 1.05 Any one of FePO4 or any value within the range of any one of x, y, or z.

[0067] In some embodiments, the chemical general formula of the second slurry is Li a Fe b P c O4, where a:b:c = (1.0~1.05):(0.955~0.97):1.0.

[0068] The chemical general formula of the second slurry can be LiFe 0.955 PO4, Li 1.01 Fe 0.956 PO4, Li 1.02 Fe 0.96 PO4, Li 1.04 Fe 0.965 PO4 or Li 1.05 Fe 0.97 PO4 or any value within the range of any one of a, b, or c.

[0069] In some embodiments, the solid content of the first slurry is 20%~50%.

[0070] The solid content in the first slurry can be any one of 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, or 50% or any value between any two of them. By controlling the solid content of the first slurry within the above range, it can ensure that the first slurry has good evaporation efficiency and can make various raw materials in the first slurry mix evenly; if the solid content of the first slurry is too large, it will cause uneven dispersion of the raw material components in the first slurry, the viscosity of the first slurry is too large, which is likely to lead to adhesion of the first spray material, resulting in extreme sizes at both ends of the particle size of the subsequent second spray material; if the solid content of the first slurry is too small, it will reduce the evaporation efficiency and cause a long drying time for the subsequent primary drying.

[0071] In some embodiments, the particle size of the first slurry is 0.2μm ≤ D 50 ≤ 4μm.

[0072] Specifically, the particle size of the first slurry can be any one of 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3.0μm, 3.2μm, 3.4μm, 3.6μm, 3.8μm or 4.0μm, or any point value between any two of them. By controlling the particle size of the first slurry within this range, the particle size of the final product lithium iron phosphate can be controlled from the source, ensuring the formation of particle gradation of the lithium iron phosphate cathode material; for example, in terms of the lithium iron phosphate preparation process, reducing the particle size of the sanding slurry can reduce the particle size of the generated finished product. The particles inside the first spray balls formed by the first slurry need to generate small particle size particles. The particle size of the finished product can be regulated by controlling the particle size of the sanding slurry to achieve the gradation effect.

[0073] In some embodiments, the water content of the first spray material is 0.5% - 5%.

[0074] Specifically, the water content of the first spray material can be any one of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5% or 5.0%, or any point value between any two of them. By controlling the water content of the first spray material within the above range, it can effectively ensure that during subsequent sintering, holes and cracks do not appear inside the first spray material particles and the second spray material particles; generally, the water content of the first spray material will also affect the distribution of each raw material component inside the first spray material. If the water content of the first spray material is too high, it will cause holes and cracks inside the particles due to the generation of water vapor during the subsequent high-temperature sintering process; at the same time, it is likely to cause insufficient subsequent drying, and then the particles of the first spray material will not be dense enough, affecting the sintering process. If the water content of the first spray material is too low, it will cause cracking of the second spray material particles during subsequent fluidized granulation; at the same time, too low water content of the first spray material indicates that the evaporation rate is too fast during the spraying process, and the rapidly evaporated water vapor will carry soluble components (such as soluble carbon sources) to migrate to the particle surface, forming a surface carbon film. The raw materials inside the first spray material may promote particle growth due to lack of carbon source reaction / surface coating reduction, making it difficult to control the formation of particle gradation of the subsequent lithium iron phosphate.

[0075] In some embodiments, the particle size of the first spray material is 15μm ≤ D 50 ≤ 30μm.

[0076] Specifically, the particle size of the first spray material can be any one of 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, or 30μm, or any point value between any two of them. Within this range of the particle size of the first spray material, the particle size of the subsequent second spray material can be controlled, and then the particle size of the product lithium iron phosphate and the tap density of the lithium iron phosphate cathode material can be controlled. If the particle size of the first spray material is too small, it may cause abnormal growth of the particles inside it, resulting in abnormal growth of the particle size of the subsequent second spray material, which has an adverse effect on the subsequent sintering, such as carbon deposition inside the sphere to form floating carbon, affecting the tap density.

[0077] In some embodiments, the solid content of the second slurry is 20% - 50%.

[0078] Specifically, the solid content in the second slurry can be any one of 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, or 50%, or any point value between any two of them. Within this range of the solid content of the second slurry, it ensures that the second slurry coated on the surface of the first spray material and the first spray material have good evaporation efficiency; at the same time, the raw materials in the second slurry are evenly dispersed and coated on the surface of the first spray material. If the solid content of the second slurry is too small, it may lead to a decrease in the evaporation efficiency of the second slurry and also affect the first spray material, causing cracking of the first spray material; if the solid content of the second slurry is too large, it will affect the uneven distribution of the raw materials in the second slurry, and the coating on the surface of the first spray material is also uneven, and it is easy to make the second spray materials stick together. After subsequent secondary drying, it may cause the second spray materials not to have a core-shell structure, and it is difficult to control the particle size of the subsequent product during sintering.

[0079] In some embodiments, the particle size of the second slurry is 0.2μm ≤ D 50 ≤ 2μm.

[0080] Specifically, the particle size of the second slurry can be any one of 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, or 2.0μm, or any point value between any two of them. By controlling the particle size of the second slurry within this range, the particle size of the final product lithium iron phosphate can be further precisely controlled, ensuring the formation of particle gradation of the lithium iron phosphate cathode material.

[0081] In some embodiments, the water content of the second spray material is 0.5% - 2.5%.

[0082] Specifically, the water content of the second spray material can be any one of 0.5%, 1.0%, 1.5%, 2.0%, or 2.5% or any point value between any two of them. Limiting the water content of the second spray material within this range can ensure that no holes or cracks will occur inside the second spray material during the sintering process, and can make the distribution of each raw material in the second spray material uniform, forming stable and uniform core-shell structure particles. If it is greater than this range, cracks and the like are likely to occur during the subsequent sintering process, causing abnormal growth of the particles and affecting the compaction of the subsequent lithium iron phosphate cathode material. If it is less than this range, cracking of the second spray material shell is likely to occur, making it difficult to control the particle size of the subsequent product.

[0083] In some embodiments, the particle size of the second spray material is 20μm ≤ D 50 ≤ 100μm.

[0084] Specifically, the particle size of the second spray material can be any one of 20μm, 22μm, 24μm, 26μm, 27μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, or 100μm or any point value between any two of them. When the particle size of the second spray material is within this range, the particle size of the subsequent product can be effectively controlled, enabling the final lithium iron phosphate cathode material to form a particle size distribution.

[0085] In some embodiments, the lithium source includes but is not limited to one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium chloride, or lithium nitrate.

[0086] In some embodiments, the iron source includes but is not limited to one or more of iron phosphate, iron(III) oxide, ferrous oxalate, iron nitrate, or iron chloride.

[0087] In some embodiments, the phosphorus source includes but is not limited to one or more of iron phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, or lithium dihydrogen phosphate.

[0088] In some embodiments, the carbon source includes but is not limited to one or more of glucose, polyethylene glycol, sucrose, starch, or citric acid.

[0089] In some embodiments, the solvent includes but is not limited to one or more of ultrapure water, methanol, or ethanol.

[0090] In some embodiments, the addition amount of the carbon source in the first slurry accounts for 1.0 wt% - 1.5 wt% of the mass of the lithium iron phosphate cathode material.

[0091] Specifically, the mass ratio of the carbon source added in the first slurry to the lithium iron phosphate cathode material can be any one of 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt% or 1.5 wt% or any point value between any two of them. By limiting the addition amount of the carbon source in the first slurry, a certain amount of lithium, iron and phosphorus can be contained in the lithium iron phosphate particles. While the carbon improves the conductivity of the lithium iron phosphate cathode material, it can also form a carbon coating layer on the surface of the lithium iron phosphate particles, and the lithium iron phosphate particles are adhered together through the carbon coating layer on their surface, thereby improving the tap density of the lithium iron phosphate cathode material.

[0092] In some embodiments, the mass ratio of the carbon source added in the second slurry to the lithium iron phosphate cathode material is 1.0 wt% - 1.5 wt%.

[0093] Specifically, the mass ratio of the carbon source added in the second slurry to the lithium iron phosphate cathode material can be any one of 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt% or 1.5 wt% or any point value between any two of them. By limiting the addition amount of the carbon source in the second slurry, the second slurry can be stably coated on the surface of the first sprayed material, and the carbon source also acts as a binder to promote the formation of a core-shell structure of the second sprayed material.

[0094] In some embodiments, the mass ratio of the solid substances of the first sprayed material to the second slurry is 0.15 - 1.5:1.0.

[0095] Specifically, the mass ratio of the solid substances of the first sprayed material to the second slurry can be any one of 0.15:1.0, 0.2:1.0, 0.25:1.0, 0.5:1.0, 0.7:1.0, 1.0:1.0, 1.2:1.0 or 1.5:1.0 or any ratio between any two of them. By limiting the mass ratio of the solid substances of the first sprayed material to the second slurry, a mixed ratio of large and small particles can be formed, so that the subsequent obtained lithium iron phosphate particles achieve grading, and the tap density of the lithium iron phosphate cathode material is improved.

[0096] In some embodiments, during the sintering process, the sintering temperature is 700°C - 800°C, the heating rate is 1°C / min - 5°C / min, and the sintering time is 6 h - 20 h.

[0097] During the sintering process, the sintering temperature can be any one of 700°C, 720°C, 750°C, or 800°C or any point value between any two of them; the heating rate from room temperature to the sintering temperature can be any one of 1°C / min, 2°C / min, 3°C / min, 4°C / min, or 5°C / min or any point value between any two of them; the sintering time can be any one of 6h, 7h, 8h, 9h, 10h, 15h, or 20h or any point value between any two of them.

[0098] The above sintering process can be carried out in an inert gas atmosphere, such as in an atmosphere of nitrogen, helium, argon, etc., which can avoid side reactions between the second spray material and some gases (such as oxygen, carbon dioxide, etc.), affect the particle size of the product lithium iron phosphate, etc., and have an adverse impact on the comprehensive performance of the battery.

[0099] In some embodiments, the drying process uses spray drying. Among them, the inlet air temperature for the primary drying is 200°C to 280°C, and the outlet air temperature is 80°C to 120°C; the inlet air temperature for the secondary drying is 150°C to 200°C, and the outlet air temperature is 60°C to 120°C.

[0100] When using spray drying, the inlet air temperature for the primary drying can be any one of 200°C, 220°C, 240°C, 260°C, or 280°C or any point value between any two of them; the outlet air temperature can be any one of 80°C, 90°C, 100°C, 110°C, or 120°C or any point value between any two of them. When using spray drying, the inlet air temperature for the secondary drying can be any one of 150°C, 160°C, 170°C, 180°C, or 200°C or any point value between any two of them; the outlet air temperature can be any one of 60°C, 70°C, 80°C, 100°C, 110°C, or 120°C or any point value between any two of them; by respectively limiting the inlet air temperature and outlet air temperature of the primary drying and the secondary drying, the evaporation efficiency of the first slurry and the second slurry can be ensured, and it can also ensure that the first spray material and the second spray material will not crack, effectively controlling the particle size distribution effect of the subsequent product.

[0101] In some embodiments, the first slurry further includes a first doping element; the first doping element includes one or more of a metal element, a non-metal element, or a metalloid element.

[0102] The first doping element includes, but is not limited to, one or more of titanium, vanadium, magnesium, niobium, manganese, or boron; by adding the first doping element to the first slurry, the capacity of the lithium iron phosphate cathode material can be further improved, and at the same time, it can also play a role in refining the particles of the lithium iron phosphate cathode material.

[0103] In some embodiments, the addition amount of the first doping element is 500 ppm to 5000 ppm;

[0104] Specifically, the addition amount of the first doping element can be any one of 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm or 5000 ppm or any point value between any two of them; by limiting the addition amount of the first doping element, it shows that the addition amount of the first doping element does not reach a very high level, enabling the first doping element to refine the particles and increase the capacity while not affecting the performance of the lithium iron phosphate particles.

[0105] In some embodiments, the second slurry further includes a second doping element; the second doping element includes one or more of a metal element, a non-metal element or a metalloid element;

[0106] The second doping element includes, but is not limited to, one or more of titanium, vanadium, magnesium, niobium, manganese or boron; by adding the second doping element to the second slurry, the particles in the second slurry can be refined, enabling the second slurry to tightly coat the surface of the first spray material, and further increasing the capacity of the lithium iron phosphate cathode material.

[0107] In some embodiments, the addition amount of the second doping element is 500 ppm to 5000 ppm.

[0108] Specifically, the addition amount of the second doping element can be any one of 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm or 5000 ppm or any point value between any two of them; by limiting the addition amount of the second doping element, it shows that the addition amount of the second doping element does not reach a very high level, enabling the second doping element to refine the particles and increase the battery capacity while not affecting the performance of the lithium iron phosphate particles.

[0109] [Lithium iron phosphate cathode material]

[0110] In some embodiments of the present invention, a lithium iron phosphate cathode material is provided, and the lithium iron phosphate cathode material is obtained by using the preparation method in any one of the above-mentioned embodiments.

[0111] In some embodiments, the lithium iron phosphate cathode material includes first lithium iron phosphate particles and second lithium iron phosphate particles; a first carbon coating layer is coated on the surface of the first lithium iron phosphate particles, and a second carbon coating layer is coated on the surface of the second lithium iron phosphate particles; wherein, the iron-to-phosphorus ratio in the first lithium iron phosphate particles is greater than or equal to the iron-to-phosphorus ratio in the second lithium iron phosphate particles.

[0112] The particle sizes of the first lithium iron phosphate particles and the second lithium iron phosphate particles can be the same or different. For example, the particle size of the first lithium iron phosphate particles can be in the micron range, and the particle size of the second lithium iron phosphate particles can be in the nanometer range or also in the micron range. The iron-to-phosphorus ratio of the first lithium iron phosphate particles is greater than or equal to that of the second lithium iron phosphate particles, enabling the lithium iron phosphate cathode material to have a particle grading effect, a higher tap density, and thus a higher capacity and excellent cycling performance.

[0113] The surface of the first lithium iron phosphate particles is coated with a first carbon coating layer, and the surface of the second lithium iron phosphate particles is coated with a second carbon coating layer. The first carbon coating layer and the second carbon coating layer can adhere to each other, making the first lithium iron phosphate particles and the second lithium iron phosphate particles closer, and thus increasing the tap density of the lithium iron phosphate cathode material.

[0114] In some embodiments, the particle size D of the lithium iron phosphate cathode material 10 ≥0.30 μm, 0.7 μm ≤ D 50 ≤1.5 μm, D 99 ≤12 μm.

[0115] The particle size D of the lithium iron phosphate cathode material 10 can be any value of 0.30 μm, 0.35 μm, 0.40 μm, 0.45 μm, or 0.50 μm and above; D 50 can be any one of 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, or 1.5 μm or any value between any two of them; D 99 can be any value of 12 μm, 11 μm, 10 μm, 9 μm, or 8 μm and below.

[0116] In some embodiments, the tap density of the lithium iron phosphate cathode material is 2.55 g / cm 3 ~2.70 g / cm 3 .

[0117] The tap density of the lithium iron phosphate cathode material can be 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.65 g / cm 3 or 2.70 g / cm 3Any point value between any one or any two of them. By defining the tap density of the lithium iron phosphate cathode material, excellent capacity and cycle performance of the lithium iron phosphate cathode material can be ensured; of course, those skilled in the art are familiar that under current conditions, on the premise of taking into account the battery capacity, the upper limit of the tap density of the lithium iron phosphate cathode material is about 2.7 g / cm 3 However, as the tap technology level is constantly improving, this upper limit may also increase; so it is considered here that an upper limit should not be set for the tap density of the lithium iron phosphate cathode material. Generally, the tap density of lithium iron phosphate is 3.6 g / cm 3 , and even if an upper limit is given, it should be below 3.6 g / cm 3 . Currently, only the lower limit of high tap density needs to be given.

[0118] In some embodiments, the particle size of the first lithium iron phosphate particles is 0.2 μm ≤ D 50 ≤ 0.8 μm, and the particle size of the second lithium iron phosphate particles is 1.0 μm ≤ D 50 ≤ 2.0 μm.

[0119] The particle size of the first lithium iron phosphate particles can be any one or any point value between any two of 0.20 μm, 0.22 μm, 0.24 μm, 0.26 μm, 0.28 μm, 0.30 μm, 0.40 μm, 0.50 μm, 0.60 μm, 0.70 μm or 0.80 μm; the particle size of the second lithium iron phosphate particles can be any one or any point value between any two of 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2.0 μm. By respectively defining the particle sizes of the first lithium iron phosphate particles and the second lithium iron phosphate particles to achieve particle grading and uniform particle size distribution, the tap density of the lithium iron phosphate cathode material is improved.

[0120] In some embodiments, the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 0.1~1.5:1.

[0121] Specifically, the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles can be any one or any ratio between any two of 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 1.0:1 or 1.5:1. By matching the two kinds of lithium iron phosphate particles with different particle sizes, their distribution can be made uniform, the tap density of the lithium iron phosphate cathode material is improved, and the battery has a higher capacity.

[0122] In some embodiments, the iron-to-phosphorus ratio in the first lithium iron phosphate particles is 0.97~1.

[0123] Specifically, the iron-to-phosphorus ratio in the first lithium iron phosphate particles can be any one of 0.97, 0.975, 0.98, 0.985, 0.99, 0.995 or 1, or any point value between any two of them.

[0124] In some embodiments, the iron-to-phosphorus ratio in the second lithium iron phosphate particles is 0.955 to 0.97.

[0125] Specifically, the iron-to-phosphorus ratio in the second lithium iron phosphate particles can be any one of 0.955, 0.958, 0.96, 0.965 or 0.97, or any point value between any two of them. By limiting the iron-to-phosphorus ratios of the two lithium iron phosphate particles, the particle sizes of the first lithium iron phosphate particles and the second lithium iron phosphate particles can be further regulated, so that the lithium iron phosphate cathode material forms a particle size grading, further improving the capacity of the lithium iron phosphate cathode material.

[0126] In some embodiments, the thickness of the first carbon coating layer is 2 to 20 nm; the thickness of the second carbon coating layer is 2 to 20 nm.

[0127] Specifically, the thickness of the first carbon coating layer can be any one of 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 14 nm, 16 nm or 20 nm, or any point value between any two of them. The thickness of the second carbon coating layer can be any one of 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 14 nm, 16 nm or 20 nm, or any point value between any two of them. The thicknesses of the first carbon coating layer and the second carbon coating layer can be the same or different. By limiting the thicknesses of the first carbon coating layer and the second carbon coating layer, the conductivity of the lithium iron phosphate particles can be improved. At the same time, the first carbon coating layer and the second carbon coating layer can promote the close fitting of the first lithium iron phosphate particles and the second lithium iron phosphate particles, thereby improving the tap density of the lithium iron phosphate cathode material.

[0128] [Pole piece]

[0129] Some embodiments of the present invention further provide a pole piece, including the lithium iron phosphate cathode material obtained by the preparation method in any one of the above embodiments.

[0130] In some embodiments, the pole piece includes the lithium iron phosphate cathode material in any one of the above embodiments.

[0131] The pole piece further includes a current collector, and the current collector includes, but is not limited to, any one of aluminum foil, copper foil or aluminum alloy foil; in the thickness direction, the above lithium iron phosphate cathode material is coated on one side or both sides of the current collector.

[0132] The pole piece further includes a conductive agent and a binder, and no specific limitation is made on the conductive agent and the binder in this embodiment.

[0133] [Battery]

[0134] In some embodiments of the present invention, a battery is further provided, including a lithium iron phosphate cathode material obtained by the preparation method in any one of the above embodiments.

[0135] In some embodiments, the battery includes the lithium iron phosphate cathode material in any one of the above embodiments.

[0136] In some embodiments, the battery includes the electrode sheet in any one of the above embodiments.

[0137] The above battery further includes a negative electrode sheet, a separator, and an electrolyte. The separator includes any one of polypropylene (PP), polyethylene (PE), polyimide (PI), and polypropylene / polyethylene composite (PP / PE). The lithium salt of the electrolyte is selected as lithium hexafluorophosphate (LiPF6), and the solvent includes cyclic carbonate (EC) and chain carbonate dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC).

[0138] Since the battery provided by the embodiments of the present invention adopts all the technical solutions of the above embodiments, therefore, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be elaborated here one by one.

[0139] The following specifically describes the present application with reference to the drawings and examples, but the implementation and protection of the present invention are not limited thereto. The following examples are only some embodiments of the present application and do not limit the present application.

[0140] Example 1

[0141] S11. After adding lithium dihydrogen phosphate, ferrous oxalate, and titanium dioxide according to the stoichiometric ratio of Li 1.05 Fe 0.985 PO4, an appropriate amount of glucose is added. The addition amount of glucose is 1.1% of the total mass after calcination. An appropriate amount of methanol is added to make the solid content of the slurry 20%, and it is stirred and dispersed for 0.5 h; the slurry is milled using a sand mill, the diameter of the zirconia balls is 0.3 - 0.4 mm, and it is milled until the particle size of the slurry is D 50 to about 2 μm to obtain the first slurry.

[0142] S12. The milled first slurry is spray-dried using two-fluid spraying, controlling the inlet air temperature at 200 °C and the outlet air temperature at 80 °C, and D 50 to 19.4 μm to obtain the first spray material.

[0143] S13. According to the stoichiometric ratio of Li 1.01 Fe 0.965After adding iron phosphate, lithium carbonate, and borohydride to PO4, appropriate amounts of glucose and polyethylene glycol were added. The addition amounts of glucose and polyethylene glycol were 1.15% of the total mass after calcination. Appropriate amounts of deionized water were added to make the solid content of the slurry 30%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry reached D 50 to about 0.75 μm to obtain a second slurry; the ratio of the solid substances of the first spray material to the second slurry in this embodiment was 0.3:1.0. The first spray material was used as the bottom material and placed in a fluidized bed spray granulation drying device, and the second slurry was used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature was controlled at 200 °C, the granulation temperature was 90 °C, and the moisture content of the spray material was 1.28%, D 50 to 84.8 μm to obtain a second spray material.

[0144] S14. The second spray material was placed in an atmosphere tube furnace for sintering. The sintering atmosphere was argon, the oxygen content in the furnace was controlled to drop below 50 ppm, the heating rate was controlled at 1.5 °C / min, the sintering temperature was 700 °C, and the sintering time was 18 h. After sintering, it was cooled to room temperature with the furnace to obtain a sintered material. The sintered material was crushed using a jet mill, and parameters such as the classification frequency and crushing pressure were adjusted to control the particle size of the finished product within D 10 ≥0.4 μm, 1.1 μm ≤ D 50 ≤1.3 μm, D 99 ≤10 μm. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0145] Example 2

[0146] S21. According to the stoichiometric ratio Li 1.03 Fe 0.983 PO4, after adding diammonium hydrogen phosphate, iron(III) oxide, lithium carbonate, and ammonium metavanadate, appropriate amounts of starch and polyethylene glycol were added. The addition amounts of starch and polyethylene glycol were 1.05% of the total mass after calcination. Appropriate amounts of deionized water were added to make the solid content of the slurry 40%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.5 mm until the particle size of the slurry reached D 50 to about 0.2 μm to obtain a first slurry.

[0147] S22. The milled first slurry was spray-dried using centrifugal spraying. The inlet air temperature was controlled at 260 °C, the outlet air temperature was 90 °C, the moisture content of the spray material was 2.87%, D 50 to 18.6 μm to obtain a first spray material.

[0148] S23. According to the stoichiometric ratio Li 1.0 Fe 0.963After adding iron phosphate, ferrous oxalate, lithium hydrogen phosphate, lithium carbonate, and titanium dioxide to PO4, an appropriate amount of citric acid and polyethylene glycol were added. The addition amounts of citric acid and polyethylene glycol were 1.21% of the total mass after calcination. An appropriate amount of methanol was added to make the solid content of the slurry 30%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was D 50 to about 0.6 μm to obtain a second slurry. The ratio of the solid substances of the first spray material to the second slurry in this embodiment was 0.5:1.0. The first spray material was used as the bottom material and placed in a fluidized bed spray drying device, and the second slurry was used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature was controlled at 180 °C, the granulation temperature was 80 °C, and the moisture content of the spray material was 1.8%. D 50 to 93.4 μm to obtain a second spray material.

[0149] S24. The second spray material was loaded into a crucible and placed in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere was nitrogen, the oxygen content in the furnace was controlled to drop below 50 ppm, the heating rate was controlled at 2 °C / min, the sintering temperature was 780 °C, and the sintering time was 12 h. After sintering, it was cooled to room temperature with the furnace to obtain a sintered material. The sintered material was pulverized using a jet mill, and parameters such as the classification frequency and pulverization pressure were adjusted to control the particle size of the finished product within D 10 ≥0.39 μm, 1.2 μm ≤ D 50 ≤1.4 μm, D 99 ≤10 μm range. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0150] Example 3

[0151] S31. According to the stoichiometric ratio Li 1.02 Fe 0.977 PO4, after adding iron phosphate, lithium carbonate, and niobium pentoxide, an appropriate amount of sucrose was added. The addition amount of sucrose was 1.23% of the total mass after calcination. An appropriate amount of deionized water was added to make the solid content of the slurry 50%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was D 50 to about 0.2 μm to obtain a first slurry.

[0152] S32. The milled first slurry was spray dried using pressure spraying. The inlet air temperature was controlled at 230 °C, the outlet air temperature was 95 °C, and the moisture content of the spray material was 1.77%. D 50 to 21.3 μm to obtain a first spray material.

[0153] S33. According to the stoichiometric ratio Li 1.01 Fe 0.961After adding PO4 to iron oxide, iron phosphate, ammonium hydrogen phosphate, and lithium carbonate, an appropriate amount of polyethylene glycol is added. The addition amount of polyethylene glycol is 1.2% of the total mass after calcination. An appropriate amount of deionized water is added to make the solid content of the slurry 30%, and it is stirred and dispersed for 0.5 h; the slurry is milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry is D 50 to about 0.4 μm to obtain a second slurry. The ratio of the solid substances of the first spray material to the second slurry in this embodiment is 0.2:1.0. The first spray material is used as the bottom material and placed in a fluidized bed spray granulation drying device. The second slurry is used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature is controlled at 190°C, the granulation temperature is 85°C, and the moisture content of the spray material is 1.10%, D 50 to 89.7 μm to obtain a second spray material.

[0154] S34: The second spray material is placed in an atmosphere rotary sintering furnace for sintering. The sintering atmosphere is carbon dioxide gas. The oxygen content in the furnace is controlled to be below 50 ppm, the heating rate is controlled at 3°C / min, the sintering temperature is 750°C, and the sintering time is 10 h. After sintering, it is cooled to room temperature with the furnace to obtain a sintered material; the sintered material is pulverized using an air stream pulverizer, and parameters such as the classification frequency and pulverization pressure are adjusted to control the particle size of the finished product within D 10 ≥0.38 μm, 1.0 μm ≤ D 50 ≤1.2 μm, D 99 ≤12 μm range. After sieving to remove impurities, the lithium iron phosphate cathode material can be obtained.

[0155] Example 4

[0156] S41: According to the stoichiometric ratio Li 1.03 Fe 0.99 PO4, after adding iron phosphate, lithium carbonate, iron oxide, phosphoric acid, and magnesium oxide, an appropriate amount of citric acid and glucose are added. The addition amounts of citric acid and glucose are 1.27% of the total mass after calcination. An appropriate amount of deionized water is added to make the solid content of the slurry 30%, and it is stirred and dispersed for 0.5 h; the slurry is milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry is D 50 to about 0.2 μm to obtain a first slurry.

[0157] S42: The milled first slurry is spray dried using centrifugal spraying. The inlet air temperature is controlled at 200°C, the outlet air temperature is 80°C, and the moisture content of the spray material is 3.44%, D 50 to 24.5 μm to obtain a first spray material.

[0158] S43: According to the stoichiometric ratio Li 1.0 Fe 0.96After adding iron phosphate, lithium carbonate, and tetrabutyl titanate to PO4, an appropriate amount of glucose and polyethylene glycol were added. The addition amounts of glucose and polyethylene glycol were 1.19% of the total mass after calcination. An appropriate amount of deionized water was added to make the solid content of the slurry 40%. After stirring and dispersing for 0.5 h, the second slurry was obtained. The ratio of the solid substances of the first spray material to the second slurry in this embodiment was 0.5:1.0; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was D 50 to about 0.6 μm; the first spray material was used as the bottom material and placed in a fluidized bed spray granulation drying device. The second slurry was used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature was controlled at 200 °C, the granulation temperature was 95 °C, and the moisture content of the spray material was 0.84%, D 50 to 91.6 μm to obtain the second spray material.

[0159] S44. The second spray material was placed in an atmosphere tube furnace for sintering. The sintering atmosphere was argon. The oxygen content in the furnace was controlled to drop below 50 ppm. The heating rate was controlled at 1.5 °C / min, the sintering temperature was 790 °C, and the sintering time was 12 h. After sintering, the furnace was cooled to room temperature with the furnace to obtain the sintered material. The sintered material was crushed using a jet mill, and parameters such as the classification frequency and crushing pressure were adjusted to control the particle size of the finished product within the range of D 10 ≥0.45 μm, 1.3 μm ≤ D 50 ≤1.5 μm, D 99 ≤9 μm. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0160] Example 5

[0161] S51. According to the stoichiometric ratio Li 1.0 Fe 0.97 PO4, after adding iron phosphate, lithium carbonate, and ammonium metavanadate, an appropriate amount of glucose was added. The addition amount of glucose was 1.26% of the total mass after calcination. An appropriate amount of water was added to make the solid content of the slurry 40%. After stirring and dispersing for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was D 50 to about 0.3 μm to obtain the first slurry.

[0162] S52. The milled first slurry was spray-dried using centrifugal spraying. The inlet air temperature was controlled at 230 °C, the outlet air temperature was 85 °C, and the moisture content of the spray material was 3.2%, D 50 to 29.6 μm to obtain the first spray material.

[0163] S53. According to the stoichiometric ratio Li 1.02 Fe 0.96After adding iron phosphate, lithium carbonate, and tetrabutyl titanate to PO4, an appropriate amount of glucose and polyethylene glycol were added. The addition amounts of glucose and polyethylene glycol were 1.28% of the total mass after calcination. An appropriate amount of deionized water was added to make the solid content of the slurry 40%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was D 50 to about 0.35 μm to obtain a second slurry; the ratio of the solid substances of the first spray material to the second slurry in this embodiment was 0.4:1.0. The first spray material was used as the bottom material and placed in a fluidized bed spray granulation drying device, and the second slurry was used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature was controlled at 200 °C, the granulation temperature was 95 °C, the moisture content of the spray material was 1.27%, D 50 to 97.5 μm to obtain a second spray material.

[0164] S54、The second spray material was placed in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere was nitrogen, the oxygen content in the furnace was controlled to drop below 50 ppm, the heating rate was controlled at 1 °C / min, the sintering temperature was 800 °C, and the sintering time was 10 h. After sintering, it was cooled to room temperature with the furnace to obtain a sintered material. The sintered material was pulverized using a jet mill, and parameters such as the classification frequency and pulverization pressure were adjusted to control the particle size of the finished product within D 10 ≥0.35 μm, 1.3 μm ≤ D 50 ≤1.5 μm, D 99 ≤10 μm range. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0165] Example 6

[0166] S61、According to the stoichiometric ratio Li 1.0 Fe 0.97 PO4, after adding ferric oxide, lithium carbonate, ammonium hydrogen phosphate, and titanium dioxide, an appropriate amount of starch and polyethylene glycol were added. The addition amounts of starch and polyethylene glycol were 1.12% of the total mass after calcination. An appropriate amount of water was added to make the solid content of the slurry 40%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was D 50 to about 0.2 μm to obtain a first slurry.

[0167] S62、The milled first slurry was spray-dried using centrifugal spraying. The inlet air temperature was controlled at 240 °C, the outlet air temperature was 90 °C, the moisture content of the spray material was 3.35%, D 50 to 26.7 μm to obtain a first spray material.

[0168] S63、According to the stoichiometric ratio Li 1.0 Fe 0.955After adding iron phosphate, lithium carbonate, and titanium dioxide to PO4, an appropriate amount of glucose and polyethylene glycol are added. The addition amounts of glucose and polyethylene glycol are 1.38% of the total mass after calcination. An appropriate amount of deionized water is added to make the solid content of the slurry 40%, and it is stirred and dispersed for 0.5 h; the slurry is milled using a sand mill, and the diameter of the zirconia balls is 0.3 - 0.4 mm. The slurry is milled until the particle size is D 50 to about 0.4 μm to obtain the second slurry; the ratio of the solid substances of the first spray material to the second slurry in this embodiment is 0.15:1.0. The first spray material is used as the bottom material and placed in a fluidized bed spray granulation drying device. The second slurry is used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature is controlled at 195 °C, the granulation temperature is 90 °C, and the moisture content of the spray material is 1.31%. D 50 to 94.3 μm to obtain the second spray material.

[0169] S64. The second spray material is placed in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere is nitrogen. The oxygen content in the furnace is controlled to drop below 50 ppm. The heating rate is controlled at 1 °C / min, the sintering temperature is 780 °C, and the sintering time is 10 h. After sintering, it is cooled to room temperature with the furnace to obtain the sintered material. The sintered material is crushed using a jet mill, and parameters such as the classification frequency and crushing pressure are adjusted to control the particle size of the finished product within the range of D 10 ≥0.3 μm, 1.2 μm ≤ D 50 ≤1.4 μm, D 99 ≤10 μm, and after sieving to remove impurities, the lithium iron phosphate cathode material can be obtained.

[0170] Example 7

[0171] S71. After adding ferrous oxalate, lithium dihydrogen phosphate, and titanium dioxide according to the stoichiometric ratio Li 1.05 Fe 0.975 PO4, an appropriate amount of glucose is added. The addition amount of glucose is 1.19% of the total mass after calcination. An appropriate amount of methanol is added to make the solid content of the slurry 20%, and it is stirred and dispersed for 0.5 h; the slurry is milled using a sand mill, and the diameter of the zirconia balls is 0.3 - 0.4 mm. The slurry is milled until the particle size is D 50 to about 2 μm to obtain the first slurry.

[0172] S72. The milled first slurry is spray-dried using centrifugal spraying. The inlet air temperature is controlled at 200 °C, the outlet air temperature is 80 °C, D 50 to 18.9 μm to obtain the first spray material.

[0173] S73. According to the stoichiometric ratio Li 1.05 Fe 0.97After adding iron phosphate, iron oxide, lithium dihydrogen phosphate, lithium carbonate, and titanium dioxide to PO4, an appropriate amount of glucose and polyethylene glycol were added. The addition amounts of glucose and polyethylene glycol were 1.215% of the total mass after calcination. An appropriate amount of deionized water was added to make the solid content of the slurry 30%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill with zirconia balls having a diameter of 0.3 - 0.4 mm until the particle size of the slurry was about D 50 to 0.4 μm to obtain a second slurry; the ratio of the solid substances of the first spray material to the second slurry in this embodiment was 0.3:1.0. The first spray material was used as the bottom material and placed in a fluidized bed spray granulation drying device, and the second slurry was used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature was controlled at 200 °C, the granulation temperature was 80 °C, the moisture content of the spray material was 1.01%, and D 50 to 95.6 μm to obtain a second spray material.

[0174] S74. The second spray material was placed in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere was nitrogen, the oxygen content in the furnace was controlled to drop below 50 ppm, the heating rate was controlled at 5 °C / min, the sintering temperature was 750 °C, and the sintering time was 20 h. After sintering, it was cooled to room temperature with the furnace to obtain a sintered material. The sintered material was crushed using an air jet mill, and parameters such as the classification frequency and crushing pressure were adjusted to control the finished product particle size within D 10 ≥0.39 μm, 1.3 μm ≤ D 50 ≤1.6 μm, D 99 ≤10 μm range. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0175] Comparative Example 1

[0176] S81. Two kinds of slurries (the first slurry and the second slurry) in Example 4 were mixed in the same proportion and subjected to centrifugal spray drying. The inlet air temperature was controlled at 260 °C, the granulation temperature was 95 °C, the moisture content of the spray material was 1.14%, and D 50 to 21.9 μm to obtain a spray material.

[0177] S82. The spray material was placed in an atmosphere tube-type sintering furnace for sintering. The sintering atmosphere was argon, the oxygen content in the furnace was controlled to drop below 50 ppm, the heating rate was controlled at 1.5 °C / min, the sintering temperature was 790 °C, and the sintering time was 12 h. After sintering, it was cooled to room temperature with the furnace to obtain a sintered material.

[0178] S83. The sintered material was crushed using an air jet mill, and parameters such as the classification frequency and crushing pressure were adjusted to control the finished product particle size within D 10 ≥0.45 μm, 1.3 μm ≤ D 50 ≤1.5 μm, D 99 ≤12 μm range. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0179] Comparative Example 2

[0180] S91, according to the stoichiometric ratio Li 1.02 Fe 0.963 After adding iron phosphate, lithium carbonate and titanium dioxide to PO4, add appropriate amount of polyethylene glycol, glucose and polyethylene glycol. The amount of glucose added is 1.34% of the total mass after calcination. Add appropriate amount of deionized water to make the slurry solid content at 40%, stir and disperse for 0.5h. Use a sand mill to sand grind the slurry. The diameter of the zirconium oxide ball is 0.3~0.4mm. Sand grind to the slurry particle size D 50 to about 0.4 μm, and a first slurry was obtained.

[0181] S92, spray dry the sand-milled first slurry using centrifugal spray, control the air inlet temperature at 245°C, the granulation temperature at 90°C, and the spray material moisture at 1.38%. 50 To 22.4 μm, the first spray material was obtained.

[0182] S93. The first spray material is loaded into a sagger and placed in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere is nitrogen. The oxygen content in the furnace is controlled to be below 50 ppm. The heating rate is controlled to be 1.5°C / min, the sintering temperature is 790°C, and the sintering time is 10 hours. After the sintering is completed, the temperature is lowered to room temperature with the furnace to obtain a sintered material.

[0183] S94, crush the sintered material with a jet mill, adjust the parameters such as classification frequency and crushing pressure, and control the finished product particle size to D 10 ≥0.45μm, 1.0μm≤D 50 ≤1.4μm, D 99 The lithium iron phosphate positive electrode material can be obtained after sieving and removing impurities within the range of ≤10μm.

[0184] Comparative Example 3

[0185] S101, according to the stoichiometric ratio Li 1.00 Fe 0.96 After adding iron phosphate, lithium carbonate and titanium dioxide to PO4, add appropriate amount of sucrose, the amount of sucrose added is 1.37% of the total mass after calcination, add appropriate amount of water to make the slurry solid content at 40%, stir and disperse for 0.5h; use a sand mill to sand grind the slurry, the diameter of the zirconium oxide ball is 0.3~0.4mm, sand grind to the slurry particle size D 50 to about 0.4 μm, and a first slurry was obtained.

[0186] S102, spray-dry the sand-ground first slurry using pressure spray, controlling the air inlet temperature at 245°C, the air outlet temperature at 90°C, and the spray material moisture at 2.33%. 50 To 27.3 μm, the first spray material was obtained.

[0187] S103, according to the stoichiometric ratio Li 1.02 Fe 0.97 After adding ferric phosphate, ferric oxide, lithium dihydrogen phosphate, lithium carbonate, and tetrabutyl titanate to PO4, add appropriate amounts of glucose and polyethylene glycol. The addition amount of glucose and polyethylene glycol is 1.13% of the total mass after calcination. Add appropriate amount of deionized water to make the slurry solid content at 30%, stir and disperse for 0.5h; use a sand mill to sand grind the slurry. The diameter of the zirconia ball is 0.3~0.4mm. Sand grind to a slurry particle size of D 50 To about 0.6μm, the second slurry is obtained; the solid matter ratio of the first spray material to the second slurry in this embodiment is 0.3:1.0. The first spray material is used as the base material and placed in the fluidized bed spray granulation drying equipment, and the second slurry is used as the mother liquid for fluidized bed spray drying granulation. The air inlet temperature is controlled at 200℃, the granulation temperature is 95℃, the spray material moisture is 0.96%, D 50 To 96.8 μm, the second spray material was obtained.

[0188] S104, the second spray material is placed in an atmosphere box sintering furnace for sintering, the sintering atmosphere is nitrogen, the oxygen content in the furnace is controlled to be below 50ppm, the heating rate is controlled to be 1.5℃ / min, the sintering temperature is 790℃, the sintering time is 10h, and after the sintering is completed, the temperature is cooled to room temperature with the furnace to obtain a sintered material. The sintered material is crushed with a jet mill, and the parameters such as the classification frequency and crushing pressure are adjusted to control the finished product particle size to be within D 10 ≥0.38μm, 1.3μm≤D 50 ≤1.5μm, D 99 The lithium iron phosphate positive electrode material can be obtained after sieving and removing impurities within the range of ≤10μm.

[0189] Comparative Example 4

[0190] S111, according to the stoichiometric ratio Li 1.00 Fe 0.97 After adding iron phosphate and lithium carbonate to PO4, add an appropriate amount of glucose, the amount of glucose added is 1.31% of the total mass after calcination, add an appropriate amount of water to make the slurry solid content at 40%, stir and disperse for 0.5h; use a sand mill to sand grind the slurry, the diameter of the zirconia ball is 0.3~0.4mm, and sand grind to the slurry particle size D 50 to about 0.4 μm, and a first slurry was obtained.

[0191] S112, spray-dry the sand-milled first slurry using centrifugal spray, controlling the air inlet temperature at 230°C, the air outlet temperature at 85°C, and the spray material moisture at 3.52%. 50 To 26.4 μm, the first spray material was obtained.

[0192] S113. According to the stoichiometric ratio of Li 1.02 Fe 0.96 PO4, after adding iron phosphate, lithium carbonate, and titanium dioxide, add appropriate amounts of glucose and polyethylene glycol. The addition amounts of glucose and polyethylene glycol are 1.16% of the total mass after calcination. Add appropriate amounts of deionized water to make the solid content of the slurry 40%, and stir and disperse for 0.5 h; use a sand mill to sand the slurry. The diameter of the zirconia balls is 0.3 - 0.4 mm, and sand until the particle size of the slurry is D 50 to about 0.35 μm to obtain the second slurry; the ratio of the solid substances of the first spray material to the second slurry in this embodiment is 0.3:1.0. Take the first spray material as the bottom material and place it in a fluidized bed spray granulation drying device. Take the second slurry as the mother liquor and perform fluidized bed spray drying granulation. Control the inlet air temperature at 200 °C, the granulation temperature at 95 °C, and the moisture content of the spray material at 1.23%, D 50 to 96.8 μm to obtain the second spray material.

[0193] S114. Place the second spray material in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere is nitrogen. Control the oxygen content in the furnace to drop below 50 ppm, the heating rate to 1 °C / min, the sintering temperature to 800 °C, and the sintering time to 10 h. After sintering, cool with the furnace to room temperature to obtain the sintered material. Crush the sintered material with an air flow crusher, adjust parameters such as the classification frequency and crushing pressure, and control the finished product particle size within D 10 ≥0.38 μm, 1.3 μm ≤ D 50 ≤1.5 μm, D 99 ≤10 μm range. After sieving to remove impurities, the lithium iron phosphate cathode material can be obtained.

[0194] Comparative Example 5

[0195] S121. According to the stoichiometric ratio of Li 1.00 Fe 0.97 PO4, after adding iron phosphate, lithium carbonate, and magnesium oxide, add appropriate amounts of glucose. The addition amount of glucose is 1.23% of the total mass after calcination. Add appropriate amounts of water to make the solid content of the slurry 40%, and stir and disperse for 0.5 h; use a sand mill to sand the slurry. The diameter of the zirconia balls is 0.3 - 0.4 mm, and sand until the particle size of the slurry is D 50 to about 0.4 μm to obtain the first slurry.

[0196] S122. Perform spray drying on the sanded first slurry. Use two-fluid spraying, control the inlet air temperature at 230 °C, the outlet air temperature at 85 °C, the moisture content of the spray material at 3.16%, D 50 to 23.9 μm to obtain the first spray material.

[0197] S123. According to the stoichiometric ratio of Li 1.02 Fe0.96 After adding iron phosphate and lithium carbonate to PO4, appropriate amounts of glucose and polyethylene glycol were added. The addition amounts of glucose and polyethylene glycol were 1.3% of the total mass after calcination. Appropriate amounts of deionized water were added to make the solid content of the slurry 40%, and it was stirred and dispersed for 0.5 h; the slurry was milled using a sand mill, and the diameter of the zirconia balls was 0.3 - 0.4 mm. The slurry was milled until the particle size of the slurry was D 50 to about 0.35 μm to obtain a second slurry; the ratio of the solid substances of the first spray material and the second slurry in this embodiment was 0.3:1.0. The first spray material was used as the bottom material and placed in a fluidized bed spray granulation drying device. The second slurry was used as the mother liquor for fluidized bed spray drying granulation. The inlet air temperature was controlled at 200 °C, the granulation temperature was 95 °C, the moisture content of the spray material was at 1.28%, D 50 to 93.2 μm to obtain a second spray material.

[0198] S124. The second spray material was placed in an atmosphere box-type sintering furnace for sintering. The sintering atmosphere was nitrogen. The oxygen content in the furnace was controlled to drop below 50 ppm, the heating rate was controlled at 1 °C / min, the sintering temperature was 800 °C, and the sintering time was 10 h. After sintering, it was cooled to room temperature with the furnace to obtain a sintered material. The sintered material was crushed using a jet mill, and parameters such as the classification frequency and crushing pressure were adjusted to control the particle size of the finished product at D 10 ≥0.4 μm, 1.3 μm ≤ D 50 ≤1.5 μm, D 99 ≤10 μm. After sieving to remove impurities, the lithium iron phosphate cathode material could be obtained.

[0199] Performance test:

[0200] The carbon content, powder tap density, specific surface area, 0.1C and 1C charge-discharge specific capacities of the products obtained in Examples 1 - 7 and Comparative Examples 1 - 5 were further tested. The results are shown in Table 1.

[0201] 1. The battery assembly process and test conditions are as follows: Battery assembly: Using NMP (N-methylpyrrolidone) as the solvent, the mass ratios of lithium iron phosphate, binder, and conductive agent (SP (conductive carbon black)) were adjusted to 90:5:5 respectively for homogenization. After the slurry was appropriate, it was coated on an infrared flat coater with aluminum foil as the current collector. After coating, the electrode sheet was vacuum dried, rolled, die-cut, selected, and weighed to obtain a positive electrode sheet. The weighed electrode sheet was transferred to a MIKRONA glove box, and a coin cell was assembled while controlling the water and oxygen content in the glove box < 0.1 ppm. The coin cell case was of the CR2032 model, the negative electrode was a lithium sheet, and the electrolyte was a conventional lithium iron phosphate electrolyte.

[0202] 2. Electrochemical performance test: The assembled coin cell was subjected to an electrochemical performance test using a Blue Power test system. The temperature of the constant temperature oven was set at room temperature of 25 °C. The voltage range for the coin cell test was 2.0 - 3.75 V, and 0.1C and 1C charge-discharge tests were respectively carried out according to the mass of the active material in each electrode sheet. The charge-discharge specific capacity was calculated based on the measured rate; specific process: Constant current charge at 0.1C to 3.75 V, stand for 30 min, constant current discharge at 0.1C to 2.0 V, record the discharge capacity, discharge specific capacity = discharge capacity / mass of active material; similarly, change the current to 1C and calculate the 1C discharge specific capacity.

[0203] 3. Carbon content test: It was obtained by testing with a carbon-sulfur analyzer in accordance with GB / T 20123-2006.

[0204] 4. Specific surface area (BET) test: It was determined by the BET method using a specific surface area tester in accordance with GB / T 19587-2017.

[0205] 5. Powder tap density test: A Sansi powder tap densimeter was used to conduct the tap density test according to the national standard GB / T 24533-2009, test conditions: 4T. After the instrument was zeroed, 1 g ± 0.0050 g of the sample was weighed, and the constant pressure time was 30 s, and the result was recorded.

[0206] Table 1. Test results of carbon content, powder tap density, specific surface area, and 0.1C and 1C charge-discharge specific capacities:

[0207]

[0208] As can be seen from Table 1, compared with Comparative Example 1, the discharge specific capacity of Examples 1-7 is higher, which proves that the lithium iron phosphate prepared by the present invention has a higher specific capacity. Compared with Comparative Examples 1-2, the tap density of Examples 1-7 is higher, which proves that the lithium iron phosphate prepared by the present invention has a higher tap density.

[0209] Figure 1 It is the SEM image of the lithium iron phosphate cathode material in Example 1. It can be seen that the lithium iron phosphate cathode material exists in the form of a graded mixture of large and small particles of lithium iron phosphate. The proportion of large and small particles of lithium iron phosphate is appropriate. The large particles of lithium iron phosphate serve as the high-tap density framework, and the small particles of lithium iron phosphate serve as the gap-filling material, and improve the electrochemical performance of the lithium iron phosphate cathode material.

[0210] Figure 2 It is the XRD pattern of the lithium iron phosphate cathode material in Example 2. It can be seen from the XRD pattern that the lithium iron phosphate cathode material has good crystallinity and no obvious impurity phases.

[0211] Figure 3It is the 0.1C charge-discharge curve graph of the lithium iron phosphate obtained from the lithium iron phosphate cathode material in Example 3 (the abscissa is specific capacity). It can be seen that the lithium iron phosphate cathode material has excellent charge-discharge capacity.

[0212] The parts not detailed in this invention are well-known technologies to those skilled in the art.

[0213] The basic principles of the present invention have been described above in conjunction with specific embodiments. However, it should be noted that the advantages, advantages, effects, etc. mentioned in the present invention are only examples and not limitations. It cannot be considered that these advantages, advantages, effects, etc. are essential for each embodiment of the present invention. In addition, the above-disclosed specific details are only for the purpose of illustration and facilitating understanding, rather than limitations. The above details do not limit the present invention to necessarily adopt the above specific details to implement.

[0214] It should be noted that the term "and / or" or " / " used herein is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may mean: A exists alone, A and B exist simultaneously, and B exists alone. The singular forms of "a", "the" and "said" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0215] In the specific embodiments and the claims, the list of items connected by the terms "at least one of", "at least one of", "at least one kind of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0216] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps include: S1: mixing a lithium source, an iron source, a phosphorus source, a carbon source and a solvent, and grinding the mixture to obtain a first slurry; wherein the molar ratio of the elements Fe and P in the first slurry is y:z; A lithium source, an iron source, a phosphorus source, a carbon source and a solvent are mixed uniformly and ground to obtain a second slurry; wherein the molar ratio of the elements Fe and P in the second slurry is b:c; and y and b satisfy: y:z>b:c; S2: drying the first slurry once to obtain a first spray material; S3: using the first spray material as a base material and the second slurry as a coating liquid to coat the first spray material, and performing secondary drying to obtain a second spray material; S4: Sintering and crushing the second spray material to obtain the lithium iron phosphate positive electrode material.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The chemical formula of the first slurry is Li x Fe y P z O4, where x:y:z=(1.0~1.05):(0.97~1):1.0; And / or, the chemical formula of the second slurry is Li a Fe b P c O4, where a:b:c=(1.0~1.05):(0.955~0.97):1.

0.

3. The method for preparing the lithium iron phosphate cathode material according to claim 2, wherein: Satisfy at least one of the following characteristics (1) to (8): (1) The solid content of the first slurry is 20% to 50%; (2) The particle size of the first slurry is 0.2 μm ≤ D 50 ≤4μm; (3) The water content of the first spray material is 0.5% to 5%; (4) The particle size of the first spray material is 15 μm ≤ D 50 ≤30μm; (5) The solid content of the second slurry is between 20% and 50%; (6) The particle size of the second slurry is 0.5 μm ≤ D 50 ≤2μm; (7) The water content of the second spray material is 0.5% to 2.5%; (8) The particle size of the second spray material is 20 μm ≤ D 50 ≤100μm.

4. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: Satisfy at least one of the characteristics (1) to (5): (1) The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium chloride or lithium nitrate; (2) The iron source includes one or more of ferric phosphate, ferric oxide, ferrous oxalate, ferric nitrate or ferric chloride; (3) The phosphorus source includes one or more of ferric phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid or lithium dihydrogen phosphate; (4) The carbon source includes one or more of glucose, polyethylene glycol, sucrose, starch or citric acid; (5) The solvent includes one or more of ultrapure water, methanol or ethanol.

5. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: Satisfy at least one of the characteristics (1) to (5): (1) In the first slurry, the amount of the carbon source added is 1.0 wt% to 1.5 wt% of the mass of the lithium iron phosphate positive electrode material; (2) In the second slurry, the amount of the carbon source added is 1.0 wt% to 1.5 wt% of the mass of the lithium iron phosphate positive electrode material; (3) The mass ratio of the solid matter of the first spray material to that of the second slurry is 0.15-1.5:1.0; (4) During the sintering process, the sintering temperature is 700°C to 800°C, the heating rate is 1°C / min to 5°C / min, and the sintering time is 6h to 20h; (5) The drying process adopts spray drying, wherein the air inlet temperature of the primary drying is 200℃~280℃, and the air outlet temperature is 80℃~120℃; the air inlet temperature of the secondary drying is 150℃~200℃, and the air outlet temperature is 60℃~120℃.

6. The method for preparing a lithium iron phosphate cathode material according to claim 1, wherein: Satisfy at least one of the characteristics (1) to (2): (1) The first slurry further includes a first doping element; The first doping element includes one or more of a metal element, a non-metal element or a metalloid element; and / or the addition amount of the first doping element is 500ppm~5000ppm; (2) The second slurry further includes a second doping element; The second doping element includes one or more of a metal element, a non-metal element or a metalloid element; and / or, the added amount of the second doping element is 500 ppm to 5000 ppm.

7. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is obtained by the preparation method according to any one of claims 1 to 6; The lithium iron phosphate positive electrode material includes a first lithium iron phosphate particle and a second lithium iron phosphate particle; the first lithium iron phosphate particle is coated with a first carbon coating layer, and the second lithium iron phosphate particle is coated with a second carbon coating layer; Wherein, the iron-to-phosphorus ratio in the first lithium iron phosphate particles is greater than or equal to the iron-to-phosphorus ratio in the second lithium iron phosphate particles.

8. The lithium iron phosphate cathode material according to claim 7, characterized in that Satisfy at least one of the following characteristics (1) to (8): (1) Particle size D of the lithium iron phosphate positive electrode material 10 ≥0.30μm, 0.7μm≤D 50 ≤1.5μm, D 99 ≤12μm; (2) The compaction density of the lithium iron phosphate positive electrode material is 2.55 g / cm 3 ~2.70g / cm 3 ; (3) The particle size of the first lithium iron phosphate particles is 0.2 μm ≤ D 50 ≤0.8μm, the particle size of the second lithium iron phosphate particles is 1.0μm≤D 50 ≤2.0μm; (4) The mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 0.1-1.5:1; (5) The iron-phosphorus ratio in the first lithium iron phosphate particles is 0.97-1; (6) The iron-phosphorus ratio in the second lithium iron phosphate particles is 0.955-0.97; (7) The thickness of the first carbon coating layer is 2 to 20 nm; (8) The thickness of the second carbon coating layer is 2-20 nm.

9. A pole piece, characterized in that: include: The lithium iron phosphate positive electrode material obtained by the preparation method according to any one of claims 1 to 6, and / or the lithium iron phosphate positive electrode material according to any one of claims 7 to 8.

10. A battery, characterized in that: include: The lithium iron phosphate positive electrode material obtained by the preparation method according to any one of claims 1 to 6; And / or the lithium iron phosphate positive electrode material according to any one of claims 7 to 8, and / or the pole piece according to claim 9.

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