Negative electrode material and preparation method and application thereof

Abstract

In order to improve the electrochemical performance of a negative electrode material and relieve silicon-based volume expansion, the invention provides a negative electrode material and a preparation method and application thereof.The negative electrode material comprises silicon carbon particles and an organic coating layer, the organic coating layer is arranged on the surfaces of the silicon carbon particles, and the organic coating layer is of a porous structure; in the direction away from the surfaces of the silicon carbon particles, the density of functional groups in the organic coating layer is increased in a gradient mode on the whole, and the aperture of the porous structure is decreased in a gradient mode on the whole.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of electronic equipment and electric vehicles, higher requirements are put forward for the energy density and cycle performance of lithium ion batteries. Silicon-carbon composite material has become a potential negative electrode material due to its high theoretical specific capacity. Silicon-carbon material prepared by chemical vapor deposition (CVD silicon-carbon) combines the high capacity of silicon and the conductivity and stability of carbon. However, during the charging and discharging process, the volume expansion of silicon is as high as 300%, which leads to the pulverization of the material structure, the instability of the electrode and electrolyte interface, the rapid decay of the battery capacity, the shortening of the cycle life, and greatly limits its practical application.

[0003] At present, it is a common method to improve the electrochemical performance of silicon-carbon material by coating modification. However, there are some problems in traditional coating materials and methods, such as the single structure of ordinary polymers, the limited optimization of ion and electron transmission, and the difficulty of effectively buffering the volume change of silicon, which leads to the reduction of cycle stability.

[0004] Therefore, there is an urgent need for a negative electrode material to solve the technical problems of volume expansion, poor cycle stability and low ion transmission efficiency of silicon-carbon negative electrode material. SUMMARY

[0005] The purpose of the present application is to provide a negative electrode material and a preparation method and application thereof to solve the above technical problems.

[0006] The technical solution adopted by the present application to solve the above technical problems is as follows: In a first aspect of the present application, a negative electrode material is provided, comprising silicon-carbon particles and an organic coating layer, the organic coating layer being on the surface of the silicon-carbon particles, the organic coating layer having a porous structure; In the direction away from the surface of the silicon-carbon particles, the density of functional groups in the organic coating layer as a whole increases in gradient, the pore size of the porous structure as a whole decreases in gradient, and the organic coating layer comprises at least two coating layers.

[0007] Optionally, the organic coating layer comprises a first coating layer and a second coating layer, the first coating layer comprises a first covalent organic framework, and the second coating layer comprises a second covalent organic framework, the second covalent organic framework being coated on the surface of the first covalent organic framework. The pore size of the first covalent organic framework is larger than that of the second covalent organic framework, and the density of functional groups in the first covalent organic framework is smaller than that of the second covalent organic framework.

[0008] Optionally, the first covalent organic framework has a pore size ranging from 5 to 20 nm, and the second covalent organic framework has a pore size ranging from 1 to 5 nm.

[0009] Optionally, the first covalent organic framework has a functional group density ranging from 5 to 10 per nm 2 , and the second covalent organic framework has a functional group density ranging from 15 to 25 per nm 2 .

[0010] Optionally, the silicon-carbon particles have a particle size ranging from 4 to 20 µm.

[0011] Optionally, the silicon-carbon particles have a silicon mass fraction of 30 to 70% and a carbon mass fraction of 30 to 70%.

[0012] Optionally, the organic coating layer has a thickness of 50 to 500 nm.

[0013] In a second aspect, the present application provides a preparation method of a negative electrode material, comprising the following steps: dissolving an organic monomer A, an organic monomer B, and a catalyst in an organic solvent to prepare a pre-assembly solution; adding silicon-carbon particles to the pre-assembly solution to perform a first polymerization reaction at a first temperature, to obtain a first reaction liquid, and a first coating layer is formed on the surface of the silicon-carbon particles in the first reaction liquid; adding an organic monomer C, an organic monomer D, and a promoter to the first reaction liquid to perform a second polymerization reaction at a second temperature, to form a second coating layer outside the first coating layer, and obtain a second reaction liquid containing a crude product; sequentially performing post-treatment and vacuum drying on the crude product to obtain a negative electrode material; the first temperature is less than the second temperature; the organic monomer A comprises at least one of a phenyl-containing aldehyde compound and a phenyl-containing acid anhydride compound; the organic monomer B comprises an amine compound; the organic monomer C comprises at least one of a carboxylic acid compound, an aldehyde compound, a boric acid compound, and a phenol compound; the organic monomer D comprises at least one of an amine compound and a phenol compound.

[0014] Optionally, the first temperature is 40 to 60°C, and the first polymerization reaction has a reaction time of 1 to 3 h; the second temperature is 60 to 80°C, and the second polymerization reaction has a reaction time of 2 to 5 h.

[0015] Optionally, the organic monomer A comprises at least one of triformylphenyl, pyromellitic dianhydride, terephthaldehyde, 2,5-dimethoxyterephthaldehyde; The organic monomer B comprises at least one of p-phenylenediamine, tartaric acid dihydrazide, 1,3,5-tris (4-aminophenyl) benzene; And / or, the organic monomer C comprises at least one of 2,5-dihydroxyterephthalic acid, 2,5-dihydroxyterephthaldehyde, 2-amino terephthalic acid, 2,5-dicyanoterephthaldehyde, tetra (4-formylphenyl) methane, 1,4-benzenediboronic acid, 2,3,6,7,10,11-hexahydroxytriphenylene; The organic monomer D comprises at least one of ethylenediamine, hexamethylenediamine, 1,3,5-tris (4-aminophenyl) triazine, 4,4', 4''-triaminotriphenylamine, benzidine-3,3', 5,5'-tetramine, hydroquinone, 1,3,5-trihydroxybenzene.

[0016] Optionally, the molar ratio of the organic monomer A to the organic monomer B is 1:1-1:2; And / or, the molar ratio of the organic monomer C to the organic monomer D is 1:1-1:1.5.

[0017] Optionally, the catalyst comprises at least one of acetic acid, trifluoroacetic acid, p-toluenesulfonic acid, scandium triflate, tris (pentafluorophenyl) borane; The promoter comprises at least one of piperidine, triethylamine, 1,8-diazabicycloundec-7-ene, 1,4-diazabicyclo [2.2.2] octane, zinc acetate, scandium triflate, tetrabutylammonium bromide.

[0018] Optionally, the amount of the catalyst is 5-15% of the total molar amount of the organic monomer A and the organic monomer B; And / or, the amount of the promoter is 3-8% of the total molar amount of the organic monomer C and the organic monomer D.

[0019] In a third aspect of the present application, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer is arranged on at least one side of the negative electrode current collector, and the negative electrode active material layer comprises the negative electrode material described above or is prepared by the preparation method described above.

[0020] In a fourth aspect of the present application, a battery is provided, comprising the negative electrode sheet described above.

[0021] The organic coating layer of the anode material of the present invention has a gradient porous structure. The pore size is larger near the silicon-carbon particles, which has good flexibility and deformability, and can provide sufficient space for the volume expansion of silicon, thereby effectively relieving the stress generated by the volume expansion of silicon. In the direction away from the surface of silicon-carbon particles, the pore size is smaller and the density of functional groups is larger. This structure can limit the excessive expansion of silicon, thereby maintaining the overall structural stability of the anode material, reducing the pulverization and shedding of the anode material, and thus improving the cycle performance of the battery. The gradient porous structure of the organic coating layer is conducive to the rapid diffusion and transport of lithium ions in the organic coating layer, reducing ion transport resistance. The abundant functional groups with a gradient increasing density can not only provide more active sites, but also enhance the interaction between the negative electrode material and lithium ions, improve the internal conduction of electrons in the negative electrode material, and improve the charge and discharge efficiency of the material. The organic coating layer and silicon-carbon particles have good interfacial bonding force, and the gradient structure can make the stress distribution at the interface more uniform, reduce interface cracking and electrolyte penetration, thereby forming a stable electrode-electrolyte interface, reducing irreversible capacity loss in the battery, and improving battery cycle life and safety. The method for preparing the negative electrode material of the present invention constructs a gradient structure organic coating layer through stepwise assembly. The preparation process is simple, the reaction conditions are mild and easy to control, it is suitable for large-scale production, and the gradient structure of the organic coating layer can be precisely controlled to meet different application requirements. Detailed Implementation

[0022] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] The present invention provides a negative electrode material comprising silicon carbon particles and an organic coating layer, wherein the organic coating layer is on the surface of the silicon carbon particles and the organic coating layer has a porous structure. Along the direction away from the surface of the silicon-carbon particles, the density of functional groups in the organic coating layer generally increases in a gradient, and the pore size of the porous structure generally decreases in a gradient. The organic coating layer contains at least two coating layers.

[0024] Specifically, the gradient porous structure of the organic coating layer facilitates the rapid diffusion and transport of lithium ions within the organic coating layer, reducing ion transport resistance. The abundant functional groups with a gradient increasing density not only provide more active sites, enhancing the interaction between the negative electrode material and lithium ions, but also improve electron conduction within the negative electrode material, thereby increasing the charge and discharge efficiency of the material. Furthermore, the organic coating layer exhibits good interfacial bonding with silicon-carbon particles, and the gradient structure enables a more uniform stress distribution at the interface, reducing interfacial rupture and electrolyte penetration, thus forming a stable electrode-electrolyte interface. This reduces irreversible capacity loss in the battery, improving battery cycle life and safety.

[0025] In some embodiments, the organic coating layer includes a first coating layer and a second coating layer, the first coating layer includes a first covalent organic framework, the second coating layer includes a second covalent organic framework, and the second covalent organic framework coats the surface of the first covalent organic framework; The pore size of the first covalent organic framework is larger than that of the second covalent organic framework, and the functional group density in the first covalent organic framework is smaller than that in the second covalent organic framework.

[0026] Specifically, the organic coating layer of the anode material has a gradient porous structure. The portion near the silicon-carbon particles, i.e., the first covalent organic framework, has a larger pore size, exhibiting good flexibility and deformability. This provides sufficient space for the volume expansion of silicon, effectively alleviating the stress generated by silicon volume expansion. In the direction away from the surface of the silicon-carbon particles, the second covalent organic framework has a smaller pore size and a higher density of functional groups. This structure can limit the excessive expansion of silicon, thereby maintaining the overall structural stability of the anode material, reducing the pulverization and shedding of the anode material, and thus improving the cycle performance of the battery. Furthermore, the higher density of functional groups can enhance the initial discharge specific capacity of the battery.

[0027] In some embodiments, the pore size of the first covalent organic framework is in the range of 5 to 20 nm, and the pore size of the second covalent organic framework is in the range of 1 to 5 nm.

[0028] Specifically, the pore size of the first covalent organic framework includes, but is not limited to, 5nm, 7nm, 9nm, 10nm, 12nm, 15nm, 16nm, 18nm, and 20nm. The larger pore size of the first covalent organic framework provides good flexibility and deformability. The pore size of the second covalent organic framework includes, but is not limited to, 1nm, 2nm, 3nm, 4nm, and 5nm. The smaller pore size of the second covalent organic framework provides good support.

[0029] In some embodiments, the functional group density of the first covalent organic framework is 5-10 groups / nm. 2The functional group density of the second covalent organic framework is 15~25 per nm. 2 .

[0030] Specifically, the functional group density of the first covalent organic framework includes, but is not limited to, 5 groups / nm. 2 6 7 / nm 2 8 per nm 2 9 / nm 2 10 per nm 2 The functional group density of the second covalent organic framework includes, but is not limited to, 15 groups / nm. 2 16 per nm 2 17 per nm 2 18 per nm 2 19 per nm 2 20 units / nm 2 21 per nm 2 22 per nm 2 23 per nm 2 24 per nm 2 25 units / nm 2 The second covalent organic framework has a high functional group density, which can limit the excessive expansion of silicon and thus maintain the overall structural stability of the anode material.

[0031] In some embodiments, the particle size of the silicon-carbon particles ranges from 4 to 20 µm.

[0032] Specifically, the particle size of silicon-carbon particles includes, but is not limited to, 4µm, 5µm, 8µm, 11µm, 13µm, 16µm, 17µm, 19µm, and 20µm. Within this range, the particle size of silicon-carbon particles can better balance volume expansion and electrical conductivity.

[0033] In some embodiments, the silicon-carbon particles contain 30-70% silicon by mass and 30-70% carbon by mass.

[0034] Specifically, the mass percentage of silicon in silicon-carbon particles includes, but is not limited to, 30%, 36%, 43%, 49%, 54%, 58%, 61%, 67%, and 70%; the mass percentage of carbon includes, but is not limited to, 30%, 36%, 43%, 49%, 54%, 58%, 61%, 67%, and 70%. A high silicon mass percentage leads to a significantly increased overall expansion rate of the silicon-carbon particles, easily causing electrode cracking and a sharp decrease in cycle life. It also reduces the overall conductivity of the negative electrode material and increases charge transport resistance. Conversely, a low silicon mass percentage can extend cycle life, but due to the reduced silicon content, it cannot meet the requirements of high-energy-density batteries. A low carbon mass percentage prevents the carbon matrix from completely encapsulating the silicon particles, failing to effectively suppress silicon volume expansion, reducing conductivity, and deteriorating cycle stability. Conversely, an excessively high carbon mass percentage affects the silicon content and thus the battery's energy density.

[0035] In some embodiments, the thickness of the organic coating layer is 50~500 nm.

[0036] Specifically, the thickness of the organic coating layer includes, but is not limited to, 50nm, 85nm, 130nm, 175nm, 220nm, 280nm, 315nm, 390nm, 455nm, and 500nm. If the coating layer is too thin, the space to accommodate the expansion of silicon volume becomes smaller, making it prone to cracks, voids, or discontinuous regions, leading to structural collapse; if the coating layer is too thick, it will increase charge transport resistance and weaken electrocatalytic performance.

[0037] This invention provides a method for preparing a negative electrode material, comprising the following steps: Organic monomer A, organic monomer B, and catalyst are dissolved in an organic solvent to prepare a pre-assembled solution; Silicon carbon particles are added to the pre-assembled solution, and a first polymerization reaction is carried out at a first temperature to obtain a first reaction solution, wherein a first coating layer is formed on the surface of the silicon carbon particles in the first reaction solution; Organic monomer C, organic monomer D and accelerator are added to the first reaction solution, and a second polymerization reaction is carried out at a second temperature. A second coating layer is formed outside the first coating layer, and a second reaction solution containing crude product is obtained. The crude product was sequentially post-processed and vacuum dried to obtain the negative electrode material. The first temperature is lower than the second temperature; The organic monomer A includes at least one of phenyl-containing aldehyde compounds and phenyl-containing acid anhydride compounds; The organic monomer B includes amine compounds; The organic monomer C includes at least one of carboxylic acid compounds, aldehyde compounds, boric acid compounds, and phenolic compounds; The organic monomer D includes at least one of amine compounds and phenolic compounds.

[0038] Specifically, organic monomer A and organic monomer B are monomers with large steric hindrance and long chain segments, while organic monomer C and organic monomer D are monomers with small steric hindrance and high-density functional groups.

[0039] Specifically, the formation of gradient structures is mainly related to the structure of organic monomers, reaction conditions, and assembly methods. The structure of organic monomers is primarily related to steric hindrance and the number of functional groups. Steric hindrance manifests as the length of chain segments and side chains, and the number of benzene rings. Larger steric hindrance, longer chain segments, more benzene rings, and longer chain segments and side chains result in a more extended structure, leading to a larger pore size in the first coating layer. Conversely, smaller steric hindrance, higher-density functional groups, shorter chain segments (such as short-chain aromatic compounds), and higher functional group density result in a more compact structure, leading to a smaller pore size in the second coating layer. Reaction conditions mainly reflect… At the reaction temperatures, the first temperature is lower, resulting in a slower reaction rate and thus a first coating layer with lower crystallinity and larger pore size. The second temperature is higher, resulting in a faster reaction rate, more complete diffusion of organic monomers, and more complete polymerization, leading to a second coating layer with higher crystallinity and smaller pore size. The assembly mechanism involves a synergistic effect of self-polymerization and cross-linking reactions. The first coating layer formed by the first polymerization reaction may have skeletal gaps or incompletely reacted sites. The polymerization products of the second polymerization reaction can fill these gaps or combine with the first coating layer through cross-linking reactions, increasing the number of functional groups (such as amino, aldehyde, and hydroxyl groups) per unit volume. Constructing gradient-structured organic coating layers through stepwise assembly is a simple process with mild and easily controllable reaction conditions, suitable for large-scale production. The gradient structure of the organic coating layer can be precisely controlled to meet different application requirements. In some embodiments, the first temperature is 40~60°C, and the reaction time of the first polymerization reaction is 1~3h; The second temperature is 60~80℃, and the reaction time of the second polymerization reaction is 2~5h.

[0040] Specifically, the first temperature is lower and the time is shorter, resulting in a slower reaction rate and thus a first coating layer with lower crystallinity and larger pore size. The second temperature is higher and the time is longer, resulting in a faster reaction rate, more complete diffusion of organic monomers, and more complete polymerization, resulting in a second coating layer with higher crystallinity and smaller pore size.

[0041] In some embodiments, the organic monomer A includes at least one of pyromellitic dianhydride, terephthalaldehyde, and 2,5-dimethoxyterephthalaldehyde; The organic monomer B includes at least one of p-phenylenediamine, dihydrazide tartrate, and 1,3,5-tris(4-aminophenyl)benzene; Specifically, organic monomers A and B are monomers with large steric hindrance and long chain segments. In particular, organic monomers A and B have a large number of benzene rings, longer chain segments and side chains, and a more extended structure, which leads to a larger pore size in the first coating layer formed later.

[0042] In some embodiments, the organic monomer C includes at least one of 2,5-dihydroxyterephthalic acid, 2,5-dihydroxyterephthalaldehyde, 2-aminoterephthalic acid, 2,5-dicyanoterephthalaldehyde, tetra(4-formylphenyl)methane, 1,4-phenylboronic acid, and 2,3,6,7,10,11-hexahydroxytriphenylene. The organic monomer D includes at least one of ethylenediamine, hexamethylenediamine, 1,3,5-tris(4-aminophenyl)triazine, 4,4',4''-triaminotriphenylamine, benzidine-3,3',5,5'-tetramine, hydroquinone, and 1,3,5-trihydroxybenzene.

[0043] Specifically, organic monomers C and D are monomers with low steric hindrance and high-density functional groups. In particular, organic monomers C and D have shorter chain segments and higher functional group density, forming a compact structure, which leads to a smaller pore size in the second coating layer.

[0044] In some embodiments, the molar ratio of organic monomer A to organic monomer B is 1:1 to 1:2; Specifically, the molar ratio of organic monomer A to organic monomer B includes, but is not limited to, 1:1, 1:1.2, 1:1.5, 1:1.8, and 1:2, which promotes the complete polycondensation reaction of organic monomer A and organic monomer B.

[0045] In some embodiments, the molar ratio of organic monomer C to organic monomer D is 1:1 to 1:1.5.

[0046] Specifically, the molar ratio of organic monomer C to organic monomer D includes, but is not limited to, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, and 1:1.5, which promotes the complete condensation reaction between organic monomers.

[0047] In some embodiments, the catalyst comprises at least one selected from acetic acid, trifluoroacetic acid, p-toluenesulfonic acid, scandium trifluoromethanesulfonate, and tris(pentafluorophenyl)borane; The accelerator includes at least one of piperidine, triethylamine, 1,8-diazabicycloundec-7-ene, 1,4-diazabicyclo[2.2.2]octane, zinc acetate, scandium trifluoromethanesulfonate, and tetrabutylammonium bromide.

[0048] Specifically, the catalyst helps control the reaction rate between organic monomer A and organic monomer B, thereby regulating the pore size of the first coating layer; the promoter can increase the reaction rate, which is beneficial for the formation of a smaller pore size in the second coating layer.

[0049] In some embodiments, the amount of catalyst used is 5 to 15% of the total molar amount of organic monomer A and organic monomer B.

[0050] Specifically, the amount of catalyst used helps to control the reaction rate between organic monomer A and organic monomer B, thereby regulating the pore size range of the first coating layer.

[0051] In some embodiments, the amount of the promoter is 3 to 8% of the total molar amount of organic monomer C and organic monomer D.

[0052] Specifically, the amount of accelerator can control the rate of the second polymerization reaction, which is beneficial for the formation of a smaller pore size range in the second coating layer.

[0053] As those skilled in the art will know, the reactions in the above steps are conventional polycondensation reactions, and the specific methods and reaction conditions are commonly used in the prior art, and will not be described in detail in this invention.

[0054] The present invention provides a battery comprising a negative electrode as described above.

[0055] The present invention will be further illustrated by the following examples.

[0056] Example 1 This embodiment illustrates a negative electrode material disclosed in this invention, its preparation method, and its application.

[0057] 1) Preparation of silicon-carbon particles: Porous carbon material was placed in a sintering furnace, and silane gas was introduced into the porous carbon material in an inert atmosphere at 500°C for 10 hours to deposit silicon material. After deposition, the silane gas was switched to acetylene gas, and the temperature was increased to 600°C to coat the surface of the porous carbon material with amorphous carbon, resulting in silicon-carbon particles of 8µm in size, with silicon accounting for 40% by mass and carbon accounting for 60% by mass.

[0058] 2). Preparation of gradient structure organic coating layer: Preparation of pre-assembled solution: Trimethylbenzaldehyde (organic monomer A) and p-phenylenediamine (organic monomer B) are mixed in a molar ratio of 1:1.2, and trifluoroacetic acid, accounting for 10% of the total molar amount of organic monomers, is added as a catalyst and dissolved in N,N-dimethylformamide to prepare a pre-assembled solution.

[0059] First coating layer (i.e., first covalent organic framework, hereinafter the same): Silicon carbon particles are added to the pre-assembly solution and stirred at 50℃ (first temperature) for 2 hours to allow organic monomers to be initially assembled on the surface of silicon carbon particles.

[0060] The second coating layer (i.e., the second covalent organic framework, the same below): 2,5-dihydroxyterephthalic acid (organic monomer C) and 1,3,5-tris(4-aminophenyl)triazine (organic monomer D) are added to the above reaction system in a molar ratio of 1:1.2. Then, piperidine, accounting for 5% of the total molar amount of organic monomers C and D, is added as a promoter, and the reaction is continued at 70°C (second temperature) for 3 hours to form an organic coating layer with a gradient structure.

[0061] 3) Post-processing: The coated product was centrifuged, washed three times with N,N-dimethylformamide, and then vacuum dried at 70°C for 18 hours to obtain a gradient-structured covalent organic framework compound-coated CVD silicon-carbon material (i.e., anode material). The organic coating thickness was measured to be 200 nm, with a pore size of 10 nm in the first coating layer and 3 nm in the second coating layer; the functional group density of the first coating layer was 7 groups / nm. 2 The functional group density of the second coating layer is 20 per nm. 2 .

[0062] 4) Negative electrode sheet: The negative electrode main material, carbon black, lithium carboxymethyl cellulose (CMC), and polystyrene butadiene (SBR) are mixed evenly in a ratio of 80:10:5:5 and evenly coated on a 10µm copper foil. The mixture is then dried in a vacuum oven at 80℃ for 12 hours to obtain the negative electrode sheet. 5) Positive electrode sheet: Lithium cobalt oxide, carbon black and polyvinylidene fluoride are mixed evenly in a ratio of 80:10:10 and evenly coated on 12µm aluminum foil. The mixture is then dried in a vacuum oven at 80℃ for 12h to obtain the positive electrode sheet. 6) Electrolyte: The electrolyte is 1 mol / L lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC) / diethyl carbonate (DEC) solvent (volume ratio: v / v=1:1). 7) Battery: The negative electrode, separator, and positive electrode are stacked in sequence to form a battery cell. The battery cell is placed in an aluminum-plastic film, injected with electrolyte, and sealed. After processes such as electrolyte injection, formation, and degassing, a lithium-ion battery is obtained.

[0063] Example 2 This embodiment illustrates a negative electrode material, its preparation method, and its application disclosed in this invention. Most of the operational steps are the same as in Example 1, except that: 1) Preparation of silicon-carbon particles: Porous carbon material was placed in a sintering furnace, and silane gas was introduced into the porous carbon material in an inert atmosphere at 600°C. Silicon material was deposited in the porous carbon material for 13 hours. After deposition, the silane gas was switched to acetylene gas, and the temperature was increased to 700°C. Amorphous carbon was coated on the surface of the porous carbon material to obtain silicon-carbon particles with a mass ratio of 9µm, in which the mass ratio of silicon was 50% and the mass ratio of carbon was 50%.

[0064] 2). Preparation of gradient structure organic coating layer: Preparation of pre-assembled solution: Pyromellitic dianhydride (organic monomer A) and 1,3,5-tris(4-aminophenyl)benzene (organic monomer B) are mixed in a molar ratio of 1:1.5. p-Toluenesulfonic acid, accounting for 12% of the total molar amount of organic monomers, is added as a catalyst and dissolved in dimethyl sulfoxide to prepare a pre-assembled solution.

[0065] First coating layer: Silicon carbon particles are added to the pre-assembly solution and stirred at 45℃ (first temperature) for 2.5h to allow organic monomers to be initially assembled on the surface of silicon carbon particles.

[0066] Second coating layer: 2-aminoterephthalic acid (organic monomer C) and hexamethylenediamine (organic monomer D) are added to the above reaction system in a molar ratio of 1:1.3. Triethylamine, accounting for 6% of the total molar amount of organic monomers C and D, is then added as a promoter. The reaction is continued at 75°C (second temperature) for 4 hours to form an organic coating layer with a gradient structure.

[0067] 3) Post-processing: The coated product was centrifuged, washed three times with dimethyl sulfoxide, and then vacuum dried at 75°C for 20 h to obtain a gradient-structured covalent organic framework compound-coated CVD silicon-carbon material (i.e., anode material). The organic coating thickness was measured to be 300 nm, with a pore size of 12 nm in the first coating layer and 2 nm in the second coating layer; the functional group density of the first coating layer was 6 groups / nm. 2 The functional group density of the second coating layer is 22 per nm. 2 .

[0068] Example 3 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that: organic monomer C is 2,5-dicyano-terephthalaldehyde, and organic monomer D is benzidine-3,3',5,5'-tetraamine.

[0069] Example 4 This embodiment illustrates a negative electrode material disclosed in this invention, its preparation method, and its application. Most of the operation steps are the same as in Example 1, except that the reaction temperature for preparing the first coating layer is 40°C and the time is 3 hours; the reaction temperature for preparing the second coating layer is 60°C and the time is 5 hours.

[0070] Example 5 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Embodiment 1, except that: the reaction temperature for preparing the first coating layer is 60°C and the time is 1 hour; the reaction temperature for preparing the second coating layer is 80°C and the time is 2 hours.

[0071] Example 6 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that: the reaction temperature for preparing the first coating layer is 30°C and the time is 4 hours; the reaction temperature for preparing the second coating layer is 50°C and the time is 6 hours.

[0072] Example 7 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Embodiment 1, except that: the reaction temperature for preparing the first coating layer is 70°C and the time is 0.5h; the reaction temperature for preparing the second coating layer is 90°C and the time is 1h.

[0073] Example 8 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer A and organic monomer B is 1:1.

[0074] Example 9 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer A to organic monomer B is 1:2.

[0075] Example 10 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer A and organic monomer B is 1:0.5.

[0076] Example 11 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer A to organic monomer B is 1:3.

[0077] Example 12 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer C to organic monomer D is 1:1.

[0078] Example 13 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that the molar ratio of organic monomer C to organic monomer D is 1:1.5.

[0079] Example 14 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer C and organic monomer D is 1:0.5.

[0080] Example 15 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the molar ratio of organic monomer C to organic monomer D is 1:2.

[0081] Example 16 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the amount of catalyst is 5% and the amount of promoter is 3%.

[0082] Example 17 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the amount of catalyst is 15% and the amount of promoter is 8%.

[0083] Example 18 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the amount of catalyst is 2% and the amount of promoter is 1%.

[0084] Example 19 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Example 1, except that the amount of catalyst is 18% and the amount of promoter is 10%.

[0085] Example 20 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as in Embodiment 1, except that when preparing silicon-carbon particles, the mass ratio of silicon is 30% and the mass ratio of carbon is 70%.

[0086] Example 21 This embodiment is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Embodiment 1, except that when preparing silicon-carbon particles, the mass ratio of silicon is 70% and the mass ratio of carbon is 30%.

[0087] Comparative Example 1 This comparative example is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that the negative electrode material is silicon-carbon particles without a coating layer.

[0088] Comparative Example 2 This comparative example is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that the silicon carbon particles do not contain a second coating layer.

[0089] Comparative Example 3 This comparative example is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that the silicon carbon particles do not contain a first coating layer.

[0090] Comparative Example 4 This comparative example is used to illustrate a negative electrode material disclosed in this invention, its preparation method and application. Most of the operation steps are the same as those in Example 1, except that the outer surface of the silicon carbon particles is a second coating layer, and the first coating layer is disposed on the surface of the second coating layer.

[0091] Silicon carbon particles were added to 2,5-dihydroxyterephthalic acid (organic monomer C) and 1,3,5-tris(4-aminophenyl)triazine (organic monomer D), with a molar ratio of organic monomer C to D of 1:1.2. Piperidine, accounting for 5% of the total molar amount of organic monomers C and D, was then added as a promoter, and the reaction was continued at 70°C for 3 hours. The organic monomers formed a second coating layer on the surface of the silicon carbon particles.

[0092] Add pyromellitic aldehyde (organic monomer A) and p-phenylenediamine (organic monomer B) to the above reaction system. Mix organic monomers A and B in a molar ratio of 1:1.2. Add trifluoroacetic acid, accounting for 10% of the total molar amount of organic monomers, as a catalyst and dissolve it in N,N-dimethylformamide. Then stir the reaction at 50°C for 2 hours to form the first coating layer on the surface of the second coating layer.

[0093] Comparative Example 5 This comparative example illustrates a negative electrode material, its preparation method, and its application disclosed in this invention. Most of the operational steps are the same as in Example 1, except that the first temperature is 70°C and the second temperature is 50°C. This material was used as a negative electrode material for lithium-ion batteries and assembled with a positive electrode material, a separator, and an electrolyte to form a lithium-ion battery for performance testing.

[0094] Phenotypic testing The negative electrode materials obtained in the above embodiments and comparative examples were tested. High-resolution TEM (HRTEM) can clearly show the interlayer structure of the coating layer and directly observe the size and distribution of the pores. The pore size of the first coating layer and the second coating layer was tested by transmission electron microscopy (TEM): the micromorphology and pore structure of the coating layer can be directly observed by penetrating the material with an electron beam, and the pore size can be quantified by combining image analysis.

[0095] The functional group density of the first and second coating layers was measured using X-ray photoelectron spectroscopy (XPS) and XPS imaging. First, the atomic percentage of characteristic elements related to functional groups in the coating layer (such as hydroxyl and carboxyl groups containing O, and amine groups containing N) was determined through XPS full-spectrum analysis. Then, high-resolution XPS peak fitting (such as C1s, O1s, and N1s spectra) was used to distinguish elements in different chemical environments (i.e., corresponding to specific functional groups). The atomic percentage of elements in the target functional group was calculated. Combined with the specific surface area of ​​the coating layer (obtained through BET testing), the functional group density (e.g., "units / nm") was calculated using a formula. 2 "); to distinguish between the first and second coating layers: using XPS imaging with spatial resolution down to the micrometer level, the cross-section or surface of the material is scanned, and the spatial resolution of the two coating layers is quantitatively determined by the differences in the distribution of characteristic elements (the higher the functional group density, the stronger the corresponding element signal), thereby calculating their functional group densities separately. The thickness of the coating layer was tested using STEM (Scanning Transmission Electron Microscopy)-EDS mapping: while observing the morphology, the boundaries of the coating layer were confirmed by elemental distribution (such as the distribution of specific elements in COF), thereby accurately locating the regions of the first and second coating layers. Combined with image analysis, the thickness of the coating layer could be measured. The particle size of silicon-carbon particles was tested using laser diffraction (LD), and the specific test results are shown in Table 1.

[0096] Table 1. Phenotypic test results of the negative electrode materials prepared in Examples 1-21 and Comparative Examples 1-5 Performance testing The negative electrode sheets prepared in the above examples and comparative examples were assembled into lithium-ion half-cells. They were charged and discharged at 0.005V-2V at a current density of 0.1C (1C=1800mAh / g) at 25°C for 100 cycles. The discharge capacity of each cycle was recorded and the capacity retention rate was calculated.

[0097] First discharge specific capacity: can be directly read using the Xinwei testing machine and testing software; Negative electrode expansion rate: Measure the initial electrode thickness. Disassemble the battery after 100 cycles (charged to 2.0V) in a glove box and measure the thickness of the negative electrode. Negative electrode expansion rate = electrode thickness after cycling / initial electrode thickness - 1.

[0098] The performance test results of Examples 1-21 and Comparative Examples 1-5 are shown in Table 2.

[0099] Table 2 Performance test results of lithium-ion batteries prepared in Examples 1-21 and Comparative Examples 1-5 The data in Tables 1 and 2 show that, compared with Comparative Example 1, Examples 1-21 have the following results: When the silicon-carbon particles do not contain a coating layer, the lithium-ion transport efficiency is low, the first discharge specific capacity of the battery decreases, and the capacity retention rate drops to 52%. The volume expansion of silicon leads to a significant increase in the expansion rate of the negative electrode. Compared with Comparative Example 2, Examples 1-21 have the following results: When the silicon-carbon particles do not contain a second coating layer, the excessive expansion of the negative electrode cannot be limited, resulting in a high expansion rate of the negative electrode and a decrease in capacity retention. Due to the low electron conduction efficiency inside the negative electrode material, the first discharge specific capacity decreases. Compared with Comparative Example 3, Examples 1-21 have the following results: When the silicon-carbon particles do not contain a first coating layer, the second coating layer has a higher functional group density. Although this improves the internal electron conduction and the first discharge specific capacity is higher than that of Comparative Example 2, the small-pore coating layer cannot provide sufficient space for the volume expansion of silicon, resulting in a worse expansion rate and capacity retention rate of the negative electrode than that of Comparative Example 2. Compared with Comparative Example 4, Examples 1-21 have the following results. This indicates that when the outer surface of silicon-carbon particles is a second coating layer, i.e., the density of functional groups decreases gradually while the pore size of the porous structure increases gradually, the second coating layer cannot provide support for the expansion of the first coating layer, greatly reducing the overall structural stability of the negative electrode material. This leads to an increase in the expansion rate of the negative electrode sheet and a decrease in the capacity retention rate. However, because the outer surface of silicon-carbon particles is a second coating layer with a larger density of functional groups, its initial discharge specific capacity is higher than that of Comparative Example 3. The results of Examples 1-21 compared with Comparative Example 5 show that when the first temperature is higher than the second temperature, the reaction rate of the first coating layer is too fast, the pore size is too small, and the space for silicon volume expansion is reduced. The reaction rate of the second coating layer is relatively slower, and the pore size is too large, which easily causes the pulverization and shedding of the negative electrode material, increases the expansion rate of the negative electrode sheet, and further leads to a decrease in its capacity retention rate. However, because the pore sizes of its first and second coating layers are larger than those of Comparative Example 4, lithium ions can reach the interior of the particles relatively smoothly, so the initial discharge specific capacity is higher than that of Comparative Example 4.

[0100] The results of Examples 1-2, 4-7 show that when the first temperature is in the range of 40-60°C and the reaction time is in the range of 1-3 hours, the pore size of the first coating layer can be controlled to be in the range of 5-20 nm, and the functional group density can be controlled to be in the range of 5-10 groups / nm. 2When the first temperature is below 40℃ and the reaction time is less than 1 hour, the reaction rate is too slow, resulting in excessively large pores in the first coating layer. This significantly reduces the overall structural stability of the negative electrode material, increases the expansion rate of the negative electrode sheet, and further leads to a decrease in its capacity retention rate and initial discharge specific capacity. When the first temperature is above 60℃ and the reaction time is greater than 3 hours, the reaction rate is too fast and too long, resulting in excessively small pores in the first coating layer. This reduces flexibility and deformability, causes a sharp increase in expansion rate, and makes the overall structure of the negative electrode material prone to collapse, leading to a rapid decay in its capacity retention rate and initial discharge specific capacity. When the second temperature is in the range of 60~80℃ and the reaction time is in the range of 2~5 hours, the pore size of the second coating layer ranges from 1~5 nm, and the functional group density ranges from 15~25 per nm. 2 When the second temperature is below 60℃ and the reaction time is less than 2 hours, the reaction rate is too slow, resulting in an excessively large pore size in the second coating layer, which reduces the effect of relieving expansion stress. When the second temperature is above 80℃ and the reaction time is greater than 5 hours, the pore size in the second coating layer is too small, which excessively restricts the deformation of the first coating layer. When the stress range of the second coating layer is exceeded, the expansion rate increases significantly, the capacity retention rate decreases, and the initial discharge specific capacity decreases.

[0101] The results of Examples 20-21 compared with Example 1 show that when the mass percentage of silicon in silicon-carbon particles is 30-70% and the mass percentage of carbon is 30-70%, the volume expansion and conductivity of the negative electrode can be better balanced, thereby improving the cycle performance of the battery.

[0102] The results of Examples 8-15 compared with Example 1 show that when the molar ratio of organic monomer A to organic monomer B is in the range of 1:1 to 1:2, the condensation reaction between organic monomer A and organic monomer B is complete. When the molar ratio exceeds this range, there is too much organic monomer, and the reaction is incomplete. Excessive monomer residue can easily clog the pore size, reduce the buffer expansion capacity, increase the expansion rate, and decrease the initial discharge specific capacity and capacity retention rate. When the molar ratio of organic monomer C to organic monomer D is in the range of 1:1 to 1:1.5, the condensation reaction between organic monomers is complete. When the molar ratio exceeds this range, the condensation reaction is incomplete, resulting in short molecular chains, poor crystallinity, and inability to form a complete, long-range ordered covalent organic framework structure. This seriously affects its ion transport efficiency, reduces the initial discharge specific capacity, and the structure of the second coating layer is loose, the rigidity is reduced, the expansion rate increases, and the capacity retention rate decreases.

[0103] The results of Examples 16-19 compared with Example 1 show that when the amount of catalyst is in the range of 5-15%, the pore size range of the first coating layer can be controlled to be 5-20 nm. When the amount of catalyst is less than 5%, the pore size of the first coating layer is too large. When the amount of catalyst is more than 15%, the pore size of the first coating layer is too small, which reduces the space for silicon volume expansion, increases the expansion rate, and makes the overall structure of the negative electrode material prone to collapse, resulting in a rapid decay of its capacity retention rate and first discharge specific capacity. When the amount of promoter is in the range of 3-8%, the pore size range of the second coating layer is 1-5 nm. When the amount of promoter is less than 3%, the pore size of the second coating layer is too large, which reduces the effect of relieving expansion stress, decreases rigidity, increases expansion rate, and decreases capacity retention rate. When the amount of promoter is more than 8%, the pore size of the second coating layer is too small, which excessively restricts the deformation of the first coating layer. When the stress range of the second coating layer is exceeded, the expansion rate increases significantly, the capacity retention rate decreases, and the first discharge specific capacity decreases.

[0104] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A negative electrode material, characterized in that, It includes silicon carbon particles and an organic coating layer, wherein the organic coating layer is on the surface of the silicon carbon particles and the organic coating layer has a porous structure; Along the direction away from the surface of the silicon-carbon particles, the density of functional groups in the organic coating layer generally increases in a gradient, while the pore size of the porous structure generally decreases in a gradient.

2. The negative electrode material according to claim 1, characterized in that, The organic coating layer includes a first coating layer and a second coating layer. The first coating layer includes a first covalent organic framework, and the second coating layer includes a second covalent organic framework. The second covalent organic framework coats the surface of the first covalent organic framework. The pore size of the first covalent organic framework is larger than that of the second covalent organic framework, and the functional group density in the first covalent organic framework is smaller than that in the second covalent organic framework.

3. The negative electrode material according to claim 2, characterized in that, The first covalent organic framework has a pore size range of 5~20nm, and the second covalent organic framework has a pore size range of 1~5nm.

4. The negative electrode material according to claim 2, characterized in that, The functional group density of the first covalent organic framework is 5~10 per nm. 2 The functional group density of the second covalent organic framework is 15~25 per nm. 2 .

5. The negative electrode material according to claim 1, characterized in that, The particle size of the silicon-carbon particles ranges from 4 to 20 µm.

6. The negative electrode material according to claim 1, characterized in that, The silicon-carbon particles contain 30-70% silicon and 30-70% carbon by mass.

7. The negative electrode material according to claim 1, characterized in that, The thickness of the organic coating layer is 50~500nm.

8. The method for preparing the negative electrode material according to any one of claims 1-7, characterized in that, Includes the following steps: Organic monomer A, organic monomer B, and catalyst are dissolved in an organic solvent to prepare a pre-assembled solution; Silicon carbon particles are added to the pre-assembled solution, and a first polymerization reaction is carried out at a first temperature to obtain a first reaction solution, wherein a first coating layer is formed on the surface of the silicon carbon particles in the first reaction solution; Organic monomer C, organic monomer D and accelerator are added to the first reaction solution, and a second polymerization reaction is carried out at a second temperature. A second coating layer is formed outside the first coating layer, and a second reaction solution containing crude product is obtained. The crude product was sequentially post-processed and vacuum dried to obtain the negative electrode material. The first temperature is lower than the second temperature; The organic monomer A includes at least one of phenyl-containing aldehyde compounds and phenyl-containing acid anhydride compounds; The organic monomer B includes amine compounds; The organic monomer C includes at least one of carboxylic acid compounds, aldehyde compounds, boric acid compounds, and phenolic compounds; The organic monomer D includes at least one of amine compounds and phenolic compounds.

9. The method for preparing the negative electrode material according to claim 8, characterized in that, The first temperature is 40~60℃, and the reaction time of the first polymerization reaction is 1~3h; The second temperature is 60~80℃, and the reaction time of the second polymerization reaction is 2~5h.

10. The method for preparing the negative electrode material according to claim 8, characterized in that, The organic monomer A includes at least one of pyromellitic trimethylaldehyde, pyromellitic dianhydride, terephthalaldehyde, and 2,5-dimethoxyterephthalaldehyde. The organic monomer B includes at least one of p-phenylenediamine, dihydrazide tartrate, and 1,3,5-tris(4-aminophenyl)benzene; And / or, the organic monomer C comprises at least one of 2,5-dihydroxyterephthalic acid, 2,5-dihydroxyterephthalaldehyde, 2-aminoterephthalic acid, 2,5-dicyanoterephthalaldehyde, tetra(4-formylphenyl)methane, 1,4-phenylenediboronic acid, and 2,3,6,7,10,11-hexahydroxytriphenylene. The organic monomer D includes at least one of ethylenediamine, hexamethylenediamine, 1,3,5-tris(4-aminophenyl)triazine, 4,4',4''-triaminotriphenylamine, benzidine-3,3',5,5'-tetramine, hydroquinone, and 1,3,5-trihydroxybenzene.

11. The method for preparing the negative electrode material according to claim 10, characterized in that, The molar ratio of organic monomer A to organic monomer B is 1:1 to 1:2; And / or, the molar ratio of the organic monomer C to the organic monomer D is 1:1 to 1:1.

5.

12. The method for preparing the negative electrode material according to claim 8, characterized in that, The catalyst includes at least one of acetic acid, trifluoroacetic acid, p-toluenesulfonic acid, scandium trifluoromethanesulfonate, and tris(pentafluorophenyl)borane; The accelerator includes at least one of piperidine, triethylamine, 1,8-diazabicycloundec-7-ene, 1,4-diazabicyclo[2.2.2]octane, zinc acetate, scandium trifluoromethanesulfonate, and tetrabutylammonium bromide.

13. The method for preparing the negative electrode material according to claim 12, characterized in that, The amount of catalyst used is 5-15% of the total molar amount of organic monomer A and organic monomer B; And / or, the amount of the promoter is 3 to 8% of the total molar amount of organic monomer C and organic monomer D.

14. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer is disposed on at least one side of the negative electrode current collector, and the negative electrode active material layer includes the negative electrode material as described in any one of claims 1-7, or the negative electrode material prepared by the preparation method described in any one of claims 8-13.

15. A battery, characterized in that, Includes the negative electrode sheet as described in claim 14.

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