Positive electrode active material and preparation method thereof, positive pole piece, secondary battery and electrical device

Abstract

A positive electrode active material, the positive electrode active material comprises lithium iron phosphate particles and lithium nickel cobalt manganate particles, wherein the primary average particle size of the lithium iron phosphate particles is 500-3000 nm, the specific surface area BET of the lithium iron phosphate particles is 3m 2 / g-8m 2 / g, and the specific surface area BET of the lithium nickel cobalt manganate particles is 0.4m 2 / g-2.0m 2 / g. The positive electrode active material has good processability, and also has good gram capacity.

Description

Positive electrode active material and preparation method thereof, positive electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202410026192.7, filed on January 8, 2024, entitled “Positive Electrode Active Material and Preparation Method Thereof, Positive Electrode Sheet, Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a positive electrode active material and a preparation method thereof, a positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0004] In recent years, lithium-ion batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] Lithium nickel cobalt manganese oxide and lithium iron phosphate are commonly used lithium-ion cathode active materials. To reduce the cost of lithium-ion batteries, these materials can be combined. However, there are a number of processing issues associated with mixing these materials to prepare cathode slurry.

[0006] Summary of the Invention

[0007] This application is made in light of the above-mentioned issues and aims to solve at least one of the technical problems existing in the prior art. To this end, this application provides a positive electrode active material and a preparation method thereof, a positive electrode sheet, a secondary battery, and an electrical device. The positive electrode active material of this application has good processing performance while also taking into account the gram capacity of the secondary battery.

[0008] The first aspect of the present application provides a positive electrode active material, the positive electrode active material includes lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles, the primary average particle size of the lithium iron phosphate salt particles is 500-3000nm, and the specific surface area BET of the lithium iron phosphate salt particles is 3m 2 / g-8m 2 / g, and the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m 2 / g-2.0m 2 / g.

[0009] In order to improve the electrochemical properties of lithium iron phosphate salt particles, they are usually nano-sized to improve their ability to transmit electrons. However, the specific surface area of ​​nano-sized lithium iron phosphate salt particles is too large, which leads to serious water absorption. In actual application, in order to reduce costs, they are often mixed with lithium nickel cobalt manganese oxide particles, but in the process of preparing the mixed slurry, lithium iron phosphate salt particles are prone to form gels due to high water absorption, which brings a series of processing problems such as low solid content and poor filtration performance. Furthermore, the high molecular weight binders used in the lithium nickel cobalt manganese oxide positive electrode slurry, such as polyvinylidene fluoride, have poor compatibility with nano-scale lithium iron phosphate. In this type of binder system, lithium iron phosphate salt particles are prone to agglomeration, which brings processing problems. The primary average particle size of the lithium iron phosphate salt particles of the present application is 500-3000nm, and the specific surface area is 3m 2 / g-8m 2 / g, the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m 2 / g-2.0m 2 / g. The reduced specific surface area is conducive to reducing the water absorption performance of the positive electrode slurry, thereby reducing the gelation and agglomeration phenomena during the slurry processing, and can also improve the drying speed and degree of the electrode. At the same time, a larger primary average particle size is conducive to better dispersion of the lithium iron phosphate salt particles in the high molecular weight binder system, further reducing the occurrence of agglomeration. However, excessively large particle size and too low specific surface area will affect the kinetic performance and reduce the gram capacity. The positive electrode active material of the present application increases the primary average particle size of the lithium iron phosphate salt particles and reduces the specific surface area thereof, and controls the two within a reasonable range, while obtaining better processing performance while taking into account the gram capacity of the secondary battery.

[0010] In any embodiment, the lithium iron phosphate particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q q1 , wherein Q comprises at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, 0.9≤x1≤1, 0.95≤y1≤1, 3.5≤z1≤4, 0<q1≤0.1, and / or

[0011] Lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2 Mn1-x2-y2-z2O2, wherein R includes at least one of Cr, Ti, V, Mg, Al, and Nb, and 0≤x2<1, 0<y2<1, 0<z2<1, and 0<x2+y2+z2<1.

[0012] Doping lithium iron phosphate particles and / or lithium nickel cobalt manganese oxide particles with the above elements helps improve the ion transport capacity of the positive electrode active material. These elements can create vacancies in the particle lattice or change the interatomic bond lengths, facilitating the movement of lithium ions within the lattice, thereby effectively improving the conductivity of the particles themselves and enhancing the kinetic performance of the positive electrode active material.

[0013] In any embodiment, the primary average particle size of the lithium iron phosphate salt particles is 650-2500 nm.

[0014] Controlling the primary average particle size of lithium iron phosphate salt particles within a reasonable range will help further improve the processing performance of the positive electrode slurry while taking into account the gram capacity of the secondary battery.

[0015] In any embodiment, the BET of the lithium iron phosphate particles is 4m 2 / g-7m 2 / g, the BET of lithium nickel cobalt manganese oxide particles is 0.6m 2 / g-2.0m 2 / g.

[0016] If the BET of lithium iron phosphate particles and / or lithium nickel cobalt manganese oxide particles is too high, the particles will increase their water absorption performance and affect the processing performance of the mixed positive electrode slurry. However, if the BET of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles is too low, their gram capacity and kinetic performance will be reduced. Control the BET of lithium iron phosphate particles within 4m 2 / g-7m 2 / g, and control the BET of lithium nickel cobalt manganese oxide particles to 0.6m 2 / g-2.0m 2 / g, which is beneficial to further balance the processing performance of the positive electrode slurry and the gram capacity of the secondary battery.

[0017] In any embodiment, the carbon content of the lithium iron phosphate salt particles is Cx weight %, calculated based on the total weight of the lithium iron phosphate salt particles, and the ratio z of BET to Cx of the lithium iron phosphate salt particles satisfies 1.5≤z≤8.5. In some embodiments, z satisfies 3≤z≤6.

[0018] In the present application, the ratio z of the specific surface area BET to Cx of the lithium iron phosphate particles can characterize the uniform density of the carbon contained in the lithium iron phosphate particles. When the primary average particle size and carbon content of the lithium iron phosphate particles remain unchanged, the lower the ratio z, the more uniform and dense the carbon contained in the lithium iron phosphate particles. Improving the uniform density of the contained carbon is conducive to further improving the kinetic performance and gram capacity of the lithium iron phosphate particles, but too high a carbon coating density will affect the insertion and extraction of lithium ions, and to a certain extent, affect the kinetic performance and gram capacity of the secondary battery. The range of the ratio z in the embodiment of the present application is conducive to the carbon contained in the lithium iron phosphate particles having a suitable uniform density, which is conducive to improving the conductive properties of the particle surface, and is conducive to further improving the kinetic performance and gram capacity of the secondary battery.

[0019] In any embodiment, the carbon content of the lithium iron phosphate salt particles is 0.8 wt %-2.0 wt % based on the total weight of the lithium iron phosphate salt particles. In some embodiments, the carbon content of the lithium iron phosphate salt particles is 1.0 wt %-1.6 wt % based on the total weight of the lithium iron phosphate salt particles.

[0020] Increasing the carbon content of lithium iron phosphate particles helps improve their conductivity, thereby improving kinetic performance and gram capacity. However, excessive carbon content may affect the insertion and extraction of lithium ions, affecting the gram capacity of the secondary battery to a certain extent. The carbon content range of this application helps further achieve better kinetic performance and gram capacity.

[0021] In any embodiment, Q comprises at least one of Ti, V, Mg, and Nb. In some embodiments, Q is Ti;

[0022] Calculated based on the total weight of the lithium iron phosphate particles, the content of Q in the lithium iron phosphate particles is 1000-10000 ppm. In some embodiments, based on the total mass of the lithium iron phosphate particles, the content of Q in the lithium iron phosphate particles is 2500-6000 ppm.

[0023] The content of doping elements in existing lithium iron phosphate particles is generally low or no doping elements are added. Increasing the content of Q element in lithium iron phosphate particles helps to further improve the bulk ion transport capacity of lithium iron phosphate particles and improve its kinetic performance. However, as the content of Q element further increases, the bulk ion transport capacity may not continue to increase, and it may also occupy the position of lithium ions, affecting the performance of gram capacity. The range of Q element in this application helps to further obtain better kinetic performance and gram capacity.

[0024] In any embodiment, the ratio of Dv50 of the lithium iron phosphate salt particles to the Dv50 of the lithium nickel cobalt manganese oxide particles is 1:(1-25). In some embodiments, the ratio of D50 value of the lithium iron phosphate salt particles to the Dv50 of the lithium nickel cobalt manganese oxide particles is 1:(1-20).

[0025] In any embodiment, the Dv50 of the lithium iron phosphate salt particles is 500-5000 nm. In some embodiments, the Dv50 of the lithium iron phosphate salt particles is 700-2500 nm.

[0026] In any embodiment, the Dv50 of the lithium nickel manganese cobalt oxide particles is 1200-20000 nm. In some embodiments, the Dv50 of the lithium nickel manganese cobalt oxide particles is 3000-15000 nm.

[0027] Controlling the Dv50 ratio range and value range of the lithium iron phosphate salt particles and the lithium nickel cobalt manganese oxide particles in the embodiment of the present application within the above-mentioned range is conducive to the lithium iron phosphate salt particles being evenly and tightly filled between the lithium nickel cobalt manganese oxide particles, alleviating the agglomeration phenomenon of the single-component particles in the composite positive electrode active material, and improving the compaction density of the positive electrode active material.

[0028] In any embodiment, the Dv50 of the lithium iron phosphate particles is 870-1500 nm, and the Dv50 of the lithium nickel cobalt manganese oxide particles is 1800-4200 nm.

[0029] In any embodiment, the mass percentage of the lithium iron phosphate salt particles is 5%-80% based on the total mass of the positive electrode active material, and in some embodiments, 15%-50%.

[0030] In any embodiment, the mass percentage of the lithium nickel cobalt manganese oxide particles is 20% to 95% based on the total mass of the positive electrode active material, and in some embodiments, is 50% to 85%.

[0031] Controlling the mass percentage of the lithium iron phosphate particles and the lithium nickel cobalt manganese oxide particles within the above range helps to further balance the electrical properties and cost of the positive electrode active material.

[0032] In any embodiment, the capacity ratio of the lithium iron phosphate particles is η ≥ 88%, where η is defined as:

[0033] A battery using lithium iron phosphate particles as the positive electrode active material was charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a single charge and discharge at a constant current rate of 1C. During the 1C charge and discharge test, the capacity at a discharge voltage of 3.2V was recorded as C1, and the capacity at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50uA.

[0034] The capacity ratio η of the lithium iron phosphate salt particles can reflect its kinetic performance and platform retention performance, and can be adjusted by adjusting the primary average particle size, carbon content, ratio of carbon source and carbon film-forming agent, modifier and its content of the lithium iron phosphate salt particles. The η value of the lithium iron phosphate salt particles in the embodiment of the present application is ≥88%, which has good kinetic performance. At the same time, when the η value is high, the secondary battery can still maintain good power performance when discharged to a low SOC (battery state of charge), that is, the battery with a high η value has a smaller voltage drop when discharged at low power and high current. When the positive electrode active material of the present application is discharged, the lithium nickel cobalt manganese oxide particles have a high voltage and participate in the discharge first. The lithium iron phosphate salt particles participate in the discharge later, will withstand a larger current, and are prone to polarization. The lithium iron phosphate particles of the present application have higher kinetic performance, which is beneficial to improving the discharge power performance of the secondary battery.

[0035] In any embodiment, the lithium iron phosphate particles satisfy at least one of (a)-(e):

[0036] a) Dv10 of lithium iron phosphate particles ≥ 200 nm;

[0037] b) Dv90 of lithium iron phosphate particles ≤ 10000m;

[0038] c) Dv99 of lithium iron phosphate particles ≤ 12000nm;

[0039] d) The powder compaction density of lithium iron phosphate at 3T pressure is ≥2.25g / cm 3 ;

[0040] e) The powder resistivity of the lithium iron phosphate salt is less than 60Ω·cm.

[0041] By making the lithium iron phosphate salt particles satisfy at least one of (a) to (e), the lithium iron phosphate salt particles can better achieve the technical effects of the present application.

[0042] The second aspect of the present application provides a method for preparing a positive electrode active material, the method comprising: mixing lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles;

[0043] The preparation process of lithium iron phosphate particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, optionally a carbon source, optionally a carbon film-forming agent, and optionally a modifier, and performing at least two sintering operations, wherein:

[0044] The temperature of the first sintering is 500° C. to 760° C., and in some embodiments, 550° C. to 720° C.;

[0045] The temperature of the second sintering is 700°C to 800°C, and in some embodiments, 720°C to 780°C.

[0046] Compared to traditional preparation methods that use high temperatures to achieve particle growth, the lithium iron phosphate salt particles of the present application undergo two sintering processes. Controlling the temperatures of the two sintering processes is beneficial for preparing lithium iron phosphate salt particles with the primary average particle size and specific surface area of ​​the present application. Furthermore, during conventional high-temperature sintering processes, the carbon coating on the particle surface is prone to cracking, reducing the integrity of the carbon coating. The present application synthesizes large particles at low temperatures, which helps reduce floating carbon and improves the consistency and uniformity of the surface carbon coating.

[0047] In any embodiment, the lithium iron phosphate particles are mainly obtained by the following preparation method: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein,

[0048] The carbon content of the material after the first sintering is 0.01 wt% to 0.79 wt%, and in some embodiments, 0.05 wt% to 0.4 wt%;

[0049] The carbon content of the material after the second sintering is 0.8 wt% to 2.0 wt%, and in some embodiments, 1.0 wt% to 1.6 wt%.

[0050] Adding a carbon source before the first sintering can effectively reduce the trivalent iron in the raw material and improve the purity and stability of the product. Furthermore, adding a lower carbon source content during the first sintering process is beneficial to reducing the barrier effect of the carbon layer on the growth process of lithium iron phosphate particles, which is beneficial to the crystallization growth of the lithium iron phosphate salt particle precursor at a lower temperature. At the same time, the low carbon content during the first sintering process is also beneficial to the solid-phase diffusion reaction between the modifier that may be added and the lithium iron phosphate salt material, which is beneficial to achieve a higher concentration of metal ion doping. Increasing the added carbon content in the second sintering further improves the uniformity and integrity of the coating, which is beneficial to obtaining lithium iron phosphate salt particles with the carbon content of the embodiment of the present application.

[0051] In any embodiment, the lithium iron phosphate salt particles contain at least one of Ti, V, Mg, and / or Nb elements, and the content of the element is 1000ppm-10000ppm, and in some embodiments, 2500ppm-6000ppm, calculated based on the total weight of the lithium iron phosphate salt particles.

[0052] In any embodiment, the first pulverization is performed after the first sintering, and the second pulverization is performed after the second sintering, wherein:

[0053] The Dv50 of the product after the first pulverization is 300 nm to 1200 nm, and in some embodiments, 400 nm to 1100 nm;

[0054] The Dv50 of the product after the second pulverization is 500 nm to 5000 nm, and in some embodiments, 700 nm to 2500 nm.

[0055] Controlling the Dv50 of the product after the first pulverization within the above range helps reduce the growth barrier effect of the added carbon source and any doping elements on the lithium iron phosphate salt particle precursor crystals, thereby facilitating the preparation of micron-sized lithium iron phosphate salt particle precursors. Controlling the Dv50 value of the product after the second pulverization within the above range helps to obtain lithium iron phosphate salt particles having the primary average particle size of the present application.

[0056] A third aspect of the present application provides a positive electrode plate, which includes the positive electrode active material of the present application or the positive electrode active material obtained by the preparation method of the present application.

[0057] In any embodiment, the compacted density of the positive electrode sheet is 3.0 g / cm 3 -3.5g / cm 3 .

[0058] A fourth aspect of the present application provides a secondary battery, which includes the positive electrode sheet of the present application.

[0059] A fifth aspect of the present application provides an electrical device, which includes the secondary battery of the present application.

[0060] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] FIG1 is a cross-sectional view of the positive electrode sheet of Example 1 of the present application, FIG1a is a cross-sectional view of the positive electrode sheet under 5k times magnification, and FIG1b is a cross-sectional view of the positive electrode sheet under 10k times magnification.

[0062] FIG2 is a scanning electron microscope image of the lithium iron phosphate particles of Example 1 of the present application.

[0063] FIG3 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0064] FIG. 4 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 3 .

[0065] FIG5 is a schematic diagram of a battery module according to an embodiment of the present application.

[0066] FIG6 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0067] FIG. 7 is an exploded view of the battery pack shown in FIG. 6 according to an embodiment of the present application.

[0068] FIG8 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0069] FIG9 is a scanning electron microscope image of lithium iron phosphate particles with their primary particle sizes marked according to an embodiment of the present application.

[0070] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0071] Below, the embodiments of the positive electrode active material and its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack and electrical device of the present application are specifically disclosed in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0072] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0073] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0074] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0075] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0076] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0077] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0078] In order to balance the cost of the battery and the electrical performance of the battery, lithium iron phosphate materials can be mixed with lithium nickel cobalt manganese oxide materials to prepare the positive electrode active material. Lithium iron phosphate particles have the problem of poor electrical performance. In order to improve the electrical properties of lithium iron phosphate particles, they are usually nano-sized to improve their ability to transmit electrons. However, in the process of mixing lithium iron phosphate particles with lithium nickel cobalt manganese oxide particles to prepare the slurry, the positive electrode slurry easily forms a gel, and the agglomeration phenomenon is serious, which brings a series of processing problems such as low solid content, poor filtration performance, strong water absorption of the electrode, and difficulty in drying. The present application increases the primary average particle size of the lithium iron phosphate particles and reduces its specific surface area and controls it within a reasonable range, thereby obtaining better processing performance while taking into account the gram capacity of the secondary battery. At the same time, the reduction in BET and water absorption performance of the lithium iron phosphate particles is also conducive to reducing the occurrence of side reactions and improving the life of the battery cell.

[0079] [Positive electrode active material]

[0080] Based on this, the present application provides a positive electrode active material, which includes lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles, the primary average particle size of the lithium iron phosphate particles is 500-3000 nanometers (nm), and the specific surface area BET of the lithium iron phosphate particles is 3 square meters / gram (m 2 / g)-8m 2 / g, and the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m 2 / g-2.0m 2 / g.

[0081] In this application, the term "primary average particle size" refers to the average primary particle size of all particles, where the primary particle size is the longest distance between two points on the edge of a cross-sectional view. In a specific embodiment, as shown in the scanning electron micrograph of lithium iron phosphate particles in Figure 9, the line segment indicated by the double arrow in the particle represents the primary particle size as defined in this application.

[0082] In this application, the term "specific surface area" or "BET" refers to the total area per unit mass of particles. In this application, the BET of lithium iron phosphate particles is related to factors such as the primary average particle size of the lithium iron phosphate particles, the carbon content, the density of the carbon coating, the degree of fit between the carbon and the particles, and the looseness and porosity of the particles.

[0083] In some embodiments, the lithium iron phosphate salt particles are primary particles.

[0084] In this article, "primary particles" refer to particles that do not have obvious agglomeration interfaces in the particle scanning electron microscope image, but may have tiny pores and point or line defects, which are different from the smallest unit powder particles without structures such as stacking and flocculation.

[0085] In some embodiments, the lithium iron phosphate salt particles are single crystal particles and / or polycrystalline particles.

[0086] In this article, the term "single crystal" refers to a structurally complete crystal grown from a single nucleus. The single crystal of the present application is a whole in the field of view of a transmission electron microscope photograph, and there are no grain boundaries within the single crystal.

[0087] In some embodiments, the single crystal of the present application may have tiny defects, for example, micropores inside, a small number of points and surfaces, or a small number of particles adhering to each other on the surface of a particle.

[0088] As used herein, the term "polycrystalline" refers to a crystal composed of small single crystal particles randomly oriented together, with grain boundaries existing within the polycrystalline.

[0089] In some embodiments, the proportion of single crystal particles is greater than or equal to 90% based on the total number of lithium iron phosphate particles. Controlling the proportion of single crystal particles within the above range helps reduce BET and improve processing performance compared to polycrystalline and secondary agglomerates.

[0090] The primary average particle size of the lithium iron phosphate salt particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode plate to expose the cross section. The cross section is photographed by a scanning electron microscope, and the particle size of the lithium iron phosphate salt particles is statistically analyzed by the long diameter statistical method. Specifically, the total number of lithium iron phosphate salt particles with a primary particle size greater than 80nm and the sum of the primary particle sizes of the lithium iron phosphate salt particles with a primary particle size greater than 80nm can be counted in the electron microscope scanning photo. The primary average particle size of the lithium iron phosphate salt particles = the sum of the primary particle sizes of the lithium iron phosphate salt particles / the total number of lithium iron phosphate salt particles. Among them, in the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 80nm are not within the statistical range.

[0091] In order to improve the electrical properties of lithium iron phosphate particles, they are usually nanosized to improve their ability to transmit electrons. However, the specific surface area of ​​nanosized lithium iron phosphate particles is too large, which causes them to absorb water seriously. Therefore, in the process of mixing with lithium nickel cobalt manganese oxide particles to prepare slurry, lithium iron phosphate particles are prone to form gels due to high water absorption, resulting in a series of processing problems such as low solid content and poor filtration performance. Furthermore, nanoscale lithium iron phosphate particles and high molecular weight binders commonly used in the preparation of lithium nickel cobalt manganese oxide particles positive active materials, such as polyvinylidene fluoride, have a problem of poor compatibility. In this type of binder system, lithium iron phosphate particles are prone to agglomeration, which brings about processing problems. The primary average particle size of the lithium iron phosphate particles of the present application is 500-3000nm, and the specific surface area of ​​the lithium iron phosphate particles is further adjusted to 3m 2 / g-8m 2 / g, the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m 2 / g-2.0m 2 / g. The reduced specific surface area is conducive to reducing the water absorption performance of the positive electrode slurry, thereby reducing the gelation and agglomeration phenomena during the slurry processing, and can also improve the drying speed and degree of the slurry. At the same time, a larger primary average particle size is conducive to better dispersion of the lithium iron phosphate salt particles in the high molecular weight binder system, further reducing the occurrence of agglomeration. However, too large a particle size and too low a specific surface area will reduce the gram capacity of the positive electrode active material. The positive electrode active material of the present application increases the primary average particle size of the lithium iron phosphate salt particles and reduces the specific surface area thereof, and reduces the specific surface area of ​​the lithium nickel cobalt manganese oxide particles, and controls the two within a reasonable range, while obtaining better processing performance and taking into account the gram capacity of the secondary battery.

[0092] In some embodiments, the lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q q1 , wherein Q comprises at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, 0.9≤x1≤1, 0.95≤y1≤1, 3.5≤z1≤4, 0<q1≤0.1, and / or

[0093] Lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2Mn1-x2-y2-z2O2, wherein R includes at least one of Cr, Ti, V, Mg, Al, and Nb, and 0≤x2<1, 0<y2<1, 0<z2<1, and 0<x2+y2+z2<1.

[0094] In some embodiments, the lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q q1 , m1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, x1 can be 0.9, 1.0, y1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, z1 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, and q1 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0095] In some embodiments, the lithium nickel cobalt manganese oxide particles have the formula LiR x2 Ni y2 Co z2 Mn1-x2-y2-z2O2, R includes at least one of Ti, V, Mg, and Nb.

[0096] In some embodiments, the lithium nickel cobalt manganese oxide particles have the formula LiR x2 Ni y2 Co z2 Mn1-x2-y2-z2O2, x2 can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.5, 0.7, 0.8, 0.9, y2 can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.5, 0.7, 0.8, 0.9, and z2 can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.5, 0.7, 0.8, 0.9. In some embodiments, the ratio of lithium and oxygen in the lithium nickel cobalt manganese oxide particles fluctuates during battery formation and cycling.

[0097] Doping lithium iron phosphate particles and / or lithium nickel cobalt manganese oxide particles with the above elements helps improve the ion transport capacity of the positive electrode active material. These elements can create vacancies in the particle lattice or change the interatomic bond lengths, facilitating the movement of lithium ions within the lattice, thereby effectively improving the conductivity of the particles themselves and enhancing the kinetic performance of the positive electrode active material.

[0098] In some embodiments, the primary average particle size of the lithium iron phosphate salt particles is 650-2500 nm.

[0099] In some embodiments, the primary average particle size of the lithium iron phosphate particles can be 500nm, 550nm, 600nm, 650nm, 700nm, 730nm, 750nm, 780nm, 800nm, 830nm, 850nm, 870nm, 900nm, 950nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 2000nm, 2100nm, 2200nm, 2300nm, 2400nm, 2500nm, 2600nm, 2700nm, 2800nm, 2900nm, 3000nm, or a value in the range composed of any two of the above primary average particle sizes.

[0100] Controlling the primary average particle size of lithium iron phosphate salt particles within a reasonable range will help further improve the processing performance of the positive electrode slurry while taking into account the gram capacity of the secondary battery.

[0101] In some embodiments, the BET of the lithium iron phosphate particles is 4m 2 / g-7m 2 / g, the BET of lithium nickel cobalt manganese oxide particles is 0.6m 2 / g-2.0m 2 / g.

[0102] In some embodiments, the specific surface area BET of the lithium iron phosphate particles can be 3 m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g、7m 2 / g, 7.5m 2 / g、8m 2 / g, or a value within the range of the BET composition of the specific surface areas of any two of the above lithium iron phosphate particles.

[0103] In some embodiments, the specific surface area of ​​the lithium nickel cobalt manganese oxide particles can be 0.6 m 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m2 / g, 2.0m 2 / g, or a value within the range of the BET composition of the specific surface area of ​​any two of the above lithium nickel cobalt manganese oxide particles.

[0104] The BET specific surface area of ​​particles can be measured using methods and equipment known in the art. For example, the test can be performed using the gas adsorption method in accordance with GB / T 19587-2017. As an example, lithium iron phosphate particles are placed as a sample in a sample tube. The sample tube is immersed in liquid nitrogen at -196°C. The amount of nitrogen adsorbed on the solid surface at different pressures is measured at a relative pressure of 0.05-0.30. The monolayer adsorption of the sample is calculated based on the BET multilayer adsorption theory and its formula, thereby obtaining the specific surface area of ​​the sample.

[0105] If the BET of lithium iron phosphate particles and / or lithium nickel cobalt manganese oxide particles is too high, the particles will increase their water absorption performance and affect the processing performance of the mixed positive electrode slurry. If the BET of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles is too low, their gram capacity will be reduced. Control the BET of lithium iron phosphate particles within 4m 2 / g-7m 2 / g, and control the BET of lithium nickel cobalt manganese oxide particles to 0.6m 2 / g-2.0m 2 / g, which is beneficial to further balance the processing performance of the positive electrode slurry and the gram capacity of the secondary battery.

[0106] In some embodiments, the carbon content of the lithium iron phosphate particles is Cx wt % based on the total weight of the lithium iron phosphate particles, and the ratio z of BET to Cx of the lithium iron phosphate particles satisfies 1.5≤z≤8.5.

[0107] In some embodiments, z satisfies 3≤z≤6.

[0108] In some embodiments, z can be 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.3, 4.5, 4.8, 5, 5.3, 5.5, 5.8, 6, 6.3, 6.5, 6.8, 7, 7.3, 7.5, 7.8, 8, 8.3, 8.5, or a value in a range consisting of any two of the above values ​​of z.

[0109] In the present application, the ratio z of the specific surface area BET to Cx of the lithium iron phosphate particles can characterize the uniform density of the carbon contained in the lithium iron phosphate particles. When the primary average particle size and carbon content of the lithium iron phosphate particles remain unchanged, the lower the ratio z, the higher the coating utilization rate of the carbon in the particles, the less floating carbon, and the more uniform and dense the carbon contained in the lithium iron phosphate particles. Improving the uniform density of the contained carbon is beneficial to improving the kinetic performance and gram capacity of the lithium iron phosphate particles. However, too high a carbon coating density may affect the insertion and extraction of lithium ions, and to a certain extent affect the kinetic performance and gram capacity of the secondary battery. The range of the ratio z in the embodiment of the present application is conducive to the carbon contained in the lithium iron phosphate particles having a suitable uniform density, which is beneficial to improving the conductivity of the particle surface, and is beneficial to further improving the kinetic performance and gram capacity of the secondary battery.

[0110] In some embodiments, the carbon content of the lithium iron phosphate salt particles is 0.8 wt % to 2.0 wt %, calculated based on the total weight of the lithium iron phosphate salt particles.

[0111] In some embodiments, the carbon content of the lithium iron phosphate salt particles is 1.0 wt % to 1.6 wt %, based on the total weight of the lithium iron phosphate salt particles.

[0112] In some embodiments, the carbon content of the lithium iron phosphate salt particles is 0.8 weight %, 0.9 weight %, 1.0 weight %, 1.1 weight %, 1.2 weight %, 1.3 weight %, 1.4 weight %, 1.5 weight %, 1.6 weight %, 1.7 weight %, 1.8 weight %, 1.9 weight %, 2.0 weight %, or a value in the range composed of any two of the above carbon contents, calculated based on the total weight of the lithium iron phosphate salt particles.

[0113] In some embodiments, the carbon contained in the lithium iron phosphate salt particles is coated on the surface of the particles. In some embodiments, the carbon contained in the lithium iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the lithium iron phosphate salt particles is partially coated on the surface of the particles and partially embedded in the particles.

[0114] The carbon content of the ferric phosphate particles can be measured by methods and equipment known in the art, for example, by referring to GB / T 20123-2006 / ISO 15350:2000.

[0115] Increasing the carbon content of lithium iron phosphate particles helps improve their conductivity, thereby improving kinetic performance and gram capacity. However, excessive carbon content may affect the insertion and extraction of lithium ions, affecting the gram capacity of the secondary battery to a certain extent. The carbon content range of the embodiments of the present application helps further achieve better kinetic performance and gram capacity.

[0116] In some embodiments, Q comprises at least one of Ti, V, Mg, and Nb. In some embodiments, Q is Ti.

[0117] In some embodiments, the content of Q in the lithium iron phosphate particles is 1000-10000 parts per million (ppm) calculated based on the total weight of the lithium iron phosphate particles.

[0118] In some embodiments, the content of Q in the lithium iron phosphate salt particles is 2500-6000 ppm based on the total mass of the lithium iron phosphate salt particles.

[0119] In some embodiments, the content of Q in the lithium iron phosphate salt particles is 1000 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm, 6500 ppm, 7000 ppm, 7500 ppm, 8000 ppm, 8500 ppm, 9000 ppm, 9500 ppm, 10000 ppm, or a value in the range consisting of the contents of any two of the above Qs.

[0120] The Q content, for example, the Ti content, in the lithium iron phosphate particles can be measured using methods and equipment known in the art. For example, the test can be performed with reference to GB / T 33822-2017.

[0121] The content of doping elements in existing lithium iron phosphate particles is generally low or no doping elements are added. Increasing the content of Q element in lithium iron phosphate particles helps to further improve the bulk ion transport capacity of lithium iron phosphate particles and improve its kinetic performance. However, as the content of Q element further increases, the bulk ion transport capacity may not continue to increase, and it may also occupy the position of lithium ions, affecting the performance of gram capacity. The range of Q element in this application helps to further obtain better kinetic performance and gram capacity.

[0122] In some embodiments, the ratio of Dv50 of the lithium iron phosphate salt particles to Dv50 of the lithium nickel cobalt manganese oxide particles is 1:(1-25).

[0123] In some embodiments, the ratio of the D50 value of the lithium iron phosphate salt particles to the Dv50 value of the lithium nickel cobalt manganese oxide particles is 1:(1-20).

[0124] In some embodiments, the ratio of Dv50 of the lithium iron phosphate salt particles to the Dv50 of the lithium nickel cobalt manganese oxide particles can be 1:1, 1:3, 1:5, 1:7, 1:10, 1:13, 1:15, 1:20, 1:23 or 1:25, or a value in the range consisting of the ratios of Dv50 of any two of the above lithium iron phosphate salt particles to Dv50 of the lithium nickel cobalt manganese oxide particles.

[0125] In some embodiments, the lithium iron phosphate salt particles have a Dv50 of 500-5000 nm.

[0126] In some embodiments, the lithium iron phosphate salt particles have a Dv50 of 700-2500 nm.

[0127] In some embodiments, the Dv50 of the lithium iron phosphate salt particles is 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, 2300 nm, 2500 nm, 2700 nm, 3000 nm, 3300 nm, 3500 nm, 3700 nm, 4000 nm, 4300 nm, 4500 nm, 4700 nm, 5000 nm, or a value in the range composed of the Dv50 of any two of the above lithium iron phosphate salt particles.

[0128] In some embodiments, the lithium nickel cobalt manganese oxide particles have a Dv50 of 1200-20000 nm.

[0129] In some embodiments, the lithium nickel cobalt manganese oxide particles have a Dv50 of 3000-15000 nm.

[0130] In some embodiments, the Dv50 of the lithium nickel cobalt manganese oxide particles can be 1200 nm, 1500 nm, 1700 nm, 2000 nm, 2300 nm, 2500 nm, 2700 nm, 3000 nm, 3300 nm, 3500 nm, 4000 nm, 4300 nm, 4500 nm, 4700 nm, 5000 nm, 6000 nm, 7000 nm, 8000 nm, 9000 nm, 10000 nm, 11000 nm, 12000 nm, 13000 nm, 14000 nm, 15000 nm, 16000 nm, 17000 nm, 18000 nm, 19000 nm, 20000 nm, or a value in a range consisting of the Dv50 of any two of the foregoing lithium nickel cobalt manganese oxide particles.

[0131] In this application, the term "Dv50" refers to the particle size corresponding to when the volume cumulative particle size distribution percentage in the particles reaches 50%.

[0132] The Dv50 of the lithium iron phosphate particles and the Dv50 of the lithium nickel cobalt manganese oxide particles can be measured by methods and equipment known in the art. For example, they can be measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0133] Controlling the Dv50 ratio range and value range of the lithium iron phosphate salt particles and the lithium nickel cobalt manganese oxide particles in the embodiment of the present application within the above-mentioned range is conducive to the lithium iron phosphate salt particles being evenly and tightly filled between the lithium nickel cobalt manganese oxide particles, alleviating the agglomeration phenomenon of the single-component particles in the composite positive electrode active material, and improving the compaction density of the positive electrode active material.

[0134] In some embodiments, the Dv50 of the lithium iron phosphate particles is 870-1500 nm, and the Dv50 of the lithium nickel cobalt manganese oxide particles is 1800-4200 nm.

[0135] In some embodiments, the mass percentage of the lithium iron phosphate salt particles is 5%-80% based on the total mass of the positive electrode active material.

[0136] In some embodiments, the mass percentage of the lithium iron phosphate salt particles is 15%-50% based on the total mass of the positive electrode active material.

[0137] In some embodiments, based on the total mass of the positive electrode active material, the mass percentage of the lithium iron phosphate salt particles can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or a value within the range of the mass percentages of any two of the above-mentioned lithium iron phosphate salt particles.

[0138] In some embodiments, the mass percentage of the lithium nickel cobalt manganese oxide particles is 20%-95% based on the total mass of the positive electrode active material.

[0139] In some embodiments, the mass percentage of the lithium nickel cobalt manganese oxide particles is 50%-85% based on the total mass of the positive electrode active material.

[0140] In some embodiments, based on the total mass of the positive electrode active material, the mass percentage of the lithium nickel cobalt manganese oxide particles can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or a value in the range composed of the mass percentages of any two of the above lithium nickel cobalt manganese oxide particles.

[0141] Controlling the mass percentage of the lithium iron phosphate particles and the lithium nickel cobalt manganese oxide particles within the above range helps to further balance the electrical properties and cost of the positive electrode active material.

[0142] In some embodiments, the capacity ratio of the lithium iron phosphate particles is η≥88%, where η is defined as:

[0143] A battery containing lithium iron phosphate particles as the positive electrode active material was charged and discharged twice at a constant current rate of 0.1 coulomb (C) over a voltage range of 2.0 volts (V) to 3.75V, followed by a single charge and discharge at a constant current rate of 1C. During the 1C charge and discharge test, the capacity at a discharge voltage of 3.2V was recorded as C1, and the capacity at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50 microamperes (uA).

[0144] In some embodiments, n can be 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or a value in the range formed by any two of the above n values.

[0145] The η value of lithium iron phosphate salt particles can be measured using methods and equipment known in the art. As an example, a button cell was first prepared using lithium iron phosphate salt particles as the positive electrode active material. The specific button cell preparation process was as follows: 2.0000 grams (g) of lithium iron phosphate salt particles were mixed with 0.1111 g of conductive carbon black and 0.1111 g of polyvinylidene fluoride, and then added to 2.5 g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry was formed. The slurry was coated on aluminum foil to a thickness of 140 microns (μm), dried under vacuum at 120 degrees Celsius (°C) for 2 hours (h), and punched into discs with a diameter of 13 millimeters (mm). The discs were pressed using a tablet press at 10 megapascals (MPa) and kept in vacuum at 120°C for 12 hours to obtain a positive electrode sheet. The positive electrode sheet was weighed, and the loading of lithium iron phosphate salt particles was 11-12 milligrams (mg). Button cells were assembled in an argon-protected glove box, with a metallic lithium sheet as the negative electrode, an electrolyte consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, LiPF6 electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0146] The prepared button cell was tested for electrical performance on a blue battery tester. Specifically, the button cell was charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a single charge and discharge at a constant current rate of 1C. During the 1C charge and discharge test, the capacity value at a discharge voltage of 3.2V was recorded as C1, and the capacity value at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50uA.

[0147] The capacity ratio η of the lithium iron phosphate salt particles can reflect its kinetic performance and platform retention performance, and can be adjusted by adjusting the primary average particle size, carbon content, ratio of carbon source and carbon film-forming agent, modifier and its content of the lithium iron phosphate salt particles. The η value of the lithium iron phosphate salt particles in the embodiment of the present application is ≥88%, which has good kinetic performance. At the same time, when the η value is high, the secondary battery can still maintain good power performance when discharged to a low SOC, that is, the battery with a high η value has a smaller voltage drop when the battery is discharged at a low power and a large current. When the positive active material of the embodiment of the present application is discharged, the lithium nickel cobalt manganese oxide particles have a high voltage and participate in the discharge first. The lithium iron phosphate salt particles participate in the discharge later and will withstand a larger current and are prone to polarization. The lithium iron phosphate particles of the present application have higher kinetic performance, which is beneficial to improving the discharge power performance of the secondary battery.

[0148] In some embodiments, the lithium iron phosphate salt particles satisfy at least one of (a)-(e):

[0149] a) Dv10 of lithium iron phosphate particles ≥ 200 nm;

[0150] b) Dv90 of lithium iron phosphate particles ≤ 10000m;

[0151] c) Dv99 of lithium iron phosphate particles ≤ 12000nm;

[0152] d) The powder compaction density of lithium iron phosphate at 3T pressure is ≥ 2.25 g / cm3 (g / cm 3 );

[0153] e) The powder resistivity of the lithium iron phosphate salt is less than 60 ohm·cm (Ω·cm).

[0154] As used herein, the term "Dv10" refers to the particle size at which the volume cumulative particle size distribution percentage in the particles reaches 10%.

[0155] As used herein, the term "Dv90" refers to the particle size at which the volume cumulative particle size distribution percentage reaches 90%.

[0156] As used herein, the term "Dv99" refers to the particle size at which the volume cumulative particle size distribution percentage reaches 99%.

[0157] In this article, the term "powder compaction density" refers to the density of a compact with a certain density and strength formed during the external compression process. As the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced. The unit is g / cm 3 .

[0158] In some embodiments, the lithium iron phosphate salt particles have a Dv10 < Dv50.

[0159] In some embodiments, the lithium iron phosphate salt particles have a Dv90>Dv50.

[0160] The Dv10, Dv90, and Dv99 of the lithium iron phosphate particles can be measured by methods and equipment known in the art. For example, they can be measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0161] Under different pressures, the powder compaction density of lithium iron phosphate salt particles can be measured by methods and equipment known in the art. For example, it can be measured using a compaction density instrument with reference to GB / T 24533-2009. Specifically, a certain amount of lithium iron phosphate salt particles are placed on a special compaction mold (the mold diameter is known), and the mold is hollow in the middle with a metal disc on the top and bottom. The lithium iron phosphate salt particles are placed between the metal discs, a metal cylinder is placed on the top, and the mold is placed on the compaction density instrument. Different pressures are set (for example, 3T). The thickness of the lithium iron phosphate salt particles under different pressures can be read on the device. The powder compaction density of the lithium iron phosphate salt particles is ρ=m / v, where v=(S×H), m is the mass of the lithium iron phosphate salt particles, S is the bottom area of ​​the mold, and H is the thickness of the lithium iron phosphate salt particles after compaction.

[0162] The powder resistivity of the lithium iron phosphate salt particles can be measured using methods and equipment known in the art. For example, it can be measured using a powder resistivity meter (Suzhou Jingge, ST2722 model) with reference to GB / T 33822-2017. Specifically, a certain amount of lithium iron phosphate salt particles (e.g., 1 g) is weighed and added to the feeding chamber of the powder resistivity meter. A pressure of 8 MPa is applied, and the forward and reverse resistivities of the lithium iron phosphate salt particles are measured respectively. The average of the two values ​​is taken as the powder resistivity of the lithium iron phosphate salt particles.

[0163] By making the lithium iron phosphate salt particles satisfy at least one of (a)-(e), the lithium iron phosphate salt particles can better achieve the technical effect of the present application.

[0164] The present application provides a method for preparing a positive electrode active material, the preparation method comprising: mixing lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles,

[0165] The preparation process of lithium iron phosphate particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, optionally a carbon source, optionally a carbon film-forming agent, and optionally a modifier, and performing at least two sintering operations, wherein:

[0166] The temperature of the first sintering is 500° C. to 760° C., and in some embodiments, 550° C. to 720° C.;

[0167] The temperature of the second sintering is 700°C to 800°C, and in some embodiments, 720°C to 780°C.

[0168] In some embodiments, the temperature of the first sintering may be 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, or a value in a range consisting of any two of the above first sintering temperatures; the temperature of the second sintering may be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, or a value in a range consisting of any two of the above second sintering temperatures.

[0169] In some embodiments, the heating rates of the first sintering and the second sintering are each independently 2 degrees Celsius / minute (°C / min) to 20°C / min.

[0170] In some embodiments, the heating rates in the first sintering and the second sintering are each independently 2°C / min, 5°C / min, 7°C / min, 10°C / min, 13°C / min, 15°C / min, 17°C / min, or 20°C / min.

[0171] In some embodiments, the constant temperature sintering time of the first sintering is 1-6 hours. In some embodiments, the constant temperature sintering time of the first sintering can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0172] In some embodiments, the constant temperature sintering time of the second sintering is 2-12 hours. In some embodiments, the constant temperature sintering time of the second sintering can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours or 12 hours.

[0173] Compared to the traditional method of using high temperature to achieve particle growth, the lithium iron phosphate salt particles of the present application are sintered twice. Controlling the temperature of the two sinterings is conducive to preparing lithium iron phosphate salt particles having the primary average particle size and specific surface area of ​​the present application. Furthermore, controlling the heating rate, sintering temperature and constant temperature sintering time of the first sintering and / or the second sintering helps to reduce side reactions, thereby better preparing the lithium iron phosphate salt particles of the present application. Furthermore, in the conventional high-temperature sintering process, the carbon coating layer on the surface of the particles is prone to cracking, reducing the integrity of the carbon coating. The present application performs large particle synthesis at low temperature, which is conducive to improving the consistency and uniformity of the surface carbon coating.

[0174] In some embodiments, the lithium iron phosphate particles are mainly prepared by the following method: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sintering steps, wherein:

[0175] The carbon content of the material after the first sintering is 0.01 wt% to 0.79 wt%, and in some embodiments, 0.05 wt% to 0.4 wt%;

[0176] The carbon content of the material after the second sintering is 0.8 wt% to 2.0 wt%, and in some embodiments, 1.0 wt% to 1.6 wt%.

[0177] In some embodiments, the carbon content of the material after the first sintering may be 0.01 weight %, 0.05 weight %, 0.1 weight %, 0.15 weight %, 0.20 weight %, 0.25 weight %, 0.30 weight %, 0.35 weight %, 0.40 weight %, 0.45 weight %, 0.50 weight %, 0.55 weight %, 0.60 weight %, 0.65 weight %, 0.70 weight %, 0.75 weight %, 0.79 weight %, or a value in a range consisting of any two of the above carbon contents; the carbon content of the material after the second sintering may be 0.8 weight %, 0.9 weight %, 1.0 weight %, 1.1 weight %, 1.2 weight %, 1.3 weight %, 1.4 weight %, 1.5 weight %, 1.6 weight %, 1.7 weight %, 1.8 weight %, 1.9 weight %, 2.0 weight %, or a value in a range consisting of any two of the above carbon contents.

[0178] Adding a carbon source before the first sintering can effectively reduce the trivalent iron in the raw material and improve the purity and stability of the product. Furthermore, adding a lower carbon source content during the first sintering process is beneficial to reducing the barrier effect of the carbon layer on the growth process of lithium iron phosphate particles, which is beneficial to the crystallization growth of the lithium iron phosphate salt particle precursor at a lower temperature. At the same time, the low carbon content during the first sintering process is also beneficial to the solid-phase diffusion reaction between the modifier that may be added and the lithium iron phosphate salt material, which is beneficial to achieve a higher concentration of metal ion doping. Increasing the added carbon content in the second sintering further improves the uniformity and integrity of the coating, which is beneficial to obtaining lithium iron phosphate salt particles with the carbon content of the embodiment of the present application.

[0179] In some embodiments, raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent are provided and sintered at least twice.

[0180] In some embodiments, the mixing ratio of the lithium source, the iron source, and the phosphorus source satisfies, based on the atomic moles of each element, Fe:P=(0.96-0.985):1, and Li:Fe=(1.0-0.95):1.1.

[0181] In some embodiments, the mixing ratio of the iron source and the phosphorus source, calculated on the basis of the atomic moles of each element, satisfies Fe:P=0.96:1, Fe:P=0.965:1, Fe:P=0.97:1, Fe:P=0.975:1, Fe:P=0.98:1 or Fe:P=0.985:1.

[0182] In some embodiments, the mixing ratio of the lithium source and the iron source, calculated on the basis of the atomic moles of each element, satisfies Li:Fe=1.0:1.1, Li:Fe=0.99:1.1, Li:Fe=0.98:1.1, Li:Fe=0.97:1.1, Li:Fe=0.96:1.1 or Li:Fe=0.95:1.1.

[0183] In some embodiments, the weight ratio of the carbon source to the carbon film-forming agent is (9-0.25) : 1. In some embodiments, the weight ratio of the carbon source to the carbon film-forming agent may be 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 0.8:1, 0.5:1 or 0.25:1.

[0184] In some embodiments, the lithium source is a lithium-containing compound. In some embodiments, the lithium source includes at least one of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate. In some embodiments, the lithium source includes lithium carbonate.

[0185] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferric oxide, and ferric oxyhydroxide. In some embodiments, the iron source includes ferric oxide.

[0186] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In some embodiments, the phosphorus source includes phosphoric acid.

[0187] In some embodiments, the carbon source comprises at least one of citric acid, glucose, sucrose, starch, fructose, and lactose. In some embodiments, the carbon source comprises glucose.

[0188] In some embodiments, the carbon film-forming agent comprises at least one of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinyl pyrrolidone, and polyvinyl alcohol. In some embodiments, the carbon film-forming agent comprises polyaniline.

[0189] In some embodiments, the modifier comprises at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate. In some embodiments, the modifier comprises titanium dioxide.

[0190] By using the raw materials in the above ratio, it is advantageous to form the lithium iron phosphate salt particles of the present application.

[0191] In some embodiments, the lithium iron phosphate salt particles contain at least one of Ti, V, Mg, and / or Nb elements, and the content of the element is 1000ppm-10000ppm, and in some embodiments, 2500ppm-6000ppm, calculated based on the total weight of the lithium iron phosphate salt particles.

[0192] In some embodiments, the content of the element may be selected from 2500 ppm to 6000 ppm. Specifically, it can be selected from 2500ppm, 2600ppm, 2700ppm, 2800ppm, 2900ppm, 3000ppm, 3100ppm, 3200ppm, 3300ppm, 3400ppm, 3500ppm, 3600ppm, 3700ppm, 3800ppm, 3900ppm, 4000ppm, 4100ppm, 4200ppm, 4300ppm, 4400ppm, 4500ppm, 4600ppm, 4700ppm, 4800ppm, 4900ppm, 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm or 6000ppm, or the range between any two of the above values.

[0193] By doping the lithium iron phosphate salt of the embodiment of the present application with one or more of the above elements and making their content within the above range, it is possible to better achieve metal bulk modification of the lithium iron phosphate salt particles while improving the primary average particle size of the lithium iron phosphate salt particles, further enhancing the bulk ion transport capacity of the lithium iron phosphate salt particles, and improving the kinetic performance and gram capacity.

[0194] In some embodiments, a first pulverization is performed after a first sintering, and a second pulverization is performed after a second sintering, wherein:

[0195] The Dv50 of the product after the first pulverization is 300 nm to 1200 nm, and in some embodiments, 400 nm to 1100 nm;

[0196] The Dv50 of the product after the second pulverization is 500 nm to 5000 nm, and in some embodiments, 700 nm to 2500 nm.

[0197] In some embodiments, the Dv50 of the product after the first crushing can be 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, or a value in the range composed of the Dv50 of any two of the above products after the first crushing.

[0198] In some embodiments, the Dv50 of the product after the second crushing can be 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1700nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 2900nm, 3000nm, 3200nm, 3400nm, 3600nm, 3800nm, 4000nm, 4200nm, 4400nm, 4600nm, 4800nm, 5000nm, or a value in the range composed of the Dv50 of any two of the above products after the second crushing.

[0199] In some embodiments, comminution includes one or more of mechanical crushing, grinding, sand milling, and air flow crushing.

[0200] Controlling the Dv50 of the product after the first pulverization within the above range helps reduce the growth barrier effect of the added carbon source and any doping elements on the lithium iron phosphate salt particle precursor crystals, thereby facilitating the preparation of micron-sized lithium iron phosphate salt particle precursors. Controlling the Dv50 value of the product after the second pulverization within the above range helps to obtain lithium iron phosphate salt particles having the primary average particle size of the present application.

[0201] [Positive electrode]

[0202] The present application provides a positive electrode plate, which includes the positive electrode active material as described in the present application or the positive electrode active material obtained by the preparation method as described in the present application.

[0203] In some embodiments, the compacted density of the positive electrode sheet is 3.0 g / cm 3 -3.5g / cm 3 .

[0204] In some embodiments, the compaction density of the positive electrode sheet can be 3.0 g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 Or it is a value within the range consisting of the compaction densities of any two of the above-mentioned positive electrode sheets.

[0205] The compaction density of the pole piece can be tested by methods and equipment known in the art. As an example, when the pole piece is coated on one side, the compaction density of the film layer on one side of the pole piece = m / (V1-V2); when the pole piece is coated on both sides, the compaction density of the film layer on one side of the pole piece = m / [2×(V1-V2)], where m represents the weight of the film layer, V1 represents the volume of the pole piece, and V2 represents the volume of the current collector. m can be obtained by subtracting the weight of the current collector from the weight of the pole piece. The product of the surface area of ​​the pole piece and the thickness of the pole piece is the volume V1 of the pole piece, and the product of the surface area of ​​the pole piece and the thickness of the current collector is V2. The thickness of the current collector and the thickness of the pole piece are obtained by measuring the thickness of the empty foil in the tab area with a micrometer.

[0206] The compacted density of the positive electrode active material in the embodiment of the present application is 3.0 g / cm 3 -3.5g / cm 3 , higher compaction density is beneficial to improving the energy density of secondary batteries.

[0207] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, a binder and a conductive agent, and the positive electrode active material is the positive electrode active material of the embodiment of the present application or the positive electrode active material prepared by the preparation method of the embodiment of the present application.

[0208] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0209] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0210] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0211] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0212] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0213] [Negative electrode]

[0214] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0215] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0216] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0217] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0218] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0219] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0220] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0221] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0222] [Electrolytes]

[0223] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0224] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0225] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0226] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0227] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0228] [Isolation film]

[0229] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0230] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0231] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0232] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0233] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0234] [Secondary battery]

[0235] The present application provides a secondary battery, which includes the positive electrode sheet of the embodiment of the present application.

[0236] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG3 shows a secondary battery 5 having a square structure as an example.

[0237] In some embodiments, referring to Figure 4, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0238] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0239] Figure 5 shows an example battery module 4. Referring to Figure 5 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0240] In some embodiments, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 may be received in the receiving space.

[0241] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0242] Figures 6 and 7 illustrate an example battery pack 1. Referring to Figures 6 and 7 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0243] [Electrical devices]

[0244] In addition, the present application also provides an electrical device, which includes the secondary battery of the embodiment of the present application.

[0245] In some embodiments, the electrical device of the embodiments of the present application may further include at least one of a battery module or a battery pack. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0246] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0247] Figure 8 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0248] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0249] Example

[0250] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0251] Example 1

[0252] 1. Preparation method

[0253] 1) Preparation of positive electrode active materials

[0254] Preparation of lithium iron phosphate particles:

[0255] Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (the amount of titanium dioxide added, based on the total weight of the lithium iron phosphate particles, is sufficient to ensure a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline are weighed separately. The weight ratios of Li, Fe, and P satisfy the following: Fe:P = 0.968:1; Li:Fe = 1:0.98. The weight ratio of glucose to polyaniline satisfies the following: glucose:polyaniline = 1:2. The amount of glucose and polyaniline added is sufficient to ensure that the carbon content of the lithium iron phosphate precursor after the first sintering step is 0.15%. Water is added to the above substances to form a mixture slurry.

[0256] The mixture was mixed in a ball mill and ground in a sand mill to a slurry with a solids content of 38% and a Dv50 of 400 nm. The mixture was then spray-dried (the negative pressure of the high-speed spray dryer was -650 to -200 Pa, the inlet temperature was 300°C to 360°C, and the outlet temperature was 100°C to 140°C). The dried reactants were placed in a sintering furnace for the first sintering step. The heating rate was controlled at 5°C / min, the holding temperature was controlled at 650°C, and the holding time was 4 hours. After cooling, the material was pulverized using a mechanical mill to obtain a powder.

[0257] The resulting powder, glucose, polyaniline, and water were mixed (based on the total weight of the lithium iron phosphate particles, the amount of glucose and polyaniline added was such that the carbon content of the product after the second sintering was 1.2%, and the weight ratio of glucose to polyaniline was 1:2) to achieve a solids content of 40%. After uniform mixing, the mixture was processed using a ball mill and a sand mill to obtain a slurry with a Dv50 value of insoluble matter of 550 nm. The slurry was then spray-dried (using a high-speed spray dryer with a negative pressure of -650 to -200 Pa, an inlet temperature of 300°C to 360°C, and an outlet temperature of 100°C to 140°C). The dried reactants were then placed in a sintering furnace for a second low-temperature sintering (heating rate controlled at 5°C / min, sintering temperature of 750°C, and sintering time of 4 hours). After cooling, the material was pulverized a second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate particles were obtained. The carbon content was 1.2%, and the Ti content in the lithium iron phosphate particles was 5000 ppm. The obtained lithium iron phosphate particles were observed under a scanning electron microscope, as shown in Figure 2.

[0258] Lithium nickel cobalt manganese oxide particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd., model CPE-16, and the BET value and Dv50 value are shown in Table 1.

[0259] The lithium iron phosphate particles and the lithium nickel cobalt manganese oxide particles were mixed according to the mass listed in Table 1 to obtain a positive electrode active material.

[0260] 2) Preparation of positive electrode sheet

[0261] 2.0 wt% of polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone (NMP), and then 1.0 wt% of Super P, 0.5 wt% of carbon nanotubes and 96.5 wt% of the above-mentioned positive electrode material were added and stirred and mixed to obtain a positive electrode slurry;

[0262] The positive electrode slurry was prepared at 252mg / 1540mm 2The single-sided weight is evenly coated on the surface of the current collector aluminum foil, and then transferred to a vacuum drying oven to dry completely. The dried electrode is rolled and punched to obtain the positive electrode.

[0263] 3) Preparation of negative electrode sheet

[0264] Add artificial graphite (a negative electrode active material), carbon black (a conductive agent), carboxymethyl cellulose (a thickener), and styrene-butadiene rubber (a binder) into deionized water at a mass ratio of 0.9735:0.007:0.01:0.0095, and stir evenly to obtain a negative electrode slurry.

[0265] Then the negative electrode slurry was mixed with 139mg / 1540mm 2 The single-side weight is evenly coated on the copper foil, and the negative electrode sheet is obtained through drying, cold pressing, and slitting.

[0266] 4) Preparation of isolation membrane

[0267] Polypropylene film is used as the isolation film.

[0268] 5) Battery Preparation

[0269] The above-mentioned positive electrode sheet, isolation film, and negative electrode sheet are stacked in order, so that the isolation film is between the positive and negative electrode sheets to play an isolating role, and then wound to obtain a bare battery cell, the bare battery cell is welded with a pole ear, and the bare battery cell is placed in an aluminum shell and baked in a vacuum oven at 100°C for 8 hours. Then, the electrolyte is injected and sealed to obtain an uncharged battery. The uncharged battery is then subjected to the processes of static standing, hot and cold pressing, formation, shaping, capacity testing, etc. in sequence to obtain the lithium-ion battery of Example 1.

[0270] The preparation methods of the batteries of Examples 2-5 are similar to those of Example 1, but the primary average particle size of the lithium iron phosphate salt particles is adjusted, as shown in Table 1. The preparation methods of Examples 2-5 differ from those of Example 1 in that:

[0271] Example 2: Lowering the Dv50 of the mixed slurry after the first grinding to 350 nm;

[0272] Example 3: Increasing the Dv50 of the mixed slurry after the first grinding to 1200 nm;

[0273] Example 4: Lowering the Dv50 of the mixed slurry after the first grinding to 450 nm;

[0274] Example 5: Increasing the Dv50 of the mixed slurry after the first grinding to 1100 nm.

[0275] The preparation method of the battery of Example 6-7 is similar to that of Example 1, but the specific surface area value and carbon content of the lithium iron phosphate salt particles are adjusted, as shown in Table 1. The preparation method of Example 6-7 differs from that of Example 1 in that:

[0276] Example 6: The carbon content during the second sintering process is adjusted to 0.8%;

[0277] Example 7: The carbon content during the second sintering process was adjusted to 1.4%.

[0278] The preparation method of the battery of Example 8-9 is similar to that of Example 1, but the specific surface area value of the lithium iron phosphate salt particles is adjusted, as shown in Table 1. The preparation method of Example 8-9 differs from that of Example 1 in that:

[0279] Example 8: During the first sintering process, the weight ratio of glucose to polyaniline was 1:4;

[0280] Example 9: During the first sintering process, the weight ratio of glucose to polyaniline was 1:0.5.

[0281] The preparation methods of the batteries of Examples 10-11 were similar to those of Example 1, except that the specific surface area of ​​the lithium nickel cobalt manganese oxide was adjusted, as shown in Table 1. The lithium nickel cobalt manganese oxide particles of Example 10 were purchased from Guangdong Brunp Recycling Technology Co., Ltd., model CPE17; the lithium nickel cobalt manganese oxide particles of Example 11 were purchased from Guangdong Brunp Recycling Technology Co., Ltd., model CPE18.

[0282] The preparation methods of the batteries of Examples 12-14 are similar to those of Example 1, but the doping amount of the Q element is adjusted, as shown in Table 1. The difference from Example 1 in the process of preparing the lithium iron phosphate particles is that;

[0283] Example 12: The temperature of the first sintering was controlled to be 590°C, the temperature of the second sintering was controlled to be 720°C, and the Ti content was controlled to be 0 ppm;

[0284] Example 13: The temperature of the first sintering was controlled to be 620°C, the temperature of the second sintering was controlled to be 740°C, and the Ti content was controlled to be 2500ppm;

[0285] Example 14: The temperature of the first sintering was controlled to be 655° C., the temperature of the second sintering was controlled to be 755° C., and the Ti content was controlled to be 6000 ppm.

[0286] The preparation methods of the batteries of Examples 15-18 are similar to those of Example 1, but the Dv50 values ​​of the lithium iron phosphate particles or the lithium nickel cobalt manganese oxide particles are adjusted, as shown in Table 1. The preparation methods of Examples 15-18 differ from those of Example 1 in that:

[0287] Example 15: Controlling the Dv50 of the crushed product after the second sintering to be 870m;

[0288] Example 16: Controlling the Dv50 of the pulverized product after the second sintering to be 3500 nm;

[0289] The lithium nickel cobalt manganese oxide particles of Example 17 were purchased from Guangdong Brunp Recycling Technology Co., Ltd., model number CPE19;

[0290] The lithium nickel cobalt manganese oxide particles of Example 18 were purchased from Guangdong Brunp Recycling Technology Co., Ltd., model number CPE20.

[0291] The preparation method of the batteries of Examples 19-20 is similar to that of Example 1, but the mass blending ratio of the lithium iron phosphate salt particles and the lithium nickel cobalt manganese oxide particles is adjusted, as shown in Table 1.

[0292] The preparation method of the battery of Comparative Example 1 is similar to that of Example 1, but the lithium iron phosphate salt particles used are different from those in Example 1. The lithium iron phosphate salt particles used in Comparative Example 1 are purchased from Hunan Yuneng New Energy Battery Materials Co., Ltd., model CPF-087-1, and specific parameters are shown in Table 1.

[0293] The preparation method of the battery of Comparative Example 2 is similar to that of Example 1, but the lithium iron phosphate salt particles used are different from those in Example 1. The lithium iron phosphate salt particles used in Comparative Example 2 are purchased from Hunan Yuneng New Energy Battery Materials Co., Ltd., model CPF-087-2, and specific parameters are shown in Table 1.

[0294] The preparation method of the battery of Comparative Example 3 is similar to that of Example 1, except that the primary average particle size and specific surface area of ​​the lithium iron phosphate particles are adjusted, as shown in Table 1. The preparation method of Comparative Example 2 differs from that of Example 1 in that only one sintering process is used in Comparative Example 2, the sintering temperature is 750°C, and the carbon content of the sintered product is 1.2%.

[0295] The preparation method of the battery of Comparative Example 4 is similar to that of Example 1, except that the primary average particle size, specific surface area, and carbon content of the lithium iron phosphate particles are adjusted, as shown in Table 1. The preparation method of Comparative Example 3 differs from that of Example 1 in that the carbon content of the second sintering is reduced to 0.7%, thereby adjusting the BET and primary average particle size.

[0296] 2. Battery performance test

[0297] 1. Performance test of positive electrode active materials

[0298] 1) Test method for the primary average particle size of lithium iron phosphate particles

[0299] An argon ion beam is used to cut the positive electrode plate perpendicular to the large surface to expose the cross section. The cross section is photographed using a scanning electron microscope, and the particle size of the lithium iron phosphate salt particles is statistically analyzed using the length diameter statistical method. Specifically, the total number of lithium iron phosphate salt particles with a primary particle size greater than 80nm and the sum of the primary particle sizes of lithium iron phosphate salt particles with a primary particle size greater than 80nm can be counted in the electron microscope scanning photograph. The primary average particle size of the lithium iron phosphate salt particles = the primary particle size of the total lithium iron phosphate salt particles / the total number of lithium iron phosphate salt particles. In the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 80nm are not included in the statistical range.

[0300] 2) Dv50 value test method

[0301] Lithium iron phosphate particles: The Dv50 value of the lithium iron phosphate particles can be measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0302] Lithium nickel cobalt manganese oxide particles: The Dv50 value of the lithium nickel cobalt manganese oxide particles can be measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0303] 3) Specific surface area (BET) test method

[0304] According to the GB / T19587-2017 test standard method, the gas adsorption method is used to test the specific surface area of ​​lithium iron phosphate particles and nickel cobalt manganese oxide particles, as follows: lithium iron phosphate particles or lithium nickel cobalt manganese oxide particles are taken as samples, the sample tube is immersed in -196 ° C liquid nitrogen, and the adsorption amount of nitrogen on the solid surface at different pressures is measured at a relative pressure of 0.05-0.30. The sample monolayer adsorption amount is obtained based on the BET multilayer adsorption theory and its formula, and the specific surface area of ​​the sample is calculated.

[0305] 4) Test method for carbon content of lithium iron phosphate particles

[0306] The carbon content of lithium iron phosphate pellets was tested using an infrared absorption method after burning them in a high-frequency induction furnace. The specific testing process was in accordance with the standard GB / T 20123-2006 / ISO 15350:2000.

[0307] 5) Test method for the amount of Q element (such as Ti element) doped in lithium iron phosphate particles

[0308] The test method for the Q element content of lithium iron phosphate particles is carried out in accordance with GB / T 33822-2017.

[0309] 6) Test method for solid content of positive electrode slurry

[0310] Use an electronic balance (accuracy 0.0001), a drying oven, and a glass drying tray. Evenly spread 8-10g of the positive electrode slurry sample on the sample tray. Record the mass of the positive electrode slurry before drying as A. Place the sample tray with the positive electrode slurry sample in the drying oven and heat it at 130°C for 5 hours. After drying and cooling, remove the sample and record the mass of the positive electrode slurry after drying. Repeat the drying process several times until the mass of the dried positive electrode slurry sample reaches a constant weight, recording the mass as B. Positive electrode slurry solids content = (A / B) × 100%.

[0311] 7) Cathode slurry filtration performance test method

[0312] Fold a 200-mesh filter into an inverted triangle, place it in a 500-ml beaker, and pour in 500 ml of the prepared positive electrode slurry (pour all of it). Start timing when the slurry begins to flow out from the tip of the filter, and record the filtration time when the slurry in the beaker reaches 300 ml.

[0313] 8) Measurement of η value of lithium iron phosphate particles

[0314] First, a button cell was prepared using lithium iron phosphate particles as the positive electrode active material. The specific button cell preparation process was as follows: 2.0000g of lithium iron phosphate particles were mixed with 0.1111g of conductive carbon black and 0.1111g of polyvinylidene fluoride, and then added to 2.5g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry was formed. The slurry was coated on aluminum foil with a coating thickness of 140μm, dried under vacuum at 120°C for 2h, and punched into discs with a diameter of 13mm using a punch. A tablet press was used to press the tablets at 10Mpa and vacuum-insulated at 120°C for 12h to obtain the positive electrode sheet. The positive electrode sheet was weighed, and the loading amount of lithium iron phosphate particles was 11-12mg. Button cells were assembled in an argon-protected glove box, with a metallic lithium sheet as the negative electrode, an electrolyte consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, LiPF6 electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0315] The prepared button cell was tested for electrical performance on a blue battery tester. Specifically, the button cell was charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a single charge and discharge at a constant current rate of 1C. During the 1C charge and discharge test, the capacity value at a discharge voltage of 3.2V was recorded as C1, and the capacity value at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50uA.

[0316] 2. Battery performance test

[0317] 1) Battery capacity in grams

[0318] Place the battery in a 25°C oven environment and let it stand for 2 hours until the battery temperature remains at 25°C; discharge the battery at a constant current of 1 / 3C to 2.0V; pause for 5 minutes; charge the battery at a constant current of 1 / 3C to 4.35V, then charge at a constant voltage of 4.35V to a cutoff current of 0.05C; pause for 5 minutes.

[0319] Discharge the battery at a constant current of 1 / 3C to 2.0V. This step is the actual battery cell capacity test. Combined with the mass of the positive electrode active material, the gram capacity of the battery can be calculated. Gram capacity = capacity / mass of the positive electrode active material.

[0320] 2) Positive electrode compaction density test method

[0321] When the electrode is coated on one side, the compaction density of the film layer on one side of the electrode = m / (V1-V2). When the electrode is coated on both sides, the compaction density of the film layer on one side of the electrode = m / [2×(V1-V2)], where m represents the weight of the film layer, V1 represents the volume of the electrode, and V2 represents the volume of the current collector. m can be obtained by subtracting the weight of the current collector from the weight of the electrode. The product of the surface area of ​​the electrode and the thickness of the electrode is the volume V1 of the electrode, and the product of the surface area of ​​the electrode and the thickness of the current collector is V2. The thickness of the current collector and the thickness of the electrode are obtained by measuring the thickness of the empty foil in the tab area with a micrometer.

[0322] 3. Analysis of test results of various embodiments and comparative examples

[0323] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The positive electrode active material parameter table is shown in Table 1, and the performance test results table is shown in Table 2.

[0324] Table 2 Performance test results

[0325] According to the results in the above table, it can be seen from Examples 1-20 and Comparative Example 1 that increasing the primary average particle size of the lithium iron phosphate salt particles and reducing their specific surface area can improve the solid content and filtration performance of the positive electrode slurry, which is beneficial to improving processing problems. It can be seen from Examples 1-20 and Comparative Example 2 that reducing the specific surface area of ​​the lithium iron phosphate salt particles is beneficial to improving the solid content and filtration performance of the positive electrode slurry, which is beneficial to improving processing problems. It can be seen from Examples 1-18 and Comparative Example 3 that the primary average particle size of the lithium iron phosphate salt particles of the present application is beneficial to maintaining a good gram capacity. It can be seen from Examples 1-18 and Comparative Example 4 that when the BET of the lithium iron phosphate salt particles is too small, the gram capacity of the secondary battery will be reduced. In summary, when the primary average particle size of the lithium iron phosphate salt particles is 500-3000nm and the specific surface area BET is 3m 2 / g-8m 2 / g, and the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m 2 / g-2.0m 2 / g, which can improve the processing performance of the positive electrode slurry while taking into account the gram capacity of the secondary battery.

[0326] As shown in Figures 1a and 1b, the lithium nickel manganese oxide particles and the lithium iron phosphate particles are uniformly mixed, and the lithium iron phosphate particles densely fill the pores of the lithium nickel manganese oxide particles. The scanning electron microscope image in Figure 2 shows the microscopic morphology of the lithium iron phosphate particles of Example 1 of the present application. As can be seen from this image, the lithium iron phosphate of Example 1 of the present application exhibits the morphology of large single crystals.

[0327] It can be seen from Examples 1-5 that when the primary average particle size of the lithium iron phosphate particles is controlled to be 500-3000 nm, the processing performance of the positive electrode slurry can be improved while taking into account the gram capacity of the secondary battery.

[0328] From Examples 1, 6-7 and 10-11, it can be seen that the BET value of the lithium iron phosphate particles is controlled to be 3m 2 / g-8m 2 / g, and control the BET value of lithium nickel cobalt manganese oxide particles to 0.4m 2 / g-2.0m 2 / g, the gram capacity of the secondary battery can be taken into account while improving the processing performance of the positive electrode slurry.

[0329] It can be seen from Example 1 and Examples 8-9 that increasing the proportion of the carbon source film-forming agent is conducive to achieving dense coating of the carbon layer. The dense carbon coating of the present application is conducive to further improving the dynamic performance and gram capacity of the secondary battery.

[0330] As shown in Examples 1 and 12-14, adding metal elements, such as titanium, to lithium iron phosphate particles can improve the kinetic performance of the positive electrode active material. Furthermore, increasing the metal element content, for example within the range of 1,000-10,000 ppm, further improves the kinetic performance while minimizing the impact on the gram capacity of the lithium iron phosphate particles.

[0331] It can be seen from Example 1 and Examples 15-18 that when the Dv50 of the lithium iron phosphate particles is 500-5000nm, the Dv50 of the lithium nickel cobalt manganese oxide particles is 1200-20000nm, and the ratio of the two satisfies 1:(1-50), it is beneficial to improve the compaction density of the positive electrode sheet.

[0332] It can be seen from Examples 1 and 19-20 that controlling the mass percentage of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles within an appropriate range, for example, when the mass percentage of lithium iron phosphate particles is 5%-80% and the mass percentage of lithium nickel cobalt manganese oxide particles is 95%-20%, helps to further balance the processing performance, electrical properties and cost of the positive electrode active material.

[0333] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles. The primary average particle size of the lithium iron phosphate salt particles is 500 - 3000 nm, and the BET specific surface area of the lithium iron phosphate salt particles is 3 m 2 / g - 8 m 2 / g. The BET specific surface area of the lithium nickel cobalt manganese oxide particles is 0.4 m 2 / g - 2.0 m 2 / g.

2. The positive electrode active material according to claim 1, wherein the lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q q1 , where Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95 ≤ m1 ≤ 1.15, 0.9 ≤ x1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ z1 ≤ 4, 0 < q1 ≤ 0.1, and / or The lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2 Mn1-x2-y2-z2O2, where R includes at least one of Cr, Ti, V, Mg, Al, and Nb, 0 ≤ x2 < 1, 0 < y2 < 1, 0 < z2 < 1, 0 < x2 + y2 + z2 < 1.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The primary average particle size of the lithium iron phosphate salt particles is 650 - 2500 nm.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, The BET of the lithium iron phosphate particles is 4 m 2 / g - 7 m 2 / g, and the BET of the lithium nickel cobalt manganese oxide particles is 0.6 m 2 / g - 2.0 m 2 / g.

5. The cathode active material according to any one of claims 1-4, characterized in that, Based on the total weight of the lithium iron phosphate salt particles, the carbon content of the lithium iron phosphate salt particles is 0.8 wt% - 2.0 wt%.

6. The cathode active material according to any one of claims 1-5, characterized in that, Based on the total weight of the lithium iron phosphate salt particles, the carbon content of the lithium iron phosphate salt particles is 1.0 wt% - 1.6 wt%.

7. The cathode active material according to any one of claims 2-6, characterized in that, Q includes at least one of Ti, V, Mg, and Nb; Based on the total weight of the lithium iron phosphate salt particles, the content of Q in the lithium iron phosphate salt particles is 1000 - 10000 ppm.

8. The cathode active material according to any one of claims 1-7, characterized in that, The Dv50 of the lithium iron phosphate salt particles is 500 - 5000 nm.

9. The cathode active material according to any one of claims 1-8, characterized in that, The Dv50 of the lithium nickel cobalt manganese oxide particles is 1200 - 20000 nm.

10. The cathode active material according to any one of claims 1-9, characterized in that, The Dv50 of the lithium iron phosphate salt particles is 870 - 1500 nm, and the Dv50 of the lithium nickel cobalt manganese oxide particles is 1800 - 4200 nm.

11. The positive electrode active material according to any one of claims 1-10, characterized in that, Based on the total mass of the positive electrode active material, the mass percentage content of the lithium iron phosphate salt particles is 5% - 80%.

12. The cathode active material according to any one of claims 1-11, characterized in that, Based on the total mass of the positive electrode active material, the mass percentage content of the lithium nickel cobalt manganese oxide particles is 20% - 95%.

13. The positive electrode active material according to any one of claims 1-12, characterized in that, The capacity ratio η of the lithium iron phosphate salt particles ≥ 88%, and η is defined as: A battery having the lithium iron phosphate salt particles as the positive electrode active material is subjected to constant current charge and discharge twice at a rate of 0.1C within a voltage range of 2.0V - 3.75V, and then subjected to constant current charge and discharge once at a rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value when the discharge voltage is 3.2V is extracted and denoted as C1, and the capacity value when discharged to 2.0V is C2. And η = C1 / C2. Among them, the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

14. The positive electrode active material according to any one of claims 1-13, characterized in that, The lithium iron phosphate salt particles satisfy at least one of (a) - (e): a) The Dv10 of the lithium iron phosphate salt particles ≥ 200 nm; b) The Dv90 of the lithium iron phosphate salt particles ≤ 10000 m; c) The Dv99 of the lithium iron phosphate salt particles ≤ 12000 nm; d) The powder compaction density of the lithium iron phosphate salt under 3T pressure ≥ 2.25 g / cm 3 ; e) The powder resistivity of the lithium iron phosphate is less than 60 Ω·cm.

15. A method for preparing a positive electrode active material, characterized in that, The preparation method includes: mixing the lithium iron phosphate salt particles and the lithium nickel cobalt manganese oxide particles. Among them, the preparation process of the lithium iron phosphate salt particles includes: providing raw materials containing at least a lithium source, an iron source, and a phosphorus source, and performing at least two sinterings. Among them, The temperature of the first sintering is 500°C - 760°C; The temperature of the second sintering is 700°C - 800°C.

16. The preparation method according to claim 15, characterized in that, The lithium iron phosphate salt particles are mainly obtained by the following preparation method: Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings. Among them, The carbon content of the material after the first sintering is 0.01 wt% - 0.79 wt%; The carbon content of the material after the second sintering is 0.8 wt% - 2.0 wt%.

17. The preparation method according to claim 15 or 16, characterized in that, The lithium iron phosphate salt particles contain at least one of the elements Ti, V, Mg, and / or Nb, and based on the total weight of the lithium iron phosphate salt particles, the content of the element is 1000 ppm - 10000 ppm.

18. The preparation method according to any one of claims 15-17, characterized in that, After the first sintering, the first pulverization is carried out, and after the second sintering, the second pulverization is carried out, wherein, The Dv50 of the product after the first pulverization is 300 nm - 1200 nm; The Dv50 of the product after the second pulverization is 500 nm - 5000 nm.

19. A positive electrode plate, characterized in that, The positive electrode sheet includes the positive electrode active material according to any one of claims 1-14 or the positive electrode active material obtained by the preparation method according to any one of claims 15-18.

20. The positive electrode sheet according to claim 19, characterized in that, The compaction density of the positive electrode sheet is 3.0 g / cm 3 - 3.5 g / cm 3 .

21. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet according to claim 19 or 20.

22. An electrical device, characterized in that, The electrical device includes the secondary battery according to claim 21.

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