Composite phosphate positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a composite phosphate positive electrode material and a preparation method and application thereof. The composite phosphate-based positive electrode material comprises a carbon-coated first phosphate-based active material and a carbon-coated second phosphate-based active material which are graded in particle size, wherein the total doping concentration of metal ions in the first phosphate-based active material is higher than the total doping concentration of metal ions in the second phosphate-based active material. According to the composite phosphate positive electrode material disclosed by the invention, a grading effect is formed between carbon-coated phosphate active materials with different metal ion doping concentrations, so that a positive electrode powder material with relatively high compaction density can be obtained. In addition, the doped metal cations not only can play a role in regulating and controlling the particle size, but also can play a role in improving the discharge capacity.

Description

Technical Field

[0001] The present application belongs to the technical field of battery materials, and in particular relates to a composite phosphate-based positive electrode material and a preparation method and application thereof. Background Art

[0002] With the rapid development of electric vehicles and clean energy, the demand for high-performance lithium-ion batteries is growing. The electrochemical performance of lithium-ion batteries, including energy density, cost and safety, depends largely on the cathode materials used. Lithium iron manganese phosphate (LiMn x Fe y Lithium iron phosphate (LiFePO4, LFP) is an important and cost-effective cathode material for lithium-ion batteries. Its operating voltage and theoretical energy density are higher than those of lithium iron phosphate (LiFePO4, LFP), and it has better safety than layered oxide cathode materials. However, with the increase of manganese content, the compaction density of LMFP powder is difficult to increase, thus limiting its volume energy density.

[0003] Therefore, innovative synthesis processes are needed to improve the compaction density of phosphate-based cathode materials, especially high manganese-lithium manganese iron phosphate (G-LMFP) powders with a manganese content / iron content > 1. Summary of the invention

[0004] The purpose of the present application is to provide a composite phosphate-based positive electrode material and a preparation method and application thereof, aiming to solve the problem that the compaction density of existing phosphate-based positive electrode material powders is difficult to improve to a certain extent.

[0005] In order to achieve the above application purpose, the technical solution adopted in this application is as follows:

[0006] In a first aspect, the present application provides a composite phosphate-based positive electrode material, comprising a first carbon-coated phosphate-based active material and a second carbon-coated phosphate-based active material having graded particle sizes, wherein the total doping concentration of metal ions in the first phosphate-based active material is higher than the total doping concentration of metal ions in the second phosphate-based active material.

[0007] The composite phosphate positive electrode material of the present application includes carbon-coated phosphate active materials with different metal ion doping concentrations and different particle sizes. The carbon coating can not only improve the structural stability of the active material, but also improve the ionic and electronic conductivity of the material. Among them, the total doping concentration of metal ions in the first phosphate active material is relatively high, and the high metal ion doping concentration can reduce the critical size required for crystal nucleation, increase the number of initial nucleation, and the particle size is relatively dispersed, which is easier to grow into small particles. The total doping concentration of metal ions in the second phosphate active material is relatively low, and the low metal ion doping concentration has little effect on crystal nucleation, and it is easier to become large particles. Small particles can be filled between large particles, and the particles of carbon-coated phosphate active materials with different particle sizes can form a grading effect, so that a positive electrode powder material with a higher compaction density can be obtained. In addition, the doped metal cations can not only play a role in regulating the particle size, but also play a role in improving the discharge capacity. Therefore, the composite phosphate positive electrode material of the present application has a higher compaction density, and the volume energy density can be better exerted, so that the composite phosphate positive electrode material has a higher charge and discharge capacity.

[0008] In a second aspect, the present application provides a method for preparing a composite phosphate-based positive electrode material, comprising the following steps:

[0009] Separately preparing first pre-sintered particles and second pre-sintered particles of a phosphate-based positive electrode material doped with metal ions; wherein the particle size of the first pre-sintered particles is smaller than the particle size of the second pre-sintered particles; and the total concentration of the metal ions doped in the first pre-sintered particles is higher than the total concentration of the metal ions doped in the second pre-sintered particles;

[0010] The first pre-sintered particles and the second pre-sintered particles are mixed with a carbon source to obtain first pre-sintered particles coated with the carbon source and second pre-sintered particles coated with the carbon source, respectively;

[0011] In an inert atmosphere, the first pre-sintered particles and the second pre-sintered particles coated with the carbon source are placed in the same sintering container for sintering to obtain a composite phosphate-based positive electrode material.

[0012] The preparation method of the composite phosphate positive electrode material provided in the second aspect of the present application, after preparing the first pre-sintered particles and the second pre-sintered particles of the phosphate positive electrode material with different particle sizes and different metal ion doping concentrations, respectively, and then mixing them with the carbon source, and then placing the first pre-sintered particles coated with the carbon source and the second pre-sintered particles coated with the carbon source in the same sintering container in an inert atmosphere for sintering treatment, the total doping concentration of metal ions in the first phosphate active material is relatively high, and the high metal ion doping concentration can reduce the critical size required for crystal nucleation, increase the number of initial nucleation, and the particle size is relatively dispersed, and it is easier to grow into small particles. The total doping concentration of metal ions in the second phosphate active material is relatively low, and the low metal ion doping concentration has little effect on crystal nucleation, and it is easier to become large particles. A better grading effect can be formed between particles of different particle sizes, so that a positive electrode powder material with a higher compaction density can be obtained. And the doped metal cations can not only play a role in regulating the particle size, but also play a role in improving the discharge capacity.

[0013] In a third aspect, the present application provides a secondary battery, which includes the above-mentioned positive electrode sheet, the positive electrode sheet 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 above-mentioned composite phosphate-based positive electrode material.

[0014] The secondary battery provided in the present application includes the composite phosphate-based positive electrode material with high compaction density, high discharge capacity and low resistivity, thereby improving the electrochemical properties of the secondary battery, such as energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 It is a schematic diagram of a process for preparing a composite phosphate-based positive electrode material provided in an embodiment of the present application;

[0017] Figure 2 1-3 and 1-3 are XRD patterns of the products prepared in Examples 1-3 and Comparative Examples 1-3 of the present application, wherein the left figure (b) is a partial enlarged view of the strongest diffraction peak in the right figure (a);

[0018] Figure 3 are SEM images of the products prepared in Examples 1-3 and Comparative Examples 1-3 of the present application;

[0019] Figure 4It is the charge-discharge curve of the products prepared in Examples 1-3 and Comparative Examples 1-3 of the present application;

[0020] Figure 5 It is the electrochemical impedance spectra prepared by Examples 1-3 and Comparative Examples 1-3 of the present application. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0022] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0023] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b or c", or "at least one of a, b and c", can all represent: a, b, c, ab (i.e. a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0024] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0025] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0026] The weight of the relevant components mentioned in the embodiments of the present specification can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components in the embodiments of the present specification is proportionally enlarged or reduced, it is within the scope disclosed in the embodiments of the present specification. Specifically, the mass described in the embodiments of the present specification can be μg, mg, g, kg and other mass units known in the chemical industry.

[0027] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0028] A first aspect of an embodiment of the present application provides a composite phosphate-based positive electrode material, comprising a first phosphate-based active material coated with carbon and a second phosphate-based active material coated with carbon, the particle sizes of which are graded, wherein the total doping concentration of metal ions in the first phosphate-based active material is higher than the total doping concentration of metal ions in the second phosphate-based active material.

[0029] The composite phosphate positive electrode material of the embodiment of the present application includes a carbon-coated phosphate active material with different doping concentrations of metal ions. The carbon coating can not only improve the structural stability of the active material, but also improve the ionic and electronic conductivity of the material. Among them, the total doping concentration of metal ions in the first phosphate active material is relatively high, and the high metal ion doping concentration can reduce the critical size required for crystal nucleation, increase the number of initial nucleation, and the particle size is relatively dispersed, which is easier to grow into small particles. The total doping concentration of metal ions in the second phosphate active material is relatively low, and the low metal ion doping concentration has little effect on crystal nucleation, and it is easier to become large particles. Therefore, in the composite phosphate positive electrode material, the particles of different particle sizes of the carbon-coated first phosphate active material and the carbon-coated second phosphate active material can form a grading effect, so that a positive electrode powder material with a higher compaction density can be obtained. In addition, the doped metal cations can not only play a role in regulating the particle size, but also play a role in improving the discharge capacity. Therefore, the composite phosphate-based positive electrode material of the embodiment of the present application has a higher compaction density, and the volume energy density can be better exerted, so that the composite phosphate-based positive electrode material has a higher charge and discharge capacity.

[0030] In some possible implementations, the first phosphate-based active material and the second phosphate-based active material are doped with the same type of metal ions. In this case, the first phosphate-based active material and the second phosphate-based active material of different particle sizes are doped with the same type of metal ions, which is conducive to ensuring the relative uniformity of the metal elements doped in the finished composite phosphate-based positive electrode material powder, and can also effectively suppress the voltage hysteresis and other side effects caused by the asynchronous reaction of the positive electrode material powder due to the doping of different metal ions, thereby improving the utilization rate of the particles.

[0031] In some possible implementations, the doped metal ions include at least one of Ti ions, V ions, Mg ions, Ni ions, Zn ions, Al ions, and Ca ions; these metal ions can regulate the particle size of phosphate-based active materials and enhance the discharge capacity. Specifically, doping with Ti can optimize the crystal structure of phosphate-based positive electrode materials, improve electrochemical properties such as capacity, ion conductivity, and cycle life, and accelerate the charge and discharge speed. Doping with V can improve the ion conductivity, structural stability, and electrochemical properties of phosphate-based materials, such as by forming a fast ion conductor coating layer to improve the rate performance and cycle performance of phosphate-based positive electrode materials. Doping with Mg mainly focuses on improving the conductivity and ion transfer rate of phosphate-based positive electrode materials, and optimizing their electrochemical properties, including reversible capacity and cycle stability. At the same time, Mg doping also helps to reduce costs and improve the stability and high temperature resistance of materials. When Ni ions and Co ions exist at the same time, binary transition metal synergy is generated in the electrode material, which helps to improve the electrochemical properties of the electrode material. The radius of Al ions is similar to that of Li ions, and they can replace some Li ions and enter the lattice of the positive electrode material, thereby enhancing the structural stability of the material and reducing the volume change of the battery during the charge and discharge process. Appropriate Zn ion doping may enhance the structural stability of the positive electrode material, reduce the volume change of the battery during the charge and discharge process, and thus improve the cycle stability of the battery. Doping with Ca ions may optimize the ion channels inside the positive electrode material and improve the ion conductivity. This helps to improve the electrochemical performance and power density of the battery.

[0032] In some possible implementations, the total doping mass percentage of metal ions in the first phosphate-based active material is 0.49% to 0.98%; the total doping mass percentage of metal ions in the second phosphate-based active material is 0.04% to 0.19%. In this case, the different doping concentrations of metal ions in the phosphate-based active material ensure that phosphate-based active materials with different particle sizes are obtained, and the compaction density of the positive electrode material is increased by forming a gradation between large and small particles, and the capacity of the positive electrode material is improved.

[0033] Exemplarily, the total doping mass percentage of metal ions in the first phosphate-based active material can be 0.49%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 0.98%, or the like, typical but non-limiting arbitrary point values ​​or an interval value between any two point values; the total doping mass percentage of metal ions in the second phosphate-based active material can be 0.04%, 0.05%, 0.06%, 0.08%, 0.1%, 0.12%, 0.14%, 0.15%, 0.16%, 0.18%, 0.19%, or the like, typical but non-limiting arbitrary point values ​​or an interval value between any two point values.

[0034] In some possible implementations, the composite phosphate-based positive electrode material includes small particles with a particle size of less than 200nm, medium particles with a particle size of 200nm to 300nm, and large particles with a particle size greater than 300nm. In this case, the composite phosphate-based positive electrode material includes large particles, medium particles, and small particles in different particle size ranges. The three different particle sizes of particles can form a better grading effect, so that a positive electrode powder material with a higher compaction density can be obtained, and the capacity of the positive electrode material can be better improved. The first phosphate-based active material and the second phosphate-based active material both contain small particles, medium particles, and large particles, among which the small particles account for a large proportion of the first phosphate-based active material with a high metal ion doping concentration, and the large particles account for a large proportion of the second phosphate-based active material with a low metal ion doping concentration.

[0035] Exemplarily, the composite phosphate-based positive electrode material includes small particles with a particle size of 30nm, 50nm, 100nm, 150nm, 200nm, etc., which are typical but non-limiting values ​​at any point or an interval between any two point values; medium particles with a particle size of 200nm, 230nm, 250nm, 280nm, 300nm, etc., which are typical but non-limiting values ​​at any point or an interval between any two point values; and large particles with a particle size of 300nm, 320nm, 350nm, 380nm, 400nm, 450nm, 500nm, 800nm, 1000nm, etc., which are typical but non-limiting values ​​at any point or an interval between any two point values.

[0036] In some possible implementations, in the composite phosphate-based positive electrode material, the mass ratio of small particles, medium particles, and large particles is (20-30): (30-40): (30-50). Under this ratio, it can better ensure that the three different particle sizes of particles can form a better grading effect, thereby obtaining a positive electrode powder material with a higher compaction density and better improving the capacity of the positive electrode material.

[0037] Exemplarily, in the composite phosphate-based positive electrode material, the mass ratio of small particles, medium particles and large particles can be 20:40:40, 20:30:50, 25:35:40, 25:40:35, 30:30:40, 30:40:30, or any other typical but non-limiting point value or an interval value between any two point values.

[0038] In some possible implementations, the first phosphate-based active material and the second phosphate-based active material have the same type and content of other elements except the doped metal element, that is, the phosphate-based positive electrode material substrates in the two phosphate-based active materials are the same, which is beneficial to improving the stability of the composite phosphate-based positive electrode material.

[0039] In some possible implementations, among the first phosphate-based active material and the second phosphate-based active material, the phosphate-based positive electrode material substrate includes at least one of lithium iron manganese phosphate and lithium iron phosphate. Among them, the phosphate-based positive electrode material substrate refers to an active material that is not doped with metal ions and is not coated with carbon materials. In this case, phosphate-based positive electrode materials such as lithium iron manganese phosphate and lithium iron phosphate are faced with the problem of further improving the compaction density. By forming a gradation of a variety of phosphate-based active materials with different particle sizes and different metal ion doping concentrations in the embodiments of the present application, the compaction density and capacity of the composite phosphate-based positive electrode material can be effectively improved.

[0040] In some possible implementations, the carbon content of the composite phosphate cathode material is 1% to 2%, and exemplary values ​​may be 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2.0%, and other typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values. In this case, the carbon content can fully improve the structural stability of the composite phosphate cathode material, while improving the ionic and electronic conductivity of the composite phosphate cathode material.

[0041] In some embodiments, the carbon material in the composite phosphate-based positive electrode material can form a carbon coating layer on the surface of the active particles, and can also form a three-dimensional conductive network structure inside the active particles and between the active particles.

[0042] In some possible implementations, in the composite phosphate-based cathode material, the thickness of the carbon coating layer on the particle surface is 3nm to 5nm, and can be exemplified by typical but non-limiting arbitrary point values ​​such as 3nm, 4nm, and 5nm, or an interval value between any two point values. In this case, the thickness of the carbon coating layer can fully improve the structural stability of the composite phosphate-based cathode material, while improving the ionic and electronic conductivity of the composite phosphate-based cathode material.

[0043] In some possible implementations, the resistivity of the composite phosphate-based positive electrode material is lower than 37Ω·m, and exemplary values ​​may be 37Ω·m, 35Ω·m, 30Ω·m, 25Ω·m, 20Ω·m, 18Ω·m, 16.8Ω·m, 15Ω·m, 14Ω·m, 13.7Ω·m, 13.3Ω·m, 13Ω·m, 12Ω·m, and other typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values. In this case, the resistivity of the composite phosphate-based positive electrode material is low, and the ion electron migration and transmission efficiency is high, which is conducive to improving the charge and discharge performance of the composite phosphate-based positive electrode material.

[0044] In a second aspect, the present invention provides a method for preparing a composite phosphate positive electrode material. Figure 1 As shown, the following steps are included:

[0045] S10. preparing first and second pre-sintered particles of a phosphate-based positive electrode material doped with metal ions, respectively; wherein the particle size of the first pre-sintered particles is smaller than the particle size of the second pre-sintered particles; and the total concentration of the metal ions doped in the first pre-sintered particles is higher than the total concentration of the metal ions doped in the second pre-sintered particles;

[0046] S20. The first pre-sintered particles and the second pre-sintered particles are mixed with a carbon source to obtain first pre-sintered particles coated with a carbon source and second pre-sintered particles coated with a carbon source, respectively;

[0047] S30. In an inert atmosphere, the first pre-sintered particles and the second pre-sintered particles coated with a carbon source are placed in the same sintering container for sintering to obtain a composite phosphate-based positive electrode material.

[0048] The preparation method of the composite phosphate positive electrode material provided in the second aspect of the embodiment of the present application, after preparing the first pre-sintered particles and the second pre-sintered particles of the phosphate positive electrode material with different particle sizes and different metal ion doping concentrations, respectively, and then mixing them with the carbon source, and then placing the first pre-sintered particles coated with the carbon source and the second pre-sintered particles coated with the carbon source in the same sintering container for sintering in an inert atmosphere, the total doping concentration of metal ions in the first phosphate active material is high, and the high metal ion doping concentration can reduce the critical size required for crystal nucleation, the number of initial nucleation increases, the particle size is relatively dispersed, and it is easier to grow into small particles. The total doping concentration of metal ions in the second phosphate active material is relatively low, and the low metal ion doping concentration has little effect on crystal nucleation, and it is easier to become large particles. A better grading effect can be formed between particles of different particle sizes, so that a positive electrode powder material with a higher compaction density can be obtained. And the doped metal cations can not only play a role in regulating the particle size, but also play a role in improving the discharge capacity.

[0049] In the above step S10:

[0050] In some possible implementations, the phosphate-based positive electrode material includes at least one of lithium iron manganese phosphate and lithium iron phosphate. In this case, by preparing phosphate-based positive electrode materials such as lithium iron manganese phosphate and lithium iron phosphate with different particle sizes and different metal ion doping concentrations through the method of the embodiment of the present application to form a graded distribution, the compaction density and capacity of the composite phosphate-based positive electrode material can be effectively improved.

[0051] In some possible implementations, the calcination temperature for preparing the first pre-sintered particles is 400°C to 550°C, and exemplary values ​​may be 400°C, 450°C, 500°C, 550°C, and other typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values. In this case, the metal ion doping concentration in the first pre-sintered particles is high, and high-concentration doping can reduce the critical size required for crystal nucleation, increase the number of initial nucleations, and make the particle size relatively dispersed, making it easier to grow into small particles. In addition, low-temperature sintering provides lower energy, which is not conducive to particle growth. Thus, first pre-sintered particles with smaller particle sizes are obtained.

[0052] In some possible implementations, the calcination temperature for preparing the second pre-sintered particles is 650°C to 700°C, and can be 650°C, 680°C, 700°C, or any other typical but non-limiting point value or an interval value between any two point values. In this case, the metal ion doping concentration in the second pre-sintered particles is low, and the low metal ion doping concentration has little effect on crystal nucleation, and the sintering temperature is high, making it easier to form large particles. Thus, second pre-sintered particles with larger particle sizes are obtained.

[0053] In some possible implementations, the particle size D50 of the first pre-sintered particles is 0.5 μm to 1.0 μm, and can be 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, etc., which are typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values. In some possible implementations, the particle size D50 of the second pre-sintered particles is 1.5 μm to 3.0 μm, and can be 1.5 μm, 1.8 μm, 2.0 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3.0 μm, etc., which are typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values. In this case, the first pre-sintered particles and the second pre-sintered particles of the phosphate-based positive electrode material with different particle sizes and different metal ion doping concentrations are conducive to the inheritance of their morphological characteristics to the composite phosphate-based positive electrode material product. By forming a grading effect between particles of different sizes, a positive electrode powder material with a higher compaction density can be obtained.

[0054] In some possible implementations, the first pre-sintered particles and the second pre-sintered particles are doped with the same type of metal ions. In this case, the first pre-sintered particles and the second pre-sintered particles of different sizes are preferably doped with the same type of metal cations to ensure the relative uniformity of the metal elements doped in the finished powder, and to effectively suppress the side effects such as voltage hysteresis caused by the asynchronous reaction of the powder due to the doping of different metal ions, thereby improving the utilization rate of the particles.

[0055] In some possible implementations, the doped metal ions include at least one of Ti ions, V ions, Mg ions, Ni ions, Zn ions, Al ions, and Ca ions; these metal ions can regulate the particle size of the phosphate-based active material and enhance the discharge capacity.

[0056] In some possible implementations, the first pre-sintered particles and the second pre-sintered particles have the same type and content of elements except the doped metal element, so that the phosphate-based cathode material substrates in the two prepared phosphate-based active materials are the same, which is beneficial to improving the stability of the composite phosphate-based cathode material.

[0057] In some possible implementations, the total concentration of metal ions doped by mass in the first pre-sintered particles is 0.5% to 1.0%, and may be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or any other typical but non-limiting point values ​​or interval values ​​between any two point values. In some possible implementations, the total concentration of metal ions doped by mass in the second pre-sintered particles is 0.05% to 0.2%, and may be 0.05%, 0.1%, 0.15%, 0.18%, 0.2%, or any other typical but non-limiting point values ​​or interval values ​​between any two point values. In addition, due to the loss in the preparation process, the total concentration of metal ions doped in the first phosphate active material prepared is slightly lower than that in the first pre-sintered particles; similarly, the total concentration of metal ions doped in the second phosphate active material prepared is slightly lower than that in the second pre-sintered particles. In this case, the doping concentration of metal ions in the first pre-sintered particles and the second pre-sintered particles ensures that particle products of different particle sizes are obtained, ensures the grading effect between the product particles, improves the compaction density of the positive electrode material product, and ensures the discharge capacity of the positive electrode material.

[0058] In the above step S20:

[0059] In some possible implementations, the mixing step includes: wet ball milling the first pre-sintered particles or the second pre-sintered particles with the carbon source and water, and then spray drying to obtain the first pre-sintered particles coated with the carbon source or the second pre-sintered particles coated with the carbon source. In the subsequent sintering process, the carbon source can play a reducing role and form a carbon coating layer and a three-dimensional conductive network structure to improve the structural stability and conductive properties of the material.

[0060] In the embodiment of the present application, a high-concentration metal cation-doped precursor is first used to prepare pre-sintered small particles at a relatively low calcination temperature, and a low-concentration metal cation-doped precursor is used to prepare pre-sintered large particles at a relatively high calcination temperature. The pre-sintered small particles and large particles obtained by calcination are then dispersed in an appropriate amount of water together with a certain amount of carbon source for ball milling mixing, and are spray-dried separately to obtain first pre-sintered particles coated with a carbon source or second pre-sintered particles coated with a carbon source, respectively.

[0061] In some possible implementations, the carbon source includes at least one of glucose, citric acid, sucrose, polyethylene glycol PEG, and polyvinyl alcohol PVA.

[0062] In some embodiments, the amount of carbon source added is based on the carbon content in the prepared composite phosphate positive electrode material being 1% to 2%. In this case, the carbon content can sufficiently improve the structural stability of the composite phosphate positive electrode material and simultaneously improve the ionic and electronic conductivity of the composite phosphate positive electrode material.

[0063] In some embodiments, the amount of carbon source added is determined based on the difference between the carbon content of the pre-sintered small particles and large particles and the target carbon content after high-temperature sintering. For example: the carbon content of the pre-sintered small particles is 0.5%, the target carbon content is 1.5%, the difference is 1%, and the carbon source can be added by about 5%. The added carbon source is converted into an amorphous carbon material to form a coating layer during the subsequent sintering process. During this process, there will be a loss in the conversion of the carbon source into the carbon material, and the carbon content of the finished product is close to 1.5%.

[0064] In the above step S30:

[0065] In some possible implementations, in an inert atmosphere, the step of placing the first pre-sintered particles and the second pre-sintered particles coated with a carbon source in the same sintering container for sintering includes: in an inert atmosphere, after laying the second pre-sintered particles coated with a carbon source on the bottom layer of the sintering container, laying the first pre-sintered particles coated with a carbon source to form an upper and lower layer structure, and sintering to obtain a composite phosphate-based positive electrode material. That is, firstly lay the pre-sintered large particles flat on the lower layer of the graphite crucible, and then place the pre-sintered small particles on the upper layer of the large particles, and lay them flat in layers to form a stacked structure. In this case, the second pre-sintered particles coated with carbon source with relatively large particle size at the bottom of the sintering container are directly in contact with the sintering container, while the first pre-sintered particles coated with carbon source laid on the upper layer are in contact with the inert atmosphere in the sintering furnace. The heat transfer effect of the sintering container is better than that of the inert atmosphere, which leads to the sintering temperature of the lower layer of the sintering container being slightly higher than that of the upper layer. The higher sintering temperature is conducive to the growth of particles. In addition, the temperature of the middle contact layer is relatively the lowest, and the first pre-sintered particles coated with carbon source and the second pre-sintered particles coated with carbon source are also easy to melt with each other and grow into medium-sized particles. By utilizing this longitudinal temperature gradient during the sintering process, the sintered product naturally produces particles with small average particle size in the upper layer, particles with moderate average particle size in the middle contact layer, and particles with large average particle size in the lower layer. The effect of stacking and distributing large, medium and small average particle sizes will eventually be naturally produced in the longitudinal gradient. The obtained sintered material is evenly crushed to obtain a composite phosphate-based positive electrode material with good particle grading effect, so that a composite phosphate-based positive electrode material with a higher compaction density can be obtained and the capacity can be maintained normally.

[0066] In some possible implementations, the sintering container includes a graphite sagger. In this case, the graphite sagger as the sintering container has a better heat transfer performance.

[0067] In some possible implementations, in the sintering container, the mass ratio of the first pre-sintered particles coated with the carbon source to the second pre-sintered particles coated with the carbon source is (1:9) to (9:1), and exemplary values ​​may be 1:9, 2:8, 7:3, 6:4, 5:5, 4:6, 3:7, 8:2, 9:1, and other typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values. In this case, the ratio of the first pre-sintered particles coated with the carbon source to the second pre-sintered particles coated with the carbon source ensures the powder compaction density and discharge capacity of the composite phosphate-based positive electrode material.

[0068] In some possible implementations, in the sintering container, the mass ratio of the first pre-sintered particles coated with the carbon source to the second pre-sintered particles coated with the carbon source is preferably (3:7) to (7:3).

[0069] In some possible implementations, the heating rate of the sintering process is 3°C / min to 8°C / min, the temperature is 750°C to 800°C, and the duration is 12h to 16h. Under the sintering conditions, during the high-temperature sintering process, the first pre-sintered particles coated with small carbon source particles and the second pre-sintered particles coated with large carbon source particles undergo the same type of reaction, which is the process of crystal growth to form a phosphate-based active material crystal form, and the carbon source forms a carbon coating layer and a conductive network structure.

[0070] Exemplarily, the heating rate of the sintering treatment can be 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, etc., typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values; the temperature can be 750°C, 780°C, 800°C, etc., typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values; the duration can be 12h, 13h, 14h, 15h, 16h, etc., typical but non-limiting arbitrary point values ​​or interval values ​​between any two point values.

[0071] In some embodiments, the inert shielding gas flow is argon or nitrogen.

[0072] In some possible implementations, after the sintering process is completed, the average particle size of the sintered product obtained in the sintering container increases from top to bottom. Further, in some possible implementations, the composite phosphate-based positive electrode material includes small particles with a particle size less than 200nm, medium particles with a particle size of 200nm to 300nm, and large particles with a particle size greater than 300nm. Because the bottom layer is in contact with the sintering container during the sintering process, and the top layer is in contact with the inert gas in the kiln, the heat transfer effect of the sintering container is better than the inert gas, resulting in the temperature of the lower layer of the sintering container being higher than the temperature of the upper layer. Since the temperature of the lower layer is the highest, the large pre-sintered particles in the lower layer are more likely to grow into larger particles, while the temperature of the upper layer is slightly lower. Although the small pre-sintered particles in the upper layer will also grow, it is not obvious. The temperature of the middle contact layer is the lowest, and the first pre-sintered particles coated with the carbon source and the second pre-sintered particles coated with the carbon source are also easy to melt with each other and grow into medium-sized particles. By utilizing the longitudinal temperature gradient during the sintering process, the sintered product naturally produces particles with a small average particle size in the upper layer, particles with a moderate average particle size in the middle contact layer, and particles with a large average particle size in the lower layer, which will eventually naturally produce a stacked distribution of large, medium and small average particle sizes in the longitudinal gradient. After subsequent pulverization and uniform mixing, a natural grading effect is formed, which improves the high compaction density of the composite phosphate-based positive electrode material.

[0073] In some possible implementations, in the composite phosphate-based positive electrode material, the mass ratio of small particles, medium particles, and large particles is (20-30): (30-40): (30-50). Under this ratio, it can better ensure that the three different particle sizes of particles can form a better grading effect, thereby obtaining a positive electrode powder material with a higher compaction density and better improving the capacity of the positive electrode material.

[0074] In some possible implementations, in the composite phosphate-based cathode material, the thickness of the carbon coating layer on the particle surface is 3 nm to 5 nm. In this case, the thickness of the carbon coating layer can fully improve the structural stability of the composite phosphate-based cathode material, while improving the ionic and electronic conductivity of the composite phosphate-based cathode material.

[0075] In a third aspect, an embodiment of the present application provides a secondary battery, which includes the above-mentioned positive electrode sheet, the positive electrode sheet 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 above-mentioned composite phosphate positive electrode material.

[0076] The secondary battery provided in the embodiment of the present application includes the composite phosphate-based positive electrode material with high compaction density, high discharge capacity and low resistivity, thereby improving the electrochemical properties of the secondary battery, such as energy density.

[0077] The present application does not specifically limit the negative electrode sheet, electrolyte, separator, etc. in the secondary battery of the embodiment, and can be applied to any battery system.

[0078] In some possible implementations, the preparation of the positive electrode active layer includes the steps of: mixing the above-mentioned composite phosphate positive electrode material, conductive agent and binder to form an electrode slurry, coating the electrode slurry on the current collector, and preparing the positive electrode sheet through the steps of drying, rolling, die cutting, etc.

[0079] In some possible implementations, the mass percentage of the composite phosphate-based positive electrode material in the positive electrode active layer of the positive electrode sheet is 90% to 95%. Specifically, the mass percentage of the composite phosphate-based positive electrode material in the positive electrode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0080] In some possible implementations, the current collector of the positive electrode sheet includes but is not limited to any one of copper foil and aluminum foil.

[0081] In some possible implementations, the content of the binder in the active material layer of the positive electrode sheet is 2 wt% to 5 wt%. In specific embodiments, the content of the binder can be 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc., which are typical but not limiting contents.

[0082] In some possible implementations, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0083] In some possible implementations, the content of the conductive agent in the active material layer of the positive electrode sheet is 1 wt% to 5 wt%. In specific embodiments, the content of the conductive agent can be 3 wt%, 4 wt%, 5 wt%, etc., which are typical but not limiting contents.

[0084] In some possible implementations, the conductive agent includes graphite, carbon black, acetylene black, graphene, carbon fiber, C 60 and one or more of carbon nanotubes.

[0085] In some possible implementations, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (such as coke), hard carbon, or nitrides, tin-based oxides, tin-based oxides, tin alloys, and nano-negative electrode materials, etc. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0086] In some possible embodiments, the steps of making the negative electrode sheet include: mixing the negative electrode active material with a conductive agent such as conductive carbon black, a binder such as carboxymethyl cellulose and styrene-butadiene rubber, and a solvent such as water in a mass ratio of (80-99):(1-5):(2-10):100 to make a positive electrode mixed slurry, vacuum degassing, discharging, coating on a coating machine, and obtaining a negative electrode sheet after rolling, slitting, and die-cutting.

[0087] In some possible implementations, the separator can block the passage of electrons and allow the passage of ions. Exemplarily, the separator includes, but is not limited to, at least one material selected from polypropylene fiber, polyacrylonitrile fiber, polyvinyl formal fiber, poly(ethylene terephthalate), polyethylene terephthalate, polyamide fiber, and poly(p-phenylene terephthalamide).

[0088] In some possible implementations, the electrolyte includes at least one soluble metal salt. In some specific embodiments, the metal salt includes LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 SO2)N], wherein m and n are natural numbers. These electrolytic salts can ensure high ionic conductivity of the electrolyte, and do not cause harmful side reactions with electrode materials, electrolytes, diaphragms, etc., and have good chemical stability.

[0089] In some possible implementations, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0090] In some possible implementations, the battery cell types include lithium-ion batteries, as well as new batteries such as lithium-air batteries and lithium metal batteries.

[0091] In some possible implementations, the battery cells of the present application can be assembled into a battery module, and the battery module can contain multiple battery cells, and the specific number can be adjusted according to the application and capacity of the battery module. The battery module can also include a housing with a storage space, and multiple battery cells are stored in the storage space.

[0092] In a possible implementation, the battery cells and / or battery modules may also be assembled into a battery pack, and the number of battery cells or battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0093] In order to enable the above implementation details and operations of the present application to be clearly understood by those skilled in the art, as well as to demonstrate the significant improvement in performance of the composite phosphate-based positive electrode material and its preparation method and application in the embodiments of the present application, the above technical scheme is illustrated by multiple embodiments below.

[0094] Example 1

[0095] A powder with high compaction density lithium iron manganese phosphate G-LM 0.7 F 0.3 P-7 / 3 positive electrode material, comprising the following steps:

[0096] (1) Lithium carbonate as a lithium source, manganese nitrate as a manganese source, ferric nitrate as an iron source, and ammonium dihydrogen phosphate as a phosphorus source were mixed in a molar ratio of 1.06:0.7:0.3:1.03, and 5% glucose was added for reduction and initial coating. 0.01 mol of titanium dioxide and magnesium oxide were used as dopants for the high-concentration doping precursor, respectively. The high-concentration doping precursor was prepared by the sol-gel method (the metal cations doped in the precursor were Ti+Mg, and Ti / Mg=4, the total concentration of metal cation doping was 1.0%, and manganese / iron in the precursor was 7 / 3). The high-concentration doping precursor was loaded into a 5 kg graphite sagger and calcined in a roller kiln at a calcination temperature of 450°C for 10 hours to obtain pre-sintered small particles. After the material was cooled, it was crushed by a jet mill (crushing frequency was 50 Hz) to obtain the first pre-sintered small particles, marked as G-LM 0.7 F 0.3 PG.

[0097] Similarly, lithium carbonate as a lithium source, manganese nitrate as a manganese source, ferric nitrate as an iron source, and ammonium dihydrogen phosphate as a phosphorus source were mixed in a molar ratio of 1.06:0.7:0.3:1.03, and 5% glucose was added for reduction and initial coating. 0.002 mol of titanium dioxide and magnesium oxide were used as dopants for the low-concentration doping precursor, respectively. The low-concentration doping precursor was prepared by the sol-gel method (the metal cations doped in the precursor were Ti+Mg, and Ti / Mg=4, the total concentration of metal cation doping was 0.1%, and manganese / iron in the precursor was 7 / 3). The low-concentration doping precursor was loaded into a 5kg graphite sagger and calcined in a roller kiln at a calcination temperature of 700°C for 10 hours to obtain pre-sintered large particles. After the discharge was cooled, it was crushed by a jet mill (crushing frequency was 50Hz) to obtain the second pre-sintered large particles, marked as G-LM 0.7 F 0.3 PD.

[0098] (2) The first pre-sintered small particles G-LM prepared in step (1) are 0.7 F 0.3 PG and second pre-sintered large particles G-LM 0.7 F 0.3 PD and a certain amount of polyethylene glycol PEG solid carbon source (the amount added is to make the carbon content of the finished product reach about 1.4%) are dispersed in an appropriate amount of water, and ball milling is performed (the amount of water added is to ensure that the solid content of the slurry reaches 50%, and the particle size D50 of the ball milling output is controlled at 0.4 μm). After the particle size is qualified, spray drying is performed to obtain the first pre-sintered small particles coated with carbon source and the second pre-sintered large particles coated with carbon source, which are marked as carbon source coated G-LM 0.7 F 0.3 PG and carbon source coated G-LM 0.7 F 0.3 PD.

[0099] (3) Spray-drying the carbon source obtained in step (2) to coat G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 PD powder is packed in a single graphite sagger with a weight of 5 kg, and the carbon source is coated with G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 The ratio of PD = 7 / 3 was placed on the upper and lower layers of the graphite crucible, respectively, and sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with inert protective gas N2 continuously introduced. After the sintered material was cooled, it was crushed with a jet mill to obtain G-LM 0.7 F 0.3P-7 / 3 black powder positive electrode material, that is, composite phosphate positive electrode material.

[0100] Example 2

[0101] A powder with high compaction density lithium iron manganese phosphate G-LM 0.7 F 0.3 P-5 / 5 positive electrode material, including the following steps: (1) (2) Same as Example 1.

[0102] (3) Spray-drying the carbon source obtained in step (2) to coat G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 PD powder is packed in a single graphite sagger with a weight of 5 kg, and the carbon source is coated with G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 The ratio of PD = 5 / 5 was placed on the upper and lower layers of the graphite sagger respectively and sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with a continuous inert protective gas N2. The sintered material was cooled and crushed with a jet mill to obtain G-LM 0.7 F 0.3 P-5 / 5 black powder positive electrode material, that is, composite phosphate positive electrode material.

[0103] Example 3

[0104] A powder with high compaction density lithium iron manganese phosphate G-LM 0.7 F 0.3 P-3 / 7 positive electrode material, including the following steps: (1) (2) Same as Example 1.

[0105] (3) Spray-drying the carbon source obtained in step (2) to coat G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 The total amount of PD powder in a single graphite sagger is 5 kg, and the carbon source is coated with G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 The ratio of PD = 3 / 7 was placed in the upper and lower layers of the graphite crucible respectively and sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with inert protective gas N2 continuously introduced. The sintered material was cooled and crushed with a jet mill to obtain G-LM 0.7 F 0.3P-3 / 7 black powder positive electrode material, that is, composite phosphate positive electrode material.

[0106] Example 4

[0107] A powder with high compaction density lithium iron manganese phosphate G-LM 0.7 F 0.3 P-H5 / 5 positive electrode material, including the following steps: (1) (2) Same as Example 1.

[0108] (3) Spray-drying the carbon source obtained in step (2) to coat G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 PD powder is packed in a single graphite sagger with a weight of 5 kg, and the carbon source is coated with G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 After mixing in a ratio of PD=5 / 5, the mixture was sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with inert protective gas N2 continuously introduced. The sintered material was cooled and crushed with a jet mill to obtain G-LM 0.7 F 0.3 P-H5 / 5 black powder positive electrode material, that is, composite phosphate positive electrode material.

[0109] Example 5

[0110] A powder with high compaction density lithium iron manganese phosphate G-LM 0.7 F 0.3 P-C5 / 5 positive electrode material, comprising the following steps: (1) (2) Same as Example 1, except that: the carbon source in (1) is coated with G-LM 0.7 F 0.3 The cations used as PD dopants are V ions and Ca ions (the raw materials used are the corresponding metal oxides or nitrates).

[0111] (3) Spray-drying the carbon source obtained in step (2) to coat G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 PD powder is packed in a single graphite sagger with a weight of 5 kg, and the carbon source is coated with G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3The ratio of PD = 5 / 5 was placed on the upper and lower layers of the graphite sagger respectively and sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with a continuous inert protective gas N2. The sintered material was cooled and crushed with a jet mill to obtain G-LM 0.7 F 0.3 P-C5 / 5 black powder positive electrode material, that is, composite phosphate positive electrode material.

[0112] Comparative Example 1

[0113] A lithium iron manganese phosphate G-LM 0.7 F 0.3 PD positive electrode material, comprising the following steps:

[0114] (1) Lithium carbonate as a lithium source, manganese nitrate as a manganese source, ferric nitrate as an iron source, and ammonium dihydrogen phosphate as a phosphorus source were mixed in a molar ratio of 1.06:0.7:0.3:1.03, and 5% glucose was added for reduction and initial coating. 0.01 mol of titanium dioxide and magnesium oxide were used as dopants for the high-concentration doping precursor, respectively. A low-concentration doping precursor was prepared by a sol-gel method (the metal cations doped in the precursor were Ti+Mg, and Ti / Mg=4, the total concentration of metal cation doping was 0.1%, and manganese / iron in the precursor was 7 / 3). The low-concentration doping precursor was loaded into a 5 kg graphite sagger and calcined in a roller kiln at a calcination temperature of 700°C for 10 hours to obtain pre-sintered large particles. After the material was cooled, it was crushed by a jet mill (crushing frequency was 50 Hz) to obtain pre-sintered large particles, which were marked as G-LM. 0.7 F 0.3 PD.

[0115] (2) crushing the pre-sintered large particles G-LM obtained in step (1) 0.7 F 0.3 PD and a certain amount of polyethylene glycol PEG solid carbon source (the amount added is to make the carbon content of the finished product reach about 1.4%) are dispersed in an appropriate amount of water for ball milling (the amount of water added is to ensure that the solid content of the slurry reaches 50%, and the particle size D50 of the ball milling output is controlled at 0.5μm). After the particle size is qualified, it is spray dried to obtain carbon source-coated pre-sintered large particles, which are marked as carbon source-coated G-LM 0.7 F 0.3 PD.

[0116] (3) Coating the carbon source spray-dried in step (2) with G-LM 0.7 F 0.3PD powder was loaded into a single graphite crucible with a weight of 5 kg, and sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with inert protective gas N2 continuously introduced. After the sintered material was cooled, it was crushed with a jet mill to obtain G-LM 0.7 F 0.3 PD black powder positive electrode material.

[0117] Comparative Example 2

[0118] A lithium iron manganese phosphate G-LM 0.7 F 0.3 PG positive electrode material, comprising the following steps:

[0119] (1) Lithium carbonate as a lithium source, manganese nitrate as a manganese source, ferric nitrate as an iron source, and ammonium dihydrogen phosphate as a phosphorus source were mixed in a molar ratio of 1.06:0.7:0.3:1.03, and 5% glucose was added for reduction and initial coating. 0.01 mol of titanium dioxide and magnesium oxide were used as dopants for the high-concentration doping precursor, respectively. The high-concentration doping precursor was prepared by the sol-gel method (the metal cations doped in the precursor were Ti+Mg, and Ti / Mg=4, the total concentration of metal cation doping was 1.0%, and manganese / iron in the precursor was 7 / 3). The high-concentration doping precursor was loaded into a 5 kg graphite sagger and calcined in a roller kiln at a calcination temperature of 450°C for 10 hours to obtain pre-sintered small particles. After the material was cooled, it was crushed by a jet mill (crushing frequency was 50 Hz) to obtain the first pre-sintered small particles, marked as G-LM 0.7 F 0.3 PG.

[0120] (2) crushing the pre-sintered small particles G-LM obtained in step (1) 0.7 F 0.3 PG and a certain amount of polyethylene glycol PEG solid carbon source (the amount added is to make the carbon content of the finished product reach about 1.4%) are dispersed in an appropriate amount of water for ball milling (the amount of water added is to ensure that the solid content of the slurry reaches 50%, and the particle size D50 of the ball milling output is controlled at 0.5μm). After the particle size is qualified, it is spray dried to obtain pre-sintered small particles coated with carbon source, which are marked as carbon source coated G-LM 0.7 F 0.3 PG.

[0121] (3) Coating the carbon source spray-dried in step (2) with G-LM 0.7 F 0.3 PG powder was loaded into a single graphite crucible with a weight of 5 kg, and sintered for 16 hours at a temperature of 780°C at a heating rate of 8°C / min in a roller kiln with inert protective gas N2 continuously introduced. After the sintered material was cooled, it was crushed with a jet mill to obtain G-LM 0.7 F0.3 PG black powder positive electrode material.

[0122] Comparative Example 3

[0123] A lithium iron manganese phosphate G-LM 0.7 F 0.3 P-5 / 5H positive electrode material, including the following steps: (1) (2) Same as Example 2.

[0124] (3) Spray-dry the carbon source from step (2) and coat the G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 PD powder is packed in a single graphite sagger with a weight of 5 kg, and the carbon source is coated with G-LM 0.7 F 0.3 PG / Carbon source coated G-LM 0.7 F 0.3 The ratio of PD = 5 / 5 was placed in two graphite saggers and in a roller kiln with inert protective gas N2 continuously introduced. The temperature was raised to 780°C at a heating rate of 8°C / min and sintered for 16 hours. After the sintered material was cooled, it was crushed and mixed evenly with a jet mill to obtain G-LM 0.7 F 0.3 P-5 / 5H black powder positive electrode material.

[0125] Furthermore, in order to verify the progress of the embodiments of the present application, the following performance tests were performed:

[0126] 1. X-ray diffraction tests were performed on the composite phosphate positive electrode materials prepared in the above examples and comparative examples, and their XRD spectra are shown in the attached Figure 2 As shown, it can be seen that the products obtained are all lithium manganese iron phosphate LiMn 0.7 Fe 0.3 PO4, and highly doped with G-LM 0.7 F 0.3 PG black powder doped with G-LM at a lower concentration 0.7 F 0.3 The diffraction peak of PD black powder shifts to the large angle direction, and G-LM 0.7 F 0.3 P-7 / 3, G-LM 0.7 F 0.3 P-5 / 5, G-LM 0.7 F 0.3 The diffraction peak of P-3 / 7 black powder is between the two. 0.7 F 0.3 Diffraction peak position of P-H5 / 5 black powder and high concentration doped G-LM 0.7 F0.3 PG's approach.

[0127] 2. The composite phosphate positive electrode materials prepared in the embodiment and the comparative example were observed by scanning electron microscope, and their SEM images are shown in the attached figure. Figure 3 As shown, it can be seen that the G-LM prepared in Examples 1-3 of the present application 0.7 F 0.3 P-7 / 3, G-LM 0.7 F 0.3 P-5 / 5, G-LM 0.7 F 0.3 P-3 / 7 black powder, with uniform distribution of large and small particles, and an average particle size of about 250nm. 0.7 F 0.3 The PD black powder has a concentrated size distribution and relatively large particles, with an average primary particle size of 350-400 nm. 0.7 F 0.3 The PG black powder has a concentrated size distribution and small particles, mainly small particles, with an average primary particle size of 150-200 nm. The lithium manganese iron phosphate black powder prepared in Comparative Example 3 has uniform particle size distribution, but the proportion of large and small particles is not as good as that of the embodiment.

[0128] 3. G-LM prepared in each embodiment and comparative example 0.7 F 0.3 The carbon content, resistivity, specific surface area, and compaction of the P cathode material were measured.

[0129] The above test results are shown in Table 1 below:

[0130] Table 1

[0131]

[0132] It can be seen from the above test results that by comparing the particle size and compaction of the positive electrode materials obtained in the comparative examples and the embodiments, the large and small particles of the embodiments 1-3 of the present application are evenly distributed and the proportion of large and small particles is relatively ideal (small particles of 0-200nm account for about 25%, medium particles of 200nm-300nm account for about 35%, and large particles of 300nm+ account for about 40%), with an average particle size of 200nm-300nm. The compaction of the embodiments is greatly improved, and the particle size is reduced, indicating that it is precisely because of the uniform distribution of large and small particles that the compaction density of the powder is improved.

[0133] 4. The positive electrode materials prepared in each embodiment and comparative example were tested for their charge-discharge performance. Figure 4 The charge and discharge curves of the buckled battery show that compared with the comparative example, the discharge capacity of the embodiment of the present application has not decreased significantly, but has increased.

[0134] 5. Electrochemical impedance spectroscopy (EIS) tests were performed on the positive electrode materials prepared in each embodiment and comparative example. Figure 5 The electrochemical impedance spectrum shown in the figure shows that compared with the comparative example, the charge transfer impedance Rct (the semicircle radius in the high-frequency region becomes smaller) of the positive electrode material of the embodiment of the present application becomes smaller and is similar to that of the comparative example 2, indicating that the Li + The diffusion rate is increased, resulting in an increase in its discharge capacity.

[0135] The performance test results of the positive electrode materials prepared in the above embodiments and comparative examples at 25°C, 0.1C and 1C charge-discharge capacity, rate performance, cycle performance, etc. are shown in Table 2 below:

[0136] Table 2

[0137]

[0138] It can be seen from the above test results that compared with Comparative Example 3, the powder compaction, low-rate discharge and 3C rate performance of Examples 1-3 are improved to varying degrees, which shows that layered stacking sintering has a better effect than mixing large and small particles after sintering. Compared with Example 4, the powder compaction, low-rate discharge and 3C rate performance of Example 2 are significantly improved, which shows that layered stacking sintering has a better effect than sintering after mixing. Compared with Example 5, the powder compaction, low-rate discharge and 3C rate performance of Example 2 are significantly improved. This shows that the use of the same type of metal cations for high and low concentration doped large and small particle precursors has a better effect than using heterogeneous cations. This may be attributed to the fact that the metal elements doped in the finished powder made by doping large and small particles with the same metal cations are relatively uniform, and at the same time, the side effects such as voltage hysteresis caused by the asynchronous reaction of the powder due to the doping of different metal ions are effectively suppressed, thereby improving the utilization rate of the particles.

[0139] In summary, the present invention uses a method in which a high-concentration doped precursor is calcined at a relatively low temperature to obtain pre-sintered small particles, a low-concentration doped precursor is calcined at a relatively high temperature to obtain pre-sintered large particles, and then the pre-sintered small particles and the pre-sintered large particles are spray-dried with a carbon source, and then placed in any proportion, sintered at a high temperature, crushed, and mixed evenly to obtain a composite phosphate-based positive electrode material G-LM. 0.7 F 0.3 P-7 / 3, G-LM 0.7 F 0.3 P-5 / 5, G-LM 0.7 F 0.3 P-3 / 7, compared with G-LM prepared in the comparative example 0.7 F 0.3 PG or G-LM 0.7 F 0.3PD lithium manganese iron phosphate positive electrode material has a higher powder compaction density and improved discharge capacity, which can improve the battery's energy density and other electrochemical properties.

[0140] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A composite phosphate positive electrode material, characterized in that: The invention comprises a first phosphate-based active material coated with carbon and a second phosphate-based active material coated with carbon, wherein the total doping concentration of metal ions in the first phosphate-based active material is higher than the total doping concentration of metal ions in the second phosphate-based active material.

2. The composite phosphate positive electrode material according to claim 1, characterized in that The first phosphate-based active material and the second phosphate-based active material have the same types and contents of other elements except the doped metal element; and / or the first phosphate-based active material and the second phosphate-based active material are doped with the same type of metal ions; And / or, the doped metal ions include at least one of Ti ions, V ions, Mg ions, Ni ions, Zn ions, Al ions, and Ca ions; And / or, the composite phosphate-based positive electrode material includes small particles with a particle size less than 200 nm, medium particles with a particle size of 200 nm to 300 nm, and large particles with a particle size greater than 300 nm.

3. The composite phosphate positive electrode material according to claim 2, characterized in that: In the first phosphate-based active material and the second phosphate-based active material, the phosphate-based positive electrode material substrate includes at least one of lithium iron manganese phosphate and lithium iron phosphate; And / or, the total doping mass percentage of metal ions in the first phosphate-based active material is 0.49% to 0.98%; the total doping mass percentage of metal ions in the second phosphate-based active material is 0.04% to 0.19%; And / or, in the composite phosphate-based positive electrode material, the mass ratio of the small particles, the medium particles and the large particles is (20-30): (30-40): (30-50); And / or, the carbon content of the composite phosphate positive electrode material is 1% to 2%; And / or, the resistivity of the composite phosphate-based positive electrode material is lower than 37Ω·m.

4. A method for preparing a composite phosphate positive electrode material, characterized in that: The following steps are involved: Separately preparing first pre-sintered particles and second pre-sintered particles of a phosphate-based positive electrode material doped with metal ions; wherein the particle size of the first pre-sintered particles is smaller than the particle size of the second pre-sintered particles; and the total concentration of the metal ions doped in the first pre-sintered particles is higher than the total concentration of the metal ions doped in the second pre-sintered particles; The first pre-sintered particles and the second pre-sintered particles are mixed with a carbon source to obtain first pre-sintered particles coated with the carbon source and second pre-sintered particles coated with the carbon source, respectively; In an inert atmosphere, the first pre-sintered particles and the second pre-sintered particles coated with the carbon source are placed in the same sintering container for sintering to obtain a composite phosphate-based positive electrode material.

5. The method for preparing a composite phosphate positive electrode material according to claim 4, characterized in that: The particle size D50 of the first pre-sintered particles is 0.5 μm to 1.0 μm; and / or, the calcination temperature for preparing the first pre-sintered particles is 400° C. to 550° C.; and / or, the particle size D50 of the second pre-sintered particles is 1.5 μm to 3.0 μm; And / or, the calcination temperature for preparing the second pre-sintered particles is 650°C to 700°C.

6. The method for preparing a composite phosphate positive electrode material according to claim 4, characterized in that: The doped metal ions include at least one of Ti ions, V ions, Mg ions, Ni ions, Zn ions, Al ions, and Ca ions; and / or, the first pre-sintered particles and the second pre-sintered particles are doped with the same type of metal ions; and / or, the total doping mass percentage concentration of metal ions in the first pre-sintered particles is 0.5% to 1.0%; and / or, the total doping mass percentage concentration of metal ions in the second pre-sintered particles is 0.05% to 0.2%; And / or, except for the doped metal element, the types and contents of other elements in the first pre-sintered particles and the second pre-sintered particles are the same.

7. The method for preparing a composite phosphate positive electrode material according to any one of claims 4 to 6, characterized in that: The mixing step comprises: wet ball milling the first pre-sintered particles or the second pre-sintered particles with the carbon source and water, and then spray drying to obtain the first pre-sintered particles coated with the carbon source or the second pre-sintered particles coated with the carbon source; And / or, the sintering step includes: after laying the second pre-sintered particles coated with the carbon source on the bottom layer of the sintering container, laying the first pre-sintered particles coated with the carbon source to form an upper and lower layer structure, and performing sintering to obtain the composite phosphate-based positive electrode material; And / or, the sintering container comprises a graphite sagger; And / or, the heating rate of the sintering treatment is 3°C / min to 8°C / min, the temperature is 750°C to 800°C, and the duration is 12h to 16h.

8. The method for preparing a composite phosphate positive electrode material according to claim 7, characterized in that: In the sintering container, the mass ratio of the first pre-sintered particles coated with the carbon source to the second pre-sintered particles coated with the carbon source is (1:9) to (9:1), preferably (3:7) to (7:3); and / or, after the sintering process is completed, the average particle size of the sintered product obtained in the sintering container increases from the top to the bottom; And / or, the composite phosphate-based positive electrode material includes small particles with a particle size less than 200 nm, medium particles with a particle size of 200 nm to 300 nm, and large particles with a particle size greater than 300 nm.

9. The method for preparing a composite phosphate positive electrode material according to claim 8, characterized in that: In the composite phosphate-based positive electrode material, the mass ratio of the small particles, the medium particles and the large particles is (20-30): (30-40): (30-50).

10. A secondary battery, characterized in that: The positive electrode sheet in the secondary battery includes a current collector and a positive electrode active layer formed on the surface of the current collector, and the positive electrode active layer contains the composite phosphate positive electrode material as described in any one of claims 1-3, or the composite phosphate positive electrode material prepared by the method as described in any one of claims 4 to 9.

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