CVD (Chemical Vapor Deposition) silicon-carbon composite material and preparation method and application thereof

Abstract

The invention relates to the technical field of battery negative electrode materials, in particular to a CVD silicon-carbon composite material and a preparation method and application thereof. The problems that the battery assembled by using the carbon-silicon composite material as the negative electrode material is poor in cycling stability and the negative electrode plate is high in expansion rate are solved. Phenolic resin is used under the action of a dispersing agent and a curing agent, and the silicon-carbon composite material is prepared through stepped heating curing, carbonization treatment, activation impurity removal, nanometer silicon deposition through a chemical vapor deposition method and carbon material coating. By changing the raw material ratio of the phenolic resin, the specific capacity of a battery assembled by the composite material is improved; the multiplying power retention rate of the battery is improved by adjusting the types of the dispersing agent and the curing agent and controlling the carbonization temperature; the cycling stability of the battery is improved by adjusting the specific surface area of the high-micropore porous carbon; the ion conductivity of the battery is improved by adjusting the silicon deposition temperature, the gas flow and the volume ratio; the expansion rate of the battery negative pole piece is reduced by controlling the thickness of the carbon coating layer and the deposition temperature.

Description

Technical Field

[0001] This invention relates to the field of battery anode material technology, specifically to a CVD silicon-carbon composite material, its preparation method, and its application. Background Technology

[0002] With the widespread application and rapid development of lithium batteries, people have higher and higher requirements for the performance of lithium-ion batteries. Not only are high capacity required, but also good capacity retention during repeated charge and discharge processes, good cycle performance, and long service life are required.

[0003] Silicon (Si) is considered a candidate material for next-generation lithium-ion battery (LIB) anode materials due to its high theoretical specific capacity (4200 mAh / g), low lithium intercalation potential, fast reaction kinetics, and abundant reserves. Current synthetic routes for preparing silicon-carbon materials mainly utilize specialized processes such as chemical vapor deposition, liquid-phase impregnation, and hydrothermal self-assembly. However, the significant volume expansion of silicon leads to electrode failure during cycling, including both mechanical and electrochemical failures. Current methods to reduce electrode expansion include reducing particle size to prepare nano-silicon, constructing core-shell structures, and silicon coating. However, the preparation of nano-silicon, core-shell structures, and silicon coatings is difficult and costly, and existing silicon-carbon supports (porous carbon) cannot meet the required performance indicators.

[0004] Patent CN102769139B reports a method for preparing a high-capacity lithium-ion battery anode silicon-carbon composite material. This method involves using natural spherical graphite as raw material, concentrated sulfuric acid as an intercalating agent, and potassium permanganate as an oxidant. The graphite is then expanded at high temperature to obtain micro-expanded graphite, mixed with nano-silicon powder, and then coated with a carbon source precursor in a certain proportion. Finally, it is carbonized and sintered under inert gas protection to obtain a silicon-carbon composite anode material, thus improving the battery's electrochemical performance. However, this method fails to achieve uniform dispersion of nano-silicon particles, resulting in a poor cycle performance and low initial coulombic efficiency for the anode material.

[0005] Patent CN103367727B reports a silicon-carbon anode material for lithium-ion batteries and its preparation method, comprising nano-silicon, graphite polymer, and organic pyrolysis carbon. By mixing nano-silicon, dispersant, binder, and particulate graphite in an organic solvent, adding it to a dispersion of carbon source precursor, drying, and heat treatment, the silicon-carbon anode material for lithium-ion batteries is obtained, which improves the electrochemical performance of the battery. However, the use of organic solvents in the preparation process results in poor dispersibility, making it impossible to achieve nanoscale dispersion of silicon. At the same time, the initial efficiency and specific capacity performance of the battery are poor, affecting the battery's lifespan.

[0006] Patent CN105226285B reports a porous silicon-carbon composite material and its preparation method. The method uses silicon-active metal alloy as raw material to obtain nanoporous silicon through etching. The nanoporous silicon and polymer are mixed and ball-milled to prepare a porous silicon-carbon composite material. The assembled battery material has good cycle stability and coulombic efficiency. However, during long-term cycling, the huge volume change of silicon will cause the carbon coating layer on its surface to break, the composite material structure to collapse, the ionic conductivity to decrease, and affect the cycle performance of the battery material.

[0007] In summary, the silicon-carbon composite materials prepared by existing technologies, when used as negative electrode materials in lithium batteries, suffer from poor cycle stability due to low specific capacity and poor rate retention performance. Furthermore, the high volume expansion rate of nano-silicon leads to structural collapse of the composite material, further affecting the ionic conductivity and cycle stability, and ultimately reducing the battery's lifespan.

[0008] To this end, a CVD silicon-carbon composite material, its preparation method, and its application are proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a CVD silicon-carbon composite material, its preparation method, and its application. The method involves mixing phenols, formaldehyde aqueous solution, and concentrated hydrochloric acid in a certain proportion, heating and stirring to obtain a first mixture; preparing a dispersant solution; mixing the first mixture with the dispersant solution to form a third mixture, and then stirring and heating it to obtain a porous carbon precursor; sequentially subjecting the porous carbon precursor to heating curing, medium-temperature carbonization, and chemical activation to obtain highly microporous porous carbon; and then depositing nano-silicon and coating the carbon layer within a highly microporous porous carbon matrix via chemical vapor deposition. A silicon-carbon composite material with grown nano-silicon was obtained and assembled into a CR2032 coin cell as the negative electrode material. The specific capacity of the composite material-assembled battery was improved by changing the raw material ratio of phenolic resin. The rate retention rate of the battery was improved by adjusting the types of dispersants and curing agents and controlling the carbonization temperature. The cycle stability of the battery was improved by improving the specific surface area of ​​the high microporous porous carbon. The ionic conductivity of the battery was improved by improving the silicon deposition temperature, gas flow rate and volume ratio. The expansion rate of the negative electrode sheet of the battery was reduced by adjusting the thickness of the carbon coating layer and improving the deposition temperature.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] This invention provides a method for preparing CVD silicon-carbon composite materials, comprising the following steps:

[0012] S1. Phenols, formaldehyde aqueous solution, and concentrated sulfuric acid are mixed in a mass ratio of 0.8-2.2:0.5-0.8:0.2-0.6 and heated to react, resulting in a mixed solution one.

[0013] S2. Add the dispersant to the solvent, heat and stir, and mix evenly to obtain mixture two;

[0014] S3 mixes the first mixture and the second mixture, adds a curing agent and heats at 60-100℃ to obtain the third mixture; the third mixture is sieved to obtain a spherical sample; the spherical sample is washed and dried to obtain a porous carbon precursor;

[0015] S4 The porous carbon precursor is uniformly dispersed in a sagger and solidified by step heating to obtain a solid composite.

[0016] S5 The solid composite is placed in a kiln and carbonized in nitrogen at 400-700℃ to obtain a carbonized composite.

[0017] S6 After the carbonized compound is mixed with caustic soda flakes, it is placed in a kiln and activated to obtain an activated product; the mass ratio of caustic soda flakes to carbonized compound is 1-10:1-4; the activated product is cooled to room temperature, acid washed and dried to obtain highly microporous porous carbon.

[0018] S7 Place the highly microporous porous carbon in a kiln, introduce silicon-containing gas, and deposit silicon for 3 hours to obtain a porous carbon matrix with nano-silicon deposition; cool the porous carbon matrix, introduce carbon-containing gas, and deposit carbon for 4 hours to coat the surface of the porous carbon matrix with a carbon layer to obtain the CVD silicon-carbon composite material.

[0019] The silicon-containing gas is a mixture of hydrogen and tetramethylsilane in a volume ratio of 1-10:1;

[0020] The carbon-containing gas is a mixture of argon and methane in a volume ratio of 1-10:1;

[0021] The nano-silicon accounts for 5%-35% of the total mass of the silicon-carbon composite material; the thickness of the carbon coating layer is 20-200 nm.

[0022] Preferably, the porous carbon precursor has a particle size range of Dv00 > 1 μm, Dv50 of 5 μm-10 μm, and Dv100 < 18 μm;

[0023] Preferably, the stepped heating curing procedure is as follows: heating to 60℃-120℃ at a heating rate of 1℃ / min-5℃ / min, holding at that temperature for 10h-24h, then heating to 150℃-250℃ at a heating rate of 1℃ / min-5℃ / min, holding at that temperature for 6h-12h, and then allowing it to cool naturally to room temperature.

[0024] Preferably, the stepped heating and curing process involves heating to 90°C at a rate of 1°C / min, holding at that temperature for 16 hours, then heating to 220°C at a rate of 3°C / min, holding at that temperature for 6 hours, and then allowing it to cool naturally to room temperature.

[0025] Preferably, the mass of nano-silicon accounts for 5%-35% of the total mass of the silicon-carbon composite material, and the size of the nano-silicon particles is 0nm-2nm.

[0026] Preferably, the phenol is selected from one of phenol, resorcinol, and p-phenylphenol.

[0027] Preferably, the formaldehyde aqueous solution has a mass percentage concentration of 35%-45%.

[0028] Preferably, the volume ratio of the first mixture to the second mixture is 1-3:1-3.

[0029] Preferably, the dispersant is selected from one of polyvinyl alcohol, polyethylene, sodium hexametaphosphate, and polyethylene glycol.

[0030] Preferably, the curing agent is selected from one of hexamethylenetetramine, 2-methylimidazole, and triethanolamine.

[0031] Preferably, the activation reaction temperature is 600-1000℃; the activation reaction time is 3-14h.

[0032] Preferably, the flow rate of the silicon-containing gas is 8-50 L / h; the flow rate of the carbon-containing gas is 20-40 L / h.

[0033] Another aspect of the present invention provides the application of CVD silicon-carbon composite materials obtained by any of the preparation methods described above, wherein the CVD silicon-carbon composite materials are used as battery anode materials in lithium-ion batteries.

[0034] This invention also provides a CVD silicon-carbon composite material, comprising phenols, formaldehyde aqueous solution, dispersant, curing agent, silicon-containing gas, and carbon-containing gas; the CVD silicon-carbon composite material is prepared by any one of the methods described above; the first-cycle efficiency of the battery assembled from the CVD silicon-carbon composite material is 97.2%, and the discharge capacity is 964.5 mAh / g; the 1C rate retention rate of the battery assembled from the CVD silicon-carbon composite material is 98.5%, and the 5C rate retention rate is 96.8%; the capacity retention rate of the battery assembled from the CVD silicon-carbon composite material after 300 cycles is 95.2%; the ionic conductivity of the battery assembled from the CVD silicon-carbon composite material is 8.2 × 10⁻⁶. -2 S / cm; The electrode expansion rate of the battery negative electrode in the fully intercalated state of the CVD silicon-carbon composite material assembled battery is 41.17%.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1. In this invention, phenolic compounds react with formaldehyde aqueous solution to generate phenolic resin, which serves as the carbon source skeleton. The introduction of benzene rings can provide more carbon binding sites for silicon-carbon composite materials, allowing for thorough mixing with dispersants and the formation of a uniform and stable network structure under the action of curing agents. The carbonization process facilitates the formation of a porous structure, which is conducive to the adsorption and deposition of nano-silicon in the later stage, thereby improving the discharge capacity of the battery. By controlling the type of phenol, the concentration of formaldehyde aqueous solution, and the mass ratio of raw materials, the silicon-carbon composite material obtained is assembled into a CR2032 coin cell. The battery's first-time efficiency test is 97.2%, and the discharge capacity is 964.5 mAh / g, showing a significant improvement in specific capacity.

[0037] 2. This invention utilizes hexamethylenetetramine as a curing agent to provide a stable skeletal structure for phenolic resin. Under high-temperature conditions, calcination and oxidation form graphitic nitrogen, which fuses with the carbon skeleton in the original components. Simultaneously, nitrogen is uniformly distributed within it. The doping of nitrogen increases the conductivity of the negative electrode material and improves the battery rate performance. By controlling the types of dispersants and curing agents, heating temperature, and carbonization temperature, the assembled CR2032 coin cell achieves a 1C rate retention rate of 98.5% and a 5C rate retention rate of 96.8%, demonstrating improved rate retention performance.

[0038] 3. This invention activates the carbonized composite to obtain highly microporous carbon with a significantly increased specific surface area and a higher micropore ratio. This provides more nano-silicon deposition sites during vapor-phase deposition of nano-silicon and carbon coating, increasing the binding and adsorption capacity with nano-silicon and making the deposition and adsorption of nano-silicon more stable. By controlling the activation reaction temperature, activation reaction time, and alkali-to-carbon mass ratio to adjust the specific surface area of ​​the highly microporous carbon, the assembled CR2032 coin cell exhibits a capacity retention rate of 95.2% after 300 cycles, demonstrating significantly improved cycle stability.

[0039] 4. This invention utilizes chemical vapor deposition to deposit and adsorb nano-silicon particles into the interior of highly microporous carbon. Leveraging the high theoretical specific capacity of silicon, the overall capacity of the silicon-carbon anode material is improved, thereby enhancing the ionic conductivity of the battery. By controlling the silicon flow rate and silicon deposition temperature, and improving the mass ratio of nano-silicon to silicon-carbon composite materials, the assembled CR2032 coin cell exhibits an ionic conductivity of 8.2 × 10⁻⁶. -2 With a S / cm, the cycle capacity retention rate is 95.3%, indicating that the battery material's conductivity is improved while cycle stability is guaranteed.

[0040] 5. This invention uses chemical vapor deposition to coat organic carbon onto the surface of a porous carbon matrix containing deposited nano-silicon, which acts as a bond between the nano-silicon and the carbon framework matrix. Simultaneously, the carbon coating reduces the surface area of ​​the material after ball milling, minimizing side reactions and improving ionic conductivity. Furthermore, after carbon coating, the pyrolysis of lithium carbonate increases the porosity of the surface carbon coating layer, buffering the volume expansion of silicon and reducing the volume expansion rate of the electrode. It also isolates the core from the electrolyte, which is beneficial for improving the cycle performance of the silicon-carbon anode material. By controlling the carbon-containing gas flow rate and carbon deposition temperature, and improving the thickness of the coated carbon layer, the assembled CR2032 coin cell exhibits a battery anode expansion rate of 41.17%, demonstrating a significantly reduced expansion rate. Attached Figure Description

[0041] Figure 1 This is a flowchart of the preparation method of the silicon-carbon composite material obtained by the present invention;

[0042] Figure 2 This is a scanning electron microscope image of the porous carbon precursor prepared in Example 14 of the present invention. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] Please see Figures 1 to 2 This invention provides a CVD silicon-carbon composite material, its preparation method, and its application. The technical solution is as follows:

[0045] Example 1

[0046] S1. Phenol, a 35% aqueous solution of formaldehyde, and concentrated hydrochloric acid (36%) are mixed in a mass ratio of 1.5:0.6:0.3. The mixture is heated to 90°C and stirred for 80 minutes at a stirring speed of 1000 rpm to obtain mixture one.

[0047] S2. Polyvinyl alcohol is added to deionized water, heated to 85°C, and stirred at 900 rpm for 80 minutes to obtain mixture two.

[0048] S3. Mixture 1 and Mixture 2 are blended at a volume ratio of 1:1 to obtain a mixed solution; 8% of hexamethylenetetramine is added to the mixed solution, heated to 85°C, and stirred at 400 rpm for 200 min to obtain Mixture 3; Mixture 3 is sieved to collect small spherical samples; The small spherical samples are washed and dried to obtain a porous carbon precursor; The precursor is collected, washed, and the porous carbon precursor is obtained.

[0049] S4 The porous carbon precursor was uniformly dispersed in a crucible and placed in an oven. It was cured by step heating under a nitrogen atmosphere (0.4 L / min). The curing procedure was as follows: heating to 90°C at a rate of 1°C / min, then holding for 16 hours, then heating to 220°C at a rate of 3°C / min, holding for 6 hours, and then naturally cooling to room temperature of 30°C to obtain a solid composite.

[0050] S5 places the solid composite in a kiln and carbonizes it under a nitrogen atmosphere (0.4 L / min). The carbonization process is as follows: heat up to 550℃ at 5℃ / min, hold for 4 hours, and cool naturally to room temperature to obtain the carbonized composite.

[0051] S6 After blending the carbonized composite with flake KOH, it was placed in a kiln and activated by chemical activation under a nitrogen atmosphere. The activation reaction method was as follows: heating to 850℃ at 3℃ / min, holding for 7h, with an alkali-to-carbon mass ratio of 5:2, and cooling to room temperature under a nitrogen atmosphere to obtain a highly microporous porous carbon matrix; the highly microporous porous carbon was then acid-washed to remove impurities and dried to obtain highly microporous porous carbon.

[0052] S7 involves placing a highly microporous porous carbon material in a rotary furnace, introducing a silicon-containing gas, and depositing nano-silicon inside the porous carbon matrix to obtain a porous carbon matrix with grown nano-silicon. The matrix is ​​then naturally cooled to 300°C under a nitrogen atmosphere, and the gas source is switched to introduce argon and methane. The mixture is held at this temperature for 4 hours to obtain a silicon-carbon composite material. The silicon-containing gas is a mixture of hydrogen and tetramethylsilane with a volume ratio of 3.5:1, a silicon source gas flow rate of 28 L / h, a heating temperature of 850°C, and a holding time of 3 hours. The volume ratio of argon to methane is 4:1, and the flow rate of the mixed gas is 32 L / h. The preparation process of the silicon-carbon composite material is as follows: Figure 1 As shown.

[0053] Graphite, the silicon-carbon composite material, carbon black, and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 65:30:1.5:3.5 and dispersed in deionized water to form a uniform slurry. The slurry is cast onto copper foil using a scraper method and baked at 85°C for 3.5 hours. Then, it is cold-pressed, trimmed, and cut into sheets. Finally, it is dried under vacuum at 90°C for 18 hours to obtain the negative electrode sheet.

[0054] Assemble CR2032 coin cell batteries: In a glove box filled with protective gas (where the content of H2O and O2 is less than 0.1ppm), lithium metal sheet is used as the counter electrode, PP / PE is used as the separator, and 1mol / L LiPF6 solution is used as the electrolyte. The solvent in the electrolyte is a mixture of ethylene carbonate EC, methyl ethyl carbonate EMC, and dimethyl carbonate DMC (where the volume ratio of EC, EMC, and DMC is 1:1:1) and fluoroethylene carbonate at a volume ratio of 22.5:1.

[0055] Examples 2-12 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 1.

[0056] Table 1. Variations in component dosage in Examples 1-12

[0057]

[0058] Comparative Example 1 follows the same preparation method and parameters as Example 2, except that no dispersant is added for dispersion treatment.

[0059] Comparative Example 2 follows the same preparation method and parameters as Example 2, except that it uses commercially available thermoplastic phenolic resin.

[0060] Example 13 First Charge Specific Capacity Test

[0061] The CR2032 coin cells assembled using the carbon-silicon composite materials of Examples 1-12 and Comparative Examples 1-2 as the negative electrode material were tested using an electrochemical testing system. First-cycle efficiency test: 0.2C discharge capacity divided by the first 0.1C charge capacity. First-cycle specific capacity: In an environment of 25°C, the cells were first discharged to 5mV at 0.5C, then to 5mV at 0.05C, then to 5mV at 0.02C, then to 5mV at 0.01C, and finally charged to 1.5V at 0.1C. The specific capacity of the first cycle was recorded. The test results are shown in Table 2.

[0062] Table 2 Discharge capacity tests of Examples 1-12 and Comparative Examples 1-2

[0063]

[0064]

[0065] As shown in Table 2, the silicon-carbon composite material prepared with phenolic resin (without dispersant treatment) in Comparative Example 1, used as the negative electrode material for lithium batteries, showed significantly lower first-efficiency and charge specific capacity compared to Examples 1-12. The discharge capacity of the coin cell prepared using commercially available phenolic resin as the carbon source resin in Comparative Example 2 was 482.3 mAh / g, significantly lower than both Examples 1-12 and Comparative Example 1. The results from Examples 1-3 indicate that changing the type of phenol in the phenolic resin significantly affects the discharge capacity; the introduction of benzene rings can provide more carbon bonding sites for the silicon-carbon composite material. Due to the rigid conjugated structure of the benzene ring, it can be fully mixed with the dispersant during the curing process and form a uniform and stable network structure under the action of the curing agent. This ensures the stability of the carbon skeleton during subsequent carbonization. Simultaneously, the dispersant and curing agent volatilize at high temperatures, forming a porous structure that facilitates the adsorption and deposition of nano-silicon, providing a stable lithium-ion pathway and improving the battery's discharge capacity. The results of Examples 3-5 show that the battery's discharge capacity gradually decreases with increasing formaldehyde aqueous solution concentration. With increasing formaldehyde concentration, the methylene content in the phenolic resin increases. During calcination, the methylene content increases at high temperatures. Destruction occurs at low temperatures, affecting the formation of the carbon skeleton, further manifesting as a reduction in pathways and a decrease in discharge capacity. Examples 3 and 6-8 show that the battery discharge capacity exhibits significant changes with alterations in the mass ratio of phenolic resin and concentrated hydrochloric acid. The use of more formaldehyde solution reduces the formation of the carbon skeleton network structure, affecting the formation of nano-silicon and further reducing the battery discharge capacity. Examples 3 and 9-12 show that as the amount of phenolic resin added gradually decreases, the battery discharge capacity exhibits a trend of first increasing and then decreasing. When the dispersant content is low, the dispersibility of the phenolic resin is poor, and the number of free crosslinking sites decreases. The cross-linked structure formed during the curing process with the curing agent has poor stability, which reduces the stability of the carbon skeleton during high-temperature carbonization. If the dispersant content is too high, the combination of phenolic resin and dispersant increases. After high-temperature carbonization, the dispersant volatilizes, and the pore size structure inside the carbon skeleton becomes larger, which further affects the adsorption and deposition of nano-silicon, resulting in a decrease in the discharge capacity of the battery. According to the results in Table 2, by adjusting the type of phenol, the concentration of formaldehyde, and controlling the mass ratio of phenol, aldehyde, and concentrated hydrochloric acid, the silicon-carbon composite material prepared and applied to the CR2032 coin cell showed a first-efficiency test of 97.2% and a discharge capacity of 964.5 mAh / g, which showed a significant improvement in specific capacity.

[0066] Example 14

[0067] S1. Resorcinol, a 35% aqueous solution of formaldehyde, and concentrated hydrochloric acid (36%) are mixed in a mass ratio of 1.5:0.6:0.3. The mixture is heated to 90°C and stirred for 80 minutes at a stirring speed of 1000 rpm to obtain mixture one.

[0068] S2. Polyvinyl alcohol is added to deionized water, heated to 85°C, and stirred at 900 rpm for 80 minutes to obtain mixture two.

[0069] S3. Mixture 1 and Mixture 2 are blended at a volume ratio of 1:1 to obtain a mixed solution; 8% of hexamethylenetetramine is added to the mixed solution, heated to 85°C, and stirred at 400 rpm for 200 min to obtain Mixture 3; Mixture 3 is sieved to collect small spherical samples; the small spherical samples are washed and dried to obtain a porous carbon precursor; the precursor is collected, washed, and the porous carbon precursor is obtained; the scanning electron microscope image of the porous carbon precursor is shown below. Figure 1 As shown.

[0070] S4 The porous carbon precursor was uniformly dispersed in a crucible and placed in an oven. It was cured by step heating under a nitrogen atmosphere (0.4 L / min). The curing procedure was as follows: heating to 90°C at a rate of 1°C / min, then holding for 16 hours, then heating to 220°C at a rate of 3°C / min, holding for 6 hours, and then naturally cooling to room temperature of 30°C to obtain a solid composite.

[0071] S5 places the solid composite in a kiln and carbonizes it under a nitrogen atmosphere (0.4 L / min). The carbonization process is as follows: heat up to 550℃ at 5℃ / min, hold for 4 hours, and cool naturally to room temperature to obtain the carbonized composite.

[0072] The CR2032 coin cell was prepared according to the preparation method and parameters of Example 2.

[0073] Examples 15-26 follow the same preparation method and parameter conditions as in Example 14, with differences shown in Table 3.

[0074] Comparative Example 3 follows the same preparation method and parameters as Example 2, except that no dispersant is added.

[0075] Comparative Example 4 follows the same preparation method and parameters as Example 2, except that no curing agent is added.

[0076] Comparative Example 5 follows the same preparation method and parameters as Example 2, except that it does not involve step-by-step temperature increase curing.

[0077] Example 27: Ratio Retention Rate Determination

[0078] The CR2032 coin cells prepared in Examples 14-26 and Comparative Examples 3-5 were tested for rate retention using an electrochemical testing system. 1C rate retention: After being fully charged at 0.2C, the cells were discharged at 1C and 0.2C respectively; the 1C discharge capacity was divided by the 0.2C discharge capacity. 2C rate retention: After being fully charged at 0.2C, the cells were discharged at 5C and 0.2C respectively; the 5C discharge capacity was divided by the 0.2C discharge capacity. The test results are shown in Table 3.

[0079] Table 3. Tests on the retention rate of Examples 14-26 and Comparative Examples 3-5x.

[0080]

[0081]

[0082] As shown in Table 3, in Comparative Example 3, without the addition of a dispersant, the phenolic resin could not be fully cured with the curing agent during the curing process. The resulting carbon skeleton structure after carbonization had poor stability, leading to a significant decrease in the rate performance of the battery. In Comparative Example 4, without the addition of a curing agent, the rate performance was significantly lower than in Comparative Example 3, further demonstrating that the cured phenolic resin maintained a stable carbon source skeleton. During carbonization, the dispersant and curing agent decomposed at high temperatures, maintaining a stable pore size skeleton structure, facilitating the adsorption and deposition of nano-silicon particles, and improving the rate performance of the battery material. In Comparative Example 5, without step-by-step temperature curing, the cross-linking degree between the curing agent and the phenolic resin was poor, resulting in a carbon skeleton structure with poor stability after carbonization, failing to maintain the battery's rate performance. The results of Examples 14-16 show that using a polar dispersant is beneficial for the uniform dispersion of the phenolic resin in the solution. Under the action of the curing agent, it rapidly cross-links with the polar groups of the curing agent to form a stable skeleton structure. Figure 1The results show that the porous carbon precursor has a uniform particle size distribution, with a Dv50 of 5-10 μm. After heating and high-temperature carbonization, the framework structure is stabilized and provides a uniform pore size distribution, increasing the adsorption and deposition of nano-silicon and improving the rate performance of the battery. Examples 16 and 18-20 show that the rate performance changes significantly with the type of curing agent. Aliphatic amines and tertiary amine curing agents decompose at lower temperatures during the high-temperature carbonization process, leading to a decrease in the stability of the carbon framework. The formation of impurities occupies the pore structure of the carbon framework, affecting the deposition and adsorption of nano-silicon, and reducing the specific capacity and rate performance of the battery. The use of hexamethylenetetramine provides a stable framework structure for the phenolic resin. Simultaneously, the aliphatic ring structure prevents the curing agent from decomposing and carbonizing at lower temperatures, avoiding the introduction of carbon impurities. Furthermore, calcination and oxidation at high temperatures form graphitic nitrogen, which fuses with the carbon framework in the original components, while nitrogen is uniformly distributed within it. In this study, nitrogen doping increases the conductivity of the negative electrode material and improves its rate performance. The results of Examples 16 and 21-23 show that as the heating temperature increases, the battery's rate performance initially increases and then decreases. Higher temperatures result in more uniform dispersion of the dispersant and phenolic resin, increasing the degree of crosslinking during curing and making the framework structure more stable. However, excessively high temperatures further increase the degree of crosslinking, increasing the overall heat resistance of the phenolic resin framework. During carbonization, the dispersant and curing agent cannot be fully decomposed, resulting in a carbonized composite with a porous structure, further affecting the deposition of nano-silicon. The results of Examples 16 and 24-26 show that as the carbonization temperature increases, the 1C rate retention rate of the battery tends to remain constant, while the 5C rate retention rate initially increases and then decreases. Excessively high carbonization temperatures increase the pore size during carbon framework formation, reducing the adsorption capacity of nano-silicon and leading to dissociation of nano-silicon during rate cycling, further reducing the battery's rate performance. (See Table 3). Figure 1 The results show that by controlling the types of dispersants and curing agents, heating temperature and carbonization temperature, the CR2032 coin cell produced has a 1C rate retention rate of 98.5% and a 5C rate retention rate of 96.8%, demonstrating a significantly improved rate retention performance.

[0083] Example 28

[0084] The preparation of the carbonized composite was carried out according to Example 16. The carbonized composite was mixed with flake KOH and placed in a kiln. The kiln was activated by chemical activation under a nitrogen atmosphere. The temperature was increased to 850°C at 3°C / min and held for 7 hours. The alkali-to-carbon mass ratio was 5:2. The mixture was cooled to room temperature under a nitrogen atmosphere to obtain a highly microporous porous carbon matrix. The highly microporous porous carbon matrix was acid washed to remove impurities and dried to obtain highly microporous porous carbon. The preparation of the CR2032 coin cell was carried out according to Example 16.

[0085] Examples 29-38 follow the same preparation method and parameter conditions as Example 28, with differences shown in Table 4.

[0086] Example 39 Cyclic Performance Test

[0087] The batteries prepared in Examples 29-38 were discharged to 5mV at 0.5C, 0.05C, 0.02C, and 0.01C in an environment of 25°C, followed by charging to 1.5V at 0.1C. The specific capacity of the first cycle was recorded. Then, 300 discharge and charge cycles were performed, with a charging rate of 0.5C and a discharging rate of 1C. The specific capacity of the 300th cycle was recorded. The cycle capacity retention rate was calculated as (first cycle specific capacity / 300th cycle specific capacity) × 100%. Three groups of batteries were tested, and the average capacity retention rate of the three groups after 300 cycles was calculated. The test results are shown in Table 4.

[0088] Table 4 Cyclic performance tests of Examples 29-38

[0089]

[0090]

[0091] As shown in Table 4, the specific surface area of ​​the highly microporous porous carbon prepared in Examples 29-38 is 1400-3050 m². 2 / g, with a micropore content of 85%-100%, exhibits a significantly improved specific surface area. This high specific surface area provides more deposition sites for nano-silicon during vapor deposition and carbon coating, increasing the binding and adsorption capacity with nano-silicon, making the deposition and adsorption of nano-silicon more stable. This prevents the battery's cycle capacity retention rate from being affected by nano-silicon shedding during cycling. Simultaneously, the high empty content further restricts the movement of nano-silicon, improving the battery's cycle stability. The results of Examples 29-32 show that as the activation reaction temperature increases, the battery's cycle capacity retention rate shows a trend of increasing and then decreasing. Excessively high activation temperatures cause over-activation of the carbonized composite. Due to corrosion, the specific surface area of ​​the highly microporous porous carbon increases significantly. However, the excessively high specific surface area causes the battery electrolyte and metal ions to accumulate in the battery's negative electrode material during charging and discharging. Long-term cycling further reduces the battery's ionic conductivity and cycle stability. The results of Examples 29 and 33-35 show that as the activation reaction time is extended, the activation process becomes more complete, which also causes... An excessive increase in specific surface area also affects the battery's cycle capacity retention. Results from Examples 29 and 36-38 show that as the alkali-to-carbon mass ratio increases, the battery's cycle performance initially increases and then decreases. When the alkali-to-carbon ratio is low, the activation of the carbonized composite is insufficient, resulting in a low specific surface area of ​​the highly microporous carbon. This reduces the deposition and adsorption of nano-silicon during chemical vapor deposition, thus lowering the battery's cycle performance. As the alkali-to-carbon ratio increases, the specific surface area gradually increases, leading to increased adsorption of nano-silicon. Simultaneously, more adsorption occurs during the carbon coating process. The abundance of carbon binding sites improves battery cycle performance and increases ionic conductivity. However, an excessively high alkali-to-carbon ratio leads to a significant increase in the specific surface area of ​​the highly porous carbon, resulting in the accumulation of ions and electrolyte during battery cycling, further affecting the battery's cycle performance. As shown in Table 4, by controlling the activation reaction temperature, activation reaction time, and alkali-to-carbon mass ratio to adjust the specific surface area of ​​the highly porous carbon, the CR2032 coin cell obtained exhibited a capacity retention rate of 95.2% after 300 cycles, demonstrating a significantly improved cycle stability.

[0092] Example 40

[0093] The preparation of highly microporous porous carbon is described in Example 29.

[0094] Highly microporous porous carbon material was placed in a rotary kiln, and silicon-containing gas was introduced to deposit nano-silicon inside the porous carbon matrix, resulting in a porous carbon matrix with deposited nano-silicon. The matrix was then naturally cooled to 300°C under a nitrogen atmosphere, and the gas source was switched to introduce a protective gas and an alkane gas. The mixture was held at this temperature for 4 hours to obtain a silicon-carbon composite material with grown nano-silicon. The silicon-containing gas was a mixture of hydrogen and tetramethylsilane with a volume ratio of 3.5:1, a flow rate of 28 L / h, a heating temperature of 850°C, and a holding time of 3 hours. The protective gas was hydrogen, and the silicon source gas was tetramethylsilane. The volume ratio of the protective gas to the carbon source gas was 4:1, the flow rate of the mixed gas was 32 L / h, and the carbon source gas was methane.

[0095] The preparation of the CR2032 coin cell is described in Example 29.

[0096] Examples 41-52 follow the same preparation method and parameter conditions as Example 40, with differences shown in Table 5.

[0097] Table 5. Component dosages for Examples 40-52 and Comparative Example 6

[0098]

[0099] Comparative Example 6 follows the same preparation method and parameters as Example 40, except that nano-silicon deposition is not performed.

[0100] Example 53 Ionic conductivity test

[0101] The blocking electrode was brought into close contact with the CR2032 coin cells prepared in Examples 41-52 and Comparative Example 6 to ensure good contact between the electrode and the sample, free of bubbles and impurities. The temperature was kept constant at 25°C. The frequency range of the test was set from 13MHz to 10Hz for AC impedance testing. The ionic conductivity was calculated based on the measured impedance value and the sample size. The test results are shown in Table 6.

[0102] The batteries prepared in Examples 41-53 and Comparative Example 6 were subjected to discharge at 0.5C to 5mV, 0.05C to 5mV, 0.02C to 5mV, and 0.01C to 5mV in an environment of 25°C, followed by charging at 0.1C to 1.5V. The specific capacity of the first cycle was recorded. Then, 300 discharge and charge cycles were performed, with a charge rate of 0.5C and a discharge rate of 1C during the cycles. The specific capacity of the 300th cycle was recorded. The test results are shown in Table 6.

[0103] Table 6 Ionic conductivity tests of Examples 41-52 and Comparative Example 6

[0104] Example Ionic conductivity / S / cm Cyclic capacity retention rate / % Example 40 <![CDATA[8.2×10 -2 ]]> 95.3 Example 41 <![CDATA[5.2×10 -2 ]]> 90.2 Example 42 <![CDATA[7.6×10 -2 ]]> 94.5 Example 43 <![CDATA[6.8×10 -2 ]]> 93.7 Example 44 <![CDATA[5.6×10 -2 ]]> 91.4 Example 45 <![CDATA[6.4×10 -2 ]]> 93.5 Example 46 <![CDATA[8.5×10 -2 ]]> 91.2 Example 47 <![CDATA[5.4×10 -2 ]]> 90.5 Example 48 <![CDATA[6.2×10 -2 ]]> 93.1 Example 49 <![CDATA[8.0×10 -2 ]]> 95.2 Example 50 <![CDATA[6.3×10 -2 ]]> 93.3 Example 51 <![CDATA[7.6×10 -2 ]]> 94.6 Example 52 <![CDATA[8.8×10 -2 ]]> 90.2 Comparative Example 6 <![CDATA[3.2×10 -2 ]]> 65.6

[0105] As shown in Table 6, the ionic conductivity of the CR2032 coin cell obtained in Comparative Example 6 without nano-silicon deposition is 3.2 × 10⁻⁶. -5 The S / cm and 300-cycle capacity retention rate were 65.6%, showing a significant decrease in ionic conductivity compared to Examples 41-53, which also reduced the battery's cycle stability. Results from Examples 40-43 show that as the amount of silicon source gas decreased, the battery's ionic conductivity initially increased and then decreased. When the silicon source gas content decreased, fewer nano-silicon particles were deposited within the highly microporous carbon during vapor deposition, resulting in less electron exchange between metal ions and less migration of nano-silicon during battery use, leading to low ionic conductivity and poor cycle stability. Conversely, the introduction of excessive silicon source gas rapidly increased the amount of nano-silicon deposited, occupying the pore structure of the highly microporous carbon and affecting nano-silicon migration. Simultaneously, the contact area between the electrolyte and the battery's negative electrode decreased, reducing ion exchange and conductivity. Furthermore, excessive nano-silicon particles agglomerated, further reducing the battery's cycle stability. Results from Examples 40 and 44-46 show... It is known that the change in silicon-containing gas flow rate controls the deposition rate of nano-silicon inside highly microporous carbon. Increasing the gas flow rate causes nano-silicon to deposit rapidly inside the porous carbon, affecting the carbon coating process and inducing nano-silicon agglomeration, further reducing the battery's conductivity and cycle performance. The results of Examples 40 and 47-49 show that with increasing deposition temperature, the battery's ionic conductivity and cycle capacity retention initially increase and then tend to remain constant; further increasing the deposition temperature has little impact on battery performance. The results of Examples 40 and 50-52 show that with increasing mass ratio of nano-silicon to silicon-carbon composite material, the battery's ionic conductivity gradually increases; however, excessive nano-silicon deposition reduces the battery's cycle capacity retention and decreases cycle stability. In summary, the results in Table 6 show that controlling the silicon-containing gas flow rate, silicon deposition temperature, and improving the mass ratio of nano-silicon to silicon-carbon composite material resulted in a CR2032 coin cell with an ionic conductivity of 8.2 × 10⁻⁶. -2 With an S / cm ratio and a cycle capacity retention rate of 95.3%, the battery material's conductivity has been improved while its cycle stability has been further enhanced.

[0106] Examples 54-66 follow the same preparation method and parameters as Example 41, with differences shown in Table 7.

[0107] Table 7. Changes in preparation parameters in Examples 54-66

[0108] Example Argon:methane volume ratio Carbon-containing gas flow rate / L / h Carbon deposition temperature / °C Carbon coating thickness / nm Example 54 4:1 32 300 120 Example 55 1:1 32 300 120 Example 56 8:1 32 300 120 Example 57 10:1 32 300 120 Example 58 4:1 20 300 120 Example 59 4:1 25 300 120 Example 60 4:1 40 300 120 Example 61 4:1 32 200 120 Example 62 4:1 32 400 120 Example 63 4:1 32 500 120 Example 64 4:1 32 300 20 Example 65 4:1 32 300 80 Example 66 4:1 32 300 200

[0109] Comparative Example 7

[0110] S1 is prepared by adding silicon particles, carbon source precursor polyvinylpyrrolidone, phenolic resin and antioxidant citric acid to an ethanol solution, and grinding them in a mass ratio of 1:0.8:1:0.1 to obtain a dispersion.

[0111] S2 spray-dry the dispersion to obtain the precursor material.

[0112] S3 involves vapor-phase coating of the precursor material. Specifically, 1 kg of the precursor material is placed in a rotary kiln and heated to 900°C under a protective atmosphere. Argon and acetylene gases are introduced at a volume ratio of 2:1 for vapor-phase coating. The mixture is held at this temperature for 1.5 hours, then the organic gas source is turned off. After cooling to room temperature, the material is discharged and graded to obtain the silicon-carbon anode material. The prepared conventional silicon-carbon anode material is used to fabricate electrodes and assemble coin cells for testing, following the same process as in Example 41.

[0113] Example 67 Negative electrode sheet thickness expansion rate test

[0114] The thickness of the negative electrode sheet before battery assembly was measured and denoted as D1. The assembled battery was discharged to 5mV at 0.1C and then to 5mV at 0.02C in a 25°C environment to ensure the negative electrode sheet was fully embedded. The battery was then disassembled, and the thickness of the fully embedded negative electrode sheet was measured and denoted as D2. The thickness of the foil used was 9μm. The thickness expansion rate was calculated according to the following formula: Thickness expansion rate = (D2-D1) / (D1-9)×100%. The test results of Examples 54-66 and Comparative Example 7 are shown in Table 8.

[0115] Table 8 Expansion Rate Tests for Examples 54-66 and Comparative Example 7

[0116] Example Thickness before expansion (μm) Thickness after expansion (μm) Electrode expansion rate (%) Example 54 43 57 41.17 Example 55 44 59 42.85 Example 56 43 58 44.11 Example 57 42 57 45.45 Example 58 44 61 48.57 Example 59 45 62 47.22 Example 60 44 59 42.85 Example 61 44 61 48.57 Example 62 46 62 43.24 Example 63 45 61 44.44 Example 64 41 58 53.12 Example 65 42 58 48.48 Example 66 49 68 47.5 Comparative Example 7 46 74 75.67

[0117] As shown in Table 8, the expansion rate of the negative electrode sheet in Comparative Example 7, prepared using conventional silicon-carbon anode material obtained by spray drying and chemical vapor deposition, and assembled into a coin cell, is significantly increased compared to Examples 54-66. The higher expansion rate of the negative electrode sheet during use further affects the cycle stability of the battery, and significantly reduces battery life under long-term use. The results of Examples 54-57 show that as the volume of methane gas decreases while the carbon coating thickness remains constant, the uniformity of carbon coating in the silicon-carbon composite material decreases significantly. Chemical vapor deposition coats organic carbon onto the porous carbon matrix surface of deposited nano-silicon, acting as a bond between the nano-silicon and the carbon framework matrix. Simultaneously, carbon coating reduces the surface area of ​​the material after ball milling, reducing side reactions. Furthermore, the carbon material also improves the conductivity of the nano-silicon. In Comparative Example 7, since nano-silicon deposition was not performed, and nano-silicon was loaded into the precursor material in a one-step process, the nano-silicon easily expands in volume during battery use, thus increasing the expansion rate of the negative electrode sheet. After carbon coating, the conductivity of lithium carbonate... Pyrolysis can increase the porosity of the surface carbon coating layer, buffer the volume expansion of silicon, and reduce the volume expansion rate of the electrode. The results of Examples 54 and 58-63 show that with the increase of carbon-containing gas flow rate and carbon deposition temperature, the electrode expansion rate exhibits a trend of first increasing and then decreasing. With the increase of flow rate and deposition temperature, the density of the carbon deposit gradually increases. Excessively high carbon layer density further affects the contact area between the battery negative electrode and the electrolyte, further reducing the specific capacity and cycle performance of the battery material. The results of Examples 54 and 64-66 show that with the increase of carbon layer thickness, the uniformity of the carbon coating material gradually decreases, and the buffering effect on the expansion of nano-silicon gradually weakens, resulting in an increase in the electrode expansion rate. In summary, the results in Table 8 show that by controlling the carbon-containing gas flow rate and carbon deposition temperature, and improving the thickness of the carbon coating layer, the electrode expansion rate of the CR2032 coin cell in the fully embedded state of the battery negative electrode is 41.17%, showing a significantly reduced expansion rate, further improving the battery's specific capacity and cycle performance, and increasing battery life.

[0118] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a CVD silicon-carbon composite material, characterized in that: The preparation of the CVD silicon-carbon composite material includes the following steps: S1. Phenols, formaldehyde aqueous solution, and concentrated hydrochloric acid are mixed in a mass ratio of 0.8-2.2:0.5-0.8:0.2-0.6 and heated to react to obtain mixture one; S2. Add the dispersant to the solvent, heat and stir, and mix evenly to obtain mixture two; S3 mixes the first mixture and the second mixture, adds a curing agent and heats at 60-100℃ to obtain the third mixture; the third mixture is sieved to obtain a spherical sample; the spherical sample is washed and dried to obtain a porous carbon precursor; S4 The porous carbon precursor is uniformly dispersed in a sagger and solidified by step heating to obtain a solid composite. S5 The solid composite is placed in a kiln and carbonized in nitrogen at 400-700℃ to obtain a carbonized composite. S6 After blending the carbonized composite with caustic soda flakes, the mixture is placed in a kiln and activated to obtain an activated product; the mass ratio of caustic soda flakes to the carbonized composite is 1-10:1-4; the activated product is cooled to room temperature, acid-washed, and dried to obtain highly microporous porous carbon. S7 Place the highly microporous porous carbon in a kiln, introduce silicon-containing gas, and deposit silicon for 3 hours to obtain a porous carbon matrix with nano-silicon deposition; cool the porous carbon matrix, introduce carbon-containing gas, and deposit carbon for 4 hours to coat the surface of the porous carbon matrix with a carbon layer to obtain the CVD silicon-carbon composite material. The silicon-containing gas is a mixture of hydrogen and tetramethylsilane in a volume ratio of 1-10:1; The carbon-containing gas is a mixture of argon and methane in a volume ratio of 1-10:1; The nano-silicon accounts for 5%-35% of the total mass of the silicon-carbon composite material; The thickness of the carbon coating layer is 20-200 nm.

2. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The phenols are selected from one of phenol, resorcinol, and p-phenylphenol.

3. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The formaldehyde aqueous solution has a mass percentage concentration of 35%-45%.

4. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The volume ratio of the first mixture to the second mixture is 1-3:1-3.

5. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The dispersant is selected from one of polyvinyl alcohol, polyethylene, sodium hexametaphosphate, and polyethylene glycol.

6. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The curing agent is selected from one of hexamethylenetetramine, 2-methylimidazole, and triethanolamine.

7. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The activation reaction temperature is 600-1000℃; the activation reaction time is 3-14h.

8. The method for preparing a CVD silicon-carbon composite material according to claim 1, characterized in that: The flow rate of the silicon-containing gas is 8-50 L / h; the flow rate of the carbon-containing gas is 20-40 L / h.

9. The application of the CVD silicon-carbon composite material obtained by any one of the preparation methods according to claims 1-8, characterized in that: The CVD silicon-carbon composite material is used as a negative electrode material in lithium-ion batteries.

10. A CVD silicon-carbon composite material, characterized in that: The CVD silicon-carbon composite material comprises phenols, formaldehyde aqueous solution, dispersant, curing agent, silicon-containing gas, and carbon-containing gas; the CVD silicon-carbon composite material is prepared by the method described in any one of claims 1-8; the first-cycle efficiency of the battery assembled from the CVD silicon-carbon composite material is 97.2%, and the discharge capacity is 964.5 mAh / g; the 1C rate retention rate of the battery assembled from the CVD silicon-carbon composite material is 98.5%, and the 5C rate retention rate is 96.8%; the capacity retention rate of the battery assembled from the CVD silicon-carbon composite material after 300 cycles is 95.2%; the ionic conductivity of the battery assembled from the CVD silicon-carbon composite material is 8.2 × 10⁻⁶. -2 S / cm; The electrode expansion rate of the battery negative electrode in the fully intercalated state of the CVD silicon-carbon composite material assembled battery is 41.17%.

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