Silicon / carbon nanotube composite material, preparation method thereof, lithium battery negative electrode and lithium battery

Abstract

The present invention provides a silicon / carbon nanotube composite material, a preparation method thereof, a negative electrode of a lithium battery, and a lithium battery. The preparation method includes: Step S1, dispersing carbon nanotubes and polyvinylpyrrolidone in an alcohol solvent to obtain a carbon nanotube dispersion; Step S2, mixing tetraethyl orthosilicate and the carbon nanotube dispersion under stirring conditions to obtain a mixed solution; Step S3, using ammonia water to catalyze the hydrolysis of tetraethyl orthosilicate in the mixed solution to obtain an alcohol dispersion of carbon nanotube / silica; Step S4, performing solid-liquid separation on the alcohol dispersion and drying the obtained solid to obtain a carbon nanotube / silica composite material; Step S5, reducing the carbon nanotube / silica composite material to obtain a carbon nanotube / silicon composite material; and Step S6, setting a carbon source on the surface of the carbon nanotube / silicon composite material and carbonizing the carbon source to obtain a silicon / carbon nanotube composite material. The problem that the poor mixing uniformity of silicon and carbon leads to poor binding force between the two is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon-carbon composite material preparation, and in particular, to a silicon / carbon nanotube composite material, a preparation method thereof, a negative electrode of a lithium battery, and a lithium battery. Background Art

[0002] Lithium-ion batteries with graphite as the negative electrode have been widely used in the fields of electric vehicles, energy storage, digital, etc. due to their excellent performance. Among them, the ternary NCM / graphite system is considered to have good application prospects. However, with the gradual adjustment of national subsidy policies, lithium-ion batteries are gradually developing in the two directions of high energy density and high power density. At present, the widely favored NCM811 / graphite system has become the main direction of enterprise research and development, and some enterprises have achieved mass production.

[0003] Silicon has a theoretical specific capacity of 4200 mAh / g, which has good application potential compared with 372 mAh / g of graphite. It is expected to replace graphite as the best choice for the negative electrode material of the new generation of high energy density batteries and has recently received the attention of many researchers. However, silicon undergoes a huge volume change (300%) during charge and discharge, causing the SEI film to rupture repeatedly, the electrolyte to be continuously consumed, the material structure to be damaged, and the battery capacity to decay rapidly. This has become an obstacle to the application of silicon as a new generation of negative electrode material.

[0004] In order to make up for the defects of silicon materials, researchers have adopted the idea of material compounding, coating a layer of carbon or other materials on the outside of silicon materials to avoid direct contact between silicon and the electrolyte, reduce the occurrence of side reactions, relieve the volume change of silicon, and improve the structural stability of the material, thereby realizing the improvement of the cycle life of the NCM811 / Si system.

[0005] One-dimensional multi-walled carbon nanotube (MWCNTs) materials are widely used in various electrode materials due to their ultra-high specific surface area, large aspect ratio, excellent electrical conductivity, and good mechanical stability. Many studies have shown that the compounding of multi-walled carbon nanotube materials can effectively improve the electrochemical performance of electrodes. Usually, the ball milling method is used to ball mill and mix single-walled carbon nanotubes and silicon nanoparticles. The prepared composite material of carbon nanotubes and silicon as the negative electrode of a lithium battery has obtained good electrochemical performance. However, the disadvantage of this method is that the ball milling mixing method has poor uniformity, the binding force between carbon nanotubes and silicon is poor, and silicon particles are easy to fall off from the carbon tubes; moreover, silicon particles adhere to the surface of the carbon tubes, and part of the silicon surface is still exposed, and the volume change during the cycle is not completely and effectively suppressed, resulting in the inability to improve the cycle stability of the battery. Summary of the Invention

[0006] The main object of the present invention is to provide a silicon / carbon nanotube composite material, a preparation method thereof, a negative electrode of a lithium battery, and a lithium battery, so as to solve the problem that the binding force between silicon and carbon nanotubes is poor due to the poor mixing uniformity of the silicon / carbon nanotube composite material in the prior art.

[0007] To achieve the above object, according to one aspect of the present invention, a preparation method of a silicon / carbon nanotube composite material is provided, including: Step S1, dispersing carbon nanotubes and polyvinylpyrrolidone in an alcohol solvent to obtain a carbon nanotube dispersion; Step S2, mixing tetraethyl orthosilicate and the carbon nanotube dispersion under stirring to obtain a mixed solution; Step S3, using ammonia water to catalyze the hydrolysis of tetraethyl orthosilicate in the mixed solution to obtain an alcohol dispersion of carbon nanotube / silica; Step S4, performing solid-liquid separation on the alcohol dispersion and then drying the obtained solid to obtain a carbon nanotube / silica composite material; Step S5, reducing the carbon nanotube / silica composite material to obtain a carbon nanotube / silicon composite material; and Step S6, setting a carbon source on the surface of the carbon nanotube / silicon composite material and carbonizing the carbon source to obtain a silicon / carbon nanotube composite material.

[0008] Further, the mass ratio of the above carbon nanotubes to polyvinylpyrrolidone is 1.5:1 to 5:1, preferably 1.5:1 to 2.5:1, and the mass content of carbon nanotubes in the carbon nanotube dispersion is 1 to 10%, preferably 1 to 1.25%; preferably, the alcohol solvent is ethanol or isopropanol.

[0009] Further, both Step S1 and Step S2 are stirred, preferably at a stirring speed of 200 to 1000 rpm, preferably the stirring time of Step S1 is 0.5 to 8 hours, and preferably the stirring time of Step S2 is 10 to 30 min.

[0010] Further, the above carbon source is pitch, and the mass ratio of carbon nanotubes, tetraethyl orthosilicate, and pitch is 32.0 to 58.0:25.0 to 59.0:8.0 to 17.0.

[0011] Further, the addition amount of the above ammonia water relative to tetraethyl orthosilicate is 0.1 to 5 mL / g, preferably the hydrolysis temperature is 50 to 70 °C, and preferably the hydrolysis time is 8 to 24 hours.

[0012] Further, the above Step S4 includes: washing and deammoniating the alcohol dispersion with an alcohol solvent and then performing centrifugal separation to obtain a solid; drying the solid at 50 to 80 °C for 12 to 48 hours to obtain a carbon nanotube / silica composite material.

[0013] Further, the above-mentioned step S5 includes: in an inert gas atmosphere or an argon gas atmosphere, reducing the silica in the carbon nanotube / silica composite material with a reducing powder to obtain a reduced product; performing acid treatment on the reduced product to obtain a carbon nanotube / silicon composite material. Preferably, the reducing powder is magnesium powder or carbon powder, the reduction temperature is preferably 500-1200°C, the reduction time is preferably 2-24 hours, the acid used for acid treatment is preferably hydrochloric acid, and more preferably hydrochloric acid with a concentration of 5-20%.

[0014] Further, the above-mentioned step S6 includes: mixing the carbon nanotube / silicon composite material and pitch to form a mixture; carbonizing the mixture in an inert gas atmosphere or a nitrogen gas atmosphere to obtain a silicon / carbon nanotube composite material; preferably, the carbonization temperature is 400-1000°C and the carbonization time is 6-24 hours.

[0015] According to another aspect of the present invention, there is provided a silicon / carbon nanotube composite material prepared by using any one of the above preparation methods.

[0016] According to still another aspect of the present invention, there is provided a negative electrode of a lithium battery, including a silicon / carbon nanotube composite material, and the silicon / carbon nanotube composite material is the above-mentioned silicon / carbon nanotube composite material.

[0017] According to yet another aspect of the present invention, there is provided a lithium battery, including a positive electrode and a negative electrode, and the negative electrode is the above-mentioned negative electrode of the lithium battery.

[0018] Applying the technical solution of the present invention, polyvinylpyrrolidone is used as a surfactant in step S1 of the present application to disperse carbon nanotubes as evenly as possible in an alcohol solvent; and tetraethyl orthosilicate used in step S2 can be well-compatible and mixed with polyvinylpyrrolidone, so the carbon nanotubes and tetraethyl orthosilicate in the obtained mixed solution are relatively evenly mixed. In the alcohol dispersion of carbon nanotube / silica obtained by hydrolysis in step S3, since the hydrolysis of tetraethyl orthosilicate is a relatively mild chemical reaction, it will not affect the uniform system already formed in step S2, and then the silicon obtained by reducing the silica in the carbon nanotube / silica composite material can be evenly coated on the carbon nanotubes; finally, carbonizing the carbon source provided on the surface of the carbon nanotube / silicon composite material realizes the coating of silicon.

[0019] The raw materials used in the above preparation method of the present invention are widely sourced, the process is simple, easy to operate, efficient, green and environmentally friendly; the silicon / carbon nanotube composite material obtained by the above preparation method is a hollow silicon-carbon nanotube composite structure. The nanotubes can alleviate the volume expansion of silicon, while making up for the disadvantage of poor conductivity of silicon. It has a large specific surface area, which improves the conductivity of the material and avoids material agglomeration. Since the silicon in the silicon / carbon nanotube composite material of the present application is evenly distributed and in close contact with the carbon nanotubes, the carbon layer formed after carbonization also coats the silicon, enabling the silicon to stably exist in the composite material. Therefore, the silicon / carbon nanotube composite anode material has good cycle stability. After testing, some of the silicon / carbon nanotube composite materials prepared in the present application are applied to lithium batteries. The lithium battery has an initial discharge specific capacity of 634.3 mAh / g at a rate of 0.5C. After 50 cycles, the discharge specific capacity is 547.4 mAh / g, and the capacity retention rate is 86.3%, indicating that the problems of poor material conductivity and poor cycle performance are effectively solved. Brief Description of the Drawings

[0020] The specification drawings forming a part of the present application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0021] Figure 1 It is the SEM picture of the silicon / carbon nanotube composite material prepared in Example 1 of the present application;

[0022] Figure 2 It is the cycle performance graph of the silicon-carbon / nanotube composite material prepared in Example 1 of the present application during charge and discharge at a rate of 0.5C. Detailed Embodiments

[0023] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0024] As analyzed in the background art of the present application, in the existing silicon / carbon nanotube composite material, due to the poor mixing uniformity of silicon and carbon nanotubes, the binding force between the two is poor. To solve this problem, the present application provides a silicon / carbon nanotube composite material, a preparation method thereof, a lithium battery anode, and a lithium battery.

[0025] In a typical embodiment of the present application, a method for preparing a silicon / carbon nanotube composite material is provided. The preparation method includes: Step S1, dispersing carbon nanotubes and polyvinylpyrrolidone in an alcohol solvent to obtain a carbon nanotube dispersion; Step S2, mixing tetraethyl orthosilicate and the carbon nanotube dispersion under stirring conditions to obtain a mixed solution; Step S3, using ammonia water to catalyze the hydrolysis of tetraethyl orthosilicate in the mixed solution to obtain an alcohol dispersion of carbon nanotube / silica; Step S4, performing solid-liquid separation on the alcohol dispersion and drying the obtained solid to obtain a carbon nanotube / silica composite material; Step S5, reducing the carbon nanotube / silica composite material to obtain a carbon nanotube / silicon composite material; and Step S6, setting a carbon source on the surface of the carbon nanotube / silicon composite material and carbonizing the carbon source to obtain a silicon / carbon nanotube composite material.

[0026] In Step S1 of the present application, polyvinylpyrrolidone is used as a surfactant to disperse carbon nanotubes as evenly as possible in the alcohol solvent; moreover, tetraethyl orthosilicate used in Step S2 can be well-compatible and mixed with polyvinylpyrrolidone, so the carbon nanotubes and tetraethyl orthosilicate in the obtained mixed solution are relatively evenly mixed. In the alcohol dispersion of carbon nanotube / silica obtained by hydrolysis in Step S3, since the hydrolysis of tetraethyl orthosilicate is a relatively mild chemical reaction, it will not affect the uniform system formed in Step S2. Then, the silicon obtained by reducing the silica in the carbon nanotube / silica composite material can be evenly coated on the carbon nanotubes; finally, carbonizing the carbon source set on the surface of the carbon nanotube / silicon composite material realizes the coating of silicon.

[0027] The raw materials used in the above preparation method of the present invention are widely sourced, the process is simple, easy to operate, efficient, green and environmentally friendly; the silicon / carbon nanotube composite material obtained by the above preparation method is a hollow silicon-carbon nanotube composite structure. The nanotubes can alleviate the volume expansion of silicon, and at the same time make up for the disadvantage of poor conductivity of silicon. It has a large specific surface area, which improves the conductivity of the material and avoids material agglomeration. Since silicon in the silicon / carbon nanotube composite material of the present application is evenly distributed and in close contact with the carbon nanotubes, and the carbon layer formed after carbonization also coats the silicon, silicon can stably exist in the composite material. Therefore, the silicon / carbon nanotube composite anode material has good cycle stability. After testing, some silicon / carbon nanotube composite materials prepared in the present application are applied to lithium batteries. The lithium battery has an initial discharge specific capacity of 634.3 mAh / g at a 0.5C rate. After 50 cycles, the discharge specific capacity is 547.4 mAh / g, and the capacity retention rate is 86.3%, indicating that the problems of poor material conductivity and poor cycle performance are effectively solved.

[0028] The carbon nanotubes of the present application are carbon nanotubes in a conventional form in the prior art, such as multi-walled carbon nanotubes or short single-walled carbon nanotubes, preferably carbon nanotubes with a diameter greater than 30 nm.

[0029] In a preferred embodiment of the present application, in order to improve the dispersion uniformity of carbon nanotubes in an alcohol solvent, it is preferred that the mass ratio of the above-mentioned carbon nanotubes to polyvinylpyrrolidone is 1.5:1 to 5:1, preferably 1.5:1 to 2.5:1, and the mass content of carbon nanotubes in the carbon nanotube dispersion is 1 to 10%, preferably 1 to 1.25%; preferably the alcohol solvent is ethanol or isopropanol.

[0030] In order to improve the mixing uniformity of polyvinylpyrrolidone and carbon nanotubes, it is preferred that the above-mentioned step S1 includes dissolving polyvinylpyrrolidone in isopropanol to form a primary solution; adding carbon nanotubes to the primary solution in batches under continuous stirring, and after adding all the carbon nanotubes, increasing the stirring speed and continuing to stir to obtain a carbon nanotube dispersion.

[0031] In addition, in order to improve the mixing uniformity and mixing efficiency of each solid material in the mixed solution, it is preferred that both the above-mentioned step S1 and step S2 are stirred, preferably the stirring speed is 200 to 1000 rpm, preferably the stirring time of step S1 is 0.5 to 8 hours, and preferably the stirring time of step S2 is 10 to 30 min.

[0032] By controlling the mass ratio of carbon nanotubes to tetraethyl orthosilicate, the thickness of the silicon layer coated on the carbon nanotubes is adjusted to obtain a composite structure with high capacity and stable structure. Asphalt is used as the carbon source, and of course, carbon sources of other materials can also be used, such as acetylene. Preferably, the mass ratio of the above-mentioned carbon nanotubes, tetraethyl orthosilicate and asphalt is 32.0 to 58.0:25.0 to 59.0:8.0 to 17.0, preferably 32.9 to 57.9:25.5 to 58.3:8.0 to 16.6.

[0033] Tetraethyl orthosilicate hydrolyzes in an alcohol solvent under the action of ammonia water to form silicon dioxide. In order to accelerate the above hydrolysis reaction and avoid the silicon from falling off the carbon nanotubes due to too fast hydrolysis, it is preferred that the addition amount of the above ammonia water relative to tetraethyl orthosilicate is 0.1 to 5 mL / g, preferably the hydrolysis temperature is 50 to 70 °C, and preferably the hydrolysis time is 8 to 24 hours.

[0034] In one embodiment of the present application, the above step S4 includes: washing and deammoniating the alcohol dispersion with an alcohol solvent and then performing centrifugal separation to obtain a solid; drying the solid at 50-80 °C for 12-48 hours to obtain a carbon nanotube / silica composite material. In order to ensure that tetraethyl orthosilicate is hydrolyzed as completely as possible, generally, ammonia water may be in excess. After the above washing and centrifugal separation, the ammonia water is separated from the alcohol dispersion to avoid its influence on subsequent reduction; then slow drying is carried out at the above temperature to avoid excessive movement of substances due to too fast drying, resulting in a large amount of silica falling off the carbon nanotubes.

[0035] The reducing agent used for reducing the carbon nanotube / silica composite material in the present application mainly plays a reducing role on silica. Therefore, it can be selected from the common reducing agents for silica reduction in the prior art. In a preferred embodiment, the above step S5 includes: in an inert gas atmosphere or an argon gas atmosphere, reducing the silica in the carbon nanotube / silica composite material with a reducing powder to obtain a reduced product; performing acid treatment on the reduced product to obtain a carbon nanotube / silicon composite material. After the above reduction, in order to further remove the reducing powder, acid treatment is performed on the reduced product.

[0036] In a preferred embodiment, the preferred reducing powder is magnesium powder or carbon powder, the preferred reduction temperature is 500-1200 °C, and the preferred reduction time is 2-24 hours to achieve sufficient reduction of silica; the preferred acid used for acid treatment is hydrochloric acid, and further preferably hydrochloric acid with a concentration of 5-20% to remove the reducing powder under relatively mild conditions and avoid excessive acid treatment reaction resulting in the silicon on the carbon nanotubes falling off.

[0037] There are various ways to set a carbon source on the surface of the carbon nanotube / silicon composite material and carbonize the carbon source, such as carbonizing after chemically depositing the carbon source. In order to achieve more sufficient coating of silicon with simple operations in the present application, the above step S6 preferably includes: mixing the carbon nanotube / silicon composite material and asphalt to form a mixture; carbonizing the mixture in an inert gas atmosphere or a nitrogen gas atmosphere to obtain a silicon / carbon nanotube composite material, and preferably the carbonization temperature is 400-1000 °C and the carbonization time is 6-24 hours.

[0038] In another typical embodiment of the present application, a silicon / carbon nanotube composite material is provided, and the silicon / carbon nanotube composite material is prepared by using any one of the above preparation methods.

[0039] The silicon / carbon nanotube composite material obtained by the above preparation method is a hollow silicon-carbon nanotube composite structure. The nanotubes can alleviate the volume expansion of silicon, while compensating for the disadvantage of poor conductivity of silicon. It has a large specific surface area, which improves the conductivity of the material and avoids material agglomeration. Since the silicon in the silicon / carbon nanotube composite material of the present application is uniformly distributed and in close contact with the carbon nanotubes, and the carbon layer formed after carbonization also coats the silicon, the silicon can stably exist in the composite material. Therefore, the silicon / carbon nanotube composite anode material has good cycle stability. After testing, some of the silicon / carbon nanotube composite materials prepared in the present application are applied to lithium batteries. The lithium battery has an initial discharge specific capacity of 634.3 mAh / g at a rate of 0.5C. After 50 cycles, the discharge specific capacity is 547.4 mAh / g, and the capacity retention rate is 86.3%, indicating that the problems of poor material conductivity and poor cycle performance are effectively solved.

[0040] In another typical embodiment of the present application, a lithium battery anode is provided, including a silicon / carbon nanotube composite material, and the silicon / carbon nanotube composite material is the above-mentioned silicon / carbon nanotube composite material. Since the silicon / carbon nanotube composite material of the present application has good cycle stability and rate performance, the lithium battery anode having the same also has good cycle stability and rate performance.

[0041] In still another typical embodiment of the present application, a lithium battery is provided, including a positive electrode and a negative electrode, and the negative electrode is the above-mentioned lithium battery anode. The lithium battery having the lithium battery anode of the present application also has good cycle stability and rate performance.

[0042] The beneficial effects of the present application will be further described below in conjunction with examples and comparative examples.

[0043] Example 1

[0044] 0.2 g of PVP was added to 45 ml of isopropanol and dissolved. Under continuous stirring, 0.375 g of carbon nanotubes was added in small portions several times, and then stirred at 800 rpm for 2 h to obtain a carbon nanotube dispersion.

[0045] While maintaining high-speed stirring, 0.32 g of TEOS was slowly added dropwise to the carbon nanotube dispersion, and stirred for 30 min to obtain a mixed solution.

[0046] While maintaining high-speed stirring, 0.8 ml of ammonia water was slowly added dropwise to the mixed solution, sealed with plastic wrap, and the temperature of the reaction system was maintained at 50 °C for 24 h to obtain an ethanol dispersion of carbon nanotubes / silica.

[0047] The ethanol dispersion of carbon nanotubes / silica was washed several times with absolute ethanol until the washing liquid was neutral, centrifuged, and dried at 70 °C for 24 h to obtain a carbon nanotube / SiO2 composite material.

[0048] 0.47 g of the carbon nanotube / SiO2 composite material and magnesium powder were reacted at 650 °C for 4 h in an argon atmosphere to obtain a reduced product. Then, the reduced product was reacted in 10% dilute hydrochloric acid, washed, centrifuged, and dried to obtain the carbon nanotube / Si composite material.

[0049] 0.42 g of the carbon nanotube / Si composite material and 0.1 g of pitch were mixed to form a mixture, which was heat-treated at 600 °C for 20 h in a high-purity N2 atmosphere to carbonize the pitch, obtaining the final product, the silicon / carbon nanotube composite material.

[0050] Example 2

[0051] 0.2 g of PVP was added to 35 ml of an ethanol dispersion (1%) containing 0.35 g of carbon nanotubes, and stirred until dissolved to obtain a carbon nanotube dispersion.

[0052] While maintaining high-speed stirring, 0.371 g of TEOS was slowly added dropwise to the carbon nanotube dispersion, and stirred for 20 min to obtain a mixed solution.

[0053] While maintaining high-speed stirring, 1.0 ml of ammonia water was slowly added dropwise to the mixed solution, sealed with plastic wrap, and the temperature of the reaction system was maintained at 55 °C for 20 h to obtain an ethanol dispersion of carbon nanotube / SiO2;

[0054] The ethanol dispersion of carbon nanotube / SiO2 was washed several times with anhydrous ethanol, centrifuged, and dried at 80 °C for 12 h to obtain the carbon nanotube / SiO2 composite material.

[0055] 0.46 g of the carbon nanotube / SiO2 composite material and carbon powder were reacted at 550 °C for 8 h in an argon atmosphere to obtain a reduced product. Then, the reduced product was reacted in 8% dilute hydrochloric acid, washed, centrifuged, and dried to obtain the carbon nanotube / Si composite material.

[0056] 0.4 g of the carbon nanotube / Si composite material and 0.125 g of pitch were mixed, and heat-treated at 700 °C for 12 h in a high-purity N2 atmosphere to carbonize the pitch, obtaining the final product, the silicon / carbon nanotube composite material.

[0057] Example 3

[0058] 0.2 g of PVP was added to 35 ml of anhydrous ethanol and dissolved. While continuously stirring, 0.42 g of carbon nanotubes was added in small amounts several times, and then stirred at high speed for 4 h to obtain a carbon nanotube dispersion.

[0059] While maintaining high-speed stirring, 0.297 g of TEOS was slowly added dropwise to the carbon nanotube dispersion, and stirred for 25 min to obtain a mixed solution.

[0060] Keep stirring at high speed, slowly add 0.7 ml of ammonia water drop by drop to the mixed solution, seal it with plastic wrap, keep the temperature of the reaction system at 60 °C, and react for 18 h to obtain an ethanol dispersion of carbon nanotube / SiO2.

[0061] Wash the ethanol dispersion of carbon nanotube / SiO2 several times with absolute ethanol, centrifuge and separate, and dry at 60 °C for 48 h to obtain a carbon nanotube / SiO2 composite material.

[0062] React 0.506 g of the carbon nanotube / SiO2 composite material with magnesium powder in a nitrogen atmosphere at a high temperature of 750 °C for 2 h to obtain a reduced product, and then react the reduced product in 9% dilute hydrochloric acid, wash, centrifuge, and dry to obtain a carbon nanotube / Si composite material.

[0063] Mix 0.46 g of the carbon nanotube / Si composite material with 0.063 g of asphalt, and perform heat treatment at 800 °C for 8 h in a high-purity argon atmosphere to carbonize the asphalt and obtain the final product, the silicon / carbon nanotube composite material.

[0064] Example 4

[0065] Dissolve 0.2 g of PVP in 45 ml of isopropanol. Under continuous stirring, add 1.860 g of carbon nanotubes in small amounts several times, and stir at 800 rpm for 2 h to obtain a carbon nanotube dispersion.

[0066] Keep stirring at high speed, slowly add 1.587 g of TEOS drop by drop to the carbon nanotube dispersion, and stir for 30 min to obtain a mixed solution.

[0067] Keep stirring at high speed, slowly add 1 mL of ammonia water drop by drop to the mixed solution, seal it with plastic wrap, keep the temperature of the reaction system at 50 °C, and react for 24 h to obtain an ethanol dispersion of carbon nanotube / silica.

[0068] Wash the ethanol dispersion of carbon nanotube / silica several times with absolute ethanol until the washing liquid is neutral, centrifuge and separate, and dry at 70 °C for 24 h to obtain a carbon nanotube / SiO2 composite material.

[0069] React 2.318 g of the carbon nanotube / SiO2 composite material with magnesium powder in an argon atmosphere at a high temperature of 650 °C for 4 h to obtain a reduced product, and then react the reduced product in 10% dilute hydrochloric acid, wash, centrifuge, and dry to obtain a carbon nanotube / Si composite material.

[0070] Mix 2.074 g of the carbon nanotube / Si composite material with 0.496 g of asphalt to form a mixture, and perform heat treatment at 600 °C for 20 h in a high-purity N2 atmosphere to carbonize the asphalt and obtain the final product, the silicon / carbon nanotube composite material.

[0071] Example 5

[0072] Dissolve 0.2 g of PVP in 45 ml of isopropanol. With continuous stirring, add 4.820 g of carbon nanotubes in several small portions. Then, stir at 1000 rpm for 8 h to obtain a carbon nanotube dispersion.

[0073] While maintaining high-speed stirring, slowly add 4.115 g of TEOS dropwise to the carbon nanotube dispersion and stir for 30 min to obtain a mixed solution.

[0074] While maintaining high-speed stirring, slowly add 3 mL of ammonia water dropwise to the mixed solution, seal it with plastic wrap, and keep the temperature of the reaction system at 50 °C. React for 24 h to obtain an ethanol dispersion of carbon nanotube / silica.

[0075] Wash the ethanol dispersion of carbon nanotube / silica several times with absolute ethanol until the washing solution is neutral, then centrifuge and dry at 70 °C for 24 h to obtain a carbon nanotube / SiO2 composite material.

[0076] React 6.007 g of the carbon nanotube / SiO2 composite material with magnesium powder in an argon atmosphere at a high temperature of 650 °C for 4 h to obtain a reduced product. Then, react the reduced product with 10% dilute hydrochloric acid, wash, centrifuge, and dry to obtain a carbon nanotube / Si composite material.

[0077] Mix 5.374 g of the carbon nanotube / Si composite material with 1.285 g of pitch to form a mixture. Heat-treat it at 600 °C for 20 h in a high-purity N2 atmosphere to carbonize the pitch and obtain the final product, a silicon / carbon nanotube composite material.

[0078] Example 6

[0079] Dissolve 0.2 g of PVP in 45 ml of isopropanol. With continuous stirring, add 0.375 g of carbon nanotubes in several small portions. Then, stir at 800 rpm for 2 h to obtain a carbon nanotube dispersion.

[0080] While maintaining high-speed stirring, slowly add 0.664 g of TEOS dropwise to the carbon nanotube dispersion and stir for 30 min to obtain a mixed solution.

[0081] While maintaining high-speed stirring, slowly add 1 mL of ammonia water dropwise to the mixed solution, seal it with plastic wrap, and keep the temperature of the reaction system at 50 °C. React for 24 h to obtain an ethanol dispersion of carbon nanotube / silica.

[0082] Wash the ethanol dispersion of carbon nanotube / silica several times with absolute ethanol until the washing solution is neutral, then centrifuge and dry at 70 °C for 24 h to obtain a carbon nanotube / SiO2 composite material.

[0083] 0.855 g of the carbon nanotube / SiO2 composite material and magnesium powder were reacted in an argon atmosphere at a high temperature of 650 °C for 4 h to obtain a reduction product. Then, the reduction product was reacted in 10% dilute hydrochloric acid, washed, centrifuged, and dried to obtain the carbon nanotube / Si composite material.

[0084] 0.599 g of the carbon nanotube / Si composite material and 0.1 g of asphalt were mixed to form a mixture, which was heat-treated at 600 °C for 20 h in a high-purity N2 atmosphere to carbonize the asphalt and obtain the final product, the silicon / carbon nanotube composite material.

[0085] Example 7

[0086] 0.2 g of PVP was added to 45 ml of isopropanol and dissolved. Under continuous stirring, 0.375 g of carbon nanotubes was added in small portions several times, and then stirred at 800 rpm for 2 h to obtain a carbon nanotube dispersion.

[0087] While maintaining high-speed stirring, 0.154 g of TEOS was slowly added dropwise to the carbon nanotube dispersion, and stirred for 30 min to obtain a mixed solution.

[0088] While maintaining high-speed stirring, 0.5 mL of ammonia water was slowly added dropwise to the mixed solution, sealed with plastic wrap, and the temperature of the reaction system was maintained at 50 °C and reacted for 24 h to obtain an ethanol dispersion of carbon nanotube / silica.

[0089] The ethanol dispersion of carbon nanotube / silica was washed several times with absolute ethanol until the washing solution was neutral, centrifuged, and dried at 70 °C for 24 h to obtain the carbon nanotube / SiO2 composite material.

[0090] 0.42 g of the carbon nanotube / SiO2 composite material and magnesium powder were reacted in an argon atmosphere at a high temperature of 650 °C for 4 h to obtain a reduction product. Then, the reduction product was reacted in 10% dilute hydrochloric acid, washed, centrifuged, and dried to obtain the carbon nanotube / Si composite material.

[0091] 0.396 g of the carbon nanotube / Si composite material and 0.1 g of asphalt were mixed to form a mixture, which was heat-treated at 600 °C for 20 h in a high-purity N2 atmosphere to carbonize the asphalt and obtain the final product, the silicon / carbon nanotube composite material.

[0092] Example 8

[0093] The difference from Example 1 was that the hydrolysis of tetraethyl orthosilicate was as follows: While maintaining high-speed stirring, 0.5 mL of ammonia water was slowly added dropwise to the mixed solution, sealed with plastic wrap, and the temperature of the reaction system was maintained at 70 °C and reacted for 8 h to obtain an ethanol dispersion of carbon nanotube / silica.

[0094] Example 9

[0095] The difference from Example 1 is that the hydrolysis of tetraethyl orthosilicate is as follows: maintaining high-speed stirring, slowly adding 0.5 mL of ammonia water dropwise to the mixed solution, sealing it with plastic wrap, keeping the temperature of the reaction system at 75 °C, and reacting for 5 h to obtain an ethanol dispersion of carbon nanotube / silica.

[0096] Example 10

[0097] The difference from Example 1 is that 0.47 g of carbon nanotube / SiO2 composite material and magnesium powder are reacted in an argon atmosphere at a high temperature of 500 °C for 24 h to obtain a reduction product, and then the reduction product is reacted in 10% dilute hydrochloric acid, washed, centrifuged, and dried to obtain a carbon nanotube / Si composite material.

[0098] Example 11

[0099] The difference from Example 1 is that 0.47 g of carbon nanotube / SiO2 composite material and magnesium powder are reacted in an argon atmosphere at a high temperature of 1200 °C for 2 h to obtain a reduction product, and then the reduction product is reacted in 10% dilute hydrochloric acid, washed, centrifuged, and dried to obtain a carbon nanotube / Si composite material.

[0100] Example 12

[0101] The difference from Example 1 is that 0.47 g of carbon nanotube / SiO2 composite material and magnesium powder are reacted in an argon atmosphere at a high temperature of 1300 °C for 1 h to obtain a reduction product, and then the reduction product is reacted in 10% dilute hydrochloric acid, washed, centrifuged, and dried to obtain a carbon nanotube / Si composite material.

[0102] Example 13

[0103] The difference from Example 1 is that 0.42 g of carbon nanotube / Si composite material and 0.1 g of asphalt are mixed to form a mixture, and the mixture is heat-treated at 400 °C for 24 h in a high-purity N2 atmosphere to carbonize the asphalt, obtaining the final product of silicon / carbon nanotube composite material.

[0104] Example 14

[0105] The difference from Example 1 is that 0.42 g of carbon nanotube / Si composite material and 0.1 g of asphalt are mixed to form a mixture, and the mixture is heat-treated at 1000 °C for 6 h in a high-purity N2 atmosphere to carbonize the asphalt, obtaining the final product of silicon / carbon nanotube composite material.

[0106] Example 15

[0107] The difference from Example 1 is that 0.42 g of carbon nanotube / Si composite material and 0.1 g of asphalt are mixed to form a mixture, and the mixture is heat-treated at 1200 °C for 4 h in a high-purity N2 atmosphere to carbonize the asphalt, obtaining the final product of silicon / carbon nanotube composite material.

[0108] Comparative Example 1

[0109] Different from Example 1, after mixing 0.375 g of carbon nanotubes and 45 ml of isopropanol, stir at 800 rpm for 2 h to obtain a carbon nanotube dispersion. While maintaining high-speed stirring, slowly add 0.32 g of TEOS dropwise to the carbon nanotube dispersion and stir for 60 min to obtain a mixed solution.

[0110] The rest is the same as in Example 1 to obtain the final product silicon / carbon nanotube composite.

[0111] Performance Test

[0112] The silicon-carbon nanotube composites prepared in each example and comparative example are uniformly mixed with CMC and SBR according to a mass percentage of 90:5:5, made into a slurry with deionized water, coated on a copper foil to make an electrode sheet with a diameter of 14 mm, and then assembled with a metal lithium sheet with a diameter of 14 mm, a polyethylene separator with a diameter of 16 mm, 1 M LiPF6, and an electrolyte with a molar ratio of EC / DMC / EMC of 1:1:1 into a button battery for electrochemical performance testing. The voltage range is 0.05 - 1.5 V and the current density is 400 mA / g. The test results are shown in Table 1. In addition, Figure 1 is the SEM image of the silicon-carbon nanotube composite prepared in Example 1. Figure 2 is the cycle performance graph of the silicon-carbon nanotube composite prepared in Example 1 at a charge-discharge rate of 0.5C. As can be seen from the figure, the initial discharge specific capacity is 634.3 mAh / g. After 50 charge-discharge cycles, the discharge specific capacity is 547.4 mAh / g, and the capacity retention rate is 90.87%.

[0113] Table 1

[0114]

[0115]

[0116] Among them, the proportion of carbon nanotubes, tetraethyl orthosilicate and pitch in Example 7 is slightly unsatisfactory, resulting in a smaller initial specific capacity. However, due to the preparation method of the present application, the components are more uniformly dispersed, so the capacity retention rate is relatively high.

[0117] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0118] In step S1 of the present application, polyvinylpyrrolidone is used as a surfactant to disperse carbon nanotubes as evenly as possible in an alcohol solvent; moreover, tetraethyl orthosilicate used in step S2 can be well-compatible and mixed with polyvinylpyrrolidone, so the carbon nanotubes and tetraethyl orthosilicate in the obtained mixed solution are relatively evenly mixed. In the alcohol dispersion of carbon nanotube / silica obtained by hydrolysis in step S3, since the hydrolysis of tetraethyl orthosilicate is a relatively mild chemical reaction, it will not affect the uniform system already formed in step S2. Then, the silicon obtained by reducing the silica in the carbon nanotube / silica composite material can be evenly coated on the carbon nanotubes; finally, the carbon source provided on the surface of the carbon nanotube / silicon composite material is carbonized to achieve the coating of silicon.

[0119] The raw materials used in the above preparation method of the present invention are widely sourced, the process is simple, easy to operate, highly efficient, green and environmentally friendly; the silicon / carbon nanotube composite material obtained by the above preparation method is a hollow silicon-carbon nanotube composite structure. The nanotubes can relieve the volume expansion of silicon, while making up for the disadvantage of poor conductivity of silicon. It has a large specific surface area, which improves the conductivity of the material and avoids material agglomeration. Since silicon in the silicon / carbon nanotube composite material of the present application is evenly distributed and in close contact with the carbon nanotubes, and the carbon layer formed after carbonization also coats the silicon, silicon can stably exist in the composite material. Therefore, the silicon / carbon nanotube composite anode material has good cycle stability. After testing, some of the silicon / carbon nanotube composite materials prepared in the present application are applied to lithium batteries. The lithium battery has an initial discharge specific capacity of 634.3 mAh / g at a rate of 0.5C. After 50 weeks of cycling, the discharge specific capacity is 547.4 mAh / g, and the capacity retention rate is 86.3%, indicating that the problems of poor material conductivity and poor cycle performance are effectively solved.

[0120] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a silicon / carbon nanotube composite material, characterized in that, Including: Step S1: Dispersing carbon nanotubes and polyvinylpyrrolidone in an alcohol solvent to obtain a carbon nanotube dispersion; The mass ratio of the carbon nanotubes to the polyvinylpyrrolidone is 1.5:1 to 5:1, and the mass content of the carbon nanotubes in the carbon nanotube dispersion is 1% to 10%; Step S2: Mixing tetraethyl orthosilicate and the carbon nanotube dispersion under stirring to obtain a mixed solution; Step S3: Catalyzing the hydrolysis of tetraethyl orthosilicate in the mixed solution with ammonia water to obtain an alcohol dispersion of carbon nanotube / silica; the addition amount of the ammonia water relative to the tetraethyl orthosilicate is 0.1 to 5 mL / g, the hydrolysis temperature is 50 to 70 °C, and the hydrolysis time is 8 to 24 hours; Step S4: Performing solid-liquid separation on the alcohol dispersion and then drying the obtained solid to obtain a carbon nanotube / silica composite; Step S5: Reducing the carbon nanotube / silica composite to obtain a carbon nanotube / silicon composite; And Step S6: Setting a carbon source on the surface of the carbon nanotube / silicon composite and carbonizing the carbon source to obtain a silicon / carbon nanotube composite, the carbon source is pitch, and the mass ratio of the carbon nanotubes, the tetraethyl orthosilicate, and the pitch is 32.0 to 58.0:25.0 to 59.0:8.0 to 17.

0.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the carbon nanotubes to the polyvinylpyrrolidone is 1.5:1 to 2.5:1, and the mass content of the carbon nanotubes in the carbon nanotube dispersion is 1% to 1.25%.

3. The preparation method according to claim 2, characterized in that, The alcohol solvent is ethanol or isopropanol.

4. The preparation method according to claim 1, characterized in that, Both Step S1 and Step S2 are stirred.

5. The preparation method according to claim 4, characterized in that, The stirring speed is 200 to 1000 rpm.

6. The preparation method according to claim 4, wherein The stirring time of Step S1 is 0.5 to 8 hours.

7. The preparation method according to claim 4, characterized in that, The stirring time of Step S2 is 10 to 30 min.

8. The preparation method according to claim 1, wherein Step S4 includes: Washing and deammoniating the alcohol dispersion with an alcohol solvent and then performing centrifugal separation to obtain a solid; Drying the solid at 50 to 80 °C for 12 to 48 hours to obtain the carbon nanotube / silica composite.

9. The preparation method according to claim 1, characterized in that, Step S5 includes: In an inert atmosphere, reducing the silica in the carbon nanotube / silica composite with a reducing powder to obtain a reduced product; Performing acid treatment on the reduced product to obtain the carbon nanotube / silicon composite.

10. The preparation method according to claim 9, characterized in that, The reducing powder is magnesium powder or carbon powder.

11. The preparation method according to claim 9, characterized in that, The reduction temperature is 500 to 1200 °C.

12. The preparation method according to claim 9, characterized in that, The reduction time is 2 to 24 hours.

13. The preparation method according to claim 9, wherein, The acid used for the acid treatment is hydrochloric acid.

14. The preparation method according to claim 13, characterized in that, The acid used for the acid treatment is hydrochloric acid with a concentration of 5% to 20%.

15. The preparation method according to claim 1, wherein Step S6 includes: Mixing the carbon nanotube / silicon composite and pitch to form a mixture; In an inert atmosphere, carbonizing the mixture to obtain the silicon / carbon nanotube composite.

16. The preparation method according to claim 15, characterized in that, The carbonization temperature is 400 to 1000 °C, and the carbonization time is 6 to 24 hours.

17. A silicon / carbon nanotube composite material, characterized in that, The silicon / carbon nanotube composite is prepared by the preparation method described in any one of claims 1 to 16.

18. A lithium battery anode, comprising a silicon / carbon nanotube composite material, characterized in that, The silicon / carbon nanotube composite is the silicon / carbon nanotube composite described in claim 17.

19. A lithium battery, comprising a positive electrode and a negative electrode, characterized in that, The negative electrode is the negative electrode of the lithium battery described in claim 18.

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