Positive electrode active material, method for manufacturing the same, positive electrode sheet, secondary battery, and power using device

Abstract

The application provides a positive electrode active material, which comprises 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, the specific surface area BET of the lithium iron phosphate salt particles is 3 m 2 / g-8 m 2 / g, and the specific surface area BET of the lithium nickel cobalt manganese oxide particles is 0.4 m 2 / g-2.0 m 2 / g. The positive electrode active material has good processing performance and can also consider the specific capacity.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a positive electrode active material and its preparation method, a positive electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] 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 plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace.

[0003] Lithium nickel cobalt manganese oxide (LCO) and lithium iron phosphate (LFP) are commonly used active materials for lithium-ion cathodes. To reduce the cost of lithium-ion batteries, LCO and LFP can be used in combination. However, a series of processing problems exist in the preparation of cathode slurries by mixing LCO and LFP. Summary of the Invention

[0004] This application was made in view of the aforementioned issues and aims to at least solve one of the technical problems existing in the prior art. To this end, this application provides a positive electrode active material and its preparation method, 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 achieving a good specific capacity of the secondary battery.

[0005] The first aspect of this application provides a positive electrode active material, which 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 specific surface area (BET) of the lithium iron phosphate salt particles is 3 m². 2 / g-8m 2 / g, the specific surface area (BET) of the lithium nickel cobalt manganese oxide particles is 0.4m². 2 / g-2.0m 2 / g.

[0006] To improve the electrochemical performance of lithium iron phosphate (LFP) particles, they are typically nano-sized to enhance their electron transport capabilities. However, nano-sized LFP particles have an excessively large specific surface area, leading to severe water absorption. In practical applications, to reduce costs, they are often mixed with lithium nickel cobalt manganese oxide (LCO) particles. However, during the preparation of the mixed slurry, the LFP particles are prone to gelation due to their high water absorption, resulting in a series of processing problems such as low solids content and poor filtration performance. Furthermore, the high molecular weight binders used in LCO cathode slurries, such as polyvinylidene fluoride (PVDF), have poor compatibility with nano-sized LFP. In such binder systems, LFP particles are prone to agglomeration, causing further processing issues. The LFP particles in this application have a primary average particle size of 500-3000 nm and a specific surface area of ​​3 m².2 / g-8m 2 / g, the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m². 2 / g-2.0m 2 / g. A reduced specific surface area helps decrease the water absorption of the cathode slurry, thereby reducing gelation and agglomeration during slurry processing and improving the drying speed and degree of the electrode. Simultaneously, a larger primary average particle size facilitates better dispersion of lithium iron phosphate particles in a high molecular weight binder system, further reducing agglomeration. However, excessively large particle size and excessively low specific surface area can affect kinetic performance and reduce specific capacity. The cathode active material of this application increases the primary average particle size of lithium iron phosphate particles and reduces their specific surface area, controlling both within a reasonable range, achieving good processing performance while also considering the specific capacity of the secondary battery.

[0007] In any embodiment, the lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q q1 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, and Br, with 0.95 ≤ m1 ≤ 1.15, 0.9 ≤ x1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ z1 ≤ 4, 0 < q1 ≤ 0.1, and / or

[0008] The lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2 Mn 1-x2-y2-z2 O2, wherein R includes at least one of Cr, Ti, V, Mg, Al, and Nb, and 0 ≤ x2 < 1, 0 < y2 < 1, 0 < z2 < 1, 0 < x2 + y2 + z2 < 1.

[0009] Doping lithium iron phosphate particles and / or lithium nickel cobalt manganese oxide particles with the aforementioned elements helps to improve the ion transport capability of the cathode active material. These elements can create vacancies or alter interatomic bond lengths in the particle lattice, facilitating the movement of lithium ions within the lattice and thus effectively improving the conductivity of the particles themselves and enhancing the kinetic performance of the cathode active material.

[0010] In any embodiment, the primary average particle size of the lithium iron phosphate salt particles is 650-2500 nm.

[0011] Controlling the primary average particle size of lithium iron phosphate salt particles within a reasonable range helps to further improve the processing performance of the cathode slurry while also taking into account the specific capacity of the secondary battery.

[0012] In any embodiment, the BET of the lithium iron phosphate salt particles is 4m. 2 / g-7m 2 / g, the BET of the lithium nickel cobalt manganese oxide particles is 0.6m. 2 / g-2.0m 2 / g.

[0013] Excessively high BET values ​​for lithium iron phosphate (LFP) salt particles and / or lithium nickel cobalt manganese oxide (LCO) particles will increase their water absorption capacity, affecting the processing performance of the mixed cathode slurry. However, excessively low BET values ​​for LFP and LCO particles will reduce their specific capacity and kinetic performance. Maintaining the BET of LFP particles at 4m is recommended. 2 / g-7m 2 Within / g, the BET of lithium nickel cobalt manganese oxide particles is controlled at 0.6m. 2 / g-2.0m 2 Within / g, it is beneficial to further balance the processing performance of the positive electrode slurry and the specific capacity of the secondary battery.

[0014] In any embodiment, the carbon content of the lithium iron phosphate particles is calculated as Cx by weight%, based on the total weight of the lithium iron phosphate particles. The ratio z of the BET of the lithium iron phosphate particles to Cx satisfies 1.5 ≤ z ≤ 8.5. Optionally, z satisfies 3 ≤ z ≤ 6.

[0015] In this application, the ratio z of the specific surface area (BET) to Cx of lithium iron phosphate (LFP) particles can characterize the uniformity and density of the carbon contained in the LFP particles. When the primary average particle size and carbon content of the LFP particles remain constant, a lower ratio z indicates a more uniform and dense carbon composition. Improving the uniformity and density of the contained carbon is beneficial for further improving the kinetic performance and specific capacity of the LFP particles. However, excessively high carbon coating density can affect lithium-ion intercalation and deintercalation, thus impacting the kinetic performance and specific capacity of the secondary battery to some extent. The ratio z range in the embodiments of this application is conducive to the LFP particles having a suitable uniform density of carbon, thereby improving the conductivity of the particle surface and further enhancing the kinetic performance and specific capacity of the secondary battery.

[0016] In any embodiment, the carbon content of the lithium iron phosphate particles is calculated based on the total weight of the lithium iron phosphate particles, which is 0.8% to 2.0% by weight. Optionally, the carbon content of the lithium iron phosphate particles is 1.0% to 1.6% by weight, based on the total weight of the lithium iron phosphate particles.

[0017] Increasing the carbon content of lithium iron phosphate particles helps improve their conductivity, thereby enhancing kinetic performance and specific capacity. However, excessively high carbon content may affect lithium-ion insertion and extraction, thus impacting the specific capacity of the secondary battery to some extent. The carbon content range specified in this application helps to further achieve better kinetic performance and specific capacity.

[0018] In any embodiment, Q includes at least one of Ti, V, Mg, and Nb, and optionally, Q is Ti;

[0019] Based on the total weight of the lithium iron phosphate particles, the Q content in the lithium iron phosphate particles is 1000-10000 ppm. Optionally, based on the total mass of the lithium iron phosphate particles, the Q content in the lithium iron phosphate particles is 2500-6000 ppm.

[0020] Existing lithium iron phosphate (LFP) salt particles generally have low or no doping element content. Increasing the Q element content in LFP salt particles helps to further improve their bulk ion transport capability and kinetic performance. However, with further increases in the Q element content, the bulk ion transport capability may not continue to increase, and it may even occupy lithium ion positions, affecting the specific capacity. The Q element range specified in this application helps to achieve better kinetic performance and specific capacity.

[0021] In any embodiment, the ratio of the Dv50 of the lithium iron phosphate particles to the Dv50 of the lithium nickel cobalt manganese oxide particles is 1:(1-25). Optionally, the ratio of the D50 value of the lithium iron phosphate particles to the Dv50 of the lithium nickel cobalt manganese oxide particles is 1:(1-20).

[0022] In any embodiment, the Dv50 of the lithium iron phosphate particles is 500-5000 nm, and optionally, the Dv50 of the lithium iron phosphate particles is 700-2500 nm.

[0023] In any embodiment, the Dv50 of the lithium nickel cobalt manganese oxide particles is 1200-20000 nm, and optionally, the Dv50 of the lithium nickel cobalt manganese oxide particles is 3000-15000 nm.

[0024] By controlling the Dv50 ratio range and value range of lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles in the embodiments of this application within the above-mentioned range, it is beneficial for lithium iron phosphate salt particles to be uniformly and tightly filled between lithium nickel cobalt manganese oxide particles, thereby alleviating the agglomeration phenomenon of single-component particles in the composite positive electrode active material and improving the compaction density of the positive electrode active material.

[0025] 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.

[0026] In any embodiment, the lithium iron phosphate salt particles have a mass percentage content of 5%-80% based on the total mass of the positive electrode active material, and can be optionally 15%-50%.

[0027] In any embodiment, 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, and can be optionally 50%-85%.

[0028] Controlling the mass percentage of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles within the above range helps to further balance the electrical performance and cost of the positive electrode active material.

[0029] In any embodiment, the capacity percentage η of the lithium iron phosphate salt particles is ≥ 88%, where η is defined as:

[0030] The battery, using the lithium iron phosphate particles as the positive electrode active material, was subjected to two constant current charge-discharge cycles at a rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by one constant current charge-discharge cycle at a rate of 1C. In the 1C charge-discharge test, the capacity value extracted at a discharge voltage of 3.2V was recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V was recorded as C2, where η = C1 / C2. The charging process included constant voltage charging at 3.75V and a constant voltage cutoff current of 50µA.

[0031] The capacity ratio η of lithium iron phosphate (LFP) particles reflects their kinetic performance and plateau retention performance, and can be adjusted by modifying the primary average particle size, carbon content, carbon source and carbon film-forming agent ratio, and modifier and its content. In this application, the LFP particles have an η value ≥ 88%, exhibiting good kinetic performance. Furthermore, when the η value is high, the secondary battery can still maintain good power performance even when discharged to a low SOC; that is, a battery with a high η value experiences a smaller voltage drop during low-charge, high-current discharge. In this application, the lithium nickel cobalt manganese oxide (LCO) particles, with their high voltage, participate in the discharge first, while the LFP particles participate later, bearing a larger current and prone to polarization. The LFP particles in this application possess high kinetic performance, which is beneficial for improving the discharge power performance of the secondary battery.

[0032] In any embodiment, the lithium iron phosphate salt particles satisfy at least one of (a)-(e):

[0033] a) The Dv10 of the lithium iron phosphate salt particles is ≥200nm;

[0034] b) The Dv90 of the lithium iron phosphate salt particles is ≤10000m;

[0035] c) The Dv99 of the lithium iron phosphate particles is ≤12000nm;

[0036] d) The compacted density of the lithium iron phosphate powder under 3T pressure is ≥2.25 g / cm³. 3 ;

[0037] e) The resistivity of the lithium iron phosphate powder is less than 60 Ω·cm.

[0038] By ensuring that the lithium iron phosphate particles satisfy at least one of (a)-(e), the lithium iron phosphate particles can achieve the technical effects of this application more effectively.

[0039] A second aspect of this application provides a method for preparing a positive electrode active material, the method comprising: mixing the lithium iron phosphate salt particles and the lithium nickel cobalt manganese oxide particles.

[0040] The preparation process of the lithium iron phosphate salt 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 processes.

[0041] The temperature for the first sintering is 500℃-760℃, and can be selected as 550℃-720℃;

[0042] The temperature for the second sintering is 700℃-800℃, and can be selected as 720℃-780℃.

[0043] Compared to traditional methods that use high temperatures to grow particles, the lithium iron phosphate particles in this application undergo two sintering processes. Controlling the temperatures of these two sintering processes is beneficial for obtaining lithium iron phosphate particles with the same average particle size and specific surface area as described in this application. Furthermore, in conventional high-temperature sintering processes, the carbon coating layer on the particle surface is prone to cracking, reducing the integrity of the carbon coating. This application synthesizes large particles at low temperatures, which helps reduce floating carbon and improves the consistency and uniformity of the surface carbon coating.

[0044] In any embodiment, 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 sintering processes, wherein...

[0045] The carbon content of the material after the first sintering is 0.01%-0.79% by weight, and can be selected as 0.05%-0.4% by weight.

[0046] The carbon content of the material after the second sintering is 0.8%-2.0% by weight, and can be selected as 1.0%-1.6% by weight.

[0047] Adding a carbon source before the first sintering can effectively reduce the trivalent iron in the raw materials, improving product purity and stability. Furthermore, adding a lower carbon source content during the first sintering process helps reduce the barrier effect of the carbon layer on the growth of lithium iron phosphate particles, facilitating the crystallization growth of the lithium iron phosphate salt particle precursor at lower temperatures. Simultaneously, the low carbon content during the first sintering process also promotes the solid-phase diffusion reaction between any added modifiers and the lithium iron phosphate salt material, thereby facilitating the achievement of higher concentrations of metal ion doping. Increasing the added carbon content during the second sintering further enhances the uniformity and integrity of the coating, resulting in lithium iron phosphate salt particles with the carbon content described in the embodiments of this application.

[0048] In any embodiment, the lithium iron phosphate particles contain at least one of the elements Ti, V, Mg, and / or Nb, and the content of the element is 1000ppm-10000ppm, optionally 2500ppm-6000ppm, based on the total weight of the lithium iron phosphate particles.

[0049] In any embodiment, a first pulverization is performed after the first sintering, and a second pulverization is performed after the second sintering, wherein...

[0050] The Dv50 of the product after the first pulverization is 300nm-1200nm, and can be selected as 400nm-1100nm;

[0051] The Dv50 of the product after the second pulverization is 500nm-5000nm, and can be selected as 700nm-2500nm.

[0052] Controlling the Dv50 of the product after the first pulverization within the aforementioned range helps reduce the growth barrier effect of added carbon sources and potentially added dopants on the lithium iron phosphate precursor crystals, thus facilitating the preparation of micron-sized lithium iron phosphate precursor particles. Controlling the Dv50 value of the product after the second pulverization within the aforementioned range helps obtain lithium iron phosphate particles with the average particle size of the first pulverization step described in this application.

[0053] A third aspect of this application provides a positive electrode sheet, the positive electrode sheet comprising the positive electrode active material as described in this application or the positive electrode active material obtained by the preparation method described in this application.

[0054] In any embodiment, the compaction density of the positive electrode sheet is 3.0 g / cm³. 3 -3.5g / cm 3 .

[0055] A fourth aspect of this application provides a secondary battery comprising a positive electrode as described in this application.

[0056] The fifth aspect of this application provides an electrical device comprising a secondary battery as described in this application.

[0057] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0058] Figure 1 This is a cross-sectional view of the positive electrode sheet of Embodiment 1 of this application. Figure 1 a is a slice of the positive electrode under 5kx magnification and Figure 1 b is a slice of the positive electrode under 10kx magnification.

[0059] Figure 2 This is a scanning electron microscope image of lithium iron phosphate salt particles from Example 1 of this application.

[0060] Figure 3 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0061] Figure 4 yes Figure 3 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0062] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application.

[0063] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0064] Figure 7 yes Figure 6 An exploded view of a battery pack according to one embodiment of this application is shown.

[0065] Figure 8 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0066] Figure 9 This is a scanning electron microscope image of lithium iron phosphate salt particles with primary particle size labeled according to an embodiment of this application.

[0067] Explanation of reference numerals in the attached figures:

[0068] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0069] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0070] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0071] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0072] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0073] Unless otherwise specified, all steps in this 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 it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0074] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0075] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: 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).

[0076] To balance battery cost and electrical performance, lithium iron phosphate (LFP) materials can be mixed with lithium nickel cobalt manganese oxide (LCO) materials to prepare positive electrode active materials. However, LFP particles suffer from poor electrical performance. To improve their electrical properties, they are typically nano-sized to enhance their electron transport capacity. However, during the preparation of the slurry by mixing LFP and LCO particles, the positive electrode slurry is prone to gelation and severe agglomeration, leading to a series of processing problems such as low solids content, poor filtration performance, high water absorption of the electrode, and difficulty in drying. This application addresses these issues by increasing the primary average particle size of LFP particles and reducing their specific surface area within a reasonable range, achieving better processing performance while maintaining the specific capacity of the secondary battery. Furthermore, the reduced BET and water absorption of LFP particles help reduce side reactions and improve cell lifespan.

[0077] [Positive electrode active material]

[0078] Based on this, this application provides a positive electrode active material, which 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 specific surface area (BET) of the lithium iron phosphate salt particles is 3 m². 2 / g-8m 2 / g, the specific surface area (BET) of lithium nickel cobalt manganese oxide particles is 0.4m². 2 / g-2.0m 2 / g.

[0079] 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 connecting two points on the edge in a cross-sectional view. In specific embodiments, as shown in the appendix... Figure 9 The scanning electron microscope image of lithium iron phosphate particles is shown. The line segment marked by the double arrow in the particle is the primary particle size as defined in this application.

[0080] In this application, the term "specific surface area" or "BET" refers to the total surface 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, carbon content, density of carbon coating, degree of adhesion between carbon and particles, and porosity of particles.

[0081] In some implementations, the lithium iron phosphate salt particles are primary particles.

[0082] In this paper, "primary particles" refers to particles that do not have obvious agglomeration interfaces in particle scanning electron micrographs, but may have tiny pores and point or line defects, which are different from the smallest unit of powder particles that do not have structures such as stacking and flocculation.

[0083] In some embodiments, the lithium iron phosphate salt particles are monocrystalline particles and / or polycrystalline particles.

[0084] In this document, the term "single crystal" refers to a structurally complete crystal grown from a single crystal nucleus. The single crystal of this application appears as a single entity in the field of view of a transmission electron microscope, and there are no grain boundaries within the single crystal.

[0085] In some embodiments, the single crystal of this application may have minor defects, such as internal micropores, a small number of points and surfaces, or a small number of particles adhering to each other on the surface of a particle.

[0086] In this paper, the term "polycrystalline" refers to a crystal composed of small, randomly oriented single-crystal grains, with grain boundaries present within the polycrystalline structure.

[0087] In some implementations, the proportion of monocrystalline particles is greater than or equal to 90% based on the total number of lithium iron phosphate particles. Controlling the proportion of monocrystalline particles within the above range, compared to polycrystalline and secondary agglomerates, results in a higher proportion of monocrystalline particles, which is beneficial for reducing BET and improving processing performance.

[0088] The primary average particle size of lithium iron phosphate (LFP) salt particles can be measured using methods and equipment known in the art. For example, it can be tested using scanning electron microscopy (SEM) and arithmetic length-diameter (ALT) statistics. As an example, an argon ion beam is used to cut the electrode perpendicular to its large surface, exposing the cross-section. The cross-section is photographed using an SEM, and the ALT statistics method is used to statistically analyze the particle size of the LFP salt particles. Specifically, the total number of LFP salt particles with a primary diameter greater than 80 nm can be counted from the SEM images, as well as the sum of the primary diameters of LFP salt particles with a primary diameter greater than 80 nm. The primary average particle size of LFP salt particles is calculated as: (sum of primary diameters of LFP salt particles / total number of LFP salt particles). During the primary diameter statistics process, particles with a primary diameter between 0 and 80 nm are not included in the statistical scope.

[0089] To improve the electrical properties of lithium iron phosphate (LFP) particles, they are typically nano-sized to enhance their electron transport capabilities. However, nano-sized LFP particles have an excessively large specific surface area, leading to severe water absorption. Therefore, during the preparation of slurries with lithium nickel cobalt manganese oxide (LCO) particles, the LFP particles easily form gels due to their high water absorption, resulting in low solids content and poor filtration performance. Furthermore, nano-sized LFP particles have poor compatibility with high-molecular-weight binders commonly used in the preparation of LCO particle cathode active materials, such as polyvinylidene fluoride (PVDF). In such binder systems, LFP particles are prone to agglomeration, causing further processing problems. The LFP particles in this application have a primary average particle size of 500-3000 nm, and the specific surface area of ​​the LFP particles has been further adjusted to 3 m² / s. 2 / g-8m 2 / g, the specific surface area of ​​lithium nickel cobalt manganese oxide particles is 0.4m². 2 / g-2.0m 2 / g. A reduced specific surface area helps decrease the water absorption of the cathode slurry, thereby reducing gelation and agglomeration during slurry processing and improving the drying speed and degree of the slurry. Simultaneously, a larger primary average particle size facilitates better dispersion of lithium iron phosphate particles in a high molecular weight binder system, further reducing agglomeration. However, excessively large particle size and excessively low specific surface area will reduce the specific capacity of the cathode active material. The cathode active material of this application increases the primary average particle size of lithium iron phosphate particles and reduces their specific surface area, as well as the specific surface area of ​​lithium nickel cobalt manganese oxide particles, keeping both within a reasonable range. This achieves good processing performance while also considering the specific capacity of the secondary battery.

[0090] In some embodiments, lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Qq1 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, and Br, with 0.95 ≤ m1 ≤ 1.15, 0.9 ≤ x1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ z1 ≤ 4, 0 < q1 ≤ 0.1, and / or

[0091] Lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2 Mn 1-x2-y2-z2 O2, wherein R includes at least one of Cr, Ti, V, Mg, Al, and Nb, and 0 ≤ x2 < 1, 0 < y2 < 1, 0 < z2 < 1, 0 < x2 + y2 + z2 < 1.

[0092] In some embodiments, 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, or 1.15; x1 can be 0.9 or 1.0; y1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; z1 can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4; and q1 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1.

[0093] In some embodiments, lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2 Mn 1-x2-y2-z2 O2, R includes at least one of Ti, V, Mg, and Nb.

[0094] In some embodiments, lithium nickel cobalt manganese oxide particles have the molecular formula LiR x2 Ni y2 Co z2 Mn 1-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 lithium and oxygen content ratio in the lithium nickel cobalt manganese oxide particles may fluctuate during battery formation and cycling.

[0095] Doping lithium iron phosphate particles and / or lithium nickel cobalt manganese oxide particles with the aforementioned elements helps to improve the ion transport capability of the cathode active material. These elements can create vacancies or alter interatomic bond lengths in the particle lattice, facilitating the movement of lithium ions within the lattice and thus effectively improving the conductivity of the particles themselves and enhancing the kinetic performance of the cathode active material.

[0096] In some implementations, the primary average particle size of lithium iron phosphate salt particles is 650-2500 nm.

[0097] In some embodiments, the primary average particle size of lithium iron phosphate salt 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 within the range of any two of the above primary average particle sizes.

[0098] Controlling the primary average particle size of lithium iron phosphate salt particles within a reasonable range helps to further improve the processing performance of the cathode slurry while also taking into account the specific capacity of the secondary battery.

[0099] In some embodiments, the BET of lithium iron phosphate salt 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.

[0100] In some implementations, the specific surface area (BET) of lithium iron phosphate salt particles can be 3 m². 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m2 / 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 specific surface areas (BET) of any two lithium iron phosphate particles mentioned above.

[0101] In some embodiments, the specific surface area (BET) of lithium nickel cobalt manganese oxide particles can be 0.6 m². 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g, or a value within the range of the specific surface areas (BET) of any two of the above lithium nickel cobalt manganese oxide particles.

[0102] The specific surface area (BET) of particles can be measured using methods and equipment known in the art. For example, it can be tested according to the gas adsorption method, referring to GB / T 19587-2017. As an example, lithium iron phosphate particles are placed in a sample tube, which is then immersed in liquid nitrogen at -196°C. The amount of nitrogen adsorbed on the solid surface at different pressures (0.05-0.30) is measured. Based on the BET multilayer adsorption theory and its formula, the monolayer adsorption amount of the sample is calculated, thus obtaining the specific surface area of ​​the sample.

[0103] Excessively high BET values ​​for lithium iron phosphate (LFP) salt particles and / or lithium nickel cobalt manganese oxide (LCO) particles will increase their water absorption capacity, affecting the processing performance of the mixed cathode slurry. Conversely, excessively low BET values ​​for LFP and LCO particles will reduce their specific capacity. Maintaining the BET of LFP salt particles at 4m is recommended. 2 / g-7m 2 Within / g, the BET of lithium nickel cobalt manganese oxide particles is controlled at 0.6m. 2 / g-2.0m 2 Within / g, it is beneficial to further balance the processing performance of the positive electrode slurry and the specific capacity of the secondary battery.

[0104] In some implementations, the carbon content of the lithium iron phosphate particles is calculated as Cx by weight%, 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.

[0105] In some implementations, z satisfies 3≤z≤6.

[0106] In some implementations, 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 within a range consisting of any two of the above z values.

[0107] In this application, the ratio z of the specific surface area (BET) to Cx of lithium iron phosphate (LFP) particles can characterize the uniformity and density of the carbon contained in the LFP particles. When the primary average particle size and carbon content of the LFP particles remain constant, a lower ratio z indicates higher carbon coating utilization, less floating carbon, and more uniform and dense carbon content within the LFP particles. Improving the uniformity and density of the contained carbon is beneficial for enhancing the kinetic performance and specific capacity of the LFP particles. However, excessively high carbon coating density may affect lithium-ion intercalation / deintercalation, thus impacting the kinetic performance and specific capacity of the secondary battery to some extent. The ratio z range in the embodiments of this application is conducive to the LFP particles having a suitable uniform density of carbon, thereby improving the conductivity of the particle surface and further enhancing the kinetic performance and specific capacity of the secondary battery.

[0108] In some implementations, the carbon content of the lithium iron phosphate particles is calculated to be 0.8% to 2.0% by weight, based on the total weight of the lithium iron phosphate particles.

[0109] In some embodiments, the carbon content of the lithium iron phosphate particles is 1.0% to 1.6% by weight, based on the total weight of the lithium iron phosphate particles.

[0110] In some embodiments, the carbon content of the lithium iron phosphate particles is calculated based on the total weight of the particles, and is 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, or a value within any two of the above-mentioned carbon content ranges.

[0111] In some embodiments, the carbon contained in the lithium iron phosphate particles is coated on the surface of the particles. In some embodiments, the carbon contained in the lithium iron phosphate particles is embedded within the particles. In some embodiments, the carbon contained in the lithium iron phosphate particles is partially coated on the surface of the particles and partially embedded within the particles.

[0112] The carbon content of iron phosphate particles can be measured using methods and equipment known in the art. For example, it can be tested in accordance with GB / T 20123-2006 / ISO 15350:2000.

[0113] Increasing the carbon content of lithium iron phosphate salt particles helps improve their conductivity, thereby enhancing kinetic performance and specific capacity. However, excessively high carbon content may affect lithium-ion insertion / extraction, impacting the specific capacity of the secondary battery to some extent. The carbon content range described in this application's embodiments helps to further achieve better kinetic performance and specific capacity.

[0114] In some embodiments, Q includes at least one of Ti, V, Mg, and Nb. In other embodiments, Q is Ti.

[0115] In some embodiments, the Q content in the lithium iron phosphate particles is calculated to be 1000-10000 ppm based on the total weight of the lithium iron phosphate particles.

[0116] In some embodiments, the Q content in the lithium iron phosphate particles is 2500-6000 ppm based on the total mass of the lithium iron phosphate particles.

[0117] In some embodiments, the content of Q in the lithium iron phosphate salt particles is calculated based on the total weight of the lithium iron phosphate salt particles, and is 1000ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, 10000ppm, or a value within a range consisting of any two of the above-mentioned contents of Q.

[0118] The content of Q in lithium iron phosphate particles, for example, the content of Ti, can be measured using methods and equipment known in the art. For example, it can be tested with reference to GB / T 33822-2017.

[0119] Existing lithium iron phosphate (LFP) salt particles generally have low or no doping element content. Increasing the Q element content in LFP salt particles helps to further improve their bulk ion transport capability and kinetic performance. However, with further increases in the Q element content, the bulk ion transport capability may not continue to increase, and it may even occupy lithium ion positions, affecting the specific capacity. The Q element range specified in this application helps to achieve better kinetic performance and specific capacity.

[0120] In some embodiments, the ratio of the Dv50 of lithium iron phosphate particles to the Dv50 of lithium nickel cobalt manganese oxide particles is 1:(1-25).

[0121] In some embodiments, the ratio of the D50 value of lithium iron phosphate particles to the Dv50 value of lithium nickel cobalt manganese oxide particles is 1:(1-20).

[0122] In some embodiments, the ratio of the Dv50 of lithium iron phosphate particles to the Dv50 of 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 within the range formed by any two of the above ratios of the Dv50 of lithium iron phosphate particles to the Dv50 of lithium nickel cobalt manganese oxide particles.

[0123] In some implementations, the Dv50 of lithium iron phosphate particles is 500-5000 nm.

[0124] In some implementations, the Dv50 of lithium iron phosphate particles is 700-2500 nm.

[0125] In some embodiments, the Dv50 of lithium iron phosphate particles is 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1600nm, 1700nm, 1800nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 3000nm, 3300nm, 3500nm, 3700nm, 4000nm, 4300nm, 4500nm, 4700nm, or 5000nm, or a value within the range formed by any two of the above-mentioned lithium iron phosphate particles' Dv50.

[0126] In some implementations, the Dv50 of lithium nickel cobalt manganese oxide particles is 1200-20000 nm.

[0127] In some implementations, the Dv50 of lithium nickel cobalt manganese oxide particles is 3000-15000 nm.

[0128] In some embodiments, the Dv50 of lithium nickel cobalt manganese oxide particles can be 1200nm, 1500nm, 1700nm, 2000nm, 2300nm, 2500nm, 2700nm, 3000nm, 3300nm, 3500nm, 4000nm, 4300nm, 4500nm, 4700nm, 5000nm, 6000nm, 7000nm, 8000nm, 9000nm, 10000nm, 11000nm, 12000nm, 13000nm, 14000nm, 15000nm, 16000nm, 17000nm, 18000nm, 19000nm, 20000nm, or a value within a range consisting of the Dv50 of any two of the above lithium nickel cobalt manganese oxide particles.

[0129] In this application, the term "Dv50" refers to the particle size at which the cumulative volumetric particle size distribution percentage in the particle reaches 50%.

[0130] The Dv50 of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles can be measured using methods and equipment known in the art. For example, it can be determined using a laser particle size analyzer (Malvern MasterSize 3000) with reference to GB / T19077.1-2016.

[0131] By controlling the Dv50 ratio range and value range of lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles in the embodiments of this application within the above-mentioned range, it is beneficial for lithium iron phosphate salt particles to be uniformly and tightly filled between lithium nickel cobalt manganese oxide particles, thereby alleviating the agglomeration phenomenon of single-component particles in the composite positive electrode active material and improving the compaction density of the positive electrode active material.

[0132] 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.

[0133] In some embodiments, the mass percentage of lithium iron phosphate particles is 5%-80% based on the total mass of the positive electrode active material.

[0134] In some embodiments, the mass percentage of lithium iron phosphate particles is 15%-50% based on the total mass of the positive electrode active material.

[0135] In some embodiments, the mass percentage of lithium iron phosphate particles, based on the total mass of the positive electrode active material, 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 any two of the above-mentioned lithium iron phosphate particles mass percentages.

[0136] In some embodiments, the mass percentage of lithium nickel cobalt manganese oxide particles is 20%-95% based on the total mass of the positive electrode active material.

[0137] In some embodiments, the mass percentage of lithium nickel cobalt manganese oxide particles is 50%-85% based on the total mass of the positive electrode active material.

[0138] In some embodiments, the mass percentage of lithium nickel cobalt manganese oxide particles, based on the total mass of the positive electrode active material, can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or a value within the range of any two of the above-mentioned mass percentages of lithium nickel cobalt manganese oxide particles.

[0139] Controlling the mass percentage of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles within the above range helps to further balance the electrical performance and cost of the positive electrode active material.

[0140] In some implementations, the capacity percentage η of lithium iron phosphate salt particles is ≥ 88%, where η is defined as:

[0141] A battery using lithium iron phosphate particles as the positive electrode active material was subjected to two constant current charge-discharge cycles at a rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by one constant current charge-discharge cycle at a rate of 1C. In the 1C charge-discharge test, the capacity value extracted at a discharge voltage of 3.2V was recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at 3.75V and a constant voltage cutoff current of 50uA.

[0142] In some implementations, η can be 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or a value within a range of any two of the above η values.

[0143] The η value of lithium iron phosphate particles can be measured using methods and equipment known in the art. As an example, a coin cell was first prepared using lithium iron phosphate particles as the positive electrode active material. The specific coin 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 onto aluminum foil to a thickness of 140 micrometers, vacuum dried at 120°C for 2 hours, and then punched into discs with a diameter of 13mm. The discs were then pressed using a tablet press at 10 MPa and vacuum-insulated at 120°C for 12 hours to obtain the positive electrode sheet. The weight of the positive electrode sheet was measured, revealing a lithium iron phosphate particle loading of 11-12mg. A button cell battery is assembled in an argon-protected glove box, with a lithium metal sheet as the negative electrode, a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent as the electrolyte, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0144] The prepared coin cells were tested for electrical performance using a blue-light tester. Specifically, the coin cells were charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by one charge and discharge cycle at a constant current rate of 1C. In 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.

[0145] The capacity ratio η of lithium iron phosphate (LFP) particles reflects their kinetic performance and plateau retention performance, and can be adjusted by modifying the primary average particle size, carbon content, carbon source and carbon film-forming agent ratio, and modifier and its content. In this application embodiment, the LFP particles have an η value ≥ 88%, exhibiting good kinetic performance. Furthermore, when the η value is high, the secondary battery can still maintain good power performance when discharged to a low SOC; that is, a battery with a high η value experiences a smaller voltage drop during low-charge, high-current discharge. In this application embodiment, during discharge, the lithium nickel cobalt manganese oxide (LCO) particles have a high voltage and participate in the discharge first, while the LFP particles participate later, bearing a larger current and easily becoming polarized. The LFP particles of this application possess high kinetic performance, which is beneficial for improving the discharge power performance of the secondary battery.

[0146] In some embodiments, the lithium iron phosphate salt particles satisfy at least one of (a)-(e):

[0147] a) The Dv10 of lithium iron phosphate particles is ≥200nm;

[0148] b) The Dv90 of lithium iron phosphate particles is ≤10000m;

[0149] c) The Dv99 of lithium iron phosphate particles is ≤12000nm;

[0150] d) The compacted density of lithium iron phosphate powder under 3T pressure is ≥2.25 g / cm³. 3 ;

[0151] e) The resistivity of lithium iron phosphate powder is less than 60 Ω·cm.

[0152] In this paper, the term "Dv10" refers to the particle size at which the cumulative volumetric size distribution percentage in the particle reaches 10%.

[0153] In this paper, the term "Dv90" refers to the particle size at which the cumulative volumetric size distribution percentage in the particle reaches 90%.

[0154] In this paper, the term "Dv99" refers to the particle size at which the cumulative volumetric size distribution percentage in the particle reaches 99%.

[0155] In this paper, the term "powder compaction density" refers to the density of a compacted powder with a certain density and strength, formed during the external force compression process. This density is measured in g / cm³ as the powder moves and deforms, filling larger voids, increasing the contact area between particles, generating attractive forces between atoms, and enhancing the mechanical cohesion between particles. 3 .

[0156] In some implementations, the Dv10 of the lithium iron phosphate salt particles is less than the Dv50.

[0157] In some implementations, the Dv90 of lithium iron phosphate particles is greater than the Dv50.

[0158] The Dv10, Dv90, and Dv99 of lithium iron phosphate particles can be measured using methods and equipment known in the art. For example, they can be determined using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0159] The compacted density of lithium iron phosphate particles under different pressures can be measured using methods and equipment known in the art. For example, it can be measured using a compaction density instrument, referring to GB / T 24533-2009. Specifically, a certain amount of lithium iron phosphate particles is placed on a compaction mold (the mold diameter is known). The mold is hollow in the middle and has a metal disc at the top and bottom. The lithium iron phosphate particles are placed between the metal discs, and a metal cylinder is placed on top. The mold is placed on a compaction density instrument, and different pressures are set (e.g., 3T). The thickness of the lithium iron phosphate particles under different pressures can be read on the instrument. The compacted density of the lithium iron phosphate particles is ρ = m / v, where v = (S × H), m is the mass of the lithium iron phosphate particles, S is the bottom area of ​​the mold, and H is the thickness of the lithium iron phosphate particles after compaction.

[0160] The powder resistivity of lithium iron phosphate 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 type) according to GB / T 33822-2017. Specifically, a certain amount of lithium iron phosphate particles (e.g., 1g) is weighed and added to the feeding chamber of the powder resistivity meter. A pressure of 8MPa is applied, and the forward and reverse resistivity of the lithium iron phosphate particles are measured separately. The average value of the two is taken as the powder resistivity of the lithium iron phosphate particles.

[0161] By ensuring that the lithium iron phosphate salt particles satisfy at least one of (a)-(e), the lithium iron phosphate salt particles can more effectively achieve the technical effects of this application.

[0162] This application provides a method for preparing a positive electrode active material, the method comprising: mixing lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles.

[0163] The preparation process of lithium iron phosphate salt 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 processes.

[0164] The temperature for the first sintering is 500℃-760℃, and can be selected as 550℃-720℃;

[0165] The temperature for the second sintering is 700℃-800℃, and can be selected as 720℃-780℃.

[0166] In some embodiments, the temperature for the first sintering can be 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, or 710°C.

[0167] The temperatures for the first sintering can be 720℃, 730℃, 740℃, 750℃, 760℃, or any value within a range consisting of any two of the above first sintering temperatures; the temperatures for the second sintering can be 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, or any value within a range consisting of any two of the above second sintering temperatures.

[0168] In some embodiments, the heating rates for the first and second sintering are each independently 2°C / min to 20°C / min.

[0169] In some embodiments, the heating rates in the first and 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.

[0170] In some embodiments, the isothermal sintering time for the first sintering is 1-6 hours. In some embodiments, the isothermal sintering time for 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.

[0171] In some embodiments, the isothermal sintering time for the second sintering is 2-12 hours. In some embodiments, the isothermal sintering time for 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.

[0172] Compared to traditional methods that use high temperatures to grow particles, the lithium iron phosphate particles in this application undergo two sintering processes. Controlling the temperatures of the two sintering processes is beneficial for obtaining lithium iron phosphate particles with the primary average particle size and specific surface area of ​​this application. Furthermore, controlling the heating rate, sintering temperature, and isothermal sintering time of the first and / or second sintering processes helps reduce side reactions, thereby better preparing the lithium iron phosphate particles of this application. Furthermore, in conventional high-temperature sintering processes, the carbon coating layer on the particle surface is prone to cracking, reducing the integrity of the carbon coating. This application synthesizes large particles at low temperatures, which is beneficial for improving the consistency and uniformity of the surface carbon coating.

[0173] In some embodiments, 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 sintering processes, wherein...

[0174] The carbon content of the material after the first sintering is 0.01%-0.79% by weight, and can be selected as 0.05%-0.4% by weight.

[0175] The carbon content of the material after the second sintering is 0.8%-2.0% by weight, and can be selected as 1.0%-1.6% by weight.

[0176] In some embodiments, the carbon content of the material after the first sintering can be 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt%, 0.50 wt%, 0.55 wt%, 0.60 wt%, 0.65 wt%, 0.70 wt%, 0.75 wt%, or 0.79 wt%, or a value within a range consisting of any two of the above carbon contents; the carbon content of the material after the second sintering can be 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, or 2.0 wt%, or a value within a range consisting of any two of the above carbon contents.

[0177] Adding a carbon source before the first sintering can effectively reduce the trivalent iron in the raw materials, improving product purity and stability. Furthermore, adding a lower carbon source content during the first sintering process helps reduce the barrier effect of the carbon layer on the growth of lithium iron phosphate particles, facilitating the crystallization growth of the lithium iron phosphate salt particle precursor at lower temperatures. Simultaneously, the low carbon content during the first sintering process also promotes the solid-phase diffusion reaction between any added modifiers and the lithium iron phosphate salt material, thereby facilitating the achievement of higher concentrations of metal ion doping. Increasing the added carbon content during the second sintering further enhances the uniformity and integrity of the coating, resulting in lithium iron phosphate salt particles with the carbon content described in the embodiments of this application.

[0178] 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 sintering is performed at least twice.

[0179] In some implementations, the mixing ratios of lithium, iron, and phosphorus sources, based on the atomic molar number of each element, satisfy the following: Fe:P = (0.96-0.985):1 and Li:Fe = (1.0-0.95):1.1.

[0180] In some implementations, the mixing ratio of the iron source and the phosphorus source, based on the atomic molar number of each element, satisfies the following: 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.

[0181] In some embodiments, the mixing ratio of lithium source and iron source, based on the atomic molar number of each element, satisfies the following: 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.

[0182] In some embodiments, the weight ratio of carbon source to carbon film-forming agent is (9-0.25):1. In some embodiments, the weight ratio of carbon source to carbon film-forming agent can 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.

[0183] In some embodiments, the lithium source is a lithium-containing compound. In some embodiments, the lithium source includes at least one selected from lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate. In some embodiments, the lithium source includes lithium carbonate.

[0184] 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, magnetite, and ferric hydroxide. In some embodiments, the iron source includes ferric oxide.

[0185] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one selected from phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In some embodiments, the phosphorus source includes phosphoric acid.

[0186] In some embodiments, the carbon source includes at least one selected from citric acid, glucose, sucrose, starch, fructose, and lactose. In some embodiments, the carbon source includes glucose.

[0187] In some embodiments, the carbon film-forming agent includes at least one selected from polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol. In some embodiments, the carbon film-forming agent includes polyaniline.

[0188] In some embodiments, the modifier includes at least one selected from 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 includes titanium dioxide.

[0189] By using raw materials in the above proportions, it is beneficial to form the lithium iron phosphate particles of this application.

[0190] In some embodiments, the lithium iron phosphate particles contain at least one of the elements Ti, V, Mg, and / or Nb, with the element content calculated based on the total weight of the lithium iron phosphate particles at 1000ppm-10000ppm, optionally 2500ppm-6000ppm.

[0191] In some implementations, the element content can be selected as 2500ppm-6000ppm. Specifically, the values ​​can be 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 any range between any two of the above values.

[0192] By doping the lithium iron phosphate salts of the embodiments of this application with one or more of the above-mentioned elements and ensuring that their content is within the above-mentioned range, it is possible to achieve better bulk metal phase modification of the lithium iron phosphate salt particles while improving the primary average particle size, thereby further enhancing the bulk ion transport capability of the lithium iron phosphate salt particles and improving kinetic performance and specific capacity.

[0193] In some embodiments, a first pulverization is performed after the first sintering, and a second pulverization is performed after the second sintering, wherein...

[0194] The Dv50 of the product after the first pulverization is 300nm-1200nm, and can be selected as 400nm-1100nm;

[0195] The Dv50 of the product after the second pulverization is 500nm-5000nm, and can be selected as 700nm-2500nm.

[0196] In some embodiments, the Dv50 of the product after the first pulverization can be 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, or a value within the range of any two of the above-mentioned products after the first pulverization.

[0197] In some embodiments, the Dv50 of the product after the second pulverization 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, or 5000nm, or a value within the range of any two of the above-mentioned products after the second pulverization.

[0198] In some embodiments, pulverization includes one or more of mechanical crushing, grinding, sand milling, and air jet milling.

[0199] Controlling the Dv50 of the product after the first pulverization within the aforementioned range helps reduce the growth barrier effect of added carbon sources and potentially added dopants on the lithium iron phosphate precursor crystals, thus facilitating the preparation of micron-sized lithium iron phosphate precursor particles. Controlling the Dv50 value of the product after the second pulverization within the aforementioned range helps obtain lithium iron phosphate particles with the average particle size of the first pulverization step described in this application.

[0200] [Positive electrode plate]

[0201] This application provides a positive electrode sheet, which includes the positive electrode active material as described in this application or the positive electrode active material obtained by the preparation method described in this application.

[0202] In some embodiments, the compaction density of the positive electrode sheet is 3.0 g / cm³. 3 -3.5g / cm 3 .

[0203] In some embodiments, the compaction density of the positive electrode sheet can be 3.0 g / cm³. 3 3.1g / cm 3 3.2g / cm 3 3.3g / cm 3 3.4g / cm3 3.5g / cm 3 Or it can be a value within a range consisting of the compaction densities of any two of the above positive electrode plates.

[0204] The compaction density of the electrode can be tested using methods and equipment known in the art. As an example, when the electrode is single-sided coated, the compaction density of the film layer on one side of the electrode is m / (V1-V2), and when the electrode is double-sided coated, the compaction density of the film layer on one side of the electrode is 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 using a micrometer.

[0205] The compaction density of the positive electrode active material in this embodiment is 3.0 g / cm³. 3 -3.5g / cm 3 Higher compaction density is beneficial to improving the energy density of secondary batteries.

[0206] In some embodiments, the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material is the positive electrode active material of the present application embodiment or the positive electrode active material prepared by the preparation method of the present application embodiment.

[0207] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0208] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0209] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0210] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0211] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0212] [Negative electrode plate]

[0213] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0214] As an example, the negative electrode current collector has two surfaces opposite each other in its own 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.

[0215] 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0216] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. 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, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional 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.

[0217] In some embodiments, the negative electrode film layer may optionally 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).

[0218] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0219] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0220] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0221] [Electrolytes]

[0222] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0223] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0224] 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0225] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0226] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0227] [Isolation membrane]

[0228] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0229] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven 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.

[0230] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0231] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0232] 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. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0233] [Rechargeable Battery]

[0234] This application provides a secondary battery, which includes the positive electrode sheet according to the embodiments of this application.

[0235] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 3 This is an example of a square-structured secondary battery 5.

[0236] In some implementations, refer to Figure 4 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0237] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0238] Figure 5 This is battery module 4, used as an example. (See reference...) Figure 5 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0239] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0240] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0241] Figure 6 and Figure 7 This is battery pack 1 as an example. (See reference...) Figure 6 and Figure 7 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0242] [Electrical appliances]

[0243] In addition, this application also provides an electrical device, which includes a secondary battery according to the embodiments of this application.

[0244] In some embodiments, the electrical device of this application may further include at least one of a battery module or a battery pack. A secondary battery, battery module, or battery pack can be used as the power source for the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0245] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0246] Figure 8 This is an example of an electrical device. The device could be 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 for this device, a battery pack or battery module can be used.

[0247] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0248] Example

[0249] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0250] Example 1

[0251] I. Preparation Method

[0252] 1) Preparation of positive electrode active materials

[0253] Preparation of lithium iron phosphate salt particles:

[0254] Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (based on the total weight of lithium iron phosphate particles, with the amount of titanium dioxide added ensuring a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline were weighed separately. The weight ratios of Li, Fe, and P elements were: Fe:P = 0.968:1; Li:Fe = 1:0.98, and the weight ratio of glucose to polyaniline was: glucose:polyaniline = 1:2. The amounts of glucose and polyaniline added were such that, after the first sintering, the carbon content accounted for 0.15% of the weight of the lithium iron phosphate precursor. Water was added to the above substances to obtain a slurry mixture.

[0255] The mixture was homogenized using a ball mill and then ground using a sand mill to obtain a slurry with a solid content of 38% and a Dv50 of 400 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℃ to 360℃, and an outlet temperature of 100℃ to 140℃). The dried reactants were then loaded into a sintering furnace for the first sintering process. The heating rate was controlled at 5℃ / min, the holding temperature was controlled at 650℃, and the holding time was 4 hours. After cooling, the material was mechanically ground to obtain powder.

[0256] The obtained powder, glucose, polyaniline, and water were mixed (based on the total weight of lithium iron phosphate particles, the amount of glucose and polyaniline added met the requirement 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), with a solid content of 40%. After uniform mixing, the mixture was processed using a ball mill and a sand mill to obtain a slurry. The Dv50 of the insoluble matter in the slurry was 550 nm. The slurry was then spray-dried (the negative pressure of the high-speed spray dryer was -650 to -200 Pa, the inlet temperature was 300℃ to 360℃, and the outlet temperature was 100℃ to 140℃). The dried reactants were then loaded into a sintering furnace for a second low-temperature sintering (the heating rate was controlled at 5℃ / min, the sintering temperature was 750℃, and the sintering time was 4 h). After the material cooled, it was pulverized a second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate particles were obtained with a carbon content of 1.2% and a Ti element content of 5000 ppm. The obtained lithium iron phosphate particles were observed under a scanning electron microscope, and the results are shown in the attached figure. Figure 2 As shown.

[0257] The lithium nickel cobalt manganese oxide particles were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., model CPE-16. The BET value and Dv50 value are shown in Table 1.

[0258] The positive electrode active material was obtained by mixing lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles according to the mass listed in Table 1.

[0259] 2) Preparation of positive electrode sheet

[0260] 2.0 wt% polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone (NMP), and then 1.0 wt% Super P, 0.5 wt% carbon nanotubes and 96.5 wt% of the above cathode material were added and stirred to mix evenly to obtain cathode slurry;

[0261] The positive electrode slurry was prepared at 252 mg / 1540 mm. 2 The single-sided weight is evenly coated on the surface of the current collector aluminum foil, and then transferred to a vacuum drying oven for complete drying. The dried electrode is then rolled and punched to obtain the positive electrode.

[0262] 3) Preparation of negative electrode sheet

[0263] Artificial graphite (anode active material), carbon black (conductive agent), carboxymethyl cellulose (thickener), and styrene-butadiene rubber (binder) were added to deionized water in a mass ratio of 0.9735:0.007:0.01:0.0095 and stirred evenly to obtain the anode slurry.

[0264] Then the negative electrode slurry was prepared at 139 mg / 1540 mm. 2 The single-sided weight is evenly coated on copper foil, and after drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0265] 4) Preparation of the separating membrane

[0266] Polypropylene film is used as the separator.

[0267] 5) Battery manufacturing

[0268] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. Tabs are welded onto the bare cell, and the cell is placed in an aluminum casing and baked in a vacuum oven at 100°C for 8 hours. Electrolyte is then injected and the casing is sealed to obtain a non-charged battery. The non-charged battery is then subjected to a series of processes, including settling, hot and cold pressing, formation, shaping, and capacity testing, to obtain the lithium-ion battery of Example 1.

[0269] The batteries in Examples 2-5 were prepared using a method similar to that in Example 1, but the average particle size of the lithium iron phosphate salt particles was adjusted, as shown in Table 1. The differences between the preparation methods of Examples 2-5 and Example 1 are as follows:

[0270] Example 2: Reduce the Dv50 of the mixed slurry after the first grinding to 350nm;

[0271] Example 3: Increase the Dv50 of the mixed slurry after the first grinding to 1200nm;

[0272] Example 4: Reduce the Dv50 of the mixed slurry after the first grinding to 450nm;

[0273] Example 5: Increase the Dv50 of the mixed slurry after the first grinding to 1100nm.

[0274] The batteries in Examples 6-7 were prepared using a similar method to those in Example 1, but the specific surface area and carbon content of the lithium iron phosphate particles were adjusted, as shown in Table 1. The difference between the preparation methods of Examples 6-7 and Example 1 lies in:

[0275] Example 6: Adjust the carbon content during the second sintering process to 0.8%;

[0276] Example 7: The carbon content during the second sintering process was adjusted to 1.4%.

[0277] The batteries in Examples 8-9 were prepared using a method similar to that in Example 1, but the specific surface area of ​​the lithium iron phosphate salt particles was adjusted, as shown in Table 1. The difference between the preparation methods of Examples 8-9 and Example 1 lies in:

[0278] Example 8: During the first sintering process, the weight ratio of glucose to polyaniline was 1:4;

[0279] Example 9: During the first sintering process, the weight ratio of glucose to polyaniline was 1:0.5.

[0280] The batteries in Examples 10-11 were prepared using a similar method to those in Example 1, but the specific surface area of ​​lithium nickel cobalt manganese oxide was adjusted, as shown in Table 1. The lithium nickel cobalt manganese oxide particles in Example 10 were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., model CPE17; the lithium nickel cobalt manganese oxide particles in Example 11 were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., model CPE18.

[0281] The batteries in Examples 12-14 were prepared using a method similar to that in Example 1, but the doping amount of the Q element was adjusted, as shown in Table 1. The difference from Example 1 lies in the preparation of the lithium iron phosphate salt particles;

[0282] Example 12: The temperature of the first sintering was controlled at 590°C, the temperature of the second sintering was 720°C, and the Ti content was 0 ppm;

[0283] Example 13: The temperature of the first sintering was controlled at 620°C, the temperature of the second sintering was 740°C, and the Ti content was 2500 ppm;

[0284] Example 14: The temperature of the first sintering was controlled at 655°C, the temperature of the second sintering was controlled at 755°C, and the Ti content was 6000ppm.

[0285] The batteries in Examples 15-18 were prepared using a method similar to that in Example 1, but the Dv50 values ​​of the lithium iron phosphate or lithium nickel cobalt manganese oxide particles were adjusted, as shown in Table 1. The difference between the preparation methods of Examples 15-18 and Example 1 lies in:

[0286] Example 15: Controlling the Dv50 of the pulverized material after the second sintering to be 870m;

[0287] Example 16: Controlling the Dv50 of the pulverized material after the second sintering to be 3500 nm;

[0288] The lithium nickel cobalt manganese oxide particles in Example 17 were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., model CPE19;

[0289] The lithium nickel cobalt manganese oxide particles in Example 18 were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., model CPE20.

[0290] The batteries in Examples 19-20 were prepared using a method similar to that in Example 1, but the mass mixing ratio of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles was adjusted, as shown in Table 1.

[0291] The battery of Comparative Example 1 was prepared in a similar manner to that of Example 1, but the lithium iron phosphate particles used were different from those used in Example 1. The lithium iron phosphate particles used in Comparative Example 1 were purchased from Hunan Yuneng New Energy Battery Materials Co., Ltd., model CPF-087-1, and the specific parameters are shown in Table 1.

[0292] The battery of Comparative Example 2 was prepared in a similar manner to that of Example 1, but the lithium iron phosphate particles used were different from those used in Example 1. The lithium iron phosphate particles used in Comparative Example 2 were purchased from Hunan Yuneng New Energy Battery Materials Co., Ltd., model CPF-087-2, and the specific parameters are shown in Table 1.

[0293] The battery of Comparative Example 3 was prepared using a similar method to that of Example 1, but the primary average particle size and specific surface area of ​​the lithium iron phosphate salt particles were adjusted, as shown in Table 1. The preparation method of Comparative Example 2 differed from that of Example 1 in that Comparative Example 2 used only a single sintering process at a sintering temperature of 750°C, and the carbon content of the sintered product was 1.2%.

[0294] The battery of Comparative Example 4 was prepared using a method similar to that of Example 1, but the primary average particle size, specific surface area, and carbon content of the lithium iron phosphate particles were adjusted, as shown in Table 1. The preparation method of Comparative Example 3 differed from that of Example 1 in that the carbon content in the second sintering was reduced to 0.7%, thereby adjusting the BET and primary average particle size.

[0295] II. Battery Performance Testing

[0296] 1. Performance testing of positive electrode active materials

[0297] 1) Method for testing the average particle size of lithium iron phosphate salt particles in one step

[0298] The electrode was cut open perpendicular to the large surface of the positive electrode using an argon ion beam, exposing the cross-section. Scanning electron microscopy (SEM) images were taken of the cross-section, and the particle size of lithium iron phosphate (LFP) salts was statistically analyzed using a major diameter statistical method. Specifically, the total number of LFP salt particles with a primary diameter greater than 80 nm and the primary diameter of these particles were counted from the SEM images. The primary average particle size of LFP salts was calculated as: primary average particle size = total primary diameter of LFP salts / total number of LFP salt particles. Particles with a primary diameter between 0 and 80 nm were excluded from the primary diameter statistics.

[0299] 2) Dv50 value test method

[0300] Lithium iron phosphate particles: Refer to GB / T19077.1-2016 and use a laser particle size analyzer (MalvernMaster Size 3000) to determine the Dv50 value of lithium iron phosphate particles.

[0301] Lithium nickel cobalt manganese oxide particles: Refer to GB / T19077.1-2016 and use a laser particle size analyzer (MalvernMaster Size 3000) to determine the Dv50 value of lithium nickel cobalt manganese oxide particles.

[0302] 3) Specific surface area (BET) test method

[0303] According to the GB / T19587-2017 standard test method, the specific surface area of ​​lithium iron phosphate particles and nickel cobalt manganese oxide particles was tested by gas adsorption method, as follows: Lithium iron phosphate particles or nickel cobalt manganese oxide particles were taken as samples, and the sample tubes were immersed in liquid nitrogen at -196℃. The amount of nitrogen adsorbed on the solid surface under different pressures of 0.05-0.30 was measured. Based on the BET multilayer adsorption theory and its formula, the monolayer adsorption amount of the sample was obtained, and the specific surface area of ​​the sample was calculated.

[0304] 4) Test method for carbon content of lithium iron phosphate particles

[0305] After burning lithium iron phosphate particles in a high-frequency induction furnace, their carbon content was tested by infrared absorption method. The specific testing procedure was in accordance with standard GB / T 20123-2006 / ISO 15350:2000.

[0306] 5) Test method for Q element (e.g., Ti element) doping content of lithium iron phosphate salt particles

[0307] The test method for the Q element content of lithium iron phosphate salt particles shall be in accordance with GB / T 33822-2017.

[0308] 6) Test method for solid content of positive electrode slurry

[0309] Electronic balance (accuracy 0.0001), oven, glass drying tray. Take 8-10g of positive electrode slurry sample and evenly spread it 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 oven and heat it at 130℃ for 5 hours. After drying and cooling, remove it and record the mass of the positive electrode slurry after drying. Repeat the drying process multiple times until the mass of the positive electrode slurry sample reaches constant weight, and record the mass as B. Solid content of the positive electrode slurry = (A / B) × 100%.

[0310] 7) Test method for the filtration performance of positive electrode slurry

[0311] Fold the 200-mesh filter into an inverted triangle and place it in a 500ml beaker. Pour in 500ml of the prepared positive electrode slurry (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 300ml.

[0312] 8) Measurement of η value of lithium iron phosphate particles

[0313] First, coin cells were prepared using lithium iron phosphate (LFP) particles as the positive electrode active material. The specific preparation process was as follows: 2.0000g of LFP particles were mixed with 0.1111g of conductive carbon black and 0.1111g of polyvinylidene fluoride (PVDF), and then added to 2.5g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry was formed. The slurry was coated onto aluminum foil to a thickness of 140 micrometers, dried under vacuum at 120℃ for 2 hours, and then punched into discs with a diameter of 13mm. The discs were then pressed using a tablet press at 10 MPa and vacuum-treated at 120℃ for 12 hours to obtain the positive electrode sheet. The weight of the positive electrode sheet was measured, revealing a LFP particle loading of 11-12mg. A button cell battery is assembled in an argon-protected glove box, with a lithium metal sheet as the negative electrode, a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent as the electrolyte, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0314] The prepared coin cells were tested for electrical performance using a blue-light tester. Specifically, the coin cells were charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by one charge and discharge cycle at a constant current rate of 1C. In 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.

[0315] 2. Battery performance testing

[0316] 1) Battery capacity

[0317] Place the battery in a 25°C oven 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 it at a constant voltage of 4.35V until the cutoff current is 0.05C; pause for 5 minutes.

[0318] Discharge the battery at a constant current of 1 / 3C to 2.0V. This step is the actual cell capacity test. Combined with the mass of the positive electrode active material, the specific capacity of the battery can be calculated. Specific capacity = capacity / mass of positive electrode active material.

[0319] 2) Test method for compacted density of positive electrode sheet

[0320] When the electrode is coated on one side only, the compaction density of the film layer on one side of the electrode is m / (V1-V2). When the electrode is coated on both sides only, the compaction density of the film layer on one side of the electrode is 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 of the electrode, V1. 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.

[0321] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0322] Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The parameters of the positive electrode active material are shown in Table 1, and the performance test results are shown in Table 2.

[0323]

[0324] Table 2 Performance Test Results

[0325]

[0326] Based on the results in the table above, it can be seen from Examples 1-20 and Comparative Example 1 that increasing the primary average particle size of lithium iron phosphate particles and reducing their specific surface area can improve the solid content and filtration performance of the cathode slurry, which is beneficial for improving processing issues. It can be seen from Examples 1-20 and Comparative Example 2 that reducing the specific surface area of ​​lithium iron phosphate particles is beneficial for improving the solid content and filtration performance of the cathode slurry, which is beneficial for improving processing issues. It can be seen from Examples 1-18 and Comparative Example 3 that the primary average particle size of the lithium iron phosphate particles in this application is beneficial for maintaining a good specific capacity. It can be seen from Examples 1-18 and Comparative Example 4 that when the BET of lithium iron phosphate particles is too small, the specific capacity of the secondary battery will decrease. In summary, when the primary average particle size of lithium iron phosphate particles is 500-3000 nm and the specific surface area BET is 3 m², the desired effect is achieved. 2 / g-8m 2 / g, the specific surface area (BET) of lithium nickel cobalt manganese oxide particles is 0.4m². 2 / g-2.0m 2 / g can improve the processing performance of the positive electrode slurry while taking into account the specific capacity of the secondary battery.

[0327] From the appendix Figure 1 As shown in a and 1b, the lithium nickel cobalt manganese oxide particles and lithium iron phosphate salt particles are uniformly mixed, and the lithium iron phosphate salt particles densely fill the pores of the lithium nickel cobalt manganese oxide particles. Figure 2 The scanning electron microscope image shows the microstructure of the lithium iron phosphate particles of Example 1 of this application. As can be seen from the image, the lithium iron phosphate of Example 1 of this application exhibits the morphology of large single crystals.

[0328] As can be seen from Examples 1-5, when the primary average particle size of lithium iron phosphate particles is controlled to be 500-3000 nm, the processing performance of the cathode slurry can be improved while the specific capacity of the secondary battery can be taken into account.

[0329] As shown in Examples 1, 6-7, and 10-11, the BET value of lithium iron phosphate salt 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 At / g, the specific capacity of the secondary battery can be considered while improving the processing performance of the positive electrode slurry.

[0330] As can be seen from Examples 1 and 8-9, increasing the proportion of carbon source film-forming agent is beneficial to achieving dense carbon coating. The density of carbon coating in this application is beneficial to further improving the kinetic performance and specific capacity of the secondary battery.

[0331] As demonstrated in Examples 1 and 12-14, adding a metal element, 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 1000-10000 ppm, is beneficial for further improving kinetic performance while minimizing the impact on the specific capacity of the lithium iron phosphate particles.

[0332] As can be seen from Examples 1 and 15-18, when the Dv50 of lithium iron phosphate particles is 500-5000nm and the Dv50 of 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.

[0333] As can be seen from Examples 1 and 19-20, controlling the mass percentage of lithium iron phosphate particles and lithium nickel cobalt manganese oxide particles within a suitable 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%, it helps to further balance the processing performance, electrical performance and cost of the positive electrode active material.

[0334] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this 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 diameter of the lithium iron phosphate salt particles is 500-3000 nm, the specific surface area BET of the lithium iron phosphate salt particles is 3 m 2 / g-7 m 2 / g, and the specific surface area BET of the lithium nickel cobalt manganese oxide particles is 0.4 m 2 / g-2.0 m 2 / g. Wherein, the lithium iron phosphate salt particles are primary particles; Based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is Cx by weight%, and the ratio z of the BET of the lithium iron phosphate particles to Cx satisfies 3 ≤ z ≤ 6.

2. The positive electrode active material according to claim 1, characterized in that, 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 Mn 1-x2-y2-z2 O2, wherein R includes at least one of Cr, Ti, V, Mg, Al, and Nb, and 0 ≤ x2 < 1, 0 < y2 < 1, 0 < z2 < 1, 0 < x2 + y2 + z2 < 1.

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

5. The positive electrode active material according to claim 1, characterized in that, Based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is 0.8%-2.0% by weight.

6. The positive electrode active material according to claim 1, characterized in that, Based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is 1.0%-1.6% by weight.

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

8. The positive electrode 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 positive electrode active material according to any one of claims 1-7, characterized in that, The Dv50 of the lithium nickel cobalt manganese oxide particles is 1200-20000 nm.

10. The positive electrode active material according to any one of claims 1-7, characterized in that, The Dv50 of the lithium iron phosphate salt particles is 870-1500nm, and the Dv50 of the lithium nickel cobalt manganese oxide particles is 1800-4200nm.

11. The positive electrode active material according to any one of claims 1-7, characterized in that, The lithium iron phosphate salt particles comprise 5%-80% of the total mass of the positive electrode active material.

12. The positive electrode active material according to any one of claims 1-7, characterized in that, 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.

13. The positive electrode active material according to any one of claims 1-7, characterized in that, The capacity percentage of the lithium iron phosphate salt particles η is ≥ 88%, where η is defined as: The battery with the 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, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the 1C rate, the capacity value extracted at the discharge voltage of 3.2V was recorded as C1, and the capacity value extracted at the discharge voltage of 2.0V was recorded as C2, and η=C1 / C2; wherein, the charging process includes constant voltage charging, constant voltage of 3.75V, and constant voltage cutoff current of 50uA.

14. The positive electrode active material according to any one of claims 1-7, 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 is ≥200nm; b) The Dv90 of the lithium iron phosphate salt particles is ≤10000m; c) The Dv99 of the lithium iron phosphate salt particles is ≤12000nm; d) The compacted density of the lithium iron phosphate powder under 3T pressure is ≥2.25 g / cm³. 3 ; e) The resistivity of the lithium iron phosphate powder is less than 60 Ω·cm.

15. A method for preparing a positive electrode active material, characterized in that, The preparation method includes: mixing lithium iron phosphate salt particles and lithium nickel cobalt manganese oxide particles. The lithium iron phosphate salt particles are mainly obtained by the following preparation methods: The raw materials provided contain at least lithium source, iron source, phosphorus source, carbon source, carbon film-forming agent, and modifier, and are subjected to at least two sintering processes. The temperature for the first sintering is 500℃-760℃; The second sintering temperature is 700℃-800℃; The lithium iron phosphate salt particles are primary particles; The carbon content of the material after the first sintering is 0.01%-0.79% by weight. After the second sintering, the carbon content of the material is 0.8%-1.2% by weight. The primary average particle size of the lithium iron phosphate salt particles is 500-3000 nm, and the specific surface area (BET) of the lithium iron phosphate salt particles is 3 m². 2 / g-7m 2 / g, the specific surface area (BET) of the lithium nickel cobalt manganese oxide particles is 0.4m². 2 / g-2.0m 2 / g; Based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is Cx by weight%, and the ratio z of the BET of the lithium iron phosphate particles to Cx satisfies 3 ≤ z ≤ 6.

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

17. The method for preparing the positive electrode active material according to claim 15 or 16, characterized in that, After the first sintering, a first pulverization is performed; after the second sintering, a second pulverization is performed. The Dv50 of the product after the first pulverization is 300nm-1200nm; The Dv50 of the product after the second pulverization is 500nm-5000nm.

18. A positive electrode plate, characterized in that, The positive electrode sheet comprises the positive electrode active material as described in any one of claims 1-14 or the positive electrode active material obtained by the preparation method of the positive electrode active material as described in any one of claims 15-17.

19. The positive electrode sheet according to claim 18, characterized in that, The compaction density of the positive electrode sheet is 3.0 g / cm³. 3 -3.5g / cm 3 .

20. A secondary battery, characterized in that, The secondary battery includes the positive electrode as described in claim 18 or 19.

21. An electrical appliance, characterized in that, The electrical device includes the secondary battery as described in claim 20.

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