Silicon-carbon negative electrode material and preparation method thereof

Abstract

The invention provides a silicon-carbon negative electrode material and a preparation method thereof, and particularly relates to the technical field of lithium battery negative electrode materials. The silicon-carbon negative electrode material comprises coating porous carbon, coating silicon deposited in channels of the coating porous carbon and an amorphous carbon layer coating the outer layer of the coating porous carbon, the coating porous carbon comprises porous carbon and a first silicon nitride coating; the first silicon nitride coating coats the interiors of pore channels of the porous carbon and the outer surface of the porous carbon; and the coating silicon comprises silicon and a second silicon nitride coating coated on the surface of the silicon. In the silicon-carbon negative electrode material provided by the invention, the porous carbon is protected through the first silicon nitride coating, side reaction is reduced, charge and discharge stress is buffered, and structural damage is prevented. The second silicon nitride coating relieves the stress of the silicon material and protects the structural integrity. The layered silicon nitride enhances the electrochemical performance, and the amorphous carbon layer reduces the side reaction, thereby improving the efficiency and service life of the battery, and ensuring the double improvement of the battery performance and structural stability.

Description

Technical Field

[0001] The present invention relates to the technical field of negative electrode materials for lithium batteries, and more particularly to a silicon-carbon negative electrode material and a preparation method thereof. Background Art

[0002] With the rapid growth of the consumer electronics and new energy vehicle industries, the market demand for lithium-ion batteries with high energy density and long cycle life is continuously increasing. Against this background, silicon negative electrode materials exhibit great application potential due to their excellent theoretical specific capacity (4200 mAh / g), which is approximately ten times that of commercial graphite negative electrodes (372 mAh / g), and their moderate working voltage (0.4 V).

[0003] However, during the charge-discharge cycling process, silicon negative electrodes undergo significant volume changes (exceeding 300%) due to the insertion and extraction of lithium ions. This repeated expansion and contraction of volume leads to the continuous destruction and thickening of the solid electrolyte interface membrane (SEI), thereby accelerating the consumption of the electrolyte and the loss of lithium ions, ultimately resulting in a sharp decline in the battery cycle life. Therefore, the volume expansion problem of silicon negative electrodes during the charge-discharge process is the main technical obstacle limiting their widespread application.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] One object of the present invention is to provide a silicon-carbon negative electrode material, aiming to solve at least one of the above technical problems in the prior art.

[0006] Another object of the present invention is to provide a preparation method for the silicon-carbon negative electrode material.

[0007] To achieve the above objects of the present invention, the following technical solutions are specifically adopted:

[0008] The first aspect of the present invention provides a silicon-carbon negative electrode material, comprising coated porous carbon, coated silicon deposited in the pores of the coated porous carbon, and an amorphous carbon layer coated on the outer layer of the coated porous carbon;

[0009] The coated porous carbon comprises porous carbon and a first silicon nitride coating;

[0010] The first silicon nitride coating is coated on the inner pores and the outer surface of the porous carbon;

[0011] The coated silicon comprises silicon and a second silicon nitride coating coated on the surface of the silicon;

[0012] The silicon nitride has a hexagonal layered structure with the chemical formula Si3Nx, where 1 < x < 4.

[0013] Further, by mass percentage, it includes 15% - 30% of nano - silicon, 20% - 30% of silicon nitride, 1% - 10% of amorphous carbon coating layer, and the balance is porous carbon.

[0014] Further, at least part of the porous carbon has a hollow structure; the diameter of the hollow structure of the porous carbon is 0.5 - 5 μm, and the hollow structure is a closed pore.

[0015] Further, the ratio of the porous carbon with a hollow structure to all the porous carbon ranges from 10% to 100%.

[0016] Further, the average particle size of the silicon is ≤ 5 nm.

[0017] Further, the Dv of the silicon - carbon negative electrode material 10 <6 μm, Dv 50 <10 μm, Dv 90 <20 μm.

[0018] The second aspect of the present invention provides a preparation method of the silicon - carbon negative electrode material, including the following steps:

[0019] A. Mix and heat a water - soluble carbon source in water to carry out a hydrothermal reaction to obtain a porous carbon precursor;

[0020] B. First carbonize the porous carbon precursor, and then carry out activation pore - forming treatment to obtain porous carbon;

[0021] C. Deposit a first silicon nitride coating on the porous carbon to obtain coated porous carbon;

[0022] D. Deposit silicon and a second silicon nitride coating on the coated porous carbon in sequence, and finally carry out carbon coating to obtain the silicon - carbon negative electrode material.

[0023] Further, step A further includes adding a polymer compound, mixing the polymer compound and the water - soluble carbon source in water and heating to carry out a hydrothermal reaction to obtain a porous carbon precursor;

[0024] The polymer compound includes at least one of polystyrene, polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polyimide, polymethyl methacrylate, and methyl methacrylate. The water - soluble carbon source includes one or several of potato starch, mung bean starch, wheat starch, sweet potato starch, water chestnut starch, lotus root starch, cassava starch, pea starch, and sucrose.

[0025] The feeding ratio of the polymer compound, water - soluble carbon source, and water is: 0 - 0.25:1:2.

[0026] Further, the preparation method includes at least one of the following features:

[0027] (1) In step A, the temperature of the hydrothermal reaction is 150 - 250 °C and the time is 2 - 5 h;

[0028] (2) In step B, the temperature of the carbonization is 700 - 900 °C and the time is 1 - 4 h;

[0029] (3) In step B, the temperature of the activation and pore formation is 700 - 900 °C and the time is 8 - 16 h;

[0030] (4) In step C, the method for depositing the first silicon nitride coating includes: introducing a silicon source and a nitrogen source into a reactor, depositing on the porous carbon, with the deposition temperature being 400 - 600 °C and the time being 1 - 3 h, to obtain the first silicon nitride coating;

[0031] (5) In step D, the method for depositing the second silicon nitride coating includes: introducing a silicon source and a nitrogen source into a reactor, depositing on the material to be deposited, with the deposition temperature being 400 - 600 °C and the time being 1 - 3 h, to obtain the second silicon nitride coating;

[0032] (6) In step D, the method for depositing silicon includes: introducing a silicon source into a reactor, depositing on the coated porous carbon, with the deposition temperature being 400 - 600 °C and the time being 2 - 5 h;

[0033] (7) In step D, the method for carbon coating includes: introducing a carbon source into a reactor, coating the material to be coated, with the deposition temperature being 400 - 600 °C and the time being 0.5 - 2 h, to obtain an amorphous carbon layer.

[0034] Further, the reactor includes but is not limited to a fluidized bed, a rotary kiln, a pyrolysis deposition furnace, a chemical vapor deposition furnace, etc.

[0035] Further, the silicon source includes at least one of silane, disilane, dimethylsilane, difluorosilane, trifluorosilane, tetrafluorosilane, chlorosilane, dichlorosilane, and trichlorosilane.

[0036] The carbon source includes at least one of acetylene, methane, ethane, propane, ethylene, and propylene.

[0037] Compared with the prior art, the present invention has at least the following beneficial effects:

[0038] In the silicon-carbon anode material provided by the present invention, the first silicon nitride coating is directly coated on the inner and outer surfaces of the pores of the porous carbon, avoiding the direct contact between the porous carbon and the external medium, thereby reducing the occurrence of side reactions; the first silicon nitride coating also plays a buffering role, absorbing the stress generated by volume expansion or contraction during the charge and discharge process, effectively preventing the destruction of the porous carbon structure. The second silicon nitride coating is used to relieve the stress generated by the internal silicon material during the charge and discharge process, reducing the influence of the stress on the external structure, thereby protecting the integrity of the overall structure. Both the first silicon nitride coating and the second silicon nitride coating adopt layered silicon nitride, which not only provides additional mechanical support but also provides additional capacity for the insertion and extraction of lithium ions, enhancing the electrochemical performance of the anode material.

[0039] In the present invention, the porous carbon with a hollow structure has an internal space that can relieve the volume expansion of the silicon material. The amorphous carbon layer as the outermost coating further reduces the occurrence of side reactions and also helps to improve the initial efficiency and cycle life of the lithium-ion secondary battery. Through this progressive structure design, the silicon-carbon anode material of the present invention enhances its structural stability and durability while improving the battery performance.

[0040] The preparation method provided by the present invention, during the preparation of the porous carbon precursor, by introducing a high molecular compound and performing carbonization pyrolysis and pore-forming treatment, a porous carbon with a hollow structure is successfully prepared. Subsequently, the chemical vapor deposition technique is used to precisely deposit layered silicon nitride, nanosilicon, and a carbon coating layer, which not only effectively inhibits the volume expansion of the silicon-carbon anode material during the charge and discharge process, but also the controllability and repeatability of the process make it very suitable for industrial-scale production. In addition, this fine structure design significantly improves the initial charge and discharge efficiency and cycle stability of the silicon-carbon anode material, thereby providing a strong material basis for the performance optimization of lithium-ion batteries. Description of the Drawings

[0041] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0042] Figure 1 It is a 13,000-fold cross-sectional scanning electron microscope image of the silicon-carbon anode material provided in Example 1 of the present invention;

[0043] Figure 2 It is a 13,000-fold cross-sectional elemental analysis diagram of the silicon-carbon anode material provided in Example 1 of the present invention;

[0044] Figure 3 The pore size distribution diagram of the porous carbon provided in Embodiment 1 of the present invention;

[0045] Figure 4 The first charge-discharge curve diagram of the lithium-ion button battery of the silicon-carbon negative electrode material obtained in Embodiment 1 of the present invention. Detailed implementation manners

[0046] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0047] The first aspect of the present invention provides a silicon-carbon negative electrode material, which includes coated porous carbon, coated silicon deposited in the pores of the coated porous carbon, and an amorphous carbon layer coated on the outer layer of the coated porous carbon;

[0048] The coated porous carbon includes porous carbon and a first silicon nitride coating;

[0049] The first silicon nitride coating is coated inside the pores and on the outer surface of the porous carbon;

[0050] The coated silicon includes silicon and a second silicon nitride coating coated on the surface of the silicon;

[0051] The silicon nitride has a hexagonal layered structure with the chemical formula Si3Nx, where 1 < x < 4.

[0052] In the silicon-carbon negative electrode material provided by the present invention, the first silicon nitride coating is directly coated inside the pores and on the outer surface of the porous carbon, avoiding the direct contact between the porous carbon and the external medium, thereby reducing the occurrence of side reactions; the first silicon nitride coating also plays a buffering role, absorbing the stress generated by volume expansion or contraction during the charge-discharge process, effectively preventing the destruction of the porous carbon structure. The second silicon nitride coating is used to relieve the stress generated by the internal silicon material during the charge-discharge process, reducing the influence of the stress on the external structure, thereby protecting the integrity of the overall structure. Both the first silicon nitride coating and the second silicon nitride coating adopt layered silicon nitride, which not only provides additional mechanical support, but also provides additional capacity for the insertion and extraction of lithium ions, enhancing the electrochemical performance of the negative electrode material. The amorphous carbon layer as the outermost coating further reduces the occurrence of side reactions and also helps to improve the first efficiency and cycle life of the lithium-ion secondary battery. Through this progressive structure design, the silicon-carbon negative electrode material of the present invention enhances its structural stability and durability while improving the battery performance.

[0053] It should be noted that the first silicon nitride coating covers the inner walls and outer surfaces of the pores of the porous carbon, ensuring that all surfaces of the porous carbon in contact with the outside world are coated with this coating.

[0054] Silicon nitride, with its high hardness and good mechanical strength, provides stability to the battery structure, effectively preventing structural damage caused by volume expansion and contraction during charge and discharge processes. This physical property is crucial for maintaining the long-term stability of the battery. The presence of the silicon nitride layer avoids the direct contact between the internal silicon and porous carbon materials and the electrolyte, thereby reducing side reactions that may shorten the battery life. This protective effect not only extends the service life of the battery but also provides guarantee for the continuous operation of the battery. In addition, the chemical stability of silicon nitride reduces the erosion of the electrolyte on the electrode materials, maintains the electrochemical activity of the electrode materials, and thus improves the charge and discharge efficiency of the battery.

[0055] At the same time, the thermal stability of silicon nitride means that it can maintain its performance at higher working temperatures, reducing the risk of thermal runaway, and thus improving the safety of the battery. The corrosion resistance of silicon nitride provides additional protection for the carbon layer, protecting it from erosion in acidic or alkaline environments that may be encountered during battery use. This property ensures the reliability and durability of the battery in different environments.

[0056] Furthermore, the silicon-carbon negative electrode material, by mass percentage, includes 15% - 30% of nano-silicon, 20% - 30% of silicon nitride, 1% - 10% of amorphous carbon coating layer, and the balance is porous carbon.

[0057] Typically but not restrictively, in the silicon-carbon negative electrode material, the content of nano-silicon can be 15%, 20%, 25%, 30%, or any value within the range of 15% - 30%; the content of silicon nitride can be 20%, 22%, 24%, 26%, 28%, or 30%, or any value within the range of 20% - 30%; the content of the amorphous carbon coating layer can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or 10%, or any value within the range of 1% - 10%. The balance is porous carbon to meet the requirement that the sum of the total mass percentages is 100%.

[0058] Furthermore, at least part of the porous carbon has a hollow structure; the diameter of the hollow structure of the porous carbon is 0.5 - 5 μm, and the hollow structure is a closed pore.

[0059] The hollow structure can, to a certain extent, improve the structural stability of the material, relieve volume expansion, and reduce structural damage caused by volume changes during charge and discharge processes.

[0060] The hollow diameter of the porous carbon is 0.5 - 5 μm.

[0061] The ratio of the porous carbon with a hollow structure to all the porous carbon ranges from 10% to 100%.

[0062] Further, the average particle size of the silicon ≤ 5 nm.

[0063] Further, in the silicon-carbon negative electrode material, Dv 10 < 6 μm, Dv 50 < 10 μm, Dv 90 < 20 μm.

[0064] The second aspect of the present invention provides a preparation method of the silicon-carbon negative electrode material, comprising the following steps:

[0065] A. Mix and heat a water-soluble carbon source in water to carry out a hydrothermal reaction to obtain a porous carbon precursor;

[0066] B. First carbonize the porous carbon precursor, and then carry out activation pore-forming treatment to obtain porous carbon;

[0067] C. Deposit a first silicon nitride coating on the porous carbon to obtain coated porous carbon;

[0068] D. Sequentially deposit silicon and a second silicon nitride coating on the coated porous carbon, and finally carry out carbon coating to obtain the silicon-carbon negative electrode material.

[0069] In the preparation method provided by the present invention, in the process of preparing the porous carbon precursor, by introducing a high molecular compound and carrying out carbonization pyrolysis and pore-forming treatment, the high molecular compound in the center of the porous carbon precursor undergoes dehydrogenation and deoxidation during carbonization to form a hollow structure, and carbon dioxide performs activation pore-forming from the outside to the inside, successfully obtaining porous carbon with a hollow structure. Subsequently, chemical vapor deposition technology is used to precisely deposit a layered structure of silicon nitride, nano-silicon, and a carbon coating layer. This continuous process flow not only effectively inhibits the volume expansion of the silicon-carbon negative electrode material during charge and discharge, but also the controllability and repeatability of the process make it very suitable for industrial-scale production. In addition, this fine structure design significantly improves the first charge-discharge efficiency and cycle stability of the silicon-carbon negative electrode material, thus providing a strong material basis for optimizing the performance of lithium-ion batteries.

[0070] Further, step A further includes adding a high molecular compound, mixing the high molecular compound and the water-soluble carbon source in water and heating to carry out a hydrothermal reaction to obtain a porous carbon precursor.

[0071] The high molecular compound includes at least one of polystyrene, polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polyimide, polymethyl methacrylate, and methyl methacrylate.

[0072] The water-soluble carbon source includes one or more of potato starch, mung bean starch, wheat starch, sweet potato starch, water chestnut starch, lotus root starch, cassava starch, pea starch, and sucrose.

[0073] Through the hydrothermal reaction of the water-soluble carbon source precursor, the carbon source first dissolves into the liquid phase, and then the liquid-phase carbon source is uniformly distributed outside the polymer compound and forms a porous carbon precursor during the hydrothermal process. In the subsequent carbonization process of step B, the internal polymer compound (such as polystyrene) undergoes dehydrogenation and deoxygenation reactions during carbonization, gradually forming an initial framework with a hollow structure. Finally, this process leaves a hollow cavity in the carbon matrix, thus realizing the preparation of the porous carbon material.

[0074] The feeding ratio of the polymer compound, water-soluble carbon source, and water is: 0 - 0.25:1:2, preferably 0.03 - 0.25:1:2. If the proportion of the polymer compound is too low, the hollow structure cannot be formed. If the proportion of the polymer compound is too high, the diameter of the hollow structure is too large, and through holes are easily formed during the subsequent activation and pore formation process. On the one hand, the structural strength is insufficient, and on the other hand, it leads to agglomeration during the subsequent nano-silicon deposition process, reducing the electrochemical performance.

[0075] The described preparation method includes at least one of the following features:

[0076] (1) In step A, the temperature of the hydrothermal reaction is 150 - 250 °C, and the time is 2 - 5 h. Typically but not restrictively, the temperature of the hydrothermal reaction in step A can be 150 °C, 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, 250 °C, or any value within the range of 150 °C - 250 °C; the reaction time can be 2 hours, 3 hours, 4 hours, 5 hours, or any value within the range of 2 hours - 5 hours.

[0077] (2) In step B, the temperature of the carbonization is 700 - 900 °C, and the time is 1 - 4 h. Typically but not restrictively, the temperature of the carbonization in step B can be 700 °C, 710 °C, 720 °C, 730 °C, 740 °C, 750 °C, 760 °C, 770 °C, 780 °C, 790 °C, 800 °C, 810 °C, 820 °C, 830 °C, 840 °C, 850 °C, 860 °C, 870 °C, 880 °C, 890 °C, 900 °C, or any value within the range of 700 °C - 900 °C; the carbonization time can be 1 hour, 2 hours, 3 hours, 4 hours, or any value within the range of 1 hour - 4 hours.

[0078] (3) In step B, the temperature for activation and pore formation is 700 - 900 °C, and the time is 8 - 16 h. Typically but not restrictively, the activation and pore formation temperature in step B can be 700 °C, 710 °C, 720 °C, 730 °C, 740 °C, 750 °C, 760 °C, 770 °C, 780 °C, 790 °C, 800 °C, 810 °C, 820 °C, 830 °C, 840 °C, 850 °C, 860 °C, 870 °C, 880 °C, 890 °C, 900 °C, or any value within the range of 700 °C - 900 °C; the activation and pore formation time can be 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, or any value within the range of 8 hours - 16 hours. A CO2 / N2 mixed atmosphere (such as 80% N2 and 20% CO2) is used in the activation and pore formation stage, and activation and pore formation are carried out at 700 - 900 °C for 8 - 16 hours. When CO2 etches the carbon material from the outside to the inside, the activation reaction preferentially etches the peripheral area. By controlling the reaction time, the internal hollow structure is not completely penetrated, and finally closed pores are formed.

[0079] The core control factors for closed pore formation include: the feeding ratio of the polymer compound to the carbon source (0.03 - 0.25:1). If it is too low, cavities cannot be formed; if it is too high, through holes will be caused. The synergistic effect of the activation temperature (700 - 900 °C) and time (8 - 16 h), the diameter range of the hollow structure is 0.5 - 5 μm, ensuring that the inside of the cavity is not completely penetrated by the activation gas. This kind of closed pore structure (such as the 1.5 - μm - diameter closed pores in Example 1) can effectively limit the agglomeration and expansion of nanosilicon and prevent the penetration of the electrolyte, thereby improving the cycle stability of the battery.

[0080] (4) In step C, the method for depositing the first silicon nitride coating includes: introducing a silicon source and a nitrogen source into a reactor, and depositing on the porous carbon. The deposition temperature is 400 - 600 °C, and the time is 1 - 3 h to obtain the first silicon nitride coating.

[0081] (5) In step D, the method for depositing the second silicon nitride coating includes: introducing a silicon source and a nitrogen source into a reactor, and depositing on the material to be deposited. The deposition temperature is 400 - 600 °C, and the time is 1 - 3 h to obtain the second silicon nitride coating.

[0082] (6) In step D, the method for depositing silicon includes: introducing a silicon source into a reactor, and depositing on the coated porous carbon. The deposition temperature is 400 - 600 °C, and the time is 2 - 5 h.

[0083] (7) In step D, the method for carbon coating includes: introducing a carbon source into a reactor, and coating the material to be coated. The deposition temperature is 400 - 600 °C, and the time is 0.5 - 2 h to obtain an amorphous carbon layer.

[0084] Further, the reactor includes, but is not limited to, a fluidized bed, a rotary kiln, a pyrolysis deposition furnace, a chemical vapor deposition furnace, etc.

[0085] Further, the silicon source includes at least one of silane, disilane, dimethylsilane, difluorosilane, trifluorosilane, tetrafluorosilane, chlorosilane, dichlorosilane, and trichlorosilane.

[0086] The carbon source includes at least one of acetylene, methane, ethane, propane, ethylene, and propylene.

[0087] The present invention will be further described below through specific examples and comparative examples. However, it should be understood that these examples are only for more detailed illustration and should not be construed as limiting the present invention in any way. For the raw materials used in the examples and comparative examples of the present invention, unless otherwise specified, they are carried out under conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments used without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0088] Example 1

[0089] This example provides a silicon-carbon anode material, and the preparation process is as follows:

[0090] 1. Mix 3 kg of potato starch, 0.15 kg of polystyrene, and 6 kg of pure water in a reaction kettle, and react at a temperature of 200 °C for 3 h to obtain a porous carbon precursor filled with polystyrene inside.

[0091] 2. Heat up to 850 °C in a nitrogen atmosphere and hold for 2 h for carbonization, then switch to a mixed atmosphere of 80% nitrogen and 20% carbon dioxide. The porous carbon precursor is activated and pore-formed in a rotary kiln. The activation and pore-formation temperature is 850 °C, and the activation and pore-formation time is 12 h. After pulverization, a porous carbon with a hollow structure inside is obtained.

[0092] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.5 h to obtain a first silicon nitride coating.

[0093] Stop the mixed gas of silane and ammonia, displace it with nitrogen 3 times, then introduce silane gas, and pyrolyze at 500 °C for 3 h to obtain nano-silicon.

[0094] Stop the silane gas, displace it with nitrogen 3 times, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.5 h to obtain a second silicon nitride coating.

[0095] Stop the mixed gas of silane and ammonia, displace it with nitrogen 3 times, then introduce acetylene gas, and pyrolyze at 500 °C for 1 h to obtain the silicon-carbon anode material.

[0096] Example 2

[0097] This example provides a silicon-carbon anode material, and the preparation process is as follows:

[0098] 1. Mix 3 kg of potato starch, 0.75 kg of polyvinylpyrrolidone, and 6 kg of pure water in a reaction kettle, and react at a temperature of 200 °C for 3 h to obtain a porous carbon precursor filled with polyvinylpyrrolidone inside.

[0099] 2. The same as this step in Example 1.

[0100] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.25 h to obtain a first silicon nitride coating.

[0101] Stop the mixed gas of silane and ammonia, displace it with nitrogen 3 times, then introduce silane gas, and pyrolyze at 500 °C for 3.5 h to obtain nanosilicon.

[0102] Stop silane gas, displace it with nitrogen 3 times, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.25 h to obtain a second silicon nitride coating.

[0103] Stop the mixed gas of silane and ammonia, displace it with nitrogen 3 times, then introduce acetylene gas, and pyrolyze at 500 °C for 2 h to obtain the silicon-carbon anode material.

[0104] Example 3

[0105] This example provides a silicon-carbon anode material, and the preparation process is as follows:

[0106] 1. Mix 3 kg of potato starch, 0.09 kg of polyethylene glycol, and 6 kg of pure water in a reaction kettle, and react at a temperature of 200 °C for 3 h to obtain a porous carbon precursor filled with polyethylene glycol inside.

[0107] 2. The same as this step in Example 1.

[0108] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.25 h to obtain a first silicon nitride coating.

[0109] Stop the mixed gas of silane and ammonia, displace it with nitrogen 3 times, then introduce silane gas, and pyrolyze at 500 °C for 2 h to obtain nanosilicon.

[0110] Stop silane gas, displace it with nitrogen 3 times, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.25 h to obtain a second silicon nitride coating.

[0111] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce acetylene gas, and pyrolyze it at 500 °C for 0.5 h to obtain a silicon-carbon negative electrode material.

[0112] Example 4

[0113] This example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0114] 1. Mix 3 kg of potato starch, 0.33 kg of polystyrene and 6 kg of pure water in a reaction kettle, and react at a temperature of 200 °C for 3 h to obtain a porous carbon precursor filled with polystyrene inside.

[0115] 2. The same as this step in Example 1.

[0116] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.75 h to obtain a first silicon nitride coating.

[0117] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce silane gas, and pyrolyze it at 500 °C for 4.5 h to obtain nanosilicon.

[0118] Stop the silane gas, displace it with nitrogen three times, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.75 h to obtain a second silicon nitride coating.

[0119] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce acetylene gas, and pyrolyze it at 500 °C for 1 h to obtain a silicon-carbon negative electrode material.

[0120] Example 5

[0121] This example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0122] 1. Mix 3 kg of potato starch, 0.33 kg of polystyrene and 6 kg of pure water in a reaction kettle, and react at a temperature of 200 °C for 3 h to obtain a porous carbon precursor filled with polystyrene inside.

[0123] 2. The same as this step in Example 1.

[0124] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 2 h to obtain a first silicon nitride coating.

[0125] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce silane gas, and pyrolyze it at 500 °C for 3.75 h to obtain nanosilicon.

[0126] Stop the silane gas, displace it with nitrogen three times, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 2 h to obtain a second silicon nitride coating.

[0127] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce acetylene gas, and pyrolyze it at 500 °C for 0.5 h to obtain a silicon-carbon negative electrode material.

[0128] Example 6

[0129] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0130] 1. Mix 3 kg of potato starch and 6 kg of pure water in a reaction kettle, and react at a temperature of 200 °C for 3 h to obtain a porous carbon precursor.

[0131] 2. The same as this step in Example 1.

[0132] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.5 h to obtain a first silicon nitride coating.

[0133] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce silane gas, and pyrolyze it at 500 °C for 3 h to obtain nanosilicon.

[0134] Stop the silane gas, displace it with nitrogen three times, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.5 h to obtain a second silicon nitride coating.

[0135] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce acetylene gas, and pyrolyze it at 500 °C for 1 h to obtain a silicon-carbon negative electrode material.

[0136] Comparative Example 1

[0137] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0138] 1. The same as this step in Example 1.

[0139] 2. The same as this step in Example 1.

[0140] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce silane gas, and pyrolyze it at 500 °C for 3 h to obtain nanosilicon.

[0141] Stop the silane gas, displace it with nitrogen three times, then introduce a mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.5 h to obtain a second silicon nitride coating.

[0142] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce acetylene gas, and pyrolyze it at 500 °C for 1 h to obtain the silicon-carbon negative electrode material.

[0143] Comparative Example 2

[0144] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0145] 1. The same step as in Example 1.

[0146] 2. The same step as in Example 1.

[0147] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce the mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.5 h to obtain the first silicon nitride coating.

[0148] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce silane gas, and pyrolyze it at 500 °C for 3 h to obtain nanosilicon.

[0149] Stop the silane gas, displace it with nitrogen three times, then introduce acetylene gas, and pyrolyze it at 500 °C for 1 h to obtain the silicon-carbon negative electrode material.

[0150] Comparative Example 3

[0151] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0152] 1. The same step as in Example 1.

[0153] 2. The same step as in Example 1.

[0154] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce the mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.5 h to obtain the first silicon nitride coating.

[0155] Stop the mixed gas of silane and ammonia, displace it with nitrogen three times, then introduce silane gas, and pyrolyze it at 500 °C for 3 h to obtain nanosilicon.

[0156] Stop the silane gas, displace it with nitrogen three times, then introduce the mixed gas of silane and ammonia, and pyrolyze it at 500 °C for 1.5 h to obtain the silicon-carbon negative electrode material.

[0157] Comparative Example 4

[0158] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0159] 1. The same step as in Example 1.

[0160] 2. The same step as in Example 1.

[0161] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, introduce silane gas, and pyrolyze at 500 °C for 3 h to obtain nanosilicon.

[0162] Stop the silane gas, displace it with nitrogen 3 times, then introduce acetylene gas, and pyrolyze at 500 °C for 1 h to obtain the silicon-carbon negative electrode material.

[0163] Comparative Example 5

[0164] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0165] 1. The same step as in Example 1.

[0166] 2. The same step as in Example 1.

[0167] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce silane gas, and pyrolyze at 500 °C for 3 h to obtain nanosilicon.

[0168] Stop the silane gas, displace it with nitrogen 3 times, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.5 h to obtain the silicon-carbon negative electrode material.

[0169] Comparative Example 6

[0170] This comparative example provides a silicon-carbon negative electrode material, and the preparation process is as follows:

[0171] 1. The same step as in Example 1.

[0172] 2. The same step as in Example 1.

[0173] 3. Place the porous carbon in a fluidized bed, introduce nitrogen as a protective gas, then introduce a mixed gas of silane and ammonia, and pyrolyze at 500 °C for 1.5 h to obtain the first silicon nitride coating.

[0174] Stop the mixed gas of silane and ammonia, displace it with nitrogen 3 times, then introduce silane gas, and pyrolyze at 500 °C for 3 h to obtain the silicon-carbon negative electrode material.

[0175] Characterization Example

[0176] Perform scanning electron microscopy on the silicon-carbon negative electrode material obtained in Example 1. Using argon ion polishing and field emission scanning electron microscopy (JSM-7800F), the cross-sectional scanning electron microscopy image at 13,000 times is shown in Figure 1 . From Figure 1 It can be seen that the silicon-carbon negative electrode material has a hollow structure, and the hollow structure is a closed pore, which has the function of buffering volume expansion. The closed pore can prevent the deposition of nanosilicon from agglomerating and generating large expansion, and can prevent excessive side reactions caused by the entry of electrolyte in the lithium-ion battery.

[0177] For Figure 1Elemental analysis was performed on the cross-sectional scanning electron micrograph to obtain Figure 2 , from Figure 2 it can be seen that the ratio of Si atoms to N atoms is 3:1. The Si atoms come from nanosilicon and silicon nitride. The ratio of the time for introducing silane when forming silicon nitride to the time for introducing silane when producing nanosilicon is 1:1. It can be known that the mass ratio of silicon elements in silicon nitride and nanosilicon is 1:1. Furthermore, it can be known that the chemical formula of silicon nitride is Si3N2.

[0178] Pore size analysis was performed on the porous carbon obtained in Example 1 to obtain Figure 3 , from Figure 3 it can be known that the pore volume ratio of the porous carbon with a pore size less than 2 nm accounts for more than 90%, and it can be known that the average particle size of the deposited nanosilicon in the porous carbon is less than 5 nm.

[0179] Test Example 1

[0180] The physical and chemical index parameters of the silicon-carbon anode materials obtained in the examples and comparative examples are shown in Table 1.

[0181] The silicon nitride content, silicon content, and acetylene carbon (amorphous carbon layer) content of the materials were tested using a steel research NCS (CS-3000G) and an electronic balance.

[0182] The particle size (μm) range of the materials was tested using a Dandong BET Laser Particle Sizer BT-9300ST.

[0183] The pore size distribution of the materials was tested using a Guoyi Quantum UltraSorb 3803.

[0184] Table 1

[0185]

[0186]

[0187] As can be seen from Table 1, the silicon nitride content, silicon content, and acetylene carbon content of the silicon-carbon anode materials are determined by the deposition amount, and the average hollow diameter size is determined by the addition amount of the polymer compound. Among Examples 1 to 6, in Example 2, the relative deposition time of silicon nitride is short, the relative deposition time of acetylene is the longest, the silicon nitride content is the least, and the acetylene carbon content is the most; in Example 5, the relative deposition time of silicon nitride is long, the relative deposition time of acetylene is the shortest, the silicon nitride content is more, and the acetylene carbon content is less; in Example 2, the addition amount of the polymer compound is the largest, and the formed average hollow diameter is larger. In Example 3, the addition amount of the polymer compound is the least, and the formed average hollow diameter is the smallest; in Example 6, no polymer compound was added, and there is no hollow structure; there are varying degrees of differences in the contents of the first silicon nitride coating, the second silicon nitride coating, and the amorphous carbon layer in Comparative Examples 1 to 6.

[0188] Test Example 2

[0189] The first reversible capacity and the first efficiency of the silicon-carbon anode materials obtained from the test examples and the comparative examples were measured as follows:

[0190] The silicon-carbon anode material, conductive carbon black, and binder were mixed in pure water at a mass ratio of 94.5:1.5:4, homogenized, and the solid content was controlled at 48 wt%. It was coated on a copper foil current collector and vacuum baked at 100 °C for 8 h. After pressing into shape, it was punched into a negative electrode sheet.

[0191] A button-type half-cell was assembled in a glove box filled with argon. The counter electrode was a lithium metal sheet. The separator used was PE, and the electrolyte was 1 mol / L LiPF6 in EC / DMC (Vol 1:1).

[0192] The button-type battery was subjected to charge-discharge testing. The testing process was 0.2C DC to 0V, 0.05C DC to 0V, 0V CV 50 μA, 0.01C DC to 0V, 0V CV 20 μA, Rest 10 min, 0.2C CC to 1.5V.

[0193] The first reversible capacity and efficiency of the silicon-carbon anode material were measured. Among them, the testing equipment for the button-type battery was the LAND battery testing system of Wuhan Blue Electronic Co., Ltd.

[0194] Swelling rate test of the silicon-carbon anode material S600:

[0195] The first reversible capacity of the silicon-carbon anode material was measured according to the above-mentioned button cell testing method. Then, after calculation, a certain amount of the same graphite anode was mixed, and the silicon-carbon anode material was mixed to 600 ± 5 mA·h / g, abbreviated as S600.

[0196] The S600 mixed material, conductive carbon black, and binder (mass ratio 92:2:6) were homogenized in pure water, and the solid content was controlled at 48%. It was coated on a current collector based on copper foil. Then, it was vacuum baked at 90 °C for 8 h. After being pressed into shape by a rolling equipment, it was sliced by a slicing equipment to prepare a negative electrode sheet. The thickness of the negative electrode sheet was measured using a micrometer and recorded as T1, and the thickness of the substrate was measured and recorded as T2, and the data was recorded.

[0197] The coin-type half-cells were assembled in a glove box filled with argon. The counter electrode was a lithium metal sheet. The separator used was made of PE material, and the electrolyte was 1 mol / L LiPF6 in EC / DMC (Vol 1:1). Charge-discharge tests were conducted on the coin-type cells. The test procedure was from 0.1 C DC to 0.005 V, 0.05 C DC to 0.005 V, 0.02 C DC to 0.005 V, Rest for 10 min, 0.1 C CC to 1.5 V, 0.1 C DC to 0.005 V, 0.05 C DC to 0.005 V, 0.02 C DC to 0.005 V. The first reversible capacity and efficiency of the anode material were measured.

[0198] The battery was disassembled, and the thickness of the anode material obtained from the disassembly was measured and denoted as T3. The first full-charge swelling data of the anode material was calculated according to the formula F = (T3 - T1) / (T1 - T2), where F is the first full-charge swelling rate.

[0199] The test equipment for the coin-type cells was the LAND battery test system of Wuhan Blue Electronic Co., Ltd. The slicing equipment was the MSK-T10 coin-type half-cell slicing equipment of Kejing. The micrometer detection equipment was the Mitutoyo 293-100-10 made in Japan. The rolling equipment was the MSK-HRP-05 coin-type half-cell slicing equipment of Kejing.

[0200] Capacity retention rate test:

[0201] Mix the prepared silicon-carbon anode material powder with graphite anode (mass ratio 10 - 80:20 - 90) to obtain a mixed anode powder. Then, mix the anode powder, Super p, and BP-7 in pure water at a mass ratio of 92:4:4, control the solid content at 48%, homogenize and coat it on a current collector with a copper foil substrate, place it in a vacuum baking oven at 90°C for vacuum baking for 4 h, cold press the electrode sheet using a Kejing commercial rolling equipment, and use a Kejing commercial slicing equipment to slice the electrode sheet into small round pieces with a diameter of 22 mm to prepare a silicon composite anode electrode sheet. Then, mix the commercial 523 cathode material, SP, and PVDF (polyvinylidene fluoride) in an appropriate amount of NMP (N-methylpyrrolidone) solvent at a mass ratio of 90:7:3, control the solid content at 55%, coat the mixed slurry on a current collector with an aluminum foil substrate, place it in a vacuum baking oven at 120°C for vacuum baking for 4 h, cold press the electrode sheet using a Kejing commercial rolling equipment, and use a Kejing commercial slicing equipment to slice the electrode sheet into small round pieces with a diameter of 22 mm to prepare a cathode electrode sheet. Dry it under vacuum (-0.1 MPa) conditions at 85°C for 8 h, weigh it and calculate the weight of the active material. The separator used is PE, and the electrolyte is 1 mol / L LiPF6 in EC / DMC (Vol 1:1). Assemble it into a CR2430 type button full cell in a glove box, let the button full cell stand at room temperature for 2 h, perform charge-discharge activation at 0.1C on a Blue Energy test system, and then perform charge-discharge cycling test at 1C (voltage range 3.0 - 4.2V) for 500 cycles. The capacity retention rate of the material = discharge capacity of the last cycle / discharge capacity of the first cycle × 100%.

[0202] The above test results are shown in Table 2. The first charge-discharge curve diagram of the lithium-ion button battery with the silicon-carbon anode material in Example 1 is shown in Figure 4 .

[0203] Table 2

[0204]

[0205] As can be seen from Table 2, Examples 1 to 6 have a high initial efficiency, mainly because they have appropriate proportions of the first silicon nitride coating, the second silicon nitride coating, and acetylene carbon content. The layered silicon nitride can provide reversible capacity and has high mechanical strength, which can prevent the nano-silicon particles from breaking. Acetylene carbon can reduce the occurrence of side reactions. Having an appropriate average hollow diameter results in a lower initial expansion rate and a better cycle capacity retention rate. The difference in the initial reversible capacity mainly comes from the difference in the ratio of nano-silicon to silicon nitride. The initial efficiency of Example 6 is lower than that of other examples because there is no hollow structure, and the nano-silicon is more likely to break during charge and discharge, resulting in more irreversible side reactions. The absence of a hollow structure leads to a larger initial expansion rate, thereby reducing the cycle capacity retention rate. In Comparative Example 1, there is no first silicon nitride coating, and the mechanical strength of the particle hollow structure is low, making it easy to break during charge and discharge. The electrolyte enters the interior, increasing side reactions, resulting in a lower initial efficiency and a larger initial expansion rate, thereby reducing the cycle capacity retention rate. In Comparative Example 2, there is no second silicon nitride coating, and the mechanical strength of the outer surface of the particles is low, making it easy to break during electrode rolling and charge and discharge, increasing side reactions, resulting in a lower initial efficiency and a larger initial expansion rate, thereby reducing the cycle capacity retention rate. In Comparative Example 3, there is no amorphous carbon layer, and side reactions are likely to increase during charge and discharge, resulting in a decrease in the initial efficiency and a larger initial expansion rate, thereby reducing the cycle capacity retention rate. In Comparative Example 4, there are no first silicon nitride coating and second silicon nitride coating, and the mechanical strength of the interior and outer surface of the particles is very low, making it easy to break during electrode rolling and charge and discharge, and side reactions are likely to increase during charge and discharge, resulting in a lower initial efficiency and a larger initial expansion rate, thereby reducing the cycle capacity retention rate. In Comparative Examples 5 to 6, the silicon nitride coating and the amorphous carbon layer are missing respectively, and the mechanical strength of the interior and outer surface of the particles is low, making it easy to break during electrode rolling and charge and discharge, and side reactions are likely to increase during charge and discharge, resulting in a lower initial efficiency and a larger initial expansion rate, thereby reducing the cycle capacity retention rate.

[0206] Finally, it should be noted that the above-described embodiments are only specific embodiments of the present invention, which are used to illustrate the technical solutions of the present invention and are not intended to limit them. The protection scope of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that any person skilled in the art within the technical scope disclosed by the present invention can still modify the technical solutions described in the foregoing embodiments or can easily conceive of changes, or perform equivalent replacements on some of the technical features; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A silicon-carbon negative electrode material, characterized in that: The invention comprises a coated porous carbon, a coated silicon deposited in the pores of the coated porous carbon, and an amorphous carbon layer coated on the outer layer of the coated porous carbon; The coated porous carbon includes porous carbon and a first silicon nitride coating; The first silicon nitride coating is coated inside the pores of the porous carbon and on the outer surface of the porous carbon; The coated silicon includes silicon and a second silicon nitride coating coated on the surface of the silicon; The silicon nitride has a hexagonal layered structure and a chemical formula of Si3Nx, where 1<x<4.

2. The silicon-carbon negative electrode material according to claim 1, characterized in that: According to mass percentage, it comprises 15% to 30% of nano silicon, 20% to 30% of silicon nitride, 1% to 10% of an amorphous carbon coating layer, and the remainder is porous carbon.

3. The silicon-carbon negative electrode material according to claim 1, characterized in that: The porous carbon at least partially has a hollow structure; the diameter of the hollow structure of the porous carbon is 0.5 to 5 μm, and the hollow structure is a closed hole.

4. The silicon-carbon negative electrode material according to claim 3, characterized in that: The porous carbon with a hollow structure accounts for 10% to 100% of the total porous carbon.

5. The silicon-carbon negative electrode material according to any one of claims 1 to 4, characterized in that: The average particle size of the silicon is ≤5 nm.

6. The silicon-carbon negative electrode material according to any one of claims 1 to 4, characterized in that: The Dv of the silicon-carbon negative electrode material 10 <6μm、Dv 50 <10μm、Dv 90 <20μm.

7. A method for preparing the silicon-carbon negative electrode material according to any one of claims 1 to 6, characterized in that: The following steps are involved: A. mixing a water-soluble carbon source in water and heating it to generate a hydrothermal reaction to obtain a porous carbon precursor; B. carbonizing the porous carbon precursor first, and then performing an activation pore-forming treatment to obtain porous carbon; C. depositing a first silicon nitride coating on the porous carbon to obtain a coated porous carbon; D. Sequentially depositing silicon and a second silicon nitride coating on the coated porous carbon, and finally performing carbon coating to obtain the silicon-carbon negative electrode material.

8. The preparation method according to claim 7, characterized in that: The step A further comprises adding a polymer compound, mixing the polymer compound and a water-soluble carbon source in water and heating them to generate a hydrothermal reaction, thereby obtaining a porous carbon precursor; The polymer compound includes at least one of polystyrene, polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polyimide, polymethyl methacrylate and methyl methacrylate.

9. The preparation method according to claim 7, characterized in that: The water-soluble carbon source includes one or more of potato starch, mung bean starch, wheat starch, sweet potato starch, water chestnut starch, lotus root starch, cassava starch, pea starch and sucrose.

10. The preparation method according to any one of claims 7 to 9, characterized in that: Include at least one of the following features: (1) In step A, the temperature of the hydrothermal reaction is 150-250° C. and the time is 2-5 hours; (2) In step B, the carbonization temperature is 700 to 900° C. and the time is 1 to 4 hours; (3) In step B, the activation pore-forming temperature is 700-900° C. and the time is 8-16 hours; (4) In the step C, the method for depositing the first silicon nitride coating comprises: introducing a silicon source and a nitrogen source into a reactor, depositing the porous carbon at a deposition temperature of 400 to 600° C. for 1 to 3 hours, to obtain a first silicon nitride coating; (5) In the step D, the method for depositing the second silicon nitride coating comprises: introducing a silicon source and a nitrogen source into a reactor, depositing the material to be deposited, the deposition temperature is 400 to 600° C., the time is 1 to 3 hours, and the second silicon nitride coating is obtained; (6) In the step D, the method for depositing silicon includes: introducing a silicon source into a reactor to deposit the coated porous carbon at a temperature of 400 to 600° C. for 2 to 5 hours; (7) In the step D, the carbon coating method includes: introducing a carbon source into a reactor to coat the material to be coated, the deposition temperature is 400 to 600° C., the time is 0.5 to 2 hours, and an amorphous carbon layer is obtained.

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