A silicon-carbon composite material with core-shell structure and a preparation method and application thereof

Abstract

The present application relates to a kind of silicon-carbon composite materials of core-shell structure and its preparation method and application, silicon-carbon composite material includes core and shell;Wherein, core is silicon-carbon particle, including: porous carbon and the nanoparticle of uniform dispersion in the pore of porous carbon;Shell includes: the buffer layer of coating in the outer surface of silicon-carbon particle, and the carbon coating layer of coating in the outer surface of buffer layer;Buffer layer includes silicon-iron alloy, carbide and ordered arrangement carbon nanotube;Carbide is the non-dense amorphous carbon of organic carbon source carbonization after formation;Carbon nanotube is by wrapping in the outer surface of silicon-carbon particle silicon-iron alloy and the mixture of organic carbon source after coating in the outer surface of silicon-carbon particle, form after carbonization;Silicon-carbon composite material of core-shell structure is used in lithium battery, can improve the first week coulomb efficiency of battery, inhibit the expansion of battery and improve cycle stability.

Description

Technical Field

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

[0002] As the requirements for range of new energy vehicles continue to increase, battery anode materials are also developing towards higher energy density. Silicon has gradually come into people's view due to its ultra-high specific capacity (4200mAh / g) and low lithium intercalation potential (0.4V), but the significant volume expansion (about 300%) of silicon anodes during charging and discharging greatly limits its large-scale commercialization.

[0003] Existing technologies have improved the volume expansion problem by reducing the size of silicon particles from the micrometer level to the nanometer level. Although this method can effectively reduce volume expansion to a certain extent, nano-silicon is prone to agglomeration and cannot be effectively dispersed. Furthermore, it is difficult to obtain smaller nano-silicon particles. At the same time, as the particle size decreases, the cost increases rapidly.

[0004] Chemical vapor deposition (CVD) offers unique advantages in silicon nanostructuring, such as effective control over silicon nanoparticle size, obtaining smaller particle sizes, and ease of large-scale production. However, uneven deposition is inevitable during the process, leading to uneven stress on the particles. This results in significant volume changes in the negative electrode material during charging and discharging, causing instability in the electrode structure and affecting the battery's cycle life.

[0005] To overcome these defects, composite technology can be used to buffer the volume expansion of silicon. Among them, carbon materials are often regarded as the preferred matrix for composite with silicon due to their advantages such as small volume change during charging and discharging, good cycle stability and high conductivity. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a core-shell structured silicon-carbon composite material, its preparation method, and its applications.

[0007] To achieve the above objectives, in a first aspect, embodiments of the present invention provide a core-shell structured silicon-carbon composite material, the silicon-carbon composite material comprising a core and a shell;

[0008] The core is a silicon-carbon particle, comprising: porous carbon and nano-silicon particles uniformly dispersed in the pores of the porous carbon.

[0009] The outer shell includes: a buffer layer covering the outer surface of the silicon-carbon particles, and a carbon coating layer covering the outer surface of the buffer layer;

[0010] The buffer layer comprises a silicon-iron alloy, carbides, and ordered carbon nanotubes; the carbides are non-dense carbon formed by carbonizing an organic carbon source; the carbides are amorphous carbon.

[0011] The carbon nanotubes are formed by coating the outer surface of the silicon-carbon particles with a mixture of ferrosilicon alloy and an organic carbon source, followed by carbonization.

[0012] Preferably, the pore size of the porous carbon is between 0.1 nm and 20 nm; the specific surface area of ​​the porous carbon is 100 m². 2 / g~3000m 2 Between / g;

[0013] The organic carbon source is an organic polymer, including one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone;

[0014] The particle size Dv50 of the silicon-carbon composite material is between 0.5 μm and 100 μm.

[0015] Preferably, the thickness of the buffer layer is 0.5 μm to 50 μm.

[0016] The thickness of the carbon coating layer is between 2 nm and 50 nm.

[0017] Preferably, the mass percentage of the nano-silicon particles in the total mass of the silicon-carbon composite material is 30% to 60%.

[0018] The carbon nanotubes account for 1% to 5% of the total mass of the silicon-carbon composite material;

[0019] The porous carbon accounts for 35% to 65% of the total mass of the silicon-carbon composite material.

[0020] Secondly, embodiments of the present invention provide a method for preparing the silicon-carbon composite material described in the first aspect above, the method comprising:

[0021] Step S1: Place porous carbon in a reactor and raise it to a first temperature under a protective gas environment. The silicon source gas is introduced into the reactor through the protective gas and subjected to a first heat preservation. The silicon element obtained by the dissociation of the silicon source gas is deposited in the pores of the porous carbon and grows into nanoscale silicon particles to obtain a porous silicon-carbon composite.

[0022] Step S2: The porous carbon composite is mixed with an organic carbon source and a silicon-iron alloy, so that the organic carbon source and the silicon-iron alloy coat the surface of the porous carbon composite. Then, it is placed in the reactor for high-temperature carbonization treatment. After cooling to room temperature, the precursor material is obtained.

[0023] Step S3: Place the precursor material in a rotary kiln and heat it to a second temperature. Under a protective gas environment, bring the carbon source gas into the rotary kiln through the protective gas for a second heat preservation and perform gas phase carbon coating treatment. A carbon coating layer is formed on the outermost surface of the precursor material to obtain a core-shell structured silicon-carbon composite material.

[0024] Preferably, in step S1, the porous carbon includes one or more of the following: biomass porous carbon, resin porous carbon, coke-based porous carbon, and graphite porous carbon.

[0025] The silicon source gas includes one or more of the following: metallic silicon vapor, silane, propane, dichlorosilane, trichlorosilane, and tetrachlorosilane; the flow rate of the silicon source gas is 1 L / min to 15 L / min.

[0026] The protective gas includes nitrogen and / or argon; the volumetric flow rate ratio of the protective gas to the silicon source gas is 10:1 to 1:10;

[0027] The first temperature is 400℃~700℃, and the first heat preservation time is 1 hour~8 hours.

[0028] Preferably, in step S2, the organic carbon source includes one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone.

[0029] The mixing process specifically involves placing the porous carbon composite, the organic carbon source, and the ferrosilicon alloy in a fusion machine and fusing them at a speed of 200 rpm to 800 rpm for 10 to 60 minutes.

[0030] The silicon content in the ferrosilicon alloy is 15wt% to 90wt%;

[0031] The mass ratio of the porous carbon composite, the organic carbon source, and the ferrosilicon alloy is 5-8:1-3:1-3;

[0032] The high-temperature carbonization treatment has a carbonization temperature of 500℃ to 1100℃ and a carbonization time of 2 hours to 10 hours.

[0033] Preferably, in step S3, the carbon source gas includes one or more of methane, ethane, acetylene, propane, propylene, and carbon dioxide; the flow rate of the carbon source gas is 1 L / min to 15 L / min.

[0034] The protective gas includes nitrogen and / or argon; the volume flow ratio of the protective gas to the carbon source gas is 20:1 to 1:1;

[0035] The second temperature is 400℃~900℃, and the second heat preservation time is 1 hour~15 hours;

[0036] The rotary kiln rotates at a speed of 0.25 rpm to 1.5 rpm.

[0037] Thirdly, embodiments of the present invention provide a negative electrode sheet, the negative electrode sheet comprising the core-shell structured silicon-carbon composite material described in the first aspect above.

[0038] Fourthly, embodiments of the present invention provide a lithium battery, the lithium battery including the negative electrode sheet described in the third aspect above.

[0039] This invention provides a method for preparing a core-shell structured silicon-carbon composite material. First, nano-silicon particles are deposited in the pores of porous carbon to obtain a porous silicon-carbon composite. Then, the porous silicon-carbon composite is fused with a silicon-iron alloy and an organic carbon source, followed by high-temperature carbonization to form a buffer layer on the surface of the porous silicon-carbon composite. Finally, a gas-phase carbon coating treatment is performed to obtain the core-shell structured silicon-carbon composite material. This preparation method is simple to operate, low in cost, and suitable for large-scale production.

[0040] The core-shell silicon-carbon composite material prepared by the above-mentioned preparation method provided in this embodiment of the invention includes ordered carbon nanotubes and non-dense carbon formed by the carbonization of ferrosilicon alloy and organic carbon source in the buffer layer. The catalytic effect of ferrosilicon alloy promotes the uniform and orderly growth of carbon nanotubes. The obtained carbon nanotubes can provide buffer space for volume changes during particle expansion. At the same time, the non-dense carbon formed after carbonization of organic carbon source can also provide buffer space, thus suppressing the overall expansion of silicon-carbon composite material particles. The secondary carbon coating layer can effectively reduce the overall specific surface area of ​​silicon-carbon composite material particles, reduce the generation of solid electrolyte interphase (SEI) film, ensure the first-cycle coulombic efficiency, and the carbon coating layer formed by secondary coating also provides support for suppressing particle expansion.

[0041] Using the core-shell structured silicon-carbon composite material provided in this invention in lithium batteries can improve the first-cycle coulombic efficiency of the battery, suppress battery expansion, and improve cycle stability. Attached Figure Description

[0042] Figure 1 A flowchart illustrating the preparation method of a core-shell structured silicon-carbon composite material provided in this embodiment of the invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0044] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0045] This invention provides a core-shell structured silicon-carbon composite material, comprising a core and an outer shell; wherein the core is silicon-carbon particles, comprising: porous carbon and nano-silicon particles uniformly dispersed in the pores of the porous carbon; the outer shell comprises: a buffer layer covering the outer surface of the silicon-carbon particles, and a carbon coating layer covering the outer surface of the buffer layer.

[0046] Specifically, the buffer layer comprises a silicon-iron alloy, carbides, and ordered carbon nanotubes; the carbides are non-dense carbon formed by the carbonization of organic carbon sources, and the carbides are amorphous carbon.

[0047] Carbon nanotubes are formed by coating the outer surface of silicon-carbon particles with a mixture of ferrosilicon alloy and an organic carbon source, followed by carbonization.

[0048] The organic carbon source is an organic polymer, including one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone.

[0049] The pore size of the porous carbon is between 0.1 nm and 20 nm, preferably between 1.5 nm and 4.8 nm; the specific surface area of ​​the porous carbon is 100 m² / m³. 2 / g~3000m 2 Between / g, preferably 1200m 2 / g~2500m 2 / g.

[0050] The particle size Dv50 of the silicon-carbon composite material is between 0.5 μm and 100 μm, preferably between 3 μm and 12 μm.

[0051] The thickness of the buffer layer is 0.5μm to 50μm, preferably 1.5μm to 6μm.

[0052] The thickness of the carbon coating is between 2 nm and 50 nm.

[0053] The mass of nano-silicon particles accounts for 30% to 60% of the total mass of silicon-carbon composite materials.

[0054] The mass of carbon nanotubes accounts for 1% to 5% of the total mass of silicon-carbon composite materials.

[0055] The mass percentage of porous carbon in the total mass of silicon-carbon composites is 35% to 65%.

[0056] This invention provides a method for preparing the above-mentioned core-shell structured silicon-carbon composite material, such as... Figure 1 As shown, the specific steps include:

[0057] Step S1: Place porous carbon in a reactor and raise it to the first temperature under a protective gas environment. The silicon source gas is introduced into the reactor through the protective gas and kept at the first temperature. The silicon element obtained by the dissociation of the silicon source gas is deposited in the pores of the porous carbon and grows into nanoscale silicon particles to obtain a porous silicon-carbon composite.

[0058] Porous carbon includes one or more of the following: biomass porous carbon, resin porous carbon, coke-based porous carbon, and graphite porous carbon; the pore size of the porous carbon is between 0.1 nm and 20 nm, and the specific surface area is 100 m². 2 / g~3000m 2 Between / g;

[0059] The silicon source gas includes one or more of the following: metallic silicon vapor, silane, propane, dichlorosilane, trichlorosilane, and tetrachlorosilane; the flow rate of the silicon source gas is 1 L / min to 15 L / min;

[0060] The protective gas includes nitrogen and / or argon; the volumetric flow rate ratio of the protective gas to the silicon source gas is 10:1 to 1:10.

[0061] The first temperature is 400℃~700℃, and the first heat preservation time is 1 hour~8 hours.

[0062] Step S2: The porous carbon composite is mixed with an organic carbon source and a silicon-iron alloy, so that the organic carbon source and the silicon-iron alloy are coated on the surface of the porous carbon composite. Then, it is placed in a rotary kiln for high-temperature carbonization under a protective atmosphere. After cooling to room temperature, the precursor material is obtained.

[0063] The organic carbon sources include one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone.

[0064] The mixing process involves placing the porous carbon composite, organic carbon source, and ferrosilicon alloy in a fusion machine and fusing them at a speed of 200 rpm to 800 rpm for 10 to 60 minutes.

[0065] The silicon content in the ferrosilicon alloy is 15wt% to 90wt%; the ferrosilicon alloy phase is conducive to promoting the uniform and orderly growth of carbon nanotubes on the particle surface, resulting in a buffer layer of uniform thickness.

[0066] The mass ratio of porous carbon composite, organic carbon source and ferrosilicon alloy is 5-8:1-3:1-3.

[0067] The protective gas includes nitrogen and / or argon; the carbonization temperature of the high-temperature carbonization treatment is 500℃~1100℃, and the carbonization time is 2 hours~10 hours.

[0068] Step S3: Place the precursor material in a rotary kiln and heat it to a second temperature. Under a protective gas environment, bring the carbon source gas into the rotary kiln through the protective gas for a second heat preservation and perform gas phase carbon coating treatment. A carbon coating layer is formed on the outermost surface of the precursor material to obtain a core-shell structured silicon-carbon composite material.

[0069] The carbon source gas includes one or more of methane, ethane, acetylene, propane, propylene, and carbon dioxide; the flow rate of the carbon source gas is 1 L / min to 15 L / min.

[0070] The protective gas includes nitrogen and / or argon; the volumetric flow rate ratio of the protective gas to the carbon source gas is 20:1 to 1:1.

[0071] The second temperature is 400℃~900℃, and the second holding time is 1 hour~15 hours;

[0072] The rotary kiln used above has a rotation speed of 0.25 rpm to 1.5 rpm.

[0073] The organic carbon source used in this invention is one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone. These organic carbon sources can form loose carbon after carbonization, which provides a buffer for the volume expansion of silicon, helps to suppress the volume expansion of silicon nanoparticles, avoids material pulverization or breakage, and also helps to provide space conditions for the vertical growth of carbon nanotubes.

[0074] The core-shell structured silicon-carbon composite material provided in this invention can be used to prepare negative electrode sheets, which can be used to assemble lithium batteries.

[0075] To better understand the technical solution provided by this invention, the following uses several specific examples to illustrate the preparation process and characteristics of the silicon-carbon composite material with a core-shell structure of this invention.

[0076] Example 1

[0077] This embodiment provides a preparation process for a core-shell structured silicon-carbon composite material, the specific process of which is as follows.

[0078] (1) Place 1 kg of biomass porous carbon in the chamber of a reactor. First, purge the chamber with nitrogen gas at a flow rate of 5 L / min for 30 minutes to remove the air from the chamber. Under nitrogen gas conditions, raise the temperature in the chamber to 500°C at a heating rate of 5°C / min. Use nitrogen gas at a flow rate of 3 L / min as a carrier gas to introduce silane at a flow rate of 3 L / min into the chamber of the reactor. Keep the chamber at this temperature for 3 hours to allow the silicon elements obtained from the dissociation of the silicon source gas to deposit in the pores of the biomass porous carbon and grow into nanoscale silicon particles, thus obtaining a porous silicon-carbon composite. The biomass porous carbon has a particle size Dv50 of 10 μm and a specific surface area of ​​1800 m².2 / g, with an average pore size of 1.95nm.

[0079] (2) The porous carbon composite, ferrosilicon alloy and polysaccharide alcohol are placed in a fusion machine at a mass ratio of 7:1:2 and fused at 400 rpm for 30 minutes to coat the surface of the porous carbon composite with polysaccharide alcohol and ferrosilicon alloy. Then, it is placed in a rotary furnace at 1 rpm and subjected to high-temperature carbonization treatment at 800°C for 4 hours. After cooling to room temperature, a precursor material containing a buffer layer is obtained.

[0080] (3) Under a nitrogen atmosphere, the rotary furnace with a rotation speed of 1 rpm is heated to 660°C. Nitrogen with a flow rate of 3 L / min is used as the carrier gas to carry acetylene with a flow rate of 8 L / min into the rotary furnace. The furnace is kept at this temperature for 2 hours to perform gas phase carbon coating treatment, forming a carbon coating layer on the outermost surface of the precursor material. The material is then cooled to room temperature and discharged to finally obtain a core-shell structured silicon-carbon composite material.

[0081] The silicon-carbon composite material with a core-shell structure used in this embodiment was used as the negative electrode active material to prepare the electrode sheet and assemble it into a coin cell for testing. The specific process is as follows.

[0082] Electrode preparation process: Silicon-carbon composite negative electrode material, conductive additive carbon black, and binder (sodium carboxymethyl cellulose and styrene-butadiene rubber at a mass ratio of 95:2:3) were weighed and slurry was prepared using a pulping machine at room temperature. The prepared slurry was uniformly coated onto copper foil to a thickness of 220 μm and dried in a forced-air drying oven at 55°C for 2 hours. The coated electrode was then cut into 14 mm diameter circular electrodes and vacuum-dried in a vacuum drying oven at 100°C for 8 hours. The dried electrodes were then immediately transferred to a glove box for use in battery assembly.

[0083] Assembly process of button half cell: The assembly of the simulated battery is carried out in a glove box containing a high-purity Ar atmosphere. Lithium metal is used as the counter electrode, and the electrolyte is LiPF6 with a molar concentration of 1 mol / L. The solvent of the electrolyte is ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1. The separator is polyethylene (PE) membrane, and the battery is assembled into a button half cell.

[0084] Testing procedure for coin cell half-cells: Constant current charge-discharge mode testing was performed using a charge-discharge apparatus. The discharge cutoff voltage was 0.005V, and the charge cutoff voltage was 2V. Routine charge-discharge tests were conducted at a current density of C / 10. Detailed test data for the lithium intercalation specific capacity and first-cycle coulombic efficiency of the coin cell half-cells are shown in Table 1.

[0085] The silicon-carbon composite material with a core-shell structure of this embodiment was used as the negative electrode active material to prepare a negative electrode sheet and assembled into a full cell for testing. The specific process is as follows.

[0086] Negative electrode preparation process: A composite negative electrode material is prepared by combining silicon-carbon composite material with graphite with a specific capacity of 350 mAh / g. The theoretical capacity of the composite negative electrode material is 480 mAh / g. The negative electrode active material (composite negative electrode material), conductive agent (Super P), binder carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBr) are mixed and ground into a slurry at a mass ratio of 94.5:2:2:1.5. The slurry is then coated onto copper foil, dried, cut, and prepared into a negative electrode sheet. The sheet is then placed in an argon-filled glove box for later use.

[0087] Positive electrode preparation process: The positive active material (811 type high nickel ternary material), conductive agent 1 (Super P), conductive agent 2 (single-walled CNT), and binder (PVDF) are mixed and ground into a slurry at a mass ratio of 95.2:1.8:1:2. The slurry is then coated onto aluminum foil, dried, cut, and prepared into a positive electrode sheet. The sheet is then placed in an argon-filled glove box for later use.

[0088] The process of assembling a 5085 type small soft-pack full cell is as follows: 11 negative electrode plates and 10 positive electrode plates are assembled into a 5085 type small soft-pack cell. The electrolyte is LiPF6 with a molar concentration of 1 mol / L (the solvent is ethylene carbonate, dimethyl carbonate and diethyl carbonate in a volume ratio of 1:1:1).

[0089] Full battery testing process: Under the conditions of a charging cutoff voltage of 4.25V and a discharging cutoff voltage of 2.75V, the battery was fully charged at a rate of 0.5C. The battery was then disassembled, the electrode thickness was measured, and the average value was calculated to determine the expansion rate. The specific method for measuring the expansion rate is as follows:

[0090] 1) Measure the initial thickness h1 of the negative electrode sheet;

[0091] 2) Measure the thickness h2 of the negative electrode after lithium insertion / extraction or after cyclic charge / discharge;

[0092] 3) The formula for calculating the expansion rate is (h2-h1) / h1. See Table 1 for the expansion rate test results.

[0093] The powder conductivity test process for the core-shell structured silicon-carbon composite material prepared in this embodiment was as follows: the conductivity was measured according to the four-probe method Q / JSGL 006—2014. Detailed conductivity test data are shown in Table 1.

[0094] Example 2

[0095] This embodiment provides a preparation process for a core-shell structured silicon-carbon composite material, the specific process of which is as follows.

[0096] (1) Place 1 kg of porous carbon resin in the chamber of the reactor. First, purge the chamber with nitrogen gas at a flow rate of 5 L / min for 30 minutes to remove the air from the chamber. Under nitrogen gas conditions, raise the temperature in the chamber to 500°C at a heating rate of 5°C / min. Use nitrogen gas at a flow rate of 3 L / min as a carrier gas to introduce silane at a flow rate of 3 L / min into the chamber of the reactor. Keep the chamber at this temperature for 3 hours to allow the silicon elements obtained from the dissociation of the silicon source gas to deposit in the pores of the biomass porous carbon and grow into nanoscale silicon particles, thus obtaining a porous silicon-carbon composite. Among them, the particle size Dv50 of the micron-sized porous carbon resin is 6 μm, and the specific surface area is 1600 m². 2 / g, with an average pore size of 1.88nm.

[0097] (2) The porous carbon composite, ferrosilicon alloy and polydopamine are placed in a fusion machine at a mass ratio of 7:1:2 and fused at 300 rpm for 60 minutes to coat the surface of the porous carbon composite with polydopamine and ferrosilicon alloy. Then, it is placed in a rotary kiln at a speed of 0.8 rpm and subjected to high-temperature carbonization treatment at 650°C for 5 hours. After cooling to room temperature, a precursor material containing a buffer layer is obtained.

[0098] (3) Under a nitrogen atmosphere, the rotary kiln with a rotation speed of 0.8 rpm was heated to 550°C. Nitrogen with a flow rate of 3 L / min was used as the carrier gas to carry ethylene with a flow rate of 10 L / min into the rotary kiln. The kiln was kept at the temperature for 4 hours to carry out gas phase carbon coating treatment, forming a carbon coating layer on the outermost surface of the precursor material. The material was then cooled to room temperature and discharged to finally obtain a core-shell structured silicon-carbon composite material.

[0099] The silicon-carbon composite material with a core-shell structure used in this embodiment was used as the negative electrode active material to prepare the electrode sheet and assemble it into a coin cell for testing. The specific process was the same as in Example 1. The test data of the lithium intercalation specific capacity and the first-cycle coulombic efficiency of the coin cell are detailed in Table 1.

[0100] The silicon-carbon composite material with a core-shell structure used in this embodiment was used as the negative electrode active material to prepare the negative electrode sheet and assemble it into a full cell for testing. The specific process was the same as in Example 1. The expansion rate test results are detailed in Table 1.

[0101] The powder conductivity test method for the core-shell structured silicon-carbon composite material prepared in this embodiment is the same as that in Example 1, and the test data are detailed in Table 1.

[0102] Example 3

[0103] This embodiment provides a preparation process for a core-shell structured silicon-carbon composite material, the specific process of which is as follows.

[0104] (1) Place 1 kg of biomass porous carbon in the chamber of a reactor. First, purge the chamber with nitrogen gas at a flow rate of 5 L / min for 30 minutes to remove the air from the chamber. Under nitrogen gas conditions, raise the temperature in the chamber to 500°C at a heating rate of 5°C / min. Use nitrogen gas at a flow rate of 3 L / min as a carrier gas to introduce silane at a flow rate of 3 L / min into the chamber of the reactor. Keep the chamber at this temperature for 3 hours to allow the silicon elements obtained from the dissociation of the silicon source gas to deposit in the pores of the biomass porous carbon and grow into nanoscale silicon particles, thus obtaining a porous silicon-carbon composite. The biomass porous carbon has a particle size Dv50 of 9 μm and a specific surface area of ​​300 m². 2 / g, with an average pore size of 10.61nm.

[0105] (2) The porous carbon composite, ferrosilicon alloy and polysaccharide alcohol are placed in a fusion machine at a mass ratio of 7:1:2 and fused at 400 rpm for 60 minutes to coat the surface of the porous carbon composite with polysaccharide alcohol and ferrosilicon alloy. Then, it is placed in a rotary furnace at a speed of 0.8 rpm and subjected to high-temperature carbonization treatment at 800℃ for 4 hours. After cooling to room temperature, a precursor material containing a buffer layer is obtained.

[0106] (3) Under a nitrogen atmosphere, the rotary kiln with a rotation speed of 1 rpm was heated to 660°C. Propane with a flow rate of 6 L / min was introduced into the rotary kiln by using nitrogen with a flow rate of 3 L / min as the carrier gas. The kiln was kept at the temperature for 15 hours to perform gas phase carbon coating treatment, forming a carbon coating layer on the outermost surface of the precursor material. The material was then cooled to room temperature and discharged to finally obtain a core-shell structured silicon-carbon composite material.

[0107] The silicon-carbon composite material with a core-shell structure used in this embodiment was used as the negative electrode active material to prepare the electrode sheet and assemble it into a coin cell for testing. The specific process was the same as in Example 1. The test data of the lithium intercalation specific capacity and the first-cycle coulombic efficiency of the coin cell are detailed in Table 1.

[0108] The silicon-carbon composite material with a core-shell structure used in this embodiment was used as the negative electrode active material to prepare the negative electrode sheet and assemble it into a full cell for testing. The specific process was the same as in Example 1. The expansion rate test results are detailed in Table 1.

[0109] The powder conductivity test method for the core-shell structured silicon-carbon composite material prepared in this embodiment is the same as that in Example 1, and the test data are detailed in Table 1.

[0110] To better illustrate the effects of the embodiments of the present invention, Comparative Examples 1-2 are compared with Example 1 above.

[0111] Comparative Example 1

[0112] This comparative example provides a process for preparing a silicon-carbon composite material. The difference from Example 1 is that step (2) does not use a silicon-iron alloy. Specifically, a porous carbon composite and polysaccharide are placed in a fusion machine at a mass ratio of 7:2 and fused at 400 rpm for 30 minutes, allowing the polysaccharide and silicon-iron alloy to coat the surface of the porous carbon composite. Then, it is placed in a rotary kiln at 1 rpm and subjected to high-temperature carbonization at 800°C for 4 hours. After cooling to room temperature, a precursor material with a single-layer carbon coating is obtained. Other preparation processes are the same as in Example 1. Finally, a silicon-carbon composite material with a double-layer carbon coating is obtained.

[0113] The silicon-carbon composite material prepared in the comparative example was used as the negative electrode active material to prepare the electrode sheet, which was then assembled into a coin cell for testing. The specific process was the same as in Example 1. The test data of the lithium intercalation specific capacity and the first-cycle coulombic efficiency of the coin cell are detailed in Table 1.

[0114] The silicon-carbon composite material prepared in this comparative example was used as the negative electrode active material to prepare a negative electrode sheet and assembled into a full cell for testing. The specific process was the same as in Example 1. The expansion rate test results are detailed in Table 1.

[0115] The powder conductivity test method for the silicon-carbon composite material prepared in this comparative example is the same as that in Example 1, and the test data are detailed in Table 1.

[0116] Comparative Example 2

[0117] This comparative example provides a process for preparing a silicon-carbon composite material. The difference from Example 1 is that iron powder is used in step (2). Specifically, a porous carbon composite, iron powder, and polysaccharide alcohol are placed in a fusion machine at a mass ratio of 7:1:2 and fused at 400 rpm for 30 minutes, allowing the polysaccharide alcohol and ferrosilicon alloy to coat the surface of the porous carbon composite. Then, it is placed in a rotary kiln at 1 rpm and subjected to high-temperature carbonization treatment at 800°C for 4 hours. After cooling to room temperature, a precursor material containing a single layer of carbon coating is obtained. Other preparation processes are the same as in Example 1. Finally, a silicon-carbon composite material is obtained.

[0118] The silicon-carbon composite material prepared in the comparative example was used as the negative electrode active material to prepare the electrode sheet, which was then assembled into a coin cell for testing. The specific process was the same as in Example 1. The test data of the lithium intercalation specific capacity and the first-cycle coulombic efficiency of the coin cell are detailed in Table 1.

[0119] The silicon-carbon composite material prepared in this comparative example was used as the negative electrode active material to prepare a negative electrode sheet and assembled into a full cell for testing. The specific process was the same as in Example 1. The expansion rate test results are detailed in Table 1.

[0120] The powder conductivity test method for the silicon-carbon composite material prepared in this comparative example is the same as that in Example 1, and the test data are detailed in Table 1.

[0121] Table 1 summarizes the test data for Examples 1-3 and Comparative Examples 1-2:

[0122]

[0123] Table 1

[0124] As can be seen from the test data in Table 1, compared with Comparative Example 1, the electrode prepared by the core-shell silicon-carbon composite material provided in Example 1 of this invention has a lower expansion rate and higher conductivity. This is because the core-shell silicon-carbon composite material prepared in Example 1 of this invention has a tough structure formed by the synergistic effect of the polysaccharide carbide buffer medium in the buffer layer and the catalytically formed carbon nanotubes, which effectively ensures the suppression of silicon-carbon expansion. In addition, the presence of carbon nanotubes also provides support for the internal conductivity of this core-shell structure. Since the coin cell assembled in Example 1 uses the silicon-carbon composite material of this invention, the coin cell of Example 1 has a higher first-cycle coulombic efficiency compared with the coin cell of Comparative Example 1.

[0125] Compared to Comparative Example 2, the silicon-carbon composite material with a core-shell structure provided in Example 1 of this invention has a lower expansion rate. This is because the silicon-iron alloy, compared to elemental iron, is conducive to promoting the uniform and orderly growth of carbon nanotubes on the particle surface, resulting in a uniform buffer layer thickness and reducing the expansion rate of the silicon-carbon composite material.

[0126] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A core-shell structured silicon-carbon composite material, characterized in that, The silicon-carbon composite material includes a core and a shell; The core is a silicon-carbon particle, comprising: porous carbon and nano-silicon particles uniformly dispersed in the pores of the porous carbon. The outer shell includes: a buffer layer covering the outer surface of the silicon-carbon particles, and a carbon coating layer covering the outer surface of the buffer layer; The buffer layer comprises a silicon-iron alloy, carbides, and ordered carbon nanotubes; the carbides are non-dense carbon formed by carbonizing an organic carbon source; the carbides are amorphous carbon. The carbon nanotubes are formed by coating the outer surface of the silicon-carbon particles with a mixture of ferrosilicon alloy and an organic carbon source, followed by carbonization.

2. The silicon-carbon composite material according to claim 1, characterized in that, The pore size of the porous carbon is between 0.1 nm and 20 nm; the specific surface area of ​​the porous carbon is 100 m². 2 / g~3000m 2 Between / g; The organic carbon source is an organic polymer, including one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone; The particle size Dv50 of the silicon-carbon composite material is between 0.5 μm and 100 μm.

3. The silicon-carbon composite material according to claim 1, characterized in that, The thickness of the buffer layer is 0.5 μm to 50 μm. The thickness of the carbon coating layer is between 2 nm and 50 nm.

4. The silicon-carbon composite material according to claim 1, characterized in that, The mass percentage of the nano-silicon particles in the total mass of the silicon-carbon composite material is 30% to 60%. The carbon nanotubes account for 1% to 5% of the total mass of the silicon-carbon composite material; The porous carbon accounts for 35% to 65% of the total mass of the silicon-carbon composite material.

5. A method for preparing the silicon-carbon composite material according to any one of claims 1-4, characterized in that, The preparation method includes: Step S1: Place porous carbon in a reactor and raise it to a first temperature under a protective gas environment. The silicon source gas is introduced into the reactor through the protective gas and subjected to a first heat preservation. The silicon element obtained by the dissociation of the silicon source gas is deposited in the pores of the porous carbon and grows into nanoscale silicon particles to obtain a porous silicon-carbon composite. Step S2: The porous carbon composite is mixed with an organic carbon source and a silicon-iron alloy, so that the organic carbon source and the silicon-iron alloy coat the surface of the porous carbon composite. Then, it is placed in the reactor for high-temperature carbonization treatment. After cooling to room temperature, the precursor material is obtained. Step S3: Place the precursor material in a rotary kiln and heat it to a second temperature. Under a protective gas environment, bring the carbon source gas into the rotary kiln through the protective gas for a second heat preservation and perform gas phase carbon coating treatment. A carbon coating layer is formed on the outermost surface of the precursor material to obtain a core-shell structured silicon-carbon composite material.

6. The preparation method according to claim 5, characterized in that, In step S1, the porous carbon includes one or more of the following: biomass porous carbon, resin porous carbon, coke-based porous carbon, and graphite porous carbon. The silicon source gas includes one or more of the following: metallic silicon vapor, silane, propane, dichlorosilane, trichlorosilane, and tetrachlorosilane; the flow rate of the silicon source gas is 1 L / min to 15 L / min. The protective gas includes nitrogen and / or argon; the volumetric flow rate ratio of the protective gas to the silicon source gas is 10:1 to 1:10; The first temperature is 400℃~700℃, and the first heat preservation time is 1 hour~8 hours.

7. The preparation method according to claim 5, characterized in that, In step S2, the organic carbon source includes one or more of polysaccharide alcohol, polydopamine, and polyvinylpyrrolidone. The mixing process specifically involves placing the porous carbon composite, the organic carbon source, and the ferrosilicon alloy in a fusion machine and fusing them at a speed of 200 rpm to 800 rpm for 10 to 60 minutes. The silicon content in the ferrosilicon alloy is 15wt% to 90wt%; The mass ratio of the porous carbon composite, the organic carbon source, and the ferrosilicon alloy is 5-8:1-3:1-3; The high-temperature carbonization treatment has a carbonization temperature of 500℃ to 1100℃ and a carbonization time of 2 hours to 10 hours.

8. The preparation method according to claim 5, characterized in that, In step S3, the carbon source gas includes one or more of methane, ethane, acetylene, propane, propylene, and carbon dioxide; the flow rate of the carbon source gas is 1 L / min to 15 L / min. The protective gas includes nitrogen and / or argon; the volume flow ratio of the protective gas to the carbon source gas is 20:1 to 1:1; The second temperature is 400℃~900℃, and the second heat preservation time is 1 hour~15 hours; The rotary kiln rotates at a speed of 0.25 rpm to 1.5 rpm.

9. A negative electrode sheet, characterized in that, The negative electrode sheet comprises a silicon-carbon composite material with a core-shell structure as described in any one of claims 1-4.

10. A lithium battery, characterized in that, The lithium battery includes the negative electrode sheet as described in claim 9.

Images