Silicon-carbon composite material and preparation method therefor, and secondary battery

Abstract

A silicon-carbon composite material and a preparation method therefor, and a secondary battery. The silicon-carbon composite material comprises a core and a shell that coats the core, wherein the core comprises porous carbon and silicon dispersed in the porous carbon, and the shell comprises a carbon material. The Raman spectral characteristics of the silicon-carbon composite material satisfy: 0.77≤A≤0.95, wherein the value of A is the ratio of IS3 to I2d, with I2d representing the intensity of the peak at 2700±5 cm-1 in the Raman spectrum, and IS3 representing the intensity of the peak at 2960±5 cm-1 in the Raman spectrum. The silicon-carbon composite material facilitates the intercalation of active ions into the silicon-carbon composite material from all directions, and can improve the uniformity of the intercalation of the active ions into the silicon-carbon composite material, thereby improving the specific capacity, initial coulombic efficiency, cycling performance, rate capability and low-temperature performance of the silicon-carbon composite material.

Description

Silicon-carbon composite material, preparation method thereof and secondary battery Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a secondary battery using the silicon-carbon composite material. Background Art

[0002] Graphite is the most widely used anode material, offering advantages such as high efficiency and a stable charge-discharge platform. However, its low specific capacity hinders its further application. Compared to graphite, elemental silicon is considered an ideal anode material to replace graphite due to its higher theoretical specific capacity and suitable operating voltage.

[0003] However, as a semiconductor material, silicon has low conductivity and undergoes huge volume expansion during the alloying / de-alloying process, resulting in poor cycle performance of secondary batteries.

[0004] Summary of the Invention

[0005] The present application provides a silicon-carbon composite material, and also provides a method for preparing the silicon-carbon composite material and a secondary battery using the silicon-carbon composite material.

[0006] The first aspect of the present application provides a silicon-carbon composite material, comprising a core and a shell covering the core, wherein the core comprises porous carbon and silicon dispersed in the porous carbon, and the shell comprises a carbon material, wherein the Raman spectrum characteristics of the silicon-carbon composite material satisfy: 0.77≤A≤0.95, and the A value is 1 S3 and I 2d The ratio of I 2d Indicates that the Raman spectrum is at 2700±5cm -1 The peak intensity at I S3 Indicates that the Raman spectrum is at 2960±5cm -1 The peak intensity at .

[0007] In this application, the S3 peak of the Raman spectrum of the silicon-carbon composite material is related to the defects of the silicon-carbon composite material, the 2d peak is used to reflect the number of graphene layers in the silicon-carbon composite material, and the size of the A value can reflect the defect distribution density of the silicon-carbon composite material. When the A value is within the above range, it is conducive to the embedding of active ions into the silicon-carbon composite material from all directions, which can improve the uniformity of the embedding of active ions into the silicon-carbon composite material and also facilitate the embedding of more active ions into the silicon-carbon composite material, thereby improving the specific capacity, first coulombic efficiency, cycle performance, rate performance and low-temperature performance of the silicon-carbon composite material.

[0008] Based on the first aspect, in some possible embodiments, the Raman spectrum characteristics of the silicon-carbon composite material further satisfy: 0.91≤B≤1.62, and the B value is 1 d and Ig The ratio of I d Indicates that the Raman spectrum is at 1350±5cm -1 The peak intensity at I g Indicates that the Raman spectrum is at 1580±5cm -1 The peak intensity at . Set I d and I g The ratio of B to C can reflect the defectivity of the silicon-carbon composite material. When the B value is within the above range, it is beneficial to increase the rate of active ion embedding into the silicon-carbon composite material and reduce the resistance encountered by the active ions embedded in the silicon-carbon composite material, thereby improving the rate performance and cycle performance of the silicon-carbon composite material.

[0009] Based on the first aspect, in some possible embodiments, the Raman spectrum characteristics of the silicon-carbon composite material further satisfy: 0.24≤C≤0.5, and the C value is 1 2d and I g The ratio of I g Indicates that the Raman spectrum is at 1580±5cm -1 The peak intensity at . Set I 2d and I g The ratio can reflect the degree of graphitization of the silicon-carbon composite material, which is beneficial to improving the electronic conductivity and the rate capability of the silicon-carbon composite material.

[0010] Based on the first aspect, in some possible embodiments, the Raman spectral characteristics of the silicon-carbon composite material further satisfy the following: 0.5≤A / B≤0.87. This facilitates regulating the balance between the defect distribution density and defect degree of the silicon-carbon composite material, thereby increasing the rate of active ion embedding into the silicon-carbon composite material while also improving the uniformity of active ion embedding into the silicon-carbon composite material. This improves the specific capacity, rate capability, low-temperature performance, and cycling performance of the silicon-carbon composite material while maintaining excellent electronic and ionic conductivity.

[0011] Based on the first aspect, in some possible embodiments, the silicon-carbon composite material further satisfies the following: 0.01≤a / b≤0.07, where a represents the mass fraction of oxygen in the silicon-carbon composite material, and b represents the mass fraction of silicon in the silicon-carbon composite material. Regulating the relationship between oxygen and silicon in the silicon-carbon composite material is beneficial to the utilization of silicon in the silicon-carbon composite material and also helps improve the volume expansion of silicon in the silicon-carbon composite material, thereby enabling the silicon-carbon composite material to have good specific capacity and cycle performance.

[0012] Based on the first aspect, in some possible implementations, 0.3%≤a≤2.5%, which is beneficial to the utilization of silicon in the silicon-carbon composite material and can also reduce the generation of SiOx. A small amount of SiOx can react with a small amount of lithium ions to generate lithium silicate with buffering capacity, thereby improving the specific capacity, cycle performance and expansion performance of the secondary battery.

[0013] Based on the first aspect, in some possible implementations, 30%≤b≤60%. When the silicon-carbon composite material has good expansion properties, it is beneficial to improve the specific capacity of the silicon-carbon composite material.

[0014] Based on the first aspect, in some possible embodiments, the powder conductivity of the silicon-carbon composite material is 0.7 S / cm to 3.1 S / cm, which facilitates electron conduction within the silicon-carbon composite material and improves the specific capacity, cycle performance, and rate performance of the silicon-carbon composite material.

[0015] Based on the first aspect, in some possible embodiments, the specific surface area of ​​the silicon-carbon composite material is 1.1 m 2 / g to 15.1m 2 / g. It is beneficial to reduce the side reaction between the silicon-carbon composite material and the electrolyte, and improve the first coulombic efficiency and cycle performance of the silicon-carbon composite material.

[0016] Based on the first aspect, in some possible embodiments, the silicon-carbon composite material has a particle size Dv50 of 5.0 μm to 13.0 μm, and a particle size Dv99 of 12.0 μm to 25.0 μm. This helps reduce the risks of increased electrolyte consumption and low material compaction density, as well as the risks of low ionic conductivity and reduced rate performance. It also helps shorten the silicon deposition path, reduces the transmission path of electrons and active ions, and improves the uniformity of silicon deposition, thereby reducing the volume expansion of the silicon-carbon composite material when lithium is inserted.

[0017] Based on the first aspect, in some possible embodiments, the shell has a thickness of 5 nm to 100 nm, which facilitates the transmission of electrons and active ions and improves the rate capability and cycle performance of the silicon-carbon composite material.

[0018] The second aspect of the present application provides a method for preparing a silicon-carbon composite material, comprising: providing porous carbon; introducing a silicon source gas into the porous carbon to form silicon on the porous carbon to obtain a first intermediate; introducing a first carbon source into the first intermediate and reacting at a first reaction temperature to obtain a second intermediate, the second intermediate comprising a core body and a first carbon layer formed on the core body, the core body comprising porous carbon and silicon dispersed in the porous carbon; introducing a second carbon source into the second intermediate and reacting at a second reaction temperature to form a second carbon layer on the second intermediate to obtain a silicon-carbon composite material, the second reaction temperature being higher than the first reaction temperature.

[0019] In the present application, a first carbon layer with certain defects is formed on a first intermediate, and a second carbon source is carbon-coated on the second intermediate at a second reaction temperature to regulate the defect distribution density of the silicon-carbon composite material to be compatible with the electronic conductivity of the silicon-carbon composite material and the rate of inward diffusion of active ions, which is beneficial for the active ions to be embedded in the silicon-carbon composite material from all directions and for more active ions to be embedded in the silicon-carbon composite material, thereby further improving the specific capacity, cycle performance, rate performance and low-temperature performance of the secondary battery.

[0020] Based on the second aspect, in some possible embodiments, the first reaction temperature is 500°C to 600°C, which is conducive to regulating the defects on the surface of the first carbon layer during the carbon deposition of the first carbon source, thereby facilitating the regulation of the defect distribution density and defect degree of the silicon-carbon composite material. The second reaction temperature is 600°C to 800°C. The higher second reaction temperature promotes the deposition of the second carbon source and fills the defects in the second intermediate, and at the same time promotes the decomposition and volatilization of hydrogen in the first carbon layer, further regulating the defect distribution density and defect degree on the surface of the silicon-carbon composite material, and improving the uniformity of the carbon material on the surface of the core body, forming a carbon coating layer with a certain density.

[0021] Based on the second aspect, in some possible embodiments, before the second carbon source is introduced into the second intermediate, carbon dioxide is also introduced into the second intermediate, and the time for introducing carbon dioxide is 1 hour to 3 hours, and the flow rate of introducing carbon dioxide is 50 sccm to 150 sccm. When carbon dioxide is introduced at the second reaction temperature, carbon dioxide can not only regulate the defects on the first intermediate, but also reduce the formation of silicon carbide and lower the temperature at which the graphite carbon layer is formed.

[0022] A third aspect of the present application provides a secondary battery comprising a negative electrode plate, the negative electrode plate comprising a silicon-carbon composite material. The secondary battery comprises the silicon-carbon composite material, wherein the A value of the silicon-carbon composite material satisfies 0.77≤A≤0.95, thereby facilitating the embedding of active ions into the silicon-carbon composite material from all directions, improving the uniformity of the embedding of active ions into the silicon-carbon composite material, and facilitating the embedding of more active ions into the silicon-carbon composite material, thereby improving the specific capacity, cycle performance, rate performance, and low-temperature performance of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0024] FIG1 is a scanning electron microscope image of the silicon-carbon composite material prepared in Example 1.

[0025] FIG2 is a Raman spectrum of the silicon-carbon composite material prepared in Example 1.

[0026] FIG3 is a Raman spectrum of the silicon-carbon composite material prepared in Comparative Example 3. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application are described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0028] Below, embodiments of the present application will be described in detail. However, the present application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments illustrated herein. Rather, these exemplary embodiments are provided to make the present application thorough and detailed for those skilled in the art.

[0029] One embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0030] The outer shell can be a packaging bag encapsulated by an encapsulating film (such as an aluminum-plastic film), such as when the electrochemical device is a soft-pack battery. In other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0031] The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator, with the separator being disposed between the positive and negative electrode sheets. The electrode assembly can be a laminated structure, formed by stacking the positive electrode sheet, separator, and negative electrode sheet. In other embodiments, the electrode assembly can also be a wound structure, formed by stacking the positive electrode sheet, separator, and negative electrode sheet and then winding them.

[0032] Negative electrode

[0033] The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on the negative electrode current collector. The negative electrode current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector. It can also be any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode active layer contains a negative electrode active material, which includes a silicon-carbon composite material.

[0034] The silicon-carbon composite material includes a core and a shell covering the core, the core includes porous carbon and silicon dispersed in the porous carbon, the shell includes a carbon material, and the Raman spectrum characteristics of the silicon-carbon composite material meet the following requirements: 0.77≤A≤0.95, and the A value is 1 S3 and I 2d The ratio of I 2dIndicates that the Raman spectrum is at 2700±5cm -1 The peak intensity at I S3 Indicates that the Raman spectrum is at 2960±5cm -1 The peak intensity at .

[0035] In this application, the S3 peak of the silicon-carbon composite material is related to the defects of the silicon-carbon composite material, the 2d peak is used to reflect the number of graphene layers of the silicon-carbon composite material, and the size of the A value can reflect the defect distribution density of the silicon-carbon composite material. When the A value is within the above range, it is beneficial for active ions to be embedded in the silicon-carbon composite material from all directions, which can improve the uniformity of active ions embedded in the silicon-carbon composite material, and is also beneficial for more active ions to be embedded in the silicon-carbon composite material, thereby improving the rate performance and low-temperature performance of the silicon-carbon composite material. In addition, the active ions are more uniformly embedded in the silicon-carbon composite material, which increases the active ions embedded in the silicon-carbon composite material, which is beneficial to improving the specific capacity and first coulomb efficiency of the silicon-carbon composite material; during the charge and discharge process of the silicon-carbon composite material, the active ions embedded in the silicon-carbon composite material are more uniform, so that the expansion force on the silicon-carbon composite material is more uniform, which is beneficial to improving the cycle performance of the silicon-carbon composite material.

[0036] If the A value is large, the defect distribution density of the silicon-carbon composite material is large, which will affect the structural stability of the silicon-carbon composite material, cause the silicon-carbon composite material to collapse during the cycle, and reduce the electronic conductivity, thereby reducing the specific capacity, cycle performance and rate performance of the silicon-carbon composite material. If the A value is small, the defect distribution density of the silicon-carbon composite material is small, which will increase the resistance of active ions to be embedded in the silicon-carbon composite material, and will also affect the uniformity of active ions embedded in the silicon-carbon composite material, thereby affecting the specific capacity, first coulomb efficiency, rate and low-temperature performance of the silicon-carbon composite material. For example, the A value can be 0.77, 0.78, 0.80, 0.82, 0.84, 0.86, 0.88, 0.90, 0.92, 0.94, 0.95 or any value within the range composed of any two of the above values.

[0037] In some embodiments, the Raman spectrum characteristics of the silicon-carbon composite material further satisfy the following: 0.91≤B≤1.62, and the B value is 1 d and I g The ratio of I d Indicates that the Raman spectrum is at 1350±5cm -1 The peak intensity at D represents the defect peak of the carbon material, and the defect can be located on the surface or edge of the carbon material layer. d and I gThe ratio of A to B can reflect the defectivity of the silicon-carbon composite material. When the B value is within the above range, it is obviously beneficial to increase the rate of active ion embedding in the silicon-carbon composite material and reduce the resistance encountered by the active ions embedded in the silicon-carbon composite material, thereby improving the rate performance and cycle performance of the silicon-carbon composite material. When both A and B meet the above conditions, the defect distribution and defectivity of the silicon-carbon composite material can be improved at the same time, so as to further improve the uniformity and rate of active ion embedding in the silicon-carbon composite material, thereby improving the specific capacity, rate performance, cycle performance and low-temperature performance of the silicon-carbon composite material.

[0038] If the B value is large, the defect degree is large, which increases the contact area between the silicon-carbon composite material and the electrolyte, increases the occurrence of side reactions, and also affects the stability of the silicon-carbon composite material structure, thereby affecting the specific capacity, low temperature performance and cycle performance of the silicon-carbon composite material. If the B value is small, the defect degree is small, which reduces the channel for active ions to be embedded in the silicon-carbon composite material, affects the transmission of active ions, and affects the rate performance and cycle performance of the silicon-carbon composite material. For example, the B value can be 0.91, 0.93, 0.95, 0.97, 0.99, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.28, 1.3, 1.32, 1.35, 1.38, 1.4, 1.45, 1.49, 1.5, 1.55, 1.6, 1.62 or any value within the range composed of any two of the above values.

[0039] In some embodiments, the Raman spectrum characteristics of the silicon-carbon composite material further satisfy the following: 0.24≤C≤0.5, and the C value is 1 2d and I g The ratio of I g Indicates that the Raman spectrum is at 1580±5cm -1 The peak intensity at G represents the order of carbon materials. 2d and I g The ratio can reflect the degree of graphitization of the silicon-carbon composite material. When the C value is within the above range, the electronic conductivity is significantly improved, which is beneficial to improving the rate capability of the silicon-carbon composite material. When both A and C values ​​meet the above conditions, the electronic conductivity of the silicon-carbon composite material can be improved by adjusting the defect distribution of the silicon-carbon composite material to further improve the rate capability of the silicon-carbon composite material. When the A, B and C values ​​meet the above conditions, the defect distribution, graphitization degree and defectivity of the silicon-carbon composite material can be simultaneously regulated to improve the electronic conductivity and ionic conductivity of the silicon-carbon composite material, and significantly improve the rate capability, low temperature performance and cycle performance of the silicon-carbon composite material.

[0040] If the C value is large, I 2dThe peak intensity is larger, the order of the silicon-carbon composite material is large, and the defects are fewer, which affects the embedding of more active ions into the silicon-carbon composite material. If the C value is smaller, the disorder of the silicon-carbon composite material is large, which affects the electron transport ability of the silicon-carbon composite material. The above C value being larger or smaller will affect the cycle performance, rate performance and low temperature performance of the silicon-carbon composite material. For example, the C value can be 0.24, 0.25, 0.27, 0.28, 0.3, 0.32, 0.35, 0.39, 0.41, 0.43, 0.49, 0.5 or any value within the range composed of any two of the above values.

[0041] In some embodiments, the Raman spectral characteristics of the silicon-carbon composite material also satisfy: 0.5≤A / B≤0.87. The defect distribution density and defectivity of the silicon-carbon composite material also have the above relationship. When A / B meets the above conditions, it is beneficial to regulate the balance between the defect distribution density and defectivity of the silicon-carbon composite material. While increasing the rate of active ion embedding into the silicon-carbon composite material, it can also improve the uniformity of active ion embedding into the silicon-carbon composite material, so as to improve the specific capacity, rate capability, low temperature performance and cycle performance of the silicon-carbon composite material under the condition that the silicon-carbon composite material has better electronic conductivity and ionic conductivity. For example, the ratio of A / B can be 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.87 or any value within the range composed of any two of the above values. More preferably, 0.7≤A / B≤0.87.

[0042] In some embodiments, the silicon-carbon composite material further satisfies: 0.01≤a / b≤0.07, where a represents the mass ratio of oxygen in the silicon-carbon composite material, and b represents the mass ratio of silicon in the silicon-carbon composite material. Under the above conditions, controlling the relationship between oxygen and silicon in the silicon-carbon composite material is beneficial to the function of silicon in the silicon-carbon composite material, and is also beneficial to improving the volume expansion of silicon in the silicon-carbon composite material, so that the silicon-carbon composite material has good specific capacity and cycle performance. If the ratio of a / b is large, the mass ratio of oxygen in the silicon-carbon composite material is large, and the mass ratio of silicon is small, which is not conducive to improving the specific capacity of the silicon-carbon composite material, and the large amount of oxygen will also affect the dynamics of the silicon-carbon composite material; if the ratio of a / b is small, the mass ratio of silicon in the silicon-carbon composite material is large, which will affect the cycle performance of the silicon-carbon composite material. For example, the ratio of a / b can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, or any value within the range of any two of the above values.

[0043] In some embodiments, 0.3% ≤ a ≤ 2.5%, which facilitates the utilization of silicon in the silicon-carbon composite material and reduces the generation of SiOx. A small amount of SiOx can react with a small amount of lithium ions to form lithium silicate with buffering capacity, thereby improving the specific capacity, cycling performance, and expansion performance of the secondary battery. For example, a can be 0.3%, 0.5%, 0.7%, 0.9%, 1%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9%, 2%, 2.1%, 2.2%, 2.5%, or any value within a range formed by any two of the foregoing values.

[0044] In some embodiments, 30% ≤ b ≤ 60%, which is beneficial for improving the specific capacity of the silicon-carbon composite material when the silicon-carbon composite material has good expansion properties. For example, b can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, or any value within a range formed by any two of the foregoing values.

[0045] In some embodiments, 37% ≤ c ≤ 70%, where c represents the mass fraction of carbon in the silicon-carbon composite material. This improves the electrical conductivity of the silicon-carbon composite material, resulting in good cycling stability. c can be 37%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any value within a range formed by any two of the foregoing values.

[0046] In some embodiments, the powder conductivity of the silicon-carbon composite material is 0.7 S / cm to 3.1 S / cm, which is conducive to the conduction of electrons within the silicon-carbon composite material, thereby improving the specific capacity, cycle performance, and rate performance of the silicon-carbon composite material. The powder conductivity of the silicon-carbon composite material can be 0.7 S / cm, 0.9 S / cm, 1.1 S / cm, 1.3 S / cm, 1.5 S / cm, 1.7 S / cm, 1.9 S / cm, 2.1 S / cm, 2.3 S / cm, 2.5 S / cm, 2.7 S / cm, 2.9 S / cm, 3.1 S / cm, or any value within the range formed by any two of the above values.

[0047] In some embodiments, the specific surface area of ​​the silicon-carbon composite material is 1.1 m 2 / g to 15.1m 2 / g. It is beneficial to reduce the side reaction between silicon-carbon composite materials and electrolyte, and improve the initial coulombic efficiency and cycle performance of silicon-carbon composite materials. The specific surface area of ​​silicon-carbon composite materials can be 1.1m 2 / g, 2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m2 / g、9m 2 / g、10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15.1m 2 / g or any value within the range formed by any two of the above values.

[0048] In some embodiments, the particle size D of the silicon-carbon composite material V 50 is 5μm to 13μm, particle size D V 99 is 12μm to 25μm. The silicon-carbon composite material within the above particle size range can, on the one hand, not only reduce the risk of increased electrolyte consumption and low material compaction density, but also reduce the risk of low ion conductivity and reduced rate performance; on the other hand, it is also beneficial to shorten the silicon deposition path, reduce the transmission path of electrons and active ions, and improve the uniformity of silicon deposition, thereby reducing the volume expansion of the silicon-carbon composite material when lithium is inserted. In particular, when the silicon-carbon composite material prepared from the silicon-carbon composite material is used in the negative electrode sheet, the uniform dispersion in the negative electrode slurry preparation process can be improved. For example, the particle size D of the silicon-carbon composite material V 50 can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm or any value within the range formed by any two of the above values. V 99 can be 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm or any value within the range formed by any two of the above values.

[0049] Among them, D V 50 is also called the "median particle size", which means the particle size at which the volume of the silicon-carbon composite material particles reaches 50% of the cumulative volume measured from the smallest particle size in the volume-based particle size distribution. That is, the volume of the silicon-carbon composite material particles smaller than this particle size accounts for 50% of the total volume of the silicon-carbon composite material particles. V 99 indicates the particle size that reaches 99% of the cumulative volume, measured from the smallest particle size, in the volume-based particle size distribution of the silicon-carbon composite material particles.

[0050] In some embodiments, the thickness of the shell is 5 nm to 100 nm, which is beneficial to the transmission of electrons and active ions and improves the rate performance and cycle performance of the silicon-carbon composite material. The thickness of the shell can be 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, or any value within the range formed by any two of the above values.

[0051] The present application also provides a method for preparing a silicon-carbon composite material, comprising:

[0052] S1, providing porous carbon.

[0053] The porous carbon can be at least one of activated carbon, template porous carbon, carbon molecular sieve, carbon nanofiber, carbon nanotube, expanded graphite, etc. Preferably, the activated carbon is obtained by activating biochar, resin carbon, coke, etc.

[0054] S2, introducing silicon source gas into the porous carbon to form silicon on the porous carbon to obtain a first intermediate.

[0055] Under a certain temperature and pressure, silicon source gas deposits silicon in the pores or on the surface of porous carbon. The silicon source gas includes at least one of monosilane, disilane and trisilane. In this step, the silicon source gas and the inert gas are mixed or introduced into the chemical vapor deposition furnace at the same time for silicon deposition, and the inert gas includes at least one of nitrogen or argon. The mixture of silicon source gas and inert gas (referred to as mixed gas) is conducive to obtaining a silicon deposition layer with uniform silicon size. Among them, the flow rate of the mixed gas is 300 sccm to 600 sccm. In the mixed gas, the volume proportion of the silicon source gas is 5% to 20%.

[0056] In this step, the reaction temperature of the silicon source gas is 420°C to 470°C, and the reaction time is 6h to 10h. The above reaction temperature and reaction time can promote the decomposition of the silicon source gas, and regulate the mass proportion of silicon in the silicon-carbon composite material, thereby improving the specific capacity and initial efficiency of the silicon-carbon composite material.

[0057] S3, introducing a first carbon source into the first intermediate and reacting at a first reaction temperature to obtain a second intermediate, wherein the second intermediate includes a core body and a first carbon layer formed on the core body, and the core body includes porous carbon and silicon dispersed in the porous carbon.

[0058] In this step, the first carbon source and the inert gas are mixed or introduced simultaneously into a chemical vapor deposition furnace for the first carbon deposition, and the inert gas includes at least one of nitrogen or argon. The first carbon source includes at least one of acetylene, propylene or ethylene. At the first reaction temperature, the first carbon source is deposited on the surface of the core to form a first carbon layer. In the mixed gas of the first carbon source and the inert gas, the volume proportion of the first carbon source is 10% to 50%. For example, the volume proportion of the first carbon source can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or any value within the range formed by any two of the above values.

[0059] The first reaction temperature is 500°C to 600°C, and the reaction time is 3h to 10h. Under these temperature and time conditions, the first carbon source forms a first carbon layer on the surface of the core body. The surface of the formed first carbon layer has certain defects, which is conducive to the subsequent second carbon deposition to control the defects on the surface of the carbon material. In some embodiments, the first reaction temperature can be 500°C, 520°C, 550°C, 580°C, 600°C, or any value within the range of any two of the above values. The reaction time is 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or any value within the range of any two of the above values.

[0060] In some embodiments, the flow rate of the first carbon source / inert gas is 200 sccm to 600 sccm. Under these flow rate conditions, the first carbon source and the first intermediate react sufficiently and production costs are reduced. For example, the flow rate of the first carbon source can be 200 sccm, 300 sccm, 400 sccm, 500 sccm, 600 sccm, or any value within a range formed by any two of the foregoing values.

[0061] By regulating the flow rate, time, and reaction temperature of the first carbon source, defects in the silicon-carbon composite can be adjusted. Increasing the flow rate of the first carbon source increases the amount of decomposed carbon source, while increasing the amount of carbon deposited over the same period. The deposited carbon becomes looser and the carbon layer thicker, with a slight increase in the A and B values ​​and a negligible change in the C value. Reducing the flow rate of the first carbon source reduces the amount of carbon deposited, slightly decreasing the A and B values, and negligible changes in the C value.

[0062] If the carbonization temperature of the first carbon source is increased, the decomposition rate of the carbon source increases, and the volatilized hydrogen-containing substances also increase, the degree of graphitization increases, and the defects slightly decrease, thereby reducing the A and B values ​​and increasing the C value. If the carbonization temperature of the first carbon source is decreased, the degree of graphitization decreases, the defects slightly increase, the A and B values ​​increase, and the C value decreases.

[0063] Increasing the reaction time of the first carbon source increased the overall deposition amount, while the A and B values ​​gradually decreased and the C value remained unchanged. Decreasing the reaction time of the first carbon source slightly increased the A and B values ​​and maintained the C value unchanged.

[0064] S4, introducing carbon dioxide into the second intermediate at a second reaction temperature and reacting to obtain a modified material.

[0065] The introduction of carbon dioxide into the second intermediate can, on the one hand, increase the defect distribution density on the surface of the first carbon layer, improving the defect distribution density on the surface of the first carbon layer, which is beneficial to improving the cycle performance, rate performance, and low-temperature performance of the silicon-carbon composite material. On the other hand, the added carbon dioxide can also act as a mild oxidant and catalyst, reducing the active silicon on the surface of the first carbon layer to form SiC, which is beneficial to improving the specific capacity of the silicon-carbon composite material. Carbon dioxide can also catalytically reduce the decomposition temperature of the carbon source, lowering the formation temperature of the graphite carbon layer in the silicon-carbon composite material, and improving the degree of graphitization.

[0066] In some embodiments, the second reaction temperature is 600° C. to 800° C. At this temperature, carbon dioxide acts on the surface of the first carbon layer, thereby facilitating the control of the defect density distribution on the surface of the first carbon layer. For example, the second reaction temperature can be 600° C., 650° C., 700° C., 750° C., 800° C., or any value within a range formed by any two of the foregoing values.

[0067] The reaction time for introducing carbon dioxide is 1 hour to 3 hours, and the flow rate of carbon dioxide is 50 sccm to 150 sccm. Under the above conditions, the carbon dioxide is fully reacted on the first carbon layer, and the defect distribution density on the surface of the first carbon layer is fully controlled. For example, the reaction time for carbon dioxide is 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or any value within the range of any two of the above values. The flow rate of carbon dioxide can be 50 sccm, 75 sccm, 100 sccm, 125 sccm, 150 sccm, or any value within the range of any two of the above values.

[0068] The time and flow rate of carbon dioxide injection affect the defect distribution density of the silicon-carbon composite material. If the carbon dioxide injection time is increased, the A and B values ​​decrease, and the C value increases slightly. If the carbon dioxide injection time is reduced, the A and B values ​​increase slightly, and the C value decreases slightly. If the carbon dioxide injection flow rate is increased, the A and B values ​​decrease, and the C value increases slightly; if the carbon dioxide injection flow rate is reduced, the A and B values ​​increase slightly, and the C value decreases slightly.

[0069] S5, introducing a second carbon source into the modified material and reacting at a second reaction temperature to form a second carbon layer on the modified material to obtain a silicon-carbon composite material, wherein the second reaction temperature is higher than the first reaction temperature.

[0070] After stopping the introduction of carbon dioxide into the second intermediate, at the second reaction temperature, after the second carbon source is introduced, the second carbon source is deposited on the modified material and forms a second carbon layer, thereby forming a shell on the surface of the core body. Compared with the first reaction temperature, the higher second reaction temperature promotes the deposition of the second carbon source and fills the internal and external defects of the porous carbon in the modified material, and at the same time promotes the removal of more hydrogen during the decomposition of the second carbon source, and increases the volatilization of hydrogen and oxygen-containing substances in the first carbon layer, so as to fully regulate the defects on the surface of the second carbon layer, further regulate the defect distribution density and defect degree on the surface of the silicon-carbon composite material, and improve the uniformity of the carbon material on the surface of the core body, forming a carbon coating layer with a certain density. At the same time, the second reaction temperature can also increase the decomposition rate of the second carbon source and promote the decomposition of the second carbon source.

[0071] In this step, the second carbon source and an inert gas are mixed or introduced simultaneously into a chemical vapor deposition furnace for a second carbon deposition. The inert gas includes at least one of nitrogen or argon. The second carbon source includes at least one of methane, acetylene, propylene, or ethylene. The volume percentage of the second carbon source in the second carbon source and the inert gas is 5% to 20%. For example, the volume percentage of the second carbon source can be 5%, 8%, 10%, 12%, 18%, 20%, or any value within a range formed by any two of the foregoing values.

[0072] In some embodiments, the reaction time of the second carbon source is 4h to 11h, and the flow rate of the second carbon source / inert gas is 300sccm to 700sccm. Under the above conditions, a second carbon layer of a certain thickness is formed on the first carbon layer to obtain a carbon coating layer with a certain density, and the defect distribution density and defect degree on the surface of the silicon-carbon composite material are regulated by controlling the reaction temperature, reaction time and flow rate of the second carbon source, which is beneficial to improving the cycle performance, rate performance and low temperature performance of the silicon-carbon composite material. For example, the reaction time of the second carbon source can be 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h or any value within the range of any two of the above values. The flow rate of the second carbon source can be 300sccm, 400sccm, 500sccm, 600sccm, 700sccm or any value within the range of any two of the above values.

[0073] After the second carbon layer is formed on the modified material, the material is further ground and sieved through 400 meshes to finally obtain a silicon-carbon composite material.

[0074] The second reaction temperature affects the defect distribution density, defect degree, and graphitization degree of the silicon-carbon composite material. If the second reaction temperature is high, the decomposition rate of the carbon source increases, the degree of graphitization increases, the amount of hydrogen removed by the carbon source itself also increases, and the amount of hydrogen and oxygen-containing substances volatilized from the first carbon layer also increases. When the temperature increases to a certain level, the specific surface area increases significantly. At the same time, excessive carbon coating also easily forms pores, which in turn increases defects. As a result, the A and B values ​​first decrease and then increase, while the C value increases.

[0075] If the second reaction temperature is low, the decomposition rate of the second carbon source decreases, the degree of graphitization decreases, and the defects increase relatively, so that the A value is slightly increased, the B value increases, and the C value decreases.

[0076] If the reaction time of the second carbon source is longer, the overall deposition amount can be increased, the A and B values ​​gradually decrease, and the C value does not change significantly. If the reaction time of the second carbon source is shorter, the A and B values ​​increase slightly, and the C value does not change significantly.

[0077] In the present application, by forming a first carbon layer with certain defects on the first intermediate, and by passing carbon dioxide at a second reaction temperature, carbon dioxide can further regulate the defects on the first intermediate. At the second reaction temperature, the second carbon source is carbon-coated again on the third intermediate to regulate the defect distribution density of the silicon-carbon composite material, so as to be compatible with the electronic conductivity of the silicon-carbon composite material and the rate at which the active ions diffuse inward, thereby facilitating the embedding of active ions into the silicon-carbon composite material from all directions and facilitating the embedding of more active ions into the silicon-carbon composite material, thereby further improving the specific capacity, cycle performance, rate performance and low temperature performance of the secondary battery. In the above process, the introduction of carbon dioxide can also reduce the generation of silicon carbide and lower the temperature at which the graphite carbon layer is formed.

[0078] In some embodiments, the negative electrode active material layer further comprises a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0079] In some embodiments, the conductive agent includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0080] Isolation film

[0081] The material and shape of the separator used in the secondary battery of the present application are not particularly limited and can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0082] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film, or composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric, or polypropylene-polyethylene-polypropylene porous composite film may be used.

[0083] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by a mixture of a polymer and an inorganic material. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

[0084] The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer comprises a polymer, and the polymer material is selected from at least one of polyamide, polyacrylonitrile, an acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0085] electrolyte

[0086] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and optional additives.

[0087] The organic solvent in the electrolyte of the present application may be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. The electrolyte used in the electrolyte according to the present application is not limited, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application may be any additive known in the prior art that can be used as an electrolyte additive. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) or ethyl propionate.

[0088] In some embodiments, the organic solvent includes an ether solvent, for example, at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate) LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0089] The secondary battery provided by the present application includes a negative electrode plate, and the negative electrode plate includes a silicon-carbon composite material. The silicon-carbon composite material is applied to the secondary battery. In the silicon-carbon composite material, 0.77≤A≤0.95, within this range, can improve the uniformity of active ion embedding into the silicon-carbon composite material, facilitate the embedding of active ions into the silicon-carbon composite material from all directions, thereby improving the cycle performance, rate performance, and low-temperature performance of the silicon-carbon composite material. Thus, while the secondary battery has excellent energy density, it also improves the rate performance, rate performance, and cycle performance of the secondary battery. The preparation method provided by the present application is simple, easy to operate, and suitable for industrial production.

[0090] According to some embodiments of the present application, the secondary battery of the present application includes, but is not limited to: a lithium-ion battery or a sodium-ion battery. In some embodiments, the secondary battery includes a lithium-ion battery.

[0091] This application also applies secondary batteries to electronic devices to power loads within the electronic devices. The electronic devices are coated with silicon-carbon composite materials, which, while having excellent specific capacity, also exhibit good cycle performance, rate performance, and low-temperature performance, thereby increasing the service life, charge-discharge efficiency, and low-temperature applicability of the electronic devices.

[0092] The electronic devices or devices of the present application are not particularly limited. In some embodiments, the electronic devices of the present application include, but are not limited to, laptop computers, pen-type computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and lithium-ion capacitors, etc.

[0093] The present application is described below by way of specific examples and comparative examples. It should be understood by those skilled in the art that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0094] Example 1

[0095] 1) For porous carbon (pore volume 0.9cm 3 / g, specific surface area 1800m 2 / g) was dried and sieved through 400 mesh, 50 g of the treated porous carbon material was weighed and placed in a chemical vapor deposition furnace, the temperature was raised to 450° C. under an argon protective atmosphere, and monosilane / argon was used as a silicon source under a slightly positive gas pressure of 0.5 kPa, wherein the volume proportion of monosilane in the monosilane / argon was 5%, and monosilane / argon was introduced into the chemical vapor deposition furnace at a flow rate of 500 sccm. The reaction was continued for 10 hours, and the elemental nanosilicon was adsorbed and deposited in the pores of the porous carbon.

[0096] 2) In the same vapor deposition furnace, the temperature was continued to be raised to 500°C under an argon protective atmosphere. At a slightly positive gas pressure of 0.5 kPa, acetylene / argon was used as the first carbon source, wherein the volume proportion of acetylene in the acetylene / argon was 50%. The acetylene / argon mixed gas was introduced into the vapor deposition furnace at a flow rate of 500 sccm. The reaction was continued for 5 hours, and acetylene-derived carbon was deposited on the outer surface of the material described in step 1) to form a carbon coating layer.

[0097] 3) In the same vapor deposition furnace, continue to raise the temperature to 800° C. under an argon protective atmosphere, keep the temperature for 1 hour, and then introduce carbon dioxide into the vapor deposition furnace at a flow rate of 100 sccm. After continuing to introduce carbon dioxide for 1 hour, stop introducing carbon dioxide.

[0098] 4) Under a slightly positive gas pressure of 0.5 kPa, methane / argon is used as a second carbon source, wherein the volume proportion of methane in the methane / argon is 10%, and methane / argon is introduced into the vapor deposition furnace at a flow rate of 650 sccm. The reaction is continued for 6 hours, and methane-derived carbon is deposited again on the carbon surface of the material described in step 3) to form a second carbon coating layer.

[0099] 5) Grinding and sieving the sediment sample obtained in step 4) to obtain a silicon-carbon composite material.

[0100] Examples 2 to 13 and Comparative Examples 1 to 3 are substantially the same as Example 1, with the only difference being that different silicon-carbon composite materials were prepared according to the relevant preparation parameters in Table 1, wherein the relevant parameters for testing the silicon-carbon composite materials are shown in Tables 2 and 3.

[0101] Table 1

[0102] The oxygen, carbon, and silicon elements in the silicon-carbon composite materials prepared in Examples 1 to 13 and Comparative Examples 1 to 3 were tested. The powder conductivity, specific surface area, Dv50 / Dv99, Raman spectroscopy, and shell thickness of the silicon-carbon composite materials were also tested. The test data are reported in Tables 2 and 3.

[0103] Test method:

[0104] (1) The testing steps for silicon, oxygen and carbon in silicon-carbon composite materials include:

[0105] The Si content (in wt.%) in the silicon-carbon composite material can be tested with the help of ICP (inductively coupled plasma spectrometer) characterization.

[0106] With the help of EDS characterization, the content of C and O elements in silicon-carbon composite materials can be tested (in wt.%).

[0107] (2) The conductivity test procedure includes: using a powder conductivity meter (model FT-8100) based on the four-probe test principle and the test standard GB / T1552-1995. A known amount of silicon-carbon composite solid powder is compressed to a set pressure value or pressure under hydraulic power, and the conductivity of the solid powder is measured online.

[0108] (3) The specific surface area test steps include: taking a solid powder of the silicon-carbon composite material in a sample tube and vacuum degassing it at 100°C for 12 hours. The adsorption amount of nitrogen by the material at different pressures is tested using a physical adsorption analyzer (model ASAP1460). The adsorption-desorption isotherm is plotted. The shape of the pores in the porous carbon composite material is determined based on the shape of the hysteresis loop. The pore size distribution curve is fitted using the DFT model, and the specific surface area of ​​the material is calculated. The test is carried out in accordance with the national standard GB / T 19587-2017 "Determination of the Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0109] (4)D V 50 / D V The 99 test steps include:

[0110] 0.02 g of the powder sample of the silicon-carbon composite material in each embodiment and comparative example was added to a 50 ml clean beaker, 20 ml of deionized water was added, and a surfactant was added dropwise to completely disperse the powder sample in the water. The sample was ultrasonically cleaned in a 120 W ultrasonic cleaning machine for 5 min. The silicon-carbon composite material was tested using a MasterSizer 2000 device at a detection angle of 0° to 135°. The particle size distribution was measured according to the particle size distribution laser diffraction method GB / T19077-2016 to obtain the D of the silicon-carbon composite material. V 50 and D V 99 value.

[0111] (5) Raman testing steps include:

[0112] An area of ​​100 μm × 100 μm of silicon-carbon composite solid powder was selected, and the particles within the area were scanned using a laser microconfocal Raman spectrometer (Raman, HR Evolution, HORIBA Scientific Instruments Division). The laser wavelength of the Raman spectrometer can be in the range of 532 nm to 785 nm, and the D peak, G peak, 2d peak, and S3 peak of all particles within the area were obtained. The data were processed using LabSpec software to obtain the peak intensities of the D peak, G peak, 2D peak, and S3 peak of each particle, which were recorded as I d , I g , I 2d , I S3 , count the I of all particles in the area S3 and I 2d The average value of the ratio, I d and I g The average value of the ratio and I 2d and I g The average value of the ratio is obtained to obtain the corresponding A, B, C and A / B.

[0113] (6) The test steps for shell thickness include:

[0114] Testing was performed using a FEI TitanThemis 200TEM, Bruker super-X EDS, and FEI Helios 450S dual-beam FIBT instrument. The specific testing steps were as follows: The sample was placed on the test stage, oriented to the target area, and the FIB test voltage was set to 30 kV. The sample stage was horizontally adjusted, and testing began at different magnifications. At a high magnification of 5 nm, the shell within the particle was visible, and its thickness was measured using a ruler.

[0115] (7) SEM test steps include:

[0116] The silicon-carbon composite material was tested using a JEOL-JSM-6700F scanning electron microscope at a voltage of 5 kV and a current of 0.8 nA.

[0117] Referring to FIG. 1 , it can be seen from FIG. 1 that the surface of the silicon-carbon composite material is rough and has low surface flatness.

[0118] 2 and 3, the Raman spectra of the silicon-carbon composite materials prepared in Example 1 and Comparative Example 3 are at 2700 cm -1 The 2d peak at 2960 cm -1 Peak characteristics appeared at the S3 peak.

[0119] Table 2

[0120] The preparation process of button half-cell includes:

[0121] (1) Silicon-carbon composite material, conductive carbon black and sodium alginate were added to deionized water in a mass ratio of 80:10:10, and stirred thoroughly to form a slurry. A coating with a thickness of 100 μm was applied using a scraper. After drying in a vacuum drying oven at 85°C for 12 hours, the coating was cut into discs with a diameter of 1 cm using a punch in a dry environment to obtain a negative electrode sheet.

[0122] Electrolyte and separator: In an argon-filled glove box (moisture <10ppm, oxygen <1ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a 1:1 volume ratio to obtain a mixed solution. 1 mol / L LiPF6 and 5 vol.% fluoroethylene carbonate (FEC) were added to the mixed solution and stirred to obtain the electrolyte. Celgard 2400 separator was used as the separator.

[0123] (2) Assembly of button half-cell:

[0124] In the glove box, a button-type half-cell was assembled in the order of negative electrode sheet, separator, and lithium sheet, using the lithium sheet as the counter electrode.

[0125] The preparation process of lithium-ion batteries includes:

[0126] Positive electrode preparation: LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF) were thoroughly mixed in an N-methylpyrrolidone solvent system at a weight ratio of approximately 95%:2.5%:2.5% to prepare a positive electrode slurry. The slurry was coated onto aluminum foil, dried, and cold-pressed to produce a positive electrode sheet.

[0127] Preparation of negative electrode: Graphite, silicon-carbon composite material prepared in Example and Comparative Example, conductive agent (conductive carbon black, Super ) and binder PAA are mixed in a weight ratio of approximately 70%:15%:5%:10%, and an appropriate amount of water is added. The mixture is kneaded to a solids content of approximately 55% to 70% by weight. An appropriate amount of water is then added to adjust the slurry viscosity to approximately 4000 to 6000 Pa·s to prepare a negative electrode slurry. The prepared negative electrode slurry is coated onto a negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode sheet.

[0128] Preparation of electrolyte: Under a dry argon environment, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed and stirred in a weight ratio of 1:1:1 to obtain a solvent, and 1.15 mol / L LiPF6 is added to the solvent and mixed evenly. Then, about 12.5 wt% of fluoroethylene carbonate (FEC) is added and mixed evenly to obtain an electrolyte.

[0129] Preparation of isolation membrane: PE composite film is used as isolation membrane.

[0130] Lithium-ion battery assembly: The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The winding process yields a bare cell. The bare cell is then placed in an outer packaging, injected with electrolyte, and encapsulated. After formation, degassing, and trimming, the soft-pack lithium-ion battery is obtained.

[0131] The LAND series was used to test the initial lithium removal capacity, capacity retention after 50 cycles, initial coulombic efficiency, low-temperature discharge capacity retention, lithium ion diffusion coefficient range, and the number of cycles to 90% at 25°C for button-type half-cells assembled with silicon-carbon composite materials prepared in each embodiment and comparative example. A 2C / 0.2C discharge rate was tested on soft-pack full-cells assembled with silicon-carbon composite materials prepared in each embodiment and comparative example. The specific testing methods are as follows:

[0132] 1. The test methods for the first discharge specific capacity, first coulombic efficiency and capacity retention rate after 50 cycles of half-cell include:

[0133] At 25°C and normal pressure, the prepared button half-cell was discharged at a constant current of 0.1C to 0.01V, and then allowed to stand for 5 minutes. The discharge specific capacity at this time was recorded, which was the first discharge specific capacity; then it was charged at a constant current of 0.1C to 1.5V, and then charged at a constant voltage to a current of 0.05C, and then allowed to stand for 5 minutes. This was a cyclic charge and discharge process, and the charge capacity at this time was recorded, which was the first charge specific capacity.

[0134] First coulombic efficiency (%) = (first lithium removal capacity / first lithium insertion capacity) × 100%.

[0135] The button half-cell was subjected to 50 cycle charge and discharge tests according to the above method, and the discharge specific capacity of each cycle was recorded.

[0136] 50-cycle capacity retention rate (%) of the coin-type half-cell = discharge specific capacity at the 50th cycle / discharge specific capacity at the 2nd cycle × 100%.

[0137] 2. Test of low temperature discharge capacity retention rate:

[0138] The assembled half-cell was discharged at 0.5C constant current to 0.01V at 25°C, allowed to rest for 5 minutes, then discharged at 50μA constant current to 0.01V, allowed to rest for 5 minutes, and then charged at 0.5C constant current to 2.0V, allowed to rest for 5 minutes. This cycle was repeated three times, and the discharge capacity of the third cycle was recorded.

[0139] Then, it was placed at -10℃ for 24h, discharged at a constant current of 0.5C to 0.01V, left to stand for 5min, and then discharged at a constant current of 50μA to 0.01V, and the discharge capacity at low temperature was recorded.

[0140] Low-temperature discharge capacity retention rate (%)=(discharge capacity under low-temperature conditions / discharge capacity of the third cycle)×100%.

[0141] 3. Discharge rate test:

[0142] The soft-pack full battery prepared above was discharged to 3.0V at 0.2C at 25°C, allowed to stand for 5 minutes, charged to 4.45V at 0.5C, charged to 0.05C at a constant voltage and allowed to stand for 5 minutes. The discharge rate was adjusted and discharge tests were performed at 0.2C, 0.5C, 1C, 1.5C, and 2.0C, respectively, to obtain the discharge capacity. The capacity obtained at each rate was compared with the capacity obtained at 0.2C, and the rate performance of the full battery was recorded by comparing the ratio of 2C to 0.2C.

[0143] 4. The steps for testing the lithium ion diffusion coefficient by constant current intermittent titration method include:

[0144] Using the Xinwei constant current charge and discharge tester, the assembled button half-cell was left at room temperature for 12 hours, then discharged at a constant current of 0.1C until the voltage was less than or equal to 0.005V. After standing for 1 hour, if the voltage was greater than 0.005V, continue to discharge at a constant current of 0.1C until the voltage was less than or equal to 0.005V, then stand for 1 hour, charge at a constant current of 0.1C until the voltage was greater than or equal to 1.5V, and then stand for 1 hour; if the voltage was less than 1.5V, continue to charge at a constant current of 0.1C until the voltage was greater than or equal to 1.5V, then stand for 1 hour, and then discharge at a constant current of 0.1C until the voltage was less than or equal to 0.005V. Repeat the above steps for two cycles and the test is complete. The tester software obtains the lithium ion diffusion coefficient according to the following calculation formula:

[0145] Where m and M represent the mass and molar mass of the active substance, respectively, and V m is the molar volume, τ is the relaxation time, A is the electrode surface area, ΔE s is the voltage change caused by the pulse, ΔE τ The voltage change during constant current charging and discharging.

[0146] 5. Cycle performance test

[0147] The test temperature was 25°C. The battery was charged at a constant current of 0.7C to 4.4V, then charged at a constant voltage of 0.025C. After a 5-minute rest, the battery was discharged at 0.5C to 3.0V. The capacity obtained in this step was used as the initial capacity. Cycling tests were repeated using a 0.7C charge / 0.5C discharge cycle. The capacity at each step was compared to the initial capacity to generate a capacity decay curve. The number of cycles at 25°C until the capacity retention reached 90% was recorded as the room temperature cycling performance of the button cell.

[0148] Table 3

[0149] As shown in Tables 1 to 3 above, compared with Comparative Examples 1 to 3, the Raman spectra of the silicon-carbon composite materials prepared in Examples 1 to 6 are A(I S3 and I 2d The ratio of ), B value and C value are all within the appropriate range, and the corresponding silicon-carbon composite material's first delithiation specific capacity, capacity retention rate after 50 cycles, first coulombic efficiency, low-temperature discharge capacity retention rate, discharge rate, lithium ion diffusion coefficient range and number of cycles to 90% are all significantly improved.

[0150] Compared to Comparative Example 1, Examples 1 to 6 adjust the carbonization temperature of the first carbon source, acetylene, so that the calculated A value of the peak intensity of the Raman spectrum of the silicon-carbon composite material is within an appropriate range, thereby improving the lithium removal specific capacity, cycle performance, first coulombic efficiency, rate performance, and low-temperature performance of the silicon-carbon composite material. Comparative Example 1 has a smaller A value, the defect distribution density of the silicon-carbon composite material is smaller, and there are fewer channels for active ions to be embedded in the silicon-carbon composite material, which affects the specific capacity, first coulombic efficiency, cycle performance, rate performance, and low-temperature performance of the silicon-carbon composite material.

[0151] Compared with Comparative Examples 2 and 3, Examples 1 and 4 to 6 adjusted the carbonization temperature of methane in the second carbon source, and the peak intensity of the Raman spectrum of the silicon-carbon composite material was reduced after calculation, and was within the set range, which correspondingly improved the delithiation specific capacity, cycle performance, first coulomb efficiency, rate performance, and low-temperature performance of the silicon-carbon composite material. In Comparative Example 2, the defect distribution density of the silicon-carbon composite material was large, and the structural stability was poor, which affected the first delithiation specific capacity, first coulomb efficiency, cycle performance, rate performance, and low-temperature performance of the silicon-carbon composite material.

[0152] The second reaction temperature in Comparative Example 3 is lower, and the decomposition amount of the second carbon source is less, resulting in less coating amount of the second carbon layer and less impact on the first carbon layer and the modified material. Therefore, it has no obvious effect on improving the rate performance and cycle performance of the silicon-carbon composite material.

[0153] In Examples 1 and 4 to 6, as the methane carbonization temperature gradually decreases, the corresponding A value gradually increases. This may be because as the methane carbonization temperature decreases, at lower temperatures (>600°C), the rate of methane decomposition decreases and H-containing substances volatilize, forming more defects, thereby increasing the A value.

[0154] In Examples 8 to 10, the silicon-carbon composite materials prepared under different second carbon sources and different first carbon source temperatures all had A values ​​within a suitable range.

[0155] Compared with Comparative Example 2, the carbonization temperature of methane in Example 1 and Examples 4 to 6 was adjusted, and the calculated B value of the peak intensity of the Raman spectrum of the silicon-carbon composite material was reduced and within the set range, balancing the defect distribution density and defect degree of the silicon-carbon composite material, and improving the lithium removal specific capacity, cycle performance, first coulomb efficiency, rate performance and low-temperature performance of the silicon-carbon composite material. In Comparative Example 2, the defect degree in the silicon-carbon composite material is large, which will reduce the stability of the structure, and the contact area between the silicon-carbon composite material and the electrolyte is large, which will affect the specific capacity, first coulomb efficiency, cycle performance, rate performance and low-temperature performance of the silicon-carbon composite material.

[0156] Compared with Comparative Example 1, the carbonization temperature of acetylene is lowered in Examples 1 to 3, and the calculated C values ​​of the peak intensities of the silicon-carbon composite materials prepared in Examples 1 to 3 are all reduced and are within the set range, which is compatible with the defect distribution density and degree of graphitization of the silicon-carbon composite materials, and correspondingly improves the delithiation specific capacity, cycle performance, first coulombic efficiency, rate performance and low-temperature performance of the silicon-carbon composite materials.

[0157] In the embodiment, the A value, B value, C value and A / B value are within the set range, so that the silicon-carbon composite material has a certain defect distribution density, defect degree and degree of graphitization. The shell inhibits the direct contact between the silicon in the core and the electrolyte, and is also conducive to electron conduction and ion diffusion to improve the electronic conductivity and ion conductivity, thereby improving the cycle performance, rate performance and low-temperature performance of the secondary battery.

[0158] Compared with Example 13, the introduction of carbon dioxide in Examples 1 and 7 increased the range of the lithium ion diffusion coefficient and increased the initial delithiation specific capacity, cycle performance, rate performance, and low-temperature performance of the silicon-carbon composite material. In Example 12, the flow rate of the carbon dioxide was too high, which affected the cycle performance, rate performance, and low-temperature performance of the silicon-carbon composite material.

[0159] The above disclosure is only a preferred embodiment of the present application and certainly cannot be used to limit the present application. Therefore, equivalent changes made based on the present application are still within the scope covered by the present application.

Claims

1. A silicon-carbon composite material, wherein: The invention comprises a core body and a shell covering the core body, wherein the core body comprises porous carbon and silicon dispersed in the porous carbon, the shell comprises a carbon material, and the Raman spectrum characteristics of the silicon-carbon composite material meet the following requirements: 0.77≤A≤0.95, and the A value is 1 S3 and I 2d The ratio of I 2d Indicates that the Raman spectrum is at 2700±5cm -1 The peak intensity at I S3 Indicates that the Raman spectrum is at 2960±5cm -1 The peak intensity at .

2. The silicon-carbon composite material according to claim 1, wherein The Raman spectrum characteristics of the silicon-carbon composite material also meet the following requirements: 0.91≤B≤1.62, and the B value is 1 d and I g The ratio of I d Indicates that the Raman spectrum is at 1350±5cm -1 The peak intensity at I g Indicates that the Raman spectrum is at 1580±5cm -1 The peak intensity at .

3. The silicon-carbon composite material according to claim 1 or 2, wherein: The Raman spectrum characteristics of the silicon-carbon composite material also meet the following requirements: 0.24≤C≤0.5, and the C value is 1 2d and I g The ratio of I g Indicates that the Raman spectrum is at 1580±5cm -1 The peak intensity at .

4. The silicon-carbon composite material according to claim 2 or 3, wherein: The Raman spectrum characteristics of the silicon-carbon composite material also satisfy the following: 0.50≤A / B≤0.

87.

5. The silicon-carbon composite material according to any one of claims 1 to 4, wherein The silicon-carbon composite material further satisfies the following conditions: 0.01≤a / b≤0.07, where a represents the mass ratio of oxygen in the silicon-carbon composite material, and b represents the mass ratio of silicon in the silicon-carbon composite material.

6. The silicon-carbon composite material according to claim 5, wherein: 0.3%≤a≤2.5%, 30%≤b≤60%.

7. The silicon-carbon composite material according to any one of claims 1 to 6, wherein The silicon-carbon composite material satisfies at least one of the following conditions: (1) The powder conductivity of the silicon-carbon composite material is 0.7 S / cm to 3.1 S / cm; (2) The specific surface area of ​​the silicon-carbon composite material is 1.1 m 2 / g to 15.1m 2 / g; (3) The particle size Dv50 of the silicon-carbon composite material is 5.0 μm to 13.0 μm, and the particle size Dv99 is 12.0 μm to 25.0 μm; (4) The thickness of the shell is 5 nm to 100 nm.

8. A method for preparing the silicon-carbon composite material according to any one of claims 1 to 7, wherein: include: Providing porous carbon; introducing a silicon source gas into the porous carbon to form silicon on the porous carbon to obtain a first intermediate; introducing a first carbon source into the first intermediate and reacting at a first reaction temperature to obtain a second intermediate, wherein the second intermediate comprises the core body and a first carbon layer formed on the core body, wherein the core body comprises the porous carbon and silicon dispersed in the porous carbon; At a second reaction temperature, a second carbon source is introduced into the second intermediate and reacts to form a second carbon layer on the second intermediate to obtain the silicon-carbon composite material. The second reaction temperature is higher than the first reaction temperature.

9. The preparation method according to claim 8, wherein The first reaction temperature is 500°C to 600°C, and the second reaction temperature is 600°C to 800°C.

10. The preparation method according to claim 8 or 9, wherein Before the second carbon source is introduced into the second intermediate, carbon dioxide is also introduced into the second intermediate. The time for introducing the carbon dioxide is 1 hour to 3 hours, and the flow rate of the carbon dioxide is 50 sccm to 150 sccm.

11. A secondary battery, wherein: The invention comprises a negative electrode plate, wherein the negative electrode plate comprises the silicon-carbon composite material according to any one of claims 1 to 7.

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