A silicon-carbon composite anode material and its preparation method

Abstract

This invention relates to a silicon-carbon composite anode material and its preparation method, belonging to the technical field of silicon-carbon composite materials. The material has a three-level structure: a core layer, a composite intermediate layer, and an outer protective layer. The core is... FeSi2 The composite intermediate layer consists of 50-80 nm iron-doped silicon particles with a volume fraction ≥15%; the intermediate layer is composed of a 5-10 nm pitch pyrolysis carbon layer and a single-walled carbon nanotube conductive network, with the carbon nanotubes penetrating the carbon layer to form a linear density ≥3 tubes / μm. 2 The three-dimensional conductive bridge has a Raman value ≤0.18; the outer layer is a nitrogen-doped carbon shell with an N-C bond ratio >60%, which is fused together with the middle layer.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of silicon-carbon composite materials and relates to a silicon-carbon composite negative electrode material and a preparation method thereof. BACKGROUND

[0002] With the continuous increase in the energy density demand of lithium ion batteries, silicon-based negative electrode materials are considered as the most potential next-generation negative electrode system due to their high specific capacity, but their industrialization process is still subject to multiple technical bottlenecks. The mainstream silane chemical vapor deposition method relies on high-risk raw materials. As a class 1 dangerous chemical, the explosion limit of silane is as wide as 1.37%-100%. Special explosion-proof facilities need to be equipped during the production process, resulting in an investment of more than 20 million yuan for kiloton-scale production line equipment, of which about 40% of the cost is used for the construction of explosion-proof systems in the high-temperature deposition link. In addition, silane cracking needs precise temperature control (> 400℃) to avoid side reactions, and the process fault tolerance is low, further increasing the safety risk and difficulty of large-scale production.

[0003] Elemental nanosilicon has a high volume expansion rate of 280% during lithium intercalation. Traditional carbon coating schemes, such as graphene or hard carbon layer coating, can initially alleviate stress, but are difficult to inhibit the expansion of deep cracks in silicon particles during the cycling process. This structural failure directly leads to electrode pulverization and capacity drop, which seriously restricts practical application. At the same time, existing conductive network construction techniques mostly rely on adding carbon nanotubes by dry mixing. Mechanical mixing leads to serious agglomeration of carbon nanotubes, which cannot form a three-dimensional conductive channel penetrating through the particles. Local conductivity differences not only result in low utilization of active materials, but also exacerbate uneven distribution of lithium ions, causing local overcharging and other safety hazards.

[0004] Therefore, it is urgent to develop a silicon-carbon composite negative electrode material that has a safe process path, efficient expansion inhibition, and stable conductive network construction. SUMMARY

[0005] The application aims to provide a silicon-carbon composite negative electrode material and a preparation method thereof, which have the characteristics of high cycle stability.

[0006] The object of the application can be achieved by the following technical solutions.

[0007] A silicon-carbon composite negative electrode material has a three-level structure of an inner core layer, a composite intermediate layer, and an outer protective layer,

[0008] The inner core is an iron-doped silicon particle with a FeSi2 phase volume ratio of ≥15%;

[0009] The composite intermediate layer comprises a pitch pyrolytic carbon layer continuously wrapping the inner core and a single-walled carbon nanotube conductive network. The single-walled carbon nanotube penetrates the pitch pyrolytic carbon layer and forms a three-dimensional conductive bridge with the surface of the silicon particle, with a linear density of ≥3 tubes / μm 2;

[0010] The outermost layer is a nitrogen-doped carbon shell, which completely covers the conductive network and is fused together with the carbon layer, with an NC bond ratio of >60%.

[0011] Furthermore, the kernel particle size D50 is 50~80nm.

[0012] Furthermore, the thickness of the pyrolysis carbon layer of the asphalt is 5~10 nm.

[0013] Furthermore, the single-walled carbon nanotubes form a through-network within the composite particles, and their Raman characteristic peak intensity is higher than that of I. D / I G ≤0.18.

[0014] A method for preparing a silicon-carbon composite anode material includes the following preparation steps:

[0015] S1: A silicon-iron alloy with an iron content of 9.5~10.5wt% is ball-milled with a ball-milling energy input of ≥350kW·h / t to obtain iron-doped silicon particles in the core layer;

[0016] S2: The iron-doped silicon particles in the core layer prepared in S1 are coated with pitch in an inert atmosphere, and the pitch with a softening point of 180~220℃ is used to form a continuous pyrolytic carbon layer with a thickness of 5~10nm.

[0017] S3: The particles coated with S2 are uniformly mixed with single-walled carbon nanotube slurry to obtain a mixed slurry, which is then granulated by pressure spray drying to form a conductive network preform.

[0018] S4: The conductive network preform obtained in S3 is placed in a rotary kiln, and a mixed gas with a volume ratio of NH3 to C2H2 of 3.0~4.8% is introduced. The mixture is then subjected to a pyrolysis reaction at 620~680℃ to generate a nitrogen-doped carbon shell.

[0019] Furthermore, in S1, the ball milling medium is zirconia balls, the ball-to-material ratio is 8:1 to 12:1, and the ball milling time is 1.5 to 3.5 hours.

[0020] Furthermore, the heating rate of the asphalt melt coating in S2 is 8~15℃ / min, and the pyrolysis carbonization is completed by holding at 450~500℃ for 30~60min.

[0021] Furthermore, the solid content of the mixed slurry in S3 is 15~25wt%, the spray drying inlet air temperature is 180~220℃, and the nozzle pressure is 0.8~1.2MPa;

[0022] The formulation of the single-walled carbon nanotube slurry in S3 is as follows, by mass percentage: 5.5-6.5 parts single-walled carbon nanotubes, 82-85 parts N-methylpyrrolidone, and 2.5-5 parts polyvinylidene fluoride.

[0023] Furthermore, the total flow rate of the mixed gas in S4 is 50~80L / min, and the rotary kiln speed is 1.5~3r / min.

[0024] Furthermore, in S4, the mixed gas is input through a vortex-enhanced distributor with an opening ratio of 35-40% and a gradient distribution of orifice diameters of Φ0.8-1.2mm.

[0025] The silicon-carbon composite anode material of this invention, through a three-level structural design of core layer-composite intermediate layer-outer protective layer, combined with refined parameter control, effectively solves the problems of high volume expansion rate, insufficient electronic conductivity, and numerous interfacial side reactions inherent in silicon-based anode materials. In the core layer, the FeSi2 phase of iron-doped silicon particles accounts for ≥15% of the volume. Its mechanism lies in the fact that FeSi2, with a particle size D50 of 50~80nm, acts as a highly conductive intermetallic compound, forming a three-dimensional continuous conductive network penetrating the silicon particles. During silicon lithium insertion / extraction, it accelerates electron transfer and acts as a rigid support framework to disperse stress concentration caused by silicon volume expansion. The coherent interface formed between FeSi2 and the silicon substrate enhances the stability of the internal structure of the particles through metallic bonding, preventing particle breakage or interface debonding due to excessive local stress. The core particle size is strictly limited to the range of 50~80nm. This scale reduces the lithium-ion diffusion path length, further ensuring the uniformity of the FeSi2 phase distribution within the silicon substrate and preventing local interruptions in the conductive network.

[0026] The thickness of the pitch pyrolysis carbon layer in the composite intermediate layer is controlled at 5-10 nm, which balances ion transport resistance and interface protection. A thickness <5 nm may lead to insufficient carbon layer continuity, failing to effectively isolate silicon from the electrolyte; while a thickness >10 nm will significantly increase lithium-ion diffusion resistance and reduce the material's rate performance. This carbon layer, formed by chemical vapor deposition, covers the silicon particle surface with a dense structure, inhibiting the direct reaction between silicon and the fluoride solvent in the electrolyte, reducing irreversible lithium loss and excessive growth of the solid electrolyte interphase (SEI) film. Simultaneously, single-walled carbon nanotubes with a linear density ≥3 tubes / μm... 2 The carbon nanotubes penetrate the pitch layer, forming covalently bonded conductive bridging points with the silicon surface during the penetration process, thus constructing a three-dimensional conductive network throughout the composite particles. This linear density design ensures that the carbon nanotubes form a uniform coverage on the silicon particle surface, with a high-density distribution of bridging points (≥3 tubes / μm). 2This enables rapid electron migration between silicon particles, carbon layers, and carbon nanotubes, maintaining a stable conductive path even when silicon undergoes volume expansion and deformation. The Raman characteristic peak intensity of carbon nanotubes is higher than that of I... D / I G ≤0.18 indicates that carbon nanotubes themselves have an extremely low structural defect rate, and their sp... 2 The highly ordered arrangement of hybrid carbon atoms significantly enhances the axial conductivity and mechanical strength of carbon nanotubes, thereby absorbing stress through their own tough deformation when silicon expands, reducing the risk of breakage of the conductive network.

[0027] The nitrogen-doped carbon shell of the outer protective layer forms a seamless interface with the intermediate carbon material through a fusion bonding process. Its NC bond ratio >60% achieves dual optimization of interface stability and lithium-ion kinetics. The high proportion of NC bonds can alter the electronic distribution characteristics of the carbon shell. In the nitrogen-doped carbon material of this invention, nitrogen exists primarily in the forms of pyridine nitrogen and graphitic nitrogen. The lone pair electrons of pyridine nitrogen enhance the adsorption capacity for lithium ions, while the conjugated system of graphitic nitrogen improves the intrinsic conductivity of the carbon shell. This is because pyridine nitrogen, in sp... 2 The hybrid form exists in a six-membered heterocycle. Its unbonded lone pairs of electrons possess strong Lewis basicity, allowing it to form coordinate bonds with lithium ions, significantly enhancing the adsorption capacity of the carbon shell for lithium ions and thus improving the material's lithium storage capacity. Graphite nitrogen directly replaces carbon atoms in the graphite lattice, forming sp... 2 The hybrid conjugated system optimizes the electron transport path through the π-electron delocalization effect, significantly improving the intrinsic conductivity of the carbon shell and enhancing its structural stability. Therefore, in this invention, when the two work synergistically, pyridine nitrogen is responsible for efficient lithium storage, while graphitic nitrogen ensures rapid conductivity, jointly optimizing the electrochemical performance of the carbon shell and exhibiting significant advantages in lithium-ion battery anode materials. Nitrogen doping-induced polar surfaces promote uniform nucleation and rapid diffusion of lithium ions on the carbon shell surface, while simultaneously suppressing the continuous decomposition of the electrolyte by regulating the SEI film composition. The interface between the carbon shell and the intermediate layer, fused together, eliminates microcracks or voids that may exist in traditional physical coatings. Its dense structure completely blocks the penetration of electrolyte into the internal conductive network, reducing the consumption of active materials and the increase in interfacial impedance caused by side reactions. The high chemical inertness of the nitrogen-doped carbon shell also resists the oxidative decomposition of the electrolyte under high-voltage cycling conditions, preventing carbon material corrosion or gas evolution at high potentials.

[0028] The synergistic mechanism of the three-tiered structure manifests as a progressive protection and functional enhancement: the core FeSi2 phase provides rigid support and a conductive matrix; the intermediate layer of pitched carbon and carbon nanotubes forms a dynamic stress buffer and electron transport network; and the outer nitrogen-doped carbon shell constructs a stable ion / electron dual-conductivity interface. The matching of the core particle size to the FeSi2 phase ratio ensures controllable volume change during lithium intercalation; the combination of the linear density and structural integrity of the intermediate carbon nanotubes guarantees electrical contact stability during long-term cycling; and the bond state modulation of the outer nitrogen-doped carbon shell fundamentally improves interfacial compatibility. This parametric synergy enables the material to maintain the high specific capacity of silicon while simultaneously achieving excellent cycle life and rate performance, providing a reliable solution for the commercial application of silicon-carbon composite anodes.

[0029] In step S1, the ferrosilicon alloy with an iron content of 9.5–10.5 wt% undergoes high-energy ball milling. By controlling the parameters of the milling medium (zirconia), the ball-to-material ratio (8:1–12:1), and the milling time (1.5–3.5 h), sufficient mechanical alloying is ensured. During ball milling, Fe elements are uniformly dispersed and embedded into the silicon lattice through intense collisions, resulting in a FeSi2 phase volume percentage ≥15%. A ball milling energy input of ≥350 kW·h / t controls the core particle size (D50) to be within the range of 50–80 nm. This ensures the formation of a continuous conductive framework for the FeSi2 phase while utilizing the size effect of nanoscale silicon particles to suppress mechanical stress concentration during lithiation. By controlling the ball milling energy input to ≥350 kW·h / t, the degree of atomic diffusion and reaction during ball milling can be adjusted. Sufficient energy input ensures a sufficient number of iron and silicon atoms participate in the formation of the FeSi2 phase, resulting in a FeSi2 phase volume percentage of ≥15% in the final iron-doped silicon particles. If the energy input is insufficient, atomic diffusion will be inadequate, resulting in a smaller amount of FeSi2 phase formed, which will not meet the volume ratio requirements. On the other hand, if the energy input is too high, although it can promote the formation of FeSi2 phase, it may trigger other unnecessary side reactions or cause excessive particle agglomeration, thus affecting the performance of the particles.

[0030] In step S2, asphalt with a softening point of 180-220℃ is used for melt coating. This parameter range ensures that the asphalt has suitable fluidity in the molten state to completely cover the silicon core surface. When the temperature is below 180℃, the asphalt melts insufficiently, which may lead to discontinuous coating; above 220℃, the asphalt decomposes excessively, resulting in porosity defects. By controlling the heating rate to 8-15℃ / min, thermal shock is prevented from causing carbon layer peeling. Holding at 450-500℃ for 30-60 min ensures that the thickness of the asphalt pyrolysis carbon layer is precisely stabilized at 5-10 nm. The slower heating rate allows the asphalt molecules to arrange themselves in an orderly manner, and the carbonization temperature range of 450-500℃ matches the optimal thermodynamic conditions for the graphitization transformation of asphalt, forming a dense, continuous carbon layer with a certain graphite microcrystal orientation. This thickness of carbon layer can both prevent the electrolyte from directly contacting the silicon core and avoid excessive thickness that would hinder lithium-ion transport.

[0031] In step S3, the design of a 15-25 wt% solid content in the single-walled carbon nanotube slurry balances slurry flowability with carbon nanotube dispersion uniformity: a solid content below 15 wt% results in insufficient conductive network density after spray drying, while a solid content above 25 wt% leads to carbon nanotube agglomeration. Pressure spray drying employs an inlet air temperature of 180-220℃ and a nozzle pressure of 0.8-1.2 MPa. This parameter combination controls the droplet drying rate, ensuring that carbon nanotubes form a three-dimensional bridging structure penetrating the pitch carbon layer on the surface of silicon-carbon core-shell particles. When the nozzle pressure is ≥0.8 MPa, the droplet size decreases to the micrometer level, promoting uniform adhesion of carbon nanotubes to the particle surface; while the upper limit of the inlet air temperature of 220℃ prevents localized overheating of the carbon nanotubes, which could cause structural damage.

[0032] In step S4, during the pyrolysis of a mixed gas with a NH3 to C2H2 volume ratio of 3.0–4.8% at 620–680℃, the ratio of carbon to nitrogen sources regulates the type and content of nitrogen doping in the carbon shell: when the NH3 content is >4.8%, inactive amino nitrogen is easily formed, while <3.0% results in insufficient nitrogen doping. At the pyrolysis temperature of 620–680℃, the pyrolysis of C2H2 generates a carbon framework, and the active nitrogen atoms generated by the decomposition of NH3 penetrate into the carbon layer through gas phase infiltration, forming a doped structure with an NC bond ratio >60%. A rotary kiln speed of 1.5–3 r / min, combined with a total mixed gas flow rate of 50–80 L / min, ensures sufficient contact between the material and the gas during dynamic tumbling. A speed >3 r / min leads to increased particle collisions and damage to the intermediate carbon structure, while a flow rate <50 L / min reduces reaction efficiency. The eddy current enhanced distributor features an opening ratio of 35-40% and a gradient distribution of apertures Φ0.8-1.2mm. This design improves the uniformity of gas distribution through turbulence, ensuring the integrity of the carbon shell coating and the consistency of nitrogen doping.

[0033] The synergistic effect of parameters in each step ultimately results in precise control of the material's core layer-composite intermediate layer-outer protective layer structure: S1 ball milling parameters establish a synergistic volume buffer substrate between the FeSi2 conductive phase and the nano-silicon core; S2 pitch melt coating parameters construct an ion / electron selectively permeable interfacial isolation layer; S3 spray drying process achieves the directional distribution and structural preservation of the carbon nanotube conductive network; S4 vapor deposition parameters complete the chemically inert interface construction of the nitrogen-doped carbon shell. For example, the carbon nanotube linear density is ≥3 tubes / μm. 2 The requirements are achieved through a combination of the solid content of the S3 slurry and the spray drying parameters, while the fusion bonding between the outer carbon shell and the middle layer depends on the thermodynamic conditions of local graphitization of the carbon material at the S4 rotary kiln temperature (620~680℃). The preparation method systematically solves bottleneck problems such as poor cycle stability, low rate performance, and uncontrollable interfacial side reactions in silicon-based anodes through the mutual constraints and verification of parameters at each stage, achieving highly controllable preparation of the material's structure and properties. Attached Figure Description

[0034] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0035] Figure 1 Electron micrograph of the silicon-carbon composite anode material prepared in Example 1. Detailed Implementation

[0036] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided.

[0037] Example 1

[0038] A method for preparing a silicon-carbon composite anode material includes the following preparation steps:

[0039] S1: A silicon-iron alloy with an iron content of 10wt% is subjected to ball milling. The ball milling media is zirconia balls, the ball-to-material ratio is 10:1, the ball milling time is 2h, and the ball milling energy input is ≥350kW·h / t to obtain iron-doped silicon core particles in the core layer.

[0040] S2: The iron-doped silicon core particles of the core layer prepared in S1 are coated with pitch by pitch melting in an inert atmosphere. The softening point of the pitch is controlled at 200℃ to form a continuous pyrolytic carbon layer with a thickness of 8nm. The heating rate of pitch melting coating is 8℃ / min, and the pyrolytic carbonization is completed by holding at 450℃ for 45min.

[0041] S3: The particles coated with S2 are uniformly mixed with single-walled carbon nanotube slurry to obtain a mixed slurry with a solid content of 20wt%. The slurry is then granulated by pressure spray drying to form a conductive network preform. The spray drying inlet air temperature is 180℃ and the nozzle pressure is 0.8MPa.

[0042] The formulation of the single-walled carbon nanotube slurry is as follows, by mass percentage: 5.5 parts single-walled carbon nanotubes, 82 parts N-methylpyrrolidone, and 2.5 parts polyvinylidene fluoride.

[0043] S4: The conductive network preform obtained in S3 is placed in a rotary kiln, and a mixed gas with a volume ratio of 4.2% of NH3 and C2H2 is introduced. The total flow rate of the mixed gas is 65 L / min, and the rotary kiln speed is 2 r / min. The mixed gas is input through an eddy current enhanced distributor with an opening ratio of 40% and a gradient distribution of pore size Φ0.8 mm. The mixture undergoes a pyrolysis reaction at 650℃ to generate a nitrogen-doped carbon shell.

[0044] The silicon-carbon composite anode material prepared in this embodiment has a FeSi2 phase volume ratio of 16.3%, a pitch pyrolysis carbon layer thickness of 8 nm, and a single-walled carbon nanotube linear density of 3.8 nanotubes / μm. 2 Raman I D / I G With a carbon shell NC bond ratio of 0.15 and a nitrogen-doped carbon shell NC bond ratio of 63.2%, the resulting silicon-carbon composite anode material is as follows: Figure 1 As shown.

[0045] Example 2

[0046] A method for preparing a silicon-carbon composite anode material includes the following preparation steps:

[0047] S1: A silicon-iron alloy with an iron content of 9.5 wt% was ball-milled with zirconium oxide balls as the milling medium, a ball-to-material ratio of 8:1, a milling time of 1.5 h, and a milling energy input of ≥350 kW·h / t to obtain iron-doped silicon core particles in the core layer.

[0048] S2: The iron-doped silicon core particles of the core layer prepared in S1 are coated with pitch by pitch melting in an inert atmosphere. The pitch softening point is controlled at 180℃ to form a continuous pyrolytic carbon layer with a thickness of 5nm. The heating rate of pitch melting coating is 8℃ / min, and pyrolytic carbonization is completed by holding at 450℃ for 30min.

[0049] S3: The particles coated with S2 are uniformly mixed with single-walled carbon nanotube slurry to obtain a mixed slurry with a solid content of 15wt%. The slurry is then granulated by pressure spray drying to form a conductive network preform. The spray drying inlet air temperature is 180℃ and the nozzle pressure is 0.8MPa.

[0050] The formulation of the single-walled carbon nanotube slurry is as follows, by mass percentage: 5.5 parts single-walled carbon nanotubes, 82 parts N-methylpyrrolidone, and 2.5 parts polyvinylidene fluoride.

[0051] S4: The conductive network preform obtained in S3 is placed in a rotary kiln, and a mixed gas with a volume ratio of NH3 to C2H2 of 3.0% is introduced. The total flow rate of the mixed gas is 50 L / min, and the rotary kiln speed is 1.5 r / min. The mixed gas is input through an eddy current enhanced distributor with an opening ratio of 35% and a gradient distribution of pore size Φ0.8 mm. The gas is then subjected to a pyrolysis reaction at 620℃ to generate a nitrogen-doped carbon shell.

[0052] The silicon-carbon composite anode material prepared in this embodiment has a FeSi2 phase volume ratio of 15.8%, a pitch pyrolysis carbon layer thickness of 5 nm, and a single-walled carbon nanotube linear density of 3.5 nanotubes / μm. 2 Raman I D / I G The value is 0.16 and the proportion of nitrogen-doped carbon shell NC bonds is 62.5%.

[0053] Example 3

[0054] A method for preparing a silicon-carbon composite anode material includes the following preparation steps:

[0055] S1: A silicon-iron alloy with an iron content of 10.5 wt% was ball-milled with zirconium oxide balls as the milling medium, a ball-to-material ratio of 12:1, a milling time of 3.5 h, and a milling energy input of ≥350 kW·h / t to obtain iron-doped silicon core particles in the core layer.

[0056] S2: The iron-doped silicon core particles of the core layer prepared in S1 are coated with pitch by pitch melting in an inert atmosphere. The pitch softening point is controlled at 220℃ to form a continuous pyrolytic carbon layer with a thickness of 10nm. The heating rate of pitch melting coating is 15℃ / min, and the pyrolytic carbonization is completed by holding at 500℃ for 60min.

[0057] S3: The particles coated with S2 are uniformly mixed with single-walled carbon nanotube slurry to obtain a mixed slurry with a solid content of 25wt%. The slurry is then granulated by pressure spray drying to form a conductive network preform. The spray drying inlet air temperature is 20℃ and the nozzle pressure is 1.2MPa.

[0058] The formulation of the single-walled carbon nanotube slurry is as follows, by mass percentage: 6.5 parts single-walled carbon nanotubes, 85 parts N-methylpyrrolidone, and 5 parts polyvinylidene fluoride.

[0059] S4: The conductive network preform obtained in S3 is placed in a rotary kiln, and a mixed gas with a volume ratio of 4.8% of NH3 and C2H2 is introduced. The total flow rate of the mixed gas is 80 L / min, and the rotary kiln speed is 3 r / min. The mixed gas is input through an eddy current enhanced distributor with an opening ratio of 40% and a pore size of Φ1.2 mm in a gradient distribution. The gas is decomposed at 680℃ to generate a nitrogen-doped carbon shell.

[0060] The silicon-carbon composite anode material prepared in this embodiment has a FeSi2 phase volume ratio of 16.2%, a pitch pyrolysis carbon layer thickness of 10 nm, and a single-walled carbon nanotube linear density of 3.7 nanotubes / μm. 2 Raman I D / I G The value is 0.16 and the proportion of nitrogen-doped carbon shell NC bonds is 63.1%.

[0061] Comparative Example 1

[0062] This comparative example does not involve S2 asphalt melt coating; the remaining steps are the same as in Example 1.

[0063] The silicon-carbon composite anode material prepared in this comparative example has a FeSi2 phase volume fraction of 16.3% and a single-walled carbon nanotube linear density of 3.8 nanotubes / μm. 2 Raman I D / I G The value is 0.24 and the proportion of nitrogen-doped carbon shell NC bonds is 63.2%.

[0064] Comparative Example 2

[0065] This comparative example does not involve mixing S3 single-walled carbon nanotube slurry; the remaining steps are the same as in Example 1.

[0066] The silicon-carbon composite anode material prepared in this comparative example has a FeSi2 phase volume ratio of 16.3%, a pitch pyrolysis carbon layer thickness of 8 nm, and Raman spectroscopy performance of [missing information]. D / I G The value is 0.31 and the proportion of nitrogen-doped carbon shell NC bonds is 63.2%.

[0067] Comparative Example 3

[0068] This comparative example does not involve S4 nitrogen-doped carbon shell coating; the remaining steps are the same as in Example 1.

[0069] The silicon-carbon composite anode material prepared in this comparative example has a FeSi2 phase volume fraction of 16.3%, a pitch pyrolysis carbon layer thickness of 8 nm, and a single-walled carbon nanotube linear density of 3.8 nanotubes / μm. 2 Raman I D / I G Value 0.28.

[0070] The silicon-carbon composite anode materials prepared in the examples and comparative examples were subjected to 300 cycles to test their capacity retention, volume expansion rate, and specific capacity. The testing methods followed the standard GB / T 30835-2014. The experimental results are summarized in the table below.

[0071]

[0072] The experimental data above show that the silicon-carbon composite anode material prepared by this invention has the best performance.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A silicon-carbon composite negative electrode material, characterized by, The silicon-carbon composite negative electrode material has a three-level structure of a core layer, a composite intermediate layer and an outer protective layer, The core layer is iron-doped silicon particles with a FeSi2 phase volume ratio of ≥15%; The composite interlayer comprises a pitch pyrocarbon layer continuously wrapping the core and a single-walled carbon nanotube conductive network, the single-walled carbon nanotubes penetrating the pitch pyrocarbon layer and forming a three-dimensional conductive bridge with the surface of the silicon particles, linear density ≥ 3 tubes / μm 2 ; The outer protective layer is a nitrogen-doped carbon shell, which completely covers the conductive network and is fusedly connected with the carbon layer, and the N-C bond ratio is >60%; The silicon-carbon composite negative electrode material comprises the following preparation steps, S1: Ball milling silicon-iron alloy with iron content of 9.5-10.5wt%, ball milling energy input ≥350kW·h / t, to obtain iron-doped silicon particles in the core layer; S2: Inert atmosphere, S1 prepared core layer iron-doped silicon particles are coated with pitch, control the softening point 180-220℃ pitch to form a continuous pyrolytic carbon layer with a thickness of 5-10nm; S3: The coated particles of S2 are uniformly mixed with single-walled carbon nanotube slurry to obtain a mixed slurry, and a conductive network preform is formed by pressure spray drying; S4: The conductive network preform prepared in S3 is placed in a rotary kiln, and a mixed gas with a volume ratio of NH3 to C2H2 of 3.0-4.8% is introduced, and a nitrogen-doped carbon shell is generated by cracking reaction at 620-680℃.

2. The silicon-carbon composite negative electrode material of claim 1, wherein, The particle size D50 of the core is 50-80nm. 3.The silicon-carbon composite negative electrode material of claim 1, characterized in that, The single-walled carbon nanotubes form a through-network in the composite particles, with a Raman characteristic peak intensity ratio I D / I G ≤ 0.

18.

4. The silicon-carbon composite negative electrode material of claim 1, wherein the carbon material is a carbon material having a graphitic structure. The ball milling medium in S1 is zirconia ball, the ball-to-material ratio is 8:1-12:1, and the ball milling time is 1.5-3.5h.

5. The silicon-carbon composite negative electrode material of claim 1, wherein the carbon material is a carbon material having a graphitic structure. The heating rate of pitch melting coating in S2 is 8-15℃ / min, and the pyrolysis carbonization is completed at 450-500℃ for 30-60min. 6.The silicon-carbon composite negative electrode material of claim 1, characterized in that, The solid content of the mixed slurry in S3 is 15-25wt%, the spray drying inlet temperature is 180-220℃, and the nozzle pressure is 0.8-1.2MPa; The formula of the single-walled carbon nanotube slurry in S3 is as follows: single-walled carbon nanotube 5.5-6.5 parts, N-methyl pyrrolidone 82-85 parts, and polyvinylidene fluoride 2.5-5 parts by mass percentage. 7.The silicon-carbon composite negative electrode material of claim 1, characterized in that, The total flow of the mixed gas in S4 is 50-80L / min, and the rotary kiln speed is 1.5-3r / min. 8.The silicon-carbon composite negative electrode material of claim 1, characterized in that, The mixed gas in S4 is input through a vortex intensifier distributor, the opening rate of the distributor is 35-40%, and the hole diameter Φ0.8-1.2mm is gradient distributed.

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