A method for modifying a graphite negative electrode by CVD continuous carbon coating

Abstract

The application discloses a CVD continuous carbon-coating modification method of graphite negative electrode and belongs to the technical field of lithium ion battery negative electrode materials. The method adopts a rotary kiln continuous reaction device. After graphite raw materials are pretreated and dehydrated, the internal temperature gradient of the particles is eliminated by subsection gradient heating homogenization. The mixed reaction gas of acetylene and inert gas is introduced, and the temperature is activated and pre-deposited to 780-820 DEG C. The volume concentration of acetylene is controlled to be 5%-20%, and the constant-temperature continuous carbon-coating is carried out for 300-1200 min. Based on Fick diffusion mass transfer and double-reaction competition kinetics regulation, the gas phase self-polymerization reaction rate is lower than the surface heterogeneous deposition rate. After the acetylene is stopped, the temperature is raised to 850-900 DEG C for high-temperature hydrogen removal aftertreatment. The application realizes the continuous controllable deposition of the carbon layer on the graphite surface, solves the problems of acetylene self-polymerization, uneven coating and poor consistency, and the product has excellent fast-charging performance and long cycle stability, and is suitable for power batteries and energy storage fields.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material modification technology, specifically to a CVD continuous carbon coating modification method for graphite anodes. Background Technology

[0002] Graphite is currently the most widely used anode material in lithium-ion batteries due to its excellent conductivity, low lithium intercalation / deintercalation potential, long cycle life, and low cost. However, the layered structure of graphite itself results in a high activation energy barrier for lithium-ion intercalation between graphite layers, leading to poor interfacial lithium-ion transport kinetics. Under high-current charge-discharge conditions, severe electrode polarization hinders the rapid intercalation of lithium ions between graphite layers, easily causing lithium dendrites to precipitate on the graphite surface. This results in insufficient rate performance and rapid capacity decay, making it difficult to meet the high-power, fast-response requirements of electric vehicle fast charging and large-scale energy storage. Therefore, improving the interfacial structure and ion transport kinetics of graphite anodes has become a key technical challenge restricting the upgrading of the high-performance lithium-ion battery industry.

[0003] Carbon coating is a core modification technique for improving the interfacial structure of graphite anodes and enhancing electronic conductivity and ion transport efficiency. Among these techniques, chemical vapor deposition (CVD) carbon coating technology, with its advantages of strong coating layer adhesion, good uniformity, controllable thickness, and stable modification effect, has greater potential for industrial application compared to liquid-phase coating techniques such as resin coating and pitch coating. CVD carbon coating grows a carbon coating layer in situ on the graphite surface by pyrolyzing the carbon source gas at high temperatures. This effectively repairs graphite surface defects, reduces interfacial charge transfer resistance, and improves the lithium-ion diffusion rate.

[0004] However, existing CVD carbon coating technologies still face many problems that urgently need to be addressed. First, existing technologies, such as the scheme disclosed in CN113851614A, mostly employ intermittent batch CVD processes, which are only suitable for small-batch experimental preparation and cannot achieve a continuous industrial production mode with continuous feeding and discharging. This results in low production efficiency, large batch-to-batch variations, and difficulty in meeting the needs of large-scale mass production. Second, existing CVD coating processes generally use high-concentration acetylene as a carbon source for short-term coating. High-concentration acetylene is prone to homogeneous nucleation and gas-phase self-polymerization side reactions in a high-temperature gas phase environment, generating impurities such as high-carbon organic matter and carbon black particles. These impurities adhere to the graphite surface, not only destroying the density and uniformity of the coating layer but also leading to an abnormal increase in the specific surface area of ​​the material, exacerbating interfacial side reactions, and significantly deteriorating electrochemical rate performance and cycle stability. Secondly, existing CVD coating processes mostly employ short-time coating modes (typically 30-300 minutes), resulting in an extremely narrow process time window. Even minor adjustments to parameters such as reaction temperature, gas flow rate, and material residence time can cause drastic fluctuations in coating quality. Product performance is highly sensitive to process conditions, leading to extremely low production tolerance and making it unsuitable for large-scale, continuous, and stable industrial production. Furthermore, the uneven distribution of axial and radial temperature and flow fields in batch reactors results in significant differences in the degree of reaction of materials in different regions of the kiln. This leads to poor uniformity and insufficient batch stability in continuously discharged products, resulting in large performance deviations and severely hindering the industrial application of CVD carbon-coated graphite anode materials. Finally, existing CVD coating processes lack post-processing steps for structural optimization of the coated carbon layer. This results in high residual hydrogen content within the carbon layer, insufficient conductivity and structural stability, affecting the rate performance and cycle life of the final material.

[0005] Therefore, in order to address the above problems, this invention provides a CVD continuous carbon coating modification method for graphite anodes, in order to solve the technical problems that existing batch CVD processes cannot be adapted to continuous mass production, high concentrations of acetylene are prone to gas-phase self-polymerization side reactions leading to coating layer deterioration, and the narrow short-time coating process window leads to poor product consistency and mass production stability. Summary of the Invention

[0006] The purpose of this invention is to provide a CVD continuous carbon coating modification method for graphite anodes. This method utilizes a rotary kiln continuous reaction system to achieve continuous feeding and discharging of carbon coating material. A segmented gradient heating homogenization process is combined to eliminate internal temperature gradients within graphite particles, ensuring uniform coating across the entire surface. A low-concentration acetylene and inert gas mixture is used for long-term continuous carbon coating at a constant temperature range of 780-820℃ for 300-1200 minutes. Based on the Fick diffusion mass transfer and acetylene dual-reaction competition kinetics, the acetylene gas-phase self-polymerization reaction rate is controlled to be lower than the heterogeneous deposition reaction rate on the graphite surface, thus suppressing gas-phase self-polymerization side reactions. After coating, a high-temperature dehydrogenation post-treatment at 850-900℃ is performed to remove residual hydrogen components within the carbon layer and optimize the carbon layer's microstructure. This results in a thin and dense amorphous carbon coating layer on the graphite surface, comprehensively improving the lithium-ion diffusion rate, interfacial charge transfer resistance, fast-charge rate performance, long-cycle stability, and consistency in continuous mass production of the graphite anode material.

[0007] The objective of this invention is achieved through the following technical solution: A continuous carbon coating modification method for graphite anodes via CVD, employing a rotary kiln continuous reaction system for continuous carbon coating modification, includes the following steps: S1. Graphite pretreatment and dehydration: Place the graphite raw material in a rotary kiln and heat it to 90~110℃ at a heating rate of ≤5℃ / min. Hold the temperature for 50~70min to remove the adsorbed moisture and residual trace solvent impurities from the graphite surface. S2. Segmented gradient heating and homogenization: After dehydration, the temperature is increased to 280~320℃ at a heating rate of ≤3℃ / min and held at the temperature for no less than 60min. Then, the temperature is increased to 580~620℃ at a heating rate of ≤3℃ / min and held at the temperature for no less than 60min to achieve overall temperature homogenization of the graphite matrix and eliminate the internal temperature gradient of the particles. S3. Acetylene activation pre-deposition: After the isothermal preheating is completed, a mixture of acetylene and inert gas is continuously introduced into the rotary kiln, and the temperature is raised to 780~820℃ at a heating rate of 0.8~1.2℃ / min. S4. Continuous isothermal carbon coating: Continuous carbon coating is carried out in the isothermal range of 780~820℃, the volume concentration of acetylene in the mixed gas is controlled at 5%~20%, and the material is kept at the isothermal position in the rotary kiln for 300~1200 min. Based on the competitive kinetic regulation of Fick diffusion mass transfer and acetylene dual reaction, the rate of acetylene gas phase self-polymerization reaction is controlled to be lower than the rate of heterogeneous deposition reaction on the graphite surface. S5. High-temperature dehydrogenation post-treatment: Stop the acetylene supply, heat to 850~900℃ at a heating rate of 0.8~1.2℃ / min under an inert atmosphere, hold at the temperature for 40~80min to remove residual hydrogen components inside the coated carbon layer, and cool to room temperature to obtain continuously modified graphite anode material.

[0008] In the above scheme, step S1 limits the heating rate to ≤5℃ / min to avoid excessive heating that could cause the graphite particles to break apart due to violent vaporization of moisture. The graphite particles are then kept at a constant temperature of 90~110℃ for 50~70min to fully remove adsorbed moisture and residual solvent from the surface, ensuring the cleanliness of the graphite surface and laying the foundation for subsequent carbon layer deposition.

[0009] In the above scheme, if the graphite raw material is directly heated to the high-temperature CVD coating temperature in step S2, a significant temperature gradient will form inside the particles, leading to thermal stress and particle cracking. Simultaneously, temperature differences in different areas of the same particle surface will cause inconsistent carbon source decomposition rates, resulting in uneven coating thickness. This invention employs a two-stage gradient heating homogenization process, maintaining constant temperatures of 280~320℃ and 580~620℃ for at least 60 minutes each, ensuring sufficient temperature uniformity inside and outside the particles, eliminating temperature gradients. The heating rate in each stage is ≤3℃ / min, ensuring a smooth and controllable heating process, providing a prerequisite for uniform coating across the entire surface.

[0010] In the above scheme, the timing of acetylene introduction in step S3 is advanced to the heating process. The acetylene is gradually activated as the temperature rises, forming uniform carbon nucleation sites on the graphite surface, due to a low heating rate of 0.8~1.2℃ / min. This "simultaneous heating and pre-deposition" method avoids direct exposure of graphite to a high-temperature environment without carbon source protection, which could lead to surface oxidation. Simultaneously, the uniform nucleation sites formed by pre-deposition provide a uniform growth starting point for subsequent long-term continuous coating, avoiding coating defects caused by uneven nucleation.

[0011] In the above scheme, step S4, 780~820℃, represents the mild pyrolysis temperature window for acetylene. Within this temperature range, acetylene possesses sufficient activation energy to undergo pyrolysis and deposition, while the pyrolysis rate is controllable. A low acetylene concentration of 5%~20% increases the mean free path of acetylene molecules, significantly reducing the gas-phase collision frequency and the probability of homogeneous nucleation. The vast majority of acetylene molecules can diffuse to the graphite surface to undergo heterogeneous deposition. Combined with a long-term isothermal coating of 300~1200 min, slow, uniform, and dense carbon layer growth is achieved. Based on Fick's diffusion mass transfer law, the diffusion flux is moderate under low concentration conditions, ensuring that acetylene does not undergo homogeneous nucleation due to gas-phase supersaturation before reaching the surface. Through the competitive kinetics of acetylene's two reactions, the gas-phase self-polymerization reaction rate is made much lower than the heterogeneous deposition reaction rate on the graphite surface, ensuring preferential deposition of the carbon source on the graphite surface from a reaction kinetics perspective, completely avoiding the generation of high-carbon byproducts.

[0012] In the above scheme, the carbon layer formed by acetylene cracking and deposition in step S5 contains residual hydrogen atoms, which exist in the form of CH bonds within the amorphous carbon layer. This reduces the graphitization degree and conductivity of the carbon layer, and increases the interfacial impedance. In this invention, after coating, the temperature is raised to 850-900℃ and held for 40-80 minutes to break the CH bonds, remove hydrogen atoms as hydrogen gas, reduce the residual hydrogen content in the carbon layer, improve the graphitization degree and conductivity, further reduce the interfacial impedance, and simultaneously make the internal structure of the carbon layer more dense and ordered, enhancing its bonding force with the graphite matrix. The heating rate of 0.8-1.2℃ / min ensures a smooth transition of the carbon layer structure, avoiding cracking or peeling.

[0013] Preferably, the acetylene volume concentration in step S4 is 9%~11%, and the material is continuously coated at a constant temperature in the rotary kiln for 600~900 min. The 9%~11% concentration provides the most ideal diffusion mass transfer state, and the gas-phase self-polymerization side reaction is suppressed to an extremely low level; the 600~900 min duration ensures moderate carbon layer thickness and optimal density, achieving the best overall performance in terms of fast charging rate, long-cycle stability, and product consistency.

[0014] Preferably, the rotary kiln rotates at a speed of 0.5~5 r / min, and the system pressure is 0~500 Pa. The rotary kiln operates continuously throughout the entire process, with continuous feeding and continuous discharging. The rotation speed of 0.5~5 r / min ensures that the material continuously tumbles and its surface is constantly renewed, allowing for full contact with the reactant gas. The slightly positive pressure environment prevents outside air from entering and causing graphite oxidation, while also facilitating the uniform axial flow of the reactant gas within the kiln, avoiding localized concentration differences.

[0015] Preferably, the graphite raw material is any one of natural graphite, artificial graphite, or recycled graphite, and the particle size of the graphite raw material is 5~25μm. The method of the present invention has wide applicability to a variety of graphite raw materials, and the particle size range of 5~25μm ensures ideal fluidization behavior of the particles in the rotary kiln and the highest contact efficiency with the reactant gas.

[0016] Preferably, the inert gas is either nitrogen or argon, which does not react with graphite and carbon layers under high temperature conditions, effectively isolating oxygen and ensuring a pure and stable reaction atmosphere.

[0017] Preferably, the heating rate in step S1 is 3~5℃ / min; the heating rate in step S2 is 1~3℃ / min; the heating rate in step S3 is 0.8~1.2℃ / min; and the heating rate in step S5 is 0.8~1.2℃ / min. The dehydration stage uses 3~5℃ / min to ensure dehydration efficiency; the homogenization stage uses 1~3℃ / min to ensure sufficient uniformity of the internal temperature of the particles; and the activation pre-deposition and high-temperature hydrogen removal stages use 0.8~1.2℃ / min for precise control.

[0018] Preferably, after stopping the acetylene supply in step S5, the temperature is increased to 850-900°C at a heating rate of 0.8-1.2°C / min under an inert atmosphere, held at that temperature for 40-80 minutes, and then allowed to cool naturally to room temperature in the furnace. Natural cooling avoids the thermal stress cracks caused by the difference in thermal expansion coefficients between the carbon layer and the graphite matrix due to forced cooling, thus ensuring the integrity of the coating layer and the structural stability.

[0019] Preferably, in step S4, the material is kept at a constant temperature of 780~820℃ for 600~900 min in the rotary kiln, the rotary kiln speed is 1~3 r / min, and the acetylene volume concentration in the mixed gas is 9%~11%. The three core parameters are synergistically matched to achieve optimal performance, the gas-phase self-polymerization side reaction is suppressed to the lowest level, the carbon layer deposition rate and diffusion mass transfer rate reach dynamic equilibrium, and the carbon layer is uniform, dense, and defect-free.

[0020] This invention also provides a modified graphite anode material prepared by the above-described CVD continuous carbon coating modification method, wherein the modified graphite anode material has a thin and dense amorphous carbon layer coated on its surface, and the specific surface area of ​​the modified graphite anode material is 1.5~3.5m². 2 / g, 2C / 0.2C capacity ratio ≥25%, lithium-ion diffusion coefficient improved by more than 30% compared to raw materials, 2C high-current long-cycle 500-cycle capacity retention ≥87%, 2C constant-current fast charging ratio ≥63%, and product performance deviation ≤2% for continuous rotary kiln output. This modified graphite anode material's carbon layer repairs graphite surface defects, reduces interfacial charge transfer impedance, and its low specific surface area indicates a dense coating layer without carbon black byproducts. The 2C / 0.2C capacity ratio ≥25% demonstrates excellent fast-charging rate performance; the more than 30% improvement in lithium-ion diffusion coefficient confirms the carbon layer's promoting effect on ion transport; the 2C long-cycle 500-cycle capacity retention ≥87% indicates stable material structure and minimal performance degradation; the 2C constant-current fast charging ratio ≥63% indicates excellent charging efficiency and low polarization; and the product performance deviation ≤2% verifies product consistency under continuous mass production conditions.

[0021] This invention also provides a lithium-ion battery comprising the aforementioned modified graphite anode material. Due to the excellent lithium-ion diffusion rate and low interfacial charge transfer impedance of the anode material, the battery's fast-charging performance is significantly improved, exhibiting high charge-discharge efficiency and full capacity utilization at 2C rate, and high capacity retention during long-cycle high-current operation. It can be widely applied in new energy storage devices such as on-board fast-charging power batteries, large-scale energy storage lithium batteries, and portable digital fast-charging batteries.

[0022] Due to the application of the above technical solution, the present invention has the following beneficial effects compared with the prior art: 1. This invention eliminates the internal temperature gradient of graphite particles by using a rotary kiln continuous reaction equipment and a segmented gradient heating homogenization process, ensuring that all particles in the rotary kiln are at the same temperature when entering the CVD coating zone. This solves the technical problems of existing intermittent batch CVD processes being unable to adapt to continuous mass production, poor product uniformity, and large batch fluctuations. The performance deviation of continuously discharged products is ≤2%, realizing the industrial continuous production of highly consistent graphite anode materials.

[0023] 2. This invention controls the acetylene volume concentration within a low range of 5% to 20%, combined with a long-term isothermal coating process of 300 to 1200 minutes. Based on the competitive kinetics of Fick diffusion mass transfer and acetylene dual reaction, the rate of acetylene gas-phase self-polymerization is lower than that of heterogeneous deposition on the graphite surface. From a reaction kinetic perspective, this ensures that the carbon source preferentially deposits on the graphite surface rather than undergoes gas-phase self-polymerization, completely avoiding the formation of high-carbon byproducts. This solves the technical problem in existing high-concentration acetylene CVD processes where acetylene self-polymerization leads to an abnormal increase in material specific surface area, coating layer deterioration, and rate performance degradation. The material specific surface area is stably controlled at 1.5 to 3.5 m². 2 / g.

[0024] 3. This invention overcomes the technical bottleneck of the narrow time window in existing CVD coating processes. By utilizing a wide acetylene concentration range of 5% to 20% and a coating duration of 300 to 1200 minutes, the coating process becomes less sensitive to minor fluctuations in operating parameters during continuous rotary kiln production. This results in a high process tolerance and solves the problems of high production risk and inability to adapt to large-scale continuous and stable production in existing processes. Furthermore, this invention eliminates the need for pre-treatment with ultra-high temperature graphitization, simplifying the process flow and significantly reducing energy consumption and manufacturing costs.

[0025] 4. This invention forms a thin and dense amorphous carbon coating layer on the graphite surface, which differs from the composite coating structure of carbon nanotubes and amorphous carbon in the prior art. Combined with high-temperature dehydrogenation treatment at 850~900℃ to remove residual hydrogen inside the carbon layer and optimize the microstructure of the carbon layer, the modified graphite anode has a lithium-ion diffusion coefficient that is more than 30% higher than that of the raw material, a 2C / 0.2C capacity ratio of ≥25%, a 2C constant current fast charging ratio of ≥63%, and a 2C high current long cycle capacity retention rate of ≥87% after 500 cycles, thus taking into account both excellent fast charging performance and long cycle stability.

[0026] 5. The method of the present invention has wide applicability to natural graphite, artificial graphite and recycled graphite, and does not require pre-treatment of ultra-high temperature graphitization. The process is simple and efficient. Combined with the continuous operation of the rotary kiln and the mass production mode of automatic feeding and discharging, it is suitable for large-scale application in the field of new energy storage devices such as vehicle fast charging power batteries, large-scale energy storage lithium batteries and portable digital fast charging batteries. Attached Figure Description

[0027] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, some of the drawings in the following description are some embodiments of the present invention. For those skilled in the art, other drawings can be made based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the process of the graphite anode CVD continuous carbon coating modification method in Example 1 of the present invention.

[0029] Figure 2 This is a schematic diagram of the process steps of the graphite anode CVD continuous carbon coating modification method in Example 1 of the present invention.

[0030] Figure 3 This is a schematic diagram of the rotary kiln continuous reaction equipment used in the graphite anode CVD continuous carbon coating modification method of Embodiment 1 of the present invention.

[0031] Figure 4 This is a scanning electron microscope (SEM) image of the modified graphite anode material prepared in Example 1 of the present invention. Detailed Implementation

[0032] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific implementation schemes are now described in detail.

[0033] The present invention will be further described below with reference to embodiments, but the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0034] In this invention, unless otherwise stated, directional terms such as "upper," "lower," "top," and "bottom" are generally used in relation to the direction shown in the accompanying drawings, or in relation to the vertical, perpendicular, or gravitational direction of the component itself; similarly, for ease of understanding and description, "inner" and "outer" refer to the inner and outer contours of each component itself, but the above directional terms are not intended to limit this invention.

[0035] The following embodiments all adopt the following... Figure 3The dual rotary kiln series continuous reaction equipment shown is used for carbon coating modification. The upper rotary kiln is used to complete step S1 pretreatment dehydration and step S2 segmented gradient temperature homogenization, while the lower rotary kiln is used to complete step S3 acetylene activation pre-deposition, step S4 continuous isothermal carbon coating, and step S5 high-temperature dehydrogenation post-treatment. The material is continuously fed from the top feed end, continuously processed by the two series rotary kilns, and continuously discharged from the bottom discharge end.

[0036] Example 1 See appendix Figure 1 - Appendix Figure 4 This embodiment provides a continuous carbon coating modification method for graphite anodes using CVD. The method employs a rotary kiln continuous reaction equipment for continuous carbon coating modification and includes the following steps: S1. Graphite pretreatment and dehydration: Place the graphite raw material (artificial graphite, particle size D50 is 15μm) in a rotary kiln, heat it to 100℃ at a heating rate of 5℃ / min, and keep it at a constant temperature for 60min to remove the adsorbed water and residual trace solvent impurities on the graphite surface. S2. Segmented gradient heating and homogenization: After dehydration, the temperature is increased to 300℃ at a heating rate of 3℃ / min and held for 60min. Then, the temperature is increased to 600℃ at a heating rate of 3℃ / min and held for 60min to achieve overall temperature homogenization of the graphite matrix and eliminate the internal temperature gradient of the particles. S3. Acetylene activation pre-deposition: After the isothermal preheating is completed, a mixture of acetylene and nitrogen reaction gas is continuously introduced into the rotary kiln, and the temperature is raised to 800℃ at a heating rate of 1℃ / min. S4. Continuous isothermal carbon coating: Continuous carbon coating is carried out in an isothermal range of 800℃, the volume concentration of acetylene in the mixed gas is controlled at 10%, the material is kept at the isothermal position in the rotary kiln for 600 min, the rotary kiln speed is 2 r / min, the system pressure is 200 Pa, and based on the competitive kinetic regulation of Fick diffusion mass transfer and acetylene dual reaction, the rate of acetylene gas phase self-polymerization reaction is controlled to be lower than the rate of heterogeneous deposition reaction on the graphite surface. S5. High-temperature dehydrogenation post-treatment: Stop the acetylene supply, heat to 850℃ at a heating rate of 1℃ / min under nitrogen atmosphere, hold at the temperature for 60min to remove residual hydrogen components inside the coated carbon layer, and allow the furnace to cool naturally to room temperature to obtain continuously modified graphite anode material.

[0037] Example 2 This embodiment is based on the above embodiment 1. The similarities with embodiment 1 will not be repeated. The difference between this embodiment and embodiment 1 is that in step S4, the volume concentration of acetylene in the mixed gas is 10%, and the material stays at a constant temperature in the rotary kiln for 900 minutes. The remaining steps are the same as in embodiment 1.

[0038] Example 3 This embodiment is based on the above embodiment 1. The similarities with embodiment 1 will not be repeated. The difference between this embodiment and embodiment 1 is that in step S4, the volume concentration of acetylene in the mixed gas is 8%, and the remaining steps are the same as in embodiment 1.

[0039] Example 4 This embodiment is based on the above embodiment 1. The similarities with embodiment 1 will not be repeated. The difference between this embodiment and embodiment 1 is that in step S4, the volume concentration of acetylene in the mixed gas is 15%, and the remaining steps are the same as in embodiment 1.

[0040] Example 5 This embodiment is based on the above embodiment 1. The similarities with embodiment 1 will not be repeated. The difference between this embodiment and embodiment 1 is that the rotary kiln speed is 0.5 r / min, and the remaining steps are the same as in embodiment 1.

[0041] Example 6 This embodiment is based on the above embodiment 1. The similarities with embodiment 1 will not be repeated. The difference between this embodiment and embodiment 1 is that the rotary kiln speed is 5 r / min, and the remaining steps are the same as in embodiment 1.

[0042] Example 7 This embodiment is based on the above embodiment 1. The similarities with embodiment 1 will not be repeated. The difference between this embodiment and embodiment 1 is that in step S4, the volume concentration of acetylene in the mixed gas is 11%, and the material stays at a constant temperature in the rotary kiln for 600 minutes for coating. The remaining steps are the same as in embodiment 1.

[0043] Comparative Example 1 The difference between this comparative example and Example 1 is that the graphite raw material was not subjected to CVD carbon coating modification treatment, that is, steps S2 to S5 were omitted. The same batch of graphite raw material (artificial graphite with a particle size D50 of 15 μm) used in Example 1 was directly used as the negative electrode material after being dehydrated and pretreated under the same temperature conditions.

[0044] Comparative Example 2 This comparative example is based on Example 1 above. The similarities with Example 1 will not be repeated. The difference between this comparative example and Example 1 is that in step S4, the acetylene volume concentration in the mixed gas is 30%, the coating time is 120 min, and the remaining steps are the same as in Example 1.

[0045] Comparative Example 3 This comparative example is based on Example 1 above. The similarities with Example 1 will not be repeated. The difference between this comparative example and Example 1 is that in step S4, the acetylene volume concentration in the mixed gas is 15% and the coating time is 600 min. The remaining steps are the same as in Example 1.

[0046] Comparative Example 4 This comparative example is based on Example 1 above. The similarities with Example 1 will not be repeated. The difference between this comparative example and Example 1 is that in step S4, the acetylene volume concentration in the mixed gas is 15% and the coating time is 300 min. The remaining steps are the same as in Example 1.

[0047] Comparative Example 5 This comparative example is based on Example 1 above. The similarities with Example 1 will not be repeated. The difference between this comparative example and Example 1 is that in step S4, the acetylene volume concentration in the mixed gas is 10%, the coating time is 150 min, and the remaining steps are the same as in Example 1.

[0048] Comparative Example 6 This comparative example is based on Example 1 above. The similarities with Example 1 will not be repeated. The difference between this comparative example and Example 1 is that in step S4, the constant temperature coating temperature is 850°C, and in step S5, the high temperature dehydrogenation temperature is 900°C. The remaining steps are the same as in Example 1.

[0049] Comparative Example 7 This comparative example uses an intermittent batch CVD process for carbon coating modification, including the following steps: S1. Graphite pretreatment and dehydration: Place the same batch of graphite raw materials (artificial graphite, particle size D50 is 15μm) in an intermittent vacuum furnace, heat to 100℃ at a heating rate of 5℃ / min, and hold at the temperature for 60min. S2, CVD carbon coating: Raise the furnace temperature to 800℃, introduce a mixture of acetylene and nitrogen gas with an acetylene volume concentration of 30%, and coat at a constant temperature for 120 minutes. S3. Cooling: Stop the acetylene supply and cool the furnace to room temperature under nitrogen atmosphere protection to obtain the modified graphite anode material.

[0050] Comparative Example 7 does not use a rotary kiln continuous equipment and does not involve segmented gradient heating homogenization and high-temperature dehydrogenation post-processing, in order to simulate the characteristics of typical intermittent batch short-time high-concentration CVD coating processes in existing technologies.

[0051] The graphite anode materials prepared in the above embodiments and comparative examples were subjected to electrochemical performance tests under the following conditions: A coin cell half-cell was assembled using modified graphite as the working electrode and lithium metal sheet as the counter electrode. The electrolyte was prepared by dissolving 1 mol / L LiPF6 lithium salt in a 1:1 volume ratio EC / DEC mixed solvent. The battery was first discharged at a constant current of 0.1C to 5mV, allowed to stand for 10 min, then discharged at a constant current of 0.05C to 5mV, allowed to stand for 10 min, and subsequently charged at a constant current of 0.1C to 2V. This cycle was repeated twice to activate the battery. Rate performance testing involved charging at a constant current of 0.05C to a specific capacity of 350 mAh / g, allowing to stand for 15 min, and then discharging at constant currents of 0.2C and 2C to a cutoff voltage of 5mV. The 2C / 0.2C discharge capacity ratio was used to quantitatively characterize the material's fast-charging rate performance. Specific surface area was determined using the nitrogen adsorption BET method. Product performance deviation is defined as the maximum relative deviation of performance indicators between different sampling points from the same batch of continuously discharged material. Test results are shown in Table 1.

[0052] Table 1

[0053] Note: "—" indicates that the relevant parameters cannot be effectively measured due to severe deterioration of material properties or excessive polarization.

[0054] As shown in Table 1, the modified graphite anode materials prepared within the optimal process range (Examples 1, 2, and 7) with an acetylene concentration of 9%–11% and a coating time of 600–900 min exhibit significantly superior performance compared to other examples and all comparative examples. The specific surface area of ​​Examples 1, 2, and 7 was controlled between 1.87 and 1.94 m². 2 The low level of / g indicates that the coating layer is dense and defect-free; the 2C / 0.2C capacity ratio reaches 27.4%~29.1%, the lithium-ion diffusion coefficient is improved by 32%~35% compared with the raw material, the capacity retention rate after 500 cycles of 2C reaches 88.7%~90.4%, the proportion of 2C constant current fast charging reaches 66.2%~69.5%, the product performance deviation is ≤1%, and the overall performance is optimal.

[0055] Acetylene concentration is a core parameter determining the reaction mechanism of the system. A comparison of Example 3 (8% acetylene concentration), Example 4 (15% acetylene concentration), and Example 1 (10% acetylene concentration) shows that when the concentration is between 8% and 15% (Examples 1, 3, and 4), the system primarily exhibits heterogeneous deposition on the graphite surface, with no significant formation of carbon black or high-carbon byproducts, and the material's specific surface area is controllable (1.79–2.62 m²). 2 / g), exhibiting good fast-charging performance; however, when the concentration reaches 30% (Comparative Example 2), the system exceeds the diffusion mass transfer saturation critical value, and excess acetylene undergoes a high-order self-polymerization reaction in the gas phase, generating a large number of defective impurities, causing the specific surface area to soar to 10.33m². 2 / g, the rate performance and cycling performance deteriorated significantly, and the 2C / 0.2C capacity ratio and 2C retention rate after 500 cycles could not be effectively measured; when the concentration was below 8%, the carbon source supply was insufficient, the coating was incomplete, and the rate performance improvement was limited.

[0056] The coating duration directly affects the carbon layer thickness and density. A comparison of Comparative Example 5 (150 min) and Comparative Example 4 (300 min) with Example 1 (600 min) shows that short-term coating (150-300 min) results in insufficient carbon layer growth, with a 2C / 0.2C capacity ratio of only 21.3%-24.1% and a cycle retention rate below 83.4%. The preferred long-term continuous coating duration of 600-900 min (Examples 1, 2, and 7) adapts to the dynamic reaction characteristics of the rotary kiln, resulting in a uniform and dense carbon layer, and achieving optimal fast-charging kinetics and cycle stability. Beyond 900 min, an excessively thick carbon layer increases the ion transport distance, leading to a slight decrease in rate performance.

[0057] The rotary kiln rotation speed determines the material agitation and surface renewal rate. A comparison of Example 5 (0.5 r / min), Example 6 (5 r / min) and Example 1 (2 r / min) shows that within the rotation speed range of 0.5 to 5 r / min, the material agitation is uniform, the gas-solid contact is sufficient, and the product performance deviation is ≤2%. However, when the rotation speed is too low (0.5 r / min), the uniformity of the material reaction in the kiln decreases slightly, and when the rotation speed is too high (5 r / min), the gas-solid contact time is insufficient, the carbon layer coverage rate decreases slightly, and the rate performance decreases slightly.

[0058] The isothermal temperature directly affects the acetylene activation and cracking rate and the quality of carbon layer nucleation. A comparison between Comparative Example 6 (coating temperature 850℃, dehydrogenation temperature 900℃) and Example 1 (coating temperature 800℃, dehydrogenation temperature 850℃) shows that at excessively high temperatures, acetylene undergoes intense high-temperature pyrolysis, leading to disordered carbon deposition and an increase in specific surface area to 2.35 m². 2 / g, performance deviation ≥2%, product consistency decreased, proving that the 780~820℃ isothermal range is the key temperature window for achieving mild pyrolysis and uniform nucleation.

[0059] The uncoated raw graphite raw material (Comparative Example 1) had a 2C / 0.2C capacity ratio of only 17.2%, a 2C constant current fast charging ratio of only 44.2%, and a 2C cycle 500-cycle capacity retention rate of only 79.7%. The fast charging performance and cycle stability were both poor, which confirms the significant effect of the CVD continuous carbon coating modification of this invention.

[0060] Comparative Example 7, which employs an intermittent batch short-time high-concentration CVD process, has a specific surface area as high as 9.85 m². 2The 2C / 0.2C capacity ratio is only 15.3%, the 2C constant current fast charging rate is only 41.7%, and the product performance deviation is ≥5%, all of which are significantly inferior to all embodiments of this invention. This indicates that the traditional intermittent batch high-concentration short-time CVD coating process suffers from severe acetylene gas-phase self-polymerization side reactions, resulting in poor coating quality and poor product consistency, and cannot be compared with the rotary kiln continuous low-concentration long-term coating process of this invention.

[0061] In summary, within the wide process protection range defined by this invention—5%–20% acetylene concentration, 300–1200 min coating time, 0.5–5 r / min rotation speed, and 780–820℃ temperature—all embodiments showed significantly better performance than the raw materials, the comparative examples exceeding the range, and the intermittent batch process comparative examples. This demonstrates that the process range of this invention is scientifically effective, has a wide process window, and is suitable for continuous mass production in rotary kilns. The optimal range of 9%–11% concentration and 600–900 min duration allows for optimal fast-charging performance, long-cycle stability, and product consistency. High-concentration, high-temperature, short-time processes, and intermittent batch processes exceeding the parameter range of this invention all exhibited varying degrees of performance degradation, further validating the rationality and inventiveness of the process parameter range of this invention.

[0062] In summary, this invention, through the synergistic matching of continuous rotary kiln equipment and low-concentration, long-lasting CVD coating process, combined with segmented gradient heating homogenization and high-temperature dehydrogenation post-treatment, constructs a graphite anode carbon coating modification technology system suitable for industrial continuous production. This technical solution addresses core challenges in traditional CVD processes, such as acetylene self-polymerization, uneven coating, narrow process window, and poor mass production consistency, at the source of diffusion mass transfer and reaction kinetics. It achieves continuous and controllable deposition of a thin, dense, amorphous carbon layer on the graphite surface. The process of this invention is simple to operate, highly adaptable to raw materials, and exhibits high mass production stability. The modified graphite anode material obtained possesses both excellent fast-charging performance and long cycle life, providing a reliable anode material solution for high-power lithium-ion batteries. It has broad application prospects and industrialization value in fields such as new energy vehicles and large-scale energy storage.

[0063] The embodiments described above are merely illustrative of more specific and detailed implementations of the present invention, and should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A method for continuous carbon coating modification of graphite anodes via CVD, characterized in that, Continuous carbon coating modification using a rotary kiln continuous reaction equipment includes the following steps: S1. Graphite pretreatment and dehydration: Place the graphite raw material in a rotary kiln and heat it to 90~110℃ at a heating rate of ≤5℃ / min. Hold the temperature for 50~70min to remove the adsorbed moisture and residual trace solvent impurities from the graphite surface. S2. Segmented gradient heating and homogenization: After dehydration, the temperature is increased to 280~320℃ at a heating rate of ≤3℃ / min and held at the temperature for no less than 60min. Then, the temperature is increased to 580~620℃ at a heating rate of ≤3℃ / min and held at the temperature for no less than 60min to achieve overall temperature homogenization of the graphite matrix and eliminate the internal temperature gradient of the particles. S3. Acetylene activation pre-deposition: After the isothermal preheating is completed, a mixture of acetylene and inert gas is continuously introduced into the rotary kiln, and the temperature is raised to 780~820℃ at a heating rate of 0.8~1.2℃ / min. S4. Continuous isothermal carbon coating: Continuous carbon coating is carried out in the isothermal range of 780~820℃, the volume concentration of acetylene in the mixed gas is controlled at 5%~20%, and the material is kept at the isothermal position in the rotary kiln for 300~1200 min. Based on the competitive kinetic regulation of Fick diffusion mass transfer and acetylene dual reaction, the rate of acetylene gas phase self-polymerization reaction is controlled to be lower than the rate of heterogeneous deposition reaction on the graphite surface. S5. High-temperature dehydrogenation post-treatment: Stop the acetylene supply, heat to 850~900℃ at a heating rate of 0.8~1.2℃ / min under an inert atmosphere, hold at the temperature for 40~80min to remove residual hydrogen components inside the coated carbon layer, and cool to room temperature to obtain continuously modified graphite anode material.

2. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, The acetylene volume concentration in step S4 is 9%~11%, and the material is continuously coated in the rotary kiln at a constant temperature for 600~900 minutes.

3. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, The rotary kiln rotates at a speed of 0.5~5 r / min, and the system pressure is 0~500 Pa. The rotary kiln operates continuously throughout the entire process, with continuous feeding and continuous discharging.

4. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, The graphite raw material is any one of natural graphite, artificial graphite, or recycled graphite, and the particle size of the graphite raw material is 5~25μm.

5. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, The inert gas is either nitrogen or argon.

6. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, The heating rate in step S1 is 3~5℃ / min; the heating rate in step S2 is 1~3℃ / min; the heating rate in step S3 is 0.8~1.2℃ / min; and the heating rate in step S5 is 0.8~1.2℃ / min.

7. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, After stopping the acetylene supply in step S5, the temperature is increased to 850-900℃ at a heating rate of 0.8-1.2℃ / min under an inert atmosphere, held at the temperature for 40-80 minutes, and then allowed to cool naturally to room temperature with the furnace.

8. The method for continuous carbon coating modification of graphite anode by CVD according to claim 1, characterized in that, In step S4, the material is kept at a constant temperature of 780~820℃ for 600~900 min in the rotary kiln, the rotary kiln speed is 1~3 r / min, and the acetylene volume concentration in the mixed gas is 9%~11%.

9. A modified graphite anode material prepared by the CVD continuous carbon coating modification method according to any one of claims 1 to 8, characterized in that, The modified graphite anode material has a thin and dense amorphous carbon layer coated on its surface, and the specific surface area of ​​the modified graphite anode material is 1.5~3.5m². 2 / g, 2C / 0.2C capacity ratio ≥25%, lithium-ion diffusion coefficient improved by more than 30% compared with raw materials, 2C high current long cycle 500 cycles capacity retention rate ≥87%, 2C constant current fast charging ratio ≥63%, rotary kiln continuous discharge product performance deviation ≤2%.

10. A lithium-ion battery, characterized in that, It includes the modified graphite anode material as described in claim 9.

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