Artificial graphite and preparation method thereof, method for testing isotropic degree of raw material coke, and secondary battery

Abstract

The invention provides artificial graphite and a preparation method thereof, a method for testing the isotropic degree of raw material coke, and a secondary battery. The preparation raw materials of the artificial graphite comprise raw material coke, the microstructure isotropic index MI of the raw material coke is greater than or equal to 2.3 and less than or equal to 6; optionally 2.5 < = MI < = 6; the calculation formula of the MI is shown in the specification; in the formula, alpha i is the isotropic contribution assignment of the ith microstructure in the raw material coke, delta i is the volume fraction of the ith microstructure in the raw material coke, and m is the variety number of the microstructures in the raw material coke; the microstructure in the raw material coke comprises an isotropic microstructure and an anisotropic microstructure, the isotropic contribution assignment of the isotropic microstructure is 3-6, and the isotropic contribution assignment of the anisotropic microstructure is 1-2. According to the method for testing the isotropic degree of the raw material coke, the isotropic degree of the raw material coke can be accurately tested, and the artificial graphite and the secondary battery have good fast charging performance.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, and in particular to artificial graphite and its preparation method, a test method for the isotropic degree of raw material coke, and secondary batteries. Background Technology

[0002] Lithium-ion rechargeable batteries have advantages such as high energy density, high operating voltage, long cycle life, and environmental friendliness, and are widely used in 3C consumer products, power batteries, and energy storage batteries. Commonly used negative electrode materials in rechargeable batteries include graphite and silicon-based materials. Among them, artificial graphite has become the most commonly used negative electrode material in commercially available lithium-ion batteries due to its high energy density, low voltage, good conductivity, abundant resources, and low price.

[0003] During the charging process of a secondary battery, lithium ions preferentially insert into the graphite layer from the end face and then diffuse into the interior of the graphite particles. This characteristic limits the reactive sites in the material and lengthens the diffusion path, hindering the rapid insertion of lithium ions. Consequently, the stability and safety of artificial graphite materials under high-rate charging cannot meet the application requirements of fast-charging batteries. Therefore, modifying artificial graphite materials to improve the fast-charging capability of secondary batteries has become a research hotspot for artificial graphite anode materials in recent years.

[0004] Currently, methods to improve the fast-charging performance of artificial graphite materials mainly include surface coating, doping modification, and aggregate particle design. These methods all involve further modification of the artificial graphite product without considering the raw material coke used; furthermore, these methods typically involve complex processing steps. Summary of the Invention

[0005] Based on this, this application provides an artificial graphite and its preparation method, a method for testing the isotropic degree of the raw material coke, and a secondary battery; the artificial graphite has good fast charging performance and a simple process.

[0006] The technical solution proposed in this application is as follows:

[0007] According to a first aspect of this application, an artificial graphite is provided, wherein the raw material for preparing the artificial graphite includes raw material coke, and the microstructure isotropic index of the raw material coke is MI, wherein 2.3≤MI≤6; optionally, 2.5≤MI≤6;

[0008] The formula for calculating MI is: ;

[0009] In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i is the volume fraction of the i-th microstructure in the raw coke; m is the number of types of microstructures in the raw coke;

[0010] The microstructure of the raw coke includes one or more of isotropic and anisotropic structures, with the isotropic contribution value of the isotropic structure being 3 to 6 and the isotropic contribution value of the anisotropic structure being 1 to 2.

[0011] In some embodiments, the isotropic tissue includes one or more of coarse-grained mosaic tissue, medium-grained mosaic tissue, fine-grained mosaic tissue, and isotropic tissue; the anisotropic tissue includes one or more of sheet-like tissue, fully fibrous tissue, and incomplete fibrous tissue.

[0012] The isotropic contribution of the coarse-grained mosaic structure is assigned a value of 3; the isotropic contribution of the medium-grained mosaic structure is assigned a value of 3; the isotropic contribution of the fine-grained mosaic structure is assigned a value of 4; and the isotropic contribution of the isotropic structure is assigned a value of 6.

[0013] The isotropic contribution of the sheet-like tissue is assigned a value of 1; the isotropic contribution of the fully fibrous tissue is assigned a value of 1; and the isotropic contribution of the incomplete fibrous tissue is assigned a value of 2.

[0014] In some embodiments, the volume fraction δ of the i-th microstructure in the raw coke i The calculation formula is: ;

[0015] In the formula, D i is the number of valid test points for the i-th microstructure; the sum of the valid test points for all microstructures in the raw coke is greater than or equal to 400; m is the number of types of microstructures in the raw coke.

[0016] According to a second aspect of this application, a method for preparing artificial graphite is provided, comprising the following steps: pre-oxidizing and graphitizing raw material coke sequentially to obtain the artificial graphite;

[0017] Wherein, the microstructure isotropic index of the raw coke is MI, where 2.3≤MI≤6; or optionally 2.5≤MI≤6;

[0018] The formula for calculating MI is: ;

[0019] In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i is the volume fraction of the i-th microstructure in the raw coke; m is the number of types of microstructures in the raw coke;

[0020] The microstructure of the raw coke includes one or more of isotropic and anisotropic structures, with the isotropic contribution value of the isotropic structure being 3 to 6 and the isotropic contribution value of the anisotropic structure being 1 to 2.

[0021] In some embodiments, the pre-carbonization temperature is 900℃~1200℃, and the pre-carbonization time is 0.5h~2h; the graphitization temperature is 2800℃~3100℃, and the graphitization time is 0.5h~2h.

[0022] According to a third aspect of this application, a method for testing the isotropy of raw material coke is provided, comprising the following steps:

[0023] The raw coke is crushed and sieved to obtain coke powder;

[0024] The coke powder is mixed with a binder to prepare a polarization test sample;

[0025] After grinding and polishing the polarized light test sample in sequence, the polarized light test sample was examined under a polarizing microscope. The volume fraction of each microstructure was counted and the isotropic index MI of the raw material coke was calculated.

[0026] The formula for calculating MI is as follows: ;

[0027] In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i is the volume fraction of the i-th microstructure in the raw coke; m is the number of types of microstructures in the raw coke;

[0028] The microstructure of the raw coke includes one or more of isotropic and anisotropic structures, with the isotropic contribution value of the isotropic structure being 3 to 6 and the isotropic contribution value of the anisotropic structure being 1 to 2.

[0029] In some embodiments, the particle size of the coke powder is less than 1 mm, and the mass percentage of particles with a particle size of less than 0.1 mm in the coke powder does not exceed 15%.

[0030] In some embodiments, grinding the polarization test sample includes the following steps: grinding the polarization test sample sequentially on a polishing machine using sandpaper with mesh sizes of 600, 800, 1200, and 1500; the center pressure of the polishing machine is 60N~90N, the pressure of the pressure head is 15N~25N, the rotation speed of the rotating head is 30rpm~50rpm, and the rotation speed of the grinding disc is 200rpm~400rpm; the grinding time for 600-mesh and 800-mesh sandpaper is 20s~40s, the grinding time for 1200-mesh sandpaper is 30s~60s, and the grinding time for 1500-mesh sandpaper is 60s~90s.

[0031] In some embodiments, polishing the polarized test sample includes the following steps: polishing the ground polarized test sample with a polishing cloth and a polishing liquid; the polishing cloth includes one or more of gray velvet, red velvet and black velvet; the fineness of the polishing liquid is 1μm~5μm; and the polishing time is 200s~300s.

[0032] According to a fourth aspect of this application, a secondary battery is provided, including a negative electrode sheet, the negative electrode sheet including a negative electrode active layer, the negative electrode active layer including artificial graphite of the first aspect of this application, or including artificial graphite prepared by the preparation method of the second aspect of this application.

[0033] Compared with traditional technologies, this application has at least the following beneficial effects:

[0034] This application assigns isotropic contribution values ​​to each microstructure in the raw coke based on their contribution to the coke's isotropy, and determines the microstructure isotropy index of the raw coke by combining the volume fraction of each microstructure. This method allows for relatively accurate testing of the isotropy of the raw coke. Furthermore, raw coke with a microstructure isotropy index MI ≥ 2.3 is selected, and the artificial graphite prepared using this raw coke exhibits good fast-charging performance. Moreover, compared to traditional methods of improving the fast-charging performance of artificial graphite through surface coating, doping modification, and aggregate particle design, this application improves the fast-charging performance of artificial graphite from the perspective of raw coke selection, resulting in a simpler process.

[0035] Furthermore, when testing the isotropy of the raw coke, this application controls the particle size of the coke powder used to prepare the polarized test sample to be below 1 mm, and the mass proportion of particles with a particle size of less than 0.1 mm in the coke powder does not exceed 15%; this enables the polarized test sample to better reflect the original structure of the raw coke and improves the accuracy of the test results of the isotropy of the raw coke.

[0036] Furthermore, when testing the isotropic properties of raw coke, this application can create a smoother surface on the polarized light test sample by controlling specific grinding and polishing process parameters, reducing scratches and pits on the sample surface, and making the microstructure of the raw coke more clearly exposed, thereby further improving the accuracy of the isotropic property test results of the raw coke. Attached Figure Description

[0037] To better describe and illustrate the embodiments or examples provided in this application, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the currently described embodiments or examples, or the best mode of conduct of these applications as currently understood. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0038] Figure 1 The microstructure of the isotropic structure of the raw coke is shown.

[0039] Figure 2 The microstructure of the fine-grained mosaic structure of the raw coke is shown.

[0040] Figure 3 The microstructure of the grain-embedded structure in the raw coke is shown.

[0041] Figure 4 The microstructure of the coarse-grained mosaic structure of the raw coke is shown.

[0042] Figure 5 The microstructure of the incomplete fibrous structure of the raw material coke is shown.

[0043] Figure 6 The microstructure of the complete fibrous structure of the raw material coke is shown.

[0044] Figure 7 The microstructure of the raw material coke flakes is shown. Detailed Implementation

[0045] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, a detailed description of specific embodiments of this application is provided. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise specifically stated, all raw materials, reagents, instruments, and equipment used in this application are commercially available or can be prepared by existing methods.

[0047] One embodiment of this application provides an artificial graphite, the raw material for preparing the artificial graphite including raw material coke, the microstructure isotropic index of the raw material coke being MI, 2.3≤MI≤6;

[0048] The formula for calculating MI is: ;

[0049] In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i denoted as volume fraction of the i-th microstructure in the raw coke; m represents the number of microstructure types in the raw coke; the microstructures in the raw coke include one or more of isotropic and anisotropic microstructures, with isotropic contribution values ​​of 3 to 6 for isotropic microstructures and isotropic contribution values ​​of 1 to 2 for anisotropic microstructures.

[0050] The artificial graphite of this application is prepared using the aforementioned raw coke as a raw material. Based on the contribution of various microstructures in the raw coke to its isotropy, an isotropy contribution value is assigned to each microstructure in the raw coke, and the microstructure isotropy index of the raw coke is determined by combining the volume fraction of each microstructure. This method allows for relatively accurate testing of the isotropy of the raw coke. Furthermore, selecting raw coke with a microstructure isotropy index MI ≥ 2.3 is beneficial for obtaining artificial graphite with good fast-charging performance.

[0051] Furthermore, compared to traditional methods of improving the fast-charging performance of artificial graphite through surface coating, doping modification, and aggregate particle design, this application improves the fast-charging performance of artificial graphite from the perspective of raw material coke screening, and its method is simpler.

[0052] It should be noted that the isotropy of raw coke is a quantitative indicator that measures the consistency of the microstructure and physicochemical properties of raw coke in different spatial directions, reflecting the random uniformity of the carbon microcrystal arrangement within the raw coke. The isotropy index (MI) can reflect the isotropy of the raw coke; the larger the MI, the higher the isotropy of the raw coke. When the isotropy of the raw coke is high, the isotropy of the corresponding artificial graphite will also be relatively high. Internal lithium ions can rapidly insert and extract from multiple directions in the artificial graphite material, and there are more lithium ion transport channels and shorter transport paths, resulting in better fast-charging performance of the artificial graphite.

[0053] Understandable, formula This means: multiply the isotropic contribution of each microstructure by the corresponding volume fraction of that microstructure to obtain the product; then sum the products of all types of microstructures.

[0054] Alternatively, the isotropic index of the raw coke microstructure is 2.5 ≤ MI ≤ 6. Using raw coke with an isotropic index MI ≥ 2.5 as the raw material for preparing artificial graphite is beneficial for obtaining artificial graphite products with better fast-charging performance.

[0055] In some embodiments, isotropic tissues include one or more of coarse-grained mosaic tissue, medium-grained mosaic tissue, fine-grained mosaic tissue, and isotropic tissue; anisotropic tissues include one or more of sheet-like tissue, fully fibrous tissue, and incomplete fibrous tissue.

[0056] In this application, the isotropic contribution of coarse-grained mosaic tissue is assigned a value of 3, the isotropic contribution of medium-grained mosaic tissue is assigned a value of 3, the isotropic contribution of fine-grained mosaic tissue is assigned a value of 4, and the isotropic contribution of isotropic tissue is assigned a value of 6; the isotropic contribution of lamellar tissue is assigned a value of 1, the isotropic contribution of fully fibrous tissue is assigned a value of 1, and the isotropic contribution of incomplete fibrous tissue is assigned a value of 2.

[0057] By further subdividing the isotropic and anisotropic microstructures in the raw coke and assigning isotropic contribution values ​​to each subdivided microstructure type, the accuracy of the isotropic degree test of the raw coke can be improved, which in turn can improve the fast-charging performance of artificial graphite.

[0058] It should be noted that the various microstructures of the raw material coke can be tested using the methods commonly used in this application. Without limitation, the raw material coke can be prepared as a polarized light test sample, and the content of various microstructures in the sample can be tested using a polarized light microscope. Then, the microstructure isotropic index (MI) of the raw material coke can be determined by combining the isotropic contribution values ​​of each microstructure.

[0059] In this application, the various microstructures in the raw coke can be distinguished by the following optical characteristics:

[0060] The optical characteristics of isotropic tissues are smooth pore edges, flat surface, and a first-order red color under a polarizing microscope. Their microstructure is as follows: Figure 1 As shown. The optical characteristics of the fine-grained mosaic structure are anisotropic unit size <1.0 μm, and its microstructure morphology is as follows. Figure 2 As shown. The optical characteristics of the medium-grained mosaic structure are that the anisotropic unit size is between 1.0 μm and 5.0 μm (excluding 5.0 μm), and its microstructure morphology is as follows. Figure 3 As shown. The optical characteristics of the coarse-grained mosaic structure are that the anisotropic unit size is between 5.0 μm and 10.0 μm, and its microstructure morphology is as follows. Figure 4 As shown. The optical characteristics of the incomplete fibrous tissue are anisotropic unit size <10.0 μm, length between 10.0 μm and 30.0 μm, and unidirectional flow. Its microstructure is as follows. Figure 5 As shown. The optical characteristics of the fully fibrous tissue are that the anisotropic units are <10.0 μm in size, ≥30 μm in length, and arranged in multiple parallel bundles. Its microstructure is as follows. Figure 6 As shown. The optical characteristics of the lamellar tissue are anisotropic unit size >30.0 μm, and its microstructure is as follows. Figure 7 As shown.

[0061] In some embodiments, the raw material coke is at least one selected from petroleum coke, coal-based needle coke, oil-based needle coke, coal-based pitch coke, and oil-based pitch coke. The ash content of the raw material coke is no more than 10%, and the sulfur content is no more than 5%. The volume average particle size Dv50 of the artificial graphite is 6μm~18μm, and the specific surface area is less than 2cm². 3 / g, with a magnetic material content of less than 1000ppb.

[0062] In some embodiments, the volume fraction δ of the i-th microstructure in the feedstock coke i The calculation formula is: In the formula, D i is the number of valid test points for the i-th microstructure; the sum of the valid test points for all microstructures in the raw coke is greater than or equal to 400; m is the number of types of microstructures in the raw coke.

[0063] For example, suppose the microstructure tested in the raw coke is one of seven types: coarse-grained mosaic, medium-grained mosaic, fine-grained mosaic, isotropic structure, lamellar structure, fully fibrous structure, and incomplete fibrous structure, i.e., m=7; suppose the effective test points for each type of microstructure are 80, i.e., D i=80; therefore, the sum of the effective test points for various microstructures in the raw coke is 80 × 7 = 560; therefore, the volume fraction δ of any one of the above microstructures in the raw coke is... i The result is 80 / 560×100%=14.29 (rounded to the nearest whole number).

[0064] The volume fraction δ of each microstructure in the raw coke is determined using the above formula. i This makes it easier to accurately test the isotropic degree of raw material coke, thereby more accurately screening out raw material coke with a high degree of isotropic degree and improving the fast-charging performance of artificial graphite.

[0065] Understandably, the test area for each valid test point is consistent. A valid test point refers to a test point where the microstructure under the crosshair of the polarizing microscope during polarization testing does not exhibit pores, gaps, or microcavities. If pores, gaps, or microcavities are present in the microstructure under the crosshair of the polarizing microscope, it is considered an invalid test point and is not included in the calculation of the microstructure volume fraction of the raw coke.

[0066] One embodiment of this application provides a method for preparing artificial graphite, which includes the following steps: pre-oxidizing and graphitizing the raw material coke in sequence to obtain artificial graphite;

[0067] The isotropic index of the microstructure of the raw coke is MI, where 2.3≤MI≤6; or 2.5≤MI≤6.

[0068] The formula for calculating MI is: ;

[0069] In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i denoted as volume fraction of the i-th microstructure in the raw coke; m represents the number of microstructure types in the raw coke; the microstructures in the raw coke include one or more of isotropic and anisotropic microstructures, with isotropic contribution values ​​of 3 to 6 for isotropic microstructures and isotropic contribution values ​​of 1 to 2 for anisotropic microstructures.

[0070] By using coke with a high isotropic index (MI) selected in this application as the raw material for preparing artificial graphite, and sequentially performing pre-oxidation and graphitization treatments, artificial graphite with excellent fast-charging performance can be prepared. Compared with traditional methods that improve the fast-charging performance of artificial graphite through surface coating, doping modification, and aggregate particle design, the preparation method of this application improves the fast-charging performance of artificial graphite from the perspective of coke selection, and its preparation method is simpler.

[0071] In some embodiments, the pre-carbonization temperature is 900℃~1200℃, and the pre-carbonization time is 0.5h~2h. Pre-carbonizing the raw coke under the above temperature and time conditions is beneficial for removing impurities from the raw coke and constructing a preliminary carbon skeleton.

[0072] Understandably, the pre-carbonization temperature can be 900℃, 950℃, 1000℃, 1050℃, 1100℃, 1150℃, 1200℃, or any value within the range formed by any two of the above values. The pre-carbonization time can be 0.5h, 0.8h, 1h, 1.2h, 1.5h, 1.8h, 2h, or any value within the range formed by any two of the above values.

[0073] In some embodiments, the graphitization temperature is 2800℃~3100℃, and the graphitization time is 0.5h~2h. Performing graphitization treatment on the pre-carbonized raw coke under the above temperature and time conditions helps to improve the orderliness of the carbon structure and ensures thorough graphitization of the raw coke.

[0074] Understandably, the graphitization temperature can be 2800℃, 2820℃, 2850℃, 2880℃, 2900℃, 2920℃, 2950℃, 2980℃, 3000℃, 3020℃, 3050℃, 3080℃, 3100℃, or any value within the range formed by any two of the above values. The graphitization time can be 0.5h, 0.8h, 1h, 1.2h, 1.5h, 1.8h, 2h, or any value within the range formed by any two of the above values.

[0075] In some embodiments, the process includes crushing, drying, and grinding the raw coke before pre-oxidation treatment. Crushing, drying, and grinding the raw coke allows it to reach a suitable particle size, thereby ensuring the prepared artificial graphite conforms to the desired particle size range.

[0076] In some specific examples, the raw coke can be crushed using jaw crushers, roller crushers, or hammer crushers. The raw coke can be ground using air jet mills, mechanical mills, or other grinding methods. The pre-carbonization process of the raw coke can be carried out in at least one of a box furnace, tunnel kiln, or roller kiln. The graphitization of the raw coke can be carried out in at least one of a medium-frequency furnace, an Atchison furnace, or a continuous graphitization furnace.

[0077] In some embodiments, after grinding, the raw coke has a Dv10 of 1 μm to 5 μm, a Dv50 of 7 μm to 20 μm, and a Dv90 of 25 μm to 50 μm. The prepared artificial graphite powder has a Dv50 of 6 μm to 18 μm and a specific surface area of ​​less than 2 cm². 3 / g, with a magnetic material content of less than 1000ppb.

[0078] Understandably, Dv10 refers to the particle size corresponding to a cumulative volume distribution percentage of 10%, meaning that particles smaller than or equal to this size account for 10% of the total particle volume. Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, meaning that particles smaller than or equal to this size account for 50% of the total particle volume; and Dv90 refers to the particle size corresponding to a cumulative volume distribution percentage of 90%, meaning that particles smaller than or equal to this size account for 90% of the total particle volume.

[0079] Dv10, Dv50, and Dv90 can be tested using methods known in the art. For example, they can be determined using a laser particle size analyzer such as the Malvern Master Size 3000.

[0080] Particle size distribution can be obtained by the following method: Take a clean beaker, add an appropriate amount of particle sample, and sonicate thoroughly to ensure complete dispersion of the particle sample. The testing instrument is a Malvern 2000 (USA). After the particle sample is poured into the injection tower, it circulates with the solution to the test optical path system. Under the illumination of the laser beam, the particle size distribution characteristics can be obtained by receiving and measuring the energy distribution of the scattered light (opause: 8%~12%). Particle size distribution diagrams are plotted based on the test data.

[0081] In some embodiments, the raw material coke used to prepare artificial graphite is at least one selected from petroleum coke, coal-based needle coke, oil-based needle coke, coal-based pitch coke, and oil-based pitch coke. The ash content of the raw material coke is no more than 10%, and the sulfur content is no more than 5%.

[0082] Furthermore, in this application, isotropic tissues include one or more of coarse-grained mosaic tissue, medium-grained mosaic tissue, fine-grained mosaic tissue, and isotropic tissues; anisotropic tissues include one or more of sheet-like tissues, fully fibrous tissues, and incomplete fibrous tissues.

[0083] In this application, the isotropic contribution of coarse-grained mosaic tissue is assigned as 3, the isotropic contribution of medium-grained mosaic tissue is assigned as 3, the isotropic contribution of fine-grained mosaic tissue is assigned as 4, and the isotropic contribution of isotropic tissue is assigned as 6; the isotropic contribution of lamellar tissue is assigned as 1, the isotropic contribution of fully fibrous tissue is assigned as 1, and the isotropic contribution of incomplete fibrous tissue is assigned as 2.

[0084] By further subdividing the isotropic and anisotropic microstructures in the raw coke and assigning isotropic contribution values ​​to each subdivided microstructure type, the accuracy of the isotropic degree test of the raw coke can be improved, which in turn can improve the fast-charging performance of artificial graphite.

[0085] In some embodiments, the volume fraction δ of the i-th microstructure in the feedstock coke i The calculation formula is: In the formula, D i is the number of valid test points for the i-th microstructure; the sum of the valid test points for all microstructures in the raw coke is greater than or equal to 400; m is the number of types of microstructures in the raw coke.

[0086] The volume fraction δ of each microstructure in the raw coke is determined using the above formula. i This makes it easier to accurately test the isotropic degree of raw material coke, thereby more accurately screening out raw material coke with a high degree of isotropic degree and improving the fast-charging performance of artificial graphite.

[0087] One embodiment of this application provides a method for testing the isotropy of raw coke, the method comprising the following steps S100 to S300:

[0088] Step S100: The raw coke is crushed and sieved to obtain coke powder.

[0089] Step S200: Mix the coke powder with the binder to prepare a polarization test sample.

[0090] Step S300: After grinding and polishing the polarized light test sample in sequence, the polarized light test sample is examined under a polarizing microscope. The volume fraction of each microstructure is counted and the isotropic index MI of the raw material coke is calculated.

[0091] The formula for calculating MI is as follows: ;

[0092] In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i denoted as volume fraction of the i-th microstructure in the raw coke; m represents the number of microstructure types in the raw coke; the microstructures in the raw coke include one or more of isotropic and anisotropic microstructures, with isotropic contribution values ​​of 3 to 6 for isotropic microstructures and isotropic contribution values ​​of 1 to 2 for anisotropic microstructures.

[0093] The method for testing the isotropic degree of raw coke described in this application first involves crushing and sieving the raw coke to obtain coke powder of the desired particle size; then mixing the coke powder with a binder to prepare a polarized light test sample; after grinding and polishing the polarized light test sample in sequence, the polarized light test sample is examined under a polarizing microscope, the volume fraction of each microstructure is counted, and the isotropic index (MI) of the raw coke is calculated based on the isotropic contribution of each microstructure.

[0094] This testing method assigns an isotropic contribution value to each microstructure in the raw coke based on their contribution to the coke's isotropy. The isotropy index of the raw coke is then determined by combining the volume fraction of each microstructure with the isotropy index, which reflects the degree of isotropy of the raw coke. This method allows for relatively accurate testing of the isotropy of the raw coke.

[0095] In some embodiments, the particle size of the coke powder is less than 1 mm, and the mass percentage of particles smaller than 0.1 mm in the coke powder does not exceed 15%. By crushing and sieving the raw coke, the particle size of the coke powder is made to be within the above-mentioned range, which can meet the testing requirements. At the same time, it is beneficial to better preserve the structure of the raw coke, so as not to destroy the structure of the raw coke due to excessively small particle size. This allows the polarized light test sample to better reflect the original structure of the raw coke, which is beneficial to improving the accuracy of the isotropic test results of the raw coke.

[0096] In some embodiments, shellac is used as the binder. The sieved coke powder is mixed with the shellac binder, and then a columnar polarization test sample is prepared using a thermal embedding method. Shellac is a natural resin, primarily extracted and processed from the resin secreted by lac insects, and possesses excellent adhesive, film-forming, and solubility properties. Using it as a binder to mix with coke powder to prepare columnar polarization test samples will not cause optical interference to the test results.

[0097] In some embodiments, grinding the polarization test sample includes the following steps: grinding the polarization test sample sequentially on an automatic polishing machine using sandpaper with mesh sizes of 600, 800, 1200, and 1500; the center pressure of the automatic polishing machine is controlled at 60N~90N, the pressure of the pressure head is controlled at 15N~25N, the rotation speed of the rotating head is controlled at 30rpm~50rpm, and the rotation speed of the grinding disc is controlled at 200rpm~400rpm. The grinding time for 600-mesh and 800-mesh sandpaper is 20s~40s, the grinding time for 1200-mesh sandpaper is 30s~60s, and the grinding time for 1500-mesh sandpaper is 60s~90s.

[0098] By using sandpaper of different grits from coarse to fine to grind the polarization test sample, and controlling the grinding process parameters within the above range, it is beneficial to create a smoother surface on the polarization test sample and reduce scratches and pits on the sample surface; thus, the microstructure of the raw material coke is more clearly exposed.

[0099] In some embodiments, polishing the polarization test sample includes the following steps: polishing the ground polarization test sample on an automatic polishing machine using a polishing cloth and polishing liquid; the polishing cloth is one or more of gray velvet, red velvet and black velvet; the fineness of the polishing liquid is 1μm~5μm; the polishing time is 200s~300s.

[0100] The above-described polishing process effectively polishes the surface of polarized test samples, further improving surface smoothness, eliminating surface defects, and allowing the arrangement and morphology (such as isotropic / anisotropic structures) of carbon microcrystals within the raw coke to be more clearly exposed. Understandably, conventional polishing slurries in the art, such as alumina polishing slurries, can be used.

[0101] In some embodiments, isotropic tissues include one or more of coarse-grained mosaic tissue, medium-grained mosaic tissue, fine-grained mosaic tissue, and isotropic tissue; anisotropic tissues include one or more of sheet-like tissue, fully fibrous tissue, and incomplete fibrous tissue.

[0102] In this application, the isotropic contribution of coarse-grained mosaic tissue is assigned as 3, the isotropic contribution of medium-grained mosaic tissue is assigned as 3, the isotropic contribution of fine-grained mosaic tissue is assigned as 4, and the isotropic contribution of isotropic tissue is assigned as 6; the isotropic contribution of lamellar tissue is assigned as 1, the isotropic contribution of fully fibrous tissue is assigned as 1, and the isotropic contribution of incomplete fibrous tissue is assigned as 2.

[0103] By further subdividing the isotropic and anisotropic microstructures in the raw coke and assigning isotropic contribution values ​​to each subdivided microstructure type, the accuracy of the isotropic degree test of the raw coke can be improved.

[0104] In some embodiments, the volume fraction δ of the i-th microstructure in the feedstock coke i The calculation formula is: In the formula, D i is the number of valid test points for the i-th microstructure; the sum of the valid test points for all microstructures in the raw coke is greater than or equal to 400; m is the number of types of microstructures in the raw coke.

[0105] The volume fraction δ of each microstructure in the raw coke is determined using the above formula. iThis makes it easier to accurately test the isotropy of the raw coke, thereby more accurately screening out raw coke with a high degree of isotropy.

[0106] In some specific examples, when performing polarization testing on the microstructure of polarized light test samples, the sample is fixed on the stage of a polarizing microscope. Starting from the upper left corner of the sample, the stage movement step is determined to be 30μm~50μm, and the type of microstructure of the sample under the crosshair of the polarizing microscope is determined. Defects such as pores, gaps, and microcavities on the sample surface are considered invalid points. When the crosshair falls on a blank area or the edge of a different microstructure, according to the quadrant principle, starting from the upper right quadrant and moving clockwise, a certain microstructure filling the quadrant area is counted as a valid test point. The above method is used to classify the microstructure of the polarized light test samples and count the number of each type of microstructure. Based on the volume fraction δ mentioned above... i The degree of isotropy of the raw coke can be determined by the calculation formula and the isotropic index (MI) calculation formula.

[0107] One embodiment of this application provides a secondary battery, which includes a negative electrode sheet, a negative electrode active layer, and the negative electrode active layer comprising the artificial graphite described above. By using the artificial graphite of this application, the secondary battery can achieve better fast-charging performance.

[0108] Typically, a secondary battery also includes a positive electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0109] The present application will be further described below with reference to specific embodiments and comparative examples, but should not be construed as limiting the scope of protection of the present application.

[0110] Example 1:

[0111] A method for preparing a fast-charging artificial graphite anode material, comprising the following steps:

[0112] (1) Raw material coke polarization test

[0113] Petroleum coke produced by CNOOC Huizhou Petrochemical Co., Ltd. was crushed to less than 1 mm using a double-roll crusher and dried. The crushed and dried sample was then sieved to obtain samples with a particle size between 0.1 mm and 1 mm. 10 g of the sieved sample was weighed and placed in a long-handled crucible. 5 g of shellac was added and mixed evenly. The mixture was then placed in a small heating furnace and heated until it softened and melted. The mixture was then transferred to a thermal embedding apparatus and held under a pressure of 10 N for 120 seconds before being removed to obtain a columnar polarized light test sample.

[0114] The polarizing test samples were ground and polished using a fully automatic grinding and polishing machine. The center pressure was set to 70N, the pressure head to 20N, the rotor speed to 35rpm, and the grinding disc speed to 300rpm. Grinding was performed sequentially using 600-grit, 800-grit, 1200-grit, and 1500-grit sandpaper. The grinding time for 600-grit and 800-grit sandpaper was 30s each, for 1200-grit sandpaper 45s, and for 1500-grit sandpaper 60s. After grinding, a red velvet polishing cloth was used. During polishing, 5μm fine alumina polishing slurry was added, and the polishing time was 300s, until the columnar sample surface was mirror-like and free of pits and scratches. After rinsing with clean water and drying, the sample was examined under a polarizing microscope.

[0115] Starting from the upper left corner of the polarized sample, the stage movement step was set to 30 μm. The microstructure type under the crosshair intersection was determined, and 450 valid test points were obtained through sequential testing. The microstructure type of each valid test point was recorded, and the number of valid test points for each microstructure type was counted.

[0116] (2) Evaluation of the isotropy of raw coke

[0117] According to the formula Calculate the volume fraction δ of each microstructure in the raw coke. i Then according to the formula The isotropic index (MI) of the raw coke was calculated; the isotropic index of the microstructure of the raw coke was obtained through testing, as shown in Table 1. This raw coke was used as a raw material for the preparation of artificial graphite.

[0118] (3) Pretreatment, precarbonization and graphitization

[0119] The selected raw coke was successively crushed by roller crushing and then mechanically ground to obtain process product powder. The powder was pre-carbonized in a box furnace at 1000℃ for 1.5 hours; then graphitized at 3000℃ for 2 hours to obtain artificial graphite powder. Finally, the powder was passed through a 325-mesh standard sieve, and the undersize material was collected to obtain artificial graphite anode material with a Dv50 particle size of 11.3 μm.

[0120] Example 2:

[0121] This embodiment is basically the same as Embodiment 1, except that the raw coke used is different. This embodiment uses asphalt coke produced by Shanghai Baosteel Chemical Co., Ltd., and the isotropic index of the raw coke is different. The isotropic index is shown in Table 1. After pretreatment, precarbonization and graphitization of the raw coke, artificial graphite anode material with a Dv50 particle size of 11.2 μm is obtained. The pretreatment, precarbonization and graphitization process conditions are the same as those in Embodiment 1.

[0122] Example 3:

[0123] This embodiment is basically the same as Embodiment 1, except that the petroleum coke used is from a different manufacturer, and the isotropic index of the raw coke is different accordingly. This embodiment uses petroleum coke produced by Zhenjiang Coking Coal Gas Group Co., Ltd., and its isotropic index is shown in Table 1. After pretreatment, precarbonization and graphitization of the raw coke, artificial graphite anode material with a Dv50 particle size of 11.5μm is obtained. The pretreatment, precarbonization and graphitization process conditions are the same as those in Embodiment 1.

[0124] Comparative Example 1:

[0125] This comparative example is basically the same as Example 1, except that the raw material coke used is different. This comparative example uses pitch needle coke produced by Shanghai Baosteel Chemical Co., Ltd., and the isotropic index of the raw material coke is different. The isotropic index is shown in Table 1. After pretreatment, precarbonization and graphitization of the raw material coke, artificial graphite anode material with a Dv50 particle size of 11.3 μm is obtained. The process conditions for pretreatment, precarbonization and graphitization are the same as those in Example 1.

[0126] Comparative Example 2:

[0127] This comparative example is basically the same as Example 1, except that the raw material coke used is different. This comparative example uses petroleum needle coke produced by Jinzhou Petrochemical Branch of China National Petroleum Corporation. Accordingly, the isotropic index of the raw material coke is different, and its isotropic index is shown in Table 1. After pretreatment, precarbonization and graphitization of the raw material coke, artificial graphite anode material with a Dv50 particle size of 11.4 μm is obtained. The process conditions for pretreatment, precarbonization and graphitization are the same as those in Example 1.

[0128] Test method:

[0129] (1) Volume average particle size Dv10, Dv50 and Dv90 test

[0130] The volume average particle size Dv10, Dv50 and Dv90 can be tested using a laser particle size analyzer (Malvern Master Size 3000) in accordance with the standard GB / T19077-2016 / ISO13320:2009.

[0131] (2) Half-cell test

[0132] Conductive carbon black (SP) is added to an aqueous solution of carboxymethyl cellulose (CMC), followed by finished graphite, and finally styrene-butadiene rubber (SBR). The mixture is stirred until homogeneous to obtain a negative electrode slurry. The ratio of finished graphite to SP to CMC to SBR is 95:1:1.5:2.5 (mass ratio). The negative electrode slurry is then evenly coated onto copper foil using a coating machine to form a negative electrode sheet. The coated negative electrode sheet is then vacuum-dried in a vacuum drying oven at 110°C for 4 hours. After drying, the negative electrode sheet is rolled on a roller press for later use.

[0133] The battery assembly was conducted in an argon-filled Braun glove box from Germany. The electrolyte volume ratio was EC:DEC:DMC = 1:1:1, and the lithium salt in the electrolyte was 1 mol / L LiPF6. A lithium metal sheet was used as the counter electrode to form a half-cell. Electrochemical performance testing was performed on an Arbin BT2000 battery tester from the USA, with a charge / discharge voltage range of 0.005V to 2.0V and a charge / discharge rate of 0.1C. Rate performance (i.e., capacity retention at 3C / 0.2C) was also tested; a higher capacity retention indicates better rate performance.

[0134] The steps for the 3C / 0.2C capacity retention test are as follows: 2 min rest; constant current 0.1C discharge to 0.01V; constant voltage 0.01V discharge with a cutoff current of 0.01C; 1 min rest; constant current 0.1C charging to 1.5V; 1 min rest; constant current 0.2C discharge to 0.01V; constant voltage 0.01V discharge with a cutoff current of 0.01C; 1 min rest; constant current 0.2C charging to 1.5V; 1 min rest; constant current 3.0C discharge to 0.01V; constant voltage 0.01V discharge with a cutoff current of 0.01C; 1 min rest; constant current 0.2C charging to 1.5V; 1 min rest.

[0135] 3C / 0.2C capacity retention rate = (3C discharge capacity / 0.2C discharge capacity) × 100%. Wherein, the 0.2C discharge capacity is the total discharge capacity during the constant current 0.2C discharge to 0.01V and constant voltage 0.01V discharge to the cutoff current of 0.01C; the 3C discharge capacity is the total discharge capacity during the constant current 3.0C discharge to 0.01V and constant voltage 0.01V discharge to the cutoff current of 0.01C.

[0136] The particle size and capacity retention of the fast-charging artificial graphite anode materials in Examples 1-3 and Comparative Examples 1-2 were tested at 3C / 0.2C. The test results are shown in Table 1. In Table 1, the isotropic contribution of the isotropic structure is assigned as 6, the isotropic contribution of the fine-grained mosaic structure is assigned as 4, the isotropic contribution of the medium-grained mosaic structure is assigned as 3, the isotropic contribution of the coarse-grained mosaic structure is assigned as 3, the isotropic contribution of the incomplete fibrous structure is assigned as 2, the isotropic contribution of the complete fibrous structure is assigned as 1, and the isotropic contribution of the lamellar structure is assigned as 1.

[0137] Table 1

[0138]

[0139] A comparison of the results from the above examples and comparative examples shows that, under the same process conditions, the artificial graphite anode materials prepared using raw material coke with a microstructure isotropy index MI ≥ 2.3 (Examples 1-3) exhibit superior fast-charging performance. Compared with Comparative Examples 1-2, the capacity retention rate of batteries using the artificial graphite materials obtained in Examples 1-3 at 3C / 0.2C is greater than 70%, significantly higher than that of the comparative examples. This indicates that the artificial graphite anode materials prepared by the method provided in this application exhibit superior fast-charging performance, and the process flow of this method is simple.

[0140] Furthermore, compared to Example 1, the isotropic index (MI) of the raw material coke in Examples 2 and 3 is greater than 2.5. The capacity retention rate of batteries using the artificial graphite anode materials prepared in Examples 2 and 3 at 3C / 0.2C is greater than 75%, which is higher than that of batteries using the artificial graphite anode material in Example 1. This indicates that using raw material coke with an isotropic index (MI) greater than 2.5 is more beneficial for improving the fast-charging performance of the prepared artificial graphite.

[0141] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0142] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A type of artificial graphite, characterized in that, The raw materials for preparing the artificial graphite include raw material coke, wherein the microstructure isotropic index of the raw material coke is MI, 2.3≤MI≤6; or optionally 2.5≤MI≤6. The formula for calculating MI is: ; In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i is the volume fraction of the i-th microstructure in the raw coke; m is the number of types of microstructures in the raw coke; The microstructure of the raw coke includes one or more of isotropic and anisotropic structures, with the isotropic contribution value of the isotropic structure being 3 to 6 and the isotropic contribution value of the anisotropic structure being 1 to 2.

2. The artificial graphite according to claim 1, characterized in that, The isotropic tissues include one or more of coarse-grained mosaic tissue, medium-grained mosaic tissue, fine-grained mosaic tissue, and isotropic tissue; the anisotropic tissues include one or more of sheet-like tissue, complete fibrous tissue, and incomplete fibrous tissue. The isotropic contribution of the coarse-grained mosaic structure is assigned a value of 3; the isotropic contribution of the medium-grained mosaic structure is assigned a value of 3; the isotropic contribution of the fine-grained mosaic structure is assigned a value of 4; and the isotropic contribution of the isotropic structure is assigned a value of 6. The isotropic contribution of the sheet-like tissue is assigned a value of 1; the isotropic contribution of the fully fibrous tissue is assigned a value of 1; and the isotropic contribution of the incomplete fibrous tissue is assigned a value of 2.

3. The artificial graphite according to claim 1 or 2, characterized in that, The volume fraction δ of the i-th microstructure in the raw coke i The calculation formula is: ; In the formula, D i is the number of valid test points for the i-th microstructure; the sum of the valid test points for all microstructures in the raw coke is greater than or equal to 400; m is the number of types of microstructures in the raw coke.

4. A method for preparing artificial graphite, characterized in that, Includes the following steps: The raw coke is pre-carbonized and graphitized sequentially to obtain the artificial graphite; Wherein, the microstructure isotropic index of the raw coke is MI, where 2.3≤MI≤6; or optionally 2.5≤MI≤6; The formula for calculating MI is: ; In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i is the volume fraction of the i-th microstructure in the raw coke; m is the number of types of microstructures in the raw coke; The microstructure of the raw coke includes one or more of isotropic and anisotropic structures, with the isotropic contribution value of the isotropic structure being 3 to 6 and the isotropic contribution value of the anisotropic structure being 1 to 2.

5. The method for preparing artificial graphite according to claim 4, characterized in that, Meet one or more of the following: (1) The pre-carbonization temperature is 900℃~1200℃, and the pre-carbonization time is 0.5h~2h; (2) The graphitization temperature is 2800℃~3100℃ and the graphitization time is 0.5h~2h.

6. A method for testing the isotropy of raw coke, characterized in that, Includes the following steps: The raw coke is crushed and sieved to obtain coke powder; The coke powder is mixed with a binder to prepare a polarization test sample; After grinding and polishing the polarized light test sample in sequence, the polarized light test sample was examined under a polarizing microscope. The volume fraction of each microstructure was counted and the isotropic index MI of the raw material coke was calculated. The formula for calculating MI is as follows: ; In the formula, α i Assign a value δ to the isotropic contribution of the i-th microstructure in the raw coke. i is the volume fraction of the i-th microstructure in the raw coke; m is the number of types of microstructures in the raw coke; The microstructure of the raw coke includes one or more of isotropic and anisotropic structures, with the isotropic contribution value of the isotropic structure being 3 to 6 and the isotropic contribution value of the anisotropic structure being 1 to 2.

7. The method for testing the isotropy of raw coke according to claim 6, characterized in that, The coke powder has a particle size of less than 1 mm, and the mass percentage of particles with a particle size of less than 0.1 mm in the coke powder does not exceed 15%.

8. The method for testing the isotropy of raw coke according to claim 6, characterized in that, Grinding the polarized test sample includes the following steps: The polarization test sample was sequentially ground on a polishing machine using sandpaper with mesh sizes of 600, 800, 1200, and 1500. The center pressure of the polishing machine was 60N~90N, the pressure of the pressure head was 15N~25N, the rotation speed of the head was 30rpm~50rpm, and the rotation speed of the grinding disc was 200rpm~400rpm. The grinding time for 600-mesh and 800-mesh sandpaper was 20s~40s, the grinding time for 1200-mesh sandpaper was 30s~60s, and the grinding time for 1500-mesh sandpaper was 60s~90s.

9. The method for testing the isotropy of raw coke according to any one of claims 6 to 8, characterized in that, Polishing the polarized test sample includes the following steps: The polarized test sample after grinding is polished using a polishing cloth and a polishing liquid; the polishing cloth includes one or more of gray velvet, red velvet and black velvet; the fineness of the polishing liquid is 1μm~5μm; the polishing time is 200s~300s.

10. A secondary battery, characterized in that, It includes a negative electrode sheet, the negative electrode sheet including a negative electrode active layer, the negative electrode active layer including artificial graphite according to any one of claims 1 to 3, or including artificial graphite prepared by the preparation method according to any one of claims 4 to 5.

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