Silicon-carbon composite materials, secondary batteries and electronic devices

Abstract

The application provides a silicon-carbon composite material, a secondary battery and an electronic device. The silicon-carbon composite material comprises a silicon matrix and a first layer at least partially on the silicon matrix, and the first layer comprises a conductive agent and a silicon-oxygen organic compound. The silicon-carbon composite material provided by the application improves the interface stability of the silicon matrix and the electrolyte, improves the cycle performance of the secondary battery, reduces the decomposition gas of the SEI film during over-discharge of the secondary battery, and improves the gas production problem during over-discharge of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon composite material, a secondary battery using the silicon-carbon composite material, and an electronic device using the secondary battery. Background Technology

[0002] In recent years, rechargeable batteries (such as lithium-ion batteries) have seen rapid development in the fields of new energy vehicles and large-scale energy storage. However, during the charging process of rechargeable batteries, the SEI film formed on the negative electrode surface decomposes and produces gas, which affects the cycle performance of lithium-ion batteries. Summary of the Invention

[0003] This application provides a silicon-carbon composite material that can improve cycle performance.

[0004] Additionally, this application also provides a secondary battery and electronic device using silicon-carbon composite materials.

[0005] This application provides a silicon-carbon composite material, including a silicon matrix and a first layer at least partially located on the silicon matrix, the first layer including a conductive agent and a silicon-oxygen organic compound.

[0006] When the silicon-carbon composite material of this application is applied to a secondary battery, the siloxane groups (SiO-) in the organic silicon oxide can react with the hydrofluoric acid (HF) in the electrolyte, thereby reducing or eliminating the etching of the silicon substrate by HF, improving the interfacial stability between the silicon substrate and the electrolyte, and enhancing the cycle performance of the secondary battery. Simultaneously, it can also reduce the corrosion of the SEI film by HF, thus improving the stability of the SEI film. Furthermore, while improving the stability of the SEI film, the organic silicon oxide can also inhibit the decomposition of the solvent in the SEI film, reducing the organic components in the SEI film, thereby achieving high-voltage stability of the SEI film, reducing lithium plating, and reducing the decomposition and gas generation of the SEI film during over-discharge of the secondary battery, thus improving the gas generation problem during over-discharge. Adding a conductive agent to the first layer helps improve the interfacial conductivity of the silicon-carbon composite material, thereby improving its kinetic performance. The combination of organic silicon oxide and conductive agent in the silicon-carbon composite material results in excellent cycle performance, expansion performance, and kinetic performance. Simultaneously, organic silicon-oxygen compounds can form a network structure, which improves the toughness and strength of the silicon-carbon composite surface, thereby further enhancing the interfacial stability between SEI films and ultimately improving the cycling and expansion performance of the silicon-carbon composite. Conductive agents dispersed within the network structure formed by organic silicon-oxygen compounds can further improve the interfacial conductivity of the silicon-carbon composite, thus enhancing its kinetic properties.

[0007] Based on the first aspect, in some possible embodiments, the siloxane organic material includes at least one of polymethylhydrosiloxane, polysilsesquioxane, vinyltrimethoxysilane, or vinyltriethoxysilane. This siloxane and silane can reduce or eliminate HF etching of the silicon substrate, improve the interfacial stability between the silicon substrate and the electrolyte, thereby enhancing the cycle performance of the secondary battery. It can also reduce lithium plating and reduce the decomposition and gas generation of the SEI film during over-discharge of the secondary battery, thus improving the gas generation problem during over-discharge.

[0008] Based on the first aspect, in some possible embodiments, the conductive agent includes at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. This conductive agent has good conductivity, which is beneficial for improving the conductivity of silicon-carbon composite materials, thus providing a good transport channel for active ions and improving the charge-discharge efficiency and cycle life of the secondary battery. Based on the first aspect, in some possible embodiments, the silicon matrix includes a silicon-carbon material. The silicon-carbon material may include porous carbon material and elemental silicon dispersed on the porous carbon material, wherein the porous carbon material is used to suppress the volume expansion of elemental silicon during cycling.

[0009] Based on the first aspect, in some possible embodiments, the mass ratio of silicon matrix to conductive agent is 100:(0.5~0.9). This is beneficial for improving the conductivity of silicon-carbon composite materials while also having good specific capacity, and for giving the coated silicon-carbon composite materials excellent kinetic properties.

[0010] Based on the first aspect, in some possible embodiments, the mass ratio of the conductive agent to the organic silicon oxide is (0.5-5):2. This facilitates the full coating of the organic silicon oxide onto the silicon substrate, improves the integrity of the coating, thereby significantly reducing the etching of the silicon substrate by the electrolyte, and giving the silicon-carbon composite material good conductivity, resulting in good kinetic and cycling performance.

[0011] Based on the first aspect, in some possible embodiments, the conductive agent has a linear structure with an average diameter of y nm, where 0.5 ≤ y ≤ 20. This is beneficial for improving the conductivity of the silicon-carbon composite material while simultaneously enhancing the dispersibility of the conductive agent, thereby improving the rate performance and cycle performance of the silicon-carbon composite material. The average length of the conductive agent is L nm, where 500 ≤ L ≤ 1000. This is beneficial for improving the dispersibility of the conductive agent while ensuring that the active ions have good long-range conductivity, thus improving the kinetic properties of the silicon-carbon composite material.

[0012] Based on the first aspect, in some possible implementations, the particle size Dv50 of the silicon-carbon composite material is Dμm, 4≤D≤13, which helps to reduce the side reactions between the silicon-carbon composite material and the electrolyte, improve the cycle performance of the silicon-carbon composite material, and improve the gas release problem during overcharging of the secondary battery.

[0013] Based on the first aspect, in some possible implementation manners, the thickness of the first layer is H nm, where 5 ≤ H ≤ 100. This is beneficial for enabling the silicon-carbon composite material to have a good specific capacity while also enabling the first layer to protect the silicon matrix and improve the gas evolution problem during overcharging of the secondary battery.

[0014] Based on the first aspect, in some possible implementation manners, the relationship between H and Dv50 satisfies: 0.55 ≤ H / D ≤ 12.5. This is beneficial for enabling the silicon-carbon composite material to have both good rate performance and cycling performance, and is also beneficial for enhancing the interfacial stability between the silicon-carbon composite material and the electrolyte, thereby improving the gas production problem of the SEI film during overcharging of the secondary battery.

[0015] Based on the first aspect, the silicon matrix includes a silicon-carbon material, and the silicon-carbon material includes silicon and carbon elements. Based on the sum of the masses of silicon and carbon elements in the silicon matrix, the mass percentage of silicon is 41.8% to 57.9%. This is beneficial for enabling the silicon matrix to have good electrical conductivity while also facilitating the capacity contribution of silicon in the silicon matrix and enabling the silicon-carbon composite material to have good cycling performance.

[0016] Based on the first aspect, in some possible implementation manners, the average diameter of the conductive agent is y nm, where 1 ≤ H / y ≤ 25. This is beneficial for the first layer to have a certain thickness while also having good electrical conductivity, which is beneficial for improving the cycling stability performance and energy density of the secondary battery and for improving the over-discharge gas production of the secondary battery.

[0017] The second aspect of the present application further provides a secondary battery, including an electric core, and the electric core includes a negative electrode plate, a positive electrode plate, and an electrolyte. The negative electrode plate further includes a silicon-carbon composite material, and the silicon-oxygen organic matter in the silicon-carbon composite material can improve the interfacial stability between the silicon matrix and the electrolyte and reduce the decomposition gas production of the SEI film during over-discharge of the secondary battery, enhancing the cycling performance of the secondary battery and improving the gas production performance of the secondary battery.

[0018] Based on the second aspect, in some possible implementation manners, when the voltage of the electric core is 3V to 3.95V, the thickness of the electric core is H0, and when the electric core discharges to 0.5V, the thickness of the electric core is H2, where 0.58 < H2 / H0 ≤ 1.36. The gas production problem inside the electric core is improved, and it can maintain good stability, thereby improving the cycling performance and service life of the secondary battery.

[0019] The third aspect of the present application provides an electronic device, including a secondary battery. The secondary battery powers the electronic device, and the secondary battery includes a silicon-carbon composite material, which can improve the cycling life of the secondary battery and thus improve the service life of the electronic device. Description of the Drawings

[0020] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0021] Figure 1 The image shows a scanning electron microscope image of the silicon-carbon composite material prepared in Example 1.

[0022] Figure 2 This is a scanning electron microscope image of the uncoated silicon substrate in Comparative Example 1.

[0023] Figure 3 The graph shows the changes in cell thickness and voltage when the cell assembled from the silicon-carbon composite material prepared in Example 1 is discharged from 3.19V to 0.5V. Detailed Implementation

[0024] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0025] In related technologies, during the over-discharge process of lithium-ion batteries, the negative electrode potential continues to rise to the oxidation potential of the solid electrolyte interphase (SEI) membrane, which causes the SEI membrane to oxidize and decompose, generating a large amount of gas, thus affecting the cycle performance and safety performance of the lithium-ion battery.

[0026] This application provides a silicon-carbon composite material, comprising a silicon matrix and a first layer at least partially located on the silicon matrix. The first layer includes a conductive agent and a silicon-oxygen organic compound containing SiO2. When the silicon-carbon composite material of this application is applied to a secondary battery, the siloxane groups (SiO-) in the silicon-oxygen organic compound can react with hydrofluoric acid (HF) in the electrolyte, thereby reducing or eliminating the etching of the silicon matrix by HF, improving the interfacial stability between the silicon matrix and the electrolyte, and enhancing the cycle performance of the secondary battery. Simultaneously, it can also reduce the corrosion of the SEI film by HF, thereby improving the stability of the SEI film. While improving the stability of the SEI film, the silicon-oxygen organic compound can also inhibit the decomposition of the solvent in the SEI film, reducing the organic components in the SEI film, thereby achieving high-voltage stability of the SEI film, reducing lithium plating, and reducing the decomposition and gas generation of the SEI film during over-discharge of the secondary battery, thus improving the gas generation problem during over-discharge of the secondary battery. The addition of a conductive agent to the first layer is beneficial for improving the interfacial conductivity of the silicon-carbon composite material, thereby improving the kinetic performance of the silicon-carbon composite material. The combination of silicon-oxygen organic compounds and conductive agents in silicon-carbon composites gives them excellent cycling performance, expansion performance, and kinetic performance.

[0027] In some embodiments, the first layer may completely cover the outer surface of the silicon substrate, or the first layer may only cover a portion of the outer surface of the silicon substrate. For example, the surface area of ​​the first layer may account for one-half or one-third of the outer surface area of ​​the silicon substrate.

[0028] In some embodiments, the siloxane organic material includes at least one of polymethylhydrosiloxane, polysilsesquioxane, vinyltrimethoxysilane, or vinyltriethoxysilane. The siloxane or SiO bonds in this siloxane organic material can reduce or eliminate the etching of the Si or silicon compound material interface in the silicon matrix by HF, improving the interfacial stability between the silicon matrix and the electrolyte, thereby enhancing the cycle performance of the secondary battery. It can also reduce the decomposition and gas generation of the SEI film during over-discharge of the secondary battery, improving the gas generation problem during over-discharge. Simultaneously, the network structure formed by the aforementioned siloxane organic material can also improve the toughness and strength of the silicon-carbon composite surface, thereby further improving the stability of the SEI film, and thus improving the cycle performance and expansion performance of the silicon-carbon composite material. The conductive agent dispersed in the network structure formed by the siloxane organic material can further improve the interfacial conductivity of the silicon-carbon composite material, thereby improving the kinetic properties of the silicon-carbon composite material.

[0029] In some embodiments, the conductive agent includes at least one of single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes. This conductive agent exhibits good conductivity, which is beneficial for improving the conductivity of silicon-carbon composite materials. Therefore, it can provide a good transport channel for active lithium ions, reduce the internal resistance of the secondary battery, and thus improve the charge-discharge efficiency and cycle life of the secondary battery.

[0030] In some embodiments, the silicon matrix comprises a silicon-carbon material. The silicon-carbon material may include a porous carbon material and elemental silicon dispersed on the porous carbon material, the porous carbon material being used to suppress the volume expansion of the elemental silicon during cycling.

[0031] In some embodiments, the mass ratio of silicon substrate to conductive agent is 100:

[0032] (0.5~0.9). A mass ratio of conductive agent to silicone organic compound within the above range is beneficial for improving the conductivity of the silicon-carbon composite material while also maintaining a good specific capacity, resulting in excellent kinetic properties in the coated silicon-carbon composite material. In some embodiments, the mass ratio of conductive agent to silicone organic compound can be 100:0.5, 100:0.6, 100:0.65, 100:0.7, 100:0.75, 100:0.8, 100:0.85, etc.

[0033] 100:0.9 or any ratio within the range of any two of the above ratios.

[0034] In some embodiments, the mass ratio of the conductive agent to the organic silicon oxide is (0.5–5):2. This mass ratio within the above range ensures that the organic silicon oxide can adequately coat the silicon substrate, improving the integrity of the coating and thus minimizing electrolyte etching of the silicon substrate. This also results in good conductivity in the silicon-carbon composite material, giving it both good kinetic and cycling performance. In some embodiments, the mass ratio of the conductive agent to the organic silicon oxide can be 0.5:2, 1:2, 1.5:2, 2:2, 2.5:2, 3:2, 3.5:2, 4:2, 4.5:2, 5:2, or any ratio within the range of any two of the above ratios.

[0035] In some embodiments, the conductive agent has a linear structure with an average diameter of y nm, where 0.5 ≤ y ≤ 20. Within this length range, it is beneficial to improve the conductivity of the silicon-carbon composite material while simultaneously enhancing the dispersibility of the conductive agent, thereby improving the rate performance and cycle performance of the silicon-carbon composite material. If the average diameter of the conductive agent is too large, such as y > 20, it will reduce the conductivity of the silicon-carbon composite material and decrease its cycle performance. If the average diameter of the conductive agent is too small, such as y < 0.5, the dispersion of the conductive agent in the first layer will be poor, resulting in uneven charge distribution on the surface of the silicon-carbon composite material, reducing interfacial stability, thereby reducing the cycle performance of the silicon-carbon composite material, and worsening the over-emission gas generation. In some embodiments, the average diameter y of the conductive agent can be 0.5, 1, 2, 3, 5, 7, 9, 10, 12, 15, 17, 18, 20, or any value within the range of any two of the above values.

[0036] In some embodiments, the average length of the conductive agent is L nm, where 500 ≤ L ≤ 1000. An average length within this range is beneficial for improving the dispersibility of the conductive agent while ensuring good long-range conductivity of the active lithium ions. The conductive agent is dispersed on the network structure of the silicon-oxygen organic compound, providing an excellent conductive network and improving the rate performance of the silicon-carbon composite material. In some embodiments, the average length of the conductive agent can be 500, 600, 700, 800, 900, 1000, or any value within the range of any two of the above values.

[0037] In some embodiments, the particle size Dv50 of the silicon-carbon composite material is D μm, where 4 ≤ D ≤ 13. Within this range, it is beneficial to reduce side reactions between the silicon-carbon composite material and the electrolyte, improve the cycle performance of the silicon-carbon composite material, and alleviate the gas release problem during overcharging of the secondary battery. In some embodiments, the particle size D of the silicon-carbon composite material can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or any value within the range of any two of the above values.

[0038] In some embodiments, the thickness of the first layer of the silicon-carbon composite material is H nm, where 5 ≤ H ≤ 100. A thickness within this range is beneficial for achieving good specific capacity in the silicon-carbon composite material while also protecting the silicon matrix and mitigating gas release during overcharging of the secondary battery. In some embodiments, the thickness H of the first layer can be 5, 10, 15, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, or any value within the range of any two of the above values. Preferably, 6 ≤ H ≤ 63.

[0039] In some embodiments, H and Dv50 satisfy the relationship: 0.55 ≤ H / D ≤ 12.5. Adjusting the relationship between the thickness of the first layer and the particle size D of the silicon-carbon composite material within a suitable range is beneficial for the silicon-carbon composite material to possess both good rate performance and cycle performance. For example, silicon-based composite materials have relatively large particle sizes and strong stability, but weak kinetic performance. By reducing the thickness of the first layer, the silicon-based composite material can maintain good kinetic performance. Simultaneously, it also helps improve the interfacial stability between the silicon-carbon composite material and the electrolyte, thereby mitigating the gas generation problem of the SEI film during overcharging of the secondary battery. In some embodiments, the ratio between H and D can be 0.55, 0.8, 0.9, 1, 2, 5, 8, 10, 11, 12, 12.5, or any value within the range of any two of the above values.

[0040] In some embodiments, 1 ≤ H / y ≤ 25. When the relationship between the thickness of the first layer and the average diameter of the conductive agent satisfies the above relationship, a balance is achieved between the thickness of the first layer and the average diameter of the conductive agent. This ensures that the first layer has a certain thickness while also possessing good conductivity, which is beneficial for improving the cycle stability, energy density, and over-discharge gas generation of the secondary battery. In some embodiments, H / y can be 1, 2, 5, 7, 8, 10, 12, 15, 17, 20, 23, 25, or any value within the range of any two of the above values.

[0041] In some embodiments, based on the sum of the masses of silicon and carbon elements in the silicon matrix, the mass percentage of carbon is 38.2% to 61.3%, and the mass percentage of silicon in the silicon matrix is ​​38.7% to 61.8%. The carbon and silicon content in the silicon matrix within these ranges is beneficial for the silicon matrix to possess good electrical conductivity while also maximizing the capacity contribution of silicon in the silicon matrix, resulting in good cycle performance of the silicon-carbon composite material. In some embodiments, the mass percentage of carbon elements in the silicon matrix can be 38.2%, 39%, 41%, 45%, 47%, 49%, 53%, 55%, 58%, 60%, 61.3%, or any value within the range of any two of the above values. The mass percentage of silicon in the silicon matrix can be 38.7%, 39%, 41.8%, 43%, 45%, 48%, 51%, 54%, 56%, 57%, 57.9%, 59%, 61%, 61.8%, or any value within the range of any two of the above values. Preferably, the mass percentage of silicon is between 41.8% and 57.9%.

[0042] This application does not impose any particular restrictions on the preparation method of silicon-carbon composite materials, as long as the purpose of this application can be achieved. For example, the preparation method of silicon-carbon composite materials may include, but is not limited to, the following steps: (1) dissolving and dispersing the conductive agent and the silicon-oxygen organic compound containing Si-OR in a solvent and stirring evenly; (2) adding the silicon matrix material to step (1), stirring and dispersing evenly, and drying to obtain the silicon-carbon composite material.

[0043] The solvent used in step (1) includes at least one of water, ethanol, ethylene glycol, propylene glycol, and tetrahydrofuran.

[0044] In step (2), the stirring time is 1h to 24h, the stirring speed is 500r / min to 2500r / min, the drying temperature is 92℃ to 198℃, and the drying time is 2h to 24h.

[0045] This application does not impose any particular limitation on the preparation method of the silicon substrate, as long as it achieves the purpose of this application. For example, the above-mentioned method for preparing the silicon substrate includes: using chemical vapor deposition, a silicon source gas is deposited on porous carbon to form silicon, and then a carbon source is introduced, causing the carbon source to form amorphous carbon on the surface of the silicon substrate, thereby obtaining the silicon substrate. The carbon source gas may include, but is not limited to, at least one of methane, acetylene, ethylene, ethane, propyne, propylene, propane, butyne, butene, or butane. The silicon source gas may include, but is not limited to, one or more of silane, disilane, propane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane. The silicon content in the silicon substrate can be adjusted by changing the introduction time of the silicon source gas.

[0046] In the preparation method of silicon-carbon composite materials, the mass ratio of silicon to carbon in the silicon matrix and the mass ratio of silicon matrix, conductive agent, and siloxane organic compound affect the cycle performance, lithium plating performance, and over-discharge gas generation of the silicon-carbon composite material. The mass ratio of silicon matrix to conductive agent is 100:(0.5-0.9); the mass ratio of conductive agent to siloxane organic compound is (0.5-5):2. Preferably, the mass ratio of silicon matrix, conductive agent, and siloxane organic compound is 100:0.5:2. Increasing the mass ratio of silicon matrix to conductive agent will reduce the cycle performance of the silicon-carbon composite material; decreasing the mass ratio of silicon matrix to conductive agent will reduce the specific capacity of the silicon-carbon composite material and reduce the energy density of the secondary battery. Increasing the mass ratio of silicon matrix to siloxane organic compound will reduce the effect of the silicon-carbon composite material on improving over-discharge gas generation. Decreasing the mass ratio of silicon matrix to siloxane organic compound will reduce the kinetic properties (such as lithium plating performance) and cycle performance of the silicon-carbon composite material. Increasing the mass ratio of organic silicon oxides to conductive agents will reduce the kinetic and cycle performance of silicon-carbon composite materials, while decreasing the mass ratio of organic silicon oxides to conductive agents will reduce the performance of silicon-carbon composite materials in improving the over-discharge gas generation of secondary batteries.

[0047] Based on the mass ratio of the silicon matrix, conductive agent, and organic silicon oxide mentioned above, the thickness H of the first layer can be changed by adjusting the stirring time and rate when the silicon matrix content is changed.

[0048] The average diameter y and average length L of the conductive agent can be changed by selecting conductive agent raw materials with different average diameters and average lengths.

[0049] One embodiment of this application provides a secondary battery, which includes a casing and a battery cell. The battery cell is located inside the casing.

[0050] The outer casing can be a packaging bag sealed with an encapsulating film (such as aluminum-plastic film), for example, a soft-pack secondary battery. In other embodiments, the secondary battery can also be a steel-cased secondary battery, an aluminum-cased secondary battery, etc.

[0051] The battery cell includes an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator, with the separator disposed between the positive and negative electrode. The electrode assembly can be a stacked structure, formed by stacking the positive electrode, separator, and negative electrode. In other embodiments, the electrode assembly can also be a wound structure, formed by winding the stacked positive electrode, separator, and negative electrode.

[0052] Negative electrode sheet

[0053] The negative electrode includes a negative current collector and a negative active layer disposed on the negative current collector. The negative current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The negative active layer contains a negative active material, including the silicon-carbon composite material described above.

[0054] The negative electrode active layer also includes a binder to bond the negative electrode active material particles, thereby facilitating the formation of the film layer and improving the bonding force between the negative electrode active layer and the negative electrode current collector. In some embodiments, the binder may include, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0055] The negative electrode active layer may further include a conductive material, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0056] The negative electrode active layer can further contain graphite. Due to the flexibility of graphite, its combination with silicon-carbon composite materials can alleviate the overall volume expansion of the negative electrode active layer. At the same time, using graphite and silicon-carbon composite materials as negative electrode active materials can fully utilize the advantages of both silicon-carbon composite materials and graphite to achieve better electrochemical performance.

[0057] Positive electrode sheet

[0058] The positive electrode includes a positive current collector and a positive active layer disposed on the positive current collector. The positive current collector can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The positive active layer contains a positive active material, which includes compounds that reversibly insert and extract lithium ions (i.e., lithiation intercalation compounds). In some embodiments, the positive active material may include a lithium transition metal composite oxide. This lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive active material may include, but is not limited to, lithium cobalt oxide, lithium nickel manganese cobalt oxide, 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-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, and lithium titanate.

[0059] The positive electrode active layer also includes an adhesive for bonding the positive electrode active material particles to facilitate the formation of the film layer, and also to improve the bonding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the adhesive may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0060] The positive electrode active layer may also include a conductive material, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0061] Separating membrane

[0062] The material and shape of the separator used in the electrochemical device of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application.

[0063] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected.

[0064] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixture of polymer and inorganic materials. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, 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.

[0065] The binder is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, the polymer material of which is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0066] electrolyte

[0067] According to some embodiments of this application, the electrolyte includes an organic solvent, a lithium salt, and optional additives.

[0068] The organic solvent in the electrolyte of this application may be any organic solvent known in the prior art that can be used as an electrolyte solvent. There are no limitations on the electrolyte used in the electrolyte according to this application; it may be any electrolyte known in the prior art. The additives in the electrolyte according to this application may be any additives known in the prior art that can be used as electrolyte additives. 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.

[0069] In some embodiments, the organic solvent includes ether solvents, such as at least one of 1,3-dioxapentane (DOL) and dimethyl ethylene glycol (DME).

[0070] 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(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0071] 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.

[0072] In the battery cell formed by combining the above-mentioned negative electrode sheets, when the voltage of the battery cell is 3V to 3.95V, the thickness of the battery cell is H0, and when the battery cell is discharged to 0.5V, the thickness of the battery cell is H2, 0.58 < H2 / H0 ≤ 1.36. The negative electrode sheet in the battery cell includes the silicon-carbon composite material described above. When the ratio of the thickness of the battery cell before and after discharge is within the above range, the gas generation problem inside the battery cell is improved, and it can maintain good stability, thereby improving the cycle performance and service life of the battery cell. When the thickness of the battery cell is H0, the corresponding voltage can be 3V, 3.1V, 3.2V, 3.3V, 3.4V, 3.5V, 3.6V, 3.7V, 3V, 3.8V, 3.95V or any value within the range composed of any two of the above values. In some embodiments, H2 / H0 can be 0.59, 0.6, 0.63, 0.7, 0.8, 0.9, Any value within the range composed of any two of the above values. Preferably, 0.63 ≤ H2 / H0 ≤ 1.23, and more preferably, 0.9 ≤ H2 / H0 ≤ 1.16.

[0073] The thickness change of the above-mentioned battery cell is related to the interfacial stability in the battery cell. The better the interfacial stability between the silicon-carbon composite material and the electrolyte, the smaller the thickness of the battery cell when the battery cell is discharged to 0.5V. The content of Si element, the content of Si-O bond, and the conductivity of the conductive agent in the silicon-carbon composite material affect the interfacial stability of the battery cell. The present application adjusts the above influencing factors to change the thickness of the battery cell when it is discharged to 0.5V, so that the ratio of H2 / H0 is within a suitable range, so that the battery cell has good interfacial stability.

[0074] When the battery cell discharges from its initial voltage to 2.5V, the volume change of its silicon-carbon composite material is affected. If the cell continues to discharge from 2.5V, the SEI film is primarily affected. When the cell discharges from its initial voltage of 3V-3.95V to 0.5V, lithium ions are extracted from the negative electrode active layer, reducing the thickness of the negative electrode. Combined with the fact that the first layer of the silicon-carbon composite material is formed on a silicon substrate, and the good stability of the silicon-carbon composite material itself, as well as the good interfacial stability between the silicon-carbon composite material and the electrolyte, reduces or eliminates the decomposition and gas production of the SEI film, thus reducing the cell thickness. However, if the cell discharges from its initial voltage to 0.5V, the SEI film's stability is poor, and it will decompose and produce gas, significantly increasing the cell thickness and thus affecting the H2 / H0 ratio.

[0075] This application also applies secondary batteries to electronic devices, whereby the secondary batteries power the load of the electronic devices. The secondary batteries in the aforementioned electronic devices contain a negative electrode active material, which includes a silicon-carbon composite material. The first layer of the silicon-carbon composite material can reduce the etching of the silicon substrate by HF, improve the stability of the SEI film, thereby improving the cycle performance and charging efficiency of the secondary battery, and consequently improving the lifespan and charging efficiency of the electronic device.

[0076] Electronic devices may include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0077] The present application will be described below through specific embodiments and comparative examples. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0078] Example 1

[0079] (1) Preparation of silicon-carbon composite materials:

[0080] Single-walled carbon nanotubes and polymethylhydrosiloxane were dissolved and dispersed in a mixed solvent of water and ethanol at a mass ratio of 0.5:2 and stirred evenly to obtain a mixed slurry; (2) 100g of silicon matrix (based on the sum of silicon and carbon elements in the silicon matrix, the mass ratio of carbon element is 50.7% and the mass ratio of silicon element is 49.3%) was added to the mixed slurry in step (1), the stirring speed was 1000r / min, and after stirring for 4h, the mixture was evenly dispersed and dried at 168℃ to obtain a silicon-carbon composite material. The Dv50 of the silicon-carbon composite material was 9μm, the average diameter of the single-walled carbon nanotubes was 5nm, and the average length was 800nm.

[0081] (2) Preparation of lithium-ion batteries:

[0082] Preparation of the negative electrode sheet: The silicon-carbon composite material, artificial graphite, styrene-butadiene rubber (SBR) as the negative electrode binder, and carboxymethyl cellulose (CMC) as the negative electrode dispersant were mixed in a mass ratio of 10:88:1.6:0.4. Deionized water was then added as a solvent and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as the negative electrode current collector. The copper foil was dried at 85°C for 4 hours to obtain a negative electrode sheet with a single-sided coating of the negative electrode active material layer, with a coating thickness of 80 μm. After cold pressing, cutting, and slitting, the sheet was dried under vacuum at 120°C for 12 hours to obtain a negative electrode sheet with dimensions of 76.6 mm × 875 mm. The cold pressing pressure was 20 tons (t), and the compaction density of the negative electrode sheet was 1.78 g / cm³. 3 .

[0083] Preparation of the positive electrode sheet: Lithium cobalt oxide (positive electrode active material), acetylene black (positive electrode conductive agent), and polyvinylidene fluoride (PVDF) (positive electrode binder) were mixed in a mass ratio of 96.3:2.2:1.5. N-methylpyrrolidone (NMP) was added as a solvent and stirred evenly to prepare a positive electrode slurry with a solid content of 75 wt%. The positive electrode slurry was uniformly coated on one surface of a 13 μm thick aluminum foil for the positive electrode current collector and dried at 85 °C to obtain a single-sided positive electrode sheet with a positive electrode active material layer thickness of 130 μm. After cold pressing, cutting, and slitting, the positive electrode sheet was dried under vacuum at 85 °C for 4 hours to obtain a positive electrode sheet with a size of 74 mm × 867 mm. The cold pressing pressure was 20 t, and the compaction density of the positive electrode sheet was 4.15 g / cm³. 3 .

[0084] Electrolyte preparation: In a dry argon-atmospheric glove box, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl propionate (EP) were mixed in a mass ratio of 3:1:3:3. Then, lithium salt LiPF6 was added and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the lithium salt content was 12.5%, with the remainder being organic solvent.

[0085] Preparation of the separator: A porous polyethylene film with a thickness of 15 μm (provided by Celgard) was used as the separator.

[0086] Assembling lithium-ion batteries: The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to form the electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 80°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected. The battery undergoes vacuum sealing, settling, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), degassing, and edge trimming processes to obtain the lithium-ion battery (i.e., the cell).

[0087] Examples 2 to 7

[0088] The difference between Examples 2 to 7 and Example 1 is that the type of organic silicon oxide or the mass ratio of silicon matrix to conductive agent or the mass ratio of conductive agent to organic silicon oxide are changed, while the rest of the preparation process is the same as in Example 1.

[0089] Examples 8 to 10

[0090] The difference between Examples 8 to 10 and Example 1 is that the mass ratio of silicon in the silicon matrix is ​​changed, while the rest of the preparation process is the same as that of Example 1.

[0091] Examples 11 to 12

[0092] The difference between Examples 11 and 12 and Example 1 is that the particle size Dv50 of the silicon-carbon composite material is changed, while the rest of the preparation process is the same as that of Example 1.

[0093] Examples 13 to 16

[0094] The difference between Examples 13 to 16 and Example 1 is that the average diameter or average length of the conductive agent in the silicon-carbon composite material is changed, while the rest of the preparation process is the same as in Example 1.

[0095] Example 17

[0096] The difference between Example 17 and Example 1 is that the stirring time in step (2) of preparing the silicon-carbon composite material is changed. The stirring time is 10 min at a stirring speed of 1000 r / min, and the thickness of the first layer is 5 nm. The rest of the preparation process is the same as in Example 1.

[0097] Example 18

[0098] The difference between Example 18 and Example 1 is that the stirring time in step (2) of preparing the silicon-carbon composite material is changed. The stirring speed is 1000 r / min and the stirring time is 1 h, resulting in a first layer with a thickness of 36 nm. The rest of the preparation process is the same as in Example 1.

[0099] Example 19

[0100] The difference between Example 19 and Example 1 is that the stirring time in step (2) of preparing the silicon-carbon composite material is changed. The stirring time is 8 hours at a stirring speed of 1000 r / min, and the thickness of the first layer is 72 nm. The rest of the preparation process is the same as in Example 1.

[0101] Example 20

[0102] The difference between Example 20 and Example 1 is that the stirring time in step (2) of preparing the silicon-carbon composite material is changed. The stirring speed is 1000 r / min and the stirring time is 24 h, resulting in a first layer with a thickness of 100 nm. The rest of the preparation process is the same as in Example 1.

[0103] Comparative Example 1

[0104] The difference from Example 1 is that the surface of the silicon matrix was not coated during the preparation of the silicon-carbon composite material. The specific material composition is shown in Table 1.

[0105] Comparative Example 2

[0106] The difference from Example 1 is that in the process of preparing silicon-carbon composite material, the surface of the silicon matrix is ​​only coated with a conductive agent and not with silicon-oxygen organic matter. The specific material composition is shown in Table 1.

[0107] Comparative Example 3

[0108] The difference from Example 1 is that in the process of preparing silicon-carbon composite material, the surface of the silicon matrix is ​​only coated with silicon-oxygen organic matter and not with conductive agent. The specific material composition is shown in Table 1.

[0109] The silicon-carbon composite materials and lithium-ion batteries prepared in the examples and comparative examples were subjected to the following tests, as shown in Tables 1 to 4.

[0110] Performance testing

[0111] (1) Test method for powder particle size Dv50:

[0112] The particle size distribution of the silicon-based composite material was tested using a Malvern particle size analyzer (Master Sizer 2000). The sample preparation method was as follows: approximately 0.02 g of powder sample was added to a 50 ml clean beaker, followed by approximately 20 ml of deionized water. Then, 3 drops of sodium dodecyl sulfate surfactant were added to completely disperse the powder in the water. The mixture was then ultrasonically cleaned for 5 minutes using a 120W ultrasonic cleaner to obtain the powder particle size distribution sample. In the volumetric particle size distribution of the material, the particle size reaching 50% of the cumulative volume was defined as Dv50, starting from the smallest particle size.

[0113] (2) Method for measuring the thickness of the first layer:

[0114] The silicon-carbon composite material was sliced ​​using focused ion beam (FIB) and characterized by high-resolution transmission electron microscopy (HRTEM, model Talos F200X). Within the same selected area (500,000x magnification), the first layer in the particles was observed, and the thickness of the first layer in the silicon-carbon composite material particles was measured. The thickness at fifty different locations in the first layer was randomly selected. The thickness can be obtained by measuring with a scale, and the average value of the fifty thickness values ​​was calculated to obtain the thickness of the first layer.

[0115] (3) Test methods for the average diameter and average length of conductive agents:

[0116] A 30,000kx magnified image was captured using a scanning electron microscope (SEM). The image was processed using the image analysis software ImageJ. The length and diameter of 20 CNTs were then measured. The average length of the CNTs was calculated by taking the average of the length values ​​of the 20 CNTs, and the average diameter of the CNTs was calculated by taking the average of the diameter values ​​of the 20 CNTs.

[0117] (4) Test method for the mass ratio of silicon and carbon elements in silicon matrix:

[0118] By capturing a 1000x magnified image using a scanning electron microscope (SEM), selecting the silicon matrix portion of the silicon-carbon material, and performing EDS elemental analysis, the mass percentages of silicon and carbon can be obtained.

[0119] (5) Testing methods for scanning electron microscopy (SEM):

[0120] Silicon-based composite materials were tested using a JEOL-JSM-6700F scanning electron microscope at a voltage of 5 kV and a current of 0.8 nA.

[0121] Lithium-ion battery performance testing

[0122] (1) Cyclic performance testing methods:

[0123] The lithium-ion battery was placed in a constant temperature chamber at 25℃±1℃ for 30 minutes, then charged at a constant current of 0.5C to 4.45V, and then charged at a constant voltage of 4.45V to 0.025C. After being placed in a constant temperature chamber for 5 minutes, it was discharged at 0.5C to 3.0V. This constitutes one charge-discharge cycle. The initial cycle discharge capacity C0 of the lithium-ion battery was recorded. This cycle was then repeated 500 times. The cycle discharge capacity C1 of the 500th cycle was recorded. The 500-cycle capacity retention rate = C1 / C0 × 100%.

[0124] (2) Method for measuring cell thickness before and after discharge:

[0125] The initial thickness of the battery cell (with a voltage of 3V to 3.95V) is measured using a micrometer and recorded as H0. Then, the cell is discharged to 0.5V using 0.01C and the thickness of the cell is measured using a micrometer and recorded as H2. The ratio of H2 to H0 is then calculated.

[0126] (3) Test methods or calculation formulas for energy density:

[0127] Five lithium-ion batteries were selected from each group of tested batteries and subjected to their first charge and discharge cycles at 25°C. Constant current charging was performed at 0.5C until the upper voltage limit was reached, followed by constant voltage charging to 0.02C. Then, constant current discharging was performed at 0.2C until the cutoff voltage was reached. The discharge capacity of the lithium-ion batteries was obtained, and the average discharge voltage was calculated.

[0128] The lithium-ion batteries were charged to 50% SOC at 0.5C to obtain the 50% SOC state. The length, width, and thickness of each lithium-ion battery at 50% SOC were measured, and the volume of the lithium-ion battery was calculated. The volumetric energy density of the lithium-ion battery was calculated as follows: lithium-ion battery discharge capacity × average discharge voltage of lithium-ion battery / lithium-ion battery volume.

[0129] The upper limit voltage for charging the lithium-ion battery is 4.45V, and the discharge cutoff voltage is 3.0V.

[0130] (4) Test method for lithium plating performance:

[0131] The lithium-ion battery was placed in a constant temperature chamber at 25℃±1℃ for 30 minutes, then charged at a constant current of 1.2C to 4.45V, followed by constant voltage charging at 4.45V to 0.025C. After a 5-minute rest, it was discharged at 0.5C to 3.0V, completing one charge-discharge cycle. This cycle was repeated 10 times. Then, the battery was charged at a constant current of 4C to 4.45V, followed by constant voltage charging at 4.45V to 0.025C, and then rested for 5 minutes. The lithium-ion battery was then disassembled to observe lithium plating. Following the same method, the 1.2C was adjusted to 1.6C, 2C, 2.4C, 2.6C, 2.8C, 3C, 3.1C, 3.2C, 3.4C, 3.8C, or 4C, and lithium plating was observed at different charging rates. The higher the charging rate, the better the lithium-ion battery's lithium plating performance. The determination is based on the contamination status of the separator in contact with the negative electrode during full-charge disassembly. If the separator in contact with the negative electrode is white as a whole and the area showing gray is less than 2%, it is determined that there is no lithium plating.

[0132] Figure 1 This is a scanning electron microscope image of the silicon-carbon composite material prepared in Example 1. Figure 2 This is a scanning electron microscope image of the uncoated silicon substrate in Comparative Example 1. From... Figure 1 and Figure 2 In the process, single-walled carbon nanotubes with a linear structure are formed on the surface of the silicon substrate, and the surface of the silicon substrate is coated with a first layer. Figure 3 The battery cell assembled in Example 1 shows the trend of the initial voltage of the battery cell from 3.19V to 0.5V and the trend of the thickness of the battery cell. During the discharge process, the thickness of the battery cell gradually decreases. When the discharge reaches 0.5V, the thickness of the battery cell is 4mm.

[0133] Table 1

[0134]

[0135] Table 2

[0136]

[0137]

[0138] Note: In Tables 1 and 2, " / " indicates that no parameters have been added or there are no related parameters.

[0139] As shown in Tables 1 and 2, compared to Comparative Examples 1 to 3, in Examples 1 to 7, the first layer simultaneously contains a conductive agent and an organic silicon oxide compound, which improves the cycle retention rate of the lithium-ion battery and reduces lithium plating. This indicates that the first layer on the silicon substrate needs to simultaneously coat a conductive agent and an organic silicon oxide compound, as only one of these two compounds will reduce the cycle retention rate or lithium plating performance of the lithium-ion battery. In Examples 1 to 7, when the silicon-carbon composite material has both an organic silicon oxide compound and a conductive agent on the surface of the silicon substrate, the cycle performance of the silicon-carbon composite material is improved and lithium plating is reduced. Furthermore, in the above examples, adjusting the mass ratio of the silicon substrate to the conductive agent, the mass ratio of the conductive agent to the organic silicon oxide compound, or using different types of organic silicon oxide compounds affects the SiO bond content in the silicon-carbon composite material, which in turn affects the stability of the SEI film in the cell, thereby affecting the H2 / H0 ratio. Compared to Comparative Examples 1 to 3, the H2 / H0 ratio decreased in Examples 1 to 7, which indicates that the presence of both silicon-oxygen organic matter and conductive agent in the silicon-carbon composite material reduces or eliminates the decomposition gas generation of the SEI film.

[0140] Table 3

[0141]

[0142]

[0143] Table 4

[0144]

[0145] Referring to Tables 1 to 4, in Examples 8 to 9, adjusting the silicon and carbon content in the silicon matrix affects the cycle performance and lithium plating performance of the silicon-carbon composite material. A suitable silicon content in the silicon matrix contributes to good cycle performance and lithium plating performance in the secondary battery. In Examples 1, 11, and 12, adjusting the particle size Dv50 of the silicon-carbon composite material, and ensuring that the particle size Dv50 and H / D ratio are within suitable ranges, results in secondary batteries with good energy density, as well as good cycle performance and lithium plating performance.

[0146] In Examples 1, 13 to 16, by adjusting the average diameter and average length of the conductive agent, the corresponding secondary batteries not only have good energy density but also good cycle performance and lithium plating performance.

[0147] In Examples 1, 17 to 20, during the preparation of silicon-carbon composite materials, the stirring time was adjusted to adjust the thickness of the first layer in the silicon-carbon composite material. When the thickness of the first layer is within a suitable range, the corresponding secondary battery has good energy density and also good cycle performance and lithium plating performance.

[0148] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Therefore, any equivalent variations made in accordance with this application are still within the scope of this application.

Claims

1. A silicon-carbon composite material, characterized by, The silicon base and the first layer at least partially located on the silicon base, the first layer comprising a conductive agent and a siloxane organic matter, the conductive agent being in a linear structure, the average diameter of the conductive agent being y nm, 0.5≤y≤20, the average length of the conductive agent being L nm, 500≤L≤1000.

2. The silicon-carbon composite of claim 1, wherein, The siloxane organic matter comprises at least one of polymethylhydrosiloxane, polysilsesquioxane, vinyltrimethoxysilane or vinyltriethoxysilane.

3. The silicon-carbon composite of claim 1, wherein, The conductive agent comprises at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. And / or, The silicon base comprises a silicon-carbon material.

4. The silicon-carbon composite of claim 1, wherein, The mass ratio of the silicon base to the conductive agent is 100:(0.5~0.9); the mass ratio of the conductive agent to the siloxane organic matter is (0.5~5):

2.

5. The silicon-carbon composite of any one of claims 1 to 4, wherein, The silicon-carbon composite material satisfies at least one of the following conditions: (1) The particle size Dv50 of the silicon-carbon composite material is D μm, 4≤D≤13; (2) The thickness of the first layer is H nm, 5≤H≤100; (3) The particle size Dv50 of the silicon-carbon composite material is D μm, the thickness of the first layer is H nm, 0.55≤H / D≤12.5; (4) The silicon base comprises a silicon-carbon material, the silicon-carbon material comprising silicon elements and carbon elements, the mass ratio of the silicon elements being 41.8% to 57.9% based on the sum of the mass of the silicon elements and the carbon elements in the silicon base.

6. The silicon-carbon composite of claim 1, wherein, The thickness of the first layer is H nm, 1≤H / y≤25.

7. A secondary battery, comprising a cell, characterized in that, The battery cell comprises a negative electrode sheet, a positive electrode sheet and an electrolyte, the negative electrode sheet further comprising the silicon-carbon composite material according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, wherein The thickness of the battery cell when the voltage of the battery cell is 3V to 3.95V is H0, the thickness of the battery cell when the battery cell is discharged to 0.5V is H2, 0.58<H2 / H0≤1.

36.

9. An electronic device, comprising: The secondary battery comprises the battery cell according to claim 8.

Images