Boron and phosphorus co-doped silicon-carbon composite material, preparation method thereof, negative electrode material and lithium ion battery

Abstract

The invention provides a boron and phosphorus co-doped silicon-carbon composite material and a preparation method thereof, a negative electrode material and a lithium ion battery. The negative electrode material comprises a porous carbon matrix, boron and phosphorus co-doped nano silicon particles located in a porous structure of porous carbon, and a carbon layer modified on the surface of the porous carbon, and further comprises a solid electrolyte layer modified on the outer side of the carbon layer. According to the invention, boron and phosphorus elements are doped in the nano silicon particles to form amorphous nano silicon particles, and the doping of boron and phosphorus elements can effectively improve the conductivity of the nano silicon particles; the design of the porous carbon structure can provide a certain buffer space for the expansion of the nano silicon in the lithium intercalation process, and meanwhile, the modification of the solid electrolyte layer on the surface of the porous carbon provides a coating layer with high elastic modulus for the material, so that a buffer effect is achieved to avoid the fragmentation of the porous carbon material caused by the expansion of part of nano silicon particles, and the performance of the material is improved. And the effect of recovering broken particles can also be achieved.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion batteries, and specifically relates to a boron- and phosphorus-co-doped silicon-carbon composite material, a preparation method thereof, a negative electrode material, and a lithium-ion battery. Background Art

[0002] Lithium-ion battery is a secondary battery or rechargeable battery, which mainly relies on the reciprocating movement of lithium ions between the positive and negative electrodes to work. + Insertion and deinsertion back and forth between the two electrodes: When charging, Li + During discharge, the opposite occurs. Lithium-ion batteries, due to their environmental compatibility, long cycle life, and low self-discharge rates, have become the most commonly used energy storage device, widely used in portable devices and electric vehicles.

[0003] Among them, Si-based materials have the highest theoretical specific capacity among the materials studied so far, and the alloys formed by them are Li x For Si, where x ranges from 0 to 4.4, the theoretical specific capacity of pure silicon is 4200 mAh / g. Silicon's voltage platform is slightly higher than that of graphite, making it less susceptible to surface lithium deposition during charging, offering safety advantages over graphite electrodes. Furthermore, silicon is one of the most abundant elements in the Earth's crust, making it widely available and inexpensive. Furthermore, unlike graphite, silicon alloys exhibit a solvation effect. Their low lithium insertion potential, low atomic weight, high energy density, and high lithium mole fraction in Li-Si alloys have drawn considerable attention for their greater stability compared to other metals and materials. Consequently, silicon is considered the most likely anode material to replace graphite in the near term. Consequently, the development of silicon-based anodes is highly attractive.

[0004] However, silicon negative electrode materials have been slow to achieve large-scale commercial applications. While possessing many advantages, silicon anode materials also have several disadvantages. First, silicon anode materials undergo a volume change of up to 300% or more during the charge and discharge process. Such high volume expansion and contraction can easily cause the electrode material to shatter and lose contact with the current collector and the electrode conductive network. At the same time, the volume change brings about the generation of a new surface, requiring the formation of a new solid-electrolyte interface (SEI), which leads to a large consumption of electrolyte and a significant reduction in cycle life. On the other hand, the electrical conductivity and lithium ion diffusion rate of silicon are lower than those of graphite, which will limit the performance of silicon under high current and high power conditions. Summary of the Invention

[0005] The present invention aims to address the problem of delamination between the silicon-based anode material and the current collector, as well as between the material and the conductive network, caused by the expansion of the material during charge and discharge. It also aims to overcome the rupture of the SEI film caused by material expansion and the reduction in material capacity caused by irreversible lithium ion consumption.

[0006] The present invention provides a boron and phosphorus co-doped silicon-carbon composite material.

[0007] The boron and phosphorus co-doped silicon-carbon composite material provided by the present invention comprises: a porous carbon matrix, boron and phosphorus co-doped nano-silicon particles located in the pore structure of the porous carbon, and a carbon layer modified on the surface of the porous carbon.

[0008] Furthermore, the boron and phosphorus co-doped silicon-carbon composite material further includes a solid electrolyte layer, and the solid electrolyte layer is modified on the outside of the carbon layer.

[0009] The present invention dopes boron and phosphorus elements inside the nano-silicon particles to form amorphous nano-silicon particles. At the same time, the doping of boron and phosphorus elements can effectively improve the conductive properties of the nano-silicon particles. The design of the porous carbon structure can also provide a certain buffer space for the expansion of nano-silicon during the lithium insertion process. At the same time, the modification of the solid electrolyte layer on the surface of the porous carbon provides the material with a coating layer with a high elastic modulus, which plays a buffering role to avoid the fragmentation of the porous carbon material caused by the expansion of some nano-silicon particles, and can also play a role in restoring broken particles. The present invention also provides a method for preparing the boron and phosphorus co-doped silicon-carbon composite material.

[0010] The method for preparing a boron-phosphorus co-doped silicon-carbon composite material provided by the present invention comprises the following steps: 1) Boron- and phosphorus-co-doped nano-silicon particles A are prepared on a substrate surface by radio frequency plasma enhanced chemical vapor deposition (RFPED) using silane, borane, and phosphine as raw materials in an inert atmosphere. 2) coating the obtained nano-silicon particles A with a pore-forming agent to obtain pore-forming agent-coated nano-silicon particles B; 3) Using the obtained nano-silicon particles B, phenol and aldehyde monomers as raw materials, spherical phenolic resin-coated composite particles C are prepared by a hydrothermal method; 4) carbonizing the obtained composite particles C, and washing and drying the carbonized composite material to remove excess impurity atoms, thereby obtaining a spherical silicon-carbon material D; 5) In-situ carbon coating is performed on the spherical silicon-carbon material D to obtain a boron- and phosphorus-coated silicon-carbon composite material.

[0011] The operation of step 1) of the above method is as follows: a cleaned single crystal silicon and a high-melting quartz substrate are placed in a reaction chamber of a radio frequency plasma-enhanced chemical vapor deposition system; after the chamber is evacuated, an inert gas is introduced; silane, borane, and phosphine diluted with high-purity hydrogen are used as raw materials; low-temperature plasma generated by glow discharge is used as a heat source to decompose, dehydrogenate, and condense the silane, borane, and phosphine, thereby obtaining boron- and phosphorus-co-doped nano-silicon particles A on the substrate surface; During the deposition process, the RF power is between 20W and 100W, and the deposition temperature is controlled between 250°C and 700°C; During the deposition process, the concentration of diluted silane gas is 50%-80%, and the concentration of borane and phosphine gas is 1%-5%; The deposition time is controlled within 2h-10h; The doped boron and phosphorus elements account for 2wt‰-2wt% of the mass of the boron and phosphorus co-doped nano-silicon particles A; The average particle size D of the boron and phosphorus co-doped nano-silicon particles A 50 At 10-20nm, D max ≤50nm.

[0012] The operation of step 2) of the above method is: dispersing the pore-forming agent and the nano-silicon particles A in an organic solvent by ultrasonication, and then drying the particles by spray drying; The pore-forming agent is a mixture of one or more of magnesium carbonate, iron carbonate, and ammonium carbonate; The mass ratio of the pore-forming agent to the nano-silicon particles A is 2:3-7; The thickness of the pore-forming agent coating layer is 20-50 nm; The operation of step 3) of the above method is: ultrasonically dispersing the obtained nano-silicon particles B and phenol and aldehyde monomers in an organic solvent, and hydrothermally reacting them to obtain spherical composite particles C coated with phenolic resin; The mass ratio of the phenol monomer, aldehyde monomer and nano-silicon particles B can be 4-10:3-7:2 respectively; The phenol monomer is selected from at least one of phenol, cresol, and resorcinol; The aldehyde monomer is selected from at least one of formaldehyde, acetaldehyde, and paraformaldehyde; The temperature of the hydrothermal reaction can be 100-200°C, and the time can be 2-7 hours; The average particle size of the spherical phenolic resin coated composite particles C is D 50 3-10μm, Dmax≤30μm.

[0013] In step 4) of the above method, the carbonization adopts a dynamic carbonization process, the carbonization temperature is 600° C.-800° C., and the carbonization time can be 2-7 hours; The carbonization is carried out in an inert atmosphere at a heating rate of 2-10°C / min; Acid washing is performed, wherein the acid is one or more of hydrochloric acid, acetic acid, and trifluoroacetic acid. The washing can be performed multiple times, and after acid washing, the product is dried and then washed with deionized water to remove excess acidic substances, ensuring that the product pH is between 7 and 10; The drying is low-temperature drying, and the drying temperature can be 200°C-400°C; In step 5) of the above method, a liquid phase method is used for in-situ carbon coating, and the carbon source is pitch, glucose, polyaniline, polypyrrole, polythiophene, polyacrylonitrile, and mixtures or copolymers thereof; In-situ carbon coating is performed using a gas phase method, and the carbon source is at least one of acetylene, methane, ethane, propane, ethylene, and propylene; The carbon coating accounts for 3%-10% of the final product mass.

[0014] The above method further includes, after step 5), coating the obtained carbon layer-coated composite material with a solid electrolyte to obtain a boron-phosphorus co-doped silicon-carbon composite material. The specific operation is: mixing and dispersing the obtained carbon layer-coated composite material and the solid electrolyte under the protection of an inert atmosphere, and placing the mixed product in a quartz tube. After the inert atmosphere is introduced to remove the air in the tube, low-temperature sintering is performed, and the obtained product is ground and sieved to obtain the final product, that is, a boron-phosphorus co-doped silicon-carbon composite material.

[0015] The mixing and dispersing method adopts one of 3D, VC or high mixing machine methods; The solid electrolyte is one or more of PAN (polyacrylonitrile), PI (polyimide), PMMA (polymethyl methacrylate), PPO (polypropylene oxide), and PVDC (polyvinylidene chloride); The solid electrolyte accounts for 0.5%-5% of the mass of the final product; The temperature of the low-temperature sintering is 200-500°C, and the time can be 2-5 hours; The average particle size D of the obtained boron and phosphorus co-doped silicon-carbon composite material 50 3-10μm, Dmax≤50μm.

[0016] The present invention also provides a negative electrode material, which contains the boron and phosphorus co-doped silicon-carbon composite material.

[0017] The present invention also provides a lithium ion battery.

[0018] The negative electrode of the lithium-ion battery is made from the above-mentioned negative electrode material. Lithium-ion batteries using this boron-phosphorus co-doped silicon-carbon composite material as the negative electrode material can effectively alleviate the problem of traditional silicon-carbon materials in lithium-ion batteries, which can lead to material particle fragmentation due to their own expansion effect, continuously consume the battery's electrolyte to form new interfaces, and affect the material's rate and cycling performance. This is of great significance to the material's electrical performance.

[0019] The advantages of the present invention are: The present invention designs and produces a silicon-based negative electrode material and lithium-ion battery with excellent electrochemical performance. Boron is doped into nano-silicon particles in a silicon-carbon composite material, changing the crystal order within the nano-silicon particles to form amorphous particles while also improving their electrical conductivity. Since the nano-silicon particles are coated with a pore-forming agent, a buffer space of a certain volume is formed around the particles after being coated with the carbon matrix. Finally, the particles are modified with a solid electrolyte and a carbon layer to form a coating with a high elastic modulus, creating a CP-coated negative electrode. During the charge and discharge process, the CP layer can induce the formation of a CP-integrated re-ad-SEI. The CP combines electrical conductivity with the elasticity of a polymer. The unique CP-integrated structure imparts excellent elasticity to the re-ad-SEI. The dipole-dipole interaction between the CP and the FEC in the electrolyte induces the generation of a large amount of uniformly distributed LiF with a high elastic modulus within the re-ad-SEI. Together, these two interactions ensure the structural strength and integrity of the SEI and the particles within it. At the same time, the CP's integrated re-ad-SEI structure and its π-π interaction with the internal particle carbon layer ensures continued high adhesion between the SEI and the active particles and efficient lithium ion transport, further alleviating the problem of material expansion and significantly improving the cycle and rate performance of lithium-ion batteries using silicon anode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of the silicon-based negative electrode material of the present invention.

[0021] Figure 2 This is a comparison chart of the charge and discharge curves of a lithium-ion battery assembled with the silicon-based negative electrode material prepared in Example 1 of the present invention and a lithium-ion battery assembled with the commercially available silicon-based negative electrode material in Comparative Example 2.

[0022] Figure 3 This is a full electrical cycle diagram of the silicon-based negative electrode material prepared in Example 1 of the present invention and the traditional silicon-carbon negative electrode material on the market in Comparative Example 2.

[0023] Figure 4(a, b, d, e) TEM images of the active material after encapsulation by re-ad-SEI and re-ab-SEI, respectively. (c, f) EIS spectra of the SiOx@C−CP (prepared in Example 1) and SiOx@C (prepared in Example 2) anodes after different cycles. (gj) and (kn) Two-dimensional modulus mapping of the re-ad-SEI and re-ab-SEI at different depths ranging from 5 to 20 nm. DETAILED DESCRIPTION

[0024] The present invention will be further described in detail below in conjunction with specific embodiments. The examples provided are only for illustrating the present invention and are not intended to limit the scope of the present invention. The examples provided below can serve as a guide for further improvements by those skilled in the art and are not intended to limit the present invention in any way.

[0025] Unless otherwise specified, the experimental methods in the following examples are conventional methods and were performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials and reagents used in the following examples, unless otherwise specified, were all commercially available.

[0026] Example 1. Preparation of Boron-Phosphorus Co-doped Silicon-Carbon Composite Material The cleaned single crystal silicon and high melting quartz substrate were placed in the reaction chamber of the radio frequency plasma enhanced chemical vapor deposition system. After the chamber was evacuated, high-purity argon gas was introduced. The reaction chamber temperature was controlled at 250°C, the radio frequency power was adjusted to 50W, and silane (SiH4) (concentration of 50%) was introduced to deposit nano-silicon. The silane gas flow rate was controlled at 5 mL / min, and the deposition time was 5 hours. While depositing nano-silicon, phosphine (PH3) and borane (B2H6) diluted with hydrogen to a concentration of 1% were introduced as doping gases to dope the nano-silicon, obtaining boron-phosphorus co-doped D 50 At 20 nm nano-silicon particles A (D max is 40nm); The obtained nano-silicon particles A and the pore-forming agent magnesium carbonate were ultrasonically dispersed in an ethanol solution at a mass ratio of 5:2 for 10 minutes, and then spray-dried at 350°C to prepare nano-silicon particles B coated with the pore-forming agent (the thickness of the pore-forming agent coating layer was 10 nm). The obtained nano-silicon particles B were ultrasonically dispersed with phenol (phenol) and aldehyde monomer (formaldehyde) in a solvent (a mixed solvent of water and ethanol, pure water:ethanol = 1:2) at a mass ratio of 2:10:7. Spherical phenolic resin-coated composite particles C were prepared by a hydrothermal method (reaction temperature at 150°C, reaction time for 7 hours, and the reaction pH was controlled at around 8). The particle size D50 was about 5 μm and Dmax was about 15 μm. The obtained composite particles C were placed in a carbonization furnace, and high-purity argon gas was introduced to exclude air. The carbonization was carried out at 800°C for 2 hours at a heating rate of 5°C / min to obtain a spherical silicon-carbon composite material. The spherical silicon-carbon composite material obtained above was added to a 1 mol / L hydrochloric acid solution, stirred and washed at a material-liquid mass ratio of 1:4, and dried by centrifugation and drying at 150°C. The obtained material was then washed with water at a water-to-material ratio of 1:3 to remove excess impurity atoms in the material, and the pH of the material was ensured to be around 8. The material was placed in a box furnace and dried at a low temperature of 300°C, and high-purity argon gas was introduced for atmosphere protection; The obtained material was in-situ coated with a low-temperature carbon layer by CVD using acetylene as a carbon source at a coating temperature of 600°C and high-purity argon gas was introduced for atmosphere protection. The coated carbon layer accounted for 5% of the final material weight. The carbon-coated spherical silicon-carbon material obtained above was mixed with the solid electrolyte PAN at a mass ratio of 100:5 at a speed of 1500 r / min. High-purity argon was introduced into the mixing process for atmosphere protection, and cooling water was passed through to ensure that the material temperature did not exceed 50°C during the mixing process. The resulting mixture was placed in a quartz tube and sintered at 300°C for 2 hours under high-purity argon gas to obtain the final product, a boron- and phosphorus-doped silicon-carbon composite material (sample 1).

[0027] The boron and phosphorus-doped silicon-carbon composite material prepared above was mixed with commercially available graphite material at a gram capacity of 600 mAh / g, and then slurried with sodium carboxymethyl cellulose (CMC) and polyacrylic acid (PAA) in a mass ratio of 95:3:2. The mixture was then coated, rolled, and slit to form a lithium-ion battery.

[0028] Among them, the positive electrode sheet slurry is Umicore ternary material (WXO4Mg), conductive agent SP, and binder is PVDF in a ratio of 97:2:1; the electrolyte composition used is EC:DMC:EMC= 3:4:3, lithium salt: 1.0 M LiPF6+0.2 M LiFSI, additives: 10% FEC (silicon negative electrode film formation), 1% VC (SEI strengthening), 1% DTD (suppressing gas production), 1% PS (positive electrode protection).

[0029] Example 2: Preparation of Boron- and Phosphorus-Co-Doped Silicon-Carbon Composite Material The cleaned single crystal silicon and high melting quartz substrate were placed in the reaction chamber of the radio frequency plasma enhanced chemical vapor deposition system. After the chamber was evacuated, high-purity argon gas was introduced. The reaction chamber temperature was controlled at 250°C, the radio frequency power was adjusted to 50W, and silane (SiH4) (concentration of 50%) was introduced to deposit nano-silicon. The silane gas flow rate was controlled at 5 mL / min, and the deposition time was 5 hours. While depositing nano-silicon, phosphine (PH3) and borane (B2H6) diluted with hydrogen to a concentration of 1% were introduced as doping gases to dope the nano-silicon, obtaining boron-phosphorus co-doped D 50 At 20 nm nano-silicon particles A (D max is 40nm); The obtained nano-silicon particles A and the pore-forming agent magnesium carbonate were ultrasonically dispersed in an ethanol solution at a mass ratio of 5:2 for 10 minutes, and then spray-dried at 350°C to prepare nano-silicon particles B coated with the pore-forming agent (the thickness of the pore-forming agent coating layer was 10 nm). The obtained nano-silicon particles B were ultrasonically dispersed with phenol (phenol) and aldehyde monomer (formaldehyde) in a solvent (water and ethanol mixed solvent, pure water: ethanol = 1:2) at a mass ratio of 2:10:7. Spherical composite particles C coated with phenolic resin were prepared by a hydrothermal method (reaction temperature at 150 ° C, reaction time 7 h, and the reaction pH was controlled at about 8). The particle size D 50 About 5μm, D max About 15 μm; The obtained composite particles C were placed in a carbonization furnace, and high-purity argon gas was introduced to exclude air. The carbonization was carried out at 800°C for 2 hours at a heating rate of 5°C / min to obtain a spherical silicon-carbon composite material. The spherical silicon-carbon composite material obtained above was added to a 1 mol / L hydrochloric acid solution, stirred and washed at a material-liquid mass ratio of 1:4, and dried by centrifugation and drying at 150°C. The obtained material was then washed with water at a water-to-material ratio of 1:3 to remove excess impurity atoms in the material, and the pH of the material was ensured to be around 8. The material was placed in a box furnace and dried at a low temperature of 300°C, and high-purity argon gas was introduced for atmosphere protection; The above-obtained material was in-situ coated with a low-temperature carbon layer using acetylene as a carbon source by CVD. The coating temperature was 600°C, and high-purity argon was introduced for atmosphere protection. The coating carbon layer accounted for 5% of the final material mass, and the average particle size was obtained. 50 7μm, D max 20μm boron and phosphorus co-doped silicon-carbon composite material (sample 2).

[0030] The boron and phosphorus-doped silicon-carbon composite material prepared above was mixed with commercially available graphite material at a gram capacity of 600 mAh / g, and then slurried with sodium carboxymethyl cellulose (CMC) and polyacrylic acid (PAA) in a mass ratio of 95:3:2. The mixture was then coated, rolled, and slit to form a lithium-ion battery.

[0031] Among them, the positive electrode sheet slurry is Umicore ternary material (WXO4Mg), conductive agent SP, and binder is PVDF in a ratio of 97:2:1; the electrolyte composition used is solvent: EC:DMC:EMC= 3:4:3, lithium salt: 1.0 M LiPF6+ 0.2 M LiFSI, additives: 10% FEC (silicon negative electrode film formation), 1% VC (SEI strengthening), 1% DTD (suppressing gas production), 1% PS (positive electrode protection).

[0032] Comparative Example 1: Preparation of silicon-carbon composite material The cleaned single crystal silicon and high melting quartz substrate were placed in the reaction chamber of the radio frequency plasma enhanced chemical vapor deposition system. After the chamber was evacuated, high-purity argon gas was introduced. The reaction chamber temperature was controlled at 250°C, the radio frequency power was adjusted to 50W, and silane (SiH4) (concentration of 50%) was introduced to deposit nano-silicon. The silane gas flow rate was controlled at 5 mL / min, and the deposition time was 5 hours. D 50 Nano-silicon particles A at 20 nm; The obtained nano-silicon particles A and the pore-forming agent magnesium carbonate were ultrasonically dispersed in an ethanol solution at a mass ratio of 5:2 for 10 minutes, and then spray-dried at 350°C to obtain nano-silicon particles B coated with the pore-forming agent. The obtained nano-silicon particles B were ultrasonically dispersed with phenol (phenol) and aldehyde monomer (formaldehyde) in a solvent (a mixed solvent of water and ethanol, pure water:ethanol = 1:2) at a mass ratio of 2:10:7. Spherical phenolic resin-coated composite particles C were prepared by a hydrothermal method (reaction temperature at 150 ° C, reaction time 7 h, and the reaction pH was controlled at about 8). The particle size D 50 About 5μm, D max About 15 μm; The obtained composite particles C were placed in a carbonization furnace, and high-purity argon gas was introduced to exclude air. The carbonization was carried out at 800°C for 2 hours at a heating rate of 5°C / min to obtain a spherical silicon-carbon composite material. The spherical silicon-carbon composite material obtained above was added to a 1 mol / L hydrochloric acid solution, stirred and washed at a material-liquid mass ratio of 1:4, and dried by centrifugation and drying at 150°C. The obtained material was then washed with water at a water-to-material ratio of 1:3 to remove excess impurity atoms in the material, and the pH of the material was ensured to be around 8. The material was placed in a box furnace and dried at a low temperature of 300°C, and high-purity argon gas was introduced for atmosphere protection; The obtained material was in-situ coated with a low-temperature carbon layer by CVD using acetylene as a carbon source at a coating temperature of 600°C and high-purity argon gas was introduced for atmosphere protection. The coated carbon layer accounted for 5% of the final material weight. The carbon-coated spherical silicon-carbon material was mixed with PAN at a mass ratio of 100:5 at a speed of 1500 r / min. High-purity argon was introduced into the mixing process for atmosphere protection, and cooling water was passed through the mixing process to ensure that the material temperature did not exceed 50°C. The mixture obtained above was placed in a quartz tube, and high-purity argon was introduced to perform low-temperature sintering at 300° C. for 2 h to obtain the final product, a silicon-carbon composite material.

[0033] The silicon-carbon composite material prepared above was mixed with commercially available graphite material at a gram capacity of 600 mAh / g, and then slurried with sodium carboxymethyl cellulose (CMC) and polyacrylic acid (PAA) in a mass ratio of 95:3:2. The mixture was then coated, rolled, and slit to form a lithium-ion battery.

[0034] Among them, the positive electrode sheet slurry is Umicore ternary material (WXO4Mg), conductive agent SP, and binder is PVDF in a ratio of 97:2:1; the electrolyte composition used is solvent: EC:DMC:EMC= 3:4:3, lithium salt: 1.0 M LiPF6+ 0.2 M LiFSI, additives: 10% FEC (silicon negative electrode film formation), 1% VC (SEI strengthening), 1% DTD (suppressing gas production), 1% PS (positive electrode protection).

[0035] Table 1 Resistivity of materials prepared in Example 1 and Comparative Example 1

[0036] Figure 4 (a, b, d, e) show TEM images of the active material after encapsulation by re-ad-SEI and re-ab-SEI, respectively. (c, f) show EIS spectra of the SiOx@C−CP (prepared in Example 1) and SiOx@C (prepared in Example 2) anodes after different cycles. (gj) and (kn) show two-dimensional modulus mapping of the re-ad-SEI and re-ab-SEI at different depths ranging from 5 to 20 nm, respectively.

[0037] Depend on Figure 4 It can be seen that by coating the material surface with solid electrolyte PAN, a SEI film with a high elastic modulus can be induced to form on the material surface in the battery, thereby reducing the breakage of material particles caused by material expansion.

[0038] Comparative Example 2 The commercially available silicon-carbon negative electrode material (AX-10 (Lanxi Zhide Silicon Carbon Material)) and the commercially available graphite material (FC-12G (Xiangfenghua Graphite)) were mixed with a gram capacity of 600mAh / g, and then slurried with sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and conductive carbon black (SP) in a mass ratio of 94:3:2:1. The mixture was then coated, rolled, and slit to form a lithium-ion battery.

[0039] Among them, the positive electrode sheet slurry is Umicore ternary material (WXO4Mg), conductive agent SP, and binder is PVDF in a ratio of 97:2:1; the electrolyte composition used is solvent: EC:DMC:EMC= 3:4:3, lithium salt: 1.0 M LiPF6+ 0.2 M LiFSI, additives: 10% FEC (silicon negative electrode film formation), 1% VC (SEI strengthening), 1% DTD (suppressing gas production), 1% PS (positive electrode protection).

[0040] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that further improvements can be made to the present invention. In short, according to the principles of the present invention, this application is intended to include any changes, uses or improvements to the present invention, including changes that depart from the disclosed scope in this application and are made using conventional techniques known in the art.

Claims

1. A boron- and phosphorus-co-doped silicon-carbon composite material, comprising a porous carbon matrix, boron- and phosphorus-co-doped nano-silicon particles located in the pore structure of the porous carbon, and a carbon layer modified on the surface of the porous carbon.

2. The boron and phosphorus co-doped silicon-carbon composite material according to claim 1, characterized in that: The silicon-carbon composite material further includes a solid electrolyte layer, and the solid electrolyte layer is modified on the outside of the carbon layer.

3. A method for preparing the boron and phosphorus co-doped silicon-carbon composite material according to claim 1, comprising the following steps: 1) Boron- and phosphorus-co-doped nano-silicon particles A are prepared on a substrate surface by radio frequency plasma enhanced chemical vapor deposition (RFPED) using silane, borane, and phosphine as raw materials in an inert atmosphere. 2) coating the obtained nano-silicon particles A with a pore-forming agent to obtain pore-forming agent-coated nano-silicon particles B; 3) Using the obtained nano-silicon particles B, phenol and aldehyde monomers as raw materials, spherical phenolic resin-coated composite particles C are prepared by a hydrothermal method; 4) carbonizing the obtained composite particles C, and washing and drying the carbonized composite material to remove excess impurity atoms, thereby obtaining a spherical silicon-carbon material D; 5) In-situ carbon coating is performed on the spherical silicon-carbon material D to obtain a boron- and phosphorus-coated silicon-carbon composite material.

4. The method according to claim 3, characterized in that The operation of step 1) is as follows: single crystal silicon and a high-melting quartz substrate are placed in a reaction chamber of a radio frequency plasma enhanced chemical vapor deposition system. After the chamber is evacuated, an inert gas is introduced. Silane, borane, and phosphine diluted with high-purity hydrogen are used as raw materials. Low-temperature plasma generated by glow discharge is used as a heat source to decompose, dehydrogenate, and condense the silane, borane, and phosphine, thereby obtaining boron- and phosphorus-co-doped nano-silicon particles A on the surface of the substrate. During the deposition process, the radio frequency power is between 20W and 100W, and the deposition temperature is controlled between 250°C and 700°C. During the deposition process, the concentration of diluted silane gas is 50%-80%, and the concentration of borane and phosphine gas is 1%-5%; The deposition time is controlled within 2h-10h; The doped boron and phosphorus elements account for 2wt‰-2wt% of the mass of the boron and phosphorus co-doped nano-silicon particles A; The average particle size D of the boron and phosphorus co-doped nano-silicon particles A 50 At 10-20nm, D max ≤50nm.

5. The method according to claim 3, characterized in that The operation of step 2) is: dispersing the pore-forming agent and nano-silicon particles A in an organic solvent by ultrasonication, and then drying the particles by spray drying; The pore-forming agent is a mixture of one or more of magnesium carbonate, iron carbonate, and ammonium carbonate; The mass ratio of the pore-forming agent to the nano-silicon particles A is 3-7:2; The thickness of the pore-forming agent coating layer is 20-50 nm.

6. The method according to claim 3, characterized in that The operation of step 3) is: ultrasonically dispersing the obtained nano-silicon particles B and phenol and aldehyde monomers in an organic solvent, and hydrothermally reacting them to obtain spherical composite particles C coated with phenolic resin; The mass ratios of the phenol monomer, aldehyde monomer and nano-silicon particles B are 4-10:3-7:2 respectively; The phenol monomer is selected from at least one of phenol, cresol, and resorcinol; The aldehyde monomer is selected from at least one of formaldehyde, acetaldehyde, and paraformaldehyde; The temperature of the hydrothermal reaction is 100-200°C and the time is 2-7 hours; The average particle size of the spherical phenolic resin coated composite particles C is D 50 3-10μm, Dmax≤30μm.

7. The method according to claim 3, characterized in that In step 4), the carbonization adopts a dynamic carbonization process, the carbonization temperature is 600° C.-800° C., and the carbonization time is 2-7 hours; The carbonization is carried out in an inert atmosphere at a heating rate of 2-10°C / min; Acid washing is performed, wherein the acid is one or more of hydrochloric acid, acetic acid, and trifluoroacetic acid. The washing is performed multiple times, and after acid washing, the product is dried and then washed with deionized water to remove excess acidic substances, ensuring that the product pH is between 7 and 10.

8. The method according to claim 3, characterized in that In step 5), in-situ carbon coating is performed by a liquid phase method, and the carbon source is asphalt, glucose, polyaniline, polypyrrole, polythiophene, polyacrylonitrile, and mixtures or copolymers thereof; In-situ carbon coating is performed using a gas phase method, and the carbon source is at least one of acetylene, methane, ethane, propane, ethylene, and propylene; The carbon coating accounts for 3%-10% of the final product mass; or The method further comprises, after step 5), coating the obtained carbon layer-coated composite material with a solid electrolyte, comprising: mixing and dispersing the obtained carbon layer-coated composite material with a solid electrolyte under the protection of an inert atmosphere, placing the mixed product in a quartz tube, sintering the mixed product at a low temperature in the inert atmosphere, and grinding and sieving the obtained product to obtain a final product, i.e., a boron-phosphorus co-doped silicon-carbon composite material; Wherein, the mixing and dispersing method adopts one of 3D, VC or high mixing machine methods; The solid electrolyte is one or more of PAN (polyacrylonitrile), PI (polyimide), PMMA (polymethyl methacrylate), PPO (polypropylene oxide), and PVDC (polyvinylidene chloride); The solid electrolyte accounts for 0.5%-5% of the mass of the final product; The low-temperature sintering temperature is 200-500°C and the time is 2-5h; The average particle size D of the obtained boron and phosphorus co-doped silicon-carbon composite material 50 3-10μm, Dmax≤50μm.

9. A negative electrode material, characterized in that The negative electrode material contains the boron and phosphorus co-doped silicon-carbon composite material according to claim 1 or 2.

10. A lithium ion battery, characterized in that: The negative electrode of the lithium-ion battery is made of the negative electrode material according to claim 9.

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