Silicon-carbon composite material, method for preparing the same, and negative electrode material

Abstract

The application relates to a silicon-carbon composite material, a preparation method thereof and a negative electrode material, and belongs to the technical field of lithium ion batteries. The preparation method of the silicon-carbon composite material comprises the following steps: mixing carbonizable resin, a pore-forming agent and silicon-containing particles in an organic solvent, uniformly stirring to obtain a mixed slurry; coating the mixed slurry to obtain a first preformed film; removing the pore-forming agent in the first preformed film to obtain a porous composite film; and performing carbonization treatment on the porous composite film under the protection of an inert gas atmosphere to obtain a silicon-carbon composite film; the pore-forming agent is selected from at least one of cyclohexanol, polyvinylpyrrolidone or glycerol. The pore-forming agent has an adsorption effect on the silicon-containing particles, and after the pore-forming agent is removed and the carbonization treatment is performed, the silicon-containing particles with a particle size of 50-300 nm are located in pores with a pore size of 0.5-2 mu m in the porous carbon film. The silicon-carbon composite film can relieve the volume expansion of silicon, and improve the cycle stability and capacity retention rate of a lithium ion battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and more specifically, to a silicon-carbon composite material, its preparation method, and a negative electrode material. Background Technology

[0002] Against the backdrop of dual-carbon policies, the sales of new energy vehicles have experienced exponential explosive growth. As the power system for new energy vehicles, lithium-ion batteries (LIBs) require higher energy density and lower cost to meet the advancements in battery technology.

[0003] Currently, graphite is the primary anode material used in lithium-ion batteries. However, graphite's relatively low theoretical specific capacity (372 mAh / g) does not meet the higher energy density requirements of new energy vehicles. Researchers have conducted extensive exploration of novel anode materials, with silicon and its oxides, metals (Ge, Sn, etc.), transition metal sulfides, transition metal oxides, and graphene being developed. Among these anode materials, silicon is one of the most promising candidates. Its advantages include: a theoretical capacity of 4200 mAh / g at room temperature, far exceeding the 372 mAh / g of graphite anodes; and a discharge plateau (~0.2V) higher than the potential for lithium dendrite formation, preventing the formation of lithium dendrites and thus improving the safety of lithium-ion batteries (LIBs). Furthermore, its low discharge voltage allows for a higher operating potential when matched with the positive electrode, thereby increasing the energy density of LIBs. Simultaneously, silicon's high abundance and low toxicity make it more competitive in commercial applications.

[0004] However, raw silicon powder exhibits volume expansion during charge and discharge. Repeated volume changes generate significant stress in the silicon particles, eventually leading to breakage and detachment from the current collector surface, resulting in rapid capacity decay during battery cycling. Simultaneously, silicon itself has poor electrical conductivity, increasing the battery's internal resistance and restricting current flow, thus impairing its rate performance. Summary of the Invention

[0005] Based on the above-mentioned shortcomings, this application provides a silicon-carbon composite material, its preparation method, and anode material to partially or completely improve the volume expansion problem of silicon-containing anode materials in related technologies.

[0006] This application is implemented as follows:

[0007] In a first aspect, an example of this application provides a silicon-carbon composite membrane comprising a porous carbon membrane and silicon-containing particles. The porous carbon membrane contains a first pore with a pore size of 0.5-2 μm, and the silicon-containing particles have a particle size of 50-300 nm, with the silicon-containing particles located within the first pore.

[0008] In the above process, silicon-containing particles with a particle size of 50-300nm are located in the first pore of the porous carbon film. The pore size of the first pore is 0.5-2μm. The first pore of the porous carbon film can reserve space for the volume expansion of silicon-containing particles, alleviate the volume expansion of silicon during cycling, reduce the probability of the porous carbon film breaking during charging and discharging, and improve the cycle stability and capacity retention of lithium-ion batteries containing silicon-carbon composite films.

[0009] In conjunction with the first aspect, in optional embodiments of this application, the silicon-containing particles are selected from at least one of nano-silicon powder, carbon-coated nano-silicon powder, or vertically graphene-coated nano-silicon powder.

[0010] In the above process, silicon-containing particles such as nano-silicon powder, carbon-coated nano-silicon powder, or vertically graphene-coated nano-silicon powder are composited with a porous carbon film. The first pore in the porous carbon film can alleviate the volume expansion of silicon in the silicon-carbon composite film during cycling. Carbon coating of nano-sized silicon powder can prevent direct contact between the silicon powder and the electrolyte, reduce side reactions, and improve the conductivity of the silicon powder, thereby enhancing rate performance. Vertically graphene-coating of nano-sized silicon powder increases the contact sites between the particles and the porous carbon film. Simultaneously, the vertical structure of graphene provides channels for lithium-ion transport, improving the rate performance of the material.

[0011] In conjunction with the first aspect, in an optional embodiment of this application, silicon-containing particles account for 40-60% of the total weight of the silicon-carbon composite film.

[0012] In the above process, silicon-containing particles account for 40-60% of the total weight of the silicon-carbon composite film, which can alleviate the volume expansion of the silicon-carbon composite film and improve its conductivity, energy density and lithium storage capacity.

[0013] In conjunction with the first aspect, in an optional embodiment of this application, the thickness of the silicon-carbon composite membrane is 10-50 μm; optionally, the volume of the first pore accounts for more than 60% of the total pore volume in the porous carbon membrane.

[0014] In the above implementation process, silicon-carbon composite films with a thickness of 10-50μm have flexible self-supporting properties and high areal capacity. They can be used directly as electrodes without the need to add non-active materials such as binders and conductive agents.

[0015] In a second aspect, an example of this application provides a method for preparing a silicon-carbon composite material according to the first aspect, comprising:

[0016] A carbonizable resin, a pore-forming agent, and silicon-containing particles are mixed in an organic solvent and stirred until homogeneous to obtain a mixed slurry. The mixed slurry is then coated to obtain a first pre-film. The pore-forming agent in the first pre-film is removed to obtain a porous composite film. The porous composite film is then carbonized under an inert gas atmosphere to obtain a silicon-carbon composite film. The pore-forming agent is selected from at least one of cyclohexanol, polyvinylpyrrolidone, or glycerol.

[0017] In the above process, a carbonizable resin, a pore-forming agent, and silicon-containing particles are mixed in an organic solvent to form a mixed slurry. The mixed slurry is then coated to obtain a first preform. After removing the pore-forming agent from the first preform, pores are formed in the carbonizable resin corresponding to the positions of the pore-forming agent. Since pore-forming agents such as cyclohexanol, polyvinylpyrrolidone, or glycerol are adsorbed onto the silicon-containing particles, the silicon-containing particles are located within the pores of the carbonizable resin after the pore-forming agent is removed. The porous composite membrane after the pore-forming agent is removed is then carbonized, and the carbonizable resin is carbonized to form a porous carbon membrane, resulting in a silicon-carbon composite membrane in which the silicon-containing particles are located within the pores of the porous carbon membrane.

[0018] In conjunction with the second aspect, in an optional embodiment of this application, the method for removing the pore-forming agent from the first preform includes: placing the first preform in water to dissolve the pore-forming agent in the first preform in the water; optionally, placing the first preform in water at a temperature of 50-100°C and keeping it at that temperature for 2-7 hours; optionally, the water is selected from deionized water.

[0019] In the above process, pore-forming agents such as cyclohexanol, polyvinylpyrrolidone, or glycerol have good solubility in deionized water. The first pre-formed membrane is transferred to deionized water and kept at 50-100°C for 2-7 hours. The phase exchange between the pore-forming agent and the carbonizable resin and the deionized water can be utilized to form pores in the carbonizable resin. The silicon-containing particles adsorbed by the pore-forming agent are located in the pores, thus obtaining a porous composite membrane.

[0020] In conjunction with the second aspect, in an optional embodiment of this application, the method for obtaining a silicon-carbon composite membrane by carbonizing a porous composite membrane under an inert gas atmosphere includes: pre-oxidizing the porous composite membrane in an air atmosphere, and then carbonizing the pre-oxidized porous composite membrane under an inert gas atmosphere; optionally, the temperature of the pre-oxidation treatment is 200–400°C, the heating rate is 1–10°C / min, and the holding time is 1–3 h; optionally, the temperature of the carbonization treatment is 600–1100°C, the heating rate is 1–10°C / min, and the holding time is 0.5–4 h; optionally, the inert gas is selected from at least one of nitrogen or argon.

[0021] In the above process, the porous composite membrane is pre-oxidized in an air atmosphere at a temperature of 200-400℃. This pre-oxidation allows the carbonizable resin to form a more uniform and continuous porous carbon membrane during subsequent carbonization. The carbonization temperature is 600-1100℃, which prevents non-carbon elements such as H and N from being completely removed from the carbonizable resin material, resulting in a porous carbon membrane with good mechanical properties.

[0022] In conjunction with the second aspect, in an optional embodiment of this application, the method of coating the mixed slurry to obtain a first preform includes: coating the mixed slurry, and removing the organic solvent after coating to obtain the first preform; optionally, the organic solvent is selected from at least one of N,N-dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide; optionally, the method of removing the organic solvent after coating includes: allowing the coated mixed slurry to stand for 10-30 minutes to allow the organic solvent to evaporate. In conjunction with the second aspect, in an optional embodiment of this application, the mass ratio of the carbonizable resin, the pore-forming agent, and the silicon-containing particles in the mixed slurry is 1:(0.2-1):(0.2-0.6); optionally, the carbonizable resin is selected from at least one of polyacrylonitrile, polyvinyl alcohol, or lignin; optionally, the carbonizable resin is selected from polyacrylonitrile.

[0023] In the above-mentioned process, organic solvents such as N,N-dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide exhibit good dispersibility for carbonizable resins such as polyacrylonitrile, polyvinyl alcohol, or lignin, and pore-forming agents such as cyclohexanol, polyvinylpyrrolidone, or glycerol, resulting in a uniformly dispersed slurry. Since these organic solvents are volatile, removing them from the first pre-formed membrane before removing the pore-forming agent reduces the likelihood of rapid solvent efflux during the removal process, leading to the formation of finger-like macropores. In the slurry, the mass ratio of carbonizable resin, pore-forming agent, and silicon-containing particles is 1:(0.2-1):(0.2-0.6), which allows for the acquisition of a porous structure with suitable pore size after subsequent pore-forming agent removal. Using polyacrylonitrile as the carbonizable resin, subsequent carbonization yields a more continuous and uniform porous carbon membrane with lower impurity content.

[0024] In a third aspect, an example of this application provides a negative electrode material comprising the silicon-carbon composite film provided in the first aspect.

[0025] In the above implementation process, the negative electrode material containing the silicon-carbon composite film provided in the first aspect can alleviate the volume expansion during cycling, reduce the probability of the porous carbon film breaking during charging and discharging, and improve the cycle stability and capacity retention of the lithium-ion battery containing the silicon-carbon composite film. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0027] Figure 1 A schematic diagram of the preparation process of the silicon-carbon composite membrane provided as an example in this application;

[0028] Figure 2 This is a low-magnification SEM image of the silicon-carbon composite film in Example 1 of this application;

[0029] Figure 3 This is a high-magnification SEM image of the silicon-carbon composite film in Example 1 of this application;

[0030] Figure 4 This is a Raman diagram of the silicon-carbon composite film in Example 1 of this application;

[0031] Figure 5 The image shown is the XRD pattern of the silicon-carbon composite film in Example 1 of this application.

[0032] Figure 6 This is an XPS image of the silicon-carbon composite film in Example 1 of this application;

[0033] Figure 7 This is the XPS spectrum of the Si 2p layer of the silicon-carbon composite film in Example 1 of this application;

[0034] Figure 8 This is a TGA image of the silicon-carbon composite film in Example 1 of this application;

[0035] Figure 9 This is a SEM image of the silicon-carbon composite in Example 2 of this application;

[0036] Figure 10 Here is a SEM image of the silicon-carbon composite film in Example 3 of this application;

[0037] Figure 11 Here is a SEM image of the silicon-carbon composite film in Example 4 of this application;

[0038] Figure 12 This is a photograph of the silicon-carbon composite film in Example 4 of this application;

[0039] Figure 13 This is a SEM image of the multi-silicon-carbon composite film provided in Example 5 of this application;

[0040] Figure 14 This is a SEM image of the silicon-carbon composite film provided in Example 6 of this application;

[0041] Figure 15 This is a low-magnification SEM image of the silicon-carbon composite film provided in Example 7 of this application;

[0042] Figure 16 This is a high-magnification SEM image of the silicon-carbon composite film provided in Example 7 of this application;

[0043] Figure 17 SEM image of the silicon-carbon composite film provided in Comparative Example 1 of this application;

[0044] Figure 18 The first charge-discharge curve of the lithium battery negative electrode made using the silicon-carbon composite film provided in Example 1 in the test examples of this application;

[0045] Figure 19 In the test examples of this application, the lithium battery negative electrode made using the silicon-carbon composite film provided in Example 1 has a 0.2Ag content. -1 Cyclic stability curves at current density;

[0046] Figure 20 The rate performance curve of the lithium battery negative electrode made using the silicon-carbon composite film provided in Example 1 in the test examples of this application;

[0047] Figure 21 The rate performance curve of the silicon-carbon composite film provided in Example 3 in the test examples of this application;

[0048] Figure 22 The test examples in this application utilize the first charge-discharge curve of the silicon-carbon composite film provided in Example 4. Detailed Implementation

[0049] The embodiments of this application will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0050] Among numerous anode materials, silicon is one of the most promising candidates, with the advantages of a theoretical capacity of 4200 mAh g⁻¹ at room temperature. -1 It is much higher than the 372mAh g of the graphite anode. -1 Its discharge plateau (~0.2V) is higher than the potential for lithium dendrite formation, preventing the formation of lithium dendrites and thus improving the safety of LIBs. Furthermore, its low discharge voltage allows for a higher operating potential when matched with the positive electrode, thereby increasing the energy density of LIBs. Simultaneously, Si's high abundance and low toxicity make it more competitive in commercial applications.

[0051] However, raw silicon powder exhibits volume expansion during charging and discharging. Repeated volume changes generate significant stress on the silicon particles, eventually leading to breakage and detachment from the current collector surface, resulting in rapid capacity decay during battery cycling. Simultaneously, the poor conductivity of Si material itself increases the battery's internal resistance, restricting current flow and causing poor rate performance.

[0052] To address the issues of poor volume expansion, cycle performance, and rate capability in silicon anode materials, the inventors attempted to mitigate silicon volume expansion by incorporating nano-sized silicon powder with carbon. This involved designing a suitable electrode structure to reduce the contact area between silicon and the electrolyte. For example, they designed a core-shell, yolk-like silicon-carbon composite thin film material.

[0053] However, the core-shell silicon-carbon composite thin film materials prepared by existing methods such as ball milling, spray granulation, and chemical vapor deposition suffer from cracking of the carbon layer on the surface of silicon particles during cycling due to the volume expansion and contraction of silicon.

[0054] The inventors further attempted to reserve a space between the carbon layer and silicon particles through etching to provide a buffer for the volume expansion of silicon. For example, this could be achieved by the redox reaction of alkali metal Mg / Al with SiO2, using the Mg / Al thermal reduction method; or by using HF etching to etch away the SiO2 layer on the Si surface to create a buffer space for volume expansion.

[0055] However, in silicon-carbon anode materials prepared by magnesian thermal, aluminothermic reduction, or HF etching methods, the gap size between silicon particles and carbon shells is mostly between 10-50 nm, which is insufficient to alleviate the 300% volume expansion of silicon during cycling. This still leads to the cracking of the carbon layer on the surface during charging and discharging, reducing cycle stability and capacity retention.

[0056] Furthermore, the inventors discovered that materials synthesized using the Mg / Al thermal reduction method suffer from several drawbacks. The Mg / Al reduction reaction requires high temperatures and is exothermic, leading to a reaction between Si and C. Additionally, Mg / Al impurities are introduced during the preparation process and need to be removed in subsequent steps. Moreover, the Mg / Al thermal reduction reaction is energy-intensive, and the reactants are difficult to mix uniformly, resulting in incomplete SiO2 reduction and low silicon purity. Furthermore, the HF etching process is problematic because HF is a highly corrosive acid, and the reaction between HF and SiO2 during etching produces the highly toxic gas SiF4, posing challenges to the safety of the production equipment and processes. Therefore, a simpler and more efficient preparation process is needed to construct the porous structure between silicon and carbon.

[0057] Based on this, this application provides a silicon-carbon composite material, its preparation method, and an anode material, which can improve the pore structure of the silicon-carbon composite material to a certain extent through a simple, efficient, and safer method, thereby improving the cycle stability and capacity retention of the anode material. To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0058] Please see Figure 1 The method for preparing silicon-carbon composite materials provided in this application includes:

[0059] S1. The carbonizable resin, pore-forming agent, and silicon-containing particles are mixed in an organic solvent and stirred until homogeneous to obtain a mixed slurry. The pore-forming agent is selected from at least one of cyclohexanol, polyvinylpyrrolidone, or glycerol.

[0060] A homogeneous slurry can be obtained by mixing a carbonizable resin, a pore-forming agent, and silicon-containing particles in an organic solvent and stirring until homogeneous. This slurry can then be used to coat films.

[0061] After the pore-forming agent is removed, the carbonizable resin can form a porous resin membrane loaded with silicon particles. After subsequent carbonization treatment, a porous carbon membrane can be obtained.

[0062] This application does not limit the specific type of carbonizable resin, and relevant personnel can make the appropriate selection as needed.

[0063] In one possible embodiment, the carbonizable resin may be selected from at least one of polyacrylonitrile, polyvinyl alcohol, or lignin.

[0064] For example, the carbonizable resin can be selected from polyacrylonitrile. Using polyacrylonitrile, a more continuous and uniform porous carbon film with less impurities can be obtained after subsequent carbonization treatment.

[0065] The pore-forming agent is selected from at least one of cyclohexanol, polyvinylpyrrolidone, or glycerol, and can adsorb silicon-containing particles in the mixed slurry, so that the silicon-containing particles are located in the pores formed after the pore-forming agent is removed.

[0066] For example, the pore-forming agent may be selected from cyclohexanol; or, the pore-forming agent may be selected from polyvinylpyrrolidone; or, the pore-forming agent may be selected from glycerol.

[0067] Adding silicon-containing particles can improve the energy density of silicon-carbon composite membranes. After subsequent carbonization, the silicon-containing particles are located in the first pore of the porous carbon membrane, which provides expansion space for the silicon-containing particles.

[0068] This application does not limit the specific type of silicon-containing particles. In one possible embodiment, the silicon-containing particles may be nano-silicon powder.

[0069] Alternatively, in one possible embodiment, the silicon-containing particles may be carbon-coated nano-silicon powder.

[0070] Coating nano-sized silicon powder with a carbon layer can almost completely block direct contact between the silicon powder and the electrolyte, reduce the occurrence of side reactions, and also improve the conductivity of the silicon powder, thereby enhancing rate performance.

[0071] Alternatively, in one possible embodiment, the silicon-containing particles can be vertically graphene-coated nano-silicon powder.

[0072] Vertical graphene carbon coating on nano-sized silicon powder increases the contact sites between silicon particles and the film. At the same time, the vertical structure of graphene provides channels for lithium-ion transport, improving the rate performance of the material.

[0073] Furthermore, the particle size of the silicon-containing particles can be 50–300 nm.

[0074] For example, the particle size of the silicon-containing particles can be one of 50nm, 80nm, 100nm, 150nm, 200nm, 250nm or 300nm or any combination thereof.

[0075] Organic solvents are used to uniformly disperse carbonizable resins and pore-forming agents.

[0076] This application does not limit the specific type of organic solvent. In one possible embodiment, the organic solvent may be selected from at least one of N,N-dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide.

[0077] For example, the organic solvent may be selected from N,N-dimethylformamide; or, the organic solvent may be selected from N-methylpyrrolidone; or, the organic solvent may be selected from dimethyl sulfoxide.

[0078] To improve the dispersion uniformity of the mixed slurry, in one possible embodiment, the carbonizable resin, pore-forming agent and silicon-containing particles can be mixed in an organic solvent and stirred at 50°C for a period of time to obtain the mixed slurry.

[0079] Furthermore, this application does not limit the mass ratio between the carbonizable resin, the pore-forming agent, and the silicon-containing particles in the mixed slurry; the pore size in the porous carbon membrane can be adjusted by adjusting the amount of pore-forming agent added.

[0080] The mass ratio of carbonizable resin, pore-forming agent, and silicon-containing particles, as well as their amount in organic solvents, need to be controlled within appropriate ranges.

[0081] If the amount of pore-forming agent used is too small, such as polyvinylpyrrolidone (PVP), the porous composite membrane obtained after removing the pore-forming agent will have a small pore size, which may lead to a structure in which the carbonizable resin tightly encapsulates the silicon-containing particles. Due to the small pore size between the carbon walls and the silicon-containing particles, this type of material has a poor effect on mitigating the volume expansion of the silicon-containing particles during charging and discharging.

[0082] If a large amount of PVP is used, in addition to creating a porous structure that can alleviate the volume expansion of silicon particles, the high local concentration of PVP in the coating will also form many unfilled redundant porous structures. Such structures are detrimental to the overall mechanical properties of the membrane. Furthermore, the dense porous structure will increase the specific surface area of ​​the membrane, resulting in more irreversible capacity during the first charge-discharge process.

[0083] The amount of organic solvent used, such as N,N-dimethylformamide (DMF), directly affects the viscosity of the mixed slurry. When a large amount of DMF is used, the viscosity of the resulting mixed slurry is low. During phase separation, due to the rapid departure of the solvent, micron-sized finger-like macropores are easily formed in the membrane, and silicon-containing particles do not fill the macropores. Such a pore structure is detrimental to the overall performance of the membrane.

[0084] Carbonizable resins are organic materials that form the membrane skeleton. If the content of carbonizable resin is low, for example, if the content of polyacrylonitrile (PAN) is low, a discontinuous organic phase may be formed, which cannot form a film after subsequent carbonization treatment. If the content of PAN is high, it is not conducive to improving the lithium storage capacity of the composite material. This is because carbon mainly comes from the pre-oxidation and carbonization process of PAN, and silicon has a much higher capacity than carbon materials. The higher the carbon content, the lower the lithium storage capacity.

[0085] In one possible embodiment, the mass ratio of the carbonizable resin, the pore-forming agent, and the silicon-containing particles is 1:(0.2-1):(0.2-0.6).

[0086] For example, the mass ratio of carbonizable resin, pore-forming agent and silicon-containing particles can be 1:0.2:0.2; the mass ratio of carbonizable resin, pore-forming agent and silicon-containing particles can be 1:0.2:0.6; or, the mass ratio of carbonizable resin, pore-forming agent and silicon-containing particles can be 1:1:0.2; or, the mass ratio of carbonizable resin, pore-forming agent and silicon-containing particles can be 1:1:0.6.

[0087] For further information, please refer to [link / reference]. Figure 1 The method for preparing the silicon-carbon composite film provided in this application includes:

[0088] S2. Coat the mixed slurry to obtain the first preform.

[0089] Furthermore, the mixed slurry obtained in step S1 can be coated onto a substrate, such as a glass plate, and then the organic solvent can be removed to obtain a relatively stable first preform. The carbonizable resin, pore-forming agent, and silicon-containing particles in the first preform are relatively fixed in position, facilitating the formation of a continuous porous structure by the carbonizable resin after the pore-forming agent is removed. The silicon-containing particles are located within the pores of the porous structure due to the adsorption effect of the pore-forming agent. Moreover, removing the organic solvent avoids the possibility of excessive macropore formation caused by rapid removal of the organic solvent during the pore-forming agent removal process.

[0090] This application does not limit how to remove organic solvents. In one possible embodiment, organic solvents such as dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide are volatile. The coated mixed slurry can be left to stand for a certain period of time to allow the organic solvents to evaporate.

[0091] The settling time should not be too short. For example, the coated slurry can be settling for 10-30 minutes. When the settling time is too short, the first pre-formed membrane contains solvent that has not been removed. During the subsequent pore-forming process, the excess solvent will quickly precipitate out, producing finger-shaped macropores of tens of micrometers in size, which will reduce the mechanical properties of the membrane.

[0092] For example, the mixed slurry obtained in step S1 can be coated onto a glass plate with a scraper and left to stand for 15 minutes. Under the induction of the natural evaporation of the organic solvent, the polymer phase and the solvent phase in the mixed slurry will slowly separate to obtain the first preform.

[0093] Furthermore, this application does not limit the specific thickness of the coating; relevant personnel can make the appropriate selection based on the required film thickness.

[0094] In one possible embodiment, the coating thickness can be 10-50 μm.

[0095] For example, the thickness of the coating can be one of 10 μm, 20 μm, 30 μm, 40 μm or 50 μm or any combination thereof.

[0096] For further information, please refer to [link / reference]. Figure 1 The method for preparing silicon-carbon composite materials provided in this application includes:

[0097] S3. Remove the pore-forming agent from the first pre-formed membrane to obtain a porous composite membrane.

[0098] In the mixed slurry, silicon-containing particles are adsorbed onto the surface of the pore-forming agent. During the removal of the organic solvent, the silicon-containing particles are retained in the first preform. After the pore-forming agent in the first preform is removed, the silicon-containing particles are located within the pores of the carbonizable resin membrane.

[0099] This application does not limit how the pore-forming agent in the first preform can be removed. In one possible embodiment, the first preform can be placed in water so that the pore-forming agent in the first preform can be dissolved in the water.

[0100] Compared to carbonizable resins, pore-forming agents such as cyclohexanol, polyvinylpyrrolidone, or glycerol have superior water solubility. Therefore, when the first pre-formed membrane is placed in deionized water, PVP will gradually dissolve in the deionized water due to its superior water solubility compared to PAN. The PVP sites within the first pre-formed membrane will be replaced by water, resulting in a phase exchange. By drying the membrane to remove the water, a porous composite membrane with a porous structure can be obtained. Since PVP adsorbs silicon-containing particles, after removing the pore-forming agent to create pores, the silicon-containing particles will remain within the pore structure.

[0101] Furthermore, in order to improve the removal efficiency of the pore-forming agent, in one possible embodiment, the deionized water can be heated to 50-100°C.

[0102] For example, the temperature of deionized water can be one of 50°C, 60°C, 70°C, 80°C, 90°C or 100°C or any combination thereof.

[0103] The heat preservation process is the process of detaching the pore-forming agent PVP from the membrane to create pores. If the heat preservation time is too short, the PVP cannot detach completely, resulting in excess PVP particles in the pore structure; if the heat preservation time is too long, water absorption and bubbling will occur, affecting its mechanical properties. Therefore, it is necessary to appropriately adjust the heat preservation time of placing the first pre-formed membrane in deionized water.

[0104] In one possible embodiment, the heat preservation time can be 2 to 7 hours.

[0105] For example, the heat preservation time can be one of 2h, 3h, 4h, 5h, 6h or 7h or any range between two of them.

[0106] For further information, please refer to [link / reference]. Figure 1 In one possible embodiment, the method for preparing the silicon-carbon composite film includes:

[0107] S4. The porous composite membrane is carbonized under an inert gas atmosphere to obtain a silicon-carbon composite membrane.

[0108] Carbonization treatment of porous composite membranes can carbonize carbonizable resin materials to form a porous carbon skeleton, thereby obtaining silicon-carbon composite membranes.

[0109] To improve the continuity and mechanical properties of the porous carbon framework, in one possible embodiment, the porous composite membrane can be pre-oxidized in an air atmosphere, and the pre-oxidized porous composite membrane can be carbonized in an inert gas atmosphere.

[0110] Furthermore, the pre-oxidation treatment temperature is 200–400℃, the heating rate is 1–10℃ / min, and the holding time is 1–3h.

[0111] For example, the temperature of the pre-oxidation treatment can be one of 200°C, 250°C, 300°C, 350°C, or any combination thereof.

[0112] For example, the heating rate can be one or a range of any two of 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min.

[0113] For example, the heat preservation time can be one of 1 hour, 2 hours, or 3 hours, or any range between two of them.

[0114] If the pre-oxidation temperature is below 200℃, the pre-oxidation reaction may not be completed, affecting the film-forming performance of PAN in the subsequent carbonization process; if the pre-oxidation temperature is above 400℃, the organic molecular chains are easily oxidized and broken, and the decomposition of functional groups on the molecular chains may produce toxic gases.

[0115] Furthermore, the carbonization treatment temperature is 600–1100℃, the heating rate is 1–10℃ / min, and the holding time is 0.5–4h.

[0116] For example, the temperature of the pre-oxidation treatment can be one of 600°C, 700°C, 800°C, 900°C, 1000°C or 1100°C or any combination thereof.

[0117] For example, the heating rate can be one or a range of any two of 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min.

[0118] For example, the heat preservation time can be one of 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h or 4h or any two of them.

[0119] If the carbonization temperature is not higher than 600℃, non-carbon elements such as H and N cannot be completely removed and cannot be completely converted into carbon materials, resulting in impurities. If the carbonization temperature is higher than 1100℃ and the time is greater than 4 hours, PAN will be over-carbonized, affecting the mechanical properties of the porous carbon skeleton.

[0120] Furthermore, this application does not limit the specific type of inert gas. In one possible embodiment, the inert gas may be selected from at least one of nitrogen or argon.

[0121] Furthermore, through the above preparation method, this application provides a silicon-carbon composite membrane. The silicon-carbon composite membrane includes a porous carbon membrane and silicon-containing particles. The silicon-containing particles are located in the first pore of the porous carbon membrane, the pore size of the first pore is 0.5-2 μm, and the particle size of the silicon-containing particles is 50-300 nm.

[0122] The silicon-carbon composite membrane provided in this application example may contain macropores with larger particle sizes or micropores with smaller particle sizes in addition to the first pore.

[0123] The flexible self-supporting porous silicon-carbon composite membrane prepared by the above method has nanoscale silicon-containing particles filling the first pores of the micron-sized porous carbon membrane. The space reserved in the first pore can effectively alleviate the volume expansion of silicon-containing particles during the cycling process, thereby improving the cycling stability.

[0124] Furthermore, this application example provides a negative electrode material, including the aforementioned silicon-carbon composite film.

[0125] The silicon-carbon composite film prepared by the above method possesses flexible and self-supporting properties, and can be directly used as an electrode without the need for adding inactive materials such as binders and conductive agents. That is, it can reduce the mass of inactive materials, thereby increasing the battery energy density. The prepared film thickness can be reduced to 10 micrometers, achieving an areal capacity as high as 2.0 mAh / cm². 2 In comparison, for the same areal capacity, the thickness of a commercial graphite anode is approximately 35 micrometers. Therefore, using a silicon-carbon composite film as the anode can improve the volumetric energy density of LIBs.

[0126] Meanwhile, when the silicon-containing particles are nano-silicon powder composite materials coated with vertical graphene or carbon layers, the carbon or vertical graphene coating structure can provide a good electron and lithium-ion transport channel. When used as the negative electrode of lithium-ion batteries, it can improve the rate performance of LIBs.

[0127] Furthermore, the flexible silicon-carbon composite film electrode makes it possible for LIBs assembled from it to be used in wearable electronic devices, thereby contributing to the development of high-energy-density flexible wearable electronic devices. Moreover, the preparation process of the silicon-carbon composite film involves phase separation, pre-oxidation, and carbonization, which are simple to operate, have low energy consumption, do not use strong acids or alkalis, and are easy to scale up.

[0128] Furthermore, this application example also provides a lithium-ion battery including the aforementioned negative electrode material.

[0129] The lithium-ion battery provided in this application example has high energy density and long cycle life.

[0130] The silicon-carbon composite material of this application will be further described in detail below with reference to the embodiments.

[0131] Example 1

[0132] Example 1 provides a silicon-carbon composite film, the preparation method of which is as follows:

[0133] Step (1): First, mix 0.5g of nano-sized silicon powder and 0.5g of PVP in 10ml of DMF and stir at 50℃ for 2h to make the silicon particles uniformly dispersed in the solution; then add 1.5g of PAN in three portions and stir for 2h to obtain a mixed slurry.

[0134] Step (2): Apply the mixed slurry obtained in step (1) to the surface of a glass plate using a 100μm blade, let it stand for 10 minutes to remove the organic solvent, and obtain the first preform.

[0135] Step (3): Remove the first preform from the glass plate in step (2), place it in deionized water, boil it at 70°C for 7 hours to remove the pore-forming agent PVP in the first preform by dissolution, and obtain a porous composite membrane with a porous structure.

[0136] Step (4): The porous composite membrane obtained in step (3) is sandwiched between two corundum plates weighing 100g each, placed in a muffle furnace, heated to 230℃ at a heating rate of 1℃ / min and held for 80min for pre-oxidation; then the pre-oxidized membrane is transferred to a tube furnace, and held at 850℃ and 1050℃ for 45min at a heating rate of 5℃ / min under an argon atmosphere to obtain a silicon-carbon composite membrane with a flexible self-supporting porous structure.

[0137] After preparation, the structure and composition of the silicon-carbon composite film in Example 1 were characterized using SEM (Scanning Electron Microscope), Raman spectroscopy, XRD (X-ray Diffraction), XPS (X-ray Photoelectron Spectroscopy), and Thermogravimetric Analyzer (TGA). The characterization results are as follows: Figures 2-7 As shown.

[0138] from Figure 2 It can be seen that the thickness of the obtained silicon-carbon composite film is 15-20 μm, and irregular pores are uniformly distributed in the film; Figure 3 yes Figure 2 The magnified SEM images show that the obtained silicon-carbon composite membrane is composed of a porous carbon framework with a pore size of 0.5-1 μm and silicon particles with a size of 50-150 nm, and the nano-silicon particles are uniformly distributed in the pore structure.

[0139] from Figure 4 It can be seen that at a Raman displacement of 507 cm -1 The presence of distinct silicon characteristic peaks indicates the presence of silicon in the material. Furthermore, at 1326.5 cm⁻¹... -1 and 1593.1cm -1 The presence of typical D and G peaks found in carbon materials confirms the successful carbonization of PAN. Figure 5 It can be seen that the diffraction peaks at 28.6°, 47.8°, and 56.1° are attributed to the (111), (220), and (311) crystal planes of silicon, respectively, further demonstrating the presence of silicon material. From... Figure 6 and Figure 7 It can be seen that the sample is mainly composed of silicon, carbon, and oxygen, and a clear Si 2p XPS spectrum is displayed at 99.1 eV, further confirming the presence of silicon. The peak appearing at 102.2 eV is attributed to the surface oxidation of the nano-Si powder in the raw material or the surface oxidation of the nano-Si powder during the pre-oxidation process. Figure 8 The TGA curve shows that the mass fraction of Si in the material is about 40%.

[0140] Example 2

[0141] Example 2 provides a silicon-carbon composite film, which differs from Example 1 in that:

[0142] In step (1), the organic solvent is N-methylpyrrolidone.

[0143] from Figure 9 It can be seen that Example 2 has a hole structure similar to that of Example 1.

[0144] Example 3

[0145] Example 3 provides a silicon-carbon composite film, which differs from Example 1 in that:

[0146] In step (1), the pore-forming agent is cyclohexanol.

[0147] from Figure 10 It can be seen that Example 3 has a similar hole structure to Example 1.

[0148] Example 4

[0149] Example 4 provides a silicon-carbon composite film, which differs from Example 1 in that:

[0150] In step (1), the mass ratio of PAN:PVP:vertical graphene-coated nano-silicon powder is 1:0.2:0.4.

[0151] After preparation, the cross-section of the porous silicon-carbon composite thin film material in this embodiment was characterized using SEM, and the results are as follows. Figure 11 As shown, when silicon powder is replaced by vertically graphene-coated nano-silicon powder with a particle size of 100-200 nm, the carbon nanosheets on the vertically graphene-coated nano-silicon powder will form good contact with the pore walls, enhancing charge transport. At the same time, there is a certain space between the pores and the vertically graphene composite silicon powder, which can help alleviate volume expansion. Figure 12 The images show actual materials, where (A) represents the silicon-carbon composite film before bending, and (B) represents the silicon-carbon composite film during bending. Figure 12 It can be seen that the obtained silicon-carbon composite film can be bent at large angles and has good flexibility.

[0152] Example 5

[0153] Example 5 provides a silicon-carbon composite film, which differs from Example 1 in that the mass ratio of PAN:PVP:nanosilicon powder is 1:0.1:0.3.

[0154] After preparation, the cross-section of the silicon-carbon composite film of Example 5 was characterized using SEM, and the results are as follows. Figure 13 As shown. By Figure 13 It can be seen that nanoscale silicon particles are uniformly distributed in the pore structure with a diameter of 500-600 nm. In addition, after adding a small amount of PVP, some incompletely formed pores of 5-10 nm are also formed in the porous carbon film.

[0155] Example 6

[0156] Example 6 provides a silicon-carbon composite film, the main difference of which is that the mass ratio of PAN:PVP:nanosilicon powder is 1:0.6:0.3 compared with Example 1.

[0157] After preparation, the cross-section of the porous silicon-carbon composite thin film material in this comparative example was characterized using SEM, and the results are as follows. Figure 14 As shown. From Figure 14 It can be seen that macropores of 0.5-1 μm have formed in the silicon-carbon composite film, with silicon particles encapsulated within these macropores. Furthermore, compared to… Figure 3 It also forms numerous micropores of 50-100 nm, resulting in a high pore density. Although macropores capable of accommodating nano-silicon particles have formed in the silicon-carbon composite film, there are also many nanoscale hollow pores, which are attributed to the local aggregation of excessive PVP in the polymer phase.

[0158] Example 7

[0159] Example 7 provides a silicon-carbon composite film. The main difference between the preparation method of Example 1 and that of Example 1 is that: in step (2), the mixed slurry obtained in step (1) is coated on the surface of a glass plate with a doctor blade of size 100 μm, and the organic solvent is not removed by standing. Step (3) is carried out directly.

[0160] After preparation, the cross-section of the silicon-carbon composite film in Example 7 was characterized using SEM, and the results are as follows: Figure 15 and Figure 16 As shown.

[0161] from Figure 15 It can be seen that the porous carbon framework exhibits an asymmetrical pore structure on both sides, which is consistent with... Figure 2 In contrast, finger-shaped macropores of 30-40 μm appeared in the middle of the membrane, which was caused by the rapid detachment of the solvent phase DMF during the phase separation process of the pore-forming agent. Figure 16 yes Figure 15 The magnified SEM image of a portion of the carbon framework shows that silicon particles are uniformly distributed in the porous structure formed by PVP.

[0162] Comparative Example 1

[0163] Comparative Example 1 provides a silicon-carbon composite membrane. The main difference between its preparation method and Example 1 is that step (3) is not performed, and the first preform obtained in step (2) is directly subjected to step (4).

[0164] After preparation, the cross-section of the silicon-carbon composite film in Comparative Example 2 was characterized using SEM, and the results are as follows: Figure 17 As shown.

[0165] from Figure 17 It can be seen that, with Figure 1 In contrast, the silicon-carbon composite membrane provided in Comparative Example 2 did not form a porous structure.

[0166] Test case

[0167] Lithium-ion battery anodes were prepared using the silicon-carbon composite film materials obtained in Examples 1, 3, and 4, respectively, and the performance of the lithium-ion battery anodes was tested.

[0168] The specific preparation process of the lithium battery anode is as follows:

[0169] The silicon-carbon composite films obtained in Examples 1, 3, and 4 were cut into 14mm electrode sheets, and then assembled into batteries for testing.

[0170] Electrochemical performance testing of lithium battery anodes: The performance of lithium battery anodes made from the materials obtained in Examples 1, 3, and 4 was tested using a constant current charge-discharge method. The test results for Example 1 are as follows: Figures 18-20 As shown.

[0171] from Figure 18 The first charge-discharge curves show that the reversible capacity of the negative electrode of the battery corresponding to Example 1 after the first charge-discharge is 1164.5 mAh / g, and the first charge-discharge efficiency is 86.1%.

[0172] from Figure 19 The cycling curves show that at a current density of 0.2 A / g, after 100 cycles, the reversible capacity of the negative electrode of the battery in Example 1 is as high as 1028.9 mAh / g, with a capacity retention rate of 99.6%.

[0173] from Figure 20 The rate curves show that at current densities of 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, 2 A / g, 5 A / g, and 10 A / g, the capacities obtained by the battery anode are 1127.7 mAh / g, 1066.0 mAh / g, 934.4 mAh / g, 771.8 mAh / g, 588.2 mAh / g, 304.9 mAh / g, and 93.1 mAh / g, respectively. Compared to the capacity obtained at 0.1 A / g, the capacity retention rate at 10 A / g is 8.2%, and when the current density returns to 0.1 A / g, its reversible capacity is still as high as 1100.9 mAh / g.

[0174] Depend on Figures 18-20 It can be seen that the battery anode using the silicon-carbon composite film provided in Example 1 has high lithium storage capacity, initial coulombic efficiency, cycle life and rate performance.

[0175] from Figure 21 As can be seen, the lithium battery anode prepared with the silicon-carbon composite film provided in Example 3 exhibits capacities of 824.1 mAh / g, 777.3 mAh / g, 657.5 mAh / g, 517.5 mAh / g, 347.4 mAh / g, 193.6 mAh / g, and 105.0 mAh / g at current densities of 0.1 A / g, 10 A / g, and 105.0 mAh / g, respectively. The capacity retention rate at 10 A / g is 11.9% compared to the capacity obtained at 0.1 A / g, which is higher than the 8.2% retention rate of the porous silicon-carbon composite film material with pure silicon particles as the silicon material in Example 1. This indicates that introducing vertical graphene coating can improve the rate performance of the battery anode.

[0176] from Figure 22It can be seen that the lithium battery anode prepared with the silicon-carbon composite film provided in Example 4 has a reversible capacity of 1218.4 mAh / g after the first charge-discharge cycle, and an initial charge-discharge efficiency of 81.3%. The initial charge-discharge efficiency is lower than that in Example 1, indicating that the excess pore structure leads to the formation of more solid-phase electrolyte interface layers during the first charge-discharge process, which will generate more irreversible capacity and affect the initial efficiency.

[0177] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a silicon-carbon composite membrane, characterized in that, include: Carbonizable resin, pore-forming agent and silicon-containing particles are mixed in an organic solvent and stirred evenly to obtain a mixed slurry. The mixed slurry is coated, and the organic solvent is removed after coating to obtain a first pre-film. The first pre-film is placed in water at a temperature of 50-100°C and kept at that temperature for 2-7 hours to dissolve the pore-forming agent in the first pre-film. The pore-forming agent in the first pre-film is then removed to obtain a porous composite film. The porous composite film is subjected to pre-oxidation treatment in an air atmosphere at a temperature of 200-400°C. The pre-oxidized porous composite film is subjected to carbonization treatment in an inert gas atmosphere to obtain the silicon-carbon composite film. The pore-forming agent is selected from at least one of cyclohexanol, polyvinylpyrrolidone, or glycerol. The silicon-carbon composite film includes a porous carbon film and silicon-containing particles. The porous carbon film contains a first pore with a pore size of 0.5-2 μm, and the silicon-containing particles have a particle size of 50-300 nm. The silicon-containing particles are located within the first pore. The volume of the first pore accounts for more than 60% of the total pore volume of the porous carbon film.

2. The preparation method according to claim 1, characterized in that, The silicon-containing particles are selected from at least one of nano-silicon powder, carbon-coated nano-silicon powder, or vertically graphene-coated nano-silicon powder.

3. The preparation method according to claim 1, characterized in that, The silicon-containing particles account for 40-60% of the total weight of the silicon-carbon composite film.

4. The preparation method according to claim 1, characterized in that, The thickness of the silicon-carbon composite membrane is 10-50 μm.

5. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, The water is selected from deionized water.

6. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, The heating rate of the pre-oxidation treatment is 1~10 °C / min, and the holding time is 1~3 h.

7. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, The carbonization treatment is carried out at a temperature of 600~1100 °C, a heating rate of 1~10 °C / min, and a holding time of 0.5~4 h.

8. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, The inert gas is selected from at least one of nitrogen or argon.

9. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, The organic solvent is selected from at least one of N,N-dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide.

10. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, The method for removing the organic solvent after coating includes: allowing the coated mixed slurry to stand for 10-30 minutes to allow the organic solvent to evaporate.

11. The method for preparing the silicon-carbon composite film according to claim 1, characterized in that, In the mixed slurry, the mass ratio of the carbonizable resin, the pore-forming agent, and the silicon-containing particles is 1:(0.2-1):(0.2-0.6).

12. The method for preparing the silicon-carbon composite film according to claim 11, characterized in that, The carbonizable resin is selected from at least one of polyacrylonitrile, polyvinyl alcohol, or lignin.

13. The method for preparing the silicon-carbon composite film according to claim 11, characterized in that, The carbonizable resin is selected from polyacrylonitrile.

14. A silicon-carbon composite membrane prepared by the preparation method according to any one of claims 1 to 13.

15. A negative electrode material, characterized in that, The negative electrode material includes the silicon-carbon composite film as described in claim 14.

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