A negative electrode material, a preparation method therefor, and an application thereof

Abstract

The application relates to a negative electrode material and a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The negative electrode material comprises: an inner core comprising graphite particles; the sphericity of the graphite particles is 0.5-0.8; the graphite particles have basal planes and end faces; a coating layer coats part of the surface of the graphite particles; the part of the surface comprises all the end faces and part of the basal planes of the graphite particles; the coating layer contains a coating agent; and the ratio P of the area of the graphite particles coated by the coating layer to the surface area of the graphite particles satisfies 50% < P < 100%. The different chemical bonds between the layers and in the layers of the graphite particles are utilized, so that the magnetization direction and degree under the action of a magnetic field are different, the graphite particles are kept in the same orientation, and then liquid-phase coating is carried out; and the feature that the lithium intercalation capacity of the end faces of the graphite particles is much higher than that of the basal planes is utilized, so that coating all the end faces does not reduce the charging capacity of the battery, and coating part of the basal planes can effectively improve the high-temperature performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a negative electrode material, its preparation method and application. Background Technology

[0002] The rapid development of electric vehicles has placed higher demands on both the high-temperature performance and fast-charging performance of batteries. High-temperature performance mainly refers to the battery's calendar life, while fast-charging performance mainly refers to the battery's fast-charging time. Insufficient high-temperature performance is primarily caused by excessively high reactivity at the electrolyte-negative electrode interface, while insufficient fast-charging performance is also largely related to insufficient reactivity at the electrolyte-negative electrode interface. Existing methods to improve fast-charging performance through negative electrode materials typically involve coating the negative electrode surface with other materials with higher reactivity (including soft carbon, hard carbon, etc.) or reducing the particle size of the material. However, these methods usually deteriorate high-temperature performance while improving fast-charging performance; conversely, measures to improve high-temperature performance have the opposite effect. Therefore, how to simultaneously mitigate or solve these problems remains a key design challenge for negative electrode materials. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides an anode material, its preparation method, and its application. It utilizes the layered structure of graphite particles to achieve interlayer lithium intercalation. Furthermore, leveraging the orientation of graphite particles, lithium intercalation can only occur on the end faces of the graphite particles, while the basal surface lacks this capability. Unlike the fully coated designs in existing technologies, this invention only coats the end faces capable of lithium intercalation, reducing coating on the basal surface. This reduces the amount of coating agent used, lowers costs, and decreases the overall coating dosage on the material surface, significantly improving the high-temperature performance of the anode material while maintaining fast-charging performance.

[0004] The first objective of this invention is to provide a negative electrode material, the negative electrode material comprising:

[0005] The core comprises graphite particles; the sphericity of the graphite particles is 0.5-0.8; the graphite particles have a basal surface and an end face;

[0006] A coating layer covers a portion of the surface of the graphite particles; the portion of the surface includes all end faces and a portion of the base face of the graphite particles; the coating layer contains a coating agent;

[0007] The ratio P of the area of ​​the graphite particle covered by the coating layer to the surface area of ​​the graphite particle satisfies 50% < P < 100%.

[0008] In one embodiment of the present invention, the D50 of the graphite particles is 5μm-15μm;

[0009] And / or, the thickness of the coating layer is 5nm-50nm.

[0010] In one embodiment of the present invention, the D50 of the negative electrode material is 7μm-17μm.

[0011] In one embodiment of the present invention, the coating agent is selected from bitumen and / or resin.

[0012] The second objective of this invention is to provide a method for preparing a negative electrode material, comprising the following steps:

[0013] S1. Graphite particles are dispersed in a coating agent solution. Under the action of a magnetic field and airflow, the graphite particles undergo orientational movement and are then passed through a spray drying device to coat part of the graphite particles with the coating agent, resulting in incompletely coated graphite. The coated portion includes all end faces and part of the base face of the graphite particles.

[0014] S2. Under a protective atmosphere, the incompletely coated graphite described in S1 is subjected to carbonization treatment to obtain the negative electrode material.

[0015] In one embodiment of the present invention, in S1, the viscosity of the coating agent solution is 500 Pa·s-10000 Pa·s;

[0016] And / or, the coating agent in the coating agent solution is selected from bitumen and / or resin;

[0017] And / or, the solvent in the coating agent solution is selected from one or more of gasoline, tetrafluorohydroran, benzene, and alcohol.

[0018] In one embodiment of the present invention, in S1, the magnetic field strength of the magnetic field is 1T-6T;

[0019] The gas flow rate of the airflow force is 0.5L / min-5L / min.

[0020] In one embodiment of the present invention, in S2, the protective atmosphere is selected from nitrogen, argon or helium.

[0021] In one embodiment of the present invention, in S2, the carbonization process involves heating to 1000℃-1200℃ at a rate of 4℃ / min-6℃ / min and holding at that temperature for 2h-6h.

[0022] A third objective of this invention is to provide a lithium-ion battery comprising the aforementioned negative electrode material or a negative electrode material prepared by the method thereof.

[0023] The technical solution of the present invention has the following advantages compared with the prior art:

[0024] The preparation method described in this invention utilizes the different lamellar and interlayer chemical bonds of graphite particles, resulting in different magnetization directions and degrees under the action of a magnetic field, so that the graphite particles maintain the same orientation (i.e. all end faces face one direction and all base faces one direction), and then perform liquid phase coating; and utilizes the characteristic that the lithium intercalation capacity of the end faces of graphite particles is much higher than that of their base faces, so that coating all end faces does not reduce the charging capacity of the battery, while coating part of the base face can effectively improve the high-temperature performance of the battery. Attached Figure Description

[0025] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0026] Figure 1 This is a schematic diagram of the base surface and end surface of the graphite particles of the present invention;

[0027] Figure 2 This is a schematic diagram illustrating the preparation of the negative electrode material of the present invention;

[0028] Figure 3 This is a schematic diagram of the coating of the negative electrode material in the comparative examples and embodiments of the present invention;

[0029] Figure 4 This is a schematic diagram illustrating the principle of the negative electrode material in the comparative examples and embodiments of the present invention. Detailed Implementation

[0030] As described in the background section, the fast-charging performance of negative electrode materials is usually improved by coating the surface of the negative electrode with other materials with higher reactivity or by reducing the particle size of the materials. However, although these methods can improve the fast-charging performance to a certain extent, they also lead to the deterioration of high-temperature performance. The commonly used measures to improve high-temperature performance are basically opposite to those to improve fast-charging performance. Therefore, how to improve both fast-charging performance and high-temperature performance is an urgent problem to be solved.

[0031] To address the aforementioned technical problems, this invention provides an anode material, its preparation method, and its application. It utilizes the layered structure of graphite particles to achieve interlayer lithium intercalation. Furthermore, leveraging the orientation of graphite particles, lithium intercalation can only occur on the end faces of the graphite particles, while the basal surface lacks this capability. Unlike the fully coated designs in existing technologies, this invention only coats the end faces capable of lithium intercalation, reducing coating on the basal surface. This reduces the amount of coating agent used, lowers costs, and decreases the overall coating dosage on the material surface, significantly improving the high-temperature performance of the anode material while maintaining fast-charging performance.

[0032] The first objective of this invention is to provide a negative electrode material, the negative electrode material comprising:

[0033] The core comprises graphite particles; the sphericity of the graphite particles is 0.5-0.8; the graphite particles have a basal surface and an end face;

[0034] A coating layer covers a portion of the surface of the graphite particles; the portion of the surface includes all end faces and a portion of the base face of the graphite particles; the coating layer contains a coating agent;

[0035] The ratio P of the area of ​​the graphite particles covered by the coating layer to the surface area of ​​the graphite particles satisfies 50% < P < 100%. Generally, coating layers offer good fast-charging capability but poor high-temperature performance. To achieve a balance between fast charging and high-temperature performance, the coating layer should be minimized while maintaining fast charging capability. Considering that graphite particles have a basal surface and an end face, the ion diffusion coefficient of the basal surface is orders of magnitude lower than that of the end face, meaning that lithium intercalation mainly occurs from the end face. Therefore, reducing the coating of the basal surface while leaving the end face normal achieves a balance between fast charging and high temperature performance. Specifically, to completely cover the end face, at least 50% of the specific surface area needs to be coated, while the basal surface area should be minimized, i.e., less than 100% of the specific surface area.

[0036] It should be noted that: such as Figure 1 As shown, the base plane is a plane parallel to the graphite layer inside the graphite crystal; the end face is a plane on the base plane that cuts off the graphite crystal; during the charging process of a lithium-ion graphite battery, lithium ions are mainly embedded into the graphite layer through the end face and gradually diffuse into the interior of the particles; the embedding of the end face is a key step in the charging and discharging process of a lithium-ion battery, which affects the charging and discharging performance of the battery.

[0037] In one embodiment of the present invention, the D50 of the graphite particles is 5μm-15μm; if the D50 is less than 5μm, the graphite particle yield is low and the cost is high; if the D50 is greater than 15μm, the fast charging capability is poor and the actual application effect is poor.

[0038] And / or, the thickness of the coating layer is 5nm-50nm; excessive thickness leads to poor high-temperature performance.

[0039] In one embodiment of the present invention, the D50 of the negative electrode material is 7μm-17μm; compared with the D50 of graphite particles, it is increased for two reasons: first, the presence of the coating layer slightly increases the D50; second, after coating, there will be adhesion, resulting in slight granulation, which increases the D50.

[0040] In one embodiment of the present invention, the coating agent is selected from asphalt and / or resin; firstly, it is widely available and has a low cost; secondly, it has a certain degree of fluidity and good coating effect.

[0041] The second objective of this invention is to provide a method for preparing a negative electrode material, comprising the following steps:

[0042] S1. Graphite particles are dispersed in a coating agent solution. Under the action of a magnetic field and airflow, the graphite particles undergo orientational movement and are then passed through a spray drying device to coat part of the graphite particles with the coating agent, resulting in incompletely coated graphite. The coated portion includes all end faces and part of the base face of the graphite particles.

[0043] S2. Under a protective atmosphere, the incompletely coated graphite described in S1 is subjected to carbonization treatment to obtain the negative electrode material.

[0044] In one embodiment of the present invention, in S1, the viscosity of the coating agent solution is 500 Pa·s-10000 Pa·s; when the viscosity is too low, the fluidity is good, which will result in a thin coating layer and poor fast charging capability; when the viscosity is too high, the fluidity is poor, which will result in a thick coating layer and poor high-temperature performance.

[0045] And / or, the coating agent in the coating agent solution is selected from bitumen and / or resin;

[0046] And / or, the solvent in the coating agent solution is selected from one or more of gasoline, tetrafluorohydroran, benzene, and alcohol.

[0047] In one embodiment of the present invention, in S1, the magnetic field strength of the magnetic field is 1T-6T;

[0048] The gas flow rate of the airflow force is 0.5L / min-5L / min.

[0049] In one embodiment of the present invention, in S1, the preparation of the graphite particles includes the following steps: coarsely crushing, graphitizing and classifying carbon particles to obtain the graphite particles.

[0050] Furthermore, the carbon particles are selected from one or more of petroleum coke, needle coke, and pitch coke.

[0051] Furthermore, the graphitization process includes two stages: the first stage involves heating to 1000℃-1200℃ at a rate of 4℃ / min-6℃ / min; the second stage involves heating to 2800℃-3300℃ at a rate of 13℃ / min-17℃ / min, and holding at that temperature for 24h-50h.

[0052] In one embodiment of the present invention, in S2, the protective atmosphere is selected from nitrogen, argon or helium.

[0053] In one embodiment of the present invention, in S2, the carbonization process involves heating to 1000℃-1200℃ at a rate of 4℃ / min-6℃ / min and holding at that temperature for 2h-6h.

[0054] A third objective of this invention is to provide a lithium-ion battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the active material of the negative electrode comprises the aforementioned negative electrode material or a negative electrode material prepared by the method described above.

[0055] In one embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector, wherein the negative electrode material layer includes a negative electrode active material, a conductive agent, and a binder. The negative electrode sheet can be prepared using conventional methods in the art, one illustrative preparation method being: preparing an electrode slurry by mixing the negative electrode active material, binder, and conductive agent in a certain proportion, then coating it onto at least one surface of the negative electrode current collector, and after drying and pressing, obtaining the negative electrode sheet.

[0056] The type of negative electrode current collector described above is not specifically limited and can be selected according to actual needs. In some embodiments, the negative electrode current collector may be copper foil, carbon-coated copper foil, or a polymer conductive film.

[0057] The type and content of the aforementioned conductive agent are not specifically limited and can be selected according to actual needs. In some embodiments, the conductive agent includes one or more of conductive carbon black, carbon nanotubes, acetylene black, graphene, Ketjen black, carbon nanofibers, etc. It should be understood that, without departing from the spirit of this application, other conductive agents capable of achieving the functions of this application can be selected according to specific needs, and are not limited thereto.

[0058] The type and content of the above-mentioned adhesives are not specifically limited and can be selected according to actual needs. In some embodiments, the above-mentioned adhesives include one or more of the following: polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, sodium carboxymethyl cellulose, polymethacrylamide, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyamide, polyimide, polyacrylate, styrene-butadiene rubber, sodium alginate, chitosan, polyethylene glycol, guar gum, etc.

[0059] In one embodiment of the present invention, the positive electrode sheet is prepared in the same way as the negative electrode sheet, wherein the positive electrode active material, binder and conductive agent are prepared into an electrode slurry in a certain proportion, and then coated on at least one surface of the positive electrode current collector, and obtained after drying and pressing.

[0060] The types and amounts of the aforementioned positive electrode active materials are not specifically limited and can be selected according to actual needs. In some embodiments, the aforementioned positive electrode active materials include one or more of lithium iron phosphate, ternary lithium, lithium iron manganese, lithium cobalt oxide, and lithium manganese oxide.

[0061] The type of positive current collector is not specifically limited and can be selected according to actual needs. For example, the positive current collector can be aluminum foil, nickel foil, or a polymer conductive film. The types of conductive agents and binders in the positive electrode sheet are the same as those in the negative electrode sheet, and will not be repeated here.

[0062] In one embodiment of the present invention, the electrolyte includes one or more of the following: organic liquid electrolyte, organic solid electrolyte, solid ceramic electrolyte, gel electrolyte, etc. Preferably, the electrolyte is an organic liquid electrolyte, which is obtained by dissolving a lithium salt in a non-aqueous organic solvent; wherein the lithium salt includes one or more of lithium difluorophosphate, lithium hexafluorophosphate, lithium difluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, and lithium difluorooxalate borate. The non-aqueous organic solvent may include one or more of cyclic carbonates, chain carbonates, and carboxylic acid esters. The cyclic carbonate may be selected from one or more of ethylene carbonate, propylene carbonate, butenyl carbonate, and γ-butyrolactone; the chain carbonate may be selected from one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, and ethyl propionate, etc.

[0063] In one embodiment of the present invention, the separator is polyethylene, polypropylene, polyvinylidene fluoride or nonwoven fabric, or their multilayer composite membranes, as well as the modified separators such as ceramic-modified or PVDF-modified membranes, which can be selected according to actual needs.

[0064] It should be understood that the preparation methods of the positive electrode, negative electrode, separator, and electrolyte in this application can be adapted to achieve the functions of this application, without departing from the spirit of this application, and other preparation methods that can achieve the functions of this application can be selected according to specific needs, without being limited thereto. In one embodiment of the method for manufacturing a lithium-ion battery, the preparation method includes: sequentially winding, folding, or stacking the above-mentioned negative electrode, separator, and positive electrode into an electrode assembly; placing the electrode assembly into, for example, an aluminum-plastic film; injecting electrolyte; and then performing vacuum sealing, settling, formation, shaping, and other processes to obtain a lithium-ion battery.

[0065] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0066] In this invention, unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0067] In this invention, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0068] In this invention, unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0069] In this invention, unless otherwise stated, the incompletely coated graphite prepared in the embodiments of this invention is the graphite particles whose end faces and part of their base faces are coated.

[0070] Example 1

[0071] Reference Figure 2 As shown, the negative electrode material and its preparation method in this embodiment specifically include the following steps:

[0072] S1. The petroleum coke is coarsely crushed to below 5 mesh, and then crushed and shaped using a roller mill-shaping integrated machine to obtain precursor A with an average particle size D50 of about 20 μm and an actual particle size D50 of 20.3 μm;

[0073] S2. Graphitize precursor A by first heating it to 1100℃ at 5℃ / min, then heating it to 3000℃ at 15℃ / min, and holding it at that temperature for 24 hours to obtain precursor B.

[0074] S3. Precursor B is crushed and classified to an average particle size D50 of about 13 μm, an actual particle size D50 of 12.7 μm, and a sphericity of about 0.6, to obtain graphite particles.

[0075] S4. Graphite particles are dispersed in a coating agent solution. An external magnetic field of 5T and a gas flow rate of 3L / min are applied to push the graphite particles through the surface of the coating agent solution to obtain incompletely coated graphite. The coating agent solution is prepared by dissolving asphalt in tetrafluorohydroran and has a viscosity of 8000 Pa·s.

[0076] S5. The incompletely coated graphite is carbonized under nitrogen protection, heated to 1000℃ at a heating rate of 5℃ / min, and held at a constant temperature for 4h. Then, the negative electrode material is obtained by grading and demagnetizing. The ratio of the area of ​​graphite particles coated by the coating layer to the surface area of ​​the graphite particles is 71%. The thickness of the coating layer is about 10nm.

[0077] Example 2

[0078] Reference Figure 2 As shown, the negative electrode material and its preparation method in this embodiment specifically include the following steps:

[0079] S1. The petroleum coke is coarsely crushed to below 5 mesh, and then crushed and shaped using a roller mill-shaping integrated machine to obtain precursor A with an average particle size D50 of about 15 μm and an actual particle size D50 of 15.5 μm.

[0080] S2. Graphitize precursor A by first heating it to 1100℃ at 5℃ / min, then heating it to 3000℃ at 15℃ / min, and holding it at that temperature for 24 hours to obtain precursor B.

[0081] S3. Precursor B is crushed and classified to an average particle size D50 of about 10 μm, an actual particle size D50 of 10.1 μm, and a sphericity of about 0.7, to obtain graphite particles.

[0082] S4. Graphite particles are dispersed in a coating agent solution. An external magnetic field of 5T and a gas flow rate of 3L / min are applied to push the graphite particles through the surface of the coating agent solution to obtain incompletely coated graphite. The coating agent solution is prepared by dissolving asphalt in tetrafluorohydroran and has a viscosity of 8000 Pa·s.

[0083] S5. The incompletely coated graphite is carbonized under nitrogen protection, heated to 1000℃ at a heating rate of 5℃ / min, and held at a constant temperature for 4h. Then, the negative electrode material is obtained by grading and demagnetizing. The ratio of the area of ​​graphite particles coated by the coating layer to the surface area of ​​the graphite particles is 77%. The thickness of the coating layer is about 10nm.

[0084] Example 3

[0085] Reference Figure 2 As shown, the negative electrode material and its preparation method in this embodiment specifically include the following steps:

[0086] S1. The needle-shaped coke is coarsely crushed to below 5 mesh, and then crushed and shaped using a roller mill-shaping integrated machine to obtain precursor A with an average particle size D50 of about 15 μm and an actual particle size D50 of 15.2 μm;

[0087] S2. Graphitize precursor A by first heating it to 1100℃ at 5℃ / min, then heating it to 3300℃ at 15℃ / min, and holding it at that temperature for 24 hours to obtain precursor B.

[0088] S3. The precursor B is crushed and classified to an average particle size D50 of about 10 μm, an actual particle size D50 of 10.4 μm, and a sphericity of about 0.6, to obtain graphite particles.

[0089] S4. Graphite particles are dispersed in a coating agent solution. An external magnetic field of 3T and a gas flow rate of 3L / min are applied to push the graphite particles through the surface of the coating agent solution to obtain incompletely coated graphite. The coating agent solution is prepared by dissolving asphalt in tetrafluorohydroran and has a viscosity of 8000 Pa·s.

[0090] S5. The incompletely coated graphite is carbonized under nitrogen protection, heated to 1000℃ at a heating rate of 5℃ / min, and held at a constant temperature for 4h. Then, the negative electrode material is obtained by grading and demagnetizing. The ratio of the area of ​​graphite particles coated by the coating layer to the surface area of ​​the graphite particles is 82%. The thickness of the coating layer is about 13nm.

[0091] Example 4

[0092] The method is basically the same as in Example 1, except that the sphericity of the graphite particles is adjusted to about 0.5, and an external magnetic field of 6T and a gas flow rate of 5L / min are applied so that the ratio of the area of ​​the graphite particles covered by the coating layer to the surface area of ​​the graphite particles is 51%.

[0093] Example 5

[0094] The method is basically the same as in Example 1, except that the sphericity of the graphite particles is adjusted to about 0.8, and the external magnetic field and airflow force are adjusted to 1T and 0.5L / min, so that the ratio of the area of ​​the graphite particles covered by the coating layer to the surface area of ​​the graphite particles is 98%.

[0095] Comparative Example 1

[0096] The process is basically the same as in Example 1, except that graphite particles and coating agent solution are mixed in a mechanical fusion machine at a weight ratio of 100:10 and mixed at 800 r / min and 200°C for 60 min to obtain fully coated graphite.

[0097] Comparative Example 2

[0098] The process is basically the same as in Example 2, except that graphite particles and coating agent solution are mixed in a mechanical fusion machine at a weight ratio of 100:10 and mixed at 800 r / min and 200°C for 60 min to obtain fully coated graphite.

[0099] Comparative Example 3

[0100] The process is basically the same as in Example 3, except that graphite particles and coating agent solution are mixed in a mechanical fusion machine at a weight ratio of 100:10 and mixed at 800 r / min and 200°C for 60 min to obtain fully coated graphite.

[0101] Comparative Example 4

[0102] The basic principle is the same as in Example 1, except that the sphericity of the graphite particles is adjusted to approximately 0.49.

[0103] Comparative Example 5

[0104] The basic principle is the same as in Example 1, except that the sphericity of the graphite particles is adjusted to approximately 0.82.

[0105] Performance testing

[0106] The anode materials prepared in Examples 1-5 and Comparative Examples 1-5 were tested for particle size D50, tap density, specific surface area, etc.

[0107] (1) Particle size D50: A certain amount of sample is placed in a Malvern 3000 laser particle size analyzer. After sufficient dispersion, the particle size distribution can be read. D50 is the particle size value corresponding to 50% of the particle size volume distribution.

[0108] (2) Tap density: Take a certain amount of sample and place it in the tap density meter. The instrument vibrates at high frequency to make the material compacted. After the instrument reading stabilizes, read the tap density data.

[0109] (3) Specific surface area: The sample is placed in a BET tester and nitrogen (N2) is introduced. The specific surface area is measured by the N2 adsorption method. The specific surface area of ​​the material is characterized by the amount of gas adsorbed. That is, after nitrogen is introduced for a period of time, the specific surface area of ​​the composite material is represented by the amount of gas adsorbed.

[0110] Battery assembly:

[0111] Negative electrode sheet: The negative electrode active materials are the negative electrode active materials prepared in Examples 1-3 and Comparative Examples 1-3, respectively. The conductive agent is conductive carbon black (SP), the binder is polyacrylic acid (PAA) and styrene-butadiene rubber (SBR) (mass ratio 2:1), and the negative electrode current collector is 6μm copper foil. The negative electrode active material, conductive agent and binder are mixed in a mass ratio of 95.5:1.5:3, deionized water is added and stirred to form a uniform and stable negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector and dried to obtain the negative electrode sheet.

[0112] Positive electrode sheet: The positive active material is lithium iron phosphate, the conductive agent is conductive carbon black (SP) and carbon nanotubes (mass ratio 2:1), the binder is PVDF5130, and the positive current collector is aluminum foil; after mixing the positive active material, conductive agent and binder at a mass ratio of 96.5:2.5:1, NMP is added and stirred to form a uniform and stable positive slurry. The positive slurry is uniformly coated on the positive current collector, dried and cold pressed to obtain the positive electrode sheet.

[0113] Separator membrane: The base membrane of the separator membrane is a polypropylene membrane with a thickness of 9+3+3+3μm.

[0114] Electrolyte: Ethyl carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) are mixed in a volume ratio of 1:1:1. Then, fully dried LiPF6 is dissolved in the mixed organic solvent at a ratio of 1.2 mol / L. Ethylene carbonate is added to prepare the electrolyte.

[0115] Assembly of lithium-ion batteries: The positive electrode, separator, negative electrode, and separator are arranged in sequence and wound to obtain the battery cell; the tabs are ultrasonically welded, the top and side are sealed with aluminum-plastic film, dried to remove moisture, and then sealed again after injecting electrolyte. The electrolyte is injected into the dried battery cell and soaked for 24 hours. After that, formation is carried out at 45°C. The formation process is as follows: charge to 3.4V at 0.05C, then charge to 3.75V at 0.2C; and finally age at room temperature for 24 hours to obtain the lithium-ion battery.

[0116] Performance testing:

[0117] (1) 4C charging process: The battery was cycled for 3 weeks in a 25℃ chamber at a rate of 0.33C / 0.33C (charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage of 3.65V until the current is 0.05C and then discharged at a constant current of 0.33C until the voltage drops to 2.5V). The discharge capacity of the last week was taken as the initial capacity C0. Then, a multi-channel recorder was connected to the negative electrode and the reference electrode to fully charge the battery at a rate of 4C0 and record the minimum voltage value of the multi-channel recorder during the charging process.

[0118] (2) High-temperature storage process: The battery is cycled for 3 weeks at a rate of 0.33C / 0.33C in a 25℃ chamber, and the discharge capacity of the last week is taken as the initial capacity C0; then the battery is fully charged at a rate of 0.33C, and after full charging, it is stored in a 60℃ chamber for 30 days; after storage, the battery is placed in a 25℃ chamber and the battery temperature is stabilized at 25℃. First, it is discharged at 0.33C to obtain the discharge capacity C1, and then cycled for 3 weeks at a rate of 0.33C, and the discharge capacity of the last week is taken as C2; then the capacity retention rate is C1 / C0*100%, and the capacity recovery rate is C2 / C0*100%.

[0119] (3) Specific capacity: First, charge at a constant current of 0.05C to 5.0mV, then charge at a constant current of 0.1C to 2.0V for 2 cycles, and then discharge at 0.1C to 0.005V; the ratio of the discharge specific capacity to the weight of the active material (i.e. the weight of the graphite particles with the coating layer in this application) is the discharge specific capacity.

[0120] (4) First effect: First charge at 0.05C constant current to 5.0mV, then charge at 0.1C constant current to 2.0V for 2 cycles, then discharge at 0.1C to 0.005V; the first effect is calculated by the ratio of discharge capacity to charge capacity (the ratio of the discharge capacity of the third step to the total charge capacity of the first and second steps is the first effect).

[0121] Table 1 shows the relevant test results:

[0122] Table 1

[0123]

[0124]

[0125] As can be seen from Table 1, comparing Examples 1-3 and Comparative Examples 1-3, the negative electrode materials of the examples exhibit excellent specific capacity and first-time efficiency; this is because partial coating reduces the amount of coating on the material surface ( Figure 3 The coating layer itself, due to its low orderliness and high interfacial activity, leads to a decrease in specific capacity and initial efficiency. Therefore, reducing the coating amount is beneficial to improving specific capacity and initial efficiency. Furthermore, compared to the comparative example, the charging capacity is not significantly different because the examples completely coat the graphite end faces, allowing the graphite end faces to charge and discharge, resulting in comparable charging capacity. However, the high-temperature performance is significantly improved because the examples reduce the coating of the substrate, lowering the coating amount. The high reactivity of the coating layer leads to deterioration in high-temperature performance; therefore, the high-temperature performance of the examples is significantly improved. Figure 4 ).

[0126] Comparing Example 1 and Comparative Examples 4-5, it can be seen that the reduction in graphite sphericity worsens fast charging performance and high-temperature performance. The improvement in sphericity has limited effect on improving fast charging and high-temperature performance beyond a certain range, and it also significantly increases production efficiency and cost.

[0127] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A negative electrode material, characterized in that, The negative electrode material includes: The core comprises graphite particles; the sphericity of the graphite particles is 0.5-0.8; the graphite particles have a basal surface and an end face; A coating layer covers a portion of the surface of the graphite particles; the portion of the surface includes all end faces and a portion of the base face of the graphite particles; the coating layer contains a coating agent; The ratio P of the area of ​​the graphite particle covered by the coating layer to the surface area of ​​the graphite particle satisfies 50% < P < 100%.

2. The negative electrode material according to claim 1, characterized in that, The D50 of the graphite particles is 5μm-15μm; And / or, the thickness of the coating layer is 5nm-50nm.

3. The negative electrode material according to claim 1, characterized in that, The D50 of the negative electrode material is 7μm-17μm.

4. The negative electrode material according to claim 1, characterized in that, The coating agent is selected from bitumen and / or resin.

5. A method for preparing a negative electrode material, characterized in that, Includes the following steps: S1. Graphite particles are dispersed in a coating agent solution. Under the action of a magnetic field and airflow, the graphite particles undergo orientational movement. The particles are then passed through a spray drying device to coat some of the graphite particles with the coating agent, resulting in incompletely coated graphite. The sphericity of the graphite particles is 0.5-0.

8. The graphite particles have a basal surface and end faces. The coated portion includes all end faces and part of the basal surface of the graphite particles. The ratio P of the area of ​​the graphite particles coated by the coating layer to the surface area of ​​the graphite particles satisfies 50% < P < 100%. S2. Under a protective atmosphere, the incompletely coated graphite described in S1 is subjected to carbonization treatment to obtain the negative electrode material.

6. The method for preparing the negative electrode material according to claim 5, characterized in that, In S1, the viscosity of the coating agent solution is 500 Pa·s-10000 Pa·s; And / or, the coating agent in the coating agent solution is selected from bitumen and / or resin; And / or, the solvent in the coating agent solution is selected from one or more of gasoline, tetrafluorohydroran, benzene, and alcohol.

7. The method for preparing the negative electrode material according to claim 5, characterized in that, In S1, the magnetic field strength is 1T-6T; The gas flow rate of the airflow force is 0.5L / min-5L / min.

8. The method for preparing the negative electrode material according to claim 5, characterized in that, In S2, the protective atmosphere is selected from nitrogen, argon or helium.

9. The method for preparing the negative electrode material according to claim 5, characterized in that, In S2, the carbonization process involves heating to 1000℃-1200℃ at a rate of 4℃ / min-6℃ / min and holding at that temperature for 2h-6h.

10. A lithium-ion battery, characterized in that, Includes the negative electrode material according to any one of claims 1-4 or the negative electrode material prepared by the method according to any one of claims 5-9.

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