Silicon-carbon composite material, method for preparing the same, and use thereof

Abstract

The application discloses a silicon-carbon composite material and a preparation method and application thereof. In a first aspect of the application, a silicon-carbon composite material is provided, which comprises carbon-silicon composite particles and a second coating layer. The carbon-silicon composite particles comprise silicon particles and a first coating layer covering the silicon particles, and the first coating layer comprises elastic carbon. The carbon-silicon composite particles have a deformation recovery S≥80% after load removal. The second coating layer covers the carbon-silicon composite particles, and the second coating layer comprises a porous carbon framework. The silicon-carbon composite material adopts a three-layer composite structure, wherein the first coating layer serves as a buffer layer and has high elasticity and high conductivity, and the second coating layer serves as a protective layer and has high hardness, high conductivity and high specific surface area. In this way, the material performance of silicon can be improved, and the integrity of the silicon particles in the process and working process is ensured, and breakage is effectively prevented.

Description

Technical Field

[0001] This application relates to the field of battery anode material technology, and in particular to silicon-carbon composite materials, their preparation methods and applications. Background Technology

[0002] With increasing awareness of low-carbon and environmental protection, the new energy industry has become an important development direction in the energy sector, with continuous advancements in new energy technologies and further expansion of the industry scale. Currently, whether for conventional consumer batteries or the increasingly demanding power batteries, the entire industry is placing higher requirements on battery energy density. The anode material is directly related to the battery's energy density, but the actual capacity of conventional graphite anodes is approaching their theoretical limit, failing to meet current demands. Compared to graphite, silicon's theoretical specific capacity can reach 3579 mAh / g, more than ten times that of graphite, making it a highly promising anode material.

[0003] Due to factors such as lithium intercalation capacity and phase transition, the volume expansion of silicon materials has become a significant problem for researchers. This substantial volume change can cause silicon particles to break down, leading to a rapid decline in the reversible capacity of the battery and affecting its lifespan. Nanostructuring is a major approach to addressing the volume expansion of silicon materials, but nanomaterials with large specific surface areas are prone to aggregation, hindering performance. Furthermore, attempts have been made to combine silicon with graphite or other carbon materials to improve silicon performance, but these methods often struggle to support the long-term expansion and contraction of silicon, resulting in limited improvement in cycle performance. Moreover, these methods often focus on the selection of raw materials; the design of material structures to prevent silicon particle breakage remains unresolved. Summary of the Invention

[0004] This application aims to address at least one of the technical problems existing in the prior art. To this end, this application proposes a silicon-carbon composite material, its preparation method, and its application. This silicon-carbon composite material employs a novel structural design that can effectively solve the problem of silicon particle breakage.

[0005] A first aspect of this application provides a silicon-carbon composite material, the silicon-carbon composite material comprising:

[0006] The carbon-silicon composite particles include silicon particles and a first coating layer covering the silicon particles, the first coating layer including elastic carbon, and the deformation recovery S of the carbon-silicon composite particles after removing the load is ≥80%.

[0007] A second coating layer is applied to the carbon-silicon composite particles, and the second coating layer includes a porous carbon framework.

[0008] The deformation recovery of the carbon-silicon composite particles was detected by nanoindentation, and any nanoindenter can be selected for the detection.

[0009] In some embodiments of the present application, the silicon particles are at least one of nano-silicon particles and micro-silicon particles.

[0010] In some embodiments of the present application, the particle size of the silicon particles is 0.1 nm to 500 μm, for example, 0.1 nm, 0.2 nm, 0.5 nm, 1 nm, 2 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 200 nm, 500 nm, 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, 500 μm. In some embodiments, the particle size of the silicon particles is 1 nm to 100 μm, 2 nm to 10 μm, 5 nm to 500 nm, 5 nm to 300 nm, 10 nm to 200 nm, 100 nm to 10 μm.

[0011] In some embodiments of the present application, the thickness H1 of the first coating layer is 20 to 40 nm, for example, it can be 20 nm, 25 nm, 30 nm, 35 nm, 40 nm.

[0012] In some embodiments of the present application, the thickness H2 of the second coating layer is 100 to 200 nm, for example, it can be 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm.

[0013] In some embodiments of the present application, the silicon particles are doped SiC x (0 < x ≤ 1) Si particles.

[0014] In some embodiments of the present application, the elastic modulus of the first coating layer is T1, the elastic modulus of the second coating layer is T2, the self-expansion rate Z of the silicon particles ≥ 300%, and the silicon-carbon composite material satisfies the following relationship: In the relationship, H1 and H2 are numerical values omitting the unit of measurement in nm, T1 and T2 are numerical values omitting the unit of measurement in GPa, and Z is the actual value in percentage.

[0015] Among them, the measurement methods of the elastic modulus T1 and the elastic modulus T2 of the first coating layer and the second coating layer are to measure the surface elastic modulus of the silicon-carbon composite particles and the silicon-carbon composite material respectively by nanoindentation method, which are the elastic moduli of the first coating layer and the second coating layer.

[0016] By observing the formed silicon-carbon composite material with a high-resolution transmission electron microscope, it can be seen that there are obvious boundaries between the silicon particles, the first coating layer and the second coating layer. Therefore, the thickness H1 of the first coating layer and the thickness H2 of the second coating layer can be directly measured by a high-resolution transmission electron microscope.

[0017] The self-expansion rate Z of silicon particles is measured as follows:

[0018] In the preparation and testing of the pouch cell in the reference embodiment, pure silicon was used instead of silicon-carbon composite material to prepare the same specification battery. The battery was disassembled in a glove box after 300 cycles at 25°C. The negative electrode was cleaned with diethyl carbonate (DEC) and the thickness of the electrode was measured. Self-expansion rate = (electrode thickness after cycling - electrode thickness before cycling) / electrode thickness before cycling × 100%.

[0019] In some embodiments of this application, Z is 300%, as in Li Chunxiao's "Research Progress of Lithium-ion Battery Materials", where silicon materials expand significantly during charging / discharging, with a volume change of up to 300%.

[0020] In some embodiments of this application, the elastic modulus T1 of the first coating layer is 40 to 80 GPa, for example, it can be 40 GPa, 50 GPa, 60 GPa, 70 GPa, or 80 GPa.

[0021] In some embodiments of this application, the elastic modulus T2 of the second coating layer is 8 to 15 GPa, for example, it can be 8 GPa, 9 GPa, 10 GPa, 11 GPa, 12 GPa, 13 GPa, 14 GPa, or 15 GPa.

[0022] The silicon-carbon composite material according to the embodiments of this application has at least the following beneficial effects:

[0023] The silicon-carbon composite material in this embodiment employs a three-layer composite structure. The first coating layer serves as a buffer layer, possessing both high elasticity and high conductivity, while the second coating layer acts as a protective layer, exhibiting high hardness, high conductivity, and high specific surface area. This not only improves the material properties of silicon but also ensures the integrity of the silicon particles during processing and operation, effectively preventing breakage.

[0024] Specifically, the elastic carbon in the first coating layer has high elasticity and high Li content. +The high diffusion coefficient and elasticity provide a certain degree of buffering, protecting the silicon particles and ensuring stable contact between the silicon and carbon layers. This maintains a stable electrode / electrolyte interface during long-term cycling, preventing excessive SEI growth. In the second coating layer, the carbon framework possesses high hardness and stability, providing structural support and preventing material breakage during processing. The porous structure increases the specific surface area and improves ionic conductivity. During rolling, the outermost porous carbon framework is stressed first; if it breaks due to excessive stress, the highly elastic carbon layer acts as a protective layer, providing buffering capacity to prevent silicon core breakage and also restraining silicon expansion during the reaction. By constructing two protective layers around the silicon particle core in this way to prevent breakage during processing, the resulting silicon-carbon composite material not only possesses excellent electrochemical performance but also exhibits better cycling performance due to particle integrity.

[0025] A second aspect of this application provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0026] The first carbon source is coated on the surface of silicon particles and carbonized to obtain carbon-silicon composite particles.

[0027] The silicon-carbon composite particles are mixed with a second carbon source and a pore-forming agent, freeze-dried, and calcined to form a second coating layer, thereby obtaining the silicon-carbon composite material.

[0028] In some embodiments of this application, coating the surface of the silicon particles with the first carbon source includes:

[0029] A coating solution is obtained by mixing a first carbon source with a first solvent.

[0030] The silicon particles are mixed and stirred with the coating solution. After the first solvent evaporates, the first carbon source is coated on the surface of the silicon particles.

[0031] In some embodiments of this application, the mass-to-volume ratio of the first carbon source to the first solvent is (0.1–0.5) g: 15 mL. For example, 0.1 g, 0.15 g, 0.2 g, 0.25 g, 0.3 g, 0.35 g, 0.4 g, 0.45 g, or 0.5 g of the first carbon source can be mixed with every 15 mL of the first solvent. Decreasing the proportion of the first carbon source and reducing the proportion of carbon material will degrade performance; increasing the proportion of the first carbon source and increasing the amount of carbon material will cause elastic failure, forming a hardened carbon framework that hinders the performance of silicon particles.

[0032] In some embodiments of this application, silicon particles are mixed and stirred with the coating solution for 8 to 16 hours.

[0033] In some embodiments of this application, the mass-to-volume ratio of the first carbon source to the first solvent is approximately 0.2 g: 15 mL.

[0034] In some embodiments of this application, the first solvent includes toluene.

[0035] In some embodiments of this application, the first carbon source is pitch.

[0036] In some embodiments of this application, the asphalt is selected from at least one of petroleum asphalt, coal tar pitch, and natural asphalt.

[0037] In some embodiments of this application, the mass ratio of the first carbon source to silicon particles is (1-9):3, for example, it can be 1:3, 2:3, 1:1, 4:3, 5:3, 2:1, 7:3, 8:3, or 3:1.

[0038] In some embodiments of this application, the carbonization temperature is 500 to 1000°C, for example, 500°C, 600°C, 700°C, 800°C, 900°C, or 1000°C.

[0039] In some embodiments of this application, the carbonization time is 15 min to 4 h, for example, it can be 15 min, 30 min, 45 min, 1 h, 2 h, 3 h, or 4 h.

[0040] In some embodiments of this application, the mass ratio of the second carbon source to the pore-forming agent is 1:(0.1 to 0.5), for example, it can be 1:0.1, 1:0.2, 1:0.3, 1:0.4, or 1:0.5.

[0041] In some embodiments of this application, the second carbon source includes at least one of polyvinylpyrrolidone, polyvinyl alcohol, sucrose, glucose, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyethylene glycol.

[0042] In some embodiments of this application, the pore-forming agent includes at least one of NaCl, KCl, urea, and ammonium bicarbonate.

[0043] In some embodiments of this application, the pore-forming agent includes NaCl and urea.

[0044] In some embodiments of this application, the freeze-drying temperature is -90 to -20°C, for example, it can be -90°C, -80°C, -70°C, -60°C, -50°C, -40°C, -30°C, or -20°C.

[0045] In some embodiments of this application, freeze drying includes freezing at the aforementioned temperature for 8 to 24 hours, followed by vacuum drying.

[0046] In some embodiments of this application, the vacuum drying time is 24 to 72 hours.

[0047] In some embodiments of the present application, the calcination temperature is 500 - 800 °C, for example, it can be 500 °C, 600 °C, 700 °C, 800 °C.

[0048] In some embodiments of the present application, the calcination time is 3 - 12 h.

[0049] In some embodiments of the present application, the silicon-carbon composite material prepared by the above preparation method includes a multi-layer structure. The multi-layer structure includes, from the inside out, silicon particles, a first coating layer, and a second coating layer. The first coating layer includes elastic carbon, and the second coating layer includes a porous carbon framework.

[0050] In some embodiments of the present application, the silicon particles are at least one of nano-silicon particles and micro-silicon particles.

[0051] In some embodiments of the present application, the particle size of the silicon particles is 0.1 nm - 500 μm, for example, it is 0.1 nm, 0.2 nm, 0.5 nm, 1 nm, 2 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 200 nm, 500 nm, 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, 500 μm. In some embodiments, the particle size of the silicon particles is 1 nm - 100 μm, 2 nm - 10 μm, 5 nm - 500 nm, 5 nm - 300 nm, 10 nm - 200 nm, 100 nm - 10 μm.

[0052] In some embodiments of the present application, the thickness H1 of the first coating layer is 20 - 40 nm, for example, it can be 20 nm, 25 nm, 30 nm, 35 nm, 40 nm.

[0053] In some embodiments of the present application, the thickness H2 of the second coating layer is 100 - 200 nm, for example, it can be 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm.

[0054] In some embodiments of the present application, the silicon particles are doped SiC x (0 < x ≤ 1) Si particles.

[0055] In some embodiments of the present application, the elastic modulus of the first coating layer is T1, the elastic modulus of the second coating layer is T2, the self-expansion rate Z of the silicon particles ≥ 300%, and the silicon-carbon composite material satisfies the following relationship: In the relationship, H1 and H2 are numerical values omitting the unit of measurement in nm, T1 and T2 are numerical values omitting the unit of measurement in GPa, and Z is the actual value in percentage.

[0056] In some embodiments of the present application, the elastic modulus T1 of the first coating layer is 40 to 80 GPa, and for example, it can be 40 GPa, 50 GPa, 60 GPa, 70 GPa, 80 GPa.

[0057] In some embodiments of the present application, the elastic modulus T2 of the second coating layer is 8 to 15 GPa, and for example, it can be 8 GPa, 9 GPa, 10 GPa, 11 GPa, 12 GPa, 13 GPa, 14 GPa, 15 GPa.

[0058] It can be understood that during the carbonization process of forming the first coating layer, some of the elastic carbon will further react with the silicon particles to generate SiC x (0 < x ≤ 1), therefore, the internal silicon particles are actually Si particles doped with SiC x particles.

[0059] The preparation method of the silicon-carbon composite material according to the embodiment of the present application has at least the following beneficial effects:

[0060] The carbon framework precursor is prepared by freeze-drying. Compared with evaporation, this method is carried out in a low-temperature environment, avoiding over-baking that affects the quality of substances, so that the humidity of the material can be better controlled, and the occurrence of side reactions is prevented, reducing impurities in the precursor. In this way, many properties including cycle performance can be effectively improved, especially the cycle performance in long cycles.

[0061] The finally prepared silicon-carbon composite material has a three-layer composite structure. The first coating layer serves as a buffer layer, having both high elasticity and high conductivity. The second coating layer serves as a protective layer and simultaneously has high hardness, high conductivity and high specific surface area. This can not only improve the material properties of the silicon particles as the innermost core, but also the two coating layers ensure the integrity of the silicon particles during the process and working process, effectively preventing breakage. During roll pressing, the outermost porous carbon framework is stressed first. If it breaks due to excessive stress, the high-elasticity carbon layer as a protective layer will provide a certain buffering ability to prevent the Si core from breaking, and can also restrain the expansion of Si during the reaction.

[0062] In the third aspect of the present application, a secondary battery is provided, and the secondary battery includes the aforementioned silicon-carbon composite material, or the silicon-carbon composite material prepared by the aforementioned preparation method.

[0063] In some embodiments of the present application, the secondary battery includes a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode.

[0064] The negative electrode includes a negative current collector and a negative active layer on the negative current collector. The negative active layer includes a positive active material, which includes any of the aforementioned silicon-carbon composite materials. The positive electrode includes a positive current collector and a positive active layer on the positive current collector. The positive active layer includes a positive active material, such as at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium nickel manganese aluminum oxide.

[0065] In some embodiments of this application, the secondary battery includes a positive electrode, a negative electrode, a separator located between the positive and negative electrodes, and an electrolyte.

[0066] In some embodiments of this application, the electrolyte is a solid electrolyte or an electrolyte solution.

[0067] In some embodiments of this application, the positive / negative active layer further includes at least one of a conductive agent and a binder. The conductive agent includes, but is not limited to, at least one of graphite, acetylene black, carbon black, carbon nanotubes, and carbon fibers. The binder includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyurethane (PU), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0068] In some embodiments of this application, the positive electrode active layer / negative electrode active layer comprises 70–99 wt% positive electrode active material / negative electrode active material, 0.5–6 wt% conductive agent, and 0.5–20 wt% binder. In some embodiments, the mass fraction of the positive electrode active material in the positive electrode active layer / negative electrode active layer is 80–99 wt%, 90–99 wt%, or 95–99 wt%; the mass fraction of the conductive agent in the positive electrode active layer / negative electrode active layer is 1–6 wt% or 2–5 wt%; and the mass fraction of the binder in the positive electrode active layer / negative electrode active layer is 1–10 wt% or 1–5 wt%.

[0069] In some embodiments of this application, when the positive / negative active material, conductive agent, and binder are used to form the positive / negative active layer, the process includes dispersing them in a solvent, coating them onto the positive / negative current collector, and drying them to obtain the positive / negative active layer. In some embodiments, the solvent may be N-methylpyrrolidone (NMP) or other optional solvents.

[0070] In some embodiments of this application, the positive electrode current collector / negative electrode current collector includes at least one of the following: metal foil (such as aluminum foil, silver foil, tin foil, iron foil, titanium foil, nickel foil, copper foil, or alloy foil of the above metals) and metal mesh (such as aluminum mesh, silver mesh, tin mesh, iron mesh, titanium mesh, nickel mesh, copper mesh, or alloy mesh of the above metals).

[0071] In some embodiments of this application, the diaphragm includes, but is not limited to, a single-layer or multi-layer film of one or more materials selected from polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF).

[0072] In some embodiments of this application, the secondary battery includes a bare cell, which is formed from a positive electrode, a negative electrode, and a separator through at least one of the following processes: winding, stacking, etc.

[0073] In some embodiments of this application, the secondary battery includes a bare cell and a casing that encloses the bare cell.

[0074] In some embodiments of this application, the outer shell is a soft shell or a hard shell.

[0075] In some embodiments of this application, the secondary battery is obtained by injecting electrolyte into a casing that encloses the bare battery cell and then sealing it.

[0076] A fourth aspect of this application provides an electrical device that includes the aforementioned secondary battery.

[0077] Electrical equipment refers to any device that can utilize electrical energy and convert it into mechanical energy, thermal energy, light energy, or one or more other forms of energy, such as electric motors, electric heaters, and electric light sources. This includes, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, and energy storage systems. Mobile devices can include mobile phones, laptops, drones, robot vacuum cleaners, and e-cigarettes; electric vehicles can include pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, and electric trucks.

[0078] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0079] Figure 1 This is a cross-sectional view of the silicon-carbon composite material in the embodiments of this application.

[0080] Reference numerals: silicon particle 100, first coating layer 200, second coating layer 300. Detailed Implementation

[0081] The following will clearly and completely describe the concept and technical effects of this application in conjunction with embodiments, so as to fully understand the purpose, features and effects of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are all within the scope of protection of this application.

[0082] The embodiments of this application are described in detail below. The described embodiments are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0083] In the description of this application, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number, and "approximately" means within the range of ±20%, 10%, 8%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.2%, 0.1%, etc. of the stated number. The use of "first" and "second" is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0084] In the description of this application, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0085] The following examples will further illustrate this application.

[0086] Example 1

[0087] This embodiment provides a silicon-carbon composite material, referenced... Figure 1 The image shows a multilayer structure of a silicon-carbon composite material in an embodiment of this application, which includes, from the inside out, an innermost silicon particle 100, a first coating layer 200 formed by elastic carbon on the surface of the silicon particle 100, the two of which constitute a silicon-carbon composite particle, and a second coating layer 300 formed by a porous carbon framework on the surface of the first coating layer 200 of the silicon-carbon composite particle.

[0088] The preparation method of the silicon-carbon composite material in this embodiment is as follows:

[0089] (1) Dissolve 0.2g of asphalt in 15mL of toluene to form an asphalt coating solution.

[0090] (2) Disperse 0.6g of nano-silicon particles in the coating solution and stir for 12h. Wait for the toluene to evaporate so that the asphalt is coated on the surface of the nano-silicon particles.

[0091] (3) The nano-silicon particles coated with asphalt were carbonized at 800°C for 1 hour in an argon atmosphere to obtain silicon-carbon composite particles Si / C1.

[0092] (4) Polyvinylpyrrolidone (PVP), urea, NaCl and Si / C1 were added to deionized water in a mass ratio of 1:1:1:2 and stirred for 2 hours. Then, the mixture was frozen at -65°C for 12 hours in a freeze dryer until it was completely solid. After that, it was vacuum dried at -65°C for 24 hours to prepare the precursor of Si / C1 / C2.

[0093] (5) Argon gas was introduced into the tube furnace as a protective gas, and the obtained Si / C1 / C2 precursor was calcined at 600℃ for 6h.

[0094] (6) The calcined product was washed three times with deionized water and then dried in a drying oven at 45°C for 12 hours to obtain the silicon-carbon composite material Si / C1 / C2.

[0095] In the final product, the silicon-carbon composite material Si / C1 / C2, the thickness of the first coating layer is 30 nm, and the thickness of the second coating layer is 150 nm.

[0096] The elastic modulus of the first and second coating layers was characterized by measuring the elastic modulus of individual particles in silicon-carbon composite particles (Si / C1) and silicon-carbon composite materials (Si / C1 / C2). Specifically, a nanoindenter (Hysitron TI 950) was used for testing according to JB / T 12721-2016. Before testing, the particle powder was dispersed in epoxy resin and cured. The cured resin was cut using ion polishing. The nanoindenter's nanoprobe was used to apply pressure to individual particles, and the indentation depth on the particle surface was monitored. The strength of the individual particle was calculated, and the degree of deformation recovery after removing the load (nanopertor) was measured. The elastic modulus of five particles from the same sample were tested in parallel, and the average value was taken.

[0097] In this embodiment, the Si / C1 silicon-carbon composite particles recover 89.4% of their deformation after the load is removed. The elastic modulus T1 of the first coating layer is 60 GPa, and the elastic modulus T2 of the second coating layer is 10 GPa.

[0098] Examples 2, 3, 6, and 7

[0099] Examples 2, 3, 6, and 7 provide a silicon-carbon composite material, which is obtained by adjusting the amount of different raw materials, with some parameters of the product shown in Table 1.

[0100] Examples 4, 5, 8, and 9

[0101] Examples 4, 5, 8, and 9 provide a silicon-carbon composite material. Referring to Examples 1, Examples 4 and 5 employ depressurization and pressurization treatments respectively during carbonization in step (3), and Examples 8 and 9 employ depressurization and pressurization treatments respectively during calcination in step (5).

[0102] Comparative Example 1

[0103] This comparative example provides a silicon-carbon composite material, which differs from Example 1 in that it does not have a first coating layer. The preparation process is the same as in Example 1, but steps (1) to (3) are omitted, and the silicon-carbon composite material Si / C2 is finally obtained.

[0104] Comparative Example 2

[0105] This comparative example provides a silicon-carbon composite material, which differs from Example 1 in that it does not have a second coating layer. The preparation process is the same as in Example 1, but step (4) is omitted, and the silicon-carbon composite material Si / C1 is finally obtained.

[0106] Comparative Examples 3-6

[0107] Comparative Examples 3-6 each provide a silicon-carbon composite material, which differs from Example 1 in that it is obtained by adjusting the amount of different raw materials and the preparation process. Some parameters of the product are shown in Table 1.

[0108] Table 1. Some parameters of the examples and comparative examples

[0109]

[0110] Performance testing

[0111] The batteries prepared in the examples and comparative examples were subjected to relevant performance tests, as detailed below:

[0112] LiNiCoMnO2, Super P and PVDF were added to N-methylpyrrolidone (NMP) in a mass ratio of 97:1.7:1.3 and mixed evenly. The mixture was then coated onto a current collector copper foil, dried, cold-pressed, trimmed, cut into sheets, slit, and dried a second time to obtain the positive electrode sheet.

[0113] The silicon-carbon composite material, CNT, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber of the examples or comparative examples were added to NMP in a mass ratio of 96:1.3:1.2:1.5 and mixed evenly. The mixture was then coated onto the current collector aluminum foil, dried, trimmed, cut into sheets, slit, and dried a second time for 12 hours to obtain the negative electrode sheet.

[0114] The prepared positive electrode sheet, PE separator and negative electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrode sheets, and wound to obtain a bare cell. The bare cell is placed in an aluminum-plastic film outer packaging, and the electrolyte (1M lithium hexafluorophosphate, ethylene carbonate and dimethyl carbonate volume ratio 1:1) is injected into the dried battery. The battery is then encapsulated, left to stand, formed, shaped and capacity tested to prepare a soft-pack battery.

[0115] In a 25℃ environment, the capacity-graded battery is first discharged at a constant current of 0.2C to 3.0V; then charged at a constant current and constant voltage of 3C to 4.25V, with a cutoff current of 1.8C; charged at a constant current and constant voltage of 3C to 4.4V, with a cutoff current of 1.5C; charged at a constant current and constant voltage of 1.5C to 4.5V, with a cutoff current of 0.05C; subsequently discharged at a constant current of 0.7C to 3.0V. A 5-minute interval is required between each charge / discharge cycle. This cycle is repeated 300 times. After this process, the 300-cycle capacity retention rate is calculated using the following formula:

[0116] 300-cycle capacity retention = (300-cycle discharge specific capacity / initial discharge specific capacity) × 100%.

[0117] Negative electrode sheet full charge expansion rate test: After the battery is divided into two cycles, it is discharged to 5mV again according to the above process to make the negative electrode sheet fully charged. Disassemble it in an inert atmosphere and measure the thickness of the negative electrode sheet with a multimeter. Negative electrode sheet full charge expansion rate = negative electrode sheet full charge thickness / negative electrode sheet initial thickness after pressing.

[0118] The test results are shown in Table 2.

[0119] Table 2. Performance Test Results

[0120]

[0121] As can be seen from the table, Comparative Example 1 omitted the first coating layer, and Comparative Example 2 omitted the second coating layer. Both showed a decrease in deformation recovery rate after load removal compared to Example 1, a significant decrease in cycle performance, and a higher expansion rate. Examples 2 and 3 adjusted the ratio of the first carbon source and solvent. While the cycle performance decreased to some extent compared to Example 1, the decrease was relatively small, and the fully charged expansion rate fluctuated. In Example 4, the second coating layer was prepared using an evaporation-drying method. It can be seen that the effect on the deformation recovery rate after load removal was minimal, and the cycle performance decreased slightly compared to Example 1. The first coating layer in Example 5 had a higher elastic modulus. Examples 6 and 7 showed a change in the thickness of the second coating layer by adjusting the amount of PVP, resulting in changes in cycle performance and fully charged expansion rate. In Examples 8 and 9, the elastic modulus of the second coating layer was lower or higher than that of Example 1, and the cycle performance and fully charged expansion rate also changed accordingly.

[0122] The embodiment adjusts the thickness of the first coating layer by controlling the mass ratio of asphalt to silica particles, and adjusts the thickness of the second coating layer by controlling the mass ratio of polyvinylpyrrolidone. Ultimately, the thickness H1, elastic modulus T1, thickness H2, and elastic modulus T2 of the first coating layer are adjusted to satisfy 5Z ≤ (H1×T1+H2×T2) / (H1+H2) ≤ 8Z, i.e., the value is between 15 and 24. This formula signifies that the sum of the elastic buffering capacity contributed by the first elastic carbon coating layer and the elastic buffering capacity contributed by the second porous carbon coating layer synergistically balances the self-expansion of the silica particles.

[0123] Furthermore, comparing Examples 1 and Examples 5-8, an excessively thick first carbon coating layer transforms into a rigid framework, losing its elastic characteristics and hindering lithium-ion transport; an excessively thin layer affects conductivity and reduces buffering capacity, weakening protection for the silicon core; an excessively thick second carbon coating layer leads to carbon framework aggregation, affecting the formation of porous structures, thereby reducing the specific surface area of ​​the material and impacting cycle life retention; an excessively thin layer makes the carbon framework prone to breakage, and the reduced carbon content also decreases conductivity. Comparing Examples 1 and Comparative Example 3, the relatively lower proportion of asphalt in Comparative Example 3 results in an excessively thin first coating layer, insufficient elastic buffering capacity, weakened protection for the silicon core, a significant decrease in cycle capacity retention, and a significant increase in expansion rate. Comparing Examples 1 and Comparative Example 4, the excessively high proportion of asphalt in Comparative Example 4 results in an excessively thick first coating layer that transforms into a rigid framework, losing its elastic characteristics and hindering lithium-ion transport; the deterioration in cycle capacity retention and expansion rate is even more severe than in Comparative Example 3. Comparing Example 1 and Comparative Example 5, the amount of PVP used in Comparative Example 5 was too high, which greatly increased the thickness of the second carbon coating layer. The polymerization of the carbon framework affected the formation of the porous structure, and the specific surface area of ​​the material was reduced, which had a significant impact on the cycle performance. Comparing Example 1 and Comparative Example 6, the amount of PVP used in Comparative Example 6 was too low, which made the second carbon coating layer too thin. The carbon framework structure was easy to break, and the reduced carbon content also reduced the conductivity. Therefore, the cycle capacity retention rate and expansion rate were far inferior to those of Example 1.

[0124] The present application has been described in detail above with reference to the embodiments. However, the present application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present application. Furthermore, unless otherwise specified, the embodiments and features in the embodiments of the present application can be combined with each other.

Claims

1. A silicon-carbon composite material, characterized in that, include: The carbon-silicon composite particles include silicon particles and a first coating layer covering the silicon particles, the first coating layer including elastic carbon, and the deformation recovery S of the carbon-silicon composite particles after removing the load is ≥80%. A second coating layer is applied to the carbon-silicon composite particles, and the second coating layer includes a porous carbon framework. The thickness H1 of the first coating layer is 20-40 nm, and the thickness H2 of the second coating layer is 100-200 nm; The elastic modulus of the first coating layer is T1, the elastic modulus of the second coating layer is T2, the self-expansion rate Z of the silicon particles is ≥300%, and the silicon-carbon composite material satisfies the following relationship: In the given formula, H1 and H2 are numerical values ​​in nm without measurement units, T1 and T2 are numerical values ​​in GPa without measurement units, and Z is the actual value of the percentage. The elastic modulus T1 of the first coating layer is 40-80 GPa, and the elastic modulus T2 of the second coating layer is 8-15 GPa; The preparation method of the silicon-carbon composite material includes the following steps: The first carbon source is coated on the surface of silicon particles and carbonized to obtain carbon-silicon composite particles. The carbon-silicon composite particles are mixed with a second carbon source and a pore-forming agent, freeze-dried, and calcined to form a second coating layer, thereby obtaining the silicon-carbon composite material. The carbonization temperature is 800–1000℃; The calcination temperature is 500–600°C.

2. The silicon-carbon composite material according to claim 1, characterized in that, The silicon particles are at least one of nano-silicon particles and micron-silicon particles.

3. The silicon-carbon composite material according to claim 1, characterized in that, The step of coating the first carbon source onto the surface of the silicon particles includes: The first carbon source is mixed with the first solvent to obtain a coating solution; the silicon particles are mixed and stirred with the coating solution, and the first solvent is allowed to evaporate, so that the first carbon source coats the surface of the silicon particles; The mass-to-volume ratio of the first carbon source to the first solvent is (0.1–0.5) g: 15 mL; The first solvent includes toluene.

4. The silicon-carbon composite material according to claim 1, characterized in that, The first carbon source is asphalt; The asphalt is selected from at least one of petroleum asphalt, coal tar pitch, and natural asphalt; The mass ratio of the first carbon source to the silicon particles is (1-9):3; The carbonization time is 15 minutes to 4 hours.

5. The silicon-carbon composite material according to claim 1, characterized in that, The mass ratio of the second carbon source to the pore-forming agent is 1:(0.1~0.5); The second carbon source includes at least one of polyvinylpyrrolidone, polyvinyl alcohol, sucrose, glucose, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyethylene glycol; The pore-forming agent includes at least one of NaCl, KCl, urea, and ammonium bicarbonate.

6. The silicon-carbon composite material according to claim 1, characterized in that, The freeze-drying temperature is -90 to -20°C; The freeze-drying process includes freezing at the stated temperature for 8–24 hours, followed by vacuum drying. The vacuum drying time is 24–72 h; the calcination time is 3–12 h.

7. A secondary battery, characterized in that, Including the silicon-carbon composite material according to any one of claims 1 to 6.

8. Electrical equipment, characterized in that, Includes the secondary battery as described in claim 7.

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