Method for improving structural stability of silicon-carbon material

Abstract

This application provides a method for improving the structural stability of silicon-carbon materials, comprising the following steps: hydrolyzing a siloxane compound in an aqueous solution, and adding a catalyst precursor and a carbon source material to form a silicon source sol; immersing a porous carbon substrate in the silicon source sol, causing the silicon source sol to adhere to the pores of the porous carbon substrate and converting the silicon source sol into silicon oxide; placing the porous carbon substrate in a reaction atmosphere containing a carbon source gas and a first reducing gas and performing a first heat treatment, reducing the catalyst precursor to a catalyst for catalyzing carbon nanotube deposition, and depositing carbon nanotubes on the surface of the porous carbon substrate; and placing the porous carbon substrate in a reaction atmosphere containing a protective gas and a second reducing gas and performing a second heat treatment at a temperature above 1500°C. This method can improve the structural stability of the prepared silicon-carbon material while ensuring its conductivity.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, specifically to a method for improving the structural stability of silicon-carbon materials. Background Technology

[0002] Silicon-carbon materials are composites of silicon-based and carbon-based materials. Silicon-carbon materials have a significantly higher specific capacity than traditional graphite materials. Furthermore, the good conductivity of carbon-based materials facilitates electron extraction from silicon-based materials. Therefore, silicon-carbon materials are considered a promising lithium-ion anode material. However, silicon itself has poor conductivity, and it undergoes significant volume expansion and contraction during charging and discharging. Frequent volume expansion and contraction can destabilize the overall structure of silicon-carbon materials, thus limiting the cycle performance of batteries using silicon-carbon materials as the anode active material. Summary of the Invention

[0003] Therefore, it is necessary to provide a method to improve the structural stability of silicon-carbon materials. This method can improve the structural stability of the prepared silicon-carbon materials, and thus enable the battery materials prepared from them to have better cycle stability.

[0004] According to some embodiments of this application, a method for improving the structural stability of silicon-carbon materials is provided, which includes the following steps: A siloxane compound is placed in a solution containing water for hydrolysis, and a catalyst precursor and a carbon source material are added to the solution to form a silicon source sol containing the catalyst precursor and the carbon source material. The porous carbon substrate is immersed in the silicon source sol, so that the silicon source sol adheres to the pores of the porous carbon substrate and the silicon source sol is converted into silicon oxide. The porous carbon substrate is placed in a reaction atmosphere containing a carbon source gas and a first reducing gas and subjected to a first heating treatment to reduce the catalyst precursor to a catalyst for catalyzing carbon nanotube deposition, and carbon nanotubes are deposited on the surface of the porous carbon substrate. Furthermore, the porous carbon substrate is placed in a reaction atmosphere containing a protective gas and a second reducing gas and subjected to a second heat treatment at a temperature above 1500°C.

[0005] In some embodiments of this application, the molar ratio of carbon in the carbon source material to silicon in the silicon source sol is (1.6~2.4):1; and / or, the mass ratio of the siloxane compound to the catalyst precursor is 100:(1~10).

[0006] In some embodiments of this application, the carbon source material is selected from one or more of glucose, sucrose, fructose, ascorbic acid, and citric acid; the siloxane compound is selected from one or more of methyl orthosilicate, ethyl orthosilicate, methyl ethyl orthosilicate, and isopropyl orthosilicate; and the catalyst precursor is selected from one or more of soluble iron salts, soluble nickel salts, and soluble cobalt salts.

[0007] In some embodiments of this application, before immersing the porous carbon substrate in the silicon source sol, the following steps are further included: The porous carbon substrate is subjected to an oxidation treatment to form oxygen-containing groups on the surface of the pores of the porous carbon substrate; The porous carbon substrate after oxidation treatment is placed in a negative pressure environment, and then the porous carbon substrate is immersed in the silicon source sol.

[0008] In some embodiments of this application, the porous carbon substrate is placed in an oxidizing solution to oxidize the porous carbon substrate. The oxidation treatment temperature is 60°C to 100°C, and the oxidation treatment time is 6h to 12h. The oxidizing solution includes one or more of nitric acid, sulfuric acid, hydrogen peroxide, potassium permanganate, and potassium dichromate.

[0009] In some embodiments of this application, the second heat treatment process includes the following steps: The ambient temperature of the porous carbon substrate is controlled at 1500℃~1600℃ and kept at that temperature for 1h~3h. Then, the ambient temperature of the porous carbon substrate is controlled at 1700℃~1800℃ and kept at that temperature for 0.5h~1h.

[0010] In some embodiments of this application, during the first heat treatment, the ambient temperature of the porous carbon substrate is controlled to be 600°C to 1000°C.

[0011] In some embodiments of this application, during the first heating treatment, the carbon source gas is selected from one or more of methane, ethylene, acetylene, and ethane; the first reducing gas is selected from hydrogen; and the flow rate ratio of the carbon source gas to the first reducing gas is (2~10):1.

[0012] In some embodiments of this application, the average pore size of the porous carbon substrate is 2nm~50nm, and the porosity of the porous carbon substrate is 40%~70%.

[0013] In some embodiments of this application, a silicon-carbon composite material is formed after a second heat treatment, wherein the mass percentage of silicon is 20% to 50% and the mass percentage of carbon is 50% to 80%.

[0014] During the conversion of silicon source sol into silicon oxide, there is a significant volume shrinkage phenomenon. Therefore, the actual volume of the resulting silicon oxide is usually smaller than the pores of the porous carbon substrate. This can lead to two problems: firstly, the final silicon material cannot make close contact with the porous carbon substrate, making it difficult to fully charge and discharge; secondly, the final silicon material is prone to detaching from the pores of the porous carbon substrate, resulting in a sharp decline in battery capacity.

[0015] In at least one embodiment of the method for improving the structural stability of silicon-carbon materials in this application, a catalyst precursor and a carbon source material are first directly dispersed in the silicon source sol during the formation of the silicon source sol. The silicon source sol adheres to the pores of the porous carbon substrate and is then converted into silicon oxide, achieving in-situ mixing of the catalyst precursor, carbon source material, and silicon oxide. Subsequently, during a first heat treatment, the catalyst precursor is reduced by a first reducing gas to form a catalyst, the carbon source material is carbonized to form carbon material, and the carbon source gas is cracked and, under the catalysis of the catalyst, carbon nanotubes are generated in-situ within the silicon oxide. Therefore, the carbon nanotubes can stably bond with the subsequently generated silicon material. After the generation of carbon nanotubes, a second heat treatment is performed in a second reducing gas, causing the silicon oxide to be reduced to silicon material by the carbon material and the second reducing gas. In the silicon-carbon composite material prepared by this method, the finally generated silicon material is located within the pores of the porous carbon substrate. Furthermore, during deposition, carbon nanotubes can extend from the pores of the porous carbon substrate to its outer surface, and during the subsequent high-temperature second heat treatment, they bond with carbon atoms in the porous carbon substrate and silicon atoms in the silicon material to ensure structural stability. Therefore, carbon nanotubes can lock silicon material within the pores of the porous carbon substrate, ensuring the structural stability of the silicon-carbon composite material. Simultaneously, carbon nanotubes grown in situ on the surface of the silicon material can also act as conductive bridges, improving the conductivity of the silicon material within the pores.

[0016] In a further embodiment of this application, the porous carbon substrate is pre-oxidized before being immersed in the silicon source sol. During the oxidation process, oxygen-containing groups are generated on the pore surface of the porous carbon substrate. During the conversion of the silicon source sol into silicon oxide, the silicon oxide is tightly bound to the pore surface of the porous carbon substrate through these oxygen-containing groups, maintaining a relatively stable structure within the porous carbon substrate.

[0017] In a further embodiment of this application, during the second heat treatment, the porous carbon substrate is first heated at a temperature of 1500°C to 1600°C to initially reduce silicon oxide to silicon material, and then further heated at a temperature of 1700°C to 1800°C to generate silicon carbide at the interface between the silicon material and the porous carbon substrate, thereby increasing the bonding strength between the silicon material and the porous carbon substrate, further fixing the silicon material, and further improving the structural stability of the silicon-carbon material. Detailed Implementation

[0018] To facilitate understanding of this application, a more complete description will be provided below. Preferred embodiments of this application are shown below. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be thorough and complete.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0020] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0021] As described in the background section, silicon itself has poor electrical conductivity. Silicon experiences significant volume expansion and contraction during charging and discharging, and these frequent volume changes can cause silicon to detach from carbon materials, leading to a sharp decline in battery capacity retention. Some related technologies attach silicon to porous carbon, using the porous carbon to slow down silicon detachment, and the carbon material can also improve the conductivity of silicon. However, the pores in porous carbon are not closed; during long-term cycling, silicon particles will still gradually detach from the pores of the porous carbon due to volume expansion and contraction. Even if an attempt is made to further deposit a carbon film on the porous carbon to seal or block the pores, the carbon film will still be gradually destroyed during the volume changes of silicon, and the carbon film will also, to some extent, prevent ion diffusion to the silicon surface, adversely affecting the charging and discharging speed of silicon.

[0022] To address the problems in related technologies, this application proposes the following technical concept: When attaching silicon material to the pores of a porous carbon substrate, the silicon material only fills a portion of the pores, avoiding complete filling and allowing space for volume expansion and contraction, thus reducing stress during volume changes. Simultaneously, carbon nanotubes are used to bond both the silicon material and the porous carbon substrate, fixing the silicon material to the substrate and enhancing the bonding strength. The in-situ bonding of carbon nanotubes to silicon aids in electron extraction from the silicon, and the gaps between the linear carbon nanotubes do not completely block the pores of the porous carbon substrate, thus minimally hindering ion diffusion to the silicon surface.

[0023] This application provides a method for improving the structural stability of silicon-carbon materials, which includes the following steps S1 to S4.

[0024] Step S1: The siloxane compound is placed in a solution containing water for hydrolysis, and a catalyst precursor and a carbon source material are added to the solution to form a silicon source sol containing the catalyst precursor and the carbon source material.

[0025] In some embodiments of this application, in step S1, the siloxane compound is selected from one or more of methyl orthosilicate, ethyl orthosilicate, methyl ethyl orthosilicate, and isopropyl orthosilicate. The orthosilicate compound can undergo hydrolysis to generate a silicon source sol, which contains the hydrolysis product orthosilicic acid. During aging, the silicon source sol undergoes dehydration and condensation to form a gel, which, after further drying and curing, forms silicon oxide. The catalyst precursor is also dispersed in the silicon source sol.

[0026] In some embodiments of this application, the siloxane compound is first placed in an aqueous solution for hydrolysis, and then the carbon source material and catalyst precursor are added to the solution. Alternatively, in other embodiments, the siloxane compound, carbon source material, and catalyst precursor may be added to an aqueous solution first, and then the siloxane compound is allowed to complete the hydrolysis reaction and form a silicon source sol.

[0027] It is understandable that this carbon source material can be used to generate carbon material during the subsequent first heat treatment process. This carbon material is uniformly mixed with the silicon oxide generated by the silicon source sol, and can reduce the silicon oxide during the subsequent second heat treatment process, thereby rapidly and fully reducing the silicon oxide to elemental silicon. Although the porous carbon substrate also contains carbon, its surface state is relatively stable, its reactivity is low, and the contact between the porous carbon substrate and the silicon oxide is insufficient, resulting in a slow reaction rate. Furthermore, the amount of carbon material actually participating in the reaction is not easily controlled.

[0028] In some embodiments of this application, the carbon source material may be selected from organic compounds that are soluble in water, so as to ensure that the carbon source material is uniformly and sufficiently dispersed in the silicon source sol. For example, the carbon source material may be selected from one or more of glucose, sucrose, fructose, ascorbic acid, and citric acid.

[0029] In some embodiments of this application, the molar ratio of carbon in the carbon source material to silicon in the silicon source sol is (1.6~2.4):1. For example, the molar ratio of carbon in the carbon source material to silicon in the silicon source sol is 1.6:1, 1.8:1, 2:1, 2.2:1, 2.4:1, or the molar ratio of carbon to silicon can be between any two of the above ratios. The molar ratio of carbon in the carbon source material to silicon in the silicon source sol should not be too high or too low. If it is too high, more carbon material will be generated, which is more likely to undergo further side reactions with the reduced silicon material. If it is too low, it is not easy to fully reduce silicon oxide, resulting in insufficient silicon material generation.

[0030] It is understood that this catalyst precursor can be used to generate a catalyst for catalyzing the deposition of carbon nanotubes. The catalyst precursor is selected from soluble metal salts. In some embodiments of this application, the catalyst precursor is selected from one or more of soluble iron salts, soluble nickel salts, and soluble cobalt salts. Specifically, the soluble iron salt is selected from one or more of ferric nitrate and ferric chloride, the soluble nickel salt is selected from one or more of nickel nitrate and nickel chloride, and the soluble cobalt salt is selected from one or more of cobalt nitrate and cobalt chloride.

[0031] In some embodiments of this application, the mass ratio of the siloxane compound to the catalyst precursor is 100:(1~10). For example, the mass ratio of the siloxane compound to the catalyst precursor is 100:1, 100:2, 100:3, 100:5, 100:7, or 100:10, or the mass ratio of the siloxane compound to the catalyst precursor can be between any two of the above mass ratios. In this embodiment, if the mass proportion of the catalyst precursor is too low, the catalyst may not be able to be exposed from the subsequently prepared silicon oxide, resulting in difficulty in carbon nanotube deposition or insufficient carbon nanotube deposition. If the mass proportion of the catalyst precursor is too high, it will affect the mass proportion of the final silicon material.

[0032] In some embodiments of this application, the solution may further contain alcohol compounds in step S1. The alcohol compounds may be selected from one or more of methanol, ethanol, and isopropanol. Alcohol compounds can be used to control the hydrolysis rate of siloxane compounds, preventing the silicon source sol from gelling too quickly, so that the silicon source sol can fully penetrate the pores of the porous carbon substrate. In some embodiments of this application, the molar ratio of water to alcohol compounds in the solution is 1:(0.5~5).

[0033] In some embodiments of this application, in step S1, the solution may further contain an acidic catalyst to promote the hydrolysis reaction. The acidic catalyst can promote the hydrolysis of siloxane compounds while inhibiting the agglomeration of the catalyst precursor due to hydrolysis. This is beneficial for the uniform dispersion of the catalyst precursor in the silicon source sol, so as to facilitate the subsequent catalytic reaction.

[0034] Siloxane compounds exhibit significant volume shrinkage during their conversion to silicon oxides, resulting in silicon oxides with a volume typically smaller than the pores of the porous carbon substrate. This leads to the silicon material easily detaching from the pores of the porous carbon substrate, causing a sharp decline in battery capacity. Therefore, siloxane compounds are generally not used in related technologies to prepare silicon materials within the pores of porous carbon substrates. This application cleverly utilizes the volume shrinkage characteristic of siloxane compounds during their conversion to silicon oxides. The spontaneous volume shrinkage during silicon oxide formation prevents the silicon oxide from completely occupying the pores, thus achieving silicon material filling only a portion of the pores. In-situ grown carbon nanotubes, after a second heat treatment, can be used to lock the silicon material within the pores of the porous carbon substrate, preventing silicon material detachment, as detailed below.

[0035] Step S2: Immerse the porous carbon substrate in silicon source sol, so that the silicon source sol adheres to the pores of the porous carbon substrate and transforms the silicon source sol into silicon oxide.

[0036] In some embodiments of this application, the average pore size of the porous carbon substrate is 2 nm to 50 nm, and the porosity of the porous carbon substrate is 40% to 70%. The porous carbon substrate with this average pore size and porosity is beneficial for the full infiltration of silicon source sol in subsequent processes, and also for the infiltration of gas into the pores of the porous carbon substrate to promote the full progress of the reaction.

[0037] In some embodiments of this application, before immersing the porous carbon substrate in the silicon source sol, the following step is further included: oxidizing the porous carbon substrate to form oxygen-containing groups on the pore surface of the porous carbon substrate. During the oxidation process, oxygen-containing groups are generated on the pore surface of the porous carbon substrate. During the conversion of the silicon source sol into silicon oxide, the silicon oxide can be tightly bound to the pore surface of the porous carbon substrate through these oxygen-containing groups, maintaining a relatively stable structure of the silicon oxide within the porous carbon substrate.

[0038] In some embodiments of this application, the porous carbon substrate is placed in an oxidizing solution to oxidize it. The oxidizing solution includes one or more of nitric acid, sulfuric acid, hydrogen peroxide, potassium permanganate, and potassium dichromate. In this embodiment, the oxidizing solution can fully penetrate into the pores of the porous carbon substrate, thereby ensuring that oxygen-containing functional groups are formed as fully and uniformly as possible on the surface of the pores. Furthermore, the oxidizing solution can also be used to remove impurities such as metals from the porous carbon substrate.

[0039] In some embodiments of this application, the oxidation treatment temperature is 60°C to 100°C. For example, the oxidation treatment temperature can be 60°C, 70°C, 80°C, 90°C, or 100°C, or the oxidation treatment temperature can be between any two of the above temperatures.

[0040] In some embodiments of this application, the oxidation treatment time is 6h to 12h. For example, the oxidation treatment time can be 6h, 7h, 8h, 9h, 10h, 11h, or 12h, or the oxidation treatment time can be between any two of the above times.

[0041] In some embodiments of this application, after oxidation treatment, the porous carbon substrate can be cleaned to remove the oxidizing solution adhering to the surface of the porous carbon substrate, and then the porous carbon substrate can be dried for later use.

[0042] In some embodiments of this application, the oxidized porous carbon substrate can be placed in a negative pressure environment, and then the porous carbon substrate can be immersed in a silicon source sol. Optionally, the negative pressure environment is a vacuum environment. Placing the oxidized porous carbon substrate in a negative pressure environment can remove the gas inside the pores of the porous carbon substrate, and then the silicon source sol can fully penetrate into the pores of the porous carbon substrate under the action of external atmospheric pressure.

[0043] In some embodiments of this application, after the silicon source sol is attached to the pores of the porous carbon substrate, the porous carbon substrate can be removed and dried to remove the solvent from the silicon source sol. The drying process can be heat drying, vacuum drying, or freeze drying. During solvent removal, the silicon source sol gradually forms a non-flowing gel, which contains silicon oxide components. As mentioned above, in some embodiments, because the surface of the porous carbon substrate has oxygen-containing groups, the silicon oxide can also combine with these groups during the process, thereby allowing the silicon oxide to stably adhere to the pore surface of the porous carbon substrate through chemical bonds.

[0044] Step S3: The porous carbon substrate is placed in a reaction atmosphere containing carbon source gas and a first reducing gas and subjected to a first heating treatment to reduce the catalyst precursor to a catalyst for catalyzing carbon nanotube deposition, and carbon nanotubes are deposited on the surface of the porous carbon substrate.

[0045] In some embodiments of this application, the porous carbon substrate is pre-sintered before the first heat treatment, and the pre-sintering temperature is 150°C to 500°C. The purpose of the pre-sintering treatment is to completely condense the silicon oxide in the pores of the porous carbon substrate and expose at least part of the catalyst precursor to facilitate the subsequent deposition of carbon nanotubes.

[0046] In some embodiments of this application, during the first heat treatment, the ambient temperature of the porous carbon substrate is controlled to be between 600°C and 1000°C. For example, the ambient temperature of the porous carbon substrate can be 600°C, 700°C, 800°C, 900°C, or 1000°C, or it can be between any two of the above temperatures. This temperature range is beneficial for both the reduction of the catalyst precursor to the catalyst and the deposition of carbon nanotubes. It can be understood that the catalyst precursor is a metal salt, which can form a corresponding metal oxide in the previous treatment, and the final catalyst is the corresponding metal. For example, if the catalyst precursor is an iron salt, then the final catalyst is iron. Since the catalyst precursor is directly embedded in the silicon source sol, the formed catalyst is also embedded in the silicon oxide.

[0047] In some embodiments of this application, the first reducing gas is selected from hydrogen. The first reducing gas is capable of converting the catalyst precursor into a catalyst during the first heat treatment.

[0048] In some embodiments of this application, the carbon source gas is selected from one or more of methane, ethylene, acetylene, and ethane. Methane, ethylene, acetylene, and ethane are cracked during the first heat treatment to produce carbon atoms, which are then deposited based on a catalyst to form carbon nanotubes. Because the catalyst is embedded in silicon oxide, and the carbon nanotubes are deposited based on the catalyst, the formed carbon nanotubes can be in situ bonded to the silicon oxide through the catalyst.

[0049] In some embodiments of this application, the flow ratio of the carbon source gas to the first reducing gas is (2~10):1. For example, the flow ratio of the carbon source gas to the first reducing gas can be 2:1, 3:1, 4:1, 5:1, 6:1, 8:1, or 10:1. Using this flow ratio is beneficial for ensuring that the catalyst precursor is fully converted to produce sufficient catalyst, and also for ensuring that carbon nanotubes are fully deposited based on the catalyst.

[0050] In some embodiments of this application, the duration of the first heating treatment is 0.5h to 6h. For example, the duration of the first heating treatment is 0.5h, 1h, 2h, 4h, or 6h, or the duration of the first heating treatment may be between any two of the above times.

[0051] In some embodiments of this application, a protective gas may also be included in the reaction atmosphere during the first heating treatment. The protective gas may be selected from one or more of nitrogen and argon.

[0052] Step S4: The porous carbon substrate is placed in a reaction atmosphere containing a protective gas and a second reducing gas and subjected to a second heat treatment at a temperature above 1500°C.

[0053] In this embodiment, when the temperature is above 1500°C, carbon and the second reducing gas gradually reduce the silicon oxide in the pores to silicon material. Simultaneously, the carbon atoms in the carbon nanotubes can rebond with the porous carbon substrate surface, resulting in a stable bond between the carbon nanotubes and the porous carbon substrate. Thus, the carbon nanotubes can lock the silicon material within the pores of the porous carbon substrate, ensuring the structural stability of the silicon-carbon composite material. The carbon nanotubes grown in situ on the surface of the silicon material can also act as conductive bridges for the silicon, improving the conductivity of the silicon material within the pores.

[0054] In some embodiments of this application, the temperature during the second heat treatment is 1500℃~2000℃.

[0055] In some embodiments of this application, the second heat treatment process includes the following steps: controlling the ambient temperature of the porous carbon substrate to 1500℃~1600℃ and holding it at that temperature for 1h~3h; then controlling the ambient temperature of the porous carbon substrate to 1700℃~1800℃ and holding it at that temperature for 0.5h~1h. Specifically, the porous carbon substrate is first heated at 1500℃~1600℃ to initially reduce silicon oxide to silicon material, and then further heated at 1700℃~1800℃ to generate silicon carbide at the interface between the silicon material and the porous carbon substrate, thereby increasing the bonding strength between the silicon material and the porous carbon substrate, further fixing the silicon material, and further improving the structural stability of the silicon-carbon material.

[0056] In some embodiments of this application, a silicon-carbon composite material is formed after a second heat treatment. In the silicon-carbon composite material, the mass percentage of silicon is 20% to 50%, and the mass percentage of carbon is 50% to 80%. It is understood that the carbon element includes not only the carbon element in the porous carbon substrate, but also the carbon element in carbon nanotubes and other carbon materials.

[0057] In some embodiments of this application, the first heating treatment and the second heating treatment are performed sequentially in the same device.

[0058] Through the above steps S1 to S4, the method for improving the structural stability of silicon-carbon materials can be completed. In the embodiments of this application, a silicon-carbon composite material can be prepared by using this method for improving the structural stability of silicon-carbon materials.

[0059] In at least one embodiment of the method for improving the structural stability of silicon-carbon materials in this application, a catalyst precursor and a carbon source material are first directly dispersed in the silicon source sol during the formation of the silicon source sol. The silicon source sol adheres to the pores of the porous carbon substrate and is then converted into silicon oxide, achieving in-situ mixing of the catalyst precursor, carbon source material, and silicon oxide. Subsequently, during a first heat treatment, the catalyst precursor is reduced by a first reducing gas to form a catalyst, the carbon source material is carbonized to form carbon material, and the carbon source gas is cracked and, under the catalysis of the catalyst, carbon nanotubes are generated in-situ within the silicon oxide. Therefore, the carbon nanotubes can stably bond with the subsequently generated silicon material. After the generation of carbon nanotubes, a second heat treatment is performed in a second reducing gas, causing the silicon oxide to be reduced to silicon material by the carbon material and the second reducing gas. In the silicon-carbon composite material prepared by this method, the finally generated silicon material is located within the pores of the porous carbon substrate. Furthermore, during deposition, carbon nanotubes can extend from the pores of the porous carbon substrate to its outer surface, and during the subsequent high-temperature second heat treatment, they bond with carbon atoms in the porous carbon substrate and silicon atoms in the silicon material to ensure structural stability. Therefore, carbon nanotubes can lock silicon material within the pores of the porous carbon substrate, ensuring the structural stability of the silicon-carbon composite material. Simultaneously, carbon nanotubes grown in situ on the surface of the silicon material can also act as conductive bridges, improving the conductivity of the silicon material within the pores.

[0060] Furthermore, one embodiment of this application also provides a silicon-carbon composite material, which is prepared by the method for improving the structural stability of silicon-carbon materials described in the above embodiments.

[0061] Furthermore, one embodiment of this application also provides an electrode comprising the silicon-carbon composite material described in the above embodiments.

[0062] In some embodiments of this application, the electrode includes a current collector and an active material layer disposed on the current collector. The active material layer includes a binder and the aforementioned silicon-carbon composite material. Further, the active material layer may also include a conductive agent selected from one or more of conductive carbon black, carbon fibers, carbon nanotubes, and graphene.

[0063] Furthermore, one embodiment of this application also provides a lithium-ion battery, which includes the electrode described in the above embodiment. In this embodiment, the electrode can serve as the negative electrode of the lithium-ion battery. The lithium-ion battery can be any of the following forms: a battery cell, a pouch cell, a cylindrical cell, a battery case, or a battery module.

[0064] This application also provides the following embodiments and comparative examples to further illustrate the implementation and advantages of the circuit board hole wall modification method of this application.

[0065] Example 1 (1) Tetraethyl orthosilicate (TEOS), ethanol and deionized water are mixed evenly in a molar ratio of 1:4:2, and then a mixed solution of glucose, hydrochloric acid and ferric chloride is added and stirred evenly to form a silicon source sol; wherein the mass ratio of tetraethyl orthosilicate to ferric chloride is 20:1, and the molar ratio of carbon in glucose to silicon in tetraethyl orthosilicate is 2:1.

[0066] (2) A mesoporous carbon material with a porosity of 55% was used as the porous carbon substrate. The porous carbon substrate was placed in a mixed solution of concentrated sulfuric acid and potassium permanganate, heated to 90°C and stirred for 10 hours. Then it was taken out, cleaned and dried. The oxidized porous carbon substrate was placed in a vacuum chamber for vacuum treatment, and then immersed in silicon source sol and stirred. After 2 hours, it was taken out and transferred to a constant temperature oven at 60°C for slow drying, so that the silicon source sol gelled and formed silicon oxide.

[0067] (3) Transfer the porous carbon substrate containing silicon oxide to a sintering furnace, first introduce argon gas and gradually raise the temperature to 300℃ and hold for 2 hours; then introduce a mixture of hydrogen, ethylene and argon gas and raise the temperature to 800℃ and hold for 1 hour, wherein the flow ratio of ethylene and hydrogen is 5:1.

[0068] (4) Stop the ethylene supply, continue the supply of hydrogen and argon, and continue to heat the sintering furnace. First, heat the sintering furnace to 1550℃ and hold for 2 hours, then heat the sintering furnace to 1750℃ and hold for 0.5 hours, and then control the sintering furnace to cool naturally to room temperature, and take out the prepared silicon-carbon composite material.

[0069] Example 2 The main difference between Example 2 and Example 1 is that in step (2), no oxidation treatment was performed beforehand; the porous carbon substrate was simply cleaned and set aside. The other steps are basically the same.

[0070] Example 3 The main difference between Example 3 and Example 1 is that in step (4), the sintering furnace was only heated to 1550°C and held for 2 hours before being cooled, without any further heating. The other steps are basically the same.

[0071] Example 4 The main difference between Example 4 and Example 1 is that in step (4), the sintering furnace was only heated to 1750°C and held for 2 hours before being cooled, without any further heating. The other steps are basically the same.

[0072] Example 5 The main difference between Example 5 and Example 1 is that the mass ratio of tetraethyl orthosilicate to the catalyst precursor is 5:1. The other steps are basically the same.

[0073] Example 6 The main difference between Example 6 and Example 1 is that in step (1), methyl orthosilicate is used instead of tetraethyl orthosilicate, and nickel chloride is used instead of ferric chloride. The other steps are basically the same.

[0074] Comparative Example 1 The main difference between Comparative Example 1 and Example 1 is that ferric chloride is not added in step (1). The other steps are basically the same. A carbon film is deposited on the surface of the silicon-carbon composite material of Comparative Example 1, but carbon nanotubes are not grown in situ.

[0075] Comparative Example 2 The main difference between Comparative Example 2 and Example 1 is that step (4) is omitted. The other steps are basically the same. Since step (4) was not performed in Comparative Example 2, the silicon oxide could not be reduced, and X-ray diffraction results showed that no silicon material was generated. Therefore, Comparative Example 2 was not used to prepare the battery.

[0076] Comparative Example 3 The main difference between Comparative Example 3 and Example 1 is that the order of steps (3) and (4) is reversed, that is, step (4) is performed first, and then step (3) is performed. In Comparative Example 3, silicon oxide is reduced first, but during the reduction process, iron has already alloyed with silicon, and carbon nanotubes cannot be grown again.

[0077] Comparative Example 4 The main difference between Comparative Example 4 and Example 1 is that ferric chloride is not added in step (1), and the prepared silicon-carbon composite material is further subjected to the following step (5) after step (4), while the other steps are basically the same.

[0078] (5) The silicon-carbon composite material was immersed in ferric chloride solution, dried, and then transferred to a sintering furnace. A mixture of hydrogen, ethylene, and argon was introduced and the temperature was raised to 800°C and held for 1 hour. The flow rate ratio of ethylene to hydrogen was 5:1. The main purpose of step (5) is to deposit carbon nanotubes on the surface of the silicon-carbon composite material. In Comparative Example 4, silicon oxide was first reduced and then carbon nanotubes were prepared separately. The carbon nanotubes were not grown in situ on the silicon material in the pores of the porous carbon substrate.

[0079] Comparative Example 5 The main steps of Comparative Example 5 are as follows.

[0080] (1) A mesoporous carbon material with a porosity of 55% was used as a porous carbon substrate. The porous carbon substrate was placed in a mixed solution of concentrated sulfuric acid and potassium permanganate, heated to 90°C and stirred for 10 hours. Then it was taken out, cleaned and dried.

[0081] (2) Transfer the porous carbon substrate to a sintering furnace, introduce a mixture of silane and argon, heat to 800°C and hold for 1 hour, and uniformly deposit a layer of silicon material on the surface of the porous carbon substrate.

[0082] (3) The silicon-carbon composite material is immersed in ferric chloride solution, dried, and then transferred to a sintering furnace. A mixture of hydrogen, ethylene, and argon is introduced and heated to 800°C for 1 hour. The flow ratio of ethylene to hydrogen is 5:1. The main purpose of step (5) is to deposit carbon nanotubes on the surface of the silicon-carbon composite material.

[0083] Comparative Example 5 uses silane as a raw material to deposit silicon material in a porous carbon substrate, and then deposits carbon nanotubes separately.

[0084] Preparation of the negative electrode sheet: The silicon-carbon composite materials of the above embodiments and comparative examples are mixed evenly with the negative electrode conductive agent and the negative electrode binder at a mass ratio of 8:1:1 and dispersed in a solvent. After stirring evenly, the mixture is coated onto the surface of copper foil, dried, and stamped into a negative electrode sheet. The negative electrode conductive agent is conductive carbon black, the negative electrode binder is carboxymethyl cellulose and styrene-butadiene rubber, and the solvent is deionized water.

[0085] Preparation of half-cell: The negative electrode and lithium metal sheet are assembled into a half-cell in a glove box. The electrolyte is a 1.0 mol / L LiPF6 solution. The solvent of the electrolyte is a mixed solvent of EC, EMC and DEC in a volume ratio of 1:1:1.

[0086] Preparation of the full cell: The positive electrode of the battery is made by uniformly mixing NCM811, positive electrode conductive agent, and positive electrode binder in a mass ratio of 8:1:1 and dispersing them in a solvent. After stirring evenly, the mixture is coated onto the surface of aluminum foil, dried, and stamped to form the positive electrode sheet. The positive electrode conductive agent is conductive carbon black, the positive electrode binder is polyvinylidene fluoride, and the solvent is N-methylpyrrolidone. The negative electrode sheet and the positive electrode sheet are then assembled into a full cell. The electrolyte is a 1.0 mol / L LiPF6 solution, and the solvent of the electrolyte is a mixed solvent prepared by EC, EMC, and DEC in a volume ratio of 1:1:1.

[0087] Testing: After the half-cell and full-cell were left to rest for 24 hours, charge-discharge cycle tests were performed on a constant current charge-discharge tester. The test voltage range was 0.01V~1.5V, and the first cycle was activated at a current density of 0.05C. The reversible specific capacity of the half-cell during the first charge-discharge cycle was tested. The full-cell was cycled 500 times at current densities of 0.5C and 1C respectively. The reversible specific capacity, 0.5C cycle capacity retention rate, and 1C cycle capacity retention rate of the half-cell are shown in Table 1 below.

[0088] Table 1 Example 1 involves the in-situ doping of iron salts into the pores of a porous carbon substrate while preparing silicon oxide. Carbon nanotubes are then deposited at a lower temperature, followed by reduction of the silicon oxide at a higher temperature to form a silicon-carbon composite material. Simultaneously, the carbon nanotubes bond with carbon atoms in the porous carbon substrate to immobilize the silicon material. Referring to Table 1, the half-cell of Example 1 exhibited a high specific capacity of 2215.4 mAh / g, primarily due to the efficient extraction of electrons from the silicon material by the in-situ grown carbon nanotubes. The full-cell of Example 1 exhibited a high capacity retention of 95.5% at a discharge rate of 0.5C, mainly due to the confinement of the silicon structure by the porous carbon substrate and carbon nanotubes. Furthermore, the full cell of Example 1 still exhibited a capacity retention rate as high as 92.7% at a discharge rate of 1C. This is mainly due to the atomic-level bonding between the carbon nanotubes and the porous carbon substrate and silicon material during high-temperature treatment. Even if the silicon material undergoes rapid volume changes during high-rate charging and discharging, the silicon material can still be stably confined in the pores of the porous carbon substrate and is difficult to detach.

[0089] Referring to Table 1, the discharge specific capacity of Comparative Example 1 was only 1902.7 mAh / g, and the cycle capacity retention rates at 0.5C and 1C were 79.2% and 69.7%, respectively, both significantly lower than those of Example 1. This indicates that the silicon material of Comparative Example 1 has poor conductivity and structural stability. This is mainly because, compared with Example 1, Comparative Example 1 did not grow carbon nanotubes.

[0090] The discharge specific capacity of Comparative Example 3 was only 1511.2 mAh / g, and the cycle capacity retention rates at 0.5C and 1C were 76.2% and 68.9%, respectively, both slightly lower than those of Comparative Example 1. This is mainly because Comparative Example 3 underwent step (4) first, which caused the iron in the catalyst precursor to alloy with the silicon material, making it unable to catalyze the deposition of carbon nanotubes and also leading to a reduction in active materials.

[0091] Comparative Example 4 maintained a capacity retention rate of 88.1% at 0.5C, but only 72.4% at 1C. This means that Comparative Example 4 exhibits less capacity decay at lower charge-discharge rates, but still suffers significant capacity loss at higher rates. This is mainly because Comparative Example 4, after step (4), prepared a catalyst and deposited carbon nanotubes. The catalyst failed to embed into the silicon material, affecting the conductivity between the carbon nanotubes and the silicon material. Furthermore, the carbon nanotubes did not undergo high-temperature treatment and therefore did not bond with the silicon material and the porous carbon substrate. This resulted in poor bonding between the silicon material, carbon nanotubes, and the porous carbon substrate. Consequently, the silicon material still detached during higher charge-discharge cycles, leading to significant irreversible capacity loss.

[0092] Similar to Comparative Example 4, Comparative Example 5 exhibits capacity retention rates of 85.6% and 67.1% at 0.5C and 1C, respectively. This indicates that Comparative Example 5 shows less capacity decay at lower charge-discharge rates, but still suffers significant capacity loss at higher rates. Furthermore, the capacity retention rate of Comparative Example 5 at 1C is lower than that of Comparative Example 4. This is primarily because the silicon material in Comparative Example 5 fills the pores of the porous carbon substrate via vapor deposition. This process makes it easy for the silicon material to clog the pores of the porous carbon substrate during deposition, resulting in a lack of space for volumetric changes.

[0093] Compared to Example 1, Example 2 did not undergo pre-oxidation treatment, resulting in poor bonding performance between silicon oxide and porous carbon substrate in step (2), which in turn made it difficult for silicon material and porous carbon substrate to bond effectively in step (4), thus reducing the capacity retention rate of Example 2. In Example 3, silicon oxide was only reduced at a lower temperature in step (4), without further sintering the silicon material and porous carbon substrate, resulting in poor bonding performance between the silicon material and porous carbon substrate, thus reducing the capacity retention rate of Example 3. In Example 4, sintering was performed directly at a higher temperature in step (4), affecting the sufficient reduction and sintering of silicon oxide, resulting in a decrease in discharge specific capacity and a decrease in cycle capacity retention rate. In Example 5, a catalyst precursor with a high mass ratio was used, which had a certain negative impact on the discharge specific capacity of silicon material.

[0094] The above detailed description is a specific description of the feasible embodiments of this application. These embodiments are not intended to limit the patent scope of this application. All equivalent implementations or modifications that do not depart from the scope of this application should be included in the patent scope of this application.

Claims

1. A method for improving the structural stability of silicon-carbon materials, characterized in that, Includes the following steps: A siloxane compound is placed in a solution containing water for hydrolysis, and a catalyst precursor and a carbon source material are added to the solution to form a silicon source sol containing the catalyst precursor and the carbon source material. The porous carbon substrate is immersed in the silicon source sol, so that the silicon source sol adheres to the pores of the porous carbon substrate and the silicon source sol is converted into silicon oxide. The porous carbon substrate is placed in a reaction atmosphere containing a carbon source gas and a first reducing gas and subjected to a first heating treatment to reduce the catalyst precursor to a catalyst for catalyzing carbon nanotube deposition, and carbon nanotubes are deposited on the surface of the porous carbon substrate. Furthermore, the porous carbon substrate is placed in a reaction atmosphere containing a protective gas and a second reducing gas and subjected to a second heat treatment at a temperature above 1500°C.

2. The method according to claim 1, characterized in that, The molar ratio of carbon in the carbon source material to silicon in the silicon source sol is (1.6~2.4):1; and / or, the mass ratio of the siloxane compound to the catalyst precursor is 100:(1~10).

3. The method according to claim 2, characterized in that, The carbon source material is selected from one or more of glucose, sucrose, fructose, ascorbic acid, and citric acid; the siloxane compound is selected from one or more of methyl orthosilicate, ethyl orthosilicate, methyl ethyl orthosilicate, and isopropyl orthosilicate; the catalyst precursor is selected from one or more of soluble iron salt, soluble nickel salt, and soluble cobalt salt.

4. The method according to any one of claims 1 to 3, characterized in that, Before immersing the porous carbon substrate in the silicon source sol, the following steps are also included: The porous carbon substrate is subjected to an oxidation treatment to form oxygen-containing groups on the surface of the pores of the porous carbon substrate; The porous carbon substrate after oxidation treatment is placed in a negative pressure environment, and then the porous carbon substrate is immersed in the silicon source sol.

5. The method according to claim 4, characterized in that, The porous carbon substrate is placed in an oxidizing solution to oxidize it. The oxidation temperature is 60℃~100℃ and the oxidation time is 6h~12h. The oxidizing solution includes one or more of nitric acid, sulfuric acid, hydrogen peroxide, potassium permanganate and potassium dichromate.

6. The method according to any one of claims 1 to 3, characterized in that, The second heat treatment process includes the following steps: The ambient temperature of the porous carbon substrate is controlled at 1500℃~1600℃ and kept at that temperature for 1h~3h. Then, the ambient temperature of the porous carbon substrate is controlled at 1700℃~1800℃ and kept at that temperature for 0.5h~1h.

7. The method according to any one of claims 1 to 3, characterized in that, During the first heat treatment, the ambient temperature of the porous carbon substrate is controlled to be 600℃~1000℃.

8. The method according to any one of claims 1 to 3, characterized in that, During the first heating treatment, the carbon source gas is selected from one or more of methane, ethylene, acetylene, and ethane; the first reducing gas is selected from hydrogen; and the flow rate ratio of the carbon source gas to the first reducing gas is (2~10):

1.

9. The method according to any one of claims 1 to 3, characterized in that, The porous carbon substrate has an average pore size of 2 nm to 50 nm and a porosity of 40% to 70%.

10. The method according to any one of claims 1 to 3, characterized in that, After a second heat treatment, a silicon-carbon composite material is formed, wherein the mass percentage of silicon is 20% to 50% and the mass percentage of carbon is 50% to 80%.

Images