CN107785541B

CN107785541B - Silicon-carbon composite material for lithium ion battery and preparation method thereof

Info

Publication number: CN107785541B
Application number: CN201610750365.5A
Authority: CN
Inventors: 张和宝; 王岑; 李喆; 叶兰; 丁烨
Original assignee: Amprius Nanjing Co ltd
Current assignee: Boselis Hefei Co ltd; Bosellis Nanjing Co ltd
Priority date: 2016-08-29
Filing date: 2016-08-29
Publication date: 2021-06-01
Anticipated expiration: 2036-08-29
Also published as: CN107785541A

Abstract

The invention relates to a silicon-carbon composite material for a lithium ion battery and a preparation method thereof, wherein the silicon-carbon composite material is a secondary particle structure formed by uniformly dispersing and embedding silicon materials on the surface of a graphite material and between the graphite materials; the graphite material and the silicon material on the surface and inside of the secondary particles are coated with a layer of amorphous carbon, the graphite materials forming the secondary particles are mutually randomly oriented, and the secondary particles are isotropically oriented. Through the steric hindrance and the adhesive effect of the high molecular polymer, the floating and agglomeration of the silicon material and the layering of the silicon material and graphite are effectively inhibited, and the silicon material is uniformly dispersed in the graphite flake and effectively compounded with the graphite. The silicon-carbon composite material prepared by the method is used for the lithium ion battery, has the advantages of high efficiency, small expansion and good circulation, and the method has simple process and is easy to realize industrial production.

Description

Silicon-carbon composite material for lithium ion battery and preparation method thereof

Technical Field

The invention relates to the technical field of lithium ion batteries, in particular to a silicon-carbon composite material for a lithium ion battery and a preparation method thereof.

Background

The lithium ion battery has wide application as an energy storage element, and as a material for a negative electrode, the theoretical specific capacity of graphite is only 372 mAh/g. Nowadays, with the demand of increasing the energy density of lithium ion batteries, the actual specific capacity of graphite negative electrodes has been developed to the limit, and the development potential thereof is extremely limited.

The theoretical specific capacity of the silicon material reaches 4200mAh/g, and the silicon material has huge reserve in the crust and is a candidate material of the next generation of high-energy density lithium ion battery. However, when the silicon is inserted and removed from the lithium, the volume of the silicon expands and contracts by 300%, and the silicon is pulverized, falls off and loses activity in the charge and discharge cycle process of the battery, so that the application of the silicon in the lithium ion battery is limited. The key difficulty is that the volume expansion of silicon during lithium intercalation is inhibited, and the cycle of the silicon cathode is improved. At present, silicon is subjected to nanocrystallization and is compounded with a carbon material, so that the expansion and cycle performance of a silicon cathode can be effectively improved.

Chinese patent CN 103682287A discloses a method for preparing a silicon-based composite material, which comprises the steps of firstly grinding graphite into hollows, embedding nano-silicon particles between the hollows, coating an organic carbon precursor by fusion and VC mixing, carrying out isotropic pressure treatment and carbonizing to obtain the silicon-based composite material. The preparation method has complex process and harsh technological conditions, and industrial production is difficult to realize. The silica-based material is extruded to be compact, and the rate capability of the material is greatly limited.

Chinese patent CN 102891297A discloses a silicon-carbon composite material and a preparation method thereof, the composite material is a graphite, asphalt and nano-silicon composite structure, and a nano-level silicon-carbon composite material precursor is obtained by adding graphite, asphalt and micron silicon into an aqueous solution of sodium carboxymethylcellulose for ball milling. And carrying out spray drying and carbonization on the precursor to obtain the silicon-carbon composite material. According to the method, sodium carboxymethylcellulose is used as the adhesive, so that the pulverization phenomenon caused by the silicon in the charging and discharging processes is prevented, and the cycle performance of the silicon is effectively improved. However, ball milling and spray drying granulation are carried out in a sodium carboxymethylcellulose aqueous solution, and during high-energy ball milling, a sodium carboxymethylcellulose long molecular chain is easily broken, so that the viscosity of a slurry system is rapidly reduced, nano silicon and graphite flakes are layered, and uniform dispersion of the nano silicon and the graphite flakes is not realized in the ball milling process. Secondly, the asphalt is an oily material and is not infiltrated with water, so that the uniform compounding of silicon and carbon is difficult to realize. And thirdly, the nano silicon without an oxide layer on the surface has stronger hydrophobicity, is spray-dried in a pure water system, and is easy to float to the surface through a graphite gap due to the absence of steric hindrance and adhesion of a high-molecular polymer, so that the nano silicon is agglomerated, and after the nano silicon is manufactured into a battery, local over-expansion, pulverization and poor cycle performance are easily caused.

Chinese patent CN 104425802A discloses a preparation method of a silicon-based composite material. The method mixes the base material with the conductive agent and carries out granulation by a spray drying mode. The base material used is the mechanical mixing of emulsified asphalt, graphite and silicon powder. Similarly, when the method is used for spray drying by using water-based slurry, nano silicon particles are easy to float and agglomerate, and the circulation performance is deteriorated.

In the existing system, the nano silicon is unevenly dispersed along with the agglomeration and upward floating of the nano silicon in the compounding process of the nano silicon and graphite, so that the lithium ion battery prepared from the existing silicon material has low first efficiency, poor circulation and large expansion. Therefore, the key point of preparing the silicon-carbon composite material taking graphite and nano silicon as matrixes lies in how to inhibit the agglomeration and floating of the nano silicon and control the uniform dispersion of the nano silicon in a graphite system.

Disclosure of Invention

The invention aims to provide a silicon-carbon composite material for a lithium ion battery, wherein silicon materials are uniformly dispersed in a graphite sheet, and the composite material is used for the lithium ion battery and has the performances of high efficiency, small expansion and good cycle.

In order to achieve the purpose, the technical scheme provided by the invention is as follows:

a silicon-carbon composite material for a lithium ion battery is a secondary particle structure formed by uniformly dispersing and embedding a silicon material on the surface of a graphite material and between the graphite materials; the graphite material and the silicon material on the surface and inside of the secondary particles are coated with a layer of amorphous carbon, the graphite materials forming the secondary particles are mutually randomly oriented, and the secondary particles are isotropically oriented.

The median diameter of the secondary particles is between 2 and 60 mu m, preferably between 4 and 30 mu m, and more preferably between 5 and 15 mu m.

The median particle diameter of the single graphite sheet in the graphite material is 1-15 μm, and the length of the graphite sheet is preferably 1-10 μm.

The median diameter of the silicon particles in the silicon material is between 0.01 and 5 mu m, preferably, the median diameter of the silicon particles is between 0.02 and 1 mu m, and more preferably, the median diameter of the silicon particles is between 0.05 and 0.5 mu m.

The thickness of the amorphous carbon layer is between 0.001 and 2 μm, preferably between 0.002 and 0.1 μm, and more preferably between 0.005 and 0.05 μm.

In the silicon-carbon composite material, the graphite content is 10-99 wt%, preferably, the graphite content is 50-80 wt%.

The silicon content is 0.01 to 80 wt%, preferably 0.1 to 40 wt%, more preferably 5 to 35 wt%.

The amorphous carbon content is 1 to 50 wt%, preferably 15 to 40 wt%.

The invention provides a preparation method of the silicon-carbon composite material for the lithium ion battery, which is characterized by comprising the following steps of: the method comprises the following steps:

(1) carrying out wet grinding on a graphite material and a silicon material respectively with a dispersant and a solvent to obtain graphite slurry and silicon slurry, and mixing the two slurries to obtain graphite/silicon mixed slurry; or carrying out wet grinding on the graphite material and the silicon material, the dispersant and the solvent simultaneously to obtain graphite/silicon mixed slurry;

(2) preparing a high molecular polymer solution, dissolving a first carbon precursor by using a solvent, adding the dissolved first carbon precursor and the high molecular polymer solution into the slurry prepared in the step (1), and carrying out wet grinding to obtain graphite/silicon/high molecular polymer/first carbon precursor mixed slurry; or dissolving the first carbon precursor by using a solvent, adding the dissolved first carbon precursor and high polymer powder into the slurry prepared in the step (1) together, and carrying out wet grinding to obtain graphite/silicon/high polymer/first carbon precursor mixed slurry;

(3) drying and granulating the mixed slurry obtained in the step (2), and then carrying out high-temperature carbonization treatment in a non-oxidizing atmosphere;

(4) performing second carbon precursor coating treatment on the product obtained in the step (3), and then performing high-temperature carbonization in a non-oxidizing atmosphere;

(5) and (4) crushing, screening and demagnetizing the product obtained in the step (4) to obtain the silicon-carbon composite material.

Wherein, in step (1):

the graphite material is one or the combination of at least two of artificial graphite, natural graphite, surface-coated natural graphite, expanded graphite, conductive graphite and mesocarbon microbeads;

the silicon material is crystalline silicon or amorphous silicon; the silicon material is silicon nano-particles, silicon nano-wires, silicon nano-tubes, silicon nano-rods, silicon nano-cones, silicon micro-particles, silicon micro-rods and silicon micro-wires;

the wet grinding adopts any one of a high-speed stirring mill, a ball mill, a tube mill, a cone mill, a rod mill or a sand mill; preferably, the wet grinding employs a sand mill. The sanding of the sand mill is carried out in a high-energy mode, and the effective linear speed is 10-15 m/s.

The solvent used for wet grinding is an organic solvent; preferably, the organic solvent is one or a combination of at least two of methanol, ethanol, isopropanol, N-butanol, ethylene glycol, diethyl ether, acetone, N-methylpyrrolidone, methyl butanone, tetrahydrofuran, benzene, toluene, xylene, N-dimethylformamide, N-dimethylacetamide, or chloroform.

The dispersant used for wet grinding is one or the combination of at least two of sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate, cetyl trimethyl ammonium bromide, polyacrylic acid, polyvinylpyrrolidone and polysorbate-80.

Wherein, in step (2):

the high molecular polymer is one or a combination of at least two of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinylpyrrolidone, hydroxymethyl cellulose, sodium hydroxymethyl cellulose, hydroxyethyl cellulose, methyl hydroxyethyl cellulose, ethyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, gelatin, carrageenan, pectin, propylene glycol alginate, alginic acid, sodium alginate, lithium alginate and xanthan gum;

in the high molecular polymer, the molecular weight of polyacrylic acid, sodium polyacrylate and lithium polyacrylate is preferably 400,000-80,000,000, the molecular weight of carboxymethyl cellulose and sodium carboxymethyl cellulose is preferably 100,000-1,000,000, the molecular weight of pectin is preferably 50,000-150,000, the molecular weight of gelatin is preferably 10,000-300,000, the molecular weight of xanthan gum is preferably 200,000-5,000,000, and the molecular weight of alginic acid, sodium alginate and lithium alginate is preferably 10,000-600,000.

The high molecular polymer accounts for 0.5-10 wt% of the solid in the graphite/silicon mixed slurry, and preferably accounts for 1-5 wt%.

The first carbon precursor is one or a combination of at least two of glucose, sucrose, chitosan, starch, citric acid, coal pitch, petroleum pitch, mesophase pitch, phenolic resin, tar, naphthalene oil, anthracene oil, polyvinyl chloride, polystyrene, polyvinylidene fluoride, polyethylene oxide, polyvinyl alcohol, epoxy resin, polyacrylonitrile and polymethyl methacrylate, or the high molecular polymer in the step (2);

the solvent for dissolving the first carbon precursor is one or the combination of at least two of water, methanol, ethanol, isopropanol, N-butanol, ethylene glycol, diethyl ether, acetone, N-methylpyrrolidone, methyl butanone, tetrahydrofuran, benzene, toluene, xylene, N-dimethylformamide, N-dimethylacetamide and chloroform;

the wet grinding adopts any one of a high-speed stirring mill, a ball mill, a tube mill, a cone mill, a rod mill and a sand mill; preferably, the wet grinding is performed by a sand mill; the sanding of the sand mill is carried out at a low speed on the premise of not damaging the macromolecular molecular chain structure, and the effective linear speed of the sand mill is 2-5 m/s.

In the grinding process, the size of silicon particles is reduced, more fresh surfaces without oxide layers are exposed, the hydrophobicity is stronger, and small silicon particles are easy to float upwards and are layered with graphite. The wet grinding is carried out in an organic solvent system, so that the phenomenon can be effectively relieved. The high molecular polymer is added in the form of solution or powder, the rotating speed of a sand mill is reduced, the molecular chain structure of the high molecular polymer is not damaged, the high molecular polymer is uniformly dispersed in a graphite/silicon/organic solvent system, a huge network structure is formed on the surfaces of graphite and silicon, the floating and agglomeration of silicon are further inhibited by utilizing the steric hindrance effect of the high molecular polymer, and the effective compounding of the silicon and the graphite is realized.

Wherein, in step (3):

the drying mode adopts spray drying, and the spray drying equipment adopts a spray dryer;

the temperature of the high-temperature carbonization reaction is 500-1400 ℃;

the heating time of the high-temperature carbonization is 0.5-24 hours;

the non-oxidizing atmosphere in the high-temperature carbonization process is provided by at least one of the following gases: nitrogen, argon, hydrogen, helium, neon, or krypton.

In the spray drying process, the spray head of the spray dryer atomizes the slurry into small droplets, the dispersion medium in the droplets is rapidly evaporated at the interface of the droplets and hot air under the high-temperature condition, small-sized silicon particles tend to diffuse to the surfaces of the droplets along with the dispersion medium, and the silicon particles are enriched on the surfaces of secondary particles after the dispersion medium is dried by distillation. Before spray drying, high molecular polymer is added into a slurry system to form a huge high molecular network structure, and silicon particles are firmly locked in graphite gaps by virtue of steric hindrance and bonding effects of the high molecular network structure, so that the silicon particles are not easy to diffuse and migrate along with a dispersion medium in the spray drying process, and a structure that the silicon particles are uniformly dispersed in the interior and the outer surface of secondary particles is formed.

Wherein, in step (4):

the second carbon precursor coating equipment adopts any one of a mechanical fusion machine, a VC mixer or a high-speed dispersion machine;

the second carbon precursor is one or a combination of more of coal pitch, petroleum pitch, mesophase pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile and polymethyl methacrylate; if a VC mixer and a high-speed dispersion machine are adopted for coating treatment, a solvent capable of dissolving a carbon precursor can be selected to improve the coating effect, and the selected solvent is at least two combinations of water, methanol, ethanol, isopropanol, N-butanol, ethylene glycol, diethyl ether, acetone, N-methylpyrrolidone, tetrahydrofuran, benzene, toluene, xylene, N-dimethylformamide, N-dimethylacetamide or trichloromethane;

the reaction temperature of the high-temperature carbonization is 500-1400 ℃; preferably, the high-temperature carbonization temperature is 700-1000 ℃;

the heating time of high-temperature carbonization is 0.5-24 hours;

the non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen, helium, neon, or krypton.

The invention also protects the lithium ion battery cathode material prepared by the silicon-carbon composite material, the lithium ion battery cathode prepared by the lithium ion battery cathode material, and the lithium ion battery prepared by the lithium ion battery cathode.

Has the advantages that:

the invention adopts high molecular polymer, which is added during low-speed grinding and uniformly dispersed in a graphite/silicon system, and silicon particles are still uniformly dispersed in graphite gaps by virtue of the steric hindrance and the bonding effect of a network structure formed by the high molecular polymer. The obtained secondary particles are of a structure with uniformly dispersed nano-silicon particles, so that the problems that the electrode plate part area is excessively expanded due to the enrichment of nano-silicon on the surface of the secondary particles, an SEI film is repeatedly damaged and re-formed, the electrolyte is excessively quickly consumed, and the battery cycle is poor are solved. The silicon-carbon composite material prepared by the invention and the method have simple process steps and are easy to realize industrial production.

Drawings

Fig. 1 is a schematic view of the silicon-carbon composite material.

Fig. 2 is a Scanning Electron Microscope (SEM) photograph of the silicon carbon composite prepared in example 1 under back scattering.

Fig. 3 is an SEM photograph of the silicon carbon composite prepared in example 2 under back scattering.

Fig. 4 is an X-ray diffraction pattern of the silicon carbon composite material prepared in example 1.

Fig. 5 is a cycle curve at 0.5C of the full cell prepared in example 1.

Fig. 6 is an SEM photograph of the silicon carbon composite prepared in comparative example 1.

Fig. 7 is an SEM photograph of the silicon carbon composite prepared in comparative example 1.

Detailed Description

The present invention will be further described with reference to the following specific examples.

Example 1

317g of artificial graphite with the median particle size of 19 microns, 422g of nano silicon powder with the median particle size of 200nm, 2400g of ethanol and 22.17g of sorbitan-80 are weighed and added into a sand mill to be ball-milled for 4 hours by using zirconia balls with the median particle size of 0.8mm, and the linear speed of the sand mill is 14m/s, so that graphite/silicon mixed slurry with the median particle size of 5 microns is obtained. 739g of a PAA-Na solution having a molecular weight of 500,000 was weighed and the solid content was diluted from 1 wt% to 0.1 wt%. 222g of starch are weighed and dissolved in 2000g of water. Adding 0.1 wt% of PAA-Na slurry and a starch solution into a sand mill, further diluting the slurry to 5 wt% by using deionized water, reducing the linear speed of the sand mill to 3m/s, and continuing ball milling for 1h to obtain graphite/silicon/starch/PAA-Na mixed slurry. And (3) carrying out spray drying treatment on the slurry, wherein the air inlet temperature of a spray dryer is 150 ℃, the air outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 400Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 15 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 900 ℃ under the argon atmosphere, and carbonizing starch at the heating rate of 5 ℃/min to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. 740g of the composite material and 317g of the coal pitch are weighed and added into a VC mixer to be mixed for 10min, then the mixture is added into a fusion machine to be fused for 30min, the material coated by the pitch is heated to 900 ℃ in the inert atmosphere of argon to be carbonized for 2h, and the carbonized material is naturally cooled to room temperature and then crushed and sieved to obtain the silicon/graphite composite material secondarily coated by the amorphous carbon.

The silicon-carbon composite material prepared above was characterized using the following equipment, which was used in the following examples.

The particle size distribution of the silicon-carbon composite material is tested by a Dandongbott BetterSize 2000 laser particle size analyzer.

And observing the surface morphology of the silicon-carbon composite material by using a Hitachi SU8010 scanning electron microscope.

And testing the crystal structure of the silicon-carbon composite material by using a Rigaku MiniFlex 600X-ray diffractometer.

The natural graphite and silicon-carbon composite material, the thickening agent and the binder are uniformly mixed according to the ratio of 87:10:1.5:1.5, and the mixture is coated on a copper foil to prepare the electrode. The obtained negative electrode sheet and lithium sheet were assembled into a CR2016 type half cell, and the capacity and discharge efficiency of the half cell were tested using a CT2001A type test apparatus, product of wuhan blue electronics gmbh. The capacity of the half cell reaches 473.7mAh/g, the first charge-discharge efficiency is 93.0%, and the expansion rate of the negative plate after ten-week circulation is 49.0%.

The obtained negative pole piece is cut, vacuum-baked, wound together with the matched positive pole piece and the diaphragm and filled into an aluminum plastic shell with corresponding size, then a certain amount of electrolyte is injected and sealed, so that a complete silicon-containing negative pole lithium ion full battery can be obtained, and the full battery capacity and the discharge efficiency are tested by adopting BTS79 testing equipment of New Will electronics Limited company, Shenzhen. The energy density of the full cell reaches 718Wh/L, the capacity retention rate is 85.1 percent after 500 times of charging and discharging, and the expansion of the cell is 8.0 percent.

In the following examples, the obtained negative electrode sheets were fabricated into half cells and full cells in the same manner as in example 1, and the specific capacities and the charge and discharge efficiencies of the half cells and the full cells were tested on the same equipment.

Example 2

633g of natural graphite with the median particle size of 19 mu m, 90g of silicon nanowire with the median particle size of 200nm, 2400g of ethanol and 25g of hexadecyl trimethyl ammonium bromide are weighed and added into a sand mill to be ball-milled for 4 hours by using zirconia balls with the median particle size of 0.8mm, and the linear velocity of the sand mill is 14 m/s. The graphite/silicon mixed slurry with the median particle size of 5 mu m is obtained. 833g of a PAA-Na solution having a molecular weight of 1,000,000 are weighed out and the solids content is diluted from 1% to 0.5%. 250g of sucrose was weighed and dissolved in 2000g of water. Adding the PAA-Na solution and the glucose solution into a sand mill, further diluting the slurry to 5 wt% by using deionized water, reducing the linear speed of the sand mill to 3m/s, and continuing ball milling for 1h to obtain the mixed slurry of graphite/silicon/sucrose/PAA-Na. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 150 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 400Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 15 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace, carbonizing at 900 ℃ for 2h under the argon atmosphere, and carbonizing sucrose at the heating rate of 5 ℃/min to obtain the graphite/silicon composite material bonded and coated by the amorphous carbon. 700g of the composite particles and 174g of petroleum asphalt were mechanically mixed in a VC mixer for 10 minutes, and then treated in a mechanical fusion machine for 30 minutes. And (3) heating the obtained composite material to 1000 ℃ in an argon inert atmosphere, carbonizing for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the graphite/silicon composite material secondarily coated by the amorphous carbon.

The natural graphite, the artificial graphite, the silicon-carbon composite material, the thickening agent and the binder are uniformly mixed according to the ratio of 31:31:35:1.5:1.5, and the mixture is coated on a copper foil to prepare the pole piece. Half cells and full cells were prepared in the same manner as in example 1 and tested for electrochemical properties.

The capacity of the half cell is 466.8mAh/g, the first charge-discharge efficiency is 92.1%, and the expansion rate of the negative plate after ten-week circulation is 46.5%. The energy density of the full-cell test is 712Wh/L, the capacity retention rate is 85.0 percent after 500 times of charge and discharge, and the expansion of the cell is 7.6 percent.

Example 3

Weighing 475g of expanded graphite with the median particle size of 21 microns, 309g of silicon nanocone with the median particle size of 500nm, 2400g of ethylene glycol and 25g of polyvinylpyrrolidone, adding into a sand mill, and ball-milling for 6 hours by using a zirconia ball with the median particle size of 0.8mm, wherein the linear speed of the sand mill is 15m/s, so as to obtain graphite/silicon mixed slurry with the median particle size of 4 microns. 250g of phenolic resin was weighed and dissolved in 2000g of ethylene glycol. Weighing 3.92g of pectin powder with the molecular weight of 10,000, adding the pectin powder and a phenolic resin solution into a sand mill, further diluting the slurry to 5 wt% by using ethylene glycol, reducing the linear speed of the sand mill to 3m/s, and continuing ball milling for 1h to obtain the graphite/silicon/phenolic resin/pectin mixed slurry. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 150 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 500Hz, and the feeding speed is 80 g/min. Spray drying gave spherical or ellipsoidal secondary particles having a median particle size of 10 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace, carbonizing for 2h at 1000 ℃ under the argon atmosphere, and carbonizing phenolic resin at the heating rate of 5 ℃/min to obtain the graphite/nano silicon composite material bonded and coated by the amorphous carbon. 784g of the composite particles are taken, 320g of coal pitch is taken, 1280g N and N-Dimethylacetamide (DMAC) are added, mechanical mixing is carried out for 10 minutes by a VC mixer, vacuumizing is carried out, air in the equipment is replaced by nitrogen, the equipment is heated to 200 ℃, stirring is carried out for 30 minutes, the rotating speed of the equipment is reduced, vacuum is started until the materials are dried, and the materials are cooled to room temperature. And (3) heating the obtained asphalt-coated silicon/graphite/amorphous carbon composite particles to 1100 ℃ in an argon inert atmosphere, carbonizing for 3 hours, naturally cooling to room temperature, crushing and sieving to obtain the graphite/silicon composite material secondarily coated by the amorphous carbon.

The natural graphite, the silicon-carbon composite material, the thickening agent and the binder are uniformly mixed according to the ratio of 62:35:1.5:1.5, the mixture is coated on copper foil to prepare a pole piece, and a half battery and a full battery are prepared and tested for electrochemical performance in the same method as the embodiment 1.

The prepared half cell has the capacity of 637.5mAh/g, the first charge-discharge efficiency of 89.1 percent and the expansion rate of the negative plate after ten-week circulation of 70.7 percent. The energy density of the full cell is 783Wh/L, the capacity retention rate after 500 times of charge and discharge is 81.6%, and the cell expansion is 11.5%.

Example 4

317g of surface-coated natural graphite with the median particle size of 20 mu m, 500g of N-methyl pyrrolidone (NMP) and 12.5g of PVP are weighed and added into a sand mill, and are ball-milled for 2 hours by using zirconia balls with the diameter of 0.8mm to obtain graphite slurry with the median particle size of 8 mu m, wherein the linear speed of the sand mill is 10 m/s. 385g of micron silicon powder with the median particle size of 10 microns, 600g of NMP and 12.5g of PVP are weighed and added into a sand mill, the rotation speed of the sand mill is 3000rpm, 0.8mm of zirconia balls are used for ball milling for 1 hour to obtain silicon slurry with the median particle size of 1 micron, the silicon slurry is added into the graphite slurry and uniformly mixed to obtain the graphite/silicon mixed slurry. Weighing 7020g of sodium carboxymethylcellulose (CMC-Na) slurry with the molecular weight of 500 ten thousand and the solid content of 1 percent, adding the slurry into a sand mill, further diluting the slurry to 5 percent by weight by using deionized water, reducing the rotating speed of the sand mill to 3m/s, and continuing ball milling for 1h to obtain the silicon/graphite/CMC-Na mixed slurry. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 180 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 350Hz, and the feeding speed is 100 g/min. Spray drying to obtain spherical or ellipsoidal secondary particles with a median particle size of 20 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 900 ℃ under the argon atmosphere, and carbonizing CMC-Na at the heating rate of 5 ℃/min to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. 43g of phenolic resin is weighed and added into 100g of DMF, and the phenolic resin is dissolved in the DMF by ultrasonic stirring. 70g of the above graphite sheet/silicon particle/amorphous carbon composite powder was added with stirring, the linear velocity of the dispersion plate was raised to 6m/s, and the temperature of the stirring vessel was raised to 150 ℃. The dispersion was continued for 10min after the temperature reached 150 ℃. Then the temperature was raised to 200 ℃ and stirring was slowly maintained until the DMF was completely evaporated to dryness. And (3) heating the material coated with the phenolic resin to 900 ℃ in an argon inert atmosphere, carbonizing for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the silicon/graphite composite material secondarily coated with the amorphous carbon.

Uniformly mixing the artificial graphite, the silicon-carbon composite material, the thickening agent and the binder according to the ratio of 87:10:1.5:1.5, and coating the mixture on a copper foil to prepare the pole piece. Half cells and full cells were prepared and tested for electrochemical properties according to the procedure of example 1.

The prepared half-cell has the capacity of 457.4mAh/g, the first charge-discharge efficiency of 93.2 percent and the expansion rate of the negative plate after ten-week circulation of 51.7 percent. The energy density of the full cell is 715Wh/L, the capacity retention rate after 500 times of charge and discharge is 84.7%, and the cell swelling is 8.4%.

Example 5

633g of conductive graphite with the median particle size of 4 mu m, 341g of nanotubes with the median particle size of 200nm, 2400g of DMF and 29g of CTAB are weighed and added into a sand mill to be ball-milled for 4 hours by using 0.4mm of zirconia balls, and the linear speed of the sand mill is 15m/s, so that silicon/graphite mixed slurry with the median particle size of 2 mu m is obtained. 292g of coal tar pitch was weighed and added to 2000g of DMF to prepare a coal tar pitch suspension. Weighing 9.74g of PAA-Na powder with the molecular weight of 500,000, dissolving the PAA-Na powder in ethanol to prepare 1 wt% solution, uniformly mixing the PAA-Na solution and the coal tar pitch suspension, further diluting the slurry to 5 wt% by using DMF, reducing the rotation speed of a sand mill to 3m/s, and continuing ball milling for 1h to obtain the mixed slurry of silicon/graphite/coal tar pitch/PAA-Na. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 150 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 500Hz, and the feeding speed is 50 g/min. Spray drying to obtain spherical or ellipsoidal secondary particles with a median particle size of 5 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 1100 ℃ under the argon atmosphere, and at the heating rate of 5 ℃/min, so that the coal pitch is carbonized to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. Weighing 30g of petroleum asphalt, adding into 150g of DMAC, and ultrasonically stirring to prepare a petroleum asphalt suspension. 100g of the above graphite sheet/silicon particle/amorphous carbon composite powder was added with stirring, the linear velocity of the dispersion plate was raised to 6m/s, and the temperature of the stirring vessel was raised to 160 ℃. The dispersion was continued for 10min after the temperature reached 160 ℃. The temperature was then raised to 200 ℃ and slow stirring was maintained until the DMAC was completely evaporated to dryness. And (3) preserving the heat of the petroleum asphalt coated material for 2 hours at 400 ℃ in an inert atmosphere of argon, then heating to 1100 ℃ for carbonization for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the amorphous carbon secondary coated silicon/graphite composite material.

The natural graphite, the MCMB, the silicon-carbon composite material, the thickening agent and the binder are uniformly mixed according to the ratio of 41:41:15:1.5:1.5, and the mixture is coated on a copper foil to prepare the electrode. The electrochemical performance of the half-cell and full-cell test materials were prepared according to the method of example 1.

The half-cell test capacity is 481.7mAh/g, the first efficiency is 92.5%, and the expansion rate of the negative plate after ten-week circulation is 35.8%. The energy density of the full cell is 719Wh/L, the capacity retention rate after 500 times of charge and discharge is 84.2%, and the expansion is 5.8%.

Example 6

633g of mesocarbon microbeads with the median particle size of 19 microns, 200g of micron silicon with the median particle size of 8 microns, 2400g of ethanol and 25g of polyvinylpyrrolidone are weighed and added into a sand mill to be ball-milled for 1.5 hours by using zirconia balls with the median particle size of 0.8mm, and the linear speed of the sand mill is 14m/s, so that silicon/graphite slurry with the median particle size of 10 microns is obtained. Weighing 83.3g of sodium alginate powder, adding the sodium alginate powder into a sand mill, diluting the slurry to 5 wt% by using deionized water, reducing the linear velocity of the sand mill to 3m/s, and continuing ball milling for 1h to obtain the graphite/silicon/sodium alginate mixed slurry. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 150 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 350Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 25 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace, carbonizing at 900 ℃ for 2h under the argon atmosphere, heating at the rate of 5 ℃/min, and carbonizing sodium alginate to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. 600g of the composite particles and 122g of petroleum asphalt are taken, mechanically mixed for 10 minutes by a VC mixer, and then treated for 30 minutes by a mechanical fusion machine. And (3) heating the obtained graphite flake/silicon particle/amorphous carbon composite particle coated with the asphalt to 900 ℃ in an argon inert atmosphere, carbonizing for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the silicon/graphite composite material coated with the amorphous carbon secondarily.

And uniformly mixing the natural graphite, the silicon-carbon composite material, the thickening agent and the binder according to the ratio of 77:20:1.5:1.5, and coating the mixture on a copper foil to prepare the pole piece. Half cells and full cells were prepared according to the method of example 1 to test their electrochemical properties.

The capacity of the half cell is 478.6mAh/g, the first charge-discharge efficiency is 92.6%, and the expansion rate of the negative plate after ten-week circulation is 62.3%. The energy density of the full cell is 710Wh/L, the capacity retention rate after 500 times of charge-discharge cycles is 84.2%, and the expansion of the cell is 10.1%.

Example 7

158g of artificial graphite having a median particle size of 19 μm, 200g of N-methylpyrrolidone (NMP) and 4.74g of PVP were weighed into a sand mill, and ball-milled for 4 hours with 0.8mm zirconia balls at a linear velocity of 14m/s to obtain a graphite slurry having a median particle size of 5 μm. 791g of micron silicon rod with the median particle size of 5 microns, 1000g of NMP and 23.73g of PVP are weighed and added into a sand mill, 0.8mm of zirconia balls are used for ball milling for 1 hour, the linear speed of the sand mill is 14m/s, silicon slurry with the median particle size of 500nm is obtained, and the silicon slurry is added into the graphite slurry and mixed uniformly. 285g of citric acid was weighed and dissolved in deionized water. Weighing 9.49g of hydroxyethyl cellulose powder, adding the hydroxyethyl cellulose powder and a citric acid solution into a sand mill, further diluting the slurry to 5 wt% by using deionized water, reducing the rotation speed of the sand mill to 600rpm, and continuing ball milling for 1h to obtain the silicon/graphite/citric acid/hydroxyethyl cellulose mixed slurry. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 180 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 400Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 15 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 900 ℃ under the argon atmosphere, and carbonizing citric acid at the heating rate of 5 ℃/min to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. 10g of polyvinyl alcohol is weighed and added into 150g of ethylene glycol, and the polyvinyl alcohol is dissolved in the ethylene glycol by ultrasonic stirring. 95g of the above graphite sheet/silicon particle/amorphous carbon composite powder was added with stirring, the linear velocity of the dispersion plate was raised to 6m/s, and the temperature of the stirring vessel was raised to 150 ℃. After the temperature reaches 150 ℃, the dispersion is continued for 10min, and the stirring is slowly kept until the glycol is completely evaporated to dryness. And (3) heating the polyvinyl alcohol coated material to 900 ℃ in an inert atmosphere of argon gas, carbonizing for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the amorphous carbon secondary coated silicon/graphite composite material.

Uniformly mixing the artificial graphite, the silicon-carbon composite material, the thickening agent and the binder according to the ratio of 92:5:1.5:1.5, and coating the mixture on a copper foil to prepare the pole piece. Half cells and full cells were prepared and tested for electrochemical properties according to the procedure of example 1.

According to the test of the prepared half-cell, the capacity reaches 466.9mAh/g, the first charge-discharge efficiency is 93.1%, and the expansion rate of the negative plate after ten-week circulation is 47.8%. The energy density of the full cell is 718Wh/L, the capacity retention rate after 500 times of charge and discharge is 84.4%, and the battery swelling is 7.8%.

Example 8

633g of artificial graphite with the median particle size of 19 microns, 200g of nano silicon powder with the median particle size of 200nm, 2400g of ethanol and 25g of polyvinylpyrrolidone are weighed and added into a sand mill to be ball-milled for 4 hours by using a zirconia ball with the median particle size of 0.8mm, and the linear velocity of the sand mill is 14m/s, so that graphite/silicon slurry with the median particle size of 5 microns is obtained. 833g of a gelatin solution having a relative molecular weight of 300,000 are weighed out and the solids content is diluted from 1% to 0.1% by weight. 250g of glucose was weighed and dissolved in 2000g of water. Adding 0.1 wt% of PAA-Na solution and glucose solution into a sand mill, further diluting the slurry to 5 wt% with water, reducing the linear velocity of the sand mill to 4m/s, and continuing ball milling for 1h to obtain the mixed slurry of graphite/silicon/glucose/PAA-Na. And (3) carrying out spray drying treatment on the slurry, wherein the air inlet temperature of a spray dryer is 150 ℃, the air outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 400Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 15 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 900 ℃ under the argon atmosphere, and carbonizing glucose at the heating rate of 5 ℃/min to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. 17g of coal tar pitch was added to 70g N, N-Dimethylformamide (DMF), and stirred with ultrasound to form a partially soluble stable suspension of coal tar pitch in DMF. 84g of the above graphite sheet/silicon particle/amorphous carbon composite powder was added with stirring, the linear velocity of the dispersion board was raised to 6m/s, and the temperature of the stirring vessel was raised to 150 ℃. The dispersion was continued for 10min after the temperature reached 150 ℃. Then the temperature was raised to 200 ℃ and stirring was slowly maintained until the DMF was completely evaporated to dryness. And (3) heating the asphalt-coated material to 900 ℃ in an argon inert atmosphere, carbonizing for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the amorphous carbon secondary-coated silicon/graphite composite material.

Uniformly mixing the artificial graphite, the silicon-carbon composite material, the thickening agent and the binder according to the ratio of 77:20:1.5:1.5, and coating the mixture on a copper foil to prepare the pole piece. Half cells and full cells were prepared according to the method of example 1 to test their electrochemical properties.

The capacity of the half cell is 480.4mAh/g, the first charge-discharge efficiency is 92.2%, and the expansion rate of the negative plate after ten-week circulation is 49.0%. The energy density of the full cell is 713Wh/L, the capacity retention rate after 500 times of charge and discharge is 84.6, and the battery swelling is 8.0%.

Example 9

633g of Mesophase Carbon Microbeads (MCMB) with the median particle size of 21 microns, 3g of micron silicon powder with the median particle size of 10 microns, 1200g of ethanol and 19g of polyvinylpyrrolidone are weighed and added into a sand mill, the linear speed of the sand mill is 12m/s, and 1mm of zirconia balls are used for ball milling for 1h to obtain graphite/silicon mixed slurry with the median particle size of 15 microns. 190g of glucose was weighed and dissolved in 1000g of deionized water. 633g of PAA-Na solution with the molecular weight of 40,000 and the solid content of 1 percent is weighed and added into a sand mill together with the glucose solution, the slurry is further diluted to 5 percent by weight by using glycol, the linear velocity of the sand mill is reduced to 3m/s, and the ball milling is continued for 1h, so that the mixed slurry of graphite/silicon/glucose/PAA-Na is obtained. And (3) carrying out spray drying treatment on the slurry, wherein the inlet air temperature of a spray dryer is 150 ℃, the outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 250Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 50 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 900 ℃ under the argon atmosphere, and carbonizing glucose at the heating rate of 5 ℃/min to obtain the graphite/silicon composite material bonded and coated by the amorphous carbon. And (2) taking 633g of the composite particles, taking 30g of coal tar pitch, adding into 1000g of DMF, mechanically mixing for 10 minutes by using a VC mixer, vacuumizing, replacing air in the equipment with nitrogen, heating the equipment to 200 ℃, continuously stirring for 30 minutes, then reducing the rotation speed of the equipment, starting vacuum until the materials are dried, and cooling to room temperature. And (3) heating the obtained asphalt-coated silicon/graphite/amorphous carbon composite particles to 1100 ℃ in an argon inert atmosphere, carbonizing for 3 hours, naturally cooling to room temperature, crushing and sieving to obtain the graphite/silicon composite material secondarily coated by the amorphous carbon.

The obtained silicon-carbon composite material, the thickening agent and the binder are uniformly mixed according to the ratio of 97:1.5:1.5, coated on a copper foil to prepare a pole piece, and a half battery and a full battery are prepared and tested for electrochemical performance in the same method as the embodiment 1.

The prepared half-cell test shows that the capacity is 378.2mAh/g, the first charge-discharge efficiency is 94.8%, and the expansion rate of the negative plate after ten-week circulation is 37.1%. The energy density of the whole battery is 681Wh/L, the capacity retention rate is 86.0% after 500 times of charge and discharge, and the battery swelling is 6.0%.

Comparative example 10

106g of natural graphite with the median particle size of 20 microns, 500g of ethanol and 3.18g of PVP are weighed and added into a sand mill, the linear speed of the sand mill is 14m/s, and the graphite slurry with the median particle size of 10 microns is obtained by ball milling for 1.5h through a zirconia ball with the diameter of 0.8 mm. 528g of micron silicon wires with the median particle size of 5 microns, 800g of ethanol and 15.84g of PVP are weighed and added into a sand mill, the linear speed of the sand mill is 14m/s, 0.8mm of zirconia balls are used for ball milling for 1h to obtain silicon slurry with the median particle size of 1 micron, and the silicon slurry is added into the graphite slurry and mixed uniformly. Weighing 3170g of xanthan gum solution with the molecular weight of 200 ten thousand and the solid content of 1%, adding the xanthan gum solution into a sand mill, further diluting the slurry to 5 wt% by using deionized water, reducing the linear speed of the sand mill to 3m/s, and continuing ball milling for 1h to obtain the graphite/silicon/xanthan gum mixed slurry. And (3) carrying out spray drying treatment on the slurry, wherein the air inlet temperature of a spray dryer is 180 ℃, the air outlet temperature is 100 ℃, the rotating speed of a rotary atomizing nozzle is 350Hz, and the feeding speed is 100 g/min. Spray drying to give spherical or ellipsoidal secondary particles having a median particle size of 25 μm. And (3) putting the spherical or ellipsoidal secondary particles into an electric heating furnace to carbonize for 2h at 900 ℃ under the argon atmosphere, and carbonizing xanthan gum at the heating rate of 5 ℃/min to obtain the graphite/nano-silicon composite material bonded and coated by the amorphous carbon. 42g of coal tar pitch is weighed and added into 100g of DMF, and the mixture is stirred by ultrasound, so that the coal tar pitch can be well dissolved and dispersed in the DMF to form stable suspension. 63g of the above graphite sheet/silicon particle/amorphous carbon composite powder was added with stirring, the linear velocity of the dispersion plate was raised to 6m/s, and the temperature of the stirring vessel was raised to 150 ℃. The dispersion was continued for 10min after the temperature reached 150 ℃. Then the temperature was raised to 200 ℃ and stirring was slowly maintained until the DMF was completely evaporated to dryness. And (3) heating the asphalt-coated material to 900 ℃ in an argon inert atmosphere, carbonizing for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the amorphous carbon secondary-coated silicon/graphite composite material.

And uniformly mixing the artificial graphite, the silicon-carbon composite material and the binder according to the proportion of 89:8:1.5:1.5, and coating the mixture on a copper foil to prepare the pole piece. Half cells and full cells were prepared and tested for electrochemical properties according to the procedure of example 1.

According to the test of the prepared half-cell, the capacity reaches 472.2mAh/g, the first charge-discharge efficiency is 92.9%, and the expansion rate of the negative plate after ten-week circulation is 62.3%. The energy density of the full cell is 711Wh/L, the capacity retention rate after 500 times of charge and discharge is 84.3%, and the battery swelling is 10.1%.

Comparative example 1

A silicon carbon composite was prepared in substantially the same manner as in example 1, except that: no PAA-Na slurry was added during the sanding. A battery was produced in the same manner as in example 1.

According to a half-cell test, the capacity is 475.6mAh/g, the first charge-discharge efficiency is 92.5%, and the expansion rate of the negative plate after ten-week circulation is 55.0%. The energy density of the full cell is 713Wh/L, the capacity retention rate after 500 times of charge and discharge is 76.3 percent, and the expansion rate is 8.9 percent. Comparative example 2

A silicon carbon composite was prepared in substantially the same manner as in example 2, except that: the solvent is changed into deionized water in the sanding process, and the subsequent whole process is carried out under a deionized water system. A battery was produced in the same manner as in example 2.

According to a half-cell test, the capacity is 469.0mAh/g, the first charge-discharge efficiency is 91.9%, and the expansion rate of the negative plate after ten-week circulation is 52.5%. The energy density of the full cell was 712Wh/L, the capacity retention rate after 500 charges and discharges was 77.1%, and the expansion rate was 8.5%. Comparative example 3

A silicon carbon composite was prepared in substantially the same manner as in example 5, except that: adding PAA-Na slurry when sanding the graphite and the silicon particles, and carrying out high-energy ball milling in the whole process. A battery was produced in the same manner as in example 5.

According to a half-cell test, the capacity is 484.0mAh/g, the first charge-discharge efficiency is 92.1%, and the expansion rate of the negative plate after ten-week circulation is 41.8%. The energy density of the full-cell is 714Wh/L, the capacity retention rate after 500 times of charge and discharge is 76.5%, and the expansion rate is 6.8%.

Comparative example 4

Mixing natural graphite, a thickening agent and a conductive agent uniformly according to a ratio of 97:1.5:1.5, and coating the mixture on a copper foil to prepare the pole piece. Half cells and full cells were prepared and tested for electrochemical properties in the same manner as in example 1.

The test capacity of the half cell is 370.0mAh/g, the first charge-discharge efficiency is 94.2%, and the expansion rate of the negative plate after ten-week circulation is 29.0%. The energy density of the full cell is 678Wh/L, the capacity retention rate after 500 cycles is 87.0%, and the expansion of the cell is 6.0%. The data of examples 1 to 10 and comparative examples 1 to 3 are shown in Table 1.

TABLE 1

The above description is only a preferred embodiment of the present invention, and should not be taken as limiting the invention in any way, and any person skilled in the art can make any simple modification, equivalent replacement, and improvement on the above embodiment without departing from the technical spirit of the present invention, and still fall within the protection scope of the technical solution of the present invention.

Claims

1. The silicon-carbon composite material for the lithium ion battery is characterized in that: the silicon-carbon composite material is a secondary particle structure formed by uniformly dispersing and embedding a silicon material on the surface of a graphite material and between the graphite materials; the graphite material and the silicon material on the surface and inside of the secondary particles are coated with a layer of amorphous carbon, the graphite materials forming the secondary particles are mutually randomly oriented, and the secondary particles are isotropically oriented;

the preparation method of the silicon-carbon composite material for the lithium ion battery comprises the following steps:

(1) respectively carrying out wet grinding on a graphite material and a silicon material with a dispersant and an organic solvent to obtain graphite slurry and silicon slurry, and mixing the two slurries to obtain graphite/silicon mixed slurry; or carrying out wet grinding on the graphite material and the silicon material, the dispersant and the organic solvent simultaneously to obtain graphite/silicon mixed slurry;

(3) drying and granulating the mixed slurry obtained in the step (2), and then carbonizing at 500-1400 ℃ in a non-oxidizing atmosphere;

(4) performing second carbon precursor coating treatment on the product obtained in the step (3), and then carbonizing at 500-1400 ℃ in a non-oxidizing atmosphere;

(5) crushing, screening and demagnetizing the product obtained in the step (4) to obtain the silicon-carbon composite material;

wherein the effective linear velocity of wet grinding in the step (1) is 10-15 m/s, and the effective linear velocity of wet grinding in the step (2) is 2-5 m/s;

the high molecular polymer is one or the combination of at least two of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, hydroxymethyl cellulose, sodium hydroxymethyl cellulose, hydroxyethyl cellulose, methyl hydroxyethyl cellulose, ethyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, gelatin, carrageenan, pectin, propylene glycol alginate, alginic acid, sodium alginate, lithium alginate and xanthan gum.

2. The silicon-carbon composite material for a lithium ion battery according to claim 1, wherein: the median particle size of the secondary particles is between 2 and 60 mu m, the median particle size of a single graphite sheet in the graphite material is between 1 and 15 mu m, the median particle size of silicon particles in the silicon material is between 0.01 and 5 mu m, and the thickness of the amorphous carbon layer is between 0.001 and 2 mu m.

3. The silicon-carbon composite material for a lithium ion battery according to claim 1, wherein: in the silicon-carbon composite material, the content of graphite is 50-80 wt%, the content of silicon is 0.01-80 wt%, and the content of amorphous carbon is 15-40 wt%.

4. A preparation method of a silicon-carbon composite material for a lithium ion battery is characterized by comprising the following steps: the method comprises the following steps:

5. The method for preparing the silicon-carbon composite material for the lithium ion battery according to claim 4, wherein the method comprises the following steps:

in the step (1):

the graphite material is one or the combination of at least two of artificial graphite, natural graphite, surface-coated natural graphite, expanded graphite and conductive graphite;

the silicon material is crystalline silicon or amorphous silicon;

the wet grinding adopts any one of a high-speed stirring mill, a ball mill, a tube mill, a cone mill, a rod mill or a sand mill;

the dispersant used for wet grinding is one or the combination of at least two of cetyl trimethyl ammonium bromide, polyacrylic acid, polyvinylpyrrolidone and polysorbate-80.

6. The method for preparing the silicon-carbon composite material for the lithium ion battery according to claim 4, wherein the method comprises the following steps:

in the step (2):

the high molecular polymer accounts for 0.5-10 wt% of the solid in the graphite/silicon mixed slurry;

the wet grinding adopts any one of a high-speed stirring mill, a ball mill, a tube mill, a cone mill, a rod mill and a sand mill.

7. The method for preparing the silicon-carbon composite material for the lithium ion battery according to claim 4, wherein the method comprises the following steps: in the step (3):

the heating time of the carbonization treatment is 0.5-24 hours;

8. The preparation method of the silicon-carbon composite material for the lithium ion battery according to any one of claims 4 to 7, characterized by comprising the following steps: in the step (4):

the second carbon precursor is one or a combination of more of coal pitch, petroleum pitch, mesophase pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile and polymethyl methacrylate;

the heating time of the carbonization treatment is 0.5-24 hours;

9. A lithium ion battery negative electrode, characterized in that: the silicon-carbon composite material is prepared by adopting the silicon-carbon composite material as defined in any one of claims 1-3.

10. A lithium ion battery, characterized by: prepared using the lithium ion battery negative electrode of claim 9.