Method for in-situ construction of natural graphite-based composite negative electrode material on current collector

Abstract

The invention relates to the technical field of lithium ion battery negative electrode materials, in particular to a method for in-situ construction of a natural graphite-based composite negative electrode material on a current collector. The preparation method comprises the following steps: uniformly distributing polyamide acid in pore structures among crystalline flake graphite with submicron particle size in spherical graphite particles by adopting an impregnation method to prepare a polyamide acid / powder 2 compound; mixing the polyamic acid / powder 2 compound with polyamic acid, coating the mixture on a current collector, and curing to obtain a polyimide / powder 2 compound composite film loaded on the current collector; and finally, converting the polyimide inside and on the surface of the powder 2 into graphene by utilizing a laser-induced composite film, namely generating graphene in situ in a pore structure between crystalline flake graphite with submicron particle size inside spherical graphite particles and on the surface of the spherical graphite particles to prepare the natural graphite-based composite negative electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery negative electrode materials, in particular to a method for in-situ constructing a natural graphite-based composite negative electrode material on a current collector. Background Art

[0002] Lithium-ion batteries offer a range of advantages, including high specific capacity, high operating voltage, excellent safety, and no memory effect. They are widely used in consumer electronics, power systems, and energy storage devices. In recent years, with the increasing demand for miniaturization, lightweighting, multi-functionality, and long-term operation in electronic products, vehicles, and energy storage devices, the demand for lithium-ion batteries with high energy density, high rate performance, and long cycle life has continued to increase.

[0003] As a crucial component of lithium-ion batteries, the performance of the anode directly impacts the overall performance of the battery system. Natural graphite is widely used due to its high charge and discharge capacity, excellent charge and discharge performance, wide availability, and low cost. However, natural graphite suffers from significant initial irreversible capacity loss and rapid capacity decay during cycling, which are its key drawbacks.

[0004] In order to solve this problem, the existing technology mainly uses natural graphite to be crushed and spheroidized into spherical graphite. Such treatment improves the isotropy of natural graphite and can alleviate its volume expansion during charge and discharge, thereby improving its cycle performance. However, the size of the spherical graphite particles prepared by this method is large, which will cause the migration path of lithium ions to become longer, resulting in low rate performance of natural graphite, affecting the fast charge performance of the battery. In order to improve the fast charge performance of natural graphite, the existing technology usually crushes or ball-mills natural graphite into submicron graphite particles (0.1-1 μm), and then obtains a regular spherical graphite structure by spray granulation. Since small primary particles help to shorten the migration path of lithium ions, this operation significantly improves the rate performance of spherical graphite. However, the submicron graphite particles inside the spherical graphite particles prepared by this method are still oriented as anisotropic flakes. During charge and discharge, due to the different expansion degrees of submicron graphite particles in all directions, the capacity decays quickly during the cycle, thereby significantly reducing the cycle performance of natural graphite. Therefore, how to simultaneously improve the cycle performance and rate performance of natural graphite negative electrode materials is a problem that still needs to be solved. Summary of the Invention

[0005] In order to solve the problem in the prior art that natural graphite cannot achieve both cycle performance and rate performance, the present invention provides a method for in-situ construction of a natural graphite-based composite negative electrode material on a current collector. The lithium-ion battery assembled with the natural graphite-based composite negative electrode material prepared by the method has high cycle performance and rate performance.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] A method for in-situ fabrication of a natural graphite-based composite negative electrode material on a current collector, wherein the method comprises the following steps:

[0008] (1) crushing natural flake graphite to prepare flake graphite with a particle size of submicron level;

[0009] (2) mixing submicron-sized flake graphite, asphalt, and a solvent, and spray granulating the mixture to prepare powder 1;

[0010] (3) heat-treating powder 1 under a protective atmosphere to obtain powder 2;

[0011] (4) mixing a diamine compound, a dianhydride compound, a catalyst, and water, and reacting them to prepare a polyamic acid solution;

[0012] (5) mixing the powder 2 and the polyamic acid solution, impregnating, filtering, and drying to prepare a polyamic acid / powder 2 composite;

[0013] (6) The polyamic acid / powder 2 complex and the polyamic acid solution are mixed and coated on the current collector, and after curing, a composite film loaded on the current collector is obtained. The natural graphite-based composite negative electrode material is prepared by laser induction of the composite film, that is, the natural graphite-based composite negative electrode material is constructed in situ on the current collector.

[0014] According to an embodiment of the present invention, in step (1), the median particle size of the natural flake graphite is not particularly defined, and flake graphite with a particle size of submicron level can be obtained by crushing it.

[0015] According to an embodiment of the present invention, in step (1), the natural flake graphite is preferably irregular flake graphite tailings (median particle size such as 1-3 μm) produced by preparing spherical graphite; by selecting irregular flake graphite tailings produced by preparing spherical graphite as raw material, the utilization rate of natural flake graphite can be improved and the production cost of the natural graphite-based composite negative electrode material can be reduced.

[0016] According to an embodiment of the present invention, in step (1), the purity of the natural flake graphite is ≥99%.

[0017] According to an embodiment of the present invention, in step (1), the median particle size of the submicron flake graphite is 0.1 μm-1 μm, such as 0.2 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.8 μm or 1 μm.

[0018] According to an embodiment of the present invention, in step (1), the pulverization is carried out using methods and equipment known in the art, and there is no special definition of pulverization parameters, as long as flake graphite with a particle size of submicron level can be obtained.

[0019] According to an embodiment of the present invention, in step (2), the residual carbon value of the asphalt is 20%-80%, for example, 20%, 30%, 40%, 50%, 60%, 70% or 80%.

[0020] According to an embodiment of the present invention, in step (2), the solvent is selected from ethanol and / or water.

[0021] According to an embodiment of the present invention, in step (2), the mass ratio of the asphalt to the flake graphite with a particle size of submicron size is (10-20):100, for example, 10:100, 11:100, 12:100, 13:100, 14:100, 15:100, 16:100, 17:100, 18:100, 19:100 or 20:100.

[0022] According to an embodiment of the present invention, in step (2), the mass ratio of the solvent to the submicron flake graphite is (30-100):100, for example, 30:100, 35:100, 40:100, 45:100, 50:100, 55:100, 60:100, 65:100, 70:100, 75:100, 80:100, 85:100, 90:100, 95:100 or 100:100.

[0023] According to an embodiment of the present invention, in step (2), the mixing time is not particularly defined, and the flake graphite with submicron particle size, asphalt and solvent can be evenly mixed.

[0024] According to an embodiment of the present invention, in step (2), the inlet temperature of the spray granulation is 150-210°C, for example, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C or 210°C; the outlet temperature of the spray granulation is 90-110°C, for example, 90°C, 92°C, 95°C, 98°C, 100°C, 102°C, 105°C, 106°C, 108°C or 110°C; and the spray granulation is carried out under an air atmosphere.

[0025] According to an embodiment of the present invention, in step (2), the powder 1 is spherical in shape.

[0026] According to an embodiment of the present invention, in step (2), the median particle size of the powder 1 is 5 μm-16 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm or 16 μm.

[0027] According to an embodiment of the present invention, in step (2), the powder 1 is spherical graphite particles having a plurality of pore structures; preferably, the powder 1 is spherical graphite particles having a plurality of pore structures with a pore size of 10-40 nm. Preferably, the powder 1 comprises flake graphite with a particle size of submicron and asphalt; further preferably, the powder 1 is spherical graphite particles having a pore structure formed by flake graphite with a particle size of submicron and asphalt, and asphalt is filled between some flake graphite particles with a particle size of submicron, and a pore structure is formed between some flake graphite particles with a particle size of submicron, and the pore size of the pore structure is 10-40 nm.

[0028] According to an embodiment of the present invention, in step (2), during the spray granulation process, the flake graphite with a particle size of submicron will undergo a stacking effect, so that asphalt is filled between some flake graphite particles with a particle size of submicron, and a pore structure is formed between some flake graphite particles with a particle size of submicron, thereby obtaining spherical graphite particles with a pore structure having a plurality of pore sizes of 10-40nm, i.e., powder 1.

[0029] According to an embodiment of the present invention, in step (3), the temperature of the heat treatment is 800-1200°C, for example, 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, 1150°C or 1200°C; the time of the heat treatment is 2-10 hours, for example, 3-8 hours, exemplified by 3 hours, 4 hours, 5 hours or 6 hours.

[0030] According to an embodiment of the present invention, in step (3), the protective atmosphere is nitrogen or argon.

[0031] According to an embodiment of the present invention, in step (3), the powder 2 is spherical graphite particles having a plurality of pore structures; preferably, the powder 2 is spherical graphite particles having a plurality of pore structures with a pore size of 10-40 nm. Preferably, the powder 2 comprises flake graphite and amorphous carbon with a particle size of submicron order; further preferably, the powder 2 is spherical graphite particles having a pore structure formed by flake graphite and amorphous carbon with a particle size of submicron order, and amorphous carbon is in situ formed between some flake graphite particles with a particle size of submicron order, and a pore structure is formed between some flake graphite particles with a particle size of submicron order, and the pore structure has a pore size of 10-40 nm.

[0032] According to an embodiment of the present invention, in step (4), the diamine compound is selected from one or a mixture of p-phenylenediamine, 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-p-phenylenediamine, 1,5-diaminonaphthalene, 1,4-diaminopyrene and 1,4-diaminoanthracene.

[0033] According to an embodiment of the present invention, in step (4), the catalyst is selected from one or a mixture of more than one of 1,2-dimethylimidazole, triethylamine and dodecyltrimethylammonium chloride.

[0034] According to an embodiment of the present invention, in step (4), the dianhydride compound is selected from one or a mixture of more than one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxybenzene)hexafluoropropane dianhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

[0035] According to an embodiment of the present invention, in step (4), the molar ratio of the diamine compound to the dianhydride compound is 1:1.

[0036] According to an embodiment of the present invention, in step (4), the molar ratio of the diamine compound to the catalyst is 1:(1.8-2.2), for example, 1:1.8, 1:1.9, 1:2, 1:2.1 or 1:2.2.

[0037] According to an embodiment of the present invention, in step (4), the mass ratio of the diamine compound to water is 100:(100-120), for example, 100:100, 100:105, 100:110, 100:115 or 100:120.

[0038] According to an embodiment of the present invention, in step (4), the reaction temperature is 70-90°C, for example, 70°C, 75°C, 80°C, 85°C or 90°C; the reaction time is 3-24 hours, for example, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, 15 hours, 18 hours or 20 hours.

[0039] According to an embodiment of the present invention, in step (4), the diamine compound, the catalyst and water are mixed, heated to 40-50°C, and stirred for 30-120 minutes to obtain a mixed solution; then the temperature of the mixed solution is raised to 70-90°C, the dianhydride compound is added, and the mixture is stirred for 3-24 hours to react to obtain a polyamic acid solution.

[0040] According to an embodiment of the present invention, in step (4), the polyamic acid solution is an aqueous solution of polyamic acid.

[0041] According to an embodiment of the present invention, in step (4), the concentration of the polyamic acid solution is 1-18wt%, for example, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 12wt%, 15wt%, 16wt%, or 18wt%; the concentration of the polyamic acid solution can be controlled by controlling the reaction time.

[0042] According to an embodiment of the present invention, in step (5), the mass ratio of the polyamic acid solution to the powder 2 is 100:(10-30), for example, 100:10, 100:15, 100:20, 100:25 or 100:30.

[0043] According to an embodiment of the present invention, in step (5), the concentration of the polyamic acid solution is 3-8wt%, for example, 3wt%, 4wt%, 5wt%, 6wt%, 7wt% or 8wt%. Selecting a low concentration of the polyamic acid solution is advantageous in that the polyamic acid is impregnated into the pore structure between the submicron-sized flake graphite particles within the spherical graphite particles by an impregnation method.

[0044] According to an embodiment of the present invention, in step (5), the immersion temperature is room temperature, and the immersion time is 30 min-120 min, for example, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min or 120 min.

[0045] According to an embodiment of the present invention, in step (5), the drying temperature is 60-100°C.

[0046] According to an embodiment of the present invention, in step (6), the mass ratio of the polyamic acid / powder 2 composite to the polyamic acid solution is 10:(20-30), for example, 10:20, 10:22, 10:24, 10:25, 10:26, 10:28 or 10:30.

[0047] According to an embodiment of the present invention, in step (6), the concentration of the polyamic acid solution is 10-18wt%, for example, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt% or 18wt%. Selecting a high concentration of polyamic acid solution is beneficial for coating the surface of the spherical graphite particles with polyamic acid, thereby in situ generating graphene with a certain thickness.

[0048] According to an embodiment of the present invention, in step (6), the current collector is copper foil.

[0049] According to an embodiment of the present invention, in step (6), the curing is performed in a vacuum oven.

[0050] According to an embodiment of the present invention, in step (6), the curing temperature is 100-200°C, for example, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C or 200°C; the curing time is 1-20 hours, for example, 2 hours, 3 hours, 5 hours, 6 hours, 8 hours, 10 hours, 12 hours, 15 hours or 18 hours.

[0051] According to an embodiment of the present invention, in step (6), during the curing process, polyamic acid generates polyimide through imidization reaction.

[0052] According to an embodiment of the present invention, in step (6), the thickness of the composite film is 10-30 μm.

[0053] According to an embodiment of the present invention, in step (6), the laser-induced light source includes but is not limited to one or more of solid laser, liquid laser, gas laser, semiconductor laser, fiber laser, pulsed laser, continuous laser and excimer laser.

[0054] According to an embodiment of the present invention, in step (6), the control parameters of the laser induction include laser wavelength, laser power, scanning rate, scanning spacing, pulse frequency, focal length and voltage; preferably, the laser wavelength is 0.1-1000 μm, the laser power is 0.1-1000 W, the scanning rate is 0.1-5000 mm / s, the scanning spacing is 0.001-100 mm, the pulse frequency is 0.001-2000 kHz, the focal length is 0.1-1000 cm, and the voltage is 1-10000 kV.

[0055] The present invention provides a natural graphite-based composite negative electrode material prepared by the above method.

[0056] According to an embodiment of the present invention, the natural graphite-based composite negative electrode material includes graphene, flake graphite with a particle size of submicron order, and amorphous carbon; preferably, the natural graphite-based composite negative electrode material is spherical graphite particles formed by graphene, flake graphite with a particle size of submicron order, and amorphous carbon, and amorphous carbon is in situ formed between some flake graphites with a particle size of submicron order, and graphene is in situ formed in the pore structure between some flake graphites with a particle size of submicron order; the surface of the natural graphite-based composite negative electrode material is coated with graphene.

[0057] According to an embodiment of the present invention, the natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a particle size of submicron order, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a particle size of submicron order; the graphene is distributed in the pore structure between the flake graphite with a particle size of submicron order; and the shell layer includes graphene.

[0058] The present invention also provides the use of the natural graphite-based composite negative electrode material as a negative electrode of a lithium ion battery.

[0059] The present invention also provides a negative electrode sheet, which comprises the above-mentioned natural graphite-based composite negative electrode material.

[0060] The present invention also provides a lithium-ion battery, which comprises the above-mentioned natural graphite-based composite negative electrode material, or comprises the above-mentioned negative electrode sheet.

[0061] Beneficial effects of the present invention:

[0062] The present invention first granulates submicron-sized flake graphite particles into spherical graphite particles with a porous structure (i.e., powder 1) by spray granulation. The submicron-sized flake graphite particles form a relatively large number of pore structures inside the spherical graphite particles through stacking during the spray granulation process, specifically, a pore structure is formed between the submicron-sized flake graphite particles. Then, heat treatment is performed under a protective atmosphere to convert the asphalt into amorphous carbon, and the submicron-sized flake graphite particles are tightly bonded by the amorphous carbon to prepare powder 2. Then, the polyamic acid is evenly distributed in the spherical graphite particles by an impregnation method. A polyamic acid / powder 2 composite is prepared in the pore structure between submicron flake graphite particles inside spherical graphite particles; the polyamic acid / powder 2 composite is then mixed with polyamic acid and coated on a current collector, and after curing, a polyimide / powder 2 composite film loaded on the current collector is obtained; finally, a laser is used to induce the composite film to convert the polyimide inside and on the surface of powder 2 into graphene, that is, graphene is in situ generated in the pore structure between submicron flake graphite particles inside the spherical graphite particles and on the surface of the spherical graphite particles, thereby preparing a natural graphite-based composite negative electrode material.

[0063] In the natural graphite-based composite negative electrode material, the graphene inside the natural graphite and the graphene on the surface form a complete three-dimensional skeleton, which together stabilize the structure of the natural graphite-based composite negative electrode material. During the cycle, the volume expansion of the submicron-sized flake graphite in the process of lithium ion insertion and extraction can be buffered, thereby improving the cycle performance of the natural graphite-based composite negative electrode material. At the same time, the three-dimensional network conductive channel structure of the graphene can not only accelerate the conduction rate of lithium ions in the graphite, but also enhance the overall conductivity of the natural graphite-based composite negative electrode material. Through the combined effect of the graphene inside and outside the natural graphite-based composite negative electrode material, the lithium-ion battery can have high rate performance and long cycle life. In addition, the natural graphite-based composite negative electrode material is directly formed in situ on the current collector without the need to add additional conductive agents and binders. Compared with the traditional wet slurry method, the method of directly constructing the natural graphite-based composite negative electrode material in situ on the current collector of the present invention ensures the integrity of the graphene network structure, so that the lithium-ion battery exhibits better cycle performance and rate performance. DETAILED DESCRIPTION

[0064] The preparation method of the present invention will be described in further detail below with reference to specific examples. It should be understood that the following examples are merely illustrative and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are encompassed within the scope of protection intended by the present invention.

[0065] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the reagents, materials, etc. used in the following examples are all commercially available unless otherwise specified.

[0066] The purity of the natural flake graphite in the following examples and comparative examples is ≥99%, and the natural flake graphite is derived from the irregular flake graphite tailings produced in the preparation of spherical graphite.

[0067] Example 1

[0068] (1) For 1000g natural flake graphite (D 50 2μm) were crushed to obtain submicron-sized flake graphite (D 50 0.95 μm);

[0069] (2) 100 g of submicron flake graphite from step (1), 12 g of asphalt (carbon residue value 60%) and 60 g of ethanol were mixed to obtain a slurry, and the slurry was spray granulated (inlet temperature 180° C., outlet temperature 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0070] (3) heat treating the powder 1 obtained in step (2) at 1000° C. for 3 hours under a nitrogen atmosphere and cooling to room temperature to obtain powder 2;

[0071] (4) 162 g of p-phenylenediamine, 288 g of 1,2-dimethylimidazole, and 170 g of water were added to a three-necked flask, heated to 40° C., and stirred for 70 min to obtain a mixed solution; then the temperature was raised to 75° C., 441 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added to the mixed solution in 5 portions, and the mixture was stirred for 6 h to react to obtain a polyamic acid solution (concentration of 4 wt %). A portion of the polyamic acid solution was taken out from the three-necked flask, cooled to room temperature, and sealed for storage; then the remaining polyamic acid solution was stirred for 10 h to react to obtain a polyamic acid solution (concentration of 10 wt %), cooled to room temperature, and sealed for storage;

[0072] (5) immersing 10 g of the powder 2 obtained in step (3) into 100 g of the polyamic acid solution (4 wt %) obtained in step (4), immersing for 50 min, filtering, and drying at 80° C. to obtain a polyamic acid / powder 2 composite;

[0073] (6) After uniformly mixing 10 g of the polyamic acid / powder 2 composite of step (5) and 20 g of the polyamic acid solution (concentration of 10 wt%) of step (4), the mixture was uniformly coated on a copper foil and cured in a vacuum oven at 100° C. for 20 h to obtain a polyimide / powder 2 composite film (thickness of 20 μm) loaded on a current collector; the composite film was irradiated with a carbon dioxide infrared laser having a wavelength of 1064 nm, a laser power of 10 W, a scanning rate of 200 mm / s, a scanning spacing of 0.1 mm, a pulse frequency of 100 kHz, a focal length of 20 cm, and a voltage of 300 kV to directly construct the natural graphite-based composite negative electrode material in situ on the copper foil.

[0074] The natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a submicron particle size, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a submicron particle size; the graphene is distributed in the pore structure between the flake graphite with a submicron particle size; and the shell layer includes graphene.

[0075] Example 2

[0076] (1) For 1000g natural flake graphite (D 50 1.5 μm) was crushed to obtain submicron flake graphite (D 50 0.9 μm);

[0077] (2) 100 g of submicron flake graphite from step (1), 15 g of asphalt (residual carbon value: 60%) and 70 g of ethanol were mixed to obtain a slurry. The slurry was subjected to spray granulation (inlet temperature: 180° C., outlet temperature: 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0078] (3) heat treating the powder 1 obtained in step (2) at 900° C. for 6 hours under a nitrogen atmosphere and cooling the mixture to room temperature to obtain powder 2;

[0079] (4) 162 g of p-phenylenediamine, 288 g of 1,2-dimethylimidazole, and 170 g of water were added to a three-necked flask, heated to 40° C., and stirred for 70 min to obtain a mixed solution; then the temperature was raised to 80° C., 441 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added to the mixed solution in 5 portions, and the mixture was stirred for 6 h to react to obtain a polyamic acid solution (concentration of 5 wt%). A portion of the polyamic acid solution was taken out from the three-necked flask, cooled to room temperature, and sealed for storage; then the remaining polyamic acid solution was stirred for 12 h to react to obtain a polyamic acid solution (concentration of 12 wt%), cooled to room temperature, and sealed for storage;

[0080] (5) immersing 10 g of the powder 2 obtained in step (3) into 100 g of the polyamic acid solution (5 wt %) obtained in step (4), immersing for 60 min, filtering, and drying at 80° C. to obtain a polyamic acid / powder 2 composite;

[0081] (6) After uniformly mixing 10 g of the polyamic acid / powder 2 composite of step (5) and 24 g of the polyamic acid solution (concentration of 12 wt%) of step (4), the mixture was uniformly coated on a copper foil and cured in a vacuum oven at 120° C. for 18 h to obtain a polyimide / powder 2 composite film (thickness of 22 μm) loaded on a current collector; the composite film was irradiated with a carbon dioxide infrared laser having a wavelength of 1064 nm, a laser power of 10 W, a scanning rate of 200 mm / s, a scanning spacing of 0.1 mm, a pulse frequency of 100 kHz, a focal length of 20 cm, and a voltage of 300 kV to directly construct the natural graphite-based composite negative electrode material in situ on the copper foil.

[0082] The natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a submicron particle size, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a submicron particle size; the graphene is distributed in the pore structure between the flake graphite with a submicron particle size; and the shell layer includes graphene.

[0083] Example 3

[0084] (1) For 1000g natural flake graphite (D 50 1.2 μm) was crushed to obtain submicron flake graphite (D 50 0.85 μm);

[0085] (2) 100 g of submicron flake graphite obtained in step (1), 18 g of asphalt (residual carbon value 60%) and 60 g of ethanol were mixed to obtain a slurry, and the slurry was spray granulated (inlet temperature 180° C., outlet temperature 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0086] (3) heat treating the powder 1 obtained in step (2) at 1000° C. for 3 hours under a nitrogen atmosphere and cooling to room temperature to obtain powder 2;

[0087] (4) 162 g of p-phenylenediamine, 288 g of 1,2-dimethylimidazole, and 170 g of water were added to a three-necked flask, heated to 40° C., and stirred for 70 min to obtain a mixed solution; then the temperature was raised to 85° C., 441 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added to the mixed solution in 5 portions, and the mixture was stirred for 6 h to react to obtain a polyamic acid solution (concentration of 6 wt%). A portion of the polyamic acid solution was taken out from the three-necked flask, cooled to room temperature, and sealed for storage; then the remaining polyamic acid solution was stirred for 13 h to obtain a polyamic acid solution (concentration of 15 wt%), cooled to room temperature, and sealed for storage.

[0088] (5) immersing 10 g of the powder 2 obtained in step (3) into 100 g of the polyamic acid solution (concentration: 6 wt%) obtained in step (4), immersing for 70 min, filtering, and drying at 80° C. to obtain a polyamic acid / powder 2 composite;

[0089] (6) After uniformly mixing 10 g of the polyamic acid / powder 2 composite of step (5) and 26 g of the polyamic acid solution (concentration of 15 wt%) of step (4), the mixture was uniformly coated on a copper foil and cured in a vacuum oven at 150° C. for 12 h to obtain a polyimide / powder 2 composite film (thickness of 25 μm) loaded on a current collector; the composite film was irradiated with a carbon dioxide infrared laser having a wavelength of 1064 nm, a laser power of 10 W, a scanning rate of 200 mm / s, a scanning spacing of 0.1 mm, a pulse frequency of 100 kHz, a focal length of 20 cm, and a voltage of 300 kV to directly construct the natural graphite-based composite negative electrode material in situ on the copper foil.

[0090] The natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a submicron particle size, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a submicron particle size; the graphene is distributed in the pore structure between the flake graphite with a submicron particle size; and the shell layer includes graphene.

[0091] Example 4

[0092] (1) For 1000g natural flake graphite (D 50 1 μm) was crushed to obtain submicron-sized flake graphite (D 50 0.8 μm);

[0093] (2) 100 g of submicron flake graphite from step (1), 20 g of asphalt (carbon residue value 60%) and 80 g of ethanol were mixed to obtain a slurry, and the slurry was spray granulated (inlet temperature 180° C., outlet temperature 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0094] (3) heat treating the powder 1 obtained in step (2) at 1000° C. for 3 hours under a nitrogen atmosphere and cooling to room temperature to obtain powder 2;

[0095] (4) 162 g of p-phenylenediamine, 288 g of 1,2-dimethylimidazole, and 170 g of water were added to a three-necked flask, heated to 40° C., and stirred for 70 min to obtain a mixed solution; then the temperature was raised to 90° C., 441 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added to the mixed solution in 5 portions, and the mixture was stirred for 5 h to react to obtain a polyamic acid solution (concentration of 7 wt%). A portion of the polyamic acid solution was taken out from the three-necked flask, cooled to room temperature, and sealed for storage; then the remaining polyamic acid solution was stirred for 14 h to react to obtain a polyamic acid solution (concentration of 18 wt%), cooled to room temperature, and sealed for storage;

[0096] (5) immersing 10 g of the powder 2 obtained in step (3) into 100 g of the polyamic acid solution (7 wt %) obtained in step (4), immersing for 50 min, filtering, and drying at 80° C. to obtain a polyamic acid / powder 2 composite;

[0097] (6) After uniformly mixing 10 g of the polyamic acid / powder 2 complex of step (5) and 28 g of the polyamic acid solution (concentration of 18 wt%) of step (4), the mixture was uniformly coated on a copper foil and cured in a vacuum oven at 180° C. for 6 h to obtain a polyimide / powder 2 complex composite film (thickness of 28 μm) loaded on a current collector; the composite film was irradiated with a carbon dioxide infrared laser having a wavelength of 1064 nm, a laser power of 10 W, a scanning rate of 200 mm / s, a scanning spacing of 0.1 mm, a pulse frequency of 100 kHz, a focal length of 20 cm, and a voltage of 300 kV to directly construct the natural graphite-based composite negative electrode material in situ on the copper foil.

[0098] The natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a submicron particle size, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a submicron particle size; the graphene is distributed in the pore structure between the flake graphite with a submicron particle size; and the shell layer includes graphene.

[0099] Comparative Example 1

[0100] (1) For 1000g natural flake graphite (D 50 2μm) were crushed to obtain submicron-sized flake graphite (D 50 0.95 μm);

[0101] (2) 100 g of submicron flake graphite obtained in step (1), 12 g of asphalt (carbon residue value 60%) and 60 g of ethanol were mixed to obtain a slurry. The slurry was subjected to spray granulation (inlet temperature 180° C., outlet temperature 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0102] (3) The powder 1 of step (2) was heat treated at 1000° C. for 3 hours under a nitrogen atmosphere and cooled to room temperature to obtain a natural graphite negative electrode material.

[0103] Comparative Example 2

[0104] (1) For 1000g natural flake graphite (D 50 2μm) were crushed to obtain submicron-sized flake graphite (D 50 0.95 μm);

[0105] (2) 100 g of submicron flake graphite from step (1), 12 g of asphalt (carbon residue value 60%) and 60 g of ethanol were mixed to obtain a slurry, and the slurry was spray granulated (inlet temperature 180° C., outlet temperature 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0106] (3) heat treating the powder 1 obtained in step (2) at 1000° C. for 3 hours under a nitrogen atmosphere and cooling to room temperature to obtain powder 2;

[0107] (4) 162 g of p-phenylenediamine, 288 g of 1,2-dimethylimidazole, and 170 g of water were added to a three-necked flask, heated to 40° C., and stirred for 70 min to obtain a mixed solution; then the temperature was raised to 75° C., 441 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added to the mixed solution in 5 portions, and the mixture was stirred for 16 h to react to obtain a polyamic acid solution (concentration of 10 wt%), which was cooled to room temperature and sealed for storage;

[0108] (5) After uniformly mixing 10 g of the powder 2 of step (2) and 20 g of the polyamic acid solution (concentration of 10 wt%) of step (4), the mixture was uniformly coated on a copper foil, and cured in a vacuum oven at 100° C. for 20 h to obtain a composite film of powder 2 loaded on a current collector (thickness of 20 μm); the composite film was irradiated with a carbon dioxide infrared laser having a wavelength of 1064 nm, a laser power of 10 W, a scanning rate of 200 mm / s, a scanning spacing of 0.1 mm, a pulse frequency of 100 kHz, a focal length of 20 cm, and a voltage of 300 kV, to obtain the natural graphite-based composite negative electrode material directly in situ on the copper foil.

[0109] The natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a particle size of submicron level and amorphous carbon; the amorphous carbon is distributed between the flake graphite with a particle size of submicron level; and the shell layer includes graphene.

[0110] Comparative Example 3

[0111] (1) For 1000g natural flake graphite (D 50 2μm) were crushed to obtain submicron-sized flake graphite (D 50 0.95 μm);

[0112] (2) 100 g of submicron flake graphite from step (1), 12 g of asphalt (carbon residue value 60%) and 60 g of ethanol were mixed to obtain a slurry, and the slurry was spray granulated (inlet temperature 180° C., outlet temperature 100° C.; air atmosphere) to obtain powder 1 (D 50 8μm);

[0113] (3) heat treating the powder 1 obtained in step (2) at 1000° C. for 3 hours under a nitrogen atmosphere and cooling to room temperature to obtain powder 2;

[0114] (4) 162 g of p-phenylenediamine, 288 g of 1,2-dimethylimidazole, and 170 g of water were added to a three-necked flask, heated to 40° C., and stirred for 70 min to obtain a mixed solution; then the temperature was raised to 75° C., 441 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added to the mixed solution in 5 portions, and the mixture was stirred for 6 h to react to obtain a polyamic acid solution (concentration of 4 wt%), which was then cooled to room temperature and sealed for storage;

[0115] (5) immersing 10 g of the powder 2 obtained in step (3) into 100 g of the polyamic acid solution (with a concentration of 4 wt%) obtained in step (4) for 50 min to obtain a polyamic acid / powder 2 composite solution;

[0116] (6) The polyamic acid / powder 2 complex solution of step (5) is evenly coated on a copper foil, and cured in a vacuum oven at 100° C. for 20 h to obtain a polyimide / powder 2 complex composite film (thickness 20 μm) loaded on a current collector; the composite film is irradiated with a carbon dioxide infrared laser with a wavelength of 1064 nm, a laser power of 10 W, a scanning rate of 200 mm / s, a scanning spacing of 0.1 mm, a pulse frequency of 100 kHz, a focal length of 20 cm, and a voltage of 300 kV to directly construct the natural graphite-based composite negative electrode material in situ on the copper foil.

[0117] The natural graphite-based composite negative electrode material includes flake graphite with a submicron particle size, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a submicron particle size; the graphene is distributed in the pore structure between the flake graphite with a submicron particle size, and a very small amount of graphene is distributed on the surface of the composite negative electrode material.

[0118] Electrochemical performance test

[0119] The natural graphite-based composite material prepared in Comparative Example 1: conductive carbon black (SP): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) = 95:1:1.5:2.5 (mass ratio) was mixed evenly and applied on copper foil. The coated electrode was placed in a vacuum drying oven at 120°C and dried for 12 hours to obtain a natural graphite-based / copper foil.

[0120] Full battery test method: The natural graphite-based / copper foil prepared in Examples 1-4 and Comparative Examples 1-3 was used as the negative electrode, lithium cobalt oxide was used as the positive electrode, and 1M-LiPF6+EC:DEC:DMC (volume ratio 1:1:1) solution was used as the electrolyte to assemble a full battery with a voltage range of 0.01-1.5V. The rate performance, cycle performance, and electrode expansion rate obtained from the test are listed in Table 1.

[0121] Table 1 Electrochemical performance test results

[0122]

[0123] Compared with Examples 1-4, the natural graphite-based composite negative electrode material of Comparative Example 1 has pores formed between the submicron flake graphite particles inside and no graphene on its surface. During the cycle, it is unable to buffer the volume expansion of the submicron flake graphite particles during the process of lithium ion insertion and extraction, resulting in a significant decrease in the cycle performance of the battery and a significant increase in the expansion rate of the electrode.

[0124] Compared with Example 1, the concentration of the polyamic acid solution in Comparative Example 2 is high, which prevents it from entering the pores formed between the submicron-sized flake graphite particles in the natural graphite-based composite negative electrode material. During the subsequent laser induction process, a graphene coating is only generated on the surface of the natural graphite. The shell graphene can only be relied upon to mitigate the volume expansion of the submicron-sized flake graphite particles in the core during the cycle. However, this structure is unstable and easily broken during the cycle, resulting in a significant decrease in the cycle performance of the obtained battery and a significant increase in the pole piece expansion rate. In addition, the electrical conductivity of the natural graphite-based composite negative electrode material is reduced, resulting in a decrease in the rate performance of the battery.

[0125] Compared with Example 1, although the polyamic acid solution of Comparative Example 3 can enter the pores formed between the submicron-sized flake graphite in the natural graphite-based composite negative electrode material, due to the low concentration of the polyamic acid solution, it is impossible to form a coating with a certain thickness on the natural graphite surface. In the subsequent laser induction process, the polyamide in the pores formed between the submicron-sized flake graphite is induced into graphene. The natural graphite surface contains only a small amount of graphene, and a complete graphene coating cannot be formed on the surface of the natural graphite-based composite negative electrode material. This structure is also unstable and easily broken during the cycle process, resulting in a decrease in the cycle performance of the obtained battery and an increase in the pole piece expansion rate. In addition, the electrical conductivity of the natural graphite-based composite negative electrode material decreases, causing the rate performance of the battery to also decrease.

[0126] As can be seen from Examples 1-4, in the natural graphite-based composite negative electrode material prepared by the present invention, the graphene inside the natural graphite and the graphene on the surface form a complete three-dimensional skeleton, which together stabilize the structure of the natural graphite-based composite negative electrode material. During the cycle, the volume expansion of the flake graphite with a submicron particle size during the deintercalation and intercalation of lithium ions can be buffered, thereby improving the cycle performance of the natural graphite-based composite negative electrode material. At the same time, the three-dimensional network conductive channel structure of graphene can not only accelerate the conduction rate of lithium ions inside the graphite, but also enhance the overall conductive properties of the natural graphite-based composite negative electrode material. Through the combined action of the graphene inside and outside the natural graphite-based composite negative electrode material, the lithium-ion battery can have the characteristics of high rate performance and long cycle life. In addition, the natural graphite-based composite negative electrode material prepared by the present invention is directly formed in situ on the current collector without the need to add additional conductive agents and binders. Compared with the traditional wet slurry method, the present invention's method of directly constructing the natural graphite-based composite negative electrode material in situ on the current collector ensures the integrity of the graphene network structure, enabling the lithium-ion battery to exhibit better cycle performance and rate performance.

[0127] The above describes the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for in-situ construction of a natural graphite-based composite negative electrode material on a current collector, wherein: The method comprises the following steps: (1) crushing natural flake graphite to prepare flake graphite with a particle size of submicron level; (2) mixing submicron-sized flake graphite, asphalt, and a solvent, and spray granulating the mixture to prepare powder 1; (3) heat-treating powder 1 under a protective atmosphere to obtain powder 2; (4) mixing a diamine compound, a dianhydride compound, a catalyst, and water, and reacting them to prepare a polyamic acid solution; (5) mixing the powder 2 and the polyamic acid solution, impregnating, filtering, and drying to prepare a polyamic acid / powder 2 composite; (6) The polyamic acid / powder 2 complex and the polyamic acid solution are mixed and coated on the current collector, and after curing, a composite film loaded on the current collector is obtained. The natural graphite-based composite negative electrode material is prepared by laser induction of the composite film, that is, the natural graphite-based composite negative electrode material is constructed in situ on the current collector.

2. The method according to claim 1, wherein In step (1), the median particle size of the submicron flake graphite is 0.1 μm-1 μm. Preferably, in step (2), the mass ratio of the asphalt to the submicron flake graphite is (10-20):100; and / or, in step (2), the mass ratio of the solvent to the submicron flake graphite is (30-100):100; And / or, in step (2), the inlet temperature of the spray granulation is 150-210°C; the outlet temperature of the spray granulation is 90-110°C; and the spray granulation is carried out in an air atmosphere; And / or, in step (2), the powder 1 is spherical graphite particles with a porous structure formed by flake graphite with a particle size of submicron order and asphalt, and asphalt is filled between some flake graphite with a particle size of submicron order, and a porous structure is formed between some flake graphite with a particle size of submicron order, and the pore size of the porous structure is 10-40nm.

3. The method according to claim 1 or 2, wherein: In step (3), the heat treatment temperature is 800-1200° C. and the heat treatment time is 2-10 hours; And / or, in step (3), the powder 2 is spherical graphite particles with a porous structure formed by flake graphite with a particle size of submicron order and amorphous carbon, and amorphous carbon is in situ formed between some flake graphite with a particle size of submicron order, and a porous structure is formed between some flake graphite with a particle size of submicron order, and the pore size of the porous structure is 10-40 nm.

4. The method according to any one of claims 1 to 3, wherein: In step (4), the diamine compound is selected from one or more of p-phenylenediamine, 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-p-phenylenediamine, 1,5-diaminonaphthalene, 1,4-diaminopyrene and 1,4-diaminoanthracene; and / or, in step (4), the catalyst is selected from one or more of 1,2-dimethylimidazole, triethylamine and dodecyltrimethylammonium chloride; and / or, in step (4), the dianhydride compound is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride; and / or, in step (4), the molar ratio of the diamine compound to the dianhydride compound is 1:1; and / or, in step (4), the molar ratio of the diamine compound to the catalyst is 1:(1.8-2.2); and / or, in step (4), the mass ratio of the diamine compound to water is 100:(100-120); And / or, in step (4), the reaction temperature is 70-90° C.; the reaction time is 3-24 hours.

5. The method according to any one of claims 1 to 4, wherein: In step (5), the mass ratio of the polyamic acid solution to the powder 2 is 100:(10-30); and / or, in step (5), the concentration of the polyamic acid solution is 3-8 wt%; And / or, in step (5), the immersion temperature is room temperature, and the immersion time is 30 min-120 min.

6. The method according to any one of claims 1 to 5, wherein: In step (6), the mass ratio of the polyamic acid / powder 2 composite to the polyamic acid solution is 10:(20-30); and / or, in step (6), the concentration of the polyamic acid solution is 10-18 wt %; And / or, in step (6), the curing temperature is 100-200° C.; the curing time is 1-20 hours; and / or, in step (6), the thickness of the composite film is 10-30 μm; And / or, in step (6), the control parameters of the laser induction include laser wavelength, laser power, scanning rate, scanning spacing, pulse frequency, focal length and voltage; the laser wavelength is 0.1-1000 μm, the laser power is 0.1-1000 W, the scanning rate is 0.1-5000 mm / s, the scanning spacing is 0.001-100 mm, the pulse frequency is 0.001-2000 kHz, the focal length is 0.1-1000 cm, and the voltage is 1-10000 kV.

7. A natural graphite-based composite negative electrode material prepared by the method according to any one of claims 1 to 6.

8. The natural graphite-based composite negative electrode material according to claim 7, wherein: The natural graphite-based composite negative electrode material includes graphene, flake graphite with a particle size of submicron level and amorphous carbon; Preferably, the natural graphite-based composite negative electrode material is spherical graphite particles formed by graphene, flake graphite with a particle size of submicron level, and amorphous carbon, and amorphous carbon is in situ formed between some flake graphite particles with a particle size of submicron level, and graphene is in situ formed in the pore structure between some flake graphite particles with a particle size of submicron level; the surface of the natural graphite-based composite negative electrode material is coated with graphene; Preferably, the natural graphite-based composite negative electrode material has a core-shell structure, including a core and a shell layer; the core includes flake graphite with a submicron particle size, amorphous carbon and graphene; the amorphous carbon is distributed between the flake graphite with a submicron particle size; the graphene is distributed in the pore structure between the flake graphite with a submicron particle size; and the shell layer includes graphene.

9. A negative electrode sheet, comprising the natural graphite-based composite negative electrode material according to claim 7 or 8. 10 . A lithium-ion battery comprising the natural graphite-based composite negative electrode material according to claim 7 or 8 , or the negative electrode sheet according to claim 9 .

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