Microcrystalline graphite material, preparation method and test method thereof, negative electrode sheet, secondary battery and electric device

Abstract

The application provides a microcrystalline graphite material and a preparation method and a test method thereof, a negative pole piece, a secondary battery and an electric device, and belongs to the technical field of batteries. The surface roughness Sa of the microcrystalline graphite material is less than or equal to 52 nm. When the microcrystalline graphite material is used as a negative active material of a battery, the opportunity of the edge surface being directly exposed to an electrolyte can be reduced, thereby helping to reduce the side reaction problem with the electrolyte, and further helping to improve the initial coulomb efficiency of the battery.

Description

Technical Field

[0001] This application belongs to the field of battery technology, and in particular relates to a microcrystalline graphite material and its preparation and testing methods, a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] Graphite is currently the mainstream material with a high proportion of anodes in commercial lithium-ion batteries. Its theoretical specific capacity is 372 mAh / g, which can balance battery life and stability, meeting the needs of all application scenarios such as mobile phones, electric vehicles, and energy storage. Among them, microcrystalline graphite is considered one of the ideal anode materials for high-energy-density and long-cycle-life lithium-ion batteries due to its isotropic and dense structure.

[0003] However, the side reactions between microcrystalline graphite and the electrolyte are more intense, resulting in a lower initial coulombic efficiency of the battery. Summary of the Invention

[0004] This invention provides a microcrystalline graphite material, its preparation and testing methods, a negative electrode sheet, a secondary battery, and an electrical device, aiming to improve the initial coulombic efficiency of the battery.

[0005] In a first aspect, embodiments of the present invention provide a microcrystalline graphite material, wherein the surface roughness Sa of the microcrystalline graphite material is ≤52nm.

[0006] In some embodiments, the volumetric density ρ of the microcrystalline graphite material v ≥1.85 g / cm 3 .

[0007] In some embodiments, the surface roughness Sa of the microcrystalline graphite material is ≥20 nm; and / or, the volumetric density ρ of the microcrystalline graphite material is... v ≤2.05g / cm 3 .

[0008] In some embodiments, the reflectance at a wavelength of 550 nm in the diffuse reflectance spectrum of the microcrystalline graphite material is R550, wherein R550 satisfies: 6.6% ≤ R550 ≤ 9.0%; and / or, the lightness value L* of the microcrystalline graphite material satisfies: 24.5 ≤ L* ≤ 30.

[0009] In some embodiments, the reflectance at a wavelength of 450 nm in the diffuse reflectance spectrum of the microcrystalline graphite material is R450, and R450 and R550 satisfy: 0.75≤R450 / R550≤0.85.

[0010] In some embodiments, the chromaticity angle h of the microcrystalline graphite material ab Satisfy: -172 ° ≤h ab≤-167 ° Optionally, the a* of the microcrystalline graphite material satisfies -0.3≤a*<0, and the b* of the microcrystalline graphite material satisfies -2.0≤b*≤-1.0.

[0011] In some embodiments, the microcrystalline graphite material includes a core and a shell covering at least a portion of the surface of the core, the core comprising microcrystalline graphite and the shell comprising amorphous carbon.

[0012] In some embodiments, the thickness of the shell layer is 100nm~300nm.

[0013] In some embodiments, the particle size Dv50 of the microcrystalline graphite material is 8μm~20μm, and the SPAN is 0.8~1.6.

[0014] Secondly, embodiments of the present invention provide a method for preparing microcrystalline graphite material, comprising: providing microcrystalline graphite concentrate; subjecting the microcrystalline graphite concentrate to spheroidization treatment to obtain spheroidized microcrystalline graphite powder; and purifying the spheroidized microcrystalline graphite powder to obtain microcrystalline graphite material; wherein the spheroidized microcrystalline graphite powder has a Dv50 of 6μm to 19μm and a SPAN ≤ 1.6.

[0015] In some embodiments, the step of purifying the spherical microcrystalline graphite powder includes: heating the spherical microcrystalline graphite powder to a first temperature and holding it in an inert atmosphere, then naturally cooling it to room temperature after the holding period, and finally sieving it through an airflow pulverization process to obtain microcrystalline graphite material, wherein the first temperature is greater than or equal to 2700°C.

[0016] In some embodiments, before the step of heating the spherical microcrystalline graphite powder to a first temperature and holding it at that temperature, the method further includes: adding the spherical microcrystalline graphite powder to an acid solution and stirring to react; after the reaction is completed, washing the filtrate with deionized water until the pH of the filtrate is 6-7; and then vacuum drying to obtain spherical microcrystalline graphite powder with a fixed carbon content of ≥99%.

[0017] In some embodiments, after the step of purifying the spherical microcrystalline graphite powder, the method further includes: providing a carbon source; mixing the spherical microcrystalline graphite powder and the carbon source at a mass ratio of (85~95):(5~15), adding a solvent and stirring evenly, then vacuum drying to remove the solvent, heating the dried mixture to a second temperature under an inert atmosphere for heat preservation and carbonization, and after the heat preservation is completed, naturally cooling to room temperature, and sieving to obtain microcrystalline graphite material, wherein the second temperature is 900℃~1200℃.

[0018] Thirdly, embodiments of the present invention provide a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector, the negative electrode film layer comprising a negative electrode active material, the negative electrode active material comprising the microcrystalline graphite material described in any one of the above claims or the microcrystalline graphite material prepared by the preparation method described in any one of the above claims.

[0019] Fourthly, an embodiment of the present invention provides a secondary battery comprising the aforementioned negative electrode sheet.

[0020] Fifthly, embodiments of the present invention provide a testing method for microcrystalline graphite materials, comprising: testing the diffuse reflectance spectrum of the microcrystalline graphite material; obtaining optical parameters in the diffuse reflectance spectrum, wherein the optical parameters include at least one of reflectance R550 at 550 nm and lightness value L*; and characterizing the surface roughness of the microcrystalline graphite material based on the optical parameters.

[0021] In some embodiments, the step of testing the diffuse reflectance spectrum of a microcrystalline graphite material includes: providing a microcrystalline graphite material; providing a binder; mixing the microcrystalline graphite material and the binder uniformly to form a preform, and curing the preform at room temperature; polishing the cured preform to obtain a graphite sheet; cleaning the graphite sheet and then vacuum drying it; and performing diffuse reflectance spectrum testing on the graphite sheet using a spectrophotometer configured with a D65 standard light source and a 10° observation angle.

[0022] In some embodiments, the mass ratio of the microcrystalline graphite to the binder is (90~98):(2~10).

[0023] In some embodiments, the polishing process includes polishing the cured blank sequentially with diamond polishing paste of 800 mesh, 1500 mesh, and 3000 mesh, for 2 to 5 minutes each time.

[0024] Sixthly, the present invention provides an electrical device, characterized in that it includes the aforementioned secondary battery.

[0025] The beneficial effects of the embodiments of the present invention are as follows: This invention provides a microcrystalline graphite material. Since the microcrystals of microcrystalline graphite are randomly packed, their particle surfaces typically expose unstable edge faces. This invention specifies that the surface roughness Sa of the microcrystalline graphite material should be less than or equal to 52 nm, meaning that the surface undulations of the microcrystalline graphite are smaller, and the particle morphology is smoother. This relatively smooth morphology allows the particle surface to be covered by more stable basal surfaces. When used as a negative electrode active material in batteries, it can reduce the chance of edge faces being directly exposed to the electrolyte, thereby helping to reduce side reactions with the electrolyte and thus helping to improve the battery's initial coulombic efficiency. Furthermore, a surface roughness Sa of less than or equal to 52 nm also helps to construct a stable SEI film (solid electrolyte interface film), thereby reducing the continuous consumption of active lithium by side reactions during cycling, and thus helping to improve the battery's cycle performance.

[0026] This invention further defines the volumetric density ρ of microcrystalline graphite materials. v ≥1.85 g / cm 3 This helps reduce ineffective pores, thereby reducing the continuous irreversible reactions caused by excessive electrolyte seepage into the pores, which in turn helps suppress excessive side reactions, reduce electrolyte loss, and improve the battery's initial coulombic efficiency and cycle performance.

[0027] The testing method for microcrystalline graphite provided by this invention characterizes the surface roughness Ra of microcrystalline graphite by collecting optical parameters in the diffuse reflectance spectrum of microcrystalline graphite materials. It can achieve rapid and non-destructive screening of microcrystalline graphite materials, realize full inspection of production lines, and can be used for testing bulk materials. Attached Figure Description

[0028] Figure 1 This is a SEM image of the microcrystalline graphite material in an embodiment of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the present invention. In the present invention, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in actual use or operation, specifically the drawing directions in the accompanying drawings; while "inner" and "outer" refer to the outline of the device.

[0030] Microcrystalline graphite is composed of numerous tiny and randomly oriented microcrystals, which exposes a large number of highly reactive edge planes on the surface of the microcrystalline graphite particles. As a result, during the first charge, a large number of lithium ions are consumed in the formation of the SEI film, leading to a high irreversible capacity loss and thus affecting the first coulombic efficiency of the secondary battery.

[0031] Therefore, this application provides a microcrystalline graphite material with a surface roughness Sa of less than or equal to 52 nm.

[0032] Microcrystalline graphite is a polycrystalline aggregate composed of numerous tiny graphite crystallites linked by van der Waals forces or disordered carbon, arranged in a random, stacked manner. The average stacking height (Lc) of these crystallites along the c-axis and the average width of the basal plane (La) along the a-axis are typically much less than 100 nm, with typical values ​​ranging from a few nanometers to tens of nanometers. In contrast, the Lc and La of highly graphitized artificial graphite or large flake graphite can reach hundreds of nanometers or even micrometers. The random stacking of crystallites in microcrystalline graphite gives it macroscopic isotropy, and these randomly oriented crystallites are tightly interlocked, forming a dense aggregate.

[0033] In some embodiments, microcrystalline graphite can be identified by X-ray diffraction (XRD). If the (002) peak of its XRD pattern is significantly broadened, with a peak position of about 2θ≈26.5°, and clear, even separated, three-dimensional diffraction peaks (100) and (101) double peaks are present near 2θ≈42°~46°, then the graphite can be confirmed as microcrystalline graphite by calculating Lc(002) and La(100) using the Scherrer formula. If both Lc(002) and La(100) are on the order of tens of nanometers, then the graphite can be confirmed as microcrystalline graphite.

[0034] Surface roughness Sa, short for arithmetic mean height, refers to the arithmetic mean of the distances from the heights of points Z(x,y) on the surface to the reference plane (least square mid-plane) within the sampling area. A smaller Sa value indicates a smoother surface; a larger Sa value indicates more pronounced surface undulations. Because it's a surface scan, Sa is unaffected by the direction of individual scratches and can reflect the texture of the entire surface (such as pits, particle protrusions, etc.), offering far better data repeatability than Ra.

[0035] When the surface roughness Sa of microcrystalline graphite is less than or equal to 52 nm, it means that the surface of the microcrystalline graphite has small undulations and a relatively smooth particle morphology. This relatively smooth morphology allows the particle surface to be covered by more stable basal surfaces. When used as a negative electrode active material in batteries, it can reduce the chance of the edge surfaces being directly exposed to the electrolyte, thereby helping to reduce side reactions with the electrolyte and thus helping to improve the first coulombic efficiency of the battery. In addition, a surface roughness Sa of less than or equal to 52 nm also helps to construct a stable SEI film (solid electrolyte interface film), thereby reducing the consumption of active lithium by continuous side reactions during cycling, and thus helping to improve the cycle performance of the battery.

[0036] For example, the surface roughness Sa of microcrystalline graphite can be 52nm, 50nm, 45nm, 40nm, 35nm, 30nm, 25nm, 20nm, 15nm or 10nm.

[0037] In some embodiments, the surface roughness Sa of the microcrystalline graphite material can be obtained by testing with an atomic force microscope (AFM) or a white light interferometer.

[0038] According to some embodiments of the present invention, the volumetric density ρ of microcrystalline graphite materials v ≥1.85g / cm 3 .

[0039] The volumetric density ρ of microcrystalline graphite materials v The volumetric density ρ of microcrystalline graphite material refers to the ratio of the mass of pure graphite within the block to its total volume after the powder is pressed into a standard dense block. The total volume includes the closed micropores within the graphite but excludes external macroscopic gaps. If the volumetric density of the microcrystalline graphite material is too low, there are too many open pores, which may allow a large amount of electrolyte to seep into the pores, leading to continuous irreversible reactions. Therefore, the volumetric density ρ of the microcrystalline graphite material is limited. v ≥1.85g / cm 3 This helps reduce ineffective pores, thereby reducing the continuous irreversible reactions caused by excessive electrolyte seepage into the pores, which in turn helps suppress excessive side reactions, reduce electrolyte loss, and improve the battery's initial coulombic efficiency and cycle performance.

[0040] In addition, the volumetric density ρ of microcrystalline graphite materials v ≥1.85g / cm 3 This means that more microcrystalline graphite material can be loaded within the same electrode volume, which helps to improve battery capacity and volumetric energy density. In addition, high-density microcrystalline graphite particles can be tightly stacked, increasing the number of contact sites between particles and creating a continuous electron conduction pathway, which helps to significantly improve electronic conductivity, reduce polarization, and improve the rate performance of the battery.

[0041] For example, the volumetric density ρ of microcrystalline graphite materials v It can be 1.85g / cm 3 1.90g / cm 3 1.95g / cm 3 2.00g / cm 3 2.05g / cm 3 2.10 g / cm 3 2.15g / cm 3 Or 2.20g / cm 3 .

[0042] In some embodiments, the true density / skeletal density of the powder can be determined by Archimedes' displacement method or gas displacement method, and the compaction density can be calculated.

[0043] According to some embodiments of the present invention, the surface roughness Sa of the microcrystalline graphite material is ≥20 nm; and / or, the volumetric density ρ of the microcrystalline graphite material is... v ≤2.05g / cm 3 .

[0044] When the surface roughness of microcrystalline graphite material is too low, the material surface is too smooth and has almost no trace active lithium intercalation sites, which may hinder the lithium-ion intercalation kinetics. Therefore, this invention limits the surface roughness Sa of microcrystalline graphite material to ≥20nm to reduce the polarization of the battery at high charge and discharge rates, thereby taking into account the rate performance of the battery.

[0045] When the volumetric density of microcrystalline graphite materials is too high, there is no adequate micropores inside the material, resulting in insufficient space for lithium ion intercalation and extraction, and increased intercalation and extraction resistance. Therefore, this invention limits the volumetric density ρ of the microcrystalline graphite material. v ≤2.05g / cm 3 It also helps to reduce the polarization of the battery during high-rate charging and discharging, thus balancing the battery's rate performance.

[0046] According to some embodiments of the present invention, in the diffuse reflectance spectrum of the microcrystalline graphite material, the reflectance at a wavelength of 550 nm is R550, and R550 satisfies: 6.6%≤R550≤9.0%; and / or, the lightness value L* of the microcrystalline graphite material satisfies: 24.5≤L*≤30.

[0047] The diffuse reflectance spectrum of microcrystalline graphite refers to the absorption spectrum obtained by collecting the diffuse reflectance light of microcrystalline graphite using ultraviolet-visible-near-infrared (UV-Vis-NIR) diffuse reflectance spectroscopy and converting it using the Kubelka-Munk function.

[0048] R550 corresponds to the reflectivity at a wavelength of 550nm, also known as green light reflectivity. A lower R550 indicates more green light is scattered and absorbed by the pores of the microcrystalline graphite material. This implies a rougher surface and more pore defects in the microcrystalline graphite material. This is because a smooth surface results in strong specular diffuse reflection and low green light reflectivity. Numerous open micropores and uneven defects cause diffuse scattering and absorption of visible light, thus weakening R550. Therefore, an R550 value of 6.6% ≤ R550 ≤ 9.0% indicates a relatively smooth surface of the microcrystalline graphite material, reducing side reactions and improving the initial coulombic efficiency of the battery.

[0049] R550 can be any value between 6.6% and 9.0%. For example, R550 can be 6.6%, 6.8%, 7.0%, 7.2%, 7.5%, 7.8%, 8.0%, 8.3%, 8.5%, 8.8%, or 9.0%.

[0050] The lightness value L* refers to the lightness coordinate in the CIE-Lab color space, ranging from 0 to 100, where 0 represents ideal black and 100 represents ideal white. A lower lightness value L* for microcrystalline graphite materials indicates a more uneven surface and more defects. This is because numerous open micropores and defects cause diffuse scattering and absorption of visible light, thus reducing the lightness value L*. Therefore, a lightness value L* of 24.5 ≤ L* ≤ 30 for microcrystalline graphite materials indicates a relatively smooth surface, reducing side reactions and improving the initial coulombic efficiency of the battery.

[0051] The lightness value L* of the microcrystalline graphite material can be any value between 24.5 and 30. For example, the lightness value L* of the microcrystalline graphite material can be 24.5, 25, 26, 27, 28, 29 or 30.

[0052] In some embodiments, the diffuse reflectance spectrum of microcrystalline graphite materials can be obtained by spectrophotometry.

[0053] According to some embodiments of the present invention, in the diffuse reflectance spectrum of the microcrystalline graphite material, the reflectance at a wavelength of 450 nm is R450, and R450 and R550 satisfy: 0.75≤R450 / R550≤0.85.

[0054] R450 corresponds to the reflectance at a wavelength of 450nm, and also refers to the blue light reflectance.

[0055] A ratio of R450 / R550 of 0.75–0.85 indicates stable blue light reflection in microcrystalline graphite, suggesting high volumetric density, numerous closed micropores internally, and few open pores on the surface. When R450 / R550 deviates from this range, it indicates enhanced blue light scattering, a surge in open pores on the surface, and lower density. This is because high-density microcrystalline graphite materials have predominantly closed nanopores internally, which do not absorb large amounts of blue light; low-density microcrystalline graphite materials have numerous interconnected open pores, making it easier for 450nm short-wavelength blue light to be scattered and lost, resulting in a significant decrease in R450 and a lower R450 / R550 ratio. Therefore, an R450 / R550 ratio of 0.75–0.85 can reduce side reactions and improve the initial coulombic efficiency.

[0056] R450 / R550 can be any value between 0.75 and 0.85. For example, R450 / R550 can be 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84 or 0.85.

[0057] Furthermore, R450 / R550 can help determine whether the pores are open or closed, thus reflecting the surface roughness of the microcrystalline graphite material. An excessively high R450 / R550 indicates the presence of open pores on the surface of the microcrystalline graphite material, which can easily worsen side reactions and thus affect the initial coulombic efficiency.

[0058] According to some embodiments of the present invention, the chromaticity angle h of microcrystalline graphite ab Satisfy: -172 ° ≤h ab ≤-167 ° Optionally, the a* of microcrystalline graphite satisfies -0.3≤a*<0, and the b* of microcrystalline graphite satisfies -2.0≤b*≤-1.0.

[0059] The chromaticity angle h of microcrystalline graphite ° This refers to the angle value in the CIE-Lab color space used to precisely define the hue of microcrystalline graphite powder, which can be expressed by the formula h. ab =arctan(b* / a*) is used to calculate the color intensity of microcrystalline graphite, where a* represents the red-green hue and b* represents the yellow-blue hue. -172 ° ≤h ab ≤-167 °It adopts the [-180°, 180°] labeling mode, which is equivalent to 188°~193° in the standard 0~360° range. It falls in the blue-green third quadrant where a*<0 and b*<0, which can eliminate the influence of impurities and oxidation on the detection of R450 / R550, so that the correlation between R450 / R550 is not affected by impurities.

[0060] Among them, the chromaticity angle h of microcrystalline graphite ab It can be -172 ° to -167 ° Any value between these ranges; for example, the chromaticity angle h of microcrystalline graphite. ab It can be -172 ° -171 ° -170 ° -169 ° -168 ° or -167 ° .

[0061] According to some embodiments of the present invention, the microcrystalline graphite material includes a core and a shell covering at least a portion of the surface of the core, the core comprising microcrystalline graphite and the shell comprising amorphous carbon.

[0062] The shell refers to the continuous or nearly continuous structure located on the surface of the core. In transmission electron microscopy (TEM), especially in high-resolution mode (HRTEM), the core and shell exhibit different contrasts due to the different degrees of order in their atomic arrangement, thus creating a boundary between them. The core appears as a cluster of alternating bright and dark parallel stripes, most commonly lattice stripes on the (002) crystal plane, with a spacing of approximately 0.336 nm to 0.34 nm. These stripes may be locally curved or interrupted, but overall maintain a clear parallel relationship and periodicity, forming an abrupt change with the shell. In contrast, the amorphous shell shows no long-range ordered stripes or lattice in the image, only countless randomly distributed bright and dark tiny spots.

[0063] Microcrystalline graphite has a small interlayer spacing, which makes it easy for electrolyte solvent molecules to embed into the graphite layers along with lithium ions during the first charge, leading to graphite layer peeling and irreversible capacity loss. This invention coats the microcrystalline graphite, and the amorphous carbon shell acts as a physical barrier, blocking solvent molecules and thus helping to improve the battery's first charge-discharge efficiency.

[0064] According to some embodiments of the present invention, the thickness of the shell layer is 100nm~300nm.

[0065] The thickness of the shell layer affects both the protective effect and ion transport. When the shell layer thickness is greater than or equal to 100 nm, it provides sufficient thickness to cover the active defects and catalytic sites on the surface of the microcrystalline graphite, reducing direct contact between the electrolyte and graphite and preventing irreversible decomposition. This also reduces the lithium consumed in forming the SEI film, contributing to improved initial coulombic efficiency. A shell layer thickness of 300 nm or less can balance ion transport while minimizing the impact on fast-charging performance.

[0066] The thickness of the shell can be any value between 100nm and 300nm. For example, the thickness of the shell can be 100nm, 125nm, 150nm, 175nm, 200nm, 225nm, 250nm, 275nm or 300nm.

[0067] In some embodiments, the thickness of the shell can be obtained by taking the average of multiple measurements of the shell thickness under TEM.

[0068] According to some embodiments of the present invention, the particle size Dv50 of the microcrystalline graphite material is 8μm~20μm, and the SPAN is 0.8~1.6.

[0069] Dv50 is the median particle size, meaning that half the volume of particles in a microcrystalline graphite material has a particle size smaller than this value, and the other half has a particle size larger than this value. The particle size Dv50 of microcrystalline graphite materials is 8μm to 20μm, which effectively controls the specific surface area of ​​the material. Combined with an amorphous carbon shell of 100nm to 300nm, it can maximally suppress irreversible decomposition of the electrolyte, thereby achieving a higher first coulombic efficiency. The particle size Dv50 of the microcrystalline graphite material can be any value between 8μm and 20μm; for example, the particle size Dv50 of the microcrystalline graphite material can be 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, or 20μm.

[0070] SPAN is the particle size distribution span, calculated as SPAN = (Dv90 - Dv10) / Dv50. When the SPAN of microcrystalline graphite material is between 0.8 and 1.6, the particle size is uniform, allowing for a denser and more regular packing during rolling, resulting in higher electrode compaction density and thus contributing to improved volumetric energy density of the battery. The SPAN of microcrystalline graphite material can be any value between 0.8 and 1.6; for example, the SPAN of microcrystalline graphite material can be 0.8, 1.0, 1.2, 1.4, or 1.6.

[0071] In some embodiments, the particle size Dv50 and SPAN of the microcrystalline graphite material can be obtained by laser particle size analyzer.

[0072] According to some embodiments of the present invention, the present invention also provides a method for preparing microcrystalline graphite material, comprising: providing microcrystalline graphite concentrate; subjecting the microcrystalline graphite concentrate to spheroidization treatment to obtain spheroidized microcrystalline graphite powder; and purifying the spheroidized microcrystalline graphite powder to obtain microcrystalline graphite material; wherein the Dv50 of the spheroidized microcrystalline graphite powder is 6μm~19μm, and the SPAN is ≤1.6.

[0073] Microcrystalline graphite concentrate refers to the concentrate obtained by purifying microcrystalline graphite ore through flotation, wherein the fixed carbon content of the microcrystalline graphite concentrate is greater than or equal to 90%. In some embodiments, the steps of providing microcrystalline graphite concentrate include: grinding the microcrystalline graphite ore to a suitable fineness, adding water to a stirred tank to make a slurry with a concentration generally around 25% to 35%, and sequentially adding flotation reagents (collector and frother). Commonly used collectors are kerosene or diesel oil, which can coat graphite particles to make their surface hydrophobic; commonly used frothers are No. 2 oil (pine oil), which can produce stable and fine bubbles; vigorous stirring and air introduction in a flotation machine, the hydrophobic graphite particles adhere to the bubbles, float to the surface of the slurry to form a black foam layer, which is continuously scraped off by a scraper to obtain a rough concentrate (foam product). The foam scraped off from the roughing process is pumped into the next vertical mill. After further separation in the vertical mill, it is pumped into the flotation machine. The process is repeated 6 times for grinding and 10 times for cleaning. The foam scraped off from the 10th cleaning process is dried to obtain microcrystalline graphite concentrate.

[0074] Spheroidization is a process used to shape irregularly shaped, rough-surfaced, and sharp-edged microcrystalline graphite particles into spherical or near-spherical particles. In some embodiments, spheroidization can be performed in an integrated air jet mill-spheroidization device, using high-pressure air as the working fluid and adjusting the speed of the classifying wheel to simultaneously pulverize and shape the particles in the milling chamber, thereby obtaining spherical microcrystalline graphite powder.

[0075] When the Dv50 of spheroidized microcrystalline graphite powder is between 6 μm and 19 μm, the particles have a regular shape and few sharp edges, which helps to reduce the surface roughness Ra of the finished microcrystalline graphite material. A SPAN ≤ 1.6 indicates that the spheroidized microcrystalline graphite powder has a uniform particle size, few sharp edges and protrusions, and few surface defects, which also helps to reduce the surface roughness Ra. Therefore, this invention can control the surface roughness Ra of the finished microcrystalline graphite material by controlling the particle size Dv50 and SPAN value of the spheroidized microcrystalline graphite powder.

[0076] The particle size Dv50 of the spheroidized microcrystalline graphite powder can be any value between 6 μm and 19 μm. For example, the particle size Dv50 of the spheroidized microcrystalline graphite powder can be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or 19 μm. The SPAN of the spheroidized microcrystalline graphite powder can be any value less than or equal to 1.6. For example, the SPAN of the spheroidized microcrystalline graphite can be 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, or 0.7.

[0077] Purification is performed to remove residual impurities and increase the fixed carbon content of the finished product. Typically, after purification, the fixed carbon content of spheroidized microcrystalline graphite powder can reach over 99%. In some embodiments, purification may include at least one of chemical purification and high-temperature purification.

[0078] According to some embodiments of the present invention, the purification process of spherical microcrystalline graphite powder includes: adding spherical microcrystalline graphite powder to an acid solution and stirring to react; after the reaction is completed, washing with deionized water until the pH of the filtrate is 6-7, and then vacuum drying for later use; in an inert atmosphere, heating the spherical microcrystalline graphite powder purified by the acid solution to a first temperature and holding it at that temperature; after the holding temperature is completed, naturally cooling it to room temperature; and then sieving it after airflow pulverization to obtain microcrystalline graphite material, wherein the first temperature is greater than or equal to 2700°C.

[0079] Adding spherical microcrystalline graphite powder to an acid solution and stirring it is a chemical purification process aimed at dissolving gangue such as silicates / metal oxides. To improve dissolution efficiency, the stirring reaction can be continued at 50°C to 70°C for 3 to 6 hours. In some embodiments, the acid solution includes at least one of hydrofluoric acid, hydrochloric acid, and sulfuric acid. In some embodiments, the fixed carbon content of the chemically purified microcrystalline graphite powder reaches 99.9%, meaning that impurities such as metals and silicates are essentially completely removed. This reduces the risk of impurities vaporizing and creating internal pores during subsequent high-temperature purification. Insufficient purity and high impurity content can easily lead to the escape of impurities at high temperatures, forming micropores and causing a decrease in bulk density.

[0080] Heating spherical microcrystalline graphite powder, after acid purification, to a first temperature and holding it there constitutes high-temperature purification. High-temperature purification aims to evaporate low-boiling-point impurities and repair crystal defects. In this invention, setting the first temperature to be greater than or equal to 2700°C can improve the layered stacking structure of graphite, significantly reducing internal voids and increasing internal density, thereby contributing to higher bulk density of the microcrystalline graphite material. Exemplarily, the first temperature can be 2700°C, 2750°C, 2800°C, 2850°C, 2900°C, 2950°C, or 3000°C. In some embodiments, the holding time can be 2 hours to 5 hours. In some embodiments, the inert gas includes at least one of argon, helium, and nitrogen. In some embodiments, high-temperature purification can be carried out in a graphitization furnace.

[0081] According to some embodiments of the present invention, after the step of purifying the spherical microcrystalline graphite powder, the method further includes: providing a carbon source; mixing the spherical microcrystalline graphite powder and the carbon source at a mass ratio of (85~95):(5~15), adding a solvent and stirring evenly, then vacuum drying to remove the solvent, heating the dried mixture to a second temperature under an inert atmosphere and holding it at that temperature, then naturally cooling it to room temperature after the holding period, and finally sieving it to obtain the microcrystalline graphite material, wherein the second temperature is 900℃~1200℃.

[0082] A carbon source is used to form a carbon coating shell on the surface of microcrystalline graphite powder. In some embodiments, the carbon source includes at least one of asphalt, polymer resin (phenolic resin), biomass raw materials (sucrose, glucose, starch), and water-soluble polymer (polyvinyl alcohol).

[0083] The mass ratio of spherical microcrystalline graphite powder to carbon source affects the coating amount. If the coating amount is too low, it cannot fill the microscopic defects on the surface, leading to increased surface roughness in the finished material. If the coating amount is too high, the coating layer becomes too thick and stacked, also increasing surface undulations and surface roughness. Therefore, a mass ratio of spherical microcrystalline graphite powder to carbon source of (85~95):(5~15) helps reduce the surface roughness of the finished microcrystalline graphite material. The mass ratio of spherical microcrystalline graphite powder to carbon source can be any value between 85:15 and 95:5. For example, the mass ratio of spherical microcrystalline graphite powder to carbon source can be 85:15, 88:12, 90:10, 93:7, or 95:5.

[0084] The solvent is used to uniformly disperse the spherical microcrystalline graphite powder and the carbon source. In some embodiments, the solvent includes at least one of tetrahydrofuran, N-methylpyrrolidone, toluene, and xylene.

[0085] Heating the dried mixture to a second temperature under an inert atmosphere and holding it there allows the carbon source to pyrolyze and carbonize, forming an amorphous carbon shell. The second temperature can be any value between 900°C and 1200°C; for example, it can be 900°C, 950°C, 1000°C, 1050°C, 1100°C, 1150°C, or 1200°C. To ensure maximum carbonization of the carbon source, the holding time can be 2–5 hours. In some embodiments, the carbonization process can be carried out in a tube furnace.

[0086] According to some embodiments of the present invention, the present invention also provides a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector, the negative electrode film layer including a negative electrode active material, the negative electrode active material including any of the above-mentioned microcrystalline graphite materials or microcrystalline graphite materials prepared by any of the above-mentioned preparation methods.

[0087] In some embodiments, the negative electrode active material may further include silicon-based materials. Silicon-based materials have a high theoretical specific capacity, which helps to improve the energy density of the battery. However, silicon-based materials have a large volume expansion, which can affect the cycle performance of the battery. The isotropic nature of microcrystalline graphite helps to uniformly distribute the expansion stress of the silicon-based material in all directions, thereby helping to reduce problems such as particle cracking and negative electrode film peeling. Therefore, combining microcrystalline graphite materials and silicon-based materials helps to improve the battery's energy density while maintaining good cycle performance. In some embodiments, the silicon-based material includes at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0088] The negative electrode current collector can be a metal foil or a composite current collector. For example, copper foil can be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0089] According to some embodiments of the present invention, the compaction density of the negative electrode sheet is 1.60 g / cm³. 3 ~1.75g / cm 3 ; and / or, the porosity of the negative electrode sheet is 18% to 26%.

[0090] The compaction density of the negative electrode sheet refers to the total mass of all substances (negative electrode active material, conductive agent, binder, etc.) per unit volume of the electrode coating (negative electrode film). It can be obtained by the ratio of the total mass of the negative electrode film to its apparent volume, where the apparent volume of the negative electrode film = electrode area × thickness of the negative electrode film. Based on the high bulk density of microcrystalline graphite material, the compaction density of the negative electrode sheet of this invention corresponds to 1.60 g / cm³. 3 ~1.75g / cm 3 This spacing helps to improve the energy density of the battery. For example, the compaction density of the negative electrode sheet can be 1.60 g / cm³. 3 1.62g / cm 3 1.65g / cm 3 1.68g / cm 3 1.70g / cm 3 1.72g / cm 3 Or 1.75g / cm 3 .

[0091] In some embodiments, the compaction density of the negative electrode sheet can be tested by the following method: After discharging the battery to 0% SOC, disassemble it, remove the electrode assembly and separate the negative electrode sheet, clean the negative electrode sheet with dimethyl carbonate and dry it for later use; take a negative electrode sheet sample with a known area S, accurately weigh its total mass and record it as m, take a negative current collector with the same area and thickness, accurately weigh it and record it as m0; observe the cross-section of the electrode sample under an electron microscope. Based on the different compositions of the negative current collector and the negative electrode film, the negative current collector and the negative electrode film will show a clear boundary under the electron microscope. Measure the thickness of the negative electrode film at multiple points using the software provided with the electron microscope, and record the average value as d. Then, the compaction density of the negative electrode sheet = (m-m0) / (S×d).

[0092] The porosity of the negative electrode refers to the porosity of the electrode coating (negative electrode film). Due to the high density of the negative electrode current collector, the porosity of the negative electrode is generally considered to be the same as the porosity of the negative electrode film. Because of the high density and low surface roughness of microcrystalline graphite materials, the negative electrode needs to maintain a certain porosity to provide lithium-ion transport channels. A porosity below 18% will result in insufficient electrolyte wetting and a decrease in rate capability; a porosity above 26% will lead to a decrease in electrode energy density and an increase in side reactions. Therefore, a porosity of 18% to 26% for the negative electrode helps to balance the initial coulombic efficiency and rate performance of the battery.

[0093] In some embodiments, the porosity of the negative electrode sheet can be tested by the following method: Standardized testing using the helium displacement true density method, referring to the relevant porous material characterization specification GB / T 21650.1, with the following steps: Discharge the battery to 0% SOC and disassemble; remove the negative electrode sheet; repeatedly soak and clean it with dimethyl carbonate to remove electrolyte residue; vacuum dry at 60℃ for 12 h to completely remove solvent; cut a complete, powder-free rectangular active coating; peel off the copper foil; measure the average coating thickness at multiple points using a high-precision micrometer; and calculate the apparent total volume of the coating based on the length and width. V 表观 The dry mass of the coating was precisely weighed; the dried coating sample was placed in a high-purity helium-replacement true density analyzer, evacuated, and then filled with helium to achieve equilibrium. The true volume of the solid skeleton of the coating was automatically calculated based on the gas state equation. V 骨架 ; Three parallel tests were conducted on samples from the same batch, and the average value was taken to reduce equipment error; Porosity calculation: Porosity = ( V 表观 - V 骨架 ) / V 表观 ×100%.

[0094] According to some embodiments of the present invention, the negative electrode film layer includes a negative electrode conductive agent, and the mass percentage of the negative electrode conductive agent is 2% to 2.5% based on the total mass of the negative electrode film layer.

[0095] Due to the high bulk density and good interparticle contact of microcrystalline graphite materials, a small amount of negative electrode conductive agent can build a long-range conductive network. However, if the amount of negative electrode conductive agent is too high, it may lead to excessive porosity, thereby undermining the high density advantage. In some embodiments, the mass percentage of the negative electrode conductive agent can be 2%, 2.1%, 2.2%, 2.3%, 2.4%, or 2.5%.

[0096] In some embodiments, the negative electrode conductive agent includes at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, and graphene. In some embodiments, the negative electrode conductive agent includes carbon black and carbon nanotubes, wherein the mass percentage of carbon black is 1.5% to 2.0%, a small amount of carbon nanotubes can build a long-range conductive network, and carbon black fills the gaps between particles, taking into account electron conduction without excessively increasing the porosity of the electrode. According to some embodiments of the present invention, the negative electrode film layer includes a negative electrode binder, and the mass percentage of the negative electrode binder is less than or equal to 6% based on the total mass of the negative electrode film layer.

[0097] When the mass percentage of the negative electrode binder is less than or equal to 6%, it helps reduce the problem of clogging the pores on the graphite surface, and helps provide more lithium-ion transport channels, thus ensuring rate performance. For example, the mass percentage of the negative electrode binder can be 6%, 5.5%, 5%, 4.5%, or 4%.

[0098] In some embodiments, the negative electrode binder includes at least one of styrene-butadiene rubber, carboxymethyl cellulose, and polyvinylidene fluoride.

[0099] According to some embodiments of the present invention, the present invention also provides a secondary battery, including the negative electrode sheet described in any of the above claims.

[0100] According to some embodiments of the present invention, the secondary battery includes an electrolyte, the electrolyte includes additives, and the mass percentage of the additives is less than or equal to 4% based on the total mass of the electrolyte.

[0101] Due to the low surface roughness (few surface defects) of microcrystalline graphite materials, only a small amount of additives is needed to form a thin and stable SEI film. Therefore, setting the mass percentage of the additive to less than or equal to 4% in this invention can reduce the impedance rise problem. For example, the mass percentage of the additive can be 4%, 3.5%, 3%, 2.5%, or 2%.

[0102] In some embodiments, the additive includes at least one of vinylene carbonate and fluoroethylene carbonate.

[0103] Typically, the electrolyte also includes an electrolyte salt and a solvent. In some embodiments, the electrolyte salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate. In some embodiments, the solvent includes at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

[0104] According to some embodiments of the present invention, the present invention provides a testing method for microcrystalline graphite materials, including: testing the diffuse reflectance spectrum of the microcrystalline graphite material; obtaining optical parameters in the diffuse reflectance spectrum, the optical parameters including at least one of reflectance R550 at 550 nm and lightness value L*; and characterizing the surface roughness of the microcrystalline graphite material according to the optical parameters.

[0105] The optical parameters strongly correlated with the surface roughness of microcrystalline graphite materials are R550 and the brightness value L*. The greater the surface roughness Sa (the rougher the surface and the more pores and defects), the more green light is scattered and absorbed by the pores, and the lower the R550 value. At the same time, uneven surface and defects will reduce the reflective brightness, and L* will decrease accordingly.

[0106] In some embodiments, the optical parameters also include R450 / R550. The internal pores of high-volume-density microcrystalline graphite are mainly closed nanopores, which do not absorb a large amount of blue light; low-volume-density microcrystalline graphite has a large number of interconnected open pores, and 450nm short-wavelength blue light is easily scattered and lost by the pores, resulting in a significant decrease in R450 and a corresponding decrease in the R450 / R550 ratio.

[0107] Therefore, based on the specific surface microstructure (low roughness, high density) of microcrystalline graphite materials, they exhibit unique optical characteristics, and materials can be quickly tested and screened through their optical parameters.

[0108] According to some embodiments of the present invention, the step of testing the diffuse reflectance spectrum of microcrystalline graphite material includes: providing microcrystalline graphite material; providing a binder; mixing the microcrystalline graphite material and the binder uniformly to form a preform, and curing the preform at room temperature; polishing the cured preform to obtain a graphite sheet; cleaning the graphite sheet and then vacuum drying it; and using a spectrophotometer equipped with a D65 standard light source and a 10° observation angle to test the diffuse reflectance spectrum of the graphite sheet.

[0109] The present invention does not impose specific limitations on the adhesive. In some embodiments, the adhesive includes at least one of polyvinylidene fluoride and phenolic resin. In some embodiments, the preform can be naturally air-dried and cured at room temperature for 10 to 20 hours.

[0110] Polishing is performed to give the blank surface a mirror-like finish and to remove any scratches visible to the naked eye, so as to obtain accurate optical parameters.

[0111] The cleaning process is to remove residual abrasive and debris from the surface of the graphite sheet. In some embodiments, the graphite sheet can be ultrasonically cleaned in anhydrous ethanol for 5 to 15 minutes, followed by vacuum drying at 50°C to 70°C for 1 to 3 hours.

[0112] According to some embodiments of the present invention, the mass ratio of microcrystalline graphite material to binder is (90~98):(2~10).

[0113] The spherical microcrystalline graphite of this invention has a D50 of 6μm to 19μm and a particle size distribution SPAN ≤ 1.6. The particles exhibit high sphericity, and after close packing, only tiny contact gaps exist between the particles. The microcrystalline graphite material has a large specific surface area, but the total amount of binder (2% to 10%) is limited. After solvent evaporation, it only fills the tiny gaps at the particle contact necks. Due to the low surface energy of the graphite basal surface, the binder cannot completely spread and cover the particle surface. Therefore, when the mass ratio of microcrystalline graphite material to binder is (90~98):(2~10), the binder will not form a continuous and complete coating film on the graphite particle surface. This means that most of the graphite basal surface and microporous structure are exposed. The diffuse reflectance spectrum collected by the spectrophotometer is dominated by the graphite bulk signal, and the optical parameter fluctuations caused by the binder are minimal, reducing the impact on the measurement of the material's optical parameters.

[0114] The mass ratio of microcrystalline graphite material to binder can be any value between 90:10 and 98:2. For example, the mass ratio of microcrystalline graphite material to binder can be 90:10, 91:9, 92:8, 93:7, 94:6, 95:5, 96:4, 97:3 or 98:2.

[0115] According to some embodiments of the present invention, the polishing process includes: polishing the cured blank sequentially with diamond polishing paste of 800 mesh, 1500 mesh and 3000 mesh, for 2 min to 5 min each time.

[0116] According to some embodiments of the present invention, the present invention provides an electrical device, characterized in that it includes any of the above-mentioned secondary batteries.

[0117] The present application will be further described below through specific embodiments. Unless otherwise specified, the experimental materials used in the embodiments can be purchased from conventional biochemical reagent companies.

[0118] Example 1 [Microcrystalline Graphite Materials] Step 1: Flotation for rough purification Microcrystalline graphite ore was taken and crudely purified using conventional flotation. After one roughing and six cleaning processes, a flotation concentrate with a fixed carbon content of 94% was obtained.

[0119] Step 2: Spheroidization treatment The flotation concentrate obtained in step 1 is fed into an integrated air jet mill-spheroidizing device. High-pressure air is used as the working fluid, and the speed of the classifying wheel is adjusted to pulverize and shape the particles simultaneously in the grinding chamber, resulting in spheroidized microcrystalline graphite powder. The obtained powder has a Dv50 of 14 μm and a particle size distribution SPAN of 1.1.

[0120] Step 3: Chemical purification The spherical microcrystalline graphite powder obtained in step 2 was added to a reaction vessel, and a mixed acid solution of 15% hydrochloric acid (HCl) and 5% hydrofluoric acid (HF) was added. The mixture was stirred and reacted at 60°C for 4 hours. After the reaction was completed, the mixture was repeatedly washed with deionized water until the pH of the filtrate was 6.5. The filtrate was then vacuum dried at 120°C for 2 hours to obtain chemically purified microcrystalline graphite with a fixed carbon content of 99.9%.

[0121] Step 4: High-temperature purification The powder obtained in step 3 is placed in a graphite crucible and then placed in a high-temperature graphitization furnace. First, a vacuum is drawn to below 10 Pa, and then high-purity argon (Ar, purity ≥ 99.999%) is introduced to a slightly positive pressure. The temperature is raised to 2800℃ at a heating rate of 5℃ / min and held at this temperature for 3 hours. After the holding period, the powder is naturally cooled to room temperature under the protection of continuous high-purity argon and then removed from the furnace. After being removed from the furnace, the powder is crushed by airflow pulverization and passed through a 300-mesh sieve to obtain microcrystalline graphite powder after high-temperature purification.

[0122] Step 5: Carbon Coating Treatment The powder obtained in step 4 was mixed with asphalt (coal tar pitch with a softening point of about 110℃ and a fixed carbon content of about 65%) at a mass ratio of 95:5, that is, the amount of coating agent asphalt added accounted for 5% of the total mass of the mixture; an appropriate amount of tetrahydrofuran solvent was added and stirred to disperse evenly, and then vacuum dried at 80℃ to remove the solvent; the dried mixture was placed in a tube furnace and carbonized under the protection of high-purity argon (Ar, purity ≥99.999%) at a rate of 3℃ / min to 1000℃ and held for 2 hours to convert the asphalt into an amorphous carbon coating layer; after naturally cooling to room temperature, it was passed through a 200-mesh sieve to obtain microcrystalline graphite material.

[0123] The prepared microcrystalline graphite material was tested using a scanning electron microscope, and the results are as follows: Figure 1 As shown. From Figure 1 It can be seen that the microcrystalline graphite material has a core, and the surface of the core is covered with a continuous granular shell.

[0124] [Negative electrode plate] The above-mentioned microcrystalline graphite material was mixed with conductive agent carbon black, binder carboxymethyl cellulose, and styrene-butadiene rubber in a mass ratio of 92:2:2:4. Deionized water was added and the mixture was stirred evenly to obtain a negative electrode slurry. The negative electrode slurry was then uniformly coated on both sides of the negative electrode current collector copper foil, with a coating surface density of 160 g / m². 2 After drying and cold pressing, the negative electrode sheet is obtained.

[0125] [Positive electrode plate] The positive electrode active material NCM523, conductive agent carbon black, and polyvinylidene fluoride are mixed in a mass ratio of 96:2:2. N-methylpyrrolidone is added and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on both sides of the positive electrode current collector aluminum foil. After drying and cold pressing, a positive electrode sheet is obtained.

[0126] [Isolation membrane] Polyethylene (PE) porous polymer film is used as the separator.

[0127] Electrolyte In a dry argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a mass ratio of 3:5:2 to obtain a solvent. NaPF6 was then added and mixed thoroughly. Next, film-forming additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added and stirred until homogeneous to obtain the electrolyte. The concentration of NaPF6 was 1.05 mol / L, the mass percentage of VC based on the electrolyte was 2%, and the mass percentage of FEC based on the electrolyte was 1%.

[0128] [Assemble] Stack the positive electrode, separator, and negative electrode in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. Then assemble the CR2032 button cell.

[0129] Example 2 The difference between Example 2 and Example 1 is that by reducing the particle size span and grinding away all the tiny sharp edges of the particles, the Dv50 of the spherical powder is 14 μm and the particle size distribution SPAN is 1.0. The rest is the same as in Example 1.

[0130] Example 3 The difference between Example 3 and Example 1 is that the Dv50 of the powder after spheroidization treatment is 14 μm, and the particle size distribution SPAN is 1.3. The rest is the same as in Example 1.

[0131] Example 4 The difference between Example 4 and Example 1 is that the Dv50 of the powder after spheroidization treatment is 14 μm, and the particle size distribution SPAN is 1.4. The rest is the same as in Example 1.

[0132] Example 5 The difference between Example 5 and Example 1 is that the Dv50 of the powder after spheroidization treatment is 14 μm and the particle size distribution SPAN is 1.6. The rest is the same as in Example 1.

[0133] Example 6 The difference between Example 6 and Example 1 is that in the coating process, the mass ratio of microcrystalline graphite powder to asphalt is 90:10. Everything else is the same as in Example 1.

[0134] Example 7 The difference between Example 7 and Example 1 is that in the coating process, the mass ratio of microcrystalline graphite powder to asphalt is 85:15. Everything else is the same as in Example 1.

[0135] Example 8 The difference between Example 8 and Example 1 is that in the coating process, the mass ratio of microcrystalline graphite powder to asphalt is 82:18. Everything else is the same as in Example 1.

[0136] Example 9 The difference between Example 7 and Example 1 is that in the coating process, the mass ratio of microcrystalline graphite powder to asphalt is 98:2. Everything else is the same as in Example 1.

[0137] Example 10 The difference between Example 10 and Example 1 is that the temperature in the high-temperature purification step is 2900℃. The rest is the same as in Example 1.

[0138] Example 11 The difference between Example 11 and Example 1 is that the temperature in the high-temperature purification step is 3000℃. The rest is the same as in Example 1.

[0139] Example 12 The difference between Example 12 and Example 1 is that the temperature in the high-temperature purification step is 2700℃. The rest is the same as in Example 1.

[0140] Example 13 The difference between Example 13 and Example 1 is that the temperature in the high-temperature purification step is 2600℃. The rest is the same as in Example 1.

[0141] Example 14 The difference between Example 14 and Example 1 is that the microcrystalline graphite material is not coated. Everything else is the same as in Example 1.

[0142] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the Dv50 of the powder after spheroidization treatment is 20 μm, the particle size distribution SPAN is 2.0, and the temperature in the high-temperature purification step is 2700℃. The rest is the same as Example 1.

[0143] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the Dv50 of the powder after spheroidization treatment is 20 μm and the particle size distribution SPAN is 1.8. The rest is the same as Example 1.

[0144] Test methods Surface roughness Sa of microcrystalline graphite material: determined using a laser confocal three-dimensional topology analyzer, referring to GB / T 33523-2017 and GB / T Test 42671-23: Microcrystalline graphite material and binder polyvinylidene fluoride were mixed evenly at a mass ratio of 95:5. The mixture was then pressed into cubes (20mm long × 20mm wide × 5mm thick) using a powder press at 20MPa pressure. The cubes were then allowed to air-dry and cure at room temperature for 12 hours. The dried cubes were then polished sequentially on a polishing cloth using diamond abrasive paste of 800 mesh, 1500 mesh, and 3000 mesh, for 3 minutes at each level, until the surface achieved a mirror finish without visible scratches. The polished cubes were then ultrasonically cleaned in anhydrous ethanol for 10 minutes to remove residual abrasive and debris. After removal, they were vacuum-dried at 60℃ for 2 hours to obtain the graphite sheet for testing. The graphite sheet was then tested using a laser confocal 3D topography instrument with a 20x objective lens and a diameter of 500μm × 500μm. The scanning area was filtered by Gaussian filter with λc=0.8mm; the sample center, left, and right were scanned independently, and the arithmetic mean height Sa of the three-dimensional surface was taken as the test result.

[0145] The volumetric density ρv of microcrystalline graphite material was determined using the Archimedes' anhydrous ethanol displacement method (GB / T 24851-2010): The dry weight of a graphite sheet was measured. m 0. After complete immersion in ethanol and vacuum degassing, the buoyant weight is measured by suspension. m 1. Calculate the total volume of the block material based on the density of ethanol at 23℃, ρv= m 0 / [( m 0- m 1) / ρ 液 The same sample was tested in parallel three times and the average value was taken.

[0146] Optical parameters of microcrystalline graphite material: The diffuse reflectance spectrum and CIE-Lab colorimetry of the graphite sheet were measured using a spectrophotometer (model X-Rite Ci7800) with a D65 standard light source and a 10° observation angle. The test aperture was selected as medium aperture (10 mm), including specular reflection mode. The optical parameters were recorded and calculated.

[0147] Compacted density of negative electrode sheet: Cut regular coatings and weigh the total mass of the electrode sheet and the mass of blank copper foil respectively; measure the coating thickness at multiple points with a micrometer to calculate the apparent volume. Compacted density = (net mass of coating) / (length and width of coating × average thickness). Take the average value of three slices for each sample group.

[0148] Porosity of the negative electrode sheet: The porosity of the negative electrode sheet was detected by the helium displacement true density method, which conforms to the relevant porous material characterization specifications in GB / T21650.1. The average value was calculated after testing three groups of samples.

[0149] The battery's initial coulombic efficiency was determined by the following conditions: 12 hours of constant temperature at 25°C; constant current charging to 4.3V at 0.1C, cutoff at 0.01C, 10 minutes of rest, followed by discharge at 0.1C to 2.8V; Initial coulombic efficiency = (Initial discharge capacity / Initial charge capacity) × 100%.

[0150] Battery cycle performance: 25℃ constant temperature, 1C constant current charging to 4.3V, constant voltage cutoff at 0.01C, rest for 5 minutes; 1C discharge to 2.8V to complete one cycle; continuous cycle of 100 cycles, capacity retention rate = (discharge capacity of the 100th cycle / discharge capacity of the 1st cycle) × 100%.

[0151] Battery fast charging performance: Under 25℃ conditions, first charge at 0.1C constant current to 4.3V, then cut off at 0.01C constant voltage, and let stand for 5 minutes; then discharge at 1C to 2.8V, and record the 0.1C discharge capacity; after standing, charge at 1C constant current to 4.3V, then cut off at 0.01C constant voltage, and let stand for 5 minutes; then discharge at 1C to 2.8V, and record the 1C discharge capacity; fast charging performance = (1C discharge capacity ÷ 0.1C discharge capacity) × 100%.

[0152] Experimental data Table 1. Performance of Microcrystalline Graphite Materials

[0153] Table 2 Battery Performance Table

[0154] As can be seen from Tables 1 and 2, compared with Comparative Examples 1 and 2, the batteries of the present invention all exhibit higher initial coulombic efficiency. This is because the surface roughness Sa of the microcrystalline graphite material in the present invention is less than or equal to 52 nm, and the particle morphology is relatively smooth, which reduces the chance of the edge surfaces being directly exposed to the electrolyte, thereby helping to reduce side reactions with the electrolyte and thus contributing to improving the initial coulombic efficiency of the battery. At the same time, a surface roughness Sa of less than or equal to 52 nm also helps to construct a stable SEI film, thereby reducing the consumption of active lithium by continuous side reactions during cycling, and thus contributing to improving the cycle performance of the battery.

[0155] Comparing Example 13 with other examples, it can be seen that, compared to Example 13, when the surface roughness Sa of the microcrystalline graphite material is less than or equal to 52 nm, and the volume density ρ v ≥1.85 g / cm 3 The higher initial coulombic efficiency and cycle performance of the battery are due to the higher volumetric density ρ of the microcrystalline graphite material. v ≥1.85 g / cm 3This helps reduce ineffective pores, thereby reducing the continuous irreversible reactions caused by excessive electrolyte seepage into the pores, which in turn helps suppress excessive side reactions, reduce electrolyte loss, and improve the battery's initial coulombic efficiency and cycle performance.

[0156] Comparing Examples 1 and 14, it can be seen that the surface coating treatment affects the surface roughness Sa of the microcrystalline graphite material, and the first coulombic efficiency and cycle performance of the battery are better after the surface coating treatment. This is because the coating layer, as a physical barrier, can reduce direct contact with the electrolyte, thereby reducing side reactions and reducing losses.

[0157] As can be seen from Table 1, the surface roughness Sa of microcrystalline graphite material is negatively correlated with the optical parameters R550 and lightness value. That is, the larger the surface roughness Sa is, the smaller the R550 and lightness values ​​are. Therefore, the surface roughness of microcrystalline graphite material can be characterized by R550 and lightness value, which can realize rapid and non-destructive screening of microcrystalline graphite material, realize full inspection of production line, and can be used for testing of bulk materials.

[0158] The embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A microcrystalline graphite material, characterized in that, The surface roughness Sa of the microcrystalline graphite material is ≤52nm.

2. The microcrystalline graphite material according to claim 1, characterized in that, The volume density ρ of the microcrystalline graphite material v ≥1.85 g / cm 3 .

3. The microcrystalline graphite material according to claim 1 or 2, characterized in that, The surface roughness Sa of the microcrystalline graphite material is ≥20 nm; and / or, the volumetric density ρ of the microcrystalline graphite material is ≥20 nm. v ≤2.05g / cm 3 .

4. The microcrystalline graphite material according to any one of claims 1 to 3, characterized in that, In the diffuse reflectance spectrum of the microcrystalline graphite material, the reflectance at a wavelength of 550 nm is R550, where R550 satisfies: 6.6% ≤ R550 ≤ 9.0%; and / or, The lightness value L* of the microcrystalline graphite material satisfies: 24.5≤L*≤30.

5. The microcrystalline graphite material according to claim 4, characterized in that, In the diffuse reflectance spectrum of the microcrystalline graphite material, the reflectance at a wavelength of 450 nm is R450, and R450 and R550 satisfy: 0.75≤R450 / R550≤0.

85.

6. The microcrystalline graphite material according to claim 4 or 5, characterized in that, The chromaticity angle h of the microcrystalline graphite material ab Satisfy: -172 ° ≤h ab ≤-167 ° ; Optionally, the a* of the microcrystalline graphite material satisfies -0.3≤a*<0, and the b* of the microcrystalline graphite material satisfies -2.0≤b*≤-1.

0.

7. The microcrystalline graphite material according to any one of claims 1 to 6, characterized in that, The microcrystalline graphite material includes a core and a shell covering at least a portion of the surface of the core, the core comprising microcrystalline graphite and the shell comprising amorphous carbon.

8. The microcrystalline graphite material according to claim 7, characterized in that, The thickness of the shell is 100nm~300nm.

9. The microcrystalline graphite material according to any one of claims 1 to 8, characterized in that, The microcrystalline graphite material has a particle size Dv50 of 8μm to 20μm and a SPAN of 0.8 to 1.

6.

10. A method for preparing a microcrystalline graphite material, characterized in that, include: Provide microcrystalline graphite concentrate; The microcrystalline graphite concentrate is spheroidized to obtain spheroidized microcrystalline graphite powder. The spherical microcrystalline graphite powder was purified to obtain microcrystalline graphite material; The spherical microcrystalline graphite powder has a Dv50 of 6μm to 19μm and a SPAN of ≤1.

6.

11. The method for preparing microcrystalline graphite material according to claim 10, characterized in that, The purification process for the spherical microcrystalline graphite powder includes: The spherical microcrystalline graphite powder was added to the acid solution and stirred to react. After the reaction was completed, the solution was washed with deionized water until the pH of the filtrate was 6-7, and then vacuum dried for later use. In an inert atmosphere, spherical microcrystalline graphite powder that has been purified by acid is heated to a first temperature and held at that temperature. After the holding period, it is naturally cooled to room temperature, and then sieved after being subjected to airflow pulverization to obtain microcrystalline graphite material. The first temperature is greater than or equal to 2700℃.

12. The method for preparing microcrystalline graphite material according to claim 10 or 11, characterized in that, The step of purifying the spherical microcrystalline graphite powder further includes: Provide carbon source; The spherical microcrystalline graphite powder and the carbon source are mixed at a mass ratio of (85~95):(5~15), a solvent is added and stirred evenly, and then the solvent is removed by vacuum drying. The dried mixture is heated to a second temperature under an inert atmosphere and kept at that temperature. After the holding period, it is naturally cooled to room temperature and then sieved to obtain microcrystalline graphite material. The second temperature is 900℃~1200℃.

13. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes the microcrystalline graphite material according to any one of claims 1 to 9 or the microcrystalline graphite material prepared by the preparation method according to any one of claims 10 to 12.

14. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 13.

15. A method for testing microcrystalline graphite materials, characterized in that, include: Testing the diffuse reflectance spectrum of microcrystalline graphite materials; Obtain optical parameters from the diffuse reflectance spectrum, the optical parameters including at least one of reflectance R550 at 550 nm and lightness value L*; The surface roughness of the microcrystalline graphite material is characterized by the optical parameters described above.

16. The test method for microcrystalline graphite materials according to claim 15, characterized in that, The steps for testing the diffuse reflectance spectrum of microcrystalline graphite materials include: Provide microcrystalline graphite materials; Provide adhesive; The microcrystalline graphite material and the binder are mixed evenly to form a preform, which is then cured at room temperature. The cured preform is polished to obtain a graphite sheet. The graphite sheet is then cleaned and vacuum dried. The diffuse reflectance spectrum of the graphite sheet was tested using a spectrophotometer equipped with a D65 standard light source and a 10° observation angle.

17. The test method for microcrystalline graphite materials according to claim 16, characterized in that, The mass ratio of the microcrystalline graphite to the binder is (90~98):(2~10).

18. The test method for microcrystalline graphite according to claim 16 or 17, characterized in that, The polishing process includes the following steps: The cured blank was polished sequentially using diamond polishing paste of 800 mesh, 1500 mesh, and 3000 mesh, for 2 to 5 minutes each time.

19. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 14.

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