Negative active material, method of preparation and corresponding secondary battery

Abstract

The present application relates to a kind of negative electrode active material, it includes silicon carbon material particle and the carbon coating layer of doped nitrogen coated in the surface of the silicon carbon material particle, the silicon carbon material particle is by biomass preparation and includes the particle of silicon, carbon;Wherein in the negative electrode active material, the weight ratio of nitrogen, silicon and carbon is 1:(1-15):(20-60), by respective element;And its preparation method and corresponding secondary battery.The negative electrode active material is prepared with the carbon silicon material particle of biomass as core, on its surface, coating carbon coating layer of doped nitrogen, and limit the weight ratio of nitrogen, silicon and carbon therein, improve the structural stability of the negative electrode active material, in turn, improve the first coulomb efficiency and cycle performance of corresponding battery.

Description

Technical Field

[0001] This invention relates to the field of secondary batteries, and more specifically, to a negative electrode active material, a preparation method thereof, and a corresponding secondary battery. Background Technology

[0002] With the increasing market demand for battery capacity, developing high-capacity lithium-ion battery cathode and anode materials is a primary approach. Graphite is currently the main material for commercial lithium-ion battery anodes, with a theoretical capacity of approximately 370 mAh / g. Silicon-based materials are a promising class of lithium-ion battery anode materials, with a theoretical capacity reaching 4200 mAh / g; however, their volume expansion during lithium-ion intercalation / deintercalation exceeds 300%, resulting in low structural stability. Therefore, research on combining graphite and silicon materials to improve the capacity and stability of these materials has been ongoing.

[0003] However, silicon-based materials, especially high-purity silicon, are expensive, resulting in high production costs and limiting their further application. Summary of the Invention

[0004] Based on this, one of the objectives of this application is to provide a negative electrode active material, its preparation method, and a corresponding secondary battery. The negative electrode active material uses carbon-silicon material particles prepared from biomass as the core, and a nitrogen-doped carbon coating layer is coated on its surface. The weight ratio of nitrogen, silicon, and carbon is limited, which improves the structural stability of the negative electrode active material and thus improves the first coulombic efficiency and cycle performance of the corresponding battery.

[0005] The first aspect of this application provides a negative electrode active material comprising silicon-carbon material particles and a nitrogen-doped carbon coating layer covering the surface of the silicon-carbon material particles, wherein the silicon-carbon material particles are particles containing silicon and carbon prepared from biomass; wherein in the negative electrode active material, the weight ratio of nitrogen, silicon and carbon is 1:(1-15):(20-60), based on their respective elements.

[0006] A second aspect of this application provides a method for preparing the above-mentioned negative electrode active material, comprising:

[0007] (1) Carbonize biomass in an inert atmosphere at 500-1100℃ to obtain carbonization products;

[0008] (2) The carbonized product is washed with acid, filtered and dried;

[0009] (3) The carbonized product obtained in step (2) is mixed with magnesium powder and aluminum chloride and reduced under an inert atmosphere to obtain silicon carbide material particles.

[0010] (4) The silicon carbon material particles are mixed with nitrogen-containing materials and graphite powder in a solvent to form a uniform suspension, and then dried to obtain a composite spherical material;

[0011] (5) The composite spherical material is then sintered in an inert atmosphere at a temperature of 650-1000°C to obtain the negative electrode active material.

[0012] A third aspect of this application provides a secondary battery comprising the negative electrode active material as described above and the negative electrode active material prepared according to the method described above.

[0013] The negative electrode active material of this application uses biomass-prepared carbon-silicon material particles as the core, and coats the surface with a nitrogen-doped carbon coating layer. At the same time, the weight ratio of nitrogen, silicon and carbon is limited, which reduces the economic cost of the negative electrode active material, improves the structural stability of the negative electrode active material, and thus improves the first coulombic efficiency and cycle performance of the corresponding battery. Attached Figure Description

[0014] Figure 1 This is a scanning electron microscope image of the negative electrode active material of Preparation Example 1 of this application.

[0015] Figure 2 This is a graph showing the first charge-discharge curve of the battery in Embodiment 1 of this application. Detailed Implementation

[0016] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0018] Graphite is currently the primary material for anodes in commercial lithium-ion batteries, with a theoretical capacity of only about 370 mAh / g. Silicon-based materials are a promising class of anode materials for lithium-ion batteries, with a theoretical capacity of up to 4200 mAh / g. However, their volume expansion during lithium-ion intercalation / deintercalation exceeds 300%, resulting in low structural stability and posing significant safety risks to rechargeable batteries. Therefore, research on combining graphite and silicon materials to improve capacity and stability has been ongoing. However, silicon-based materials, especially high-purity silicon, are expensive, while biomass such as rice husks are widely available and inexpensive, but burning them in farmland can lead to safety accidents such as fires. Therefore, researchers are seeking methods to prepare silicon from biomass materials. The inventors discovered that using biomass-based carbon-silicon materials as the core and nitrogen-doped carbon as the coating layer to prepare anode active materials, while limiting the weight ratio of nitrogen, silicon, and carbon, can improve the structural stability of the anode active material and reduce its economic cost, thereby improving the initial coulombic efficiency and cycle performance of the corresponding battery.

[0019] In one embodiment, a first aspect of this application provides a negative electrode active material comprising silicon-carbon material particles and a nitrogen-doped carbon coating layer covering the surface of the silicon-carbon material particles, wherein the silicon-carbon material particles are particles containing silicon and carbon prepared from biomass; wherein in the negative electrode active material, the weight ratio of nitrogen, silicon and carbon is 1:(1-15):(20-60), based on their respective elements.

[0020] The negative electrode active material of this application uses biomass-prepared carbon-silicon material particles as the core, and coats the surface with a nitrogen-doped carbon coating layer, and limits the weight ratio of nitrogen:silicon:carbon, which improves the structural stability of the negative electrode active material and reduces its economic cost. At the same time, the nitrogen doping improves the volume expansion of silicon and carbon, thereby improving the first coulombic efficiency and cycle performance of the corresponding battery.

[0021] In some preferred embodiments, the weight ratio of nitrogen, silicon, and carbon in the negative electrode active material is 1:(1.5-6):(30-50), optionally 1:(4.3-5):(40-50), based on their respective elements. The content of nitrogen, silicon, and carbon is typically determined by ICP testing.

[0022] In some embodiments, the negative electrode active material is composed of oxygen, in addition to nitrogen, silicon, and carbon. The negative electrode active material typically contains 0-6% oxygen based on its total weight.

[0023] In some embodiments, the coating layer can be uniformly or patchily coated on the surface of silicon carbide particles, preferably uniformly coated on the surface of silicon carbide particles. When the coating layer is patchily coated on the surface of silicon carbide particles, the surface area of ​​the coating layer accounts for more than 80% of the surface area of ​​silicon carbide particles, preferably more than 95%, and more preferably more than 99%.

[0024] In some embodiments, the nitrogen content in the carbon coating is 10-50%, preferably 20-40%, more preferably 25-33%, based on the weight of the carbon coating; the carbon coating content is 1-12%, preferably 2-9%, more preferably 4-7%, based on the total weight of the negative electrode active material.

[0025] In some embodiments, the silicon-carbon material particles contain 55-75% silicon and 25-45% carbon, based on the weight of the silicon-carbon material particles; the biomass includes rice husks, straw, diatomaceous earth, diatoms, or combinations thereof.

[0026] In some embodiments, the silicon carbide material particles may optionally contain oxygen, wherein the oxygen content is 0-10% by weight, based on the weight of the silicon carbide material particles.

[0027] In some embodiments, the average volumetric particle size D of the negative electrode active material V50 The particle size is 10-35 μm, preferably 15-25 μm, and the specific surface area is 2-6 m² according to particle size analyzer measurements. 2 / g, preferably 3-4m 2 / g, measured using a specific surface area analyzer; tap density is 0.2-3 g / cm³. 3 The preferred concentration is 0.5-1.5 g / cm³. 3 Further optimization was performed using 0.8-1.2 g / cm³. 3 Measured using a tap density tester.

[0028] The second aspect of this application also provides a method for preparing the negative electrode active material of the first aspect of this application, comprising:

[0029] (1) Carbonize biomass in an inert atmosphere at 500-1100℃ to obtain carbonization products;

[0030] (2) The carbonized product is washed with acid, filtered and dried;

[0031] (3) The carbonized product obtained in step (2) is mixed with magnesium powder and aluminum chloride, and reduced under an inert atmosphere. The reduced product is washed with dilute hydrochloric acid, filtered and dried to obtain silicon carbide particles.

[0032] (4) The silicon carbon material particles are mixed with nitrogen-containing materials and graphite powder in a solvent to form a uniform suspension, and then dried to obtain a composite spherical material;

[0033] (5) The composite spherical material is then sintered in an inert atmosphere at a temperature of 650-1000°C to obtain the negative electrode active material.

[0034] In some implementations, in step (1), the biomass is first dried in an oven at 50-120°C until the weight change of the biomass within ten minutes is less than 1%, and then carbonized in an inert atmosphere at 500-1100°C for 0.5-5 hours to obtain the carbonized product.

[0035] In some embodiments, in step (2), the carbonized product is added to an excess of 0.5-2 mol / L acid solution and mixed for 10-90 minutes, preferably 20-40 minutes, then filtered, and the filtrate is dried in an oven at 50-120°C for 5-20 hours, preferably 8-16 hours. The acid may be one or more of hydrochloric acid, sulfuric acid, methanesulfonic acid, and phosphoric acid. The volume of the acid solution used is at least 10 times the volume of the carbonized product.

[0036] In some embodiments, in step (3), the carbonized product obtained in step (2) is mixed with magnesium powder and aluminum chloride by grinding in a grinding device for 0.1-1.5 hours, and then transferred to a high-pressure reactor. Under an inert atmosphere, the temperature is raised to 100-300°C, preferably 160-240°C, at a heating rate of 1-10°C / min, and reduced for 1-15 hours, preferably 3-8 hours, and more preferably 4-6 hours. The reduced product is washed with excess 1 mol / L hydrochloric acid (to ensure that the remaining magnesium powder is removed), then filtered, and the filtrate is dried in an oven at 50-120°C for 5-20 hours, preferably 8-16 hours. In some embodiments, in step (3), the weight ratio of the carbonized product obtained in step (2) to magnesium powder and aluminum chloride is 1:(0.20-0.98):(3-12), which can be selected as 1:(0.5-0.7):(5-7).

[0037] In some embodiments, step (4) includes mixing the silicon carbon material particles with nitrogen-containing materials and graphite powder in a solvent, ultrasonically dispersing them at room temperature for 0.5-5 hours, preferably 0.5-1.5 hours, and then stirring and mixing them at a temperature of 30-90°C, preferably 50-70°C, for 5-20 hours, preferably 8-16 hours, and more preferably 10-14 hours, to form a uniform suspension, and then drying them to obtain the composite spherical material.

[0038] In some implementations, in step (4), the graphite is artificial graphite or natural graphite.

[0039] In some implementations, in step (4), the mass ratio of the silicon carbide material particles to graphite is 1:(2-15), optionally 1:(3-6), and further optionally 1:(5-6).

[0040] In some embodiments, in step (4), the nitrogen-containing material is selected from melamine or dopamine, preferably melamine. The melamine has a nitrogen content of up to 66.6% and is low in cost. After carbonization, it can form a cross-linked carbon layer structure with graphite, further reducing the volume expansion of silicon and graphite, while increasing the conductivity of the negative electrode active material.

[0041] In some embodiments, in step (4), the amount of nitrogen-containing material is 1-10% of the sum of the mass of the silicon carbide material particles and graphite, preferably 3-8%, and more preferably 3-6%.

[0042] In some implementations, in step (4), the solvent includes water.

[0043] In some implementations, in step (4), the drying is a conventional technique known to those skilled in the art, preferably spray drying.

[0044] In some embodiments, in step (5), the composite spherical material is sintered in an inert atmosphere at a temperature of 650-1000°C at a heating rate of 5°C / min for 0.5-6 hours, preferably 1-3 hours, to obtain the negative electrode active material. Step (5) is preferably carried out in a tube furnace.

[0045] In some embodiments, the inert atmosphere described in steps (1) and (5) is selected from one or more of nitrogen, argon, and helium, with nitrogen being preferred.

[0046] A third aspect of this application provides a secondary battery comprising the negative electrode active material as described above and the negative electrode active material prepared according to the method described above.

[0047] Therefore, the resulting battery exhibits excellent high-temperature cycling performance.

[0048] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0049] The positive electrode sheet includes a positive current collector and a positive electrode film layer containing a positive active material coated on at least one surface of the positive current collector.

[0050] The positive electrode active material is a positive electrode active material known to those skilled in the art, such as lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, or nickel-cobalt-manganese ternary materials. The weight ratio of the positive electrode active material in the positive electrode film layer is 80-100% by weight, based on the total weight of the positive electrode film layer.

[0051] The negative electrode sheet comprises a negative current collector and a positive electrode film layer containing a negative active material coated on at least one surface of the negative current collector. The negative active material is the negative active material described in the first aspect of the present invention. The weight percentage of the negative active material in the negative electrode film layer is 70-100% by weight, based on the total weight of the negative electrode film layer.

[0052] The electrolyte typically consists of an organic solvent, a lithium electrolyte salt, and optional additives. The concentration of the electrolyte salt is typically 0.5-5 mol / L.

[0053] The separator is a material known in the art, typically a polyolefin membrane. The thickness of the separator is 6-40 μm, optionally 12-20 μm; the porosity is 30-75%, preferably 30-60%; and the average pore size is submicron, for example 100 nm-1000 nm.

[0054] Example

[0055] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0056] Unless otherwise specified, all techniques or conditions described in the examples were performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products. Unless otherwise stated, if the test conditions do not mention temperature or pressure, it is understood that the operation was carried out at room temperature or pressure.

[0057] I. Preparation Examples

[0058] Preparation Example 1:

[0059] (1) After drying 50g of rice husks in an oven at 80℃ for 12 hours, put them into a crucible and heat them in a tube furnace and nitrogen atmosphere to 800℃ for 2 hours to obtain carbonized products.

[0060] (2) Add the above carbonized product to 1L of 1mol / L dilute hydrochloric acid and mix for 30min. Filter the mixture using a vacuum filter and dry the filter in an 80℃ oven for 12h.

[0061] (3) Mix and grind 10g of the dried filtrate from step (2) with 6.4g of magnesium powder and 64g of aluminum chloride in a mortar. Transfer the mixture into a 50mL stainless steel high-pressure reactor. Under an argon atmosphere, heat the mixture to 200℃ at a heating rate of 5℃ / min and hold for 5h to obtain the reduction product. Wash the original product with 1L of 1mol / L hydrochloric acid, then filter it. Dry the filtrate in an oven at 80℃ for 12 hours to obtain silicon carbon material particles containing 65% silicon and 35% carbon, based on the weight of the silicon carbon material.

[0062] (4) 5.7g of the obtained silicon carbon material particles, 31.6g of artificial graphite, and 1.87g of melamine were added to 200mL of deionized water and ultrasonically dispersed in an ultrasonic cleaner at room temperature for 1h. The mixture was then placed in a 60℃ water bath and stirred at 200rpm in a constant temperature magnetic stirrer for 12h to obtain a uniform suspension. The obtained suspension was spray-dried using a spray dryer to prepare composite spherical materials.

[0063] (5) The composite spherical material is placed in a crucible and heated to 800°C in a tube furnace under N2 atmosphere at a heating rate of 5°C / min and held for 2 hours to obtain the negative electrode active material.

[0064] The scanning electron microscope image of the particles of the negative electrode active material is shown below. Figure 1 .

[0065] The negative electrode active material contains 2.1% nitrogen, 9.1% silicon, and 85.1% carbon, as determined by ICP testing.

[0066] The weight ratio of nitrogen:silicon:carbon is 1:4.33:40.52, calculated on an element-by-element basis.

[0067] Preparation Example 2:

[0068] The preparation of Example 1 was repeated, except that in step (4), 68.9 g of artificial graphite was used.

[0069] Preparation Example 3:

[0070] The preparation of Example 1 was repeated, except that in step (4), 19.1 g of artificial graphite was used.

[0071] Preparation Example 4:

[0072] The preparation of Example 1 was repeated, except that in step (4), melamine was replaced with dopamine.

[0073] Preparation Example 5: Changing the preparation conditions of key steps

[0074] The preparation of Example 1 was repeated, except that in step (4), each substance was added to deionized water and then placed directly into a water bath at 60°C and stirred at 200 rpm for 13 hours in a constant temperature magnetic stirrer to obtain a uniform suspension.

[0075] Preparation Example 6:

[0076] The preparation of Example 1 was repeated, except that in step (5), the temperature was raised to 700°C and held for 2 hours.

[0077] Preparation Example 7:

[0078] The preparation of Example 1 was repeated, except that in step (5), the temperature was raised to 750°C and held for 2 hours.

[0079] Preparation Example 8:

[0080] The preparation of Example 1 was repeated, except that in step (5), the temperature was raised to 900°C and held for 2 hours.

[0081] Preparation Example 9:

[0082] The preparation of Example 1 was repeated, except that an additional 50g of activated carbon powder was added in step (4).

[0083] Preparation Example 10:

[0084] The preparation of Example 1 was repeated, except that 5.7g of the obtained silicon carbide material and 1.87g of melamine were added in step (4).

[0085] Comparative Example 1

[0086] The preparation of Example 1 was repeated, except that in step (4), melamine was replaced with 1.87g of corn starch.

[0087] Comparative Example 2

[0088] The preparation of Example 1 was repeated, except that steps (4) and (5) were omitted.

[0089] Comparative Example 3:

[0090] The preparation of Example 1 was repeated, except that melamine was not added in step (4).

[0091] The different parameters of the products of the preparation examples 1-10 and comparative examples 1-3 are summarized in Table 1.

[0092] Table 1. Different parameters of the products prepared in Examples 1-10 and Comparative Examples 1-3

[0093]

[0094] II. Application Examples

[0095] Example 1

[0096] 1) Preparation of positive electrode sheet

[0097] Lithium iron phosphate (LiFePO4), conductive carbon black (SP), and PVDF binder were dispersed in NMP solvent at a weight ratio of 98:1:1 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto an aluminum foil current collector. After drying and cold pressing, a positive electrode sheet was obtained, with a coating weight of 0.27 g / 1540.25 mm². 2 .

[0098] 2) Preparation of negative electrode sheet

[0099] The negative electrode active material prepared in Example 1, the thickener sodium carboxymethyl cellulose, the binder styrene-butadiene rubber, and the conductive agent acetylene black were mixed in a mass ratio of 97:1:1:1. Deionized water was added, and the mixture was stirred in a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto copper foil. After the copper foil was dried at room temperature, it was transferred to a 120°C oven and dried for 1 hour. Then, it was cold-pressed and slit to obtain a negative electrode sheet with a coating weight of 0.17 g / 1540.25 mm². 2 .

[0100] 3) Separating membrane

[0101] A 12μm thick polypropylene separator with an average pore size of 50nm was selected.

[0102] 4) Preparation of electrolyte

[0103] The organic solvent was a mixture containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), with a volume ratio of EC:20:20:60. Thoroughly dried lithium salt LiPF6 was dissolved in the organic solvent and mixed thoroughly in an argon-atmosphere glove box with a water content of <10 ppm to obtain the electrolyte. The concentration of the lithium salt was 1 mol / L.

[0104] 5) Battery manufacturing

[0105] The positive electrode, separator, and negative electrode are stacked in sequence, then wound into a square bare cell, and then placed in an aluminum-plastic film. After baking at 80°C to remove water, 10g of the corresponding electrolyte is injected, and the cell is sealed. After processes such as standing, hot and cold pressing, formation, clamping, and capacity testing, a finished battery with a capacity of 4000mAh is obtained, with an initial discharge capacity of approximately 519mAh / g.

[0106] The initial charge-discharge curve of the battery in Example 1 is shown below. Figure 2 .

[0107] The batteries in Examples 2-10 and Comparative Examples 1-3 were prepared using methods similar to those in Example 1, but the corresponding negative electrode active materials from the preparation examples / comparative examples were used.

[0108] III. Performance Testing

[0109] The lithium-ion battery was left to stand at a constant temperature of 25°C for 2 hours, then charged at a constant current of 1C to 3.65V, and continued to be charged at a constant voltage until the charging current was less than 0.05C. The initial charging capacity was recorded. The charge was then paused for 5 minutes. The battery was then discharged at a constant current of 1C to 2.5V, and the initial discharge capacity was recorded. The charge was then paused for 5 minutes. This constituted one charge-discharge cycle of the battery. This process was repeated until the discharge capacity of the 100th cycle was recorded.

[0110] Initial coulombic efficiency = (initial discharge capacity / initial charge capacity) * 100%;

[0111] 100-cycle capacity retention = 100-cycle discharge capacity (mAh) / first discharge capacity (mAh).

[0112] The performance parameters of the batteries in each embodiment and comparative example were measured according to the above method, and the results are shown in Table 2 below.

[0113] Table 2. Properties of the batteries in each embodiment and comparative example.

[0114] Example number First Coulomb efficiency 100-week capacity retention 1 91％ 96.75％ 2 92％ 97.63％ 3 84％ 92.43％ 4 91％ 93.78％ 5 90％ 92.89％ 6 91％ 96.63％ 7 90％ 96.80％ 8 91％ 93.22％ 9 94％ 98.35％ 10 82％ 82.9％ Comparative Example 1 88％ 87.96％ Comparative Example 2 80％ 78.20％ Comparative Example 3 85％ 83.84％

[0115] Depend on Figure 1 The scanning electron microscope images show that the experiment successfully synthesized microspherical negative electrode active materials with a composite shell structure on the surface.

[0116] Depend on Figure 2 The initial charge-discharge curves of the battery show that the initial discharge capacity of the battery in Example 1 is approximately 519 mAh / g, and its initial efficiency is approximately 91%. Table 2 shows that the battery in Example 1 retains more than 94% of its discharge capacity after 100 cycles, indicating that the graphite / silicon composite material not only achieves improved initial coulombic efficiency but also exhibits considerable long-term cycling performance (capacity retention after 100 cycles).

[0117] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0118] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A negative electrode active material comprising silicon-carbon material particles and a nitrogen-doped carbon coating layer covering the surface of the silicon-carbon material particles, wherein the silicon-carbon material particles are particles containing silicon and carbon prepared from biomass; wherein in the negative electrode active material, the weight ratio of nitrogen, silicon and carbon is 1:(1-15):(20-60), based on their respective elements; The negative electrode active material is prepared by the following method: (1) Carbonize biomass in an inert atmosphere at 500-1100℃ to obtain carbonized products; the biomass includes rice husks, straw, diatomaceous earth, diatoms or combinations thereof; (2) The carbonized product is washed with acid, filtered and dried; (3) The carbonized product obtained in step (2) is mixed with magnesium powder and aluminum chloride and reduced under an inert atmosphere to obtain silicon carbide material particles. (4) The silicon carbon material particles are mixed with nitrogen-containing materials and graphite powder in a solvent to form a uniform suspension, and then dried to obtain a composite spherical material; the nitrogen-containing material is selected from melamine or dopamine; (5) The composite spherical material is sintered in an inert atmosphere at a temperature of 650-1000℃ to obtain the negative electrode active material.

2. The negative electrode active material according to claim 1, characterized in that, The nitrogen content in the carbon coating layer is 10-50% based on the weight of the carbon coating layer; the carbon coating layer content is 1-12% based on the total weight of the negative electrode active material.

3. The negative electrode active material according to claim 1 or 2, characterized in that, The silicon-carbon material particles contain 55-75% silicon and 25-45% carbon, based on the weight of the silicon-carbon material particles.

4. The negative electrode active material according to claim 1 or 2, characterized in that, The average volume particle size D of the negative electrode active material V50 Its thickness is 10-35 μm, and its specific surface area is 2-6 m². 2 / g, tap density is 0.2-3g / cm³ 3 .

5. A method for preparing the negative electrode active material according to any one of claims 1-4, comprising: (1) Carbonize biomass in an inert atmosphere at 500-1100℃ to obtain carbonized products; (2) The carbonized product is washed with acid, filtered and dried; (3) The carbonized product obtained in step (2) is mixed with magnesium powder and aluminum chloride and reduced under an inert atmosphere to obtain silicon carbide material particles. (4) The silicon-carbon material particles are mixed with nitrogen-containing materials and graphite powder in a solvent to form a uniform suspension, and then dried to obtain a composite spherical material; (5) The composite spherical material is sintered in an inert atmosphere at a temperature of 650-1000℃ to obtain the negative electrode active material.

6. The method according to claim 5, characterized in that, In step (3), the weight ratio of the carbonized product obtained in step (2) to magnesium powder and aluminum chloride is 1: (0.20-0.98): (3-12).

7. The method according to claim 5 or 6, characterized in that, In step (4), the mass ratio of the silicon carbide material particles to graphite is 1:(2-15).

8. The method according to claim 5 or 6, characterized in that, In step (4), the amount of nitrogen-containing material used is 1-10% of the sum of the mass of the silicon carbide material particles and graphite.

9. The method according to claim 5 or 6, characterized in that, The inert atmosphere described in steps (1) and (5) is selected from one or more of nitrogen, argon and helium.

10. A secondary battery comprising the negative electrode active material according to any one of claims 1-4.

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