Fast charging graphite, preparation method thereof and lithium ion battery negative electrode material

Abstract

The invention discloses fast charge graphite, a preparation method thereof and a lithium ion battery negative electrode material, and the preparation method comprises the following steps: carrying out acid leaching elution and alkali liquor neutralization on natural graphite fine powder to passivate the surface of the natural graphite fine powder; mixing the passivated graphite fine powder with boric acid and acetic acid, and then carrying out heating reaction to obtain modified graphite fine powder; mixing the modified graphite fine powder with a binder, and granulating to obtain secondary particles; after the secondary particles are subjected to graphitization treatment, the graphitized secondary particles are coated with a carbon source coating agent, and then carbonization treatment is conducted in the inert atmosphere; and carrying out vacuum impregnation on the carbonized secondary particles in a lithium source solution, and drying to obtain the lithium ion battery positive electrode material. The preparation method has the advantages that the interlayer spacing of the natural graphite can be expanded by modifying the natural graphite fine powder, and then the natural graphite is subjected to granulation, graphitization and carbon coating treatment and then is subjected to gradient coating, so that the prepared fast-charging graphite has more excellent dynamic performance.

Description

Technical Field

[0001] This invention belongs to the field of electrode material preparation technology, specifically relating to a fast-charging graphite, its preparation method, and a lithium-ion battery anode material. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Fast-charging graphite refers to graphite-based anode materials used to support rapid charging of lithium-ion batteries (typically ≥3C or even higher charging rates). Its key kinetic performance requirements are: high lithium-ion diffusion coefficient, short lithium-ion diffusion path, excellent electronic conductivity, low interfacial impedance, and high reaction uniformity. Lithium-ion insertion / deintercalation is an electrochemical process that requires electrons to simultaneously flow into and out of graphite particles. Excellent electronic conductivity ensures that the charge transfer reaction proceeds efficiently and uniformly throughout the electrode, reducing ohmic polarization.

[0004] Natural graphite is widely used as a negative electrode material for lithium-ion batteries due to its high capacity and low cost. In the process of preparing negative electrode materials, graphite needs to be pulverized. During the pulverization process, a lot of fine powder (D50: 2~8μm) will be generated. These natural graphite fine powders cannot be used as lithium battery negative electrode materials because of their small particle size, high specific surface area and low tap density, resulting in waste of resources and increased cost of negative electrode material preparation.

[0005] In addition, natural graphite has problems such as low interlayer lithium-ion transport rate, high interfacial impedance, high polarization, and large initial irreversible capacity loss. When the lithium-ion insertion rate cannot keep up with the electron supply rate, lithium-ions will be reduced to metallic lithium on the graphite surface (lithium plating), causing safety hazards and damaging battery life.

[0006] Existing technologies have begun to explore the modification of natural graphite materials, using boric acid as an intercalating agent and acetic acid as a solvent. During the reaction, boron and boric acid enter the interior of natural graphite in different forms, forming interlayer compounds between the graphite layers and increasing the interlayer distance to effectively improve the migration rate of lithium ions. However, this method has a relatively weak effect on improving the conductivity of natural graphite. When a mismatch occurs between ion and electron transport rates, the following adverse effects can easily arise: Increased polarization and voltage lag lead to decreased energy efficiency and reduced usable capacity (especially at high rates). Increased reaction inhomogeneity: Regions with poor electronic conductivity (such as inside the electrode or regions far from the current collector) cannot obtain enough electrons, resulting in low utilization of active materials; The risk of lithium precipitation surges: when Li +When lithium reaches the graphite surface but lacks electrons, it cannot be embedded between graphite layers and is forced to be reduced to metallic lithium at the graphite / electrolyte interface. The resulting lithium dendrites may pierce the separator, causing short circuits and thermal runaway. Lithium plating consumes active lithium, increasing irreversible capacity loss. Lithium plating exacerbates SEI rupture / regeneration, and the electrolyte continues to decompose, causing a sharp drop in the battery's cycle life.

[0007] Deterioration of electrode dynamics: Although intrinsic Li + It diffuses quickly, but due to the bottleneck of electron conduction, its overall rate performance is still poor; Increased risk of thermal runaway: Poor conductivity leads to increased ohmic resistance of the electrodes. When current flows through the high-resistance path, a large amount of heat is generated, which can easily cause local overheating. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a fast-charging graphite, its preparation method, and a lithium-ion battery anode material.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing fast-charging graphite, comprising the following steps: The surface of the natural graphite fine powder is passivated by acid leaching and alkaline neutralization. Passivated graphite powder was mixed with boric acid and acetic acid, and then heated to react, resulting in modified graphite powder. Modified graphite powder is mixed with a binder and granulated to obtain secondary particles; After the secondary particles are graphitized, the graphitized secondary particles are coated with a carbon coating agent and then carbonized in an inert atmosphere. The carbonized secondary particles are vacuum impregnated in a lithium source solution and then dried to obtain the final product.

[0010] Secondly, the present invention provides a fast-charging graphite, which is prepared by the aforementioned preparation method.

[0011] Thirdly, the present invention provides a lithium-ion battery anode material prepared from the fast-charging graphite.

[0012] Fourthly, the present invention provides a lithium-ion battery, wherein the negative electrode is made of the lithium-ion battery negative electrode material.

[0013] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: The method for preparing fast-charging graphite of the present invention effectively utilizes fine natural graphite powder with small particle size, enabling high-value utilization of graphite resources and effectively reducing the preparation cost of fast-charging graphite, thus achieving ultra-low-cost preparation of fast-charging graphite.

[0014] This invention modifies natural graphite fine powder to expand the interlayer spacing of natural graphite. After granulation, graphitization, and carbon coating, gradient coating is performed to prepare fast-charging graphite with superior kinetic properties. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0016] Figure 1 This is a schematic diagram of the fast-charging graphite structure prepared by passivating and modifying natural graphite fine powder, followed by granulation and coating.

[0017] Among them, 1 is a secondary granular graphite base layer prepared by reacting natural graphite fine powder with boric acid and acetic acid after passivation treatment and then undergoing granulation, graphitization and carbonization treatment; 2 is the oxygen coating layer in the gradient coating; and 3 is the lithium coating layer in the gradient coating. Detailed Implementation

[0018] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0019] In a first aspect, the present invention provides a method for preparing fast-charging graphite, comprising the following steps: The surface of the natural graphite fine powder is passivated by acid leaching and alkaline neutralization. Passivated graphite powder was mixed with boric acid and acetic acid, and then heated to react, resulting in modified graphite powder. Modified graphite powder is mixed with a binder and granulated to obtain secondary particles; After the secondary particles are graphitized, the graphitized secondary particles are coated with a carbon coating agent and then carbonized in an inert atmosphere. The carbonized secondary particles are vacuum impregnated in a lithium source solution and then dried to obtain the final product.

[0020] The main benefits of acid leaching and washing of natural graphite fine powder are as follows: Dissolves metal oxide / salt impurities, such as Fe2O3, SiO2, Al2O3, CaO, MgO, etc.; removes harmful ionic elements, such as Fe. 3+ Cu 2+ Mn 2+ Na + K + wait.

[0021] By washing away the above impurities and disordered carbon at the edges, the interlayer channels of graphite are opened, forming micron-sized pores and exposing more lithium-ion diffusion channels, effectively improving Li-ion diffusion. + The diffusion rate is increased, enhancing wettability with the electrolyte.

[0022] Passivated graphite powder is mixed with boric acid and acetic acid, and then heated to react. Acetic acid acts as a solvent, and boric acid acts as an intercalating agent. Upon heating, boric acid decomposes into B₂O₃ vapor, which permeates into the interlayer spaces of the graphite, occupying the interlayer space and increasing the interlayer spacing. This increased interlayer spacing effectively lowers the lithium-ion intercalation barrier and improves the lithium-ion diffusion coefficient.

[0023] There is a certain synergistic effect between the acid leaching and boric acid layer expansion steps, as detailed below: Acid etching of the graphite edges opens interlayer channels, making it easier for subsequent B2O3 vapor to penetrate; Acid leaching can activate the surface functional groups of graphite, generating oxygen-containing groups such as -COOH and -OH on the graphite surface. These oxygen-containing groups form hydrogen bonds with B-OH in boric acid, anchoring the intercalating agent and increasing the loading of the intercalating agent.

[0024] Metallic impurities in natural graphite can block interlayer channels. Acid leaching can dissolve these impurities, which helps improve the uniformity of intercalation.

[0025] B2O3, which penetrates between graphite layers, adheres to the surface pits created by acid etching. During subsequent heat treatment, it forms a BC bond network, which can repair structural defects. Boron atom doping strengthens the interlayer bonding energy and counteracts the interlayer weakening caused by acid etching.

[0026] The effect of mixing modified graphite fine powder with a binder and granulating is as follows: Granulation allows modified graphite powder to bind tightly together, reducing contact resistance between particles and thus improving overall conductivity. The formation of secondary particles enhances the material's structural strength and stability, making it less prone to breakage and pulverization during subsequent graphitization, carbonization, and electrochemical reactions, thereby extending the material's lifespan.

[0027] Proper granulation can give secondary particles a suitable specific surface area, increase the adsorption and diffusion sites of lithium ions, and help improve the lithium ion storage capacity and rate performance of the material.

[0028] The regular shape and appropriate size of secondary particles can improve the wettability of the electrolyte on the material, allowing lithium ions to diffuse more evenly on the surface and inside the material, which is beneficial to improving the efficiency of electrochemical reactions.

[0029] When graphitized secondary particles are coated with a carbon source coating agent, the regular shape and uniform size of the secondary particles result in a more uniform coating layer, effectively improving the material's oxidation resistance and cycle stability. The secondary particles also exhibit better flowability and processability, facilitating mixing and coating processes in subsequent electrode fabrication, thus enhancing electrode quality and consistency.

[0030] The effect of secondary graphitization of particles is: The graphitization process makes carbon atoms more ordered, forming a highly ordered graphite crystal structure, which greatly increases the migration path and migration rate of free electrons, thereby significantly improving the conductivity of the material.

[0031] After graphitization, the carbon atoms inside the secondary particles are arranged in a more stable hexagonal layered structure. The layers interact closely with each other through van der Waals forces, making the structure of the entire particle more robust and less prone to breakage, pulverization, or other structural damage during subsequent processing or use. This helps maintain the integrity and stability of the material.

[0032] Graphitization can reduce defects and disorder within particles, decrease capacity decay caused by structural changes during charge-discharge cycles, and extend the lifespan of materials.

[0033] The reduced surface energy of graphitized secondary particles improves their wettability with substances such as electrolytes, allowing the electrolyte to better penetrate and distribute on and inside the particles. This promotes the transport and reaction of lithium ions at the electrode / electrolyte interface, increases the efficiency of electrochemical reactions, and ultimately enhances the overall performance of batteries and other electrochemical devices.

[0034] Graphitized carbon materials have higher chemical stability and are less likely to react chemically with other substances. For example, during battery charging and discharging, they can reduce side reactions with the electrolyte, reduce irreversible capacity loss, and improve battery safety and reliability.

[0035] The effect of using a carbon-based coating agent to coat and carbonize the graphitized secondary particles is as follows: After carbonization, the carbon coating agent forms a uniform carbon coating layer on the surface of the secondary particles. This carbon coating layer further increases the conductive sites of the electrode material, creating a more efficient electron conduction network. This makes electron transport in the lithium-ion battery smoother during charging and discharging, reduces the battery's internal resistance, and thus significantly improves the battery's rate performance and overall conductivity.

[0036] In the subsequent lithium-ion insertion / extraction process and battery cycle charging and discharging, the carbon coating can effectively mitigate particle volume changes, reduce particle pulverization and breakage, maintain the integrity of the particle structure, thereby improving the battery's cycle stability and extending its service life.

[0037] Carbon coating can optimize the interfacial properties between secondary particles and the electrolyte. On the one hand, it can reduce the number of active sites on the particle surface, reducing the possibility of side reactions with the electrolyte, thereby reducing irreversible capacity loss and improving the first charge-discharge efficiency of the battery. On the other hand, it can improve the wettability of the electrode / electrolyte interface, which is conducive to better penetration and distribution of the electrolyte around the particles, promoting the transport and reaction of lithium ions at the interface.

[0038] The carbon coating has good chemical stability and oxidation resistance, which can resist the erosion of secondary particles by external oxidizing substances such as oxygen to a certain extent, avoid oxidation and deterioration of particles during storage or use, and ensure the stability and durability of material performance.

[0039] Vacuum impregnation of carbonized secondary particles in a lithium source solution can construct a Li-OC composite conductive network on the surface and subsurface layers of natural graphite, thereby improving the initial coulombic efficiency and reducing interfacial impedance. Specifically: During the initial charge-discharge cycle, a solid electrolyte interphase (SEI) film forms on the surface of natural graphite, consuming a large number of lithium ions and leading to an increase in irreversible capacity and a decrease in initial coulombic efficiency. By constructing a Li-OC composite conductive network, Li-OC can form a more stable interface layer on the electrode surface. This interface layer effectively reduces the direct contact between lithium ions and the electrolyte, inhibits excessive growth of the SEI film, and thus reduces irreversible lithium loss.

[0040] The lithium source in the Li-OC network can provide new active sites for lithium-ion storage, increase the adsorption and diffusion channels of lithium-ions, enable lithium-ions to participate in electrochemical reactions more efficiently, improve the effective utilization rate of lithium-ions, and thus improve the first coulombic efficiency.

[0041] Constructing a Li-OC composite conductive network on the surface of natural graphite can improve the microstructure of the electrode / electrolyte interface, making the interface more uniform and smooth. This optimized interface structure helps reduce interface defects and roughness, and lowers the charge transfer resistance and diffusion resistance at the interface.

[0042] The Li-OC composite conductive network itself possesses good conductivity, and it can synergize with the conductive network of natural graphite to form a more efficient electron transport channel, thereby improving the overall conductivity of the electrode. Furthermore, the oxygen element in the Li-OC network can act as a dopant, regulating the electronic structure of the graphite surface, further enhancing the conductivity of the interface and reducing interfacial impedance.

[0043] By reducing interfacial resistance and improving initial coulombic efficiency, the battery's internal resistance decreases, charge transfer becomes faster, thereby improving charge and discharge efficiency, shortening charge and discharge time, and enabling the battery to complete the charge and discharge process more quickly.

[0044] Constructing a Li-OC composite conductive network can improve the structural and chemical stability of the electrode. During cyclic charging and discharging, it can effectively alleviate the volume expansion and contraction of graphite, reduce particle pulverization and shedding, suppress side reactions, and extend the cycle life of the battery.

[0045] The Li-OC composite conductive network constructs a three-dimensional conductive network structure on the surface and subsurface of natural graphite, providing more shortcuts and channels for lithium ion diffusion. This enables lithium ions to be transported more quickly between the electrode and the electrolyte, reducing the distance and time required for lithium ion diffusion.

[0046] Oxygen in the Li-OC network has a certain degree of lithium affinity, which can adsorb lithium ions and promote their diffusion. At the same time, the conductivity of carbon can provide a stable electric field environment for lithium ion diffusion, further enhancing the diffusion kinetics of lithium ions and improving the rate performance of the battery.

[0047] In some embodiments, the particle size D50 of the natural graphite powder is 2-8 μm.

[0048] In some embodiments, a sodium hydroxide solution is used for alkali neutralization, and the pH value of the sodium hydroxide solution is 10-12.

[0049] In some embodiments, the mass ratio of natural graphite powder, boric acid, and acetic acid is 5~20:1:5~20.

[0050] Preferably, the heating reaction temperature is 150-180℃, and the heating reaction time is 6-48h.

[0051] The reactor can be a hydrothermal reactor, a vertical reactor, or a horizontal reactor.

[0052] In some embodiments, the binder is kerosene coke or pitch coke.

[0053] The reactor used is a horizontal or vertical reactor; the softening temperature of the binder is 300-600℃.

[0054] Kerosene coke and pitch coke exhibit good binding properties after heating, effectively binding together particles such as fine graphite powder to form secondary particles with a certain strength. This helps maintain the integrity and stability of the material during subsequent processing and use, preventing particle detachment and scattering.

[0055] The high carbon content of kerosene coke and pitch coke allows for the attainment of high carbon purity during secondary particle graphitization, which improves the conductivity and graphitization degree of the material, thereby enhancing the performance and quality of the final product.

[0056] Kerosene coke and pitch coke have high coking values. A high coking value means that more carbon skeleton structures can be formed during the carbonization process, which helps to increase the packing density and mechanical strength of secondary particles. At the same time, it also helps to reduce the resistivity of the material and improve its electrochemical performance.

[0057] Kerosene coke and pitch coke have good wettability on aggregates such as graphite powder, and can be uniformly coated on the particle surface during the mixing process, so as to form a good interfacial bond between the binder and the aggregate, thereby improving the overall performance and consistency of the material.

[0058] Kerosene coke and pitch coke have moderate melting points and viscosities, and exhibit good fluidity and plasticity at conventional processing temperatures, making them easy to mix and granulate with materials such as graphite powder.

[0059] During the subsequent graphitization and carbonization processes, kerosene coke and pitch coke are transformed into carbonaceous materials with high conductivity, forming a good conductive network together with fine graphite powder, further improving the material's conductivity. Furthermore, the formed carbonaceous network structure enhances the structural stability of secondary particles, enabling them to better resist volume changes and particle breakage during charge and discharge, thereby improving the material's cycle stability and service life.

[0060] In some embodiments, the graphitization temperature is 2800-3000°C.

[0061] In some embodiments, the coating agent is asphalt, phenolic resin, epoxy resin, or polyfurfuryl alcohol.

[0062] Preferably, the carbonization temperature is 1000-1300℃.

[0063] In some embodiments, during the vacuum impregnation process of the carbonized secondary particles in a lithium source solution, the lithium source is LiOH and LiCl, and the molar ratio of LiOH to LiCl is 0.3:0.1-0.3.

[0064] Preferably, the solvent of the lithium source solution is ethanol, the concentration of LiOH is 0.3 mol / L, and the concentration of LiCl is 0.1-0.3 mol / L.

[0065] Secondly, the present invention provides a fast-charging graphite, which is prepared by the aforementioned preparation method.

[0066] Thirdly, the present invention provides a lithium-ion battery anode material prepared from the fast-charging graphite.

[0067] Fourthly, the present invention provides a lithium-ion battery, wherein the negative electrode is made of the lithium-ion battery negative electrode material.

[0068] The present invention will be further described below with reference to the embodiments.

[0069] Example 1 (1) Disperse natural graphite fine powder with D50 of 2 μm in an acid solution with a mass percentage of 5%, the acid being a mixture of sulfuric acid and nitric acid, with a volume ratio of concentrated H2SO4 to concentrated HNO3 of 2:1. Let it stand at 80℃ for 2 h, then wash it clean. Disperse the acid-treated natural graphite fine powder in a NaOH solution with a pH of 10, sonicate it for 30 min, wash it until neutral, and dry it in a vacuum drying oven at 80℃ to obtain passivated natural graphite fine powder. (2) The passivated natural graphite powder in (1) is mixed with boric acid and acetic acid in a mass ratio of 5:1:5 to obtain a mixture; the mixture is placed in a hydrothermal reactor and reacted at 170°C for 12 hours. After cooling, it is taken out, filtered, and dried to obtain a semi-finished product. (3) The semi-finished product obtained in (2) and asphalt are added to the mixer at a mass ratio of 85:15, heated to 80°C, and mixed. Then the mixture is placed in a vertical reactor and heated. The heating process is as follows: 30 min to 300°C, then 100 min to 500°C, then 60 min to 600°C, and finally kept at 650°C for 2 h. (The temperature curve changes during the granulation process. The heating process is as follows: 30 min to 300°C is mainly to remove moisture and low molecular weight alkanes; then 100 min to 500°C is mainly to use asphalt coke as a binder in this temperature range, filling the gaps between graphite particles through plastic flow and bonding the graphite particles; 60 min to 600°C, and finally kept at 650°C for 2 h: the asphalt coke is violently pyrolyzed to generate semi-coke and form a dense carbon layer.) Then, granulation is carried out under a nitrogen atmosphere to obtain secondary granules, and the secondary granules are then shaped to 14μm. (4) The shaped secondary granules from (3) are placed in an Atchison furnace for graphitization treatment at a temperature of 2500℃ to obtain graphitized granules.

[0070] Then, the secondary granules are coated with pitch coke, which accounts for 10% of the mass of the graphitized granules. The granules are then placed in a box furnace and carbonized at 1000°C under a nitrogen atmosphere to obtain the carbonized secondary granules.

[0071] (5) The secondary particulate carbon obtained in (4) is placed in a mixture of 0.3 mol / L LiOH and 0.1 mol / L LiCl (the solvent is an ethanol solution with a volume percentage of 10%), and the natural graphite fine powder is subjected to gradient coating treatment by negative pressure permeation (5 Pa → normal pressure alternating 3 times) to obtain the product.

[0072] Example 2 (1) Disperse natural graphite fine powder with D50 of 5 μm in a 10% H2SO4+HNO3 mixed solution (as above), let it stand at 80℃ for 2 h, then wash it clean, place the acid-treated natural graphite fine powder in a NaOH solution with pH of 11, sonicate it for 30 min, wash it until neutral, and dry it in a vacuum drying oven at 80℃ to obtain passivated natural graphite fine powder; (2) The passivated natural graphite powder in (1) is mixed with boric acid and acetic acid in a mass ratio of 10:1:10 to obtain a mixture; the mixture is placed in a hydrothermal reactor and reacted at 170°C for 12 hours. After cooling, it is taken out, filtered, and dried to obtain a semi-finished product. (3) The semi-finished product obtained in (2) and asphalt are added to the mixer at a mass ratio of 85:15, heated to 80°C, and mixed. Then the mixture is placed in a vertical reactor and heated. The heating process is as follows: 30 min to 300°C, then 100 min to 500°C, then 60 min to 600°C, and finally held at 650°C for 2 h. Then granulation is carried out under a nitrogen atmosphere to obtain secondary granules. The secondary granules are then shaped to 14 μm. (4) The shaped secondary granules from (3) are placed in an Atchison furnace for graphitization treatment at a temperature of 2500℃ to obtain graphitized granules.

[0073] Then, the secondary granules are coated with pitch coke, which accounts for 10% of the mass of the graphitized granules. The granules are then placed in a box furnace and carbonized at 1000°C under a nitrogen atmosphere to obtain the carbonized secondary granules.

[0074] (5) The secondary particulate carbon obtained in (4) is placed in a mixture of 0.3 mol / L LiOH + 0.2 mol / L LiCl in 10% ethanol, and the natural graphite fine powder is subjected to gradient coating treatment by negative pressure permeation (5 Pa → normal pressure alternating 3 times) to obtain the product.

[0075] Example 3 (1) Disperse natural graphite fine powder with D50 of 8 μm in a 15% H2SO4+HNO3 mixed solution, let it stand at 80℃ for 2 h, then wash it clean, disperse the acid-treated natural graphite fine powder in a NaOH solution with pH of 12, sonicate it for 30 min, wash it until neutral, and dry it in a vacuum drying oven at 80℃ to obtain passivated natural graphite fine powder; (2) The passivated natural graphite powder in (1) is mixed with boric acid and acetic acid in a mass ratio of 10:1:10 to obtain a mixture; the mixture is placed in a hydrothermal reactor and reacted at 170°C for 12 hours. After cooling, it is taken out, filtered, and dried to obtain a semi-finished product. (3) The semi-finished product obtained in (2) and asphalt are added to the mixer at a mass ratio of 85:15, heated to 80°C, and mixed. Then the mixture is placed in a vertical reactor and heated. The heating process is as follows: 30 min to 300°C, then 100 min to 500°C, then 60 min to 600°C, and finally held at 650°C for 2 h. Then granulation is carried out under a nitrogen atmosphere to obtain secondary granules. Then the secondary granules are shaped to 14 μm. (4) The shaped secondary granules from (3) are placed in an Atchison furnace for graphitization at a temperature of 2500℃ to obtain graphitized granules.

[0076] Then, the secondary granules are coated with pitch coke, which accounts for 10% of the mass of the graphitized granules. The granules are then placed in a box furnace and carbonized at 1000°C under a nitrogen atmosphere to obtain the carbonized secondary granules. (5) The secondary particulate carbon obtained in (4) is placed in a mixture of 0.3 mol / L LiOH and 0.3 mol / L LiCl in 10% ethanol, and the natural graphite fine powder is subjected to gradient coating treatment by negative pressure permeation (5 Pa → normal pressure alternating 3 times) to obtain the product.

[0077] Example 4 The only difference between this embodiment and embodiment 1 is that in step (2), the mass ratio of natural graphite powder: boric acid: acetic acid is 10:1:10.

[0078] Example 5 The only difference between this embodiment and embodiment 1 is that in step (2), the mass ratio of natural graphite powder: boric acid: acetic acid is 20:1:20.

[0079] Example 6 The only difference between this embodiment and Example 1 is that in step (2), the mass ratio of natural graphite powder to boric acid and acetic acid is 10:1:10, and in step (5), a mixture of 0.3 mol / L LiOH + 0.3 mol / L LiCl (with 10% ethanol as the solvent) is used.

[0080] Example 7 The only difference between this embodiment and Example 2 is that in step (5), a gradient coating is performed using a mixture of 0.3 mol / L LiOH and 0.3 mol / L LiCl (with 10% ethanol as the solvent).

[0081] Comparative Example 1 The only difference between this comparative example and Example 1 is that in step (2), the mass ratio of natural graphite powder: boric acid: acetic acid is 10:1:10, and step (5) for gradient coating is not performed.

[0082] Comparative Example 2 The only difference between this comparative example and Example 2 is that step (5) was not performed to conduct the gradient coating experiment.

[0083] Comparative Example 3 The only difference between this comparative example and Example 3 is that step (5) was not performed to conduct the gradient coating experiment.

[0084] Comparative Example 4 (1) Add natural graphite fine powder with D50 of 2μm and asphalt in a ratio of 85:15 to a mixer and heat to 80°C. Mix the natural graphite fine powder and then place the mixture in a vertical reactor for heating. The heating process is as follows: heat to 300°C in 30min, then heat to 500°C in 100min, then heat to 600°C in 60min, and finally keep at 650°C for 2h. Then granulate under a nitrogen atmosphere to obtain secondary granules. Then shape the secondary granules to 14μm. (2) The secondary granules obtained in (1) are placed in an Atchison furnace for graphitization treatment at a temperature of 2500℃ to obtain graphitized granules. Then, the granules are coated with pitch coke at a mass percentage of 10% of the graphitized granules. The granules are then placed in a box furnace and carbonized at 1000℃ to obtain carbon-coated secondary granular graphite.

[0085] Comparative Example 5 (1) Add natural graphite fine powder with D50 of 5μm and asphalt to a mixer at a mass ratio of 85:15, heat to 80℃, mix, and then place the mixture in a vertical reactor for heating. The heating process is as follows: heat to 300℃ in 30min, then heat to 500℃ in 100min, then heat to 600℃ in 60min, and finally keep at 650℃ for 2h. Then granulate under nitrogen atmosphere to obtain granulated secondary granules. Then shape the secondary granules to 14μm. (2) The shaped secondary granules in (1) are placed in an Atchison furnace for graphitization at a temperature of 2500℃ to obtain graphitized granules. Then, the granules are coated with pitch coke at a mass percentage of 10% of the graphitized granules. The granules are then placed in a box furnace and carbonized at 1000℃ to obtain carbon-coated secondary granular graphite.

[0086] Comparative Example 6 (1) Add natural graphite fine powder with D50 of 8μm and asphalt to a mixer at a mass ratio of 85:15, heat to 80℃, mix, and then place the mixture in a vertical reactor for heating. The heating process is as follows: heat to 300℃ in 30min, then heat to 500℃ in 100min, then heat to 600℃ in 60min, and finally keep at 650℃ for 2h. Then granulate under nitrogen atmosphere to obtain granulated secondary granules. Then shape the secondary granules to 14μm. (3) The secondary granules after (2) are placed in an Atchison furnace for graphitization at a temperature of 2500℃ to obtain graphitized granules. Then, the granules are coated with pitch coke at a mass percentage of 10% of the graphitized granules. The granules are then placed in a box furnace and carbonized at 1000℃ to obtain carbon-coated secondary granular graphite.

[0087] Comparative Example 7 The difference from Example 1 is that step (1) is omitted, while everything else is the same as in Example 1.

[0088] Comparative Example 8 The difference from Example 1 is that step (2) is omitted, while everything else is the same as in Example 1.

[0089] Comparative Example 9 The difference from Example 1 is that the graphitization step in step (4) is omitted, while all other steps are the same as in Example 1.

[0090] Comparative Example 10 The difference from Example 1 is that the steps of coating with pitch coke and carbonizing in an inert atmosphere in step (4) are omitted, while the rest are the same as in Example 1.

[0091] The testing method is as follows: (I) Physicochemical tests: 1. Powder resistivity test: The resistivity of the powdered graphite in the above examples and comparative examples was measured using the four-probe method; 2. Powder compaction: The powder compaction of the above examples and comparative examples was measured using the ten-point method with a pressure of 5 tons. (II) Electrical Performance Testing: The graphite, conductive carbon black, CMC, and SBR prepared in the above examples and comparative examples were dissolved in water and stirred to form a negative electrode slurry. The appropriate ratio of graphite, conductive carbon black, CMC, and SBR was 96.5%:0.5%:1.2%:1.8%. The stirred negative electrode slurry was coated onto a negative electrode current collector, dried to form a negative electrode sheet, and then rolled, slit, and die-cut. The resistivity of the electrode sheet was tested. Testing (using the four-probe method); Lithium iron sulfate, conductive agent, and PVDF were dissolved in NMP and stirred to form a slurry. The appropriate ratio of lithium iron sulfate positive electrode, conductive agent, and PVDF was 96%:1.5%:2.5%. The prepared positive electrode slurry was coated on the positive electrode current collector and dried to form a positive electrode sheet. The positive electrode sheet was then rolled, slit, and die-cut. The positive electrode sheet, negative electrode sheet, separator, and electrolyte were assembled into a pouch battery.

[0092] 3. Capacity utilization: The soft-pack battery was subjected to a process at 45°C (step 1: rest for 30 minutes; step 2: constant current charging at 0.02C for 5 hours; step 3: rest for 10 minutes; step 4: constant current charging at 0.15C for 2 hours; step 5: rest for 10 minutes). Capacity testing at 25℃ (Step 1: Let stand for 10 minutes; Step 2: Constant current and constant voltage charging: constant current: 0.05C, voltage: 3.65V, cutoff current: 0.05C; Step 3: Constant current discharge: 1.0C, cutoff voltage: 2.0V; Step 4: Let stand for 10 minutes; Step 5: Constant current discharge: 0.1C, cutoff voltage: 2.0V; Step 6: Let stand for 10 minutes; Step 7: Constant current charging at 0.33C for 1 hour). Specific capacity calculation = (1.0C discharge capacity + 0.1C discharge capacity) * 1000 / weight of positive electrode active material; First-efficiency calculation = (1.0C discharge capacity + 0.1C discharge capacity) / (0.02C charging capacity after formation + 0.15C charging capacity after formation + step 2 charging capacity after capacity division) * 100%.

[0093] 4. Rate charging test: 1) Let stand at 25℃ for 30 minutes; 2) Discharge at a constant current of 1.0C to 2.0V; 3) Let stand for 30 minutes; 4) Charge to 3.65V using a constant current and constant voltage at 0.5C, with a cutoff current of 0.05C; 5) Let stand for 30 minutes; 6) Discharge at a constant current of 1.0C to 2.0V, and record the discharge capacity as C0; 7) Let stand for 10 minutes; 8) 1.0C0 constant current discharge 2.0V; 9) Charge at 3.0C0 constant current and constant voltage until 3.65V is reached, with a cutoff current of 0.05C0. 10) Cycle: Repeat steps 7) to 9) 5 times and record the constant current charging capacity of step 9) 5 times as C1 / C2 / C3 / C4 / C5). Calculate the capacity retention rate of 3C rate charging after 5 cycles = (C1+C2+C3+C4+C5) / 5 / C0*100%.

[0094] 5. DCR test: 1) Let stand at 25℃ for 30 minutes; 2) Discharge at a constant current of 1.0C to 2.0V; 3) Let stand for 30 minutes; 4) Charge to 3.65V using a constant current and constant voltage at 0.5C, with a cutoff current of 0.05C; 5) Let stand for 30 minutes; 6) Discharge at a constant current of 1.0C to 2.0V, and record the discharge capacity as C0; 7) Let stand for 30 minutes; 8) 1C0 constant current and constant voltage charging to 3.65V, cutoff current 0.05C0; 9) Let stand for 30 minutes; 10) Discharge at a constant current of 0.5C0 to 50%C0 (adjust to 50%SOC); 11) Let it stand for 60 minutes and record the voltage V1 at the end of the standing period; 12) Discharge at a constant current of 3.0C0 for 30s, and record the voltage V2 after discharge; 13) Let stand for 40 seconds; 14) 2.2C0 constant current charging for 15s; 15) Let stand for 60 minutes.

[0095] Then calculate DCR = (V1 - V2) / discharge current.

[0096] 6. Loop testing: 1) Let stand at 25℃ for 30 minutes; 2) Charge at 0.5C0 constant current and constant voltage until 3.65V is reached, with a cutoff current of 0.05C. 3) Let stand for 30 minutes; 4) 1C constant current discharge 2.0V; 5) Let stand for 30 minutes; 6) Repeat steps 2) to 5) 500 times.

[0097] The discharge capacities recorded for the 3rd and 500th cycles are C3 and C500, respectively. Therefore, the capacity retention rate after 500 cycles is (C500 / C3)*100%.

[0098] Table 1. Physicochemical and Electrical Performance Test Results of Graphite Anode

[0099] The data in the table above shows that after the natural graphite fine powder is modified by a mixture of boric acid and acetic acid, and then granulated, graphitized, coated and carbonized, and then subjected to gradient coating treatment, the kinetic properties of the material can be effectively improved. The data shows that gradient coating with lithium salt assisted by different concentrations of ethanol can significantly improve the kinetic properties and efficiency of natural graphite. Furthermore, the reaction of boric acid and acetic acid can also improve the kinetic properties of natural graphite to some extent by expanding the interlayer spacing.

[0100] Overall, while lower particle size natural graphite fine powder has better kinetic properties, its aggregate particle size is too small, which will reduce its energy density and powder compaction density. Therefore, in order to achieve a balanced development of fast charging capability and energy density, it is recommended to use natural graphite fine powder with slightly larger aggregate particle size for modification treatment to prepare fast charging negative electrode graphite.

[0101] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing fast-charging graphite, characterized in that: Includes the following steps: The surface of the natural graphite fine powder is passivated by acid leaching and alkaline neutralization. Passivated graphite powder was mixed with boric acid and acetic acid, and then heated to react, resulting in modified graphite powder. Modified graphite powder is mixed with a binder and granulated to obtain secondary particles; After the secondary particles are graphitized, the graphitized secondary particles are coated with a carbon coating agent and then carbonized in an inert atmosphere. The carbonized secondary particles are vacuum impregnated in a lithium source solution and then dried to obtain the final product.

2. The method for preparing fast-charging graphite according to claim 1, characterized in that: The particle size D50 of the natural graphite fine powder is 2-8 μm.

3. The method for preparing fast-charging graphite according to claim 1, characterized in that: During acid leaching and elution, a mixture of concentrated sulfuric acid and concentrated nitric acid is used, with a volume ratio of concentrated H2SO4 to concentrated HNO3 of 1.5-2.5:

1.

4. The method for preparing fast-charging graphite according to claim 1, characterized in that: The mass ratio of natural graphite powder, boric acid, and acetic acid is 5-20:1:5-20; Preferably, the heating reaction temperature is 150-180℃, and the heating reaction time is 6-48h.

5. The method for preparing fast-charging graphite according to claim 1, characterized in that: The binder is kerosene coke or pitch coke.

6. The method for preparing fast-charging graphite according to claim 1, characterized in that: The coating agent is asphalt, phenolic resin, epoxy resin or polyfurfuryl alcohol; Preferably, the carbonization temperature is 1000-1300℃.

7. The method for preparing fast-charging graphite according to claim 1, characterized in that: During the vacuum impregnation process of the carbonized secondary particles in a lithium source solution, the lithium source is LiOH and LiCl, and the molar ratio of LiOH to LiCl is 0.3:0.1-0.3; Preferably, the solvent of the lithium source solution is ethanol, the concentration of LiOH is 0.3 mol / L, and the concentration of LiCl is 0.1-0.3 mol / L.

8. A fast-charging graphite, characterized in that: It is prepared by any of the preparation methods described in claims 1-7.

9. A lithium-ion battery anode material, characterized in that: It is prepared from the fast-charging graphite described in claim 8.

10. A lithium-ion battery, characterized in that: Its negative electrode is made from the lithium-ion battery negative electrode material as described in claim 9.

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