Composite positive electrode active material and preparation method thereof, secondary battery and electric device

Abstract

The invention provides a composite positive electrode active material and a preparation method thereof, a secondary battery and an electric device, and belongs to the technical field of secondary batteries. The composite positive electrode active material comprises lithium iron phosphate containing doping elements and a coating layer coating the surface of the lithium iron phosphate containing the doping elements, the coating layer comprises a first carbon layer, a fast ion conductor layer and a second carbon layer, the first carbon layer is close to the lithium iron phosphate containing the doping elements, and the second carbon layer is close to the fast ion conductor layer. The first carbon layer is far away from lithium iron phosphate containing doping elements, the second carbon layer is far away from lithium iron phosphate containing doping elements, the fast ion conductor layer is located between the first carbon layer and the second carbon layer, the first carbon layer and the second carbon layer are continuous film layers, and the fast ion conductor layer is a continuous film layer or a discontinuous film layer. The composite positive electrode active material has good conductivity and excellent specific capacity.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, specifically relating to a composite positive electrode active material and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Secondary batteries are widely used due to their advantages such as high voltage, good cycle performance, high energy density, and no memory effect. The cathode material is a crucial factor determining the energy density, power density, price, and safety of secondary batteries. Among them, secondary batteries using lithium iron phosphate (LFP) as the cathode material have rapidly become a hot topic in secondary battery research due to their high capacity, low price, and environmental friendliness. However, due to inherent structural limitations, LFP suffers from low electronic and ionic conductivity, leading to low conductivity and high internal resistance in the cathode material. To improve this, metal or carbon elements are typically doped into the LFP matrix, or a continuous carbon coating layer and / or a continuous fast-ion conductor layer are coated onto the LFP matrix.

[0003] However, the conductivity of current lithium iron phosphate cathode active materials is still relatively low, the internal resistance is still relatively high, and there is still room for further improvement in specific capacity. Summary of the Invention

[0004] This application aims to at least partially address one of the technical problems in the related art. To this end, embodiments of this application propose a composite positive electrode active material, a method for preparing the same, a secondary battery, and an electrical device.

[0005] In a first aspect, this application provides a composite positive electrode active material, comprising lithium iron phosphate containing doped elements and a coating layer covering the surface of the lithium iron phosphate containing doped elements. The coating layer comprises a first carbon layer, a fast ion conductor layer and a second carbon layer. The first carbon layer is close to the lithium iron phosphate containing doped elements, and the second carbon layer is far from the lithium iron phosphate containing doped elements. The fast ion conductor layer is located between the first carbon layer and the second carbon layer. The first carbon layer and the second carbon layer are continuous films, and the fast ion conductor layer is either a continuous film or a discontinuous film.

[0006] In some embodiments, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the doping element is 0.12–0.72 wt.%, and / or the mass percentage of lithium iron phosphate is 97.5–98.5 wt.%, and / or the mass percentage of the first carbon layer is 0.01–0.5 wt.%, and / or the mass percentage of the fast ion conductor layer is 0.1–3 wt.%, and / or the mass percentage of the second carbon layer is 0.8–1.3 wt.%.

[0007] In some embodiments, the fast ion conductor layer has a mass percentage of 0.1 to 0.5 wt.%.

[0008] In some embodiments, the fast ion conductor layer is a discontinuous film layer in which fast ion conductors are distributed in an island-like pattern.

[0009] In some embodiments, the average particle size of the primary particles of the lithium iron phosphate containing doped elements is 50–2000 nm, and / or the average particle size of the primary particles of the composite positive electrode active material is 100–2500 nm.

[0010] In some embodiments, the thickness of the first carbon layer is 0.01 to 1 nm, and / or the thickness of the second carbon layer is 1 to 10 nm.

[0011] Secondly, this application provides a method for preparing a composite positive electrode active material, comprising the following steps:

[0012] S1. Mix lithium source, iron source, phosphorus source, dopant compound and first carbon source to obtain first dry powder; subject the first dry powder to first sintering treatment under inert atmosphere to obtain first pre-product.

[0013] S2. The first preform and the fast ion conductor are mixed to obtain the second dry powder; the second dry powder is subjected to a second sintering treatment under an inert atmosphere to obtain the second preform.

[0014] S3. The second preform and the second carbon source are mixed to obtain the third dry powder; the third dry powder is subjected to a third sintering treatment under an inert atmosphere to obtain the composite positive electrode active material.

[0015] In some embodiments, the lithium source is at least one of lithium phosphate, lithium hydrogen phosphate, and lithium dihydrogen phosphate.

[0016] In some embodiments, the mass ratio of the first preform to the fast ion conductor is 1:(0.001 to 0.05).

[0017] In some embodiments, the mass ratio of the first preform to the fast ion conductor is 1:(0.001 to 0.005).

[0018] Thirdly, this application provides a secondary battery, wherein the positive electrode of the secondary battery includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, and the positive electrode film layer includes the composite positive electrode active material described in the first aspect.

[0019] Fourthly, this application provides an electrical device including the secondary battery described in the third aspect.

[0020] The advantages and technical effects of the composite positive electrode active material in this application are as follows:

[0021] (1) In the composite positive electrode active material of this application, the doping elements, the first carbon layer and the second carbon layer in lithium iron phosphate can improve the electronic conductivity of the composite positive electrode active material, and the fast ion conductor layer can improve the ionic conductivity of the composite positive electrode active material. The above two aspects can jointly improve the conductivity of the composite positive electrode active material, reduce its internal resistance, and thus improve its specific capacity.

[0022] (2) In the composite positive electrode active material of this application, the fast ion conductor layer is located between the first carbon layer and the second carbon layer. This structure of the coating layer is more effective in improving the conductivity of the composite positive electrode active material, reducing its internal resistance, and ultimately increasing its specific capacity than the coating structure in related technologies where the inner layer is a carbon layer and the outer layer is a fast ion conductor layer or the inner layer is a fast ion conductor layer and the outer layer is a coating layer.

[0023] (3) In the composite positive electrode active material of this application, the fast ion conductor layer can be a continuous film layer or a discontinuous film layer. When the fast ion conductor layer is a discontinuous film layer, the first carbon layer and the second carbon layer can be in contact with each other. Therefore, in this case, electron transport channels can be formed inside the lithium iron phosphate particles, between the particles and the particle surface, and between particles. This can more effectively improve the conductivity of the composite positive electrode active material and reduce its internal resistance than when the fast ion conductor layer is a continuous film layer, and ultimately further improve its specific capacity. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the composite positive electrode active material of this application;

[0025] Figure 2 This is a SEM image of the composite positive electrode active material of Example 1 of this application;

[0026] Figure 3 This is a SEM image of the composite positive electrode active material of Example 2 of this application;

[0027] Explanation of reference numerals in the attached figures:

[0028] 1- Lithium iron phosphate containing doped elements; 2- First carbon layer; 3- Fast ion conductor layer; 4- Second carbon layer. Detailed Implementation

[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0031] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0032] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0033] 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 understood that ranges of 60–110 and 80–120 are also expected. 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 "a–b" 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.

[0034] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0035] Firstly, embodiments of this application provide a composite positive electrode active material, such as... Figure 1 As shown, it includes a lithium iron phosphate 1 containing doped elements and a coating layer covering the surface of the lithium iron phosphate 1 containing doped elements. The coating layer includes a first carbon layer 2, a fast ion conductor layer 3 and a second carbon layer 4. The first carbon layer 2 is close to the lithium iron phosphate 1 containing doped elements, and the second carbon layer 4 is far away from the lithium iron phosphate 1 containing doped elements. The fast ion conductor layer 3 is located between the first carbon layer 2 and the second carbon layer 4. The first carbon layer 2 and the second carbon layer 4 are continuous films. The fast ion conductor layer 3 at least partially covers the surface of the first carbon layer.

[0036] In the composite positive electrode active material of this application embodiment, the doping elements, the first carbon layer and the second carbon layer in lithium iron phosphate can improve the electronic conductivity of the composite positive electrode active material, and the fast ion conductor layer can improve the ionic conductivity of the composite positive electrode active material. The above two aspects can jointly improve the conductivity of the composite positive electrode active material, reduce its internal resistance, and thus improve its specific capacity.

[0037] In the composite positive electrode active material of this application embodiment, the fast ion conductor layer is located between the first carbon layer and the second carbon layer. This structure of the coating layer is more effective in improving the conductivity of the composite positive electrode active material, reducing its internal resistance, and ultimately increasing its specific capacity than the coating structure in related technologies where the inner layer is a carbon layer and the outer layer is a fast ion conductor layer or the inner layer is a fast ion conductor layer and the outer layer is a coating layer.

[0038] In the composite cathode active material of this application embodiment, the fast ion conductor layer can be a continuous film layer, that is, completely covering the surface of the first carbon layer, or it can be a discontinuous film layer, that is, only partially covering the surface of the first carbon layer. When the fast ion conductor layer is a discontinuous film layer, the first carbon layer and the second carbon layer can be in contact with each other. Therefore, in this case, electron transport channels can be formed inside the lithium iron phosphate particles, between the particle interior and the particle surface, and between particles. This can more effectively improve the conductivity of the composite cathode active material, reduce its internal resistance, and ultimately further improve its specific capacity than when the fast ion conductor layer is a continuous film layer.

[0039] It should be understood that the first dry powder, first preform, second dry powder, second preform, third dry powder, and negative electrode active material in the embodiments of this application can all include primary particles and / or secondary particles formed by the agglomeration of primary particles. The average particle size in the embodiments of this application refers to the average diameter of a single particle, which can be obtained statistically by high-magnification scanning electron microscopy images.

[0040] In some embodiments, the fast ion conductor layer is a discontinuous film layer in which the fast ion conductors are distributed in an island-like pattern. When the fast ion conductors are distributed in an island-like pattern, the distribution on the surface of the first carbon layer is relatively more uniform, and the distribution of the second carbon layer in contact with the first carbon layer through the exposed portion of the fast ion conductor layer is also relatively more uniform. Therefore, it is more beneficial to improve the conductivity of the composite positive electrode active material, reduce its internal resistance, and thus further improve its specific capacity.

[0041] In some embodiments, based on a total mass of 100 wt.% of the composite cathode active material, the mass percentage of the dopant element is 0.12–0.72 wt.%, for example, 0.12 wt.%, 0.22 wt.%, 0.32 wt.%, 0.42 wt.%, 0.52 wt.%, 0.62 wt.%, 0.72 wt.%, etc. When the mass percentage of the dopant element is too low, it is not conducive to the formation of electron transport channels within the lithium iron phosphate particles, thus hindering the improvement of the conductivity of the composite cathode active material, hindering the reduction of its internal resistance, and ultimately hindering the improvement of its specific capacity. When the mass percentage of the dopant element is too high, the mass percentage of lithium iron phosphate may be too low, resulting in a low content of active material, which is also not conducive to improving the specific capacity of the composite cathode active material.

[0042] In some embodiments, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of lithium iron phosphate is 97.5–98.5 wt.%, for example, 97.5 wt.%, 97.6 wt.%, 97.7 wt.%, 97.8 wt.%, 97.9 wt.%, 98.0 wt.%, 98.1 wt.%, 98.2 wt.%, 98.3 wt.%, 98.4 wt.%, 98.5 wt.%, etc. When the mass percentage of lithium iron phosphate is too low, the content of active material is too low, which is not conducive to improving the specific capacity of the composite positive electrode active material. When the mass percentage of lithium iron phosphate is too high, the mass percentages of doping elements, the first carbon layer, the fast ion conductor layer, and the second carbon layer may be too low, which is not conducive to improving the conductivity of the composite positive electrode active material, nor is it conducive to improving its specific capacity.

[0043] In some embodiments, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the first carbon layer is 0.01–0.5 wt.%, for example, 0.01 wt.%, 0.05 wt.%, 0.1 wt.%, 0.15 wt.%, 0.2 wt.%, 0.25 wt.%, 0.3 wt.%, 0.35 wt.%, 0.4 wt.%, 0.45 wt.%, 0.5 wt.%, etc. When the mass percentage of the first carbon layer is too low, it is not conducive to the formation of electron transport channels between the interior and surface of the lithium iron phosphate particles, thereby hindering the improvement of the conductivity of the composite positive electrode active material, hindering the reduction of its internal resistance, and ultimately hindering the improvement of its specific capacity. When the mass percentage of the first carbon layer is too high, the mass percentage of lithium iron phosphate may be too low, resulting in a low content of active material, which is also not conducive to improving the specific capacity of the composite positive electrode active material.

[0044] In some embodiments, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the fast ion conductor layer can be 0.1 to 3 wt.%.

[0045] Optionally, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the fast ion conductor layer is 1–3 wt.%, for example, 1 wt.%, 1.2 wt.%, 1.4 wt.%, 1.6 wt.%, 1.8 wt.%, 2 wt.%, 2.2 wt.%, 2.4 wt.%, 2.6 wt.%, 2.8 wt.%, 3 wt.%, etc. When the mass percentage of the fast ion conductor layer is within this range, the fast ion conductor layer is a continuous film.

[0046] Optionally, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the fast ion conductor layer is 0.1–0.5 wt.%, for example, 0.1 wt.%, 0.15 wt.%, 0.2 wt.%, 0.25 wt.%, 0.3 wt.%, 0.35 wt.%, 0.4 wt.%, 0.45 wt.%, 0.5 wt.%, etc. When the mass percentage of the fast ion conductor layer is within this range, the fast ion conductor layer is a discontinuous film layer. When the mass percentage of the fast ion conductor layer is too low, it is not conducive to improving the ionic conductivity of the composite positive electrode active material, thus not conducive to improving the conductivity of the composite positive electrode active material, not conducive to reducing its internal resistance, and ultimately not conducive to improving its specific capacity. When the mass percentage of the fast ion conductor layer is too high, it is easy to form a continuous fast ion conductor layer. Compared to forming a continuous fast ion conductor layer, a discontinuous fast ion conductor layer does not significantly reduce the ionic conductivity of the composite cathode active material. However, due to the contact between the first and second carbon layers, it significantly improves the electronic conductivity of the composite cathode active material, thus being more conducive to improving the conductivity of the composite cathode active material, reducing its internal resistance, and ultimately improving its specific capacity.

[0047] In some embodiments, based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the second carbon layer is 0.8–1.3 wt.%, for example, 0.8 wt.%, 0.9 wt.%, 1 wt.%, 1.1 wt.%, 1.2 wt.%, 1.3 wt.%, etc. When the mass percentage of the second carbon layer is too low, it is not conducive to the formation of electron transport channels between lithium iron phosphate particles, thereby hindering the improvement of the conductivity of the composite positive electrode active material, hindering the reduction of its internal resistance, and ultimately hindering the improvement of its specific capacity. When the mass percentage of the second carbon layer is too high, the mass percentage of lithium iron phosphate may be too low, resulting in a low content of active material, which is also not conducive to improving the specific capacity of the composite positive electrode active material.

[0048] In some embodiments, the average particle size of the doped lithium iron phosphate is 50–2000 nm, such as 50 nm, 100 nm, 150 nm, 200 nm, 400 nm, 600 nm, 1000 nm, 1500 nm, 2000 nm, etc. When the average particle size of the doped lithium iron phosphate is too small, it is not conducive to the dispersion of lithium iron phosphate, and it is easy to form agglomeration between small particles, which affects both the carbon layer coating quality and the material compaction density. When the average particle size of the doped lithium iron phosphate is too large, it is not conducive to the transport of lithium ions in lithium iron phosphate, and ultimately it is not conducive to improving its specific capacity.

[0049] In some embodiments, the thickness of the first carbon layer is 0.01–1 nm, such as 0.01 nm, 0.1 nm, 0.2 nm, 0.3 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1 nm, etc. When the thickness of the first carbon layer is too small, it hinders the transport of electrons between the interior and surface of the lithium iron phosphate, thus negatively impacting the conductivity of the composite cathode active material, reducing its internal resistance, and ultimately hindering the improvement of its specific capacity. When the thickness of the first carbon layer is too large, the overall thickness of the coating layer may be too large, and the content of active material may be too low, which is also detrimental to improving the specific capacity of the composite cathode active material.

[0050] In some embodiments, the thickness of the second carbon layer is 1–10 nm, such as 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, etc. When the thickness of the second carbon layer is too small, it hinders the transport of electrons between lithium iron phosphate particles, thus negatively impacting the conductivity of the composite cathode active material, reducing its internal resistance, and ultimately hindering the improvement of its specific capacity. When the thickness of the second carbon layer is too large, the overall thickness of the coating layer may be too large, and the content of active material may be too low, which also hinders the improvement of the specific capacity of the composite cathode active material.

[0051] In some embodiments, the average particle size of the primary particles of the composite positive electrode active material is 100–2500 nm, such as 100 nm, 150 nm, 200 nm, 400 nm, 600 nm, 1000 nm, 1500 nm, 2000 nm, 2200 nm, 2500 nm, etc. When the average particle size of the primary particles of the composite positive electrode active material is too small, it is not conducive to electrolyte wetting, which can easily lead to high internal resistance of the battery and is not conducive to improving the specific capacity of the composite positive electrode active material. When the average particle size of the primary particles of the composite positive electrode active material is too large, it is not conducive to lithium ion transport and is not conducive to improving the specific capacity of the composite positive electrode active material.

[0052] In some embodiments, the doping element is at least one selected from Mg, Nb, Ti, Zr, V, N, and F. These types of doping elements are beneficial for improving the electronic conductivity of the composite cathode active material, thereby increasing its electrical conductivity, reducing its internal resistance, and ultimately improving the specific capacity of the composite cathode active material.

[0053] In some embodiments, the fast ion conductor is at least one selected from lithium aluminum titanium phosphate, lithium vanadate, lithium titanate, lithium lanthanum titanium oxide, lithium lanthanum zirconium oxide, and lithium silicon vanadium oxide. These types of fast ion conductors are beneficial for improving the ionic conductivity of the composite cathode active material, thereby increasing its conductivity, reducing its internal resistance, and ultimately increasing the specific capacity of the composite cathode active material.

[0054] Secondly, embodiments of this application provide a method for preparing a composite positive electrode active material, comprising the following steps:

[0055] S1. Mix lithium source, iron source, phosphorus source, dopant compound and first carbon source to obtain first dry powder; subject the first dry powder to first sintering treatment under inert atmosphere to obtain first pre-product.

[0056] S2. The first preform and the fast ion conductor are mixed to obtain the second dry powder; the second dry powder is subjected to a second sintering treatment under an inert atmosphere to obtain the second preform.

[0057] S3. The second preform and the second carbon source are mixed to obtain the third dry powder; the third dry powder is subjected to a third sintering treatment under an inert atmosphere to obtain the composite positive electrode active material.

[0058] In step S1 of the preparation method of this application embodiment, the first carbon source serves two purposes: firstly, as a reducing agent to ensure that the lithium source, iron source, phosphorus source, and dopant compound react to generate lithium iron phosphate containing dopant elements after the first sintering treatment; secondly, as a carbon source for forming the first carbon layer. Therefore, the first preform is a lithium iron phosphate / first carbon layer structure containing dopant elements.

[0059] In step S2 of the preparation method of this application embodiment, after the fast ion conductor undergoes a second sintering treatment, a fast ion conductor layer can be formed on the surface of the first preform, that is, the second preform is a lithium iron phosphate / first carbon layer / fast ion conductor layer structure containing doped elements.

[0060] In step S3 of the preparation method of this application embodiment, the second carbon source can form a second carbon layer on the surface of the second preform after the third sintering treatment. That is, the composite positive electrode active material is a lithium iron phosphate / first carbon layer / fast ion conductor layer / second carbon layer structure containing doped elements.

[0061] The preparation method of this application embodiment does not particularly limit the type of lithium source mentioned in step S1, and it can be at least one of lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, lithium carbonate, lithium acetate, and lithium hydroxide. In some embodiments, the lithium source can be at least one of lithium phosphate, dilithium hydrogen phosphate, and lithium dihydrogen phosphate. These lithium sources also serve as phosphorus sources, and they are more likely to react to form lithium iron phosphate, thus increasing the yield of the composite positive electrode active material and reducing its production cost.

[0062] The preparation method of this application embodiment does not have any particular limitation on the type of iron source mentioned in step S1. For example, the iron source can be at least one of anhydrous ferric phosphate, ferric phosphate dihydrate, ferric oxide, ferric oxide and elemental iron powder.

[0063] The preparation method of this application embodiment does not have any particular limitation on the type of phosphorus source mentioned in step S1. For example, the phosphorus source can be at least one of anhydrous iron phosphate, iron phosphate dihydrate, phosphoric acid, ammonium dihydrogen phosphate, diamine hydrogen phosphate, lithium dihydrogen phosphate, lithium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0064] The preparation method of this application embodiment does not have any particular limitation on the type of dopant compound in step S1, as long as the target dopant element can be provided. For example, the dopant compound can be at least one of Mg, Nb, Ti, Zr, V, N and F. The dopant compound can be at least one of oxides, salts, hydroxides, etc. containing the dopant element, and is generally a compound containing the above-mentioned dopant element and elements such as C, H, O, and N.

[0065] The first carbon source in step S1 of the preparation method of this application embodiment is mainly to provide a reducing atmosphere to ensure that trivalent iron can be converted into divalent iron to obtain lithium iron phosphate. The preparation method of this application embodiment does not have particular limitations on the type of the first carbon source in step S1. The first carbon source can be at least one of glucose, granulated sugar, citric acid, sucrose, polyethylene glycol, phenolic resin, polyvinylpyrrolidone, polyphenylene ether, polyacrylamide, and polyethylene glycol. Since the second carbon source in step S3 involves carbon coating on the particle surface, in order to improve the conductivity of the composite positive electrode active material, the second carbon source can be at least one of phenolic resin, polyvinylpyrrolidone, polyphenylene ether, polyacrylamide, and polyethylene glycol.

[0066] In some embodiments, the lithium source, the iron source, the phosphorus source, and the dopant compound in step S1 are mixed in a molar ratio of lithium, iron, phosphorus, and dopant of (1.02–1.06):1:(1.02–1.05):(0.01–0.02). The amounts of these raw materials within this range are beneficial for the composite cathode active material to form stable channels conducive to lithium-ion transport in its structure.

[0067] In some embodiments, the mass ratio of the iron source to the first carbon source in step S1 is 1:(0.04~0.08). When the mass ratio of the iron source to the first carbon source is too small, the thickness of the first carbon layer is too large. When the mass ratio of the iron source to the first carbon source is too large, the thickness of the first carbon layer is too small.

[0068] The preparation method of this application embodiment does not have any particular limitation on the mixing method described in step S1, as long as the various raw materials are mixed evenly. For example, a dry mixing followed by pulverization method can be used, or a wet sand milling followed by spray drying method can be used. In some embodiments, the mixing of lithium source, iron source, phosphorus source and dopant compound in step S1 to obtain the first dry powder specifically involves: mixing lithium source, iron source, phosphorus source and dopant compound, and then pulverizing and mixing them using a universal crusher to obtain the first dry powder.

[0069] In some embodiments, the mixing of the lithium source, iron source, phosphorus source, and dopant compound in step S1 to obtain the first dry powder specifically involves: mixing the lithium source, iron source, phosphorus source, dopant compound, and dispersant to obtain a slurry; then milling the slurry and spray-drying it to obtain the first dry powder. Compared to the former mixing method, the latter mixing method is more conducive to uniformly mixing the various raw materials, thereby making the dopant element more evenly distributed within the lithium iron phosphate matrix.

[0070] The preparation method of this application embodiment does not have any particular limitation on the type of dispersant in step S1, as long as it can achieve uniform dispersion of various raw materials. For example, the dispersant can be at least one selected from water, methanol, ethanol, ethylene glycol, n-butanol, ethyl acetate, methyl acetate, and butyl acetate.

[0071] In some embodiments, the solid content of the slurry in step S1 is 20–50 wt.%, for example, 20 wt.%, 25 wt.%, 30 wt.%, 35 wt.%, 40 wt.%, 45 wt.%, 50 wt.%, etc., preferably 30–40 wt.%. When the solid content of the slurry is too low, the throughput of spray drying is too large. When the solid content of the slurry is too high, it is not conducive to the uniform mixing of the lithium source, iron source, phosphorus source, dopant compound, and first carbon source.

[0072] The spray drying in step S1 of the preparation method of this application embodiment serves to achieve solid-liquid separation, thereby achieving the drying effect. The preparation method of this application embodiment does not have particular limitations on the specific process parameters of the spray drying in step S1, as long as the above-described effect is achieved.

[0073] In some embodiments, the average particle size of the primary particles of the first dry powder in step S1 is 0.2–0.6 μm, for example, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, etc. When the average particle size of the primary particles of the first dry powder is too small, it is not conducive to cost reduction and efficiency improvement. When the average particle size of the primary particles of the first dry powder is too large, it is not conducive to increasing the contact area between the lithium source, iron source, phosphorus source, and dopant compound to ensure sufficient reaction.

[0074] The first sintering treatment in step S1 of the preparation method of this application embodiment is carried out under an inert atmosphere because the iron in lithium iron phosphate is divalent iron, and sintering under an inert atmosphere is necessary to prevent the lithium iron phosphate from being oxidized. The inert atmosphere can be nitrogen and / or argon, etc.

[0075] In some embodiments, the holding temperature for the first sintering treatment in step S1 is 300–800°C, such as 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, etc., preferably 450–750°C, and the holding time is 1–10 hours, such as 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, etc. When the temperature or time of the first sintering treatment is too low, it is not conducive to improving the crystallinity of the first preform. When the temperature or time of the first sintering treatment is too high, it is not conducive to cost reduction and efficiency improvement.

[0076] In some embodiments, the average particle size of the primary particles of the first preform in step S1 is 50 to 2000 nm, such as 50 nm, 100 nm, 200 nm, 400 nm, 600 nm, 800 nm, 1000 nm, 1200 nm, 1400 nm, 1600 nm, 1800 nm, 2000 nm, etc.

[0077] The preparation method of this application embodiment does not have any particular limitation on the type of fast ion conductor in step S2. For example, the fast ion conductor can be lithium aluminum titanium phosphate (LATP). 1.4 Al 0.4 Ti 1.6 (PO4)3), lithium vanadate LVO (LiVO3, Li3VO4), lithium titanate LTO (Li2TiO3, Li4Ti5O4) 12 ), Lithium lanthanum titanium oxide LLZO (Li7La3Zr2O) 12 ), Lithium lanthanum zirconium oxide LLTO (Li 0.34 La 0.51 TiO 2.94 ) and lithium silicon vanadium oxide LVSO (Li 3.6 Si 0.6 V 0.4 At least one of O4). The above types of fast ion conductors help to improve the ionic conductivity of composite positive electrode active materials, thereby improving their conductivity and ultimately their specific capacity.

[0078] In some embodiments, the mass ratio of the first preform to the fast ion conductor in step S2 can be 1:

[0079] (0.001~0.05), for example 1:0.001, 1:0.005, 1:0.01, 1:0.05, etc.

[0080] Optionally, in step S2, the mass ratio of the first preform to the fast ion conductor is 1:(0.01 to 0.05), for example, 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, etc. When the mass ratio is within the above range, the formed fast ion conductor layer is a continuous film layer.

[0081] Optionally, in step S2, the mass ratio of the first preform to the fast ion conductor is 1:(0.001~0.005), for example, 1:0.001, 1:0.002, 1:0.003, 1:0.004, 1:0.005, etc. When this mass ratio is within the above range, the formed fast ion conductor layer is a discontinuous film layer. When the mass ratio is too low, the mass percentage of the fast ion conductor layer is too high, and the gaps between the fast ion conductor particles are too small, which is not conducive to the contact between the first carbon layer and the second carbon layer, thus not conducive to improving the conductivity of the composite positive electrode active material, and ultimately not conducive to improving its specific capacity. When the mass ratio is too high, the mass percentage of the fast ion conductor layer is too low, which is not conducive to the formation of ion transport channels, not conducive to improving the conductivity of the composite positive electrode active material, not conducive to reducing its internal resistance, and thus also not conducive to improving its specific capacity.

[0082] The preparation method of this application embodiment does not have any particular limitation on the mixing method described in step S2, as long as the various raw materials are mixed evenly. For example, a dry mixing followed by pulverization can be used, or a wet sand milling followed by spray drying can be used. In some embodiments, mixing the first preform and the fast ion conductor in step S2 to obtain the second dry powder specifically involves: mixing the first preform and the fast ion conductor, and then pulverizing and mixing them using a universal crusher to obtain the second dry powder. In some embodiments, mixing the first preform and the fast ion conductor in step S2 to obtain the second dry powder specifically involves: mixing the first preform, the fast ion conductor, and the dispersant to obtain a slurry; then sand milling the slurry and then spray drying it to obtain the second dry powder. The type of dispersant and the solid content of the slurry in step S2 can be set with reference to step S1.

[0083] In some embodiments, the average particle size of the primary particles of the second dry powder in step S2 is 0.3–1 μm, for example, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 0.55 μm, 1 μm, etc. When the average particle size of the primary particles of the second dry powder is too small, it is not conducive to cost reduction and efficiency improvement. When the average particle size of the primary particles of the second dry powder is too large, it is not conducive to increasing the contact area between the first preform and the fast ion conductor to ensure sufficient reaction.

[0084] The second sintering process in step S2 of the preparation method of this application embodiment is carried out under an inert atmosphere because the iron in lithium iron phosphate is divalent iron. Sintering under an inert atmosphere is necessary to prevent the lithium iron phosphate from being oxidized.

[0085] In some embodiments, the holding temperature for the second sintering treatment in step S2 is 700–800°C, such as 700°C, 720°C, 740°C, 760°C, 780°C, 800°C, etc., preferably 720–780°C, and the holding time is 1–10 hours, such as 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, etc. When the temperature or time of the second sintering treatment is too low, it is not conducive to forming a uniform fast ion conductor layer on the surface of the first preform. When the temperature or time of the second sintering treatment is too high, it is not conducive to cost reduction and efficiency improvement.

[0086] In some embodiments, the average particle size of the primary particles of the second preform in step S2 is 50 to 2000 nm, such as 50 nm, 100 nm, 150 nm, 200 nm, 400 nm, 600 nm, 1000 nm, 1500 nm, 2000 nm, etc.

[0087] In step S3 of the preparation method of this application embodiment, the second carbon source is used to prepare a second carbon layer on the surface of the second preform. The preparation method of this application embodiment does not particularly limit the type of the second carbon source in step S3. For example, the second carbon source can be at least one of glucose, granulated sugar, citric acid, sucrose, polyethylene glycol, phenolic resin, polyvinylpyrrolidone, polyphenylene ether, polyacrylamide, and polyethylene glycol. Since the second carbon source in step S3 involves carbon coating on the particle surface, in order to improve the conductivity of the composite positive electrode active material, the second carbon source can be at least one of phenolic resin, polyvinylpyrrolidone, polyphenylene ether, polyacrylamide, and polyethylene glycol.

[0088] In some embodiments, the mass ratio of the second preform to the second carbon source in step S3 is 1:(0.07~0.15), for example, 1:0.07, 1:0.08, 1:0.09, 1:0.10, 1:0.11, 1:0.12, 1:0.13, 1:0.14, 1:0.15, etc. When this mass ratio is too low, the mass percentage of the second carbon layer is too high, the mass percentage of lithium iron phosphate may be too low, and the mass percentage of the active material is too low, which is not conducive to improving the specific capacity of the composite cathode active material. When this mass ratio is too high, the mass percentage of the second carbon layer is too low, which is not conducive to forming electron transport channels between lithium iron phosphate particles, not conducive to improving the conductivity of the composite cathode active material, not conducive to reducing its internal resistance, and thus also not conducive to improving its specific capacity.

[0089] The preparation method of this application embodiment does not have any particular limitation on the mixing method described in step S3, as long as the various raw materials are mixed evenly. For example, a dry mixing followed by pulverization method can be used, or a wet sand milling followed by spray drying method can be used. In some embodiments, the mixing of the second preform and the second carbon source in step S3 to obtain the third dry powder is specifically: the second preform and the second carbon source are mixed and then pulverized and mixed using a universal crusher to obtain the third dry powder. In some embodiments, the mixing of the second preform and the second carbon source in step S3 to obtain the third dry powder is specifically: the second preform, the second carbon source, and the dispersant are mixed to obtain a slurry; the slurry is then sand milled and then spray dried to obtain the third dry powder. As for the type of dispersant and the solid content of the slurry in step S3, they can be set with reference to step S1.

[0090] In some embodiments, the average particle size of the primary particles of the third dry powder in step S3 is 0.3–1 μm, for example, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 0.55 μm, 1 μm, etc. When the average particle size of the primary particles of the third dry powder is too small, it is not conducive to cost reduction and efficiency improvement. When the average particle size of the primary particles of the third dry powder is too large, it is not conducive to increasing the contact area between the second preform and the second carbon source to ensure sufficient reaction.

[0091] The third sintering process in step S3 of the preparation method of this application embodiment is carried out under an inert atmosphere because the iron in lithium iron phosphate is divalent iron. Sintering under an inert atmosphere is necessary to prevent the lithium iron phosphate from being oxidized.

[0092] In some embodiments, the holding temperature for the third sintering treatment in step S3 is 700–800°C, such as 700°C, 720°C, 740°C, 760°C, 780°C, 800°C, etc., preferably 720–780°C, and the holding time is 1–10 hours, such as 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, etc. When the temperature or time of the third sintering treatment is too low, it is not conducive to the formation of a uniform second carbon layer on the surface of the second preform. When the temperature or time of the third sintering treatment is too high, it is not conducive to cost reduction and efficiency improvement.

[0093] In some embodiments, the average particle size of the primary particles of the composite positive electrode active material in step S3 is 100-2500 nm, such as 100 nm, 150 nm, 200 nm, 400 nm, 600 nm, 1000 nm, 1500 nm, 2000 nm, 2200 nm, 2500 nm, etc.

[0094] Thirdly, embodiments of this application provide a secondary battery, wherein the positive electrode of the secondary battery includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, and the positive electrode film layer includes the composite positive electrode active material described in the first aspect.

[0095] Because of the use of the composite positive electrode active material described in the first aspect, the secondary battery of the present application embodiment has excellent specific capacity.

[0096] 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.

[0097] 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.).

[0098] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0099] 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.

[0100] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as composite positive electrode 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.

[0101] 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.

[0102] 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.

[0103] 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 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 (copper, copper 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.).

[0104] 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. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material 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.

[0105] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive 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).

[0106] 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.

[0107] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0108] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as 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.

[0109] 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.

[0110] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] In some embodiments, the secondary battery further 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.

[0115] 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.

[0116] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and the electrolyte.

[0117] 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.

[0118] The embodiments of this application do not impose any particular restrictions on the shape of the secondary battery, which can be cylindrical, square, or any other arbitrary shape.

[0119] Fourthly, embodiments of this application provide an electrical device including the secondary battery described in the third aspect.

[0120] Because of the use of the secondary battery described in the third aspect, the power supply device of this application embodiment has excellent energy storage capacity or battery life.

[0121] This application is described in detail below with reference to the embodiments and accompanying drawings.

[0122] Example 1

[0123] A method for preparing a composite positive electrode active material, comprising the following steps:

[0124] (1) Weigh 1000g (8.64mol) of lithium phosphate raw material and 1368.8g (24.51mol) of iron powder. Add 10kg of pure water and stir evenly to form a slurry. Slowly add 1932.8g (16.8mol) of phosphoric acid with a mass concentration of 85wt% to the slurry while stirring. After all the slurry has been added, continue stirring for 30min. Then add 60g of polyvinylpyrrolidone (PVP), 75g of glucose, and 15g of magnesium acetate and continue stirring for 30min.

[0125] (2) The slurry obtained in step (1) is fined by sand milling to an average particle size of 0.3 μm, and then spray-dried to obtain the first dry powder.

[0126] (3) The first dry powder obtained in step (2) is sintered in a tube furnace at a heating rate of 5℃ / min and a sintering temperature of 500℃ for 3 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process and the gas flow rate is controlled at 0.2L / min. The first pre-product is obtained by sintering. The average particle size of the primary particles of the first pre-product is 150nm.

[0127] (4) Add 1000g of the first pre-product obtained in step (3) and 2g of lithium vanadate to 3kg of pure water, stir evenly to form a slurry.

[0128] (5) The slurry obtained in step (4) is fined by sand milling to an average particle size of 0.3 μm for primary particles, and then spray-dried to obtain a second dry powder.

[0129] (6) The second dry powder obtained in step (5) is sintered in a tube furnace at a heating rate of 3℃ / min and a sintering temperature of 745℃ for 6 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process and the gas flow rate is controlled at 0.2L / min. The second pre-product is obtained by sintering. The average particle size of the primary particles of the second pre-product is 600nm.

[0130] (7) Add 1000g of the second pre-product obtained in step (6) and 100g of PVP to 3kg of pure water, stir evenly to form a slurry.

[0131] (8) The slurry obtained in step (7) is fined by sand milling to an average particle size of 0.3 μm for the primary particles, and then spray-dried to obtain the third dry powder.

[0132] (9) The third dry powder obtained in step (8) is sintered in a tube furnace at a heating rate of 3℃ / min and a sintering temperature of 745℃. The temperature is held for 6 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process, and the gas flow rate is controlled at 0.2L / min. The composite positive electrode active material coarse powder is obtained by sintering.

[0133] (10) The coarse powder of the composite positive electrode active material obtained in step (9) is pulverized by airflow to an average particle size of 1.0 μm to obtain the composite positive electrode active material. The average particle size of the primary particles of the composite positive electrode active material is 700 nm.

[0134] Based on a total mass of 100 wt.% for the composite positive electrode active material, the mass percentage of the doping element is 0.25 wt.%, the mass percentage of the first carbon layer is 0.15 wt.%, the mass percentage of the fast ion conductor layer is 0.2 wt.%, the mass percentage of the second carbon layer is 1.0 wt.%, and the balance is lithium iron phosphate. The thickness of the first carbon layer is 0.1 nm, the thickness of the second carbon layer is 6 nm, the doping element is magnesium, and the fast ion conductor is lithium vanadate.

[0135] Example 2

[0136] The composite positive electrode active material in this embodiment is the same as that in Example 1, except that the mass percentage of the first carbon layer is 0.01 wt.% and the mass percentage of the second carbon layer is 1.3 wt.%. The thickness of the first carbon layer is 0.01 nm and the thickness of the second carbon layer is 10 nm.

[0137] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 1, except that the carbon source and amount used in step (1) are 30g polyvinylpyrrolidone (PVP) and 35g glucose, respectively. The amount of PVP used in (7) is 180g.

[0138] Example 3

[0139] The composite positive electrode active material in this embodiment is the same as that in Example 1, except that the mass percentage of the first carbon layer is 0.5 wt.% and the mass percentage of the second carbon layer is 0.8 wt.%. The thickness of the first carbon layer is 1 nm and the thickness of the second carbon layer is 2 nm.

[0140] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 1, except that the carbon source and amount used in step (1) are 75g of polyvinylpyrrolidone (PVP) and 90g of glucose, respectively. The amount of PVP used in step (7) is 50g.

[0141] Example 4

[0142] The composite positive electrode active material in this embodiment is the same as that in Example 1, except that the mass percentage of the fast ion conductor layer is 0.1 wt.%, the mass percentage of the second carbon layer is 1.0 wt.%, and the thickness of the second carbon layer is 6 nm.

[0143] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 1, except that the amount of lithium vanadate added in step (4) is 1g.

[0144] Example 5

[0145] The composite positive electrode active material in this embodiment is the same as that in Example 1, except that the mass percentage of the fast ion conductor layer is 0.5 wt.%, the mass percentage of the second carbon layer is 1.0 wt.%, and the thickness of the second carbon layer is 6 nm.

[0146] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 1, except that the amount of lithium vanadate added in step (4) is 5g.

[0147] Example 6

[0148] The composite positive electrode active material in this embodiment is the same as that in Example 1, except that the doping element is V and the fast ion conductor is lithium titanium aluminum phosphate.

[0149] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 1. The difference is that the lithium source, iron source and phosphorus source in step (1) are replaced with 2647.7g of lithium dihydrogen phosphate, 18.8g of lithium carbonate and 1368.8g of iron powder, respectively, and the doping source is ammonium metavanadate (NH4VO3).

[0150] Example 7

[0151] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 1. The difference is that in step (1), 1000g (8.64mol) of lithium phosphate and 1368.8g (24.51mol) of iron powder are weighed and added to 10kg of pure water, and instead, 4000g (26.53mol) of iron phosphate and 1015.4g (13.74mol) of lithium carbonate are weighed and added to 10kg of pure water.

[0152] Example 8

[0153] The preparation method of the composite positive electrode active material in this embodiment is the same as that in Example 6. The difference is that in step (1), 2647.7g of lithium dihydrogen phosphate, 18.8g of lithium carbonate, and 1368.8g of iron powder are weighed and added to 10kg of pure water. Instead, 4000g of iron phosphate (26.53mol) and 1015.4g of lithium carbonate (13.74mol) are weighed and added to 10kg of pure water.

[0154] Example 9

[0155] A method for preparing a composite positive electrode active material, comprising the following steps:

[0156] (1) Weigh 1000g (8.64mol) of lithium phosphate raw material and 1368.8g (24.51mol) of iron powder. Add 10kg of pure water and stir evenly to form a slurry. Slowly add 1932.8g (16.8mol) of phosphoric acid with a mass concentration of 85wt% to the slurry while stirring. After all the slurry has been added, continue stirring for 30min. Then add 60g of polyvinylpyrrolidone (PVP), 75g of glucose, and 15g of magnesium acetate and continue stirring for 30min.

[0157] (2) The slurry obtained in step (1) is fined by sand milling to an average particle size of 0.3 μm, and then spray-dried to obtain the first dry powder.

[0158] (3) The first dry powder obtained in step (2) is sintered in a tube furnace at a heating rate of 5℃ / min and a sintering temperature of 500℃ for 3 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process and the gas flow rate is controlled at 0.2L / min. The first pre-product is obtained by sintering. The average particle size of the primary particles of the first pre-product is 150nm.

[0159] (4) Add 1000g of the first pre-process obtained in step (3) and 20g of lithium vanadate to 3kg of pure water, stir evenly to form a slurry.

[0160] (5) The slurry obtained in step (4) is fined by sand milling to an average particle size of 0.3 μm for primary particles, and then spray-dried to obtain a second dry powder.

[0161] (6) The second dry powder obtained in step (5) is sintered in a tube furnace at a heating rate of 3℃ / min and a sintering temperature of 745℃ for 6 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process and the gas flow rate is controlled at 0.2L / min. The second pre-product is obtained by sintering. The average particle size of the primary particles of the second pre-product is 600nm.

[0162] (7) Add 1000g of the second pre-product obtained in step (6) and 100g of PVP to 3kg of pure water, stir evenly to form a slurry.

[0163] (8) The slurry obtained in step (7) is fined by sand milling to an average particle size of 0.3 μm for the primary particles, and then spray-dried to obtain the third dry powder.

[0164] (9) The third dry powder obtained in step (8) is sintered in a tube furnace at a heating rate of 3℃ / min and a sintering temperature of 745℃. The temperature is held for 6 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process, and the gas flow rate is controlled at 0.2L / min. The composite positive electrode active material coarse powder is obtained by sintering.

[0165] (10) The coarse powder of the composite positive electrode active material obtained in step (9) is pulverized by airflow to an average particle size of 1.0 μm to obtain the composite positive electrode active material. The average particle size of the primary particles of the composite positive electrode active material is 700 nm.

[0166] Based on a total mass of 100 wt.% for the composite positive electrode active material, the mass percentage of the doping element is 0.25 wt.%, the mass percentage of the first carbon layer is 0.15 wt.%, the mass percentage of the fast ion conductor layer is 2 wt.%, the mass percentage of the second carbon layer is 1.0 wt.%, and the balance is lithium iron phosphate. The thickness of the first carbon layer is 0.1 nm, the thickness of the second carbon layer is 6 nm, the doping element is magnesium, and the fast ion conductor is lithium vanadate.

[0167] Comparative Example 1

[0168] A method for preparing a composite positive electrode active material, comprising the following steps:

[0169] The lithium iron phosphate cathode material in this embodiment is the same as that in Example 1, except that the lithium iron phosphate does not contain the doping element magnesium.

[0170] The preparation method of the lithium iron phosphate cathode material in this embodiment is the same as that in Example 1, except that magnesium acetate is not added in step (1).

[0171] Comparative Example 2

[0172] A method for preparing a composite positive electrode active material, comprising the following steps:

[0173] (1) Weigh 1000g (8.64mol) of lithium phosphate raw material and 1368.8g (24.51mol) of iron powder. Add 10kg of pure water and stir evenly to form a slurry. Slowly add 1932.8g (16.8mol) of phosphoric acid with a mass concentration of 85wt% to the slurry while stirring. After all the slurry has been added, continue stirring for 30min. Then add 25g of polyvinylpyrrolidone (PVP), 30g of glucose, and 15g of magnesium acetate and continue stirring for 30min.

[0174] (2) The slurry obtained in step (1) is fined by sand milling to an average particle size of 0.3 μm, and then spray-dried to obtain the first dry powder.

[0175] (3) The first dry powder obtained in step (2) is sintered in a tube furnace at a heating rate of 5℃ / min and a sintering temperature of 500℃ for 3 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process and the gas flow rate is controlled at 0.2L / min. The first pre-product is obtained by sintering. The average particle size of the primary particles of the first pre-product is 300nm.

[0176] (4) Add 1000g of the first pre-process obtained in step (6) and 20g of lithium vanadate to 3kg of pure water, stir evenly to form a slurry.

[0177] (5) The slurry obtained in step (4) is fined by sand milling to an average particle size of 0.3 μm, and then spray-dried to obtain a second dry powder.

[0178] (6) The second dry powder obtained in step (5) is sintered in a tube furnace at a heating rate of 3℃ / min and a sintering temperature of 745℃ for 6 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process and the gas flow rate is controlled at 0.2L / min. The second pre-product is obtained by sintering. The average particle size of the primary particles of the second pre-product is 800nm.

[0179] (7) Add 1000g of the second pre-product obtained in step (6) and 140g of PVP to 3kg of pure water, stir evenly, and form a slurry.

[0180] (8) The slurry obtained in step (7) is fined by sand milling to an average particle size of 0.3 μm, and then spray-dried to obtain the third dry powder.

[0181] (9) The third dry powder obtained in step (8) is sintered in a tube furnace at a heating rate of 3℃ / min and a sintering temperature of 745℃. The temperature is held for 6 hours. Nitrogen gas is introduced into the protective atmosphere during the entire sintering process, and the gas flow rate is controlled at 0.2L / min. The composite positive electrode active material coarse powder is obtained by sintering.

[0182] (10) The coarse powder of the composite positive electrode active material obtained in step (9) is pulverized by airflow to an average particle size of 1.0 μm to obtain the composite positive electrode active material. The average particle size of the primary particles of the composite positive electrode active material is 900 nm.

[0183] Based on a total mass of 100 wt.% for the composite positive electrode active material, the mass percentage of the doping element is 0.25 wt.%, the mass percentage of lithium iron phosphate is 98.4 wt.%, the mass percentage of the fast ion conductor layer is 0.2 wt.%, and the mass percentage of the carbon layer is 1.15 wt.%. The carbon layer has a thickness of 6.5 nm, the doping element is magnesium, and the fast ion conductor is lithium vanadate.

[0184] Performance testing:

[0185] (1) The SEM images of the composite positive electrode active materials of the above embodiments were tested using a scanning electron microscope. The SEM image of the composite positive electrode active material of Example 1 is shown below. Figure 2 As shown, the SEM image of the composite positive electrode active material in Example 2 is as follows. Figure 3 As shown.

[0186] (2) The compaction density and powder resistivity of the composite positive electrode active materials of the above embodiments and comparative examples were tested. The compaction density was tested using a powder compaction tester (pressure 3t) / GB / T 30835-2014; the powder resistivity was tested using a volume resistivity tester at a test temperature of 25℃. The test results are shown in Table 1.

[0187] (3) Assemble the positive electrode active materials of each embodiment and comparative example into coin cells, and perform electrochemical performance tests on the coin cells:

[0188] 1) Battery Assembly: ① Dissolve the positive electrode material / acetylene black / polyvinylidene fluoride in N-methyl-pyrrolidone at a weight ratio of 80:10:10, stir evenly, coat it onto aluminum foil, and dry it in a forced-air drying oven at 100℃ to obtain the positive electrode sheet. ② Cut the dried electrode sheet into small round pieces with a diameter of 12mm as the positive electrode sheet. ③ Assemble the CR2025 coin cell in an argon-filled glove box, using lithium metal sheet as the negative electrode sheet, polypropylene microporous membrane as the separator, and 1mol / L LiPF6 / EC+DMC (volume ratio 1:1:1) as the electrolyte.

[0189] 2) Electrochemical performance testing

[0190] The coin cell capacity was tested using a coin cell tester 2.0-3.75V / 20202915-T-610. A BTS-5V / 5mA battery testing system was used to test the charge-discharge performance of the battery, with a voltage range of 2.0-3.75V. The specific test steps were as follows: In a 25℃ constant temperature chamber, the assembled battery was charged at a constant current rate of 0.1C to 3.75V, then charged at a constant voltage until the cutoff current was 0.05C, and then discharged at a rate of 0.1C to 2.0V. The initial charge specific capacity and the initial discharge specific capacity at 0.1C were obtained. The initial coulombic efficiency was calculated as: initial discharge specific capacity at 0.1C / initial charge specific capacity at 0.1C. The test results are shown in Table 1.

[0191] Table 1. Performance data of composite positive electrode active materials prepared in each embodiment and comparative example

[0192]

[0193] As can be seen from Examples 1 to 9, the powder resistivity of the composite positive electrode active material in this application is low, and it has good conductivity and excellent specific capacity.

[0194] A comparison of Example 1 and Comparative Example 1 shows that when the fast ion conductor layer in the sandwich structure coating layer of Comparative Example 1 is a continuous film layer, the powder resistivity of the composite positive electrode active material is high, that is, the conductivity is poor, resulting in a low specific capacity of the composite positive electrode active material.

[0195] From the comparison between Example 1 and Comparative Example 2, it can be seen that when the coating layer in Comparative Example 2 is a double coating layer NC, the powder resistivity of the composite positive electrode active material is high, that is, the conductivity is poor, resulting in a low specific capacity of the composite positive electrode active material.

[0196] Table 2. Yield of composite positive electrode active materials in Examples 1, 6-8

[0197]

[0198] The yield is calculated as follows: if the mass of the final composite positive electrode active material corresponding to all powders with a mass of m1 in the first dry powder is m2, then the yield = m2 / m1 × 100%.

[0199] The comparison between Examples 1 and 7, and between Examples 6 and 8, shows that when the lithium source is lithium phosphate or lithium dihydrogen phosphate, the yield of the composite positive electrode active material can be significantly improved.

[0200] In this application, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0201] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A composite positive electrode active material, characterized in that, The invention includes lithium iron phosphate containing doped elements and a coating layer covering the surface of the lithium iron phosphate containing doped elements. The coating layer includes a first carbon layer, a fast ion conductor layer and a second carbon layer. The first carbon layer is close to the lithium iron phosphate containing doped elements, and the second carbon layer is away from the lithium iron phosphate containing doped elements. The fast ion conductor layer is located between the first carbon layer and the second carbon layer. The first carbon layer and the second carbon layer are continuous films, and the fast ion conductor layer is either a continuous film or a discontinuous film.

2. The composite positive electrode active material according to claim 1, characterized in that, Based on a total mass of 100 wt.% of the composite positive electrode active material, the mass percentage of the doping element is 0.12–0.72 wt.%, and / or the mass percentage of lithium iron phosphate is 97.5–98.5 wt.%, and / or the mass percentage of the first carbon layer is 0.01–0.5 wt.%, and / or the mass percentage of the fast ion conductor layer is 0.1–3 wt.%, and / or the mass percentage of the second carbon layer is 0.8–1.3 wt.%.

3. The composite positive electrode active material according to claim 2, characterized in that, The fast ion conductor layer has a mass percentage of 0.1–0.5 wt.%.

4. The composite positive electrode active material according to claim 1, characterized in that, The fast ion conductor layer is a discontinuous film layer in which fast ion conductors are distributed in an island-like pattern.

5. The composite positive electrode active material according to any one of claims 1 to 4, characterized in that, The average particle size of the primary particles of the lithium iron phosphate containing doped elements is 50–2000 nm, and / or the average particle size of the primary particles of the composite positive electrode active material is 100–2500 nm.

6. The composite positive electrode active material according to any one of claims 1 to 4, characterized in that, The thickness of the first carbon layer is 0.01 to 1 nm, and / or the thickness of the second carbon layer is 1 to 10 nm.

7. The method for preparing the composite positive electrode active material according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Mix lithium source, iron source, phosphorus source, doped element compound, and first carbon source to obtain first dry powder; The first dry powder was subjected to a first sintering treatment under an inert atmosphere to obtain the first pre-product; S2. Mix the first preform and the fast ion conductor to obtain the second dry powder; The second dry powder is subjected to a second sintering treatment under an inert atmosphere to obtain the second pre-product; S3. The second preform and the second carbon source are mixed to obtain the third dry powder; the third dry powder is subjected to a third sintering treatment under an inert atmosphere to obtain the composite positive electrode active material.

8. The method for preparing the composite positive electrode active material according to claim 7, characterized in that, The lithium source is at least one of lithium phosphate, lithium hydrogen phosphate, and lithium dihydrogen phosphate.

9. The method for preparing the composite positive electrode active material according to claim 7, characterized in that, The mass ratio of the first preform to the fast ion conductor is 1:(0.001 to 0.05).

10. The method for preparing the composite positive electrode active material according to claim 9, characterized in that, The mass ratio of the first preform to the fast ion conductor is 1:(0.001 to 0.005).

11. A secondary battery, characterized in that, The positive electrode of the secondary battery includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein the positive electrode film layer includes the composite positive electrode active material according to any one of claims 1 to 6.

12. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 11.

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