Composite phosphate-based positive electrode material, preparation method therefor, and use thereof

Abstract

The present application relates to the technical field of positive electrode materials for lithium-ion batteries, and in particular relates to a composite phosphate-based positive electrode material, a preparation method therefor and the use thereof. The provided composite phosphate-based positive electrode material comprises a phosphate-based positive electrode material core and a carbon coating layer, wherein the particle size of the composite phosphate-based positive electrode material exhibits a normal distribution, and the carbon coating layer has a thickness of 2.0-3.0 nm, with a variance of 0.01-0.30. The resulting product exhibits larger particles having a uniform particle size and a remarkably improved tap density. In addition, the surface of the core material is uniformly coated with the carbon coating layer, and the carbon coating layer has a moderate thickness, thus not only increasing the electronic conductivity of the positive electrode material, but also improving the electrochemical performances such as the discharge capacity and coulombic efficiency of the composite phosphate-based positive electrode material.

Description

Composite phosphate-based positive electrode material, preparation method and application thereof

[0001] The present application claims priority to the Chinese patent application No. 202411338473.2, filed on September 24, 2024 in the China Patent Office, and entitled "Composite phosphate-based positive electrode material, preparation method and application thereof", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium ion battery positive electrode materials, in particular to a composite phosphate-based positive electrode material, a preparation method and application thereof. BACKGROUND

[0003] With the continuous development of lithium ion battery technology, people have higher requirements for the energy density and low temperature performance of phosphate-based positive electrode materials. Phosphate-based positive electrode materials commonly use lithium iron phosphate, lithium manganese iron phosphate and the like. The structures of these two materials are both olivine-type structures, and both have the same theoretical specific capacity and good safety performance. Among them, lithium manganese iron phosphate has a higher voltage platform than lithium iron phosphate due to the presence of Mn elements, so it has a higher energy density, which is theoretically 10-20% higher than that of lithium iron phosphate, and has better low temperature performance. Based on the above advantages, in recent years, lithium manganese iron phosphate has received widespread attention and research from the academic and industrial communities, and has high application potential.

[0004] The voltage platform of lithium manganese iron phosphate is affected by the proportion of manganese elements in the material. The more the proportion of manganese elements, the more obvious the voltage platform of the material is improved, and the higher the energy density is. Therefore, lithium manganese iron phosphate materials with high manganese content are one of the current hotspots for the development of phosphate-based positive electrode materials. However, in the actual production process, lithium manganese iron phosphate materials are not easy to form large particles, and their particle size is smaller than that of lithium iron phosphate, resulting in a lower tap density of lithium manganese iron phosphate materials than that of lithium iron phosphate. Moreover, the higher the manganese content in the material, the more difficult it is for the particles to grow. These factors greatly affect the performance of the material and the application of the material. Therefore, the particle size of lithium manganese iron phosphate material is one of the key factors affecting its performance and application.

[0005] Current phosphate-based positive electrode materials are mainly prepared by high-temperature solid-phase sintering method. However, the particle size and particle distribution of phosphate-based positive electrode materials obtained by conventional high-temperature solid-phase sintering method are not uniform, and the carbon coating uniformity is poor, resulting in poor carbon coating quality, low tap density and small specific capacity of the obtained phosphate-based positive electrode material, which is not conducive to wide application. TECHNICAL PROBLEM

[0006] One of the purposes of the embodiments of the present application is to provide a composite phosphate-based positive electrode material and a preparation method and application thereof, aiming at solving the problems of poor carbon coating quality, low compaction density and small specific capacity of the phosphate-based positive electrode material in the prior art. Technical solutions

[0007] The technical solutions adopted by the embodiments of the present application are as follows:

[0008] In a first aspect, a composite phosphate-based positive electrode material is provided, comprising a phosphate-based positive electrode material core and a carbon coating layer, wherein the particle size of the composite phosphate-based positive electrode material is normally distributed, the thickness of the carbon coating layer is 2.0-3.0 nm, and the variance is 0.01-0.30.

[0009] In some embodiments, the chemical formula of the phosphate-based positive electrode material core is Li x Mn a Fe b M c PO4, 0.9≤x≤1, 0.6≤a<1, 0.4<b<1, 0.001≤c≤0.004, and a+b+c=1; M includes any one of Ti, Mg, V, Ni, Nb, Co, La, Sc, and Ce.

[0010] In some embodiments, the particle size of the composite phosphate-based positive electrode material is 180-230 nm, and the compaction density is greater than 2.37 g / cm 3 .

[0011] In a second aspect, a preparation method of a composite phosphate-based positive electrode material is provided, comprising the following steps:

[0012] Providing a precursor mixture of the composite phosphate-based positive electrode material;

[0013] Ball-milling the precursor mixture to obtain a powder-like precursor, and monitoring the particle size of the powder-like precursor in real time until it is normally distributed;

[0014] Carrying out two-stage sintering treatment on the powder-like precursor to obtain the composite phosphate-based positive electrode material.

[0015] In some embodiments, the step of ball-milling the precursor mixture to obtain a powder-like precursor comprises: ball-milling the precursor mixture multiple times according to a ball-to-material ratio of 5:1-15:1 and monitoring in real time, so as to control the particle size of the powder-like precursor to be normally distributed, and the particle size D 90 is 1.0-3.0 μm.

[0016] In some embodiments, the rotation speed of the ball-milling is 450-650 rpm, the time is 7-8 hours, the number of times is 3-5, and the diameter of the grinding ball is 6-7 mm.

[0017] In some embodiments, the step of performing a two-stage sintering process on the powdered precursor includes: providing an inert gas atmosphere, heating the powdered precursor to a constant temperature of 520-560°C for 2-5 hours at a heating rate of 3-8°C / min, and then heating to a constant temperature of 720-780°C for 8-12 hours at a heating rate of 3-8°C / min.

[0018] In some embodiments, the step of providing a precursor mixture of the composite phosphate-based positive electrode material includes: obtaining a lithium source, a manganese source, an iron source, a phosphorus source, and a M source according to the chemical formula Li x Mn a Fe b M c PO4, and mixing the lithium source, the manganese source, the iron source, the phosphorus source, and the M source with a carbon source to obtain the precursor mixture; wherein M includes any one of Ti, Mg, V, Ni, Nb, Co, La, Sc, and Ce; 0.9≤x≤1, 0.6≤a<1, 0.4

[0019] In some embodiments, the lithium source is any one of lithium carbonate, lithium acetate, and lithium hydroxide.

[0020] In some embodiments, the manganese source is at least one of manganese dioxide, trimanganese tetroxide, and manganese oxalate.

[0021] In some embodiments, the iron source is at least one of iron phosphate, ferrous nitrate, ferrous oxalate dihydrate, and ferrous sulfate.

[0022] In some embodiments, the phosphorus source is at least one of iron phosphate, ammonium dihydrogen phosphate, and di-ammonium hydrogen phosphate.

[0023] In some embodiments, the M source includes an oxide containing an M element.

[0024] In some embodiments, the carbon source is at least one of glucose, polyethylene glycol, citric acid, sucrose, glycine, and starch.

[0025] In some embodiments, the amount of the carbon source added is 5-10 wt% based on the total mass of the precursor mixture being 100%.

[0026] In a third aspect, a positive electrode tab is provided, which includes a current collector and a positive electrode active layer formed on the surface of the current collector, and the positive electrode active layer includes the composite phosphate-based positive electrode material described above or the composite phosphate-based positive electrode material prepared by the preparation method described above.

[0027] In a fourth aspect, a secondary battery is provided, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the positive electrode sheet is selected from the positive electrode sheets described above. Advantages

[0028] The composite phosphate-based positive electrode material provided in the first aspect of the present application includes a phosphate-based positive electrode material core and a carbon coating layer. The particle size of the composite phosphate-based positive electrode material is normally distributed. The finished product has large and uniform particle size, and the compaction density is significantly improved. The carbon coating layer is uniformly coated on the surface of the core material. The carbon coating layer has moderate thickness. The carbon coating layer can improve the electronic conductivity of the positive electrode material, and can improve the discharge capacity and coulombic efficiency of the composite phosphate-based positive electrode material.

[0029] The preparation method of the composite phosphate-based positive electrode material provided in the second aspect of the present application includes ball milling the obtained precursor mixture to obtain a powder-like precursor. The particle size of the powder-like precursor is controlled by controlling the ball milling conditions. The particle size distribution curve of the powder-like precursor is monitored in real time until an approximately normal distribution curve is obtained. The composite phosphate-based positive electrode material has large overall particle size, moderate and uniform carbon coating thickness, and high compaction density and specific capacity. The preparation method is simple and fast, does not require large-scale instruments and equipment, and is conducive to wide application.

[0030] The positive electrode sheet provided in the third aspect of the present application includes a composite phosphate-based positive electrode material having excellent carbon coating quality, high compaction density, and high specific capacity. Therefore, the obtained positive electrode sheet has excellent cycle performance and capacity effect.

[0031] The secondary battery provided in the fourth aspect of the present application includes the positive electrode sheet described above. The positive electrode sheet has excellent cycle performance and capacity effect. Therefore, the obtained secondary battery has high charge-discharge specific capacity, high cycle efficiency, and excellent performance. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments or exemplary technical descriptions will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application. Those skilled in the art can also obtain other drawings according to these drawings without any creative effort.

[0033] FIG. 1 is an SEM analysis diagram (A) and a primary particle size statistical diagram (B) of the material finished product provided in the first embodiment of the present application.

[0034] FIG. 2 is an SEM analysis diagram (A) and a primary particle size statistical diagram (B) of the material finished product provided in the second embodiment of the present application.

[0035] Figure 3 is an SEM analysis graph (A) and a primary particle size statistical graph (B) of the material product provided by Example 3 of the present application.

[0036] Figure 4 is an SEM analysis graph (A) and a primary particle size statistical graph (B) of the material product provided by Comparative Example 1 of the present application.

[0037] Figure 5 is an SEM analysis graph (A) and a primary particle size statistical graph (B) of the material product provided by Comparative Example 2 of the present application.

[0038] Figure 6 is an SEM analysis graph (A) and a primary particle size statistical graph (B) of the material product provided by Comparative Example 3 of the present application.

[0039] Figure 7 is an SEM analysis graph (A) and a primary particle size statistical graph (B) of the material product provided by Comparative Example 4 of the present application.

[0040] Figure 8 is a precursor particle size distribution graph provided by Examples 1-3 and Comparative Examples 1-4 of the present application.

[0041] Figure 9 is a TEM characterization graph of the material product provided by Example 1 of the present application.

[0042] Figure 10 is a TEM characterization graph of the material product provided by Example 2 of the present application.

[0043] Figure 11 is a TEM characterization graph of the material product provided by Example 3 of the present application.

[0044] Figure 12 is a TEM characterization graph of the material product provided by Comparative Example 1 of the present application.

[0045] Figure 13 is a TEM characterization graph of the material product provided by Comparative Example 2 of the present application.

[0046] Figure 14 is a TEM characterization graph of the material product provided by Comparative Example 3 of the present application.

[0047] Figure 15 is a TEM characterization graph of the material product provided by Comparative Example 4 of the present application.

[0048] Figure 16 is a charge-discharge curve graph of the material at 0.1C provided by Examples 1-3 and Comparative Examples 1-4 of the present application. Embodiments of the present application

[0049] In order to make the purposes, technical solutions and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and are not used to limit the present application.

[0050] It is to be noted that when a component is referred to as being "fixed" or "set" on another component, it can be directly on the other component or indirectly on the other component. When a component is referred to as being "connected" to another component, it can be directly or indirectly connected to the other component. The terms "upper", "lower", "left", "right", and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. For those skilled in the art, the specific meanings of the above terms can be understood according to the specific circumstances. The terms "first", "second" are only for the convenience of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features.

[0051] In order to illustrate the technical solutions provided in the present application, the following will be described in detail in combination with specific drawings and examples.

[0052] The first aspect of the embodiment of the present application provides a composite phosphate-based positive electrode material, which comprises a phosphate-based positive electrode material core and a carbon coating layer. The particle size of the composite phosphate-based positive electrode material is normally distributed. The thickness of the carbon coating layer is 2.0-3.0 nm, and the variance is 0.01-0.30.

[0053] The composite phosphate-based positive electrode material provided by the first aspect of the embodiment of the present application comprises a phosphate-based positive electrode material core and a carbon coating layer. The particle size of the composite phosphate-based positive electrode material is normally distributed. The finished product has large particles and uniform particle size, and the compaction density is significantly improved. Moreover, the surface of the core material is uniformly coated with a carbon coating layer, and the thickness of the carbon coating layer is moderate. The carbon coating layer not only improves the electronic conductivity of the positive electrode material, but also improves the discharge capacity and coulombic efficiency electrochemical performance of the composite phosphate-based positive electrode material.

[0054] The particle size of the composite phosphate-based positive electrode material is normally distributed, which ensures uniform particle size and is conducive to improving the compaction density and capacity of the material as a whole. In the embodiment of the present application, the particle size of the composite phosphate-based positive electrode material also includes the case of approximate normal distribution. If the particle size distribution of the composite phosphate-based positive electrode material deviates greatly from the normal distribution, the particle agglomeration will be serious, the boundary will not be obvious, and it will be difficult to improve the compaction density of the material.

[0055] Further, the thickness of the carbon coating layer is 2.0-3.0 nm, and the variance is 0.01-0.30. The carbon coating layer of the obtained composite phosphate-based positive electrode material is uniformly coated, and the thickness is moderate, which helps to improve the capacity of the composite phosphate-based positive electrode material as a whole. The better the uniformity of the carbon coating layer, the larger the particle size and the thicker the coating layer, the smaller the particle size and the thinner the coating layer, and the moderate particle size and the moderate thickness of the carbon layer. If the carbon coating layer is too thick, the transmission of lithium ions is hindered to a certain extent; if the carbon coating layer is too thin and uneven, the electrolyte and the inner core of the phosphate-based positive electrode material directly contact, which increases the side reaction and causes the loss of capacity. In some specific embodiments, the thickness of the carbon coating layer includes but is not limited to 2.0 nm, 2.1 nm, 2.2 nm, 2.3 nm, 2.4 nm, 2.5 nm, 2.6 nm, 2.7 nm, 2.8 nm, 2.9 nm, 3.0 nm, and the like, which are typical but not limited to any point value or interval value between any two point values.

[0056] In some embodiments, the chemical formula of the inner core of the phosphate-based positive electrode material is Li x Mn a Fe b M c PO4, 0.9≤x≤1, 0.6≤a<1, 0.4<b<1, 0.001≤c≤0.004, and a+b+c=1; M includes any one of Ti, Mg, V, Ni, Nb, Co, La, Sc, and Ce. The main purpose of adding the doping element M is to introduce certain defects in the crystal lattice of the phosphate-based positive electrode material, to widen the transmission channel of lithium ions, to improve the lithium ion migration rate of the material, and to further improve the performance of the discharge specific capacity of the material.

[0057] wherein the addition amount of the doping element M is 0.001-0.004 mol. The doping amount of the doping element M has a significant influence on the capacity of the positive electrode material and the particle size. Within the range of the doping amount, the lithium ion migration rate can be effectively improved, thereby improving the discharge specific capacity of the material; and the obtained positive electrode material has a moderate particle size, which can maintain a high compaction density. If the addition amount of the doping component is too much, on the one hand, it will affect the particle size and the compaction density, and on the other hand, it will form impurities in the crystal lattice of the material, which will affect the capacity and rate performance of the material; if the addition amount is too small, the doping effect is not obvious, and the capacity is very low, which is not conducive to the application of the product.

[0058] In some embodiments, the particle size of the composite phosphate-based positive electrode material is 180-230 nm, and the compaction density is greater than 2.37 g / cm 3In some embodiments, the particle size of the composite phosphate-based positive electrode material includes, but is not limited to, 180 nm, 190 nm, 200 nm, 205 nm, 210 nm, 215 nm, 220 nm, 225 nm, 230 nm, and the like typical but non-limiting any point value or interval value between any two point values. In some embodiments, the compaction density of the composite phosphate-based positive electrode material includes, but is not limited to, 2.37 g / cm 3 , 2.38 g / cm 3 , 2.39 g / cm 3 , 2.40 g / cm 3 , 2.41 g / cm 3 , 2.42 g / cm 3 , 2.43 g / cm 3 , 2.44 g / cm 3 , 2.45 g / cm 3 , and the like.

[0059] In a second aspect, the application provides a preparation method of a composite phosphate-based positive electrode material, comprising the following steps:

[0060] S01. Providing a precursor mixture of a composite phosphate-based positive electrode material;

[0061] S02. Ball milling the precursor mixture to obtain a powder precursor, and monitoring the particle size of the powder precursor in real time until it is normally distributed;

[0062] S03. Two-stage sintering the powder precursor to obtain a composite phosphate-based positive electrode material.

[0063] The preparation method of the composite phosphate-based positive electrode material provided in the second aspect of the embodiments of the application, which obtains a powder precursor by ball milling the mixed precursor mixture, controls the particle size of the powder precursor by controlling the ball milling conditions, monitors the particle size distribution curve of the powder precursor in real time until it is approximately normally distributed, so as to obtain a composite phosphate-based positive electrode material with large overall particle size, moderate and uniform carbon coating thickness, and high compaction density and specific capacity. The preparation method is simple and fast, does not require large-scale instruments and equipment, and is conducive to wide application.

[0064] In step S01, a precursor mixture of a composite phosphate-based positive electrode material is provided.

[0065] In some embodiments, the step of providing a precursor mixture of a composite phosphate-based positive electrode material includes: according to the chemical formula Li x Mn a Fe b M cThe PO4 obtains a lithium source, a manganese source, an iron source, a phosphorus source, an M source, and then is mixed with a carbon source to obtain a precursor mixture; wherein M includes any one of Ti, Mg, V, Ni, Nb, Co, La, Sc, and Ce; 0.9≤x≤1, 0.6≤a<1, 0.4<b<1, 0.001≤c≤0.004, and a+b+c=1. In the preparation process of the precursor mixture, a carbon source is additionally added, and a carbon coating layer is prepared in situ in the sintering process. The preparation of the carbon coating layer can improve the electronic conductivity of the material, avoid direct contact between the core material and the electrolyte, and improve the specific capacity of the material.

[0066] In some embodiments, the lithium source is any one of lithium carbonate, lithium acetate, and lithium hydroxide.

[0067] In some embodiments, the manganese source is at least one of manganese dioxide, trimanganese tetroxide, and manganese oxalate.

[0068] In some embodiments, the iron source is at least one of iron phosphate, ferrous nitrate, ferrous oxalate dihydrate, and ferrous sulfate.

[0069] In some embodiments, the phosphorus source is at least one of iron phosphate, ammonium dihydrogen phosphate, and di-ammonium hydrogen phosphate.

[0070] In some embodiments, the M source includes an oxide containing an M element.

[0071] In some embodiments, the carbon source is at least one of glucose, polyethylene glycol, citric acid, sucrose, glycine, and starch.

[0072] In some embodiments, the total mass of the precursor mixture is 100%, and the addition amount of the carbon source is 5-10wt%. If the addition amount of the carbon source is too much, the carbon content of the finished product will increase, the thickness of the carbon coating layer will be large, the specific surface area of the material will increase, which will affect the processing of the material, and in addition, the thick carbon layer will hinder the transmission of lithium ions; if the addition amount is too small, the carbon coating will be insufficient, the effect of improving the electrical conductivity of the material will be reduced, and at the same time, due to the low thickness of the coating layer, the side reaction between the material and the electrolyte will increase, which will affect the capacity performance.

[0073] In some specific embodiments, the total mass of the precursor mixture is 100%, and the addition amount of the carbon source includes but is not limited to 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, and the like, which are typical but not limited to any point value or interval value between any two point values.

[0074] In step S02, the precursor mixture is subjected to ball milling to obtain a powder precursor, and the particle size of the powder precursor is monitored in real time until it is normally distributed. In the process of crushing the precursor mixture, the precursor mixture is ground by a specific ball milling method, and the particle size of the material is monitored and analyzed in real time after each ball milling until the particle size of the powder precursor is normally distributed.

[0075] In some embodiments, in the step of ball milling the precursor mixture to obtain a powder precursor, the precursor mixture is subjected to multiple ball milling with a ball-to-material ratio of 5:1 to 15:1 and real-time monitoring, the precursor mixture is controlled to be normally distributed, and the particle size D 90 is 1.0 to 3.0 μm; wherein the rotation speed of the ball milling is 450 to 650 rpm, the time is 7 to 8 hours, the number of times is 3 to 5 times, and the diameter of the grinding ball is 6 to 7 mm.

[0076] The ball-to-material ratio of the ball milling is 5:1 to 15:1, and a suitable range of the ball-to-material ratio helps to obtain a precursor with a suitable particle size. If the amount of grinding balls added is too much, the particle size of the obtained precursor will be too small, resulting in smaller particles of the finished product; on the contrary, if the amount of grinding balls added is too small, the particle size of the precursor will be too large, and the grinding will not be sufficient, and the particle size curve of the precursor will not be qualified. In some specific embodiments, the ball-to-material ratio of the ball milling includes but is not limited to 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, and other typical but non-limiting point values or interval values between any two point values.

[0077] Furthermore, the diameter of the grinding ball is 6 to 7 mm. If the diameter of the grinding ball is too large, the impact force is stronger during the grinding process, the packing void ratio of the grinding body is larger, and the material flow rate is faster, resulting in a product with a particle size that is too large, which is not conducive to controlling the particle size of the product. In some specific embodiments, the diameter of the grinding ball includes but is not limited to 6 nm, 6.1 nm, 6.2 nm, 6.3 nm, 6.4 nm, 6.5 nm, 6.6 nm, 6.7 nm, 6.8 nm, 6.9 nm, 7.0 nm, and other typical but non-limiting point values or interval values between any two point values.

[0078] Further, the rotation speed of the ball milling treatment is 450-650 rpm, and the time is 7-8 hours. The rotation speed of the ball milling treatment directly affects the movement state of the grinding ball in the ball mill. If the rotation speed of the ball milling treatment is too slow, the grinding ball does not rise high and slides down, the crushing effect is small, the particle size of the obtained powder precursor is too large and is not uniform, and the powder precursor with normal particle size distribution cannot be obtained. If the rotation speed of the ball milling treatment is too fast, the grinding ball is attached to the inner wall of the grinding machine and rotates, and the grinding effect on the raw material is weak. When the rotation speed of the ball milling treatment is controlled to be 450-650 rpm, the impact force and the grinding effect on the precursor mixture are the largest, and the crushing efficiency is the highest. In some specific embodiments, the rotation speed of the ball milling treatment includes but is not limited to 450 rpm, 500 rpm, 500 rpm, 550 rpm, 600 rpm, 650 rpm, and the like, which are typical but not limited to any point value or interval value between any two point values.

[0079] The time of the ball milling treatment is 7-8 hours, and the time of the ball milling treatment is controlled to be moderate, which can ensure that the ball milling treatment has better effect and ensure that the grinding degree is high. If the time of the ball milling treatment is too low, the grinding effect is poor and the grinding degree is weak. If the time of the ball milling treatment is too high, the particle size of the precursor is too small, and the product particles are prone to agglomeration. In some specific embodiments, the time of the ball milling treatment includes but is not limited to 7 hours, 7.5 hours, 8 hours, and the like, which are typical but not limited to any point value or interval value between any two point values.

[0080] The ball milling times are generally 3-5 times, and the optimal is 3 times. If the ball milling times are too large, the particle size of the precursor particles is too small, and the particle size of the product is difficult to grow. If the ball milling times are too small, the particle size distribution of the precursor is not uniform, which affects the performance of the product. In some specific embodiments, the ball milling times include but are not limited to 3 times, 4 times, or 5 times.

[0081] Further, in the process of the ball milling treatment, real-time monitoring is performed, and the particle size of the powder precursor is controlled until it is normally distributed. Further, the particle size D 90 of the powder precursor is controlled to be 1.0-3.0 μm. Controlling the particle size D 90 of the precursor to be 1.0-3.0 μm, when the particle size of the obtained product can be controlled within the target range, the product particle size meets the standard, which helps to improve the compaction density and capacity performance. If the particle size of the precursor is too large, the particle size of the product is too high, the particle size distribution curve is not normally distributed, the carbon coating is thin and uneven, and the compaction density and capacity performance of the material are affected. If the particle size of the precursor is too small, the product particles are difficult to grow, the carbon coating layer is thick, and the compaction density of the material is low. In some specific embodiments, the particle size D 90including but not limited to 1.0 μm, 1.5 μm, 2 μm, 2.5 μm, 3.0 μm, etc. typical but non-limiting any point value or interval value between any two point values.

[0082] In step S03, the powdery precursor is subjected to two-stage sintering treatment to obtain the composite phosphate-based positive electrode material.

[0083] In some embodiments, in the step of subjecting the powdery precursor to two-stage sintering treatment, the following steps are included: providing an inert gas atmosphere, heating the powdery precursor at a heating rate of 3-8 ℃ / min to 520-560 ℃ for constant temperature calcination for 2-5 hours, and then heating at a heating rate of 3-8 ℃ / min to 720-780 ℃ for constant temperature calcination for 8-12 hours.

[0084] The main purpose of providing an inert gas atmosphere is to control the generation of impurity molecules in the entire reaction environment and ensure that there is no side reaction and no impurity molecules are generated during the reaction process. The inert gas atmosphere includes but is not limited to nitrogen atmosphere, argon atmosphere, helium atmosphere, etc.

[0085] The sintering treatment is two-stage sintering treatment. In the first stage, the powdery precursor is heated at a heating rate of 3-8 ℃ / min to 520-560 ℃ for constant temperature calcination for 2-5 hours. Low-temperature short-time calcination conditions are mainly used. This stage is mainly the nucleation process of the phosphate-based positive electrode material, and the crystal lattice of the phosphate-based positive electrode material is generated in this stage. If the reaction temperature exceeds 560 ℃ in this stage, a side reaction will occur and a heterogeneous phase will be formed. If the temperature is lower than 520 ℃, the reaction energy is not enough and the crystal lattice of the phosphate-based positive electrode material cannot be fully formed. The calcination time in this stage is controlled to be 2-5 hours. If the reaction time is too long, overburning will occur and a heterogeneous phase will be formed. If the reaction time is too short, the reaction is not sufficient and the crystal lattice of the phosphate-based positive electrode material cannot be fully formed, and the crystal lattice is not stable.

[0086] In the specific embodiment, under the sintering conditions of the first stage, the heating rate can be selected as 3 ℃ / min, 4 ℃ / min, 5 ℃ / min, 6 ℃ / min, 7 ℃ / min, 8 ℃ / min, etc. typical but non-limiting any point value or interval value between any two point values; the heating temperature can be selected as 520 ℃, 530 ℃, 540 ℃, 550 ℃, 560 ℃, etc. typical but non-limiting any point value or interval value between any two point values; and the heating time can be selected as 2 hours, 3 hours, 4 hours, 5 hours, etc. typical but non-limiting any point value or interval value between any two point values.

[0087] The second stage sintering process is to heat to 720-780℃ at a heating rate of 3-8℃ / min and keep constant temperature for 8-12 hours. Mainly, high temperature and long time calcination conditions are adopted, and this stage is mainly the process of crystal nucleus growth and crystallinity enhancement. The particles of the phosphate-based positive electrode material are further grown in this process. In this process, if the temperature exceeds 780℃, the particles will grow too large due to the excessively high temperature, which will affect the capacity performance of the material and may generate impurities. If the temperature is lower than 720℃, the crystallinity is poor and the particles do not grow sufficiently. If the reaction time is too long, the particles will grow too large, which will affect the capacity performance of the material and may generate impurities. If the time is too short, the crystallinity is poor and the particles do not grow sufficiently.

[0088] In the specific embodiments, under the second stage sintering conditions, the heating rate can be selected as 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, etc., typical but non-limiting point values or interval values between any two point values; the heating temperature can be selected as 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, etc., typical but non-limiting point values or interval values between any two point values; and the heating time can be selected as 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, etc., typical but non-limiting point values or interval values between any two point values.

[0089] In a third aspect, the application provides a positive electrode tab, which comprises a current collector and a positive electrode active layer formed on the surface of the current collector. The positive electrode active layer comprises the composite phosphate-based positive electrode material described above or the composite phosphate-based positive electrode material prepared by the preparation method described above.

[0090] The positive electrode tab provided in the third aspect of the embodiments of the application comprises the composite phosphate-based positive electrode material with excellent carbon coating quality, high tap density and high specific capacity in the positive electrode active layer, and therefore the obtained positive electrode tab has excellent cycle performance and capacity effect.

[0091] In some specific embodiments, the preparation method of the positive electrode tab comprises the following steps: mixing the composite phosphate-based positive electrode material, the conductive agent and the binder to obtain a mixed slurry; coating the mixed slurry on the surface of the current collector, and then performing drying, rolling, die cutting and other post-processing to obtain the positive electrode tab.

[0092] In some embodiments, the composite phosphate-based positive electrode material accounts for 90wt%-95wt% of the total mass of the positive electrode active layer, with the total mass of the positive electrode active layer being 100%. The content of the composite phosphate-based positive electrode material can be 90wt%, 91wt%, 92wt%, 93wt%, 94wt%, 95wt%, etc., typical but non-limiting content.

[0093] In some embodiments, the conductive agent accounts for 1wt% to 5wt% of the total mass of the positive electrode active layer, with the total mass of the positive electrode active layer being 100%. The content of the conductive agent can be 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, etc. The conductive agent is selected from at least one of graphite, carbon black, acetylene black, graphene, carbon fiber, and carbon nanotube.

[0094] In some embodiments, the binder accounts for 1wt% to 5wt% of the total mass of the positive electrode active layer, with the total mass of the positive electrode active layer being 100%. The content of the binder can be 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, etc. The binder is selected from at least one of polyvinylidene chloride, soluble polytetrafluoroethylene, butadiene styrene rubber, hydroxypropyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0095] Further, the current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0096] In a fourth aspect, the application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet is selected from the positive electrode sheet described above.

[0097] The secondary battery provided in the fourth aspect of the embodiments of the application includes the positive electrode sheet described above. The positive electrode sheet has excellent cycle performance and capacity effect, so that the secondary battery obtained has high charge-discharge specific capacity, high cycle efficiency, and excellent performance.

[0098] The negative electrode sheet, the separator, and the electrolyte of the secondary battery can be selected from corresponding materials that are conventionally used in secondary batteries, and will not be described here.

[0099] The embodiments will be described below in conjunction with specific examples.

[0100] Example 1

[0101] Composite phosphate-based positive electrode material and preparation method thereof

[0102] The composite phosphate-based positive electrode material is LiMn 0.798 Fe 0.199 Ti 0.003 PO4. The preparation method thereof includes the following steps:

[0103] ① Mixing of raw materials: 37.0g of lithium carbonate, 69.6g of manganese dioxide, 30.2g of iron phosphate, and 92.0g of ammonium dihydrogen phosphate are mixed, and then 18.3g of glucose and 0.12g of titanium dioxide are added;

[0104] ② Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 10:1, a rotation speed of 500 rpm, and a ball milling time of 8 h, repeated three times, to obtain a powder-like precursor;

[0105] ③ Sintering of the precursor: the powder-like precursor is heated to 550°C at a heating rate of 5°C / min, and then heated to 780°C at a heating rate of 5°C / min, under a nitrogen or argon atmosphere, and held for 4 h and 10 h, respectively. After cooling, a composite lithium manganese iron phosphate material is obtained.

[0106] Example 2

[0107] Composite phosphate-based positive electrode material and preparation method thereof

[0108] The composite phosphate-based positive electrode material is LiMn 0.698 Fe 0.299 Ti 0.003 PO4. The preparation method thereof comprises the following steps:

[0109] ① Mixing of raw materials: lithium carbonate 37.0 g, manganese dioxide 60.9 g, iron phosphate 45.2 g, ammonium dihydrogen phosphate 80.5 g, and glucose 17.9 g, and titanium dioxide 0.12 g are mixed;

[0110] ② Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 10:1, a rotation speed of 500 rpm, and a ball milling time of 8 h, repeated three times, to obtain a powder-like precursor;

[0111] ③ Sintering of the precursor: the powder-like precursor is heated to 550°C at a heating rate of 5°C / min, and then heated to 780°C at a heating rate of 5°C / min, under a nitrogen or argon atmosphere, and held for 4 h and 10 h, respectively. After cooling, a composite lithium manganese iron phosphate material is obtained.

[0112] Example 3

[0113] Composite phosphate-based positive electrode material and preparation method thereof

[0114] The composite phosphate-based positive electrode material is LiMn 0.598 Fe 0.399 Ti 0.003 PO4. The preparation method thereof comprises the following steps:

[0115] ① Mixing of raw materials: lithium carbonate 37.0 g, manganese dioxide 52.2 g, iron phosphate 60.3 g, ammonium dihydrogen phosphate 69.0 g, and glucose 17.5 g, and titanium dioxide 0.12 g are mixed;

[0116] ② Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 10:1, a rotation speed of 500 rpm, and a ball milling time of 8 h, repeated three times, to obtain a powder-like precursor;

[0117] ③ Sintering of the precursor: the powder-like precursor is heated to 550°C at a heating rate of 5°C / min, and then heated to 780°C at a heating rate of 5°C / min, under a nitrogen or argon atmosphere, and held for 4 h and 10 h, respectively. After cooling, a composite lithium manganese iron phosphate material is obtained.

[0118] Comparative Example 1

[0119] Composite phosphate-based positive electrode material and preparation method thereof

[0120] The composite phosphate-based positive electrode material is LiMn 0.798 Fe 0.199 Ti 0.003 PO4. The preparation method comprises the following steps:

[0121] ① Mixing of raw materials: lithium carbonate 37.0 g, manganese dioxide 69.6 g, iron phosphate 30.2 g, and ammonium dihydrogen phosphate 92.0 g are mixed, and then glucose 18.3 g and titanium dioxide 0.12 g are added;

[0122] ② Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 20:1, a rotation speed of 500 rpm, and a ball milling time of 8 h, repeated three times, to obtain a powder-like precursor;

[0123] ③ Sintering of the precursor: the powder-like precursor is heated to 550°C at a heating rate of 5°C / min, and then heated to 780°C at a heating rate of 5°C / min, under a nitrogen or argon atmosphere, and held for 4 h and 10 h, respectively. After cooling, a composite lithium manganese iron phosphate material is obtained.

[0124] Comparative Example 2

[0125] Composite phosphate-based positive electrode material and preparation method thereof

[0126] The composite phosphate-based positive electrode material is LiMn 0.798 Fe 0.199 Ti 0.003 PO4. The preparation method comprises the following steps:

[0127] ① Mixing of raw materials: lithium carbonate 37.0 g, manganese dioxide 69.6 g, iron phosphate 30.2 g, and ammonium dihydrogen phosphate 92.0 g are mixed, and then glucose 18.3 g and titanium dioxide 0.12 g are added;

[0128] ② Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 10:1 at a rotation speed of 500 rpm for 8 h to obtain a powder-like precursor;

[0129] ③ Sintering of the precursor: the powder-like precursor is heated to 550°C at a heating rate of 5°C / min under nitrogen or argon atmosphere, and then heated to 780°C at a heating rate of 5°C / min under nitrogen or argon atmosphere for 10 h. After cooling, a composite lithium manganese iron phosphate material is obtained.

[0130] Comparative Example 3

[0131] Composite phosphate-based positive electrode material and preparation method thereof

[0132] The composite phosphate-based positive electrode material is LiMn 0.794 Fe 0.198 Ti 0.008 PO4. The preparation method comprises the following steps:

[0133] ① Mixing of raw materials: lithium carbonate 37.0 g, manganese dioxide 69.6 g, iron phosphate 30.2 g, and ammonium dihydrogen phosphate 92.0 g are mixed, and then glucose 18.3 g and titanium dioxide 0.32 g are added;

[0134] ② Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 20:1 at a rotation speed of 500 rpm for 8 h, and the process is repeated three times to obtain a powder-like precursor;

[0135] ③ Sintering of the precursor: the powder-like precursor is heated to 550°C at a heating rate of 5°C / min under nitrogen or argon atmosphere, and then heated to 780°C at a heating rate of 5°C / min under nitrogen or argon atmosphere for 10 h. After cooling, a composite lithium manganese iron phosphate material is obtained.

[0136] Comparative Example 4

[0137] Composite phosphate-based positive electrode material and preparation method thereof

[0138] The composite phosphate-based positive electrode material is LiMn 0.8 Fe 0.2 PO4. The preparation method comprises the following steps:

[0139] ① Mixing of raw materials: lithium carbonate 37.0 g, manganese dioxide 69.6 g, iron phosphate 30.2 g, and ammonium dihydrogen phosphate 92.0 g are mixed, and then glucose 18.3 g and titanium dioxide 0.32 g are added;

[0140] (ii) Preparation of the precursor: the precursor mixture is ball milled at a ball-to-material ratio of 20:1, a rotation speed of 500 rpm, and for 8 h, repeated three times, to obtain a powder precursor;

[0141] (iii) Sintering of the precursor: the powder precursor is heated to 550°C at a heating rate of 5°C / min, and then heated to 780°C at a heating rate of 5°C / min, and held for 10 h, under a nitrogen or argon atmosphere, to obtain the composite lithium manganese iron phosphate material.

[0142] Performance test

[0143] (i) The powder precursor and the finished product of the composite phosphate-based positive electrode material obtained in Examples 1-3 and Comparative Examples 1-4 are analyzed by scanning electron microscopy, the finished product particles are analyzed by transmission electron microscopy, and the D 50 50% particle size, D 90 50% particle size of each powder precursor is measured, and the tap density of the finished product particles is measured.

[0144] (ii) The composite phosphate-based positive electrode material obtained in Examples 1-3 and Comparative Examples 1-4 is used as the active material for the positive electrode sheet of a secondary battery to prepare a coin cell. The specific preparation method of the coin cell includes the following steps: 90 g of the composite phosphate-based positive electrode material prepared according to the above method, 10 g of a conductive agent, and 10 g of a binder are added to 80 g of an N-methyl pyrrolidone solution (NMP solution), and stirred in a vacuum stirrer for 2 h to prepare a positive electrode slurry. The slurry is uniformly coated on an aluminum foil, then placed in a vacuum drying oven at 115°C for 13 h, and then cut to obtain a positive electrode sheet. The positive electrode sheet, a negative electrode sheet (a metal lithium sheet with a diameter of 14.5 mm), a separator (Celgard 2400 microporous polypropylene film), and an electrolyte (1 mol / L LiPF6 / EC+DMC (volume ratio 1:1)) are assembled into a CR2025 type coin lithium ion battery in a hydrogen-filled glove box.

[0145] The lithium ion battery prepared above is subjected to electrochemical performance tests such as charge-discharge specific capacity, average voltage, and efficiency at 25°C and 0.1C.

[0146] Results analysis

[0147] (1) The powdery precursor and finished product of the composite phosphate-based positive electrode material obtained in Examples 1-3 and Comparative Examples 1-4 were analyzed by scanning electron microscopy, and the scanning electron microscopy analysis diagrams are shown in Figures 1-7, respectively. It can be seen that the composite phosphate-based positive electrode material obtained in Example 3 is uniform in size, has a moderate and uniform particle coating layer, and is helpful to improve the electronic conductivity of the material. The tap density of each finished product particle was determined, as shown in Table 1. The tap densities of the finished product particles obtained in Examples 1-3 were 2.37 g / cm 3 , 2.41 g / cm 3 , and 2.40 g / cm 3 , respectively. The tap densities of the finished product particles obtained in Comparative Examples 1-4 were basically lower than 2.21 g / cm 3 . It can be seen that by controlling the particle size of the powdery precursor and the normal distribution of the particles using the preparation method of the present application, the particle size of the finished product particles can be controlled, and the tap density of the material can be improved.

[0148] Further analysis of the powdery precursor of each example and comparative example was performed. The D 50 particle size and D 90 particle size of the precursor were analyzed, as shown in Table 1, and in combination with Figures 1-8. It can be analyzed that in Example 1, the D 50 particle size of the precursor was 0.59 μm, the D 90 particle size was 2.34 μm, and the overall particle size was normally distributed; in Example 2, the D 50 particle size of the precursor was 0.57 μm, the D 90 particle size was 1.76 μm, and the overall particle size was normally distributed; in Example 3, the D 50 particle size of the precursor was 0.52 μm, the D 90 particle size was 1.64 μm, and the overall particle size was normally distributed. In Comparative Example 1, the D 50 particle size of the precursor was 0.40 μm, the D 90 particle size was 0.66 μm, and the particle size of the precursor was too small. The finished product particles prepared therefrom were small and prone to agglomeration. The reason was that the ball-to-material ratio was too high during the ball milling process, the number of grinding balls was large, the grinding degree was too large, and the product particle size could not be controlled within the appropriate range. In Comparative Example 2, the D 50 particle size of the precursor was 2.4 μm, the D 90 particle size was 16.4 μm, and the particle size of the precursor was not normally distributed. Therefore, the tap density of the finished product particles was small, the carbon coating was thin and uneven, and the tap density capacity of the material was affected. In Comparative Example 3, the D 50 particle size of the precursor was 0.71 μm, the D 90The particle size is 2.52 μm; in Comparative Example 3, the amount of doped ion Ti is too much, the particles are difficult to grow, and the compaction density is low, and the impurity phase is easily generated in the crystal lattice of the finished product, which will affect the capacity and rate performance of the material. In Comparative Example 4, no Ti ion is doped, the D 50 The particle size is 0.54 μm, and the D 90 The particle size is 1.94 μm; the precursor does not have a normal distribution, and the compaction density is low, which will affect the electrochemical performance of the finished material.

[0149] Table 1

[0150] Further, the finished particles of Examples 1-3 and Comparative Examples 1-4 are analyzed by transmission electron microscopy; as shown in Figures 9-15, the carbon layer thickness is measured by the transmission electron microscopy photograph of the lithium manganese iron phosphate particles, and ten different positions are selected for each example for measurement, the purpose is to observe and verify the thickness and uniformity of the particle coating layer, and the data obtained are shown in Table 2. According to the analysis of the carbon layer thickness in Figures 9-15 and Table 2, it can be seen that in the samples of Examples 1-3, the carbon layer coating is uniform, and the thickness is moderate, being 1.66-2.3 nm, and the variance is 0.03-0.23. It can be seen that the carbon coating effect of the finished particles is good, the thickness is moderate, which is beneficial to improve the material capacity. On the contrary, in Comparative Examples 1-4, the carbon coating layer of the obtained material is not uniform, some positions are thicker, some positions are thinner, the uniformity is poor, the carbon coating quality is poor, which will affect the specific capacity and other electrochemical characteristics of the material as a whole.

[0151] Table 2

[0152] (II) The lithium ion batteries prepared are respectively tested for electrochemical performance such as 0.1C charge-discharge specific capacity, average voltage, and efficiency at 25°C, as shown in Table 3 and Figure 16. It can be seen that the lithium ion batteries prepared from the composite phosphate-based positive electrode material prepared by the preparation method of the present application can exhibit better 0.1C discharge specific capacity. The lithium ion battery of Example 1 has a 0.1C discharge specific capacity of 146.5 mAh / g, the lithium ion battery of Example 2 has a 0.1C discharge specific capacity of 152.3 mAh / g, and the lithium ion battery of Example 3 has a 0.1C discharge specific capacity of 154.1 mAh / g.

[0153] (III) The prepared lithium ion batteries were respectively tested for electrochemical performance such as specific charge-discharge capacity, average voltage, efficiency at 25°C and 0.1C, and the results are shown in Table 3 and FIG. 16. It can be seen that the lithium ion batteries prepared by using the composite phosphate-based cathode material prepared by the preparation method of the application in Examples 1-3 can exhibit better discharge capacity at 0.1C. The lithium ion battery of Example 1 has a discharge capacity of 146.5 mAh / g at 0.1C, the lithium ion battery of Example 2 has a discharge capacity of 152.3 mAh / g at 0.1C, and the lithium ion battery of Example 3 has a discharge capacity of 154.1 mAh / g at 0.1C.

[0154] Table 3

[0155] According to the above analysis, it can be seen that the particle size and distribution of the precursor particles have a significant influence on the particle size and compaction density of the finished product, which is specifically manifested as follows: compared to D 90 For the precursor in the range of 1.0-3.0 μm, the particle size D 90 When the particle size of the precursor is less than 1.0 μm, the primary particle size of the finished product will be reduced by about 100 nm, and at this time the discharge capacity of the material is improved to a certain extent. When the particle size distribution of the precursor is greatly different from the normal distribution, the size distribution of the particles is poor, resulting in serious agglomeration of the finished product particles and unclear boundaries, which is not conducive to the improvement of the compaction density of the material. When the particle distribution of the precursor is good, the carbon coating thickness of the finished product is moderate and uniform: when the carbon coating layer is thick, the transmission of lithium ions is hindered to a certain extent; when the carbon coating thickness is thin and uneven, the electrolyte will be in direct contact with the LMFP, increasing the side reaction and causing capacity loss, so the carbon coating layer of the LMFP material in the application is helpful for the capacity of the material. In addition, the doping has a significant influence on the capacity of the material and the particle size: within a suitable doping range, the material can have both compaction density and capacity; when the doping is too high, the specific capacity of the material is high, but the particle size and compaction density are at a low level; and when the doping is too low or no doping, the capacity of the material is very poor.

[0156] In summary, the composite phosphate-based cathode material provided by the application includes a phosphate-based cathode material core and a carbon coating layer, wherein the particles of the composite phosphate-based cathode material are normally distributed, the finished product particles are large and uniform in size, and the compaction density is significantly improved; and the surface of the core material is uniformly coated with a carbon coating layer, and the carbon coating layer has a moderate thickness, which can not only improve the electronic conductivity of the cathode material, but also improve the discharge capacity and coulombic efficiency and other electrochemical properties of the composite phosphate-based cathode material.

[0157] The above merely provides optional embodiments of the present application, but not for limiting the present application. For those skilled in the art, the present application can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc., made within the principles and technical scope of the present application, should be included in the scope of the claims of the present application.

Claims

1. A composite phosphate-based positive electrode material, characterized by, The composite phosphate-based positive electrode material comprises a phosphate-based positive electrode material core and a carbon coating layer, wherein the particle size of the composite phosphate-based positive electrode material is normally distributed, the thickness of the carbon coating layer is 2.0-3.0 nm, and the variance is 0.01-0.

30.

2. The composite phosphate-based cathode material according to claim 1, characterized in that, The chemical formula of the phosphate-based positive electrode material core is Li x Mn a Fe b M c PO4, 0.9≤x≤1, 0.6≤a<1, 0.4<b<1, 0.001≤c≤0.004, and a+b+c=1; M includes any one of Ti, Mg, V, Ni, Nb, Co, La, Sc, Ce.

3. The composite phosphate-based cathode material according to claim 1 or 2, characterized in that, The composite phosphate-based positive electrode material has a particle size of 180-230 nm and a compaction density of greater than 2.37 g / cm 3 .

4. A method for producing a composite phosphate-based positive electrode material, characterized by, The method comprises the following steps: A precursor mixture of the composite phosphate-based positive electrode material is provided; The precursor mixture is subjected to ball milling to obtain a powdered precursor, and the particle size of the powdered precursor is monitored in real time until it is normally distributed; The powdered precursor is subjected to two-stage sintering to obtain the composite phosphate-based positive electrode material.

5. The method for preparing the composite phosphate-based cathode material according to claim 4, characterized in that, In the step of ball-milling the precursor mixture to obtain the powder precursor, the following steps are included: the precursor mixture is ball-milled for multiple times with a ball-to-material ratio of 5:1-15:1 and monitored in real time, the particle size of the powder precursor is controlled to be normally distributed, and the particle size D 90 is 1.0-3.0 μm.

6. The method for preparing the composite phosphate-based cathode material according to claim 5, characterized in that, The ball milling is performed at a speed of 450-650 rpm for 7-8 hours, 3-5 times, and the diameter of the grinding balls is 6-7 mm.

7. The method for preparing the composite phosphate-based cathode material according to claim 4, characterized in that, In the step of subjecting the powdered precursor to two-stage sintering, the powdered precursor is heated to a constant temperature of 520-560 ℃ at a heating rate of 3-8 ℃ / min and calcined for 2-5 hours, and then heated to a constant temperature of 720-780 ℃ at a heating rate of 3-8 ℃ / min and calcined for 8-12 hours in an inert gas atmosphere.

8. The method for preparing the composite phosphate-based cathode material according to claim 4, characterized in that, The step of providing the precursor mixture of the composite phosphate-based positive electrode material comprises: obtaining a lithium source, a manganese source, an iron source, a phosphorus source, and a M source according to the chemical formula Li x Mn a Fe b M c PO4, and mixing the lithium source, the manganese source, the iron source, the phosphorus source, and the M source with a carbon source to obtain the precursor mixture; wherein M comprises any one of Ti, Mg, V, Ni, Nb, Co, La, Sc, and Ce; 0.9≤x≤1, 0.6≤a<1, 0.4<b<1, 0.001≤c≤0.004, and a+b+c=1.

9. The method of claim 8, wherein the composite phosphate-based cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphorus source; and heating the mixture to a temperature of 600 to 800°C for 6 to 24 hours in a non-oxidizing atmosphere. The lithium source is any one of lithium carbonate, lithium acetate and lithium hydroxide; and / or, The manganese source is at least one of manganese dioxide, trimanganese tetroxide and manganese oxalate; and / or, The iron source is at least one of iron phosphate, ferrous nitrate, ferrous oxalate dihydrate and ferrous sulfate; and / or, The phosphorus source is at least one of iron phosphate, ammonium dihydrogen phosphate and di-ammonium hydrogen phosphate.

10. The method of claim 8, wherein the composite phosphate-based cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphorus source; and heating the mixture to a temperature of 600 to 800°C for 6 to 24 hours. The M source comprises an oxide containing an M element.

11. The method of claim 8, wherein the composite phosphate-based cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphorus source; and heating the mixture to a temperature of 600 to 800°C for 6 to 24 hours. The carbon source is at least one of glucose, polyethylene glycol, citric acid, sucrose, glycine and starch.

12. The method of claim 8, wherein the composite phosphate-based cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphorus source; and heating the mixture to a temperature of 600 to 800°C for 6 to 24 hours in a non-oxidizing atmosphere. The addition amount of the carbon source is 5-10 wt% based on the total mass of the precursor mixture.

13. A positive electrode sheet characterized by comprising: The positive electrode tab comprises a current collector and a positive electrode active layer formed on the surface of the current collector, and the positive electrode active layer comprises the composite phosphate-based positive electrode material according to any one of claims 1-3 or the composite phosphate-based positive electrode material prepared by the preparation method of the composite phosphate-based positive electrode material according to any one of claims 4-12.

14. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode tab, a negative electrode tab, a separator and an electrolyte, wherein the positive electrode tab is selected from the positive electrode tab according to claim 13.

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