A preparation method of silicon-carbon negative electrode material and silicon-carbon negative electrode material

Abstract

The present invention relates to the technical field of lithium-ion battery negative electrode materials, and specifically to a preparation method and silicon-carbon negative electrode material. The preparation method comprises the following steps: preparing porous silicon using silicon dioxide and magnesium powder as raw materials; uniformly dispersing porous silicon, polyvinyl pyrrolidone and mesophase carbon microspheres in ethanol, then adding expanded graphite for mixing and carbonizing; adding phenolic resin into ethanol for dispersion treatment, then adding carbonizing material for dispersion and carbonization treatment to obtain a Si@C precursor; passing acetylene gas into the Si@C precursor for chemical vapor deposition carbon coating to prepare a silicon-carbon negative electrode material. The silicon-carbon negative electrode material prepared in the present application is used to prepare a negative electrode material for lithium-ion batteries, has good cycle performance, and has both high reversible specific capacity and first coulombic efficiency, and excellent overall electrochemical performance.

Description

Technical Field

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

[0002] The application of silicon-carbon anode materials in lithium-ion batteries has been a key research topic in battery technology in recent years. Graphite is the primary anode material for conventional lithium-ion batteries, but its low theoretical specific capacity (approximately 372 mAh / g) makes it difficult to meet the growing demand for higher energy density. Silicon, due to its extremely high theoretical specific capacity (approximately 4200 mAh / g, more than 10 times that of graphite) and moderate operating voltage, is considered an ideal candidate for the next generation of high-capacity anode materials. However, silicon exhibits severe volume expansion during charge and discharge (expansion rates exceeding 300%), leading to structural fracture and electrode pulverization, resulting in rapid capacity decay and shortened cycle life.

[0003] To address the volume expansion problem of silicon materials, researchers have proposed a design concept for silicon-carbon composite materials. Silicon-carbon composite materials combine silicon with carbon materials, utilizing 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. The difficulty in combining silicon with carbon materials is that the bonding between silicon and carbon materials is weak, making them prone to peeling off under cyclic expansion stress, leading to rapid capacity decay. In addition, the expansion of silicon easily produces local stress concentration, which accelerates capacity decay and results in a shorter cycle life. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for preparing 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 silicon volume expansion.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention provides a method for preparing a silicon-carbon negative electrode material, comprising the following steps:

[0007] S1, reacting silicon dioxide and magnesium powder as raw materials, and obtaining porous silicon by acid leaching and drying;

[0008] S2, adding the porous silicon in S1 to ethanol for dispersion, adding polyvinyl pyrrolidone and mesophase carbon microspheres and uniformly dispersing them, then adding expanded graphite and mixing, removing the solvent and performing carbonization treatment to obtain a carbonized material;

[0009] S3, adding phenolic resin to ethanol for dispersion treatment, then adding the carbonized material in S2 for dispersion, removing the solvent and then carbonizing to obtain a Si@C precursor;

[0010] S4. The Si@C precursor obtained in S3 is subjected to chemical vapor deposition carbon coating using acetylene gas to prepare a silicon-carbon negative electrode material.

[0011] In the preparation method of the silicon-carbon negative electrode material provided by the present invention, a porous silicon material is first prepared using silicon dioxide as a raw material, and a porous silicon dispersion is made by mechanical stirring in ethanol. Polyvinyl pyrrolidone (PVP, the proportion of residue after carbonization is about 8%) and mesophase carbon microspheres (MCMB) are added to the porous silicon dispersion. MCMB has a spherical flexible structure and oxygen-containing functional groups on the surface. It can enter the pores of porous silicon and combine with porous silicon, playing a flexible supporting role during the expansion and contraction of porous silicon, alleviating structural damage caused by stress concentration during the expansion and contraction of porous silicon. PVP can improve the dispersion uniformity of MCMB and can also be adsorbed to the surface of porous silicon. After sintering, a cavity is formed to provide expansion space for porous silicon. Treating the carbonized material with an ethanol solution of phenolic resin can act as a bond and improve the bonding strength between silicon and expanded graphite. After carbonization, the phenolic resin can form a high-strength rigid coating layer, which has an impact-resistant effect on the external force of silicon. Finally, chemical vapor deposition (CVD) treatment is used to further reduce surface defects of the material. The above method can effectively inhibit the structural damage of the material caused by the expansion and contraction process of silicon, and improve the cycle performance of the silicon-carbon negative electrode material.

[0012] Furthermore, the D of the silicon dioxide described in S1 50 The value is 10μm~20μm; the molar ratio of the silicon dioxide to the magnesium powder is 1:(1~3).

[0013] Furthermore, 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 polyvinyl pyrrolidone, and the mesophase carbon microspheres is 3:(8-12):(8-12).

[0014] Furthermore, the carbonization treatment in S2 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 3 h to 5 h;

[0015] In S2, after the carbonization treatment, the carbonized material is crushed and sieved and classified using a 325-mesh molecular sieve to obtain a carbonized material.

[0016] Furthermore, 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.

[0017] Furthermore, the carbonization treatment in S3 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 3 h to 5 h;

[0018] In S2, after the carbonization treatment, the carbonized material is crushed and sieved with a 325-mesh molecular sieve to obtain a Si@C precursor.

[0019] Furthermore, the chemical vapor deposition carbon coating method described in S4 is to put the Si@C precursor into a rotary furnace, introduce nitrogen for gas replacement, control the rotation speed to 1rpm~5rpm, raise the temperature to 850℃~950℃, keep it warm for 20min~40min, and then introduce acetylene to treat the Si@C precursor at a constant temperature for 3h~5h.

[0020] Furthermore, during the chemical vapor deposition carbon coating process 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.

[0021] Furthermore, 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.

[0022] The present invention also provides a silicon-carbon negative electrode material prepared by the above-mentioned method for preparing the silicon-carbon negative electrode material.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. In the above-mentioned preparation scheme proposed by the present invention, porous silicon is first prepared using silicon dioxide as a raw material. Then, during the silicon-carbon composite process, the porous silicon and expanded graphite are firmly bonded by PVP, mesophase carbon microspheres, and phenolic resin. PVP is sintered to form a cavity, mesophase carbon microspheres are sintered to form a spherical flexible support, phenolic resin is sintered to form a high-strength rigid coating, and chemical vapor deposition coating is performed. The silicon-carbon negative electrode material obtained by the above-mentioned preparation method not only has a buffering effect on the outside of the silicon, but also provides a buffer inside the porous structure of the silicon, thereby effectively suppressing the material structure damage caused by the expansion and contraction process of the silicon, and improving the cycle performance of the negative electrode material.

[0025] 2. The expanded graphite selected in this application is lower in cost than porous carbon and has higher preparation safety and good processing performance. The porous silicon prepared using silica as the raw material does not undergo crystalline phase transition at high temperatures compared to amorphous silicon produced by conventional silane cracking, resulting in better electrochemical performance, and better preparation process safety and processing performance.

[0026] 3. The silicon-carbon negative electrode material provided in this application is used to prepare the negative electrode material of lithium-ion batteries, has good cycle performance, and has both high reversible specific capacity and first coulombic efficiency, and excellent overall electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is an SEM image of the silicon-carbon negative electrode material prepared in Example 1;

[0028] Figure 2 This is a transmission electron microscope image of the silicon-carbon negative electrode material prepared in Example 1. DETAILED DESCRIPTION

[0029] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. The embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] Unless otherwise specified, the methods are conventional methods, and the raw materials can be obtained from public commercial channels unless otherwise specified.

[0031] Example 1

[0032] Step 1: Silicon dioxide (D 50 =10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0033] Step 2: Take 3.5 parts of porous silicon powder, add it to 35 parts of anhydrous ethanol, and ultrasonically disperse it. Add PVP and mesophase carbon microspheres to the dispersion at a mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres). After fully stirring, add 96.5 parts of expanded graphite and mix at high speed for 60 minutes. Then transfer it to a rotary evaporator and dry it to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0034] Step 3: Dissolve the phenolic resin in anhydrous ethanol (the mass ratio of the phenolic resin to anhydrous ethanol is 1:5, and the mass ratio of the 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 minutes, and then transfer to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and then the temperature is raised to 800°C and carbonized at a constant temperature for 4 hours. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a Si@C precursor.

[0035] Step 4: The Si@C precursor obtained in step 3 is put into a rotary kiln, N2 is introduced for gas replacement, the speed is 3 rpm, the temperature is raised to 900°C, and after maintaining the temperature for half an hour, acetylene gas is introduced for CVD carbon coating to reduce surface defects of the material. The flow rates of nitrogen and acetylene are 8 L / min and 2 L / min, respectively. After maintaining the temperature for 4 hours, Si@C powder is discharged and the obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0036] Example 2

[0037] Step 1: Silicon dioxide (D 50 =10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0038] Step 2: Take 3.5 parts of porous silicon powder, add it to 35 parts of anhydrous ethanol, and ultrasonically disperse it. Add PVP and mesophase carbon microspheres to the dispersion at a mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres). After fully stirring, add 96.5 parts of expanded graphite and mix at high speed for 60 minutes. Then transfer it to a rotary evaporator and dry it to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0039] Step 3: Dissolve the phenolic resin in anhydrous ethanol (the mass ratio of the phenolic resin to anhydrous ethanol is 1:5, and the mass ratio of the phenolic resin to the porous silicon powder in step 2 is 1:0.12), add the carbonized material obtained in step 2, mix at high speed for 60 minutes, and then transfer to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and then the temperature is raised to 800°C and carbonized at a constant temperature for 4 hours. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a Si@C precursor.

[0040] Step 4: The Si@C precursor obtained in step 3 is put into a rotary kiln, N2 is introduced for gas replacement, the speed is 3 rpm, the temperature is raised to 900°C, and after maintaining the temperature for half an hour, acetylene gas is introduced for CVD carbon coating to reduce surface defects of the material. The flow rates of nitrogen and acetylene are 8 L / min and 2 L / min, respectively. After maintaining the temperature for 4 hours, Si@C powder is discharged and the obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0041] Example 3

[0042] Step 1: Silicon dioxide (D 50 =10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0043] Step 2: Take 10 parts of porous silicon powder, add it to 100 parts of anhydrous ethanol, and ultrasonically disperse it. Add PVP and mesophase carbon microspheres to the dispersion at a mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres). After fully stirring, add 90 parts of expanded graphite and mix at high speed for 60 minutes. Then transfer it to a rotary evaporator and dry it to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0044] Step 3: Dissolve the phenolic resin in anhydrous ethanol (the mass ratio of the phenolic resin to anhydrous ethanol is 1:5, and the mass ratio of the 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 minutes, and then transfer to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and then the temperature is raised to 800°C and carbonized at a constant temperature for 4 hours. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a Si@C precursor.

[0045] Step 4: The Si@C precursor obtained in step 3 is put into a rotary kiln, N2 is introduced for gas replacement, the speed is 3 rpm, the temperature is raised to 900°C, and after maintaining the temperature for half an hour, acetylene gas is introduced for CVD carbon coating to reduce surface defects of the material. The flow rates of nitrogen and acetylene are 8 L / min and 2 L / min, respectively. After maintaining the temperature for 4 hours, Si@C powder is discharged and the obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0046] Comparative Example 1

[0047] Step 1: Silicon dioxide (D 50 =10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0048] Step 2: 3.5 parts of porous silicon powder were added to 35 parts of anhydrous ethanol and ultrasonically dispersed. 96.5 parts of expanded graphite were added and mixed at high speed for 60 minutes. The mixture was then transferred to a rotary evaporator and dried to obtain a Si@C precursor.

[0049] Step 3: The Si@C precursor obtained in step 2 is put into a rotary kiln, N2 is introduced for gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 ° C, and acetylene gas is introduced after maintaining the temperature for half an hour 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 maintaining the temperature for 4 hours, the Si@C powder is discharged and the obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0050] Comparative Example 2

[0051] Step 1: Silicon dioxide (D 50=10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0052] Step 2: First, 3.5 parts of porous silicon powder are added to 35 parts of anhydrous ethanol and ultrasonically dispersed. PVP and mesophase carbon microspheres are added to the dispersion in a mass ratio of 0.3:1:2 (porous silicon: PVP: mesophase carbon microspheres). After fully stirring, 96.5 parts of expanded graphite are added and mixed at high speed for 60 minutes, and then transferred to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0053] Step 3: The carbonized material obtained in step 2 is put into a rotary kiln, N2 is introduced for gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 ° C, and acetylene gas is introduced after maintaining the temperature for half an hour 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 maintaining the temperature for 4 hours, the material is discharged to obtain Si@C powder. The obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0054] Comparative Example 3

[0055] Step 1: Silicon dioxide (D 50 =10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0056] Step 2: First, 3.5 parts of porous silicon powder are added to 35 parts of anhydrous ethanol and ultrasonically dispersed. PVP and mesophase carbon microspheres are added to the dispersion in a mass ratio of 0.3:1:1 (porous silicon: PVP: mesophase carbon microspheres). After fully stirring, 96.5 parts of expanded graphite are added and mixed at high speed for 60 minutes, and then transferred to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0057] Step 3: Dissolve the phenolic resin in anhydrous ethanol (the mass ratio of the phenolic resin to anhydrous ethanol is 1:5, and the mass ratio of the 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 minutes, and then transfer to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and then the temperature is raised to 800°C and carbonized at a constant temperature for 4 hours. The obtained material is crushed and sieved with a 325-mesh molecular sieve to obtain a Si@C negative electrode material.

[0058] Comparative Example 4

[0059] Step 1: Silicon dioxide (D 50 =10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0060] Step 2: First, 3.5 parts of porous silicon powder are added to 35 parts of anhydrous ethanol and ultrasonically dispersed. PVP, mesophase carbon microspheres and phenolic resin are added to the dispersion in a mass ratio of 0.3:1:1:1 (porous silicon: PVP: mesophase carbon microspheres: phenolic resin). After fully stirring, 96.5 parts of expanded graphite are added and mixed at high speed for 60 minutes, and then transferred to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment, and then the powder is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0061] Step 3: The carbonized material obtained in step 2 is put into a rotary kiln, N2 is introduced for gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 ° C, and acetylene gas is introduced after maintaining the temperature for half an hour 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 maintaining the temperature for 4 hours, the material is discharged to obtain Si@C powder. The obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0062] Comparative Example 5

[0063] Step 1: Silicon dioxide (D 50=10μm~20μm) and magnesium powder are put into a high-pressure mixer in a molar ratio of 1:2, mixed for 3 hours, and then transferred to an atmosphere furnace. After nitrogen is introduced for gas replacement, the temperature is raised to 700℃ and kept constant for 5 hours to obtain a powder material, which is immersed in a 2mol / L hydrochloric acid solution for washing, and then washed with deionized water and ethanol, and then vacuum dried at 60℃ for 12 hours to obtain porous silicon.

[0064] Step 2: First, 3.5 parts of porous silicon powder are added to 35 parts of anhydrous ethanol and ultrasonically dispersed. PVP and phenolic resin are added to the dispersion in a mass ratio of 0.3:1:2 (porous silicon: PVP: phenolic resin). After fully stirring, 96.5 parts of expanded graphite are added and mixed at high speed for 60 minutes, and then transferred to a rotary evaporator for drying to obtain a powder. The obtained powder is transferred to an atmosphere furnace, N2 is introduced for gas replacement, and the temperature is raised to 800°C. The temperature is kept constant for 4 hours for carbonization treatment, and then the material is crushed and sieved with a 325-mesh molecular sieve to obtain a carbonized material.

[0065] Step 3: The carbonized material obtained in step 2 is put into a rotary kiln, N2 is introduced for gas replacement, the rotation speed is 3 rpm, the temperature is raised to 900 ° C, and acetylene gas is introduced after maintaining the temperature for half an hour 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 maintaining the temperature for 4 hours, the material is discharged to obtain Si@C powder. The obtained Si@C material is sieved and graded with a 325-mesh molecular sieve to obtain a finished silicon-carbon negative electrode material.

[0066] Test example

[0067] The physical indexes of the materials were tested using a desktop scanning electron microscope, a fully automatic specific surface area tester, and a laser particle size analyzer; the electrochemical indexes of the materials were tested using a button battery packaging machine, a button battery test control cabinet, an electrochemical analyzer, and a resistance measuring instrument.

[0068] The half-cell testing method uses the prepared silicon-carbon anode material as the anode active material to form a slurry. The slurry ratio is: active material: CNTs (including dispersant): CMC: SBR = 89%: 4.5%: 1.5%: 5%. The slurry is then coated on copper foil and vacuum-dried for 12 hours to form the anode sheet. The electrolyte is commercially available, the separator is PE film, and the lithium sheet serves as the counter electrode. Half-cells are assembled in a glove box. Constant current charge and discharge experiments are conducted on a LAND battery test system. Unless otherwise specified, the following data are based on 1C charge and discharge, with charge and discharge voltages limited to 0.005V to 2V. A computer-controlled charge and discharge cabinet is used for data acquisition and control.

[0069] Table 1: Half-cell test results

[0070]

[0071] The half-cell test data in Table 1 show that the silicon-carbon anode material provided in Example 1 has overall superior performance in terms of capacity, initial efficiency, and cycle stability compared to the silicon-carbon anode materials provided in Examples 2-3 and Comparative Examples 1-5. In Example 2, the increased phenolic resin content resulted in an excessively thick carbon coating layer, reducing capacity. In Example 3, increasing the porous silicon content increased capacity, but resulted in more side reactions and made it more difficult to limit silicon volume expansion, resulting in poorer cycle performance. The silicon-carbon anode material in Comparative Example 1 lacked a three-layer buffer structure, resulting in a lack of force between the porous silicon and the expanded graphite. Consequently, silicon detached during the experiment, reducing capacity. The carbon layer deposited solely by CVD was insufficient to limit silicon expansion, resulting in a poor cycle life. In Comparative Example 2, only low-strength mesocarbon microbeads were used as the flexible coating layer, resulting in a significant decrease in cycle life, indicating that the strength of the mesocarbon microbeads was insufficient to limit silicon volume expansion. At the same time, the effect of CVD on reducing surface defects on the first effect and cycle of the material was also explored. It can be seen that in Comparative Example 3 without a CVD step, the first effect is poor, the surface functional groups are not completely removed, and during the cycle, the electrolyte reacts with the surface functional groups to continuously generate an SEI film, resulting in continuous consumption of lithium ions and poor cycle. In Comparative Example 4, the effect of the one-step mixing method on the electrochemical properties of the material was explored. Since silicon is mixed with PVP, mesophase carbon microspheres, and phenolic resin in the same container, part of the silicon is exposed to the surface of the material. Although the capacity is slightly improved, its cycle performance is poor. In Comparative Example 5, the mesophase carbon microspheres are replaced with the same proportion of phenolic resin for mixing, and it is found that the cycle performance is worse than that of Comparative Example 4. This may be because silicon does not have the flexible support and buffering provided by the mesophase carbon microspheres, but only the limitation of the rigid carbon structure provided by the phenolic resin, which causes the silicon to break and pulverize after expansion and contraction during long-term charge and discharge.

[0072] The microscopic morphology of the silicon-carbon negative electrode material prepared in Example 1 was further analyzed. Figure 1 This is the SEM image of the silicon-carbon negative electrode material prepared in Example 1. Figure 2 This is a transmission electron microscope image of the finished silicon-carbon negative electrode material prepared in Example 1. Figure 1 It can be seen that the porous silicon is uniformly covered by the carbon layer, and there is no free or agglomerated porous silicon. Figure 2 From the transmission electron microscope image, it can be seen that there is a CVD vapor-deposited carbon coating on the surface of the material, and the surface is smooth without obvious defects.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them. Although the present application has been described in detail with reference to preferred embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present application can still be modified or some technical features can be replaced by equivalents, which should all be included in the scope of the technical solutions requested for protection in this application.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: The following steps are involved: S1, reacting silicon dioxide and magnesium powder as raw materials, 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 and uniformly dispersing them, then adding expanded graphite and 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 carbonizing to obtain a Si@C precursor; S4, using acetylene gas to perform chemical vapor deposition carbon coating on the Si@C precursor obtained in S3 to prepare a silicon-carbon negative electrode material; 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 polyvinyl pyrrolidone, and the mesophase carbon microspheres is 3:(8-12):(8-12); The mass ratio of the phenolic resin described in S3 to the porous silicon described in S2 is 1:(0.12-0.3).

2. The method for preparing a silicon-carbon negative electrode material according to claim 1, wherein: S1 silica 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, wherein: The carbonization treatment in step S2 is carried out under an inert atmosphere at a temperature of 750° C. to 850° C. and for a time of 3 to 5 hours; In S2, after the carbonization treatment, the carbonized material is crushed and sieved and classified using a 325-mesh molecular sieve to obtain a carbonized material.

4. The method for preparing a silicon-carbon negative electrode material according to claim 1, wherein: The carbonization treatment in S3 is carried out under an inert atmosphere at a temperature of 750° C. to 850° C. and for a time of 3 to 5 hours; 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.

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

6. The method for preparing a silicon-carbon negative electrode material according to claim 5, characterized in that: During the chemical vapor deposition carbon coating process described 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.

7. 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.

8. A 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 7.

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