Positive electrode active material, method for manufacturing the same, positive electrode sheet, secondary battery, battery module, battery pack, and power tool

Abstract

The application discloses a positive electrode active material, comprising: a positive electrode active material base, a first carbon coating layer and a second carbon coating layer, the first carbon coating layer is coated on the surface of the positive electrode active material base, and the second carbon coating layer is coated on the surface of the first carbon coating layer; the interface between the first carbon coating layer and the second carbon coating layer has a crystal lattice defect, and the second carbon coating layer has a jagged surface.

Description

Technical Field

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

[0002] Ternary cathode materials are the mainstream material for power lithium-ion batteries due to their high discharge platform and energy density. However, their cycle performance is poor, mainly due to the decomposition of the electrolyte on the electrode surface at high potentials and the dissolution of Ni, Mn, and Co elements. Lithium iron phosphate (LFP), on the other hand, has advantages such as low cost, good cycle performance, and good safety, and is widely used in lithium-ion battery cathode materials. However, LFP has a low tap density, so it is generally mixed with ternary cathode materials to increase the packing density of the cathode active material layer. However, when using slurries with different particle sizes, especially when using nano-sized LFP as filler particles, LFP exhibits poor pressure resistance, conductivity, dispersibility, and lithium-ion diffusion performance, failing to achieve the ideal particle size distribution. While conventional carbon-coated LFP can improve electronic conductivity, the resulting nano-sized LFP particles still suffer from easy agglomeration, poor lithium-ion diffusion performance, and poor pressure resistance. Summary of the Invention

[0003] Based on this, this application provides a positive electrode active material with excellent pressure resistance, conductivity, dispersibility and lithium ion diffusion.

[0004] In addition, this application also provides a method for preparing a positive electrode active material, a positive electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device.

[0005] According to a first aspect of this application, a positive electrode active material is provided, comprising: a positive electrode active material matrix, a first carbon coating layer and a second carbon coating layer, wherein the first carbon coating layer coats the surface of the positive electrode active material matrix, and the second carbon coating layer coats the surface of the first carbon coating layer;

[0006] The interface between the first carbon coating layer and the second carbon coating layer has lattice defects, and the second carbon coating layer has a serrated surface.

[0007] The aforementioned positive electrode active material, by coating the surface of the positive electrode active material matrix with two layers, inner and outer, creates a double-layer structure. This structure prevents particle breakage of the positive electrode active material matrix, such as lithium iron phosphate particles, due to compression during charging and discharging, thus improving the voltage resistance of the positive electrode active material matrix. The presence of lattice defects at the interface between the first and second carbon coating layers significantly improves the diffusion rate of lithium ions within the lattice. Simultaneously, the unique serrated structure of the second carbon coating layer overcomes material fracture problems at the edges, further enhancing voltage resistance, and also better disperses the positive electrode active material matrix particles, protecting them from aggregation during the reaction process.

[0008] Therefore, the above-mentioned positive electrode active material can be used in combination with ternary positive electrode material. While reducing porosity and electrolyte injection volume, it also has excellent conductivity and lithium-ion diffusion performance, thereby improving battery capacity retention and reducing battery impedance while reducing the electrolyte injection coefficient.

[0009] In some embodiments, the lattice defects include line defects. Line defects are dislocation lines in the lattice, which can be considered as the result of local slippage of one part of the crystal relative to another. They are obtained through calcination production using serrated, alternating dislocation structures. Introducing lattice defects in the crystal effectively improves the lithium-ion diffusion coefficient and ionic conductivity, thereby reducing the lithium-ion conductivity as the lithium-ion migration rate increases. + Resistance during the insertion / extraction process.

[0010] In some embodiments, the second carbon coating layer has an atomically serrated surface. The unique serrated structure of the second carbon coating layer may result in atomically serrated edges on the outermost carbon lattice, which can overcome the problem of material fracture at the edges and better disperse the active particles, protecting them from aggregation during the reaction.

[0011] In some embodiments, at least one of the first carbon coating layer and the second carbon coating layer is a nitrogen-doped carbon layer. Setting the first and second carbon coating layers as nitrogen-doped coating layers can, on the one hand, further improve their conductivity; on the other hand, the bond angles of CNC are non-linear, which facilitates the formation of a serrated structure on the surface of the second carbon coating layer and the formation of lattice defects at the interface between the first and second carbon coating layers.

[0012] In some embodiments, the total thickness of the first carbon coating layer and the second carbon coating layer is 10 nm to 30 nm. A total thickness that is too thin or too thick will affect the conductivity of the coating layer, hindering electron transfer and ion diffusion.

[0013] In some embodiments, the thickness ratio of the first carbon coating layer to the second carbon coating layer is (1-10):(1-5). When the thickness of the first carbon coating layer is too low, on the one hand, it cannot achieve the effect of an electronic conductor, and on the other hand, it cannot form lattice defects with the second carbon coating layer during the secondary calcination process, thus failing to improve the effect of lithium-ion transport. Conversely, when the thickness of the first carbon coating layer is too high, the excessively long path will hinder the transport rate of lithium ions from the first carbon coating layer into the positive electrode active material matrix, or directly cause lithium ions to be deposited in the carbon coating layer, making it difficult for lithium ions to be released or inserted, thereby causing ineffective lithium-ion diffusion.

[0014] In some embodiments, the positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate. Lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, as positive electrode active materials, suffer from problems such as low electronic conductivity and low lithium-ion diffusion rate. The aforementioned coating layer can mitigate these problems.

[0015] In some embodiments, the volume average particle size Dv50 of the positive electrode active material matrix is ​​100 nm to 500 nm. A suitable particle size of the positive electrode active material matrix is ​​beneficial when used in conjunction with other positive electrode materials, such as ternary positive electrode materials, to fully fill the excess pores of the ternary positive electrode material, increase the compaction density of the positive electrode sheet, reduce the consumption of electrolyte by the electrode pores, and thus improve the volumetric energy density of the battery.

[0016] According to a second aspect of this application, a method for preparing a positive electrode active material is provided, comprising the following steps:

[0017] The first carbon source is coated onto the surface of the positive electrode active material matrix and then calcined once to obtain a carbon-coated matrix.

[0018] The second carbon source is coated onto the surface of the carbon-coated matrix and then subjected to secondary calcination.

[0019] Both the first carbon source and the second carbon source have serrated spatial structures in their molecules.

[0020] The preparation method of the above-mentioned positive electrode active material coats the surface of the positive electrode active material matrix with a first carbon source and a second carbon source having a serrated molecular structure after calcination. The double-layer coating structure can prevent the positive electrode active material matrix, such as lithium iron phosphate particles, from being damaged due to the extrusion of ternary particles during charge and discharge, and improve the pressure resistance of the positive electrode active material matrix. In addition, the first carbon source is calcined twice, and the serrated structure at the interface of the inner and outer coating layers causes strain in the carbon lattice during the secondary calcination process, generating lattice defects, which greatly improves the diffusion rate of lithium ions inside the lattice. After calcination, the second carbon source with a serrated molecular structure forms a structure with a serrated surface, which can overcome the problem of material rupture appearing at the edge, and can better disperse the active particles and protect them from aggregation during the reaction process.

[0021] Therefore, the positive electrode active material prepared by the above method can be used in combination with the ternary positive electrode material. While reducing the porosity and the amount of electrolyte injection, it also has excellent conductivity and lithium ion diffusion performance, thereby improving the capacity retention rate of the battery and reducing the battery impedance while reducing the injection coefficient.

[0022] In some embodiments, both the first carbon source and the second carbon source include components having a conjugated molecular structure. The first carbon source and the second carbon source including components having a conjugated molecular structure can further improve the conductive characteristics of the coating layer, and thus improve the conductivity of the positive electrode active material.

[0023] In some embodiments, both the first carbon source and the second carbon source independently include at least one of polybenzylamine, polyaniline, polypyrrole, and polyethylene diamine. Taking polybenzylamine as an example, the structural formula of polybenzylamine is

[0024] -(A-B-A) n -, <0OO0091>where n is an integer, A includes at least one of an aromatic group and an alkenyl group, B includes an N atom, and 90° < bond angle of A-B-A < 180°.

[0026] In some embodiments, both the first carbon source and the second carbon source independently include at least one of polybenzylamine, polyaniline, polypyrrole, and polyethylene diamine. Taking polybenzylamine as an example, the polybenzylamine structural formula has an inherent twisted molecular chain, in which the benzene ring and the amino group are alternately connected in a non-linear manner to form a serrated molecular chain. This serrated structure may cause atomic-level edges and strain in the carbon lattice. During the secondary calcination process, the strain at the carbon lattice edge at the interface between the first carbon coating layer and the second carbon coating layer will induce distortion of the carbon lattice at high temperature, generating lattice defects, which greatly improves the diffusion rate of lithium ions inside the carbon lattice. In addition, the outermost atomic-level serrated structure can promote the stability of the material to reduce the damage to the electrode structure caused by the volume change of the material, which is beneficial to improving the cycle performance. ​​

[0027] In some embodiments, the mass ratio of the first carbon source to the positive electrode active material matrix is ​​(0.001 to 0.5):100.

[0028] In some embodiments, the mass ratio of the first carbon source to the positive electrode active material matrix is ​​(0.003 to 0.2):100.

[0029] In some embodiments, the mass ratio of the first carbon source to the positive electrode active material matrix is ​​(0.01 to 0.2):100.

[0030] By adjusting the mass ratio of the first carbon source to the cathode active material matrix, the thickness of the first carbon coating layer can be controlled. When the thickness of the first carbon coating layer is too low, it fails to achieve the effect of an electronic conductor and cannot form lattice defects with the second carbon coating layer during the secondary calcination process, thus failing to improve lithium-ion transport. Conversely, when the thickness of the first carbon coating layer is too high, the excessively long path will hinder the transport rate of lithium ions from the first carbon coating layer into the cathode active material matrix, or it may directly cause lithium ions to deposit in the carbon layer, making it difficult for lithium ions to be released or inserted, thus resulting in ineffective lithium-ion diffusion.

[0031] In some embodiments, the mass ratio of the first carbon source to the second carbon source is (1-10):(1-5). By adjusting the mass ratio of the first carbon source to the second carbon source, and adjusting the thickness ratio of the first carbon coating layer to the second carbon coating layer, the tap density, conductivity, and lithium-ion diffusion coefficient of the positive electrode active material can be further improved.

[0032] In some embodiments, the temperatures of the primary calcination and the secondary calcination are each independently between 200°C and 1000°C.

[0033] In some embodiments, the temperatures of the primary calcination and the secondary calcination are each independently 600°C to 800°C.

[0034] Calcination temperature affects the ion diffusion rate and electrical conductivity of the positive electrode active material. A suitable calcination temperature will make the lattice defects at the interface between the first carbon coating layer and the second carbon coating layer more uniform, thereby improving the electrical performance of the coated positive electrode active material.

[0035] In some embodiments, the time for the first calcination and the second calcination are each independently 1h to 10h.

[0036] In some embodiments, the time for the first calcination and the second calcination are each independently 3h to 5h.

[0037] Calcination time also affects the ion diffusion rate and electrical conductivity of the positive electrode active material. A suitable calcination time will make the lattice defects at the interface between the first carbon coating layer and the second carbon coating layer more uniform, thereby improving the electrical performance of the coated positive electrode active material.

[0038] In some embodiments, the steps of coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface each independently include:

[0039] The monomer and the core material are mixed and ball-milled, and then a doping acid and an initiator are added to polymerize the monomer, forming a carbon source coating the core material on the surface of the core material. The core material is the positive electrode active material matrix or the carbon-coated matrix, and the carbon source is the first carbon source or the second carbon source.

[0040] By mixing monomers with core materials and initiating monomer polymerization, the first and second carbon sources are coated onto the surface of the positive electrode active material matrix. The resulting coating layer is denser, and when used to prepare batteries, the DC resistance (DCR) value is lower, which is beneficial for improving the capacity value.

[0041] In some embodiments, the doped acid includes at least one of hydrochloric acid, sulfuric acid, and nitric acid.

[0042] In some embodiments, the initiator includes at least one of ammonium persulfate, potassium persulfate, and sodium persulfate.

[0043] In some embodiments, the molar ratio of the doped acid, the initiator, and the monomer is (1-3):(0.5-2.5):(1-5).

[0044] In some embodiments, the steps of polymerizing the monomers during the processes of coating the first carbon source onto the surface of the positive electrode active material matrix and coating the second carbon source onto the carbon-coated matrix surface each independently include:

[0045] The monomer, the core material, the doped acid, and the initiator are reacted at 4°C to 10°C for 14 to 18 hours, then washed and vacuum dried at 60°C to 80°C for 12 to 18 hours.

[0046] In some embodiments, the steps of coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface each independently include:

[0047] The carbon source and the core material are mixed and ball-milled for 10-16 hours, then washed and vacuum-dried at 60-80°C for 12-18 hours. The carbon source is either the first carbon source or the second carbon source, and the core material is either the positive electrode active material matrix or the carbon-coated matrix.

[0048] The method is simpler: directly mix the first and second carbon sources with the core material.

[0049] In some embodiments, the positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate. Lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, as positive electrode materials, suffer from problems such as low electronic conductivity and low lithium-ion diffusion rate. The aforementioned coating layer can mitigate these problems.

[0050] In some embodiments, the volume average particle size Dv50 of the positive electrode active material matrix is ​​100 nm to 500 nm. A suitable particle size of the positive electrode active material matrix is ​​beneficial when used in conjunction with other positive electrode materials, such as ternary positive electrode materials, to fully fill the excess pores of the ternary positive electrode material, increase the compaction density of the positive electrode sheet, reduce the consumption of electrolyte by the electrode pores, and thus improve the volumetric energy density of the battery.

[0051] According to a third aspect of this application, a positive electrode sheet is provided, comprising:

[0052] Positive current collector; and

[0053] A positive electrode active material layer is located on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the positive electrode active material prepared by the preparation method of the positive electrode active material of the first aspect of this application or the positive electrode active material of the second aspect of this application.

[0054] In some embodiments, the positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, and the positive electrode active material layer further includes a ternary positive electrode material. By mixing the positive electrode active material with the ternary positive electrode material, the compaction density of the positive electrode sheet is increased, and the consumption of electrolyte by the electrode pores is reduced.

[0055] In some embodiments, the mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1-20):100.

[0056] In some embodiments, the mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1-10):100.

[0057] In some embodiments, the ratio of the volume average particle size Dv50 of the positive electrode active material to that of the ternary positive electrode material is (0.1-20):100.

[0058] In some embodiments, the ratio of the volume average particle size Dv50 of the positive electrode active material to that of the ternary positive electrode material is (0.1-12):100.

[0059] A suitable mass ratio of positive active material to ternary cathode material and a suitable Dv50 particle size ratio are beneficial for fully filling the excess pores of the ternary cathode material, increasing the compaction density of the cathode sheet, reducing electrolyte consumption due to electrode pores, and thus improving the volumetric energy density of the battery. When the mass ratio of positive active material to ternary cathode material is too low or the Dv50 particle size ratio is too high, the ternary cathode material has a large number of pores, resulting in low particle packing efficiency and uneven material distribution. These pore spaces in the cathode sheet, on the one hand, mean that the electrolyte floating within them contributes nothing to the capacity, and the extra electrolyte weight reduces the battery's energy density and increases the manufacturing cost of the cell; on the other hand, in ternary cathode materials such as high-nickel systems, a large amount of electrolyte in the pores can easily penetrate into the cracks of the ternary cathode material and react, further leading to a decline in battery performance. When the mass ratio of the positive electrode active material to the ternary positive electrode material is too high or the Dv50 particle size ratio is too low, the addition of a large number of positive electrode active material particles will affect the theoretical specific capacity of the ternary positive electrode material and reduce the capacity value.

[0060] In some embodiments, the porosity of the positive electrode sheet is 20% to 25%. The lower porosity of the positive electrode sheet is beneficial for increasing the compaction density of the positive electrode sheet, reducing the consumption of electrolyte by the electrode pores, and thus increasing the volumetric energy density of the battery.

[0061] A secondary battery is provided according to a fourth aspect of this application, including the positive electrode sheet of the third aspect of this application.

[0062] In some embodiments, the electrolyte injection coefficient of the secondary battery is 2.1 g / Ah to 2.2 g / Ah.

[0063] Under the above electrolyte injection coefficient, the capacity and cycle retention rate of the secondary battery are still relatively high, but the amount of electrolyte used is reduced, which is beneficial to improving the energy density of the battery.

[0064] According to a fifth aspect of this application, a battery module is provided, including a secondary battery according to a fourth aspect of this application.

[0065] A battery pack is provided according to a sixth aspect of this application, including the battery module of the fifth aspect of this application.

[0066] According to a seventh aspect of this application, an electrical device is provided, comprising at least one selected from the fourth aspect of this application, the fifth aspect of this application, and the sixth aspect of this application.

[0067] Details of one or more embodiments of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims. Attached Figure Description

[0068] To better describe and illustrate embodiments or examples of the applications disclosed herein, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the embodiments or examples currently described, or the best mode of conduct for these applications as currently understood.

[0069] Figure 1 This is a schematic diagram of the structure of the positive electrode active material according to one embodiment of this application;

[0070] Figure 2 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0071] Figure 3 yes Figure 2 An exploded view of a secondary battery according to an embodiment of this application is shown;

[0072] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application;

[0073] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0074] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown;

[0075] Figure 7 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;

[0076] Figure 8 This is a scanning electron microscope image of the positive electrode active material prepared in Example 1 of this application;

[0077] Figure 9 This is an HRTEM image of the positive electrode active material prepared in Example 1 of this application;

[0078] Figure 10 This is a mapping diagram of the positive electrode sheet prepared in Example 1 of this application;

[0079] Figure 11 This is a capacity cycling decay diagram of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 of this application.

[0080] Explanation of reference numerals in the attached figures:

[0081] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate; 6 Electrical device. Detailed Implementation

[0082] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0083] 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 belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0084] 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.

[0085] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0086] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0087] 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.

[0088] 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.

[0089] 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).

[0090] 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.

[0091] 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.

[0092] As described in the background section, due to the inherent problems of ternary cathode materials and lithium iron phosphate materials, they are usually mixed together to increase the packing density of the cathode active material layer. However, when lithium iron phosphate is used as a filler particle, it still suffers from poor voltage resistance, conductivity, dispersibility, and lithium-ion diffusion performance. Although conventional carbon-coated lithium iron phosphate can improve electronic conductivity, when lithium iron phosphate particles are used in combination with ternary cathode materials, the requirements for its dispersion performance, conductivity, voltage resistance, and lithium-ion diffusion performance are even higher, which is difficult to achieve with traditional carbon coating.

[0093] Based on this, the inventors of this application have obtained the positive electrode active material with excellent pressure resistance, conductivity, dispersibility and lithium ion diffusion through a large number of experiments.

[0094] Please see Figure 1 The first aspect of this application provides a positive electrode active material, comprising: a positive electrode active material matrix, a first carbon coating layer and a second carbon coating layer, wherein the first carbon coating layer is coated on the surface of the positive electrode active material matrix and the second carbon coating layer is coated on the surface of the first carbon coating layer.

[0095] The interface between the first carbon coating layer and the second carbon coating layer has lattice defects, and the second carbon coating layer has a serrated surface.

[0096] The aforementioned positive electrode active material, by coating the surface of the positive electrode active material matrix with two layers, inner and outer, creates a double-layer structure. This structure prevents particle breakage of the positive electrode active material matrix, such as lithium iron phosphate particles, due to compression during charging and discharging, thus improving the voltage resistance of the positive electrode active material matrix. The presence of lattice defects at the interface between the first and second carbon coating layers significantly improves the diffusion rate of lithium ions within the lattice. Simultaneously, the unique serrated structure of the second carbon coating layer overcomes material fracture problems at the edges, further enhancing voltage resistance, and also better disperses the active positive electrode active material matrix particles, protecting them from aggregation during the reaction process.

[0097] Therefore, the above-mentioned positive electrode active material can be used in combination with ternary positive electrode material. While reducing porosity and electrolyte injection volume, it also has excellent conductivity and lithium-ion diffusion performance, thereby improving battery capacity retention and reducing battery impedance while reducing the electrolyte injection coefficient.

[0098] In some embodiments, lattice defects include line defects. Line defects are dislocation lines in the lattice, which can be considered as the result of local slippage of one part of the crystal relative to another. They are obtained through calcination production using serrated, alternating dislocation structures. Introducing lattice defects in the crystal effectively improves the lithium-ion diffusion coefficient and ionic conductivity, thereby reducing the lithium-ion conductivity as the lithium-ion migration rate increases. +Resistance during the insertion / extraction process.

[0099] A serrated surface refers to a non-smooth surface of the second carbon coating layer. The carbon source molecules used to form the second carbon coating layer have a serrated spatial structure, and the carbon source molecules form a second carbon coating layer with a serrated surface after calcination.

[0100] In some embodiments, the second carbon coating layer has an atomically serrated surface. The unique serrated structure of the second carbon coating layer can result in atomically sized edges on the outermost carbon lattice, overcoming material fracture problems at these edges and better dispersing the active particles, protecting them from aggregation during the reaction. It is understood that the serrated surface is not limited to the atomic scale and can also be of other sizes; an atomically sized serrated surface provides better dispersion of the active particles.

[0101] In some embodiments, a serrated surface refers to a surface having one or more triangular, circular, elliptical, or irregularly shaped teeth.

[0102] In some embodiments, the atomic-level dimensions are 0.1 nm to 1 nm.

[0103] In some embodiments, at least one of the first carbon coating layer and the second carbon coating layer is a nitrogen-doped carbon layer. Further, both the first and second carbon coating layers are nitrogen-doped carbon layers. Setting the first and second carbon coating layers as nitrogen-doped coating layers can, on the one hand, further improve their conductivity; on the other hand, the bond angles of CNC are non-linear, which facilitates the formation of a serrated structure on the surface of the second carbon coating layer and the formation of lattice defects at the interface between the first and second carbon coating layers.

[0104] In some embodiments, the total thickness of the first carbon coating layer and the second carbon coating layer is 10 nm to 30 nm. In a specific example, the total thickness of the first carbon coating layer and the second carbon coating layer can be 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, 22 nm, 25 nm, 28 nm, 30 nm, or any range of these values. Further, the total thickness of the first carbon coating layer and the second carbon coating layer is 10 nm to 20 nm.

[0105] If the total thickness of the first and second carbon coating layers is too thin or too thick, it will affect the conductivity of the coating layers and be detrimental to electron transfer and ion diffusion.

[0106] In some embodiments, the thickness ratio of the first carbon coating layer to the second carbon coating layer is (1-10):(1-5). In a specific example, the thickness ratio of the first carbon coating layer to the second carbon coating layer is 1:1, 1:2, 1:3, 1:4, 1:5, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, or any range of two of these values.

[0107] Furthermore, the thickness ratio of the first carbon coating layer to the second carbon coating layer is 4:1 to 1:4. Even further, the thickness ratio of the first carbon coating layer to the second carbon coating layer is 4:1 to 1:2. Still further, the thickness ratio of the first carbon coating layer to the second carbon coating layer is 2:1 to 1:2.

[0108] When the thickness of the first carbon coating layer is too low, it fails to achieve the effect of an electronic conductor and cannot form lattice defects with the second carbon coating layer during the secondary calcination process, thus failing to improve lithium-ion transport. Conversely, when the thickness of the first carbon coating layer is too high, the excessively long path will hinder the transport rate of lithium ions from the first carbon coating layer into the positive electrode active material matrix, or it may directly cause lithium ions to deposit in the carbon coating layer, making it difficult for lithium ions to be released or inserted, thus resulting in ineffective lithium-ion diffusion.

[0109] In some embodiments, the positive electrode active material matrix includes at least one of lithium iron phosphate (such as LiFePO4, also referred to as LFP), lithium manganese phosphate (such as LiMnPO4), and lithium manganese iron phosphate. In a specific example, the positive electrode active material matrix includes at least one of olivine-structured lithium iron phosphate, olivine-structured lithium manganese phosphate, and olivine-structured lithium manganese iron phosphate. Lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, as positive electrode active materials, suffer from problems such as low electronic conductivity and low lithium-ion diffusion rate. The above-mentioned coating layer can improve these problems.

[0110] In some embodiments, the volume average particle size Dv50 of the positive electrode active material matrix is ​​100 nm to 500 nm. Dv50 refers to the particle size corresponding to 50% of the volume distribution. As an example, Dv50 can be conveniently determined using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method. In a specific example, the volume average particle size Dv50 of the positive electrode active material matrix is ​​100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or any combination of these values.

[0111] Furthermore, the volume average particle size (Dv50) of the positive electrode active material matrix is ​​200 nm to 400 nm. A suitable particle size of the positive electrode active material matrix is ​​beneficial when used in conjunction with other positive electrode materials, such as ternary positive electrode materials, to fully fill the excess pores of the ternary positive electrode material, increase the compaction density of the positive electrode sheet, reduce the consumption of electrolyte by the electrode pores, and thus improve the volumetric energy density of the battery.

[0112] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0113] The first carbon source is coated onto the surface of the positive electrode active material matrix and then calcined once to obtain a carbon-coated matrix.

[0114] The second carbon source is coated onto the surface of the carbon-coated matrix and then subjected to secondary calcination.

[0115] Both the first and second carbon sources have serrated spatial structures.

[0116] The above-mentioned method for preparing the positive electrode active material involves calcining a first carbon source and a second carbon source with a serrated spatial structure onto the surface of the positive electrode active material matrix. This double-layer coating structure prevents particle breakage of the positive electrode active material matrix, such as lithium iron phosphate particles, due to compression during charging and discharging, thus improving the pressure resistance of the positive electrode active material matrix. Furthermore, the first carbon source undergoes two calcinations, and the serrated structure at the interface between the inner and outer coating layers causes strain in the carbon lattice during the second calcination, generating lattice defects. This significantly improves the diffusion rate of lithium ions within the lattice. The second carbon source, with its serrated molecular structure, forms a serrated surface structure after calcination, which overcomes the material breakage problem at the edges and better disperses the active particles, protecting them from aggregation during the reaction.

[0117] Therefore, the positive electrode active material prepared by the above method can be used in combination with ternary positive electrode materials. While reducing porosity and electrolyte injection volume, it also has excellent conductivity and lithium ion diffusion performance, thereby improving battery capacity retention and reducing battery impedance while reducing the electrolyte injection coefficient.

[0118] In some embodiments, both the first carbon source and the second carbon source include components with conjugated molecular structures. For example, the molecular structures of the first carbon source and the second carbon source contain conjugated structures such as aromatic groups and alkenyl groups. The inclusion of components with conjugated molecular structures in the first carbon source and the second carbon source can further improve the conductivity of the coating layer, thereby improving the conductivity of the positive electrode active material.

[0119] In some embodiments, both the first carbon source and the second carbon source comprise materials having the following structure:

[0120] -(A-B-A) n -,

[0121] where n is an integer, A includes at least one of an aryl group and an alkenyl group, B includes an N atom, and 90° < bond angle of A-B-A < 180°. In a specific example, the bond angle can be measured by a single crystal X-ray diffraction method.

[0122] In some embodiments, the first carbon source and the second carbon source each independently include at least one of polybenzylamine, polyaniline, polypyrrole, and polyethylene diamine. It can be understood that the first carbon source and the second carbon source can be the same or different.

[0123] Taking polybenzylamine as an example, the structural formula of polybenzylamine is It has an inherent twisted molecular chain, in which benzene rings and amino groups are alternately connected in a non-linear manner to form a zigzag molecular chain. This zigzag structure may cause atomic-level edges and strains in the carbon lattice. During the second calcination process, the strain at the carbon lattice edge at the interface between the first carbon coating layer and the second carbon coating layer will induce distortion of the carbon lattice at high temperature, greatly improving the diffusion rate of lithium ions inside the carbon lattice. In addition, the outermost atomic-level zigzag structure can promote the stability of the material to reduce the damage to the electrode structure caused by the volume change of the material, which is beneficial to improving the cycle performance.

[0124] In some embodiments, the mass ratio of the first carbon source to the cathode active material matrix is (0.001 - 0.5):100. In a specific example, the mass ratio of the first carbon source to the cathode active material matrix is 0.001:100, 0.005:100, 0.01:100, 0.02:100, 0.03:100, 0.04:100, 0.05:100, 0.06:100, 0.07:100, 0.08:100, 0.09:100, 0.1:100, 0.12:100, 0.15:100, 0.18:100, 0.2:100, 0.25:100, 0.3:100, 0.35:100, 0.4:100, 0.45:100, 0.5:100 or the range composed of any two of these values.

[0125] Furthermore, the mass ratio of the first carbon source to the cathode active material matrix is (0.003 - 0.2):100. Further still, the mass ratio of the first carbon source to the cathode active material matrix is (0.01 - 0.2):100. Further still, the mass ratio of the first carbon source to the cathode active material matrix is (0.01 - 0.05):100.

[0126] By adjusting the mass ratio of the first carbon source to the cathode active material matrix, the thickness of the first carbon coating layer can be controlled. When the thickness of the first carbon coating layer is too low, it fails to achieve the effect of an electronic conductor and cannot form lattice defects with the second carbon coating layer during the secondary calcination process, thus failing to improve lithium-ion transport. Conversely, when the thickness of the first carbon coating layer is too high, the excessively long path will hinder the transport rate of lithium ions from the first carbon coating layer into the cathode active material matrix, or it may directly cause lithium ions to deposit in the carbon layer, making it difficult for lithium ions to be released or inserted, thus resulting in ineffective lithium-ion diffusion.

[0127] In some embodiments, the mass ratio of the first carbon source to the second carbon source is (1 to 10):(1 to 5). In a specific example, the mass ratio of the first carbon source to the second carbon source is 1:1, 1:2, 1:3, 1:4, 1:5, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, or any range of two of these values.

[0128] Furthermore, the mass ratio of the first carbon source to the second carbon source is 4:1 to 1:4. Even further, the mass ratio of the first carbon source to the second carbon source is 4:1 to 1:2. Even more further, the mass ratio of the first carbon source to the second carbon source is 2:1 to 1:2.

[0129] By adjusting the mass ratio of the first carbon source and the second carbon source, and by adjusting the thickness ratio of the first carbon coating layer and the second carbon coating layer, the tap density, conductivity, and lithium-ion diffusion coefficient of the positive electrode active material can be further improved.

[0130] In some embodiments, the temperatures for the primary and secondary calcinations are each independently between 200°C and 1000°C. In a specific example, the temperatures for the primary and secondary calcinations are each independently 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 620°C, 650°C, 680°C, 700°C, 720°C, 750°C, 780°C, 800°C, 850°C, 900°C, 950°C, 1000°C, or any range of two of these values. It is understood that the temperatures for the primary and secondary calcinations can be the same or different.

[0131] Furthermore, the temperatures for the first and second calcinations are each independently set at 600℃ to 800℃. The calcination temperature affects the ion diffusion rate and conductivity of the positive electrode active material. A suitable calcination temperature will make the lattice defects at the interface between the first and second carbon coating layers more uniform, thereby improving the electrical performance of the coated positive electrode active material.

[0132] In some embodiments, the times for the first and second calcinations are each independently 1 hour to 10 hours. In a specific example, the times for the first and second calcinations are each independently 1 hour, 2 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any range of two of these values. It is understood that the times for the first and second calcinations can be the same or different.

[0133] Furthermore, the calcination time for the first and second calcinations is independently set to 3 to 5 hours. Calcination time also affects the ion diffusion rate and conductivity of the positive electrode active material. A suitable calcination time will make the lattice defects at the interface between the first and second carbon coating layers more uniform, thereby improving the electrical performance of the coated positive electrode active material.

[0134] In some embodiments, the heating rate during the primary calcination and secondary calcination processes is independently 1°C / min to 10°C / min. In a specific example, the heating rate during the primary calcination and secondary calcination processes is independently 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or a range of any two of these values.

[0135] In some embodiments, coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface each independently include:

[0136] The monomer and the core material are mixed and ball-milled, and then doped acid and initiator are added to polymerize the monomer and form a carbon source coating the core material on the surface of the core material. The core material is a positive electrode active material matrix or a carbon-coated matrix, and the carbon source is a first carbon source or a second carbon source.

[0137] By mixing monomers with core materials and initiating monomer polymerization, the first and second carbon sources are coated onto the surface of the positive electrode active material matrix. The resulting coating layer is denser, and when used to prepare batteries, the DC resistance (DCR) value is lower, which is beneficial for improving the capacity value.

[0138] The monomer and the core material are thoroughly mixed by ball milling.

[0139] In one specific example, the solvent used for ball milling includes at least one of water, ethanol, isopropanone, and acetone.

[0140] In a specific example, the mixing and ball milling time for the monomer and core material is 10h to 16h. For example, the mixing and ball milling time is 10h, 11h, 12h, 13h, 14h, 15h, 16h, or any combination of these values.

[0141] In some embodiments, the doping acid includes at least one of hydrochloric acid, sulfuric acid, and nitric acid. It is understood that the doping acid used in coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface may be the same or different.

[0142] In some embodiments, the initiator includes a persulfate. Further, the initiator includes at least one selected from ammonium persulfate, potassium persulfate, and sodium persulfate. It is understood that the initiators used in coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface may be the same or different.

[0143] In some embodiments, the molar ratio of dopant acid, initiator, and monomer is (1-3):(0.5-2.5):(1-5). It is understood that the molar ratio of dopant acid, initiator, and monomer used in coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface can be the same or different.

[0144] In some embodiments, during the processes of coating the first carbon source onto the surface of the positive electrode active material matrix and coating the second carbon source onto the carbon-coated matrix surface, the monomer polymerization steps each independently include:

[0145] The monomer, core material, doped acid and initiator are reacted at 4℃~10℃ for 14h~18h, then washed and vacuum dried at 60℃~80℃ for 12h~18h.

[0146] In a specific example, the reaction temperature of the monomer, core material, doped acid, and initiator is 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, or any combination of these values. The reaction time of the monomer, core material, doped acid, and initiator is 14h, 15h, 16h, 17h, 18h, or any combination of these values.

[0147] In a specific example, the vacuum drying temperature is 60°C, 65°C, 70°C, 75°C, 80°C, or any combination of these values. The vacuum drying time is 12h, 13h, 14h, 15h, 16h, 17h, 18h, or any combination of these values.

[0148] In a specific example, the washing was performed with anhydrous ethanol and deionized water, respectively.

[0149] In other embodiments, coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface each independently include:

[0150] The carbon source and the core material are mixed and ball-milled for 10-16 hours, then washed and vacuum-dried at 60-80°C for 12-18 hours. The carbon source is either the first or the second carbon source, and the core material is either the positive electrode active material matrix or a carbon-coated matrix.

[0151] The method of coating by directly mixing the first and second carbon sources with the core material is simpler.

[0152] The carbon source and the core material are thoroughly mixed by ball milling. In a specific example, the solvent used for ball milling includes at least one of water, ethanol, isoacetone, and acetone.

[0153] In a specific example, the washing was performed with anhydrous ethanol and deionized water, respectively.

[0154] In a specific example, the ball milling time is 10h, 11h, 12h, 13h, 14h, 15h, 16h, or any combination of these values. The vacuum drying temperature is 60℃, 65℃, 70℃, 75℃, 80℃, or any combination of these values. The vacuum drying time is 12h, 13h, 14h, 15h, 16h, 17h, 18h, or any combination of these values.

[0155] In some embodiments, the positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate. Lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, as positive electrode active materials, suffer from problems such as low electronic conductivity and low lithium-ion diffusion rate. The aforementioned coating layer can mitigate these problems.

[0156] In some embodiments, the volume average particle size Dv50 of the positive electrode active material matrix is ​​100 nm to 500 nm. In a specific example, the volume average particle size Dv50 of the positive electrode active material matrix is ​​100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or any combination of these values.

[0157] Furthermore, the volume average particle size (Dv50) of the positive electrode active material matrix is ​​200 nm to 400 nm. A suitable matrix particle size is beneficial when the positive electrode active material matrix is ​​used in conjunction with other positive electrode materials, such as ternary positive electrode materials, to fully fill the excess pores of the ternary positive electrode material, increase the compaction density of the positive electrode sheet, reduce the consumption of electrolyte by the electrode pores, and thus improve the volumetric energy density of the battery.

[0158] A third aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes the positive active material of the first aspect of this application or the positive active material prepared by the preparation method of the positive active material of the second aspect of this application.

[0159] In some embodiments, the positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, and the positive electrode active material layer further includes a ternary positive electrode material with the molecular formula LiNi. x Co y Mn 1-x-y O2, x>0, y>0, or LiNi 0.8+a+b Co 0.15-a Al 0.05-b O2, 0≤a<0.15, 0≤b<0.05. For example, ternary cathode materials can be lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide (such as LiNi), etc. 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. In a specific example, lithium nickel cobalt manganese oxide can be LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 )wait.

[0160] By mixing the positive electrode active material with the ternary positive electrode material, the compaction density of the positive electrode sheet is increased, and the consumption of electrolyte by the electrode pores is reduced.

[0161] In some embodiments, the mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1–20):100. In a specific example, the mass ratio of the positive electrode active material to the ternary positive electrode material is 0.1:100, 0.2:100, 0.5:100, 1:100, 1.5:100, 2:100, 2.5:100, 3:100, 3.5:100, 4:100, 4.5:100, 5:100, 6:100, 7:100, 8:100, 9:100, 10:100, 12:100, 15:100, 18:100, 20:100, or any range of two of these values.

[0162] Furthermore, the mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1–10):100. Even further, the mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1–5):100. Even more specifically, the mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1–3):100.

[0163] In some embodiments, the ratio of the volume average particle size Dv50 of the positive electrode active material to that of the ternary positive electrode material is (0.1–20):100. In a specific example, the ratio of the volume average particle size Dv50 of the positive electrode active material to that of the ternary positive electrode material is 0.1:100, 0.2:100, 0.5:100, 1:100, 1.5:100, 2:100, 2.5:100, 3:100, 3.5:100, 4:100, 4.5:100, 5:100, 5.5:100, 6:100, 6.5:100, 7:100, 8:100, 9:100, 10:100, 12:100, 15:100, 18:100, 20:100, or a range of any two of these values.

[0164] Furthermore, the ratio of the volume average particle size (Dv50) of the positive electrode active material to that of the ternary positive electrode material is (0.1–12):100. Even further, the ratio of the volume average particle size (Dv50) of the positive electrode active material to that of the ternary positive electrode material is (0.1–10):100. Even further, the ratio of the volume average particle size (Dv50) of the positive electrode active material to that of the ternary positive electrode material is (1–10):100. Even further, the ratio of the volume average particle size (Dv50) of the positive electrode active material to that of the ternary positive electrode material is (3–7):100.

[0165] A suitable mass ratio of positive active material to ternary cathode material and a suitable Dv50 particle size ratio are beneficial for fully filling the excess pores of the ternary cathode material, increasing the compaction density of the cathode sheet, reducing electrolyte consumption due to electrode pores, and thus improving the volumetric energy density of the battery. When the mass ratio of positive active material to ternary cathode material is too low or the Dv50 particle size ratio is too high, the ternary cathode material has a large number of pores, resulting in low particle packing efficiency and uneven material distribution. These pore spaces in the cathode sheet, on the one hand, mean that the electrolyte floating within them contributes nothing to the capacity, and the extra electrolyte weight reduces the battery's energy density and increases the manufacturing cost of the cell; on the other hand, in ternary cathode materials such as high-nickel systems, a large amount of electrolyte in the pores can easily penetrate into the cracks of the ternary cathode material and react, further leading to a decline in battery performance. When the mass ratio of the positive electrode active material to the ternary positive electrode material is too high or the Dv50 particle size ratio is too low, the addition of a large number of positive electrode active material particles will affect the theoretical specific capacity of the ternary positive electrode material and reduce the capacity value.

[0166] In some embodiments, the porosity of the positive electrode sheet is 20% to 25%. In a specific example, the porosity of the positive electrode sheet is 20%, 21%, 21.2%, 21.4%, 21.6%, 21.8%, 22%, 22.1%, 22.2%, 22.4%, 22.6%, 22.8%, 23%, 24%, 25%, or any combination of these values. Further, the porosity of the positive electrode sheet is 21% to 23%. The lower porosity of the above-mentioned positive electrode sheet is beneficial to increasing the compaction density of the positive electrode sheet, reducing the consumption of electrolyte by the electrode pores, thereby increasing the volumetric energy density of the battery.

[0167] In some embodiments, the positive electrode active material 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.

[0168] In some embodiments, the positive electrode active material layer 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.

[0169] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0170] 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.).

[0171] 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, ternary positive electrode 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 after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0172] The secondary battery, battery module, battery pack, and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0173] The fourth aspect of this application provides a secondary battery.

[0174] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0175] In some embodiments, the electrolyte injection coefficient of the secondary battery is 2.1 g / Ah to 2.2 g / Ah. The electrolyte injection coefficient is a fixed constant determined during battery design, referring to the ratio of the injected electrolyte mass to the battery's designed discharge capacity. In a specific example, the electrolyte injection coefficient of the secondary battery is 2.1 g / Ah, 2.11 g / Ah, 2.12 g / Ah, 2.13 g / Ah, 2.14 g / Ah, 2.15 g / Ah, 2.16 g / Ah, 2.17 g / Ah, 2.18 g / Ah, 2.19 g / Ah, 2.2 g / Ah, or any range of two of these values. At the above electrolyte injection coefficients, the secondary battery still maintains a high capacity and cycle retention rate, but requires less electrolyte, which is beneficial for improving the battery's energy density.

[0176] Positive electrode sheet

[0177] The positive electrode sheet is the positive electrode sheet provided in the third aspect of this application, and will not be described in detail here.

[0178] Negative electrode sheet

[0179] 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.

[0180] 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.

[0181] 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.).

[0182] 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.

[0183] 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).

[0184] 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.

[0185] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0186] 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.

[0187] electrolytes

[0188] 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.

[0189] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0190] 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.

[0191] 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.

[0192] 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.

[0193] Separating membrane

[0194] 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.

[0195] 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.

[0196] In some embodiments, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0197] 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.

[0198] 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.

[0199] 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 2 This is an example of a square-structured secondary battery 5.

[0200] In some embodiments, refer to Figure 3 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. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a 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.

[0201] A fifth aspect of this application provides a battery module. The battery module includes a secondary battery as described in the fourth aspect of this application. In one example, the battery module is assembled using the secondary battery.

[0202] In some embodiments, the battery module may contain one or more secondary batteries, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0203] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 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.

[0204] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0205] A sixth aspect of this application provides a battery pack including the battery module of the fifth aspect of this application. In one example, the battery pack is assembled from the battery modules. In some embodiments, the battery pack may contain one or more battery modules, the specific number of which can be selected by those skilled in the art based on the application and capacity of the battery pack.

[0206] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 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.

[0207] The seventh aspect of this application also provides an electrical device, which includes at least one selected from the secondary battery provided in the fourth aspect of this application, the battery module provided in the fifth aspect of this application, and the battery pack provided in the sixth aspect of this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices may include, for example, mobile phones, laptops, etc.; electric vehicles may include, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0208] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0209] Figure 7 Here is an example of an electrical device 6. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0210] 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.

[0211] 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.

[0212] Example 1

[0213] Preparation of positive electrode active materials:

[0214] (1) Benzylamine and lithium iron phosphate with a Dv50 of 300 nm were mixed at a mass ratio of 0.01:100 and added to a mixed solution of ethanol and water. The mixture was ball-milled for 10 h. The concentration of benzylamine monomer in the mixed solution was 0.5 M. 0.5 M hydrochloric acid was added as a doping acid and 1 M ammonium persulfate was added as an initiator to oxidize the benzylamine monomer. The reaction system was then placed in an ice-water bath at 4 °C and stirred for 14 h. After the reaction was completed, the mixture was washed with anhydrous ethanol and deionized water, and dried under vacuum at 60 °C for 12 h to obtain a polybenzylamine-coated lithium iron phosphate precursor. The polybenzylamine-coated lithium iron phosphate precursor was calcined at 700 °C for 3 h under a nitrogen atmosphere at a heating rate of 5 °C / min to obtain N-doped carbon-coated lithium iron phosphate.

[0215] (2) The N-doped carbon-coated lithium iron phosphate obtained in (1) was further mixed with benzylamine. The mass ratio of benzylamine to that added in (1) was 2:1. Then, it was added to a mixed solution of ethanol and water and ball-milled for 2 hours. The concentration of benzylamine monomer in the mixed solution was 0.5 M. 0.5 M hydrochloric acid was added as a doping acid and 1 M ammonium persulfate was added as an initiator to oxidize the benzylamine monomer. The reaction system was then placed in an ice-water bath at 4 °C and stirred for 14 hours. After the reaction was completed, it was vacuum dried at 60 °C for 12 hours to obtain polybenzylamine-coated carbon-coated lithium iron phosphate. Then, it was calcined under a nitrogen atmosphere with a heating rate of 5 °C / min, a calcination temperature of 700 °C, and a calcination time of 3 hours. A second carbon coating layer with serrated edges was coated on the outside of the N-doped carbon-coated lithium iron phosphate. At the same time, lattice defects were formed at the interface between the N-doped carbon layer and the second carbon coating layer. Finally, the resulting coating was ground to obtain the lithium iron phosphate cathode active material of Example 1.

[0216] Preparation of the positive electrode sheet:

[0217] The lithium iron phosphate positive electrode active material (Dv50 = 300 nm), nickel cobalt manganese (NCM) ternary positive electrode material (Dv50 = 6 μm), conductive agent carbon black, binder polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) were mixed evenly in a weight ratio of 2.88:92.96:1.47:1.55:1.14 to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto the positive electrode current collector, and after drying, cold pressing and slitting, the positive electrode sheet was obtained.

[0218] Preparation of negative electrode sheet:

[0219] The active material artificial graphite, silicon dioxide, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC) were dissolved in deionized water at a weight ratio of 72.3:24.9:0.8:0.8:1.2 and mixed evenly to prepare a negative electrode slurry. The negative electrode slurry was coated onto a copper foil negative electrode current collector, and the negative electrode sheet was obtained after cold pressing.

[0220] Electrolyte preparation:

[0221] In an argon-atmospheric glove box, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed evenly in a volume ratio of 36:29:35. 1M LiPF6 lithium salt was added and dissolved in the organic solvent, and the mixture was stirred evenly to obtain the electrolyte.

[0222] Separating membrane:

[0223] Polypropylene film is used as the separator.

[0224] Preparation of lithium-ion batteries:

[0225] The positive electrode, separator, and negative electrode from Example 1 are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The resulting bare cell is then wound, tabs are welded onto it, and the cell is placed in an aluminum casing. It is then baked at 100°C to remove moisture, followed by the injection of electrolyte and sealing to obtain a non-charged battery. This non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing to obtain the lithium-ion battery of Example 1.

[0226] Examples 2 to 7

[0227] The difference between Examples 2 to 7 and Example 1 is that the mass ratio of benzylamine added in step (1) of the preparation of the positive electrode active material is different from that in step (2), and is adjusted to 3:1, 4:1, 1:1, 1:2, 1:3, and 1:4 respectively. The other steps are the same as those in Example 1.

[0228] Examples 8 to 9

[0229] The difference between Examples 8 and 9 and Example 1 is that in the preparation step (1) of the positive electrode active material, the mass ratio of lithium iron phosphate to benzylamine is different, and is adjusted to 100:0.005 and 100:0.02 respectively. The other steps are the same as in Example 1.

[0230] Examples 10 to 14

[0231] The difference between Examples 10 to 14 and Example 1 is that the calcination temperatures in steps (1) and (2) of the preparation of the positive electrode active material are different, and are adjusted to 300℃, 500℃, 600℃, 800℃ and 900℃ respectively. The other steps are the same as in Example 1.

[0232] Examples 15 to 19

[0233] The difference between Examples 15 to 19 and Example 1 is that the calcination time in steps (1) and (2) of the preparation of the positive electrode active material is different, and is adjusted to 1h, 2h, 4h and 5h respectively. The other steps are the same as those in Example 1.

[0234] Example 20

[0235] The difference between Example 20 and Example 1 lies in the preparation process of the positive electrode active material; all other steps are the same as in Example 1. Specifically, the preparation process of the positive electrode active material is as follows:

[0236] (1) Polybenzylamine and lithium iron phosphate with a Dv50 of 300 nm were mixed at a mass ratio of 0.01:100 and added to a mixed solution of ethanol and water. The mixture was ball-milled for 10 h to ensure uniform mixing of polybenzylamine and lithium iron phosphate. The mixture was then washed with anhydrous ethanol and deionized water and vacuum-dried at 60 °C for 12 h to obtain a polybenzylamine-coated lithium iron phosphate precursor. The polybenzylamine-coated lithium iron phosphate precursor was heated to 700 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 3 h to obtain N-doped carbon-coated lithium iron phosphate.

[0237] (2) The nitrogen-doped carbon-coated lithium iron phosphate obtained in (1) was further mixed with polyaniline. The mass ratio of polybenzylamine to the polybenzylamine added in (1) was 2:1. Then, it was added to a mixed solution of ethanol and water and ball-milled for 2 hours. After washing with anhydrous ethanol and deionized water, it was vacuum dried at 60°C and then calcined under nitrogen. The heating rate was 5°C / min, the calcination temperature was 700°C, and the calcination time was 3 hours. A second carbon coating layer with serrated edges was formed on the outside of the carbon-coated lithium iron phosphate. At the same time, lattice defects were formed at the interface between the N-doped carbon layer and the second carbon coating layer. Finally, the resulting coating was ground to obtain the lithium iron phosphate positive electrode active material of Example 20.

[0238] Examples 21-22

[0239] The difference between Examples 21 and 22 and Example 1 is that the monomers in steps (1) and (2) of the preparation process of the positive electrode active material are different, and are respectively adjusted to aniline and ethylenediamine. The other steps are the same as those in Example 1.

[0240] Example 23

[0241] The difference between Example 23 and Example 1 is that in step (1) of the preparation process of the positive electrode active material, the first monomer is different; pyrrole is used instead of benzylamine in Example 1. The other steps are the same as in Example 1.

[0242] Example 24

[0243] The difference between Example 24 and Example 1 is that the matrix of the positive electrode active material is different; the lithium iron phosphate matrix in Example 1 is replaced with a lithium manganese phosphate matrix. All other steps are the same as in Example 1.

[0244] Examples 25-26

[0245] The difference between Examples 25 and 26 and Example 1 is that the volume average particle size Dv50 of the lithium iron phosphate matrix is ​​different during the preparation of the positive electrode active material, and is adjusted to 200 nm and 400 nm respectively. Correspondingly, the volume average particle size Dv50 of the positive electrode active material is adjusted to 200 nm and 400 nm respectively during the preparation of the positive electrode sheet. All other steps are the same as in Example 1.

[0246] Examples 27 to 31

[0247] The difference between Examples 27 to 31 and Example 1 is that the mass ratio of lithium iron phosphate positive electrode active material to ternary positive electrode material is different during the preparation of the positive electrode sheet, and is adjusted to 0.005:1, 0.02:1, 0.05:1, 0.1:1, and 0.2:1 respectively. The other steps are the same as in Example 1.

[0248] Examples 32 to 34

[0249] The difference between Examples 32-34 and Example 1 lies in the electrolyte injection coefficient during the lithium-ion battery preparation process, which was adjusted from 2.2 g / Ah to 2.15 g / Ah, 2.10 g / Ah, and 2.05 g / Ah, respectively. All other steps are the same as in Example 1.

[0250] Comparative Example 1

[0251] The difference between Comparative Example 1 and Example 1 lies in the different positive electrode active materials; correspondingly, the preparation process of the positive electrode sheet differs. All other steps are the same as in Example 1. The specific preparation process of the positive electrode sheet is as follows:

[0252] Using uncoated lithium iron phosphate as the positive electrode active material, lithium iron phosphate (Dv50 = 300 nm), NCM (nickel-cobalt-manganese) ternary positive electrode material (Dv50 = 6 μm), conductive agent carbon black, binder polyvinylidene fluoride (PVDF), and solvent N-methylpyrrolidone (NMP) were thoroughly mixed in a weight ratio of 2.88:92.96:1.47:1.55:1.14 to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto a positive electrode current collector, followed by drying, cold pressing, and slitting to obtain the positive electrode sheet of Comparative Example 1.

[0253] Comparative Example 2

[0254] The difference between Comparative Example 2 and Example 1 lies in the preparation process of the lithium iron phosphate cathode active material. All other steps are the same as in Example 1. Specifically, the preparation process of the lithium iron phosphate cathode active material in Comparative Example 2 is as follows:

[0255] Benzylamine and lithium iron phosphate with a Dv50 of 300 nm were mixed at a mass ratio of 0.02:100 and added to a mixed solution of ethanol and water. The mixture was ball-milled for 10 h. The concentration of benzylamine monomer in the mixed solution was 0.5 M. 0.5 M hydrochloric acid was added as a doping acid and 1 M ammonium persulfate as an initiator to oxidize the benzylamine monomer. The reaction system was then placed in an ice-water bath at 4 °C and stirred for another 14 h. After the reaction, the mixture was washed with anhydrous ethanol and deionized water, and then vacuum-dried at 60 °C for 12 h to obtain a polybenzylamine-coated lithium iron phosphate precursor. The polybenzylamine-coated lithium iron phosphate precursor was calcined at 700 °C for 3 h under a nitrogen atmosphere at a heating rate of 5 °C / min to obtain N-doped carbon-coated lithium iron phosphate.

[0256] Comparative Example 3

[0257] The difference between Comparative Example 3 and Example 1 lies in the preparation process of the lithium iron phosphate cathode active material. All other steps are the same as in Example 1. Specifically, the preparation process of the lithium iron phosphate cathode active material in Comparative Example 3 is as follows:

[0258] (1) Sucrose and lithium iron phosphate with a Dv50 of 300 nm were mixed at a mass ratio of 0.01:100 and added to a mixed solution of ethanol and water. The mixture was ball-milled for 10 h to ensure uniform mixing of sucrose and lithium iron phosphate. The mixture was then washed with anhydrous ethanol and deionized water and vacuum-dried at 60 °C for 12 h to obtain a sucrose-coated lithium iron phosphate precursor. The sucrose-coated lithium iron phosphate precursor was heated to 700 °C under a nitrogen atmosphere at a heating rate of 5 °C / min and calcined for 3 h to obtain carbon-coated lithium iron phosphate.

[0259] (2) The carbon-coated lithium iron phosphate obtained in (1) was further mixed with sucrose, with the mass ratio of sucrose to that added in (1) being 2:1. The mixture was then added to a mixed solution of ethanol and water and ball-milled for 2 hours. After washing with anhydrous ethanol and deionized water, it was vacuum dried at 60°C and calcined under nitrogen at a heating rate of 5°C / min, a calcination temperature of 700°C, and a calcination time of 3 hours, thus coating the carbon-coated lithium iron phosphate with a second carbon coating layer. Finally, the resulting coating was ground to obtain the lithium iron phosphate positive electrode active material of Comparative Example 3.

[0260] Comparative Example 4

[0261] The difference between Comparative Example 4 and Example 1 lies in the preparation process of the lithium iron phosphate cathode active material. All other steps are the same as in Example 1. Specifically, the preparation process of the lithium iron phosphate cathode active material in Comparative Example 4 is as follows:

[0262] (1) Benzylamine and lithium iron phosphate with a Dv50 of 300 nm were mixed at a mass ratio of 0.01:100 and added to a mixed solution of ethanol and water. The mixture was ball-milled for 10 h. The concentration of benzylamine monomer in the mixed solution was 0.5 M. 0.5 M hydrochloric acid was added as a doping acid and 1 M ammonium persulfate was added as an initiator to oxidize the benzylamine monomer. The reaction system was then placed in an ice-water bath at 4 °C and stirred for 14 h. After the reaction was completed, the mixture was washed with anhydrous ethanol and deionized water, and dried under vacuum at 60 °C for 12 h to obtain a polybenzylamine-coated lithium iron phosphate precursor. The polybenzylamine-coated lithium iron phosphate precursor was calcined at 700 °C for 3 h under a nitrogen atmosphere at a heating rate of 5 °C / min to obtain N-doped carbon-coated lithium iron phosphate.

[0263] (2) The N-doped carbon-coated lithium iron phosphate obtained in (1) was further added to a mixed solution of ethanol and water and ball-milled for 2 hours. Then it was vacuum dried at 60°C for 12 hours and calcined under a nitrogen atmosphere. The heating rate was 5°C / min, the calcination temperature was 700°C, and the calcination time was 3 hours. Finally, the calcined product was ground to obtain the lithium iron phosphate positive electrode active material of Comparative Example 4.

[0264] Comparative Example 5

[0265] The difference between Comparative Example 5 and Example 1 is that step (1) in the preparation process of the lithium iron phosphate cathode active material is different, while the remaining steps are the same as in Example 1. Specifically, step (1) in the preparation process of the lithium iron phosphate cathode active material in Comparative Example 5 is as follows:

[0266] (1) Starch and lithium iron phosphate with a Dv50 of 300 nm were mixed at a mass ratio of 0.01:100 and added to a mixed solution of ethanol and water. The mixture was ball-milled for 10 h to ensure uniform mixing of starch and lithium iron phosphate. The mixture was then washed with anhydrous ethanol and deionized water and vacuum-dried at 60 °C for 12 h to obtain a starch-coated lithium iron phosphate precursor. The starch-coated lithium iron phosphate precursor was then heated to 700 °C under a nitrogen atmosphere at a heating rate of 5 °C / min and calcined for 3 h to obtain carbon-coated lithium iron phosphate.

[0267] Comparative Example 6

[0268] The difference between Comparative Example 6 and Example 1 is that step (2) in the preparation process of the lithium iron phosphate cathode active material is different, while the remaining steps are the same as in Example 1. Specifically, step (2) in the preparation process of the lithium iron phosphate cathode active material in Comparative Example 6 is as follows:

[0269] (2) The nitrogen-doped carbon-coated lithium iron phosphate obtained in (1) was further mixed with starch. The mass ratio of starch to benzylamine added in (1) was 2:1. Then, it was added to a mixed solution of ethanol and water and ball-milled for 2 hours. After washing with anhydrous ethanol and deionized water, it was vacuum dried at 60°C and then calcined under nitrogen. The heating rate was 5°C / min, the calcination temperature was 700°C, and the calcination time was 3 hours. A second carbon coating layer was formed on the outside of the carbon-coated lithium iron phosphate. Finally, the resulting coating was ground to obtain the lithium iron phosphate positive electrode active material of Comparative Example 6.

[0270] Comparative Example 7

[0271] The difference between Comparative Example 7 and Example 24 lies in the different positive electrode active materials; correspondingly, the preparation process of the positive electrode sheet differs. All other steps are the same as in Example 24. The specific preparation process of the positive electrode sheet is as follows:

[0272] Using uncoated lithium manganese phosphate as the positive electrode active material, lithium manganese phosphate (Dv50 = 300 nm), NCM (nickel-cobalt-manganese) ternary positive electrode material (Dv50 = 6 μm), conductive agent carbon black, binder polyvinylidene fluoride (PVDF), and solvent N-methylpyrrolidone (NMP) were thoroughly mixed in a weight ratio of 2.88:92.96:1.47:1.55:1.14 to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto a positive electrode current collector, followed by drying, cold pressing, and slitting to obtain the positive electrode sheet of Comparative Example 7.

[0273] Test section:

[0274] (1) The relevant parameters of the positive electrode active material and the positive electrode sheet were tested as follows:

[0275] 1. Morphological test

[0276] The positive electrode active material was tested using a ZEISS Sigma 300 scanning electron microscope, and then the morphology of the sample was observed in accordance with the standard JY / T010-1996. Figure 8 This is a SEM image of the positive electrode active material prepared in Example 1.

[0277] 2. Test of the total thickness of the first and second carbon coating layers

[0278] The electrode sheet before cold pressing was cut into 6cm × 6cm samples with scissors, and then polished using an IB-19500CP ion section polisher to obtain polished samples with cut surfaces. The samples were then tested using a ZEISS sigma 300 instrument according to standard JY / T010-1996. Ten different locations were randomly selected from the test samples for testing, and the average value was used to obtain the thickness of the first carbon coating layer, the second carbon coating layer, and the total thickness.

[0279] 3. Lattice defect testing

[0280] Taking Example 1 as an example, 2 mg of the positive electrode active material prepared in Example 1 was added to 10 mL of ethylene glycol solution for ultrasonic dispersion. The dispersed solution was then dropped onto a copper grid, dried, and observed using a high-resolution transmission electron microscope (HRTEM, JEM-2010). Figure 9 The image shows an HRTEM image of the positive electrode active material prepared in Example 1. It can be seen from the image that the lattice arrangement at the interface between the first carbon coating layer and the second carbon coating layer is irregular, which proves the existence of lattice defects.

[0281] 4. Tap density test

[0282] The test can be performed using a powder tap density tester (such as Dandong Baite BT-300) and a 25mL graduated cylinder, in accordance with standard GB / T 5162-2006.

[0283] 5. Conductivity test

[0284] The impedance of the powder (pressed into discs) was tested using an electrochemical workstation. The conductivity of the sample was calculated using the following formula:

[0285] σ=l / (S×R) (1)

[0286] In the formula, R—resistance, Ω; l—thickness of the disc, cm; S—cross-sectional area of ​​the disc, cm² 2 σ - Conductivity, S / cm. Before testing, Ag paste was symmetrically coated on both sides of the sample as point electrodes, with an area of ​​0.5 cm². 2 During the test, the AC bias amplitude was 20mV, and the frequency range was 10-10. 6 Hz.

[0287] 6. Test of lithium-ion diffusion coefficient

[0288] Taking the GITT method as an example, the steps for testing the lithium-ion diffusion coefficient of the positive electrode active material are as follows:

[0289] The positive electrode active material was ground into a powder microelectrode; the powder microelectrode was then connected to an electrochemical workstation for coulometric titration. A pulsed current of 20 μA was used, with a titration time of 1 h followed by a 4 h interval. (Note: To compare the effects of pulsed current and time, parallel experiments at 10 μA for 10 min can be performed). After obtaining the GITT curve, the lithium-ion diffusion coefficient was calculated using the following formula:

[0290] Where D is the lithium-ion diffusion coefficient; i is the applied current of 20 μA; Vm is the molar volume of the positive electrode active material; F is the Faraday constant; A is the electrode surface area; dE / dδ is the slope of the coulometric titration curve, i.e., the slope of the open-circuit potential versus Li concentration curve at a certain concentration in the electrode; (dE) / (dt) 1 / 2 ) represents the polarization voltage relative to t 1 / 2 The slope of the curve.

[0291] 7. Porosity test

[0292] Taking Example 1 as an example, the porosity test process of the positive electrode sheet is as follows:

[0293] Testing Environment: Pre-treatment was performed in a standard laboratory: More than 20 round discs with good appearance and no powder shedding from the edges were selected using tweezers and placed into a sample cup. The number of discs was recorded, and the apparent volume was calculated. Testing: The sample cup containing the sample was placed in a true density analyzer. The testing system was sealed, and helium gas was introduced according to the procedure. The gas pressure in the sample chamber and expansion chamber was measured, and the true volume was calculated according to Bohr's Law (PV = nRT), thus obtaining the porosity of the sample. Sample cup volume: 3.5 cm³. 3 Analyzed gas: Helium.

[0294] 8. Element Mapping Test

[0295] Sample preparation: Cut the positive electrode sheet into 5mm×5mm pieces with scissors and stick it on the sample stage with conductive adhesive;

[0296] Parameter settings: Mode: In-lens, Voltage: 20kV, Aperture: 60μm, Working distance: 8.5mm

[0297] Test procedure: Select 3 locations for point scanning, refer to standard GB / T 17359-2012, and use a Hitachi scanning electron microscope SU390.

[0298] Figure 10 The mapping image of the positive electrode sheet prepared in Example 1 is shown below. Figure 10 As can be seen, lithium iron phosphate is uniformly dispersed on the positive electrode.

[0299] (2) Performance testing of lithium-ion batteries

[0300] 1. Capacity test

[0301] Taking Example 1 as an example, the battery capacity test process is as follows: At 25°C, the battery corresponding to Example 1 is charged to 4.25V at a constant current of 1 / 3C, then charged to a current of 0.05C at a constant voltage of 4.25V, left to stand for 5 minutes, and then discharged to 2.8V at 1 / 3C. The resulting capacity is recorded as the initial capacity C0. The capacity ratio is the ratio of the capacity value obtained from different examples or comparative examples to the capacity value of Example 1.

[0302] 2. Battery capacity retention test

[0303] Taking Example 1 as an example, the battery capacity retention rate test process is as follows: At 25°C, the battery corresponding to Example 1 is charged to 4.25V at a constant current of 1 / 3C, then charged to a current of 0.05C at a constant voltage of 4.25V, left to rest for 5 minutes, and then discharged to 2.8V at 1 / 3C. The resulting capacity is recorded as the initial capacity C0. The above steps are repeated for the same battery, and the discharge capacity Cn of the battery after the nth cycle is recorded. Then, the battery capacity retention rate Pn after each cycle is Pn = Cn / C0 × 100%. With the 190 points P1, P2...P190 as the vertical axis and the corresponding number of cycles as the horizontal axis, the following results are obtained: Figure 11 The graph shown is a curve of capacity retention versus cycle number for the lithium-ion battery of Example 1.

[0304] During this test, the first cycle corresponds to n=1, the second cycle to n=2, ..., the 190th cycle to n=190. The battery capacity retention rate data corresponding to Example 1 in Table 1 is the data measured after 190 cycles under the above test conditions, i.e., the value of P190. The test process for the comparative example and other examples is the same as above.

[0305] 3. DC resistance test for DCR

[0306] Taking Example 1 as an example, the DCR DC impedance test process is as follows: At 25°C, the battery corresponding to Example 1 is charged to 4.25V with a constant current of 1 / 3C, the SOC is adjusted to 50% with a rate of 1 / 3C, and then pulsed discharged with a constant current of 4C for 30s. After resting for 5min, the ratio of the voltage difference before and after the pulse discharge to the current difference is the DCR DC impedance value.

[0307] 4. Energy density test

[0308] At room temperature, the capacitor was charged to 4.2V at a standard rate of 0.33C, then charged to 0.05C at a constant voltage of 4.2V. After standing for 10 minutes, it was discharged to 2.8V at 0.33C. The discharge energy was recorded, and then the energy density during discharge was calculated.

[0309] Energy density (Wh / L) = Discharge energy (Wh) / Lithium-ion secondary battery volume (L)

[0310] The process parameters and test data for the preparation of the above embodiments and comparative examples are shown in Tables 1 and 2 below:

[0311] Table 1

[0312]

[0313]

[0314] As can be seen from Table 1, the thickness ratio of the first carbon coating layer and the second carbon coating layer is different in Examples 1 to 7. By adjusting the thickness ratio of the first carbon coating layer and the second carbon coating layer, the tap density, conductivity and lithium-ion diffusion coefficient of the lithium iron phosphate cathode material can be further improved.

[0315] The total thickness of the first carbon coating layer and the second carbon coating layer in Examples 8 to 9 is different from that in Example 1. The total thickness affects the conductivity, and thus affects electron transfer and ion diffusion.

[0316] Examples 10-14 differ from Example 1 in calcination temperature, and Examples 15-19 differ from Example 1 in calcination time. Table 1 shows that calcination temperature and time affect the ion diffusion rate and conductivity of the positive electrode active material. Suitable calcination temperature and time result in more uniform lattice defects at the interface between the first and second carbon coating layers, thereby improving the electrical performance of the coated positive electrode active material.

[0317] In Comparative Example 1, the lithium iron phosphate was not coated; in Comparative Example 2, the positive electrode active material was only coated with one layer; and in Comparative Example 3, conventional double-layer coating was used. The tap density, conductivity, and ion diffusion coefficient of the positive electrode active materials in Comparative Examples 1, 2, and 3 were all significantly lower than those in the examples.

[0318] Table 2

[0319]

[0320] As can be seen from Table 2, the difference between Example 20 and Example 1 is that Example 20 directly uses polybenzylamine for coating, without starting from polymerization. The carbon layer directly coated with polybenzylamine may not be as dense as the carbon layer obtained by monomer polymerization, resulting in a larger DCR value, which affects the capacity value, but the performance is still better than the comparative example.

[0321] Examples 21 to 23 use different materials for the coating layer than Example 1, and their electrical performance is better than that of Comparative Example 1. This shows that using other materials with serrated spatial structures as carbon sources to coat the surface of the positive electrode active material matrix can also improve battery performance.

[0322] Example 24 uses a different positive electrode active material matrix than Example 1, and its electrical performance is better than that of Comparative Example 7, indicating that using other positive electrode active material matrices can also improve battery performance.

[0323] Examples 25 and 26 use a different Dv50 of the positive electrode active material matrix than Example 1, and their electrical performance is better than that of Comparative Example 1, indicating that using positive electrode active material matrices of different sizes can also improve battery performance.

[0324] Compared with Example 1, Examples 27 to 31 show different proportions of LFP particles added when making the positive electrode sheet. The mass ratio of LFP to NCM will affect the capacity of the cell to a certain extent. Therefore, it is necessary to select an appropriate amount of LFP to match with ternary particles.

[0325] Examples 32 to 34 differ from Example 1 in their electrolyte injection coefficients. As the electrolyte injection coefficient decreases, the cell capacity and cycle retention rate also decrease. When the electrolyte injection coefficient is 2.15 g / Ah, the capacity and cycle retention rate are basically the same as in Example 1, but the amount of electrolyte used is reduced, and the energy density increases from 201 Wh / L to 203 Wh / L, which is beneficial to improving the cell's energy density.

[0326] The positive electrode active material of Comparative Example 1 was not carbon coated, the positive electrode active material of Comparative Example 2 was only coated with one layer, and the positive electrode active material of Comparative Example 3 was coated with a traditional carbon coating layer. The capacity retention, DCR value and energy density of Comparative Examples 1, 2 and 3 were significantly lower than those of the Example.

[0327] The positive electrode active material of Comparative Example 4 was coated once with polybenzylamine, which has a serrated spatial structure, as the carbon source. However, after two calcinations, the capacity retention rate of the prepared battery was significantly lower. This is because with only one carbon coating layer, even after two calcinations, lattice defects cannot be formed, resulting in lower lithium-ion diffusion rate and electronic conductivity.

[0328] In Comparative Example 5, the molecules of the first carbon source do not have a serrated spatial structure, and in Comparative Example 6, the molecules of the second carbon source do not have a serrated spatial structure. The capacity retention rates of the positive electrode active materials in Comparative Examples 5 and 6 are significantly lower. This is because when either the molecules of the first carbon source or the molecules of the second carbon source do not have a serrated spatial structure, no lattice defects are formed at the interface between the first carbon coating layer and the second carbon coating layer, resulting in lower lithium-ion diffusion rate and electronic conductivity.

[0329] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0330] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A positive electrode active material, comprising: A cathode active material matrix, a first carbon coating layer, and a second carbon coating layer, wherein the first carbon coating layer coats the surface of the cathode active material matrix, and the second carbon coating layer coats the surface of the first carbon coating layer; There are lattice defects at the interface between the first carbon coating layer and the second carbon coating layer, and the second carbon coating layer has a serrated surface.

2. The positive electrode active material according to claim 1, characterized in that, The lattice defects include line defects.

3. The positive electrode active material according to claim 1, characterized in that, The second carbon coating layer has an atomic-level serrated surface.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that, At least one of the first carbon coating layer and the second carbon coating layer is a nitrogen-doped carbon layer.

5. The positive electrode active material according to any one of claims 1 to 3, characterized in that, The total thickness of the first carbon coating layer and the second carbon coating layer is 10 nm to 30 nm.

6. The positive electrode active material according to any one of claims 1 to 3, characterized in that, The thickness ratio of the first carbon coating layer to the second carbon coating layer is (1 to 10):(1 to 5).

7. The positive electrode active material according to any one of claims 1 to 3, characterized in that, The cathode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate.

8. The positive electrode active material according to any one of claims 1 to 3, characterized in that, The volume average particle size Dv50 of the cathode active material matrix is 100 nm to 500 nm.

9. A preparation method of a cathode active material, comprising the following steps: Coat a first carbon source on the surface of the cathode active material matrix, and perform a first calcination to obtain a carbon-coated matrix; Coat a second carbon source on the surface of the carbon-coated matrix, and perform a second calcination; the second carbon coating layer coats the surface of the first carbon coating layer; there are lattice defects at the interface between the first carbon coating layer and the second carbon coating layer; The molecules of the first carbon source and the second carbon source both have a serrated spatial structure.

10. The method for preparing the positive electrode active material according to claim 9, characterized in that, Both the first carbon source and the second carbon source include components with a conjugated molecular structure.

11. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, Both the first carbon source and the second carbon source include materials with the following structure: -(A-B-A) n -, Wherein, n is an integer, A includes at least one of an aromatic group and an alkenyl group, B includes an N atom, and 90° < the bond angle of A - B - A < 180°.

12. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The first carbon source and the second carbon source each independently include at least one of polybenzylamine, polyaniline, polypyrrole, and polyethylene diamine.

13. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The mass ratio of the first carbon source to the cathode active material matrix is (0.001 to 0.5):

100.

14. The method for preparing the positive electrode active material according to claim 13, characterized in that, The mass ratio of the first carbon source to the cathode active material matrix is (0.003 to 0.2):

100.

15. The method for preparing the positive electrode active material according to claim 13, characterized in that, The mass ratio of the first carbon source to the cathode active material matrix is (0.01 to 0.2):

100.

16. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The mass ratio of the first carbon source to the second carbon source is (1 to 10):(1 to 5).

17. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The temperature of the first calcination and the second calcination are each independently 200°C to 1000°C.

18. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The temperature of the first calcination and the second calcination are each independently 600°C to 800°C.

19. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The time of the first calcination and the second calcination are each independently 1 h to 10 h.

20. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The time of the first calcination and the second calcination are each independently 3 h to 5 h.

21. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, [[ID=]]The step of coating the first carbon source on the surface of the cathode active material matrix and the step of coating the second carbon source on the surface of the carbon-coated matrix each independently include: The monomer and the core material are mixed and ball-milled, and then a doping acid and an initiator are added to polymerize the monomer, forming a carbon source coating the core material on the surface of the core material. The core material is the positive electrode active material matrix or the carbon-coated matrix, and the carbon source is the first carbon source or the second carbon source.

22. The method for preparing the positive electrode active material according to claim 21, characterized in that, The doped acid includes at least one of hydrochloric acid, sulfuric acid, and nitric acid.

23. The method for preparing the positive electrode active material according to claim 21, characterized in that, The initiator includes at least one of ammonium persulfate, potassium persulfate, and sodium persulfate.

24. The method for preparing the positive electrode active material according to claim 21, characterized in that, The molar ratio of the doped acid, the initiator and the monomer is (1~3):(0.5~2.5):(1~5).

25. The method for preparing the positive electrode active material according to claim 21, characterized in that, In the processes of coating the first carbon source onto the surface of the positive electrode active material matrix and coating the second carbon source onto the carbon-coated matrix surface, the steps of monomer polymerization each independently include: The monomer, the core material, the doped acid, and the initiator are reacted at 4°C to 10°C for 14 to 18 hours, then washed and vacuum dried at 60°C to 80°C for 12 to 18 hours.

26. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The steps of coating the first carbon source onto the surface of the positive electrode active material substrate and coating the second carbon source onto the carbon-coated substrate surface each independently include: The carbon source and the core material are mixed and ball-milled for 10-16 hours, then washed and vacuum-dried at 60-80°C for 12-18 hours. The carbon source is either the first carbon source or the second carbon source, and the core material is either the positive electrode active material matrix or the carbon-coated matrix.

27. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate.

28. The method for preparing the positive electrode active material according to any one of claims 9 to 10, characterized in that, The volume average particle size Dv50 of the positive electrode active material matrix is ​​100nm~500nm.

29. A positive electrode plate, comprising: Positive current collector; as well as A positive electrode active material layer is located on at least one surface of the positive electrode current collector, and the positive electrode active material layer comprises the positive electrode active material as described in any one of claims 1 to 8 or the positive electrode active material prepared by the preparation method of the positive electrode active material as described in any one of claims 9 to 28.

30. The positive electrode sheet according to claim 29, characterized in that, The positive electrode active material matrix includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, and the positive electrode active material layer further includes a ternary positive electrode material with the molecular formula LiNi. x Co y Mn 1-x-y O2, x>0, y>0, or LiNi 0.8+a+b Co 0.15-a Al 0.05-b O2, 0≤a<0.15, 0≤b<0.

05.

31. The positive electrode sheet according to claim 30, characterized in that, The mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1~20):

100.

32. The positive electrode sheet according to claim 30, characterized in that, The mass ratio of the positive electrode active material to the ternary positive electrode material is (0.1~10):

100.

33. The positive electrode sheet according to claim 30, characterized in that, The ratio of the volume average particle size Dv50 of the positive electrode active material to that of the ternary positive electrode material is (0.1~20):

100.

34. The positive electrode sheet according to claim 30, characterized in that, The ratio of the volume average particle size Dv50 of the positive electrode active material to that of the ternary positive electrode material is (0.1~12):

100.

35. The positive electrode sheet according to any one of claims 29 to 34, characterized in that, The porosity of the positive electrode sheet is 20%~25%.

36. A secondary battery comprising the positive electrode sheet as described in any one of claims 29 to 35.

37. The secondary battery according to claim 36, characterized in that, The electrolyte injection coefficient of the secondary battery is 2.1 g / Ah to 2.2 g / Ah.

38. A battery module comprising the secondary battery as described in any one of claims 36 to 37.

39. A battery pack comprising the battery module of claim 38.

40. An electrical device comprising at least one selected from the secondary battery of any one of claims 36-37, the battery module of claim 38, and the battery pack of claim 39.

Images