Negative active material and preparation method thereof, lithium ion battery and electric equipment

Abstract

The invention belongs to the technical field of battery negative electrode materials, and particularly relates to a negative electrode active material and a preparation method thereof, a lithium ion battery and electric equipment. The preparation method comprises the following steps: uniformly mixing a silicon negative electrode material in a solution containing a metal salt and an organic ligand, standing for reaction, and then performing centrifugal separation and drying to obtain a powdery premixed material; carrying out heat treatment on the powdery premixed material and a carbon source under inert gas to obtain a negative electrode active material of which the surface is sequentially coated with a metal compound layer and a carbon nanotube; wherein the carbon source contains nitrogen and / or boron elements. Through the synergistic effect of the compound layer containing cobalt, nickel or aluminum and the carbon nanotube layer, the conductivity and mechanical stability of the material are improved, and the lithium ion diffusion efficiency is also enhanced; and meanwhile, the doping of nitrogen or boron further optimizes the interface characteristics of the carbon nanotubes, so that the cycle performance and the rate capability of the negative electrode active material are remarkably improved. The lithium ion battery adopting the material has the advantages of high capacity, long service life and quick charge and discharge.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery negative electrode materials, and in particular relates to a negative electrode active material and a preparation method thereof, a lithium ion battery and an electrical device. Background Art

[0002] Lithium-ion batteries are widely used in electric vehicles and consumer electronics due to their high energy density, long cycle life, and low self-discharge rate. However, traditional graphite anode materials are no longer able to meet the growing demand for energy storage. For example, the theoretical specific capacity of graphite anodes is only 372 mAh / g, which limits the improvement of overall battery performance. In addition, graphite anodes have certain volume expansion problems during the charge and discharge process. Although the expansion rate is relatively low, it can still lead to unstable electrode structure and reduced battery performance under high-rate charge and discharge conditions. Therefore, the development of new high-performance anode materials has become a top priority.

[0003] Silicon-based materials are considered to be highly promising anode materials for next-generation lithium-ion batteries due to their high theoretical specific capacity (4200 mAh / g) and suitable operating voltage (0.4 V). However, silicon-based materials experience significant volume expansion during the charge and discharge process, with the maximum expansion rate reaching 300%-400%. This volume expansion can lead to pulverization of silicon particles, unstable electrode structure, and repeated formation of SEI films, thereby accelerating battery capacity decay and aging. In addition, silicon-based materials themselves have poor electrical conductivity, which further limits their application in lithium-ion batteries.

[0004] At present, the scientific research team has developed a variety of strategies for the preparation of various types of silicon-carbon composite materials to address the key issues such as volume expansion and low conductivity faced by silicon-based negative electrode materials during application. However, after in-depth research and practical application, it was found that the existing technical solutions still have relatively significant limitations. Specifically, under fast charging conditions, traditional carbon-coated structures are prone to conductive network breakage, causing the conductive performance of the material to drop sharply, thereby affecting the overall performance of the battery. Although some technological breakthroughs have been achieved, such as optimizing the composite structure design, which has alleviated the volume expansion problem of silicon materials to a certain extent and improved the conductivity of the material, its large-scale production still faces many technical bottlenecks. Therefore, the development of a silicon-carbon composite material with both excellent performance and good economy and its production process has become a key issue that needs to be urgently addressed in this field. Summary of the Invention

[0005] The object of the present invention is to provide a negative electrode active material, a preparation method thereof, a lithium ion battery and an electrical device. Through the synergistic effect of the metal compound layer and the carbon nanotube layer, the conductivity and mechanical stability of the material are improved, and at the same time, the lithium ion diffusion efficiency is enhanced, so that the negative electrode active material is significantly improved in cycle performance and rate performance.

[0006] To achieve the above object, a first aspect of the present invention provides a preparation method of a negative electrode active material, comprising the following steps:

[0007] S1. Mix the silicon negative electrode material evenly in a solution containing a metal salt and an organic ligand, then let it stand for reaction, and then centrifuge and dry to obtain a powdery premixed material;

[0008] S2. Heat-treat the powdery premixed material and a carbon source under an inert gas to obtain a negative electrode active material with a metal compound layer and a carbon nanotube layer coated on the surface in sequence; wherein, the carbon source contains nitrogen and / or boron elements.

[0009] Further, the silicon negative electrode material is one or more of silicon powder, silicon monoxide, silicon powder coated with a carbon layer, and silicon monoxide coated with a carbon layer, and preferably silicon powder coated with a carbon layer. The thickness of the carbon layer is 1-15 nm, and the ID / IG in the Raman spectrum is 0.5-1.2. For example, the thickness of the coated carbon layer can be 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm or a range composed of any two of the above values. Thereby, the electrochemical performance of the negative electrode material can be further improved.

[0010] Further, the average particle size of the silicon negative electrode material is 2-15 μm. For example, the average particle size of the silicon-based negative electrode material particles can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm or a range composed of any two of the above values. The silicon negative electrode material contains silicon nanocrystalline domains, and the average particle size of the silicon nanocrystalline domains is 4 nm-10 nm. For example, the average particle size of the silicon nanocrystalline domains can be 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm or a range composed of any two of the above values.

[0011] Further, the diameter of the carbon nanotube is 1-5 nm, and the length-to-diameter ratio is (2-50):1.

[0012] Further, the metal salt is one or more of cobalt salts, nickel salts, and aluminum salts; the organic ligand is dimethylimidazole or dicyandiamide.

[0013] Further, the carbon source is one or more of melamine, cyanuric acid, dopamine hydrochloride, chitosan, and polyaniline.

[0014] Further, in step S1, the mass ratio of the silicon negative electrode material to the organic ligand is 1:(8 - 15), and the molar ratio of the organic ligand to the metal salt is (5 - 8):1.

[0015] Further, the time for the static reaction is 6 to 48 hours; a binder is further added to the solution, and the binder includes one or more of polyvinylpyrrolidone, polyacrylonitrile, polydopamine, and carboxymethyl cellulose; the mass ratio of the binder to the silicon negative electrode material is (0.5 - 1):1. The drying is freeze-drying at -30°C to -20°C.

[0016] Preferably, the silicon negative electrode material is first added to the solution containing the binder, and then the organic ligand and the metal salt are added in sequence. After stirring, the mixture is allowed to react statically, centrifuged, washed, frozen, and dried to obtain a powdery premixed material. The solvent of the solution is preferably water.

[0017] Further, in step S2, the temperature of the heat treatment is 650°C to 850°C, and the heat preservation time is 1 - 3 hours.

[0018] Further, the mass ratio of the carbon source to the silicon negative electrode material in step S1 is (8 - 12):1.

[0019] Preferably, the powdery premixed material and the carbon source are placed in a tubular furnace, heated to a high temperature for carbonization at a certain heating rate under an argon atmosphere, kept at a certain temperature for a period of time, and then cooled to room temperature and taken out to obtain a negative electrode active material with a metal compound layer and a carbon nanotube layer coated on the surface in sequence.

[0020] Compared with the conventional silicon-carbon composite material, in the above preparation method of the present invention, by introducing a solid product containing cobalt or nickel metal-organic framework (MOF), under high temperature and argon atmosphere, the carbon source is catalytically in-situ grown into carbon nanotubes and doped with nitrogen elements, obtaining a silicon-carbon negative electrode material with carbon nanotubes on the surface. At the same time, the MOF layer is also pyrolyzed into a metal compound layer. Among them, the distribution state of the metal compound layer is active sites or a layer of compounds on the surface.

[0021] The second aspect of the present invention provides a negative electrode active material obtained by the preparation method described in any one of the above. It includes silicon-based negative electrode material particles, a carbon coating layer provided on the surface of the silicon-based negative electrode material particles; a cobalt, nickel, or aluminum-containing compound layer provided on the surface of the carbon coating layer; and a carbon nanotube layer provided on the surface of the compound layer, and the carbon nanotubes can be doped with nitrogen or boron elements.

[0022] Through the synergistic effect of the cobalt-, nickel- or aluminum-containing compound layer and the carbon nanotube layer, the present invention not only improves the electrical conductivity and mechanical stability of the material, but also enhances the lithium ion diffusion efficiency. At the same time, the doping of nitrogen or boron elements further optimizes the interfacial properties of the carbon nanotubes, significantly improving the cycling performance and rate performance of the anode active material. The lithium ion battery using this material has the advantages of high capacity, long life and fast charge and discharge.

[0023] In the third aspect of the present invention, a lithium ion battery is provided, including a negative electrode sheet, the negative electrode sheet including a current collector and a negative electrode active material layer provided on the surface of the current collector, and the negative electrode active material layer including the above-mentioned negative electrode active material.

[0024] In the fourth aspect of the present invention, an electrical device is provided, including the above-mentioned lithium ion battery. The electrical device includes, but is not limited to, electric vehicles and energy storage devices, etc.

[0025] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following technical advantages are mainly possessed:

[0026] 1. The negative electrode active material provided by the present invention effectively improves the electrical performance through a multi-layer structure design. Among them, the metal compound layer enhances the structural stability, and the carbon nanotube layer further strengthens and improves the conductivity. The doping of nitrogen and / or boron elements in the carbon nanotubes can optimize the electronic structure, improve the conductivity and chemical stability, promote the adsorption and diffusion of lithium ions, enhance the interfacial compatibility, and reduce the stress concentration, so as to maintain the structural integrity of the negative electrode material during the charge and discharge process, improve the cycling stability and rate performance of the battery, and extend the battery cycle life.

[0027] 2. The present invention selects a silicon negative electrode material coated with a carbon layer on the surface. The carbon coating layer on the particle surface can buffer the volume change, reduce the influence of volume expansion on the structure and performance of the material, improve the stability of the electrode structure, and inhibit the formation of the SEI film.

[0028] 3. By regulating the amounts of the silicon negative electrode material, metal salt, organic ligand and carbon source, the present invention regulates the structure and thickness of the metal compound layer and the carbon nanotube layer, thereby significantly improving the cycling performance and rate performance of the negative electrode active material.

[0029] 4. By regulating the static reaction time of the metal organic ligand, the present invention regulates the growth amount of the MOF layer, and further regulates the catalytic growth process of the carbon nanotube layer, making the performance of the negative electrode active material better, having the advantages of high capacity, long life and fast charge and discharge. Description of the Drawings

[0030] Figure 1It is a comparison chart of the EIS test results of the button cells with and without in-situ grown CNTs in the anode material prepared in Example 1 of the present invention.

[0031] Figure 2 It is a comparison chart of the rate test results of the button cells with and without in-situ grown CNTs in the anode material prepared in Example 1 of the present invention. Detailed implementation manners

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0033] For those experimental steps or conditions not specified in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in the art can be followed. For the reagents or instruments not indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0034] Example 1

[0035] This example provides an anode material, which is prepared by the following method:

[0036] (1) Place 0.02 g of polyvinylpyrrolidone in 50 ml of deionized water, and let it stand and disperse evenly to obtain a first solution;

[0037] (2) Add 0.325 g of dimethylimidazole and 0.03 g of SiO material with a carbon layer on the surface to the first solution obtained in operation (1) successively. After stirring, a second solution is obtained;

[0038] (3) Add 0.145 g of cobalt nitrate hexahydrate to the second solution obtained in operation (2), and stir for 3 minutes to obtain a third solution;

[0039] (4) Seal the container of the third solution, and let it stand and react for 24 hours to obtain a fourth solution;

[0040] (5) Centrifuge the fourth solution at a speed of 8000 rpm for the first time. The solid obtained from the first centrifugation is fully resuspended with deionized water, then centrifuged at a speed of 8000 rpm for the second time. The solid obtained from the second centrifugation is fully resuspended with ethanol, then centrifuged at a speed of 8000 rpm for the third time. The solid obtained from the third centrifugation is fully resuspended with deionized water, then centrifuged at a speed of 8000 rpm for the fourth time. The solid obtained from the fourth centrifugation is fully resuspended with ethanol, then centrifuged at a speed of 8000 rpm for the fifth time. The solid obtained from the fifth centrifugation is fully resuspended with deionized water to obtain a premixed material;

[0041] (6) placing the premixed material obtained in operation (5) in a low-temperature freezer and freezing it at -26°C for 24 hours to obtain a solid premixed material;

[0042] (7) placing the solid frozen premixed material in a freeze drying chamber and freeze drying to obtain a powdered premixed material;

[0043] (8) The powdered premixed material and 0.3 g of melamine were placed in a porcelain boat, and the porcelain boat was placed in a tube furnace. In an argon atmosphere, the temperature was increased to 800 °C at a heating rate of 5 °C / min and kept warm for 2 h. After the insulation was completed, the material was cooled to room temperature in an argon atmosphere and taken out to obtain a silicon-carbon negative electrode material with carbon nanotubes on the surface, and the carbon nanotubes contained nitrogen elements.

[0044] The negative electrode material provided in this embodiment comprises an 8μm SiO-based anode material and a 15nm-thick carbon coating layer coated on the surface of the silicon-based material. The SiO particles contain silicon nanocrystals with an average particle size of 5nm. Calculations show that the cobalt content in the compound accounts for 0.001-0.05% of the total mass of the active material, the nitrogen content in the carbon nanotubes is 0.001-0.01%, the diameter of the carbon nanotube layer is 1-3nm, and the aspect ratio is 3:1-30:1.

[0045] Example 2

[0046] The negative electrode material was prepared according to the method of Example 1, except that in operation (4) of this example, the reaction was allowed to stand for 12 hours.

[0047] In the negative electrode material of this embodiment, the size of the silicon nanocrystal domain contained in SiO is 3nm, the particle size of SiO is 8μm, and the carbon coating layer with a thickness of 15nm is coated on the surface of the silicon-based material. The thickness of the carbon coating layer obtained by heat treatment is 3nm. D / I G The value of is 0.91.

[0048] Example 3

[0049] The negative electrode material was prepared according to the method of Example 1, except that in operation (4) of this example, the reaction was allowed to stand for 20 hours.

[0050] The negative electrode material of this embodiment has a silicon nanocrystal domain size of 3 nm, a SiO particle size of 8 μm, and a 15 nm thick carbon coating layer coated on the surface of the silicon-based material. The thickness of the carbon coating layer after heat treatment is 4 nm. The nitrogen content of the carbon nanotubes is 0.02-0.1%, the diameter of the carbon nanotube layer is 2-5 nm, and the aspect ratio is 5:1-25:1. The ID / IG value of the negative electrode material in the Raman spectrum is 0.97.

[0051] Example 4

[0052] The negative electrode material was prepared according to the method of Example 1, except that in operation (8) of this example, the static reaction was carried out for 28 hours.

[0053] In the negative electrode material of this example, the size of the silicon nanocrystalline domains is 3 nm, the particle size of SiO is 8 μm, and the thickness of the carbon coating layer is 15 nm. The mass of cobalt element in the compound accounts for 0.003% - 0.02% of the total mass of the active material, the nitrogen element content in the carbon nanotubes is 0.05 - 0.01%, the diameter of the carbon nanotube layer is 2 - 5 nm, and the aspect ratio is 12:1 - 30:1. The value of ID / IG in the Raman spectrum of the negative electrode material is 0.96.

[0054] Example 5

[0055] The negative electrode material was prepared according to the method of Example 1, except that in operation (8) of this example, 0.4 g of melamine was added.

[0056] In the negative electrode material of this example, the size of the silicon nanocrystalline domains is 3 nm, the particle size of SiO is 8 μm, and there is a carbon coating layer with a thickness of 15 nm covering the surface of the silicon-based material, and the thickness of the carbon coating layer obtained by heat treatment is 8 nm. The nitrogen element content in the carbon nanotubes is 0.07 - 0.2%, the diameter of the carbon nanotube layer is 3 - 6 nm, and the aspect ratio is 20:1 - 35:1. The value of ID / IG in the Raman spectrum of the negative electrode material is 0.91.

[0057] Example 6

[0058] The negative electrode material was prepared according to the method of Example 1, except that in operation (8) of this example, the heat preservation temperature was 700 °C.

[0059] In the negative electrode material of this example, the size of the silicon nanocrystalline domains is 3 nm, the particle size of SiO is 8 μm, and there is a carbon coating layer with a thickness of 15 nm covering the surface of the silicon-based material, and the thickness of the carbon coating layer obtained by heat treatment is 15 nm. The diameter of the carbon nanotube layer is 2 - 5 nm, and the aspect ratio is 8:1 - 45:1. The value of ID / IG in the Raman spectrum of the negative electrode material is 0.93.

[0060] Example 7

[0061] The negative electrode material was prepared according to the method of Example 1, except that in operation (8) of this example, the heat preservation temperature was 650 °C.

[0062] In the anode material of this embodiment, the size of the silicon nanocrystalline domains is 3 nm, the particle size of SiO is 8 μm, and there is a carbon coating layer with a thickness of 15 nm coated on the surface of the silicon-based material, and the thickness of the carbon coating layer obtained by heat treatment is 2 nm. The nitrogen element content in the carbon nanotubes is 0.07 - 0.2%, the diameter of the carbon nanotube layer is 2 - 5 nm, and the aspect ratio is 5:1 - 15:1. The value of ID / IG in the Raman spectrum of the anode material is 0.98.

[0063] Example 8

[0064] The anode material was prepared according to the method of Example 1, except that in operation (8) of this example, the heat preservation time was 1 hour.

[0065] In the anode material of this embodiment, the size of the silicon nanocrystalline domains is 3 nm, the particle size of SiO is 8 μm, and there is a carbon coating layer with a thickness of 15 nm coated on the surface of the silicon-based material, and the thickness of the carbon coating layer obtained by heat treatment is 7 nm. The nitrogen element content in the carbon nanotubes is 0.07 - 0.2%, the diameter of the carbon nanotube layer is 2 - 5 nm, and the aspect ratio is 5:1 - 15:1. The value of ID / IG in the Raman spectrum of the anode material is 0.91.

[0066] Example 9

[0067] The anode material was prepared according to the method of Example 1, except that in operation (8) of this example, the heat preservation time was 3 hours.

[0068] In the anode material of this embodiment, the size of the silicon nanocrystalline domains is 8 nm, the particle size of SiO is 8 μm, and there is a carbon coating layer with a thickness of 15 nm coated on the surface of the silicon-based material, and the thickness of the carbon coating layer obtained by heat treatment is 15 nm. The nitrogen element content in the carbon nanotubes is 0.09 - 0.25%, the diameter of the carbon nanotube layer is 2 - 6 nm, and the aspect ratio is 15:1 - 45:1. The value of ID / IG in the Raman spectrum of the anode material is 0.95.

[0069] Comparative Example 1

[0070] The anode material was prepared according to the method of Example 1, except that in operation (8) of this example, the heat preservation temperature was 900 °C.

[0071] In the anode material of this comparative example, the size of the silicon nanocrystalline domains is 12 nm, the particle size of SiO is 8 μm, and there is a carbon coating layer with a thickness of 15 nm coated on the surface of the silicon-based material, and the thickness of the carbon coating layer obtained by heat treatment is 15 nm. The diameter of the carbon nanotube layer is 3 nm, and the aspect ratio is 35:1 - 50%:1. The value of ID / IG in the Raman spectrum of the anode material is 0.95.

[0072] Comparative Example 2

[0073] The negative electrode material was prepared according to the method of Example 1, except that in operation (2) of this example, the SiO material selected did not have a carbon layer on its surface.

[0074] In the negative electrode material of this comparative example, the size of the silicon nanocrystalline domains was 5 nm, the particle size of SiO was 8 μm, the thickness of the carbon coating layer obtained by heat treatment was 16 nm. The diameter of the carbon nanotube layer was 3 nm, and the aspect ratio was 150%:1 to 600%:1. The value of ID / IG in the Raman spectrum of the negative electrode material was 0.95.

[0075] Comparative Example 3

[0076] The negative electrode material was prepared according to the method of Example 1, except that in operation (3) of this example, 2-methylimidazole was not added.

[0077] In the negative electrode material of this comparative example, the size of the silicon nanocrystalline domains was 5 nm, the particle size of SiO was 8 μm, and there was a carbon coating layer with a thickness of 15 nm coated on the surface of the silicon-based material, and no carbon coating layer was obtained by heat treatment. No carbon nanotubes grew. The value of ID / IG in the Raman spectrum of the negative electrode material was 0.92.

[0078] Experimental Example

[0079] Button cells and CR2032 and 3 Ah soft-pack batteries were respectively prepared using the negative electrode materials of Examples 1 to 9 and the negative electrode materials of Comparative Examples 1 to 3, and the electrical properties of each battery were detected.

[0080] Among them, the button cell was composed as follows:

[0081] Negative electrode: A slurry was prepared according to the weight ratio of negative electrode material, conductive agent SP, and modified polyacrylic acid of 93:3:4, and coated on an 8-μm copper foil current collector to prepare the negative electrode.

[0082] Positive electrode: Lithium metal.

[0083] Electrolyte: 1.2 mol / L LiPF6, and the solvent was EC / DMC with a molar ratio of 1:1 plus 10 wt% FEC (EC: ethylene carbonate, DMC: dimethyl carbonate, FEC: fluoroethylene carbonate).

[0084] Separator: Celgard 2500.

[0085] The soft-pack battery was composed as follows:

[0086] Negative electrode: 12 wt% negative electrode material + 88 wt% artificial graphite was used as the negative electrode active material. A slurry was prepared according to the weight ratio of negative electrode active material, conductive agent SP, and modified polyacrylic acid of 93:3:4, and coated on an 8-μm copper foil current collector to prepare the negative electrode.

[0087] Positive electrode: Lithium nickel cobalt manganese oxide NCM811 is used as the positive electrode material. A slurry is prepared by mixing the positive electrode material, conductive agent SP, and PVDF (binder) in a weight ratio of 97:2:1, and then coated on a carbon-coated aluminum foil current collector to prepare the positive electrode.

[0088] Electrolyte: 1.2 mol / L LiPF6, and the solvent is a mixture of EC / DMC with a molar ratio of 1:1 plus 10 wt% FEC (EC: ethylene carbonate, DMC: dimethyl carbonate, FEC: fluoroethylene carbonate).

[0089] Separator: Celgard 2500.

[0090] The method for detecting electrical performance is as follows:

[0091] (1) Initial cycle negative electrode specific capacity utilization test: Initial discharge capacity (mAh) / mass of negative electrode active material (g);

[0092] (2) Initial cycle Coulombic efficiency: Initial charge capacity / initial discharge capacity * 100%;

[0093] (3) Initial charge specific capacity: Initial charge capacity / mass of active material (mAh / g);

[0094] (4) 4C rate discharge capacity retention test: 4C discharge capacity / 1C discharge capacity;

[0095] (5) DC internal resistance DCR test: Divide the battery capacity for grading and adjust it to 50% SOC, discharge at 5C for 10s, and measure the discharge resistance. Resistance DCR = (V0 - V10) / I, where V0 is the potential before discharge, V10 is the potential at the 10th second of discharge, and I is the discharge current of 5C;

[0096] (6) Capacity retention test: ① Charging: Constant current charge at a current density of 1C until 4.2V and then stand for 10 min; ② Discharging: Constant current discharge at a current density of 1C until 2.5V and stand for 0 min. Record the discharge capacity as Qn (n = 1, 2, 3... 400); ③ Repeat "①, ②" for 400 cycles; The capacity retention rate of the soft-pack battery after 400 cycles is: Q400 / Q1;

[0097] (7) Volume expansion rate test: After 400 cycles, disassemble the battery when it is fully charged. Use a micrometer to measure the thickness as d2, and the thickness of the fresh electrode after rolling is d1. The calculation formula for the 400-cycle full charge expansion rate of the soft-pack battery is: (d2 - d1) / (d1 - 8), where the thickness of the copper foil is 8μm;

[0098] (8) Lithium plating test: ① Charging: Constant current charging at a current density of 2.4C until 4.2V, then standing for 10 minutes; ② Discharging: Constant current discharging at a current density of 1C until 2.5V, then standing for 10 minutes; After 10 cycles, charge to 4.2V full charge at a constant current density of 1.6C and disassemble to observe the negative electrode interface.

[0099] (9) Electrochemical impedance spectroscopy (EIS) test: EIS measurement is carried out before charge-discharge cycling, and the frequency range is from 1MHz to 0.01Hz.

[0100] The test results are shown in Table 1 and Table 2.

[0101] Table 1 Detection results of the electrical performance of each button cell

[0102]

[0103]

[0104] As can be seen from Table 1, the first-cycle negative electrode specific capacity and Coulomb efficiency of the button cells obtained from the negative electrode material prepared by the present invention are significantly improved.

[0105] Table 2 Detection results of the electrical performance of each soft-pack cell

[0106]

[0107] As can be seen from Table 2, too high heat treatment temperature will affect the growth of CNT and the growth of silicon crystal domain size. The overall conductivity and lithium ion transport ability of the material both decrease, and the volume expansion of the electrode sheet increases. Therefore, the capacity retention rate of long cycles decreases sharply.

[0108] Such as Figure 1 and 2 shown, no CNT in the figure means the original silicon-based material with a carbon layer on the surface, and in-situ CNT growth means the negative electrode material prepared in Example 1. As can be seen from Figure 1 the negative electrode material prepared in Example 1 has a smaller ohmic impedance and obvious advantages in the lithium ion transport rate.

[0109] From Figure 2 it can be seen that the negative electrode material prepared in Example 1 has stronger conductivity, larger specific capacity performance under battery rate charge and discharge, and higher capacity retention rate after high-rate current. The conductive network of in-situ grown CNT significantly promotes the rate performance of the material.

[0110] Those skilled in the art can easily understand that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a negative electrode active material, characterized in that It includes the following steps: S1. Mix the silicon negative electrode material evenly in a solution containing metal salt and organic ligand, then let it stand for reaction, and then perform centrifugal separation and drying to obtain a powdery premixed material; S2. Heat-treat the powdery premixed material and the carbon source under an inert gas to obtain a negative electrode active material with a metal compound layer and carbon nanotubes coated on the surface in sequence; wherein, the carbon source contains nitrogen and / or boron elements.

2. The preparation method of the negative electrode active material according to claim 1, wherein The silicon negative electrode material is one or more of silicon powder, silicon monoxide, silicon powder coated with a carbon layer, and silicon monoxide coated with a carbon layer; and / or, the average particle size of the silicon negative electrode material is 2 - 15 μm, and the thickness of the carbon layer coated on the surface is 1 - 15 nm.

3. The preparation method of the negative electrode active material according to claim 1, characterized in that, The diameter of the carbon nanotubes is 1 - 5 nm, and the aspect ratio is (2 - 50):

1.

4. The preparation method of the negative electrode active material according to claim 1, characterized in that, The metal salt is one or more of cobalt salt, nickel salt, and aluminum salt; the organic ligand is dimethylimidazole or dicyandiamide; and / or, the carbon source is one or more of melamine, cyanuric acid, dopamine hydrochloride, chitosan, and polyaniline.

5. The preparation method of the negative electrode active material according to any one of claims 1-4, characterized in that, In step S1, the mass ratio of the silicon negative electrode material to the organic ligand is 1:(8 - 15), and the molar ratio of the organic ligand to the metal salt is (5 - 8):

1.

6. The preparation method of the negative electrode active material according to claim 5, wherein, The time for the standing reaction is 6 - 48 hours; A binder is further added to the solution, and the binder includes one or more of polyvinylpyrrolidone, polyacrylonitrile, polydopamine, and carboxymethyl cellulose; The drying is freeze-drying at -30°C to -20°C.

7. The preparation method of the negative electrode active material according to any one of claims 1-4, characterized in that, In step S2, the temperature of the heat treatment is 650°C to 850°C, and the heat preservation time is 1 - 3 hours; and / or, the mass ratio of the carbon source to the silicon negative electrode material in step S1 is (8 - 12):

1.

8. A negative electrode active material, characterized in that, Obtained by the preparation method according to any one of claims 1 - 7.

9. A lithium-ion battery, characterized in that, It includes a negative electrode plate, the negative electrode plate includes a current collector and a negative electrode active material layer provided on the surface of the current collector, and the negative electrode active material layer includes the negative electrode active material according to claim 8.

10. An electrical device, characterized in that, It includes the lithium-ion battery according to claim 9.

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