Natural graphite negative electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion battery negative electrode materials, in particular to a natural graphite negative electrode material and a preparation method and application thereof. The negative electrode material comprises porous graphite containing interconnected buffer pores, and a carbon coating layer and a fluorinated layer on the surface of the porous graphite; the preparation method comprises the following steps: pretreating natural flake graphite to construct porous graphite, performing in-situ catalytic carbon coating to form a graphitized carbon layer, performing surface fluorination to passivate defects, and performing post-treatment to obtain the product. The first coulombic efficiency of the material is not lower than 94%, the 500-time cycle capacity retention ratio under 1C multiplying power is not lower than 90%, the raw materials are cheap, the process is integrated, the cost advantage of natural graphite is reserved, and the material is suitable for high-performance lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, specifically to a natural graphite anode material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage systems due to their high energy density and long cycle life. Anode materials, as their core component, are currently mainly classified into artificial graphite, natural graphite, silicon-based materials, and lithium titanate. Among them, natural graphite has attracted much attention due to its abundant reserves, low cost, and high theoretical capacity (~372 mAh / g).

[0003] However, using natural graphite directly as a negative electrode material has significant drawbacks. Therefore, existing technologies have proposed various improvement schemes, mainly including: Oxidation treatment: using gaseous (e.g., air, O2) or liquid (e.g., concentrated sulfuric acid, concentrated nitric acid) oxidants to lightly oxidize the surface of natural graphite, generating oxygen-containing functional groups and micropores to enhance electrolyte wettability and partially alleviate the problem of solvent molecule co-intercalation. Surface coating: coating the surface of natural graphite particles with a layer of amorphous carbon (e.g., soft or hard carbon formed after the carbonization of pitch or resin). This amorphous carbon layer can act as a "buffer layer," reducing direct contact between the graphene layer and the electrolyte, inhibiting excessive growth of the SEI film, and mitigating volume changes during cycling. Element doping: introducing non-metallic elements (e.g., nitrogen, sulfur, phosphorus) or metallic elements into the graphite lattice or surface to change its electronic structure and surface chemical properties, thereby improving ionic conductivity and rate performance. Particle spheroidization: Natural flake graphite is mechanically shaped into approximately spherical shapes to improve its tap density, electrode processing performance, and volumetric energy density.

[0004] Despite some progress made by the above methods, the following significant drawbacks remain, preventing a complete solution to the problem of natural graphite, especially its first-effect issue, from being fully replaced by high-performance synthetic graphite: Limited and unstable improvement in first-time efficiency: While oxidation creates ion channels, it introduces numerous defects and irreversible lithium intercalation sites. Although this increases capacity, it often comes at the cost of sacrificing first-time efficiency. The degree of oxidation is difficult to control precisely, resulting in poor batch-to-batch consistency. Weak adhesion between the coating layer and the matrix: After traditional pitch coating and carbonization, the carbon layer formed is mostly physically bonded to the natural graphite crystals. This bond is prone to peeling during long-term cycling and volume expansion / contraction, leading to protection failure and deterioration in cycle performance. Complex process and increased cost: Multi-step processing (such as oxidation followed by coating) or the use of expensive precursors (such as certain resins) significantly increases the complexity and time cost of the production process, weakening the low-cost advantage of natural graphite. Poor improvement in rate performance: Simple surface modification has limited effect on improving the intrinsic ion diffusion rate of natural graphite, especially during high-current charging, where polarization remains significant.

[0005] Existing technologies have failed to systematically solve the above problems. They mainly focus on improving a single method and fail to design in a coordinated manner from multiple dimensions of "structure-interface-chemistry", thus failing to simultaneously achieve high first-efficiency, long cycle and excellent rate performance. Summary of the Invention

[0006] To overcome the shortcomings of the existing technology, the present invention aims to provide a natural graphite anode material, its preparation method, and its applications. The core idea of ​​this invention is to simultaneously solve problems such as low initial efficiency, poor cycle life, and inadequate rate capability by constructing a multi-level synergistic structure of "internal buffer pores - robust interface coating - surface active modification" without damaging the core crystal structure of natural graphite.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a natural graphite anode material, the anode material comprising: Porous graphite, which has an internal network of interconnected buffered pores; A carbon coating layer is applied to the surface of the porous graphite. A fluorinated layer is formed on the surface of the carbon coating layer.

[0008] As a preferred embodiment of the present invention, the negative electrode material has at least one of the following characteristics a1) to a5): a1) The expansion rate of the porous graphite is 1.35~1.45 times; a2) The thickness of the carbon coating layer is 80~120nm; a3) The fluorine atom content in the fluorinated layer is 5.8~7.2 at%; a4) The initial coulombic efficiency of the negative electrode material is not less than 94%; a5) The capacity retention rate of the negative electrode material after 500 cycles at 1C rate is not less than 90%.

[0009] In the above schemes, if the expansion ratio of porous graphite is less than 1.35, the internal pores are insufficient, the structure is still relatively dense, the improvement in first-time efficiency is limited, the cycle life is poor, and the rate capability is not good, failing to fundamentally solve the problems of natural graphite; if it is greater than 1.45, the graphite crystals are destroyed, becoming loose and disordered, with low capacity, low first-time efficiency, high resistance, and poor cycle life, losing the fundamental advantages of graphite as a negative electrode material. Therefore, a ratio of 1.35 to 1.45 is preferred.

[0010] In the above scheme, in a3), the unit at% means atomic percentage, specifically referring to the percentage of a certain specific atom among all the atoms being measured.

[0011] Secondly, the present invention provides a method for preparing a natural graphite anode material, comprising the following steps: Porous graphite is obtained by pretreating natural flake graphite and constructing an interconnected buffer pore network. In-situ catalytic carbon coating is applied to porous graphite to form porous graphite with a carbon coating layer on its surface. Surface fluorination and defect passivation are performed on coated porous graphite to achieve selective fluorination of the carbon coating surface. The fluorinated porous graphite is washed and post-treated to obtain the negative electrode material.

[0012] As a preferred embodiment of the present invention, the preprocessing and interconnection buffer porous network construction steps include: Natural flake graphite was mixed with an intercalating agent and reacted. After the reaction was completed, the mixture was washed until neutral and then dried. The porous graphite is obtained by heat treatment under an inert atmosphere to achieve controllable micro-expansion.

[0013] As a preferred embodiment of the present invention, the reaction of natural flake graphite with the intercalating agent has at least one of the following characteristics b1) to b4): b1) The mass ratio of the natural flake graphite to the intercalating agent is (8~11):1; b2) The intercalating agent is a mixed solution of nickel acetate, nitric acid and deionized water; b3) The reaction temperature of the natural flake graphite with the intercalating agent is 42~48℃; b4) The reaction time between the natural flake graphite and the intercalating agent is 85-95 min; The heat treatment under an inert atmosphere is as follows: under an inert atmosphere, the temperature is increased to 500~600℃ at a rate of 15℃ / min and held for 20-30min.

[0014] In the above scheme, the strong oxidizing and intercalation effects of nitric acid are: Oxidation reaction: Concentrated nitric acid (HNO3), as a strong oxidizing agent, attacks carbon atoms at the edges and defects of graphite sheets, generating oxygen-containing functional groups (such as carboxyl-COOH and hydroxyl-OH), and may produce carbon dioxide (CO2) or carbon monoxide (CO). This process partially disrupts the crystal structure of the graphite edges, making them easier to intercalate.

[0015] Intercalation reaction: Nitric acid molecules (or ions formed in an acidic environment, such as NO2) + NO + These species insert themselves into the spaces between graphite layers. The insertion of these species expands the graphene sheets that were originally tightly bound together by van der Waals forces, forming "graphite intercalation compounds".

[0016] The role and decomposition of nickel acetate: Formation of intercalated species: In a strongly acidic environment in the presence of nitric acid, nickel acetate decomposes and provides Ni. 2+ Ions. These metal ions may be introduced into the graphite interlayer in some complexed form (e.g., combined with nitrate ions or water molecules). More importantly, the introduced nickel compounds play a crucial role in subsequent high-temperature processing. They decompose, and their products are the core substances that subsequently generate gas and cause expansion.

[0017] Synergistic effect: Nitric acid first opens the interlayer spaces of graphite and initiates initial intercalation, creating space for the insertion of larger nickel complex ions. The two work synergistically to form an unstable, "expanded" intermediate compound. Washing to neutrality removes surface and some interlayer free acidic substances and ions, but a considerable number of intercalated species remain within the graphite layers.

[0018] Thermal decomposition of intercalating agents: When the temperature rises, the nitric acid intercalating compounds and nickel compounds (such as nickel nitrate, basic nickel nitrate, etc.) remaining between the graphite layers undergo violent thermal decomposition reactions.

[0019] Achieving micro-expansion: 1. Temperature control: 500~600℃ is a carefully selected temperature range. If the temperature is too low, the intercalator will not decompose completely, resulting in insufficient expansion; if the temperature is too high, it will lead to excessive reaction of the carbon structure, a severe decrease in the degree of graphitization, and even structural collapse. At this temperature, the gas generation rate and the strength of the graphite structure reach a balance, achieving controllable interlayer pushing rather than explosive peeling. 2. Expansion rate control (1.35~1.45 times): By precisely controlling the ratio of precursor intercalator (graphite:intercalator solid mass ratio (8~11):1) and heat treatment parameters (heating rate, temperature, time), the amount of intercalator contained in a unit volume of graphite can be controlled, thereby controlling the amount of gas generated, and ultimately precisely regulating the expansion ratio. 3. Formation of "interconnected buffer pores": This method, where internal gas pressure expands the interlayer, does not create isolated pores, but rather interlayer gaps that expand along the c-axis (vertical direction) of the graphite crystal, as well as cracks and pores caused by uneven stress within the ab plane (horizontal direction). These pores are interconnected, forming a three-dimensional, interconnected buffer network.

[0020] More preferably, the intercalating agent is prepared by mixing nickel acetate, 70% concentrated nitric acid and deionized water in a ratio of 1g:1.5mL:3mL.

[0021] As a preferred embodiment of the present invention, the in-situ catalytic carbon coating step includes: Porous graphite was impregnated in a sucrose solution containing a metal catalyst and stirred. After drying, cross-linking and catalytic carbonization are carried out under an inert atmosphere to form a graphitized carbon coating layer.

[0022] As a preferred embodiment of the present invention, the step of impregnating porous graphite in a sucrose solution containing a metal catalyst and stirring has at least one of the following characteristics c1) to c5): c1) The metal catalyst is a transition metal salt; c2) The metal catalyst is ferric nitrate; c3) The mass ratio of the metal catalyst to sucrose is 1:6~9; c4) The mass ratio of graphite to sucrose is 4~6:1; c5) The stirring time is 5~7h and the stirring temperature is 22~28℃.

[0023] In the above scheme, the stirring time is 5~7h to allow sucrose and ferric nitrate precursors to fully and uniformly penetrate into the internal pores and surface of porous graphite. The reactions involved are mainly hydrolysis and complexation reactions.

[0024] As a preferred embodiment of the present invention, the crosslinking and catalytic carbonization performed after drying under an inert atmosphere has at least one of the following characteristics d1) to d3): d1) The drying temperature is 75~85℃, and the drying time is 7.5~8.5h; d2) The crosslinking temperature is 330~430℃, and the crosslinking time is 1.8~2.2h; d3) The catalytic carbonization is carried out at 10℃ / min to 900~1000℃ for 1.8~2.2h.

[0025] In the above scheme, polymer crosslinking is a "stabilization treatment" step. Its purpose is to allow sucrose and its hydrolysis products to polymerize and crosslink, forming a thermosetting resin and preventing melting and volatilization at subsequent high temperatures. Dehydration reaction: Sucrose undergoes intramolecular and intermolecular dehydration under heating, forming ether bonds (-O-). Polymerization and aromatization: The dehydrated products further cyclize and condense, forming polycyclic aromatic hydrocarbon macromolecules containing benzene ring structures. This process is similar to the chemical process of "caramelization," but it tends to form a thermosetting polymer with a three-dimensional network structure. This step determines the basic framework of the coating layer.

[0026] Decomposition of ferric nitrate: Ferric nitrate decomposes first during the heating process.

[0027] 4Fe(NO3)3·9H2O→2Fe2O3+12NO2↑+O2↑+36H2O↑ The generated nanoscale iron oxide (Fe2O3) acts as a catalyst for subsequent reactions. Carbonization and catalytic graphitization: Carbonization: At high temperatures, non-carbon elements (H2O) escape from the cross-linked polymer in the form of small molecules such as H2O, CO, CO2, and CH4, leaving behind an amorphous carbon skeleton mainly composed of carbon atoms. Catalytic graphitization (core mechanism): Iron oxide (Fe2O3) is partially reduced by carbon to elemental iron (Fe), or forms Fe3C (iron carbide).

[0028] Fe₂O₃ + 3C → 2Fe + 3CO↑ 3Fe + C → Fe3C Elemental iron or Fe3C exists as liquid droplets at high temperatures, exhibiting high solubility for amorphous carbon. The dissolution-precipitation mechanism involves the liquid Fe / Fe3C droplets dissolving the surrounding amorphous carbon. Since the solubility of carbon in iron varies with temperature, and the presence of iron lowers the energy barrier for carbon atom rearrangement, carbon atoms precipitate from the supersaturated metal droplets in a more thermodynamically stable graphite lattice form, depositing on the epitaxial layer of natural graphite. This process significantly increases the graphitization of the coated carbon material, transforming it from disordered amorphous carbon into highly ordered graphitized carbon. This graphitized carbon coating possesses excellent electrical conductivity and mechanical strength, effectively promoting electron transport and suppressing electrolyte co-intercalation.

[0029] As a preferred embodiment of the present invention, the surface fluorination and defect passivation treatment of the coated porous graphite has at least one of the following features e1) to e5): e1) The processing temperature is 130~170℃; e2) The pressure is 90~110Pa; e3) RF power is 190~210W; e4) The processing time is 40~50 minutes; e5) The fluorinated gas is a mixture of NF3 and N2, with the volume fraction of NF3 being 10~20%.

[0030] In the above scheme, plasma is generated by ionizing the NF3 / N2 mixed gas under the excitation of radio frequency power (190~210W) to produce plasma containing a large number of highly active species, including free electrons, fluorine radicals (F·), excited N2 molecules, and possible free radicals such as NF2· and NF·.

[0031] Surface activation and attack: These high-energy reactive species, especially the electrically neutral fluorine radicals (F·), possess extremely strong reactivity. They bombard and attack the carbon atoms on the surface of the carbon coating layer. Formation of CF bonds: The fluorine radicals react with the carbon atoms to form covalent bonds.

[0032] C (surface) + F → CF Selective fluorination: The conditions of plasma treatment (low temperature, low power, short time) determine that this is a surface reaction. Fluorination mainly occurs at: 1. defect sites in the carbon coating layer; 2. edges and corners of the carbon layer (these areas are more reactive). Unlike high-temperature fluorination, it does not lead to the formation of CF4 and the complete destruction of the carbon skeleton, but achieves "selective fluorination," that is, introducing CF bonds on the surface while maintaining the integrity of the carbon matrix.

[0033] As a preferred embodiment of the present invention, the washing and post-treatment of the fluorinated porous graphite has at least one of the following features f1) to f4): f1) The washing process involves sequentially using dilute hydrochloric acid solution and deionized water. f2) The washing process first involves washing with 0.5 mol / L dilute hydrochloric acid solution at 55-65°C for 3-5 hours to remove metal residue, followed by washing with deionized water. f3) The post-processing involves drying and sieving sequentially; f4) The post-processing is vacuum drying at 100~140℃ for 10~14h, followed by passing through a 400-mesh sieve to obtain the final product.

[0034] Thirdly, the present invention provides a lithium-ion battery comprising the aforementioned natural graphite anode material.

[0035] This invention creatively designs and implements a three-in-one synergistic modification strategy of in vivo porosity building, in vitro strong coating, and surface passivation. Its core improvements are specifically reflected in: Controlled micro-expansion constructs an internal buffer network: By employing a specific metal salt-acid composite intercalator system and precisely controlling the pyrolysis process, a non-destructive, three-dimensional interconnected nano / micro pore network is created within natural graphite particles. This structure differs from simple surface etching or pore creation; it provides both a "highway" for ion transport and a "buffer pool" for volumetric stress, thus improving the basic structure.

[0036] In-situ catalytic graphitization coating technology: By introducing a transition metal catalyst, a tough carbon layer with a high degree of graphitization is induced in situ from amorphous carbon precursors during the carbon coating process. This carbon layer is not only dense and uniform, but its graphite microcrystals also have a good lattice matching degree with the natural graphite matrix, forming a strong and tough chemical-physical composite interface, which solves the core problem of easy peeling off of traditional coating layers.

[0037] Plasma-assisted selective surface fluorination: This is the first time that low-temperature plasma-assisted gas-phase fluorination technology has been applied to the modification of natural graphite anodes. This technology can precisely and selectively react with defects and dangling bonds only on the outermost surface of the material to form stable CF bonds, without destroying the overall structure as in liquid-phase fluorination or high-temperature fluorination. This is a "surgical" precision passivation, which greatly reduces lithium source consumption caused by side reactions during the first charge-discharge cycle. This is the most direct and crucial reason for achieving high initial efficiency.

[0038] The process is integrated and innovative: the three core processes of intercalation expansion, catalytic coating, and plasma fluorination are organically integrated to form a highly efficient, continuous, and controllable preparation route. Each step is interconnected and complementary (for example, the internal pores provide anchoring points for coating, and the coating layer provides a stable substrate for fluorination), achieving a synergistic effect of 1+1+1>3, which is unattainable by single modification technologies.

[0039] Compared with the prior art, the present invention has the following significant advantages and effects: This invention utilizes a multi-level synergistic design to create an internal porous network that provides a rapid transport channel for lithium ions and buffers volume changes; a robust in-situ catalytic carbon coating effectively isolates the electrolyte and maintains structural integrity; and a surface fluorinated layer precisely passivates highly active sites. These three elements work synergistically to systematically solve the problem from the bulk phase to the interface. Specifically: the internal three-dimensional interconnected porous structure significantly shortens the ion diffusion path and improves ionic conductivity, giving the material excellent fast-charging capability and outstanding rate performance. The catalytically generated graphitized carbon coating has a much stronger bond with the substrate than traditional pitch carbon layers, and the internal porous structure provides anchor points for the coating, preventing detachment. Plasma-assisted fluorination is a low-temperature, dry, and controllable surface treatment technology that can precisely target and passivate defect sites, avoiding the damage to the material structure caused by strong acid oxidation or high-temperature treatment. Because the surface active sites are effectively passivated, irreversible side reactions are significantly reduced.

[0040] The present invention effectively inhibits the continuous thickening of the SEI film and the pulverization of the graphite structure during cycling by the combined action of a robust coating layer and internal buffer pores. After 500 cycles at 1C rate, the capacity retention rate of the material exceeds 90%.

[0041] The first coulombic efficiency of the natural graphite anode material prepared by this invention can stably reach over 94%, which is even better than some artificial graphite products, demonstrating extremely high first-efficiency.

[0042] The present invention has controllable costs, the raw materials used (sucrose, ferric nitrate, etc.) are inexpensive and readily available, and the main process steps can be integrated and continuous. While significantly improving performance, it also retains the cost advantage of natural graphite. Detailed Implementation

[0043] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0044] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0045] Example 1 This embodiment provides a method for preparing a natural graphite anode material, including the following steps: (1) Natural flake graphite (particle size D50 of 12 μm) and a mixed solution of nickel acetate and nitric acid (nickel acetate: 70% concentrated nitric acid: deionized water = 1 g: 1.5 mL: 3 mL) were uniformly mixed at a graphite:intercalant solid mass ratio of 10:1. The mixture was reacted at 45 °C for 90 minutes, washed with deionized water until neutral, and then dried. Subsequently, the mixture was heated to 550 °C at a rate of 15 °C / min under an argon atmosphere and held for 25 minutes to achieve controllable micro-expansion, forming internally interconnected buffer pores (the expansion rate was controlled at 1.4 times), thus obtaining a porous graphite intermediate.

[0046] (2) The porous graphite was impregnated in a sucrose solution containing ferric nitrate (Fe(NO3)3·9H2O) (sucrose concentration 200 g / L, metal salt to sucrose mass ratio 1:8), with a graphite to sucrose mass ratio of 5:1. After stirring for 6 hours, it was dried at 80 °C. The polymer was first crosslinked by holding at 380 °C for 2 hours in argon atmosphere, and then catalytically carbonized at 950 °C at 10 °C / min for 2 hours to form a dense graphitized carbon coating layer (coating layer thickness approximately 100 nm).

[0047] (3) The coated graphite was placed in a plasma gas phase fluorination device, and an NF3 / N2 mixed gas (NF3 volume fraction 15%) was introduced. The surface was treated at 150℃ and 100Pa pressure with 200W radio frequency power for 45 minutes to achieve selective fluorination (controlling the F atom content to 6.5at%).

[0048] (4) Wash with 0.5 mol / L dilute hydrochloric acid solution at 60°C for 4 hours to completely remove metal residue. After washing with deionized water, vacuum drying at 120°C for 12 hours, and passing through a 400-mesh sieve, the final product is obtained.

[0049] Example 2 This embodiment provides a method for preparing a natural graphite anode material, including the following steps: (1) Natural flake graphite (particle size D50 of 12 μm) and intercalating agent (nickel acetate: 70% concentrated nitric acid: deionized water = 1 g: 1.5 mL: 3 mL) were uniformly mixed at a graphite:intercalating agent solid mass ratio of 8:1. The mixture was reacted at 42 °C for 85 min, washed with deionized water until neutral, and then dried. Subsequently, the mixture was heated to 500 °C at a rate of 15 °C / min under an argon atmosphere and held for 20 min to achieve controllable micro-expansion. The expansion rate was controlled at 1.35 times, thus obtaining a porous graphite intermediate.

[0050] (2) The porous graphite was impregnated in a sucrose solution containing ferric nitrate (Fe(NO3)3・9H2O) (sucrose concentration 200 g / L, metal catalyst to sucrose mass ratio 1:6), with a graphite to sucrose mass ratio of 6:1. After stirring for 5 h, it was dried at 80 °C. The polymer was first crosslinked by holding at 330 °C for 1.8 h in argon atmosphere, and then catalytically carbonized at 900 °C at a rate of 10 °C / min for 1.8 h to form a dense graphitized carbon coating layer, with the coating layer thickness controlled at 80 nm.

[0051] (3) The coated graphite was placed in a plasma gas phase fluorination device, and an NF3 / N2 mixed gas (NF3 volume fraction 10%) was introduced. The surface was treated at 130℃ and 90Pa pressure with 190W radio frequency power for 40 minutes to achieve selective fluorination and control the fluorine atom content to 5.8at.

[0052] (4) Wash with 0.5 mol / L dilute hydrochloric acid solution at 60℃ for 3 h to completely remove metal residue. After washing with deionized water, vacuum dry at 100℃ for 10 h, and pass through a 400 mesh sieve to obtain the final product.

[0053] Example 3 This embodiment provides a method for preparing a natural graphite anode material, including the following steps: (1) Natural flake graphite (particle size D50 of 12 μm) and intercalating agent (nickel acetate: 70% concentrated nitric acid: deionized water = 1 g: 1.5 mL: 3 mL) were uniformly mixed at a graphite:intercalating agent solid mass ratio of 11:1. The mixture was reacted at 48 °C for 95 min, washed with deionized water until neutral, and then dried. Subsequently, the mixture was heated to 600 °C at a rate of 15 °C / min under an argon atmosphere and held for 30 min to achieve controllable micro-expansion. The expansion rate was controlled at 1.45 times, thus obtaining a porous graphite intermediate.

[0054] (2) The porous graphite was impregnated in a sucrose solution containing ferric nitrate (Fe(NO3)3・9H2O) (sucrose concentration 200 g / L, metal catalyst to sucrose mass ratio 1:9), with a graphite to sucrose mass ratio of 4:1. After stirring for 7 h, it was dried at 80 °C. The polymer was first crosslinked by holding at 430 °C for 2.2 h in argon atmosphere, and then catalytically carbonized at 10 °C / min to 1000 °C for 2.2 h to form a dense graphitized carbon coating layer, with the coating layer thickness controlled at 120 nm.

[0055] (3) The coated graphite was placed in a plasma gas phase fluorination device, and an NF3 / N2 mixed gas (NF3 volume fraction 20%) was introduced. The surface was treated at 170℃ and 110Pa pressure with 210W radio frequency power for 50min to achieve selective fluorination and control the fluorine atom content to 7.2at.

[0056] (4) Wash with 0.5 mol / L dilute hydrochloric acid solution at 60℃ for 5 h to completely remove metal residue. After washing with deionized water, vacuum dry at 140℃ for 14 h, and pass through a 400 mesh sieve to obtain the final product.

[0057] Example 4 This embodiment provides a method for preparing a natural graphite anode material, including the following steps: (1) Natural flake graphite (particle size D50 of 12 μm) and intercalating agent (nickel acetate: 70% concentrated nitric acid: deionized water = 1 g: 1.5 mL: 3 mL) were uniformly mixed at a graphite:intercalating agent solid mass ratio of 9:1. The mixture was reacted at 46 °C for 92 min, washed with deionized water until neutral, and then dried. Subsequently, the mixture was heated to 580 °C at a rate of 15 °C / min under an argon atmosphere and held at that temperature for 28 min to achieve controllable micro-expansion. The expansion rate was controlled at 1.4 times, thus obtaining a porous graphite intermediate.

[0058] (2) The porous graphite was impregnated in a sucrose solution containing ferric nitrate (Fe(NO3)3・9H2O) (sucrose concentration 200 g / L, metal catalyst to sucrose mass ratio 1:7), with a graphite to sucrose mass ratio of 5:1. After stirring for 6.5 h, it was dried at 80 °C. The polymer was first crosslinked by holding at 400 °C for 2.0 h in argon atmosphere, and then catalytically carbonized at 980 °C at 10 °C / min for 2.0 h to form a dense graphitized carbon coating layer, with the coating layer thickness controlled to be 100 nm.

[0059] (3) The coated graphite was placed in a plasma gas phase fluorination device, and an NF3 / N2 mixed gas (NF3 volume fraction 18%) was introduced. The surface was treated at 160℃ and 105Pa pressure with 205W radio frequency power for 48 minutes to achieve selective fluorination and control the fluorine atom content to 6.0at.

[0060] (4) Wash with 0.5 mol / L dilute hydrochloric acid solution at 60℃ for 4.5 h to completely remove metal residue. After washing with deionized water, vacuum drying at 130℃ for 13 h, and passing through a 400-mesh sieve, the final product is obtained.

[0061] Comparative Example 1 This comparative example uses raw, natural graphite without any processing.

[0062] Comparative Example 2 Compared with Example 1, this comparative example only involves the mixing reaction of graphite and intercalating agent in step 1, without controlled micro-expansion. The rest are the same as steps 2-4 of Example 1.

[0063] Comparative Example 3 Compared with Example 1, step 3 is omitted in this comparative example, while the rest are the same as steps 1, 2 and 4 of Example 1.

[0064] Comparative Example 4 Compared with Example 1, steps 1 and 2 are omitted in this comparative example, while steps 3 and 4 are the same as in Example 1.

[0065] Assembly and testing of lithium-ion batteries: The negative electrode materials of each embodiment and comparative example were assembled with a positive electrode (LiFePO4), an electrolyte (1 mol / L LiPF6-EC / DEC / DMC = 1:1:1), and a separator (polypropylene porous membrane) to form CR2032 coin-type lithium-ion batteries. Charge-discharge cycle tests were performed on the batteries (charge-discharge voltage range 2.75-4.2V, 1C rate) to evaluate their DC resistance (DCR), initial coulombic efficiency, and capacity retention after 500 cycles. The test data are shown in Table 1 below. Table 1 The following conclusions can be drawn from Example 1, Comparative Examples 1-4, and Table 1: Comparative Example 1, which used untreated raw natural graphite, had the worst performance among all tested samples: a DCR (direct current resistance) as high as 85 mΩ, an initial efficiency of only 88.52%, and a capacity retention rate of 92.93% after 500 cycles. This confirms that raw natural graphite cannot meet the requirements of high-performance lithium-ion batteries and highlights the necessity of the modification method of this invention.

[0066] The complete process of Example 1 is controllable micro-expansion (step 1) → in-situ catalytic carbon coating (step 2) → surface fluorination (step 3). Comparative Examples 2-4 are missing one or two steps respectively. Data comparison clearly shows the irreplaceability of each step: 1. Comparative Example 2, lacking only the controllable micro-expansion step (the rest being identical to Example 1), exhibited a significant performance degradation compared to Example 1: the DCR increased from 70 mΩ to 80 mΩ, the initial efficiency decreased from 94.50% to 89.00%, and the 500-cycle capacity retention decreased from 96.66% to 93.97%. This confirms the design purpose of controllable micro-expansion in constructing an internally interconnected buffer pore network. This structure provides a rapid transport channel for lithium ions (reducing DCR) and buffers volume changes during cycling (improving capacity retention), while also reducing side reactions to improve initial efficiency, forming the fundamental structure for performance enhancement. Without this step, the internal structure of graphite becomes dense, hindering ion transport and preventing the release of volumetric stress, leading to deterioration in various performance aspects.

[0067] 2. Comparative Example 4, lacking both the controllable micro-expansion and in-situ catalytic carbon coating steps (only surface fluorination is retained), exhibited significantly lower DCR (78 mΩ), first-cycle efficiency (90.00%), and 500-cycle capacity retention (95.45%) compared to Example 1. In-situ catalytic carbon coating forms a graphitized carbon layer with a high lattice matching degree to the natural graphite matrix. This layer not only isolates the electrolyte from direct contact with the graphite matrix (reducing excessive SEI film growth) but also enhances the integrity of the particle structure (resisting pulverization), solving the problem of easy detachment of traditional coating layers. Without this step, the graphite surface lacks robust protection, making it susceptible to performance degradation during cycling due to electrolyte erosion and structural damage.

[0068] 3. Comparative Example 3, lacking only the surface fluorination step (the rest being the same as Example 1), saw its initial efficiency decrease from 94.50% to 90.50%, and its 500-cycle capacity retention decrease from 96.66% to 95.09%. This aligns with the mechanism of plasma-assisted selective fluorination precisely passivating surface defects. Fluorination forms stable CF bonds on the carbon coating surface, reducing side reactions (such as lithium source consumption) initiated by active sites during the first charge-discharge cycle, which is a direct key factor in improving initial efficiency. Simultaneously, the passivated surface also inhibits the continuous thickening of the SEI film, indirectly improving cycle stability. Without this step, highly active surface sites are not eliminated, side reactions intensify, leading to a decline in initial efficiency and cycle performance.

[0069] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A natural graphite negative electrode material, characterized by, The negative electrode material comprises: porous graphite having an interconnected buffer pore network inside; a carbon coating layer coated on the surface of the porous graphite; a fluorinated layer formed on the surface of the carbon coating layer. 2.The natural graphite negative electrode material of claim 1, characterized in that, The negative electrode material has at least one of the following characteristics a1)~a5): a1) the expansion rate of the porous graphite is 1.35~1.45 times; a2) the thickness of the carbon coating layer is 80~120 nm; a3) the content of fluorine atoms in the fluorinated layer is 5.8~7.2 at%; a4) the first coulombic efficiency of the negative electrode material is not less than 94%; a5) the capacity retention rate of the negative electrode material after 500 cycles at 1C rate is not less than 90%.

3. A method for preparing a natural graphite negative electrode material, characterized by, The method comprises the following steps: pretreating and constructing an interconnected buffer pore network on natural flake graphite to obtain porous graphite; in-situ catalytic carbon coating on the porous graphite to form porous graphite with a carbon coating layer on the surface; surface fluorination and defect passivation on the coated porous graphite to achieve selective fluorination on the surface of the carbon coating layer; washing and post-treatment on the fluorinated porous graphite to obtain the negative electrode material.

4. The production method according to claim 3, characterized by, The pretreatment and interconnected buffer pore network construction step comprises: mixing and reacting natural flake graphite with intercalation agent, washing to neutral and drying after the reaction; heat treatment in an inert atmosphere to achieve controllable micro-expansion to obtain the porous graphite.

5. The preparation method according to claim 4, characterized in that, The mixing and reacting of the natural flake graphite with the intercalation agent has at least one of the following characteristics b1)~b4): b1) the mass ratio of the natural flake graphite to the intercalation agent is (8~11):1; b2) the intercalation agent is a mixed solution of nickel acetate, nitric acid and deionized water; b3) the reaction temperature of the natural flake graphite and the intercalation agent is 42~48℃; b4) the reaction time of the natural flake graphite and the intercalation agent is 85~95 min; The heat treatment in an inert atmosphere is: heating to 500~600℃ at a rate of 15℃ / min under an inert atmosphere for 20-30 min.

6. The preparation method according to claim 3, characterized in that, The in-situ catalytic carbon coating step comprises: immersing the porous graphite in a sucrose solution containing a metal catalyst and stirring; after drying, cross-linking and catalytic carbonization in an inert atmosphere to form a graphitized carbon coating layer.

7. The production method according to claim 6, wherein The immersing the porous graphite in a sucrose solution containing a metal catalyst and stirring has at least one of the following characteristics c1)~c5): c1) the metal catalyst is a transition metal salt; c2) the metal catalyst is iron nitrate; c3) the mass ratio of the metal catalyst to sucrose is 1:6~9; c4) the mass ratio of the graphite to sucrose is 4~6:1; c5) the stirring time is 5~7 h and the stirring temperature is 22~28℃.

8. The preparation method according to claim 6, characterized in that, The cross-linking and catalytic carbonization after drying in an inert atmosphere has at least one of the following characteristics d1)~d3): d1) the drying temperature is 75~85℃ and the drying time is 7.5~8.5 h; d2) the cross-linking temperature is 330~430℃ and the cross-linking time is 1.8~2.2 h; d3) the catalytic carbonization is raised to 900~1000℃ at a rate of 10℃ / min for 1.8~2.2 h.

9. The preparation method according to claim 3, characterized in that, The surface fluorination and defect passivation treatment of the coated porous graphite has at least one of the following characteristics e1) to e5): e1) the treatment temperature is 130 to 170 DEG C; e2) the pressure is 90 to 110 Pa; e3) the radio frequency power is 190 to 210 W; e4) the treatment time is 40 to 50 min; e5) the fluorination gas is a mixture of NF3 and N2, and the volume fraction of NF3 is 10 to 20%.

10. A lithium-ion battery, characterized by, The natural graphite negative electrode material according to claim 1 or 2.

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