CN113659125B - Silicon-carbon composite material and preparation method thereof - Google Patents

Silicon-carbon composite material and preparation method thereof Download PDF

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CN113659125B
CN113659125B CN202110748966.3A CN202110748966A CN113659125B CN 113659125 B CN113659125 B CN 113659125B CN 202110748966 A CN202110748966 A CN 202110748966A CN 113659125 B CN113659125 B CN 113659125B
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silicon
temperature
carbon
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composite material
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CN113659125A (en
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蔡金明
汤小辉
梁惠明
陈其赞
彭杨城
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Guangdong Morion Nanotech Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/386—Silicon or alloys based on silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
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    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/05—Preparation or purification of carbon not covered by groups C01B32/15, C01B32/20, C01B32/25, C01B32/30
    • C—CHEMISTRY; METALLURGY
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    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
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    • C01B32/182—Graphene
    • C01B32/184—Preparation
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    • C01B33/00—Silicon; Compounds thereof
    • C01B33/02—Silicon
    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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    • H01M4/625—Carbon or graphite
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    • C01P2004/00—Particle morphology
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    • C01P2004/03—Particle morphology depicted by an image obtained by SEM
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    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
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    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
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    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention aims to disclose a silicon-carbon composite material and a preparation method thereof, wherein the silicon-carbon composite material is a triple core structure which coats three layers of carbon shells on the surface of a nanometer silicon; the inner core of the silicon-carbon negative electrode material is nano silicon particles, and the surface of the nano silicon particles is sequentially coated with a hard carbon layer, a graphene layer and a soft carbon layer; the hard carbon layer is formed by pyrolysis of sweet potato starch, and the surface of the hard carbon layer has a loose porous structure; the soft carbon layer is formed by mixing and carbonizing asphalt and artificial graphite. The silicon-carbon composite material prepared by the method has the characteristics of low specific surface area, high tap density and the like in physical and chemical indexes, and has the characteristics of high first-time charge-discharge coulombic efficiency, high gram capacity, stable cycle performance, excellent rate performance and the like in electrochemical performance indexes.

Description

Silicon-carbon composite material and preparation method thereof
Technical Field
The invention relates to the field of lithium ion batteries, in particular to a silicon-carbon composite material and a preparation method thereof.
Background
The lithium ion battery has the advantages of high working voltage, long cycle life, environmental protection and the like, and the market space is larger and larger along with the development of the technology in recent years. Meanwhile, the market also puts higher demands on the cruising ability of the power supply product. At present, the negative electrode material for the lithium ion battery mainly takes graphite as a main material, and the theoretical capacity of the negative electrode material is very close to the theoretical capacity 372mAh/g of the graphite in actual use. In order to improve the energy density of the lithium ion battery, a novel negative electrode material with high specific capacity needs to be developed urgently. The theoretical capacity of silicon is 4200mAh/g, the lithium intercalation potential is about 0.4V higher than that of graphite, and the silicon is one of the most developed lithium ion battery negative electrode materials at present. However, when silicon is used alone as a negative electrode of a lithium ion battery, the following problems mainly exist: 1. volume expansion > 300%; 2. the first charge-discharge coulomb efficiency is low; 3. the electron conductivity is poor. Therefore, solving the three major problems of silicon currently is making silicon the first premise for commercialization.
Patent with application number CN108899488A discloses a modified carbon-coated silica composite material, a preparation method and application thereof, wherein the preparation method comprises the following steps: dissolving phenolic resin in ethanol to obtain a material A; adding graphene oxide into the material A, uniformly mixing, adding silicon monoxide powder, heating and stirring until the mixture is evaporated to dryness, and then carrying out vacuum drying to obtain a material B; and carbonizing the material B to obtain the modified carbon-coated silicon monoxide composite material. The obtained composite material has a three-dimensional layered three-dimensional structure, can relieve huge stress change, is used in a lithium ion battery, and has high theoretical capacity and first charge-discharge efficiency. According to the invention, the problem of volume expansion and electronic conductivity of silicon is solved to a certain extent by coating the silicon monoxide with the phenolic resin and the graphene oxide in the ethanol solvent, but because the graphene oxide lamella contains rich oxygen-containing functional groups such as carboxyl, hydroxyl, epoxy and the like, and the functional groups have very strong polarity, the graphene lamella is easy to shrink and agglomerate in the ethanol solvent with weak polarity, uniform dispersion is difficult to achieve, and performance of graphene is not facilitated. Meanwhile, the surface of the material is coated by graphene oxide, and the material has the characteristics of large specific surface area and low tap density.
Patent application No. CN201910346290.8 discloses a silicon-carbon negative electrode material, a preparation method thereof and a battery, wherein the preparation method comprises the following steps: mixing silicon dioxide nanoparticles, graphene oxide and a first solvent to form a first mixed solution; dissolving chitosan in a second solvent to form a second mixed solution; mixing the first mixed solution and the second mixed solution to form a third mixed solution; and drying the third mixed solution to form a chitosan-graphene oxide-silicon dioxide composite material, and sintering the chitosan-graphene oxide-silicon dioxide composite material to form an organic carbon-graphene-silicon dioxide composite material and a silicon carbon negative electrode material. The method has simple process flow, and the prepared silicon-carbon material has low expansion rate, high gram capacity, good cycle performance and rate capability, but still has the characteristics of large specific surface area and low tap density. In addition, because the graphene oxide is of a single-layer or few-layer structure, a large amount of wrinkles are formed on the surface, so that the silicon dioxide nanoparticles are easy to gather locally, and the carbon coating uniformity of the nano silicon dioxide material is influenced.
Disclosure of Invention
Based on the above, the invention provides a preparation method of a silicon-carbon composite material, and the silicon-carbon composite material prepared by the method has the characteristics of low specific surface area, high tap density and the like in physical and chemical indexes, and has the characteristics of high first charge-discharge coulombic efficiency, high gram capacity, stable cycle performance, excellent rate performance and the like in electrochemical performance indexes. The silicon carbon in the prepared silicon carbon composite material is in a hard carbon-graphene-soft carbon triple nuclear structure, has the characteristics of uniform graphene coating, compact external structure and porous internal structure, and can solve the problems of high volume expansion, low initial charge-discharge coulomb efficiency and poor electronic conductivity of silicon to a certain extent. In addition, in the coating process of the soft carbon asphalt, the traditional method is mainly implemented by adopting an organic solvent or solid phase mixing technical route. The organic solvent can realize good coating effect, but the characteristics of toxicity, large safety risk coefficient, high recovery process and cost and the like are not beneficial to large-scale production; the solid phase mixing process can well solve the problem of industrialization, but cannot ensure the consistency of submicron products and the stability among batches in the process. The invention adopts a liquid phase hydrosolvent mixing process technical route, and utilizes the functions of bonding, thickening and dispersing of CMC, so that the asphalt can be uniformly dispersed in a water phase, the agglomeration and sedimentation of the asphalt can not occur, and the asphalt can be uniformly coated on the surface of silicon carbon material particles. The technical process of the invention has the characteristics of simple whole manufacturing process and suitability for batch and stable production of enterprises.
The invention aims to disclose a silicon-carbon composite material, which is a triple core structure with a nano silicon surface coated with three layers of carbon shells; the inner core of the silicon-carbon negative electrode material is nano silicon particles, and the surface of the nano silicon particles is sequentially coated with a hard carbon layer, a graphene layer and a soft carbon layer; the hard carbon layer is formed by pyrolysis of sweet potato starch, and the surface of the hard carbon layer has a loose porous structure; the soft carbon layer is formed by mixing and carbonizing asphalt and artificial graphite.
The tap density of the silicon-carbon composite material is more than or equal to 0.8g/cm3The specific surface area is less than or equal to 4.0m2/g。
Another objective of the present invention is to provide a method for preparing a silicon-carbon composite material, comprising the following steps: 1) in an absolute ethyl alcohol solvent, firstly adding sweet potato starch for dissolving, secondly adding nano silicon powder for uniformly stirring, and then carrying out first spray drying and first high-temperature carbonization to obtain a precursor A; 2) adding polydopamine and a precursor A into an aqueous solution of graphene oxide, uniformly stirring, and then carrying out secondary spray drying to obtain a precursor B; 3) sequentially adding asphalt, a precursor B and artificial graphite into a sodium carboxymethylcellulose (CMC) aqueous solution, mixing and stirring, spray-drying for the third time, and carrying out high-temperature carbonization for the second time to finally obtain the novel silicon-carbon composite negative electrode material.
Specifically, in step 1, sweet potato starch: the mass ratio of the nano silicon powder is 1:6-1: 1. The particle size of the nano silicon powder is 30-500nm, and the preferred particle size is 30-150 nm. The concentration of the nano silicon in water is 3-10%; the dispersion equipment adopts double-planet stirring or wet grinding equipment. The silicon with small particle size has relatively low volume expansion and stress in the lithium releasing and embedding process, can effectively avoid the problem of particle pulverization of the silicon in the charging and discharging cycle process, and is greatly helpful for prolonging the service life of the silicon material. The sweet potato starch has a molecular chain length, can swell in an aqueous solution and generate certain viscosity, forms a suspension in a low-concentration nano silicon solution, further has good adsorption and dispersion effects on nano silicon, prevents the nano silicon from agglomerating and settling in an aqueous solvent, and is beneficial to realizing the close contact between the sweet potato starch and the nano silicon.
In a further technical scheme, spray granulation equipment is used for spray drying in the step 1, the temperature of a feed inlet of the equipment is controlled to be 190-210 ℃, and the temperature of a discharge outlet of the equipment is controlled to be 90-100 ℃. The high-temperature carbonization equipment is a box furnace or a tube furnace, and the required gas environment is inert atmosphere, such as argon or nitrogen; the temperature of the first high-temperature carbonization is 500-. Spray drying and high-temperature carbonization are two common process methods in the material industry. The powder after spray drying treatment is relatively regular and uniform in particle size, morphology and the like. The high-temperature carbonization is to crack the sweet potato starch at high temperature under the inert atmosphere condition to form amorphous carbon which is uniformly covered on the surface of the nano silicon, so that the electronic and ionic conductivity of the nano silicon can be improved.
In the step 2, the mass ratio of the graphene oxide to the polydopamine to the precursor A is as follows: the graphene oxide accounts for 10% -30%; the proportion of polydopamine is 10-30%; the proportion of the precursor A is 40-80%. The concentration of the graphene oxide in the aqueous solution is 0.5% -3%, the carbon content of the graphene oxide is more than or equal to 80%, and the number of layers is less than or equal to 10. Spray granulation equipment is used for spray drying, the feeding temperature of the equipment is controlled at 190-210 ℃, and the discharging temperature is controlled at 90-100 ℃. The powder prepared by spray drying is called precursor B. The graphene oxide sheet layer contains rich oxygen-containing functional groups such as carboxyl, hydroxyl, epoxy and the like, and the solution of the graphene oxide sheet layer is weakly acidic and has a small amount of negative charges; the polydopamine is easy to agglomerate under an alkaline condition, but can be inserted between graphene oxide lamella layers in a small molecule form in a solution under a weak acid condition, and meanwhile, the polydopamine also has a polar functional group, so that a hydrogen bond is easy to form between the polydopamine and graphene oxide, and the stability of the graphene lamella structure can be effectively supported.
In the step 3, the mass ratio of the CMC, the pitch, the precursor B and the artificial graphite is as follows: 1-10% of CMC, 5-20% of asphalt, 40-80% of artificial graphite and 5-30% of precursor B. The molecular weight of CMC is 50000-100000, and is a common binder in the current lithium ion battery cathode material. The asphalt is petroleum or coal-based asphalt, the softening point temperature is 150-300 ℃, and the particle size is 0.5-20 um; the artificial graphite is a commercialized common lithium ion battery cathode material, and the requirements of particle size, specific surface area and specific capacity index of the artificial graphite meet the national technical standard. The invention adopts a liquid phase hydrosolvent mixing process technical route, and utilizes the functions of bonding, thickening and dispersing of CMC, so that the asphalt can be uniformly dispersed in a water phase, the agglomeration and sedimentation of the asphalt can not occur, and the asphalt can be uniformly coated on the surface of silicon carbon material particles.
In the step 3, the carbonization process is carried out in two steps, wherein firstly, the temperature is kept for 1-3h at the temperature of 150 ℃ plus 300 ℃ of the asphalt softening point, so that the precursor B and the artificial graphite are fully wrapped by the asphalt; secondly, raising the temperature to the temperature of 900 ℃ and 1100 ℃, and keeping the temperature for 2-5 h; the temperature rise speed in the whole process is controlled to be 2-15 ℃/min.
According to the invention, a triple core structure consisting of hard carbon, graphene and soft carbon is respectively formed on the surface of the nano silicon particle from inside to outside. Hard carbon is formed by pyrolysis of sweet potato starch, the surface of the hard carbon has a loose porous structure, the problems of volume expansion and stress of silicon can be relieved, and meanwhile, the porous structure is favorable for rapid migration of lithium ions, and the rate capability is improved. The graphene has excellent electronic conductivity and mechanical strength, has the function of enhancing the electronic conductivity of a silicon material, reduces the impedance of the material, and can further solve the problems of deformation and stress release of the silicon due to the excellent elastic deformation capability and the fold structure of the graphene. Therefore, the specific surface area of the material can be obviously reduced by coating the asphalt on the surface of the graphene; meanwhile, the artificial graphite with a certain proportion is compounded, so that the tap density of the material can be increased, and the first charge-discharge efficiency and the cycle stability of the silicon-carbon can be further improved, thereby achieving the commercialization of the silicon-carbon cathode.
Drawings
FIG. 1 is an SEM of a hard carbon-graphene-soft carbon coated nano silicon carbon material
FIG. 2 is the first charge-discharge curve of the Si-C material in example 1
FIG. 3 is a 0.1C charge-discharge cycle performance curve for examples 1-3 and comparative examples 1-2
Detailed Description
To facilitate an understanding of the invention, the invention will now be described more fully with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
The following are specific examples:
example 1
The invention provides a preparation method of a silicon-carbon composite material, which comprises the following steps:
(1) at room temperature, 200g of sweet potato starch is added into 10Kg of aqueous solvent, and after the mixture is stirred to be fully dissolved, 500g of nano silicon powder is added, wherein the D50 particle size is 80 nm. And after the nano silicon powder is uniformly dispersed, carrying out spray drying and high-temperature carbonization treatment on the nano silicon powder to obtain a precursor A. The temperature of the feed inlet of the spray drying equipment is 195 ℃, the temperature of the discharge outlet is 90 ℃, and the atomization frequency is 50 Hz. The high-temperature carbonization equipment is a tubular furnace and protected by argon; the carbonization temperature is 600 ℃, the carbonization time is 4h, and the temperature rise rate is 5 ℃/min.
(2) Slowly adding 50g of graphene oxide into 4.95Kg of pure water solvent at room temperature to prepare a graphene oxide solution with the concentration of 1.0%; 50g of polydopamine and 250g of precursor A are respectively added into the solution, fully and uniformly stirred, and then spray-dried. The temperature of the feed inlet of the spray drying equipment is 195 ℃, the temperature of the discharge outlet is 90 ℃, and the atomization frequency is 50 Hz. The powder prepared by spray drying is called precursor B.
(3) At room temperature, 25g of CMC was slowly added to 4.975Kg of pure water solvent to prepare a 0.5% CMC gum solution. And adding 75g of pitch into the glue solution, fully stirring, adding 150g of the precursor B by mass, stirring again, and finally adding 325g of artificial graphite. And finally, adjusting the solid content of the slurry to be within 5-10% by using pure water.
(4) After the stirring was completed, spray drying was performed. The temperature of the feed inlet of the spray drying equipment is 195 ℃, the temperature of the discharge outlet is 90 ℃, and the atomization frequency is 50 Hz.
(5) And 4, carbonizing the powder prepared in the step 4 at a high temperature to obtain the novel silicon-carbon composite material. The high-temperature carbonization equipment is a tubular furnace and is protected by nitrogen; the high-temperature carbonization process is carried out in two steps, wherein the first step is that the temperature is kept at 200 ℃ for 2 hours, so that the asphalt is melted and uniformly distributed on the surfaces and gaps of surrounding particles; secondly, raising the temperature to 900 ℃ for asphalt carbonization, and keeping the temperature for 4 hours; the temperature rise speed in the whole process is controlled at 5 ℃/min.
Example 2
(1) At room temperature, 300g of sweet potato starch is added into 10Kg of aqueous solvent, and after the mixture is stirred to be fully dissolved, 500g of nano silicon powder is added. And after the nano silicon powder is uniformly dispersed, carrying out spray drying and high-temperature carbonization treatment on the nano silicon powder to obtain a precursor A. The temperature of the feed inlet of the spray drying equipment is 200 ℃, the temperature of the discharge outlet is 95 ℃, and the atomization frequency is 50 Hz. The high-temperature carbonization equipment is a tubular furnace and protected by argon; the carbonization temperature is 700 ℃, the carbonization time is 3h, and the heating rate is 8 ℃/min.
(2) Slowly adding 75g of graphene oxide into 4.95Kg of pure water solvent at room temperature to prepare a graphene oxide solution with the concentration of 1.5%; 56.25g of polydopamine and 243.75g of precursor A are respectively added into the solution, fully and uniformly stirred, and then spray-dried. The temperature of the feed inlet of the spray drying equipment is 200 ℃, the temperature of the discharge outlet is 95 ℃, and the atomization frequency is 50 Hz. The powder prepared by spray drying is called precursor B.
(3) At room temperature, 25g of CMC was slowly added to 4.975Kg of pure water solvent to prepare a 0.5% CMC gum solution. And adding 75g of pitch into the glue solution, fully stirring, adding 100g of the precursor B, stirring again, and finally adding 300g of artificial graphite. And finally, adjusting the solid content of the slurry to be within 5-10% by using pure water.
(4) After the stirring was completed, spray drying was performed. The temperature of the feed inlet of the spray drying equipment is 200 ℃, the temperature of the discharge outlet is 95 ℃, and the atomization frequency is 50 Hz.
(5) And carrying out high-temperature carbonization treatment to obtain the novel silicon-carbon composite material. The high-temperature carbonization equipment is a tubular furnace and is protected by nitrogen; the high-temperature carbonization process is carried out in two steps, wherein firstly, the temperature is kept for 2 hours at 250 ℃, so that the asphalt is melted and uniformly distributed on the surfaces and gaps of surrounding particles; secondly, raising the temperature to 1000 ℃ for asphalt carbonization, and keeping the temperature for 3 hours; the temperature rise speed in the whole process is controlled at 8 ℃/min.
Example 3
(1) At room temperature, 400g of sweet potato starch is added into 10Kg of aqueous solvent, stirred to be fully dissolved, and then 500g of nano silicon powder is added. And after the nano silicon powder is uniformly dispersed, carrying out spray drying and high-temperature carbonization treatment on the nano silicon powder to obtain a precursor A. The temperature of the feed inlet of the spray drying equipment is 90 ℃, the temperature of the discharge outlet is 60 ℃, and the atomization frequency is 50 Hz. The high-temperature carbonization equipment is a tubular furnace and protected by argon; the carbonization temperature is 800 ℃, the carbonization time is 2h, and the heating rate is 10 ℃/min.
(2) Slowly adding 100g of graphene oxide into 9.9Kg of pure water solvent at room temperature to prepare a graphene oxide solution with the concentration of 1.0%; and respectively adding 60g of polydopamine and 220g of precursor A into the solution, fully and uniformly stirring, and then spraying and drying. The temperature of the feed inlet of the spray drying equipment is 205 ℃, the temperature of the discharge outlet is 100 ℃, and the atomization frequency is 50 Hz. The powder prepared by spray drying is called precursor B.
(3) At room temperature, slowly adding 25g of CMC into 4.975Kg of pure water solvent to prepare 0.5% CMC glue solution. And adding 100g of asphalt into the glue solution, fully stirring, adding 100g of the precursor B by mass, stirring again, and finally adding 275g of artificial graphite. And finally, adjusting the solid content of the slurry to be within 5-10% by using pure water.
(4) After the stirring was completed, spray drying was performed. The temperature of the feed inlet of the spray drying equipment is 205 ℃, the temperature of the discharge outlet is 100 ℃, and the atomization frequency is 50 Hz.
(5) And carrying out high-temperature carbonization treatment to obtain the novel silicon-carbon composite material. The high-temperature carbonization equipment is a tubular furnace and is protected by nitrogen; the high-temperature carbonization process is carried out in two steps, wherein firstly, the temperature is kept for 2 hours at 300 ℃ to ensure that the asphalt is melted and uniformly distributed on the surfaces and gaps of surrounding particles; secondly, raising the temperature to 1100 ℃ of the carbonization temperature of the asphalt and keeping the temperature for 2 hours; the temperature rise speed in the whole process is controlled at 10 ℃/min.
Comparative example 1
Comparative example 1 the example 1 scheme was operated in the same manner except that no bitumen was added;
in the scheme of the comparative example 2, no graphene is added in the scheme of the example 1, and other operation modes are the same.
Effect example 1SEM
The silicon carbon composite materials of examples 1 to 3 were observed, and the results are shown in FIG. 1. The silicon-carbon composite material is similar to spherical particles, is coated by three layers of hard carbon, graphene and soft carbon, has a compact structure and uniform carbon source coating, and has the particle size of 5-30 um.
Effect example 2 performance test
Slurries were prepared in pure water solvent in a mass ratio of 80:5:10:5 of the silicon-carbon composite materials of examples 1-3 and comparative examples 1-2 to CMC, SP, SBR, as described above, the mass of pure water being 1.0-2.0 times the mass of solids. And coating the slurry on a copper foil with the thickness of 12um, and drying, rolling and punching to form a buckle type wafer. And assembling the button 2032 battery by using lithium foil as a counter electrode and the button wafer prepared above. The electrolyte used by the battery comprises the following main components: lithium hexafluorophosphate is adopted as lithium salt, and the concentration is 1 mol/L; the solvent is ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate, and the mass ratio is 1:1: 1. The thickness of the diaphragm is 20um, and the polypropylene/polyethylene (PP/PE/PP) three-layer microporous diaphragm is formed. Test the charging and discharging current of 0.6mA/cm2Charging and dischargingThe cut-off voltage is 0.001V-2V. The button cells were tested for initial capacity and coulombic efficiency, and charge/discharge tests were performed on the cells for 30 cycles by repeating the above operations. In addition, the slurries prepared in examples 1 to 3 and comparative examples 1 to 2 were coated on a PET film, dried and punched into a 12mm diameter round piece, and the resistivity thereof was measured. The test results are shown in Table 1.
TABLE 1 table of results of physical and chemical properties and electrical properties tests of examples 1-3 and comparative examples 1-2
Test object Tap density (g/cm)3) Specific surface area (m)2/g) Resistivity (m omega/cm) Initial capacity (mAh/g) First coulomb efficiency Capacity retention at week 30
Example 1 1.05 1.35 35.35 827.5 89.5% 101.5%
Example 2 0.98 1.29 30.33 780.7 90.3% 99.4%
Example 3 0.93 1.33 28.57 746.6 90.5% 98.9%
Comparative example 1 0.35 12.8 32.17 723.8 77.6% 87.6%
Comparative example 2 1.13 0.89 124.4 750.2 83.9% 81.2%
As is clear from the data in Table 1, the specific surface areas of the silicon carbon materials obtained in examples 1 to 3 were 1.35m2/g、1.29m2G and 1.33m2(ii)/g; in comparative example 1 without coating pitch, a silicon carbon materialHas a specific surface area of 12.8m2(ii)/g; therefore, the specific surface area of the silicon-carbon material can be effectively reduced by coating the asphalt on the surface of the graphene. The asphalt soft carbon has higher carbon residue rate, less gas production, less structural defects and compactness in the high-temperature carbonization process, and can greatly reduce the specific surface area of the material. Meanwhile, in comparative example 2 to which no graphene was added, the resistivity value was 124.4m Ω/cm; the resistivity values in examples 1 to 3 were 40 m.OMEGA./cm or less. The graphene has high electronic conductivity, good chemical stability and small charge transfer impedance, and a continuous conductive network formed by a two-dimensional plane of the graphene can greatly reduce the resistivity of the silicon-carbon material. The coated asphalt also has performance improvement of nearly 10% in the aspects of gram capacity, first effect, cycle performance and the like of the silicon-carbon material. In the aspect of the cycling stability of the silicon-carbon material, the surface coating layer of the asphalt soft carbon can prevent the inside of electrolyte particles from generating side reaction with active substances, and the generation of an unstable SEI film is reduced; meanwhile, the graphene has good mechanical strength and flexibility, and can be reversibly deformed by the volume change of silicon so as to keep the electrolytic contact among nano silicon, thereby being beneficial to the circulation stability of the silicon-carbon material. The silicon-carbon cathode prepared by the technology has the advantages of high tap density, small specific surface area, high coulombic efficiency, good cycle stability and the like.
Button cells were made using the silicon carbon composite of example 1 as the electrode and lithium foil as the counter electrode. The first charge-discharge curve and the cycle performance curve are respectively shown in fig. 2 and fig. 3. From the results in FIG. 2, the charge-discharge voltage curve of the material is stable, the gram capacity of the electrical property of the material is more than or equal to 800mAh/g, and the first effect is more than or equal to 89%. From the results of fig. 3, the examples showed excellent cycle performance, capacity retention after 30-week cycle, 101.5% for example 1, 99.4% for example 2, and 98.9% for example 3. In comparative examples 1 and 2, however, the capacity fade was relatively fast, and the capacity retention rates were 87.6% and 81.2%, respectively. Therefore, under the protection of the triple nuclear structure of hard carbon, graphene and soft carbon, the problems of volume expansion, electronic conduction, contact with electrolyte and the like of nano-silicon are effectively improved, and the method plays a key role in greatly improving the electrical property of the material.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (5)

1. A preparation method of a silicon-carbon composite material is characterized in that,
at least comprises the following steps:
1) adding nano silicon powder into the sweet potato starch solution, uniformly stirring, and performing first spray drying and first high-temperature carbonization to obtain a precursor A;
2) adding polydopamine and the precursor A into a graphene oxide aqueous solution, uniformly stirring, and then carrying out secondary spray drying to obtain a precursor B;
3) and sequentially adding asphalt, the precursor B and the artificial graphite into the aqueous solution of sodium carboxymethylcellulose, mixing and stirring to obtain a mixed solution, and performing third spray drying and second high-temperature carbonization treatment on the mixed solution to obtain the silicon-carbon negative electrode material.
2. The method for preparing a silicon-carbon composite material according to claim 1, wherein:
in the step 1), the sweet potato starch and the nano silicon powder are added into water according to the mass ratio of 1:1-1:6, and stirred, dissolved and dispersed; the particle size of the nano silicon powder is 30-150nm, and the concentration of the nano silicon powder in the water is 3-10%; the temperature of the equipment feed inlet of the nano silicon powder in the first spray drying step is controlled to be 190-210 ℃, and the temperature of the equipment discharge outlet is controlled to be 90-100 ℃; the temperature of the first high-temperature carbonization is 500-800 ℃, the carbonization time is 2-6h, and the heating rate in the carbonization process is 2-15 ℃/min.
3. The method for preparing a silicon-carbon composite material according to claim 1, wherein:
in the step 2), the adding proportion of the graphene oxide, the polydopamine and the precursor A is as follows according to the mass percentage: the graphene oxide accounts for 10% -30%, the polydopamine accounts for 10% -30%, and the precursor A accounts for 40% -80%; the carbon content of the graphene oxide is more than or equal to 80%, and the number of layers is less than or equal to 10.
4. The method for preparing a silicon-carbon composite material according to claim 1, wherein:
in the step 3), the adding proportion of the sodium carboxymethylcellulose, the pitch, the precursor B and the artificial graphite is as follows according to the mass percentage: the proportion of the sodium carboxymethylcellulose is 1-10%, the proportion of the asphalt is 5-20%, the proportion of the precursor B is 5-30%, and the proportion of the artificial graphite is 40-80%.
5. The method for preparing a silicon-carbon composite material according to claim 1, wherein:
in step 3), the specific process of the second high-temperature carbonization treatment comprises: when the temperature reaches the asphalt softening point temperature of 150-300 ℃, preserving the heat for 1-3 h; continuously heating to the temperature of 900-1100 ℃ asphalt carbonization, and keeping the temperature for 2-5 h; the temperature rise speed in the whole process is controlled to be 2-15 ℃/min.
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