A negative electrode composite material and its preparation method

Abstract

This invention provides a negative electrode composite material and its preparation method. The negative electrode composite material has a three-level gradient stable structure; it includes nitrogen-boron co-doped carbon, with nitrogen and boron co-doped chemical anchoring sites constructed on the carbon surface; amorphous or nanocrystalline red phosphorus is uniformly loaded onto the pores and surface of the nitrogen-boron co-doped carbon matrix, forming P–N, P–B, and P–C covalent bond structures; the surface of the active material loading layer is coated with a continuous and dense conductive polymer flexible layer; this invention pre-dops nitrogen and boron into the hard carbon skeleton, which serves as a precursor for the phosphorus-carbon composite, and by controlling the doping sites and contents of nitrogen and boron, stable phosphorus-nitrogen covalent bonds and phosphorus-boron bonds can be formed in situ between phosphorus and nitrogen and boron atoms on the carbon skeleton surface during the subsequent red phosphorus deposition process. This multiple chemical bonding can effectively anchor the phosphorus active material, inhibiting its shedding and aggregation during cycling, thereby significantly improving the cycling stability of the composite material while maintaining high capacity.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery materials technology, and in particular to a negative electrode composite material and its preparation method. Background Technology

[0002] Graphite, as the mainstream anode material for commercial lithium-ion batteries, has a relatively low theoretical specific capacity (only about 372 mAh / g), poor rate performance, and is difficult to apply directly to sodium-ion battery systems. To improve the electrochemical performance of graphite anodes, elemental doping is a common modification method. Among them, nitrogen (N) doping can improve the conductivity and electrolyte wettability of graphite, but its effect on capacity improvement is limited.

[0003] On the other hand, elemental red phosphorus (P) possesses an extremely high theoretical capacity (approximately 2596 mAh / g), but its intrinsic conductivity is poor, and its volume expansion during charge and discharge is enormous, easily leading to electrode structure collapse and extremely poor cycle stability. To improve the electrochemical behavior of red phosphorus, the industry commonly employs a strategy of combining it with carbon materials. This strategy mainly falls into two categories: one is to fill the pores of porous carbon (such as activated carbon, porous carbon fibers, etc.); the other is to attach red phosphorus to the surface or gaps of highly conductive carbon (such as carbon black, carbon nanotubes, etc.). Among these, the former utilizes the porous structure to effectively buffer the volume expansion of red phosphorus; therefore, phosphorus / carbon composites based on porous carbon generally exhibit better cycle stability than pure phosphorus.

[0004] However, existing phosphorus / carbon composite materials still suffer from key drawbacks: the bonding between phosphorus and carbon relies mainly on physical adsorption or weak interfacial adhesion, resulting in weak bonding and a failure to form stable and robust chemical bonds at the interface. During repeated charge-discharge cycles, the volume changes of red phosphorus continuously disrupt this weak adhesion interface, causing the active material to gradually detach from the conductive carbon framework, lose electrical contact, and ultimately lead to rapid capacity decay. Therefore, how to significantly enhance the interfacial bonding stability between phosphorus and the carbon substrate while leveraging the high capacity advantage of phosphorus has become a pressing technical challenge in this field. Summary of the Invention

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A negative electrode composite material having a three-level gradient stable structure: The first level is a chemically bonded anchoring layer: nitrogen and boron co-doped carbon. Through the co-pyrolysis of nitrogen-containing polymers and boron-containing precursors with the carbon matrix, nitrogen and boron co-doped chemical anchoring sites are constructed on the carbon surface. The second level is the active material support layer: amorphous or nanocrystalline red phosphorus is uniformly loaded on the pores and surface of the nitrogen-boron co-doped carbon matrix. By vaporizing the red phosphorus at low temperature under vacuum, it undergoes in-situ P-N, P-B, and P-C bonding reactions with the nitrogen and boron anchoring points to form an amorphous anchoring layer. The third level is a flexible buffer confinement layer: the surface of the active material loading layer is coated with a continuous and dense flexible conductive polymer layer; by forming a conductive polymer coating layer on the outer surface of the composite material, the volume expansion is buffered and the SEI film is stabilized.

[0006] The three-level gradient stable structure constructed in this invention forms a multi-synergistic protection system. The first-level chemical bonding layer solves the problem of red phosphorus shedding from the microscopic interface, the second-level active loading layer ensures the uniform utilization of active materials, and the third-level flexible coating layer buffers volume stress from the macroscopic perspective. It completely overcomes the shortcomings of traditional phosphorus-carbon composite materials, such as weak interfacial bonding, easy structural collapse, and rapid cycle decay. It has high conductivity, high structural stability, and high ion transport efficiency, and its comprehensive electrochemical performance is significantly better than that of traditional materials.

[0007] A method for preparing a negative electrode composite material includes the following steps: S1: Preparation of nitrogen-boron co-doped carbon matrix: The carbon matrix, nitrogen-containing polymer and boron-containing precursor are uniformly mixed at a weight ratio of 100:(1~20):(0.5~5), and subjected to high-temperature heat treatment under an inert atmosphere. After cooling, the mixture is ground to obtain the nitrogen-boron co-doped carbon matrix. The nitrogen-boron co-doped carbon matrix prepared in step S1 has the following core functions: First, it has a multi-layered chemical anchoring effect. Through high-temperature pyrolysis, pyridine nitrogen, pyrrole nitrogen and electron-deficient boron active sites are introduced into the carbon skeleton at the same time. In the subsequent red phosphorus deposition process, strong P-N bonds, P-B bonds and P-C bonds can be induced to form a multi-toothed chemical anchoring, which can more effectively inhibit the volume expansion and shedding of red phosphorus during the charging and discharging process. Second, the electronic structure is optimized in a synergistic manner. The co-doping of electron-rich nitrogen and electron-deficient boron can regulate the Fermi level and charge density distribution of the carbon matrix, further improving the intrinsic conductivity and providing a more efficient electron transport pathway for insulator red phosphorus. Third, it enriches defects and nucleation centers. The carbon planar lattice distortion and uneven charge distribution caused by nitrogen and boron co-doping provide high-density and highly dispersed nucleation sites for red phosphorus, promoting the highly uniform loading of red phosphorus in amorphous or nanocrystalline states.

[0008] S2: Red phosphorus vapor deposition: The nitrogen-boron co-doped carbon matrix prepared in step S1 and elemental red phosphorus are placed at both ends of a vacuum-sealed container at a weight ratio of 1:(1~2). After evacuating to a pressure <100 Pa, the container is sealed. Under an inert atmosphere, the container is heated to 400~500℃ and kept at that temperature for 10~30 hours to carry out the deposition reaction. After cooling, a phosphorus-carbon composite material with nitrogen-boron co-doped carbon matrix is ​​obtained. Explanation of the function of step S2: Leveraging the high-density nucleation sites provided by the S1 nitrogen-boron co-doped carbon matrix, phosphorus atoms in the gas phase can uniformly nucleate and grow on the surface and within the pores of the carbon matrix, ultimately forming amorphous / nanocrystalline red phosphorus. This avoids the aggregation and local accumulation of red phosphorus particles, alleviating the overall volume expansion problem during the charging and discharging process of red phosphorus from the source. The high-temperature vapor deposition environment facilitates the full contact and reaction between phosphorus atoms and the pyridine nitrogen, pyrrole nitrogen, and boron active sites on the carbon matrix, stably generating P–N, P–B, and P–C covalent bonds. Relying on the multidentate chemical anchoring effect, the red phosphorus is firmly fixed on the carbon skeleton, significantly reducing the risk of red phosphorus shedding and pulverization during cycling and ensuring the integrity of the electrode structure.

[0009] By employing a two-end partitioned vacuum phase deposition method, red phosphorus migrates and deposits in the form of gaseous molecules, which can fully penetrate into the micropores and mesopores of the carbon matrix, achieving close contact between red phosphorus and the carbon matrix throughout the entire process. This shortens the electron and ion transport path, improves interfacial impedance, and enhances the overall electrochemical kinetic performance of the material.

[0010] The vacuum + inert atmosphere environment avoids side reactions. The vacuum degree is <100 Pa and the entire process is protected by an inert atmosphere, which can isolate oxygen and water vapor, prevent red phosphorus oxidation and carbon matrix corrosion at high temperature, and avoid the introduction of impurities, ensuring the purity of the phosphorus-carbon composite system and reducing the occurrence of electrochemical side reactions.

[0011] S3. Conductive polymer coating: The phosphorus-carbon composite material obtained in step S2 is dispersed in a conductive polymer solution or dispersion, and a conductive polymer coating layer is formed on the material surface through a coating process, finally obtaining a multi-level interface-stable phosphorus-carbon composite material.

[0012] Explanation of the function of step S3: Without the coating layer, red phosphorus is directly exposed to the electrolyte. The volume expansion during charge and discharge can easily lead to repeated rupture and regeneration of the SEI film, resulting in reduced first efficiency and rapid capacity decay. The conductive polymer coating layer in this step has both flexibility and interface confinement function, which can effectively buffer volume stress and isolate electrolyte side reactions, thereby improving first efficiency and cycle life.

[0013] This invention employs a stepwise, controllable process: first, doping the matrix; then, vapor deposition of the active material; and finally, coating with a conductive polymer. Through nitrogen-boron dual-doping modification and multi-level structural synergistic design, it fundamentally solves the defects of traditional phosphorus-carbon composite materials, such as easy aggregation of red phosphorus, weak interfacial bonding, severe charge-discharge volume expansion, and poor conductivity. The process steps are clear, and the reaction conditions are controllable, enabling the stable construction of a multi-level stable structure with chemical bonding and flexible confinement, which significantly improves the electrochemical stability and conductivity of the material. At the same time, the process has strong compatibility and is suitable for large-scale preparation.

[0014] More preferably, in step S1, the carbon matrix, nitrogen-containing polymer and boron-containing precursor are preferably in a weight ratio of 100:(5-15):(2-4).

[0015] This invention, by limiting the optimal doping ratio range, can precisely control the nitrogen and boron doping amounts of the carbon matrix, avoiding the problems of uneven charge distribution and local red phosphorus agglomeration caused by excessive nitrogen doping, as well as the problems of insufficient anchoring points and poor conductivity modification caused by excessive doping. Under this ratio, the nitrogen-boron doping synergistic effect is optimal, which can build high-density and uniformly distributed active anchoring points on the carbon matrix surface, taking into account both the matrix conductivity and the uniformity of red phosphorus loading, and maximizing the improvement of material cycle life.

[0016] In a further preferred embodiment, in step S1, the high-temperature heat treatment temperature is 800–1200°C and the time is 1–4 hours; this heat treatment temperature range and holding time can fully realize uniform doping of nitrogen and boron elements, effectively construct lattice defects and chemically active sites, ensure that the bonding ability and conductivity of the carbon matrix meet the standards, and avoid problems such as insufficient doping or collapse of the carbon matrix structure.

[0017] In a further preferred embodiment, in step S3, the coating process is one of physical mixing, in-situ polymerization, or vapor deposition. Multiple coating processes are suitable for different production conditions and raw material systems, and can all form a complete coating layer, flexibly balancing preparation efficiency and coating quality.

[0018] More preferably, the conductive polymer is one or more of poly(3,4-ethylenedioxythiophene), polypyrrole, polyaniline, or polythiophene.

[0019] The conductive polymer selected in this invention has both excellent conductivity and flexible deformation capability, which can not only construct electron transport channels, but also adapt to the volume expansion and contraction of red phosphorus, and maintain the integrity of the interface structure for a long time.

[0020] More preferably, the nitrogen-containing polymer is at least one of polyacrylonitrile, polypyrrole, and polyaniline.

[0021] The nitrogen-containing polymer selected in this invention can stably generate active nitrogen sites such as pyridine nitrogen and pyrrole nitrogen after pyrolysis, which can effectively participate in chemical bonding and enhance the anchoring and binding ability of the matrix.

[0022] More preferably, the boron-containing precursor is at least one selected from boric acid, sodium borohydride, phenylboronic acid, and borate esters.

[0023] The boron-containing precursors of this invention can efficiently introduce electron-deficient boron active sites, synergistically optimize the electronic structure with nitrogen, strengthen PB bond bonding, and further stabilize the active material.

[0024] Further preferably, in step S3, the mass of the conductive polymer coating layer accounts for 3-8 wt% of the total mass of the final composite material; preferably 5 wt%. This invention limits the coating amount range, which can form a continuous, dense, and thin flexible conductive coating layer; it avoids the problem that the coating amount is too low, which leads to the inability to form a complete protective layer and effectively buffer volume expansion and isolate electrolyte side reactions. At the same time, it avoids the defects of excessive coating amount, which block material pores, hinder ion transport, and reduce material capacity and rate performance. The optimal 5 wt% coating amount can simultaneously achieve the triple effects of structural confinement, conductivity enhancement, and SEI film stabilization, significantly improving the battery's first efficiency and cycle life.

[0025] More preferably, the inert gas is one of argon, nitrogen, helium, and neon.

[0026] Inert gases can isolate oxygen and water vapor from the air, preventing the raw materials from oxidizing and deteriorating during high-temperature heat treatment and vapor deposition stages, ensuring the purity of doping and deposition reactions, and improving the yield and electrochemical performance of finished products.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention first obtains a carbon matrix with good electrical conductivity and a surface rich in nitrogen-containing functional groups and electron-deficient boron through the pyrolysis of a nitrogen-containing polymer and a boron-containing precursor. This matrix not only provides a conductive network, but the nitrogen and boron atoms on its surface also serve as highly efficient "anchoring sites," laying the foundation for the subsequent strong chemical bonding of red phosphorus.

[0028] 2. Through vapor deposition, gaseous phosphorus molecules can penetrate deep into the matrix and react with electron-deficient carbon atoms on the surface of the nitrogen-boron-doped carbon matrix to form strong PC, PB, and PN bonds. This strong chemical bonding effectively mitigates the volume expansion and stripping of red phosphorus during charge and discharge, maintaining the integrity of the electrode structure and thus greatly improving cycle life.

[0029] 3. Furthermore, the nitrogen-boron-doped carbon matrix itself possesses excellent electronic conductivity, providing a highly efficient electron transport channel for red phosphorus. Simultaneously, the defects introduced by doping and the expanded interlayer spacing also facilitate rapid ion diffusion, thereby improving the material's rate performance.

[0030] 4. Finally, the method has a clear process route, and the two key processes (pyrolysis and vapor deposition) are mature and reliable, making it easy to achieve large-scale production and showing significant industrial application prospects. Attached Figure Description

[0031] Figure 1 This is a comparison chart of lithium removal curves of coin cells prepared for embodiments and comparative examples of the present invention; Figure 2 The graphs show the cycle life test results of the button cells produced in the embodiments and comparative examples of this invention. Detailed Implementation

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Example 1

[0033] Preparation of nitrogen-boron co-doped carbon matrix: Hard carbon powder (carbon matrix), polyacrylonitrile (PAN), and boric acid (H3BO3) were mixed in a weight percentage ratio of 100:5:2. N-methylpyrrolidone (NMP) was added as a dispersion medium (the amount of NMP added was 2 to 5 times the total mass of the hard carbon powder, polyacrylonitrile, and boric acid). The mixture was ball-milled for 4 hours to obtain a uniform slurry. Subsequently, it was vacuum dried at 80°C to completely remove the solvent and obtain the precursor powder. The precursor was placed in a tube furnace and heated to 800°C at a rate of 5°C / min under an argon atmosphere, held at that temperature for 2 hours, and then naturally cooled to room temperature. After grinding, the nitrogen-boron co-doped carbon matrix was obtained, denoted as NBC-5.

[0034] During pyrolysis, boric acid decomposes, providing boron atoms to be incorporated into the carbon framework. These boron atoms, together with nitrogen atoms introduced during PAN pyrolysis, form N and B co-doped active sites. The hole-type doping of boron and the electronic doping of nitrogen produce a synergistic effect, further optimizing the charge distribution and defect structure on the carbon surface.

[0035] Vapor-phase deposition of red phosphorus: NBC-5 and commercial red phosphorus powder were placed at both ends of a vacuum-sealed tube at a weight ratio of 1:1.5. The tube was then evacuated to a system pressure <100 Pa and sealed. The tube was placed in a dual-temperature zone tube furnace: the red phosphorus end was heated to 400°C, and the NBC-5 substrate end was heated to 450°C, and the temperature was maintained for 10 hours. At 400°C, the red phosphorus sublimated into a gaseous state and migrated to the surface and pores of the NBC-5 at 450°C for deposition. After the reaction was completed and the mixture was allowed to cool naturally, the red phosphorus-supported nitrogen-boron co-doped carbon composite material, denoted as RP@NBC-5, was obtained.

[0036] Polyaniline coating: Take 1.0 g of the prepared RP@NBC-5 powder, add 0.05 g of hydrochloric acid-doped polyaniline (PANI, conductivity ≥10 S / cm) and 10 mL of NMP, and ultrasonically disperse for 15 minutes to fully dissolve the PANI and mix it evenly with the powder. Place the mixture in an 80℃ vacuum drying oven and dry under reduced pressure (reduce the absolute pressure in the vacuum drying oven and maintain it stably within the range of 1 kPa to 10 kPa) for 6 hours until the solvent is completely evaporated. Remove and grind to obtain the polyaniline-coated composite material, denoted as PANI@RP@NBC-5. The polyaniline coating amount accounts for approximately 5 wt% of the total mass.

[0037] Assembling lithium-ion batteries: 0.1 g of PANI@RP@NBC-5 and 1.9 g of graphite were mixed evenly to form a composite negative electrode active material. The slurry was prepared by adding deionized water to adjust the solid content to 30–60 wt% according to a mass ratio of 80% active material, 10% conductive carbon black, and 10% sodium polyacrylate (PAANa) binder. The mixture was then evenly coated onto a copper foil current collector and vacuum dried at 90°C for 5 hours to form the secondary battery anode electrode. The secondary battery was then assembled using this electrode. A lithium metal sheet was used as the counter electrode, PP / PE / PP as the separator, and the electrolyte was 1.0 M LiPF6 in EC:DMC = 47:50 Vol%, 3% FEC. CR2032 coin cells were assembled in an argon-filled glove box. Charge-discharge capacity tests and the first coulombic efficiency test were performed on the coin cells within a voltage range of 0.01 V–2.0 V at a charge-discharge rate of 0.05 C / 0.05 C (see [link to relevant documentation]). Figure 1 Cycle life tests were performed on the coin cells according to the 1C / 1C charge / discharge standard (see...). Figure 2 ). Example 2

[0038] Preparation of nitrogen-boron co-doped carbon matrix: Hard carbon powder (carbon matrix), polyacrylonitrile (PAN), and boric acid (H3BO3) were mixed in a weight percentage ratio of 100:10:2. N-methylpyrrolidone (NMP) was added as a dispersion medium (the amount of NMP added was 2 to 5 times the total mass of the hard carbon powder, polyacrylonitrile, and boric acid). The mixture was ball-milled for 4 hours to obtain a uniform slurry. Subsequently, it was vacuum dried at 80°C to completely remove the solvent and obtain the precursor powder. The precursor was placed in a tube furnace and heated to 800°C at a rate of 5°C / min under an argon atmosphere, held at that temperature for 2 hours, and then naturally cooled to room temperature. After grinding, the nitrogen-boron co-doped carbon matrix was obtained, denoted as NBC-10.

[0039] Vapor-phase deposition of red phosphorus: NBC-10 and commercial red phosphorus powder were placed at both ends of a vacuum-sealed tube at a weight ratio of 1:1.5. The tube was then evacuated to a system pressure <100 Pa and sealed. The tube was placed in a dual-temperature zone tube furnace: the red phosphorus end was heated to 400°C, and the NBC-10 substrate end was heated to 450°C, and the temperature was maintained for 10 hours. At 400°C, the red phosphorus sublimated into a gaseous state and migrated to the surface and pores of the NBC-10 at 450°C for deposition. After the reaction was completed and the mixture was allowed to cool naturally, the red phosphorus-supported nitrogen-boron co-doped carbon composite material, denoted as RP@NBC-10, was obtained.

[0040] Polyaniline coating: Take 1.0 g of the prepared RP@NBC-10 powder, add 0.05 g of hydrochloric acid-doped polyaniline (PANI, conductivity ≥10 S / cm) and 10 mL of NMP, and ultrasonically disperse for 15 minutes to fully dissolve the PANI and mix it evenly with the powder. Place the mixture in an 80℃ vacuum drying oven and dry under reduced pressure (reduce the absolute pressure in the vacuum drying oven and maintain it stably within the range of 1 kPa to 10 kPa) for 6 hours until the solvent is completely evaporated. Remove and grind to obtain the polyaniline-coated composite material, denoted as PANI@RP@NBC-10. The polyaniline coating amount accounts for approximately 5 wt% of the total mass.

[0041] Assembly and testing of lithium-ion batteries: consistent with the method in Example 1. Example 3

[0042] Preparation of nitrogen-boron co-doped carbon matrix: Hard carbon powder (carbon matrix), polyacrylonitrile (PAN), and boric acid (H3BO3) were mixed in a weight percentage ratio of 100:15:2. N-methylpyrrolidone (NMP) was added as a dispersion medium (the amount of NMP added was 2 to 5 times the total mass of the hard carbon powder, polyacrylonitrile, and boric acid). The mixture was ball-milled for 4 hours to obtain a uniform slurry. Subsequently, it was vacuum dried at 80°C to completely remove the solvent and obtain the precursor powder. The precursor was placed in a tube furnace and heated to 800°C at a rate of 5°C / min under an argon atmosphere, held at that temperature for 2 hours, and then naturally cooled to room temperature. After grinding, the nitrogen-boron co-doped carbon matrix was obtained, denoted as NBC-15.

[0043] Vapor-phase deposition of red phosphorus: NBC-15 and commercial red phosphorus powder were placed at both ends of a vacuum-sealed tube at a weight ratio of 1:1.5. The tube was then evacuated to a system pressure <100 Pa and sealed. The tube was placed in a dual-temperature zone tube furnace: the red phosphorus end was heated to 400°C, and the NBC-10 substrate end was heated to 450°C, and the temperature was maintained for 10 hours. At 400°C, the red phosphorus sublimated into a gaseous state and migrated to the surface and pores of the NBC-10 at 450°C for deposition. After the reaction was completed and the mixture was allowed to cool naturally, the red phosphorus-supported nitrogen-boron co-doped carbon composite material, denoted as RP@NBC-15, was obtained.

[0044] Polyaniline coating: Take 1.0 g of the prepared RP@NBC-15 powder, add 0.05 g of hydrochloric acid-doped polyaniline (PANI, conductivity ≥10 S / cm) and 10 mL of NMP, and ultrasonically disperse for 15 minutes to fully dissolve the PANI and mix it evenly with the powder. Place the mixture in an 80℃ vacuum drying oven and dry under reduced pressure (reduce the absolute pressure in the vacuum drying oven and maintain it stably within the range of 1 kPa to 10 kPa) for 6 hours until the solvent is completely evaporated. Remove and grind to obtain the polyaniline-coated composite material, denoted as PANI@RP@NBC-15. The polyaniline coating amount accounts for approximately 5 wt% of the total mass.

[0045] Assembly and testing of lithium-ion batteries: consistent with the method in Example 1.

[0046] Comparative Example 1 Unmodified carbon matrix (C) and red phosphorus powder were placed at both ends of a vacuum-sealed tube in a weight ratio of 1:1.5. The tube was then evacuated to <100 Pa and sealed. The end containing red phosphorus was placed in a 400°C heating zone, and the end containing the C matrix was placed in a 450°C heating zone, and maintained for 10 hours. During this time, the red phosphorus sublimated and migrated to the C matrix, undergoing a deposition reaction. After the reaction was complete, the mixture was cooled to room temperature and coated with PANI to obtain the final composite material (denoted as PANI@RP@C). (The red phosphorus vapor deposition method and the conductive polymer coating method are consistent with those in Example 1.) Assembly and testing of lithium-ion batteries: consistent with the method in Example 1.

[0047] from Figure 1 The test data shows that: Compared with Comparative Example 1, Examples 1-3 showed higher initial efficiency and a smaller slope in the 1.0-1.5V delithiation range, indicating that nitrogen-boron co-doping effectively reduced polarization.

[0048] from Figure 2 The test data shows that: When the nitrogen doping density is too high, the difference in bonding energies between PN, PC, and PB bonds leads to a severely uneven charge distribution. This, in turn, induces localized preferential growth and aggregation of red phosphorus during deposition. Because this aggregated red phosphorus loses its tight contact with the conductive substrate, it is prone to pulverization and detachment during cycling, resulting in accelerated capacity decay.

[0049] This invention pre-dops nitrogen (N) and boron (B) into a hard carbon framework, which serves as a precursor for phosphorus-carbon composites. By controlling the doping sites and contents of nitrogen and boron, stable phosphorus-nitrogen (PN) covalent bonds and phosphorus-boron (PB) bonds can be formed in situ between phosphorus and nitrogen and boron atoms on the carbon framework surface during subsequent red phosphorus deposition. This multiple chemical bonding effectively anchors the phosphorus-active material, inhibiting its shedding and aggregation during cycling, thereby significantly improving the cycling stability of the composite material while maintaining high capacity.

Claims

1. A negative electrode composite material, characterized in that, The negative electrode composite material has a three-level gradient stable structure: The first level is a chemically bonded anchoring layer: nitrogen and boron co-doped carbon, with nitrogen and boron co-doped chemical anchoring points constructed on the carbon surface; The second level is the active material support layer: amorphous or nanocrystalline red phosphorus is uniformly loaded onto the pores and surface of the nitrogen-boron co-doped carbon matrix to form P–N, P–B, and P–C covalent bonded structures. The third level is a flexible buffer confinement layer: the surface of the active material loading layer is covered with a continuous and dense flexible conductive polymer layer.

2. A method for preparing the negative electrode composite material according to claim 1, characterized in that, Includes the following steps: S1: Preparation of nitrogen-boron co-doped carbon matrix: The carbon matrix, nitrogen-containing polymer and boron-containing precursor are uniformly mixed at a weight ratio of 100:(1~20):(0.5~5), and subjected to high-temperature heat treatment under an inert atmosphere. After cooling, the mixture is ground to obtain the nitrogen-boron co-doped carbon matrix. S2: Red phosphorus vapor deposition: The nitrogen-boron co-doped carbon matrix prepared in step S1 and elemental red phosphorus are placed at both ends of a vacuum-sealed container at a weight ratio of 1:(1~2). After evacuating to a pressure <100 Pa, the container is sealed. Under an inert atmosphere, the container is heated to 400~500℃ and kept at that temperature for 10~30 hours to carry out the deposition reaction. After cooling, a phosphorus-carbon composite material with nitrogen-boron co-doped carbon matrix is ​​obtained. S3. Conductive polymer coating: The phosphorus-carbon composite material obtained in step S2 is dispersed in a conductive polymer solution or dispersion, and a conductive polymer coating layer is formed on the material surface through a coating process, finally obtaining a multi-level interface-stable phosphorus-carbon composite material.

3. The method for preparing a negative electrode composite material according to claim 2, characterized in that, In step S1, the high-temperature heat treatment temperature is 800-1200℃ and the time is 1-4 hours.

4. The method for preparing a negative electrode composite material according to claim 2, characterized in that, In step S3, the coating process is one of physical mixing, in-situ polymerization, or vapor deposition.

5. The method for preparing a negative electrode composite material according to claim 2, characterized in that, The conductive polymer is one or more of poly(3,4-ethylenedioxythiophene), polypyrrole, polyaniline, or polythiophene.

6. The method for preparing a negative electrode composite material according to claim 2, characterized in that, The nitrogen-containing polymer is at least one of polyacrylonitrile, polypyrrole, and polyaniline.

7. The method for preparing a negative electrode composite material according to claim 2, characterized in that, The boron-containing precursor is at least one of boric acid, sodium borohydride, phenylboronic acid, and borate ester.

8. The method for preparing a negative electrode composite material according to claim 2, characterized in that, In step S3, the mass of the conductive polymer coating layer accounts for 3 to 8 wt% of the total mass of the final composite material.

9. The method for preparing a negative electrode composite material according to claim 2, characterized in that, The inert gas is one of argon, nitrogen, helium, or neon.

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