Graphite-based silicon-carbon composite negative electrode material and preparation method and application thereof

Abstract

The invention relates to a graphite-based silicon-carbon composite negative electrode material as well as a preparation method and application thereof. The graphite-based silicon-carbon composite negative electrode material comprises a composite silicon-carbon material and a carbon coating layer coating the outer surface of the composite silicon-carbon material, the composite silicon-carbon material comprises a composite carbon matrix and a silicon nano material uniformly dispersed in holes of the composite carbon matrix, the composite carbon matrix comprises a porous soft carbon matrix and graphite microcrystals which are formed in situ and are uniformly distributed inside and on the surface of the porous soft carbon matrix; the graphitization degree of the composite carbon matrix is 10%-95%; 2 theta of a crystal face of a graphite peak (002) in XRD (X-Ray Diffraction) peaks of the composite silicon carbon material is in a range of 26.08-26.35 degrees; the lithium ion battery using the graphite-based silicon-carbon composite negative electrode material provided by the invention has relatively good cycle performance and rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery materials, and particularly to a graphite-based silicon-carbon composite anode material, a preparation method thereof, and an application thereof. Background Art

[0002] In the context of the global advocacy for green travel and the booming development of the electric vehicle industry, consumers' expectations for the driving range of electric vehicles have been continuously rising. This demand has directly promoted the research and development of high-energy-density batteries to become the core task in the current battery field. Among the studies on various anode materials, silicon has long been regarded as one of the most promising anode materials due to its theoretical capacity of up to 3579 mAh / g, which is about 10 times that of traditional graphite anodes.

[0003] However, silicon materials face severe challenges in practical applications. During the lithium deintercalation and intercalation process, silicon undergoes a volume change of up to 300%. Such a large volume expansion and contraction are extremely likely to cause silicon particles to break and pulverize, and then detach from the current collector, ultimately leading to a serious attenuation of the battery capacity. To solve the problems of large volume expansion and pulverization related to silicon particles, a large number of explorations have been carried out by researchers, and combining carbon with silicon to form a silicon-carbon composite material has become an effective approach.

[0004] For example, a Chinese invention patent with the publication number CN116598464A (publication date: August 15, 2023) discloses a porous graphite / amorphous silicon / soft carbon lithium-ion battery anode material and a preparation method thereof, which adopts a method of uniformly depositing a silicon layer on porous graphite and then coating a carbon layer outside the silicon layer. The method of this patent enhances the ion and electron transport efficiency, reduces the contact between amorphous silicon and the electrolyte through the surface soft carbon, and effectively improves the first-cycle Coulombic efficiency of the material.

[0005] Another example is a Chinese invention patent with the publication number CN114497551A (publication date: May 13, 2022), which discloses a method of preparing a multi-level microcrystalline buffer structure from a silicon dioxide / silicon / silicon carbide / graphene composite material / soft carbon, significantly improving the specific capacity and cycle stability of the lithium-ion battery anode, and the material has high stability, simple process, and strong environmental friendliness.

[0006] Although the above technical solution prepares a silicon-carbon composite material by combining graphite material or soft carbon with silicon, which alleviates the volume expansion problem of pure silicon material to a certain extent and improves the cycle stability of the battery, it is still difficult to meet the increasingly stringent requirements of the current market for the rate performance and cycle performance of the silicon-carbon battery anode. For the silicon-carbon anode prepared with graphite carbon material, the graphite is brittle and the interlayer elastic modulus is extremely low (about 5-50 GPa). During the lithium insertion expansion process of silicon, its ability to buffer the expansion of silicon material is limited, and the binding interface with silicon is unstable, which easily leads to rapid capacity decay and poor cycle performance. For the silicon-carbon anode material prepared entirely based on soft carbon matrix, due to the lack of a fast electron transfer channel, the internal resistance of the material is too large, the rate performance is poor, and a heterogeneous SEI layer will be generated during the charge and discharge process, resulting in a low Coulomb efficiency.

[0007] Therefore, developing a silicon-carbon anode composite material that can effectively buffer the expansion of silicon material, has a good electron transport channel, and can improve the cycle performance and rate performance at the same time has become a key problem to be solved urgently. Summary of the Invention

[0008] The object of the present invention is to address the deficiencies of the prior art and propose a graphite-based silicon-carbon composite anode material, its preparation method and application. First, a composite carbon matrix containing graphite microcrystals and soft carbon with a graphitization degree between 10% and 95% is prepared, and then the composite carbon matrix is subjected to silicon deposition and carbon coating treatment to obtain the graphite-based silicon-carbon composite anode material. In the graphite-based silicon-carbon composite anode material, due to the control of the graphitization degree of the matrix, the synergistic effect of soft carbon and graphite is better exerted. The high toughness of soft carbon can effectively inhibit the volume expansion of silicon and reduce the possibility of cracking during the silicon expansion process in the battery cycle, while the graphite microcrystals provide fast ion and electron transport channels, which can reduce the overall resistance of the material and improve the rate performance of the material. Applying the graphite-based silicon-carbon composite anode material provided by the embodiments of the present invention as the anode active material in a lithium-ion battery can provide the rate performance and cycle performance of the lithium-ion battery.

[0009] To achieve the above object, in a first aspect, an embodiment of the present invention provides a graphite-based silicon-carbon composite anode material, which includes: a composite silicon-carbon material, and a carbon coating layer coated on the outer surface of the composite silicon-carbon material;

[0010] The composite silicon-carbon material includes: a composite carbon matrix, and silicon nanomaterials uniformly dispersed in the pores of the composite carbon matrix;

[0011] The composite carbon matrix includes: a porous soft carbon matrix, and graphite microcrystals formed in-situ and uniformly distributed on the inside and surface of the porous soft carbon matrix; the graphitization degree of the composite carbon matrix is 10% - 95%;

[0012] The 2θ of the (002) crystal plane of the graphite peak in the XRD diffraction peak of the composite silicon carbide material is in the range of 26.08° to 26.35°.

[0013] Preferably, in the graphite-based silicon carbide composite anode material, the carbon content is 50 wt% to 90 wt%, and the content of the silicon nanomaterial is 15 wt% to 60 wt%.

[0014] The thickness of the carbon coating layer is between 1 nm and 50 nm; the percentage of the mass of the carbon coating layer in the total mass of the graphite-based silicon carbide composite anode material is 1% to 24%.

[0015] Preferably, the pore volume of the composite carbon matrix is 0.1 cm 3 / g to 0.8 cm 3 / g, and the specific surface area is 200 cm 2 / g to 2000 cm 2 / g;

[0016] The specific surface area of the graphite-based silicon carbide composite anode material is between 0.3 m 2 / g and 20 m 2 / g; the volume median diameter Dv50 of the graphite-based silicon carbide composite anode material is between 2 μm and 30 μm.

[0017] In a first aspect, an embodiment of the present invention provides a preparation method of the graphite-based silicon carbide composite anode material described in the first aspect above, and the preparation method includes:

[0018] Step S1, preparing a porous soft carbon matrix;

[0019] Step S2, heat-treating the porous soft carbon matrix to graphitize the porous soft carbon matrix to obtain a composite carbon matrix;

[0020] Step S3, placing the composite carbon matrix in a reaction device, and depositing silicon nanomaterials on the pores and surfaces of the composite carbon matrix to obtain a silicon carbide composite material semi-finished product;

[0021] Step S4, performing carbon coating treatment on the silicon carbide composite material semi-finished product to obtain a graphite-based silicon carbide composite anode material.

[0022] Preferably, the preparation of the porous soft carbon matrix in step S1 specifically includes:

[0023] The coke-based bulk material is first coarsely crushed by a jaw crusher and then ground by a planetary ball mill for 3 to 8 hours, so that the volume median diameter Dv50 of the coke-based bulk material is reduced to less than 20 μm. After discharging, a coke-based powder is obtained. Among them, the grinding speed of the planetary ball mill is 400 rpm to 600 rpm. The coke-based bulk material includes one or more of petroleum coke, needle coke, coke or pitch coke.

[0024] The coke-based powder is added to a hydrochloric acid solution, and stirred at 50°C to 90°C for 5 to 10 hours for impurity removal. Subsequently, it is washed with deionized water until neutral, dried and then dispersed to obtain a pure coke-based powder. Among them, the content of the solute in the hydrochloric acid solution is 5 wt% to 15 wt%, and the solvent is deionized water. The mass ratio of the coke-based powder to the hydrochloric acid solution is 1:10 to 1:3.

[0025] The pure coke-based powder and potassium hydroxide are added to a mixed solvent of ethanol and water to form a mixed solution. After the mixed solution is subjected to ultrasonic treatment and magnetic stirring in sequence, it is placed in a vacuum oven and dried at a temperature of 60°C to 80°C to obtain a black mixed dried body. Among them, the volume ratio of the pure coke-based powder to the potassium hydroxide is 1:3 to 1:5. The volume ratio of the ethanol to the water is 1:1 to 1:3, and the solid content in the mixed solution is 10 wt% to 30 wt%.

[0026] The black mixed dried body is subjected to stepwise carbonization activation. Specifically, the black mixed dried body is placed in a high-temperature furnace. First, it is heated at a heating rate of 1°C / min to 5°C / min to 200°C and kept warm for 30 min to 120 min to remove the moisture and volatile components in the black mixed dried body. Then, it is heated at a heating rate of 1°C / min to 5°C / min to 200°C to 400°C and kept warm for 30 min to 120 min for pre-activation treatment, so that the potassium hydroxide is intercalated into the carbon matrix gap of the black mixed dried body. Then, it is heated at a heating rate of 1°C / min to 5°C / min to 400°C to 800°C and kept warm for 30 min to 240 min for main activation treatment, so that the potassium hydroxide reacts with carbon. Finally, it is heated at a heating rate of 1°C / min to 5°C / min to 800°C to 900°C and kept warm for 30 min to 240 min for pore expansion treatment to obtain a carbonized product.

[0027] The carbonized product is soaked in a hydrochloric acid solution with a molar concentration of 1 mol / L for 24 hours for pickling and purification, then washed with deionized water until neutral, and finally dried to obtain the porous soft carbon matrix.

[0028] Preferably, in the step S2, the heat treatment of the porous soft carbon matrix to obtain a composite carbon matrix specifically includes:

[0029] Place the porous soft carbon matrix in a high-temperature furnace and perform heat treatment at a temperature of 1000°C to 2200°C for 1 hour to 10 hours to obtain a composite carbon matrix; wherein, the high-temperature furnace includes any one of a box furnace, a tube furnace, or a graphite felt insulation furnace;

[0030] The composite carbon matrix includes a porous soft carbon matrix, and graphite microcrystals formed in-situ and uniformly distributed inside and on the surface of the porous soft carbon matrix; the graphitization degree of the composite carbon matrix is 10% to 95%.

[0031] Preferably, in step S3, it specifically includes: placing the composite carbon matrix in a reaction device, heating it to 500°C to 700°C under a protective atmosphere, introducing a silicon source gas, and keeping it warm for 2 hours to 4 hours, so that the silicon element decomposed from the silicon source gas is deposited in the pores and on the surface of the composite carbon matrix, and then stopping introducing the silicon source gas to obtain a semi-finished silicon-carbon composite material;

[0032] The protective gas of the protective atmosphere includes nitrogen and / or argon; the flow rate of the protective gas is 1 L / min to 60 L / min;

[0033] The reaction device includes any one of a tube furnace, a box furnace, a chemical vapor deposition furnace, or a fluidized bed;

[0034] The silicon source gas includes one or more gases among silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, and hexachloroethylsilane; the flow rate of the silicon source gas is 10 L / min to 60 L / min.

[0035] Preferably, in step S4, the carbon coating treatment is chemical vapor carbon coating, including: under a protective atmosphere, adjusting the temperature of the reaction device to 500°C to 580°C, introducing a carbon source gas into the reaction device, and keeping it warm for 3 hours to 5 hours, so that the carbon element decomposed from the carbon source gas is deposited on the surface of the semi-finished silicon-carbon composite material to form a carbon coating layer, and finally obtaining a graphite-based silicon-carbon composite negative electrode material;

[0036] Among them, the carbon source gas includes at least one of methane, acetylene, ethylene, and propylene; the flow rate of the carbon source gas is 5 L / min to 60 L / min.

[0037] In a third aspect, an embodiment of the present invention provides a negative electrode sheet, and the negative electrode sheet includes the graphite-based silicon-carbon composite negative electrode material described in the first aspect above.

[0038] In a fourth aspect, an embodiment of the present invention provides a lithium-ion battery, and the lithium-ion battery includes the negative electrode sheet described in the third aspect.

[0039] The present invention provides a graphite-based silicon-carbon composite anode material, a preparation method and an application thereof, having the following beneficial effects:

[0040] (1) A preparation method of a graphite-based silicon-carbon composite anode material provided by an embodiment of the present invention first obtains a purified porous soft carbon matrix, and then controls the graphitization degree of the obtained composite carbon matrix to be between 10% and 95% by controlling the graphitization temperature and time. After that, silicon deposition and carbon coating treatments are carried out on the composite carbon matrix, and finally a graphite-based silicon-carbon composite anode material is obtained; this method has low preparation cost, simple process, is easy to implement, and is suitable for popularization and application of large-scale production.

[0041] (2) For the graphite-based silicon-carbon composite anode material prepared by the preparation method provided by an embodiment of the present invention, due to the control of the graphitization degree of the matrix, the synergistic effect of soft carbon and graphite is better exerted. On the one hand, graphite is uniformly embedded in the soft carbon structure, providing fast ion and electron transport channels for the material, which can reduce the overall resistance of the material and improve the rate performance of the material; on the other hand, soft carbon forms a tough skeleton structure. Graphite is embedded in the skeleton, which can limit excessive expansion, prevent the rupture of the overall structure of the material, and enhance the overall cycle performance of the material.

[0042] The present invention constructs a soft carbon / graphite composite carbon matrix with a specific structure to effectively buffer the volume expansion of silicon materials and improve the electron transport efficiency of the materials, so as to solve the deficiencies of existing silicon-carbon anode materials in rate performance and cycle performance.

[0043] (3) Applying the graphite-based silicon-carbon composite anode material provided by an embodiment of the present invention in a lithium-ion battery can improve the comprehensive performance of the lithium-ion battery, such as rate performance and cycle performance. Description of the Drawings

[0044] Figure 1 It is a flowchart of the preparation method of the graphite-based silicon-carbon composite anode material provided by an embodiment of the present invention.

[0045] Figure 2 It is an X-ray diffraction (XRD) pattern of the composite carbon matrix provided by Embodiment 4 of the present invention. Detailed Embodiments

[0046] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0047] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0048] An embodiment of the present invention provides a graphite-based silicon-carbon composite anode material, which includes: a composite silicon-carbon material, and a carbon coating layer coated on the outer surface of the composite silicon-carbon material;

[0049] Among them, the composite silicon-carbon material includes: a composite carbon matrix, and silicon nanomaterials uniformly dispersed in the pores of the composite carbon matrix.

[0050] Specifically, the composite carbon matrix includes: a porous soft carbon matrix, and graphite microcrystals formed in-situ and uniformly distributed inside and on the surface of the porous soft carbon matrix; the 2θ of the graphite peak (002) plane in the XRD diffraction peak of the composite silicon-carbon material is in the range of 26.08° to 26.35°; the graphitization degree of the composite carbon matrix is 10% to 95%, which can be any value within this range, for example: 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable. The graphitization degree of the composite carbon matrix is preferably 30% to 70%.

[0051] The graphitization degree of the present invention is measured for the composite carbon matrix by X-ray diffraction method. According to the XRD pattern of the measured composite carbon matrix, the diffraction angle 2θ of the (002) plane of the composite carbon matrix is obtained, and the interplanar spacing d(002) is calculated through the Bragg formula. The Bragg formula is:

[0052] nλ = 2dsinθ 弧度 ;

[0053] Among them, θ 弧度 is the radian value after converting the angle θ between the incident X-ray and the corresponding crystal plane into radians. d represents the interplanar spacing d(002), λ is the wavelength of the X-ray, and n is the reflection order; it should be noted that to ensure the calculation accuracy and avoid errors caused by environmental differences, θ should be converted into radians before substituting it into the Bragg formula, θ 弧度 = (π / 180)×θ, where π is the pi;

[0054] Then, the graphitization degree is calculated according to the Franklin formula. The Franklin formula is:

[0055] G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%;

[0056] Among them, G in Franklin's formula is the graphitization degree, 0.3440 nm is the interlayer spacing of completely non-graphitized carbon, 0.3354 nm is the interlayer spacing of an ideal graphite crystal, and d(002) is d in Bragg's formula.

[0057] When the graphitization degree of the composite carbon matrix provided by the present invention is between 10% and 95%, there are sufficient graphite microcrystals in the soft carbon matrix framework. These graphite microcrystals build rich and continuous fast ion / electron transport channels, greatly improving the rate performance of the battery. When the graphitization is lower than 10%, the number of graphite microcrystals is scarce, resulting in a significant increase in the overall resistance of the battery. This may not only cause local overheating of the battery, but also promote the decomposition of the electrolyte, thereby forming a relatively thick solid electrolyte interface (SEI) film, ultimately leading to a rapid decay of the battery capacity. When the graphitization is greater than 95%, the role of soft carbon as a buffer layer between graphite layers in the carbon matrix weakens. During the process of silicon lithium insertion and expansion, due to the lack of effective buffering, the graphite layers are extremely prone to cracking, resulting in the cracking and collapse of the main structure of the matrix. This causes the silicon nanomaterial to trigger side reactions in the electrolyte, leading to a rapid decay of the capacity.

[0058] The pore volume of the composite carbon matrix is 0.1 cm 3 / g to 0.8 cm 3 / g.

[0059] The specific surface area of the composite carbon matrix is 200 cm 2 / g to 2000 cm 2 / g.

[0060] The silicon nanomaterial includes but is not limited to: nano-silicon grains, island-shaped silicon nanomaterials composed of closely arranged nano-silicon grains, or linear silicon nanomaterials composed of closely arranged nano-silicon grains.

[0061] The average particle size of the nano-silicon grains is between 0.5 nm and 3 nm, and can be any value within this range. For example: 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, 2 nm, 2.1 nm, 2.2 nm, 2.3 nm, 2.4 nm, 2.5 nm, 2.6 nm, 2.7 nm, 2.8 nm, 2.9 nm, 3.0 nm, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0062] The average size of the island-shaped or linear silicon nanomaterials ranges from 1 nm to 100 nm and can be any value within this range, such as 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0063] The carbon content in the graphite-based silicon-carbon composite anode material is 50 wt% to 90 wt% and can be any value within this range, such as: 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0064] The content of the silicon nanomaterials in the graphite-based silicon-carbon composite anode material is 15 wt% to 60 wt% and can be any value within this range, such as: 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0065] The thickness of the carbon coating layer ranges from 1 nm to 50 nm and can be any value within this range, such as: 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0066] The percentage of the mass of the carbon coating layer in the total mass of the graphite-based silicon-carbon composite anode material is 1% to 24% and can be any value within this range, such as: 1%, 5%, 10%, 15%, 20%, 24%, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0067] The specific surface area of the graphite-based silicon-carbon composite anode material is between 0.3 m 2 / g and 20 m 2 / g.

[0068] The volume median diameter Dv50 of the graphite-based silicon-carbon composite anode material is between 2 μm and 30 μm.

[0069] In the present invention, the test method for specific surface area, average pore diameter, and pore volume is to use a specific surface area analyzer of model Micromeritics ASAP2460 for testing. The specific method is as follows: Take a porous carbon material as the sample to be tested, and first screen the sample to be tested using a 200-mesh sieve; then, perform degassing treatment. Usually, place the sample to be tested in a vacuum and heat it at a high temperature of 200 °C for degassing treatment for several hours, 6 hours. Then, in an environment of constant low temperature (-196 °C), introduce nitrogen, control the nitrogen gas pressure, and let the sample adsorb and desorb at different gas pressures to obtain the isothermal adsorption and desorption curve of the sample; after that, according to the isothermal adsorption and desorption curve, use the BET fitting to calculate the specific surface area of the sample, and use the t-plot model fitting to calculate the average pore diameter and pore volume.

[0070] In the present invention, the volume median diameter Dv50 refers to the volume median diameter of the material, which represents the particle diameter corresponding to 50% of the volume distribution of the material. This is a well-known meaning in the art. The test method for the volume median diameter Dv50 adopts a conventional method, such as laser diffraction method. According to the standard of "ISO 13320:2020 Particle Size Analysis - Laser Diffraction Method", use a Mastersizer 3000 type laser particle size analyzer of Malvern Instruments Limited, UK, to measure the particle diameter Dv50.

[0071] An embodiment of the present invention provides a preparation method of the above-mentioned graphite-based silicon-carbon composite negative electrode material, as Figure 1 shown, which specifically includes the following steps.

[0072] Step S1, prepare a porous soft carbon matrix;

[0073] Among them, the specific method for preparing the porous soft carbon matrix includes:

[0074] Coarsely crush the coke-based bulk material with a jaw crusher and grind it with a planetary ball mill for 3 to 8 hours to reduce the volume median diameter Dv50 of the coke-based bulk material to less than 20 μm. After discharging, obtain a coke-based powder; among them, the grinding speed of the planetary ball mill is 400 rpm to 600 rpm; the coke-based bulk material includes one or more of petroleum coke, needle coke, coke, or pitch coke;

[0075] Add the coke-based powder to a hydrochloric acid solution, stir at 50 °C to 90 °C for 5 to 10 hours for impurity removal, then wash it with deionized water until neutral, dry it, and disperse it to obtain a pure coke-based powder; among them, the content of the solute in the hydrochloric acid solution is 5 wt% to 15 wt%, and the solvent is deionized water; the mass ratio of the coke-based powder to the hydrochloric acid solution is 1:10 to 1:3;

[0076] Add pure coke-based powder and potassium hydroxide to a mixed solvent of ethanol and water to form a mixed solution. After subjecting the mixed solution to ultrasonic treatment and magnetic stirring in sequence, place it in a vacuum oven and perform drying treatment at a temperature of 60°C to 80°C to obtain a black mixed dried body; wherein, the volume ratio of the pure coke-based powder to potassium hydroxide is 1:3 to 1:5; the volume ratio of ethanol to water is 1:1 to 1:3, and the solid content in the mixed solution is 10 wt% to 30 wt%.

[0077] Perform stepwise carbonization activation on the black mixed dried body, specifically: place the black mixed dried body in a high-temperature furnace, generally a rotary furnace is selected for the high-temperature furnace. First, heat it at a heating rate of 1°C / min to 5°C / min to 200°C and hold for 30 min to 120 min to remove the moisture and volatile components in the black mixed dried body; then heat it at a heating rate of 1°C / min to 5°C / min to 200°C to 400°C and hold for 30 min to 120 min for pre-activation treatment. In this process, KOH is intercalated into the carbon matrix gap; then heat it at a heating rate of 1°C / min to 5°C / min to 400°C to 800°C and hold for 30 min to 240 min for main activation treatment to realize the reaction between KOH and carbon. The reaction equation for this process is: 6KOH + 2C → 2K + 3H2↑ + 2K2CO3; finally, heat it at a heating rate of 1°C / min to 5°C / min to 800°C to 900°C and hold for 30 min to 240 min for pore expansion treatment to realize the pore expansion effect. The reaction equation is K2CO3 → K2O + CO2↑, and finally obtain a carbonized product.

[0078] Soak the carbonized product in a hydrochloric acid solution with a molar concentration of 1 mol / L for 24 hours for pickling and purification, then wash it with deionized water until neutral, and finally obtain a porous soft carbon matrix after drying treatment.

[0079] Step S2, perform heat treatment on the porous soft carbon matrix. After graphitizing the porous soft carbon matrix, obtain a composite carbon matrix.

[0080] Among them, performing heat treatment on the porous soft carbon matrix to obtain a composite carbon matrix specifically includes:

[0081] Place the porous soft carbon matrix in a high-temperature furnace and perform heat treatment at a temperature of 1000°C to 2200°C for 1 hour to 10 hours to obtain a composite carbon matrix; among them, the high-temperature furnace includes any one of a box furnace, a tube furnace, or a graphite felt insulation furnace.

[0082] The heat treatment temperature is 1000°C to 2200°C, and it can be any temperature within this range, such as: 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C, 1600°C, 1700°C, 1800°C, 1900°C, 2000°C, 2100°C, 2200°C, etc., but it is not limited to the listed temperatures, and other unlisted temperatures within this numerical range are equally applicable;

[0083] The heat treatment time is 1 hour to 10 hours, and it can be any value within this range, such as: 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, etc., but it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable;

[0084] The composite carbon matrix includes: a porous soft carbon matrix, and graphite microcrystals formed in-situ and uniformly distributed inside and on the surface of the porous soft carbon matrix; the graphitization degree of the composite carbon matrix is 10% to 95%, preferably 30% to 70%.

[0085] Step S3: Place the composite carbon matrix in a reaction device, and deposit silicon nanomaterials on the pores and surface of the composite carbon matrix to obtain a semi-finished silicon-carbon composite material;

[0086] Among them, the specific process of depositing silicon nanomaterials includes: placing the composite carbon matrix in a reaction device, heating it to 500°C to 700°C under a protective atmosphere, introducing a silicon source gas, and keeping it warm for 2 hours to 4 hours, so that the silicon element decomposed from the silicon source gas is deposited in the pores and on the surface of the composite carbon matrix, and then stop introducing the silicon source gas to obtain a semi-finished silicon-carbon composite material;

[0087] The protective gas of the protective atmosphere includes: nitrogen and / or argon; the flow rate of the protective gas is 1 L / min to 60 L / min;

[0088] The reaction device includes any one of a tube furnace, a box furnace, a chemical vapor deposition furnace, or a fluidized bed;

[0089] The silicon source gas includes one or more gases among silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, and hexachloroethylsilane; the flow rate of the silicon source gas is 10 L / min to 60 L / min.

[0090] Step S4: Perform carbon coating treatment on the semi-finished silicon-carbon composite material to obtain a graphite-based silicon-carbon composite negative electrode material;

[0091] Specifically, the carbon coating treatment is a gas-phase carbon coating, including: under a protective atmosphere, adjusting the temperature of the reaction equipment to 500°C to 580°C, introducing a carbon source gas into the reaction equipment, and keeping it warm for 3 to 5 hours, so that the carbon element decomposed from the carbon source gas is deposited on the surface of the semi-finished silicon-carbon composite material to form a carbon coating layer, and finally obtaining a graphite-based silicon-carbon composite negative electrode material;

[0092] Among them, the reaction equipment includes but is not limited to any one of a box furnace, a tube furnace, and a chemical vapor deposition furnace; the carbon source gas includes at least one of methane, acetylene, ethylene, and propylene; the flow rate of the carbon source gas is 5 L / min to 60 L / min.

[0093] The graphite-based silicon-carbon composite negative electrode material prepared by the preparation method provided by the embodiment of the present invention can be used as a negative electrode active material to prepare a negative electrode sheet, and the negative electrode sheet can be assembled with an electrolyte and / or a solid electrolyte, a separator, and a positive electrode into a lithium-ion battery.

[0094] The above lithium-ion battery includes any one of a liquid lithium-ion battery, a semi-solid lithium-ion battery, a all-solid-state lithium metal battery, or a all-solid-state lithium-ion battery.

[0095] To better understand the technical solution provided by the present invention, the following uses multiple specific examples to separately illustrate the preparation process and characteristics of the graphite-based silicon-carbon composite negative electrode material of the present invention.

[0096] Example 1

[0097] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite negative electrode material, which are specifically as follows.

[0098] (1) Preparation of a porous soft carbon matrix:

[0099] The petroleum coke bulk material is coarsely crushed by a jaw crusher to reduce the volume median diameter Dv50 to 5 mm, and then ground by a planetary ball mill at 600 rpm for 6 hours and finally subjected to air flow crushing to reduce the volume median diameter Dv50 to 9 μm, and the petroleum coke powder is obtained after discharging.

[0100] 100 g of the petroleum coke powder is added to 300 g of a 5% dilute hydrochloric acid solution, stirred at 60°C for 8 hours for impurity removal, then washed with deionized water until neutral, dried and dispersed to obtain a pure coke-based powder.

[0101] Pure coke-based powder and potassium hydroxide with a mass ratio of 1:3 were added to a mixed solvent of ethanol and water to form a mixed solution. The mixed solution was first ultrasonically treated for 15 min, then magnetically stirred for 3 hours, placed in a vacuum oven, and dried at 80 °C to obtain a black mixed dried body; among them, the volume ratio of ethanol to water was 1:1, and the solid content in the mixed solution was 20 wt%.

[0102] The black mixed dried body was placed in a rotary kiln for stepped carbonization activation, specifically: heated to 200 °C at a heating rate of 5 °C / min and held for 30 min to remove the moisture and volatile components therein; then heated to 400 °C at a heating rate of 2 °C / min and held for 60 min, and in this process, KOH was intercalated into the carbon matrix gap; then heated to 800 °C at a heating rate of 5 °C / min and held for 120 min to realize the reaction of KOH with carbon; finally heated to 900 °C at a heating rate of 3 °C / min and held for 60 min to realize the pore expansion effect, and a carbonized product was obtained.

[0103] The carbonized product was soaked in a hydrochloric acid solution with a molar concentration of 1 mol / L for 24 hours for pickling and purification, then washed with deionized water until neutral, and finally dried to obtain a porous soft carbon matrix.

[0104] (2) The porous soft carbon matrix was placed in a box furnace, and under the environment of nitrogen protective gas, the temperature was raised to 1600 °C at a heating rate of 5 °C / min and held for 4 hours for heat treatment to obtain a composite carbon matrix.

[0105] After mixing the composite carbon matrix with a small amount of silicon powder, the peak position was measured on an X-ray diffractometer. After calibration with silicon powder, the 2θ of the graphite peak d(002) plane of the composite carbon matrix was 26.25°. According to the Bragg equation nλ = 2dsinθ 弧度 (where n is 1 and λ is 0.15406 nm) and the Franklin formula G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix was calculated to be 55.52%.

[0106] (3) The composite carbon matrix was placed in the cavity of a chemical vapor deposition furnace. N2 with a flow rate of 10 L / min was introduced for 30 min to remove the oxygen in the chamber and avoid oxidation of the material. Then, at a heating rate of 5 °C / min, the temperature in the chamber was raised to 600 °C. After that, under the protection of nitrogen gas with a flow rate of 8 L / min, SiH4 with a flow rate of 8 L / min was carried, and silicon deposition was carried out for 3 hours to deposit silicon nanomaterials on the pores and surface of the composite carbon matrix, and a silicon-carbon composite material semi-finished product was obtained.

[0107] (4) Place the semi-finished silicon-carbon composite material in a coating furnace, introduce nitrogen with a flow rate of 5 L / min for 30 min to remove the air in the furnace, then increase the temperature at a rate of 5 °C / min to 540 °C. At this temperature, introduce acetylene with a flow rate of 10 L / min and keep it warm for 4 hours for carbon coating. After natural cooling, a graphite-based silicon-carbon composite anode material with a specific surface area less than 3 m 2 / g is obtained.

[0108] Use the graphite-based silicon-carbon composite anode material prepared in this example to prepare a pole piece and assemble it into a button-type half-cell for testing. The specific process is as follows:

[0109] 1) Preparation of the pole piece: Weigh the graphite-based silicon-carbon composite anode material, conductive additive carbon black, and binder (where the binder is sodium carboxymethyl cellulose and styrene-butadiene rubber with a mass ratio of 1:0.8) according to a mass ratio of 9:0.5:0.5. Prepare the slurry with a beater at room temperature for 3 hours. Coat the prepared slurry evenly on the copper foil with a coating thickness of 150 μm, and place it in a vacuum oven to dry at 70 °C for 5 hours. Cut the dried pole piece into circular pole pieces with a diameter of 12 mm. Immediately transfer the cut circular pole pieces into the glove box for standby to assemble the battery.

[0110] 2) Assembly of the button-type half-cell: It is carried out in a glove box containing a high-purity Ar atmosphere. Use metallic lithium as the counter electrode, use a solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / diethyl carbonate (DEC) containing 1 mol / L of LiPF6 (where the volume ratio of EC, DMC, and DEC is 1:1:1) as the electrolyte, use a polyethylene (PE) diaphragm, and use a 2032-type battery case to assemble the battery.

[0111] 3) Test the assembled button-type half-cell: Use a charge-discharge instrument to conduct a constant current charge-discharge mode test. The discharge cut-off voltage is 0.005 V, and the charge cut-off voltage is 2 V. The charge-discharge rate tests are carried out at current densities of 0.1 C and 2 C respectively. Take the average value of the discharge specific capacity of the first 10 cycles to obtain the capacity at the corresponding rate. Use an electrochemical workstation to conduct an impedance test. The test frequency range is 0.01 Hz - 100 KHz, and the frequency fluctuates within this range during the test to obtain the impedance of 10 cycles. Disassemble the battery after the first cycle of discharge at 0.1 C, take out the electrode piece, and use an optical profiler to measure the swelling of the first cycle of the battery. Compare the thickness of the electrode piece before and after discharge, that is, the percentage of the thickness of the electrode piece after the first cycle of discharge to the initial thickness of the electrode piece, to obtain the first cycle swelling rate of the electrode piece.

[0112] The test data of the discharge specific capacity values, the first cycle swelling rate of the electrode piece, and the impedance values of 10 cycles at current densities of 0.1 C and 2 C are shown in Table 1.

[0113] 4) Full cell assembly: The graphite-based silicon-carbon composite anode material and graphite with a specific capacity of 350 mAh / g are combined to form a composite anode material, and the theoretical capacity of the composite anode material is 520 mAh / g. The obtained composite anode material is assembled into a 5085-type small soft-pack full cell for electrochemical performance testing. The negative electrode active material (composite anode material), conductive agent (Super P), binder carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBr) are mixed and ground into a slurry in a mass ratio of 94.5:2:2:1.5, and then coated on a copper foil to prepare a negative electrode sheet; the positive electrode active material (lithium cobaltate), conductive agent 1 (Super P), conductive agent 2 (single-walled carbon nanotube CNT), and binder (PVDF) are mixed and ground into a slurry in a mass ratio of 95.2:1.8:1:2, and then coated on an aluminum foil to prepare a positive electrode sheet. An 5085-type small soft-pack battery is assembled with 11 negative electrode sheets and 10 positive electrode sheets, and the electrolyte is 1 mol / L LiPF6 (where the solvent is ethylene carbonate, dimethyl carbonate, and diethyl carbonate with a volume ratio of 1:1:1).

[0114] 5) Full cell testing: The soft-pack full cell is electrochemically tested using a Blue Electric test system. Under a charge-discharge cut-off voltage of 2.5V - 4.20V, constant current charge-discharge tests are performed on the battery at different current densities. The battery formation is carried out at a rate of 0.05C, and then the battery is divided into capacities at a rate of 0.1C. The cycle performance is tested at a rate of 1C or 3C. The test data of the cycle capacity retention rate of the full cell after 500 cycles at a rate of 1C or 3C are shown in Table 1.

[0115] Example 2

[0116] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 is that when preparing the composite carbon matrix in step (2), the porous soft carbon matrix is placed in a box furnace, and under a nitrogen protection gas environment, the temperature is raised to 1800°C at a heating rate of 5°C / min and kept for 6 hours for heat treatment to obtain the composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0117] After mixing the composite carbon matrix with a small amount of silicon powder, the peak positions are measured on an X-ray diffractometer. After calibration with silicon powder, the 2θ of the graphite peak d(002) plane of the composite carbon matrix is 26.34°. According to the Bragg equation nλ = 2dsinθ 弧度 (where n is 1, λ is 0.15406 nm) and the Franklin formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 68.76%.

[0118] The graphite-based silicon-carbon composite anode material prepared in this example was used to prepare an electrode sheet and assembled into a button cell for testing. The assembly and testing processes of the button cell were the same as those in Example 1. The test data are shown in Table 1 in detail.

[0119] Example 3

[0120] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 lies in the different heat treatment temperature and time in step (2) for preparing the composite carbon matrix. The porous soft carbon matrix was placed in a graphite felt insulation furnace, and under a nitrogen protective gas environment, the temperature was raised to 2200 °C at a heating rate of 5 °C / min and held for 4.5 hours for heat treatment to obtain the composite carbon matrix. Other preparation steps were the same as those in Example 1.

[0121] After mixing the composite carbon matrix with a small amount of silicon powder, the peak positions were measured on an X-ray diffractometer. After calibration with silicon powder, it was obtained that the 2θ of the graphite peak d(002) plane of the composite carbon matrix was 26.52°. Through the Bragg equation nλ = 2dsinθ 弧度 (where n is 1, λ is 0.15406 nm) and the Franklin formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix was calculated to be 94.97%.

[0122] The graphite-based silicon-carbon composite anode material prepared in this example was used to prepare an electrode sheet and assembled into a button cell for testing. The assembly and testing processes of the button cell were the same as those in Example 1. The test data are shown in Table 1 in detail.

[0123] Example 4

[0124] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 lies in the different heat treatment temperature and time in step (2) for preparing the composite carbon matrix. The porous soft carbon matrix was placed in a box furnace, and under a nitrogen protective gas environment, the temperature was raised to 1600 °C at a heating rate of 5 °C / min and held for 2 hours for heat treatment to obtain the composite carbon matrix. Other preparation steps were the same as those in Example 1.

[0125] After mixing the composite carbon matrix with a small amount of silicon powder, the peak positions were measured on an X-ray diffractometer. After calibration with silicon powder, it was obtained that the diffraction angle 2θ of the graphite peak d(002) plane of the composite carbon matrix was 26.08°. The XRD pattern is as Figure 2 shown, the abscissa is the diffraction angle 2θ (Two-Theta) of the graphite peak d(002) plane, in degrees (°, deg), and the ordinate is the number of X-ray photons I (Counts); through the Bragg equation nλ = 2dsinθ 弧度(where n is 1 and λ is 0.15406 nm) and Franklin's formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 30.20%.

[0126] The graphite-based silicon-carbon composite anode material prepared in this example is used to prepare a pole piece and assembled into a button cell for testing. The assembly and testing processes of the button cell are the same as those in Example 1. The test data are shown in Table 1.

[0127] Example 5

[0128] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 is that the heat treatment temperature and time are different when preparing the composite carbon matrix in step (2). The porous soft carbon matrix is placed in a box furnace, and under a nitrogen protective gas environment, the temperature is raised to 1600 °C at a heating rate of 5 °C / min and held for 1 hour for heat treatment to obtain the composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0129] After mixing the composite carbon matrix with a small amount of silicon powder, the peak position is measured on an X-ray diffractometer. After calibration with silicon powder, it is obtained that the 2θ of the d(002) crystal plane of the graphite peak of the composite carbon matrix is 25.96°. After Bragg's equation nλ = 2dsinθ 弧度 and Franklin's formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 12.23%.

[0130] The graphite-based silicon-carbon composite anode material prepared in this example is used to prepare a pole piece and assembled into a button cell for testing. The assembly and testing processes of the button cell are the same as those in Example 1. The test data are shown in Table 1.

[0131] Example 6

[0132] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 is that the heat treatment temperature and time are different when preparing the composite carbon matrix in step (2). The porous soft carbon matrix is placed in a box furnace, and under a nitrogen protective gas environment, the temperature is raised to 1450 °C at a heating rate of 5 °C / min and held for 4 hours for heat treatment to obtain the composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0133] After mixing the composite carbon matrix with a small amount of silicon powder, the peak position is measured on an X-ray diffractometer. After calibration with silicon powder, it is obtained that the 2θ of the d(002) crystal plane of the graphite peak of the composite carbon matrix is 26.13°. After Bragg's equation nλ = 2dsinθ 弧度(where n is 1 and λ is 0.15406 nm) and Franklin's formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354) × 100%, the graphitization degree of the composite carbon matrix is calculated to be 37.72%.

[0134] The graphite-based silicon-carbon composite anode material prepared in this example was used to prepare a pole piece and assembled into a button cell for testing. The assembly and testing processes of the button cell were the same as those in Example 1. The test data are shown in Table 1 for details.

[0135] Example 7

[0136] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 is that the temperature and time of heat treatment are different when preparing the composite carbon matrix in step (2). The porous soft carbon matrix was placed in a box furnace, and under the environment of nitrogen protective gas, the temperature was raised to 1500 °C at a heating rate of 5 °C / min and kept for 8 hours for heat treatment to obtain the composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0137] After mixing the composite carbon matrix with a small amount of silicon powder, the peak position was measured on an X-ray diffractometer. After calibration with silicon powder, the 2θ of the d(002) crystal plane of the graphite peak of the composite carbon matrix was 26.29°. After the Bragg equation nλ = 2dsinθ 弧度 (where n is 1 and λ is 0.15406 nm) and Franklin's formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354) × 100%, the graphitization degree of the composite carbon matrix is calculated to be 61.42%.

[0138] The graphite-based silicon-carbon composite anode material prepared in this example was used to prepare a pole piece and assembled into a button cell for testing. The assembly and testing processes of the button cell were the same as those in Example 1. The test data are shown in Table 1 for details.

[0139] Example 8

[0140] This example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 1 is in steps (1) and (2), and other preparation steps are the same as those in Example 1.

[0141] In step (1), the raw material is different from that in Example 1 when preparing the porous soft carbon matrix. Needle coke is used. The needle coke lump is coarsely crushed and medium-crushed by a jaw crusher, then ground by a planetary ball mill at a speed of 500 rpm for 6 hours, and then its volume median diameter Dv50 is reduced to 9 μm using a jet milling equipment.

[0142] In step (2), when preparing the composite carbon matrix, the heat treatment temperature is different from that in Example 1. The porous soft carbon matrix is placed in a box furnace, and under the environment of nitrogen protective gas, the temperature is raised to 1600 °C at a heating rate of 5 °C / min and kept for 6 hours for heat treatment to obtain the composite carbon matrix.

[0143] After mixing the composite carbon matrix with a small amount of silicon powder, the peak positions are measured on an X-ray diffractometer. After calibration with silicon powder, it is obtained that the 2θ of the graphite peak d(002) plane of the composite carbon matrix is 26.32°. According to the Bragg equation nλ = 2dsinθ 弧度 (where n is 1, λ is 0.15406 nm) and the Franklin formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 65.83%.

[0144] The graphite-based silicon-carbon composite anode material prepared in this example is used to prepare a pole piece and assembled into a button cell for testing. The assembly and testing processes of the button cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0145] To better illustrate the effects of the embodiments of the present invention, a comparative example is compared with the above embodiments.

[0146] Comparative Example 1

[0147] This comparative example provides a preparation process of a soft carbon-based silicon-carbon composite anode material, which is different from Example 1 in that the heat treatment temperature in step (2) is lower. The porous soft carbon matrix is placed in a box furnace, and under the environment of nitrogen protective gas, the temperature is raised to 900 °C at a heating rate of 5 °C / min and kept for 2 hours for heat treatment to obtain the composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0148] The peak positions of the composite carbon matrix are measured on an X-ray diffractometer. After calibration with silicon powder, it is obtained that the 2θ of the graphite peak d(002) plane of the composite carbon matrix is 25.92°. According to the Bragg equation nλ = 2dsinθ 弧度 (where n is 1, λ is 0.15406 nm) and the Franklin formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 6.18%, which is lower than that in Example 1. Other preparation processes are the same as those in Example 1.

[0149] The soft carbon-based silicon-carbon composite anode material prepared in this comparative example is used to prepare a pole piece and assembled into a button cell for testing. The assembly and testing processes of the button cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0150] Comparative Example 2

[0151] This comparative example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 3 lies in that the heat treatment time is increased in step (2), and the graphitization degree is relatively high. Specifically, the porous soft carbon matrix is placed in a graphite felt heat-insulating furnace, and under the environment of nitrogen protective gas, the temperature is raised to 2200 °C at a heating rate of 5 °C / min and held for 5 hours for heat treatment to obtain a composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0152] After mixing the composite carbon matrix with a small amount of silicon powder, the peak position is measured on an X-ray diffractometer. After calibration with silicon powder, it is obtained that the 2θ of the graphite peak d(002) plane of the composite carbon matrix is 26.54°. Through the Bragg equation nλ = 2dsinθ 弧度 (where n is 1, λ is 0.15406 nm) and the Franklin formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 97.86%.

[0153] Comparative Example 3

[0154] This comparative example provides a preparation process and performance test of a graphite-based silicon-carbon composite anode material. The difference from Example 8 lies in that the heat treatment time is increased in step (2), and the graphitization degree is relatively high. Specifically, the porous soft carbon matrix is placed in a graphite felt heat-insulating furnace, and under the environment of nitrogen protective gas, the temperature is raised to 2200 °C at a heating rate of 5 °C / min and held for 5 hours for heat treatment to obtain a composite carbon matrix. Other preparation steps are the same as those in Example 1.

[0155] After mixing the composite carbon matrix with a small amount of silicon powder, the peak position is measured on an X-ray diffractometer. After calibration with silicon powder, it is obtained that the 2θ of the graphite peak d(002) plane of the composite carbon matrix is 26.53°. Through the Bragg equation nλ = 2dsinθ 弧度 (where n is 1, λ is 0.15406 nm) and the Franklin formula: G = (0.3440 - d(002)) / (0.3440 - 0.3354)×100%, the graphitization degree of the composite carbon matrix is calculated to be 96.42%.

[0156] Table 1 shows the summary of test data of the coin cells assembled with Examples 1-4 and Comparative Examples 1-3:

[0157]

[0158]

[0159] Table 1

[0160] By comparing and analyzing the data of Examples 1-8 and Comparative Examples 1-3 in Table 1, it can be seen that when the graphitization degree of the composite carbon matrix containing soft carbon and graphite is in the range of 10% to 95%, the full battery prepared with the finally obtained graphite-based silicon-carbon composite anode material shows a good cycle capacity retention rate after 500 cycles, indicating that the lithium-ion battery using the graphite-based silicon-carbon composite anode material provided in the examples of the present invention has good cycle performance.

[0161] Analysis of the advantages of the examples: When the graphitization degree of the composite carbon matrix prepared in Examples 1-8 is in the range of 10% to 95%, there are sufficient graphite microcrystals in the soft carbon matrix framework. These graphite microcrystals build rich and continuous fast ion / electron transport channels, greatly improving the rate performance of the battery. For example, in the coin cell test, the capacity retention rates of Examples 1-8 at a discharge rate of 2C relative to 0.1C are all above 88%, verifying that the battery using the graphite-based silicon-carbon composite anode material provided in the examples of the present invention has good rate performance.

[0162] Analysis of the disadvantages of Comparative Example 1: The graphitization degree of Comparative Example 1 is relatively low, only 6.18%. A lower graphitization degree means a lack of graphite microcrystals, resulting in a significant increase in the overall resistance of the battery. This may not only cause local overheating of the battery but also promote the decomposition of the electrolyte, thereby forming a relatively thick solid electrolyte interface (SEI) film. The combined effect of these adverse factors ultimately leads to a rapid decay of the battery capacity. For example, the capacity retention rate of Comparative Example 1 at a discharge rate of 2C relative to 0.1C is only 75.14%, far lower than the level of the examples, indicating that the rate performance of the battery using Comparative Example 1 is much lower than that of Example 1.

[0163] Analysis of the disadvantages of Comparative Example 2 and Comparative Example 3: The graphitization degrees of Comparative Examples 2-3 are relatively high, reaching 97.86% and 96.42% respectively. In this case, the role of soft carbon as a buffer layer between the graphite layers in the carbon matrix is weakened. During the process of silicon lithium insertion and expansion, due to the lack of effective buffering, the graphite layers are prone to cracking, and then the main structure of the matrix collapses. This exposes the silicon to the electrolyte, triggering side reactions and seriously affecting the battery capacity, resulting in a rapid decay of the capacity. During the 500-cycle process of the full battery of Comparative Example 3 and Comparative Example 4, the capacity retention rates at 1C and 5C are significantly lower than the retention rates at the corresponding rates in the examples.

[0164] In summary, when the graphitization degree of the composite carbon matrix provided by the present invention is in the range of 10% to 95%, the synergistic combination of soft carbon and graphite microcrystals significantly enhances the overall anti-expansion performance of the matrix, effectively reduces the overall expansion rate, greatly improves the stability of the anode material, and thus realizes excellent cycle stability and excellent cycle performance.

[0165] The specific embodiments described above further elaborate on the objective, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, 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 graphite-based silicon-carbon composite anode material, characterized in that, The graphite-based silicon-carbon composite anode material includes: a composite silicon-carbon material, and a carbon coating layer coated on the outer surface of the composite silicon-carbon material; The composite silicon-carbon material includes: a composite carbon matrix, and silicon nanomaterials uniformly dispersed in the pores of the composite carbon matrix; The composite carbon matrix includes: a porous soft carbon matrix, and graphite microcrystals formed in-situ and uniformly distributed inside and on the surface of the porous soft carbon matrix; the graphitization degree of the composite carbon matrix is 10% to 95%; The 2θ of the (002) crystal plane of the graphite peak in the XRD diffraction peak of the composite silicon-carbon material is in the range of 26.08° to 26.35°.

2. The graphite-based silicon-carbon composite anode material according to claim 1, wherein In the graphite-based silicon-carbon composite anode material, the carbon content is 50wt% to 90wt%, and the content of the silicon nanomaterials is 15wt% to 60wt%; The thickness of the carbon coating layer is between 1nm and 50nm; the percentage of the mass of the carbon coating layer in the total mass of the graphite-based silicon-carbon composite anode material is 1% to 24%.

3. The graphite-based silicon-carbon composite anode material according to claim 1, wherein The pore volume of the composite carbon matrix is 0.1 cm 3 / g to 0.8 cm 3 / g, and the specific surface area is 200 cm 2 / g to 2000 cm 2 / g; The specific surface area of the graphite-based silicon-carbon composite anode material is between 0.3 m 2 / g and 20 m 2 / g; the volume median diameter Dv50 of the graphite-based silicon-carbon composite anode material is between 2 μm and 30 μm.

4. A method for preparing the graphite-based silicon-carbon composite anode material according to any one of claims 1-3, characterized in that, The preparation method includes: Step S1, preparing a porous soft carbon matrix; Step S2, heat-treating the porous soft carbon matrix to graphitize the porous soft carbon matrix to obtain a composite carbon matrix; Step S3, placing the composite carbon matrix in a reaction device, depositing silicon nanomaterials on the pores and surface of the composite carbon matrix to obtain a semi-finished silicon-carbon composite material; Step S4, performing carbon coating treatment on the semi-finished silicon-carbon composite material to obtain a graphite-based silicon-carbon composite anode material.

5. The preparation method according to claim 4, wherein The specific preparation of the porous soft carbon matrix in step S1 includes: Coarsely crushing the coke-based bulk material with a jaw crusher and grinding it with a planetary ball mill for 3 to 8 hours to reduce the volume median diameter Dv50 of the coke-based bulk material to less than 20μm, and discharging to obtain a coke-based powder; wherein, the grinding speed of the planetary ball mill is 400rpm to 600rpm; the coke-based bulk material includes one or more of petroleum coke, needle coke, coke or pitch coke; Adding the coke-based powder to a hydrochloric acid solution, stirring at 50°C to 90°C for 5 to 10 hours for impurity removal, then washing with deionized water until neutral, drying and dispersing to obtain a pure coke-based powder; wherein, the content of the solute in the hydrochloric acid solution is 5wt% to 15wt%, and the solvent is deionized water; the mass ratio of the coke-based powder to the hydrochloric acid solution is 1:10 to 1:3; Adding the pure coke-based powder and potassium hydroxide to a mixed solvent of ethanol and water to form a mixed solution, subjecting the mixed solution to ultrasonic treatment and magnetic stirring in sequence, and then placing it in a vacuum oven for drying treatment at a temperature of 60°C to 80°C to obtain a black mixed dried body; wherein, the volume ratio of the pure coke-based powder to the potassium hydroxide is 1:3 to 1:5; the volume ratio of the ethanol to the water is 1:1 to 1:3, and the solid content in the mixed solution is 10wt% to 30%; The black mixed dried body is subjected to stepped carbonization activation, specifically: placing the black mixed dried body in a high-temperature furnace, first heating it at a heating rate of 1 °C / min to 5 °C / min to 200 °C and holding for 30 min to 120 min to remove moisture and volatile components in the black mixed dried body; then heating it at a heating rate of 1 °C / min to 5 °C / min to 200 °C to 400 °C and holding for 30 min to 120 min for pre-activation treatment to insert the potassium hydroxide into the carbon matrix gaps of the black mixed dried body; further heating it at a heating rate of 1 °C / min to 5 °C / min to 400 °C to 800 °C and holding for 30 min to 240 min for main activation treatment to react the potassium hydroxide with carbon; finally heating it at a heating rate of 1 °C / min to 5 °C / min to 800 °C to 900 °C and holding for 30 min to 240 min for pore expansion treatment to obtain a carbonized product; The carbonized product is soaked in a hydrochloric acid solution with a molar concentration of 1 mol / L for 24 hours for pickling purification, then washed with deionized water until neutral, and finally dried to obtain the porous soft carbon matrix.

6. The preparation method according to claim 4, characterized in that, In step S2, the heat treatment of the porous soft carbon matrix to obtain a composite carbon matrix specifically includes: Placing the porous soft carbon matrix in a high-temperature furnace and performing heat treatment at a temperature of 1000 °C to 2200 °C for 1 hour to 10 hours to obtain a composite carbon matrix; wherein, the high-temperature furnace includes any one of a box furnace, a tube furnace, or a graphite felt insulation furnace; The composite carbon matrix includes: a porous soft carbon matrix, and graphite microcrystals formed in-situ and uniformly distributed inside and on the surface of the porous soft carbon matrix; the graphitization degree of the composite carbon matrix is 10% to 95%.

7. The preparation method according to claim 4, characterized in that, In step S3, it specifically includes: placing the composite carbon matrix in a reaction device, heating it to 500 °C to 700 °C under a protective atmosphere, introducing a silicon source gas, and holding for 2 hours to 4 hours to deposit silicon elements decomposed from the silicon source gas in the pores and on the surface of the composite carbon matrix, and stopping introducing the silicon source gas to obtain a silicon-carbon composite material semi-finished product; The protective gas of the protective atmosphere includes: nitrogen and / or argon; the flow rate of the protective gas is 1 L / min to 60 L / min; The reaction device includes any one of a tube furnace, a box furnace, a chemical vapor deposition furnace, or a fluidized bed; The silicon source gas includes one or more gases among silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, and hexachloroethylsilane; the flow rate of the silicon source gas is 10 L / min to 60 L / min.

8. The preparation method according to claim 4, characterized in that, In step S4, the carbon coating treatment is chemical vapor carbon coating, including: under a protective atmosphere, adjusting the temperature of the reaction device to 500 °C to 580 °C, introducing a carbon source gas into the reaction device, and holding for 3 hours to 5 hours to deposit carbon elements decomposed from the carbon source gas on the surface of the silicon-carbon composite material semi-finished product to form a carbon coating layer, and finally obtaining a graphite-based silicon-carbon composite negative electrode material; Among them, the carbon source gas includes at least one of methane, acetylene, ethylene, and propylene; the flow rate of the carbon source gas is 5 L / min to 60 L / min.

9. A negative electrode plate, characterized in that, The negative electrode sheet includes the graphite-based silicon-carbon composite negative electrode material described in any one of claims 1-3 above.

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

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