Silicon-carbon composite material as well as preparation method and application thereof

Abstract

The invention discloses a silicon-carbon composite material as well as a preparation method and application thereof, and relates to the technical field of silicon-carbon materials. The silicon-carbon inner layer particles are coated with the first carbon layer and the second carbon layer, the second carbon layer is the nitrogen and phosphorus co-doped amorphous carbon layer, the strength is high, silicon volume expansion in the discharging process can be limited, and the conductivity of the amorphous carbon layer can be improved through nitrogen and phosphorus co-doping; nitrogen, phosphorus and carbon elements can be subjected to dehydration condensation in the sintering process to form a heterocyclic structure, a three-dimensional network is formed, and the structural stability of the carbon layer can be further improved. Therefore, the silicon-carbon composite material prepared by the invention has the advantages of low expansion rate, high strength and high conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon-carbon materials, and in particular, to a silicon-carbon composite material, a preparation method thereof, and an application thereof. Background Art

[0002] The theoretical specific capacity of traditional graphite anodes is about 372 mAh / g, and that of silicon anodes is about 4200 mAh / g, which is significantly higher than that of traditional graphite anodes. Therefore, among lithium-ion battery anode materials, silicon-based anodes are now regarded as the next-generation lithium battery anode materials most likely to replace traditional graphite. However, due to a series of problems inherent in silicon particles themselves, such as volume expansion effect and poor conductivity, the wide application of silicon-carbon composite materials is limited.

[0003] In recent years, the composite structure design of newly developed silicon-carbon materials has received extensive attention in the industry. By depositing nano-silicon on porous carbon to form a silicon-carbon composite structure, this composite structure utilizes the rich pore structure inside the porous carbon to deposit gaseous silicon sources into the pores of the porous carbon at a certain temperature, solving the problem of silicon particle agglomeration and pulverization. However, the large volume expansion of current silicon-based materials still restricts the improvement of their electrochemical cycle stability.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a silicon-carbon composite material, a preparation method thereof, and an application thereof, aiming to inhibit the expansion of the silicon-carbon composite material while improving the strength and conductivity of the material.

[0006] The present invention is implemented as follows:

[0007] In a first aspect, the present invention provides a silicon-carbon composite material, including silicon-carbon inner layer particles, a first carbon layer, and a second carbon layer. The first carbon layer coats the silicon-carbon inner layer particles to form carbon-coated particles, and the second carbon layer coats the carbon-coated particles;

[0008] Among them, the silicon-carbon inner layer particles include porous carbon, and nano-silicon particles are deposited in the pores of the porous carbon; nitrogen and phosphorus are doped on the second carbon layer.

[0009] In an alternative embodiment, the first carbon layer is formed by gas-phase coating, and the second carbon layer is formed by solid-phase coating; each silicon-carbon inner layer particle is coated with a first carbon layer, and the second carbon layer contains multiple carbon-coated particles;

[0010] Preferably, the particle size D50 of the silicon-carbon inner layer particles is 5.00 μm - 8.00 μm, the particle size D50 of the carbon-coated particles is 5.01 μm - 8.10 μm, and the particle size D50 of the silicon-carbon composite material is 6 μm - 12 μm; preferably, the specific surface area of the porous carbon is greater than 1200 m2 / g; More preferably, the specific surface area of the porous carbon is 1200 m 2 / g - 1400 m 2 / g, and the micropore proportion is 50% - 70%.

[0011] In an alternative embodiment, by mass percentage, in the silicon-carbon composite material, the silicon content is 35% - 45%, the carbon content is 54% - 64%, the nitrogen content is 0.05% - 0.5%, and the phosphorus content is 0.05% - 0.5%.

[0012] In a second aspect, the present invention provides a method for preparing the silicon-carbon composite material according to any one of the foregoing embodiments, including: depositing nano-silicon particles on porous carbon to obtain silicon-carbon inner layer particles, coating a first carbon layer on the silicon-carbon inner layer particles to obtain carbon-coated particles, and coating a second carbon layer on the carbon-coated particles.

[0013] In an alternative embodiment, the preparation process of the silicon-carbon inner layer particles includes: placing the porous carbon in a reactor, heating to the reaction temperature under an inert atmosphere, and then introducing a silicon source gas for reaction; wherein, the silicon source gas contains silane;

[0014] Preferably, first heat to the first reaction temperature for reaction under an inert atmosphere, and then cool to the second reaction temperature for reaction under an inert atmosphere. The first reaction temperature is 500°C - 600°C, and the second reaction temperature is 400°C - 500°C;

[0015] More preferably, when reacting at the first reaction temperature, control the apparent gas velocity of the introduced silicon source gas to be 0.10 m / s - 0.20 m / s, and control the mass ratio of the introduced silane to the mass of the porous carbon to be (1 - 5):100;

[0016] More preferably, when reacting at the second reaction temperature, control the apparent gas velocity of the introduced silicon source gas to be 0.02 m / s - 0.10 m / s, and control the total mass ratio of the silane introduced in the two-step reaction to the mass of the porous carbon to be (50 - 120):100;

[0017] More preferably, when reacting at the first reaction temperature, control the volume fraction of silane in the introduced silicon source gas to be 1% - 5%; when reacting at the second reaction temperature, control the volume fraction of silane in the introduced silicon source gas to be 5% - 50%;

[0018] Further preferably, the silicon source gas introduced during the two-step reaction process further contains an optional gas, and the optional gas is selected from at least one of phosphine, borane, hydrogen, ammonia, nitrogen, and argon.

[0019] In an alternative embodiment, a first carbon layer is formed by gas-phase coating to obtain carbon-coated particles, and a second carbon layer is formed on the carbon-coated particles by solid-phase coating.

[0020] In an alternative embodiment, the process of forming the first carbon layer includes: after the preparation of the inner silicon-carbon layer particles is completed, heating to a third reaction temperature under an inert atmosphere, introducing a reaction gas containing a carbon source into the reactor for reaction, and controlling the mass ratio of the carbon source content in the introduced reaction gas to the mass of the porous carbon to be (1-20):100;

[0021] Preferably, the third reaction temperature is 600°C - 800°C;

[0022] Preferably, the superficial gas velocity of the reaction gas containing a carbon source is 0.02 m / s - 0.10 m / s;

[0023] Preferably, the reaction gas containing a carbon source is selected from at least one of ethanol, ethylene, benzene, methane, ethane, acetylene, phosphine, borane, hydrogen, ammonia, nitrogen, and argon, and the volume fraction of the carbon source gas is 5% - 50%.

[0024] In an alternative embodiment, the process of forming the second carbon layer includes: mixing the carbon-coated particles with an organic carbon source, a phosphorus source, and a nitrogen source, and performing sintering after hot pressing and forming;

[0025] Preferably, the mass ratio of the organic carbon source to the carbon-coated particles is (10-20):100, and the mass ratio of the amounts of the organic carbon source, the phosphorus source, and the nitrogen source is (10-20):(0.5-3.0):(0.5-3.0); more preferably, the organic carbon source is selected from at least one of phenolic resin and pitch; the phosphorus source is selected from at least one of phosphine, phosphorus pentoxide, triphenylphosphine, and phytic acid; the nitrogen source is selected from at least one of melamine, polydopamine, urea, and thiourea;

[0026] Preferably, during the hot pressing and forming process, the operating pressure is controlled to be 10 MPa - 100 MPa, the operating temperature is 120°C - 300°C, and the hot pressing time is 0.5 min - 10 min;

[0027] Preferably, during the sintering process, the sintering temperature is controlled to be 600°C - 1000°C, and the sintering time is 3 h - 8 h; more preferably, during the sintering process, the protective gas used is selected from at least one of nitrogen, argon, hydrogen, and carbon monoxide;

[0028] More preferably, after sintering, pulverization and classification are performed to obtain a product with a D50 particle size of 6 μm - 12 μm.

[0029] In a third aspect, the present invention provides a negative electrode tab, including a negative electrode current collector, and the silicon-carbon composite material according to any one of the foregoing embodiments is attached to the negative electrode current collector.

[0030] In a fourth aspect, the present invention provides a secondary battery, including the negative electrode tab according to the foregoing embodiment.

[0031] The present invention has the following beneficial effects: By coating the inner silicon-carbon particles with a first carbon layer and a second carbon layer, the second carbon layer is a nitrogen and phosphorus co-doped amorphous carbon layer, which has a relatively high strength and can limit the volume expansion of silicon during the discharge process. The nitrogen and phosphorus co-doping can also improve the conductivity of the amorphous carbon layer. The nitrogen, phosphorus, and carbon elements can undergo dehydration condensation during the sintering process to form a heterocyclic structure, constituting a three-dimensional network, which can further improve the structural stability of the carbon layer. Therefore, the silicon-carbon composite material prepared by the present invention has the advantages of low expansion rate, high strength, and high conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 It is a schematic structural diagram of the silicon-carbon composite material provided by the embodiment of the present invention;

[0034] Figure 2 It is a dQ / dV curve graph of a button battery. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] In order to make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0036] The embodiment of the present invention also provides a preparation method of a silicon-carbon composite material, including the following steps:

[0037] S1. Prepare inner silicon-carbon particles

[0038] Deposit nano-silicon particles on porous carbon to obtain inner silicon-carbon particles, and a chemical vapor deposition method can be used, and the nano-silicon particles adhere to the pores of the porous carbon.

[0039] In the actual operation process, the preparation process of the inner silicon-carbon particles includes: placing the porous carbon in a reactor, heating to the reaction temperature under an inert atmosphere, and then introducing a silicon source gas for reaction; wherein, the silicon source gas contains silane, and the silane forms nano-silicon particles during the reaction and deposits in the pores of the porous carbon.

[0040] In a preferred embodiment, it is first heated to a first reaction temperature for reaction under an inert atmosphere, and then cooled to a second reaction temperature for reaction under the inert atmosphere. The first reaction temperature is 500°C - 600°C, and the second reaction temperature is 400°C - 500°C; when reacting at the first reaction temperature, the apparent gas velocity of the silicon source gas introduced is controlled to be 0.10 m / s - 0.20 m / s, and the mass ratio of the introduced silane to the mass of the porous carbon is controlled to be (1 - 5):100; when reacting at the second reaction temperature, the apparent gas velocity of the silicon source gas introduced is controlled to be 0.02 m / s - 0.10 m / s, and the total mass ratio of the silane introduced in the two-step reaction to the mass of the porous carbon is controlled to be (50 - 120):100.

[0041] It should be noted that by dividing the reaction into two stages and controlling the reaction temperature, gas velocity, and silane introduction amount, reacting at a higher gas velocity and temperature first can obtain monodisperse nano-silicon crystal nuclei deeply embedded in the pores of the porous carbon; then reacting at a lower gas velocity and lower temperature enables the monodisperse nano-silicon crystal nuclei to grow uniformly in the micropores. By controlling the nucleation and growth reaction rate of the nano-silicon and the silicon-carbon ratio, a silicon-carbon composite material with a suitable porosity is obtained. The silicon grain growth process can obtain a silicon-carbon composite material with a certain porosity by controlling the reaction time to accommodate the volume expansion of the nano-silicon during charge and discharge.

[0042] Specifically, in the first reaction stage, the controlled first reaction temperature can be 500°C, 530°C, 550°C, 580°C, 600°C, etc., and the apparent gas velocity of the silicon source gas introduced can be 0.10 m / s, 0.12 m / s, 0.13 m / s, 0.14 m / s, 0.15 m / s, 0.20 m / s, etc. By controlling the introduction amount of the silicon source gas, the mass ratio of the introduced silane to the mass of the porous carbon can be 1:100, 2:100, 3:100, 4:100, 5:100, etc. In the second reaction stage, the controlled second reaction temperature can be 400°C, 420°C, 440°C, 460°C, 480°C, 500°C, etc., and the apparent gas velocity of the silicon source gas introduced can be 0.02 m / s, 0.04 m / s, 0.06 m / s, 0.08 m / s, 0.10 m / s, etc. By controlling the introduction amount of the silicon source gas, the total mass ratio of the silane introduced in the two-step reaction to the mass of the porous carbon can be 50:100, 60:100, 70:100, 80:100, 90:100, 100:100, 110:100, 120:100, etc.

[0043] In some embodiments, when reacting at the first reaction temperature, the volume fraction of silane in the introduced silicon source gas is controlled to be 1% - 5%, such as 1%, 2%, 3%, 4%, 5%, etc.; when reacting at the second reaction temperature, the volume fraction of silane in the introduced silicon source gas is controlled to be 5% - 50%, such as 5%, 10%, 20%, 30%, 40%, 50%, etc.

[0044] Further, the silicon source gas introduced during the two-step reaction further contains an optional gas, and the optional gas is selected from at least one of phosphine, borane, hydrogen, ammonia, nitrogen, and argon. The optional gas can be any one or several of the above, such as hydrogen alone, or phosphine and borane can be introduced for doping. The composition of the silicon source gas introduced during the two-step reaction can be the same or different.

[0045] S2. Prepare the first carbon layer

[0046] Coat the first carbon layer on the inner silicon-carbon particles to obtain carbon-coated particles. The first carbon layer can be formed by gas-phase coating to make the coating of the first carbon layer more uniform and form carbon-coated particles with small particle sizes.

[0047] In some embodiments, the process of forming the first carbon layer includes: after the inner silicon-carbon particles are prepared, heating to the third reaction temperature in an inert atmosphere, and introducing a reaction gas containing a carbon source into the reactor to continue the reaction to form carbon coating. The mass ratio of the carbon source content in the introduced reaction gas to the porous carbon is controlled to be (1 - 20):100, such as 1:100, 5:100, 10:100, 15:100, 20:100, etc. The thickness of the first carbon layer is controlled by controlling the amount of the carbon source.

[0048] Further, the third reaction temperature is controlled to be 600°C - 800°C, and the apparent gas velocity of the reaction gas containing the carbon source is 0.02 m / s - 0.10 m / s. The reaction conditions are optimized to make the formed first carbon layer more uniform. Specifically, the third reaction temperature can be 600°C, 650°C, 700°C, 750°C, 800°C, etc., and the apparent gas velocity of the reaction gas containing the carbon source can be 0.02 m / s, 0.03 m / s, 0.04 m / s, 0.05 m / s, 0.06 m / s, 0.07 m / s, 0.08 m / s, 0.09 m / s, 0.10 m / s, etc.

[0049] Further, the reaction gas containing the carbon source is selected from at least one of ethanol, ethylene, benzene, methane, ethane, acetylene, phosphine, borane, hydrogen, ammonia, nitrogen, and argon, and the volume ratio of the carbon source gas is preferably 5% - 50%. In addition to the carbon source gas, gases such as nitrogen or argon are also contained to control the volume ratio of the carbon source gas to be 5% - 50%, such as 5%, 10%, 20%, 30%, 40%, 50%, etc.

[0050] S3. Prepare the second carbon layer

[0051] Coat the second carbon layer on the carbon-coated particles. The second carbon layer can be formed on the carbon-coated particles by solid-phase coating. The thickness of the second carbon layer is relatively thick, and a product with higher density can be obtained through a sintering process.

[0052] In some embodiments, the process of forming the second carbon layer includes: mixing the carbon-coated particles with an organic carbon source, a phosphorus source, and a nitrogen source, performing hot pressing and then sintering. By introducing nitrogen and phosphorus element doping into the second carbon layer, an amorphous carbon layer with high strength and high conductivity is formed, which can inhibit the expansion of the silicon-carbon composite material and prevent the electrolyte from contacting the silicon particles at the same time. Relying on the thickness advantage, this carbon layer can limit the volume expansion of silicon during the discharge process. On the other hand, the co-doping of nitrogen and phosphorus can improve the conductivity of the relatively thick amorphous carbon layer. Nitrogen, phosphorus, and carbon elements can undergo dehydration condensation to form a heterocyclic structure during the sintering process, constituting a three-dimensional network, which can further improve the structural stability of this carbon layer.

[0053] Furthermore, the mass ratio of the organic carbon source to the carbon-coated particles is (10 - 20):100, and the mass ratio of the amounts of the organic carbon source, the phosphorus source, and the nitrogen source is (10 - 20):(0.5 - 3.0):(0.5 - 3.0). By controlling the amounts of the organic carbon source, the phosphorus source, and the nitrogen source, the thickness of the second carbon layer and the doping amounts of nitrogen and phosphorus are controlled to better limit the volume expansion of silicon during the discharge process and improve the structural stability of the carbon layer.

[0054] Specifically, the mass ratio of the organic carbon source to the carbon-coated particles can be 10:100, 15:100, 20:100, etc., and the mass ratio of the amounts of the organic carbon source, the phosphorus source, and the nitrogen source can be 10:0.5:0.5, 10:1.0:1.0, 15:1.5:1.5, 15:2.0:1.5, 15:1.5:2.0, 20:2.5:2.5, 20:3.0:3.0, etc.

[0055] In some embodiments, the organic carbon source is selected from at least one of phenolic resin and asphalt, can be any one or both of the above, or can also be other organic substances with high residual carbon; the phosphorus source is selected from at least one of phosphine, phosphorus pentoxide, triphenylphosphine, and phytic acid, can be any one or several of the above; the nitrogen source is selected from at least one of melamine, polydopamine, urea, and thiourea, can be any one or several of the above.

[0056] Further, during the hot pressing process, control the operating pressure to be 10 MPa - 100 MPa, the operating temperature to be 120 °C - 300 °C, and the hot pressing time to be 0.5 min - 10 min, so that the material can be better pressed into shape. Using the hot pressing process for solid-phase coating can improve the contact strength between the organic polymer and the material and soften it to form a coating layer. Subsequently, through the processes of sintering, crushing, and screening, a silicon-carbon composite material uniformly coated with carbon is obtained. Under the electron microscope, an amorphous carbon layer with a thickness of about 30 nm - 80 nm and a core-shell structure composed of the silicon-carbon composite material can be clearly seen. Specifically, the operating pressure can be 10 MPa, 30 MPa, 50 MPa, 80 MPa, 100 MPa, etc.

[0057] Further, during the sintering process, control the sintering temperature to be 600 °C - 1000 °C and the sintering time to be 3 h - 8 h to form a uniform and dense carbon coating layer. During the sintering process, the protective gas used is selected from at least one of nitrogen, argon, hydrogen, and carbon monoxide, and can be any one or several of the above. Specifically, the sintering temperature can be 600 °C, 700 °C, 800 °C, 900 °C, 1000 °C, etc.

[0058] In some embodiments, after sintering, pulverization and classification are carried out to obtain a product with a D50 particle size of 6 μm - 12 μm. For example, the D50 particle size can be 6 μm, 8 μm, 10 μm, 12 μm, etc. The pulverization method is not limited and can be mechanical pulverization, air flow pulverization, jaw crushing, roll crushing, etc.

[0059] The embodiment of the present invention also provides a silicon-carbon composite material, which includes silicon-carbon inner layer particles, a first carbon layer, and a second carbon layer. The first carbon layer coats the silicon-carbon inner layer particles to form carbon-coated particles, and the second carbon layer coats the carbon-coated particles. Among them, the silicon-carbon inner layer particles include porous carbon, and nano-silicon particles are deposited in the pores of the porous carbon; at the same time, nitrogen and phosphorus are doped on the second carbon layer, and it can be prepared by the above preparation method.

[0060] It should be noted that the structure of the silicon-carbon composite material is as Figure 1 shown, having a garnet structure of ABB’. A represents the silicon-carbon inner layer particles, which are composed of complex porous carbon materials and nano-silicon particles attached in the pores; B represents the first carbon layer, which is a thin carbon layer coated on the surface of A; B’ represents the second carbon layer, which is the outermost nitrogen and phosphorus co-doped amorphous carbon layer with high strength and conductivity.

[0061] In some embodiments, the first carbon layer is formed by gas-phase coating, and the second carbon layer is formed by solid-phase coating; each silicon-carbon inner layer particle is coated with the first carbon layer, and the second carbon layer contains a plurality of carbon-coated particles; the particle size D50 of the silicon-carbon inner layer particles is 5.00 μm - 8.00 μm (such as 5.00 μm, 6.00 μm, 7.00 μm, 8.00 μm, etc.), the particle size D50 of the carbon-coated particles is 5.01 μm - 8.10 μm (such as 5.01 μm, 6.00 μm, 7.00 μm, 8.00 μm, 8.10 μm, etc.), and the particle size D50 of the silicon-carbon composite material is 6 μm - 12 μm (such as 6 μm, 8 μm, 10 μm, 12 μm, etc.). The thickness of the first carbon layer is relatively thin, and the thickness of the second carbon layer is relatively thick, which can better limit the volume expansion of silicon during discharge.

[0062] Further, the specific surface area of the porous carbon is greater than 1200 m 2 / g, such as 1200 m 2 / g, 1300 m 2 / g, 1400 m 2 / g, 1500 m 2 / g, 1600 m 2 / g. Preferably, the specific surface area of the porous carbon is 1200 m 2 / g - 1400 m 2 / g, and the micropore proportion is 50% - 70% (such as 50%, 60%, 70%, etc.), having a relatively high specific surface area and micropore proportion.

[0063] In some embodiments, by mass percentage, in the silicon-carbon composite material, the silicon content is 35% - 45%, the carbon content is 54% - 64%, the nitrogen content is 0.05% - 0.5%, and the phosphorus content is 0.05% - 0.5%. By controlling the content of each element, the prepared product has the characteristic of low expansion, which is beneficial to improving the electrochemical performance of the product. Specifically, the silicon content can be 35%, 40%, 45%, etc., the carbon content can be 54%, 56%, 58%, 60%, 62%, 64%, etc., the nitrogen content can be 0.05%, 0.10%, 0.20%, 0.30%, 0.40%, 0.50%, etc., and the phosphorus content can be 0.05%, 0.10%, 0.20%, 0.30%, 0.40%, 0.50%, etc.

[0064] It should be added that the silicon-carbon composite material prepared in the embodiments of the present invention has the characteristic of low expansion mainly because: the inner core has pores with a certain volume that can accommodate the expansion of nano-silicon, and at the same time, it has a high-strength outer shell that can further limit the expansion of the composite material. The reversible capacity of this material at 0.8V is 1200-1400 mAh / g, the Coulomb efficiency of the first cycle at 0.8V is 82-84%, the expansion rate of the electrode sheet is 30%-35%, and the Y value ≤ 0.52.

[0065] The embodiments of the present invention also provide a negative electrode sheet, which includes a negative electrode current collector, and the above-mentioned silicon-carbon composite material is attached to the negative electrode current collector. The type of the negative electrode current collector is not limited and can be copper foil, but is not limited thereto.

[0066] Furthermore, the embodiments of the present invention also provide a secondary battery, which includes the above-mentioned negative electrode sheet, and also includes a positive electrode sheet, an electrolyte, a separator, etc., forming a complete battery structure.

[0067] The features and properties of the present invention will be further described in detail below in conjunction with embodiments.

[0068] Example 1

[0069] This embodiment provides a preparation method of a silicon-carbon composite material, which includes the following steps:

[0070] (1) Add 500 g of porous carbon raw material into a fluidized bed reactor, and introduce nitrogen at a flow rate of 20 L / min for replacement and purging. When the oxygen content is reduced to 500 ppm, heat the reactor at a heating rate of 10 °C / min. After the reactor temperature rises to 550 °C, introduce silane gas at a flow rate of 1 L / min, hydrogen at a flow rate of 30 L / min, and phosphine gas at a flow rate of 0.1 L / min, and the silane introduction amount is set to 25 g. After the reaction ends, set the silane flow rate to 0 and stop heating. When the reactor temperature drops to 500 °C, introduce silane at a flow rate of 3.5 L / min, nitrogen at a flow rate of 14 L / min, and phosphine at a flow rate of 0.1 L / min, and the silane introduction amount is set to 425 g.

[0071] (2) After the silicon deposition ends, raise the temperature of the reactor to 600 °C, introduce acetylene at a flow rate of 4 L / min and nitrogen at a flow rate of 15 L / min, and the acetylene introduction amount is set to 50 g. After the reaction ends, cool down and discharge the material under a nitrogen atmosphere with a flow rate of 20 L / min to obtain an intermediate product P1.

[0072] (3) After discharging, mix the intermediate product P1 with 100 g of asphalt, 20 g of phytic acid, and 20 g of polydopamine at high speed. Load the mixed material into a mold and perform hot pressing. Set the molding pressure to 100 MPa, the temperature to 200 °C, and the hot pressing time to 2 min to obtain the hot-pressed block. Load the hot-pressed block into a box furnace, displace the furnace with nitrogen, heat the furnace when the oxygen content in the furnace is reduced to 500 ppm, heat it up to 900 °C at a rate of 5 °C / min, and hold for 4 h. After cooling and discharging, use a pair-roll crusher to reduce the particle size of the block to less than 2 mm, and then use a jet mill to further crush the particulate material below 2 mm to obtain the product silicon-carbon composite material.

[0073] Example 2

[0074] This example provides a method for preparing a silicon-carbon composite material, which includes the following steps:

[0075] (1) Add 500 g of porous carbon raw material into a fluidized bed reactor, and introduce nitrogen at a flow rate of 20 L / min for displacement and purging. When the oxygen content is reduced to 500 ppm, heat the reactor at a heating rate of 10 °C / min. After the reactor temperature rises to 500 °C, introduce silane gas at a flow rate of 0.4 L / min, hydrogen at a flow rate of 40 L / min, and ammonia at a flow rate of 0.01 L / min, and set the silane introduction amount to 5 g. After the reaction ends, set the silane flow rate to 0 and stop heating. When the reactor temperature drops to 450 °C, introduce silane at a flow rate of 4 L / min, nitrogen at a flow rate of 12 L / min, and ammonia at a flow rate of 0.1 L / min, and set the silane introduction amount to 445 g.

[0076] (2) After the silicon deposition ends, raise the temperature of the reactor to 650 °C, introduce acetylene at a flow rate of 3 L / min and nitrogen at a flow rate of 8 L / min, and set the acetylene introduction amount to 100 g. After the reaction ends, cool and discharge in a nitrogen atmosphere of 20 L / min to obtain the intermediate product P1.

[0077] (3) After discharging, mix the intermediate product P1 with 157 g of asphalt, 16 g of triphenylphosphine, and 16 g of thiourea at high speed. Load the mixed material into a mold and perform hot pressing. Set the molding pressure to 100 MPa, the temperature to 150 °C, and the hot pressing time to 5 min to obtain the hot-pressed block. Load the hot-pressed block into a box furnace, displace the furnace with nitrogen, heat the furnace when the oxygen content in the furnace is reduced to 500 ppm, heat it up to 1000 °C at a rate of 5 °C / min, and hold for 4 h. After cooling and discharging, use a pair-roll crusher to reduce the particle size of the block to less than 2 mm, and then use a jet mill to further crush the particulate material below 2 mm to obtain the product silicon-carbon composite material.

[0078] Example 3

[0079] This embodiment provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0080] (1) Add 500 g of porous carbon raw material into a fluidized bed reactor, and introduce nitrogen at a flow rate of 20 L / min for displacement and purging. When the oxygen content is reduced to 500 ppm, heat the reactor at a heating rate of 10 °C / min. After the reactor temperature rises to 600 °C, introduce silane gas at a flow rate of 2 L / min, hydrogen at a flow rate of 38 L / min, and ammonia at a flow rate of 0.1 L / min. The silane introduction amount is set to 10 g. After the reaction ends, set the silane flow rate to 0 and stop heating. When the reactor temperature drops to 520 °C, introduce silane at a flow rate of 2 L / min and nitrogen at a flow rate of 18 L / min. The silane introduction amount is set to 490 g.

[0081] (2) After the silicon deposition ends, raise the reactor temperature to 650 °C, introduce acetylene at a flow rate of 2 L / min, argon at a flow rate of 18 L / min, and ammonia at a flow rate of 0.1 L / min. The acetylene introduction amount is set to 25 g. After the reaction ends, cool down and discharge the material under a nitrogen atmosphere at a flow rate of 20 L / min to obtain an intermediate product P1.

[0082] (3) After discharging, mix the intermediate product P1 with 205 g of asphalt, 31 g of phosphine, and 31 g of melamine at high speed. Load the mixed material into a mold and perform hot pressing. Set the hot pressing pressure to 50 MPa, the temperature to 200 °C, and hot press for 0.5 min to obtain a hot-pressed block. Load the hot-pressed block into a box furnace, displace the furnace with nitrogen. When the oxygen content in the furnace is reduced to 500 ppm, heat the furnace, raise the temperature to 900 °C at a rate of 5 °C / min, and keep it warm for 4 h. After cooling down and discharging, use a pair-roll crusher to reduce the particle size of the block to less than 2 mm, and then use a jet mill to further crush the particulate material below 2 mm to obtain the silicon-carbon composite material product.

[0083] Example 4

[0084] This embodiment provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0085] (1) Add 500 g of porous carbon raw material into a fluidized bed reactor, and introduce nitrogen at a flow rate of 20 L / min for displacement and purging. When the oxygen content is reduced to 500 ppm, heat the reactor at a heating rate of 10 °C / min. After the reactor temperature rises to 600 °C, introduce silane gas at a flow rate of 1 L / min, hydrogen at a flow rate of 32 L / min, and phosphine at a flow rate of 0.05 L / min. The silane introduction amount is set to 5 g. After the reaction ends, set the silane flow rate to 0 and stop heating. When the reactor temperature drops to 520 °C, introduce silane at a flow rate of 7 L / min and nitrogen at a flow rate of 7 L / min. The silane introduction amount is set to 545 g.

[0086] (2) After the silicon deposition is completed, the reactor is heated to 700 °C, and acetylene at a flow rate of 7 L / min, argon at a flow rate of 7 L / min, and ammonia at a flow rate of 0.1 L / min are introduced. The acetylene feed amount is set to 50 g. After the reaction is completed, the temperature is decreased and the product is discharged under a nitrogen atmosphere of 20 L / min to obtain the intermediate product P1.

[0087] (3) After discharging, the intermediate product P1 is mixed with 110 g of asphalt, 11 g of phosphine, and 11 g of melamine at high speed. The mixed material is loaded into a mold and hot-pressed. The molding pressure is set to 20 MPa, the temperature is 200 °C, and the hot-pressing is carried out for 3 min to obtain the hot-pressed block. The hot-pressed block is loaded into a box furnace, and the furnace is purged with nitrogen. When the oxygen content in the furnace is reduced to 500 ppm, the furnace is heated. The temperature is increased at a rate of 5 °C / min to 900 °C and held for 4 h. After cooling and discharging, the particle size of the block is reduced to less than 2 mm by using a pair-roll crusher, and then the particulate material below 2 mm is further pulverized by using a jet mill to obtain the silicon-carbon composite material product.

[0088] Comparative Example 1

[0089] A conventional preparation process is adopted, and the specific steps are as follows: 500 g of porous carbon raw material is added into a fluidized bed reactor, and nitrogen at a flow rate of 20 L / min is introduced for replacement and purging. When the oxygen content is reduced to 500 ppm, the reactor is heated at a heating rate of 10 °C / min. After the reactor temperature rises to 500 °C, silane gas at a flow rate of 2 L / min and nitrogen at a flow rate of 30 L / min are introduced. The silane feed amount is set to 500 g. After the reaction is completed, the reactor is heated to 650 °C, and acetylene at a flow rate of 1 L / min and nitrogen at a flow rate of 15 L / min are introduced. The acetylene feed amount is set to 50 g. After the reaction is completed, the temperature is decreased and the product is discharged under a nitrogen atmosphere of 20 L / min to obtain the silicon-carbon composite material product.

[0090] Comparative Example 2

[0091] The difference between this comparative example and Example 1 is only that: step (3) is not carried out. The specific steps are as follows:

[0092] (1) 500 g of porous carbon raw material is added into a fluidized bed reactor, and nitrogen at a flow rate of 20 L / min is introduced for replacement and purging. When the oxygen content is reduced to 500 ppm, the reactor is heated at a heating rate of 10 °C / min. After the reactor temperature rises to 550 °C, silane gas at a flow rate of 1 L / min, hydrogen at a flow rate of 30 L / min, and phosphine gas at a flow rate of 0.1 L / min are introduced. The silane feed amount is set to 25 g. After the reaction is completed, the silane flow rate is set to 0 and the heating is stopped. When the reactor temperature is decreased to 500 °C, silane at a flow rate of 3.5 L / min, nitrogen at a flow rate of 14 L / min, and phosphine at a flow rate of 0.1 L / min are introduced. The silane feed amount is set to 425 g.

[0093] (2) After the silicon deposition is completed, the reactor is heated to 600 °C, and acetylene at a flow rate of 4 L / min and nitrogen at a flow rate of 15 L / min are introduced. The introduced amount of acetylene is set to 50 g. After the reaction is completed, the temperature is decreased and the product is discharged under a nitrogen atmosphere with a flow rate of 20 L / min to obtain the silicon-carbon composite material product.

[0094] Comparative Example 3

[0095] The difference between this comparative example and Example 1 is only that: in step (3), nitrogen and phosphorus are not introduced.

[0096] (1) 500 g of porous carbon raw material is added into a fluidized bed reactor, and nitrogen with a flow rate of 20 L / min is introduced for replacement and purging. When the oxygen content is reduced to 500 ppm, the reactor is heated at a heating rate of 10 °C / min. After the reactor temperature rises to 550 °C, silane gas with a flow rate of 1 L / min, hydrogen with a flow rate of 30 L / min, and phosphine gas with a flow rate of 0.1 L / min are introduced. The introduced amount of silane is set to 25 g. After the reaction is completed, the silane flow rate is set to 0 and the heating is stopped. When the reactor temperature is decreased to 500 °C, silane with a flow rate of 3.5 L / min, nitrogen with a flow rate of 14 L / min, and phosphine with a flow rate of 0.1 L / min are introduced. The introduced amount of silane is set to 425 g.

[0097] (2) After the silicon deposition is completed, the reactor is heated to 600 °C, and acetylene at a flow rate of 4 L / min and nitrogen at a flow rate of 15 L / min are introduced. The introduced amount of acetylene is set to 50 g. After the reaction is completed, the temperature is decreased and the intermediate product P1 is obtained under a nitrogen atmosphere with a flow rate of 20 L / min.

[0098] (3) After discharging, the intermediate product P1 is mixed with 100 g of asphalt at a high speed. The mixed material is loaded into a mold and hot-pressed. The mold pressure is set to 100 MPa, the temperature is 200 °C, and the hot-pressing time is 2 min to obtain the hot-pressed block. The hot-pressed block is loaded into a box furnace, and the furnace is replaced with nitrogen. When the oxygen content in the furnace is reduced to 500 ppm, the furnace is heated, and the temperature is increased to 900 °C at a rate of 5 °C / min and held for 4 h. After the temperature is decreased and the product is discharged, the particle size of the block is reduced to less than 2 mm by using a pair-roll crusher, and then the particulate material with a particle size of less than 2 mm is further pulverized by using a jet mill to obtain the silicon-carbon composite material product.

[0099] Comparative Example 4

[0100] The difference from Example 1 is only that: solid-phase coating is not performed, and fluorine and nitrogen are doped during gas-phase coating. The specific steps are as follows.

[0101] (1) Add 500 g of porous carbon raw material into a fluidized bed reactor, and introduce nitrogen at a flow rate of 20 L / min for displacement and purging. When the oxygen content drops to 500 ppm, heat the reactor at a heating rate of 10 °C / min. After the reactor temperature rises to 550 °C, introduce silane gas at a flow rate of 1 L / min, hydrogen at a flow rate of 30 L / min, and phosphine gas at a flow rate of 0.1 L / min. The introduced amount of silane is set to 25 g. After the reaction ends, set the silane flow rate to 0 and stop heating. When the reactor temperature drops to 500 °C, introduce silane at a flow rate of 3.5 L / min, nitrogen at a flow rate of 14 L / min, and phosphine at a flow rate of 0.1 L / min. The introduced amount of silane is set to 425 g.

[0102] (2) After the silicon deposition ends, heat the reactor to 700 °C, introduce nitrogen trifluoride at a flow rate of 1 L / min and nitrogen at a flow rate of 15 L / min. The introduced amount of nitrogen trifluoride is set to 20 g; then stop introducing the above gases, introduce acetylene at a flow rate of 4 L / min and nitrogen at a flow rate of 15 L / min. The introduced amount of acetylene is set to 50 g. After the reaction ends, cool down and discharge the product under a nitrogen atmosphere with a flow rate of 20 L / min to obtain the silicon-carbon composite material.

[0103] Test Example 1

[0104] Test the oxygen content, carbon-silicon mass ratio, specific surface area, Dv50 particle size, and true density of the silicon-carbon composite materials prepared in the test examples and comparative examples.

[0105] Testing method: (1) Use a carbon-sulfur analyzer to test the carbon content in the silicon-carbon composite materials prepared in each example and comparative example; (2) Use an inductively coupled plasma mass spectrometer to test the silicon content in the silicon-carbon composite materials prepared in each example and comparative example; (3) Use a specific surface area analyzer to test the specific surface area of the silicon-carbon composite materials prepared in each example and comparative example; (4) Use a laser diffraction particle size analyzer to test the Dv50 particle size of the silicon-carbon composite materials prepared in each example and comparative example; (5) Use a true density meter to test the true density of the silicon-carbon composite materials prepared in each example and comparative example with reference to GB / T 24533.

[0106] Table 1 Summary table of parameters of silicon-carbon composite materials

[0107]

[0108]

[0109] Test Example 2

[0110] Test the performance of the silicon-carbon composite materials prepared in the test examples and comparative examples. The testing method is as follows:

[0111] (1) The finished silicon-carbon composite material is used as the negative electrode active material. A negative electrode slurry is prepared by mixing the negative electrode active material, sodium carboxymethyl cellulose (CMC), conductive carbon black (SP), and styrene-butadiene rubber (SBR) according to a mass ratio of 94.5:1.5:1.5:2.5. The negative electrode slurry is coated on both surfaces of the copper foil current collector, vacuum dried, and a negative electrode plate is obtained. The negative electrode plate is assembled with a lithium sheet, an electrolyte, and a Celgard 2400 separator, and a button battery is assembled using a conventional production process for the outer shell. The above button battery is charged and discharged cyclically at a rate of 0.1C at 25°C, the initial charge specific capacity and discharge specific capacity are recorded, and the first Coulomb efficiency is calculated. Using the initial discharge specific capacity as the initial capacity, the above cyclic operation is repeated. After 5 cycles and 50 cycles, the thickness of the electrode plate is measured respectively and compared with the initial electrode plate thickness to calculate the electrode plate swelling rate.

[0112] Table 2 Summary of the electrochemical performance of the silicon-carbon composite material

[0113]

[0114] Test Example 3

[0115] The dQ / dV curve is plotted for the charge-discharge curve of the button battery assembled with the materials of Example 1. The peak height in the range of 0.25 - 0.3V in the curve is denoted as h1, and the peak height in the region of 0.4 - 0.5V in the curve is denoted as h2. Then, Y = h2 / h1 is defined.

[0116] As Figure 2 shown, the silicon-carbon composite material satisfies Y ≤ 0.55, and further Y ≤ 0.52, as Figure 2 shown. The Y value can reflect the degree of silicon enrichment on the surface of the sample. The larger the Y value, the more serious the silicon enrichment on the surface and the larger the swelling rate of the electrode plate.

[0117] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, It includes silicon-carbon inner layer particles, a first carbon layer and a second carbon layer. The first carbon layer coats the silicon-carbon inner layer particles to form carbon-coated particles, and the second carbon layer coats the carbon-coated particles. Among them, the silicon-carbon inner layer particles include porous carbon, and nano-silicon particles are deposited in the pores of the porous carbon; nitrogen and phosphorus are doped on the second carbon layer.

2. The silicon-carbon composite material according to claim 1, characterized in that The first carbon layer is formed by gas-phase coating, and the second carbon layer is formed by solid-phase coating; each of the silicon-carbon inner layer particles is coated with the first carbon layer, and the second carbon layer contains a plurality of the carbon-coated particles. Preferably, the particle size D50 of the silicon-carbon inner layer particles is 5.00μm - 8.00μm, the particle size D50 of the carbon-coated particles is 5.01μm - 8.10μm, and the particle size D50 of the silicon-carbon composite material is 6μm - 12μm. Preferably, the specific surface area of the porous carbon is greater than 1200 m 2 / g; more preferably, the specific surface area of the porous carbon is 1200 m 2 / g - 1400 m 2 / g, and the proportion of micropores is 50% - 70%.

3. The silicon-carbon composite material according to claim 1 or 2, characterized in that, By mass percentage, in the silicon-carbon composite material, the silicon content is 35% - 45%, the carbon content is 54% - 64%, the nitrogen content is 0.05% - 0.5%, and the phosphorus content is 0.05% - 0.5%.

4. The preparation method of the silicon-carbon composite material according to any one of claims 1-3, characterized in that, It includes: Depositing nano-silicon particles on the porous carbon to obtain the silicon-carbon inner layer particles, coating the first carbon layer on the silicon-carbon inner layer particles to obtain carbon-coated particles, and coating the second carbon layer on the carbon-coated particles.

5. The preparation method according to claim 4, characterized in that, The preparation process of the silicon-carbon inner layer particles includes: placing the porous carbon in a reactor, heating to the reaction temperature under an inert atmosphere, and then introducing a silicon source gas for reaction; wherein, the silicon source gas contains silane. Preferably, first heat to the first reaction temperature for reaction under an inert atmosphere, and then cool to the second reaction temperature for reaction under an inert atmosphere. The first reaction temperature is 500℃ - 600℃, and the second reaction temperature is 400℃ - 500℃. More preferably, when reacting at the first reaction temperature, control the apparent gas velocity of the introduced silicon source gas to be 0.10m / s - 0.2m / s, and control the mass ratio of the introduced silane to the mass of the porous carbon to be (1 - 5):

100. More preferably, when reacting at the second reaction temperature, control the apparent gas velocity of the introduced silicon source gas to be 0.02m / s - 0.1m / s, and control the total mass ratio of the silane introduced in the two-step reaction to the mass of the porous carbon to be (50 - 120):

100. More preferably, when reacting at the first reaction temperature, control the volume fraction of silane in the introduced silicon source gas to be 1% - 5%; when reacting at the second reaction temperature, control the volume fraction of silane in the introduced silicon source gas to be 5% - 50%. Further preferably, the silicon source gas introduced in the two-step reaction process further contains an optional gas, and the optional gas is selected from at least one of phosphine, borane, hydrogen, ammonia, nitrogen and argon.

6. The preparation method according to claim 4, characterized in that, Use the gas-phase coating method to form the first carbon layer to obtain the carbon-coated particles, and use the solid-phase coating method to form the second carbon layer on the carbon-coated particles.

7. The preparation method according to claim 6, characterized in that, The process of forming the first carbon layer includes: after the preparation of the silicon-carbon inner layer particles is completed, heating to a third reaction temperature under an inert atmosphere, introducing a reaction gas containing a carbon source into the reactor for reaction, and controlling the mass ratio of the carbon source content in the introduced reaction gas to the mass of the porous carbon to be (1-20):100; Preferably, the third reaction temperature is 600°C - 800°C; Preferably, the apparent gas velocity of the reaction gas containing a carbon source is 0.02 m / s - 0.1 m / s; Preferably, the reaction gas containing a carbon source is selected from at least one of ethanol, ethylene, benzene, methane, ethane, acetylene, phosphine, borane, hydrogen, ammonia, nitrogen, and argon, and the volume fraction of the carbon source gas is 5% - 50%.

8. The preparation method according to claim 6, characterized in that, The process of forming the second carbon layer includes: mixing the carbon-coated particles with an organic carbon source, a phosphorus source, and a nitrogen source, and performing sintering after hot pressing and forming; Preferably, the mass ratio of the organic carbon source to the carbon-coated particles is (10-20):100, and the mass ratio of the amounts of the organic carbon source, the phosphorus source, and the nitrogen source is (10-20):(0.5-3.0):(0.5-3.0); more preferably, the organic carbon source is selected from at least one of phenolic resin and pitch; the phosphorus source is selected from at least one of phosphine, phosphorus pentoxide, triphenylphosphine, and phytic acid; the nitrogen source is selected from at least one of melamine, polydopamine, urea, and thiourea; Preferably, during the hot pressing and forming process, the operating pressure is controlled to be 10 MPa - 100 MPa, the operating temperature is 120°C - 300°C, and the hot pressing time is 0.5 min - 10 min; Preferably, during the sintering process, the sintering temperature is controlled to be 600°C - 1000°C, and the sintering time is 3 h - 8 h; more preferably, during the sintering process, the protective gas used is selected from at least one of nitrogen, argon, hydrogen, and carbon monoxide; More preferably, after sintering, crushing and grading are performed to obtain a product with a D50 particle size of 6 μm - 12 μm.

9. A negative electrode sheet, characterized in that, It includes a negative electrode current collector, and the silicon-carbon composite material according to any one of claims 1-3 is attached to the negative electrode current collector.

10. A secondary battery, characterized in that, It includes the negative electrode plate according to claim 9.

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