Silicon-carbon composite material, preparation method and application thereof

Abstract

The application provides a silicon-carbon composite material and a preparation method and application thereof, and relates to the technical field of lithium ion battery negative electrode materials. The specific preparation method is as follows: silicon powder, pitch and graphite are mixed in stages to obtain mixed powder; the mixed powder is pressed and formed in a mold with a needle structure to obtain a silicon-carbon composite material precursor; and the precursor is subjected to staged reduced pressure carbonization treatment to obtain the silicon-carbon composite material. The preparation method is simple and efficient, the interface stability of the silicon particles and the carbon matrix can be improved by adjusting the pressure of the pressing and forming, the expansion of the silicon particles is alleviated, the pores formed on the precursor by the mold in combination with the reduced pressure carbonization can promote the escape of unstable volatile components during carbonization. The prepared silicon-carbon composite material has a high tap density, and as a lithium ion battery electrode material, can realize the synchronous improvement of the battery capacity, the initial coulomb efficiency, the rate performance and the cycle stability.

Description

Technical Field

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

[0002] Currently, commercially available batteries are still dominated by lithium-ion batteries, which are widely used in various portable electronic devices (such as smartphones, laptops, and cameras) and transportation equipment (such as drones and electric vehicles). However, traditional lithium-ion batteries are increasingly unable to meet the requirements of modern society for battery energy density and power density.

[0003] Commercially available lithium-ion batteries mostly use graphite as the anode material. However, compared to the theoretical capacity of graphite (approximately 372 mAh / g), silicon has a much larger theoretical capacity (where silicon's maximum lithium intercalation state is Li). 22Si5 (with a theoretical capacity of 4200 mAh / g) is abundant and has the potential for development. However, silicon's poor conductivity makes it difficult to fully utilize its lithium storage capacity, reducing the battery's initial coulombic efficiency. Furthermore, during lithium ion absorption and release, silicon undergoes approximately 400% volume change, causing silicon particles to break down and pulverize, leading to electrode structure damage and loss of electrical contact between the electrode material and the current collector. This also results in repeated formation and rupture of the SEI (solid electrolyte interphase), all of which contribute to battery capacity decay and severely impact battery cycle performance and lifespan. Currently, research on the above problems at home and abroad mainly focuses on the following aspects: First, nano-sizing, using the size effect of nano-sizing to alleviate the volume effect caused by silicon expansion [Liu Y, Zhou G, Liu K, et al. Design of complex nanomaterials for energy storage: past success and future opportunity [J]. Accounts of Chemical Research, 2017, 50(12): 2895-2905.], such as silicon nanowires, silicon nanotubes, double-walled silicon nanotubes, hollow silicon nanospheres, etc. These nanostructured silicon anodes have improved the electrochemical reaction kinetics and structural stability of the electrode due to the nano-sizing design, but their low conductivity problem still exists; Second, preparing silicon-carbon composite materials with special structures by combining silicon and carbon, such as core-shell structure, yolk-shell structure, sandwich structure, 3D porous structure, embedded structure and secondary structure, etc. [Luo W, Wang Y, Chou S, et al. Critical thickness of phenolic resin-based carbon interfacial layer for improving long cycling stability of silicon nanoparticle anodes, Nano [Energy, 2016, 27:255-264] Carbon materials possess certain mechanical strength and high electrical conductivity, which can both alleviate the expansion of silicon materials and increase the conductivity of composite materials. Designing silicon-carbon composite materials can effectively solve the problems existing in silicon anodes. However, silicon-carbon composite structures may have defects such as poor dispersion of silicon in the carbon matrix and poor interfacial contact between the two, affecting their performance.Li et al. proposed a porous carbon microsphere with embedded Si particles and encapsulated in a carbon nanoshell [Wu XR, Yu CH, Li C C. Carbon-encapsulated gigaporous microsphere as potential Si anode-active material for lithium-ion batteries[J]. Carbon, 2020, 160: 255-264.]; Zhang et al. designed a robust layered 3D Si / CNT composite material [Zhang M, Zhang T, Ma Y, et al. Latest development of nanostructured Si / C materials for lithium anode studies and applications[J]. Energy Storage Materials, 2016: 1-14.]; Qin et al. coated silicon particles with graphene sheets that have high mechanical strength and extremely high conductivity [Qin J, Wu M, Feng T, et al. High rate capability and long cycling life of graphene-coated silicon composite anodes for lithium-ion batteries[J]. Electrochimica Acta, 2017, 256: 259-266.]. While these solutions have achieved good results, their operation processes are complex, difficult to control, and costly. Furthermore, the introduction of nanomaterials often leads to a decrease in tap density, hindering commercial applications.

[0004] Therefore, how to provide a simple, efficient, and highly controllable silicon-carbon composite material and its preparation method and application, so as to achieve synergistic improvement in properties such as tap density, capacity, first coulombic efficiency, rate performance, and cycle stability, is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a silicon-carbon composite material, its preparation method, and its application, so as to solve the problem that the tap density, volume, initial coulombic efficiency, rate performance, and cycle stability of existing silicon-carbon composite materials cannot be improved simultaneously using a simple and controllable technology.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0008] 1) Mix silica powder, pitch and graphite to obtain a mixed powder;

[0009] 2) Press the mixed powder obtained in step 1) into a mold to obtain a silicon-carbon composite material precursor;

[0010] 3) Carbonize the silicon-carbon composite material precursor obtained in step 2) to obtain the silicon-carbon composite material.

[0011] Preferably, in step 1), the mass ratio of silica powder, pitch and graphite is 1-1.5:1-3:6-9.

[0012] Preferably, in step 1), the particle size of the silicon powder is 20-1000 nm; the softening point of the asphalt is 150-280 °C; the particle size of the graphite is 1-10 μm, and the carbon content of the graphite is >90%.

[0013] Preferably, the mixing in step 1) is a staged ball milling and blending process. Specifically, the silica powder and pitch are first ball-milled and dry-blended, and then graphite is added and the ball milling and dry-blending process is continued.

[0014] The silica powder and pitch are dry-mixed by ball milling at a speed of 100-500 r / min for 2-10 h.

[0015] The ball milling dry mixing after adding graphite is performed at a speed of 50–300 r / min for a time of 0.5–5 h.

[0016] Preferably, the mold used in the pressing process in step 2) has a needle-punched structure.

[0017] Preferably, in step 2), the pressure during the pressing process is 0-50 MPa and the pressing time is 0.05-2 h.

[0018] Preferably, the carbonization process in step 3) involves first heating to 250–350°C and holding for 2–3 hours, then heating to 600–1200°C and holding for 0.5–10 hours.

[0019] The carbonization process is carried out at a pressure of -10 to 0 kPa.

[0020] The present invention also provides a silicon-carbon composite material and its application in lithium-ion batteries.

[0021] The present invention has at least the following beneficial effects:

[0022] 1. This invention improves the dispersibility and interfacial compatibility of silicon particles, pitch media and graphite by controlling the feeding sequence, speed and time of the staged ball mill.

[0023] Adjusting the pressing pressure improves the contact between silicon particles and the carbon matrix, thereby improving the toughness of the amorphous carbon interface formed by asphalt, enhancing the electrical conductivity of the material, and mitigating the expansion of silicon particles. Furthermore, utilizing the needle-like structure of the mold during briquetting creates open pores in the silicon-carbon composite precursor, supplemented by reduced-pressure carbonization, thus promoting the release of volatiles during carbonization and improving initial efficiency and stability.

[0024] The briquetting process is compatible with the carbonization process. By reducing the briquetting pressure and increasing the carbonization temperature, high-capacity, high-efficiency silicon-carbon anode materials can be obtained; conversely, by increasing the briquetting pressure and decreasing the carbonization temperature, silicon-carbon anode materials with good cycle stability and high density can be obtained. The preparation process is simple, easy to operate, and suitable for industrial production.

[0025] 2. This invention uses silicon powder, pitch and graphite as raw materials to prepare silicon-carbon composite materials. The raw material resources are abundant, the preparation method is simple, and it is green and pollution-free. When used as a negative electrode material for lithium-ion batteries, the silicon-carbon composite material prepared by the preparation method described in this invention exhibits high tap density, reversible capacity, rate performance and good cycle stability. Attached Figure Description

[0026] Figure 1 The image shows a scanning electron microscope (SEM) image of the silicon-carbon composite material prepared in Example 1.

[0027] Figure 2 The cycling performance curve of the lithium-ion battery prepared in Example 1 at a current density of 100 mA / g is shown.

[0028] Figure 3 The cycling performance curve of the lithium-ion battery prepared in Example 2 at a current density of 100 mA / g is shown.

[0029] Figure 4 The cycling performance curve of the lithium-ion battery prepared in Example 8 at a current density of 100 mA / g is shown.

[0030] Figure 5 The cycling performance curve of the lithium-ion battery prepared in Comparative Example 3 at a current density of 100 mA / g;

[0031] Figure 6 The cycling performance curve of the lithium-ion battery prepared in Comparative Example 4 at a current density of 100 mA / g.

[0032] Figure 7 The image shows a scanning electron microscope (SEM) image of the silicon-carbon composite material prepared in Comparative Example 4.

[0033] Figure 8 The image shows a scanning electron microscope (SEM) image of the silicon-carbon composite material prepared in Comparative Example 5.

[0034] Figure 9 This is a schematic diagram illustrating the modification principle of the preparation scheme disclosed in this invention. Detailed Implementation

[0035] This invention provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0036] 1) Mix silica powder, pitch and graphite to obtain a mixed powder;

[0037] 2) Press the mixed powder obtained in step 1) into a mold to obtain a silicon-carbon composite material precursor;

[0038] 3) Carbonize the silicon-carbon composite material precursor obtained in step 2) to obtain the silicon-carbon composite material.

[0039] In this invention, the mass ratio of silicon powder, pitch and graphite in step 1) is 1-1.5:1-3:6-9, preferably 1.1-1.4:1.5-2.5:6.5-8.5, and more preferably 1.2-1.3:2:7-8.

[0040] In this invention, the particle size of the silicon powder in step 1) is 20-1000 nm, preferably 200-800 nm, more preferably 400-600 nm, and even more preferably 450-550 nm; the softening point of the asphalt is 80℃-300℃, preferably 120℃-280℃, more preferably 160℃-260℃, and even more preferably 200℃-250℃; the particle size of the graphite is 1-20 μm, preferably 1-15 μm, more preferably 1-10 μm, and even more preferably 2-5 μm.

[0041] In this invention, the mixing in step 1) is a staged ball milling and blending. The specific operation steps are: first, the silicon powder and the asphalt are ball milled and dry-blended, and then graphite is added and the ball milling and dry-blending is continued.

[0042] The rotation speed for ball milling the silica powder and asphalt is 100-500 r / min, preferably 150-450 r / min, more preferably 200-400 r / min, and even more preferably 250-350 r / min; the ball milling time for ball milling the silica powder and asphalt is 2-10 h, preferably 3-9 h, more preferably 4-8 h, and even more preferably 5-7 h.

[0043] The rotation speed of the ball mill after adding graphite is 50-300 r / min, preferably 100-250 r / min, more preferably 150-200 r / min, and even more preferably 160-180 r / min; the ball milling time after adding graphite is 0.5-5 h, preferably 1-4 h, more preferably 1.5-3.5 h, and even more preferably 2-3 h.

[0044] In this invention, the mold used in the pressing process of step 2) has a needle-punched structure.

[0045] In this invention, the pressure during the pressing process in step 2) is 0-50 MPa, preferably 10-40 MPa, more preferably 15-35 MPa, and even more preferably 20-30 MPa; the pressing time is 0.05-2 h, preferably 0.5-1.5 h, more preferably 0.75-1.25 h, and even more preferably 1-1.25 h.

[0046] The pressure of the carbonization treatment is -10 to 0 kPa, preferably -8 to -2 kPa, more preferably -7 to -4 kPa, and even more preferably -6 to -5 kPa;

[0047] In this invention, the carbonization process in step 3) is preferably characterized by first heating to 250–350°C, holding at that temperature for 2–3 hours, and then heating to 600–1200°C and holding at that temperature for 0.5–10 hours; preferably, heating to 250–350°C at a rate of 4–7°C / min, holding at that temperature for 2–3 hours, and then heating to 600–1200°C at a rate of 4–7°C / min and holding at that temperature for 0.5–10 hours; more preferably, heating to 270–330°C at a rate of 5–6°C / min and holding at that temperature for 2 hours. After 3 hours, the temperature is increased to 700-1100℃ at a rate of 5-6℃ / min and held for 2-8 hours; more preferably, the temperature is increased to 290-310℃ at a rate of 5-6℃ / min and held for 2.3-2.8 hours, then increased to 800-1000℃ at a rate of 5-6℃ / min and held for 3-7 hours; more preferably, the temperature is increased to 300℃ at a rate of 5-6℃ / min and held for 2.5 hours, then increased to 850-950℃ at a rate of 5-6℃ / min and held for 4-6 hours.

[0048] This invention provides a silicon-carbon composite material.

[0049] This invention also provides an application of silicon-carbon composite materials in lithium-ion batteries.

[0050] In this invention, the specific application of the silicon-carbon composite material in lithium-ion batteries is as follows:

[0051] Silicon-carbon composite material, sodium carboxymethyl cellulose, carbon black, and single-walled carbon nanotubes were weighed in a mass ratio of 95:3:1:1, and a suitable amount of distilled water was added to form a slurry. The slurry was then uniformly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery.

[0052] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0053] Example 1

[0054] Weigh 0.4g of 50nm silicon powder and 0.6g of 250℃ pitch at a mass ratio of 1:1.5 and add them to a zirconia ball mill jar. Simultaneously, add 8 6mm and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm graphite and continue ball milling at 200 rpm for 4 hours. After milling, weigh 2g of the mixture and pour it into a cylindrical mold with needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and then placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0055] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0056] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. A lithium-ion battery was assembled using the obtained battery electrode as the working electrode and a lithium sheet as the counter electrode. Electrochemical performance tests were performed, and the material tap density was found to be 1.1 g / cm³. 3 The battery exhibits a coulombic efficiency of 72% in the first cycle, a reversible specific capacity of 658 mAh / g, and a capacity retention rate of 99.4% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0057] Simultaneously, the prepared silicon-carbon composite material was used to fabricate a lithium-ion battery using the following method:

[0058] Silicon-carbon composite material, sodium carboxymethyl cellulose, carbon black, and single-walled carbon nanotubes were weighed in a mass ratio of 95:3:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then uniformly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. A lithium-ion battery was assembled using the obtained battery electrode as the working electrode and a lithium foil as the counter electrode. Electrochemical performance tests showed that the initial efficiency of the half-cell was 83%, the capacity was 932 mAh / g, and the capacity retention rate was 99.6% from 5 to 50 cycles. The lithium-ion battery prepared by this method exhibits higher capacity and better cycle performance.

[0059] Example 2

[0060] Weigh 0.4g of 50nm silicon powder and 0.6g of 250℃ pitch at a mass ratio of 1:1.5 and add them to a zirconia ball mill jar. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 500 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 500 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold with needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to form the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0061] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0062] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 1.2 g / cm³. 3 The battery exhibits a coulombic efficiency of 71% in the first cycle, a reversible specific capacity of 665.2 mAh / g, and a capacity retention of 102% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0063] Simultaneously, the prepared silicon-carbon composite material was used to fabricate a lithium-ion battery using the following method:

[0064] Silicon-carbon composite material, sodium carboxymethyl cellulose, carbon black, and single-walled carbon nanotubes were weighed in a mass ratio of 95:3:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then uniformly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. A lithium-ion battery was assembled using the obtained battery electrode as the working electrode, and its electrochemical performance was tested. The initial efficiency of the half-cell was 81%, the capacity was 912 mAh / g, and the capacity retention rate was 99.5% from 5 to 50 cycles. The lithium-ion battery prepared by this method exhibits higher capacity and better cycle performance.

[0065] Example 3

[0066] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 200℃ softening pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 10 hours. After 10 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 5 hours. After ball milling, weigh 2g of the mixture and pour it into a spherical mold with needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0067] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0068] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 1.0 g / cm³. 3 The battery exhibits a coulombic efficiency of 70% in the first cycle, a reversible specific capacity of 620.1 mAh / g, and a capacity retention rate of 99.1% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0069] Simultaneously, the prepared silicon-carbon composite material was used to fabricate a lithium-ion battery using the following method:

[0070] Silicon-carbon composite material, sodium carboxymethyl cellulose, carbon black, and single-walled carbon nanotubes were weighed in a mass ratio of 95:3:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then uniformly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. A lithium-ion battery was assembled using the obtained battery electrode as the working electrode, and its electrochemical performance was tested. The initial efficiency of the half-cell was 81%, the capacity was 923 mAh / g, and the capacity retention rate was 99.2% from 5 to 50 cycles. The lithium-ion battery prepared by this method exhibits higher capacity and better cycle performance.

[0071] Example 4

[0072] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold with a needle-like structure for pressing. Apply a pressure of 50MPa to the mold and hold for 10 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0073] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0074] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 1.4 g / cm³. 3 The battery exhibits a coulombic efficiency of 69% in the first cycle, a reversible specific capacity of 600.5 mAh / g, and a capacity retention rate of 98.9% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0075] Example 5

[0076] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter and 16 12mm diameter zirconia ball milling beads to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold with needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 120 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 750°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0077] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0078] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 1.15 g / cm³. 3 The battery exhibits a coulombic efficiency of 74% in the first cycle, a reversible specific capacity of 650.6 mAh / g, and a capacity retention rate of 99.4% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0079] Example 6

[0080] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold with needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for low-pressure carbonization (pressure of -10 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then naturally cooled to room temperature to obtain the silicon-carbon composite material.

[0081] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0082] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.98 g / cm³. 3 The battery exhibits a coulombic efficiency of 68% in the first cycle, a reversible specific capacity of 590.4 mAh / g, and a capacity retention rate of 98.1% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0083] Example 7

[0084] Weigh 0.4g of 50nm diameter silicon powder and 0.4g of 250℃ softening pitch into a zirconia ball mill jar at a mass ratio of 1:1. Simultaneously, add 8 6mm diameter and 16 12mm diameter zirconia ball milling beads to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3.2g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold with needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 600°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0085] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0086] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.95 g / cm³. 3 The battery exhibits a coulombic efficiency of 67% in the first cycle, a reversible specific capacity of 550.3 mAh / g, and a capacity retention rate of 97% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0087] Example 8

[0088] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter and 16 12mm diameter zirconia ball milling beads to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold with needle-like protrusions for pressing. Apply a pressure of 50MPa to the mold and hold for 120 minutes to obtain the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for vacuum carbonization (pressure -10 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 600°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0089] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0090] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum dried at 60℃ for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 1.45 g / cm³. 3 The battery exhibits a coulombic efficiency of 68.6% in the first cycle, a reversible specific capacity of 580 mAh / g, and a capacity retention rate of 99.8% from 5 to 50 cycles. This silicon-carbon composite material demonstrates high reversible capacity and good cycle stability.

[0091] Comparative Example 1

[0092] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold without needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to form the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and then placed in a tube furnace for vacuum carbonization (pressure -2 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0093] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0094] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.98 g / cm³. 3 The battery exhibits a coulombic efficiency of 63% in the first cycle, a reversible specific capacity of 440.5 mAh / g, and a capacity retention rate of 89.1% from 5 to 50 cycles. Compared with silicon-carbon composite materials prepared by briquetting and carbonization, this silicon-carbon composite material shows significantly reduced capacity and cycle performance.

[0095] Comparative Example 2

[0096] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold without needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to form the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and placed in a tube furnace for atmospheric pressure carbonization (pressure 101.3 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then naturally cooled to room temperature to obtain the silicon-carbon composite material.

[0097] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0098] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.95 g / cm³. 3 The battery exhibits a coulombic efficiency of 65% in the first cycle, a reversible specific capacity of 430.7 mAh / g, and a capacity retention rate of 89.2% from 5 to 50 cycles. Compared with silicon-carbon composite materials prepared by briquetting and carbonization, this silicon-carbon composite material shows significantly reduced capacity and cycle performance.

[0099] Comparative Example 3

[0100] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter zirconia grinding balls and 16 12mm diameter zirconia grinding balls to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours. After ball milling, weigh 2g of the mixture and pour it into a cylindrical mold without needle-like protrusions for pressing. Apply a pressure of 2MPa to the mold and hold for 10 minutes to form the silicon-carbon composite material precursor. The silicon-carbon composite material precursor was placed in a magnetic boat and carbonized in a tube furnace at atmospheric pressure (101.3 kPa). The temperature was first increased to 300°C at a rate of 5°C / min and held for 3 hours. Then, the temperature was increased to 1000°C at a rate of 5°C / min and held for 3 hours. The mixture was then allowed to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0101] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0102] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.94 g / cm³. 3 The battery's initial coulombic efficiency was 66.2%, its reversible specific capacity was 425.2 mAh / g, and its capacity retention rate from 5 to 50 cycles was 88.1%. Compared with silicon-carbon composite materials prepared by briquetting and carbonization, this silicon-carbon composite material exhibits significantly lower capacity and reduced cycle performance.

[0103] Comparative Example 4

[0104] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter and 16 12mm diameter zirconia ball milling beads to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours to obtain a mixed powder (unpressed). Place the mixed powder directly into a magnetic boat and vacuum carbonize it in a tube furnace (pressure -2KPa). First, heat to 300℃ at a rate of 5℃ / min and hold for 3 hours. Then, continue heating to 1000℃ at a rate of 5℃ / min and hold for 3 hours. Allow to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0105] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0106] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.90 g / cm³. 3 The battery exhibits a coulombic efficiency of 70% in the first cycle, a reversible specific capacity of 423.9 mAh / g, and a capacity retention rate of 89.0% from 5 to 50 cycles. Compared with silicon-carbon composite materials prepared by briquetting and carbonization, this silicon-carbon composite material shows significantly reduced capacity and cycle performance.

[0107] Comparative Example 5

[0108] Weigh 0.4g of 50nm diameter silicon powder and 0.6g of 250℃ softening point pitch into a zirconia ball mill jar at a mass ratio of 1:1.5. Simultaneously, add 8 6mm diameter and 16 12mm diameter zirconia ball milling beads to the jar and ball mill at 200 rpm for 2 hours. After 2 hours, add 3g of 5μm diameter graphite and continue ball milling at 200 rpm for 4 hours to obtain a mixed powder (unpressed). Place the mixed powder directly into a magnetic boat and place it in a tube furnace for atmospheric pressure carbonization (101.3 kPa). First, heat to 300℃ at a rate of 5℃ / min and hold for 3 hours. Then, continue heating to 1000℃ at a rate of 5℃ / min and hold for 3 hours. Allow to cool naturally to room temperature to obtain the silicon-carbon composite material.

[0109] The steps for preparing lithium-ion batteries using the obtained silicon-carbon composite material are as follows:

[0110] Silicon-carbon composite material, sodium carboxymethyl cellulose, and carbon black were weighed in a mass ratio of 8:1:1, and a suitable amount of distilled water was added to form a slurry. This slurry was then evenly coated with copper foil and vacuum-dried at 60°C for 24 hours to obtain the battery electrode. The obtained battery electrode was used as the working electrode to assemble a lithium-ion battery, and its electrochemical performance was tested. The tap density of the material was 0.89 g / cm³. 3 The battery exhibits a coulombic efficiency of 65% in the first cycle, a reversible specific capacity of 382.5 mAh / g, and a capacity retention rate of 89.1% from 5 to 50 cycles. Compared with silicon-carbon composite materials prepared by briquetting and carbonization, this silicon-carbon composite material shows significantly reduced capacity and cycle performance.

[0111] The electrochemical performance of the lithium-ion button batteries prepared in the above examples and comparative examples was tested using a CT2001A blue battery testing system at room temperature (25°C). The charge-discharge specific capacity results are shown in Table 1.

[0112] Table 1. Test results of charge-discharge performance of lithium-ion button batteries prepared in the examples and comparative examples.

[0113]

[0114] Through Example 1 and Comparative Example 1, it can be seen that the briquettes with needles can better promote the escape of small gas molecules generated during carbonization, reduce irreversible reactions, and improve material properties. Through Example 1 and Comparative Example 2, it can be seen that vacuum carbonization, compared with atmospheric pressure carbonization, can promote the escape of volatile gases and significantly improve material properties. Through Example 1 and Comparative Example 4, it can be seen that pressing into briquettes helps the carbon medium to knead silicon and graphite, thereby improving material properties. In Comparative Example 3, which uses briquettes without needles and is carbonized at atmospheric pressure, the material properties are significantly reduced. In Comparative Example 5, which is not pressed into briquettes and is carbonized at atmospheric pressure, the material properties are further reduced.

[0115] Figure 1 The scanning electron microscope image of the material prepared in Example 1 shows that there are no exposed silicon particles. Figure 2 , Figure 3 , Figure 4 The cycling performance curves are shown for the materials prepared in Examples 1, 2, and 8, respectively. It can be seen that the silicon-carbon composite material described in this invention exhibits excellent cycling performance. After 50 cycles at a current density of 100 A / g, the capacity reaches 654 mAh / g, with a capacity retention of 99.4% (compared to the 50th cycle). Figure 5 The cycling performance curves of the material prepared for Comparative Example 4 at a current density of 100 A / g are shown. It can be seen that the material performance is significantly reduced after unpacking, with a capacity of only 423.9 mAh / g. Figure 6 The cycling performance curves of the material prepared for Comparative Example 5 at a current density of 100 A / g are shown. It can be seen that without any technical improvements, the material performance is significantly reduced, with a capacity of only 382.5 mAh / g. Figure 7 , Figure 8 These are scanning electron microscope (SEM) images of the materials prepared in Comparative Examples 4 and 5, respectively. Figure 1 In contrast, it is evident that silicon particles agglomerate together and detach from the graphite matrix, resulting in a decrease in its electrochemical performance.

[0116] The lithium-ion button batteries prepared in Examples 1-8 and Comparative Examples 1-5 were subjected to charge-discharge cycle performance tests. The test results are shown in Table 1. It can be seen that the silicon-carbon material prepared by briquetting carbonization according to the present invention has a capacity of up to 654 mAh / g after 50 cycles at a charge-discharge rate of 100 mA / g, with a capacity retention rate of 99.4% (compared to the 50th cycle). In contrast, the silicon-carbon composite material prepared without briquetting carbonization has a reversible specific capacity of 377.5 mAh / g at the same charge-discharge rate, with a capacity retention rate of only 89.0%. The charge-discharge cycle performance test figures for the lithium-ion button batteries prepared in Examples 1, 2, 8, Comparative Examples 4, and 5 are shown in Figures 2-6 of the specification. It can be seen from the figures that the lithium-ion button batteries made from the silicon-carbon composite material prepared by briquetting carbonization according to the present invention have higher capacity and better cycle stability compared to those made from silicon-carbon composite material prepared without briquetting carbonization.

[0117] Figure 9 The diagram shows the preparation principle of silicon-carbon composite materials. As can be seen from the diagram, the existing technology cannot achieve effective composite and interface design of silicon, graphite and carbon media. The carbon media cannot uniformly bond silicon powder and graphite particles together, resulting in severe phase separation. Based on the improved powder pressing and depressurized carbonization technology proposed in this invention, the phases of the silicon-carbon composite material are tightly and uniformly distributed. The tough carbon media buffer layer tightly and uniformly composites silicon powder and graphite particles.

[0118] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a silicon-carbon composite material, characterized in that, Includes the following steps: 1) Mix silica powder, pitch and graphite to obtain a mixed powder; 2) Press the mixed powder obtained in step 1) into a mold to obtain a silicon-carbon composite material precursor; 3) The silicon-carbon composite material precursor obtained in step 2) is carbonized to obtain the silicon-carbon composite material; The mold used in the pressing process in step 2) has a needle-punched structure; In step 2), the pressure during the pressing process is 0-50 MPa, and the pressing time is 0.05-2 h. The carbonization process is carried out at a pressure of -10 to 0 kPa.

2. The method for preparing a silicon-carbon composite material according to claim 1, characterized in that, In step 1), the mass ratio of silica powder, pitch and graphite is 1-1.5:1-3:6-9.

3. The method for preparing a silicon-carbon composite material according to claim 1, characterized in that, In step 1), the particle size of the silicon powder is 20–1000 nm; the softening point of the asphalt is 150–280 °C; the particle size of the graphite is 1–10 μm, and the carbon content of the graphite is >90%.

4. A method for preparing a silicon-carbon composite material according to claim 2 or 3, characterized in that, In step 1), the mixing is a staged ball milling and blending process. The specific steps are: first, the silica powder and pitch are ball milled and dry-blended, and then graphite is added and the ball milling and dry-blending process is continued. The silica powder and pitch are dry-mixed by ball milling at a speed of 100-500 r / min for 2-10 h. The ball milling dry mixing after adding graphite is performed at a speed of 50–300 r / min for a time of 0.5–5 h.

5. The method for preparing a silicon-carbon composite material according to claim 1, characterized in that, The carbonization process in step 3) involves first heating to 250–350°C and holding for 2–3 hours, then heating to 600–1200°C and holding for 0.5–10 hours.

6. The silicon-carbon composite material obtained by the preparation method of any one of claims 1 to 5.

7. The application of the silicon-carbon composite material according to claim 6 in lithium-ion batteries.

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