Nanometer silicon / graphite carbon composite negative electrode material, preparation method thereof and battery

Abstract

The invention belongs to the technical field of composite negative electrode materials, and discloses a nanometer silicon / graphite carbon composite negative electrode material, a preparation method thereof and a battery, and the preparation method comprises the following steps: carrying out heat treatment on expanded graphite in an inert atmosphere to fully expand the expanded graphite, crushing, and screening to obtain a porous graphite material; the preparation method comprises the following steps: uniformly mixing micron silicon and a porous graphite material according to a ratio to ensure that the micron silicon accounts for 6-10wt%, then carrying out microwave irradiation in an inert atmosphere to crush the micron silicon into nano silicon, and quenching to anchor nano silicon particles on the porous graphite material to obtain an n-Si (at) G composite material; and uniformly mixing the n-Si (at) G composite material with carbon source slurry, drying, and carbonizing to obtain the n-Si (at) G-C composite material. According to the invention, through a microwave-induced carbon thermal shock treatment process, rapid and uniform combination of nano silicon particles and graphite carbon is realized, and the structural stability and cycle performance of the negative electrode material are effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode material preparation, and in particular relates to a nano-silicon / graphite carbon composite negative electrode material and a preparation method thereof and a battery. Background Art

[0002] The statements herein merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Currently, commercial anodes primarily utilize graphite-based carbon materials, but their limited theoretical capacity makes them difficult to meet high energy density requirements. Silicon-based materials offer an ideal alternative due to their ultra-high theoretical capacity, but their dramatic volume expansion during lithium insertion and removal can easily damage the electrode structure and dramatically reduce the battery's cycle life.

[0004] Nano-silicon-carbon composites, by combining the high capacity of nano-silicon with the buffering properties of carbon materials, have become a key approach to breaking through bottlenecks. However, the current mainstream processes, which use chemical vapor deposition or liquid phase methods, require the use of highly toxic silane and silicon tetrachloride. The multi-step synthesis process leads to low production efficiency and high production costs. Furthermore, the prepared nano-silicon-carbon composites are prone to problems such as nano-silicon agglomeration and uneven carbon coating.

[0005] When nano-silicon agglomerates, the local volume expansion of the agglomerates will be superimposed when lithium is inserted, generating stress far exceeding the design value, which can easily cause microcracks inside the electrode. The cracks will extend along the boundaries of the agglomerates, eventually leading to the powdering and falling off of the electrode; the agglomerates can also easily block the lithium ion diffusion channels, resulting in the aggravation of local concentration polarization. Large-sized agglomerates will split the carbon-based conductive network, reducing the utilization rate of silicon.

[0006] When carbon coating is uneven, the coating thickness varies, making weak areas susceptible to rupture under expansion stress. This also leads to direct exposure of silicon particles, where the exposed silicon surface reacts with the electrolyte to form a non-homogeneous SEI film (solid electrolyte interface film, a passivation layer covering the surface of the electrode material formed by the reaction between the electrode material and the electrolyte at the solid-liquid interface during the initial charge and discharge of a lithium-ion battery), which continuously consumes active lithium and electrolyte. Differences in silicon expansion cause the coating to peel off from the silicon interface, exposing new silicon surfaces and causing side reactions that continue to consume active lithium. Furthermore, side reactions produce gases such as CO and C2H4, which can easily lead to battery bulging and increased internal pressure.

[0007] Another preparation process uses hard carbon to coat nanosilicon, forming a hard carbon layer to buffer the volume expansion of silicon. This is then compounded with graphite to create a dual conductive / buffering network of hard carbon + graphite, improving cycling stability. However, this requires two carbonization steps, which is energy-intensive and time-consuming. The low degree of graphitization leads to insufficient conductivity and accelerated capacity decay. Furthermore, this preparation process is limited in the amount of silicon loaded, limiting the potential for energy density improvement.

[0008] In short, existing nano-silicon-carbon composite materials are difficult to simultaneously meet the requirements of high capacity, high rate performance, high cycle performance, simple preparation process and low preparation cost. Summary of the Invention

[0009] In view of the shortcomings of the prior art, the present invention aims to provide a nano-silicon / graphite carbon composite negative electrode material and a preparation method thereof and a battery.

[0010] In order to achieve the above object, the present invention is implemented through the following technical solutions: In a first aspect, the present invention provides a method for preparing a nano-silicon / graphite carbon composite negative electrode material, comprising the following steps: The expanded graphite is subjected to heat treatment in an inert atmosphere to fully expand, and then crushed and sieved to obtain a porous graphite material; The micron silicon and porous graphite material are mixed uniformly in proportion to make the micron silicon account for 6-10wt%, and then microwave irradiated in an inert atmosphere to break the micron silicon into nano-silicon. The mixture is then quenched to anchor the nano-silicon particles on the porous graphite material to obtain an n-Si@G composite material. The n-Si@G composite material is mixed evenly with a carbon source slurry, dried, and carbonized to obtain the n-Si@GC composite material.

[0011] In a second aspect, the present invention provides a nano-silicon / graphite carbon composite negative electrode material prepared by the preparation method.

[0012] In a third aspect, the present invention provides a battery, wherein the negative electrode of the battery is prepared from the nano-silicon / graphite carbon composite negative electrode material.

[0013] The beneficial effects achieved by one or more embodiments of the present invention are as follows: Micron-sized silicon particles (m-Si) in a porous graphite matrix generate instantaneous high temperatures through microwave induction, and the difference in thermal expansion coefficients between graphite and silicon triggers stress fracture, generating nano-silicon particles (n-Si, ~50nm). These are then anchored in the pores and surface of the graphite particles through quenching, achieving rapid and uniform bonding between the nano-silicon particles and graphite carbon, effectively improving the structural stability and cycle performance of the negative electrode material.

[0014] Microwave processing offers advantages such as fast and uniform heating and low energy consumption, significantly improving production efficiency and reducing costs. The oxygen-free microwave environment reduces silicon oxide impurities (purity >99.9%), allowing the graphite matrix and carbon coating to form a three-dimensional conductive network, improving rate performance.

[0015] This method has the advantages of simple process, high production efficiency, low cost, and excellent negative electrode material performance. It improves the cycle stability and capacity performance of n-Si@GC negative electrode materials and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0017] Figure 1 This is a schematic diagram of the synthesis principle of the nano-silicon / graphite carbon negative electrode material according to an embodiment of the present invention; Figure 2 This is a SEM electron microscope image of the nano-silicon / graphite carbon negative electrode material prepared in Example 1 of the present invention; Figure 3 1 is a comparison chart of the rate retention rates of Example 1, Example 2 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0018] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0019] In a first aspect, the present invention provides a method for preparing a nano-silicon / graphite carbon composite negative electrode material, comprising the following steps: The expanded graphite is subjected to heat treatment in an inert atmosphere to fully expand, and then crushed and sieved to obtain a porous graphite material; The micron silicon and porous graphite material are mixed uniformly in proportion to make the micron silicon account for 6-10wt%, and then microwave irradiated in an inert atmosphere to break the micron silicon into nano-silicon. The mixture is then quenched to anchor the nano-silicon particles on the porous graphite material to obtain an n-Si@G composite material. The n-Si@G composite material is mixed evenly with a carbon source slurry, dried, and carbonized to obtain the n-Si@GC composite material.

[0020] In the mixed material of micron silicon and porous graphite material, the proportion of micron silicon can be 6wt%, 7wt%, 8wt%, 9wt% or 10wt%.

[0021] Graphite is a strong microwave absorber, while silicon absorbs almost no microwaves. The energy of microwave radiation is preferentially absorbed by graphite, causing the local temperature of graphite to soar rapidly, while silicon remains at a low temperature. Micro-area thermal bridges are formed at the contact points between high-temperature graphite and silicon, which instantly transfer heat to the silicon particles, causing the silicon particles to heat up rapidly, and then cause the silicon particles to expand and break rapidly.

[0022] Under high temperature conditions, the nano-silicon particles enter the pores of the porous graphite. The particle fluidization caused by microwave excitation further mixes the crushed nano-silicon particles with the porous graphite, allowing the nano-silicon particles to be evenly embedded in the porous graphite. During quenching, the mixed solid particles cool rapidly, causing the porous graphite to shrink dramatically, while the nano-silicon particles shrink relatively little. Therefore, the contracted porous graphite tightly holds the nano-silicon particles, producing a nano-silicon-porous graphite composite material.

[0023] In the nano-silicon-porous graphite composite material, since the nano-silicon particles are tightly fixed by the pores of the porous graphite, it is difficult for them to escape from the pores and difficult for them to agglomerate again, so that the nano-silicon particles can be evenly dispersed in the composite material.

[0024] The n-Si@G composite material is uniformly mixed with a carbon source slurry, dried, and carbonized to form a carbon coating on the surface of the n-Si@G composite material. The carbon coating forms a rigid shell. During lithium insertion, the carbon coating absorbs most of the strain energy, effectively preventing silicon particle breakage and suppressing excessive expansion of the negative electrode material.

[0025] The carbon coating layer can also isolate silicon from the electrolyte, preventing the nano-silicon particles from directly contacting the electrolyte and reducing side reactions.

[0026] The carbon source is prepared into a slurry and mixed with the n-Si@G composite material, which can effectively improve the mixing uniformity of the carbon source and the n-Si@G composite material, thereby ensuring the uniformity of carbon coating.

[0027] In some embodiments, the frequency of microwave irradiation is 2.45-2.60 GHz, the power is 2.5-3.5 kW, and the time of microwave irradiation is 40-60 s.

[0028] Setting the microwave irradiation frequency to 2.45-2.60 GHz can effectively improve the graphite's absorption efficiency of microwave energy. In addition, 2.45-2.60 GHz microwave irradiation can also stimulate silicon lattice resonance, generating subsurface thermal stress, further promoting the fragmentation efficiency of micron silicon.

[0029] The inventors have discovered through experiments that microwave irradiation power significantly affects the particle size of nano-silicon particles obtained by breaking down micron-silicon. When the microwave irradiation power is 2.5-3.5kW, the resulting nano-silicon particles have an appropriate and uniform particle size, and the resulting composite negative electrode material has good performance. However, when the power is too high, graphite ablation and silicon sintering are likely to occur, and the broken nano-silicon particles sinter to form micron-sized silicon spheres, which adversely affects the performance of the composite negative electrode material.

[0030] When microwave irradiation power is low, the heating rate of the porous graphite is slow, making it difficult to effectively break up the micron-sized silicon. This not only makes it difficult to achieve the appropriate nano-sized silicon particle size, but also seriously affects processing efficiency. Furthermore, when the micron-sized silicon cannot be effectively broken up, the edges of the micron-sized silicon are broken, but the core remains intact, which easily forms sharp edges and corners that can easily puncture the SEI film and damage the porous graphite structure.

[0031] In some embodiments, the porous graphite material has a particle size D50 of 6-9 μm and a pore size distribution of 4-1000 nm.

[0032] In lithium-ion battery negative electrode materials, appropriate particle size is conducive to the embedding and extraction of lithium ions. When the particle size D50 of the porous graphite material is 6-9μm, it can provide a moderate ion transmission channel to a certain extent. If the particle size is too large, the diffusion distance of lithium ions will increase, resulting in a decrease in the battery's rate performance.

[0033] Porous graphite materials within this particle size range, when mixed with micron-sized silicon, provide suitable physical space for the silicon to break up and anchor. During microwave irradiation, porous graphite with a particle size D50 of 6-9 μm can evenly disperse silicon particles, allowing the silicon to be better distributed on the graphite surface after being broken into nano-silicon. Furthermore, porous graphite of the appropriate particle size facilitates good electrical contact, as the larger particle size prevents the materials from packing too tightly, creating a good electron transport path between the nano-silicon and the graphite.

[0034] In some embodiments, the expanded graphite is heat-treated at a temperature of 800-1000° C. for a time of 1.5-3 hours. The heat-treatment temperature may be 800° C., 850° C., 900° C., 950° C., or 1000° C., and the heat-treatment time may be 1.5 hours, 2 hours, 2.5 hours, or 3 hours.

[0035] In some embodiments, the heating temperature of the quenching is 1150-1200° C., and the heating time of the quenching is 10-30 seconds.

[0036] In some embodiments, the average particle size of the micron silicon is 2-8 μm, preferably 3-5 μm, and can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, or 8 μm.

[0037] When the average particle size of micron silicon is too large, the nano silicon particles formed by crushing are likely to aggregate, making it difficult to fully and evenly disperse them.

[0038] In some embodiments, in the carbon source slurry, the carbon source is coal tar pitch, petroleum-based mesophase pitch or phenolic resin; The solvent is tetrahydrofuran or sulfolane / 3-sulfolene.

[0039] Petroleum-based mesophase pitch is a transitional substance with optical anisotropy formed during the carbonization process of petroleum-based heavy oil or coal tar pitch.

[0040] Coal tar pitch is a byproduct of the coal coking process. It is readily available, relatively inexpensive, and readily available, reducing the production cost of negative electrode materials. Coal tar pitch has a high carbon content and forms a structurally stable carbon coating during the carbonization process, effectively improving the conductivity and structural stability of the negative electrode material, thereby enhancing the battery's electrochemical performance. The carbonized product exhibits excellent conductivity and mechanical strength, helping to improve the negative electrode material's conductive network, reduce electrode resistance, and enhance the battery's rate capability and cycling stability. During the carbonization process, coal tar pitch undergoes a series of processes, including softening, salivation, and hardening, as the temperature rises, ultimately forming a carbon material with a rich porous structure. These pores not only provide a buffer for the volume expansion of the silicon material during charge and discharge, alleviating silicon expansion stress, but also shorten the diffusion path of lithium ions, improving battery performance. Coal tar pitch can evenly coat the surface of porous graphite materials, forming a good bonding interface and enhancing the integrity and stability of the composite material. This uniform coating enhances the electrochemical performance of the negative electrode material, allowing the silicon, porous graphite, and carbon coating to work synergistically, leveraging their respective strengths.

[0041] Tetrahydrofuran and sulfolane / 3-sulfolene are excellent organic solvents with good solubility for carbon sources such as coal tar pitch, ensuring uniformity during the subsequent mixing process and facilitating the formation of a uniform carbon coating on the surface of the nano-silicon / porous graphite composite. Tetrahydrofuran has a relatively moderate boiling point, allowing it to evaporate slowly during the drying process, avoiding problems such as structural damage or uneven carbon coating caused by rapid solvent evaporation. Its moderate boiling point also facilitates control of the drying temperature and rate, ensuring material quality and performance. Tetrahydrofuran is stable at room temperature and pressure and is not prone to decomposition or chemical reactions with other substances. This ensures that it maintains excellent performance during the preparation process, without introducing impurities or adversely affecting the anode material, ensuring a smooth carbon coating process and the quality of the anode material. Compared to some other organic solvents, tetrahydrofuran is relatively less toxic and poses fewer safety risks during use and operation, contributing to an environmentally friendly and safe production process. Furthermore, tetrahydrofuran can be recycled and reused through appropriate recovery processes, reducing production costs and environmental pollution.

[0042] Preferably, the concentration of the carbon source in the carbon source slurry is 0.02-0.03 g / mL.

[0043] Preferably, the mass ratio of the n-Si@GC composite material to the carbon source is 8-10:1.

[0044] In some embodiments, the carbonization temperature is 900-1100° C., and the carbonization time is 1.5-2 hours. The carbonization temperature can be 900° C., 950° C., 1000° C., 1050° C., or 1100° C.; and the carbonization time can be 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours, or 2 hours.

[0045] In a second aspect, the present invention provides a nano-silicon / graphite carbon composite negative electrode material prepared by the preparation method.

[0046] In a third aspect, the present invention provides a battery, wherein the negative electrode of the battery is prepared from the nano-silicon / graphite carbon composite negative electrode material.

[0047] The present invention will be further described below with reference to the embodiments.

[0048] Example 1 A method for preparing a nano-silicon / graphite carbon composite negative electrode material comprises the following steps: (1) Preparation of porous graphite: The expanded graphite was placed in a reaction furnace and heated to 900°C at a rate of 20°C / min under argon atmosphere for 2.0 hours to fully expand. After cooling, the expanded graphite was crushed and sieved to obtain a porous graphite material with a D50 of about 8 μm and a pore size of 5-900 nm. (2) Mixing and molding: Micronized silicon (m-Si) (average particle size approximately 4 μm) and porous graphite (average particle size approximately 8 μm) were thoroughly mixed in a mass ratio of 1:9. The mixed powder was then placed in a graphite container and subjected to microwave irradiation at a frequency of 2.45 GHz and a power of 3 kW for 45 seconds in an inert atmosphere. After microwave irradiation, the m-Si particles underwent stress fracture due to rapid thermal expansion, producing a large number of nano-Si particles. The hot mixed powder was then quenched at a post-quenching temperature of 1200°C for 20 seconds. After quenching, a large number of n-Si particles were anchored within the pores of the porous graphite, resulting in the production of an n-Si@G composite material.

[0049] (3) Preparation of n-Si@GC composite materials: The n-Si@G composite and coal tar pitch were stirred at 400 rpm in a tetrahydrofuran solvent at a mass ratio of 9:1 to obtain a mixed solution with a solid content of 0.25 g / mL. The mixture was then dried at 80°C. The resulting powder was then heated in a furnace under an Ar flow at 1000°C for 1.5 hours. After cooling, it was ground and sieved to obtain the n-Si@GC composite.

[0050] Schematic diagram of synthesis principle of nano-silicon / graphite carbon negative electrode materials, such as Figure 1 As shown in the SEM electron microscope image of the nano-silicon / graphite carbon negative electrode material prepared in Example 1, Figure 2 shown.

[0051] Example 2 A method for preparing a nano-silicon / graphite carbon composite negative electrode material comprises the following steps: (1) Preparation of porous graphite: The expanded graphite was placed in a reactor and heated to 800°C at a rate of 15°C / min under argon atmosphere. After sufficient expansion for 2 hours, the material was cooled, crushed and sieved to obtain a porous graphite material with a D50 of about 9μm and a pore size of 4-1000nm.

[0052] (2) Mixing and molding: Micronized silicon (m-Si) (average particle size approximately 4 μm) and porous graphite (average particle size approximately 9 μm) were thoroughly mixed in a mass ratio of 1:9. The mixed powder was then placed in a graphite container and subjected to microwave irradiation in an inert atmosphere at a frequency of 2.5 GHz and a power of 3 kW for 50 seconds. After microwave irradiation, the m-Si particles underwent stress fracture due to rapid thermal expansion, producing a large number of nano-Si particles. The hot mixed powder was then quenched at 1200°C for 20 seconds. After quenching, a large number of n-Si particles were anchored in the pores of the porous graphite, resulting in the production of an n-Si@G composite material.

[0053] (3) Preparation of n-Si@GC composite materials: The n-Si@G composite and phenolic resin were stirred at 400 rpm in a sulfolane solvent at a mass ratio of 9:1 to a solids content of 0.25 g / mL and then dried at 80°C. The resulting powder was then heated in a furnace under an Ar flow at 1000°C for 2 h. The resulting n-Si@GC composite was ground and sieved before use.

[0054] Example 3 A method for preparing a nano-silicon / graphite carbon composite negative electrode material comprises the following steps: (1) Preparation of porous graphite: The expanded graphite was placed in a reactor and heated to 850°C at a rate of 15°C / min under argon atmosphere for 2.5 hours to fully expand. After cooling, the material was crushed and sieved to obtain a porous graphite material with a D50 of about 8μm and a pore size of 4-1000nm.

[0055] (2) Mixing and molding: Micronized silicon (m-Si) (average particle size approximately 5 μm) and porous graphite (average particle size approximately 8 μm) were thoroughly mixed in a mass ratio of 1:12. The mixed powder was then placed in a graphite container and subjected to microwave irradiation in an inert atmosphere at a frequency of 2.6 GHz, a power of 3.5 kW, and a carbon thermal shock for 55 seconds. After microwave irradiation, the m-Si particles underwent stress fracture due to rapid thermal expansion, producing a large number of nano-Si particles. The hot mixed powder was then quenched at 1200°C for 20 seconds. After quenching, a large number of n-Si particles were anchored in the pores of the porous graphite, resulting in the production of an n-Si@G composite material.

[0056] (3) Preparation of n-Si@GC composite materials: The n-Si@G composite and petroleum-based mesophase pitch were stirred at 400 rpm in tetrahydrofuran (THF) at a mass ratio of 8:1 to a solids content of 0.25 g / mL and then dried at 80°C. The resulting powder was then heated in a furnace at 950°C under an Ar flow for 1.5 hours. The resulting n-Si@GC composite was ground and sieved before use.

[0057] Comparative Example 1 The difference from Example 1 is that the preparation step of porous graphite in step (1) is omitted, and the rest is the same as Example 1. The details are as follows: A method for preparing a nano-silicon / graphite carbon composite negative electrode material comprises the following steps: (1) Mixing and molding: Micron silicon m-Si (average particle size of about 4 μm) and natural graphite (average particle size of about 8 μm) are thoroughly mixed at a mass ratio of 1:9 and loaded into a container.

[0058] The mixed powder was irradiated with microwaves at a frequency of 2.45 GHz and a power of 3 kW in an inert atmosphere for 45 seconds. After microwave irradiation, the m-Si particles underwent stress fracture due to rapid thermal expansion, producing a large number of nano-Si particles. The high-temperature mixed powder was then quenched at 1200°C for 20 seconds. After quenching, a large number of n-Si particles were anchored in the pores of the porous graphite, resulting in the production of an n-Si@G composite material.

[0059] (2) Preparation of n-Si@GC composite materials: The n-Si@G composite and coal tar pitch were stirred at 400 rpm in a tetrahydrofuran solvent at a mass ratio of 9:1 to a solid content of 0.25 g / mL, and then dried at 80°C. Subsequently, the prepared powder was heated in a furnace under an Ar flow at 1000°C for 1.5 h. After cooling, it was ground and sieved to obtain the n-Si@GC composite.

[0060] Through comparative experiments, it was found that the graphite matrix in Comparative Example 1 used natural graphite. This structure mainly coated the surface of graphite particles with nano-silicon, and the internal space of the graphite particles could not be effectively utilized, making it difficult to further increase the loading amount of nano-silicon.

[0061] Comparative Example 2 The difference from Example 1 is that in step (2), the frequency of microwave irradiation is 2.40 GHz, and the rest is the same as Example 1.

[0062] Comparative Example 3 The difference from Example 1 is that in step (2), the frequency of microwave irradiation is 2.65 GHz, and the rest is the same as Example 1.

[0063] Comparative Example 4 The difference from Example 1 is that in step (2), the power of microwave irradiation is 2 kW, and the rest is the same as Example 1.

[0064] Comparative Example 5 The difference from Example 1 is that in step (2), the power of microwave irradiation is 4 kW, and the rest is the same as Example 1.

[0065] Comparative Example 6 The difference from Example 1 is that in step (2), the quenching step is omitted, and the high-temperature mixed powder is slowly cooled in an inert gas flow. The rest is the same as Example 1.

[0066] Comparative Example 7 The difference from Example 1 is that in step (2), the average particle size of the micron silicon is 10 μm, and the rest is the same as Example 1.

[0067] Comparative Example 8 The difference from Example 1 is that in step (1), the D50 of the porous graphite material is about 5 μm, and the rest is the same as Example 1.

[0068] Performance testing: Batteries were prepared using the composite negative electrode materials prepared in Examples 1-3 and Comparative Examples 1-8, and the specific steps included: The composite negative electrode material, SP, SBR, and CMC were mixed and dissolved in deionized water at a mass ratio of 96:1:2:1, with the solid content controlled at 50%, coated on a copper foil current collector, and vacuum dried to prepare a negative electrode sheet; The negative electrode, electrolyte, separator (Enjie, 9+2+2), lithium sheet and shell are assembled into button cells using conventional production processes; The solvent of the electrolyte is ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1; the solute is LiPF6, and the concentration of the solute is 1 mol / L; the electrochemical performance of the battery is tested on a battery testing system.

[0069] The test conditions are: at room temperature, 0.1C constant current charge and discharge, charge and discharge cut-off voltage 0.01V-2V, test results are shown in Table 1 and Figure 3 shown.

[0070] Double charge test method: At room temperature, constant current and constant voltage charging at different rates (0.5C, 1C, 2C, 3C), 0.1C discharge, charge and discharge cut-off voltage 0.01V-2V.

[0071] Table 1 Comparison of the performance of the composite negative electrode materials of Examples 1-3 and Comparative Examples 1-8

[0072] Table 1 demonstrates the necessity of a porous graphite structure. Comparative Example 1 (using natural graphite instead of porous graphite) exhibits significantly lower capacity (1126 mAh / g) and cycle stability (84.08% capacity retention after 300 cycles) than all other examples. This demonstrates that the internal pores of porous graphite are key to anchoring nanosilicon and increasing the loading capacity, effectively mitigating volume expansion and improving reversible capacity.

[0073] Optimization range of microwave irradiation parameters, frequency influence: Comparison of Example 1 (2.45 GHz) with Comparative Example 2 (2.40 GHz) and Comparative Example 3 (2.65 GHz): The high-frequency (2.65 GHz) capacity (1245 mAh / g) of Comparative Example 3 decreased significantly, indicating that excessively high frequency may lead to uneven heat distribution and weaken the silicon particle crushing effect.

[0074] Comparative Example 4 (2 kW), Example 1 (3 kW), and Comparative Example 5 (4 kW): Comparative Example 5 has the worst capacity (1204 mAh / g) and rate performance (3C retention rate 75.63%), demonstrating that excessive power causes silicon sintering and graphite ablation, destroying the material structure. Comparative Example 4 (2 kW) has a capacity (1366 mAh / g) close to Example 1, but slightly worse rate performance, indicating that insufficient power can maintain capacity but affects crushing efficiency and rate performance.

[0075] The capacity (937 mAh / g) and cycle retention (80.17% after 500 cycles) of Comparative Example 6 (quenching omitted) dropped drastically, demonstrating that quenching can quickly fix nano-silicon in the pores, preventing agglomeration and structural relaxation, and is a key step in maintaining high dispersion and structural stability.

[0076] The capacity of Comparative Example 7 (micron silicon particle size 10 μm) (1254 mAh / g) is lower than that of Example 1 (1356 mAh / g), indicating that overly large silicon particles are difficult to fully break down into nanometer size, and the residual micron structure reduces the effective load and exacerbates the volume effect.

[0077] The capacity (1086 mAh / g) of Comparative Example 8 (porous graphite D50 = 5 μm) is significantly lower, indicating that the smaller graphite particle size limits the pore space and silicon loading capacity, and needs to be matched with the silicon particle size (4-5 μm) to optimize the anchoring effect.

[0078] The n-Si@GC composite material is produced from a mixture of m-Si and graphite powder through a carbon thermal shock process, followed by carbon coating with coal tar pitch. The introduction of n-Si particles into and on the surface of the carbon-coated porous graphite has been shown to be effective in improving reversible capacity and fast charging capability. These improvements are primarily attributed to the effective volume regulation and enhanced conductivity of the n-Si particles. This method offers advantages such as simple process, high production efficiency, low cost, and excellent negative electrode material performance, and has broad application prospects.

[0079] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a nano-silicon / graphite carbon composite negative electrode material, characterized in that: The steps include: The expanded graphite is subjected to heat treatment in an inert atmosphere to fully expand, and then crushed and sieved to obtain a porous graphite material; The micron silicon and porous graphite material are mixed uniformly in proportion to make the micron silicon account for 6-10wt%, and then microwave irradiated in an inert atmosphere to break the micron silicon into nano-silicon. The mixture is then quenched to anchor the nano-silicon particles on the porous graphite material to obtain an n-Si@G composite material. The n-Si@G composite material is mixed evenly with a carbon source slurry, dried, and carbonized to obtain the n-Si@GC composite material.

2. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, wherein: The frequency of microwave irradiation is 2.45-2.60 GHz, the power is 2.5-3.5 kW, and the time of microwave irradiation is 40-60 s.

3. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, wherein: The particle size D50 of the porous graphite material is 6-9 μm, and the pore size distribution is 4-1000 nm.

4. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, wherein: The temperature for heat treatment of expanded graphite is 800-1000° C., and the heat treatment time is 1.5-3 hours.

5. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, wherein: The average particle size of the micron silicon is 2-8 μm.

6. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, characterized in that: In the carbon source slurry, the carbon source is coal tar, petroleum-based mesophase asphalt or bio-based asphaltene; and the solvent is tetrahydrofuran or sulfolane.

7. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, wherein: The mass ratio of n-Si@GC composite material to carbon source is 8-10:

1.

8. The method for preparing the nano-silicon / graphite carbon composite negative electrode material according to claim 1, wherein: The carbonization temperature is 900-1100° C., and the carbonization time is 1.5-2 hours.

9. A nano-silicon / graphite carbon composite negative electrode material, characterized by: Prepared by the preparation method according to any one of claims 1 to 8.

10. A battery, characterized in that: The negative electrode of the battery is prepared from the nano-silicon / graphite carbon composite negative electrode material according to claim 9.

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