A hollow silicon-carbon composite material and preparation method thereof

Abstract

The present invention discloses a hollow silicon-carbon composite material and a preparation method thereof. The material has hollow silicon nanoparticles, SiO x The intermediate layer and coating carbon layer are prepared by mixing and dispersing nanosilicon powder and a carbon source in a solvent, followed by a two-stage, one-step solvothermal process. This environmentally friendly, streamlined process is particularly suitable for large-scale production. The resulting material is used in lithium-ion battery anodes, effectively improving the cycling stability of silicon-based materials, optimizing electrochemical performance, and increasing the energy density of lithium-ion batteries.

Description

Technical Field

[0001] The present invention relates to the field of battery electrodes, in particular to a hollow silicon-carbon composite material and a preparation method thereof, which is particularly suitable for negative electrode materials of high-energy-density lithium-ion batteries. Background Art

[0002] Graphite, the most widely used negative electrode material for lithium-ion batteries, has a theoretical capacity of only 372mAh / g. Currently, the maximum capacity of commercial graphite can reach 360mAh / g, which makes it difficult to break through the bottleneck. As a result, the energy density of lithium-ion batteries cannot meet the market's pursuit of high energy density.

[0003] Silicon has the highest known theoretical lithium storage capacity of 3580 mAh / g, nearly ten times that of graphite. However, silicon experiences significant volume changes during lithium insertion and extraction, resulting in poor cycling performance. Existing technical solutions aim to improve cycling performance through nanomaterialization, composite materials, and structural design.

[0004] Literature indicates that reducing silicon particle size to below 150nm significantly reduces the volume expansion effect of silicon materials. Industrial methods for preparing nano-silicon include laser etching, vapor deposition, and fluidized bed reactors (FBR). These methods are complex, require high-quality equipment, and are prone to environmental pollution.

[0005] Composites are mainly coated or loaded with carbon-based materials or non-carbon-based materials. On the one hand, carbon coating can optimize the silicon surface structure, stabilize the surface SEI film structure, and improve the cycle performance; on the other hand, it can improve the electrical conductivity of silicon and further improve the rate performance of the battery. At present, the main methods of carbon coating include vapor deposition, asphalt carbon coating, ball milling, etc. These methods also have defects such as complex process, high equipment requirements, and uneven carbon coating. Some existing technologies also use non-carbon-based composites, such as TiO2 coating to provide a shell to restrain the expansion of silicon, thereby improving electrochemical performance.

[0006] Silicon structural designs primarily include core-shell, hollow, Yolk-Shell, and porous structures. Acid-base etching methods involve the use of toxic reagents such as HF and cause contamination, sacrificial template methods offer low yields, and magnesium thermal reduction methods generate significant heat. Specialized structural designs can provide a buffer for silicon volume expansion, thereby improving its cyclic stability.

[0007] Chinese patent CN104143629A (published November 12, 2014) discloses a silicon-carbon-graphite composite anode material with a graphite core and a coating of nano-silicon particles and amorphous carbon. This technology, with the graphite as the core material and silicon coated on the outer layer, suffers from relatively low capacity and cycle capacity retention.

[0008] Chinese invention patent application CN103107317A (published May 15, 2013) discloses a carbon-silicon composite material and its preparation method. First, nano-silicon powder is calcined and oxidized at 400°C, and then a carbon source is added. The reaction is carried out at 180°C for 12 hours to obtain a brown product of a silicon dioxide-coated silicon outer-coated carbon precursor. The product is then reacted at 900°C for 2 hours to obtain a final black product. This technology does not construct a hollow silicon structure. Conventional technical means are used to oxidize the silicon surface, involving steps such as high-temperature oxidation and high-temperature calcination, which consumes a lot of energy. The introduction of the oxide layer on the silicon surface is a very conventional high-temperature oxidation and has no innovation.

[0009] Chinese patent application CN103346324A (published October 9, 2013) discloses a carbon-silicon composite core and a carbon composite shell. The core is prepared by ball-milling silicon powder and a carbon source twice, spray-drying twice, and then calcining at 700-1000°C. The carbon-silicon composite has a core particle size of 2-10 μm, and a shell particle size of 1-5 μm. This process involves multiple steps of ball milling, spray drying, and high-temperature calcination, resulting in a product with low specific capacity and poor cycling performance. Summary of the Invention

[0010] Purpose of the invention: The purpose of the present invention is to provide a hollow silicon-carbon composite material suitable for use as a negative electrode material for high-energy-density lithium-ion batteries; another purpose of the present invention is to combine silicon material structural design with composite modification methods to provide a more efficient and streamlined method for preparing the aforementioned hollow silicon-carbon composite material; another purpose of the present invention is to provide the application of the aforementioned hollow silicon-carbon composite material as a negative electrode material for lithium-ion batteries.

[0011] Technical solution: In order to achieve the above-mentioned purpose of the invention, the present invention provides a hollow silicon-carbon composite material having a three-layer structure (Si@SiO x @C), from the inside to the outside, it includes: silicon nanoparticles with hollow structure, SiO x Intermediate layer, and coated carbon layer; wherein the SiO x The intermediate layer has a thickness of 2-50 nm and 0<x<2, and the coating carbon layer has a thickness of 2-30 nm.

[0012] The hollow silicon nanoparticles are simple silicon and have a hollow structure, which can provide a buffer space for the volume expansion of silicon during lithium insertion. xDuring the lithium insertion process, it can be converted into irreversible lithium oxide and lithium silicate. These irreversible phases can effectively mitigate the volume expansion of silicon materials. The outermost carbon coating layer can not only mitigate the volume expansion of silicon materials, but also improve the conductivity and electrochemical properties of the material. These advantages combined can increase the energy density of lithium-ion batteries and improve the cycling stability and electrochemical performance of silicon materials. Traditional silicon-carbon materials prepared from silicon and graphite, nano-silicon, lack special structural design and still experience a large volume expansion rate during the cycle. Moreover, simply mixing with graphite, there is no chemical bonding force, which makes it difficult to restrain the volume expansion of silicon materials, and the cycling performance cannot be improved.

[0013] Furthermore, the silicon nanoparticles are derived from nano-silicon powder with a particle size between 20-500nm, and their morphology is any one or more combinations of spherical, ellipsoidal, and peanut-shaped. They are converted into a hollow core-shell structure through a corresponding preparation process, wherein the core is hollow nano-silicon and the shell is a SiOx intermediate layer.

[0014] Wherein, the intermediate layer is an amorphous SiO with a thickness of 2-50 nm. x , with a thickness of 2-50nm and 0<x<2. Preferably, by optimizing the process parameters, a thickness of 5-20nm can be further achieved; most preferably, the thickness of the intermediate layer is 5-15nm. The irreversible phase of lithium oxide or lithium silicate generated during the first lithium insertion process can effectively alleviate the expansion of the silicon material. An excessively thick intermediate layer will hinder the lithium ion reaction and reduce the lithium battery activity.

[0015] Furthermore, the carbon coating layer has a thickness of 2-20 nm. By further optimizing the process parameters, a thickness of 5-10 nm can be obtained, and good structural properties can be maintained even after silicon expands.

[0016] The present invention adopts nano silicon powder with a particle size of 20-500nm to obtain hollow silicon nanoparticles with a particle size of 20-500nm, the corresponding wall thickness of which is 5-100nm, and the shape is uniform and regular, which is significantly better than silicon materials prepared by ball milling.

[0017] The present invention also provides a method for preparing a hollow silicon-carbon composite material, comprising mixing nano-silicon powder and a carbon source and dispersing them in a solvent; reacting them at 30-100°C for 0.1-5 hours and then at 120-220°C for 3-18 hours under a pressurized environment with a pH of 7-9; and obtaining the hollow silicon-carbon composite material after centrifugation, washing, and drying; wherein the mass ratio of the silicon powder to the carbon source is 1:0.1-10.

[0018] The present invention obtains a three-layer composite structure in one process by optimizing the process, which is an innovative discovery in this field and has not been recorded in the prior art. In a weak base and pressurized environment, the temperature is divided into two stages for reaction: the first stage forms a hollow structure of silicon nano core-shell, and mainly forms SiO x The carbon source in this stage is not carbonized. The second stage is used to form the carbon coating layer. The entire reaction is controlled within 220°C. Of these two stages, the reaction control in the first stage is particularly important. The Si should not be rapidly corroded or directly dissolved; instead, the Si surface should be slowly oxidized and the pores on the Si surface should be etched from the inside to form a hollow structure.

[0019] Therefore, as a further optimization of the present invention, the first stage is a reaction at 50-100°C for 1-3 hours to form a two-layer silicon oxide core-shell structure (Si@SiOx); the second stage is a reaction at 140-180°C for 6-12 hours to form a three-layer hollow silicon-carbon composite structure (Si@SiOx@C). The pH is controlled at 7.2-8.5, and the reaction is carried out under a pressure of 0.1-3 MPa.

[0020] Furthermore, the mass ratio of the silicon powder to the solvent is 1:10-80. The solvent is selected from a combination of any one or more polar solvents including, but not limited to, water, ethanol, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, N-methylpyrrolidone, and ethylene glycol dimethyl ether. As a further optimization of the present invention, the mass ratio of the silicon powder to the solvent is 1:30-60.

[0021] Furthermore, the carbon source is selected from any one or more combinations including but not limited to citric acid, glucose, sucrose, polyvinyl pyrrolidone, polyvinylidene fluoride, and phenolic resin.

[0022] More preferably, the mass ratio of the silicon powder to the carbon source is 1:0.5-5, and the carbon source is selected from any one or more combinations of glucose, sucrose, fructose, and citric acid.

[0023] Preferably, the present invention adds silicon powder, a carbon source, and a solvent to a hydrothermal reactor, sequentially performs magnetic stirring and ultrasonic dispersion treatments, and then places the hydrothermal reactor in an oven to carry out the first and second hydrothermal reactions. The magnetic stirring time is 0.5-6 hours, preferably 1-3 hours; the ultrasonic dispersion time is 0.5-6 hours, preferably 1-3 hours.

[0024] The hollow silicon-carbon composite material obtained by the preparation process provided by the present invention is used for lithium battery negative electrode, effectively improving the cycle stability of silicon-based materials, optimizing electrochemical performance, and increasing the energy density of lithium-ion batteries. The preparation method is environmentally friendly, streamlined, and particularly suitable for large-scale production. Compared with the traditional process of precursor preparation, mid-stage treatment, high-temperature calcination, spray drying and other process routes, the production cycle is greatly shortened. At 0.5Ag -1 The capacity of the battery is 1176 mAh g after 300 cycles at a current density of 100. -1 , the capacity retention rate is 92%. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the XRD pattern of the hollow silicon-carbon composite material of Example 2 of the present invention;

[0026] Figure 2 This is a TEM image of the hollow silicon-carbon composite material of Example 2 of the present invention;

[0027] Figure 3 This is a lithium storage performance diagram of the hollow silicon-carbon composite material according to Example 2 of the present invention;

[0028] Figure 4 This is a TEM image of silicon powder after etching with ammonia water in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0030] Example 1

[0031] This embodiment provides a type of hollow silicon-carbon composite material, the preparation method of which is as follows: silicon powder, a carbon source, and a solvent are added to a hydrothermal kettle, the carbon source is selected from any one of glucose, sucrose, fructose, and citric acid, and the mass ratio of silicon powder to carbon source is 1:1-3; the solvent is selected from any one or more combinations of water, ethanol, ethylene glycol, and polyethylene glycol, and the mass ratio of silicon powder to solvent is 1:40-50. After the above raw materials are mixed in the hydrothermal kettle, they are magnetically stirred and ultrasonically dispersed, and then placed in an oven. The reaction pH is 7.2-8.5, and under a pressure of 0.1-3MPa, the reaction is first carried out at 70-80°C for 1.5-2.5 hours, and then at 180-200°C for 8-10 hours.

[0032] The mixture was centrifuged at a speed of 6000-12000 rpm / min for 3-8 minutes, washed with distilled water 1-3 times, washed with ethanol 1-3 times, placed in a vacuum drying oven, and dried at a temperature of 60-120 degrees for 5-12 hours to obtain a hollow silicon-carbon composite material powder.

[0033] The hollow silicon-carbon composite material prepared by the above method has an obvious three-layer structure (Si@SiOx@C), the innermost layer is a hollow nano-Si core-shell structure with a particle size of 20-100nm and a wall thickness of 5-20nm; the middle layer is SiO x , 0<x<2, with a thickness of 5-15nm; the outermost layer is a 2-15nm thick carbon layer. This material has excellent electrochemical properties, significantly improves the cycle stability of silicon-based materials, and increases the energy density of lithium-ion batteries. -1 The capacity of the battery after 300 cycles at a current density of not less than 800 mAh g -1 , the capacity retention rate is not less than 80%.

[0034] Example 2

[0035] This embodiment provides a hollow silicon-carbon composite material. Silicon powder, glucose, and a solvent are added to a hydrothermal kettle. The mass ratio of silicon powder to glucose is 1:1. A mixed solvent of ethanol and water is selected, and the mass ratio of silicon powder to solvent is 1:50. After mixing in the lining of the hydrothermal kettle, it is magnetically stirred and ultrasonically dispersed, and then placed in an oven. The reaction pH is 7.8, and the reaction is carried out at 70°C for 2 hours under a pressure of 2MPa, and then at 180°C for 10 hours. After the reaction is completed, the hollow silicon-carbon composite material is obtained by centrifugation, washing, and drying.

[0036] The Si@SiOx@C prepared by the above method has an obvious three-layer structure. The innermost hollow nano-Si core-shell particle size is 50nm and the wall thickness is 20nm. x The thickness of the intermediate layer is 5 nm; the thickness of the coating carbon layer is 8 nm.

[0037] Battery production: The above-mentioned hollow silicon-carbon composite material, acetylene black, and adhesive are mixed in a mass ratio of 8:1:1, and an appropriate amount of water is added to prepare a slurry, which is coated on the current collector copper foil and dried at 60°C for 10 hours under vacuum conditions to obtain a pole piece. Metal lithium sheets are used as counter electrodes and reference electrodes, 1M LiPF6 is dissolved in a mixed solution of EC / DEC (volume ratio 1:1) as an electrolyte, and button cells are assembled in an argon glove box. The assembled button cells are charged and discharged in a voltage range of 0.01-1V at a certain current density. At 0.5A g -1 The capacity of the battery is 1176 mAh g after 300 cycles at a current density of 100. -1 , the capacity retention rate is 92%.

[0038] Figure 1 The XRD pattern of the hollow silicon-carbon composite material in this embodiment is shown. It can be seen that the peak position is completely consistent with the peak position of crystalline silicon, indicating that the main body of the material is still silicon. The bulge at 20-30 degrees is the amorphous carbon layer and the amorphous SiOx layer.

[0039] Figure 2 The TEM image of the hollow silicon-carbon composite material in this embodiment is provided, showing an obvious hollow structure. The outer ring structure is the hollow nano-silicon layer, SiO x A three-layer structure (Si@SiOx@C) formed by the middle layer and the covering carbon layer.

[0040] Figure 3 This is a diagram showing the lithium storage performance of the lithium battery negative electrode prepared from the hollow silicon-carbon composite material in this embodiment. As can be seen from the figure, thanks to the good structure, the volume expansion problem of the silicon material can be well buffered, thereby greatly improving the electrochemical cycle stability of silicon.

[0041] Example 3

[0042] This embodiment provides a hollow silicon-carbon composite material. Silicon powder, sucrose, and a solvent are added to a hydrothermal autoclave, with a silicon powder to sucrose mass ratio of 1:2. The solvent is a mixture of ethylene glycol and water, with a silicon powder to solvent mass ratio of 1:50. After mixing in the autoclave liner, the mixture is subjected to magnetic stirring and ultrasonic dispersion, and then placed in an oven. The reaction is carried out at a pH of 8.2, under a pressure of 2 MPa, at 80°C for 2 hours, and then at 180°C for 10 hours. After completion of the reaction, the mixture is centrifuged, washed, and dried to obtain the hollow silicon-carbon composite material.

[0043] The Si@SiOx@C prepared by the above method has a distinct three-layer structure: the innermost layer is a hollow nano-Si core-shell particle with a diameter of 50nm and a wall thickness of 15nm; the SiOx middle layer has a thickness of 8nm; and the outermost layer is a 10nm thick coating carbon layer.

[0044] The electrode and battery were manufactured and tested using the method described in Example 2. The obtained product was -1 The capacity of the battery is 1020 mAh g after 300 cycles at a current density of 1000 mAh g -1 , the capacity retention rate is 88%.

[0045] Example 4

[0046] This embodiment provides a hollow silicon-carbon composite material. Silicon powder, sucrose, and a solvent are added to a hydrothermal autoclave, with a silicon powder to sucrose mass ratio of 1:1. The solvent is a mixture of ethanol and ethylene glycol, with a silicon powder to solvent mass ratio of 1:50. After mixing in the autoclave liner, the mixture is subjected to magnetic stirring and ultrasonic dispersion, and then placed in an oven. The reaction is carried out at a pH of 7.1, under a pressure of 1 MPa, at 80°C for 2 hours, then at 180°C for 10 hours. After completion of the reaction, the mixture is centrifuged, washed, and dried to obtain the hollow silicon-carbon composite material.

[0047] The Si@SiOx@C prepared by the above method has a distinct three-layer structure. The innermost layer is hollow nano-Si with a particle size of 50nm and a wall thickness of 40nm; the SiOx middle layer has a thickness of 2nm; and the outermost layer is an 8nm thick coating carbon layer.

[0048] The electrode and battery were manufactured and tested using the method described in Example 2. The obtained product was -1 The capacity of the battery is 997 mAh g after 300 cycles at a current density of 100. -1 , the capacity retention rate is 85%.

[0049] Example 5

[0050] This embodiment provides a hollow silicon-carbon composite material. Silicon powder, citric acid, and a solvent are added to a hydrothermal autoclave, with the mass ratio of silicon powder to citric acid being 1:2. The solvent is a mixture of ethanol and water, with the mass ratio of silicon powder to solvent being 1:70. After mixing in the lining of the hydrothermal autoclave, the mixture is subjected to magnetic stirring and ultrasonic dispersion, and then placed in an oven. The reaction is carried out at a pH of 8.5, under a pressure of 1 MPa, at 80°C for 3 hours, then at 180°C for 12 hours. After completion of the reaction, the mixture is centrifuged, washed, and dried to obtain the hollow silicon-carbon composite material.

[0051] The Si@SiOx@C prepared by the above method has a distinct three-layer structure: the innermost layer is hollow nano-Si with a particle size of 100nm and a wall thickness of 50nm; the SiOx middle layer has a thickness of 10nm; and the outermost layer is a 15nm thick coating carbon layer.

[0052] The electrode and battery were manufactured and tested using the method described in Example 2. The obtained product was -1 The capacity of the battery is 860 mAh g after 300 cycles at a current density of 1000. -1 , the capacity retention rate is 83%.

[0053] Example 6

[0054] This embodiment provides a hollow silicon-carbon composite material. Silicon powder, citric acid, and a solvent are added to a hydrothermal autoclave, with a mass ratio of silicon powder to citric acid of 1:2. The solvent is ethylene glycol, with a mass ratio of silicon powder to solvent of 1:70. The mixture is mixed in the autoclave liner, subjected to magnetic stirring and ultrasonic dispersion, and then placed in an oven. The reaction is carried out at a pH of 9, under a pressure of 2 MPa, at 100°C for 5 hours, then at 220°C for 18 hours. After completion of the reaction, the hollow silicon-carbon composite material is obtained by centrifugation, washing, and drying.

[0055] The Si@SiOx@C prepared by the above method has a distinct three-layer structure. The innermost layer is hollow nano-Si with a particle size of 100nm and a wall thickness of 50nm; the SiOx middle layer has a thickness of 8nm; and the outermost layer is a 30nm thick coating carbon layer.

[0056] The electrode and battery were manufactured and tested using the method described in Example 2. The obtained product was -1 The capacity of the battery is 750 mAh g after 300 cycles at a current density of 1000. -1 , the capacity retention rate is 80%.

[0057] Comparative Example 1

[0058] The key to the present invention's process lies in the first-stage, low-temperature hydrothermal reaction. This comparative example provides a hollow silicon-carbon composite material that undergoes only the second-stage hydrothermal reaction: silicon powder, glucose, and a solvent are added to a hydrothermal kettle, with a silicon powder to glucose mass ratio of 1:1. The solvent is a mixture of ethanol and water, with a silicon powder to solvent mass ratio of 1:50. After mixing in the hydrothermal kettle liner, the mixture is subjected to magnetic stirring and ultrasonic dispersion, followed by reaction at 180°C for 10 hours. After completion of the reaction, the Si@C composite material is centrifuged, washed, and dried.

[0059] The Si@C composite material prepared by the above method has a double-layer structure, with the middle nano-Si particle size being 50 nm and the outermost layer being a coated carbon layer with a thickness of 8 nm.

[0060] The electrode and battery were manufactured and tested using the method described in Example 2. The obtained product was -1 The capacity of the battery is 514.7 mAh g after 300 cycles at a current density of 100. -1 , the capacity retention rate is only 43%. It can be seen that the hollow structure and SiO x The formation of the intermediate layer can significantly improve the cycle stability and increase the discharge capacity of the battery.

[0061] Comparative Example 2

[0062] Silicon powder, glucose, and solvent were added to a hydrothermal autoclave at a 1:1 silicon powder to glucose mass ratio. The solvent was an ammonia solution with a pH of 11, with a silicon powder to solvent mass ratio of 1:50. After mixing in the autoclave's lining, the mixture was subjected to magnetic stirring and ultrasonic dispersion. The mixture was then placed in an oven and reacted at 70°C for 2 hours, followed by 180°C for 10 hours. After completion of the reaction, the mixture was centrifuged, washed, and dried to obtain a powder material.

[0063] from Figure 4It can be seen that the silicon powder has completely lost its spherical structure and has agglomerated, indicating that the etching by ammonia is carried out from the outside, causing the silicon powder to lose its original spherical structure and agglomerate, and unable to form a hollow structure from the inside. The method described in Example 2 was used to make and test the electrode and battery. The obtained product was 0.5A g -1 The capacity of the battery is 204 mAh g after 300 cycles at a current density of 100. -1 , the capacity retention rate is only 23%.

[0064] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a hollow silicon-carbon composite material, characterized in that: The nano-silicon powder and a carbon source are mixed and dispersed in a solvent, wherein the carbon source is selected from any one or more combinations of glucose, sucrose, fructose, and citric acid, and the mass ratio of the silicon powder to the carbon source is 1:0.1-10; The pH value is controlled at 7.2-8.5, and the reaction is carried out at 50-100° C. for 1-3 hours and then at 140-180° C. for 6-12 hours in a pressurized environment of 0.1-3 MPa; the product is obtained after centrifugation, washing and drying.

2. The method for preparing a hollow silicon-carbon composite material according to claim 1, wherein: The mass ratio of the silicon powder to the solvent is 1:10-80.

3. The method for preparing a hollow silicon-carbon composite material according to claim 2, wherein: The solvent includes any one or more combinations of water, ethanol, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, N-methyl pyrrolidone, and ethylene glycol dimethyl ether.

4. The method for preparing a hollow silicon-carbon composite material according to claim 3, wherein: The dispersing step includes magnetic stirring and / or ultrasonic dispersion.

5. The hollow silicon-carbon composite material prepared by the method according to any one of claims 1 to 4, characterized in that: From the inside to the outside, they include: silicon nanoparticles with hollow structure, SiO x Intermediate layer, and coated carbon layer; wherein the SiO x The intermediate layer has a thickness of 2-50 nm and 0 < x <2, the coating carbon layer has a thickness of 2-30 nm.

6. The hollow silicon-carbon composite material according to claim 5, characterized in that: The coating carbon layer has a thickness of 2-20 nm.

7. A hollow silicon-carbon composite material according to claim 5 or 6, characterized in that: The silicon nanoparticles have a particle size of 20-500 nm and a wall thickness of 5-100 nm.

8. Use of the hollow silicon-carbon composite material as claimed in claim 5 in preparing negative electrode materials for lithium-ion batteries.

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