Soft carbon-coated hard carbon anode material with a pinning structure, preparation method and application

Abstract

The present invention provides a soft carbon-coated hard carbon anode material with a pinning structure, a preparation method thereof and an application thereof. The purpose is to fill and deposit carbon in and on the surface of micropores of porous hard carbon by chemical vapor deposition to form a soft carbon-coated hard carbon anode material with a pinning structure. The preparation method mainly includes the following steps: using porous hard carbon rich in long-range graphite domains as a substrate, adopting a chemical vapor deposition process to deposit carbon into relatively large micropores. As time goes on, after partial pores are deposited, carbon will gradually continue to be deposited on the surface of the hard carbon to form a soft carbon coating layer with a pinning structure. Since the carbon deposited in the pores is radially forced or covalently bonded to the outer coating layer, the structural stability of the coating material is significantly improved. The soft carbon layer with a pinning structure on the surface of the hard carbon has few defects, high electronic conductivity, a weakly solvated interface, and at the same time generates more closed pores inside the hard carbon, enabling the soft carbon-coated hard carbon anode material with a pinning structure to have both high capacity and high first-cycle efficiency for sodium storage characteristics, and has good application prospects in the field of high specific energy and long-life sodium-ion batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of energy material preparation, and particularly relates to a soft carbon-coated hard carbon anode material with a pinning structure, a preparation method thereof, and an application thereof. Background Art

[0002] Since the first commercialization of lithium-ion batteries in 1991, they have developed vigorously in the past few decades due to their excellent energy density and power density. However, due to the shortage of lithium resources and the relatively small domestic distribution, the price of lithium carbonate fluctuates greatly, resulting in an increase in the cost of lithium batteries. Therefore, it is necessary to explore alternative secondary battery systems based on low cost, uniform distribution, and relatively abundant elements. Compared with lithium, the crustal abundance of sodium accounts for about 2.75%, and its chemical properties are similar. Therefore, compared with lithium-ion batteries, sodium-ion batteries have a huge cost advantage and can be a reliable substitute for lithium-ion batteries in the energy storage field.

[0003] Graphite is the preferred anode material for lithium-ion batteries. Although there are many other options, carbon-based materials are still the most promising anode materials for large-scale applications of sodium-ion batteries. Since the radius of Na+ (1.02 Å) is larger than that of Li+ (0.76 Å), graphite cannot achieve satisfactory performance in sodium-ion batteries. Hard carbon materials are highly disordered in structure and difficult to graphitize, and have abundant sodium ion storage sites (such as the edges of short-range graphite domains, porous structures, functional groups, and heteroatom defects), making them the preferred anode materials for sodium-ion batteries. However, due to side reactions caused by the defect-rich surface and irreversible insertion of graphene layers, their first Coulomb efficiency is poor and the capacity is low. In addition, the low degree of order results in a lower electronic conductivity of hard carbon than that of graphite, seriously hindering the further commercial application of hard carbon anode materials.

[0004] To solve the problems of low initial Coulombic efficiency and low capacity caused by surface defects / open pores, it is urgent to coat the surface of hard carbon so that hard carbon has a faster and closer ion transport structure and also prevents the defective sites from directly contacting the electrolyte. In the recent literature "Weakly solvating few-layer-carbon interface toward high initial coulombic efficiency and cyclability hard carbon anodes (ACS Nano, 2024, 18, 1733-1743)", the authors coated a carbon layer structure with sp2 carbon on the surface of hard carbon using asphalt waste to construct a weakly solvating interface, effectively improving the electronic conductivity and initial Coulombic efficiency. However, simple coating easily leads to slippage at the interface and the presence of an air layer, resulting in unstable material structure and thus affecting the cycle stability. In the literature "Growing curly graphene layer boosts hard carbon with superior sodium-ion storage (Nano Research, 2023, 16, 9299-9309)", the authors successfully improved the electronic conductivity and shielded the surface defects by growing curly graphene layers on the surface of hard carbon, but the surface adsorption of curly graphene easily leads to a decrease in the plateau capacity. In the patent "A soft carbon-coated hard carbon composite material and its preparation method" (publication number CN117117126A), it is pointed out that a coupling agent is used to achieve chemical bond connection to coat soft carbon on the surface of hard carbon materials, improving the electronic conductivity of the materials and the cycle performance. However, although surface chemical bond connection can improve the material stability, it does not regulate the closed pore structure and increase the capacity.

[0005] Therefore, how to adopt a suitable coating strategy to simultaneously meet the requirements of high first efficiency, high capacity, and long cycle stability for hard carbon anode materials is one of the research focuses in the field of hard carbon anode materials. Summary of the Invention

[0006] The purpose of the present invention is to provide a soft carbon-coated hard carbon negative electrode material with a pinning structure, a preparation method and its application, using a porous hard carbon material rich in long-range graphite domains prepared by mixed salt catalysis-activation modification in the early stage as a matrix, using a chemical vapor deposition process, carbon is deposited into relatively large micropores, and as time goes on, after some holes are deposited, they will gradually continue to deposit on the hard carbon surface to form a soft carbon coating layer with a pinning structure. The soft carbon layer with a pinning structure on the hard carbon surface has fewer defects, high electronic conductivity, and a weakly solvated interface. At the same time, more closed pores are generated inside the hard carbon, and sodium ions can effectively enter and store during the charge and discharge process, achieving excellent sodium storage performance.

[0007] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0008] The present invention provides a soft carbon-coated hard carbon negative electrode material with a pinning structure, a preparation method and an application thereof, comprising the following steps:

[0009] (1) Weigh a certain mass of hard carbon matrix material and grind it thoroughly to reduce the agglomeration of hard carbon particles and ensure sufficient and uniform vapor deposition;

[0010] (2) placing the porous hard carbon material obtained in step (1) in a tube furnace, and performing a chemical vapor deposition coating reaction in an atmosphere of argon or nitrogen and a mixed gas containing carbon, so that carbon is fully deposited in the micropores on the surface of the porous hard carbon material;

[0011] (3) The precursor of step (2) is further heat treated for a certain period of time to further deposit carbon on the surface. The carbon deposited in the pores is bonded to the external coating layer through covalent bonds to form a pinning structure coating layer, thereby preparing a soft carbon-coated hard carbon negative electrode material with a pinning structure.

[0012] Preferably, in step (1), the grinding is carried out in the same direction and the grinding time is greater than 30 min.

[0013] Preferably, the particle size of the porous hard carbon matrix in step (1) is 3-20 microns, preferably 5-10 microns.

[0014] Preferably, the chemical vapor deposition coating reaction temperature in step (2) is 600-1000°C (preferably 800°C), and the heating rate is 5-20°C / min. The microcrystalline structure of the coating layer can be regulated by controlling the reaction temperature to obtain a suitable coating layer.

[0015] Preferably, the coating time is 1-4 h. By controlling the coating time, the thickness of the coating layer and the depth of pinning in the pores can be adjusted to obtain a suitable coating layer thickness and pore size of closed pores.

[0016] Preferably, acetylene, methane, toluene, and methyl ethyl sulfide are used as carbon sources. By changing the gas components, the microcrystalline structure of the carbon in the coating layer can be changed, and at the same time, heteroatom doping of the coating layer can be achieved. Due to its appropriate molecular size and carbonophilicity, acetylene forms a suitable pinning coating layer.

[0017] Preferably, in step (2), the carrier gas is argon or nitrogen, the gas flow rate is 50 - 150 mL / min, and the volume concentration of the carbon-containing gas in the mixed atmosphere is not less than 5% to ensure the formation of a soft carbon coating layer.

[0018] Preferably, the thickness of the soft carbon coating layer with a pinning structure is 2 - 10 nm, and the carbon content in the coating layer is 5 - 20%.

[0019] Preferably, the specific surface area of the soft carbon-coated hard carbon negative electrode material with a pinning structure is 2 - 15 m2 / g.

[0020] The present invention also provides a soft carbon-coated hard carbon negative electrode material with a pinning structure for a sodium-ion battery, which is characterized in that a soft carbon-coated hard carbon negative electrode material with a pinning structure is prepared by coating using any combination of the above technical solutions.

[0021] The present invention uses the above soft carbon-coated hard carbon negative electrode material with a pinning structure to prepare an electrode plate and assemble it into a sodium-ion battery.

[0022] The technical effects achieved by the present invention are as follows:

[0023] 1. The present invention uses porous hard carbon rich in long-range graphite domains as the matrix material. The method is simple and feasible, and can be used for large-scale production of a soft carbon coating layer with a pinning structure and a hard carbon negative electrode material with abundant closed pores.

[0024] 2. The soft carbon-coated hard carbon negative electrode material with a pinning structure prepared by the present invention uses porous hard carbon as the matrix material. Through chemical vapor deposition and pore filling, pore deposition is completed in some open pores and gradually continues to deposit on the surface to form a soft carbon coating layer with a pinning structure. A hard carbon material with a soft carbon coating layer structure and abundant closed pores is obtained.

[0025] 3. The soft carbon-coated hard carbon negative electrode material with a pinning structure of the present invention has the following advantages: Since the carbon deposited in the pores is radially forced or covalently bonded to the external coating layer, the soft carbon coating layer is tightly attached to the surface of the hard carbon, significantly improving the structural stability of the coating material. The soft carbon layer with a pinning structure has an interface with few defects and weak solvation, high electronic conductivity, and is rich in abundant closed pores, making the soft carbon-coated hard carbon negative electrode material with a pinning structure have both high capacity and high initial efficiency. Description of the Drawings

[0026] Figure 1Schematic diagram of the hard carbon negative electrode material before / after coating in Embodiment 1 of the present invention;

[0027] Figure 2 XRD pattern of the hard carbon negative electrode material before / after coating in Embodiment 1 of the present invention;

[0028] Figure 3 Raman pattern of the hard carbon negative electrode material before / after coating in Embodiment 1 of the present invention;

[0029] Figure 4 Nitrogen adsorption - desorption curve and pore size distribution diagram of the hard carbon negative electrode material before / after coating in Embodiment 1 of the present invention, where Figure 4 A is the nitrogen adsorption - desorption curve, Figure 4 B is the pore size distribution diagram;

[0030] Figure 5 is the transmission electron microscope image of the hard carbon negative electrode material after coating in Embodiment 1 of the present invention, where Figure 5A is Figure 5B the partial enlarged view;

[0031] Figure 6 Charge - discharge curve of the hard carbon negative electrode material before / after coating in Embodiment 1 of the present invention;

[0032] Figure 7 Rate performance of the hard carbon negative electrode material after coating in Embodiment 1 of the present invention;

[0033] Figure 8 Cycling performance of the hard carbon negative electrode material after coating in Embodiment 1 of the present invention. Detailed implementation manners

[0034] The present invention uses porous hard carbon rich in long - range graphite domains as the matrix material, and adopts a simple chemical vapor deposition method to deposit carbon into relatively large micropores. As the deposition time progresses, after some pores are deposited, the carbon will gradually continue to deposit on the surface of the hard carbon, forming a soft carbon coating layer with a pinned structure. Since the carbon deposited in the pore part is radially forced or covalently bonded to the external coating layer, the soft carbon coating layer is closely attached to the surface of the hard carbon, significantly improving the structural stability of the coated material.

[0035] The soft carbon - coated hard carbon negative electrode material with a pinned structure prepared by the present invention has an interface with few defects and weak solvation, high electronic conductivity and abundant closed pores; the interface with few defects and weak solvation realizes a high first - cycle Coulombic efficiency; the long - range graphite domains and abundant closed pores provide a good platform capacity for sodium ion storage; making the soft carbon - coated hard carbon negative electrode material with a pinned structure have both high capacity and high first - cycle efficiency.

[0036] The present invention provides a soft carbon - coated hard carbon negative electrode material with a pinned structure, a preparation method and its application, including the following steps:

[0037] (1) Weigh a certain mass of hard carbon matrix material and grind it thoroughly to reduce the agglomeration of hard carbon particles and ensure sufficient and uniform vapor deposition;

[0038] (2) placing the porous hard carbon material obtained in step (1) in a tube furnace, and performing a chemical vapor deposition coating reaction in an atmosphere of argon or nitrogen and a mixed gas containing carbon, so that carbon is fully deposited in the micropores on the surface of the porous hard carbon material;

[0039] (3) The precursor of step (2) is further heat treated for a certain period of time to further deposit carbon on the surface. The carbon deposited in the pores is bonded to the external coating layer through covalent bonds to form a pinning structure coating layer, thereby preparing a soft carbon-coated hard carbon negative electrode material with a pinning structure.

[0040] (4) One of the difficulties in the preparation process of the soft carbon-coated hard carbon negative electrode material with a pinning structure of the present invention is the need to control the rate of introducing argon or nitrogen and a carbon-containing mixed gas. If the rate of introducing the carbon-carrying mixed gas is too fast, the pores will not be fully filled, the closed pore diameter will be too large, and the coating layer will be uneven. If it is too slow, the pinning depth will be too deep, the pores will be blocked, and the closed porosity will be reduced. The flow rate of the carbon-containing carrier gas of the present invention is 50-150 mL / min, and the volume concentration of the carbon-containing gas in the mixed atmosphere is not less than 5%, ensuring a suitable soft carbon coating layer.

[0041] (5) The selection of coating reaction temperature is also critical. If the temperature is too low, the graphitization degree of the coating layer is too low and the defects are serious; if the temperature is too high, the graphitization degree is high and the interlayer spacing is reduced. The appropriate temperature range is 600-1000 ℃.

[0042] (6) A soft carbon-coated hard carbon material with a pinning structure prepared according to the above method, wherein the thickness of the soft carbon coating layer of the pinning structure is 2-10 nm, the carbon content of the coating layer is 5-20%, and the remainder is a porous hard carbon matrix, and the specific surface area of ​​the soft carbon-coated hard carbon negative electrode material with a pinning structure is 5-15 m2 / g. The soft carbon-coated hard carbon with a pinning structure can be used as a negative electrode material for sodium ion batteries. Experiments have shown that the soft carbon-coated hard carbon material with a pinning structure prepared by the present invention is used as a negative electrode material for sodium ion batteries. The rivets in the porous hard carbon micropores and the external soft carbon coating layer have radial forces or covalent bonds, which can make the soft carbon coating layer fit more closely to the hard carbon surface, significantly improving the structural stability of the material. The interface with few defects and weak solvation improves the first coulomb efficiency, and the abundant closed pores improve the platform capacity, thereby showing excellent electrochemical performance. Example

[0043] (1)Weigh 500 g of crushed coconut shells for pre-carbonization treatment. The carbonization temperature is 650 °C, the carbonization time is 3 h, the heating rate is 10 °C / min, and after cooling to room temperature, the carbonized material is obtained.

[0044] (2)Weigh 10 g of the pre-carbonized material and mix it evenly with the mixed salts Na2CO3 and K2CO3 according to the mass percentage. Among them, Na2CO3 and K2CO3 account for 50% of the total mass. The most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C. Carry out the catalytic-activation reaction under an argon or nitrogen atmosphere. The reaction temperature is 750 °C, the reaction time is 5 h, and after washing, the intermediate is obtained for standby.

[0045] (3)Place the intermediate obtained in step (2) in a high-temperature furnace under an argon or nitrogen atmosphere, heat it to 1300 °C for high-temperature reaction, with a heating rate of 10 °C / min and a holding time of 3 h. After cooling to room temperature, a porous hard carbon material is obtained.

[0046] (4)Place the porous hard carbon material obtained in step (3) in a tubular furnace under an atmosphere of a mixed gas of argon or nitrogen and acetylene for chemical vapor deposition coating reaction. The reaction temperature is 800 °C, the reaction time is 2 h, and the heating rate is 10 °C / min. Through experiments, it is verified that 10 °C / min is the optimal heating rate. The 2-h coating reaction at 800 °C mainly ensures the full deposition of acetylene molecules. Too fast a heating rate, too high a temperature, and too long a time will all affect the formation of the coating layer. After cooling to room temperature, a soft carbon-coated hard carbon material with a pinning structure is obtained.

[0047] (5)Fabrication of the electrode plate. Weigh the prepared soft carbon-coated hard carbon material with a pinning structure, conductive carbon black, and binder PVDF in a mass ratio of 80:10:10 and place them in a mortar and mix evenly. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to prepare a slurry, evenly coat it on an aluminum (Al) foil, dry it in a vacuum drying oven at 120 °C for 12 h, and cut it into a 12-mm circular negative electrode plate for standby.

[0048] (6)The coin cell assembly is carried out in a double-glove box filled with an argon atmosphere. The prepared hard carbon material electrode plate is used as the negative electrode plate, a commercial electrolyte 1M NaPF6 DME = 100 Vol% is used as the electrolyte, and a metal Na sheet is used as the counter electrode to assemble the coin cell.

[0049] (7)Perform charge-discharge tests on the assembled 2032-type coin cell on a constant current test system, with a voltage range of 0.001 - 2.5 V.

[0050] Figure 1This is a schematic diagram of the negative electrode material model before and after the soft carbon with pinning structure is coated with hard carbon; it can be seen from the figure that the hard carbon before coating is porous hard carbon rich in long-range graphite domains, and after the soft carbon with pinning structure is coated, the open pores on the hard carbon surface are blocked by the soft carbon layer, further increasing its closed porosity, thereby improving its capacity and initial efficiency.

[0051] Figure 2 XRD diagrams of soft carbon with pinning structure before and after coating hard carbon prepared in Example 1 of the present invention and the comparative example; it can be seen from the figure that after coating the soft carbon with pinning structure, the (002) peak position moves to a high angle, the interlayer spacing decreases, the degree of graphitization significantly increases, and a hard carbon material with fewer defects is formed, indicating the successful coating of the soft carbon coating layer.

[0052] Figure 3 These are Raman images of soft carbon with a pinning structure before and after being coated with hard carbon prepared in Example 1 of the present invention and the comparative example; as can be seen from the figure, the D band peak intensity decreases and the corresponding G band peak intensity increases after the soft carbon with a pinning structure is coated with hard carbon, indicating that the ordered graphite domains of the soft carbon coating layer lead to a decrease in its defect degree and a significant increase in the degree of graphitization.

[0053] Figure 4 Nitrogen adsorption-desorption curves and pore size distribution diagrams before and after the soft carbon-coated hard carbon with a pinning structure prepared in Example 1 of the present invention and the comparative example; the specific surface area of ​​the soft carbon with a pinning structure is significantly reduced after coating, and the pore size distribution does not show the existence of open pores, indicating that the pores of the soft carbon-coated hard carbon material with a pinning structure are in a closed-pore state.

[0054] Figure 5 is a TEM image of the soft carbon with a pinning structure before and after coating the hard carbon prepared in Example 1 of the present invention; it can be clearly seen that the long-range graphite domain microstructure is turbine-shaped, and the thickness of the soft carbon coating layer is about 3 nm, forming abundant closed pores, which is conducive to sodium storage in the platform area.

[0055] Figure 6 The charge-discharge curves before and after the soft carbon with pinning structure coated with hard carbon prepared in Example 1 of the present invention and the comparative example; at a current density of 20 mA g-1, the charge capacity before coating is 343 mAh g-1, and the first efficiency is 73%. After coating, the charge capacity is as high as 425 mAh g-1, and the corresponding first efficiency is increased to 81%. The high capacity and first efficiency are attributed to the soft carbon with pinning structure coated with hard carbon to form a soft carbon coating layer with rich closed-pore structure and fewer defects.

[0056] Figure 7Rate performance of the soft carbon-coated hard carbon with a pinning structure before and after preparation in Example 1 of the present invention; as can be seen from the figure, at a high current density of 2 A g-1, the reversible specific capacity is 249 mAh g-1. The excellent rate performance is attributed to the high conductivity and fast kinetics of the soft carbon coating layer with a pinning structure.

[0057] Figure 8 Cycling stability of the soft carbon-coated hard carbon with a pinning structure before and after preparation in Example 1 of the present invention; as can be seen from the figure, at a current density of 500 mA g-1, the capacity retention rate is 85% after 500 cycles. The excellent cycling stability is attributed to the fact that the soft carbon coating layer with a pinning structure increases the structural stability of the material, and at the same time, the abundant closed pore structure alleviates the volume shrinkage during repeated charge and discharge processes, enabling its structure to be retained. Example

[0058] (1) Weigh 500 g of crushed coconut shell for pre-carbonization treatment, with a carbonization temperature of 650 °C, a carbonization time of 3 h, a heating rate of 10 °C / min, and obtain the carbonized material after cooling to room temperature.

[0059] (2) Weigh 10 g of the pre-carbonized material and uniformly mix it with the mixed salts Na2CO3 and K2CO3 according to the mass percentage. Among them, Na2CO3 and K2CO3 account for 50% of the total mass, and the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, with a melting point of 710 °C. Carry out the catalytic-activation reaction under an argon or nitrogen atmosphere, with a reaction temperature of 750 °C and a reaction time of 5 h, and obtain the intermediate for standby after washing.

[0060] (3) Place the intermediate obtained in step (2) in a high-temperature furnace under an argon or nitrogen atmosphere, heat it to 1300 °C for a high-temperature reaction, with a heating rate of 10 °C / min and a holding time of 3 h, and obtain the porous hard carbon material after cooling to room temperature.

[0061] (4) Fabrication of the electrode sheet. Weigh the prepared porous hard carbon material for comparison, conductive carbon black, and binder PVDF in a mass ratio of 80:10:10 and place them in a mortar for uniform mixing. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry, uniformly coat it on an aluminum (Al) foil, dry it in a vacuum drying oven at 120 °C for 12 h, and cut it into a 12 mm circular negative electrode sheet for standby.

[0062] (5) The coin cell assembly is carried out in a double-glove box filled with an argon atmosphere. The prepared porous hard carbon material electrode sheet for comparison is used as the negative electrode sheet, 1M NaPF6 DME = 100 Vol% of the commercial electrolyte is used as the electrolyte, and a metal Na sheet is used as the counter electrode to assemble the coin cell.

[0063] (6) The assembled 2032 button cell was subjected to charge and discharge tests on a constant current test system, with the voltage range being 0.001 - 2.5 V. Example

[0064] (1) Weigh 500 g of crushed coconut shell and perform pre-carbonization treatment. The carbonization temperature is 650 °C, the carbonization time is 3 h, the heating rate is 10 °C / min, and after cooling to room temperature, the carbonized material is obtained.

[0065] (2) Weigh 10 g of the pre-carbonized material and uniformly mix it with the mixed salts Na2CO3 and K2CO3 by mass percentage. Among them, Na2CO3 and K2CO3 account for 50% of the total mass, the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C. Perform a catalytic-activation reaction under an argon or nitrogen atmosphere. The reaction temperature is 750 °C, the reaction time is 5 h, and after washing, the intermediate is obtained for standby.

[0066] (3) Place the intermediate obtained in step (2) in a high-temperature furnace under an argon or nitrogen atmosphere, heat it to 1300 °C for a high-temperature reaction, with a heating rate of 10 °C / min and a holding time of 3 h. After cooling to room temperature, a porous hard carbon material is obtained.

[0067] (4) Place the porous hard carbon material obtained in step (3) in a tubular furnace under an atmosphere of a mixed gas of argon or nitrogen and methane, and carry out a chemical vapor deposition coating reaction. The reaction temperature is 600 °C, the reaction time is 2 h, and the heating rate is 10 °C / min. After cooling to room temperature, a soft carbon-coated hard carbon material with a pinned structure is obtained.

[0068] Through the analysis of the above examples, since methane molecules are too small, the open pores are filled, and more closed pores cannot be formed, resulting in an insignificant increase in the plateau capacity. Example

[0069] (1) Weigh 500 g of crushed coconut shell and perform pre-carbonization treatment. The carbonization temperature is 650 °C, the carbonization time is 3 h, the heating rate is 10 °C / min, and after cooling to room temperature, the carbonized material is obtained.

[0070] (2) Weigh 10 g of the pre-carbonized material and uniformly mix it with the mixed salts Na2CO3 and K2CO3 by mass percentage. Among them, Na2CO3 and K2CO3 account for 50% of the total mass, the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C. Perform a catalytic-activation reaction under an argon or nitrogen atmosphere. The reaction temperature is 750 °C, the reaction time is 5 h, and after washing, the intermediate is obtained for standby.

[0071] (3) Place the intermediate obtained in step (2) in a high-temperature furnace under an argon or nitrogen atmosphere, heat it to 1300 °C for a high-temperature reaction, with a heating rate of 10 °C / min, a holding time of 3 h, and cool it to room temperature to obtain a porous hard carbon material.

[0072] (4) Place the porous hard carbon material obtained in step (3) in a tubular furnace under a mixed gas atmosphere of argon or nitrogen and toluene, and carry out a chemical vapor deposition coating reaction at a reaction temperature of 800 °C, a reaction time of 1 h, and a heating rate of 10 °C / min. After cooling to room temperature, a soft carbon-coated hard carbon material with a pinning structure is obtained.

[0073] Through the analysis of the above examples, due to the carbonophobicity of toluene molecules, toluene molecules tend to enter the open pores for deposition, and it is impossible to form a soft carbon coating layer. Although the plateau capacity has increased, the initial Coulombic efficiency is relatively low. Example

[0074] (1) Weigh 500 g of crushed coconut shell for pre-carbonization treatment, with a carbonization temperature of 650 °C, a carbonization time of 3 h, a heating rate of 10 °C / min, and cool it to room temperature to obtain a carbonized material.

[0075] (2) Weigh 10 g of the pre-carbonized material and uniformly mix it with the mixed salts Na2CO3 and K2CO3 according to the mass percentage. Among them, Na2CO3 and K2CO3 account for 50% of the total mass, and the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, with a melting point of 710 °C. Carry out a catalytic-activation reaction under an argon or nitrogen atmosphere at a reaction temperature of 750 °C, a reaction time of 5 h, and wash it to obtain an intermediate for standby.

[0076] (3) Place the intermediate obtained in step (2) in a high-temperature furnace under an argon atmosphere, heat it to 1300 °C for a high-temperature reaction, with a heating rate of 10 °C / min, a holding time of 3 h, and cool it to room temperature to obtain a porous hard carbon material.

[0077] (4) Place the porous hard carbon material obtained in step (3) in a tubular furnace under a mixed gas atmosphere of argon or nitrogen and methyl ethyl sulfide, and carry out a chemical vapor deposition coating reaction at a reaction temperature of 700 °C, a reaction time of 1 h, and a heating rate of 10 °C / min. After cooling to room temperature, a soft carbon-coated hard carbon material with a pinning structure is obtained.

[0078] Through the analysis of the above examples, since methyl ethyl sulfide molecules are easily pyrolyzed into heteroatom-doped carbon materials, the catalytic effect of heteroatoms easily leads to the decomposition of the electrolyte, affecting its cycle stability. At the same time, heteroatom doping easily leads to a decrease in the graphitization degree of the soft carbon layer and serious defects, resulting in a low initial Coulombic efficiency.

[0079] The foregoing has shown and described the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and what is described in the above embodiments and the specification is only the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection required by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a soft carbon-coated hard carbon negative electrode material with a pinning structure, characterized in that: The following steps are involved: (1) Weigh a porous hard carbon matrix material with a particle size of 5-10 μm and grind it for more than 30 minutes to reduce particle agglomeration; (2) placing the hard carbon material obtained in step (1) in a tube furnace, and carrying out a chemical vapor deposition coating reaction in a mixed gas atmosphere of argon and acetylene, so that carbon is fully deposited in the micropores on the surface of the porous hard carbon material; heating the temperature to 800-1000°C at a heating rate of 10-20°C / min, and carrying out a chemical vapor deposition reaction for 2-4 hours; wherein the volume concentration of acetylene is 5%-20% of the mixed gas, and the gas flow rate is 50-150 mL / min; (3) Based on step (2), heat treatment is continued for 1-2 hours, so that the carbon produced by acetylene cracking is deposited in the micropores on the surface of the hard carbon to form closed pores, and at the same time, it is combined with the external soft carbon coating layer through covalent bonds to form a pinned structure coating layer with a thickness of 2-10 nm; the carbon content of the soft carbon coating layer is 5-20%, and the specific surface area of ​​the hard carbon after coating is 2-15 m² / g.

2. A soft carbon-coated hard carbon negative electrode material having a pinning structure, the material being obtained by the preparation method according to claim 1; wherein: The soft carbon-coated hard carbon negative electrode material has a defect-free, weakly solvated interface, and the coating layer is tightly adhered to the hard carbon surface.

3. Sodium ion battery, characterized by: The invention comprises a negative electrode material for a sodium ion battery, wherein the negative electrode material is a soft carbon-coated hard carbon negative electrode material having a pinning structure and prepared by the preparation method according to claim 1.

Images