Microcrystalline graphite composite material and preparation method thereof

Abstract

The invention provides a microcrystalline graphite composite material and a preparation method thereof, and relates to the technical field of lithium batteries, the microcrystalline graphite composite material comprises a core layer, a transition layer coated on the surface of the core layer, and a coating layer coated on the surface of the transition layer; the core layer comprises first microcrystalline graphite; the transition layer comprises second microcrystalline graphite and first amorphous carbon which are mixed with each other; the coating layer comprises second amorphous carbon; the second microcrystalline graphite is nitrogen-doped microcrystalline graphite, and the particle size of the second microcrystalline graphite is smaller than that of the first microcrystalline graphite. According to the composite material, a nitrogen-doped small-particle-size transition layer is used for constructing a gradient structure, stress is relieved, strength is enhanced, crushing and stripping are inhibited, and the compaction density is improved; and meanwhile, conductive transmission is optimized, and circulation and rate capability are synergistically improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and more specifically, to a microcrystalline graphite composite material and its preparation method. Background Technology

[0002] With societal development and the increasing demand for new energy sources, lithium-ion batteries, as efficient and environmentally friendly chemical energy storage devices, have been widely used in consumer electronics, electric vehicles, and energy storage power stations. As one of the four key materials for lithium-ion batteries, the anode material has a crucial impact on the battery's energy density, cycle life, and rate performance. Among numerous anode materials, graphite, with its high conductivity, good layered structure stability, and low lithium insertion / extraction potential, has become the most commercially mature anode material for lithium-ion batteries.

[0003] Traditional graphite anode materials mainly include artificial graphite and natural graphite. While artificial graphite possesses excellent rate performance and electrolyte compatibility, it suffers from complex processing, high cost, and supply shortages. In contrast, microcrystalline graphite (also known as amorphous graphite), as an important natural graphite resource, is composed of randomly stacked tiny graphite grains. It boasts significant advantages such as abundant reserves, low cost, and good isotropic structure, demonstrating enormous application potential in the field of lithium-ion battery anode materials. To improve the surface properties and electrochemical performance of microcrystalline graphite, existing technologies typically employ various modification methods to enhance its performance, such as surface coating, morphology control, or doping modification. The most common method is to coat the surface of microcrystalline graphite particles with a layer of amorphous carbon to reduce electrolyte side reactions and improve material stability.

[0004] However, natural microcrystalline graphite still faces many challenges in practical industrial applications. Due to its small crystal size (typically less than 1 μm) and numerous internal pores, microcrystalline graphite exhibits low mechanical strength. During the processing of lithium-ion battery electrodes, especially in high-pressure rolling processes, the insufficiently strong microcrystalline graphite particles are prone to breakage, generating a large amount of fine powder. This not only affects the consistency of electrode quality but also leads to poor slurry stability. Furthermore, microcrystalline graphite has a low tap density (typically only 0.4 g / cm³). 3 ~1.5g / cm 3 This also directly limits the compaction density of the electrode and the overall volumetric energy density of the battery.

[0005] More importantly, existing coating modification technologies often fail to address the fundamental issue of material structural stability. Traditional coating methods typically form an amorphous carbon layer directly on the surface of microcrystalline graphite. However, there are significant differences in physical properties between the microcrystalline graphite matrix and the outer amorphous carbon layer, particularly in their elastic moduli. During long-term charge-discharge cycles, the active particles undergo volume expansion and contraction due to repeated lithium-ion insertion and extraction. This mismatch in mechanical properties between the matrix and the coating layer makes the coating layer highly susceptible to peeling, breakage, or detachment. This structural damage not only leads to the loss of active materials but also causes electrolyte re-penetration into the particles, triggering continuous side reactions and ultimately significantly shortening the battery's cycle life. Therefore, overcoming the low mechanical strength of microcrystalline graphite and the structural instability caused by the mismatch in interfacial mechanical properties has become a pressing technical challenge in this field.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a microcrystalline graphite composite material and its preparation method. The microcrystalline graphite composite material constructs a three-layer gradient structure by introducing a nitrogen-doped small-particle-size transition layer, which effectively relieves interfacial stress and enhances mechanical strength. While suppressing particle breakage and peeling, it optimizes electrical conductivity and synergistically improves the compaction density, cycle life and rate performance of the material.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a microcrystalline graphite composite material, comprising: a core layer, a transition layer covering the surface of the core layer, and a covering layer covering the surface of the transition layer; The core layer comprises a first microcrystalline graphite; The transition layer comprises a mixture of a second microcrystalline graphite and a first amorphous carbon. The coating layer contains a second amorphous carbon; In an optional embodiment, the particle size D50 of the first microcrystalline graphite is 6 μm to 20 μm; and / or, The first microcrystalline graphite has a particle size D50 of 11 μm to 17 μm; and / or, The particle size D50 of the second microcrystalline graphite is 1.5 μm to 5 μm; and / or, The ratio of the particle size D50 of the second microcrystalline graphite to the particle size D50 of the first microcrystalline graphite is (0.2~0.4):1; and / or, The mass ratio of the second microcrystalline graphite to the first microcrystalline graphite is (0.05–0.2):1; and / or, The thickness ratio of the transition layer to the core layer is (0.2–0.4):1; and / or, The nitrogen content in the second microcrystalline graphite is 0.05%~5%; and / or, The nitrogen content in the second microcrystalline graphite is 0.1% to 1%.

[0009] In an optional embodiment, the specific surface area of ​​the first microcrystalline graphite is 5 m² / g to 20 m² / g; and / or, The tap density of the first microcrystalline graphite is 0.6 g / mL to 1.2 g / mL; and / or, The carbon purity of the first microcrystalline graphite is 85%–99.999%; and / or, The particle size distribution SPAN value of the first microcrystalline graphite is 1.0~1.3; and / or, The particle size distribution SPAN value of the second microcrystalline graphite is 0.8~1.2.

[0010] In an optional embodiment, the coating layer is an asphalt carbonization layer; and / or, The elastic modulus of the microcrystalline graphite composite material varies in a gradient from the inside to the outside.

[0011] Secondly, the present invention provides a method for preparing a microcrystalline graphite composite material as described in any of the foregoing embodiments, comprising: A first microcrystalline graphite and a second microcrystalline graphite are provided respectively; wherein the second microcrystalline graphite is nitrogen-doped and has a smaller particle size than the first microcrystalline graphite. The first microcrystalline graphite, the second microcrystalline graphite, and the binder are granulated to form a transition layer on the surface of the first microcrystalline graphite, thus obtaining an intermediate composite material. A carbon source is coated onto the surface of the intermediate composite material and then carbonized to form a coating layer.

[0012] In an optional embodiment, the step of providing the second microcrystalline graphite includes: ball milling or high-pressure homogenization of the microcrystalline graphite raw material to obtain small-particle-size microcrystalline graphite, and mixing the small-particle-size microcrystalline graphite with a nitrogen source and then performing carbonization treatment. Preferably, the carbonization temperature is 800℃~1200℃; Preferably, the nitrogen source includes at least one of melamine and urea; Preferably, the microcrystalline graphite raw material is the first microcrystalline graphite.

[0013] In an optional embodiment, the mass ratio of the microcrystalline graphite raw material to the nitrogen source is 1:(0.1~0.3); and / or, The granulation process is carried out at a temperature of 600℃~800℃; and / or, The adhesive includes bitumen; and / or, Following the step of forming the coating layer, the process further includes sieving and demagnetizing the material; and / or, The mass ratio of the binder to the second microcrystalline graphite is 3% to 20%; preferably, the mass ratio of the binder to the second microcrystalline graphite is 5% to 10%.

[0014] Thirdly, the present invention provides a battery negative electrode comprising a microcrystalline graphite composite material as described in any of the foregoing embodiments; or comprising the microcrystalline graphite composite material prepared by the preparation method described in any of the foregoing embodiments.

[0015] Fourthly, the present invention provides a battery including a battery negative electrode as described in the foregoing embodiments.

[0016] Fifthly, the present invention provides an electrical device including a battery as described in the foregoing embodiments.

[0017] This invention provides a microcrystalline graphite composite material and its preparation method. Compared with existing technologies, this microcrystalline graphite composite material, based on its structural characteristics, adopts a three-layer composite structure: a microcrystalline graphite core layer, a nitrogen-doped small-particle-size microcrystalline graphite transition layer, and a coating layer. By introducing a transition layer composed of small-particle-size nitrogen-doped microcrystalline graphite and heat-treated amorphous carbon from a binder, a buffer region for physical and chemical properties is constructed between the microcrystalline graphite core and the outer amorphous carbon layer. Nitrogen doping enhances the electrochemical performance of the microcrystalline graphite. Furthermore, by optimizing the composition ratio of nitrogen-doped microcrystalline graphite to the binder in the transition layer, the elastic modulus of the transition layer is adjusted, placing it between that of the microcrystalline graphite core layer and the coating layer. This design effectively improves the interface mismatch problem caused by the large difference in elastic modulus between microcrystalline graphite and the coating layer in existing technologies, achieving a smooth transition of the overall elastic modulus of the particles, thereby significantly enhancing the mechanical strength and structural stability of the material. During high-pressure processing such as battery electrode rolling, this structure can effectively disperse stress, reduce particle breakage and fine powder generation, and improve the compaction density and processing performance of the electrode.

[0018] Meanwhile, the microcrystalline graphite particles used in the transition layer are smaller than those in the core layer, allowing the small particles to tightly fill the surface defects and pores of the large-particle core layer. This not only modifies the core layer surface but also increases interlayer bonding, suppressing the coating peeling phenomenon caused by volume expansion during charging and discharging. Furthermore, nitrogen doping in the transition layer introduces additional active sites and electron transport channels, enhancing the material's conductivity and facilitating rapid lithium-ion transport. This synergistically improves the material's rate performance and cycle life, solving the problems of easy pulverization and rapid capacity decay of microcrystalline graphite anode materials during long cycles. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic flowchart illustrating the preparation method of microcrystalline graphite composite material in the embodiments of this application. Detailed Implementation

[0021] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0022] This application provides a microcrystalline graphite composite material, comprising: a core layer, a transition layer covering the surface of the core layer, and a coating layer covering the surface of the transition layer; the core layer comprises a first microcrystalline graphite; the transition layer comprises a mixture of a second microcrystalline graphite and a first amorphous carbon; the coating layer comprises a second amorphous carbon; wherein the second microcrystalline graphite is nitrogen-doped microcrystalline graphite, and the particle size of the second microcrystalline graphite is smaller than the particle size of the first microcrystalline graphite.

[0023] The overall structure of the aforementioned microcrystalline graphite composite material is a three-layer composite structure consisting of a core layer, a transition layer, and a coating layer. Specifically, it may include: (1) Core layer: As the central part of the composite material, it is the main structure of the material. Its composition includes "first microcrystalline graphite". It can be a natural graphite resource with the characteristics of low cost and good isotropic structure.

[0024] (2) Transition layer: The intermediate layer located between the core layer and the outermost coating layer, covering the surface of the core layer. Its composition includes a mixture of "second microcrystalline graphite" and "first amorphous carbon". The "second microcrystalline graphite" consists of microcrystalline graphite particles with smaller particle size, and has undergone nitrogen doping treatment (nitrogen atoms are incorporated into the graphite carbon skeleton); the "first amorphous carbon" is distributed between the second microcrystalline graphite particles and between the second microcrystalline graphite and the core layer. It is usually formed by the carbonization of a binder (such as pitch) added during the preparation process through heat treatment, which plays a role in firmly bonding and anchoring the small-sized second microcrystalline graphite to the surface of the core layer.

[0025] (3) Coating layer: Located on the outermost layer of the composite material, covering the surface of the transition layer. Its composition includes "secondary amorphous carbon". This layer is usually formed by independent carbonization of the coating carbon source (such as petroleum bitumen) to build a dense outer protective shell.

[0026] It should be noted that this structural design achieves a synergistic effect from the inside (core layer) to the outside (coating layer). This design aims to solve the problem of a large difference in elastic modulus between traditional microcrystalline graphite and the outer amorphous carbon. By introducing a transition layer composed of a mixture of "second microcrystalline graphite" and "first amorphous carbon", the overall particles form a structure in which the elastic modulus gradually changes from the inside to the outside.

[0027] By utilizing a smaller-particle-size second microcrystalline graphite as a transition layer framework, combined with the bonding effect of the first amorphous carbon, it can tightly coat the larger-particle-size first microcrystalline graphite (core layer), forming a large contact area and strong bonding force. Nitrogen atoms, after being incorporated into the graphitic carbon framework, contribute additional electrons, thereby improving the material's electronic conductivity. Nitrogen doping introduces defects and active sites into the carbon material; these sites can strongly adsorb lithium ions and even generate pseudocapacitive effects, altering the simple insertion / extraction mechanism. Nitrogen atoms act as "bridges," enhancing the interaction forces between graphite layers.

[0028] In summary, this microcrystalline graphite composite material, by constructing a three-layer structure of "core layer-transition layer-coating layer," effectively solves the problem of easy peeling and breakage of the coating layer due to large modulus differences by utilizing the gradient transition of elastic modulus. This endows the material with excellent mechanical strength and structural stability, and can buffer the volume changes caused by repeated lithium ion insertion / extraction. This structural stability effectively prevents the pulverization and peeling of particles during charge and discharge, significantly extending the cycle life. At the same time, nitrogen doping in the transition layer not only improves the electronic conductivity and rate performance of the material by constructing efficient electron transport channels, but also increases the lithium storage sites by introducing active sites, significantly improving the specific capacity of the material.

[0029] In some embodiments, the particle size D50 of the first microcrystalline graphite is 6 μm to 20 μm. For example, it can be 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm, 16 μm, 18 μm, 19 μm, 20 μm, etc.

[0030] In some embodiments, the particle size D50 of the first microcrystalline graphite is 11 μm to 17 μm. For example, it can be 11 μm, 12 μm, 13 μm, 13.5 μm, 14 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, etc.

[0031] In some embodiments, the particle size D50 of the second microcrystalline graphite is 1.5 μm to 5 μm. For example, it can be 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, etc.

[0032] In some embodiments, the ratio of the particle size D50 of the second microcrystalline graphite to the particle size D50 of the first microcrystalline graphite is (0.2 to 0.4):1. For example, it can be 0.2:1, 0.22:1, 0.25:1, 0.28:1, 0.3:1, 0.32:1, 0.35:1, 0.38:1, 0.4:1, etc.

[0033] This specific particle size ratio (0.2~0.4:1) is key to achieving tight coating. The small-sized microcrystals in the transition layer need to be small enough to tightly coat the large core, forming a large contact area and strong bonding force.

[0034] If the ratio is too large (e.g., greater than 0.5:1), meaning the particle sizes of the two particles are similar, segregation is likely to occur during mixing and subsequent processing, making it difficult to form a uniform core-shell structure. Furthermore, the transition layer cannot effectively "anchor" the core, resulting in weak interlayer bonding and easy peeling during charging and discharging.

[0035] If the ratio is too small (e.g., less than 0.1:1), that is, the core is too large or the transition layer particles are too small, the transition layer will be too thick or completely encapsulate the core, which will significantly lengthen the diffusion path of lithium ions from the outer layer to the core, affecting the rate performance; at the same time, it is easy to cause uneven thickness of the transition layer, resulting in excessively high local current density and side reactions.

[0036] The aforementioned limits achieve the best balance between providing sufficient interfacial bonding strength (to prevent pulverization) and ensuring efficient lithium-ion transport (to improve rate capability).

[0037] In some embodiments, the mass ratio of the second microcrystalline graphite to the first microcrystalline graphite is (0.05 to 0.2):1. For example, it can be 0.05:1, 0.06:1, 0.08:1, 0.1:1, 0.12:1, 0.14:1, 0.15:1, 0.16:1, 0.18:1, 0.2:1, etc.

[0038] In some embodiments, the thickness ratio of the transition layer to the core layer is (0.2–0.4):1. For example, it can be 0.2:1, 0.23:1, 0.25:1, 0.27:1, 0.3:1, 0.33:1, 0.35:1, 0.37:1, 0.4:1, etc. Here, the thickness of the core layer refers to its particle size.

[0039] The mass ratios described above define the mass percentage of the transition layer material in the overall composite material. The thickness ratios define the relative thickness of the transition layer on a geometric scale.

[0040] This ratio range ensures that the transition layer has sufficient thickness to buffer the volume expansion of the core layer during charging and discharging, thus acting as a mechanical buffer layer.

[0041] If the ratio is too low, a complete buffer network cannot be formed; if the ratio is too high, the interface volume contributed by the lower capacity increases, or the penetration distance of lithium ions increases, which may reduce the overall volumetric energy density or ion conductivity. This limitation, together with the aforementioned particle size ratio, maintains the integrity of the "core-transition layer" structure.

[0042] In some embodiments, the nitrogen content in the second microcrystalline graphite is 0.05% to 5%. For example, it can be 0.05%, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 3.0%, 3.5%, 4.0%, 5.0%, etc.

[0043] In some embodiments, the nitrogen content in the second microcrystalline graphite is 0.1% to 1%. For example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc.

[0044] The nitrogen content mentioned above refers to the mass percentage of nitrogen atoms incorporated into the carbon skeleton of the second microcrystalline graphite (transition layer).

[0045] Nitrogen doping contributes additional electrons, significantly improving the electronic conductivity of the material. This creates more efficient electron transport channels between microcrystalline graphite layers and at the interfaces with the core / coating layers, thereby enhancing the material's rate performance. Nitrogen doping introduces defects and active sites into carbon materials, which can strongly adsorb lithium ions and even generate pseudocapacitive effects, allowing lithium-ion storage to move beyond traditional insertion / extraction mechanisms and significantly improving the material's specific capacity. Nitrogen atoms can also act as "bridges" to enhance the interaction forces between graphite layers, helping to stabilize the layered structure of graphite, buffering volume changes caused by repeated lithium-ion insertion / extraction, and improving cycle stability.

[0046] If the nitrogen content is below 0.05%, sufficient active sites cannot be effectively formed, resulting in limited improvement in rate performance and specific capacity. If the nitrogen content is above 5%, excessive nitrogen will severely damage the regularity of the graphite carbon layer, leading to a significant reduction in graphitization and a decrease in conductivity. At the same time, it will cause an excessively high specific surface area, resulting in a sharp drop in the initial coulombic efficiency and accelerated capacity decay.

[0047] The range of 0.1% to 1% achieves the best balance between introducing a large number of active sites and maintaining the good conductivity of graphite crystals.

[0048] In some embodiments, the specific surface area of ​​the first microcrystalline graphite is 5 m² / g to 20 m² / g. For example, it can be 5 m² / g, 6.5 m² / g, 8 m² / g, 10 m² / g, 12 m² / g, 14 m² / g, 15 m² / g, 16 m² / g, 18 m² / g, 20 m² / g, etc.

[0049] Specific surface area refers to the total surface area per unit mass of the first microcrystalline graphite particle. This range defines the effective area of ​​the microcrystalline graphite particle surface, which serves as the core layer, in contact with the external environment.

[0050] The size of the specific surface area directly affects the electrochemical performance of lithium-ion batteries. This range (5~20 m² / g) is an optimized range, which provides sufficient electrochemical active sites to ensure rapid lithium-ion insertion and extraction, avoiding poor kinetic performance due to excessively small specific surface area; at the same time, it prevents excessive side reactions of the electrolyte on the material surface due to excessively large specific surface area, thereby reducing irreversible capacity loss during the first charge and discharge process (i.e., improving the first coulombic efficiency) and helping to form a stable solid electrolyte interphase (SEI) film.

[0051] In some embodiments, the tap density of the first microcrystalline graphite is 0.6 g / mL to 1.2 g / mL. For example, it can be 0.6 g / mL, 0.7 g / mL, 0.8 g / mL, 0.85 g / mL, 0.9 g / mL, 1.0 g / mL, 1.05 g / mL, 1.1 g / mL, 1.15 g / mL, 1.2 g / mL, etc.

[0052] Tap density refers to the density of powder after it has been mechanically vibrated and compacted in a container, reflecting the compactness of the powder particles in their packed state. This range defines the filling capacity of the core layer material.

[0053] A higher tap density means lower porosity between particles and a more compact packing. By controlling the tap density of the core layer between 0.6 g / mL and 1.2 g / mL, it is directly beneficial to improve the compaction density during subsequent electrode processing, thereby significantly improving the volumetric energy density of lithium-ion batteries. A suitable tap density also helps to improve the flowability and dispersibility of materials during mixing and coating processes, reduce the generation of fine powder during processing, and ensure the consistency of electrode quality.

[0054] In some embodiments, the carbon purity of the first microcrystalline graphite is 85% to 99.999%. For example, it can be 85%, 90%, 95%, 98%, 99%, 99.5%, 99.9%, 99.95%, 99.99%, 99.99%, etc.

[0055] Carbon purity refers to the percentage of carbon in a material by mass. This range covers microcrystalline graphite specifications from high purity to ultra-high purity, with a preferred range of over 99.5%.

[0056] High purity means low impurity content. Impurities (such as metal ions and ash) not only do not contribute to lithium storage capacity, but may also catalyze electrolyte decomposition or cause micro-short circuits. Controlling carbon purity to above 85%, especially close to 99.999%, can effectively reduce side reactions and ensure that the material has high reversible specific capacity and excellent cycle stability.

[0057] In some embodiments, the particle size distribution SPAN value of the first microcrystalline graphite is 1.0 to 1.3. For example, it can be 1.0, 1.05, 1.08, 1.1, 1.12, 1.15, 1.2, 1.22, 1.25, 1.3, etc.

[0058] In some embodiments, the particle size distribution SPAN value of the second microcrystalline graphite is 0.8 to 1.2. For example, it can be 0.8, 0.85, 0.9, 0.92, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, etc.

[0059] The SPAN value is used to characterize the width of the particle size distribution, and the calculation formula is usually (D90-D10) / D50. The smaller the value, the narrower the particle size distribution and the more uniform the particle size. The first SPAN value of microcrystalline graphite (1.0~1.3): defines the uniformity of the core layer particle size; the second SPAN value of microcrystalline graphite (0.8~1.2): defines the uniformity of the transition layer small particle size.

[0060] A smaller SPAN value implies a concentrated particle size distribution. For both the core and transition layers, a uniform particle size distribution helps prevent segregation caused by excessive particle size differences during mixing and fusion. The narrower distribution (0.8–1.2) in the transition layer ensures that small particles are uniformly coated on the core surface, avoiding localized over-coating or exposure, thus forming a uniform and stable core-shell structure and guaranteeing the consistency of the material's overall mechanical and electrochemical properties.

[0061] In some embodiments, the coating layer is an asphalt carbonization layer.

[0062] The aforementioned "coating layer" refers to the structure located on the outermost layer of the composite material. The definition of it as "asphalt carbonization layer" means that this layer is an amorphous carbon layer formed by coating the surface of the intermediate composite material (core layer + transition layer) with asphalt as a carbon source precursor and then undergoing high-temperature carbonization treatment.

[0063] It should be noted that during the high-temperature carbonization process, asphalt undergoes pyrolysis and condensation reactions, forming amorphous carbon with a disordered layer structure. This structure differs from the ordered crystalline structure of the internal microcrystalline graphite, and can form a dense protective film on the particle surface.

[0064] The asphalt carbonization layer can reduce the direct exposure of microcrystalline graphite to the electrolyte, thereby stabilizing the interface between the material and the electrolyte and inhibiting the excessive growth of the solid electrolyte interphase (SEI) film. As the outermost shell, it works synergistically with the internal structure to help maintain the integrity of the particle morphology and inhibit pulverization during cycling.

[0065] In some embodiments, the elastic modulus of the microcrystalline graphite composite material varies in a gradient from the inside to the outside.

[0066] The aforementioned "elastic modulus" is a physical quantity that measures a material's resistance to elastic deformation. "Gradual change from the inside to the outside" means that the elastic modulus of the composite material particles is neither uniform nor abrupt, but rather exhibits a gradual transition along the radial direction (from the core layer to the outer layer). This indicates that the material, through its three-layer structural design, achieves a smooth transition of mechanical properties between different layers.

[0067] This gradient change is achieved based on a three-layer composite structure of "core layer-transition layer-cladding layer". By introducing a transition layer with physical properties (such as particle size and nitrogen doping characteristics) between the core microcrystalline graphite and the outer cladding layer, or one that can act as a connector, the interfacial stress mismatch caused by excessive difference in elastic modulus between the core and the outer shell is avoided.

[0068] This modulus gradient structure endows the material with excellent mechanical strength. Under high-stress environments such as the rolling process in battery electrode manufacturing, it can effectively disperse stress, prevent particle breakage and fine powder generation, thereby improving the compaction density and quality consistency of the electrode. The gradient mechanical structure can effectively buffer the volume expansion and contraction stress generated during repeated lithium-ion insertion / extraction, preventing coating peeling, thus significantly improving the structural stability of the material during long-cycle processes and extending battery life.

[0069] refer to Figure 1 This application also provides a method for preparing a microcrystalline graphite composite material as described in any of the foregoing embodiments, comprising: Step S1: Provide a first microcrystalline graphite and a second microcrystalline graphite; wherein the second microcrystalline graphite is nitrogen-doped and has a smaller particle size than the first microcrystalline graphite.

[0070] This step involves preparing two microcrystalline graphite raw materials with different physical and chemical properties as the basis for constructing the composite material. The first microcrystalline graphite serves as the raw material for the core layer, while the second microcrystalline graphite serves as the raw material for the transition layer, and must meet two limiting conditions: it has undergone "nitrogen doping treatment" and its "particle size is smaller than that of the first microcrystalline graphite".

[0071] The aforementioned first microcrystalline graphite can be obtained through conventional processes such as flotation, crushing and shaping and purification of microcrystalline graphite ore to achieve a specific purity (e.g. 99.96%) and particle size requirements (e.g. D50 of 11 μm).

[0072] The aforementioned second microcrystalline graphite can be obtained by refining the microcrystalline graphite raw material through mechanical means such as ball milling or high-pressure homogenization to obtain small-particle-size microcrystalline graphite (e.g., D50 of 3μm); then it is mixed with a nitrogen source (such as melamine, urea, etc.) and subjected to high-temperature (e.g., 800~1200℃) carbonization treatment to achieve nitrogen doping.

[0073] This step yields large-particle pure microcrystalline graphite (core material) and modified small-particle nitrogen-doped microcrystalline graphite (transition layer material).

[0074] Small-particle-size second microcrystalline graphite can more easily coat the surface of the first microcrystalline graphite; nitrogen doping treatment introduces active sites and improves electronic conductivity, laying the foundation for improving the rate performance and specific capacity of the material.

[0075] Step S2 involves granulating the first microcrystalline graphite, the second microcrystalline graphite, and the binder to form a transition layer on the surface of the first microcrystalline graphite, thereby obtaining an intermediate composite material.

[0076] This step involves combining the core layer material (first microcrystalline graphite), the transition layer material (second microcrystalline graphite), and a binder to construct a "core-transition layer" structure through physical / chemical interactions.

[0077] Specifically, the first and second microcrystalline graphite particles can be mixed with an appropriate amount of binder (such as asphalt) and granulated at a certain temperature (such as 600~800℃). During this process, relying on the bonding effect of the binder and mechanical force, the smaller-sized second microcrystalline graphite particles are uniformly attached and anchored to the surface of the larger-sized first microcrystalline graphite particles, thus obtaining an "intermediate composite material". Its structural feature is that the surface of the large-particle microcrystalline graphite is coated with a transition layer composed of small-particle nitrogen-doped microcrystalline graphite.

[0078] This step creates a transition region with physical properties (such as elastic modulus) between the core and the shell, which can modify defects and porosity on the core surface. The tight coating of small particles increases the contact area and bonding force, providing structural support to mitigate volume expansion and stress mismatch.

[0079] Step S3: Coat the surface of the intermediate composite material with a carbon source and perform carbonization treatment to form a coating layer.

[0080] This step is the final encapsulation step, where an amorphous carbon shell is built on the outermost layer of the intermediate composite material.

[0081] Specifically, the intermediate composite material obtained in step S2 can be mixed with a carbon source (such as asphalt) and then subjected to high-temperature carbonization treatment. The carbon source is pyrolyzed at high temperature and transformed into amorphous carbon, thus obtaining the final "microcrystalline graphite composite material" (i.e., microcrystalline graphite composite material D), which has a three-layer structure of "core layer - transition layer - coating layer".

[0082] The outermost amorphous carbon layer can stabilize the material interface and inhibit the erosion of the graphite interior by the electrolyte and side reactions. In synergy with the transition layer, it further improves the gradient distribution of the elastic modulus, thereby significantly improving the mechanical strength, processing performance (compacted density) and cycle life of the material.

[0083] In some embodiments, the step of providing the second microcrystalline graphite includes: ball milling or high-pressure homogenizing the microcrystalline graphite raw material to obtain small-particle-size microcrystalline graphite, and then mixing the small-particle-size microcrystalline graphite with a nitrogen source and performing carbonization treatment.

[0084] The aforementioned "ball milling or high-pressure homogenization of microcrystalline graphite raw materials to obtain small-particle-size microcrystalline graphite" is a physical refining step in the preparation of the transition layer raw material (second microcrystalline graphite). It refers to using specific mechanical force to break down and refine the initial microcrystalline graphite raw material from a larger particle size to the target small particle size range.

[0085] Specifically, grinding media (such as zirconia balls) in a ball mill can be used to impact and grind microcrystalline graphite raw materials. Particle size can be precisely controlled by adjusting the ball mill speed, time, and ball-to-material ratio. Furthermore, a high-pressure homogenizer can be used to force the microcrystalline graphite slurry through a narrow slit under high pressure, utilizing shear force, impact force, and cavitation effect to pulverize and refine the particles, thereby obtaining microcrystalline graphite particles with smaller particle sizes (e.g., D50 of 1.5μm to 5μm).

[0086] Small-diameter particles obtained through mechanical refinement can more densely and uniformly cover the surface of the core layer compared to large particles, providing a geometrical basis for constructing a dense transition layer and increasing the contact area and bonding force with the core layer.

[0087] The aforementioned step of "mixing the small-particle-size microcrystalline graphite with a nitrogen source and then performing carbonization treatment" is a chemical modification step for physically refined microcrystalline graphite. By introducing nitrogen and performing lattice reconstruction or surface modification at high temperature, "nitrogen-doped" microcrystalline graphite is prepared.

[0088] Specifically, the small-particle-size microcrystalline graphite powder obtained in the above steps can be uniformly mixed with a nitrogen source (solid powder) in a certain proportion, and then placed in a heating device under inert atmosphere protection for high-temperature heat treatment, so that nitrogen atoms can be successfully incorporated into the graphite carbon skeleton or bonded to the edge, thus obtaining nitrogen-doped second microcrystalline graphite.

[0089] The incorporation of nitrogen atoms contributes additional electrons, significantly improving the electronic conductivity of the material and creating efficient electron transport channels between the layers of microcrystalline graphite sheets. It also introduces defects and active sites, enhancing the adsorption capacity for lithium ions and even generating pseudocapacitive effects, thereby improving the specific capacity. Nitrogen atoms act as "bridges" to enhance interlayer forces, which helps to buffer volume changes.

[0090] Furthermore, the carbonization temperature is 800℃~1200℃; for example, it can be 800℃, 850℃, 900℃, 950℃, 1000℃, 1050℃, 1100℃, 1150℃, 1180℃, 1200℃, etc.

[0091] The carbonization temperature described above is a limit on key heat treatment parameters in the nitrogen doping process. During the heating of the mixture, the temperature in the isothermal zone can be controlled between 800℃ and 1200℃.

[0092] This temperature range is crucial for achieving effective nitrogen doping. Too low a temperature may lead to incomplete precursor decomposition or difficulty in nitrogen atoms entering the carbon lattice; too high a temperature may result in excessive nitrogen loss or excessive graphitization of the carbon structure, reducing active sites. This range ensures an optimal balance between introducing active sites and maintaining conductivity.

[0093] Furthermore, the nitrogen source includes at least one of melamine and urea.

[0094] Melamine and urea are nitrogen-rich solid organic compounds that are widely available and inexpensive. They readily decompose at high temperatures to produce nitrogen-containing active atmospheres or intermediates, which is beneficial for achieving uniform doping of microcrystalline graphite.

[0095] Furthermore, the microcrystalline graphite raw material is the first microcrystalline graphite.

[0096] Since the core layer and the transition layer originate from the same mineral raw material, their crystal structure, coefficient of thermal expansion and other physical properties are most similar, which is conducive to forming a strong interface bond in the subsequent granulation process, reducing interface stress caused by material differences; and also reducing the management of material types and lowering production costs.

[0097] In some embodiments, the mass ratio of the microcrystalline graphite raw material to the nitrogen source is 1:(0.1~0.3). For example, it can be 1:0.1, 1:0.12, 1:0.14, 1:0.15, 1:0.18, 1:0.2, 1:0.22, 1:0.25, 1:0.28, 1:0.3, etc.

[0098] This refers to the formulation parameter limitation in the process of preparing the second microcrystalline graphite (i.e., the transition layer raw material), which refers to the weight ratio range between the microcrystalline graphite matrix material (usually refined small-particle-size microcrystalline graphite) used to prepare the transition layer and the raw material (nitrogen source) that provides nitrogen.

[0099] Specifically, before high-temperature carbonization doping, the refined microcrystalline graphite powder can be mechanically mixed with nitrogen source powders such as melamine and urea at a ratio of 0.1 to 0.3 parts nitrogen source to 1 part graphite, so as to achieve a uniform dispersion and obtain a precursor mixture with uniform composition, which lays the foundation for subsequent high-temperature solid-phase reaction.

[0100] This specific ratio range ensures the controllability of nitrogen doping. An appropriate amount of nitrogen source can ensure the introduction of sufficient active sites after carbonization to improve specific capacity and conductivity, while avoiding the destruction of carbon structure or excessive specific surface area due to excessive nitrogen source, or the insignificant doping modification effect due to insufficient nitrogen source.

[0101] In some embodiments, the granulation process is carried out at a temperature of 600°C to 800°C. For example, it can be 600°C, 620°C, 640°C, 650°C, 680°C, 700°C, 720°C, 750°C, 780°C, 800°C, etc.

[0102] These are the key process temperature parameters for constructing the "core-transition layer" intermediate composite structure. The first microcrystalline graphite (core), the second microcrystalline graphite (transition layer powder), and the binder can be mixed and then mechanically granulated or fused at a temperature of 600℃ to 800℃. At this temperature, the binder (such as asphalt) softens or melts, firmly bonding and anchoring the small-diameter second microcrystalline graphite to the surface of the large-diameter first microcrystalline graphite, forming a structurally stable intermediate composite material (modified microcrystalline graphite C).

[0103] This temperature range ensures that the binder fully exerts its bonding effect, allowing the transition layer and the core layer to bond tightly and preventing peeling during subsequent processing. At the same time, it does not reach the level of complete high-temperature graphitization or violent reaction, thus preserving the structural characteristics of the intermediate and preparing for the final coating and carbonization process.

[0104] In some embodiments, the adhesive includes bitumen.

[0105] Asphalt has good wettability and a high residual carbon content. At the granulation temperature of 600~800℃, asphalt can effectively wet the surface of graphite particles, acting as "glue" to fix small particles to the surface of large particles; at the same time, asphalt itself is also a carbon source, which is transformed into carbon material after subsequent heat treatment, has good compatibility with the graphite matrix, and will not introduce impurity elements.

[0106] In some embodiments, after the step of forming the coating layer, the material is further subjected to sieving and demagnetization treatment.

[0107] This step is a post-processing step at the end of the preparation process, used to ensure the physical specifications and purity of the final product.

[0108] Specifically, this may include: (1) Screening: Physical screening of the carbonized final product is carried out using a sieve with a specific mesh size to remove particles that are too large (agglomerates) or too small.

[0109] (2) Demagnetization: The powder is processed by magnetic separation equipment to adsorb and remove any magnetic metal impurities (such as iron filings) that may be mixed in.

[0110] This step yields a final anode material product with uniform particle size distribution and extremely low magnetic material content.

[0111] The aforementioned sieving process ensures the uniformity of material particle size (Dmax control), which is beneficial for the uniformity of downstream battery slurry coating. Demagnetization removes metallic impurities that could puncture the separator, leading to micro-short circuits or self-discharge in the battery, significantly improving the battery's safety performance and electrochemical stability.

[0112] In some embodiments, the mass ratio of the binder to the second microcrystalline graphite is 3% to 20% (e.g., 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, etc.); preferably, the mass ratio of the binder to the second microcrystalline graphite is 5% to 10% (e.g., 5%, 6%, 7%, 8%, 9%, 10%, etc.).

[0113] This application also provides a battery negative electrode, comprising the microcrystalline graphite composite material as described in any of the foregoing embodiments; or comprising the microcrystalline graphite composite material prepared by the preparation method described in any of the foregoing embodiments.

[0114] This application also provides a battery, including a battery negative electrode as described in the foregoing embodiments.

[0115] The core feature of this battery is the use of the microcrystalline graphite composite material described in the foregoing embodiments or the material prepared by the foregoing method as the battery negative electrode. In addition to the negative electrode, the battery typically includes a positive electrode (whose active material may include, but is not limited to, lithium iron phosphate, lithium cobalt oxide, ternary materials, etc.), a separator (membrane) located between the positive and negative electrodes to isolate electrons but allow ions to pass through, an electrolyte (including liquid electrolyte, gel electrolyte, or solid electrolyte) for conducting ions, and a battery casing. Specific types of this battery mainly include, but are not limited to, various types of lithium-ion secondary batteries, lithium polymer batteries, solid-state lithium batteries, and other chemical energy storage devices.

[0116] This application also provides an electrical device, including a battery as described in the foregoing embodiments.

[0117] The term "device" refers to any device that uses the aforementioned battery as a power source or energy storage component. The scope of this "electrical device" is very broad, including but not limited to various consumer electronics products (such as smartphones, tablets, laptops, smart wearable devices, digital cameras, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, electric bicycles, electric scooters, etc.), power tools (such as electric drills, chainsaws, etc.), energy storage systems (such as home energy storage stations, industrial energy storage cabinets, etc.), and any terminal device requiring mobile power support, such as drones and toys.

[0118] The present invention will be further illustrated below with specific embodiments. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0119] Example 1 In this embodiment, a microcrystalline graphite composite material is prepared.

[0120] Experimental methods: Step S1: Provide the first microcrystalline graphite and the second microcrystalline graphite: The first microcrystalline graphite was obtained by selecting microcrystalline graphite ore and processing it through flotation, crushing, shaping, and purification to achieve a purity of 99.96%. This first microcrystalline graphite has a particle size D50 of 11 μm and a specific surface area of ​​6.5 m². 2 The first microcrystalline graphite was used as the core layer material. It had a tap density of 1.0 g / mL, a carbon purity of 99.96%, and a SPAN value of 1.1.

[0121] Preparation of the second microcrystalline graphite: Refining treatment: Using microcrystalline graphite from the same source as the first microcrystalline graphite (i.e., microcrystalline graphite A), a high-speed ball mill was used for physical refining treatment. The ball milling process parameters were controlled to achieve a particle size D50 of 3.0 μm for the treated microcrystalline graphite. This particle size satisfies the ratio of the particle size of the second microcrystalline graphite to that of the first microcrystalline graphite being approximately 0.27:1, which is within the range of (0.2~0.4):1.

[0122] Nitrogen doping treatment: The refined small-particle-size microcrystalline graphite was mechanically mixed with a nitrogen source (melamine powder) at a mass ratio of 1:0.2 until homogeneous. The mixture was placed in a furnace under an inert atmosphere and heated to 1000℃ for carbonization treatment for 2 hours. After cooling, nitrogen-doped microcrystalline graphite, i.e., the second microcrystalline graphite, was obtained. Testing showed that its nitrogen content was approximately 0.5%, its particle size D50 was approximately 3.5 μm (slight agglomeration occurred after carbonization, but it was still within the preferred range), and its SPAN value was 1.0.

[0123] Step S2: Granulation process to form a transition layer (preparation of intermediate composite material): (1) The first microcrystalline graphite, the prepared second microcrystalline graphite, and the binder (asphalt) are mixed according to the mass ratio. The mass ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.12:1, and the mass ratio of the binder to the second microcrystalline graphite is 7%:1.

[0124] (2) The mixture is fed into a granulation reactor and granulated at 700°C. At this temperature, the asphalt softens and acts as a binder, allowing the small-diameter second microcrystalline graphite to uniformly and firmly coat the surface of the first microcrystalline graphite, forming an intermediate composite material with a "core layer-transition layer" structure. Characterization showed that the thickness ratio of the transition layer to the core layer was approximately 0.3:1.

[0125] Step S3: Forming the coating layer: (1) The above intermediate composite material is mixed with the coated carbon source (petroleum asphalt), and the amount of carbon source added is 8% of the mass of the intermediate composite material.

[0126] (2) The mixture is subjected to high-temperature carbonization treatment under an inert atmosphere at a carbonization temperature of 1100℃, so that the asphalt is pyrolyzed into amorphous carbon and a dense coating layer is formed on the surface of the transition layer.

[0127] (3) The carbonized product is sieved (through a 300-mesh sieve) and demagnetized (to remove magnetic foreign matter) to finally obtain microcrystalline graphite composite material.

[0128] Example 2 In this embodiment, a microcrystalline graphite composite material is prepared.

[0129] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the particle size and ratio parameters of the first and second microcrystalline graphite are adjusted to reflect the lower end of the important parameter range. The specific differences are as follows: First microcrystalline graphite: particle size D50 is 6μm.

[0130] Second microcrystalline graphite: By controlling the ball milling time, its particle size D50 is made to be 1.5μm.

[0131] Particle size ratio: The particle size ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.25:1 (close to the lower limit region).

[0132] Mixing ratio: The mass ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.05:1.

[0133] Transition layer thickness ratio: The thickness ratio of the transition layer to the core layer is controlled at 0.2:1.

[0134] Nitrogen content: The nitrogen content of the second microcrystalline graphite was controlled to be 0.19% by adjusting the nitrogen source ratio (1:0.1) and carbonization conditions.

[0135] The remaining steps are the same as in Example 1.

[0136] Example 3 In this embodiment, a microcrystalline graphite composite material is prepared.

[0137] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the parameters are adjusted to reflect the upper end of the important parameter range. The specific differences are as follows: First microcrystalline graphite: particle size D50 is 20μm.

[0138] Second microcrystalline graphite: particle size D50 is 5μm.

[0139] Particle size ratio: The particle size ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.25:1.

[0140] Mixing ratio: The mass ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.2:1.

[0141] Transition layer thickness ratio: The thickness ratio of the transition layer to the core layer is controlled at 0.32:1.

[0142] Nitrogen content: The nitrogen content of the second microcrystalline graphite was controlled to be 1.0% by adjusting the nitrogen source ratio (1:0.3) and carbonization conditions.

[0143] The remaining steps are the same as in Example 1.

[0144] Example 4 In this embodiment, a microcrystalline graphite composite material is prepared.

[0145] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that it reflects the lower end of the range of the important parameter, particle size ratio. The specific differences are as follows: First microcrystalline graphite: Particle size D50 is selected as 17μm.

[0146] Second microcrystalline graphite: The particle size D50 is selected as 3.4μm.

[0147] Particle size ratio: The particle size ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.2:1.

[0148] The remaining steps are the same as in Example 1.

[0149] Example 5 In this embodiment, a microcrystalline graphite composite material is prepared.

[0150] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 lies in that it reflects the upper end of the range of the important parameter, particle size ratio. The specific differences are as follows: First microcrystalline graphite: Particle size D50 is selected as 10μm.

[0151] Second microcrystalline graphite: The particle size D50 is selected as 4.0μm.

[0152] Particle size ratio: The particle size ratio of the second microcrystalline graphite to the first microcrystalline graphite is 0.4:1.

[0153] The remaining steps are the same as in Example 1.

[0154] Example 6 In this embodiment, a microcrystalline graphite composite material is prepared.

[0155] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the type of nitrogen source is changed. The specific differences are as follows: Nitrogen source: Urea was used as the nitrogen source in the preparation of the second microcrystalline graphite, and the mass ratio of urea to the small-particle-size microcrystalline graphite was 1:0.25.

[0156] The remaining steps are the same as in Example 1.

[0157] Example 7 In this embodiment, a microcrystalline graphite composite material is prepared.

[0158] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the refining treatment method of the second microcrystalline graphite is changed. The specific differences are as follows: Refining process: Small-particle-size microcrystalline graphite was prepared using a high-pressure homogenization process. The microcrystalline graphite raw material was dispersed in a medium to form a slurry, which was then processed by a high-pressure homogenizer and dried to obtain microcrystalline graphite powder with a D50 of 3.0 μm.

[0159] The remaining steps are the same as in Example 1.

[0160] Example 8 In this embodiment, a microcrystalline graphite composite material is prepared.

[0161] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the granulation temperature is changed. The specific differences are as follows: Granulation temperature: The granulation temperature in step S2 is adjusted to 600℃ (lower limit of parameter range).

[0162] The remaining steps are the same as in Example 1.

[0163] Example 9 In this embodiment, a microcrystalline graphite composite material is prepared.

[0164] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the amount of binder used in the granulation process is adjusted to reflect the lower end of the range of the mass ratio of binder to second microcrystalline graphite. The specific differences are as follows: Granulation binder ratio: In step S2 granulation treatment, the mass ratio of the binder (asphalt) to the second microcrystalline graphite is adjusted to 3%:1.

[0165] The remaining steps are the same as in Example 1.

[0166] Example 10 In this embodiment, a microcrystalline graphite composite material is prepared.

[0167] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the amount of binder used in the granulation process is adjusted to reflect the upper end of the range of the mass ratio of binder to second microcrystalline graphite. The specific differences are as follows: Granulation binder ratio: In step S2 granulation treatment, the mass ratio of the binder (asphalt) to the second microcrystalline graphite is adjusted to 20%:1.

[0168] The remaining steps are the same as in Example 1.

[0169] Example 11 In this embodiment, a microcrystalline graphite composite material is prepared.

[0170] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the amount of binder used in the granulation process is adjusted to reflect the preferred range of the mass ratio of binder to second microcrystalline graphite. The specific differences are as follows: Granulation binder ratio: In step S2 granulation treatment, the mass ratio of the binder (asphalt) to the second microcrystalline graphite is adjusted to 5%:1.

[0171] The remaining steps are the same as in Example 1.

[0172] Example 12 In this embodiment, a microcrystalline graphite composite material is prepared.

[0173] The experimental method is basically the same as in Example 1. The difference between this example and Example 1 is that the amount of binder used in the granulation process is adjusted to reflect the preferred range of the mass ratio of binder to second microcrystalline graphite. The specific differences are as follows: Granulation binder ratio: In step S2 granulation treatment, the mass ratio of the binder (asphalt) to the second microcrystalline graphite is adjusted to 10%:1.

[0174] The remaining steps are the same as in Example 1.

[0175] Example 13 In this embodiment, a microcrystalline graphite composite material is prepared.

[0176] The experimental method is basically the same as in Example 1. This comparative example is used to verify the effect of "excessive particle size ratio" (conversely demonstrating the importance of particle size matching). Specific differences are as follows: First microcrystalline graphite: particle size D50 is 11μm.

[0177] Second microcrystalline graphite: The particle size D50 is selected as 6μm.

[0178] Particle size ratio: approximately 0.55:1 (outside the range of 0.2 to 0.4:1).

[0179] The remaining steps are the same as in Example 1.

[0180] Example 14 In this embodiment, a microcrystalline graphite composite material is prepared.

[0181] The experimental method is basically the same as in Example 1. This comparative example is used to verify the effect of "too small particle size ratio". The specific differences are as follows: First microcrystalline graphite: particle size D50 is 15μm.

[0182] Second microcrystalline graphite: Particle size D50 is selected as 1.0μm.

[0183] Particle size ratio: approximately 0.067:1 (below the range of 0.2 to 0.4:1). The remaining steps are the same as in Example 1.

[0184] Comparative Example 1 In this comparative example, a microcrystalline graphite composite material was prepared.

[0185] The experimental method is basically the same as in Example 1. This comparative example is used to verify the effect of the "no transition layer structure" (conversely demonstrating the importance of the three-layer structure). The specific differences are as follows: The preparation of the second microcrystalline graphite in step S1 and the granulation in step S2 are omitted.

[0186] The first microcrystalline graphite (core layer) is directly mixed with the coated carbon source (petroleum pitch), with the amount of carbon source being 12% of the graphite mass (to keep the total carbon content similar). Then, the high-temperature carbonization treatment in step S3 is directly performed.

[0187] Result: The prepared material is a traditional "core-cladding layer" bilayer structure material.

[0188] Comparative Example 2 In this comparative example, a microcrystalline graphite composite material was prepared.

[0189] The experimental method is basically the same as in Example 1. This comparative example is used to verify the effect of "the second microcrystalline graphite not being nitrogen-doped" (conversely demonstrating the importance of nitrogen doping). Specific differences are as follows: In step S1: the second microcrystalline graphite is only ball-milled to 3.0 μm, without being mixed with a nitrogen source or subjected to nitrogen doping and carbonization treatment. Undoped small-particle-size microcrystalline graphite is used directly for granulation in step S2. The remaining steps are the same as in Example 1.

[0190] Results: The transition layer is ordinary small-particle-size microcrystalline graphite with no nitrogen doping characteristics.

[0191] Comparative Example 3 In this comparative example, a microcrystalline graphite composite material was prepared.

[0192] The experimental methods are basically the same as in Example 1. This comparative example is used to verify the difference between "simple physical mixing" and "granulation to form a transition layer" (conversely demonstrating the importance of the preparation method). The specific differences are as follows: Step S2 is modified: The first microcrystalline graphite, the nitrogen-doped second microcrystalline graphite, and the asphalt are simply mechanically mixed at room temperature without high-temperature granulation at 600-800℃. The process proceeds directly to step S3 for coating and carbonization.

[0193] Comparative Example 4 In this comparative example, a microcrystalline graphite composite material was prepared.

[0194] The experimental methods are basically the same as in Example 1. This comparative example is used to verify the effect of "excessive nitrogen doping". The specific differences are as follows: Small-particle-size microcrystalline graphite and a nitrogen source (melamine powder) were mechanically mixed uniformly at a mass ratio of 1:1.6. The mixture was then placed in a furnace under an inert atmosphere and heated to 1150℃ for carbonization treatment for 10 hours. Testing revealed a nitrogen content of approximately 5.5%.

[0195] The remaining steps are the same as in Example 1.

[0196] Comparative Example 5 In this comparative example, a microcrystalline graphite composite material was prepared.

[0197] The experimental methods are basically the same as in Example 1. This comparative example is used to verify the effect of "insufficient nitrogen doping". The specific differences are as follows: Small-particle-size microcrystalline graphite and a nitrogen source (melamine powder) were mechanically mixed uniformly at a mass ratio of 1:0.03. The mixture was then placed in a furnace under an inert atmosphere and heated to 950°C for carbonization treatment for 1 hour. Testing revealed a nitrogen content of approximately 0.04%.

[0198] The remaining steps are the same as in Example 1.

[0199] Test Experiment 1. Testing method: (1) Physicochemical index testing: A. Particle size distribution (D50 and SPAN values): Tested using a laser particle size analyzer (Malvern Mastersizer 3000, UK). The sample was dispersed in deionized water and ultrasonically dispersed before measurement.

[0200] D50: The particle size at which the cumulative particle size distribution percentage of the sample reaches 50%.

[0201] SPAN value calculation formula: SPAN=(D90-D10) / D50.

[0202] B. Specific surface area (BET): The specific surface area and porosity were tested using a fully automated specific surface area and porosity analyzer (MicromeriticsTriStar II 3020) via nitrogen adsorption / desorption.

[0203] C. Tap density: Tested using a tap density meter (BT-301).

[0204] D. Carbon purity (fixed carbon content): The fixed carbon content is calculated by measuring moisture, volatile matter, and ash content.

[0205] E. Nitrogen content: Determined using an oxygen, nitrogen, and hydrogen analyzer (LECOONH836) or an elemental analyzer.

[0206] (2) Characterization of mechanical properties: Powder compaction density: Tested using an electronic pressure testing machine and a special compaction density mold. A certain mass (1.0g) of powder sample was weighed and placed in a cylindrical mold. A specific pressure (5T) was applied and held for a certain time. The height of the powder was then measured, and the compaction density under that pressure was calculated. Formula: Compaction density = Sample mass / (Mold bottom area × Compacted height).

[0207] (3) Electrochemical performance testing: To evaluate the electrochemical performance of the negative electrode materials prepared in the examples and comparative examples, they were assembled into CR2032 coin cells for testing.

[0208] Negative electrode preparation: The prepared microcrystalline graphite composite material (active material), conductive agent (SuperP), binder (styrene-butadiene rubber SBR), and thickener (sodium carboxymethyl cellulose CMC) are mixed in a mass ratio of 96:1:1.5:1.5. Using deionized water as a solvent, the mixture is stirred until homogeneous to prepare a negative electrode slurry. The slurry is uniformly coated onto a copper foil current collector, and then vacuum dried, rolled, and stamped to obtain the negative electrode sheet.

[0209] Battery assembly: Assembly is carried out in a glove box filled with argon gas (water and oxygen content are both <0.1ppm).

[0210] Counter electrode: Lithium metal sheet.

[0211] Membrane: Polypropylene (PP) microporous membrane (Celgard2400).

[0212] Electrolyte: 1 mol / L LiPF6 solution, with a solvent of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1:1:1, and 1% by mass of vesyl carbonate (VC) added.

[0213] Charge / discharge test: The battery test system (LANDCT2001A) was used to conduct the test in a constant temperature environment of 25℃. The voltage test range was 0.005V~1.5V (vs. Li / Li). + ).

[0214] Initial charge / discharge and coulombic efficiency: Discharge at a constant current rate of 0.1C (lithium insertion) to 0.005V, then discharge at a constant voltage rate until the current drops to 0.01C; after resting, charge at a constant current rate of 0.1C (lithium removal) to 1.5V. Record the initial lithium insertion capacity and the initial lithium removal capacity. Initial coulombic efficiency = (initial lithium removal capacity / initial lithium insertion capacity) × 100%.

[0215] Rate performance testing: Charge and discharge tests were conducted at different current rates (e.g., 0.1C, 0.2C, 0.5C, 1C, 2C), and the discharge capacity at each rate was recorded. Rate performance = (2C discharge capacity / 0.2C discharge capacity) × 100%.

[0216] Cyclic performance test: Perform cyclic charge-discharge tests at a charge-discharge rate of 1C. Record the discharge capacity of the Nth cycle.

[0217] Cycle capacity retention = (500th cycle discharge capacity / first cycle discharge capacity) × 100%.

[0218] 2. Test Results: Table 1. Performance test results of examples and comparative examples

[0219] 3. Analysis: Based on the test results in Table 1, and by comparing the data from Examples 1-14 with those from Comparative Examples 1-5, it can be concluded that the microcrystalline graphite composite material with a three-layer structure of "core layer-transition layer-coating layer" proposed in this application exhibits significant comprehensive advantages in terms of compaction density, cycle stability, and rate performance. Specific analysis is as follows: (1) Synergistic effect analysis of the three-layer gradient structure of "core layer-transition layer-coating layer" (Example 1 vs. Comparative Example 1): Comparing Example 1 and Comparative Example 1, it can be seen that when both use the same first microcrystalline graphite as the core layer and have similar total carbon content, Example 1 introduces a transition layer composed of nitrogen-doped small-particle-size microcrystalline graphite and first amorphous carbon, while Comparative Example 1 is only a traditional "core layer-coating layer" bilayer structure.

[0220] In terms of cycle performance, Example 1 achieved a capacity retention of 90.8% after 500 cycles at 1C, significantly better than Comparative Example 1's 86.1%. This strongly demonstrates that the introduction of the transition layer creates an effective buffer zone between the microcrystalline graphite hard core and the external soft carbon coating, alleviating volume expansion stress during charge and discharge and preventing coating peeling.

[0221] In terms of rate performance, Example 1 achieved a 2C / 0.2C rate performance of 40.9%, significantly higher than Comparative Example 1's 32.9%. This indicates that the nitrogen-doped sites and small particles in the transition layer created more efficient electron and ion transport channels.

[0222] (2) Analysis of the key role of nitrogen doping in the transition layer (Example 1 vs. Comparative Example 2): Comparing Example 1 and Comparative Example 2, the only difference between them is that the small-particle-size graphite in the transition layer of Comparative Example 2 was not nitrogen-doped.

[0223] The initial discharge specific capacity (362.4 mAh / g) and rate performance (40.9%) of Example 1 were both higher than those of Comparative Example 2 (capacity 356.5 mAh / g, rate performance 37.4%). This confirms that the introduction of nitrogen provides additional active lithium storage sites for the material, thereby improving capacity; at the same time, nitrogen atoms improve the electronic conductivity at the microcrystalline graphite interface, thus enhancing fast charging performance.

[0224] (3) Analysis of the influence of the proportion of binder (first amorphous carbon) in the transition layer (Examples 1, 9-12): This application investigated the effect of the mass ratio of binder to second microcrystalline graphite during the granulation process through Examples 9-12.

[0225] Insufficient binder content (Example 9, 3%): Cycle retention decreased to 87.2% (Example 1, 90.8%). This is because insufficient binder resulted in insufficient "first amorphous carbon" to firmly anchor the small-particle-size second microcrystalline graphite to the core layer surface, leading to loosening and peeling of the transition layer structure during cycling.

[0226] Excessive binder content (Example 10, 20%): The compaction density decreased to 1.57 g / cm³, and the cycle retention (85.6%) and specific capacity (356.7 mAh / g) were the lowest in the group. This is because the excessively thick binder carbonization layer caused particle agglomeration, hindering the liquid-phase transport of lithium ions, and the excessively high proportion of amorphous carbon reduced the overall energy density.

[0227] Preferred range (Examples 11, 12, 5%~10%): When the proportion is in the range of 5%~10%, the properties of the material are the same as those in Example 1 (7%), and all show excellent performance. This confirms that the preferred proportion can balance the interfacial bonding strength and ion transport efficiency.

[0228] (4) Analysis of the influence of particle size matching between the core layer and the transition layer (Example 1 vs Examples 13 and 14): Excessive particle size ratio (Example 13, 0.55:1): Cycle retention rate dropped sharply to 83.2%. This is because the transition layer particles are too large and cannot form a dense packing on the core surface, instead creating porosity and failing to provide stress buffering, making the structure extremely prone to collapse.

[0229] The particle size ratio was too small (Example 14, 0.067:1): The initial coulombic efficiency decreased to 90.0% (93.8% in Example 1). This is because the transition layer particles were too fine, resulting in a sharp increase in specific surface area, which consumed a large number of lithium ions to form the SEI film, and also prolonged the diffusion path of lithium ions.

[0230] The particle size ratios of Examples 1, 4, and 5 are all within the range of (0.2~0.4):1, and all performance characteristics remain at a high level, proving the necessity of this particle size matching range.

[0231] (5) Analysis of the influence of granulation process and nitrogen content (Example 1 vs. Comparative Examples 3, 4, and 5): Granulation process (Comparative Example 3): Omitting high-temperature granulation and only performing physical mixing resulted in a cycle retention rate dropping to 82.5% (the lowest in the entire group). This clearly demonstrates that the granulation process is necessary to "anchor" the transition layer to the core layer with the first amorphous carbon converted from the binder, thus resisting expansion and contraction.

[0232] Nitrogen content (Comparative Examples 4 and 5): Excessive nitrogen doping (Comparative Example 4) leads to the destruction of the carbon structure, resulting in low first-time efficiency (89.9%) and poor cycle performance; insufficient nitrogen doping (Comparative Example 5) results in insignificant modification effect and limited performance improvement.

[0233] (6) Verification analysis of gradient elastic modulus structure: The microcrystalline graphite core layer is highly rigid (high modulus), while the outer coating layer is soft (low modulus). Direct contact easily generates huge interfacial stress, leading to delamination (as shown in Comparative Example 1, the cycle retention rate is only 86.1%). However, Example 1 of this invention introduces a transition layer composed of a mixture of "second microcrystalline graphite" and "first amorphous carbon," whose physical properties lie between a hard core and a soft shell. Test results show that the cycle life of Example 1 is significantly improved to 90.8%. This qualitative change in macroscopic performance strongly demonstrates that a gradient structure with a smooth transition in elastic modulus from the inside to the outside of the material has been successfully constructed, effectively eliminating interfacial stress concentration and thus achieving long-term structural stability.

[0234] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A microcrystalline graphite composite material, characterized in that, include: The core layer, the transition layer covering the surface of the core layer, and the coating layer covering the surface of the transition layer; The core layer comprises a first microcrystalline graphite; The transition layer comprises a mixture of a second microcrystalline graphite and a first amorphous carbon. The coating layer contains a second amorphous carbon; The second microcrystalline graphite is nitrogen-doped microcrystalline graphite, and the particle size of the second microcrystalline graphite is smaller than that of the first microcrystalline graphite.

2. The microcrystalline graphite composite material as described in claim 1, characterized in that, The first microcrystalline graphite has a particle size D50 of 6 μm to 20 μm; and / or, The first microcrystalline graphite has a particle size D50 of 11 μm to 17 μm; and / or, The particle size D50 of the second microcrystalline graphite is 1.5 μm to 5 μm; and / or, The ratio of the particle size D50 of the second microcrystalline graphite to the particle size D50 of the first microcrystalline graphite is (0.2~0.4):1; and / or, The mass ratio of the second microcrystalline graphite to the first microcrystalline graphite is (0.05–0.2):1; and / or, The thickness ratio of the transition layer to the core layer is (0.2–0.4):1; and / or, The nitrogen content in the second microcrystalline graphite is 0.05%~5%; and / or, The nitrogen content in the second microcrystalline graphite is 0.1% to 1%.

3. The microcrystalline graphite composite material as described in claim 1, characterized in that, The specific surface area of ​​the first microcrystalline graphite is 5 m² / g to 20 m² / g; and / or, The tap density of the first microcrystalline graphite is 0.6 g / mL to 1.2 g / mL; and / or, The carbon purity of the first microcrystalline graphite is 85%–99.999%; and / or, The particle size distribution SPAN value of the first microcrystalline graphite is 1.0~1.3; and / or, The particle size distribution SPAN value of the second microcrystalline graphite is 0.8~1.

2.

4. The microcrystalline graphite composite material as described in claim 1, characterized in that, The covering layer is an asphalt carbonization layer; and / or, The elastic modulus of the microcrystalline graphite composite material varies in a gradient from the inside to the outside.

5. A method for preparing the microcrystalline graphite composite material according to any one of claims 1-4, characterized in that, include: A first microcrystalline graphite and a second microcrystalline graphite are provided respectively; wherein the second microcrystalline graphite is nitrogen-doped and has a smaller particle size than the first microcrystalline graphite; The first microcrystalline graphite, the second microcrystalline graphite, and the binder are granulated to form a transition layer on the surface of the first microcrystalline graphite, thus obtaining an intermediate composite material. A carbon source is coated onto the surface of the intermediate composite material and then carbonized to form a coating layer.

6. The method for preparing the microcrystalline graphite composite material as described in claim 5, characterized in that, The step of providing the second microcrystalline graphite includes: ball milling or high-pressure homogenization of the microcrystalline graphite raw material to obtain small-particle-size microcrystalline graphite, and mixing the small-particle-size microcrystalline graphite with a nitrogen source and then performing carbonization treatment. Preferably, the carbonization temperature is 800℃~1200℃; Preferably, the nitrogen source includes at least one of melamine and urea; Preferably, the microcrystalline graphite raw material is the first microcrystalline graphite.

7. The method for preparing the microcrystalline graphite composite material as described in claim 5, characterized in that, The mass ratio of the microcrystalline graphite raw material to the nitrogen source is 1:(0.1~0.3); and / or, The granulation process is carried out at a temperature of 600℃~800℃; and / or, The adhesive includes bitumen; and / or, Following the step of forming the coating layer, the process further includes sieving and demagnetizing the material; and / or, The mass ratio of the binder to the second microcrystalline graphite is 3% to 20%; preferably, the mass ratio of the binder to the second microcrystalline graphite is 5% to 10%.

8. A battery negative electrode, characterized in that, Includes the microcrystalline graphite composite material as described in any one of claims 1-4; or includes the microcrystalline graphite composite material prepared by the preparation method as described in any one of claims 5-7.

9. A battery, characterized in that, Includes the battery negative electrode as described in claim 8.

10. An electrical-related device, characterized in that, Includes the battery as described in claim 9.

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