Secondary battery, positive electrode material and preparation method therefor, and electric device

Abstract

A secondary battery, a positive electrode material and a preparation method therefor, and an electric device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet comprises a positive electrode active material. The positive electrode active material comprises a phosphate, a carbon layer, and a hydrophobic layer. The carbon layer covers the surface of the phosphate. The hydrophobic layer covers the surface of the carbon layer. The infrared absorption spectrum of the hydrophobic layer has characteristic peaks at the following positions: 1451 cm-1 to 1491 cm-1, and / or 900 cm1 to 850 cm-1. By coating the surface of the phosphate having the coating of the carbon layer with the hydrophobic layer, the water absorption of a phosphate material can be reduced, thereby improving the production efficiency of secondary batteries.

Description

Secondary batteries, cathode materials and preparation methods, and electrical equipment

[0001] Cross-references to related applications

[0002] This application claims priority to patent application No. 2025100963017, filed on January 21, 2025, entitled "Secondary Battery, Positive Electrode Material and Preparation Method, Electrical Device", the entirety of which is incorporated herein by reference.

[0003] Technical Field

[0004] This application relates to the field of secondary battery technology, and in particular to secondary batteries, cathode materials and preparation methods, and electrical equipment. Background Technology

[0005] The cathode material is a crucial factor determining the performance of a rechargeable battery; its energy density, cycle life, and other properties are closely related to the cathode material. Among various cathode materials for rechargeable batteries, phosphate cathode materials have attracted widespread attention due to their advantages such as structural stability, good thermal stability, and high specific power. However, phosphates are highly hygroscopic, and during battery manufacturing, prolonged vacuum drying is typically used to remove moisture from the phosphates, leading to extended production cycles, increased energy consumption, and reduced production efficiency. Summary of the Invention

[0006] In view of this, the secondary battery, positive electrode material, preparation method, and electrical equipment provided in this application can reduce the water absorption of phosphate materials, thereby improving the production efficiency of secondary batteries.

[0007] The first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the positive electrode includes a positive active material, which comprises a phosphate, a carbon layer, and a hydrophobic layer, wherein the carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0008] In the technical solution of this application embodiment, a hydrophobic layer is coated on the surface of a phosphate material with a carbon layer coating. The hydrophobic properties of the hydrophobic layer are used to transform the phosphate material into a hydrophobic material, thereby reducing the water absorption of the phosphate material. The positive electrode active material provided in this application exhibits significantly reduced water absorption during transportation and storage. In the manufacturing process of secondary batteries, it can reduce the time required for vacuum drying to remove moisture, greatly reducing the water absorption of the phosphate positive electrode active material. This significantly reduces the moisture adsorbed on the positive electrode sheet during secondary battery manufacturing, improves the efficiency of the secondary battery dehydration process, shortens the secondary battery production cycle, reduces energy consumption, and increases production efficiency. The material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. The hydrophobic layer possesses the aforementioned characteristic peaks of C-C and CH bonds, exhibiting strong hydrophobicity. Coating this layer onto the carbon-coated phosphate surface can reduce the phosphate's water absorption. Phosphates possess advantages such as structural stability and good thermal stability; their application in positive electrode active materials can improve the cycle performance of secondary batteries. The carbon layer coating the phosphate surface can enhance the phosphate's electronic conductivity, thereby improving the charge-discharge efficiency of the secondary battery.

[0009] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin have extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer with a polyolefin, the hydrophobicity of the CH groups can be used to hydrophobize the surface of the phosphate material with a carbon layer, thereby reducing the water absorption of the phosphate material.

[0010] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0011] In any embodiment, the number average molecular weight of the polyolefin is 2000-4000. By controlling the number average molecular weight of the polyolefin within the above range, the polyolefin can be made to have good solubility in the solvent, and when the polyolefin is coated onto the surface of the phosphate material with a carbon layer by liquid phase method, the coating uniformity of the polyolefin on the phosphate material with a carbon layer is good.

[0012] In any embodiment, the mass percentage of the hydrophobic layer is 0.1% to 5% based on the mass of the positive electrode active material. By adjusting the mass percentage of the hydrophobic layer within the above range, the hydrophobic layer can reduce the water absorption of the phosphate material while improving the electrochemical performance of the secondary battery.

[0013] In any embodiment, the mass percentage of the hydrophobic layer is 0.5% to 1% based on the mass of the positive electrode active material. By adjusting the mass percentage of the hydrophobic layer within the above range, the hydrophobic layer can reduce the water absorption of the phosphate material while improving the electrochemical performance of the secondary battery.

[0014] In any embodiment, the mass percentage of the carbon layer is 0.8% to 5%, based on the mass of the phosphate and the carbon layer. When the mass percentage of the carbon layer is within the above range, on the one hand, the carbon layer can improve the electronic conductivity of the phosphate, thereby improving the charge and discharge efficiency of the secondary battery; on the other hand, the carbon layer can reduce the side reactions between the phosphate and the electrolyte, reduce the loss of phosphate, and thus extend the cycle life of the secondary battery.

[0015] In any embodiment, the phosphate includes the structure LiFe (1-x) Mn x M y For PO4 materials, 0≤x≤1, 0≤y≤0.1, and M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA. Doping with element M can improve its charge-discharge performance and enhance its cycle stability.

[0016] In any embodiment, the phosphate includes the structure Na4Fe a N b For (PO4)2P2O7 materials, 2.8 ≤ a ≤ 3.0, 0 ≤ b ≤ 0.2, and N includes one or more of V, Mg, and Ti. Doping with Fe and V can improve its electrochemical performance. Doping with V can enhance the electrochemical properties of Na4Fe. a N b The crystal structure of (PO4)2P2O7 changes, forming channels that are more conducive to sodium ion diffusion, shortening the sodium ion insertion / extraction path, and accelerating the sodium ion diffusion rate, thereby improving the rate performance of lithium-ion secondary batteries. Since the radius of Mg ions is similar to that of Fe ions, doping with Mg can partially replace Fe ions in Na4Fe. a N b In the (PO4)2P2O7 crystal lattice, Ti plays a role in stabilizing the crystal structure, reducing changes and collapses in the crystal structure during charging and discharging, improving the cycle stability of the material, and thus increasing the cycle life of lithium-ion secondary batteries. Doping with Ti further stabilizes the crystal structure, which is beneficial for improving the structural stability of the material during charging and discharging, thereby increasing the cycle life of lithium-ion secondary batteries.

[0017] In any embodiment, the volume average particle size (DV50) of the positive electrode active material is 100 nm to 5 μm. In this application, the hydrophobic coating enhances the hydrophobicity of the positive electrode active material with a volume average particle size (DV50) of 100 nm to 5 μm.

[0018] In any embodiment, the volume average particle size (DV50) of the positive electrode active material is 100 nm to 800 nm. In this application, the hydrophobic coating enhances the hydrophobicity of the positive electrode active material with a volume average particle size (DV50) of 100 nm to 800 nm.

[0019] In any embodiment, after 0.1g of the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 15% to 100% of the positive electrode active material floats on the water surface. The hydrophobic layer coating the surface of the carbon-coated phosphate material makes the carbon-coated phosphate material hydrophobic, allowing it to float when placed on the water surface. In the embodiments of this application, after the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 15% to 100% of the positive electrode active material floats on the water surface, indicating that the hydrophobic layer has good uniformity of coating on the carbon-coated phosphate material.

[0020] In any embodiment, after 0.1g of the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 80%~100% of the positive electrode active material floats on the water surface. The hydrophobic layer coating on the surface of the carbon-coated phosphate material makes the carbon-coated phosphate material hydrophobic, allowing it to float when placed on the water surface. In the embodiments of this application, after 30 minutes of standing on the water surface, 80%~100% of the positive electrode active material floats, indicating that the hydrophobic layer has good uniformity of coating on the carbon-coated phosphate material.

[0021] The second aspect of this application provides a method for preparing a secondary battery, comprising: coating a slurry containing a positive electrode active material onto a positive electrode current collector to obtain a positive electrode sheet, wherein the positive electrode active material comprises a phosphate, a carbon layer, and a hydrophobic layer, the carbon layer coating the surface of the phosphate, and the hydrophobic layer coating the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 The negative electrode, separator, and positive electrode are assembled into a secondary battery.

[0022] In the technical solution of this application embodiment, a hydrophobic layer is coated on the surface of a phosphate material with a carbon layer coating. The hydrophobic properties of the hydrophobic layer are used to transform the phosphate material into a hydrophobic material, thereby reducing the water absorption of the phosphate material. The positive electrode active material provided in this application exhibits significantly reduced water absorption during transportation and storage. In the manufacturing process of secondary batteries, it can reduce the time required for vacuum drying to remove moisture, greatly reducing the water absorption of the phosphate positive electrode active material. This significantly reduces the moisture adsorbed on the positive electrode sheet during secondary battery manufacturing, improves the efficiency of the secondary battery dehydration process, shortens the secondary battery production cycle, reduces energy consumption, and increases production efficiency. The material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. When the hydrophobic layer has the aforementioned characteristic peaks of C-C and CH bonds, it exhibits strong hydrophobicity. Coating it onto the surface of a phosphate with a carbon coating can reduce the phosphate's water absorption. Phosphates possess advantages such as structural stability and good thermal stability; their application in positive electrode active materials can improve the cycle performance of secondary batteries. The carbon layer coating the phosphate surface can enhance the electronic conductivity of the phosphate, thereby improving the charge-discharge efficiency of the secondary battery.

[0023] In any embodiment, the method for preparing the positive electrode active material includes: dissolving a hydrophobic material in a solvent, including a benzene-based solvent, at a first preset temperature to obtain a first intermediate product; mixing a phosphate with a carbon layer coated on its surface with the first intermediate product to obtain a second intermediate product; cooling the second intermediate product to a second preset temperature to obtain a third intermediate product, the second preset temperature being lower than the first preset temperature; and removing the solvent from the third intermediate product to obtain the positive electrode active material, which includes a phosphate, a carbon layer, and a hydrophobic layer, wherein the carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer. In this application, a liquid-phase method is used to coat the hydrophobic material onto the phosphate material with a carbon layer coated on its surface. The structure of the hydrophobic material is not destroyed, and the bonding force between the hydrophobic material and the carbon layer is weak. The hydrophobic material does not hinder the electrolyte from penetrating into the phosphate particles. Therefore, while reducing the water absorption of the phosphate material, the hydrophobic material also improves the electrochemical performance of the secondary battery. This application utilizes the solubility characteristics of hydrophobic materials in benzene-based solvents. When the temperature of benzene-based solvents is at a first preset temperature, the hydrophobic materials have a high solubility. When the temperature is reduced to a second preset temperature, the benzene-based solvents can hardly dissolve the hydrophobic materials. This property can be used to uniformly coat the hydrophobic materials onto the phosphate materials with a carbon layer on the surface. The liquid phase coating method used in this application is relatively simple and easy to scale up production.

[0024] In any embodiment, the first preset temperature is 60°C to 100°C. When the first preset temperature is within the above range, the solvent can dissolve the hydrophobic material into a solution.

[0025] In any embodiment, the second preset temperature is 15°C to 30°C. When the second preset temperature is within the above range, the solvent can hardly dissolve the hydrophobic material, and the hydrophobic material will recrystallize. The phosphate with a carbon layer on its surface serves as a crystallization site, and the hydrophobic material is uniformly coated on the phosphate with a carbon layer on its surface.

[0026] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin have extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer with a polyolefin, the hydrophobicity of the CH groups can be used to hydrophobize the surface of the phosphate material with a carbon layer, thereby reducing the water absorption of the phosphate material.

[0027] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0028] In any embodiment, the mass ratio of the hydrophobic material to the carbon-coated phosphate in the first intermediate product is 0.001 to 0.053. By controlling the mass ratio of the hydrophobic material to the carbon-coated phosphate within the above range, the hydrophobic material can reduce the water absorption of the phosphate material while improving the electrochemical performance of the secondary battery.

[0029] In any embodiment, the mass ratio of the solvent to the carbon-coated phosphate in the first intermediate product is 2:1 to 5:1. When the mass ratio of the solvent to the carbon-coated phosphate in the first intermediate product is within the above range, the carbon-coated phosphate can be uniformly dispersed in the solvent, which is beneficial for the hydrophobic material to coat the surface of the carbon-coated phosphate material.

[0030] In any embodiment, the phosphate includes the structure LiFe (1-x) Mn x M y For PO4 materials, 0≤x≤1, 0≤y≤0.1, and M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA. Doping with element M can improve its charge-discharge performance and enhance its cycle stability.

[0031] In any embodiment, the phosphate includes the structure Na4Fe a N bFor (PO4)2P2O7 materials, 2.8 ≤ a ≤ 3.0, 0 ≤ b ≤ 0.2, and N includes one or more of V, Mg, and Ti. Doping with Fe and V can improve its electrochemical performance. Doping with V can enhance the electrochemical properties of Na4Fe. a N b The crystal structure of (PO4)2P2O7 changes, forming channels that are more conducive to sodium ion diffusion, shortening the sodium ion insertion / extraction path, and accelerating the sodium ion diffusion rate, thereby improving the rate performance of lithium-ion secondary batteries. Since the radius of Mg ions is similar to that of Fe ions, doping with Mg can partially replace Fe ions in Na4Fe. a N b In the (PO4)2P2O7 crystal lattice, Ti plays a role in stabilizing the crystal structure, reducing changes and collapses in the crystal structure during charging and discharging, improving the cycle stability of the material, and thus increasing the cycle life of lithium-ion secondary batteries. Doping with Ti further stabilizes the crystal structure, which is beneficial for improving the structural stability of the material during charging and discharging, thereby increasing the cycle life of lithium-ion secondary batteries.

[0032] A third aspect of this application provides a positive electrode active material, comprising a phosphate, a carbon layer, and a hydrophobic layer, wherein the carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0033] In the technical solution of this application embodiment, a hydrophobic layer is coated on the surface of a phosphate material with a carbon layer coating. The hydrophobic properties of the hydrophobic layer are used to transform the phosphate material into a hydrophobic material, thereby reducing the water absorption of the phosphate material. The positive electrode active material provided in this application exhibits significantly reduced water absorption during transportation and storage. In the manufacturing process of secondary batteries, it can reduce the time required for vacuum drying to remove moisture, greatly reducing the water absorption of the phosphate positive electrode active material. This significantly reduces the moisture adsorbed on the positive electrode sheet during secondary battery manufacturing, improves the efficiency of the secondary battery dehydration process, shortens the secondary battery production cycle, reduces energy consumption, and increases production efficiency. The material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. When the hydrophobic layer has the aforementioned characteristic peaks of C-C and CH bonds, it exhibits strong hydrophobicity. Coating it onto the surface of a phosphate with a carbon coating can reduce the phosphate's water absorption. Phosphates possess advantages such as structural stability and good thermal stability; their application in positive electrode active materials can improve the cycle performance of secondary batteries. The carbon layer coating the phosphate surface can enhance the electronic conductivity of the phosphate, thereby improving the charge-discharge efficiency of the secondary battery.

[0034] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin have extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer with a polyolefin, the hydrophobicity of the CH groups can be used to hydrophobize the surface of the phosphate material with a carbon layer, thereby reducing the water absorption of the phosphate material.

[0035] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0036] In any embodiment, the phosphate includes the structure LiFe (1-x) Mn x M y For PO4 materials, 0≤x≤1, 0≤y≤0.1, and M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA. Doping with element M can improve its charge-discharge performance and enhance its cycle stability.

[0037] In any embodiment, the phosphate includes the structure Na4Fe a N b For (PO4)2P2O7 materials, 2.8 ≤ a ≤ 3.0, 0 ≤ b ≤ 0.2, and N includes one or more of V, Mg, and Ti. Doping with Fe and V can improve its electrochemical performance. Doping with V can enhance the electrochemical properties of Na4Fe. a N b The crystal structure of (PO4)2P2O7 changes, forming channels that are more conducive to sodium ion diffusion, shortening the sodium ion insertion / extraction path, and accelerating the sodium ion diffusion rate, thereby improving the rate performance of lithium-ion secondary batteries. Since the radius of Mg ions is similar to that of Fe ions, doping with Mg can partially replace Fe ions in Na4Fe. a N bIn the (PO4)2P2O7 crystal lattice, Ti plays a role in stabilizing the crystal structure, reducing changes and collapses in the crystal structure during charging and discharging, improving the cycle stability of the material, and thus increasing the cycle life of lithium-ion secondary batteries. Doping with Ti further stabilizes the crystal structure, which is beneficial for improving the structural stability of the material during charging and discharging, thereby increasing the cycle life of lithium-ion secondary batteries.

[0038] The fourth aspect of this application provides a method for preparing a positive electrode active material, comprising: dissolving a hydrophobic material in a solvent at a first preset temperature to obtain a first intermediate product, the solvent including a benzene-based solvent; mixing a phosphate with a carbon layer coated on its surface with the first intermediate product to obtain a second intermediate product; cooling the second intermediate product to a second preset temperature to obtain a third intermediate product, the second preset temperature being lower than the first preset temperature; removing the solvent from the third intermediate product to obtain a positive electrode active material, the positive electrode active material comprising a phosphate, a carbon layer, and a hydrophobic layer, the carbon layer coating the surface of the phosphate, and the hydrophobic layer coating the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer having a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0039] In the technical solution of this application embodiment, a hydrophobic material is coated onto a phosphate material with a carbon layer on its surface using a liquid-phase method. The structure of the hydrophobic material is not destroyed, and the bonding force between the hydrophobic material and the carbon layer is weak. The hydrophobic material does not hinder the electrolyte from penetrating into the phosphate particles. Therefore, while reducing the water absorption of the phosphate material, the hydrophobic material also improves the electrochemical performance of the secondary battery. Specifically, the phosphate material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. When the hydrophobic layer has the aforementioned characteristic peaks of C-C and CH bonds, it exhibits strong hydrophobicity. Coating it onto the surface of a phosphate with a carbon coating layer can reduce the water absorption of the phosphate. This application utilizes the solubility characteristics of hydrophobic materials in benzene-based solvents. When the temperature of benzene-based solvents is at a first preset temperature, the hydrophobic material has high solubility. When the temperature is lowered to a second preset temperature, the benzene-based solvents can hardly dissolve the hydrophobic material. This property can be used to uniformly coat the hydrophobic material onto the phosphate material with a carbon coating layer on its surface. The liquid-phase coating process used in this application is relatively simple and easy to scale up for production.

[0040] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin have extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer with a polyolefin, the hydrophobicity of the CH groups can be used to hydrophobize the surface of the phosphate material with a carbon layer, thereby reducing the water absorption of the phosphate material.

[0041] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0042] In any embodiment, the first preset temperature is 60°C to 100°C. When the first preset temperature is within the above range, the solvent can dissolve the hydrophobic material into a solution.

[0043] In any embodiment, the second preset temperature is 15°C to 30°C. When the second preset temperature is within the above range, the solvent can hardly dissolve the hydrophobic material, and the hydrophobic material will recrystallize. The phosphate with a carbon layer on its surface serves as a crystallization site, and the hydrophobic material is uniformly coated on the phosphate with a carbon layer on its surface.

[0044] The fifth aspect of this application provides an electrical device comprising a secondary battery according to the first aspect of this application, and / or a secondary battery prepared by the method for preparing a secondary battery according to the second aspect, and / or a positive electrode active material according to the third aspect, and / or a positive electrode active material prepared by the method for preparing a positive electrode active material according to the fourth aspect. In embodiments of this application, the electrical device possesses at least the same advantages as the secondary battery of the first aspect.

[0045] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0046] Figure 1 is a structural schematic diagram of a vehicle according to an embodiment of this application;

[0047] Figure 2 is an exploded structural diagram of a battery according to an embodiment of this application;

[0048] Figure 3 is an exploded structural diagram of a battery cell according to an embodiment of this application;

[0049] Figure 4 shows the positive electrode active materials prepared in Examples 3, 10, Comparative Example 1, and Comparative Example 2 of this application at 600 cm⁻¹. -1 ~1500 -1 Infrared spectrum in the specified band;

[0050] Figure 5 shows the positive electrode active materials prepared in Examples 3, 10, Comparative Example 1, and Comparative Example 2 of this application at 1200 cm⁻¹. -1 ~3500 -1 Infrared spectrum in the specified band;

[0051] Figure 6 shows the positive electrode active material prepared in Example 3 of this application at 1200 cm⁻¹. -1 ~1700 -1 Infrared spectrum in the band. Embodiments of the present invention

[0052] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, positive electrode material, preparation method, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0053] 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 also 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 "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0054] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0055] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0056] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means 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.

[0057] 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.

[0058] 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).

[0059] Phosphate materials are highly hygroscopic. Using them in the positive electrode sheet can lead to excessive moisture absorption during the production of secondary batteries, failing to meet required moisture content. To meet the moisture content requirements of secondary batteries, a dehydration process is necessary. However, to avoid damaging the separator membrane, the dehydration process must be carried out at relatively low temperatures for extended periods. This prolongs the production cycle and reduces the overall production efficiency of the secondary batteries.

[0060] The relevant technology employs a solid-state method, calcining a carbon-coated phosphate (phosphate / carbon) with a hydrophobic material at high temperatures (e.g., 200°C~250°C), causing a polymer (e.g., polyethylene) to melt and coat the phosphate / carbon material surface. At 200°C~250°C, the polymer, such as polyethylene, undergoes denaturation, experiencing some thermal decomposition and releasing some hydrogen. The hydrocarbon peaks change, making the polyethylene coating layer difficult to detect in infrared absorption spectroscopy, or at least some characteristic peaks undetectable. At temperatures above 200°C, the overall chain structure of polyethylene is disrupted, potentially causing some polyethylene material to volatilize. This results in poor uniformity of the polyethylene coating on the phosphate / carbon surface, leaving the phosphate / carbon cathode active material still hygroscopic. On the other hand, while solid-state methods can be used to coat polymers such as polyethylene onto the surface of phosphate / carbon, resulting in a tight bond between the polyethylene and the phosphate / carbon layer, the strong adhesion between polyethylene and the carbon layer leads to poor wettability of polyethylene in the electrolyte. This hinders the penetration of the electrolyte into the positive electrode active material particles after the secondary battery is manufactured, and the phosphate cannot come into contact with the electrolyte, thus limiting the battery's capacity and reducing its overall performance. Therefore, reducing the water absorption of phosphate materials to improve the production efficiency of secondary batteries is a crucial technical problem that needs to be solved.

[0061] Based on this, the first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the positive electrode includes a positive active material, which includes a phosphate, a carbon layer, and a hydrophobic layer, wherein the carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0062] In the technical solution of this application embodiment, a hydrophobic layer is coated on the surface of a phosphate material with a carbon layer coating. The hydrophobic properties of the hydrophobic layer are used to transform the phosphate material into a hydrophobic material, thereby reducing the water absorption of the phosphate material. The positive electrode active material provided in this application exhibits significantly reduced water absorption during transportation and storage. In the manufacturing process of secondary batteries, it can reduce the time required for vacuum drying to remove moisture, greatly reducing the water absorption of the phosphate positive electrode active material. This significantly reduces the moisture adsorbed on the positive electrode sheet during secondary battery manufacturing, improves the efficiency of the secondary battery dehydration process, shortens the secondary battery production cycle, reduces energy consumption, and increases production efficiency. The material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm-1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. In the embodiments of this application, the above-mentioned characteristic peak can be detected in the infrared absorption spectrum, indicating that the hydrophobic layer in the embodiments of this application is coated on the phosphate / carbon surface by a liquid phase method. The hydrophobic layer has the C-C and CH bonds of the above-mentioned characteristic peak, and the hydrophobic layer has strong hydrophobicity. Coating it on the phosphate surface with a carbon coating layer can reduce the water absorption of the phosphate. Phosphate has advantages such as structural stability and good thermal stability. Its application in positive electrode active materials can improve the cycle performance of secondary batteries. The carbon layer coated on the phosphate surface can improve the electronic conductivity of the phosphate, thereby improving the charge and discharge efficiency of the secondary battery.

[0063] Infrared absorption spectroscopy is common knowledge in the field and has a common meaning in the field. It can be measured by methods and instruments in the field.

[0064] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin possess extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer using the polyolefin, the hydrophobicity of the CH groups can hydrophobize the surface of the phosphate material with the carbon layer, thereby reducing the water absorption of the phosphate material. The polyolefin includes polyethylene, polypropylene, polybutene, etc.

[0065] It should be noted that when the hydrophobic layer is made of polyolefin, the infrared absorption spectrum of the hydrophobic layer, except at 1451 cm⁻¹, will exhibit certain characteristics. -1 ~1491cm -1 and 900cm -1 ~850cm -1 Apart from having a characteristic peak at 2900 cm⁻¹, it also has a characteristic peak at 2900 cm⁻¹. -1 ~2930cm -1 2840cm -1 ~2870cm -1 It also has a characteristic peak at 2900cm. -1 ~2930cm -1 2840cm -1 ~2870cm -1 This is the stretching vibration peak of the methylene group.

[0066] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0067] In any embodiment, the number average molecular weight of the polyolefin is 2000-4000. By controlling the number average molecular weight of the polyolefin within the above range, the polyolefin has better solubility in the solvent, and when the polyolefin is coated onto the surface of a phosphate material with a carbon layer using a liquid-phase method, the coating uniformity of the polyolefin on the phosphate material with the carbon layer is better. Specifically, the number average molecular weight of the polyolefin is 2000, 2400, 2800, 3200, 3600, 4000, etc., or a range consisting of any two of the above values, such as 2000-2400, 2400-2800, 2800-3200, 3200-3600, 3600-4000, etc.

[0068] Among them, number-average molecular weight is common knowledge in the field, has a common meaning in the field, and can be measured by methods and instruments in the field.

[0069] In any embodiment, the mass percentage of the hydrophobic layer, based on the mass of the positive electrode active material, is 0.1% to 5%. By controlling the mass percentage of the hydrophobic layer within the above range, the hydrophobic layer can reduce the water absorption of the phosphate material while simultaneously improving the electrochemical performance of the secondary battery. Specifically, the mass percentage of the hydrophobic layer, based on the mass of the positive electrode active material, is 0.1%, 1%, 1.7%, 2.1%, 2.8%, 3.5%, 4.2%, 5%, etc., or a range consisting of any two of the above values, such as 0.1% to 1%, 1% to 1.7%, 1.7% to 2.1%, 2.1% to 2.8%, 2.8% to 3.5%, 3.5% to 4.2%, 4.2% to 5%, etc.

[0070] In any embodiment, the mass percentage of the hydrophobic layer, based on the mass of the positive electrode active material, is 0.5% to 1%. By controlling the mass percentage of the hydrophobic layer within the above range, the hydrophobic layer can reduce the water absorption of the phosphate material while simultaneously improving the electrochemical performance of the secondary battery. Specifically, the mass percentage of the hydrophobic layer, based on the mass of the positive electrode active material, is 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, etc., or a range consisting of any two of the above values, such as 0.5% to 0.6%, 0.6% to 0.7%, 0.7% to 0.8%, 0.8% to 0.9%, 0.9% to 1%, etc.

[0071] In any embodiment, the mass percentage of the carbon layer is 0.8% to 5%, based on the mass of the phosphate and the carbon layer. When the mass percentage of the carbon layer is within the above range, on the one hand, the carbon layer can improve the electronic conductivity of the phosphate, thereby improving the charge and discharge efficiency of the secondary battery; on the other hand, the carbon layer can reduce the side reactions between the phosphate and the electrolyte, reducing phosphate loss and thus extending the cycle life of the secondary battery. Specifically, based on the mass of the positive electrode active material, the mass percentage of the carbon layer is 0.8%, 1.6%, 2.5%, 3.2%, 4.1%, 5%, etc., or a range consisting of any two of the above values, such as 0.8% to 1.6%, 1.6% to 2.5%, 2.5% to 3.2%, 3.2% to 4.1%, 4.1% to 5%, etc.

[0072] In any embodiment, the phosphate includes the structure LiFe (1-x) Mn x M y For PO4 materials, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.1, and M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA. Doping with element M can improve its charge-discharge performance and enhance its cycle stability. For example, M includes one or more of Al, Cu, Cr, V, Zn, Co, Ni, and Mg.

[0073] In any embodiment, the phosphate includes the structure Na4Fe a N b For (PO4)2P2O7 materials, 2.8 ≤ a ≤ 3.0, 0 ≤ b ≤ 0.2, and N includes one or more of V, Mg, and Ti. Doping with Fe and V can improve its electrochemical performance. Doping with V can enhance the electrochemical properties of Na4Fe. a N b The crystal structure of (PO4)2P2O7 changes, forming channels that are more conducive to sodium ion diffusion, shortening the sodium ion insertion / extraction path, and accelerating the sodium ion diffusion rate, thereby improving the rate performance of lithium-ion secondary batteries. Since the radius of Mg ions is similar to that of Fe ions, doping with Mg can partially replace Fe ions in Na4Fe. a N b In the (PO4)2P2O7 crystal lattice, Ti plays a role in stabilizing the crystal structure, reducing changes and collapses in the crystal structure during charging and discharging, improving the cycle stability of the material, and thus increasing the cycle life of lithium-ion secondary batteries. Doping with Ti further stabilizes the crystal structure, which is beneficial for improving the structural stability of the material during charging and discharging, thereby increasing the cycle life of lithium-ion secondary batteries.

[0074] In any embodiment, the volume average particle size (DV50) of the positive electrode active material is 100 nm to 5 μm. In this application, the hydrophobic coating enhances the hydrophobicity of the positive electrode active material with a volume average particle size (DV50) of 100 nm to 5 μm. The volume average particle size (DV50) of the positive electrode active material is 100 nm, 560 nm, 1.1 μm, 2.3 μm, 3.5 μm, 4.1 μm, 5 μm, etc., or a range of any two of the above values, such as 100 nm to 560 nm, 560 nm to 1.1 μm, 1.1 μm to 2.3 μm, 2.3 μm to 3.5 μm, 3.5 μm to 4.1 μm, 4.1 μm to 5 μm, etc.

[0075] In any embodiment, the volume average particle size (DV50) of the positive electrode active material is 100 nm to 800 nm. In this application, the hydrophobic coating enhances the hydrophobicity of the positive electrode active material with a volume average particle size (DV50) of 100 nm to 800 nm. The volume average particle size (DV50) of the positive electrode active material is 100 nm, 304 nm, 512 nm, 630 nm, 711 nm, 800 nm, etc., or a range of any two of the above values, such as 100 nm to 304 nm, 304 nm to 512 nm, 512 nm to 630 nm, 630 nm to 711 nm, 711 nm to 800 nm, etc.

[0076] In any embodiment, when the phosphate in the positive electrode active material includes the structure LiFe (1-x) Mn x M y When using PO4 material, under conditions of 60% relative humidity and after standing for 24 hours, the water absorption of the positive electrode active material is between 1000ppm and 3200ppm. By controlling the water absorption of the positive electrode active material within this range, the water absorption of the positive electrode sheet during the production of secondary batteries can be reduced, thereby improving the water removal efficiency of the secondary batteries and thus increasing the production efficiency of the secondary batteries. Specifically, the water absorption of the positive electrode active material can be 1000ppm, 1400ppm, 1800ppm, 2200ppm, 2500ppm, 2900ppm, 3200ppm, or any range of two of the above values, such as 1000ppm~1400ppm, 1400ppm~1800ppm, 1800ppm~2200ppm, 2200ppm~2500ppm, 2500ppm~2900ppm, 2900ppm~3200ppm, etc.

[0077] In any embodiment, when the phosphate in the positive electrode active material includes the structure Na4Fe a N bWhen using (PO4)2P2O7 material, under conditions of 60% relative humidity and after standing for 24 hours, the water absorption of the positive electrode active material is 5000ppm~8000ppm. By controlling the water absorption of the positive electrode active material within this range, the water absorption of the positive electrode sheet during the production of secondary batteries can be reduced, thereby improving the water removal efficiency of the secondary batteries and thus increasing the production efficiency of the secondary batteries. Specifically, the water absorption of the positive electrode active material can be 5000ppm, 5400ppm, 6300ppm, 7100ppm, 8000ppm, or any range of two of the above values, such as 5000ppm~5400ppm, 5400ppm~6300ppm, 6300ppm~7100ppm, 7100ppm~8000ppm, etc.

[0078] In any embodiment, after 0.1g of the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 15% to 100% of the positive electrode active material floats on the water surface. The hydrophobic layer coating the surface of the carbon-coated phosphate material makes the carbon-coated phosphate material hydrophobic, allowing it to float when placed on the water surface. In the embodiments of this application, after the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 15% to 100% of the positive electrode active material floats on the water surface, indicating that the hydrophobic layer has good uniformity of coating on the carbon-coated phosphate material. Among them, after the positive electrode active material is placed on the water surface and left to stand for 30 minutes, the amount of positive electrode active material floating on the water surface is 15%, 28%, 46%, 62%, 78%, 100%, etc., or any two of the above values ​​are combined into a range of values, such as 15%~28%, 28%~46%, 46%~62%, 62%~78%, 78%~100%, etc.

[0079] In any embodiment, after 0.1g of the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 80%~100% of the positive electrode active material floats on the water surface. The hydrophobic layer coating on the surface of the carbon-coated phosphate material makes the carbon-coated phosphate material hydrophobic, allowing it to float when placed on the water surface. In the embodiments of this application, after 30 minutes of standing on the water surface, 80%~100% of the positive electrode active material floats, indicating that the hydrophobic layer has good uniformity of coating on the carbon-coated phosphate material. Among them, after the positive electrode active material is placed on the water surface and left to stand for 30 minutes, the amount of positive electrode active material floating on the water surface is 80%, 84%, 89%, 92%, 97%, 100%, etc., or any two of the above values ​​are combined into a range of values, such as 80%~84%, 84%~89%, 89%~92%, 92%~97%, 97%~100%, etc.

[0080] A second aspect of this application provides a method for preparing a secondary battery, comprising:

[0081] S11: A slurry containing positive electrode active material is coated onto a positive electrode current collector to obtain a positive electrode sheet. The positive electrode active material includes a phosphate, a carbon layer, and a hydrophobic layer. The carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer. The infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0082] S12: Assemble the negative electrode, separator and positive electrode into a secondary battery.

[0083] In the technical solution of this application embodiment, a hydrophobic layer is coated on the surface of a phosphate material with a carbon layer coating. The hydrophobic properties of the hydrophobic layer are used to transform the phosphate material into a hydrophobic material, thereby reducing the water absorption of the phosphate material. The positive electrode active material provided in this application exhibits significantly reduced water absorption during transportation and storage. In the manufacturing process of secondary batteries, it can reduce the time required for vacuum drying to remove moisture, greatly reducing the water absorption of the phosphate positive electrode active material. This significantly reduces the moisture adsorbed on the positive electrode sheet during secondary battery manufacturing, improves the efficiency of the secondary battery dehydration process, shortens the secondary battery production cycle, reduces energy consumption, and increases production efficiency. The material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. When the hydrophobic layer has the aforementioned characteristic peaks of C-C and CH bonds, it exhibits strong hydrophobicity. Coating it onto the surface of a phosphate with a carbon coating can reduce the phosphate's water absorption. Phosphates possess advantages such as structural stability and good thermal stability; their application in positive electrode active materials can improve the cycle performance of secondary batteries. The carbon layer coating the phosphate surface can enhance the electronic conductivity of the phosphate, thereby improving the charge-discharge efficiency of the secondary battery.

[0084] In any embodiment, the method for preparing the positive electrode active material includes:

[0085] S21: At a first preset temperature, the hydrophobic material is dissolved in a solvent to obtain a first intermediate product, the solvent including benzene-based solvents.

[0086] In this embodiment, the first preset temperature is higher than room temperature, and the hydrophobic material can dissolve in a solvent at a higher temperature to obtain the first intermediate product.

[0087] S22: Mix the phosphate with a carbon layer on its surface with the first intermediate to obtain the second intermediate.

[0088] In this embodiment of the application, at a first preset temperature, the first intermediate product is a solution. The phosphate (phosphate / carbon) with a carbon layer on its surface is mixed with the first intermediate product at the first preset temperature to achieve the effect of wet mixing, so that the hydrophobic material in the first intermediate product is evenly distributed around the phosphate / carbon.

[0089] S23: Cool the second intermediate product to a second preset temperature to obtain a third intermediate product, wherein the second preset temperature is lower than the first preset temperature.

[0090] In this embodiment, at a second preset temperature, the hydrophobic material is slightly soluble or insoluble in solvents such as benzene. The first intermediate product is cooled to the second preset temperature, causing the hydrophobic material to precipitate and uniformly distribute on the phosphate / carbon surface.

[0091] S24: Remove the solvent from the third intermediate to obtain the positive electrode active material, which includes a phosphate, a carbon layer and a hydrophobic layer. The carbon layer is coated on the surface of the phosphate, and the hydrophobic layer is coated on the surface of the carbon layer.

[0092] In this application, a liquid-phase method is used to coat a hydrophobic material onto a phosphate material with a carbon layer on its surface. The structure of the hydrophobic material remains intact, and the bonding force between the hydrophobic material and the carbon layer is weak. The hydrophobic material does not hinder the electrolyte from penetrating into the phosphate particles. Therefore, while reducing the water absorption of the phosphate material, the hydrophobic material also improves the electrochemical performance of the secondary battery. This application utilizes the solubility of the hydrophobic material in benzene-based solvents. At a first preset temperature, benzene-based solvents exhibit high solubility for the hydrophobic material; when the temperature drops to a second preset temperature, the benzene-based solvents can hardly dissolve the hydrophobic material. This property allows for the uniform coating of the hydrophobic material onto the phosphate material with a carbon layer. The liquid-phase coating process used in this application is relatively simple and easily scaled up for production.

[0093] In any embodiment, the first preset temperature is 60°C to 100°C. When the first preset temperature is within the above range, the solvent can dissolve the hydrophobic material into a solution. The first preset temperature can be 60°C, 70°C, 80°C, 90°C, 100°C, etc., or a range consisting of any two of the above values, such as 60°C to 70°C, 70°C to 80°C, 80°C to 90°C, 90°C to 100°C, etc.

[0094] In any embodiment, the second preset temperature is 15℃ to 30℃. When the second preset temperature is within the above range, the solvent can hardly dissolve the hydrophobic material, and the hydrophobic material will recrystallize. The phosphate with a carbon layer on its surface serves as a crystallization site, and the hydrophobic material is uniformly coated on the phosphate with a carbon layer on its surface. The second preset temperature is 15℃, 18℃, 20℃, 25℃, 30℃, etc., or a range consisting of any two of the above values, such as 15℃~18℃, 18℃~20℃, 20℃~25℃, 25℃~30℃, etc.

[0095] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin have extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer with a polyolefin, the hydrophobicity of the CH groups can be used to hydrophobize the surface of the phosphate material with a carbon layer, thereby reducing the water absorption of the phosphate material.

[0096] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0097] In any embodiment, the mass ratio of the hydrophobic material to the carbon-coated phosphate in the first intermediate product is 0.001 to 0.053. By controlling the mass ratio of the hydrophobic material to the carbon-coated phosphate within the above range, the hydrophobic material can reduce the water absorption of the phosphate material while improving the electrochemical performance of the secondary battery. Specifically, the mass ratio of the hydrophobic material to the carbon-coated phosphate in the first intermediate product is 5 × 10⁻⁶. -4 0.002, 0.008, 0.03, 0.053, etc., or a range of values ​​consisting of any two of the above, for example, 5 × 10 -4 ~0.002, 0.002~0.008, 0.008~0.03, 0.03~0.053, etc.

[0098] In any embodiment, the mass ratio of the solvent to the carbon-coated phosphate in the first intermediate product is 2:1 to 5:1. When the mass ratio of the solvent to the carbon-coated phosphate in the first intermediate product is within the above range, the carbon-coated phosphate can be uniformly dispersed in the solvent, which is beneficial for the hydrophobic material to coat the surface of the carbon-coated phosphate material. The mass ratio of the solvent to the carbon-coated phosphate in the first intermediate product is 2:1, 2.8:1, 3.2:1, 4.3:1, 5:1, etc., or a range of any two of the above values, such as 2:1 to 2.8:1, 2.8:1 to 3.2:1, 3.2:1 to 4.3:1, 4.3:1 to 5:1, etc.

[0099] In any embodiment, the phosphate includes the structure LiFe (1-x) Mn x M y For PO4 materials, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.1, and M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA. Doping with element M can improve its charge-discharge performance and enhance its cycle stability. For example, M includes one or more of Al, Cu, Cr, V, Zn, Co, Ni, and Mg.

[0100] In any embodiment, the phosphate includes the structure Na4Fe a N b For (PO4)2P2O7 materials, 2.8 ≤ a ≤ 3.0, 0 ≤ b ≤ 0.2, and N includes one or more of V, Mg, and Ti. Doping with Fe and V can improve its electrochemical performance. Doping with V can enhance the electrochemical properties of Na4Fe. a N b The crystal structure of (PO4)2P2O7 changes, forming channels that are more conducive to sodium ion diffusion, shortening the sodium ion insertion / extraction path, and accelerating the sodium ion diffusion rate, thereby improving the rate performance of lithium-ion secondary batteries. Since the radius of Mg ions is similar to that of Fe ions, doping with Mg can partially replace Fe ions in Na4Fe. a N b In the (PO4)2P2O7 crystal lattice, Ti plays a role in stabilizing the crystal structure, reducing changes and collapses in the crystal structure during charging and discharging, improving the cycle stability of the material, and thus increasing the cycle life of lithium-ion secondary batteries. Doping with Ti further stabilizes the crystal structure, which is beneficial for improving the structural stability of the material during charging and discharging, thereby increasing the cycle life of lithium-ion secondary batteries.

[0101] A third aspect of this application provides a positive electrode active material, comprising a phosphate, a carbon layer, and a hydrophobic layer, wherein the carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0102] In the technical solution of this application embodiment, a hydrophobic layer is coated on the surface of a phosphate material with a carbon layer coating. The hydrophobic properties of the hydrophobic layer are used to transform the phosphate material into a hydrophobic material, thereby reducing the water absorption of the phosphate material. The positive electrode active material provided in this application exhibits significantly reduced water absorption during transportation and storage. In the manufacturing process of secondary batteries, it can reduce the time required for vacuum drying to remove moisture, greatly reducing the water absorption of the phosphate positive electrode active material. This significantly reduces the moisture adsorbed on the positive electrode sheet during secondary battery manufacturing, improves the efficiency of the secondary battery dehydration process, shortens the secondary battery production cycle, reduces energy consumption, and increases production efficiency. The material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. When the hydrophobic layer has the aforementioned characteristic peaks of C-C and CH bonds, it exhibits strong hydrophobicity. Coating it onto the surface of a phosphate with a carbon coating can reduce the phosphate's water absorption. Phosphates possess advantages such as structural stability and good thermal stability; their application in positive electrode active materials can improve the cycle performance of secondary batteries. The carbon layer coating the phosphate surface can enhance the electronic conductivity of the phosphate, thereby improving the charge-discharge efficiency of the secondary battery.

[0103] In any embodiment, the hydrophobic layer material includes a polyolefin. The CH groups in the polyolefin possess extremely strong hydrophobicity. By coating the surface of the phosphate material with a carbon layer using the polyolefin, the hydrophobicity of the CH groups can hydrophobize the surface of the phosphate material with the carbon layer, thereby reducing the water absorption of the phosphate material. The polyolefin includes polyethylene, polypropylene, polybutene, etc.

[0104] In any embodiment, the hydrophobic layer material includes bitumen. Bitumen has good hydrophobic properties, and coating the surface of the carbon-coated phosphate material with it can reduce the contact between moisture and the carbon-coated phosphate material, thereby reducing the water absorption of the phosphate material.

[0105] In any embodiment, the phosphate includes the structure LiFe (1-x) Mnx M y For PO4 materials, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.1, and M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA. Doping with element M can improve its charge-discharge performance and enhance its cycle stability. For example, M includes one or more of Al, Cu, Cr, V, Zn, Co, Ni, and Mg.

[0106] In any embodiment, the phosphate includes the structure Na4Fe a N b For (PO4)2P2O7 materials, 2.8 ≤ a ≤ 3.0, 0 ≤ b ≤ 0.2, and N includes one or more of V, Mg, and Ti. Doping with Fe and V can improve its electrochemical performance. Doping with V can enhance the electrochemical properties of Na4Fe. a N b The crystal structure of (PO4)2P2O7 changes, forming channels that are more conducive to sodium ion diffusion, shortening the sodium ion insertion / extraction path, and accelerating the sodium ion diffusion rate, thereby improving the rate performance of lithium-ion secondary batteries. Since the radius of Mg ions is similar to that of Fe ions, doping with Mg can partially replace Fe ions in Na4Fe. a N b In the (PO4)2P2O7 crystal lattice, Ti plays a role in stabilizing the crystal structure, reducing changes and collapses in the crystal structure during charging and discharging, improving the cycle stability of the material, and thus increasing the cycle life of lithium-ion secondary batteries. Doping with Ti further stabilizes the crystal structure, which is beneficial for improving the structural stability of the material during charging and discharging, thereby increasing the cycle life of lithium-ion secondary batteries.

[0107] The fourth aspect of this application provides a method for preparing a positive electrode active material. The method for preparing the positive electrode active material in this application is the same as the method for preparing the positive electrode active material in the method for preparing a secondary battery in the second aspect. The method for preparing the positive electrode active material includes:

[0108] S21: At a first preset temperature, the hydrophobic material is dissolved in a solvent to obtain a first intermediate product, the solvent including benzene-based solvents.

[0109] S22: The phosphate with a carbon layer on its surface is mixed with the first intermediate to obtain the second intermediate.

[0110] S23: Cool the second intermediate product to a second preset temperature to obtain a third intermediate product, wherein the second preset temperature is lower than the first preset temperature.

[0111] S24: Removing the solvent from the third intermediate yields the positive electrode active material, which comprises a phosphate, a carbon layer, and a hydrophobic layer. The carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer. The infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

[0112] In the technical solution of this application embodiment, a hydrophobic material is coated onto a phosphate material with a carbon layer on its surface using a liquid-phase method. The structure of the hydrophobic material is not destroyed, and the bonding force between the hydrophobic material and the carbon layer is weak. The hydrophobic material does not hinder the electrolyte from penetrating into the phosphate particles. Therefore, while reducing the water absorption of the phosphate material, the hydrophobic material also improves the electrochemical performance of the secondary battery. Specifically, the phosphate material is located at 1451 cm⁻¹. -1 ~1491cm -1 The characteristic peak is the bending vibration peak of the methylene group, located at 900 cm⁻¹. -1 ~850cm -1 The characteristic peak is the ring-symmetric stretching vibration peak of the C-C bond. When the hydrophobic layer has the aforementioned characteristic peaks of C-C and CH bonds, the hydrophobic layer has strong hydrophobicity. Coating it onto the surface of a phosphate with a carbon coating layer can reduce the water absorption of the phosphate. This application utilizes the solubility characteristics of hydrophobic materials in benzene solvents. When the temperature of benzene solvent is at a first preset temperature (higher temperature), the hydrophobic material has high solubility. When the temperature is lowered to a second preset temperature (lower temperature), the benzene solvent can hardly dissolve the hydrophobic material. Using this property, the hydrophobic material can be uniformly coated onto the phosphate material with a carbon coating layer on the surface. The liquid-phase coating process used in this application is relatively simple and easy to scale up for production.

[0113] In any embodiment, the carbon source of the carbon layer includes one or more of starch, sucrose, glucose, citric acid, and polyethylene glycol. Starch, sucrose, glucose, citric acid, and polyethylene glycol, as carbon sources, are converted into carbon through pyrolysis to form a carbon coating layer. Starch and citric acid are widely available and inexpensive, thus reducing the cost of secondary batteries; sucrose and glucose are easy to control during pyrolysis, thus improving the production efficiency of secondary batteries; and polyethylene glycol has good stability, thus improving the stability of secondary batteries.

[0114] In any embodiment, a phosphate with a carbon-coated surface is added to the first intermediate product and stirred for 0.5 to 8 hours to obtain a second intermediate product. The stirring time is within the above range, which allows the phosphate with the carbon-coated surface to be dispersed more uniformly in the first intermediate product.

[0115] In any embodiment, the solvent in the third intermediate product is removed by vacuum drying at 80°C to obtain the positive electrode active material. Vacuum drying is used to remove the solvent to prevent it from evaporating into the air and causing environmental pollution.

[0116] In this embodiment, the electrolyte acts as a conductor of ions between the positive and negative electrode plates. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0117] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution comprises an electrolyte salt and a solvent. After the electrolyte salt dissolves, it forms electrolyte ions, and conduction occurs through the movement of these electrolyte ions within the electrolyte salt.

[0118] In some embodiments, in sodium batteries, the electrolyte salt includes sodium salts such as sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium trifluoromethanesulfonate (CF3NaO3S), and sodium sulfide (Na2S). In lithium batteries, the electrolyte salt includes at least one lithium salt selected from lithium perchlorate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalateborate, lithium tetrafluoroborate, and lithium trifluoromethanesulfonate.

[0119] In one embodiment, the solvent includes one or more solvents selected from chain ethers, ethylene glycol dimethyl ether and its derivatives, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and cyclic ethers. Specifically, it includes dimethyl ether (DME), diethylene glycol diethyl ether, tetraethylene glycol dimethyl ether, 2,2,2,2-trifluoroethyl ether, ethylene glycol diethyl ether, triethylene glycol dimethyl ether, ethylene glycol dimethyl ether derivatives, methyltrifluoroethyl carbonate (FEMC), dioxolane (DOL), acetonitrile (AN), fluorobenzene, triethyl phosphate (TEP), sulfolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethyl sulfoxide, N,N-dimethylacetamide, etc.

[0120] In some embodiments, the electrolyte may also 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.

[0121] In some embodiments, a separator is disposed between the positive and negative electrode plates, primarily to prevent short circuits between the positive and negative electrodes while allowing ions to pass through. 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.

[0122] 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.

[0123] In some embodiments, the separator is a polypropylene membrane with an alumina coating on one side. Coating one side of the polypropylene membrane with an alumina coating can improve the electrolyte wettability of the polypropylene membrane.

[0124] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0125] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0126] 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.).

[0127] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0128] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0129] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0130] In some embodiments, 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. In this embodiment, the battery cell is an ion battery, and during the battery charging and discharging process, active ions (such as Li) + Na + Insertion / deintercalation in the negative electrode active material.

[0131] 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.

[0132] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0133] 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.

[0134] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0135] 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.

[0136] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0137] 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 a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0138] The fifth aspect of this application provides an electrical device, including a secondary battery according to the first aspect of this application, and / or a secondary battery prepared by the method for preparing a secondary battery according to the second aspect, and / or a positive electrode active material according to the third aspect, and / or a positive electrode active material prepared by the method for preparing a positive electrode active material according to the fourth aspect. In embodiments of this application, the electrical device has at least the same advantages as the secondary battery of the first aspect. The electrical device may include mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0139] For ease of explanation, we will take a vehicle 1000 as an example of an electrical device.

[0140] Please refer to Figure 1, which is a structural schematic diagram of an embodiment of the vehicle 1000 of this application. The vehicle 1000 can be a gasoline vehicle, a natural gas vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. A battery 100 is disposed inside the vehicle 1000, and the battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The battery 100 can be used to power the vehicle 1000; for example, the battery 100 can serve as the operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 is used to control the battery 100 to supply power to the motor 300, for example, to meet the power requirements of the vehicle 1000 during startup, navigation, and driving.

[0141] In some embodiments of this application, the battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0142] Please refer to Figure 2, which is an exploded structural diagram of an embodiment of the battery 100 of this application. The battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10. The housing 10 provides a space for the battery cell 20 and can have various structures. In some embodiments, the housing 10 may include a first portion 11 and a second portion 12, which overlap each other, jointly defining a space for accommodating the battery cell 20. The second portion 12 may be a hollow structure with one open end, and the first portion 11 may be a plate-like structure, covering the open side of the second portion 12 so that the first portion 11 and the second portion 12 jointly define the space. Alternatively, the first portion 11 and the second portion 12 may both be hollow structures with one open side, with the open side of the first portion 11 covering the open side of the second portion 12. Of course, the housing 10 formed by the first portion 11 and the second portion 12 can have various shapes, such as a cylinder, a cuboid, etc.

[0143] In battery 100, there can be multiple battery cells 20. These multiple battery cells 20 can be connected in series, in parallel, or in a mixed manner. A mixed connection means that multiple battery cells 20 are connected in both series and parallel. Multiple battery cells 20 can be directly connected in series, in parallel, or in a mixed manner, and then the whole assembly of multiple battery cells 20 is housed in the housing 10. Alternatively, battery 100 can also be composed of multiple battery cells 20 first connected in series, in parallel, or in a mixed manner to form a battery module, and then multiple battery modules are connected in series, in parallel, or in a mixed manner to form a whole, which is then housed in the housing 10.

[0144] The battery cell 20 includes the lithium-ion secondary battery provided in this application. There can be multiple battery cells 20. Besides the lithium-ion secondary battery provided in this application, the battery cell 20 may also include lithium-sulfur batteries, sodium-ion batteries, or magnesium-ion batteries, but is not limited to these. The battery cell 20 may be cylindrical, flat, cuboid, or other shapes.

[0145] Please refer to Figure 3, which is an exploded structural diagram of an embodiment of the battery cell 20 of this application. In Figure 3, X, Y, and Z represent the directions of the three-dimensional spatial coordinate axes. The battery cell 20 refers to the smallest unit that makes up the battery 100. The battery cell 20 includes an end cap 21, a housing 22, a cell assembly 23, and other functional components.

[0146] End cap 21 refers to a component that covers the opening of housing 22 to isolate the internal environment of battery cell 20 from the external environment. The shape of end cap 21 can be adapted to the shape of housing 22 to fit it. Optionally, end cap 21 can be made of a material with certain hardness and strength (such as aluminum alloy), so that end cap 21 is not easily deformed under pressure and impact, allowing battery cell 20 to have higher structural strength and improved safety performance. Functional components such as electrode terminals 21a can be provided on end cap 21. Electrode terminals 21a can be used for electrical connection with cell assembly 23 to output or input electrical energy to battery cell 20. In some embodiments, end cap 21 can also be provided with a pressure relief mechanism for releasing internal pressure when the internal pressure or temperature of battery cell 20 reaches a threshold. The material of end cap 21 can also be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and this application embodiment does not impose special limitations on this. In some embodiments, an insulating element may be provided on the inner side of the end cap 21. The insulating element can be used to isolate the electrical connection components within the housing 22 from the end cap 21 to reduce the risk of short circuits. For example, the insulating element may be made of plastic, rubber, etc.

[0147] The housing 22 is a component used to cooperate with the end cap 21 to form the internal environment of the battery cell 20. This internal environment can accommodate the cell assembly 23, electrolyte, and other components. The housing 22 and the end cap 21 can be independent components. An opening can be provided on the housing 22, and the end cap 21 can be used to close the opening to form the internal environment of the battery cell 20. Alternatively, the end cap 21 and the housing 22 can be integrated. Specifically, the end cap 21 and the housing 22 can form a common connecting surface before other components are inserted into the housing. When it is necessary to encapsulate the interior of the housing 22, the end cap 21 closes the housing 22. The housing 22 can be of various shapes and sizes, such as cuboid, cylindrical, hexagonal prism, etc. Specifically, the shape of the housing 22 can be determined according to the specific shape and size of the cell assembly 23. The material of the housing 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special limitations on this.

[0148] The cell assembly 23 is the component in the battery cell 20 where the electrochemical reaction occurs. The casing 22 may contain one or more cell assemblies 23. The cell assembly 23 is mainly formed by winding or stacking positive and negative electrode plates, and typically a separator is provided between the positive and negative electrode plates. The portions of the positive and negative electrode plates containing active material constitute the main body of the cell assembly, while the portions of the positive and negative electrode plates without active material each constitute a tab 23a. The positive and negative tabs may be located together at one end of the main body or separately at both ends of the main body. During the charging and discharging process of the battery 100, the positive and negative active materials react with the electrolyte, and the tabs 23a connect to the electrode terminals 21a to form a current loop.

[0149] The beneficial effects of this application are further illustrated below with reference to the embodiments.

[0150] To make the technical problems, technical solutions, and beneficial effects solved by the embodiments of this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0151] Example 1

[0152] 1) Preparation of positive electrode active materials

[0153] Polyethylene was added to toluene and heated and stirred at 80°C for 0.5 hours to dissolve the polyethylene in the toluene, yielding the first intermediate product. Subsequently, lithium manganese iron phosphate (LiFe) with a carbon-coated surface was added... 0.4 Mn 0.6 After adding PO4 to the first intermediate and stirring for 2 hours, a second intermediate was obtained. Subsequently, while maintaining stirring, the temperature of the second intermediate was lowered to 25°C to obtain a third intermediate. In the third intermediate, polyethylene re-aggregated and coated onto lithium manganese iron phosphate (LiFe2+) with a carbon layer on its surface. 0.4 Mn 0.6 Finally, the toluene solution was completely removed by filtration and vacuum drying at 80°C to obtain the positive electrode active material. The positive electrode active material is lithium manganese iron phosphate with a carbon layer coated on the surface, and the carbon layer is coated with polyethylene (LiFe) 0.4 Mn 0.6 PO4 / C / polyethylene). Among them, based on LiFe 0.4 Mn 0.6The mass percentage of PO4 and the carbon layer is 2%; based on the mass of the positive electrode active material, the mass percentage of polyethylene is 0.05%; lithium manganese iron phosphate (LiFe) with a carbon layer coated on the surface. 0.4 Mn 0.6 The mass ratio of PO4 to toluene is 1:3.

[0154] 2) Preparation of positive electrode sheet

[0155] The above-prepared positive electrode active material, positive electrode binder polyvinylidene fluoride (PVDF), and positive electrode conductive agent acetylene black were dissolved in solvent N-methylpyrrolidone (NMP) at a weight ratio of 92:5.5:2.5. After thorough stirring and mixing, a positive electrode slurry was prepared. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0156] 3) Preparation of negative electrode sheet

[0157] Lithium metal sheets are used as the negative electrode.

[0158] 4) Separating membrane

[0159] A 12μm thick polypropylene membrane is used as the separator, and one side of the polypropylene membrane is coated with an alumina coating.

[0160] 5) Electrolyte

[0161] Ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the mixture to obtain the electrolyte. The concentration of LiPF6 in the electrolyte was 1 mol / L.

[0162] 6) Assembly of secondary batteries

[0163] The positive electrode, separator, and negative electrode are assembled in a button cell and injected with electrolyte to obtain a button cell.

[0164] Examples 2-13 are similar to Example 1, except that the process parameters are different, as detailed in Tables 1-5.

[0165] Example 14

[0166] 1) Preparation of positive electrode active materials

[0167] Add 240ml of deionized water to a beaker, then dissolve 96.96g of ferric nitrate nonahydrate in the 240ml of deionized water. Stir thoroughly until the ferric nitrate nonahydrate particles dissolve. At this point, the solution turns transparent red.

[0168] b. Add 38.4 g of anhydrous citric acid to the solution prepared in step a, and stir thoroughly until the anhydrous citric acid particles dissolve. At this point, the solution turns into a uniform yellow color.

[0169] c. Add 40.32 g of anhydrous sodium dihydrogen phosphate (or 52.4 g of sodium dihydrogen phosphate dihydrate) to the solution prepared in step b, and stir thoroughly for four hours to obtain the process product. During the stirring process, seal the beaker.

[0170] d. The process product is spray-dried to obtain a yellow powder. Since the yellow powder is hygroscopic, it needs to be quickly bagged and sealed.

[0171] e. Place the yellow powder into a glove box muffle furnace and heat it to 500℃ at a heating rate of 2℃ / min. After holding it at 500℃ for 12h, Na4Fe3(PO4)2P2O7 with a carbon layer on the surface is obtained.

[0172] Polyethylene was added to toluene and heated and stirred at 80°C for 0.5 hours to dissolve the polyethylene in the toluene, obtaining a first intermediate product. Then, Na4Fe3(PO4)2P2O7 with a carbon layer coated on its surface was added to the first intermediate product and stirred for 2 hours to obtain a second intermediate product. Subsequently, while maintaining stirring, the temperature of the second intermediate product was lowered to 25°C to obtain a third intermediate product. In the third intermediate product, the polyethylene re-aggregated and coated onto the Na4Fe3(PO4)2P2O7 with the carbon layer. Finally, the toluene solution was completely removed by filtration and vacuum drying at 80°C to obtain the positive electrode active material. The positive electrode active material is Na4Fe3(PO4)2P2O7 with a carbon layer coated on its surface, and the carbon layer is coated with polyethylene (Na4Fe3(PO4)2P2O7 / C / polyethylene). Among them, based on the mass of Na4Fe3(PO4)2P2O7 and the carbon layer, the mass ratio of the carbon layer is 3%; based on the mass of the positive electrode active material, the mass ratio of polyethylene is 1%; and the mass ratio of Na4Fe3(PO4)2P2O7 with a carbon layer on the surface to toluene is 1:3.

[0173] 2) Preparation of positive electrode sheet

[0174] The positive electrode active material prepared in step 1), the positive electrode binder polyvinylidene fluoride (PVDF), and the positive electrode conductive agent acetylene black were dissolved in the solvent N-methylpyrrolidone (NMP) at a weight ratio of 92:5.5:2.5. After thorough stirring and mixing, a positive electrode slurry was prepared. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0175] 3) Preparation of negative electrode sheet

[0176] The negative electrode is directly selected from sodium metal sheets.

[0177] 4) Separating membrane

[0178] A 12μm thick polypropylene membrane was used as the separator.

[0179] 5) Electrolyte

[0180] An organic solvent was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30:70. The thoroughly dried sodium salt NaPF6 was dissolved in the organic solvent in an argon-atmosphere glove box with a water content of <10 ppm, and the mixture was thoroughly homogenized to obtain the electrolyte. The sodium salt constituted 12.5% ​​of the electrolyte by mass.

[0181] 6) Assembly of secondary batteries

[0182] Stack the positive electrode, separator, and negative electrode in sequence, with the separator positioned between the positive and negative electrodes, and add the electrolyte to assemble a secondary battery.

[0183] Comparative Example 1

[0184] The difference between Comparative Example 1 and Example 1 is that step 1) of preparing the positive electrode active material is omitted. In step 2) of Comparative Example 1, the positive electrode active material is lithium manganese iron phosphate (LiFe3+) with a carbon layer coated on its surface. 0.4 Mn 0.6 PO4), meaning that the surface of the positive electrode active material does not contain polyethylene.

[0185] Comparative Example 2

[0186] The difference between Comparative Example 2 and Example 1 is that the preparation methods of the positive electrode active material in step 1) are different.

[0187] Specifically as follows:

[0188] 1) Preparation of positive electrode active materials

[0189] Lithium manganese iron phosphate (LiFe) with a carbon layer on its surface 0.4 Mn 0.6 PO4) and polyethylene are mixed at a mass ratio of 100:1 and calcined at 200°C for 3 hours under a nitrogen atmosphere to obtain the positive electrode active material.

[0190] Everything else is the same as in Example 1, and will not be repeated here.

[0191] Comparative Example 3

[0192] The difference between Comparative Example 3 and Example 1 is that step 1) of preparing the positive electrode active material is omitted. In the preparation of the positive electrode sheet in step 2) of Comparative Example 3, the positive electrode active material is lithium iron phosphate (LiFePO4) with a carbon layer coated on the surface, and the surface of the positive electrode active material does not have polyethylene.

[0193] Comparative Example 4

[0194] The difference between Comparative Example 4 and Example 1 is that step 1) of preparing the positive electrode active material is omitted. In step 2) of Comparative Example 4, the positive electrode active material is lithium manganese titanium iron phosphate (LiFe) with a carbon layer coated on its surface. 0.4 Mn 0.59 Ti 0.01 PO4), the surface of the positive electrode active material does not contain polyethylene.

[0195] Comparative Example 5

[0196] The difference between Comparative Example 5 and Example 1 is that step 1) of preparing the positive electrode active material is not included. In the preparation of the positive electrode sheet in step 2) of Comparative Example 5, the positive electrode active material is Na4Fe3(PO4)2P2O7 with a carbon layer coated on the surface, and the surface of the positive electrode active material does not have polyethylene.

[0197] The process parameters and performance of Examples 1-14 and Comparative Examples 1-5 are detailed in Tables 1-5.

[0198] The specific testing methods for the relevant parameters are as follows:

[0199] 1) Infrared spectrum test of positive electrode active material.

[0200] The infrared spectra of the positive electrode active material prepared in this application were tested using an IS10 Fourier transform infrared spectrometer from Nicolet Corporation, USA, in accordance with the general rules of infrared spectroscopy analysis method in GB / T6040-2002.

[0201] 2) Testing of the water absorption and dehydration properties of the positive electrode active material.

[0202] Under a relative humidity of 60%, the positive electrode active material to be tested was left to stand for 24 hours, and the moisture content of the positive electrode active material was tested at 170℃ using a Karl Fischer solid moisture analyzer to obtain the water absorption performance of the positive electrode active material to be tested.

[0203] Under a relative humidity of 60%, the positive electrode active material to be tested was left to stand for 24 hours, and then transferred to a 110℃ oven for vacuum drying for 5 hours. The moisture content of the material was then tested at 170℃ using a Karl Fischer solids moisture analyzer to obtain the dehydration performance of the material.

[0204] 3) Test method for the mass ratio of hydrophobic layer in positive electrode active material.

[0205] First, scrape off the positive electrode material from the positive electrode sheet and weigh it to obtain the mass of the positive electrode material as M1. Wash the scraped positive electrode material three times with N-methyl-2-pyrrolidone (NMP) solution at room temperature to obtain the washed powder. Note: The purpose of washing the positive electrode material with N-methyl-2-pyrrolidone (NMP) solution is to remove the binder in the positive electrode material.

[0206] After washing and drying, the powder was weighed to obtain mass M2 (Note: M2 is the total mass of the positive electrode active material (phosphate, coated carbon layer, coated hydrophobic layer) and conductive agent). The dried powder was then dissolved in toluene at 80°C and stirred at a constant temperature of 80°C for at least 30 minutes to ensure complete dissolution of the hydrophobic layer in the toluene. Next, the powder was quickly filtered out. The toluene solution with the powder removed was placed in a beaker with a known weight of M3. The beaker was placed in a vacuum drying oven at 100°C to evaporate the toluene. After complete evaporation, the beaker mass M4 was measured. M4 - M3 is the mass of the coated hydrophobic layer.

[0207] The mass percentage of the hydrophobic layer is approximately (M4-M3) / M2. (Note: Since M2 is the total mass of the positive electrode active material and the conductive agent, this method actually tests the mass percentage of the hydrophobic layer in the positive electrode active material and the conductive agent. However, since the mass of the conductive agent added is extremely small, its impact on the test results is negligible. Therefore, the mass percentage of the hydrophobic layer in the positive electrode active material can be calculated using (M4-M3) / M2.)

[0208] 4) Test method for determining the mass percentage of carbon layer in the total mass of phosphate and carbon layer.

[0209] First, scrape off the positive electrode material from the positive electrode sheet and weigh it to obtain the mass of the positive electrode material as M1. Wash the scraped positive electrode material three times with N-methyl-2-pyrrolidone (NMP) solution at room temperature to obtain the washed powder. Note: The purpose of washing the positive electrode material with N-methyl-2-pyrrolidone (NMP) solution is to remove the binder in the positive electrode material.

[0210] After washing and drying, the powder was weighed to obtain mass M2 (Note: M2 is the total mass of the positive electrode active material (phosphate, coated carbon layer, coated hydrophobic layer) and conductive agent). The dried powder was then dissolved in toluene at 80°C and stirred at a constant temperature of 80°C for at least 30 minutes to ensure complete dissolution of the hydrophobic layer in the toluene. The powder was then rapidly filtered to obtain the final powder, which was placed in a beaker with a known weight of M3. The beaker was placed in a vacuum drying oven at 100°C to evaporate the toluene. After complete evaporation of the toluene, the beaker mass M4 was measured. M4 - M3 represents the total mass of the phosphate, coated carbon layer, and conductive agent.

[0211] The separated powders (phosphate, coated carbon layer, and conductive agent) were combusted in a high-frequency induction furnace, and their carbon content was tested using infrared absorption spectroscopy. The specific testing procedure followed standard GB / T 20123-2006 / ISO 15350:2000. (Note: The carbon content tested by this method is the sum of the carbon content of the coated carbon layer and the conductive agent. However, since the mass of the conductive agent added is extremely small, its impact on the test results is negligible. Therefore, the carbon content tested in this application is approximately equal to the mass percentage of the coated carbon layer.)

[0212] 5) Number-average molecular weight test.

[0213] Polyolefins were dissolved or dispersed in polar solvents such as ethanol, methanol, or tetrahydrofuran, and their number-average molecular weights were determined by gel permeation chromatography (GPC).

[0214] 6) Volume average particle size (DV50) test of positive electrode active material.

[0215] Pretreatment: Take a clean beaker, weigh an appropriate amount of the positive electrode active material to be tested, add surfactant and then add 20 ml of dispersant, sonicate at 120 W for 5 min to ensure that the sample is completely dispersed in the dispersant.

[0216] Test: After pretreatment, the positive electrode active material is poured into the sample column of the laser particle size analyzer (Malvin Company, model: Mastersizer3000) and circulated with the solution to the test optical path system. Under the irradiation of the laser beam, the particle size distribution characteristics (shading degree: 15-20%) can be obtained by receiving and measuring the energy distribution of the scattered light, and the corresponding DV50 value is read.

[0217] 7) Button capacity test.

[0218] At a voltage range of 2V to 4.3V, the coin cells prepared in Examples 1-10, Example 13, Comparative Examples 1, 2, and 4 were charged to 4.3V at 0.1C, then charged at 4.3V at a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at 0.1C to 2V. The ratio of the discharge capacity to the mass of the positive electrode active material at this point is the coin cell capacity (mAh / g).

[0219] At 2.5~3.65V, the coin cells prepared in Examples 11, 12 and Comparative Example 3 were charged to 3.65V at 0.1C, then charged at 3.65V at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at 0.1C to 2.5V. The ratio of the discharge capacity to the mass of the positive electrode active material at this time is the coin cell capacity (mAh / g).

[0220] At 2V~3.45V, the coin cells prepared in Example 14 and Comparative Example 5 were charged to 3.45V at 0.1C, then charged at 3.45V at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at 0.1C to 2V. The ratio of the discharge capacity to the mass of the positive electrode active material at this time is the coin cell capacity (mAh / g).

[0221] Table 1. Process and performance parameters of the positive electrode active materials in Examples 1 to 10, Comparative Example 1, and Comparative Example 2.

[0222]

[0223] Note: DV50 represents the volume average particle size DV50; the mass percentage floating on the water surface represents the mass percentage of the positive electrode active material floating on the water surface after being placed on the water surface for 30 minutes; the number average molecular weight represents the number average molecular weight of polyolefins; the specific capacity represents the coin cell specific capacity.

[0224] Table 2. Process and performance parameters of the positive electrode active materials in Examples 11, 12 and Comparative Example 3.

[0225]

[0226] Table 3. Process and performance parameters of the positive electrode active material in Example 13 and Comparative Example 4.

[0227]

[0228] Table 4. Process and performance parameters of the positive electrode active material in Example 14 and Comparative Example 5.

[0229]

[0230] Table 5. Preparation conditions of the positive electrode active material in Examples 1 to 14.

[0231]

[0232] 1) Under the same test conditions, in Comparative Example 1, the water absorption of the positive electrode active material (i.e., the water absorption performance in Table 1) was 3500 ppm, and the moisture content of the positive electrode active material after drying (i.e., the dehydration performance in Table 1) was 460 ppm; while in Examples 1 to 10, the water absorption of the positive electrode active material was 1000 ppm to 3200 ppm, and the moisture content of the positive electrode active material after drying was 200 ppm to 409 ppm. It can be seen that the water absorption of the positive electrode active materials prepared in Examples 1 to 10 is lower than that of the positive electrode active material in Comparative Example 1. This is because in Examples 1 to 10, a hydrophobic layer was coated on the surface of the phosphate / carbon material. Utilizing the hydrophobic properties of the hydrophobic layer, the phosphate / carbon material was modified into a hydrophobic material, which can reduce the water absorption of the phosphate / carbon material and result in better electrochemical performance of the secondary battery.

[0233] Similarly, compared to Comparative Example 3, the positive electrode active materials prepared in Examples 11 and 12 had lower water absorption (i.e., water absorption performance in Table 2) and lower moisture content (i.e., dehydration performance in Table 2) after drying. It is evident that the water absorption of the positive electrode active materials prepared in Examples 11 and 12 is lower than that of the positive electrode active material in Comparative Example 3.

[0234] Similarly, compared to Comparative Example 4, the positive electrode active material prepared in Example 13 had a lower water absorption (i.e., water absorption performance in Table 3) and a lower moisture content (i.e., dehydration performance in Table 3) after drying. It is evident that the water absorption of the positive electrode active material prepared in Example 13 is lower than that of the positive electrode active material in Comparative Example 4.

[0235] Similarly, compared to Comparative Example 5, the positive electrode active material prepared in Example 14 had a lower water absorption (i.e., water absorption performance in Table 4) and a lower moisture content (i.e., dehydration performance in Table 4) after drying. It is evident that the water absorption of the positive electrode active material prepared in Example 14 is lower than that of the positive electrode active material in Comparative Example 5.

[0236] 2) Under the same test conditions, in Comparative Example 2, the water absorption of the positive electrode active material (i.e., the water absorption performance in Table 1) was 3300 ppm, and the moisture content of the positive electrode active material after drying (i.e., the dehydration performance in Table 1) was 410 ppm; while in Example 3, the water absorption of the positive electrode active material was 2200 ppm, and the moisture content of the positive electrode active material after drying was 316 ppm. It can be seen that, with the mass percentage of the hydrophobic layer being 1%, compared to the solid-phase method (Comparative Example 2 used a solid-phase method to coat the hydrophobic material onto the surface of the phosphate / carbon material), the positive electrode active material prepared by the liquid-phase method in Example 3 has lower water absorption. This is because, by using the liquid-phase method to coat the hydrophobic material onto the surface of the phosphate / carbon material, the structure of the hydrophobic material is not destroyed, thus significantly reducing the water absorption of the phosphate / carbon material. Comparative Example 3 used a solid-phase method to coat the hydrophobic material onto the surface of the phosphate / carbon material. Under high-temperature conditions, the overall chain structure of polyethylene was disrupted, which may cause some polyethylene material to volatilize, resulting in poor uniformity of polyethylene coating on the phosphate / carbon material surface, thus allowing the phosphate / carbon material to retain strong water absorption. In Comparative Example 2, after 0.1g of the positive electrode active material was placed on the water surface and left to stand for 30 minutes, only 10% of the positive electrode active material floated on the water surface (i.e., the mass percentage floating on the water surface in Table 1). In Example 3, after 0.1g of the positive electrode active material was placed on the water surface and left to stand for 30 minutes, 85% of the positive electrode active material floated on the water surface (i.e., the mass percentage floating on the water surface in Table 1). This indicates that in Example 3, the positive electrode active material prepared by the liquid-phase method had better uniformity of hydrophobic coating on the phosphate material with carbon layer coating.

[0237] 3) Figure 4 shows the positive electrode active materials prepared in Examples 3, 10, Comparative Example 1, and Comparative Example 2 of this application at 600 cm⁻¹. -1 ~1500 -1 Infrared spectra in the specified bands; Figure 5 shows the infrared spectra of the positive electrode active materials prepared in Examples 3, 10, Comparative Examples 1 and 2 of this application at 1200 cm⁻¹. -1 ~3500 -1 Infrared spectrum of the band; Figure 6 shows the infrared spectrum of the positive electrode active material prepared in Example 3 of this application at 1200 cm⁻¹. -1 ~1700 -1 Infrared spectrum in the band.

[0238] Example 3 involved coating polyethylene onto the surface of a phosphate / carbon material using a liquid-phase method. Figures 4-6 show that at a depth of 2916 cm⁻¹... -1 2850cm -1 1471cm -1 900cm -1 It has a characteristic peak at 2916 cm⁻¹.-1 and 2850cm -1 The peak represents the stretching vibration of the methylene group, at 1471 cm⁻¹. -1 The peak is the bending vibration peak of the methylene group, at 900 cm⁻¹. -1 The peak represents the ring-symmetric stretching vibration of the C-C bond. The presence of the aforementioned characteristic peaks of polyethylene in the positive electrode active material prepared in Example 3 indicates that the polyethylene structure was not destroyed when the polyethylene was coated onto the surface of the phosphate / carbon material using a liquid-phase method.

[0239] Example 10 involves coating asphalt onto the surface of a phosphate / carbon material using a liquid-phase method. Figures 4 and 5 show that at a depth of 900 cm... -1 It has a characteristic peak at 900 cm⁻¹ -1 The peaks are circum-symmetric stretching vibrations of C-C bonds, indicating that the asphalt is coated on the surface of the phosphate / carbon material.

[0240] Comparative Example 1 shows the infrared spectrum of a phosphate / carbon material without a hydrophobic layer. As can be seen from the infrared spectrum, the positive electrode active material prepared in Comparative Example 1 does not have the aforementioned characteristic peaks.

[0241] Comparative Example 2 shows the application of a solid-state method to coat polyethylene onto the surface of a phosphate / carbon material. Figures 4 and 5 show that the coating is only applied at a depth of 2916 cm⁻¹. -1 and 2850cm -1 It exhibits a weak characteristic peak at 1471 cm⁻¹, while at 1471 cm⁻¹... -1 and 900cm -1 The absence of characteristic peaks indicates that during solid-phase coating at high temperatures, polyethylene undergoes some thermal decomposition, disrupting its overall chain structure and thus altering its hydrocarbon peaks. The structural changes in polyethylene at high temperatures also alter its hydrophobic properties. Therefore, coating polyethylene onto the surface of phosphate / carbon materials using a solid-phase method allows the phosphate / carbon materials to retain strong water absorption.

[0242] 4) As can be seen from Examples 1 to 6, when the mass ratio of the hydrophobic layer is 0.1% to 5% based on the mass of the positive electrode active material, the hydrophobic layer can reduce the water absorption of the phosphate material while improving the electrochemical performance of the secondary battery. This is because the mass ratio of the hydrophobic layer in Examples 1 to 5 of this application is optimal, which can significantly reduce the water absorption of the phosphate / carbon material while improving the electrochemical performance of the secondary battery.

[0243] In Example 6, the hydrophobic layer accounts for 8% of the total mass, which is greater than 5%. It can coat the surface of the phosphate / carbon material, making the phosphate / carbon material hydrophobic and thus reducing its water absorption. However, the hydrophobic layer has poor conductivity, and excessive hydrophobic coating can affect the electrochemical performance of the secondary battery. Therefore, while significantly reducing the water absorption of the phosphate / carbon material, the electrochemical performance of the secondary battery is still better than that of Examples 1-5.

[0244] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, wherein, It includes positive electrode, negative electrode, and electrolyte; The positive electrode sheet includes a positive electrode active material, which comprises a phosphate, a carbon layer, and a hydrophobic layer. The carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer. The infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

2. The secondary battery according to claim 1, wherein, The hydrophobic layer is made of polyolefins and / or bitumen.

3. The secondary battery according to claim 2, wherein, The number average molecular weight of the polyolefin is 2000~4000.

4. The secondary battery according to any one of claims 1 to 3, wherein, Based on the mass of the positive electrode active material, the mass percentage of the hydrophobic layer is 0.1% to 5%.

5. The secondary battery according to any one of claims 1 to 4, wherein, Based on the mass of the positive electrode active material, the hydrophobic layer accounts for 0.5% to 1% of the mass.

6. The secondary battery according to any one of claims 1 to 5, wherein, Based on the mass of the phosphate and the carbon layer, the mass percentage of the carbon layer is 0.8% to 5%.

7. The secondary battery according to any one of claims 1 to 6, wherein, The phosphate includes the structural formula LiFe. (1-x) Mn x M y For materials containing PO4, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.1, M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA; and / or, the structural formula is Na4Fe. a N b For (PO4)2P2O7 materials, 2.8≤a≤3.0, 0≤b≤0.2, and N includes one or more of V, Mg, and Ti.

8. The secondary battery according to any one of claims 1 to 7, wherein, The volume average particle size (DV50) of the positive electrode active material is 100 nm to 5 μm.

9. The secondary battery according to any one of claims 1 to 8, wherein, The volume average particle size (DV50) of the positive electrode active material is 100 nm to 800 nm.

10. The secondary battery according to any one of claims 1 to 9, wherein, After 0.1g of the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 15% to 100% of the positive electrode active material floats on the water surface.

11. The secondary battery according to any one of claims 1 to 10, wherein, After 0.1g of the positive electrode active material is placed on the water surface and left to stand for 30 minutes, 80%~100% of the positive electrode active material floats on the water surface.

12. A method for preparing a secondary battery, wherein: include: A slurry containing a positive electrode active material is coated onto a positive electrode current collector to obtain a positive electrode sheet. The positive electrode active material comprises a phosphate, a carbon layer, and a hydrophobic layer. The carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer. The infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 ; The negative electrode, the separator, and the positive electrode are assembled into a secondary battery.

13. The method for preparing a secondary battery according to claim 12, wherein, The method for preparing the positive electrode active material includes: At a first preset temperature, a hydrophobic material is dissolved in a solvent to obtain a first intermediate product, wherein the solvent includes benzene-based solvents; The phosphate with a carbon layer on its surface is mixed with the first intermediate product to obtain the second intermediate product; The second intermediate product is cooled to a second preset temperature to obtain a third intermediate product, wherein the second preset temperature is lower than the first preset temperature; The positive electrode active material is obtained by removing the solvent from the third intermediate product. The positive electrode active material includes a phosphate, a carbon layer and a hydrophobic layer. The carbon layer coats the surface of the phosphate and the hydrophobic layer coats the surface of the carbon layer.

14. The method for preparing a secondary battery according to claim 13, wherein, The first preset temperature is 60℃-100℃; And / or, The second preset temperature is 15℃-30℃.

15. The method for preparing a secondary battery according to claim 13 or 14, wherein, The hydrophobic material includes polyolefins and / or bitumen; And / or, The benzene-based solvents include benzene and / or toluene.

16. The method for preparing a secondary battery according to any one of claims 13 to 15, wherein, The mass ratio of the hydrophobic material to the carbon-coated phosphate in the first intermediate product is 0.001 to 0.

053. And / or, The mass ratio of the solvent to the carbon-coated phosphate in the first intermediate product is 2:1 to 5:

1.

17. The method for preparing a secondary battery according to any one of claims 12 to 16, wherein, The phosphate includes the structural formula LiFe. (1-x) Mn x M y For materials containing PO4, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.1, M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA; and / or, the structural formula is Na4Fe. a N b For (PO4)2P2O7 materials, 2.8≤a≤3.0, 0≤b≤0.2, and N includes one or more of V, Mg, and Ti.

18. A positive electrode active material, wherein, It comprises a phosphate, a carbon layer, and a hydrophobic layer, wherein the carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer; the infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

19. The positive electrode active material according to claim 18, wherein, The hydrophobic layer is made of polyolefins and / or bitumen.

20. The positive electrode active material according to claim 18 or 19, wherein, The phosphate includes the structural formula LiFe. (1-x) Mn x M y For materials containing PO4, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.1, M includes one or more metallic elements from Groups IVB, VB, VIII, IIA, IIIA, IVA, and VA; and / or, the structural formula is Na4Fe. a N b For (PO4)2P2O7 materials, 2.8≤a≤3.0, 0≤b≤0.2, and N includes one or more of V, Mg, and Ti.

21. A method for preparing a positive electrode active material, wherein, include: At a first preset temperature, a hydrophobic material is dissolved in a solvent to obtain a first intermediate product, wherein the solvent includes benzene-based solvents; A second intermediate product is obtained by mixing a phosphate with a carbon-coated surface with the first intermediate product. The second intermediate product is cooled to a second preset temperature to obtain a third intermediate product, wherein the second preset temperature is lower than the first preset temperature; The positive electrode active material is obtained by removing the solvent from the third intermediate product. The positive electrode active material comprises a phosphate, a carbon layer, and a hydrophobic layer. The carbon layer coats the surface of the phosphate, and the hydrophobic layer coats the surface of the carbon layer. The infrared absorption spectrum of the hydrophobic layer has a characteristic peak at 1451 cm⁻¹. -1 ~1491cm -1 , and / or 900cm -1 ~850cm -1 .

22. The method for preparing the positive electrode active material according to claim 21, wherein, The hydrophobic material includes polyolefins and / or bitumen; And / or, The benzene-based solvents include benzene or toluene.

23. The method for preparing the positive electrode active material according to claim 21 or 22, wherein, The first preset temperature is 60℃~100℃; And / or, The second preset temperature is 15℃~30℃.

24. An electrical appliance, wherein, Includes the secondary battery according to any one of claims 1 to 11, and / or the secondary battery prepared by the method of preparing the secondary battery according to any one of claims 12 to 17, and / or the positive electrode active material according to any one of claims 18 to 20; and / or the positive electrode active material prepared by the method of preparing the positive electrode active material according to any one of claims 21 to 23.

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