Preparation method of silicon-carbon negative electrode material and silicon-carbon negative electrode material

Abstract

The invention relates to the technical field of lithium ion battery negative electrode materials, in particular to a preparation method of a silicon-carbon negative electrode material and the silicon-carbon negative electrode material. The preparation method comprises the following steps: preparing porous silicon by taking silicon dioxide and magnesium powder as raw materials; the preparation method comprises the following steps: uniformly dispersing porous silicon, polyvinylpyrrolidone and mesocarbon microbeads in ethanol, then adding expanded graphite, mixing and carbonizing; phenolic resin is added into ethyl alcohol for dispersion treatment, then the carbonized material is added for dispersion, carbonization treatment is carried out, and a Si-C precursor is obtained; and introducing acetylene gas into the Si-coated C precursor for chemical vapor deposition carbon coating to prepare the silicon-carbon negative electrode material. The silicon-carbon negative electrode material prepared by the invention is used for preparing a negative electrode material of a lithium ion battery, has good cycle performance, has relatively high reversible specific capacity and first coulombic efficiency, and is excellent in overall electrochemical performance.

Description

Technical Field

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

[0002] The application of silicon-carbon negative electrode materials in lithium-ion batteries is one of the important research directions in the field of battery technology in recent years. The negative electrode materials of traditional lithium-ion batteries are mainly graphite, but its theoretical specific capacity is relatively low (about 372 mAh / g), which is difficult to meet the increasing demand for high energy density. Silicon is considered an ideal choice for the next-generation high-capacity negative electrode material due to its extremely high theoretical specific capacity (about 4200 mAh / g, more than 10 times that of graphite) and moderate working voltage. However, silicon has a serious volume expansion problem during charge and discharge (the expansion rate is as high as more than 300%), resulting in the rupture of the material structure and the pulverization of the electrode, which in turn causes rapid capacity decay and shortened cycle life.

[0003] In order to solve the volume expansion problem of silicon materials, researchers have proposed the design idea of silicon-carbon composite materials. By compounding silicon with carbon materials, silicon-carbon composite materials utilize the high conductivity and structural stability of carbon materials to alleviate the volume expansion effect of silicon, while improving the conductivity and mechanical strength of the electrode. When compounding silicon with carbon materials, the difficult problem is that the binding force between silicon and carbon materials is weak and it is easy to peel off under the action of cyclic expansion stress, resulting in rapid capacity decay. In addition, the expansion of silicon is prone to local stress concentration, which accelerates capacity decay and results in a short cycle life. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a preparation method of a silicon-carbon negative electrode material and a silicon-carbon negative electrode material, which can effectively alleviate the problem of rapid capacity decay caused by the volume expansion of silicon.

[0005] In order to achieve the above purpose, the present invention adopts the following technical solutions: The present invention provides a preparation method of a silicon-carbon negative electrode material, comprising the following steps: S1. Reacting silicon dioxide and magnesium powder as raw materials, and obtaining porous silicon through acid leaching and drying; S2. Dispersing the porous silicon in S1 in ethanol, adding polyvinylpyrrolidone and mesophase carbon microspheres and dispersing evenly, then adding expanded graphite and mixing, removing the solvent and performing carbonization treatment to obtain a carbonized material; S3. Dispersing phenolic resin in ethanol for treatment, then adding the carbonized material in S2 for dispersion, removing the solvent and performing carbonization treatment to obtain a Si@C precursor; S4. The Si@C precursor obtained in S3 is subjected to chemical vapor deposition carbon coating with acetylene gas to prepare a silicon-carbon negative electrode material.

[0006] In the preparation method of the silicon-carbon negative electrode material provided by the present invention, first, a porous silicon material is prepared using silicon dioxide as a raw material, and a porous silicon dispersion is made by mechanical stirring in ethanol. Polyvinylpyrrolidone (PVP, the proportion of residues after carbonization is about 8%) and mesocarbon microbeads (MCMB) are added to the porous silicon dispersion. Among them, MCMB has a spherical flexible structure and oxygen-containing functional groups on its surface, can enter the pores of the porous silicon and combine with the porous silicon, play a flexible support role during the expansion and contraction process of the porous silicon, and relieve the structural damage caused by stress concentration during the expansion and contraction process of the porous silicon. PVP can improve the dispersion uniformity of MCMB and can also be adsorbed on the surface of the porous silicon, forming a cavity after sintering, providing an expansion space for the porous silicon. Treating the carbonized material with an ethanol solution of phenolic resin can play a binding role, improve the bonding force between silicon and expanded graphite, and after the phenolic resin is carbonized, it can form a rigid coating layer with high strength, playing an impact-resistant role for the external force on the silicon. Finally, chemical vapor deposition (CVD) treatment is used to further reduce the surface defects of the material. Through the above method, the structural damage caused by the expansion and contraction process of silicon can be effectively inhibited, and the cycle performance of the silicon-carbon negative electrode material can be improved.

[0007] Further, the D 50 value of the silicon dioxide in S1 is 10 μm to 20 μm; the molar ratio of the silicon dioxide to the magnesium powder is 1:(1 - 3).

[0008] Further, the mass ratio of the porous silicon to the expanded graphite in S2 is 3 - 15:85 - 97; the mass ratio of the porous silicon, the polyvinylpyrrolidone, and the mesocarbon microbeads is 3:(8 - 12):(8 - 12).

[0009] Further, the carbonization treatment in S2 is carried out in an inert atmosphere, the temperature of the carbonization treatment is 750 °C to 850 °C, and the time of the carbonization treatment is 3 h to 5 h; In S2, after the carbonization treatment, it further includes crushing the carbonized material and screening and classifying it with a 325-mesh molecular sieve to obtain a carbonized material.

[0010] Further, the mass ratio of the phenolic resin in S3 to the porous silicon in S2 is 1:(0.12 - 0.3). The phenolic resin used in the present invention is an alcohol-soluble phenolic resin, such as PF4012 produced by Jinan Shengquan Group Co., Ltd.

[0011] Further, the carbonization treatment in S3 is carried out in an inert atmosphere. The temperature of the carbonization treatment is 750°C to 850°C, and the time of the carbonization treatment is 3h to 5h. In S2, after the carbonization treatment, it further includes crushing the carbonized material and screening and classifying it with a 325-mesh molecular sieve to obtain the Si@C precursor.

[0012] Further, the method for chemical vapor deposition of carbon coating in S4 is as follows: Put the Si@C precursor into a rotary furnace. After purging with nitrogen for gas replacement, control the rotation speed to be 1 rpm to 5 rpm, heat up to 850°C to 950°C, keep the temperature for 20 min to 40 min, and then introduce acetylene to carry out constant-temperature treatment of the Si@C precursor for 3h to 5h.

[0013] Further, during the chemical vapor deposition of carbon coating in S4, the nitrogen flow rate is 6 L / min to 10 L / min, and the acetylene flow rate is 1 L / min to 3 L / min.

[0014] Further, in S4, it further includes screening and classifying the powder after chemical vapor deposition of carbon coating with a 325-mesh molecular sieve to obtain the silicon-carbon negative electrode material.

[0015] The present invention also provides a silicon-carbon negative electrode material prepared by the above-mentioned preparation method of the silicon-carbon negative electrode material.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the above-mentioned preparation scheme proposed by the present invention, using silicon dioxide as the raw material to prepare porous silicon first, and then during the silicon-carbon composite process, through PVP, mesophase carbon microspheres, and phenolic resin, the porous silicon can be firmly bonded to the expanded graphite; PVP sintering forms cavities, mesophase carbon microspheres sintering form spherical flexible supports, phenolic resin sintering forms a high-strength rigid coating, and chemical vapor deposition coating. The silicon-carbon negative electrode material obtained by the above preparation method not only has a buffering effect on the outside of silicon, but also provides buffering inside the porous structure of silicon, thereby effectively inhibiting the material structure damage caused by the silicon expansion and contraction process and improving the cycle performance of the negative electrode material.

[0017] 2. The expanded graphite selected in this application has a lower cost compared to porous carbon, and has higher preparation safety and good processing performance; using silicon dioxide as the raw material to prepare porous silicon, compared with amorphous silicon prepared by conventional silane cracking, the porous silicon prepared by silicon dioxide will not undergo a phase transformation at high temperature, has better electrochemical performance, and better safety and processing performance during the preparation process.

[0018] 3. The silicon-carbon negative electrode material provided by this application is used to prepare the negative electrode material of a lithium-ion battery, which has good cycling performance, and also has a relatively high reversible specific capacity and first Coulomb efficiency, and the overall electrochemical performance is excellent. Description of the Drawings

[0019] Figure 1 SEM image of the silicon-carbon negative electrode material prepared in Example 1; Figure 2 Transmission electron microscope image of the silicon-carbon negative electrode material prepared in Example 1. Detailed Description of the Invention

[0020] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present invention.

[0021] Unless otherwise specified, the methods are all conventional methods, and unless otherwise specified, the raw materials can all be obtained from public commercial channels.

[0022] Example 1 Step 1: Put silicon dioxide (D 50 = 10μm - 20μm) and magnesium powder into a high-speed mixer in a molar ratio of 1:2, mix for 3 hours, then transfer to an atmosphere furnace, and introduce N 2 After gas replacement, heat up to 700°C and keep it at a constant temperature for 5 hours. The obtained powder material is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol and vacuum dried at 60°C for 12 hours to obtain porous silicon.

[0023] Step 2: Take 3.5 parts of porous silicon powder and add it to 35 parts of absolute ethanol, and disperse it by ultrasonic wave. According to the mass ratio of 0.3:1:1 (porous silicon:PVP:mesophase carbon microspheres), add PVP and mesophase carbon microspheres to the dispersion liquid. After stirring evenly, add 96.5 parts of expanded graphite and mix at high speed for 60 minutes, then transfer to a rotary evaporator for drying to obtain a powder material. Transfer the obtained powder material into an atmosphere furnace, introduce N 2 After gas replacement, heat up to 800°C and keep it at a constant temperature for 4 hours for carbonization treatment. Crush the obtained material and screen and classify it with a 325-mesh molecular sieve to obtain a carbonized material.

[0024] Step 3: Dissolve phenolic resin in absolute ethanol (the mass ratio of phenolic resin to absolute ethanol is 1:5, and the mass ratio of phenolic resin to the porous silicon powder obtained in Step 2 is 1:0.3), add the carbonized material obtained in Step 2, mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain a powder. Transfer the obtained powder into an atmosphere furnace, and introduce N 2 After gas replacement, heat up to 800 °C and keep the temperature constant for 4 hours for carbonization treatment. Crush the obtained material and screen and classify it with a 325-mesh molecular sieve to obtain the Si@C precursor.

[0025] Step 4: Put the Si@C precursor obtained in Step 3 into a rotary kiln, and introduce N 2 After gas replacement, rotate at a speed of 3 rpm, heat up to 900 °C, keep the temperature constant for half an hour, then introduce acetylene gas for CVD carbon coating to reduce the surface defects of the material. The flow rates of nitrogen and acetylene are 8 L / min and 2 L / min respectively. After keeping the temperature constant for 4 h, discharge to obtain the Si@C powder. Screen and classify the obtained Si@C material with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0026] Example 2 Step 1: Put silicon dioxide (D 50 = 10 μm - 20 μm) and magnesium powder into a high-speed mixer in a molar ratio of 1:2, mix for 3 h, then transfer to an atmosphere furnace, and introduce N 2 After gas replacement, heat up to 700 °C and keep the temperature constant for 5 hours to obtain a powder material. Immerse it in a 2 mol / L hydrochloric acid solution for washing, then wash with deionized water and ethanol, and vacuum dry at 60 °C for 12 hours to obtain porous silicon.

[0027] Step 2: Take 3.5 parts of porous silicon powder and add it to 35 parts of absolute ethanol, and disperse it by ultrasonic. Add PVP and mesophase carbon microspheres to the dispersion liquid in a mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres). After stirring evenly, add 96.5 parts of expanded graphite and mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain a powder. Transfer the obtained powder into an atmosphere furnace, and introduce N 2 After gas replacement, heat up to 800 °C and keep the temperature constant for 4 hours for carbonization treatment. Crush the obtained material and screen and classify it with a 325-mesh molecular sieve to obtain the carbonized material.

[0028] Step 3: Dissolve phenolic resin in absolute ethanol (the mass ratio of phenolic resin to absolute ethanol is 1:5, and the mass ratio of phenolic resin to the porous silicon powder obtained in Step 2 is 1:0.12), add the carbonized material obtained in Step 2, mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain a powder. Transfer the obtained powder into an atmosphere furnace, and introduce N 2After gas replacement, the temperature is raised to 800 °C and carbonization treatment is carried out at a constant temperature for 4 hours. The obtained material is pulverized and screened and classified with a 325-mesh molecular sieve to obtain the Si@C precursor.

[0029] Step 4: Put the Si@C precursor obtained in Step 3 into a rotary furnace, and introduce N 2 After gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 °C, and after half an hour of constant temperature, acetylene gas is introduced for CVD carbon coating to reduce the surface defects of the material. The nitrogen and acetylene flow rates are 8 L / min and 2 L / min respectively. After 4 h of constant temperature, the material is discharged to obtain Si@C powder. The obtained Si@C material is screened and classified with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0030] Example 3 Step 1: Put silicon dioxide (D 50 = 10 μm to 20 μm) and magnesium powder into a high-speed mixer in a molar ratio of 1:2, mix for 3 h, then transfer to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 700 °C and kept at a constant temperature for 5 hours. The obtained powder material is immersed in a 2 mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol and vacuum dried at 60 °C for 12 hours to obtain porous silicon.

[0031] Step 2: Take 10 parts of porous silicon powder and add it to 100 parts of absolute ethanol, and ultrasonically disperse it. According to the mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres), add PVP and mesophase carbon microspheres to the dispersion liquid. After stirring evenly, add 90 parts of expanded graphite and mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain a powder material. Transfer the obtained powder material to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 800 °C and carbonization treatment is carried out at a constant temperature for 4 hours. The obtained material is pulverized and screened and classified with a 325-mesh molecular sieve to obtain the carbonized material.

[0032] Step 3: Dissolve phenolic resin in absolute ethanol (the mass ratio of phenolic resin to absolute ethanol is 1:5, and the mass ratio of phenolic resin to the porous silicon powder in Step 2 is 1:0.3), add the carbonized material obtained in Step 2, mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain a powder material. Transfer the obtained powder material to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 800 °C and carbonization treatment is carried out at a constant temperature for 4 hours. The obtained material is pulverized and screened and classified with a 325-mesh molecular sieve to obtain the Si@C precursor.

[0033] Step 4: Put the Si@C precursor obtained in Step 3 into a rotary furnace, and introduce N 2After gas replacement, the rotation speed is 3 rpm, and the temperature is raised to 900 °C. After holding the temperature constant for half an hour, acetylene gas is introduced for CVD carbon coating to reduce the surface defects of the material. The nitrogen and acetylene flow rates are 8 L / min and 2 L / min respectively. After holding the temperature constant for 4 h, the material is discharged to obtain Si@C powder. The obtained Si@C material is screened and classified with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0034] Comparative Example 1 Step 1: Put silicon dioxide (D 50 = 10 μm to 20 μm) and magnesium powder into a high-speed mixer in a molar ratio of 1:2, mix for 3 h, then transfer to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 700 °C and held constant for 5 hours. The obtained powder material is immersed in a 2 mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol and vacuum dried at 60 °C for 12 hours to obtain porous silicon.

[0035] Step 2: Take 3.5 parts of porous silicon powder and add it to 35 parts of absolute ethanol, and ultrasonically disperse it. Add 96.5 parts of expanded graphite and mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain a Si@C precursor.

[0036] Step 3: Put the Si@C precursor obtained in Step 2 into a rotary kiln, introduce N 2 After gas replacement, the rotation speed is 3 rpm, and the temperature is raised to 900 °C. After holding the temperature constant for half an hour, acetylene gas is introduced for CVD carbon coating to reduce the surface defects of the material. The nitrogen and acetylene flow rates are 8 L / min and 2 L / min respectively. After holding the temperature constant for 4 h, the material is discharged to obtain Si@C powder. The obtained Si@C material is screened and classified with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0037] Comparative Example 2 Step 1: Put silicon dioxide (D 50 = 10 μm to 20 μm) and magnesium powder into a high-speed mixer in a molar ratio of 1:2, mix for 3 h, then transfer to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 700 °C and held constant for 5 hours. The obtained powder material is immersed in a 2 mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol and vacuum dried at 60 °C for 12 hours to obtain porous silicon.

[0038] Step 2: First, add 3.5 parts of porous silicon powder to 35 parts of absolute ethanol and ultrasonically disperse it. According to the mass ratio of 0.3:1:2 (porous silicon: PVP: mesophase carbon microspheres), add PVP and mesophase carbon microspheres to the dispersion liquid, stir well, then add 96.5 parts of expanded graphite and mix at high speed for 60 min, and then transfer to a rotary evaporator for drying to obtain a powder. Transfer the obtained powder to an atmosphere furnace and introduce N2 After gas replacement, the temperature is raised to 800 °C and carbonization treatment is carried out at a constant temperature for 4 hours. The obtained material is pulverized and screened and classified with a 325-mesh molecular sieve to obtain carbonized material.

[0039] Step 3: Put the carbonized material obtained in Step 2 into a rotary kiln, and introduce N 2 After gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 °C, and after half an hour of constant temperature, acetylene gas is introduced for CVD carbon coating to reduce the surface defects of the material. The flow rates of nitrogen and acetylene are 8 L / min and 2 L / min respectively. After 4 h of constant temperature, the material is discharged to obtain Si@C powder. The obtained Si@C material is screened and classified with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0040] Comparative Example 3 Step 1: Put silicon dioxide (D 50 = 10 μm - 20 μm) and magnesium powder into a high-speed mixer in a molar ratio of 1:2, mix for 3 h, then transfer to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 700 °C and kept at a constant temperature for 5 hours. The obtained powder material is immersed in a 2 mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol and vacuum dried at 60 °C for 12 hours to obtain porous silicon.

[0041] Step 2: First, add 3.5 parts of porous silicon powder to 35 parts of absolute ethanol and disperse it ultrasonically. According to the mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres), add PVP and mesophase carbon microspheres to the dispersion liquid. After stirring evenly, add 96.5 parts of expanded graphite and mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain powder material. Transfer the obtained powder material to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 800 °C and carbonization treatment is carried out at a constant temperature for 4 hours. The obtained material is pulverized and screened and classified with a 325-mesh molecular sieve to obtain carbonized material.

[0042] Step 3: Dissolve phenolic resin in absolute ethanol (the mass ratio of phenolic resin to absolute ethanol is 1:5, and the mass ratio of phenolic resin to the porous silicon powder in Step 2 is 1:0.3), add the carbonized material obtained in Step 2, mix at high speed for 60 min, then transfer to a rotary evaporator for drying to obtain powder material. Transfer the obtained powder material to an atmosphere furnace, and introduce N 2 After gas replacement, the temperature is raised to 800 °C and carbonization treatment is carried out at a constant temperature for 4 hours. The obtained material is pulverized and screened and classified with a 325-mesh molecular sieve to obtain the Si@C anode material.

[0043] Comparative Example 4 Step 1: Put silicon dioxide (D 50(with a particle size of 10 μm to 20 μm) and magnesium powder were added to a high-speed mixer at a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After purging with N 2 After gas replacement, the temperature was raised to 700 °C and held for 5 hours. The resulting powder material was immersed in a 2 mol / L hydrochloric acid solution for washing, then washed with deionized water and ethanol, and dried in vacuo at 60 °C for 12 hours to obtain porous silicon.

[0044] Step 2: First, 3.5 parts of porous silicon powder were added to 35 parts of absolute ethanol and ultrasonically dispersed. PVP, mesophase carbon microspheres, and phenolic resin were added to the dispersion at a mass ratio of 0.3:1:1:1 (porous silicon:PVP:mesophase carbon microspheres:phenolic resin). After stirring well, 96.5 parts of expanded graphite were added and mixed at high speed for 60 minutes, then transferred to a rotary evaporator for drying to obtain a powder material. The obtained powder material was transferred into an atmosphere furnace, and N 2 After gas replacement, the temperature was raised to 800 °C and held for 4 hours for carbonization treatment, then pulverized and sieved and classified with a 325-mesh molecular sieve to obtain a carbonized material.

[0045] Step 3: The carbonized material obtained in Step 2 was put into a rotary kiln, and N 2 After gas replacement, the rotation speed was 3 rpm, the temperature was raised to 900 °C, and after holding for half an hour, acetylene gas was introduced for CVD carbon coating to reduce the surface defects of the material. The flow rates of nitrogen and acetylene were 8 L / min and 2 L / min respectively. After holding for 4 hours, the product was discharged to obtain Si@C powder. The obtained Si@C material was sieved and classified with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0046] Comparative Example 5 Step 1: Silicon dioxide (D 50 = 10 μm to 20 μm) and magnesium powder were added to a high-speed mixer at a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After purging with N 2 After gas replacement, the temperature was raised to 700 °C and held for 5 hours. The resulting powder material was immersed in a 2 mol / L hydrochloric acid solution for washing, then washed with deionized water and ethanol, and dried in vacuo at 60 °C for 12 hours to obtain porous silicon.

[0047] Step 2: First, 3.5 parts of porous silicon powder were added to 35 parts of absolute ethanol and ultrasonically dispersed. PVP and phenolic resin were added to the dispersion at a mass ratio of 0.3:1:2 (porous silicon:PVP:phenolic resin). After stirring well, 96.5 parts of expanded graphite were added and mixed at high speed for 60 minutes, then transferred to a rotary evaporator for drying to obtain a powder material. The obtained powder material was transferred into an atmosphere furnace, and N 2After gas replacement, the temperature is raised to 800 °C, and carbonization treatment is carried out at a constant temperature for 4 hours. Then, it is pulverized and screened and classified with a 325-mesh molecular sieve to obtain carbonized material.

[0048] Step 3: Put the carbonized material obtained in Step 2 into a rotary furnace, and introduce N 2 After gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 °C, and after half an hour of constant temperature, acetylene gas is introduced for CVD carbon coating to reduce the surface defects of the material. The flow rates of nitrogen and acetylene are 8 L / min and 2 L / min respectively. After 4 h of constant temperature, the material is discharged to obtain Si@C powder. The obtained Si@C material is screened and classified with a 325-mesh molecular sieve to obtain the finished silicon-carbon anode material.

[0049] Test Example A bench-top scanning electron microscope, a full-automatic specific surface area tester, and a laser particle size analyzer are used to test the physical indexes of the material; a button battery encapsulation machine, a button cell test control cabinet, an electrochemical analyzer, and a resistance measuring instrument are used to test the electrochemical indexes of the material.

[0050] The half-cell test method is to prepare a slurry with the prepared silicon-carbon anode material as the negative electrode active material. The slurry ratio is: active material: CNTs (including dispersant): CMC: SBR = 89%: 4.5%: 1.5%: 5%. Then, the slurry is coated on a copper foil and vacuum dried for 12 h to make a negative electrode sheet. The electrolyte is commercially purchased, the separator is a PE membrane, and the lithium sheet is the counter electrode. A half-cell is assembled in a glove box. A constant current charge-discharge experiment is carried out on a LAND battery test system. Unless otherwise specified, the following data are all charged and discharged at 1C, and the charge-discharge voltage is limited to 0.005V - 2V. A computer-controlled charge-discharge cabinet is used for data acquisition and control.

[0051] Table 1: Half-cell test results

[0052] It can be seen from the half-cell test data in Table 1 that, in terms of capacity, initial efficiency, and cycle stability, the overall performance of the silicon-carbon anode material provided in Example 1 is superior to that of the silicon-carbon anode materials provided in Examples 2-3 and Comparative Examples 1-5. Among them, in Example 2, the increase in the content of phenolic resin led to an overly thick carbon coating layer and a decrease in capacity; in Example 3, the capacity increased after increasing the content of porous silicon, but there were relatively more side reactions. At the same time, it was more difficult to limit the volume expansion of silicon, and its cycle performance was worse; in Comparative Example 1, the three-layer buffer structure was not constructed for the silicon-carbon anode material, and there was no force to fix the porous silicon on the surface and the expanded graphite. As a result, silicon shedding occurred during the experiment, the capacity decreased, and the carbon layer deposited only by CVD was not sufficient to limit the silicon expansion, and the cycle life was poor. In Comparative Example 2, only low-strength mesophase carbon microspheres were used as the flexible layer for coating. It can be seen that the cycle life decreased significantly, indicating that the strength of the mesophase carbon microspheres was not sufficient to limit the volume expansion of silicon. At the same time, the influence of CVD on reducing surface defects on the initial efficiency and cycle of the material was also explored. It can be seen that in Comparative Example 3 without the CVD step, its initial efficiency was poor, and the surface functional groups were not completely removed. During the cycle, the electrolyte reacted with the surface functional groups, continuously generating the SEI film, resulting in continuous consumption of lithium ions and poor cycle performance. In Comparative Example 4, the influence of the one-step mixing method on the electrochemical performance of the material was explored. Since silicon was mixed with PVP, mesophase carbon microspheres, and phenolic resin in the same container, part of the silicon was exposed on the surface of the material. Although the capacity increased slightly, its cycle performance was poor. In Comparative Example 5, the mesophase carbon microspheres were replaced with the same proportion of phenolic resin for mixing, and it was found that the cycle performance was worse than that in Comparative Example 4. This may be because silicon did not have the flexible support buffer provided by the mesophase carbon microspheres and was only restricted by the rigid carbon structure provided by the phenolic resin, resulting in breakage and pulverization of silicon after expansion and contraction during long-term charge and discharge.

[0053] Further analyze the microscopic morphology of the silicon-carbon anode material prepared in Example 1, Figure 1 is the SEM image of the silicon-carbon anode material prepared in Example 1, Figure 2 is the transmission electron microscope image of the finished silicon-carbon anode material prepared in Example 1. From Figure 1 it can be seen that the porous silicon is uniformly coated with a carbon layer, and there is no free or agglomerated porous silicon; Figure 2 It can be seen from the transmission electron microscope image that there is a carbon coating layer deposited by CVD gas phase on the surface of the material, and the surface is flat without obvious defects.

[0054] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application and not to limit them; although the present application has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: the specific implementation manners of the present application can still be modified or some technical features can be equivalently replaced, and they should all be covered by the scope of the technical solutions claimed in the present application.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: The following steps are involved: S1, using silicon dioxide and magnesium powder as raw materials to react, and obtaining porous silicon by acid leaching and drying; S2, adding the porous silicon in S1 to ethanol for dispersion, adding polyvinyl pyrrolidone and mesophase carbon microspheres for uniform dispersion, adding expanded graphite for mixing, removing the solvent and performing carbonization treatment to obtain a carbonized material; S3, adding phenolic resin to ethanol for dispersion treatment, then adding the carbonized material in S2 for dispersion, removing the solvent and then performing carbonization treatment to obtain a Si@C precursor; S4. The Si@C precursor obtained in S3 is carbon-coated by chemical vapor deposition using acetylene gas to prepare a silicon-carbon negative electrode material.

2. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: S1 SiO2 D 50 The value is 10μm~20μm; the molar ratio of the silicon dioxide to the magnesium powder is 1:(1~3).

3. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: S2: The mass ratio of the porous silicon to the expanded graphite is 3-15:85-97; the mass ratio of the porous silicon, the polyvinyl pyrrolidone and the mesophase carbon microspheres is 3:(8-12):(8-12).

4. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: S2: The carbonization treatment is carried out under an inert atmosphere, the temperature of the carbonization treatment is 750°C to 850°C, and the time of the carbonization treatment is 3h to 5h; In S2, after the carbonization treatment, the carbonized material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

5. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: The mass ratio of the phenolic resin in S3 to the porous silicon in S2 is 1:(0.12-0.3).

6. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: S3: the carbonization treatment is carried out under an inert atmosphere, the temperature of the carbonization treatment is 750°C to 850°C, and the time of the carbonization treatment is 3h to 5h; In S3, after the carbonization treatment, the carbonized material is crushed and sieved with a 325-mesh molecular sieve to obtain a Si@C precursor.

7. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: S4 The chemical vapor deposition carbon coating method is to put the Si@C precursor into a rotary furnace, introduce nitrogen for gas replacement, control the rotation speed to 1rpm~5rpm, increase the temperature to 850℃~950℃, keep the temperature for 20min~40min, then introduce acetylene to treat the Si@C precursor at a constant temperature for 3h~5h.

8. The method for preparing a silicon-carbon negative electrode material according to claim 7, characterized in that: In the chemical vapor deposition carbon coating process described in S4, the nitrogen flow rate is 6L / min to 10L / min, and the acetylene flow rate is 1L / min to 3L / min.

9. The method for preparing a silicon-carbon negative electrode material according to claim 1, characterized in that: S4 also includes sieving and grading the chemical vapor deposition carbon-coated powder using a 325-mesh molecular sieve to obtain a silicon-carbon negative electrode material.

10. The silicon-carbon negative electrode material prepared by the method for preparing a silicon-carbon negative electrode material according to any one of claims 1 to 9.

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