Negative active material, method for preparing the same, and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, and discloses a negative electrode active material, a preparation method thereof and a lithium ion battery. The negative electrode active material comprises nano silicon and a three-dimensional carbon conductive grid, the nano silicon is embedded in the three-dimensional carbon conductive grid, the three-dimensional carbon conductive grid comprises uniformly mixed carbon nanotubes, layered graphene and graphite powder, the mass percentage of the nano silicon is 15%-35%, the mass percentage of the graphite powder is 32.5%-59.5%, and the mass ratio of the carbon nanotubes to the layered graphene is 1:2-8. The application uniformly mixes the carbon material with multiple shapes including carbon nanotubes, layered graphene and graphite powder with the nano silicon, forms a three-dimensional carbon conductive network structure, effectively absorbs the volume expansion of Si, improves the isotropic conductive performance and conductive efficiency of the active material, and has high structural stability.

Description

Technical Field

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

[0002] Silicon (Si) has a theoretical specific capacity of up to 3579 mAh / g (Li 15 Si4, with a capacity nearly 10 times that of graphite (372 mAh / g), is considered one of the most promising candidates for next-generation lithium-ion battery anode materials. However, silicon exhibits a volume expansion effect of up to 300% during charge and discharge, leading to the pulverization of active particles and the continuous rupture and reconstruction of the solid electrolyte interphase (SEI) film, ultimately resulting in rapid capacity decay and severely restricting its commercial application.

[0003] Existing technologies have disclosed various technical approaches to improve the cycle stability of silicon-based anodes, such as the porous carbon host approach: depositing silicon inside a prefabricated porous carbon support can constrain silicon expansion, but the chemical vapor deposition (CVD) process is complex, the preparation of porous carbon is difficult, and stress concentration occurs at the silicon / carbon interface; at the same time, the entire conductivity method is relatively simple, with only carbon coating or graphite mixing, resulting in low conductivity efficiency.

[0004] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of the present invention, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] To provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This summary is not intended as a general commentary, nor is it intended to identify key / important components or describe the scope of these embodiments, but rather as a prelude to the detailed description that follows.

[0006] The existing technologies for improving the cycle stability of silicon-based anodes mainly include the following approaches: 1. Nanoscale approach: Silicon is prepared into nanoparticles, silicon nanowires or porous silicon to physically reduce the size of the expanded monomer, but there are problems such as high preparation cost and low volumetric energy density. 2. Porous carbon host path: Depositing silicon inside a pre-fabricated porous carbon scaffold can constrain silicon expansion, but the chemical vapor deposition (CVD) process is complex, the preparation of porous carbon is difficult, and the stress concentration problem at the silicon / carbon interface has not been effectively solved. 3. External coating path: Coating the surface of silicon particles with a carbon layer (such as a pitch carbonization layer) only solves the conductivity problem, but cannot alleviate the volume expansion structurally; 4. Simple silicon-carbon mixing path: Silicon particles are mechanically mixed with graphite. There is a lack of effective three-dimensional network connection between graphite and silicon. The conductive network is prone to breakage after cycling, and the buffering effect is limited.

[0007] The common problem with the above-mentioned technical approaches is that the conductive network design is singular (only carbon coating or only graphite mixing), and it is impossible to simultaneously achieve the triple functions of electronic conduction, ion transport and mechanical buffering in three-dimensional space.

[0008] This invention addresses the limitation of existing technologies by providing a more comprehensive solution. It offers a negative electrode active material, its preparation method, and a lithium-ion battery. This solves the problem of existing negative electrode active materials having a single conductivity mode, failing to form an isotropic three-dimensional conductive network, and resulting in poor cycle stability. The solution is achieved through a complex combination of techniques, including improving the composition of the negative electrode active material, optimizing the structure and ratio of its components, and optimizing the reaction modes of each component.

[0009] In some embodiments, a negative electrode active material includes: Nano-silicon and a three-dimensional carbon conductive mesh, wherein the nano-silicon is embedded in the three-dimensional carbon conductive mesh; the three-dimensional carbon conductive mesh comprises a mixture of carbon nanotubes, layered graphene and graphite powder; The mass percentage of the nano-silicon is set to 15%-35%; the mass percentage of the graphite powder is set to 32.5%-59.5%; and the mass ratio of the carbon nanotubes to the layered graphene is set to 1:2-8.

[0010] In some embodiments, the carbon nanotubes have an outer diameter of 8 nm-15 nm and an aspect ratio of 50 or greater.

[0011] In some embodiments, the carbon nanotubes are multi-walled carbon nanotubes with 3 to 10 walls.

[0012] In some embodiments, the layered graphene has 2 to 5 layers and the interlayer spacing d002 is 3.35 Å to 3.42 Å.

[0013] In some embodiments, the layered graphene is tightly adhered to the surface of the nano-silicon in a wrinkled state.

[0014] In some embodiments, the D of the nano-silicon 50 The particle size ranges from 80 nm to 200 nm, with a particle size distribution span (D). 90 -D 10 ) / D 50 ≤ 1.5.

[0015] In some embodiments, the negative electrode active material further includes: A coating layer is applied to the outer surface of the three-dimensional carbon conductive mesh, with a thickness of 5nm-30nm.

[0016] In some embodiments, the BET specific surface area of ​​the negative electrode active material satisfies the following relationship with the first-cycle coulombic efficiency: ICE ≥ -0.8 × S + 98.4; where ICE is the first-cycle coulombic efficiency in %; and S is the BET specific surface area in m² / g; or, The negative electrode active material is granular with a median particle size D. 50 The thickness ranges from 8 μm to 20 μm, and the compacted density measured at 10 MPa is 1.4 g / cm³ to 1.8 g / cm³, while the vibratory density is 0.8 g / cm³ to 1.2 g / cm³.

[0017] In some embodiments, a method for preparing a negative electrode active material as described above includes: Step a): Disperse nano-silicon, carbon nanotubes, layered graphene and graphite powder in a certain proportion in an N-methylpyrrolidone solution containing polyvinylpyrrolidone, and disperse under high shear until the particles are uniform. Step b): Use a two-fluid spray dryer with an inlet temperature of 150℃-200℃ to collect spherical precursor particles; Step c): Under an inert atmosphere, the material is heated to 700℃-900℃ at a rate of 2℃-5℃ / min for 1h-3h, and then naturally cooled to obtain the negative electrode active material.

[0018] In some embodiments, a lithium-ion battery includes a negative electrode and an electrolyte. The negative electrode is coated with a negative electrode active material as described above, or a negative electrode active material obtained by the preparation method described above. The electrolyte contains 0.5 wt%-3 wt% fluoroethylene carbonate, and the electrolyte forms a fluorine-containing film layer on the negative electrode active material. The molar ratio of fluorine to silicon in the film layer is 0.3-0.8.

[0019] The negative electrode active material, its preparation method, and the lithium-ion battery provided in the embodiments of the invention can achieve the following technical effects: By employing carbon materials of various shapes, including carbon nanotubes, layered graphene, and graphite powder, and mixing them with nano-silicon, the nano-silicon particles are encapsulated by the carbon precursor dispersion system during the mixing and spray drying process. After high-temperature carbonization, the carbon phase shrinks and solidifies, forming a rigid three-dimensional network. The silicon particles are fixed in the network pores, achieving an embedded structure and creating a three-dimensional carbon conductive network structure. This structure can effectively absorb the volume expansion of Si, keeping the electrode expansion rate below 17.8%. It improves the isotropic conductivity and conductivity efficiency of the active material, and has strong structural stability, controllable structure, stable performance, and low cycle decay, making it suitable for high-performance lithium-ion battery applications.

[0020] The above general description and the description below are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0021] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations and drawings do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are shown as similar elements. The drawings are not to be scaled. And wherein: Figure 1 This is a schematic diagram of the structure of a negative electrode active material particle provided in an embodiment of the present invention.

[0022] Figure label: 1. Nano-silicon; 2. Carbon nanotubes; 3. Layered graphene; 4. Graphite powder; 5. Coating layer. Detailed Implementation

[0023] To provide a more detailed understanding of the features and technical content of the embodiments of the present invention, the implementation of the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The accompanying drawings are for illustrative purposes only and are not intended to limit the embodiments of the present invention. In the following technical description, for ease of explanation, several details are used to provide a full understanding of the disclosed embodiments. However, one or more embodiments may still be implemented without these details. In other cases, well-known structures and devices may be shown in a simplified manner to simplify the drawings.

[0024] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of the invention described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0025] In this embodiment of the invention, the terms "upper," "lower," "inner," "middle," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for better describing the embodiments of the invention and their implementations, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to require them to be constructed and operated in a specific orientation. Furthermore, some of the above terms may be used to indicate other meanings besides orientation or positional relationship; for example, the term "upper" may in some cases indicate a dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in the embodiments of the invention according to the specific circumstances.

[0026] Furthermore, the terms "set up," "connect," and "fix" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this invention according to the specific circumstances.

[0027] Unless otherwise stated, the term "multiple" means two or more.

[0028] In this embodiment of the invention, the character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0029] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0030] It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0031] like Figure 1 As shown, an embodiment of the present invention provides a negative electrode active material comprising: Nano-silicon 1 and a three-dimensional carbon conductive mesh, wherein the nano-silicon 1 is embedded in the three-dimensional carbon conductive mesh; the three-dimensional carbon conductive mesh comprises a mixture of carbon nanotubes 2, layered graphene 3 and graphite powder 4. The mass percentage of the nano-silicon 1 is set to 15%-35%; the mass percentage of the graphite powder 4 is set to 32.5%-59.5%; and the mass ratio of the carbon nanotubes 2 to the layered graphene 3 is set to 1:2-8.

[0032] The negative electrode active material provided in this embodiment of the invention is composed of nano-silicon 1 and a three-dimensional carbon conductive network, wherein nano-silicon 1 is embedded in the three-dimensional carbon conductive network; the three-dimensional carbon conductive network is composed of carbon nanotubes 2, layered graphene 3 and graphite powder 4 mixed together, wherein the carbon nanotubes 2 are straight tubular structures, the layered graphene 3 are multi-layered sheet structures, and the graphite powder 4 are granular. The mixing of these three types of carbon materials utilizes the high conductivity of each structure itself and conducts to each other in three dimensions, improving the rapid transport of lithium ions, so that the first-cycle coulombic efficiency of the entire silicon-carbon negative electrode active material is ≥88%, or even ≥93%.

[0033] In this invention, nano-silicon 1 refers to silicon particles with a size of less than 200 nanometers. In the entire negative electrode active material, the mass percentage of nano-silicon 1 is set to 15%-35%. This embodiment utilizes the small size of nano-silicon 1, which results in a small expansion rate and a low degree of physical expansion of nano-silicon, thereby improving the cycle life of the product.

[0034] In comparison, the three-dimensional carbon conductive network accounts for a relatively large proportion of the negative electrode active material, with a mass percentage of 65%-85%. The three-dimensional carbon conductive network mainly consists of three types of carbon materials with different structures. Among them, the mass percentage of graphite powder 4 is set at 32.5%-59.5%, and the remaining percentage is shared by carbon nanotubes 2 and layered graphene 3. The mass ratio of carbon nanotubes 2 and layered graphene 3 is set at 1:2-8. The above percentages are all relative to the entire negative electrode active material.

[0035] Since the three-dimensional carbon conductive network is made up of three carbon materials with different structures, and combined with nano-silicon embedded in the three-dimensional carbon conductive network, in the entire negative electrode active material, the layered graphene 3 is in sheet form, which can be easily attached to the nano-silicon 1. At the same time, multiple layered graphene 3 form partitions, and graphite powder 4 fills each partition. The carbon nanotube 2 is a straight tube structure, but it has a certain degree of elasticity, which can easily conduct each partition, thus realizing the entire three-dimensional carbon conductive network structure.

[0036] In some embodiments, the carbon nanotube 2 has an outer diameter of 8nm-15nm and an aspect ratio of 50 or greater.

[0037] In this embodiment of the invention, the structural dimensions of carbon nanotube 2 are limited. Carbon nanotube 2 is basically a straight tube structure with an outer diameter of 8nm-15nm. The aspect ratio is the ratio of its axial length to its outer diameter, which can be set to be greater than or equal to 50. This ensures that it can maintain its extension to the two adjacent partitions, that is, conduct graphite powder separated by layered graphene, thereby improving the overall conductivity.

[0038] In some embodiments, the carbon nanotube 2 is a multi-walled carbon nanotube with 3 to 10 walls.

[0039] More specifically, in this embodiment of the invention, the carbon nanotubes used are limited to multi-walled carbon nanotubes. Multi-walled carbon nanotubes are seamless tubular nanomaterials formed by coaxially rolling multiple layers of graphene sheets, with an interlayer spacing of approximately 0.34 nanometers, exhibiting a Russian nesting structure. The two ends of multi-walled carbon nanotubes are typically not open. The wall number refers to the number of layers of graphene sheets that constitute the walls of the carbon nanotube.

[0040] The embodiments of the present invention employ multi-walled carbon nanotubes 2 with 3 to 10 wall layers, which gives the entire carbon nanotube sufficient mechanical strength to prevent collapse during the carbonization process during manufacturing, and appropriate bending flexibility to ensure uniform distribution inside the particles.

[0041] In some embodiments, the layered graphene 3 has 2 to 5 layers and an interlayer spacing d002 of 3.35 Å to 3.42 Å.

[0042] In this embodiment of the invention, the number of layers of layered graphene 3 used is limited to 2-5 layers, and the interlayer spacing d002 is set to 3.35 Å-3.42 Å. The interlayer spacing d002 refers to the vertical distance between adjacent carbon atom layers in graphite crystals and is a key parameter characterizing the structural features of graphite materials. The ideal interlayer spacing is 0.3354 nanometers (i.e., 3.354 Å). The interlayer size of 3.35 Å-3.42 Å used in this invention is more conducive to the absorption of expansion of nano-silicon 1 by layered graphene 3, so that the expansion rate of the negative electrode sheet using the negative electrode active material of this invention is maintained below 17.8%.

[0043] In some embodiments, the layered graphene 3 is tightly adhered to the surface of the nano-silicon 1 in a wrinkled state.

[0044] In this embodiment of the invention, the wrinkled layer in the layered graphene 3 is tightly attached to the nano-silicon 1, which is more conducive to absorbing the volume expansion of silicon during use, thereby reducing the bulging phenomenon of the entire negative electrode sheet.

[0045] In some embodiments, the D of the nano-silicon 1 50 The particle size ranges from 80 nm to 200 nm, with a particle size distribution span (D). 90 -D 10 ) / D 50 ≤ 1.5.

[0046] In this embodiment of the invention, the size of the nano-silicon 1 used in the negative electrode active material is defined. The nano-silicon 1 is granular and contains particles of various sizes. The D of the nano-silicon in this invention... 50 Particle size set to 80nm-200nm, D 50 Particle size refers to the particle size value corresponding to the cumulative particle size distribution reaching 50%; similar to D. 90 and D 10 These refer to the particle size values ​​corresponding to when the cumulative particle size distribution reaches 90% and 10%, respectively; the overall particle size distribution is expressed by the particle size distribution span formula (D). 90 -D 10 ) / D 50 The particle size distribution is limited to ≤1.5 to ensure uniformity of particle size distribution and stability of performance.

[0047] In some embodiments, the negative electrode active material further includes: The coating layer 5 is coated on the outer surface of the three-dimensional carbon conductive mesh, with a thickness of 5nm-30nm.

[0048] In this embodiment of the invention, a coating layer 5, typically a carbide layer with a thickness of 5nm-30nm, is also coated on the outer surface of the three-dimensional carbon conductive network. The carbide layer primarily plays three core roles: enhanced conductivity, volume buffering, and interface stabilization. For intrinsically insulating or low-conductivity active materials such as silicon, the carbide layer constructs a continuous electron transport network, significantly reducing electrode internal resistance and improving rate performance. During charge and discharge (especially for silicon-based anodes), the carbide layer absorbs internal stress due to its high elastic modulus, inhibiting the pulverization and shedding of active particles and maintaining structural integrity. The dense carbide layer prevents direct contact between the active material and the electrolyte, reducing irreversible side reactions and guiding the formation of a more stable and thinner SEI film, thereby improving the initial coulombic efficiency and cycle life. Some amorphous carbon coating layers have large interlayer spacing and abundant pores, which can serve as rapid lithium-ion diffusion channels, guiding ion intercalation and inhibiting graphite exfoliation caused by solvent co-intercalation.

[0049] In some embodiments, the BET specific surface area of ​​the negative electrode active material satisfies the following relationship with the first-cycle coulombic efficiency: ICE ≥ -0.8 × S + 98.4; where ICE is the first-cycle coulombic efficiency in %; and S is the BET specific surface area in m² / g; or, The negative electrode active material is granular with a median particle size D. 50 The thickness ranges from 8 μm to 20 μm, and the compacted density measured at 10 MPa is 1.4 g / cm³ to 1.8 g / cm³, while the vibratory density is 0.8 g / cm³ to 1.2 g / cm³.

[0050] In this embodiment of the invention, the negative electrode active material can be spherical or near-spherical particles, especially in a structure coated with a carbonization layer. The entire negative electrode active material particle satisfies the following relationship between the BET specific surface area and the first-cycle coulombic efficiency: ICE ≥ -0.8 × S + 98.4; where ICE is the first-cycle coulombic efficiency in %; and S is the BET specific surface area in m² / g.

[0051] BET specific surface area refers to the total internal and external surface area per unit mass of material; first-cycle coulombic efficiency refers to the energy conversion rate of the battery during the first charge and discharge cycle, calculated as a percentage. A higher value indicates less energy wasted during the first charge and discharge cycle. The first-cycle coulombic efficiency was calculated using an ion battery employing the negative electrode active material of this invention.

[0052] The median particle size D of the entire negative electrode active material 50The thickness ranges from 8 μm to 20 μm, and the compacted density measured at 10 MPa is 1.4 g / cm³ to 1.8 g / cm³, while the vibratory density is 0.8 g / cm³ to 1.2 g / cm³.

[0053] In some embodiments, a method for preparing a negative electrode active material as described above includes: Step a): Disperse nano-silicon, carbon nanotubes, layered graphene and graphite powder in a certain proportion in an N-methylpyrrolidone solution containing polyvinylpyrrolidone, and disperse under high shear until the particles are uniform. Step b): Use a two-fluid spray dryer with an inlet temperature of 150℃-200℃ to collect spherical precursor particles; Step c): Under an inert atmosphere, the material is heated to 700℃-900℃ at a rate of 2℃-5℃ / min for 1h-3h, and then naturally cooled to obtain the negative electrode active material.

[0054] The present invention provides an operational process for preparing negative electrode active materials, which mainly adopts high-shear operation in the liquid phase. By using a high-shear emulsifier or disperser, strong mechanical shear force, centrifugal extrusion, impact tearing and cavitation effect are generated in the liquid phase system through the high-speed relative motion between the rotor and stator, thereby realizing the process of refining, dispersing, homogenizing and emulsifying the material.

[0055] In some embodiments, a lithium-ion battery includes a negative electrode and an electrolyte. The negative electrode is coated with a negative electrode active material as described above, or a negative electrode active material obtained by the preparation method described above. The electrolyte contains 0.5 wt%-3 wt% fluoroethylene carbonate, and the electrolyte forms a fluorine-containing film layer on the negative electrode active material. The molar ratio of fluorine to silicon in the film layer is 0.3-0.8.

[0056] Traditional lithium-ion batteries, especially those using a mixture of graphite and silicon particles as the active material on the negative electrode, typically have a fluorine to silicon molar ratio of less than 0.1 in the electrolyte film layer on this active material. This invention uses the aforementioned negative electrode active material, combined with an electrolyte containing 0.5wt%-3wt% fluoroethylene carbonate, which allows the fluorine to silicon molar ratio in the film layer to be set at 0.3-0.8. This improves the thermal stability of the film and enhances battery safety.

[0057] The lithium-ion battery described above can be used in electrical equipment including hand-held power tools, such as hand drills, angle grinders, and electric shears, thereby improving safety.

[0058] The present invention will be further described below with reference to specific embodiments. The following preparation process and performance testing method are used in the following embodiments and comparative examples.

[0059] Example 1 This embodiment prepares a negative electrode active material, a silicon-carbon composite negative electrode material comprising nano-silicon and a three-dimensional carbon conductive network, with the following parameters: Silicon nanoparticles: 25 wt%; d 50 = 120 nm, purity ≥ 99.9%; Multi-walled carbon nanotubes (MWCNTs, outer diameter 10 nm, aspect ratio ≥ 100, 5-walled); mass percentage 3.75%; The layered graphene consists of 2-3 layers, with a lateral dimension of 2-5 μm and a purity ≥ 99%; the mass percentage content is 15%. Graphite powder (d 50 = 3 μm), accounting for 40% of the mass percentage of the negative electrode active material; CNT:graphene mass ratio = 1:4; Polyvinylidene fluoride (PVDF, precursor binder, which is converted into an amorphous carbon layer after carbonization, with a thickness of about 10 nm): 15% by mass.

[0060] Preparation steps: (a) The above raw materials were dispersed in an N-methylpyrrolidone (NMP) solution containing 1 wt% polyvinylpyrrolidone (PVP) (solid content 15%), and dispersed using a high-speed homogenizer (12000 rpm, 60 min) with ultrasonic assistance (300 W, 30 min). TEM analysis showed that the carbon nanotube (CNT) agglomeration rate was <3%. (b) Two-fluid spray drying (inlet temperature 180℃, outlet temperature 90℃, feed rate 10 mL / min), collecting D 50 = 13 μm spherical precursor; (c) Under an inert atmosphere (Ar), the temperature was increased to 800°C at 3°C / min and held for 2 h for carbonization. The sample was then cooled to room temperature in the furnace to obtain the sample of Example 1.

[0061] Characterization results: ID / IG = 0.72, I²D / IG = 1.38; d(002) = 3.37 Å; CNT outer diameter 9.8 nm (TEM statistics, n=50); BET specific surface area = 8.2 m² / g; compacted density = 1.62 g / cm³; D 50 = 13.1 μm.

[0062] Electrochemical performance: First-cycle ICE = 93.5%, 500-cycle capacity retention = 91.2%, full-charge electrode expansion rate = 12.3%, 10C / 0.1C rate ratio = 82.1%.

[0063] Examples 2-5 (Changes in the mass fraction of nano-silicon) The difference between this set of examples and Example 1 is that the mass fraction of silicon nanoparticles is changed to 15%, 20%, 30%, and 35%, respectively, and the proportions of other raw materials (CNT:graphene = 1:4, CNT outer diameter 10 nm, graphite powder 40%) are adjusted proportionally, while the rest are the same as in Example 1.

[0064] Examples 6-9 (Changes in the mass ratio of carbon nanotubes to layered graphene) The difference between this set of examples and Example 1 is that, while keeping Si = 25 wt%, CNT outer diameter 10 nm, and graphite powder accounting for 40% of the negative electrode active material unchanged, the mass ratio of carbon nanotubes to graphene is adjusted to 1:2, 1:3, 1:6, and 1:8, respectively. All other aspects are the same as in Example 1.

[0065] Examples 10-13 (CNT outer diameter variation) The difference between this set of examples and Example 1 is that multi-walled carbon nanotubes with outer diameters of 8 nm, 9 nm, 12 nm, and 15 nm (all aspect ratios ≥ 80) were selected, while other parameters (Si=25%, CNT:graphene=1:4, graphite=40%) were the same as in Example 1.

[0066] Examples 14-17 (Changes in the mass fraction of graphite powder) The difference between this set of examples and Example 1 is that the mass fraction of graphite powder in the negative electrode active material is adjusted to 32.5%, 41.2%, 48.7%, and 59.5%, respectively, and the absolute mass of graphene and CNT is adjusted accordingly. The CNT:graphene ratio of 1:4 and Si ratio of 25% are kept unchanged. All other aspects are the same as in Example 1.

[0067] Comparative Example 1 (Si mass fraction = 10%) The difference between this comparative example and Example 1 is that the mass fraction of nano-silicon is reduced to 10%, while the rest are the same as in Example 1. The results show that although the first-cycle ICE is improved to 94.5%, the specific capacity of the material is significantly reduced, the 10C rate performance deteriorates to 73.2%, and the overall energy density is insufficient, thus negating the significance of silicon-carbon composite.

[0068] Comparative Example 2 (Si mass fraction = 42%) The difference between this comparative example and Example 1 is that the mass fraction of silicon nanoparticles is increased to 42%, while the rest are the same as in Example 1. The ternary carbon network cannot completely encapsulate the excess Si particles, resulting in a sharp drop in the first-cycle ICE to 85.2%, a 500-cycle retention rate to 72.3%, and an electrode expansion rate soaring to 28.5%, leading to severe cycle failure.

[0069] Comparative Example 3 (CNT:Graphene = 1:1) The difference between this comparative example and Example 1 is that the mass ratio of carbon nanotubes to graphene was adjusted to 1:1 (with a higher CNT content), while the rest were the same as in Example 1. Excessive CNTs caused CNT agglomeration inside the particles (obvious CNT bundles were visible during TEM), the isotropy of the three-dimensional network was destroyed, the 500-cycle retention rate dropped to 83.5%, and the expansion rate increased to 15.8%.

[0070] Comparative Example 4 (CNT:Graphene = 1:10) The difference between this comparative example and Example 1 is that the mass ratio of carbon nanotubes to graphene was adjusted to 1:10, while all other aspects remained the same as in Example 1. Insufficient CNTs resulted in inadequate bridging between graphite microregions, poor continuity of the electron conduction network, and a rate performance of only 75.8% at 10C / 0.1C, with a 500-cycle retention rate of 82.8%.

[0071] Comparative Example 5 (CNT outer diameter = 5 nm) The difference between this comparative example and Example 1 is that single-walled carbon nanotubes (SWCNTs) with an outer diameter of 5 nm were used instead of MWCNTs; otherwise, they were the same as in Example 1. The poor dispersion of the fine-diameter CNTs (SDI = 18%) and severe aggregation led to an increase in local defects in the three-dimensional network, reducing the 500-cycle retention rate to 84.2% and the expansion rate to 15.5%.

[0072] Comparative Example 6 (CNT outer diameter = 20 nm) The difference between this comparative example and Example 1 is that a coarse-diameter MWCNT with an outer diameter of 20 nm is used; otherwise, they are the same as in Example 1. Coarse-diameter CNTs have high rigidity, reduced elastic buffering capacity, and a smaller contact area with graphene, resulting in insufficient network elasticity and a 500-cycle retention rate of only 84.8%.

[0073] Comparative Example 7 (Graphite powder content = 30%) The difference between this comparative example and Example 1 is that the mass fraction of graphite is reduced to 30% (supplemented by graphene), while the rest is the same as in Example 1. Graphite Li + With fewer embedded channels, rate performance decreases (10C / 0.1C = 76.8%), cycle retention drops to 83.6%, and overall dynamics deteriorate.

[0074] Comparative Example 8 (Graphite powder content = 60%) The difference between this comparative example and Example 1 is that the mass fraction of graphite is increased to 60%, and CNTs and graphene are significantly diluted; otherwise, they are the same as in Example 1. The insufficient CNT+graphene content leads to a lower density of the three-dimensional elastic network, weakened buffering effect, and a decrease in the 10C / 0.1C rate ratio to 74.5%, with a retention rate of 82.1%.

[0075] The above-mentioned negative electrode active materials were characterized as follows: The characterization methods involved in this invention cover multiple dimensions, including spectroscopy, diffraction, microscopy, thermal analysis, adsorption, and electrochemistry, which together constitute a complete characterization system for the structural fingerprint, morphological characteristics, and electrochemical performance of internally mixed three-dimensional carbon network silicon-carbon composite anode materials.

[0076] For spectral characterization, Raman spectroscopy with a laser wavelength of 532 nm was used to perform micro-spot scanning on individual composite particles. The intensity ratio of the D peak to the G peak (ID / IG ≤ 0.8) was used to characterize the degree of carbon phase ordering, and the intensity ratio of the 2D peak to the G peak (I²D / IG ≥ 1.2) was used to identify the presence of few-layer graphene. Furthermore, the G peak of multi-walled carbon nanotubes could be identified. + The peak and radial breathing mode (RBM), the three Raman features mentioned above, together constitute the low-cost, identifiable spectral fingerprint of this invention. X-ray photoelectron spectroscopy (XPS) was used to perform elemental analysis on the carbonized layer on the outer surface of the particles, determining the sp² hybridization ratio of C (≥85%) to evaluate the graphitization quality of the carbonized layer. Energy dispersive X-ray diffraction (EDX) was used to scan the cross-section of the negative electrode sheet after disassembly of the finished battery, quantitatively characterizing the F / Si molar ratio (0.3–0.8) at the silicon-carbon particle interface to verify the formation and composition range of the fluorine-containing SEI film. For diffraction characterization, X-ray diffraction (XRD) was used to determine the graphite interlayer spacing d(002) (3.35–3.42 Å) of the carbon phase and the integrated diffraction intensity of the Si(111) crystal plane. The grain size Lc(Si) (20–50 nm) of the silicon nanoparticles was calculated using the Scherrer equation to eliminate interference from amorphous silicon and confirm the crystal structure of the nano-silicon.

[0077] In terms of microscopic characterization, after preparing thin slices of particle cross-section using focused ion beam (FIB), the fine internal structure of the particles was directly observed using transmission electron microscopy (TEM) bright-field imaging. The outer diameter (8–15 nm), number of walls (3–10 layers), and aspect ratio (≥ 50) of multi-walled carbon nanotubes (MWCNTs) were statistically analyzed to confirm the structural feature of CNTs bridging and conducting across at least two graphite microregions. The number of graphene layers (2–5 layers) and their wrinkled coating state were verified by interlayer spacing measurement. The morphology of the electrode cross-section after 500 cycles was observed using scanning electron microscopy (SEM). The morphology retention rate was defined as the proportion of particles with clearly distinguishable particle boundaries to the total number of particles (≥ 80%), and the electrode thickness expansion rate (≤ 18%) was measured.

[0078] In terms of thermal analysis, thermogravimetric analysis (TGA) was used to program the temperature of the samples in an air atmosphere (up to 700℃). The characteristic oxidation exothermic peak in the 500-650℃ range (peak temperature 560-620℃) was used to identify multi-walled carbon nanotubes, which are different from single-walled CNTs (about 450℃) and ordinary carbon black, providing a verifiable thermogravimetric fingerprint of the carbon phase composition of the material.

[0079] For physical property characterization, the specific surface area (5–15 m² / g) of the material was determined at 77 K using the nitrogen adsorption-desorption (BET) method, and the mesopore size distribution (concentrated in the range of 2–10 nm) was analyzed using the BJH method. The proportion of mesopore volume to total pore volume (≥ 70%) and the total pore volume (≤ 0.08 cm³ / g) were calculated to prove that there was no pre-fabricated hollow cavity structure inside the particles. The density (D) of the composite particles was determined using a laser particle size analyzer. 50 (8~20 μm), D 10 and D 90 Particle size, calculate the particle size distribution span (D) 90 -D 10 ) / D 50 (≤ 1.5); the compacted density was determined by a powder compaction density meter under a pressure of 10 MPa (1.4~1.8 g / cm³), and the vibratory density was determined by a vibratory density meter (0.8~1.2 g / cm³).

[0080] In terms of electrochemical performance characterization, the material is made into a negative electrode sheet and assembled into a coin cell. The first-cycle coulombic efficiency (ICE, ≥ 88%, preferably ≥ 93%) is measured at 0.1C using a constant current charge-discharge tester. The capacity retention rate is evaluated after 500 cycles at 0.1C (≥ 85%, preferably ≥ 90%). The high-rate performance is characterized by the ratio of 10C to 0.1C charge-discharge capacity (≥ 75%, preferably ≥ 80%). The electrode thickness expansion rate is directly measured under full charge (≤ 15%). The above electrochemical indicators and BET specific surface area satisfy the quantitative correlation formula proposed in claim 3 of this invention (ICE ≥ -0.8 × SBET + 98.4), which can be used as an auxiliary criterion for technical identification.

[0081] The performance of the above embodiments and comparative examples is summarized in Table 1 below.

[0082] Table 1

[0083] Note: X1, X2, X3...X17 represent Example 1, Example 2, Example 3...Example 17 respectively; D1, D2, D3...D8 represent Comparative Example 1, Comparative Example 2, Comparative Example 3...Comparative Example 8 respectively.

[0084] As shown in Table 1, the first-cycle coulombic efficiency decreased (94.5% → 85.2%) as the Si mass fraction increased from 10% to 42%, while the electrode expansion rate increased sharply from 7.5% to 28.5%. Within the range of 15% to 35%, the volume fraction of the ternary carbon network (CNT + graphene + graphite) was sufficient to completely encapsulate the silicon particles in three dimensions. The CNT elastic framework and graphene wrinkled layer effectively absorbed the volume expansion of Si, keeping the electrode expansion rate below 17.8%. When Si > 35%, the carbon phase volume fraction was insufficient to provide complete three-dimensional encapsulation and buffering. The exposed area of ​​the silicon particles was in direct contact with the electrolyte during cycling, and the continuous growth of the SEI film consumed active lithium, causing the first-cycle ICE to drop below 90%, and accelerating cycle degradation. When Si < 15%, the specific capacity of the material decreased significantly, the volume effect of silicon became negligible, the design advantages of the ternary carbon network were not fully realized, rate performance deteriorated, and commercial value decreased. Therefore, 15% to 35% is the optimal balance range between the ternary carbon network system and the capacity and stability of Si, with the optimal point located around 25%.

[0085] CNTs and graphene play different structural functions in the three-dimensional network: CNTs act as an elastic "spring" framework, providing three-dimensional isotropic elastic stress buffering and long-range electronic conduction; graphene adheres closely to the surface of Si particles, providing a highly conductive interfacial contact layer and in-plane conductive pathways. When the CNT:graphene ratio is <1:2 (i.e., Comparative Example 3, 1:1), the CNT content is relatively too high, leading to CNT aggregation within the particles (CNT bundles visible in TEM), disrupting the isotropic elastic network, exacerbating local stress concentration, and increasing the expansion rate. When the CNT:graphene ratio is >1:8 (i.e., Comparative Example 4, 1:10), the CNT content is too low, resulting in insufficient bridging and conduction between graphite microregions, forming electron transport "islands," and causing a simultaneous decrease in rate performance and cycle retention. Within the range of 1:2 to 1:8, the synergistic effect of CNTs and graphene is optimal, with the best ratio being 1:4. At this ratio, I2D / IG = 1.38, indicating that graphene maintains a good few-layer structure and that the CNT-graphene interface has sufficient contact.

[0086] The outer diameter of CNTs determines the balance between their axial stiffness and radial elasticity: If the outer diameter is too small (<8 nm, such as the 5.5 nm SWCNT in comparative example 5), the CNTs are prone to agglomeration during NMP dispersion, resulting in a dispersion index (SDI) as high as 18%, leading to numerous local defects in the three-dimensional network and a decrease in both cycle retention and expansion control. If the outer diameter is too large (>15 nm, such as the 20 nm in comparative example 6), the CNTs have excessive bending stiffness, making it difficult to form a tight contact network with graphene and fine graphite powder, and reducing the elastic buffering capacity per unit mass (lower specific elastic modulus), thus weakening the cycle expansion control. In the range of 8–15 nm, MWCNTs possess both sufficient mechanical strength (preventing collapse during carbonization) and appropriate bending flexibility (ensuring uniform distribution within the particles), and the characteristic oxidation peak of TGA is located at 560–620 °C, clearly distinguishable from other carbon phases, forming an identifiable thermogravimetric fingerprint. The optimal value is 10 nm.

[0087] Graphite powder as a contributor to lithium intercalation capacity and Li in ternary carbon networks + The framework of the transport channel also serves to maintain the integrity of the particle structure. When the graphite powder content is below 32.5% (Comparative Example 7, 30%), the Li within the particles... + Insufficient transmission channels lead to increased electrochemical polarization, deteriorated kinetics, and a significant decrease in rate performance (10C / 0.1C = 76.8%), with a corresponding decrease in cycle retention. When the graphite powder content exceeds 59.5% (Comparative Example 8, 60%), the absolute content of CNTs and graphene is significantly diluted, resulting in insufficient density of the three-dimensional elastic buffer network, weakened Si expansion buffering effect, and a similar decrease in rate performance (10C / 0.1C = 74.5%). Within the range of 32.5%–59.5%, the Li provided by graphite powder... +The optimal balance is achieved between the channel and the elastic buffer provided by CNT / graphene, with the optimal point being 59.5%, at which point the ICE-BET relationship satisfies the quantitative correlation.

[0088] Compared with conventional technology, the present invention has the following advantages: 1. Compared with the traditional porous silicon / carbon CVD system, see Table 2; Table 2

[0089] 2. Comparison with other structures is shown in Table 3. Table 3

[0090] Based on the above comparison, this invention possesses sufficient inventiveness in the following three aspects: (1) Structural innovation: The concept of "internal hybrid three-dimensional elastic carbon conductive network" was proposed for the first time, which combines CNT (elastic framework), graphene (interface buffer layer), and graphite (Li) + The three components (channel, anode, and cathode) are designed collaboratively at the single-particle scale, rather than being simply stacked or coated on the surface, which is a structural innovation in the field of silicon-carbon composite anodes.

[0091] (2) Parameter innovation: The combination of parameters such as CNT:graphene mass ratio of 1:2 to 1:8, CNT outer diameter of 8 to 15 nm, and graphite mass percentage of 32.5% to 59.5% has not been disclosed in existing literature and patents, and experimental data show that this combination has unexpected effects that are superior to the single carbon phase system (the cycle retention rate is increased by 3 to 5 percentage points and the expansion rate is reduced by 3 to 5 percentage points).

[0092] (3) Detectability innovation: The product of this invention can be verified by Raman fingerprint (ID / IG ≤ 0.8, I2D / IG ≥ 1.2), which makes the detection cost extremely low.

[0093] The foregoing description and accompanying drawings fully illustrate embodiments of the invention to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Individual components and functions are optional unless explicitly required, and the order of operation may vary. Some portions and features of some embodiments may be included or substituted for portions and features of other embodiments. Embodiments of the invention are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from their scope. The scope of the invention is limited only by the appended claims.

Claims

1. A negative electrode active material, characterized in that, include: Nano-silicon and a three-dimensional carbon conductive mesh, wherein the nano-silicon is embedded in the three-dimensional carbon conductive mesh; the three-dimensional carbon conductive mesh comprises a mixture of carbon nanotubes, layered graphene and graphite powder; The mass percentage of the nano-silicon is set to 15%-35%; the mass percentage of the graphite powder is set to 32.5%-59.5%; and the mass ratio of the carbon nanotubes to the layered graphene is set to 1:2-8.

2. The negative electrode active material according to claim 1, characterized in that, The carbon nanotubes have an outer diameter of 8nm-15nm and an aspect ratio of 50 or greater.

3. The negative electrode active material according to claim 2, characterized in that, The carbon nanotubes are multi-walled carbon nanotubes with 3 to 10 walls.

4. The negative electrode active material according to any one of claims 1-3, characterized in that, The layered graphene has 2 to 5 layers, and the interlayer spacing d002 is 3.35 Å to 3.42 Å.

5. The negative electrode active material according to claim 4, characterized in that, The layered graphene adheres tightly to the surface of the nano-silicon in a wrinkled state.

6. The negative electrode active material according to claim 5, characterized in that, The D of the nano-silicon 50 The particle size ranges from 80 nm to 200 nm, with a particle size distribution span (D). 90 -D 10 ) / D 50 ≤ 1.

5.

7. The negative electrode active material according to claim 1, characterized in that, Also includes: A coating layer is applied to the outer surface of the three-dimensional carbon conductive mesh, with a thickness of 5nm-30nm.

8. The negative electrode active material according to claim 7, characterized in that, The relationship between the BET specific surface area and the first-cycle coulombic efficiency of the negative electrode active material is: ICE ≥ -0.8 × S + 98.4; where ICE is the first-cycle coulombic efficiency (%), and S is the BET specific surface area (m² / g); or, The negative electrode active material is granular with a median particle size D. 50 The thickness ranges from 8 μm to 20 μm, and the compacted density measured at 10 MPa is 1.4 g / cm³ to 1.8 g / cm³, while the vibratory density is 0.8 g / cm³ to 1.2 g / cm³.

9. A method for preparing the negative electrode active material according to any one of claims 1-8, characterized in that, include: Step a): Disperse nano-silicon, carbon nanotubes, layered graphene and graphite powder in a certain proportion in an N-methylpyrrolidone solution containing polyvinylpyrrolidone, and disperse under high shear until the particles are uniform. Step b): Use a two-fluid spray dryer with an inlet temperature of 150℃-200℃ to collect spherical precursor particles; Step c): Under an inert atmosphere, the material is heated to 700℃-900℃ at a rate of 2℃-5℃ / min for 1h-3h, and then naturally cooled to obtain the negative electrode active material.

10. A lithium-ion battery, comprising a negative electrode and an electrolyte, characterized in that, The negative electrode sheet is coated with the negative electrode active material as described in any one of claims 1-8, or the negative electrode active material obtained by the preparation method as described in claim 9; the electrolyte contains 0.5wt%-3wt% fluoroethylene carbonate, and the electrolyte forms a fluorine-containing film layer on the negative electrode active material, wherein the molar ratio of fluorine to silicon in the film layer is 0.3-0.8.

Images