A carbon-coated preparation method of a fast-charging lithium ion battery graphite negative electrode material

Abstract

The application relates to a carbon-coated preparation method of a graphite negative electrode material for a fast-charging lithium ion battery. The method comprises the following steps: preparing expanded graphite (EG), preparing ball-milled expanded graphite (BMEG), and carbon-coating the ball-milled expanded graphite (C@BMEG). The application creatively combines a liquid phase method, a normal-temperature stirring intercalation method and a mechanical ball-milling method, constructs an amorphous carbon-coated layer, effectively constructs a stable SEI film, effectively slows down the volume expansion effect caused by the solvation co-embedded lithium in the charging and discharging process, and further improves the rate performance and the cycle stability of the material. The application has the advantages of high thermal stability, high conductivity, non-toxicity, non-harmfulness and wide source.

Description

Technical Field

[0001] This invention relates to a method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries, and particularly to the field of new energy lithium-ion batteries. Background Technology

[0002] With the worsening environmental pollution caused by motor vehicles and other means of transportation, clean energy has become a hot topic in scientific research. In recent years, significant progress has been made in sustainable energy development technologies, such as improved wind turbines, solar thermal radiation receivers, and photovoltaic cells. However, research on energy storage and conversion devices and equipment has lagged behind. Consequently, the demand for research and development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has emerged. Considering factors such as performance, cost, and safety, rechargeable lithium-ion batteries (LIBs) are widely recognized as the most likely option for realizing this production application, and they also represent a promising power device in the field of electrochemical energy storage.

[0003] Graphite is a type of graphite composed of countless sps 2 Hybridized carbon atoms, with their hexagonal rings forming a unique layered structure and interlayered bonds via van der Waals forces, exhibit excellent electrical conductivity due to the delocalized π bonds formed by the 2p electrons in each layer. This has led to their increasingly widespread use as anode materials in lithium-ion batteries. Currently, graphite is highly favored by the lithium-ion battery industry due to its excellent thermal stability, high electrical conductivity, non-toxicity, and wide availability. With the large-scale application and development of electric vehicles and other electric products, the energy storage market is placing higher demands on the use of sustainable energy. Research on high-capacity lithium-ion battery anode materials is in full swing, but many research methods for modified graphite remain at the laboratory stage. Therefore, finding a simple, feasible, and easily mass-producible process to prepare high-capacity modified graphite is urgently needed. Summary of the Invention

[0004] In order to overcome the above-mentioned technical problems in the existing technology field, the purpose of this invention is to provide a method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries.

[0005] This invention provides a method for preparing carbon coating on graphite anode materials for fast-charging lithium-ion batteries, comprising the following steps: Step (1) Preparation of expanded graphite (EG) S11 Weigh an appropriate amount of (NH4)2S2O8 powder and place it in a glass beaker. Slowly add H2SO4 to the beaker, then sonicate the mixture for a period of time, and then place it on a magnetic stirrer to stir until the powder is completely dissolved. S12 is slowly and evenly added to natural graphite (G), the rotation speed is reduced, and the mixture is stirred for a period of time to allow the natural graphite powder to undergo a slight expansion during this pre-oxidation process. When the natural graphite powder exhibits slight expansion, turn off the magnetic stirrer and slowly and evenly add H2O2. During the settling process, the graphite begins to expand violently and its volume expands, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for a period of time, then filter the black product obtained by suction, wash it with ultrapure water until the filtrate is neutral, collect the solid and put it into a forced-air drying oven, and dry it to obtain the target product expanded graphite (EG). Step (2) Preparation of ball-milled expanded graphite (BMEG) S21 Weigh a certain mass of expanded graphite and place it into a clean ball mill steel jar. Then weigh a certain amount of steel balls and place them into the jar. Tighten the jar lid and place the ball mill jar in the vacuum chamber of the glove box for evacuation / filling. After a period of time, take it out and place it into a high-speed vibrating ball mill. Tighten the screws, close the door, set the time parameters, and start the machine. After the machine runs for the time required, take out the powder from the jar, weigh it, and put it into a sample bottle. Record it as an intermediate product. S22 Weigh a certain mass of intermediate product into a beaker, add hydrochloric acid, ethanol, and ultrapure water in sequence, place in an oil bath and stir for a period of time. Then, divide the black turbid liquid into four centrifuge tubes, add ultrapure water dropwise until each tube has an equal mass, place in a centrifuge and centrifuge. Discard the upper yellow-green liquid in the tube, add ultrapure water again to balance, repeat centrifugation, and obtain black powder at the bottom of the tube. Then place in a refrigerator to freeze. After the solid and residual liquid in the tube are completely frozen, place in a freeze dryer. After a period of time, take it out and obtain black powder. Collect the powder, weigh it and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG). Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) S31 adopts chemical vapor deposition method, using acetylene gas cracking as carbon source for carbon coating experiment. A certain amount of ball milled expanded graphite (BMEG) and pitch powder are weighed and ground in an agate mortar until uniform. Then, it is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S32, first introduce high-purity argon to purge the air from the tube, then set the appropriate heating program to raise the temperature to a suitable level at a certain heating rate, hold the temperature for a period of time, then raise the temperature to the required level at a certain heating rate, open the acetylene / argon mixed gas cylinder valve, close the high-purity argon gas cylinder valve, hold the temperature for a period of time, then open the high-purity argon gas cylinder valve again, close the mixed gas cylinder valve, and after cooling to room temperature with the furnace, take out the prepared carbon-coated ball-milled expanded graphite (C@BMEG).

[0006] Preferably, step (1) preparation of expanded graphite (EG) includes: S11 Weighing 12-18g of (NH4)2S2O8 powder into a 4000-5500mL glass beaker, slowly adding 80-140mL of H2SO4 to the beaker, then sonicating the mixture for 0.2-1h, and then stirring on a magnetic stirrer until the powder is completely dissolved; S12 Slowly and evenly adding 2-5g of natural graphite (G), reducing the rotation speed to 160-200r / min, and stirring for 2-8h, so that the natural graphite powder exhibits a slight expansion during this pre-oxidation process; S13 When the natural graphite powder exhibits a slight expansion, turning off the magnetic stirrer, and slowly and evenly adding 8-14mL of H2SO4. H2O2, during the standing process, causes graphite to begin to expand violently, with its volume expanding to 800~1600mL, accompanied by the generation of a large amount of gas; S14 is added with H2O2 and left to stand for 2~5h, then the obtained black product is filtered, washed with ultrapure water until the filtrate is neutral, the solid is collected and placed in a 60~90℃ forced-air drying oven, dried for 18~24h to obtain the target product expanded graphite (EG).

[0007] In this method, expanded graphite (EG) is prepared from natural graphite using a liquid-phase method and a room-temperature stirring intercalation method. This method is beneficial for increasing the interlayer spacing of graphite. During the preparation of expanded graphite, the interlayer spacing is effectively increased. This structural change facilitates the rapid insertion and extraction of lithium ions, thereby improving the charging and discharging rate and the energy density of the battery. Furthermore, the prepared expanded graphite has excellent thermal conductivity, effectively transferring heat generated inside the battery to the outside and preventing thermal runaway. This is particularly important for fast-charging lithium-ion batteries, as high-rate charging and discharging generate a large amount of heat, which increases the thermal conductivity of the material. Additionally, during fast charging, the rapid insertion and extraction of lithium ions in the graphite anode leads to internal stress. The structural characteristics of expanded graphite give it good elasticity and buffering properties, effectively absorbing and releasing these internal stresses, thus extending the battery's cycle life.

[0008] Preferably, step (2) preparation of ball-milled expanded graphite (BMEG) includes: S21 Weighing 1-2.5g of expanded graphite into a clean ball mill steel jar, then weighing 100-140g of steel balls into the jar, tightening the jar lid, and then placing the ball mill jar in the vacuum chamber of the glove box for evacuation / inflation 2-5 times. After 0.5-1h, take it out and place it in a high-speed vibrating ball mill, tighten the screws, close the chamber door, set the time parameter to 10-40h, and start the machine; after the machine running time ends, take out the powder from the jar, weigh it and put it into a sample bottle, and record it as the intermediate product; S22 Weighing 0.5-1.5g of the intermediate product into a beaker, and adding 16-24mL of hydrochloric acid in sequence. Mix 6-10 mL of ethanol and 6-10 mL of ultrapure water in an oil bath at 50-80°C and stir for 12-48 hours. Then, divide the black turbid liquid into four 40-70 mL centrifuge tubes, add ultrapure water dropwise to each tube until the mass of each tube is equal, and centrifuge at 7000-9000 r / min. Discard the upper yellow-green liquid in the tube, add ultrapure water dropwise again to balance the liquid, and repeat the centrifugation 3-6 times to obtain black powder at the bottom of the tube. Then, freeze the tube for 1-4 hours. After all the solid and residual liquid in the tube are frozen, place it in a freeze dryer and remove it after 6-15 hours to obtain black powder. Collect the powder, weigh it, and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG).

[0009] This method innovatively further processes expanded graphite using mechanical ball milling to obtain ball-milled expanded graphite. The aim is to effectively increase the specific surface area of ​​expanded graphite through ball milling, thereby providing more reaction sites for active materials. This is beneficial for increasing the reaction rate of lithium-ion insertion and extraction, improving the charge-discharge performance of the battery. Furthermore, ball milling effectively breaks down the expanded graphite particles, reducing their particle size and shortening the diffusion path of lithium ions in the negative electrode material. This helps accelerate the diffusion rate of lithium ions, improving the rate performance of the battery. Ball milling also improves the electronic conductivity of expanded graphite, making it easier to transfer electrons. This is beneficial for improving the charge-discharge rate and efficiency of the battery. Moreover, ball milling allows the expanded graphite particles to be more tightly packed together, increasing its tap density. This is beneficial for improving the energy density and power density of the battery.

[0010] As a preferred embodiment, step (3) of preparing carbon-coated ball-milled expanded graphite (C@BMEG) includes: S31 Using chemical vapor deposition, acetylene gas cracking is used as the carbon source for carbon coating experiments. 120-150 mg of ball-milled expanded graphite (BMEG) and 300-400 mg of pitch powder are weighed and placed in an agate mortar and ground for 10-40 min until uniform. Then, the powder is taken out and spread to cover the bottom of a small porcelain boat and placed in a tube furnace; S32 After connecting the gas path of the tube furnace, high-purity argon is first introduced for 20-50 min. Remove the air from the tube, then set the appropriate heating program, raising the temperature to 250-500℃ at a rate of 3-6℃ / min, and then holding it at that temperature for 15-30 minutes. Next, raise the temperature to 700-950℃ at a rate of 3-6℃ / min, open the acetylene / argon mixed gas cylinder valve, close the high-purity argon gas cylinder valve, hold the temperature for 0.5-1.5 hours, then open the high-purity argon gas cylinder valve again, close the mixed gas cylinder valve, and cool it to room temperature with the furnace before taking out the prepared carbon-coated ball-milled expanded graphite (C@BMEG).

[0011] In this method, carbon-coated spherically expanded graphite is prepared as the active material for the battery anode by processing spherically expanded graphite. Asphalt powder is added during the carbon coating process; its role is to fill the voids in the spherically expanded graphite, increasing the material's density and compressive strength. This helps improve the structural stability and conductivity of the electrode material. Furthermore, as a conductive material, the asphalt powder works with the carbon coating layer to form conductive pathways, enhancing the conductivity of the composite material. This helps accelerate the electrochemical reaction rate and improve the battery's charge-discharge performance. Moreover, the carbon-coated spherically expanded graphite prepared using this method allows for more precise control of the carbon source decomposition and deposition process through segmented heating and holding, resulting in a uniform and dense carbon coating layer. This method ensures that the quality and thickness of the carbon coating layer reach optimal levels, improving the electrochemical performance of the electrode material. The carbon coating layer also helps form a stable solid electrolyte interphase (SEI) film, which plays a crucial role in protecting the electrode material, preventing electrolyte decomposition, and improving battery performance.

[0012] This invention discloses a carbon-coated preparation method for graphite anode materials used in fast-charging lithium-ion batteries. The carbon-coated ball-milled expanded graphite prepared by this invention can effectively construct a stable SEI film by building an amorphous carbon coating layer. At the same time, it can effectively mitigate the volume expansion effect caused by solvation co-intercalation of lithium during the charging and discharging process, thereby further improving the rate performance and cycle stability of the material. The advantages of this invention are its thermal stability, high conductivity, non-toxicity and harmlessness, and it can be widely used in the field of new energy lithium-ion batteries. Attached Figure Description

[0013] Figure 1 This is a process for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries.

[0014] Figure 2 This is a graph showing the room-temperature cycling performance of a pouch cell assembled from carbon-coated ball-milled expanded graphite (C@BMEG) as the negative electrode material. The C@BMEG prepared in Example 4 retained 89.83% of its capacity after 1000 cycles at room temperature.

[0015] Figure 3 This is a graph showing the rate charging performance of the C@BMEG assembled pouch cell prepared in Example 4. The capacity percentage in the 4C constant current range is 93.54%, and the capacity percentage in the 8C constant current range is 31.50%.

[0016] Figure 4 This is a graph showing the rate discharge performance of the C@BMEG assembled pouch cell prepared in Example 4. The 4C capacity retention is 99.52%, and the 8C capacity retention is 69.72%. Detailed Implementation Example 1

[0017] This invention provides a method for preparing carbon coating on graphite anode materials for fast-charging lithium-ion batteries, comprising the following steps: Step (1) Preparation of expanded graphite (EG) Weigh 12g of (NH4)2S2O8 powder into a 4000mL glass beaker, slowly add 80mL of H2SO4 into the beaker, then sonicate the mixture for 0.2h, and then stir it on a magnetic stirrer until the powder is completely dissolved. S12 slowly and evenly add 2g of natural graphite (G), reduce the rotation speed to 160r / min, and stir for 2h, so that the natural graphite powder will exhibit a slight expansion phenomenon during this pre-oxidation process; When the natural graphite powder underwent a slight expansion, turn off the magnetic stirrer and slowly and evenly add 8 mL of H2O2. During the standing process, the graphite began to expand violently, and its volume expanded to 800 mL, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for 2 hours. Then, filter the black product and wash it with ultrapure water until the filtrate is neutral. Collect the solid and put it into a 60°C forced-air drying oven. After drying for 18 hours, the target product expanded graphite (EG) is obtained. Step (2) Preparation of ball-milled expanded graphite (BMEG) S21 Weigh 1g of expanded graphite and place it into a clean ball mill jar. Then weigh 100g of steel balls and place them into the jar. Tighten the jar lid and place the ball mill jar in the vacuum chamber of the glove box for evacuation / inflation twice. After 0.5 hours, remove the jar and place it in a high-speed vibrating ball mill. Tighten the screws, close the chamber door, set the time parameter to 10 hours, and start the machine. After the machine runs for the required time, remove the powder from the jar, weigh it, and put it into a sample bottle. This powder is recorded as an intermediate product. S22 Weigh 0.5g of intermediate product into a beaker, add 16mL of hydrochloric acid, 6mL of ethanol, and 6mL of ultrapure water in sequence, and stir in a 50℃ oil bath for 12h. Then, divide the black turbid liquid into four 40mL centrifuge tubes, add ultrapure water dropwise until each tube has an equal mass, and centrifuge at 7000r / min. Discard the upper yellow-green liquid in the tube, add ultrapure water dropwise again to balance the liquid, and repeat the centrifugation 3 times to obtain black powder at the bottom of the tube. Then freeze in a refrigerator for 1h. After the solid and residual liquid in the tube are completely frozen, put it into a freeze dryer. After 6h, take it out to obtain black powder. Collect the powder, weigh it, and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG). Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) S31 adopts chemical vapor deposition method and uses acetylene gas cracking as carbon source to carry out carbon coating experiment. 120mg ball milled expanded graphite (BMEG) and 300mg pitch powder are weighed and placed in an agate mortar and ground for 10min until uniform. Then, the mixture is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S32, first introduce high-purity argon for 20 minutes to purge the air from the tube. Then, set the appropriate heating program and raise the temperature to 350°C at a rate of 3°C / min. Hold the temperature for 15 minutes, then raise the temperature to 800°C at a rate of 3°C / min. Open the acetylene / argon mixed gas cylinder valve and close the high-purity argon gas cylinder valve. Hold the temperature for 0.5 hours, then open the high-purity argon gas cylinder valve again and close the mixed gas cylinder valve. After cooling to room temperature with the furnace, take out the prepared carbon-coated ball-milled expanded graphite (C@BMEG). Example 2

[0018] Step (1) Preparation of expanded graphite (EG) Weigh 14g of (NH4)2S2O8 powder into a 4500mL glass beaker, slowly add 100mL of H2SO4 into the beaker, then sonicate the mixture for 0.5h, and then stir it on a magnetic stirrer until the powder is completely dissolved. S12 slowly and evenly add 3g of natural graphite (G), reduce the rotation speed to 170r / min, and stir for 4h, so that the natural graphite powder will exhibit a slight expansion phenomenon during this pre-oxidation process. When the natural graphite powder exhibits slight expansion, turn off the magnetic stirrer and slowly and evenly add 10 mL of H2O2. During the standing process, the graphite begins to expand violently, increasing in volume to 1000 mL, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for 3 hours. Then, filter the black product and wash it with ultrapure water until the filtrate is neutral. Collect the solid and put it into a 70°C forced-air drying oven. After drying for 20 hours, the target product expanded graphite (EG) is obtained. Step (2) Preparation of ball-milled expanded graphite (BMEG) S21 Weigh 1.5g of expanded graphite and place it into a clean ball mill jar. Then weigh 110g of steel balls and place them into the jar. Tighten the jar lid and place the ball mill jar in the vacuum chamber of the glove box for evacuation / inflation three times. After 0.6 hours, remove the jar and place it into a high-speed vibrating ball mill. Tighten the screws, close the chamber door, set the time parameter to 20 hours, and start the machine. After the machine runs for the required time, remove the powder from the jar, weigh it, and put it into a sample bottle. This powder is recorded as an intermediate product. S22 Weigh 0.8g of intermediate product into a beaker, add 18mL of hydrochloric acid, 7mL of ethanol, and 7mL of ultrapure water in sequence, and stir in a 60℃ oil bath for 24h. Then, divide the black turbid liquid into four 50mL centrifuge tubes, add ultrapure water dropwise until each tube has an equal mass, and centrifuge at 7500r / min. Discard the upper yellow-green liquid in the tube, add ultrapure water dropwise again to balance the liquid, and repeat the centrifugation 4 times to obtain black powder at the bottom of the tube. Then freeze in a refrigerator for 2h. After all the solid and residual liquid in the tube are frozen, put it into a freeze dryer. After 9h, take it out to obtain black powder. Collect the powder, weigh it, and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG). Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) S31 adopts chemical vapor deposition method and uses acetylene gas cracking as carbon source to carry out carbon coating experiment. 130mg ball milled expanded graphite (BMEG) and 320mg pitch powder are weighed and placed in an agate mortar and ground for 20min until uniform. Then, the mixture is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S32, first introduce high-purity argon for 30 minutes to purge the air from the tube. Then, set the appropriate heating program and raise the temperature to 400°C at a rate of 4°C / min. Hold the temperature for 20 minutes, then raise the temperature to 850°C at a rate of 4°C / min. Open the valve of the acetylene / argon mixed gas cylinder, close the valve of the high-purity argon gas cylinder, hold the temperature for 0.8 hours, then open the valve of the high-purity argon gas cylinder again, close the valve of the mixed gas cylinder, and remove the carbon-coated ball-milled expanded graphite (C@BMEG) after the furnace has cooled to room temperature. Example 3

[0019] Step (1) Preparation of expanded graphite (EG) Weigh 16g of (NH4)2S2O8 powder into a 5000mL glass beaker, slowly add 120mL of H2SO4 into the beaker, then sonicate the mixture for 0.8h, and then stir it on a magnetic stirrer until the powder is completely dissolved. S12 slowly and evenly add 4g of natural graphite (G), reduce the rotation speed to 180r / min, and stir for 6h, so that the natural graphite powder will exhibit a slight expansion phenomenon during this pre-oxidation process; When the natural graphite powder exhibits slight expansion, turn off the magnetic stirrer and slowly and evenly add 12 mL of H2O2. During the standing process, the graphite begins to expand violently, increasing in volume to 1200 mL, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for 4 hours. Then, filter the black product and wash it with ultrapure water until the filtrate is neutral. Collect the solid and put it into an 80℃ forced-air drying oven. After drying for 20 hours, the target product expanded graphite (EG) is obtained. Step (2) Preparation of ball-milled expanded graphite (BMEG) S21 Weigh 2.0g of expanded graphite and place it into a clean ball mill jar. Then weigh 120g of steel balls and place them into the jar. Tighten the jar lid and place the ball mill jar in the vacuum chamber of the glove box for evacuation / inflation four times. After 0.8h, remove the jar and place it in a high-speed vibrating ball mill. Tighten the screws, close the chamber door, set the time parameter to 30h, and start the machine. After the machine runs for the required time, remove the powder from the jar, weigh it, and put it into a sample bottle. This powder is recorded as an intermediate product. S22 Weigh 1.2g of intermediate product into a beaker, add 20mL hydrochloric acid, 8mL ethanol, and 8mL ultrapure water in sequence, and stir in a 70℃ oil bath for 36h. Then, divide the black turbid liquid into four 60mL centrifuge tubes, add ultrapure water dropwise until each tube has an equal mass, and centrifuge at 8000r / min. Discard the upper yellow-green liquid in the tube, add ultrapure water dropwise again to balance the liquid, and repeat the centrifugation 5 times to obtain black powder at the bottom of the tube. Then freeze in a refrigerator for 3h. After all the solid and residual liquid in the tube are frozen, put it into a freeze dryer. After 12h, take it out to obtain black powder. Collect the powder, weigh it, and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG). Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) S31 adopts chemical vapor deposition method and uses acetylene gas cracking as carbon source to carry out carbon coating experiment. 140mg ball milled expanded graphite (BMEG) and 350mg pitch powder are weighed and placed in an agate mortar and ground for 30min until uniform. Then, the mixture is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S32, first introduce high-purity argon for 40 minutes to purge the air from the tube. Then, set the appropriate heating program and raise the temperature to 450°C at a rate of 5°C / min. Hold the temperature for 25 minutes, then raise the temperature to 900°C at a rate of 5°C / min. Open the valve of the acetylene / argon mixed gas cylinder and close the valve of the high-purity argon gas cylinder. Hold the temperature for 1.2 hours, then open the valve of the high-purity argon gas cylinder again and close the valve of the mixed gas cylinder. After cooling to room temperature with the furnace, take out the prepared carbon-coated ball-milled expanded graphite (C@BMEG). Example 4

[0020] Step (1) Preparation of expanded graphite (EG) Weigh 18g of (NH4)2S2O8 powder into a 5500mL glass beaker, slowly add 140mL of H2SO4 into the beaker, then sonicate the mixture for 1h, and then stir it on a magnetic stirrer until the powder is completely dissolved. S12 slowly and evenly add 5g of natural graphite (G), reduce the rotation speed to 200r / min, and stir for 8h, so that the natural graphite powder will exhibit a slight expansion phenomenon during this pre-oxidation process; When the natural graphite powder exhibits slight expansion, turn off the magnetic stirrer and slowly and evenly add 14 mL of H2O2. During the standing process, the graphite begins to expand violently, increasing in volume to 1600 mL, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for 5 hours. Then, filter the black product and wash it with ultrapure water until the filtrate is neutral. Collect the solid and put it into a 90°C forced-air drying oven. After drying for 24 hours, the target product expanded graphite (EG) is obtained. Step (2) Preparation of ball-milled expanded graphite (BMEG) S21 Weigh 2.5g of expanded graphite and place it into a clean ball mill jar. Then weigh 140g of steel balls and place them into the jar. Tighten the jar lid and place the ball mill jar in the vacuum chamber of the glove box for evacuation / inflation 5 times. After 1 hour, take it out and place it in a high-speed vibrating ball mill. Tighten the screws, close the door, set the time parameter to 40 hours, and start the machine. After the machine runs for the required time, take out the powder from the jar, weigh it, and put it into a sample bottle. Record this as the intermediate product. S22 Weigh 1.5g of intermediate product into a beaker, add 24mL hydrochloric acid, 10mL ethanol, and 10mL ultrapure water in sequence, and stir in an 80℃ oil bath for 48h. Then, divide the black turbid liquid into four 70mL centrifuge tubes, add ultrapure water dropwise until each tube has an equal mass, and centrifuge at 9000r / min. Discard the upper yellow-green liquid in the tube, add ultrapure water dropwise again to balance the liquid, and repeat the centrifugation 6 times to obtain black powder at the bottom of the tube. Then freeze in a refrigerator for 4h. After all the solid and residual liquid in the tube are frozen, put it into a freeze dryer. After 15h, take it out to obtain black powder. Collect the powder, weigh it, and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG). Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) S31 adopts chemical vapor deposition method and uses acetylene gas cracking as carbon source for carbon coating experiment. 150mg ball milled expanded graphite (BMEG) and 400mg pitch powder are weighed and placed in an agate mortar and ground for 40min until uniform. Then, the mixture is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S32, first introduce high-purity argon for 50 minutes to purge the air from the tube. Then, set the appropriate heating program and raise the temperature to 500°C at a rate of 6°C / min. Hold the temperature for 30 minutes, then raise the temperature to 950°C at a rate of 6°C / min. Open the acetylene / argon mixed gas cylinder valve and close the high-purity argon gas cylinder valve. Hold the temperature for 1.5 hours, then open the high-purity argon gas cylinder valve again and close the mixed gas cylinder valve. After cooling to room temperature with the furnace, remove the prepared carbon-coated ball-milled expanded graphite (C@BMEG). Comparative Example 1

[0021] Step (1) Preparation of expanded graphite (EG) Weigh 12g of (NH4)2S2O8 powder into a 4000mL glass beaker, slowly add 80mL of H2SO4 into the beaker, then sonicate the mixture for 0.2h, and then stir it on a magnetic stirrer until the powder is completely dissolved. S12 slowly and evenly add 2g of natural graphite (G), reduce the rotation speed to 160r / min, and stir for 2h, so that the natural graphite powder will exhibit a slight expansion phenomenon during this pre-oxidation process; When the natural graphite powder underwent a slight expansion, turn off the magnetic stirrer and slowly and evenly add 8 mL of H2O2. During the standing process, the graphite began to expand violently, and its volume expanded to 800 mL, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for 2 hours. Then, filter the black product and wash it with ultrapure water until the filtrate is neutral. Collect the solid and put it into a 60°C forced-air drying oven. After drying for 18 hours, the target product expanded graphite (EG) is obtained. Step (2) Preparation of carbon-coated expanded graphite (C@EG) S21 adopts chemical vapor deposition method and uses acetylene gas cracking as carbon source to carry out carbon coating experiment. 120mg of expanded graphite (EG) is weighed and placed in an agate mortar and ground for 10min until uniform. Then it is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S22, first introduce high-purity argon for 20 minutes to purge the air from the tube. Then set the appropriate heating program, raising the temperature to 250°C at a rate of 3°C / min, and then holding it at that temperature for 15 minutes. Next, raise the temperature to 700°C at a rate of 3°C / min. Open the valve of the acetylene / argon mixed gas cylinder, close the valve of the high-purity argon gas cylinder, hold the temperature for 0.5 hours, then open the valve of the high-purity argon gas cylinder again, close the valve of the mixed gas cylinder, and remove the carbon-coated expanded graphite (C@EG) after cooling to room temperature with the furnace. Comparative Example 2

[0022] Step (1) Preparation of expanded graphite (EG) Weigh 18g of (NH4)2S2O8 powder into a 5500mL glass beaker, slowly add 140mL of H2SO4 into the beaker, then sonicate the mixture for 1h, and then stir it on a magnetic stirrer until the powder is completely dissolved. S12 slowly and evenly add 5g of natural graphite (G), reduce the rotation speed to 200r / min, and stir for 8h, so that the natural graphite powder will exhibit a slight expansion phenomenon during this pre-oxidation process; When the natural graphite powder exhibits slight expansion, turn off the magnetic stirrer and slowly and evenly add 14 mL of H2O2. During the standing process, the graphite begins to expand violently, increasing in volume to 1600 mL, accompanied by the generation of a large amount of gas. After adding H2O2 to S14, let it stand for 5 hours. Then, filter the black product and wash it with ultrapure water until the filtrate is neutral. Collect the solid and put it into a 90°C forced-air drying oven. After drying for 24 hours, the target product expanded graphite (EG) is obtained. Step (2) Preparation of carbon-coated expanded graphite (C@EG) S21 adopts chemical vapor deposition method and uses acetylene gas cracking as carbon source to carry out carbon coating experiment. 150mg of expanded graphite (EG) is weighed and placed in an agate mortar and ground for 40min until uniform. Then it is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S22, first introduce high-purity argon for 50 minutes to purge the air from the tube. Then set the appropriate heating program, raising the temperature to 300°C at a rate of 6°C / min, and then holding it at that temperature for 30 minutes. Next, raise the temperature to 750°C at a rate of 6°C / min. Open the valve of the acetylene / argon mixed gas cylinder, close the valve of the high-purity argon gas cylinder, and hold it at that temperature for 1.5 hours. Then open the valve of the high-purity argon gas cylinder again, close the valve of the mixed gas cylinder, and let the furnace cool to room temperature before taking out the prepared carbon-coated expanded graphite (C@EG).

[0023] The negative electrode materials obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were each packaged into button batteries for testing. The specific testing methods are as follows: Constant current charge and discharge test After the packaged button batteries were left to stand for 12 hours, they were connected to the Xinwei Battery Testing System for constant current charge-discharge testing. The voltage was set to 0.01~3 V, and the current was calculated based on the charge-discharge rate and the theoretical capacity of the graphite anode material.

[0024] Soft-pack battery testing The negative electrode materials (CMC:SBR:SP) were uniformly mixed in an aqueous solution at a mass ratio of 96:1.3:1.5:1.2, coated onto copper foil, dried at 105℃, and then rolled and slit to form a negative electrode sheet. The positive electrode materials (PVDF:CNTs) were uniformly mixed in an NMP solution at a mass ratio of 97.5:1.25:1.25, coated onto aluminum foil, dried at 108℃, and then rolled and slit to form a positive electrode sheet. The positive and negative electrode sheets were assembled into a 700mAh soft-pack battery cell. Charge-discharge tests were conducted at 4C-8C rates, with a charge-discharge voltage range of 3-4.2V.

[0025] Specific surface area test The specific surface area, pore volume, and pore size distribution of carbon materials generally have a significant impact on their electrochemical performance. Specific surface area is typically measured using a low-temperature nitrogen adsorption / desorption method. In this experiment, an ASAP2020M fully automated specific surface area and porosity analyzer (McMc Instruments, Inc., USA) was used to determine the nitrogen adsorption / desorption curves of the samples at liquid nitrogen temperature.

[0026]

[0027] From Table 1 and Figure 2It can be seen that the negative electrode materials prepared in Examples 1-4 have higher cycle stability than those prepared in the comparative examples, and the cycle stability after 200 cycles is greater than 95%. This is because asphalt powder was added during the carbon coating process in the examples. The addition of asphalt powder can fill the voids, increase the density, and improve the compactness and compressive strength of the material, thus helping to improve the structural stability and conductivity of the electrode material. Moreover, the carbon atoms in the asphalt powder undergo mutual diffusion and bonding with the ball-milled expanded graphite matrix, thereby enhancing the bonding force between the carbon coating layer and the graphite matrix. This is beneficial to improving the cycle stability and service life of the electrode material, significantly reducing the loss rate during recycling, and increasing the electrochemical stability of the negative electrode material.

[0028]

[0029] As shown in Table 2, the specific surface area of ​​the negative electrode materials prepared in Examples 1-4 is larger than that of the negative electrode materials prepared in the comparative examples. Figure 3 and Figure 4 It is evident that the negative electrode material prepared in Example 4 exhibits excellent rate performance. In the preparation process of this example, expanded graphite was further processed into ball-milled expanded graphite through ball milling. Ball milling effectively breaks down the expanded graphite particles, reducing their size and increasing the specific surface area of ​​the negative electrode material. This shortens the diffusion path of lithium ions within the negative electrode material. Furthermore, a larger specific surface area results in a larger contact area between the material and the electrolyte, accelerating electron transport and improving the battery's charge-discharge performance. A larger specific surface area also provides more reaction sites, increasing the reaction area and improving the utilization rate of the active material, which contributes to achieving high capacity and high rate performance in the battery.

[0030] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but as long as they are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for preparing carbon-coated graphite anode material for fast-charging lithium-ion batteries, characterized in that... include: Step (1) Preparation of expanded graphite (EG) S11 Weigh an appropriate amount of (NH4)2S2O8 powder and place it in a glass beaker. Slowly add H2SO4 to the beaker, then sonicate the mixture for a period of time, and then place it on a magnetic stirrer to stir until the powder is completely dissolved. S12 is slowly and evenly added to natural graphite (G), the rotation speed is reduced, and the mixture is stirred for a period of time to allow the natural graphite powder to undergo a slight expansion during this pre-oxidation process. When the natural graphite powder exhibits slight expansion, turn off the magnetic stirrer and slowly and evenly add H2O2. During the settling process, the graphite begins to expand violently and its volume expands, accompanied by the generation of gas. After adding H2O2 to S14, let it stand for a period of time, then filter the black product obtained by suction, wash it with ultrapure water until the filtrate is neutral, collect the solid and put it into a forced-air drying oven, and dry it to obtain the target product expanded graphite (EG). Step (2) Preparation of ball-milled expanded graphite (BMEG) S21 Weigh a certain mass of expanded graphite and place it into a clean ball mill steel jar. Then weigh a certain amount of steel balls and place them into the jar. Tighten the jar lid and place the ball mill jar in the vacuum chamber of the glove box for evacuation / filling. After a period of time, take it out and place it into a high-speed vibrating ball mill. Tighten the screws, close the door, set the time parameters, and start the machine. After the machine runs for the time required, take out the powder from the jar, weigh it, and put it into a sample bottle. Record it as an intermediate product. S22 Weigh a certain mass of intermediate product into a beaker, add hydrochloric acid, ethanol, and ultrapure water in sequence, place in an oil bath and stir for a period of time. Then, divide the black turbid liquid into four centrifuge tubes, add ultrapure water dropwise until each tube has an equal mass, place in a centrifuge and centrifuge. Discard the upper yellow-green liquid in the tube, add ultrapure water again to balance, repeat centrifugation, and obtain black powder at the bottom of the tube. Then place in a refrigerator to freeze. After the solid and residual liquid in the tube are completely frozen, place in a freeze dryer. After a period of time, take it out and obtain black powder. Collect the powder, weigh it and put it into a sample bottle to obtain ball-milled expanded graphite (BMEG). Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) S31 adopts chemical vapor deposition method, using acetylene gas cracking as carbon source for carbon coating experiment. A certain amount of ball milled expanded graphite (BMEG) and pitch powder are weighed and ground in an agate mortar until uniform. Then, it is taken out and spread on the bottom of a small porcelain boat and placed in a tube furnace. After connecting the gas line of the tube furnace to S32, first introduce high-purity argon to purge the air from the tube, then set the appropriate heating program to raise the temperature to a suitable level at a certain heating rate, hold the temperature for a period of time, then raise the temperature to the required level at a certain heating rate, open the acetylene / argon mixed gas cylinder valve, close the high-purity argon gas cylinder valve, hold the temperature for a period of time, then open the high-purity argon gas cylinder valve again, close the mixed gas cylinder valve, and after cooling to room temperature with the furnace, take out the prepared carbon-coated ball-milled expanded graphite (C@BMEG).

2. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (1) Preparation of expanded graphite (EG) includes weighing 12~18g of (NH4)2S2O8 powder and placing it in a 4000~5500mL glass beaker. Slowly add 80~140mL of H2SO4 to the beaker, and then sonicate the mixture for 0.2~1h. Then place it on a magnetic stirrer and stir until the powder is completely dissolved.

3. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (1) Preparation of expanded graphite (EG) includes slowly and uniformly adding 2~5g of natural graphite (G) to S12, reducing the rotation speed to 160~200r / min, and stirring for 2~8h, so that the natural graphite powder will exhibit micro-expansion during this pre-oxidation process.

4. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (1) Preparation of expanded graphite (EG) includes S13 When the natural graphite powder shows a slight expansion phenomenon, turn off the magnetic stirrer switch, slowly and evenly add 8~14mL H2O2, and let it stand so that the graphite begins to expand violently and the volume expands to 800~1600mL, accompanied by a large amount of gas.

5. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (1) Preparation of expanded graphite (EG) includes adding H2O2 to S14 and letting it stand for 2-5 hours. Then, the obtained black product is filtered and washed with ultrapure water until the filtrate is neutral. The solid is collected and placed in a 60-90℃ forced-air drying oven and dried for 18-24 hours to obtain the target product expanded graphite (EG).

6. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (2) Preparation of ball-milled expanded graphite (BMEG) includes S21 Weighing 1~2.5g of expanded graphite into a clean ball mill steel jar, then weighing 100~140g of steel balls into the jar, tightening the jar lid, and then placing the ball mill jar in the vacuum chamber of the glove box for evacuation / filling 2~5 times. After 0.5~1h, take it out and put it into a high-speed vibrating ball mill, tighten the screws, close the door, set the time parameter to 10~40h, and start the machine; after the machine running time ends, take out the powder in the jar, weigh it and put it into a sample bottle, and record it as intermediate product.

7. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (2) Preparation of ball-milled expanded graphite (BMEG) includes S22 weighing 0.5~1.5g of intermediate product and placing it in a beaker, adding 16~24mL of hydrochloric acid, 6~10mL of ethanol and 6~10mL of ultrapure water in sequence, and stirring in an oil bath at 50~80℃ for 12~48h. Then, the black turbid liquid is divided into four 40~70mL centrifuge tubes, and ultrapure water is added dropwise until each tube has an equal mass. The tubes are centrifuged at 7000~9000r / min. The upper yellow-green liquid in the tubes is discarded, and ultrapure water is added dropwise again to balance the liquid. The centrifugation is repeated 3~6 times to obtain black powder at the bottom of the tubes. Then, the tubes are frozen for 1~4h. After all the solid and residual liquid in the tubes are frozen, they are placed in a freeze dryer and removed after 6~15h to obtain black powder. The powder is collected, weighed and placed in a sample bottle to obtain ball-milled expanded graphite (BMEG).

8. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) includes S31 using chemical vapor deposition method, using acetylene gas cracking as carbon source for carbon coating experiment, weighing 120~150mg ball-milled expanded graphite (BMEG) and 300~400mg pitch powder and placing them in an agate mortar and grinding for 10~40min until uniform, then taking it out and spreading it on the bottom surface of a small porcelain boat and placing it in a tube furnace.

9. The method for preparing carbon coating of graphite anode material for fast-charging lithium-ion batteries according to claim 1, characterized in that: Step (3) Preparation of carbon-coated ball-milled expanded graphite (C@BMEG) includes connecting the gas path of the tube furnace to S32, first introducing high-purity argon for 20-50 min to remove air from the tube, then setting the corresponding heating program, heating to 250-500℃ at a heating rate of 3-6℃ / min, then holding for 15-30 min, then heating to 700-950℃ at a heating rate of 3-6℃ / min, opening the acetylene / argon mixed gas cylinder valve, closing the high-purity argon gas cylinder valve, holding for 0.5-1.5 h, then opening the high-purity argon gas cylinder valve again, closing the mixed gas cylinder valve, and cooling to room temperature with the furnace before taking out the prepared carbon-coated ball-milled expanded graphite (C@BMEG).

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