Graphite-based negative electrode material and preparation method and application thereof

Abstract

The application belongs to the technical field of secondary batteries, and particularly relates to a graphite-based negative electrode material and a preparation method and application thereof. The graphite-based negative electrode material provided by the application comprises: a graphite core, the interlayer spacing of the graphite core is 0.337-0.342 nm; the average particle size of the graphite core is 6-9 microns; a first shell layer, the first shell layer is a hard carbon coating layer; a second shell layer, the second shell layer is a soft carbon coating layer; wherein the ratio of the average particle size of the graphite-based negative electrode material to the average particle size of the graphite core is 1.5-1.8. Through the control of the interlayer spacing of the graphite core, the limitation of the specific composition of the first shell layer and the second shell layer, and the limitation of the ratio of the particle size of the graphite core to the graphite-based negative electrode material, the rate performance and the fast-charging performance of the material are improved, the compaction density and the mechanical strength of the material are improved, and the volume expansion rate in the cycle process is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a graphite-based negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the development of society, the new energy industry develops rapidly, and the lithium ion battery production is continuously improved driven by new energy vehicles. With the demand of customers for fast charging of lithium ion batteries, the negative electrode with fast charging capacity is currently a research hotspot.

[0003] At present, graphite as a negative active material still has the following problems when fast charging: (1) Graphite is an anisotropic material as a whole, which cannot embed lithium ions from all directions, but only diffuses from the layered edge to the material interior, and the long diffusion path limits its rate performance; (2) The layered structure of graphite can realize the embedding and extraction of lithium ions, but due to the small interlayer spacing of graphite (0.335 nm), the diffusion resistance of lithium ions is large, and the ideal rate performance cannot be achieved; (3) The interlayer spacing of graphite is expanded due to the embedding of lithium ions, and the volume expansion caused by long-term cycling will break the complete solid electrolyte interface film (SEI film), and the generation of new SEI film will reduce the amount of active lithium, ultimately leading to a sharp decline in cycling.

[0004] In order to solve the above problems, the prior art discloses a method for preparing micro-swelling graphite by chemical intercalation method, which can expand the interlayer spacing of graphite, improve the solid phase diffusion rate of lithium ions in graphite, and thus improve the rate performance of graphite negative electrode material. However, in the preparation process of micro-swelling graphite, large pores will inevitably be generated in the graphite material, which will reduce the tap density of graphite; and the mechanical strength of the large-porosity graphite is low, and the ability to resist volume change is weak, and the particle structure is easy to be severely broken during the cycling process, so that the isostatic pressing treatment is needed to solve the problem of low tap density. In addition, the methods in the prior art are mainly suitable for natural graphite, because the natural graphite raw material is flaky graphite, and the artificial graphite is blocky graphite, and the acid washing and chemical intercalation agent cannot enter the inside of the blocky graphite; in addition, the use of strong acid as an intercalation agent is not easy to control, and the use of strong acid intercalation agent is a severe challenge for the corrosion of processing equipment and waste acid treatment. SUMMARY

[0005] Therefore, the technical problem to be solved by the present application is to overcome the above-mentioned defects in the micro-swelling graphite negative electrode material and its preparation process in the prior art, so as to provide a graphite-based negative electrode material and a preparation method and application thereof.

[0006] To this end, the present application provides the following technical solutions:

[0007] According to one aspect of the present application, a graphite-based negative electrode material is provided, comprising:

[0008] a graphite core, the interlayer spacing of the graphite core being 0.337-0.342 nm; the average particle size of the graphite core being 6-9 μm;

[0009] a first shell layer, coated on at least part of the surface of the graphite core, the first shell layer being a hard carbon coating layer;

[0010] a second shell layer, coated on at least part of the surface of the first shell layer, the second shell layer being a soft carbon coating layer;

[0011] wherein the ratio of the average particle size of the graphite-based negative electrode material to the average particle size of the graphite core is 1.5-1.8.

[0012] In the present application, the first shell layer and the second shell layer can be distinguished by means of conventional detection methods in the field. As an example, Raman and XRD can be used for the distinction, and the peak positions and peak intensities of the two are different. The first shell layer has a peak position of 1580 cm -1 (G peak) and 1360 cm -1 (D peak) in the Raman spectrum, the intensity ratio of the D peak to the G peak (ID / IG) being 0.9-1.2; the interlayer spacing of the (002) crystal plane calculated by the Bragg equation in the XRD test being 0.35-0.37 nm. The second shell layer has a peak position of 1590 cm -1 (G peak) and 1350 cm -1 (D peak) in the Raman spectrum, the intensity ratio of the D peak to the G peak (ID / IG) being 0.5-0.9; the interlayer spacing of the (002) crystal plane calculated by the Bragg equation in the XRD test being 0.342-0.350 nm.

[0013] In the present application, the graphite interlayer spacing can be tested by methods and equipment known in the field. As an example, the specific testing method comprises the following steps: 1.2 g of material and 0.8 g of silicon powder are mixed and ground uniformly, an appropriate amount is taken and placed on a sample holder to flatten, an X-RAY device is turned on for testing, and the highScore software is used to fit and calculate the interlayer spacing d 002 .

[0014] In the present application, the average particle size of the graphite raw material core and the final product is tested by using the methods and devices known in the art. As an example, the specific test method includes the following steps: about 0.02 g of the powder sample is added into a 50 ml clean beaker, about 20 ml of deionized water is added, about 0.2 mL of surfactant NP-40 with a concentration of 1 wt% is added dropwise, the powder is completely dispersed in the water, ultrasonic cleaning is performed in a 120 W ultrasonic cleaner for 1 minute, and the particle size distribution is tested by using a MasterSizer 3000, and the volume particle size distribution D50 is the average particle size.

[0015] In some alternative embodiments, the mass of the first shell layer accounts for 1% to 3% of the total mass of the graphite-based negative electrode material.

[0016] In some alternative embodiments, the mass of the second shell layer accounts for 1% to 3% of the total mass of the graphite-based negative electrode material.

[0017] In the present application, the mass ratio of the first shell layer can be tested by using the methods and devices known in the art. As an example, the specific test method includes the following steps: the material to be weighed is placed lightly on the platform of an electronic scale, and the center of gravity of the article is ensured to be located in the center of the scale table. The result can be recorded after the value on the display screen of the electronic scale is stable. The material is increased or decreased according to the difference between the displayed result and the required mass.

[0018] In the present application, the mass ratio of the second shell layer can be tested by using the methods and devices known in the art. As an example, the specific test method includes the following steps: the material to be weighed is placed lightly on the platform of an electronic scale, and the center of gravity of the article is ensured to be located in the center of the scale table. The result can be recorded after the value on the display screen of the electronic scale is stable. The material is increased or decreased according to the difference between the displayed result and the required mass.

[0019] In some alternative embodiments, the graphite core includes at least one of artificial graphite.

[0020] In the present application, the compaction density of the graphite-based negative electrode material is tested by using the methods and devices known in the art. As an example, the specific test method includes the following steps: 1.0000 g ± 0.0100 g of the sample is weighed, the weighed sample is added into the mold of a press machine, the sample name and the sample mass are input by using the software interface, the powder compaction density is measured by using a powder compaction machine UTM7305, and the test pressure in the present application is 5T.

[0021] According to still another aspect of the present application, a preparation method of a graphite-based negative electrode material is provided, including the following steps:

[0022] S1, the graphite raw material is crushed, and the average particle size of the crushed material is controlled to be A;

[0023] S2, mixing the pulverized material with high aromatic oil, pyrolyzing to obtain an intermediate with a first shell layer;

[0024] S3, performing low-temperature graphitization treatment on the intermediate, the treatment temperature being 1800-2400℃, to obtain a graphitized product;

[0025] S4, performing heat treatment on the graphitized product in a protective atmosphere, the heat treatment pressure being 0.2-2Mpa, the temperature being 300-600℃, and the time being 3-12h;

[0026] S5, mixing the product after heat treatment with soft carbon raw material, performing carbonization, magnetic removal, and screening, and controlling the average particle size B of the graphite-based negative electrode material to satisfy the following relationship: 1.5≤B / A≤1.8.

[0027] In some optional embodiments, in step S1, the average particle size A of the pulverized material is controlled to be 6-9μm;

[0028] And / or, in step S2, the mass ratio of the pulverized material to high aromatic oil is 100:5-20;

[0029] And / or, in step S5, the mass ratio of the product after heat treatment to soft carbon raw material is 100:5-20.

[0030] In some optional embodiments, the preparation method of the graphite-based negative electrode material satisfies at least one of the following (1)-(7):

[0031] (1) In step S2, the pyrolysis temperature is 300-800℃, and the time is 0.5-2h;

[0032] (2) In step S3, the low-temperature graphitization treatment time is 8-40h;

[0033] (3) In step S5, the carbonization treatment temperature is 800-1200℃, and the time is 6-12h;

[0034] (4) In step S5, the carbonization treatment temperature is 800-1200℃, and the time is 6-12h;

[0035] (5) The high aromatic oil includes at least one of creosote, naphthalene oil, or anthracene oil;

[0036] (6) The graphite raw material includes at least one of needle coke, petroleum coke, or pitch coke;

[0037] (7) The soft carbon raw material includes at least one of pitch, rubber plasticizer, carbon fiber, or carbon microsphere.

[0038] In the present application, the crushing device in step S1 is conventionally comprised of one or more of a roll mill, a mechanical mill, but not limited to.

[0039] In the present application, the mixing device used in the mixing step is conventionally comprised of one or more of a fusion machine, a mixer, but not limited to.

[0040] According to another aspect of the present application, a negative electrode sheet is provided, comprising the above-mentioned graphite-based negative electrode material or the graphite-based negative electrode material prepared by the above-mentioned preparation method.

[0041] According to another aspect of the present application, a secondary battery is provided, comprising the above-mentioned negative electrode sheet.

[0042] According to another aspect of the present application, an electrical equipment is provided, comprising the above-mentioned secondary battery.

[0043] In the present application, the high aromatic oil refers to a component with a high aromatic content, for example, the mass percentage of aromatic hydrocarbons is 60-85%.

[0044] In the present application, the composition and preparation method of the negative electrode sheet are conventionally comprised of a negative electrode composite material, conductive carbon black, a binder (styrene-butadiene rubber SBR), and a thickening agent (sodium carboxymethyl cellulose CMC) in a weight ratio of about 95-97:0.5-1.5:1-2:1-2, and an appropriate amount of deionized water is added to prepare a slurry with a solid content of 50-70 wt% for kneading to form a dough-like material; then an appropriate amount of deionized water is added to adjust the viscosity of the slurry to 3000-8000 Pa·S to prepare a negative electrode slurry. The prepared negative electrode slurry is coated on a negative electrode current collector copper foil (thickness of 8 μm), dried at room temperature, and then transferred to a 100-140℃ oven for drying for 0.5-2 h, followed by cold pressing and cutting to obtain a negative electrode sheet.

[0045] In the present application, the secondary battery further comprises a positive electrode sheet, an electrolyte, and a separator, and the specific composition and preparation method are conventionally comprised of a positive electrode material LiFePO4, a binder PVDF, and a conductive agent acetylene black in a mass ratio of 93-96:2-4:2-4, and N-methyl pyrrolidone (NMP) solvent is added until the system becomes homogeneous and transparent, and a positive electrode slurry is prepared after stirring with a vacuum stirrer, and then uniformly coated on a current collector aluminum foil (thickness of 12 μm), dried at room temperature, and then transferred to a 100-140℃ oven for drying for 0.5-2 h, followed by cold pressing (compaction density of 2.3-2.6 g / cm 3The positive electrode sheet is obtained by slitting the positive electrode sheet. The positive electrode sheet, the negative electrode sheet and the polypropylene porous polymer film are wound, wrapped with an aluminum plastic film, dried, injected with electrolyte and sealed after baking. The soft package lithium ion battery is prepared through the processes of standing, hot and cold pressing, formation, clamp, and separate capacity.

[0046] The electrolyte used in the secondary battery of the present application can include any of the techniques disclosed in the prior art.

[0047] In the present application, the secondary battery can be used as a power supply for the power consuming device, and can also be used as an energy storage unit for the power consuming device. The power consuming device can be, but is not limited to, a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc.

[0048] The technical solution of the present application has the following advantages:

[0049] The graphite-based negative electrode material provided by the present application comprises: a graphite core, the interlayer spacing of the graphite core is 0.337-0.342 nm; a first shell layer, coated on at least part of the surface of the graphite core, the first shell layer is a hard carbon coating layer; a second shell layer, coated on at least part of the surface of the first shell layer, the second shell layer is a soft carbon coating layer, wherein the ratio of the average particle size of the graphite-based negative electrode material to the average particle size of the graphite core is 1.5-1.8. By controlling the interlayer spacing of the graphite core, the present application makes the lithium intercalation / sodium intercalation resistance smaller, which is helpful for the rapid charging and discharging process of the graphite negative electrode material and is beneficial to the cycle stability; by coating the hard carbon layer on the surface of the graphite core, the mechanical strength of the graphite can be improved, the volume change can be reduced, and the particle structure rupture and the generation of new SEI film caused by volume expansion during the cycle process can be avoided; the outermost layer is coated and granulated with a soft carbon layer, which improves the compaction density of the negative electrode material, and forms secondary particles with higher isotropy, forms amorphous carbon structure, avoids direct contact between the graphite core and the electrolyte, and protects the internal graphite. By controlling the interlayer spacing of the graphite core and limiting the specific composition of the first shell layer and the second shell layer, and by limiting the ratio of the particle size of the graphite core to the particle size of the graphite-based negative electrode material, the present application improves the rate performance and fast charging performance of the material, improves the compaction density and mechanical strength of the material, and reduces the volume expansion rate during the cycle process.

[0050] The application provides a preparation method of a graphite-based negative electrode material, S1, crushing a graphite raw material, and controlling the average particle size of the crushed material to be A; S2, mixing the crushed material with a high-aromatic oil material, pyrolyzing to obtain an intermediate with a first shell layer; S3, performing low-temperature graphitization treatment on the intermediate, and the treatment temperature is 1800-2400 DEG C, to obtain a graphitized product; S4, performing heat treatment on the graphitized product in a protective atmosphere, the pressure of the heat treatment is 0.2-2 Mpa, the temperature is 300-600 DEG C, and the time is 3-12 h; S5, mixing the product after the heat treatment with pitch, performing carbonization, magnetic removal and screening, and controlling the average particle size B of the graphite-based negative electrode material to satisfy the following relationship: 1.5<=B / A<=1.8. The application is designed according to the interlayer spacing of the graphite core and the material size, improves the kinetic performance and cycle life of the graphite-based negative electrode material, meanwhile, considering the manufacturing possibility of the material and the processing performance of the material, strong acid is not used, the corrosion of the strong acid material to the processing equipment and the treatment and discharge problem of the waste acid liquid are avoided. The large interlayer spacing of the graphite core reduces the volume expansion caused by the lithium intercalation process to a certain extent, reduces the consumption of lithium in the cathode, improves the cycle performance, and effectively inhibits the capacity decay. Moreover, the process is basically the same as the conventional graphite synthesis process, without the need of large-scale modification of the production line, the manufacturing cost is low, and the application is convenient. BRIEF DESCRIPTION OF DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without any creative effort.

[0052] Figure 1 It is a structural schematic diagram of the graphite-based negative electrode material provided by the application.

[0053] Reference signs:

[0054] 1, graphite core; 2, first shell layer; 3, second shell layer. EMBODIMENT

[0055] The following embodiments are provided to better further understand the present application, and do not limit the best embodiments, and do not limit the content and protection scope of the present application. Any person who obtains any product same or similar to the present application under the enlightenment of the present application or by combining the present application with other prior art features falls within the protection scope of the present application.

[0056] The specific experimental steps or conditions are not specified in the examples, which can be carried out according to the conventional experimental steps described in the literature or the operation or conditions. The reagents or instruments used are not specified by the manufacturer, which are conventional reagent products that can be obtained by purchase.

[0057] Example 1

[0058] This embodiment provides a graphite-based negative electrode material, the structural schematic diagram of which is shown in Figure 1 The specific preparation method comprises the following steps:

[0059] (1) The graphite raw material needle coke (needle coke GHBA-1 of Qingdao Phillips Six Six Energy Co., Ltd.) is crushed, and the average particle size A of the crushed particles is 7.8 μm;

[0060] (2) The crushed powder particles are mixed with naphthalene oil (an organic substance evaporated at about 220°C from coal tar, the same below) at a mass ratio of 100:15;

[0061] (3) The reaction furnace is heated and combusted at a combustion temperature of 600°C, and the mixed material is sprayed into the combusting flame for reaction for 1 h, and a hard carbon coating layer (i.e. the first shell layer) is formed on the surface by rapid pyrolysis at high temperature;

[0062] (4) The pyrolyzed particles are subjected to low-temperature graphitization at a graphitization temperature of 2000°C for 20 h;

[0063] (5) The dried powder obtained after graphitization in step (4) is loaded into a sealed reaction kettle, nitrogen protection is added, the furnace body pressure is set to 0.4 Mpa, heated at 400°C for 6 h, and then the reaction kettle is naturally cooled to room temperature to obtain a large interlayer spacing graphite core;

[0064] (6) The product obtained in step (5) and pitch (softening point 200°C, the same below) are mixed at a mass ratio of 100:10 in a mechanical fusion device for mixing treatment;

[0065] (7) The mixed mixture in step (6) is subjected to carbonization treatment at a temperature of 1000°C for 8 h, and the heating rate is 10°C / min, and the magnetic field is removed and sieved to control the ratio relationship between the average particle size B of the graphite-based negative electrode material (the average particle size of the final product) and the average particle size A of the negative electrode material after crushing, i.e. B / A = 1.72.

[0066] Example 2

[0067] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 1, the difference lies in that the average particle size control of graphite raw materials and final products is different, and the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0068] Embodiment 3

[0069] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 1, the difference lies in that the graphitization temperature is different, the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0070] Embodiment 4

[0071] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 1, the difference lies in that anthracene oil is used instead of naphthalene oil, the crushing particle size control of graphite raw materials is different, the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0072] Embodiment 5

[0073] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 4, the difference lies in that the crushing particle size control of graphite raw materials is different, the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0074] Embodiment 6

[0075] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 1, the difference lies in that the graphitization temperature is different, the same mass of creosote oil (boiling point is 220 DEG C, the same below) is used instead of naphthalene oil, the crushing particle size control of graphite raw materials is different, the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0076] Embodiment 7

[0077] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 1, the difference lies in that the graphitization temperature and the crushing particle size control of graphite raw materials are different, the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0078] Embodiment 8

[0079] The embodiment provides a graphite-based negative electrode material, compared with the embodiment 1, the difference lies in that the graphitization temperature and the crushing particle size control of graphite raw materials are different, the specific difference of parameters and the difference of the final graphite-based negative electrode material end product are shown in Table 1.

[0080] Embodiment 9

[0081] The embodiment provides a graphite-based negative electrode material, wherein, compared with the embodiment 6, the difference lies in that the crushing particle size control of a graphite raw material is different, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0082] Embodiment 10

[0083] The embodiment provides a graphite-based negative electrode material, wherein, compared with the embodiment 7, the difference lies in that the crushing particle size control of a graphite raw material is different, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0084] Embodiment 11

[0085] The embodiment provides a graphite-based negative electrode material, wherein, compared with the embodiment 1, the difference lies in that the same mass of petroleum coke (upper petroleum coke of Shandong Yidaxin Material Co., Ltd.) is used to replace the acicular coke in the embodiment 1, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0086] Embodiment 12

[0087] The embodiment provides a graphite-based negative electrode material, wherein, compared with the embodiment 1, the difference lies in that the same mass of a rubber plasticizer (Liaoning Xinxin New Material, model XD-Y) is used to replace the asphalt in the step S6, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0088] Comparative Example 1

[0089] The comparative example provides a graphite-based negative electrode material, wherein, compared with the embodiment 1, the difference lies in that the interlayer spacing and particle size control of a graphite core are different by adjusting a graphitization temperature, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0090] Comparative Example 2

[0091] The comparative example provides a graphite-based negative electrode material, wherein, compared with the embodiment 1, the difference lies in that the interlayer spacing and particle size control of a graphite core are different by adjusting a graphitization temperature, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0092] Comparative Example 3

[0093] The comparative example provides a graphite-based negative electrode material, wherein, compared with the embodiment 1, the difference lies in that the particle size control is different, and the specific difference of parameters and the difference of the final obtained graphite-based negative electrode material end product are shown in Table 1.

[0094] Comparative Example 4

[0095] The comparative example provides a graphite-based negative electrode material, which is different from Example 4 in that the particle size control is different, and the specific differences of the parameters and the final graphite-based negative electrode material end product are shown in Table 1.

[0096] Comparative Example 5

[0097] The comparative example provides a graphite-based negative electrode material, which is different from Example 1 in that the same mass of pitch is used instead of naphthalene oil in step (2), and the specific differences of the parameters and the final graphite-based negative electrode material end product are shown in Table 1.

[0098] Comparative Example 6

[0099] The comparative example provides a graphite-based negative electrode material, which is different from Example 1 in that step (2) is not included, and the crushed graphite raw material is directly subjected to subsequent steps such as step (3), and the specific differences of the parameters and the final graphite-based negative electrode material end product are shown in Table 1.

[0100] Comparative Example 7

[0101] The comparative example provides a graphite-based negative electrode material, which is different from Example 1 in that step (6) is not included, and the product obtained in step (5) is directly subjected to subsequent steps such as carbonization in step (7), and the specific differences of the parameters and the final graphite-based negative electrode material end product are shown in Table 1.

[0102] Table 1

[0103]

[0104]

[0105] Test Example

[0106] Battery Test

[0107] Preparation of negative electrode sheet:

[0108] The graphite-based negative electrode material, conductive carbon black, binder (styrene-butadiene rubber SBR), and thickener (sodium carboxymethyl cellulose CMC) provided by each example and comparative example are mixed in a weight ratio of about 96:1:1.6:1.4 to obtain 100 kg of a mixture, and an appropriate amount of deionized water is added to prepare a slurry with a solid content of 60 wt% for kneading to form a dough-like material; then an appropriate amount of deionized water is added to adjust the viscosity of the slurry to about 5000 Pa·S to prepare a negative electrode slurry. The prepared negative electrode slurry is coated on a negative electrode current collector copper foil (thickness of 8 μm), and the negative electrode area density is 120-160 g / m 2 (the area density in this test example is 140 g / m 2), and then transferred to a 120°C oven to dry for 1 h, followed by cold pressing, slitting to obtain the negative electrode sheet.

[0109] Preparation of the positive electrode sheet: the positive electrode material LiFePO4, the binder PVDF, and the conductive agent acetylene black were mixed in a mass ratio of 94:3:3, and N-methyl pyrrolidone (NMP) solvent was added until the system became homogeneous and transparent. The positive electrode slurry was prepared by stirring with a vacuum stirrer, and then uniformly coated on the current collector aluminum foil (thickness of 12 pm). After air drying at room temperature, the positive electrode sheet was obtained by transferring to a 120°C oven to dry for 1 h, followed by cold pressing (compaction density of 2.5 g / cm 3 ), slitting.

[0110] Battery assembly: the positive electrode sheet, the negative electrode sheet, and the polypropylene porous polymer film (AS9-1, Enjie Co., Ltd.) were wound, wrapped with an aluminum plastic film, and then baked to remove water. After injecting the electrolyte (LD-124B, Dongguan Shan Shan Battery Material Co., Ltd.) and sealing, the soft package lithium ion battery (capacity of 3 Ah, length, width, and height of 120 mm x 70 mm x 30 mm) was prepared through the processes of standing, hot and cold pressing, formation, clamping, and capacity distribution.

[0111] 1. 3C charge rate test:

[0112] At 25°C, discharge to 2.0 V at 0.33C, charge to 3.65 V at 3C, and then stand for 5 minutes after constant voltage charging to 0.05C. Read the constant current charging capacity and the total charging capacity, and the ratio of the two is the 3C rate performance.

[0113] 2. High temperature cycle performance test:

[0114] The test temperature was 45°C, and the capacity obtained by discharging to 2.0 V at 1C after constant voltage charging to 3.65 V at 1C and constant voltage charging to 0.05C was the initial capacity. The 1C charge / 1C discharge cycle test was performed. The capacity retention rate at 45°C after 500 cycles was recorded to compare the high temperature cycle performance of the battery.

[0115] 3. Electrode assembly full charge expansion rate test:

[0116] The thickness of the fresh electrode assembly was tested using a screw micrometer. When cycled to 500 cycles, the electrode assembly was in a full charge state, and the thickness of the electrode assembly at this time was tested using a screw micrometer. The thickness of the fresh electrode assembly was compared with the thickness of the electrode assembly at this time, and the full charge electrode assembly expansion rate at this time was obtained.

[0117] The specific test results are shown in the following table:

[0118] Table 2

[0119]

[0120]

[0121] From the data in the above table, it can be seen that, compared with Examples 1-3 and Comparative Examples 1-2, by adjusting the interlayer spacing of the graphite core within the limited range, the 3C rate performance and 45°C cycle retention rate of the negative electrode material are both significantly improved, and the cell expansion rate is reduced. According to Examples 1-3 and Comparative Examples 3-4, by special size design, the 3C rate performance and 45°C cycle retention rate of the obtained graphite-based negative electrode material are both significantly improved. Comparative Example 5 uses different types of coating layers with equal mass, compared with Examples 1-3, the interlayer spacing of the graphite cannot be increased in the subsequent heat treatment step, and the 3C rate performance and 45°C cycle retention rate of the obtained graphite-based negative electrode material are both reduced. Comparative Example 6 cannot obtain the graphite core interlayer spacing within the limited range because it does not have a first shell layer, compared with Examples 1-2, the 3C rate performance and 45°C cycle retention rate and cycle expansion performance are all deteriorated. Comparative Example 7 cannot obtain the graphite size within the limited range because it does not have a second shell layer, compared with Examples 1-2, the 3C rate performance and 45°C cycle retention rate and cycle expansion performance are deteriorated more obviously.

[0122] Obviously, the above examples are only examples for clarity, and are not limitations on the embodiments. Based on the above description, other different forms of changes or variations can also be made by those of ordinary skill in the art. It is not necessary and impossible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A graphite-based negative electrode material, characterized by, Comprising: a graphite core, an interlayer spacing of the graphite core being 0.337-0.342 nm; an average particle size of the graphite core being 6-9 μm; a first shell layer, coated on at least part of a surface of the graphite core, the first shell layer being a hard carbon coating layer; a second shell layer, coated on at least part of a surface of the first shell layer, the second shell layer being a soft carbon coating layer; wherein a ratio of an average particle size of the graphite-based negative electrode material to an average particle size of the graphite core is 1.5-1.

8.

2. The graphite-based negative electrode material according to claim 1, characterized in that, A mass of the first shell layer accounts for 1%-3% of a total mass of the graphite-based negative electrode material.

3. The graphite-based negative electrode material according to claim 1, characterized in that, A mass of the second shell layer accounts for 1%-3% of the total mass of the graphite-based negative electrode material.

4. The graphite-based negative electrode material according to any one of claims 1 to 3, characterized in that, The graphite core comprises artificial graphite.

5. A method for producing the graphite-based negative electrode material according to any one of claims 1 to 4, characterized by, Comprising the following steps: S1, crushing a graphite raw material, and controlling an average particle size of a crushed material to be A; S2, mixing the crushed material with a high aromatic oil material, pyrolyzing to obtain an intermediate with a first shell layer; S3, performing low-temperature graphitization treatment on the intermediate, a treatment temperature being 1800-2400 ℃, to obtain a graphitized product; S4, performing heat treatment on the graphitized product in a protective atmosphere, a pressure of the heat treatment being 0.2-2 MPa, a temperature being 300-600 ℃, and a time being 3-12 h; S5, mixing the product after the heat treatment with a soft carbon raw material, performing carbonization, magnetic removal, and screening, and controlling an average particle size B of the graphite-based negative electrode material to satisfy the following relationship: 1.5≤B / A≤1.

8.

6. The method of claim 5, wherein the graphite-based negative electrode material is prepared by the steps of: mixing graphite powder, a binder, and a solvent to prepare a slurry; coating the slurry on a current collector; and drying the coated current collector. In step S1, the average particle size A of the crushed material is controlled to be 6-9 μm; And / or, in step S2, a mass ratio of the crushed material to the high aromatic oil material is 100:5-20; And / or, in step S5, a mass ratio of the product after the heat treatment to pitch is 100:5-20.

7. The method of producing a graphite-based negative electrode material according to claim 5 or 6, characterized by, At least one of the following (1)-(7) is satisfied: (1) In step S2, a temperature of the pyrolysis is 300-800 ℃, and a time is 0.5-2 h; (2) In step S3, a time of the low-temperature graphitization treatment is 8-40 h; (3) In step S5, a heating rate of the carbonization treatment is 1-5 ℃ / min; (4) A temperature of the carbonization treatment is 800-1200 ℃, and a time of the carbonization treatment is 6-12 h; (5) The high aromatic oil material comprises at least one of creosote, naphthalene oil, or anthracene oil; (6) The graphite raw material comprises at least one of needle coke, petroleum coke, or pitch coke; (7) The soft carbon raw material comprises at least one of pitch, rubber plasticizer, carbon fiber, or carbon microsphere.

8. A negative electrode sheet characterized by comprising: The graphite-based negative electrode material of any one of claims 1-4 or the graphite-based negative electrode material prepared by the preparation method of any one of claims 5-7.

9. A secondary battery characterized by comprising: The negative electrode sheet of claim 8.

10. An electric device, characterized by The secondary battery of claim 9.

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