CN116646482B

CN116646482B - Silicon-carbon composite material, preparation method of silicon-carbon composite material and secondary battery

Info

Publication number: CN116646482B
Application number: CN202310440237.0A
Authority: CN
Inventors: 陈振宇; 郑安华; 余德馨; 傅儒生; 仰韻霖
Original assignee: Guangdong Kaijin New Energy Technology Co Ltd
Current assignee: Guangdong Kaijin New Energy Technology Co Ltd
Priority date: 2023-04-21
Filing date: 2023-04-21
Publication date: 2024-04-05
Anticipated expiration: 2043-04-21
Also published as: CN116646482A

Abstract

The invention relates to the technical field of battery materials, and discloses a silicon-carbon composite material, a preparation method of the silicon-carbon composite material and a secondary battery. The silicon-carbon composite material comprises a carbon coating outer layer, a silicon-based inner core and a carbon filling body arranged between the silicon-based inner core and the carbon coating outer layer. The silicon-based core comprises a silicon core from inside to outside, a first carbon layer, a cavity and a second carbon layer. The first carbon layer and the second carbon layer enclose a cavity and at least point contact is made between the first carbon layer and the second carbon layer. The silicon core comprises nano silicon and SiO coated with the nano silicon _x Layer, x is more than 0 and less than or equal to 2. The silicon-carbon composite material can effectively relieve the huge volume effect generated in the lithium intercalation and deintercalation process of silicon, and can ensure the stability of the material structure and improve the conductivity of the material.

Description

Silicon-carbon composite material, preparation method of silicon-carbon composite material and secondary battery

Technical Field

The invention relates to the technical field of battery materials, in particular to a silicon-carbon composite material, a preparation method of the silicon-carbon composite material and a secondary battery.

Background

At present, graphite is mainly used as a negative electrode material of a commercial lithium ion battery, but the specific capacity of the graphite is close to the theoretical specific capacity (372 mAh/g), so that the development of high-energy-density and high-cycle lithium ion battery products is severely limited.

Silicon is used as a novel negative electrode material of a lithium ion battery, has the advantages of higher theoretical specific capacity (4200 mAh/g), very wide source, low cost and the like, and is hopeful to become one of the next-generation negative electrode materials. However, silicon also has the obvious disadvantages of low conductivity, serious volume expansion in the charge and discharge processes, and the like. Currently, many researchers mainly start to solve the current defects of silicon-based materials from the structure, and three directions are mainly: material nanocrystallization, material compounding and material pore structure design.

Material nanocrystallization is typically the preparation of silicon materials into nano-silicon particles, nano-silicon films, silicon nanotubes, and the like. The material compounding can adopt a coating material such as carbon or oxide to carry out surface coating on silicon, or compound silicon and other materials to prepare a silicon composite material. Although the above-mentioned method can improve the cycle performance of the silicon material to a certain extent, the capacity attenuation is still obvious, and the cycle performance after modification is poor. The pore structure of the design material can better relieve the huge volume effect of silicon in the lithium removal and intercalation process, but the current main design flow adopts an acid etching mode to form a hollow structure on the surface of nano silicon, and the mode causes acid pollution and increases the mass production cost, so that the method is not beneficial to industrialization. Meanwhile, the acid etching process can acid-wash the SiO on the surface of the nano silicon _x Layers result in poor material properties, especially with respect to cycle performance.

Therefore, how to effectively relieve the volume expansion of the silicon material, and to obtain the silicon negative electrode material with high specific capacity and long cycle life with more environmental protection and low energy consumption is a problem to be solved in the current lithium battery field.

Disclosure of Invention

In view of the foregoing, an object of the present invention is to provide a silicon-carbon composite material, a method for producing the silicon-carbon composite material, and a secondary battery. The silicon-carbon composite material can effectively relieve the huge volume effect generated in the lithium intercalation and deintercalation process of silicon, and can ensure the stability of the material structure and improve the conductivity of the material. Therefore, the secondary battery prepared by adopting the silicon-carbon composite material as the lithium ion battery anode material has excellent cycle performance, lower volume expansion effect, higher first coulomb efficiency and first charge-discharge capacity.

To achieve the above object, a first aspect of the present invention provides a silicon carbon composite material. The silicon-carbon composite material comprises a carbon coating outer layer, a silicon-based inner core and a silicon-based intermediate layerA carbon filler between the inner core and the carbon-coated outer layer. The silicon-based core comprises a silicon core from inside to outside, a first carbon layer, a cavity and a second carbon layer. The first carbon layer and the second carbon layer enclose a cavity and at least point contact is made between the first carbon layer and the second carbon layer. The silicon core comprises nano silicon and SiO coated with the nano silicon _x And a layer, wherein x is more than 0 and less than or equal to 2.

The silicon-carbon composite material is characterized in that a carbon filler and a carbon cladding outer layer are arranged outside a silicon-based core, and a first carbon layer, a second carbon layer and a cavity between the first carbon layer and the second carbon layer are arranged outside the silicon-based core. Therefore, in the silicon-carbon composite material, the first carbon layer, the cavity, the second carbon layer, the carbon filling body and the carbon coating outer layer are arranged outside the nano silicon. The 5-layer coating structure can synergistically relieve the huge volume effect of nano silicon in the lithium removal and intercalation process, can inhibit side reaction between active silicon and electrolyte, and can further improve the conductivity of the material by contact between the first carbon layer and the second carbon layer. Therefore, the secondary battery adopting the silicon-carbon composite material has excellent cycle performance, lower volume expansion effect, higher first coulomb efficiency and first charge-discharge capacity.

In some embodiments, the nanosilicon is polycrystalline nanosilicon and/or amorphous nanosilicon.

In some embodiments, siO _x The thickness of the layer is 0.1nm to 20.0nm.

In some embodiments, the volume of the cavity is 10% to 50% of the volume of the silicon-based core.

In some embodiments, the thickness of the first carbon layer and the second carbon layer are each independently from 2nm to 50nm.

In some embodiments, the carbon-coated outer layer has a thickness of 10nm to 500nm.

In some embodiments, the first carbon layer coats at least 90% of the area of the outer surface of the silicon core.

In some embodiments, the ratio of closed cell volume to apparent volume of the silicon carbon composite is m, m being from 10% to 50%.

In some embodiments, the total carbon content of the silicon carbon composite is 10wt.% to 40wt.%.

In some embodiments, the carbon filler is amorphous carbon.

In some embodiments, the silicon carbon composite is not additionally doped with N, P, S, B element.

In a second aspect, the invention provides a method for preparing a silicon-carbon composite material, comprising the following steps.

(I) Preparation of nano silicon slurry

Adding the nano silicon powder and the surfactant into a solvent for grinding.

(II) preparation of a pore-Forming agent solution

And adding the pore-forming agent into an organic solvent, wherein the pore-forming agent is a high polymer with a carbon residue rate of 1-10% after sintering.

(III) preparation of the first precursor

And mixing the nano silicon slurry with the pore-forming agent solution and drying.

(IV) preparation of the second precursor

And mixing the first precursor and a carbon source, and then performing high-temperature heat treatment.

(V) carbon coating

And coating the second precursor with carbon.

According to the preparation method of the silicon-carbon composite material, the surfactant and the nano silicon powder are mixed, the electrostatic repulsion principle is combined with mechanical dispersion to improve the single particle dispersibility of the nano silicon powder in a mixed solvent, and the surface of the nano silicon powder is oxidized to form SiO _x A layer; the pore-forming agent solution can be uniformly coated on the surface of the nanometer silicon powder in advance through stirring and drying; the carbon source can be decomposed firstly by high-temperature heat treatment to form a carbon filling body, then the pore-forming agent is decomposed, the pore-forming agent can be melted and flowed in the heating process to be respectively adhered to the surface of the silicon core and the surface of the carbon filling body, and along with the continuous heating, a large amount of micromolecular gas can be generated by cracking in the sintering process of the pore-forming agent, and the pore-forming agent occupies a certain space and has extremely low carbon residue rate (only 1 to 10 percent), so that a first carbon layer is formed on the surface of the silicon core, a second carbon layer is formed near the surface of the carbon filling body, a cavity is formed between the first carbon layer and the second carbon layer, and the porous carbon material is subjected to the heatingThe first carbon layer coated silicon core is submerged under gravity into at least point contact with the second carbon layer. According to the invention, on the basis of the silicon-carbon composite material, the material is subjected to preset pore forming and a plurality of carbon coating layers are formed, the preparation process is environment-friendly and simple, and the huge volume effect of nano silicon in the lithium removal and intercalation process can be effectively relieved. In addition, in the preparation method, acid etching is not used, so that the performance of the material can be improved, acid pollution is avoided, and the cost of mass production is increased.

In some embodiments, the nano-silicon slurry and the pore former solution are mixed to adjust the solids content to 10% to 20%.

In some embodiments, the solids content is adjusted using at least one of the following solvents: methanol, ethanol, isopropanol, benzene, toluene, xylene, acetone, methyl butanone, methyl isobutyl ketone, cyclohexane, isoheptane, 3-dimethylpentane, N-methylpyrrolidone, tetrahydrofuran and dimethylformamide.

In some embodiments, the pore-forming agent is an alcohol-soluble high molecular polymer.

In some embodiments, the high molecular polymer comprises at least one of an alcohol-soluble polyamide resin, an alcohol-soluble nylon, an alcohol-soluble acrylic resin, and a rosin-modified alkyd resin.

In some embodiments, the nanosilicon has a particle size D50 of from 30nm to 150nm.

In some embodiments, the mass ratio of the nano silicon powder to the surfactant is 5-20:1.

In some embodiments, the mass ratio of the nano silicon powder to the pore-forming agent is 1-100:1.

In some embodiments, the surfactant is an anionic surfactant.

In some embodiments, the surfactant comprises at least one of sodium lauryl sulfate, sodium dodecylbenzene sulfonate, sodium lignin sulfonate, sodium p-methylstyrene sulfonate, sodium isooctyl alcohol sulfate, and naphthalene sulfonic acid formaldehyde condensate.

In some embodiments, the solvent in step (I) is a mixture of water and an organic solvent, the organic solvent in the mixture being present in an amount of not less than 95wt.%. Alternatively, the organic solvent includes at least one of methanol, ethanol, isopropanol, benzene, toluene, xylene, acetone, methyl butanone, methyl isobutyl ketone, cyclohexane, isoheptane, 3-dimethylpentane, N-methylpyrrolidone, tetrahydrofuran, and dimethylformamide.

In some embodiments, the nano-silicon slurry prepared in step (I) is stirred for a period of 10min to 60min.

In some embodiments, the milling is performed with a mill for a period of time ranging from 2 hours to 6 hours.

In some embodiments, the organic solvent in the step (II) preparation of the pore-former solution comprises at least one of methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, and propylene glycol.

In some embodiments, the drying is spray drying. The temperature of the spray-dried feed inlet is 120-200 ℃, and the temperature of the spray-dried discharge outlet is 70-100 ℃.

In some embodiments, the drying is by evaporation. The heating temperature for the evaporation drying is 50 ℃ to 100 ℃.

In some embodiments, the carbon source comprises at least one of pitch, phenolic resin, sucrose, glucose, starch, polyvinyl alcohol, epoxy resin, polydopamine, lignin, and citric acid.

In some embodiments, step (IV) is performed by mechanical mixing during the preparation of the second precursor. Mixing is VC mixing, ball milling mixing or grinding mixing.

In some embodiments, step (IV) is performed at a rate of 1000r/min to 1500r/min for mixing in preparing the second precursor.

In some embodiments, the high temperature heat treatment is performed under a protective atmosphere. The protective atmosphere comprises at least one of argon, neon, nitrogen and helium.

In some embodiments, the temperature of the high temperature heat treatment is 600 ℃ to 800 ℃.

In some embodiments, the elevated temperature rate of the high temperature heat treatment is from 1 ℃/min to 10 ℃/min.

In some embodiments, the time of the high temperature heat treatment is from 2 hours to 8 hours.

In some embodiments, the carbon coating is by gas phase coating, liquid phase coating, or solid phase coating.

In some embodiments, the carbon coating is also followed by sieving.

In some embodiments, the method of preparation is not additionally doped with N, P, S, B element.

In some embodiments, no acid is used in the preparation process, or an acid etching process is not included.

The third aspect of the invention provides application of the silicon-carbon composite material in a negative electrode material.

A fourth aspect of the present invention provides a secondary battery comprising a positive electrode material and a negative electrode material, the negative electrode material comprising the aforementioned silicon-carbon composite material, or a silicon-carbon composite material prepared by the aforementioned method for preparing a silicon-carbon composite material.

Drawings

FIG. 1 is a schematic structural view of a silicon carbon composite material of the present invention;

fig. 2 is a FESEM view of the silicon-carbon composite material of example 1.

Description of the embodiments

The silicon-carbon composite material can be used as a negative electrode active material to be applied to secondary batteries (such as sodium ion batteries, lithium ion batteries or potassium ion batteries). The negative electrode active material can be used alone or in combination with other negative electrode active materials (e.g., natural graphite, artificial graphite, soft carbon, hard carbon, etc.). The silicon carbon composite material of the present invention may be free of additional doping atoms, such as free of heteroatoms such as N, P, S, B.

The ratio of closed cell volume to apparent volume of the inventive silicon carbon composite is m, m being 10% to 50%, m may be, by way of example and not limitation, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%.

In actual operation, the value of m= (V ₁ -V ₀ )/V ₂ M can be calculated. Wherein V is ₀ Is the real volume, i.e. the volume of the dried sample tested by helium displacement method; v (V) ₁ Is the effective volume, i.e. mass m of the dried sample ₁ Subtracting the mass m of the saturated sample in water ₂ Ratio of post to density of water, mass m of saturated sample in water ₂ Can be measured by a drainage method; v (V) ₂ For apparent volume, i.e. mass m of saturated sample in air ₃ Subtracting the mass m of saturated sample in water ₂ Ratio of the latter to the density of water.

The total carbon content of the silicon-carbon composite is 10wt.% to 40wt.%, as an example, the total carbon content of the silicon-carbon composite may be, but is not limited to, 10wt.%, 13wt.%, 15wt.%, 18wt.%, 20wt.%, 23wt.%, 25wt.%, 27wt.%, 30wt.%, 32wt.%, 35wt.%, 37wt.%, 40wt.%. The carbon in the silicon-carbon composite material is derived from a carbon-coated outer layer, a carbon filler, a first carbon layer and a second carbon layer.

A schematic structural diagram of the silicon carbon composite material of the present invention can be shown in fig. 1. The silicon carbon composite material includes a carbon-coated outer layer 10, a silicon-based core 30, and a carbon filler 50 interposed between the silicon-based core 30 and the carbon-coated outer layer 10. The silicon-based core 30 in the silicon-carbon composite of the present invention may be one or more, such as at least 2, at least 3, at least 4, at least 5, dispersed in the carbon filler 50. The silicon-based core 30 includes a silicon core 31 from inside to outside, a first carbon layer 33, a cavity 35, and a second carbon layer 37. The silicon core 31 includes nano silicon and SiO coated with nano silicon _x Layer, x is more than 0 and less than or equal to 2.

Wherein the nano silicon is polycrystalline nano silicon and/or amorphous nano silicon. SiO (SiO) _x The layer is formed by oxidizing the surface of the nano silicon with high activity in the preparation process, and can cover the whole surface of the nano silicon. SiO (SiO) _x X may be, but is not limited to, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, in certain embodiments 0 < x < 2, or 0.7.ltoreq.x.ltoreq.1.3. SiO (SiO) _x The thickness of the layer is 0.1nm to 20.0nm, as an example, siO _x The thickness of the layer may be, but is not limited to, 0.1nm, 1.0nm, 2.0nm, 4.0nm, 6.0nm, 8.0nm, 10.0nm, 12.0nm,14.0nm、16.0nm、18.0nm、20.0nm。

The first carbon layer and the second carbon layer are obtained by sintering pore-forming agents. The thicknesses of the first and second carbon layers are each independently 2nm to 50nm, and as an example, the thicknesses of the first and second carbon layers are each independently 2nm, 4nm, 6nm, 8nm, 10nm, 12nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm. The first carbon layer coats at least 90% of the outer surface area of the silicon core, as an example, the first carbon layer may coat at least 95%, 96%, 97%, 98%, 99%, 100% of the outer surface area of the silicon core. In practice, the first carbon layer may cover only a portion of SiO _x The outer surface of the layer, or coating all SiO _x The outer surface of the layer. At least point contact between the first carbon layer and the second carbon layer, in practice, there may be some surface contact between the first carbon layer and the second carbon layer.

The pore-forming agent adopted by the invention has lower carbon residue rate, and a cavity is formed between the first carbon layer and the second carbon layer after the pore-forming agent is sintered. The volume of the cavity is 10% to 50%, or 10% to 40%, or 10% to 30% of the volume of the silicon-based core. By way of example, the volume fraction of the cavity may be, but is not limited to, 10%, 13%, 15%, 17%, 20%, 22%, 25%, 27%, 30%, 33%, 35%, 37%, 40%, 42%, 44%, 46%, 48%, 50%.

The carbon coating outer layer is formed by gas phase coating, liquid phase coating or solid phase coating of a carbon material. The thickness of the carbon-coated outer layer is 10nm to 500nm, and as an example, the thickness of the carbon-coated outer layer may be, but is not limited to, 10nm, 30nm, 50nm, 70nm, 100nm, 120nm, 140nm, 160nm, 180nm, 200nm, 230nm, 260nm, 300nm, 330nm, 370nm, 400nm, 430nm, 450nm, 470nm, 500nm. The carbon filler is amorphous carbon, is filled in gaps between adjacent silicon-based inner cores and on the surfaces of the silicon-based inner cores, and is obtained by sintering asphalt, phenolic resin, sucrose, glucose, starch, polyvinyl alcohol, epoxy resin, polydopamine, lignin, citric acid and other organic matters.

The preparation method of the silicon-carbon composite material comprises the steps of preparing nano silicon slurry in the step (I), preparing pore-forming agent solution in the step (II), preparing a first precursor in the step (III), preparing a second precursor in the step (IV) and coating carbon in the step (V).

The step (I) of preparing the nano silicon slurry comprises the steps of adding nano silicon powder and a surfactant into a solvent for grinding.

As an embodiment of the invention, the particle size D50 of the nano silicon powder is 30nm to 150nm, or 60nm to 120nm. By way of example, the particle size D50 of the nano-silicon powder may be, but is not limited to, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm.

As an embodiment of the invention, the mass ratio of the nano silicon powder to the surfactant is 5-20:1. As an example, the mass ratio of the nano silicon powder to the surfactant may be, but is not limited to, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1.

As an embodiment of the present invention, the surfactant is an anionic surfactant. The surfactant includes at least one of sodium lauryl sulfate, sodium dodecylbenzene sulfonate, sodium lignin sulfonate, sodium p-methylstyrene sulfonate, sodium isooctyl alcohol sulfate and naphthalene sulfonic acid formaldehyde condensate.

As an embodiment of the present invention, the solvent in step (I) is a mixture of water and an organic solvent, the organic solvent in the mixture having a ratio of not less than 95wt.%, and as an example, the organic solvent in the mixture may have a ratio of not less than 95wt.%, 96wt.%, 97wt.%, 98wt.%, 99wt.%. The organic solvent comprises at least one of methanol, ethanol, isopropanol, benzene, toluene, xylene, acetone, methyl butanone, methyl isobutyl ketone, cyclohexane, isoheptane, 3-dimethylpentane, N-methylpyrrolidone, tetrahydrofuran and dimethylformamide.

As an embodiment of the present invention, the stirring is performed for a period of 10min to 60min before the grinding in the step (I). By way of example, the stirring time may be, but is not limited to, 10min, 20min, 30min, 40min, 50min, 60min. Grinding is performed using a grinder for a period of 2h to 6h, and as an example, the grinding period may be, but not limited to, 2h, 3h, 4h, 5h, 6h.

As an embodiment of the present invention, the nano-silicon slurry in step (I) has a solid content of 10% to 30%, or 10% to 25%, or 10% to 20%. By way of example, the solids content may be, but is not limited to, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%.

Because the anionic surfactant is ionized in the solvent and can be partially negatively charged and simultaneously adsorbed on the nano silicon, the repulsive force of the nano silicon surface is enhanced due to the same charge, so that the dispersing effect is achieved; and then, by applying mechanical force to the dispersion system, the physical and chemical properties of substances in the system can be changed, and the dispersion of the nano silicon powder is realized along with a series of chemical reactions.

Step (II) preparing a pore-former solution includes adding a pore-former to an organic solvent.

As an embodiment of the present invention, the pore-forming agent is a high molecular polymer having a carbon residue rate of 1% to 10% after sintering. In certain embodiments, the char yield is from 1% to 8%, or from 1% to 5%. As an example, the char yield may be, but is not limited to, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%. In certain embodiments, the pore former has a sintering temperature of 200 ℃ to 450 ℃, or 250 ℃ to 400 ℃, or 300 ℃ to 350 ℃. By way of example, the sintering temperature of the pore former may be, but is not limited to, 200 ℃, 230 ℃, 250 ℃, 270 ℃, 300 ℃, 330 ℃, 350 ℃, 370 ℃, 400 ℃, 420 ℃, 430 ℃, 450 ℃. In practice, the sintering temperature of the pore-forming agent is determined according to the specific material thereof so as to satisfy the carbon residue rate of 1 to 10% remaining after sintering.

As one embodiment of the invention, the pore-forming agent is an alcohol-soluble high molecular polymer. The high molecular polymer comprises at least one of alcohol-soluble polyamide resin, alcohol-soluble nylon, alcohol-soluble acrylic resin and rosin modified alkyd resin. The organic solvent in the pore-forming agent solution is prepared from at least one of methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol and propylene glycol.

The step (III) of preparing the first precursor comprises the steps of mixing nano-silicon slurry and pore-forming agent solution and drying.

As an embodiment of the invention, the mass ratio of the nano silicon powder to the pore-forming agent is 1-100:1, or 1-50:1, or 1-10:1. As an example, the mass ratio of the nano silicon powder to the pore former may be, but is not limited to, 1:1, 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1.

As an embodiment of the present invention, the solid content is adjusted to 10% to 20% after the nano-silicon slurry and the pore-forming agent solution are mixed. Adjusting the solids content using at least one of the following solvents: methanol, ethanol, isopropanol, benzene, toluene, xylene, acetone, methyl butanone, methyl isobutyl ketone, cyclohexane, isoheptane, 3-dimethylpentane, N-methylpyrrolidone, tetrahydrofuran and dimethylformamide. In certain embodiments, the solids content is 12% to 18%, or 14% to 16%. By way of example, the solids content may be, but is not limited to, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%.

As an embodiment of the present invention, spray drying or evaporation drying may be used for drying. The feed inlet temperature of spray drying is 120 ℃ to 200 ℃, or 120 ℃ to 170 ℃, or 130 ℃ to 150 ℃. By way of example, the feed inlet temperature of spray drying may be, but is not limited to, 120 ℃, 130 ℃, 140 ℃, 150 ℃, 160 ℃, 170 ℃, 180 ℃, 190 ℃, 200 ℃. The spray-dried discharge temperature is 70 ℃ to 100 ℃, or 70 ℃ to 90 ℃, or 70 ℃ to 80 ℃. By way of example, the exit temperature of spray drying may be, but is not limited to, 70 ℃, 75 ℃, 80 ℃, 85 ℃, 90 ℃, 95 ℃, 100 ℃. The drying is carried out by evaporation, and the heating temperature of evaporation drying is 50 ℃ to 100 ℃, or 50 ℃ to 90 ℃, or 50 ℃ to 80 ℃. As an example, the heating temperature of the evaporation drying may be, but is not limited to, 50 ℃, 55 ℃, 60 ℃, 65 ℃, 70 ℃, 75 ℃, 80 ℃, 85 ℃, 90 ℃, 95 ℃, 100 ℃. The stirring speed in the evaporation drying is 100r/min to 300r/min, or 100r/min to 250r/min, or 100r/min to 200r/min. By way of example, the stirring speed at the time of evaporative drying may be, but is not limited to, 100r/min, 120r/min, 150r/min, 180r/min, 200r/min, 220r/min, 250r/min, 270r/min, 300r/min.

The step (IV) of preparing the second precursor includes mixing the first precursor and the carbon source and then performing a high temperature heat treatment.

As an embodiment of the present invention, the carbon source includes at least one of pitch, phenolic resin, sucrose, glucose, starch, polyvinyl alcohol, epoxy resin, polydopamine, lignin, and citric acid, and the carbon source forms a carbon filler after sintering.

As an embodiment of the invention, mechanical mixing is employed for mixing, such as VC mixing, ball milling or grinding. The mixing is carried out at a rate of 1000r/min to 1500r/min, or 1200r/min to 1500r/min, or 1300r/min to 1500r/min. By way of example, the rate at which mixing is employed may be, but is not limited to, 1000r/min, 1100r/min, 1200r/min, 1300r/min, 1400r/min, 1500r/min.

As an embodiment of the present invention, the high temperature heat treatment is performed under a protective atmosphere including at least one of argon, neon, nitrogen and helium. The temperature of the high-temperature heat treatment is 600 ℃ to 800 ℃, or 650 ℃ to 750 ℃. By way of example, the temperature of the high temperature heat treatment may be, but is not limited to, 600 ℃, 620 ℃, 640 ℃, 660 ℃, 680 ℃, 700 ℃, 720 ℃, 740 ℃, 760 ℃, 780 ℃, 800 ℃. The temperature rising rate of the high-temperature heat treatment is 1 ℃/min to 10 ℃/min, or 1 ℃/min to 5 ℃/min, or 1 ℃/min to 3 ℃/min. By way of example, the heating rate of the high temperature heat treatment may be, but is not limited to, 1 ℃/min, 2 ℃/min, 3 ℃/min, 4 ℃/min, 5 ℃/min, 6 ℃/min, 7 ℃/min, 8 ℃/min, 9 ℃/min, 10 ℃/min. The time of the high-temperature heat treatment is 2 to 8 hours, or 2 to 6 hours, or 3 to 5 hours. As examples, the time of the high temperature heat treatment may be, but is not limited to, 2h, 3h, 4h, 5h, 6h, 7h, 8h.

Step (V) carbon coating includes subjecting the second precursor to carbon coating followed by sieving. The carbon coating can be gas phase coating, liquid phase coating or solid phase coating. Of course, other coating methods such as plasma may be used as long as the coating forms a carbon-coated outer layer. The carbon-coated outer layer formed by the method can be one layer, two layers, three layers and the like. The silicon-carbon composite material is not limited by a carbon coating mode, and is also not limited by the number of layers of the carbon coating outer layer. Among them, the gas phase coating, the liquid phase coating or the solid phase coating can be operated in a conventional manner.

As an example, the gas phase cladding may be, but is not limited to, placing the second precursor in a CVD furnace, heating to 700 ℃ to 900 ℃ at a rate of 5 ℃/min to 10 ℃/min, introducing inert gas at a rate of 4L/min to 10L/min, introducing a gas phase carbon source at a rate of 0.1L/min to 1.0L/min for 2h to 7h, and naturally cooling to room temperature. Wherein the heating rate can be, but is not limited to, 5 ℃/min, 6 ℃/min, 7 ℃/min, 8 ℃/min, 9 ℃/min, 10 ℃/min. The reaction temperature may be, but is not limited to, 700 ℃, 750 ℃, 800 ℃, 850 ℃, 900 ℃. The inert gas includes at least one of nitrogen, argon and helium. The flow rate of the inert gas may be, but is not limited to, 4L/min, 5L/min, 6L/min, 7L/min, 8L/min, 9L/min, 10L/min. The gaseous carbon source may be, but is not limited to, at least one of methane, ethane, ethylene, acetylene, propane, and propylene. The gas flow rate of the gas phase carbon source may be, but is not limited to, 0.1L/min, 0.2L/min, 0.3L/min, 0.4L/min, 0.5L/min, 0.6/min, 0.7L/min, 0.8L/min, 0.9L/min, 1.0/min. The time for introducing the gaseous carbon source may be, but is not limited to, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours.

As an example, the liquid phase coating may be, but not limited to, mixing and dispersing the second precursor, the organic carbon source and the solvent in a certain ratio, then spray-drying, then heating to 700 to 900 ℃ at a rate of 1 to 5 ℃ per minute under an inert gas atmosphere, performing a heat treatment, and naturally cooling to room temperature. Wherein the organic carbon source may be, but is not limited to, at least one of polyvinyl alcohol, phenolic resin, epoxy resin, polydopamine, lignin, citric acid, glucose and sucrose. The solvent may be, but is not limited to, at least one of water, ethanol, acetone, and isopropanol. The inert gas includes at least one of nitrogen, argon and helium. The heating rate may be, but is not limited to, 1 ℃/min, 2 ℃/min, 3 ℃/min, 4 ℃/min, 5 ℃/min. The temperature of the heat treatment may be, but is not limited to, 700 ℃, 750 ℃, 800 ℃, 850 ℃, 900 ℃.

As an example, the solid phase coating may be, but is not limited to, dispersing the second precursor and the organic carbon source by high-speed mixing, heating to 700 ℃ to 900 ℃ at a rate of 1 ℃/min to 5 ℃/min under an inert gas environment, performing heat treatment, and naturally cooling to room temperature. Wherein the organic carbon source may be, but is not limited to, at least one of solid phase pitch, glucose, sucrose, phenolic resin and starch. The high speed mixing dispersion may be performed using general purpose equipment and the parameters employed for mixing may be conventional parameters. The heating rate may be, but is not limited to, 1 ℃/min, 2 ℃/min, 3 ℃/min, 4 ℃/min, 5 ℃/min. The temperature of the heat treatment may be, but is not limited to, 700 ℃, 750 ℃, 800 ℃, 850 ℃, 900 ℃.

As an embodiment of the present invention, the carbon coating may be followed by sieving using a 100 mesh to 500 mesh sieve, which may be, by way of example and not limitation, 100 mesh, 130 mesh, 150 mesh, 170 mesh, 200 mesh, 230 mesh, 250 mesh, 300 mesh, 350 mesh, 400 mesh, 450 mesh, 500 mesh.

As an embodiment of the present invention, the preparation method is not additionally doped with N, P, S, B element.

As an embodiment of the present invention, no acid is used in the preparation process.

For better illustrating the objects, technical solutions and advantageous effects of the present invention, the present invention will be further described with reference to examples. It should be noted that the following implementation of the method is a further explanation of the present invention and should not be taken as limiting the present invention.

Example 1

This example is a method for preparing a silicon carbon composite material, comprising the following steps.

(I) Preparation of nano silicon slurry

800g of nano silicon powder with the granularity D50 of 120nm and 80g of sodium dodecyl benzene sulfonate are added into a mixed solvent of n-butyl alcohol and water (96 wt.% of n-butyl alcohol) to be stirred for 30min and then ground for 4h by a grinder.

(II) preparation of a pore-Forming agent solution

80g of the alcohol-soluble polyamide resin was dissolved in ethanol and stirred.

(III) preparation of the first precursor

Mixing the nano silicon slurry and the pore-forming agent solution, regulating the solid content to 12% by adopting ethanol, continuously stirring for 30min, and then evaporating and drying the dispersed slurry, wherein the heating temperature of the evaporating and drying is 80 ℃, and the stirring speed during the evaporating and drying is 150r/min.

(IV) preparation of the second precursor

VC mixing the first precursor and asphalt according to the mass ratio of 10:6 at the speed of 1500r/min, performing heat treatment in nitrogen atmosphere after mixing, heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 4h, and naturally cooling to room temperature in a furnace.

(V) carbon coating

And (3) placing the second precursor in a CVD furnace, heating to 800 ℃ at a speed of 5 ℃/min, respectively introducing high-purity nitrogen at a speed of 4L/min, introducing acetylene gas at a speed of 0.5L/min for 4h, naturally cooling to room temperature, and performing scattering and screening treatment.

As can be seen from FIG. 2, the silicon-carbon composite material prepared in example 1 has a smooth surface, which indicates that the coating uniformity is better. The silicon-carbon composite material comprises a carbon-coated outer layer, a plurality of silicon-based inner cores and an amorphous carbon filler between the silicon-based inner cores and the carbon-coated outer layer. The silicon-based core comprises a silicon core, a first carbon layer, a cavity and a second carbon layer from inside to outside, and the silicon core comprises nano silicon and SiO coated with the nano silicon _x (0 < x < 0.8) layer, the first carbon layer and the second carbon layer are in point contact. Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0 < x < 0.8) layer is 5nm, the thickness of the carbon coating outer layer is about 110nm, the thickness of the first carbon layer and the second carbon layer are about 30nm, the volume of the cavity accounts for 20% of the volume of the silicon-based core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon core, the carbon residue rate of the alcohol-soluble polyamide resin is 5%, the total carbon content of the silicon-carbon composite material is 34 wt%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 18%.

Example 2

(I) Preparation of nano silicon slurry

(II) preparation of a pore-Forming agent solution

160g of the alcohol-soluble polyamide resin was dissolved in ethanol and stirred.

(III) preparation of the first precursor

(IV) preparation of the second precursor

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0.2 < x < 0.8) layer is 6nm, the thickness of the carbon coating outer layer is about 108nm, the thickness of the first carbon layer and the second carbon layer are about 21nm, the volume of the cavity accounts for 22% of the volume of the silicon-based inner core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon-based inner core, the carbon residue rate of the alcohol-soluble polyamide resin is 5%, the total carbon content of the silicon-carbon composite material is 35wt.%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 21.9%.

Example 3

(I) Preparation of nano silicon slurry

(II) preparation of a pore-Forming agent solution

(III) preparation of the first precursor

(IV) preparation of the second precursor

VC mixing the first precursor and asphalt according to the mass ratio of 10:3 at the speed of 1500r/min, performing heat treatment in nitrogen atmosphere after mixing, heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 4h, and naturally cooling to room temperature in a furnace.

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0.2 < x < 0.8) layer is 6nm, the thickness of the carbon coating outer layer is about 110nm, the thickness of the first carbon layer and the second carbon layer are about 20nm, the volume of the cavity accounts for 23% of the volume of the silicon-based inner core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon-based inner core, the carbon residue rate of the alcohol-soluble polyamide resin is 5%, the total carbon content of the silicon-carbon composite material is 23wt.%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 24%.

Example 4

(I) Preparation of nano silicon slurry

(II) preparation of a pore-Forming agent solution

80g of alcohol-soluble nylon was dissolved in ethanol and stirred.

(III) preparation of the first precursor

(IV) preparation of the second precursor

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0.1 < x < 0.7) layer is 4nm, the thickness of the carbon coating outer layer is about 115nm, the thickness of the first carbon layer and the second carbon layer are about 15nm, the volume of the cavity accounts for 18% of the volume of the silicon-based inner core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon-based inner core, the carbon residue rate of alcohol-soluble nylon is 1.91%, the total carbon content of the silicon-carbon composite material is 35wt.%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 18.9%.

Example 5

(I) Preparation of nano silicon slurry

(II) preparation of a pore-Forming agent solution

160g of alcohol-soluble nylon was dissolved in ethanol and stirred.

(III) preparation of the first precursor

(IV) preparation of the second precursor

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0.2 < x < 0.7) layer is 5nm, the thickness of the carbon coating outer layer is about 112nm, the thickness of the first carbon layer and the second carbon layer are about 10nm, the volume of the cavity accounts for 19% of the volume of the silicon-based inner core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon-based inner core, the carbon residue rate of alcohol-soluble nylon is 1.91%, the total carbon content of the silicon-carbon composite material is 35wt.%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 20.9%.

Example 6

(I) Preparation of nano silicon slurry

(II) preparation of a pore-Forming agent solution

160g of alcohol-soluble nylon was dissolved in ethanol and stirred.

(III) preparation of the first precursor

(IV) preparation of the second precursor

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0 < x < 0.7) layer is 3nm, the thickness of the carbon coating outer layer is about 108nm, the thickness of the first carbon layer and the second carbon layer are about 11nm, the volume of the cavity accounts for 24% of the volume of the silicon-based core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon-based core, the carbon residue rate of the alcohol-soluble nylon is 1.91%, the total carbon content of the silicon-carbon composite material is 22 wt%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 25.0%.

Example 7

(I) Preparation of nano silicon slurry

(II) preparation of a pore-Forming agent solution

(III) preparation of the first precursor

(IV) preparation of the second precursor

(V) carbon coating

And mixing and dispersing the second precursor, the polyvinyl alcohol and the ethanol according to a certain proportion, then carrying out spray drying, then heating to 800 ℃ at a speed of 3 ℃/min under a nitrogen environment for heat treatment, and naturally cooling to room temperature. And then carrying out scattering and screening treatment.

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0 < x < 0.8) layer is 5nm, the thickness of the carbon coating outer layer is about 111nm, the thickness of the first carbon layer and the second carbon layer are about 29nm, the volume of the cavity accounts for 20% of the volume of the silicon-based core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon core, the carbon residue rate of the alcohol-soluble polyamide resin is 5%, the total carbon content of the silicon-carbon composite material is 34 wt%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 18%.

Example 8

(I) Preparation of nano silicon slurry

800g of nano silicon powder with the particle size D50 of 150nm and 120g of sodium lauryl sulfate are added into a mixed solvent of isopropanol and water (the isopropanol accounts for 90 wt.%) and stirred for 50min, and then ground for 3h by a grinder.

(II) preparation of a pore-Forming agent solution

80g of the alcohol-soluble polyamide resin was dissolved in isopropanol and stirred.

(III) preparation of the first precursor

Mixing nano silicon slurry and pore-forming agent solution, regulating the solid content to 12% by adopting isopropanol, continuously stirring for 30min, and then evaporating and drying the dispersed slurry, wherein the heating temperature of the evaporating and drying is 90 ℃, and the stirring speed during the evaporating and drying is 200r/min.

(IV) preparation of the second precursor

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0 < x < 0.8) layer is 7nm, the thickness of the carbon coating outer layer is about 110nm, the thickness of the first carbon layer and the second carbon layer are about 30nm, the volume of the cavity accounts for 19% of the volume of the silicon-based core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon core, the carbon residue rate of the alcohol-soluble polyamide resin is 5%, the total carbon content of the silicon-carbon composite material is 34wt.%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 17%.

Example 9

(I) Preparation of nano silicon slurry

800g of nano silicon powder with the particle size D50 of 120nm and 80g of sodium lauryl sulfate are added into a mixed solvent of n-butanol and water (96 wt.% of n-butanol) and stirred for 30min, and then ground for 4h by a grinder.

(II) preparation of a pore-Forming agent solution

(III) preparation of the first precursor

Mixing nano silicon slurry and pore-forming agent solution, regulating the solid content to 20% by adopting ethanol, continuously stirring for 40min, and then spray-drying the dispersed slurry, wherein the temperature of a feed inlet of spray-drying is 190 ℃, and the temperature of a discharge outlet of spray-drying is 90 ℃.

(IV) preparation of the second precursor

Ball-milling and mixing the first precursor and starch according to a mass ratio of 10:6, wherein the mixing speed is 1300r/min, performing heat treatment in an argon atmosphere after mixing, heating to 750 ℃ at a speed of 5 ℃/min, preserving heat for 5h, and naturally cooling to room temperature in a furnace.

(V) carbon coating

Through detection and calculation, the nano silicon is polycrystalline nano silicon, siO _x The thickness of the (0 < x < 0.8) layer is 6nm, the thickness of the carbon coating outer layer is about 109nm, the thickness of the first carbon layer and the second carbon layer are about 31nm, the volume of the cavity accounts for 19% of the volume of the silicon-based core, the first carbon layer at least coats 90% of the area of the outer surface of the silicon core, the carbon residue rate of the alcohol-soluble polyamide resin is 5%, the total carbon content of the silicon-carbon composite material is 21wt.%, and the ratio m of the closed pore volume to the appearance volume of the silicon-carbon composite material is 18%.

Comparative example 1

(I) Preparation of the precursor

Mixing 800g of nano silicon powder with the granularity D50 of 120nm with asphalt according to the mass ratio of 10:6 at the speed of 1500r/min, performing heat treatment in nitrogen atmosphere after mixing, heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 4 hours, and naturally cooling to room temperature in a furnace.

(II) carbon coating

And (3) placing the precursor in a CVD furnace, heating to 800 ℃ at a speed of 5 ℃/min, respectively introducing high-purity nitrogen at a speed of 4L/min, introducing acetylene gas at a speed of 0.5L/min for 4h, naturally cooling to room temperature, and performing scattering and screening treatment.

By way of examination, the silicon-carbon composite material prepared in comparative example 1 includes a carbon-coated outer layer, a plurality of silicon-based cores, and an amorphous carbon filler interposed between the silicon-based cores and the carbon-coated outer layer. The silicon-based core comprises nano silicon and SiO coated with the nano silicon _x (0.1 < x < 0.8) layer, which has no cavity, a first carbon layer and a second carbon layer, silicon-based kernels in the formed structure are randomly distributed in the carbon filling body, and meanwhile, part of nano silicon also has agglomeration phenomenon.

Comparative example 2

(I) Preparation of nano silicon slurry

800g of nano silicon powder with the granularity D50 of 120nm is added into a mixed solvent of n-butyl alcohol and water (96 wt.% of n-butyl alcohol) to be stirred for 30min and then ground for 4h by a grinder.

(II) preparation of a pore-Forming agent solution

(III) preparation of the first precursor

(IV) preparation of the second precursor

(V) carbon coating

By way of examination, the silicon-carbon composite material prepared in comparative example 2 includes a carbon-coated outer layer, a plurality of silicon-based cores, and an amorphous carbon filler interposed between the silicon-based cores and the carbon-coated outer layer. The silicon-based core comprises a silicon core, wherein the silicon core comprises nano silicon and SiO coated with the nano silicon _x ((0 < x < 0.8) layer) the first and second carbon layers are present on only a portion (about 60% to 70% surface area) of the silicon core surface, most of the silicon core surface being in direct contact with the carbon filler.

Comparative example 3

(I) Preparation of nano silicon slurry

(II) preparation of precursor

VC mixing the nano silicon slurry and asphalt according to the mass ratio of 10:6, wherein the mixing speed is 1500r/min, carrying out heat treatment in nitrogen atmosphere after mixing, heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 4 hours, and naturally cooling to room temperature in a furnace.

(III) carbon coating

By way of examination, the silicon-carbon composite material prepared in comparative example 3 includes a carbon-coated outer layer, a plurality of silicon-based cores, and an amorphous carbon filler interposed between the silicon-based cores and the carbon-coated outer layer. The silicon-based core comprises nano silicon and SiO coated with the nano silicon _x (0.1 < x < 0.8) layers without cavities, a first carbon layer and a second carbon layer, the silicon-based core in the formed structure is relatively moreThe order is distributed in the carbon packing.

Comparative example 4

(1) 800g of nano silicon powder with the granularity D50 of 120nm is placed in a CVD furnace, the temperature is increased to 800 ℃ at the speed of 5 ℃/min, high-purity argon gas is respectively introduced at the speed of 4L/min, ethane gas is introduced at the speed of 0.5L/min, and the gas introduction time is 2h, so that the carbon-coated silicon material (Si@C) is obtained.

(2) Coating a SiO layer on the surface of a carbon coated silicon material (Si@C) by tetraethoxysilane ₂ A layer, obtaining Si@C@SiO ₂ The material is then placed in a CVD furnace, the temperature is increased to 800 ℃ at a speed of 5 ℃/min, high-purity argon is respectively introduced at a speed of 4L/min, ethane gas is introduced at a speed of 0.5L/min, and the gas introduction time is 2h, so that Si@C@SiO is obtained ₂ @ C precursor material.

(3) Firstly, si@C@SiO ₂ Placing the @ C precursor material in 5.0wt% hydrofluoric acid for 3 hours, washing and drying, then carrying out VC mixing with asphalt according to a mass ratio of 10:6, then carrying out heat treatment in an argon atmosphere, raising the temperature to 700 ℃ at a heating rate of 1 ℃/min, preserving heat for 4 hours, and naturally cooling to room temperature in a furnace to obtain a silicon-carbon composite precursor;

(4) And (3) placing the obtained silicon-carbon composite precursor in a CVD furnace, heating to 800 ℃ at a speed of 5 ℃/min, respectively introducing high-purity argon at a speed of 4L/min, introducing ethane gas at a speed of 0.5L/min, introducing the gas for 4 hours, naturally cooling to room temperature, and performing scattering and screening treatment to obtain the silicon-carbon composite material.

Through detection, the silicon-carbon composite material prepared in the comparative example 4 comprises an outer carbon coating layer, an amorphous carbon filling body, a hollow double-carbon layer structure and nano silicon from outside to inside. The material structure of comparative example 4 is free of SiO due to the acid etching process _x The layer needs to be etched by acid in the preparation process, and is easy to cause environmental pollution.

The silicon carbon composites prepared in examples 1 to 9 and comparative examples 1 to 4 were subjected to an electrical property test with reference to the following test conditions, and the test results thereof are shown in table 1.

And (3) testing electrical properties: examples 1 to 9 and comparative examples 1 to 4 were describedThe prepared silicon-carbon composite material is respectively used as an active substance, is mixed with a thickening agent (CMC), a binder (SBR) and a conductive agent (Super-P) according to the mass ratio of 85:2:3:10, is added with a proper amount of deionized water as a solvent to prepare slurry, is coated on a copper foil, and is subjected to vacuum drying and rolling to prepare the negative plate. Lithium metal was used as a counter electrode, and 1mol/L LiPF was used ₆ And mixing the three components of mixed solvents according to the ratio of EC to DMC to emc=1:1:1 (v/v) to form an electrolyte, and adopting a polypropylene microporous membrane as a diaphragm to assemble the CR2032 button cell in a glove box filled with inert gas. The charge and discharge test of the button cell is carried out on a cell test system of blue electric power electronic Co-Ltd in Wuhan city, under the normal temperature condition, 0.1C constant current charge and discharge is carried out to 0.01V, then 0.02C constant current discharge is carried out to 0.005V, finally 0.1C constant current charge is carried out to 1.5V, the first charge capacity and the first discharge capacity are recorded, the ratio of the first charge capacity to the first discharge capacity is the first coulomb efficiency, and the expansion rate of the pole piece after 100 weeks of circulation is calculated.

In addition, 500-week cycle performance test was performed.

Table 1 results of electrical performance testing for each example and comparative example

From the results in table 1, it is understood that the silicon-carbon composite materials prepared in examples 1 to 9 are high in both the first charge and discharge capacity and the first coulombic efficiency. The silicon carbon composites prepared in examples 1 to 9 have lower pole piece expansion and higher capacity retention than those prepared in comparative examples 1 to 4. This is mainly the silicon carbon composite material prepared in examples 1 to 9, in which the outside of the nano silicon is provided with a first carbon layer, a cavity, a second carbon layer, a carbon filler and a carbon-coated outer layer. The 5-layer coating structure can synergistically relieve the huge volume effect of nano silicon in the lithium removal and intercalation process, and can inhibit side reaction between active silicon and electrolyte, so that the cycle performance of the material is improved.

In contrast, in comparative examples 1 and 3, the prepared silicon-carbon composite material has no cavity, the first carbon layer and the second carbon layer, and the huge volume effect of nano silicon in the lithium intercalation process cannot be effectively relieved, so the cycle performance is poor.

The comparative example 2, in which no surfactant was added, resulted in agglomeration of nano silicon, and in the actual charge and discharge process, was liable to cause crushing of powder due to the volume expansion effect of silicon, so that the cycle performance was poor.

The silicon-carbon composite material prepared by acid etching in comparative example 4 has a hollow double-carbon layer structure, and can improve the cycle performance, but has no SiO _x The layer has the performance still inferior to that of the material obtained by pore-forming by adopting the pore-forming agent.

Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the scope of the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the present invention can be modified or substituted without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. The silicon-carbon composite material is characterized by comprising a carbon coating outer layer, a silicon-based core and a carbon filling body between the silicon-based core and the carbon coating outer layer, wherein the silicon-based core comprises a silicon core, a first carbon layer, a cavity and a second carbon layer, the first carbon layer and the second carbon layer are enclosed to form the cavity, at least point contact is formed between the first carbon layer and the second carbon layer, and the silicon core comprises nano silicon and coatingSiO of the nano silicon _x And the volume of the cavity accounts for 10-50% of the volume of the silicon-based core, and the silicon-carbon composite material is not additionally doped with N, P, S, B element.

2. The silicon-carbon composite according to claim 1, comprising at least one of the following features (1) to (8):

(1) the nano silicon is polycrystalline nano silicon and/or amorphous nano silicon;

(2) the SiO is _x The thickness of the layer is 0.1nm to 20.0nm;

(3) the thickness of the first carbon layer and the second carbon layer is 2nm to 50nm independently;

(4) the thickness of the carbon-coated outer layer is 10nm to 500nm;

(5) the first carbon layer coats at least 90% of the outer surface area of the silicon core;

(6) the ratio of the closed pore volume to the apparent volume of the silicon-carbon composite material is m, and m is 10 to 50 percent;

(7) the total carbon content of the silicon carbon composite is 10wt.% to 40wt.%;

(8) the carbon filler is amorphous carbon.

3. The preparation method of the silicon-carbon composite material is characterized by comprising the following steps:

(I) Preparation of nano silicon slurry

Adding nano silicon powder and a surfactant into a solvent for grinding;

(II) preparation of a pore-Forming agent solution

Adding a pore-forming agent into an organic solvent, wherein the pore-forming agent is a high molecular polymer with a carbon residue rate of 1-10% after sintering, the pore-forming agent is an alcohol-soluble high molecular polymer, and the high molecular polymer comprises at least one of alcohol-soluble polyamide resin, alcohol-soluble nylon, alcohol-soluble acrylic resin and rosin modified alkyd resin;

(III) preparation of the first precursor

Mixing the nano silicon slurry and the pore-forming agent solution and drying;

(IV) preparation of the second precursor

Mixing the first precursor with a carbon source and then carrying out high-temperature heat treatment;

(V) carbon coating

And coating the second precursor with carbon.

4. A method of producing a silicon-carbon composite material as defined in claim 3, wherein the nano-silicon slurry and the pore-forming agent solution are mixed and then adjusted to have a solids content of 10% to 20%.

5. The method of producing a silicon carbon composite material as defined in claim 4, wherein the solid content is adjusted using at least one of the following solvents: methanol, ethanol, isopropanol, benzene, toluene, xylene, acetone, methyl butanone, methyl isobutyl ketone, cyclohexane, isoheptane, 3-dimethylpentane, N-methylpyrrolidone, tetrahydrofuran and dimethylformamide.

6. A method of producing a silicon carbon composite material as claimed in claim 3, comprising at least one of the following features (1) to (22):

(1) The granularity D50 of the nano silicon powder is 30nm to 150nm;

(2) The mass ratio of the nano silicon powder to the surfactant is 5-20:1;

(3) The mass ratio of the nano silicon powder to the pore-forming agent is 1-100:1;

(4) The surfactant is an anionic surfactant;

(5) The surfactant comprises at least one of sodium lauryl sulfate, sodium dodecyl benzene sulfonate, sodium lignin sulfonate, sodium p-methylstyrene sulfonate, sodium isooctyl alcohol sulfate and naphthalene sulfonic acid formaldehyde condensate;

(6) The solvent in the step (I) is a mixture of water and an organic solvent, wherein the organic solvent accounts for not less than 95wt.% in the mixture;

(7) Stirring is carried out before grinding in the step (I), wherein the stirring time is 10-60 min;

(8) The grinding is carried out by adopting a grinder, and the grinding time is 2-6 hours;

(9) Preparing a pore-forming agent solution, wherein the organic solvent comprises at least one of methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol and propylene glycol;

(10) The drying adopts spray drying, the temperature of a feed inlet of the spray drying is 120 ℃ to 200 ℃, and the temperature of a discharge outlet of the spray drying is 70 ℃ to 100 ℃;

(11) The drying adopts evaporation drying, and the heating temperature of the evaporation drying is 50-100 ℃;

(12) The carbon source comprises at least one of asphalt, phenolic resin, sucrose, glucose, starch, polyvinyl alcohol, epoxy resin, polydopamine, lignin and citric acid;

(13) Step (IV) preparing a second precursor, wherein mechanical mixing is adopted for mixing, and the mixing is VC mixing, ball milling mixing or grinding mixing;

(14) The mixing in the step (IV) for preparing the second precursor is carried out at a speed of 1000r/min to 1500r/min;

(15) The high-temperature heat treatment is performed in a protective atmosphere, wherein the protective atmosphere comprises at least one of argon, neon, nitrogen and helium;

(16) The temperature of the high-temperature heat treatment is 600-800 ℃;

(17) The heating rate of the high-temperature heat treatment is 1 ℃/min to 10 ℃/min;

(18) The high-temperature heat treatment time is 2 to 8 hours;

(19) The carbon coating mode is gas phase coating, liquid phase coating or solid phase coating;

(20) The carbon is coated and then screened;

(21) The N, P, S, B element is not additionally doped in the preparation method;

(22) No acid is used in the preparation process.

7. Use of the silicon-carbon composite material according to claim 1 or 2, or the silicon-carbon composite material prepared by the method for preparing a silicon-carbon composite material according to any one of claims 3 to 6, in a negative electrode material.

8. A secondary battery comprising a positive electrode material and a negative electrode material, characterized in that the negative electrode material comprises the silicon-carbon composite material according to claim 1 or 2, or the silicon-carbon composite material produced by the production method of the silicon-carbon composite material according to any one of claims 3 to 6.