CN112652742B

CN112652742B - Silicon-carbon composite material and preparation method and application thereof

Info

Publication number: CN112652742B
Application number: CN201910959660.5A
Authority: CN
Inventors: 陈旭; 冯晓磊; 杨文胜; 孙伟航; 林伟国; 荣峻峰; 杜泽学
Original assignee: Sinopec Research Institute of Petroleum Processing; China Petroleum and Chemical Corp
Current assignee: Sinopec Research Institute of Petroleum Processing; China Petroleum and Chemical Corp
Priority date: 2019-10-10
Filing date: 2019-10-10
Publication date: 2022-07-12
Anticipated expiration: 2039-10-10
Also published as: CN112652742A

Abstract

The invention provides a preparation method of a silicon-carbon composite material, which comprises the following steps: crushing and dealloying the silicon-based alloy to obtain micron silicon powder; dispersing micron silicon powder in a solution containing a first carbon source, and crushing to obtain primary coated nano silicon slurry; mixing the nano silicon slurry with a second carbon source, uniformly stirring, drying and roasting to obtain a secondary coated precursor; and carrying out chemical vapor deposition on the precursor in a third carbon source atmosphere to obtain the three-stage coated silicon-carbon composite material. The silicon-carbon composite material can be used as a good lithium ion battery cathode active material, and by constructing a multi-level buffer coating layer, the long cycle stability is effectively improved while the high capacity and the first coulombic efficiency of the battery are ensured, and meanwhile, the tap density can also be kept at a higher level, so that the silicon-carbon composite material has good comprehensive performance. The preparation method is simple, the cost is low, the preparation process is suitable for large-scale production, and the method has good industrial prospect.

Description

Silicon-carbon composite material and preparation method and application thereof

Technical Field

The invention relates to a battery material technology, in particular to a silicon-carbon composite material and a preparation method and application thereof.

Background

The lithium ion battery has the advantages of high specific energy, long charging and discharging life, no memory effect, low self-discharging rate, quick charging, no pollution, wide working temperature range, safety, reliability and the like, and thus, the lithium ion battery becomes an ideal chemical power source for modern communication, portable electronic products, hybrid electric vehicles and the like. The current commercialized negative electrode material is graphite, and the theoretical specific capacity is 372mAh g^-1The demand for high energy density batteries has not been satisfied, and therefore, development of a high-capacity anode active material is urgently required.

Silicon can be alloyed with lithium at normal temperature to generate Li₁₅Si₄The theoretical specific capacity of the phase is up to 3572 mAh.g^-1And the silicon anode material has rich reserves in the earth crust, low cost and environmental protection, so the silicon anode material is one of the most potential next-generation lithium ion battery anode materials, but the silicon generates huge volume expansion in the charge and discharge process and seriously influences the cycle performance and the service life of the battery. At present, the cycling stability of silicon is mainly improved through the nanocrystallization and silicon-carbon recombination of silicon. Although the silicon nano wire and the silicon nano tube in the silicon material with the nano structure can effectively slow down the volume expansion, the preparation process is complex, the yield is low, and the practical application is difficult to meet. The nano silicon powder is technically produced on a large scale, but the cost is high. In addition, the hollow structure of the nano-silicon causes the general low tap density of the composite material, which is also a problem to be solved. Therefore, cheap silicon sources are searched for preparing the nano silicon, and the volume expansion of the silicon is relieved by means of the good conductivity and mechanical strength of the carbon material, so that the cycle stability of the silicon-carbon composite material is improved, a certain tap density is ensured, and the silicon-carbon composite material has important commercial value.

Chinese patent CN201410276413.2 discloses a preparation method of a porous silicon-carbon composite material, which takes silicon-active metal alloy as a raw material to obtain nano porous silicon through etching, and the nano porous silicon and a polymer are mixed and ball-milled to prepare the porous silicon-carbon composite material. Although the electrochemical performance of silicon can be improved by simply coating carbon on the surface of the silicon material, the carbon coating on the surface of the silicon material is cracked due to the huge volume change of the silicon in the long-term circulation process, the structure of the composite material is collapsed, and the circulation stability is rapidly reduced.

It is noted that the information disclosed in the foregoing background section is only for enhancement of background understanding of the invention and therefore it may contain information that does not constitute prior art that is already known to a person of ordinary skill in the art.

Disclosure of Invention

The present invention is directed to overcoming at least one of the above-mentioned drawbacks of the prior art, and providing a silicon-carbon composite material and a method for preparing the same, wherein the silicon-carbon composite material adopts a design of a multi-layer buffer structure, and when the silicon-carbon composite material is used as a negative electrode material of a lithium ion battery, the problems of low tap density, poor cycle stability, high cost, unsuitability for industrial production, and the like of the existing silicon-carbon composite material can be effectively solved.

In order to achieve the purpose, the invention adopts the following technical scheme:

the first aspect of the invention provides a preparation method of a silicon-carbon composite material, which comprises the following steps: crushing and dealloying the silicon-based alloy to obtain micron silicon powder; dispersing micron silicon powder in a solution containing a first carbon source, and crushing to obtain primary coated nano silicon slurry; mixing the nano silicon slurry with a second carbon source, uniformly stirring, drying and roasting to obtain a secondary coated precursor; and carrying out chemical vapor deposition on the precursor in a third carbon source atmosphere to obtain the three-stage coated silicon-carbon composite material.

According to one embodiment of the invention, the silicon-based alloy is a silicon-aluminum alloy, a silicon-iron alloy or a silicon-copper alloy, and the silicon content in the silicon-based alloy is 40 wt% to 80 wt%.

According to one embodiment of the invention, the silicon-based alloy is subjected to ball milling and crushing under a protective atmosphere, the ball milling rotation speed is 200-500 rpm, the ball milling time is 12-24 h, and the ball material mass ratio is 10: 1-20: 1.

According to one embodiment of the invention, the dealloying is dealloyed by acid etching, the acid being selected from hydrochloric acid, sulfuric acid or nitric acid.

According to one embodiment of the invention, the first carbon source is selected from one or more of citric acid, glucose and polyvinylpyrrolidone (PVP), and the first carbon source accounts for 50-120 wt% of the content of the micron silicon powder.

According to one embodiment of the invention, the solvent in the solution containing the first carbon source is selected from one or more of ethanol, isopropanol and n-heptane, after the micro silicon powder is dispersed in the solution containing the first carbon source, the micro silicon powder is subjected to sand grinding and crushing under a protective atmosphere, the rotation speed of the sand grinding is 1800 rpm-2500 rpm, the sand grinding time is 240 min-720 min, and the solid content in the sand grinding process is maintained at 5 wt% -15 wt%.

According to one embodiment of the invention, the second carbon source is a mixture of graphite and asphalt, the mass ratio of the asphalt to the graphite is 1 (3-5), and the mass ratio of the nano silicon to the graphite is 1: 3-5: 8. The solid content of the nano silicon slurry mixed with the second carbon source is 10-15 wt%.

The graphite may be selected from spherical graphite, flake graphite or artificial graphite. Preferably, spherical graphite.

According to one embodiment of the invention, the graphite is spheroidal graphite having a tap density of 0.8g cm^-3～1.1g cm^-3The median particle size is 10-25 μm; the softening point of the asphalt is 200-300 ℃, and the average grain diameter of the asphalt is 1-5 μm.

According to one embodiment of the invention, the firing is carried out in a non-oxidizing atmosphere, the firing temperature being between 700 ℃ and 900 ℃.

According to one embodiment of the invention, the calcination comprises at 3 ℃ min^-1～5℃·min^-1The temperature is raised to 250 to 350 ℃ at the temperature raising rate for pre-carbonization, and after the temperature is kept for 1 to 3 hours, the temperature is raised to 5 ℃ for min^-1～10℃·min^-1The temperature is raised to 700-900 ℃ at the temperature raising rate, and the temperature is kept for 2-4 h.

According to one embodiment of the present invention, the third carbon source is selected from one or more of acetylene, methane, ethanol and ethylene, and the flow rate of the third carbon source is 100sccm to 200 sccm.

According to one embodiment of the invention, the deposition temperature of the chemical vapor deposition is 700-900 ℃, and the deposition time is 10-30 min.

The second aspect of the invention provides a silicon-carbon composite material which is obtained by adopting the preparation method.

According to one embodiment of the present invention, the silicon content of the silicon-carbon composite material is 10 wt% to 30 wt%, and the carbon content is 70 wt% to 90 wt%.

According to one embodiment of the present invention, the silicon-carbon composite material comprises silicon-carbon microspheres having three carbon coating layers and a silicon core, and the particle size of the silicon-carbon microspheres is 10 μm to 35 μm.

The third aspect of the invention provides the application of the silicon-carbon composite material as a negative electrode of a lithium ion battery.

According to the technical scheme, the silicon-carbon composite material and the preparation method thereof have the advantages and positive effects that:

the silicon-carbon composite material provided by the invention can be used as a good lithium ion battery cathode active material, and by constructing a multi-layer buffer coating layer, the high capacity and the first coulombic efficiency of the battery are ensured, the long-cycle stability is effectively improved, and the tap density can be kept at a higher level, so that the silicon-carbon composite material has good comprehensive performance. The preparation method of the silicon-carbon composite material is simple, low in cost, suitable for large-scale production and good in industrialization prospect.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.

FIG. 1 is a scanning electron microscope image of the primary coated nano-silicon obtained in step (2) of example 1;

FIGS. 2a and 2b are scanning electron micrographs of the silicon-carbon composite material obtained in example 1 at different magnifications, respectively;

FIG. 3 is an XRD spectrum of the silicon carbon composite of example 1;

fig. 4 is a first charge-discharge curve of the silicon carbon composite of example 1.

Detailed Description

The following presents various embodiments, or examples, in order to enable one of ordinary skill in the art to practice the invention with reference to the description herein. These are, of course, merely examples and are not intended to limit the invention. The endpoints of the ranges and any values disclosed in the present application are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to yield one or more new ranges of values, which ranges of values should be considered as specifically disclosed herein.

According to the present invention, as mentioned above, the existing technology of silicon-carbon composite by nano-crystallization of silicon is adopted to improve the volume expansion problem of silicon during the charge and discharge process, so as to improve the cycle stability of the material. However, the nano-silicon often has the problems of complex preparation process, low yield or high cost, and the like, and in addition, in order to improve the battery capacity or the cycle stability, the existing silicon-carbon composite material may not reach a higher tap density, so that the active material amount per unit area is less, the volume capacity is too low, and the product requirement cannot be met.

The inventor of the invention finds that the silicon-carbon composite material obtained by using silicon-based alloy as a raw material to perform dealloying and nanocrystallization on prepared nano silicon and then compounding the nano silicon with a carbon material can effectively relieve the problem of silicon volume expansion. Furthermore, the invention further improves the structural reliability of the whole material by constructing a three-level buffering carbon coating structure on the obtained nano silicon on the basis, so that the material can still keep the structure intact in long-term cyclic use, and cannot collapse due to the expansion of silicon, thereby ensuring the cyclic stability of the material. Wherein, the primary coating is in-situ coating in the process of further crushing the micron silicon powder; the secondary coating is to further mix and stir the solution containing the carbon source and the solution containing the carbon source on the basis of in-situ coating and roast the solution at high temperature, so that the carbon coating layer on the silicon surface is more compact, and the tap density and the reliability of the material are further improved; and finally, performing three-stage coating by a chemical vapor deposition method to ensure that the carbon layer on the surface of the material is more uniform and compact. The three-stage coated silicon-carbon composite material obtained by the method effectively improves the long-cycle stability, and the capacity retention rate can reach over 90 percent after 100 cycles. More importantly, the method can still ensure the performances of high capacity, first coulombic efficiency and the like of the battery, and simultaneously the tap density can also be kept at a higher level, so that the method has good comprehensive performance.

In some embodiments, the aforementioned silicon-based alloy is a silicon-aluminum alloy, a silicon-iron alloy, or a silicon-copper alloy, preferably a silicon-aluminum alloy or a silicon-iron alloy, and more preferably a silicon-aluminum alloy. Wherein the silicon content in the silicon-based alloy is 40 wt% to 80 wt%, optionally 50 wt% to 80 wt%.

In some embodiments, the silicon-based alloy is preferably comminuted by ball milling, wherein a shielding gas, such as one or more of nitrogen or argon, is added during the ball milling process. The rotation speed of the ball mill can be 200-500 rpm, the ball milling time is 12-24 h, and the ball material mass ratio is 10: 1-20: 1. Under this condition, the silicon-based alloy can be more favorably pulverized.

And then, carrying out acid etching on the silicon-based alloy scraps subjected to ball milling to remove alloying so as to obtain micron silicon powder. Wherein the acid selected in the acid etching process can react with the active metal in the silicon-based alloyBut should not react with silicon. The acid can be hydrochloric acid, dilute sulfuric acid (concentration less than 3mol L)^-1) Or dilute nitric acid. When the acid is hydrochloric acid, it may be concentrated hydrochloric acid, with a concentration of about 28mol/L, but will react violently, giving off a large amount of hydrogen and heat; or dilute hydrochloric acid with the concentration of 1mol L^-1～3mol L^-1. When the silicon-based alloy is silicon-aluminum alloy or silicon-iron alloy, preferably, acid etching is performed using hydrochloric acid.

Compared with nano silicon prepared by other methods, the nano silicon prepared by taking the silicon-based alloy as the raw material after dealloying has fewer impurities and low cost. In addition, the micron silicon powder obtained after the alloying removal by acid etching has a certain pore structure and is easier to crush, so that the subsequent silicon nanocrystallization is facilitated.

In some embodiments, the micro silicon powder obtained after dealloying is dispersed in a solution containing a first carbon source and then subjected to a crushing treatment to obtain a primary coated nano silicon slurry. Specifically, the first carbon source is an organic carbon source, preferably one or more of citric acid, glucose and polyvinylpyrrolidone (PVP), and preferably, sodium carboxymethylcellulose (CMC) may also be added as a binder, so that the organic carbon source is better coated on the silicon surface. The first carbon source accounts for 50-120 wt% of the content of the micron silicon powder.

In some embodiments, the solvent in the solution containing the first carbon source is selected from one or more of ethanol, isopropanol, and n-heptane. In some embodiments, the nano silicon slurry is obtained by grinding and pulverizing. The in-situ coating of silicon is carried out in the sanding process, and the isolation of the organic solvent helps to reduce the oxidation of the carbon layer by air, so that a better carbon coating layer is obtained.

In some embodiments, the sanding speed is 1800 rpm-2500 rpm, the sanding time is 240 min-720 min, and the solid content of the sanding process is maintained at 5 wt% -15 wt%. Under the condition, the micron silicon powder can be better ground and crushed, and the average particle size of the nano silicon obtained by the treatment of the method is 2 nm-150 nm. In addition, in some embodiments, the sanding process described above is purged with argon or nitrogen as a shielding gas to prevent oxidation of the carbon layer during sanding.

According to the invention, the primary coated nano silicon slurry obtained by the method is subjected to secondary coating. Specifically, the nano silicon slurry and a second carbon source are mixed, uniformly stirred, dried and roasted to obtain a secondary coated precursor.

In some embodiments, the second carbon source is a mixture of graphite and pitch, wherein the graphite can be spheroidal graphite, flake graphite, and the like, preferably spheroidal graphite, and the spheroidal graphite has a tap density of 0.8g cm^-3～1.1g cm^-3The median particle size is 10-25 μm; the softening point of the asphalt is 200-300 ℃, and the average grain diameter of the asphalt is 1-5 μm. By adopting graphite and asphalt to be mixed as a second carbon source for coating, on one hand, the adhesiveness of asphalt after pyrolysis and carbonization in the high-temperature process can be utilized, and the carbon layer can be better coated on the molecular level, and on the other hand, the good conductivity of graphite is combined, so that the overall conductivity of the silicon-carbon composite material is improved, and the tap density is also favorably improved.

In some embodiments, the mass ratio of pitch to graphite is 1 (3-5), and the pitch contributes little to the contrast capacity, and mainly plays a role in coating and bonding in the composite material. The specific capacity of the finally obtained composite material is greatly reduced due to excessive addition of the asphalt; the asphalt addition is too small, and the coating and bonding effects are poor. Therefore, the mass ratio is preferably as described above.

In some embodiments, the solid content of the nano-silicon slurry mixed with the second carbon source is 10 wt% to 15 wt%, wherein the solid content refers to the mass percentage of the asphalt, the graphite and the primary coated nano-silicon in the mixed slurry.

In some embodiments, the mass ratio of the nano-silicon to the graphite is 1:3 to 5:8, wherein the nano-silicon refers to the mass of silicon and does not include a coating layer. Specifically, since the volume of liquid and the mass of solid added during sanding are determined, the solid content of nano silicon in the slurry after sanding can be calculated (only the mass of micro silicon is calculated, and carbon sources used for carbon coating are not included). The actual mass of nanosilicon in the slurry (nanosilicon that the microsilicon was sanded to, and no carbon source contained) was calculated by taking the volume of the slurry during the next compounding process. The mass ratio of the nano silicon to the graphite is controlled by controlling the volume of the measured liquid and the mass of the added graphite.

In some embodiments, firing is carried out under a non-oxidizing atmosphere, e.g., under an argon or nitrogen atmosphere, at a temperature of 700 ℃ to 900 ℃. Preferably, the calcination comprises at 3 ℃ min^-1～5℃·min^-1The temperature is raised to 250 to 350 ℃ at the temperature raising rate for pre-carbonization, the temperature is kept for 1 to 3 hours to fully melt the asphalt, and then the temperature is raised to 5 ℃ for min^-1～10℃·min^-1The temperature is raised to 700-900 ℃ at the heating rate, the temperature is kept for 2-4 h, and then the silicon-carbon composite material precursor is naturally cooled to room temperature, thus obtaining the secondary coated silicon-carbon composite material precursor.

According to the invention, the obtained secondary coated silicon-carbon composite material precursor is subjected to chemical vapor deposition in a third carbon source atmosphere to obtain a tertiary coated silicon-carbon composite material.

In some embodiments, the third carbon source is selected from one or more of acetylene, methane, ethanol, ethylene, preferably acetylene. Acetylene is used as a vapor deposition carbon source, which is more beneficial to forming a graphitized carbon layer and improving the electrical property of the material. In some embodiments, the third carbon source has a flow rate of 100sccm to 200 sccm. The deposition temperature of the chemical vapor deposition is 700-900 ℃, and the deposition time is 10-30 min. Under the conditions, a uniform and compact graphitized carbon layer is further formed on the precursor, and the three-stage coated silicon-carbon composite material is obtained.

The preparation method of the silicon-carbon composite material provided by the invention has the advantages of low raw material cost, simple process and easiness in large-scale production. The silicon content of the obtained silicon-carbon composite material can reach 10 wt% -30 wt%, and the carbon content is 70 wt% -90 wt%. The tap density of the silicon-carbon composite material is 0.8g cm^-3～0.9g cm^-3The silicon-carbon microsphere comprises silicon-carbon microsphere particles with three carbon coating layers, wherein the median particle diameter is 10-35 mu m, and through constructing the multi-layer buffer coating layers, the high capacity and the first coulombic efficiency of the battery are ensured, the long-circulation stability is effectively improved, and simultaneously the tap density can be kept at a higher levelHas good comprehensive performance, is applied as a lithium ion battery cathode material, and has good industrialization prospect.

The invention will be further illustrated by the following examples, but is not to be construed as being limited thereto. Unless otherwise specified, all reagents used in the invention are analytically pure.

The scanning electron microscope adopted by the invention is a German Zeiss scanning electron microscope (Zeiss Supra 55), and the test conditions are as follows: and fixing the powder sample on a sample table through conductive adhesive, wherein the accelerating voltage is 20kV, and the magnification is 1000-20000.

The X-ray diffraction (XRD) adopted by the invention is a Japan Shimadzu X-ray diffractometer (XRD-6000), and the test conditions are as follows: the Cu target was irradiated with K α rays (wavelength λ is 0.154nm), tube voltage was 40KV, tube current was 200mA, and scanning speed was 10 ° (2 θ)/min in a 2 θ scanning range of 20 ° to 80 °.

Example 1

(1) Putting 50g of Al-Si alloy block (the Si content is 60 wt%) into a ball milling tank, putting stainless steel grinding balls according to the ball-material mass ratio of 15:1, introducing nitrogen as protective gas, and then carrying out ball milling at the rotating speed of 200rpm for 24 hours to obtain Al-Si alloy powder. Adding Al-Si alloy powder to the mixture with the concentration of 1mol L^-1Etching in hydrochloric acid, filtering, washing, and drying at 50 ℃ in vacuum to obtain micron silicon powder;

(2) dissolving 100g of citric acid in 2L of isopropanol, adding 100g of the micron silicon powder prepared in the step (2), ultrasonically mixing uniformly, pouring into a sand mill dispersion tank, sanding at 2300rpm for 8 hours under the protection of nitrogen, and taking out to obtain citric acid coated nano silicon slurry;

(3) putting 14g of spherical graphite and 4g of asphalt into 150mL of the slurry prepared in the step (2), uniformly stirring and mixing, drying, and finally heating at 3 ℃ for min in an argon atmosphere^-1Heating to 300 deg.C, maintaining for 2 hr to melt asphalt, and heating at 5 deg.C for 5min^-1Heating to 800 ℃, preserving heat for 3h, and naturally cooling to room temperature to obtain a silicon-carbon composite material precursor;

(4) taking 1g of the silicon-carbon composite material precursor prepared in the step (3) under the nitrogen atmosphere for 10 ℃ min^-1Heating to 800 deg.CThen changing to acetylene gas for depositing for 20min at the flow of 150sccm, changing to nitrogen gas for natural cooling to room temperature, and obtaining the silicon-carbon composite material. Wherein, the mass proportion of the nano silicon in the composite material is 25 percent, and the mass proportion of the carbon in the composite material is 75 percent.

Material characterization:

the silicon-aluminum alloy raw material is silver and about 1-5cm long. Fig. 1 is a scanning electron microscope image of the primarily coated nano-silicon obtained in step (2) of example 1, and as shown in fig. 1, a sheet structure of about 80nm can be observed. FIGS. 2a and 2b are scanning electron micrographs of the silicon-carbon composite material obtained in example 1 at different magnifications, respectively, as shown in FIGS. 2a and 2b, which are spheroidal structures with particle sizes mainly concentrated between 10-20 μm.

Fig. 3 is an XRD spectrum of the silicon carbon composite material of example 1, which corresponds to characteristic peaks of silicon at positions of 28.55 °, 47.44 °, 56.12 ° in 2 θ, and is characteristic peaks of graphite at positions of 26.38 °, 42.22 °, 44.39 °, 54.54 °, 59.69 °, 77.24 ° in 2 θ, as shown in fig. 3. As can be seen, the silicon-carbon composite material of the present invention is prepared by the foregoing method.

Example 2

(1) Putting 50g of Fe-Si alloy block (the Si content is 75 wt%) into a ball milling tank, putting stainless steel grinding balls according to the ball-material mass ratio of 20:1, introducing nitrogen as protective gas, and then carrying out ball milling for 12h at the rotating speed of 400rpm to obtain Fe-Si alloy powder. Adding Fe-Si alloy powder into the mixture with the concentration of 2mol L^-1Etching in hydrochloric acid, filtering, washing, and drying at 50 ℃ in vacuum to obtain micron silicon powder;

(2) dissolving 100g of glucose in 2L of ethanol, adding 100g of the micron silicon powder prepared in the step (2), ultrasonically mixing uniformly, pouring into a sand mill dispersion tank, sanding at the rotating speed of 2500rpm for 4 hours under the protection of nitrogen, and taking out to obtain glucose-coated nano silicon slurry;

(3) putting 15g of spherical graphite and 5g of asphalt into 150mL of the slurry prepared in the step (2), uniformly stirring and mixing, drying, and finally, firstly carrying out 5 ℃ min in an argon atmosphere^-1Heating to 350 deg.C, maintaining for 1 hr to melt asphalt, and heating at 10 deg.C for 10min^-1Heating to 900 ℃, preserving the heat for 2 hours, and naturally cooling to room temperature to obtain a silicon-carbon composite material precursor;

(4) taking 1g of the silicon-carbon composite material precursor prepared in the step (3) in the nitrogen atmosphere at 10 ℃ for min^-1And raising the temperature to 700 ℃, then changing acetylene gas to deposit for 30min at the flow of 100sccm, then changing nitrogen gas to naturally cool to room temperature, and obtaining the silicon-carbon composite negative electrode material. Wherein, the mass proportion of the nano silicon in the composite material is 23 percent, and the mass proportion of the carbon in the composite material is 77 percent.

Example 3

(1) 100g of Al-Si alloy block (the Si content is 50 wt%) is put into a ball milling tank, stainless steel grinding balls are put into the ball milling tank according to the ball material mass ratio of 10:1, nitrogen is introduced to be used as protective gas, and then ball milling is carried out for 16 hours at the rotating speed of 500rpm, so that Al-Si alloy powder is obtained. Adding Al-Si alloy powder to the mixture with the concentration of 3mol L^-1Etching in hydrochloric acid, filtering, washing, and drying at 50 ℃ in vacuum to obtain micron silicon powder;

(2) dissolving 50g of polyvinylpyrrolidone in 2L of n-heptane, adding 100g of micron silicon powder prepared in the step (2), ultrasonically mixing uniformly, pouring into a sand mill dispersion tank, sanding at 1800rpm for 12h under the protection of nitrogen, and taking out to obtain polyvinylpyrrolidone-coated nano silicon slurry;

(3) putting 12g of spherical graphite and 4g of asphalt into 100mL of the slurry prepared in the step (2), uniformly stirring and mixing, drying, and finally performing heating at 4 ℃ for min in an argon atmosphere^-1Heating to 250 deg.C, maintaining for 3 hr to melt asphalt, and heating at 8 deg.C for min^-1Heating to 700 ℃, preserving the heat for 4 hours, and naturally cooling to room temperature to obtain a silicon-carbon composite material precursor;

(4) taking 1g of the silicon-carbon composite material precursor prepared in the step (3) in the nitrogen atmosphere at 10 ℃ for min^-1And raising the temperature to 900 ℃, then changing acetylene gas to deposit for 10min at the flow of 200sccm, then changing nitrogen gas to naturally cool to room temperature, and obtaining the silicon-carbon composite negative electrode material. Wherein, the mass proportion of the nano silicon in the composite material is 26 percent, and the mass proportion of the carbon in the composite material is 74 percent.

Example 4

(1) Putting 50g of Al-Si alloy block (with Si content of 60 wt%) into a ball milling tank, putting stainless steel grinding balls according to the ball-material mass ratio of 20:1, introducing nitrogen as protective gas, and then carrying out ball milling for 15 hours at the rotating speed of 300rpm to obtain Al-Si alloy powder. Adding Al-Si alloy powder into the mixture with the concentration of 2mol L^-1Etching in hydrochloric acid, filtering, washing, and drying at 50 ℃ in vacuum to obtain micron silicon powder;

(2) dissolving 120g of citric acid in 2L of mixed solution of isopropanol and ethanol, adding 100g of micron silicon powder prepared in the step (2), ultrasonically mixing uniformly, pouring into a sand mill dispersion tank, sanding at 2200rpm for 10 hours under the protection of nitrogen, and taking out to obtain citric acid-coated nano silicon slurry;

(3) putting 15g of spherical graphite and 4g of asphalt into 150mL of the slurry prepared in the step (2), uniformly stirring and mixing, drying, and finally, firstly heating at 3 ℃ for min under the argon atmosphere^-1Heating to 300 deg.C, maintaining for 2 hr to melt asphalt, and heating at 5 deg.C for min^-1Heating to 800 ℃, preserving heat for 3h, and naturally cooling to room temperature to obtain a silicon-carbon composite material precursor;

(4) taking 1g of the silicon-carbon composite material precursor prepared in the step (3) in the nitrogen atmosphere at 10 ℃ for min^-1And raising the temperature to 800 ℃, then changing to acetylene gas for deposition for 15min at the flow of 150sccm, then changing to nitrogen gas for natural cooling to room temperature, and obtaining the silicon-carbon composite negative electrode material. Wherein, the mass proportion of the nano silicon in the composite material is 22 percent, and the mass proportion of the carbon in the composite material is 78 percent.

Example 5

(2) dissolving 100g of polyvinylpyrrolidone in 2L of n-heptane, adding 100g of the micron silicon powder prepared in the step (2), ultrasonically mixing uniformly, pouring into a sand mill dispersion tank, sanding for 6 hours at a rotating speed of 2500rpm under the protection of nitrogen, and taking out to obtain polyvinylpyrrolidone-coated nano silicon slurry;

(3) putting 14g of spherical graphite and 4g of asphalt into 150mL of the slurry prepared in the step (2), uniformly stirring and mixing, drying, and finally performing heating at 4 ℃ for min in an argon atmosphere^-1Heating to 300 deg.C, maintaining for 1h to melt asphalt, and heating at 5 deg.C for 5min^-1Heating to 800 ℃, preserving the heat for 2 hours, and naturally cooling to room temperature to obtain a silicon-carbon composite material precursor;

(4) taking 1g of the silicon-carbon composite material precursor prepared in the step (3) under the nitrogen atmosphere for 10 ℃ min^-1And raising the temperature to 900 ℃, then changing acetylene gas to deposit for 30min at the flow of 100sccm, then changing nitrogen gas to naturally cool to room temperature, and obtaining the silicon-carbon composite negative electrode material. Wherein, the mass proportion of the nano silicon in the composite material is 23 percent, and the mass proportion of the carbon in the composite material is 77 percent.

Comparative example 1

The silicon-carbon composite material was prepared according to the method of example 1, except that graphite and pitch were not added, and the vapor deposition process of step (4) was not performed, and finally the carbon-coated nano-silicon composite material was obtained. Wherein, the mass ratio of the nano silicon in the composite material is 96 percent, and the mass ratio of the carbon in the composite material is 4 percent.

Comparative example 2

The silicon-carbon composite material was prepared according to the method of example 1, except that citric acid was not added during sanding, and the vapor deposition process of step (4) was not performed, and finally the nano silicon-carbon composite material was obtained. Wherein, the mass proportion of the nano silicon in the composite material is 22 percent, and the mass proportion of the carbon in the composite material is 78 percent.

Test example 1

The silicon-carbon composite prepared in example 1 was mixed with acetylene black, binder (sodium carboxymethylcellulose/styrene-butadiene rubber/deionized water 1 wt%: 1 wt%: 48 wt%) according to 7: 2: 1, coating the slurry on a copper foil, and drying to obtain the electrode plate. The prepared electrode plate is used as a positive electrode, a metal lithium plate is used as a negative electrode, a Celgard 2400 type diaphragm is selected, and 1mol L of the diaphragm is selected^-1LiPF₆And (the volume ratio of ethylene carbonate to dimethyl carbonate to diethyl carbonate is 1: 1: 1) assembling the electrolyte into a button half-cell in a glove box, and carrying out charge-discharge test on the cell by using a blue-ray system. The parameters are set as follows: the current density was 200mA g^-1The voltage interval is 0.01-1.5V, and the first charge-discharge curve is shown in figure 4.

The specific first discharge capacity of the silicon-carbon composite material prepared in the example 1 is 1079.0mA h g^-1The first charging specific capacity is 874.0mA h g^-1The first coulombic efficiency was 81.0%, the capacity retention rate after 100 weeks of circulation was 92.3%, the capacity retention rate after 300 weeks of circulation was 80.6%, and the tap density was 0.82g cm^-3. Therefore, the silicon-carbon composite material effectively improves the long cycle stability while ensuring the high capacity and the first coulombic efficiency of the battery, and simultaneously the tap density can be kept at a higher level, so that the silicon-carbon composite material has good comprehensive performance.

Test example 2

Electrochemical performance tests were performed on the composite materials of examples 2 to 5 and comparative examples 1 to 2 according to the method of test example 1, and the results are shown in table 1 below.

TABLE 1

From the above table 1, it can be seen that the silicon-carbon composite material of the present invention has good cycling stability, the capacity retention rate can reach more than 90% after 100 cycles, and higher first charge specific capacity, first coulombic efficiency and tap density can be ensured.

The material prepared in comparative example 1 was coated in situ only once, i.e. with only a buffered coating of organic carbon source, without graphite, pitch and fumed carbon buffer matrix, and had a capacity retention of only 49.3% after 100 cycles. Therefore, only by coating a carbon layer on the silicon surface, the improvement of the cycle stability is limited.

The material prepared in the comparative example 2 is not coated with an organic carbon source and is not coated with vapor deposition, only the graphite and asphalt buffer layers are adopted, the capacity retention rate is 79.8 percent after the circulation is performed for 100 weeks, and compared with the silicon-carbon composite material with the multilayer buffer structure, the material has a certain difference in circulation stability.

Therefore, the multi-layer buffer structure has an obvious effect of improving the long-term circulation stability of the battery.

In conclusion, the silicon-carbon composite material with three-level coating is prepared by the specific preparation method, the long-cycle stability is effectively improved, and the capacity retention rate can reach over 90% after 100-week cycle. More importantly, the method can still ensure the performances of high capacity, first coulombic efficiency and the like of the battery, and simultaneously the tap density can also be kept at a higher level, so that the method has good comprehensive performance and has good industrial application prospect when being used as a lithium ion battery cathode material.

It should be noted by those skilled in the art that the described embodiments of the present invention are merely exemplary and that various other substitutions, alterations, and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the above-described embodiments, but is only limited by the claims.

Claims

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

crushing and dealloying the silicon-based alloy to obtain micron silicon powder;

dispersing the micron silicon powder in a solution containing a first carbon source, and crushing to obtain primary coated nano silicon slurry, wherein the first carbon source accounts for 50-120 wt% of the content of the micron silicon powder;

mixing the nano silicon slurry with a second carbon source, uniformly stirring, drying and roasting to obtain a secondary coated precursor;

performing chemical vapor deposition on the precursor in a third carbon source atmosphere to obtain the three-level coated silicon-carbon composite material, wherein the silicon content of the silicon-carbon composite material is 10-30 wt%, the carbon content of the silicon-carbon composite material is 70-90 wt%, and the tap density of the silicon-carbon composite material is 0.8 g-cm^-3～0.9g·cm^-3；

The second carbon source is a mixture of graphite and asphalt, the mass ratio of the asphalt to the graphite in the second carbon source is 1 (3-5), the mass ratio of the nano-silicon to the graphite is 1: 3-5: 8, the graphite is spherical graphite, the tap density of the spherical graphite is 0.8g cm < -3 > to 1.1g cm < -3 >, and the median particle size is 10 mu m-25 mu m.

2. The method according to claim 1, wherein the silicon-based alloy is a silicon-aluminum alloy, a silicon-iron alloy, or a silicon-copper alloy, and the silicon content in the silicon-based alloy is 40 wt% to 80 wt%.

3. The preparation method of claim 1, wherein the silicon-based alloy is subjected to ball milling and crushing in a protective atmosphere, the rotation speed of the ball milling is 200-500 rpm, the ball milling time is 12-24 h, and the mass ratio of the ball to the material is 10: 1-20: 1.

4. The method of claim 1, wherein the dealloying is dealloyed using an acid etch, the acid being selected from hydrochloric acid, nitric acid, or sulfuric acid.

5. The method according to claim 1, wherein the first carbon source is one or more selected from the group consisting of citric acid, glucose, and polyvinylpyrrolidone.

6. The preparation method according to claim 1, wherein the solvent in the solution containing the first carbon source is selected from one or more of ethanol, isopropanol and n-heptane, the micro silicon powder is dispersed in the solution containing the first carbon source and then subjected to sand grinding under a protective atmosphere, the rotation speed of the sand grinding is 1800-2500 rpm, the sand grinding time is 240-720 min, and the solid content of the sand grinding process is maintained at 5-15 wt%.

7. The preparation method according to claim 1, wherein the solid content of the nano silicon slurry after mixing with the second carbon source is 10 to 15 wt%.

8. The method according to claim 7, wherein the asphalt has a softening point of 200 to 300 ℃ and an average particle diameter of 1 to 5 μm.

9. The method according to claim 1, wherein the firing is performed in a non-oxidizing atmosphere, and the firing temperature is 700 ℃ to 900 ℃.

10. The method of claim 9, wherein the firing comprises at 3 ℃. min^-1～5℃·min^-1The temperature is raised to 250 to 350 ℃ at the temperature raising rate for pre-carbonization, and after the temperature is kept for 1 to 3 hours, the temperature is raised to 5 ℃ for min^-1～10℃·min^-1The temperature is raised to 700-900 ℃ at the temperature raising rate, and the temperature is kept for 2-4 h.

11. The method according to claim 1, wherein the third carbon source is selected from one or more of acetylene, methane, ethanol and ethylene, and the flow rate of the third carbon source is 100sccm to 200 sccm.

12. The preparation method according to claim 1, wherein the deposition temperature of the chemical vapor deposition is 700 ℃ to 900 ℃ and the deposition time is 10min to 30 min.

13. A silicon-carbon composite material obtained by the preparation method of any one of claims 1 to 12.

14. The silicon-carbon composite material according to claim 13, comprising silicon-carbon microspheres having three carbon coating layers and a silicon core, wherein the particle size of the silicon-carbon microspheres is 10 μm to 35 μm.

15. Use of the silicon carbon composite material according to claim 13 or 14 as a negative electrode for a lithium ion battery.