Silicon-based negative electrode material with core-shell structure, preparation method of silicon-based negative electrode material and lithium ion battery

Abstract

The invention discloses a core-shell structure silicon-based negative electrode material, a preparation method thereof and a lithium ion battery, and the preparation method comprises the following steps: (1) depositing an amorphous silicon oxide layer on the surface of a silicon particle to obtain a first product; (2) coating the first product by utilizing a carbon source and a biomass carbon source, and calcining to obtain a second product; (3) depositing a carbon layer on the outer layer of the second product to obtain core-shell structure carbon-coated silicon nanoparticles; and (4) dispersing and mixing the core-shell structure carbon-coated silicon nanoparticles and a carbon source, and carrying out vacuum drying, roasting, crushing and screening to obtain the core-shell structure silicon-based negative electrode material. The deposition in the step (1) and the deposition in the step (3) are both LPCVD (low pressure chemical vapor deposition). The silicon surface is uniformly coated and modified, and a good ion pathway is constructed, so that the silicon-based negative electrode material which is uniform in coating, small in volume expansion, high in ion transmission capability and good in comprehensive performance is formed.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a core-shell structured silicon-based negative electrode material and a preparation method thereof, and a lithium ion battery. Background Art

[0002] Lithium-ion batteries have the advantages of high operating voltage, long cycle life, no memory effect, low self-discharge, and environmental friendliness, and they are playing an increasingly important role in electric vehicles, wearable devices, mobile communications, and large-scale energy storage. As one of the key components of lithium batteries, the negative electrode material directly affects the capacity, power, and cycle stability of the battery. The commonly used negative electrode material for lithium-ion batteries is graphite-based negative electrode material, which has a low theoretical specific capacity (372mAh·g -1 ), which is far from meeting the needs of future new generation battery research and development. Silicon-based anode has a very high theoretical specific capacity (4200mAh g -1 ) and its low delithiation potential platform is considered to be one of the most promising negative electrode materials, attracting a lot of research attention, and gradually becoming a strong competitor to replace the traditional graphite negative electrode. It is considered to be a key material for the next generation of high energy density lithium batteries.

[0003] However, although the energy density of silicon-based negative electrodes has been greatly improved, there are still many problems in their large-scale application. Silicon will have a huge volume effect during the charging and discharging process, causing the electrode material to pulverize and fall off, and the active material loses effective electrical contact, showing poor cycle stability. In addition, the direct contact between the silicon material and the electrolyte leads to the continuous growth of the interfacial SEI film. In this process, the limited electrolyte in the battery and the lithium from the positive electrode will be continuously consumed, eventually leading to a rapid decay of the battery capacity. Due to the low intrinsic conductivity of silicon, it is not conducive to the rapid transfer of electrons and high current and high rate charging and discharging, which limits the commercial application of silicon-carbon negative electrodes.

[0004] In recent years, researchers have tried to solve a series of problems faced by silicon-based negative electrodes from many aspects, such as surface coating, artificial SEI film and other surface modification technologies; constructing microstructures such as yolk-eggshell structure, porous structure and loose skeleton structure can effectively improve the electrochemical performance of silicon-based negative electrodes. For example, the patent document with publication number CN115986121B discloses a method for preparing a silicon-based negative electrode material with two coating layers, forming a first coating layer of a metal organic framework compound on the surface of silicon-based material particles, and then forming a second coating layer of carbon material on the first coating layer through ball milling. Its structure can inhibit the volume expansion of silicon while increasing conductivity. The patent document with publication number CN118448608A discloses a hollow structure carbon / lithium aluminum sulfide coated composite material, the outermost shell of which is a uniform carbon layer doped with nitrogen and copper elements, the secondary inner shell is a lithium aluminum sulfide layer, and the core structure is nano-silicon. The hollow structure and double-layer shell coating characteristics in the material structure are conducive to eliminating the huge volume effect of Si electrode materials, thereby effectively improving the electrochemical performance of electrode materials.

[0005] Although many silicon / silicon-carbon composite surface modifications have achieved certain results, the actual effect is still mediocre. Ball milling / hydrothermal coating cannot achieve uniform coating on the silicon surface. The exposed silicon still has a serious volume expansion after several cycles. At the same time, the ion transmission capacity is low, and the cycle and rate performance are limited, which still does not meet the commercial needs. In view of this, this patent application is filed. Summary of the invention

[0006] In order to solve the technical problems of uneven coating on the silicon surface, still serious volume expansion and low ion transmission capacity existing in the current silicon-based negative electrode materials mentioned above, the present invention provides a method for preparing a core-shell structured silicon-based negative electrode material, provides a negative electrode material prepared by the above preparation method, and also provides a lithium-ion battery.

[0007] The present invention adopts the following technical solutions: The first object of the present invention is to provide a method for preparing a core-shell structured silicon-based negative electrode material, comprising the following steps: (1) depositing an amorphous silicon oxide layer on the surface of silicon particles to obtain a first product; (2) using a carbon source and a biomass carbon source to coat the first product and then calcine it to obtain a second product; (3) depositing a carbon layer on the outer layer of the second product to obtain core-shell structured carbon-coated silicon nanoparticles; (4) After the core-shell structure carbon-coated silicon nanoparticles are dispersed and mixed with the carbon source, vacuum drying, calcining, crushing, and screening are performed to obtain the core-shell structure silicon-based negative electrode material; The deposition in step (1) and step (3) is LPCVD deposition.

[0008] like Figure 1 As shown in the present invention, firstly, an amorphous silicon oxide layer (i.e., Figure 1 The LPCVD amorphous oxide layer in the electrolyte has good covering ability, high uniformity of deposition, and high deposition rate. The dense amorphous oxide layer formed can promote the formation of a stable SEI film at the electrode / electrolyte interface; then the first product is coated by combining a carbon source with a biomass carbon source to form a layer of porous carbon-coated silicon particles as the second product. The carbon coating in this layer can slow down the volume expansion of the silicon negative electrode. At the same time, the carbon coating using amorphous porous carbon can also enhance the ion transmission ability and reduce the impedance. After calcination, a three-dimensional conductive network can be formed; then LPCVD is used to uniformly deposit a carbon layer on the outer layer of the second product, and finally it is mixed with the carbon source for calcination, so as to form a coated carbon layer (i.e. Figure 1 The LPCVD carbon coating in the process can greatly reduce the specific surface area of ​​the material, slow down the side reaction between the electrolyte and the material, and enhance the mechanical strength of the entire silicon particle negative electrode, thereby achieving a high compaction density. The core-shell structure has a three-dimensional carbon skeleton inside, which still has good ion passage at high compaction density and tap density, thereby improving the cycle performance of silicon-based negative electrode materials.

[0009] As a preferred design, during the deposition in step (1), silane and oxygen are used as reaction gases, and an inert gas is introduced into the deposition chamber for deposition, so that a layer of oxide is uniformly deposited on the surface of the silicon particles; The deposition temperature is 500-1000° C., the silane flow rate is 100-200 sccm, the oxygen flow rate is 50-100 sccm, and the inert gas flow rate is 400-500 sccm. Nitrogen is preferably selected as the inert gas.

[0010] As a preferred design, before the deposition in step (1), the silicon particles are cleaned according to the RCA standard (a wet chemical cleaning method), cleaned with deionized water, and then immersed in hydrofluoric acid and then vacuum dried; The hydrofluoric acid concentration is 1-3%, the immersion time is 30-60s, the vacuum drying temperature is 60-90°C, and the drying time is 12-24h.

[0011] As a preferred design, the process of step (2) is: Mixing a quinone monomer solution with an amine monomer solution to obtain a quinoneamine prepolymer; Then, adding biomass carbon fiber to the quinoneamine prepolymer and dispersing it evenly to obtain a dispersed solution, and soaking the first product in the dispersed solution; The material is then allowed to dry, and the dried material is calcined to obtain a second product.

[0012] As a preferred design, the quinone monomer is any one of benzoquinone, naphthoquinone, and anthraquinone, such as p-benzoquinone; the solvent selected in the quinone monomer is any one of ethanol, methanol, and ether, and the mass concentration of the quinone monomer is 4-12 g / L; The amine monomer is any one of phenylenediamine, naphthylamine and anthraceneamine, such as o-phenylenediamine and p-phenylenediamine; the solvent selected in the amine monomer is any one of ethanol, methanol and ether, and the mass concentration of the amine monomer is 10-18 g / L; The biomass carbon fiber element is one or more of renewable cotton, bamboo pulp, paper pulp, and methyl cellulose, and the mass of the biomass carbon fiber element is 0.5-3g.

[0013] In the present invention, quinone amine prepolymer formed by quinone and amine monomers is used as the carbon source, which has a planar structure, while biomass carbon fiber has a linear structure. The surface-line combination greatly enhances the ion transmission capacity of the material, and a three-dimensional conductive structure can be formed after calcination. At high compaction density and tap density, it still has good ion passage, thereby improving the cycle performance of silicon-based negative electrode materials.

[0014] As a preferred design, the calcination temperature in step (2) is 300-800°C, and the calcination is followed by grinding and crushing to obtain a second product.

[0015] As a preferred design, in step (3), when depositing the carbon layer on the outer layer of the second product, acetylene gas is used as the reaction gas and nitrogen is used as the inert gas; The acetylene flow rate is 100-200 sccm, the deposition pressure is 0.2-0.7 Torr, and the deposition time is 30-120 min.

[0016] As a preferred design, the time for dispersing and mixing the silicon nanoparticles and the carbon source in step (4) is 3-8 hours, and the temperature for vacuum drying is 60-100°C; The calcination temperature is 800-1200℃, the heating rate is 2-20℃ / min, and the high temperature holding time is 1-3h; The carbon source used in step (4) is any one of natural graphite, artificial graphite, and mesophase carbon microspheres.

[0017] The second object of the present invention is to provide a core-shell structured silicon-based negative electrode material, which is prepared by any of the preparation methods described above.

[0018] The third object of the present invention is to provide a lithium-ion battery, whose negative electrode sheet includes the core-shell structure silicon-based negative electrode material as described above, or includes the core-shell structure silicon-based negative electrode material obtained by the preparation method as described in any of the above items.

[0019] Compared with the prior art, the present invention has the following advantages and beneficial effects: The silicon-based negative electrode material provided by the present invention has a multi-layer core-shell structure, the inner layer is high-capacity micron silicon, an amorphous oxide layer is formed on the silicon surface, and a stable SEI film is formed at the electrode / electrolyte interface. The middle layer is coated with silicon by a carbon source and a biomass carbon source, which slows down the volume expansion and forms a three-dimensional conductive network. The outermost layer is then uniformly deposited with a carbon layer to greatly reduce the material specific surface area, slow down the side reaction between the electrolyte and the material, and ensure the stability of the material structure. The silicon-based negative electrode material with this structure of the present invention reduces the expansion of the pole piece by 20% in lithium-ion battery applications, and the cycle performance of the silicon-based negative electrode is improved by 300 cycles (capacity retention rate 80%). BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings: Figure 1 A schematic flow chart of a method for preparing a silicon-based negative electrode material provided by the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The illustrative embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention. Example

[0022] A method for preparing a core-shell structured silicon-based negative electrode material is provided, which adopts the following process: (1) Clean the nano-silicon particles using the RCA standard to remove organic matter, inorganic salts, metal ions, etc. on their surface. After RCA cleaning, rinse thoroughly with deionized water, then soak in 2% hydrofluoric acid for 60 seconds to remove the surface oxide layer, and then dry in a vacuum oven at 80°C for 24 hours to obtain the treated nano-silicon; (2) Place the treated nano-silicon into a quartz tube in the deposition chamber of the LPCVD equipment, draw a vacuum in the furnace chamber, set the LPCVD deposition temperature to 800°C, introduce silane and oxygen as reaction gases, and deposit nitrogen as an inert gas in the chamber. The silane flow rate is 150 sccm, the oxygen flow rate is 80 sccm, and the nitrogen flow rate is 400 sccm. Cool and obtain the first product.

[0023] (3) Measure 100 ml of anhydrous ethanol in a beaker, weigh 0.6 g of p-benzoquinone powder and add it to the anhydrous ethanol, and stir evenly; then measure 30 ml of anhydrous ethanol in another beaker, weigh 0.5 g of o-phenylenediamine powder and add it to the ethanol, and stir evenly. Use a dropper to drop the o-phenylenediamine-ethanol mixed solution into the p-benzoquinone-ethanol solution, continue stirring, and after complete dropwise addition, a black quinoneamine prepolymer is formed. Then, weigh 1 g of renewable cotton and add it to the quinoneamine prepolymer. Ultrasonicate to evenly disperse the cotton. Place the first product in a beaker, use a dropper to slowly drop the ultrasonically dispersed solution onto the first product material, so that the solution completely soaks the powdered first product, and let it stand; then place the material that has been left to stand in a muffle furnace, heat it to 700 ° C under nitrogen protection, calcine it for 2-3 hours, cool it to room temperature, grind it, and obtain the second product.

[0024] (4) The second product is placed in a quartz tube in the deposition chamber of the LPCVD equipment, the chamber is vacuumed, acetylene gas is introduced as a reaction gas, and nitrogen is deposited in the chamber as an inert gas. The acetylene flow rate is 150 sccm, the deposition pressure is 0.6 Torr, and the deposition time is 60 min. After the deposition is completed and cooled, a core-shell carbon-coated nano-silicon material is obtained.

[0025] (5) The obtained core-shell carbon-coated nano-silicon material and graphite are fully dispersed and mixed in an ethanol solvent, and then dried in a vacuum drying oven. The dried mixed material is placed in a muffle furnace and calcined at 1000°C for 2 h in a nitrogen atmosphere. After crushing and screening, a silicon-carbon negative electrode material is obtained. Example

[0026] The difference between this embodiment and embodiment 1 is that when a carbon layer is deposited on the outer layer of the second product in step (4), the LPCVD deposition pressure is 0.4 Torr, and the rest is the same as embodiment 1. Example

[0027] The difference between this embodiment and embodiment 1 is that when depositing a carbon layer on the outer layer of the second product in step (4), the LPCVD deposition time is 120 min, and the rest is the same as embodiment 1. Example

[0028] The difference between this embodiment and embodiment 1 is that when the amorphous silicon oxide layer is deposited on the surface of the silicon particles in step (2), the LPCVD deposition temperature is set to 600° C., and the rest is the same as embodiment 1. Example

[0029] The difference between this embodiment and embodiment 1 is that when the amorphous silicon oxide layer is deposited on the surface of the silicon particles in step (2), the LPCVD deposition temperature is 900° C., and the rest is the same as embodiment 1. Example

[0030] The difference between this embodiment and embodiment 1 is that the added amount of p-benzoquinone powder is 1.2 g, and the rest is the same as embodiment 1. Example

[0031] The difference between this embodiment and embodiment 1 is that the added amount of o-phenylenediamine powder is 0.3 g, and the rest is the same as embodiment 1. Example

[0032] The difference between this embodiment and embodiment 1 is that the added amount of p-benzoquinone powder is 1.2 g, the added amount of cotton is 3 g, and the rest is the same as embodiment 1. Example

[0033] The difference between this embodiment and embodiment 1 is that the amount of p-benzoquinone powder added is 1.2 g, the amount of o-phenylenediamine powder added is 0.3 g, the amount of cotton added is 3 g, and the rest is the same as embodiment 1.

[0034] Comparative Example 1: The difference between this comparative example and Example 1 is that no amorphous silicon oxide layer is deposited on the surface of the silicon particles, that is, the silicon particles are not oxidized, and the rest is the same as Example 1.

[0035] Comparative Example 2: The difference between this comparative example and Example 1 is that no carbon source and biomass carbon coating are performed, and the rest is the same as Example 1.

[0036] Comparative Example 3: The difference between this comparative example and Example 1 is that after depositing the amorphous silicon oxide layer on the surface of the silicon particles, only carbon source coating is performed without adding biomass carbon fiber for coating, and the rest is the same as Example 1.

[0037] Comparative Example 4: The difference between this comparative example and Example 1 is that carbon deposition is not performed on the outer layer of the second product, that is, steps (4) and (5) in Example 1 are not performed, and the rest are the same as Example 1.

[0038] Comparative Example 5: The difference between this comparative example and Example 1 is that after obtaining the second product, the method used for carbon coating on the outside is a fusion coating method.

[0039] Specifically, the second product and asphalt are put into a fusion machine for fusion, the fused materials are carbonized at 800°C for 2 hours and then cooled to room temperature, and the rest of the process for obtaining the first product and the second product is the same as in Example 1.

[0040] Comparative Example 6: The difference between this comparative example and Example 1 is that the nano silicon particles are cleaned using the RCA standard to remove organic matter, inorganic salts, metal ions, etc. on the surface thereof, and then thoroughly cleaned with deionized water after RCA cleaning, and then immersed in 2% hydrofluoric acid for 60s, and then dried in a vacuum oven at 80°C for 24h. A certain amount of silicon powder is weighed and placed in a crucible and transferred to a high-purity argon atmosphere tube furnace, and the temperature is raised to 100°C at a heating rate of 2°C / min, and the argon flow rate is maintained at 200sccm. At the same time, the oxygen valve is opened, and oxygen is introduced at a flow rate of 50sccm for 1h, the oxygen valve is closed and heating is stopped, and the first product is obtained after natural cooling, and the rest is the same as Example 1.

[0041] Performance Test: The negative electrode sheets were prepared using the materials obtained in each embodiment and comparative example as raw materials.

[0042] Specifically, the dried negative electrode sheet was cut into discs with a diameter of 12 mm to obtain a silicon-based negative electrode sheet suitable for assembling the battery, which was weighed and the mass of the active material was calculated. In an inert atmosphere glove box, a metal lithium sheet was used as the counter electrode, LiPF6 was used as the electrolyte, the solvent was ethylene carbonate and diethyl carbonate (EC:EMC=3:7) solution, and the electrolyte was prepared with 8% fluoroethylene carbonate (FEC), and polypropylene (PP) was used as the diaphragm to assemble the Si / C metal Li half-cell. After standing for 8 hours, the electrochemical performance was tested under 0.1C charge and discharge. The results are shown in Table 1.

[0043] Table 1

[0044] As can be seen from Table 1, the deposition temperature, deposition pressure and deposition time in the LPCVD deposition technology all have an impact on the performance of the negative electrode material, and the concentration of the carbon source and biomass carbon is also closely related to the battery performance. In Examples 1 to 5, the preparation technology of the present invention is adopted to change the temperature, pressure and deposition time in the deposition process, and the negative electrode materials obtained have good overall performance in terms of the volume expansion rate of the pole piece, the first charge and discharge efficiency and the capacity retention rate. In Examples 6 to 9, the concentration of the carbon source and biomass carbon is changed, and good performance is also achieved in terms of the volume expansion rate of the pole piece, the first charge and discharge efficiency and the capacity retention rate. Among them, Example 1 of the present invention has a relatively obvious leading advantage in terms of the volume expansion rate of the pole piece, the first charge and discharge efficiency and the capacity retention rate.

[0045] Compared with Example 1, Example 1 does not oxidize the inner silicon, and no stable SEI film is formed on the surface. Silicon is in direct contact with the electrolyte, resulting in lower first efficiency and poor cycle performance. Compared with Example 1, Example 2 does not carry out carbon source and biomass carbon coating, and the volume expansion is large. At the same time, the core does not form a three-dimensional ion path, and the material conductivity is poor, reflecting poor cycle performance. Compared with Example 1, Comparative Example 3 does not add biomass carbon cellulose coating, and the volume expansion is larger than the coating with the addition of carbon source and biomass carbon. It shows that the biomass carbon source and the carbon source combined coating have a better effect of reducing volume expansion for silicon-based materials. At the same time, the two jointly form a three-dimensional ion path, and a good conductive network is constructed by combining the surface and the line, which enhances the ion transmission capacity and reflects excellent cycle performance. Compared with Example 1, Comparative Example 4 does not carry out the outermost carbon deposition, showing lower first efficiency and cycle performance, indicating that the outermost carbon deposition can reduce the contact between the electrolyte and the silicon material, avoid the occurrence of side reactions in the cycle, further improve the stability of the material structure, and prolong the life of the battery cell. Compared with Example 1, in Comparative Example 5, the outermost layer is carbon-coated by the fusion method, which shows lower first efficiency and cycle performance, indicating that the use of the fusion method results in uneven carbon coating of the outermost layer, which is partially exposed to the outside and contacts with the electrolyte during the cycle, resulting in too many side reactions during the cycle and poor cycle performance. Compared with Example 1, in Comparative Example 6, a tube furnace is used to pass gas to oxidize the silicon surface. Since the gas passed through the tube furnace causes uneven oxidation of the silicon powder, the silicon powder placed in the lower layer is not fully oxidized. The uneven oxidation results in no formation of a stable SEI film, and the silicon is in direct contact with the electrolyte, resulting in lower first efficiency and poor cycle performance. At the same time, the silicon powder on the surface is too fully exposed to oxygen, which makes the oxide layer too thick, resulting in reduced activity of nano-silicon and affecting the overall capacity.

[0046] In summary, the present invention uniformly coats and modifies the silicon surface, constructs a good ion path, forms an SEI film through LPCVD deposition technology, and forms a carbon source and biomass carbon source co-coating, and the outer carbon coating cooperates with each other to form a silicon-based negative electrode material with good overall performance, uniform silicon surface coating, small volume expansion, and high ion transmission capacity. The silicon-based negative electrode material of this structure in the present invention reduces the expansion of the pole piece by 20% in the application of lithium-ion batteries, and the cycle performance of the silicon-based negative electrode is improved by 300 cycles (capacity retention rate 80%).

[0047] The above specific implementation methods further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation methods of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a core-shell structured silicon-based negative electrode material, characterized in that: The following steps are involved: (1) depositing an amorphous silicon oxide layer on the surface of silicon particles to obtain a first product; (2) using a carbon source and a biomass carbon source to coat the first product and then calcine it to obtain a second product; (3) depositing a carbon layer on the outer layer of the second product to obtain core-shell structured carbon-coated silicon nanoparticles; (4) After the core-shell structure carbon-coated silicon nanoparticles are dispersed and mixed with the carbon source, vacuum drying, calcining, crushing, and screening are performed to obtain the core-shell structure silicon-based negative electrode material; The deposition in step (1) and step (3) is LPCVD deposition.

2. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 1, characterized in that: During the deposition in step (1), silane and oxygen are used as reaction gases, and an inert gas is introduced into the deposition chamber for deposition, so that a layer of oxide is uniformly deposited on the surface of the silicon particles; The deposition temperature is 500-1000° C., the silane flow rate is 100-200 sccm, the oxygen flow rate is 50-100 sccm, and the inert gas flow rate is 400-500 sccm.

3. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 1, characterized in that: Before deposition in step (1), the silicon particles are cleaned according to RCA standards, cleaned with deionized water, immersed in hydrofluoric acid, and then vacuum dried; The hydrofluoric acid concentration is 1-3%, the immersion time is 30-60s, the vacuum drying temperature is 60-90°C, and the drying time is 12-24h.

4. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 1, characterized in that: The process of step (2) is: Mixing a quinone monomer solution with an amine monomer solution to obtain a quinoneamine prepolymer; Then, adding biomass carbon fiber to the quinoneamine prepolymer and dispersing it evenly to obtain a dispersed solution, and soaking the first product in the dispersed solution; The material is then allowed to dry, and the dried material is calcined to obtain a second product.

5. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 4, characterized in that: The quinone monomer is any one of benzoquinone, naphthoquinone and anthraquinone, the solvent selected in the quinone monomer is any one of ethanol, methanol and ether, and the mass concentration of the quinone monomer is 4-12 g / L; The amine monomer is any one of phenylenediamine, naphthylamine and anthraceneamine, the solvent selected in the amine monomer is any one of ethanol, methanol and ether, and the mass concentration of the amine monomer is 10-18 g / L; The biomass carbon fiber element is one or more of renewable cotton, bamboo pulp, paper pulp, and methyl cellulose, and the mass of the biomass carbon fiber element is 0.5-3g.

6. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 4, characterized in that: The calcination temperature in step (2) is 300-800°C, and the calcination is followed by grinding and crushing to obtain a second product.

7. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 1, characterized in that: In step (3), when a carbon layer is deposited on the outer layer of the second product, acetylene gas is used as a reaction gas and nitrogen gas is used as an inert gas; The acetylene flow rate is 100-200 sccm, the deposition pressure is 0.2-0.7 Torr, and the deposition time is 30-120 min.

8. The method for preparing a core-shell structured silicon-based negative electrode material according to claim 1, characterized in that: In step (4), the time for dispersing and mixing the silicon nanoparticles and the carbon source is 3-8 hours, and the temperature for vacuum drying is 60-100°C; The calcination temperature is 800-1200℃, the heating rate is 2-20℃ / min, and the high temperature holding time is 1-3h; The carbon source used in step (4) is any one of natural graphite, artificial graphite, and mesophase carbon microspheres.

9. A core-shell structured silicon-based negative electrode material, prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: The negative electrode sheet includes the core-shell structure silicon-based negative electrode material as claimed in claim 9, or includes the core-shell structure silicon-based negative electrode material prepared by the preparation method as claimed in any one of claims 1 to 8.

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