Double-layer coated silicon-based composite negative electrode material, preparation method thereof and secondary battery

Abstract

The invention provides a double-layer coated silicon-based composite negative electrode material, a preparation method thereof and a secondary battery. The double-layer coated silicon-based composite negative electrode material comprises a silicon-based material core, and a copolymer coating layer and an oxide coating layer which are sequentially arranged on the surface of the silicon-based material core from inside to outside, the material of the copolymer coating layer comprises a copolymer formed by an unsaturated phosphate ester monomer and an unsaturated siloxane monomer containing an ester group. The silicon-based material is subjected to multi-level surface modification treatment, so that the interface performance of the silicon-based material is further optimized, the safety performance of the secondary battery is improved, meanwhile, acidic impurities in an electrolyte can be adsorbed, and the volume expansion phenomenon of the silicon material is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrode materials, and in particular relates to a double-layer coated silicon-based composite negative electrode material, a preparation method thereof, and a secondary battery. Background Art

[0002] With the rapid development of portable electronic products, new energy vehicles, and energy storage systems, different industries have placed higher demands on the energy density of lithium-ion batteries. Improving energy density is one of the key directions in the current development of lithium-ion battery technology. For example, by optimizing positive and negative electrode materials (such as high-nickel ternary positive electrode materials and silicon-based negative electrode materials) and battery structure design (such as CTP technology and blade battery technology), the energy density of batteries can be significantly improved.

[0003] In recent years, silicon-based negative electrode materials have been considered as important candidates for the next generation of lithium-ion battery negative electrode materials due to their advantages such as high specific capacity, low delithiation potential and low cost. However, silicon-based negative electrode materials will undergo significant volume expansion (up to 300%) during the charge and discharge process, causing the SEI film to continuously rupture and reform, further increasing the interfacial resistance, thereby affecting the cycle life and stability of the secondary battery. At the same time, the volume expansion of silicon materials may cause damage to the electrode structure, thereby causing battery short circuit or thermal runaway. In addition, silicon materials themselves have poor conductivity, and most existing technologies improve their conductivity by doping with carbon materials or other conductive additives.

[0004] In order to solve the above problems, coating technology is widely used on the surface of silicon-based negative electrode materials to form a buffer layer to relieve the stress caused by volume expansion, thereby improving the structural stability of the material. Common coating materials include carbon materials (such as graphene, carbon nanotubes), polymers (such as polyvinyl alcohol, polyacrylic acid) and inorganic materials (such as lithium fluoride). Among them, the carbon coating layer is widely used due to its excellent conductivity and mechanical stability. Although the surface coating technology of silicon-based negative electrode materials has significant advantages in improving its cycle stability and electrochemical performance, there are still problems such as difficulty in controlling the thickness of the coating layer, insufficient stability, complex selection and preparation, adverse effects on conductivity, and reaction with the electrolyte.

[0005] In addition, the traditional SEI film has poor stability on the surface of silicon-based negative electrode materials and is easily decomposed under high voltage. In addition, the acidic impurities (such as hydrogen fluoride) present in the electrolyte will have a destructive effect on the SEI film, and thus cannot effectively inhibit the volume expansion of the silicon material and affect the electrochemical performance of the secondary battery. This seriously limits the application of silicon-based negative electrode materials in high-performance batteries.

[0006] Therefore, there is an urgent need to develop a silicon-based negative electrode material with good coating effect, so as to improve the corrosion resistance, structural stability and safety performance of the electrode material, thereby improving the electrochemical performance of the silicon-based negative electrode material. Summary of the Invention

[0007] To address the shortcomings of the existing technology, the present invention aims to provide a double-layered silicon-based composite negative electrode material, its preparation method, and a secondary battery. This invention further optimizes its interfacial properties and improves the safety of the secondary battery by subjecting the silicon-based material to multi-level surface modification. It also absorbs acidic impurities in the electrolyte and reduces the volume expansion of the silicon material.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a double-layer coated silicon-based composite negative electrode material, the double-layer coated silicon-based composite negative electrode material comprising a silicon-based material core, and a copolymer coating layer and an oxide coating layer sequentially arranged on the surface of the silicon-based material core from the inside to the outside;

[0010] The material of the copolymer coating layer includes a copolymer formed by an unsaturated phosphate monomer and an unsaturated siloxane monomer containing an ester group.

[0011] The present invention provides a multi-level coating structure on the surface of the silicon-based material core, which not only optimizes the interface structure between the electrolyte and the negative electrode material, reduces the hydrogen fluoride content in the electrolyte and the occurrence of side reactions, but also enables the coating layer to have appropriate mechanical strength, thereby effectively suppressing the volume expansion of the silicon-based material and promoting the safety performance of the secondary battery.

[0012] On one hand, the present invention directly coats a copolymer of an unsaturated phosphate monomer and an ester-containing unsaturated siloxane monomer on the surface of a silicon-based core, thereby acting as an artificial SEI membrane. Specifically, the phosphate functional groups in the copolymer structure interact with lithium ions in the electrolyte, optimizing the interface structure between the electrolyte and the negative electrode. This helps reduce interfacial resistance and increase lithium ion transport rates. Furthermore, the phosphate functional groups can enhance the safety of the secondary battery.

[0013] In addition, the silicon-oxygen bonds introduced into the copolymer structure can absorb acidic impurities such as hydrogen fluoride in the electrolyte, thereby improving the structural stability of the silicon-based negative electrode material. The ester groups in the unsaturated siloxane monomer structure containing ester groups can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, thereby enhancing the bonding force between the silicon-based negative electrode material and the copolymer coating, and reducing the volume expansion of the silicon-based negative electrode material. The copolymer coating provided by the present invention can also effectively isolate the electrolyte, preventing it from directly contacting the silicon-based negative electrode material.

[0014] On the other hand, the present invention provides an oxide coating layer on the outermost layer of the silicon-based material core, which can not only further improve the rigidity and safety performance of the double-layer coating layer, but also absorb hydrogen fluoride in the electrolyte, thereby synergistically protecting the silicon-based negative electrode material with the copolymer coating layer.

[0015] Preferably, the unsaturated phosphate monomer includes a phosphate monomer containing at least one alkenyl group.

[0016] Preferably, the phosphate monomer containing at least one alkenyl group includes triallyl phosphate.

[0017] Preferably, the unsaturated siloxane monomer containing an ester group includes a siloxane monomer containing at least one alkenyl group and at least one ester group.

[0018] Preferably, the siloxane monomer containing at least one alkenyl group and at least one ester group includes any one or a combination of at least two of methacryloxypropyltrimethoxysilane (CAS No.: 2530-85-0), methacryloxypropyltriethoxysilane (CAS No.: 21142-29-0) or 3-[tris(1-methylethoxy)silyl]propyl methacrylate (CAS No.: 80750-05-6).

[0019] Preferably, the number average molecular weight of the copolymer is 10,000 Da to 80,000 Da, preferably 30,000 Da to 60,000 Da, for example, it can be 10,000 Da, 15,000 Da, 20,000 Da, 25,000 Da, 30,000 Da, 35,000 Da, 40,000 Da, 45,000 Da, 50,000 Da, 55,000 Da, 60,000 Da, 65,000 Da, 70,000 Da, 75,000 Da or 80,000 Da, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0020] The present invention regulates the number-average molecular weight of the copolymer to achieve copolymer films with excellent processing, film-forming, and mechanical properties. Using a copolymer with a lower number-average molecular weight results in poor film-forming and mechanical properties, making it ineffective in protecting silicon-based anode materials. Using a copolymer with a higher number-average molecular weight increases the viscosity of the copolymer solution, resulting in poor processing performance and ultimately affecting the coating effect.

[0021] Preferably, the crystallinity of the copolymer is 10% to 40%, preferably 20% to 30%, for example, it can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38% or 40%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0022] The present invention effectively mitigates the volume expansion of silicon-based anode materials by regulating the crystallinity of the copolymer, resulting in a copolymer film with both good flexibility and structural stability. Using a copolymer with a lower crystallinity would result in insufficient stability of the copolymer film, while using a copolymer with a higher crystallinity would result in poor flexibility and difficulty adapting to the volume expansion of the silicon-based anode material.

[0023] Preferably, the material of the oxide coating layer includes aluminum oxide.

[0024] Preferably, the average particle size of the aluminum oxide is 10 nm to 100 nm, preferably 20 nm to 50 nm, for example, it can be 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm or 100 nm, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0025] Preferably, the mass ratio of the material of the copolymer coating layer to the material of the oxide coating layer is 1:(0.1-1), preferably 1:(0.3-0.5), for example, it can be 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.32, 1:0.35, 1:0.38, 1:0.4, 1:0.42, 1:0.45, 1:0.48, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0026] The present invention regulates the mass ratio of the copolymer coating material to the oxide coating material to ensure that the resulting double coating layer possesses suitable mechanical strength, thereby further improving the structural stability of the silicon-based composite negative electrode material. Using a lower mass ratio of the copolymer coating material to the oxide coating material results in poor flexibility of the double coating layer and the resulting copolymer artificial SEI film is prone to rupture. Using a higher mass ratio of the copolymer coating material to the oxide coating material results in poor rigidity of the double coating layer.

[0027] Preferably, the total thickness of the copolymer coating layer and the oxide coating layer is 40nm to 200nm, for example, it can be 40nm, 50nm, 60nm, 80nm, 100nm, 120nm, 150nm, 180nm or 200nm, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0028] By regulating the combined thickness of the copolymer coating and the oxide coating, the present invention further improves the cycling and rate performance of silicon-based composite anode materials while ensuring the double-layer coating has good structural stability and high ion transport performance. A thinner double-layer coating is prone to rupture, ultimately leading to loss of active lithium. A thicker double-layer coating deteriorates its ion transport performance, resulting in poor cycling and rate performance of the silicon-based composite anode material.

[0029] In the present invention, the silicon-based material core exemplarily includes a silicon oxide material core and / or a silicon-carbon material core.

[0030] In the present invention, the average particle size of the silicon-carbon material core is 4μm to 10μm, for example, it can be 4μm, 4.2μm, 4.5μm, 4.8μm, 5μm, 5.2μm, 5.5μm, 5.8μm, 6μm, 6.2μm, 6.5μm, 6.8μm, 7μm, 7.2μm, 7.5μm, 7.8μm, 8μm, 8.2μm, 8.5μm, 8.8μm, 9μm, 9.2μm, 9.5μm, 9.8μm or 10μm, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] The present invention regulates the average particle size of the silicon-carbon material core, facilitating subsequent processing and improving the ion transport performance of the silicon-based composite anode material. Using a silicon-carbon material core with a smaller average particle size makes it difficult to disperse the silicon-carbon material particles during the subsequent homogenization process. Using a silicon-carbon material core with a larger average particle size degrades the ion transport performance of the resulting silicon-based composite anode material, and the rate performance of the resulting secondary battery is subsequently impaired.

[0032] In the present invention, the mass percentage of silicon material in the silicon-carbon material core is 40% to 50%, for example, it can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49% or 50%, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0033] By regulating the mass percentage of silicon in the silicon-carbon core, the present invention achieves a high gram capacity and high initial coulombic efficiency while minimizing volume expansion. Using a silicon-carbon core with a lower silicon content results in lower gram capacity and initial coulombic efficiency; using a silicon-carbon core with a higher silicon content results in greater volume expansion.

[0034] In a second aspect, the present invention provides a method for preparing the double-layer coated silicon-based composite negative electrode material according to the first aspect, the method comprising the following steps:

[0035] S1. Mixing an unsaturated phosphate monomer, an unsaturated siloxane monomer containing an ester group, an initiator and a solvent to obtain a copolymer by copolymerization;

[0036] S2. preparing a precursor solution containing the copolymer and a silicon-based material, and electrospinning the precursor solution to obtain a silicon-based material having a surface-coated copolymer;

[0037] S3. Performing oxide coating treatment on the silicon-based material with the copolymer coated on the surface to obtain the double-layer coated silicon-based composite negative electrode material.

[0038] Preferably, the molar ratio of the unsaturated phosphate monomer to the unsaturated siloxane monomer containing an ester group in step S1 is (1 to 5):1, preferably 3:1, for example, it can be 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, 3.2:1, 3.5:1, 3.8:1, 4:1, 4.2:1, 4.5:1, 4.8:1 or 5:1, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0039] The present invention adjusts the molar ratio of unsaturated phosphate monomers to unsaturated siloxane monomers containing ester groups so that the formed silicon-based negative electrode material interface film has good structural stability and interface performance, thereby improving the rate performance and thermal safety performance of the secondary battery. If a smaller molar ratio of unsaturated phosphate monomers to unsaturated siloxane monomers containing ester groups is used, the content of phosphate groups will be lower, and the thermal safety performance of the electrolyte will be poor. In addition, due to the extremely high hydrophobicity of the siloxane chain segments, excessive introduction will cause the compatibility between the copolymer coating and the electrolyte to decrease, resulting in poor wettability at the electrode / electrolyte interface and limited number of lithium ion transmission channels. If a larger molar ratio of unsaturated phosphate monomers to unsaturated siloxane monomers containing ester groups is used, the content of silicon-oxygen bonds will be lower, and the HF generated by the electrolyte cannot be fully absorbed, which has an adverse effect on the stability of the interfacial film. At the same time, the excessive content of phosphate groups leads to greater rigidity of the copolymer coating, making it unable to dissipate stress through chain segment movement, generating local stress at the interface, and ultimately leading to poor interface stability.

[0040] Preferably, the initiator in step S1 exemplarily includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, or dibenzoyl peroxide (BPO). Furthermore, based on the total mass of the unsaturated phosphate monomer and the unsaturated siloxane monomer containing an ester group as 100%, the mass percentage of the initiator is 0.5% to 1%, preferably 0.7%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0041] Preferably, the solvent in step S1 illustratively includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0042] In the present invention, after the copolymerization reaction, the method further comprises the following steps: adding the reaction solution after the copolymerization reaction into a precipitation solvent to obtain a copolymer precipitate, and then washing and drying the copolymer precipitate to obtain the copolymer.

[0043] In the present invention, the precipitation solvent illustratively includes at least one of propanol, isopropanol or acetone.

[0044] Preferably, the temperature of the copolymerization reaction in step S1 is 50° C. to 80° C., and the time of the copolymerization reaction is 6 h to 12 h.

[0045] Specifically, the temperature of the copolymerization reaction can be, for example, 50°C, 60°C, 70°C or 80°C, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the time of the copolymerization reaction can be, for example, 6h, 7h, 8h, 9h, 10h, 11h or 12h, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0046] Preferably, the mass concentration of the solute in the precursor solution in step S2 is 5% to 15%, preferably 8% to 12%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0047] In the present invention, the organic solvent in the precursor solution in step S2 illustratively includes at least one of benzene, tetrahydrofuran (THF), acetone, or N,N-dimethylformamide (DMF).

[0048] Preferably, the operating voltage of the electrospinning in step S2 is 10 kV to 30 kV, preferably 15 kV to 20 kV, for example, it can be 10 kV, 12 kV, 15 kV, 16 kV, 17 kV, 18 kV, 19 kV, 20 kV, 22 kV, 25 kV, 28 kV or 30 kV, etc., not limited to the listed values, other values ​​not listed within the numerical range are also applicable.

[0049] Preferably, the receiving distance of the electrospinning in step S2 is 10 cm to 30 cm, preferably 15 cm to 20 cm, for example, it can be 10 cm, 12 cm, 15 cm, 16 cm, 17 cm, 18 cm, 19 cm, 20 cm, 22 cm, 25 cm, 28 cm or 30 cm, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0050] Compared to traditional liquid-phase coating methods, the present invention uses electrospinning to coat the silicon-based material with a copolymer. This is because electrospinning offers significant advantages over traditional liquid-phase coating methods, particularly in terms of coating consistency, structural controllability, and functional optimization. The present invention uses electrospinning to form a continuous, uniform nanofiber network on the surface of silicon particles, with adjustable fiber diameter, and utilizes parameter optimization (such as voltage, solution concentration, and receiving distance) to achieve uniform coating thickness.

[0051] Preferably, the oxide coating process in step S3 includes ball milling the silicon-based material of the surface-coated copolymer and the oxide, so that the oxide is evenly attached to the surface of the silicon-based material of the surface-coated copolymer, thereby obtaining the double-layer coated silicon-based composite negative electrode material.

[0052] During the ball milling process, the impact and friction of the milling balls promote the thorough mixing of the oxide and the silicon-based material coated with the copolymer.

[0053] Preferably, based on the total mass of the uncoated silicon-based material as 100%, the mass percentage of the oxide is 1% to 5%, for example, it can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8% or 5%, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0054] In the present invention, a ball milling solvent needs to be added before the ball milling treatment, and the ball milling solvent exemplarily includes ethanol or acetone.

[0055] Preferably, the ball-to-material ratio of the ball milling treatment is (5-10):1, for example, it can be 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0056] Preferably, the rotation speed of the ball milling treatment is 200 rpm to 500 rpm, and the time of the ball milling treatment is 10 h to 24 h.

[0057] Specifically, the rotation speed of the ball milling treatment can be, for example, 200 rpm, 300 rpm, 400 rpm or 500 rpm, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the time of the ball milling treatment can be, for example, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h or 24 h, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0058] In a third aspect, the present invention provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises a double-layer coated silicon-based composite negative electrode material prepared according to the preparation method described in the first aspect or according to the preparation method described in the second aspect.

[0059] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] The present invention provides a double-layer coated silicon-based composite negative electrode material. By arranging a multi-level coating structure on the surface of the silicon-based material core, it can not only optimize the interface structure between the electrolyte and the negative electrode material, reduce the hydrogen fluoride content in the electrolyte and the occurrence of side reactions, but also enable the coating layer to have appropriate mechanical strength, thereby effectively suppressing the volume expansion of the silicon-based material and promoting the safety performance of the secondary battery.

[0062] On one hand, the present invention directly coats a copolymer of an unsaturated phosphate monomer and an ester-containing unsaturated siloxane monomer on the surface of a silicon-based core, thereby acting as an artificial SEI membrane. Specifically, the phosphate functional groups in the copolymer structure interact with lithium ions in the electrolyte, optimizing the interface structure between the electrolyte and the negative electrode. This helps reduce interfacial resistance and increase lithium ion transport rates. Furthermore, the phosphate functional groups can enhance the safety of the secondary battery.

[0063] In addition, the silicon-oxygen bonds introduced into the copolymer structure can absorb acidic impurities such as hydrogen fluoride in the electrolyte, thereby improving the structural stability of the silicon-based negative electrode material. The ester groups in the unsaturated siloxane monomer structure containing ester groups can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, thereby enhancing the bonding force between the silicon-based negative electrode material and the copolymer coating, and reducing the volume expansion of the silicon-based negative electrode material. The copolymer coating provided by the present invention can also effectively isolate the electrolyte, preventing it from directly contacting the silicon-based negative electrode material.

[0064] On the other hand, the present invention provides an oxide coating layer on the outermost layer of the silicon-based material core, which can not only further improve the rigidity and safety performance of the double-layer coating layer, but also absorb hydrogen fluoride in the electrolyte, thereby synergistically protecting the silicon-based negative electrode material with the copolymer coating layer. DETAILED DESCRIPTION

[0065] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0066] Example 1

[0067] This embodiment provides a double-layer coated silicon-based composite negative electrode material, which includes a silicon-carbon material core, and a copolymer coating layer and a nano-alumina coating layer sequentially arranged on the surface of the silicon-carbon material core from the inside to the outside.

[0068] Among them, the material of the copolymer coating layer is a copolymer formed by triallyl phosphate and methacryloyloxypropyltrimethoxysilane, and the number average molecular weight of the copolymer is 45,000 Da and the crystallinity is 25%; the average particle size of the nano-alumina in the nano-alumina coating layer is 35 nm; and the mass ratio of the material of the copolymer coating layer to the material of the nano-alumina coating layer is 1:0.4, and the total thickness of the copolymer coating layer and the nano-alumina coating layer is 120 nm.

[0069] The average particle size of the silicon-carbon material core is 7 μm, and the mass percentage of silicon material in the silicon-carbon material core is 45%.

[0070] This embodiment also provides a method for preparing the double-layer coated silicon-based composite negative electrode material, the preparation method comprising the following steps:

[0071] S1. Triallyl phosphate and methacryloxypropyltrimethoxysilane in a molar ratio of 3:1 were added to tetrahydrofuran, and azobisisobutyronitrile was added as an initiator. The mixture was heated to 70° C. under an argon atmosphere for 9 hours to carry out a copolymerization reaction to obtain a reaction solution; the reaction solution was added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate was then washed and dried to obtain a copolymer;

[0072] S2. The copolymer and the silicon-carbon material were dissolved in N,N-dimethylformamide solvent to prepare a precursor solution having a solute mass concentration of 10%, and the precursor solution was electrospun to obtain a silicon-carbon material having a surface coated with the copolymer, wherein the electrospinning voltage was 18 kV and the receiving distance was 18 cm;

[0073] S3. Add the surface-coated copolymer silicon-carbon material and nano-alumina (the total mass of the silicon-carbon material core is 100%, and the mass percentage of nano-alumina is 3%) to a ball milling jar filled with ethanol organic solvent for ball milling, wherein the ball-to-material ratio is 7:1. Turn on the ball mill and ball mill at a speed of 350 rpm for 18 hours. After the ball milling is completed, transfer the mixed slurry to an evaporating dish and heat and stir at a temperature of 70°C to volatilize the organic solvent. After the organic solvent has basically evaporated, place the sample in a vacuum drying oven and dry it at 90°C for 18 hours to remove residual moisture and organic solvent to obtain the double-layer coated silicon-based composite negative electrode material.

[0074] Example 2

[0075] This embodiment provides a double-layer coated silicon-based composite negative electrode material, which includes a silicon-carbon material core, and a copolymer coating layer and a nano-alumina coating layer sequentially arranged on the surface of the silicon-carbon material core from the inside to the outside.

[0076] Among them, the material of the copolymer coating layer is a copolymer formed by triallyl phosphate and methacryloyloxypropyltrimethoxysilane, and the number average molecular weight of the copolymer is 55,000 Da and the crystallinity is 20%; the average particle size of the nano-alumina in the nano-alumina coating layer is 20 nm; and the mass ratio of the material of the copolymer coating layer to the material of the nano-alumina coating layer is 1:0.3, and the total thickness of the copolymer coating layer and the nano-alumina coating layer is 120 nm.

[0077] The average particle size of the silicon-carbon material core is 7 μm, and the mass percentage of silicon material in the silicon-carbon material core is 47%.

[0078] This embodiment also provides a method for preparing the double-layer coated silicon-based composite negative electrode material, the preparation method comprising the following steps:

[0079] S1. Triallyl phosphate and methacryloxypropyltrimethoxysilane in a molar ratio of 3:1 were added to tetrahydrofuran, and azobisisobutyronitrile was added as an initiator. The mixture was heated to 75° C. under an argon atmosphere for a copolymerization reaction for 8 hours to obtain a reaction solution. The reaction solution was added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate was washed and dried to obtain a copolymer.

[0080] S2. The copolymer and the silicon-carbon material were dissolved in N,N-dimethylformamide solvent to prepare a precursor solution having a solute mass concentration of 8%, and the precursor solution was electrospun to obtain a silicon-carbon material having a surface coated with the copolymer, wherein the electrospinning voltage was 15 kV and the receiving distance was 15 cm;

[0081] S3. Add the surface-coated copolymer silicon-carbon material and nano-alumina (the total mass of the silicon-carbon material core is 100%, and the mass percentage of nano-alumina is 2.5%) to a ball mill filled with ethanol organic solvent for ball milling, wherein the ball-to-material ratio is 7:1. Turn on the ball mill and ball mill at a speed of 350 rpm for 18 hours. After the ball milling is completed, transfer the mixed slurry to an evaporating dish and heat and stir at a temperature of 70°C to volatilize the organic solvent. After the organic solvent has basically evaporated, place the sample in a vacuum drying oven and dry it at 90°C for 18 hours to remove residual moisture and organic solvent to obtain the double-layer coated silicon-based composite negative electrode material.

[0082] Example 3

[0083] This embodiment provides a double-layer coated silicon-based composite negative electrode material, which includes a silicon-carbon material core, and a copolymer coating layer and a nano-alumina coating layer sequentially arranged on the surface of the silicon-carbon material core from the inside to the outside.

[0084] Among them, the material of the copolymer coating layer is a copolymer formed by triallyl phosphate and methacryloyloxypropyltrimethoxysilane, and the number average molecular weight of the copolymer is 32000Da and the crystallinity is 30%; the average particle size of the nano-alumina in the nano-alumina coating layer is 50nm; and the mass ratio of the material of the copolymer coating layer to the material of the nano-alumina coating layer is 1:0.5, and the total thickness of the copolymer coating layer and the nano-alumina coating layer is 120nm.

[0085] The average particle size of the silicon-carbon material core is 7 μm, and the mass percentage of silicon material in the silicon-carbon material core is 42%.

[0086] This embodiment also provides a method for preparing the double-layer coated silicon-based composite negative electrode material, the preparation method comprising the following steps:

[0087] S1. Triallyl phosphate and methacryloxypropyltrimethoxysilane in a molar ratio of 3:1 were added to tetrahydrofuran, and azobisisobutyronitrile was added as an initiator. The mixture was heated to 60° C. under an argon atmosphere for 10 hours to obtain a copolymer solution. The reaction solution was added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate was washed and dried to obtain a copolymer.

[0088] S2. The copolymer and the silicon-carbon material were dissolved in N,N-dimethylformamide solvent to prepare a precursor solution having a solute mass concentration of 12%, and the precursor solution was electrospun to obtain a silicon-carbon material having a surface coated with the copolymer, wherein the electrospinning operating voltage was 20 kV and the receiving distance was 20 cm;

[0089] S3. Add the surface-coated copolymer silicon-carbon material and nano-alumina (the total mass of the silicon-carbon material core is 100%, and the mass percentage of nano-alumina is 3.5%) to a ball mill filled with ethanol organic solvent for ball milling, wherein the ball-to-material ratio is 7:1. Turn on the ball mill and ball mill at a speed of 350 rpm for 18 hours. After the ball milling is completed, transfer the mixed slurry to an evaporating dish and heat and stir at a temperature of 70°C to volatilize the organic solvent. After the organic solvent has basically evaporated, place the sample in a vacuum drying oven and dry it at 90°C for 18 hours to remove residual moisture and organic solvent to obtain the double-layer coated silicon-based composite negative electrode material.

[0090] Example 4

[0091] The difference between this embodiment and embodiment 1 is that the molar ratio of triallyl phosphate to methacryloxypropyltrimethoxysilane in step S1 is 0.5:1, and the other steps are the same as those in embodiment 1.

[0092] Example 5

[0093] The difference between this embodiment and embodiment 1 is that the molar ratio of triallyl phosphate to methacryloxypropyltrimethoxysilane in step S1 is 10:1, and the other steps are the same as those in embodiment 1.

[0094] Example 6

[0095] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the copolymer in step S1 is 5000 Da, wherein the change in the number average molecular weight of the copolymer is achieved by adjusting the temperature and time of the copolymerization reaction. The rest is the same as embodiment 1.

[0096] Example 7

[0097] The difference between this embodiment and embodiment 1 is that the crystallinity of the copolymer in step S1 is 5%, wherein the change in the crystallinity of the copolymer is achieved by adjusting the temperature and time of the copolymerization reaction. The rest is the same as embodiment 1.

[0098] Example 8

[0099] The difference between this embodiment and embodiment 1 is that the crystallinity of the copolymer in step S1 is 50%, wherein the change in the crystallinity of the copolymer is achieved by adjusting the temperature and time of the copolymerization reaction. Other aspects are the same as those in embodiment 1.

[0100] Example 9

[0101] The difference between this embodiment and embodiment 1 is that the mass ratio of the material of the copolymer coating layer to the material of the nano-alumina coating layer is 1:0.05, which is obtained by adjusting the content of nano-alumina in the precursor solution. The rest is the same as embodiment 1.

[0102] Example 10

[0103] The difference between this embodiment and embodiment 1 is that the mass ratio of the material of the copolymer coating layer to the material of the nano-alumina coating layer is 1:5, which is obtained by adjusting the content of nano-alumina in the precursor solution. Others are the same as embodiment 1.

[0104] Example 11

[0105] The difference between this embodiment and embodiment 1 is that in step S2, the silicon-carbon material with the surface coated with the copolymer is not obtained by electrospinning, but is obtained by liquid phase coating treatment, specifically, the copolymer is dissolved in an organic solvent to prepare a copolymer solution; the silicon-carbon material is added to the above copolymer solution, and stirred at 90°C for 9 hours to obtain a mixed solution; the above mixed solution is spray-dried, wherein the inlet temperature is 150°C and the outlet temperature is 80°C, and the spray-dried product is collected to obtain the silicon-carbon material with the surface coated with the copolymer. The rest is the same as in embodiment 1.

[0106] Comparative Example 1

[0107] The difference between this comparative example and Example 1 is that the silicon-carbon material is not subjected to double-layer coating treatment, and only one silicon-carbon material is provided, and the average particle size of the provided silicon-carbon material and the mass percentage of the silicon material are the same as those in Example 1.

[0108] Comparative Example 2

[0109] The difference between this comparative example and Example 1 is that the silicon-carbon core material is not subjected to copolymer coating treatment, that is, the surface of the silicon-carbon core material is directly coated with a nano-aluminum oxide layer. The specific step of coating the nano-aluminum oxide layer is the same as step S3, and the rest is the same as Example 1.

[0110] Comparative Example 3

[0111] The difference between this comparative example and Example 1 is that the silicon-carbon core material is not subjected to nano-alumina coating treatment, that is, the copolymer layer is directly coated on the surface of the silicon-carbon core material. The specific steps of coating the copolymer layer are the same as Step S1-Step S2, and the rest are the same as Example 1.

[0112] Comparative Example 4

[0113] The difference between this comparative example and Example 1 is that the triallyl phosphate in step S1 is replaced by methacryloxypropyltrimethoxysilane in an equal molar content, and the other conditions are the same as those in Example 1.

[0114] Comparative Example 5

[0115] The difference between this comparative example and Example 1 is that the methacryloxypropyltrimethoxysilane in step S1 is replaced by triallyl phosphate in an equimolar content, and the other conditions are the same as those in Example 1.

[0116] Application Examples 1-11 and Comparative Application Examples 1-5

[0117] The silicon-based negative electrode materials provided in Examples 1 to 11 and Comparative Examples 1 to 5 were used to prepare negative electrode sheets, and then assembled to obtain lithium-ion batteries. The specific preparation method is as follows:

[0118] Preparation of negative electrode sheet:

[0119] The silicon-based negative electrode material, conductive carbon black Super-P, single-walled carbon nanotubes (SWCNTs) and polyacrylic acid binder (PAA) provided in the above embodiments and comparative examples were mixed and stirred uniformly with water in a mass ratio of 80:9:1:10 to obtain a negative electrode slurry, and the solid content was controlled to be 30%. The negative electrode slurry was then coated on a copper foil current collector through a coating process, and then vacuum dried and cold pressed to obtain a negative electrode sheet.

[0120] Preparation of positive electrode:

[0121] The ternary cathode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 O2) active material, polyvinylidene fluoride binder (PVDF) and conductive carbon black Super-P are mixed and stirred evenly with N-methylpyrrolidone solvent in a mass ratio of 95:3:2 to obtain positive electrode slurry, and then the positive electrode slurry is coated on aluminum foil through a coating process, and then dried and cold pressed to obtain a positive electrode sheet.

[0122] Electrolyte:

[0123] Ethylene carbonate, dimethyl carbonate, diethyl carbonate and fluoroethylene carbonate are mixed in a volume ratio of 20:40:30:10 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0124] Preparation of lithium-ion batteries:

[0125] The positive electrode sheet, separator (including a polyethylene-based film and a ceramic coating provided on one side of the polyethylene-based film) and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming and shaping processes, a lithium-ion battery is obtained.

[0126] Test conditions

[0127] The lithium-ion batteries provided in Application Examples 1 to 11 and Comparative Application Examples 1 to 5 were subjected to performance tests. The rate performance of the batteries was tested under lithium-ion battery test conditions. The tests were conducted on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd. at room temperature (25°C). The charge and discharge voltages were limited to 2.5V to 4.2V. The test conditions are as follows:

[0128] (1) First Coulombic efficiency

[0129] At 25°C, the lithium-ion battery was charged to 4.2V at a constant current and constant voltage rate of 0.33C and allowed to stand for 10 minutes. Then, the lithium-ion battery was discharged to 2.5V at a constant current rate of 0.33C and allowed to stand for 10 minutes. The first coulombic efficiency of the lithium-ion battery was calculated.

[0130] First coulombic efficiency (%)=(first discharge total capacity of the lithium-ion battery at a rate of 0.33C / first charge total capacity of the lithium-ion battery at a rate of 0.33C)×100%.

[0131] (2) Capacity retention after 1000 cycles at 1C / 2C at room temperature

[0132] At 25°C, the lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C, and allowed to stand for 10 minutes. Then, the lithium-ion battery is discharged to 2.5V at a constant current rate of 2C and allowed to stand for 10 minutes. This is considered one charge and discharge cycle. The lithium-ion battery is charged and discharged 1200 times according to the above method. The capacity retention rate of the lithium-ion battery after 1000 charge and discharge cycles at 1C / 2C is calculated.

[0133] Capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity at the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0134] (3) Room temperature 6C rate performance - constant current charging ratio

[0135] At 25°C, the lithium-ion battery was discharged at a constant current rate of 1C to 2.5V and allowed to stand for 10 minutes. The lithium-ion battery was then charged at a constant current and constant voltage rate of 6C to 4.2V with a cut-off current of 0.05C and allowed to stand for 10 minutes. The constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion battery were recorded. The constant current charging ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charging ratio = (constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2) × 100%.

[0136] (4) 1C / 8C discharge capacity retention rate at room temperature

[0137] At 25°C, the divided lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; it is left to stand for 10 minutes; then the lithium-ion battery is discharged to 2.5V at a constant current rate of 1C, and its discharge capacity Q is recorded. 1CAs the initial discharge capacity; then at 25 ℃, the lithium ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; let it stand for 10 minutes; then the fully charged battery is discharged to 2.5V at a constant current rate of 8C, and its discharge capacity Q is recorded. 8C ; Calculate the discharge capacity retention rate (%) of lithium-ion batteries at 1C / 8C rate = (discharge capacity Q at 8C rate) 8C / Discharge capacity Q at 1C rate 1C )×100%.

[0138] (5) Initial expansion rate of negative electrode

[0139] ① Before assembling the lithium-ion battery, use a micrometer to measure the initial thickness of the negative electrode sheet and record it as h1. The thickness of the negative electrode current collector is recorded as h0.

[0140] ② Fully charged: At 25°C, after assembling the battery cells, charge the lithium-ion battery at a constant current and constant voltage rate of 0.33C to 4.2V, with a cut-off current of 0.05C, and let it stand for 120 minutes;

[0141] ③ Disassemble the fully charged lithium-ion battery to obtain the negative electrode sheet, clean the negative electrode sheet with dimethyl carbonate, and measure the thickness of the cleaned negative electrode sheet, which is recorded as h2;

[0142] ④The initial expansion rate of the negative electrode is: (h2-h1) / (h1-h0)×100%.

[0143] (6) Battery cell thermal runaway ARC test: Start the ARC adiabatic thermal runaway test (the test sample is heated from room temperature to 45±2℃ in the chamber, and after being placed for 90 minutes, the temperature rise rate of the lithium-ion battery is detected). If the temperature rise exceeds 0.2℃ within 10 minutes (i.e., the self-heating temperature rise rate, referred to as SHR, SHR>0.02℃ / min), it is considered that a self-exothermic reaction occurs inside the lithium-ion battery, and the adiabatic environment is maintained until the lithium-ion battery thermal runaway occurs; if the temperature rise does not exceed 0.2℃ within 10 minutes (i.e., SHR≤0.02℃ / min), continue to the next step temperature rise test; each temperature step is 5℃, and the steps are repeated on each temperature step. The ARC test temperature range is 45℃~300℃, the self-heating starting temperature is T1 (temperature rise rate SHR>0.02℃ / min), and the thermal runaway starting temperature is T2 (temperature rise rate SHR>1℃ / min).

[0144] The test results are shown in Table 1:

[0145] Table 1

[0146]

[0147]

[0148]

[0149] As can be seen from Table 1, compared with Comparative Application Example 1, Application Examples 1 to Application Examples 3 of the present invention can optimize the interface structure between the electrolyte and the negative electrode material, reduce the hydrogen fluoride content in the electrolyte and the occurrence of side reactions, and at the same time, enable the coating layer to have suitable mechanical strength, thereby effectively suppressing the volume expansion of the silicon-carbon material and improving the electrochemical performance and safety performance of the lithium-ion battery.

[0150] Comparing Application Examples 1, 4, and 5 shows that the molar ratio of triallyl phosphate to methacryloxypropyltrimethoxysilane significantly impacts the stability and thermal safety of the copolymer coating. A low content of phosphate groups in the copolymer structure can impair the safety of lithium-ion batteries. A low content of siloxane groups can prevent the copolymer from fully absorbing HF from the electrolyte, resulting in poor cycling and fast-charging performance.

[0151] By comparing Application Examples 1, 6, 7 and 8, it can be seen that the present invention further regulates the relevant physical parameters of the copolymer (such as number average molecular weight and crystallinity) so that the formed copolymer coating layer has both good mechanical properties and structural stability, thereby better inhibiting the volume expansion of silicon particles and improving the overall performance of lithium-ion batteries.

[0152] Comparing Application Example 1, Application Example 9 and Application Example 10, it can be seen that the mass ratio of the material of the copolymer coating layer to the material of the nano-alumina coating layer has an important influence on the structural stability and mechanical properties of the silicon-based composite negative electrode material, while ensuring that the lithium-ion battery has good thermal safety performance.

[0153] Comparing Application Example 1 with Application Example 11, it can be seen that the present invention improves the coating effect by optimizing the coating method, for example, the uniformity of the coating layer thickness is improved.

[0154] Comparing Application Examples 1, 2 and 3, it can be seen that if only one type of coating layer is used, it is impossible to take into account the comprehensive improvement of the structural stability, electrochemical properties and safety performance of the silicon-based composite negative electrode material, which further proves that there is a synergistic effect between the copolymer coating layer and the nano-alumina coating layer.

[0155] From Comparative Application Examples 1, 4 and 5, it can be seen that if the material of the copolymer coating layer is only a homopolymer prepared using a certain monomer, the structural stability of the silicon-based composite negative electrode material cannot be guaranteed, and the thermal safety performance of the lithium-ion battery deteriorates. This further proves that there is a synergistic effect between the phosphate functional group and the siloxane group in the copolymer structure.

[0156] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A double-layer coated silicon-based composite negative electrode material, characterized in that: The double-layer coated silicon-based composite negative electrode material comprises a silicon-based material core, and a copolymer coating layer and an oxide coating layer sequentially arranged on the surface of the silicon-based material core from the inside to the outside; The material of the copolymer coating layer includes a copolymer formed by an unsaturated phosphate monomer and an unsaturated siloxane monomer containing an ester group.

2. The double-layer coated silicon-based composite negative electrode material according to claim 1, characterized in that: The unsaturated phosphate monomer includes a phosphate monomer containing at least one alkenyl group; Preferably, the phosphate monomer containing at least one alkenyl group comprises triallyl phosphate; Preferably, the unsaturated siloxane monomer containing an ester group includes a siloxane monomer containing at least one alkenyl group and at least one ester group; Preferably, the siloxane monomer containing at least one alkenyl group and at least one ester group includes any one of methacryloxypropyltrimethoxysilane, methacryloxypropyltriethoxysilane or 3-[tris(1-methylethoxy)silyl]propyl methacrylate or a combination of at least two thereof.

3. The double-layer coated silicon-based composite negative electrode material according to claim 1 or 2, characterized in that: The number average molecular weight of the copolymer is 10,000 Da to 80,000 Da, preferably 30,000 Da to 60,000 Da; Preferably, the crystallinity of the copolymer is 10% to 40%, preferably 20% to 30%.

4. The double-layer coated silicon-based composite negative electrode material according to any one of claims 1 to 3, characterized in that: The material of the oxide coating layer includes aluminum oxide; Preferably, the average particle size of the aluminum oxide is 10 nm to 100 nm, preferably 20 nm to 50 nm.

5. The double-layer coated silicon-based composite negative electrode material according to any one of claims 1 to 4, characterized in that: The mass ratio of the material of the copolymer coating layer to the material of the oxide coating layer is 1:(0.1-1), preferably 1:(0.3-0.5); Preferably, the total thickness of the copolymer coating layer and the oxide coating layer is 40 nm to 200 nm.

6. A method for preparing a double-layer coated silicon-based composite negative electrode material according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: S1. Mixing an unsaturated phosphate monomer, an unsaturated siloxane monomer containing an ester group, an initiator and a solvent to obtain a copolymer by copolymerization; S2. preparing a precursor solution containing the copolymer and a silicon-based material, and electrospinning the precursor solution to obtain a silicon-based material having a surface-coated copolymer; S3. Performing oxide coating treatment on the silicon-based material with the copolymer coated on the surface to obtain the double-layer coated silicon-based composite negative electrode material.

7. The method according to claim 6, characterized in that In step S1, the molar ratio of the unsaturated phosphate monomer to the unsaturated siloxane monomer containing an ester group is (1-5):1, preferably 3:1; Preferably, the temperature of the copolymerization reaction in step S1 is 50° C. to 80° C., and the time of the copolymerization reaction is 6 h to 12 h.

8. The method according to claim 6 or 7, characterized in that The mass concentration of the solute in the precursor solution in step S2 is 5% to 15%, preferably 8% to 12%; Preferably, the operating voltage of the electrospinning in step S2 is 10 kV to 30 kV, preferably 15 kV to 20 kV; Preferably, the receiving distance of the electrospinning in step S2 is 10 cm to 30 cm, preferably 15 cm to 20 cm.

9. The method according to any one of claims 6 to 8, characterized in that The oxide coating process in step S3 includes ball milling the surface-coated copolymer silicon-based material and the oxide to obtain the double-layer coated silicon-based composite negative electrode material; Preferably, the mass percentage of the oxide is 1% to 5% based on the total mass of the uncoated silicon-based material as 100%; Preferably, the ball-to-material ratio of the ball milling process is (5-10):1; Preferably, the rotation speed of the ball milling treatment is 200 rpm to 500 rpm, and the time of the ball milling treatment is 10 h to 24 h.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes a double-layer coated silicon-based composite negative electrode material prepared by the preparation method according to any one of claims 1-5 or any one of claims 6-9.

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