A screening method for a graphite material of a silicon-carbon composite negative electrode system, a silicon-carbon composite negative electrode, and a secondary battery

Abstract

This invention relates to the field of lithium-ion battery technology, specifically a method for screening graphite materials for silicon-carbon composite anode systems, a silicon-carbon composite anode, and a secondary battery. The screening process tests the initial efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles and graphite materials. The requirements are: initial efficiency of graphite materials ≥94% with a difference of 2%-5% compared to silicon-carbon composite particles; median particle size ratio of graphite materials to silicon-carbon composite particles of 0.3-2; and specific surface area ratio of graphite materials to silicon-carbon composite particles of 0.2-1.4. The differential capacity processing of the graphite material discharge curves shows that the differential capacity area ratios for 0.02V-0.15V and 0V-0.3V, 0.02V-0.15V and 0.15V-0.25V, and 0.25V-0.5V and 0V-0.5V are >65%, >400%, and <5%, respectively. The graphite materials obtained using this screening method, when blended with silicon-carbon composite particles, significantly improve the overall electrochemical performance of the silicon-carbon composite anode system.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for screening graphite materials for silicon-carbon composite anode systems, a silicon-carbon composite anode, and a secondary battery. Background Technology

[0002] The rapid development of the lithium battery industry has placed increasingly higher demands on the energy density of lithium-ion batteries, thus promoting the rapid development of silicon-carbon composite anode materials. In existing technologies, the common preparation method for silicon-carbon composite anode materials is as follows: first, structurally optimized silicon-carbon composite particles are prepared, such as nano-silicon embedded in a carbon matrix, or a carbon layer coated on a silicon suboxide surface, or a porous silicon substrate with a carbon source deposited on it. Then, the silicon-carbon composite particles are physically mixed with graphite, finally obtaining the silicon-carbon anode material. Graphite, as a continuous conductive matrix and volume buffer phase, plays a crucial role in the anode material.

[0003] Currently, when selecting graphite for blending, the industry typically focuses on the structural parameters of graphite itself, such as particle size, specific surface area, morphology, purity, and degree of graphitization, or performance parameters such as volatile matter. However, it often neglects the degree of matching between graphite and the electrochemical performance of silicon-carbon composite particles. This screening method often results in the anode materials exhibiting problems such as substandard initial efficiency, rapid cycle decay, and poor rate performance, which is detrimental to the excellent electrochemical performance of silicon-carbon composite anode materials.

[0004] Therefore, there is an urgent need to propose a systematic method for screening graphite materials to obtain graphite materials with high matching degree with silicon-carbon composite particles, thereby promoting the full utilization of the electrochemical performance of silicon-carbon composite anode materials. Summary of the Invention

[0005] Based on the background technology, this invention provides a method for screening graphite materials for silicon-carbon composite anode systems, a silicon-carbon composite anode, and a secondary battery, aiming to overcome the problems of poor electrochemical performance caused by the poor effect of existing graphite screening methods, resulting in anode materials exhibiting substandard initial efficiency, rapid cycle decay, and poor rate performance.

[0006] To achieve the above objectives, the main technical solutions adopted by the present invention are as follows.

[0007] Firstly, this invention proposes a method for screening graphite materials for silicon-carbon composite anode systems, comprising the following steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and represent the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC respectively. S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G≥94% and (ICE-G)-(ICE-SC)≥2%-5%; (2) 0.3≤D50-G / D50-SC≤2, 0.2≤BET-G / BET-SC≤1.4; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A significant main peak appears in the 0.02V-0.15V voltage range, and the percentage of the main peak area to the total differential capacity area in the 0V-0.3V voltage range is >65%; ii. The percentage of the differential capacity area in the 0.02V-0.15V voltage range to the differential capacity area in the 0.15V-0.25V voltage range is >400%; iii. The percentage of the differential capacity area in the 0.25V-0.5V voltage range to the total differential capacity area in the 0V-0.5V voltage range is <5%.

[0008] S3. The graphite material obtained in step S2 is uniformly mixed with the silicon-carbon composite particles to be matched according to a preset mass ratio and the negative electrode sheet is prepared and its electrochemical performance is verified.

[0009] The above condition (1) is mainly to achieve electrochemical window matching between graphite material and silicon-carbon composite particles. By setting the first efficiency of graphite material to be ≥94% and the difference between the first efficiency of graphite material and silicon-carbon composite particles to be ≥2%-5%, it is ensured that the doped graphite has a sufficiently high reversible capacity, which can effectively compensate for the high irreversible capacity loss of silicon-carbon particles during the first lithium insertion process, thereby ensuring the first coulombic efficiency of the silicon-carbon composite anode system.

[0010] The above condition (2) is mainly to achieve a physical spatial structure match between graphite materials and silicon-carbon composite particles. By ensuring that the ratio of the median particle size of graphite materials to silicon-carbon composite particles is 0.3-2 and the ratio of their specific surface areas is 0.2-1.4, an ideal pore structure is constructed inside the electrode, avoiding the problem of simply pursuing particle size distribution while ignoring interfacial side reactions. Generally speaking, the common median particle size D50-SC of silicon-carbon composite particles is 5-10 μm (preferably 7-9 μm), and the common specific surface area BET-SC is 1-5 m². 2 / g (preferably 2-3m) 2 / g), combined with the test results of step S1 and the limitations of condition (2), graphite materials with median particle size and specific surface area that meet the requirements can be screened out.

[0011] The above conditions (3) are used to process the discharge curve by differential capacity dQ / dV, and the percentage of differential capacity area in multiple voltage ranges is used to measure the crystal structure order and lithium intercalation behavior of graphite materials. Condition i defines the position and purity of the main peak. Graphite materials have a highly ordered layered structure, and the main lithium intercalation process occurs in the characteristic low-voltage plateau region. Condition ii defines the percentage of the differential capacity area between the characteristic low-voltage plateau region and the 0.15V-0.25V voltage range as >400%, further strengthening the purity requirements and excluding graphite materials with impurities or defective lithium intercalation in slightly higher voltage regions, ensuring the concentration of energy output. Condition iii defines the percentage of the differential capacity area between the 0.25V-0.5V and 0V-0.5V voltage ranges as <5%, to screen graphite materials with fewer surface defects and side reactions, ensuring that they do not introduce additional irreversible capacity after being mixed with silicon-carbon composite particles. The screened low-side-reaction graphite materials can effectively suppress parasitic reactions between the entire negative electrode system and the electrolyte, reduce gas generation, and improve safety and first coulombic efficiency.

[0012] Compared with the prior art, the present invention solves the problems of the prior art relying on structural parameters to screen graphite materials, resulting in poor matching between graphite materials and silicon-carbon composite particles, and the resulting silicon-carbon composite anode system having many interfacial side reactions, failing to meet the first efficiency standard, fast cycle decay, poor rate performance, and short cycle life.

[0013] By constructing a three-dimensional synergistic screening mechanism through initial coulombic efficiency constraints, particle size-specific surface area joint constraints, and differential capacity quantification, this invention transforms graphite materials from simple fillers into functional components for regulating silicon-carbon composite anodes. The screened graphite materials exhibit good crystal structure order and lithium intercalation behavior characteristics, and match the electrochemical window and physical spatial structure of silicon-carbon composite particles. After forming a silicon-carbon composite anode system, it can significantly enhance the comprehensive electrochemical performance, including initial coulombic efficiency and long-term cycle stability.

[0014] Furthermore, in condition (2), 0.3 ≤ D50-G / D50-SC ≤ 1. Preferably, 0.6 ≤ D50-G / D50-SC ≤ 0.8, 0.4 ≤ BET-G / BET-SC ≤ 1.4, and 1.2m 2 / g≤BET-G≤2m 2 / g. Preferably, in condition (2), 0.4≤D50-G / D50-SC≤0.6, 0.6≤BET-G / BET-SC≤1.4 and 2m 2 / g≤BET-G≤2.8m 2 / g.

[0015] In this technical solution, the particle size of the graphite material is smaller than that of the silicon-carbon composite particles. This invention unexpectedly discovered that when graphite is blended with silicon-carbon composite particles, different application scenarios can be met by adjusting the ranges of D50-G / D50-SC, BET-G / BET-SC, and BET-G. Specifically, when the graphite material satisfies: 0.6 ≤ D50-G / D50-SC ≤ 0.8, 0.4 ≤ BET-G / BET-SC ≤ 1.4, and 1.2 μm... 2 / g≤BET-G≤2m 2 When the density is 0.4 / g, the screened graphite material is called fine-filled graphite. It can achieve high-density fine filling and avoid the problem of increased side reactions caused by the excessive specific surface area of ​​small graphite particles. Furthermore, when the graphite material satisfies the following conditions: 0.4 ≤ D50-G / D50-SC ≤ 0.6, 0.6 ≤ BET-G / BET-SC ≤ 1.4 and 2m 2 / g≤BET-G≤2.8m 2 When the density is / g, the screened graphite material is called conductive network reinforced graphite, which can provide more conductive contact points and strengthen the conductive network. It is suitable for scenarios where the conductivity of silicon-carbon composite particles themselves is poor and the interface contact needs to be strengthened.

[0016] In practical applications, finely filled graphite can be selected for high energy density scenarios to maximize electrode compaction density and improve volumetric energy density; while for high power scenarios, conductive network-reinforced graphite can be selected to utilize its high specific surface area to construct a rich conductive network and improve rate performance.

[0017] Furthermore, in condition (2), 1 ≤ D50-G / D50-SC ≤ 2. Preferably, 1.3 ≤ D50-G / D50-SC ≤ 1.8, 0.2 ≤ BET-G / BET-SC ≤ 1, and 1m 2 / g≤BET-G≤2m 2 / g, graphite that meets this condition is called framework-supported graphite.

[0018] In this technical solution, the particle size of the graphite material is larger than that of the silicon-carbon composite particles. The large graphite particles form a stable conductive framework, and the appropriate specific surface area helps to anchor the silicon-carbon composite particles, ensuring good interfacial bonding. The screened graphite material, after being blended, helps to reduce side reactions with the electrolyte and improves cycle life.

[0019] In practical applications, for long cycle life scenarios, framework-supported graphite can be selected. Large-particle, low-specific-surface-area graphite is used to construct a stable conductive framework, which buffers the volume expansion of silicon-carbon particles, maintains the integrity of the electrode structure, and significantly extends the cycle life.

[0020] Furthermore, in condition (3), the graphite material satisfies iv: the full width at half maximum (FWHM) of the main peak is ≤35mV.

[0021] In this technical solution, to pursue the ultimate performance of the silicon-carbon composite anode system, such as ultra-long cycle performance or ultra-low temperature performance, graphite materials with a full width at half maximum (FWHM) of ≤35mV are further screened based on the previous screening conditions. The smaller the FWHM, the higher the crystallinity of the graphite material, the more uniform the particle size distribution, the more concentrated the lithium intercalation process, and the more stable the performance. Its higher quality helps to pursue more extreme performance of the silicon-carbon composite anode.

[0022] Secondly, this invention proposes a silicon-carbon composite negative electrode, comprising a negative electrode current collector and a negative electrode active material disposed on at least one surface of the negative electrode current collector. The negative electrode active material comprises silicon-carbon composite particles and graphite material obtained by the above screening method. The mass ratio of silicon-carbon composite particles to graphite material is usually (5-50):(95-50), for example, it can be 5:95, 15:85, 20:80, 30:70, 40:60, 50:50, etc.

[0023] It should be noted that both the negative electrode current collector and the silicon-carbon composite particles are existing technologies. For example, the negative electrode current collector can be selected from any one of copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal. Specifically, in terms of preparation process, the silicon-carbon composite particles can be any one or more of nano-silicon-carbon composite particles, porous carbon with vapor-deposited nano-silicon, silicon-oxygen composite particles, and porous silicon-carbon composite particles. In terms of structural composition, they can be any one or more of coated silicon-carbon materials, doped silicon-carbon materials, and surface-modified silicon-carbon materials. In terms of morphology, they can be porous silicon-carbon materials, spherical silicon-carbon materials, or near-spherical silicon-carbon materials.

[0024] In addition, the negative electrode active material also includes a binder and a conductive agent, both of which are existing technologies. For example, the binder is selected from any one or more of the following: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, styrene-butadiene rubber (SBR), acrylated SBR, and epoxy resin. The conductive agent is typically selected from carbon-based or metal-based materials; carbon-based materials are selected from any one or more of the following: natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber; metal-based materials are selected from any one or more of the following: metal powder, metal fiber, copper, nickel, aluminum, and silver.

[0025] Furthermore, the silicon-carbon composite anode exhibits the following performance in soft-pack full-cell testing: initial coulombic efficiency ≥86%, and capacity retention ≥80% after 1C rate and 800 cycles.

[0026] The high initial coulombic efficiency and excellent long-term cycling stability of the silicon-carbon composite anode indicate that the graphite material screening method proposed in this invention is scientific and effective. It not only effectively alleviates the problem of conductive network failure caused by the volume expansion of silicon-carbon composite particles, but also suppresses continuous interfacial side reactions. The screened graphite material is precisely matched with the silicon-carbon composite particles, which significantly improves the comprehensive electrochemical performance of the silicon-carbon composite anode.

[0027] Thirdly, this invention also proposes a secondary battery, including a silicon-carbon composite negative electrode. In addition, the secondary battery also includes a positive electrode, an electrolyte, and a separator. The positive electrode, the electrolyte, and the separator all employ existing technologies, and those skilled in the art can flexibly select them according to actual needs.

[0028] The screening method proposed in this invention is scientific, systematic, and refined, providing a standardized screening approach for silicon-carbon composite particles from different sources and with varying performance requirements. Compared to the traditional method of comparing the compatibility between graphite materials and silicon-carbon composite particles by testing the performance of battery cells, the screening method proposed in this invention has a shorter evaluation cycle and higher efficiency. This helps to significantly shorten the R&D cycle and reduce the R&D cost of silicon-carbon composite anodes, providing crucial technical support for the industrial application of high-performance silicon-carbon composite anodes. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 The diagram shows the cycle performance test results of the pouch cells assembled in Examples 1, 5, 4, 5, and 6.

[0031] Figure 2 The discharge curves and dQ / dV curves of the graphite material screened in Example 1 during half-cell charge-discharge testing are shown: a) dQ / dV curve; b) discharge curve. Detailed Implementation

[0032] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0033] All chemical raw materials used in the following examples and comparative examples are commercially available, and all apparatus and operations involved are conventional in the art.

[0034] Example 1

[0035] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=4%; (2) D50-G=9.6μm, that is, D50-G / D50-SC=1.2, BET-G=2m 2 / g, that is, BET-G / BET-SC=0.8; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 85% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 425% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 31mV. S3. The graphite material obtained in step S2 is uniformly mixed with silicon-carbon composite particles at a mass ratio of 70:30 to prepare a negative electrode active material. Then, the negative electrode active material, binder polyacrylic acid, and conductive agent carbon black are mixed at a mass ratio of 94:3:3 and NMP solvent is added to prepare a negative electrode slurry. The negative electrode slurry is then uniformly coated on both sides of the negative electrode current collector copper foil. Finally, it is dried and cold-pressed to obtain a negative electrode sheet.

[0036] Example 2

[0037] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92.3%, D50-SC is 8.5 μm, and BET-SC is 1.4 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.3%, that is, (ICE-G)-(ICE-SC)=2%; (2) D50-G=8.8μm, that is, D50-G / D50-SC=1, BET-G=1.9m 2 / g, that is, BET-G / BET-SC=1.4; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 69% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 415% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.6% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0038] Example 3

[0039] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.5%, that is, (ICE-G)-(ICE-SC)=4.5%; (2) D50-G=10.4μm, that is, D50-G / D50-SC=1.3, BET-G=2.1m 2 / g, that is, BET-G / BET-SC=0.8; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 73% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 435% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.1% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 30mV. S3, the same as step S3 in Example 1.

[0040] Example 4

[0041] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=95%, that is, (ICE-G)-(ICE-SC)=5%; (2) D50-G=8μm, that is, D50-G / D50-SC=1, BET-G=2.3m 2 / g, that is, BET-G / BET-SC=0.9; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 75% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 445% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.8% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 29mV. S3, the same as step S3 in Example 1.

[0042] Example 5

[0043] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.2%, that is, (ICE-G)-(ICE-SC)=4.2%; (2) D50-G=6.4μm, that is, D50-G / D50-SC=0.8, BET-G=2.4m 2 / g, that is, BET-G / BET-SC=1; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 70% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 423% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.2% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 40mV. S3, the same as step S3 in Example 1.

[0044] Example 6

[0045] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.5%, that is, (ICE-G)-(ICE-SC)=4.5%; (2) D50-G=5.6μm, that is, D50-G / D50-SC=0.7, BET-G=2.5m 2 / g, that is, BET-G / BET-SC=1; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 68% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 430% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0046] Example 7

[0047] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92%, D50-SC is 8.5 μm, and BET-SC is 3 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.7%, that is, (ICE-G)-(ICE-SC)=2.7%; (2) D50-G=2.6μm, that is, D50-G / D50-SC=0.3, BET-G=3m 2 / g, that is, BET-G / BET-SC=1; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 72% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 430% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.2% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 33mV. S3, the same as step S3 in Example 1.

[0048] Example 8

[0049] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 91.2%, D50-SC is 8.3 μm, and BET-SC is 2 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=2.8%; (2) D50-G=3.3μm, that is, D50-G / D50-SC=0.4, BET-G=2.8m 2 / g, that is, BET-G / BET-SC=1.4; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 74% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 440% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.9% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 30mV. S3, the same as step S3 in Example 1.

[0050] Example 9

[0051] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90.5%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=3.5%; (2) D50-G=4.1μm, that is, D50-G / D50-SC=0.5, BET-G=2.5m 2 / g, that is, BET-G / BET-SC=1; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 75% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 445% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.8% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 29mV. S3, the same as step S3 in Example 1.

[0052] Example 10

[0053] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92%, D50-SC is 7.9 μm, and BET-SC is 5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=2%; (2) D50-G=4.7μm, that is, D50-G / D50-SC=0.6, BET-G=2m 2 / g, that is, BET-G / BET-SC=0.4; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 76% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 450% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.7% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 28mV. S3, the same as step S3 in Example 1.

[0054] Example 11

[0055] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92%, D50-SC is 7.7 μm, and BET-SC is 1 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=2%; (2) D50-G=5.4μm, that is, D50-G / D50-SC=0.7, BET-G=1.4m 2 / g, that is, BET-G / BET-SC=1.4; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 78% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 460% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.5% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 28mV. S3, the same as step S3 in Example 1.

[0056] Example 12

[0057] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92%, D50-SC is 7.5 μm, and BET-SC is 1.3 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=2%; (2) D50-G=6μm, that is, D50-G / D50-SC=0.8, BET-G=1.2m 2 / g, that is, BET-G / BET-SC=0.9; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 79% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 465% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.4% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 27mV. S3, the same as step S3 in Example 1.

[0058] Example 13

[0059] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92%, D50-SC is 7.3 μm, and BET-SC is 5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=2%; (2) D50-G=7.3μm, that is, D50-G / D50-SC=1, BET-G=1m 2 / g, that is, BET-G / BET-SC=0.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 80% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 470% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 26mV. S3, the same as step S3 in Example 1.

[0060] Example 14

[0061] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92.3%, D50-SC is 7.1 μm, and BET-SC is 2 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=95.5%, that is, (ICE-G)-(ICE-SC)=3.2%; (2) D50-G=9.2μm, that is, D50-G / D50-SC=1.3, BET-G=2m 2 / g, that is, BET-G / BET-SC=1; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 84% ​​of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 490% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 2.7% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 25mV. S3, the same as step S3 in Example 1.

[0062] Example 15

[0063] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92.3%, D50-SC is 7 μm, and BET-SC is 3 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=95.6%, that is, (ICE-G)-(ICE-SC)=3.3%; (2) D50-G=10.5μm, that is, D50-G / D50-SC=1.5, BET-G=1.5m 2 / g, that is, BET-G / BET-SC=0.5; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 85% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 495% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 2.6% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 24mV. S3, the same as step S3 in Example 1.

[0064] Example 16

[0065] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92.3%, D50-SC is 7.2 μm, and BET-SC is 5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=95.1%, that is, (ICE-G)-(ICE-SC)=2.8%; (2) D50-G=13μm, that is, D50-G / D50-SC=1.8, BET-G=1m 2 / g, that is, BET-G / BET-SC=0.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 86% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 500% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 2.5% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 23mV. S3, the same as step S3 in Example 1.

[0066] Example 17

[0067] Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 92.5%, D50-SC is 7.4 μm, and BET-SC is 2 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=95.7%, that is, (ICE-G)-(ICE-SC)=3.2%; (2) D50-G=14.8μm, that is, D50-G / D50-SC=2, BET-G=2.4m 2 / g, that is, BET-G / BET-SC=1.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 80% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 470% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 3.2% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 28mV. S3, the same as step S3 in Example 1.

[0068] Comparative Example 1 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=89.5%, that is, (ICE-G)-(ICE-SC)=-0.5%; (2) D50-G=9.3μm, that is, D50-G / D50-SC=1.1, BET-G=2.2m 2 / g, that is, BET-G / BET-SC=0.9; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 64% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 380% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 5.6% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 39mV. S3, the same as step S3 in Example 1.

[0069] Comparative Example 2 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=96%, that is, (ICE-G)-(ICE-SC)=6%; (2) D50-G=8.7μm, that is, D50-G / D50-SC=1.1, BET-G=2m 2 / g, that is, BET-G / BET-SC=0.8; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 62% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 385% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 5.9% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 38mV. S3, the same as step S3 in Example 1.

[0070] Comparative Example 3 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=93%, that is, (ICE-G)-(ICE-SC)=3%; (2) D50-G=10.2μm, that is, D50-G / D50-SC=1.3, BET-G=2.4m 2 / g, that is, BET-G / BET-SC=1; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 61% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 380% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 6% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 39mV. S3, the same as step S3 in Example 1.

[0071] Comparative Example 4 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94%, that is, (ICE-G)-(ICE-SC)=4%; (2) D50-G=3.2μm, that is, D50-G / D50-SC=0.4, BET-G=2.9m 2 / g, that is, BET-G / BET-SC=1.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 58% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 365% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 6.9% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 42mV. S3, the same as step S3 in Example 1.

[0072] Comparative Example 5 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=95%, that is, (ICE-G)-(ICE-SC)=5%; (2) D50-G=18.5μm, that is, D50-G / D50-SC=2.3, BET-G=2.1m 2 / g, that is, BET-G / BET-SC=0.8; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 57% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 345% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 7.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 43mV. S3, the same as step S3 in Example 1.

[0073] Comparative Example 6 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=92.4%, that is, (ICE-G)-(ICE-SC)=2.4%; (2) D50-G=9.5μm, that is, D50-G / D50-SC=1.2, BET-G=0.5m 2 / g, that is, BET-G / BET-SC=0.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 70% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 420% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.4% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0074] Comparative Example 7 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=92.5%, that is, (ICE-G)-(ICE-SC)=2.5%; (2) D50-G=9.8μm, that is, D50-G / D50-SC=1.2, BET-G=4.2m 2 / g, that is, BET-G / BET-SC=1.7; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 68% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 412% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0075] Comparative Example 8 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.3%, that is, (ICE-G)-(ICE-SC)=4.3%; (2) D50-G=9μm, that is, D50-G / D50-SC=1.1, BET-G=0.5m 2 / g, that is, BET-G / BET-SC=0.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 67% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 410% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0076] Comparative Example 9 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=94.7%, that is, (ICE-G)-(ICE-SC)=4.7%; (2) D50-G=9.2μm, that is, D50-G / D50-SC=1.1, BET-G=0.4m 2 / g, that is, BET-G / BET-SC=0.2; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 71% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 420% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0077] Comparative Example 10 Select the graphite materials that meet the requirements by following these steps: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and denote the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC, respectively, where ICE-SC is 90%, D50-SC is 8.1 μm, and BET-SC is 2.5 μm. 2 / g; S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G=92.2%, that is, (ICE-G)-(ICE-SC)=2.2%; (2) D50-G=2.2μm, that is, D50-G / D50-SC=0.3, BET-G=2m 2 / g, that is, BET-G / BET-SC=0.8; (3) When conducting half-cell charge and discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the area of ​​the main peak is 72% of the total differential capacity area in the 0V-0.3V voltage range; ii. The differential capacity area in the 0.02V-0.15V voltage range is 432% of the total differential capacity area in the 0.15V-0.25V voltage range; iii. The differential capacity area in the 0.25V-0.5V voltage range is 4.3% of the total differential capacity area in the 0V-0.5V voltage range; iv. The full width at half maximum (FWHM) of the main peak is 32mV. S3, the same as step S3 in Example 1.

[0078] Lithium cobalt oxide was used as the positive electrode active material. It was mixed with conductive agent carbon black and binder polyvinylidene fluoride at a mass ratio of 98:1:1, and NMP solvent was added to prepare a positive electrode slurry. This slurry was then uniformly and double-sidedly coated onto a positive electrode current collector aluminum foil. After drying and cold pressing, a positive electrode sheet was obtained. The negative electrode sheets obtained in each embodiment and comparative example were stacked with the positive electrode sheet and separator, respectively. After encapsulation, electrolyte injection, and composition testing, a soft-pack battery cell was obtained. The electrolyte was a 1 M LiPF6 solution (the solvent was a 1:1:1 volume ratio of EC, DEC, and EMC mixed solvents), and the separator was a 9 μm thick double-sided ceramic polyolefin microporous membrane.

[0079] The electrochemical performance of the obtained soft-pack cells was tested at 25±2℃. The electrochemical performance mainly involved the negative electrode specific capacity, initial coulombic efficiency, cycle performance and rate performance. The test results are shown in Table 1. The test conditions are as follows: (1) First coulombic efficiency test: charge at 0.5C constant current to 4.53V, then charge at constant voltage to current ≤0.025C, and after resting, discharge at 0.2C constant current to 3.0V. The first coulombic efficiency is the ratio of the first discharge capacity to the charging capacity; (2) Cyclic performance test: charge and discharge at 1C constant current for 800 cycles. The capacity retention rate is the ratio of the discharge capacity after the cycle to the initial discharge capacity; (3) Rate performance test: charge and discharge at 0.2C and 3C respectively. The 3C capacity retention rate (vs 0.2C) is the ratio of the discharge capacity of the 3C charge and discharge test to the discharge capacity of the 0.2C charge and discharge test; (4) Negative electrode specific capacity test: charge and discharge at 0.2C, take the discharge capacity of the 3rd cycle, and divide it by the mass g of the negative electrode active material (i.e., the sum of the mass of graphite material and silicon carbon composite particles).

[0080]

[0081] As shown in Table 1, Figure 1 ,Figure 2 As shown in the following: (1) From the comparison of the test results of Examples 1-4 and Comparative Examples 1-3, it can be seen that when the first-efficiency of the graphite material and the difference in first-efficiency with the silicon-carbon composite particles meet the screening condition (1), the specific capacity of the obtained negative electrode is 176.5-178 mAh / g, the first-efficiency of the assembled soft-pack cell is 85%-88.5%, the capacity retention rate after 800 cycles is 82.5%-88%, and the 3C capacity retention rate (vs 0.2C) is 86%-94%; taking Example 1 as an example, Figure 2 As shown, it can stably release a capacity of 358.5 mAh / g. The dQ / dV curve has a clear main peak near 0.07V. Combined with the differential capacity-area percentage within a specific voltage range, it can be seen that the obtained graphite material has high purity, stable layered structure, few surface defects, and few side reactions. Figure 1 As shown: The screening method shown in Example 1 is stable and reliable, with good screening effect. The resulting soft-pack battery cell has good cycle performance, and the capacity retention rate is as high as 88% after 800 cycles. However, when the first efficiency of graphite material, the difference between the first efficiency of graphite material and the first efficiency of silicon-carbon composite particles does not meet the requirements, the specific capacity of the negative electrode drops to 173-175 mAh / g, the first efficiency drops to 78.2%-83%, the capacity retention rate drops to 72%-78% after 800 cycles, and the 0.3C capacity retention rate (vs 0.2C) drops to 75%-80%. The energy density, first efficiency, cycle performance and rate performance are all significantly degraded. Moreover, from the percentage results of the differential capacity area, it can be seen that the crystal structure order and lithium intercalation behavior of the obtained graphite material at this time are far from meeting the screening conditions (3). To put it another way, a comparison of the test results of Example 1 with those of Comparative Examples 6, 7 and 10 shows that when the graphite material does not meet the screening condition (1), even if it meets the requirements of screening condition (3) and the voltage platform characteristics meet the requirements in some cases, the overall electrochemical performance of the negative electrode formed by mixing it with silicon-carbon composite particles is not good.

[0082] Obviously, condition (1) helps to achieve the matching of the electrochemical window of graphite materials and silicon-carbon composite particles, and is the primary condition for graphite material screening.

[0083] (2) A comparison of the test results of Examples 1, 7, 13 and Examples 8-12 shows that when the particle size of the graphite material is smaller than that of the silicon-carbon composite particles, it can play a fine filling or conductive role after filling. This is achieved when the graphite material satisfies 0.6≤D50-G / D50-SC≤0.8, 0.4≤BET-G / BET-SC≤1.4 and 1.2m 2 / g≤BET-G≤2m 2 / g helps to achieve fine filling and high density filling, thereby improving energy density; while when the graphite material satisfies: 0.4≤D50-G / D50-SC≤0.6, 0.6≤BET-G / BET-SC≤1.4 and 2m 2 / g≤BET-G≤2.8m 2 At / g, it helps to strengthen the conductive network and interface contact, and improve rate performance.

[0084] Comparing the test results of Examples 1, 13-17 with Comparative Examples 7, 8 and 9, it can be seen that when the particle size of the graphite material is larger than that of the silicon-carbon composite particles, and it plays a skeleton role after filling, its electrochemical performance is better when its structural parameters meet condition (2), and its long cycle life and rate performance are particularly excellent. It can be seen that the conductive skeleton constructed by the graphite material obtained at this time is more stable and can more effectively buffer the volume expansion of silicon-carbon particles and maintain the integrity of the electrode structure.

[0085] (3) A comparison of the test results of Examples 1, 6 and Comparative Example 4 shows that, based on the graphite materials meeting screening conditions (1) and (2), the graphite materials meeting conditions i, ii and iii in screening condition (3) exhibit significantly better electrochemical performance than those not meeting these conditions. It is evident that screening graphite materials based on their lithium intercalation characteristics, purity, and structural stability using conditions i, ii and iii to obtain graphite materials with fewer surface defects and side reactions significantly improves the electrochemical performance of lithium batteries. This conclusion is also evident from the test results of Comparative Example 5.

[0086] Furthermore, a comparison of the test results of Example 1 and Example 5 shows that: based on setting screening conditions (1), (2), i, ii, and iii, setting screening condition iv: FWHM≤35mV helps to screen for higher quality graphite materials; by fabricating soft-pack cells and testing cell performance, it is confirmed that setting condition iv helps to pursue more extreme electrochemical performance.

[0087] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions, and variations to the above embodiments within the scope of the present invention. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples.

Claims

1. A method for screening graphite materials for silicon-carbon composite anode systems, characterized in that: The following steps are included: S1. Test the initial coulombic efficiency, median particle size, and specific surface area of ​​the silicon-carbon composite particles to be matched, and represent the measured initial coulombic efficiency, median particle size, and specific surface area as ICE-SC, D50-SC, and BET-SC respectively. S2. Provide a variety of graphite materials and test their initial coulombic efficiency, median particle size, and specific surface area. The measured initial coulombic efficiency, median particle size, and specific surface area are respectively denoted as ICE-G, D50-G, and BET-G. Screen out the graphite materials that meet conditions (1) to (3): (1) ICE-G≥94% and (ICE-G)-(ICE-SC)≥2%-5%; (2) 0.3≤D50-G / D50-SC≤2, 0.2≤BET-G / BET-SC≤1.4; (3) When conducting half-cell charge-discharge tests on graphite materials, the discharge curves are processed using differential capacity dQ / dV. Graphite materials whose dQ / dV curves meet the following requirements are selected: i. A distinct main peak appears in the 0.02V-0.15V voltage range, and the percentage of the main peak area to the total differential capacity area in the 0V-0.3V voltage range is >65%; ii. The percentage of the differential capacity area in the 0.02V-0.15V voltage range to the differential capacity area in the 0.15V-0.25V voltage range is >400%; iii. The percentage of the differential capacity area in the 0.25V-0.5V voltage range to the total differential capacity area in the 0V-0.5V voltage range is <5%. S3. The graphite material obtained in step S2 is uniformly mixed with the silicon-carbon composite particles to be matched according to a preset mass ratio and the negative electrode sheet is prepared and its electrochemical performance is verified.

2. The method for screening graphite materials for silicon-carbon composite anode systems according to claim 1, characterized in that: In condition (2), 0.3≤D50-G / D50-SC≤1.

3. The method for screening graphite materials for silicon-carbon composite anode systems according to claim 2, characterized in that: In condition (2), 0.6 ≤ D50-G / D50-SC ≤ 0.8, 0.4 ≤ BET-G / BET-SC ≤ 1.4, and 1.2m 2 / g≤BET-G≤2m 2 / g.

4. The method for screening graphite materials for silicon-carbon composite anode systems according to claim 2, characterized in that: In condition (2), 0.4 ≤ D50-G / D50-SC ≤ 0.6, 0.6 ≤ BET-G / BET-SC ≤ 1.4 and 2m 2 / g≤BET-G≤2.8m 2 / g.

5. The method for screening graphite materials for silicon-carbon composite anode systems according to claim 1, characterized in that: In condition (2), 1≤D50-G / D50-SC≤2.

6. The method for screening graphite materials for silicon-carbon composite anode systems according to claim 5, characterized in that: In condition (2), 1.3≤D50-G / D50-SC≤1.8, 0.2≤BET-G / BET-SC≤1 and 1m 2 / g≤BET-G≤2m 2 / g.

7. The method for screening graphite materials for silicon-carbon composite anode systems according to claim 1, characterized in that: In condition (3), the graphite material also satisfies iv: the full width at half maximum (FWHM) of the main peak is ≤35mV.

8. A silicon-carbon composite negative electrode, comprising a negative electrode current collector and a negative electrode active material disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material comprises silicon-carbon composite particles and graphite material obtained by the screening method according to any one of claims 1-7.

9. The silicon-carbon composite negative electrode according to claim 8, characterized in that: The silicon-carbon composite anode exhibits the following performance in soft-pack full-cell testing: initial coulombic efficiency ≥86%, and capacity retention ≥80% after 1C rate and 800 cycles.

10. A secondary battery comprising a silicon-carbon composite negative electrode as described in claim 8 or 9.

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