Battery negative electrode composite material, preparation method and application

Abstract

The application discloses a battery negative electrode composite material, a preparation method and application. The battery negative electrode composite material comprises: nanometer graphite crystals; primary graphite crystal clusters comprising the nanometer graphite crystals and transition structure one adjacent to the nanometer graphite crystals; secondary graphite crystal clusters comprising the primary graphite crystal clusters and transition structure two adjacent to the primary graphite crystal clusters; silicon nanoparticles distributed in the secondary graphite crystal clusters; and doped elements distributed in the secondary graphite crystal clusters. The battery negative electrode composite material of the application takes nanometer graphite crystals with high graphitization degree and small particle size as a stable matrix, introduces doped elements to optimize the electronic structure and interface characteristics, and loads silicon active substances on the basis, so as to construct a new type of heteroatom-doped graphite crystal-based silicon-carbon negative electrode material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to negative electrode composite materials for batteries, their preparation methods, and their applications. Background Technology

[0002] Silicon-based anode materials, due to their extremely high theoretical specific capacity, have become a research focus for overcoming the current energy density bottleneck of graphite anodes. However, the dramatic volume expansion of silicon during charging and discharging can cause electrode material pulverization, repeated rupture and proliferation of the solid electrolyte interphase (SEI) film, ultimately leading to rapid capacity decay. In addition, the poor intrinsic conductivity of silicon and the slow lithium-ion diffusion also severely restrict its practical applications.

[0003] To address these issues, researchers have proposed various strategies, including nanostructuring, composite materials, and doping. Among these, combining silicon with carbon materials (especially graphite) to form silicon-carbon anodes is the mainstream technical approach. Graphite, as a matrix, can both buffer the volume changes of silicon and provide a conductive network. However, in traditional physical composite methods, the interfacial bonding between graphite and silicon is weak (mainly van der Waals forces), making it difficult to effectively constrain the expansion of silicon particles along the graphite sheet direction during cycling. This leads to interfacial delamination and capacity decay, resulting in limited performance improvement.

[0004] To address this, existing technologies have further developed strategies for porous carbon-supported silicon, aiming to utilize the pore space to accommodate the volume expansion of silicon. However, in-depth research has revealed that electrochemical performance and porosity are not simply positively correlated. Excessively high porosity and specific surface area can lead to insufficient electrolyte wetting, reduced coulombic efficiency in the first cycle, and persistent side reactions, ultimately impairing overall performance. This indicates that simply optimizing the physical structure is no longer sufficient to achieve breakthroughs.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a negative electrode composite material for batteries, a preparation method thereof, and its application, which is beneficial for improving the cycle stability of the negative electrode composite material.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a negative electrode composite material for batteries, comprising: Nano-graphite crystals; A primary graphite crystal cluster, comprising nano-graphite crystals and a transition structure adjacent to the nano-graphite crystals; A secondary graphite cluster includes a primary graphite cluster and a transition structure II adjacent to the primary graphite cluster; Silicon nanoparticles are distributed within the secondary graphite crystal clusters; The doping elements are distributed within the secondary graphite cluster.

[0008] In an optional embodiment, the nano-graphite crystals comprise uniformly oriented graphene sheets; And / or, the doping element is selected from at least one of nitrogen, phosphorus, boron, sulfur, fluorine, magnesium, aluminum, titanium, germanium, tin, copper, iron, zinc, sodium and potassium; And / or, the mass fraction of the doping element in the negative electrode composite material for the battery is 0.1% to 10%; And / or, the types and mass fractions of doping elements in different nano-graphite crystals, primary graphite crystal clusters or secondary graphite crystal clusters can be selected independently; And / or, the doping element is located on the surface of the nano-graphite crystals and / or between adjacent graphene sheets; And / or, at least one of intermolecular interaction forces, covalent bonds, and carbon-metal bonds exists between the doping element and the nano-graphite crystals; And / or, the types and / or mass fractions of doping elements in the nanographite crystals, the first transition structure, and the second transition structure are the same or different; And / or, each of the transition structure one and / or the transition structure two is independently selected from at least one of amorphous carbon and graphitized carbon.

[0009] In an optional embodiment, the mass fraction of dopant elements in the nanographite crystals, the first transition structure, and the second transition structure increases or decreases in a gradient. And / or, the graphene sheets in the nano-graphite crystals are doped with doping elements, and the interlayer spacing between the graphene sheets is 0.335 nm to 0.42 nm; after lithium intercalation between the graphene sheets, the interlayer spacing is increased to 0.37 nm to 0.45 nm. And / or, the surface of the nano-graphite crystal is provided with a doped modification layer, the thickness of which is 0.5 nm to 5 nm.

[0010] In an optional embodiment, the defect density in the nanographite crystals is less than the defect density of the first transition structure and the second transition structure. And / or, the nanographite crystals I D / I G ≤ 1.0; And / or, the first transition structure and / or the second transition structure I D / I G The value is 0.5~1.5; And / or, both the first transition structure and the second transition structure include a channel structure, wherein the mass of silicon nanoparticles located in the interlayer voids of the nanographite crystals, the channel structure of the first transition structure and the channel structure of the second transition structure accounts for more than 50% of the total mass of silicon nanoparticles. And / or, both the first transition structure and the second transition structure include a pore structure, wherein the pore structure includes mesopores of 2 nm to 20 nm.

[0011] In an optional embodiment, the silicon nanoparticles form chemical bonds with at least one of the nano-graphite crystals, transition structure one, and transition structure two, wherein the chemical bonds include at least one of Si-C bonds, Si-OC bonds, Si-N bonds, Si-P bonds, and Si-B bonds. And / or, the particle size of the silicon nanoparticles is 1 nm to 100 nm; And / or, the average particle size of the nanographite crystals is 1 nm to 100 nm; And / or, the average particle size of the primary graphite cluster is 50 nm to 500 nm; And / or, the average particle size of the secondary graphite clusters is 200 nm to 5 μm; And / or, the mass fraction of silicon nanoparticles in the negative electrode composite material for the battery is 1% to 60%.

[0012] In an optional embodiment, a carbon coating layer covering the secondary graphite cluster is also included; And / or, the average thickness of the carbon coating layer is 1 nm to 50 nm; And / or, the carbon coating layer accounts for 0.1% to 2% of the mass fraction of the secondary graphite cluster; And / or, a continuous conductive network is formed between the carbon coating layer and the secondary graphite cluster; And / or, the carbon coating layer is a doped carbon coating layer.

[0013] Secondly, the present invention provides a method for preparing the negative electrode composite material for batteries according to any one of the foregoing embodiments, comprising: The nano-graphite crystals are hierarchically assembled to prepare the secondary graphite crystal cluster precursor, which is a doped secondary graphite crystal cluster loaded with silicon nanoparticles. The secondary graphite cluster precursor is heat-treated to obtain the negative electrode composite material for the battery.

[0014] In an optional embodiment, the nanographite crystals are doped nanographite crystals, and the preparation method of the doped nanographite crystals includes: mixing a gaseous precursor containing dopant elements into a carbon source gas during the preparation of nanographite crystals by catalytic chemical vapor deposition; or mixing a nanographite crystal substrate with a compound containing dopant elements and treating it at 300 ℃~1200 ℃ for 1 h~10 h in an inert or reducing atmosphere. And / or, the graded assembly includes at least one of spray drying, template-induced assembly, and solvent evaporation-induced self-assembly, wherein, during the graded assembly process, transition structure one and transition structure two may or may not introduce doping elements; And / or, the loading method of the silicon nanoparticles includes at least one of chemical vapor deposition, liquid phase impregnation-reduction, and mechanical ball milling.

[0015] Thirdly, the present invention provides an electrode comprising the negative electrode composite material for batteries described in any of the foregoing embodiments.

[0016] Fourthly, the present invention provides an electrochemical energy storage device, including the electrode described in the foregoing embodiments.

[0017] The present invention has the following beneficial effects: The battery anode composite material of this application uses high graphitization degree and small particle size nano-graphite crystals as a stable matrix, and introduces doping elements to optimize electronic structure and interface characteristics, and loads silicon active material on this basis, thereby constructing a new type of heteroatom-doped graphite crystal-based silicon-carbon anode material. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of a negative electrode composite material for batteries.

[0020] Figure 2 HRTEM image of nitrogen-doped graphite nanocrystals obtained in Example 1; Figure 3 The SEM-EDS spectrum of the nitrogen-doped graphite nanocrystals obtained in Example 1; Figure 4 Raman spectra of nitrogen-doped graphite nanocrystals obtained in Example 1; Figure 5 The SEM-EDS spectrum of the phosphorus-doped graphite nanocrystals obtained in Example 2; Figure 6 Raman spectra of phosphorus-doped graphite nanocrystals obtained in Example 2; Figure 7 The SEM-EDS spectrum of the boron-doped graphite nanocrystals obtained in Example 3; Figure 8 Raman spectra of boron-doped graphite nanocrystals obtained in Example 3; Figure 9 The SEM-EDS spectrum of the nitrogen- and boron-doped nanographite crystals obtained in Example 4; Figure 10 Raman spectra of nitrogen- and boron-doped nanographite crystals obtained in Example 4; Figure 11 The image shows the SEM-EDS spectrum of undoped hierarchical graphite clusters loaded with silicon nanoparticles obtained in Comparative Example 8. Figure 12 The Raman spectrum of silicon nanoparticles loaded on undoped hierarchical graphite clusters obtained in Comparative Example 8 is shown.

[0021] Illustration: 001-Silicon nanoparticles; 002-Nanographite crystals; 003-Primary graphite crystal clusters; 004-Secondary graphite crystal clusters; 005-Transition structure one; 006-Transition structure two; 007-Doping element. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0023] In existing technologies, the composite of silicon and carbon materials is mostly achieved through simple physical mixing or single-layer coating, which lacks an effective buffering mechanism for silicon expansion stress. The uneven distribution of doping elements and the lack of spatial selectivity make it impossible to synergistically optimize electronic conductivity and ion transport dynamics, resulting in the battery's specific capacity, initial coulombic efficiency, and cycle life failing to meet the requirements of high-energy-density energy storage devices.

[0024] Therefore, embodiments of the present invention provide a negative electrode composite material for batteries, such as... Figure 1 As shown, it includes: Nano-graphite crystal 002; The primary graphite crystal cluster 003 includes nano-graphite crystals 002 and a transition structure 005 adjacent to the nano-graphite crystals 002; The secondary graphite cluster 004 includes a primary graphite cluster 003 and a transition structure 2 006 adjacent to the primary graphite cluster 003; Silicon nanoparticles 001 are distributed within the secondary graphite crystal cluster 004; Dopant element 007 is distributed within the secondary graphite cluster 004.

[0025] The present application uses high-graphitization, small-particle-size nano-graphite crystals 002 as a stable matrix for the negative electrode composite material for batteries, and introduces dopant element 007 to optimize the electronic structure and interface characteristics. On this basis, silicon active material is loaded, thereby constructing a new type of heteroatom-doped graphite crystal-based silicon-carbon negative electrode material.

[0026] It should be noted that the silicon nanoparticles 001 in this application can be loaded in at least one region of the nanographite crystals 002, transition structure one 005, and transition structure two 006. For example, they can be loaded only in the nanographite crystals 002, only in the transition structure one 005, only in the transition structure two 006, only in the nanographite crystals 002 and transition structure one 005, only in the nanographite crystals 002 and transition structure two 006, only in the two regions of transition structure one 005 and transition structure two 006, or loaded in the three regions of the nanographite crystals 002, transition structure one 005, and transition structure two 006, thereby achieving precise anchoring of silicon active materials at multiple scales.

[0027] It should be noted that the dopant element 007 in this application can be distributed in at least one region of the nano-graphite crystal 002, transition structure one 005, and transition structure two 006. For example, it can be distributed only in the nano-graphite crystal 002, only in the transition structure one 005, only in the transition structure two 006, only in the nano-graphite crystal 002 and transition structure one 005, only in the nano-graphite crystal 002 and transition structure two 006, only in the two regions of transition structure one 005 and transition structure two 006, or distributed in the three regions of the nano-graphite crystal 002, transition structure one 005, and transition structure two 006.

[0028] In an optional embodiment, the nano-graphite crystal 002 comprises graphene sheets with consistent orientation; the consistent orientation of the graphene sheets in the nano-graphite crystal 002 ensures efficient electron transport at the nanoscale.

[0029] In optional embodiments, the dopant element 007 is selected from at least one of nitrogen, phosphorus, boron, sulfur, fluorine, magnesium, aluminum, titanium, germanium, tin, copper, iron, zinc, sodium, and potassium. Introducing non-metallic elements into the graphite phase can alter the electronic state of graphite, enhancing its ability to acquire electrons and thus increasing the amount of lithium-ion insertion (e.g., dopant elements such as B and N can occupy lattice positions in carbon materials, forming substitutional doping. While introducing structural defects, it also regulates the electronic structure within the graphite sheets, thereby affecting the electrochemical performance of the graphite anode in terms of specific capacity, rate capability, and cycle life). Introducing metallic elements can enhance the material's conductivity, promote the uniform distribution of electrons on the surface of graphite particles, reduce polarization effects, and thus improve high-current charge-discharge performance.

[0030] In an optional embodiment, the mass fraction of dopant element 007 in the negative electrode composite material for the battery is 0.1% to 10%; this optimizes electron / ion transport kinetics, improves electrode reaction reversibility, and suppresses structural pulverization caused by silicon volume expansion.

[0031] In optional embodiments, the type and mass fraction of dopant element 007 in different nano-graphite crystals 002, primary graphite clusters 003, or secondary graphite clusters 004 can be selected independently; that is, the type and mass fraction of dopant element 007 in different nano-graphite crystals 002 can be the same or different, the type and mass fraction of dopant element 007 in different primary graphite clusters 003 can be the same or different, and the type and mass fraction of dopant element 007 in different secondary graphite clusters 004 can be the same or different.

[0032] In an optional embodiment, the dopant element 007 is located on the surface of the nanographite crystal 002 and / or between adjacent graphene sheets.

[0033] In an optional embodiment, at least one of intermolecular interaction forces, covalent bonds, and carbon-metal bonds exists between the dopant element 007 and the nanographite crystal 002, ensuring the stability of the dopant and its effective control over the electronic structure.

[0034] In an optional embodiment, the type and / or mass fraction of dopant element 007 in the nanographite crystal 002, the first transition structure 005, and the second transition structure 006 are the same or different; for example, the doping concentration of dopant element 007 in the nanographite crystal 002 is higher than the doping concentration in the first transition structure 005 and / or the second transition structure 006.

[0035] For example, the doping concentration of the dopant element 007 in the second transition structure 006 is higher than the doping concentration in the first transition structure 005 and / or the nanographite crystal 002.

[0036] For example, the nano-graphite crystal 002 is doped with a first dopant element 007, and the transition structure 1 005 and / or the transition structure 2 006 is doped with a second dopant element 007, wherein the first dopant element 007 and the second dopant element 007 are different.

[0037] For example, the doping concentration increases or decreases in a gradient along the directions of the nanographite crystal 002, transition structure 1 005 and transition structure 2 006.

[0038] In an optional embodiment, the transition structure 005 and / or the transition structure 006 are each independently selected from at least one of amorphous carbon and graphitized carbon. This balances strain buffering and conductive pathway construction, alleviating interfacial stress concentration during charging and discharging.

[0039] In an optional embodiment, the mass fraction of dopant element 007 in the nanographite crystal 002, the first transition structure 005, and the second transition structure 006 increases or decreases in a gradient.

[0040] In an optional embodiment, the graphene sheets in the nano-graphite crystal 002 are doped with dopant element 007, and the interlayer spacing between the graphene sheets is 0.335 nm to 0.42 nm. This interlayer doping not only provides more space for lithium-ion intercalation but also effectively alleviates the volume expansion stress during charging and discharging. After lithium intercalation between the graphene sheets, the interlayer spacing increases to 0.37 nm to 0.45 nm. The uniformly oriented graphene sheets form an "electron highway" from the inside to the outside of the particle, significantly reducing the internal resistance and polarization of the electrode.

[0041] In an optional embodiment, a doped modification layer is disposed on the surface of the nano-graphite crystals 002, the thickness of which is 0.5 nm to 5 nm. This surface doping modification layer can enhance interface stability and optimize the interfacial bonding with silicon nanoparticles 001.

[0042] In an optional embodiment, the defect density in the nano-graphite crystal 002 is less than the defect density of the first transition structure 005 and the second transition structure 006; a moderate defect density is beneficial for enhancing the wettability of the electrolyte and the ion adsorption capacity.

[0043] In an optional embodiment, the nano-graphite crystals 002 I D / I G ≤ 1.0 indicates that the graphite microcrystals have high order and few defects, which is conducive to rapid electron conduction and enhances structural stability.

[0044] In an optional embodiment, the transition structure 1 005 and / or the transition structure 2 006 I D / I G It has a strength of 0.5~1.5; it can balance conductivity and elastic buffering capacity, and take into account lithium-ion intercalation / deintercalation kinetics and volume change adaptability.

[0045] In an optional embodiment, both the first transition structure 005 and the second transition structure 006 include a channel structure, and the mass of silicon nanoparticles 001 located in the interlayer voids of the nanographite crystals 002, the channel structure of the first transition structure 005, and the channel structure of the second transition structure 006 accounts for more than 50% of the total mass of silicon nanoparticles 001; this multi-scale confinement design effectively prevents the migration and aggregation of silicon nanoparticles 001 during the cycling process.

[0046] In an optional embodiment, both transition structure 1 (005) and transition structure 2 (006) include a pore structure comprising mesopores of 2 nm to 20 nm. These mesopores provide channels for the rapid diffusion of lithium ions while reserving buffer space for the volume expansion of silicon.

[0047] In an optional embodiment, the silicon nanoparticles 001 form chemical bonds with at least one of the nanographite crystals 002, transition structure one 005, and transition structure two 006, the chemical bonds including at least one of Si-C bonds, Si-OC bonds, Si-N bonds, Si-P bonds, and Si-B bonds; this chemical bonding significantly enhances the interfacial bonding strength between silicon and the carbon matrix, preventing silicon from detaching from the carbon skeleton during volume expansion.

[0048] In an optional embodiment, the silicon nanoparticles 001 have a particle size of 1 nm to 100 nm; this shortens the lithium-ion diffusion path, fully releases the high capacity advantage of silicon, and simultaneously inhibits particle aggregation and breakage.

[0049] In an optional embodiment, the average particle size of the nano-graphite crystals 002 is 1 nm to 100 nm; together with silicon nanoparticles 001, a multi-level conductive framework is constructed to improve the overall electronic connectivity of the composite material.

[0050] In an optional embodiment, the average particle size of the primary graphite cluster 003 is 50 nm to 500 nm; forming locally conductive network nodes, supporting silicon active components and improving the uniformity of current distribution.

[0051] In an optional embodiment, the average particle size of the secondary graphite cluster 004 is 200 nm to 5 μm; as a macroscopic framework carrier, it provides mechanical support and maintains the overall structural integrity of the electrode.

[0052] In an optional embodiment, the mass fraction of silicon nanoparticles in the negative electrode composite material for the battery is 1% to 60%.

[0053] In an optional embodiment, the area ratio of the transition structure one in the cross-section of the negative electrode composite material particles for the battery is 2-6%, preferably 3-4%.

[0054] In an optional embodiment, the area ratio of the transition structure two in the cross-section of the negative electrode composite material particles for the battery is 5-10%, preferably 7-9%.

[0055] In an optional embodiment, the mass fraction of silicon nanoparticles 001 in the negative electrode composite material for the battery is 1%-60%; by adjusting the proportion of active material, a synergistic fit is achieved between capacity contribution and cycle stability.

[0056] In an optional embodiment, a carbon coating layer is also included to cover the secondary graphite cluster 004; this physically confines the volume change of the silicon component, suppresses electrolyte side reactions, and improves the first efficiency and cycle retention rate.

[0057] In an optional embodiment, the average thickness of the carbon coating layer is 1 nm to 50 nm; ensuring complete coating while maintaining lithium ion penetration efficiency, thus balancing protection and kinetic performance.

[0058] In an optional embodiment, the carbon coating layer accounts for 0.1% to 2% of the mass fraction of the secondary graphite cluster.

[0059] In an optional embodiment, a continuous conductive network is formed between the carbon coating layer and the secondary graphite cluster 004; this eliminates interfacial contact impedance and ensures efficient electron transport in the multi-scale structure.

[0060] In an optional embodiment, the carbon coating layer is a doped carbon coating layer, wherein the dopant element 007 in the doped carbon coating layer is the same as or different from the dopant element 007 in the nano-graphite crystal 002, the first transition structure 005 and / or the second transition structure 006.

[0061] The present invention also provides a method for preparing the negative electrode composite material for batteries according to any one of the foregoing embodiments, comprising: The nano-graphite crystals 002 are hierarchically assembled to prepare the precursor of the secondary graphite crystal cluster 004. The precursor of the secondary graphite crystal cluster 004 is a doped secondary graphite crystal cluster 004, which is loaded with silicon nanoparticles 001. The secondary graphite cluster 004 precursor is heat-treated to form a chemical bond between the silicon nanoparticles 001 and the multi-level graphite cluster structure, thereby obtaining the negative electrode composite material for the battery.

[0062] In an optional embodiment, the nano-graphite crystal 002 is a doped nano-graphite crystal 002, and the preparation method of the doped nano-graphite crystal 002 includes: when preparing nano-graphite crystal 002 by catalytic chemical vapor deposition, mixing a gas precursor containing dopant element 007 into the carbon source gas; or, mixing the nano-graphite crystal 002 substrate with a compound containing dopant element 007 and treating it at 300 ℃~1200 ℃ for 1 h~10 h in an inert or reducing atmosphere.

[0063] In an optional embodiment, the graded assembly includes at least one of spray drying, template-induced assembly, and solvent evaporation-induced self-assembly, wherein during the graded assembly process, the first transition structure 005 and the second transition structure 006 may or may not introduce dopant element 007.

[0064] In an optional embodiment, the loading method of the silicon nanoparticles 001 includes at least one of chemical vapor deposition, liquid phase impregnation-reduction, and mechanical ball milling.

[0065] The present invention also provides an electrode comprising the negative electrode composite material for batteries described in any of the foregoing embodiments.

[0066] This invention also provides an electrochemical energy storage device, including the electrodes described in the preceding embodiments. By differentially doping at three levels—nanographite crystals, transition regions, and graphite clusters—a gradient buffer network from microscopic to macroscopic is constructed: lower-level dopants enter the interlayer spaces of graphene sheets, widening the interlayer spacing through covalent bonds or carbon-metal bonds, providing pre-stored space for silicon expansion; the types and proportions of dopants in the transition regions exhibit a gradient change, utilizing the bonding between different elements to form a flexible interface capable of absorbing stress, dispersing and redirecting expansion stress, and preventing crack propagation along grain boundaries. Simultaneously, this multi-level doping strategy also constructs multi-dimensional conductive pathways: the uniformly oriented graphene sheets in the nanographite crystal core region, combined with orbital hybridization doping, lower the electron transition barrier, forming an efficient electron transport channel; the dopants in the transition regions introduce polar sites and defects, reducing lithium-ion interface impedance and accelerating ion transport kinetics. Furthermore, the dopants enriched in the transition region react with the silicon surface oxide layer to form an ion-conducting layer, which, together with the carbon coating layer, suppresses electrolyte side reactions and improves the first coulombic efficiency. Meanwhile, the large-sized dopants embedded between the graphite layers can appropriately widen the interlayer spacing, accommodate more lithium ions, and alleviate the volume fluctuations of the graphite crystals themselves. Through the synergistic effect of multi-level doping, the destructive energy of silicon volume expansion is converted into uniformly distributed reversible deformation, maintaining the integrity of the electrode structure.

[0067] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0068] I. Preparation of Doped Nanographite 002 Substrate Preparation Example 1: Preparation of Nitrogen-Doped Nanographite Crystals 002 A catalytic chemical vapor deposition (CVD) method was employed, using methane as the carbon source and nickel nanoparticles as the catalyst, to obtain nano-graphite crystal 002 substrates at 650 °C for 2 hours in a fluidized bed reactor. During the reaction, ammonia (NH3) was introduced into the carbon source gas as a nitrogen source, with a volume ratio of ammonia to methane of 1:10. The resulting nitrogen-doped nano-graphite crystal 002 particles were mainly distributed in the range of 5–20 nm, and Raman spectroscopy analysis was performed. I D / I G The value was 0.49, and ICP analysis showed that the nitrogen doping content was 3.2 at%, with nitrogen mainly existing in the form of pyridine nitrogen and graphitic nitrogen. Figure 2 The HRTEM image of Preparation Example 1 clearly shows graphite lattice fringes. The SEM-EDS and Raman spectra are shown below. Figure 3-4 As shown.

[0069] Preparation Example 2: Preparation of Phosphorus-Doped Nanographite Crystals 002 A high-temperature graphitization method was employed, using mesophase pitch as a precursor and adding 5% (w / w) triphenylphosphine as a phosphorus source. Treatment was carried out at 2800 °C for 5 hours under an argon atmosphere to obtain phosphorus-doped nano-graphite crystals (002). The resulting nano-graphite crystals (002) had a particle size mainly distributed in the range of 10–30 nm, and Raman spectroscopy was performed. I D / I G The value was 0.98, and ICP analysis showed that the phosphorus doping content was 1.8 at%. SEM-EDS and Raman spectra are as follows: Figure 5-6 As shown.

[0070] Preparation Example 3: Preparation of Boron-Doped Nanographite Crystals 002 A catalytic graphitization conversion method was employed, using phenolic resin as the carbon source, ferric nitrate as the catalyst precursor, and 3% (w / w) boric acid as the boron source. Carbonization was carried out at 900 °C for 2 hours under a nitrogen atmosphere, followed by treatment at 1100 °C for 1 hour to obtain boron-doped nano-graphite crystals (002). The particle size of the obtained nano-graphite crystals (002) was [determined by the following parameters in Raman spectroscopy]. I D / I G The value was 0.85, and ICP analysis showed a boron doping content of 2.1 at%. SEM-EDS and Raman spectra are as follows: Figure 7-8 As shown.

[0071] Preparation Example 4: Preparation of Multi-element Co-doped Nanoscale Graphite Crystals 002 Following the method of Preparation Example 1, ammonia (nitrogen source) and diborane (boron source) were simultaneously introduced during the reaction, controlling the molar ratio of nitrogen source to boron source to be 2:1, to obtain nitrogen-boron co-doped nano-graphite crystals 002. Raman spectroscopy. I D / I G The value was 0.58. ICP analysis showed that the nitrogen doping content was 2.5 at%, and the boron doping content was 1.2 at%. SEM-EDS and Raman spectra are as follows: Figure 9-10 As shown.

[0072] II. Hierarchical Assembly of Multilevel Graphite Clusters Preparation Example 5: Assembly of Nitrogen-Doped Hierarchical Graphite Clusters 1.5 g of nitrogen-doped nanographite crystals 002 prepared in Preparation Example 1 were dispersed in 500 mL of ethanol and ultrasonically dispersed for 30 minutes. A hierarchical assembly was performed using spray drying with an inlet air temperature of 200 ℃ and an outlet air temperature of 90 ℃, resulting in a multi-level graphite cluster structure composed of doped primary graphite clusters 003 and doped secondary graphite clusters 004.

[0073] During the spray drying process, ethanol evaporates rapidly, and nano-graphite crystals (5... Driven by surface tension, 20 nm crystals are sequentially stacked to form primary crystal clusters (80 nm). 200 nm) and secondary clusters (300 nm) 1 μm). Grain contact edges form incomplete transition regions due to orientation misalignment or defects, where disordered carbon mainly originates from incompletely graphitized organic matter or amorphous carbon at grain boundaries in the raw material. Within primary clusters, the graphite lattice is continuous and dense, while significant gaps exist between clusters. Transmission electron microscopy reveals that transition region one exhibits a gradual transition from lattice fringes to amorphous regions, while transition region two is a loose connecting layer between clusters. Nitrogen adsorption-desorption measurements of 3... The 15 nm mesopores are contributed by the stacked pores between these crystal clusters and the interstitial spaces of amorphous carbon.

[0074] Preparation Example 6: Assembly of Phosphorus-Doped Hierarchical Graphite Clusters Take 1.5 g of phosphorus-doped graphite nanocrystals 002 obtained in Preparation Example 2 and disperse them in 300 mL N In methylpyrrolidone, 5 g of polyvinylpyrrolidone was added as a structure directing agent, and solvent evaporation induced self-assembly was used: the solvent was slowly evaporated at 60 °C to form an assembly, and then heat-treated at 800 °C for 2 hours to obtain a phosphorus-doped multi-level graphite cluster structure.

[0075] Preparation Example 7: Assembly of Gradient-Doped Multilevel Graphite Clusters The nitrogen-doped nanographite crystals 002 of Preparation Example 1 and the boron-doped nanographite crystals 002 of Preparation Example 3 were mixed at a mass ratio of 3:1, dispersed in 500 mL of ethanol according to the method of Preparation Example 5, and graded assembly was performed. A nitrogen-containing organic amine (2 wt%) was added to the ethanol as a doping supplement to form a gradient doping distribution between transition structure 1 005 and transition structure 2 006.

[0076] III. Loading of Silicon Nanoparticles 001 Example 1: Nitrogen-doped hierarchical graphite clusters supporting Si composite nanoparticles 50 g of the nitrogen-doped hierarchical graphite cluster structure prepared in Preparation Example 5 was placed in a fluidized bed reactor. Under argon protection, the temperature was raised to 600 °C, and silane (SiH4) gas was introduced at a flow rate of 20 L / min and the reaction was maintained at this temperature for 3 hours. After the reaction was completed, the temperature was lowered to room temperature under an argon atmosphere to obtain a composite material loaded with silicon nanoparticles 001. ICP-MS analysis showed that the total content of silicon nanoparticles 001 was 36 wt%.

[0077] Example 2: Phosphorus-doped hierarchical graphite clusters supporting Si composite nanoparticles 50 g of the phosphorus-doped hierarchical graphite cluster structure prepared in Preparation Example 6 was placed in a fluidized bed reactor. The reactor was heated to 580 °C under argon protection, and silane (SiH4) gas was introduced at a flow rate of 15 L / min. The reaction was maintained at this temperature for 4 hours. After the reaction was completed, a composite material loaded with silicon nanoparticles 001 was obtained.

[0078] Example 3: Gradient-doped multi-level graphite clusters supporting Si composite nanoparticles 50 g of the gradient-doped hierarchical graphite cluster structure prepared in Example 7 was placed in a fluidized bed reactor. Under argon protection, the temperature was raised to 620 °C, and silane (SiH4) gas was introduced at a flow rate of 25 L / min. The reaction was maintained at this temperature for 3.5 hours. After the reaction was completed, a composite material loaded with silicon nanoparticles 001 was obtained.

[0079] Example 4: Carbon Coating Treatment Take 50 g of the composite material obtained in Example 1 and place it in a fluidized bed reactor. Under argon protection, heat to 550°C, introduce acetylene gas at a flow rate of 15 L / min, and maintain the temperature for 2 hours to form a carbon coating layer on the surface of the composite material, wherein the mass fraction of the carbon coating layer is 0.5 wt%.

[0080] IV. Comparative Example Comparative Example 1: Silicon nanoparticles supported on undoped hierarchical graphite clusters 001 Following the method of Preparation Example 5, but using undoped nanographite crystals 002 (prepared according to the method of Preparation Example 1 but without ammonia) for hierarchical assembly. Silicon nanoparticles 001 were loaded according to the method of Example 1. The resulting material contained 35 wt% silicon but no doping elements 007. SEM-EDS and Raman spectra are shown below. Figure 11-12 As shown.

[0081] Comparative Example 2: Uniformly doped nanographite crystals 002 (without hierarchical structure) loaded with silicon nanoparticles 001 Take g of nitrogen-doped graphite nanocrystals 002 (without hierarchical assembly) prepared in Preparation Example 1, and directly load silicon nanoparticles 001 onto them according to the method in Example 1. The resulting material does not have a hierarchical graphite cluster structure, and the silicon nanoparticles 001 are only randomly distributed on the surface of the graphite nanocrystals 002.

[0082] Comparative Example 3: Multi-level graphite clusters (undoped) physically mixed with silicon nanoparticles 001 50 g of the nitrogen-doped hierarchical graphite cluster structure prepared in Example 5 was mechanically mixed with 15 g of silicon powder with a particle size of 30-50 nm, ball-milled for 4 hours, and then heat-treated at 800 °C for 2 hours under an argon atmosphere. The resulting material contained 33 wt% silicon, but the silicon and the carbon matrix were only in physical contact and there was no chemical bonding.

[0083] Comparative Example 4: Multilevel graphite clusters with only single-element doping (without gradient distribution) The method of Preparation Example 5 was followed, but only a single dopant element 007 (nitrogen doping only) was used, and doping control was not performed in the transition region. Silicon nanoparticles 001 were loaded according to the method of Example 1.

[0084] Comparative Example 5: Composite materials with high silicon content but no transition region design Following the method of Preparation Example 5, the inlet air temperature was adjusted to 300 °C, resulting in over-assembly during the graded assembly process. This caused the disappearance of transition structure 1 005 and transition structure 2 006, forming a dense structure. Silicon nanoparticles 001 were loaded according to the method of Example 1, with the silicon content controlled at 45 wt%.

[0085] Comparative Example 6: Commercial Graphite / Silicon Physical Hybrids 50 g of commercially available artificial graphite was mechanically mixed with 15 g of nano-silicon powder and ground for 4 hours to obtain a composite material. This material does not possess the hierarchical structure and interfacial chemical bonding of this invention.

[0086] Comparative Example 7: Silicon nanoparticles loaded only by primary graphite clusters (no secondary assembly) 1.5 g of nitrogen-doped nanographite crystals 002 prepared in Example 1 were dispersed in 500 mL of ethanol and ultrasonically dispersed for 30 minutes. A spray-drying method was used for graded assembly, but by adjusting the process parameters (inlet air temperature 120℃, outlet air temperature 50℃, solid content reduced to 0.5 wt%), the nanographite crystals were made to assemble only into primary graphite clusters 003, suppressing the formation of secondary graphite clusters 004. Silicon nanoparticles 001 were loaded according to the method of Example 1 to obtain a silicon-carbon composite material with only primary structure.

[0087] Comparative Example 8: Composite materials in which mesoporous carbon spheres replace nano-graphite crystals (1) Preparation of nitrogen-doped mesoporous carbon spheres: Using commercially available phenolic resin (20 g) as the carbon source and F127 as the template (5 g), ordered mesoporous carbon spheres (average particle size 200 nm, mesopore size 6 nm) were prepared by evaporation-induced self-assembly. Nitrogen doping treatment (heat treatment with ammonia at 900 °C) was performed according to the method of Preparation Example 1 to obtain nitrogen-doped mesoporous carbon spheres (nitrogen content approximately 2.5 at%).

[0088] (2) Assembly and Silicon Deposition: 1.5 g of the nitrogen-doped mesoporous carbon spheres were taken and spray-dried according to the method of Preparation Example 5. Since the mesoporous carbon spheres are spherical particles, they are difficult to further self-assemble into secondary crystal clusters. The product remains dispersed spherical particles (particle size 200-300 nm) without obvious secondary structure or transition regions. Silicon deposition and carbon coating were performed according to the method of Example 1 to obtain a mesoporous carbon sphere-based silicon-carbon composite material. SEM-EDS and Raman spectra are shown below. Figure 11-12 As shown. In Raman spectra I D / I G = 1.38.

[0089] V. Characterization and Testing (1) Structural parameter testing.

[0090] The doping level of nano-graphite crystals 002 was determined by ICP analysis; The average particle size of nano-graphite crystals 002, primary graphite crystal clusters 003, and secondary graphite crystal clusters 004 was obtained by statistical analysis of no fewer than 100 particles using transmission electron microscopy. The proportion of silicon in the pore structure was determined by ICP to measure the total silicon content; The area ratios of transition structure 1 (005) and transition structure 2 (006) in the cross-section were observed and statistically analyzed using a transmission electron microscope.

[0091] (2) Electrode preparation and electrochemical performance testing Electrode preparation: The composite materials obtained in each example and comparative example were mixed with conductive carbon black and sodium alginate binder at a mass ratio of 80:10:10. An appropriate amount of deionized water was added to form a slurry, which was then coated onto copper foil. The slurry was dried in a vacuum drying oven at 120°C for 12 hours and cut into circular electrode sheets with a diameter of 12 mm. The active material loading was approximately 1.2~1.5 mg / cm³. 2 .

[0092] Battery assembly: Using lithium metal sheets as the counter electrode, Celgard 2400 as the separator, and 1M LiPF6 EC / DMC (volume ratio 1:1, containing 5% FEC) solution as the electrolyte, CR2032 coin cells were assembled in an argon glove box.

[0093] Electrochemical testing: Constant current charge-discharge tests were conducted on the Blue Battery testing system, with a voltage range of 0.01~1.5V and a test temperature of 25℃. Cyclic performance testing current density was 0.1C (1C=1000 mA / g), and rate performance testing current densities were 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C.

[0094] VI. Test Results Table 1. Composition and structural parameters of composite materials in each embodiment and comparative example.

[0095] Table 2. Electrochemical performance test results of each embodiment and comparative example.

[0096] VII. Results Analysis As can be seen from the data in Tables 1 and 2, this invention significantly improves the electrochemical performance of silicon-carbon anodes by constructing a multi-level graphite cluster structure and achieving gradient doping. Example 1 (nitrogen-doped, with a transition region) shows significantly better first-cycle coulombic efficiency (89.2%), 100-cycle capacity retention (94.5%), and rate performance (81.2%) than the undoped Comparative Example 1 (73.6%, 68.2%, 58.3%) and the Comparative Example 7 (78.4%, 46.8%, 46.7%) with only a primary structure, indicating that the mesoporous structure of the secondary clusters and transition region is crucial for buffering silicon expansion and stabilizing the SEI film. Example 3 (nitrogen-boron co-doped, with a gradient distribution in the transition region) further improves the first-cycle efficiency to 90.4%, the 200-cycle capacity retention to 92.4%, and the 5C rate ratio to 85.7%, which is superior to the uniformly doped Examples 1 and 2, demonstrating that the gradient change of dopant elements in the transition region can more effectively disperse stress and optimize ion transport. Example 4, with carbon coating, achieved an initial efficiency of 92.1% and a 100-cycle retention of 97.2%, verifying the interfacial stabilizing effect of the outer carbon shell. Conversely, Comparative Example 4 (doped only with nano-graphite crystals and no doping in the transition region) showed significantly lower initial efficiency (81.1%) and cycle retention (72.5%) compared to the gradient-doped samples. Comparative Example 5 (without a transition region), despite its high silicon content, exhibited the worst cycle performance (45.6%), confirming that the transition region is crucial for the multi-level buffer network. Comparative Example 8, which replaced nano-graphite crystals with mesoporous carbon spheres, showed a complete performance degradation, demonstrating that the graphene sheet structure with uniformly oriented nano-graphite crystals is irreplaceable. In summary, the multi-level gradient doping design of this invention transforms silicon expansion stress into reversible strain while simultaneously constructing an efficient electron / ion pathway, achieving synergistic optimization of high capacity, high initial efficiency, and long cycle life.

[0097] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A negative electrode composite material for batteries, characterized in that, include: Nano-graphite crystals; A primary graphite crystal cluster, comprising nano-graphite crystals and a transition structure adjacent to the nano-graphite crystals; A secondary graphite cluster includes a primary graphite cluster and a transition structure II adjacent to the primary graphite cluster; Silicon nanoparticles are distributed within the secondary graphite crystal clusters; The doping elements are distributed within the secondary graphite cluster.

2. The negative electrode composite material for batteries according to claim 1, characterized in that, The nano-graphite crystals comprise graphene sheets with consistent orientation; And / or, the doping element is selected from at least one of nitrogen, phosphorus, boron, sulfur, fluorine, magnesium, aluminum, titanium, germanium, tin, copper, iron, zinc, sodium and potassium; And / or, the mass fraction of the doping element in the negative electrode composite material for the battery is 0.1% to 10%; And / or, the types and mass fractions of doping elements in different nano-graphite crystals, primary graphite crystal clusters or secondary graphite crystal clusters are selected independently; And / or, the doping element is located on the surface of the nano-graphite crystals and / or between adjacent graphene sheets; And / or, at least one of intermolecular interaction forces, covalent bonds, and carbon-metal bonds exists between the doping element and the nano-graphite crystals; And / or, the types and / or mass fractions of doping elements in the nanographite crystals, the first transition structure, and the second transition structure are the same or different; And / or, each of the transition structure one and / or the transition structure two is independently selected from at least one of amorphous carbon and graphitized carbon.

3. The negative electrode composite material for batteries according to claim 2, characterized in that, The mass fraction of doping elements in the nanographite crystals, transition structure one, and transition structure two increases or decreases in a gradient. And / or, the graphene sheets in the nano-graphite crystals are doped with doping elements, and the interlayer spacing between the graphene sheets is 0.335 nm to 0.42 nm; after lithium intercalation between the graphene sheets, the interlayer spacing is increased to 0.37 nm to 0.45 nm. And / or, the surface of the nano-graphite crystal is provided with a doped modification layer, the thickness of which is 0.5 nm to 5 nm.

4. The negative electrode composite material for batteries according to claim 1, characterized in that, The defect density in the nano-graphite crystals is less than the defect density of transition structure one and transition structure two. And / or, the nanographite crystals I D / I G ≤ 1.0; And / or, the first transition structure and / or the second transition structure I D / I G The value is 0.5~1.5; And / or, both the first transition structure and the second transition structure include a channel structure, wherein the mass of silicon nanoparticles located in the interlayer voids of the nanographite crystals, the channel structure of the first transition structure and the channel structure of the second transition structure accounts for more than 50% of the total mass of silicon nanoparticles. And / or, both the first transition structure and the second transition structure include a pore structure, wherein the pore structure includes mesopores of 2 nm to 20 nm.

5. The negative electrode composite material for batteries according to claim 1, characterized in that, The silicon nanoparticles form chemical bonds with at least one of the nano-graphite crystals, transition structure one, and transition structure two, wherein the chemical bonds include at least one of Si-C bonds, Si-OC bonds, Si-N bonds, Si-P bonds, and Si-B bonds. And / or, the particle size of the silicon nanoparticles is 1 nm to 100 nm; And / or, the average particle size of the nanographite crystals is 1 nm to 100 nm; And / or, the average particle size of the primary graphite cluster is 50 nm to 500 nm; And / or, the average particle size of the secondary graphite clusters is 200 nm to 5 μm; And / or, the mass fraction of silicon nanoparticles in the negative electrode composite material for the battery is 1% to 60%.

6. The negative electrode composite material for batteries according to claim 1, characterized in that, It also includes a carbon coating layer that covers the secondary graphite crystal clusters; And / or, the average thickness of the carbon coating layer is 1 nm to 50 nm; And / or, the carbon coating layer accounts for 0.1% to 2% of the mass fraction of the secondary graphite cluster; And / or, a continuous conductive network is formed between the carbon coating layer and the secondary graphite cluster; And / or, the carbon coating layer is a doped carbon coating layer.

7. A method for preparing a negative electrode composite material for batteries according to any one of claims 1-6, characterized in that, include: The nano-graphite crystals are hierarchically assembled to prepare the secondary graphite crystal cluster precursor, which is a doped secondary graphite crystal cluster loaded with silicon nanoparticles. The secondary graphite cluster precursor is heat-treated to obtain the negative electrode composite material for the battery.

8. The method for preparing the negative electrode composite material for batteries according to claim 1, characterized in that, The nano-graphite crystals are doped nano-graphite crystals, and the preparation method of the doped nano-graphite crystals includes: mixing a gas precursor containing doped elements into a carbon source gas during the preparation of nano-graphite crystals by catalytic chemical vapor deposition; or mixing a nano-graphite crystal substrate with a compound containing doped elements and treating it at 300 ℃~1200 ℃ for 1 h~10 h in an inert or reducing atmosphere. And / or, the graded assembly includes at least one of spray drying, template-induced assembly, and solvent evaporation-induced self-assembly, wherein, during the graded assembly process, transition structure one and transition structure two may or may not introduce doping elements; And / or, the loading method of the silicon nanoparticles includes at least one of chemical vapor deposition, liquid phase impregnation-reduction, and mechanical ball milling.

9. An electrode, characterized in that, Includes the negative electrode composite material for batteries as described in any one of claims 1-6.

10. An electrochemical energy storage device, characterized in that, Includes the electrode as described in claim 9.

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