Preparation method of improved silicon-carbon composite negative electrode material and lithium ion battery

Abstract

The invention belongs to the field of silicon-carbon negative electrode material manufacturing, and particularly relates to a preparation method of an improved silicon-carbon material and a lithium ion battery. The preparation method comprises the following steps: (S1) soaking porous carbon in a solution containing boron and transition metal, taking out, and drying to obtain boron-doped porous carbon; (S2) carrying out vacuum degassing on the boron-doped porous carbon under a heating condition; (S3) putting the porous carbon subjected to vacuum degassing into a reactor, introducing a mixed fluid of supercritical carbon dioxide and silane gas, and introducing the mixed fluid under an ultrasonic vibration condition to complete the adsorption of the porous carbon on the silane; (S4) after adsorption, raising the temperature of the reactor, forming a uniform silicon layer in a porous carbon channel by the silane gas, and cooling; (S5) repeating the steps S3 and S4 until the material reaches the required silicon content; and (S6) coating the surface of the silicon deposition layer with an amorphous carbon layer to form the silicon-carbon composite material.

Description

Technical Field

[0001] This invention belongs to the field of silicon-carbon anode material manufacturing, specifically relating to a method for preparing an improved silicon-carbon composite anode material and a lithium-ion battery. Background Technology

[0002] Silicon, due to its extremely high theoretical specific capacity (approximately 4200 mAh / g), is considered an important candidate for next-generation high-energy-density lithium-ion battery anode materials. However, silicon undergoes significant volume changes (approximately 300%) during charge and discharge, leading to material structure damage, continuous reconstruction of the solid-state electrolyte interface (SEI), and rapid degradation of cycle performance, severely limiting its practical applications. To alleviate these problems, researchers generally adopt a strategy of embedding nano-silicon into porous carbon matrices to utilize the buffering effect of carbon to suppress the volume effect. Chemical vapor deposition (CVD) is a commonly used silicon deposition method, but it still faces several key challenges in practical applications: the diffusion of gas molecules within micropores is restricted, especially when the pore size is small, making it difficult for gaseous precursors to penetrate deep pores, resulting in uneven silicon deposition and low actual pore utilization; residual gases adsorbed on porous carbon hinder the adsorption and reaction of silicon source gases, causing local nucleation and uneven deposition; and the low gas-solid mass transfer efficiency and gas film blocking effect in traditional fluidized bed reactors further exacerbate the unevenness of silicon deposition, affecting the consistency of material structure and electrochemical performance. Therefore, there is an urgent need to develop a deposition method that can achieve uniform silicon distribution and high bonding strength within porous carbon, while maintaining process control, in order to improve the initial coulombic efficiency and long-term cycling stability of silicon-carbon composites. It is evident that achieving high silicon content and uniform distribution within porous carbon has been a significant technical challenge in the preparation of silicon-carbon composites via silane deposition.

[0003] CN119008920A, CN119674026A, CN119683604A, and CN119208563A reported methods for preparing silicon-carbon composite anode materials based on silicon deposition. However, the problems of insufficient silane vapor deposition capability, residual gas in the pores hindering silane diffusion, and low deposition efficiency have not been solved.

[0004] The inventor's previous patent disclosed a method for preparing silicon-carbon materials by vapor deposition based on supercritical carrier gas. This method utilizes supercritical fluidization to enhance the dispersion of silicon source gas in the reactor and the diffusion of porous carbon nanopores, helping the silicon source gas to rapidly reach the deep pores of the porous carbon framework and significantly enhancing its adsorption effect in ultramicropores <1 nm. However, because the volume of silane gas adsorbed in the porous carbon pores is limited, this patent requires repeated adsorption and heating deposition steps, which is time-consuming and labor-intensive, and still presents inconveniences for large-scale industrial production. Summary of the Invention

[0005] The purpose of this invention is to improve silicon deposition efficiency, structural stability, and electrochemical performance of silicon-carbon composite materials through supercritical carrier gas technology. It provides an improved method for preparing silicon-carbon materials. First, boron- and transition metal-doped porous carbon is prepared. Boron doping provides Lewis acid sites that interact with silane molecules. The transition metal has d orbitals that can interact with the empty orbitals of silane gas, forming complex adsorption. The synergistic effect of boron and transition metal doping increases the adsorption capacity of the doped porous carbon for silanes. The pore structure of the porous carbon is optimized to increase the micropore ratio. Supercritical carbon dioxide and silane gas are premixed, and the mixture is introduced under high pressure. The pressure is then rapidly reduced, causing the silane compounds to be supersaturated and adsorbed within the porous carbon channels. Silane deposition and carbon coating are then performed, reducing the number of silane adsorption and deposition cycles, which is more conducive to large-scale industrial production. Specifically, this invention provides the following technical solutions: A method for preparing an improved silicon-carbon material includes the following steps: (S1) Porous carbon is impregnated in a solution containing boron and transition metals, removed, and dried to obtain boron-doped porous carbon; (S2) Vacuum degassing of boron-doped porous carbon under heating conditions; (S3) The vacuum-degassed porous carbon is put into the reactor, and a mixed fluid of supercritical carbon dioxide and silane gas is introduced. Under ultrasonic vibration conditions, the mixed fluid is introduced to complete the adsorption of silane by the porous carbon. (S4) After adsorption, the reactor is heated, and the silane gas forms a uniform silicon layer in the porous carbon channels, and then the temperature is lowered. (S5) Repeat steps S3 and S4 until the material reaches the required silicon content. (S6) An amorphous carbon layer is coated on the surface of the silicon deposition layer to form a silicon-carbon composite material.

[0006] Further, in step (S1), the median particle size D50 of the porous carbon is 3-10 μm, and the BET specific surface area is 1600-2400 m². 2 / g, pore volume 0.8-0.95 cm³ 3 / g, with a micropore content of 85-95%, where micropore content is the percentage of micropore volume to total pore volume. Using porous carbon with a large BET specific surface area and a high micropore content can increase the saturated adsorption capacity of silane gases, increase adsorption efficiency, and reduce the number of adsorption and deposition cycles.

[0007] Further, in step (S1), the solution containing boron and transition metal is a mixture of boric acid, water-soluble salts of transition metal, and polyol and water in a mass ratio of 1-2:0.3-0.5:1-3:10; the mass-volume ratio of porous carbon to boron-containing solution is 1g:5-10mL; the immersion time is 30-100min; and the drying is not particularly limited, such as oven drying or vacuum drying.

[0008] Furthermore, the water-soluble salt of the transition metal is a halide salt of Sn, Ni, Cu, or Fe, preferably at least one of SnCl4, NiCl2, CuCl2, or FeCl3.

[0009] Furthermore, the polyol is at least one of polyethylene glycol and polypropylene glycol with a number average molecular weight of 400-800.

[0010] This invention involves doping porous carbon with boron and transition metals. The two doping elements work together synergistically, significantly improving the adsorption capacity of the porous carbon for silanes. This reduces the need for subsequent silane gas adsorption and silane deposition cycles, thereby reducing process steps, saving costs, and enabling the use of supercritical fluid as a carrier gas for silane adsorption. This further advances the industrialization of silane deposition technology in the field of silicon-carbon composite anode material manufacturing technology.

[0011] Further, in step (S2), the purpose of vacuum degassing under heating conditions is to remove the gas originally adsorbed in the porous carbon channel structure and increase the adsorption capacity of silane gas. This invention employs a multi-stage heating vacuum degassing process, with the temperature gradually increasing and the pressure gradually decreasing. This thoroughly removes the gas originally adsorbed by the porous carbon, which is beneficial for the adsorption of silane gas by the porous carbon while maintaining the integrity of the porous carbon channel structure. This avoids the collapse and shrinkage of the channel structure caused by rapid degassing of local channels in the porous carbon framework, leading to local pressure imbalances. In a preferred embodiment of this invention, vacuum degassing under heating conditions includes the following steps: the temperature in the first stage is 80-120℃, and the pressure is reduced to 1×10⁻⁶. 3 Pa to 5×10 3 The first stage is at 120-180℃, with the pressure reduced to 1 Pa to 100 Pa, maintained for 10-60 min; the second stage is at 180-225℃, with the pressure reduced to 0.1 Pa to 1 Pa, maintained for 20-60 min; the third stage is at 225-290℃, with the pressure reduced to 0.01 Pa to 0.1 Pa, maintained for 20-60 min; furthermore, the heating rate in the vacuum degassing process is 1-5℃ / min.

[0012] Further, in step (S3), the mixed fluid is a mixture of carbon dioxide and silane at a volume ratio of 0.8-1.2:1, for example, 1:1; the flow rate of the mixed fluid is 100-150 L / min; the temperature of the mixed fluid is >35℃, and the pressure is >8MPa. At this point, the carbon dioxide is in a supercritical state, and supercritical carbon dioxide has good solubility for silane. In the inventor's previous scheme for the uniform deposition of silane on porous carbon based on supercritical fluid as a carrier gas, the conventional silane deposition method involves introducing supercritical carbon dioxide fluid and silane gas, and using a conventional carrier gas to silane gas ratio. On the one hand, the adsorption efficiency is low, increasing the time for porous carbon to reach saturation adsorption; on the other hand, although porous carbon has weak adsorption of carbon dioxide, there is still competitive adsorption. When the volume of carbon dioxide fluid is much larger than that of silane gas, the amount of carbon dioxide adsorbed by porous carbon cannot be ignored, and carbon dioxide adsorption is ineffective for this invention and should be minimized. The silane gas includes, but is not limited to, at least one of silane, disilane, propane, monochlorosilane, dichlorosilane, trichlorosilane, and silicon tetrachloride; preferably silane or disilane. Porous carbon adsorbs it rapidly and has a large adsorption capacity, which is more conducive to achieving the purpose of this invention.

[0013] Furthermore, in step (S3), the frequency of the ultrasonic vibration is 30-60 kHz, and the ultrasonic power is 150-200 W / m. 3 Within the aforementioned frequency and power range, ultrasound can effectively break up bubbles and particle clusters in the reactor, thereby achieving more uniform gas-solid contact, improving the uniformity of silicon deposition, while ensuring sufficient penetration depth to avoid power attenuation at the center of the device, thus ensuring more uniform gas-solid contact at every point in the reactor.

[0014] Furthermore, in step (S4), the heating temperature is 500-650℃, and after heating to the target temperature, the holding time is 30-60 minutes. There is no particular limitation on the heating rate, such as 1-10℃ / min.

[0015] Furthermore, in step (S5), the number of repetitions is such that the silicon mass content of the composite material obtained in step (S5) is 45-55%. Generally, silicon-carbon composite materials with silicon content within the above range have the best electrochemical performance.

[0016] Further, in step (S6), the amorphous carbon layer is deposited by pyrolyzing a carbon source gas on the surface. The carbon source gas is at least one of methane, acetylene, ethylene, propylene, propyne, methane, ethane, and propane. The coating process temperature is preferably 450-750℃, the carbon source gas flow rate is preferably 3-20 L / min, and the holding time is preferably 3-12 h. The thickness of the carbon coating layer after coating is 3-7 nm, for example, 4-6 nm, or even 5 nm.

[0017] The present invention also provides a lithium-ion battery, wherein the negative electrode comprises the silicon-carbon composite negative electrode material prepared by the above preparation method.

[0018] Compared with the prior art, the present invention has the following technical advantages: This invention involves doping porous carbon with boron and transition metals. The synergistic effect of boron and transition metals significantly enhances the adsorption capacity of porous carbon for silane gas, reducing the number of cycles for silane adsorption and deposition in steps S3 and S4, saving process time, and further paving the way for the industrialization of supercritical fluid-assisted silane deposition for the preparation of silicon-carbon composite materials. This invention uses supercritical fluid fluidization to enhance the dispersion of silicon source gas in the reactor and the diffusion of the porous carbon nanopore structure, helping the silicon source gas to quickly reach the deep pore structure of the porous carbon framework. This achieves ultra-uniform deposition of silicon source gas within the porous carbon framework, thereby reducing side reactions on the particle surface, improving initial coulombic efficiency, reducing stress concentration and volume expansion during charge and discharge, and improving cycle stability. Attached Figure Description

[0019] Figure 1 This is a SEM image of the silicon-carbon composite anode material prepared in Example 1; Figure 2 This is a SEM image of the silicon-carbon composite anode material prepared in Comparative Example 4. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0021] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are commercially available.

[0022] The scanning electron microscope (SEM) used was a Regulus 8100, and the transmission electron microscope (TEM) used was a JEM-2100F.

[0023] The porous carbon material used in this invention was purchased from Beijing Yijin New Energy Technology Co., Ltd., with a D50 of 7.2 μm and a BET specific surface area of ​​2045 m². 2 / g, micropores account for 90.2%, and the pore volume is 0.91 cm³. 3 / g.

[0024] Example 1 (S1) Boric acid, ferric chloride, PEG400 and water were mixed in a mass ratio of 10:5:15:100 to prepare a mixed solution. 10 kg of porous carbon was immersed in 50 L of the above mixed solution for 60 min. After immersion, the carbon was removed and vacuum dried to obtain doped porous carbon. (S2) 10 kg of doped porous carbon was added to the vacuum chamber, heated to 100°C, and then evacuated to remove gas until it reached 100°C. 3 Pa, maintain for 15 min, then heat to 150℃, vacuum degas to 10 Pa, maintain for 20 min, then heat to 200℃, vacuum degas to 0.1 Pa, maintain for 25 min, finally heat to 260℃, vacuum degas to 0.01 Pa, maintain for 30 min to complete degassing, and obtain the degassed porous carbon material. (S3) 10 kg of vacuum-degassed porous carbon was added to the reactor. A mixture of supercritical carbon dioxide and silane gas (volume ratio 1:1) was introduced at a flow rate of 100 L / min under conditions of 50 °C and 10 MPa. The mixture was introduced under ultrasonic vibration conditions. An ultrasonic generator was installed in the reactor, with an ultrasonic vibration frequency of 30 kHz and an ultrasonic power of 150 W / m. 3 The adsorption of silane gas by porous carbon is completed in 8 minutes, after which the ventilation and ultrasonic vibration are stopped. (S4) The reactor is heated to 560°C at a heating rate of 5°C / min, held for 30 min, and then cooled to 50°C. Silane gas forms a uniform silicon layer in the porous carbon channels. (S5) Repeat step S3 (silane adsorption) and step S4 (silane deposition) 9 times. The silicon content of the final material is 51.8 wt%. (S6) The reactor is heated to 650°C and methane gas at a rate of 13 L / min is introduced and kept at this temperature for 4 h to obtain a silicon-carbon composite anode material with a carbon shell coating on the surface. The carbon coating thickness is about 5 nm.

[0025] Example 2 The other conditions and operations are the same as in Example 1, except that in step (S1), the mixed solution is prepared by mixing boric acid, copper chloride, PPG800 and water in a mass ratio of 15:3:20:100.

[0026] Example 3 The other conditions and operations are the same as in Example 1, except that in step (S1), the mixed solution is prepared by mixing boric acid, tin chloride, PEG400 and water in a mass ratio of 20:4:30:100.

[0027] Example 4 The other conditions and operations are the same as in Example 1, except that in step (S3), the mixed fluid is a mixture of supercritical carbon dioxide and silane gas in a volume ratio of 0.8:1.

[0028] Example 5 The other conditions and operations are the same as in Example 1, except that in step (S3), the mixed fluid is a mixture of supercritical carbon dioxide and silane gas in a volume ratio of 1.2:1.

[0029] Comparative Example 1 The other operations are the same as in Example 1, except that in step (S1): boric acid, PEG400, and water are mixed in a mass ratio of 10:15:100 to form a solution, meaning the porous carbon is not doped with Fe. In step (S5), steps S3 and S4 need to be repeated 12 times to achieve a silicon content of 51.8 wt% in the final material. Comparative Example 2 The other operations are the same as in Example 1, except that in step (S1): ferric chloride, PEG400, and water are mixed in a mass ratio of 5:15:100 to form a mixed solution, meaning that the porous carbon is not doped with boron. In step (S5), steps S3 and S4 need to be repeated 11 times to achieve a silicon content of 51.8 wt% in the final material.

[0030] Comparative Example 3 The other operations are the same as in Example 1, except that step (S1) is omitted, and in step (S2), porous carbon is directly heated and degassed. Then, steps S3 and S4 are repeated 15 times, and the silicon content of the final material is 51.8 wt%.

[0031] Comparative Example 4 (S1) 10 kg of porous carbon material was put into the reactor and heated to 580°C at a heating rate of 5°C / min. A mixture of silane and argon in a volume ratio of 1:4 was introduced and the flow rate of the mixed gas was 20 L / min. Silane deposition was carried out. The silicon mass content of the obtained silicon-carbon material was 51.8 wt% by controlling the deposition time, which is the same as in Example 1. (S2) Coating of silicon-carbon composite material: The reactor was heated to 650℃ and methane gas was introduced at a rate of 13 L / min and kept at the temperature for 4 h to obtain silicon-carbon composite anode material with a carbon shell coating on the surface.

[0032] Figure 1 This is a SEM image of the silicon-carbon composite anode material prepared in Example 1. Figure 2 This is a SEM image of the silicon-carbon composite anode material prepared in Comparative Example 4.

[0033] Application examples The electrochemical performance of the silicon-based anode materials prepared in the above examples and comparative examples was tested according to the following method: The prepared silicon-carbon composite material, carbon black, and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) composite binder were mixed in a mass ratio of 80:10:10 to form a slurry (where the mass ratio of CMC and SBR was 1:1). The slurry was uniformly coated onto a copper foil current collector and dried under vacuum for 12 h to form a working electrode. A lithium sheet was used as the counter electrode, a glass fiber membrane (purchased from Whatman Ltd., UK) was used as the separator, and 1 mol / L LiPF6 (the solvent was a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1) was used as the electrolyte. 1% VC and 5% FEC were added to the electrolyte. The cells were assembled into coin cells in an argon-atmosphere inert gas glove box from Braun, Germany.

[0034] Electrochemical analysis and testing were performed on the silicon-carbon composite materials prepared in the examples and comparative examples. The charge-discharge range was 0-1.5 V, and the charge-discharge was performed at a current density of 0.2 C, which proved that the silicon-carbon composite material obtained in this invention has good cycle stability.

[0035] Charge-discharge tests were conducted on the negative electrode materials of other embodiments and comparative examples using the method described above, and the results are shown in Table 1 below: Table 1 Electrochemical performance test .

[0036] In summary, the preparation method provided by this invention achieves ultra-uniform deposition of porous carbon using a mixed fluid of supercritical carbon dioxide and silane, thereby improving the cycle stability of silicon-carbon composite materials. Furthermore, this invention reduces the number of cycles for silane adsorption and deposition by doping the porous carbon with boron and transition metals, and by pre-mixing the supercritical fluid and silane gas, thus shortening the process time, reducing the number of steps, and making it more suitable for industrial production.

Claims

1. A method for preparing an improved silicon-carbon material, characterized in that, Includes the following steps: (S1) Porous carbon is impregnated in a solution containing boron and transition metals, removed, and dried to obtain boron-doped porous carbon; (S2) Vacuum degassing of boron-doped porous carbon under heating conditions; (S3) The vacuum-degassed porous carbon is put into the reactor, and a mixed fluid of supercritical carbon dioxide and silane gas is introduced. Under ultrasonic vibration conditions, the mixed fluid is introduced to complete the adsorption of silane by the porous carbon. (S4) After adsorption, the reactor is heated, and the silane gas forms a uniform silicon layer in the porous carbon channels, and then the temperature is lowered. (S5) Repeat steps S3 and S4 until the material reaches the required silicon content; (S6) An amorphous carbon layer is coated on the surface of the silicon deposition layer to form a silicon-carbon composite material.

2. The preparation method according to claim 1, characterized in that, In step (S1), the median particle size D50 of the porous carbon is 3-10 μm, and the BET specific surface area is 1600-2400 m². 2 / g, pore volume 0.8-0.95 cm³ 3 / g, with a micropore content of 85-95%.

3. The preparation method according to claim 1, characterized in that, In step (S1), the solution containing boron and transition metals is a mixture of boric acid, water-soluble salts of transition metals, polyol and water in a mass ratio of 1-2:0.3-0.5:1-3:

10.

4. The preparation method according to claim 1, characterized in that, The mass-to-volume ratio of porous carbon to boron-containing solution is 1 g: 5-10 mL; the impregnation time is 30-100 min.

5. The preparation method according to claim 3, characterized in that, Water-soluble salts of transition metals are halides of Sn, Ni, Cu, and Fe.

6. The preparation method according to claim 3, characterized in that, The polyol is at least one of polyethylene glycol and polypropylene glycol with a number average molecular weight of 400-800.

7. The preparation method according to claim 1, characterized in that, In step (S3), the frequency of ultrasonic vibration is 30-60 kHz, and the ultrasonic power is 150-200 W / m. 3 .

8. The preparation method according to claim 1, characterized in that, In step (S4), the heating temperature is 500-650℃, and after heating to the target temperature, the holding time is 30-60 minutes. There is no particular limitation on the heating rate, such as 1-10℃ / min.

9. The preparation method according to claim 1, characterized in that, In step (S5), the number of repetitions is such that the silicon mass content of the composite material obtained in step (S5) is 45-55%; and / or In step (S6), the amorphous carbon layer is deposited by pyrolyzing a carbon source gas on the surface. The carbon source gas is at least one of methane, acetylene, ethylene, propylene, propyne, methane, ethane, and propane. The coating process temperature is 450-750℃, the carbon source gas flow rate is 3-20 L / min, and the holding time is 3-12 h. The thickness of the carbon coating layer after coating is 3-7 nm.

10. A lithium-ion battery, characterized in that, Its negative electrode includes the silicon-carbon composite negative electrode material prepared by the preparation method according to any one of claims 1-9.

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