Negative electrode material and preparation method and application thereof

Abstract

The invention 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 of the present invention comprises: an inner core comprising graphite particles; the sphericity degree of the graphite particles is 0.5-0.8; each graphite particle has a base surface and an end surface; the coating layer is used for coating part of the surfaces of the graphite particles; the partial surface comprises all end surfaces and partial base surfaces 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 graphite particles are kept in the same orientation by utilizing different lamellar and interlayer chemical bonds of the graphite particles and different magnetization directions and degrees under the action of a magnetic field, and then liquid phase coating is carried out; and by utilizing the characteristic that the lithium intercalation capability of the end surfaces of the graphite particles is far higher than that of the base surfaces, the charging capability of the battery is not reduced by coating all the end surfaces, and the high-temperature performance of the battery can be effectively improved by coating part of the base surfaces.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a negative electrode material and a preparation method and application thereof. Background Art

[0002] The rapid development of electric vehicles has put forward better requirements for both high-temperature performance and fast-charging performance of batteries. High-temperature performance mainly refers to the calendar life of the battery, and fast-charging performance mainly refers to the fast-charging time of the battery. The inadequate high-temperature performance of the battery is mainly caused by the excessively high active reaction ability of the electrolyte and the negative electrode interface, while the inadequate fast-charging performance is also largely related to the inadequate active reaction ability of the electrolyte and the negative electrode interface. Existing methods of improving fast-charging performance through negative electrode materials usually involve coating the negative electrode surface with other materials with higher reaction activity (including soft carbon, hard carbon, etc.) or reducing the particle size of the material, etc. However, these methods usually lead to a deterioration of high-temperature performance while improving fast-charging performance; and measures to improve high-temperature performance are just the opposite; therefore, how to simultaneously mitigate or solve the above problems remains the key to the design of negative electrode materials. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention provides a negative electrode material and its preparation method and application, which utilizes the lamellar structure of graphite particles to achieve interlayer lithium insertion; and utilizes the orientation of graphite particles so that lithium insertion can only be carried out on the end faces of graphite particles, while the basal surface has no lithium insertion ability; different from the fully coated design in the prior art, the present invention will only coat the end faces with lithium insertion ability, and reduce the coating on the basal surface, thereby reducing the amount of coating agent used, reducing costs and the coating amount on the overall material surface, and greatly improving the high-temperature performance of the negative electrode material while ensuring fast charging performance.

[0004] A first object of the present invention is to provide a negative electrode material, comprising:

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

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

[0007] A 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%.

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

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

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

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

[0012] A second object of the present invention is to provide a method for preparing a negative electrode material, comprising the following steps:

[0013] S1. Dispersing graphite particles in a coating agent solution, causing the graphite particles to orient and move under the action of a magnetic field and airflow force, and passing the graphite particles through a spray drying device so that the coating agent covers a portion of the graphite particles to obtain incompletely coated graphite; the coated portion includes all end faces and a portion of the basal surface of the graphite particles;

[0014] S2. Under a protective atmosphere, carbonize the incompletely coated graphite described in S1 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 asphalt and / or resin;

[0017] And / or, the solvent in the coating agent solution is selected from one or more of gasoline, tetrafluoroethylene, 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 air flow 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 treatment is performed by heating the temperature to 1000-1200°C at a rate of 4-6°C / min and keeping the temperature for 2-6 hours.

[0022] The third object of the present invention is to provide a lithium ion battery comprising the negative electrode material or the negative electrode material prepared by the method described above.

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

[0024] The preparation method described in the present invention utilizes the different chemical bonds between the flakes and layers 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 the end faces are facing in one direction, and all the basal faces are facing in one direction), and then liquid phase coating is performed; and the characteristic that the lithium insertion capacity of the end faces of graphite particles is much higher than that of their basal faces is utilized, so that coating all the end faces will not reduce the charging capacity of the battery, while coating part of the basal face can effectively improve the high-temperature performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0026] Figure 1 Schematic diagram of the basal plane and end face of the graphite particles of the present invention;

[0027] Figure 2 Schematic diagram of the preparation of the negative electrode material of the present invention;

[0028] Figure 3 Schematic diagram of coating of negative electrode materials of comparative examples and embodiments of the present invention;

[0029] Figure 4 Schematic diagram of the principle of the negative electrode material of the comparative example and embodiment of the present invention. DETAILED DESCRIPTION

[0030] As described in the background technology, the fast charging performance of the negative electrode material is usually improved by coating the negative electrode surface with other materials with higher reaction activity or reducing the particle size of the material. However, although these methods can improve the fast charging performance to a certain extent, they will also bring about the deterioration of high-temperature performance. The commonly used measures to improve high-temperature performance and the measures to improve fast charging performance are basically opposite. Therefore, how to achieve both fast charging performance and high-temperature performance at the same time is an urgent problem that needs to be solved.

[0031] In order to solve the above technical problems, the present invention provides a negative electrode material and its preparation method and application, which utilizes the lamellar structure of graphite particles to achieve interlayer lithium insertion; and utilizes the orientation of graphite particles so that lithium insertion can only be carried out on the end faces of graphite particles, while the basal surface has no lithium insertion ability; different from the fully coated design in the prior art, the present invention will only coat the end faces with lithium insertion ability, and reduce the coating on the basal surface, thereby reducing the amount of coating agent used, reducing costs and the coating amount on the overall material surface, and greatly improving the high-temperature performance of the negative electrode material while ensuring fast charging performance.

[0032] A first object of the present invention is to provide a negative electrode material, comprising:

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

[0034] A coating layer covering a portion of the surface of the graphite particles; the portion of the surface includes all end faces and a portion of the basal surface 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%; the coating layer usually has good fast charging capability but poor high temperature performance; the coating layer should be reduced as much as possible while ensuring fast charging capability to achieve a balance between fast charging and high temperature; considering that graphite particles have a basal plane and an end plane, the ion diffusion coefficient of the basal plane is an order of magnitude lower than that of the end plane, that is, lithium insertion is mainly carried out from the end plane; therefore, the coating of the basal plane is reduced, while the end plane is normal, to achieve a balance between fast charging and high temperature. Specifically, to completely cover the end plane, at least 50% of the surface area needs to be covered, while the basal plane coating is reduced as much as possible, that is, the surface area is less than 100%.

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

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

[0038] And / or, the coating layer has a thickness of 5 nm to 50 nm; excessively thick coating layer results in poor high temperature performance.

[0039] In one embodiment of the present invention, the D50 of the negative electrode material is 7 μm-17 μm, which is an increase compared to the D50 of graphite particles. There are two reasons: first, the presence of the coating layer causes a slight increase in D50; second, there is adhesion after coating, resulting in slight granulation and an increase in D50.

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

[0041] A second object of the present invention is to provide a method for preparing a negative electrode material, comprising the following steps:

[0042] S1. Dispersing graphite particles in a coating agent solution, causing the graphite particles to orient and move under the action of a magnetic field and airflow force, and passing the graphite particles through a spray drying device so that the coating agent covers a portion of the graphite particles to obtain incompletely coated graphite; the coated portion includes all end faces and a portion of the basal surface of the graphite particles;

[0043] S2. Under a protective atmosphere, carbonize the incompletely coated graphite described in S1 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, the coating layer thickness is too thin, and the fast charging capability is poor; when the viscosity is too high, the fluidity is poor, the coating layer thickness is too thick, and the high temperature performance is poor;

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

[0046] And / or, the solvent in the coating agent solution is selected from one or more of gasoline, tetrafluoroethylene, 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 air flow force is 0.5L / min-5L / min.

[0049] In one embodiment of the present invention, in S1, the preparation of the graphite particles comprises 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 treatment includes two stages. The first stage is to heat the temperature to 1000-1200°C at a rate of 4°C / min-6°C / min; the second stage is to heat the temperature to 2800-3300°C at a rate of 13°C / min-17°C / min and keep it warm 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 treatment is performed by heating the temperature to 1000-1200°C at a rate of 4-6°C / min and keeping the temperature for 2-6 hours.

[0054] The third object of the present invention is to provide a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the active material of the negative electrode sheet comprises the negative electrode material or the negative electrode material prepared by the method described.

[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. 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. An exemplary preparation method includes preparing an electrode slurry containing the negative electrode active material, the binder, and the conductive agent in a certain proportion. The slurry is then coated onto at least one surface of the negative electrode current collector. After drying and pressing, the negative electrode sheet is obtained.

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

[0057] The type and content of the conductive agent are not specifically limited and can be selected based on 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 other conductive agents that can achieve the functions of the present application can be selected based on specific needs without departing from the spirit of the present application, without being limited by these.

[0058] The type and content of the above-mentioned binder are not specifically limited and can be selected according to actual needs. In some embodiments, the above-mentioned binder includes one or more of 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 preparation method of the positive electrode sheet is the same as that of the negative electrode sheet, which is to prepare the positive electrode active material, binder and conductive agent into an electrode slurry in a certain proportion, and then apply it on at least one surface of the positive electrode collector, and obtain it after drying and pressing.

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

[0061] The type of positive electrode current collector is not specifically limited and can be selected based on actual needs. For example, the positive electrode current collector can be aluminum foil, nickel foil, or a polymer conductive film. The types of conductive agent and binder in the positive electrode sheet refer to those in the negative electrode sheet and will not be further described in this invention.

[0062] In one embodiment of the present invention, the electrolyte comprises one or more of an organic liquid electrolyte, an organic solid electrolyte, a solid ceramic electrolyte, a gel electrolyte, and the like. Preferably, the electrolyte is an organic liquid electrolyte, obtained by dissolving a lithium salt in a non-aqueous organic solvent; wherein the lithium salt comprises one or more of lithium difluorophosphate, lithium hexafluorophosphate, lithium difluorooxalatophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, and lithium difluorooxalatoborate. The non-aqueous organic solvent may comprise one or more of cyclic carbonates, chain carbonates, and carboxylates. The cyclic carbonate may be selected from one or more of ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone; the chain carbonate may be selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, methyl acetate, ethyl acetate, and ethyl propionate.

[0063] In one embodiment of the present invention, the isolation membrane is polyethylene, polypropylene, polyvinylidene fluoride or non-woven fabric, and their multilayer composite membranes and modified membranes such as ceramic modification and PVDF modification of the above membranes can be selected according to actual needs.

[0064] It should be understood that the preparation methods of the positive electrode sheet, negative electrode sheet, separator, and electrolyte in this application can be selected according to specific needs without violating the spirit of this application. Other preparation methods that can achieve the functions of this application can be selected without limitation. In one embodiment of the method for manufacturing a lithium-ion battery, the preparation method includes: winding, folding, or stacking the above-mentioned negative electrode sheet, separator, and positive electrode sheet in sequence into an electrode assembly, encapsulating the electrode assembly in, for example, an aluminum-plastic film, and injecting an electrolyte, followed by vacuum packaging, standing, forming, shaping, and other processes to obtain a lithium-ion battery.

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

[0066] In the present invention, unless otherwise defined, technical and scientific terms used herein have the same meanings as commonly understood by one skilled in the art to which the present invention belongs.

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

[0068] In the present invention, unless otherwise stated, the experimental methods used in the examples of the present invention are conventional methods unless otherwise stated, and the materials, reagents, etc. used are all commercially available unless otherwise stated.

[0069] In the present invention, unless otherwise specified, the incompletely coated graphite prepared in the embodiments of the present invention is the entire end surface and a portion of the basal surface of the coated graphite particles.

[0070] Example 1

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

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

[0073] S2. Graphitizing the precursor A by first heating it to 1100°C at 5°C / min, then heating it to 3000°C at 15°C / min, and keeping the temperature for 24 hours to obtain the precursor B.

[0074] S3, crushing and classifying the precursor B 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. Dispersing graphite particles in a coating agent solution, applying an external magnetic field of 5 T and a gas flow rate of 3 L / min, pushing the graphite particles through the coating agent solution surface to obtain incompletely coated graphite; wherein the coating agent solution is prepared by dissolving asphalt in tetrafluoroethylene and has a viscosity of 8000 Pa.s;

[0076] S5. The incompletely coated graphite is carbonized and heated to 1000°C at a heating rate of 5°C / min under nitrogen protection, maintained at a constant temperature for 4 hours, and then classified and demagnetized to obtain a negative electrode material; wherein the ratio of the area of the graphite particles coated by the coating layer to the surface area of the graphite particles is 71%; and the thickness of the coating layer is about 10 nm.

[0077] Example 2

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

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

[0080] S2. Graphitizing the precursor A by first heating it to 1100°C at 5°C / min, then heating it to 3000°C at 15°C / min, and keeping the temperature for 24 hours to obtain the precursor B.

[0081] S3, crushing and classifying the precursor B 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. Dispersing graphite particles in a coating agent solution, applying an external magnetic field of 5 T and a gas flow rate of 3 L / min, pushing the graphite particles through the coating agent solution surface to obtain incompletely coated graphite; wherein the coating agent solution is prepared by dissolving asphalt in tetrafluoroethylene and has a viscosity of 8000 Pa.s;

[0083] S5. The incompletely coated graphite is carbonized and heated to 1000°C at a heating rate of 5°C / min under nitrogen protection, maintained at a constant temperature for 4 hours, and then classified and demagnetized to obtain a negative electrode material; wherein the ratio of the area of the graphite particles coated by the coating layer to the surface area of the graphite particles is 77%; and the thickness of the coating layer is about 10 nm.

[0084] Example 3

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

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

[0087] S2. Graphitizing the precursor A by first heating it to 1100°C at 5°C / min, then heating it to 3300°C at 15°C / min, and keeping the temperature for 24 hours to obtain the precursor B.

[0088] S3, crushing and classifying the precursor B 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. Dispersing graphite particles in a coating agent solution, applying an external magnetic field of 3 T and a gas flow rate of 3 L / min, pushing the graphite particles through the surface of the coating agent solution to obtain incompletely coated graphite; wherein the coating agent solution is prepared by dissolving asphalt in tetrafluoroethylene and has a viscosity of 8000 Pa.s;

[0090] S5. The incompletely coated graphite is carbonized and heated to 1000°C at a heating rate of 5°C / min under nitrogen protection, maintained at a constant temperature for 4 hours, and then classified and demagnetized to obtain a negative electrode material; wherein the ratio of the area of the graphite particles coated by the coating layer to the surface area of the graphite particles is 82%; and the thickness of the coating layer is about 13 nm.

[0091] Example 4

[0092] The method is basically the same as Example 1, except that the sphericity of the graphite particles is adjusted to about 0.5, an external magnetic field of 6 T and a gas flow rate of 5 L / 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 Example 1, except that the sphericity of the graphite particles is adjusted to about 0.8, the external magnetic field is adjusted to 1 T, and the air flow force is adjusted to 0.5 L / 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 method is basically the same as Example 1, except that the graphite particles and the 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 minutes to obtain completely coated graphite.

[0097] Comparative Example 2

[0098] The method is basically the same as Example 2, except that the graphite particles and the 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 minutes to obtain completely coated graphite.

[0099] Comparative Example 3

[0100] The method is basically the same as Example 3, except that the graphite particles and the 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 minutes to obtain completely coated graphite.

[0101] Comparative Example 4

[0102] The process is basically the same as Example 1, except that the sphericity of the graphite particles is adjusted to be approximately 0.49.

[0103] Comparative Example 5

[0104] The process is basically the same as Example 1, except that the sphericity of the graphite particles is adjusted to be approximately 0.82.

[0105] Performance Testing

[0106] The negative electrode 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: Take a certain amount of sample and place it in the 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 a tap density meter. The instrument vibrates at a high frequency to compact the material. 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 adsorption. 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 adsorption collected together;

[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 6 μm copper foil; the negative electrode active material, the conductive agent and the binder are mixed in a mass ratio of 95.5:1.5:3, deionized water is added and stirred to form a uniformly mixed and stable negative electrode slurry, the negative electrode slurry is evenly coated on the negative electrode current collector, and dried to obtain the negative electrode sheet.

[0112] Positive electrode sheet: The positive electrode 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 electrode current collector is aluminum foil; the positive electrode active material, conductive agent and binder are mixed in a mass ratio of 96.5:2.5:1, NMP is added and stirred into a uniform and stable positive electrode slurry, the positive electrode slurry is evenly coated on the positive electrode current collector, and the positive electrode sheet is obtained after drying and cold pressing.

[0113] Isolation film: The base film of the isolation film is polypropylene film with a thickness of 9+3+3+3μm.

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

[0115] Assembly of lithium-ion batteries: Arrange the positive electrode sheet, separator, negative electrode sheet, and separator in order and wind them to obtain a battery cell; ultrasonically weld the tabs, seal the top and side of the aluminum-plastic film, dry to remove moisture, inject electrolyte and then seal again, inject the electrolyte into the dried battery cell, soak it for 24 hours, and then form it at 45°C. The formation process is: charge at 0.05C to 3.4V, then charge at 0.2C to 3.75V; after aging at room temperature for 24 hours, the lithium-ion battery is completed.

[0116] Performance testing:

[0117] (1) 4C charging process: The battery was cycled for 3 weeks in a 25°C incubator 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 reached 0.05C; then discharged at a constant current of 0.33C until the voltage dropped to 2.5V), with the discharge capacity of the last week as the initial capacity C0; then a multi-channel recorder was used to connect the negative electrode and the reference electrode, and the battery was fully charged at a rate of 4C0, and the minimum voltage value of the multi-channel recorder during the charging process was recorded.

[0118] (2) High-temperature storage process: The battery is cycled at a rate of 0.33C / 0.33C for 3 weeks in a 25°C incubator, with the discharge capacity in the last week being the initial capacity C0; the battery is then fully charged at a rate of 0.33C, and then stored in a 60°C incubator for 30 days; the stored battery is discharged at 0.33C in a 25°C incubator until the battery temperature stabilizes at 25°C, to obtain a discharge capacity C1, and then cycled at a rate of 0.33C for 3 weeks, with the discharge capacity in the last week being C2; the capacity retention rate is C1 / C0*100%, and the capacity recovery rate is C2 / C0*100%.

[0119] (3) Gram 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 discharge capacity in grams is calculated based on the discharge capacity in the third step and the weight of the active material (i.e., the weight of the graphite particles with a coating layer in this application) as the discharge capacity in grams;

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

[0121] Table 1 shows the relevant test results:

[0122] Table 1

[0123]

[0124]

[0125] As can be seen from Table 1, by comparing Examples 1-3 with Comparative Examples 1-3, it can be seen that the negative electrode materials of the Examples have excellent gram capacity and first efficiency; this is because the coating amount on the surface of the material is reduced by partial coating ( Figure 3 ); The coating layer itself causes a decrease in gram capacity and first efficiency due to its low order and high interfacial activity; therefore, a reduction in the coating amount is beneficial to an increase in gram capacity and first efficiency. In addition, compared with the comparative example, the charging capacity of the embodiment is not much different. This is because the embodiment completely coats the graphite end faces, and the graphite end faces are charged and discharged, so the charging capacity is equivalent; and the high-temperature performance is significantly improved. This is because the embodiment reduces the coating of the base surface and reduces the coating amount. The high reactivity of the coating layer will lead to deterioration of the high-temperature performance. Therefore, the high-temperature performance of the embodiment is significantly improved ( Figure 4 ).

[0126] Comparing Example 1 and Comparative Examples 4-5, it can be seen that the reduction of graphite sphericity deteriorates the fast charging performance and high temperature performance, and the improvement of sphericity has limited improvement on fast charging and high temperature beyond a certain range, and the production efficiency and cost are significantly increased.

[0127] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A negative electrode material, characterized in that The negative electrode material includes: The core comprises graphite particles; the graphite particles have a sphericity of 0.5-0.8; and the graphite particles have a basal surface and an end surface; A coating layer covering a portion of the surface of the graphite particles; the portion of the surface includes all end faces and a portion of the basal surface of the graphite particles; the coating layer contains a coating agent; A 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%.

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

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

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

5. A method for preparing a negative electrode material, characterized in that: The following steps are involved: S1. Dispersing graphite particles in a coating agent solution, causing the graphite particles to orient and move under the action of a magnetic field and airflow force, and passing the graphite particles through a spray drying device so that the coating agent covers a portion of the graphite particles to obtain incompletely coated graphite; the coated portion includes all end faces and a portion of the basal surface of the graphite particles; S2. Under a protective atmosphere, carbonize the incompletely coated graphite described in S1 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 asphalt and / or resin; And / or, the solvent in the coating agent solution is selected from one or more of gasoline, tetrafluoroethylene, benzene and alcohol.

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

8. The method for preparing the negative electrode material according to claim 5, wherein: 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, wherein: In S2, the carbonization treatment is performed by heating the temperature to 1000-1200°C at a rate of 4-6°C / min and keeping the temperature for 2-6 hours.

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

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