Coating modification method of negative electrode material, negative electrode material and application of negative electrode material

Abstract

The invention relates to the technical field of lithium ion battery negative electrode materials, in particular to a coating modification method of a negative electrode material, the negative electrode material and application of the negative electrode material, and the negative electrode material sequentially comprises a silicon core, a gradient interface layer, a self-repairing elastic layer and a hierarchical porous carbon skeleton from inside to outside; the gradient interface layer coats the surface of the silicon core and sequentially comprises an amino silane coupling agent modification layer and a conductive polymer layer from inside to outside; the self-repairing elastic layer covers the outer side of the gradient interface layer and comprises a dynamic covalent bond cross-linked network; the hierarchical porous carbon skeleton coats the outer side of the self-repairing elastic layer, a strongly-combined gradient interface layer is firstly constructed on a silicon core through hydroxylation, amino silane coupling and conducting polymer in-situ polymerization, then a reversible cross-linked network is formed outside the gradient interface layer based on Diels-Alder dynamic covalent bonds, and in-situ crack repairing is achieved. And finally, biomass carbon and CVD carbon nanotubes are compounded, and a micropore / mesopore synergistic conductive buffer structure is formed outside the reversible cross-linked network, so that the cycling stability of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of anode materials for lithium-ion batteries, and specifically relates to a method for coating and modifying anode materials, anode materials and their applications. Background Art

[0002] Silicon-based materials are regarded as ideal anode materials for next-generation high-energy-density lithium-ion batteries due to their high theoretical specific capacity (4200 mAh / g). However, during the charge and discharge process, their volume expands (>300%), leading to problems such as particle pulverization and repeated rupture of the SEI film. Existing carbon coating technologies (such as graphene, carbon nanotubes, etc.) can alleviate the expansion, but there are problems such as easy cracking of the coating layer and insufficient interfacial bonding force with the silicon matrix.

[0003] For example, in the Chinese invention patent with the patent application number 202410352158.9 applied by Zhuhai Guanyu, it specifically discloses "a silicon-carbon composite material, anode sheet and battery", which uses the pores of a hierarchical porous carbon material to disperse silicon nanoparticles and coats an amorphous carbon layer on the surface of the hierarchical porous carbon material to inhibit the aggregation of silicon nanoparticles through the pore structure. However, this patented technology still cannot achieve self-repair of the cracking of the coating layer.

[0004] Furthermore, in the Chinese invention patent with the patent application number 201910918320.8 applied by BETRAY New Materials Group Co., Ltd., it specifically discloses "a composite anode material, its preparation method and a lithium-ion battery", which adopts the method of coating a carbon material layer on the core of a silicon-containing material to improve performance such as the cyclic capacity retention rate. It is also a surface coating technology, but it also cannot achieve self-repair of the cracking of the coating layer.

[0005] In addition, in the Chinese invention patent with the patent application number 202411153164.8 applied by Wanxiang Yisaner Co., Ltd., it specifically discloses "a silicon anode material coated with metal nitride and porous carbon, preparation method and application", which obtains a silicon anode material coated with metal nitride and porous carbon by mixing nano-silicon particles, etc. with a polymer compound carbon source, etc., and through grinding and carbonization treatment. It also cannot achieve self-repair of the cracking of the coating layer.

[0006] Therefore, there is an urgent need for an anode material that can achieve self-repair of the cracking of the coating layer, thereby improving the cycling stability of the battery. Summary of the Invention

[0007] To address the above problems, the present invention provides a method for coating and modifying a negative electrode material, the negative electrode material and its application. Through hydroxylation → amino-silane coupling → in-situ polymerization of a conductive polymer, a strongly bonded gradient interface layer is first constructed on the silicon core, and then a reversible cross-linked network is formed outside the gradient interface layer based on Diels-Alder dynamic covalent bonds to achieve in-situ crack repair. Finally, the biomass carbon is combined with CVD carbon nanotubes to form a microporous / mesoporous synergistic conductive buffer structure outside the reversible cross-linked network, thereby improving the cycle stability of the battery.

[0008] To achieve the above object, the present invention provides the following technical solutions: A negative electrode material, which sequentially includes from the inside to the outside: A silicon core, a gradient interface layer, a self-healing elastic layer, and a hierarchical pore carbon skeleton; The silicon core is a nano-silicon particle with a particle size of 50 - 200 nm; The gradient interface layer is coated on the surface of the silicon core and sequentially includes an amino-silane coupling agent modification layer and a conductive polymer layer from the inside to the outside; The self-healing elastic layer is coated outside the gradient interface layer and contains a dynamic covalent bond cross-linked network; The hierarchical pore carbon skeleton is coated outside the self-healing elastic layer and is composed of a composite of biomass carbon and vapor-deposited carbon nanotubes, with a porosity of 50 - 70%.

[0009] Gradient interface layer: A gradient interface layer is constructed on the surface of the silicon particles through multi-step chemical modification to enhance the binding force between silicon and the carbon layer.

[0010] Self-healing elastic layer: A polymer elastic layer containing dynamic covalent bonds is introduced to repair cracks through thermal or electrochemical stimulation during the cycling process.

[0011] Hierarchical pore carbon skeleton: Combining biomass-derived carbon and vapor-deposited carbon to form a hierarchical pore structure, adapting to the volume expansion of silicon and improving the ion / electron transport efficiency.

[0012] Among them, the total thickness of the gradient interface layer is 10 - 30 nm, the thickness of the amino-silane modification layer is 2 - 5 nm, the conductive polymer layer is polypyrrole, and the thickness is 5 - 15 nm.

[0013] If the total thickness of the gradient interface layer < 10 mm, it will result in an overly thin gradient interface layer, which cannot effectively transfer and disperse the mechanical stress generated by silicon expansion, easily leading to cracking of the coating layer or direct exposure of silicon particles, causing repeated rupture of the SEI film.

[0014] If the total thickness of the gradient interface layer > 30 mm, it will result in an overly thick gradient interface layer, prolonging the lithium ion diffusion path, significantly increasing the internal resistance, leading to a decline in rate performance, increasing the rigidity of the thick interface layer, reducing the adaptability to silicon expansion, and instead accelerating failure due to stress concentration.

[0015] The minimum thickness of the amino-silane modified layer is 2 nm, ensuring sufficient reaction between the hydroxyl groups (-OH) on the silicon surface and the amino-silane (such as KH550) to form a dense Si-O-Si covalent bond network with a coverage rate of ≥95%; If the thickness is <2 nm, the surface modification is incomplete, the interfacial binding energy decreases, and the measured shear strength drops from 50 MPa to 30 MPa; The maximum thickness is 5 nm. The length of the amino-silane molecular chain is about 1-2 nm. Multilayer stacking may lead to loose molecular arrangement, which instead reduces the binding strength. Moreover, an overly thick amino-silane layer will hinder the in-situ polymerization uniformity of the subsequent conductive polymer (PPy).

[0016] The minimum thickness of the conductive polymer layer is 5 nm, forming a continuous conductive network, increasing the surface electron conductivity of the silicon particles from <10 -5 S / cm to ≥10² S / cm (four-probe test). If the thickness is <5 nm, the conductive path is discontinuous, resulting in increased polarization (the charge transfer resistance Rct shown by EIS increases from 25 Ω to 50 Ω); The maximum thickness is 15 nm. The flexibility of PPy can buffer the initial stress of silicon expansion (the elastic modulus is about 1 GPa), but being overly thick will reduce the elasticity of the overall interfacial layer and occupy too much thickness budget (the total thickness needs to be ≤30 nm).

[0017] In addition, the dynamic covalent bond is a reversible cross-linked structure formed through the Diels-Alder reaction, composed of a polyurethane prepolymer containing furan groups and a bismaleimide cross-linking agent. The cross-linking point spacing is 2-5 nm. The furan group (Dienes) serves as an electron donor (such as the furan side group in the polyurethane chain segment), and the maleimide group (Dienophiles) serves as an electron acceptor (such as the bismaleimide cross-linking agent).

[0018] Reversibility of the Diels-Alder reaction 1. Forward reaction (low temperature): At a lower temperature (<60 °C), the furan group (diene) reacts with the maleimide group (dienophile) through a DA addition reaction to form a six-membered cyclic covalent bond (DA adduct) and construct a cross-linked network.

[0019] Chemical formula: Furan + Maleimide DA Adduct 2. Reverse reaction (high temperature): When the temperature rises to 60-100 °C, the DA adduct undergoes a retro-Diels-Alder (retro-DA) reaction, the covalent bond breaks, and the cross-linked network dissociates temporarily, and the material exhibits fluidity.

[0020] Chemical formula: DA Adduct Furan + Maleimide Specifically, during the charge and discharge process, silicon expands in volume (such as lithium insertion expansion), resulting in local stress concentration in the elastic layer and generating microcracks. When the battery cycles, internal heat is generated (40 - 60 °C), and the local temperature reaches the threshold of the retro-DA reaction (such as 60 °C). The DA adduct at the crack dissociates, the crosslinking points are temporarily disconnected, the fluidity of the material increases, and the broken molecular chains (containing furan / maleimide groups) diffuse towards the crack region under thermal drive to fill the crack gap. The temperature slightly decreases (such as the temperature drops back to < 60 °C after charging and discharging stops), and the DA reaction occurs again, forming new covalent bonds at the crack interface to achieve in-situ repair.

[0021] Preferably, the self-healing elastic layer is 20 - 50 nm. The self-healing elastic layer needs to absorb the expansion stress through deformation to prevent the stress from being transmitted to the outer carbon skeleton and causing cracking. The elastic modulus (about 0.5 - 1 GPa) and the thickness act synergistically. When the thickness is too thin (< 20 nm), the modulus is insufficient and it is easily broken through by silicon expansion; when it is too thick (> 50 nm), the rigidity increases and the buffering ability is lost.

[0022] Moreover, when the thickness of the self-healing elastic layer < 20 nm, the density of dynamic bonds is insufficient (the crosslinking point spacing > 5 nm), and the repair efficiency significantly decreases (< 70%); when the thickness ≥ 20 nm, the crosslinking point spacing ≤ 5 nm, and the repair efficiency ≥ 90%; An overly thick self-healing elastic layer (> 50 nm) will extend the Li⁺ diffusion path, resulting in a decrease in ionic conductivity (from 10 -4 S / cm to 10 -5 S / cm) and an increase in internal resistance (Rct increases from 25 Ω to 60 Ω).

[0023] In addition, in the multi-level pore carbon skeleton, the biomass carbon is porous carbon formed by carbonizing sodium lignosulfonate, and the pore size distribution is micropores < 2 nm and mesopores of 2 - 50 nm; the length of the vapor-deposited carbon nanotubes is 1 - 5 μm, and the density ≥ 104 tubes / μm².

[0024] The biomass carbon is porous carbon formed by carbonizing sodium lignosulfonate. The micropores buffer the expansion, the mesopores promote the infiltration of the electrolyte and ion transport, and the vapor-deposited carbon nanotubes form a continuous conductive network to reduce the internal resistance of the electrode.

[0025] The present invention also provides a modification method for coating the above-mentioned negative electrode material, including the following steps: Step a, gradient interface treatment of silicon particles: a1. Hydroxylate the silicon particles in a mixed solution of concentrated sulfuric acid and hydrogen peroxide. After hydroxylation, the dispersibility of the silicon particles is enhanced, avoiding agglomeration, and at the same time laying a chemical foundation for subsequent modification with silane coupling agents; a2. Modify to form an amino-functional layer in the ethanol solution of the amino-silane coupling agent, which can enhance the interfacial binding energy, inhibit the peeling of silicon and the carbon layer caused by the expansion stress, and at the same time, the amino group can serve as a polymerization initiation site for conductive polymers (such as polypyrrole); a3. Coating a polypyrrole layer on the silicon surface by in-situ polymerization, the conductivity of PPy (about 100 S / cm) can improve the electron transport efficiency of silicon particles, and its flexibility can buffer part of the volume expansion stress; Step b. Coating with a self-healing elastic layer: b1. Mix the polyurethane prepolymer containing furan groups with the bismaleimide cross-linking agent to prepare a dynamic covalent elastomer through the Diels-Alder reaction. The dynamic covalent elastomer can spontaneously repair cracks at the battery operating temperature (40 - 60 °C), maintain the integrity of the coating layer, and prevent the continuous decomposition of the electrolyte; b2. Spin-coat the elastomer solution on the surface of the silicon particles after gradient interface treatment and cure to form a self-healing elastic layer with a thickness of 20 - 50 nm. The self-healing elastic layer (with a thickness of 20 - 50 nm) provides mechanical buffering, disperses the silicon expansion stress to the carbon skeleton, and inhibits particle pulverization; Step c. Construction of a hierarchical pore carbon skeleton: c1. Mix the silicon particles coated with the elastic layer with the sodium lignosulfonate solution and spray-dry to form a precursor. The porosity of the biomass carbon (50 - 70%) provides a volume expansion buffer space, and at the same time, its surface functional groups (such as -SO 3 - ) enhance the electrolyte wettability; c2. Gradually heat up to 800 °C in an argon atmosphere for carbonization to form a porous carbon skeleton. The conductivity of the carbon skeleton (about 10 2 S / m) and mechanical strength (elastic modulus about 10 GPa) are improved simultaneously, and nitrogen doping enhances the lithium-ion diffusion kinetics; c3. Grow a carbon nanotube network on the surface of the carbon skeleton by chemical vapor deposition (CVD). The CNT network (with a length of 1 - 5 μm) reduces the internal resistance of the electrode, and the capacity retention rate at 5C rate is increased to ≥80%.

[0026] Among them, in the step a, the hydroxylation treatment uses a mixed solution of concentrated sulfuric acid and hydrogen peroxide with a volume ratio of 3:1, and ultrasonically treat for 30 - 60 minutes. Concentrated sulfuric acid (H 2 SO 4 ) and hydrogen peroxide (H 2 O 2 ) mixed solution (such as piranha solution) has strong oxidizing property, which can oxidize the silicon surface to generate Si-OH bonds and form a hydrophilic surface.

[0027] Furthermore, in the step b, the rotation speed of the spin coating process is 2000 - 5000 rpm, the curing temperature is 60 - 80 °C, and the curing time is 1 - 2 hours.

[0028] In addition, in the step c, the carbon source for chemical vapor deposition is acetylene, the deposition temperature is 600 - 800 °C, and the deposition time is 10 - 30 minutes.

[0029] The present invention also provides an application of the above-mentioned anode material, and the anode material is used as the anode active material of a lithium-ion battery.

[0030] The beneficial effects of the present invention are as follows: (1) The present invention enhances the bonding force between silicon and the carbon layer through gradient interface design, and the self-healing elastic layer (Diels-Alder dynamic covalent bond) can spontaneously repair cracks at the battery operating temperature (40 - 60 °C), with a crack closure rate ≥ 90% and a mechanical recovery rate ≥ 85%; (2) Through the synergistic effect of gradient interface design (hydroxylation → amino silane coupling → conductive polymer), self-healing elastic layer (Diels-Alder dynamic covalent bond) and hierarchical pore carbon skeleton (biomass carbon + CVD-CNT), the present invention significantly improves the cycle stability (capacity retention rate ≥ 90% after 500 cycles under 1C charge and discharge conditions) and rate performance (capacity ≥ 2500 mAh / g at 5C) of the silicon-based anode.

[0031] In summary, the present invention has the advantages of synergistically solving the silicon expansion problem from atomic-level interface to macroscopic structure, while taking into account conductivity and cycle life, and has significant technological breakthroughs, etc., and is particularly suitable for the field of coating modification technology of anode materials. Description of the Drawings

[0032] Figure 1 It is the SEM image of the anode material in Example 1 of the invention; Figure 2 It is the schematic flow chart of the modification method in Example 2 of the present invention; Figure 3 It is the SEM image of the anode material in Preparation Example 1 of the present invention before repair; Figure 4 It is the SEM image of the anode material in Preparation Example 1 of the present invention after repair; Figure 5 It is the SEM image of the anode material in Preparation Example 3 of the present invention before repair; Figure 6 It is the SEM image of the anode material in Preparation Example 3 of the present invention after repair; Figure 7 It is the SEM image of the anode material in Preparation Example 6 of the present invention before repair; Figure 8 It is the SEM image of the anode material in Preparation Example 6 of the present invention after repair. Detailed implementation mode

[0033] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work shall fall within the protection scope of the present invention.

[0034] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0035] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality" means two or more, unless otherwise specifically defined.

[0036] Embodiment 1: As Figure 1 shown, a negative electrode material includes, from the inside to the outside in sequence: a silicon core, a gradient interface layer, a self-healing elastic layer, and a hierarchical pore carbon skeleton; The silicon core is nano-silicon particles with a particle size of 50 - 200 nm; The gradient interface layer is coated on the surface of the silicon core and includes, from the inside to the outside in sequence, an amino-silane coupling agent modification layer and a conductive polymer layer; The self-healing elastic layer is coated on the outside of the gradient interface layer and contains a dynamic covalent bond crosslinking network; The hierarchical pore carbon skeleton is coated on the outside of the self-healing elastic layer and is composed of a composite of biomass carbon and vapor-deposited carbon nanotubes, with a porosity of 50 - 70%.

[0037] Among them, the total thickness of the gradient interface layer is 10 - 30 nm, the thickness of the amino-silane modification layer is 2 - 5 nm, the conductive polymer layer is polypyrrole, and the thickness is 5 - 15 nm.

[0038] Furthermore, the dynamic covalent bond is a reversible cross-linked structure formed by the Diels-Alder reaction, which is composed of a polyurethane prepolymer containing a furan group and a bismaleimide cross-linking agent, and the cross-linking point spacing is 2-5 nm.

[0039] In addition, in the hierarchical pore carbon skeleton, the biomass carbon is porous carbon formed by the carbonization of sodium lignosulfonate, and the pore size distribution is micropores with a pore diameter of <2 nm and mesopores with a pore diameter of 2-50 nm; the length of the vapor-deposited carbon nanotubes is 1-5 μm, and the density is ≥10 4 tubes / μm². The lignin replaces the traditional graphite, reducing the raw material cost by more than 30% and the carbon emission by 50%.

[0040] First of all, it is worth emphasizing that the -NH 2 of the aminosilane forms a hydrogen bond with the furan group in the elastic layer, enhancing the adhesion between the elastic layer and the silicon core to form an interfacial anchor. At the same time, the flexible-rigid gradient transition of the gradient interfacial layer (PPy + coupling agent) avoids the premature failure of the elastic layer due to stress concentration, achieving a synergistic effect that after 500 battery cycles, the interfacial peeling area is reduced from 30% of the traditional material to <5%.

[0041] Furthermore, it is necessary to emphasize that the elastic layer repairs microcracks, prevents cracks from spreading to the carbon skeleton, and maintains the integrity of the pore structure. At the same time, the flexibility of the elastic layer and the rigidity of the carbon skeleton complement each other, forming a "rigid-flexible combination" composite structure (the overall elastic modulus is about 5 GPa), achieving a synergistic effect of improving the porosity utilization rate of the carbon skeleton and increasing the 5C capacity by 40% (compared with the design without the elastic layer).

[0042] In addition, it is also necessary to emphasize that the CNT network is connected to the PPy conductive layer to form a "point-line-plane" conduction path through the electrode, and the electronic conductivity is increased from 10 -5 S / cm to 10 2 S / cm, and the hierarchical pores shorten the Li⁺ diffusion path, and the ion diffusion coefficient is increased from 10 -14 cm² / s to 10 -12 cm² / s, achieving a synergistic effect of reducing electrode polarization (ΔV≤0.1 V) and increasing the Coulomb efficiency to ≥99.5%.

[0043] Example 2: As Figure 2 shown, a modification method for coating the negative electrode material described in Example 1 includes the following steps: Step a. Gradient interfacial treatment of silicon particles: a1. Hydroxylate the silicon particles in a mixed solution of concentrated sulfuric acid and hydrogen peroxide; a2. Modify to form an amino functional layer in an ethanol solution of an aminosilane coupling agent; a3. Coating a polypyrrole layer on the silicon surface by in-situ polymerization; Step b. Coating with a self-healing elastic layer: b1. Mix a polyurethane prepolymer containing furan groups with a bismaleimide crosslinking agent to prepare a dynamic covalent elastomer through the Diels-Alder reaction; b2. Spin-coat the elastomer solution on the surface of the silicon particles after gradient interface treatment and cure to form a self-healing elastic layer; Step c. Construction of a hierarchical pore carbon skeleton: c1. Mix the silicon particles coated with the elastic layer with a sodium lignosulfonate solution and spray-dry to form a precursor; c2. Gradually heat to 800 °C in an argon atmosphere for carbonization to form a porous carbon skeleton; c3. Grow a carbon nanotube network on the surface of the carbon skeleton by chemical vapor deposition.

[0044] Among them, in the step a, the hydroxylation treatment uses a mixed solution of concentrated sulfuric acid and hydrogen peroxide with a volume ratio of 3:1, and ultrasonic treatment is carried out for 30 - 60 minutes.

[0045] The -Si-OH introduced by hydroxylation on the silicon core surface forms a Si-O-Si covalent bond (bond energy is about 450 kJ / mol) with an amino silane (such as KH550), and the interfacial bonding strength is increased to ≥50 MPa (traditional physical adsorption is only about 5 MPa). At the same time, the amino group (-NH 2 ) serves as a polymerization initiation site for the conductive polymer (PPy) to ensure continuous coating of the PPy layer. Moreover, the dispersibility of the hydroxylated silicon particles in the solution is improved, avoiding local stress concentration caused by agglomeration.

[0046] Furthermore, in the step b, the rotation speed of the spin-coating process is 2000 - 5000 rpm, the curing temperature is 60 - 80 °C, and the curing time is 1 - 2 hours.

[0047] Based on the Diels-Alder dynamic covalent bond, the crack closure rate ≥90% is achieved at 40 - 80 °C to inhibit the repeated rupture of the SEI film; the elastic modulus (about 0.5 - 1 GPa) matches the silicon expansion stress to disperse mechanical shocks; and the self-healing temperature window overlaps with the battery operating temperature (40 - 60 °C) without external intervention.

[0048] In addition, in the step c, the carbon source for chemical vapor deposition is acetylene, the deposition temperature is 600 - 800 °C, and the deposition time is 10 - 30 minutes.

[0049] The micropores (<2 nm) and mesopores (2 - 50 nm) of the biomass carbon provide a hierarchical buffering space, and the expansion rate is reduced from 300% to ≤120%; CVD-CNT (density ≥10 4Reduce the internal resistance of the electrode (Rct ≤ 25 Ω) and improve the rate performance (5C capacity ≥ 2500 mAh / g); the elastic modulus of the carbon skeleton is ≥ 10 GPa to support the silicon particles and prevent pulverization.

[0050] Example 3: An application of the negative electrode material described in Example 1, where the negative electrode material is used as the negative electrode active material of a lithium-ion battery.

[0051] Under the 1C charge-discharge conditions of the prepared lithium battery, the capacity retention rate after 500 cycles is ≥ 90%, which is +28% higher than that of traditional silicon materials. The silicon volume expansion rate is ≤ 120%, the 5C rate capacity is ≥ 2500 mAh / g, the self-healing efficiency is ≥ 90%, and the interfacial bonding strength is ≥ 50 MPa.

[0052] Preparation Example 1: Using 100 nm diameter nano-silicon particles as the silicon core, according to the preparation method of Example 2, a negative electrode material with a total interfacial layer thickness of 10 nm (PPy 7 nm + amino-silane modified layer 3 nm), a self-healing elastic layer thickness of 30 nm, a crosslinking point spacing of 3 nm, a multi-level pore carbon skeleton porosity of 60%, and a CNT density of 1.2×10 4 tubes / μm² was prepared.

[0053] Preparation Example 2: Using 50 nm diameter nano-silicon particles as the silicon core, according to the preparation method of Example 2, a negative electrode material with a total interfacial layer thickness of 7 nm (PPy 5 nm + amino-silane modified layer 2 nm), a self-healing elastic layer thickness of 20 nm, a crosslinking point spacing of 2 nm, a multi-level pore carbon skeleton porosity of 70%, and a CNT density of 1.5×10 4 tubes / μm² was prepared.

[0054] Preparation Example 3: Using 200 nm diameter nano-silicon particles as the silicon core, according to the preparation method of Example 2, a negative electrode material with a total interfacial layer thickness of 20 nm (PPy 15 nm + amino-silane modified layer 5 nm), a self-healing elastic layer thickness of 50 nm, a crosslinking point spacing of 5 nm, a multi-level pore carbon skeleton porosity of 50%, and a CNT density of 1.0×10 4 tubes / μm² was prepared.

[0055] Preparation Example 4: Using 150 nm diameter nano-silicon particles as the silicon core, according to the preparation method of Example 2, a negative electrode material with a total interfacial layer thickness of 15 nm (PPy 11 nm + amino-silane modified layer 4 nm), a self-healing elastic layer thickness of 40 nm, a crosslinking point spacing of 4 nm, a multi-level pore carbon skeleton porosity of 65%, and a CNT density of 1.3×10 4The anode material of tubes / μm².

[0056] Preparation Example 5: Using nano-silicon particles with a particle size of 80 nm as the silicon core, according to the preparation method of Example 2, an anode material with a total interfacial layer thickness of 12 nm (PPy 9 nm + amino-silane modification layer 3 nm), a self-healing elastic layer thickness of 25 nm, a crosslinking point spacing of 3 nm, a multi-channel carbon skeleton porosity of 55%, and a CNT density of 1.1×10 4 tubes / μm² was prepared.

[0057] Preparation Example 6: Using nano-silicon particles with a particle size of 50 nm as the silicon core, according to the preparation method of Example 2, an anode material with a total interfacial layer thickness of 7 nm (PPy 5 nm + amino-silane modification layer 2 nm), a self-healing elastic layer thickness of 20 nm, a crosslinking point spacing of 2 nm, a multi-channel carbon skeleton porosity of 70%, and a CNT density of 2.0×10 4 tubes / μm² was prepared.

[0058] Preparation Example 7: Using nano-silicon particles with a particle size of 200 nm as the silicon core, according to the preparation method of Example 2, an anode material with a total interfacial layer thickness of 20 nm (PPy 15 nm + amino-silane modification layer 5 nm), a self-healing elastic layer thickness of 50 nm, a crosslinking point spacing of 5 nm, a multi-channel carbon skeleton porosity of 50%, and a CNT density of 2.0×10 4 tubes / μm² was prepared.

[0059] Comparative Example 1: Using nano-silicon particles with a particle size of 100 nm as the silicon core, according to the preparation method of Example 2, an anode material with a 15-nm PPy layer without an amino-silane modification layer, a self-healing elastic layer thickness of 30 nm, a multi-channel carbon skeleton porosity of 60%, and a CNT density of 1.2×10 4 tubes / μm² was prepared.

[0060] Comparative Example 2: Using nano-silicon particles with a particle size of 50 nm as the silicon core, according to the preparation method of Example 2, an anode material with a total interfacial layer thickness of 7 nm, no self-healing elastic layer thickness, a multi-channel carbon skeleton porosity of 70%, and a CNT density of 2.0×10 4 tubes / μm² was prepared.

[0061] Comparative Example 3: Using nanosilicon particles with a particle size of 200 nm as the silicon core, according to the preparation method of Example 2 in real time, a negative electrode material with an overall interface layer thickness of 20 nm (15 nm of PPy + 5 nm of amino-silane modified layer), a self-healing elastic layer thickness of 50 nm, a crosslinking point spacing of 5 nm, a porosity of 40% in the multi-stage pore carbon skeleton, and a CNT density of 0.5×10 4 tubes / μm² was prepared.

[0062] For Preparation Examples 1-7 and Comparative Examples 1-3, through the existing methods of making electrode sheets, assembling batteries, and testing batteries in sequence, the capacity retention rate after 500 cycles under the condition of 1C charge and discharge was tested for these 4 performance parameters. The test results are shown in Table 1 below: Table 1 From the comparison between Preparation Examples 1-7 and Comparative Examples 1-3, it can be seen that the capacity retention rates of Examples 1-7 are significantly higher than those of the comparative examples, reflecting the synergistic effect of the gradient interface layer, elastic layer, and carbon skeleton. Among them, Example 6 has the best capacity retention rate performance due to the high porosity + high CNT density compensating for the thin interface layer; Comparative Example 2 has the worst performance because there is no self-healing mechanism.

[0063] The swelling rate of Examples 1-7 ≤ 115%, and that of the comparative examples is ≥ 200%, which proves the synergistic buffering effect of the multi-stage carbon skeleton (porosity ≥ 50%) and the elastic layer. Examples 2 (85%) and 6 (80%) have the best swelling inhibition due to the high porosity + small particle size silicon.

[0064] The 5C capacity of Examples 1-7 ≥ 2200 mAh / g, which is much higher than that of the comparative examples (≤ 1800 mAh / g), benefiting from the synergy of the CNT network (density ≥ 10 4 / μm²) and the PPy conductive layer. Among them, Example 6 (2900 mAh / g) has the best rate performance due to the ultra-high CNT density (2×10 4 / μm²).

[0065] As Figures 3 to 8 shown, the repair efficiency of Examples 1-7 ≥ 82%, while in Comparative Example 1, the interface peeling cannot trigger the repair, and Comparative Example 2 has no repair ability. Example 6 (96%) achieves the ultimate repair efficiency due to the high dynamic bond density (crosslinking spacing of 2 nm).

[0066] In summary, chemical bonding (Si-O-Si) and conductive network (PPy) are used to solve the problems of interface stripping and electron transmission, dynamic covalent bonds (DA reactions) are used to achieve in-situ crack repair, buffer expansion stress, micropores buffer expansion, mesopores accelerate ion transmission, and CNTs enhance conductivity. Example 6 proves that structural optimization (high porosity + high CNT density) can break through the thickness limitation and achieve optimal performance, and even if the parameters are degraded as in Example 7, it is still significantly better than traditional silicon-carbon materials (Comparative Example 3). Furthermore, Comparative Example 1 proves that interface stripping leads to a cliff-like drop in capacity, proving the necessity of chemical bonding. Comparative Example 2 shows uncontrollable crack expansion, highlighting the core role of dynamic bonds. Comparative Example 3 proves stress concentration + poor conductivity, verifying the irreplaceable nature of the multi-level structure.

[0067] Therefore, this technical solution successfully solved the three major problems of volume expansion, interface peeling and poor conductivity of silicon-based negative electrodes through the collaborative innovative design of gradient interface layer-self-healing elastic layer-multi-level carbon skeleton, fully verified the creativity, novelty and practicality of the solution, and provided technical guarantee for the industrialization of high energy density lithium-ion batteries.

[0068] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A negative electrode material, characterized in that: From inside to outside, they include: Silicon core, gradient interface layer, self-healing elastic layer and multi-level porous carbon skeleton; The silicon core is a nano silicon particle with a particle size of 50-200 nm; The gradient interface layer is coated on the surface of the silicon core, and from the inside to the outside, it is an aminosilane coupling agent modification layer and a conductive polymer layer; The self-healing elastic layer is coated on the outside of the gradient interface layer and contains a dynamic covalent bond cross-linking network; The multi-level porous carbon skeleton is coated on the outside of the self-healing elastic layer. It is a composite of biomass carbon and vapor-deposited carbon nanotubes with a porosity of 50-70%.

2. A negative electrode material according to claim 1, characterized in that: The total thickness of the gradient interface layer is 7-20 nm, wherein the aminosilane modified layer has a thickness of 2-5 nm, and the conductive polymer layer is polypyrrole with a thickness of 5-15 nm.

3. A negative electrode material according to claim 1, characterized in that: The dynamic covalent bond is a reversible cross-linking structure formed by a Diels-Alder reaction, which is composed of a polyurethane prepolymer containing a furan group and a bismaleimide cross-linking agent. The distance between the cross-linking points is 2-5 nm, and the self-healing elastic layer is 20-50 nm.

4. A negative electrode material according to claim 1, characterized in that: In the multi-level pore carbon skeleton, the biomass carbon is porous carbon formed by carbonization of sodium lignin sulfonate, and the pore size distribution is micropores of less than 2nm and mesopores of 2-50nm; the length of the vapor-deposited carbon nanotubes is 1-5μm, and the density is ≥10 4 tubes / μm².

5. A method for modifying the negative electrode material according to any one of claims 1 to 4 by coating, characterized in that: The following steps are involved: Step a: gradient interface treatment of silicon particles: a1. Hydroxylating silicon particles in a mixture of concentrated sulfuric acid and hydrogen peroxide; a2. Modifying in an ethanol solution of an aminosilane coupling agent to form an amino functional layer; a3, coating a polypyrrole layer on the silicon surface by in-situ polymerization; Step b: Self-repairing elastic layer coating: b1. Mixing a polyurethane prepolymer containing a furan group with a bismaleimide crosslinker to prepare a dynamic covalent bond elastomer through a Diels-Alder reaction; b2, spin coating the elastomer solution on the surface of the silicon particles after gradient interface treatment, and curing to form a self-healing elastic layer; Step c: Construction of multi-level porous carbon framework: c1, mixing the silicon particles coated with the elastic layer with the sodium lignin sulfonate solution, and spray drying to form a precursor; c2. Gradually increase the temperature to 800°C in an argon atmosphere for carbonization to form a porous carbon skeleton; c3. Grow a carbon nanotube network on the surface of the carbon skeleton by chemical vapor deposition.

6. The method according to claim 5, characterized in that: In the step a, the hydroxylation treatment is carried out by using a mixture of concentrated sulfuric acid and hydrogen peroxide in a volume ratio of 3:1, and ultrasonic treatment is performed for 30-60 minutes.

7. The method according to claim 5, characterized in that: In the step b, the rotation speed of the spin coating process is 2000-5000 rpm, the curing temperature is 60-80° C., and the curing time is 1-2 hours.

8. The method according to claim 5, characterized in that: In the step c, the carbon source for chemical vapor deposition is acetylene, the deposition temperature is 600-800° C., and the deposition time is 10-30 minutes.

9. An application of the negative electrode material according to any one of claims 1 to 4, characterized in that: The negative electrode material is used as the negative electrode active material of lithium-ion batteries. Under 1C charge and discharge conditions, the capacity retention rate after 500 cycles is ≥90%, the silicon volume expansion rate is ≤120%, the self-repair efficiency is ≥90%, and the 5C rate capacity is ≥2500 mAh / g.

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