Graphite negative electrode material, negative electrode sheet and lithium ion battery

Abstract

The application belongs to the technical field of batteries, and particularly relates to a graphite negative electrode material, a negative electrode sheet and a lithium ion battery. The graphite negative electrode material is obtained by graphitizing needle coke, and the needle coke comprises needle coke with a fine fiber structure with a mass ratio of x and needle coke with a granular inlaid structure with a mass ratio of y; the x and y satisfy: 0.1<=x<=0.9, 0.2<=y<=0.8, and 1.00<=x / y<=2.25. By limiting the structure form and content of the needle coke, the application constructs a negative electrode material with different structure forms, effectively solves the problem of fast capacity attenuation of the current high-capacity negative electrode, reduces the expansion rate of the obtained negative electrode sheet, and improves the long cycle performance, high-low temperature performance and kinetic performance of the lithium ion battery by using the improved negative electrode material in the lithium ion battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a graphite anode material, anode sheet, and lithium-ion battery. Background Technology

[0002] To meet the growing demand for ultra-high energy density, the next generation of high-energy-density secondary batteries (LIBs) needs to be developed that are safe, widely applicable, and low-cost (e.g., for electric vehicles and hybrid electric vehicles). This requires high-capacity electrode materials with long cycle life and excellent rate performance.

[0003] Lithium-ion batteries undergo numerous complex physical and chemical reactions during charging and discharging, resulting in a multitude of factors affecting their cycle life. The changes in expansion force during lithium-ion battery cycling have a significant impact on module and system design. The accelerated capacity decay in the later stages of battery cycling is related to excessive compressive stress on the battery. Within the module, this expansion force is further amplified. Therefore, researching ways to reduce expansion force during cycling is crucial for extending cell cycle life.

[0004] Needle coke is a byproduct of coal chemical and petrochemical industries, obtained through airflow coking in delayed coking. Its microstructure mainly consists of needle coke with a fine fibrous structure and a particle size <30μm. Figure 1 a) mainly composed of slender fibrous structures, with high anisotropy, and conducive to graphitization; granular mosaic structure of needle-like coke ( Figure 1 b) mainly uses mosaic optical structures, which have high isotropy but are not conducive to graphitization; needle-shaped foci with flow deformation structures ( Figure 1 c) Predominantly composed of sheet-like coarse fibers, needle coke exhibits high anisotropy, which facilitates graphitization; however, it suffers from a large expansion rate during charge and discharge. Needle coke contains many different microstructures, and its thermal expansion coefficient decreases with increasing microstructure content. Fine-fiber needle coke has a high orientation degree, is more easily graphitized, and possesses higher capacity. However, its anisotropy results in a large cyclic expansion rate and relatively low cycle life for graphitized products. Therefore, screening materials with high capacity and low expansion is of significant research importance. Summary of the Invention

[0005] This invention aims to solve the above problems and provides a graphite anode material, anode sheet, and lithium-ion battery based on needle coke with different structural morphologies.

[0006] According to the technical solution of the present invention, the graphite anode material is obtained by graphitization of needle coke, wherein the needle coke includes needle coke with a fine fiber structure having a mass percentage of x and needle coke with a granular embedded structure having a mass percentage of y.

[0007] The x and y satisfy: 0.1≤x≤0.9, 0.2≤y≤0.8, and 1.00≤x / y≤2.25.

[0008] The theory of needle coke formation with different structural morphologies provided by this invention is as follows: directional petroleum residue and coal tar pitch with multiple components are planar lamellar cyclic aromatic molecules generated by pyrolysis and polycondensation reactions during the medium-temperature (400-450℃) liquid-phase carbonization process of chemical and petrochemical industries. Under the action of van der Waals forces, they undergo lamination, and the resulting laminated body reaches the lowest energy state of the system—small spheres—under the action of its surface tension, thus forming mesophase spheres. As the system reaction progresses, the number and volume of mesophase spheres increase. The collisions and contact between spheres and the accumulation of mesophase building units lead to the bonding between sphere molecules. Under the synergistic effect of the system's chemical reaction and physical adjustment, the molecules inside the spheres rearrange to form fused bodies, causing the original spheres to disappear and form larger spheres. The fusion and collision between large spheres cause disintegration, forming a non-spherical mesophase stage. By using the directional flow of airflow in delayed coking to apply sufficient shear force to the bulk mesophase pitch—airflow coking—the mesophase pitch molecules are solidified in a nematic ordered arrangement, thus forming a wide-area streamlined structure, fibrous or granular mosaic mesophase structure with different structures, and then solidifying into fibrous or granular mosaic forms of coke.

[0009] Among them, needle coke with a fine fiber structure has high orientation and high graphitization, which can provide high capacity in graphite anode materials. However, due to the anisotropy of this type of material, the expansion rate increases, which affects the cycle life of the battery. Needle coke with a granular mosaic structure is isotropic, has small expansion, and has good cycle life, but its capacity utilization is lower. When needle coke with both fine fiber structure and granular mosaic structure is included, the higher the content of fine fiber structure and the lower the content of granular mosaic structure, the better the dynamic performance of the battery and the higher the capacity. At the same time, the expansion during cycling will increase and the cycle life will decrease. The higher the content of granular mosaic structure, the higher the cycle life, but the corresponding decrease in capacity. This is because the needle coke with a fine fiber structure has better graphite crystallinity.

[0010] Specifically, needle focal lengths with different structural morphologies are distinguished using a polarizing microscope. To obtain the most suitable mass ratio, a number of photographs (e.g., 1000) of needle focal lengths can be taken at different locations using a polarizing microscope, and the proportion of different structural morphologies can be statistically analyzed from the photographs.

[0011] Preferably, x and y satisfy: 0.5≤x≤0.7, 0.3≤y≤0.5.

[0012] Furthermore, the needle coke includes needle coke with a flow deformation structure having a mass percentage of z; wherein, z ≤ 0.1, and x + y + z = 1; preferably, z ≤ 0.05. The flow deformation structure of the needle coke is mainly composed of sheet-like coarse fiber structure, with high anisotropy, which is beneficial for graphitization, but has a large expansion rate during charging and discharging.

[0013] Specifically, the needle coke is composed of needle coke with a fine fiber structure, needle coke with a granular mosaic structure, and needle coke with a flow deformation structure, with a mass ratio of 10-90:20-80:0-10; preferably, the mass ratio is 55-70:30-45:0-1.

[0014] Furthermore, the needle coke also includes shaping and grading steps and fine powder removal steps before graphitization.

[0015] Furthermore, the graphitization temperature is 2500-3000℃, and the graphitization treatment time is 12-60h.

[0016] Furthermore, the needle coke is also mixed with a binder before graphitization.

[0017] Furthermore, the adhesive satisfies at least one of the following conditions:

[0018] The adhesive is selected from one or more of medium-temperature asphalt, phenolic resin, and epoxy resin.

[0019] The amount of binder added is 2%-8% of the mass of the needle coke.

[0020] Furthermore, the particle size distribution of the graphite anode material is as follows: D10 is 4-7 μm; D50 is 11-16 μm; D90 is 20-27 μm; and D99 is 32-39 μm.

[0021] Furthermore, the particle size of the needle coke is 8-25 μm, for example, it can be 8-15 μm, 15-25 μm, etc.; the carbon content is ≥88%.

[0022] A second aspect of the present invention provides a method for preparing the above-mentioned graphite anode material, comprising the following steps:

[0023] S1: Shape and classify the needle coke to remove fine powder and obtain needle coke particles of uniform size;

[0024] S2: Graphitize the needle-shaped coke particles;

[0025] S3: The graphitized needle-shaped coke particles are screened and demagnetized to obtain the graphite anode material.

[0026] Furthermore, the particle size of the needle-shaped coke particles can be 8-15μm, 15-25μm, etc., and the carbon content is ≥88%.

[0027] Furthermore, in step S2, the graphitization temperature is 2500-3000℃, and the graphitization time is 12-60h.

[0028] A third aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises the above-described graphite negative electrode material or the graphite negative electrode material prepared by the above-described preparation method.

[0029] Furthermore, the content of graphite anode material in the negative electrode active material layer is 80-96.5 wt%.

[0030] A fourth aspect of the present invention provides a lithium-ion battery comprising the aforementioned negative electrode.

[0031] The technical solution of the present invention has the following advantages compared with the prior art:

[0032] 1) The graphite anode material provided by the present invention uses needle coke with different contents of fine fiber structure, granular embedded structure and flow deformation structure as raw materials. Compared with conventional fiber structure needle coke, the raw materials of the present invention have different forms. Among them, needle coke with obvious fiber structure can ensure the capacity of the anode material, and needle coke with embedded structure can effectively alleviate the full-charge expansion and cycle expansion of the anode material. The combination of the two structural forms effectively solves the problem of fast capacity decay of high-capacity anodes.

[0033] 2) By limiting the morphology and content of needle coke structure, this invention constructs anode materials with different structural morphologies, resulting in anode sheets with reduced expansion rates. Furthermore, the improved anode materials are used in lithium-ion batteries, effectively enhancing the long-cycle performance, high and low temperature performance, and kinetic performance of lithium-ion batteries. Attached Figure Description

[0034] Figure 1 These are partial microscopic images of needle foci with different structural morphologies, where a is a needle foci with a fine fibrous structure; b is a needle foci with a granular mosaic structure; and c is a needle foci with a flow deformation structure.

[0035] Figure 2 The expansion rate is given for the graphite negative electrode sheets in each embodiment and comparative example. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0037] This invention provides a graphite anode material, the preparation method of which is as follows:

[0038] (1) Using needle coke as raw material, the particle size of the crushed material can be 8-15μm, 15-25μm, etc., and the carbon content is ≥88%;

[0039] Needle coke includes various structural forms, mainly fine fibrous needle coke and granular mosaic needle coke, with contents of x and y, respectively, where x and y satisfy: 0.1≤x≤0.9, 0.2≤y≤0.8, and 1.00≤x / y≤2.25. Needle coke can also include flow-deformation needle coke with a mass percentage of z, where z≤0.1, and x+y+z=1;

[0040] Preferably, x, y, and z satisfy: 0.5≤x≤0.7, 0.3≤y≤0.5, z≤0.05;

[0041] (2) Shape and classify the raw materials to remove fine powder and obtain uniformly sized needle-shaped coke particles;

[0042] (3) Graphitize the needle-shaped coke particles. The graphitization temperature can be 2500-2600℃, 2600-2700℃, 2700-2800℃, 2800-2900℃, 2900-3000℃, etc.

[0043] (4) The graphitized needle-shaped coke particles are screened and demagnetized to obtain graphite finished products, namely graphite anode materials.

[0044] The particle size distribution of finished graphite products is as follows: D10: 4-7μm; D50: 11-16μm; D90: 20-27μm; D99: 32-39μm.

[0045] Since needle coke particles with different structural forms have no binding ability, a binder needs to be added to bind the various dispersed particles together to form a whole. The binder can be medium-temperature asphalt, phenolic resin and epoxy resin, etc., and the amount added is 2%-8% of the mass of needle coke.

[0046] The obtained graphite anode material can be used to prepare anode sheets, as follows:

[0047] Graphite anode material, conductive material, and binder are mixed at mass percentages of 80-96.5 wt%, 1.7-9.5 wt%, and 1.8-10.5 wt%, respectively. Water is added until the graphite anode material content reaches 40-65%, and stirring is continued for 60-300 minutes. The viscosity is adjusted to between 1.0-6.0 Pa·s to obtain anode slurry. The anode slurry is coated onto anode current collector, dried at 70-115℃ to remove moisture, pressed into sheets, and slit to obtain anode sheets. The surface density of the active material layer on the anode sheet is 90-140 g / m². 2The coating width is 90-130mm and the thickness is 0.130-0.190mm.

[0048] Furthermore, the conductive material is at least one of conductive carbon black (SP), acetylene black, graphite, graphene, carbon microwires, carbon nanowires, carbon microtubes, and carbon nanotubes.

[0049] The binder is at least one of the following: polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, sodium carboxymethyl cellulose (CMC), polymethacrylamide, polyacrylic acid, lithium polyacrylate, lithium polyacrylate, carboxymethyl cellulose, styrene-butadiene rubber (SBR), fluororubber, ethylene propylene diene, etc.

[0050] The negative electrode current collector is one of the following: copper foil, porous copper foil, foamed nickel / copper foil, zinc-plated copper foil, nickel-plated copper foil, carbon-coated copper foil, nickel foil, and titanium foil.

[0051] The resulting negative electrode can be used in a lithium-ion battery, which also includes a positive electrode, a separator, and an electrolyte. The method for preparing this lithium-ion battery should be known to those skilled in the art. For example, the positive electrode, separator, and negative electrode can each be a layer, which can be cut to the target size and stacked sequentially, or further wound to the target size to form a cell, and further combined with an electrolyte to form a lithium-ion battery. There are no specific limitations on the specific type of lithium-ion battery; for example, it can be, but is not limited to, cylindrical batteries, aluminum-cased batteries, or pouch batteries.

[0052] The positive electrode sheet includes a positive current collector and a positive electrode coating disposed on the positive current collector. The positive electrode coating contains a positive electrode active material, and the specific type of positive electrode active material is not specifically limited and can be selected according to requirements. For example, the positive electrode active material of the positive electrode coating can be one or more combinations of, but not limited to, layered positive electrode active materials, spinel-type positive electrode active materials, olivine-type positive electrode active materials, and metal sulfides.

[0053] The separator can be any material suitable for lithium-ion battery separators in the art, for example, it can be one or more of the following: polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fibers.

[0054] The electrolyte comprises an organic solvent, a lithium electrolyte salt, and additives. The lithium electrolyte salt can be LiPF6 and / or LiBOB (lithium bis(oxalato)borate) used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI (lithium bis(trifluoromethanesulfonate)imide) used in overcharge-resistant electrolytes; or it can be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC (propylene carbonate) and EC (ethylene carbonate); it can also be a chain carbonate, including DEC (diethyl carbonate), DMC (dimethyl carbonate), or EMC (ethyl methyl carbonate); or it can be a carboxylic acid ester, including MF (methyl p-toluenesulfonate), MA (methyl acrylate), EA (ethyl acetate), and MP (methyl propyl carbonate), etc. The additives include, but are not limited to, at least one of the following: film-forming additives, conductive additives, flame-retardant additives, overcharge prevention additives, additives for controlling the H2O and HF content in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0055] Example 1

[0056] This embodiment provides a graphite anode material, and the preparation method is as follows:

[0057] (1) Using coke B as raw material, mechanical crushing and shaping were carried out to form particles with D50 of 4-20μm. The content of needle coke with different structural morphologies in coke A is shown in Table 1.

[0058] (2) Mix the A coke of different particle sizes processed in step (1), then add liquid asphalt binder, and use mechanical stirring to mix the materials evenly.

[0059] (3) Place the mixture obtained in step (2) in a graphitization furnace (Acheson furnace) and heat it to 2500-3000℃, so that the hexagonal carbon atom planar network changes from a two-dimensional network structure to a three-dimensional ordered graphite structure, and its expansion rate is as follows. Figure 2 As shown.

[0060] The graphite anode material obtained above was mixed with 1 wt% sodium carboxymethyl methyl cellulose (CMC), 1.0 wt% styrene-butadiene rubber (SBR), and 1.5 wt% conductive carbon black (SP), dissolved in an aqueous solution, stirred evenly, and uniformly coated onto the surface of copper foil. After drying and cold rolling, a cathode sheet was obtained. The amount of graphite anode material added was 96.5 wt%, and the coating surface density was 110 g / m². 2 The coating width of the negative electrode is 90mm and the thickness is 0.137mm.

[0061] Positive electrode sheet: Using NMC (nickel-cobalt-manganese 613 series) as the positive electrode active material, 95wt% NMC, 3wt% PVDF and 2wt% conductive carbon black are dissolved in NMP, stirred evenly, coated onto aluminum foil, dried and cold-rolled to obtain the positive electrode sheet, wherein the coating weight of the positive electrode sheet is 174g / m 2 The positive electrode coating has a width of 80 mm and a thickness of 0.110 mm. The separator is a PE polymer porous membrane.

[0062] Electrolyte: In the 1M-LiPF6 system, an EC:DMC:EMC solution with a volume ratio of 1:1:1 was used as the electrolyte.

[0063] The negative electrode, separator, and positive electrode (the positive electrode active material is lithium nickel cobalt manganese oxide) are stacked and wound in sequence to obtain a bare cell. The tabs are welded, and the bare cell is placed in the battery aluminum shell / soft-pack aluminum-plastic film, sealed on the top and side, dried to remove moisture, injected with electrolyte into the battery shell, formed, determined capacity, and vented and sealed to obtain a secondary battery.

[0064] Example 2

[0065] The difference from Example 1 is that the raw material is C coke; the other steps are the same as in Example 1.

[0066] Example 3

[0067] The difference from Example 1 is that the raw materials are 55% F coke mixed with 45% G coke; the other steps are the same as in Example 1.

[0068] Example 4

[0069] The difference from Example 1 is that the raw materials are 65% F coke mixed with 35% G coke; the other steps are the same as in Example 1.

[0070] Comparative Example 1

[0071] The difference from Example 1 is that the raw material is coke A; the other steps are the same as in Example 1.

[0072] Comparative Example 2

[0073] Unlike Example 1, the raw material is D coke; the other steps are the same as in Example 1.

[0074] Comparative Example 3

[0075] Unlike Example 1, the raw material is E-coke; the other steps are the same as in Example 1.

[0076] Comparative Example 4

[0077] Unlike Example 1, the raw materials are 25% F coke mixed with 75% G coke; the other steps are the same as in Example 1.

[0078] Comparative Example 5

[0079] Unlike Example 1, the raw materials are 35% F coke mixed with 65% G coke; the other steps are the same as in Example 1.

[0080] Comparative Example 6

[0081] Unlike Example 1, the raw materials are 75% F coke mixed with 25% G coke; the other steps are the same as in Example 1.

[0082] Comparative Example 7

[0083] Unlike Example 1, the raw material is F coke (100% fine fiber needle coke); the other steps are the same as in Example 1.

[0084] Comparative Example 8

[0085] Unlike Example 1, the raw material is G coke (100% granular needle coke with an inlaid structure); the other steps are the same as in Example 1.

[0086] The contents of needle coke with fine fiber structure, needle coke with granular mosaic structure, and needle coke with flow deformation structure in the raw materials (coke A-coke-coke or a combination thereof) in each embodiment and comparative example are shown in Table 1 (in Table 1, needle coke with fine fiber structure is referred to as fine fiber structure, needle coke with granular mosaic structure is referred to as granular mosaic structure, and needle coke with flow deformation structure is referred to as flow deformation structure).

[0087] Table 1

[0088]

[0089] Results Analysis

[0090] The secondary batteries obtained in Examples 1-4 and Comparative Examples 1-8 were tested, as follows:

[0091] 1. Battery electrical performance testing

[0092] (1) At room temperature of 25°C, the lithium-ion secondary batteries of Examples 1-4 and Comparative Examples 1-8 were formed and capacitated. The first charge and discharge charge of the battery during the formation and capacitation process was recorded, and the first coulombic efficiency was calculated (first coulombic efficiency = first discharge charge / first charge charge * 100%).

[0093] (2) Cyclic capacity retention test at different temperatures: The standard for determining capacity decay is 80%. The test method involves charging the battery to 4.3V with a constant current and then a constant voltage at 1C. The battery is then placed in high and low temperature chambers, left to stand for 30 minutes, and then subjected to 1C / 1C charge and discharge.

[0094] 2. Electrode Expansion Rate Test

[0095] At room temperature of 25°C, the thickness d1 of the negative electrode sheet was measured before the battery was wound in both the example and the comparative example. After the lithium-ion secondary battery was charged by constant current charging and constant voltage charging, the battery was disassembled and the thickness d2 of the negative electrode sheet was measured. The expansion rate of the electrode sheet was obtained by the formula: (d1-d2) / (d1-d)*100% (d is the thickness of the current collector of the negative electrode sheet).

[0096] The results are shown in Table 2 and 2 respectively. Figure 2 As shown.

[0097] Table 2

[0098]

[0099] The results showed that the graphitization process and secondary battery manufacturing conditions were the same for the examples and the comparative examples. The raw material microstructure of Examples 1-4 contained fine fibrous needle-like coke (content x) and interlocking needle-like coke (content y), with both components satisfying 1.00 ≤ x / y ≤ 2.25. This resulted in higher specific capacity, better rate performance, and better cycle stability. This was attributed to the high orientation and graphitization degree of the fine fibrous structure component, leading to higher capacity utilization. Simultaneously, the interlocking granular structure in the raw material exhibited good anisotropy, effectively mitigating expansion and improving cycle stability during cycling. In the comparative examples, the high content of fine fibrous structure in the raw material resulted in higher capacity utilization, but the anisotropy of the fine fibrous structure led to increased expansion of the negative electrode and faster degradation in the later stages of cycling. The raw material with a lower content of fine fibrous structure and a higher content of interlocking granular structure component, along with the isotropy of the interlocking granular structure component, improved the expansion rate of the negative electrode, but resulted in lower graphitization, lower capacity utilization, and significant loss of active lithium, with no significant improvement in cycle life. Test results show that the mixing ratio of fine fibers and granular embedded components in the raw materials used in this invention is between 1.00 ≤ x / y ≤ 2.25, which increases the material's specific capacity and effectively improves the material's expansion rate and cycle stability.

[0100] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A graphite anode material, characterized in that, The needle coke is obtained by graphitization, and the needle coke includes needle coke with a fine fiber structure having a mass percentage of x and needle coke with a granular mosaic structure having a mass percentage of y. The x and y satisfy: 0.1≤x≤0.9, 0.2≤y≤0.8, and 1.00≤x / y≤2.25; The x and y satisfy: 0.5 ≤ x ≤ 0.7, 0.3 ≤ y ≤ 0.5; The needle coke includes needle coke with a flow deformation structure having a mass percentage of z; wherein, z≤0.1, and x+y+z=1; The needle coke is also mixed with a binder before graphitization; the amount of binder added is 2%-8% of the mass of the needle coke.

2. The graphite anode material as described in claim 1, characterized in that, The graphitization temperature is 2500-3000℃, and the graphitization time is 12-60h.

3. The graphite anode material as described in claim 1, characterized in that, The adhesive is selected from one or more of medium-temperature asphalt, phenolic resin, and epoxy resin.

4. The graphite anode material as described in claim 1, characterized in that, The particle size distribution of the graphite anode material is as follows: D10 is 4-7 μm; D50 is 11-16 μm; D90 is 20-27 μm; and D99 is 32-39 μm.

5. A method for preparing a graphite anode material according to any one of claims 1-4, characterized in that, Includes the following steps, S1: Shape and classify the needle coke to remove fine powder and obtain needle coke particles of uniform size; S2: Graphitize the needle-shaped coke particles; S3: The graphitized needle-shaped coke particles are screened and demagnetized to obtain the graphite anode material.

6. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, characterized in that, The negative electrode active material layer comprises the graphite negative electrode material according to any one of claims 1-4, or the graphite negative electrode material prepared by the preparation method according to claim 5.

7. A lithium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 6.

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