Negative plate and battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a battery. The negative plate comprises a negative active material, the negative active material comprises a silicon-carbon composite material with an inner core and a shell, the inner core comprises a porous carbon matrix and silicon particles distributed in inner pore channels of the porous carbon matrix, and the first carbon nanotubes are distributed in the inner core; the cross section of the inner core is provided with a central area, a transition area and a surface layer area; the area proportions of the orthographic projection areas of the first carbon nanotubes in the surface layer area, the transition area and the central area are S1, S2 and S3 respectively, S1 < S2 < S3, and 0.1 < = S1 / S3 < = 0.9. The battery provided by the invention has good cycling stability and rate capability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a negative electrode and a battery. Background Technology

[0002] In recent years, lithium-ion batteries have been considered the most promising energy storage devices due to their high operating voltage, high energy density, long cycle life, and good safety performance. With industrial transformation and social development, higher demands are being placed on lithium-ion batteries in terms of energy density, cycle life, and safety performance. However, the low theoretical capacity of traditional commercial graphite anodes (372 mAh / g) limits the improvement of battery energy density. While silicon-based anode materials have an ultra-high theoretical capacity (4200 mAh / g) and have become a highly promising anode material for next-generation lithium-ion batteries, their high capacity is often accompanied by significant volume expansion and internal stress. This leads to the destruction of the anode active material particle structure, loss of conductive structure, and repeated formation and destruction of the solid electrolyte interface film, affecting the battery's cycle stability and rate performance. Therefore, providing a silicon-based anode system for lithium-ion batteries that combines high energy density, cycle stability, and rate performance is of great significance for promoting the commercialization of silicon-based anodes. Summary of the Invention

[0003] During battery cycling, silicon-carbon composite materials undergo significant volume expansion and generate expansion stress, which disrupts the structural integrity and stability of the material. This results in discontinuous and incomplete conductive networks in the negative electrode. The broken silicon-carbon composite material exposes fresh silicon-carbon composite material surfaces that come into contact with the electrolyte and react, causing repeated rupture and growth of the SEI film on the surface of the silicon-carbon composite material, leading to an increase in the SEI film thickness. In addition, the porous carbon matrix in the core and the silicon particles within it have poor conductivity, which affects the insertion and extraction of lithium ions. As a result, during the battery insertion and extraction process, the migration rate of lithium ions inside the silicon-carbon composite material is low, which is detrimental to the improvement of battery cycle stability and rate performance.

[0004] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a negative electrode and a battery including the negative electrode. The negative electrode of this invention employs a silicon-carbon composite material with a core-shell structure. The core comprises a porous carbon matrix and silicon particles located in the pores within the porous carbon matrix. The cross-section of the core of this silicon-carbon composite material has a central region, a transition region, and a surface region. The geometric shape formed by the cross-section of the core is equidistantly scaled to a point O. The line segment between point O and any point F on the edge of the cross-section of the core has a dimension of 0.5L. On this line segment, the region 0-0.1L from the edge of the cross-section constitutes the surface region, the region greater than 0.1L and less than or equal to 0.3L constitutes the transition region, and the region greater than 0.3L and less than or equal to 0.5L constitutes the central region. The area ratios of the projected area of ​​the first carbon nanotube in the surface region, transition region, and central region are S1, S2, and S3, respectively, satisfying: S1 < S2 < S3, and 0.1 ≤ S1 / S3 ≤ 0.9. In this silicon-carbon composite material, the content of the first carbon nanotube increases from the surface area towards the center, which improves the conductivity of the silicon-carbon composite material while also taking into account its strength. In particular, it improves the conductivity and strength of the silicon particles inside the silicon-carbon composite material, thereby improving the cycle stability and rate performance of the battery.

[0005] Carbon nanotubes possess excellent mechanical strength and electrical conductivity. Introducing carbon nanotubes into the core of silicon-carbon composite materials can form a three-dimensional conductive network within the core, providing more conductive pathways for ion / electron transport, improving the ion / electron conduction performance of the silicon-carbon composite material, and enhancing the rate performance of the battery. Introducing carbon nanotubes into the core of silicon-carbon composite materials can also enhance the mechanical strength of the porous carbon matrix, improve the ability of the porous carbon matrix framework to withstand the volume expansion stress of the silicon-carbon composite material, and increase its resistance to rolling during the electrode rolling process. This prevents the silicon-carbon composite material particles from cracking or even breaking during rolling, which would lead to incomplete capacity utilization of the silicon-carbon composite material and a decrease in the cycle stability of the battery. During lithium intercalation, the lithium ion intercalation rate and resistance differ within the silicon-carbon composite core. Because the surface region of the core is closest to the electrolyte, lithium ions migrate a short distance and have a high deintercalation rate, enabling rapid deintercalation. Compared to the surface region, the transition region has an increased ion diffusion path, further reducing the deintercalation rate. In the central region of the core, the increased channel tortuosity further increases the deintercalation path. The surface region's earlier deintercalation exerts a squeezing effect on the central region or causes channel narrowing and blockage, further reducing the deintercalation rate. Furthermore, the insufficient conductivity of the silicon particles and porous carbon matrix in the silicon-carbon composite core leads to decreased kinetics during deintercalation and intercalation, reducing lithium ion migration efficiency and hindering improvements in battery rate and cycle performance.Based on this, this application introduces a first carbon nanotube into the core of a silicon-carbon composite material. The first carbon nanotube is a three-dimensional material with good conductivity and rigidity. It can form a network structure between the pores of the porous carbon matrix in the core and the silicon particles within those pores. This not only improves the overall conductivity of the silicon-carbon composite material but also acts as a framework. During lithium-ion insertion / extraction in the core, some of the resulting expansion and contraction stress is dispersed onto the first carbon nanotube, reducing the probability of the silicon-carbon composite material breaking during battery cycling and improving battery cycle performance. Furthermore, this application designs a gradient distribution of the first carbon nanotube in the core, resulting in an increasing distribution from the core surface towards the center of the silicon-carbon composite material core. Near the core center, the larger the contact area between the first carbon nanotube and the center, the better the conductivity of the silicon-carbon composite core can be, based on the differences in lithium insertion / extraction capabilities in different regions of the core. This enhances the conductivity of electrons and ions in different regions, ensuring that lithium ions can be smoothly and efficiently extracted / inserted within the silicon-carbon composite, thereby improving the cycle life and rate performance of the battery. In addition, this gradient distribution design can also reduce the overall content of the first carbon nanotube in the silicon-carbon composite core, avoiding excessive content of the first carbon nanotube in the core, which could lead to excessive compression of silicon particles in the pores of the porous carbon matrix or the formation of a large number of pores in the porous carbon matrix, reducing the strength of the porous carbon matrix, reducing its binding force on silicon particles, and thus reducing the cycle life and rate performance of the battery.

[0006] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0007] Figure 1 The figure shown is a schematic cross-sectional view of the core structure of the silicon-carbon composite material in one embodiment of the present invention.

[0008] Figure 2 The figure shown is a schematic diagram of the cross-sectional structure of the transition zone in one embodiment of the present invention.

[0009] Attached label: 1 is the central area, 2 is the transition area, 3 is the surface area, 21 is region D1, 22 is region D (x-1) Region 23 is D x area. Detailed Implementation

[0010] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0011] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode active layer, the negative electrode active layer comprising a negative electrode active material, the negative electrode active material comprising a silicon-carbon composite material, the silicon-carbon composite material having a core and a shell, the core comprising a porous carbon matrix and silicon particles distributed in the internal channels of the porous carbon matrix, the silicon-carbon composite material further comprising a first carbon nanotube, the first carbon nanotube being distributed in the core.

[0012] In this invention, the cross-section of the core has a central region, a transition region, and a surface region. The geometric shape formed by the cross-section of the core is equidistantly scaled to a point O. The line segment between point O and any point F on the edge of the cross-section of the core has a dimension of 0.5L. On this line segment, the region 0-0.1L from the edge of the cross-section constitutes the surface region, the region greater than 0.1L and less than or equal to 0.3L constitutes the transition region, and the region greater than 0.3L and less than or equal to 0.5L constitutes the central region. The projected area of ​​the first carbon nanotube is distributed as S1, S2, and S3 in the surface region, the transition region, and the central region, respectively, satisfying: S1 < S2 < S3, and 0.1 ≤ S1 / S3 ≤ 0.9, for example, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9.

[0013] In one embodiment, 0.3 ≤ S1 / S3 ≤ 0.55.

[0014] In this invention, 2%≤S1≤10%, for example, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.5%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0015] In this invention, 10% < S3 ≤ 20%, for example, 10%, 10.2%, 10.4%, 10.6%, 10.8%, 11%, 11.5%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%.

[0016] like Figure 1 The diagram shows a cross-sectional structure of the core of a silicon-carbon composite material in one embodiment of the present invention, where 1 is the central region, 2 is the transition region, and 3 is the surface region. It can be understood that the cross-section of the core can be any geometric shape, and the diagram uses a circular cross-section as an example.

[0017] Carbon nanotubes possess excellent electrical conductivity and mechanical strength. Introducing carbon nanotubes into the core of silicon-carbon composites can form a three-dimensional conductive network within the core, thereby enhancing the electron and ion conduction properties within the particles. Furthermore, introducing carbon nanotubes into the core of silicon-carbon composites can strengthen the mechanical strength of the porous carbon matrix, thus improving the porous carbon matrix's ability to withstand volume expansion stresses and its resistance to rolling during the rolling process. This prevents the silicon-carbon composite from cracking, which could lead to incomplete capacity utilization, and improves the structural stability of the silicon-carbon composite, ultimately enhancing the battery's cycle stability and rate performance. However, if the carbon nanotube content in the silicon-carbon composite material is too high (e.g., S1 > 10% and / or S3 > 20%), the coverage area of ​​carbon nanotubes in the core of the silicon-carbon composite material will increase. On the one hand, this will reduce the strength of the porous carbon matrix. This is because the excessive introduction of carbon nanotubes will cause entanglement and aggregation between the carbon nanotubes due to van der Waals forces. These agglomerates have weak physical bonding with the carbon walls of the porous carbon matrix. Under the stress generated by high-temperature carbonization or subsequent silicon deposition, local debonding will occur between the agglomerates and the porous carbon matrix interface, resulting in the formation of larger pores inside the porous carbon matrix. This reduces the mechanical strength of the porous carbon matrix framework, decreases its resistance to rolling pressure and its ability to withstand expansion stress, leading to a deterioration in the cycle performance of the battery. On the other hand, if the aforementioned agglomerates and the porous carbon matrix have a high degree of entanglement and aggregation due to van der Waals forces, the composite material will become increasingly porous. If local debonding does not occur between the porous carbon matrix interfaces, the excessive introduction of carbon nanotubes may block the internal pores of the porous carbon matrix and form an excessively strong interfacial bond with the porous carbon matrix. As a result, when the silicon-carbon composite material undergoes volume expansion, the porous carbon matrix cannot disperse the volume expansion stress in time, and instead, it will undergo sudden brittle fracture, reducing the strength of the silicon-carbon composite material. On the other hand, if the content of carbon nanotubes is too low (e.g., S1 < 2% and / or S3 < 10%), the carbon nanotubes cannot form a continuous conductive path, which is not conducive to improving the electron / ion conduction performance of the silicon-carbon composite material. Moreover, too few carbon nanotubes have little effect on improving the mechanical strength of the porous carbon matrix, and are also not conducive to improving the structural stability of the silicon-carbon composite material, thus leading to a decrease in the rate performance and cycle stability of the battery.

[0018] Furthermore, in silicon-carbon composites, lithium ions migrate a longer distance closer to the core center and a shorter distance closer to the core surface. Considering the consistency of lithium ion migration kinetics, setting the distribution of carbon nanotubes in the silicon-carbon composite core to decrease from the center to the surface can improve the problem of large differences in lithium ion migration rates in the core, transition region, and surface region of the silicon-carbon composite core. It can also improve the effective utilization rate of carbon nanotubes. The higher carbon nanotube content in the core region increases the migration rate of lithium ions inside the silicon-carbon composite, avoiding insufficient internal kinetics. The lower carbon nanotube content in the surface region can meet the migration kinetics requirements of the surface region while reducing the overall carbon nanotube content in the silicon-carbon composite. This avoids the decrease in the strength of the porous carbon matrix due to excessive carbon nanotubes and the reduction in the subsequent deposition of silicon particles due to excessive carbon nanotubes inside the silicon-carbon composite, thus reducing the capacity of the silicon-carbon composite. When S1 / S3 is too small (e.g., <0.1), S1 is too small and / or S3 is too large. When S1 is too small, there are too few carbon nanotubes in the surface region, resulting in insufficient lithium-ion migration kinetics. The efficiency of lithium-ion migration into the particle interior decreases, which is not conducive to improving the battery's kinetic performance and leads to a decrease in the battery's rate performance. At the same time, when S3 is too large, the excessive carbon nanotubes in the central region will lead to a decrease in the internal strength of the porous carbon matrix, which is not conducive to improving the battery's cycle stability. When S1 / S3 is too large (e.g., >0.9), S1 is too large and / or S3 is too small. The kinetics in the central region are insufficient, while the carbon nanotubes in the surface region are redundant. The overall kinetic performance of the battery decreases, affecting the battery's rate performance and cycle performance.

[0019] In this invention, S1, S2, and S3 can be obtained using conventional testing methods in the art, such as transmission electron microscopy (TEM). Specifically, silicon-carbon composite material particles are cut into concentric thin slices with a thickness of approximately 100 nm using a polyion beam. A slice near the center of the particle or its maximum diameter is selected for TEM testing. The sample is placed on the TEM stage, and the distribution of the first carbon nanotubes inside the particle (i.e., the core) is observed and images are acquired in high-resolution TEM mode. The maximum diameter of the cross-section is L. Within the range of 0-0.1 L, a cross-sectional area of ​​500 nm is randomly selected. 2 Three test areas were selected. The first carbon nanotubes exhibited distinct TEM lattice fringes, while the porous carbon matrix and silicon particles were amorphous, allowing direct calculation of the area percentage of the first carbon nanotubes. The average area percentage of the first carbon nanotubes in all three test areas was S1. Similarly, cross-sectional areas of 500 nm were randomly selected within a range greater than 0.1 L and less than or equal to 0.3 L. 2 Three test areas were used to obtain S2 using the method described above. A cross-sectional area of ​​500 nm was randomly selected within the range of greater than 0.3 L and less than or equal to 0.5 L. 2The test area consists of three blocks, and S3 is obtained by following the above method.

[0020] In this invention, the outer shell includes an amorphous carbon layer that covers at least a portion of the outer surface of the core. The outer shell also includes a second carbon nanotube located on at least a portion of the surface of the amorphous carbon layer.

[0021] It is understood that when the amorphous carbon completely covers the outer surface of the core, the second carbon nanotube is located on at least a portion of the outer surface of the amorphous carbon; when the amorphous carbon partially covers the outer surface of the core, a portion of the second carbon nanotube is located on the outer surface of the amorphous carbon, and another portion of the second carbon nanotube is located on the outer surface of the core.

[0022] In this invention, "the amorphous carbon layer at least covers a portion of the outer surface of the core" means that the area of ​​the orthographic projection of the amorphous carbon layer on the outer surface of the core can be equal to 100% or less than 100%; "the second carbon nanotube is located on at least a portion of the surface of the amorphous carbon layer" means that the area of ​​the orthographic projection of the second carbon nanotube on the surface of the amorphous carbon layer can be equal to 100% or less than 100%.

[0023] In this invention, the diameter of the second carbon nanotube is larger than the diameter of the first carbon nanotube.

[0024] It is understood that the “tube diameter” refers to the outer diameter of the first carbon nanotube and the second carbon nanotube.

[0025] In this invention, the mass content of the second carbon nanotube in the silicon-carbon composite material is 0.1%-2%, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%.

[0026] An outer shell is formed on the outer surface of the core. The amorphous carbon layer in the shell reduces the direct contact between the silicon particles in the core and the electrolyte. This significantly reduces the amount of new surface exposed by the silicon particles during cycling due to expansion, thus minimizing large-area contact with the electrolyte and preventing side reactions. This improves the structural stability of the negative electrode active material and reduces excessive thickening of the SEI film on the surface of the silicon-carbon composite material. Simultaneously, the amorphous carbon layer also inhibits the expansion of the silicon-carbon composite material, preventing stress concentration-induced failure and improving the cycle stability of the battery. Furthermore, since silicon is a semiconductor, the conductivity is significantly reduced after the porous carbon matrix is ​​combined with silicon particles. The outer shell containing the amorphous carbon layer and second carbon nanotubes helps improve the overall conductivity of the silicon-carbon composite material, thereby enhancing the rate performance of the battery and enabling rapid lithium-ion transfer between the inside and outside of the silicon-carbon composite material.

[0027] Furthermore, by adjusting the diameters of the first and second carbon nanotubes, the smaller diameter of the first carbon nanotubes distributed in the core facilitates the formation of a dense conductive network in the porous carbon matrix, providing a fast electron channel for silicon particles and improving the overall conductivity of the silicon-carbon composite material, which is beneficial for further improving the rate performance of the battery. The larger diameter of the second carbon nanotubes located on the surface of the core enhances the rigidity of the surface second carbon nanotubes, further enhancing the overall mechanical strength of the silicon-carbon composite material to resist the expansion stress of the silicon-carbon composite material during battery cycling, which is beneficial for further improving the cycle stability of the battery.

[0028] In this invention, the diameters of the first and second carbon nanotubes can be obtained by conventional testing methods in the art, such as by transmission electron microscopy (TEM). Specifically, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours, followed by rinsing with DMC solvent to remove the lithium salts adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active layer can then be peeled off from the negative electrode current collector. The negative electrode active layer is collected as a test sample. After sample preparation, it is observed using transmission electron microscopy (TEM). The diameters (outer diameters) of the carbon nanotubes with clear morphology are measured. Ten different carbon nanotubes are selected and measured separately, and the average value is taken.

[0029] In this invention, the transition region comprises x regions, which are sequentially named D1 region, D2 region, ... D1 region from the center outwards towards the surface. (x-1) Region and D x The region, 1≤x≤5, for example, is 1, 2, 3, 4 or 5, referred to as region D1, region D2...D (x-1) Region and D xThe thickness of the region is selected from 0.02L to 0.2L, for example, 0.02L, 0.04L, 0.06L, 0.1L, 0.15L, or 0.2L. It is understood that the thickness has a conventional meaning in the art; when x=1, the thickness is the thickness of the transition zone, i.e., the shortest distance between the boundary lines of the transition zone and the central / surface zone; when x>1, the thickness is the shortest distance between the boundary lines of this region and two adjacent regions.

[0030] In this invention, the thickness of the D1 region is 0.02L-0.2L.

[0031] In this invention, the thickness of the D2 region is 0.02L-0.2L.

[0032] In this invention, the D (x-1) The thickness of the region is 0.02L-0.2L.

[0033] In this invention, the D x The thickness of the region is 0.02L-0.2L.

[0034] It is understandable that the D1 region, D2 region...D (x-1) Region and D x The sum of the thicknesses of the regions is 0.2L. When x=1, the thickness of region D1 is 0.2L. When x>1, the thickness of each region is in the range of 0.02L-0.2L, and can be the same or different.

[0035] In this invention, the area of ​​the first carbon nanotube near the D1 region in the transition region is S4, and the area near the D1 region is S4. x The area of ​​the first carbon nanotube in the region that is projected onto the transition zone is S5, and S4≥S5.

[0036] When x=1, the transition region contains one region, S4=S5.

[0037] like Figure 2 The diagram shown is a cross-sectional view of the transition zone in one embodiment of the present invention, where 21 is region D1 and 22 is region D. (x-1) Region 23 is D x area.

[0038] In this invention, the test methods for S4 and S5 can be performed with reference to the test methods for S1, S2 and S3, using TEM.

[0039] In this invention, the silicon-carbon composite material has a silicon content of 30%-70% by mass, for example, 30%, 32%, 34%, 36%, 38%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%; and a carbon content of 30%-70% by mass, for example, 30%, 32%, 34%, 36%, 38%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%.

[0040] In this invention, the silicon content, based on the total mass of the negative electrode active layer, is 1.5%-50%. For example, it is 1.5%, 2%, 6%, 10%, 12%, 15%, 20%, 25%, 30%, 32%, 34%, 36%, 38%, 40%, 45%, or 50%. When the mass content of silicon in the silicon-carbon composite material satisfies the above relationship, the silicon-carbon composite material has high specific capacity, high initial coulombic efficiency, and low expansion rate. The appropriate silicon content can ensure that the silicon-carbon composite material has high specific capacity and ensure the energy density of the battery. When the mass content of silicon is too high (e.g., >70%), the silicon content in the silicon-carbon composite material is too high, and the volume expansion of silicon particles during battery cycling is more significant. Although carbon nanotubes are introduced into the porous carbon matrix, they are still insufficient to buffer the volume change caused by high silicon content, which easily leads to the breakage of the silicon-carbon composite material. This results in repeated growth of the surface SEI film, which continuously consumes electrolyte and active lithium. This is not only detrimental to the improvement of the initial coulombic efficiency of the battery, but also leads to a decrease in the battery cycle stability. When the mass content of silicon is too low (e.g., <30%), the silicon content in the silicon-carbon composite material is too low to meet the high capacity requirements and contributes little to the improvement of battery energy density.

[0041] In this invention, the mass content of silicon and carbon elements in the silicon-carbon composite material can be obtained by conventional testing methods in the art, such as the following method: using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS); disassembling the battery after discharging it to 0% SOC, removing the negative electrode, and processing it with argon ion polishing technology to obtain a cross-sectional sample of the negative electrode; then testing the obtained cross-section using the backscatter mode of scanning electron microscopy at a magnification of 10K; selecting the middle region of a single silicon-carbon composite material particle for EDS analysis, testing only the content of carbon and silicon elements; and then taking a weighted average of the measurements of 20 silicon-carbon composite material particles to obtain the mass content of silicon and carbon elements in the silicon-carbon composite material.

[0042] In this invention, the mass content of silicon in the total mass of the negative electrode active layer can be determined by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and soaked in dimethyl carbonate (DMC) solvent for 12 hours, then rinsed with DMC solvent to remove lithium salts adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can then be peeled off from the negative electrode current collector, and the negative electrode active coating is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes. This allows the non-silicon components in the negative electrode active coating to volatilize while the silicon is fully oxidized to silicon dioxide. The remaining substance is the ash content of the negative electrode active coating. The mass content of silicon in the negative electrode active coating can be calculated based on the mass of ash. The calculation formula is as follows: Based on the total mass of the negative electrode active material layer, the mass percentage of silicon = 7 × mass of ash / (15 × mass of test sample).

[0043] In this invention, the median particle size Dv50 of the silicon-carbon composite material is 5μm-15μm, for example, 5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm.

[0044] In one embodiment, the median particle size Dv50 of the silicon-carbon composite material is 7 μm-10 μm.

[0045] In this invention, the particle size Dv10, median particle size Dv50, and particle size Dv90 of the silicon-carbon composite material satisfy the following: (Dv90-Dv10) / Dv50 is 0.6-2, for example, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.

[0046] It is understood that the volume distribution particle sizes Dv10, Dv50, and Dv90 have conventional meanings in the art, referring to the following: when the particles are arranged from smallest to largest, the particle size corresponding to 10% of the total volume is Dv10, the particle size corresponding to 50% of the total volume is Dv50, and the particle size corresponding to 90% of the total volume is Dv90; at this time, the particle size distribution = (Dv90 - Dv10) / Dv50.

[0047] When the particle size distribution of the silicon-carbon composite material meets the above conditions, it is beneficial to improve the cycle stability and rate performance of the battery. When the median particle size Dv50 of the silicon-carbon composite material is too small (e.g., <5μm), the particles have a large specific surface area, increasing the contact area with the electrolyte. This intensifies side reactions between the particles and the electrolyte and causes more loss of active lithium, which is detrimental to the improvement of the battery's initial coulombic efficiency and cycle performance. When the median particle size Dv50 of the silicon-carbon composite material is too large (e.g., >12μm), the diffusion path of lithium ions within the silicon-carbon composite material is longer, resulting in poor kinetic performance of the negative electrode active material, which is detrimental to further improvement of the battery's rate performance. Furthermore, adjusting the (Dv90-Dv10) / Dv50 of the silicon-carbon composite material within a suitable range can avoid the existence of extremely large or extremely small particles in the silicon-carbon composite material while ensuring that the processing difficulty of the silicon-carbon composite material is not too high. This improves the consistency of the chemical reaction of the silicon-carbon composite material and the consistency of lithium ion insertion / extraction and transport within the battery, thereby further improving the overall cycle performance and rate performance of the battery.

[0048] In this invention, the particle size Dv10, median particle size Dv50, and particle size Dv90 of the silicon-carbon composite material can be obtained by conventional testing methods in the art, such as measuring with a Malvern particle size analyzer. The testing steps are as follows: disperse the silicon-carbon composite material in deionized water containing a dispersant (such as isopropanol, content 0.02wt%-0.03wt%) to form a mixture, sonicate the mixture for 2 minutes, and then place it in a Malvern particle size analyzer for testing.

[0049] In this invention, the specific surface area of ​​the silicon-carbon composite material is 0.01 m². 2 / g-8m 2 / g, for example, 0.01m 2 / g, 0.05m 2 / g, 0.1m 2 / g, 0.2m 2 / g, 0.4m 2 / g, 0.6m 2 / g, 0.8m 2 / g, 1m 2 / g、2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g or 8m 2 / g.

[0050] In one embodiment, the specific surface area of ​​the silicon-carbon composite material is 0.05 m². 2 / g-6m2 / g.

[0051] When the specific surface area of ​​the silicon-carbon composite material is within a suitable range, it enables the negative electrode active material to have a high specific capacity while effectively reducing the contact between the negative electrode active material and the electrolyte, reducing the repeated formation of the SEI film on the surface of the silicon-carbon composite material, and lowering the risk of an excessively thick SEI film, thereby improving the battery's initial efficiency and capacity retention. However, when the specific surface area of ​​the silicon-carbon composite material is too low (e.g., <0.01m²), it is detrimental. 2 / g), the effective contact area between the silicon-carbon composite material and the electrolyte decreases, resulting in poor electrolyte wetting. This leads to increased resistance to lithium-ion transport within the silicon-carbon composite material, which is detrimental to improving the dynamic performance of the negative electrode and thus affects the cycle and efficiency performance of the battery. When the specific surface area of ​​the silicon-carbon composite material is too high (e.g., >8m²), 2 / g), the increased contact area between the silicon-carbon composite material and the electrolyte leads to intensified side reactions between them, continuously depleting active lithium. The excessive growth of the SEI film on the surface of the silicon-carbon composite material increases the SEI thickness on the surface of the negative electrode, resulting in increased battery internal resistance, reduced lithium-ion transport rate, and deterioration of battery cycle performance and rate performance.

[0052] In this invention, the specific surface area of ​​the silicon-carbon composite material can be determined by conventional testing methods in the art, such as the Brunauer-Emmett-Teller (BET) test method, or by using a TriStar II specific surface area analyzer, with N2 being the adsorbed gas.

[0053] In this invention, the sphericity of the silicon-carbon composite material is 0.6-1, for example, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 or 1.

[0054] In one embodiment, the sphericity of the silicon-carbon composite material is 0.8-0.98.

[0055] When the sphericity of the silicon-carbon composite material is within the above-mentioned range, the silicon-carbon composite material particles have fewer sharp edges and a smooth appearance, resulting in higher compressive strength. This avoids excessive pressure concentration that could cause particle breakage and reduces the probability of cracks or even breakage of the silicon-carbon composite material during the rolling process of the negative electrode sheet. In this case, the silicon-carbon composite material can withstand a higher compaction density, thereby improving the energy density of the battery. When the sphericity of the silicon-carbon composite material is too low (e.g., <0.6), the surface of the silicon-carbon composite material has many sharp edges, resulting in uneven distribution of internal expansion stress. When the surface is subjected to rolling stress, stress concentration is likely to occur, leading to the cracking and pulverization of the silicon-carbon composite material. The surface SEI film continuously breaks down and regenerates, exacerbating the consumption of electrolyte and active lithium, which is detrimental to further improvement of battery rate performance and initial coulombic efficiency.

[0056] In this invention, the sphericity of the silicon-carbon composite material can be obtained by conventional testing methods in the art, such as the following method: discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet, or directly remove the negative electrode sheet, polish its cross-section with an argon ion mill, and image the obtained cross-section using a scanning electron microscope (SEM) in backscatter imaging mode; analyze the image of each silicon-carbon composite material particle in the SEM image (backscatter mode) of the negative electrode active layer at a certain magnification (e.g., 2500x) using image processing software such as Image Pro Plus, obtain the perimeter and area of ​​each particle, calculate the perimeter equivalent radius r1 and area equivalent radius r2 of each silicon-carbon particle, then the sphericity S of each silicon-carbon composite material particle is S=r2 / r1, and then the sphericity of each particle is weighted averaged to obtain the sphericity of the silicon-carbon composite material.

[0057] In this invention, the negative electrode active material further includes a carbon-based material, which includes artificial graphite and / or natural graphite.

[0058] In one embodiment, the carbon-based material includes artificial graphite.

[0059] In one embodiment, the carbon-based material includes natural graphite.

[0060] In yet another embodiment, the carbon-based material includes artificial graphite and natural graphite.

[0061] In this invention, the particle size Dv10 of the carbon-based material is 2μm-7μm, for example, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, or 7μm; the median particle size Dv50 of the carbon-based material is 5μm-14μm, for example, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 8μm, or 9μm. The particle size Dv90 of the carbon-based material is 11μm-25μm, for example, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm or 25μm.

[0062] When the particle sizes Dv10, Dv50, and Dv90 of the carbon-based material are within the aforementioned ranges, the carbon-based material exhibits good fast-charging performance, which is beneficial for further improving the battery's energy density and achieving superior rate performance. Using fast-charging graphite combined with silicon-carbon composite materials incorporating carbon nanotubes in both the internal and external shells allows lithium ions to migrate efficiently within both the silicon-carbon composite and the carbon-based material. This facilitates the combined performance of the two negative electrode active materials, enhancing the overall kinetic performance of the negative electrode. Furthermore, when the particle sizes Dv10, Dv50, and Dv90 of the carbon-based material are within the aforementioned ranges, the active material particles within the entire negative electrode can form a more compact packing, further increasing the contact area between the silicon-carbon composite material and the carbon-based material (with a sphericity between 0.6 and 1), which is beneficial for further improving the battery's energy density.

[0063] In this invention, the particle size Dv10, median particle size Dv50, and particle size Dv90 of the carbon-based material can be determined by referring to the testing methods for silicon-carbon composite materials.

[0064] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector.

[0065] In one embodiment, the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer, the second negative electrode active layer being located between the first negative electrode active layer and the negative electrode current collector. The first negative electrode active layer includes a first negative electrode active material, which includes hard carbon, artificial graphite, and the silicon-carbon negative electrode material. The second negative electrode active layer includes a second negative electrode active material, which includes artificial graphite and the silicon-carbon negative electrode material.

[0066] When the negative electrode uses a double-layer coating and the first negative electrode active layer contains hard carbon, the porous characteristics of hard carbon can be used to increase the liquid retention of the electrode. At the same time, the large interlayer spacing and isotropy of hard carbon can effectively shorten the lithium-ion transport path and improve the surface kinetics, thereby mitigating the risk of lithium plating at the top, bottom and corners of the negative electrode.

[0067] This invention also provides a method for preparing a silicon-carbon composite material, comprising at least the following steps: Step 1: Mix phenolic resin and anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 0; Take the first carbon nanotube, hexamethylenetetramine (HMTA), and anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 0; Solution C 1 0 and solution C 2 Mixing yields a mixture M0, which is then subjected to a first spray drying process to obtain material W0; Step 2: Mix benzyl silicone oil and sodium dodecyl sulfonate to obtain a dispersed phase. Disperse the material W0 in the dispersed phase and carry out a curing reaction to obtain particles K0, which is the central region. Step 3: Mix phenolic resin and anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 1; Take the first carbon nanotube, HMTA and anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 1; Particle K0, solution C 1 1 and solution C 2 1. Mix to obtain mixture M1, perform first spray drying to obtain material W1, then repeat step 2 to obtain particles K1, thus obtaining region D1; mix phenolic resin and anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 2; Take the first carbon nanotube, HMTA and anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 2; Particle K1 and solution C 1 2 and solution C 2 2. Mix to obtain mixture M2, perform first spray drying to obtain material W2, then repeat step 2 to obtain particles K2, thus obtaining region D2; ...; Mix phenolic resin and anhydrous ethanol, stir until completely dissolved to obtain solution C. 1 x Take the first carbon nanotube, hexamethylenetetramine (HMTA), and anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 x ; to place particles K x-1 Solution C 1 x and solution C 2 x Mixing yields mixture M x The first spray drying process is performed to obtain material W.x Repeat step 2 to obtain particle K. x D x Region; Phenolic resin and anhydrous ethanol are mixed and stirred until completely dissolved to obtain solution C. 1 x+1 Take the first carbon nanotube, hexamethylenetetramine (HMTA), and anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 x+1 ; to place particles K x-1 Solution C 1 x+1 and solution C 2 x+1 Mixing yields mixture M x+1 The first spray drying process is performed to obtain material W. x+1 Repeat step 2 to obtain particle K. x+1 The surface region is obtained; among which, the region from D1 to D... x The region constitutes a transition zone, where x is an integer between 1 and 5, for example, 1, 2, 3, 4, or 5; the particles K obtained in step 3 x+1 That is, a porous carbon precursor containing first carbon nanotubes; Step 4: Place the porous carbon precursor containing the first carbon nanotubes in a tube furnace, first perform a first treatment under a nitrogen atmosphere, and then switch to a water vapor-nitrogen mixed gas for a second treatment to obtain a porous carbon matrix. Step 5: Add the porous carbon matrix to the fluidized bed, first introduce nitrogen gas and then switch to silicon source-nitrogen mixed gas for the first deposition. After the silicon source gas is exhausted, introduce carbon source-nitrogen mixed gas for the second deposition to obtain a core with an amorphous carbon layer on the surface. Mix the second carbon nanotube, anhydrous ethanol and the core and then perform a second spray drying to obtain a silicon-carbon composite material with the second carbon nanotube coated on the surface.

[0068] In this invention, in the solution C 1 0, ... and C 1 x+1 In this process, the mass ratio of the phenolic resin to anhydrous ethanol is in the range of 1:(1-10) (e.g., 1:1, 1:2, 1:4, 1:6, 1:8 or 1:10).

[0069] In this invention, solution C 2 0, ... and C 2 x+1 In the process, the mass ratio of the first carbon nanotube, HMTA and anhydrous ethanol is 1:(1-10):(8-70), for example, "1-10" can be 1, 2, 4, 6, 8 or 10, and "10-70" can be 8, 10, 15, 20, 30, 40, 50, 60 or 70.

[0070] In this invention, solution C 1 0 and C 2 0. The solution C 1 1 and C 2 1. ..., the solution C 1 x+1 and C 2 x+1 In this context, the method of mixing the two solutions is not limited; for example, the latter can be added to the former, with C... 1 0 and C 2 For example, 0 means: [The solution C is...] 2 0. Add solution C dropwise. 1 0 in.

[0071] In this invention, the outlet temperature of the first spray dryer is 60°C-80°C (e.g., 60°C, 65°C, 70°C, 75°C, or 80°C).

[0072] In this invention, the mass ratio of benzyl silicone oil to sodium dodecyl sulfonate in the dispersed phase is (100-300):1, for example, 100:1, 150:1, 200:1, 250:1 or 300:1.

[0073] In this invention, the curing reaction temperature is 80℃-150℃ (e.g., 80℃, 90℃, 100℃, 120℃, 140℃ or 150℃), and the time is 1h-5h (e.g., 1h, 2h, 3h, 4h or 5h).

[0074] In this invention, the homogenization can be performed in a manner conventional in the art, such as by ultrasonic oscillation and / or mechanical stirring.

[0075] In this invention, step 2 may further include filtration, washing, and drying after the curing reaction is completed.

[0076] In this invention, in the first process, the heating rate is 1℃ / min-10℃ / min (e.g., 1℃ / min, 4℃ / min, 6℃ / min, 8℃ / min or 10℃ / min), first heating to 400℃-800℃ (e.g., 400℃, 500℃, 600℃ or 800℃) and holding for 1h-5h (e.g., 1h, 2h, 3h, 4h or 5h), then heating to 500℃-1500℃ (e.g., 500℃, 600℃, 800℃, 1000℃, 1200℃ or 1500℃) and holding for 1h-5h (e.g., 1h, 2h, 3h, 4h or 5h).

[0077] In this invention, the volume fraction of water vapor in the water vapor-nitrogen mixed gas is 1%-20% (e.g., 1%, 2%, 5%, 10%, 15% or 20%), and the second treatment refers to keeping the mixture at a temperature of water vapor-nitrogen mixed gas for 1h-10h (e.g., 1h, 2h, 4h, 8h or 10h).

[0078] In this invention, step 4 further includes switching to nitrogen gas and cooling to room temperature after the second treatment is completed.

[0079] In this invention, the silicon source includes at least one of silane (SiH4), dichlorosilane, trichlorosilane, and tetrachlorosilane.

[0080] In this invention, the conditions for the first deposition are as follows: under a nitrogen atmosphere, the temperature is first increased to 200℃-700℃ (e.g., 200℃, 400℃, 600℃, or 700℃) at a heating rate of 5℃ / min-15℃ / min (e.g., 5℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, or 15℃ / min), and then switched to a silicon source-nitrogen mixed gas for holding for 1h-10h (e.g., 1h, 5h, or 10h), with a silicon source-nitrogen mixed gas flow rate of 10L / min-20L / min (e.g., 10L / min, 12L / min, 14L / min, 16L / min, or 20L / min).

[0081] In this invention, the volume fraction of the silicon source in the silicon-nitrogen mixed gas is 1%-50% (e.g., 1%, 10%, 20%, 30%, 40% or 50%).

[0082] In this invention, the conditions for the second deposition are as follows: first, the temperature is increased to 300℃-800℃ (e.g., 300℃, 500℃, 600℃, 700℃ or 800℃) at a heating rate of 5℃ / min-15℃ / min (e.g., 5℃ / min, 6℃ / min, 8℃ / min, 10℃ / min or 15℃ / min), then the carbon source gas is switched to hold the temperature for 1h-10h (e.g., 1h, 5h or 10h), and the flow rate of the carbon source-nitrogen mixed gas is 10L / min-20L / min (e.g., 10L / min, 12L / min, 14L / min, 16L / min or 20L / min).

[0083] In this invention, the volume fraction of the carbon source gas in the carbon source-nitrogen mixture is 1%-50% (e.g., 1%, 10%, 20%, 30%, 40% or 50%).

[0084] In this invention, the carbon source gas includes at least one of methane, ethylene, and acetylene.

[0085] In this invention, step 5 further includes switching to nitrogen gas and cooling to room temperature after the second deposition is completed.

[0086] In this invention, the mass ratio of the second carbon nanotube, anhydrous ethanol, and the core containing the amorphous carbon layer is 1:(500-1500):(50-150), for example, "500-1500" can be 500, 600, 800, 1000, 1200, or 1500. "50-150" can be 50, 60, 80, 100, 120, or 150. The mixing order of the second carbon nanotube, anhydrous ethanol, and the core is not limited; for example, the second carbon nanotube and anhydrous ethanol can be mixed first, and then the core can be added.

[0087] In this invention, the outlet temperature of the second spray dryer is 60°C-80°C (e.g., 60°C, 65°C, 70°C, 75°C, or 80°C).

[0088] A second aspect of the present invention provides a battery comprising an electrolyte comprising fluoroethylene carbonate (FEC), wherein the mass content of the fluoroethylene carbonate is A based on the total mass of the electrolyte, and the mass content of A and S1 satisfies: 0.5 ≤ A / S1 ≤ 7, for example, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6 or 7.

[0089] In one embodiment, 0.9 ≤ A / S1 ≤ 5.6.

[0090] In this invention, the range of A is 5%-25%. For example, it is 5%, 5.2%, 5.4%, 5.6%, 5.8%, 6%, 7%, 8%, 9%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, or 25%.

[0091] In one embodiment, the range of A is 8%-20%.

[0092] FEC (Fluorescent electrolyte emulsion) is used to form a structurally stable and elastic LiF-rich SEI film on the surface of silicon-carbon composite materials. This film can prevent continuous electrolyte decomposition and accommodate the volume expansion during silicon lithium intercalation, thus avoiding SEI film rupture. Due to the higher electronic conductivity of carbon nanotubes, they exhibit higher reactivity compared to other components in the silicon-carbon composite material. Variations in carbon nanotube content affect the SEI film distribution. When the ratio of the projected area S1 of the first carbon nanotube in the core-surface region of the silicon-carbon composite material (A) is within the specified range, the FEC in the electrolyte synergistically interacts with the first carbon nanotube in the core-surface region, forming a highly tough SEI film rich in inorganic compounds such as LiF on the core-surface layer of the silicon-carbon composite material. This further enhances the stability of the SEI film on the core-surface layer of the silicon-carbon composite material, thereby improving interfacial stability and reducing electrolyte consumption and active lithium loss caused by repeated SEI film growth during cycling, thus further improving the cycle stability of the battery. If A / S1 is too low (e.g., <0.5), it indicates that there is an excess of FEC in the electrolyte. It will be over-reduced in the surface area, forming an excessively thick SEI film, which will increase impedance and reduce the rate performance of the battery. At the same time, the excess FEC will also increase the probability of decomposition at high temperature to produce HF and CO2, resulting in continuous gas production at the interface in the later stages of cycling and deteriorating the cycle performance of the battery. If A / S1 is too high (e.g., >7), it indicates that the first carbon nanotube coverage area in the surface area of ​​the silicon-carbon composite core is large, that is, the first carbon nanotube network on the surface of the silicon-carbon composite core is too dense, while the content of FEC in the electrolyte is relatively small. The LiF-rich SEI film generated by FEC reduction is thin and locally missing, which will cause the SEI film to rupture and regenerate repeatedly when the battery expands, which is not conducive to further improvement of the cycle performance of the battery.

[0093] In this invention, the electrolyte further includes ethyl difluoroacetate, which includes at least one of 2,2-difluoroethyl acetate (DFEA) and ethyl 2,2-difluoroacetate.

[0094] In one embodiment, the ethyl difluoroacetate comprises 2,2-difluoroethyl acetate (DFEA).

[0095] In this invention, based on the total mass of the electrolyte, the mass content of the ethyl difluoroacetate is 5%-65%, for example, 5%, 6%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% or 65%.

[0096] In one embodiment, the mass content of the ethyl difluoroacetate is 10%-55% based on the total mass of the electrolyte.

[0097] Ethyl difluoroethyl acetate (such as DFEA) has a high reduction potential, providing an additional F source for the formation of the SEI film. This further increases the abundance and density of LiF in the SEI film, forming a more continuous and uniform LiF network, making the SEI film less prone to brittle fracture under mechanical stress. When the content of ethyl difluoroethyl acetate is too high (e.g., >65%), its dielectric constant is low, leading to a decrease in the overall ionic conductivity of the electrolyte, which is detrimental to further improvement of the battery's rate performance. When the content of ethyl difluoroethyl acetate is too low (e.g., <5%), its effect on improving the density and mechanical properties of the negative electrode SEI film is not significant.

[0098] In this invention, the contents of FEC and ethyl difluoroacetate in the electrolyte can be obtained by conventional testing methods in the art, such as by gas chromatography or gas chromatography coupled with mass spectrometry.

[0099] In this invention, the battery further includes a positive electrode sheet. The positive electrode sheet also includes a positive current collector, a positive conductive agent, and a positive binder, all of which are conventional choices for those skilled in the art. For example, the positive current collector may include aluminum foil, the positive dispersant may include carboxymethyl cellulose (CMC), the positive binder may include at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene-butadiene rubber, and the positive conductive agent may include at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0100] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0101] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0102] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0103] The following preparation examples illustrate the silicon-carbon composite material of the present invention.

[0104] Preparation Example 1: Step 1: Mix 50g of phenolic resin and 150g of anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 0; Take 3.5g of first carbon nanotubes, 7.5g of HMTA and 50g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 0; Solution C 20. Add solution C dropwise. 1 The mixture M0 obtained from 0 is subjected to first spray drying (outlet temperature is 70℃) to obtain material W0; Step 2: Mix 10 kg of benzyl silicone oil and 50 g of sodium dodecyl sulfonate to obtain a dispersed phase. Disperse the material WO in the dispersed phase and cure at 120°C for 2 h. After filtration, washing and drying, obtain particles K0, which is the central region. Step 3: Mix 350g of phenolic resin and 1750g of anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 1. Take 17.5g of first carbon nanotubes, 52.5g of HMTA and 350g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 1. Prepare solution C 2 1. Add solution C dropwise 1 1. Disperse particles K0 in the mixture and stir for 60 minutes to obtain a mixture M1. Perform a first spray drying (outlet temperature 70℃) to obtain material W1. Repeat step 2 to obtain particles K1, thus obtaining region D1. Mix 350g of phenolic resin and 2800g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 2. Take 10.5g of first carbon nanotubes, 52.5g of HMTA and 350g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 2. Prepare solution C 2 2. Add solution C dropwise 1 2. Polyvinylpyrrolidone (PVP, based on the total mass of phenolic resin, the PVP content is 3%) and particles K1 are dispersed in it and stirred for 60 min to obtain a mixture M2. The mixture is then spray-dried for the first time (outlet temperature is 70℃) to obtain material W2. Step 2 is repeated to obtain particles K2, resulting in a porous carbon matrix with a surface region. Step 4: Place the porous carbon precursor containing the first carbon nanotube in a tube furnace, first heat it to 600℃ at 3℃ / min under a nitrogen atmosphere and hold it for 2h, then heat it to 900℃ and hold it for 3h, then switch to a 10% water vapor-90% nitrogen mixed gas and hold it for 2h to obtain a porous carbon matrix. Step 5: Add the porous carbon matrix to a fluidized bed and heat it to 550°C at a rate of 10°C / min under a nitrogen atmosphere. Then switch to a 20% silicon source-80% nitrogen mixed gas with a flow rate of 10L / min and keep it at that temperature for 3 hours. After the silicon source gas is exhausted, heat it to 650°C at a rate of 10°C / min and switch to a 30% carbon source-70% nitrogen mixed gas with a flow rate of 15L / min and keep it at that temperature for 2 hours to obtain a core with an amorphous carbon layer on the surface. Mix 10g of the second carbon nanotube, 5000g of anhydrous ethanol and 1000g of the core and then perform a second spray drying (outlet temperature of 70°C) to obtain a silicon-carbon composite material with the second carbon nanotube coated on the surface, denoted as SC-1.

[0105] At this point, S1 is 6.2%, S2 is 10.4%, S3 is 14.6%, S1 / S3 is 0.425, S1 < S2 < S3, x = 1, S4 = S5, the mass content of the second carbon nanotube in the silicon-carbon composite material is 1.2%, the diameter of the second carbon nanotube is larger than that of the first carbon nanotube, the mass content of silicon in the silicon-carbon composite material is 45%, the median particle size Dv50 of the silicon-carbon composite material is 9.6 μm, (Dv90-Dv10) / Dv50 is 1.3, and the specific surface area is 1.02 m². 2 / g, sphericity is 0.953.

[0106] Preparation Example 2: Step 1: Mix 50g of phenolic resin and 150g of anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 0; Take 3g of first carbon nanotubes, 7.5g of HMTA and 50g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 0; Solution C 2 0. Add solution C dropwise. 1 The mixture M0 obtained from 0 is subjected to first spray drying (outlet temperature is 70℃) to obtain material W0; Step 2: Mix 10 kg of benzyl silicone oil and 50 g of sodium dodecyl sulfonate to obtain a dispersed phase. Disperse the material WO in the dispersed phase and cure at 120°C for 2 h. After filtration, washing and drying, obtain particles K0, which is the central region. Step 3: Mix 150g of phenolic resin and 1050g of anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 1. Mix 8.4g of first carbon nanotubes, 21.5g of HMTA, and 150g of anhydrous ethanol, and homogenize to obtain solution C. 2 1. Prepare solution C 2 1. Add solution C dropwise 11. Disperse particles K0 in the mixture and stir for 60 minutes to obtain a mixture M1. Perform a first spray drying (outlet temperature 70℃) to obtain material W1. Repeat step 2 to obtain particles K1, thus obtaining region D1. Mix 200g of phenolic resin and 1400g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 2. Take 8g of the first carbon nanotube, 30g of HMTA and 200g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 2. Prepare solution C 2 2. Add solution C dropwise 1 2. Disperse particles K1 within the mixture and stir for 60 minutes to obtain a mixture M2. Perform a first spray drying (outlet temperature 70℃) to obtain material W2. Repeat step 2 to obtain particles K2, thus obtaining region D2. Mix 350g of phenolic resin and 2800g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 3. Take 6.3g of the first carbon nanotube, 52.5g of HMTA and 350g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 3. Prepare solution C 2 3. Add solution C dropwise 1 3. Polyvinylpyrrolidone (PVP, based on the total mass of phenolic resin, the PVP content is 2%) and particles K3 are dispersed in it and stirred for 60 min to obtain a mixture M3. The mixture is then spray-dried for the first time (outlet temperature is 70℃) to obtain material W3. Step 2 is repeated to obtain particles K3, resulting in a porous carbon matrix with a surface region. Step 4: Place the porous carbon precursor containing the first carbon nanotube in a tube furnace, first heat it to 600℃ at 3℃ / min under a nitrogen atmosphere and hold it for 2h, then heat it to 900℃ and hold it for 3h, then switch to a 10% water vapor-90% nitrogen mixed gas and hold it for 2h to obtain a porous carbon matrix. Step 5: Add the porous carbon matrix to a fluidized bed and heat it to 550°C at a rate of 10°C / min under a nitrogen atmosphere. Then switch to a 20% silicon source-80% nitrogen mixed gas with a flow rate of 10L / min and keep it at that temperature for 3 hours. After the silicon source gas is exhausted, heat it to 650°C at a rate of 10°C / min and switch to a 30% carbon source-70% nitrogen mixed gas with a flow rate of 15L / min and keep it at that temperature for 2 hours to obtain a core with an amorphous carbon layer on the surface. Mix 1g of the second carbon nanotube, 5000g of anhydrous ethanol and 1000g of the core and perform a second spray drying (outlet temperature of 70°C) to obtain a silicon-carbon composite material with the second carbon nanotube coated on the surface, denoted as SC-2.

[0107] At this point, S1 is 3.6%, S2 is 10.4%, S3 is 12%, S1 / S3 is 0.3, S1 < S2 < S3, x = 2, the projected area of ​​the first carbon nanotube in regions D1 and D2 is 11.6% and 8.3% respectively, S4 > S5, the mass content of the second carbon nanotube in the silicon-carbon composite material is 0.1%, the diameter of the second carbon nanotube is larger than that of the first carbon nanotube, the mass content of silicon in the silicon-carbon composite material is 45%, the median particle size Dv50 of the silicon-carbon composite material is 5.2 μm, (Dv90-Dv10) / Dv50 is 0.6, and the specific surface area is 0.85 m². 2 / g, sphericity is 0.812.

[0108] Preparation Example 3: Step 1: Mix 50g of phenolic resin and 150g of anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 0; Take 4.2g of first carbon nanotubes, 7.5g of HMTA and 50g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 0; Solution C 2 0. Add solution C dropwise. 1 The mixture M0 obtained from 0 is subjected to first spray drying (outlet temperature is 70℃) to obtain material W0; Step 2: Mix 10 kg of benzyl silicone oil and 50 g of sodium dodecyl sulfonate to obtain a dispersed phase. Disperse the material WO in the dispersed phase and cure at 120°C for 2 h. After filtration, washing and drying, obtain particles K0, which is the central region. Step 3: Mix 150g of phenolic resin and 600g of anhydrous ethanol, and stir until completely dissolved to obtain solution C. 1 1. Take 9g of first carbon nanotubes, 22.5g of HMTA and 150g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 1. Prepare solution C 2 1. Add solution C dropwise 1 1. Disperse particles K0 in the mixture and stir for 60 minutes to obtain a mixture M1. Perform a first spray drying (outlet temperature 70℃) to obtain material W1. Repeat step 2 to obtain particles K1, thus obtaining region D1. Mix 200g of phenolic resin and 800g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 2. Take 8.8g of the first carbon nanotube, 30g of HMTA and 200g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 2. Prepare solution C 2 2. Add solution C dropwise 12. Disperse particles K1 within the mixture and stir for 60 minutes to obtain a mixture M2. Perform a first spray drying (outlet temperature 70℃) to obtain material W2. Repeat step 2 to obtain particles K2, thus obtaining region D2. Mix 350g of phenolic resin and 2100g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 3. Take 14.7g of the first carbon nanotube, 52.5g of HMTA and 350g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 3. Prepare solution C 2 3. Add solution C dropwise 1 3. Polyvinylpyrrolidone (PVP, based on the total mass of phenolic resin, the PVP content is 4%) and particles K3 are dispersed in it and stirred for 60 min to obtain a mixture M3. The mixture is then spray-dried for the first time (outlet temperature is 70℃) to obtain material W3. Step 2 is repeated to obtain particles K3, resulting in a porous carbon matrix with a surface region. Step 4: Place the porous carbon precursor containing the first carbon nanotube in a tube furnace, first heat it to 600℃ at 3℃ / min under a nitrogen atmosphere and hold it for 2h, then heat it to 900℃ and hold it for 3h, then switch to a 10% water vapor-90% nitrogen mixed gas and hold it for 2h to obtain a porous carbon matrix. Step 5: Add the porous carbon matrix to a fluidized bed and heat it to 550°C at a rate of 10°C / min under a nitrogen atmosphere. Then switch to a 20% silicon source-80% nitrogen mixed gas with a flow rate of 10L / min and keep it at that temperature for 3 hours. After the silicon source gas is exhausted, heat it to 650°C at a rate of 10°C / min and switch to a 30% carbon source-70% nitrogen mixed gas with a flow rate of 15L / min and keep it at that temperature for 2 hours to obtain a core with an amorphous carbon layer on the surface. Mix 18g of the second carbon nanotube, 5000g of anhydrous ethanol and 1000g of the core and then perform a second spray drying (outlet temperature of 70°C) to obtain a silicon-carbon composite material with the second carbon nanotube coated on the surface, denoted as SC-3.

[0109] At this point, S1 is 8.8%, S2 is 10.8%, S3 is 17.5%, S1 / S3 is 0.503, S1 < S2 < S3, x = 2, the projected area of ​​the first carbon nanotube in regions D1 and D2 is 12.5% ​​and 9.1% respectively, S4 > S5, the mass content of the second carbon nanotube in the silicon-carbon composite material is 1.8%, the diameter of the second carbon nanotube is larger than that of the first carbon nanotube, the mass content of silicon in the silicon-carbon composite material is 45%, the median particle size Dv50 of the silicon-carbon composite material is 14.7 μm, (Dv90-Dv10) / Dv50 is 1.8, and the specific surface area is 0.93 m². 2 / g, sphericity is 0.975.

[0110] Preparation Example 4: This set of preparation examples is used to verify the effect of changes in "S1 and S3", which is achieved by changing the content of the first carbon nanotube in each region, as follows: Preparation Example 4a, based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in solution C is 4.8 g. 2 The mass of the first carbon nanotube in 2 is 3.85 g. The resulting silicon-carbon composite material is denoted as SC-4a. The content of S1 is 2.2%, S3 is 19.7%, S1 / S3 is 0.112, the median particle size Dv50 of the silicon-carbon composite material is 9.5 μm, and the sphericity is 0.952. Preparation Example 4b is based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in solution C is 2.7 g. 2 The mass of the first carbon nanotube in 2 is 16.8g. The resulting silicon-carbon composite material is denoted as SC-4b. The content of S1 is 9.7%, S3 is 11%, the S1 / S3 ratio is 0.882, the median particle size Dv50 of the silicon-carbon composite material is 9.4μm, and the sphericity is 0.95. Preparation Example 4c is based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in 0 is 2.7g. The resulting silicon-carbon composite material is denoted as SC-4c, with S3 being 11%, S1 / S3 being 0.564, and the median particle size Dv50 of the silicon-carbon composite material being 9.6μm and the sphericity being 0.954. Preparation Example 4d, based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in 0 is 4.8g. The resulting silicon-carbon composite material is denoted as SC-4d. The S3 content is 19.7%, the S1 / S3 ratio is 0.315, the median particle size Dv50 of the silicon-carbon composite material is 9.7μm, and the sphericity is 0.953. Preparation Example 4e is based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in 2 is 3.85g. The resulting silicon-carbon composite material is denoted as SC-4e, with S1 of 2.2%, S1 / S3 of 0.151, median particle size Dv50 of the silicon-carbon composite material of 9.6μm, and sphericity of 0.951. Preparation Example 4f, based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in solution 1 is 19.6 g, and the solution is C. 2 The mass of the first carbon nanotube in 2 is 16.8g. The resulting silicon-carbon composite material is denoted as SC-4f. The content of S1 is 9.7%, S2 is 11.6%, S1 / S3 is 0.664, the median particle size Dv50 of the silicon-carbon composite material is 9.8μm, and the sphericity is 0.954. Preparation Example 4g, based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in solution C is 2.3g. 2 The mass of the first carbon nanotube in 2 is 2.94 g. The resulting silicon-carbon composite material is denoted as SC-4 g. The content of S1 is 1.7%, S3 is 9.5%, S1 / S3 is 0.179, the median particle size Dv50 of the silicon-carbon composite material is 9.4 μm, and the sphericity is 0.955. Preparation Example 4h, based on Preparation Example 1, except that solution C 2 The mass of the first carbon nanotube in solution C is 5.2 g. 2 The mass of the first carbon nanotube in solution 1 is 22.4 g, and the solution is C. 2 The mass of the first carbon nanotube in 2 is 19.6g. The resulting silicon-carbon composite material is denoted as SC-4g. The content of S1 is 11.7%, S2 is 13.3%, S3 is 21.7%, and the ratio of S1 to S3 is 0.539. The median particle size Dv50 of the silicon-carbon composite material is 9.5μm, and the sphericity is 0.956.

[0111] Preparation Example 5: The examples prepared in this group are used to verify the effect of changes in "x", as detailed below: Preparation Example 5a is based on Preparation Example 1, except that in step 3, 20g of phenolic resin and 100g of anhydrous ethanol are mixed and stirred until completely dissolved to obtain solution C. 1 1. Mix 1.28g of the first carbon nanotube, 3g of HMTA, and 20g of anhydrous ethanol, and homogenize to obtain solution C. 2 1. Prepare solution C 2 1. Add solution C dropwise 1 1. Disperse particles K0 in the mixture and stir for 60 minutes to obtain a mixture M1. Perform a first spray drying (outlet temperature 70℃) to obtain material W1. Repeat step 2 to obtain particles K1, thus obtaining region D1. Mix 30g of phenolic resin and 150g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 2. Take 1.74g of the first carbon nanotube, 4.5g of HMTA and 30g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 2. Prepare solution C 2 2. Add solution C dropwise 1 2. Disperse particles K1 within the mixture and stir for 60 minutes to obtain a mixture M2. Perform a first spray drying (outlet temperature 70℃) to obtain material W2. Repeat step 2 to obtain particles K2, thus obtaining region D2. Mix 50g of phenolic resin and 250g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 13. Take 2.6g of the first carbon nanotube, 7.5g of HMTA and 50g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 3. Prepare solution C 2 3. Add solution C dropwise 1 3. Disperse particles K2 within the mixture and stir for 60 minutes to obtain a mixture M3. Perform a first spray drying (outlet temperature 70℃) to obtain material W3. Repeat step 2 to obtain particles K3, thus obtaining region D3. Mix 100g of phenolic resin and 500g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 4. Take 4.6g of the first carbon nanotube, 15g of HMTA and 100g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 4. Prepare solution C 2 4. Add solution C dropwise 1 4. Disperse particles K3 within the mixture and stir for 60 minutes to obtain mixture M4. Perform a first spray drying (outlet temperature 70℃) to obtain material W4. Repeat step 2 to obtain particles K4, thus obtaining region D4. Mix 150g of phenolic resin and 750g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 5. Take 6g of the first carbon nanotube, 22.5g of HMTA and 150g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 5. Prepare solution C 2 5. Add solution C dropwise 1 5. Disperse particles K4 in the mixture and stir for 60 min to obtain a mixture M5. Perform the first spray drying (outlet temperature is 70℃) to obtain material W5. Repeat step 2 to obtain particles K5 and obtain region D5. The resulting silicon-carbon composite material is denoted as SC-5a. At this time, S2 is 10.8%. The projected areas of the first carbon nanotubes in regions D1, D2, D3, D4 and D5 are 13.3%, 12%, 10.8%, 9.5% and 8.3% respectively. The median particle size Dv50 of the silicon-carbon composite material is 9.3 μm and the sphericity is 0.96. Preparation Example 5b is based on Preparation Example 1, except that in step 3, 50g of phenolic resin and 200g of anhydrous ethanol are mixed and stirred until completely dissolved to obtain solution C. 1 1. Take 3g of first carbon nanotubes, 7.5g of HMTA and 50g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 1. Prepare solution C 2 1. Add solution C dropwise 11. Disperse particles K0 in the mixture and stir for 60 minutes to obtain a mixture M1. Perform a first spray drying (outlet temperature 70℃) to obtain material W1. Repeat step 2 to obtain particles K1, thus obtaining region D1. Mix 100g of phenolic resin and 480g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 2. Take 5g of the first carbon nanotube, 15g of HMTA and 100g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 2. Prepare solution C 2 2. Add solution C dropwise 1 2. Disperse particles K1 within the mixture and stir for 60 minutes to obtain a mixture M2. Perform a first spray drying (outlet temperature 70℃) to obtain material W2. Repeat step 2 to obtain particles K2, thus obtaining region D2. Mix 200g of phenolic resin and 1000g of anhydrous ethanol and stir until completely dissolved to obtain solution C. 1 3. Take 8g of the first carbon nanotube, 30g of HMTA and 200g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 3. Prepare solution C 2 3. Add solution C dropwise 1 3. Disperse particles K2 in the mixture and stir for 60 min to obtain a mixture M3. Perform the first spray drying (outlet temperature is 70℃) to obtain material W3. Repeat step 2 to obtain particles K3 and obtain region D3. The resulting silicon-carbon composite material is denoted as SC-5b. At this time, S2 is 10.4%, and the projected area of ​​the first carbon nanotube in regions D1, D2 and D3 is 12.5%, 10.4% and 8.3% respectively. The median particle size Dv50 of the silicon-carbon composite material is 9.4 μm and the sphericity is 0.954.

[0112] Preparation Example 6: The examples in this group are used to verify the effects of changes in the "outer shell," as detailed below: Preparation Example 6a is based on Preparation Example 1, except that it does not coat the second carbon nanotube. The resulting silicon-carbon composite material is denoted as SC-6a. Preparation Example 6b is based on Preparation Example 1, except that no second deposition is performed, and the resulting silicon-carbon composite material is designated as SC-6b; Preparation Example 6c is based on Preparation Example 1, except that the diameter of the second carbon nanotube is smaller than that of the first carbon nanotube, and the resulting silicon-carbon composite material is denoted as SC-6c.

[0113] Preparation Example 7: This group of preparation examples was used to verify the effects of changes in the "mass content and sphericity of silicon element in silicon-carbon composite materials". This was achieved by changing the deposition time of the first deposition and the content of PVP, as follows: Preparation Example 7a is based on Preparation Example 1, except that the content of PVP is 1% based on the total mass of phenolic resin, the deposition time of the first deposition is 1h, and the resulting silicon-carbon composite material is denoted as SC-7a. In this case, the mass content of silicon element in the silicon-carbon composite material is 32%, the mass content of carbon element is 68%, the median particle size Dv50 of the silicon-carbon composite material is 9.4μm, (Dv90-Dv10) / Dv50 is 1.2, and the sphericity is 0.624. Preparation Example 7b is based on Preparation Example 1, except that the content of PVP is 5% based on the total mass of phenolic resin, the deposition time of the first deposition is 7h, and the resulting silicon-carbon composite material is denoted as SC-7b. In this case, the mass content of silicon in the silicon-carbon composite material is 69%, the mass content of carbon is 31%, the median particle size Dv50 of the silicon-carbon composite material is 9.5μm, (Dv90-Dv10) / Dv50 is 1, and the sphericity is 0.995.

[0114] Comparative preparation example 1: This set of preparation examples is used to verify the effect of changes in "S1 and S3", which is achieved by changing the content of the first carbon nanotube in each region, as follows: Compared to Preparation Example 1a, based on Preparation Example 1, the difference is that solution C 2 The mass of the first carbon nanotube in solution C is 5.2 g. 2 The mass of the first carbon nanotube in 2 is 2.8g. The resulting silicon-carbon composite material is denoted as SC-D1a. S1 is 1.7%, S3 is 21.7%, S1 / S3 is 0.078, the median particle size Dv50 of the silicon-carbon composite material is 9.5μm, and the sphericity is 0.952. Compared to Preparation Example 1b, based on Preparation Example 1, the difference is that solution C 2 The mass of the first carbon nanotube in solution C is 2.5g. 2 The mass of the first carbon nanotube in solution 1 is 16.8 g, and the solution is C. 2 The mass of the first carbon nanotube in 2 is 16g. The resulting silicon-carbon composite material is denoted as SC-D1b. The content of S1 is 9.6%, S2 is 10%, S3 is 10.4%, S1 / S3 is 0.923, the median particle size Dv50 of the silicon-carbon composite material is 9.3μm, and the sphericity is 0.954.

[0115] Comparative preparation example 2a: Based on Preparation Example 1, the difference is that 50g of phenolic resin and 270g of anhydrous ethanol were mixed and stirred until completely dissolved to obtain solution C. 1 0; Take 2.4g of first carbon nanotubes, 7.5g of HMTA and 50g of anhydrous ethanol, mix them, and homogenize to obtain solution C. 2 0; Solution C 20. Add solution C dropwise. 1 Mixture M0 was obtained from 0 and subjected to first spray drying (outlet temperature 70℃) to obtain material W0; 10kg of benzyl silicone oil and 50g of sodium dodecyl sulfonate were mixed to obtain a dispersed phase, and the material W0 was dispersed in the dispersed phase and cured at 120℃ for 2h. After filtration, washing and drying, a porous carbon matrix was obtained, and the resulting silicon-carbon composite material was denoted as SC-D2a; at this time, S1=S2=S3 was 10%, S1 / S3 was 1, the median particle size Dv50 of the silicon-carbon composite material was 9.4μm, and the sphericity was 0.956.

[0116] Comparative preparation example 2b: Based on Preparation Example 1, the difference is that solution C 2 The mass of the first carbon nanotube in solution C is 1.5g. 2 The mass of the first carbon nanotube in solution 1 is 17.5 g, and the solution is C. 2 The mass of the first carbon nanotube in 2 is 24.5g. The resulting silicon-carbon composite material is denoted as SC-D2b. The content of S1 is 14.6%, S3 is 6.2%, S1 / S3 is 2.36, S1>S2>S3, the median particle size Dv50 of the silicon-carbon composite material is 9.6μm, and the sphericity is 0.953.

[0117] The following examples illustrate the battery of the present invention.

[0118] Example 1: (1) Preparation of the positive electrode: Lithium cobalt oxide, polyvinylidene fluoride (PVDF), acetylene black, and multi-walled carbon nanotubes were mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of an 8 μm thick aluminum foil and dried in an oven at 120°C for 12 h. Then, the foil was rolled and slit to obtain a positive electrode sheet.

[0119] (2) Preparation of negative electrode: Silicon-carbon composite material SC-1, artificial graphite, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and Super P were mixed in a mass ratio of 10:86.5:1.5:1.5:0.5. Deionized water was added, and the mixture was stirred evenly in a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of a high-strength copper foil with a thickness of 8 μm and then dried in an oven at 80℃ for 12 h. After rolling and slitting, the negative electrode sheet was obtained. The artificial graphite had a particle size Dv10 of 5.5 μm, a median particle size Dv50 of 10.7 μm, and a particle size Dv90 of 16.5 μm. The silicon content in the negative electrode active layer was 4.5% by mass.

[0120] (3) Preparation of electrolyte: Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:1 to obtain a solvent. Lithium hexafluorophosphate, DFEA, and FEC were added and mixed evenly to obtain the electrolyte. Based on the total mass of the electrolyte, the concentration of lithium hexafluorophosphate was 1.2 mol / L, the content of FEC (A) was 14%, the content of DFEA was 42%, and the A / S1 ratio was 2.258.

[0121] (4) Battery fabrication: The prepared negative electrode, separator (polyethylene film with a thickness of 8μm) and prepared positive electrode are stacked in sequence to ensure that the separator is between the positive and negative electrodes to play a role in isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell and the electrolyte is injected into the dried bare cell. After vacuum sealing, standing, formation and sorting, the battery is obtained.

[0122] (4) Preparation of button half-cells: Silicon-carbon composite material SC-1, Super P, polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 88:5.0:4.0:1.2:1.8. Deionized water was added, and the mixture was stirred evenly under vacuum to obtain a coin cell negative electrode slurry. The coin cell negative electrode slurry was coated on the surface of copper foil and dried in an oven at 80°C. Then, it was dried in a vacuum oven at 100°C for 12 hours. The negative electrode sheet was then formed into a negative electrode disc with a diameter of 12 mm using a stamping machine. In a glove box, the negative electrode disc was used as the working electrode, a lithium metal sheet was used as the counter electrode, and a polyethylene membrane with a thickness of 20 μm was used as the separator. Electrolyte was added to assemble a coin cell half-cell.

[0123] Example 2: Based on Example 1, the difference is that in the preparation of the negative electrode, the silicon-carbon composite material SC-1 is replaced with SC-2 of the same mass fraction, the particle size Dv10 of the artificial graphite is 2.3 μm, the median particle size Dv50 is 5.1 μm, and the particle size Dv90 is 11.4 μm; the content of FEC (A) in the electrolyte is 20%, the content of DFEA is 10%, and the A / S1 is 5.556; in the preparation of the coin cell, the silicon-carbon composite material SC-1 is replaced with SC-2 of the same mass fraction.

[0124] Example 3: Based on Example 1, the difference is that in the preparation of the negative electrode, the silicon-carbon composite material SC-1 is replaced with SC-3 of the same mass fraction, the particle size Dv10 of the artificial graphite is 6.8 μm, the median particle size Dv50 is 13.7 μm, and the particle size Dv90 is 24.7 μm; the content of FEC (A) in the electrolyte is 8%, the content of DFEA is 55%, and the A / S1 is 0.909; in the preparation of the coin cell, the silicon-carbon composite material SC-1 is replaced with SC-3 of the same mass fraction.

[0125] Examples 4 through 7 were all based on Example 1, except that the silicon-carbon composite materials used in the battery and the coin cell were different, as detailed in Table 1. Specifically: in Example 4a, A / S1 was 6.364; in Example 4b, A / S1 was 1.443; in Example 4e, A / S1 was 6.364; in Example 4f, A / S1 was 1.443; in Example 4g, A was 8% and A / S1 was 4.706; in Example 4h, A was 20% and A / S1 was 1.709. In Example 7a, the proportion of silicon-carbon anode material in the negative electrode active layer was 5%, and the mass content of silicon was 1.6%. In Example 7b, the proportion of silicon-carbon anode material in the negative electrode active layer was 70%, and the mass content of silicon was 48.3%.

[0126] Example 8 group: This set of examples is used to verify the impact of changes in the "mass content A of fluoroethylene carbonate", as detailed below: Example 8a is based on Example 3, except that A is 5%, and A / S1 is 0.568. Example 8b is based on Example 2, except that A is 25%, and A / S1 is 6.944.

[0127] Example 9 group: This set of examples is used to verify the impact of changes to "A / S1", as detailed below: Example 9a is based on Example 1, except that A is 4.5%, and A / S1 is 0.726. Example 9b is based on Example 1, except that A is 27%, and A / S1 is 4.355. Example 9c is based on Example 4b, except that A is 20%, and A / S1 is 9.09.

[0128] Example 10 group: This set of examples includes examples used to verify the effects of changes in "ethyl difluoroacetate", as detailed below: Example 10a is based on Example 1, except that the DFEA content in the electrolyte is 5%; Example 10b is based on Example 1, except that the DFEA content in the electrolyte is 65%. Example 10c is based on Example 1, except that DFEA in the electrolyte is replaced with an equal amount of ethyl 2,2-difluoroacetate.

[0129] Example 11: Based on Example 1, the difference is that the negative electrode uses a double-layer coating, and the preparation method of the negative electrode is as follows: SC-1, artificial graphite, hard carbon, CMC-Na, SBR and Super P was mixed in a mass ratio of 10:81.5:5:1.5:1.5:0.5, and deionized water was added. The mixture was stirred in a vacuum mixer to obtain the first negative electrode slurry. SC-1, artificial graphite, CMC-Na, SBR and SuperP were mixed in a mass ratio of 10:86.5:1.5:1.5:0.5, and deionized water was added. The mixture was stirred in a vacuum mixer to obtain the second negative electrode slurry. The second negative electrode slurry was uniformly coated on both sides of a high-strength copper foil with a thickness of 8μm. The foil was then dried in an oven at 80℃ for 5 hours to obtain the second negative electrode active layer. The first negative electrode slurry was then uniformly coated on the surface of the second negative electrode active layer. After coating, the foil was dried in an oven at 80℃ for 12 hours. The thickness ratio of the second negative electrode active layer to the first negative electrode active layer was 7:3. Finally, the foil was rolled and slit to obtain the negative electrode sheet.

[0130] Comparative Examples 1 and 2 were performed in accordance with Example 1, except that the silicon-carbon composite materials in the battery and the coin cell were different, as shown in Table 1. In Comparative Example 1a, A / S1 was 8.235, in Comparative Example 1b, A / S1 was 1.458, in Comparative Example 2a, A / S1 was 1.4, and in Comparative Example 2b, A / S1 was 0.959.

[0131] Test example: (1) First Coulomb efficiency: After the coin cells prepared in the examples and comparative examples were left to stand for 3 hours, they were discharged at a constant current density of 0.1C to 5mV, left to stand for 10 minutes, and then discharged again at a current density of 0.01C to 5mV. The discharge capacity was recorded. After being left to stand for another 10 minutes, they were charged at a constant current density of 0.05C to 1.5V, and the charging capacity was recorded. The initial coulombic efficiency was calculated as (charge capacity / discharge capacity) × 100%. The test results are recorded in Table 1.

[0132] (2) Low-rate cycling capacity retention: The batteries prepared in the examples and comparative examples were charged at a constant current density of 1C to 4.5V, then charged at a constant voltage of 4.5V with a cutoff current of 0.05C, and then allowed to stand for 10 minutes. They were then discharged at a current density of 1C to 3.0V, and allowed to stand for another 10 minutes. The discharge capacity of the battery at this point was recorded as the initial capacity. This charge-discharge process was repeated until the 1000th cycle, after which the battery was discharged at a constant current density to 3.0V and allowed to stand for 10 minutes. The discharge capacity of the battery at this point was recorded as the capacity after cycling. Therefore, the low-rate cycle capacity retention rate = (capacity after cycling / initial capacity) × 100%. The test results are recorded in Table 1.

[0133] (3) High-rate cycling capacity retention: The batteries prepared in the examples and comparative examples were charged at a constant current density of 3C to 4.2V, then at a constant current density of 2C to 4.35V, then at a constant current density of 1C to 4.5V, followed by constant voltage charging at 4.5V with a cutoff current of 0.05C. After resting for 10 minutes, they were discharged at a current density of 3C to 3.0V, and then rested for 10 minutes. The discharge capacity of the battery at this point was recorded as the initial capacity. The above charge and discharge process was repeated until the 600th cycle, after which the battery was discharged at a constant current density to 3.0V and rested for 10 minutes. The discharge capacity of the battery at this point was recorded as the capacity after cycling. The high-rate cycle capacity retention rate is calculated as: (Capacity after cycling / Initial capacity) × 100%. The test results are recorded in Table 1.

[0134] Table 1: As can be seen from Table 1, the battery prepared by this invention has both good cycle stability and rate performance compared to the comparative example.

[0135] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-carbon composite material, the silicon-carbon composite material has a core and a shell, the core includes a porous carbon matrix and silicon particles distributed in the internal channels of the porous carbon matrix, the silicon-carbon composite material also includes a first carbon nanotube, the first carbon nanotube is distributed in the core; The cross-section of the core has a central region, a transition region, and a surface region. The geometry formed by the cross-section of the core is scaled equidistantly to a point O. The line segment between point O and any point F on the edge of the cross-section of the core has a dimension of 0.5L. On this line segment, the area 0-0.1L from the edge of the cross-section constitutes the surface region, the area greater than 0.1L and less than or equal to 0.3L constitutes the transition region, and the area greater than 0.3L and less than or equal to 0.5L constitutes the central region. The projected area of ​​the first carbon nanotube is S1, S2 and S3 in proportion to the area of ​​the surface region, the transition region and the central region, respectively, satisfying: S1 < S2 < S3, and 0.1 ≤ S1 / S3 ≤ 0.

9.

2. The negative electrode sheet according to claim 1, wherein, 0.3≤S1 / S3≤0.55; And / or, 2%≤S1≤10%; And / or, 10% < S3 ≤ 20%.

3. The negative electrode sheet according to claim 1 or 2, wherein, The outer shell includes an amorphous carbon layer that covers at least a portion of the outer surface of the core; And / or, the shell further includes a second carbon nanotube located on at least a portion of the surface of the amorphous carbon layer, the second carbon nanotube having a diameter larger than that of the first carbon nanotube.

4. The negative electrode sheet according to claim 3, wherein, In the silicon-carbon composite material, the mass content of the second carbon nanotube is 0.1%-2%.

5. The negative electrode sheet according to claim 1, wherein, The transition zone comprises x regions, which are sequentially named D1, D2, ... D1 as they extend from the central region towards the surface region. (x-1) Region and D x Region, 1≤x≤5, the region D1, region D2...D (x-1) Region and D x The thickness of the region is selected from 0.02L to 0.2L; Preferably, the area of ​​the first carbon nanotube near the D1 region in the transition region is S4, and the area near the D1 region is S4. x The area of ​​the first carbon nanotube in the region that is projected onto the transition zone is S5, and S4≥S5.

6. The negative electrode according to claim 1, wherein, In the silicon-carbon composite material, the mass content of silicon is 30%-70%, and the mass content of carbon is 30%-70%. And / or, the median particle size Dv50 of the silicon-carbon composite material is 5μm-15μm; And / or, the particle size Dv10, median particle size Dv50 and particle size Dv90 of the silicon-carbon composite material satisfy: (Dv90-Dv10) / Dv50 is 0.6-2; And / or, the specific surface area of ​​the silicon-carbon composite material is 0.01 m². 2 / g-8m 2 / g; Preferably, the specific surface area of ​​the silicon-carbon composite material is 0.05 m². 2 / g-6m 2 / g.

7. The negative electrode sheet according to claim 6, wherein, The sphericity of the silicon-carbon composite material is 0.6-1; Preferably, the sphericity of the silicon-carbon composite material is 0.8-0.

98.

8. The negative electrode according to claim 1, wherein, The negative electrode active material also includes carbon-based materials, which include artificial graphite and / or natural graphite; The particle size Dv10 of the carbon-based material is 2μm-7μm; And / or, the median particle size Dv50 of the carbon-based material is 5 μm-14 μm; And / or, the particle size Dv90 of the carbon-based material is 11 μm-25 μm; And / or, based on the total mass of the negative electrode active layer, the silicon content is 1.5%-50% by mass.

9. A battery, characterized in that, The battery includes a negative electrode sheet as described in any one of claims 1-8, the battery includes an electrolyte, the electrolyte includes fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate is A based on the total mass of the electrolyte, wherein A and S1 satisfy the following condition: 0.5≤A / S1≤7. And / or, the range of A is: 5%-25%; Preferably, 0.9 ≤ A / S1 ≤ 5.6; Preferably, the range of A is 8%-20%.

10. The battery according to claim 9, wherein, The electrolyte further includes ethyl difluoroacetate, which includes at least one of 2,2-difluoroethyl acetate and ethyl 2,2-difluoroacetate, and the mass content of ethyl difluoroacetate is 5%-65% based on the total mass of the electrolyte; Preferably, the mass content of the ethyl difluoroacetate is 10%-55% based on the total mass of the electrolyte.

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