Surface-modified graphite negative electrode material and preparation process thereof

Abstract

This invention relates to the field of lithium-ion battery technology and discloses a surface-modified graphite anode material and its preparation process. The process includes pretreatment, primary coating and heat treatment, wet chemical modification, secondary heat treatment, and washing and drying. By adding nano-silica powder and performing heat treatment in the primary coating and heat treatment step, a uniform preliminary modification layer is formed in situ on the surface of graphite particles. This layer structure helps to form a more stable solid electrolyte interface film in subsequent electrochemical cycles, thereby improving the first coulombic efficiency and cycle stability of the anode material. By introducing lithium salt solution and silane coupling agent to synergistically treat the primary modified graphite, the lithium salt provides an active lithium source to compensate for the irreversible capacity loss in the first cycle, while the silane coupling agent can enhance the chemical bonding between the graphite surface and the coating layer. The two synergistically improve the specific capacity and interface structure stability of the material.

Description

Technical Field

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

[0002] Graphite anode materials generally refer to graphite-based anode materials for lithium-ion batteries. Compared with conventional batteries, lithium-ion batteries have become increasingly popular in mobile phones, laptops, and power tools due to their high energy density, long cycle life, and lack of memory effect. As various products increasingly demand smaller, lighter, more multifunctional, and longer-lasting operation, the improvement of lithium-ion battery capacity mainly depends on the development and improvement of anode materials.

[0003] Currently, in the preparation of surface-modified graphite anode materials, due to the relatively simple existing processing methods or the unstable structure of the modified layer, conventional coating or doping methods are difficult to construct a uniform, stable, and firmly bonded composite modified layer on the surface of graphite particles when performing surface treatment of graphite anode materials.

[0004] Therefore, a surface-modified graphite anode material and its preparation process are proposed to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a surface-modified graphite anode material and its preparation process, which solves the problem mentioned in the background art that conventional coating or doping methods are difficult to construct a uniform, stable, and firmly bonded composite modified layer on the surface of graphite particles.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a preparation process for a surface-modified graphite anode material, comprising the following steps: Step 1: Pretreatment. Natural flake graphite is crushed and classified to obtain graphite powder with a D50 particle size of 8-25μm. The powder is then heat-treated at 300-500℃ for 1-5 hours under an inert atmosphere to obtain pretreated graphite powder. Step 2: One-time coating and heat treatment. The pretreated graphite powder and nano-silica powder are dry-mixed at a mass ratio of 100:1-10 to obtain mixed powder A. Mixed powder A is placed in a fluidized bed and heated to 600-900℃ at 2-10℃ / min under an inert atmosphere. The temperature is then maintained for 2-8 hours to allow silicon dioxide to react in situ on the graphite surface to form a preliminary modified layer, thus obtaining primary modified graphite. Step 3: Wet chemical modification. Disperse the primary modified graphite in an alcohol-water mixed solvent to prepare a slurry with a solid content of 5%-20%. Add lithium salt solution and silane coupling agent while stirring. Control the pH of the reaction system to 8-11 and react at 40-80℃ for 2-12 hours. The lithium salt is selected from at least one of lithium carbonate, lithium hydroxide, and lithium acetate. The amount of lithium salt added is 0.5%-5% of the mass of the primary modified graphite, calculated based on elemental lithium. The amount of silane coupling agent added is 0.1%-2% of the mass of the primary modified graphite. Step 4: Secondary heat treatment. The solid-liquid mixture after the reaction in Step 3 is separated. The obtained solid is washed with deionized water and organic solvent, and then vacuum dried at 80-120℃ for 6-12 hours to obtain a dried precursor. The precursor is then placed in a tube furnace and heated to 700-1000℃ at 1-5℃ / min under an inert atmosphere. It is then sintered at this temperature for 4-12 hours and then cooled to room temperature with the furnace to obtain secondary modified graphite. Step 5: Washing and drying. Immerse the secondary modified graphite in a dilute acid solution and sonicate for 0.5-2 hours to remove residual soluble impurities on the surface. Then wash with deionized water until neutral and dry under vacuum at 100-150℃ for 8-24 hours to obtain the surface-modified graphite anode material.

[0007] Preferably, in step one, the tap density of the graphite powder after crushing and grading is 0.8-1.2 g / cm³, and the specific surface area is 1.5-4.5 m² / g; The vacuum condition of the inert atmosphere is a pressure below 10 Pa.

[0008] Preferably, in step two, the average particle size of the nano-silica powder is 5-50 nm, and the purity of the silica is not less than 99.5%. The inert atmosphere is an argon-hydrogen mixture, in which the volume fraction of hydrogen is 5%-10%. During the heat treatment process, the pressure inside the furnace is maintained at a slightly positive pressure, with a gauge pressure of 0.5-5 kPa.

[0009] Preferably, in step three, the volume ratio of alcohol to water in the alcohol-water mixed solvent is 1:0.5-3, and the alcohol is one of ethanol, isopropanol, or ethylene glycol. The concentration of the lithium salt solution is 0.1-2.0 mol / L, and the silane coupling agent is at least one of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, or vinyltriethoxysilane.

[0010] Preferably, in step three, the reagent used to control the pH value of the reaction system is ammonia, lithium hydroxide aqueous solution, or tetramethylammonium hydroxide aqueous solution, and the reaction is carried out under ultrasonic assistance with an ultrasonic power of 100-500W.

[0011] Preferably, in step four, during the heat preservation and sintering stage, the furnace is kept under a slight positive pressure, with a gauge pressure of 1-10 kPa, and the cooling rate during furnace cooling is controlled at 2-8 °C / min.

[0012] Preferably, in step five, the dilute acid solution is an aqueous solution of hydrochloric acid, sulfuric acid, or oxalic acid with a concentration of 0.1-1.0 mol / L; The ultrasonic treatment has a power of 200-600W and a frequency of 20-40kHz, and the vacuum drying has a vacuum degree of less than 100Pa.

[0013] Preferably, after drying in step five, the process further includes: The dried graphite material is uniformly mixed with a carbon source precursor at a mass ratio of 100:1-8, wherein the carbon source precursor is sucrose, glucose, phenolic resin or pitch. Under an inert atmosphere, carbonization is carried out at 600-1200℃ for 2-10 hours to form a carbon coating layer with a thickness of 1-20nm, thus obtaining the final surface-modified graphite anode material.

[0014] Preferably, between step two and step three, a ball milling process is further included: The modified graphite obtained in step two is placed in a planetary ball mill and milled for 2-10 hours at a speed of 200-500 r / min under an inert atmosphere. The milling media is zirconia balls, and the ball-to-material ratio is 5:1-20:1, in order to refine the particles and activate the graphite surface.

[0015] A surface-modified graphite anode material, the graphite anode material comprising a graphite core and a composite modification layer coated on its surface; The composite modified layer has a gradient structure, with the inner layer being a mixed layer of lithium silicate, amorphous carbon and graphite carbon generated in situ, and the outer layer being an organic-inorganic hybrid layer derived from silane coupling agent and a carbon coating layer. Based on the total mass of the surface-modified graphite anode material, the mass percentage of silicon in its composite modified layer is 0.1%-3%, the mass percentage of lithium is 0.05%-2%, and the molar ratio of silicon to lithium is 0.5:1-2:1.

[0016] Compared with the prior art, the present invention provides a surface-modified graphite anode material and its preparation process, which has the following beneficial effects: 1. In this invention, by adding nano-silica powder and performing heat treatment in a single coating and heat treatment step, a uniform preliminary modified layer is formed in situ on the surface of graphite particles. This layer structure helps to form a more stable solid electrolyte interface film in subsequent electrochemical cycles, thereby improving the first coulombic efficiency and cycle stability of the negative electrode material.

[0017] 2. In this invention, lithium salt solution and silane coupling agent are introduced to synergistically treat primary modified graphite. The lithium salt provides an active lithium source to compensate for the irreversible capacity loss during the first cycle, while the silane coupling agent can enhance the chemical bonding between the graphite surface and the coating layer. The two work together to improve the specific capacity and interfacial structural stability of the material.

[0018] 3. In this invention, by performing high-temperature secondary heat treatment and ultrasonic washing with dilute acid solution in the final step, the high-temperature treatment promotes the final formation of a strong and dense composite coating layer, while acid washing removes residual impurities on the material surface. The combination of the two reduces the probability of side reactions and interfacial impedance, and together ensures the material's excellent long cycle life and fast charge and discharge capability. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1: A preparation process for a surface-modified graphite anode material, comprising the following steps: Step 1: Pretreatment. Natural flake graphite is crushed and classified to obtain graphite powder with a D50 particle size of 8μm. The powder is then heat-treated at 300℃ for 1 hour under an inert atmosphere to obtain pretreated graphite powder. Step 2: One-time coating and heat treatment. The pretreated graphite powder and nano-silica powder are dry-mixed at a mass ratio of 100:1 to obtain mixed powder A. Mixed powder A was placed in a fluidized bed and heated to 600℃ at 2℃ / min under an inert atmosphere. The temperature was then maintained for 2 hours to allow silicon dioxide to react in situ on the graphite surface to form a preliminary modified layer, thus obtaining primary modified graphite. Step 3: Wet chemical modification. The primary modified graphite is dispersed in an alcohol-water mixed solvent to prepare a slurry with a solid content of 5%. Lithium salt solution and silane coupling agent are added under stirring. The pH of the reaction system is controlled at 8, and the reaction is carried out at 40°C for 2 hours. The lithium salt is lithium carbonate, and the amount of lithium salt added is 0.5% of the mass of the primary modified graphite, calculated based on elemental lithium. The amount of silane coupling agent added is 0.1% of the mass of the primary modified graphite. Step 4: Secondary heat treatment. The solid-liquid mixture after the reaction in Step 3 is separated. The obtained solid is washed with deionized water and organic solvent, and then dried under vacuum at 80°C for 6 hours to obtain a dried precursor. It is then placed in a tube furnace and heated to 700°C at 1°C / min under an inert atmosphere. It is then sintered at this temperature for 4 hours and then cooled to room temperature with the furnace to obtain secondary modified graphite. Step 5: Washing and drying. The secondary modified graphite is immersed in a dilute acid solution and ultrasonically treated for 0.5 hours to remove residual soluble impurities on the surface. Then it is washed with deionized water until neutral and dried under vacuum at 100°C for 8 hours to obtain the surface modified graphite anode material.

[0021] In step one, the tap density of the graphite powder after crushing and grading is 0.8 g / cm³, and the specific surface area is 1.5 m² / g. The vacuum condition for an inert atmosphere is a pressure below 10 Pa.

[0022] In step two, the average particle size of the nano-silica powder is 5 nm, and the purity of the silica is not less than 99.5%. The inert atmosphere is an argon-hydrogen mixture, in which the volume fraction of hydrogen is 5%. During the heat treatment process, the pressure inside the furnace is maintained at a slightly positive pressure, with a gauge pressure of 0.5 kPa.

[0023] In step three, the volume ratio of alcohol to water in the alcohol-water mixed solvent is 1:0.5, and the alcohol is ethanol; The concentration of the lithium salt solution was 0.1 mol / L, and the silane coupling agent was γ-aminopropyltriethoxysilane.

[0024] In step three, ammonia water is used as the reagent to control the pH value of the reaction system, and the reaction is carried out under ultrasonic assistance with an ultrasonic power of 100W.

[0025] In step four, during the heat preservation and sintering stage, a slight positive pressure is maintained inside the furnace, with a gauge pressure of 1 kPa, and the cooling rate during furnace cooling is controlled at 2℃ / min.

[0026] In step five, the dilute acid solution is hydrochloric acid with a concentration of 0.1 mol / L; The ultrasonic treatment power is 200W, the frequency is 20kHz, and the vacuum degree of vacuum drying is less than 100Pa.

[0027] After drying in step five, the process also includes: The dried graphite material was uniformly mixed with a carbon source precursor at a mass ratio of 100:1. The carbon source precursor was sucrose. Under an inert atmosphere, the carbonization process was carried out at 600℃ for 2 hours to form a carbon coating layer with a thickness of 1nm, thus obtaining the final surface-modified graphite anode material.

[0028] Between step two and step three, there is also a ball milling process: The modified graphite obtained in step two was placed in a planetary ball mill and milled for 2 hours at a speed of 200 r / min under an inert atmosphere. The milling media was zirconia balls with a ball-to-material ratio of 5:1 to refine the particles and activate the graphite surface.

[0029] A surface-modified graphite anode material, comprising a graphite core and a composite modification layer coated on its surface; The composite modified layer has a gradient structure. The inner layer is a mixed layer of lithium silicate, amorphous carbon and graphite carbon generated in situ, and the outer layer is an organic-inorganic hybrid layer derived from silane coupling agent and a carbon coating layer. Based on the total mass of the surface-modified graphite anode material, the mass percentage of silicon in its composite modified layer is 0.1%, the mass percentage of lithium is 0.05%, and the molar ratio of silicon to lithium is 0.5:1.

[0030] Example 2: A preparation process for a surface-modified graphite anode material, comprising the following steps: Step 1: Pretreatment. Natural flake graphite is crushed and classified to obtain graphite powder with a D50 particle size of 16μm. The powder is then heat-treated at 400℃ for 3 hours under an inert atmosphere to obtain pretreated graphite powder. Step 2: One-time coating and heat treatment. The pretreated graphite powder and nano-silica powder are dry-mixed at a mass ratio of 100:5 to obtain mixed powder A. Mixed powder A was placed in a fluidized bed and heated to 750°C at 6°C / min under an inert atmosphere. The temperature was then maintained for 5 hours to allow silicon dioxide to react in situ on the graphite surface to form a preliminary modified layer, thus obtaining primary modified graphite. Step 3: Wet chemical modification. The primary modified graphite is dispersed in an alcohol-water mixed solvent to prepare a slurry with a solid content of 12%. Lithium salt solution and silane coupling agent are added under stirring. The pH of the reaction system is controlled at 9, and the reaction is carried out at 60°C for 7 hours. The lithium salt is lithium hydroxide, and the amount of lithium salt added is 2.7% of the mass of the primary modified graphite, calculated based on elemental lithium. The amount of silane coupling agent added is 1% of the mass of the primary modified graphite. Step 4: Secondary heat treatment. The solid-liquid mixture after the reaction in Step 3 is separated. The obtained solid is washed with deionized water and organic solvent, and dried under vacuum at 100°C for 9 hours to obtain a dried precursor. Then, it is placed in a tube furnace and heated to 850°C at 3°C / min under an inert atmosphere. It is then held at the temperature for 8 hours and sintered. After that, it is cooled to room temperature with the furnace to obtain secondary modified graphite. Step 5: Washing and drying. The secondary modified graphite is immersed in a dilute acid solution and ultrasonically treated for 1.2 hours to remove residual soluble impurities on the surface. Then it is washed with deionized water until neutral and dried under vacuum at 125°C for 16 hours to obtain the surface modified graphite anode material.

[0031] In step one, the tap density of the graphite powder after crushing and grading is 1.0 g / cm³, and the specific surface area is 3.0 m² / g. The vacuum condition for an inert atmosphere is a pressure below 10 Pa.

[0032] In step two, the average particle size of the nano-silica powder is 27 nm, and the purity of the silica is not less than 99.5%. The inert atmosphere is an argon-hydrogen mixture, in which the volume fraction of hydrogen is 7.5%. During the heat treatment process, the pressure inside the furnace is maintained at a slightly positive pressure, with a gauge pressure of 2.7 kPa.

[0033] In step three, the volume ratio of alcohol to water in the alcohol-water mixed solvent is 1:1.7, and the alcohol is isopropanol; The concentration of the lithium salt solution was 1.0 mol / L, and the silane coupling agent was γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

[0034] In step three, the reagent used to control the pH value of the reaction system is an aqueous solution of lithium hydroxide, and the reaction is carried out under ultrasonic assistance with an ultrasonic power of 300W.

[0035] In step four, during the heat preservation and sintering stage, a slight positive pressure is maintained inside the furnace, with a gauge pressure of 5 kPa, and the cooling rate during furnace cooling is controlled at 5 °C / min.

[0036] In step five, the dilute acid solution is 0.5 mol / L sulfuric acid; The ultrasonic treatment has a power of 400W and a frequency of 30kHz, and the vacuum degree of vacuum drying is less than 100Pa.

[0037] After drying in step five, the process also includes: The dried graphite material was uniformly mixed with a carbon source precursor at a mass ratio of 100:4, and the carbon source precursor was glucose. Under an inert atmosphere, the carbonization process was carried out at 900℃ for 6 hours to form a carbon coating layer with a thickness of 10nm, thus obtaining the final surface-modified graphite anode material.

[0038] Between step two and step three, there is also a ball milling process: The modified graphite obtained in step two was placed in a planetary ball mill and milled for 6 hours at a speed of 350 r / min under an inert atmosphere. The milling media was zirconia balls with a ball-to-material ratio of 12:1 to refine the particles and activate the graphite surface.

[0039] A surface-modified graphite anode material, comprising a graphite core and a composite modification layer coated on its surface; The composite modified layer has a gradient structure. The inner layer is a mixed layer of lithium silicate, amorphous carbon and graphite carbon generated in situ, and the outer layer is an organic-inorganic hybrid layer derived from silane coupling agent and a carbon coating layer. Based on the total mass of the surface-modified graphite anode material, the mass percentage of silicon in its composite modified layer is 1.5%, the mass percentage of lithium is 1%, and the molar ratio of silicon to lithium is 1.2:1.

[0040] Example 3: A preparation process for a surface-modified graphite anode material, comprising the following steps: Step 1: Pretreatment. Natural flake graphite is crushed and classified to obtain graphite powder with a D50 particle size of 25μm. The powder is then heat-treated at 500℃ for 5 hours under an inert atmosphere to obtain pretreated graphite powder. Step 2: First coating and heat treatment. The pretreated graphite powder and nano-silica powder are dry-mixed at a mass ratio of 100:10 to obtain mixed powder A. Mixed powder A was placed in a fluidized bed and heated to 900℃ at 10℃ / min under an inert atmosphere. The temperature was then maintained for 8 hours to allow silicon dioxide to react in situ on the graphite surface to form a preliminary modified layer, thus obtaining primary modified graphite. Step 3: Wet chemical modification. The primary modified graphite is dispersed in an alcohol-water mixed solvent to prepare a slurry with a solid content of 20%. Lithium salt solution and silane coupling agent are added under stirring. The pH of the reaction system is controlled at 11, and the reaction is carried out at 80°C for 12 hours. The lithium salt is lithium acetate, and the amount of lithium salt added is 5% of the mass of the primary modified graphite, calculated as lithium element; the amount of silane coupling agent added is 2% of the mass of the primary modified graphite. Step 4: Secondary heat treatment. The solid-liquid mixture after the reaction in Step 3 is separated. The obtained solid is washed with deionized water and organic solvent, and then dried under vacuum at 120°C for 12 hours to obtain a dried precursor. The precursor is then placed in a tube furnace and heated to 1000°C at 5°C / min under an inert atmosphere. It is then held at that temperature for 12 hours and sintered. Afterward, it is cooled to room temperature with the furnace to obtain secondary modified graphite. Step 5: Washing and drying. The secondary modified graphite is immersed in a dilute acid solution and ultrasonically treated for 2 hours to remove residual soluble impurities on the surface. Then it is washed with deionized water until neutral and dried under vacuum at 150°C for 24 hours to obtain the surface modified graphite anode material.

[0041] In step one, the tap density of the graphite powder after crushing and grading is 1.2 g / cm³, and the specific surface area is 4.5 m² / g. The vacuum condition for an inert atmosphere is a pressure below 10 Pa.

[0042] In step two, the average particle size of the nano-silica powder is 50 nm, and the purity of the silica is not less than 99.5%. The inert atmosphere is an argon-hydrogen mixture, in which the volume fraction of hydrogen is 10%. During the heat treatment process, the pressure inside the furnace is maintained at a slightly positive pressure, with a gauge pressure of 5 kPa.

[0043] In step three, the volume ratio of alcohol to water in the alcohol-water mixed solvent is 1:3, and the alcohol is ethylene glycol; The concentration of the lithium salt solution was 2.0 mol / L, and the silane coupling agent was vinyltriethoxysilane.

[0044] In step three, the reagent used to control the pH value of the reaction system is an aqueous solution of tetramethylammonium hydroxide, and the reaction is carried out under ultrasonic assistance with an ultrasonic power of 500W.

[0045] In step four, during the heat preservation and sintering stage, a slight positive pressure is maintained inside the furnace, with a gauge pressure of 10 kPa, and the cooling rate during furnace cooling is controlled at 8℃ / min.

[0046] In step five, the dilute acid solution is an aqueous solution of oxalic acid with a concentration of 1.0 mol / L; The ultrasonic treatment has a power of 600W and a frequency of 40kHz, and the vacuum degree of the vacuum drying is less than 100Pa.

[0047] After drying in step five, the process also includes: The dried graphite material was uniformly mixed with the carbon source precursor at a mass ratio of 100:8. The carbon source precursor was phenolic resin. Under an inert atmosphere, the carbonization process was carried out at 1200℃ for 10 hours to form a carbon coating layer with a thickness of 20nm, thus obtaining the final surface-modified graphite anode material.

[0048] Between step two and step three, there is also a ball milling process: The modified graphite obtained in step two was placed in a planetary ball mill and milled for 10 hours at a speed of 500 r / min under an inert atmosphere. The milling media was zirconia balls with a ball-to-material ratio of 20:1, in order to refine the particles and activate the graphite surface.

[0049] A surface-modified graphite anode material, comprising a graphite core and a composite modification layer coated on its surface; The composite modified layer has a gradient structure. The inner layer is a mixed layer of lithium silicate, amorphous carbon and graphite carbon generated in situ, and the outer layer is an organic-inorganic hybrid layer derived from silane coupling agent and a carbon coating layer. Based on the total mass of the surface-modified graphite anode material, the mass percentage of silicon in its composite modified layer is 3%, the mass percentage of lithium is 2%, and the molar ratio of silicon to lithium is 2:1.

[0050] Comparative Example 1 differs from Example 1 in that: in step two, no nano-silica powder was added in this comparative example, and only the pretreated graphite powder was subjected to heat treatment under the same conditions.

[0051] Comparative Example 2 differs from Example 1 in that: in step three, no lithium salt solution and silane coupling agent were added in this comparative example; only the modified graphite was stirred in an alcohol-water mixed solvent under the same conditions.

[0052] Comparative Example 3 differs from Example 1 in that the heat treatment temperature is changed to 500°C in step four of this comparative example.

[0053] Comparative Example 4 differs from Example 1 in that the ultrasonic treatment step of dilute acid solution in step five is omitted in this comparative example, and the sample is only washed with deionized water before drying.

[0054] The surface-modified graphite anode materials prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests. The test items and test methods are as follows: Specific capacity test: In the 2032 coin cell, lithium metal is used as the counter electrode, and charge and discharge tests are performed at a rate of 0.1C and a voltage of 1.5V. The specific capacity of the first discharge is taken.

[0055] First charge and discharge efficiency test: Based on the first charge capacity and discharge capacity of the above specific capacity test, calculate the first coulombic efficiency. First coulombic efficiency = first charge capacity / first discharge capacity × 100%.

[0056] Cyclic performance test: Charge and discharge cycles were performed at 1C rate, and the capacity retention rate was recorded after 300 cycles.

[0057] Rate performance test: Cycle for 5 weeks at each of the 0.2C, 0.5C, 1C and 2C rates, and record the retention rate of the discharge capacity at the 2C rate relative to the discharge capacity at the 0.2C rate.

[0058] The test data of the surface-modified graphite anode materials prepared in Examples 1-3 and Comparative Examples 1-4 are recorded in the table below: By comparing and analyzing the data in the table, it can be seen that the surface-modified graphite anode materials prepared using the processes in Examples 1-3 exhibit significantly superior performance compared to the anode materials prepared using the processes in Comparative Examples 1-4. This indicates that the present invention, through the addition of nano-silica powder in a primary coating and heat treatment step followed by heat treatment, forms a uniform preliminary modification layer in situ on the surface of graphite particles. This layer structure helps to form a more stable solid electrolyte interface film in subsequent electrochemical cycles, thereby improving the initial coulombic efficiency and cycle stability of the anode material. Furthermore, the introduction of lithium salt solution and silane coupling agent further enhances the primary modification process. Graphite undergoes synergistic processing, with lithium salt providing an active lithium source to compensate for the irreversible capacity loss during the first cycle, and silane coupling agents enhancing the chemical bonding between the graphite surface and the coating layer. Together, they improve the specific capacity and interfacial structural stability of the material. High-temperature secondary heat treatment and ultrasonic washing with dilute acid solution in the final step promote the final formation of a strong and dense composite coating layer, while acid washing removes residual impurities from the material surface. The combination of these two processes reduces the probability of side reactions and interfacial impedance, jointly ensuring the material's excellent long cycle life and rapid charge-discharge capability.

[0059] By comparing and analyzing the relevant data in the table, it can be seen that the surface-modified graphite anode material obtained by the preparation process of this invention has high specific capacity, excellent first coulombic efficiency, good long-term cycle stability and excellent rate performance.

[0060] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0061] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A preparation process for a surface-modified graphite anode material, characterized in that: Includes the following steps: Step 1: Pretreatment. Natural flake graphite is crushed and classified to obtain graphite powder with a D50 particle size of 8-25μm. The powder is then heat-treated at 300-500℃ for 1-5 hours under an inert atmosphere to obtain pretreated graphite powder. Step 2: One-time coating and heat treatment. The pretreated graphite powder and nano-silica powder are dry-mixed at a mass ratio of 100:1-10 to obtain mixed powder A. Mixed powder A is placed in a fluidized bed and heated to 600-900℃ at 2-10℃ / min under an inert atmosphere. The temperature is then maintained for 2-8 hours to allow silicon dioxide to react in situ on the graphite surface to form a preliminary modified layer, thus obtaining primary modified graphite. Step 3: Wet chemical modification. Disperse the primary modified graphite in an alcohol-water mixed solvent to prepare a slurry with a solid content of 5%-20%. Add lithium salt solution and silane coupling agent while stirring. Control the pH of the reaction system to 8-11 and react at 40-80℃ for 2-12 hours. The lithium salt is selected from at least one of lithium carbonate, lithium hydroxide, and lithium acetate. The amount of lithium salt added is 0.5%-5% of the mass of the primary modified graphite, calculated based on elemental lithium. The amount of silane coupling agent added is 0.1%-2% of the mass of the primary modified graphite. Step 4: Secondary heat treatment. The solid-liquid mixture after the reaction in Step 3 is separated. The obtained solid is washed with deionized water and organic solvent, and then vacuum dried at 80-120℃ for 6-12 hours to obtain a dried precursor. The precursor is then placed in a tube furnace and heated to 700-1000℃ at 1-5℃ / min under an inert atmosphere. It is then sintered at this temperature for 4-12 hours and then cooled to room temperature with the furnace to obtain secondary modified graphite. Step 5: Washing and drying. Immerse the secondary modified graphite in a dilute acid solution and sonicate for 0.5-2 hours to remove residual soluble impurities on the surface. Then wash with deionized water until neutral and dry under vacuum at 100-150℃ for 8-24 hours to obtain the surface-modified graphite anode material.

2. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: In step one, the tap density of the graphite powder after crushing and grading is 0.8-1.2 g / cm³, and the specific surface area is 1.5-4.5 m² / g. The vacuum condition of the inert atmosphere is a pressure below 10 Pa.

3. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: In step two, the average particle size of the nano-silica powder is 5-50 nm, and the purity of the silica is not less than 99.5%. The inert atmosphere is an argon-hydrogen mixture, in which the volume fraction of hydrogen is 5%-10%. During the heat treatment process, the pressure inside the furnace is maintained at a slightly positive pressure, with a gauge pressure of 0.5-5 kPa.

4. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: In step three, the volume ratio of alcohol to water in the alcohol-water mixed solvent is 1:0.5-3, and the alcohol is one of ethanol, isopropanol, or ethylene glycol. The concentration of the lithium salt solution is 0.1-2.0 mol / L, and the silane coupling agent is at least one of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, or vinyltriethoxysilane.

5. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: In step three, the reagents used to control the pH value of the reaction system are ammonia, lithium hydroxide aqueous solution, or tetramethylammonium hydroxide aqueous solution. The reaction is carried out under ultrasonic assistance with an ultrasonic power of 100-500W.

6. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: In step four, during the heat preservation and sintering stage, a slight positive pressure is maintained inside the furnace, with a gauge pressure of 1-10 kPa, and the cooling rate during furnace cooling is controlled at 2-8 °C / min.

7. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: In step five, the dilute acid solution is an aqueous solution of hydrochloric acid, sulfuric acid, or oxalic acid with a concentration of 0.1-1.0 mol / L. The ultrasonic treatment has a power of 200-600W and a frequency of 20-40kHz, and the vacuum drying has a vacuum degree of less than 100Pa.

8. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: After drying in step five, the process also includes: The dried graphite material is uniformly mixed with a carbon source precursor at a mass ratio of 100:1-8, wherein the carbon source precursor is sucrose, glucose, phenolic resin or pitch. Under an inert atmosphere, carbonization is carried out at 600-1200℃ for 2-10 hours to form a carbon coating layer with a thickness of 1-20nm, thus obtaining the final surface-modified graphite anode material.

9. The preparation process of a surface-modified graphite anode material according to claim 1, characterized in that: Between steps two and three, there is also a ball milling process: The modified graphite obtained in step two is placed in a planetary ball mill and milled for 2-10 hours at a speed of 200-500 r / min under an inert atmosphere. The milling media is zirconia balls, and the ball-to-material ratio is 5:1-20:1, in order to refine the particles and activate the graphite surface.

10. A surface-modified graphite anode material, prepared by the preparation process of a surface-modified graphite anode material according to any one of claims 1-9, characterized in that: The graphite anode material includes a graphite core and a composite modified layer coated on its surface; The composite modified layer has a gradient structure, with the inner layer being a mixed layer of lithium silicate, amorphous carbon and graphite carbon generated in situ, and the outer layer being an organic-inorganic hybrid layer derived from silane coupling agent and a carbon coating layer. Based on the total mass of the surface-modified graphite anode material, the mass percentage of silicon in its composite modified layer is 0.1%-3%, the mass percentage of lithium is 0.05%-2%, and the molar ratio of silicon to lithium is 0.5:1-2:1.

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