CN111348647A - Silicon-carbon composite material with multi-layer coating structure and preparation method thereof - Google Patents
Silicon-carbon composite material with multi-layer coating structure and preparation method thereof Download PDFInfo
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- H01M4/36—Selection of substances as active materials, active masses, active liquids
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- H01M4/386—Silicon or alloys based on silicon
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- H—ELECTRICITY
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Abstract
The invention discloses a multi-level cladding structure silicon-carbon composite material, which belongs to the technical field of multi-level cladding structure silicon-carbon composite materials, and particularly relates to a multi-level cladding structure silicon-carbon composite material, which can realize self-assembly of secondary particles and uniform dispersion of nano-silicon in particle gaps of graphite secondary granulation based on a cross-scale modification technology of a silicon-based cathode, can realize multi-level carbon layer cladding through fine regulation and structural design of asphalt with different softening points, effectively buffer volume expansion of silicon, and assist bonding of secondary particles in a drying process, realizes distribution of nano-silicon in the gaps of graphite matrix secondary granulation and surface cladding modification through fine screening and targeted regulation of the asphalt with different softening points, can effectively isolate direct contact of silicon and electrolyte, reduce generation of side reactions, and effectively reduce consumption of active lithium ions and the electrolyte in a battery circulation process, and improves the coulombic efficiency and the cycle retention rate of the material.
Description
Technical Field
The invention relates to the technical field of multi-level cladding structure silicon-carbon composite materials, in particular to a multi-level cladding structure silicon-carbon composite material and a preparation method thereof.
Background
In recent years, due to the dual pressure of energy crisis and environmental pollution, the importance of exploring and developing new energy sources capable of sustainable development is increasingly prominent. Meanwhile, the application of electrochemical energy storage devices represented by secondary batteries in the fields of new energy vehicles, power grid energy storage, digital products and the like is also a key point of attention in the industry. How to further improve the energy density of the lithium battery is always the leading research direction of the lithium battery industry. The traditional graphite material is the most widely applied negative electrode material in the current lithium battery, however, the theoretical capacity of the graphite is only 372mAh/g, and in order to further improve the energy density of the lithium battery, a novel negative electrode material with higher specific capacity must be developed. The theoretical specific capacity of the silicon is up to 4200mAh/g, and the silicon has lower lithium intercalation potential. Therefore, the silicon-based material is considered to be a novel high specific capacity anode material with great prospect.
However, silicon has a volume expansion of up to 400% upon intercalation of lithium, and a brittle SEI film is easily broken, resulting in direct exposure of silicon to an electrolyte. In the charge-discharge cycle process, new exposed silicon surfaces continuously contact with the electrolyte and react with the lithium ions along with the insertion and the removal of the lithium ions, and the consumption of active lithium ions and the electrolyte is continuously caused, which is expressed by low coulombic efficiency of the battery and continuous reduction of capacity. The invention provides a cross-scale modification method of a silicon-carbon composite material by a multi-level carbon coating layer generated by coking asphalt with different softening points, which effectively solves the technical problems of two silicon-based cathodes, such as self-assembly granulation of graphite secondary particles and uniform dispersion of nano-silicon in gaps of secondary particle matrixes, through fine regulation and control of the structural design of a three-dimensional composite material, thereby avoiding direct contact of silicon and electrolyte, effectively reducing consumption of active lithium ions and the electrolyte in the charging and discharging process, and improving the coulombic efficiency and the cycle retention rate of the silicon-carbon material.
Disclosure of Invention
This section is for the purpose of summarizing some aspects of embodiments of the invention and to briefly introduce some preferred embodiments. In this section, as well as in the abstract and the title of the invention of this application, simplifications or omissions may be made to avoid obscuring the purpose of the section, the abstract and the title, and such simplifications or omissions are not intended to limit the scope of the invention.
The invention is provided in view of the above and/or the problems existing in the existing silicon-carbon composite material with the multi-layer coating structure and the preparation method thereof.
Therefore, the invention aims to provide a silicon-carbon composite material with a multi-level coating structure and a preparation method thereof, which realize the distribution of nano-silicon in gaps of graphite matrix secondary granulation and the coating modification of the surface by fine screening and targeted regulation of asphalt with different softening points, can effectively isolate direct contact of silicon and electrolyte, reduce the generation of side reactions, effectively reduce the consumption of active lithium ions and electrolyte in the battery circulation process, and improve the coulomb efficiency and the circulation retention rate of the material.
To solve the above technical problem, according to an aspect of the present invention, the present invention provides the following technical solutions:
a silicon-carbon composite material with a multi-level coating structure is based on a cross-scale modification technology of a silicon-based cathode, can realize self-assembly of secondary particles and uniform dispersion of nano-silicon in particle gaps of graphite secondary granulation, can realize multi-level carbon layer coating through fine regulation and structural design of asphalt with different softening points, effectively buffers volume expansion of silicon, assists bonding of the secondary particles in a drying process, isolates multiple purposes such as direct contact of the silicon and electrolyte, and the like, thereby obviously improving the coulomb efficiency and cycle retention rate of the material.
A preparation method of a silicon-carbon composite material with a multi-level cladding structure comprises the following steps:
the method comprises the following steps: mixing silicon and the dispersion liquid, and then grinding in a sand mill to obtain the dispersion liquid of the nano silicon;
step two: adding graphite into the dispersion liquid of the nano-silicon and uniformly stirring to obtain a mixed dispersion liquid of the nano-silicon and the graphite;
step three: adding the asphalt with the low softening point into the dispersion liquid of the nano silicon and the graphite and uniformly mixing;
step four: drying and carrying out secondary granulation on the mixed dispersion liquid of nano silicon, graphite and asphalt by spray drying to obtain a dry powder-shaped single-stage coated silicon-carbon material precursor;
step five: uniformly mixing high-temperature asphalt and a single-stage coated silicon-carbon material precursor, adding the mixture into a reaction kettle, and carrying out high-temperature mixing and coating under the inert protection to obtain a multi-stage coated silicon-carbon composite material precursor;
step six: and (3) sintering the pitch-coated silicon-carbon material precursor at high temperature in an inert gas atmosphere to prepare the silicon-carbon material with the multi-layer coating structure.
As a preferable scheme of the preparation method of the silicon-carbon composite material with the multi-level coating structure, the method comprises the following steps: in the first step: the dispersion liquid is one or a mixture of ethanol, methanol and isopropanol, the ratio of the silicon to the dispersion liquid is 1:10-1:4, and the nano silicon D50 is 50-250 nm.
As a preferable scheme of the preparation method of the silicon-carbon composite material with the multi-level coating structure, the method comprises the following steps: in the second step: the graphite is natural graphite, artificial graphite or a mixture of the natural graphite and the artificial graphite, D50 is 3-8 mu m, and the ratio of silicon to graphite is 1:3-1: 20.
As a preferable scheme of the preparation method of the silicon-carbon composite material with the multi-level coating structure, the method comprises the following steps: in the third step: the softening point of the asphalt is 100-180 ℃, the particle size D50 is 0.9-2 μm, and the ratio of the asphalt to the graphite is 1:40-1: 10.
As a preferable scheme of the preparation method of the silicon-carbon composite material with the multi-level coating structure, the method comprises the following steps: in the fourth step: the temperature of spray drying is 130-220 ℃.
As a preferable scheme of the preparation method of the silicon-carbon composite material with the multi-level coating structure, the method comprises the following steps: in the fifth step: the reaction kettle is a high-temperature VC mixer which is provided with a conical mixing cavity and a central rotating shaft, wherein the product is pushed to the inner wall of the mixing cavity by centrifugal force generated by the speed of the rotating shaft, materials in the cavity are circulated in the conical cavity under the matching action of an inner wall scraper to achieve the purpose of uniform mixing, the softening point of the high-temperature asphalt is between 220 and 280 ℃, the coking value is above 60 percent, the particle size is 2-5 mu m, the ratio of the asphalt to a silicon-carbon material precursor is 1:30-1:10, the mixing temperature is 250 and 550 ℃, the rotating speed is 80-350r/min, the time is 60-180min, and the inert gas is nitrogen, argon, helium and the like.
As a preferable scheme of the preparation method of the silicon-carbon composite material with the multi-level coating structure, the method comprises the following steps: in the step 6: the high-temperature sintering temperature is 800-1100 ℃, the time is 30-180min, and the inert gas is nitrogen, argon, helium and the like.
Compared with the prior art: according to the multi-level coating structure silicon-carbon composite material and the preparation method thereof, a differential cross-scale multi-level surface modification process is adopted, distribution of nano silicon in gaps of graphite matrix secondary granulation and surface coating modification are realized through fine screening and targeted regulation and control of asphalt with different softening points, the volume expansion of silicon is effectively buffered by using a relatively flexible graphite matrix, and meanwhile, the asphalt with a high softening point forms a compact high-flexibility amorphous carbon coating layer on the surface of a silicon-carbon material.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and detailed embodiments, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise. Wherein:
fig. 1 is a test result of X-ray diffraction (XRD) of a silicon carbon composite material with a multi-layered coating structure obtained in embodiment 1 of the present invention;
fig. 2 is a Scanning Electron Microscope (SEM) test result of the silicon-carbon composite material with the multi-layered coating structure according to embodiment 1 of the present invention;
fig. 3 is a Transmission Electron Microscope (TEM) test result of the silicon-carbon composite material with the multi-layered coating structure obtained in embodiment 1 of the present invention;
fig. 4 is a test result of the particle size distribution of the silicon-carbon composite material with the multi-layer coating structure obtained in embodiment 1 of the present invention;
fig. 5 is a test result of the retention rate of the cycling capacity of the CR2016 type button cell assembled by the multilevel coated silicon-carbon composite material obtained in embodiment 1 of the present invention and the comparative silicon-carbon material;
fig. 6 is a test result of the cyclic coulomb efficiency of the CR2016 type button cell assembled by the multi-layer coating structure silicon-carbon composite material obtained in embodiment 1 of the present invention and the comparative silicon-carbon material.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described herein, and it will be apparent to those of ordinary skill in the art that the present invention may be practiced without departing from the spirit and scope of the present invention, and therefore the present invention is not limited to the specific embodiments disclosed below.
Next, the present invention will be described in detail with reference to the drawings, wherein for convenience of illustration, the cross-sectional view of the device structure is not enlarged partially according to the general scale, and the drawings are only examples, which should not limit the scope of the present invention. In addition, the three-dimensional dimensions of length, width and depth should be included in the actual fabrication.
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The invention provides a silicon-carbon composite material with a multi-layer coating structure, which can realize self-assembly of secondary particles and uniform dispersion of nano-silicon in particle gaps of graphite secondary granulation based on a cross-scale modification technology of a silicon-based cathode, can realize multi-layer carbon layer coating by fine regulation and structural design of asphalt with different softening points, effectively buffer volume expansion of silicon, assist bonding of secondary particles in a drying process, isolate direct contact of silicon and electrolyte and other multiple purposes, and thus obviously improve the coulombic efficiency and cycle retention rate of the material.
A silicon-carbon composite material with a multi-level cladding structure and a preparation method thereof are specifically implemented according to the following steps:
the method comprises the following steps: mixing silicon and the dispersion liquid, and grinding in a sand mill to obtain the silicon dispersion liquid with the particle size of D50 being 50-250 nm; the dispersion liquid is one or a mixture of ethanol, methanol and isopropanol, and the ratio of the silicon to the dispersion liquid is 1:10-1: 4;
step two: adding graphite into the dispersion liquid of the nano-silicon, and uniformly stirring to obtain a mixed dispersion liquid of the nano-silicon and the graphite; the graphite D50 is 3-8 mu m, and the ratio of silicon to graphite is 1:3-1: 20;
step three: weighing asphalt with the softening point of 100-;
step four: drying and carrying out secondary granulation on the mixed dispersion liquid of nano-silicon, graphite and asphalt by spray drying to obtain a dry powder-shaped single-stage coated silicon-carbon material precursor, wherein the spray drying temperature is set to be 130-220 ℃;
step five: uniformly mixing high-temperature asphalt and a single-stage coated silicon-carbon material precursor, and adding the mixture into a high-temperature VC (vacuum mixing) machine for high-temperature mixing to obtain a silicon-carbon composite material precursor with a multi-stage coating structure, wherein the asphalt softening point is between 220 and 280 ℃, the coking value is above 60 percent, the particle size is 2-5 mu m, the ratio of the asphalt to the silicon-carbon material precursor is 1:30-1:10, the mixing temperature is 250 and 550 ℃, the time is 60-180min, and the inert gas is nitrogen, argon, helium and the like;
step six: and sintering the pitch-coated silicon-carbon material precursor at high temperature in an inert gas atmosphere to prepare the silicon-carbon material with the multi-layer coating structure, wherein the sintering temperature is 800-1100 ℃, the sintering time is 30-180min, and the inert gas is nitrogen, argon, helium and the like.
The method comprises the following steps: weighing 1 part of nano silicon, mixing with 8 parts of ethanol, and grinding in a sand mill to obtain ethanol dispersion of D50 in 150nm silicon;
step two: weighing 8 parts of graphite with the particle size of 5 mu m of D50, adding the graphite into the ethanol dispersion liquid of the nano silicon, and uniformly stirring to obtain the ethanol dispersion liquid of the nano silicon and the graphite;
step three: adding 0.4 part of asphalt with the softening point of 160 ℃ and the particle size D50 of 1.8 mu m into the dispersion liquid of the nano silicon and the graphite, and uniformly mixing;
step four: spray drying the ethanol dispersion liquid of the nano silicon, the graphite and the asphalt at 160 ℃ to obtain a dry powder single-stage coated silicon-carbon material precursor;
step five: weighing 0.8 part of high-temperature asphalt (the softening point is 260 ℃, the coking value is 70 percent, and the particle size is 3 mu m), uniformly mixing the high-temperature asphalt with the single-stage coated silicon-carbon material precursor, adding the mixture into a high-temperature VC mixer, carrying out high-temperature mixing and secondary granulation under the protection of nitrogen, setting the mixing temperature to be 400 ℃, and carrying out 80min to obtain a multi-stage coated silicon-carbon composite material precursor;
step six: and sintering the pitch-coated silicon-carbon material precursor under the protection of nitrogen gas at the temperature of 800 ℃ for 60min to prepare the silicon-carbon material with the multi-layer coating structure.
XRD test
XRD characterization is carried out on the multi-layer coating structure silicon-carbon composite material obtained in the embodiment 1 of the invention, and the results are shown in figure 1, XRD diffraction peaks of graphite and silicon can be respectively observed, and no other impurities exist;
and (4) SEM characterization:
SEM test is performed on the silicon-carbon composite material with the multi-layered coating structure obtained in embodiment 1 of the present invention, and the result is shown in fig. 2, and the XRD test result shows that secondary granulation of particles, loading of nano-silicon and surface coating modification can be achieved by fine selection of pitches with different softening points, and no visible silicon particles are present on the surface of the material;
TEM characterization
When the multi-layer coating structure silicon-carbon composite material obtained in embodiment 1 of the present invention is subjected to TEM test, the result is shown in fig. 3, it can be observed that a uniform amorphous carbon coating layer is formed on the surface of the material due to the introduction of the pitch, no obvious crystalline region exists, and the thickness of the coating layer is 20 to 30 nm;
particle size distribution test
The result of the particle size distribution test of the silicon-carbon composite material with a multi-layer coating structure obtained in embodiment 1 of the present invention is shown in fig. 4, where the particle size of the silicon-carbon composite material is about 20 μm, the material has uniform particle size distribution, and no scattered silicon particles;
electrochemical performance test
The multi-level coating structure silicon-carbon composite material obtained in the embodiment 1 of the invention is subjected to homogenate coating to prepare a pole piece, and a lithium piece is used as a counter electrode to prepare a CR2016 type button cell to perform an electrochemical performance test, and the result is shown in fig. 5 and 6, the reversible capacity of the material is 500mAh/g, the first effect is 91.1%, the retention rate of the material is over 91% after the material is circulated for 300 weeks, the highest coulombic efficiency is over 99.9%, and the test result is obviously superior to the test result of other comparative groups of silicon-carbon materials.
| Sample (I) | |
Comparative example 1 | Comparative example 2 | Comparative example 3 |
| First effect (%) | 91.2 | 88.7 | 89.8 | 90.3 |
Comparative example 1
The method comprises the following steps: weighing 1 part of nano silicon, mixing with 8 parts of ethanol, and grinding in a sand mill to obtain ethanol dispersion of D50 in 150nm silicon;
step two: weighing 8 parts of graphite with the particle size of 5 mu m of D50, adding the graphite into the ethanol dispersion liquid of the nano silicon, and uniformly stirring to obtain the ethanol dispersion liquid of the nano silicon and the graphite;
step three: and (3) carrying out spray drying on the ethanol dispersion liquid of the nano silicon, the graphite and the asphalt to obtain the silicon-carbon composite material, wherein the spray drying temperature is 160 ℃.
Electrochemical performance test
The silicon-carbon composite material obtained in comparative example 1 of the invention was subjected to homogenate coating and prepared into a pole piece, and electrochemical performance test was performed on a CR2016 type button cell prepared using a lithium piece as a counter electrode, with the results shown in fig. 5 and 6.
Comparative example 2
The method comprises the following steps: weighing 1 part of nano silicon, mixing with 8 parts of ethanol, and grinding in a sand mill to obtain ethanol dispersion of D50 in 150nm silicon;
step two: weighing 8 parts of graphite with the particle size of 5 mu m of D50, adding the graphite into the ethanol dispersion liquid of the nano silicon, and uniformly stirring to obtain the ethanol dispersion liquid of the nano silicon and the graphite;
step three: adding 0.4 part of asphalt with the softening point of 160 ℃ and the particle size D50 of 1.8 mu m into the dispersion liquid of the nano silicon and the graphite, and uniformly mixing;
step four: spray drying the ethanol dispersion liquid of the nano silicon, the graphite and the asphalt at 160 ℃ to obtain a dry powder single-stage coated silicon-carbon material precursor;
step five: and sintering the silicon-carbon material precursor under the protection of nitrogen gas at the temperature of 800 ℃ for 60min to prepare the silicon-carbon composite material.
Electrochemical performance test
The silicon-carbon composite material obtained in comparative example 2 of the invention was subjected to homogenate coating and prepared into a pole piece, and electrochemical performance test was performed on a CR2016 type button cell prepared using a lithium piece as a counter electrode, with the results shown in fig. 5 and 6.
Comparative example 3
The method comprises the following steps: weighing 1 part of nano silicon, mixing with 8 parts of ethanol, and grinding in a sand mill to obtain ethanol dispersion of D50 in 150nm silicon;
step two: weighing 8 parts of graphite with the particle size of 5 mu m of D50, adding the graphite into the ethanol dispersion liquid of the nano silicon, and uniformly stirring to obtain the ethanol dispersion liquid of the nano silicon and the graphite;
step three: spray drying the ethanol dispersion liquid of the nano silicon and the graphite at the spray drying temperature of 160 ℃ to obtain a dry powder silicon-carbon material precursor;
step four: weighing 0.8 part of high-temperature asphalt (the softening point is 260 ℃, the coking value is 70%, and the particle size is 3 mu m), uniformly mixing the high-temperature asphalt with the silicon-carbon material precursor, adding the mixture into a high-temperature VC mixer, carrying out high-temperature mixing and secondary granulation under the nitrogen protection condition, setting the mixing temperature to be 400 ℃, and carrying out 80min to obtain a silicon-carbon composite material precursor with a single-layer coating structure;
step five: and sintering the pitch-coated silicon-carbon material precursor under the protection of nitrogen gas at the temperature of 800 ℃ for 60min to prepare the silicon-carbon material with the multi-layer coating structure.
Electrochemical performance test
The silicon-carbon composite material obtained in comparative example 2 of the invention was subjected to homogenate coating and prepared into a pole piece, and electrochemical performance test was performed on a CR2016 type button cell prepared using a lithium piece as a counter electrode, with the results shown in fig. 5 and 6.
Embodiment mode 2
The method comprises the following steps: weighing 2 parts of nano silicon, mixing with 10 parts of ethanol, and grinding in a sand mill to obtain ethanol dispersion of D50 in 200nm silicon;
step two: weighing 10 parts of graphite with the particle size of 7 mu m of D50, adding the graphite into the ethanol dispersion liquid of the nano silicon, and uniformly stirring to obtain the ethanol dispersion liquid of the nano silicon and the graphite;
step three: adding 0.5 part of asphalt with the softening point of 160 ℃ and the particle size D50 of 1.5 mu m into the dispersion liquid of the nano silicon and the graphite, and uniformly mixing;
step four: spray drying the ethanol dispersion liquid of the nano silicon, the graphite and the asphalt at 180 ℃ to obtain a dry powder single-stage coated silicon-carbon material precursor;
step five: weighing 1 part of high-temperature asphalt (the softening point is 250 ℃, the coking value is 70%, and the particle size is 3 mu m), uniformly mixing the high-temperature asphalt with the single-stage coated silicon-carbon material precursor, adding the mixture into a high-temperature VC mixer, carrying out high-temperature mixing and secondary granulation under the nitrogen protection condition, setting the mixing temperature at 500 ℃ for 120min, and obtaining the multi-stage coated silicon-carbon composite material precursor;
step six: sintering the pitch-coated silicon-carbon material precursor under the protection of nitrogen gas at 950 ℃ for 60min to prepare the silicon-carbon material with the multi-layer coating structure; .
Embodiment 3
The method comprises the following steps: weighing 1 part of nano silicon, mixing with 7 parts of methanol, and grinding in a sand mill to obtain a dispersion liquid of D50 in 150nm silicon;
step two: weighing 8 parts of graphite with the particle size of 5 mu m of D50, adding the graphite into the dispersion liquid of the nano silicon, and uniformly stirring to obtain a mixed dispersion liquid of the nano silicon and the graphite;
step three: adding 0.4 part of asphalt with the softening point of 160 ℃ and the particle size D50 of 1.3 mu m into the dispersion liquid of the nano silicon and the graphite, and uniformly mixing;
step four: spray drying the dispersion liquid of the nano silicon, the graphite and the asphalt at the spray drying temperature of 210 ℃ to obtain a dry powder-shaped single-stage coated silicon-carbon material precursor;
step five: weighing 0.8 part of high-temperature asphalt (the softening point is 260 ℃, the coking value is 70 percent, and the particle size is 3 mu m), uniformly mixing the high-temperature asphalt with the single-stage coated silicon-carbon material precursor, adding the mixture into a high-temperature VC mixer, carrying out high-temperature mixing and secondary granulation under the nitrogen protection condition, setting the mixing temperature to be 450 ℃, and carrying out time 150min to obtain a multi-stage coated silicon-carbon composite material precursor;
step six: and sintering the pitch-coated silicon-carbon material precursor under the protection of nitrogen gas at 900 ℃ for 120min to prepare the silicon-carbon material with the multi-layer coating structure.
While the invention has been described above with reference to an embodiment, various modifications may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In particular, the various features of the disclosed embodiments of the invention may be used in any combination, provided that no structural conflict exists, and the combinations are not exhaustively described in this specification merely for the sake of brevity and resource conservation. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
Claims (8)
1. The silicon-carbon composite material with the multi-layer coating structure is characterized in that:
a silicon-carbon composite material with a multi-level coating structure is based on a cross-scale modification technology of a silicon-based cathode, can realize self-assembly of secondary particles and uniform dispersion of nano-silicon in particle gaps of graphite secondary granulation, can realize multi-level carbon layer coating through fine regulation and structural design of asphalt with different softening points, effectively buffers volume expansion of silicon, assists bonding of the secondary particles in a drying process, isolates multiple purposes such as direct contact of the silicon and electrolyte, and the like, thereby obviously improving the coulomb efficiency and cycle retention rate of the material.
2. A preparation method of a silicon-carbon composite material with a multi-level cladding structure is characterized by comprising the following steps:
the method comprises the following steps: mixing silicon and the dispersion liquid, and then grinding in a sand mill to obtain the dispersion liquid of the nano silicon;
step two: adding graphite into the dispersion liquid of the nano-silicon and uniformly stirring to obtain a mixed dispersion liquid of the nano-silicon and the graphite;
step three: adding the asphalt with the low softening point into the dispersion liquid of the nano silicon and the graphite and uniformly mixing;
step four: drying and carrying out secondary granulation on the mixed dispersion liquid of nano silicon, graphite and asphalt by spray drying to obtain a dry powder-shaped single-stage coated silicon-carbon material precursor;
step five: uniformly mixing high-temperature asphalt and a single-stage coated silicon-carbon material precursor, adding the mixture into a reaction kettle, and carrying out high-temperature mixing and coating under the inert protection to obtain a multi-stage coated silicon-carbon composite material precursor;
step six: and (3) sintering the pitch-coated silicon-carbon material precursor at high temperature in an inert gas atmosphere to prepare the silicon-carbon material with the multi-layer coating structure.
3. The method for preparing the silicon-carbon composite material with the multi-layer coating structure according to claim 2, wherein in the step one: the dispersion liquid is one or a mixture of ethanol, methanol and isopropanol, the ratio of the silicon to the dispersion liquid is 1:10-1:4, and the nano silicon D50 is 50-250 nm.
4. The method for preparing the silicon-carbon composite material with the multi-layer coating structure according to claim 2, wherein in the second step: the graphite is natural graphite, artificial graphite or a mixture of the natural graphite and the artificial graphite, D50 is 3-8 mu m, and the ratio of silicon to graphite is 1:3-1: 20.
5. The method for preparing the silicon-carbon composite material with the multi-layer coating structure according to claim 2, wherein the step three is as follows: the softening point of the asphalt is 100-180 ℃, the particle size D50 is 0.9-2 μm, and the ratio of the asphalt to the graphite is 1:40-1: 10.
6. The method for preparing the silicon-carbon composite material with the multi-layer coating structure according to claim 2, wherein in the fourth step: the temperature of spray drying is 130-220 ℃.
7. The method for preparing the silicon-carbon composite material with the multi-layer coating structure according to claim 2, wherein in the step five: the reaction kettle is a high-temperature VC mixer which is provided with a conical mixing cavity and a central rotating shaft, wherein the product is pushed to the inner wall of the mixing cavity by centrifugal force generated by the speed of the rotating shaft, materials in the cavity are circulated in the conical cavity under the matching action of an inner wall scraper to achieve the purpose of uniform mixing, the softening point of the high-temperature asphalt is between 220 and 280 ℃, the coking value is above 60 percent, the particle size is 2-5 mu m, the ratio of the asphalt to a silicon-carbon material precursor is 1:30-1:10, the mixing temperature is 250 and 550 ℃, the rotating speed is 80-350r/min, the time is 60-180min, and the inert gas is nitrogen, argon, helium and the like.
8. The method for preparing the silicon-carbon composite material with the multi-layer coating structure according to claim 2, wherein in the step 6: the high-temperature sintering temperature is 800-1100 ℃, the time is 30-180min, and the inert gas is nitrogen, argon, helium and the like.
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