Manufacturing method of soft carbon-hard carbon coated graphite negative electrode material

Abstract

The invention relates to a manufacturing method of a soft carbon-hard carbon coated graphite negative electrode material. The invention provides a manufacturing method of a negative electrode material with both rate characteristics and energy density and relatively low cost. The manufacturing method comprises the following steps: step 1, performing surface pore-forming on graphite powder by adopting pulse high-pressure carbon dioxide gas phase etching; and Step 2, coating hard carbon with a water-based liquid phase: S2.1, coating a polyfurfuryl alcohol-based hard carbon precursor with a liquid phase; s2.2, carrying out in-situ thermal cross-linking curing treatment; s2.3, carrying out primary carbonization treatment; the highest process temperature of the primary carbonization treatment is 600-900 DEG C; the hard carbon coated graphite composite powder with the core-shell structure is formed, the weight gain rate of the hard carbon coated graphite composite powder ranges from 4% to 20%, and the BET specific surface area is larger than or equal to 10 square meters per gram. And step 3, performing pulse high-pressure vapor deposition to form soft carbon, performing hole sealing treatment, performing carbonization treatment in the same high-temperature and high-pressure heating furnace once, cooling the material to a temperature interval of 180-360 DEG C without discharging from the furnace, continuing pulse high-pressure vapor deposition to form soft carbon, and performing hole sealing to obtain the soft carbon-hard carbon coated graphite negative electrode material. The BET specific surface area is reduced to be less than or equal to 3.5 square meters per gram.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion secondary batteries, and in particular to an environmentally friendly, high-efficiency, and low-cost manufacturing method for anode materials that combine fast charging and high energy density. Background Technology

[0002] Graphite, as the negative electrode active material of lithium-ion batteries, has high specific capacity, good electrochemical reversibility, relatively low volume expansion rate, high electronic conductivity, and is widely available as a raw material, making it the mainstream negative electrode material for lithium-ion secondary batteries.

[0003] Commercially available graphite anode materials mainly include artificial graphite and pitch-coated modified natural graphite. Pitch-coated modified natural graphite has the advantages of low cost and high compaction density. Its main disadvantage is that the anisotropic, scaly polycrystalline nature of natural graphite means that lithium ions are inserted primarily from the end face, resulting in a small effective insertion or extraction area, poor charge / discharge rate characteristics, and a tendency for lithium plating during fast charging, leading to compromised battery safety. Pitch-coated natural graphite, after carbonization, can reduce the specific surface area of ​​pure natural graphite powder. The main technical principle is that the coated pitch forms a liquid mesophase at high temperatures, which, under capillary action, can fill the natural graphite powder. The open micropores of graphite powder significantly improve the initial efficiency of the battery; however, the cycle performance of pitch-coated natural graphite powder is far inferior to that of artificial graphite. The interfacial bonding strength between the soft carbon coating layer formed by conventional pitch-coated natural graphite and the graphite core is not high enough. During the charge and discharge cycle of the battery, the corresponding negative electrode active material undergoes expansion and contraction strain. The soft carbon coating layer formed by pitch coating is prone to cracking or local peeling, or even pulverization. The fresh fracture surface between the electrolyte and the negative electrode active material will react and grow a new SEI film, which will consume the lithium source, leading to an increase in irreversible capacity and a decrease in battery cycle performance. In addition, the traditional method of coating natural graphite with asphalt typically employs a solid-phase coating process. This involves using an air jet mill to grind the asphalt raw material into a fine powder of about 3 micrometers, pre-grinding and shaping the natural graphite raw material, then mixing the two and heating the mixture to about 50°C above the softening point of the asphalt for coating. After cooling, the material is placed in a carbonization furnace for carbonization under an inert atmosphere. The coated asphalt undergoes physicochemical changes such as mesophase formation and high-temperature chain scission and dehydrogenation, forming a soft carbon shell mainly composed of amorphous carbon on the surface of the natural graphite powder. The modified natural graphite powder obtained by solid-phase coating of asphalt often has an uneven coating layer distribution, resulting in unsatisfactory cycle life and rate performance when used in lithium batteries.

[0004] The uniformity of asphalt coated with natural graphite powder in the liquid phase is improved compared with that of asphalt coated in the solid phase. The process of liquid phase coating asphalt often requires the use of organic solvents such as tetrahydrofuran or aromatic hydrocarbon solvents such as wash oil to dissolve the asphalt and prepare it into a glue solution. After the natural graphite powder is mixed and coated in the asphalt solution, it needs to be heated to remove the solvent, which poses a risk of flammability and explosion, as well as environmental pressure.

[0005] Currently, the mainstream approach in the power battery and energy storage battery industry is to use artificial graphite powder as the negative electrode active material. Conventional artificial graphite negative electrode materials are made from calcined needle-shaped coke or pitch-modified petroleum coke as raw materials. After crushing / shaping / carbonizing, they are subjected to high-temperature graphitization treatment in an Atchison graphitization furnace at a temperature range of 2900-3200℃. Alternatively, after high-temperature graphitization, they are cooled and then crushed / shaped, or further coated with pitch / carbonized to obtain modified artificial graphite powder. The crystal orientation of these modified artificial graphite powders is anisotropic compared to flake-shaped natural graphite powders. Compared to anisotropic characteristics, the polycrystalline structure of artificial graphite powder exhibits better isotropic characteristics in its crystal orientation. The powder surface is relatively smooth, with relatively fewer active sites and a smaller specific surface area. It has a higher initial efficiency than natural graphite, a relatively longer cycle life, and better rate performance than natural graphite. The main drawback of traditional methods for manufacturing artificial graphite powder as an anode material is that it must undergo high-temperature graphitization, which has a long processing cycle, high energy consumption, and significant burn-off of the resistive material filled outside the graphite crucible during high-temperature graphitization. As a result, the overall cost of artificial graphite anode materials is relatively high.

[0006] With the market demand for electric vehicles to solve range anxiety and reduce costs, the performance requirements for fast charging of anode active materials have increased from 1.3C in the early market to 4C or even 5C fast charging. Traditional asphalt-coated natural graphite or artificial graphite anode materials have encountered bottlenecks in rate performance and are prone to lithium plating during fast charging, making it difficult to meet the market demand for 5C or even 6C fast charging performance.

[0007] Hard carbon anode materials possess superior lithium-ion transport performance and excellent rate capability. However, their drawback lies in their low true density due to their microporous structure. The true density of pure hard carbon powder is generally between 1.50 and 1.65 g / cm³. When pure hard carbon powder is used as a lithium battery anode material, while improving the battery's rate capability, it significantly reduces the electrode's compaction density and the battery's energy density. The compaction density of conventional hard carbon anode materials is generally below 1.25 g / cm³, which is significantly lower than the market expectation of a compaction density of over 1.40 g / cm³ to meet the high energy density requirements of batteries.

[0008] To meet the requirements of fast charging while improving the energy density of the battery, literature CN117012936A proposes to granulate graphite particles with modified asphalt to obtain secondary particles with a hard carbon-soft carbon composite structure. In order to improve the bonding strength of the interface between the two, it is proposed to use pore-forming agents such as sodium hydroxide, potassium hydroxide, sodium carbonate or sodium bicarbonate to be mixed with graphite particles and then calcined to create pores on the surface of graphite powder.

[0009] Literature CN114068886A proposes to mix urea, sodium carbonate, or ammonium bicarbonate into graphite raw materials for surface etching and nitriding treatment of graphite powder. The calcination treatment is carried out in a conventional roller kiln, pusher kiln, rotary kiln, or box furnace at a temperature between 850-1100℃. The above-mentioned atmospheric pressure treatment process for surface etching and pore creation of graphite powder has the disadvantage of environmental pollution. In addition, the atmospheric pressure treatment process also has the disadvantage of uneven gas phase mass transfer between powders, resulting in insufficient uniformity of reaction pore creation and product consistency.

[0010] Literature CN 111628170A proposes a method for heat-treating petroleum coke raw powder in a carbon dioxide atmosphere at a temperature range of 600-700℃ to obtain porous petroleum coke. This porous petroleum coke is then mixed with modified asphalt and subjected to oxidation, carbonization, and high-temperature graphitization to manufacture hard carbon-graphite composite secondary particle anode materials. However, this process involves heating the petroleum coke raw powder in a carbon dioxide atmosphere at a temperature range of 600-700℃. At this temperature, the petroleum coke raw powder is not fully calcined, and the pores formed by the decomposition and gas production are uneven and have large pore sizes. At this temperature, carbon and carbon dioxide cannot undergo the Bourdelle reaction to produce carbon monoxide. The process for creating pores in the petroleum coke raw powder is also crude. Furthermore, this process is lengthy, and the high-temperature graphitization process has the disadvantages of high energy consumption and high production costs.

[0011] Literature CN115312731A proposes acid activation of graphite powder and surface oxidation etching to create pores at 200 to 500°C by introducing an oxidizing atmosphere. The oxidants of the modified asphalt include hydrogen peroxide, peracetic acid, sodium dichromate, chromic acid, potassium dichromate, etc. The modified asphalt is then coated and carbonized to form a hard carbon-graphite composite negative electrode active material. However, this method involves acid washing and activation on the surface of the graphite core, which raises environmental concerns. Similarly, the atmospheric pressure oxidation process also suffers from uneven pore formation.

[0012] Literature CN109037603A proposes a novel method for modifying hard carbon anode materials using pitch-based spherical porous doped materials. The method involves pulverizing high-temperature coal-based pitch, adding terephthalaldehyde as a crosslinking agent under nitrogen protection for crosslinking, followed by the addition of ammonium persulfate as an oxidizing agent and oxygen purging for oxidation, resulting in high-temperature crosslinked oxidized pitch. This is then followed by spray granulation, carbonization, and high-temperature graphitization. The modified hard carbon anode material obtained by this method exhibits good rate performance, with a 30C / 1C capacity retention greater than 98%, a reversible specific capacity greater than 400 mAh / g, an initial efficiency greater than 80%, and a capacity retention greater than 85% after 500 cycles. However, its true density is only 1.62 g / cm³. 3 Compacted density 1.25 g / cm³ 3 The values ​​are all too low and do not meet the requirements for high energy density in lithium batteries.

[0013] In order to overcome the above-mentioned shortcomings and deficiencies of the existing technology, this invention is proposed to provide a lithium-ion battery anode material with good rate capability, high energy density, low manufacturing cost, and environmentally friendly manufacturing process. Summary of the Invention

[0014] This invention proposes a method for manufacturing anode materials that balances rate capability and energy density, has low energy consumption, high raw material utilization, and low overall cost. The technical solution is described below: The manufacturing method of soft carbon-hard carbon coated graphite anode materials mainly includes the following process steps:

[0015] Step 1: Surface pore formation using pulsed high-voltage carbon dioxide vapor phase etching.

[0016] A pulsed high-pressure carbon dioxide gas with a pressure between 0.40-1.20 MPa is used to perform surface vapor phase etching on graphite powder raw materials in a loose or compacted state at a high temperature of 750-850℃, creating numerous micropores or mesopores on the surface of the graphite powder raw materials, and increasing the BET specific surface area of ​​the graphite powder by at least 20%.

[0017] Step 2. The graphite powder after the above vapor phase etching treatment is coated with hard carbon using an aqueous liquid phase coating process, which includes the following three steps:

[0018] S2.1. Liquid-phase coated polyfurfuryl alcohol-based hard carbon precursor

[0019] First, the above-treated graphite powder is added to an aqueous solution or emulsion. The slurry in the stirring is then subjected to controlled heating polymerization. A polyfurfuryl alcohol-based polymer coating layer is formed on the surface of the graphite powder through a liquid-solid interface polymerization process, serving as a precursor for the hard carbon shell.

[0020] S2.2. In-situ thermal cross-linking curing treatment

[0021] While continuing to stir, the slurry is heated to 97-107℃ for in-situ thermal crosslinking and curing treatment, and the polyfurfuryl alcohol-based polymer coating layer formed on the surface of the graphite powder is transformed into an insoluble and infusible thermosetting state.

[0022] S2.3. Primary carbonization treatment

[0023] After the slurry is cooled to room temperature, it is filtered, washed and dried. The composite powder is then put into a high-temperature and high-pressure heating furnace in a loose or compacted state for a primary carbonization treatment under vacuum or inert atmosphere. The maximum process temperature for the primary carbonization treatment is between 600-900℃. The resulting core-shell structure hard carbon-coated graphite composite powder has a weight gain rate of 4-20% compared with the graphite raw material before liquid phase coating, and a BET specific surface area greater than or equal to 10 m² / g.

[0024] Step 3. Pulse high-pressure vapor deposition to form soft carbon for pore sealing.

[0025] After the above-mentioned carbonization treatment is carried out in the same high-temperature and high-pressure heating furnace, the material is cooled to a temperature range of 180-360℃ and does not need to be removed from the furnace. Instead, it undergoes a sealing process to form soft carbon through pulsed high-pressure vapor deposition.

[0026] An inert gas with a pressure between 0.40 and 1.20 MPa is used as the carrier gas, and naphthenic oil liquid or other hydrocarbons are used as the carbon source. The micropores or mesopores unique to the hard carbon shell material on the surface of the hard carbon-coated graphite composite powder are used to adsorb the active gas formed after the carbon source evaporates or is thermally decomposed. After the subsequent high-temperature treatment, i.e., secondary carbonization treatment, a soft carbon material of pyrolytic carbon is formed on the surface of the powder, which achieves the technical purpose of pore sealing. The resulting soft carbon-hard carbon-coated graphite anode material has a BET specific surface area of ​​less than or equal to 3.5 m² / g.

[0027] The graphite powder raw material used in this invention has the following technical characteristics before processing: initial carbon content greater than 99.9%, average particle size D50 between 4-16 micrometers, D90 less than 30 micrometers, BET specific surface area greater than or equal to 4 square meters / gram, tap density less than or equal to 0.85 g / cm³, specific capacity greater than or equal to 350 mAh / g, and initial efficiency less than or equal to 92%. The finished soft-carbon-hard-carbon coated graphite anode material has the following technical characteristics: the average particle size D50 of the finished powder increases by 2-8 micrometers compared to the average particle size D50 of the raw powder, and the particle size D90 of the finished powder... The increase in particle size (D90) of the raw material powder is less than or equal to 12 micrometers; the weight gain of the finished powder is between 4-20%; the BET specific surface area of ​​the finished powder is less than or equal to 3.5 m² / g; the tap density is greater than or equal to 0.93 g / cm³, and the compacted density is greater than or equal to 1.45 g / cm³; the lithium-ion secondary battery made from the soft carbon-hard carbon coated graphite anode material manufactured using the above method has a specific capacity greater than or equal to 345 mAh / g, an initial efficiency greater than or equal to 93%; a 5C / 1C charging capacity ratio greater than or equal to 85%, and a capacity retention rate greater than or equal to 85% after 1000 cycles of 5C charge / 1C discharge.

[0028] To better understand the manufacturing method of this invention, it is now described in detail below. The manufacturing method of soft carbon-hard carbon coated graphite anode material mainly includes the following steps:

[0029] Step 1: Perform surface vapor phase etching treatment on graphite powder raw materials using pulsed high-pressure carbon dioxide gas.

[0030] Graphite powder raw materials are loosely packed or compacted and placed in a microporous material fixture with air permeability, mass transfer, and leak-proof functions. The material is then placed into a high-pressure container. After sealing the high-pressure container, a vacuum is applied, or an inert gas purging process is used. The graphite powder raw materials are heated under vacuum or inert atmosphere protection to a temperature range of 750-850℃. Carbon dioxide gas is then introduced into the high-pressure container, and the internal carbon dioxide atmosphere pressure is adjusted to between 0.40-1.20 MPa. A high-pressure Bourdelle reaction is then carried out under pressure and temperature holding conditions for 30-120 minutes. The internal gas is then discharged, and the internal atmosphere pressure of the high-pressure container is reduced to 0.15 MPa or below. This process of gas filling, pressurization, and pressure holding is repeated. The high-pressure Bourdul reaction-exhaust depressurization process under high temperature and high pressure conditions is carried out in a pulsed high-pressure cycle. The surface of the graphite powder is etched by the Bourdul reaction under high temperature and high pressure to achieve the technical purpose of creating pores on the surface of the graphite powder. The total consumption of carbon dioxide gas inside the high-pressure container is controlled to be between 30% and 200% of the material weight. After the graphite powder raw material is etched by the above pulsed high-pressure carbon dioxide gas, the graphite powder material is cooled to below 300°C under inert gas protection. The tooling and the material inside are removed from the outside of the high-pressure container to obtain graphite powder with micropores on the surface (G1-T). The weight loss rate of the graphite powder is greater than or equal to 0.70%, and the specific surface area increase rate is at least 20%.

[0031] Step 2. The graphite powder after the above vapor phase etching treatment is coated with hard carbon in an aqueous liquid phase.

[0032] The process of water-based liquid phase coating hard carbon includes three steps: liquid-solid interface polymerization coating of polyfurfuryl alcohol-based hard carbon precursor, in-situ thermal crosslinking and curing treatment, and primary carbonization treatment.

[0033] S2.1. Liquid-solid interfacial polymerization coating of polyfurfuryl alcohol-based hard carbon precursor

[0034] The graphite powder (G1-T) with micropores on its surface was placed in an aqueous solution and subjected to liquid-phase coating of polyfurfuryl alcohol-based hard carbon precursor under stirring. The in-situ polymerization reaction of furfuryl alcohol at the liquid-solid interface, i.e., on the surface of the graphite powder in the stirred slurry, was carried out under low temperature and low furfuryl alcohol concentration. Polyfurfuryl alcohol-based polymer was grown in situ on the surface of the graphite powder and served as the precursor of the hard carbon shell.

[0035] The aqueous solution comprises purified water (B11) as the main solvent, a nonionic surfactant (B12), furfuryl alcohol monomer (B13), an acid catalyst (B14), and a dispersant (B15); wherein (B11) is used in 4500 parts by weight, (B12) in 10-45 parts by weight, (B13) in 150-450 parts by weight, (B14) in 20-90 parts by weight, (B15) in 0-5 parts by weight, and (G1-T) in 500-1300 parts by weight; the liquid-solid mixture slurry is stirred and dispersed evenly, and the interfacial polymerization is controlled to be heated and stirred continuously in at least two steps, wherein at least one step adopts a low-temperature polymerization process, using a specification between (45-60℃) * (16-48h). The process involves at least a second or third step of medium-temperature polymerization, using a specification between (61-85℃)*(6-28h). By utilizing a combination of low-temperature and medium-temperature polymerization processes, and controlling the concentration of furfuryl alcohol monomer in the aqueous solution to maintain a relatively low level, the technical objective of preventing rapid self-polymerization of furfuryl alcohol in the aqueous solution under the action of an acid catalyst is achieved. Taking advantage of the kinetics of nucleation-growth-polymerization at the liquid-solid interface, the in-situ polymerization reaction of furfuryl alcohol monomer on the surface of graphite powder (G1-T) with micropores is controlled, i.e., at the liquid-solid interface, to grow polyfurfuryl alcohol-based polymer. The polyfurfuryl alcohol-based polymer is naturally wrapped around the surface of the microporous graphite powder and partially embedded in the micropores of the graphite powder surface.

[0036] S2.2. In-situ thermal cross-linking curing treatment

[0037] After interfacial polymerization, the water-based slurry is heated to 97-107℃ while being stirred. Through in-situ thermal crosslinking and curing reaction under high temperature or combined high pressure, the polyfurfuryl alcohol-based polymer structure formed on the surface of the graphite powder (G1-T) changes, achieving an insoluble and infusible thermosetting state after thermal crosslinking. Then, it is filtered, washed with pure water 1-3 times, and dried at 105-200℃ to obtain graphite composite semi-finished powder (PFA-GCore) coated with thermosetting hard carbon precursor. The weight gain rate of graphite composite semi-finished powder (PFA-GCore) compared with that before liquid phase coating is between 8-40%, and the increase in average particle size D50 is between 2-8 micrometers, and the increase in D90 is less than or equal to 12 micrometers.

[0038] S2.3. Primary carbonization treatment

[0039] The graphite composite semi-finished powder (PFA-GCore) coated with the above-mentioned thermosetting hard carbon precursor is subjected to a single carbonization treatment under vacuum or inert atmosphere. The maximum process temperature of the single carbonization treatment is between 600-800℃, and the carbonization time at the maximum process temperature is between 2-8 hours. After the single carbonization treatment, the temperature is cooled to 180-360℃, at which point a core-shell structure hard carbon coated graphite composite powder (HC-GCore) is formed, with graphite as the core and hard carbon coating layer as the shell. The BET specific surface area of ​​the composite powder (HC-GCore) is greater than or equal to 10 m² / g, and the surface of the hard carbon coating layer has open micropores or mesopores.

[0040] Step 3. Pulse high-pressure vapor deposition to form soft carbon for pore sealing.

[0041] The composite powder (HC-GCore) cooled to 180-360℃ is then continuously injected into a high-temperature, high-pressure heating furnace using naphthenic oil as a carbon source while maintaining the high temperature of 180-360℃ without being removed from the furnace. The resulting powder is then vapor-deposited using a pulsed high-pressure variable-pressure mode to form soft carbon, thus sealing the pores on the surface of the hard carbon coating layer.

[0042] Naphthenic oil undergoes partial vaporization or partial thermal decomposition at high temperatures of 180-360℃ and subsequently above 360℃, forming a gaseous carbon source precursor. High-pressure nitrogen or argon gas with a pressure between 0.40-1.2 MPa is used as the carrier gas, and a pulsed high-pressure variable pressure mode is employed to achieve mass transfer of the gaseous carbon source precursor within the pores of the composite powder aggregate. The micropores or mesopores on the surface of the hard carbon shell of the composite powder (HC-GCore) form active adsorption on the gaseous carbon source precursor. The carbon source precursor adsorbed on the surface undergoes complex thermal polymerization or thermal decomposition reactions at high temperatures of 180-360℃ and subsequently 360-1200℃, forming a soft carbon material of pyrolytic carbon on the surface of the composite powder (HC-GCore), thus achieving the technical objective of sealing the pores on the surface of the composite powder (HC-GCore). After sealing, the material is cooled to below 300℃, passed through a 500-mesh vibrating sieve, with the undersize material accounting for more than 95%. After demagnetization, a soft carbon-hard carbon coated graphite anode material with both fast charging and energy density characteristics is obtained.

[0043] The technical solution of this invention is further explained below. The pulsed high-pressure Butoul reaction treatment of graphite powder of this invention can achieve uniform gas-phase mass transfer between the pores of loosely packed or compacted graphite powder through a high-pressure carbon dioxide atmosphere and a variable pressure mode. The uniformity of the Butoul reaction is more uniform than that of the atmospheric pressure continuous airflow process. This invention controls the temperature of both the pulsed high-pressure Butoul reaction material and the carbon dioxide gas within the temperature range of 750-850℃. This is because at excessively low temperatures, the Butoul reaction is slow or even non-existent, while at excessively high temperatures, the oxidation reaction is uncontrolled. The highly reactive carbon elements on the graphite powder surface and the crystalline carbon elements may react rapidly with the carbon dioxide gas, resulting in uneven pore formation and the formation of relatively large mesopores instead of mesopores smaller than 50 nanometers or micropores smaller than 2 nanometers. If the carbon dioxide atmosphere pressure is too low, the depth of gas-phase dissolution pore formation will be limited, and the uniformity of gas-phase mass transfer within the pores between powder particles will be poor. If the carbon dioxide atmosphere pressure is too high, the combined high temperature is detrimental to the manufacturing cost and service life of the pressure vessel.

[0044] This invention utilizes graphite powder treated with a pulsed high-pressure Bourdelle reaction. The highly active carbon on its surface reacts with carbon dioxide to generate carbon monoxide, selectively forming mesopores with nanoscale or micropores with angstrom-scale pores. This microporous graphite powder undergoes liquid-phase coating in an aqueous solution or emulsion, forming a polyfurfuryl alcohol-based hard carbon precursor on its surface. The micropores on the graphite powder surface serve as nucleation sites for liquid-solid interfacial polymerization. This invention also leverages the kinetic advantage of low-temperature, low-furfuryl alcohol concentration liquid-solid interfacial polymerization compared to the self-polymerization of furfuryl alcohol in solution. In an aqueous solution, liquid-solid interfacial polymerization enables in-situ polymerization on the surface of microporous graphite powder to generate polyfurfuryl alcohol-based polymers while maintaining the independence of the powder particles. By subjecting the stirred slurry to a higher temperature in-situ thermal crosslinking and curing reaction, the polyfurfuryl alcohol-based polymer shell formed by the liquid-phase coating on the graphite powder surface undergoes thermal crosslinking and curing, forming… The insoluble and infusible thermosetting hard carbon precursor shell has a slightly higher specific surface area after the first carbonization of the thermosetting hard carbon precursor coating layer, generally greater than 10 m² / g. This is not conducive to the stability of aqueous slurry during battery manufacturing and reduces the initial efficiency of the battery. In this invention, after hard carbon coating graphite, a pulsed high-pressure vapor deposition process and a secondary carbonization treatment are used in the same furnace to form soft carbon. This seals the nanoscale and even angstrom-scale openings on the surface of the hard carbon coating layer, which can improve the process stability during battery manufacturing and increase the initial efficiency of the battery. The pulsed high-pressure vapor deposition process used in this invention to form soft carbon for pore sealing treatment can overcome the weakness of poor mass transfer and penetration in the traditional low-pressure chemical vapor deposition (LPCVD) process. It can achieve relatively better mass transfer uniformity between pores and the surface of powder in loose or compacted state, and achieve relatively complete vapor deposition of powder. After secondary carbonization treatment, a pyrolytic carbon-based soft carbon material is formed.

[0045] The manufacturing method of this invention uses water as a process solvent, which has the advantages of being environmentally friendly, having low energy consumption, high material utilization, high manufacturing efficiency, and low overall cost. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of the high-temperature and high-pressure heating furnace of the present invention. The process involves sequentially completing the primary carbonization and cooling treatments within the same furnace, followed by pulsed high-pressure vapor deposition and secondary carbonization to form pyrolytic soft carbon. Figure 1In the diagram, 1 is the insulation layer of the side wall of the heating furnace, 2 is the resistance wire heating element, 3 is the insulation layer of the heating furnace cover, 4 is the air inlet valve, 5 is the liquid inlet valve, 6 is the liquid inlet buffer tank, 7 is the vacuum valve, and 8 is the heating furnace cover; 9 is the stainless steel fiber sintered felt of the tooling, which serves the technical purpose of permeability, mass transfer, and preventing the leakage of graphite powder inside the tooling; 10 is the flange of the tooling, 11 is the tank of the high temperature and high pressure heating furnace, 12 is the loading hopper of the tooling, 13 is the external support of the hopper, 14 is the support frame at the bottom of the hopper, 15 is the quick-opening device, 16 is the bottom movable base of the heating furnace, and 17 is the insulation layer of the base.

[0048] To better illustrate the technical objectives, technical solutions, and beneficial effects of the present invention, further explanations are provided below in conjunction with specific embodiments.

[0049] Example 1: A method for manufacturing soft carbon-hard carbon coated graphite anode material. The natural graphite powder raw material before treatment has the following technical characteristics: the specific surface area of ​​the natural graphite powder after 12 shaping processes is 6.50 m² / g, the initial carbon content is greater than 99.9%, the particle size D10 is 10.3 μm, D50 is 13.5 μm, D90 is 24.3 μm, D100 is 37.5 μm, and the tap density is 0.75 g / cm³. The manufacturing method of the soft carbon-hard carbon coated graphite anode material mainly includes the following steps:

[0050] Step 1: Perform surface vapor phase etching treatment on graphite powder raw materials using pulsed high-pressure carbon dioxide gas.

[0051] The aforementioned natural graphite powder raw material was placed in a compacted state inside a specially made 310S stainless steel fixture with a top cover featuring breathable mass transfer and leak-proof properties, using a microporous stainless steel fiber felt. (See attached diagram in the instruction manual.) Figure 1 As shown, after sealing the high-pressure container, a vacuum was drawn, and the graphite powder raw material was heated to 200℃ under vacuum. After holding at this temperature for 2 hours, nitrogen gas with a pressure between 0.30-0.35MPa was introduced for protection. The material was heated to 780℃ and held for 2 hours at a heating rate of 5℃ / min. Then, the container was purged with carbon dioxide gas three times. Carbon dioxide gas was then introduced until the furnace pressure was between 0.70-0.80MPa. The temperature was held at 780℃ for 55 minutes, and then the pressure was released to 0.10MPa. This process was repeated for 1 hour using the Bourdelle pulse high-pressure cycle mode, for a total of 30 cycles. Then, nitrogen gas was used for purging and the container was purged three times. The material was then cooled to below 150℃ under nitrogen protection and removed from the furnace, resulting in graphite powder with micropores on the surface. The weight loss rate of the graphite powder was 1.85%, and the specific surface area increase rate was 90%.

[0052] Step 2. Coat the graphite powder after the above vapor phase etching process with hard carbon using an aqueous liquid phase coating:

[0053] S2.1. Liquid-solid interfacial polymerization coating of polyfurfuryl alcohol-based hard carbon precursor

[0054] The graphite powder (G1-T) with micropores on its surface was added to an aqueous solution and subjected to liquid-phase coating of polyfurfuryl alcohol-based hard carbon precursor under stirring. The aqueous solution included 4300 parts by weight of purified water (B11) as the main solvent, 30 parts by weight of nonionic surfactant polyvinylpyrrolidone PVP-K30 (B12), 320 parts by weight of furfuryl alcohol monomer FA (B13), and 36 parts by weight of oxalic acid OA (B14) as the acid catalyst. Porous graphite powder (G1-T) --- 1200 parts by weight; the liquid-solid mixture slurry is stirred and dispersed evenly, and interfacial polymerization is controlled. The following four steps are followed by heating and continuous stirring: the first step is a low-temperature polymerization process, using a specification of 48℃*24h; the second step is a low-temperature polymerization process, using a specification of 55℃*12h; the third step is a medium-temperature polymerization process, using a specification of 66℃*9h; the fourth step is a medium-temperature polymerization process, using a specification of 80℃*9h; in-situ polycondensation reaction occurs at the liquid-solid interface, and polyfurfuryl alcohol-based polymer is grown;

[0055] S2.2. In-situ thermal cross-linking curing treatment

[0056] After interfacial polymerization, the water-based slurry was heated to 99°C while being stirred. Through in-situ thermal crosslinking and curing reaction at high temperature for 9 hours, the polyfurfuryl alcohol-based polymer structure formed on the surface of the graphite powder (G1-T) changed, achieving an insoluble and infusible thermosetting state after thermal crosslinking. Then, it was filtered, washed three times with pure water, and dried at 105-200°C to obtain graphite composite semi-finished powder (PFA-GCore) coated with thermosetting hard carbon precursor. The weight gain of graphite composite semi-finished powder (PFA-GCore) was 13% compared with the initial liquid phase before coating, and the increase in average particle size D50 was 3.7 micrometers, and the increase in D90 was 5.8 micrometers.

[0057] S2.3. Primary carbonization treatment

[0058] The graphite composite semi-finished powder (PFA-GCore) coated with the above-mentioned thermosetting hard carbon precursor was subjected to a first carbonization treatment under a vacuum atmosphere. The vacuum was maintained and the temperature was raised to 200°C and held for 3 hours. Then, the protection was changed to nitrogen atmosphere at an atmosphere pressure of 0.30 MPa. The temperature was raised to 360°C and held for 3 hours, then raised to 460°C and held for 3 hours, then raised to 600°C and held for 2 hours, and finally carbonized at the highest process temperature of 720°C for 3 hours. After the first carbonization treatment, the temperature was lowered to 200°C. At this time, a core-shell structure hard carbon coated graphite composite powder (HC-GCore) was formed, with graphite as the core and hard carbon coating as the shell. The BET specific surface area of ​​the composite powder (HC-GCore) was 28 m² / g. The surface of the hard carbon coating had open micropores or mesopores. The weight gain of the hard carbon coated graphite composite powder (HC-GCore) compared with the initial liquid phase coating was 6.7%.

[0059] Step 3. Pulse high-pressure vapor deposition to form soft carbon for pore sealing.

[0060] The composite powder (HC-GCore) cooled to 200℃ is then continuously injected into a high-temperature, high-pressure heating furnace while maintaining the high temperature of 200℃ without being removed from the furnace. Naphthenic oil is used as the carbon source, and a quantitative amount is injected into the furnace. The following pulsed high-pressure variable pressure mode is used for vapor deposition to form soft carbon, sealing the pores on the surface of the hard carbon coating layer:

[0061] Naphthenic oil undergoes partial vaporization or partial thermal decomposition in the high-temperature range of 200-360℃ and subsequently at temperatures above 360℃, forming a gaseous carbon source precursor. High-pressure nitrogen (0.60 MPa) is used as the carrier gas, and a pulsed high-pressure variable-pressure mode is employed for gas-phase deposition of the carbon source precursor: nitrogen is introduced to 0.60 MPa, and the high temperature and pressure are maintained for 1 hour. After 5 minutes, some gas is discharged from the furnace, and the furnace pressure is reduced to 0.25 MPa and maintained for approximately 55 minutes. This process is repeated for 2 hours per cycle. The pulsed pressure cycling cycle involves heating and holding at 200-600℃, followed by holding at 200℃ for 4 hours, 280℃ for 6 hours, 360℃ for 4 hours, 460℃ for 4 hours, 600℃ for 4 hours, 700℃ for 2 hours, and 900℃ for 4 hours. This allows for mass transfer of the gaseous carbon source precursor within the pores of the composite powder aggregate. The micropores or mesopores on the surface of the hard carbon shell of the composite powder (HC-GCore) facilitate mass transfer of the gaseous hard carbon precursor. Active adsorption is formed. The carbon source precursor adsorbed on the surface of the composite powder (HC-GCore) undergoes complex thermal polymerization or thermal decomposition reactions in the temperature range of 200-600℃, forming a soft carbon material of pyrolytic carbon type on the surface of the composite powder (HC-GCore), thus achieving the technical purpose of sealing the open pores on the surface of the composite powder (HC-GCore). After the pore sealing treatment, it is cooled to below 150℃, and after being taken out of the furnace, it is passed through a 500-mesh vibrating screen. The proportion of undersize material is greater than or equal to 99%. After demagnetization, it yields a product that combines fast charging and energy efficiency. The soft carbon-hard carbon coated graphite anode material exhibits the following density characteristics: a BET specific surface area of ​​1.33 m² / g and a tap density of 1.03 g / cm³; particle sizes of D10 (12.6 μm), D50 (16.8 μm), D90 (28.5 μm), and D100 (43.6 μm); a specific capacity of 358.6 mAh / g and an initial efficiency of 93.6%; a 5C / 1C charging capacity ratio of 92%; and a capacity retention rate of over 88% after 1000 cycles of 5C charge / 1C discharge.

Claims

1. A method for manufacturing a soft carbon-hard carbon coated graphite anode material, characterized in that, The manufacturing method of soft carbon-hard carbon coated graphite anode material includes the following steps: Step 1: Surface pore formation using pulsed high-voltage carbon dioxide vapor phase etching. A pulsed high-pressure carbon dioxide gas with a pressure between 0.40-1.20 MPa is used to perform surface vapor phase etching on graphite powder raw materials in a loose or compacted state at a high temperature of 750-850℃, creating numerous micropores or mesopores on the surface of the graphite powder raw materials, and increasing the BET specific surface area of ​​the graphite powder by at least 20%. Step 2. The graphite powder after the above vapor phase etching treatment is coated with hard carbon using an aqueous liquid phase coating process, which includes the following three steps: S2.

1. Liquid-phase coated polyfurfuryl alcohol-based hard carbon precursor First, the above-treated graphite powder is added to an aqueous solution or emulsion. The slurry in the stirring is then subjected to controlled heating polymerization. A polyfurfuryl alcohol-based polymer coating layer is formed on the surface of the graphite powder through a liquid-solid interface polymerization process, serving as a precursor for the hard carbon shell. S2.

2. In-situ thermal cross-linking curing treatment While continuing to stir, the slurry is heated to 97-107℃ for in-situ thermal crosslinking and curing treatment, and the polyfurfuryl alcohol-based polymer coating layer formed on the surface of the graphite powder is transformed into an insoluble and infusible thermosetting state. S2.

3. Primary carbonization treatment After the slurry is cooled to room temperature, it is filtered, washed and dried. The composite powder is then put into a high-temperature and high-pressure heating furnace in a loose or compacted state for a primary carbonization treatment under vacuum or inert atmosphere. The maximum process temperature for the primary carbonization treatment is between 600-900℃. The resulting core-shell structure hard carbon-coated graphite composite powder has a weight gain rate of 4-20% compared with the graphite raw material before liquid phase coating, and a BET specific surface area greater than or equal to 10 m² / g. Step 3. Pulse high-pressure vapor deposition to form soft carbon for pore sealing. After the above-mentioned carbonization treatment is carried out in the same high-temperature and high-pressure heating furnace, the material is cooled to a temperature range of 180-360℃ and does not need to be removed from the furnace. Instead, it undergoes a sealing process to form soft carbon through pulsed high-pressure vapor deposition. An inert gas with a pressure between 0.40 and 1.20 MPa is used as the carrier gas, and naphthenic oil liquid or other hydrocarbons are used as the carbon source. The micropores or mesopores unique to the hard carbon shell material on the surface of the hard carbon-coated graphite composite powder are used to adsorb the active gas formed after the carbon source evaporates or is thermally decomposed. After the subsequent high-temperature treatment, i.e., secondary carbonization treatment, a soft carbon material of pyrolytic carbon is formed on the surface of the powder, which achieves the technical purpose of pore sealing. The resulting soft carbon-hard carbon-coated graphite anode material has a BET specific surface area of ​​less than or equal to 3.5 m² / g.

2. The method for manufacturing the soft carbon-hard carbon coated graphite anode material according to claim 1, characterized in that, The raw graphite powder before processing has the following technical characteristics: initial carbon content greater than 99.9%, average particle size D50 between 4-16 micrometers, D90 less than 30 micrometers, BET specific surface area greater than or equal to 4 m² / g, tap density less than or equal to 0.85 g / cm³, specific capacity greater than or equal to 350 mAh / g, and initial efficiency less than or equal to 92%. The soft carbon-hard carbon coated graphite anode material has the following technical characteristics: the average particle size D50 of the finished powder is 2-8 micrometers larger than the average particle size D50 of the raw powder. The particle size D90 of the finished powder is less than or equal to 12 micrometers above the particle size D90 of the raw powder; the weight gain ratio of the finished powder is between 4-20%; the BET specific surface area of ​​the finished powder is less than or equal to 3.5 m² / g; the tap density is greater than or equal to 0.93 g / cm³, and the compacted density is greater than or equal to 1.45 g / cm³; the specific capacity is greater than or equal to 345 mAh / g, and the initial efficiency is greater than or equal to 93%; the 5C / 1C charging capacity ratio is greater than or equal to 85%, and the capacity retention rate after 1000 cycles of 5C charge / 1C discharge is greater than or equal to 85%.

3. The method for manufacturing the soft carbon-hard carbon coated graphite anode material according to claim 1, characterized in that, The manufacturing method of soft carbon-hard carbon coated graphite anode material includes the following steps: Step 1: Perform surface vapor phase etching treatment on graphite powder raw materials using pulsed high-pressure carbon dioxide gas. Graphite powder raw materials are loosely packed or compacted and placed in a microporous material fixture with permeable mass transfer and leak-proof functions. The material is then placed into a high-pressure container. After sealing the high-pressure container, a vacuum is applied, or the container is purged with inert gas. The graphite powder raw materials are heated under vacuum or inert atmosphere protection to a temperature range of 750-850℃. Carbon dioxide gas is then introduced into the high-pressure container, and the internal carbon dioxide atmosphere pressure is adjusted to between 0.40-1.20 MPa. A high-pressure Bourdelle reaction is then carried out under pressure and temperature holding conditions for 30-120 minutes. The internal gas is then discharged, and the internal atmosphere pressure of the high-pressure container is reduced to 0.15 MPa or below. This process of gas filling, pressurization, and pressure holding is repeated. The high-pressure Bourdul reaction-exhaust depressurization process under high temperature and high pressure conditions is carried out in a pulsed high-pressure cycle. The surface of the graphite powder is etched by the Bourdul reaction under high temperature and high pressure to achieve the technical purpose of creating pores on the surface of the graphite powder. The total consumption of carbon dioxide gas inside the high-pressure container is controlled to be between 30% and 200% of the material weight. After the graphite powder raw material is etched by the above pulsed high-pressure carbon dioxide gas, the graphite powder material is cooled to below 300°C under inert gas protection. The tooling and the material inside are removed from the outside of the high-pressure container to obtain graphite powder with micropores on the surface (G1-T). The weight loss rate of the graphite powder is greater than or equal to 0.70%, and the specific surface area increase rate is at least 20%. Step 2. The graphite powder after the above vapor phase etching treatment is coated with hard carbon in an aqueous liquid phase. The process of water-based liquid phase coating hard carbon includes three steps: liquid-solid interface polymerization coating of polyfurfuryl alcohol-based hard carbon precursor, in-situ thermal crosslinking and curing treatment, and primary carbonization treatment. S2.

1. Liquid-solid interfacial polymerization coating of polyfurfuryl alcohol-based hard carbon precursor The graphite powder (G1-T) with micropores on its surface was placed in an aqueous solution and subjected to liquid-phase coating of polyfurfuryl alcohol-based hard carbon precursor under stirring. The in-situ polymerization reaction of furfuryl alcohol at the liquid-solid interface, i.e., on the surface of the graphite powder in the stirred slurry, was carried out under low temperature and low furfuryl alcohol concentration. Polyfurfuryl alcohol-based polymer was grown in situ on the surface of the graphite powder and served as the precursor of the hard carbon shell. The aqueous solution comprises purified water (B11) as the main solvent, a nonionic surfactant (B12), furfuryl alcohol monomer (B13), an acid catalyst (B14), and a dispersant (B15); wherein (B11) is used in 4500 parts by weight, (B12) in 10-45 parts by weight, (B13) in 150-450 parts by weight, (B14) in 20-90 parts by weight, (B15) in 0-5 parts by weight, and (G1-T) in 500-1300 parts by weight; the liquid-solid mixture slurry is stirred and dispersed evenly, and the interfacial polymerization is controlled to be heated and stirred continuously in at least two steps, wherein at least one step adopts a low-temperature polymerization process, using a specification between (45-60℃) * (16-48h). At least one second or third step involves intermediate-temperature polymerization, using a specification between (61-85℃)*(6-28h). By utilizing a combination of low-temperature and intermediate-temperature polymerization processes, and controlling the concentration of furfuryl alcohol monomer in the aqueous solution to maintain a relatively low level, the technical objective of preventing rapid self-polymerization of furfuryl alcohol in the aqueous solution under the action of an acid catalyst is achieved. Taking advantage of the kinetics of nucleation-growth-polymerization at the liquid-solid interface, the in-situ polymerization reaction of furfuryl alcohol monomer on the surface of graphite powder (G1-T) with micropores is controlled to grow polyfurfuryl alcohol-based polymer. The polyfurfuryl alcohol-based polymer is naturally wrapped around the surface of the microporous graphite powder and partially embedded in the micropores of the graphite powder surface. S2.

2. In-situ thermal cross-linking curing treatment After interfacial polymerization, the water-based slurry is heated to 97-107℃ while being stirred. Through in-situ thermal crosslinking and curing reaction under high temperature or combined high pressure, the polyfurfuryl alcohol-based polymer structure formed on the surface of the graphite powder (G1-T) changes, achieving an insoluble and infusible thermosetting state after thermal crosslinking. Then, it is filtered, washed with pure water 1-3 times, and dried at 105-200℃ to obtain graphite composite semi-finished powder (PFA-GCore) coated with thermosetting hard carbon precursor. The weight gain rate of graphite composite semi-finished powder (PFA-GCore) compared with that before liquid phase coating is between 8-40%, and the increase in average particle size D50 is between 2-8 micrometers, and the increase in D90 is less than or equal to 12 micrometers. S2.

3. Primary carbonization treatment The graphite composite semi-finished powder (PFA-GCore) coated with the above-mentioned thermosetting hard carbon precursor is subjected to a single carbonization treatment under vacuum or inert atmosphere. The maximum process temperature of the single carbonization treatment is between 600-800℃, and the carbonization time at the maximum process temperature is between 2-8 hours. After the single carbonization treatment, the temperature is cooled to 180-360℃, at which point a core-shell structure hard carbon coated graphite composite powder (HC-GCore) is formed, with graphite as the core and hard carbon coating layer as the shell. The BET specific surface area of ​​the composite powder (HC-GCore) is greater than or equal to 10 m² / g, and the surface of the hard carbon coating layer has open micropores or mesopores. Step 3. Pulse high-pressure vapor deposition to form soft carbon for pore sealing. The composite powder (HC-GCore) cooled to 180-360℃ is then continuously injected into a high-temperature, high-pressure heating furnace using naphthenic oil as a carbon source while maintaining the high temperature of 180-360℃ without being removed from the furnace. The resulting powder is then vapor-deposited using a pulsed high-pressure variable-pressure mode to form soft carbon, thus sealing the pores on the surface of the hard carbon coating layer. Naphthenic oil undergoes partial vaporization or partial thermal decomposition at high temperatures of 180-360℃ and subsequently above 360℃, forming a gaseous carbon source precursor. High-pressure nitrogen or argon gas with a pressure between 0.40-1.2 MPa is used as the carrier gas, and a pulsed high-pressure variable pressure mode is employed to achieve mass transfer of the gaseous carbon source precursor within the pores of the composite powder aggregate. The micropores or mesopores on the surface of the hard carbon shell of the composite powder (HC-GCore) form active adsorption on the gaseous carbon source precursor. The carbon source precursor adsorbed on the surface undergoes complex thermal polymerization or thermal decomposition reactions at high temperatures of 180-360℃ and subsequently 360-1200℃, forming a soft carbon material of pyrolytic carbon on the surface of the composite powder (HC-GCore), thus achieving the technical objective of sealing the pores on the surface of the composite powder (HC-GCore). After sealing, the material is cooled to below 300℃, passed through a 500-mesh vibrating sieve, with the undersize material accounting for more than 95%. After demagnetization, a soft carbon-hard carbon coated graphite anode material with both fast charging and energy density characteristics is obtained.

4. The method for manufacturing the soft carbon-hard carbon coated graphite anode material according to claim 1, characterized in that, Lithium-ion secondary batteries are fabricated using the soft carbon-hard carbon coated graphite anode material manufactured according to claims 1-3.

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