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

Abstract

Provided in the present application are 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 coating the core, wherein the shell comprises a carbon material; and the core comprises a carbon framework, a first silicon-deposited layer formed on the carbon framework, a second silicon-deposited layer formed on the first silicon-deposited layer, and an oxide layer formed between the first silicon-deposited layer and the second silicon-deposited layer, the oxide layer comprising SiOx (0.5≤x≤2). The silicon-carbon composite material provided by the present application has a good specific capacity and initial coulombic efficiency, and also has relatively high cycle performance and rate capability.

Description

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

[0001] This application claims priority to Chinese patent application number 202410201577.2, filed with the Patent Office of China on February 23, 2024, entitled “Silicon-carbon composite material, preparation method thereof and secondary battery”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon composite material, a preparation method thereof, and a secondary battery using the silicon-carbon composite material. Background Art

[0003] 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.

[0004] However, as a semiconductor material, silicon has low intrinsic electronic conductivity and its volume expands by up to 400% after lithium insertion, which affects the fast charging capability and cycle performance of secondary batteries and restricts the large-scale application of silicon negative electrode materials in secondary batteries. Summary of the Invention

[0005] The present application provides a silicon-carbon composite material and a preparation method thereof that can improve the fast charging performance and cycle performance of a battery.

[0006] In addition, the present application also provides a secondary battery using the silicon-carbon composite material.

[0007] In a first aspect, the present application provides a silicon-carbon composite material comprising a core and a shell encapsulating the core, wherein the shell comprises a carbon material. The core comprises a carbon skeleton, a first silicon deposited layer formed on the carbon skeleton, a second silicon deposited layer formed on the first silicon deposited layer, and an oxide layer formed between the first and second silicon deposited layers. The oxide layer comprises SiOx, where 0.5≤x≤2.

[0008] In the silicon-carbon composite material provided in the present application, an oxide layer including SiOx is provided between the first silicon deposition layer and the second silicon deposition layer. After lithium insertion, SiOx is partially converted into lithium oxide (Li2O) and lithium silicate. Lithium oxide has a large Li + Diffusion coefficient (theoretical value 10 -8 ~10 -7 cm 2 / s), can be used as Li +The lithium oxide formed can also improve the diffusion dynamics within the silicon-carbon composite material, reduce internal polarization, and thus improve the rate performance and cycle performance of the silicon-carbon composite material. At the same time, because lithium oxide (Li2O) and lithium silicate also have a buffering effect, they can alleviate the volume expansion of silicon in the silicon-carbon composite material, thereby improving the rate performance and cycle performance of the secondary battery.

[0009] Based on the first aspect, in some possible implementations, the thickness of the oxide layer is 10 nm to 20 nm, which is beneficial for increasing the SiOx content while also reducing the risk of reducing the initial coulombic efficiency of the silicon-carbon composite material.

[0010] Based on the first aspect, in some possible embodiments, the specific surface area of ​​the silicon-carbon composite material is 0.5 m 2 / g to 5m 2 / g. This is beneficial for increasing the amount of silicon deposition to improve the initial efficiency of the silicon-carbon composite material, while also reducing the risk of increased electrolyte consumption during the cycle to improve cycle performance.

[0011] Based on the first aspect, in some possible embodiments, the particle size Dv50 of the silicon-carbon composite material is 5 μm to 15 μm. + The diffusion path of the silicon-carbon composite material is improved, the ionic conductivity of the silicon-carbon composite material is improved, and the initial efficiency of the silicon-carbon composite material is improved; at the same time, the defects of increased electrolyte consumption and low material compaction density can be improved, and the risks of low ionic conductivity and poor rate performance caused by large particle size can be reduced.

[0012] Based on the first aspect, in some possible embodiments, the particle size Dv90 of the silicon-carbon composite material is less than or equal to 30 μm. This is beneficial to improving the dispersion uniformity of the slurry when the silicon-carbon composite material is used to prepare the negative electrode material, and improving the Li + transport, thereby improving the cycle performance and expansion performance of the secondary battery.

[0013] Based on the first aspect, in some possible embodiments, based on the mass of the silicon-carbon composite material, the mass proportion of carbon element is 35% to 55%, which buffers the volume expansion of silicon to a certain extent, is beneficial to improving the structural stability of the silicon-carbon composite material, and is also beneficial to improving the conductivity of the silicon-carbon composite material, which is beneficial to improving the cycle performance of the secondary battery.

[0014] Based on the first aspect, in some possible embodiments, the first silicon deposited layer and the second silicon deposited layer account for a total weight ratio of 40% to 55% based on the weight of the silicon-carbon composite material. Within this range of silicon deposited weight, the silicon-carbon composite material has high reversible capacity and initial efficiency while also having good cycling performance, thereby improving the energy density and cycling performance of the secondary battery.

[0015] Based on the first aspect, in some possible implementations, the oxygen element accounts for 5% to 10% by weight of the silicon-carbon composite material, which is beneficial for improving the rate performance of the secondary battery while the silicon-carbon composite material has a high initial efficiency.

[0016] A second aspect of the present application provides a method for preparing a silicon-carbon composite material, which comprises: providing a carbon skeleton; performing a first silicon deposition on the carbon skeleton to form a first silicon deposition layer on the carbon skeleton; oxidizing the first silicon deposition layer in the carbon skeleton in an oxygen-containing atmosphere to form an oxide layer on the surface of the first silicon deposition layer, wherein the oxide layer comprises SiOx, 0.5≤x≤2, thereby obtaining a first intermediate; performing a second silicon deposition on the first intermediate to form a second silicon deposition layer on the surface of the oxide layer, thereby obtaining a second intermediate;

[0017] The second intermediate is carbon-coated to form a shell on the surface of the second intermediate, wherein the shell comprises a carbon material, thereby obtaining a silicon-carbon composite material.

[0018] In the above preparation process, the first silicon deposition layer is oxidized during the process of silicon deposition of carbon skeleton to form a SiOx oxide layer, so that the first silicon deposition layer is located between the second silicon deposition layer, so that the oxide layer is partially converted into Li2O and lithium silicate after lithium insertion, and the lithium oxide has a larger Li + The diffusion coefficient of the lithium-intercalated Li₂O and lithium silicate forms interconnected channels with excellent ionic and electronic conductivity. This not only enhances the diffusion dynamics within the silicon-carbon composite, improving its rate capability and fast charging performance, but also reduces internal polarization, which is beneficial for improving the cycling performance of the secondary battery. The above preparation method is simple and easy to operate, making it suitable for industrial production.

[0019] Based on the second aspect, in some possible embodiments, the oxidation temperature for forming the oxide layer is 25°C-100°C, and the oxidation time is 3-24 hours. These oxidation temperatures and times are beneficial to the safety of the oxidation process (excessively high temperatures may cause combustion of the carbon skeleton during oxidation), and the oxidation time can be used to adjust the thickness of the oxide layer and the oxygen content.

[0020] Based on the second aspect, in some possible embodiments, the deposition temperature of the first silicon deposition is 450-600° C., and the deposition time is 2-12 hours. This is beneficial for controlling the mass ratio of silicon in the silicon-carbon composite material, thereby increasing the gram capacity and first efficiency of the silicon-carbon composite material and reducing the volume expansion of silicon.

[0021] Based on the second aspect, in some possible embodiments, the second silicon deposition temperature is 450-600°C, and the deposition time is 2-12 hours. This is beneficial for controlling the mass ratio of silicon in the silicon-carbon composite material, thereby increasing the gram capacity and initial efficiency of the silicon-carbon composite material and reducing the volume expansion of silicon.

[0022] A third aspect of the present application provides a secondary battery, comprising a negative electrode plate, wherein the negative electrode plate comprises a silicon-carbon composite material.

[0023] Based on the third aspect, in some possible embodiments, after the secondary battery is charged and discharged, the negative electrode sheet is sliced ​​along the thickness direction, and the edge of the cut surface of the silicon-carbon composite material is radially inward in the annular area of ​​1 μm, and the annular area is tested by transmission electron microscopy and electron energy loss spectrum, and the relative intensity value of the characteristic peak of the electron energy loss spectrum at 64eV is defined as I1. With the center of the cut surface of the silicon-carbon composite material as the center of the circle, within the circular area with a diameter of 1 μm, the circular area is tested by transmission electron microscopy and electron energy loss spectrum, and the relative intensity value of the characteristic peak of the electron energy loss spectrum at 64eV is defined as I2, 0.9≤I2 / I1≤1.1. When I2 / I1 is within the above range, it indicates that a fast channel composed of Li2O and lithium silicate is formed inside the silicon-carbon composite material, which is beneficial to improve Li + The migration of carbon atoms in the silicon-carbon composite material is enhanced, which improves the diffusion dynamics inside the silicon-carbon composite material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] 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:

[0025] FIG1 is a schematic diagram of the structure of the silicon-carbon composite material prepared in Example 1 of the present application.

[0026] FIG2 is a graph showing the specific capacity of button-type half-cells assembled in Example 1 and Comparative Examples 1 to 2 at different current densities of 0.05C, 0.1C, 0.2C, 1C, 2C, 4C, and 0.05C.

[0027] FIG3 shows the lithium ion diffusion coefficient (cm2) of button-type full batteries assembled with silicon-carbon composite materials in Examples 1 to 8 and Comparative Examples 1 to 3 when the button-type full batteries are discharged to 10% DOD in the second cycle. 2 / s) histogram.

[0028] FIG4 is a schematic diagram of the cross-sectional structure of the silicon-carbon composite material particles prepared in Example 1.

[0029] Main Component Symbols Silicon-carbon composite material 100 Core 10 Carbon skeleton 11 First silicon deposition layer 12 Second silicon deposition layer 13 Oxide layer 14 Shell 20 Annular region S1 Circular region S2 DETAILED DESCRIPTION

[0030] 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.

[0031] 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.

[0032] The outer shell can be a packaging bag encapsulated by an encapsulation film (such as an aluminum-plastic film), such as a soft-pack battery. In other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0033] 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.

[0034] Positive electrode

[0035] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer includes a positive electrode active material, a binder and a conductive agent.

[0036] According to some embodiments of the present application, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or nickel foil may be used. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0037] The positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). According to some embodiments of the present application, the positive electrode active material may include, but is not limited to, at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide, and lithium titanate.

[0038] The binder is used to bond the positive electrode active material particles to facilitate the formation of a film layer, and can also improve the bonding strength between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder may include, but is not limited to, a binder polymer, such as polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane. In some embodiments, the polyolefin binder includes at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid.

[0039] In some embodiments, the conductive agent includes a carbon-based material, a metal-based material, a conductive polymer, or a mixture thereof. Carbon-based materials include natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, or carbon fiber; metal-based materials include metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; and conductive polymers include polyphenylene derivatives.

[0040] Negative electrode

[0041] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0042] The negative electrode current collector includes copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, a polymer substrate coated with a conductive metal, or any combination thereof.

[0043] The negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon composite material.

[0044] Referring to FIG. 1 , a silicon-carbon composite material 100 includes a core 10 and a shell 20 encapsulating the core 10. The core 10 includes a carbon skeleton 11, a first silicon deposition layer 12 formed on the carbon skeleton 11, a second silicon deposition layer 13 formed on the first silicon deposition layer 12, and an oxide layer 14 formed between the first silicon deposition layer 12 and the second silicon deposition layer 13. The oxide layer 14 comprises SiOx, where 0.5 ≤ x ≤ 2. The carbon skeleton 11 has a porous structure. The first silicon deposition layer 12 is deposited within or on the surface of the carbon skeleton 11. The second silicon deposition layer 13 is formed by deposition on the first silicon deposition layer 12. The oxide layer 14 comprises a silicon oxide compound with a single silicon valence state, or a mixture of silicon oxides with multiple silicon valence states. The shell 20 may encapsulate at least a portion of the surface of the core 10. The shell 20 comprises a carbon material and is used to prevent direct contact between the silicon in the silicon-carbon composite material 100 and the electrolyte, thereby reducing side reactions.

[0045] After the silicon-carbon composite material 100 is embedded with lithium, the SiOx in the oxide layer 14 will be partially converted into lithium oxide (Li2O) and lithium silicate. + The diffusion coefficient of the silicon-based composite material 100 is increased. The oxide layer 14 formed in the silicon-carbon composite material 100 is also used as a Li + The lithium oxide formed in the fast migration channel within the silicon-carbon composite material 100 can also improve the diffusion dynamics within the silicon-carbon composite material 100, reduce internal polarization, and improve the rate performance and cycle performance of the silicon-carbon composite material 100. At the same time, lithium oxide and lithium silicate also have a buffering effect, which can alleviate the volume expansion of silicon in the silicon-carbon composite material, thereby improving the rate performance and cycle performance of the secondary battery.

[0046] The oxide layer 14 may be continuous, such as the oxide layer 14 may be a network structure to form a connected channel, which is beneficial for Li + The fast migration channel further improves the rate performance and cycle performance of the silicon-carbon composite material 100, which helps to improve the rate performance and cycle performance of the secondary battery.

[0047] In the present application, the oxide layer 14 can be formed on the surface of the first silicon deposition layer 12. After the first silicon deposition layer 12 is formed, the surface of the first silicon deposition layer 12 is oxidized to obtain the oxide layer 14. Compared with the oxide layer 14 being located on the surface of the second silicon deposition layer 13, the oxide layer 14 in the present application is located between the first silicon deposition layer 12 and the second silicon deposition layer 13. The oxide layer 14 has a larger Li after lithium insertion and delithiation. +diffusion coefficient; the oxide layer 14 located between the first silicon deposition layer 12 and the second silicon deposition layer 13 forms a connected (network) channel with excellent ionic and electronic conductivity in the carbon skeleton 11 after lithium insertion and delithiation, thereby improving the internal diffusion dynamics of the silicon-carbon composite material 100, thereby improving the rate performance and significantly improving the cycle performance of the secondary battery.

[0048] In the present application, x can be 0.5, 1 or 2, and correspondingly, SiOx can be SiO 0.5 At least one of SiO, SiO, and SiO2. After lithium insertion, the above SiOx can be partially converted into Li2O. The presence of Li2O significantly increases the + The diffusion coefficient of .

[0049] Furthermore, the thickness of the oxide layer is 10nm to 20nm. It is beneficial to increase the content of SiOx, and at the same time reduces the risk of reduced first coulombic efficiency of the silicon-carbon composite material. If the thickness of the oxide layer is large and the content of oxygen increases, the first efficiency of the silicon-carbon composite material will be reduced, thereby affecting the energy density of the secondary battery. If the thickness of the oxide layer is small and a complete lithium ion rapid migration channel cannot be formed, the effect on improving the rate performance of the silicon-carbon composite material is not obvious. For example, the thickness of the oxide layer can be 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm or any value within the range formed by any two of the above values.

[0050] In some embodiments, the specific surface area of ​​the silicon-carbon composite material is 0.5 m 2 / g to 5m 2 / g. Controlling the specific surface area of ​​the silicon-carbon composite material within an appropriate range is beneficial to increasing the amount of silicon deposition to improve the initial efficiency of the silicon-carbon composite material, while also reducing the risk of increased electrolyte consumption during the cycle to improve cycle performance. The smaller specific surface area reduces the area of ​​the solid electrolyte membrane (SEI membrane) formed on the surface of the silicon-carbon composite material, reducing the effect on Li + The consumption of silicon-carbon composite materials is beneficial to improve the initial efficiency of silicon-carbon composite materials. If the specific surface area is too large, the surface activity of silicon-carbon composite materials is high and it is easy to react with the electrolyte, which will affect the initial efficiency and cycle performance of silicon-carbon composite materials. For example, the specific surface area can be 0.5m 2 / g、1m 2 / g, 2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g or any value within the range formed by any two of the above values. Optionally, the specific surface area of ​​the silicon-carbon composite material is 0.5m 2 / g to 1m 2 / g.

[0051] In some embodiments, the particle size Dv50 of the silicon-carbon composite material is 5 μm to 15 μm. Wherein, Dv50 is also called "median particle size", which means that the particle size of the silicon-carbon composite material reaches 50% of the volume cumulative from the small particle size side 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. By controlling the particle size Dv50 of the silicon-carbon composite material within the above range, it is beneficial to shorten the Li + diffusion path, increasing Li + Ionic conductivity is beneficial to improving the initial efficiency of silicon-carbon composite materials as negative electrode materials in secondary batteries. At the same time, it can also improve the defects caused by increased electrolyte consumption and low material compaction density, and can also reduce the risk of low ionic conductivity and poor rate performance caused by large particle size. Furthermore, the particle size Dv50 of the silicon-carbon composite material is 8μm to 12μm. Specifically, the particle size Dv50 can be 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm or a range consisting of any two of the above values.

[0052] In some embodiments, the particle size Dv90 of the silicon-carbon composite material is less than or equal to 30 μm. Dv90 represents the particle size at which the volume of the silicon-carbon composite material reaches 90% of the cumulative volume from the small particle size side 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 90% of the total volume of the silicon-carbon composite material particles. Regulating the particle size Dv90 of the silicon-carbon composite material within the above range can improve the dispersion uniformity of the slurry when the silicon-carbon composite material is used to prepare the negative electrode material, and improve the Li + The transport of silicon-carbon composite materials can improve the cycle performance and expansion performance of secondary batteries. A larger particle size of the silicon-carbon composite material will affect the cycle performance of the silicon-carbon composite material. For example, the particle size Dv90 of the silicon-carbon composite material can be 10μm, 12μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, or a range consisting of any two of the above values.

[0053] In some embodiments, based on the mass of the silicon-carbon composite material, the mass proportion of carbon element is 35% to 55%. To a certain extent, the volume expansion of silicon is buffered, which is beneficial to improve the structural stability of the silicon-carbon composite material. It is also beneficial to improve the conductivity of the silicon-carbon composite material and improve the cycle performance of the secondary battery. The mass proportion of carbon element can be 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55% or a range consisting of any two of the above values. Optionally, the mass proportion of carbon element is 45% to 50%.

[0054] In some embodiments, based on the mass of the silicon-carbon composite material, the total mass of the first silicon deposited layer and the second silicon deposited layer accounts for 40% to 55%. The first silicon deposited layer and the second silicon deposited layer are both amorphous silicon. By controlling the total mass of the first silicon deposited layer and the second silicon deposited layer within the above range, the silicon-carbon composite material has a high gram capacity and first efficiency while also having good cycle performance, thereby improving the energy density and cycle performance of the secondary battery. If the total mass of the first silicon deposited layer and the second silicon deposited layer is lower than the above range, the first efficiency and gram capacity of the silicon-carbon composite material will be reduced. If the total mass of the first silicon deposited layer and the second silicon deposited layer is higher than the above range, the cycle performance of the silicon-carbon composite material will be reduced. For example, the total mass of the first silicon deposited layer and the second silicon deposited layer in the silicon-carbon composite material can account for 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, or a range consisting of any two of the above values. Optionally, the total mass of the first silicon deposition layer and the second silicon deposition layer accounts for 45% to 50% of the silicon-carbon composite material.

[0055] In some embodiments, the mass of the shell accounts for 1% to 10% of the mass of the silicon-carbon composite material. This is beneficial to reduce the contact between silicon and electrolyte, reduce the side reactions of the secondary battery, and improve the cycle performance of the secondary battery. It is also beneficial to Li +By embedding and extracting the shell into the core, the cycle performance of the silicon-carbon composite material is improved, and by controlling the content of the shell to control the carbon content in the silicon-carbon composite material, the conductivity of the silicon-carbon composite material can also be improved. If the mass proportion of the shell in the silicon-carbon composite material is lower than the above range, it may cause part of the second silicon deposition layer to be exposed, affecting the cycle performance of the secondary battery; if the mass proportion of the shell in the silicon-carbon composite material is higher than the above range, it may reduce the capacity of the secondary battery. The mass proportion of the shell in the silicon-carbon composite material can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range consisting of any two of the above values. Optionally, the mass proportion of the shell in the silicon-carbon composite material is 6% to 8%.

[0056] In some embodiments, based on the mass of the silicon-carbon composite material, the mass proportion of oxygen is 5% to 10%. Among them, the oxygen element mainly comes from the oxygen in the oxide layer. During the lithium insertion process, SiOx is converted into lithium oxide, which is beneficial to improving the cycle performance and rate performance of the secondary battery. At the same time, within the above range, it is beneficial for the silicon-carbon composite material to have a higher first efficiency and improve the rate performance of the secondary battery. An appropriate amount of oxygen content is beneficial to improving the first efficiency of the silicon-carbon composite material. The higher the oxygen content, the more oxygen will react with the Li in the electrolyte. + reaction, which will reduce the initial efficiency and reversible capacity of the silicon-carbon composite material.

[0057] A second aspect of the present application further provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0058] Step S1, providing a carbon skeleton;

[0059] The porous carbon is crushed and graded, and then dried. The crushing and grading technology is used to obtain carbon skeleton materials of different particle sizes. The crushing and grading technology can be any crushing and grading means known in the art. The crushing and grading technology uses jet classification or cyclone classification for classification. After crushing and grading, the Dv50 is 5 to 15 μm, the Dv90 is less than or equal to 30 μm, and the specific surface area is 1800 to 2200 m 2 / g, pore volume is 0.8~1.2cm 3 / g, a carbon skeleton with an average pore size of 2.5 to 4 nm. The drying temperature of the carbon skeleton is 100 to 200°C, and the drying time is 12 to 24 hours.

[0060] The porous carbon may be made of at least one of biomass, resin or petroleum coke. The shape of the carbon skeleton may be at least one of spherical, quasi-spherical, flaky or massive.

[0061] Step S2 , performing a first silicon deposition on the carbon skeleton to form a first silicon deposition layer on the carbon skeleton.

[0062] The carbon skeleton is transferred to a fluidized bed, and a silicon source gas is introduced into the fluidized bed. A first silicon deposition layer is formed on the surface of the carbon skeleton via vapor deposition. The silicon source gas comprises at least one of monosilane, disilane, and trisilane. In this step, the silicon source gas is mixed with an inert gas, comprising at least one of nitrogen or argon, and then introduced into the fluidized bed for the first silicon deposition. The mixing of the silicon source gas and the inert gas facilitates obtaining a silicon deposition layer with uniform silicon size.

[0063] In some embodiments, before introducing the silicon source gas into the fluidized bed, the carbon skeleton is further held in an inert atmosphere to ensure uniform heating of the carbon skeleton, thereby facilitating more uniform deposition within the pores and on the surface of the carbon skeleton after the subsequent introduction of the silicon source gas. The holding temperature is 450-500°C for 2-3 hours.

[0064] In some embodiments, the silicon source gas comprises 5% to 50% by volume of a mixture of silicon source gas and an inert gas. A volume percentage of the silicon source gas within this range further improves the uniformity of silicon size in the deposited silicon layer. The volume percentage of the silicon source gas in the mixture can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range consisting of any two of these values.

[0065] In some embodiments, the deposition temperature of the first silicon deposition is 450-600°C. Within the above deposition temperature range, it is beneficial to regulate the mass proportion of silicon in the silicon-carbon composite material. If the deposition temperature is high, the silicon in the silicon layer is easy to crystallize, which will affect the cycle performance of the secondary battery; if the deposition temperature is low, the decomposition efficiency of the silicon source gas is reduced, which will increase the deposition time and increase the production cost. For example, the deposition temperature of the first silicon deposition can be 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C or a range consisting of any two of the above values.

[0066] In some embodiments, the deposition time of the first silicon deposition is 2 to 12 hours. Within the above deposition time, it is beneficial to regulate the mass proportion of silicon in the silicon-carbon composite material, improve the gram capacity and first efficiency of the silicon-carbon composite material, and reduce the volume expansion of silicon. If the deposition time is long, the silicon deposition layer formed in the pores or on the surface of the carbon skeleton is thicker, and the silicon content is higher, which is not conducive to the subsequent second silicon deposition; if the deposition time is short, the thickness of the silicon deposition layer is thinner, and the amount of silicon deposition is small, which will reduce the first efficiency of the silicon-carbon composite material. For example, the deposition time can be 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours or a range consisting of any two of the above values.

[0067] Step S3, oxidizing the first silicon deposited layer in the carbon skeleton in an oxygen-containing atmosphere to form an oxide layer on the surface of the first silicon deposited layer, wherein the oxide layer includes SiOx, 0.5≤x≤2, thereby obtaining a first intermediate.

[0068] The oxide layer is formed on the carbon skeleton after the first silicon deposition layer is formed. In an oxygen-containing atmosphere, the surface of the first silicon deposition layer is oxidized to obtain silicon oxide. The oxygen-containing atmosphere includes at least one of O2 and CO2. The oxygen-containing atmosphere and the inert gas are mixed and introduced into a fluidized bed for oxidation. The inert gas includes at least one of nitrogen or argon. When the first silicon deposition layer is oxidized, heat is generated during the oxidation process of the silicon deposition layer, which increases the temperature of the reaction between silicon and the oxygen-containing atmosphere. If the oxygen content of the oxygen-containing atmosphere in the mixed gas is high or the inert gas is not introduced, the carbon skeleton may burn, thereby causing safety problems. At the same time, the addition of inert gas is also beneficial to improve the uniformity of the surface oxidation of the first silicon deposition layer. Optionally, CO2 is selected as the oxygen-containing atmosphere.

[0069] The oxide layer includes SiOx, 0.5≤x≤2, where x can be 0.5, 1 or 2.

[0070] The volume percentage of the oxygen-containing atmosphere in the mixture of the oxygen-containing atmosphere and the inert gas is 5% to 10%, which helps improve the uniformity of oxidation on the surface of the first silicon deposited layer and also helps ensure safe operation. The volume percentage of the oxygen-containing atmosphere in the mixture of the oxygen-containing atmosphere and the inert gas can be 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two of the foregoing values.

[0071] In some embodiments, the oxidation temperature for forming the oxide layer is 25°C-100°C. After the first silicon deposition, the silicon source gas and the heating are turned off, and the carbon skeleton having the first silicon deposition layer formed in step S2 is cooled by introducing an inert gas (such as nitrogen). When the temperature is cooled to within the above temperature range, at least one of O2 or CO2 is introduced to form an oxide layer on the surface of the first silicon deposition layer. Within the above oxidation temperature range, the oxidation process is safe and the risk of excessive temperature and combustion of the carbon skeleton during the oxidation process is reduced. For example, the oxidation temperature for forming the oxide layer may be 25°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, or a range consisting of any two of the above values.

[0072] In some embodiments, the oxidation time for forming the oxide layer is 3 to 24 hours. Within the above oxidation time, it is conducive to oxidation to obtain an oxide layer of a certain oxidation thickness. At the same time, within this time range, the oxygen content in the oxidation process can be controlled. If the oxidation time is short, the continuity of the formed oxide layer is poor, the thickness is small, the integrity of the lithium ion rapid migration channel is poor, and the improvement of the lithium ion conductivity in the silicon-carbon composite material is not significant, and thus the effect of improving the rate performance of the silicon-carbon composite material is not obvious. If the oxidation time is long, the oxidation reaction of the first silicon deposition layer is complete, and the increase in the thickness of the oxide layer is not obvious. For example, the oxidation time for forming the oxide layer can be 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, or a range consisting of any two of the above values.

[0073] Step S4: performing a second silicon deposition on the first intermediate to form a second silicon deposition layer on the surface of the oxide layer to obtain a second intermediate.

[0074] A silicon source gas is introduced into the fluidized bed and a second silicon deposition layer is formed on the surface of the oxide layer of the first intermediate body by vapor deposition. The oxide layer may be continuous between the first silicon deposition layer and the second silicon deposition layer. + Migration provides continuous and connected channels, promoting Li + migrate.

[0075] The silicon source gas includes at least one of monosilane, disilane, and trisilane. In this step, the silicon source gas is mixed with an inert gas and introduced into the fluidized bed for a second silicon deposition step. The inert gas includes at least one of nitrogen and argon. This mixture of the silicon source gas and the inert gas facilitates obtaining a uniformly sized silicon deposited layer.

[0076] In some embodiments, the silicon source gas comprises 5% to 50% by volume of a mixture of silicon source gas and an inert gas. A volume percentage of the silicon source gas within this range further improves the uniformity of silicon size in the deposited silicon layer. The volume percentage of the silicon source gas in the mixture can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range consisting of any two of these values.

[0077] In some embodiments, the deposition temperature of the second silicon deposition is 450-600°C. Within the above deposition temperature range, it is beneficial to regulate the mass proportion of silicon in the silicon-carbon composite material. If the deposition temperature is high, the silicon in the silicon layer is easy to crystallize, which will affect the cycle performance of the secondary battery; if the deposition temperature is low, the decomposition efficiency of the silicon source gas is reduced, which will increase the deposition time and increase the production cost. For example, the deposition temperature of the second silicon deposition can be 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C or a range consisting of any two of the above values.

[0078] In some embodiments, the deposition time of the second silicon deposition is 2 to 12 hours. Within the above deposition time, it is beneficial to regulate the mass proportion of silicon in the silicon-carbon composite material, improve the gram capacity and first efficiency of the silicon-carbon composite material, and reduce the volume expansion of silicon. If the deposition time is long and the silicon deposition layer is thicker, the mass proportion of silicon in the silicon-carbon composite material will increase, affecting the cycle performance of the silicon-carbon composite material; if the deposition time is short, the thickness of the silicon deposition layer is thinner, and the amount of silicon deposition is small, which will reduce the first efficiency of the silicon-carbon composite material. For example, the deposition time can be 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or a range consisting of any two of the above values.

[0079] Step S5 , carbon-coating the second intermediate, coating the surface of the second intermediate with carbon material to form a shell, the shell comprising carbon material, thereby obtaining a silicon-carbon composite material.

[0080] The core is composed of a carbon skeleton, a first silicon deposition layer formed on the carbon skeleton, a second silicon deposition layer formed on the first silicon deposition layer, and an oxide layer formed between the first and second silicon deposition layers. The shell encapsulates the core. The shell prevents direct contact between silicon in the silicon-carbon composite material and the electrolyte, thereby reducing side reactions.

[0081] A carbon source gas is introduced into the fluidized bed and the second intermediate is carbon-coated. The carbon source gas includes at least one of methane, ethylene, and acetylene. A shell is formed by vapor deposition to control the amount of carbon material coated in the shell and improve the conductivity of the silicon-carbon composite material. The carbon source gas and an inert gas are mixed and introduced into the fluidized bed. The inert gas is provided to improve the uniformity of the carbon material distribution in the shell. The inert gas includes at least one of nitrogen and argon.

[0082] In some embodiments, the volume percentage of the carbon source gas in the mixture of the carbon source gas and the inert gas is between 10% and 50%, which helps improve the uniformity of the distribution of the carbon material in the shell. For example, the volume percentage of the carbon source gas can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range consisting of any two of the foregoing values.

[0083] In some embodiments, the coating temperature during the carbon coating process is 550°C to 750°C, and the coating time is 6 to 12 hours. Within the above temperature and time ranges, the content of the carbon material during the carbon coating process is controlled. For example, the coating temperature can be 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, or a range consisting of any two of the above values. For example, the coating time is 6h, 7h, 8h, 9h, 10h, 11h, 12h, or a range consisting of any two of the above values.

[0084] 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.

[0085] 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.

[0086] Isolation film

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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).

[0091] electrolyte

[0092] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and optional additives.

[0093] 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.

[0094] 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.

[0095] The secondary battery provided by the present application includes a negative electrode plate, the negative electrode plate includes a silicon-carbon composite material, the silicon-carbon composite material is applied to the secondary battery, and an oxide layer is constructed and formed between the first silicon deposition layer and the second silicon deposition layer in the carbon skeleton. The oxide layer forms Li2O / lithium silicate after lithium insertion and removal cycles. Li2O has a large Li + The diffusion coefficient of Li + The fast migration channel in the silicon-carbon composite material improves the diffusion dynamics inside the silicon-carbon composite material and reduces internal polarization, thereby improving the rate performance, fast charging performance and cycle performance of the secondary battery while having excellent energy density. The preparation method provided in this application is simple, easy to operate, and suitable for industrial production.

[0096] After the secondary battery provided in the present application is charged and discharged, the negative electrode sheet is sliced ​​along the thickness direction with the help of a focused ion beam (FIB). As shown in Figure 4, the edge of the cross-section of the silicon-carbon composite material is radially inward in the annular region S1 of 1 μm, and the annular region S1 is tested by transmission electron microscopy and electron energy loss spectrum (EELS spectrum). The test is performed at random points in the annular region S1. The electron energy loss spectrum shows characteristic peaks at 59eV and 64eV belonging to Li2O. The relative intensity value of the characteristic peak of the electron energy loss spectrum at 64eV is defined as I1. With the center of the cross-section of the silicon-carbon composite material as the center of the circle, in a circular region S2 with a diameter of 1 μm, the circular region S2 is tested by transmission electron microscopy and electron energy loss spectrum. The test is performed at random points in the circular region S2. The electron energy loss spectrum also shows characteristic peaks at 59eV and 64eV belonging to Li2O. The relative intensity value of the characteristic peak of the electron energy loss spectrum at 64eV is defined as I2, 0.9≤I2 / I1≤1.1. When I2 / I1 is within the above range, it can be explained that a fast channel composed of Li2O and lithium silicate is formed inside the silicon-carbon composite material, which increases the Li +The migration of carbon atoms in the silicon-carbon composite material is enhanced, which improves the diffusion dynamics inside the silicon-carbon composite material.

[0097] 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.

[0098] The present application also applies the secondary battery to an electronic device to power the load in the electronic device. An oxide layer is formed in the silicon-carbon composite material in the secondary battery in the electronic device. After lithium is inserted into the oxide layer, SiOx in the oxide layer is partially converted into Li2O and lithium silicate. The lithium oxide has a large Li + Diffusion coefficient, as Li + It provides a fast channel for migration within the silicon-carbon composite material, improving the rate performance, fast charging performance and cycle performance of the secondary battery, thereby increasing the service life and charging efficiency of the electronic device.

[0099] 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.

[0100] 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.

[0101] Example 1

[0102] <Preparation of Silicon-Carbon Composite Materials>

[0103] (1) Take 10 kg of commercial porous carbon and classify it using jet classification (or cyclone classification) to obtain the particle size D of the carbon skeleton. V 50 is 8.3μm, D V The carbon skeleton after crushing and classification was placed in an oven for drying at a temperature of 150°C for 18 hours.

[0104] (2) 1 kg of dried carbon skeleton is transferred into the fluidized bed through high-pressure transmission. After standing for about 30 minutes, the fluidized bed air inlet valve and tail gas valve are closed and vacuum is applied. When the chamber pressure reaches -101 kPa, the vacuum is closed and nitrogen is introduced at a rate of 30 L / min to positive pressure. The vacuum is repeated and nitrogen is introduced for more than 5 times, and then the oxygen content inside the chamber is detected. When the oxygen content drops below 10 ppm, the tail gas valve is opened, the fluidized bed stirring paddle is started, the speed is 100 rpm, nitrogen is introduced at a rate of 15 L / min, and the temperature is increased to 500 ° C at a heating rate of 5 ° C / min, and the temperature is kept for 1 hour. After the insulation is completed, the valve of the fluidized bed air inlet pipeline is switched, and a silane / nitrogen mixture is introduced into the fluidized bed. The volume proportion of silane is 10%, the gas flow rate is 25 L / min, and the reaction time is 160 minutes.

[0105] (3) Switch the valve of the fluidized bed inlet pipeline, close the inlet of the monosilane / nitrogen mixture, turn off the heating, adjust the speed to 100 rpm, and introduce nitrogen at a rate of 20 L / min to cool the fluidized bed. When the temperature drops to 50°C, switch the valve of the fluidized bed inlet pipeline, and introduce an oxygen / nitrogen mixture into the fluidized bed, with an oxygen volume ratio of 5% and a gas flow rate of 20 L / min. The oxidation time is 6 h.

[0106] (4) Close the air inlet valve, turn off the stirring, and after standing for about 30 minutes, close the tail gas valve and evacuate the chamber. After the chamber pressure reaches -101kPa, close the vacuum and introduce nitrogen at a rate of 30L / min to positive pressure. Repeat the vacuum and nitrogen introduction for more than 5 times, and then detect the oxygen content inside the chamber. When the oxygen content drops below 10ppm, open the tail gas valve, start the fluidized bed stirring paddle, rotate at 100 rpm, introduce nitrogen at a rate of 15L / min, heat to 500℃ at a heating rate of 5℃ / min, and keep warm for 1h. After the insulation is completed, switch the fluidized bed air inlet valve and introduce a silane / nitrogen mixture into the fluidized bed. The silane concentration is 10%, the gas flow rate is 25L / min, and the reaction time is 160min.

[0107] (5) Close the inlet valve of the monosilane / nitrogen mixture, pass nitrogen at a flow rate of 10 L / min, adjust the stirring blade speed to 100 rpm, heat to 650°C at a rate of 5°C / min, and keep warm for 30 minutes. Pass acetylene / nitrogen mixture into the fluidized bed, with acetylene accounting for 30% by volume, the mixed gas flow rate being 20 L / min, and the reaction time being 8 hours. After the reaction is completed, pass nitrogen at a flow rate of 10 L / min, adjust the stirring blade speed to 100 rpm to cool down, and discharge the material to room temperature to obtain a silicon-carbon composite material.

[0108] The preparation methods of Examples 2 to 8 are substantially the same as those of Example 1, with the differences being the oxygen-containing atmosphere, oxidation time, and oxidation temperature. The preparation parameters are recorded in Table 1.

[0109] Comparative Example 1

[0110] The difference between Comparative Example 1 and Example 1 is that in step (3), the "oxygen / nitrogen mixed gas" is replaced with "nitrogen", that is, the first silicon deposited layer is not oxidized.

[0111] Comparative Example 2

[0112] The difference between Comparative Example 2 and Example 1 is that oxidation is performed after the first silicon deposition time is increased, and the second silicon deposition is not performed.

[0113] The specific differences are as follows: in step (2), the reaction time of the monosilane / nitrogen mixture is 320 minutes. And in the process of step (4), when the fluidized bed stirring paddle is started, the speed is 100 rpm, and nitrogen is introduced at a rate of 15 L / min, "after which the temperature is raised to 650°C at a rate of 5°C / min and kept warm for 30 minutes. Acetylene / nitrogen mixture is introduced into the fluidized bed, the volume proportion of acetylene is 30%, the flow rate of the mixed gas is 20 L / min, and the reaction time is 8 hours. After the reaction is completed, nitrogen is introduced at a flow rate of 10 L / min, and the stirring paddle speed is adjusted to 100 rpm for cooling, and the material is discharged at room temperature to obtain a silicon-carbon composite material.

[0114] Comparative Example 3

[0115] The difference between Comparative Example 3 and Example 1 is that step (2) increases the time of the first silicon deposition, and silicon source gas and oxygen-containing gas are passed simultaneously during the deposition process, and steps (3) and (4) are omitted.

[0116] The specific difference is that in step (2), after the insulation is completed, the valve of the fluidized bed air inlet pipeline is switched, and a monosilane / nitrogen mixture and a carbon dioxide / nitrogen mixture are simultaneously introduced into the fluidized bed through two air inlet pipelines, wherein the volume proportion of monosilane in the monosilane / nitrogen mixture is 12.5%, and the gas flow rate is 20 L / min; the volume proportion of carbon dioxide in the carbon dioxide / nitrogen mixture is 2%, and the gas flow rate is 5 L / min; and the reaction time is 320 min.

[0117] The schematic structural diagram of the silicon-carbon composite material prepared in Example 1 is shown in Figure 1. As shown in Figure 1, an oxide layer is formed between the first silicon deposition layer and the second silicon deposition layer, and the oxide layer comprises SiOx, where 0.5≤x≤2.

[0118] In the examples and comparative examples of the present application, the following preparation methods of the positive electrode sheet, the negative electrode, the electrolyte and the separator are used to assemble the soft-pack battery, and the button battery preparation method is used to assemble the button battery.

[0119] Preparation of soft pack batteries

[0120] <Preparation of positive electrode sheet>

[0121] The positive electrode active material, LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF), were thoroughly stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 95%:2.5%:2.5% to prepare a positive electrode slurry. The prepared positive electrode slurry was coated on the positive electrode current collector aluminum foil, dried, and cold pressed to obtain a positive electrode sheet.

[0122] <Preparation of negative electrode sheet>

[0123] Graphite, silicon-carbon composite material of negative electrode active material prepared according to the embodiment and comparative example, conductive agent (conductive carbon black, Super ) and binder PAA are mixed in a weight ratio of 70%:15%:5%:10%, and an appropriate amount of water is added. The mixture is kneaded to a solids content of 55wt%-70wt%. An appropriate amount of water is added to adjust the slurry viscosity to 4000-6000 Pa·s to prepare a negative electrode slurry. The prepared negative electrode slurry is coated on a negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode sheet.

[0124] <Preparation of Electrolyte>

[0125] Under a dry argon environment, LiPF6 was added to a solvent mixed with propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) (weight ratio of 1:1:1), and the mixture was evenly mixed. Then, fluoroethylene carbonate (FEC) was added and the mixture was evenly mixed to obtain an electrolyte, wherein, based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5wt%, and the mass percentage of FEC was 12.5wt%.

[0126] <Isolation Film>

[0127] PE composite film is used as the isolation membrane.

[0128] <Method for preparing soft pack battery>

[0129] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation. Winding is performed to obtain a bare cell. The bare cell is placed in an outer packaging, injected with electrolyte, and then encapsulated. After the formation, degassing, and trimming processes, a lithium-ion soft-pack battery is obtained.

[0130] Preparation of button cells

[0131] Preparation of negative electrode: The silicon-carbon composite material prepared above is used as the active material, acetylene black as the conductive agent, and sodium alginate as the binder. The mass ratio of active material, acetylene black, and sodium alginate is 70:20:10. The active material and acetylene black are fully mixed in proportion and ground evenly, and sodium alginate aqueous solution is added in proportion and stirred for 4 hours. Finally, the mixture slurry is evenly coated on the copper foil, and vacuum dried at 70°C for 12 hours, and punched into a circular electrode sheet with a diameter of 10 mm. The loading amount of the active substance is about 1.0 mg / cm 2 Up to 1.5 mg / cm 2 .

[0132] Electrolyte Preparation: Under a dry argon atmosphere, LiPF6 was added to a solvent mixture of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) (weight ratio of 1:1:1), followed by the addition of fluoroethylene carbonate (FEC) and the mixing to obtain an electrolyte solution. The weight percentages of LiPF6 and FEC were 12.5% ​​and 12.5% ​​based on the weight of the electrolyte solution. A Celgard 2400 separator was used, and a lithium sheet was used as the counter electrode. Coin-type half-cells were assembled in a glove box.

[0133] The silicon-carbon composite materials prepared in each embodiment and comparative example were tested for silicon content, oxygen content, and carbon content. The negative electrode plates in the button batteries prepared from the silicon-carbon composite materials in each embodiment and comparative example were tested using transmission electron microscopy (TEM) and / or electron energy loss spectroscopy. The secondary batteries prepared from the silicon-carbon composite materials in each embodiment and comparative example were tested for half-cell first-cycle discharge capacity, half-cell rate performance, full-cell first-cycle discharge capacity, and cycle performance. The test results are recorded in Table 1.

[0134] Among them, Si element content test includes:

[0135] Prepare the digestion sample: Place 0.1000g of the aforementioned silicon-carbon composite material in a nickel crucible. Add 1.5g of KOH and cover the crucible. Heat the muffle furnace to 400°C. The temperature is raised from room temperature to 300°C over 2 hours, and then from 300°C to 400°C over 2 hours. The temperature is then lowered to 80°C naturally. At the end of the digestion process, remove the crucible, cool to room temperature, and remove the digested sample and place it in an F4 beaker.

[0136] Titration of the digested sample with a standard sodium hydroxide solution: Add 30 mL of boiling water to an F4 beaker and incubate for 1 hour. Then, use tweezers to clean the crucible, reducing the volume to 50 mL. Filter the solution and transfer it to a 400 mL beaker. After filtration, add 20 mL of concentrated nitric acid to the beaker to neutralize the solution, making it acidic. After the solution cools to room temperature, add solid KCl to saturation with a 2 g excess while stirring. Then, add 10 mL of potassium fluoride solution, causing a white precipitate to form. Age the solution for 15 minutes and filter through medium-speed quantitative filter paper. Wash the beaker and precipitate three times with 8 mL of potassium chloride solution each time. Remove the filter paper and return it to the original beaker. Add 20 mL of potassium chloride in ethanol and 10 drops of phenolphthalein. Then, neutralize any residual acid with a standard sodium hydroxide solution. Stir the filter paper and scrub the beaker walls until the solution turns light red. During this process, break up the paper pulp with a glass rod. Allow to react for 1 hour. Add 200 mL of neutralized boiling water into the cup (add 10 drops of phenolphthalein after boiling and neutralize with sodium hydroxide standard solution until it turns slightly red). Titrate with sodium hydroxide standard solution until it turns slightly red as the end point. Record the volume V of sodium hydroxide standard solution consumed in the titration.

[0137] Titration of blank sample with sodium hydroxide standard solution: except that no digested sample is added, the remaining steps are the same as those for titration of digested sample with sodium hydroxide standard solution. Prepare a blank sample and record the volume V0 of sodium hydroxide standard solution consumed in the titration of the blank sample.

[0138] The silicon content is calculated according to the following formula: si =(V-V0)×c×7.02 / m×100%, where: c is the concentration of sodium hydroxide standard solution, mol / L; V is the volume of sodium hydroxide standard solution consumed in titration, unit: L; V0 is the volume of sodium hydroxide standard solution consumed in blank, unit: L; 7.02 is the molar mass of 1 / 4Si, unit: g / mol; m is the mass of the sample, unit: g.

[0139] O element content tests include:

[0140] Weigh an appropriate amount of sample to the nearest 0.001 mg and test using a Shanghai Microspectrum Organic Element Analyzer (Model: PerkinElmer Series II 2400, USA) using the test method JY / T 0580-2020. Perform two or more consecutive parallel measurements according to the sample measurement procedure until the two consecutive results meet the analytical error requirements. After the sample measurement, the standard material should be measured again. If the result deviates from the instrument operating range, the sample should be re-measured. The mass percentage of oxygen in the sample is calculated according to the following formula: a = e / mk, where e is the absolute mass of hydrogen in the sample (mg) obtained from the calibration curve, m is the weighed mass of the sample (mg), and k is the daily calibration factor for oxygen.

[0141] Carbon content tests include:

[0142] The sample is heated and burned at high temperature in a high-frequency furnace under oxygen-rich conditions, oxidizing carbon and sulfur into carbon dioxide and sulfur dioxide. After treatment, the gases enter the corresponding absorption cell, where they absorb the corresponding infrared radiation and are then converted into corresponding signals by the detector. This signal is sampled by a computer and converted into a value proportional to the concentration of carbon dioxide and sulfur dioxide after linear correction. The values ​​obtained throughout the entire analysis process are then accumulated. After the analysis is completed, this accumulated value is divided by the weight value in the computer, multiplied by the correction factor, and the blank is subtracted to obtain the percentage of carbon and sulfur in the sample. Sample testing is performed using a high-frequency infrared carbon and sulfur analyzer (Shanghai Dekai HCS-140).

[0143] TEM and EELS testing include:

[0144] At 25°C, the secondary battery (full battery) was charged to 4.25V at 0.5C, and then discharged to 3.0V at 1C. After completing the first charge and discharge process, the fully discharged secondary battery was taken, disassembled, and the negative electrode was taken out and soaked in DMC (dimethyl carbonate) for 20 minutes. Then, it was rinsed with DMC and acetone in turn, and then placed in an oven and baked at 80°C for 12 hours to obtain the treated negative electrode.

[0145] The obtained negative electrode piece was tested as follows: the anode piece was sliced ​​along the thickness direction with the help of FIB (focused ion beam), and the silicon-carbon composite material particles on the cross section were characterized by transmission electron microscopy (TEM) and / or electron energy loss spectroscopy (EELS). The annular area 1 μm inward along the radial direction of the cross section edge of the silicon-carbon composite material particle was defined as annular area 1 (as shown in Figure 4). The center of the cross section of the silicon-carbon composite material particle was the center of the circle, and the circular area with a diameter of 1 μm was defined as circular area 2 (as shown in Figure 4). Any point on annular area 1 and circular area 2 was randomly selected for testing, and the EELS spectrum was recorded at 120 kV, and the EELS point spectrum was obtained at an acceleration voltage of 200 kV. The probe size for spectrum drawing was 1.5 nm, and the probe size for point collection was 0.7 nm. The incident angle and exit angle of the electron beam were 12 mrad. The energy step size was 0.3 eV and the exposure time was 1 second.

[0146] The first-cycle discharge capacity test of half-cell includes:

[0147] 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-cycle 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-cycle charge specific capacity.

[0148] Half-cell rate performance tests include:

[0149] At 25°C, the prepared button half-cell was repeatedly charged / discharged for 5 cycles at current densities of 0.05C, 0.1C, 0.2C, 1C, 2C, 4C, and 0.05C, with a voltage range of 3.0 to 0V. The capacity discharged at different rates was recorded, and the discharge capacity ratio at different rates was calculated based on the 0.05C discharge capacity.

[0150] The full battery first cycle discharge capacity test includes:

[0151] Under normal pressure at 25°C, the prepared button-type full battery was charged to 3.8V at a constant current rate of 0.1C, then charged to a current of 0.05C at a constant voltage, and then allowed to stand for 5 minutes. The charge capacity at this time was recorded, which was the first-cycle charge capacity; then the battery was discharged to 2.5V at a constant current rate of 0.1C, and allowed to stand for 5 minutes. This was a cyclic charge and discharge process, and the discharge capacity at this time was recorded, which was the first-cycle discharge capacity.

[0152] Cyclic performance tests include:

[0153] The test temperature is 25 / 45℃, charged to 4.4V at a constant current of 3.4C, charged to 0.025C at a constant voltage, and discharged to 3.0V at 0.5C after standing for 5 minutes. The capacity obtained in this step is the initial capacity of the soft-pack battery, and the cycle test is carried out at 3.4C charging / 0.5C discharge. The capacity of each step is compared with the initial capacity to obtain the capacity decay curve. The number of cycles at 25℃ until the capacity retention rate is 90% is recorded as the room temperature cycle performance of the battery, and the number of cycles at 45℃ until the capacity retention rate is 80% is recorded as the high temperature cycle performance of the battery. The cycle performance of the material is compared by comparing the number of cycles under the above two conditions.

[0154] Specific surface area tests include:

[0155] The specific surface area of ​​the silicon-carbon composite particles of each Example and Comparative Example was measured by nitrogen adsorption using a surface area analyzer (TriStar II 3020M, provided by Micromeritics, USA). The specific testing was conducted in accordance with the national standard GB / T 19587-2017, "Determination of the Specific Surface Area of ​​Solids by the BET Method for Gas Adsorption."

[0156] Diffusion coefficient test and calculation method:

[0157] The lithium ion diffusion coefficient of the button half-cell after one discharge and charge cycle was measured by constant current intermittent titration method. The procedure is as follows:

[0158] ① Leave the button half-cell at room temperature for 12 hours;

[0159] ②Discharge at a constant current of 0.05C for 30 minutes. If the voltage is less than or equal to 0.005V, jump to step ④.

[0160] ③After standing for 2 hours, go to step ②;

[0161] ④Let it stand for 30 minutes;

[0162] ⑤ Charge at 0.05C constant current for 30 minutes. If the voltage is greater than or equal to 3.0V, jump to step ⑦.

[0163] ⑥After standing for 2 hours, jump to step ⑤;

[0164] ⑦Let it stand for 30 minutes;

[0165] ⑧If the number of cycles is ≤3, jump to step ①;

[0166] ⑨End.

[0167] According to the formula D=(4 / πτ)(n m V m / S) 2 (ΔE s / ΔE t ) 2 , the diffusion coefficient D under different SOC can be obtained, where:

[0168] D is the lithium ion diffusion coefficient; τ is the relaxation time, i.e. the above-mentioned rest time; n m is the number of moles of lithium ions; V m is the molar volume of the electrode material; S is the electrode / electrolyte contact area; ΔE s is the voltage change caused by the pulse; ΔE t It is the voltage change of constant current charging (discharging).

[0169] The specific capacity diagrams of the button-type half-cells assembled in Example 1 and Comparative Examples 1-2 at different current densities of 0.05C, 0.1C, 0.2C, 1C, 2C, 4C, and 0.05C are shown in Figure 2. Under the same test conditions, as the current density increases, the specific capacity of the button-type half-cell assembled in Example 1 decreases at a slower rate than that of the button-type half-cells assembled in Comparative Examples 1 and 2. Moreover, when the current density is greater than 1C, the specific capacity of Example 1 is greater than that of Comparative Examples 1 and 2. This shows that the silicon-carbon composite material provided in this application can improve the rate capability of secondary batteries.

[0170] The lithium ion diffusion coefficient (cm2) of the button-type half-cells assembled with the silicon-carbon composite materials in Examples 1 to 8 and Comparative Examples 1 to 3 is as follows: 2As shown in FIG3 , it can be seen that the lithium ion diffusion coefficients of the button-type half-cells assembled in Examples 1 to 8 are all greater than those of Comparative Examples 1 to 3.

[0171] Table 1 shows the preparation conditions and test results of silicon-carbon composite materials prepared in Examples and Comparative Examples

[0172] Table 2 shows the test results of silicon-carbon composite materials prepared in Examples and Comparative Examples

[0173] The data in Tables 1 and 2 indicate that in Comparative Example 1, no oxidation occurred between the first and second silicon depositions, resulting in no oxide layer. After the lithium-ion battery was charged and discharged, transmission electron microscopy and electron energy loss spectroscopy (EELS) measurements of the circular region revealed no characteristic peak corresponding to I2. This indicates that in the absence of an oxide layer in the silicon-carbon composite, no Li2O was converted.

[0174] However, compared with Comparative Example 1, in Examples 1 to 3 at different oxidation times in an O2 atmosphere, in Examples 4 to 6 at different oxidation times in a CO2 atmosphere, and in Examples 7 to 8 at different oxidation temperatures, an oxide layer is formed in the core of the silicon-carbon composite material, which significantly increases the Li+ efficiency when the lithium-ion battery has a good first discharge efficiency. + The diffusion coefficient and cycle performance (combined with Figure 3) indicate, on the one hand, that the SiO2 in the oxide layer between the first and second silicon deposition layers is x The converted Li2O has a large Li + Diffusion coefficient, at the same time, part of the SiO in the oxide layer xThe converted Li₂O and lithium silicate can also mitigate the volume expansion of silicon in the silicon-carbon composite material. Furthermore, the oxide layer located between the first and second silicon deposition layers enhances the internal diffusion dynamics of the silicon-carbon composite material, improving the rate capability and cycling performance of the lithium-ion battery. In Comparative Example 2, by increasing the duration of the first silicon deposition, the total duration of the first silicon deposition in Comparative Example 2 is the same as the total duration of the first and second silicon depositions in Example 1. However, the oxide layer forms on the surface after the two silicon depositions, not between the first and second silicon deposition layers. In the silicon-carbon composite material prepared in Comparative Example 2, the oxide layer is located on the surface of the silicon deposition layer. After the lithium-ion battery was charged and discharged, transmission electron microscopy and electron energy loss spectroscopy tests of the circular region revealed no characteristic peak corresponding to I₂, indicating that no oxide layer formed in the central portion of the silicon-carbon composite particles in Comparative Example 2. This may be because, after silicon deposition on the carbon skeleton, silicon is largely or completely filled into the pores or surface of the carbon skeleton, making it difficult for silicon within the pores of the carbon skeleton to come into contact with oxygen and oxidize, resulting in the absence of a characteristic peak corresponding to I₂ within the circular region.

[0175] Compared to Comparative Example 2, the oxide layer located between the first and second silicon deposited layers in Example 1 significantly improved the cycling performance of the lithium-ion battery (25°C / 45°C). This indicates that compared to the oxide layer on the surface of the silicon deposited layer, the oxide layer formed within the first and second silicon deposited layers, after lithium insertion and delithiation, forms a connected (network) channel with excellent ionic and electronic conductivity within the carbon skeleton, thereby enhancing the internal diffusion kinetics of the silicon-carbon composite material and improving the cycling performance of the secondary battery.

[0176] In Comparative Example 3, oxygen-containing gas was introduced simultaneously during the silicon deposition process, and the total silicon deposition time was the same as in Example 1. Oxidation did not occur during the entire silicon deposition process. Since the deposited silicon exists in the form of silicon oxide, the oxygen content increases, which reduces the first discharge efficiency and first efficiency of the silicon-carbon composite material. In addition, during the deposition process, the deposited silicon is in the form of dot columns, so that the silicon oxide presents a discontinuous structure, that is, after lithium insertion and delithiation, no complete connecting channel is formed, and the diffusion kinetics inside the silicon-carbon composite material are not improved, thereby worsening the cycle performance of the secondary battery.

[0177] 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 comprising a core and a shell covering the core, wherein the shell comprises a carbon material, The core body includes a carbon skeleton, a first silicon deposition layer formed on the carbon skeleton, a second silicon deposition layer formed on the first silicon deposition layer, and an oxide layer formed between the first silicon deposition layer and the second silicon deposition layer, wherein the oxide layer includes SiOx, 0.5≤x≤2.

2. The silicon-carbon composite material according to claim 1, wherein The thickness of the oxide layer is 10 nm to 20 nm.

3. The silicon-carbon composite material according to claim 1 or 2, wherein: The silicon-carbon composite material satisfies at least one of the following conditions: (1) The specific surface area of ​​the silicon-carbon composite material is 0.5 m 2 / g to 5m 2 / g; (2) The particle size Dv50 of the silicon-carbon composite material is 5 μm to 15 μm; (3) The particle size Dv90 of the silicon-carbon composite material is less than or equal to 30 μm.

4. The silicon-carbon composite material according to any one of claims 1 to 3, wherein The silicon-carbon composite material satisfies at least one of the following conditions: (1) Based on the mass of the silicon-carbon composite material, the mass proportion of carbon element in the silicon-carbon composite material is 35% to 55%; (2) Based on the mass of the silicon-carbon composite material, the total mass of the first silicon deposition layer and the second silicon deposition layer accounts for 40% to 55%.

5. The silicon-carbon composite material according to any one of claims 1 to 4, wherein Based on the mass of the silicon-carbon composite material, the mass proportion of oxygen in the silicon-carbon composite material is 5% to 10%.

6. A method for preparing a silicon-carbon composite material, wherein: include: Provides a carbon skeleton; performing a first silicon deposition on the carbon skeleton to form a first silicon deposition layer on the carbon skeleton; In an oxygen-containing atmosphere, oxidizing the first silicon deposited layer in the carbon skeleton to form an oxide layer on a surface of the first silicon deposited layer, wherein the oxide layer comprises SiOx, 0.5≤x≤2, thereby obtaining a first intermediate; performing a second silicon deposition on the first intermediate to form a second silicon deposition layer on the surface of the oxide layer to obtain a second intermediate; The second intermediate is carbon-coated, and a carbon material is coated on the surface of the second intermediate to form a shell, thereby obtaining the silicon-carbon composite material.

7. The method for preparing the silicon-carbon composite material according to claim 6, wherein: The oxidation temperature for forming the oxide layer is 25° C.-100° C., and the oxidation time is 3-24 hours.

8. The method for preparing the silicon-carbon composite material according to claim 6 or 7, wherein: The deposition temperature of the first silicon deposition is 450-600° C. and the deposition time is 2-12 hours; or / and The deposition temperature of the second silicon deposition is 450-600° C., and the deposition time is 2-12 hours.

9. A secondary battery comprising a negative electrode plate, wherein: The negative electrode plate comprises the silicon-carbon composite material according to any one of claims 1 to 5 or is obtained by the preparation method of the silicon-carbon composite material according to any one of claims 6 to 8.

10. The secondary battery according to claim 9, wherein After the secondary battery is charged and discharged, the negative electrode plate is sliced ​​along the thickness direction, and the annular area within the radial direction of the edge of the cut surface of the silicon-carbon composite material is subjected to transmission electron microscopy and electron energy loss spectrum testing, and the relative intensity value of the characteristic peak of the electron energy loss spectrum at 64eV is defined as I1. With the center of the cut surface of the silicon-carbon composite material as the center of the circle, within a circular area with a diameter of 1μm, transmission electron microscopy and electron energy loss spectrum testing are performed on the circular area, and the relative intensity value of the characteristic peak of the electron energy loss spectrum at 64eV is defined as I2, and 0.9≤I2 / I1≤1.1.

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