Preparation method of high-purity nano-porous silicon and silicon-carbon composite negative electrode material

Abstract

The application provides a preparation method of high-purity nano-porous silicon and silicon-carbon composite negative electrode material, wherein a cast silicon alloy is ground to micron size, then placed in a muffle furnace for heat treatment in an air atmosphere, and then the heat-treated sample is placed in an acid solution for chemical corrosion, so that high-purity nano-porous silicon with continuous, open and uniformly distributed pores is formed, and the purity is 92.00-99.99%; a high-molecular compound is polymerized on the surface of the nano-porous silicon, and then the high-molecular compound is pyrolyzed in an inert protective atmosphere in a tube furnace, so that the high-purity nano-porous silicon-carbon composite negative electrode material is prepared. The method provided by the application can simplify the production process of the lithium ion battery negative electrode material and effectively improve the cycle performance and coulomb efficiency of the lithium ion battery negative electrode material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery anode materials, and relates to a method for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials. Background Technology

[0002] The continuous consumption of non-renewable energy sources has led to energy shortages, and the combustion of fossil fuels has caused severe environmental pollution. To address the current energy crisis and promote ecological civilization, achieving efficient, environmentally friendly, and sustainable development for human society, the new energy industry has garnered widespread attention. Clean energy sources such as solar and wind power are being applied to production and daily life. However, due to unstable natural conditions and imperfect technologies, the energy conversion rate is low, fluctuations are large, and the supply of energy is unsustainable. Therefore, researching and developing new energy storage technologies is crucial for the efficient development of the new energy industry. Furthermore, the development of energy storage technology can promote the development of mobile communications while reducing the dependence of industries such as automobiles on traditional fossil fuels, reducing environmental pollution, and driving new developments in ecological civilization.

[0003] Lithium-ion batteries are widely used in mobile communication devices such as mobile phones, smartwatches, and laptops due to their advantages, including high charge / discharge efficiency, absence of heavy metal pollution, and long cycle life. Furthermore, their lightweight, small size, and activation-free characteristics have led to their widespread application in emerging fields such as new energy vehicles and large-scale energy storage systems.

[0004] Currently, commercially available graphite anode materials for lithium-ion batteries have a relatively low theoretical specific capacity (only 372 mAh g⁻¹). -1 The existing materials are no longer sufficient to meet the growing performance demands of the aforementioned fields. Therefore, the development direction of new anode materials is high specific capacity, good cycle stability, and high safety. When silicon (Si) is used as an anode in lithium-ion batteries, its specific capacity reaches as high as 3579 mAh g⁻¹ when fully lithium-intercalated at room temperature. -1 While lithium ions possess advantages such as being free of heavy metal pollution and having abundant crustal reserves, the process of lithium ions intercalating and deintercalating in silicon involves a volume change of nearly 300%. The resulting internal stress can lead to instability at the solid electrolyte interface and pulverization of active materials. These problems cause rapid capacity decay, decreased conductivity, and poor cycle stability.

[0005] Y. Cui et al. disclosed a method for preparing a silicon-carbon composite anode with an eggshell structure (Liu N, WuH, McDowell MT, et al. A yolk-shell design for stabilized and scalable Li-ion battery alloy anodes [J]. Nano Letters, 2012, 12(6): 3315-3321). First, 150 mg of Si nanoparticles (less than 100 nm) were ultrasonically dispersed in a mixed solution of 240 mL ethanol and 60 mL water. Then, 3.0 mL of ammonium hydroxide solution was added to the mixed solution. Under vigorous stirring, 2.4 g of tetraethoxysilane was added dropwise and reacted at room temperature for 12 h to obtain Si nanoparticles uniformly coated with a SiO2 layer (Si@SiO2). The nanoparticles were collected using a centrifuge and washed three times with distilled water. Then, Si@SiO2 nanoparticles and dopamine hydrochloride were mixed evenly in Tris buffer solution and stirred for 24 h. The mixture was collected by centrifugation and washed to obtain Si@SiO2@polydopamine nanoparticles. These nanoparticles were then placed in a tube furnace and heated at 400℃ for 2 h under a N2 protective atmosphere, followed by heating at 800℃ for 3 h to obtain Si@SiO2@C nanoparticles. Finally, the nanoparticles were etched with a 10 wt.% hydrofluoric acid (HF) solution for 30 min to selectively remove the SiO2 sacrificial layer. The etched powder was collected by centrifugation and washed three times with ethanol to obtain a silicon-carbon composite anode material (Si@void@C) with an eggshell structure. The prepared Si@void@C anode material powder was mixed with carbon black and polyvinylidene fluoride (PVDF) at a mass ratio of 65:20:15. N-methyl-2-pyrrolidone was then added to form a slurry. The slurry was then uniformly coated onto copper foil and baked in an oven at 100 ℃ for 4 h to obtain the lithium-ion battery anode.

[0006] The lithium-ion battery anode prepared by this method has the following shortcomings: (1) The preparation process is relatively complex, costly and difficult to achieve large-scale industrial production; (2) The anode material is made into powder, then mixed with conductive agent and binder to form a slurry and coated on conductive current collector, and then dried before being used as an electrode. The binder itself is not conductive, which will hinder electron transport, increase electrode polarization and impedance, and thus lead to poor rate performance; (3) During the charging and discharging process of lithium-ion battery, the insertion and extraction of lithium ions will cause huge volume changes. The bonding force between the active components and between the active components and the copper foil is also weak. During the huge volume changes of charging and discharging, the active components are prone to fall off and fail. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, which reduces the preparation cost while ensuring that the lithium-ion battery anode materials have good cycle performance and rate performance.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows.

[0009] A method for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials is characterized by the following steps: First, a cast silicon alloy is ground to micrometer size and then heat-treated in an air atmosphere in a muffle furnace. The heat-treated sample is then chemically etched in an acidic solution to form high-purity nanoporous silicon with continuous, open, and uniformly distributed pores. Next, a polymer compound is polymerized on the surface of the nanoporous silicon, and then the polymer compound is pyrolyzed in a tube furnace under an inert protective atmosphere to obtain the high-purity nanoporous silicon-carbon composite anode material.

[0010] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode material, the nanoporous silicon obtained has a silicon purity of 92.00%~99.99% and a pore size of 50~800 nm; the nanoporous silicon-carbon composite anode material obtained has a carbon layer uniformly coated on the surface of the nanoporous silicon, and the carbon layer thickness is preferably 20~50 nm.

[0011] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, the cast silicon alloy is one or more of cast aluminum silicon, magnesium silicon, manganese silicon, nickel silicon, aluminum copper silicon, aluminum magnesium silicon, and aluminum nickel silicon alloys.

[0012] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, when the hardness of the cast silicon alloy is low, it is directly ground; when the hardness of the cast silicon alloy is high, the silicon alloy is etched with sodium hydroxide solution and then ground. The concentration of sodium hydroxide solution is preferably 1 wt.%~5 wt.%, the etching temperature is preferably 30~90℃, and the etching time is preferably 1~5 h.

[0013] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, the heat treatment temperature is preferably 300~800 ℃ and the heat treatment time is preferably 10~30 h in an air atmosphere.

[0014] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, the acidic solution used for chemical etching is preferably a mixture of one or more of hydrochloric acid, sulfuric acid, and nitric acid, the concentration of the acidic solution is preferably 1 wt.%~10 wt.%, the etching time is preferably 10~40 h, and after etching, the surface oxide layer of the sample needs to be cleaned with hydrofluoric acid solution, the concentration of the hydrofluoric acid solution is preferably 1 wt.%~5 wt.%.

[0015] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode material, when polymerizing polymer compounds on the surface of nanoporous silicon, the nanoporous silicon needs to be uniformly dispersed in a tris(hydroxymethyl)aminomethane (Tris) buffer solution by magnetic stirring, so that the polymer compounds are polymerized on the surface of nanoporous silicon in the buffer solution.

[0016] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, when the polymeric compound polymerized on the surface of the nanoporous silicon is polydextrose, polypyrrole, and polyvinylpyrrolidone, the coating layer after pyrolysis is a carbon layer; when the polymeric compound polymerized on the surface of the nanoporous silicon is polydopamine, polyacrylonitrile, and phenolic resin, the coating layer after pyrolysis is a nitrogen-doped carbon layer.

[0017] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, the inert protective atmosphere used for the pyrolysis of polymer compounds is argon or nitrogen, and the purity is preferably 99.95~99.99%.

[0018] In the above-mentioned technical solution for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials, the pyrolysis temperature of the polymer compound is preferably 400~800 ℃, the holding time is preferably 1~10 h, and the heating rate is preferably 5~15 ℃ / min.

[0019] The technical solution of this invention provides a method for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials. The formation principle of the nanoporous silicon and silicon-carbon composite anode materials is as follows: Cast silicon alloys undergo a displacement reaction with O2 in the air during high-temperature heat treatment. To increase the contact area between O2 and the silicon alloy, the silicon alloy is thoroughly ground using a ball mill or mortar before high-temperature heat treatment. As the heat treatment time increases, the silicon alloy reacts fully with O2 in the air, gradually transforming into metal oxides and elemental Si. Since elemental Si hardly reacts with acidic solutions other than hydrogen fluoride, placing the mixture containing metal oxides and elemental Si into an acidic solution without hydrogen fluoride allows for a complete reaction. The metal oxides in the mixture are completely corroded, generating H2O and water-soluble metal salts, while elemental Si is not corroded and is thus well preserved, forming high-purity nanoporous silicon with continuous, open, and uniformly distributed pores. Based on this, nanoporous silicon is uniformly dispersed in a Tris buffer solution by magnetic stirring, allowing the polymer compound to polymerize onto the surface of the nanoporous silicon in the buffer solution. Subsequently, the polymer compound is placed in a tube furnace and subjected to high-temperature pyrolysis under an inert protective atmosphere to obtain a high-purity nanoporous silicon-carbon composite anode material.

[0020] Compared with the prior art, the technical solution provided by the present invention has the following technical effects.

[0021] 1. This invention provides a method for preparing high-purity nanoporous silicon and silicon-carbon composite anode materials. The method involves grinding a cast silicon alloy to micrometer dimensions, heat-treating it in an air atmosphere in a muffle furnace, and then chemically etching the heat-treated sample in an acidic solution to obtain high-purity nanoporous silicon. After polymerizing a polymer compound on the surface of the nanoporous silicon, the polymer compound is pyrolyzed in a tube furnace under an inert protective atmosphere to obtain the high-purity nanoporous silicon-carbon composite anode material. The nanoporous silicon prepared by this method exhibits a continuous, open, and uniformly distributed nanoporous structure, and the silicon purity is high, ranging from 92.00 wt.% to 99.99 wt.%, effectively improving the electrochemical performance of silicon materials. Compared with existing technologies, the preparation method of high-purity nanoporous silicon and silicon-carbon composite anode materials described in this invention is simpler and uses inexpensive commercially available cast silicon alloys as raw materials, which greatly reduces the production cost of lithium-ion battery anode materials and makes it easier to achieve large-scale production of lithium-ion anode materials.

[0022] 2. The high-purity nanoporous silicon and silicon-carbon composite anode material of the present invention have the characteristics of continuous, open, and uniformly distributed nanoporous structure, which can effectively buffer the volume changes that occur during charging and discharging. At the same time, the uniform distribution of pores increases the specific surface area of ​​the anode material, which can increase the contact area between the electrolyte and the anode material, providing more attachment sites for lithium ions, thereby promoting the electrode reaction. The nanoporous silicon material is coated with a carbon layer after the pyrolysis of polymer, which effectively reduces the occurrence of silicon material pulverization and shedding, and significantly improves the problems of poor electrode cycle stability and low initial coulombic efficiency.

[0023] 3. The nanoporous silicon-carbon composite anode material prepared by the method described in this invention is assembled into a lithium-ion battery. At a charge / discharge current density of 0.1 C, it achieves high specific capacity and coulombic efficiency, with an initial charge / discharge specific capacity of 2794.48 mAh g⁻¹. -1 and 3806.95 mAh g -1 The initial coulombic efficiency was 73.40%. After 100 cycles, the charge specific capacity remained at 933.74 mAh g. -1 The coulombic efficiency is above 99.39%, and the cycle performance and coulombic efficiency are stable. Attached Figure Description

[0024] Figure 1 These are (a) a scanning electron microscope image and (b) an EDS spectrum of the cast magnesium-silicon alloy ingot in Example 1.

[0025] Figure 2 This is the XRD pattern of the nanoporous silicon prepared in Example 1.

[0026] Figure 3 This is the EDS spectrum of the nanoporous silicon prepared in Example 1.

[0027] Figure 4 These are scanning electron microscope images of the nanoporous silicon prepared in Example 1 at (a) low magnification and (b) high magnification.

[0028] Figure 5 These are transmission electron microscope (TEM) images of the nanoporous silicon prepared in Example 1 at (a) low magnification and (b) high magnification.

[0029] Figure 6 This is the EDS spectrum of the nanoporous silicon prepared in Example 2.

[0030] Figure 7 This is the EDS spectrum of the nanoporous silicon prepared in Example 3.

[0031] Figure 8The images show (a) XRD pattern and (b) EDS pattern of the nanoporous silicon-carbon composite anode material prepared in Example 4.

[0032] Figure 9 These are scanning electron microscope (SEM) images of the nanoporous silicon-carbon composite anode material prepared in Example 4 at (a) low magnification and (b) high magnification.

[0033] Figure 10 These are transmission electron microscope (TEM) images of the nanoporous silicon-carbon composite anode material prepared in Example 4 at (a) low magnification and (b) high magnification.

[0034] Figure 11 The constant current charge-discharge performance curves of the lithium-ion battery prepared in Example 4 are shown.

[0035] Figure 12 This is the EDS spectrum of the nanoporous silicon prepared in Example 5.

[0036] Figure 13 This is the EDS spectrum of the nanoporous silicon prepared in Example 6. Detailed Implementation

[0037] The following examples further illustrate the preparation method of high-purity nanoporous silicon and silicon-carbon composite anode materials provided by the present invention. It should be noted that the following examples are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention will still fall within the scope of protection of the present invention.

[0038] Example 1

[0039] In this embodiment, a method for preparing high-purity nanoporous silicon is provided, and the specific steps are as follows.

[0040] (1) Weigh magnesium blocks (purity greater than 99.99%) and silicon blocks (purity greater than 99.9%) according to the atomic percentage of magnesium to silicon of 45:55, and melt them in a vacuum induction furnace to obtain cast magnesium-silicon alloy ingots. Figure 1 (a) is a scanning electron microscope image of the cast magnesium-silicon alloy ingot prepared in this embodiment. Figure 1 (b) is the EDS spectrum of the cast magnesium-silicon alloy ingot prepared in this embodiment. As can be seen from the figure, the actual composition of the cast magnesium-silicon alloy ingot prepared in this embodiment is very close to the designed composition, which meets the requirements for use.

[0041] (2) The cast magnesium-silicon alloy ingot was cut into cast magnesium-silicon alloy sheets with a thickness of 600 μm using diamond wire. The surface of the cast magnesium-silicon alloy sheets was then polished with 380-mesh, 800-mesh, 1200-mesh, and 2000-mesh wet sandpaper in sequence. Then, it was polished with diamond polishing paste with a particle size of 0.5 μm. The polished cast magnesium-silicon alloy sheets were cleaned with deionized water. Subsequently, the cast magnesium-silicon alloy sheets were placed in a 5 wt.% sodium hydroxide solution for etching at a temperature of 70 ℃ for 5 h. After etching, the samples were dried and ground in a vacuum drying oven and then placed in a muffle furnace for heat treatment in an air atmosphere. The holding temperature was controlled at 550 ℃ for 20 h. Finally, the heat-treated sample was placed in a 5 wt.% hydrochloric acid solution and etched at room temperature for 24 h. The oxide layer on the sample surface was then cleaned with a 1 wt.% hydrofluoric acid solution, followed by repeated washing with deionized water and alcohol and vacuum drying to obtain high-purity nanoporous silicon.

[0042] Figure 2 This is the XRD pattern of the nanoporous silicon prepared in this embodiment. Figure 2 It can be seen that the nanoporous silicon composition consists of only a single silicon phase. Figure 3 This is the EDS spectrum of the nanoporous silicon prepared in this embodiment. Figure 3 It can be seen that the purity of this nanoporous silicon is 92.45%. Figure 4 These are scanning electron microscope (SEM) images of the nanoporous silicon prepared in this embodiment at (a) low magnification and (b) high magnification. Figure 4 It can be seen that the overall structure of nanoporous silicon is uniform, forming a continuous, open, and uniformly distributed nanoporous structure with pore sizes ranging from 300 to 600 nm. Figure 5 These are transmission electron microscope (TEM) images of the nanoporous silicon prepared in this embodiment at (a) low magnification and (b) high magnification. Figure 5 It can be seen that the nanoporous structure is uniformly distributed, and the pore and pore wall sizes are completely consistent with the results observed by scanning electron microscopy, indicating that the prepared nanoporous structure is uniformly distributed in three dimensions.

[0043] Example 2

[0044] In this embodiment, a method for preparing high-purity nanoporous silicon is provided, and the specific steps are as follows.

[0045] (1) Weigh magnesium blocks (purity greater than 99.99%) and silicon blocks (purity greater than 99.9%) according to the atomic percentage of magnesium to silicon of 64:36, and melt them in a vacuum induction furnace to obtain cast magnesium-silicon alloy ingots.

[0046] (2) Cast magnesium-silicon alloy ingots were cut into 600 μm thick sheets using diamond wire. The surfaces of the cast magnesium-silicon alloy sheets were then polished with 380-mesh, 800-mesh, 1200-mesh, and 2000-mesh wet sandpaper, and then polished with 0.5 μm diamond polishing paste. The polished cast magnesium-silicon alloy sheets were then cleaned with deionized water and dried. After grinding the dried cast magnesium-silicon alloy sheets, they were placed in a muffle furnace and heat-treated in an air atmosphere. The holding temperature was controlled at 500 °C and the holding time was 22 h. Finally, the heat-treated samples were placed in a 3 wt.% hydrochloric acid solution and etched at room temperature for 30 h. The oxide layer on the sample surface was cleaned with a 3 wt.% hydrofluoric acid solution, and then repeatedly washed with deionized water and alcohol and vacuum dried to obtain high-purity nanoporous silicon. Figure 6 This is the EDS spectrum of the nanoporous silicon prepared in this embodiment. Figure 6 It can be seen that the purity of this nanoporous silicon is 95.23%.

[0047] Example 3

[0048] In this embodiment, a method for preparing high-purity nanoporous silicon is provided, and the specific steps are as follows.

[0049] (1) Weigh aluminum blocks (purity greater than 99.99%) and silicon blocks (purity greater than 99.9%) according to the atomic percentage ratio of aluminum to silicon of 70:30, and melt them in a vacuum induction furnace to obtain cast aluminum-silicon alloy ingots.

[0050] (2) The cast aluminum-silicon alloy ingot was cut into cast aluminum-silicon alloy sheets with a thickness of 600 μm using diamond wire. The surface of the cast aluminum-silicon alloy sheets was then polished with 380-mesh, 800-mesh, 1200-mesh, and 2000-mesh wet sandpaper in sequence. Then, it was polished with diamond polishing paste with a particle size of 0.5 μm. The polished cast aluminum-silicon alloy sheets were cleaned with deionized water. Subsequently, the cast aluminum-silicon alloy sheets were placed in a 3 wt.% sodium hydroxide solution for etching at a temperature of 90 ℃ for 3 h. After etching, the samples were dried and ground in a vacuum drying oven and then placed in a muffle furnace for heat treatment in an air atmosphere. The holding temperature was controlled at 700 ℃ and the holding time was 18 h. Finally, the heat-treated sample was placed in a 7 wt.% hydrochloric acid solution and etched at room temperature for 36 h. The oxide layer on the sample surface was then cleaned with a 5 wt.% hydrofluoric acid solution, followed by repeated washing with deionized water and alcohol and vacuum drying to obtain high-purity nanoporous silicon. Figure 7 This is the EDS spectrum of the nanoporous silicon prepared in this embodiment. Figure 7 It can be seen that the purity of this nanoporous silicon is 97.02%.

[0051] Example 4

[0052] In this embodiment, a method for preparing a high-purity nanoporous silicon-carbon composite anode material is provided, and the specific steps are as follows.

[0053] (1) Weigh magnesium blocks (purity greater than 99.99%) and silicon blocks (purity greater than 99.9%) according to the atomic percentage of magnesium to silicon of 45:55, and melt them in a vacuum induction furnace to obtain cast magnesium-silicon alloy ingots.

[0054] (2) The cast magnesium-silicon alloy ingot was cut into cast magnesium-silicon alloy sheets with a thickness of 600 μm using diamond wire. The surface of the cast magnesium-silicon alloy sheets was then polished with 380-mesh, 800-mesh, 1200-mesh, and 2000-mesh wet sandpaper in sequence. Then, it was polished with diamond polishing paste with a particle size of 0.5 μm. The polished cast magnesium-silicon alloy sheets were cleaned with deionized water. Subsequently, the cast magnesium-silicon alloy sheets were placed in a 5 wt.% sodium hydroxide solution for etching at a temperature of 70 ℃ for 5 h. After etching, the samples were dried and ground in a vacuum drying oven and then placed in a muffle furnace for heat treatment in an air atmosphere. The holding temperature was controlled at 550 ℃ for 20 h. Finally, the heat-treated sample was placed in a 5 wt.% hydrochloric acid solution and etched at room temperature for 24 h. The oxide layer on the sample surface was then cleaned with a 1 wt.% hydrofluoric acid solution, followed by repeated washing with deionized water and alcohol and vacuum drying to obtain high-purity nanoporous silicon.

[0055] (3) Dissolve 0.121 g of tris(hydroxymethyl)aminomethane in 100 ml of deionized water and stir thoroughly to form a Tris buffer solution (pH=8.5). Weigh 1.0 g of nanoporous silicon sample using a balance and place it in the buffer solution. Stir magnetically for 30 min to uniformly disperse the nanoporous silicon in the solution and form a suspension. Add 1.0 g of dopamine hydrochloride to the suspension and stir magnetically for 24 h. Then wash it with deionized water and vacuum dry it to obtain a nanoporous silicon / polydopamine composite material. Place the prepared nanoporous silicon / polydopamine composite material in a tube furnace and perform pyrolysis treatment under an argon protective atmosphere. Heat the material to 800 ℃ at a heating rate of 5 ℃ / min and hold for 3 h to obtain a high-purity nanoporous silicon-carbon composite anode material.

[0056] Figure 8 (a) is the XRD pattern of the nanoporous silicon-carbon composite anode material prepared in this embodiment. Figure 5(b) is the EDS spectrum of the nanoporous silicon-carbon composite anode material prepared in this embodiment. As can be seen from the XRD spectrum, the nanoporous silicon-carbon composite anode material is composed of a single Si phase, indicating that the carbon coating exists in an amorphous form and does not change the phase structure of the final product; as can be seen from the EDS spectrum, the nanoporous silicon-carbon composite anode material is mainly composed of C, N and Si elements, confirming that the surface coating of the nanoporous silicon is a nitrogen-doped carbon layer. Figure 9 These are scanning electron microscope (SEM) images of the nanoporous silicon-carbon composite anode material prepared in this embodiment at (a) low magnification and (b) high magnification. Figure 9 It can be seen that the nitrogen-doped carbon layer is uniformly coated on the surface of the nanoporous silicon. Figure 10 These are transmission electron microscope (TEM) images of the nanoporous silicon-carbon composite anode material prepared in this embodiment at (a) low magnification and (b) high magnification. Figure 10 It can be seen that after being coated with a carbon layer, the nanoporous structure of the nanoporous silicon is well maintained, with a pore size of 300~600 nm. Meanwhile, there is a clear boundary between the amorphous carbon layer and the nanoporous silicon, with the carbon layer thickness being approximately 20 nm. By calibrating the interplanar spacing of silicon in the nanoporous silicon, the corresponding crystal plane was determined; 0.192 nm corresponds to the (220) crystal plane of Si. These results indicate that the carbon layer was successfully coated on the surface of the nanoporous silicon, and the resulting nanoporous silicon-carbon composite anode material possesses a continuous, open, and uniformly distributed nanoporous structure.

[0057] A lithium-ion battery was assembled using a lithium metal sheet as the counter electrode, the nanoporous silicon-carbon composite anode material prepared in this embodiment as the working electrode, microporous polypropylene as the separator, and a mixed solution containing 1 M LiPF6 + EC:DMC:DEC (1:1:1 V%) + 5 wt.% FEC + 1 wt.% VC as the electrolyte in a glove box filled with argon gas and with water and oxygen contents both below 0.1 ppm. The performance of the obtained lithium-ion battery was tested using a NEWARE BTS-610 constant current charge-discharge tester, with a test voltage range of 0.01~1.5 V (vs. Li / Li). + The test temperature was 15~25℃. The test results are as follows: Figure 11 As shown. By Figure 11 It can be seen that high specific capacity and coulombic efficiency can be obtained at a charge / discharge current density of 0.1 C, with initial charge / discharge specific capacities of 2794.48 mAh g⁻¹. -1 and 3806.95 mAh g -1 The initial coulombic efficiency was 73.40%, with capacity loss mainly related to the formation of the SEI film. After 100 cycles, the charge specific capacity remained at 933.74 mAh g⁻¹. -1The coulombic efficiency is above 99.39%, and the cycle performance and coulombic efficiency are stable.

[0058] Example 5

[0059] In this embodiment, a method for preparing high-purity nanoporous silicon is provided, and the specific steps are as follows.

[0060] (1) Weigh manganese flakes (purity greater than 99.99%) and silicon blocks (purity greater than 99.9%) according to the atomic percentage ratio of manganese to silicon of 50:50, and melt them in a vacuum induction furnace to obtain cast manganese-silicon alloy ingots.

[0061] (2) Cast manganese silicon alloy ingots were cut into 600 μm thick sheets using diamond wire. The surfaces of the cast manganese silicon alloy sheets were then polished with 380-mesh, 800-mesh, 1200-mesh, and 2000-mesh wet sandpaper, and then polished with 0.5 μm diamond polishing paste. The polished cast manganese silicon alloy sheets were then cleaned with deionized water and dried. After grinding the dried cast manganese silicon alloy sheets, they were placed in a muffle furnace and heat-treated in an air atmosphere. The holding temperature was controlled at 700 °C and the holding time was 26 h. Finally, the heat-treated samples were placed in a 5 wt.% hydrochloric acid solution and etched at room temperature for 24 h. The oxide layer on the sample surface was cleaned with a 3 wt.% hydrofluoric acid solution, and then repeatedly washed with deionized water and alcohol and vacuum dried to obtain high-purity nanoporous silicon. Figure 12 This is the EDS spectrum of the nanoporous silicon prepared in this embodiment. Figure 12 It can be seen that the purity of this nanoporous silicon is 93.64%.

[0062] Example 6

[0063] In this embodiment, a method for preparing high-purity nanoporous silicon is provided, and the specific steps are as follows.

[0064] (1) Weigh aluminum blocks (purity greater than 99.99%), magnesium blocks (purity greater than 99.99%) and silicon blocks (purity greater than 99.9%) according to the atomic percentage of aluminum: magnesium: silicon of 10: 60: 30, and melt them in a vacuum induction furnace to obtain cast aluminum-magnesium-silicon alloy ingots.

[0065] (2) The cast aluminum-magnesium-silicon alloy ingot was cut into 600 μm thick sheets using diamond wire. The surface of the cast aluminum-magnesium-silicon alloy sheets was then polished with 380-mesh, 800-mesh, 1200-mesh, and 2000-mesh wet sandpaper in sequence. After polishing with 0.5 μm diamond polishing paste, the polished cast aluminum-magnesium-silicon alloy sheets were cleaned with deionized water. Subsequently, the cast aluminum-magnesium-silicon alloy sheets were etched in a 5 wt.% sodium hydroxide solution at a temperature of 80 ℃ for 5 h. After etching, the samples were dried and ground in a vacuum drying oven and then placed in a muffle furnace for heat treatment in an air atmosphere. The holding temperature was controlled at 800 ℃ for 30 h. Finally, the heat-treated sample was placed in a 10 wt.% sulfuric acid solution and etched at room temperature for 24 h. The oxide layer on the sample surface was then cleaned with a 5 wt.% hydrofluoric acid solution, followed by repeated washing with deionized water and alcohol and vacuum drying to obtain high-purity nanoporous silicon. Figure 13 This is the EDS spectrum of the nanoporous silicon prepared in this embodiment. Figure 13 It can be seen that the purity of this nanoporous silicon is 94.18%.

Claims

1. A method for preparing a high-purity nanoporous silicon-carbon composite anode material, characterized in that, The steps are as follows: (1) The cast silicon alloy was etched with sodium hydroxide solution and then ground to micron size. It was then placed in a muffle furnace for heat treatment in air atmosphere. The heat-treated sample was then placed in an acidic solution for chemical etching. After chemical etching, the surface oxide layer was cleaned with hydrofluoric acid solution to obtain high-purity nanoporous silicon with continuous, open and uniformly distributed pores. The cast silicon alloy is a magnesium-silicon alloy with an atomic percentage of magnesium to silicon of 45:55 or an aluminum-magnesium-silicon alloy with an atomic percentage of aluminum, magnesium, and silicon of 10:60:30; the concentration of the sodium hydroxide solution is 1 wt.%~5 wt.%, the corrosion temperature is 30~90℃, and the corrosion time is 1~5 h; when heat treatment is performed in an air atmosphere, the heat treatment temperature is 300~800℃, and the heat treatment time is 10~30 h; the acidic solution used for chemical corrosion is one or a mixture of hydrochloric acid, sulfuric acid, and nitric acid, the concentration of the acidic solution is 1 wt.%~10 wt.%, and the corrosion time is 10~40 h; the concentration of the hydrofluoric acid solution is 1 wt.%~5 wt.%; the purity of the nanoporous silicon is 92.00%~99.99%, and the pore size is 50~800 nm; (2) Polymer compounds are polymerized on the surface of nanoporous silicon, and then placed in a tube furnace to pyrolyze the polymer compounds under an inert protective atmosphere to obtain high-purity nanoporous silicon-carbon composite anode material. In the nanoporous silicon-carbon composite anode material, a carbon layer is uniformly coated on the surface of the nanoporous silicon, and the thickness of the carbon layer is 20~50 nm.

2. The method for preparing the high-purity nanoporous silicon-carbon composite anode material according to claim 1, characterized in that, When polymerizing polymers on the surface of nanoporous silicon, the nanoporous silicon is uniformly dispersed in a tris(hydroxymethyl)aminomethane buffer solution by magnetic stirring, so that the polymers are polymerized on the surface of the nanoporous silicon in the buffer solution.

3. The method for preparing the high-purity nanoporous silicon-carbon composite anode material according to claim 1, characterized in that, When the polymeric compounds polymerized on the surface of nanoporous silicon are polydextrose, polypyrrole, and polyvinylpyrrolidone, the coating layer after pyrolysis is a carbon layer; when the polymeric compounds polymerized on the surface of nanoporous silicon are polydopamine, polyacrylonitrile, and phenolic resin, the coating layer after pyrolysis is a nitrogen-doped carbon layer.

4. The method for preparing the high-purity nanoporous silicon-carbon composite anode material according to claim 1, characterized in that, The inert protective atmosphere used for the pyrolysis of polymer compounds is argon or nitrogen with a purity of 99.95~99.99%.

5. The method for preparing the high-purity nanoporous silicon-carbon composite anode material according to claim 1, characterized in that, The pyrolysis temperature of the polymer compound is 400~800 ℃, the holding time is 1~10 h, and the heating rate is 5~15 ℃ / min.

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