Hollow nano silicon-carbon composite material, preparation method thereof and lithium ion battery

Abstract

The invention provides a hollow nano silicon-carbon composite material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of lithium ion batteries. According to the preparation method disclosed by the embodiment of the invention, the hollow nano silicon-carbon composite material is prepared by adopting a sacrificial template method, the surface of a silicon nanowire is coated with a template layer through the step S1, then the template layer is coated with a high-molecular polymer through the step S2, and then the high-molecular polymer is subjected to high-temperature carbonization through the step S3 to obtain a carbon coating layer, so that the hollow nano silicon-carbon composite material is obtained. And after the template layer is etched through the step S4, a gap layer is formed between the silicon nanowire and the carbon coating layer, and the hollow nano silicon-carbon composite material can be obtained. The gap layer can provide a space for volume expansion of the silicon nanowires, and the carbon coating layer can improve the conductivity of the composite material, so that the purpose of improving the cycle performance and the conductivity of the battery is achieved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a hollow nano-silicon-carbon composite material, its preparation method, and a lithium-ion battery. Background Technology

[0002] With the booming development of electric vehicles, power tools, consumer electronics, and new energy storage devices, the demand for high-energy-density, high-discharge-power, long-cycle-life, and environmentally friendly lithium-ion batteries is constantly increasing in the power, consumer electronics, and energy storage markets. Among these, the anode material is one of the key factors restricting the performance of lithium-ion batteries in various aspects. Currently, the mainstream commercial lithium-ion battery anode material is graphite, which is widely available, has low cost, relatively stable electrochemical performance, and good cycle performance. It can balance energy density, power density, cycle life, and rate performance, thus dominating the vast majority of the lithium-ion battery market share. However, graphite's theoretical specific capacity is only 372 mAh / g, which cannot meet the requirements of high-energy-density batteries. Furthermore, the lithium intercalation potential of graphite is close to the lithium deposition potential; charging at high currents and low temperatures can cause lithium deposition, forming lithium dendrites, posing a significant safety hazard. Therefore, more and more researchers are dedicated to developing high-energy-density lithium-ion battery anode materials.

[0003] Silicon boasts a theoretical lithium intercalation capacity as high as 4200 mAh / g, and its abundant and inexpensive reserves, along with its low lithium intercalation potential, have led many researchers to consider it a highly promising anode material for lithium-ion batteries, poised to become the mainstay of next-generation lithium-ion battery anode materials. However, silicon is prone to significant volume expansion (approximately 300%) during lithium intercalation / deintercalation, causing pulverization and cracking of the active material, resulting in poor battery cycle performance and rapid capacity decay. Furthermore, as a semiconductor material, silicon exhibits poor conductivity, hindering the full realization of its electrochemical performance. Therefore, improving the volume expansion and conductivity of silicon materials has become one of the key factors restricting the development of high-energy-density lithium-ion batteries.

[0004] There are many existing studies on combining silicon and carbon materials to create silicon-carbon composite materials. Although this can take into account the advantages of both silicon and carbon materials, such silicon-carbon composite materials still have some problems as anode materials.

[0005] For example, patent application document CN117727907A proposes a double-layer coated hollow silicon-carbon anode material and its preparation method. It uses nano-silicon as the silicon source, alumina as the rigid layer and template layer, and porous carbon as the second coating layer. Through the above double-layer coating, the volume expansion of nano-silicon is alleviated and the conductivity of the material is improved. However, for the silicon-carbon anode material prepared by this method, with porous carbon as the carbon coating layer, due to the large specific surface area of the porous carbon material, it is easy to consume more lithium ions to form the SEI film, resulting in a relatively low initial efficiency of the material, which is 76-79%, and poor cycling performance. Moreover, for the silicon-carbon anode material prepared by this method, with silicon particles as the silicon source, during the process of lithium deintercalation and intercalation, large volume expansion is likely to occur.

[0006] For example, patent application document CN117393708A proposes a preparation method of a soft-carbon coated silicon-carbon material, with a hollow silicon-carbon material as the core layer and soft carbon forming the shell layer. This kind of hollow structure provides a buffer space for the volume expansion of silicon, making it have good structural stability. However, this method uses porous silicon as the silicon source, the preparation process of porous silicon is complex and the cost is high. At the same time, too much carbon source is coated in this method, resulting in too much carbon content in the prepared silicon-carbon anode material, leading to a relatively low capacity of the material, about 800 mAh / g, and the superior properties of silicon materials cannot be reflected.

[0007] Patent application document CN115513425A proposes a hollow silicon-carbon composite material and its preparation method. Mix and disperse nano-silicon powder and carbon source in a solvent, and under the environment of weak base, by controlling the reaction temperature, use the weak base to oxidize the silicon particles to form SiO x (0 < x < 2), while etching into the oxide layer to etch silicon to form a hollow structure. The reproducibility of this method is poor, the thickness of the hollow layer and the thickness of the oxide layer are not very controllable, and there may be differences in the batch stability of the material.

[0008] Patent application document CN113745483A proposes a hollow silicon-carbon-based composite material and its preparation method. Using hollow silica as the silicon source, a carbon layer with a thickness of 10-50 nm is coated on the outer surface of the silica, and then the silica layer is reduced to elemental silicon at high temperature, thus obtaining a silicon-carbon anode material with a hollow core. For the silicon-carbon anode material prepared by this method, the production process is relatively complex, and there are great challenges for production equipment and reduction process. Otherwise, it is easy to cause problems such as insufficient reduction and low battery capacity. Summary of the Invention

[0009] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. For this reason, an embodiment of the present invention proposes a hollow nano-silicon-carbon composite material, its preparation method and a lithium-ion battery.

[0010] In a first aspect, embodiments of the present invention provide a method for preparing hollow nano-silicon-carbon composite materials, comprising the following steps:

[0011] S1. Disperse silicon nanowires in a first solvent to obtain a first dispersion. Then add a template layer precursor to the first dispersion to obtain a second dispersion. After the template layer precursor reacts, the template layer grows on the surface of the silicon nanowires. After solid-liquid separation, washing, and drying, silicon nanowire material containing a template layer is obtained.

[0012] S2. The silicon nanowire material containing the template layer is dispersed in a second solvent to obtain a third dispersion. Then, a polymer monomer and an initiator are added to the third dispersion to obtain a fourth dispersion. After the polymer monomer and the initiator react, the polymer grows on the surface of the silicon nanowire material containing the template layer. After solid-liquid separation, washing, and spray drying, a silicon nanowire material containing a polymer and a template layer is obtained.

[0013] S3. The silicon nanowire material containing the polymer and template layer is sintered at high temperature under an inert atmosphere to obtain a silicon-carbon composite material containing a carbon coating layer and a template layer.

[0014] S4. The silicon-carbon composite material containing the carbon coating layer and the template layer is dispersed in an etchant to obtain a fifth dispersion. After the silicon-carbon composite material containing the carbon coating layer and the template layer reacts with the etchant, the template layer is removed to form a void layer. After solid-liquid separation, washing, and vacuum drying, a hollow nano silicon-carbon composite material is obtained.

[0015] The advantages and technical effects of the preparation method of this invention are as follows:

[0016] (1) The preparation method of the present invention adopts the sacrificial template method to prepare hollow nano silicon-carbon composite material. First, a template layer is coated on the surface of silicon nanowires in step S1. Then, a polymer is coated on the outside of the template layer in step S2. Then, the polymer is carbonized at high temperature in step S3 to obtain a carbon coating layer. After etching away the template layer in step S4, a void layer is formed between the silicon nanowires and the carbon coating layer, and the hollow nano silicon-carbon composite material of the present invention can be obtained.

[0017] (2) The preparation method of the present invention is relatively simple and has good reproducibility. The thickness of the void layer and the thickness of the carbon coating layer are controllable, and the batch stability of the composite material is good.

[0018] (3) In step S1 of the preparation method of the present invention, a liquid phase coating-drying method is used to uniformly coat the template layer on the surface of the silicon nanowire, which makes it easier to control the thickness of the template layer and thus control the thickness of the void layer formed subsequently, thereby helping to alleviate the volume expansion of the silicon nanowire.

[0019] (4) In step S2 of the preparation method of the present invention, a liquid phase coating-spray drying granulation method is used to uniformly coat the polymer on the surface of the template layer, which makes it easier to control the thickness of the carbon coating layer, thereby improving the conductivity of the composite material and alleviating the volume expansion of silicon nanowires.

[0020] (5) The hollow nano-silicon-carbon composite material obtained by the preparation method of the present invention has a hollow core-shell structure, wherein silicon nanowires are used as the core and carbon coating layer is used as the shell layer. There is a void layer between the silicon nanowires and the carbon coating layer. The void layer can provide space for the volume expansion of silicon nanowires, and the carbon coating layer can improve the conductivity of the composite material, thereby achieving the purpose of improving battery cycle performance and conductivity.

[0021] In some embodiments, in step S1, the first solvent is selected from at least one of deionized water, ethanol, and NMP.

[0022] In some embodiments, in step S1, the mass fraction of the silicon nanowires in the first dispersion is 10-20%.

[0023] In some embodiments, in step S1, the template layer precursor is sodium carbonate and calcium chloride, and the template layer is calcium carbonate.

[0024] In some embodiments, in step S1, the molar ratio of sodium carbonate, calcium chloride and silicon nanowires in the second dispersion is 1:1:(1-8).

[0025] In some embodiments, in step S1, the thickness of the template layer is 5 to 50 nm.

[0026] In some embodiments, in step S1, the template layer precursor and the silicon nanowires are mixed by magnetic stirring at a rotation speed of 280 to 320 rpm.

[0027] In some embodiments, in step S1, the template layer precursor reacts to grow a template layer on the surface of the silicon nanowire, wherein the reaction temperature is 20–50°C and the reaction time is 1–6 h.

[0028] In some embodiments, in step S1, the drying temperature is 80–100°C.

[0029] In some embodiments, in step S2, the second solvent is selected from at least one of deionized water, ethanol, n-hexane, and NMP.

[0030] In some embodiments, in step S2, the mass fraction of the silicon nanowire material containing the template layer in the third dispersion is 5-20%.

[0031] In some embodiments, in step S2, the polymer monomer is selected from at least one of glucose, tannic acid, dopamine, and formaldehyde-resorcinol.

[0032] In some embodiments, in step S2, the molar ratio of silicon nanowires in the silicon nanowire material containing the template layer to the carbon content in the polymer monomer in the third dispersion is 1:(0.5-5).

[0033] In some embodiments, in step S2, the polymer monomer, the initiator, and the silicon nanowire material containing the template layer are mixed by magnetic stirring at a speed of 180-220 rpm.

[0034] In some embodiments, in step S2, the polymer monomer and the initiator react to allow the polymer to grow on the surface of the silicon nanowire material containing the template layer, wherein the reaction temperature is 20–50°C and the reaction time is 4–12 h.

[0035] In some embodiments, in step S2, the temperature of the spray drying is 100–120°C.

[0036] In some embodiments, in step S3, the holding time for high-temperature sintering is 700–900°C and the holding time is 2–4 hours.

[0037] In some embodiments, in step S3, the thickness of the carbon coating layer is 5–60 nm.

[0038] In some embodiments, in step S4, the etching agent is hydrochloric acid;

[0039] In some embodiments, in step S4, the molar concentration of the hydrochloric acid is 0.5–2 mol / L;

[0040] In some embodiments, in step S4, the silicon-carbon composite material containing the carbon coating layer and the template layer and the etchant are mixed by magnetic stirring at a speed of 200 to 500 rpm.

[0041] In some embodiments, in step S4, the silicon-carbon composite material containing the carbon coating layer and the template layer reacts with the etchant to remove the template layer, wherein the reaction temperature is room temperature °C and the reaction time is 2 to 4 hours.

[0042] In some embodiments, the temperature of the vacuum drying in step S4 is 80–100°C.

[0043] Secondly, embodiments of the present invention also provide a hollow nano-silicon-carbon composite material, obtained by the preparation method of the first aspect.

[0044] The advantages and technical effects of the hollow nano-silicon-carbon composite material of this invention are as follows:

[0045] The hollow nano-silicon-carbon composite material of this invention has a hollow core-shell structure, wherein silicon nanowires are used as the core and carbon coating layer is used as the shell. There is a void layer between the silicon nanowires and the carbon coating layer. The void layer can provide space for the volume expansion of the silicon nanowires, and the carbon coating layer can improve the conductivity of the composite material. Therefore, the hollow nano-silicon-carbon composite material of this invention has excellent cycle performance and conductivity.

[0046] Thirdly, embodiments of the present invention provide a lithium-ion battery, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, characterized in that the negative electrode film layer includes a hollow nano-silicon-carbon composite material as described in the second aspect.

[0047] The advantages and technical effects of the lithium-ion battery of this invention are as follows:

[0048] Due to the use of the hollow nano-silicon-carbon composite material of the second aspect, the lithium-ion battery of the present invention has excellent cycle performance. Attached Figure Description

[0049] Figure 1 This is a transmission electron microscope image of the hollow nano-silicon-carbon composite material obtained in Example 1.

[0050] Figure 2 The image shows a transmission electron microscope (TEM) image of the silicon-carbon composite material obtained in Comparative Example 1.

[0051] Figure 3 The image shows a transmission electron microscope (TEM) image of the silicon-carbon composite material obtained in Comparative Example 2. Detailed Implementation

[0052] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0053] In a first aspect, embodiments of the present invention provide a method for preparing hollow nano-silicon-carbon composite materials, comprising the following steps:

[0054] S1. Disperse silicon nanowires in a first solvent to obtain a first dispersion. Then add a template layer precursor to the first dispersion to obtain a second dispersion. After the template layer precursor reacts, the template layer grows on the surface of the silicon nanowires. After solid-liquid separation, washing, and drying, silicon nanowire material containing a template layer is obtained.

[0055] S2. Disperse the silicon nanowire material containing the template layer in the second solvent to obtain the third dispersion. Then add the polymer monomer and initiator to the third dispersion to obtain the fourth dispersion. After the polymer monomer and initiator react, the polymer grows on the surface of the silicon nanowire material containing the template layer. After solid-liquid separation, washing and spray drying, silicon nanowire material containing polymer and template layer is obtained.

[0056] S3. Silicon nanowire materials containing polymer and template layers are sintered at high temperature under an inert atmosphere to obtain silicon-carbon composite materials containing carbon coating and template layers.

[0057] S4. The silicon-carbon composite material containing a carbon coating layer and a template layer is dispersed in an etchant to obtain a fifth dispersion. After the silicon-carbon composite material containing a carbon coating layer and a template layer reacts with the etchant, the template layer is removed to form a void layer. After solid-liquid separation, washing, and vacuum drying, a hollow nano silicon-carbon composite material is obtained.

[0058] The preparation method of this invention combines the methods of generating a template layer, growing a carbon coating layer, and etching away the template layer. First, a template layer is grown on the surface of silicon nanowires, then a carbon coating layer is grown on the surface of the template layer, and finally the template layer is etched away to form a void layer between the silicon nanowires and the carbon coating layer, thus obtaining a hollow nano-silicon-carbon composite material. The presence of the void layer effectively improves the problem of low material cycling ability caused by the volume expansion of silicon nanowires; the presence of the carbon coating layer effectively improves the conductivity of the composite material.

[0059] In some embodiments, in step S1, the first solvent is selected from at least one of deionized water, ethanol, and NMP. The first solvents listed above can uniformly disperse the silicon nanowires and the template layer precursor, which helps the template layer to uniformly coat the surface of the silicon nanowires.

[0060] In some embodiments, in step S1, the mass fraction of silicon nanowires in the first dispersion is 10-20%, for example, 10%, 12%, 14%, 16%, 18%, 20%, etc. When the mass fraction of silicon nanowires in the first dispersion is too small, the amount of waste liquid that needs to be treated after subsequent solid-liquid separation is too large, resulting in low production efficiency. When the mass fraction of silicon nanowires in the first dispersion is too large, it is not conducive to improving the uniformity of silicon nanowire dispersion, which is not conducive to the uniform coating of the template layer on the surface of silicon nanowires.

[0061] In some embodiments, in step S1, the template layer precursor is sodium carbonate and calcium chloride, and the template layer is calcium carbonate. A dense, well-encapsulated template layer is grown on the surface of silicon nanowires using a hard template synthesis process. Compared to template layers grown using a soft template synthesis process, this method exhibits higher structural stability and allows for better control of the template layer thickness, providing sufficient space for the volume expansion of the silicon nanowires. This improves upon the poor cycling performance of silicon-carbon composite materials in existing technologies.

[0062] In some embodiments, in step S1, the molar ratio of sodium carbonate, calcium chloride, and silicon nanowires in the second dispersion is 1:1:(1-8), for example, 1:1:1, 1:1:2, 1:1:3, 1:1:4, 1:1:5, 1:1:6, 1:1:7, 1:1:8, etc. When the molar ratio of sodium carbonate, calcium chloride, and silicon nanowires is too small, the thickness of the template layer formed is too small, and consequently, the thickness of the subsequently formed void layer is too small, which is not conducive to providing sufficient space for the volume expansion of silicon nanowires. When the molar ratio of sodium carbonate, calcium chloride, and silicon nanowires is too large, the thickness of the template layer formed is too large, and consequently, the thickness of the subsequently formed void layer is too large, which is not conducive to the transport of lithium ions.

[0063] In some other embodiments, in step S1, the template layer precursor is tetraethyl orthosilicate (TEOS) and ammonia, and the template layer is silicon dioxide. Silicon dioxide is also a type of hard template.

[0064] In some other embodiments, in step S1, the molar ratio of silicon nanowires to tetraethyl orthosilicate (TEOS) in the second dispersion is 1:(1-5), and the mass ratio of ammonia to silicon nanowires is (10-20):1.

[0065] Preferably, in step S1, the template layer precursors are sodium carbonate and calcium chloride, and the template layer is calcium carbonate. By growing a calcium carbonate template layer on silicon nanowires, the thickness of the template layer can be flexibly and autonomously controlled, providing space for the volume expansion of the silicon nanowires. Furthermore, compared to hard templates like silicon dioxide, calcium carbonate can be etched with hydrochloric acid, while the latter can only be etched with hydrofluoric acid. Using hydrochloric acid to etch the former makes the entire etching process safer.

[0066] In some embodiments, in step S1, the thickness of the template layer is 5–50 nm, such as 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, etc. When the thickness of the template layer is too small, the thickness of the subsequently formed void layer will also be too small, which is not conducive to providing sufficient space for the volume expansion of the silicon nanowires. When the thickness of the template layer is too large, the thickness of the subsequently formed void layer will also be too large, which is not conducive to the transport of lithium ions, and thus not conducive to improving the conductivity of the composite material.

[0067] In some embodiments, in step S1, the mixing of the template layer precursor and the silicon nanowires can be carried out at room temperature.

[0068] In some embodiments, in step S1, the template layer precursor and silicon nanowires are mixed by magnetic stirring at a rotation speed of 280–320 rpm. Mixing by magnetic stirring as described above helps to ensure uniform mixing of the two materials. Reaction under stirring conditions helps the template layer grow uniformly on the surface of the silicon nanowires.

[0069] In some embodiments, in step S1, after the template layer precursor reacts, a template layer is grown on the surface of the silicon nanowires. The reaction temperature is 20–50°C, for example, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, etc., and the reaction time is 1–6 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, etc. If the reaction temperature is too low or the reaction time is too short, the thickness of the formed template layer may be too small. If the reaction temperature or reaction time is too long, the thickness of the formed template layer may be too large.

[0070] In step S1 of the preparation method of this embodiment, the washing process can be carried out by washing the solid phase after solid-liquid separation with deionized water in order to remove excess template layer precursors remaining on the surface of the silicon nanowire material containing the template layer.

[0071] In some embodiments, the drying temperature in step S1 is 80 to 100°C, such as 80°C, 85°C, 90°C, 95°C, 100°C, etc., so as to dry and remove the first solvent and washing liquid remaining on the surface of the silicon nanowire material containing the template layer.

[0072] In some embodiments, in step S2, the second solvent is selected from at least one of deionized water, ethanol, n-hexane, and NMP. The second solvents listed above can uniformly disperse the silicon nanowire material containing the template layer, the polymer monomer, and the initiator.

[0073] In some embodiments, in step S2, the mass fraction of the silicon nanowire material containing the template layer in the third dispersion is 5-20%, for example, 5%, 10%, 15%, 20%, etc. When the mass fraction of the silicon nanowire material containing the template layer in the third dispersion is too small, the amount of waste liquid to be processed subsequently is too large, resulting in low production efficiency. When the mass fraction of the silicon nanowire material containing the template layer in the third dispersion is too large, it is not conducive to the uniform coating of the polymer layer on the surface of the silicon nanowire material containing the template layer.

[0074] In some embodiments, in step S2, the polymer monomer is selected from at least one of glucose, tannic acid, dopamine, and formaldehyde-resorcinol. The carbon coating layer formed by the polymers formed from the polymer monomers listed above, after high-temperature sintering, has high electrical conductivity, which is beneficial for improving the conductivity of the composite material.

[0075] Depending on the type of polymer monomer used, a suitable initiator can be selected accordingly. For example, if the polymer monomer is tannic acid and / or dopamine, the initiator can be ammonium persulfate.

[0076] In some embodiments, in step S2, the molar ratio of silicon nanowires in the template layer to carbon in the polymer monomer in the third dispersion is 1:(0.5-5), for example, 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, etc. When the molar ratio is too small, the thickness of the formed polymer may be too small, and the thickness of the subsequently formed carbon coating layer may also be too small, which is not conducive to improving the conductivity of the composite material. When the molar ratio is too large, the thickness of the formed polymer may be too large, and the thickness of the subsequently formed carbon coating layer may also be too large, resulting in an excessively high carbon content in the composite material, which is not conducive to leveraging the excellent properties of silicon materials.

[0077] In some embodiments, in step S2, the molar ratio of polymer monomer to initiator in the third dispersion can be 1:(0.5-2), for example 1:0.5, 1:0.6, 1:0.8, 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, etc.

[0078] In some embodiments, in step S2, the polymer monomer, initiator, and silicon nanowire material containing the template layer are mixed by magnetic stirring at a rotation speed of 180–220 rpm. The magnetic stirring method described above facilitates the uniform mixing of the polymer monomer, initiator, and silicon nanowire material containing the template layer. Reaction under stirring conditions helps the polymer grow uniformly on the surface of the template layer.

[0079] In some embodiments, in step S2, the polymer monomer and initiator react to grow a polymer on the surface of a silicon nanowire material containing a template layer. The reaction temperature is 20–50°C, for example, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, etc., and the reaction time is 4–12 hours, for example, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, etc. If the reaction temperature is too low or the reaction time is too short, the thickness of the formed polymer may be too small, and the thickness of the subsequently formed carbon coating layer may also be too small, which is detrimental to improving the conductivity of the composite material. If the reaction temperature is too high or the reaction time is too long, the thickness of the formed polymer may be too large, and the thickness of the subsequently formed carbon coating layer may also be too large, resulting in an excessively high carbon content in the composite material, which is detrimental to the performance of the silicon nanowire properties. Preferably, the reaction temperature is room temperature.

[0080] In step S2 of the preparation method of this embodiment, the washing process can be carried out using a suitable solvent (such as deionized water) to remove unreacted polymer monomers and initiators remaining on the surface of the silicon nanowire material containing the template layer, as well as polymers not coated on the surface of the silicon nanowire material containing the template layer.

[0081] In some embodiments, in step S2, the spray drying temperature is 100–120°C, such as 100°C, 105°C, 110°C, 115°C, 120°C, etc. When the spray drying temperature is too low, the solvent's evaporation point is not reached, and the material will be in a semi-dry state, clumping together. When the spray drying temperature is too high, the solvent evaporates, and the sample continues to absorb heat without solvent protection, causing the material to char, melt, or even decompose.

[0082] In some embodiments, in step S3, the holding time for high-temperature sintering is 700–900°C, such as 700°C, 750°C, 800°C, 900°C, etc., and the holding time is 2–4 hours, such as 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, etc. When the holding temperature for high-temperature sintering is too low or the holding time is too short, it is not conducive to the bonding effect between carbon materials and silicon nanowires, and it is not conducive to improving the structural stability of the composite material. When the holding temperature for high-temperature sintering is too high or the holding time is too long, it is not conducive to improving production efficiency.

[0083] In step S3 of the preparation method of this embodiment, high-temperature sintering is performed under an inert atmosphere to prevent the carbon coating layer from being oxidized. In some embodiments, the inert gas used in step S3 is nitrogen and / or argon, and the flow rate of the inert gas is 500-1000 sccm.

[0084] In some embodiments, in step S3, the thickness of the carbon coating layer is 5–60 nm, such as 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, etc. When the thickness of the carbon coating layer is too small, it is not conducive to improving the conductivity of the composite material. Simultaneously, if the carbon coating layer is too thin, it is not conducive to maintaining the structural stability of the composite material during the repeated expansion and contraction of silicon. When the carbon coating layer is too thick, it is not conducive to utilizing the properties of silicon nanowires.

[0085] In some embodiments, the etchant in step S4 is hydrochloric acid. When the template layer is calcium carbonate, hydrochloric acid can be used for etching, and the etching process is safer than that of hydrofluoric acid.

[0086] In some embodiments, in step S4, the silicon-carbon composite material containing the carbon coating layer and the template layer, and the etchant are mixed by magnetic stirring at a speed of 200–500 rpm. Mixing by magnetic stirring as described above facilitates a full reaction between the etchant and the template layer, thoroughly removing the template layer.

[0087] In some embodiments, in step S4, the silicon-carbon composite material containing the carbon coating layer and the template layer reacts with the etchant to remove the template layer. The reaction temperature is room temperature, such as 20°C, 22°C, 24°C, 25°C, 26°C, 28°C, or 30°C, and the reaction time is 2–4 hours, such as 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4 hours. If the reaction temperature is too low or the reaction time is too short in this step, it is not conducive to completely etching away the template layer. If a template layer remains, it is not conducive to lithium ion insertion / extraction, thus hindering the improvement of the composite material's capacity and cycle life. If the reaction temperature is too high or the reaction time is too long in this step, it is not conducive to improving production efficiency.

[0088] In step S4 of the preparation method of this embodiment, the washing process can be carried out by washing the solid phase obtained after solid-liquid separation with deionized water until it is neutral, which facilitates the removal of the etching agent remaining on the surface of the hollow nano-silicon-carbon composite material.

[0089] In some embodiments, in step S4, the vacuum drying temperature is 80–100°C, such as 80°C, 85°C, 90°C, 95°C, 100°C, etc. Vacuum drying is used to dry and remove residual etching agent and washing liquid on the surface of the hollow nano-silicon-carbon composite material, while preventing the silicon material from being oxidized.

[0090] Secondly, embodiments of the present invention provide a hollow nano-silicon-carbon composite material, obtained by the preparation method of the first aspect.

[0091] The hollow nano-silicon-carbon composite material of this invention has a hollow core-shell structure, wherein silicon nanowires are used as the core and carbon coating layer is used as the shell. There is a void layer between the silicon nanowires and the carbon coating layer. The void layer can provide space for the volume expansion of the silicon nanowires, and the carbon coating layer can improve the conductivity of the composite material. Therefore, the hollow nano-silicon-carbon composite material of this invention has excellent cycle performance and conductivity.

[0092] Thirdly, embodiments of the present invention provide a lithium-ion battery, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, wherein the negative electrode film layer includes a hollow nano-silicon-carbon composite material as described in the second aspect.

[0093] Due to the use of the hollow nano-silicon-carbon composite material of the second aspect, the lithium-ion battery of the present invention has excellent cycle performance.

[0094] The present invention will now be described in detail with reference to the embodiments and accompanying drawings.

[0095] Example 1

[0096] This embodiment provides a method for preparing hollow nano-silicon-carbon composite materials, the method comprising:

[0097] (1) Disperse silicon nanowires in deionized water at a mass fraction of 10%, and then add 1 mol / L sodium carbonate and 1 mol / L calcium chloride solution to the water. Stir magnetically at 300 rpm for 4 h at 25 °C. After the calcium carbonate template layer is uniformly and densely coated on the silicon nanowires, wash the sample with deionized water to remove the residual sodium carbonate and calcium chloride. After drying at 90 °C, silicon nanowire material containing the template layer is obtained.

[0098] (2) The above-mentioned silicon nanowire material containing the template layer was dispersed in anhydrous ethanol at a mass fraction of 5%. Dopamine hydrochloride was added to the above dispersion at a molar ratio of 1:1 between silicon nanowires and carbon in dopamine hydrochloride. Ammonium persulfate was added to the above dispersion at a molar ratio of 1:1 between dopamine hydrochloride and ammonium persulfate. After mixing with magnetic stirring at 200 rpm at 25°C for 12 h, and after the polymer was stably grown outside the template layer, the sample was washed by centrifugation and filtration with deionized water, and then spray-dried at 100°C to obtain silicon nanowire material containing polymer and template layer.

[0099] (3) The silicon nanowire material containing the polymer and template layer was placed in a quartz tube furnace and heated to 800°C at a heating rate of 5°C / min under nitrogen protection at a flow rate of 500 sccm. After sintering for 2 hours, it was cooled to room temperature at a cooling rate of 5°C / min to obtain a silicon-carbon composite material containing a carbon coating layer and a template layer.

[0100] (4) The silicon-carbon composite material containing carbon coating layer and template layer is etched with 1 mol / L hydrochloric acid, and then magnetically stirred at 300 rpm at 25°C for 4 hours. After washing the sample with deionized water until neutral, it is vacuum dried at 80°C to obtain hollow nano silicon-carbon composite material.

[0101] Comparative Example 1

[0102] This comparative example provides a method for preparing a silicon-carbon composite material. This method omits step (1) and omits the etching process with an etchant in step (4). Except for the above, the other conditions are exactly the same as in Example 1.

[0103] Comparative Example 2

[0104] This comparative example provides a method for preparing a silicon-carbon composite material. This method omits step (1), replaces the silicon nanowires in step (2) with silicon nanoparticles, and omits the etching process with an etchant in step (4). Except for the above, the other conditions are exactly the same as in Example 1.

[0105] I. Morphological characteristics:

[0106] Figure 1 This is a transmission electron microscope (TEM) image of the hollow nano-silicon-carbon composite material obtained in Example 1. The image shows that the hollow nano-silicon-carbon composite material prepared in Example 1 has silicon nanowires with a diameter of approximately 380 nm. The surface closely adhering to the silicon nanowires is a void layer with a thickness of 20 nm, providing space for the volume expansion of the silicon nanowires. The outer layer is a carbon coating layer with a thickness of 18 nm. The boundary between the void layer and the carbon coating layer is clear, and the carbon coating layer is complete and dense, which can stably maintain the material structure due to the repeated expansion and contraction of the silicon nanowires.

[0107] Figure 2 The image shows a transmission electron microscope (TEM) image of the silicon-carbon composite material obtained in Comparative Example 1. The image shows that the silicon nanowires prepared in Comparative Example 1 have diameters ranging from 50 to 200 nm, and a dense and complete carbon coating layer with a thickness of approximately 12 nm is attached to the surface of the silicon nanowires. Figure 3 The image shows a transmission electron microscope (TEM) image of the silicon-carbon composite material obtained in Comparative Example 2. The image shows that the silicon nanoparticles prepared in Comparative Example 2 have diameters ranging from 20 to 120 nm, and a complete and dense carbon coating layer with a thickness of approximately 12 nm is closely attached to the surface of the silicon nanoparticles.

[0108] II. Battery Testing:

[0109] The composite materials prepared in Examples 1 and Comparative Examples 1-2 were used as negative electrode active materials. The negative electrode active materials, conductive agents and binders were mixed with deionized water in the corresponding proportions to form a slurry, which was then uniformly coated on copper foil. After drying in a vacuum oven at 80°C for 12 hours, the electrode was pressed into a circular electrode sheet with a diameter of 12 mm by a roller press. The electrode sheet was placed in a glove box filled with Ar, and then a button lithium-ion battery was assembled in the order of electrode sheet, separator, electrolyte, lithium sheet, gasket and spring sheet.

[0110] The obtained batteries were tested using the Xinwei CT / CTE-4000 series multi-channel battery testing system to perform charge-discharge tests and rate performance tests on the assembled half-cells. The test temperature was 30℃, and the test voltage range was 0.01~1.5V. The results are recorded in Table 1.

[0111] Table 1. Electrochemical performance of coin-type lithium-ion batteries assembled from the composite materials prepared in Example 1 and Comparative Examples 1-2

[0112] Group Capacity (mAh / g) First-time coulomb efficiency (%) Number of cycles (>80%) Example 1 1632.7 89.4 642 Comparative Example 1 1523.8 86.7 452 Comparative Example 2 1476.4 85.8 354

[0113] As shown in Table 1, the presence or absence of a hollow structure (void layer) and the morphology of silicon materials affect the structural stability of silicon materials during expansion and contraction, thus impacting performance indicators such as lithium battery capacity and cycle life. The morphology (linear, spherical) of silicon materials affects the cycle life of the anode material. Taking Comparative Examples 1 and 2 as examples, the capacity and cycle parameters of silicon nanowires are slightly higher than those of silicon particles. This is mainly due to the fact that lithium ion migration occurs perpendicular to the silicon nanowires, shortening the migration path and alleviating the stress caused by anisotropic volume expansion. During lithium ion insertion / extraction, silicon particles expand and contract, leading to particle fragmentation and subsequent detachment of the active material, resulting in poor cycle performance. The presence of a hollow structure (void layer) also significantly affects the cycle parameters of the anode material. Taking Examples 1 and 1 as examples, the capacity and cycle life of the silicon nanowire anode material containing a hollow structure are superior to the sample without a hollow structure. During lithium insertion, the presence of the void layer effectively alleviates the volume expansion of the silicon material, maintaining the structural stability of the anode material and resulting in better cycle performance.

[0114] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0115] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a hollow nano-silicon-carbon composite material, characterized in that, Includes the following steps: S1. Disperse silicon nanowires in a first solvent to obtain a first dispersion. Then add a template layer precursor to the first dispersion to obtain a second dispersion. After the template layer precursor reacts, the template layer grows on the surface of the silicon nanowires. After solid-liquid separation, washing, and drying, silicon nanowire material containing a template layer is obtained. S2. The silicon nanowire material containing the template layer is dispersed in a second solvent to obtain a third dispersion. Then, a polymer monomer and an initiator are added to the third dispersion to obtain a fourth dispersion. After the polymer monomer and the initiator react, the polymer grows on the surface of the silicon nanowire material containing the template layer. After solid-liquid separation, washing, and spray drying, a silicon nanowire material containing a polymer and a template layer is obtained. S3. The silicon nanowire material containing the polymer and template layer is sintered at high temperature under an inert atmosphere to obtain a silicon-carbon composite material containing a carbon coating layer and a template layer. S4. The silicon-carbon composite material containing the carbon coating layer and the template layer is dispersed in an etchant to obtain a fifth dispersion. After the silicon-carbon composite material containing the carbon coating layer and the template layer reacts with the etchant, the template layer is removed to form a void layer. After solid-liquid separation, washing, and vacuum drying, a hollow nano silicon-carbon composite material is obtained.

2. The preparation method according to claim 1, characterized in that, In step S1, the first solvent is selected from at least one of deionized water, ethanol, and NMP; Optionally, the mass fraction of the silicon nanowires in the first dispersion is 10-20%. Optionally, the template layer precursor is sodium carbonate and calcium chloride, and the template layer is calcium carbonate; Optionally, in the second dispersion, the molar ratio of sodium carbonate, calcium chloride and silicon nanowires is 1:1:(1-8); Optionally, the thickness of the template layer is 5–50 nm.

3. The preparation method according to claim 1, characterized in that, In step S1, the template layer precursor and the silicon nanowires are mixed by magnetic stirring at a speed of 280-320 rpm. Optionally, the template layer precursor reacts to grow a template layer on the surface of the silicon nanowire, wherein the reaction temperature is 20–50°C and the reaction time is 1–6 h. Optionally, the drying temperature is 80–100°C.

4. The preparation method according to claim 1, characterized in that, In step S2, the second solvent is selected from at least one of deionized water, ethanol, n-hexane, and NMP; Optionally, in the third dispersion, the mass fraction of the silicon nanowire material containing the template layer is 5-20%.

5. The preparation method according to claim 1, characterized in that, In step S2, the polymer monomer is selected from at least one of glucose, tannic acid, dopamine, and formaldehyde-resorcinol; Optionally, in the third dispersion, the molar ratio of silicon nanowires in the silicon nanowire material containing the template layer to the carbon content in the polymer monomer is 1:(0.5-5).

6. The preparation method according to claim 1, characterized in that, In step S2, the polymer monomer, the initiator, and the silicon nanowire material containing the template layer are mixed by magnetic stirring at a speed of 180-220 rpm. Optionally, the polymer monomer and the initiator react to allow the polymer to grow on the surface of the silicon nanowire material containing the template layer, wherein the reaction temperature is 20-50°C and the reaction time is 4-12 h. Optionally, the spray drying temperature is 100–120°C.

7. The preparation method according to claim 1, characterized in that, In step S3, the holding time for high-temperature sintering is 700-900℃ for 2-4 hours. Optionally, the thickness of the carbon coating layer is 5–60 nm.

8. The preparation method according to claim 1 or 2, characterized in that, In step S4, the etching agent is hydrochloric acid; Optionally, the molar concentration of the hydrochloric acid is 0.5–2 mol / L; Optionally, the silicon-carbon composite material containing the carbon coating layer and the template layer and the etchant are mixed by magnetic stirring at a speed of 200 to 500 rpm; Optionally, the silicon-carbon composite material containing the carbon coating layer and the template layer is reacted with the etchant to remove the template layer, wherein the reaction temperature is room temperature and the reaction time is 2 to 4 hours; Optionally, the vacuum drying temperature is 80–100°C.

9. A hollow nano-silicon-carbon composite material, characterized in that, Obtained by the preparation method according to any one of claims 1 to 8.

10. A lithium-ion battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, characterized in that, The negative electrode film layer comprises the hollow nano-silicon-carbon composite material as described in claim 9.

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